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Project manager: Prof. Dr. W. Maus-Friedrichs, M.Sc. Lienhard Wegewitz
Funding period: 01/2015 - 12/2016
Funding agency: BMWi via AiF
Funding code: IGF 18561 N
Contact: M.Sc. René Gustus, M.Sc. Maria Sonnenberg
Stipples in plastic molded parts are a widespread cause of production disruptions and interruptions as well as quality impairments. Stipples often arise from deposits that can form from the polymer melt on steel surfaces in plastics processing machines and on the surfaces of extrusion tools, for example. Particularly critical areas are preparation and production processes for optically high-quality molded parts with high demands on optical transparency or with high-gloss surfaces.
In the past, speck problems were often exacerbated by inadequate fluidic design of the melt-carrying channels in machines (e.g. non-return valves) and tools ((Müller 2002), (Wintermantel 2009), (Shelby and Caflisch 2004)). Through optimization measures, many years of experience and the possibilities of numerical flow simulation, these causes have been largely eliminated or minimized by machine and tool manufacturers. Despite these measures, problems with speck formation are still acute in certain applications. One of the main causes of the formation of flakes, which still occurs despite the optimum design in terms of flow technology, is the initially extremely thin layer of deposits adhering to the steel surfaces, which can continue to grow over time. The adhering material is thermally degraded or remodeled by the long exposure to high temperatures. The resulting layer detaches at least partially from the steel surface at a later point in time - presumably when a certain critical layer thickness is exceeded. In the case of amorphous thermoplastics, problematic cases of coating formation have been reported.
In such problematic cases, correlations with the specific additivation of individual molding compounds are suspected. As part of a completed predecessor project (IGF No. 403 ZN "Causes and mechanisms of coating formation and adhesion of plastic melts on steel surfaces"), it was already possible to gain significant insights into adhesion.
The results of this project were regularly discussed with industry representatives with practical experience in the "Materials and Surface Technology" expert group, which brings together an average of 40 participants from the fields of plastics processing, raw materials production, plastics mechanical engineering, steel production and surface coating.
At its meeting in December 2013, this group of experts proposed to build on the existing foundations and the methods developed and to extend the investigations in a follow-up project to standard coating systems. Improving the performance of surfaces with PVD coatings on components subject to tribological surface stress is now fundamental for many technical products. The influences of the process parameters on the quality of the coating have already been studied in detail for the individual processes. However, the relationship between coating parameters and coating formation in plastics processing has not yet been investigated. It can be said that the coatings and coating systems have achieved positive results in terms of coating and speck formation in individual cases, but not in others. Comprehensive findings as to why these coatings are effective in some cases but not in others are still lacking today.