1. Introduction
Polyoxymethylene (POM), commonly called acetal or by trade names such as Delrin®, is a semi-crystalline engineering thermoplastic prized for its combination of high stiffness, excellent wear and fatigue resistance, low friction, and outstanding dimensional stability.
POM is a first-choice polymer for precision mechanical parts (gears, bushings, sliders) where tight tolerances, low friction and long life are required.
This article gives a technical, data-driven review of POM’s chemistry, properties, processing, applications, limitations and future directions.
2. What Is POM?
Polyoxymethylene (POM) — often called acetal, polyacetal or by commercial names such as Delrin®, Hostaform®, and Ultraform® — is a semi-crystalline engineering thermoplastic characterized by a repeating –CH₂–O– (methylene-oxy) backbone.
It combines a high degree of crystallinity with an ether-type linkage, producing a material that is stiff, dimensionally stable, low-friction and highly resistant to wear and fatigue.
Those attributes make POM a first-choice polymer for precision mechanical components that require repeatable geometry and long service life.
Two commercial families
POM is manufactured and supplied in two principal chemistries that determine processing and performance:
- POM-homopolymer (POM-H) — produced by polymerizing formaldehyde. Homopolymer grades typically exhibit higher crystallinity, slightly higher stiffness and better creep resistance.
They deliver maximum mechanical performance, especially at room temperature, but are somewhat more sensitive to thermal oxidation during processing. - POM-copolymer (POM-C) — manufactured by copolymerizing trioxane or formaldehyde with a small fraction of stabilizing comonomer.
Copolymer grades are less prone to thermal degradation and processing discoloration, have a broader molding window and often give better dimensional control in demanding molding conditions.
3. Physical Properties of POM (typical values)
Values are typical supplier ranges and will vary by grade, filler content and test method. Use supplier datasheets for design-critical specifications.
| Property | Typical value |
| Density | ≈ 1.41 g·cm⁻³ |
| Melting point (Tm) | ~165–175 °C |
| Glass transition (Tg) | ≈ −60 °C (well below service temps) |
| Water absorption (equilibrium) | ~0.2–0.3 wt% (very low) |
| Thermal conductivity | ~0.25–0.35 W·m⁻¹·K⁻¹ |
| Coefficient of thermal expansion (linear) | ~110–130 ×10⁻⁶ K⁻¹ (amorphous direction dependent) |
| Specific heat | ~1.6–1.8 kJ·kg⁻¹·K⁻¹ |
4. Key Properties of POM: Mechanical, Thermal, and Chemical
Mechanical Properties (room temperature, 23 °C — typical engineering ranges)
| Property | Typical range (neat POM) | Practical note |
| Tensile strength (yield) | 50–75 MPa | Homopolymer grades at upper end; copolymer slightly lower |
| Tensile modulus (Young’s) | ≈ 2.8–3.5 GPa | Stiff compared with many engineering plastics |
| Flexural modulus | ≈ 2.6–3.2 GPa | Good bending stiffness |
| Elongation at break | 20–60 % | Ductile failure mode; varies by grade and test speed |
| Notched impact (Charpy) | ~2–8 kJ·m⁻² (grade dependent) | POM exhibits good toughness; fillers change behavior |
| Hardness (Rockwell R) | ~70–100 R | Good surface hardness for wear resistance |
| Fatigue strength | High — POM performs well in cyclic bending and rolling contact | Preferred for gears, bushings |
Thermal properties of POM
- Service temperature: continuous use typically up to ≈ 80–100 °C for long durations; short excursions up to 120–130 °C are possible depending on grade and environment.
- Melting/processing: melt range around 165–175 °C. Processing window is relatively narrow; thermal control in molding is important.
- Thermal degradation: prolonged exposure above ~200 °C can cause depolymerization and release of low levels of formaldehyde; avoid overheating during processing or sterilization.
Chemical resistance of POM
- Excellent: hydrocarbons, aliphatic solvents, fuels, oils, greases, many detergents and mild alkalies.
- Good: many organic solvents at moderate temperatures.
- Poor / avoid: strong oxidizers (nitric acid, chromic acid), concentrated acids, strong halogenated hydrocarbons (at temperature) and conditions that promote hydrolysis at high temperature.
- Note: POM is often used in fuel and hydraulic systems because of its resistance to fuels and oils.
Dimensional stability of POM
- Low moisture uptake (~0.2%) confers dimensional stability far superior to nylons (PA).
- High crystallinity gives low creep at room temperature; however, creep increases with temperature approaching service limits.
Design for creep in bearing and load-bearing applications, especially at elevated temperatures.
5. Processing and Manufacturing Methods
- Injection molding — the dominant method for precision parts.
Typical guidance: dry pellets (80°C for 2–4 hours), barrel/melt temperature ~190–230 °C depending on grade, mold temperature 60–100 °C to promote crystallization and reduce warpage. - Extrusion for rods, sheets and profiles (extruded rod commonly used for machining stock).
- Compression molding for large plates or specialty parts.
- Machining from bar/rod — POM machines very well: clean chips, little tool wear, tight tolerances possible; widely used for prototypes and low-volume parts.
- Joining: adhesive bonding possible with surface treatments; mechanical fastening and ultrasonic welding are common assembly methods.
Practical processing notes: POM is moisture-sensitive (surface defects) and thermally sensitive (depolymerization). Controlled drying and correct melt temperatures are essential.
6. Advantages and Limitations of POM
Key Advantages
- Superior Mechanical Balance: Combines high strength (60–75 MPa) and ductility (10–50% elongation), outperforming most engineering plastics
- Exceptional Dimensional Stability: Low water absorption and tight thermal expansion ensure consistent performance in humid/temperature-variant environments
- Self-Lubricating Properties: Low friction coefficient (0.15–0.20) reduces wear and eliminates the need for lubrication in many applications
- Excellent Machinability: Enables precision machining of custom parts with minimal tool wear
- Chemical Resistance: Inert to most solvents, acids, and bases—suitable for fluid-handling components
- Lightweight: Density (1.41 g/cm³) is 1/3 that of brass and 1/5 that of steel, reducing component weight
Limitations
- Low High-Temperature Resistance: Continuous use temperature (<110°C) limits applications in high-heat environments (e.g., engine exhaust systems)
- Flammability: Unmodified POM is flammable (UL 94 HB rating); flame-retardant grades (UL 94 V-0) require additives (e.g., magnesium hydroxide)
- Poor UV Resistance: Degrades under prolonged sunlight (yellowing, loss of strength)—requires UV stabilizers for outdoor use
- Brittleness at Low Temperatures: Homo-POM becomes brittle below –40°C (impact strength drops by 50%), limiting cryogenic applications
- Thermal Degradation Risk: Releases formaldehyde if overheated (>230°C), requiring strict processing controls
7. Applications of POM
POM’s property set fits many mechanical demands. Representative applications:
- Precision gears and racks (consumer appliances, printers, robotics)
- Bushings, bearings and slides — low friction, long life in dry or lubricated conditions
- Pumps and valve components — chemical and fuel resistance
- Fasteners and clips where dimensional stability and toughness matter
- Connector housings and electrical insulators
- Automotive trim and functional components (door hardware, locking systems)
- Medical devices (non-implant) — POM is used where cleaning/sterilization and dimensional control are required
Include fillers (glass, carbon, PTFE) changes applications: glass-filled POM for higher stiffness, PTFE-filled for lower friction and improved wear.
8. Performance Optimization and Design Considerations
Performance Optimization via Modification
- Reinforced POM: Addition of glass fibers (10–30 wt.%) increases stiffness (flexural modulus up to 5 GPa) and heat deflection temperature (up to 140°C)—used in automotive structural parts
- Wear-Resistant POM: Incorporation of PTFE (5–15 wt.%), graphite (2–5 wt.%), or molybdenum disulfide (MoS₂, 1–3 wt.%) reduces friction coefficient to 0.05–0.10—ideal for high-speed sliding components
- Flame-Retardant POM: Halogen-free flame retardants (e.g., magnesium hydroxide, 20–30 wt.%) meet UL 94 V-0, expanding use in electronic enclosures
- UV-Stabilized POM: Addition of hindered amine light stabilizers (HALS, 0.1–0.5 wt.%) prevents UV degradation—suitable for outdoor applications
Design Considerations
- Wall Thickness: Maintain uniform thickness (1–5 mm for injection molding) to avoid warpage; minimum thickness = 0.5 mm (thin-walled parts)
- Draft Angles: 1–2° for injection molding, 3–5° for extrusion to prevent mold sticking
- Fillets & Radii: Minimum fillet radius = 0.5–1.0 mm to reduce stress concentrations and improve flow during molding
- Avoid Sharp Corners: Sharp edges increase stress and risk of brittle failure—use rounded corners (radius ≥0.5 mm)
- Processing Optimization: For precision parts, use mold temperature control (60–80°C) and slow injection speed to minimize residual stress
9. Comparison with Other Engineering Plastics
| Property / Criterion | POM (Acetal) | Nylon (PA6 / PA66) | PTFE (Teflon) | PEEK | UHMW-PE | PBT |
| Density (g·cm⁻³) | ≈ 1.40–1.42 | ≈ 1.13–1.15 | ≈ 2.10–2.16 | ≈ 1.28–1.32 | ≈ 0.93–0.95 | ≈ 1.30–1.33 |
| Tensile strength (MPa) | ~50–75 | ~60–85 | ~20–35 | ~90–110 | ~20–40 | ~50–70 |
| Young’s modulus (GPa) | ~2.8–3.5 | ~2.5–3.5 | ~0.3–0.6 | ~3.6–4.1 | ~0.8–1.5 | ~2.6–3.2 |
| Melting / service temp (°C) | Tm ~165–175 / service ~80–100 | Tm ~215–265 / service ~80–120 | Tm ~327 / service up to ~260 (chem/tribo limits) | Tm ~343 / service ~200–250 | Tm ~130–135 / service ~80–100 | Tm ~220–225 / service ~120 |
| Water absorption (equilibrium) | ~0.2–0.3 wt% | ~1–3 wt% (depends on RH) | ≈ 0% | ~0.3–0.5 wt% | ~0.01–0.1 wt% | ~0.2–0.5 wt% |
| Coefficient of friction (dry) | ~0.15–0.25 | ~0.15–0.35 | ~0.04–0.15 (very low) | ~0.15–0.4 | ~0.08–0.20 | ~0.25–0.35 |
Wear / tribology |
Excellent (sliding parts, gears) | Good (improves when filled) | Poor (improves in filled grades) | Excellent (filled grades best) | Excellent for abrasion resistance | Good |
| Chemical resistance | Good (fuels/oils, many solvents) | Good / selective; sensitive to strong acids/alkalis | Outstanding (nearly universal) | Excellent (many aggressive media) | Very good (many media) | Good (hydrolysis in some conditions) |
| Machinability | Excellent (machines like metal) | Good (tool wear moderate) | Fair — machinable from billets; difficult to bond | Good (machinable, but tougher than POM) | Challenging (gummy—controls needed) | Good |
| Dimensional stability | Very good (low hygroscopic) | Moderate (moisture sensitive) | Excellent (virtually no moisture effect) | Excellent | Very good | Good |
Typical applications |
Gears, bushings, fasteners, sliding parts, fuel components | Gears, bearings, housings, cable ties | Seals, chemical linings, low-friction bearings, RF substrate | Valve components, high-temp bearings, medical implants | Liners, wear pads, conveyor parts | Connectors, housings, automotive electrical parts |
| Notes / decision guidance | Cost-effective, low-friction mechanical polymer for precision parts at moderate T | Versatile; choose when toughness needed but expect dimensional change with moisture | Use when absolute chemical inertness and lowest friction required; beware creep | Premium polymer for high-temperature, high-load use (higher cost) | Best for extreme abrasion and impact; low density | Good general-purpose engineering polymer with balanced properties |
10. Sustainability and Recycling
- Recyclability: POM is thermoplastic and recyclable by mechanical regrind; reground material is commonly used in non-critical components. Chemical recycling is less common but technically feasible.
- Lifecycle: long service life for mechanical components often improves lifecycle environmental performance vs disposable plastics.
- Safety considerations: thermal decomposition can release formaldehyde—waste processing and incineration must follow local environmental regulations.
- Recycled content: increasing in industrial practice, but designers should verify mechanical property retention for critical parts.
11. Future Trends & Innovations in POM
Advanced Modification Technologies
- High-Performance Fillers: Graphene-reinforced POM (0.1–0.5 wt.% graphene) improves tensile strength by 20% and thermal conductivity by 30%, targeting aerospace and electronics applications
- Biodegradable POM Blends: Blending POM with biodegradable polymers (e.g., PLA, PHA) improves compostability while retaining mechanical properties—suitable for single-use consumer goods
Processing Innovations
- 3D Printing Advancements: High-performance POM filaments with improved layer adhesion (strength = 95% of bulk POM) and faster print speeds (up to 100 mm/s) enable mass production of custom parts
- In-Mold Decoration (IMD): Integration of decorative films during injection molding enhances the aesthetic appeal of POM consumer goods (e.g., smartphone cases, furniture hardware)
Emerging Applications
- Electric Vehicles (EVs): POM is increasingly used in EV battery housings, motor parts, and charging connectors due to its lightweight, chemical resistance, and dimensional stability—demand expected to grow by 12% annually through 2030
- Aerospace: Low-weight, high-strength POM components (e.g., interior brackets, sensor housings) reduce aircraft fuel consumption—adoption accelerated by strict emissions regulations
- Medical Implants: Bioactive POM (coated with hydroxyapatite) promotes bone integration, expanding use in orthopedic implants (e.g., hip stems, spinal cages)
12. Conclusion
POM (polyoxymethylene) is a mature, versatile engineering thermoplastic that bridges the gap between economical commodity plastics and high-performance polymers.
Its combination of stiffness, wear resistance, low friction, low moisture pickup, and excellent dimensional stability makes it an ideal choice for precision mechanical parts and dynamic components.
Design, processing and grade selection must be aligned to the operating environment—temperature, chemical exposure and load—to maximize the material’s long service life and reliability.
FAQs
What is the difference between POM and nylon (PA6/PA66)?
POM offers better dimensional stability (low water absorption <0.2% vs. PA6’s 8%), lower friction (0.18 vs. 0.35), and superior chemical resistance.
PA6/PA66 has higher ductility (elongation up to 200%) and impact resistance but swells in moisture, reducing precision.
When should I choose Homo-POM vs. Co-POM?
Choose Homo-POM for high-strength, stiff applications (e.g., gears, fasteners) where crystallinity and rigidity are critical.
Choose Co-POM for impact-prone components (e.g., hinges, clips) or complex molding projects, as it offers better toughness and processability.
Can POM be used in fuel systems?
Yes. POM has good resistance to fuels, oils and many solvents and is widely used in fuel system components. Always validate with the specific fuel blend and temperature range.
What is a safe continuous service temperature for POM?
Design for long-term use below ~80–100 °C. Short excursions to ~120 °C are possible with appropriate grade choice and validation.
Does POM swell in water?
Very little. Equilibrium water uptake is low (~0.2–0.3%), so dimensional change from moisture is minor compared with nylon.
Is POM food contact safe?
Many POM grades are compliant with food contact regulations; specify food-grade or FDA-compliant grades when needed.
What is the maximum temperature POM can withstand?
Co-POM has a continuous use temperature of 90–110°C, while Homo-POM is limited to 80–100°C.
Short-term exposure to 120–130°C is possible, but prolonged exposure above these temperatures causes thermal degradation.
