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A Look at RIM Overmolding

Until recently, manufacturers of plastic and FRP parts had limited choices for Class A in-mold coating solutions and, as a result, had to rely heavily on post-mold, liquid-applied coatings. In the case of automotive applications, choice of in-mold coatings were virtually non-existent. Thanks to recent advancements in coating chemistry and metering technology, parts manufacturers can now deliver cost-effective, direct-from-mold Class A plastic and composite parts that meet stringent automotive OEM validation requirements while offering a broad range of attractive design options in terms of colors, textures, haptics and other attributes.

Millions of molded plastic parts requiring a Class A finish are produced by the industrial and automotive supply chain every year. Historically, post-molding application of liquid-applied coatings has been the only capable and OEM-approved method for achieving a durable, decorative appearance (color, gloss, distinctness of image, UV resistance, scratch resistance, etc.) that would meet OEMs’ targeted performance requirements. While proven, post-molding application of decorative coatings requires extensive and costly capital equipment and generates high levels of scrap and waste, such as overspray solids/filter media and VOC emissions. Post-mold coating processes also add logistics and handling costs.

Today, advancements in chemistry and metering technology allow the application of multi-component coatings for high-volume part production directly in the mold. This process is generically referred to as Reaction Injection Molding (RIM) overmolding. The RIM overmolding process involves injecting a multicomponent coating, which reacts or polymerizes directly within a closed mold at relatively low pressure.

Following the molding of the part, the tool is rotated or indexed to present the freshly-formed part to the A side tool, which includes the features and finish (e.g. matt/gloss, textured etc.) desired on the exposed surface of the part. The A side of the mold, its offset from the surface of the formed part, defines the film thickness and the A surface finish.

Pressures during the RIM overmold process at the injection port are typically between 15 psi and 30 psi, with a peak pressure at the injection port of approximately 150 psi. These pressure ranges are relatively small as compared to the clamp loads required by the substrate forming process and therefore typically do not influence press sizing.

This process of injecting a low viscosity material (similar to water) at low pressure into a closed mold has a number of distinct advantages, which are particularly relevant to composite parts producers. For example, the coating process surface being defined predominately by the A side mold allows manufacturers to overcome substrate variations that would otherwise require extensive priming and sanding operations. This feature alone enables better surface quality, reduced defects and greatly reduced operating costs.

» Author: Adam Halsband

» Publication Date: 08/01/2019

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This project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° [609149].

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