Reaction Injection Molding

February 13, 2023

While molded plastics are used in a broad range of industrial and commercial applications, selecting a specific molding process can greatly influence the quality and efficiency of a project. Broadly speaking, the injection molding process produces thermoplastic resin parts by forcing molten plastic into a design mold. The high pressures used cause the plastic to conform to the mold’s shape, thereby generating the part. Like metal die casting, custom plastic injection molding typically produces components that require little or no additional work. Reaction injection molding (or RIM) is a specialized subset of the injection molding technique that chemically bonds two or more plastics into a polymer before they are introduced into the mold.

Manufacturers considering the use of RIM may want to examine the particular attributes that set it apart from standard injection molding. One of the central differences lies in the nature of plastic polymerization, which can yield properties distinct from those of the base materials. RIM can also fabricate large parts with complex shapes, making it well-suited for industrial applications that require bulky components. Likewise, issues of cost-efficiency, production cycle rates, and machinery design can influence the decision to employ this technique.

Plastics Used in RIM Process
Polyurethanes, polyamides, and fiber composites are the most commonly used types of plastic in reaction injection molding. These materials exhibit properties that vary depending on the ratio of base substances or additive chemicals used during polymerization. Different combinations can yield a range of elastomeric qualities that determine a product’s ability to stretch at low stress and return to its original shape. This range affects the flexibility of structural, solid, and composite foams, which are the chief products used in polyurethane fabrication.

Since the process involves molten plastic, RIM may be considered a type of liquid injection molding. One of the liquids is often infused with carbon fillers or short fiber resin to increase the product’s stress-to-strain ratio and lower the degree to which it expands or contracts from thermal changes.

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