A Framework for Extrusion Screw Design

Sammanfattning

This thesis presents the development of a structured framework for extrusion screw design based on geometrical parameters, with the aim of improving understanding and supporting design decisions in industrial extrusion processes. The work was carried out in collaboration with Tetra Pak and focuses on the extrusion system used in injection compression molding of Tetra Top lids. An initial numerical simulation approach was explored, but due to limitations related to model stability and setup complexity, the study proceeded using an analytical modelling approach. The analytical models were used to describe the relationship between screw geometry and process performance, focusing on mass flow rate and power consumption in the metering section. The models were evaluated and compared with existing experimental data, showing that they capture the overall behaviour of the process, although with some deviations in absolute values. To systematically analyse the influence of geometrical parameters, a Design of Experiments approach was applied. Response surface models were developed to represent the relationships between input and output parameters, enabling further analysis of parameter trends, parameter influence and trade-offs. The results show that most geometrical parameters influence mass flow rate and power consumption in opposite directions, highlighting a clear trade-off between high throughput and low energy consumption. Additionally, only a few parameters dominate the overall behaviour, which is important for practical design decisions. Based on these results, a structured framework for extrusion screw design was proposed. The framework provides a systematic approach for analysing parameter trends, evaluating their influence, and handling trade-offs between different design objectives. It can be applied to processes similar to the studied case and serves as a decision support tool for screw design. Overall, the study demonstrates how a model-based approach can support extrusion screw design and improve understanding of the relationship between screw geometry and process performance.