Modelling IR-Heating in Stretch-Blow Moulding and Thermoforming

Abstract

The two-stage stretch-blow moulding process has been established for the large scale production of high quality PET containers with excellent mechanical and optical properties. Thermoforming is the process of choice for manufacturing thin-gauge or large-area parts for packaging or technical applications. Both processes allow lightweight thermoplastic parts to be produced rapidly and economically. In both processes thermoplastic semi-finished products are formed by pressurised air under the influence of heat. To enable forming of the thermoplastic materials, the semi-finished products need to be transferred into a thermoelastic state. IR-heating is widely used due to short heating times. From a cost perspective, about 7 % of the total production costs of a stretch-blow moulded bottle are spent for energy in order to heat and form the preform to the later bottle. Depending on machine, semi-finished product type and cycle time, energy costs in thermoforming account for around 1–5 % of the total production costs. Modern roll-fed automatic thermoforming machines use about 22 % of the energy consumption for the heating step and around 70 % for the production of pressurised air. Due to this significant share and due to increasing energy costs during recent years, the packaging industry is interested in increasing the energy efficiency of these processes. The most important quality criterion for both processes is a uniform wall thickness distribution. The production of high-quality parts requires optimised temperature profiles of the semi-finished product depending on the particular product geometry. Simulation is an approved tool for the prediction of the influence of the heater setting on the temperature profile. Over the last decade IKV has developed an integrative three-dimensional process simulation which models the complete path of a preform through a stretch-blow moulding machine. An essential first step is the heating simulation where the temperature profile of the preform is computed. Based on this data the temperature-dependent material behaviour of PET can be considered during the inflation simulation. This work shows the influence of a thoughtful temperature profile on the wall thickness distribution in stretch-blow moulding. The focus is on modelling the reheat phase of the stretch-blow moulding process in FEA. Beyond that, a purposeful heating offers the possibility to cut down energy waste.