Digital Image Correlation For On-Line Wall Thickness Measurements In Thick Gauge Thermoforming

摘要

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.