Homogenisation of the Wall Thickness Distribution of Thermoformed Cups by using Different Pre-stretch Plugs and Process Parameter Settings to Improve Material Efficiency

Abstract

In thermoforming up to 90% of the product costs are related to the material. For this reason, the efficient use of materials is essential in the production. Material efficiency of thermoformed packaging products, especially cups, can be increased by using pre-stretching plugs. These plugs affect the wall thickness distribution of the product. Thin areas, which can cause problems concerning top-load stability, can be avoided. By adjusting the shape of the pre-stretch plugs and the thermoforming process the wall thickness distribution changes significantly. Thus the distribution can be homogenised and the wall thickness can be increased more than 100% in the thinnest area. The plug design is varied in wall angle and the radius which joints the side wall and the flat top of the plug. Using DoE the effects of different geometry combinations and sheet temperatures are analysed. It can be shown, that the wall angle of the plug and the sheet temperature have a big influence on the homogenisation. Top-load stability is an important feature of packaging cups. By adjusting the wall thickness distribution, the top-load stability can be raised significantly. This high top-load stability in turn allows a reduction of the sheet’s thickness, increasing material efficiency. Additionally, the required plug forces to pre-stretch the sheet are investigated, which can improve the process control or serve as a parameter for quality management.