Polyoxymethylene (POM, Acetal)

Polyoxymethylene (POM, Acetal)

Key Takeaways

• Two families: homopolymer (Delrin type) melts near 175 °C and is stiffer; copolymer (Celcon, Hostaform type) melts near 165 °C and tolerates hot water, alkali and heat better.
• 36–38 dyne/cm plus chemical inertness make acetal one of the hardest plastics to bond, paint or print. Design for mechanical joining first.
• Natural POM barely marks under a fibre laser without an additive; dark grades foam to a light gray.
• Overheated POM releases formaldehyde. Every laser and welding cell needs extraction.

Polyoxymethylene (POM, Acetal)

Polyoxymethylene, usually called acetal or POM, is a highly crystalline engineering thermoplastic of repeating oxymethylene (CH2O) units, specified for stiffness, dimensional stability, fatigue resistance and low friction, and notorious in secondary processing because its low surface energy and chemical inertness resist adhesives, inks, paints and laser marking alike.

Two chemistries share the name. The homopolymer (DuPont Delrin is the reference) is end-capped polyformaldehyde; the copolymer (Celcon, Hostaform, Ultraform) carries a few percent of ethylene oxide units that stop the chain unzipping into formaldehyde under heat, hot water or alkali.

Homopolymer Against Copolymer

Centerline porosity matters because thick homopolymer rod cools from the outside in and the core, solidifying last, pulls voids along the section centre; a bore or weld face that opens into it leaks, bonds badly and marks unevenly. Specify copolymer or porosity-controlled stock for machined parts.

Typical Properties and Applications

• Gears, cams, bearings and bushings — stiffness, fatigue life and dry lubricity.
• Fuel systems — pump housings, sender units and valve bodies, usually copolymer.
• Drug-delivery mechanisms — dose-setting gears, ratchets and triggers in insulin pens, autoinjectors and inhalers.
• Zippers, buckles, clips and snap fits, conveyor chain, valves and plumbing fittings.

Bonding, Painting and Printing on Acetal

Acetal heads every adhesive supplier’s difficult-substrate list: surface energy of 36–38 dyne/cm sits below most inks, paints and adhesives; the smooth, highly crystalline surface offers no reactive groups; no convenient solvent attacks it, so solvent cementing is out; and it is lubricious by design. Untreated epoxy or cyanoacrylate joints typically yield under 1 N/mm² and part cleanly at the interface.

After pretreatment and a matched adhesive, 3–6 N/mm² is realistic and failure moves into the adhesive, the sign that pretreatment worked. For load the rule stands: snap fits, press fits, heat-staked inserts and overmoulding, with adhesives for sealing. Paint and pad-printed inks follow the same logic. PTFE- or silicone-lubricated grades cannot be treated into bondability.

Laser Marking and Welding Acetal

Homopolymer is the harder to mark: its narrow melt window puts the energy that creates contrast close to the energy that melts the surface, so copolymer is the usual base for laser-markable grades. Either way the mark forms above the roughly 230 °C at which POM releases formaldehyde, so extraction at the mark head is mandatory. The laser marking plastics chart gives starting wavelengths by grade.

Troubleshooting Acetal Secondary Operations

Specifying and Verifying Marks and Bonds on POM

• Name the grade in full — homopolymer or copolymer, colourant, lubricant, glass content, laser additive. “Acetal” alone is not a specification.
• State the surface energy target and method: 42 dyne/cm minimum by ASTM D2578 in production, contact angle for qualification.
• Fix the treat-to-bond window from a decay trial and re-measure aged stock.
• Define bond acceptance as lap shear to ISO 4587 or ASTM D1002 with the failure mode recorded: cohesive proves the pretreatment, interfacial condemns it.
• Define mark acceptance by verifier grade (ISO/IEC 15415, grade C or better for UDI), with a 72-hour re-inspection for crazing.
• Write fume control into every laser and welding cell specification.

Related Terms and Reading

• Plastics laser marking
• Surface pretreatment methods
• Adhesive bonding and joining
• Best practices for bonding semi-crystalline thermoplastics
• Laser marking versus inkjet printing
• Laser marking filled and reinforced plastics

Applying this in production

The Sabreen Group provides independent engineering support for acetal laser marking, surface pretreatment, bonding and welding qualification. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.

Frequently Asked Questions

Why is acetal so hard to glue?

Surface energy of 36–38 dyne/cm is below most adhesives, the crystalline surface has no reactive groups, and no common solvent attacks it, so untreated joints fail at the interface. Plasma or acid etch plus a matched adhesive brings lap shear to 3–6 N/mm²; mechanical joining remains the rule for load.

Can natural white POM be laser marked?

Only marginally with a standard fibre laser: POM is transparent at 1064 nm and has nothing to carbonise. Three routes work: a compounded laser-marking additive at 0.5–2%, a UV laser whose photochemical mark needs far less heat, or inkjet printing after pretreatment. Dark grades foam to a light gray.

Homopolymer or copolymer for parts that will be bonded, marked or welded?

Copolymer, in most cases: its wider melt window tolerates laser and welding heat with less formaldehyde, it welds more consistently, it resists hot water and alkali, and its stock shapes avoid centerline porosity. Homopolymer earns its place on stiffness, creep and fatigue; choose it where that margin matters more.

What is the formaldehyde risk when laser marking or welding POM?

Above roughly 230 °C the chain unzips into formaldehyde gas, a respiratory irritant and carcinogen with an OSHA exposure limit of 0.75 ppm over 8 hours and 2 ppm short-term. Every laser and hot-plate cell running acetal needs local extraction; a sharp odour means the surface is overheated, so reduce fluence or dwell first.

---