KEY TAKEAWAYS

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PEEK outperforms Delrin in continuous-use temperature, chemical resistance, and structural rigidity — but costs 5-10x more per pound.

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Delrin is the better default for precision mechanical parts, bushings, and gears in non-extreme environments — it machines cleanly, tolerates moisture, and is highly repeatable.

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PEEK is the correct choice for sustained temps above 150°C, aggressive chemical exposure, or load-bearing aerospace and medical implant applications.

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Both materials machine well in CNC environments, but PEEK requires sharper tooling and careful fixturing to avoid stress-induced warping on thin-wall features.

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Always validate final material selection against actual service conditions — datasheet values are measured under controlled lab conditions that rarely match real-world use.

PEEK — polyether ether ketone — is a semi-crystalline thermoplastic in the polyaryletherketone (PAEK) family. It was developed by ICI in the late 1970s and has become the benchmark high-performance engineering plastic for the aerospace, medical, and oil-and-gas industries. Its molecular backbone features alternating ether and ketone linkages that produce exceptional thermal stability and mechanical strength.

Delrin is DuPont’s trade name for acetal homopolymer (polyoxymethylene, or POM-H). The generic term is acetal or POM. It is a semi-crystalline thermoplastic known for its high stiffness, low friction, dimensional stability, and excellent machinability. Delrin and its copolymer counterpart Celcon (POM-C) are workhorses of precision mechanical components — gears, bushings, cams, valve bodies, and fasteners appear in virtually every industry.

These two materials sit at very different points on the performance-cost curve. PEEK is a premium specialty material; Delrin is an industrial commodity with exceptional engineering properties. Understanding where each belongs requires looking at their thermal, mechanical, and chemical profiles side by side.

Thermal capability is where PEEK and Delrin diverge most dramatically. PEEK has a continuous-use temperature of 250°C (482°F) and a short-term peak of around 300°C. Its glass transition temperature (Tg) sits at approximately 143°C, and its crystalline melting point is roughly 343°C. Unfilled PEEK retains meaningful structural stiffness well above temperatures that would destroy most other plastics.

Delrin’s thermal profile is far more modest. Continuous service temperature is typically rated at 82-90°C (180-195°F), and it begins to soften meaningfully at temperatures approaching its melting point of 175°C. For applications running near or above 100°C — think underhood automotive components, sterilizable medical devices, or aerospace avionics bays — Delrin is disqualified on thermal grounds alone.

This is the single most common mistake in material selection: engineers specify Delrin because it machines easily and is inexpensive, without stress-testing the thermal envelope against real operating conditions. If your part will ever see sustained heat above 90°C, PEEK, or an intermediate option like PEI (Ultem) or PVDF, deserves serious consideration.

⚠️ CRITICAL RULE: If sustained service temperature exceeds 90°C, Delrin is off the table. Do not rely on short-term exposure ratings for continuous-load applications — thermal creep will degrade dimensional accuracy over time.

Both materials are strong by plastic standards, but PEEK operates in a different performance class. Unfilled PEEK has a tensile strength of approximately 100 MPa (14,500 psi) and a flexural modulus around 3.6 GPa. When filled with 30% carbon fiber, those numbers climb to roughly 210 MPa tensile strength and 18 GPa flexural modulus — approaching the territory of aluminum alloys at a fraction of the weight.

Delrin posts respectable numbers: tensile strength in the range of 65-70 MPa and a flexural modulus of approximately 2.8-3.1 GPa. For most mechanical component applications — precision gears, bushings, sliding bearings, snap-fit housings — those values are more than adequate. Delrin also has excellent fatigue resistance and recovers well from cyclic loading, which is why it dominates gear and cam applications.

One nuance worth noting: Delrin has superior impact toughness compared to unfilled PEEK at room temperature, and it absorbs virtually no moisture (less than 0.25% water absorption vs. roughly 0.5% for PEEK). For wet or humid environments where dimensional stability at ambient temperatures is the priority, Delrin’s moisture resistance is a genuine advantage. Carbon or glass-filled PEEK grades substantially improve the impact behavior but add cost.

PEEK offers broad-spectrum chemical resistance that is nearly unmatched among thermoplastics. It is resistant to virtually all organic solvents, hydraulic fluids, fuels, acids (with some concentrated exceptions), bases, and steam. This is why PEEK dominates downhole oil-and-gas tooling, semiconductor wet-process equipment, and autoclave-sterilizable surgical instruments. It can handle steam sterilization at 134°C repeatedly without degradation — a requirement that eliminates most other plastics entirely.

Delrin’s chemical resistance is good but more limited. It performs well against most hydrocarbons, alcohols, and weak bases. However, it degrades in strong acids, oxidizing agents, and chlorinated solvents. It is also susceptible to prolonged UV exposure without stabilized grades. For food-processing or pharmaceutical equipment exposed to aggressive CIP (clean-in-place) chemicals like caustic soda or peracetic acid, Delrin requires careful vetting against the actual chemical contact list.

The practical guidance: always cross-reference your specific fluid or chemical against a resistance chart for the exact grade you are specifying. Acetal copolymer (POM-C) differs from homopolymer Delrin in some resistance profiles. Filled PEEK grades can introduce compatibility complications depending on the filler material — glass-filled grades can be attacked by hydrofluoric acid, for example.

💡 PRO TIP: Chemical resistance data from resin manufacturers is typically based on immersion at room temperature. Elevated temperature exposure dramatically accelerates chemical attack. Always derate published resistance ratings for hot fluid contact or pressurized environments.

Delrin is one of the most machinable engineering plastics in existence. It produces clean, consistent chips, holds tight tolerances well (±0.001 inch is routine), and does not require special tooling beyond sharp cutting edges and positive rake angles. It also does not require drying before machining. For prototype and production CNC work, Delrin is a machinist’s preferred material — fast cycle times, excellent surface finish, and predictable behavior.

PEEK is also machinable, but it requires more attention. It demands sharp tooling, controlled heat, and careful fixturing — residual stress in the stock material can cause thin-wall features to spring back after machining if proper annealing and stress-relieving practices are not followed. PEEK rod and plate stock should be sourced from reputable suppliers with documented processing histories; regrind or non-pedigreed stock can have inconsistent crystallinity that affects dimensional stability post-machining.

Both materials are injection moldable at scale, with PEEK requiring significantly higher barrel temperatures (360-400°C) and hardened tool steel molds due to its processing conditions. Delrin molds cleanly in standard P20 tooling at conventional melt temperatures. For low-to-medium volumes, CNC machining from stock is typically more economical for both materials. Nimble’s certified partner network regularly machines both PEEK and Delrin to aerospace and medical tolerances, with CMM inspection included on every qualified order.

Raw material cost is a significant factor in the PEEK vs. Delrin decision. Delrin rod and plate stock typically runs $3-8 per pound depending on grade, diameter, and order volume — it is a commodity material with deep supply chains and readily available inventory across dozens of distributors. For high-volume production parts, injection-molded Delrin can bring per-part costs down dramatically.

PEEK, by contrast, typically runs $40-80 per pound for unfilled stock, with carbon-fiber or glass-filled grades sometimes exceeding $100 per pound. For a 3-inch diameter by 12-inch bar of PEEK, you may be looking at $150-300 before a single cut is made. That cost premium has to be justified by genuine performance requirements. An application that only uses 10% of PEEK’s performance capability has been over-engineered and is absorbing unnecessary cost in every unit produced.

The total cost picture must also include machinability — PEEK’s slower cycle times and higher tooling wear can add 20-40% to machining costs versus an equivalent Delrin part. For programs where material cost is a significant line item, this analysis should be done explicitly, not assumed. That said, in safety-critical or regulated applications — defense, medical Class III devices, aerospace flight hardware — the cost differential is rarely the deciding factor, and specifying the correct material is non-negotiable.

💡 COST RULE OF THUMB: If your application does not require sustained temps above 150°C, resistance to aggressive solvents, or regulatory compliance mandating PEEK specifically — Delrin will perform the job at 1/10th the material cost.

The fastest way to make the right call is to run through a short set of application criteria before touching a material specification. Ask these questions in order:

• Temperature: Will the part be exposed to sustained temperatures above 90°C? If yes, PEEK (or a mid-tier option like Ultem/PEI) is required.
• Chemical exposure: Will the part contact strong acids, oxidizing agents, steam, or aggressive solvents? If yes, PEEK’s broader chemical envelope is likely needed.
• Regulatory or certification requirements: Medical implant? Aerospace flight-critical? These environments often have explicit material qualification requirements that may mandate PEEK or specific PEEK grades (e.g., PEEK-OPTIMA for implantable devices).
• Load and fatigue conditions: High-cycle fatigue in a mechanical application at ambient temperature? Delrin’s fatigue properties often outperform unfilled PEEK at room temp and cost far less.
• Precision and dimensional stability: Both materials are excellent here. Delrin’s near-zero moisture absorption gives it a slight edge in humid environments at ambient temperatures.

If the application clears all four criteria without triggering a PEEK requirement, Delrin is almost certainly the correct choice. When in doubt, a free DFM review from Nimble’s team can help validate material selection before cutting the first part — a 20-minute conversation can prevent an expensive re-spin.

Both PEEK and Delrin are available in multiple grades that significantly modify their base properties. For PEEK, the primary variants are: unfilled PEEK (maximum chemical resistance, highest ductility), glass-filled PEEK (improved stiffness, reduced thermal expansion, slight chemical resistance tradeoff), and carbon-fiber-filled PEEK (highest strength-to-weight, dramatically improved wear resistance, conductive). PEEK-OPTIMA is a registered medical-grade resin for implantable devices with full biocompatibility documentation. Each grade has distinct machining characteristics and cost points.

For Delrin, the main variants are Delrin 150 and 500 (homopolymer, general purpose), Delrin AF (PTFE-filled for reduced friction in bearing applications), Delrin 570 (glass-filled, higher stiffness), and UV-stabilized grades for outdoor applications. Acetal copolymer (Celcon, Hostaform) offers improved centerline porosity and is often preferred for thicker cross-sections. The copolymer also handles strong alkalis slightly better than homopolymer.

Procurement professionals should specify grade, not just base resin name. Calling out ‘PEEK’ or ‘Delrin’ without a grade designation leaves the supplier free to substitute — which can produce parts that pass visual and dimensional inspection but fail in service. Full material certifications with heat lot traceability are standard practice for aerospace and medical supply chains.

⚠️ SPEC WARNING: Never accept a certificate of conformance that lists only ‘PEEK’ or ‘Acetal’ without a grade designation and manufacturer lot number. Material substitution is a leading cause of field failures in precision plastic components.

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