POM vs PA6 vs PA66+GF: Which Plastic Lasts Longest for Moving Parts?
You designed a gear that should’ve lasted 500,000 cycles. At 80,000, the teeth are rounding off. Slop is showing up in the mechanism. The customer is asking questions. And you’re wondering if you picked the wrong material.
I’ve debugged this exact failure more times than I can count. Nine times out of ten, the root cause traces back to one decision: the choice between POM, PA6, and PA66+GF. They all live in the “engineering plastic” category, but their wear behavior under real loads is dramatically different. Here’s the data you need to get it right the first time.
The Core Problem: Wear in Moving Parts
Injection-molded gears, cam followers, and sliding mechanisms fail in three ways — abrasive wear (surface material gets ground off), adhesive wear (material transfers to the mating surface), and fatigue (subsurface cracks propagate until a chunk breaks loose). Each of these three materials responds differently to each failure mode.
The key metrics that predict real-world performance are wear factor (K), coefficient of friction against steel, pressure-velocity limit, and — the one nobody talks about enough — dimensional stability as humidity changes.
Let me run through each candidate.
POM (Acetal) — The Precision Standard
POM homopolymer is the benchmark for gear applications. Wear factor runs K = 1–3 × 10⁻⁶, which is the lowest of the three. Against unhardened steel, you’ll see a coefficient of friction of 0.2–0.35 dry. That’s good.
What makes POM special is what doesn’t change. Water absorption? 0.2%. That means a gear molded to 50.00 mm is still 50.00 mm after a year in a humid warehouse. For precision timing gears and indexing mechanisms, that’s the difference between reliable operation and field failures.
The tradeoff: POM’s PV limit sits at 0.10 MPa·m/s. Push past that and the surface temperature rises fast. At high loads or high speeds, it’ll fail before the PA66+GF option.
Copolymer POM (K = 2–5 × 10⁻⁶) trades a bit of wear resistance for better processing stability and reduced centerline porosity. For thick-walled gears, I lean toward copolymer every time.
PA6 — Solid When Dry, Unpredictable When Wet
PA6 unfilled gives K = 10–20 × 10⁻⁶. That’s 5–10× the wear rate of POM. The coefficient of friction against steel falls around 0.3–0.45 dry.
Here’s the thing about PA6 — it absorbs 1.8% moisture by weight at equilibrium. In a dry environment, it’s stiff and wears moderately. At 50% RH, it plasticizes, dimensions shift, and the wear rate changes. I’ve seen PA6 gears that ran beautifully in a climate-controlled lab fail within weeks in a coastal factory.
PV limit is 0.06 MPa·m/s, the lowest of the three. That makes PA6 a poor choice for continuous sliding contact under load.
When does PA6 make sense? Low-speed, low-load applications where cost is critical — it’s 0.8× the resin cost of POM. And if the gear runs fully lubricated in an oil bath, moisture absorption matters less.
PA66+30GF — Strong, Stiff, and Abrasive
PA66 with 30% glass fiber reinforcement changes the game. Wear factor K = 8–15 × 10⁻⁶ — better than unfilled PA6, still worse than POM. But the mechanical properties are in a different league: tensile strength around 170 MPa, flexural modulus above 8 GPa.
The PV limit is 0.12 MPa·m/s, the highest here. If your gear sees high loads and moderate speeds, PA66+GF handles the heat better than either POM or PA6.
But there’s a catch. The glass fibers are abrasive to steel mating surfaces. Running a PA66+GF gear against an unhardened steel shaft will wear the shaft, not the gear. That changes your failure mode — and your maintenance schedule. You need hardened steel (40+ HRC) or a different mating material.
Water absorption is 1.2% — better than PA6 but still an order of magnitude worse than POM. Expect dimensional shifts of 0.3–0.5% from dry-as-molded to equilibrium. For precision meshing, that matters.
Side-by-Side Comparison
| Property | POM (Homo) | POM (Co-poly) | PA6 | PA66+30GF |
|---|---|---|---|---|
| Wear Factor K (10⁻⁶) | 1–3 | 2–5 | 10–20 | 8–15 |
| CoF (Steel, Dry) | 0.2–0.35 | 0.25–0.40 | 0.3–0.45 | 0.3–0.50 |
| PV Limit (MPa·m/s) | 0.10 | 0.08 | 0.06 | 0.12 |
| Water Absorption | 0.2% | 0.3% | 1.8% | 1.2% |
| Dimensional Stability | Excellent | Excellent | Poor (wet) | Fair (wet) |
| Cost Index | 1.0x | 0.9x | 0.8x | 1.2x |
When to Pick Each One
Pick POM homopolymer when: You need precision — timing gears, indexing cams, sliding mechanisms where 0.05 mm matters. Water exposure is possible. Budget allows for the standard 1.0x cost index.
Pick POM copolymer when: Same requirements as homopolymer but the gear cross-section exceeds 6 mm wall thickness. Centerline porosity risk goes up with thick parts, and copolymer handles it better.
Pick PA6 when: Cost is the primary driver. The part runs lubricated or sees very light loads. Environmental humidity is controlled. Honestly, this is the “I have to use it because of the budget” choice.
Pick PA66+30GF when: The gear sees high structural loads — 100+ MPa tensile requirements. The mating surface is hardened steel. Some dimensional shift from moisture is acceptable in the design.
The Thing That Gets Overlooked
Dimensional stability from moisture absorption is the silent killer in plastic gear applications. I can’t tell you how many designs I’ve reviewed where someone spec’d PA6 for a precision gear because the tensile data looked good on paper. Six months later, the gear has grown 0.3 mm across the pitch diameter, the backlash has changed, and the mechanism feels sloppy.
POM doesn’t do that. If your part needs to stay the same size across seasons and climates, POM is the answer.
What This Means for Your Next Project
Run your application through these three questions:
- What’s the actual load and speed at the contact surface? Calculate your PV value — if it exceeds 0.08 MPa·m/s, PA66+GF becomes your best structural option.
- What’s the humidity range the part will see in service? Anything above 50% RH for extended periods rules out PA6 and makes PA66+GF a conditional choice.
- Is the mating surface hardened? If not, glass-filled materials will wear it out faster than the gear itself.
Get those answers right and you’ll land on the right material the first time instead of debugging a wear failure later.
At Corel Mould, we work with all three materials across hundreds of moving-part applications. Our engineering team can review your gear design, run wear analysis, and recommend the optimal resin before any steel gets cut. No guesswork, no expensive mold revisions.
Browse our material selection services or contact our team for help with your next gear project.