Extrusion International 5-2019

34 Extrusion International 5/2019 THERMOFORMING – FROM THE RESEARCH Optimisation of the Plug Geometry in Plug Assisted Thermoforming to increase Material Efficiency The ever-increasing demand for sustainability of plastic products, especially in packaging, calls for increasedmaterial efficiency. Additionally tomaterial savings in thermoformed products, the optimisation of the wall thickness distribution also minimises the required energy and reduces cycle time, as less material has to be heated or cooled. Thus the entire thermoforming process can be operatedmore efficiently in addition to the reduction of waste [Ede14, Mos13, TB99] There are various process-related approaches for the adjustment of the wall thickness distribution. Temper- ature profiling uses the strongly temperature depen- dent material behaviour of plastics to induce varying stretching. Pre-stretch plugs are used for production of packaging products in negative thermoforming and pull material into areas where otherwise thin spots would occur. These thin spots are considered as the weak points of a product. Thus, they have to be taken into account when designing the thermoformed prod- uct. An adjustment of the wall thickness distribution, thickening the weak spots, leads to less over dimen- sioning of thick areas, better mechanical properties and overall to the application of a thinner film. Nev- ertheless, when using pre-stretch plugs the resulting wall thickness distribution depends strongly on the ge- ometry of the plug, the process control as well as the plastic material [HM15, IS16]. Design of pre-stretching plugs in thermoforming Only a few guidelines for designing pre-stretch plugs in thermoforming exist. Furthermore, these guidelines are very general and no information is given on the resulting wall thickness distribution, as this depends on many dif- ferent factors. Although there are numerous publications on plug-assisted thermoforming with the corresponding wall thickness distributions when using different plug ge- ometries, these geometries are not directly comparable, so very few rules have been established [CHM02, HW03, IS16, MCO13, MM10, MMH06, OMS+13]. A research project at the Institute for Plastics Processing (IKV) investigates plug pre-stretching in order to analyse the influence of plug geometry on wall thickness distribu- tion. At the beginning a pre-stretch plug according to Schwar- zmann [IS16] is constructed as shown in Figure 1. Refer- ence is made to the corresponding literature and the pub- Figure 1: Selectedpre-stretchplugs for the analysis of the resulting wall thickness distribution