Measuring Thermoforming Behaviour Available to Purchase

Thermoforming is the process of choice for manufacturing thin‐gauge or large‐area parts for packaging or technical applications. The process allows low‐weight parts to be produced rapidly and economically from thermoplastic semi‐finished products. A technical and consequently economical problem is the choice of the right material in combination with the thermoformability of the product. The prediction of thermoformability includes the aspired product features and geometry and defined wall thickness distributions, depending on the specific stretchability of the semifinished product. In practice, thermoformability is estimated by empirical tests with the particular semi‐finished product using e.g. staged pyramidal moulds or model cars. With this method, it still cannot be ensured that the product can be thermoformed with the intended properties. A promising alternative is the forming simulation using finite element analysis (FEA). For the simulation, it is necessary to describe the material behaviour using defined material models and the appropriate parameters. Therefore, the stress‐/strain‐behaviour of the semi‐finished product under defined conditions is required. There are several, entirely different measurement techniques used in industry and at research facilities. This paper compares a choice of different measurement techniques to provide an objective basis for future work and research. The semi‐finished products are examined with the Membrane‐Inflation‐Rheometer (MIR), an equibiaxial strain rheometer. A flat sample is heated to the desired temperature in silicone oil. During the measurement, a servohydraulic linear drive advances a piston, thus displacing the hot silicone oil and inflating the specimen to form a sphere. Further measurements are carried out with the Karo IV Laboratory Stretching Machine at Brückner Maschinenbau GmbH & Co. KG, Siegsdorf, Germany. The samples are heated using hot air. During the biaxial stretching, the resulting forces at the clamps are measured. These techniques are compared to the stretching device developed at IKV. This measuring device is integrated into a laboratory thermoforming machine, which allows a close‐to‐real‐process heating, handling and forming of the semi‐finished products. After the heating with IR‐radiation, force‐displacement data is measured during the equibiaxial deformation. This work shows the differences between these methods and emphasises the particular benefits. Further measurement methods, like uniaxial tensile tests and plug deformation, and the effects on the forming simulation will be part of future research.