Acetal vs Nylon for CNC Machining: POM, PA and Water

Two engineering plastics that behave quite differently

Acetal and nylon are both semi-crystalline thermoplastics, both are machined from extruded or cast stock, and both are chosen for the same broad reason: the part has to be light, insulating, non-corroding and self-lubricating, and metal is not the right answer.

Under that shared purpose, the two materials behave differently in ways that matter to a machined part.

Acetal — POM, sold under trade names such as Delrin — is a stiff, hard, low-friction plastic with very low moisture absorption. It is the plastic that machines most like a metal: chips break cleanly, dimensions hold, threads cut well and the surface off the tool is close to finished.

Nylon — polyamide, PA — is a tougher, more impact-resistant plastic with better wear behaviour under high load. It also absorbs water from its surroundings, and that absorption changes both its dimensions and its mechanical properties.

	Acetal (POM)	Nylon (PA)
Family	Polyoxymethylene	Polyamide
Moisture absorption	Very low	Moderate to high (grade dependent)
Dimensional stability	Excellent	Fair — moves with moisture
Machinability	Excellent — best of the plastics	Fair — gummy, stringy chips
Stiffness	Higher	Lower
Impact toughness	Moderate	Higher
Wear under high load	Good	Better
Friction	Low, self-lubricating	Low, self-lubricating
Chemical resistance	Good against fuels and solvents; attacked by strong acids	Good against oils and fuels; affected by acids and phenols
UV / outdoor	Poor unless UV-stabilised	Better, though still not a long-life outdoor material
Service temperature	~90–100 °C continuous	Higher
Typical form	Extruded rod and plate	Extruded rod and plate; cast nylon for large sections

The row that decides most machined parts is the second one, and it deserves more than a table cell.

Moisture: the difference that decides precision work

Nylon absorbs water from the air. Not a trace — enough to change the part. Polyamide chains attract water molecules, which sit between the chains and push them apart, so the material swells. The absorbed water also acts as a plasticiser, making the part slightly softer and tougher as it grows.

The consequences for a machined part are direct and often surprising:

• The part can grow out of tolerance after machining. A nylon component cut to a tight fit in a dry workshop can be too large once it has conditioned to a humid customer site.
• It grows unevenly where the section is uneven. A thick wall takes up moisture more slowly than a thin one, so a part can distort rather than simply scaling up.
• The properties change too. Stiffness falls and toughness rises as moisture is absorbed, so a part designed on dry-condition data behaves differently in service.
• The change is reversible but slow. Drying the part brings it back, over time, and the cycle repeats with the seasons.

Acetal barely does any of this. Its moisture absorption is a small fraction of nylon’s, which is why it holds the size it was cut to and why it is the plastic of choice for precision components — gears that have to mesh, bushings that have to fit a bore, spacers that set a gap.

Three practical responses where nylon is nonetheless the right material:

1. Choose a lower-absorption grade. PA12 and cast nylon absorb far less than PA6 or PA66, and for a dimensionally critical part the grade choice is the first lever.
2. Machine from conditioned stock. If the material has already equilibrated to the service humidity before machining, the part has less room to move afterwards.
3. Design the tolerance around it. Where a nylon part can be given clearance that accommodates the movement, the problem is designed out rather than fought.

The honest rule: if the part has a tight tolerance and it is not being chosen for toughness, acetal is the material. A nylon part specified on a drawing with a precision fit is a part that will be re-cut. Tolerances are held the same way in both materials — ±0.01 mm on precision features with ISO 2768-m as the general block — but the tolerance holds on acetal and moves on nylon.

Machinability, and why acetal is the favourite

Acetal is the best-machining plastic on the shop floor, and on a complex part that is worth a great deal.

It shears cleanly and produces short chips rather than strings, so the cutting zone stays clear. It holds fine detail, sharp edges and fine threads without tearing. It does not build up on the cutting edge the way softer plastics do, so the surface off the tool is smooth and consistent. It takes a good polish. And because it is dimensionally stable, a feature cut to size stays at size, which means a complex part can be machined to a finished dimension in one pass without leaving stock for movement.

Nylon is a different experience. It is gummier, produces long stringy chips that have to be broken with the tool path, tends to burr on edges and around holes, and can smear rather than cut if the tooling is not sharp. Fine threads and sharp internal corners need more care. The finish is achievable but takes more attention.

There is a second-order effect that matters for cost. Because nylon moves after machining, a nylon part that has to hold a tolerance may need to be machined oversize, allowed to condition, and then finished — an extra operation, and an extra setup. Acetal does not need that. On a tight-tolerance part, the material difference is smaller than the processing difference.

Both are machined on the same equipment as metals — see CNC milling — but the tooling, feeds and speeds are entirely different, and plastics respond badly to the heat that metal-cutting parameters generate.

Toughness, wear and the jobs nylon takes

Where acetal wins on precision, nylon wins on abuse.

Impact. Nylon absorbs more energy before it breaks and resists impact better. A part that gets struck, dropped or subjected to shock loading is better in nylon, particularly in the grades with higher moisture content, where the absorbed water adds toughness.

Wear under load. For sliding and bearing duty under high load, nylon generally outperforms acetal. It is the material for wear pads, slide strips, sprockets, cable sheaves and heavy-duty rollers, and cast nylon in particular is used for large machined wear components.

Abrasion. Nylon resists abrasion better, which is the same property expressed differently and the reason it appears on conveyor and materials-handling components.

Fatigue. Both handle repeated loading reasonably; acetal is stiffer and holds its shape better, which matters for a gear tooth that has to stay in mesh, while nylon’s toughness suits parts that flex.

The pattern that emerges is clean: acetal for precision and stiffness, nylon for toughness and wear. Where a part needs both, the usual answer is to look at the design — a thicker acetal section, or a nylon part designed with the tolerance movement in mind — rather than to find a plastic that does both.

Temperature, chemicals and outdoor exposure

Temperature. Neither is a high-temperature material. Acetal has a useful continuous service ceiling around 90–100 °C; nylon goes higher. Both lose stiffness as they warm, so a part that is stiff enough at room temperature may not be at 80 °C. State the operating temperature on the enquiry — it is one of the three facts, along with the load and the tolerance, that decide the material.

Chemicals. Acetal resists fuels, oils, solvents and neutral chemicals well and is attacked by strong acids. Nylon handles oils and fuels well and is affected by acids and phenols. Neither is a universal chemical-handling material, and a part that sees a specific process fluid should say so rather than relying on a general reputation.

Outdoor exposure. Nylon handles UV better; acetal degrades and chalks in sunlight unless it is supplied in a UV-stabilised grade, normally a carbon-black formulation. Neither is a long-life outdoor material in the way a coated metal is, and both belong indoors or in sheltered service unless the grade has specifically been chosen for exposure — see materials for the grades we run.

Availability, and a note on how these parts are made

Both materials are stocked as extruded rod and plate in a range of sizes, and cast nylon is available in much larger sections than extruded stock, which is why large machined nylon parts are usually cut from cast rather than extruded material.

Two practical consequences follow:

• Size availability decides the material for large parts. Where a part needs a section bigger than the extruded range, cast nylon is often the only route in plastic — and the grade comes with it.
• Not every plastic part should be machined. Where the quantity is high and the geometry suits a mould, injection moulding is far cheaper per part, and the resin is chosen in DFM against the same requirements. Machining is the right answer for prototypes, low volumes, large sections and parts that would need an expensive tool.

Both materials machine without any finishing step in most applications. There is no corrosion to protect against, no coating required, and the machined surface is usually the finished surface. That absence of finishing is one of the reasons plastic parts can be economical even where the material costs more than an equivalent steel.

Where each one lives

Acetal shows up as:

• Precision gears and gear components — see custom gears
• Bushings, bearings and wear rings where the fit has to be accurate
• Rollers, cams and guides
• Electrical insulators and spacers
• Pump and valve internals
• Parts where the machined surface is the finished surface and a tight tolerance is required

Nylon shows up as:

• Wear pads, slide strips and guide rails
• Rollers and sheaves under high load
• Impact-resistant components and guards
• Sprockets and conveyor parts
• Larger machined wear components in cast nylon
• Parts that see abrasion rather than precision

The pattern is consistent: acetal takes the dimensionally critical work, and nylon takes the mechanically abusive work.

Cost

The two materials are broadly comparable in price, with acetal usually a little higher per kilo in rod and plate, and cast nylon and PA12 higher again in the larger sections. Where a price difference exists it usually reflects the product form and how well stocked it is rather than a fundamental cost gap.

As with metals, the material line is not where the decision is usually won. A machined part in acetal costs less to produce than the same part in nylon, because the cut is cleaner, the chip control is simpler and the tolerance holds without the part having to be conditioned and re-finished. On a complex or tight-tolerance component, that processing difference generally exceeds the material price difference.

The one case where nylon wins on cost is where its properties let the design be simpler — a part that can be thinner or lighter in nylon because of the toughness, or a wear part that lasts longer in service.

When neither plastic is right

Three requirements fall outside both materials, and each comes up often enough to name.

The part carries real load. Neither plastic approaches the strength or stiffness of a metal, and neither holds a tolerance under load the way aluminium or steel does. Where a part carries structural load, the answer is a metal, and the question becomes which one — covered in the material comparisons on aluminium, stainless and steel.

The part runs hot and loaded. Both materials lose stiffness as they warm, and neither is intended for sustained high-temperature service under load.

The part needs to hold a fine tolerance over years, outdoors, in varying humidity. That combination rules out nylon on the moisture movement and acetal on the UV unless a stabilised grade is used. A coated aluminium part may be the more sensible answer, and it is worth raising at the enquiry rather than after the first batch.

The callout that belongs on the drawing

Instead of	Write
Plastic	Acetal (POM) or nylon (PA) — name the material
Nylon, ±0.01 mm	Revisit — specify a low-absorption grade such as PA12, or switch to acetal
Acetal	POM-C or POM-H, whichever the duty requires
Acetal, outdoors	UV-stabilised grade, or reconsider the material
Plastic, warm and loaded	State the temperature and the load — neither plastic may suit
Plastic, high volume	Consider injection moulding rather than machining

Three lines belong on any machined plastic drawing: the material, the operating temperature, and whether the tolerance has to hold in service or only at the machine. The last one is the question that separates acetal from nylon more often than anything else.

Precision features are held to ±0.01 mm in both materials with ISO 2768-m as the general block — the difference is whether the part stays there. Send the drawing, the material and the service conditions, and the quote comes back within 24 hours with DFM feedback, including a note where the material and the tolerance requirement are working against each other. Orders start at 50 pieces per style and material, samples from a single piece in 5–10 days, and every batch ships with a material certificate tracing the grade plus pre-shipment photos.