Material selection
Nylon (PA) vs Acetal (POM)
Both are the default engineering plastics for gears, bearings and sliding parts, and both are stocked and processed here. The choice usually comes down to two things: how much the part needs to survive an impact, and how tightly it needs to hold a dimension in service. The numbers below are the same published values used on each material's own page, set side by side.
| Nylon (PA6) | Acetal (POM) | |
|---|---|---|
| Density | 1.13 g/cm³ | 1.41 g/cm³ |
| Tensile strength | 70–85 MPa (dry) | 62–70 MPa |
| Flexural modulus | 2.7 GPa (dry) | 2.6 GPa |
| Heat deflection (1.8 MPa) | 70 °C unfilled | 100 °C |
| Mold shrinkage | 0.7–1.5 % | 1.8–2.5 % |
| Water absorption | 2.5–3.5 % at saturation | 0.2 % |
| Relative cost | Medium | Medium |
Published values for unfilled grades at 23 °C unless stated, taken from each material's own datasheet page. Confirm against the supplier datasheet for the grade you specify.
What Nylon (PA) is
Nylon is the workhorse engineering thermoplastic: high strength, high toughness, excellent abrasion resistance, and good performance against fuels, oils and greases. The property you must design around is moisture. Nylon absorbs 2 to 3 percent water at equilibrium, which swells the part by roughly 0.5 percent, softens it, and roughly doubles its impact strength compared with the dry-as-molded state. Published dry data does not describe the part you will have in service.
What Acetal (POM) is
Acetal is what you specify when a plastic part has to hold a dimension and slide against something. It has the lowest coefficient of friction of the common engineering plastics, high stiffness, excellent fatigue and creep resistance, and it barely absorbs water. It machines better than any other thermoplastic, holding tolerances that would be ambitious in aluminum, which is why prototype gears, manifolds and precision fixtures are usually cut from Delrin stock.
Nylon (PA) against Acetal (POM)
| Nylon (PA6) | Acetal (POM) | |
|---|---|---|
| Toughness | Higher impact strength, especially once conditioned to equilibrium moisture; the material to choose when a sudden shock load is expected | Lower impact toughness and more notch sensitive; a sharp corner or sudden load is more likely to crack it than bend it |
| Dimensional stability | Absorbs 2.5 to 3.5 percent water at saturation, which swells a 100 mm dimension by roughly 0.5 mm; parts must be sized for the conditioned state, not as-molded | Absorbs only 0.2 percent water, so a part holds the dimension it was machined or molded to for the life of the product |
| Friction and wear | Good wear resistance, better still with a MoS₂ or PTFE-filled grade for dry-running bearings | The lowest coefficient of friction of the common engineering plastics; the default choice for unlubricated gears and slides |
| Heat resistance, unfilled | 70 °C heat deflection unfilled, but glass-filled grades (PA6 GF30) reach 220 °C and compete directly with die-cast aluminum | 100 °C heat deflection, higher than unfilled nylon, but acetal itself has no widely used glass-filled structural grade at that scale |
| Machinability | Machines well but is softer and more prone to burr and deflection under the cutter than acetal | Machines better than any other common thermoplastic, holding tolerances that are ambitious even in aluminum |
| Processing caution | Must be dried below 0.15 percent moisture before molding or the part voids and weakens; a straightforward drying step, not a hazard | Decomposes above roughly 230 °C and releases formaldehyde if it lingers in a hot barrel; needs a controlled process, not just a dried hopper |
Which one, for which part
- Gears, lead-screw nuts, and precision machined fixtures that must hold a tolerance for the life of the part favour acetal: 0.2 percent water absorption means the dimension you machined is the dimension the part keeps.
- A part that takes a sudden impact or drop, brackets, clips, housings that get dropped or knocked, favours nylon, particularly once it has equilibrated to ambient moisture and its impact strength has roughly doubled from the as-molded state.
- Unlubricated sliding parts and low-friction bushings favour acetal, which has the lowest coefficient of friction of the two and needs no grease to perform.
- Structural brackets or housings that must resist heat and compete with die-cast aluminum on stiffness favour nylon in a glass-filled grade (PA6 GF30 or PA66 GF30), reaching heat deflection figures acetal's unfilled grades do not approach.
- Cable ties, snap fits, and fasteners that flex repeatedly without cracking favour nylon for its toughness; acetal's snap fits work well too but are less forgiving of a sharp inside corner.
Our status
Trumould runs both nylon and acetal in-house through injection molding and CNC machining; nylon is also available through 3D printing in PA12 and PA11 for functional prototypes, acetal is not offered through 3D printing here. Send the part, its service environment, and target volume, and the quote states which of the two is recommended and why, not just a price for whichever one you asked about first.
Specifying this part?
Send your CAD and target volumes. We come back with a price, a lead time, and any material or design notes that would reduce either.