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.
Amorphous Sheet Thermoforming
Meaning
Secondary shaping occurs when a non-crystalline plastic substrate is heated to its softening point and drawn over a mould. Heat sensitive polymers undergo amorphous sheet thermoforming within a specific temperature range above the glass transition but below the melting point. This method relies on the lack of a defined melting temperature to maintain web strength during the draw.
Thermal Management
Heat absorption rates vary depending on the thickness and pigmentation of the polymer stock. Controlling amorphous sheet thermoforming requires infrared heater banks that apply uniform energy to prevent localized thinning. Overheating leads to webbing or sagging while underheating causes part fracture.
Structural Integrity
Mechanical properties of the finished part depend on the orientation of the polymer chains during the stretching phase. Fast cooling cycles in amorphous sheet thermoforming freeze the molecular alignment to provide impact resistance. Rapid quenching prevents the formation of unwanted crystalline structures that would otherwise turn the part brittle.
Processing Window
Operating limits are defined by the glass transition temperature of the specific resin grade. Successful amorphous sheet thermoforming demands a balance between vacuum pressure and sheet temperature. If the temperature is too high the material loses its melt strength and tears.
Modern control systems monitor the sheet sag to trigger the forming cycle at the exact moment of readiness. High scrap rates occur when the heater cycle drifts.