CNC Machining Plastics: Delrin, Nylon, PEEK and Acrylic
Short answer: the four workhorse engineering plastics machine very differently — Delrin (acetal) is the easiest and most dimensionally stable (hold ±0.02–0.05 mm routinely), glass-filled nylons cut fine but absorb moisture and move, unfilled PEEK machines well but costs bar stock like a precious metal and needs sharp tooling to avoid smearing, and acrylic cracks unless you control heat and feed. For insulators, wear pads, bushings, manifolds and transparent windows, CNC plastic beats metal on weight, cost and function — if you pick the right grade and tell the shop what the part does.
Plastic CNC work is where cheap quotes go wrong: shops that machine metal all day treat plastic the same way and produce melted edges, cracked windows and parts that grew half a millimeter overnight. Plastic is not "soft metal." It has low thermal conductivity, high thermal expansion, springiness that deflects under tool pressure, and moisture behavior that changes dimensions after machining. This guide covers the practical grades, what tolerance each honestly holds, and how to write the job so it comes back right.
The Four Workhorse Grades
| Grade | Best for | Machining behavior | Dimensional stability |
|---|---|---|---|
| Delrin / acetal (POM) | Bushings, gears, rollers, insulators, manifolds | Excellent — clean chips, tight tolerance, low stress | Excellent; low moisture uptake |
| Nylon 6/66 (cast or extruded) | Wear parts, bearings, rollers | Good — but hygroscopic: machines fine, then moves | Poor to fair; absorbs up to ~2-8% moisture |
| PEEK (unfilled / 30% GF / CF) | High-temp, chemical, medical, aerospace | Good with sharp tools; smears if tool dull | Good; low moisture, high temp capability |
| Acrylic (PMMA) | Windows, lenses, light guides, display parts | Fair — brittle, heat-sensitive, cracks easily | Good (but stress cracking under load) |
| PTFE / Teflon | Seals, chemical parts | Poor — soft, deforms; special tooling | Fair |
| Polycarbonate | Transparent + tough parts | Fair — gummy, needs sharp polished tools | Good |
Outside these, UHMWPE (abrasion), PET (low outgassing), PVC (chemical) and PSU/PEI (high temp) fill niches. For 90% of engineered plastic parts, the decision is between Delrin, nylon, PEEK and acrylic.
What Tolerance Plastics Actually Hold
| Material | Realistic machined tolerance (small parts) | Notes |
|---|---|---|
| Delrin | ±0.02–0.05 mm | Best all-round choice for precision plastic |
| Nylon (dry as-machined) | ±0.05–0.1 mm | Move after moisture absorption; measure in use condition |
| PEEK | ±0.02–0.05 mm | Stable; cost is the constraint, not tolerance |
| Acrylic | ±0.05–0.1 mm | Clamp distortion and cracking dominate |
| PTFE | ±0.1 mm+ | Too soft for tight work; machined then re-formed |
Plastics expand roughly 5-10 times more than aluminum per degree, so "hold ±0.01 mm in plastic" is usually a misunderstanding — the part will be that tolerance at the temperature you measure it, and different in the customer's warehouse. Specify tolerances at 23°C and design plastic parts with the understanding that ±0.05 mm on a 50 mm Delrin part is real engineering, while ±0.01 mm claims should be questioned.
Machining Rules That Prevent Melted and Cracked Parts
Sharp tools are the entire game. A dull end mill rubs instead of cuts, heats the plastic past its softening point, and leaves a melted smear that no finish pass fully removes. Beyond tooling: use high spindle speed with moderate feed, light depths, and coolant or air blast to carry heat away — for acrylic especially, heat is the crack starter. Clamp gently: plastics deflect and distort under the same forces a steel part ignores, and spring back after unclamping. Break sharp internal corners or let the machinist add a radius; sharp corners concentrate stress and, in acrylic and polycarbonate, start cracks weeks later. And never assume "plastic" means one material — polycarbonate is NOT acrylic, nylon is NOT Delrin, and quoting the wrong one makes a failed part.
Moisture, Stress and the Parts That Move
Nylon is the classic trap: it machines beautifully and precisely, then absorbs moisture from the air and grows — a 100 mm part can move 0.5–1.5 mm over weeks in humid conditions, depending on grade and conditioning. If the application tolerates it, machine nylon from stress-relieved or moisture-conditioned stock, or switch to Delrin where moisture is a problem. Internal stress in extruded rod is the second trap: rough-cut stock, let it sit, then finish-machine — the same discipline as metal. Acrylic and polycarbonate also carry molded-in or machined-in stress that causes crazing; anneal or avoid aggressive cuts.
When Plastic Beats Metal — and When It Does Not
Plastic wins for electrical insulation (no anodize or coating needed), weight reduction, corrosion and chemical resistance, quiet operation (nylon gears), transparency, and cost at low volume — a Delrin prototype costs a fraction of an injection mold and arrives in days. Metal still wins where load, temperature (above ~150°C continuous for most plastics; PEEK extends this but not indefinitely), wear in abrasive environments, or stiffness under thin sections matter. A common hybrid: metal frame or insert with plastic bushings, insulators and wear surfaces — both machined in one factory, assembled, and shipped as a finished component.
Frequently Asked Questions
Q: What is the best plastic for CNC machining?
A: Delrin (acetal) is the best all-round grade: it machines cleanly, holds ±0.02-0.05 mm, absorbs almost no moisture, and stays dimensionally stable. Use nylon for wear and bearing parts with moisture-tolerant designs, PEEK for high temperature or chemical duty, and acrylic for transparent parts.
Q: Can CNC machining hold tight tolerances in plastic?
A: Yes, but scale expectations: ±0.02-0.05 mm is real production work in Delrin and PEEK, while nylon moves with moisture and acrylic fights clamping distortion. Plastics expand 5-10x more than aluminum with temperature, so specify measurements at 23°C.
Q: Why did my machined nylon part change size after delivery?
A: Nylon absorbs moisture from air — up to several percent by weight depending on grade — and swells as it does. Machine from conditioned stock, design with the moisture dimension change in mind, or switch to Delrin for moisture-sensitive precision parts.
Q: Is machined PEEK worth the cost?
A: Only where its properties pay: continuous service above 150°C, aggressive chemicals, medical or aerospace traceability, or where it replaces metal with big weight savings. Unfilled PEEK bar is expensive; 30% glass-filled is cheaper and stiffer but harder to finish smooth. For ordinary precision plastic, Delrin delivers most of the benefit at a fraction of the cost.
Q: Can you machine transparent acrylic without cracking it?
A: Yes — sharp polished tooling, controlled feed, coolant or air to manage heat, gentle clamping and stress-relieved stock keep acrylic intact. Avoid sharp internal corners and heavy interrupted cuts, both of which start cracks. Polishing the cut edges afterward restores optical clarity.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
