Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of amorphous vs semi-crystalline. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Amorphous plastics — PVC, acrylic, polycarbonate, PEI, PES — have randomly arranged chains. They are usually transparent, shrink predictably, machine to a glassy finish, and are stiff at room temperature. Past their glass transition their stiffness collapses.
Semi-crystalline plastics — POM, nylon, PEEK, PTFE, polyolefins — have ordered regions that hold them stiff up to near the melting point. They resist fatigue and wear, do not stress-crack readily, and machine with a slightly waxy surface.
Common misuse
Selecting a plastic by tensile strength without checking which class it is in. The class predicts creep, wear and solvent behaviour better than the strength figure does.