Shell Moulding:

It is process in which the sand mixed with a thermosetting resin is allowed to come into contact with a heated metallic pattern plate, so that a thin and strong shell of mould is formed around the pattern. Then the shell is removed from the pattern and the cope and drag are removed together and kept in a flask with the necessary back-up material and the molten metal is poured into the mould.

Generally, dry and fine sand (90 to 140 GFN) that is completely free of clay is used for preparing the shell moulding sand. The grain size to be chosen depends on the surface finish desired on the casting.

The synthetic resins used in shell moulding are essentially thermosetting resins, which get hardened irreversibly by heat. The resins most widely used are phenol formaldehyde resins. Combined with sand, they have high strength and resistance to heat.

The first step in shell moulding is the preparation of sand mixture in such a way that each of the sand grain is thoroughly coated with resin. Since the sand resin mixture is to be cured t about 150⁰C temperature, only metal patterns with the associated gating systems are used.

The metallic pattern is heated to a temperature of 200-300 ⁰C depending on the type of the pattern. A silicon release agent is then sprayed on the pattern and the metal plate. The heated pattern is then securely fixed to a dump box, wherein coated sand is in an amount larger than required.

Then dump box is rotated so that the coated sand falls on the heated pattern. The heat from the pattern melts the resin adjacent to it thus causing the sand mixture to adhere to the pattern.

When desired shell thickness is achieved, the dump box is rotated backwards by 180 degrees so that the excess and falls back into the box, leaving the formed shell intact.

The shell along with the pattern plate is kept in an electric or gas-fired oven for curing the shell. Overcuring may cause the mould to break down as the resin would burn out, undercuring may result in blow holes in the casting.

Advantages:

1. Shell moulding castings are generally more dimensionally accurate than sand castings.

2. A smoother surface can be obtained in shell castings. This is primarily achieved by the finer grain size used.

3. Draft angles, which are lower tan the sand castings, are required in shell moulds, which considerably saves the material costs and the subsequent machining costs.

4. Sometimes, special cores may be eliminated in shell moulding. Since the sand has high strength the mould could be designed in such a manner that internal cavities can be formed directly.

5. Permeability of the shell is high and therefore no gas inclusions occur.

6. Very small amount of sand needs to be used.7. Mechanism is readily possible due to simple processing

involved.

Limitations:

1. The patters are very expensive and therefore are economical only if used in large scale production.

2. The size of the casting obtained by shell moulding is limited.3. Highly complicated shapes cannot be obtained.

4. More sophisticated equipment is needed for handling the shell mouldings such as those required for heated metal patterns.

Applications:

Cylinders and cylinder heads for air-cooled IC engines, automobile transmission parts, cast tooth bevel gears, break beam, radome hubs, track-rollers for crawler tractors, transmission planet carrier and small crank shaft are some of the common applications of shell-mould casting.

Precision Investment Casting

This is the process where the mould is prepared around an expendable pattern. The first step in this process is the presentation of the pattern for every casting to be made. To do this, molten wax, which is used as the pattern material is injected under pressure of about 2.5 MPa into a metallic die, which has the cavity of the casting to be made. The wax when allowed to solidify would produce the pattern. Then the cluster of wax patterns is attached to the gating system by applying heat.

To make the mould, the prepared pattern is dipped into slurry made by suspending fine ceramic materials in a liquid such as ethyl silicate or sodium silicate. The excess liquid is allowed to drain off from the pattern. Dry refractory grains such as fused silica or zircon are stuccoed on this liquid ceramic coating. Thus, a small shell is formed around the wax pattern. The shell is cured and then the process of dipping and stuccoing is continued with ceramic slurries of gradually increasing grain sizes.

The next step in the process is to remove the pattern from the mould, which is done by heating the mould to melt the pattern. The melted wax is completely drained through the sprue by inverting the mould.

The molten metal is poured into the mould under gravity, under slight pressure, by evacuating the mould first. The method chosen depends upon the type of casting.

Advantages:

1. Complex shapes which are difficult to produce by any other methods are possible since the pattern is withdrawn by melting it.

2. Very fine details and thin sections can be produced by this process because the mould is heated before pouring.

3. Very close tolerances and better surface finish can be produced. This is made possible because of the fine grain of sand used next to the mould cavity.

4. Castings produced by this process are ready for use with little or no machining required. This is particularly useful for hard-to-machine materials such as nimonic alloys.

5. With proper care it is possible to control grain size, grain orientation and directional solidification in this process, so that controlled mechanical properties can be obtained.

6. Since there is no parting line, dimensions across it would not vary.

Limitations:

1. The process is normally limited by the size and mass of the casting. The upper limit on the mass of a casting may be of the order of 5kg.

2. This is more expensive process because of larger manual labour involved in the preparation of the pattern and the mould.

Applications:

the process was used in the olden days for the preparation of artefacts, jewellery and surgical instruments. Presently, the products made by this process are vanes and lades for gas turbines, shuttle eyes for weaving, pawls and claws for movie cameras, wave guides for radars, bolts and triggers for fire arms and impellers for turbo chargers.

Permanent Mould Casting

For large –scale production, making a mould for every casting produced may be difficult and expensive. Therefore, a permanent mould, called die may be made from which a large number of castings anywhere between 100 and 250000 can be produced, depending on the alloy used and the complexity of the casting. This process is called permanent mould casting or gravity die casting, since the metal enters the mould under gravity.

The mould material is selected on the consideration of the pouring temperature, size of the casting and frequency of the casting cycle. They determine the total heat to be borne by the die. Fine-grained grey cast iron is the most generally used die material. The die life is less for higher melting temperature alloys such as copper or grey cast iron.

For making any hollow portions, cores are also used in permanent mould casting. The cores can be made out of metal or sand. When sand cores are used, the process is called semi-permanent moulding.

The mould cavity should normally be simple without any undesirable drafts or undercuts, which interface with the ejection of the solidified castings. The gating and risering systems used are very similar to that of the sand casting.

Under regular casting cycle, the temperature at which the mould is used depends on the pouring temperature, casting cycle frequency, casting weight, casting shape, casting wall thickness and thickness of mould coating. Mould should be heated to its operating temperature before casting.

The materials which are normally cast in permanent moulds are aluminium alloys, magnesium alloys, copper alloys, zinc alloys and

grey cast iron. Permanent mould casting is particularly suited to high volume production of small, simple castings with uniform wall thickness and no intricate details.

Advantages:

1. Because of the metallic mould used, this process produces a fine grained casting with superior mechanical properties.

2. They produce very good surface finish of the order of 4 microns and better appearance.

3. Close dimensional tolerances can be obtained.4. It is economical for large scale production as the labour

involved in the mould preparation is reduced.5. Small-cored holes may be produced compared to sand casting.6. Inserts can be readily cast in place.

Limitations:

1. The maximum size of the casting that can be produced is limited because of the equipment.

2. Complicated shapes cannot be produced.3. The cost of the die is very high and can only be justified for

large scale production.4. Not all materials are suited for permanent mould casting

essentially because of the mould material.

Applications:

Some of the components that are produced in permanent moulds are automobile pistons, stators, gear blanks, connecting rods, aircraft fittings, cylinder blocks, etc.

Die Casting

Die casting involves the preparation of components by injecting molten metal at high pressure into a metallic die. Die casting is closely related to permanent mould casting, in that both the processes use reusable metallic dies. In die casting, as the metal is forced in under pressure compared to permanent moulding, it is also called pressure die casting. Because of the high pressure involved any narrow sections, complex shapes and fine surface details can be easily produced.

In die casting, the die consists of two parts. One part is called stationary half or cover die which is fixed to the die casting machine. The second part is the moving half or ejector die that is moved out for the extraction of casting.

The casting cycles starts when the two parts of the die are apart. The lubricant is sprayed on the die cavity manually or by the auto-lubricating system so that the casting will not stick to the die. The two die halves are closed and clamped. The required amount of metal is injected into the die. After the casting is solidified under pressure, the die is opened and the casting is ejected. The die needs to have the provision of ejectors to push the casting after it gets solidified.

Hot working-tool steel is generally used for the preparation of the dies, die inserts and cores. For zinc alloys, the normal die material is AISI P20 for low volume and H13 for high volume, whereas for aluminium and magnesium, H13 and H11 are used. For copper alloys, H20,H21 and H22 are the usual die materials.

Advantages:

1. Because of the use of movable cores, it is possible to obtain fairly complex castings than that feasible by permanent mould casting.

2. Very small thickness can be easily filled because the liquid metal is injected at high pressure.

3. Very high production rates can be achieved. The typical rates can be 200 pieces per hour since the process is completely automated.

4. Because of the metallic dies, very good surface finish of the order 1 micron can be obtained. The surfaces generated by die casting can be directly electroplated without any further processing.

5. Closer dimensional tolerances of the order of +0.08 mm for small dimensions can be obtained compared to the sand castings.

6. The die has a long life, which is of the order of 300 000 pieces for zinc alloys and 150 000 for aluminium alloys.

7. Die casting gives better mechanical properties compared to sand casting, because of the fine grained skin formed during solidification.

8. Inserts can be readily cast in place.9. It is very economical for large scale production.

Limitations:

1. The maximum size of the casting is limited.2. This is not suitable for all materials because of the limitations

on the die materials. Normally, zinc, aluminium, magnesium, copper alloys are die cast.

3. The air in the die cavity gets trapped inside the casting and is therefore a problem often with the die castings.

4. The dies and the machines are very expensive and therefore, economy in production is possible only when large quantities are produced.

Applications:

The typical products made by die casting are carburetors, crank cases, magnetos, handle bar housings and other parts of scooters, motorcycles and mopeds, zip fasteners, head-lamp bezels and other decorative items on automobiles.

Vacuum Die Casting

The major problem with the die casting is the air left in the cavity when the die is closed. Since that air cannot escape, it ends up inside the casting. As a result, when the casting is heat treated, blisters appear on the surface. This problem is solved by evacuating the air from the die after the die is closed and before the metal is injected. Thus, the metal enters much faster into the die, thereby decreasing the filling time and the same time the parts do not experience any porosity due to the removal of air in the cavity.

The part is exposed to atmospheric air only after solidification and as such the oxidation of the material is avoided. It would be possible with the vacuum die casting to process parts with very thin walls, tight tolerances, fine microstructure due to rapid solidification rates and therefore have properties approaching that of wrought product and with relatively short cycle times.

Low-Pressure Die Casting

Though the process is not new, it has been adapted generally for casting aluminium and magnesium based alloys. In this process, the permanent mould and the filling system are placed over the furnace containing the molten alloy. Then, compressed gas is used at a pressure typically ranging from 0.3 to 1.5 bars to force the molten metal to rise slowly through the ceramic riser tube that is connected to the mould. Once the mould cavity is filled, the pressure in the crucible is removed and the residual molten metal in the tube flows back to the crucible. After the casting is solidified, the side dies opens and the top die is raised vertically. The casting will move with the top die owing to the shrinkage and will be ejected onto a transfer tray.

The quality of the casting is affected by the cooling rate and therefore care has to be taken to see that the casting is properly cooled before ejecting from the die. The top and the bottom dies are cooled by the means of air jets. Care needs top be exercised during the design of the mould to provide proper cooling circuits, so that the heavier sections of the casting are located close to the feeding path to make it act as a riser. Since the metal enters the mould slowly compared to die casting with less turbulence, the casting quality is improved, eliminating the defects. Close tolerance castings can be made using this process.

Centrifugal Casting

This is a process where the mould is rotated rapidly about its central axis as the metal is poured into 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 lighter, gets separated from the metal and segregates towards the centre.

This is normally used for making hollow pipes, tubes, hollow bushes, etc. , which are axi-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 axis of rotation can either be horizontal, vertical or any other angle in between. Very long pipes are normally cast with a horizontal axis, whereas short pipes are conveniently cast with a vertical axis.

First, the moulding flask is properly rammed with sand to confirm to the outer contour of the pipe to be made. Then the flask is dynamically balanced so as to reduce the occurrence of undesirable vibrations during the casting process. The finished flask is mounted in between the rollers and the mould is rotated slowly. Now, the molten metal, in requisite quantity, is poured into the mould through the movable pouring basin. The amount of metal poured determines the thickness of the pipe to be cast. After the pouring is complete, the mould is rotated at its operational speed till it solidifies to form the requisite tubing. Then the mould is replaced by a new mould machine and the process continued.

Advantages:

1. The mechanical properties of centrifugally cast jobs are better compared to other processes, because the inclusions such as slag and oxides get segregated towards the centre and can be easily removed by machining. Also, the pressure acting on the

metal throughout the solidification causes the porosity to be eliminated giving rise to dense metal.

2. Up to a certain thickness of objects, proper directional solidification can be obtained starting from the mould surface to the centre.

3. No cores are required for making concentric holes in the case of centrifugal casting.

4. There is no need for gates and runners, which increases the casting yield, reaching almost 100%.

Limitations:

1. Only certain shapes which are axi-symmetric and having concentric holes are suitable for centrifugal casting

2. The equipment is expensive and thus is only suitable for large scale production.

Continuous Casting

Generally, the starting point of any structural steel product is the ingot which is subsequently rolled through a number of mills before a final product such as a slab or a bloom is obtained. However, the wide adoption of the continuous casting has changed that scenario by directly casting slabs, billets and blooms without going through the rolling process. This process is very fast and economical.

In this process, the liquid steel is poured into a double-walled, bottomless water cooled mould where a solid skin is quickly formed and a semi-finished skin emerges from the open mould bottom. The skin formed in the mould is further solidified by intensive cooling with water sprays as the casting moves downwards.

The molten steel is collected in a ladle and kept over a refractory lined intermediate pouring vessel named tundish. The steel is then poured into water-cooled vertical copper moulds. Before starting the casting, a dummy starter bar is kept in the mould. After starting the casting process as the metal level rises in the mould to a desirable height, the starter bar is withdrawn at a rate equal to the steel pouring rate. The initial metal freezes onto the starter bar as well as the periphery of the mould. This solidified shell supports the metal a it moves downwards. This steel shell is mechanically supported as it moves down through the secondary cooling zone where water is sprayed onto the shell surface to complete the solidification process. After the casting is completely solidified, it is cut to desired lengths by suitable cutoff apparatus.

Squeeze Casting

The product quality is greatly improved in this process by solidifying the casting under heavy pressure to prevent the formation of shrinkage defects and retain dissolved gasses in solution until freezing is complete. Thus, it is a combination of casting and forging.

When the cycle is started, the punch and die portion are separated. The holding furnace holds the molten metal at the requisite temperature. Then the carefully metered charge of molten metal is poured into the die cavity. Then the punch is lowered into place forming a tight seal. The punch portion of the upper die is then forced into the cavity, displacing the molten metal under pressure until it fills the annular space between the die and the punch. The metal is then under pressure and at the same time loses heat rapidly because of its contact with the metallic die. The solidification under pressure is claimed to be responsible for the reduction in the shrinkage cavities in the resulting castings. Once the casting is completely solidified, the punch is retracted and then the casting is ejected from the die.

The squeeze-casting process has very low gas entrapment, lower shrinkage cavities, lower die costs and very high-quality surface with fine details. Also, it produces a fine grain size, which improves mechanical properties. Aluminium, magnesium and copper alloy components can be readily manufactured using this process

Manufacturing Process

Assignment: Special Casting Processes

By:

Ankit Khullar

B.Tech(C.S) 1st Year

Index

Shell Moulding Precision Investment Casting Permanent Mould Casting Die Casting Vacuum Diecasting Low Pressure Diecasting Centrifugal Casting Continuous Casting Squeeze Casting