Analysis and prevention of chill mark formation in thermoforming to increase material efficiency by means of novel stamp stretching with supporting air
In this research project, a new type of stamp stretching with targeted supporting air is being developed to prevent the formation of chill marks during thermoforming. By minimising the contact between the film and the stamp, material efficiency is to be increased and the stamp design process optimised.
Chill marks are a quality defect in the thermoforming process. If the processis not optimised, chill marks can occur locally in the moulded part during thermoforming with a pre-stretching plug. These are accumulations of material that represent optical and mechanical defects. The causes of the formation of chill marks are manifold and cannot be clearly attributed to one parameter.
© IKVThe aim of the IGF research project 22994 N is therefore to extend the existing plug design process to include the targeted introduction of supporting air at the pre-stretching plug and thus minimise contact between the film and the plug. The aim is to resolve the conflict between material-efficient wall thickness distribution and the avoidance of chill marks. First, an analytical design of the support air geometries in the pre-stretching plug is carried out with the aim of achieving the most homogeneous exit speed possible over the entire plug surface. The restrictions of additive manufacturing, for example with regard to a minimum cooling channel diameter, must be taken into account. In an iterative optimisation process, the exit speed is measured experimentally and the channel geometries are adapted. This is followed by the development of plugs with an exclusively localised support air supply in the areas of the quench marks based on various plug concepts.
Various concepts were developed to enable the introduction of supporting air in different areas of the plug. One possibility is the introduction of supporting air through nozzles on the circumference of the plug, which creates an air cushion that keeps the film away from the plug due to the resulting pressure. In addition to the simple modification of the plug by inserting a through-hole in the centre of the stamp, more complex diffuser or nozzle geometries are also possible. Fig. 1, for example, shows a more complex “diffuser” concept for introducing supporting air through a large number of air ducts on the circumference of the plug (left). On the right is the realisation of the concept as a 3D model. In order to investigate different concepts for the introduction of supporting air and the influence of new process parameters such as the switch-on point of the supporting air, the supporting air height and duration, the existing plant technology at IKV is being modified. This makes it possible to compare the plug concepts with and without the use of supporting air with regard to the formation of chill marks and the possibility of optimising the wall thickness distribution far from the conventional plug design. Initial results of the “diffuser” concept are promising and indicate a positive effect in terms of avoiding chill marks by applying an air cushion between the stamp and the film. Further investigations are planned to analyse additional stamp concepts and the further influence of various process parameters on the wall thickness distribution.
Project data and funding
We would like to thank the BMWK for funding the research project (funding code 22994 N) and the project partnersfor their cooperation.
Project duration: 01.09.2023 – 31.08.2025
Project partners and funding
