Extrusion International 1-2023

44 Extrusion International 1/2023 BLOWN FILM EXTRUSION – FROM THE RESEARCH Larger Pressure Gradient – Smaller Cells Optimisation of the Foam Structure with Adapted Die Design Author: Ch. Hopmann, M. Stieglitz T he constantly increasing economic competition as well as the demand for a good life cycle assessment of packaging materials is confronting film producers with ever greater challenges [ESE20]. Due to the intro - duction of regulations in the packaging sector, material consumption is increasingly coming into focus. At the same time, the reduction in material consumption leads to a reduction in material costs, which account up to 80% of film production costs [HHH14]. The further development of technical processes of- fers the possibility to reduce the material input and thus has great potential to increase the economic efficiency of blown film extrusion. The saving of plastics can be realised by foaming, which reduces the density. In this way, material costs and the amount of raw materials can be optimised [Sta06]. Foamed plastics are also char - acterised by improved properties in certain areas. For example, foaming increases the thermal insulating ef- fect [Tol15]. The combination of plastic foaming with blown film extrusion offers a highly efficient production process for manufacturing novel film products (see Fig. 1). Foaming in the middle layer of multilayer blown films offers sever - al advantages as the layer structure. Due to the compact outer layer, all possibilities remain open about appear- ance, printability, haptics and possible laminating or seal- ing functions. Foamed packaging is used, for example, to protect sensitive foodstuffs such as fruit, so that damage and thus premature spoilage is prevented. Because of the good thermal insulation of the foams, they are also used for packaging fast food in order to keep the tem- perature change of the food as low as possible [URL21]. The layer thickness ratio is largely responsible for the property profile of the foamed multilayer blown film. The foam structure should not be visible on the sur- face and considerable material savings must be aimed for. The resulting cavities also influence the mechanical properties of the film, which depend on the foam struc - ture. For this reason, the objective in foam extrusion is to achieve a structure, that is as fine-cell as possible, since this results in higher mechanical properties of the film compared to a large-cell structure. The foam structure depends on material parameters such as extensional viscosity and process parameters such as temperature or pressure, and also on the die technology. The process understanding of foam extru - sion is not yet sufficiently, so that further investigations and developments are needed. In the further section, the optimisation of the die technology will be discussed, which can be varied to achieve a fine-cell foam structure for blown films. In this context, the influence of the die design is of particular importance. Research on the development of a new die design for improved foam structure In order to avoid high degrees of stretching, which lead to thinning of the film and stretching of the foam cells and to further optimise the foam structure, it is necessary to investigate the influence of different die geometries in more detail. Achieving a fine-cell foam structure is important to achieve maximum density re- duction while maintaining good mechanical film prop - erties. The objective is therefore the development of a Fig. 1: Multilayer film with foamed middle layer Fig. 2: Schematic layout of the three design parameters of the die and CAD model of the die insert