Extrusion International 5-2019

36 Extrusion International 5/2019 THERMOFORMING – FROM THE RESEARCH varied process parameters are determined on the basis of the literature and the recommendations of the industrial advisory committee of the research project. Generally, the stretching distance, the plug speed and the time at which the compressed air is activated have a significant influence on the wall thickness distribution. Furthermore, the material, the film temperature and the plug geom- etry have a significant influence. The stretching distance is selected according to the design guideline. The speed of the plug is chosen as high as possible to achieve the lowest possible heat transfer in the plug as well as to mi- nimise cycle time. The activation of compressed air has no significant influence on the wall thickness distribution with the mould technology used in these tests. Reference is also made to the literature for the exact determination and the exact settings [HB18, IS16]. The selected process parameters are summarised in Table 1. In order to determine the influence of the plug geometry, the wall thickness is analysed along the measuring path over the entire cup cross-section. Afterwards one side is folded to the other side and average values are taken. Fi- nally, only one half of the cup is displayed for clarity which is shown in Figure 2. Measuring position 1 (MP 1) is located in the middle of the cup base (bottom) and MP 14 on the cups top end near the edge of the wall. Figure 2 shows a wall thickness distribution of a cup formed without a pre-stretch plug using only compressed air. The film used is polystyrene (PS) from W.u.H. Fernholz GmbH & Co. KG, Meinerzha- gen, with a thickness of 0.8 mm. The thin spot at MP 6 and MP 7 is conspicuous here. This thin spot must be specifi- cally avoided and the material has to be less thinned out in these areas by using pre-stretch plugs. Analysis of the wall thickness distribution using different pre-stretch plug geometries and film materials Since the wall thickness distribution varies if different ma- terials are used, polypropylene (PP) and amorphous poly- ethylene terephthalate (aPET) are investigated as well. These films have a thickness of 0.8 mm and are produced by W.u.H. Fernholz GmbH & Co. KG, Meinerzhagen, too. Figure 3 shows the wall thickness distribution generated by different plug geometries when using PS. The PS-film temperature is 120°C. The plug‘s wall angle is varied and the edge radius is kept constant at 9 mm. As the figure shows, the variation of the wall angles causes locally dif- ferent wall thicknesses and the thickness distribution can be influenced significantly. Figure 4: Influenceof theplugedge radius of flat plugswith constantwall anglewhenusingPP Figure 5: Influenceof theplugwall angleof flat plugswith constant radius whenusingPP Parameter pre-stretching distance 37.8 mm of 40 mm plug velocity 273 mm/s compressed air activation after 100 % of pre-stretch distance forming pressure 5 bar time of forming pressure 3 s Table 1: Processingparameters – constant parameters of plugassisted pre-stretching Figure 3: Influenceof theplugwall angleof flat plugswith constant edge radiuswhenusingPS