RIM Process
What Is the Reaction Injection Molding Process?
The reaction injection molding process uses two liquid polyurethane components: a polyol blend (resin) and isocyanate (catalyst). These components flow separately via isolated loops through a computer-controlled, precision-fed dispensing machine and a mixhead, which is attached to a temperature-controlled mold. When a part is produced, a plunger inside the mixhead retracts, allowing the chemicals to collide at high pressure—around 2000 psi—in the mixhead’s mixing chamber, then flow into the mold at lower pressure—about 100 psi. An exothermic reaction occurs, causing the chemicals to cream, gel, and solidify into a solid thermoset plastic part. Thermoset parts provide excellent strength, durability, and resistance to corrosion, impact, and heat.
This process differs significantly from traditional injection molding, which requires significantly higher operating temperatures and pressures and, therefore, more expensive steel tooling. Additionally, because injection-molded parts are not thermoset, they can be easily melted, ground, and reprocessed.
Step-by-Step Overview of the RIM Process
The RIM Process and Capabilities
Each component of the RIM system is temperature-controlled and precisely metered. The polyol and isocyanate are stored in separate tanks to maintain chemical stability and readiness.
The RIM Process and Capabilities
The two materials are pumped through a high-pressure mix head, where they impinge and blend at high velocity before being injected into the mold cavity. Despite the high mixing energy, the actual mold filling pressure remains low—typically under 100 psi—allowing for cost-effective aluminum molds.
Chemical Reaction and Curing
Inside the mold, the reactive mixture begins to expand and cure through a thermochemical reaction. The temperature can rise above 300°F as the polymers crosslink to form a solid thermoset material. This reaction provides the rigidity, dimensional stability, and durability that define RIM parts.
Demolding and Post-Processing
Once cured, the part is removed from the mold. Because it’s a thermoset, it will not melt or lose shape under heat. From there, RIM Manufacturing performs any necessary trimming, painting, assembly, or quality inspections before shipment.
Benefits of the RIM Process
The reaction injection molding process offers a range of benefits compared to conventional injection molding or composite fabrication methods:
- Lower tooling costs due to low-pressure aluminum molds
- Shorter lead times and faster development cycles
- Greater design freedom with variable wall thickness, ribs, and inserts
- Lightweight, high-strength parts with excellent dimensional stability
- Ability to encapsulate hardware or electronics directly into molded parts
- Superior chemical and thermal resistance for demanding environments
Because of these advantages, RIM Manufacturing’s process is ideal for large, complex geometric components that require durability and precision without the high cost of steel tooling.
Material Options and RIM Variations
Different RIM formulations allow for tailored performance based on the needs of the application.
RIM Manufacturing helps customers select the right material system to meet technical, structural, mechanical, aesthetic, and environmental requirements.
Structural Foam RIM
Combines rigid polyurethane with a foamed core for lightweight strength and sound absorption.
Solid RIM
Produces dense, high-strength parts with minimal foaming and excellent surface finish.
Elastomeric RIM
Creates flexible, impact-resistant components ideal for protective housings or bumpers.
High-Performance RIM Systems
Include specialty chemistries such as DCPD for enhanced heat and chemical resistance.
Self-Skinning RIM Systems
Produces a dense outer layer (or skin) providing a durable, non-porous, and aesthetically pleasing finish.
Design and Engineering Considerations
Designing for the RIM process provides engineers with tremendous freedom, but certain best practices ensure consistent results:
- Maintain even wall thickness where possible for uniform curing.
- Use ribs and bosses to add structural strength without excess weight.
- Include draft angles to aid part release from the mold.
- Design gating and venting to support balanced mold filling.
- Allow for slight part shrinkage due to thermal expansion and curing.
RIM Manufacturing’s engineering team collaborates with customers to refine part geometry and durability, optimize mold flow, and ensure manufacturability from first articles through production.
Common Applications of the Reaction Injection Molding Process
The versatility of the RIM process makes it ideal for parts that demand complex shapes, high durability, and excellent aesthetics. Typical applications include:
- Medical device, instrumentation, and laboratory enclosures
- Commercial Vehicle panels, hoods, bumpers, and other large component parts
- Industrial equipment enclosures and housings
- Technology enclosures and shrouds
- Logistics and Automation product solutions
- Energy, Environmental System Components
In each of these industries, RIM Manufacturing provides end-to-end service—from material selection and design support to full production and finishing.
Why Choose RIM Manufacturing
RIM Manufacturing is a leader in the reaction injection molding process, providing expert guidance, in-house engineering solutions, and complete manufacturing capabilities. Every project benefits from:
Decades of experience in polyurethane chemistry and RIM design
Dedicated tooling, molding, finishing, and minor assembly
Scalable production for mid-volume to high-volume manufacturing
Quality assurance systems focused on precision, consistency, and repeatability
RIM Manufacturing delivers high-quality parts and outstanding customer service.
