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.
Corner Blowout Failure
Meaning
Parison thinning during blow moulding causes a rupture at the extreme corners of a part where the plastic must stretch furthest. Corner blowout failure occurs when the inflated polymer melt cannot withstand the localized tensile stress before contacting the cooled mould wall. This issue results in scrap and halts automated production lines.
Melt Strength
Extrusion characteristics of the selected resin dictate the resistance of the parison to excessive sagging or uneven stretching. Low melt strength under high shear forces increases the likelihood of corner blowout failure during the expansion phase. Moulders often resolve this by choosing a resin with a wider molecular weight distribution.
Tool Design
Mould geometry plays a decisive role in the distribution of the expanding polymer film. Sharp corners with inadequate radii induce localized high strain rates, triggering corner blowout failure as the hot melt stretches across the metal void. Incorporating generous rounds into the cavity profile reduces this risk.
Wall Thickness
Variations in the starting parison profile affect the final material distribution in the molded component. Programmable die heads adjust the thickness of the extruded tube to ensure extra material is placed where stretching is most severe. When this adjustment is incorrect, the resulting thin spots lead to failure under pressure.
In addition, the use of parison programmer profiles must be calibrated for each resin batch to compensate for minor fluctuations in melt viscosity across production runs.