Injection Molding Residual Stress Prediction in Variable Wall Geometry
Polycarbonate parts with 2.0mm to 4.5mm wall steps generate 34 MPa thermal stresses; accurate 3D viscoelastic simulation prevents expensive re-cuts.
Flow-Induced Stress
Meaning
Mechanical phenomena occur when polymer molecules are deformed and oriented by shear forces during melt processing. The resulting flow-induced stress represents the residual tension frozen into the plastic as it solidifies. This stress is particularly high in narrow gates and thin-walled regions of a mold.
Stress Mechanism
High shear rates during injection molding stretch the coiled polymer chains along the direction of flow. Because the polymer cools rapidly upon contacting the cold mold wall, these chains cannot return to their relaxed, random-coil state. The frozen flow-induced stress remains concentrated near the surface of the molded component.
This orientation leads to anisotropic mechanical properties, where the part is stronger along the flow direction but weaker across it.
Part Defect
Unrelieved orientation causes the finished component to warp or twist when it is ejected from the tool. Moreover, regions with high flow-induced stress are highly susceptible to environmental stress cracking when exposed to active chemicals. This vulnerability can lead to premature failure of the component during assembly or field use.
Processing Adjustment
Molders reduce these internal tensions by raising the melt or mold temperature to allow faster relaxation of the chains. Lowering the injection speed also decreases the shear rate, which directly reduces flow-induced stress. However, these adjustments must be balanced against the risk of longer cycle times.