Bart Buffel

A Combined Experimental and Modelling Approach towards an Optimized Heating Strategy in Thermoforming of Thermoplastics Sheets

Abstract Determining the operational settings for the heating equipment in thermoforming is still mainly done by trial and error as well as personal experience. Depending on the type of IR heating equipment, these settings can be the consumed electrical power or the desired temperature of the heating elements. In this study, a workflow is developed, applied and validated to characterize the IR heating equipment and to determine the optimal heating strategy. The workflow starts with an on-site equipment/machine characterization, which takes all machine and environment parameters into account. This approach results in the optimal heater setting and heating duration in order to obtain a through thickness temperature distribution which lies within a predefined forming range. The proposed methodology is universally applicable as it can deal with different types of sheet material and thicknesses. Moreover it can be applied to any type of IR heating element (halogen, metal foil, ceramic or quartz). Moreover, the methodology can easily be implemented in an industrial environment. Additionally, an estimate for the thermal efficiency of halogen heater equipment can be determined.

Experimental and Computational Analysis of the Heating Step during Thermoforming of Thermoplastics

The present study addresses the difficulties in heating thermoplastic sheets for ther-moforming applications. In industrial environments, the sheets are heated in a contact free method by means of convective hot air ovens and infrared radiation. In this study the temperature evolution at the outer surface as well as the core of thermoplastic sheets as a function of time is measured by means of thermocouples. These measurements reveal significant through thickness temperature dif-ferences which need to be resolved before high quality products can be made. The temperature dif-ferences can be decreased by decreasing the radiative power. This is however not acceptable in in-dustry since it lowers the number of produced parts per unit of time.In order to gain insight in the time-temperature relationship during the heating phase, a finite differ-ence model is developed. The model clearly shows the constantly changing through thickness tem-perature distribution and can be used as a tool by the thermoforming industry to optimize the pro-duction process.

Characterisation of the mechanical behaviour of a polyurethane elastomer based on indentation and tensile creep experiments

This research focuses on the determination of the mechanical properties of a viscoelastic polyurethane material with 2 different measuring techniques on 2 different length scales. Instrumented Indentation Testing (IIT) was used to test the material on a micro scale while tensile creep experiments characterised the macro scale material behaviour. All experimental data were processed by means of a fitting procedure based on the standard linear solid material model. The experiments were performed with different loading rates and hold values. The developed fitting procedure proved to be applicable to analyse the experimental data on both length scales. FEM was used to coordinate the applied strains of both measuring techniques. A comparison between the results originating from the experiments with both techniques indicated a stiffer material response on the micro scale (up to 4x). The more complex strain field inside the material during indentation compared to the uniform tensile loading on macro scale is responsible for this large discrepancy. For this reason comparing the results of IIT with tensile creep results should be done with great care.

Modelling the elastic response of a polyurethane open cell foam based on a minimal surface energy approach

Within the present study the elastic response of a flexible open cell polyurethane foam was studied by means of experimental compression test and finite element (FE) modelling. The compression tests revealed a pronounced sample size effect which was taken into account using an analytical model. In order to eliminate the sample size and damage effects, a minimal sample size of at least 50 times the cell size was necessary in the case of the flexible foam. Surface evolver software was used to model the open cell foam structures. The FE unit cells are based on the well-known Kelvin cell and the more complex Weaire-Phelan cell topology. In both cases the cross sectional shape of the cell edges was completely determined by the minimization of the surface energy. The thus build FE-models possess a good resemblance to real open cell foam structures. The influence of relative density and shape anisotropy on the elastic properties of the cellular structures was analysed using the FE-models.