Alternative Joining Processes

The manufacture of packaging systems, for example bags, involves joining films by welding or sealing. Either thermoplastic films or films with a thermoplastic sealing layer applied to the full area of a substrate film are used. This requires a large amount of sealing material, all of which is not necessary for the sealing process. Partial application in the vicinity of the seam would suffice for this process. However, the partial application in the film fabrication process of a sealing layer is problematic for rolling up the film. This could be circumvented by application just prior to the joining process in the packaging machine.

The partial application of a thermoplastic sealing film is being studied in order to save sealing material. The objective is to obtain a totally tight seam. This requires careful selection of the sealing material with regard to its properties and the amount that is applied. It seems prudent to use low viscosity hotmelts, which should prevent the formation of capillaries especially at the fin lap seam area of the cross seam. Specific phenomena during the application, such as shrinkage effects, were taken into account. The thermal reactivation of the sealing material in the region of the cross-seam is also being studied. For this the use of the ultrasound method, which is being increasingly employed for bag sealing, will be evaluated. In addition to the sealing material, a suitable application system, which can be integrated into a vertical tubular bag forming, filling and sealing machine (VFFS), must also be chosen.

In addition to testing EVA and PE based hotmelts, ethylene copolymers and other polymers were also tested for their suitability as a partially applied sealing material. Different monolayer films and multilayer films were selected as substrates for the tests, including OPA, BOPP, OPET, PA / PE, PET / AL / PE. In order to carry out the tests an application unit for hotmelts with a application head for extrusion coating (contact method) was integrated into the web transport test rig at the Fraunhofer AVV.

Analysis of the adhesion strength showed that values in the range from 10 to 20 N/15 mm could be achieved. One hotmelt even achieved a seam-strenght up to 50 N/15mm. In general the strengths of the hot seams, the so called hot-tack was very low. In order to prevent undesired opening of the bags after the joining process, a joining technique is required which allows seam cooling under pressure. Tests with Laser or High-Frequency have been unsatisfactory. However preliminary ultrasound welding trials with selected hotmelt coatings have demonstrated the generally good reactivation properties of these sealing materials. First Tests with an unit for the cross-seam application showed good results up to 30 bags per minute. The next step will be to integrate the unit into a FFS machine.