Extrusion International 1-2023

45 Extrusion International 1/2023 novel die design. The influence of the different die ar - eas and the die gap is used to adapt the foam structure in a systematic way. Design and analytical investigations of different blown film dies In the first step, the standard die design, which was not optimised for foaming, was examined in detail and considerations were made, how the die design should be varied. The outlet of an extrusion die for blown film extrusion consists in most cases of a reduction of the di- ameter at a certain angle, a parallel zone as well as the outlet gap. These three design parameters, shown in Fig. 2, influence the behaviour of the plastic melt in the extrusion die. The parallel zone length LPa des - ignates the section in the die gap where there is no longer any reduction in the diameter. Thus, the die gap is constant over the parallel zone length. The die gap Dg corresponds to the thickness of the unfoamed film tube exit - ing the die. The angle ∠ determines how fast the diameter changes to the final die gap. Based on the three design parameters an- gle, parallel zone length and die gap, a re- design of the die was made and then also analytically calculated. For the analytical cal - culations, the die is considered as an annular gap and further simplifications are made according to [HM16]. The outer die diameter of the blow head is 80 mm and was kept constant as well as the mass throughput of 15 kg/h. The calculations are based on the material pa - rameters of PE-LD. Fig. 3 shows the die curves of all five dies. These differ from each other by the length of the parallel zone, the width of the outlet and the angle. In the process, care was taken to ensure that there was a large variance in the individual zones. The differences between the re-designed dies in the three areas of outlet gap, parallel zone and angle are shown in Table 1. In previous extrusion test with the reference die (D5) the extruder of the middle layer with a diam- eter of 35 mm (compared to the outer extruder D = 45 mm) reached its pressure limit of 300 bar at higher mass throughputs. To avoid this, the total pres - sure was reduced, especially for the small outlet gaps. Since the pressure gradient is also crucial for achieving a fine foam structure and a high nucleation rate, special attention was given to this parameter in the design, taking into it the knowledge gained previously. The total pressure loss of the individual dies is also shown in Table 1. In the following, the results of the analytical cal- culations in Fig. 4 for the die with the 0.3 mm (D1) and in Fig. 5 for the die with the 0.7 mm (D5) outlet gaps are presented as examples. If the diagrams for the 0.3 mm gap are compared with those for the 0.7 mm gap, it can be seen, that with the die D1 the pressure loss occurs in a significantly shorter range and in a shorter time. The analytical calculations show, that with die D5 even a larger pressure loss results However, this is built up over a longer period of time. For the production of foamed blown films, it is expected that especially the novel dies (D1, D5) are characterised by a good foam structure, since the pressure loss with these dies occurs particularly quickly and over a short die length. Extrusion tests with different die designs For the validation of the analytical investigation and to evaluate the process capability of the dies, extrusion tests were performed. The influence of the die design on the foam structure was carried out in a chemical foaming process. For this pur - pose, different proportions (3-15 %) of a chemi- Fig. 3: Gap shape of the re-designed dies Table 1: Analytical calculation of pressure loss at m ges = 15 kg/h Die geometry Pressure loss: ∆p [bar] Die number Die gap: D g [mm] Parallel zone length: L Pa [mm] Angle: ∠ [°] D1 0.3 0.0 70 34.4 D2 0.4 7.5 2 steps: 80 at z = 0-25 mm 76 at z = 33-41 mm 36.4 D3 0.5 3.4 45 42.4 D4 0.5 0.0 80 38.3 D5 0.7 4.0 80 47.8 Fig. 4: Influence of die design D1(D g = 0.3 mm, L Pa = 0 mm, ∠ =70) on pressure loss