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Plastics vs. Metals

From an engineering point of view, plastics differ from metallic materials in the following ways:
  - Plastics tend to have pronounced non-linear time-dependent stress/strain behavior. Below the yield point, metals typically have elastic behavior which is nearly linear.
- Instead of a single melting point, plastics typically have a
Glass Transition Temperature (TG). Metals typically have a clearly-defined melting point.
- Plastics in general exhibit pronounced creep. Metals undergo creep to a certain extent, but for most engineering configurations their creep is insignificant.
- Plastics tend to degrade or denature (due to heat) rather than corrode within a typical atmosphere. Of course, chemical degradation can occur when reactive chemicals are present. Plastics impregnated with organic fillers can be subject to bacterial infestation. Metals can corrode even in a benign atmosphere from reaction with oxygen and water, but are not affected by bacteria.
Plastics vs. Plastics

AMORPHOUS POLYMERS: e.g., PVC, Polystyrene, Acrylic, ABS, PPO, PC, Polyetherimide have
  - a wide melting range,
- low shrinkage after molding,
- better impact and lower chemical resistance than crystalline polymers,
- moderate heat resistance,
- dimensional stability,
- and superior cosmetics of outer surfaces.
CRYSTALLINE POLYMERS: e.g., PPS, PBT (Valox), Polypropylene (PP), have
- a sharply-defined melting point,
- high shrinkage after molding,
- low impact strength,
- high heat resistance,
- good fatigue endurance,
- good lubricity; wear resistance,
- good chemical resistance,
- and the ability to flow in thin-walled sections.
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