Material guide
POM, polyoxymethylene
POM, also called acetal, is the plastic you choose when the dimensions have to be right. Hard, dimensionally stable and easy to machine, with low friction. The classic base for precision parts and plain bearings, whether you go for POM C or the harder POM H.
What it is
POM stands for polyoxymethylene and is often called acetal. It is an engineering plastic that stands out for high stiffness, a hard surface and good dimensional stability, even in a damp environment. Where nylon swells with moisture, POM stays put, and that is the whole point of the material. It turns and mills easily to tight tolerances and keeps the dimensions afterward. Add low friction and good abrasion resistance, and you see why POM is a standard choice for mechanical precision parts. The group has two grades, POM C and POM H, which differ slightly in hardness and stiffness.
Properties
High dimensional stability, the dimensions hold even in moisture
Hard surface and good stiffness, carries load without deforming
Low friction and good abrasion resistance, slides quietly and smoothly
Easy to machine to tight tolerances, rewarding in the lathe and the mill
Good fatigue resistance, takes repeated loading
Limited moisture absorption compared with nylon
Common applications
POM sits in gears, plain bearings, slide rails, rollers, guides, valve components and precision parts that have to fit exactly. When a designer needs a part with tight dimensions that must not move over time, or a quiet and free-running bearing, POM is often the answer. It is also common in pump and valve parts thanks to its dimensional stability.
Limitations
POM is not a high temperature plastic, and its chemical resistance to strong acids is limited, where a fluoropolymer such as PVDF or PTFE suits better. The material should not be glued the way you would expect with other plastics either, the surface is hard to get glue to stick to, so plan for mechanical fasteners. For parts that need high impact resistance in the cold, nylon or PC can be a better choice.
Our POM grades take 100 °C in continuous service and 140 °C for short periods, go down to -40 °C and melt at around 166 °C.
Grades and variants
POM comes in two grades that solve different needs. They differ slightly in hardness and stiffness, but are typically delivered in natural unless otherwise stated.
Base grades
- POM C, copolymer. The most common grade, with good chemical resistance and stable machining. First choice for most parts.
- POM H, homopolymer. Slightly harder and stiffer, for parts that need the utmost in stiffness and abrasion resistance. It does take acids and strong bases worse than POM C, so in a chemical environment the copolymer is the safer choice.
POM grades are typically delivered in natural unless otherwise stated.
Machining
POM is perhaps the most rewarding material we sell to machine. It saws, mills, drills and turns to tight tolerances and keeps the dimensions afterward. The surface does not take glue, though, so plan for a press fit or screws. If the part has to hold really tight dimensions, it is worth stress relieving between roughing and finishing.
Formats we can supply
POM is available as sheets, round rods, tubes and hollow bars across a wide range of dimensions. You can see formats and dimensions in the range. Want a piece cut to size? Enter the dimensions in VisualCutter and you see the price right away.
In depth
POM, PA or PEEK, how do you choose?
Three materials always come up for plain bushings, gears and slide pads: POM C, PA6 G and PEEK. The price difference is a factor of 50, but the difference in performance is less dramatic. It is the PV value that decides, that is the product of surface pressure and sliding speed, not the temperature itself. Allowable PV is no constant, a material that takes high pressure at low speed can run hot at low pressure and high speed despite the same PV value.
Temperature is the second question. POM C loses about half of its PV value at 80 °C. PA6 G runs in practice to 100 °C, or 120 °C with a MoS2 additive. PEEK still holds around 70 percent of its stiffness at 200 °C. The environment decides whether you need a fluoropolymer: POM C takes oils and greases but not acids or strong bases, PA6 is hygroscopic and changes dimension by 2 to 3 percent at full moisture saturation, PEEK is inert to almost everything except concentrated sulfuric acid.
- Service below 80 °C, a dry or oily environment and PV below 0.10: start with POM C. It is a fraction of the PEEK price and proven in hundreds of thousands of applications.
- Higher temperature, varying moisture or alternating load: go to PA6 G. Cast in larger dimensions, it takes up moisture more slowly than extruded PA6 and is still economically reasonable.
- PEEK when you need high temperature, chemical resistance and low friction at the same time, or when weight is critical. Do not put PEEK on an application that POM can handle.
Feed rate when milling POM C
POM C is rewarding to mill. It is not abrasive, it goes fast and gives a fine surface, but it is heat sensitive above 120 °C and the wrong feed gives melted edges and tolerances that have gone out. Guide values with carbide tools without coolant: cutting speed 300 to 500 m/min, feed per tooth 0.05 to 0.15 mm for diameters 8 to 16 mm, depth of cut up to half the diameter. Chip evacuation matters more than chip breaking, POM gives long tough chips that like to wrap around the tool.
Cool with compressed air rather than emulsion. POM takes no harm from water, but emulsion based cooling often gives micro cracks in the surface in continuous service. Compressed air at 5 to 6 bar aimed at the edge goes a long way and keeps chip handling in check. A melted surface finish comes from too low a feed at high spindle speed, where the tool rubs instead of cutting, so raise the feed per tooth to at least 0.08 mm. A dimensional deviation of up to 0.1 mm after the operation is finished comes from thermal expansion during milling, POM expands 110 µm per meter and degree, so measure only once the part has cooled to room temperature.
Tolerances in plastic, what you need to know
A drawing that works in steel can give a press fit at 25 °C and play at 5 °C in plastic. Three areas that classic workshop handbooks barely cover decide the result: thermal expansion, moisture absorption and relaxation. Thermal expansion is 4 to 10 times higher than for steel, steel sits at about 12 µm per meter and degree, POM C at 110 µm, PE-1000 at 200 µm. A POM bushing at 80 mm diameter changes 0.7 mm between winter and summer service. Always calculate the assembly clearance at the lowest operating temperature, not at 20 °C, and add 0.5 to 1 percent of the diameter as extra clearance for gears and bearings with varying load.
Moisture absorption applies to nylon. PA6 and PA6 G take up to 3 percent of their weight at full humidity and the dimensional change is roughly the same percentage, a PA6 part at 100 mm grows 2 to 3 mm from dry delivery to moisture saturation. Condition it in a damp environment before final machining, or pick POM C, which takes less than 0.4 percent moisture. Relaxation is the third area: plastic does not spring back the way steel does. A bolted joint in POM loses preload over time, 15 to 25 percent in the first week and then slowly toward a plateau, so plan for retightening or a spring washer if the joint is critical.