PAPER PHYSICS Nordic Pulp & Paper Research Journal Vol 29 no (3) 2014 510 Optimizing shear strength profiles in paperboard for better crease formation Mikael Nygårds, Abhijit Bhattacharya, and SVR Krishnan KEYWORDS: Shear strength, Round corner cigarette package, Paperboard, converting, Creasing, Folding, Mechanical properties SUMMARY: The tensile properties and shear strength profiles have been evaluated for three paperboards having different through thickness profiles. Paperboard with a 1) strong middle ply and a weak bottom interface, 2) weak middle ply and top interface and 3) weak top and bottom interfaces. The creasing and folding performances at different crease depths were evaluated, and it was concluded that it is preferable to have a weak bottom interface to form a well-defined crease that bulged out and formed a well- shaped crease. Package formation of a cigarette package was evaluated as the ability to use multiple creases to form round corners. In addition, the in-plane crack tendency on the outside of the crease, and the forming of a smooth crease line on the inside of the crease were studied. The ability to form rounded corners coincided well with the formation of well-shaped crease lines. Creases with a weak bottom interface did also generate good looking rounded corners. If also the top interface was weak, in-plane cracks on the outside of the crease could be prevented. A well-shaped crease also resulted in a straight and well defined crease line on the inside of a folded corner. Finally, the evaluation of shear strength profiles was a good tool to confirm process changes that were made to make paperboards with different through thickness profiles. ADDRESSES OF THE AUTHORS: Mikael Nygårds (mikael.nygards@innventia.com), Innventia AB, Box 5604, 114 86 Stockholm, Sweden and BiMaC Innovation, KTH, Abhijit Bhattacharya (abhijit. bhattacharya@itc.in) and SVR Krishnan (svr.krishnan@itc.in), ITC Limited, Paperboards and Speciality papers division, Unit Bhadrachalam, Sarapaka- 507 128, Khammam Dist (A.P.), India. Corresponding author: Mikael Nygårds Paperboard is an engineered material which often consists of different plies and interfaces. This generates an out-of-plane (ZD) profile that can be designed for optimal functionality or cost performance. Often commercial paperboards are made with a bulky middle ply, since this gives high bending stiffness with a minimum amount of fibers. However, the objective of this work has been to optimize creasing and folding functionality by the engineering of different ZD profiles. Creasing and subsequent folding is necessary to convert paperboard into packages. In the creasing operation a male ruler is pushed into the paperboard, which is positioned on top of a female die, as illustrated in Fig 1. The purpose of the creasing operation is to generate purpose made damage, which will improve the subsequent folding performance. Creasing is regularly Fig 1 - Schematic illustration of the creasing operation; a sample referred to as MD sample has MD parallel with the specimen direction, a CD sample has CD parallel to the sample direction. used by different converters, and plenty of best practice knowledge exists in industry. Also in the literature experimental studies have been conducted (Carlsson et al. 1983, Nagasawa et al. 2003, Doeung et al. 2012), where the aim has been to identify deformation and damage mechanisms. It can be concluded that out-of-plane behavior, mainly ZD tension and shear, are important properties that contribute to good creasing and folding performance. In addition, the importance of well-defined delamination sites has been identified. The out-of-plane behavior is difficult to measure accurately. Often test methods rely on gluing that can penetrate the samples, and displacement needs to be measured with good accuracy. However, methods to measure ZD tensile and shear properties have been developed and presented in the literature (Byrd et al. 1975, Fellers 1977, Waterhouse 1991, Girlanda, Fellers 2007, Stenberg et al. 2001a, 2001bb) Initial numerical finite element simulations have of creasing and folding was conducted by Carlsson et al. (1982). Later, one of the aims with the simulations has been to identify and suggest material models that can be used to represent paperboard. The reason of this is that the creasing and folding operations are interesting and challenging since many deformations and damage mechanisms are activated during the operations. There are elastic-plastic deformations in both the in-plane and out-of-plane directions, delamination in the out-of-plane directions, rate dependency, sensitivity to temperature and moisture etc. From finite element simulations it has been concluded that the creasing operation is a good out- of-plane shear test (Nygårds et al. 2009a). During creasing the paperboard is shear loaded, which contributes to the breaking of bonds between the fibers. This generates delamination within the paperboard structure. During crease unloading and folding the delamination sites will open up. Due to these mechanisms a folded 90 degree paperboard corner will have a bulge that faces inwards. Huang and Nygårds (2010, 2012) have shown that the continuum response can be well captured by an elastic- plastic anisotropic Hill model, and an elastic cohesive damage interface model. The advantage with these models is that they are both available in the commercial finite element solver Abaqus (2011). Similar models have also been suggested by Beex and Peerlings (2009, 2012) Brought to you by | Uppsala University Library Authenticated Download Date | 10/23/18 6:45 AM