| Casting processes - Plaster Moulding
The plaster mould casting
process is a specialised process used for the production of non-ferrous
alloy castings with smoother surfaces, more finely reproduced detail and
greater dimensional accuracy than can be obtained from sand moulds,
including shell moulds, or gravity dies (permanent moulds). Although there
are plaster-based investment materials, which are used with expendable
patterns, they are best considered as a variation on the investment casting
process. The true plaster mould casting process is based on the use of a
permanent pattern. In the commercial production of castings three variants
of the process are used:
- The Conventional
Plaster Mould Casting Process
- The Foamed Plaster
Process
- The Antioch Process
In the conventional
process, plaster of Paris is mixed with water to produce a slurry which is
poured over a permanent pattern contained within a moulding box. Upon
setting a rigid mould is produced which, after pattern stripping, is dried
at an elevated temperature to remove free and chemically combined water
before the metal is cast into the mould. This method produces a strong,
dense, but inherently impermeable mould. Hence, metal casting must be
conducted using vacuum and/or pressure assistance to ensure complete filling
of the mould by the metal. The insulating nature of the plaster improves the
fluid life of the metal that aids mould filling and thin section production.
The foamed plaster
variant produces permeable moulds through the incorporation of air into the
slurry at the mixing stage. It is possible to produce a mould in which 50%
of the volume consists of air bubbles. This has the desired effect of
increasing permeability from 1 or 2 units to between 15 and 30 units, as
measured by the AFS permeability test (2). It also provides a more
economical use of the plaster. However, the inherent strength of the mould
is reduced and if complex shapes are to be produced then flexible patterns
traditionally rubber may be required.
The Antioch process
has, as its special feature, the requirement to process the moulds in a
steam autoclave. This produces a unique granular structure, which provides
mould permeability. The moulds produced by the Antioch process are denser
than foamed plaster moulds and weaker than conventional plaster moulds.
However, this lower strength does not manifest itself until after mould
production, which enables permanent patterns to be used.
The advantages and
disadvantages of the plaster mould casting process are summarised below.
Advantages
- the ability to
produce complex shapes
- the ability to
produce thin section castings
- the excellent
replication of pattern detail
- the ability to
produce castings which are dimensionally accurate
- the ability to
produce castings with good surface finish
- the minimisation of
residual stresses and distortion in castings
Disadvantages
- poor productivity
due to lengthy processing problems
- the need for
multiple patterns to improve moulding productivity
- the requirement for
close control of the production process
- the need for
special procedures to overcome the problems of poor mould permeability
- the possibility of
impaired mechanical properties arising as a result of slow cooling of
the casting
- the mould materials
are not reclaimable
From Precision
Casting Processes by A J Clegg. Reprinted by permission of Butterworth
Heinmann
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