MOULD OR SHELL BUILDING
This is the process where the ceramic shell is built around the wax trees / assemblies. The most economical and popular process is the dip process. This process is extremely versatile and offers very accurate control of the shell properties. The dip process; The wax tree is invested with ceramic coatings, layer by layer, until a sufficiently thick mould is formed. The wax tree is first dipped in ceramic slurry and dry sand is sprinkled (‘ stuccoed ‘) on it. A number of such coats (slurry plus sand) are applied until the desired thickness is achieved. The dipping procedure;
The step by step shell building procedure is as follows
1. Dip the wax tree into the primary ceramic slurry.
2. Stucco with fine sand.
3. Dry this coat in controlled conditions of temperature and humidity.
4. When dry, wet this tree in the primary backup ceramic slurry.
5. Stucco with slightly coarser sand and dry under same conditions.
6. Repeat this procedure using progressively coarser sands until the required thickness is reached.
7. When a particular coarseness of stucco sand is reached, secondary backup slurry is used.
8. The number of dips varies from 8 to 12 depending on the size and shape of the casting.
Properties of Shell / Mould materials:
(a) The material should have refractory properties.
(b) It should be thermally stable i.e. dimension changes on heating should be minimal.
(c) Chemical compatibility with other mould materials and metal to be cast is important.
(d) It should resist hot deformation, i.e. the material should not become soft, at molten metal temperatures, and deform.
(e) Low cost and easy availability of consistent quality material is also important.
Some of the ceramic materials used are
(a) Silica (SiO2)
(b) Zircon (ZrSiO2)
(c) Zirconia (ZrO2) etc….
Dewaxing is the process where the wax patterns are melted out. This leaves behind the monolithic ceramic shell having cavities of the exact shape and dimensions of the wax patterns. This is also the most delicate step in the entire investment casting process.
The dewaxing methods:
1. Flash heating: the shells are placed in a furnace that is preheated to a high temperature. This sudden exposure to heat enables proper dewaxing of the shells.
2. Steam autoclave dewaxing: here the shells are placed in a steam autoclave. The vessel is closed and super heated steam, at high pressure, is let into it. This causes very efficient dewaxing. Steam dewaxing is the most efficient method of dewaxing as almost no shell cracking is noticed here.
3. Boiling water/wax dewaxing: the shells are simply immersed into a bath of boiling water or wax.
The wax inside the shells is thus melted out. There are a number of disadvantages associated with this method. The only advantage being the low capital investment involved.
The shell after dewaxing is preheated. This is done to develop the high temperature bond of the shell. The preheating is usually done in a box type electrical furnace or in a directly fired oil / gas furnace. It may be noted that the shells are poured when still hot. This is done to prevent thermal shock and minimise casting defects.
There is no limitation to what metal can be cast in the investment casting process. Components in non-ferrous metals are cast regularly along with regular steel, stainless steel, cast iron, etc…. Components in titanium alloys are also investment cast. Gold ornaments are also investment cast forming an independent industry on its own. Hence, metal or metallurgy is no limitation here. Conventional casting methods like, ladle pouring or furnace pouring, are used for casting metal into the investment casting shells. Vacuum melting-casting techniques are used only where metallurgy specifications require such care. The gravity pouring method is found to be adequate for all other casting needs.
FETTLING AND CUT OFF: After the cast has solidified and the shell cooled, it is passed to the knockout stage. The shell is broken away using the pneumatic knock out machine. Additional cleaning methods like blasting and chemical leaching are used to remove shell fragments from hard to reach areas, bores and undercuts. After the cleaning operation, the castings are separated from the sprue and the runner system. Abrasive cut-off wheels, saws, oxyacetylene torch, gouging rods, etc… are used for this purpose.
The cut off witness is kept as small as possible. This is ground off in fettling, along with other casting blemishes, if any. Endless belt grinders, pedestal grinders and pneumatic die grinding tools are used for the light and delicate finishing operation. Care has to be taken to prevent damage to the casting during the fettling operation. In case there is a need the castings are further finished using glass bead/ sand/ refractory grit/ ceramic/ aluminium/ SS/ steel shot blasting techniques. Electro polishing and buffing are sometimes required on stainless steel castings. The castings are inspected at appropriate stages to ensure quality of metal, dimensions and physical properties. Standard foundry inspection methods are used here. The castings are then packed and shipped.