Acetal (POM) CNC Machining

The plastic that behaves like a metal

Machining plastics is not machining metals with different numbers. The failure modes are different: metals work-harden and conduct heat away, while plastics melt, smear, chip or stretch — and each plastic fails in its own way.

Acetal is the one that mostly does not fail. It is semi-crystalline, reasonably stiff, and has a wide enough gap between its softening point and its working temperature that cutting heat does not immediately cause problems. The practical consequences:

• Chips shear short. Acetal does not produce the long stringy ribbons that softer plastics give, and it does not wrap the tool.
• The surface comes off clean. There is no smearing across the tool edge, which is the characteristic problem with nylon.
• The tolerance holds. Acetal is stiff enough to resist cutting forces and stable enough to hold a repeatable dimension.
• The finish is often the finished finish. For most parts, no second operation is needed.

That combination is why acetal is the default plastic in a machine shop, and why the comparison with its nearest rival — nylon — comes down to a specific list of differences rather than a general preference.

What the properties buy in a part

Acetal’s value is mechanical, and the numbers explain the applications.

Property	Roughly	What it means in a part
Tensile strength	~65–70 MPa	Comparable to a soft aluminium, but at a fifth of the weight
Modulus	~3,000 MPa	Stiff enough to carry load without excessive deflection
Density	~1.41 g/cm³	Light, though denser than most plastics
Friction	Low, and self-lubricating	Runs against metal without routine lubrication
Wear resistance	Good	Survives sliding contact far better than most thermoplastics
Service temperature	~90–100 °C	Fine for mechanical and electrical duty, not for hot service

That set of properties points at one class of application: parts that move against something else. Gears, sprockets, worm wheels, bushings, rollers, cam followers, wear strips and slide pads. In each case acetal replaces a bronze or a steel part and brings three benefits — no corrosion, no lubrication schedule, and much lower weight — at the cost of strength and temperature capability.

The reason a plastic gear works at all is worth stating: acetal is softer than the steel it runs against, so it wears instead of the metal, it conforms slightly under load which spreads the contact, and it runs quietly because the material damps vibration. A metal gear pair is stronger; an acetal-steel pair is often better.

Machining acetal

The core discipline in plastics is the inverse of metals. In metal work the enemy is work-hardening and the answer is often more coolant and heavier cuts. In plastic work the enemy is heat, and the answer is a sharp edge, a fast spindle and a clear path for the chip.

Acetal is forgiving, but the rules still apply:

• Sharp, high-rake tooling. A blunt edge rubs, and rubbing generates heat exactly where the plastic will soften. In plastics more than metals, tool sharpness is the whole game.
• High spindle speeds and moderate feeds. Plastic cuts easily, so speed is available — and a fast, light cut removes material before heat accumulates in the workpiece.
• Generous chip clearance. Plastic chips are bulky for their weight and they do not conduct heat away. If the chip cannot leave, it recirculates, rubs and heats.
• Air blast rather than flood coolant on most jobs. Acetal does not need the cooling a metal does, and many plastics dislike prolonged contact with coolant — they can absorb it, stain, or suffer from the fluid’s chemistry. Compressed air clears the chip and carries away enough heat.
• Sharp, clean edges on the part. Acetal deburrs easily, but an over-aggressive tumble or brush will round edges that were machined deliberately sharp.

One caution specific to acetal stock: extruded acetal rod can carry a porous centre. The extrusion process can leave a core with voids or residual stress, and a part machined from that region may show porosity on a machined face, or move after machining as the stress releases. On a tight-tolerance or pressure-tight part, saying so at the enquiry lets the stock be checked or a different form — such as plate or a stress-relieved grade — be used instead.

Precision features are held to ±0.01 mm with ISO 2768-m as the general tolerance block — see tolerances we hold. The tolerance is achievable in acetal; what needs planning is the temperature at which it is measured.

Temperature: the design constraint, not just a machining one

Acetal’s thermal expansion is roughly ten times that of steel, and this is the property most often missed at the design stage.

Two consequences, and they point in different directions.

In the shop: a part gets warm while it is being cut. A dimension measured on a warm acetal part can read correct and come in undersized once the part cools. Critical dimensions belong on a cooled part, and the finishing cuts are best taken with the workpiece at a stable temperature.

In service: a plastic part running inside a metal housing will grow as it warms. A clearance that measures correctly at 20 °C can close up at 60 °C and bind. Where a part runs warm — and a loaded gear or bushing does, because friction heat is generated at the contact — the operating temperature belongs on the enquiry rather than being discovered in the field.

The same reasoning applies to moisture. Acetal absorbs a small amount of water from the air, less than nylon but not zero, and the part grows slightly as it does. On a general part this is irrelevant; on one held to ±0.01 mm and used in a humid environment, it is not. The practical options are to machine oversize and finish after conditioning, or to specify a tolerance band that accommodates the change — and either needs to be decided at the enquiry rather than at the inspection bench.

Where acetal is the right choice

• Gears, sprockets and worm wheels running against metal
• Bushings, bearings and thrust washers in light and moderate load service
• Rollers, cam followers and guide wheels
• Wear strips, slide pads and guide rails
• Pump and valve internals handling fuels, oils and non-aggressive fluids
• Insulators, spacers and standoffs where a mechanical rather than electrical duty dominates
• Food-handling components in the grades approved for that service
• Any machined part where a metal would corrode, need lubrication, or weigh too much

Where acetal is the wrong choice

The service temperature is above roughly 90–100 °C. Acetal softens and creeps. High-temperature plastics or a metal are the answer.

The part faces strong acids or bases. Acetal is attacked by both. The chemical needs to be stated before a grade is quoted.

The part is outdoors and exposed to sunlight. Acetal has poor UV resistance unless a stabilised grade is specified. Unstabilised material chalks and embrittles in sunlight.

The assembly has to be bonded. Acetal’s low surface energy makes adhesives and paints unreliable without surface treatment. Where bonding is central to the design, ABS solvent-bonds readily and is the easier material to build around.

The part carries sustained load at temperature. Thermoplastics creep — they deform slowly under continuous stress, and acetal is no exception however stiff it feels. Where a part carries a permanent load, that is worth reviewing against the service life.

High impact or shock loading is expected. Acetal is tough but it is not in the same class as polycarbonate for impact. Where a part must absorb a blow without cracking, the grade is different.

Finishing acetal

Acetal usually needs no finishing beyond deburring, which is part of the appeal.

• Deburring and tumbling — the standard operation, done gently to preserve machined edges
• Sand blasting — a uniform matte surface where a specific texture is wanted
• Vapour or flame polishing — used to clear a machined surface on visible parts, though acetal responds less dramatically than acrylic
• Painting and printing — possible only after a surface treatment such as flame, plasma or chemical etching; without it, the coating will not adhere reliably
• Annealing — a controlled heat cycle that relieves machining stress. Worth considering where a part will be exposed to solvents or to a warm environment, since relieving stress reduces the risk of later cracking

All finishing runs through our vetted partner network, managed under one quality process. The grades we run across every process are listed on the materials page.

What to put on the drawing

Instead of	Write
Plastic	Acetal (POM), where the part has a mechanical duty
Acetal, tight tolerance	Say which dimensions matter, and the environment they are measured in
Acetal, working temperature	State it — thermal expansion is roughly ten times steel’s
Acetal, humid or wet service	Say so — the part grows as it absorbs moisture
Acetal, will be glued or painted	Say so — acetal needs surface treatment to bond
Acetal, outdoor service	Specify a UV-stabilised grade
Acetal, strong acid or base	Wrong material — acetal is attacked by both
Acetal, above ~90–100 °C	Wrong material — it softens and creeps

The three lines that decide most acetal specifications are what the part moves against, how warm it gets, and whether it will be bonded or exposed to weather. The first is what acetal is for. The second and third are where the material’s limits bite, and both are cheap to settle at the enquiry and expensive to discover later.

Material is bought against your drawing for your job, and every batch ships with a material certificate (MTR) tracing the grade to its mill lot plus pre-shipment photos of the actual parts. Orders start at 50 pieces per style and material, samples from a single piece in 5–10 days, and production runs in 25–35 days after sample approval. Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on the tolerance, the service temperature and the stock form where any of them need settling.