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silica and moulding
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Silca uses

Foundry sand - case study

For many years there has been a requirement for high quality silica sand in the Foundry industry. Foundries use a clean, well graded sand to produce moulds for the production of metal castings which are the basis of engineering and manufacturing industries across the UK.

High purity silica sand is washed and graded to remove any impurities and then dried before it is delivered to the foundry. Because this type of sand has a higher melting point than iron, copper, aluminium and other alloys it can be mixed with a binder to produce a mould that can hold the molten metal when producing a casting. The sand is classified into different sizes which affects the amount of binder required and the final surface finish of the casting.

The sand mould is later broken away from the casting and can be recycled within the foundry or used elsewhere in the production of asphalt or concrete products.

Foundries in the UK vary in size and in type with some capable of producing extremely high quality, specialist parts for the aero space mining and automotive industries.

Some will be on a smaller scale that allows them to make individual pieces specific to their customers' needs. (Jobbing Foundries)

All of these foundries rely on the sand's specific physical and chemical properties allowing them to make their castings to the very closest tolerances.

The sand is an essential raw material for all these manufacturing companies and without the special properties of the sand, many of the vital engineering components could not be produced.

Some of the foundries specialise in supplying gearboxes, wheels, engine manifolds and brake discs to the automotive industry. (See photograph) Others will supply parts to produce water or fuel pumps that keep much of our everyday plant and equipment operating.

All these vital components that keep our industries working start off as a sand mould and core if needed and with most of them there is no other suitable alternative method of production.

Foundry applications are by far the most demanding of sand. For mould or core making, the sand is mixed with a chemical binder, or bentonite water and coal dust if required. This would be for green sand moulding, where the sand remains uncured, but rigid enough to be able to cast molten metal compacted over a shaped pattern, or into a core cavity and removed (stripped) when sufficient strength has developed.

Critical requirements include:

  • Grain (particle) Shape
  • Nominal particle size
  • Particle size distribution
  • Composition
  • Thermal stability

Grain Shape

Preferably sub angular to rounded, with reasonable sphericity. This allows the sand / binder mix to displace evenly across and throughout its section during compaction. Rounded sand particles with high sphericity have the lowest binder demand, as the surface area approaches its minimum.

Nominal Particle Size

During the casting process, the weight and pressure of the molten metal against the sand surface means that, on solidification, the casting surface will be similar to the mould or core finish. The finer the sand, the finer the casting surface finish. However, finer sands are less flow able than coarser sands, and intricate pattern geometry can therefore result in soft vertical faces in the mould. Relatively coarse sand has a lower binder demand than a finer one, but a coarser surface finish may result. Expressed as either: AFS, which is the grain fineness index, the higher the number, the finer the sand e.g. 50, 60, 70, 80 AFS or as AGS, average grain size in microns, the higher the number, the larger the particles e.g. 120, 140, 160 AGS.

Particle Size Distribution

This is of critical importance. For use with organic binder systems, particles of, or smaller than, 90microns should be present in minute amounts only. Nothing much below 100 microns will affect the surface finish, but will greatly increase binder demand. The rest of the grading should be centred pretty much on the three main required sieves for a given grading. Too narrow a grading, e.g. a two-sieve spread, can result in embrittlement.

For greensand moulding, the -90 micron fraction is not so critical. A broader distribution is desirable; as strength is heavily compaction related. A broader sieve distribution improves mould density, strength, stability and casting surface finish.

Composition and Thermal Stability

The higher the silica content i.e. the purer the sand, the higher its refractoriness. This is more important for high temperature applications such as Iron and Steel casting where temperatures may be up to 1500c, sometimes higher, close to the sintering temperature of the sand.

Crystalline silica has a higher melting point (1610 °C) than iron, copper or aluminium and other common metals. This enables castings to be made by pouring molten metal into moulds made from silica sand and a binder. The physical and chemical characteristics of the sand are important. In the past naturally clay bonded moulding sands were used. Today mainly clean sands are used with a binding agent added. The sands must have uniform grain size distribution and be generally rounded.

Sales of foundry sand have decreased in recent years due to the demise of engineering in the UK. About 11% of the sand sold in 2004 was used for foundry purposes.

Foundry moulds are mostly made from a mixture of silica sand chemically bonded with i.e.Phenolics or Furanes etc. : bentonite and water. Much of this material is now recycled for reuse. Spent foundry sand is also used as asphalt filler and in concrete manufacture.

Another example of how the sand is used can be seen in the photograph below when the BAFTA awards are cast in a bronze alloy by New Pro Foundries of West Drayton..

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