Nylon PA6 vs PA66 is the choice that faces almost every buyer specifying an engineering plastic for gears, housings, fasteners or structural parts: the two resins look similar on a pallet, share the polyamide family and overlap in half their applications, yet they behave differently where it matters - melting point, stiffness, moisture uptake and cycle time. In practice the nylon 6 vs nylon 66 decision comes down to three questions - heat, stiffness and moisture - and the rest follows from there. This guide compares PA6 and PA66 on the numbers that decide applications, shows where each resin wins, and explains how to write a specification that a supplier can quote against.
Nylon PA6 vs PA66 at a Glance
The PA6 PA66 difference traces back to the monomer: PA6, also sold as nylon 6, is built from caprolactam, while PA66 - nylon 66 - is built from hexamethylenediamine and adipic acid, and the resulting chains pack differently. That single structural difference cascades into everything a buyer notices:
| Property | PA6 | PA66 |
|---|---|---|
| Melting point | 215-225 C | 255-265 C |
| Density | 1.12-1.14 g/cm3 | 1.13-1.15 g/cm3 |
| Tensile strength, dry | 70-85 MPa | 80-95 MPa |
| Flexural modulus, dry | 2.4-2.9 GPa | 2.7-3.2 GPa |
| Notched impact, dry | 6-10 kJ/m2 | 5-8 kJ/m2 |
| Heat deflection, 1.8 MPa dry | 60-80 C | 75-95 C |
| Moisture absorption at saturation | ~9-10% | ~7-8% |
| Crystallinity and cycle | Crystallises slowly; longer cycle | Crystallises fast; shorter cycle |
Where PA6 Wins
- Toughness and dampening - PA6 absorbs shock and vibration better, which is why it is the default for gear wheels, sprockets and sliding parts that must run quietly.
- Processing window - the lower melting point widens the moulding window and reduces thermal stress on the tool and the machine.
- Sliding-contact behaviour - unfilled PA6 offers a balance of low friction and gouge resistance that suits bushings and wear pads.
- Cost - PA6 is usually the cheaper of the two per kilogram at comparable grade quality.
Where PA66 Wins
- Heat - the 40-degree-higher melting point and higher heat deflection keep stiffness in warm engine bays and under-hood duty.
- Stiffness and creep - PA66 holds load with less deformation over time, which matters for clips, fasteners and structural brackets.
- Abrasion and fatigue - glass-filled PA66 resists wear and cyclic stress better in highly loaded contacts.
- Cycle time - faster crystallisation shortens cooling, so high-cavitation tools produce more parts per hour.
Moisture: The Difference Most Buyers Underrate
Both nylons are hygroscopic, and PA6 absorbs more. As-moulded parts are dry and comparatively brittle; conditioned parts are tougher but dimensionally larger. The consequence for a buyer is that impact and dimensional specifications are meaningless without a conditioning statement - notched impact measured at 23 C and 50% relative humidity after conditioning to equilibrium is a complete specification, while notched impact on its own is not. Parts that must stay dimensionally stable in service should be specified dry-as-moulded with annealed reference dimensions, and mating tolerances should assume the conditioned size.
How to Specify PA6 or PA66
Good engineering resin selection starts with the duty, not the resin name: the load and speed of any gear or bearing contact, the maximum service temperature, whether the part sees impact at low temperature, and the conditioning state in which dimensions are measured. Filled grades change the picture again - GF30 versions of both resins multiply stiffness and creep resistance while cutting ductility - so filler content belongs in the specification, not in a later conversation. For how molecular structure drives properties in other resin families, the HDPE vs LDPE density comparison and the ABS vs PC guide are useful companions. The material guides section indexes the rest of the series, and verified plastic manufacturers can be shortlisted by region and process from the supplier directory.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
