GlossaryKind of term: Manufacturing termPOM (acetal)
What is POM?
POM (polyoxymethylene, acetal) is a stiff engineering plastic with low friction and low moisture uptake, widely used for slides, gears and rollers.
POM is semi-crystalline, and hard as well as stiff. It comes in two kinds. POM-C, the copolymer, is the usual shop grade for plate and rod and resists hot water and alkalis better. POM-H, the homopolymer, is a little stiffer and stronger but more prone to a porous core in thick rod. It is milled or turned into guides, gears, rollers, valves, insulators and grippers.
How does POM behave in the shop?
With sharp tools POM machines fast and to a clean finish. Holding size takes more care. Extruded stock can carry residual stress, even when it is supplied stress relieved, and heavy stock removal from one side can make the part move. Roughing, letting it rest and then finishing limits that, but usually needs an extra setup and more lead time. Thermal expansion is far higher than in steel. Dimensions apply at the reference temperature of 20 °C, so tight tolerances are measured only once the part has settled at that temperature after machining. Unstabilised POM discolours and embrittles under UV light; stabilised grades exist for outdoor use.
What is POM often confused with?
POM is not PA6: polyamide takes up much more moisture and changes size with it, POM hardly at all. POM-C and POM-H also differ: the same family, but POM-H is stiffer and more prone to a porous core in thick rod, which matters for a part taken from the middle of the bar.
Frequently asked questions
Why does POM warp after machining?
Because of residual stress from extrusion and heat from the cut. Take a lot off one side and that stress is released. Stress-relieved stock, roughing, a rest and then finishing to size keep the part flatter.
What is the difference between POM-C and POM-H?
POM-C is the copolymer: the common grade, less prone to a porous core and more resistant to hot water. POM-H is the homopolymer: stiffer and stronger, but more likely to have a porous core in thick rod.
Can POM be used in contact with food?
Some POM grades meet the requirements for food contact, in the EU under Regulation 10/2011. That holds per grade and per supplier; ask for the declaration of compliance for the material you order.