Abstract. Numerical simulation tools for the thermoforming of unfilled thermoplastic polymers already exist for a while, but are seldom used to full extent in industry. When it is used, it is mostly only for comparative studies and prediction of relative wall thickness. One of the major reasons is the difficulty to correctly calibrate and integrate all necessary material and process parameters into the simulation software. This paper introduces and validates a methodology, in which digital image correlation (DIC) is used as the key enabling technology that improves the knowledge of the process parameters and optimizes simulation accuracy by taking away a number of uncertainties and assumptions. DIC in combination with infrared thermal measurements and pressure monitoring is used to track sheet sagging and bubble inflation of a HIPS sheet, the two main process steps in the thermoforming of positive (male) products or the only two steps in the case of free forming. The results of these in-situ measurements are used as a guideline for selecting the correct input parameters in the commercial thermoforming simulation software T-SIM ® . A similar methodology can be further implemented for subsequent process steps such as forming and cooling or even to validate the material data used in the simulation software.

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.

This paper proposes a methodology to determine wall thickness distributions in thermoformed products derived from in-situ surface strain measurements obtained with stereo digital image correlation (DIC), under the assumption of material incompressibility. Wall thickness equations are derived for the Green-Lagrange, Hencky, Biot, logarithmic Euler-Almansi and Euler-Almansi strain definitions and validated with an analytic example.The equations are then used to calculate the wall thickness from digital image correlations with both a commercial software (VIC-3D, version 2012) and an academic software (MatchID3D). Obviously, the choice of the strain definition in the correlation software should not influence the resulting wall thickness values. The comparison reveals that the wall thickness values prove to be identical, regardless of the strain definition that was used, and manual measurements show the validity of the DIC based results. It was also found that, when using VIC-3D version 2009, an earlier release in the software series of Correlated Solutions, all but the Green-Lagrange strain definition bring forth unrealistic minimum principal strain values, resulting in different values for the wall thickness.

This paper illustrates a methodology for the characterization of textile composite reinforcements during experimental simulation of forming processes. In particular, being the shear deformation considered as the primary deformation mechanism during shaping, the evolution of shear angle distribution on the reinforcement surface is measured by means of 3D digital image correlation analyses. Two different image correlation software programs, i.e. VIC-3D and MatchID3D, are used to study the forming process of a single layer E-glass non-crimp 3D orthogonal woven reinforcement (commercialized under trademark 3WEAVE® by 3Tex Inc.). The comparison of the displacement and shear angle distributions of the reinforcement, shaped on tetrahedral and double-dome moulds, pointed out a good agreement between the results obtained with both software packages

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.