Thermoplastic
Acetal (POM, polyoxymethylene)
The precision plastic. Stiff, slippery, and dimensionally stable.
Short answer: POM is molded at a 190 to 215 °C melt into an 80 to 110 °C mold, needs little or no drying, and must never sit in the barrel above about 230 °C, where it decomposes and releases formaldehyde. It shrinks 1.8 to 2.5 % but absorbs only 0.2 % water, so it holds a dimension well. Choose homopolymer for stiffness and creep resistance, copolymer for hot water and alkalis, and PTFE or silicone filled grades for dry-running bearings.
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: acetal decomposes above roughly 230 °C. Temperatures, purging, wall thickness and shrinkage are set out step by step in the POM processing guide below.
POM processing guide
The settings and checks for molding acetal in one place. The resin supplier's data sheet for the named grade still sets the exact window.
| Step | Setting or check |
|---|---|
| Drying | Little or no drying needed; POM absorbs only 0.2 % water |
| Melt temperature | 190 to 215 °C. Above roughly 230 °C acetal decomposes and releases formaldehyde |
| Residence time | Never let the melt sit in a hot barrel |
| Mold temperature | 80 to 110 °C |
| Purging | Purge with polyethylene, never with PVC or flame-retardant grades |
| Wall thickness | Uniform and moderate, 1.5 to 3.0 mm. Thick sections form internal voids rather than sinks, because the skin freezes first |
| Shrinkage | 1.8 to 2.5 %, mainly driven by wall thickness, so the steel is sized for the grade named on the drawing |
POM grade selection guide
Pick the grade on the service environment, not the datasheet headline.
| Grade | What it gives | Choose it when |
|---|---|---|
| Homopolymer (Delrin) | Highest stiffness, strength and creep resistance | The part carries a sustained load or must stay stiff, such as gears and lead-screw nuts |
| Copolymer | Better resistance to hot water, alkalis and long-term thermal ageing | The part sees hot water, cleaning chemicals or years at temperature |
| PTFE or silicone filled | Lower friction for dry running | Bearings, bushings and slides that run without grease |
| Glass filled, 20 to 25 % | More stiffness and less creep | A structural part needs more stiffness than unfilled acetal gives |
| Impact modified | Recovers some of the toughness lost to stiffness | Clips and snap fits take knocks as well as repeated cycles |
POM dimensional stability
POM shrinks more than ABS in raw percentage terms, yet it holds a dimension well. It is highly crystalline and absorbs only 0.2 % water, so its shrinkage is uniform from shot to shot and cavity to cavity, and the molded part does not grow afterwards the way nylon does as it takes up moisture. On our own tooling, unfilled POM holds about ±0.20 mm at a 100 mm dimension under general tolerance (DIN 16742 group TG5), the same group as PC. The full table by material is in the injection molding tolerances guide.
Switching to an alternative POM grade
A homopolymer and a copolymer, or two suppliers' grades, are not automatically interchangeable in the same tool. Before approving a substitute, compare the shrinkage on both data sheets, because the steel was sized for the original grade, and check the substitute against the service environment in the table above. Then measure first article (T0/T1) samples from the existing tool against the drawing, as for any new part; what that report contains is on the quality and inspection page.
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Design notes
Keep walls uniform and moderate, 1.5 to 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, copolymer, PTFE or silicone filled, glass filled and impact modified grades are compared in the POM grade selection guide.
Alternatives
If Acetal (POM) is not the right answer
See the full Nylon (PA) vs Acetal (POM) comparison or the POM vs PBT comparison, spec by spec.
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.