Extrusion International 5-2019

37 Extrusion International 5/2019 Figure 6: Influenceof theplugwall angleof flat plugswith constant radius usingaPET This is particularly noticeable with regard to the quench- ing marks which occur at MP 8 and MP 10 using plugs with a wall angle of 4° and 8°. The quenching marks can be minimised by increasing the wall angle. Due to the higher angle the material no longer gets into contact with the plug during stretching. As there is more free space be- tween the plug and the mould the film can be stretched freely and therefore does not cool immediately by touch- ing the plug. In general, the minimum wall thickness can be more than doubled from 0.05 mm to 0.13 mm, depending on the plug geometry, compared to pure compressed air form- ing. By increasing the radius of the plug with constant wall angle, the wall thickness distributions are influenced, but their basic distribution remains the same. The differ- ences in the wall thickness profiles are rather small and differ only in local thickness [HB18]. Increasing the plug edge radius enables the material to be pulled more easily around the radius and thus more material is pulled out of the bottom area. The wall thickness decreases compared to smaller radii. The wall thickness distributions determined for PS are not easily transferred to PP and aPET, since the respec- tive stretching resistances of the materials differ. The stretching resistance influences the amount of material that will be pulled off the plug. Therefore, a change in the resistance causes a difference in the wall thickness dis- tribution, which in turn changes the cup properties, e.g. top-load stability. Figures 4 and 5 show the resulting wall thickness distributions using PP at a film temperature of 126 °C.