Polyamide (nylon)

Polyamide (PA, commonly called nylon) is a semi-crystalline thermoplastic whose chains are joined by amide bonds. The numbers in the designation give the number of carbon atoms in the building blocks: PA6 is made from caprolactam, PA66 from hexamethylenediamine and adipic acid, PA12 from laurolactam and PA11 from castor oil. The same grades are also written as nylon 6, nylon 6/6, nylon 12 and nylon 11.

Polyamides are tough, resist wear and fatigue, slide well and withstand oils, greases, fuels and most organic solvents. They do not tolerate strong acids or oxidizing agents. Their defining trait is moisture absorption: at equilibrium with normal ambient humidity, PA6 and PA66 typically take up 2.5 to 3.5% water, and up to about 8 to 10% when immersed. Water acts as a plasticizer: the modulus of unreinforced PA6 falls from about 3 GPa dry to a third or a half of that, toughness rises and the part swells slightly. PA12 absorbs far less (roughly 0.5 to 1.5%) and is more dimensionally stable.

Melting points are about 220 °C for PA6, 260 °C for PA66 and 180 °C for PA12. Unreinforced polyamides have an HDT at 1.8 MPa of only about 50 to 90 °C, but at 0.45 MPa PA6 and PA66 usually exceed 150 °C. Glass fiber reinforcement (for example PA6 GF30 or PA66 GF30 with 30% fibers) raises the modulus to about 6 to 10 GPa, depending on moisture, and the HDT to 200 °C or more.

PA6 and PA66 are among the most widely used engineering plastics for injection molding. PA6 rod and plate (extruded or cast) are common stock for machining. PA12 and PA11 are the main powder materials for SLS and MJF; PA12 parts reach a tensile modulus of about 1.6 to 1.9 GPa and a tensile strength of 45 to 50 MPa. FDM uses PA6 and PA12 filaments, often filled with carbon fiber.

Polyamide suits mechanically loaded and wear parts: gears, rollers and pulleys, plain bearings and guides, snap-fits, cable clips, under-the-hood parts and parts exposed to oils and fuels. Reinforced grades replace metal brackets and housings, cut weight and let you combine several functions in a single molding.

PA12 from SLS or MJF is the standard choice for loaded functional prototypes and small batches, and often serves as the end-use part. For precise, low-friction machined parts with tight tolerances, POM, which absorbs almost no water, is often better.

The most common mistake is ignoring moisture. Datasheets give markedly different values for the dry-as-molded and conditioned states, so calculate with values that match service conditions. Freshly molded parts are dry and more brittle, so snap-fits can crack at first assembly. For precision parts, allow for dimensional change after moisture uptake, and do not design fits around dimensions measured right after production.

Dry pellets and filament thoroughly before processing, or the material degrades and parts turn brittle. Glass-filled grades shrink differently along and across the fibers and tend to warp. In your RFQ, specify the exact grade (PA6, PA66, PA12, filler, heat stabilization, flame retardancy) and, for 3D printed parts, the process, because SLS, MJF and FDM PA12 parts differ.