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Mechanism-based strategies for the avoidance of deposit formation in plastics processing machines and tools

(Kopie 3)

Project Leader: Prof. W. Maus-Friedrichs, M.Sc. Lienhard Wegewitz

Funding period: 01/2015 - 12/2016
Funding body: BMWi via AiF
Funding reference: IGF 18561 N

Contact partner: M.Sc. René Gustus, M.Sc. Maria Sonnenberg

Specks in moulded plastic parts are a common cause of production faults and production interruptions, as well as quality impairments. Specks are frequently produced from coatings that can form on steel surfaces in plastics processing machines and on the surfaces e.g. of extrusion tools from polymer melts. Particularly critical areas are preparation and production processes for optically high-quality moulded parts with exacting requirements with regard to visual transparency or high-gloss surfaces.

in the past speck problems were frequently exacerbated by insufficient technical flow design of the melt-carrying channels in machines (e.g. non-return valves) and tools (Müller 2002), (Wintermantel 2009), (Shelby and Caflisch 2004)). Through optimisation measures, long-term experience and the ability to perform numerical flow simulation, these causes have been largely eliminated or minimised by machine and tool manufacturers. In spite of these measures, problems with speck formation are still acute in certain applications. A major cause for the formation of specks - which still occurs in spite of optimal technical flow design - are the initially extremely thinly layered coatings that adhere to the steel surfaces, which can continue growing over time. Here the adhering material is thermally removed or converted as a result of the long wait at high temperatures. The resulting layer at least partially comes off the steel surface again at a later point in time - presumably when a certain critical layer thickness is exceeded. There have been reports of problematic cases of the formation of deposits in amorphous thermoplastics.

In such problem cases, correlations with the specific addition of individual moulding compounds are presumed. Within the scope of a previously completed project (IGF No. 403 ZN "Causes and mechanisms of deposit formation and the adhesion of plastic melts on steel surfaces") significant knowledge could already be acquired with regard to adhesion.

The results of this project were regularly discussed with experienced industry representatives within the expert panel entitled, "Material and surface technology", in which an average of 40 participants from the fields of plastic processing, raw material manufacture, plastic-mechanical engineering, steel production and surface coating come together.

In its meeting in December 2013, this expert panel proposed drawing on the now available fundamentals and the developed methods and to now extend the studies to cover commonly used coating systems, within a follow-up project. An increase in the performance of surfaces using PVD layers on components that are tribologically stressed at the boundary layer is now fundamental for many technical products. The influences of the process parameters on the quality of the coatings are being examined in detail for the individual procedures. However, the correlation between coating parameters and deposit formation in plastics processing have not previously been studied. It can be said that, in application, the coatings and coating systems can have positive results in relation to deposit and speck formation in individual cases, then again this is not always the case. Comprehensive findings as to why these layers are effective in some cases but not in others have thus far proven to be elusive.

 

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