Why do plastic warp when heated

The plastic consists of very long-chain flexible polymer molecules, consisting of hundreds of molecular segments. The ordinary equilibrium configuration of these molecules are coiled into a ball, with the chains intertwined. However, during the manufacturing operation, the part undergoes very rapid deformation, and the polymer chains become elongated relative to their equilibrium configuration. Then the part is rapidly cooled (below the so-called glass transition temperature) before the molecular chains have a chance to relax back to their equilibrium configuration. So the polymer chains in the final part are "frozen in" in an extended configuration. When the part is heated again above the glass transition temperature, the chains relax, and this relaxation results in a deformation and a warping of the part. This behavior is very common in the polymer processing and man-made fiber processing industries.

Chet