Acetal (POM) vs Nylon (PA): Engineering Thermoplastic Comparison
Acetal (polyoxymethylene, POM, Delrin) and Nylon (polyamide, PA 6, PA 66, PA 12) are the two most widely used engineering thermoplastics for precision machined components. Both materials serve in gears, bearings, bushings, electrical connectors, and structural hardware — yet their machining characteristics differ substantially. Understanding the tool selection differences between these materials is essential for achieving dimensional accuracy, surface finish, and production efficiency.
Acetal (POM homopolymer) offers density 1.41 g/cm³, tensile strength 70 MPa, hardness Rockwell M94, melting point 175°C, and extremely low moisture absorption (0.2–0.25%). Nylon 6/6 has density 1.14 g/cm³, tensile strength 80–85 MPa, hardness Rockwell R118, melting point 260°C, and moisture absorption of 2.5–3.0% at equilibrium. These property differences drive distinct tooling strategies.
Why Tool Selection Differs
Acetal is harder, stiffer, more dimensionally stable, and produces clean, brittle chips. It machines more like a free-cutting metal — predictable, with excellent surface finish achievable at high speeds. Nylon is softer, tougher, more ductile, absorbs moisture from the atmosphere and coolant, and produces stringy, gummy chips that wrap around tools. Nylon also has a lower thermal conductivity and a more pronounced tendency to smear and re-weld to cutting edges.
Tool Materials for Acetal (POM)
Polished Carbide (Primary)
Fine-grain uncoated carbide with polished rake faces and sharp cutting edges (edge radius 5–15 μm) is the standard for acetal machining. Acetal’s low friction coefficient and clean chip formation mean that standard carbide delivers excellent results.
- Turning inserts: CCMT 09T304, DCMT 11T304, VBMT 160404, CNMG 120408 — all uncoated, polished.
- Milling: 2-flute or 3-flute polished carbide end mills, 35–45° helix, 1/4″–3/4″ diameter.
- Drilling: Solid carbide, 118° polished point, standard web thickness.
HSS (Secondary)
M2 or M42 HSS ground sharp and polished is acceptable for form tools, boring bars, and low-volume operations.
Tool Materials for Nylon (PA)
Polished Carbide with Aggressive Geometry (Primary)
Nylon demands sharper, more positive-rake tooling than acetal. The key differences:
- Rake angle: 25–35° positive for nylon vs. 20–25° for acetal. The higher rake reduces cutting forces and prevents the tool from pushing into the softer, more ductile nylon.
- Edge radius: 5–10 μm — sharper than for acetal. Nylon requires a keener edge to shear cleanly without plowing.
- Chip breaker geometry: Aggressive chip breakers are critical for nylon. Without proper chip control, long stringy chips wrap around the tool and workpiece. Use deep-groove chip breakers or high-positive-rake insert geometries (e.g., Kennametal -FP, Sandvik -F).
- Clearance angle: 10–15° to prevent rubbing on the elastic nylon surface.
- Turning inserts: CCMT 09T304, DCMT 11T304 — uncoated, polished, with chip breaker.
- Milling: 2-flute polished carbide, 40–45° helix, generous flute volume for chip clearance.
PCD Tools (Nylon with Glass or Carbon Fill)
Glass-filled or carbon-filled nylon grades (PA6-GF30, PA66-CF20) contain abrasive reinforcement fibers that rapidly wear carbide. For these grades, PCD or diamond-coated tools are recommended — the same strategy used for CF-PEEK and G10.
Cutting Parameters: Turning Acetal
| Operation | Speed (SFM) | Speed (m/min) | Feed (IPR) | Feed (mm/rev) | DOC (in) | DOC (mm) |
|---|---|---|---|---|---|---|
| Rough Turning | 600–900 | 183–274 | 0.008–0.012 | 0.20–0.30 | 0.060–0.100 | 1.5–2.5 |
| Semi-Finish | 800–1,200 | 244–366 | 0.005–0.008 | 0.13–0.20 | 0.020–0.060 | 0.5–1.5 |
| Finish Turning | 1,000–1,500 | 305–457 | 0.002–0.005 | 0.05–0.13 | 0.005–0.020 | 0.13–0.50 |
Cutting Parameters: Turning Nylon
| Operation | Speed (SFM) | Speed (m/min) | Feed (IPR) | Feed (mm/rev) | DOC (in) | DOC (mm) |
|---|---|---|---|---|---|---|
| Rough Turning | 500–750 | 152–229 | 0.008–0.012 | 0.20–0.30 | 0.060–0.100 | 1.5–2.5 |
| Semi-Finish | 700–1,000 | 213–305 | 0.005–0.008 | 0.13–0.20 | 0.020–0.060 | 0.5–1.5 |
| Finish Turning | 900–1,300 | 274–396 | 0.002–0.005 | 0.05–0.13 | 0.005–0.020 | 0.13–0.50 |
Nylon runs approximately 15–20% slower than acetal due to its greater ductility and tendency to smear at high speeds. Both materials tolerate high speeds with sharp tools, but nylon reaches its thermal limit sooner.
Cutting Parameters: Milling Comparison
| Operation | Acetal SFM | Nylon SFM | Feed/Tooth (IPT) | Axial DOC | Radial DOC |
|---|---|---|---|---|---|
| Face Milling | 700–1,000 | 600–850 | 0.005–0.008 | 0.040–0.080 in | 60–75% of Ø |
| End Milling | 700–1,000 | 600–850 | 0.004–0.006 | 1.0× Ø | 10–25% of Ø |
| Slot Milling | 500–800 | 450–700 | 0.003–0.005 | 0.5× Ø | Full width |
| Drilling | 300–500 | 250–400 | 0.003–0.005 IPR | — | — |
Moisture: Nylon’s Hidden Variable
The single most underappreciated factor in nylon machining is moisture content. Dry-as-molded nylon (0.5% moisture) is harder, more brittle, and machines with clean, short chips. Conditioned nylon (2.5–3.0% moisture at equilibrium) is softer, tougher, more ductile, and produces long, stringy chips. Key impacts:
- Dimensional change: Nylon expands approximately 0.5% in volume going from dry to conditioned state. Parts machined dry will grow after absorbing atmospheric moisture.
- Machining behavior: Dry nylon cuts like a harder plastic with predictable chip formation. Wet nylon is gummy and prone to BUE.
- Coolant strategy: Flood coolant adds moisture to nylon workpieces. For precision parts, use air blast or MQL. If flood coolant is necessary, condition parts to equilibrium moisture before final finishing passes.
- Tolerance control: Hold nylon parts to ±0.003″ minimum. For ±0.001″ or tighter, use acetal instead.
Acetal Advantages for Precision Work
Acetal is the clear choice when dimensional precision is critical:
- Low moisture absorption (0.2%) means dimensions remain stable regardless of humidity.
- Higher stiffness (2,800 MPa vs. 2,000 MPa for nylon) reduces deflection under cutting forces.
- Clean chip formation produces predictable surface finishes of 16–32 μin Ra as-machined.
- Lower coefficient of friction (0.2–0.3 dynamic) reduces cutting forces and tool wear.
- Tighter machined tolerances of ±0.001–0.002″ are achievable and stable.
Nylon Advantages for Toughness
Nylon outperforms acetal in applications requiring impact resistance, high-temperature performance, and fatigue life:
- Higher impact strength (Izod: 1.5–2.5 ft-lb/in vs. 1.2–1.5 for acetal).
- Higher continuous service temperature (120–150°C vs. 85–100°C for acetal).
- Superior fatigue resistance for cyclic loading applications (gears, snap fits).
- Better wear resistance in abrasive environments, especially oil-impregnated or MoS₂-filled grades.
Thermal Management
Both materials benefit from compressed air blast cooling. Acetal’s Tg is −60°C and melting point is 175°C; nylon’s Tg is 50–70°C and melting point is 220–265°C. Keep cutting temperatures below 100°C for both materials. Signs of thermal issues include glossy smeared surfaces, stringy melted chips adhering to the tool, and part warpage after unclamping.
Summary
Acetal and nylon require fundamentally similar tooling — sharp, polished carbide — but nylon demands more positive rake, more aggressive chip control, and careful moisture management. Acetal runs 15–20% faster and holds tighter tolerances. Nylon offers superior toughness and temperature resistance but requires disciplined moisture conditioning for precision work. Selecting the right material for the application and matching tooling geometry accordingly is the key to successful production machining of both engineering thermoplastics.
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