Delrin (Acetal) Parts

Delrin, acetal, POM: same family, one real choice

All three names point at the same plastic. POM, short for polyoxymethylene, is the polymer, acetal is the generic term, and Delrin is a brand name for the homopolymer version (source 1). The practical split is homopolymer versus copolymer. Boedeker puts unfilled Delrin at about 10-15% higher strength and stiffness than unfilled copolymer acetal, and says copolymer generally performs better in hot water, steam and environments containing chlorine (source 2). Wikipedia adds that large homopolymer cross-sections can show pronounced centerline porosity, a lower-density zone down the middle of the stock (source 1). Most parts are happy in either. If your spec names Delrin, or the part lives in hot water, say so in the quote notes and the quote follows the grade you name.

What gets made from it

Wear and motion parts, mostly: bushings and sleeve bearings, rollers, slides and guides, and wear pads where a plastic face meets a steel one. Wikipedia lists bearings, sliding and guiding elements, rollers, fasteners and insulators among acetal’s uses, and gives a coefficient of friction against steel of 0.31 under one set of test conditions (source 1). Parts with threads or knurls are quoted from the print too, and custom knobs are one example.

The other half of the work is lab and instrument components, insulators, seal seats, test fixtures, and food-machinery parts, where a stable plastic that holds dimension is the whole requirement. Most of it is turned or milled in small batches, with no minimum order. Plastic contact pads for fixtures are covered on Delrin and UHMW fixture details. Thin, flat Delrin wear strips and spacers can be cut from sheet instead, and Wikipedia notes POM sheet can be cut cleanly with a CO2 infrared laser (source 1); laser-cut plastic parts covers which plastics cut clean.

Acetal or nylon?

Moisture is the dividing line. Plastics Technology reports that unfilled acetal absorbs about 0.2% moisture by weight at room temperature and 50% relative humidity and grows about 0.2%, rising to almost 0.8% moisture and 0.7% growth at 100% relative humidity. Unfilled nylon 6 or 6/6 reaches roughly 1.5-2% moisture at normal room humidity, 5-8% under continuous immersion, and can grow 0.5-0.6% at room temperature (source 3). Wikipedia confirms that moisture changes nylon’s dimensions, and lists excellent abrasion resistance among nylon’s fiber properties (source 4).

The rule of thumb: if the part has a fit, a bore, or a tolerance that matters, acetal. If it’s a sacrificial wear block and a little growth is harmless, nylon can do the job. Other alloys and plastics quoted on request. Name the grade on the print.

Tolerance honesty for plastic parts

Plastic moves more than metal, and it’s better to design for that than to fight it. Wikipedia gives acetal a thermal expansion coefficient of 120 ppm/K (source 1) and steel 11-13 ppm/K (source 5), roughly ten times, so a fit dimensioned at 68 °F behaves differently in a hot enclosure. Boedeker notes that the tolerances required for thermoplastic parts are generally considerably larger than those applied to metal parts (source 6), and Wikipedia adds that machined POM can be dimensionally unstable, especially on parts with large variations in wall thickness (source 1). ±0.005 in is typical on acetal parts, with ±0.001 in on critical turned diameters, and milled features are quoted from the print. Before calling for more, it’s worth asking whether the assembly actually needs metal-grade numbers at temperature.

Two things that save revisions: call out which dimensions are critical and the temperature they apply at, and point out thin sections and uneven walls. The free design review that comes with every quote looks at them.

Sources

All accessed 30 September 2026. Where a figure here differs from your print, the print governs.

1. Wikipedia, “Polyoxymethylene”: en.wikipedia.org/wiki/Polyoxymethylene (names and brands; Delrin variants are homopolymers; homopolymer and copolymer; centerline porosity; friction against steel; thermal expansion; uses; laser cutting; machined POM stability).

2. Boedeker Plastics, “What is the difference between acetal and Delrin?”, Jim Hebel, 7 April 2026: boedeker.com/plastics-insights/knowledge-base/what-is-the-difference-between-acetal-and-delrin/ (Delrin as trademarked homopolymer acetal; 10-15% higher strength and stiffness than unfilled copolymer; copolymer in hot water, steam and chlorine).

3. Michael Sepe, “Dimensional Stability After Molding: Part 4”, Plastics Technology, 25 March 2013: ptonline.com (moisture absorption and dimensional growth of unfilled acetal and nylon).

4. Wikipedia, “Nylon”: en.wikipedia.org/wiki/Nylon (moisture changes dimensions; abrasion resistance).

5. Wikipedia, “Thermal expansion”: en.wikipedia.org/wiki/Thermal_expansion (linear expansion coefficient table, steel).

6. Boedeker Plastics, “Guide to Machining Plastics”: boedeker.com/Technical-Resources/Plastic-Machining-Guidelines (tolerances for thermoplastic parts compared with metal parts).

7. Wikipedia, “Polyether ether ketone”: en.wikipedia.org/wiki/Polyether_ether_ketone (operating temperature and resistance to organic and aqueous environments).

8. Wikipedia, “Autoclave”: en.wikipedia.org/wiki/Autoclave (steam sterilization at 121 °C and 134 °C).

9. U.S. Code of Federal Regulations, 21 CFR 177.2470, “Polyoxymethylene copolymer”, via Cornell Law School Legal Information Institute: law.cornell.edu/cfr/text/21/177.2470 (copolymer permitted for food-contact articles under stated conditions).

10. U.S. Code of Federal Regulations, 21 CFR 177.2480, “Polyoxymethylene homopolymer”, via Cornell Law School Legal Information Institute: law.cornell.edu/cfr/text/21/177.2480 (homopolymer permitted for food-contact articles under stated conditions).