Two suppliers send you hardness charts for the same five materials. One ranks polycarbonate above acetal. The other ranks acetal above polycarbonate. Both charts are genuine datasheet values, neither is a typo, and neither supplier is wrong.
That contradiction is why most engineering plastic hardness comparison exercises fail before they start. Published engineering plastic hardness charts mix Rockwell R rows with Rockwell M rows, quote unfilled grades beside 30% glass-filled ones, and print dry-as-molded nylon next to conditioned nylon. When the numbers under a table are not on the same axis, the verdict you read off the top isn’t real.
This guide gives you the four rules for a fair comparison, side-by-side values with the correct scale labelled, head-to-head verdicts for the matchups that actually get searched (POM vs PA66, POM vs polycarbonate, PA66 vs PC, ABS vs PC, HDPE vs PP), application-driven selection logic, and the sourcing step that keeps a hardness spec comparable across lots. The complete guide to plastics hardness covers the underlying scales in depth.
Need two hardness values that can actually be compared? Get a resin quote for branded prime material with the scale, standard, and condition stated on every datasheet line.
Why Engineering Plastic Hardness Comparison Charts Disagree
If two charts contradict each other on the same polymer, one of four things is happening. None of them is a data error. This is the short answer to the question every specifier eventually asks: why do plastic hardness charts disagree?
Different scales carry different letters, and R is not M
Rockwell R uses a 1/2 inch (12.7 mm) steel ball under a 60 kgf major load. Rockwell M uses a 1/4 inch ball under 100 kgf. Different ball, different load, different number.
MatWeb’s Rockwell hardness reference documents the ball diameters and loads behind each scale. A POM grade published at R120 and a PC grade published at M78 aren’t on one axis, so “which is higher” has no answer. The Rockwell hardness of plastics article works through every scale letter.
Different standards, different numbers, same polymer
Consider POM homopolymer: it is published as M94 under ASTM D785 and M88 under ISO 2039-2, and some data sheets list M89 under ASTM as well . One material, three published values, zero measurement error. The standard decides the number as much as the polymer does.
Different grades, unfilled versus glass-filled
Filler content overwhelms polymer family. Glass-filled PA66 (GF33) is reported around Rockwell M101, and glass-filled POM (GF30 with PTFE) around M113, both above the unfilled versions of the same polymer. GF nylons often move to the Rockwell E scale once the filler pushes them past the R and M ranges. Published tables such as NBK Global’s physical properties list simply place POM, nylon, and PC values in one column with no scale warning. Composition, not the family name, sets what you measure.
Different conditions, dry versus conditioned
Moisture is the sharpest example. PA66 absorbs about 2.5% water; POM absorbs under 0.25%. Dry, PA66 reads higher (roughly 160 MPa ball indentation against POM’s 140 MPa). Conditioned to 50% relative humidity, PA66 drops to roughly R104–R108 and POM becomes the harder of the two.
Same materials, ranking reversed, purely from moisture. The PA66 hardness article documents that shift.
“Harder” is three separate claims
Finally, indentation hardness, scratch resistance, and stiffness are different properties with different tests. POM is harder than HDPE on every indentation scale, yet on the sand-abrasion index HDPE wears at 11.0 against polyacetal’s 6.5: harder did not mean more wear resistant. MatWeb and the SpecialChem hardness guide both warn that Shore and Rockwell results should not be used alone to predict abrasion, wear, or strength.
The Four Rules for a Fair Engineering Plastic Hardness Comparison
- Compare only within the same scale. R to R, M to M, Shore D to Shore D.
- Compare like grades. Unfilled to unfilled, and note filler content whenever it isn’t zero.
- Compare like conditions. State dry or conditioned for every nylon value, and state the test temperature.
- Treat any cross-scale conversion as an estimate, never a spec.
So can you compare Rockwell M to Rockwell R? No, not as a measurement. You can rank the two by application, but the letters themselves have no official conversion. The same applies to Shore D and Rockwell: the Shore hardness scale explained article covers why the two don’t scale into one another.
Engineering Plastic Hardness Comparison Table (Side by Side)
Here is the engineering plastic hardness comparison chart that refuses invalid rows. The “comparable with” column is the point: it tells you which values you may legally line up.
| Material | Primary scale | Typical value | Comparable with | Best-fit application |
|---|---|---|---|---|
| LDPE | Shore D | D40–D50 | Shore D group only | Flexible film, liners, non-load-bearing parts |
| HDPE | Shore D | D60–D70 | Shore D group only | Chemical tanks, cutting boards, low-friction strips |
| PP | Shore D, Rockwell R | D70–D75, R80–R100 | Shore D and R groups | Living hinges, chemical-resistant housings |
| PTFE | Shore D | D50–D65 | Shore D group only | Chemical seals, low-friction bearings |
| PA6 (dry) | Rockwell R | R100–R115 | R group only | Gears, bushings, moisture-sensitive parts |
| PA66 (dry as molded) | Rockwell R | R119–R120 | R group only | Structural brackets, GF housings |
| PA66 (conditioned 50% RH) | Rockwell R | roughly R104–R108 | R group only | The same parts, in service |
| PBT | Rockwell R | R100–R120 | R group only | Electrical connectors, GF housings |
| ABS | Rockwell R | R90–R110 | R group only | Consumer housings, impact-first parts |
| Rigid PVC | Rockwell R | R100–R120 | R group only | Pipe and profile |
| PC | Rockwell M | M70–M80 | M group only | Transparent guards, impact-critical parts |
| PMMA | Rockwell M | M80–M100 | M group only | Optical lenses, scratch-critical displays |
| POM copolymer | Rockwell M | M76–M80 (Shore D about 80) | M group only | Gears, bushings, conveyor parts |
| POM homopolymer | Rockwell M | M88–M94 | M group only | Precision gears, high-stiffness wear parts |
| PEEK (unfilled) | Rockwell M | M99–M105 | M group only | High-temperature and chemical-duty parts |
Three lanes emerge. R-scale: PP, ABS, PA6, PA66, PBT, rigid PVC, PET. M-scale: PC, PMMA, HIPS, POM copolymer, POM homopolymer, PEEK. Shore D: LDPE, HDPE, PP, PTFE, TPU. Each lane is internally comparable; cross a lane boundary and you are ranking, not measuring.
Every figure is a starting point. Verify against the specific grade datasheet before writing a number onto a drawing. HDPE is the clearest example: many grades report no Rockwell value at all, and those that do span roughly R40 to R75.
Head-to-Head: Which Engineering Plastic Is Actually Harder?
Each verdict opens with the short answer, then the numbers on a stated scale, then the caveat that decides real parts.
POM homopolymer vs PA66
POM wins on the M scale and holds that position in moisture; PA66 wins dry on R and then gives back 20 to 40% once conditioned. POM homopolymer sits at M88–M94, a scale PA66 is not quoted on for unfilled grades. Dry, PA66’s ~160 MPa ball indentation beats POM’s ~140 MPa, so a dry-basis chart favours nylon; in service at 50% RH the two swap places. The POM hardness article explains why crystallinity keeps POM’s number stable.
POM vs polycarbonate
On the same scale, POM homopolymer reads harder: M88–M94 against PC’s M70–M80. That is a legal comparison because both are Rockwell M, and it’s also the wrong reason to pick POM over PC, whose advantage is impact toughness. Choose POM for wear and low friction, PC for impact and transparency.
PA66 vs polycarbonate
Different scales, so no direct number comparison exists; PC is the more stable hardness and PA66 the higher dry reading. Dry PA66 (R119–R120) looks dramatic, but it’s a Rockwell R value in a different lane from PC’s M70–M80, and it slides once moisture enters the part. If you need a hardness number that stays put across service life, PC is the safer specification. See polycarbonate hardness for the conditioning picture.
ABS vs polycarbonate
PC is harder on both scales it’s quoted on (M70–M80 against ABS at R90–R110), and that isn’t why anyone chooses between them. PC wins on impact strength, stiffness, and clarity; ABS wins on cost, processability, and a softer, less notch-sensitive failure mode. The matchup is decided by impact, cost, and appearance. Our PC versus ABS comparison covers the rest of the decision, and ABS plastic hardness covers the ABS grades.
HDPE vs PP
PP is harder and stiffer (Shore D 70–75 against HDPE’s 60–70); HDPE wins on toughness and chemical resistance at low temperature. The gap is real and small enough that it rarely decides a part. PP also carries a Rockwell R figure (R80–R100) that HDPE mostly does not, which is why charts putting both on R looked contradictory. Full detail is in the HDPE hardness article.
PEEK vs the high-volume family
PEEK leads the field at M99–M105 unfilled, making it the hardest engineering plastic in this comparison, and POM homopolymer leads the high-volume unfilled group behind it. PEEK is a comparison ceiling, not a cost-effective substitute: it earns its place on temperature and chemical duty, where POM and PA66 can’t compete. For most wear parts, the hardest plastic you can afford is the right answer, and that’s usually POM homopolymer.
Hardness Rankings by Application (Not by Number)
However, ranking materials by hardness alone produces a list that’s technically correct and practically useless. Rank them by what the part has to survive.
| Application | Hardness ranking | The property that decides it | Specify |
|---|---|---|---|
| Gears, bushings, wear strips | POM-H > POM-C > PA66 > HDPE | Hardness plus low friction (COF 0.2–0.35) | POM (POM resin) |
| Scratch-critical, optical | PMMA > hard-coated PC > PC | Pencil hardness and haze after abrasion | PMMA, or coated PC |
| Structural, load-bearing | PEEK > POM-H > PA66 GF > PBT GF > PC | Stiffness and creep under sustained load | PA66 GF30, PBT GF |
| Chemical exposure, low friction | PEEK > PTFE > PP > HDPE | Chemical resistance, not hardness | PTFE, HDPE |
| Impact-first | PEEK > PC > PA66 > ABS | Notched impact, not indentation | PC, ABS (PC resin) |
Two rows deserve the honest caveat. PMMA reads harder than PC on Rockwell M (M80–M100 against M70–M80) and scratches less, so here the harder material is also the better scratch performer. It is brittle, though, so an impact-exposed lens ends up as hard-coated PC instead.
PTFE, meanwhile, sits near the bottom of every hardness chart while being the only correct answer for aggressive chemical service.
What to Do When Two Sources Give You Different Numbers
In practice, this is how the confusion plays out. Tomas, a gear designer at a Portuguese motion-control company, had POM quoted at M94 from one supplier and M88 from another, and concluded one compound was soft. Both were correct homopolymer values, one to ASTM D785 and one to ISO 2039-2. Two hours of argument came down to a missing standard letter.
The polycarbonate R118 versus M70 contradiction. Some tables publish PC at R118–R133 and others at M70–M80; both are real measurements of the same polymer family. The R figure comes from the 1/2 inch, 60 kgf test and the M figure from the 1/4 inch, 100 kgf test, and the M value is the house convention in Covestro Makrolon® datasheets. If you were told PC is softer than ABS because of an M value, you were reading a different scale.
The HDPE Rockwell span. HDPE appears as R65, R40, and R75 across sources, which looks like three materials. It is one material, several grades, and a minority reporting convention: many HDPE datasheets publish no Rockwell value at all and lead with Shore D. Read HDPE on Shore D 60–70 and treat Rockwell as secondary.
Fortunately, the fix is procedural. Go to the grade datasheet, not the family average, then read the hardness line in four parts: the scale letter, the load or ball diameter, the standard (ASTM D785 or ISO 2039-2), and the condition (dry, conditioned, temperature). A number missing any of the four can’t be compared.
Making Hardness Comparisons Hold Across Suppliers
Priya runs incoming QC for an automotive tier-two supplier switching POM sources on an underhood bracket. The substitute grade read four Rockwell points below the validated spec, which on paper looked like a softer material and in practice was an unverified one. A comparison between two suppliers is only meaningful when both report comparable evidence.
Which COA values actually anchor a comparable hardness spec
Four lines on the certificate do the work:
- Melt flow index to ASTM D1238 or ISO 1133, the fastest signal that two lots came from the same process window
- Density, which moves with crystallinity and filler loading
- Filler content, because glass or mineral loading changes hardness more than the base polymer does
- Lot number, tying the sheet in your hand to the drum you received
All of which is why a branded prime grade is the only reliable baseline. Covestro Makrolon® PC, BASF Ultraform®, Celanese Hostaform® and Polyplastics DURACON® POM, and BASF Ultramid® PA66 all publish hardness with the scale, standard, and condition stated, so comparing them is comparing the same thing. A generic or regrind compound with an undeclared standard resets the whole exercise.
Comparing two grades across suppliers or lots? Contact our technical team for branded prime POM, PA66, PC, ABS, and HDPE with COA-documented MFI, density, and lot traceability.
Conclusion
In the end, a valid engineering plastic hardness comparison lives inside one scale, one grade family, and one condition. Most published charts break all three rules in a single grid, which is why they contradict each other and why the verdict you read off them doesn’t hold up in service.
The verdicts that survive scrutiny: POM homopolymer leads the high-volume unfilled group at M88–M94 and keeps that hardness in moisture; PA66 leads on a dry basis and gives ground to water; PC is stable and tough rather than hardest, at M70–M80; PP beats HDPE on hardness while HDPE wins on toughness; and PEEK sits above all of them as the ceiling. Anything you rank above POM here, you are paying for in cost, temperature capability, or chemical resistance, not in hardness alone.
Getting an engineering plastic hardness comparison right is half measurement and half sourcing. A number you can’t trace to a standard, a grade, and a lot is not comparable to anything.
Ready to specify grades that can actually be compared? Request a quote for branded prime POM, PA66, PC, ABS, or HDPE with COA-documented MFI and full lot traceability. 25 kg minimum order, FOB Shanghai, response within 24 hours.
Frequently Asked Questions
Which engineering plastic is the hardest?
Unfilled PEEK is the hardest common engineering thermoplastic at Rockwell M99–M105. Among high-volume, cost-effective unfilled grades, POM homopolymer leads at M88–M94; the plastics hardness guide charts every material individually.
Is POM harder than nylon (PA66)?
Yes in service. POM homopolymer reads M88–M94 and holds it in moisture, while PA66’s dry advantage (roughly 160 MPa ball indentation) falls to about R104–R108 once conditioned.
Is polycarbonate harder than acrylic (PMMA)?
No. On Rockwell M, PMMA reads M80–M100 against PC’s M70–M80, so PMMA is harder and scratches less. It is also brittle, which is why impact-exposed optical parts use hard-coated PC.
Is ABS harder than polycarbonate?
No. PC is harder on both scales it’s quoted on (M70–M80 versus ABS at R90–R110). ABS is chosen for cost, processability, and impact behaviour, not hardness.
Can I compare Rockwell R values with Rockwell M values?
No. Different ball diameter and different major load put them on separate axes. Rank each material within its own scale group, and treat any R-to-M conversion table as an estimate, never a specification.
Can I convert Shore D to Rockwell?
Only approximately, and never for a drawing. Use the conversion as a sanity check, then test the actual grade on the scale your specification cites.
Does glass fiber make plastic harder?
Yes, substantially. Glass-filled PA66 (GF33) reaches about Rockwell M101 and glass-filled POM (GF30 with PTFE) about M113, above the unfilled grades and sometimes far enough to move onto the Rockwell E scale.
Why does my grade datasheet disagree with the comparison chart?
Because the chart is a family average and the datasheet is a specific grade. Grade, filler content, moisture condition, and cited standard all shift the number. The datasheet wins when you specify a real part.
Which plastic is hardest for wear parts?
POM homopolymer, on the combination of hardness (M88–M94) and low friction (COF 0.2–0.35). PEEK is harder but costs far more and is usually reserved for temperature or chemical duty.
