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Foundry Sand: Part 2

Types of Sand:
There are many types of sand that one can use in moulding applications, from natural sands to man-made sands (such as ceramic sand). The purpose of sand as a moulding medium is to act as a good medium for high bonding mould strengths as well as provide a high refractoriness against heat. As a result, there will be fewer casting-related defects due to low refractoriness of the virgin sand.

It is crucial that the best quality sand is used to ensure the best quality in terms of mould production, casting definition and defect-free castings.

The several types of sand found in SA are:
  • Silica Sand
  • Chrome Sand
  • Quartz Sand
  • Olivine Sand
  • Bauxite Sand
  • Ceramic Sand
Sand Geometry:
There are several types of sand shapes.  Each shape offers a unique challenge and presents unique advantages. Very angular grains cause massive wear and tear on your capital equipment, such as pipes, sand pumps, mixers, blades, patterns, shakeout systems etc. The biggest concern with angular sand is that it flows and compacts poorly.

The several types of sand shapes are as follows:
  • Very Angular
  • Angular
  • Sub-angular
  • Sub-rounded
  • Rounded
  • Well rounded
Sand geometry is also influenced by how the sand is mined.  The shape is also influenced by Mother Nature. Alluvial sand is rounder by nature. Sand that is mined, crushed and screened into various size gradings tends to be more angular in shape. To compensate for sands which are angular or very angular in shape, one must ram harder and use more chemicals to bind the sand.

The best type of sand shape is a perfect sphere or as close to round as possible.

The benefits of well-rounded sand grains include:
  • An increase of sand flowability
  • It compacts well
  • It has excellent permeability
  • It requires less resin and catalyst to bond the sand
  • They compact more densely – they can be up to 8-10% denser than angular sand
Poor sand choice results in defects:
When selecting an adequate sand to be used in the moulding process, one must take all the above factors into consideration. However this is not always so simple. Therefore this is where sand management comes into play.  One needs to understand sand - its geometry, clay content, pH etc. Choosing the wrong type of sand can have devastating consequences for the foundry, resulting in sand-related defects.

Always go with a reputable sand supplier that can supply consistent, reliable and repeatable quality. You may pay more, however if you choose an inferior quality moulding medium, the cost of scrap castings may be even higher.
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