Key takeaways
- Acetal homopolymer is a pure polyoxymethylene chain with higher crystallinity than copolymer, translating into higher tensile strength, flexural modulus and fatigue endurance, but it is more prone to a void running down a thick part's thermal centreline.
- Acetal copolymer incorporates a few percent of a comonomer (typically ethylene oxide) that places stable C–C–O linkages along the backbone, giving markedly better resistance to thermal degradation, hot water and strong alkalis, plus a wider, more forgiving processing window.
- For precision gears, cams and bearings under continuous cyclic load, homopolymer's higher fatigue strength and lower wear are usually decisive; for snap-fit clips, hot-water plumbing fittings and parts exposed to caustic cleaning, copolymer is the safer default.
- Melting point is the cleanest field discriminator between the two — homopolymer melts near 175°C and copolymer near 165°C — and the grades must be treated as distinct materials on the drawing and Certificate of Analysis, never as a generic 'acetal' line item.
Polyoxymethylene — POM, sold as acetal under brand names such as Delrin (homopolymer) and various copolymer grades — comes in two chemistries that look identical on the shelf and behave differently in the cavity. Specifying the wrong one is a common and expensive mistake: it shows up as a gear that wears prematurely, a snap-fit that cracks in caustic wash-down, or a thick boss that leaks because of a void down its centre. The choice is application-led, not brand-led.
Homopolymer is a pure (CH₂O)ₙ chain. Copolymer breaks that chain at intervals with a small fraction of a comonomer — typically ethylene oxide, around 2% — inserting stable C–C–O linkages into the backbone. That one structural difference cascades into everything a buyer cares about: crystallinity, mechanical strength, thermal stability, chemical resistance, shrinkage and porosity. For where POM sits against nylon, PBT and the rest, see our engineering plastics compared overview; this piece is the POM-to-POM decision.
Homopolymer's uninterrupted chain packs into a more ordered crystal lattice — higher crystallinity than copolymer. That order is the source of homopolymer's headline advantages: greater tensile strength, higher flexural modulus, better fatigue endurance and lower creep at room temperature. It is also the source of its two weaknesses. The same ordered structure shrinks more on cooling, which drives centreline porosity in thick sections, and the unprotected acetal backbone is more vulnerable to chain-unzipping when attacked by heat, hot water or strong alkali.
The comonomer in copolymer acts as a chemical full-stop. When degradation starts unzipping the chain from an end, it halts at the stable C–C–O linkage rather than running the length of the molecule. That is why copolymer tolerates hot water, steam and pH well into the alkaline range that would slowly embrittle homopolymer — and why it carries a wider processing window with less sensitivity to residence time and barrel temperature.
Homopolymer wins on the datasheet; copolymer wins over the part's service life in a hostile environment. Match the resin to the failure mode you fear most.
The table below gives indicative, typical values for unfilled, natural injection-moulding grades. Always confirm against the specific grade datasheet and the batch Certificate of Analysis — melt flow rate (MFR) per ISO 1133 / ASTM D1238 and density per ISO 1183 / ASTM D792 are the values that travel on the CoA. See reading a polymer CoA for how to interrogate those numbers.
| Property (test) | Homopolymer | Copolymer | Buyer's read |
|---|---|---|---|
| Crystallinity | Higher | Lower | Homo packs tighter — stiffer, stronger |
| Density (ISO 1183) | ~1.41–1.42 g/cm³ | ~1.40–1.41 g/cm³ | Both denser than nylon |
| Melting point | ~175 °C | ~165 °C | Cleanest way to tell them apart |
| Tensile strength | Higher | Lower | Homo for load-bearing teeth/cams |
| Flexural modulus | Higher | Slightly lower | Homo for dimensional rigidity |
| Fatigue endurance | Superior | Good | Homo for continuous cyclic load |
| Hot-water / steam resistance | Moderate, degrades over time | Excellent | Copo for plumbing, sterilisation |
| Alkali / caustic resistance | Poor to moderate | Good | Copo for wash-down, detergents |
| Centreline porosity risk (thick wall) | Higher | Lower | Copo for thick pressure-tight bosses |
| Processing window | Narrower | Wider, more forgiving | Copo more tolerant of residence time |
Both acetals are dimensionally stable by engineering-plastic standards — they absorb little moisture, so unlike PA6 or PA66 their tolerances do not drift with humidity. That low moisture pickup is one reason POM displaces nylon in precision parts; the contrast is laid out in our PA6 vs PA66 buyers guide. Between the two acetals, homopolymer holds a tighter modulus and lower creep at ambient and moderate temperatures, so a homopolymer gear keeps its tooth geometry — and its backlash — more faithfully under sustained load.
Mould shrinkage differs and is anisotropic in both grades; homopolymer generally shrinks more because of its higher crystallinity, and that shrinkage is not equal in the flow and cross-flow directions. The practical consequence: a tool cut for copolymer shrinkage will not produce dimensionally correct parts in homopolymer, and vice versa. Substitution between the two is never a drop-in. Re-qualify dimensions on first articles before any switch, and record the grade change on the drawing and CoA.
- Fatigue: homopolymer's higher endurance limit suits gears, cams and springs seeing millions of cycles.
- Creep: homopolymer deforms less under constant load at room temperature — better for retained-stress snap features that must hold position for years.
- Thermal: copolymer's stability under heat and hot water makes it the safer pick where the part is steam-cleaned or runs warm continuously.
- Wear: homopolymer typically shows lower wear and friction against steel and against itself in unlubricated bearings.
Centreline porosity is the single most misunderstood POM problem. In a thick section, the high-crystallinity skin freezes first and contracts onto a molten core. Once the gate freezes, no further melt can feed the core, so the volumetric shrinkage resolves as a void — a thin tear running down the part's thermal centreline. It is invisible from outside and surfaces only when the section is machined open, sees a leak test, or fails a weld. Homopolymer is more prone because it shrinks more; copolymer's lower crystallinity reduces the risk but does not eliminate it.
It is controlled at the design and process stage, not by resin choice alone. Keep walls uniform and as thin as the function allows; core out heavy bosses; size gates to stay open through the hold phase; and sustain hold pressure long enough to pack the section. Where the part is pressure-tight or carries a structural weld, call out the porosity requirement explicitly on the drawing and require it be verified — sectioning or pressure test — on first articles and reported batch-by-batch.
Lead with the failure mode. If the part is a precision gear, cam or bearing under continuous load, start with homopolymer and a moulding strategy that manages porosity. If it sees hot water, steam, caustic cleaning or simply needs a forgiving process window, start with copolymer. Then pin the grade by MFR class — a low-MFR grade for tough, thick structural parts; a high-MFR grade for thin-wall, free-flowing precision parts — and by any glass or PTFE/silicone modification the duty demands.
On the purchasing side, treat homopolymer and copolymer as distinct materials with distinct CoAs, never as a generic "acetal" line item. Specify the test standards you will accept the resin against (ISO 1133 / ASTM D1238 for MFR, ISO 1183 / ASTM D792 for density), require melting point on the certificate as the cleanest discriminator between the two chemistries, and confirm regrind policy for any safety-critical or pressure-bearing part. OmniaStrata sources both homopolymer and copolymer acetal to qualified grade and CoA — talk to the engineering plastics desk or send a spec to our team and we'll match resin to duty rather than to brand.
Frequently asked
Questions on the desk
Is acetal homopolymer or copolymer stronger?
Homopolymer is stronger and stiffer in the as-moulded state. Its higher crystallinity gives modestly higher tensile strength and flexural modulus, plus better fatigue endurance and creep resistance at room temperature. Copolymer trades some of that mechanical headroom for far better stability in hot water, steam and alkaline environments, where homopolymer can lose strength over time.
Why does acetal homopolymer have centreline porosity?
As a thick section of homopolymer cools, the high-crystallinity skin solidifies first and shrinks onto a still-molten core. Once the gate freezes, no further melt can feed the core, so the resulting volumetric shrinkage forms a void — typically a thin tear running along the part's thermal centreline. It is controlled by uniform wall thickness, adequate gate size and sustained hold pressure, not by switching resin alone, though copolymer's lower crystallinity makes it less prone.
Which acetal is better for gears?
Homopolymer is the usual choice for precision gears and cams under continuous cyclic load. Its higher fatigue strength, stiffness and wear resistance hold tooth geometry and reduce backlash drift over millions of cycles. Copolymer gears are specified where the gearset sees hot water, steam sterilisation or caustic exposure, accepting slightly lower load capacity for chemical and thermal durability.
Can homopolymer and copolymer acetal be used interchangeably?
Not without verification. They differ in crystallinity, mould shrinkage, melting point (homopolymer around 175°C, copolymer around 165°C) and chemical resistance, so a tool cut for one grade may not produce dimensionally correct parts in the other. Always re-qualify dimensions, mechanical performance and chemical exposure before substituting, and confirm the change on the CoA and drawing.
How do I confirm which acetal grade I have received?
Check the Certificate of Analysis against the grade datasheet for melt flow rate (ISO 1133 / ASTM D1238), density (ISO 1183 / ASTM D792) and melting point. Melting point is the clearest discriminator — homopolymer melts near 175°C and copolymer near 165°C. For full assurance, see our guide to reading a polymer CoA and request batch-level test data before release.
General market commentary from the OmniaStrata desk, provided for information only. It is not legal, financial, tax, or trading advice, and it is not an offer or a commitment to any terms. Figures such as price ranges, spreads, financing costs, and credit periods are illustrative market context, not OmniaStrata's rates or terms. Actual contract terms — including price, payment instrument, credit, insurance, and Incoterms — are agreed in writing on a per-transaction basis and at OmniaStrata's discretion. Market conditions change; figures reflect the publication date.