Investigation of mechanical tearing - and how it can be applied in openability prediction

Abstract: Abstract Perforations are one of Tetra Paks oldest inventions of how to open their carton packages. They have been a part of Tetra Briks product portfolio for over 30 years and quality testing is important. Bad openability, with plastic residues covering the torn hole, is a problem that needs to be addressed. When tearing a package with perforation all people do it differently. By using a mechanical tearing method during testing it is possible to reduce this variation. A method like this is developed but it is not used widely and its results are often not satisfactory. The aim for this work is to investigate tearing done with this method. What to measure and how measured results are changed by material parameters affecting the openability. The practical part of this work is divided into two parts. The first is qualitative tests with the goal to increase the knowledge of the measuring procedure and its resulting energy curve. How is the board breaking? What energies, forces etc. should be measured to be able to extract as much valuable information as possible. The second part has a quantitative approach, to correlate the energy measurements with openability results, by measuring tearing energy on folded packaging material. This part also evaluates the method itself and which measured responses that are best to use to describe the openability problem. The results show that when using the mechanical tearing method one uses out- of- plane shear forces to break the perforation. The total energy during tearing is the best and most robust response to use if one wants to correlate tearing energy with openability. Material and process parameters that affect measured Total energy as well as the openability are the knife type, perforation engagement, and polymer grammage of the inside polymer layer. The variation in results during measurements, caused either by the equipment or the material, needs to be lowered as the method shows poor repeatability. It is most likely possible to develop a new or enhance the existing method to focus on tearing energy to perform test on perforations and predict issues with openability.