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.
Birefringence
Meaning
Optical anisotropy occurs when polymer chains freeze into a preferred orientation during injection moulding, separating refractive indices along orthogonal axes. Internal stress fields within the moulded part generate this directional variation in light velocity, directly altering the polarisation state of transmitted radiation. Polarised light microscopy measures the resulting retardation pattern across the component cross section, quantifying residual stress magnitude left by packing pressure transitions.
Part specifications establish strict retardation limits to prevent premature failure in transparent optical lenses and structural enclosures.
Resin Orientation
Polymer chains stretch along melt flow vectors inside high shear cavity zones during the filling phase. Molecular alignment increases local density parallel to flow lines while leaving transverse domains less ordered. Cooling rates lock these oriented conformations before relaxation mechanisms restore random coil configurations.
High injection speeds intensify the frozen gradient across thin walls, creating core skin differentiation that exacerbates retardation values.
Moulding Deviation
Processing shifts alter final optical performance far beyond raw material datasheet guarantees. Excessive packing pressure traps dense molecular matrices near gate sites, elevating local retardation measurements well above nominal resin baselines. Barrel temperature adjustments change melt viscosity, modifying shear histories during runner transit and cavity entry.
Virgin material processes predictably, whereas virgin regrind blends alter melt rheology, shifting local molecular orientation distributions and raising part rejection rates.
Structural Failure
Uncontrolled optical retardation reduces mechanical integrity under service loads, leading to stress crazing and premature fracture along orientation planes. Polarised light inspection stations catch high stress zones before components reach assembly lines, preventing field failures in transparent safety shields. Corrective mould adjustments balance runner geometry and gate dimensions to equalise flow velocity profiles across cavity walls.
Proper thermal control ensures uniform cooling rates, minimising internal gradients and delivering stable mechanical performance across production runs.