Research & Development In-Mould-Metal-Spraying: Surface and partial metallisation of plastic parts
Germany – This article presents a new method for the production of metallised plastic surfaces for magnetic, electrical, haptical or decorative applications.
On the one hand, established processes like electroplating or PVD coating provide high-quality coating surfaces combined with a high freedom of design [1, 2]. But they require multiple pre- and post-treatments of the plastic parts, such as laser activation or chemical pre-metallisation of the surface and the cleansing, which lead to additional expenditure of time and materials. On the other hand, it is possible to coat plastic components directly by means of thermal spraying. In this case, the poor surface quality, the plastic part’s distortion in case of thin walls caused by the molten particles, and the high temperature gas flow have to be considered [3, 4].
In order to benefit from the advantages of the aforementioned processes, such as free part design and good electrical properties, the Institute of Plastics Processing (IKV) and the Surface Engineering Institute (IOT) at Germany’s RWTH Aachen University pursue the integration of known production technologies. The so-called In-Mould-Metal-Spraying (IMMS) combines injection moulding of plastic with thermal spraying of metal coatings. First, a metal coating is applied to certain areas of a cavity insert of an injection mould via thermal spraying (Figure 2). After putting the insert into the mould, the metal coating is back-moulded with the plastic melt. The metal coating is hereby transferred onto the plastic component. The metal coating and plastic part are then demoulded as a plastic component with an integrated (partially) metallised surface.
Different thermal spraying processes can be used to apply the metal coating onto the cavity insert. Cold-gas-spraying (CGS) as well as wire-arc-spraying (WAS) were used in previous studies of IKV and IOT [5, 6] (Figure 3). WAS was chosen as the preferred process for IMMS because of the wide range of usable coating materials and the relatively low induced thermal and mechanical stresses on the inserts. WAS uses wire electrodes as feedstock material, which are melted by means of an electric arc. The molten particles are accelerated via gas stream (e.g., compressed air or nitrogen) and can reach velocities of 100 - 150 m/s in case of the conventional WAS.