FOUNDRY

Permeability is a property of foundry sand with respect to how well the sand can vent, i.e. how well gases pass through the sand. And in other words, permeability is the property by which we can know the ability of material to transmit fluid/gases. The permeability is commonly tested to see if it is correct for the casting conditions.

Affecting factors

The grain size, shape and distribution of the foundry sand, the type and quantity of bonding materials, the density to which the sand is rammed, and the percentage of moisture used for tempering the sand are important factors in regulating the degree of permeability

Significance/Important are

 An increase in permeability usually indicates a more open structure in the rammed sand, and if the increase continues, it will lead to penetration-type defects and rough castings. A decrease in permeability indicates tighter packing and could lead to blows and pinholes.

Testing procedure

The permeability number, which has no units, is determined by the rate of flow of air, under standard pressure, through a rammed cylindrical specimen. DIN standards define the specimen dimensions to be 50 mm in diameter and 50 mm tall,^[2] while the American Foundry Society defines it to be two inches in diameter and two inches tall.^[3] rammed cylindrical specimen.

METAL CASTING

Advantages

The metal casting process is extensively used in manufacturing because of its many

Advantages.

1. Molten material can flow into very small sections so that intricate shapes can be made by this process. As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated.
2. It is possible to cast practically any material that is fer rous or non-ferrous.
3. As the metal can be placed exactly where it is required, large saving in weight can be achieved.

4. The necessary tools required for casting molds are very simple and inexpensive. As a result, for production of a small lot , it is the ideal process.

5. There are certain parts made from metals and alloys that can only be processed this way.

6. Size and weight of the product is not a limitation for the casting process.

Limitations

1. Dimensional accuracy and surface finish of the castings made by sand casting processes are a limitation to this technique. Many new casting processes have been developed which can take into consideration the aspects of dimensional accuracy and surface finish. Some of these processes are die casting process, investment casting process, vacuum-sealed molding process, and shell molding process.

2. The metal casting process is a labor intensive process History Casting technology, according to biblical l records, reaches back almost 5,000 years BC.Gold, pure in nature, most likely caught Prehistoric man's fancy. as he probably hammered gold ornaments out of the gold nuggets he found. Silver would have been treated similarly. Mankind next found copper, b because it appeared in the ash of his camp fires from copper-bearing ore that he lined his fire pits with. Man soon found that copper was harder than gold or silver. Copper did not bend up when used. So copper, found antic' in man's early tools, and then marched its way into Weaponry. But, long before all this. Man found clay. So he made pottery - something to eat from. Then he thought, "Now. What else can I do with this mud." . Early man thought about it, "they used this pottery stuff, (the first pattern s), to shape metal into bowls ".

LECTURE: 1

Casting Terms

1. Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed. Depending upon the position of the flask in the  molding structure, it is  referred to by various names such as drag - lower molding flask, cope - upper molding flask, cheek - intermediate molding flask used in three piece molding.

2. Pattern: It is the replica of the final object to be made. The mold cavi ty is made with  the help of pattern.

3. Parting line: This is the dividing line between the two molding flasks that makes up the mold.

4. Molding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a mixture of silica sand, clay, and moisture in appropriate proportions.

5. Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to give a better surface finish to the castings.

6. Core: A separate part of the mold, made of sand and generally baked, which is used  to create openings and various shaped cavities in  the castings.

7. Pouring basin: A small funnel shaped cavity at the top of the mold into which the  molten metal is poured.

8. Sprue: The passage through which the molten metal, from the pouring basin, reaches  the mold cavity. In many cases it controls the flow of metal into the mold.

9. Runner: The channel through which the molten metal is carried from the sprue to the gate.
10. Gate: A channel through which the molten metal enters the mold cavity.
11. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force.

12. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as "feed head".

13. Vent: Small opening in the mold to facilitate escape of air and gases.  Figure 1 : Mold Section showing some casting terms

Steps in Making Sand Castings

There are six basic steps in making sand castings:

1. Pattern making
2. Core making
3. Mold

4. Melting and pouring

5. Cleaning

Pattern making

 The pattern is a physical model of the casting used to make the mold. The mold is made by packing some readily formed aggregate material, such as molding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become the casting. If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities.

Core making

Cores are forms, usually made of sand, which are placed into a mold cavity to form the interior surfaces of castings. Thus the void space between en the core and mold –cavity surface is what eventually becomes the casting.

Molding

Molding consists of all operations necessary to prepare a mold for receiving molten metal. Molding usually involves placing a molding aggregate around a pattern held with a supporting frame, withdrawing the pattern to leave the mold cavity, setting the cores in the mold cavity and finishing and closing the mold.

Melting and Pouring

The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the molds are filled.

Cleaning

Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. Burned-on sand and scale are removed to improve the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Inspection of the casting for defects and general quality is performed.

The pattern is the principal tool during the casting process. It is the replica of the object to be made by the casting process, with some modifications. The main modifications are the addition of pattern allowances, and the provision of core prints. If the ca sting is to be hollow, additional patterns called cores are used to create these cavities in the finished product. The quality of the casting produced depends upon the material of the pattern, its design, and construction. The costs of the pattern and the related equipment are reflected in the cost of the casting. The use of an expensive pattern is justified when the quantity of castings required is substantial.

Functions of the Pattern

1. A pattern prepares a mold cavity for the purpose of making a c asting.

2. A pattern may contain projections known as core prints if the casting requires a core and need to be made hollow.

3. Runner, gates, and risers used for feeding molten metal in the mold cavity may form a part of the pattern.

4. Patterns properly made and having finished and smooth surfaces reduce casting defects.

5. A properly constructed pattern minimizes the overall cost of the castings.

Pattern Material

Patterns may be constructed from the following materials. Each material has its own advantages, limitations, and field of application. Some materials used for making patterns are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins.

To be suitable for use, the pattern material should be:
1. Easily worked, shaped and joined
2. Light in weight
3. Strong, hard and durable
4. Resistant to wear and abrasion
5. Resistant to corrosion, and to chemical reactions
6. Dimensionally stable and unaffected by variations in temperature and humidity

7. Available at low cost

The usual pattern materials are wood, metal, and plastics. The most commonly used pattern material is wood, since it is readily available and of low weight. Also, it can be easily shaped and is relatively cheap. T he main disadvantage of wood is its absorption of moisture, which can cause distortion and dimensional changes. Hence, proper seasoning and upkeep of wood is almost a pre -requisite for large-scale use of wood as a pattern material.

Figure 2: A typical pattern attached with gating and risering system

Pattern Allowances

Pattern allowance is a vital feature as it affects the dimensional characteristics of the casting. Thus, when the pattern is produced, certain allowances must be given on the sizes specified in the finished component drawing so that a casting with the particular specification can be made. The selection of correct allowances greatly helps to reduce machining costs and avoid rejections. The allowances usually considered on patterns and core boxes are as follows:

1. Shrinkage or contraction allowance
2. Draft or taper allowance
3. Machining or finish allowance
4. Distortion or camber allowance

5. Rapping allowance

Solution 1

The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet  ( as per Table 1 )
For dimension 18 inch, allowance = 18X 0.125 / 12 = 0.1875 inch » 0.2 inch
For dimension 14 inch, allowance = 14X 0.125 / 12 = 0.146 inch » 0.15 inch
For dimension 8 inch, allowance = 8 X 0.125 / 12 = 0.0833 inch » 0. 09 inch
For dimension 6 inch, allowance = 6 X 0.125 / 12 = 0.0625 inch » 0. 07 inch
The pattern drawing with required dimension is shown below:

Lecture 4

Draft or Taper Allowance

By draft is meant the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold and without excessive rapping by the molder.Figure 3 (a) shows a pattern having no draft allowance being removed from the pattern. In this case, till the pattern is completely lifted out, its side s will remain in contact with the walls of the mold, thus tending to break it. Figure 3 (b) is an illustration of a pattern hav ing proper draft allowance. Here, the moment the pattern lifting commences, all of its surfaces are well away from the sand surface. Thus the pattern can be removed without damaging the mold cavity.

Figure 3 (a) Pattern Having No Draft on Vertical Edges

Figure 3 (b) Pattern Having Draft on Vertical Edges

Draft allowance varies with the complexity of the sand job. But in general inner details of the pattern require higher draft than outer surfaces. The amount of draft depends upon the length of the vertical side of the pattern to be extracted; the intricacy of the pattern; the method of molding; and pattern material.Table 2 provides a general guide lines for the draft allowance.

Machining or Finish Allowance

The finish and accuracy achieved in sand casting are generally poor and the refore when the casting is functionally required to be of good surface finish or dimensionally accurate, it is generally achieved by subsequent machining. Machining or finish allowances are therefore added in the pattern dimension. The amount of machining allowance to be provided for is affected by the method of molding and casting used viz. hand molding or machine molding, sand casting or metal mold casting. The amount of machining allowance is also affected by the size and shape of the casting; the castin g orientation; the metal; and the degree of accuracy and finish required. The machining allowances recommended for different metal is given inTable 3.

Table 3 : Machining Allowances of Various Metals

Exercise 2

The casting shown is to be made in cast iron using a wooden pattern. Assuming only machining allowance, calculate the dimension of the pattern. All Dimensions are in Inches

Solution 2

The machining allowance for cast iron for size, up to 12 inch is o.12 inch and from 12

inch to 20 inch is 0.20 inch ( (Table 3)
For dimension 18 inch, allowance = 0.20 inch
For dimension 14 inch, allowance = 0.20 inch

Distortion or Camber Allowance

Sometimes castings get distorted, during solidification, due to their typical shape. For example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract at the closed end caus ing the vertical legs to look slightly inclined. This can be prevented by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that the casting after distortion will have its sides vertical ( (Figure 4 ).

The distortion in casting may occur due to internal stresses. These internal stresses are caused on account of unequal cooling of different section o f the casting and hindered contraction.

Measure taken to prevent the distortion in casting include:

i.                    Modification of casting design

ii.                   Providing sufficient machining allowance to cover the distortion affect

iii.                 Providing suitable allowance on the pattern, called camber or distortion  allowance (inverse reflection)

Figure 4: Distortions in Casting

a. Lost Wax
b. Ceramics Shell Molding
c.Evaporative Pattern Casting
d. Vacuum Sealed Molding
e.Centrifugal Casting

Green Sand Molding

Green sand is the most diversified molding method used in metal casting operations. The process utilizes a mold made of compressed or compacted mois t sand. The term "green “denotes the presence of moisture in the molding sand. The mold material consists of silica sand mixed with a suitable bonding agent (usually clay) and moisture.

 Advantages

Most metals can be cast by this method.
Pattern costs and material costs are relatively low.
No Limitation with respect to size of casting and type of metal or alloy used

Disadvantages

Surface Finish of the castings obtained by this process is not good and machining is often required to achieve the finished pro duct.

Sand Mold Making Procedure

The procedure for making mold of a cast iron wheel is shown in (Figure 8 (a), (b), (c)).

  The first step in making mold is to place the pattern on the molding board.

 The drag is placed on the board ((Figure 8 (a) ).

 Dry facing sand is sprinkled over the board and pattern to provide a non sticky layer.

 Molding sand is then riddled in to cover the pattern with the fingers; then the drag  is completely filled.

 The sand is then firmly packed in the drag by means of han d rammers. The ramming must be proper i.e. it must neither be too hard or soft.

 After the ramming is over, the excess sand is leveled off with a straight bar known as a strike rod.

 With the help of vent rod, vent holes are made in the drag to the full de pth of the flask as well as to the pattern to facilitate the removal of gases during pouring and solidification. The finished drag flask is now rolled over to the bottom board exposing the pattern.

 Cope half of the pattern is then placed over the drag pa ttern with the help of locating pins. The cope flask on the drag is located aligning again with the help ofpins ( (Figure 8 (b)).

 The dry parting sand is sprinkled all over the drag and on the pattern.

 A sprue pin for making the sprue passage is located at a small distance from the  pattern. Also, riser pin, if required, is placed at an appropriate place.

 The operation of filling, ramming and venting of the cope proceed in the same  manner as performed in the drag.

 The sprue and riser pins are removed first and a pouring basin is scooped out at  the top to pour the liquid metal.

 Then pattern from the cope and drag is removed an d facing sand in the form of paste is applied all over the mold cavity and runners which would give the finished casting a good surface finish.

 The mold is now assembled. The mold now is ready for pouring (see ((Figure 8 (c) )

Figure 8 (a)

Distortion or Camber Allowance

Sometimes castings get distorted, during solidification, due to their typical shape. Forexample, if the casting has the form of the letter U, V, T, or L etc. it will tend to contractat the closed end caus ing the vertical legs to look slightly inclined. This can be prevented by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that the casting after distortion will have its sides vertical ( (Figure 4).

The distortion in casting may occur due to internal stresses. These internal stresses are caused on account of unequal cooling of different section o f the casting and hindered contraction.

Measure taken to prevent the distortion in casting includes:

i.                    Modification of casting design .

ii.                  Providing sufficient machining allowance to cover the distortion affect .

iii.                Providing suitable allowance on the pattern, called camber or distortion allowance (inverse reflection).

Figure 4: Distortions in Casting

Rapping Allowance

Before the withdrawal from the sand mold, the pattern is rapped all around the vertical
faces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges the
final casting made, it is de sirable that the original pattern dimension should be reduced to
account for this increase. There is no sure way of quantifying this allowance, since it is
highly dependent on the foundry personnel practice involved. It is a negative allowance
and is to be applied only to those dimensions that are parallel to the parting plane.

Core and Core Prints

Castings are often required to have holes, recesses, etc. of various sizes and shapes. These
impressions can be obtained by using cores. So where coring is requ ired, provision
should be made to support the core inside the mold cavity. Core prints are used to serve this purpose. The core print is an added projection on the pattern and it forms a seat in the mold on which the sand core rests during pouring of the m old. The core print must be of adequate size and shape so that it can support the weight of the core during the casting operation. Depending upon the requirement a core can be placed horizontal, vertical and can be hanged inside the mold cavity. A typical job, its pattern and the mold cavity with
core and core print is shown inFigure 5.

Rapping Allowance

Before the withdrawal from the sand mold, the pattern is rapped all around the vertical faces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges the final casting made, it is de sirable that the original pattern dimension should be reduced to account for this increase. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel practice involved. It is a negative allowance
and is to be applied only to those dimensions that are parallel to the parting plane.

Core and Core Prints  

Castings are often required to have holes, recesses, etc. of various sizes and shapes. These impressions can be obtained by using cores. So where coring is requ ired, provision should be made to support the core inside the mold cavity. Core prints are used to serve this purpose. The core print is an added projection on the pattern and it forms a seat in the mold on which the sand core rests during pouring of the m old. The core print must be of adequate size and shape so that it can support the weight of the core during the casting operation. Depending upon the requirement a core can be placed horizontal, vertical and can be hanged inside the mold cavity. A typical job, its pattern and the mold cavity with core and core print is shown inFigure 5.

Figure 5: A Typical Job, its Pattern and the Mold Cavity

Lecture 5

Types of Pattern

Patterns are of various types, each satisfying certain c asting requirements.

1. Single piece pattern
2. Split or two piece pattern
3. Match plate pattern

Single Piece Pattern

The one piece or single pattern is the most inexpensive of all types of patterns. This type of pattern is used only in cases where the job is very simple and does not create any withdrawal problems. It is also used for application in very small -scale production or in prototype development. This type of pattern is expected to be entirely in the drag and one of the surface is is expected to be flat which is used as the parting plane. A gating system is made in the mold by cut ting sand with the help of sand tools. If no such flat surface exists, the molding becomes complicated. A typical one -piece pattern is shown inFigure 6.

Figure 6: A Typical One Piece Pattern

Split or Two Piece Pattern

Split or two piece pattern is most widely used type of pattern for intricate castings. It is split along the parting surface, the position of which is determined by the shape of the casting. One half of the pattern is molded in drag and the other half in cope. T he two halves of the pattern must be aligned properly by making use of the dowel pins, which are fitted, to the cope half of the pattern. These dowel pins match with the precisely made holes in the drag half of the pattern

Dry Strength

When the molten metal is poured in the mold, the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand e evaporates due to the heat of the molten metal. At this stage the molding sand must posses the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material al.

Hot Strength

As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength.

Collapsibility

The molding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings. Besides these specific properties the molding material should be cheap, reusable and should have good thermal conductivity.

Molding Sand Composition .The main ingredients of any molding sand are:

1. Base sand,

2. Binder, and

3. Moisture

Base Sand

Silica sand is most commonly used base sand. Other base sand s that are also used for making mold are zircon sand, Chromite sand, and olivine sand. Silica sand is cheapest among all types of base sand and it is easily available.

 Binder :Binders are of many types such as:

 1. Clay binders,

2. Organic binders and

3. Inorganic binders

Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength. The most popular clay types are: Kaolinite or fire clay (Al2O3 2 SiO2 2 H2O) and Bentonite (Al2O3 4 SiO2 nH2O) Of the two the Bentonite can absorb more water which increases its bonding power.

Moisture

Clay acquires its bonding action only in the presence of the required amount of moisture. When water is added to clay, it penetrates the mixture and forms a microfilm, which coats the surface of each flake of the clay. The amount of water used should be properly controlled. This is because a part of the water, which coats the surface of the clay flakes, helps in bonding, while the remainder helps in improving the plasticity. A typical composition of molding sand is given in (Table 4).

Table 4

  ¡69.  ¡Shrinkage defects

 Shrinkage defects occur when feed metal is not available to compensate for shrinkage as the metal solidifies. Shrinkage defects can be split into two different types: open shrinkage defects and closed shrinkage defects. Open shrinkage defects are open to the atmosphere, therefore as the shrinkage cavity forms air compensates. There are two types of open air defects: pipes and caved surfaces. Pipes form at the surface of the casting and burrow into the casting, while caved surfaces are shallow cavities that form across the surface of ¡the casting. Closed shrinkage defects, also known as shrinkage porosity, are defects that form within the casting. Isolated pools of liquid form inside solidified metal, which are called hot spots. The shrinkage defect usually forms at the top of the hot spots. They require a nucleation point, so impurities and dissolved gas can induce closed shrinkage defects. The defects are broken up into macro porosity and micro porosity (or micro shrinkage), where macro porosity can be seen by the naked eye and micro porosity cannot.[4][5]

  ¡70.  ¡Gas porosity

Gas porosity is the formation of bubbles within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it ¡cools. Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal,[which reduces strength in that ¡vicinity. Nitrogen, oxygen and hydrogen are the most encountered gases in ¡cases of gas porosity. In aluminum castings, hydrogen is the only gas that dissolves in significant quantity, which can result in hydrogen gas porosity.

  ¡71.  ¡Pouring Metal defects

Pouring metal defects include misruns, cold shuts, and inclusions.  AØ misrun occurs when the liquid metal does not completely fill the mold ¡cavity, leaving an unfilled portion. Cold shuts occur when two fronts of liquid metal do not fuse properly in the mold cavity, ¡leaving a weak spot. Both are caused by either a lack of fluidity in the molten metal or ¡cross-sections that are too narrow. The fluidity can be increased by changing the chemical composition of the metal or by increasing the pouring temperature. Another possible cause is back pressure from improperly vented mold cavities

  72. ¡ ¡Metallurgical defects There are two defects in this category: hot tears and hot spots. Hot tears, also known as hot cracking, are failures in the casting that occur as the casting cools. This happens because the metal is weak when it is hot and the residual stresses in the material can cause the casting to fail as it cools. ¡Proper mold design prevents this type of defect. Hot spots are areas on the surface of casting that become very hard because they cooled more quickly than the surrounding material. This type of defect can be avoided by proper cooling practices or by changing the chemical composition of the metal.

Lecture 14

Casting Defects (Figure19)

The following are the major defects, which are likely to occur in sand castings

1. Gas defects

2. Shrinkage cavities

3. Molding material defects

4. Pouring metal defects

5. Mold shift

Gas Defects

A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal.

Causes Gas Defects

The lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability of the mold or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold.

1. Metal contains gas

2. Mold is too hot

3.Poor mold burnout

Shrinkage Cavities

These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick Sprues to avoid shrinkage cavities.

Molding Material Defect s

The defects in this category are cuts and washes, metal penetration, fusion, and swell.

 Cut and washes: These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the molding sand not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of molding sand and the latter can be overcome by the proper design of the gating system.

Metal penetration :When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration.

Fusion: This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.

Swell: Under the influence of metallostatic forces, the mold wall may move back causing a swell in the dimension of the casting. A proper ramming of the mold will correct this defect.

Inclusions

Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mold can prevent this defect.

Pouring Metal Defects: The likely defects in this category are

1. Mis-runs and 2. Cold shuts.

A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis -run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design.

A cold shut is caused when two streams while meeting in the mold cavity, do not fuse together properly thus forming a discontinuity in the casting. When the

 : A Typical Composition of Molding Sand

Molding Sand Constituent

Weight Percent

Silica sand 92

Clay (Sodium Bentonite)

8 Water 4

Lecture 7

Dry Sand Molding

When it is desired that the gas forming materials are lowered in the molds, air –dried molds are sometimes preferred to green sand molds. Two types of drying of molds are often required.

1. Skin drying and

2. Complete mold drying.

In skin drying a firm mold face is produced. Shakeout of the mold is almost as good as that obtained with green sand molding. The most common method of drying therefractory mold co ating uses hot air, gas or oil flame. Skin drying of the mold can be accomplished with the aid of torches, directed at the mold surface.

Shell Molding Process

It is a process in which, the sand mixed with a thermosetting resin is allowed to come in con tact with a heated pattern plate (200oC), this causes a skin (Shell) of about 3.5 mm of sand/plastic mixture to adhere to the pattern.. Then the shell is removed from the pattern. The cope and drag shells are kept in a flask with necessary backup material and the molten metal is poured into the mold.

This process can produce complex parts with good surface finish 1.25 µ m to 3.75 µ m, and dimensional tolerance of 0.5 %. A good surface finish and good size tolerance reduce the need for machining. The process overall is quite cost effective due to reduced machining and cleanup costs. The materials that can be used with this process are cast irons, and aluminum and copper alloys.

Molding Sand in Shell Molding Process

The molding sand is a mixture of fine grained quartz sand and powdered Bakelite. There are two methods of coating the sand grains with Bakelite. First method is Cold coating method and another one is the hot method of coating.

In the method of cold coating, quartz sand is poured into the mixer and then the solution of powdered bakelite in acetone and ethyl aldehyde are added. The typical mixtu re is 92% quartz sand, 5% bakelite, 3% ethyl aldehyde. During mixing of the ingredients, the resin envelops the sand grains and the solvent evaporates, leaving a thin film that uniformly coats the surface of sand grains, thereby imparting fluidity to the s and mixtures.

In the method of hot coating, the mixture is heated to 150 -180 o C prior to loading the sand. In the course of sand mixing, the soluble phenol formaldehyde resin is added. The mixer is allowed to cool up to 80 - 90 o C. This method gives bet ter properties to the mixtures than cold method.

Sodium Silicate Molding Process

In this process, the refractory material is coated with a sodium silicate -based binder. For molds, the sand mixture can be compacted manually, jolted or squeezed around the pattern in the flask. After compaction, CO 2 gas is passed through the core or mold. The CO 2 chemically reacts with the sodium silicate to cure, or harden, the binder. This cured binder then holds the refractory in place around the pattern. After curing, the pattern is withdrawn from the mold.

The sodium silicate process is one of the most environmentally acceptable of the chemical processes available. The major disadvantage of the process is that the binder is very hygroscopic and readily absorbs water, which causes a porosity in the castings.. Also, because the binder creates such a hard, rigid mold wall, shakeout and collapsibility characteristics can slow down production. Some of the advantages of the process are:

•A hard, rigid core and mold are ty pical of the process, which gives the casting good dimensional tolerances;

•good casting surface finishes are readily obtainable;

Permanent Mold Process  

In al the above processes, a mold need to be prepared for each of the casting produced. For large-scale production, making a mold, for every casting to be produced, may be difficult and expensive. Therefore, a permanent mold, called the die may be made from which a large number of castings can be produced. , the molds are usually made of cast iron or steel, although graphite, copper and aluminum have been used as mold materials.
The process in which we use a die to make the castings is called permanent mold casting or gravity die casting, since the metal enters the mold under gravity. Some time in die - casting we inject the molten metal with a high pressure. When we apply pressure in injecting the metal it is called pressure die casting process.

Advantages

•Permanent Molding produces a sound dense casting with superior mechanical properties.

•the castings produced are quite uniform in shape have a higher degree of dimensional accuracy than castings produced in sand

•The permanent mold process is also capable of producing a consistent quality of finish on castings

Disadvantages

•The cost of tooling is usually higher than for sand castings

•The process is generally limited to the production of small castings of simple exterior design, although complex castings such as aluminum engine blocks and heads are now commonplace.

Centrifugal Casting

In this process, the mold is rotated rapidly about its central axis as the metal is pouredinto it. Because of the centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The slag, oxides and other inclusions being l ighter, get separated from the metal and segregate towards the center. This process is normally used for the making of hollow pipes, tubes, hollow bushes, etc., which are ax symmetric with a concentric hole. Since the metal is always pushed outward because of the centrifugal force, no core needs to be used for making the concentric hole. The mold can be rotated about a vertical, horizontal or an inclined axis or about its horizontal and vertical axes simultaneously.
The length and outside diameter are fixed by the mold cavity dimensions while the inside diameter is determined by the amount of molten metal poured into the mold.Figure 9(Vertical Centrifugal Casting), Figure 10 ( Horizontal Centrifugal Casting)

Advantages

•Formation of hollow inte riors in cylinders without cores

•Less material required for gate

•Fine grained structure at the outer surface of the casting free of gas and shrinkage cavities and porosity

Disadvantages

•More segregation of alloy component during pouring under the forces of rotation

•Contamination of internal surface of castings with non -metallic inclusions

•Inaccurate internal diameter

Lecture 8

Investment Casting Process

The root of the investment casting process, the cire perdue or "lost wax" method dates back to at least the fourth millennium B.C. The artists and sculptors of ancient Egypt and Mesopotamia used the rudiments of the investment casting process to create intricately detailed jewelry, pectorals and idols. The investment casting process al os called lost wax process begins with the production of wax replicas or patterns of the desired shape of the castings. A pattern is needed for every casting to be produced. The patterns are prepared by injecting wax or polystyrene in a metal dies. A numbe r of patterns are attached to a central wax sprue to form a assembly. The mold is prepared by surrounding the pattern with refractory slurry that can set at room temperature. The mold is then heated so that pattern melts and flows out, leaving a clean cavi ty behind. The mould is further hardened by heating and the molten metal is poured while it is still hot. When the casting is solidified, the mold is broken and the casting taken out.

The basic steps of the investment casting process are (Figure 11 see below ) :

1. Production of heat-disposable wax, plastic, or polystyrene patterns
2. Assembly of these patterns onto a gating system
3. "Investing," or covering the pattern assembly with refractory slurry
4. Melting the pattern assembly to remove the pattern material
5. Firing the mold to remove the last traces of the pattern material

6. Pouring

7. Knockout, cutoff and finishing.

Advantages

•Formation of hollow interiors in cylinders without cores

•Less material required for gate

•Fine grained structure at the outer surface of the casting free of gas and shrinkage cavities and porosity

Disadvantages

•More segregation of alloy comp onent during pouring under the forces of rotation

•Contamination of internal surface of castings with non -metallic inclusions

•Inaccurate internal diameter

Ceramic Shell Investment Casting Process

The basic difference in investment casting is that in the investment casting the wax pattern is immersed in a refractory aggregate before dewaxing whereas, in ceramic shellinvestment casting a ceramic shell is built around a tree assembly by repeatedly dipping apattern into a slurry (refractory material such as zircon with binder). After each dipping and stuccoing is completed, the assembly is allowed to thoroughly dry before the next coating is applied. Thus, a shell is built up around the assembly. The thickness of this   shell is dependent on the size of the castings and temperature of the metal to be poured.

After the ceramic shell is completed, the entire assembly is placed into an autoclave or flash fire furnace at a high temperature. The shell is heated to about 982 o C to burn out any residual wax and to develop a high -temperature bond in the shell. The shell molds can then be stored for future use or molten metal can be poured into them immediately. If the shell molds are stored, they have to be preheated before molten metal is poured into
them.

Advantages

•excellent surface finish
•tight dimensional tolerances
•machining can be reduced or completely eliminated

Lecture 9

Full Mold Process / Lost Foam Process / Evaporative Pattern Casting Process

The use of foam patterns for metal casting w as patented by H.F. Shroyer on April 15, 1958. In Shroyer's patent, a pattern was machined from a block of expanded polystyrene (EPS) and supported by bonded sand during pouring. This process is known as the full mold process. With the full mold process, t he pattern is usually machined from an EPS block and is used to make primarily large, one -of-a kind castings. The full mold process was originally known as the lost foam process. However, current patents have required that the generic term for the process be full mold.

In 1964, M.C. Flemmings used unbounded sand with the process. This is known today as lost foam casting (LFC). With LFC, the foam pattern is molded from polystyrene beads. LFC is differentiated from full mold by the use of unbounded sand (LFC ) as opposed to bonded sand (full mold process).  Foam casting techniques have been referred to by a variety of generic and proprietary names. Among these are lost foam, evaporative pattern casting, cavity less casting, evaporative foam casting, and full m old casting.

In this method, the pattern, complete with gates and risers, is prepared from expanded polystyrene. This pattern is embedded in a no bake type of sand. While the pattern is inside the mold, molten metal is poured through the sprue. The heat o f the metal is sufficient to gasify the pattern and progressive displacement of pattern material by the molten metal takes place.

The EPC process is an economical method for producing complex, close –tolerance castings using an expandable polystyrene patte rn and unbonded sand. Expandable polystyrene is a thermoplastic material that can be molded into a variety of complex, rigid shapes. The EPC process involves attaching expandable polystyrene patterns to an
expandable polystyrene gating system and applying a refractory coating to the entire assembly. After the coating has dried, the foam pattern assembly is positioned on loose dry sand in a vented flask. Additional sand is then added while the flask is vibrated until the pattern assembly is completely embedd ed in sand. Molten metal is poured into the sprue, vaporizing the foam polystyrene, perfectly reproducing the pattern.

In this process, a pattern refers to the expandable polystyrene or foamed polystyrene part  that is vaporized by the molten metal. A patt ern is required for each casting.  Process Description ((Figure 12  )

1. The EPC procedure starts with the pre -expansion of beads, usually polystyrene. After the pre-expanded beads are stabilized, they are blown into a mold to form pattern sections. When the beads are in the mold, a steam cycle causes them to fully expand and fuse together.

2. The pattern sections are assembled with glue, forming a cluster. The gating  system is also attached in a similar manner.

3. The foam cluster is covered with a cer amic coating. The coating forms a barrier  so that the molten metal does not penetrate or cause sand erosion during pouring.

4. After the coating dries, the cluster is placed into a flask and backed up with bonded sand.

5. Mold compaction is then achieved by using a vibration table to ensure uniform and proper compaction. Once this procedure is complete, the cluster is packed in the flask and the mold is ready to be poured .

Figure 12: The Basic Steps of the Evaporative Pattern Casting Process

Advantages

The most important advantage of EPC process is that no cores are required. No binders or other additives are required for the sand, which is reusable. Shakeout of the castings in unbonded sand is simplified. There are no parting lines or core fins.

Lecture 10

Vacuum Sealed Molding Process

It is a process of making molds utilizing dry sand, plastic film and a physical means of binding using negative pressure or vacuum. V -process was developed in Japan in 1971. Since then it has gained considerable importanc e due to its capability to produce dimensionally accurate and smooth castings. The basic difference between the V –process and other sand molding processes is the manner in which sand is bounded to form the mold cavity. In V  process vacuum, of the order of 250 - 450 mm Hg, is imposed to bind the dry free flowing sand encapsulated in between two plastic films. The technique involves the formation of a mold cavity by vacuum forming of a plastic film over the pattern, backed by unbounded sand, which is compacte d by vibration and held rigidly in place by applying vacuum. When the metal is poured into the molds, the plastic film first melts and then gets sucked just inside the sand voids due to imposed vacuum where it condenses and forms a shell -like layer. The vacuum must be maintained until the metal solidifies, after which the vacuum is released allowing the sand to drop away leaving a casting with a smooth surface. No shakeout equipment is required and the same sand can be cooled and reused without further trea tment.

Sequence of Producing V -Process Molds

•The Pattern is set on the Pattern Plate of Pattern Box. The Pattern as well as the Pattern Plate has Numerous Small Holes. These Holes Help the Plastic Film to Adhere Closely   on Pattern When Vacuum is Applied.

•A Heater is used to Soften the Plastic Film

•The Softened Plastic Film Drapes over the Pattern. The Vacuum Suction Acts through the Vents (Pattern and Pattern Plate) to draw it so that it adheres closely to the Pattern.

•The Molding Box is Set on the Film Coated Pattern

•The Molding Box is filled with Dry Sand. Slight Vibration Compacts the Sand

•Level the Mold. Cover the Top of Molding Box with Plastic Film. Vacuum Suction Stiffens the Mold.

•Release the Vacuum on the Patter n Box and Mold Strips Easily.

•Cope and Drag are assembled and Metal is poured. During Pouring the Mold is Kept under Vacuum

•After Cooling, the Vacuum is released. Free Flowing Sand Drops Away, Leaving a

Clean Casting  Advantages

•Exceptionally Good Dimensional Accuracy

•Good Surface Finish

•Longer Pattern Life

•Consistent Reproducibility

•Low Cleaning / Finishing Cost

To view the sequence of producing V - Process Mode.

Lecture 12

Reverberatory furnace

A furnace or kiln in which the material under treatment is heated indirectly by means of a flame deflected downward from the roof. Reverberatory furnaces are used in opper, tin, and nickel production, in the production of certain concretes and cements, and in aluminum. Reverberatory furnaces heat the metal to melting temperatures with direct fired wall -mounted burners. The primary mode of heat transfer is through radia tion from the refractory brick walls to the metal, but

convective heat transfer also provides additional heating from the burner to themetal. The advantages provided by reverberatory melters is the high volume  processing rate, and low operating and mainte nance costs. The disadvantages of the reverberatory melters are the high metal oxidation rates, low efficiencies, and large floor

space requirements. A schematic of Reverberatory furnace is shown
inFigure 15 See Below

Induction furnace

Induction heating is a heating method. The heating by the induction method occurs when an electrically conductive material is placed in a varying magnetic field. Induction heating is a rapid form of heating in which a current is induced directly into the part being heated. Induction heating is a non -contact form of heating.

The heating system in an induction furnace includes:

1. Induction heating power supply,

2. Induction heating coil,

3. Water-cooling source, which cools the coil and several internal components inside the power supply.

The induction heating power supply sends alternating current through the induction coil, which generates a magnetic field. Induction furnaces work on the principle of a transformer. An alternative electromagnetic field induces eddy currents in the metal which converts the electric energy to heat without any physical contact between the induction coil and the work piece. A schematic diagram of induction furnace is shown inFigure 16. The furnace conta ins a crucible surrounded by a water cooled copper coil. The coil is called primary coil to which a high frequency current is supplied. By induction secondary currents, called eddy currents are produced in the crucible. High temperature can bemobtained by this method. Induction furnaces are of two types: cored furnace andmcoreless furnace. Cored furnaces are used almost exclusively as holdingmfurnaces. In cored furnace the electromagnetic field heats the metal between twomcoils. Coreless furnaces heat the m etal via an external primary coil.

Figure 16: Schematic of a Induction Furnace

Advantages of Induction Furnace

 Induction heating is a clean form of heating

 High rate of melting or high melting efficiency

 Alloyed steels can be melted wit hout any loss of alloying elements

 Controllable and localized heating

Disadvantages of Induction Furnace

 High capital cost of the equipment

 High operating cost

Lecture 14

Casting Defects (Figure19)

The following are the major defects, which are likely to occur in sand castings

 Gas defects

 Shrinkage cavities

 Molding material defects

 Pouring metal defects

 Mold shift

Gas Defects

A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside
the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas,which gets entrapped during heating of molten metal.

Causes

The lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability of the mold or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold.

 Metal contains gas

 Mold is too hot

 Poor mold burnout

Shrinkage Cavities

These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick Sprues to avoid shrinkage cavities.

Molding Material Defect s

The defects in this category are cuts and washes, metal penetration, fusion, and swell.

Cut and washes

These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the molding sa nd not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of molding sand and the latter
can be overcome by the proper design of the gating system.

Metal penetration

When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration.

Fusion

This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.

Swell

Under the influence of metallostatic forces, the mold wall may move back causing a swell in the dimension of the casting. A proper ramming of the mold will correct this defect.

Inclusions

Particles of slag, refractory materials, sand or deoxidation prod ucts are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mold can prevent this defect.

Pouring Metal Defects

The likely defects in this category are

 Mis-runs and

 Cold shuts.

A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis -run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design.

A cold shut is caused when two streams while meeting in the mold cavity, do not  fuse together properly thus forming a discontinuity in the casting. When the

Molding Material and Properties

A large variety of molding materials is used in foundries for manufacturing molds and cores. They include molding sand, system sand or backing sand, facing sand, parting sand, and core sand. The choice of molding materials is based on their processing properties. The properties that are generally required in molding materials are:

Refractoriness: It is the ability of the molding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the silica sand is highest.

Permeability: During pouring and subsequent solidification of a casting, a large amount of gases and steam is generated. These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the molding and core sand. If these gases are not allowed to escape from the mold, they would be entrapped inside the casting and cause casting defects. To overcome this problem the molding material must be porous. Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity.

Green Strength: The molding sand that contains moisture is termed as green sand. The green sand particles must have the ability to cling to each other to impart sufficient strength to the mold. The green sand must have enough strength so that the constructed mold retains its shape.

Dry Strength: When the molten metal is poured in the mold, the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand evaporates due to the heat of the molten metal. At this stage the molding sand must posses the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material.

Hot Strength: As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength.

Collapsibility: The molding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings. Besides these specific properties the molding material should be cheap, reusable and should have good thermal conductivity.

Molding Sand Composition

The main ingredients of any molding sand are:

• Base sand,
• Binder, and
• Moisture

Base Sand

Silica sand is most commonly used base sand. Other base sands that are also used for making mold are zircon sand, Chromite sand, and olivine sand. Silica sand is cheapest among all types of base sand and it is easily available.

Binder

Binders are of many types such as:

1. Clay binders,
2. Organic binders and
3. Inorganic binders

Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength. The most popular clay types are:

Kaolinite or fire clay (Al₂O₃ 2 SiO₂ 2 H₂O) and Bentonite (Al₂O₃ 4 SiO₂ nH₂O)

Of the two the Bentonite can absorb more water which increases its bonding power.

Moisture

Clay acquires its bonding action only in the presence of the required amount of moisture. When water is added to clay, it penetrates the mixture and forms a microfilm, which coats the surface of each flake of the clay. The amount of water used should be properly controlled. This is because a part of the water, which coats the surface of the clay flakes, helps in bonding, while the remainder helps in improving the plasticity. A typical composition of molding sand is given in

Casting defects

Defects may occur due to one or more of the following reasons:

–Fault in design of casting pattern

–Fault in design on mold and core

–Fault in design of gating system and riser

–Improper choice of molding sand

–Improper metal composition

–Inadequate melting temperature and rate of pouring

Some common defects in castings:

a) Misruns b) Cold Shut c) Cold Shot d) Shrinkage Cavity e) Micro porosity

 f) Hot Tearing Misruns:

A) Misruns

It is a casting that has solidified before completely filling the mold cavity.

Typical causes include

1) Fluidity of the molten metal is insufficient,

2) Pouring Temperature is too low,

3) Pouring is done too slowly and/or

4) Cross section of the mold cavity is too thin.

b) Cold Shut

A cold shut occurs when two portion of the metal flow together, but there is lack of fusion between them due to premature freezing, Its causes are similar to those of a Misruns.

c) Cold Shots

When splattering occurs during pouring, solid globules of the metal are formed that become entrapped in the casting. Poring procedures and gating system designs that avoid splattering can prevent these defects.

d) Shrinkage Cavity

This defect is a depression in the surface or an internal void in the casting caused by solidification shrinkage that restricts the amount of the molten metal available in the last region to freeze.

e) Micro porosity

This refers to a network of a small voids distributed throughout the casting caused by localized solidification shrinkage of the final molten metal in the dendritic structure.

f) Hot Tearing

This defect, also called hot cracking, occurs when the casting is restrained or early stages of cooling after solidification.

Sand Casting is simply melting the metal and pouring it into a preformed cavity, called mold, allowing (the metal to solidify and then breaking up the mold to remove casting. In sand casting expandable molds are used. So for each casting operation you have to form a new mold.

• Most widely used casting process.

• Parts ranging in size from small to very large

• Production quantities from one to millions

• Sand mold is used.

• Patterns and Cores

–Solid, Split, Match-plate and Cope-and-drag

 Patterns

–Cores

–achieve the internal surface of the part

Molds

–Sand with a mixture of water and bonding clay

–Typical mix: 90% sand, 3% water, and 7% clay

–to enhance strength and/or permeability Sand

–Refractory for high temperature

Size and shape of sand

Small grain size -> better surface finish

Large grain size -> to allow escape of gases during pouring

Irregular grain shapes -> strengthen molds due to interlocking but to reduce permeability

Types of sand

a) Green-sand molds - mixture of sand, clay, and water; “Green" means mold contains moisture at time of pouring.

b) Dry-sand mold - organic binders rather than clay and mold is baked to improve strength

c) Skin-dried mold - drying mold cavity surface of a green-sand –mold to a depth of 10 to 25 mm, using torches or heating.

Steps in Sand Casting

The cavity in the sand mold is formed by packing sand around a pattern, separating the mold into two halves

The mold must also contain gating and riser system, For internal cavity, a core must be included in mold.

A new sand mold must be made for each part .

1. Pour molten metal into sand mold.

2. Allow metal to solidify.

3. Break up the mold to remove casting.

4. Clean and inspect casting.

5. Heat treatment of casting is sometimes required to improve metallurgical properties

 

Module-I

Lecture Notes of Chinmay Das

It is the task of casting designer to reduce all hot spots so that no shrinkage cavities occurred. Since solidification of the casting occurs by loosing heat from the surfaces and the amount of the heat is given by the volume of the casting, the cooling characteristics of a casting can be represented by the surface area to the volume ratio. Since the riser is almost similar to the casting in its solidificationbehaviour, the riser characteristics can also be specified by the ratio of its surface area to volume. If this ratio of casting is higher, then it is expected to cool faster. According to Chvorinov, solidification time can be calculated as

ts=K {SAV } 2

Where ts= solidification time

 sV = volume of the casting,

SA = surface area

K = mould constant which depends on pouring temperature, casting & mould thermalCharacteristicsThe freezing ratio,

Xof a mould is defined as the ratio of cooling characteristics of casting to that of theriser.

X=VriserSAriser / Vcasting / SAcasting

 In order to feed the casting, the riser should solidify last and hence its freezing ratio should be greater thanunity.

CAINE’s Method

X = { a / Y-b} + c

Where Y = riser volume / casting volume a, b, c are constants whose values for different materials are given here.

Material a b c

Steel 0.10 0.03 1.00Aluminium 0.10 0.06 1.08Cast iron, Brass 0.04 0.017 1.00Grey cast iron 0.33 0.030 1.00Aluminium bronze 0.24 0.017 1.00Silicon bronze 0.24 0.017 1.00Table IV: Values of a,b,c for different materialsDesign Requirements of Risers1. Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surface area)

2

 of the riser must be greater than that of the casting. However, when this condition does not meet, the metalin the riser can be kept in liquid state by heating it externally or using exothermic materials in the risers.2. Riser placement: the spacing of risers in the casting must be considered by effectivelycalculating the feeding distance of the risers.3. Riser shape: cylindrical risers are recommended for most of the castings as spherical risers,although considers as best, are difficult to cast. To increase volume/surface area ratio the bottom of the risercan be shaped as hemispher

 

Module-I

Lecture Notes of Chinmay Das

Runner Extension:

Normally the metal which moves first into the gating system is likely to contain slag and dross which should not be allowed to get into the mould cavity. This could be achieved by extending the runner beyond the ingates so that the momentum of the metal will carry it past the gates and to a blind alley called runner extension. A runner extension having a minimum of twice the runner width is desirable.

Whirl Gate: Another method employed successfully to trap the slag from entering steel casting is a whirl gate. This utilizes the principle of centrifugal action to throw the dense metal to the periphery and retain the lighter slag at the centre. In order to achieve this action, it is necessary that entry area should be at least 1.5times the exit area so that the metal is built up at the centre quickly. Also the metal should revolve 2700 before reaching the exit gate so as to gain enough time for separating the impurities.

Figure 11: Whirl gate

Design of Riser:

The function of a riser (also called reservoir, feeders, or headers) is to feed the casting during solidification so that no shrinkage cavities are formed. The requirement of risers depends to a great extent upon the type of metal poured and the complexity of the casting. Let us consider the mould of a cube which is filled with liquid metal. As time progresses, the metal starts losing heat through all sides and as a result starts freezing from all sides equally trapping the liquid metal inside. But further solidification and subsequent volumetric shrinkage and the metal contraction due to change in temperature causes formation of a void. The solidification when complete, finally results in the shrinkage cavity as shown in the figure. The reason for the formation of the void in the cube casting is that the liquid metal in the centre which solidifies in the end is not fed during the solidification; hence the liquid shrinkage ends up as a void. Such isolated spots which remain hot till the end are called hot spots.

.Figure 12: Solidification of cube casting

Functions of Risers

•Provide extra metal to compensate for the volumetric shrinkage

•Allow mold gases to escape

•Provide extra metal pressure on the solidifying metal to reproduce mold details more exactly.

•To compensate mould expansion during pouring of hot liquid metal because of soft mould

 

Module-I

Lecture Notes of Chinmay Das

GATING SYSTEM DESIGN

Figure 1: Gating systems

 

Module-I

Lecture Notes of Chinmay Das

Elements of Gating System

The gating systems refer to all those elements which are connected with the flow of molten metal from the ladle to the mould cavity. The elements of gating systems are

•Pouring Basin

•Sprue

•Sprue Base Well

•Runner

•Runner Extension

•Ingate

RiserFigure 2: Components of a gating system

Any gating system designed should aim at providing a defect free casting.

This can be achieved by considering following requirements.

•The mould should be completely filled in the smallest possible time without having to raise neither metal temperature nor use of higher metal heads.

•The metal should flow smoothly into the mould without any turbulence. A turbulence metal flow tends to form dross in the mould.

•Unwanted materials such as slag, dross and other mould materials should not be allowed to enter the mould cavity.

•The metal entry into the mould cavity should be properly controlled in such a way that aspiration of the atmospheric air is prevented.

•A proper thermal gradient should be maintained so that the casting is cooled without any shrinkage cavities or distortions.

•Metal flow should be maintained in such a way that no gating or mould erosion takes place.

•The gating system should ensure that enough molten metal reaches the mould cavity.

•It should be economical and easy to implement and remove after casting solidification.

•The casting yield should be maximized.

 

Module-I

Lecture Notes of Chinmay Das

The liquid metal that runs through the various channels in the mould obeys the Bernoulli’s theorem which states that the total energy head remains constant at any section. Ignoring frictional losses, we have

Where

h = Potential Head,

mP = Static Pressure,

Pav = Liquid Velocity, m / s

ρg = w = Specific weight of liquid, N / m2

 g = Acceleration due to gravity, m / s2

 Though quantitatively

Bernoulli’s theorem may not be applied; it helps to understand qualitatively, the metal flow in the sand mould. As the metal enters the pouring basin, it has the highest potential energy with no kinetic or pressure energies. But as the metal moves through the gating system, aloss of energy occurs because of the friction between the molten metal and the mould walls. Heat is continuously lost through the mould material though it is not represented in the Bernoulli’s equation.

Another law of fluid mechanics, which is useful in understanding the gating system behavior, is the law of continuity which says that the volume of metal flowing at any section in the mould is constant. The same in equation form is Q = A1V1

= A

2V2 

Where Q = Rate of flow, m3/ s

A = Area of cross section, m2

 V = Velocity of metal flow, m / s

Pouring Time

The main objective for the gating system design is to fill the mould in the smallest time. The time for complete filling of a mould is called pouring time. Too long a pouring time requires a higher pouring temperature and too less a pouring time means turbulent flow in the mould which makes the casting defect prone. The pouring time depends on the casting materials, complexity of the casting, section thickness and casting size. Steels lose heat very fast , so required less pouring time while for non-ferrous materials longer pouring time is beneficial because they lose heat slowly and tend to form dross if metal is pour too quickly. Ratio of surface area to volume of casting is important in addition to the mass of the casting. Also gating mass is considered when its mass is comparable to the mass of the casting.

•For grey cast iron up to 450 KgPouring time, t = K { 1.41 +

59.14

T 

}

W 

secondsWhere K = Fluidity of iron in inches / 40T = Average section thickness, mm

W = Mass of the casting, Kg

•For grey cast iron greater than 450 KgPouring time, t = K { 1.236 +

65.16T }3W seconds Typical pouring times for cast iron are

Casting mass pouring time in seconds

20 Kg 6 to 10100 Kg 15 to 30

•Steel CastingPouring time, t = (2.4335 – 0.3953 logW )W seconds

•Shell moulded ductile iron( vertical pouring)Pouring time, t = K1W seconds

• 1. Foundry: ” Foundry or casting is the process of producing metal/alloy component parts of desired shapes by pouring the molten metal/alloy into a prepared mould (of that shape) and then allowing the metal/alloy to cool and solidify. The solidified piece of metal/alloy is known as a CASTING”.
• 2. Casting Terms:1. Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed.drag - lower molding flask,cope - upper molding flask,cheek - intermediate molding flask used in three piece molding.2. Pattern: It is the replica of the final object to be made. The mold cavity is made with the help of pattern.3. Parting line: This is the dividing line between the two molding flasks that makes up the mold.
• 3. 4. Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings.5. Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metal is poured.6. Sprue: The passage through which the molten metal, from the pouring basin, reaches the mold cavity. In many cases it controls the flow of metal into the mold.7. Runner: The channel through which the molten metal is carried from the sprue to the gate.
• 4. 8. Gate: A channel through which the molten metal enters the mold cavity.9. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force.10. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as feed head.11. Vent: Small opening in the mold to facilitate escape of air and gases.
• 5. Basic Features:Pattern and Mould ◦ A pattern is made of wood or metal, is a replica of the final product and is used for preparing mould cavity. ◦ Mould material should posses refractory characteristics and with stand the pouring temperature.
• 6. Schematic diagram of castingmould:
• 7. Sand Casting Terminology Ken Youssefi Mechanical Engineering Dept., SJSU 10
• 8. Steps involved in making a casting: 1. Make the pattern out of Wood , Metal or Plastic. 2. Prepare the necessary sand mixtures for mould and core making. 3. Prepare the Mould and necessary Cores. 4. Melt the metal/alloy to be cast. 5. Pour the molten metal/alloy into mould and remove the casting from the mould after the metal solidifies. 6. Clean and finish the casting. 7. Test and inspect the casting. 8. Remove the defects, if any. 9. Relieve the casting stresses by Heat Treatment. 10. Again inspect the casting. 11. The casting is ready for shipping.
• 9.  TransportationØApplications of Casting:  TurbineØvehicles Øvanes  Aircraft jetØ Agricultural partsØ Railway crossingsØPower generators  Communication, Construction andØ Sanitary fittingsØengine parts Atomic Energy applications, etc..
• 10. Raw Materials forFoundry: 1) Metals and alloys. 2) Fuels (for melting metals). 3) Fluxes.
• 11. Metals and alloys commonlyused in Foundries: 1. Ferrous 2. Non-Ferrous FERROUS: a. Cast irons b. Steels NON-FERROUS: a. Copper alloys b. Aluminium alloys c. Magnesium alloys d. Zinc alloys e. Nickel alloys
• Av12. Pattern Making: Pattern Except forvis a model or the replica of the object to be cast. the various allowances a pattern exactly resembles the casting to be A pattern is required even if one object has to be cast.vmade.
• 13.  A Pattern prepares a mould cavity for the§Functions of Patterns:  A Pattern may contain projections known§purpose of making a casting. as core prints if the casting requires a core and need to be made  Patterns properly made and having§hollow. finished and smooth surfaces reduce  Properly constructed patterns minimize§casting defects. overall cost of the casting.
• 14. Pattern having core prints.
• 15. Selection of Pattern Materials:The following factors assist in selecting proper pattern material: No. of castings to be produced.Ø Dimensional accuracyØ & surface finish. Shape, complexityØ and size of casting. Casting design parameters.Ø Type ofØ molding materials. The chance of repeat orders.Ø Nature ofØ molding process.
• 16. The pattern material should be:1. Easily worked, shaped and joined.2. Light in weight.3. Strong, hard and durable.4. Resistant to wear and abrasion .5. Resistant to corrosion, and to chemical reactions.6. Dimensionally stable and unaffected by variations in temperature and humidity.7. Available at low cost.
• 17. Materials for making patterns: a. Wood b. Metal c. Plastic d. Plaster e. Wax.
• 18. Types of Patterns: 1. Single piece pattern. 2. Split piece pattern. 3. Loose piece pattern. 4. Match plate pattern. 5. Sweep pattern. 6. Gated pattern. 7. Skeleton pattern 8. Follow board pattern. 9. Cope and Drag pattern.
• 19. (a)Split pattern(b) Follow-board(c) Match Plate(d) Loose-piece(e) Sweep(f) Skeletonpattern
• 20. Fig: Single piece pattern
• 21. Fig: split piece pattern
• 22. 3.Loose piece pattern:
• 23. Fig: Match plate pattern
• 24. Sweep pattern:
• 25. castings Gating systemGATED PATTRN
• 26. GATED PATTRN
• 27. Fig: Cope and drag pattern
• 28. Types of Pattern Allowances:The various pattern allowances are: 1. Shrinkage or contraction allowance. 2. Machining or finish allowance. 3. Draft of tapper allowances. 4. Distortion or chamber allowance. 5. Shake or rapping allowance.
• 29. 1.Shrinkage Allowance: All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types:1. Liquid Shrinkage:2. Solid Shrinkage:
• 30. 2. Machining Allowance: A Casting is given an allowance for machining, because:i. Castings get oxidized in the mold and during heat treatment; scales etc., thus formed need to be removed.ii. It is the intended to remove surface roughness and other imperfections from the castings.iii. It is required to achieve exact casting dimensions.iv. Surface finish is required on the casting.
• §31. 3. Draft or Taper Allowance: It is given to all surfaces perpendicular to §parting line. Draft allowance is given so that the pattern can be easily removed from the molding material tightly packed around it with out damaging the mould cavity.
• 32. Fig: taper in design
• 33. 4. Distortion or cambered allowance:A casting will distort or wrap if : i. It is of irregular shape, ii. All it parts do not shrink uniformly i.e., some parts shrinks while others are restricted from during so, iii. It is u or v-shape,
• Aq34. 5. Shake allowance: patter is shaken or rapped by striking the same with a wooden piece from side to side. This is done so that the pattern a little is loosened  Inqin the mold cavity and can be easily removed. turn, therefore, rapping enlarges the mould cavity which results in a bigger sized  Hence, a –ve allowance is provided on the pattern i.e., theqcasting. pattern dimensions are kept smaller in order to compensate the enlargement of mould cavity due to rapping.
• 35. Pattern ØLayout:Steps involved: Get the working drawing of the part for which the Øpattern is to be made. Make two views of the part drawing on a sheet, using a shrink rule. A shrink rule is modified form of an ordinary scale which has already taken care of shrinkage allowance for a particular metal Øto be cast. Add machining allowances as per the Ørequirements. Depending upon the method of molding, provide the draft allowance.
• 36. Pattern  StudyqConstruction: the pattern layout carefully and establish, a. Location of parting surface. b. No. of parts in which the pattern qwill be made. Using the various hand tools and pattern making qmachines fabricate the different parts of the pattern. Inspect the pattern as regards the alignment of different portions of the pattern and its dimensional qaccuracy. Fill wax in all the fillets in order to remove sharp qcorners. Give a shellac coatings(3 coats) to qpattern. impart suitable colors to the pattern for identification purposes and for other informations.
• 37. Moulding Materials Major part of Moulding material in sand casting are 1. 70-85% silica sand (SiO2) 2. 10-12% bonding material e.g., clay cereal etc. 3. 3-6% water Properties of molding sand are: (a) Refractoriness (b) Cohesiveness (c) Strength/Adhesiveness (d) Permeability (e) Collapsibility (f) Flowability (g) Chemical Inactiveness
• 38. Molding Sand Composition:The main ingredients of any molding sand are:  Base sand,–  Binder, and–  Moisture– – Additives
• 39. Shape of the Sand Grains
• 40. Effect of moisture, grain size and shapeon mould quality
• 41. Grain size of sandThere are three distinct sizeof sand grains:1. Fine2. Medium3. Coarse
• 42. Types of Moulding Sand1. Green Sand2. Dry Sand3. Facing Sand4. Loam Sand5. Backing Sand6. Parting Sand7. Core Sand
• 43. 2 types of moulding flask designs
• 44. SAND MOULDING PROCESS
• 45. Sand mold - opened
• 46. Sand mold - closed
• 47. Mixing moulding sand with binders & adhesives
• 48. Filling sand in moulding flasks
• 49. Melting furnace
• 50. Pouring molten liquid
• 51. Knock out
• 52. Heat treatment
• 53. Machining
• 54. final products of casting
• 55. Casting Methods• Sand Casting • Investment Casting • Die CastingHigh Temperature Alloy, High Temperature Alloy, High Temperature Alloy,Complex Geometry, Complex Geometry, Moderate Geometry,Rough Surface Finish Moderately Smooth Surface Smooth Surface Finish 65

Top of Form

Bottom of Form

  ¡64.  Permanent mold casting is a metal casting process that shares ¡similarities to both sand casting and die casting. As in sand casting, molten metal is poured into a mold which is clamped shut until the material cools and solidifies into the desired part ¡shape. However, sand casting uses an expendable mold which is destroyed after each cycle. Permanent mold casting, like die casting, uses a metal mold (die) that is typically made from steel or cast iron and can ¡be reused for several thousand cycles. Because the molten metal is poured into the die and not forcibly injected, permanent mold casting is often referred to as gravity die casting.

  65. ¡ Mould preparation - First, the mould is pre-heated to around 300-500°F (150-260°C) to allow better metal ¡flow and reduce defects. Mould assembly - The mould consists of at least two parts - the two mold halves and any cores used to form complex features. Such cores are typically made from iron or steel, but expendable sand cores are sometimes used. In this step, the cores are inserted and the mold halves are clamped together

  ¡66.  Pouring - The molten metal is poured at a slow rate from a ladle into the mold through a sprue at the top of the mold. The metal flows through a runner system and enters the ¡mold cavity. Cooling - The molten metal is allowed to cool and solidify ¡in the mould. Mold opening - After the metal has solidified, the two mold halves are opened and ¡the casting is removed. Trimming - During cooling, the metal in the runner system and sprue solidify attached to the casting. This excess material is now cut away.

  ¡67.  Advantages:  Can formØ complex shapes.  Good mechanical properties.Ø  Many materialØ options.  Low porosity.Ø  Low labor costØ  Scrap can beØ ¡recycled. Disadvantages:  High tooling cost.Ø  Long lead timeØ ¡possible. Applications: Gears, wheels, housings, engine components.

  ¡68.  For any Metal Casting Process, selection of right alloy, size, shape, thickness, tolerance, texture, and weight, is very ¡vital. Special requirements such as, magnetism, corrosion, stress distribution also influence the choice of the Metal ¡Casting Process. Views of the Tooling Designer; Foundry / Machine House needs, customers exact product requirements, and secondary operations like painting, must be taken care of before selecting the appropriate Metal Casting Process.  Tool cost.Ø  Economics ofØ machining versus process costs.  Adequate protection / packaging,Ø shipping constraints, regulations of the final components, weights and shelf life of protective coatings also play their part in the Metal Casting process.

 Permeability is a property of foundry sand with respect to how well the sand can vent, i.e. how well gases pass through the sand. And in other words, permeability is the property by which we can know the ability of material to transmit fluid/gases. The permeability is commonly tested to see if it is correct for the casting conditions.

Affecting factors

The grain size, shape and distribution of the foundry sand, the type and quantity of bonding materials, the density to which the sand is rammed, and the percentage of moisture used for tempering the sand are important factors in regulating the degree of permeability

Significance/Important are

 An increase in permeability usually indicates a more open structure in the rammed sand, and if the increase continues, it will lead to penetration-type defects and rough castings. A decrease in permeability indicates tighter packing and could lead to blows and pinholes.

Testing procedure

The permeability number, which has no units, is determined by the rate of flow of air, under standard pressure, through a rammed cylindrical specimen. DIN standards define the specimen dimensions to be 50 mm in diameter and 50 mm tall,^[2] while the American Foundry Society defines it to be two inches in diameter and two inches tall.^[3] rammed cylindrical specimen.

METAL CASTING

Advantages

The metal casting process is extensively used in manufacturing because of its many

Advantages.

1. Molten material can flow into very small sections so that intricate shapes can be made by this process. As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated.
2. It is possible to cast practically any material that is fer rous or non-ferrous.
3. As the metal can be placed exactly where it is required, large saving in weight can be achieved.

4. The necessary tools required for casting molds are very simple and inexpensive. As a result, for production of a small lot , it is the ideal process.

5. There are certain parts made from metals and alloys that can only be processed this way.

6. Size and weight of the product is not a limitation for the casting process.

Limitations

1. Dimensional accuracy and surface finish of the castings made by sand casting processes are a limitation to this technique. Many new casting processes have been developed which can take into consideration the aspects of dimensional accuracy and surface finish. Some of these processes are die casting process, investment casting process, vacuum-sealed molding process, and shell molding process.

2. The metal casting process is a labor intensive process History Casting technology, according to biblical l records, reaches back almost 5,000 years BC.Gold, pure in nature, most likely caught Prehistoric man's fancy. as he probably hammered gold ornaments out of the gold nuggets he found. Silver would have been treated similarly. Mankind next found copper, b because it appeared in the ash of his camp fires from copper-bearing ore that he lined his fire pits with. Man soon found that copper was harder than gold or silver. Copper did not bend up when used. So copper, found antic' in man's early tools, and then marched its way into Weaponry. But, long before all this. Man found clay. So he made pottery - something to eat from. Then he thought, "Now. What else can I do with this mud." . Early man thought about it, "they used this pottery stuff, (the first pattern s), to shape metal into bowls ".

LECTURE: 1

Casting Terms

1. Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed. Depending upon the position of the flask in the  molding structure, it is  referred to by various names such as drag - lower molding flask, cope - upper molding flask, cheek - intermediate molding flask used in three piece molding.

2. Pattern: It is the replica of the final object to be made. The mold cavi ty is made with  the help of pattern.

3. Parting line: This is the dividing line between the two molding flasks that makes up the mold.

4. Molding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a mixture of silica sand, clay, and moisture in appropriate proportions.

5. Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to give a better surface finish to the castings.

6. Core: A separate part of the mold, made of sand and generally baked, which is used  to create openings and various shaped cavities in  the castings.

7. Pouring basin: A small funnel shaped cavity at the top of the mold into which the  molten metal is poured.

8. Sprue: The passage through which the molten metal, from the pouring basin, reaches  the mold cavity. In many cases it controls the flow of metal into the mold.

9. Runner: The channel through which the molten metal is carried from the sprue to the gate.
10. Gate: A channel through which the molten metal enters the mold cavity.
11. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force.

12. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as "feed head".

13. Vent: Small opening in the mold to facilitate escape of air and gases.  Figure 1 : Mold Section showing some casting terms

Steps in Making Sand Castings

There are six basic steps in making sand castings:

1. Pattern making
2. Core making
3. Mold

4. Melting and pouring

5. Cleaning

Pattern making

 The pattern is a physical model of the casting used to make the mold. The mold is made by packing some readily formed aggregate material, such as molding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become the casting. If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities.

Core making

Cores are forms, usually made of sand, which are placed into a mold cavity to form the interior surfaces of castings. Thus the void space between en the core and mold –cavity surface is what eventually becomes the casting.

Molding

Molding consists of all operations necessary to prepare a mold for receiving molten metal. Molding usually involves placing a molding aggregate around a pattern held with a supporting frame, withdrawing the pattern to leave the mold cavity, setting the cores in the mold cavity and finishing and closing the mold.

Melting and Pouring

The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the molds are filled.

Cleaning

Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. Burned-on sand and scale are removed to improve the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Inspection of the casting for defects and general quality is performed.

The pattern is the principal tool during the casting process. It is the replica of the object to be made by the casting process, with some modifications. The main modifications are the addition of pattern allowances, and the provision of core prints. If the ca sting is to be hollow, additional patterns called cores are used to create these cavities in the finished product. The quality of the casting produced depends upon the material of the pattern, its design, and construction. The costs of the pattern and the related equipment are reflected in the cost of the casting. The use of an expensive pattern is justified when the quantity of castings required is substantial.

Functions of the Pattern

1. A pattern prepares a mold cavity for the purpose of making a c asting.

2. A pattern may contain projections known as core prints if the casting requires a core and need to be made hollow.

3. Runner, gates, and risers used for feeding molten metal in the mold cavity may form a part of the pattern.

4. Patterns properly made and having finished and smooth surfaces reduce casting defects.

5. A properly constructed pattern minimizes the overall cost of the castings.

Pattern Material

Patterns may be constructed from the following materials. Each material has its own advantages, limitations, and field of application. Some materials used for making patterns are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins.

To be suitable for use, the pattern material should be:
1. Easily worked, shaped and joined
2. Light in weight
3. Strong, hard and durable
4. Resistant to wear and abrasion
5. Resistant to corrosion, and to chemical reactions
6. Dimensionally stable and unaffected by variations in temperature and humidity

7. Available at low cost

The usual pattern materials are wood, metal, and plastics. The most commonly used pattern material is wood, since it is readily available and of low weight. Also, it can be easily shaped and is relatively cheap. T he main disadvantage of wood is its absorption of moisture, which can cause distortion and dimensional changes. Hence, proper seasoning and upkeep of wood is almost a pre -requisite for large-scale use of wood as a pattern material.

Figure 2: A typical pattern attached with gating and risering system

Pattern Allowances

Pattern allowance is a vital feature as it affects the dimensional characteristics of the casting. Thus, when the pattern is produced, certain allowances must be given on the sizes specified in the finished component drawing so that a casting with the particular specification can be made. The selection of correct allowances greatly helps to reduce machining costs and avoid rejections. The allowances usually considered on patterns and core boxes are as follows:

1. Shrinkage or contraction allowance
2. Draft or taper allowance
3. Machining or finish allowance
4. Distortion or camber allowance

5. Rapping allowance

Solution 1

The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet  ( as per Table 1 )
For dimension 18 inch, allowance = 18X 0.125 / 12 = 0.1875 inch » 0.2 inch
For dimension 14 inch, allowance = 14X 0.125 / 12 = 0.146 inch » 0.15 inch
For dimension 8 inch, allowance = 8 X 0.125 / 12 = 0.0833 inch » 0. 09 inch
For dimension 6 inch, allowance = 6 X 0.125 / 12 = 0.0625 inch » 0. 07 inch
The pattern drawing with required dimension is shown below:

Lecture 4

Draft or Taper Allowance

By draft is meant the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold and without excessive rapping by the molder.Figure 3 (a) shows a pattern having no draft allowance being removed from the pattern. In this case, till the pattern is completely lifted out, its side s will remain in contact with the walls of the mold, thus tending to break it. Figure 3 (b) is an illustration of a pattern hav ing proper draft allowance. Here, the moment the pattern lifting commences, all of its surfaces are well away from the sand surface. Thus the pattern can be removed without damaging the mold cavity.

Figure 3 (a) Pattern Having No Draft on Vertical Edges

Figure 3 (b) Pattern Having Draft on Vertical Edges

Draft allowance varies with the complexity of the sand job. But in general inner details of the pattern require higher draft than outer surfaces. The amount of draft depends upon the length of the vertical side of the pattern to be extracted; the intricacy of the pattern; the method of molding; and pattern material.Table 2 provides a general guide lines for the draft allowance.

Machining or Finish Allowance

The finish and accuracy achieved in sand casting are generally poor and the refore when the casting is functionally required to be of good surface finish or dimensionally accurate, it is generally achieved by subsequent machining. Machining or finish allowances are therefore added in the pattern dimension. The amount of machining allowance to be provided for is affected by the method of molding and casting used viz. hand molding or machine molding, sand casting or metal mold casting. The amount of machining allowance is also affected by the size and shape of the casting; the castin g orientation; the metal; and the degree of accuracy and finish required. The machining allowances recommended for different metal is given inTable 3.

Table 3 : Machining Allowances of Various Metals

Exercise 2

The casting shown is to be made in cast iron using a wooden pattern. Assuming only machining allowance, calculate the dimension of the pattern. All Dimensions are in Inches

Solution 2

The machining allowance for cast iron for size, up to 12 inch is o.12 inch and from 12

inch to 20 inch is 0.20 inch ( (Table 3)
For dimension 18 inch, allowance = 0.20 inch
For dimension 14 inch, allowance = 0.20 inch

Distortion or Camber Allowance

Sometimes castings get distorted, during solidification, due to their typical shape. For example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract at the closed end caus ing the vertical legs to look slightly inclined. This can be prevented by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that the casting after distortion will have its sides vertical ( (Figure 4 ).

The distortion in casting may occur due to internal stresses. These internal stresses are caused on account of unequal cooling of different section o f the casting and hindered contraction.

Measure taken to prevent the distortion in casting include:

i.                    Modification of casting design

ii.                   Providing sufficient machining allowance to cover the distortion affect

iii.                 Providing suitable allowance on the pattern, called camber or distortion  allowance (inverse reflection)

Figure 4: Distortions in Casting

a. Lost Wax
b. Ceramics Shell Molding
c.Evaporative Pattern Casting
d. Vacuum Sealed Molding
e.Centrifugal Casting

Green Sand Molding

Green sand is the most diversified molding method used in metal casting operations. The process utilizes a mold made of compressed or compacted mois t sand. The term "green “denotes the presence of moisture in the molding sand. The mold material consists of silica sand mixed with a suitable bonding agent (usually clay) and moisture.

 Advantages

Most metals can be cast by this method.
Pattern costs and material costs are relatively low.
No Limitation with respect to size of casting and type of metal or alloy used

Disadvantages

Surface Finish of the castings obtained by this process is not good and machining is often required to achieve the finished pro duct.

Sand Mold Making Procedure

The procedure for making mold of a cast iron wheel is shown in (Figure 8 (a), (b), (c)).

  The first step in making mold is to place the pattern on the molding board.

 The drag is placed on the board ((Figure 8 (a) ).

 Dry facing sand is sprinkled over the board and pattern to provide a non sticky layer.

 Molding sand is then riddled in to cover the pattern with the fingers; then the drag  is completely filled.

 The sand is then firmly packed in the drag by means of han d rammers. The ramming must be proper i.e. it must neither be too hard or soft.

 After the ramming is over, the excess sand is leveled off with a straight bar known as a strike rod.

 With the help of vent rod, vent holes are made in the drag to the full de pth of the flask as well as to the pattern to facilitate the removal of gases during pouring and solidification. The finished drag flask is now rolled over to the bottom board exposing the pattern.

 Cope half of the pattern is then placed over the drag pa ttern with the help of locating pins. The cope flask on the drag is located aligning again with the help ofpins ( (Figure 8 (b)).

 The dry parting sand is sprinkled all over the drag and on the pattern.

 A sprue pin for making the sprue passage is located at a small distance from the  pattern. Also, riser pin, if required, is placed at an appropriate place.

 The operation of filling, ramming and venting of the cope proceed in the same  manner as performed in the drag.

 The sprue and riser pins are removed first and a pouring basin is scooped out at  the top to pour the liquid metal.

 Then pattern from the cope and drag is removed an d facing sand in the form of paste is applied all over the mold cavity and runners which would give the finished casting a good surface finish.

 The mold is now assembled. The mold now is ready for pouring (see ((Figure 8 (c) )

Figure 8 (a)

Distortion or Camber Allowance

Sometimes castings get distorted, during solidification, due to their typical shape. Forexample, if the casting has the form of the letter U, V, T, or L etc. it will tend to contractat the closed end caus ing the vertical legs to look slightly inclined. This can be prevented by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that the casting after distortion will have its sides vertical ( (Figure 4).

The distortion in casting may occur due to internal stresses. These internal stresses are caused on account of unequal cooling of different section o f the casting and hindered contraction.

Measure taken to prevent the distortion in casting includes:

i.                    Modification of casting design .

ii.                  Providing sufficient machining allowance to cover the distortion affect .

iii.                Providing suitable allowance on the pattern, called camber or distortion allowance (inverse reflection).

Figure 4: Distortions in Casting

Rapping Allowance

Before the withdrawal from the sand mold, the pattern is rapped all around the vertical
faces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges the
final casting made, it is de sirable that the original pattern dimension should be reduced to
account for this increase. There is no sure way of quantifying this allowance, since it is
highly dependent on the foundry personnel practice involved. It is a negative allowance
and is to be applied only to those dimensions that are parallel to the parting plane.

Core and Core Prints

Castings are often required to have holes, recesses, etc. of various sizes and shapes. These
impressions can be obtained by using cores. So where coring is requ ired, provision
should be made to support the core inside the mold cavity. Core prints are used to serve this purpose. The core print is an added projection on the pattern and it forms a seat in the mold on which the sand core rests during pouring of the m old. The core print must be of adequate size and shape so that it can support the weight of the core during the casting operation. Depending upon the requirement a core can be placed horizontal, vertical and can be hanged inside the mold cavity. A typical job, its pattern and the mold cavity with
core and core print is shown inFigure 5.

Rapping Allowance

Before the withdrawal from the sand mold, the pattern is rapped all around the vertical faces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges the final casting made, it is de sirable that the original pattern dimension should be reduced to account for this increase. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel practice involved. It is a negative allowance
and is to be applied only to those dimensions that are parallel to the parting plane.

Core and Core Prints  

Castings are often required to have holes, recesses, etc. of various sizes and shapes. These impressions can be obtained by using cores. So where coring is requ ired, provision should be made to support the core inside the mold cavity. Core prints are used to serve this purpose. The core print is an added projection on the pattern and it forms a seat in the mold on which the sand core rests during pouring of the m old. The core print must be of adequate size and shape so that it can support the weight of the core during the casting operation. Depending upon the requirement a core can be placed horizontal, vertical and can be hanged inside the mold cavity. A typical job, its pattern and the mold cavity with core and core print is shown inFigure 5.

Figure 5: A Typical Job, its Pattern and the Mold Cavity

Lecture 5

Types of Pattern

Patterns are of various types, each satisfying certain c asting requirements.

1. Single piece pattern
2. Split or two piece pattern
3. Match plate pattern

Single Piece Pattern

The one piece or single pattern is the most inexpensive of all types of patterns. This type of pattern is used only in cases where the job is very simple and does not create any withdrawal problems. It is also used for application in very small -scale production or in prototype development. This type of pattern is expected to be entirely in the drag and one of the surface is is expected to be flat which is used as the parting plane. A gating system is made in the mold by cut ting sand with the help of sand tools. If no such flat surface exists, the molding becomes complicated. A typical one -piece pattern is shown inFigure 6.

Figure 6: A Typical One Piece Pattern

Split or Two Piece Pattern

Split or two piece pattern is most widely used type of pattern for intricate castings. It is split along the parting surface, the position of which is determined by the shape of the casting. One half of the pattern is molded in drag and the other half in cope. T he two halves of the pattern must be aligned properly by making use of the dowel pins, which are fitted, to the cope half of the pattern. These dowel pins match with the precisely made holes in the drag half of the pattern

Dry Strength

When the molten metal is poured in the mold, the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand e evaporates due to the heat of the molten metal. At this stage the molding sand must posses the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material al.

Hot Strength

As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength.

Collapsibility

The molding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings. Besides these specific properties the molding material should be cheap, reusable and should have good thermal conductivity.

Molding Sand Composition .The main ingredients of any molding sand are:

1. Base sand,

2. Binder, and

3. Moisture

Base Sand

Silica sand is most commonly used base sand. Other base sand s that are also used for making mold are zircon sand, Chromite sand, and olivine sand. Silica sand is cheapest among all types of base sand and it is easily available.

 Binder :Binders are of many types such as:

 1. Clay binders,

2. Organic binders and

3. Inorganic binders

Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength. The most popular clay types are: Kaolinite or fire clay (Al2O3 2 SiO2 2 H2O) and Bentonite (Al2O3 4 SiO2 nH2O) Of the two the Bentonite can absorb more water which increases its bonding power.

Moisture

Clay acquires its bonding action only in the presence of the required amount of moisture. When water is added to clay, it penetrates the mixture and forms a microfilm, which coats the surface of each flake of the clay. The amount of water used should be properly controlled. This is because a part of the water, which coats the surface of the clay flakes, helps in bonding, while the remainder helps in improving the plasticity. A typical composition of molding sand is given in (Table 4).

Table 4

  ¡69.  ¡Shrinkage defects

 Shrinkage defects occur when feed metal is not available to compensate for shrinkage as the metal solidifies. Shrinkage defects can be split into two different types: open shrinkage defects and closed shrinkage defects. Open shrinkage defects are open to the atmosphere, therefore as the shrinkage cavity forms air compensates. There are two types of open air defects: pipes and caved surfaces. Pipes form at the surface of the casting and burrow into the casting, while caved surfaces are shallow cavities that form across the surface of ¡the casting. Closed shrinkage defects, also known as shrinkage porosity, are defects that form within the casting. Isolated pools of liquid form inside solidified metal, which are called hot spots. The shrinkage defect usually forms at the top of the hot spots. They require a nucleation point, so impurities and dissolved gas can induce closed shrinkage defects. The defects are broken up into macro porosity and micro porosity (or micro shrinkage), where macro porosity can be seen by the naked eye and micro porosity cannot.[4][5]

  ¡70.  ¡Gas porosity

Gas porosity is the formation of bubbles within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it ¡cools. Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal,[which reduces strength in that ¡vicinity. Nitrogen, oxygen and hydrogen are the most encountered gases in ¡cases of gas porosity. In aluminum castings, hydrogen is the only gas that dissolves in significant quantity, which can result in hydrogen gas porosity.

  ¡71.  ¡Pouring Metal defects

Pouring metal defects include misruns, cold shuts, and inclusions.  AØ misrun occurs when the liquid metal does not completely fill the mold ¡cavity, leaving an unfilled portion. Cold shuts occur when two fronts of liquid metal do not fuse properly in the mold cavity, ¡leaving a weak spot. Both are caused by either a lack of fluidity in the molten metal or ¡cross-sections that are too narrow. The fluidity can be increased by changing the chemical composition of the metal or by increasing the pouring temperature. Another possible cause is back pressure from improperly vented mold cavities

  72. ¡ ¡Metallurgical defects There are two defects in this category: hot tears and hot spots. Hot tears, also known as hot cracking, are failures in the casting that occur as the casting cools. This happens because the metal is weak when it is hot and the residual stresses in the material can cause the casting to fail as it cools. ¡Proper mold design prevents this type of defect. Hot spots are areas on the surface of casting that become very hard because they cooled more quickly than the surrounding material. This type of defect can be avoided by proper cooling practices or by changing the chemical composition of the metal.

Lecture 14

Casting Defects (Figure19)

The following are the major defects, which are likely to occur in sand castings

1. Gas defects

2. Shrinkage cavities

3. Molding material defects

4. Pouring metal defects

5. Mold shift

Gas Defects

A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal.

Causes Gas Defects

The lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability of the mold or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold.

1. Metal contains gas

2. Mold is too hot

3.Poor mold burnout

Shrinkage Cavities

These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick Sprues to avoid shrinkage cavities.

Molding Material Defect s

The defects in this category are cuts and washes, metal penetration, fusion, and swell.

 Cut and washes: These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the molding sand not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of molding sand and the latter can be overcome by the proper design of the gating system.

Metal penetration :When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration.

Fusion: This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.

Swell: Under the influence of metallostatic forces, the mold wall may move back causing a swell in the dimension of the casting. A proper ramming of the mold will correct this defect.

Inclusions

Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mold can prevent this defect.

Pouring Metal Defects: The likely defects in this category are

1. Mis-runs and 2. Cold shuts.

A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis -run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design.

A cold shut is caused when two streams while meeting in the mold cavity, do not fuse together properly thus forming a discontinuity in the casting. When the

 : A Typical Composition of Molding Sand

Molding Sand Constituent

Weight Percent

Silica sand 92

Clay (Sodium Bentonite)

8 Water 4

Lecture 7

Dry Sand Molding

When it is desired that the gas forming materials are lowered in the molds, air –dried molds are sometimes preferred to green sand molds. Two types of drying of molds are often required.

1. Skin drying and

2. Complete mold drying.

In skin drying a firm mold face is produced. Shakeout of the mold is almost as good as that obtained with green sand molding. The most common method of drying therefractory mold co ating uses hot air, gas or oil flame. Skin drying of the mold can be accomplished with the aid of torches, directed at the mold surface.

Shell Molding Process

It is a process in which, the sand mixed with a thermosetting resin is allowed to come in con tact with a heated pattern plate (200oC), this causes a skin (Shell) of about 3.5 mm of sand/plastic mixture to adhere to the pattern.. Then the shell is removed from the pattern. The cope and drag shells are kept in a flask with necessary backup material and the molten metal is poured into the mold.

This process can produce complex parts with good surface finish 1.25 µ m to 3.75 µ m, and dimensional tolerance of 0.5 %. A good surface finish and good size tolerance reduce the need for machining. The process overall is quite cost effective due to reduced machining and cleanup costs. The materials that can be used with this process are cast irons, and aluminum and copper alloys.

Molding Sand in Shell Molding Process

The molding sand is a mixture of fine grained quartz sand and powdered Bakelite. There are two methods of coating the sand grains with Bakelite. First method is Cold coating method and another one is the hot method of coating.

In the method of cold coating, quartz sand is poured into the mixer and then the solution of powdered bakelite in acetone and ethyl aldehyde are added. The typical mixtu re is 92% quartz sand, 5% bakelite, 3% ethyl aldehyde. During mixing of the ingredients, the resin envelops the sand grains and the solvent evaporates, leaving a thin film that uniformly coats the surface of sand grains, thereby imparting fluidity to the s and mixtures.

In the method of hot coating, the mixture is heated to 150 -180 o C prior to loading the sand. In the course of sand mixing, the soluble phenol formaldehyde resin is added. The mixer is allowed to cool up to 80 - 90 o C. This method gives bet ter properties to the mixtures than cold method.

Sodium Silicate Molding Process

In this process, the refractory material is coated with a sodium silicate -based binder. For molds, the sand mixture can be compacted manually, jolted or squeezed around the pattern in the flask. After compaction, CO 2 gas is passed through the core or mold. The CO 2 chemically reacts with the sodium silicate to cure, or harden, the binder. This cured binder then holds the refractory in place around the pattern. After curing, the pattern is withdrawn from the mold.

The sodium silicate process is one of the most environmentally acceptable of the chemical processes available. The major disadvantage of the process is that the binder is very hygroscopic and readily absorbs water, which causes a porosity in the castings.. Also, because the binder creates such a hard, rigid mold wall, shakeout and collapsibility characteristics can slow down production. Some of the advantages of the process are:

•A hard, rigid core and mold are ty pical of the process, which gives the casting good dimensional tolerances;

•good casting surface finishes are readily obtainable;

Permanent Mold Process  

In al the above processes, a mold need to be prepared for each of the casting produced. For large-scale production, making a mold, for every casting to be produced, may be difficult and expensive. Therefore, a permanent mold, called the die may be made from which a large number of castings can be produced. , the molds are usually made of cast iron or steel, although graphite, copper and aluminum have been used as mold materials.
The process in which we use a die to make the castings is called permanent mold casting or gravity die casting, since the metal enters the mold under gravity. Some time in die - casting we inject the molten metal with a high pressure. When we apply pressure in injecting the metal it is called pressure die casting process.

Advantages

•Permanent Molding produces a sound dense casting with superior mechanical properties.

•the castings produced are quite uniform in shape have a higher degree of dimensional accuracy than castings produced in sand

•The permanent mold process is also capable of producing a consistent quality of finish on castings

Disadvantages

•The cost of tooling is usually higher than for sand castings

•The process is generally limited to the production of small castings of simple exterior design, although complex castings such as aluminum engine blocks and heads are now commonplace.

Centrifugal Casting

In this process, the mold is rotated rapidly about its central axis as the metal is pouredinto it. Because of the centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The slag, oxides and other inclusions being l ighter, get separated from the metal and segregate towards the center. This process is normally used for the making of hollow pipes, tubes, hollow bushes, etc., which are ax symmetric with a concentric hole. Since the metal is always pushed outward because of the centrifugal force, no core needs to be used for making the concentric hole. The mold can be rotated about a vertical, horizontal or an inclined axis or about its horizontal and vertical axes simultaneously.
The length and outside diameter are fixed by the mold cavity dimensions while the inside diameter is determined by the amount of molten metal poured into the mold.Figure 9(Vertical Centrifugal Casting), Figure 10 ( Horizontal Centrifugal Casting)

Advantages

•Formation of hollow inte riors in cylinders without cores

•Less material required for gate

•Fine grained structure at the outer surface of the casting free of gas and shrinkage cavities and porosity

Disadvantages

•More segregation of alloy component during pouring under the forces of rotation

•Contamination of internal surface of castings with non -metallic inclusions

•Inaccurate internal diameter

Lecture 8

Investment Casting Process

The root of the investment casting process, the cire perdue or "lost wax" method dates back to at least the fourth millennium B.C. The artists and sculptors of ancient Egypt and Mesopotamia used the rudiments of the investment casting process to create intricately detailed jewelry, pectorals and idols. The investment casting process al os called lost wax process begins with the production of wax replicas or patterns of the desired shape of the castings. A pattern is needed for every casting to be produced. The patterns are prepared by injecting wax or polystyrene in a metal dies. A numbe r of patterns are attached to a central wax sprue to form a assembly. The mold is prepared by surrounding the pattern with refractory slurry that can set at room temperature. The mold is then heated so that pattern melts and flows out, leaving a clean cavi ty behind. The mould is further hardened by heating and the molten metal is poured while it is still hot. When the casting is solidified, the mold is broken and the casting taken out.

The basic steps of the investment casting process are (Figure 11 see below ) :

1. Production of heat-disposable wax, plastic, or polystyrene patterns
2. Assembly of these patterns onto a gating system
3. "Investing," or covering the pattern assembly with refractory slurry
4. Melting the pattern assembly to remove the pattern material
5. Firing the mold to remove the last traces of the pattern material

6. Pouring

7. Knockout, cutoff and finishing.

Advantages

•Formation of hollow interiors in cylinders without cores

•Less material required for gate

•Fine grained structure at the outer surface of the casting free of gas and shrinkage cavities and porosity

Disadvantages

•More segregation of alloy comp onent during pouring under the forces of rotation

•Contamination of internal surface of castings with non -metallic inclusions

•Inaccurate internal diameter

Ceramic Shell Investment Casting Process

The basic difference in investment casting is that in the investment casting the wax pattern is immersed in a refractory aggregate before dewaxing whereas, in ceramic shellinvestment casting a ceramic shell is built around a tree assembly by repeatedly dipping apattern into a slurry (refractory material such as zircon with binder). After each dipping and stuccoing is completed, the assembly is allowed to thoroughly dry before the next coating is applied. Thus, a shell is built up around the assembly. The thickness of this   shell is dependent on the size of the castings and temperature of the metal to be poured.

After the ceramic shell is completed, the entire assembly is placed into an autoclave or flash fire furnace at a high temperature. The shell is heated to about 982 o C to burn out any residual wax and to develop a high -temperature bond in the shell. The shell molds can then be stored for future use or molten metal can be poured into them immediately. If the shell molds are stored, they have to be preheated before molten metal is poured into
them.

Advantages

•excellent surface finish
•tight dimensional tolerances
•machining can be reduced or completely eliminated

Lecture 9

Full Mold Process / Lost Foam Process / Evaporative Pattern Casting Process

The use of foam patterns for metal casting w as patented by H.F. Shroyer on April 15, 1958. In Shroyer's patent, a pattern was machined from a block of expanded polystyrene (EPS) and supported by bonded sand during pouring. This process is known as the full mold process. With the full mold process, t he pattern is usually machined from an EPS block and is used to make primarily large, one -of-a kind castings. The full mold process was originally known as the lost foam process. However, current patents have required that the generic term for the process be full mold.

In 1964, M.C. Flemmings used unbounded sand with the process. This is known today as lost foam casting (LFC). With LFC, the foam pattern is molded from polystyrene beads. LFC is differentiated from full mold by the use of unbounded sand (LFC ) as opposed to bonded sand (full mold process).  Foam casting techniques have been referred to by a variety of generic and proprietary names. Among these are lost foam, evaporative pattern casting, cavity less casting, evaporative foam casting, and full m old casting.

In this method, the pattern, complete with gates and risers, is prepared from expanded polystyrene. This pattern is embedded in a no bake type of sand. While the pattern is inside the mold, molten metal is poured through the sprue. The heat o f the metal is sufficient to gasify the pattern and progressive displacement of pattern material by the molten metal takes place.

The EPC process is an economical method for producing complex, close –tolerance castings using an expandable polystyrene patte rn and unbonded sand. Expandable polystyrene is a thermoplastic material that can be molded into a variety of complex, rigid shapes. The EPC process involves attaching expandable polystyrene patterns to an
expandable polystyrene gating system and applying a refractory coating to the entire assembly. After the coating has dried, the foam pattern assembly is positioned on loose dry sand in a vented flask. Additional sand is then added while the flask is vibrated until the pattern assembly is completely embedd ed in sand. Molten metal is poured into the sprue, vaporizing the foam polystyrene, perfectly reproducing the pattern.

In this process, a pattern refers to the expandable polystyrene or foamed polystyrene part  that is vaporized by the molten metal. A patt ern is required for each casting.  Process Description ((Figure 12  )

1. The EPC procedure starts with the pre -expansion of beads, usually polystyrene. After the pre-expanded beads are stabilized, they are blown into a mold to form pattern sections. When the beads are in the mold, a steam cycle causes them to fully expand and fuse together.

2. The pattern sections are assembled with glue, forming a cluster. The gating  system is also attached in a similar manner.

3. The foam cluster is covered with a cer amic coating. The coating forms a barrier  so that the molten metal does not penetrate or cause sand erosion during pouring.

4. After the coating dries, the cluster is placed into a flask and backed up with bonded sand.

5. Mold compaction is then achieved by using a vibration table to ensure uniform and proper compaction. Once this procedure is complete, the cluster is packed in the flask and the mold is ready to be poured .

Figure 12: The Basic Steps of the Evaporative Pattern Casting Process

Advantages

The most important advantage of EPC process is that no cores are required. No binders or other additives are required for the sand, which is reusable. Shakeout of the castings in unbonded sand is simplified. There are no parting lines or core fins.

Lecture 10

Vacuum Sealed Molding Process

It is a process of making molds utilizing dry sand, plastic film and a physical means of binding using negative pressure or vacuum. V -process was developed in Japan in 1971. Since then it has gained considerable importanc e due to its capability to produce dimensionally accurate and smooth castings. The basic difference between the V –process and other sand molding processes is the manner in which sand is bounded to form the mold cavity. In V  process vacuum, of the order of 250 - 450 mm Hg, is imposed to bind the dry free flowing sand encapsulated in between two plastic films. The technique involves the formation of a mold cavity by vacuum forming of a plastic film over the pattern, backed by unbounded sand, which is compacte d by vibration and held rigidly in place by applying vacuum. When the metal is poured into the molds, the plastic film first melts and then gets sucked just inside the sand voids due to imposed vacuum where it condenses and forms a shell -like layer. The vacuum must be maintained until the metal solidifies, after which the vacuum is released allowing the sand to drop away leaving a casting with a smooth surface. No shakeout equipment is required and the same sand can be cooled and reused without further trea tment.

Sequence of Producing V -Process Molds

•The Pattern is set on the Pattern Plate of Pattern Box. The Pattern as well as the Pattern Plate has Numerous Small Holes. These Holes Help the Plastic Film to Adhere Closely   on Pattern When Vacuum is Applied.

•A Heater is used to Soften the Plastic Film

•The Softened Plastic Film Drapes over the Pattern. The Vacuum Suction Acts through the Vents (Pattern and Pattern Plate) to draw it so that it adheres closely to the Pattern.

•The Molding Box is Set on the Film Coated Pattern

•The Molding Box is filled with Dry Sand. Slight Vibration Compacts the Sand

•Level the Mold. Cover the Top of Molding Box with Plastic Film. Vacuum Suction Stiffens the Mold.

•Release the Vacuum on the Patter n Box and Mold Strips Easily.

•Cope and Drag are assembled and Metal is poured. During Pouring the Mold is Kept under Vacuum

•After Cooling, the Vacuum is released. Free Flowing Sand Drops Away, Leaving a

Clean Casting  Advantages

•Exceptionally Good Dimensional Accuracy

•Good Surface Finish

•Longer Pattern Life

•Consistent Reproducibility

•Low Cleaning / Finishing Cost

To view the sequence of producing V - Process Mode.

Lecture 12

Reverberatory furnace

A furnace or kiln in which the material under treatment is heated indirectly by means of a flame deflected downward from the roof. Reverberatory furnaces are used in opper, tin, and nickel production, in the production of certain concretes and cements, and in aluminum. Reverberatory furnaces heat the metal to melting temperatures with direct fired wall -mounted burners. The primary mode of heat transfer is through radia tion from the refractory brick walls to the metal, but

convective heat transfer also provides additional heating from the burner to themetal. The advantages provided by reverberatory melters is the high volume  processing rate, and low operating and mainte nance costs. The disadvantages of the reverberatory melters are the high metal oxidation rates, low efficiencies, and large floor

space requirements. A schematic of Reverberatory furnace is shown
inFigure 15 See Below

Induction furnace

Induction heating is a heating method. The heating by the induction method occurs when an electrically conductive material is placed in a varying magnetic field. Induction heating is a rapid form of heating in which a current is induced directly into the part being heated. Induction heating is a non -contact form of heating.

The heating system in an induction furnace includes:

1. Induction heating power supply,

2. Induction heating coil,

3. Water-cooling source, which cools the coil and several internal components inside the power supply.

The induction heating power supply sends alternating current through the induction coil, which generates a magnetic field. Induction furnaces work on the principle of a transformer. An alternative electromagnetic field induces eddy currents in the metal which converts the electric energy to heat without any physical contact between the induction coil and the work piece. A schematic diagram of induction furnace is shown inFigure 16. The furnace conta ins a crucible surrounded by a water cooled copper coil. The coil is called primary coil to which a high frequency current is supplied. By induction secondary currents, called eddy currents are produced in the crucible. High temperature can bemobtained by this method. Induction furnaces are of two types: cored furnace andmcoreless furnace. Cored furnaces are used almost exclusively as holdingmfurnaces. In cored furnace the electromagnetic field heats the metal between twomcoils. Coreless furnaces heat the m etal via an external primary coil.

Figure 16: Schematic of a Induction Furnace

Advantages of Induction Furnace

 Induction heating is a clean form of heating

 High rate of melting or high melting efficiency

 Alloyed steels can be melted wit hout any loss of alloying elements

 Controllable and localized heating

Disadvantages of Induction Furnace

 High capital cost of the equipment

 High operating cost

Lecture 14

Casting Defects (Figure19)

The following are the major defects, which are likely to occur in sand castings

 Gas defects

 Shrinkage cavities

 Molding material defects

 Pouring metal defects

 Mold shift

Gas Defects

A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside
the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas,which gets entrapped during heating of molten metal.

Causes

The lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability of the mold or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold.

 Metal contains gas

 Mold is too hot

 Poor mold burnout

Shrinkage Cavities

These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick Sprues to avoid shrinkage cavities.

Molding Material Defect s

The defects in this category are cuts and washes, metal penetration, fusion, and swell.

Cut and washes

These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the molding sa nd not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of molding sand and the latter
can be overcome by the proper design of the gating system.

Metal penetration

When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration.

Fusion

This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.

Swell

Under the influence of metallostatic forces, the mold wall may move back causing a swell in the dimension of the casting. A proper ramming of the mold will correct this defect.

Inclusions

Particles of slag, refractory materials, sand or deoxidation prod ucts are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mold can prevent this defect.

Pouring Metal Defects

The likely defects in this category are

 Mis-runs and

 Cold shuts.

A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis -run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design.

A cold shut is caused when two streams while meeting in the mold cavity, do not  fuse together properly thus forming a discontinuity in the casting. When the

Molding Material and Properties

A large variety of molding materials is used in foundries for manufacturing molds and cores. They include molding sand, system sand or backing sand, facing sand, parting sand, and core sand. The choice of molding materials is based on their processing properties. The properties that are generally required in molding materials are:

Refractoriness: It is the ability of the molding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the silica sand is highest.

Permeability: During pouring and subsequent solidification of a casting, a large amount of gases and steam is generated. These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the molding and core sand. If these gases are not allowed to escape from the mold, they would be entrapped inside the casting and cause casting defects. To overcome this problem the molding material must be porous. Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity.

Green Strength: The molding sand that contains moisture is termed as green sand. The green sand particles must have the ability to cling to each other to impart sufficient strength to the mold. The green sand must have enough strength so that the constructed mold retains its shape.

Dry Strength: When the molten metal is poured in the mold, the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand evaporates due to the heat of the molten metal. At this stage the molding sand must posses the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material.

Hot Strength: As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength.

Collapsibility: The molding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings. Besides these specific properties the molding material should be cheap, reusable and should have good thermal conductivity.

Molding Sand Composition

The main ingredients of any molding sand are:

• Base sand,
• Binder, and
• Moisture

Base Sand

Silica sand is most commonly used base sand. Other base sands that are also used for making mold are zircon sand, Chromite sand, and olivine sand. Silica sand is cheapest among all types of base sand and it is easily available.

Binder

Binders are of many types such as:

1. Clay binders,
2. Organic binders and
3. Inorganic binders

Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength. The most popular clay types are:

Kaolinite or fire clay (Al₂O₃ 2 SiO₂ 2 H₂O) and Bentonite (Al₂O₃ 4 SiO₂ nH₂O)

Of the two the Bentonite can absorb more water which increases its bonding power.

Moisture

Clay acquires its bonding action only in the presence of the required amount of moisture. When water is added to clay, it penetrates the mixture and forms a microfilm, which coats the surface of each flake of the clay. The amount of water used should be properly controlled. This is because a part of the water, which coats the surface of the clay flakes, helps in bonding, while the remainder helps in improving the plasticity. A typical composition of molding sand is given in

Casting defects

Defects may occur due to one or more of the following reasons:

–Fault in design of casting pattern

–Fault in design on mold and core

–Fault in design of gating system and riser

–Improper choice of molding sand

–Improper metal composition

–Inadequate melting temperature and rate of pouring

Some common defects in castings:

a) Misruns b) Cold Shut c) Cold Shot d) Shrinkage Cavity e) Micro porosity

 f) Hot Tearing Misruns:

A) Misruns

It is a casting that has solidified before completely filling the mold cavity.

Typical causes include

1) Fluidity of the molten metal is insufficient,

2) Pouring Temperature is too low,

3) Pouring is done too slowly and/or

4) Cross section of the mold cavity is too thin.

b) Cold Shut

A cold shut occurs when two portion of the metal flow together, but there is lack of fusion between them due to premature freezing, Its causes are similar to those of a Misruns.

c) Cold Shots

When splattering occurs during pouring, solid globules of the metal are formed that become entrapped in the casting. Poring procedures and gating system designs that avoid splattering can prevent these defects.

d) Shrinkage Cavity

This defect is a depression in the surface or an internal void in the casting caused by solidification shrinkage that restricts the amount of the molten metal available in the last region to freeze.

e) Micro porosity

This refers to a network of a small voids distributed throughout the casting caused by localized solidification shrinkage of the final molten metal in the dendritic structure.

f) Hot Tearing

This defect, also called hot cracking, occurs when the casting is restrained or early stages of cooling after solidification.

Sand Casting is simply melting the metal and pouring it into a preformed cavity, called mold, allowing (the metal to solidify and then breaking up the mold to remove casting. In sand casting expandable molds are used. So for each casting operation you have to form a new mold.

• Most widely used casting process.

• Parts ranging in size from small to very large

• Production quantities from one to millions

• Sand mold is used.

• Patterns and Cores

–Solid, Split, Match-plate and Cope-and-drag

 Patterns

–Cores

–achieve the internal surface of the part

Molds

–Sand with a mixture of water and bonding clay

–Typical mix: 90% sand, 3% water, and 7% clay

–to enhance strength and/or permeability Sand

–Refractory for high temperature

Size and shape of sand

Small grain size -> better surface finish

Large grain size -> to allow escape of gases during pouring

Irregular grain shapes -> strengthen molds due to interlocking but to reduce permeability

Types of sand

a) Green-sand molds - mixture of sand, clay, and water; “Green" means mold contains moisture at time of pouring.

b) Dry-sand mold - organic binders rather than clay and mold is baked to improve strength

c) Skin-dried mold - drying mold cavity surface of a green-sand –mold to a depth of 10 to 25 mm, using torches or heating.

Steps in Sand Casting

The cavity in the sand mold is formed by packing sand around a pattern, separating the mold into two halves

The mold must also contain gating and riser system, For internal cavity, a core must be included in mold.

A new sand mold must be made for each part .

1. Pour molten metal into sand mold.

2. Allow metal to solidify.

3. Break up the mold to remove casting.

4. Clean and inspect casting.

5. Heat treatment of casting is sometimes required to improve metallurgical properties

 

Module-I

Lecture Notes of Chinmay Das

It is the task of casting designer to reduce all hot spots so that no shrinkage cavities occurred. Since solidification of the casting occurs by loosing heat from the surfaces and the amount of the heat is given by the volume of the casting, the cooling characteristics of a casting can be represented by the surface area to the volume ratio. Since the riser is almost similar to the casting in its solidificationbehaviour, the riser characteristics can also be specified by the ratio of its surface area to volume. If this ratio of casting is higher, then it is expected to cool faster. According to Chvorinov, solidification time can be calculated as

ts=K {SAV } 2

Where ts= solidification time

 sV = volume of the casting,

SA = surface area

K = mould constant which depends on pouring temperature, casting & mould thermalCharacteristicsThe freezing ratio,

Xof a mould is defined as the ratio of cooling characteristics of casting to that of theriser.

X=VriserSAriser / Vcasting / SAcasting

 In order to feed the casting, the riser should solidify last and hence its freezing ratio should be greater thanunity.

CAINE’s Method

X = { a / Y-b} + c

Where Y = riser volume / casting volume a, b, c are constants whose values for different materials are given here.

Material a b c

Steel 0.10 0.03 1.00Aluminium 0.10 0.06 1.08Cast iron, Brass 0.04 0.017 1.00Grey cast iron 0.33 0.030 1.00Aluminium bronze 0.24 0.017 1.00Silicon bronze 0.24 0.017 1.00Table IV: Values of a,b,c for different materialsDesign Requirements of Risers1. Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surface area)

2

 of the riser must be greater than that of the casting. However, when this condition does not meet, the metalin the riser can be kept in liquid state by heating it externally or using exothermic materials in the risers.2. Riser placement: the spacing of risers in the casting must be considered by effectivelycalculating the feeding distance of the risers.3. Riser shape: cylindrical risers are recommended for most of the castings as spherical risers,although considers as best, are difficult to cast. To increase volume/surface area ratio the bottom of the risercan be shaped as hemispher

 

Module-I

Lecture Notes of Chinmay Das

Runner Extension:

Normally the metal which moves first into the gating system is likely to contain slag and dross which should not be allowed to get into the mould cavity. This could be achieved by extending the runner beyond the ingates so that the momentum of the metal will carry it past the gates and to a blind alley called runner extension. A runner extension having a minimum of twice the runner width is desirable.

Whirl Gate: Another method employed successfully to trap the slag from entering steel casting is a whirl gate. This utilizes the principle of centrifugal action to throw the dense metal to the periphery and retain the lighter slag at the centre. In order to achieve this action, it is necessary that entry area should be at least 1.5times the exit area so that the metal is built up at the centre quickly. Also the metal should revolve 2700 before reaching the exit gate so as to gain enough time for separating the impurities.

Figure 11: Whirl gate

Design of Riser:

The function of a riser (also called reservoir, feeders, or headers) is to feed the casting during solidification so that no shrinkage cavities are formed. The requirement of risers depends to a great extent upon the type of metal poured and the complexity of the casting. Let us consider the mould of a cube which is filled with liquid metal. As time progresses, the metal starts losing heat through all sides and as a result starts freezing from all sides equally trapping the liquid metal inside. But further solidification and subsequent volumetric shrinkage and the metal contraction due to change in temperature causes formation of a void. The solidification when complete, finally results in the shrinkage cavity as shown in the figure. The reason for the formation of the void in the cube casting is that the liquid metal in the centre which solidifies in the end is not fed during the solidification; hence the liquid shrinkage ends up as a void. Such isolated spots which remain hot till the end are called hot spots.

.Figure 12: Solidification of cube casting

Functions of Risers

•Provide extra metal to compensate for the volumetric shrinkage

•Allow mold gases to escape

•Provide extra metal pressure on the solidifying metal to reproduce mold details more exactly.

•To compensate mould expansion during pouring of hot liquid metal because of soft mould

 

Module-I

Lecture Notes of Chinmay Das

GATING SYSTEM DESIGN

Figure 1: Gating systems

 

Module-I

Lecture Notes of Chinmay Das

Elements of Gating System

The gating systems refer to all those elements which are connected with the flow of molten metal from the ladle to the mould cavity. The elements of gating systems are

•Pouring Basin

•Sprue

•Sprue Base Well

•Runner

•Runner Extension

•Ingate

RiserFigure 2: Components of a gating system

Any gating system designed should aim at providing a defect free casting.

This can be achieved by considering following requirements.

•The mould should be completely filled in the smallest possible time without having to raise neither metal temperature nor use of higher metal heads.

•The metal should flow smoothly into the mould without any turbulence. A turbulence metal flow tends to form dross in the mould.

•Unwanted materials such as slag, dross and other mould materials should not be allowed to enter the mould cavity.

•The metal entry into the mould cavity should be properly controlled in such a way that aspiration of the atmospheric air is prevented.

•A proper thermal gradient should be maintained so that the casting is cooled without any shrinkage cavities or distortions.

•Metal flow should be maintained in such a way that no gating or mould erosion takes place.

•The gating system should ensure that enough molten metal reaches the mould cavity.

•It should be economical and easy to implement and remove after casting solidification.

•The casting yield should be maximized.

 

Module-I

Lecture Notes of Chinmay Das

The liquid metal that runs through the various channels in the mould obeys the Bernoulli’s theorem which states that the total energy head remains constant at any section. Ignoring frictional losses, we have

Where

h = Potential Head,

mP = Static Pressure,

Pav = Liquid Velocity, m / s

ρg = w = Specific weight of liquid, N / m2

 g = Acceleration due to gravity, m / s2

 Though quantitatively

Bernoulli’s theorem may not be applied; it helps to understand qualitatively, the metal flow in the sand mould. As the metal enters the pouring basin, it has the highest potential energy with no kinetic or pressure energies. But as the metal moves through the gating system, aloss of energy occurs because of the friction between the molten metal and the mould walls. Heat is continuously lost through the mould material though it is not represented in the Bernoulli’s equation.

Another law of fluid mechanics, which is useful in understanding the gating system behavior, is the law of continuity which says that the volume of metal flowing at any section in the mould is constant. The same in equation form is Q = A1V1

= A

2V2 

Where Q = Rate of flow, m3/ s

A = Area of cross section, m2

 V = Velocity of metal flow, m / s

Pouring Time

The main objective for the gating system design is to fill the mould in the smallest time. The time for complete filling of a mould is called pouring time. Too long a pouring time requires a higher pouring temperature and too less a pouring time means turbulent flow in the mould which makes the casting defect prone. The pouring time depends on the casting materials, complexity of the casting, section thickness and casting size. Steels lose heat very fast , so required less pouring time while for non-ferrous materials longer pouring time is beneficial because they lose heat slowly and tend to form dross if metal is pour too quickly. Ratio of surface area to volume of casting is important in addition to the mass of the casting. Also gating mass is considered when its mass is comparable to the mass of the casting.

•For grey cast iron up to 450 KgPouring time, t = K { 1.41 +

59.14

T 

}

W 

secondsWhere K = Fluidity of iron in inches / 40T = Average section thickness, mm

W = Mass of the casting, Kg

•For grey cast iron greater than 450 KgPouring time, t = K { 1.236 +

65.16T }3W seconds Typical pouring times for cast iron are

Casting mass pouring time in seconds

20 Kg 6 to 10100 Kg 15 to 30

•Steel CastingPouring time, t = (2.4335 – 0.3953 logW )W seconds

•Shell moulded ductile iron( vertical pouring)Pouring time, t = K1W seconds

• 1. Foundry: ” Foundry or casting is the process of producing metal/alloy component parts of desired shapes by pouring the molten metal/alloy into a prepared mould (of that shape) and then allowing the metal/alloy to cool and solidify. The solidified piece of metal/alloy is known as a CASTING”.
• 2. Casting Terms:1. Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed.drag - lower molding flask,cope - upper molding flask,cheek - intermediate molding flask used in three piece molding.2. Pattern: It is the replica of the final object to be made. The mold cavity is made with the help of pattern.3. Parting line: This is the dividing line between the two molding flasks that makes up the mold.
• 3. 4. Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings.5. Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metal is poured.6. Sprue: The passage through which the molten metal, from the pouring basin, reaches the mold cavity. In many cases it controls the flow of metal into the mold.7. Runner: The channel through which the molten metal is carried from the sprue to the gate.
• 4. 8. Gate: A channel through which the molten metal enters the mold cavity.9. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force.10. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as feed head.11. Vent: Small opening in the mold to facilitate escape of air and gases.
• 5. Basic Features:Pattern and Mould ◦ A pattern is made of wood or metal, is a replica of the final product and is used for preparing mould cavity. ◦ Mould material should posses refractory characteristics and with stand the pouring temperature.
• 6. Schematic diagram of castingmould:
• 7. Sand Casting Terminology Ken Youssefi Mechanical Engineering Dept., SJSU 10
• 8. Steps involved in making a casting: 1. Make the pattern out of Wood , Metal or Plastic. 2. Prepare the necessary sand mixtures for mould and core making. 3. Prepare the Mould and necessary Cores. 4. Melt the metal/alloy to be cast. 5. Pour the molten metal/alloy into mould and remove the casting from the mould after the metal solidifies. 6. Clean and finish the casting. 7. Test and inspect the casting. 8. Remove the defects, if any. 9. Relieve the casting stresses by Heat Treatment. 10. Again inspect the casting. 11. The casting is ready for shipping.
• 9.  TransportationØApplications of Casting:  TurbineØvehicles Øvanes  Aircraft jetØ Agricultural partsØ Railway crossingsØPower generators  Communication, Construction andØ Sanitary fittingsØengine parts Atomic Energy applications, etc..
• 10. Raw Materials forFoundry: 1) Metals and alloys. 2) Fuels (for melting metals). 3) Fluxes.
• 11. Metals and alloys commonlyused in Foundries: 1. Ferrous 2. Non-Ferrous FERROUS: a. Cast irons b. Steels NON-FERROUS: a. Copper alloys b. Aluminium alloys c. Magnesium alloys d. Zinc alloys e. Nickel alloys
• Av12. Pattern Making: Pattern Except forvis a model or the replica of the object to be cast. the various allowances a pattern exactly resembles the casting to be A pattern is required even if one object has to be cast.vmade.
• 13.  A Pattern prepares a mould cavity for the§Functions of Patterns:  A Pattern may contain projections known§purpose of making a casting. as core prints if the casting requires a core and need to be made  Patterns properly made and having§hollow. finished and smooth surfaces reduce  Properly constructed patterns minimize§casting defects. overall cost of the casting.
• 14. Pattern having core prints.
• 15. Selection of Pattern Materials:The following factors assist in selecting proper pattern material: No. of castings to be produced.Ø Dimensional accuracyØ & surface finish. Shape, complexityØ and size of casting. Casting design parameters.Ø Type ofØ molding materials. The chance of repeat orders.Ø Nature ofØ molding process.
• 16. The pattern material should be:1. Easily worked, shaped and joined.2. Light in weight.3. Strong, hard and durable.4. Resistant to wear and abrasion .5. Resistant to corrosion, and to chemical reactions.6. Dimensionally stable and unaffected by variations in temperature and humidity.7. Available at low cost.
• 17. Materials for making patterns: a. Wood b. Metal c. Plastic d. Plaster e. Wax.
• 18. Types of Patterns: 1. Single piece pattern. 2. Split piece pattern. 3. Loose piece pattern. 4. Match plate pattern. 5. Sweep pattern. 6. Gated pattern. 7. Skeleton pattern 8. Follow board pattern. 9. Cope and Drag pattern.
• 19. (a)Split pattern(b) Follow-board(c) Match Plate(d) Loose-piece(e) Sweep(f) Skeletonpattern
• 20. Fig: Single piece pattern
• 21. Fig: split piece pattern
• 22. 3.Loose piece pattern:
• 23. Fig: Match plate pattern
• 24. Sweep pattern:
• 25. castings Gating systemGATED PATTRN
• 26. GATED PATTRN
• 27. Fig: Cope and drag pattern
• 28. Types of Pattern Allowances:The various pattern allowances are: 1. Shrinkage or contraction allowance. 2. Machining or finish allowance. 3. Draft of tapper allowances. 4. Distortion or chamber allowance. 5. Shake or rapping allowance.
• 29. 1.Shrinkage Allowance: All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types:1. Liquid Shrinkage:2. Solid Shrinkage:
• 30. 2. Machining Allowance: A Casting is given an allowance for machining, because:i. Castings get oxidized in the mold and during heat treatment; scales etc., thus formed need to be removed.ii. It is the intended to remove surface roughness and other imperfections from the castings.iii. It is required to achieve exact casting dimensions.iv. Surface finish is required on the casting.
• §31. 3. Draft or Taper Allowance: It is given to all surfaces perpendicular to §parting line. Draft allowance is given so that the pattern can be easily removed from the molding material tightly packed around it with out damaging the mould cavity.
• 32. Fig: taper in design
• 33. 4. Distortion or cambered allowance:A casting will distort or wrap if : i. It is of irregular shape, ii. All it parts do not shrink uniformly i.e., some parts shrinks while others are restricted from during so, iii. It is u or v-shape,
• Aq34. 5. Shake allowance: patter is shaken or rapped by striking the same with a wooden piece from side to side. This is done so that the pattern a little is loosened  Inqin the mold cavity and can be easily removed. turn, therefore, rapping enlarges the mould cavity which results in a bigger sized  Hence, a –ve allowance is provided on the pattern i.e., theqcasting. pattern dimensions are kept smaller in order to compensate the enlargement of mould cavity due to rapping.
• 35. Pattern ØLayout:Steps involved: Get the working drawing of the part for which the Øpattern is to be made. Make two views of the part drawing on a sheet, using a shrink rule. A shrink rule is modified form of an ordinary scale which has already taken care of shrinkage allowance for a particular metal Øto be cast. Add machining allowances as per the Ørequirements. Depending upon the method of molding, provide the draft allowance.
• 36. Pattern  StudyqConstruction: the pattern layout carefully and establish, a. Location of parting surface. b. No. of parts in which the pattern qwill be made. Using the various hand tools and pattern making qmachines fabricate the different parts of the pattern. Inspect the pattern as regards the alignment of different portions of the pattern and its dimensional qaccuracy. Fill wax in all the fillets in order to remove sharp qcorners. Give a shellac coatings(3 coats) to qpattern. impart suitable colors to the pattern for identification purposes and for other informations.
• 37. Moulding Materials Major part of Moulding material in sand casting are 1. 70-85% silica sand (SiO2) 2. 10-12% bonding material e.g., clay cereal etc. 3. 3-6% water Properties of molding sand are: (a) Refractoriness (b) Cohesiveness (c) Strength/Adhesiveness (d) Permeability (e) Collapsibility (f) Flowability (g) Chemical Inactiveness
• 38. Molding Sand Composition:The main ingredients of any molding sand are:  Base sand,–  Binder, and–  Moisture– – Additives
• 39. Shape of the Sand Grains
• 40. Effect of moisture, grain size and shapeon mould quality
• 41. Grain size of sandThere are three distinct sizeof sand grains:1. Fine2. Medium3. Coarse
• 42. Types of Moulding Sand1. Green Sand2. Dry Sand3. Facing Sand4. Loam Sand5. Backing Sand6. Parting Sand7. Core Sand
• 43. 2 types of moulding flask designs
• 44. SAND MOULDING PROCESS
• 45. Sand mold - opened
• 46. Sand mold - closed
• 47. Mixing moulding sand with binders & adhesives
• 48. Filling sand in moulding flasks
• 49. Melting furnace
• 50. Pouring molten liquid
• 51. Knock out
• 52. Heat treatment
• 53. Machining
• 54. final products of casting
• 55. Casting Methods• Sand Casting • Investment Casting • Die CastingHigh Temperature Alloy, High Temperature Alloy, High Temperature Alloy,Complex Geometry, Complex Geometry, Moderate Geometry,Rough Surface Finish Moderately Smooth Surface Smooth Surface Finish 65

Top of Form

Bottom of Form

  ¡64.  Permanent mold casting is a metal casting process that shares ¡similarities to both sand casting and die casting. As in sand casting, molten metal is poured into a mold which is clamped shut until the material cools and solidifies into the desired part ¡shape. However, sand casting uses an expendable mold which is destroyed after each cycle. Permanent mold casting, like die casting, uses a metal mold (die) that is typically made from steel or cast iron and can ¡be reused for several thousand cycles. Because the molten metal is poured into the die and not forcibly injected, permanent mold casting is often referred to as gravity die casting.

  65. ¡ Mould preparation - First, the mould is pre-heated to around 300-500°F (150-260°C) to allow better metal ¡flow and reduce defects. Mould assembly - The mould consists of at least two parts - the two mold halves and any cores used to form complex features. Such cores are typically made from iron or steel, but expendable sand cores are sometimes used. In this step, the cores are inserted and the mold halves are clamped together

  ¡66.  Pouring - The molten metal is poured at a slow rate from a ladle into the mold through a sprue at the top of the mold. The metal flows through a runner system and enters the ¡mold cavity. Cooling - The molten metal is allowed to cool and solidify ¡in the mould. Mold opening - After the metal has solidified, the two mold halves are opened and ¡the casting is removed. Trimming - During cooling, the metal in the runner system and sprue solidify attached to the casting. This excess material is now cut away.

  ¡67.  Advantages:  Can formØ complex shapes.  Good mechanical properties.Ø  Many materialØ options.  Low porosity.Ø  Low labor costØ  Scrap can beØ ¡recycled. Disadvantages:  High tooling cost.Ø  Long lead timeØ ¡possible. Applications: Gears, wheels, housings, engine components.

  ¡68.  For any Metal Casting Process, selection of right alloy, size, shape, thickness, tolerance, texture, and weight, is very ¡vital. Special requirements such as, magnetism, corrosion, stress distribution also influence the choice of the Metal ¡Casting Process. Views of the Tooling Designer; Foundry / Machine House needs, customers exact product requirements, and secondary operations like painting, must be taken care of before selecting the appropriate Metal Casting Process.  Tool cost.Ø  Economics ofØ machining versus process costs.  Adequate protection / packaging,Ø shipping constraints, regulations of the final components, weights and shelf life of protective coatings also play their part in the Metal Casting process.