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.
High Draw Ratio
Meaning
Geometric extension occurs when a material is stretched to many times its initial size during processing. Achieving a high draw ratio is a common goal in the production of high strength fibers and oriented films. This parameter is calculated as the final length divided by the starting length.
Molecular Orientation
Alignment of the polymer chains along the direction of the stretch changes the internal structure of the material. At a high draw ratio the random coils of the melt are transformed into a highly ordered crystalline or semi-crystalline state. This arrangement maximizes the number of covalent bonds resisting tensile load.
Mechanical Enhancement
Tensile strength and stiffness increase dramatically as the stretching process continues. Implementing a high draw ratio allows for the creation of lightweight parts that can replace heavier metal components. However this process often reduces the elongation at break and the impact toughness of the final product.
Production Constraint
Limitations on the maximum stretch are imposed by the entanglement density and the temperature of the resin. Pushing for a high draw ratio beyond the natural limits of the polymer leads to fibrillation or catastrophic breakage of the web. Maintaining a consistent ratio is essential for ensuring uniform part thickness and optical clarity.
Small variations in the heater bank temperature can cause the material to fail if the ratio is near the limit. This value remains a primary target for the optimization of oriented film lines.