Quick Answer: POM (polyoxymethylene, acetal) and nylon (polyamide, PA) are both widely used engineering plastics for machined and molded mechanical components, but they serve different functional roles. POM is the correct choice when dimensional stability, low friction, and tight tolerance retention are the primary requirements — its moisture absorption of approximately 0.2% means parts maintain their machined dimensions across humidity variations. Nylon absorbs 1–3% moisture by weight, which can produce dimensional change of 0.3–0.8% and cause tolerance drift in precision fits. Nylon’s advantage is mechanical toughness and impact resistance: its notched Izod impact strength is approximately 50–100 J/m (compared to approximately 50–70 J/m for POM), making it more resistant to shock loading and crack propagation under dynamic load. The engineering rule is: parts requiring tight tolerances (±0.020–0.050 mm), low friction interfaces, and dimensional consistency in variable humidity environments belong in POM; parts requiring impact resistance, high cyclic load capacity, and wear resistance under heavy contact pressure belong in nylon.
What Actually Differentiates POM and Nylon
POM and nylon are both semi-crystalline polymers with good mechanical properties, low friction, and broad chemical resistance. Their differences emerge in four specific areas that determine suitability for demanding engineering applications.
Moisture absorption and dimensional stability: POM’s moisture absorption is approximately 0.20–0.25% (ASTM D570, 24-hour immersion). Nylon’s moisture absorption is substantially higher — PA6 absorbs approximately 2.7–3.5% moisture in equilibrium at 65% relative humidity, and PA66 absorbs approximately 2.5–3.0%. The practical consequence is dimensional change: a PA6 component exposed to ambient humidity from a dry state can change dimensions by approximately 0.5–0.8% in linear dimensions. For a precision bushing with an 8 mm bore machined to ±0.025 mm tolerance, this represents a potential bore change of 0.040–0.064 mm — exceeding the tolerance. POM’s 0.2% moisture absorption produces negligible dimensional change under normal ambient humidity variation.
Impact resistance and fracture behavior: Nylon’s higher elongation at break (50–150% vs 20–35% for POM) and impact energy absorption make it more resistant to notch-sensitive fracture under shock loads. POM behaves more like a rigid engineering plastic — it carries static loads with minimal deformation but is more prone to brittle fracture at stress concentrations under impact. For a mechanical component subject to repeated shock loads (a guide rail stop block, a roller, a heavy-duty bushing absorbing impact from a mechanism), nylon’s ability to deform slightly and absorb energy before fracture produces longer service life than POM.
Friction and wear characteristics: POM has a lower dry sliding friction coefficient (~0.20–0.35 against steel) than PA6 (~0.30–0.45 against steel) under low-to-moderate loads. This difference in dry friction makes POM preferable for precision sliding interfaces where consistent low friction force is required. However, under high contact pressure with limited or no lubrication, nylon’s wear resistance is superior to POM’s — the pressure-velocity (PV) limit for nylon (approximately 0.15–0.20 MPa·m/s) is comparable to or exceeds POM, and nylon maintains its wear performance better as contact pressure increases. For high-load bearings and wear pads where the primary failure mode is surface wear rather than friction coefficient, nylon often outlasts POM.
Machinability: POM machines more predictably than nylon for precision applications. POM forms short, well-broken chips and holds tight tolerances in turning and milling (±0.025–0.050 mm achievable with standard practice). Nylon’s higher elongation produces stringy chips that can wrap around the cutting tool, and its flexibility under clamping force causes elastic springback after machining, making it difficult to consistently hold tolerances below ±0.050–0.075 mm without specialized fixturing and process control.
Mechanical Properties Comparison
| Property | POM (Acetal/Delrin) | Nylon PA6 | Nylon PA66 | Engineering Implication |
|---|---|---|---|---|
| Tensile strength (dry) | 60–70 MPa | 75–85 MPa | 80–90 MPa | Nylon stronger in tension |
| Flexural modulus | 2.5–3.2 GPa | 2.5–3.5 GPa | 2.8–3.8 GPa | Similar stiffness, dry |
| Notched Izod impact (ASTM D256) | 50–70 J/m | 50–100 J/m | 60–100 J/m | Nylon absorbs more impact energy |
| Elongation at break | 20–35% | 50–150% | 40–90% | Nylon much more ductile |
| Moisture absorption (ASTM D570, 24hr) | 0.20–0.25% | 1.6–2.5% | 1.2–1.5% | POM dramatically lower |
| Equilibrium moisture absorption (65% RH) | ~0.20% | 2.7–3.5% | 2.5–3.0% | Nylon changes significantly with humidity |
| Friction coefficient (dry, vs steel) | 0.20–0.35 | 0.30–0.45 | 0.30–0.40 | POM lower friction |
| Rockwell hardness (M scale) | M80–M90 | M70–M80 | M75–M85 | POM harder surface |
| Continuous service temperature | -40 to +90°C | -40 to +105°C | -40 to +120°C | Nylon better high-temperature |
| Density | 1.41–1.43 g/cm³ | 1.12–1.15 g/cm³ | 1.13–1.16 g/cm³ | Nylon lighter |
The dry vs conditioned property distinction: The properties in the table above are measured on dry-as-molded specimens. Nylon’s tensile strength decreases by approximately 20–30% after moisture conditioning to equilibrium, while its impact strength and elongation increase. POM’s properties remain essentially unchanged by moisture. This means nylon’s data sheet showing higher tensile strength than POM is measured under conditions that may not represent the material’s service condition, while POM’s properties are consistent.
When POM Is the Correct Choice
Precision gears and gear trains: Plastic gear systems require consistent tooth geometry over the service life. POM’s low moisture absorption ensures the tooth pitch and profile remain within the machined or molded tolerance across humidity cycles. Low friction reduces heat generation at meshing contacts. POM’s hardness provides good surface durability against the mating gear. Standard practice for precision plastic gears in instrumentation, automation, and consumer products is POM (Delrin or Hostaform).
Precision bushings and sliding fits: A bushing maintaining a shaft clearance of 0.020–0.050 mm requires that the bore dimension remains within this range throughout the service environment. POM maintains this without adjustment for humidity. POM bushings in production applications typically achieve 0.020–0.040 mm running clearance with consistent friction torque over the service life. Nylon bushings in the same application, exposed to normal workshop humidity variation, may change bore diameter by 0.040–0.080 mm — potentially eliminating the design clearance.
Linear guides and conveyor components: Low-friction linear slides, conveyor chain guides, and linear motion guides benefit from POM’s stable friction coefficient and dimensional consistency. In food processing conveyor systems, POM’s chemical resistance to cleaning agents and its FDA food-contact compliance (for certain grades) make it suitable for these environments.
CNC machined precision components: For machined parts requiring tolerances of ±0.025–0.050 mm in features that must maintain those tolerances in service, POM is the standard engineering plastic choice. The predictable machining behavior (clean chip formation, low cutting forces, minimal springback) and dimensional stability after machining make it reliable for precision applications.
When Nylon Is the Correct Choice
Heavy-duty rollers, wheels, and impact components: Applications where the part absorbs repeated impact — conveyor rollers, forklift guide wheels, bumpers, and stop mechanisms — require a material that deforms elastically under impact and dissipates energy without fracture. Nylon’s higher elongation and impact energy absorption prevent the brittle fracture that POM can experience under shock loading, producing longer service life in these applications.
High-load bearings and wear pads under pressure: When bearing pressure is high (above approximately 10–15 MPa contact stress), nylon’s better performance under boundary lubrication conditions and its toughness under cyclic stress make it preferable to POM. Industrial plain bearings, wear strips in heavy machinery, and load-bearing pads in hydraulic machinery commonly use nylon (often filled with MoS₂ or PTFE to further reduce friction and wear).
Structural parts requiring impact resistance in controlled environments: Nylon’s higher tensile strength (dry) and toughness make it preferable for structural components that must resist occasional overloads or impact events, provided the application environment can be specified as controlled humidity or where dimensional change from moisture is acceptable within the design tolerance.
Applications where lower material density matters: Nylon’s density of 1.12–1.15 g/cm³ vs POM’s 1.41–1.43 g/cm³ means nylon parts weigh approximately 20% less for the same volume. For lightweight mechanism components where mass reduction is a design objective, nylon provides structural performance at lower weight — relevant in aerospace, automotive, and portable equipment applications.
Moisture Absorption: Why It Determines Suitability for Precision Applications
The moisture absorption difference between POM (~0.2%) and nylon (~2.7–3.5% at equilibrium) is the single most important material property distinction for engineering applications involving dimensional tolerances.
The dimensional change calculation: Linear dimensional change ≈ moisture absorption % / 3 (for isotropic approximation). For PA6 absorbing 2.7% moisture from dry to equilibrium at 65% RH: linear change ≈ 0.9%. For a 50 mm bore: 50 × 0.009 = 0.45 mm change. For POM absorbing 0.2% moisture: linear change ≈ 0.067%. For a 50 mm bore: 0.033 mm change.
The practical consequence for precision assemblies: Any assembly specification involving a shaft-to-bore clearance tighter than approximately 0.10–0.15 mm (typical for precision machine components, automation systems, and instruments) is at risk if nylon is used in environments with variable humidity, because the dimensional change from moisture absorption can exceed the designed clearance. POM’s dimensional change is an order of magnitude smaller and is within typical dimensional uncertainty for CNC machined parts.
Moisture conditioning for precision nylon parts: When nylon must be used in precision applications, conditioning the parts to equilibrium moisture content before machining the final precision features partially addresses this problem — the part dimension is stable if the service humidity matches the conditioning humidity. This is a manageable approach for controlled-environment applications but adds manufacturing complexity and is not practical for parts used across varying environments.
CNC Machining Comparison
| Machining Factor | POM | Nylon |
|---|---|---|
| Chip formation | Short, clean chips | Long, stringy chips; may wrap on tool |
| Dimensional stability under cutting | Excellent; minimal springback | Moderate; elastic recovery after clamping |
| Achievable tolerance (standard practice) | ±0.025–0.050 mm | ±0.050–0.100 mm (tighter requires care) |
| Surface finish | Ra 1.6–3.2 µm readily; Ra 0.8–1.6 µm with finishing | Ra 1.6–3.2 µm typical; more variable |
| Burr tendency | Minimal | More prone to burring and smearing |
| Post-machining dimensional change | Negligible | May change with humidity exposure |
| Clamping force sensitivity | Low (rigid) | Higher (flexible; distorts under clamping) |
Why nylon’s machinability limitations matter: For a production part machined on a CNC lathe to ±0.030 mm bore diameter, POM achieves this tolerance reliably with standard tooling and process. Nylon’s tendency to spring back elastically after clamping force is removed means the bore diameter measured after release from the chuck may differ from the diameter measured while clamped. For tight-tolerance bores, this springback — typically 0.030–0.080 mm for thin-walled nylon components — requires compensation in the machining program and careful fixturing.
Modified Grades for Demanding Applications
Both POM and nylon are available in modified grades that extend their performance envelopes:
POM modifications: PTFE-filled POM (5–20% PTFE) reduces the friction coefficient to approximately 0.10–0.15, suitable for dry bearing and slide applications. Carbon fiber-reinforced POM improves stiffness and reduces creep for structural applications. POM-H (homopolymer, e.g., Delrin) has higher crystallinity and slightly better fatigue and creep resistance than POM copolymer grades; POM copolymer has better chemical resistance and hydrolytic stability.
Nylon modifications: Glass fiber-reinforced PA66 (30% GF) increases tensile strength to 160–180 MPa and stiffness to 9–11 GPa, dramatically improving structural performance and reducing moisture-induced dimensional change (the glass fibers constrain matrix swelling). MoS₂-filled nylon improves tribological performance without external lubrication. Heat-stabilized grades extend the continuous service temperature. PA12 (nylon 12) has substantially lower moisture absorption (~0.7% equilibrium at 65% RH) than PA6 or PA66, making it a practical alternative when nylon toughness is required but moisture sensitivity must be reduced.
Common Mistakes in Material Selection
| Mistake | Consequence | Correct Approach |
|---|---|---|
| Using nylon for precision bushings in variable humidity | Bore diameter changes by 0.040–0.080 mm; clearance eliminated; seizing or binding | Use POM; or if nylon required, specify PA12 and validate dimensional stability in service conditions |
| Using POM for shock-loaded components (rollers, stops) | POM fractures under repeated impact at stress concentrations | Use nylon or nylon + GF for impact-absorbing components |
| Comparing nylon and POM on dry tensile strength only | Nylon’s real-world strength is 20–30% lower than dry datasheet values due to moisture conditioning | Test both materials in conditioned (equilibrium moisture) state; compare dimensional stability, not just dry strength |
| Applying nylon to tight-tolerance CNC machined parts | Stringy chips, springback, and post-machining moisture absorption make tolerances unreliable | Use POM for machined precision parts; nylon for molded, less precision-critical components |
| Assuming cost-per-kg determines total manufacturing cost | Nylon’s lower raw material cost is offset by higher machining cost (deburring, tighter process control) and potential scrap from tolerance non-conformance | Compare total manufactured part cost including machining, inspection, and scrap, not raw material price |
Key Takeaways
- POM is the correct choice for precision applications requiring stable dimensions across humidity variation: with moisture absorption of ~0.2%, POM maintains machined tolerances of ±0.025–0.050 mm without dimensional change in normal service environments. Nylon’s 2.7–3.5% equilibrium moisture absorption produces 0.4–0.7% linear dimensional change — 5–20× the tolerance budget for precision fits.
- Nylon is the correct choice when impact resistance and energy absorption under shock loading are the primary failure mode: notched Izod impact strength of 50–100 J/m and elongation at break of 50–150% allow nylon components to deform elastically and absorb impact energy that would fracture POM at the same stress concentrations.
- POM machines more predictably for precision CNC parts: clean chip formation, minimal springback, and dimensional stability after machining allow ±0.025–0.050 mm to be held reliably. Nylon’s stringy chips and elastic recovery require tighter process control to achieve equivalent tolerances.
- Both materials are available in modified grades: PTFE-filled POM for lower friction dry bearings; glass-reinforced nylon for structural applications with reduced moisture sensitivity; PA12 as a compromise nylon with substantially lower moisture absorption than PA6 or PA66.
- Properties should be compared in the conditioned (moisture-equilibrium) state, not from dry datasheet values. Nylon’s dry tensile strength appears higher than POM’s; conditioned, the difference is smaller and the dimensional instability of nylon may make it unsuitable regardless of strength.
- The correct selection rule: precision, dimensional stability, and low friction → POM; impact resistance, shock absorption, and high-load wear capacity → nylon. When both properties are simultaneously required (stable fit and impact resistance), evaluate modified grades or reconsider the design to separate the functional requirements.
- For OEM procurement and design teams: when specifying engineering plastic components on drawings, material should be specified by grade and standard — “POM homopolymer, ASTM D4181 Type 1” or “PA66 unfilled per ASTM D6779 Grade 66 1301” — not just “acetal” or “nylon.” This ensures the supplier uses material with the specified properties, and it identifies which material property is being relied on for function, which facilitates early identification of applications where moisture absorption or impact resistance will determine success.
Frequently Asked Questions
What is the main difference between POM and nylon for engineering parts?
The most functionally significant difference is moisture absorption. POM absorbs approximately 0.20–0.25% moisture, producing negligible dimensional change in service. Nylon (PA6) absorbs approximately 2.7–3.5% at equilibrium in ambient humidity conditions, which translates to approximately 0.4–0.7% linear dimensional change — 10–20 µm per millimeter of dimension. For precision fits and tight tolerances, this dimensional change from moisture exposure makes nylon unreliable without humidity control. POM also has lower dry friction (coefficient ~0.20–0.35 vs ~0.30–0.45 for nylon against steel) and better machinability for tight-tolerance CNC parts. Nylon’s advantage is higher impact energy absorption, better performance under high contact pressure wear, and lower density.
Which is better for plastic gears — POM or nylon?
POM is the standard material for precision plastic gears. The gear tooth profile must remain within tolerance throughout the service life; POM’s moisture stability ensures tooth pitch and profile remain consistent. POM’s lower friction reduces heat generation at meshing contacts and noise during operation. Nylon absorbs moisture and can change tooth dimensions over time, increasing backlash. The exception is high-load, low-speed gears in heavy industrial equipment where impact resistance is more critical than dimensional precision — in these cases, nylon with glass fiber reinforcement may provide better durability despite the dimensional variability.
Can POM be used outdoors or in wet environments?
POM is more suitable than nylon for outdoor and wet environments because its moisture absorption (~0.2%) is approximately 10–15× lower than nylon, meaning POM dimensions are stable regardless of rainfall, humidity, or water immersion that would cause nylon to expand significantly. POM also has adequate UV resistance for moderate outdoor exposure, though UV-stabilized grades are available for extended outdoor service. Nylon in a wet environment will absorb moisture and change dimensions — problematic for precision fits but the softening effect from moisture absorption can also improve nylon’s impact resistance in wet applications. For general outdoor mechanical applications where dimensional precision matters, POM is the preferred choice.
Why does nylon machine worse than POM for tight tolerances?
Nylon’s machining limitations for tight tolerances stem from three material characteristics: its high elongation (50–150%) causes long, stringy chip formation rather than clean chip breaking, which can wrap around cutting tools and cause surface damage; its elastic flexibility means thin walls and features spring back after clamping force is released, producing dimensions that measure differently in the machine versus after part release; and post-machining moisture absorption causes dimensional change before the final precision features reach their service conditions. POM’s lower elongation (20–35%) produces short, manageable chips; its higher rigidity means minimal springback; and its low moisture absorption means machined dimensions are stable from machining to installation.
What modified grades improve nylon’s moisture sensitivity?
PA12 (nylon 12) has equilibrium moisture absorption of approximately 0.7% at 65% RH — substantially lower than PA6 (~3.5%) or PA66 (~2.5–3.0%) — while retaining nylon’s toughness and impact resistance. PA12 is appropriate for applications where nylon toughness is required but the moisture sensitivity of PA6/PA66 is unacceptable. Glass fiber-reinforced nylon (30% GF-PA66) also shows reduced moisture-induced dimensional change because the glass fibers constrain matrix swelling; a GF-PA66 part shows approximately 40–50% less dimensional change from moisture absorption than unfilled PA66. For applications requiring both low moisture sensitivity and high toughness, these grades provide a middle path between standard nylon and POM.
Written by the CMS engineering team with 15+ years of precision CNC machining experience producing POM (acetal/Delrin) and nylon (PA6, PA66, PA12) components for precision gears, bushings, linear guides, conveyor components, and automation fixtures in industrial, medical, food processing, and consumer electronics OEM manufacturing programs. Technical references: DuPont Delrin Design Guide (Mechanical Properties), BASF Ultramid Nylon Design Guide, ASTM D4181 (POM), ASTM D6779 (Nylon PA66), ISO 1133 (Plastics — Melt Flow Properties), Brydson J.A. — Plastics Materials (Engineering Polymers chapter).
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