Delrin vs PET: Which Precision Thermoplastic Fits Your Machined Part?

Delrin (DuPont's acetal homopolymer, POM) and engineering PET (Ertalyte) are both premium semi-crystalline thermoplastics machined to precision tolerances for gears, bearings, valve components, and structural parts. They overlap broadly in application space — both are stiff, dimensionally stable, machinable, and FDA-compliant in standard grades. The choice between them hinges on specific performance requirements: Delrin has higher fatigue resistance and lower friction (better for dynamic mechanisms), while PET has lower water absorption, greater chemical resistance to aqueous environments, and better creep resistance under sustained load. Understanding where these differences become decisive — rather than treating them as fully interchangeable — prevents costly specification errors.

TL;DR

• Both are semi-crystalline precision machining plastics: Delrin = acetal/POM homopolymer; PET = polyethylene terephthalate (Ertalyte brand).
• Fatigue / cyclic duty: Delrin has the higher fatigue endurance limit — preferred for gears, cams, and cyclically loaded parts.
• Moisture: PET absorbs less moisture (0.06% vs Delrin's 0.20–0.25%) — better dimensional stability in wet environments.
• Chemical resistance: PET resists a broader range of chemicals including many solvents; Delrin is sensitive to strong acids and sustained hot-water contact.
• Friction / wear: Delrin has a lower coefficient of friction — better for sliding bearing and bushing applications.
• Temperature: Similar continuous service ratings (~110°C for Delrin, ~110°C for PET).
• Cost: Comparable pricing in rod and sheet stock; PET (Ertalyte) may carry a slight premium in specialty sizes.

Chemistry & Origin

Delrin's polyoxymethylene (POM) backbone is a simple alternating CH₂-O chain — one of the most chemically regular polymer structures in engineering plastics. This regularity produces a highly crystalline material (65–75% crystallinity) with exceptional stiffness, fatigue resistance, and low friction. The tradeoff is that the pure POM chain is susceptible to acid-initiated degradation and end-group instability in alkaline environments.

Engineering PET (Ertalyte) is a polyester — built from terephthalic acid and ethylene glycol — processed to develop controlled semi-crystallinity throughout the cross-section. The polyester linkage is chemically more diverse than POM's single ether linkage, producing a material with broader chemical resistance and significantly lower moisture absorption. PET's higher flexural modulus (stiffer than Delrin) makes it the choice for precision parts requiring high stiffness under load.

Mechanical Properties

PET (Ertalyte) has slightly higher tensile strength (~11,000 psi vs ~10,000 psi for Delrin) and considerably higher flexural modulus (~580,000 psi vs ~400,000 psi). For parts requiring high stiffness — thin-wall structures, long unsupported beams, load-bearing brackets — PET's stiffness advantage produces less deflection under the same load.

Delrin's advantage is in dynamic and fatigue loading. Its fatigue endurance limit is higher than PET's, and its lower coefficient of friction reduces heat generation in sliding and cyclic applications. For gears, cams, and components that cycle millions of times, Delrin's fatigue performance and tribological properties make it the traditional first choice. Delrin's higher impact resistance (1.4 ft·lb/in vs PET's 0.6 ft·lb/in) also makes it more tolerant of impact loading and rough handling.

Thermal Properties

Both materials carry similar continuous service ratings — approximately 105–110°C. Delrin's susceptibility to hydrolytic degradation limits its use in sustained hot-water or steam applications even within the thermal rating. PET maintains better structural stability in aqueous service within its temperature limit. Neither material approaches the temperature ceiling of PEEK or Ultem for high-temperature applications.

Chemical Resistance

PET has significantly broader chemical resistance than Delrin:

• PET (Ertalyte) resists: dilute and concentrated mineral acids (except strong oxidizing acids), organic solvents including chlorinated solvents, alcohols, esters, aromatic hydrocarbons, and aqueous solutions over a wide temperature range.
• Delrin is attacked by: concentrated mineral acids, chlorinated solvents (e.g., methylene chloride at elevated temperature), and sustained hot-water / alkaline exposure above ~160°F.

For chemical process applications, pharmaceutical machinery, or any environment with cleaning agent or solvent exposure, PET's chemical resistance is frequently the deciding factor.

Moisture and Dimensional Stability

PET absorbs roughly 3–4× less moisture than Delrin (0.06% vs 0.20–0.25% in 24-hour immersion). In wet environments, this lower absorption means PET maintains dimensional stability, electrical properties, and mechanical properties more reliably. For precision parts that must hold tight tolerances in variable humidity or wet service, PET (Ertalyte) is the better specification.

Cost & Availability

Both materials are stocked in rod and sheet form by engineering plastics distributors. Pricing is broadly comparable; Delrin is a widely distributed commodity in smaller sizes, while PET precision stock (Ertalyte) may carry a slight premium at specialty sizes. Both are available in FDA-compliant (natural/white) grades.

When to Choose Delrin vs PET

Choose Delrin when:

• The application involves cyclic loading, gears, cams, or fatigue-critical duty.
• Low coefficient of friction for dry-running bearings or sliding surfaces is required.
• Impact resistance matters (Delrin is significantly tougher at impact).
• The environment is dry or clean — no hot water or acid/alkali exposure.

Choose PET (Ertalyte) when:

• Maximum dimensional stability in wet or humid environments is needed.
• Chemical resistance to solvents, acids, or cleaning agents is required.
• High stiffness (flexural modulus) in the machined part is a design requirement.
• Long-term creep resistance under sustained static load is important.

More related guides

Applications

• Gears
• Bushings and Bearings
• Food Processing Components

Industries

• Medical
• Food & Beverage

Other comparisons

• Delrin vs Acetal Copolymer
• PEEK vs Delrin
• Acetal Copolymer vs PET

Spec sheets

• Delrin hub
• PET (Ertalyte) hub