Case Study - Sands for Foundry Use

Foundry applications are by far the most demanding of sand. For mould or coremaking, the sand is mixed with a binder, either bentonite based or one of a range of chemical binders, 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 it’s 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 flowable 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 it’s 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.