Overview
PA6 (nylon 6) is PA66’s slightly tougher, slightly cheaper, slightly softer cousin — and the dominant stock-shape engineering nylon for large industrial parts. Where PA66 dominates injection-molded parts and small extruded stock, cast PA6 dominates the large bearing, gear, sheave, and wear-pad market. If you’ve seen a giant nylon pulley sheave on a crane, a multi-foot diameter port-equipment idler wheel, or a thick wear pad in a steel mill, it was almost certainly cast PA6 (or one of its filled variants — MD for moly-disulphide-lubricated, Nyloil for oil-filled).
Mechanically PA6 and PA66 are close — strength differs by 10–15%, stiffness is similar, machining is nearly identical. The two real differences:
- PA6 melts ~30°C lower (220–230°C vs 250–265°C for PA66). This makes PA6 easier to cast monomer-into-mold from caprolactam — the process that produces large stock shapes. PA66 can’t be monomer-cast the same way; its stock shapes are limited to extrusion or compression molding.
- PA6 absorbs MORE moisture — ~9.5% saturation vs 8.5% for PA66, and ~3% at 50% RH equilibrium vs 2.6% for PA66. The dimensional growth, property shift, and Tg drop are all proportionally bigger.
For metal-replacement applications under 3-inch rod diameter, either nylon works. Above that size, cast PA6 is the practical and economic choice.
The moisture story — more extreme than PA66
The same moisture-driven property shift that defines PA66 design applies to PA6, only more so:
| Property | Dry | Conditioned (50% RH) | Saturated |
|---|---|---|---|
| Tensile strength | 80 MPa | ~40 MPa (-50%) | ~30 MPa (-63%) |
| Tensile modulus | 3.0 GPa | ~1.1 GPa (-63%) | ~0.7 GPa (-77%) |
| Elongation at break | 30–60% | ~200% | ~300% |
| Tg | 53°C | ~5°C | -10°C |
| Notched Izod | 53 J/m | ~200 J/m | ~250 J/m |
| Dimensional growth | baseline | +0.5% | +0.8% |
A 100mm cast PA6 part grows ~0.5–0.8mm in service as it equilibrates to ambient humidity. Compared to PA66’s 0.3–0.5%, this is meaningfully larger and matters more for tight-tolerance fits. Always size cast PA6 parts for the in-service conditioned dimensions, not the as-machined dry-as-machined dimensions.
The propertyStates block in the frontmatter holds both states. The
top-level mechanical properties show the dry-state numbers (the
conventional published data) — but design for the conditioned state.
Machining notes
PA6 machines nearly identically to PA66 — among the more forgiving engineering plastics on the floor. Practical recipe:
- Sharp carbide or HSS, positive rake
- Speed: 300–800 SFM
- Feed: 0.005–0.015 in/rev
- Coolant: recommended for finish work and deep cuts; light cuts can run dry
The PA6-specific machining challenges:
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Larger stock = more moisture variation. Cast PA6 shapes can be very large (20+ inch diameter rod, 6+ inch thick plate), and large sections take a long time to equilibrate. The center of a 12-inch diameter cast nylon billet may be at 1% moisture while the surface has equilibrated to 3%. Tight-tolerance machining benefits from stress-relief drying (80°C, several hours per inch of section) to normalize moisture content before final cuts.
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Residual casting stresses. Cast PA6 stock can have residual stresses from the casting/cooling process. Heavy machining cuts release these stresses and the part warps. For critical parts, rough-machine, stress-relief anneal at 150°C for 1 hour per inch of section, then finish-machine.
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Post-machining dimensional growth — 0.5–0.8% in service. Larger than PA66’s 0.3–0.5%. For tight fits to metal parts that don’t grow, undersize the cast nylon part by ~0.5% at manufacture so the conditioned fit is correct.
Cast PA6 takes a beautiful finish — better than glass-filled grades. For maximum surface quality, finish with a sharp polished-edge insert at low feed.
Variant selection guidance
- Cast PA6 (NYCAST / Sustamid 6 / MC901) — the default for stock- shape machined parts above ~3 in diameter. Large bearings, gears, sheaves, wear pads. Better economy than extruded for thick sections.
- MoS₂-filled (Cast Nylon MD / MC901) — the workhorse bearing and gear grade. Gray/black color, lower friction, better fatigue from finer crystalline structure. The standard industrial-machinery wear material.
- Oil-Filled (Nyloil) — pick for high-cycle bearings where external lubrication is impractical (food, cleanroom, sealed assemblies). Internal oil migrates to wear surfaces over service life. FDA grades available.
- Extruded PA6 — pick for smaller-diameter rod and thin sheet, where cast economy doesn’t apply. Mostly displaced by extruded PA66 in this size range.
- 30% Glass-Filled — primarily an injection-molding compound. Structural automotive and electrical components. Reduced moisture sensitivity.
- Heat-Stabilized — long-term service at 120–140°C in air.
- Food-Grade (FG) — FDA-compliant cast PA6 for food/beverage processing. Natural or blue color.
Failure modes worth designing around
Moisture-driven property shift — the dominant field failure mode for PA6 just as for PA66, only larger in magnitude. A PA6 bearing designed using only dry-state strength has a 50% reduction in service once it equilibrates with humid air. Design with conditioned-state properties as the design state.
Acid hydrolysis — PA6’s amide backbone is vulnerable. Strong acids cause chain scission and embrittlement over months/years. Not for acid-pump components or aggressive cleaning. PVDF, PP, or PEEK are better.
Hot-water hydrolysis — PA6 in continuous hot water above ~70°C degrades faster than PA66 (less crystalline, more water uptake). For continuous hot water service, PA46, PPA, or hydrolysis-stabilized PA grades are the correct choice.
Heat-aging embrittlement — sustained 100°C+ in air causes thermo-oxidative chain scission. Heat-stabilized grades extend service life ~3–5×.
UV degradation if unprotected — yellowing and embrittlement in under a year outdoors. Use heat-stabilized + UV grades.
Residual casting stress release during machining — unique to cast PA6. Heavy roughing cuts can release residual stresses and warp critical parts. Rough-machine, anneal at 150°C, finish-machine.
Applications by industry
- Industrial machinery (the #1 application) — large bearings,
bushings, gears, sheaves, wear pads. Cast PA6 dominates the
3-inch-diameter market. MD grades for high-load wear, Nyloil for self-lubricated service. Replaces bronze and steel routinely.
- Marine and offshore — hatch wheels, mooring components, anti-friction bearings, sheaves. Cast PA6 holds up to seawater exposure where other materials would corrode.
- Mining and steel mills — large sheaves, mill rolls, wear pads, chute liners. The dimensional and economic advantages of cast nylon shine at very large sizes where forged steel is heavy and expensive.
- Crane and material handling — pulley sheaves, load-bearing wear parts, idler wheels. Cast PA6 sheaves are dramatically lighter than steel (1.15 g/cm³ vs 7.8 g/cm³) and don’t corrode.
- Power transmission — large industrial gears, sprockets. MD-filled cast PA6 is the standard self-lubricating gear material in the 100mm+ pitch-diameter range.
- Pulp and paper — wear pads, idlers, scrapers, doctor blades. Cast PA6 holds up to abrasive process conditions and the slightly alkaline environment.
- Food and beverage processing — FDA-compliant cast PA6 (NYCAST FG, Nyloil FG) for gears, conveyor parts, wear pads. Self-lubricated oil-filled grades eliminate external lubrication contamination.
- Aerospace (secondary structure) — brackets, cable-management parts, interior wear pads.
- Agricultural equipment — wear parts, bushings, chain guides, pickup-reel components.