Meaning
Localized regions within a molded polymer part where internal mechanical stresses remain locked in after cooling represent the consequence of uneven thermal contraction and molecular orientation. The presence of residual stress domains can lead to dimensional instability, warping, and premature mechanical failure. These domains are created during the cooling stage of injection molding when the outer skin freezes before the inner core.
Stress Distribution
Measuring the variation of internal forces across the cross-section of a part reveals where the high-stress zones are located. These zones are usually concentrated near the gate where flow shear is highest or in thick sections that cooled slowly.
Thermal Gradient
Temperature differences between the surface of the mold and the molten polymer drive the formation of these internal stresses. The outer layer of the part cools rapidly and solidifies while the interior is still hot, and as the core subsequently cools and shrinks, it pulls against the rigid outer skin, creating tensile stresses in the center and compressive stresses on the surface.
Part Stability
Releasing these locked-in forces over time or during exposure to high temperatures causes the part to deform or warp. This behavior is particularly problematic in thin-walled components that must maintain strict flatness tolerances, making it necessary to optimize the cooling time and mold temperature to minimize the magnitude of these internal stresses.
