Reaction injection molding

Reaction injection molding

What Is reaction injecton molding (RIM)?

Reaction Injection Molding (RIM) is a unique molding technique used primarily for producing large, complex parts with specific performance characteristics. Unlike traditional injection molding, where pre-polymerized plastic is melted and injected into a mold, RIM involves the mixing of two or more liquid components directly in the mold where they chemically react to form a solid, final product.

1. Process

RIM usually involves at least two reactive liquid components. Commonly, these are a polyol and an isocyanate, especially when producing polyurethane parts.

The reactive components are mixed together, often with the help of a mix head, just before injection into the mold. The mixture is typically at a lower temperature and pressure compared to conventional injection molding.

Once inside the mold, the mixed components react chemically, leading to polymerization and solidification. This “reaction” is where the process gets its name.

The resulting polymer continues to cure and solidify within the mold, taking on its final shape.

The part is ejected from the mold after it has achieved sufficient rigidity. Some RIM parts may require additional post-curing outside the mold to achieve final properties.

2. Advantages

RIM is particularly well-suited for producing large, lightweight, complex parts with varying wall thicknesses.

Due to the reactive nature of the process, parts can solidify quickly, leading to shorter cycle times for certain applications.

RIM typically operates at lower pressures and temperatures compared to traditional injection molding, reducing wear on molds and allowing for less robust tooling.

By altering the formulation of the reactive components, manufacturers can achieve a wide range of material properties, from flexibility to rigidity, or even varied colors.

Due to the lower tooling costs, RIM can be more cost-effective than traditional injection molding for short runs or prototyping.

3. Applications

Bumpers, fenders, and other large panels or structural components.

Medical equipment housings and certain devices.

Machine casings, enclosures, and other large equipment components.

Computer housings, telecommunication equipment, and furniture.

4. Challenges

The reactive components must be stored and handled with care, as they can be sensitive to moisture, temperature, or other environmental factors.

Achieving consistent part quality requires precise control over the mixing and injection of the reactive components, as well as the curing conditions.

While the tooling for RIM can be less expensive due to the lower pressures involved, it still requires special considerations for venting and material flow, given the reactive nature of the process.


In summary, Reaction Injection Molding is a versatile and unique molding process that allows for the creation of large, complex parts with specific performance attributes. It offers distinct advantages for certain applications but also comes with its own set of considerations and challenges.

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