Reaction Injection Molding

You may know injection molding, but have you heard of reaction injection molding?

According to Thomas Net:

“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, 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.

Stages of the RIM Process
“Reaction injection molding is a multistage process involving procedures distinct from standard injection molding. To begin, two or more types of molten plastic, such as polyisocyanate and a resin compound, are poured into separate reactant tanks equipped with temperature- and feed-control mechanisms. The liquids are then released into their respective supply lines and channeled into a metering chamber that regulates pressurization and circulates the plastic into a mixing head.

“The mixing head applies pressures typically between 1,500 and 3,000 psi to blend the plastics via impingement. After the reactants mix, they are injected into the mold at high speed. The mixing head stops releasing reactants when the mold is full. An exothermic chemical reaction occurs inside the mold, resulting in plastic polymerization. After the polymer solidifies, or “cures,” it is ejected from the mold in the shape of the component.

“RIM machines accomplish this process using low temperatures. Aside from the heat needed to create the molten plastic, reaction injection molding is a relatively cool process. Rather than thermal energy, RIM produces chemical energy caused by polymerization reactions. The molds used are typically made of steel, aluminum, or nickel, and are clamped in a low-weight press. The polymer’s curing time is often under a minute, resulting in rapid turnover and quick production cycles for mid-volume runs.

Industrial Applications for RIM Products
“Due to its low processing temperatures and reduced injection pressures, RIM is well-suited for molding large components with variable wall thickness and smooth surface finishes. Manufacturers can also benefit from its relatively high turnover rate, which makes it proficient in handling low- to mid- volume projects. In addition, RIM parts exhibit high tensile strength, heat resistance, and resistance to corrosive acids or solvents. However, prolonged exposure to sunlight can degrade polymer-based parts, and the tooling or machining costs for polymer-based products are comparatively high.

“Many automotive companies use RIM-produced steering wheels, side panels, instrument gauges, dashboard consoles, armrests, body frames, and bumpers. The elastomeric properties of some RIM components also make them excellent for use in household appliances, heating and cooling units, and recreational supplies, such as water skis.

“Reaction molding enables parts consolidation, which can help to streamline the fabrication process and reduce the need for secondary tooling. Some manufacturers have used this advantage for crafting in-metal parts, such as frames and hinges. However, reaction injection molding is not ideal for high-volume projects, such as those involving smaller, mass-produced items. The metal molds are expensive, and can suffer wear under lengthy production runs. In addition, even slight pressure calibration errors in the mixing head can cause significant variances in the amount of material injected into the mold, and can lead to cracks in the product after curing

Is RIM Right For You
“Given the relatively high strength-to-weight ratio, complex designs and smooth finishes of RIM fabricated products, the RIM process lends itself to large component manufacturing. Reaction injection molding is particularly effective in reduced-volume vehicle or appliance assembly, but projects involving high numbers of small parts are usually better serviced by an alternative process.”

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