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.
High Melt Strength Polypropylene
Meaning
Modified thermoplastic resins undergo chain branching or blending to increase their resistance to sagging when in a molten state. Utilizing high melt strength polypropylene allows processors to thermoform large or complex parts without the sheet rupturing under its own weight. This resin type also enables the production of low-density foam products.
Extrusive Performance
Melt strength refers to the maximum force a polymer melt can withstand without breaking during stretching. Standard polypropylene has a linear molecular structure that exhibits low melt strength and rapid thinning. Incorporating high melt strength polypropylene into the formulation ensures that the polymer web remains stable during extrusion coating or film blowing.
Defect Prevention
Sheet sag during the heating phase of thermoforming can cause uneven wall thickness or webbing. The use of high melt strength polypropylene prevents the sheet from drooping excessively when heated above its melting point. This resistance to deformation allows for a wider processing window and reduces the cycle time required for the sheet to stabilize.
It also prevents the sheet from touching the heater elements, which would otherwise cause smoke and fire hazards. The resulting thermoformed parts have more uniform wall thickness and better structural integrity.
Economic Impact
Lower scrap rates during startup and run phases compensate for the higher initial cost of these branched resins. Processing high melt strength polypropylene reduces the need for thick sheets to compensate for thinning. This material savings directly improves the profitability of high-volume packaging runs.