PA6 and PA66 are the two most common nylons in injection molding. They look almost identical, but they behave differently with heat, moisture and cost, so choosing the right one can save money and prevent part failures.
The short answer: choose PA6 for general-purpose parts where easier processing, a better surface finish and lower cost matter. Choose PA66 when the part runs hot, carries higher loads or must stay dimensionally stable in humid conditions. PA66 melts at about 260 °C, roughly 40 °C higher than PA6.
What are PA6 and PA66?
Both are polyamides, the family commonly called nylon. The numbers describe how each is made. PA6 is made from a single monomer, caprolactam. PA66 is made from two, hexamethylene diamine and adipic acid, each with six carbon atoms. That small difference in chemistry is what gives PA66 its higher melting point.
Both share the properties that make nylon popular for mechanical parts:
- Strong and tough, with good fatigue resistance.
- Good wear resistance and low friction, which suits gears, bushings and sliding parts.
- Good resistance to oils, fuels and many chemicals.
- Much lighter than the metal parts they often replace.
PA6 vs. PA66 at a glance
| Property | PA6 (Nylon 6) | PA66 (Nylon 66) | What it means for your part |
|---|---|---|---|
| Melting point | About 220 °C | About 260 °C | PA66 keeps its shape and strength closer to heat sources |
| Typical melt temperature for molding | About 260 °C | About 290 °C | PA6 is easier to process; PA66 needs a hotter barrel |
| Stiffness and strength | Good | Slightly higher | PA66 carries a little more load before it deflects |
| Moisture absorption | Higher | Somewhat lower | PA66 holds dimensions better in humid environments |
| Surface finish | Often better on visible parts | Good | PA6 is a common pick for cosmetic housings |
| Material cost | Lower | Higher | PA6 is usually the budget-friendly choice |
Temperatures are from BASF’s Ultramid product literature (Ultramid B is PA6, Ultramid A is PA66). Exact values vary by grade, so always confirm against the datasheet of the resin you specify.
When to choose PA6
PA6 is the everyday nylon for parts that do not face extreme heat or harsh environments.
Good fits for PA6
- Gears, bearings and bushings in consumer and industrial products.
- Electrical connectors and housings where appearance matters.
- General-purpose mechanical parts that do not need the highest heat resistance.
Why teams pick it
- Lower processing temperature, which is easier on the process and the mold.
- Often a better surface finish on visible parts.
- Usually lower cost per pound.
Trade-offs
- Lower heat resistance than PA66.
- Absorbs more moisture, so dimensions shift more in very humid environments.
When to choose PA66
PA66 is the premium nylon for tougher jobs, especially where heat, load and dimensional stability matter.
Good fits for PA66
- Under-hood automotive parts near engines and other heat sources.
- High-load structural parts such as pump housings and valve bodies.
- Components exposed to heat, chemicals or demanding conditions.
Why teams pick it
- Holds strength and stiffness at higher temperatures.
- More dimensionally stable in humid environments.
- Slightly stronger and stiffer than PA6.
Trade-offs
- Needs a hotter process, which means more thermal stress on molds and machines.
- Usually costs more per pound.
- Just as sensitive to moisture during molding if it is not dried properly.
Moisture: the hidden variable in nylon parts
Both nylons absorb water from the air, and that affects the part in two different ways.
Before molding: dry the resin
Pellets that are not dried cause splay and streaks on the surface, voids inside the part, and weaker parts, because moisture can break down the polymer chains during processing. BASF recommends drying Ultramid for about 4 to 8 hours at roughly 80 °C to 120 °C, ideally in a dehumidifying dryer, and drying regrind carefully because it is particularly hygroscopic.
After molding: the part keeps changing
A molded nylon part keeps absorbing moisture from the air until it reaches equilibrium. According to BASF, as water content rises, impact resistance and elongation increase, while strength, stiffness and hardness decrease. Each 1% of absorbed water grows the part by about 0.2 to 0.3% in length. Glass-fiber reinforced grades change by less than 0.1% along the fiber direction, which is why they are often chosen when dimensions must hold. If a part is tested dry as molded, test it again after conditioning before you sign off on tolerances. Our guide to injection molding tolerances explains how to set realistic limits.
Design tips for molded nylon parts
- Keep wall thickness uniform to limit warpage; nylon’s shrinkage makes thick-to-thin transitions risky (see how cooling and geometry drive warping).
- Use ribs instead of thick sections to add stiffness without sink marks.
- Add draft so parts release cleanly from the mold.
- Consider glass-filled grades when you need more stiffness and less dimensional change, and plan for the extra tool wear they cause (see aluminum vs. steel tooling).
- Design for the conditioned state if the part will live in a humid environment, since its dimensions and stiffness will not match the dry part.
Why the choice matters for startups and small teams
The right nylon affects:
- How long your parts last under real conditions.
- How much each part costs to make.
- How consistent your dimensions are from batch to batch.
- How many parts are rejected because of heat or moisture.
For gears, housings and mechanical components, picking PA6 or PA66 can be the difference between a part that performs for years and one that warps or fails early.
Common questions about PA6 and PA66
Is PA66 stronger than PA6?
Slightly. PA66 is a little stronger and stiffer, and it keeps more of that strength at higher temperatures. For many room-temperature parts the difference is small, which is why PA6 often wins on cost.
Which nylon absorbs more water?
PA6 absorbs more moisture than PA66. Both need to be dried before molding, and both change dimensions and stiffness as they take on moisture in service.
Can PA6 replace PA66 in an existing part?
Sometimes, if the part does not run hot and small dimensional changes in humid conditions are acceptable. Check the heat exposure, the load and the tolerances first, and validate with molded samples, because shrinkage can differ between grades.
Is glass-filled nylon better?
It is stiffer and more dimensionally stable, but it is more abrasive on the mold and can leave fibers visible on the surface. Use it when the part needs the stiffness, not by default.
Key takeaways
- PA6 is easier to mold, often looks better and costs less.
- PA66 melts about 40 °C higher and handles heat and humidity better.
- Both must be dried before molding, and both change after molding as they absorb moisture.
- Design and test for the conditioned part, not only the dry one.
Not sure which nylon fits your part? Upload your CAD for a quote. Every quote includes a DFM review, and our engineers can help you choose between PA6, PA66 and glass-filled grades based on your load, heat and humidity. Learn more about our injection molding services.