Nylon / PA
CNC Machining

Nylon (Polyamide, PA) is one of the most widely used engineering thermoplastics for CNC machining. It offers high tensile strength, excellent toughness and impact resistance, good fatigue resistance, self-lubricating properties, and a broad service temperature range. Compared to POM/Acetal, nylon is tougher and better at elevated temperatures. The main consideration is moisture absorption — nylon absorbs 1–3% water at ambient humidity, causing dimensional change. For precision dimensional applications, this must be factored into fit calculations, or POM should be considered instead.

PA66 has a higher melting point (260°C vs 220°C for PA6), slightly higher stiffness and hardness, and better performance at elevated temperatures. PA6 has better impact resistance, is easier to process, and absorbs slightly more moisture. In CNC machining practice, both grades machine similarly — PA6 is more common in Europe and Asia, PA66 more common in North America. For high-temperature applications (above 100°C), PA66 is preferred. For impact-loaded structural parts, PA6 is often specified. The performance difference in most machined parts is modest.

MC Nylon (Monomer Cast Nylon, also called PA6-G or cast PA6) is produced by casting liquid caprolactam monomer directly into a mold. This produces a material with higher molecular weight, lower residual stress, and better mechanical properties than extruded nylon — tensile strength 85–100 MPa vs 70–80 MPa for extruded PA6. Key advantages: significantly higher tensile and fatigue strength, very large section sizes available (up to 1,000 mm diameter), no centerline defects, and 2–3× wear life vs extruded PA6. MC Nylon is the first choice for heavy-duty machined gears, crane wheels, pulleys, sheaves, and large wear pads.

Glass-filled nylon (PA6-GF or PA66-GF) is nylon reinforced with short glass fibers, typically 15–30% by weight. Glass reinforcement dramatically increases stiffness (flexural modulus from ~2,500 MPa to 6,000–12,000 MPa), tensile strength, creep resistance, and dimensional stability at elevated temperatures. However, glass fibers reduce impact toughness and the material is significantly more abrasive to cutting tools — carbide tooling is required. Specify PA-GF when dimensional stability at temperature is critical, when higher stiffness than unfilled nylon is needed, or when creep under sustained load is a concern.

Nylon absorbs moisture from the environment (PA6: approximately 2.5% at 50% RH). This moisture causes the material to expand — a 100 mm dimension in dry nylon can grow by 0.3–0.8 mm when fully conditioned. Best practices: machine nylon at equilibrium moisture content for the intended service environment, or account for moisture expansion in bore and shaft clearance calculations, or specify POM/Acetal if dimensional stability across humidity changes is critical. For large structural parts where exact dimensional fit is less critical, moisture absorption is managed by design clearance.

Standard CNC tolerance for unfilled nylon (PA6, PA66, MC Nylon) is ±0.05–0.10 mm, with ±0.03–0.05 mm achievable on bore fits and shaft diameters when machined at equilibrium moisture conditions. Glass-filled nylon (PA-GF) holds slightly better tolerance due to reduced thermal expansion and moisture sensitivity — ±0.05 mm standard. Note that nylon parts machined in a dry state may expand by 0.2–0.5 mm on a 50 mm dimension when exposed to ambient humidity. Always specify the operating environment when ordering precision nylon parts.

Key nylon machining challenges: (1) Moisture-induced dimensional instability — parts should be machined at or near service moisture content. (2) Heat generation — nylon has low thermal conductivity; air or mist cooling prevents thermal distortion. (3) Stringy chips — unfilled nylon produces long, stringy chips; chip-breaking geometries and peck cycles on deep bores are required. (4) Glass fiber tool wear in PA-GF — glass fibers are highly abrasive; carbide tooling with monitored tool life is mandatory. (5) Stress cracking in the presence of some cutting fluids — water-based coolants or air cooling is preferred.

PA6 and PA66 in natural (uncolored) grades are generally suitable for food contact under FDA 21 CFR 177.1500 and EU Regulation 10/2011, subject to specific conditions. Always confirm with a certificate of compliance from the raw material supplier for regulated food contact applications. For medical device applications, medical-grade nylon with ISO 10993 biocompatibility documentation is available. Black or colored nylon may contain pigments not approved for food contact — always verify the specific grade and its additive package before use in regulated applications.