PC vs POM: Which High-Performance Plastic Wins?

Which Material Should You Choose?Choose PC (Polycarbonate) if:Impact-critical componentstransparent housingselectrical insulatorsChoose POM Acetal (Delrin) if:Precision gears and bearingsvalve componentsfuel system partsPrice ComparisonPC...

Polycarbonate (PC) and POM Acetal (Delrin) are two of the most widely specified engineering thermoplastics, yet they serve fundamentally different design roles. PC is the king of impact resistance — its notched Izod impact strength of 800 J/m is among the highest of any thermoplastic, and it is one of the few transparent engineering plastics. POM is the king of precision — its low friction coefficient (0.1-0.3), excellent fatigue resistance, and outstanding dimensional stability make it the default choice for gears, bearings, and precision-molded mechanical components. Understanding the amorphous vs semicrystalline distinction between these materials is the key to selecting the right one.

Polymer Architecture: Why It Matters

PC (Amorphous): Polycarbonate has no crystalline regions — its molecular chains are randomly entangled in a glassy state. This amorphous structure gives PC its signature transparency (85-90% light transmission), isotropic shrinkage (0.5-0.7% in all directions), and exceptional impact toughness. However, it also means PC is vulnerable to chemical attack, particularly by solvents (ketones, esters, aromatic hydrocarbons) and strong bases, which can cause environmental stress cracking.

POM (Semicrystalline): POM's molecules pack into ordered crystalline regions (75-85% crystallinity), providing excellent chemical resistance to hydrocarbons, solvents, and most automotive fluids. The crystalline structure also gives POM superior fatigue resistance, low friction, and outstanding creep resistance compared to PC. POM's shrinkage is anisotropic (1.8-2.2% in the flow direction vs 1.2-1.8% transverse), requiring careful mold design. POM is opaque and cannot be made transparent.

Property Comparison

Impact Toughness: PC: 800 J/m (notched Izod) / POM: 70 J/m (notched Izod). PC is approximately 11× tougher than POM. This is the single most dramatic property difference — PC is virtually unbreakable at room temperature, while POM is relatively brittle. For any application that may experience impact loading (drops, collisions, snap-fit assembly), PC is the clear choice. Tensile Strength: PC: 65 MPa / POM: 70 MPa. Very similar, with POM having a slight edge. However, PC's much higher elongation at break (110% vs 15-25% for POM) means PC absorbs far more energy before failure. Flexural Modulus: PC: 2.4 GPa / POM: 2.8 GPa. POM is 17% stiffer, making it better for precision components where deflection must be minimized. Continuous Service Temperature: PC: 115°C / POM: 90°C. PC operates 25°C higher, though neither material is suitable for high-temperature applications above 120°C. HDT at 1.82 MPa: PC: 130°C / POM: 110°C. PC maintains structural rigidity at higher temperatures under load. Friction and Wear: POM: CoF 0.1-0.3 (excellent) / PC: CoF 0.35-0.40 (moderate). POM is one of the best bearing materials among unfilled thermoplastics — its low friction and excellent wear resistance make it the standard for unlubricated gears and bushings. PC is not recommended for tribological applications. Fatigue Resistance: POM has outstanding fatigue endurance — it can withstand millions of cyclic loading cycles without failure. This makes POM the preferred material for snap-fit closures, spring-loaded clips, and gear teeth. PC's fatigue resistance is moderate. Chemical Resistance: POM is resistant to hydrocarbons, alcohols, and most solvents — making it suitable for fuel system components, valve bodies, and pump parts. PC is attacked by ketones (acetone), esters, aromatic hydrocarbons, and strong bases. PC also undergoes hydrolysis in hot water above 60°C, limiting its use in plumbing and water-handling applications. Optical Properties: PC: 85-90% light transmission, transparent / POM: Opaque. When transparency or light transmission is required — instrument windows, sight glasses, LED lenses, medical device housings — PC is the only choice. Electrical Properties: PC has a higher dielectric strength (15 kV/mm vs POM at 10 kV/mm) and better tracking resistance, making it preferred for electrical insulators, connector housings, and switchgear components. POM's electrical properties are adequate for most non-critical applications. Processability: PC processes at 280-320°C melt / 80-100°C mold. POM processes at 185-210°C melt / 80-100°C mold. POM's lower processing temperature saves energy and enables faster cycle times. Both are easy to mold on standard machines. POM requires attention to thermal stability — it decomposes above 230°C, releasing formaldehyde, so temperature control is critical. Cost: PC: $5-12/kg / POM: $5-10/kg. Very similar cost range. The choice between them is driven by application requirements, not cost.

Decision Matrix

Choose PC When: (1) Impact toughness is the primary requirement — safety helmets, machine guards, riot shields, aircraft interior panels; (2) Transparency or light transmission is needed — instrument windows, LED lenses, medical device housings, sight gauges; (3) The component is an electrical insulator requiring high dielectric strength — connector housings, switchgear, relay bases; (4) The application involves repeated snap-fit assembly/disassembly where PC's ductility prevents crack propagation; (5) Housings for electronic devices requiring V-0 flame retardancy (PC achieves V-0 at 1.5mm with additives, or use PC/ABS blends). Choose POM When: (1) Low friction and wear resistance are critical — precision gears, bearings, bushings, conveyor components; (2) The component experiences cyclic loading — snap-fit closures, spring clips, gear teeth, latch mechanisms; (3) Chemical resistance to fuels, solvents, or hydrocarbons is required — fuel system components, valve bodies, pump impellers; (4) Dimensional stability in humid environments is important — POM's moisture absorption (0.22%) is much lower than PC's (0.15-0.20%); (5) The application requires the lowest possible creep under sustained load — POM's creep resistance is significantly better than PC's; (6) Food contact compliance is needed — POM (acetal copolymer) is FDA-compliant for repeated food contact, widely used in food processing conveyor components and beverage dispensing systems.

Comparison at a Glance

Equivalents & Cross-References

Related Diagnostics & Materials

References & Industry Standards

• ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
• UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
• MatWeb. Material Property Data for Engineering Thermoplastics. matweb.com
• ISO 1043. Plastics — Symbols and Abbreviated Terms. iso.org