POM(Polyoxymethylene) is a high-performance engineering thermoplastic with exceptional mechanical properties, dimensional stability. This article shows the key properties of POM, including its mechanical properties, thermal and electrical performance and its applications.
What is POM Plastic?
Polyoxymethylene (POM), also known as acetal, is a crystalline thermoplastic polymer. It is a high-density, highly crystalline linear polymer with excellent overall performance characteristics. It is a premium resin that emerged after nylon, ranking third among the five major engineering plastics. Based on the chemical structure of its molecular chain, POM is primarily categorized into Homopolymer POM (Delrin) and Copolymer POM (Celcon).
What distinguishes these two varieties? Delrin is a regular crystalline molecular structure polymerized from a single monomer. Celcon is an amorphous structure resulting from the copolymerization of two different monomers.
Properties of POM
A significant advantage of POM is its high elastic modulus, high stiffness and hardness, excellent fatigue resistance, and good impact strength and creep resistance.
| Property | Homopolymer (Delrin) | Copolymer (Celcon) |
| Specific Gravity | 1.42 | 1.41 |
| Tensile Strength, MPa | 68.9 | 60.6 |
| Elongation, % | 40 | 60 |
| Tensile Modulus of Elasticity, GPa | 3.10 | 2.83 |
| Flexural Strength, MPa | 97.1 | 89.6 |
| Flexural Modulus of Elasticity, GPa | 2.83 | 2.58 |
| Shear Strength, MPa | 65 | 53 |
| Notched Izod Impact Strength, J/m | 76 | 65 |
| Rockwell Hardness (M Scale) | 94 | 80 |
Tensile Strength and Stiffness
Among all plastics, POM has specific strength and specific stiffness values that are relatively close to those of metals. It can replace materials such as steel, zinc, aluminum, copper, and cast iron in many applications. As the temperature increases, the tensile strength and stiffness of POM decrease.
Impact Strength
POM exhibits good toughness, and its impact strength is minimally affected by temperature. However, the presence of a notch significantly reduces the impact strength. While Polycarbonate (PC) and ABS have substantially higher impact strength than POM, POM demonstrates higher impact strength than PC and ABS after repeated fatigue impact loading.
Fatigue Resistance
Crystalline plastics typically have higher fatigue strength than amorphous plastics. POM, with a crystallinity exceeding 70%, demonstrates excellent fatigue resistance. It possesses the highest fatigue limit strength among all thermoplastic engineering plastics.
Creep Resistance
Due to POM’s good resilience, its creep resistance is comparable to other engineering plastics like nylon. At 23°C under a 21 MPa load, the creep value is only 2.3% after 3000 hours. Furthermore, its creep value changes relatively little with temperature, allowing it to maintain good creep resistance even at elevated temperatures.
Friction and Wear Resistance
POM has a very low coefficient of friction and minimal wear rate, while its limiting pressure velocity value is quite high. This makes it suitable for components subjected to long-term sliding friction. Additionally, POM has a surface hardness similar to aluminum alloys, provides a degree of self-lubrication in dynamic friction applications, and generates very little noise, demonstrating excellent tribological properties.
Thermal Properties
POM has a high heat deflection temperature (HDT). Homopolymer POM generally has a higher HDT than copolymer POM, but homopolymer POM has lower thermal stability than the copolymer. Generally, the service temperature for POM is around 100°C.
| Property | Homopolymer (Delrin) | Copolymer (Celcon) |
| Heat Deflection Temperature, 1.82 MPa | 124 | 110 |
| Melting Point, °C | 175 | 165 |
| Freezing Temperature, °C | -50 | / |
| Linear Expansion Coefficient, -40~30 °C | 7.5 | 9.5 |
| Thermal Conductivity, W/(m · °C) | 1.47 | 1.47 |
Electrical Properties
POM has good electrical insulation properties and is largely unaffected by humidity. Across a frequency range of 102 ~107 Hz and a temperature range of 20～100°C, the dielectric constant of POM remains stable at 3.1-3.9. Its dielectric constant is minimally influenced by water absorption. The thickness of a POM product does affect its dielectric strength, thinner parts exhibit greater dielectric strength.
Chemical Resistance
POM resin offers high resistance to oils and organic solvents (such as hydrocarbons, alcohols, ketones, esters, and benzenes). Even after prolonged immersion (over half a year) at higher temperatures, it maintains a high degree of mechanical strength, with a mass change rate generally below 5%.
POM resin has good resistance to dilute acids but is susceptible to stress cracking when exposed to strong acids, particularly sulfuric, hydrochloric, nitric, sulfurous, and nitrous acids.
Weather Resistance
The weather resistance of POM resin is less than ideal. For outdoor applications, UV absorbers or antioxidants should be incorporated to enhance their durability.
Processes for POM Machining
POM resin can be shaped and processed using standard thermoplastic methods, including injection molding, extrusion, blow molding, and compression molding. Injection molding is the most common technique. Extrusion molding is primarily used to produce sheets and rods, which can then be secondary processed into final products. Blow molding supplements injection molding for manufacturing hollow articles.
Injection Molding
Equipment: Use a single-head, full-flight, abrupt-compression screw with a standard screw tip. The L/D ratio should be about 18, the compression ratio 2-3, and the metering section length 4-5D. Screw speed is typically 50-60 r/min, and back pressure is controlled at about 0.6 MPa. The nozzle should be a straight-through type with a reverse taper and must be equipped with a heating band, with temperature controlled by a voltage-regulating transformer. The nozzle bore must be sufficiently large.
Mold: The gating system consists of a sprue, runners, and gates. The sprue should be short and thick, with a diameter about 1 mm larger than the nozzle and a taper of 3° – 5°. Trapezoidal runners are often used, with a standard dimension where the top width is three times the bottom width, and the depth is two times the bottom width. Gate dimensions (e.g., thickness) must be determined based on the product thickness and other factors; various gate types are available, and the best option should be chosen according to the product design.
Molding Process: Typically, the material does not require pre-drying. However, pre-drying is necessary if the material granules were cooled by water immersion or if precision parts are being molded. Barrel temperature must be precisely controlled; excessively high temperatures can cause material discoloration and decomposition, while too low a temperature results in poor plasticization.
Extrusion
Use an equal-pitch, constant-diameter metering screw with an L/D ratio of 20−24. The metering section should be approximately 1/4 of the total screw length, and the compression ratio should be 3−4.
Screw structure varies with material viscosity. deep-channel screws for high viscosity and shallow-channel screws for medium viscosity. Pre-drying is required if the material’s moisture absorption exceeds 0.25%. Parts of the extruder in contact with the material must avoid copper and alloys that promote thermal decomposition. Specific process conditions, such as L/D ratio, compression ratio, and die temperature, vary for different products (e.g., pipes, sheets, wire jacketing).
Blow Molding
A standard blow molding machine is used, requiring precise and accurate control devices. The screw L/D ratio is 16 ~ 20, and the injection ratio 2.0 ~ 3.5. Mold material is selected based on the product. Mold temperature is generally 93°C ~127°C, and blow air pressure is 0.35 ~ 1.20 MPa. Different blow-molded products (e.g., drums, travel bags, sprayers) have their own specific process conditions, such as L/D ratio, compression ratio, barrel temperature, and screw speed.
CNC Machining
When CNC machining POM, injection-molded blanks with a 3-5 mm allowance are ideal for complex parts, while simple shapes can be cut from high-density (1.41-1.43 g/cm³) rods using water cooling, with aluminum alloy fixtures and soft-jaw vises (clamping force 5-10 kN) or vacuum suction (≥ 0.1 MPa for thin parts) to minimize deformation and stress.
At Getzshape, we provide high-precision CNC machining services for engineering plastics, ensuring exceptional quality and accuracy. Our POM (polyoxymethylene) materials are available in two color options—black and white, both offering versatility for various applications. Unless specific tolerances are specified, we adhere to the ISO 2768-c standard to guarantee consistent and reliable results.
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Secondary Processing
POM’s machining characteristics are similar to brass; it is rigid and generates little heat. It can be machined by turning, sawing, milling, drilling, stamping, and tapping.
Applications of POM
POM is mainly used to replace non-ferrous metals (such as copper, aluminum, and zinc) to manufacture various mechanical components, and is widely used in automotive, mechanical, electronic, electrical, construction, and plumbing applications.
Automotive
POM is extensively used as an energy-saving material in automotive manufacturing. This is due to its ability to facilitate vehicle lightweighting, reducing fuel consumption, and replacing various metals, which lowers the energy consumed during forming and processing. For example, using POM for car door and window handles, replacing the original chrome-plated metal or stainless steel parts, results in a 23% cost saving and reduces processing steps from 13 to 5. Compared to some other engineering plastics, POM shows outstanding resistance to a range of chemicals used in automobiles. Copolymer POM is particularly superior in this regard. For example, due to the extreme stability of copolymer POM to automotive coolants, most car radiator components are made from copolymer POM.
POM is primarily used in the following three areas in automobiles:
- Engine fuel supply system: Used to manufacture radiator drain valves, radiator caps, coolant overflow tanks, water pump impellers, water valve bodies, fuel tank caps, fuel filler necks, fuel pumps, carburetor housings, various exhaust control valves, accelerator pedals, and other parts.
- Electrical equipment system: Used to manufacture heater fans, air compressor valves, lighting switches, heater control levers, combination switches, windshield wiper motor gears and bearing supports, and washing pumps.
- Body system: Used to manufacture sun visor brackets, leaf spring bushings, speedometer housings, antenna gear housings, interior mirror supports, door lock components, window regulator handles, window glass frames and guide rollers, steering knuckle bearings, brake parts, steering wheel components, and compartment hinges.
Machinery
POM is widely used to manufacture various gears, bearings, springs, cams, bolts, nuts, as well as structural components for mechanical equipment like pump bodies, housings, and impellers. POM gears, gear couplings, and bearings are commonly used as general-purpose power transmission structural parts.
Electronics
POM has a low dissipation factor, high dielectric strength and insulation resistance, and excellent arc resistance, leading to its widespread use in electronics, electrical appliances, and OA machines. POM can be used to manufacture components for telephones, tape recorders, video recorders, televisions, computers, fax machines, printers, CD and VCD players, etc., such as telephone dials and keys, television relays and coil bobbins, computer control components, timer parts, tape recorder cassette holders, micro-switch cams and reverse sliders, CD changer switches and CD trays, printer front chassis, and speaker grilles. POM is also used to manufacture components for various power tools, such as electric wrench housings, electric sheep shear housings, coal drill housings, and switch handles.
Construction and Plumbing
In the construction and plumbing fields, POM is used to manufacture window frames, washbasins, water tanks, door and window rollers, small furniture casters, gas meter parts and covers, bathroom water heater components, water meter housings, water pipe fittings, and faucets. Faucets made from POM cost 30% less than typical metal faucets and can withstand 2×106 cycles of opening and closing under 0.5−0.6 MPa pressure, equivalent to over 40 years of use. It also possesses characteristics such as resistance to water corrosion, scale buildup, and self-lubrication. Fire hydrants and fire hose couplings made of POM can withstand burst pressures up to 5 MPa.
