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ResourcesInjection MoldingReaction Injection Molding (RIM): Definition, Importance, How it Works, and Applications
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Reaction Injection Molding (RIM): Definition, Importance, How it Works, and Applications

Xomety X
By Team Xometry
July 29, 2023
 13 min read
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Reaction Injection Molding (RIM) is a manufacturing process applicable to producing complex, lightweight, and durable polyurethane (PU), epoxy, and silicone thermoset solid and foam parts. It involves the simultaneous injection of two or more monomeric and co-reactive liquid components into a mold cavity. These components chemically react and solidify to form the desired part. These parts can be solid but are generally self-skinning foams with low density.:

This article will further discuss the reaction injection molding process, its importance, how it works, its applications, as well as its advantages and disadvantages.

What Is the Reaction Injection Molding (RIM)?

Reaction injection molding (RIM) is the process of mixing two or more liquid (monomer) components, typically a polyol (resin) and an isocyanate (hardener) in a specialized mixing head. The mixed material is then injected into a mold cavity at low pressure. The mold is typically made of two halves but can be made of more sections to allow for undercuts and additional complexity. During and after injection, the liquid components chemically react and undergo an exothermic reaction which may include degassing/foaming. The mixture then solidifies within the mold cavity. The reaction/solidification process progresses relatively quickly, allowing cycle times of 30–60 seconds, commonly.

RIM offers design freedom and flexibility, allowing for the integration of diverse features, such as ribs, bosses, curves, and undercuts, into a single part. This process can also produce lightweight and durable parts with an excellent strength-to-weight ratio. 

How Does Reaction Injection Molding (RIM) Differ From Injection Molding?

RIM and injection molding are seemingly similar manufacturing processes used to produce plastic parts, but they differ in a range of highly significant aspects. RIM typically utilizes reactive liquid polymers, such as polyurethane, epoxy, or silicone, which chemically react in the cavity and solidify to form the final part. Injection molding, on the other hand, uses thermoplastic polymers that melt when heated and solidify upon cooling. Additionally, RIM operates at low pressure and temperature, below 10 MPa and typically 60–120 °C. Injection molding requires high pressures and temperatures, up to several hundred MPa and as high as 400 °C.

RIM molds are usually made of aluminum, non-hardened steel, and even composite materials such as glass-reinforced polyester (GRP). This renders them low-cost and quick to produce. Injection molding is considerably more costly (10–20x typically), as the molds are usually required to be more durable to tolerate considerably higher pressures and temperatures. Injection molding is ideal for high-volume production, with faster cycle times and the ability to produce large quantities of identical parts. RIM is better suited to lower-volume production or prototyping and is cost-effective for smaller production runs.

For more information, see our guide on the Injection Process.

What Is the Importance of Reaction Injection Molding (RIM)?

RIM is a critically important manufacturing process in some fields. It offers several benefits and advantages in the production of plastic parts that are hard to achieve by any other means. The design flexibility that is possible with RIM enables the production of: mixed large and small wall thicknesses, complex and large undercuts, foam core inserts, and very large moldings, relative to other processes. The cross-linked materials suited to RIM generally produce parts with high strength-to-weight ratios. Low pressure/temperature processes allow RIM molds to be lower-cost and fast to build, compared with injection molding. RIM is a critically important process in various industries such as: automotive, aerospace, electronics, medical, and consumer goods. In these areas, RIM is an invaluable process for manufacturers seeking to create innovative, high-quality plastic parts.

How Does Reaction Injection Molding (RIM) Work?

RIM utilizes the simultaneous injection of two or more reactive liquid components into a mold cavity. Typically a polyol resin and an isocyanate catalyst are precisely metered and thoroughly homogenized, initiating a chemical reaction of cross-linking polymerization.

The mixture is then injected into a closed mold cavity at low pressure (up to 10 MPa). The polymerization reaction results in exothermic polymerization and solidification, reproducing the internal spaces of the mold. One of the monomer components can have a dissolved gas constituent that expands out of the solution as the polymerization happens, forming closed-cell foam rather than a solid.

The final solidified part will generally possess excellent strength, flexibility, impact resistance, and dimensional stability (due to the cross-linked polymer matrix). The specific properties of the part can be tailored by adjusting the formulation and mixture ratios of the liquid components. This allows manufacturers to achieve desired material characteristics suitable for the intended application.

How Long Does It Take for the RIM Process To Complete?

Typical cycle times for RIM parts can range from 30 seconds to several minutes. This depends on material properties, section thicknesses, and the overall size of the part. Demolding can occur when polymerization is incomplete, so long as sufficient structural integrity is achieved and/or parts are carefully supported to retain shape and critical dimensions as the cure completes.

What Is the Machine Used for Reaction Injection Molding (RIM)?

The machine used for RIM is typically referred to as a RIM machine or RIM press. This machine is specific to the requirements of the RIM process. It consists of the following:

  1. A hopper or bin system for storing the pre-mixed chemicals.
  2. A metering and mixing system that precisely meters and homogenizes the liquid components in the correct ratio.
  3. An injection system that pumps the mixed liquid into the mold cavity, usually a hydraulic or electrically-driven piston.
  4. A heating system to maintain the proper temperature of the liquid components.
  5. A mold clamping system that aligns the mold halves, secures the mold in place and resists the injection pressure during the injection process. This can be by hydraulic, pneumatic, or mechanical screw.
  6. A PLC or CNC-type control system that monitors and regulates machine parameters such as: volume/ratio of fluids, temperature, pressure, injection time, and mixing ratios. 

Does RIM and Injection Molding (RIM) Use the Same Machine?

No, these two processes have no commonality of equipment or process, despite evident similarities in an overview of the methods. RIM machines are highly specialized equipment designed specifically for the RIM process, not suited to any other task. They are quite different from conventional injection molding machines and more closely related to epoxy resin mix/metering systems for potting applications.

What Are the 2 Types of Reaction Injection Molding (RIM)?

There are two most common variations of the RIM process. These are listed and discussed below:

1. Structural Reaction Injection Molding (SRIM)

Structural Reaction Injection Molding (SRIM) is a variant of RIM that is ideally suited to producing large, structurally reinforced parts with high strength and stiffness. SRIM combines the benefits of RIM, such as design flexibility and cost-effective tooling, with the ability to incorporate structural reinforcement materials.

As is the case for RIM, the liquid components—typically a polyol and an isocyanate—are precisely metered and mixed. However, in SRIM, reinforcing materials such as glass fibers, carbon fibers, or other structural enhancers are introduced into the molding cavity before it is closed and injected. Otherwise, the process steps are essentially identical to RIM.

SRIM offers several advantages for the production of parts. The incorporation of reinforcing materials in SRIM results in parts with enhanced strength, stiffness, and resistance to deformation. SRIM also allows for the creation of lightweight parts by utilizing reinforcing materials that offer higher strength-to-weight ratios.

2. Reinforced Reaction Injection Molding (RRIM)

Reinforced Reaction Injection Molding (RRIM) is another variant of RIM that involves the incorporation of mold-inserted short-strand reinforcing materials such as glass fiber and carbon fiber into the mixing process. This is done to increase the strength, durability, and impact resilience of RIM parts.

In RRIM, the liquid components used in RIM—typically a polyol and an isocyanate—are mixed with reinforcing agents such as glass fibers, mineral fillers, or other reinforcing materials. The reinforcing agents are added to the mixture to improve the mechanical strength and impact resistance of the final part.

The process only differs from RIM in the material preparation. The liquid components—polyol and isocyanate—are accurately measured and mixed, as in RIM. Reinforcing agents are only then introduced into the mixture, to achieve uniform distribution of the reinforcing materials.

RRIM offers enhanced mechanical properties, including increased strength, impact resistance, and stiffness, compared with RIM. The process is used in the same range of industries and applications as RIM and SRIM, mainly when greater component strength, stiffness, or resilience are advantageous.

What Are the Materials Used in Reaction Injection Molding?

The common materials used in RIM are listed below:

1. Polyamides

The use of reactive polymerized precursors that form nylon materials is increasing in RIM applications. Nyrim® is an example of a nylon 6 which is elastomer-modified. It has very low viscosity during processing which results in high-molecular-weight nylon in the cured state. Varying from 7–40% elastomer allows these materials to replace advanced TPEs at lower cost, with higher strength, and with the various advantages of the RIM process.

RIM polyamides offer a beneficial stiffness/toughness balance even at low temperatures, with great fatigue and abrasion properties and good paintability. Most interestingly, RIM polyamides are thermoplastic once molded. They can be introduced into the normal recycling channels, unlike the bulk of RIM-capable materials.

2. Fiber Composites

Fiber composites are commonly used materials in RIM. Pre-formed mats of reinforcer materials such as glass fiber and carbon fiber are pre-positioned into the mold before injection. The low-viscosity precursor materials are then injected into the cavity and completely encapsulate the reinforcement components, integrating them into a composite structure.

3. Polyurethanes

Polyurethanes are among the most commonly used materials in RIM processes, due to excellent general properties and versatility. They offer a wide range of characteristics that make them suitable for various applications in RIM.

Polyurethanes, as a material family, exhibit a broad spectrum of material properties that can be tailored to specific application requirements. They can be formulated to have varying levels of hardness, flexibility, impact resistance, chemical resistance, and thermal stability. Polyurethanes can deliver high-quality surface finishes and aesthetic appearances and are easily colored, painted, or textured to meet design requirements.

Can Silicone Be Used in Reaction Injection Molding?

Yes, silicone can be used in all RIM-derived processes. Silicone RIM, also known as Liquid Silicone Rubber (LSR) injection molding, is a variant of RIM that specifically utilizes silicone-based materials. Silicone RIM offers several advantages and unique properties compared to other materials. These unique properties include: excellent resistance to high and low temperatures, excellent elongation tolerance and flexibility, good electrical insulation, excellent resistance to a wide range of chemicals, and good biocompatibility.

Silicone RIM shares similarities with other RIM processes, but requires specific equipment and processes designed for handling silicone-based materials. Injection pressures are lower than for general RIM processes and tool design requires careful attention to flow and venting, to ensure quality parts.

For more information, see our guide on Silicone Composition.

Can Polyester Be Used in Reaction Injection Molding?

Yes, polyester can be used in various RIM processes. Low-cost polyester-based materials offer certain advantages and properties that make them suitable for specific applications in RIM. Polyester-based materials allow properties to be tailored to meet application requirements. Properties such as strength, impact resistance, chemical resistance, dimensional stability, and heat resistance can be optimized by formulation and additives. These materials offer good durability and resistance to wear, making them suitable for applications in which parts are subjected to mechanical stress, abrasion, and cyclic/repeated loading.

Polyester RIM may require specific equipment and processing parameters to achieve optimal results, but many generic RIM systems can handle the necessary protocols with good results.

For more information, see our guide on the Characteristics of Polyester.

What Are the Typical Industrial Applications of the RIM Process?

RIM manufacturing finds wide and increasing applications in industrial applications. Some examples are listed below:

  1. Automotive: Body kits, body panels, spoilers, instrument panels, and door panels.
  2. Electronics: Enclosures, housings, and structural components.
  3. Marine: Fenders, engine housings, seating, storage bins.
  4. Medical: Equipment enclosures, device housings, and patient comfort components.
  5. Aerospace: Interior and exterior components, panels, and ducts.
  6. Consumer Goods: Furniture, appliances, tool handles, and sporting equipment.

What Are the Examples of Reaction Injection Molding Products?

Some examples of RIM products are listed below:

  1. Dashboards and instrument panels.
  2. Interior trims.
  3. Armrests.
  4. Overhead storage compartments.
  5. Cabin panels.
  6. Orthopedic aids.
  7. Insulation panels.

How Long Do Reaction Injection Molding Products Last?

RIM parts are among the toughest large plastic components and can have very long functional life expectancies. For example, aircraft interiors in commercial planes are commonly kept in service for 5–15 years, depending on the policies of the airline. Generally, such interiors show cosmetic wear and tear but remain serviceable for long periods. Private and light aircraft interiors can also have a much longer service life. Automotive parts made by RIM can be expected to serve for the functional life of the vehicle, generally 7–15 years depending on the market region, but potentially much longer.

Are RIM Products Thick?

It depends. The thickest sections that it is practical to manufacture by RIM are generally reported as ½ inch thick. Although localized thickening and occasional parts are molded to thicknesses approaching 5 inches without major difficulties.

Are RIM Products Durable?

Yes. RIM products are mechanically, chemically, and environmentally durable when the materials are appropriately selected for the expected application and the usage does not exceed reasonable force. 

What Are the Advantages of Reaction Injection Molding?

RIM offers various advantages, including:

  1. High strength and resilience.
  2. The complexity of parts is only limited by fine detail.
  3. Flexibility and rigidity can be specified by material selection and design.
  4. Parts tend to be lower in weight than alternative processes can achieve.
  5. Tooling costs are low compared with, for example, injection molding.
  6. Part costs can be low, as large parts with integrated inserts can serve multiple roles and be made from relatively low-cost materials.
  7. Cosmetic finishes are good to excellent straight from the mold. Most materials offered in RIM-related variant processes accept surface finishing/coating/painting.

What Are the Disadvantages of Reaction Injection Molding?

The disadvantages of RIM are listed below:

  1. The range of material options is limited but growing.
  2. Fine features can be hard to reliably reproduce.
  3. Tools are of low robustness, so they are relatively easy to damage.
  4. Very large parts can produce challenges in fill/cure processes.
  5. Many of the material options produce volatile organic compound (VOC) pollutants.

Is Reaction Injection Molding Expensive?

It depends. There are high costs associated with establishing RIM production, but overall the part and tooling costs are low compared with injection molding. The tools are lightweight and parts can be made of relatively low-cost materials for many applications.


This article presented reaction injection molding, explained what it is, and discussed how it works and its various applications. To learn more about reaction injection molding, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities, including injection molding and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.


The content appearing on this webpage is for informational purposes only. Xometry makes no representation or warranty of any kind, be it expressed or implied, as to the accuracy, completeness, or validity of the information. Any performance parameters, geometric tolerances, specific design features, quality and types of materials, or processes should not be inferred to represent what will be delivered by third-party suppliers or manufacturers through Xometry’s network. Buyers seeking quotes for parts are responsible for defining the specific requirements for those parts. Please refer to our terms and conditions for more information.

Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.

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