unit 3 specialmouldingprocesses 131218044443 phpapp02

7/10/2014 1 Hareesha N G, Asst. Prof, DSCE, Bengaluru

Special molding Processes A. Sand moulds

1. Green sand mould 2. Dry sand mould 3. Core sand mould 4. Carbon dioxide mould (CO2 mould) 5. Shell mould 6. Investment mould 7. Sweep mould 8. Full mould

B. Metal moulds 9. Gravity die casting or Permanent mould casting 10. Pressure die casting 11. Continuous casting 12. Centrifugal casting 13. Squeeze casting 14. Thixocasting process

7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru 2

1. GREEN SAND MOULDS


1. GREEN SAND MOULDS Procedure involved in making green sand moulds • Suitable proportions of silica sand (85 - 92 %), bentonite binder (6-12 %), water

(3-5 %) and additives are mixed together to prepare the green sand mixture. • The pattern is placed on a flat surface with the drag box enclosing it as shown in

figure (a). Parting sand is sprinkled on the pattern surface to avoid green sand mixture sticking to the pattern.

• The drag box is filled with green sand mixture and rammed manually till its top surface. Refer figure (b). The drag box is now inverted so that the pattern faces the top as shown in figure (c). Parting sand is sprinkled over the mould surface of the drag box.

• The cope box is placed on top of the drag box and the sprue and riser pin are placed in suitable locations. The green sand mixture is rammed to the level of cope box as shown in figure (d).

• The sprue and the riser are removed from the mould. The cope box is lifted and placed aside, and the pattern in the drag box is withdrawn by knocking it carefully so as to avoid damage to the mould. Gates are cut using hand tools to provide passage for the flow of molten metal. Refer figure (e) and (f).

• The mould cavity is cleaned and finished. Cores, if any, are placed in the mould to obtain a hollow cavity in the casting. Refer figure (g).

• The cope is now placed on the drag box and both are aligned with the help of pins. Vent holes are made to allow the free escape of gases from the mould during pouring. The mould is made ready for pouring. Refer figure (h).


Advantages

– Green sand molding is adaptable to machine molding.

– No mold baking or drying is required.

– There is less mold distortion than in dry sand molding.

– Time and cost associated with mold baking or drying is eliminated.

– Green sand molds having smaller depths permit the escape of mold gases without any difficulty.

– In green sand molding, flasks are ready for reuse in minimum amount of time.

– Being soft, green sand molds do not restrict the free contraction of the solidifying molten metal.

– Green sand molding provides good dimensional accuracy across the parting line.

Disadvantages

– Green sand molds possess lower strengths.

– They are less permeable.

– There are more chances of defects (like blow holes etc.) occurring in castings made by green sand molding.

– In green sand molding, sand control is more critical than in dry sand molding.

– Mold erosion is very common especially in the production of large sized castings.

– Surface finish deteriorates as the weight of the casting increases.

– Dimensional accuracy of the castings decreases as their weight increases. 7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru 5

2. DRY MOLDING SAND

• Dry molding sand differs from the green molding sand in the sense that it contains binders (like clay, bentonite,. molasses etc.) which harden when the mold is heated and dried.

• A typical dry sand mixture (for making non-ferrous castings) consists of floor sand 40%, new silica sand 30%, coal dust 20% and bentonite 10%.

• A dry sand mold is prepared in the same manner as a green sand mold; however, it is baked at 300 to 700°F for 8 to 48 hours depending upon binders used and the amount of sand surface to be dried.


Advantages

– Dry sand molds possess high strength.

– They are more permeable as compared to green sand molds.

– Castings produced from dry sand molds possess clean and smooth surfaces.

– As compared to green sand molding, dry sand molding turns out castings with less defects.

– Dry sand molding imparts better overall dimensional accuracy to the molds and castings as compared to green sand molding.

Disadvantages

– Dry sand molding involves more labour and consumes more time in completing the mold. Mold baking is an extra work as compared to that required in green sand molding.

– Dry sand molding is more expensive as compared to green sand molding.

– Dry sand molding involves chances of hot tears occurring in the castings.

– Because of baking, a mold may distort.

– Dry sand molding involves a longer processing cycle as compared to green sand molding.

– Dry sand molding gives a slower rate of production as compared to green sand molding.


4. CARBON DIOXIDE (CO2) MOLDING

• Carbon dioxide moulding also known as sodium silicate process is one of the widely used process for preparing moulds and cores.

• In this process, sodium silicate is used as the binder. But sodium silicate activates or tend to bind the sand particles only in the presence of carbon dioxide gas. For this reason, the process is commonly known as C02 process.


Steps involved in making carbon dioxide mould

• Suitable proportions of silica sand and sodium silicate binder (3-5% based on sand weight) are mixed together to prepare the sand mixture.

• Additives like aluminum oxide, molasses etc., are added to impart favorable properties and to improve collapsibility of the sand.

• The pattern is placed on a flat surface with the drag box enclosing it. Parting sand is sprinkled on the pattern surface to avoid sand mixture sticking to the pattern.

• The drag box is filled with the sand mixture and rammed manually till its top surface. Rest of the operations like placing sprue and riser pin and ramming the cope box are similar to that of green sand moulding process.

• Figure (a) shows the assembled cope and drag box with vent holes. At this stage, the carbon dioxide gas is passed through the vent holes for a few seconds. Refer figure (b).

• Sodium silicate reacts with carbon dioxide gas to form silica gel that binds the sand particles together. The chemical reaction is given by:

Na2Si03 + C02 -> Na2C03 + Si02

(Sodium Silicate) (silica gel)

• The sprue, riser and the pattern are withdrawn from the mould, and gates are cut in the usual manner. The mould cavity is finished and made ready for pouring. Refer figure (c). 7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru 9

Advantages • Instantaneous strength development. The development of strength takes place

immediately after carbon dioxide gassing is completed.

• Since the process uses relatively safe carbon dioxide gas, it does not present sand disposal problems or any odour while mixing and pouring. Hence, the process is safe to human operators.

• Very little gas evolution during pouring of molten metal.

Disadvantages • Poor collapsibility of moulds is a major disadvantage of this process. Although

some additives are used to improve this property for ferrous metal castings, these additives cannot be used for non-ferrous applications.

• The sand mixture has the tendency to stick to the pattern and has relatively poor flowability.

• There is a significant loss in the strength and hardness of moulds which have been stored for extended periods of time.

• Over gassing and under gassing adversely affects the properties of cured sand.


Fig: 5. SHELL MOULDING steps involved


5. SHELL MOULDING • Shell moulding is an efficient and

economical method for producing steel castings.

• The process was developed by Herr Croning in Germany during World war-II and is sometimes referred to as the Croning shell process.

Procedure involved in making shell mould a. A metallic pattern having the shape of

the desired casting is made in one half from carbon steel material. Pouring element is provided in the pattern itself. Refer figure (a).

b. The metallic pattern is heated in an oven to a suitable temperature between 180 - 250°C. The pattern is taken out from the oven and sprayed with a solution of a lubricating agent viz., silicone oil or spirit to prevent the shell (formed in later stages) from sticking to the pattern.

c. The pattern is inverted and is placed over a box as shown in figure 3.3(b). The box contains a mixture of dry silica sand or zircon sand and a resin binder (5% based on sand weight). 7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru 12

d. The box is now inverted so that the resin-sand mixture falls on the heated face of the metallic pattern. The resin-sand mixture gets heated up, softens and sticks to the surface of the pattern. Refer figure (c).

e. After a few seconds, the box is again inverted to its initial position so that the lose resin-sand mixture falls down leaving behind a thin layer of shell on the pattern face. Refer figure (d).

f. The pattern along with the shell is removed from the box and placed in an oven for a few minutes which further hardens the shell and makes it rigid. The shell is then stripped from the pattern with the help of ejector pins that are provided on the pattern. Refer figure (e).


g. Another shell half is prepared in the similar manner and both the shells are assembled, together with the help of bolts, clips or glues to form a mould. The assembled part is then placed in a box with suitable backing sand to receive the molten metal. Refer figure (f).

h. After the casting solidifies, it is removed from the mould, cleaned and finished to obtain the desired shape.

Advantages Better surface finish and dimensional tolerances. Reduced machining. Requires less foundry space. Semi-skilled operators can handle the process easily. Shells can be stored for extended periods of time.

Disadvantages Initially the metallic pattern has to be cast to the desired shape, size and finish. Size and weight range of castings is limited. Process generates noxious fumes.


6. INVESTMENT MOULD • Investment mould also called as 'Precision casting' or 'Lost

wax process' is an ancient method of casting complex shapes like impellers, turbine blades and other airplane parts that are difficult to produce by other manufacturing techniques.

The various steps involved in this process are:

Step 1 Die and Pattern making

• A wax pattern is prepared by injecting liquid wax into a pre-fabricated die having the same geometry of the cavity of the desired cast part. Refer figure.1.

• Several such patterns are produced in the similar manner and then attached to a wax gate and sprue by means of heated tools or melted wax to form a 'tree' as shown in figure 2.


Step 2 Pre-coating wax patterns

• The tree is coated by dipping into refractory slurry which is a mixture of finely ground silica flour suspended in ethyl silicate solution (binder).

• The coated tree is sprinkled with silica sand and allowed to dry. Refer figure 3 and 4.

Step 3 Investment

• The pre-coated tree is coated again (referred as 'investment') by dipping in a more viscous slurry made of refractory flour (fused silica, alumina etc.) and liquid binders (colloidal silica, sodium silicate etc.) and dusted with refractory sand.

• The process of dipping and dusting is repeated until a solid shell of desired thickness (about 6 - 10 mm) is achieved.

Note: The first coating is composed of very fine particles that produce a good surface finish, whereas the second coating which is referred as 'Investment' is coarser so as to build up the shell of desired thickness.


Step 4 De-waxing '

• The tree is placed in an inverted position and heated in a oven to about 300°F. The wax melts and drops down leaving a mould cavity that will be filled later by the molten metal. Refer figure 5.

Step 5 Reheating the mould

• The mould is heated to about 1000 - 2000°F (550-1100°C) to remove any residues of wax and at the same time to harden the binder.

Step 6 Melting and Pouring

• The mould is placed in a flask supported with a backing material and the liquid metal of the desired composition is poured under gravity or by using air pressure depending on the requirement. Refer figure.6.

• After the metal cools and solidifies, the investment is broken by using chisels or hammer and then the casting is cut from the gating

systems, cleaned and finished. Refer figure.7.


Advantages

• Gives good surface finish and dimensional tolerances to castings

• Eliminates machining of cast parts.

• Wax can be reused.

Disadvantages

• Process is expensive.

• Size and weight range of castings is limited

• In some cases, it is difficult to separate the refractory (investment) from the casting.

• Requires more processing steps.


7. SWEEP MOULD

• In sweep moulding, the cavity is formed as the pattern sweeps the sand all around the circumference.

• A thin wooden piece is attached to a spindle at one edge while the other edge has a contour depending on the desired shape of the casting.

• The spindle is placed at the center of the mould and rotated so that the wooden piece sweeps in the mould box generating the shape of the required casting.

• Green sand, loam sand or sodium silicate sand can be used symmetrical shapes.


8. FULL-MOULD PROCESS

• Full-mould casting or 'cavity less' casting is a technique similar to investment casting, but, instead of wax, polystyrene foam is used as pattern.

• The pattern can be hand cut or machined from pieces of foamed polystyrene.

• Gating and risering systems are made from the foamed material in single or multiple pieces and then assembled to the pattern with the help of paste or glue. Refer figure (a).

• The entire pattern assembly is dipped into a water based ceramic material, dried and positioned in a one piece sand mould.

• Green sand or no-bake sand is preferred for moulding. Refer figure (b).

• When the molten metal comes in contact with the foamed pattern, the foam vaporizes (melts and burns) allowing the molten metal to occupy and fill the cavity.

• The amount of gas produced by the foam is so small that it can easily escape through the sand.

• Pump housing, manifolds and auto-brake components are a few among the various products that can be made from this process.


Advantages • Withdrawal of pattern requires some form of design modifications

like providing draft allowance, loose pieces etc. Such complex processes are eliminated in full-mould process through the use of patterns that can be removed by melting and vaporization.

• No limit to size and shape of castings. • Good surface finish. Disadvantages • High cost of patterns. • More care should be taken during moulding. • Patterns being light and low in strength can be easily distorted or

damaged.


Permanent Mold Casting

Steps in permanent mold casting: (1) mold is preheated and coated

22 7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru

Permanent Mold Casting Steps in permanent mold casting: (2) cores (if used) are inserted

and mold is closed, (3) molten metal is poured into the mold, where it solidifies.

23 7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru

9. GRAVITY DIE CASTING • Gravity die casting or permanent mould casting is a casting process in which the

molten metal is poured into a metallic mould called die under the influence of gravity. Hence the name 'gravity die casting'.

• The mould or die is usually made from cast iron, tool steel, graphite, copper or aluminum alloys and the choice for a particular material depends on the type of metal to be cast.

• Gating and risering systems are machined either in one or both the mould halves.

• Figure shows a permanent mould made in two halves which resembles a book. The mould halves are hinged and can be clamped together to close the mould.


Steps involved in the process • The mould is cleaned using wire brush or compressed air to remove dust and other

particles from it. • It is preheated to a temperature of 200 - 280°C by gas or oil flame and then the

surface is sprayed with a lubricant. • The lubricant helps to control the temperature of the die thereby increasing its life

and also assist in easy removal of solidified casting. • The mould is closed tightly and the liquid metal of the desired composition is poured

into the mould under gravity. • After the metal cools and solidifies, the mould is opened and the casting is removed.

Gating and risering systems are separated from the cast part. • The mould is sprayed with lubricant and closed for next casting. The mould need not

be preheated since the heat in the previous cast is sufficient to maintain the temperature.

Advantages • Good surface finish and close dimensional tolerances can be achieved. • Suitable for mass production. • Occupies less floor space. • Thin sections can be easily cast. • Eliminates skilled operators. Disadvantages • Initial cost for manufacturing moulds (dies) is high. • Not suitable for steel and high melting point metals/alloys. • Un-economical for small productions. 7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru 25

10. PRESSURE DIE CASTING

• Pressure die casting often called 'Die casting' is a casting process in which the molten metal is injected into a 'die' under high pressures.

• The metal being cast must have a low melting point than the die material which is usually made from steel and other alloys.

• Hence, this process is best suitable for casting non-ferrous materials, although a few ferrous materials can be cast.

• The two basic methods of die casting include:

a) Hot chamber die casting process

b) Cold chamber die casting process.


10.a. Hot chamber die casting process

• Figure shows a 'goose neck' type of hot chamber die casting machine.

• In this process, the dies are made in two halves: one half called the fixed die or 'stationary die’ while the other half called 'movable die’.

• The dies are aligned in positions by means of ejector pins which also help to eject the solidified casting from the dies.

Figure: Hot chamber die casting (Submerged plunger type) 7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru 27

Steps involved in the process

• A pivoted cast iron goose neck is submerged in a reservoir of molten metal where the metal enters and fills the goose neck by gravity.

• The goose neck is raised with the help of a link and then the neck part is positioned in the sprue of the fixed part of the die.

• Compressed air is then blown from the top which forces the liquid metal into the die cavity.

• When the solidification is about to complete, the supply of compressed air is stopped and the goose neck is lowered back to receive the molten metal for the next cycle. In the meantime, the movable die half opens by means of ejector pins forcing the casting from the die cavity.

• The die halves close to receive the molten metal for the next casting.

Hot chamber process is used for casting metals like zinc, tin, magnesium and lead based alloys.

Figure: Hot chamber die casting (Goose neck or air injection type) 7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru 28

10.b: Cold chamber Die Casting Process

• In hot chamber process, the charging unit (goose neck) rests in the melting chamber, whereas in cold chamber process, the melting chamber is separate and the molten metal is charged into the machine by means of ladles.

• Cold chamber process is employed for casting materials that are not possible by the hot chamber process.

• For example, aluminum alloys react with the steel structure of the hot chamber machine and as a result there is a considerable iron pick-up by aluminum.

• This does not happen in cold chamber process, as the molten metal has a momentary contact with the structure of the machine.

• Figure shows the cold chamber die casting machine

Fig: cold chamber die casting machine

• The machine consists of a die, made in two halves: one half called the 'fixed die' or 'stationary die’ while the other half called 'movable die’.

• The dies are aligned in positions by means of ejector pins which also help to eject the solidified casting from the dies.



• A cylindrical shaped chamber called 'cold chamber' (so called because, it is not a part of melting or charging unit unlike in hot chamber process) is fitted with a freely moving piston and is operated by means of hydraulic pressure.

• A measured quantity of molten metal is poured into the cold chamber by means of ladles.

• The plunger of the piston is activated and progresses rapidly forcing the molten metal into the die cavity. The pressure is maintained during the solidification process.

• After the metal cools and solidifies, the plunger moves backward and the movable die half opens by means of ejector pins forcing the casting from the die cavity.

• The cold chamber process is slightly slower when compared to the hot chamber process.


Advantages of Die casting process

• Process is economical for large production quantities.

• Good dimensional accuracy and surface finish.

• Thin sections can be easily cast.

• Near net shape can be achieved.

Disadvantages

• High cost of dies and equipment.

• Not economical for small production quantities.

• Process not preferable for ferrous metals.

• Part geometry must allow easy removal from die cavity


11. CONTINUOUS CASTING

• Continuous casting is a casting process in which the operation of pouring, solidification and withdrawal of casting from an open mould are carried out continuously.

• Figure shows a schematic of the process.



1. The molten metal is continuously supplied from the ladle to the intermediate ladle called 'tundish' from where it is continuously poured into the mould at a controllable rate, keeping the level at a constant position.

2. The mould usually made of copper or graphite is open at the bottom and is water cooled so as to extract the heat of the metal causing its solidification. The shape of the mould corresponds to the shape of the desired casting.

3. The process is started by placing a dummy bar at the bottom of the mould upon which the first liquid metal falls.

4. The molten metal from the tundish enters the mould and takes the shape of the mould. The water cooled mould controls the cooling rate of the metal, so that it solidifies before it leaves the mould.

5. The metal after coming out of the mould is further cooled by direct water spray (or water with air) to complete solidification.

6. The solidified metal is continuously extracted (along with the dummy bar) by 'pinch rolls', bent and fed horizontally and finally cut to the desired length.

7. The dummy bar is initially placed at the bottom of the mould to receive the first liquid metal (since the bottom of the mould is open). It is later disconnected from the casting.


Advantages • Sprue, runner, riser etc., are not used. Hence, no waste metal. This

leads to 100 % casting yield*. • Capable of producing in single operation, rods, sections and tubes

with varying sizes and wall thickness. • Process is automatic. • Product has good consistent soundness. • Mechanical properties are high Disadvantages • Not suitable for small quantity production. • Continuous and efficient cooling of moulds is required, else, center-

line shrinkage develops. • Requires large floor space.

* Yield - Comparison of casting weight to the total weight of the metal poured into the mould.


12. CENTRIFUGAL CASTING • Centrifugal casting is a process in which the molten metal is poured and

allowed to solidify in a revolving mould. • The centrifugal force due to the revolving mould holds the molten metal

against the mould wall until it solidifies. • The material used for preparing moulds may be cast iron, steel, sand or

graphite (for non-ferrous castings). • The process is used for making castings of hollow cylindrical shapes. • The various centrifugal casting techniques include:

a) True centrifugal casting b) Semi-centrifugal casting and c) Centrifuge casting.


12.a. True Centrifugal casting

• True centrifugal casting is used to produce parts that are symmetrical about the axis like that of pipes, tubes, bushings, liners and rings.

• The outside shape of the casting can be round, octagonal, hexagonal etc., but the inside shape is perfectly (theoretically) round due to radially symmetric forces.

• This eliminates the need for cores for producing hollow castings.

• Figure shows the true centrifugal process.

Figure: True centrifugal process 7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru 36


1. The mould of the desired shape is prepared with metal and the walls are coated with a refractory ceramic coating.

2. The mould is rotated about its axis at high speeds in the range of 300 - 3000 rpm. A measured quantity of molten metal is poured into the rotating mould.

3. The centrifugal force of the rotating mould throws the liquid metal towards the mould wall and holds the molten metal until it solidifies.

4. The casting cools and solidifies from its outer surface towards the axis of rotation of the mould thereby promoting directional solidification.

5. The thickness of the casting obtained can be controlled by the amount of liquid metal being poured.

• An inherent quality of true centrifugal castings is based on the fact that, the non-metallic impurities in castings being less dense than the metal, are forced towards the inner surface (towards the axis) of the casting due to the centrifugal forces. These impurities can be machined later by a suitable machining process (say boring operation).

• The mould may be rotated horizontally or vertically.

• When the mould is rotated about horizontal axis, a true cylindrical inside surface is produced; if rotated on a vertical axis, parabolic inside surface is produced.

• Cores and gating/risering systems are not required for this process. 7/10/2014 Hareesha N G, Asst. Prof, DSCE, Bengaluru 37

12.b. Semi-centrifugal casting

• Semi-centrifugal casting process is used to produce solid castings and hence, requires a core to produce hollow cavities.

• The process is used only for symmetrically shaped objects and the axis of rotation of the mould is always vertical.

• Gear blanks, sheaves, wheels and pulley are the commonly produced parts by this process.

• Figure shows the process to produce a wheel shaped casting.



• The mould is prepared in the usual manner using cope and drag box.

• The mould cavity is prepared with its central axis being vertical and concentric with the axis of rotation.

• The core is placed in position and the mould is rotated at suitable speeds, usually less than true centrifugal casting process.

• The centrifugal force produced due to the rotation of the mould causes the molten metal to fill the cavity to produce the desired shape.


12.c. Centrifuging Process

• In true and semi centrifugal process, the axis of the mould/cavity coincide with the axis of rotation.

• Where as in centrifuging process, the axis of the mould cavity does not coincide with the axis of rotation.

• The mould is designed with part cavities located away from the axis of rotation.

• Hence, this process is suitable for non-symmetrical castings.

• Figure shows the centrifuging process.



1. Several mould cavities are arranged in a circle and connected to a central down sprue through gates.

2. The axis of the down sprue is common to the axis of rotation of the mould.

3. As the mould is rotated, the liquid metal is poured down the sprue which feeds the metal into the mould cavity under centrifugal force.

4. The rotational speed depends on a number of factors such as, the moulding medium (sand, metal or ceramic), size of the casting, type of metal being poured and the distance of the cavity from the central axis (sprue axis).

5. Centrifuging is done only about a vertical axis.


13. SQUEEZE CASTING

• Squeeze casting or squeeze forming or liquid metal forging is a combination of casting and forging process.

• Figure shows the sequence of operations involved in the process.



1. The process makes use of two dies: bottom die and top die, cast and machined in such a way that upon mating leaves a cavity similar to the shape of the desired casting. Refer figure (a).

2. The bottom die is preheated to around 200 - 250°C with the help of a torch and sprayed by a water based graphite lubricant to facilitate easy removal of casting after solidification. Refer figure (b).


3. Measured quantity of molten metal is poured into the bottom die as shown in figure (c). As the metal starts solidifying, pressure is applied to the top die causing it to move rapidly towards the bottom die.

4. This causes the molten metal to get squeezed and fill the mould cavity. Refer figure (d). The squeezing pressure is applied until solidification is completed.

5. The casting is ejected by operating the lift pin provided in the bottom die, and the die is then made ready for the next cycle. Refer figure (e)

• Squeeze casting is commonly used for casting aluminum and magnesium alloys. • Cores can be used in this process to produce holes and recesses.


Advantages • Metals which have poor fluidity characteristics can be cast by this

process. • Shrinkage and gas porosity will be less due to the applied pressure

during solidification. • Enhanced mechanical properties because of the fine grain structure

caused by rapid solidification. • Good surface finish. Disadvantages • Process is costlier. Manufacturing of dies to accurate dimensions

involves complex processes. • Accurate metering of molten metal is a slight difficult problem. • Un-economical for small quantity production.


14. SLUSH CASTING

• Slush casting is a process in which hollow castings are produced without the use of cores.

• The process is not preferred to produce objects for engineering use, instead, it is used to make objects like statues, toys, lamp base, candle sticks and others, where only the external features of the object are important. Refer figure (c).



• In this process, the molten metal is poured in a metallic mould and permitted to remain in the mould for a short interval of time. Refer figure (a).

• Solidification begins at the mould walls, as they are relatively cool and then progresses inward.

• When a shell of desired thickness is formed, the mould is inverted and the metal which is still in the liquid state is drained off. Refer figure (b).

• The thickness of the shell obtained depends on the time for which the metal was allowed to remain in the mould and also the thermal conductivity of the mould.

• When the mould halves are separated, a hollow casting with good features on its external surfaces, but variable wall thickness is, obtained as shown in figure (c).

Advantages

• Process is inexpensive.

• Hollow castings can be made without using cores.

Disadvantages

• Process is used for art and decorative work only.

• Only low melting point alloys with narrow freezing ranges can be used.


15. THIXOCASTING PROCESS

• Thixocasting, although similar to squeeze casting, is a more refined process in which the casting material, for example, aluminum alloy is subjected to a heating treatment to prepare a semi-molten material having solid and liquid phases co-existing therein.

• The semi-molten material is injected into a cavity whose shape resembles to the shape of the desired product and rapidly compressed at very high pressures.

• This is a high potential technology bringing together quality metallurgy, advanced mechanical properties and excellent dimensional precision.

• The yield strength of the part made by thixocasting is around 220 MPa compared to a maximum of 140 MPa, that obtained by a pressure die casting process.

• It is therefore used in the manufacture of light weight parts especially in automobiles that are subjected to severe stresses.


unit 3 specialmouldingprocesses 131218044443 phpapp02

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