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.
Tool Friction Coefficient
Meaning
Physical properties measuring the resistance to sliding between a polymer melt and the metal surface of a mould cavity affect the ease of material flow and part ejection. Tool friction coefficient determines the force required to push the molten polymer into the cavity and to eject the solidified part after cooling. A high coefficient can lead to surface defects, high cycle times, and part distortion during ejection.
Flow Behavior
Resistance to flow increases when the contact between the polymer and the steel exhibits a high friction coefficient. This requires higher injection pressures to fill the cavity completely, which can lead to flashing or tool wear. Processors can use slip agents or mould coatings to reduce this friction.
Ejection Force
Demoulding forces must be minimized to prevent cosmetic damage to the part during the ejection phase. If the tool friction coefficient is too high, the ejector pins can push through or deform the warm plastic, resulting in high scrap rates. This is especially critical for thin-walled packaging parts.
Surface Finish
Texture on the mould surface directly influences the contact area and the resulting friction. Polished surfaces generally reduce friction for amorphous polymers but can cause sticking for semi-crystalline resins due to vacuum effects. Moulders must optimize the surface finish to balance cosmetic requirements with ease of processing and ejection.
In many cases, a micro-textured finish is applied to the tool cavity to prevent sticking and to ensure consistent part release across multi-cavity tools.