CNC Machining Engineering Plastics
Precision machining of PEEK, Delrin (POM-C), Nylon, PTFE and other engineering polymers — with tolerances, tooling and fixturing chosen for each material rather than borrowed from metalwork.
- Tolerances to ±0.02 mm on stable grades
- PEEK, POM-C, Nylon, PTFE, Ultem
- Coolant and tooling matched to polymer
Quick answer
Engineering plastics machine beautifully when you respect two properties metals do not have: they move with temperature, and they relax under clamping pressure. Get those right and you can hold ±0.05 mm on PEEK, Delrin or Nylon with excellent surface finish. Get them wrong and a part that measured in tolerance on the machine is out of tolerance by morning.
We quote plastic machining within 48 hours in the normal case, and machine polymers on dedicated tooling with coolant selected for the material — never with the setup used for aluminium.
Common engineering plastics
Eight materials that cover almost every enquiry, with the trade-off spelled out.
| Material | Key strengths | Watch out for | Best for |
|---|---|---|---|
| PEEK | 250°C continuous service, excellent chemical resistance, strong and stiff | Expensive; needs stress relieving before precision work | Semiconductor fixtures, medical devices, high-temperature insulators |
| Delrin / POM-C | Excellent dimensional stability, low friction, machines like soft brass | Poor UV resistance; not for strong acids | Bushings, bearings, precision mechanical parts |
| Nylon 6 / 66 (PA) | Tough, good wear resistance, inexpensive | Absorbs moisture and swells — dimensions shift with humidity | Wear pads, rollers, bushings, structural brackets |
| PTFE (Teflon) | Near-zero friction, chemically inert, wide temperature range | Creeps under load; soft, so holding tolerance is hard | Seals, gaskets, chemical handling, low-friction guides |
| UHMW-PE | Outstanding impact and wear resistance, low friction | Very low melting point; poor dimensional stability | Guide rails, chute liners, conveyor wear strips |
| ABS | Cheap, tough, easy to bond and paint | Lower temperature and chemical resistance | Enclosures, prototypes, jigs |
| Polycarbonate | Transparent, very high impact strength | Stress-cracks with some oils and solvents | Guards, sight glasses, machine covers |
| PEI (Ultem) | Flame retardant, high strength, transparent amber | Expensive; notch-sensitive | Electrical insulators, sterilisation trays, reusable device housings |
Properties against aluminium
The thermal expansion line is the one that matters — POM moves about five times more than aluminium for the same temperature change.
| Property | PEEK | Delrin (POM-C) | Nylon 66 | Aluminium 6061 |
|---|---|---|---|---|
| Density (g/cm³) | 1.32 | 1.41 | 1.14 | 2.70 |
| Tensile strength (MPa) | 90 – 100 | 60 – 70 | 75 – 85 | ~310 |
| Max service temp (°C) | 250 | 90 – 100 | 80 – 120 | ~200 |
| Water absorption (24h) | ~0.1% | ~0.2% | ~1.2% | None |
| Thermal expansion (µm/m·°C) | ~47 | ~110 | ~90 | ~23 |
| Machining ease | Good | Excellent | Fair | Excellent |
| Relative material cost | Very high | Moderate | Low | Moderate |
Machining Capability
Realistic numbers for polymers, not metal tolerances copied across.
Standard Tolerance
±0.05 mm
Realistic and affordable on plastics.
High Precision
±0.02 mm
Achievable on stable grades such as POM and PEEK.
Surface Finish
Ra 0.8 µm
Typical as machined; polished edges available on acrylic and PC.
Processes
Mill · Turn · 5-axis
Plus drilling, tapping and threaded inserts.
Work Envelope
Up to 1000 mm
Plate, rod and tube stock depending on grade.
Inspection
Temperature controlled
Plastic dimensions are temperature-dependent — we note it on the report.
Design tips for machining plastics
Every one of these prevents a part that measures fine on the machine and drifts afterwards.
- Expect wider tolerances than metal. ±0.05 mm is realistic and affordable; ±0.01 mm requires temperature control and stress-relieved stock.
- Avoid thin walls and deep pockets — plastics deflect and spring back. Keep walls above 2 mm for PEEK and 1.5 mm for POM where possible.
- Use generous radii. Sharp internal corners concentrate stress, and notch-sensitive materials such as polycarbonate and PEI crack there later.
- Account for moisture on nylon. Nylon parts grow after machining as they absorb humidity; if the fit is critical, consider POM instead.
- Clamping matters — soft jaws and lower clamping force prevent distortion. Tell us when a part is thin or large and flat.
- Plan for heat. Sharp tools, moderate speeds and appropriate coolant keep the cut cool; melted or burnished edges are the symptom of dull tooling.
- Use threaded inserts rather than tapped holes where the assembly is opened repeatedly, especially in POM and nylon.
- Ask about stress relieving for PEEK — rough machine, relieve, then finish prevents warping.
POM, Nylon or PEEK?
These three cover most enquiries, and the choice is usually decided by temperature, moisture and budget rather than by strength.
- Choose POM-C when the part must hold a dimension, runs dry against another surface, and stays below roughly 90°C. It is the safe default for mechanical parts.
- Choose Nylon when toughness and impact resistance matter and the environment is not humidity-controlled — but accept that dimensions will move a little as the part absorbs moisture.
- Choose PEEK when the part sees temperatures above 150°C, aggressive chemicals, repeated sterilisation, or must not creep under sustained load. It costs many times more than the others, so specify it where the requirement genuinely exists and use POM everywhere else.
If you are unsure, send the service temperature, the load and the environment with the drawing. That is usually enough for us to recommend one without a round of samples.
Frequently Asked Questions
Answers to the questions we get asked most about this process.
Why not just 3D print these parts?
Printing is faster for complex one-off shapes, but machined parts come from solid stock, so they are fully dense, isotropic, and available in real production grades such as PEEK and POM-C. Tolerances and surface finish are also far better. See prototyping options for how the two compare.
Which plastic holds tolerance best?
POM-C (Delrin) is the most dimensionally stable and the easiest to machine. PEEK is excellent too, provided the stock is stress relieved. Nylon and UHMW are the least stable.
Can you machine PEEK for medical or food contact applications?
Yes, using virgin-grade stock. Tell us the compliance requirement at quote stage so the correct grade and documentation are sourced.
Will plastic parts creep under load?
All polymers creep to some degree, and PTFE and UHMW most of all. For a bearing or structural application under sustained load, share the load case and we will suggest a grade or a design change.
Do you provide inspection reports?
Yes, on request. We inspect at a controlled temperature and note it on the report, because plastic dimensions are temperature-dependent.
Can you supply metal inserts or hardware with the parts?
Yes. Threaded inserts are the standard solution for repeated assembly in POM and nylon, and we fit them as part of the job.
Do the parts need a surface finish afterwards?
Usually not — as-machined plastics are commonly used directly. Where appearance matters, polishing and vapour finishing are available; see surface finishes.
Typical applications
Insulating bushes and standoffs in electrical assemblies, wear strips and guide rails on packaging and automation equipment, lightweight replacements for metal components in corrosive environments, jigs and fixtures that must not scratch the parts they hold, semiconductor handling components, and low-friction bearings where lubricated metal is not acceptable.
Ready to Start Your Project?
Send drawings in STEP, IGES or DWG. You get a priced quote within 48 hours in the normal case, with tolerance and lead time stated up front.
Plastic soft jaws, nests and cable guides for robot cells are covered on the robotics page.