PMI edition:7 location:Gent date:22-23 September 2016
In the thermoforming process a thermoplastic sheet is heated, formed by vacuum and subsequently cooled down before demolding the part. In most cases a contact-free infrared heating method is applied by using Ceramic, Quartz or Halogen elements. Since these methods apply a constant heat flux to the sheet surface which is transmitted to the core of the thermoplastic sheet; obtaining a uniform through thickness temperature distribution remains one of the biggest challenges in the thermoforming industry.
In this study a comparison is made between different available heating strategies. The theoretical optimal heating strategy comprises two heating phases: first the outer surface of the sheet is heated to the target temperature. Secondly the surface temperature is maintained in order to allow the heat to spread through the thickness of the sheet. This strategy is however not feasible with most classical heating elements. The very low response time of more recent heating elements, allows to apply pulsed heating. In this method the heat flux is controlled by a feedback loop based on a surface temperature measurement of the sheet.
All 3 strategies are numerically modelled by means of finite difference for the case of an experimentally characterised PS material. The comparison is based on the minimal heating time in order to obtain a through thickness temperature distribution within the forming range of the material. A parameter study was conducted to determine the influence of sheet thickness, penetration depth and response time of the heating elements.
Although the results of this paper identifies the fast response heating methods as a valid approximation of the theoretical optimal heating strategy, they remain fairly unknown to the industry. The positive effect of these methods increases with the thickness of the sheet. The influence of the repeated on/off switching of the elements in pulsed heating on the lifetime of the elements is a key-subject for future studies.