Overview
POM (polyoxymethylene, commonly called acetal) is the engineering plastic that comes closest to behaving like a soft metal. High stiffness, low friction, predictable dimensional behavior, and the cleanest machining characteristics of any commercial thermoplastic. When a designer needs a plastic gear, bushing, fluid fitting, or precision mechanical part — POM is the default specification.
The biggest single design decision with POM is homopolymer (POM-H, Delrin) versus copolymer (POM-C, Hostaform/Acetron). They look and machine similarly, with property differences typically within 15%. But they fail differently:
- POM-H has slightly higher strength, stiffness, and creep resistance. It also has tendency toward centerline porosity in thick extruded sections and degrades in hot water and chlorinated environments.
- POM-C has slightly lower mechanicals but dramatically better long-term resistance to hot water, hot caustic, and thermal cycling. Cleaner porosity in stock shapes.
Default to POM-C for general use unless the application specifically benefits from POM-H’s higher stiffness AND won’t see hot water or strong base exposure.
Machining notes
POM machines like a dream — short clean chips, low gumming, holds tight tolerances without the surface smear or built-up edge issues of softer plastics. Many designers benchmark plastic machinability against POM.
Practical recipe for turning POM bar:
- Sharp carbide or polished HSS, positive rake
- Speed: 500–1500 SFM
- Feed: 0.005–0.015 in/rev
- Coolant: optional for light cuts, recommended for heavy
- Polished cutting edges critical for finish
The one thing to watch is heat-induced dimensional drift. POM has high CTE (~110 µm/m·°C, 2× aluminum) and a relatively low maximum service temperature. Uneven heating during heavy machining can warp parts after they cool. For tight-tolerance work on thick sections, either keep cuts shallow with coolant or stress-relieve anneal at 120–160°C for 1 hour per inch of section thickness after roughing.
POM does not bond well. Solvent welding doesn’t work — POM is too chemically inert. Adhesives require surface activation (plasma, flame, or chromic acid etch). Most assembled POM parts use mechanical fasteners, threaded inserts, or snap-fits — POM’s stiffness and fatigue behavior make it one of the best plastics for living hinges and snap features.
Variant selection guidance
- POM-H (Delrin) — pick when stiffness, creep resistance, or thin-section porosity matters and the part won’t see hot water or strong base. The classic “Delrin gear” application.
- POM-C (Hostaform/Acetron/TECAFORM AH) — the default. Pick for general engineering use, food-contact applications, automotive fuel systems, and any environment with thermal cycling or moisture.
- GF20 (Delrin 570) — pick when stiffness is the priority and impact resistance is acceptable. Structural gears under load.
- HPV13 / Delrin AF — pick for self-lubricating bearings and bushings. Delrin AF DE588 if you need the US Naval Sea Systems Command spec.
- FG metal-detectable — pick for food and pharma where fragments must be detectable in product streams.
Failure modes worth designing around
Acid attack is the biggest field failure mode. POM degrades in acidic environments through chain scission of the C-O backbone. Mild acids at room temperature are OK; sustained or hot acid contact will embrittle and crack POM parts. If acid exposure is even a possibility, PVDF, PP, or PEEK are better choices.
Hot-water embrittlement of POM-H is the second common failure. Delrin in continuous hot water (>60°C) or chlorinated water (pool, potable treatment) develops surface crazing and loses elongation over months to years. POM-C is dramatically better in this service.
Thermal decomposition during processing or fire produces formaldehyde, which is toxic and irritating. Stay within processing-temperature windows in injection molding (195–215°C melt), and never specify POM for fire-rated applications — it cannot achieve UL94 V-0.
Mold shrinkage is high (2.0–2.5%) and uneven on filled grades. Design with this in mind on tight-tolerance molded parts; machining from stock avoids the issue but the stock itself can have residual quench stresses (especially in thick extruded bar). Stress-relief anneal on critical parts.
Bonding doesn’t work without surface activation. Designers who assume “I’ll just glue it on” find out the hard way. Plan for mechanical fasteners from the start, or budget time for plasma/etch preparation.
Applications by industry
- General mechanical — gears, bushings, sprockets, wear pads, thrust washers. The default plastic for precision moving parts.
- Food and beverage — conveyor components, valve bodies, fittings, metering parts. POM-C with FDA grades dominates this space; metal- detectable variants are standard for parts that might fragment.
- Automotive — fuel system components (POM-C is OEM-standard for fuel rails, pumps, sender modules), seatbelt mechanisms, latches, HVAC valves.
- Plumbing and fluid handling — manifolds, fittings, pump impellers for non-acid fluids. POM-C for hot-water systems.
- Consumer products — zippers (widely used alongside PA — POM dominates the heavy-duty and outdoor segments), fasteners, locks, snap-fit hardware, lighter mechanisms.
- Industrial automation — guides, wear strips, sprockets, cam followers in conveyor and packaging machinery.
- Medical (non-implant) — surgical instrument handles, drug-delivery device components, valve bodies. Not USP Class VI — for implant use, PEEK is the go-to.