Integrating Electrical Functionalities in Plastics Parts

  IMMS CoE

The combination of plastics and metal offers a wide range of properties, especially concerning the possibility of weight reduction compared to an all-metal solution. Case components, for example, will oftentimes be made of metal in order to increase the electromagnetic tolerance of enclosed electronics. Frequently however a thin metal layer on the case suffices to ensure electromagnetic tolerance. The combined utilization of plastics and metal allows for an application-specific use of the materials. The newly developed In-Mould-Metal-Spraying (IMMS) is supposed to enable the integration of metallic areas on plastics parts. Therefore it incorporates the processes of thermal spraying of metal and plastic injection moulding in one advanced moulding tool. In the first step of the IMMS a metallic layer is applied to specific areas of the mould’s surface using wire arc spraying or cold gas spraying, which is immediately followed by the plastic injection. During this second step the metallic layer is transferred onto the plastics part similarly to the In-Mould-Labelling process. The plastics part with the transferred partial metallic surface is then removed from the mould. This new production technology for metal-plastics hybrids benefits from a shorter process chain in comparison to current methods. A compact manufacturing cell consisting of an injection moulding machine, a robotic thermal spraying unit and an appropriate mould is to render the highly integrated process possible and to enable the production-oriented fabrication of metal-plastics hybrids in shortest possible cycle times. A strong, permanent connection between the plastics and the metallic layer and an exact outline replication of the metallic layer in the pre-assigned areas are to be realized. With the help of different test parts the electromagnetic tolerance, the electric functionality and the accuracy to size are evaluated. The feasibility of the innovative approach has already been proven.

 

Practical Issues

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In order to achieve an optimal quality of the metallic coating, the layer has to stick to the mold’s surface during the plastic injection molding and may not be damaged by the melt flow. During demoulding of the part on the other hand, it has to peel off the mould easily and to adhere to the plastics. Additionally there are no previous experiences concerning the ability to reproduce more complex geometries of relevance for the application, e.g. thickness changes, cut-outs or ribs. Especially the minimal flanging radius that has to be provided at such construction elements must be experimentally determined to ensure an error-free transfer of the metallic layer from the mold to the plastics part.

 

Approach

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To solve the conflict of objectives between adherence and peeling off, it is investigated which surface qualities the mould has to possess to enable the peeling off of an applied metallic layer. The research parameters are the kind of tool steel, the surface roughness, the metal coatings and varying geometrical elements. Since the production parameters of the injection moulding greatly influence the quality and the transferability of the metallic layer, the injection molding process has to be adapted to the IMMS process. An adapted mold using changing inserts is developed to be able to examine the mentioned parameters. Additional evaluations relating to component design and process management are to be made with the help of a second test part which possesses a structured surface. The structure is repeated along the radius. Along the circumference the structure differs in shape, depth and flanging radius. The achieved adherence between tool steel and metallic layer can be determined via tensile tests before the injection molding process. In order to represent the whole product development chain simulative tools will be developed for the product and the process. On the product side the morphological simulation of the sprayed metallic layers is to predict the theoretical electrical conductivity of final metal coating. On the process side, based on the simulation of the injection molding, the probability of a successful transfer of the metallic layer is to be calculated depending on the component’s geometry and the process parameter. These simulative tools are to enable short development cycles and to reduce the number of iterations necessary in mold manufacturing.

 

Technical Challenges

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The main question is which combination of tool steel, sprayed metallic layer and spraying procedure provide for a reliable transfer of the metallic layer to the plastics part while also ensuring a long lifetime of the mould. For example one challenge is the reproducible adhesion of cold gas sprayed conductive tracks. The high kinetic energy of the particles during cold gas spraying causes a roughening of the mold surface. Therefore the conditions change with every subsequent spraying process. Furthermore the thin metallic layer can be washed away or displaced by the melt flow during the injection phase of the injection molding process. To prevent this, the roughness of the mold surface is increased in order to provide anchor points for the metallic layer. Additionally depressions can be placed in the mould surface to enhance this effect.