A comprehensive study of bubble inflation in vacuum-assisted thermoforming based on whole-field strain measurements

Abstract

Abstract. This paper focuses on the potential use of stereo-DIC in thermoforming conditions to monitor large deformations of softened thermoplastic sheets posteriori to the sagging phenomenon. The study concerns HIPS sheets which are softened by the radiative heat-transfer mode then stretched by inflation of compressed-air for 1.5 s to form a large and quasi-spherical dome of 250 mm in diameter. While the bubble-inflation operation leads to large deformations of the softened sheet, it shows transitional geometrical instabilities due to the initial surface sagging. When the temperature-induced surface deformations are inaccessible by the stereoscopic system during the heating operation, the geometrical instabilities limit the identification of the reference of displacements which affects the accuracy of results based on image-correlation computations. To compare between the principal strains assessed from bubble-inflation tests conducted at different thermal conditions, a method for filtering these instabilities is developed in this study.

Biaxial characterization of poly(ether‐ether‐ketone) for thermoforming: A comparison between bulge and in‐plane biaxial testing

“…Literature has recently emerged concerning the use of bulge testing for the characterization of polymers for thermoforming processes . These studies have identified the analogy between the bulge test and thermoforming although the data produced are yet to be used to characterize and model polymers for FE simulation applications.…”

Digital image correlation for strain measurement in plastic film blowing process

“…A time derivative of displacements was proposed to choose the reference of the stereo-DIC calculation. The derivative confirmed that the transition from sagged to the strained geometries of the sheet showed reduction of 63% of their maximal velocity amplitude [61].…”

In Situ Stereo Digital Image Correlation with Thermal Imaging as a Process Monitoring Method in Vacuum-Assisted Thermoforming

“…This heating phase is attentively controlled by setting a specific temperature, which is precisely measured using an infrared (IR) sensor. Despite the symmetrical arrangement of the heating elements aiming for uniform in-plane heat distribution, the temperature across the sheet is almost inevitably uneven [1]. When a uniform heating pattern is applied, the center of the sheet often reaches a higher temperature compared to the clamped edges, resulting in a temperature gradient from the center to the edges [2].…”

“…Here, the friction between material and mould, which is highly temperature dependent itself [14], exhibits the highest friction load in the region with initial contact [15,16]. This differential in contact timing, combined with the temperature non-uniform distribution field and material cooling rate across different regions during forming, leads to non-uniform stretching of the polymeric sheet during forming, consequently resulting in an inevitable thickness variation in the final product [1,17].…”