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 fiber 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
| Property | Homopolymer (Delrin type) | Copolymer (Celcon, Hostaform type) |
|---|---|---|
| Melting point | About 175 °C | About 165 °C |
| Crystallinity | Roughly 75–85% | Roughly 65–75% |
| Stiffness, strength, creep | About 10–15% better | Slightly lower; tougher cold |
| Hot water and steam | Hydrolyzes above about 60–80 °C | Serviceable to about 80–100 °C |
| Strong alkali | Attacked | Resistant |
| Strong acid (pH below about 4) | Attacked | Attacked |
| Melt stability | Narrow window | Wider; less formaldehyde when heated |
| Centerline porosity in stock shapes | Common in rod and plate above about 25 mm | Rare |
Centerline porosity matters because thick homopolymer rod cools from the outside in and the core, solidifying last, pulls voids along the section center; 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
| Property | Typical value, unfilled grades |
|---|---|
| Density | 1.41–1.42 g/cm³ |
| Tensile strength (ISO 527) | 60–70 MPa, homopolymer at the top |
| Tensile modulus | 2.6–3.2 GPa |
| Heat deflection, 1.8 MPa (ISO 75) | About 95–110 °C; glass-filled 150–160 °C |
| Continuous use temperature | About 90–105 °C, set by oxidative aging |
| Water absorption (ISO 62, 24 h) | About 0.2–0.25%; 0.8–0.9% saturated |
| Friction against steel | 0.2–0.35, the slipperiness that defeats adhesives |
| Surface energy | About 36–38 dyne/cm, below most inks and adhesives |
- 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.
| Pretreatment | What it does to POM | Practical notes |
|---|---|---|
| Cold gas (low-pressure) plasma | Oxidizes the top nanometers to above 50 dyne/cm | Most uniform; bond within hours, as hydrophobic recovery is fast with a glass transition near -60 °C |
| Atmospheric plasma or flame | Same oxidation, in-line on 3D parts | Brief flame passes only; overheating releases formaldehyde |
| Acid etch (“satinizing”, chromic acid) | Micro-roughens and oxidizes | Strongest bonds; hazardous chemistry, declining use |
| Primer or adhesion promoter | Amine primers for cyanoacrylate; acrylic primers under paint | Combine with plasma |
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 overmolding, 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
| Laser | Natural (white) POM | Dark POM | Notes |
|---|---|---|---|
| Fiber, 1064 nm | Poor without additive: transparent at 1064 nm, nothing to char | Light-gray foamed mark | 0.5–2% of a laser-marking additive gives gray-to-dark contrast |
| CO2, 10.6 µm | Frosty, low-contrast engraving | Low-contrast engraving | Highest heat input; melt blow-out and formaldehyde |
| UV, 355 nm | Better contrast, cold photochemical mark; additive still helps | Good | Preferred for drug-delivery parts |
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.
| Welding method | Suitability | Design and process notes |
|---|---|---|
| Ultrasonic | Fair | Sharp melting point, high damping: shear joints, near-field, 60–120 µm at 20 kHz; copolymer more consistent |
| Laser transmission | Possible | Natural POM scatters near-infrared: thin upper part, carbon black or infrared absorber below |
| Hot plate | Good | PTFE-coated plates at about 210–250 °C, short dwell, extraction; homopolymer strings more |
| Vibration and spin | Good | Friction heating; strong welds on gears and housings |
Troubleshooting Acetal Secondary Operations
| Symptom | Likely cause | Correction |
|---|---|---|
| Adhesive peels cleanly from the POM | No pretreatment, aged treatment, or a lubricated grade | Plasma or etch, verify by contact angle, bond within hours; check for PTFE or silicone |
| Printed or painted legend rubs off | Marginal wetting, no anchorage | Treat to 42 dyne/cm or better, add a primer, confirm by ASTM D3359 tape test |
| Weak laser contrast on a natural grade | Transparent at 1064 nm, nothing to char | Additive grade, UV laser, or print instead |
| Cracks at sharp corners or under a mark | Notch sensitivity plus molded-in stress; acid exposure | Radius to 0.5 mm minimum, keep marks off gates and corners, anneal at 140–160 °C |
| Dimensions drift after molding or in service | Post-crystallization from a cold tool; moisture; expansion about 110 ppm/K | Tool at 80–100 °C, anneal before final machining, state a conditioning state |
| Formaldehyde odor at the laser or welder | Surface above about 230 °C | Cut fluence or dwell, raise speed, local extraction, monitor against 0.75 ppm |
Specifying and Verifying Marks and Bonds on POM
- Name the grade in full — homopolymer or copolymer, colorant, 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 fiber laser: POM is transparent at 1064 nm and has nothing to carbonize. 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 odor means the surface is overheated, so reduce fluence or dwell first.
