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.
Sheet Sag
Meaning
Molten polymer behaviors describe the downward deflection of a plastic sheet when it is heated to its forming temperature. This sheet sag is a critical variable in thermoforming because it indicates that the polymer has reached its softened state. However, excessive deflection can lead to thinning and uneven material distribution in the final part.
Material Impact
Crystalline polymers often exhibit rapid changes in viscosity upon reaching their melting point, which makes them prone to sudden drooping. Amorphous polymers, by contrast, soften over a wider temperature range, making the sheet sag easier to control. Processors must select resins with appropriate extensional viscosity to prevent excessive sag.
Processing Defect
Excessive sag can cause the heated sheet to touch the bottom heating elements, which results in smoke, surface burns, or even fires. This issue of sheet sag also causes the sheet to stretch unevenly before the mold contact occurs. This uneven pre-stretching leads to thin spots in the finished thermoformed part, particularly at the bottom or corners.
These thin areas reduce the part’s structural strength and can lead to premature failure. Adjusting the oven temperature or using a pre-blow step to create a dome can help manage this sag.
Control Strategy
Photoelectric sensors detect when the drooping sheet crosses a specific height threshold in the thermoforming oven. This monitoring triggers the next step in the cycle or adjusts the heater output to limit the sheet sag. Automating this process ensures consistent part thickness and minimizes scrap rate.