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.
Catenary Deflection
Meaning
Physical sag measurements calculate the curve of a continuous web or strand of polymer suspended between two rollers under the influence of gravity. This catenary deflection occurs in extrusion and film-casting lines between the die exit and the take-up rolls. Controlling this curve is necessary to maintain uniform sheet dimensions.
Processing Variable
Web tension determines the severity of the sag as the polymer cools. Low tension increases the catenary deflection, which allows the molten polymer to stretch unevenly. High tension, on the other hand, can tear the hot web or cause neck-in.
Surface Defect
Incorrect tension levels cause marking or scratching on the bottom surface of the sheet if the sag touches the support framework. Minimizing the catenary deflection avoids contact with cold metal surfaces before the polymer has solidified. If the sheet touches a roller at an incorrect angle due to sag, it can entrap air bubbles between the film layers.
This results in cosmetic blemishes that render the protective film useless for optical applications.
Control Method
Optical sensors monitor the gap between the suspended sheet and the sensor head to adjust the take-off roller speed dynamically. This maintains the catenary deflection within a narrow operating range. Proper adjustment prevents wrinkling of the film.