Views: 6 Author: Site Editor Publish Time: 2026-08-19 Origin: Site
Some parts are too big, too thin-walled or too low-volume to justify a metal injection mold. Reaction injection molding fills that gap: two liquid components mix and cure in a soft tool, producing large, lightweight enclosures without the cost of hardened steel. This page covers where RIM fits, with polyurethane-system data, size limits and tooling numbers.
Table of Contents
RIM mixes two liquid reactive components (typically polyurethane systems) and injects them into a soft tool - often epoxy or aluminum - where they cure. Because the material is low-viscosity and cures at low pressure, the tool can be simple and cheap. The contrast with thermoplastic injection is detailed in our injection molding guide.
RIM wins on large parts where a steel mold would be expensive and slow. Think housings over 300 mm, low-volume automotive interior panels, and medical equipment shells. Below those sizes, conventional molding usually costs less per part; the tooling trade-off is in our rapid tooling guide.
The material system sets the part's feel and performance. Values below are typical for commercial RIM polyurethane families; exact numbers vary by formulation and filler.
PU system | Hardness | Density (g/cm3) | Typical use |
Rigid | 60-75 Shore D | ~1.05-1.15 | Structural shells, covers |
Semi-rigid | 50-65 Shore D | ~1.05-1.12 | Enclosures, panels |
Soft-touch | 60-90 Shore A | ~1.10-1.20 | Gaskets, overmold feel |
Glass-filled | 70-80 Shore D | ~1.20-1.35 | Stiffness, dimensional stability |
The full polymer selection - including the polyurethane family used here - is in our materials guide. Fillers add stiffness and flame retardancy or a painted-skin look.
RIM handles large, thin-walled geometries that would warp or sink under injection pressure. Typical wall thickness runs 2-5 mm; very large panels can go thinner. Because cure is low-pressure, designers get more freedom on ribbing and bosses than a steel mold allows. We regularly see enclosures of 400-1,200 mm on a side that would be uneconomic to injection-mold at low volume.
A soft RIM tool can be ready in about 1-2 weeks and typically yields around 50-100 parts before replacement (an aluminum RIM tool lasts longer, ~200-500). Cycle time per part is short - often 1-3 days including cure. For metal-look alternatives on smaller runs, see our metal prototyping page. Cost levers are in our cost and quoting guide.
RIM is not structural metal - it is a lightweight polymer part. Against vacuum casting, RIM scales to larger parts and longer runs for less per copy. Against machining, RIM wins on size and unit cost but loses on tight tolerance. Match the route to the test the part must pass: a 600 mm lightweight cover is RIM territory; a load-bearing bracket is machined metal (see metal prototyping).
Send the model, overall size, target hardness and quantity. The cost levers - tool, material system and volume - are spelled out in our cost and quoting guide.
Q: What size parts suit RIM?
A: Large, lightweight enclosures - typically over 300 mm - where a steel injection mold would be costly. Wall thickness usually runs 2-5 mm.
Q: How many parts can one RIM tool make?
A: Around 50-100 parts for a soft epoxy tool before replacement; an aluminum RIM tool runs 200-500.
Q: Is RIM the same as injection molding?
A: No. RIM uses liquid polyurethane in a soft, low-pressure tool; injection molding uses melted thermoplastic in a metal mold at high pressure for higher volumes.
Q: What hardness can RIM parts have?
A: From rigid (60-75 Shore D) to soft-touch (60-90 Shore A), with glass fillers for stiffness.
Q: When should I pick RIM over CNC metal?
A: When the part is large and lightweight and does not need true metal strength. For load-bearing or thermal demands, machined metal remains the right call.
CTA: Send your large-part design for a RIM quote with system data request an RIM quote.
