POM (Acetal) for Injection Molding — How Do You Design Gears, Bearings and Mechanical Parts in POM, and What Are the Key Processing Requirements?

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

POM (acetal, Delrin) is a semi-crystalline engineering resin for gears, bearings and snap fits. It has the highest rigidity, the lowest friction (0.2–0.35 against steel) and the most predictable shrinkage (1.8–2.2%) of any unfilled thermoplastic — but you must design for uniform wall, 1.0° minimum draft and a hard melt-temperature ceiling.
See also related article.

Key facts

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  • POM is the stiffest unfilled thermoplastic (flexural modulus 2,500–3,000 MPa) with the lowest friction and, unlike PA, it does not absorb moisture and swell — so its dimensions stay put.
  • Two grades: homopolymer (Delrin) for maximum rigidity, strength and fatigue; copolymer for chemical resistance and a wider processing window.
  • Design for a uniform wall of 1.0–3.0 mm, draft of at least 1.0° and the 2% shrinkage; melt at 190–220 °C and never above 240 °C, where it releases formaldehyde.

    • Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. See also related guide.

    What are the key POM properties — and how do they compare?

    POM is chosen for mechanical parts on four properties. The table sets homopolymer and copolymer against glass-filled PA66.

    Property POM homopolymer POM copolymer PA66 (GF30)
    Tensile strength 65–70 MPa 60–65 MPa 180–200 MPa
    Flexural modulus 2,800–3,000 MPa 2,500–2,700 MPa 9,000–12,000 MPa
    Impact (Izod, notched) 60–80 J/m 55–70 J/m 100–130 J/m
    Friction (vs. steel) 0.2–0.35 0.2–0.35 0.4–0.6
    Shrinkage 1.8–2.2% 1.8–2.2% 0.3–0.8%
    Continuous use temp 85–100 °C 85–100 °C 120–150 °C
    Chemical resistance Poor (acids, oxidizers) Good Good (oils, solvents)

    The combination is what matters: POM does not flex under load, slides with minimal wear, holds its dimensions across humidity swings, and resists fatigue from repeated loading. Its two limits are chemical resistance — homopolymer is attacked by strong acids and oxidizers — and temperature, since it loses rigidity and deforms above 100 °C.

    What are the design rules for POM gears and bearings?

    Four rules turn POM’s properties into a part that runs. Each one exists because POM shrinks a lot and is stiff.

    1. Gears — module, pressure angle, backlash. Use module 0.5–3.0, a 20° standard pressure angle and an involute tooth profile. Hold backlash at 0.05–0.15 mm for a module 1.0 gear and build in the 2% shrinkage — the bore shrinks onto the core pin. A POM gear carries 30–50% of a same-size steel gear’s torque, but it is lighter, quieter and runs dry.
    2. Bearings — clearance and PV limit. Design a radial clearance of 0.05–0.15 mm between shaft and bore. The 0.2–0.35 sliding friction generates heat, so keep pressure × velocity under the POM PV limit of 0.3–0.5 MPa·m/s; above it the bearing overheats, wears fast or seizes. Add bearing area, slow the shaft, or add PTFE or silicone lubricant.
    3. Wall — 1.0–3.0 mm, uniform. Keep variation within 25% of the nominal wall, because a thick section shrinks more than a thin one and the part warps. Minimum 0.6 mm (small parts) to 1.0 mm (large); above 3.0 mm you get sink and internal voids. Reinforce thin sections with ribs (1.0 mm thick, up to 3× wall height) rather than thickening the wall.
    4. Draft — 1.0° minimum, 1.5° preferred. Apply it to every cavity surface, ribs, bosses and snap fits included. POM is rigid and shrinks onto the male core, so it grips that side — locate the ejector pins on the core and size the ejection force to overcome the shrink grip.

    The hard ceiling is temperature. Run the melt at 190–220 °C (homopolymer) or 180–210 °C (copolymer) and never above 240 °C — past that POM degrades and releases formaldehyde, a flammable, toxic gas that also shows up as burn marks on the part. Mold at 60–100 °C for crystallinity and dimensional stability, and keep residence time short.

    “POM forgives very little on the way in — a thick section, a short draft or 20 °C too much melt temperature all show up on the part. In the DFM we check wall uniformity, gear backlash and bore shrinkage before we cut, and we set the process window with a margin under 240 °C, so a metal-to-plastic conversion runs dry, quiet and dimensionally stable.”

    — Project Engineer, aCatMould

    Frequently asked questions

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • What are the processing requirements — and the common defects?

    POM absorbs little moisture (below 0.25% as received), so dry it only if it has sat open more than 24 hours — then 80 °C for 2–4 hours.
    Melt at 190–220 °C, mold at 60–100 °C, and inject at medium-to-high speed because POM crystallizes fast and must fill before it freezes.

    Burn marks come from formaldehyde when the melt is too hot or residence too long — drop both. Sink comes from a non-uniform wall or low packing — even out the wall, raise packing. Warpage comes from uneven mold temperature or short cooling — lengthen cooling and balance the mold temperature.

    Can POM be welded or bonded — and which adhesives work?

    POM is hard to bond because of its low surface energy (36–38 dyne/cm) and chemical resistance. The most reliable joint is ultrasonic welding, at 60–80% of bulk strength; hot-plate welding (250–300 °C) suits large parts where the plate can be shaped to the joint.

    Adhesives need surface treatment first (corona or plasma): cyanoacrylate reaches 5–10 MPa, epoxy 3–5 MPa. Skip solvent welding — the solvents that attack POM (formic and trifluoroacetic acid) damage the part surface.

    Homopolymer (Delrin) or copolymer — which should you use?

    Homopolymer (Delrin, DuPont) is stiffer (2,800–3,000 vs 2,500–2,700 MPa), stronger (65–70 vs 60–65 MPa) and more fatigue-resistant — the choice for gears, bearings and any part that needs maximum rigidity.

    Copolymer resists the acids and oxidizers that attack homopolymer, processes in a wider window and holds up better thermally over time — the choice for fuel and chemical contact, or tight dimensional control. Maximum rigidity without chemical exposure points to homopolymer; chemical resistance or easier processing points to copolymer.

    Key takeaways

    1. POM wins on rigidity, low friction (0.2–0.35 vs steel) and moisture-free dimensional stability — the first pick for metal-to-plastic gears, bearings, bushings and snap fits.
    2. Design for a uniform 1.0–3.0 mm wall, 1.0° minimum draft, gear backlash 0.05–0.15 mm and bearing PV under 0.3–0.5 MPa·m/s, all accounting for 2% shrinkage.
    3. Homopolymer for maximum rigidity, copolymer for chemical resistance; melt at 190–220 °C, mold at 60–100 °C, never above 240 °C.

    POM is the first resin to consider when a metal gear, bearing or roller has to run quieter, lighter and without lubrication. The design rules — uniform wall, enough draft, correct backlash and clearance, and a melt window kept under 240 °C — are what turn those properties into a part that actually works. If you are converting a metal component to plastic, a DFM review confirms the geometry suits POM before the mold is cut.

    Sources
    DuPont Delrin — POM homopolymer properties and design guidance
    MatWeb — acetal (POM) material property data
    BASF — Ultraform POM copolymer and engineering plastics
    Plastics Technology — POM processing and molding practice
    Society of Plastics Engineers — plastic gear and bearing design

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