POM Hardness: Rockwell M94, Shore D & Grade Values

One acetal gear, four hardness numbers: M94, M88, R120, and Shore D 84. All four are correct.

That contradiction is the whole story of POM hardness. Polyoxymethylene, also called acetal, is the hardest high-volume unfilled engineering thermoplastic in common use. It’s also the material whose hardness gets reported most inconsistently, because the number moves with the scale, the test standard, and the grade family.

A datasheet says M94, a supplier email says R120, an online chart says M78, and a durometer says Shore D 84. Those are four measurement systems, not four materials. This guide gives you POM hardness on every scale with the test method attached, and explains why the same Delrin-class grade reads M94 in one datasheet and M88 in another. Our complete guide to plastics hardness covers the other polymers.

What Is POM Hardness?

POM hardness (also written polyoxymethylene hardness) is its resistance to indentation under a defined load. The figure on almost every POM datasheet is Rockwell M94 for homopolymer, with grades spanning M88 to M94. Copolymer sits lower, at M80 to M88 depending on grade and test standard.

The same material converts to roughly Rockwell R120, about Shore D 84 for homopolymer and 80 to 82 for copolymer, and 140 MPa by ball indentation. On a finished part you’ll more often see the shorter POM material hardness figure, a Shore D reading from a durometer.

Hardness measures resistance to indentation, not strength

Hardness is an empirical test of how far a defined indenter presses into a surface. It says nothing directly about tensile strength, impact energy, or wear life.

POM makes that easy to misread, because it’s genuinely strong: a homopolymer grade carries around 69 MPa (11,000 psi) tensile strength at yield. The complete POM material guide covers the full property set. Hardness is one line in that table, and the one most often quoted out of context.

Why POM is quoted on the Rockwell M scale

POM is a rigid, semi-crystalline plastic with a high modulus, so labs test it on the Rockwell M scale: a 1/4-inch (6.35 mm) steel ball under a 100 kgf major load and a 10 kgf minor load. ABS and unfilled nylon are softer, so they get tested on the R scale with a 1/2-inch ball under 60 kgf. Same principle, different geometry, and the results aren’t interchangeable. Our Rockwell hardness of plastics article walks through every scale letter.

Three standards sit behind the numbers you’ll meet. Rockwell M and R come from ASTM D785 or ISO 2039-2, ball indentation from ISO 2039-1 in MPa, and Shore D from ASTM D2240 or ISO 868. MatWeb’s Rockwell hardness reference documents the ball diameters and loads. All three are sensitive to specimen thickness, dwell time, and temperature, so quote a range rather than a single figure.

Need a POM hardness value you can defend on a drawing? Request a quote for branded prime acetal resin with datasheets on the correct scale and full COA documentation.

POM Hardness by Scale: M94, R120, Shore D 84, and 140 MPa

Here’s the consolidated POM hardness chart. No single datasheet gives you this view, because each one reports only the scale its customers ask for.

POM Rockwell hardness: M94 and M88 for the same material

Across unfilled homopolymer grades, POM reads M88 to M94. The number most buyers recognize is M94, the value on Delrin®-class datasheets and stock-shape listings tested to ASTM D785, the figure most people mean by Delrin hardness. Copolymer reads lower, and this is where the reporting gets messy.

One materials supplier publishes both families under both standards and gets four different answers: POM-C at 84 M under ISO 2039-2 and 88 M under ASTM D785, and POM-H at 88 M under ISO 2039-2 and 89 M under ASTM D785 . The standard alone moves a copolymer reading by four M points.

Amara caught this on a pump-components program in Pune. Her drawing called out “hardness 94” with no scale and no standard, so two qualified suppliers sent Delrin-class homopolymer that tested in her lab at M94 and M88. Both shipments were compliant, because neither datasheet named the method behind the number. The fix was one line on the drawing: Rockwell M, ASTM D785, minimum M88.

POM Shore D hardness, Rockwell R, and ball indentation

Run the same homopolymer against the R-scale geometry and you get about R120. Some property summaries, such as the AZoM acetal copolymer page, quote a bare R117 for copolymer with no standard attached. It’s not a different hardness; it’s the same material measured with a bigger ball and a lower load.

Shore D works on finished parts with almost no sample prep, so it’s the practical check when you can’t cut a specimen. Homopolymer reads about Shore D 84 and copolymer 80 to 82. Treat it as a field approximation rather than a specification, because POM sits high enough on the D scale that readings lose resolution. Our Shore hardness scale explained covers the test in detail.

European and Asian datasheets often report hardness as a pressure instead. Unfilled POM reads 140 MPa under ISO 2039-1, tested to the H 358/30 condition, where a 5 mm ball carries a 358 N load for 30 seconds.

Sourcing acetal for a wear part? Browse our POM resin grades for unfilled, glass-filled, and PTFE-modified acetal with datasheets on the scale your drawing specifies.

Homopolymer vs Copolymer: Why POM-H Reads Harder

Acetal copolymer hardness sits below homopolymer; the gap is real, not a reporting artifact.

POM homopolymer (POM-H) is built from pure formaldehyde chains. Those chains pack into highly regular crystals, which is why homopolymer reads higher on every scale: about M94 against M80 to M88, roughly 15 percent harder than copolymer by the same test, along with higher stiffness and tensile strength.

POM copolymer (POM-C) introduces small ethylene-oxide comonomer blocks into the chain. Those blocks break up the crystal regularity, which lowers crystallinity and the hardness number with it. But they also stabilize the molecule, and that buys things a harder material can’t offer:

• Better resistance to hot water and alkaline environments
• A wider processing window, which means lower scrap
• Much less centerline porosity in thick sections, a known weakness of homopolymer above roughly 6 mm wall
• A higher continuous-use temperature in air, about 100 °C against 80 °C for homopolymer

The softer family holds its properties at higher temperature, which shows how independent hardness is from everything else.

What moves a POM hardness reading

Crystallinity is the master variable, and cooling rate sits inside your control. Mold temperature for POM runs 60 to 120 °C, and a warm, evenly cooled part crystallizes more fully than a cold, fast-cooled one, which means a few points of difference on the certificate for the same grade.

Moisture barely matters here, one of POM’s strengths. Water absorption runs 0.20 to 0.25 percent across both families, against 1.3 to 2.5 percent for nylon.

Hardness by POM Grade: Mineral, MoS₂, ESD, and Glass-Filled

Unfilled acetal is the baseline for POM hardness. Filled grades move the number in both directions, and the direction matters more than the magnitude.

Glass fibre is the only filler that makes POM meaningfully harder. Glass filled POM hardness climbs from 140 MPa unfilled to 215 MPa at 40 percent glass, and one glass-filled copolymer grade with a PTFE lubricant reads M113 on ASTM D785 against M94 unfilled. The trade-offs are lower impact resistance, higher abrasion of tooling, and a tendency to shift onto the Rockwell E scale, where numbers aren’t comparable to M-scale values.

PTFE goes the other way. A PTFE-modified homopolymer reads about M87 against M94 unfilled. You gave up hardness on purpose, and you got lower friction and less stick-slip for it.

Filler data is also where bad numbers hide: a listing quoting a 20 percent glass-filled POM at “hardness 78” reports a figure below unfilled acetal, which can’t be right on the same standard.

Does Higher POM Hardness Mean Better Wear?

No, not on its own. Hardness measures how far a ball presses into a surface, while wear is a system property that depends on the counterface, load, sliding speed, lubrication, friction, and environment.

Tomás ran maintenance at a food-processing plant in Valencia and learned the difference the hard way. His washdown conveyor used POM-H bushings, chosen for being the hardest acetal available at M94, and they failed early in wet service. A switch to copolymer, softer at around M82, roughly tripled the service interval. The moisture and the lower friction coefficient mattered more than five points on the M scale.

The numbers to specify instead of hardness

For a sliding surface, these values predict performance better than hardness does:

• Coefficient of friction: typically 0.20 to 0.35. Homopolymer reads 0.2 dynamic under ASTM D3702, while supplier data for both families ranges from 0.25 to 0.45 depending on the test method.
• Wear rate: homopolymer stock records 55 × 10⁻¹⁰ in³·min/ft·lb·hr by ASTM D3702, and 45 µm/km under ISO 7148-2.
• Limiting PV: the pressure-velocity ceiling for the specific grade, counterface, and lubrication state.

Where homopolymer wins and where copolymer wins

Homopolymer takes dry-running metal-on-plastic contact, on the strength of higher crystallinity and hardness together, which is why precision gears and unlubricated bearing cages default to Delrin-class resin. Copolymer takes wet, humid, and hot-water service, where it runs a lower friction coefficient and resists hydrolysis longer.

The practical rule: choose POM for its low friction and its stable, moisture-insensitive hardness, not because it wins a hardness contest.

POM Hardness vs Other Plastics

Acetal hardness only compares to another polymer when the scale letters match, so read them before you line up any two rows. POM, PC, and PEEK are quoted on M; ABS, HDPE, and unfilled nylon are quoted on R.

Is POM harder than nylon, ABS, and polycarbonate?

For unfilled grades, POM homopolymer is the hardest of that group. Nylon needs care, because PA66 is quoted on the R scale and reads R119 to R120 dry as molded, which looks comparable to POM’s R120. Two caveats break the tie: nylon loses 20 to 40 percent of its properties to absorbed water while POM absorbs under 0.25 percent, and M94 and R120 describe different tests.

Against ABS and polycarbonate the answer is unambiguous. POM at M88 to M94 sits above PC at M70 to M80 and well above ABS at R90 to R110, which is a softer scale entirely. Our polycarbonate hardness article covers where PC’s hardness does and doesn’t serve you. To learn more about ABS Plastic Hardness, please refer to our accompanying guide.

Branded POM Grades and Their Hardness Baselines

Prime manufacturers publish hardness on a defined scale with the test method attached, which makes their data the reference point.

Two lessons sit in that table. The homopolymer band is tight, so a supplier quoting M60 or M110 for a standard Delrin-class grade is worth questioning. And the copolymer band is genuinely wide, so “M80” and “M88” can both be honest answers to the same question. One clarification: the Delrin® 100, 500, and 900 series differ by melt viscosity, not hardness, so the series isn’t a hardness lever.

Sourcing Consistency: Keeping a POM Hardness Spec Stable

Hardness is one of the fastest non-destructive checks on incoming acetal, and drift tells you what’s in the bag.

Ines was a process engineer for a medical-device molder in Barcelona. Her validated copolymer grade held its Rockwell M band for eight months, then a new campaign arrived with readings six points low on the same lot numbers. Nothing had changed in the resin. Her team had raised the mold temperature to fix a short-shot, which shifted crystallinity.

Lot-to-lot drift usually points to off-spec or substituted resin, regrind blended into virgin, a filler-content change, or a crystallinity shift from inconsistent cooling. The first three show up on a COA; the fourth is yours to control.

Confirm the grade designation, melt flow rate (ASTM D1238 or ISO 1133), lot number, and filler content on every COA, then cross-check against the manufacturer’s datasheet. POM isn’t hygroscopic, so MFI and density are the more useful substitution signals.

How Yifuhui supplies hardness-consistent POM

We hold branded prime acetal from the BASF Ultraform®, Celanese Hostaform®, Polyplastics DURACON®, Kolon KOCETAL®, and Formosa FORMOCON® families, with traceable lots and full COA and MSDS documentation, so the hardness band you validated is the band you reorder. Minimum order is 25 kg, terms are FOB Shanghai, and export documentation ships with the goods. Browse POM resin for datasheets on your grade.

Frequently Asked Questions

What is the hardness of POM?

POM hardness is Rockwell M94 for unfilled homopolymer, with grades spanning M88 to M94, and M80 to M88 for copolymer. The same material reads roughly Rockwell R120, about Shore D 84 or 80 to 82, and 140 MPa by ball indentation to ISO 2039-1.

Is POM hardness M94 or M88?

Both figures are correct, and the difference has two sources. Test standard accounts for several points, since the same grades read 88 M under ISO 2039-2 and 89 M under ASTM D785 on one published comparison. Grade family accounts for the rest, because homopolymer crystallizes more fully.

Which is harder, POM homopolymer or copolymer?

Homopolymer, by roughly 15 percent. Pure formaldehyde chains pack into more regular crystals, so homopolymer reads M94 against M80 to M88. Copolymer trades that hardness for hot-water resistance and a wider processing window.

Does glass filled POM have higher hardness?

Yes, and more than any other common filler. Ball indentation rises from 140 MPa unfilled to 215 MPa at 40 percent glass fibre, and one GF30 copolymer grade reads M113 on ASTM D785.

Does higher POM hardness mean better wear resistance?

Not on its own. Wear depends on the counterface, load, speed, lubrication, and environment as much as on hardness, and copolymer frequently outlasts harder homopolymer in wet service. Specify the friction coefficient, wear rate, and limiting PV for the real application.

Conclusion

POM hardness comes down to one scale, one standard, and one grade family. The number: Rockwell M94 for unfilled homopolymer, with copolymer at M80 to M88, and roughly R120, Shore D 84, or 140 MPa depending on the method.

Three things follow. Homopolymer is harder because it crystallizes more fully, while copolymer gives up hardness for hydrolysis resistance and processing latitude. Glass fibre is the only filler that makes acetal meaningfully harder, pushing ball indentation from 140 to 215 MPa. And hardness alone doesn’t predict wear.

That leaves the specification and the supply chain. A hardness callout without a scale letter and a test method isn’t a specification, and a reading that drifts lot to lot is one of the earliest signals of off-spec, regrind-blended, or substituted resin.

Specifying acetal for a gear, bushing, or precision part with a validated POM hardness band? Request a quote for branded prime POM (Ultraform®, Hostaform®, DURACON®), 25 kg minimum order, full COA documentation, FOB Shanghai.