Silica Grain Shape and Size Distribution

Silica sand
Most of the sand moulds and cores are based on silica sand since it is the most readily available and lowest cost moulding material. Other sands are used for special applications where higher refractoriness, higher thermal conductivity or lower thermal expansion is needed.













Table1. Properties of silica sand for foundry use.


Size distribution
The size distribution of the sand affects the quality of the castings. Coarse grained sands allow metal penetration into moulds and cores giving poor surface finish to the castings. Fine grained sands yield better surface finish but need higher binder content and the low permeability may cause gas defects in castings. Most foundry sands fall within the following size range:

Table2. Size distribution of silica sand for foundry use.


Grain shape
Grain shape is defined in terms of angularity and sphericity. Sand grains vary from well rounded to rounded, sub-rounded, sub-angular, angular and very angular. Within each angularity band, grains may have high, medium or low sphericity. The angularity of sand is estimated by visual examination with a low power microscope and comparing with published charts

Fig.1. Grain shape classification


The best foundry sands have grains which are rounded with medium to high sphericity giving good flowability and permeability with high strength at low binder additions. More angular and lower sphericity sands require higher binder additions, have lower packing density and poorer flowability.