Meaning
Structural changes in a polymer occur as the material temperature increases toward its glass transition point. The phenomenon of thermal softening allows solid resin pellets to be transformed into a flowable melt for injection or extrusion. As heat is applied, the intermolecular forces between the polymer chains weaken and the material becomes more flexible.
This state is necessary for the plastic to take the shape of the mould.
Viscosity Change
Resistance to flow decreases rapidly as the temperature rises. The rate of thermal softening depends on the molecular weight and the branching of the polymer. High viscosity resins require more energy to reach a state where they can be processed at high speeds.
Processing Temperature
Correct settings on the barrel heaters ensure the material is soft enough to flow without burning. If the temperature is too low, the plastic will not undergo enough thermal softening to fill the cavity. If it is too high, the polymer chains will break down and lose their strength.
Structural Integrity
Cooling the part reverses the softening process and locks the molecules into their final position. The speed at which this happens affects the crystallinity and the final dimensions of the part. Thermal softening is a reversible process for thermoplastics, allowing them to be reheated and recycled multiple times.
