Polyoxymethylene, commonly called POM or acetal, is an engineering plastic known for its high stiffness, low friction, and excellent dimensional stability. It belongs to a class of materials called engineering thermoplastics, meaning it can be melted and molded into precise shapes while offering mechanical performance that rivals some metals. You’ll find it in everything from gear wheels and ball bearings to knife handles and lock systems.
How POM Is Built at the Molecular Level
POM’s backbone is a repeating chain of carbon and oxygen atoms, specifically alternating formaldehyde-derived units called oxymethylene groups. This simple, highly regular structure allows the polymer chains to pack tightly together, creating a material with high crystallinity. That crystallinity is what gives POM its hardness, rigidity, and resistance to deformation under load.
POM comes in two main forms: homopolymer (POM-H) and copolymer (POM-C). The homopolymer is built entirely from those repeating oxymethylene units, made by polymerizing formaldehyde or its cyclic trimer, trioxane. The copolymer introduces a second building block, typically from a ring called dioxolane, which inserts slightly longer oxyethylene segments into the chain. This small structural difference has practical consequences.
Homopolymer vs. Copolymer: Which Is Stronger?
POM-H is the stiffer, stronger option. Its simpler, more uniform chain structure allows for greater crystallinity, which translates directly into mechanical performance. POM-H has a tensile strength of about 11,100 psi compared to 8,800 psi for POM-C, and its melting point sits higher at 178 °C (352 °F) versus 166 °C (330 °F) for the copolymer.
So why use POM-C at all? The copolymer trades a bit of strength for better thermal stability and easier processing. Those extra oxyethylene segments in the chain interrupt the regularity just enough to reduce the tendency toward a specific type of degradation called “unzipping,” where the polymer chain breaks down from its ends and releases formaldehyde gas. POM-C is generally more forgiving during manufacturing and more resistant to long-term thermal breakdown. The choice between the two depends on whether your priority is raw mechanical strength or processing reliability and chemical stability.
Why Engineers Choose POM
POM fills a specific niche: parts that need to be stiff, dimensionally precise, and low-friction, but where metal would be too heavy, too expensive, or too prone to corrosion. Its natural lubricity (slipperiness) makes it a go-to material for parts that slide against each other. The wear mechanism is primarily adhesive, meaning the surface wears gradually and predictably rather than chipping or flaking.
Common applications include:
- Mechanical components: small gear wheels, ball bearings, springs, chains, screws, nuts, and pop rivets
- Consumer products: eyeglass frames, ski bindings, knife handles, zipper mechanisms
- Industrial parts: pump components, valve bodies, fan wheels, housing parts, sliding and guiding elements
- Precision assemblies: lock systems, fasteners, gun parts
POM maintains its rigidity down to -40 °C (-40 °F), which makes it suitable for outdoor and cold-environment applications where many other plastics would become brittle. Special grades are available with enhanced toughness, increased stiffness, or even lower friction for demanding wear applications.
Chemical Resistance and Weaknesses
POM handles fuels and many common solvents extremely well. Gasoline, jet fuel, kerosene, acetone, and virtually all alcohols (ethyl, methyl, isopropyl, butyl, and others) rate as “excellent” in compatibility testing over 48-hour exposure periods. This makes POM a reliable choice for fuel system components and parts exposed to common industrial solvents.
Its weaknesses, however, are specific and worth knowing. Acids are POM’s nemesis. Acetic acid at any concentration, hydrofluoric acid, hydrogen peroxide, and strong amines all cause severe damage. Even ammonia solutions at 10% concentration are rated as having a severe effect. Hydrochloric acid is somewhat more tolerable but still only rated “fair.” Ketones, lacquer thinners, and certain esters like isopropyl acetate also cause severe degradation. If your application involves any of these chemicals, POM is not the right material.
Thermal Stability and Formaldehyde Release
One important consideration with POM is that it breaks down into formaldehyde gas when heated. The unstable end groups on the polymer chain begin to decompose above 100 °C, though meaningful degradation during processing doesn’t start until higher temperatures. In air, onset degradation temperatures range from about 269 °C to 294 °C depending on heating rate. In an oxygen-free (nitrogen) atmosphere, those temperatures are 60 to 120 °C higher, which is why processing conditions matter so much.
The optimal processing window for POM sits between 200 and 250 °C in air, or 250 to 300 °C under nitrogen. Staying within these ranges minimizes formaldehyde generation while keeping the material fluid enough to mold properly. This thermal sensitivity is one reason POM-C (copolymer) is sometimes preferred: its modified chain structure resists the unzipping degradation that releases formaldehyde.
How POM Parts Are Made
Most POM parts are either injection molded or CNC machined. Injection molding is the standard for high-volume production, where molten POM is forced into a mold cavity and cooled into its final shape. The material’s high crystallinity and low shrinkage variability make it well-suited to this process, producing parts with consistent dimensions.
For prototypes, low-volume runs, or parts with complex geometries, CNC machining is the preferred method. POM machines cleanly, forming good chips with minimal burrs when sharp tooling is used. Dimensional tolerances as tight as ±0.025 mm are achievable, and the finished surfaces come out smooth without extensive post-processing. The key considerations are tool selection and heat management. Sharp carbide tools with positive rake angles give the best results. Dull tools generate excess heat, which can deform the material and ruin tolerances. Diamond-polished carbide tools designed specifically for acetal can increase cutting speeds by 15 to 20% while maintaining surface quality. Adequate coolant application throughout the process prevents dimensional drift and reduces downtime between operations.
Food Contact and Regulatory Status
POM is used in some food-processing equipment, but its regulatory status for food contact is not a blanket approval. In the United States, any substance that contacts food, including materials like POM used in packaging or processing machinery, must be authorized through one of several FDA pathways: a food contact notification, a food additive regulation, or a threshold-of-regulation exemption. Specific POM grades from specific manufacturers may carry food-contact compliance, but this depends on the formulation, any additives used, and the intended conditions of use. If you need POM for a food-contact application, verify that the specific grade you’re using has the appropriate authorization rather than assuming all POM qualifies.