Biaxial Extensional Deformation Dynamics of Semi Crystalline Polymer Sheets
Biaxial extensional strain hardening controls sheet thinning, requiring precise melt strength, tool friction, and temperature tuning to set uniform container wall thickness.
Web Sag
Meaning
Gravitational deflection of a continuous polymer sheet or film occurs between support rollers or heater banks. In extrusion and thermoforming lines, web sag is the physical displacement of the heated polymer web due to its own weight. This phenomenon is a result of the reduced melt strength of the resin at high temperatures.
It does not apply to rigid sheet extrusion or to processes where the web is continuously supported by a conveyor belt.
Temperature Influence
As the polymer sheet passes through the preheating oven, its temperature rises above its softening point, which causes the viscosity and melt strength to drop. If the temperature is too high, the sheet will sag excessively, which can cause it to touch the heater elements and ignite. This risk requires careful monitoring of the sheet temperature and the tension of the web.
Proper tensioning can minimize the displacement, but too much tension can cause the sheet to tear or stretch unevenly.
Resin Properties
The amount of deflection depends on the molecular weight and molecular weight distribution of the polymer. Resins with long-chain branching, such as high melt strength polypropylene, exhibit less deflection because they undergo strain hardening when stretched. Linear polymers, such as standard polypropylene, are more prone to this behavior and require closer control of the heating cycle.
Resin suppliers often measure the melt elasticity to provide molders with data on how the material will behave in the oven.
Defect Prevention
Thermoforming machines utilize photoelectric sensors to monitor the sheet position and adjust the oven temperature or the sheet speed to prevent excessive deflection. Some machines also use a pre-blow system that introduces compressed air under the sheet to support it during the heating phase. By preventing excessive sag, molders can avoid defects such as webbing, thin spots, and uneven wall thickness in the thermoformed part.
In high-speed packaging lines, this prevention is necessary to maintain the fast cycle times and low scrap rates required for cost-effective production of food trays and containers.