Warping is one of the most frustrating injection moulding faults. This is because it does not always appear immediately, the causes are complex and prevention is difficult.
It is easy to say that the cause is differential shrinkage. However, the non-uniform shrinkage arising from residual stresses has two components: thermally-induced stresses and flow-induced stresses.
Thermally-induced stresses arise from non-uniform cooling, either related to thick and thin sections or from unbalanced mould cooling.
When melt is injected into a mould cavity, the polymer chains become elongated in the direction of flow and will do their best to recover before the moulding is completely cooled, causing greater shrinkage in the flow direction than the transverse direction, which will twist a centrally gated disc out of plane.
To complicate matters, shrinkage and warping are materials related. Semi-crystalline thermoplastics have a higher mould shrinkage than amorphous thermoplastics and consequently a greater tendency to warp. Warping is also a problem with fibre-filled materials because of alignment of fibres in the flow direction, giving non-isotropic properties.
As with many moulding faults, prevention requires action at different stages. Designers should avoid geometric asymmetry if possible. Toolmakers need to be careful about gate positions and concentrate on effective, balanced mould cooling. Processors have to balance the holy grail of short cycle times with the need to anneal out residual stress, either in the mould or after ejection. I am old enough to remember mould shops with areas of floor space occupied by mouldings cooling on jigs to prevent warping. In some cases warping does not show up until well into service. Today processors have to juggle injection rates, pressure dwell times and mould temperatures if designers and toolmakers have failed to do their bit.
At the risk of repeating myself, the best advice is to carry out a full simulation analysis at the design stage.