Thermoplastic
Acetal (POM)
The precision plastic. Stiff, slippery, and dimensionally stable.
Published values for unfilled grades at 23 °C unless stated. Confirm against the supplier datasheet for the grade you specify.
What it 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.
Where it is used
- Gears, worm drives, and lead-screw nuts
- Bearings, bushings, and slide components
- Precision machined fixtures and manifolds
- Fuel system and pump components
- Snap fits and clips that must survive repeated cycles
Processing notes
Little or no drying needed, but the melt is unforgiving. Above roughly 230 °C acetal decomposes and releases formaldehyde; never let it sit in a hot barrel.
Melt 190–215 °C, mold 80–110 °C. Purge with polyethylene, never with PVC or flame-retardant grades.
Shrinkage is high at 2 % and mainly thickness-driven. Thick sections form voids rather than sinking, because the skin freezes first.
Available through
Design notes
Keep walls uniform and moderate, 1.5–3.0 mm. Thick bosses will void internally and you will not see it until a section cut.
Nothing bonds to acetal. Use mechanical joints or ultrasonic welding.
Homopolymer is stiffer and stronger, copolymer resists hot water and alkalis better. Choose on the service environment, not the datasheet headline.
Grades and variants
- Homopolymer (Delrin), highest stiffness, strength, and creep resistance
- Copolymer, better resistance to hot water, alkalis, and long-term thermal ageing
- PTFE or silicone filled, for dry-running bearings
- Glass filled 20–25 %, for stiffness and reduced creep
- Impact modified, recovers some of the toughness lost to stiffness
Alternatives
If Acetal (POM) is not the right answer
Specifying this material?
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.