For previous reading, please refer to Plastic Materials (Part 9): In-depth Analysis of PEHD & PMMA.
POM (Polyoxymethylene), commonly known as "acetal," is the most rigid and wear-resistant of five major engineering plastics—combining high rigidity, high wear resistance, and high fatigue resistance, earning it title of "metal among plastics." POM boasts a tensile strength of 60-70 MPa and a flexural modulus of 2500-3200 MPa, approaching metal levels; its coefficient of friction is as low as 0.1-0.3, exhibiting excellent self-lubricating properties; and its fatigue resistance ranks among the best in plastics, capable of withstanding millions of cycles without failure.
With these unique properties, POM occupies an irreplaceable position in gears, bearings, fuel systems, precision components, and other fields.

 

POM = Polyoxymethylene, a semi-crystalline thermoplastic plastic polymerized from formaldehyde. Its molecular backbone consists of repeating -CH₂-O- units:

Core characteristics of POM: High crystallinity (70~85%) → High rigidity, high strength, and excellent wear resistance; Regular molecular chain structure → High fatigue resistance, low friction, and self-lubrication.
POM's superior properties stem from its unique molecular structure: High crystallinity → High rigidity, high strength, high heat resistance, and excellent chemical resistance; Regular molecular chains → High fatigue resistance, low friction, and self-lubrication; High density (1.41 g/cm³) → High specific strength and high specific stiffness; Ether bond structure → Easily thermally degraded (requires stabilizers)
Main classifications of POM

Homopolymer vs. Copolymer: Homopolymer POM (such as DuPont Delrin) has higher strength and modulus, but poorer thermal stability and chemical resistance; copolymer POM (such as Celcon, Hostaform) has better thermal stability and hydrolysis resistance, and is safer to process, making it mainstream in the market.
POM vs. PA66 vs. PBT: How to choose engineering plastics?

Selection logic: For wear resistance + fatigue resistance + dimensional stability, choose POM; for high strength + impact resistance, choose nylon; for electrical insulation + dimensional stability, choose PBT.

POM exhibits unique mechanical properties among engineering plastics—high rigidity, high strength, high wear resistance, and high fatigue resistance.

 

Comparison of Typical Properties of Different Types of POM
Note: Above are typical ranges for materials of same grade. Specific values may vary depending on molecular weight, toughening agent content, and additives.
Yield-to-tensile strength ratio analysis: POM's yield-to-tensile strength ratio (yield strength/tensile strength) has following characteristics: General Purpose Grade: High yield-to-tensile strength ratio (~0.9~0.95), with a distinct yield point, some cold drawing after yielding, and moderate elongation at break (10~35%). Impact Grade: High yield-to-tensile strength ratio, even greater elongation after yielding (15~20%+), and better toughness. Glass Fiber Reinforced Grade: Even higher yield-to-tensile strength ratio (~0.95+), direct brittle fracture, and very low elongation at break. Abrasion-Resistant Grade: High yield-to-tensile strength ratio, but slightly lower strength and modulus due to lubricant influence.
Engineering Conclusion: POM is a typical rigid material—high rigidity, high strength, with a distinct yield point, some plastic deformation after yielding. Its fatigue resistance is the best among plastics, making it suitable for cyclic loading applications. However, it is highly notch sensitive; therefore, fillet radius and stress concentration must be carefully considered during design.
Influence of Temperature on Performance
POM's properties are quite sensitive to temperature: Impact Strength: Moderate at room temperature (6~8 kJ/m² notch), decreases significantly at low temperatures, but impact-grade POM maintains good toughness at low temperatures. Stiffness: Decreases with increasing temperature, HDT (1.8MPa) approximately 95~105℃, long-term service temperature approximately 80~100℃. Creep: POM's creep resistance is relatively good among engineering plastics, but creep still occurs under long-term loads, becoming more pronounced at high temperatures. Fatigue: POM has excellent fatigue resistance, among the best of all thermoplastics, with fatigue strength reaching 25~30% of tensile strength at room temperature.

POM has a wide range of applications, appearing in almost all fields requiring precision, wear resistance, and fatigue resistance.
Automotive Applications

 

Fuel systems are a key application of POM in automobiles. Combination of POM's fuel resistance, low friction, and fatigue resistance makes it preferred material for fuel pump impellers, gears, and sensors. Conductive POM can meet anti-static requirements of fuel systems.
Electronic and Electrical Applications

 

Industrial and Mechanical Applications

Other Applications

POM simulation modeling requires special attention to crystallization behavior, rate dependence, and fatigue.
Five Commonly Used Constitutive Models

Four Key Points of POM Simulation: Strong Rate Dependence: Yield strength of POM increases significantly with increasing strain rate. Dynamic simulations such as collisions and drops must use rate-dependent constitutive models. Strong Temperature Sensitivity: Properties of POM vary greatly with temperature. From -40℃ to 100℃, modulus and strength may decrease by more than half. Simulations under temperature environments must consider temperature effects. Fatigue is Key: POM is most commonly used in cyclic loading applications (gears, bearings, etc.). Fatigue life prediction is focus of POM simulation. Fatigue models and S-N curves are required. Creep Cannot Be Ignored: POM exhibits significant creep under long-term loading, which is more pronounced at high temperatures. Creep needs to be considered for structural components subjected to long-term loads.
Typical Simulation Scenario

POM failure modes have unique characteristics; wear, fatigue, and thermal degradation are the most common problems.

 

1. Wear Failure
Problem: Long-term friction and wear lead to dimensional deviations, performance degradation, and increased noise. This is one of the most common failure modes of POM.
Common Scenarios: Gear tooth surface wear, bearing bushing wear, slider guide wear.
How to Solve:

2. Fatigue Fracture
Problem: Under long-term cyclic loading, POM products suddenly fracture without significant plastic deformation. Although POM has good fatigue resistance, it will still fail if fatigue limit is exceeded.
Common Scenarios: Gear tooth root fatigue fracture, spring fatigue fracture, fatigue of structural components under cyclic load.
How to Solve:

3. Thermal Degradation/Aging
Problem: At high temperatures, POM molecular chains break (depolymerization produces formaldehyde), leading to decreased strength, brittleness, and discoloration. This can occur during processing and use.
Common Scenarios: Degradation due to excessively high processing temperatures; aging due to prolonged high-temperature use; silver streaks and powdering on the product surface.
Solutions:

4. Hydrolysis/Chemical Degradation
Problem: In high-temperature water or acid/alkali environments, POM molecular chains hydrolyze and break, leading to performance degradation. Copolymer POM has better hydrolysis resistance than homopolymer POM.
Common Scenarios: Use in hot water environments, steam sterilization, contact with acid/alkali environments.
Solutions:

5. Creep Deformation
Problem: POM slowly deforms under long-term constant load, eventually leading to dimensional deviations or failure. Creep is more pronounced at high temperatures.
Common Scenarios: Deformation of structural components under long-term load, loosening of fasteners, failure of seals.
Solutions:

6. Stress Cracking/Notch Fracture
Problem: POM is very sensitive to notches and stress concentration; stress concentration at notches leads to cracking or fracture.
Common Scenarios: Fracture at sharp corners, cracking due to assembly stress, impact fracture at notches.
How to solve it: