Introduction

Thermoforming is a widely used process to produce plastic parts for various applications. The production is very cost-effective with regard to tooling and machinery costs, and achieves high output rates. To epitomize the thermoforming process, computer simulations with finite-element methods (FEM) are carried out. However, it is a big problem in thermoforming that precise material data is not available and the estimation is a very expensive and time-consuming procedure.

TMC Project

The target of the project is to develop a low-cost measurement apparatus to measure material behavior and of fitting software for the precise characterization of thermoforming materials.

The price of the TMC apparatus to measure the behavior of biaxially loaded thermoplastics under real processing conditions (large deformations, high deform. rates) should not exceed 20 kEUR. It is aimed at an official test procedure which can also be used for quality assurance.

With the measurement results, a data base will be developed for storing all relevant material data to be used for process optimization and for the design of new polymer products and tools.

In cooperation with the Institute for Polymer Sciences IKP of the University of Stuttgart as a RTD performer, the SME proposers Wagner (project coordinator), ACCUFORM (FEM-tool developer) and the thermoformers Jacob and APT, intend to develop a measuring apparatus and fitting software for thermoforming material characterization.

In the beginning of the project, measurements of polymers will be carried out as a basis for developing algorithms of reverse engineering to derive material parameters from force-deflection-curves. These measurements will be performed by the IKP, providing expert knowledge in polymer physics and measurement. The IKP is equipped with a sophisticated, fast deforming machine for biaxial tensile tests of materials. Several polymers of interest will be examined at different temperatures and deformation rates. These parametrized force-deflection-curves sufficiently describe the viscoelastic behavior of the material.

Since the material parameters (stress-strain relation) for biaxial deformation cannot be identified directly on the measurement data and no public fitting algorithm or software exists, ACCUFORM will use the data to fit material parameters until FE-simulations correspond to the measurements.

Additionally, the measurements at the IKP will give indications about how to design the new measuring apparatus, allowing both a low-end price and an accurate measurement. Difficulties with correct clamping, fast deformation and temperature control have to be mastered. The results of the tests at the IKP will serve as a reference for later measurements with the new apparatus.

The measurement results of the new apparatus will be validated in pilot applications carried out by the companies Wagner, Jacob and APT. Technical thermoforming parts will be first epitomized by simulation, manufactured with different tools on different machines and finally compared to the prediction of the simulation tools.

The project management under the coordination of Wagner as well as dissemination and exploitation will be additional tasks.

More details about the TMC project can be found in this paper (MS Word file, zipped, 535 kB).