Stress Evolution of Amorphous Thermoplastic Plate during Forming Process

Abstract

Amorphous thermoplastics, as a type of engineering plastic material, are used in various industrial sectors. In order to manufacture high-performance products, it is important to optimize their forming process to mitigate residual stresses. However, stress in a plate is difficult to measure, therefore, modeling provides a powerful way to investigate and understand the evolution of stress. In this study, the forming process of a polyetherimide (PEI) plate was modelled using finite element analysis, and then validated through a comparison with a warpage experiment. This study reveals that the whole forming process can be divided into three stages by the glass transition temperature Tg of the PEI. The second stage, corresponding to the plate cooling from above Tg to below Tg, contributes a large portion of the residual stress in a short time. The final residual stress, the magnitude of which is affected by the cooling rate and plate thickness, shows a parabolic distribution through the thickness of the plate. These important conclusions are beneficial for improving the quality of an amorphous thermoplastic plate, while allowing highly efficient production.

Investigation of residual stresses induced by composite forming using macro-micro simulation

“…This result matches the formed stress profile of a thermoplastic macro model, and can be explained by the rapid material property change of the PEI, especially the large Young’s modulus difference when PEI is at its leathery and glassy stages, as well as the rate difference between the actual strain and stress-free strain. 26 The formed stress difference of the fibers at the top and bottom of the composite layer does not decay over time. However, the stress difference of the resin at different positions becomes smaller over time, which can be explained by the viscoelasticity of the resin.…”

“…The raw data of the PEI, which are obtained from various experiments, 26 are written in the Excel file of research data and plotted as the open dots in Figure 2. The material properties fitted to equations (2) to (9) are shown as the solid lines, exhibiting a knee point when the temperature is approximately T g (210°C).…”

“…The boundary condition for the macro model is the same as that reported in previous studies. 26,31 The boundaries of the bottom, left, and back surfaces of the micro model are the same as that of the macro model. However, the other surfaces are different.…”

“…The raw data of the PEI, which are obtained from various experiments, 26 The material properties fitted to equations (2) to (9) are shown as the solid lines, exhibiting a knee point when the temperature is approximately T g (210 C). Therefore, the maximal temperature for the validation of experiments in the following sections should be higher than T g to represent the change of material properties in its leathery and glassy stages.…”

“…Although warpage experiments of a PEI plate 26 and a composite laminate 13 were conducted to examine the material properties, homogenization methods, and software performance, these experiments only compared the final stage rather than the entire process. A real-time deflection monitoring has been conducted on the PEI resin to deliver more decisive validation to the PEI resin that plays a key role in composite forming.…”