⚪ DELRIN / ACETAL

Swiss Machining Delrin and Acetal: Delrin 150, Copolymer and Homopolymer

Acetal is to plastics roughly what free-cutting brass is to metals: the material a Swiss screw machine handles so cleanly that shops reach for it whenever a small precision plastic part is needed in volume. Delrin and its acetal cousins cut crisply with low cutting force, break clean chips, hold tight tolerances for a plastic, and leave a smooth low-friction surface, which is exactly why gears, bushings, fittings, and insulators get turned in acetal by the millions.

ISO 9001ISO 13485ISO 14001
Acetal (polyoxymethylene, POM) is the standout machining plastic. It is rigid but not brittle, dimensionally stable, low-friction, and it cuts cleanly with sharp tooling at high spindle speeds, producing well-broken chips rather than the gummy strings of softer plastics. Cutting forces are low, so slender parts deflect little and the guide-bushing support of a Swiss lathe lets long, thin acetal parts turn without chatter. Finishes come off smooth and glossy straight from the tool, often without any secondary operation. The main machining cautions are heat and stress. Acetal has a relatively low melting point (around 175 degrees C), so dull tools or excessive friction can melt and smear the surface; sharp tooling and air or coolant keep the cut cool. Like all machined thermoplastics it carries some residual stress, so tight-tolerance parts may benefit from annealing, though acetal is far less stress-sensitive and warp-prone than PEEK or nylon. For the vast majority of small turned plastic parts where chemical resistance and temperature demands are moderate, acetal is the default, and a buyer specifying a fussier plastic for a simple mechanical part is often paying more and machining harder than necessary.

Homopolymer Delrin versus acetal copolymer

Delrin is DuPont's brand of acetal homopolymer, and homopolymer versus copolymer is the key grade decision. Homopolymer (Delrin 150 and similar) has slightly higher strength, stiffness, and hardness and a better surface finish and machined appearance, making it the choice for high-load mechanical parts, gears, and where maximum strength matters. Its one quirk is centerline porosity: extruded homopolymer rod can have a small low-density region at the center of the bar, so parts machined from the core of a large rod, or with a through-hole on centerline, can reveal a tiny porous zone. For critical parts this drives stock selection or a switch to copolymer. Acetal copolymer has marginally lower mechanical strength but better long-term thermal stability, better resistance to hot water and chemicals, and crucially no centerline porosity, so it machines uniformly all the way through the bar. Copolymer is the usual choice for parts exposed to hot water, certain chemicals, or where the homopolymer's centerline porosity would be a problem, and many FDA and food-contact applications favor it. Both machine beautifully; the decision comes down to whether the part needs homopolymer's extra strength and finish or copolymer's stability, chemical resistance, and freedom from centerline porosity.

Tolerances, low-friction applications, and cost

Acetal holds tighter tolerances than most plastics, commonly +/-0.001 to +/-0.002 inch on turned features and tighter on smaller diameters, because it is dimensionally stable and machines cleanly. It is still a plastic, with thermal expansion roughly ten times that of steel and some moisture and temperature sensitivity, so metal-level tenths are not realistic and designers should size tolerances accordingly. Its natural lubricity and low coefficient of friction make it ideal for gears, cams, bushings, slides, and wear parts that must run with little or no lubrication, and the smooth machined finish supports those low-friction uses directly. On cost, acetal is an inexpensive engineering plastic, far cheaper than PEEK, and its fast clean machining gives short cycle times, so acetal Swiss parts are among the most economical precision components in any material. A small turned acetal part at volume can run well under a dollar to a few dollars each. There is no corrosion finishing, and secondary operations are often minimal because the as-machined finish is good. The honest guidance for buyers is that acetal is the right default for small precision mechanical plastic parts, with homopolymer Delrin chosen for strength and copolymer for chemical and thermal stability or to avoid centerline porosity, and that stepping up to a costlier high-performance polymer is only warranted when temperature, chemical, or biocompatibility demands genuinely exceed what acetal can handle.

Frequently Asked Questions

Delrin is a brand name for acetal homopolymer, and the homopolymer versus copolymer distinction is the main grade decision. Homopolymer Delrin has slightly higher strength, stiffness, and hardness and a better surface finish and machined appearance, making it the choice for high-load gears and mechanical parts where maximum strength and a fine finish matter. Its notable quirk is centerline porosity: extruded homopolymer rod can have a small low-density region at the center of the bar, so parts machined from the rod core or with an on-centerline through-hole can reveal a tiny porous zone, which drives stock selection for critical parts. Acetal copolymer has marginally lower strength but better long-term thermal stability, better resistance to hot water and many chemicals, and no centerline porosity, so it machines uniformly through the full bar. Copolymer is preferred for hot-water and chemical exposure, for parts where centerline porosity would be a defect, and often for food-contact applications. Both machine excellently; the choice is strength and finish versus stability and porosity-free material.
Acetal (POM) is the standout machining plastic because it combines properties that suit a screw machine perfectly. It is rigid but not brittle, dimensionally stable, and low-friction, and it cuts cleanly with sharp tooling at high spindle speeds, producing well-broken chips rather than the gummy strings that softer plastics make, which keeps chip evacuation easy on a guide-bushing machine. Cutting forces are low, so slender parts deflect little and long thin parts turn without chatter, and finishes come off smooth and glossy straight from the tool, often needing no secondary operation. It holds tighter tolerances than most plastics, commonly +/-0.001 to +/-0.002 inch, and it is far less stress-sensitive and warp-prone than PEEK or nylon. It is also inexpensive and machines fast, giving short cycle times and very economical parts. The combination of clean cutting, dimensional stability, low friction, and low cost is why acetal is the default for small precision plastic parts and why shops reach for it the way they reach for free-cutting brass among metals.
Acetal holds tighter tolerances than most plastics, commonly +/-0.001 to +/-0.002 inch on turned features and tighter on smaller diameters, because it is dimensionally stable and cuts cleanly. It is still a thermoplastic, with a thermal expansion coefficient roughly ten times that of steel plus some moisture and temperature sensitivity, so metal-level tenths are not realistic and designers should size tolerances accordingly, allowing for the material's movement with temperature. For the tightest work, annealing the stock or parts improves stability, though acetal is far less stress-prone than PEEK or nylon and often needs no special treatment. Surface finishes come off smooth and glossy straight from sharp tooling, frequently good enough that no secondary finishing is required, which supports acetal's common low-friction gear, cam, and bushing applications directly. The keys to a good result are sharp tools and controlled cutting heat, since acetal's relatively low melting point near 175 degrees C means a dull tool or excessive friction can melt and smear the surface rather than cut it cleanly.
Acetal is one of the most economical engineering plastics to Swiss machine. The raw material is inexpensive, far cheaper than high-performance polymers like PEEK, and acetal's fast, clean machining gives short cycle times with minimal scrap, so the parts are among the cheapest precision components in any material. A small turned acetal part at production volume can run well under a dollar to a few dollars each depending on size and tolerance. There is no corrosion finishing as with metals, and secondary operations are often minimal because the as-machined finish is good enough to use directly. Compared with metals, acetal parts avoid plating and corrosion costs entirely; compared with other plastics, acetal beats softer or fussier materials on cycle time and scrap. The honest guidance is that acetal is the right default for small precision mechanical plastic parts, with homopolymer Delrin chosen for strength and copolymer for chemical and thermal stability, and that paying for a costlier polymer is only justified when temperature, chemical, or biocompatibility requirements genuinely exceed what acetal can deliver.

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Last updated: July 2026

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