| Casting processes - Shaw Process
The Shaw process is a
precision casting process capable of the production of accurate moulds with
excellent surface finish and metallurgical integrity. Moulds are produced
using highly refractory aggregates bonded with silica provided by a liquid
ethyl silicate binder. A high temperature firing treatment is a feature of
the production sequence and this produces an inert mould into which the
majority of commercial ferrous and non-ferrous alloys can be cast with
confidence.
The process has been
used commercially for many years; it was known before the Second World War
that silicon esters could be used as refractory aggregate binders'. As with
most processes there has been a continuous development, in particular with
respect to the binder system and the methods of mould production.
Process outline
The mould material is
prepared by blending refractory powders, containing a high proportion of
fine material, with a liquid ethyl silicate binder and a gelling agent. The
blended, mobile slurry is poured into the moulding box and around the
pattern. Within a short period of time, controlled by the amount of gelling
agent, the mould material gels to a rubbery consistency and the pattern can
be separated from the mould. On removal of the pattern the mould is torched
immediately to remove evolved alcohol. Torching also produces a very fine
network of surface and internal cracks, which do not permit metal
penetration but may improve permeability and subsequent mould breakdown.
After torching, the moulds are fired in a furnace to ensure that there are
no combustible materials in the mould and that a strong, rigid, inert,
accurate and stable mould is produced.
The advantages claimed
for moulds manufactured by the Shaw process include the following:
- good pattern
stripping characteristics: the rubbery nature of the gelled mould
provides flexibility, which enables the pattern and mould to be
separated without damage to the mould when intricate detail, or even
straight draws, are required
- dimensional
stability: the excellent reproduction of pattern detail and dimensions
is retained by the mould to a great extent after firing and during
casting, thus enabling accurate dimensional allowances to be made
- mould strength:
this is sufficient to allow moulds to be cast without the need for
moulding boxes
- collapsibility and
resistance to tears: the characteristic internal structure of the mould
material improves its breakdown properties and there is less constraint
on the casting during contraction
- resistance to
thermal shock: the characteristic internal structure permits expansion
of the mould material to occur readily, as a result moulds can safely be
poured cold
- resistance to
spalling and washing: the nature of the silica bond prevents the
generation of inclusions during mould filling
- permeability and
inertness: as the mould is inert after firing, the only gas to be
displaced is that occupying the mould cavity. The characteristic
structure provides sufficient permeability to enable the metal to
displace the gas readily through the mould.
From Precision
Casting Processes by A J Clegg. Reprinted by permission of Butterworth
Heinmann
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