Delrin vs Nylon: CNC Machining Properties, Cost & Selection Guide

Delrin (POM-H acetal homopolymer) and nylon (PA6/PA66 polyamide) dominate the engineering plastics category for CNC-machined components — gears, bushings, sliding mechanisms, and structural brackets. Both materials machine well, resist wear, and cost less than metals. Yet choosing the wrong one can mean dimensional drift in humid environments, premature gear failure, or machining costs that exceed the material savings.

This guide compares Delrin and nylon across mechanical properties in both dry and conditioned states, moisture-driven dimensional change, CNC cutting parameters, total cost of ownership, and chemical resistance. It concludes with a five-question framework that maps common application scenarios to the correct material.

Quick Property Comparison Table

The table below summarizes the most frequently referenced properties. All values represent unfilled, extruded grades tested per ASTM standards at 23°C. Where nylon properties differ between dry-as-molded (DAM) and conditioned (50% RH) states, both values appear — a distinction most comparison guides omit.

The numbers reveal a pattern that single-state comparison tables miss: nylon’s dry-state properties often exceed Delrin’s, but those advantages erode once the material absorbs ambient moisture. In a climate-controlled electronics enclosure, nylon performs well. In an outdoor hydraulic system exposed to 60–80% RH, Delrin’s property consistency becomes the decisive advantage.

Moisture-Driven Property Decay: The 50% RH Reality

Moisture absorption is the single most important differentiator between these two materials, yet most comparison guides treat it as a line item rather than the system-level concern it represents. Nylon is hygroscopic by molecular design — the amide groups (-CONH-) in its polymer backbone form hydrogen bonds with water. Delrin’s polyoxymethylene backbone contains no such sites, making it essentially non-hygroscopic.

Moisture Absorption Rates by Environment

What Moisture Does to Nylon’s Mechanical Properties

Absorbed water acts as a plasticizer in nylon. It pushes polymer chains apart, increases molecular mobility, and reduces glass transition temperature. The practical effect on CNC-machined parts is significant:

This dimensional instability is why KUSLA Prototype routinely recommends Delrin over nylon for any precision component that operates in uncontrolled humidity — CNC machining tight-tolerance features into a material that will drift defeats the purpose of precision manufacturing.

Mechanical Properties Deep Dive

Strength and Stiffness

In dry-as-molded condition, nylon 66 holds an edge in tensile strength (85 MPa vs 76 MPa) and tensile modulus (3.2–3.5 GPa vs 3.1 GPa). This advantage disappears at 50% RH, where nylon’s modulus drops to 2.0 GPa — 35% below Delrin’s stable 3.1 GPa. For parts that must maintain stiffness in service rather than on the datasheet, Delrin is the safer specification.

Impact Resistance and Toughness

Nylon’s moisture absorption, while detrimental to stiffness, dramatically improves impact resistance. Conditioned PA66 absorbs 150–200 J/m in notched Izod testing — three to four times its dry-state value and roughly double Delrin’s 60–80 J/m. Applications subject to repeated shock loading or accidental impact (protective housings, snap-fit brackets, cable guides) favor nylon for this reason.

Wear and Friction

Delrin exhibits a lower and more consistent coefficient of friction: 0.20 static / 0.15 dynamic against steel, compared to nylon’s 0.35 / 0.25 in dry conditions. Nylon’s friction drops when conditioned (absorbed water acts as a lubricant), approaching 0.15–0.20, but this improvement depends on ambient humidity — an unreliable parameter for engineering design. For sliding contact surfaces, bearings, and gear teeth where friction consistency matters, Delrin is the standard choice among materials engineers (per Machinery’s Handbook recommendations for low-friction polymer applications).

Fatigue and Creep Resistance

Delrin’s crystalline structure (approximately 75% crystallinity vs nylon’s 35–45%) provides superior creep resistance under sustained loads. A Delrin spring clip retains 90%+ of its initial deflection force after 1,000 hours at 23°C and 7 MPa stress. Nylon under identical conditions loses 15–25% of initial force due to higher creep compliance — exacerbated by any moisture uptake during service.

CNC Cutting Parameters & Surface Finish Targets

Both materials machine cleanly relative to metals, but their chip formation behavior differs enough to require distinct parameter sets. Delrin produces short, brittle chips that clear easily; nylon generates longer, stringier chips that can wrap around tooling if speeds and feeds are not optimized.

Cycle time differences compound across production runs. Delrin’s higher allowable cutting speed and deeper cuts typically result in 15–30% shorter cycle times per part compared to nylon — a factor that directly affects machining cost per unit.

Chemical Resistance Head-to-Head

Chemical environment compatibility is frequently overlooked in Delrin-vs-nylon comparisons, despite being a deciding factor in automotive, food processing, and industrial equipment applications. Neither material is universally resistant, and their vulnerabilities are complementary — meaning the chemical environment alone can dictate the choice.

A practical rule: Delrin fails in strong oxidizing acids and strong bases. Nylon fails in any acidic environment and in zinc chloride solutions (common in electroplating and flux residues). When the operating environment includes both acid and base exposure at different stages, consider PEEK or UHMW-PE as alternatives — KUSLA Prototype provides material selection support for these edge cases.

Grade Selection Matrix: Which Delrin and Which Nylon

Specifying “Delrin” or “nylon” on a drawing is insufficient — each family contains grades with meaningfully different properties. The table below maps common CNC machining applications to specific grades, helping engineers avoid the generic specification that leads to supplier substitutions.

When drawings specify “acetal” without further qualification, most CNC suppliers will default to acetal copolymer (POM-C), which has slightly lower mechanical properties than Delrin (POM-H). For critical applications, specify the exact trade name and grade number, or reference the equivalent ISO material designation (POM-H per ISO 9988 or PA66 per ISO 1874).

Total Cost of Ownership: Material Price vs Part Cost

Raw material price comparisons consistently show nylon as the cheaper option — PA6 rod stock runs $4.50–$6.50/kg versus $7.50–$11.00/kg for Delrin homopolymer. This 40–70% material cost advantage disappears and frequently reverses when total part cost is calculated.

Cost Breakdown Model: 50 mm Precision Gear, 500-Unit Run

The cost inversion happens because Delrin’s superior machinability compounds across multiple cost drivers: faster cycle times (15–30% shorter), lower scrap rates from dimensional consistency, and fewer secondary operations due to better as-machined surface finish. For precision parts with tolerances tighter than ±0.05 mm, Delrin is almost always the lower-cost option at the part level despite the material premium.

Failure Mode Comparison: How Each Material Fails

Understanding how a material fails is as important as understanding its steady-state properties. Delrin and nylon fail through different mechanisms, and matching the dominant failure mode to the application prevents in-service surprises.

In safety-critical applications where sudden brittle fracture is unacceptable — think protective guards, structural mounts, or any part where failure must be visible before catastrophic — conditioned nylon’s ductile failure mode is the safer engineering choice. Conversely, when dimensional precision must be maintained over the part’s service life, Delrin’s stability outweighs its brittleness concern.

5-Question Decision Framework

When multiple questions point to different materials, prioritize the operating environment (Question 1) and tolerance requirement (Question 3) — these two factors cause the most in-service failures. Impact and chemical resistance can often be addressed through design changes (ribbing, wall thickness) rather than material substitution.

For projects where neither Delrin nor nylon is a clear fit — high-temperature, chemically aggressive, or structurally demanding applications — consider metal alternatives or advanced polymers like PEEK. KUSLA Prototype supports material selection consultations through the engineering team.

Key Takeaways

Delrin (POM-H) is the default choice for CNC-machined precision components operating in real-world humidity conditions. Its dimensional stability, low and consistent friction, excellent fatigue resistance, and superior machinability result in lower total part cost despite a higher raw material price. Nylon (PA66) earns its place in applications requiring impact toughness, alkaline chemical resistance, or lightweight large-format parts where tight tolerances are not critical.

The most common specification error is selecting nylon for a tight-tolerance application based on its dry-state datasheet properties, then discovering dimensional drift weeks after installation. Specifying against conditioned-state properties — not dry-as-molded properties — prevents this failure mode.

Both materials machine well in the hands of an experienced CNC shop. The material choice determines the process parameters, but not the achievable quality — provided the shop understands the material’s behavior. KUSLA Prototype machines both Delrin and nylon across thousands of part geometries annually, with in-process dimensional monitoring that accounts for each material’s post-machining behavior.