Meaning
Internal stress conditions develop within a molded part due to non-uniform temperature gradients during the cooling phase of the injection molding process. Thermal stress arises when the outer skin of the part solidifies first while the hotter inner core continues to shrink and pull on the frozen surface. This differential cooling creates balanced zones of tension and compression that remain locked within the part.
Stress Generation
Rapid cooling freezes the outer layers of the polymer melt in an extended state before they can fully relax. The subsequent cooling and contraction of the interior core pull against these rigid outer layers, generating high tensile stress in the center of the wall and compressive stress at the surface.
Processing Cause
Process variables such as melt temperature, mold temperature, and cooling time directly influence the magnitude of these internal stresses. Low mold temperatures cause a large temperature differential between the plastic and the metal cavity walls, intensifying the thermal gradients. Conversely, longer cooling times allow the heat to distribute more evenly, reducing the residual stress level in the molded component.
Proper balancing of the cooling lines in the mold is essential to prevent asymmetric cooling that worsens this stress.
Molding Defect
Uncontrolled internal stress can lead to immediate part warpage or delayed stress cracking when the component is exposed to solvents or mechanical loads. In optical parts, this stress induces birefringence, which degrades the optical performance of lenses. Using annealing cycles after molding can help relieve these stresses and stabilize the part dimensions.
