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Machining plastics: what POM, nylon, PTFE and PEEK do differently

July 17, 2026

Plastics cut faster than metal and behave worse afterwards. Machining them is not the difficult part. Keeping them the size you measured is.

The four you will meet most

POM, also called acetal or by the trade name Delrin. The best all round machining plastic. Stiff, dimensionally stable, low friction, low moisture absorption. The default for gears, bushings, manifolds and wear parts.

Nylon, PA6 and PA66. Tough and wear resistant, cheaper than POM, and it absorbs moisture. Expect dimensional change with humidity. Good for non critical wear parts and rollers.

PTFE. Chemically inert, lowest friction, very soft. It cuts easily and deflects easily, so tight tolerances are difficult and thin sections are impractical. Used for seals and chemically aggressive environments.

PEEK. High temperature, chemically resistant, strong and stiff, expensive. Used where the environment rules out everything else. Stock lead time is the main practical constraint.

Also common: HDPE and UHMW for sliding surfaces, PC and acrylic for transparent parts, and glass or carbon filled versions of all of the above for stiffness and stability.

Material	Dimensional stability	Max continuous temperature	Relative cost
POM	Very good	90 C	Low
Nylon 6	Poor, absorbs moisture	100 C	Low
PTFE	Moderate, soft	250 C	Moderate
PEEK	Very good	250 C	High
Glass filled nylon	Good	120 C	Moderate

Why plastic parts move

Three mechanisms, all worth designing around.

Machining stress. Extruded and cast stock contains internal stress. Removing material unbalances it and the part warps. On large or thin parts, ask whether the shop will stress relieve the blank before finishing, and expect that to add a day.

Moisture. Nylon is the main offender. It can grow noticeably over days in a humid climate. If the part has to hold a fit, specify POM or a filled grade, or design a fit that tolerates the movement.

Thermal expansion. Plastics expand roughly ten times as much as steel per degree. A plastic part in a metal housing across a wide temperature range needs clearance designed in, not a tight fit at room temperature.

Designing for machined plastic

• Keep walls thicker than you would in metal. 1.5 mm in POM is comfortable, 1 mm is possible, thinner deflects
• Avoid sharp internal corners, which concentrate stress in a material that is already prone to cracking under load
• Watch threads. Tapped threads in plastic strip easily. Use a coarse thread, more engagement, or a heat-set insert for anything that will be disassembled
• Do not over-tolerance. A ±0.01 mm feature in unfilled plastic is not repeatable in normal factory conditions
• Mind the heat. Plastics melt locally if the tool rubs. This shows as a glazed surface and an oversize hole. Shops handle it with sharp tools and air blast, which is another reason to ask what experience the shop has with the specific material

Cost, which surprises people twice

Machine time in plastic is low, so the raw machining is cheap. Two things bring the cost back up:

1. Material price. PEEK and PTFE stock cost far more per kilogram than aluminium, and plastic parts are often machined from oversized stock because standard sheet thicknesses do not match the part
2. Handling and inspection. Soft parts deform under measurement pressure, and inspection has to be gentler and slower. On tight tolerance work this is real time

Availability in Vietnam

POM, nylon, HDPE, PC and acrylic are stocked locally in common sizes. PTFE is widely available. PEEK and specialty filled grades are imported, and the lead time on those is measured in weeks, not days.

That makes the material line in a plastics request even more important than in metal. Say whether an equivalent grade from a different manufacturer is acceptable, because insisting on one brand can add a month for no functional gain.

What to put in your request

Material: POM, natural, local stock acceptable. General tolerance ±0.1 mm, bore A 20 +0.05/0 measured at 23 C. Part runs as a bushing on a steel shaft at room temperature.

The last sentence lets a shop tell you if a different material would last longer. That is a conversation worth having before the first order rather than after the first failure.

Choosing between POM, nylon and a filled grade

Most plastic part decisions come down to three questions asked in order.

Will it hold a dimension? If yes, POM or a glass filled grade. Unfilled nylon moves with humidity and should not carry a fit that matters.

Does it run against something? POM against steel is the classic low friction pair. Nylon wears well but swells. PTFE has the lowest friction and the worst dimensional stability. UHMW is excellent as a sliding surface and poor as a structural part.

What temperature and what chemicals? Above about 100 C continuous, the cheap options drop out. PEEK and PTFE remain, at very different prices and stiffnesses.

Specifying stock, not just material

Plastic stock comes extruded or cast, and the difference matters. Cast POM and cast nylon have lower internal stress and machine flatter, which is why large plates and thin parts should be made from cast stock. Extruded rod is fine for small turned parts and is cheaper.

If your part is large, thin or tightly toleranced, write one line: “cast stock preferred for dimensional stability”. Shops that hold both will use the right one. Shops that hold only extruded will now tell you, which is also useful.

A note on colour and food contact

Natural, black and a limited palette are normal. Custom colours in engineering plastics are a stock item, not a finishing operation, so the lead time is the supplier’s lead time. For food or medical contact, name the certification you need on the material and expect a certificate with the delivery rather than a verbal assurance.

The guide on material substitution covers how to handle a shop proposing a different grade, and you can send your drawing here.

Questions we get on this

Keep reading

• Carbon and alloy steels for machined parts: 1045, 4140 and 4340 compared
• Brass and copper grades for machined parts: C360, C260, C464 and C110 compared