Choose PA66 when the part needs strength, stiffness or heat resistance and dimensional change from moisture is acceptable; choose POM when the part needs low friction, tight and stable dimensions, or resistance to creep under sustained load. That single sentence resolves most PA66 vs POM decisions. The rest of this guide gives you the numbers behind it, the two failure modes people get wrong, and the selection rule by application.
Key takeaways
- PA66 is stronger and stiffer; POM is dimensionally stable and better under sustained load.
- PA66 absorbs moisture and grows with it. POM absorbs almost nothing. This is the decision that matters most in practice.
- POM wins on wear and friction — it is the default for gears, cams and sliding parts.
- PA66 wins on temperature and on strength-to-weight, especially in glass-filled grades.
- POM is hard to bond and hard to overmold; PA66 bonds more readily.
- Both need proper drying before molding. Neither forgives a wet melt.
The short answer, by application
Gears, cams, bushings, sliding clips, conveyor components, precision mechanical parts → POM. Its low coefficient of friction, fatigue resistance and negligible moisture uptake make it the standard choice for anything that moves against something else.
Structural brackets, housings, under-hood parts, electrical connectors, anything loaded hot or exposed to oil and hydrocarbons at temperature → PA66, usually glass-filled. Its higher strength, stiffness and heat deflection temperature carry the load.
Where dimensions must hold over years in a humid environment → POM, unless the part is also hot. That combination is where the two materials genuinely conflict, and where POM usually loses.
Properties side by side
Values below are typical ranges for unfilled general-purpose grades. Glass-filled and impact-modified versions shift these substantially, so confirm the specific grade datasheet before finalising a design.
| Property | PA66 (unfilled) | POM (copolymer) |
|---|---|---|
| Tensile strength | ~75–85 MPa | ~60–70 MPa |
| Tensile modulus | ~2.8–3.2 GPa | ~2.6–3.1 GPa |
| Notched impact strength | Moderate; brittle when dry | Good, tolerant of notches |
| Heat deflection temp. (1.8 MPa) | ~70–90 °C | ~95–110 °C |
| Heat deflection temp. (0.45 MPa) | ~200–220 °C | ~150–160 °C |
| Water absorption, 24 h | ~1.0–1.5% | ~0.2–0.25% |
| Mould shrinkage | ~1.0–2.0% | ~1.8–2.5% |
| Coefficient of friction vs. steel | ~0.25–0.40 | ~0.15–0.35 |
| Creep resistance | Moderate | Excellent |
| Chemical resistance | Poor vs. strong acids; attacked by phenols | Attacked by strong acids and oxidising agents; good vs. fuels and solvents |
| Bonding / overmolding | Readily bondable | Difficult without mechanical interlock |
| Machinability | Fair | Excellent |
Moisture: the variable that decides real projects
PA66 is hygroscopic. In a humid environment it absorbs water into its amorphous regions, which plasticises the polymer: the part grows, impact strength rises, and strength and stiffness fall. An unfilled PA66 part can gain roughly 2–3% in dimension between dry-as-moulded and equilibrium in a humid climate — on a 100 mm part that is 2–3 mm, which is far beyond any reasonable tolerance.
This has three practical consequences. First, PA66 tolerances must state the moisture condition they apply to, otherwise the drawing is ambiguous. Second, a part that must fit tightly should be designed around the equilibrium dimension, not the dry dimension. Third, glass-filled grades reduce, but do not eliminate, the effect.
POM absorbs roughly an order of magnitude less moisture. That is why POM dominates precision mechanical components where fit and clearance must survive years of service in the field.
Wear, friction and creep
POM’s combination of low friction, high crystallinity and excellent fatigue and creep resistance is what makes it the standard gear and bearing material. It machines cleanly, holds tolerance, and behaves predictably under continuous load — it does not cold-flow the way many polymers do.
PA66 generates more friction heat against steel, and its creep resistance is only moderate. Where a PA66 part carries a sustained load, glass reinforcement is usually necessary, and even then it will deflect progressively at elevated temperature.
For unlubricated sliding contact, POM is almost always the right starting point. Add lubricant-filled or internally lubricated grades when the load or speed is high.
Temperature and chemical exposure
PA66 handles higher continuous temperature and holds mechanical properties at temperature better; glass-filled grades extend that further. POM’s ceiling is lower, and it degrades if held too long above its processing range.
Chemical resistance depends on the medium. PA66 is attacked by phenols, formic acid and strong mineral acids, and is affected by some chlorinated solvents. POM resists fuels, oils, alcohols and most solvents well but is attacked by strong acids and oxidising agents — and it is not suitable for contact with strong alkalis at temperature. Neither material is a good choice where you need broad chemical resistance; that usually points to PVDF, PPS or PP instead.
Cost and processing
In unfilled general-purpose grades the two materials sit in a similar price band, with POM typically slightly higher and glass-filled PA66 grades jumping well above both. In practice, cost differences between these materials rarely decide a project — a defect rate or a warranty claim will dwarf a few cents of resin per part.
Both need drying before moulding. POM is the more sensitive of the two: it is prone to degradation from excessive residence time or overheating, releasing formaldehyde, so it needs tight temperature control and consistent shot sizes. PA66 needs drying to avoid silver streaks and strength loss. Neither tolerates a sloppy process.
What this means for your mold and tolerances
POM’s higher shrinkage (roughly 1.8–2.5% versus 1.0–2.0% for PA66) means the tool must be cut with a larger shrink allowance, and because shrinkage varies with wall thickness and gate location, thick sections are harder to hold than thin ones in both materials. Plan gates and cooling so the part cools evenly — differential shrinkage is the main cause of warpage in both.
Confirm the exact shrinkage value for the grade you will run before cutting steel. Our guide to choosing the right resin for injection molding walks through the full selection process, and how tight injection molding tolerances can be gives achievable bands by material and dimension size.
DFM notes for each material
PA66: keep walls uniform, avoid sharp internal corners because it is notch-sensitive in the dry state, use generous radii, and allow for moisture growth in the tolerance scheme. Glass-filled grades wear moulds faster — specify a harder steel or accept shorter tool maintenance intervals.
POM: it is notch-sensitive, so radii matter; it machines and releases well but is prone to internal voids in thick sections, which makes uniform wall thickness more important than usual. Because it bonds poorly, design mechanical interlocks if the part must be assembled to another material.
Where these materials sit in the wider picture
PA66 and POM are two of the main engineering thermoplastics; the others you will compare against include ABS, PC, PBT, PP and the high-performance resins PEEK and PPS. The full comparison of families, with selection criteria, is in our injection molding materials and mold steel guide. Mould steel selection for glass-filled grades is covered in P20 vs NAK80 vs S136.
Still not sure which grade fits?
Tell us the load, the temperature, the chemical exposure and the tolerance the part must hold. We will recommend a resin and grade, flag the moisture or shrinkage issue if there is one, and quote the tool. Ask an engineer.
Related guides in this series
This article is part of our Injection Molding Materials & Mold Steel Guide — a full walkthrough of the topic with the numbers and checklists behind each decision.