Abstract
The deposition of physical vapor deposition (PVD) hard films is a promising approach to enhance the tribological properties of injection molds in plastic processing. However, the adhesion is influenced by the pairing of PVD film and processed plastic. For this reason, the friction behavior of different PVD films against polyamide, polypropylene, and polystyrene was investigated in tribometer tests by correlating the relation between the roughness and the adhesion. It was shown that the dispersive and polar surface energy have an impact on the work of adhesion. In particular, Cr-based nitrides with a low polar component exhibit the lowest values ranging from 65.5 to 69.4 mN/m when paired with the polar polyamide. An increased roughness leads to a lower friction due to a reduction of the adhesive friction component, whereas a higher work of adhesion results in higher friction for polyamide and polypropylene. Within this context, most Cr-based nitrides exhibited coefficients of friction below 0.4. In contrast, polystyrene leads to a friction-reducing material transfer. Therefore, a customized deposition of the injection molds with an appropriated PVD film system should be carried out according to the processed plastic.
Highlights
Injection molding is a cyclic process for the cost-effective mass production of plastic components with complex geometries at relatively low processing temperatures [1]
The chromium nitride (CrN) and chromium aluminum nitride (CrAlN) films sputtered in direct current magnetron sputtering (dcMS) were marked by a cauliflowerlike surface structure, with a smaller dimension in the case of CrAlN
This results from the smaller grain size of CrAlN in comparison to CrN [25,26]
Summary
Injection molding is a cyclic process for the cost-effective mass production of plastic components with complex geometries at relatively low processing temperatures [1]. The molded parts tend to stick on the surface of the tool, resulting in a buildup and restricting the productivity of the process as well as the quality of the plastic parts [4]. Within this context, lower adhesion strengths between injection mold and molded part enable to reduce the ejection forces, which allows reduction of the dimensions of the ejection system within the mold, exploiting the attained space by expanding the cooling circuit [5]. A suitable surface modification of the injection molding tools is necessary to optimize the tribological properties [7]
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