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

Abstract

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.

Consistency of Strain Fields and Thickness Distributions in Thermoforming Experiments Through Stereo DIC

“…This is caused by a combination of non-uniform heating and material pre-processing history. More details on this phenomenon can be found in some of our previous publications 2,25 and is hence not discussed in detail.…”

“…To assess this quality requirement during the thermoforming process and to be able to optimise the process parameters in an industrial thermoforming environment, in-situ stereo digital image correlation (DIC) is used. This technique has already proven to be valuable to assess the homogeneity of the wall thickness during the different thermoforming process steps 2 .…”

Improvements in thermoforming simulation by use of 3D digital image correlation

“…The correct calculation of the heat transfer coefficient is not possible if the thinning effect during the bubble inflation is not included, this is what the real advantage of DIC means in this case. A detailed description of the use of DIC for online thickness measurement can be found in [5]. The calculated coefficients are 26, 31 and 32 W/m#K for valve settings 3, 5 and 7 respectively (Figure 10).…”

“…Whereas DIC is widely used for other applications where full field strain measurements are required, the use of DIC in thermoforming applications is rare. Only two DIC studies for a thermoforming process are known to the authors [5,6]. In the following section, a brief overview on how DIC is implemented in the thermoforming process and the specific measures that have to be taken into consideration when using DIC for thermoforming applications are itemized.…”

“…This is a critical part of the simulation since it is impossible to obtain realistic results when starting with the wrong simulation parameters; no adequate method to define this link has been found in literature by the authors. In order to match machine settings to simulation software settings, in-situ three-dimensional Digital Image Correlation (DIC) [4], an optical non-contact full-field surface deformation measurement method, is a valuable technique [5]. DIC enables a sub-pixel measurement accuracy of displacements and strains on complex parts.…”