POM (acetal)

POM (polyoxymethylene), also called acetal or polyacetal, is a semi-crystalline thermoplastic from the family of engineering plastics. It combines high stiffness and strength, a low coefficient of friction, good wear and fatigue resistance and very low moisture absorption. As a result, it holds its dimensions better than most other engineering plastics and is one of the easiest plastics to machine.

POM comes in two variants. The copolymer, POM-C (acetal copolymer), is the more common choice for machining stock, resists hot water and alkalis better and has less centerline porosity in thick sections. The homopolymer, POM-H (best known by the trade name Delrin), is somewhat stiffer, stronger and harder and melts at a higher temperature, around 175 °C compared with roughly 165 °C for POM-C. Besides natural white and black, there are glass-filled grades, PTFE-filled grades for lower friction and food-contact grades.

Typical values for unfilled POM: density about 1.4 g/cm³, tensile strength usually 60 to 75 MPa, modulus of elasticity 2.6 to 3.2 GPa and a continuous service temperature on the order of 90 to 100 °C. Moisture absorption under normal conditions is around 0.2 %, while PA6 polyamide absorbs several percent. Thermal expansion is roughly 100 to 120 µm/(m·K), about ten times that of steel.

POM is processed mainly by injection molding and by machining stock shapes (sheet, rod and tube). In injection molding it has high shrinkage, on the order of 2 %, which the design of the injection mold must account for. It is hard to 3D print because it warps and adheres poorly to the build plate, so POM prototypes are usually milled or turned.

Choose POM for moving and precision mechanical parts that run dry or with minimal lubrication: gears, plain bushings, guides, rollers, cams, latches and snap-fit or spring features, pump and valve components, and fixtures or inspection gauges. It also suits parts in contact with water, fuels or oils that must keep their dimensions.

In the classic acetal vs nylon comparison, POM is more dimensionally stable, easier to machine to tight dimensions and has lower friction when running dry. Polyamide, on the other hand, is tougher and handles impact and higher temperatures better. For abrasive conditions and large sliding surfaces, UHMW polyethylene (UHMW-PE) is often the better choice; for temperatures above 100 °C or aggressive chemicals, consider PEEK, for example, which is considerably more expensive.

POM has poor resistance to strong acids, oxidizing agents and chlorine (disinfectants, chlorinated water) and, unless stabilized, to UV radiation. It is very difficult to bond, so design joints as bolted, press-fit or snap-fit connections. An overheated melt decomposes and releases formaldehyde, which the molder has to keep under control.

The high thermal expansion means that tolerances of a few hundredths of a millimeter only make sense at a defined temperature: a part 100 mm long grows by about 0.1 mm when heated by 10 °C. Machined parts can warp once internal stresses in the stock are released, so for thin-walled and long parts use low-stress stock and remove material evenly from both sides. In the US, "Delrin" is often used loosely for any acetal, so in your request for quote state the variant (POM-C or POM-H), the color and any food-contact or material certificate requirements.