Development and analysis of a material model based on the reptation theory for the description of the strain behaviour of PET at high strain rates

• Applicant: Professor Dr.-Ing. Christian Hopmann
• Subject Area: Plastics Engineering
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 328107189

The two-stage stretch blow moulding process is established as a method for producing high-quality plastic hollow bodies with excellent mechanical and optical properties. The stretching of the material during the process leads to a strong orientation of the molecular chains and to a formation of lamella like structures. These structures lead to a sharp increase in the strength of the material. The prediction of the material behaviour helps in the interpretation of the process and in the increase of the material efficiency in the production of hollow bodies or films. For the prediction of the material behaviour, different material models are used. Material models are currently described through spring-damper approaches and are calibrated through stress-strain curves. These models can predict the deformation within the calibrated range well, but are less accurate outside this range. The aim of this research project is the development and analysis of a material model, which depicts the stress strain behaviour of Polyethylenterephthalat (PET) based on the reptation theory. For this purpose, the equation set of the reptation theory for the description of the resulting stress in polymer melts is implemented and calibrated with the material data for amorphous (A) and semi-crystalline (C) PET determined. The first step of the project is the development of the material model. For this purpose, the equation system of the reptation theory for the description of plastic melts is fit to describe the behaviour of PET. The performance of the model is examined under different conditions for the specified PET types. The validation of the model is carried out by comparing the calculated stress strain curves for different temperatures with experiments. Furthermore, the model is applied in simulations of membrane-inflation rheometer (MIR) and the stretch blow moulding process to analyse the models accuracy by the comparison with experiments. A comparison between practical and simulated measurement of the MIR allows the analysis of the strain behaviour in small time steps during the deformation process. Therewith, the accuracy, but also weaknesses, of the model can be identified. Finally, the material model is embedded in a simulation of the two-stage stretch blow moulding process. The validation is realised through experiments and compared to an existing hyper elastic material model for PET. The complex geometry of the preforms and the inhomogeneous temperature distribution in the material make high demands on the accuracy of the model. With the acquired knowledge of the material behaviour of PET and the analysed quality of the model, the material and energy efficiency is addressed.

• DFG Programme: Research Grants