Injection-molded metal-plastic hybrids are innovative products combining dissimilar materials and their properties in the same component. The hybrids can offer benefits which are not achieved with individual material alone e.g.: savings in weight, part reduction, better dimensional stability, and manufacturing multi-functional components in a few processing steps. However, joining of physically and chemically different materials is very challenging. Bonds between metals and plastics can be achieved by chemical or physical bonding mechanisms or by mechanical interlocking like perforating a metal insert and over-molding polymer on it, which is the mainly used method in the industrial applications. In many applications like in electronic components, planar inserts without perforation are needed, but a detailed knowledge of the chemical adhesion between metal and plastic is still lacking.

In this study, metal-plastic hybrid structures were produced by injection molding and the chemical adhesion between metal and plastic was achieved with coupling agent. Stainless steel, AISI 304, and coppers, OFE-OK and Cu-DHP, were used as metallic inserts and thermoplastic urethane (TPU) was used as a plastic component and aminofunctional silane was used as coupling agent. The bonding of silane to as-received metal surfaces was poor, so active surface pre-treatments, i.e. electrolytical polishing and oxidation treatments, for metals were needed to improve the bonding. Manufacturing of the metal-plastic hybrids consisted of three steps: (1) surface modification of metal, (2) silane treatment of modified metal, and (3) injection molding of plastic onto silane-treated metal. The hybrid structures were characterized within each manufacturing step. Prior to silane treatment, metal surfaces were characterized with atomic force microscopy (AFM), transmission electron microscopy (TEM), reflection absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM), AFM, TEM, RAIRS, and XPS were used to study the silane layers. The finished hybrid parts were characterized with SEM and the adhesion strengths of the hybrids were measured with peel tests. AFM, SEM, and RAIRS were used to find out the failure types of the hybrids in peel.

According to comprehensive characterization results, a controlled oxide layer on the metal surface was needed to achieve a uniform and cross-linked silane layer with amino species on the surface to react with plastic; a smooth metal surface produced by electrolytical polishing with a native oxide layer was not enough. The hybrids manufactured with the oxidized metal inserts failed mainly cohesively in the plastic part with high peel strength values while the hybrids manufactured with the as-received or electrolytically polished inserts failed mainly inside the silane layer with poor peel strength values due to uneven formation of the silane layers. So the metal surface, prior to silane treatment, had a significant effect on the silane layer formation and hence the adhesion strength values of the related hybrids; similar strength values were achieved with similar pre-treatments of the metal surfaces regardless of metal used.