Near-net-shape parts do not leave the mould at a perfectly fixed size. Casting process capability, machining allowance, thermal contraction and post-shrinkage must be planned before functional dimensions are released.
A casting specification can combine a dimensional casting tolerance grade with a required machining allowance. Functional machined surfaces still need clearly defined finished dimensions and datums.
R = F + 2·RMA + T/2
R raw-casting size, F finished size, RMA required machining allowance and T total casting tolerance. Example notation: ISO 8062-3 · DCTG 12 · RMA 6 (RMAG H).
Processing shrinkage occurs as the part cools after moulding. Post-shrinkage can continue after demoulding, especially in crystalline plastics or during warm storage.
The Fischer examples range from about 0.45% for polystyrene to 1.7% for polyethylene, showing why one generic shrinkage factor is unsafe.
| Design input | Drawing or tooling decision |
|---|---|
| Casting process capability | Select the tolerance grade from the actual process; hand moulding, investment casting and die casting do not provide the same accuracy or surface quality. |
| Machined functional surface | Reserve enough stock for cleanup, but avoid excessive allowance, weight and machining time. |
| Plastic material and fibre direction | Use supplier data and mould-flow/tooling experience; geometry, packing, cooling and orientation affect the final result. |
| Inspection timing | Measure after the defined conditioning period so production and inspection compare the same dimensional state. |