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
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
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withdrawn after it sets in about 3 to 5 minutes.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:
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:
1. High cost
2. Difficulty in controlling dimensional tolerances across the parting line.
Applications:
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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