CNC Machining Plastics: POM, PTFE, Nylon and When Plastic Beats Metal
Plastic beats metal in a CNC part when the load is light and the enemy is friction, corrosion, weight, electrical insulation or cost — a POM bushing or PTFE seal costs a fraction of its stainless equivalent. The price you pay is thermal: plastics expand five to ten times more than metal and creep under sustained load, so tolerances and service temperature decide the design.
Most engineers first meet machined plastics as a substitute — a metal part that failed by corrosion, galling, or noise, replaced with a plastic version that simply works. But plastics are not weak metal; they are a different class of material with their own design rules. Machining them well requires sharp tooling, controlled heat, and realistic expectations about tolerance. This guide covers the plastics that actually get CNC machined, what each one is good for, and the honest numbers behind the material selection.
The Five Plastics That Dominate CNC Machining
Four engineering plastics cover most machined-plastic work, with PEEK as the premium specialist. POM (acetal, often sold as Delrin) is the default machined plastic: stiff, slippery, stable, and cheap. PTFE (Teflon) is the chemical and thermal specialist with the lowest friction of any solid. Nylon (PA6/PA66) is tough and wear-resistant but absorbs moisture. PEEK is the high-temperature, high-strength option for medical and aerospace use.
| Material | Tensile strength (typical) | Max continuous service temp | Key strength | Key weakness |
|---|---|---|---|---|
| POM (acetal) | ~65–70 MPa | ~90–100°C | Machinable, stable, low friction, low cost | UV-sensitive, limited temp |
| PTFE | ~20–30 MPa | ~260°C | Nearly universal chemical resistance, lowest friction | Soft, cold-flows, expensive to machine well |
| Nylon 66 | ~80 MPa | ~90–120°C | Tough, wear-resistant, abrasion-resistant | Absorbs moisture, dimensions drift |
| PEEK | ~95–100 MPa | ~250°C | Strong, stiff, flame-resistant, biocompatible | Expensive, ~10× POM cost |
| Acrylic (PMMA) | ~70 MPa | ~60–70°C | Optical clarity, easy to polish | Brittle, crazes under stress |
Each material's weakness defines its design envelope. PTFE's softness means it cannot hold tight tolerance on thin sections — it deforms under the cutter and creeps in service. Nylon's moisture uptake changes dimensions by fractions of a percent between dry and saturated, which kills tight fits if the designer does not account for it. PEEK's cost means it is specified only where the temperature or the regulatory environment demands it.
Machining Plastics Is Not Machining Metal
The number-one machining mistake with plastics is treating them like aluminum. Plastics are soft but thermally insulating; heat stays in the cut, melts the chip, and smears it back onto the surface. Sharp tools with positive rake, high clearance, and proper chip evacuation are non-negotiable. Dull tools push the material instead of cutting it, producing fuzzy edges, whitened surfaces, and dimensional error.
| Machining factor | Metal practice | Plastic practice |
|---|---|---|
| Tool sharpness | Important | Critical — dull tools melt, not cut |
| Cutting speed | High | Moderate–high, but watch heat |
| Feed | Wide range | Steady, never dwell |
| Coolant | Flood or through-tool | Air or mist; some plastics absorb coolant |
| Fixturing | Hard clamping fine | Soft jaws, light clamping — parts deflect |
| Surface finish after cut | As-machined often fine | Often needs deburr or polish pass |
Clamping deserves special attention. Plastics deflect under clamp pressure far more than metal, so machinists use soft jaws and light torque — then the part relaxes after unclamping and the machined dimension shifts. Good plastic work holds the part the way it will sit in service. Sharp corners are another trap: plastics notch easily, so internal radii should be generous and thread engagement kept to a sensible depth rather than driven to metal-style lengths.
Tolerances: What Is Realistic in Plastic
The honest tolerance statement for machined plastics is ±0.05 mm on well-supported features in POM at a controlled shop temperature, loosening to ±0.1 mm or more on thin walls, large parts, and moisture-sensitive nylon. Plastics expand at roughly 70–200 µm/m·K depending on grade — POM around 100 µm/m·K versus aluminum's 23 µm/m·K — so a 100 mm part changes about 0.1 mm over a 10°C shop swing. Thermal expansion alone often swamps the machining tolerance, which is why tight fits in plastic are usually designed with an interference or clearance range rather than a single number.
| Feature situation | Realistic tolerance guidance |
|---|---|
| POM small feature, stable temperature | ±0.05 mm possible |
| Large plastic part (200 mm+) | ±0.1–0.2 mm typical, thermal-dominated |
| Nylon part across humidity change | Allow 0.2–1% dimensional swing |
| PTFE thin section | Loose — material deforms under cut |
| Metal insert pressed into plastic | Design for plastic creep and expansion |
When a plastic part must mate with metal at a precise fit, the professional move is to hold the plastic side looser and let the metal side carry the tolerance, or to machine the plastic at the service temperature and humidity where possible. Our CNC milling and turning lines run plastics as routine work, and the drawing notes that matter are the same ones a metal part needs — plus the service temperature and environment, which the machinist uses to judge how tight to push.
When Plastic Beats Metal — and When It Does Not
Plastic wins on weight (roughly one-fifth to one-sixth of steel), corrosion resistance, electrical insulation, friction and wear without lubrication, and part cost. A machined POM bushing replaces a bronze one at a fraction of the price and runs dry. PTFE seals and liners handle chemicals and temperatures that destroy most metals. Nylon gears run quieter than steel and absorb shock. For prototypes and low-volume production, machining plastics from stock avoids injection mold tooling entirely — a point we make in our 5-axis vs 3-axis guide about matching process to part.
Plastic loses when the load is high, the temperature is sustained past the material's ceiling, or the stiffness requirement is absolute. Plastics creep under constant load — a plastic part under permanent stress continues to deform slowly over time, which is why springs, structural frames, and torque paths stay in metal. Strength per dollar also favors metal at scale. When the application genuinely needs metal's fatigue life, as covered in our titanium machining guide, no plastic substitute exists.
Drawing Notes That Get You Good Plastic Parts
State the material grade completely — POM-C, PTFE virgin grade, PA66 GF30 — because "plastic" is not a spec. Note the service temperature and environment: a machinist who knows the part runs at 80°C in a humid cabinet will hold different tolerances than one who assumes room temperature. Flag any mating fits with metal so the designer can allocate tolerance to the right side. And if the part needs FDA or other compliance, name it; grade selection depends on it.
Plastics also change the inspection conversation. A CMM reading at 20°C tells you the part's size in the inspection room, not necessarily at the customer's service condition. Good suppliers measure plastic parts at a documented temperature and note the condition on the report. That is the level of detail worth asking for, and it is the standard we apply across precision components work whether the material is 7075 aluminum, Ti-6Al-4V, or POM.
Email sc@bquq.com or WhatsApp +86 137 1315 7787 with your PDF/DXF/STEP file. An engineer reviews it and replies with price, lead time and DFM notes on working days.
Frequently Asked Questions
Q: What is the best plastic for CNC machining?
A: POM (acetal/Delrin) is the default: it machines cleanly, holds tolerance better than most plastics, has low friction and good stiffness, and costs little. Choose PTFE for chemicals and 260°C service, nylon for toughness and wear, PEEK where temperature and strength both matter.
Q: Can CNC machined plastic hold tight tolerances like metal?
A: Only with caveats. ±0.05 mm is achievable on small, well-supported POM features at stable temperature, but plastic's thermal expansion — five to ten times metal's — and moisture uptake in nylon mean large parts and real service conditions usually need ±0.1–0.2 mm or a designed clearance range.
Q: Is it cheaper to machine plastic parts than to injection mold them?
A: Below roughly 500–1,000 pieces, yes — CNC machining from stock has no mold tooling cost and short lead time. Above that volume, injection molding's per-part cost wins once the tool is amortized. The crossover depends on part size and complexity.
Q: Why are my machined plastic edges fuzzy or white?
A: Dull tooling or excessive heat. Plastics melt rather than cut when the tool pushes instead of shears. Sharp positive-rake tools, adequate clearance, and steady feeds without dwell produce clean edges; whitened surfaces usually mean the tool dwelled or the speed was too low.
Q: Does nylon really change size after machining?
A: Yes. Nylon absorbs atmospheric moisture and grows by roughly 0.2–1% between dry-as-molded and saturated states. Parts machined in a dry warehouse can swell enough to ruin a tight fit later. Spec nylon parts for their service humidity, or choose POM where dimensional stability matters more than toughness.
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
