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.
Webbing Defect
Meaning
Excess material bridging the gap between adjacent projections or corners on a thermoformed part results in an unacceptable finish. This bridge of plastic, called a webbing defect, occurs when the heated sheet folds over itself during the drawing process. It typically happens in moulds with high, closely spaced projections where the material has room to gather.
The defect impairs both the appearance and the dimensional accuracy of the finished piece, leading to structural failures and high scrap rates.
Forming Mechanism
Inadequate control of the sheet sag before the vacuum is applied allows the material to drape loosely over the mould peaks. This loose material is then drawn into the valleys, creating a webbing defect as it folds. The problem is exacerbated when the polymer sheet has a low melt strength.
Mould Design
Optimizing the spacing between protrusions on the forming tool helps prevent the pooling of excess plastic. When a webbing defect persists, adding a helper bar or a vacuum assist hole in the deep recess can pull the excess material flat. This design change forces the sheet to conform to the mould surface rather than bridging.
Process Adjustment
Reducing the heating temperature can make the polymer sheet less fluid and less prone to sagging. This temperature adjustment is a direct way to avoid the conditions that lead to a webbing defect. Increasing the speed of the vacuum pull can also help by drawing the material down before it has time to fold.