Influence of the Variothermal Process Control on Thermoforming of Micro-Structures

Abstract:

Functional surface effects on plastic components are a matter of current research activities. To make these surface effects usable on an industrial scale, new processes have to be developed. Forming processes such as injection moulding are currently used to produce functional surfaces, but machine and tool costs are considerably high. Therefore, it is of high importance to analyse to what extent the thermoforming process can be applied to manufacture micro-structures.The so called In-Mould-Graining (IMG) process is a promising variant of the thermoforming enabling the manufacture of micro-structures. The advantage of the IMG process is, that the structure is formed to the final shape of the part. If the structure on the film surface is set before the forming, the conversion may cause a deformation of the structure. However, the forming accuracy of the IMG process is limited by the rapid cooling of the film at the cold mould surface. A significant time interval between the heating and the forming leads to a cooling of the film, especially at its surface and thereby reduces the forming accuracy in the area of the grains.In this paper it is investigated to what extent the moulding accuracy of the IMG can be increased by the use of a variothermal mould heating. To achieve the required forming accuracy, only the surface layer of the micro-structured region of the cavity is heated by induction before the contact between film and cavity occurs. At the same time, the entire mould is cooled with a conventional water cooling system. The analysis of the process and formed parts shows, that especially the mould temperature during the forming process has a significant impact on the forming accuracy. A forming accuracy of a 50 µm high micro-structure of more than 95 % can be achieved. Furthermore, the contact of angle of a water droplet on a micro structured thermoformed surface can be increased up to + 145°.

• Periodical: Key Engineering Materials (Volumes 651-653)
• Pages: 855-862
• DOI: https://doi.org/10.4028/www.scientific.net/KEM.651-653.855
• Online since: July 2015
• Authors: Christian Hopmann, Jonathan Martens*
• Keywords: Forming Accuracy, In-Mould-Graining, Lotus Leaf Effect, Micro-Structures, Thermoforming

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