Delrin — the original homopolymer acetal (polyoxymethylene, or POM), developed by DuPont and made since 2023 by the independent Delrin company — is the plastic most machinists reach for when a part has to be precise, stiff, and low-friction without the price of PEEK. It cuts cleanly, holds a good finish, and stays put better than most plastics. If you machine engineering plastics at all, acetal is probably your bread and butter. But "easy to machine" doesn't mean "impossible to get wrong": internal stress and thermal movement will still walk a tight tolerance off spec if you rush the setup.
Here's how we approach acetal parts on the shop floor, and how to decide when it's the right material.
What is acetal?
Acetal is an engineering thermoplastic — chemically polyoxymethylene, abbreviated POM, and also called polyacetal or polyformaldehyde. It's a semi-crystalline material, which is what gives it the combination machinists care about: high stiffness, low friction, good dimensional stability, and very little moisture pickup. You'll see it sold under trade names — Delrin (the homopolymer), Celcon, Hostaform, Ultraform, and Tecaform among them — but they're all acetal.
In practice it occupies a useful middle ground. It's much stronger and stiffer than polyethylene, and while dry nylon is comparable to acetal on raw tensile strength and stiffness, acetal is far more dimensionally stable and holds its properties in humid or wet service where nylon absorbs moisture and softens. You get that stability without the cost of a high-performance polymer like PEEK — which is why acetal shows up in gears, bearings, bushings, and precision machined parts across almost every industry.
Acetal (POM) properties at a glance
Typical values for acetal homopolymer (Delrin). Copolymer grades run somewhat lower on strength and stiffness — see the next section.
| Property | Typical value |
|---|---|
| Density | ~1.41–1.42 g/cm³ |
| Tensile strength | ~10,000 psi (69 MPa) |
| Flexural modulus | ~420,000 psi (2.9 GPa) |
| Melting point | ~347–352 °F (175–178 °C) |
| Max continuous service temp | ~180 °F (82 °C) |
| Water absorption (24 hr immersion, ASTM D570) | ~0.25–0.3 % |
| Coefficient of friction (dynamic, vs. steel) | ~0.2 |
| Hardness | ~Rockwell M92 |
Treat these as typical published values for unfilled natural acetal, not a specification — grades, fillers, and stock form all shift the numbers, and you should work from the supplier's datasheet for the exact stock on a critical part. Glass-filled and PTFE-filled grades in particular trade toughness for stiffness or lubricity.
Where acetal falls short: it has poor resistance to strong acids, strong oxidizers, and strong bases — the homopolymer especially, which is generally rated for about pH 4–9 against the copolymer's pH 4–13, so caustics and chlorinated sanitizers are a copolymer job. It also isn't UV-stable in its natural form without additives, it's combustible (UL94 HB — slow-burning, not self-extinguishing), and — as covered below — it doesn't bond well with adhesives. Above roughly 180 °F of continuous service you're out of its range.
Delrin vs. acetal copolymer — a quick word
"Delrin" is often used loosely to mean any acetal, but strictly it's a homopolymer (POM-H). The other common form is acetal copolymer (POM-C). Homopolymer runs roughly 15% stronger and stiffer than copolymer and takes a marginally better finish; copolymer resists hot-water, caustic, and hot-chemical environments considerably better and is less prone to centerline porosity in large cross-sections. For most machined precision parts either works — but if you're running thick sections or parts that see hot water, it's worth specifying which one. Send us the print and we'll flag it if the grade matters for your job.
What tolerances you can realistically hold
Acetal is one of the more dimensionally cooperative plastics on a mill or lathe. Machined-plastic tolerance is best expressed proportionally — on the order of ±0.001" per inch of dimension — since a 1" part and a 12" part aren't the same problem. On well-planned acetal setups we routinely hold ±0.002", and tighter on selected critical features when the part is designed for it. It helps that Delrin absorbs very little moisture, so it doesn't swell over time the way nylon does — a real advantage for parts that need to stay in spec.
Two things still limit you:
- Thermal expansion. Acetal moves with temperature roughly ten times more than steel does. A cut that runs hot grows the dimension you just measured, and a part measured warm off the machine will read differently once it's cooled to the inspection room. Keep cuts cool and let parts normalize before you gauge them.
- Internal stress released by heavy cuts. Stock shapes generally ship annealed, so plain extrusion stress is less of a bogeyman than it's often made out to be — but removing material asymmetrically or in heavy sections (a U-shape out of plate, a bushing out of solid rod) will still let a part bow or shift after it leaves the vise. For tight-tolerance work, rough the part, let it relax (a further annealing step for the tightest jobs), then take finish cuts.
Tooling and speeds — the short version
- Sharp, positive-rake tooling. Acetal likes a keen edge that shears cleanly; dull tools rub, generate heat, and leave a fuzzy finish. Carbide holds an edge well over a run.
- High speed, steady feed. Acetal takes high surface speeds happily and produces clean, continuous chips when the feed is consistent.
- Coolant optional, chip clearing not. Many acetal jobs run dry, but clearing chips matters — recut chips mar the finish and add heat. Air or a light mist keeps things clean on longer cuts.
- Support thin walls and sharp corners. Acetal is stiff but will still flex on thin, unsupported features. Design in radii and keep drills sharp to avoid grabbing.
One planning note: acetal has a low surface energy, which is what makes it slick and self-lubricating — but it also means adhesives and solvent bonding don't hold well. Acetal assemblies are designed around machined features and mechanical fasteners, not glue.
When acetal is the right call
Acetal earns its place when a part needs precision and low friction at a sensible cost:
- Gears, bearings, bushings, and wear surfaces — low friction, good wear resistance, and enough stiffness to hold form under load.
- Precision machined parts that must stay in spec — low moisture absorption and good dimensional stability make it a safe choice for tight tolerances.
- Fluid-handling and mechanical components — manifolds, valve parts, rollers, and insulators that see room-temperature service and mild chemistry.
- A cheaper stand-in for metal — lighter than brass or aluminum, corrosion-free, quieter running, and often less expensive to machine in quantity.
Where acetal is not the answer: sustained high heat (continuous service tops out around 180 °F / 82 °C — reach for PEEK above that), strong acids, strong oxidizers, and strong bases (which attack it — and if the part sees caustics or hot water, specify copolymer over Delrin), and jobs where you truly need a bonded plastic assembly. It's also combustible, so it's a poor fit near open flame or high-temperature sources.
The takeaway
Delrin is the workhorse of precision plastic machining for good reason: it cuts clean, finishes well, and holds tolerance — provided you respect its thermal movement and relieve stress on tight parts. Match the grade to the duty, keep the setup cool and well supported, and acetal will do the job metal used to, lighter and cheaper.
If you have an acetal part — or you're weighing Delrin against nylon, PEEK, or a metal and want a straight answer — our team has been CNC machining engineering plastics since 1968. Send us the print and we'll tell you what's realistic. Request a quote and we'll take it from there.