POM (polyoxymethylene, sold as Delrin, Celcon, Hostaform) is a semi-crystalline engineering thermoplastic with 60-75 MPa tensile strength, 2,500-3,200 MPa flexural modulus, and 0.2-0.3% moisture absorption, which is the property set that puts it ahead of nylon and HDPE for precision moving parts [S2][S3]. For general fabrication, the practical reason to pick POM over commodity plastics is that it holds tolerance under load: low creep, near-zero water uptake, and a coefficient of friction of 0.1-0.3 against steel without lubrication [S2].
The decision starts with homopolymer (POM-H) versus copolymer (POM-C). POM-H runs roughly 10-15% stiffer and stronger, and it handles higher continuous service temperature; POM-C has no centerline porosity, better chemical resistance to hot water and mild bases, and is the safer default for food-contact and medical assemblies [S2][S3][S6]. Within each family, filled grades (glass fiber for stiffness, PTFE for friction, UV-stabilized for outdoor, antistatic for electronics) extend the base resin without changing the fabrication workflow [S3].
Two Resins, One Decision: POM-H vs POM-C
POM-H (Delrin) and POM-C (Hostaform, Celcon) are the two main types of acetal plastic, with slightly different property profiles suited to different applications [S2][S7]. POM-H typically reaches 60-75 MPa tensile, slightly higher continuous service temperature (~100°C vs ~90°C for POM-C), and tighter crystalline structure that translates to higher hardness and better fatigue endurance under cyclic load [S2][S3]. POM-C uses a comonomer to break up the chain, which eliminates the centerline porosity that can show up in thick POM-H stock (typically over 25 mm), and it resists hot water, bleach, and weak bases better [S2][S6].
Choose POM-H when the part is a gear, bushing, or any geometry under 15-20 mm wall where you need maximum strength and fatigue life, and you are not worried about long-term exposure to hot water or strong detergents [S3][S4]. Choose POM-C when you machine thick blocks, run parts through washdown cycles, or need FDA / food-contact compliance, and when porosity in a critical face would scrap the part [S6]. For most general fabrication shops stocking one grade, POM-C is the safer default because the porosity-free structure forgives the small mistakes in stock selection that show up on a lathe.
Spec-Anchored Comparison: POM vs PE vs PA6 vs Polycarbonate
Set against the other common engineering plastics, POM sits in a narrow window: stiffer and more dimensionally stable than PE and PA6, but lower impact than polycarbonate, and it machines cleaner than both [S1][S5]. The numbers make the trade visible: POM tensile 60-75 MPa and flexural modulus 2,500-3,200 MPa, versus PA6 (lower stiffness, ~9% moisture pickup that drifts dimensions), PE (much lower stiffness, higher impact, but softens near 80°C), and polycarbonate (higher impact, but poor wear, poor solvent resistance, and not a bearing material) [S2][S4][S5].
On friction, POM's 0.1-0.3 coefficient against steel without lubrication is the headline reason fabricators use it for sliding or rotating dry contacts, while PA6 needs lubrication or moisture to reach comparable numbers and PE is softer and tends to creep under sustained load [S2][S5]. On moisture, POM's 0.2-0.3% absorption is roughly 30x lower than PA6, so a POM-C gear that fits a 20 mm bore at 50% RH will still fit in a humid plant, where a PA6 equivalent will not [S3][S5]. For a deeper split between the two common acetal routes, see the POM types and classifications map, and for the cost side of the same decision, the POM TCO analysis for 2026 lays out what actually moves the invoice.
Who POM Is For, and Who Should Walk Past
POM is for the fabricator making precision gears, bushings, bearings, thrust washers, valve seats, conveyor components, fuel-system parts, and snap-fit assemblies that need to hold tolerance under load and slide without lubricant [S2][S3][S4][S5]. It is also the right pick for jigs, fixtures, and wear strips where metal would gall or corrode, and for any sliding interface that needs to run dry. Medical and food-contact parts almost always go POM-C over POM-H for the porosity and extractables profile [S6].
POM is the wrong material when the part sees strong acids (pH under 4), strong oxidizers, or sustained chlorinated solvents; when the part will be hot above ~100°C for POM-H or ~90°C for POM-C; when the part will be glued or painted without plasma or chemical etch treatment; or when the load case is high-impact rather than high-stress, where polycarbonate or PA6 will outlast it [S2][S3]. For electronics and semiconductor fixtures where static or outgassing is the constraint, an antistatic or static-dissipative POM grade, or a different material class entirely, becomes the right path; the engineering plastic map for electronics covers that selection in detail.
Fabrication Behavior: Machining, Tolerance, and Finishing
POM cuts cleanly with sharp carbide tools at positive rake angles; dull tools generate heat that locally softens the surface and can cause gumming or micro-cracks, so tool condition matters more than feed rate on this resin [S3]. Achievable tolerance on a CNC lathe or mill is typically ±0.05 mm on small features without special effort, and ±0.025 mm is reachable on rigid setups with light cuts, which is why POM is the default for prototype gears and small precision mechanisms that have to run before hard tooling is cut [S3]. Surface finish off the tool is already low-friction, so secondary polishing is rarely needed unless the part is a seal face.
Bonding and painting POM requires surface treatment: the polymer is too chemically inert for adhesives or inks to wet out without plasma, corona, or chemical etch, so plan for that step if the assembly calls for it [S2]. Annealing to relieve machining stress is optional for most general fabrication but worth doing for large flat parts that will sit in a tight-tolerance housing, typically 1-2 hours at 140-150°C with slow cool, which tightens dimensions and stabilizes long-term creep. Designers who want a broader material benchmark for fabricated parts can cross-check against cast iron selection for general fabrication when the geometry allows metal substitution.
Procurement and Standards Reality
For general fabrication, stock shape is the practical constraint: POM-H is widely available as rod and sheet up to ~150 mm, POM-C is the more common thick block and large-diameter rod, and both ship in natural (white), black, and a range of filled colors from distributors [S3][S4]. Specification should name the resin family (POM-H or POM-C), the grade (unfilled, GF25, PTFE-filled, UV-stabilized, antistatic), and the food-contact or medical regulatory status if the assembly carries it. FDA-grade POM-C is a stock SKU at most industrial plastics distributors; POM-H FDA grades are rarer and usually custom-run [S2][S6].
For traceability, ask for the manufacturer's data sheet with tensile, flexural modulus, deflection temperature under load, and moisture absorption numbers, and confirm the lot is from a recognized compounder (DuPont, Celanese, BASF, Mitsubishi Engineering-Plastics, Asahi Kasei). For general fabrication, the data sheet spec band is the contract; tighter than that, you are specifying a custom compound. For reference, the POM encyclopedia entry consolidates the base property window in one place for spec-writing.
Limits, Failure Modes, and What to Watch On the Floor
Three failure modes dominate field returns on POM parts. First, centerline porosity in thick POM-H sections that opens up under load or during machining; the fix is to switch to POM-C above ~25 mm wall thickness [S6]. Second, chemical attack from acids, chlorine, or strong oxidizers that the spec sheet called "resistant" but actually degrades the surface; POM is not a chemical-service material beyond mild detergents, fuels, and oils [S2][S3]. Third, thermal creep above the continuous service limit, which for unfilled POM is ~90-100°C depending on grade; above that, the part will slowly deform under sustained load even well below the melting point [S2].
For wet or food-processing service, verify the grade is rated for hot water and detergent exposure (POM-C generally yes, POM-H conditional). For sliding interfaces, derate load if the part runs above 60°C continuously, and consider a PTFE-filled or glass-filled grade if stiffness or wear life is the bottleneck rather than base cost. For assemblies with dissimilar metals in a wet environment, watch galvanic interaction at the POM interface, which POM itself does not accelerate but which can hide corrosion of a mating steel fastener.
For the broader metal-versus-plastic framing of these decisions, the cast iron grades for energy equipment comparison is a useful parallel for parts on the boundary between material classes.
Spec-level background on the components involved: pressure transmitter, and flow meter.