High temperature materials are divided into two main categories—semi-crystalline and amorphous—based on their difference in molecular structure.
Amorphous high temperature resins have a randomly ordered molecular structure which does not have a sharp melt point; instead amorphous materials soften gradually as the temperature rises.
These materials change viscosity when heated, but seldom are as easy flowing as semi-crystalline materials. They are isotropic in flow, shrinking uniformly in the direction of flow and transverse to flow. As a result, amorphous materials typically exhibit lower mold shrinkage and less tendency to warp than the semi-crystalline materials.
Amorphous resins lose their strength quickly above their glass transition temperature (Tg).