In the rotational moulding process the mould only restrains the external surface of the moulded part. The decrease in volume that takes place during the solidification process causes shrinking that is distributed in all directions, even as a function to the restrictions. A moulded part that is very restricted (due to the presence of metal inserts, stiffening ribs or other) will show shrinkage that is less than average in the direction of the restraint and higher than average in the directions where the shrinkage can take place freely.
The realisation of a good quality product requires special attention right from the early design stages: attention to detail during the concept stages of the product can eliminate some problems: for example, warpage and distortion can be reduced by inserting stiffening ribs on large flat surfaces and avoiding sharp corners and severe mould shrinkage restrictions.