Wall thickness is a critical factor. Thin sections have a shorter cooling time so less opportunity for correction within the mould or by jigging. Varying thickness within a moulding also leads to distortion due to different shrinkages. If possible a gradual transition between different sections should be used.
However, this differential shrinkage could also be used as a means of correcting warping by thickening sections at appropriate points.
Ribs can also be a problem, especially where they have been made thinner to prevent sink marks as once again there are variations in the section. Gas injection or a blowing agent may be used to overcome the problem.