POM (Acetal)

What POM Is and Why It Matters

POM is a highly crystalline thermoplastic polymer formed from repeating oxymethylene units, written chemically as a chain of carbon-oxygen acetal linkages. That regular, tightly packed backbone gives POM a degree of crystallinity around 75 to 85 percent, which is the source of its stiffness, hardness, low creep, and excellent dimensional stability. Among general-purpose engineering plastics, POM occupies the niche where a part must be hard, slippery, fatigue-tolerant, and dimensionally stable, but does not need the heat resistance of PEEK or the toughness of polycarbonate. Procurement engineers most often meet it as machined rod, sheet, and tube, or as injection-molded precision components.

The material has a clear industrial lineage. DuPont commercialized the homopolymer in 1960 under the trade name Delrin, derived from research by Robert MacDonald. Almost in parallel, Celanese introduced the copolymer under the Celcon name, and Hoechst (now part of Celanese) commercialized the European copolymer Hostaform. Those three trade names, Delrin, Celcon, and Hostaform, still dominate global acetal supply six decades later, joined by BASF Ultraform, Mitsubishi Chemical Iupital, Polyplastics Duracon, and Korea Engineering Plastics Kepital. The split between homopolymer and copolymer dates from these original two routes and is still the first technical fork in any selection.

POM stands out for a combination of traits that few other commodity-priced plastics match together. It has a high tensile strength near 60 to 70 MPa, a flexural modulus around 2,600 to 3,100 MPa for unfilled grades, a low coefficient of friction against steel of roughly 0.20 to 0.35 running dry, and water absorption of only about 0.20 to 0.25 percent at 24-hour immersion. The last property is critical: nylon absorbs 7 to 9 percent water and swells, while POM barely moves, so machined acetal parts keep their tolerance in humid plants, in water contact, and across seasons. This is why fuel-sender floats, water-meter internals, and conveyor wear strips are so often acetal.

The density of POM is about 1.41 to 1.42 grams per cubic centimetre, slightly heavier than nylon (1.13 to 1.15) or polypropylene (0.90), but it machines cleanly, chips rather than smears, and holds a fine surface finish, which is why it is a favourite on CNC and Swiss lathes for small precision parts. The trade-off for its high crystallinity is a relatively high mold and machining shrinkage of roughly 1.8 to 2.5 percent, and a high coefficient of thermal expansion, both of which the designer must budget for in tight fits.

It is important to set expectations on where POM does not belong. It has essentially no resistance to strong acids, it is flammable with only a UL 94 HB rating in unmodified form, and its long-term service temperature in air is modest, about 90 to 100 degrees Celsius. Within those boundaries, though, POM is one of the most cost-effective load-bearing, low-friction engineering plastics available, and global demand runs to well over a million tonnes per year across automotive, consumer, electrical, and industrial markets.

Homopolymer vs Copolymer

The single most consequential decision in acetal selection is homopolymer (POM-H) versus copolymer (POM-C). Both are POM under ISO 1043-1, and on a casual spec sheet they look almost identical, but their chain chemistry differs in a way that changes hot-water behaviour, thermal stability during molding, and internal soundness of thick sections. Choosing the wrong family is the classic acetal mistake: a homopolymer gear specified for a hot-water pump, or a copolymer part specified where the absolute highest stiffness and fatigue life are needed.

POM-H, the homopolymer, is built from a single repeating oxymethylene unit with reactive chain ends that are capped (acetylated) to stop depolymerization. Its higher and more uniform crystallinity gives it a higher melting point, near 175 to 178 degrees Celsius, and roughly 10 to 15 percent higher tensile strength, flexural modulus, hardness, and fatigue resistance than copolymer. POM-C, the copolymer, randomly inserts a small fraction of a second monomer (typically ethylene oxide) into the chain. Those comonomer units act as natural stoppers that halt the unzipping reaction at a stable carbon-carbon bond, which dramatically improves resistance to thermal degradation during processing and to hydrolysis in hot water and alkalis.

The practical guidance follows directly from this table. Choose POM-H where the application is dry, room-to-warm temperature, and demands maximum mechanical performance: high-load spur gears, structural snap fits, valve seats, and parts that see millions of fatigue cycles. Choose POM-C where the part meets hot water, steam, alkaline cleaners, or repeated wash-down, or where the part is a thick machined block, because copolymer extrudes with a sounder centreline and is less prone to the internal porosity that can show up at the core of large-diameter homopolymer rod.

One subtlety that catches buyers: copolymer is generally the safer default for machined semi-finished stock (sheet, rod, tube) precisely because of that centreline soundness, which is why much of the European machining-stock market (Ertacetal, Tecaform) is copolymer. Homopolymer dominates molded automotive and consumer parts where the absolute mechanical numbers and a slightly better surface finish matter and section thickness is controlled. Where a datasheet does not state which family a grade belongs to, treat the omission as a red flag and request the resin trade name before quoting.

A final note on a common misconception: POM-H is not simply better than POM-C. The two are engineered for different failure modes. Homopolymer fails the hot-water test that copolymer passes, and copolymer gives up some stiffness that homopolymer keeps. A correct selection states the temperature, the wetted environment, the load, and the section thickness first, then the family follows.

Modified and Filled Grades

Beyond the base homopolymer and copolymer, POM is sold in a family of modified compounds that trade one property for another. The base resin is already a strong, low-friction material, so modification usually targets one of four goals: more stiffness and dimensional stability, lower friction and wear, weather or UV durability, or compliance for food and water contact. Understanding what each filler does, and what it costs elsewhere, prevents over-specifying an expensive compound where a base grade would serve.

Glass-fibre reinforcement is the most dramatic modifier. Adding 20 to 30 percent short glass fibre raises the flexural modulus from roughly 2,600 MPa to about 7,000 MPa or higher and roughly halves the coefficient of thermal expansion, which is the main reason it is chosen, because dimensional movement is the enemy in precision housings. The cost is steep loss of impact strength and elongation, plus a fibre-roughened surface that abrades a metal counterface, so glass-filled POM is poor for running bearings even though it is excellent for static structural parts.

Low-friction grades pull friction the other way. PTFE-filled, silicone-filled, or solid-lubricant POM compounds reduce the dynamic friction coefficient against steel from the base 0.20 to 0.35 down toward 0.10 to 0.15, and they raise the limiting PV value so the bearing can run faster or under higher load before frictional heat softens the surface. The trade is a modest drop in tensile and flexural strength, since the lubricant phase does not carry load. For self-lubricating gears and bushings that must run dry for millions of cycles, this is usually the right family.

UV and weathering grades address one of POM's real weaknesses. Natural POM chalks, loses gloss, and embrittles under prolonged sunlight because UV attacks the chain. The standard fix is a carbon-black UV-stabilized grade, which is why outdoor acetal parts are almost always black. If a part must be a light colour outdoors, POM is often the wrong material and the designer should reconsider ASA or stabilized PBT. For food, potable-water, and pharma contact, specify grades explicitly listed under FDA 21 CFR 177.2470, EU Regulation 10/2011, and NSF/ANSI 51 or 61, and confirm the colourant masterbatch is also compliant, because an otherwise-clear base resin can be disqualified by a non-listed pigment.

Chemical, Thermal, and Standards Limits

POM is chemically a paradox: extremely resistant to one large group of media and almost defenceless against another. Knowing exactly where the cliff edge is, acids, saves more failed parts than any other piece of acetal knowledge. The chain of acetal linkages that makes POM stiff and slippery is the same chain that an acid will catalytically unzip back to formaldehyde monomer.

On the resistant side, POM tolerates a broad range of organic solvents, fuels, oils, weak alkalis, and neutral salt solutions very well at room temperature, which is why it is a staple of fuel systems, hydraulic accessories, and conveyor components. On the vulnerable side, POM has essentially no resistance to strong mineral acids, hydrochloric, sulfuric, nitric, and phosphoric, even dilute and cold, because acid protonation triggers self-accelerating depolymerization that releases formaldehyde and embrittles the part. It is also degraded by strong oxidizers and by prolonged UV without carbon-black protection. For acidic, strongly oxidizing, or chlorinated-acid media, the correct substitutes are PVDF, PTFE, or polypropylene.

Thermally, POM is a semi-crystalline material with a sharp melting transition rather than a gradual softening. It stays tough and stiff down to about minus 40 to minus 50 degrees Celsius, and tolerates short excursions toward 140 degrees Celsius, but continuous-use temperature in air is much lower because of slow oxidative degradation and creep under load. The widely cited continuous figures are about 90 degrees Celsius for POM-H and about 100 degrees Celsius for POM-C in air, dropping in hot water to roughly 60 degrees Celsius for POM-H and 85 degrees Celsius for the hydrolysis-resistant POM-C. Heat deflection temperature under the 1.8 MPa load (ISO 75 / ASTM D648) sits around 90 to 110 degrees Celsius for unfilled grades and rises substantially for glass-filled compounds.

Flammability is a genuine limitation. Unmodified POM is rated UL 94 HB, the lowest passing horizontal-burn class, with a limiting oxygen index typically around 15 percent, meaning it burns readily in air. POM is difficult to flame-retard because most halogen and phosphorus flame retardants accelerate the acid-catalyzed decomposition of the backbone, so flame-retardant POM grades are specialized and uncommon. Where a UL 94 V-0 plastic is required, POM is usually not the answer, and designers move to PC, PA, or PBT blends.

The standards landscape is worth memorizing because it makes datasheets comparable across suppliers. The ISO 1043-1 abbreviation is POM, with POM-H for homopolymer and POM-C for copolymer. ISO 9988-1 and ISO 9988-2 provide the designation system and basis for specification. In North America, ASTM D6778 is the current classification system for POM molding and extrusion materials, superseding the withdrawn ASTM D4181. The mechanical and thermal test methods that populate a POM datasheet are ISO 527 / ASTM D638 (tensile), ISO 178 / ASTM D790 (flexural), ISO 179 or ISO 180 / ASTM D256 (impact), ISO 75 / ASTM D648 (heat deflection), and ISO 62 / ASTM D570 (water absorption). When you compare two suppliers, first confirm both quote the same test method and conditioning state, because as-molded versus conditioned and ISO versus ASTM specimens shift the printed numbers.

Key Specification Parameters

A POM datasheet can list thirty values, but only a handful drive a sound selection: density, tensile strength and modulus, flexural modulus, impact strength, coefficient of friction and wear, water absorption, coefficient of thermal expansion, heat deflection temperature, and continuous-use temperature. The table below collects representative unfilled values and the test method each follows. Treat these as cross-supplier typical ranges, not as a substitute for the specific grade datasheet, since fillers and family shift every figure.

Density and mechanical strength set the load class. POM yields at about 60 to 73 MPa with a tensile modulus near 2,600 to 3,200 MPa, putting it among the stiffer unfilled engineering plastics. Homopolymer sits at the top of these ranges and copolymer near the bottom. For a part that must hold a thread, a press fit, or a gear tooth under steady load, the yield strength and the creep modulus, not the short-term tensile number, decide whether the part survives, so always check creep data for any continuously loaded acetal part.

Friction, wear, and PV are why POM exists as a bearing material. The dry friction coefficient against steel of roughly 0.20 to 0.35, combined with high surface hardness and fatigue resistance, lets plain bearings and gears run unlubricated for millions of cycles. The governing limit is PV, the product of contact pressure and sliding velocity. Unfilled POM carries a limiting PV of roughly 0.08 to 0.14 MPa times metres per second; above it, frictional heat raises the surface temperature past the material limit and the bearing fails by melting or accelerated wear. PTFE-filled grades raise this ceiling.

Water absorption and thermal expansion together define dimensional stability, the property that most distinguishes POM from nylon. POM absorbs only 0.20 to 0.25 percent water at 24 hours, so it does not swell in humid or wet service, which keeps machined tolerances. But it has a high coefficient of thermal expansion of about 100 to 130 micrometres per metre per Kelvin, roughly 10 to 13 times that of steel. A precision designer must reconcile these: POM is steady against moisture but moves significantly with temperature, so running clearances in a metal housing must accommodate that differential growth.

Heat deflection and continuous-use temperature are different numbers that buyers often confuse. Heat deflection temperature (90 to 110 degrees Celsius at 1.8 MPa) is a short-term stiffness benchmark under a defined load; continuous-use temperature (90 to 100 degrees Celsius in air) is the long-term ceiling before slow oxidation and creep degrade the part over thousands of hours. For any part under sustained load near these limits, specify against the continuous-use figure, not the heat deflection figure, and derate for stress.

Selection Decision Factors

To turn the preceding five chapters into a specific grade and supplier, follow the ordered sequence below. Most acetal selection errors come not from a single wrong value, but from deciding the grade before the environment is fully defined, then discovering a hot-water, acid, or expansion problem in the field. These eight steps work as a fixed RFQ checklist.

1. Confirm POM is the right family at all: POM fits stiff, low-friction, dimensionally stable, dry-running parts. If the duty needs UL 94 V-0 flammability, strong-acid contact, continuous service above about 100 degrees Celsius, or a light colour outdoors, stop and reconsider PA, PBT, PVDF, PEEK, or PPS before going further.
2. Homopolymer or copolymer: Choose POM-H (Delrin) for maximum stiffness, fatigue, and dry mechanical performance at room-to-warm temperature; choose POM-C (Hostaform, Celcon) for hot water, steam, alkalis, wash-down, or thick machined sections where centreline soundness matters.
3. Define the wetted and thermal environment: State media (rule out acids and strong oxidizers), maximum continuous temperature, hot-water or steam exposure, UV or outdoor exposure, and any food, potable-water, or pharma contact requiring FDA, EU 10/2011, or NSF listing.
4. Set the mechanical duty: Steady load and creep govern, not short-term tensile. For gears and bearings, calculate the PV against the grade limit; for structural housings needing stiffness and low movement, consider glass-filled; for shock and snap fits, consider impact-modified.
5. Pick the modifier grade: Glass-fibre for stiffness and low expansion (not for bearings), PTFE or silicone for low-friction wear surfaces, carbon-black UV grade for outdoor, antistatic or conductive for electronics and fuel handling.
6. Budget for shrinkage and thermal expansion: Allow for 1.8 to 2.5 percent mold or machining shrinkage and for a thermal expansion of 100 to 130 micrometres per metre per Kelvin in tolerance stack-ups and in clearance against metal mating parts across the operating temperature band.
7. Choose stock form and process: Injection molding for high-volume net-shape parts; machined semi-finished sheet, rod, or tube (often copolymer for sound centrelines) for prototypes, low volumes, and large precise parts. Confirm the stock supplier states the resin family and grade.
8. Total cost of ownership: Compare resin and machining cost against service life. A correctly specified acetal bearing or gear that runs dry for millions of cycles often beats a cheaper plastic that needs lubrication or replacement, and beats an over-specified glass-filled grade that abrades its mating part.

One often-overlooked dimension is supplier traceability and grade documentation: the exact resin trade name (Delrin, Hostaform, Celcon, Ultraform, Iupital, Duracon, Kepital, or a semi-finished brand such as Ertacetal or Tecaform), a current datasheet stating the test methods, batch certificates, and compliance declarations for food or water contact. These seem like paperwork at the quoting stage but determine whether the part can be re-sourced, re-validated, and audited years into a production run. DuPont (Delrin), Celanese (Hostaform and Celcon), BASF (Ultraform), Mitsubishi Chemical (Iupital, Ertacetal), Polyplastics (Duracon), and Korea Engineering Plastics (Kepital) all publish full datasheets and maintain global distribution, which makes them dependable anchors for a long-lived bill of materials.