Extrusion International 5-2019

38 Extrusion International 5/2019 THERMOFORMING – FROM THE RESEARCH As with PS, it can also be stated with PP that the radius has a smaller influence on the wall thickness distribution than the wall angle of the plug if all the other parameters are kept constant. Using PP, no clear statement can be made about the edge radius at a wall angle of 8°. The wall thick- ness distribution does not differ significantly in this case (Fig. 4). Wall angles of 4° and 12° with different radii lead to the same effects as using PS. Less material is pulled out of the bottom area, as the deflection around a small ra- dius requires a higher force. A larger edge radius leads to less material in the bottom area of the cup. With the 8° wall angles shown, the differences are very small. The variations of the wall angle have a greater influence on the wall thickness distribution (Fig. 5). A wall angle of 12° leads to less material in the bottom, since the material is more easily pulled around the plug compared to a 4° angle. Using pre-stretch plugs allows the wall thickness distribution to be adjusted or to be formed in general. In these trials, it is not possible to produce cups only with compressed air. The stability to evaluate the wall thick- ness distributions is not sufficient, because of a very high stretching and thus thinning of the film. When aPET is used, the described effects and influences can be observed as well. Figure 6 shows an example of a variable wall angle at a film temperature of 100 °C. The ef- fects and influences can also be observed when changing the radius and the wall thickness distribution using aPET. The observed effects are lower than those of PS but higher than those of PP. The wall thickness profile itself is slightly influenced by the change on the plug edge radius. A change of the plug wall angle allows a change of the whole distribution and thus an adjustment of the homo- geneity of the cup. The wall thickness can also be greatly increased compared to the cup produced only with com- pressed air. The thin area of the cups formed with com- pressed air only have a thickness of 0.06 mm. It can be in- creased up to 0.16 mmwith plug assisted thermoforming. Contrary to the expectations that a further increase of the plug wall angle will lead to a further improvement in ho- mogeneity, the wall thickness distribution is deteriorating significantly. The use of pointed plugs results in less mate- rial being pulled out of the bottom area, which makes it particularly thick compared to the wall thicknesses when flat plugs are used. Due to the larger freely stretchable area, different stretching resistances occur. Since more material is available for free stretching (without plug con- tact), the local stretching resistances of the film decrease and less material is pulled out of the bottom. The adhe- sive forces on the plug are higher than the forces required for stretching the freely stretchable areas. Figure 7 shows the wall thickness distribution of two pointed plugs for PP at a film temperature of 126 °C. The qualitative wall thick- ness distributions are the same when using PP and aPET. Large wall thicknesses in the bottom, which decrease to- wards the transition between bottom and wall are visible. Then the wall thickness increases again. In general the wall thicknesses can be improved (0.11 mm), but only to a lesser extent compared to the other plugs. Analysis of top-load stability Although the wall thickness distributions that are shown here give a good impression of the wall thicknesses as a function of the various measuring positions, they are not suitable for conclusions about the homogeneity of the wall thickness distribution. In order to describe the homo- geneity of the wall thickness distributions equation 1 is defined. A cup is particularly homogeneous if the maxi- mum wall thickness difference ∆s of the thickest (smax) and thinnest (smin) wall thickness is low. ∆d = dmax – dmin (eq. 1) The homogeneity can now be compared with the top- load stability of the cups. Thus, it is possible to draw con- clusions about the possible load on the cups in relation to the homogeneity. The top-load stability is determined by compression tests on a Zwick Z10, Zwick Roell GmbH & Co. KG, Ulm, Germany. The maximum wall thickness differences and the top-load determined as a function of different plug geometries are shown in Figure 8 using PP cups as examples (film temperature 126 °C). The first number describes the wall angle of the plug, the second number describes the edge radius, e.g. 4°R6 is a plug with a wall angle of 4° and a radius of 6 mm. Figure 7: Influenceof pointedplugswhenusingPP Figure 8:Maximumwall thickness differences (homogeneity) and associated top-loadas a functionof various pluggeometries