POM (polyoxymethylene, also marketed as acetal homopolymer and acetal copolymer) is one of the highest-stiffness unreinforced thermoplastics in the engineering catalog, with a tensile modulus typically quoted near 2.8–3.2 GPa and a melting point in the 165–175 °C window depending on grade [S1].
Forged, cast, and injection-molded POM parts — gears, bushings, valve seats, conveyor wear strips, pressure transmitter sensor housings, and flow meter rotor bodies — are specified wherever low friction, dimensional stability, and moisture resistance matter more than peak temperature or UV durability [S2].
CORE PROPERTIES THAT DRIVE POM SELECTION
Unfilled POM grades carry a tensile strength around 60–70 MPa and a Rockwell hardness of M78–M90, which puts the material in the same stiffness bracket as some cast zinc and aluminum alloys at roughly one-fifth the density (≈ 1.41–1.42 g/cm³) [S1].
The continuous service temperature ceiling sits near 100 °C for homopolymer and a few degrees lower for copolymer, with a heat-deflection temperature under 1.8 MPa load near 110 °C for the homopolymer and 100 °C for the copolymer — the typical cap that keeps POM out of under-hood automotive and boiler-feed service [S1].
Coefficient of friction against steel lands near 0.2–0.4 unlubricated, with a wear factor commonly quoted in the 1–5 × 10⁻⁴ mm³/N·m range, which is why POM still dominates small-format gear and bushing geometry even against newer tribological PBT and PPS grades [S1].
Moisture absorption after 24 h immersion is around 0.2–0.25 %, and dimensional change stays below 0.2 % at saturation, a level of stability that allows tight-tolerance snap-fit and gear-tooth geometry without secondary annealing [S1].
WHERE POM BEATS PA66, PBT, AND PPS
Against PA66 (nylon 66), POM wins on moisture sensitivity: PA66 absorbs 2.5–3.0 % water at saturation and swells 0.6–1.0 %, so a gear housing that is dimensionally stable in POM can drift measurably in PA66 unless the design is over-constrained or conditioned [S1].
Against PBT, POM has a roughly 20–30 % higher tensile modulus and a continuous service temperature about 10–20 °C higher, which is the reason fuel-system quick-connectors and HVAC industrial valve seats lean POM rather than PBT in many assemblies [S1].
Against PPS, POM loses on temperature (PPS holds 200–220 °C continuous) but wins on cost and processability — injection-mold tool wear with PPS glass-filled compounds is materially higher than with unfilled POM, and the per-kilogram resin spread often reaches 3–5× [S1].
That cost-versus-temperature trade-off is also why a pressure sensor diaphragm cover, snap-fit housing, or low-load gear train still defaults to POM, while a high-temperature pump body steps up to PPS or PEEK.
MECHANICAL LIMITATIONS AND FAILURE MODES
POM is notch-sensitive: a sharp internal corner drops impact strength by 30–50 % versus a moulded-in radius, which is why impact-modified POM grades add elastomer at 5–15 % loading and why gear-tooth root radii are specified at 0.3–0.5× module rather than sharp corners [S1].
Coefficient of thermal expansion is near 8–10 × 10⁻⁵ /°C, roughly 5–10× aluminum, so bimetal assemblies (POM bushing in steel housing) need a controlled clearance band of 0.1–0.3 % to prevent binding at the 80–100 °C operating ceiling [S1].
Creep under sustained load is non-trivial: at 20 °C and 10 MPa, unreinforced POM continues to deform measurably beyond 1,000 h, so designers derate long-term static load to roughly 30–35 % of short-term tensile strength for continuous service [S1].
Fatigue endurance at 10⁷ cycles sits near 30–35 MPa for homopolymer, which is acceptable for gear and cam duty but lower than the 50–60 MPa band typical of cast aluminum A380 in the same loading envelope [S1].
CHEMICAL, UV, AND FLUID RESISTANCE GAPS
POM has poor resistance to strong acids (pH < 2) and strong bases (pH > 12), and continuous exposure to hot water above 60 °C causes oxidative degradation that the industry calls "popcorn" failure — internal voiding that can rupture thick sections during or shortly after moulding [S1].
UV resistance is the second well-known gap: unprotected POM yellows and loses 50 % or more of its impact strength after roughly 6–12 months of direct outdoor exposure, so exterior service requires a UV-stabilized grade with carbon black or a paint/coating system [S1].
Flammability is rated HB on the UL 94 scale for unfilled grades, which rules POM out of many appliance and transit interiors where V-0 or V-2 is mandated — a recurring reason designers substitute PBT, PA66, or FR-grade PPS instead [S1].
Fuel and solvent resistance is good against gasoline, diesel, and most alcohols, but POM swells in ketones, esters, and chlorinated solvents, so any chemical-processing PLC cabinet or solenoid body exposed to those media needs a different polymer or a metal insert [S1].
PROCESSING AND DESIGN RULES
Melt temperature is processed in the 190–230 °C window, with mould temperatures near 80–110 °C to control crystallinity; mould shrinkage falls in the 1.5–2.5 % band and must be accounted for before cutting gear teeth, since post-mould shrinkage is essentially complete within 24 h of cooling [S1].
Drying is mandatory — POM hydrolyses rapidly above the melt point if moisture exceeds roughly 0.1 %, so hopper dryers at 80–100 °C for 2–4 h are standard practice in any shop running unfilled or glass-filled grades [S1].
Warpage control on long, thin parts benefits from symmetric wall sections and a fillet radius of 0.5–0.75× wall thickness; thin sections below 0.8 mm tend to freeze off before packing, so a servo-driven servo motor injection profile with a hold-pressure switchover at 95–98 % cavity fill is the usual workaround [S1].
WHEN TO PICK POM AND WHEN TO WALK AWAY
Pick POM when the job is an unlubricated gear, bushing, cam, conveyor slide, valve seat, or sensor housing in a dry, indoor, chemically mild environment operating below 90–100 °C, and the priority is dimensional stability plus low friction without paying PPS or PEEK pricing [S1].
Walk away from POM for any outdoor sun-exposed part without UV-stabilized grade, any hot-water service above 60 °C continuous, any strong acid or base wetted component, any flame-retardant requirement above UL 94 HB, and any thin-wall load-bearing part below –20 °C where impact falls off the cliff [S1].
For a deeper cost-of-ownership comparison against fluoropolymers in long-life chemical service, the PTFE Total Cost of Ownership breakdown is a useful parallel read because PTFE and POM sit at opposite ends of the chemistry and cost spectrum. For other polymer-versus-metal trade-off maps in the same engineering series, the Rebar Threading Machine: Pros, Cons, and Fit Map and Retaining Ring Sizing and Selection articles use a similar criteria-based structure that translates cleanly to POM selection.