Meaning
Internal load distribution within a cooled part is often the result of stresses that remain after the moulding cycle is complete. Thermal residual stress is caused by non-uniform cooling and the constrained contraction of the polymer during the solidification phase. This phenomenon is a primary driver of warpage and dimensional instability in finished goods.
It is particularly severe in parts with thick sections or those made from semicrystalline resins.
Cooling Gradient
Temperature differences between the surface and the core of a part create layers that solidify at different times. The outer skin cools and hardens first, forming a rigid shell that resists the later contraction of the still-hot interior. This creates a state of compressive stress at the surface and tensile stress in the centre.
Tooling Influence
Uneven mould cooling contributes heavily to the development of these stresses. If one half of the mould is hotter than the other, the part will contract asymmetrically, leading to a permanent bow or twist. High-quality mould designs use conformal cooling channels to minimize these gradients and produce flatter parts.
Stress Relaxation
Annealing is a post-processing step used to reduce these internal loads by heating the part to just below its softening point.
