Casting Terms (Click on thefigure 1 to view)

Casting Terms (figure 1 to view)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 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.

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 as given below:

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 When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become the casting.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 thecase 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 betwe 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 improved the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Inspection of 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 tobe made by the casting process, with some modifications. The main modifications are theaddition of pattern allowances, and the provision of core prints. If the casting is to behollow, additional patterns called cores are used to create these cavities in the finishedproduct. The quality of the casting produced depends upon the material of the pattern, itsdesign, and construction. The costs of the pattern and the related equipment are reflectedin the cost of the casting. The use of an expensive pattern is justified when the quantity ofcastings required is substantial.

Functions of the Pattern

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

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.

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 thecasting. Thus, when the pattern is produced, certain allowances must be given on thesizes specified in the finished component drawing so that a casting with the particularspecification can be made. The selection of correct allowances greatly helps to reducemachining costs and avoid rejections. The allowances usually considered on patterns andcore boxes are as follows:

Shrinkage or Contraction Allowance All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types: i. Liquid Shrinkage: it refers to the reduction in volume when the metal changes from liquid state to solid state at the solidus temperature. To account for this shrinkage; riser, which feed the liquid metal to the casting, are provided in the mold.

ii. Solid Shrinkage: it refers to the reduction in volume caused when metal loses emperature in solid state. To account for this, shrinkage allowance is provided on the patterns.

The rate of contraction with temperature is dependent on the material. For example steelcontracts to a higher degree compared to aluminum. To compensate the solid shrinkage, ashrink rule must be used in laying out the measurements for the pattern. A shrink rule forcast iron is 1/8 inch longer per foot than a standard rule. If a gear blank of 4 inch indiameter was planned to produce out of cast iron, the shrink rule in measuring it 4 inchwould actually measure 4 -1/24 inch, thus compensating for the shrinkage. Exercise 1

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

INCLUDEPICTURE "http://htmlimg3.scribdassets.com/at0py8yl25tg1q8/images/9-8f1a0c54e0/000.png" \* MERGEFORMATINET

INCLUDEPICTURE "http://htmlimg3.scribdassets.com/at0py8yl25tg1q8/images/9-8f1a0c54e0/000.png" \* MERGEFORMATINET Solution 1

The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet For dimension 18 inch, allowance = 18X 0.125 / 12 = 0.1875 inch 0.2 inchFor dimension 14 inch, allowance = 14X 0.125 / 12 = 0.146 inch 0.15 inchFor dimension 8 inch, allowance = 8 X 0.125 / 12 = 0.0833 inch 0. 09 inchFor dimension 6 inch, allowance = 6 X 0.125 / 12 = 0.0625 inch 0. 07 inch

The pattern drawing with required dimension is shown in next page.Draft or Taper Allowance

By draft is meant the taper provided by the pattern maker on all vertical surfaces of thepattern so that it can be removed from the sand without tearing away the sides of the sandmold and without excessive rapping by the molder. Figure 3 (a) above shows a pattern havingno draft allowance being removed from the pattern. In this case, till the pattern iscompletely lifted out, its side s will remain in contact with the walls of the mold, thustending to break it. Figure 3 (b) is an illustration of a pattern hav ing proper draftallowance. Here, moment the pattern lifting commences, all of its surfaces are wellaway from sand surface. Thus pattern can be removed without damaging moldcavity.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.

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

Figure 3 (b) Pattern Having Draft on Vertical Edges

Machining or Finish Allowance

The finish and accuracy achieved in sand casting are generally poor and the refore whenthe casting is functionally required to be of good surface finish or dimensionally accurate,it is generally achieved by subsequent machining. Machining or finish allowances aretherefore added in the pattern dimension. The amount of machining allowance to beprovided for is affected by the method of molding and casting used viz. hand molding ormachine molding, sand casting or metal mold casting. The amount of machiningallowance is also affected by the size and shape of the casting; the casting orientation; themetal; and the degree of accuracy and finish required. 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 For dimension 18 inch, allowance = 0.20 inchFor dimension 14 inch, allowance = 0.20 inchFor dimension 8 inch, allowance = 0.12 inchFor dimension 6 inch, allowance = 0.12 inchThe pattern drawing with required dimension is shown in Figure above.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 preventedby making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so thatthe casting after distortion will have its sides vertical.

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

Rapping Allowance

Before the withdrawal from the sand mold, the pattern is rapped all around the verticalfaces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges thefinal casting made, it is de sirable that the original pattern dimension should be reduced toaccount for this increase. There is no sure way of quantifying this allowance, since it ishighly dependent on the foundry personnel practice involved. It is a negative allowanceand 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. Theseimpressions can be obtained by using cores. So where coring is requ ired, provisionshould be made to support the core inside the mold cavity. Core prints are used to servethis purpose. The core print is an added projection on the pattern and it forms a seat in themold on which the sand core rests during pouring of the m old. The core print must be ofadequate size and shape so that it can support the weight of the core during the castingoperation. Depending upon the requirement a core can be placed horizontal, vertical andcan be hanged inside the mold cavity. A typical job, its pattern and the mold cavity withcore and core print is shown in Figure 5.

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

Core Boxes:

1. Half box: Used to form two identical halves of a symmetrical core. After they are shaped and baked, the core halves are pasted togeyher.

2. Dump box: designed to form a complete core that requires no pasting.

3. Split box: consists of two halves which are clamped together.

4. Strickle box: used when a core with an irregular shape is required. 5. Right & Left box: when two half cores made in the same box cannot be pasted together to form an entire core.

6. Gang box: where large number of cores are to be made.

7. Sweep & skeleton box: looks like a sweep & skeleton pattern. Used for large cores required in small quantities.Green Sand Molding

Green sand is the most diversified molding method used in metal casting operations. Theprocess utilizes a mold made of compressed or compacted moist sand. The term "green"denotes the presence of moisture in the molding sand. The mold material consists ofsilica 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 oflocating 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 ofpaste 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 (Figure 8 (c) )Figure 8 (a)

Figure 8 (b)

Figure 8 (c)

Molding Material and Properties

A large variety of m olding materials is used in foundries for manufacturing molds andcores. They include molding sand, system sand or backing sand, facing sand, partingsand, and core sand. The choice of molding materials is based on their processingproperties. The properti es that are generally required in molding materials are:

1. 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.2. 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 themolding material must be porous. Proper venting of the mold also helps in escaping thegases that are generated inside the mold cavity.3. Cohesiveness: This is the ability of sand particles to stick together. Insufficient strength may lead to a collapse in the mould or its partial destruction during conveying, turning over or closing.3.1 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 itsshape.3.2 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 vaporates 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.3.3 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.

4. 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 thecastings.Besides these specific properties the molding material should be cheap, reusableand should have good thermal conductivity.5. Flowability: Ability to behave like a fluid so that, when rammed it will flow to all portions of a mould and pack all-round the pattern and take up the required shape. It increases as clay and water content increase.

6. Adhesiveness: Sand particles must be capable of adhering to another body i.e. should cling to the sides of moulding boxes.Shell Molding Process ( also called Croning process or C-process)It is a process in which, the sand (100-150 mesh) mixed with a thermosetting resin is allowed to come in contact with a heated pattern plate (200 0C) (preferably made of grey CI), 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. Advantages: This process can produce complex parts with good surface finish 1.25 m to 3.75 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. Floor space and sand quality are reduced. Unskilled labour can be employed. Uses: The materials that can be used with this process are cast irons, and aluminum and copper alloys. It is used for mass production of steel casting of less than 100 kg.

Limitations: The main limitations are its high costs of pattern, resin & equipment; uneconomical for smal runs, maximum casting size & weight are limited; shrinkage factors vary with casting practice.

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 mixture 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 sand mixtures.In the method of hot coating, the mixture is heated to 150 -180oC 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 90o C. This method gives bet ter properties to the mixtures than cold method.

Permanent Mold Process

In the processes like green sand moulding / dry sand moulding, 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. For higher melting alloys such as brass and ferrous alloys, the mould must contain large proportion of stale carides.

It is a casting process involving pouring a molten metal by gravity into a steel (or cast iron) mold and is similar to the sand casting process . In distinction from sand molds, which are broken after each casting a permanent mold may be used for pouring of at least one thousand and up to 120,000 casting cycles with the rate 5-100 castings/hour. Manufacturing metal mold is much more expensive than manufacturing molds for Sand casting or investment casting process mold. Minimum number of castings for profitable use of a permanent mold is dependent on the complexity of its shape.

A mould material should have a high melting temperature, enough strength not to deform in repeated use, high thermal fatigue resistance to resist premature cracks and low adhesion.The process in which we use a die to make the castings is called permanent mold castingor 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 ininjecting the metal it is called pressure die casting process. All cast metals can be cast by permanent mould method. Zn, Cu, Al, Pb, Mg and Sn alloys, steel and CIs are most often cast by this method. Used for small & medium sized castings (upto 10 kg) non ferrous, but impractical for large , metals/ alloys with high melting temperatures. Permanent mold casting process

The interior surfaces of the two parts (cope and drag) of a permanent mold are coated with a thin ceramic coating. The mold is preheated before coating to 150-260C.

The cores are inserted and installed in the mold assembly.

The mold is closed.

The molten metal is poured into the mold.

After the casting has solidified and cooled down to the desired temperature the mold is opened and the casting is withdrawn from it.

The gating system is cut away from the casting.

The finish operations are carried out.

Permanent cores are commonly used for permanent mold castings, however if a casting has cavitys shape not allowing a withdrawal of the core it is made of chemically bonded sand or other materials used for preparation of expendable cores. New consumable cores are added after each pour. The process combining permanent mold and consumable parts (cores) are called semi-permanent casting.

Advantages: Advantages of permanent mold casting process are determined by relatively high cooling rate caused by solidification in metallic mold:

Better mechanical properties.

Homogeneous grain structure and chemical composition.

Low shrinkage and gas porosity.

Good surface quality: 40-250 inch (1-6 m) Ra.

Low dimensional tolerances: typically about 0.04 (1 mm}.

Little scrap process.

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.

Die casting

It is a process, in which the molten metal is injected into the mold cavity at an increased pressure up to 30,000 psi (200 MPa). The reusable steel mold used in the die casting process is called a die. It is a highly productive method of casting parts with low dimensions tolerance and high surface quality. The parts manufactured by die casting method are automotive connecting rods, pistons, cylinder beds, electronic enclosures, toys, plumbing fittings. This process is particularly suitable for Pb, Mg, Sn, and Zn alloys. The molten metal injection is carried out by a machine called die casting machine. If the parts are small, several parts may be cast at one time in what is known as multiple-cavity die.There are two principal die casting methods: hot chamber method and cold chamber method.

Cold chamber die casting: In the cold chamber die casting machines hydraulically operated plunger forces a molten metal to flow in the cold cylinder (chamber). A principal scheme of the cold chamber die casting machine is shown in the picture

Cold chamber process:

When the pressure chamber is filled with a molten metal the plunger starts traveling forward and builds up a pressure forcing the metal to flow through the sprue to the die cavity.

After the metal has solidified the plunger returns to its initial position allowing a new portion of the molten metal to fill the pressure chamber.

The die then opens and the ejector pins removes the casting from the die.

The casting cycle now may be repeated.

Cold chamber method is mainly used for casting Aluminum alloys, Magnesium alloys, Copper alloys and zinc alloys (including zinc-aluminum alloys).

Hot chamber die casting: In this, die casting machines the pressure chamber (cylinder) and the plunger are submerged in the molten metal in the pot (crucible).

Hot chamber machines have short casting cycle (about 1 sec.). They are capable to cast thin wall casting with good filling the cavity under precise temperature control of the molten metal.

Hot chamber process may be used for casting low melting metals, which are chemically inert to the material of the plunger and other parts of the casting machine: zinc alloys (except zinc alloys containing more than 10% of aluminum), tin alloys and Magnesium alloys.

Maintenance of these machines is more expensive as compared to the cold chamber process.

Hot chamber process:

The plunger goes up allowing the melt to fill the cylinder space. The die is closed at this stage.

The plunger goes down forcing the melt to flow through the gooseneck into the die cavity.

After the die has been filled with the melt the plunger is held under a pressure until the solidification is completed.

The die opens. The casting stays in the die part equipped with ejectors.

The plunger goes up and the melt residuals return through the gooseneck back to the pot.

The ejectors push the casting out of the die.

Advantages of die casting:

High productivity.

Good dimensional accuracy.

Good surface finish: 2-100 inch (0.5-2.5 m) Ra.

Thin wall parts may be cast.

Very economical process at high volume production.

Fine Grain structure and good mechanical properties are achieved.

Intricate shapes may be cast.

Small size parts may be produced.

Disadvantages of die casting:

Not applicable for high melting point metals and alloys (eg. steels) Large parts can not be cast.

High die cost.

Too long lead time.

Some gases my be entrapped in form of porosity.

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 metalas it solidifies. The slag, oxides and other inclusions being lighter, get separated from themetal and segregate towards the center. This process is normally used for the making ofhollow 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 needsto 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 insidediameter is determined by the amount of molten metal poured into the mold. Figure 9 (Vertical Centrifugal Casting), Figure 10 ( Horizontal Centrifugal Casting)

True Centrifugal Casting: Molten metal is poured into a rotating mould to produce tubular parts such as pipes, tubes and rings.

Semi- Centrifugal Casting: Speed is not as high as in true casting. In this method, centrifugal force is used to produce solid castings rather than tubular parts. Density of the metal in the final casting is greater in the outer sections than at the centre of rotation. The process is used on parts in which center of casting is machined away, such as wheels and pulleys.

Centrifuged: Several identical or nearly similar mouds are located radially about a vertically arranged central riser or sprue which feeds metal into cavities through a number of radially gates. The entire mould is rotated with central sprue which acts as axis of rotation. Thus, it is not purely centrifugal process. Suitable for small, intricate parts where feeding problems are encountered.

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.As the mold material steels, Cast irons, Graphite or sand may be used. The rotation speed of centrifugal mold is commonly about 1000 RPM (may vary from 250 RPM to 3600 RPM).

A centrifugal casting machine is schematically presented in the above picture:

Centrifugal casting is carried out as follows: The mold wall is coated by a refractory ceramic coating (applying ceramic slurry, spinning, drying and baking).

Starting rotation of the mold at a predetermined speed.

Pouring a molten metal directly into the mold (no gating system is employed).

The mold is stopped after the casting has solidified.

Extraction of the casting from the mold.

Centrifugal casting technology is widely used for manufacturing of iron pipes, bushings, wheels, pulleys bi-metal steel-bronze bearings and other parts possessing axial symmetry.

Squeeze casting: Squeeze casting is a method combining casting and forging technologies. In contrast to other casting techniques (sand casting, die casting), in which a molten metal is poured (injected) into the mold cavity after the two parts of the mold are assembled, squeeze casting mold is closed after a portion of molten metal has been poured into the preheated bottom die. The upper die lowers towards the bottom die causing the melt to fill the mold cavity. The squeezing pressure is applied until full solidification of the casting. A scheme of the process is shown in the picture given below. Squeeze castings are characterized by:

low shrinkage and gas porosity;

enhanced mechanical properties because of fine grain structure caused by rapid solidification ;

good surface quality.

Squeeze casting is commonly used for processing aluminum and magnesium alloys. This process is also used for fabrication of reinforced metal matrix composites where molten aluminum infiltrates a fiber reinforcing structure.

Continuous casting

It is a casting method, in which the steps of pouring, solidification and withdrawal (extraction) of the casting from an open end mold are carried out continuously. Cross-sectional dimensions of a continuous casting are constant along the casting length and they are determined only by the dimensions of the mold cavity.

The length of a continuous casting is limited by the life time of the mold.

Continuous casting technology is used for both ferrous and non-ferrous alloys.

Traditional continuous casting processes use stationary (or oscillating) molds, in which the solidified bar moves relative to the mold surface. Friction caused by the movement results in formation of micro-cracks and other defects in the surface regions of the casting. Lubricating oil supplied to the mold surface and self-lubricating graphite molds decrease the friction / sticking and reduce the defective surface zone. This defective zone is commonly machined (milled) prior to Rolling.

The alternative continuous casting methods use moving endless molds (rolls, belts, wheels) characterized by zero relative movement between the mold and casting surfaces. Strips and slabs fabricated by Continuous casting in traveling mold have low defect surface. The castings may be further processed (rolled) without surface machining. Depending on the mold position (vertical or horizontal) continuous casting machines may be vertical or horizontal.

Vertical continuous casting Steels are commonly cast in these machines. Molten metal is continuously supplied from the ladle to the intermediate ladle (tundish) from which it is continuously poured into the mold at a controllable rate keeping the melt level at a constant position.

The water-cooled copper mold (primary cooling zone) extracts the heat of the metal causing its solidification. The mold oscillates in order to prevent sticking with the casting.

When the casting goes out from the mold it is cooled in the secondary cooling zone by water (or water with air) sprayed on the casting surface.

Most of vertical continuous casting machines are equipped with strand guide units bending the casting and changing its configuration from vertical to horizontal. The casting is continuously extracted from the mold by the withdrawal unit followed by a cut-off unit.

The casting process begins from inserting a dummy (primary) bar into the mold. Then a molten metal is poured into mold where it solidifies and grips the end of the dummy bar.

The dummy bar is disconnected from the casting after passing the withdrawal unit.

Horizontal continuous casting

This is generally used for casting non-ferrous alloys. Horizontal continuous casting in stationary mold with graphite water-cooled molds, Twin-roll caster and Twin-belt caster are most popular methods of this type. Due to the water cooling (primary and secondary) solidification rate provided by continuous casting is higher than in other casting methods therefore continuous castings have more uniform and finer grain structure and enhanced mechanical properties

Investment Casting Process (also called lost wax pattern/precision cating process)The root of the investment casting process, the cire perdue or "lost wax" method datesback to at least the fourth millennium B.C. The artists and sculptors of ancient Egypt andMesopotamia used the rudiments of the investment casting process to create intricatelydetailed jewelry, pectorals and idols. The investment casting process al os called lost waxprocess begins with the production of wax replicas or patterns of the desired shape of thecastings. A pattern is needed for every casting to be produced. The patterns are preparedby injecting wax or polystyrene in a metal dies. A numbe r of patterns are attached to acentral wax sprue to form a assembly. The mold is prepared by surrounding the patternwith refractory slurry that can set at room temperature. The mold is then heated so thatpattern melts and flows out, leaving a clean cavi ty behind. The mould is further hardenedby heating and the molten metal is poured while it is still hot. When the casting issolidified, 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 patterns2. Assembly of these patterns onto a gating system3. "Investing," or covering the pattern assembly with refractory slurry4. Melting the pattern assembly to remove the pattern material5. Firing the mold to remove the last traces of the pattern material

6. Pouring

7. Knockout, cutoff and finishing

Uses: for casting turbine blades, parts of motor cars, sewing machines, typewriters etc.

Advantages

Machining can be largely reduced or eliminated since tolerances close to +0.1 to -0.1 mm and surface finish of around 1-5microne are possible.

Extremely thin sections (to the extent 0.75 mm)

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

cavities and porosity . Sound & defect free casting may be obtained.

.Suitable for mass production of small sized castings.Disadvantages

Unsuitable for casting more than 5 kg weight

Precision control ir required in all stages.

Expensive in all respect. Melting Practices

Melting is an equally important parameter for obtaining a quality castings. Anumber of furnaces can be used for melting the metal, to be used, to make ametal casting. The choice of furnace depends on the type of metal to be melted.Some of the furnaces used in metal casting are as following:.

Crucible furnaces

Cupola

Induction furnace

Reverberatory furnace

Cupola

Objective of the cupola is to produce iron of desired compositions, temperature and properties at the required rate in the most economical manner. Advantages of cupola over other types of furnaces are its simplicity of operation, continuity of production and increased output coupled with high degree of efficiency.

Cupola furnaces are tall, cylindrical furnaces used to melt iron and ferrous alloys in foundry operations. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. A schematic diagram of a cupola is shown in Figure14 . This diagram of a cupola illustrates the furnace's cylindrical shaft lined with refractory and the alternating layers of coke and metal scrap. The molten metal flows out of a spout at the bottom of the cupola.

Description of Cupola

The cupola consists of a vertical cylindrical steel sheet, 6 to 12 mm thick, and lined inside with acid refractory bricks which consist of SiO2 and Al2O3.

The lining is generally thicker in the lower portion of the cupola as the temperature are higher than in upper portion. There is a charging door through which coke, pig iron, steel scrap and flux is charged.

The shell is mounted either on brick work foundation or on steel coloumns.

In a steel coloumn arrangement, used on most modern cupolas, cupolas are provided with a drop bottom door through which debris, consisting of coke, slag etc. can be discharged at the end of the melt.

In drop bottom cupolas, the working bottom is built up with moulding sand which covers the drop doors.

Through the tap hole molten metal is poured into the ladle and it is situated at the lowest point at the front of cupola.

A slag hole is provided to remove the slag from the melt which is opposite to the tap hole, and somewhat above the tapping hole.

The blast is blown through the tuyeres. These tuyeres are arranged in one or more row around the periphery of cupola.

Hot gases which ascends from the bottom (combustion zone) preheats the iron in the preheating zone.

At the top conical cap called the spark arrest is provided to prevent the spark emerging to outside.

Operation of Cupola

The cupola is charged with wood at the bottom. On the top of the wood a bed of coke is built. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. The pur pose of adding flux is to eliminate the impurities and to protect the metal from oxidation and also to reduce melting point of slag and to increase its fluidity for easy disposal. Air blast is opened for the complete combustion of coke. When sufficient metal has been melted that slag hole is first opened to remove the slag. Tap hole is then o pened to collect the metal in the ladle.

A constant volume of air for combustion is obtained from a motorised blower. The air os carried from blower through a pipe called windpipe (air blast inlet), first to a circular jacket around the shell called windbox and then into the furnace through a number of openings called tuyeres which are provided at a height of between 450 to 500 mm above the working bottom or bed of the cupola. These tuyeres are generally 4, 6 or 8 in numbers depending on the size of cupola and may be fitted in one or more number of rows. FIGURE 14

Zones in a Cupola:

1. Crucible Zone:

between top of the sand bed and bottom of tuyeres.

Molten metal is accumulated here.

Also caled the well or hearth.

2. Combustion or oxidizing zone:

situated normally 150 to 300 mm above the top of tuyeres.

All oxygen in the air blast is consumed here and actual combustion takes place here.

Lot of heat is liberated and this is supplied to other zones. Heat is also evolved due to oxidation of silicon and manganese.

Temp being 15500C to 18500C

Molten drops of cast iron pour onto the hearth.

The chemical reactions are:

C + O2 -( CO2 + Heat

Si + O -( SiO2 + Heat

2 Mn + O2 -( 2 MnO + Heat

3. Reducing zone:

extends from the top of combustion zone to the top of the coke bed.

Reduction of CO2 to CO occur and temp drops to about 12000C at the coke bed.

Due to the reducing atmosphere, the charge is protected from any oxiding influence.

The reaction taking place is:

CO2 + C (Coke) -( 2CO Heat

4. Melting Zone:

starts from the first layer of metal charge above the coke bed and extend up to a height of 900 mm.

highest temp (16000C) is developed in this zone for complete combustion of the coke and iron is thus melted here.

A considerable carbon pickup by molten metal also occurs in this zone:

3Fe + 2 CO -( Fe3C + CO25. Preheating Zone or charging zone:

starts from above melting zone and extends up to the bottom of the charging door.

Contains cupola charge as alternate layers of coke, flux and metal

They are preheated at a temp of 11000C before coming to melting zone.

6. Stack Zone:

extends from the above the preheating zone to the top of the cupola.

Carries the gases generated within the furnace to the atomsphere.

Capacity of cupola:

Output is defined as the tonnes of molten metal obtained per hour of the heat.

Capacities vary from 1 to 15 tonnes ( or even more) of melted iron per hour.

It has been observed that 14 cm3 of cupola plan area burns about 1 kg of coke/hour.

Diameter of cupola varies from 1 to 2 m with a height of from 3 to 5 times of dia.

Cupola Operation:

1. Preparation of cupola Clean out slag & refuse on the lining and around tuyeres

Bad spots or broken bricks are repaired.

Preparation of sand bottom is begun.

Bottom doors are raisd & bottom sand is introduced through the charging doors and is rammed well.

Surface of sand bottom is sloped from all directions.

Slag hole & tap hole are formed.

Cupola should be thoroughly dried before firing.

2. Firing of cupola Wood is ignited on sand bottom. It should be done 2.5-3 hrs before molten metal is required.

A bed of coke is built on the top of wood.

Coke is added to a level slightly above tuyeres and air blast is turned on a ta lower than blowing rate to ignite coke.

As soon as red spots begin to show over the top of fuel bed, additional coke is introduced (above the upper row of tuyeres)

3. Charging the cupola As soon as coke bed is built up to the correct height and ignited uniformly throughout, alternate layers of pig iron, coke and flux are charged until cupola is filled.

Suitable scrap is also added along with pig iron to control the chemical composition of iron produced.

4. Soaking of iron Charge should soak heat for about 45 minutes after the cupola is fully charged upto the charging door.

Charge gets slowly heated.

5. Air blast When full blast is turned on. Before turning on blast, tuyeres openings and tapping hole are kept closed.

After the blast is on for a few minutes (10 mins), molten metal starts accumulating in the hearth.

At the end of melt the charging is stopped but the blast is kept on until all the metal has melted.

6. Tapping and slagging First tapping can be made 40-50 minutes after full blast turned on.

When slag accumulates in the well, the slag hole is opened and the slag is run off.

Molten metal is collected in ladles and is carried to the moulds for pouring.

7. Closing the cupola Blast is shut off and prop under the bottom door is knocked down so that bottom plates swing open.