Webbing is an unwanted fold that forms between adjacent mold features or between a feature and the clamp frame. It develops when excess sheet area is trapped in a narrowing region and cannot be redistributed before opposing surfaces seal.
Molds may be too close for their height and shape. The sheet may also be excessively fluid, too cold to stretch into the available space, or insufficiently pre-stretched. Vacuum applied too aggressively can pull material into competing features before a stable distribution is established. Local heater imbalance often makes the web region behave differently from the rest of the sheet.
Determine whether the cause is tooling or process
A fold that remains in the same location through moderate changes in temperature and timing usually requires a tooling change, downholder, or web suppressor. Webbing that changes substantially with pre-stretch or vacuum sequence is more likely process-related.
Pre-stretch should allocate material to each draw zone before final forming. Initial evacuation may need to be staged, while final air removal must remain fast enough to preserve detail. Reducing overall temperature without diagnosing the mechanism can replace webbing with poor detail or brittle tearing.