Last updated: 2026-09-01 · 5 min read

Nylon vs Acetal for Molded Gears: Which Resin Fits Your Torque and Environment?

Nylon (PA6 and PA66) wins when the gear needs high tensile strength, fatigue resistance, and can tolerate moisture absorption. Acetal (POM homopolymer and copolymer) wins when the gear needs low friction, dimensional stability in humid environments, and resistance to creep under continuous load.

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.

Key Facts

  • PA66 absorbs 2.5–3.0 % moisture at equilibrium (50 % RH, 23 °C), which increases impact strength by 200–300 % but reduces tensile modulus by 30–40 % and grows the part dimensionally by 0.5–1.0 %.
  • POM absorbs less than 0.25 % moisture and maintains dimensional stability regardless of humidity — the gear tooth profile does not change between dry and conditioned states.
  • PA66 GF30 has a tensile strength of 180–200 MPa; unfilled POM homopolymer has 70 MPa. The glass-filled nylon is roughly 2.5× stronger — but the filled resin wears the mold cavity faster and costs 20–30 % more per kilogram.

    • 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. The related guide covers adjacent topics.

    The Property Comparison That Matters for Gears

    Property PA66 (dry) PA66 (conditioned) POM Homo
    Tensile strength (MPa) 82 55–60 70
    Flexural modulus (GPa) 2.9 1.6–2.0 2.8
    Coefficient of friction 0.30–0.40 0.30–0.40 0.15–0.25
    Moisture absorption (23 °C / 50 % RH) 2.5–3.0 % 2.5–3.0 % 0.20–0.25 %
    HDT at 1.8 MPa (°C) 75–85 75–85 110–125
    Mold shrinkage (%) 1.0–1.8 1.0–1.8 1.8–2.2

    When Nylon Is the Right Choice

    Nylon is the default for high-load, high-fatigue gears. Automotive window-lift gears, power-seat adjustment gears, and power-tool planetary gears are almost always PA66 or PA66 GF30 because the torque and cyclic loading exceed what unfilled POM can handle. The fatigue endurance limit of PA66 (approximately 30–35 % of tensile strength at 10⁶ cycles) is higher than POM’s (approximately 25–30 %), which means nylon gears survive more start-stop cycles before tooth-root cracking.

    The condition-dependent properties of nylon are a design parameter, not a defect. A gear that operates in a sealed gearbox at controlled humidity performs differently from one exposed to outdoor moisture. The DFM must state the moisture condition (dry-as-molded or conditioned) for which the gear tooth stress was calculated — and the mold dimensions must account for the 0.5–1.0 % dimensional growth from moisture absorption.

    When POM Is the Right Choice

  • POM wins for low-torque, low-noise gears in consumer products: printer feed mechanisms, small appliance timers, HVAC damper actuators. The lower coefficient of friction reduces gear mesh noise and eliminates the need for external lubrication.
  • POM wins when dimensional stability matters across humidity swings.
    A gear in an outdoor-installed device that cannot afford to change backlash with the seasons needs POM’s moisture-independent dimensions.
  • POM wins when the gear operates against a metal mating gear at moderate loads. POM’s low friction and self-lubricating behavior reduce wear on both the plastic gear and the metal pinion.
  • POM loses when the gear sees high torque, high temperature (above 90 °C continuous), or high fatigue cycling.
    In those conditions, PA66 GF30 is the reliable choice — and the higher material cost is justified by the longer service life.

    • One design note: POM homopolymer (Delrin) is stronger and stiffer than POM copolymer, but the copolymer has better long-term thermal stability and lower formaldehyde outgassing. For gears in enclosed spaces (automotive interiors, medical devices), the copolymer is specified to avoid outgassing concerns.
    The strength difference is 5–10 % — enough to matter on the edge of the design envelope.

    Frequently Asked Questions

    Can POM and nylon gears mesh together?

    Yes, and it is common in multi-stage gear trains. The POM gear (lower friction, lower wear rate) is typically the smaller pinion, and the nylon gear (higher strength, higher fatigue resistance) is the larger driven gear. The different shrinkage rates must be accounted for in the center-distance calculation.

    Does glass-filled POM exist for gears?

    Yes, POM GF20 and GF25 exist, but they are less common than PA66 GF30 for gears. Glass-filled POM increases stiffness and HDT but reduces the self-lubricating advantage and increases mold wear. For gears that need both POM’s low friction and higher stiffness, PTFE-filled POM is a more common upgrade.

    How do I account for moisture-conditioned properties in the gear design?

    Use the conditioned data sheet values (50 % RH equilibrium) for the stress calculation, not the dry-as-molded values. The dry values overstate the gear’s strength in service by 30–50 % for PA66. A gear designed on dry properties will fail prematurely when it reaches moisture equilibrium in the field.

    Key Takeaways

    1. Nylon wins on strength and fatigue; POM wins on friction, dimensional stability, and moisture independence. The environment and the load profile decide which resin fits.
    2. Nylon gear design must use conditioned (not dry) property values — the 30–50 % strength reduction from moisture absorption is a real-world operating condition, not a worst-case scenario.
    3. The shrinkage difference between PA66 (1.0–1.8 %) and POM (1.8–2.2 %) affects cavity sizing and center-distance tolerance — specify the resin before the cavity is cut, not after.

    The gear resin decision drives the cavity dimensions, the mold steel grade (glass-filled nylon wears cavities faster than unfilled POM), and the expected service life. Changing the resin after the mold is built means re-cutting the cavity for different shrinkage — which is why the resin specification belongs in the first DFM review, not in the first article inspection.

    Sources
    DuPont — Delrin (POM)
    BASF — Ultramid (PA66)
    MatWeb — material property data

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