Engineering · Machining guide
How to machine PEEK, Delrin, and Ultem.
Engineering plastics are easy to cut and hard to cut well. The materials are soft, but they soften further as they heat — so the same tooling and feeds that work in metal create molten chips, dimensional creep, and stress crazing in plastic. This guide covers the three most-machined engineering thermoplastics: PEEK (high-performance, FDA-grade), Delrin / POM (general-purpose), and Ultem / PEI (high-temperature aerospace).
01 · Why plastics are different
Heat and dimensional stability.
Engineering plastics share three characteristics that drive machining strategy:
- Heat softens them. Heat from cutting friction softens the plastic ahead of the tool, leading to gummy chips, dimensional creep, and tool packing. Sharp tools and aggressive chip evacuation are essential.
- Stress crazing on Ultem and other amorphous plastics. Some coolants and solvents — chlorinated ones in particular — cause hairline environmental stress cracks in stressed amorphous polymers such as Ultem (PEI), polycarbonate and polysulfone. Semi-crystalline PEEK is largely resistant (Victrex). Use water-soluble emulsion or run dry with air blast.
- ~20× less stiff than aluminum. Modulus of elasticity is 3–4 GPa vs ~70 GPa for aluminum. Cutting forces deflect the workpiece more — fixturing must support the material, especially for thin or unsupported features.
02 · Tool selection
Sharp, polished, no coatings.
- Uncoated polished carbide
Coatings (TiN, AlTiN) drag in plastic. Uncoated polished-flute carbide gives the cleanest chip flow and best surface finish.
- Single-flute or 2-flute end mills
For pocket and slot work. Single flute moves heat away from the cutting zone better than 4-flute. Specialty plastic-cutting end mills with extra-large flute volume are worth the premium for production work.
- Sharp positive-rake inserts
Aluminum-style geometry with very sharp edges (no honing). Rake angle 15–25° for best chip flow.
- PCD inserts for high-volume PEEK
PEEK’s glass-fiber-reinforced grades (PEEK-GF30) are abrasive on carbide. PCD inserts give 10–50× the tool life on these grades.
- HSS for soft plastic and tapping
For Delrin and other soft thermoplastics, HSS with sharp edges works fine and is cheaper to replace.
03 · Cutting parameters
Numbers by material.
| Operation | Material | SFM | IPT / IPR |
|---|---|---|---|
| Turning | Delrin | 500–1000 | 0.005–0.020 IPR |
| Turning | PEEK | 300–700 | 0.005–0.012 IPR |
| Turning | Ultem | 400–800 | 0.005–0.015 IPR |
| End mill | Delrin | 400–800 | 0.003–0.010 IPT |
| End mill | PEEK | 300–600 | 0.003–0.008 IPT |
| End mill | Ultem | 350–700 | 0.003–0.008 IPT |
| Drilling | All three | 150–300 | 0.005–0.015 IPR (peck cycle) |
IPT (chip-load) values assume roughly ½–1″ end mills; scale down for smaller tools — see the chip-load chart.
04 · Coolant by material
Different rules for each.
- Delrin (POM): water-soluble flood is fine. Air blast also works for short cycles. Avoid chlorinated coolants — they can affect dimensional stability.
- PEEK: air blast or dry is cleanest; PEEK is largely resistant to coolant-induced stress cracking, so water-soluble flood is acceptable where residue isn’t a concern. Through-spindle compressed air with chip evacuation is the cleanest production approach.
- Ultem (PEI): water-soluble flood acceptable but dry with air blast preferred. As an amorphous polymer it is more susceptible than PEEK to stress cracking from some additives and solvents.
- Implant-grade PEEK (Optima LT1): dry only. Validation requires no coolant residue on finished parts. Sharp tools and aggressive air-blast chip evacuation are essential.
05 · Practical notes
What goes wrong.
- Annealed stock for PEEK: as-extruded PEEK has internal stress that releases during machining. Specify annealed stock for tight-tolerance parts; annealed PEEK holds dimensions much better.
- Clamping pressure: too much vise pressure deflects plastic stock. Soft-jaw clamping or vacuum fixturing prevents marks and avoids deflection-induced dimensional error.
- Climb mill always: conventional milling drags chips back through the cut and creates surface drag.
- Burr-free edges: single-flute end mills with sharp geometry produce burr-free edges in plastic. For deburring requirements, sharp tooling and proper chip load are the only path — flame deburring damages plastic, manual deburring is slow.
- Glass-filled grades: PEEK-GF30 and similar are abrasive. Tool life drops 50% vs unfilled grades. Plan PCD tooling for production volumes.
Keep exploring
Related tools & references
High-performance, FDA- and implant-grade thermoplastic.
General-purpose acetal properties and specs.
High-temperature aerospace thermoplastic data.
Set feeds high enough to avoid melting and packing.
Per-tooth chip loads for soft thermoplastics.
Compare PEEK, Delrin, and Ultem by application.
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