Injection Molding vs. Reaction Injection Molding (RIM) — Which Process Fits Your Large Part, and How Do the Cost and Performance Differ?

Last updated: 2026-09-03 · Reading time: about 6 min

Reaction Injection Molding mixes two liquid urethane components and cures them at low pressure; injection molding melts thermoplastic and packs it into steel at high pressure. Choose RIM for large, low-volume parts; choose injection molding for tight tolerances, high volume and a wide resin selection.
See also related article.

Key facts

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  • A RIM mold costs 60–70% less than a steel injection mold for the same large part — aluminum, low pressure, no big clamp — but the per-part cost runs 2–3× higher on a slow cure cycle and costly urethane.
  • Injection molding holds ±0.1 mm and cycles in 30–120 s; RIM holds ±0.3–0.5 mm and cures in 3–10 min, so the two cross over on volume.
  • Break-even sits at 8,000–12,000 parts/year for a typical 1 m part — below it RIM wins, above it injection molding wins.

    • Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. See also related guide.

    How do the two processes differ — and what are the key parameters?

    The gap starts with chemistry and pressure, then runs through mold material, cycle time, part size, tolerance and cost.

    Parameter Injection molding Reaction Injection Molding (RIM)
    Material Thermoplastic resins (PP, ABS, PC, PA, POM) Two-component liquid (polyol + isocyanate → polyurethane)
    Injection pressure 500–2,000 bar 2–10 bar
    Mold material Hardened steel (P20, H13, S136) Aluminum or mild steel
    Cycle time 30–120 s 3–10 min (cure)
    Part size limit Up to ~1.5 m (clamp force) Up to 2 m × 1.5 m (no clamp limit)
    Tolerance ±0.1 mm ±0.3–0.5 mm
    Mold cost (1 m part) $80,000–200,000 $20,000–60,000
    Per-part cost (10,000/yr) $5–20 $15–50

    The trade-off is mold cost against per-part cost. Injection molding buys an expensive steel tool but runs a fast cycle on inexpensive resin; RIM buys an aluminum tool that costs far less but cures slowly on costly urethane. For a 1 m part the two lines cross at 8,000–12,000 parts/year.

    When should you choose RIM — and when injection molding?

    Four rules settle most decisions. Size, volume, tolerance and material drive the call.

  • Large parts at low volume → RIM. Above 1 m × 0.8 m and below 10,000 parts/year, the 60–70% mold-cost saving outweighs the higher per-part cost. Typical: bumpers, fenders, medical-equipment enclosures, agricultural-vehicle covers.
  • Low residual stress and big-part impact → RIM. RIM cures without high pressure or shear, so residual stress is 10–20% of an injection molded part — less warp, better impact at large size, no stress cracking under chemical exposure.
  • Tight tolerance or high volume → injection molding. Below ±0.2 mm or above 20,000 parts/year, predictable thermoplastic shrinkage and a dimensionally stable steel mold win, and the 30–120 s cycle drops the per-part cost.
  • Wide material selection → injection molding. RIM is limited to polyurethane; injection molding runs hundreds of resins (PP, ABS, PC, PA, POM, PBT, PPS, PEEK). For heat above 120 °C, solvent resistance or UL94 V-0 flammability, choose injection molding.

    • “On a large cover or enclosure we run both numbers before we recommend a process — RIM tooling against injection tooling, per-part cost at your real annual volume, and the break-even in between. You get the two side by side on the quote, so the decision is arithmetic, not a sales pitch.”

    — Project Engineer, aCatMould

    Frequently asked questions

    Can RIM match the surface finish of injection molding?

    Not as molded. RIM reaches a Class B surface (3–5 μm Ra) with a slight orange-peel texture, because the urethane cures at low pressure and does not replicate the tool the way a high-pressure melt does. Injection molding reaches Class A (0.5–1.0 μm Ra).

    Painting or gel coating closes the gap — the coat fills the texture for $2–8 per part. On a painted bumper or enclosure the appearance is equivalent; for an unpainted Class A optical or gloss surface, injection molding is the choice.

    What are RIM’s material limits — and can the parts be recycled?

    RIM polyurethane is a thermoset — it cures and cannot be remelted, so it cannot be recycled like a thermoplastic. Scrap is ground into filler for low-grade uses (carpet underlay, playground surfaces), and parts cannot be welded or heat-staked, only bonded or fastened.

    PU has good impact and resists acids, bases and oils, but yellows under UV unless coated and softens above 80–120 °C. For higher heat or aggressive solvents, injection molding with PPS or PEEK is the route.

    What does the total cost look like on a typical large part?

    Take a 1 m × 0.6 m bumper (5 kg). RIM: a $30,000–50,000 aluminum tool and $25–40 per part. Injection molding: a $100,000–180,000 steel tool and $8–15 per part.

    At 5,000 parts/year RIM totals about $180,000 against $200,000 for injection molding; at 20,000/year it flips to $630,000 versus $350,000. The break-even lands at 8,000–12,000 parts/year.

    Key takeaways

    1. RIM fits large parts (>1 m) at low volume (<10K/year) — 60–70% lower mold cost and low residual stress.
    2. Injection molding fits tight tolerance (±0.1 mm), high volume (>20K/year) and a wide resin selection.
    3. Break-even is 8,000–12,000 parts/year — below it RIM is more economical, above it injection molding is.

    The call comes down to part size, annual volume and performance. For a large, low-volume cover RIM saves on tooling and holds impact; for high volume or tight tolerance injection molding wins. If you are unsure, a DFM review compares both against your exact geometry, material and volume.

    Sources
    Plastics Technology — RIM versus injection molding process comparison
    Society of Plastics Engineers — reaction injection molding fundamentals
    BASF — polyurethane systems for RIM processing
    MatWeb — polyurethane material property data
    Plastics Industry Association — process selection guidance

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet