Glass Transition Temperature (Tg): This is the critical temperature at which an amorphous polymer (or the amorphous regions of a semi-crystalline one) transitions from a hard, glassy state to a soft, rubbery state. It represents the lowest temperature at which the polymer chains have sufficient energy for segmental motion and is often considered the upper service temperature limit for rigid plastic parts.
Melting Temperature (Tm): This applies to crystalline polymers and is the temperature at which the organized crystalline structure breaks down, and the polymer transforms into a viscous liquid. The Tm represents the lower limit of the processing temperature range for these materials.
Flow Temperature (Tf): For amorphous polymers that do not have a distinct melting point, the Flow Temperature is the point at which the material transitions from a rubbery, high-elastic state into a viscous flow state, making it suitable for molding processes like injection molding.
No-Flow Temperature: This is a practical processing parameter defined as the highest temperature at which a polymer will not flow under a specific pressure. It is determined experimentally using equipment like a capillary rheometer and helps in setting the minimum barrel temperatures to prevent premature material flow, or "screw dribble."
Decomposition Temperature (Td): If a polymer is heated beyond its processing window, the molecular chains begin to break down chemically. The Decomposition Temperature is the point at which this degradation becomes significant and rapid, leading to a severe loss of properties, gas evolution, and charring. Processing must always occur well below Td.
