KEY TAKEAWAYS

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Match plastic selection to your dominant failure mode first — wear, chemical attack, thermal creep, or structural load — before comparing machinability.

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Delrin (acetal) is the default workhorse for tight-tolerance mechanical parts; only upgrade to PEEK or Torlon when temperature or chemical exposure demands it.

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Stress-relieve stock material before and after roughing passes on large or precision parts — internal stresses in plastics cause significant dimensional drift.

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Always specify tolerances realistically: most engineering plastics hold ±0.005 inch routinely; tighter than ±0.002 inch requires controlled thermal environments and careful material selection.

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Get a DFM review before finalizing wall thicknesses and deep-pocket features — plastic deflects differently than metal and toolpath strategy matters enormously.

Engineering plastics are not second-tier materials. In the right application, they outperform metals on weight, corrosion resistance, electrical isolation, and cost per part — sometimes by an order of magnitude. The problem is that ‘plastic’ gets treated as a single category when in reality the performance gap between commodity nylon and PEEK rivals the gap between aluminum and titanium.
Specifying an engineering plastic for a CNC-machined component requires the same rigor as specifying a metal alloy. You need to evaluate tensile strength, compressive yield, heat deflection temperature (HDT), chemical compatibility, moisture absorption, and machinability — all before you think about cost. Skipping that process is how programs end up with parts that creep under sustained load, swell in humid environments, or crack during assembly.
This guide covers the seven most common engineering plastics encountered in CNC machining: Acetal (Delrin), UHMW-PE, Nylon (PA6/PA66), PTFE, Polycarbonate, PEEK, and Ultem (PEI). For each material, we address mechanical properties, thermal limits, machinability, and the applications where it genuinely earns its spot on the drawing.

Acetal homopolymer (Delrin) and copolymer (POM-C) are the reference standard for tight-tolerance CNC plastic parts. Tensile strength runs 9,000–10,000 psi, compressive strength exceeds 18,000 psi, and the material machines cleanly with sharp carbide tooling. Dimensional stability is excellent — acetal absorbs less than 0.25% moisture, which means parts don’t drift between the machine and the customer’s humid assembly floor.
Heat deflection temperature sits around 257°F (125°C) under 264 psi load, which covers most mechanical service environments outside of under-hood automotive and industrial ovens. Acetal is chemically resistant to fuels, oils, and most solvents, but it degrades in strong acids and has no meaningful UV resistance without additives.
For gears, bushings, valve seats, cam followers, and food-contact components, acetal is the right call the majority of the time. It holds ±0.001 inch tolerances in controlled conditions, produces a low-friction surface without secondary finishing, and is available in rod, plate, and tube stock in virtually every size. If you are reaching for a different plastic, you should have a specific reason that acetal cannot satisfy.

⚠️ MATERIAL NOTE: Delrin (homopolymer) machines slightly better than POM-C copolymer but is not weldable and can centerline-porosity on large-diameter rod stock. For parts over 3 inches in diameter, specify POM-C or call out centerline exclusion zones on your drawing.

Nylon and UHMW-PE are the go-to materials when wear resistance and impact toughness drive the design. PA66 offers tensile strength in the 12,000 psi range and a higher HDT (roughly 194°F at 264 psi) compared to PA6, making it the preferred grade for structural components. Both grades machine well, though they produce long, stringy chips that require positive chip evacuation strategies and sharp tooling to prevent smearing.
The critical limitation of nylon is moisture absorption. PA6 can absorb up to 9% moisture by weight at saturation, which causes measurable dimensional growth — up to 0.015 inch per inch in some conditions. For tight-tolerance parts, this is a real engineering constraint, not a footnote. Either design in clearance, specify dry-as-molded (DAM) conditions with sealed packaging, or switch to acetal.
UHMW-PE trades some structural rigidity for exceptional impact resistance and a very low coefficient of friction. With a molecular weight of 3–6 million g/mol, it resists abrasion better than most engineering plastics. It machines easily but is notoriously difficult to hold tight tolerances on — the material is soft and tends to spring back. Target ±0.005 inch minimum on UHMW components. It is ideal for wear strips, guide rails, food processing equipment liners, and chain guides.

DESIGN TIP: Always note moisture conditioning requirements on nylon part drawings if the application involves press-fit or slip-fit interfaces. A PA66 bushing machined dry and installed dry will swell into interference in humid environments. Specify ‘condition to 50% RH before final inspection’ when dimensional stability at service humidity matters.

PTFE (Teflon) occupies a unique position: nothing else matches its chemical inertness and low friction coefficient, and nothing else is quite as difficult to machine precisely. PTFE has a tensile strength of only 2,000–4,000 psi and an extreme tendency to cold-flow under sustained compressive load — a loaded PTFE seal will creep and lose clamp force over time. Tolerances are difficult to hold; ±0.005 inch is achievable but requires sharp tooling, light passes, and careful fixturing. PTFE also has no meaningful structural stiffness.
Use PTFE when chemical compatibility is non-negotiable — it resists virtually every industrial chemical except alkali metals and fluorinating agents. It is the right material for valve seals, laboratory fittings, electrical insulators, and slide bearings where load is low. Never use PTFE in structural or load-bearing applications.
Polycarbonate (PC) brings optical clarity and impact resistance (roughly 16–18 ft-lb/in notched Izod) that no other common engineering plastic can match. It machines well, but it is highly sensitive to residual stress — aggressive cuts, dull tooling, or inadequate coolant cause stress cracking, which can appear hours or days after machining. PC also has moderate chemical resistance and is attacked by many common solvents including acetone, toluene, and some cutting fluids. Specify water-soluble coolant and confirm compatibility before running PC parts.

PEEK and Ultem (PEI) are the top tier of machinable engineering thermoplastics — and the price reflects it. PEEK stock in 2-inch rod runs $150–$400 per foot depending on grade. You earn that cost with a continuous service temperature of 480°F (250°C), tensile strength above 14,000 psi, inherent flame resistance, and chemical resistance that covers most aggressive aerospace and medical environments. PEEK is biocompatible in implant-grade form, radiation-resistant, and holds tolerances to ±0.001 inch reliably in a temperature-controlled shop environment.
Machining PEEK is straightforward compared to its price point — it cuts cleanly, produces manageable chips, and does not require coolant for most operations (though light air blast or misting helps). The main risk is thermal — PEEK’s high melting point means heat builds fast in deep pockets and small-diameter bores. Slow feeds with sharp tooling and proper chip evacuation prevent thermal damage.
Ultem (PEI) is a glass-transition material rated to approximately 410°F (210°C) and offers excellent flame/smoke/toxicity (FST) performance, making it a fixture in aerospace interior and semiconductor applications. It machines well but is more notch-sensitive than PEEK and benefits from conservative approach angles. Both materials are fully compatible with AS9100-level documentation requirements — material certifications, lot traceability, and first-article inspection (FAI) are standard expectations when sourcing through a qualified network like Nimble’s certified partner network.

⚠️ COST CONTROL: Don’t over-specify. If your part sees 200°F maximum service temperature, acetal or glass-filled nylon handles it at 5–10% of PEEK’s material cost. Reserve PEEK and Ultem for genuine high-temperature, high-load, or regulatory-driven requirements. A DFM review will catch over-specification before it hits your budget.

Plastic machinability is not just about cutting speed — it is about managing heat, chip evacuation, and workholding. Unlike metals, most engineering plastics have low thermal conductivity, meaning heat generated at the cutting edge stays near the part surface rather than dissipating into the chip. This causes melting, smearing, gummy surfaces, and dimensional error if feeds and speeds are not set correctly. The general rule: high surface footage, light depth of cut, sharp tooling, positive rake angles, and aggressive chip evacuation.
Here is a practical machinability ranking for the materials covered in this guide, from easiest to most demanding:

• Acetal (Delrin/POM-C): Excellent. Clean chips, stable dimensions, wide process window.
• UHMW-PE: Easy to cut, hard to hold tolerance. Soft and springy — use sharp tooling and light finishing passes.
• PTFE: Cuts easily but deflects and cold-flows. Requires rigid fixturing and fine finishing passes.
• Nylon (PA6/PA66): Good machinability, stringy chips. Moisture control is the main process variable.
• Polycarbonate: Good machinability with correct coolant. Stress cracking risk if parameters are off.
• PEEK: Machines cleanly, heat buildup in deep features. Sharp tooling mandatory.
• Ultem (PEI): Brittle in thin sections, notch-sensitive. Conservative parameters recommended.

Single-point diamond tooling is sometimes used for optical-grade PC and high-finish PEEK components, but carbide handles production requirements for all of these materials in the vast majority of applications.

Engineering plastics do not behave like metals during and after machining, and inspection must account for this. Thermal expansion coefficients for plastics are 5–10 times higher than for aluminum — a 12-inch PEEK part will expand roughly 0.006 inch for every 10°F change in temperature. This means shop temperature variation directly affects whether a part passes inspection. Most precision plastic work requires inspection in a temperature-controlled environment (68–72°F / 20–22°C) consistent with where the part was machined.
Achievable tolerances vary significantly by material. As a practical reference: acetal and PEEK can hold ±0.001–0.002 inch on critical features with good process control. Nylon and polycarbonate are reliable to ±0.003–0.005 inch in controlled conditions. UHMW-PE and PTFE are difficult below ±0.005 inch due to material compliance and creep. These are not theoretical limits — they are production-realistic targets that an experienced machinist can hit consistently.
Surface finish on CNC-machined plastics typically runs 32–125 Ra microinch as-machined. Single-pass finishing with a sharp carbide tool can reach 16–32 Ra on acetal and PEEK without secondary operations. For tight-bore or bearing-surface applications, specify the required Ra value explicitly on the drawing — do not assume ‘smooth’ communicates anything actionable to a machinist. Nimble’s certified partner network includes CMM inspection as standard on qualifying programs, which means dimensional reports and material certs travel with every order.

INSPECTION RULE: Always specify inspection temperature on drawings for plastic parts with tolerances tighter than ±0.003 inch. Write: ‘Inspect at 68°F ±2°F per ASME Y14.5’ — this single line prevents ambiguous accept/reject decisions and protects both the buyer and the shop.

A structured decision process beats gut feel every time. When selecting an engineering plastic for a CNC-machined part, work through these five filters in order, and stop when you have a clear winner:

• 1. Thermal environment: Identify maximum continuous service temperature. Below 200°F — acetal, nylon, or PC are candidates. 200–300°F — glass-filled nylon or acetal may work, verify HDT with a safety margin. Above 300°F — PEEK or Ultem only.
• 2. Chemical exposure: Map every fluid the part contacts against material chemical resistance charts. PTFE if universal inertness is required. PEEK for most aggressive environments. Eliminate nylon early if strong acids or oxidizers are present.
• 3. Mechanical load type: Static compressive load — avoid PTFE. Dynamic wear — UHMW-PE or acetal. High tensile/fatigue — PEEK or Ultem. Impact — PC or tough-grade nylon.
• 4. Regulatory or certification requirements: Medical, aerospace, or food contact applications drive material grade selection (implant-grade PEEK, FDA-compliant acetal, etc.). Confirm material certifications are available from your supply chain.
• 5. Cost reality check: If two materials satisfy filters 1–4, choose the cheaper one. This is engineering, not materials science for its own sake.

Submitting a part to Nimble with a request for a free DFM review lets an applications engineer flag material substitution opportunities before the quote is finalized — a step that has saved customers significant cost on more than a few programs.

QUICK REFERENCE — HDT at 264 psi load (approximate): Acetal: 257°F | Nylon PA66: 194°F | Polycarbonate: 270°F | PTFE: 250°F (not structural) | PEEK: 320°F | Ultem PEI: 410°F. HDT is a screening tool, not a continuous service limit — apply a 20–30% safety margin for sustained loading.

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