Evaluation Method for Stretch Blow Moulding Simulations with Process-Oriented Experiments

Abstract

In the Stretch blow moulding (SBM) process, polyethylene terephthalate (PET)-preforms are biaxially deformed to produce thin walled bottles. Finite-Element (FE)-Simulations are an important tool to optimise this process in terms of material usage and product performance. Thereby, the implementation of the thermo-mechanical material behaviour of PET plays an important role to achieve realistic simulation results. A common approach for this purpose is to calibrate a material model with stress-strain curves from biaxial stretching experiments. Thin PET-sheets are stretched under defined temperatures and strain rates. However, these experiments include process simplifications concerning geometry, heating and deformation parameters. This paper presents a method for extracting temperature dependent stress-strain-curves from experiments close to the production process. PET-Preforms receive thermal treatment with Infrared (IR)-heaters from an SBM-machine and are subsequently inflated in free air (free blow trial). A high-speed-IR-camera is used to image the axial and radial temperature distribution on the preform immediately before blowing. The deformation process is recorded via 3d-high-speed-cameras with a frame rate of 2000/s. The cameras are synchronised with a pressure sensor to consequently calculate reliable stress-strain curves at any point on the preform. In addition FE-simulations of the free blow trials are conducted using a material model calibrated with the simplified stretching experiments of thin PET sheets. Resulting stress-strain-curves from simulations and free-blow-trials are finally compared to evaluate the quality of the material model as well as the underlying testing procedure.