Biaxial Extensional Deformation Dynamics of Semi Crystalline Polymer Sheets
Biaxial extensional strain hardening controls sheet thinning, requiring precise melt strength, tool friction, and temperature tuning to set uniform container wall thickness.
Residual Stress Birefringence
Meaning
Optical anisotropy in transparent polymers arises from locked-in molecular orientation and thermal gradients during cooling. In injection molded parts, residual stress birefringence is the physical result of molecular alignment that has not had time to relax before the material solidifies. This phenomenon affects the optical performance of lenses, light guides, and transparent covers.
It stops applying to opaque materials, where light transmission is not a functional requirement, and to parts that have undergone complete thermal annealing.
Stress Generation
During the injection phase, high shear forces stretch the polymer chains along the flow direction. If the mold wall is cold, the outer layer solidifies rapidly, locking this stretched state into the part structure. In the core, slower cooling allows the polymer chains to relax and return to a random state.
This difference in cooling rates across the part thickness creates tensile and compressive stresses that alter the refractive index along different axes.
Defect Prevention
Molders can reduce this defect by optimizing the injection velocity, pack pressure, and melt temperature. Lower injection speeds and higher melt temperatures reduce the shear stress during filling, which reduces the orientation of the polymer chains. High mold temperatures allow for slower cooling and more time for molecular relaxation.
When these adjustments are insufficient, the molded parts must be annealed in a conveyor oven to relieve the internal stresses. This heat treatment must be conducted below the heat deflection temperature of the polymer to prevent deformation while allowing the molecules to reorganize. Slow cooling after annealing is also required to avoid reintroducing thermal stresses into the finished optical component.
Quality Analysis
Visualizing and measuring the stress pattern is accomplished using cross-polarized filters. When a transparent part is placed between two polarizers, areas of high stress appear as colored bands or fringes. The density and location of these fringes indicate the magnitude of the stress.
This non-destructive test is carried out in production lines to verify that optical parts meet the required clarity and do not fail prematurely from stress cracking.