| Casting processes - Green Sand
Clay-bonded sands have
provided the principal medium from which moulds for castings have been
produced for centuries. In essence the mould material consists of sand,
usually silica in the a quartz form, clay and water. The water develops the
bonding characteristics of the clay, which binds the sand grains together.
Under the application of pressure the mould material can be compacted around
a pattern to produce a mould having sufficient rigidity to enable metal to
be poured into it to produce a casting. When the mould is used in its moist
condition it is referred to as green and the method of producing the moulds
as the green sand moulding process. If the mould is dried at a temperature
just above 100°C (212°F) the majority of the free moisture will be
removed. This is the principal of the dry sand moulding process. Removal of
the free moisture is accompanied by a significant increase in the strength
and rigidity of the mould. This enables the mould to withstand much greater
pressures and so, traditionally, the dry sand process has been used in the
manufacture of large, heavy castings.
The relative abundance
of sand and clay minerals means that the material costs for the process are
low. The mould material is reclaimable, with between 90 and 95% of the sand
being recycled, although new sand and additions are required to make up for
the deliberately discarded loss. From an environmental view the process is
also acceptable, although problems may arise from the use of organic
additions to the sand and from the generation of fine silica dust. These
features, combined with the relative ease of mould production, have ensured
that the green sand moulding process has remained as the principal method by
which castings are produced. It was estimated in 1979 that 80% of casting
production in the UK was by green sand moulding.
The green sand process
remains unchallenged as the process capable of the mass production of
castings at low cost, especially in cast irons. However, the engineering
industry has progressively increased its demand for castings with improved
dimensional accuracy, surface finish and soundness. This led to the
development of the so-called high-pressure moulding machines, which produce
the high-density moulds necessary to meet the customers' demands.
Mould materials and
properties
The characteristics of
sand that influence mould properties were considered in the introduction to
this chapter. The principal addition to the sand is clay, which, in the
presence of water, provides the bond necessary to develop mould strength. In
addition to water and clay, the moulding sand may contain organic materials
to improve moulding characteristics and mould properties. In this section
the materials added to the sand will be reviewed, together with the method
of sand preparation and the development of properties.
The sand used for
green sand moulding must fulfil a number of requirements:
- It must pack
tightly around the pattern, which means that it must have flowability.
- It should be
capable of being deformed slightly without cracking, so that the pattern
can be withdrawn. In other words, it must exhibit plastic deformation.
- It must have
sufficient strength to strip from the patter and support its own weight
without deforming, and to withstand the pressure of molten metal when
the mould is cast. It must therefore have green strength.
- It must be
permeable, so that gases and steam can escape from the mould during
casting.
- It must have dry
strength, to prevent erosion of the mould surface by liquid metal during
pouring as the surface of the mould cavity dries out.
- It must have
refractoriness, to withstand the high temperature involved in pouring
without melting or fusing to the casting.
- With the exception
of refractoriness, all of these requirements are dependent on the amount
of active clay present and on the water content of the mixture.
From Precision
Casting Processes by A J Clegg. Reprinted by permission of Butterworth
Heinmann
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