Extrusion International 1-2023

47 Extrusion International 1/2023 or the volume of a single cell. To determine the total density of the three film layers, a circular sample of the film is taken. The thickness of the film is measured at 12 points equidistantly distributed on the radius. The film thickness is used to calculate the volume of the film. In addition, the weight of the sample is determined. With the presence of the two described quantities, the den- sity of the total film can be determined (see Fig. 6). Since the process points were kept constant for all the dies, the density of the total film can be compared directly with each other. It should only be noted that this is not the density of the foamed middle layer. The total density of the multilayer film varies between 0.325 g/cm 3 and 0.386 g/cm 3 for all dies, so that the den- sities differ relatively little from each other. The highest value (0.386 g/cm 3 ) determined occurs with the die D1 and the two lowest values of about 0,325 and 0.328 g/cm 3 were achieved by the dies with an outlet gap of 0.5mm (D3, D4). Since a low density can also be achieved with large cells, the next step is to examine the cell size. For the investigation of the cell size, a film sample was taken from each sample. For optical analysis images were taken with a camera COE-050-M-POE-050- IIR-C and the corresponding bi- telecentric lens TC23036 from Opto Engineering, Mantova, Italy. The foam structures were then evaluated in two dimen- sions. For this purpose, lines were placed in the cells and the number of pixels along themwas counted. The number of pixels is converted into a metric length by scaling. For the calculation of the cell area C area the cell is approxi- mated as an ellipse (Eq. 4.1). For each image, the cells were mea- sured along the length and hori- zontally to the direction of the pull-off. C a stands for the cell dimension along the pull-off direction and C trans for the cell dimension transverse to the pull-off direction. C area = C a x C trans x π/4 A comparison of the images of the foam structure (see Fig. 7) with the cell sizes determined in Fig. 8 illustrates, that the dies with the lowest outlet diameters (D1, D2) lead to the smallest cell size. The images are character - ised by a very fine and homogeneous foam structure. This was to be expected, as the pressure gradient in the die gap increases with a smaller die gap. Thedies D3 and D4, which produced the lowest film density achieve an average cell size. However, the D4 with no parallel zone is about 0.4 mm 2 below the other die. The reference die (D5) achieves the worst foam structure with the high- est cell size on the images compared to the other struc- tures. The relatively low standard deviation (see Fig 8) of all foam structures indicates a very homogeneous distribution of cell sizes. Due to the poor process stability of the dies (D1, D2), both flow channel designs are not suitable for further use in the production of blown film. Because the die (D4) is characterised by the smallest cell size (1.84 mm 2 ), suitable process stability and the lowest overall density (0.33 g/ cm 3 ) compared to the other dies, it was selected as the final die. Conclusion and outlook For the development of a novel die design for foamed multilayer films, the influences of the three design parameters die gap, parallel zone length and angle were determined After the con- struction of five die designs, the influence on the pressure loss Fig. 7: Images of the foam structure of the different die designs with a bi-telecentric lens Fig. 8: Influence of die design on the cell size