You walk into any machine shop and ask about engineering plastics, the conversation always starts the same way: "What material are you using?" And when you say nylon, half the time the response is "Have you considered Delrin?" Both are white-ish, both cut on a CNC lathe, both show up in bearing sleeves, gears, and wear pads. But they behave nothing like each other once you bolt them into a real application.

We machine thousands of plastic parts a month, and nylon and Delrin are the two materials we see most. After 20+ years of watching what works and what fails in the field, here's the no-fluff breakdown of when to pick which.

The Real Difference: Moisture Is the Deciding Factor

Delrin (POM homopolymer, typically DuPont brand) doesn't absorb water. Period. You can leave a Delrin bushing in a bucket of water for a week, pull it out, and the dimensions haven't changed. Nylon 6/6 absorbs 1.5% of its weight in moisture at 50% relative humidity — and up to 8-10% when fully saturated. That 8% moisture absorption translates to roughly 0.3% dimensional change, which is 0.15mm on a 50mm diameter bushing.

That 0.15mm is the difference between a press-fit bearing and a sloppy one. It's the difference between a gear that meshes at 0.02mm backlash and one with 0.17mm backlash. It's the difference between a part that works out of the box and one that has to be dried in an oven for 4 hours before assembly.

If your part operates in a dry, controlled environment — inside an electronics enclosure, in a cleanroom, on a factory floor with HVAC — nylon is probably fine. But if there's any chance of water exposure, humidity cycling, or outdoor installation, Delrin is the safer bet.

When Nylon Wins (Yes, It Happens)

Nylon isn't garbage. It's genuinely better than Delrin in three specific situations.

First: impact resistance. Nylon 6/6 has an Izod impact strength around 80 J/m, while Delrin sits at 120 J/m — wait, that's Delrin winning. Actually, nylon 6/6 is around 80 J/m dry, but nylon 6 (cast) can hit 150+ J/m. The catch is that nylon's impact strength drops by 50% or more when it's saturated with moisture. So the answer depends on your operating conditions.

Second: wear resistance in dry-running applications. Nylon has a lower coefficient of friction against steel than Delrin — 0.20 vs 0.30 roughly — which means nylon bearings run cooler and last longer when there's no lubrication. We've seen nylon wear pads in conveyor systems that run 2-3x longer than Delrin in the same dry-running application. The trade-off: nylon wears faster than Delrin when it's wet or lubricated, because the moisture acts as a plasticizer and softens the surface.

Third: cost. Nylon 6/6 rod stock costs about 60-70% of Delrin's price per kilogram. For a $2.50 bushing, the material cost difference is $0.15 — probably not worth switching materials for. But for a 50kg prototype run of bushings, that's $400-500 in material savings. When the volumes get big enough, nylon's cost advantage starts to matter.

When Delrin Wins (Most of the Time)

For CNC machined parts in industrial applications, Delrin wins more often than nylon. Here's why.

Dimensional stability is the big one. Delrin's moisture absorption is 0.25% at saturation — roughly 1/30th of nylon's. If you're machining bearing seats, press-fit holes, or any feature that needs to hold +/-0.01mm tolerance, Delrin gives you a fighting chance. Nylon makes you fight for it.

Machinability isn't even close. Delrin has a machinability rating that approaches free-machining brass — chips come off clean, surface finish hits Ra 0.8 without trying, and tool wear is minimal. Nylon is gummy. It melts if your cutting speed is too high, it generates long stringy chips that wrap around the tool, and the surface finish is Ra 1.6 at best without special tooling. We run Delrin at 3-4x the feed rate of nylon, which means Delrin parts cost less to machine even though the material costs more.

Stiffness favors Delrin in most comparisons. Acetal (Delrin) has a tensile modulus around 3.1 GPa, while nylon 6/6 is about 2.9 GPa — close, but Delrin is stiffer when both are dry. Nylon 6/6 gets softer when it absorbs moisture (modulus drops to about 1.0 GPa at saturation), while Delrin barely changes. For gears, bearing sleeves, and any load-bearing component, Delrin's consistent stiffness is a real advantage.

Head-to-Head Data

Numbers in this table are approximate and vary by grade and manufacturer.

What About Glass-Filled Variants?

Both materials come in glass-filled grades — typically 30% glass fiber. Glass-filled nylon gets dramatically stiffer (modulus up to 9 GPa) and loses most of its moisture sensitivity, but it becomes abrasive and destroys carbide tools 3-4x faster than unfilled nylon. Glass-filled Delrin (POM-GF) is stiffer too but less abrasive to machine. If you need the stiffness of a metal part with the weight of plastic, glass-filled Delrin is usually the better machining choice.

The downside of glass-filled grades: they're not suitable for bearing surfaces. Glass fibers exposed at the surface act as abrasives against mating metal components. We've seen glass-filled nylon bushings that destroyed the shaft they were supposed to protect. If you need a glass-filled part with a bearing surface, specify a metal sleeve insert or plan for post-machining polishing to expose the resin matrix.

Real-World Callouts We See in the Shop

A robotics company sent us a drawing for a nylon bearing sleeve — 25mm ID, 30mm OD, 40mm long, press-fit into an aluminum housing. They'd been using nylon for three design iterations and every time, the bearing was too tight or too loose depending on the weather. We suggested switching to Delrin, they approved, and the press-fit has been consistent across 10,000+ pieces over two years. Same drawing, different material, problem solved.

On the flip side, an industrial conveyor manufacturer was using Delrin wear pads on a chain-driven sorter that ran 24/7. The pads were wearing out every 3 months — faster than expected. We switched them to cast nylon 6 (not 6/6) and the wear life doubled to 6 months. The key was that the application was dry-running and the higher wear resistance of nylon (in dry conditions) outweighed Delrin's dimensional stability advantage.

Neither story proves one material is universally better. Both prove that the right answer depends on your actual operating conditions.

Our Recommendation

For CNC machined parts where you can control the operating environment: nylon is fine, and it's cheaper. For parts where moisture, humidity, or dimensional stability matters: pick Delrin. When in doubt: Delrin. The material cost premium is small, the machining cost is lower, and you won't spend 6 months debugging dimensional drift caused by humidity.

Send us your drawing with the operating conditions noted — temperature range, moisture exposure, load, speed, and mating material — and we'll give you a material recommendation backed by actual shop-floor experience, not textbook tables.