Meaning
Temperature ranges over which a semi-crystalline or amorphous polymer undergoes a fundamental shift in its physical state define the operating limits for melt processing and part performance. Each thermal transition governs the behavior of the resin as it moves between solid, rubbery, and molten phases. This material property sets the boundaries for the injection barrel temperatures and the required cooling time in the mould.
The transition is specific to each resin formulation and is influenced by the presence of plasticizers or reinforcing fibers.
Temperature Setpoint
Moulding amorphous resins requires the process temperature to be set well above the glass transition point to allow the polymer chains to flow under pressure. For semi-crystalline materials, the injection barrel must heat the resin past its crystalline melting temperature to ensure complete molecular untangling. Understanding each thermal transition allows the technician to configure the heater bands and the hot runner zones correctly to avoid thermal degradation.
If the temperature is too low, un-melted pellets will damage the screw and create cold-slug defects in the moulded parts.
Demoulding Window
Solidification of the resin inside the mould cavity occurs as the polymer cools back through its crystallization or glass transition temperature. This thermal transition determines the demoulding temperature and the minimum cycle time needed before the part can be ejected without warping. Parts exposed to high service temperatures must operate below their transition points to prevent loss of stiffness, dimensional change, and structural failure.
This limit is a critical consideration during material selection for under-hood automotive or sterilization applications.
Calorimetry Record
Differential scanning calorimetry provides the standard laboratory method to identify and record these phase changes in a polymer sample. This test measures the heat flow into or out of the sample as it is heated and cooled at a controlled rate, revealing the exact transition temperatures. Analyzing these thermal transition peaks helps the moulder detect contamination from other resins or verify the crystallinity of the molded part.
It is an essential tool for troubleshooting inconsistent shrinkage, brittle failures, and post-mould warping.
