Optimizing Thermoforming Process Windows Using Strain Hardening Rheology Metrics
Strain hardening rheology metrics define thermoforming process windows by predicting extensional melt strength to eliminate corner thinning and sheet sag.
Corner Blowout
Meaning
Localized rupture of a polymer sheet during thermoforming occurs when the material stretching exceeds its tensile limits at sharp transitions. This localized rupture, known as corner blowout, happens when the heated sheet is drawn too rapidly or too deeply into a female mould corner. The material thins excessively in these regions because the polymer cannot slide across the mould surface once it makes contact.
This phenomenon is particularly common in deep-draw ratio configurations where the raw material is stretched to its limit.
Structural Failure
Severe thinning in the corners of a finished part represents a major structural weakness that can lead to catastrophic cracking under stress. When a corner blowout occurs, the wall thickness drops to nearly zero, which permits pressurized fluids or mechanical loads to breach the part instantly.
Process Variable
Optimal control of sheet temperature is necessary to ensure uniform elongation across the mould surface. If the sheet is too cold, the polymer has insufficient elasticity to stretch into the corners without tearing, which triggers corner blowout. By adjusting the heating profiles and plug assist speed, the moulder can maintain a more balanced wall thickness.
Preventative Measure
Increasing the radius of the mould transition zones reduces localized stress concentrations during forming. Wider radiuses prevent corner blowout by distributing the tension across a larger surface area rather than concentrating it at a single point. This design adjustment allows the resin to flow more smoothly.
It ensures the final part retains its physical integrity.