Bart Van Mieghem

Digital Image Correlation for On-Line Wall Thickness Measurements in Thick Gauge Thermoforming

The homogeneity of the wall thickness (distribution) is considered to be the most critical parameter in the quality assessment of a thermoformed product. Numerous previous studies have characterized the thickness distribution by means of manual discrete tactile measurements. Such approach is slow, operator dependent and only gives results on specific points of the final product, resulting in complicated judgements on the causes of the thinning of the polymer sheet. This work presents a methodology to use digital image correlation (DIC) for on-line, full field wall thickness measurements of thick gauge thermoformed parts during and after thermoforming. Such technique offers the following advantages. Firstly, it provides the user with thickness results over a complete area instead of a discrete measuring point. Secondly, it allows on-line measurements so that a better insight can be obtained in the deformation mechanisms during the forming process. Finally, a correlation is made between each undeformed point in the base image and the same point in the deformed images during thermoforming, resulting in a full-field strain image where intermediate sheet thinning can be calculated. This makes it easier to determine a causal relation between thermoforming parameters and final thickness distribution of the product.

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.