POM Delrin properties
POM (polyoxymethylene) has a density of 1.41 g/cm³ and is an opaque white semicrystalline thermoplastic that can be colored. It is marketed in homopolymer and copolymer versions; the copolymer offers better thermo‑oxidative stability and resistance to bases, while the homopolymer has higher mechanical strength and surface hardness. Both forms share stiffness, toughness, excellent wear resistance, low friction, and good dimensional stability.
Chemical composition
Section titled “Chemical composition”The crystallinity of POM typically ranges between 70 % and 80 % due to its linear and regular chain. The homopolymer consists of repeating –(CH₂O)– units from formaldehyde, while the copolymer incorporates comonomers (e.g., ethylene oxide) that introduce C‑C bonds, giving it greater stability against thermal degradation and hydrolysis.
| Parameter | Homopolymer (Delrin) | Copolymer (Celcon/Hostaform) |
|---|---|---|
| Base structure | -(CH₂O)- | -(CH₂O)- + C-C bonds from comonomer |
| Oxygen content (%) | ≈ 40 | ≈ 40 |
| Typical crystallinity (%) | 75‑80 | 70‑75 |
Mechanical properties
Section titled “Mechanical properties”The tensile strength of injection-molded POM copolymer is 73 MPa (10 580 psi). Other representative properties are summarized below (values for copolymer at 23 °C, dry injection-molded specimens).
| Property | SI value | Imperial equivalent |
|---|---|---|
| Tensile strength | 73 MPa | 10 600 psi |
| Flexural modulus | 2,58 GPa | 374 000 psi |
| Elongation at break | 65 % | – |
| Strain at yield | 8 % | – |
| Izod impact strength (notched) | 0,069 kJ/m (69 J/m) | 1,29 ft·lb/in |
| Rockwell surface hardness | M117 (M scale) | (Similar in ASTM) |
| Dynamic friction coefficient (vs. steel) | 0,1‑0,3 | – |
Note: The homopolymer has higher values in tensile strength (≈ 78 MPa), flexural modulus (≈ 3.0 GPa), and hardness (Rockwell M120), but its elongation is lower (≈ 40 %).
Physical properties
Section titled “Physical properties”The density of POM is 1.41 to 1.42 g/cm³ (0.051 lb/in³). Other typical physical properties are shown below.
| Property | SI value | Imperial equivalent |
|---|---|---|
| Density | 1,41‑1,42 g/cm³ | 0,051‑0,0513 lb/in³ |
| Mold shrinkage | 1,8‑2,0 % | – |
| Water absorption (24 h, 23 °C) | 0,22 % | – |
| Natural color | Opaque white | – |
| Surface gloss | High (can achieve glossy finish) | – |
Thermal properties
Section titled “Thermal properties”The crystalline melting temperature of copolymer is 165 °C (329 °F), while its maximum continuous service temperature in air reaches 100 °C (212 °F). The homopolymer melts around 175 °C and withstands only 80 °C in continuous service.
| Thermal property | SI value | Imperial value |
|---|---|---|
| Melting temperature (copolymer) | 165 °C | 329 °F |
| Maximum continuous service temperature (air, copol.) | 100 °C | 212 °F |
| Heat deflection temperature (HDT, 0,45 MPa) | 160 °C | 320 °F |
| HDT (1,80 MPa) | 110 °C | 230 °F |
| Linear thermal expansion coefficient | 11×10⁻⁵ /°C | 6,1×10⁻⁵ /°F |
| Specific heat (cp) | ≈ 1,5 kJ/(kg·K) | 0,36 Btu/(lb·°F) |
Processing range: the melt temperature in the machine should be kept between 190 and 210 °C; the mold temperature between 60 and 120 °C.
Electrical properties
Section titled “Electrical properties”POM acts as an electrical insulator with a volume resistivity of 10¹⁵ Ω·cm and a dielectric strength of 20 MV/m.
| Electrical property | SI value | Comment |
|---|---|---|
| Volume resistivity (log Ω·cm) | 15 (1×10¹⁵ Ω·cm) | Measured dry |
| Dielectric strength | 20 MV/m | 3 mm thickness, alternating current |
| Dielectric constant (1 kHz) | 3,7 | – |
| Dissipation factor (1 kHz) | 0,0015 | – |
| Oxygen index (LOI) | 15 % | UL 94 HB classification |
Chemical resistance
Section titled “Chemical resistance”Water absorption in 24 h at 23 °C is only 0.22 %, giving POM excellent dimensional stability in humid environments compared to polyamides. It is resistant to most organic solvents, oils, and greases, but suffers rapid attack by concentrated mineral acids (especially nitric acid) and strong bases. Prolonged exposure to hot water (> 60 °C) is only recommended for copolymer grades. It has very low resistance to UV radiation, so parts exposed to outdoor conditions require stabilization (carbon black or UV additives).
Applications by industry
Section titled “Applications by industry”With a coefficient of friction on steel of 0,1‑0,3 dry and excellent wear resistance, POM is used for parts requiring precise sliding movements and low maintenance.
| Industry | Representative applications |
|---|---|
| Automotive | Gears for window regulators, fuel pump components, door latches, hinges |
| Appliances | Electric kettles, water jugs, telephone keypads, shower sprayers, appliance housings |
| Consumer goods | Bathroom scales, eyeglass frames, knife handles, ski bindings |
| Machinery | Bearings, conveyor chain guides, sprockets, chemical pumps |
| Electronics | Keyboards, clock mechanisms, connector parts due to low dielectric constant |
Comparison with similar materials
Section titled “Comparison with similar materials”When compared to nylon (PA 66), POM shows 40 times lower water absorption (0,22 % vs. ~8,5 %), making it preferable for precision parts in environments with variable humidity.
| Property | POM (copolymer) | Nylon 66 (PA66) | ABS | Polycarbonate (PC) |
|---|---|---|---|---|
| Density (g/cm³) | 1,41 | 1,14 | 1,05 | 1,20 |
| Tensile strength (MPa) | 73 | 60‑80 | 35‑45 | 62‑70 |
| Flexural modulus (GPa) | 2,58 | 1,8‑3,0 | 1,7‑2,4 | 2,3‑2,4 |
| Water absorption 24 h (%) | 0,22 | 8,5‑9,0 | 0,3‑0,7 | 0,15‑0,35 |
| Maximum continuous service temp. (°C) | 100 | 120 | 70 | 110 |
| Wear resistance (comparative) | Excellent | Good | Low | Low |
Processing and conversion
Section titled “Processing and conversion”Before injection molding, the material must be dried for 3 h at 90 °C to achieve a moisture content below 0.05 %. The conversion is carried out on low‑medium compression screws, with a temperature profile of 190‑210 °C and a mold heated between 60 and 120 °C. Mold shrinkage reaches 1,8‑2,0 %, requiring the cavity to be oversized. After demolding, a post‑shrinkage of ≈ 0.1 % occurs, completed within 48 hours at room temperature. It must not be processed in equipment that has been in contact with halogens (PVC, brominated flame retardants) due to the risk of explosive decomposition.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum continuous service temperature of POM Delrin?
Section titled “What is the maximum continuous service temperature of POM Delrin?”The maximum continuous service temperature in air for the copolymer is 100 °C (212 °F) and for the homopolymer is 80 °C (175 °F), exceeding many engineering thermoplastics in hot applications.
What Izod impact strength value does POM have?
Section titled “What Izod impact strength value does POM have?”The notched Izod test gives a value of 69 J/m (1.29 ft·lb/in) in injection‑molded copolymer specimens, indicating moderate toughness suitable for parts subject to impact.
How much water does POM absorb compared to nylon?
Section titled “How much water does POM absorb compared to nylon?”It absorbs only 0.22 % water in 24 h, compared to over 8 % that nylon 66 can absorb, thus reducing dimensional changes due to humidity.
What heat deflection temperature does POM support?
Section titled “What heat deflection temperature does POM support?”Under a load of 1.80 MPa (264 psi), the copolymer reaches an HDT of 110 °C (230 °F), while at 0.45 MPa it reaches 160 °C (320 °F), allowing its use in moderate thermal environments.
What is the surface hardness of POM?
Section titled “What is the surface hardness of POM?”On the Rockwell M scale, the copolymer has a hardness of M117, and the homopolymer can reach M120, equivalent to high scratch resistance in contact applications.
What linear shrinkage must be considered when molding POM?
Section titled “What linear shrinkage must be considered when molding POM?”Mold shrinkage ranges from 1.8 % to 2.0 %, depending on part thickness and processing conditions, plus a post‑shrinkage of about 0.1 % in the first 48 hours.
References
Section titled “References”- azom.com: https://www.azom.com/article.aspx?ArticleID=381
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/polymer-properties-d_1222.html
- steelnumber.com: https://www.steelnumber.com/en/steel_composition_eu.php?name_id=614