Biaxial Strain Hardening Metrics for High Draw Ratio Amorphous Sheet Thermoforming
Biaxial strain hardening metrics dictate wall thickness uniformity in high draw thermoforming; matching plug dynamics to material hardening prevents blowout.
Sheet Orientation
Meaning
Polymer chain alignment along the direction of melt flow during extrusion determines the anisotropic mechanical properties of extruded plastic sheets. Sheet orientation is set during the cooling and pulling stages of the sheet extrusion process, where the polymer is stretched while still semi-molten. This stretching aligns the molecules, increasing tensile strength in the stretch direction while decreasing it in the transverse direction.
Thermoforming Behavior
Uneven stretching during thermoforming can cause parts to warp or thin excessively in areas where the orientation is highest. Sheets with high orientation in one direction will shrink more when heated, which can cause registration issues in printed packaging. Moulders must select sheets with balanced orientation to avoid these problems.
Extrusion Control
Draw ratio and cooling rate are the primary process variables used to control this alignment. A high draw ratio increases sheet orientation, while slower cooling allows the polymer chains to relax and return to a more random state. Adjusting these parameters requires a deep understanding of the resin’s crystallization behavior.
Mechanical Properties
Impact resistance is significantly affected by the direction of the applied force relative to the polymer chain alignment. A sheet that is highly oriented will resist cracking along the orientation direction but will split easily parallel to it. This anisotropy must be considered during part design and material selection to ensure the final product can withstand the expected service loads.
For example, industrial protective guards must be cut from sheets with low orientation to prevent catastrophic splintering under impact.