Large Part Reaction Injection Molding

A Better Way to Produce Large Plastic Parts

When engineers evaluate large part injection molding or large part thermoforming, they typically run into the same limitations: high tooling costs, design constraints, or multi-part assemblies that complicate production.

Reaction injection molding (RIM) offers a better way to produce large plastic parts.

At Rimnetics, we specialize in producing large plastic parts that are structural, lightweight, and highly complex—without the cost and design constraints of traditional manufacturing methods. Instead of forcing your design to fit the process, we engineer the process around your design.

The RIM Manufacturing Process for Large Parts

Reaction injection molding produces large plastic parts through a controlled chemical reaction rather than heat and pressure. This distinction is what makes very large, structurally complex parts possible at lower tooling costs which, other more conventional processes cannot match.

Material Preparation

Polyol and isocyanate are stored in separate temperature-controlled tanks and metered through supply lines to a chamber connected to the top and bottom molding plate.

High-Pressure Impingement Mixing

The two components are combined at high pressure inside a mix head, which initiates the chemical reaction. The resulting low-viscosity mixture flows easily into large mold cavities. This polyurethane mixture flows in and through the cavity, gravity fed with little to no pressure.

Mold Fill

The reactive polyurethane mixture flows from the mix head through an aftermixer and into the mold cavity. Low injection pressure allows the material to fill large, complex geometries without the clamping force required by conventional injection molding large parts. It also eliminates flow lines, knit lines, and stresses around bosses and features. This is in sharp contrast to other molding methods.

Cure & Demold

Water lines regulate mold temperature to control the cure cycle. Once cured, the part is demolded as a single structural component.

Finishing

Trimming, machining, painting, and high-level assembly operations are performed in-house at the Loomis, CA facility.

Benefits of RIM for Large Plastic Parts

Part Consolidation

RIM enables multiple components to be combined into a single molded part, reducing assembly, hardware, failure points, and overall program cost.

Large Format Capability

Rimnetics produces large parts up to 8' x 8' x 3' in a single shot—without the high press tonnage or costly steel tooling required by traditional injection molding.

Structural Integrity Without a Frame

RIM structural foam creates self-supporting parts, eliminating the need for secondary metal frames common in thermoformed or sheet metal designs.

Design Flexibility

Low-pressure RIM allows complex geometries, variable wall thickness, integrated features, and encapsulated components—all in a single part.

Durability That Competing Materials Cannot Match

RIM polyurethane delivers superior impact resistance, surface durability, and environmental performance compared to traditional large-part materials.

Lower Tooling Investment

Aluminum RIM tooling reduces upfront cost and risk while supporting both prototyping and full production from the same tool.

Industries We Serve

Rimnetics helps engineering and sourcing teams bring large, structurally demanding parts from design through production.

Medical Devices

Laboratory Equipment

Robotics

Electronics

Industrial Equipment

Not sure if your application is a fit? Send us your CAD file and we’ll provide an initial feasibility assessment.

Why Rimnetics for Large Plastic Parts Manufacturing

Rimnetics is a single-source RIM manufacturing partner with the process expertise, tooling capability, and production infrastructure to handle large, complex structural parts.

01.

40+ Years of RIM Expertise

Deep process knowledge applied to the most demanding large part geometries

02.

Large Format Molding

Panels and parts up to 8' x 8' x 3' in a single shot

03.

Aluminum Tooling, Unlimited Shots

Lower upfront investment, faster lead times, and the same tool runs prototypes and production

04.

In-House Tooling Changes

Adjustments are made quickly without vendor handoffs

05.

Single-Source Manufacturing

Full-service design review, tooling changes, molding, finishing, painting, and assembly

06.

ISO 9001:2015 Registered, ITAR Compliant

Built for regulated industries where large part quality and traceability are non-negotiable

07.

24-72 Hour CAD Review

Submit your file and receive initial engineering feedback within one to three business days

FAQ

Large Parts Manufacturing FAQs

What is large part traditional injection molding?+

Large part injection molding produces oversized plastic components using high-tonnage presses and hardened steel tooling to withstand the pressures required to fill large mold cavities. While effective at very high volumes, the injection molding tooling investment and design constraints make it impractical for complex geometries; geometries needing variable wall sections within the same part or low-to-mid annual volume programs. Reaction injection molding (RIM) is a more cost-effective alternative at those scales.

How does RIM differ from large part injection molding?+

Conventional injection molding melts thermoplastic pellets and injects them at high pressure—often exceeding 10,000 psi—requiring hardened steel tooling and massive clamping force. RIM combines two liquid polyurethane reactants at low pressure—typically below 100 psi—allowing the material to fill large, complex mold geometries using aluminum tooling at a fraction of the cost. That pressure difference is what makes RIM a better fit for complex large parts.

What are the size limits for RIM large parts?+

Rimnetics produces panels and parts up to 8 feet by 8 feet by 3 feet in a single shot, without the press tonnage constraints of conventional injection molding. Submit your CAD file for an initial feasibility review to confirm whether your part dimensions are within scope.

Is large part thermoforming better than RIM for large parts?+

Large part thermoforming is well-suited for simple, single-surface shapes but cannot produce parts with complex 3D geometry, variable wall thickness, or integrated structural features. RIM produces self-supporting structural parts with complex geometry in a single shot, without a secondary metal frame. For parts that need to do more than serve as a cosmetic shell, RIM is typically the stronger choice. In thermoforming, the process often requires stretch plastic sheets at corners and various features of size locations which leads to variable wall sections, a constraint on quality and mismatches when assembling different components together.

What production volumes are required for RIM large parts?+

RIM tooling economics are best suited for programs running approximately 50 to several thousand units per year. Aluminum tooling costs significantly less than the steel tooling required for large part traditional injection molding, making RIM practical at volumes where traditional injection molding becomes cost-prohibitive.

How do I know if my large part is a good fit for RIM?+

Submit your CAD file and the engineering team will review it for technical and economic feasibility, with initial feedback typically within 24-72 hours. There is no commitment required.