---
type: process
name: "Reaction Injection Molding"
category: "Forming"
subcategory: "Plastics and Rubber"
materials: ["Plastic"]
tolerances: "About ±0.020 in (±0.5 mm) on features under 12 in; on large panels expect roughly ±0.1–0.2% of the dimension. Thermal expansion of polyurethane is high, so specify the inspection temperature."
volumes: "250–10,000 parts per year; below that use urethane casting, above roughly 25,000 consider injection molding"
lead_time: "4–10 weeks for cast aluminum or nickel shell tooling; 1–5 minute cycles thereafter, plus trim and paint"
url: https://manufacturingprocesses.org/processes/forming/reaction-injection-molding
---

# Reaction Injection Molding

Reaction injection molding mixes two liquid reactants that polymerize inside a low-pressure mold, producing large tough polyurethane parts on light tooling.

- **Category**: [Forming](https://manufacturingprocesses.org/processes/forming.md)
- **Family**: Plastics and Rubber
- **Materials**: Plastic
- **Typical tolerances**: About ±0.020 in (±0.5 mm) on features under 12 in; on large panels expect roughly ±0.1–0.2% of the dimension. Thermal expansion of polyurethane is high, so specify the inspection temperature.
- **Surface finish**: Integral skin is dense and paintable as molded; textures are reproduced from the tool, and in-mold coating gives a Class A painted surface
- **Typical volumes**: 250–10,000 parts per year; below that use urethane casting, above roughly 25,000 consider injection molding
- **Lead time**: 4–10 weeks for cast aluminum or nickel shell tooling; 1–5 minute cycles thereafter, plus trim and paint

## Overview

Reaction injection molding (RIM) meters two liquid reactants — normally a polyol and an isocyanate — through an impingement mixhead and into a closed mold, where they polymerize into polyurethane in the cavity itself. Because the mold is filled with low-viscosity liquid rather than polymer melt, cavity pressure is a fraction of injection molding's and the tool can be cast aluminum or nickel shell rather than hardened steel.

That single fact defines the process. RIM makes large parts — vehicle fascias and body panels, medical equipment housings, agricultural and construction enclosures — at tooling costs and volumes that sit squarely between urethane casting and injection molding. The practical window is roughly 250–10,000 parts a year.

Chemistry also does something molding cannot: RIM tolerates thick sections and thick-to-thin transitions that would sink or void in an injection molded part.

## How it works

1. **Condition the components.** Polyol (the A side, carrying catalyst, chain extenders, blowing agent, and any filler) and isocyanate (the B side) are held in temperature-controlled day tanks, typically around 100–120 °F (38–50 °C), and continuously recirculated so they stay homogeneous and gas-free.

2. **Meter and impingement mix.** High-pressure pumps drive both streams at roughly 1,500–3,000 psi (100–200 bar) into a self-cleaning mixhead where they collide head-on in a small chamber. There is no mechanical mixer — the jets themselves do the mixing in milliseconds. Ratio accuracy is critical; an off-ratio shot cures to the wrong hardness no matter what the mold does.

3. **Fill at low pressure.** The mixed liquid leaves the mixhead and enters the cavity at low pressure through a runner designed to fill without turbulence. Cavity pressure typically stays under about 100 psi (7 bar), which is why clamp forces and tooling are so much lighter than injection molding.

4. **React in the mold.** The mold is held at roughly 130–160 °F (55–70 °C). Cream time is a few seconds, gel follows, and the exothermic reaction carries the part to demold strength. This is a chemical clock, not a cooling clock — cycle time depends on the formulation, not on wall thickness squared.

5. **Demold and post-cure.** Parts release in roughly 1–5 minutes for typical formulations. An internal mold release is normally compounded into the A side; external release agents supplement it. Structural and high-modulus parts are often post-cured in an oven to complete the reaction and stabilize dimensions before paint.

### What about structural RIM and reinforced RIM?

Blowing agents produce an integral-skin foam with a dense outer surface and a cellular core, giving high stiffness per pound at densities well below solid polyurethane. Reinforced RIM adds milled glass or flake to the A side for higher modulus and lower thermal expansion, at the cost of abrasive wear on the pumps and mixhead.

## Design guidelines

### Wall thickness

The range that makes RIM interesting is 0.100–0.500 in (2.5–12 mm), and unlike injection molding, thick sections are not automatically a defect. Cure is chemically driven, so a 10 mm section next to a 3 mm section is manageable where the same transition in a molded thermoplastic would sink or void. Even so, keep transitions gradual and avoid isolated heavy masses, which run hotter during the exotherm and can scorch.

### Draft

1–3° per side. Polyurethane grips tooling — parts are removed by hand or by air poppets on cast aluminum tools, not by an ejector plate — so err generous, and add draft on textured surfaces as you would for any molded part.

### Radii and ribs

Internal radii of at least 0.125 in (3 mm) on structural corners. Ribs are practical and useful; because the fill is low-viscosity liquid, a rib fills easily. Keep the rib base near 0.6–0.75 × the nominal wall to limit sink in the skin — RIM tolerates a thicker rib than the 0.5–0.6 × wall tabulated for thermoplastics on [/charts/injection-molding-design-guidelines](/charts/injection-molding-design-guidelines), because the skin is compliant and the cure is chemical rather than conduction-limited. Use ribs freely to stiffen large panels rather than adding wall.

### Part size and gating

RIM is one of the few plastics processes that scales comfortably to parts several square meters in area, because the low fill pressure means clamp force scales gently with projected area. Gate at the lowest point and let the cavity fill upward so air is pushed ahead of the front and out through vents at the high points. Trapped air becomes a surface void that is visible after paint.

### Surface and paint

Integral-skin RIM produces a dense, paintable outer skin. Class A automotive surfaces are achievable with in-mold coating, where the paint layer is sprayed into the open cavity before the shot. Expect to specify a primer and a flexible topcoat for exterior body panels.

### Inserts

Metal inserts, threaded bosses, and reinforcing frames can be placed in the cavity and encapsulated. Because fill pressure is low, inserts need far less support against washout than in injection molding — a genuine advantage for large bonded assemblies.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Wall thickness | 0.125–0.250 in (3–6 mm) | 0.100–0.500 in (2.5–12 mm) | Chemistry-driven cure tolerates thick sections |
| Wall transition | Gradual taper | Abrupt 3:1 step | Isolated heavy masses run hot during the exotherm |
| Draft | 2–3° per side | 1° | Polyurethane grips the tool; parts are hand-pulled |
| Internal radius | 0.125 in (3 mm) or more | — | Structural corners and flow-front smoothness |
| Rib base | 0.6–0.75 × wall | — | Low-viscosity fill makes ribs easy; skin still sinks |
| Cavity pressure | Under about 100 psi (7 bar) | — | Sets the low clamp force and cheap tooling |
| Part area | Up to several square meters | — | Clamp force scales gently with projected area |

## Cost drivers

RIM exists because of a tooling arbitrage. Fill pressure under about 100 psi means a cast aluminum or electroformed nickel tool is sufficient, at a small fraction of a hardened steel injection mold for the same large part. Against that, the per-part cost is higher: two-component liquid chemistry costs more per pound than commodity pellets, cycles run minutes rather than seconds, and most parts need trimming, flash removal, and paint.

Volume breakpoints:

- Under 100 parts: urethane casting in silicone molds is cheaper and faster.
- 250–10,000 parts a year: the RIM sweet spot, especially for parts too large for economical injection molding.
- Above roughly 25,000 parts a year for a mid-size part, injection molding's short cycle usually wins despite tooling.

Cost reduction:

1. **Use structural foam where stiffness allows.** Blowing the shot to a lower density cuts chemical cost per part directly and speeds demold.
2. **Rib instead of thickening.** Ribs fill easily in a low-viscosity system, so stiffness is cheap in geometry and expensive in wall.
3. **Consolidate parts.** RIM's size capability means one molding can replace a fabricated assembly of several thermoformed or sheet metal panels plus fasteners.
4. **Specify in-mold coating only where it is seen.** It buys a Class A surface but adds a spray step to every cycle.
5. **Design flash and gate locations for easy trim.** Trimming is manual on most RIM parts and is a real per-part labor line.

## FAQ

### How is reaction injection molding different from injection molding?

RIM fills the mold with two low-viscosity liquids that polymerize in the cavity, rather than with polymer melt. Cavity pressure stays under about 100 psi (7 bar) instead of thousands, so the tool can be cast aluminum rather than hardened steel and parts can be several square meters in area. The trade is a 1–5 minute cycle instead of seconds.

### What volumes make RIM the right choice?

Roughly 250–10,000 parts a year. Below about 100 parts, urethane casting in silicone molds is cheaper and faster; above roughly 25,000 parts a year for a mid-size part, injection molding's short cycle overcomes its tooling cost. RIM occupies the gap, especially for parts too large to injection mold economically.

### How thick can a RIM part be?

Practical walls run 0.100–0.500 in (2.5–12 mm), and thick-to-thin transitions are far better tolerated than in injection molding because cure is driven by chemistry rather than by heat conduction out through the wall. Still keep transitions gradual — isolated heavy masses run hot during the exotherm and can scorch.

### What draft angle does RIM need?

1–3° per side, and lean toward the high end. Polyurethane grips tooling and parts are typically released by hand or with air poppets rather than by an ejector plate, so generous draft matters more than it does on a steel injection mold with a full ejection system.

### Can RIM parts be painted to a Class A finish?

Yes. Integral-skin RIM produces a dense, paintable outer skin, and in-mold coating — spraying the paint layer into the open cavity before the shot — achieves automotive Class A surfaces. Exterior body panels normally still require a primer and a flexible topcoat to survive substrate movement.

### What is structural RIM?

A version using a blowing agent to produce an integral-skin foam: a dense outer skin over a cellular core. It delivers high stiffness per pound at densities well below solid polyurethane, cuts chemical cost per part, and demolds faster. Reinforced RIM instead adds milled glass for higher modulus and lower thermal expansion.

## Alternative processes

- [Thermoforming](https://manufacturingprocesses.org/processes/forming/thermoforming.md): Thermoforming heats a plastic sheet until pliable and pulls or presses it over a single-sided mold, producing large thin-walled parts with low tooling cost.
- [Vacuum Casting (Urethane Casting)](https://manufacturingprocesses.org/processes/forming/vacuum-casting.md): Vacuum casting, known in the US as urethane casting, pours polyurethane resin into a silicone mold under vacuum to copy a master pattern for short runs.
- [Injection Molding](https://manufacturingprocesses.org/processes/forming/injection-molding.md): Injection molding forces molten thermoplastic into a steel mold under high pressure, producing high-volume identical plastic parts.
- [Rotation Molding](https://manufacturingprocesses.org/processes/forming/rotation-molding.md): Rotational molding tumbles powdered polymer inside a heated mold on two axes so it fuses into a seamless, stress-free hollow part with a uniform wall.

## Related processes

- [Vacuum Casting (Urethane Casting)](https://manufacturingprocesses.org/processes/forming/vacuum-casting.md): Vacuum casting, known in the US as urethane casting, pours polyurethane resin into a silicone mold under vacuum to copy a master pattern for short runs.
- [Injection Molding](https://manufacturingprocesses.org/processes/forming/injection-molding.md): Injection molding forces molten thermoplastic into a steel mold under high pressure, producing high-volume identical plastic parts.
- [Rotation Molding](https://manufacturingprocesses.org/processes/forming/rotation-molding.md): Rotational molding tumbles powdered polymer inside a heated mold on two axes so it fuses into a seamless, stress-free hollow part with a uniform wall.
- [Compression Molding](https://manufacturingprocesses.org/processes/forming/compression-molding.md): Compression molding presses a measured charge of rubber or thermoset into an open heated mold that closes to cure it into its final shape.

---

*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/forming/reaction-injection-molding)*

*Last updated: August 11, 2026*