where the sheet layers fuse or adhere together, and can exaggerate web defects. Randomisation of Cross Machine * Variations – Some flexible packaging films, either by their extrusion formation process or by their coating and laminating process, have cross machine variations of thickness too severe to be wound without exaggerating these defects. To randomise cross machine variations either the web or the slitters and winder are moved back and forth relative to the web as they are being slit and wound; this is called oscillation. The rule of thumb for the maximum oscillation speeds is 25 mm/minute per 150 m/ minute of winding speed.
Duane Smith, Process Manager – Handling & Specialty Winding, Davis-Standard
be wound tight at the core and then wound with less tightness as the roll builds in diameter. The secret to the roll hardness structure is to start with a good solid foundation and then wind with progressively less inwound tension.
* Achieving Roll Hardness – The winding tools to develop roll hardness are web tension, nip * Profiling Roll Hardness – As pressure from a pressure or a roll of flexible packaging lay-on roll or winding drum film material winds, in-wound and winding torque from a tension or residual stresses centre drive. When winding builds inside the roll. If this elastic films, web tension is stress becomes greater during the dominant principle used winding, the inner wraps to control roll hardness. When towards the core will be put winding inelastic films, nip under high compressive loads. is the dominant principle. This is what causes a defect Torque winding is the force known as ‘buckling’ of the webs induced through the centre in localised areas in the roll. of the winding roll, which is When winding non-elastic and transmitted through the web high slip films, the inner layers layers and tightens the inner will loosen; which can cause wraps of film. the roll to dish while winding or * Measuring Roll Hardness – Film telescope when unwinding. To winding is often considered prevent this, the rolls want to an art because the setting and 42 | Plastics and Rubber Asia 2018 Yearender Edition
programming of the tension, nip and torque (TNT) varies depending on the winder, type of material, roll width and winding speed. To ensure the wound rolls are produced with consistent hardness, hardness measuring devices must be used. This enables the operator to check hardness and made adjustments to keep hardness within the acceptable range for that product. Summary As it can be seen, there are several factors that must be considered to consistently wind good rolls. The winder operator’s job is not to camouflage poor quality flexible packaging products into shippable rolls. His or her responsibility is to handle films with slight imperfections to produce troublefree, quality rolls for the downstream customer. By learning more about the process and taking the right steps, it can be done! To learn more about Davis-Standard’s products and services relating to winding and unwinding, please visit: https://davis-standard.com/converting_system/windingunwinding/ We also invite you to explore the D-S Connect Blog at: https://davisstandard.com/custom_blog/making-betterfilm-rollsone-wind-at-a-time/ to learn more, and to request the full technical paper, “Challenges in Winding Flexible Packaging Films”.