Anybody has already seen too little (or too big) a polymer part disturbing a device made by assembling several parts of various materials. Sometimes, the dimension is fair but a more or less strong warpage prevents a correct assembly.
These phenomena are the consequences of:
* The coefficient of thermal expansion contributing to the shrinkage after demoulding
* The water uptake particularly known for polyamides. Other chemicals can also swell polymers but they are rather relevant from chemical behaviour.
* The desorption of humidity or additives such as plasticizers or other low molecular weight organic additives.
The variations of the above parameters are responsible for warpage and sink marks.
Moreover, plastics and rubbers are often simultaneously used with conventional materials, notably metals whose coefficients of thermal expansion can be 10 to 100 times lower. This can promote high stresses and eventually failure of the device including these different materials.