Extrusion International 1-2023

46 Extrusion International 1/2023 cal foaming agent (CFA) Hydrocerol from Avient, Avion Lake, Ohio, USA, were added to the plastic granulate in preliminary tests and the degree of foaming was investi- gated. Hydrocerol is a chemical foaming and nucleating agent, which is particularly suitable for the production of fine-cell foams. A chemical foaming agent improves the plasticisation of the plastic, which reduces the required heat input. This can save energy and improves the CO 2 balance of the process. The highest degree of foaming with a good foam structure was achieved with 7 wt.-% Hydrocerol and kept constant in the further tests. For the production of a three-layer film, with compact outer layers and foamed middle layer, a three-layer blown film line from Kuhne Anlagenbau GmbH, St. Au - gustin, Germany, is used for these tests. The melt is dis - tributed in an annular gap by a radial spiral distributor and exits through the re-designed and manufactured dies. To evaluate the influence of the die-dependent foaming behaviour during film production of the die design, the plastic material is kept constant for all film layers as well as for all tests. A low-density polyethyl - ene 2102X0 from Sabic Europe, Geleen, Netherlands, was used. The MFI (190°C, 2.16 kg) of the material is 1.9g/10min [NN21], which on the one hand re - duces the foaming of the middle layer due to the high viscosity of the outer layers, as the added foaming agent reduces the viscosity of the middle layer. On the other hand, a long foam growth of the middle layer is avoided compared to a mate- rial with lower viscosity. In order not to have to estimate the pressure loss at the die via the extruder pressure of the middle layer, a new die housing was designed and manufactured which enables the measure- ment of the melt pressure and the melt tempera- ture approx. 60 mm before the melt exits. Blown films were produced for the following analyses of the foam structures. The parameters of the test series are summarised in Table 2. For each die geometry, the mass flow rates are var - ied on two levels (8/11 kg/h), as they also have an influence on the pressure gradient and thus affect the cell size. For the individual process points, the parameters were kept constant, with the blow-up and take-up ratios playing a particu- larly important role. This ensured that films with a thickness of 125 µm were produced. The mass flow rate ratio of the two outer layers to the middle layer is two. The two outer layers were operated with the same mass flow rate of 1. With the 10 test points, the influence of the die design on the process stability could be evaluated well. The pro - cess ran trouble-free with the reference geometry with an outlet gap of 0.7 mm (D5) as well as the two new dies with 0.5 mm (D3, D4). At the different process points, a three-layer structure with a foamed middle layer could be produced. The die D4 showed good production be - haviour and better process stability compared to the die with the same outlet gap but different parallel zone and angle (D3). The dies with an outlet of 0.3mm (D1) and 0.4 mm (D2) showed an increased number of film tears and it was not possible to set up a stationary process. How - ever, film samples were still taken in order to be able to evaluate the foam structure afterwards. The very small gap meant that the expanding cells disrupted the very thin film in many places and thus no stability could be brought into the film tube. Influence of the die design on the foam structure The foam structure of foamed blown films is, next to the process stability during extrusion, the most impor- tant factor for the evaluation of the novel die designs. A fine-cell, homogeneous foam structure was aimed for. The two most important parameters for assessing foam quality are the density of the blown film and the cell size. With the help of the density, the material savings can be evaluated. However, the density of the film does not allow direct conclusions to be drawn about the cell size BLOWN FILM EXTRUSION – FROM THE RESEARCH Fig. 5: Influence of die design D5(D g = 0.7 mm, L Pa = 4 mm, ∠ =80°) on pressure loss Material Hydrocerol-percentage [wt.-%] Mass-troughput [kg/h] Mass-troughput ratio [-] Temperature [°C] Film thickness [µm] Blow-up-ratio [-] PE-LD 2102X0 7 8/11 1:2:1 All zones 170 125 2 Table 2: Process parameters used in the extrusion trials Fig. 6: Influence of die design on the total density of the multilayer film