Sand is the ideal medium to cast liquid metal into a desired form. First, a suitable sand is mixed with a binder (e. g. resins) and pressed into the desired shape by a machine. This sand core defines the structure of the future workpiece (Fig. 2). The liquid metal is cast into the mold and after the after the workpiece is completely hardened, it is released from the sand. The sand can be recycled and used for new molds by removing the remainders of the binder. For this application, it is necessary to use a high-quality sand, which is almost pure quartz (SiO2) to prevent reactions between the mold and the liquid metal.
Furthermore, the sand must have a good gas permeability and low compressibility. The grain size is carefully monitored since it has an impact on the surface structure of the cast piece, which will be too rough if the particle size is too large. Foundry sands usually have a size distribution from 0,1 mm – 0,8 mm with a mean size between 0,2 mm and 0,45 mm. A standard parameter to express the size of molding sands is the AFS fineness number (AFS = American Foundry Society). This number is calculated from the size distribution, which is determined by standard ASTM sieves. Each fraction is multiplied with a weighting factor, the results are added together and divided by 100. Note that the resulting AFS number gets bigger as the average size decreases. The AFS number is considered proportional to the number of grains per unit weight.
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