Ceramic Mould Casting (or) Cope and Drag Investment Casting (or) Plaster Moulding
The ceramic mould casting uses permanent patterns made of plaster, plastic, wood, metal or rubber and utilizes fine grain zircon and calcined, high-alumina mullite slurries for moulding. Ceramic mould casting method uses a ceramic slurry prepared by mixing fine grained refractory powders of Zircon (ZrSiO4), Alumina (Al2O3), Fused Silica (SiO2) and a liquid chemical binder (Alcohol based Silicon Ester) for making the mould. These slurries are comparable in composition to those used in investment castings. Like investment moulds, ceramic moulds are expendable. However, unlike the monolithic moulds obtained in investment castings, ceramic moulds consist of a cope and a drag setup.
Principle of Ceramic Mould Casting
The Mould is made of Plaster of Paris (Gypsum or CaSO4 1/2 H2O) with the addition of talc and Silica flour to improve strength and to control the time required for the plaster to set. These components are mixed with water and the resulting slurry is poured over the Pattern. After removing the pattern, mould is cured in an oven and it is ready to receive the molten metal.
Procedure of Ceramic Mould Casting
One of the most popular of the ceramic moulding techniques is the Shaw process. A reusable pattern is placed inside a slightly tapered flask, and slurry like mixture of refractory aggregate, hydrolyzed ethyl silicate, alcohol, and a getting agent is poured on top.
This mixture sets to a rubbery state that permits removal of the pattern and the flask, and the mould surface is then ignited with a torch (in an oven for heating to about 100 C).
The patterns used are split gated metal patterns usually mounted on a match plate. The slurry is applied over the pattern surfaces to form a thin coating around it. The slurry fills up all cavities and recesses by itself and no naming or vibration of the mould is required. The pattern is withdrawn after it sets in about 3 to 5 minutes.
During “bum-off, most of the volatiles are consumed, and a three-dimensional network of microscopic cracks (micro crazing) forms in the ceramic.
Advantages of Ceramic Mould Casting:
1. High precision and very good surface finish.
2. The process does not require any risering, venting or chilling because the rate of cooling is very slow.
3. Any patterns made of wood, metal or plastic can be used.
4. The process can be used for all types of metals including highly reactive Titanium or Uranium.
Disadvantages of Ceramic Mould Casting:
1. High cost
2. Difficulty in controlling dimensional tolerances across the parting line.
Applications of Ceramic Mould Casting:
1. The method can be used for producing precision parts like dies for drawing, extrusion, casting, forging etc., pump impellers, components of nuclear reactors and air craft.
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