1. SPECIAL MOULDING PROCESSES
Hareesha N G
Lecturer
Dept. of Aeronautical Engineering

2. Special molding Processes
A. Sand moulds
Green sand mould
Dry sand mould
Core sand mould
Carbon dioxide mould (CO2 mould)
Shell mould
Investment mould
Sweep mould
Full mould
B. Metal moulds
Gravity die casting or Permanent mould casting
Pressure die casting
Continuous casting
Centrifugal casting
Squeeze casting
Thixocasting process

3. 1. GREEN SAND MOULDS

4. 1. GREEN SAND MOULDS
Procedure involved in making green sand moulds
Suitable proportions of silica sand ( %), 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 The cope box is lifted and placed aside, and the pattern in the drag box is withdrawn by knocking it carefully so as to avoid damage to the mould.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).

5. Advantages Disadvantages
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.

6. 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.

7. 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.

8. 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.

9. (Sodium Silicate) (silica gel)
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).

10. Advantages Disadvantages
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.

11. Fig: 5. SHELL MOULDING steps involved

12. 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).
The metallic pattern is heated in an oven to a suitable temperature between °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.
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).

13. 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).
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).

14. Advantages Disadvantages
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.

15. 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.

16. 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 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.

17. 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 °F ( °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.

18. 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.

19. 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.

20. 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.

21. 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.

22. Permanent Mold Casting
Steps in permanent mold casting: (1) mold is preheated and coated

23. 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.

24. 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.

25. 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 °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.