Sand Casting requires a pattern which is commonly made from wood. Patterns can be made by hand or routed using CNC technology for exacting dimensions. Items can also be cast from originals.
Sand cast items are generally fractionally smaller than their pattern due to shrinkage, a consideration which should be taken into account during the design phase. However, for many architectural applications this shrinkage is negligable and within tolerances. Bronze actually expands on cooling, it is only aluminium, lead and copper that shrink.
Sand Casting Standard Tolerances
Castings should be within linear tolerances of ±1.0 upto 150mm and ±0.20 upto 25mm thereafter.
The draft or amount of taper given to the pattern to facilitate withdrawing from the mould depends somewhat on the size and shape of the pattern. A general guide would be 1 to 2 degrees of taper per side.
Shrinkage allowances are approximate values only, since the exact allowance depends upon the size and shape of the casting and resistance of the mould. It is, therefore, possible that more than one shrinkage allowance will be required for different parts of the same pattern / casting. A general guide for aluminium alloy castings would be 1.3%.