Sheet Sag and Pre-form Collapse in Vacuum Forming — Causes and Prevention

Sheet sag occurs when a heated thermoplastic reaches a highly viscous state and its melt strength can no longer support its own weight. As the material softens, gravity causes the sheet to stretch downward, creating excessive sag before the forming cycle begins. This issue is particularly common when processing large-format sheets or when heating times exceed the optimal forming window.

In severe cases, the pre-stretch bubble becomes unstable and collapses before the sheet can be formed over the mold. The resulting deformation leads to uneven material distribution, excessive thinning, dimensional inaccuracies, and increased scrap rates. Materials with low melt strength are especially sensitive to this defect when subjected to excessive temperatures.

• Reduce heating cycle time to prevent the sheet from reaching an excessively fluid state.
• Lower sheet temperature while maintaining sufficient formability for the application.
• Install a photo-electric sag eye system to automatically trigger air injection and maintain proper sheet position.
• Use cooling screens or aluminum mesh in the center of large-format sheets to reduce localized overheating.
• Optimize heater zoning to achieve a more uniform temperature profile across the sheet.
• Monitor sag depth during production and adjust processing parameters before instability occurs.