Injection Molding Thermal Boundary Layer Physics Basics
Thermal boundary layer growth governs active cavity flow clearance, pressure drop, and gate seal timing, setting the fundamental physical limit on cycle time.
Thermomechanical Residual Stress
Meaning
Internal forces remain locked within a solidified part due to uneven cooling and constrained shrinkage. The thermomechanical residual stress arises from the temperature gradients present as the polymer transitions from a melt to a solid. These stresses remain in the part after it is ejected from the mold and can lead to immediate or delayed warping.
Formation Mechanism
Thermal gradients cause volumetric changes that the surrounding material resists. As the core cools and tries to shrink, the already solidified skin prevents this movement, generating thermomechanical residual stress. The magnitude of this stress depends on the thermal expansion coefficient and the stiffness of the polymer.
Parts
with measurable internal stress are prone to dimensional instability and environmental stress cracking. Excessive thermomechanical residual stress can cause a component to twist or bow once it is removed from the fixture. Annealing is sometimes used as a post-processing step to relax these internal forces, although it adds cost and time.
Predicting
and measuring these stresses is a primary part of the validation process for safety-critical components. Software tools calculate the expected thermomechanical residual stress based on the cooling profile and the packing pressure. Strategic gate placement and uniform cooling minimize these stresses to ensure part reliability over a long service life.