Acetal Copolymer (POM-C)

Complete property data sheet for unfilled acetal copolymer (POM-C) — mechanical, thermal, electrical and physical values, each with its ASTM test method — plus how it differs from the acetal homopolymer used for Delrin.

Data verified 2026-09-29 · based on ASTM D6778-20

Mechanical Properties

Thermal Properties

Electrical & Physical Properties

POM-C vs POM-H (Acetal Copolymer vs Homopolymer)

Weight & Volume Calculator

Machining POM-C

Acetal copolymer is one of the two or three easiest plastics to machine well. It cuts with a clean, continuous chip similar to free-machining brass, leaves a good surface finish from a sharp cutter, holds tolerances of a few hundredths of a millimetre, and does not need coolant. Because it is dimensionally stable — 24-hour water absorption is only 0.22% — a part machined to size stays at that size, which cannot be said of nylon.

The limiting factor is heat. The melting point is 166 °C and the material softens well below that, so a dull cutter or a dwell in the cut will smear the surface, weld a chip to the edge and leave a mark that cannot be polished out. Keep the feed positive and the tool moving. Use sharp high-rake tooling with generous clearance angles, and clear chips continuously — a recut chip is the usual cause of a spoiled finish.

Clamping matters as much as cutting. Acetal is stiff enough to feel solid in a vise and compliant enough to deform under it. Use soft jaws, clamp against a rigid feature, and check the free-state dimension after unclamping — not while the part is still held.

Where POM-C Is the Right Choice

Acetal copolymer is the standard answer for a machined plastic mechanical part. It combines stiffness close to nylon's, friction far lower, water absorption roughly a fifth as high, and excellent fatigue and creep resistance. Where nylon is chosen for toughness, acetal is chosen for dimensional stability and predictability — a machined acetal part holds its size.

Its limits are heat and chemical attack. The continuous service temperature is 100 °C and the deflection temperature under load at 1.8 MPa is 110 °C, which is adequate for most mechanical applications and not adequate for anything near an engine or a hot-water line. Acetal is attacked by strong acids and by strong bases, and it is not suitable for continuous UV exposure without a stabilised grade.

Against the alternatives: POM-H (Delrin) is the same family, slightly harder and slightly better to machine; nylon 6/6 is tougher but far less dimensionally stable; PEEK covers 250 °C but costs 8–12× as much; PET-P (Ertalyte) offers better wear resistance at higher cost. Acetal remains the default because it does most of what a machined plastic part needs at the lowest cost.

How to Read This Data Sheet

Values are typical published figures for unfilled, natural-grade POM-C, measured on standard test specimens. Every row names the test method it came from, because a polymer property without its test method is not a comparable number — tensile strength measured to ASTM D638 and to ISO 527 differ systematically, and impact values depend heavily on specimen geometry.

Two caveats apply to every figure here. First, temperature: polymer properties are far more temperature-sensitive than metal properties, and a tensile strength quoted at 23 °C tells you little about the same material at 100 °C. Second, conditioning: hygroscopic plastics such as nylon absorb moisture from the air, and absorbed water acts as a plasticiser — the same grade can be stiff and strong when dry and significantly tougher but weaker when conditioned. Where a property is strongly affected by moisture, the row notes it.

Filled, reinforced, lubricated and impact-modified grades of the same base polymer differ substantially from these figures. Use this page to compare materials and to shortlist; use the specific grade's data sheet for design calculations.

Frequently Asked Questions

What is the density of POM-C?

1.41 g/cm³, which is heavier than water — about 24% denser than nylon and 15% lighter than PTFE. Density matters for two practical reasons: it sets the weight of a finished part through the material's cost per kilogram, and it is the number this page's weight calculator uses to convert a volume into a mass.

Can POM-C be machined?

POM-C machines like free-cutting brass but softer. It produces a clean, continuous chip, holds tolerance well and needs no coolant. The two failure modes to avoid are heat — it melts at only 166 °C — and over-tight clamping, which deforms the part rather than the tool.

How do I choose between POM-C and another engineering plastic?

Choose acetal copolymer when you need a stiff, low-friction, dimensionally stable plastic part that will be machined to tolerance and used at moderate temperature — gears, cams, bushings, valve bodies, insulators.

Is acetal copolymer the same as Delrin?

No. Delrin is the homopolymer (POM-H); this page is the copolymer (POM-C). Both are polyoxymethylene and both are commonly called "acetal". The homopolymer is slightly harder, stiffer and better to machine; the copolymer resists hot water and weak bases better and processes more easily. Published figures differ by only a few percent in most mechanical properties.

Can acetal be sterilised or used in autoclaves?

Not reliably in a steam autoclave. The combination of 121–134 °C steam and hot water attack degrades acetal — it hydrolyses, losing strength and dimensions. Copolymer resists this better than homopolymer, which is why POM-C is the acetal used in medical and food equipment, but the exposure still needs to be short and validated. For repeated steam sterilisation, PEEK, PPSU or PEI are the appropriate materials.

Why does my machined acetal part have a smeared finish?

Almost always heat from a dull edge or a dwell in the cut. Acetal melts at 166 °C and softens well below that, so a rubbing tool melts the surface, the chip welds to the edge and the cutter then drags it across the work. Replace the tool with a sharp high-rake edge, increase feed per tooth, reduce spindle speed, and make sure chips are cleared rather than recut.

Related

• Delrin (POM-H) Properties
• Nylon 6/6 Properties
• PTFE Properties
• PEEK Properties
• Plastic Machining Tolerance Chart
• Plastic Service Temperature Chart

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

Data Sources

Cross-checked against:

• Published POM-C stock-shape data sheets from two independent suppliers
• POM-C vs POM-H comparison values checked against a homopolymer data sheet
• Steam and hot-water resistance checked against published chemical compatibility tables

Values are typical published figures for unfilled, natural-grade resin at 23 °C, dry as molded unless the row states otherwise. They are not specification minima. Design to the specific grade's data sheet, and derate for the service temperature and moisture condition the part will actually see.

Cite This Page

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