Crimp sealing has been used for sealing flexible packaging on automatic packaging machines since they were first invented. The principle involves using a wide profiled seal jaw typically 15mm wide to apply heat and pressure for a controlled dwell time to activate the surface layer of the packaging material. The metal jaws are rigid so do not apply uniform pressure across the width of the seal so where the film thickness varies at the back seal or gussets, the film is not compressed evenly. This is particularly so when there are random, unwanted creases present in the seal area. All the pressure is taken at the thickest points i.e. where there are three layers at the back seal overlap and where there are 4 layers at the edge of the pack when gussets are present.
The rest of the seal area with only two layers receives very little pressure allowing the polymer to flow away from the heat source. Because the film is not held under pressure at these points the film suffers thermal distortion and causes pinhole leaks. Dye tests show that all crimp seals have pinhole leaks at the bag edges, the back seal overlap point and at the point of any creases. Monolayer packaging films commonly used for fresh produce are particularly prone to seal leaks because the active surface layer which makes the seal is only 2 – 3 micron thick. Hence there is little polymer present to make the seal hermetic.
Where hermetic sealing is paramount it is common to use laminated films with a thicker sealant layer of 15 -20 micron. Such films are more expensive and use more polymer simply to make a seal.
The width of a crimp seal is typically 10 % of the total film area used for the finished bag and can be as much as 20% for smaller bags typical in the snack market. As there is no guarantee that a wider seal produces hermetic seals and rigid jaw designs will always apply uneven pressure there is a need for a new approach to making hermetic seals.
Drivers for Change:
Why does the food industry need a superior seal system on packaging machines using flexible films?
- Reduce packaging material use
- Environmental pressures to reducing packaging
- Improve product quality but improving seal integrity
- Improve operational efficiencies, reduce rework and lower manufacturing costs
- Longer shelf life to reduce food wastage
- Longer shelf life to optimize production and distribution logistics