Acetal Copolymer vs PET: Hot-Water Acetal vs Stiff Polyester for Machined Parts

Acetal copolymer (Celcon, Hostaform) and engineering PET (Ertalyte) are both semi-crystalline precision machining plastics that appear frequently in the same application space: pump components, valve seats, food-contact machinery parts, and medical device components. Choosing between them requires understanding their key differences. Acetal copolymer's primary advantage over PET is hydrolytic stability — it handles hot-water, steam, and alkaline environments better than any other commodity acetal. PET's advantages are a significantly higher flexural modulus (stiffer under load), lower moisture absorption, and broader chemical resistance to solvents and acids. Neither is universally superior; the right choice depends on the specific thermal, chemical, and mechanical conditions of the application.

TL;DR

• Hot water: Acetal copolymer is the better choice — specifically formulated grades survive hot-water service where PET may begin to hydrolyze.
• Stiffness: PET (Ertalyte) flexural modulus (~580,000 psi) is markedly higher than copolymer (~375,000 psi) — PET resists deflection better in thin sections.
• Moisture absorption: PET absorbs less (0.06% vs 0.15–0.25% for copolymer) — PET is more dimensionally stable in wet environments.
• Chemical resistance: PET broader resistance to solvents and acids; copolymer better in alkaline environments.
• Weldability: Copolymer welds well; PET is challenging to weld.
• Large cross-sections: Copolymer avoids the centerline porosity that affects large-diameter Delrin (homopolymer) rod.
• Cost: Similar pricing in standard sizes.

Chemistry & Origin

Acetal copolymer incorporates a small percentage of comonomers (trioxane or ethylene oxide derivatives) into the polyoxymethylene backbone, disrupting perfect crystalline ordering and eliminating the end-group instability that causes Delrin homopolymer to degrade in alkaline and hot-water environments. The result is a more stable, weldable acetal with good hydrolytic resistance — and no centerline porosity in large rod sections.

Engineering PET (Ertalyte) is processed from polyethylene terephthalate with controlled crystallization to develop a consistent semi-crystalline structure through the full cross-section. The polyester backbone's ester linkages provide broader chemical resistance than POM's ether chemistry, while the crystallinity confers stiffness, creep resistance, and wear properties suitable for precision machined parts.

Mechanical Properties

PET (Ertalyte) has both higher tensile strength and significantly higher flexural modulus than acetal copolymer. The stiffness difference — 580,000 psi vs 375,000 psi — is the most consequential mechanical distinction in engineering design. For thin-section parts, long cantilever arms, or components where deflection under load is the failure mode, PET's stiffer cross-section allows for a smaller section size or a larger design margin.

Acetal copolymer compensates with higher impact resistance and better fatigue performance than PET. For parts subject to impact, vibration, or cyclic loading, copolymer's toughness and fatigue behavior are relevant advantages.

Thermal Properties

PET has a slightly higher continuous service rating (~110°C vs ~100°C for copolymer). More importantly, PET handles sustained elevated-temperature aqueous service better than acetal copolymer — copolymer is better than Delrin homopolymer in hot water, but PET's polyester chemistry is intrinsically more resistant to hydrolysis from boiling water. For applications above 90°C in aqueous environments, consult material data sheets for the specific grades before specifying either material.

Chemical Resistance

PET resists a broad range of organic solvents, dilute and moderate acids, alcohols, and aqueous solutions. Acetal copolymer resists oils, fuels, and weak alkalis better than Delrin, but is vulnerable to concentrated mineral acids and chlorinated solvents — similar limitations to Delrin with marginally better alkali tolerance. In broad-exposure chemical environments, PET's chemical resistance profile is more permissive.

Dimensional Stability and Moisture

PET's water absorption of 0.06% in 24-hour immersion is roughly 3× lower than acetal copolymer's 0.15–0.25%. For precision machined parts that must hold ±0.001″ in wet service or variable humidity, PET's dimensional stability advantage is meaningful. This is a common reason engineers upgrade from acetal to Ertalyte in wet-environment precision applications.

Cost & Availability

Both materials are stocked in rod and sheet form. Acetal copolymer (under various brand names including Celcon, Hostaform, and generic acetal) is widely available at competitive prices. PET precision stock (Ertalyte) is a specialty item with a moderate price premium in smaller sizes but competitive pricing in the large-diameter rod range where copolymer competes directly. FDA-compliant grades are available for both.

When to Choose Acetal Copolymer vs PET

Choose Acetal Copolymer when:

• Hot-water contact, steam exposure, or alkaline cleaning agent resistance is the requirement.
• Welding or joining the component to another acetal part is part of the assembly process.
• Large-diameter rod (>3″) without centerline porosity is needed.
• Higher impact toughness or fatigue performance than PET offers is required.

Choose PET (Ertalyte) when:

• Maximum stiffness (flexural modulus) in the machined part is the design requirement.
• Minimum moisture absorption for dimensional stability in wet service is needed.
• Broader chemical resistance to solvents and acids is required.
• Long-term creep resistance under static load is important.

More related guides

Applications

• Bushings and Bearings
• Food Processing Components
• Pump Components

Industries

• Medical
• Food & Beverage

Other comparisons

• Delrin vs Acetal Copolymer
• Delrin vs PET
• PET vs PETG

Spec sheets

• Acetal Copolymer hub
• PET (Ertalyte) hub