Film and seal testing
Use this guide to check whether the film, seal and tunnel settings can produce the finished pack standard before committing to a machine route.
Technical answer
Direct answer: The film and seal window is the range of settings where the machine can make a complete seal, transfer the pack and shrink it without burning, splitting, wrinkling, trapping excessive air or distorting the product. It should be checked with representative products, the intended POF or PE film and the finish required by the buyer.
A pack can appear to fit the sealing frame but still fail when the film, product and tunnel are tested together. The review should therefore check the unshrunk seal, the tunnel result and the cooled pack rather than judging the machine only from a specification sheet.
| Test point | Useful evidence | Warning signs |
|---|---|---|
| Film coverage | Centre-fold width, film thickness, product height and seal allowance. | Seal too close to an edge, bridging over corners or excess loose film. |
| Seal response | Continuous unshrunk seal after the cycle, with no contamination in the seal area. | Open corners, weak seams, melted holes, product debris or folded film in the seal. |
| Tunnel recovery | Finished pack after normal tunnel dwell and airflow. | Wrinkles, dog ears, ballooning, scorched film or distortion. |
| Cooled pack | Appearance and stability after normal outfeed handling. | Film moving after discharge, contact marks, softened product or barcode distortion. |
The seal window matters because a setting that seals one product or film can fail on another. Film thickness, shrink behaviour, contamination, product height, dwell time and transfer method all affect whether the same L-sealer route produces a reliable pack.
POF and PE film can behave differently through sealing and tunnel shrinking, so each should be checked as the real intended material rather than treated as interchangeable. The trial should confirm centre-fold width, perforation, seal response, tunnel recovery and the cooled pack finish.
Useful trial evidence includes representative products, pack dimensions, film specification, photos of acceptable and unacceptable packs, target output, downstream handling details and any barcode, label or display face that must remain clear after shrinking.
Related guidance: POF versus PE film · Film perforation · Sample testing · Acceptance criteria