What Is POM (Polyoxymethylene)? The Engineering Plastic Buyers Call Delrin

Polyoxymethylene (POM) is a semi-crystalline engineering thermoplastic whose backbone consists primarily of methylene oxide repeat units, typically written as –[CH₂O]–. In procurement specifications, the trade name Delrin refers specifically to acetal homopolymer, although many buyers use the term for copolymer POM as well. The distinction matters because unfilled homopolymer grades generally provide tensile stress at yield of 66–70 MPa, tensile modulus of 3000–3300 MPa, and density of 1.42 g/cm³ when tested to ISO 527-2 and ISO 1183-1. Unfilled copolymer POM typically shows tensile stress at yield of 60–65 MPa, tensile modulus of 2500–2900 MPa, and density of 1.41 g/cm³. The acetal backbone provides high fatigue endurance, low moisture absorption, and high resistance to nonpolar solvents, but homopolymer and copolymer grades differ markedly in hot-water and alkali service.

Melt-crystallized homopolymer POM typically has a degree of crystallinity of 70–85 %, while copolymer POM is often 60–75 %. The glass transition of both chemistries is reported near −60 °C. Differential scanning calorimetry at 10 °C/min records a melting peak near 175 °C for homopolymer and 165–172 °C for copolymer. High crystallinity contributes to equilibrium moisture absorption below 0.25 % in 24 h at 23 °C per ISO 62 and also to pronounced mold shrinkage. POM is not inherently flame retardant; unfilled grades usually achieve UL 94 HB. The material is frequently selected in humid service because mechanical properties change far less with moisture than those of polyamide.

Industrial production of homopolymer POM begins with aqueous formaldehyde. The monomer is concentrated, and anionic polymerization is initiated by tertiary amines or alkali metal alkoxides. The crude polymer contains terminal hydroxyl groups that are thermally unstable, so the chain ends are acetylated with acetic anhydride before processing. Without end-capping, the polymer can unzip rapidly above the melting point. Copolymer POM manufacture instead uses molten trioxane with a cyclic ether comonomer at 1–3 mol%, typically ethylene oxide or 1,3-dioxolane, and a Lewis acid catalyst such as boron trifluoride etherate. The comonomer disrupts the unzipping sequence and raises thermal and hydrolytic stability. Molecular weight is controlled with chain-transfer agents such as methylal or acetic anhydride. Commercial injection grades are classified by melt volume-flow rate at 190 °C and 2.16 kg; values from 2–14 cm³/10 min are common for general-purpose molding, while grades above 20 cm³/10 min are used for thin-wall parts with some loss in impact strength.

Table 1 lists typical property ranges for unfilled medium-viscosity grades in dry-as-molded condition. These values are not design minima; filler type, nucleant, colorant, and processing history can shift the range.

Thermal Stability Is Limited by Chain Unzipping and Acid Residues

Published heat-aging studies show that unfilled homopolymer POM can retain approximately 50 % of initial tensile strength after 500–1000 h at 140 °C in forced-air ovens, while copolymer POM retains strength longer because the comonomer interrupts the depolymerization pathway. Relative thermal index values under mechanical-without-impact loading are typically 90–105 °C for unfilled homopolymer and 100–110 °C for stabilized copolymer grades according to UL 746B. Melt processing above 230 °C for homopolymer or 220 °C for copolymer promotes formaldehyde outgassing, surface splay, and void formation. The degradation is autocatalytic once acid species are present. Strong mineral acids, chlorine, hypochlorite, and peroxide cleaners are incompatible. Copolymer POM offers higher resistance to 3–5 % sodium hydroxide at 23 °C, but hot concentrated bases can etch the surface. Stabilizer packages are compounded at 0.1–0.5 wt% and commonly include hindered phenols, trivalent organophosphites, and acid scavengers. Formaldehyde outgassing during hot-air aging can condense on mold surfaces and create deposits.

On a 120-ton hydraulic injection molding machine with a 20:1 L/D general-purpose screw, unfilled POM is typically run from hopper to nozzle at 180–200 °C, 190–210 °C, 200–215 °C, and 200–215 °C. Mold temperature is held at 60–90 °C; increasing mold temperature to 80–120 °C improves surface gloss and reduces post-mold shrinkage but extends cycle time. Back pressure is kept at 0.5–1.5 MPa, screw speed at 50–100 rpm, and injection pressure at 70–120 MPa. Hold pressure is typically 60–80 % of peak injection pressure, with a stable cushion of 3–6 mm. Because POM freezes rapidly, flow channels below 1.5 mm diameter risk premature gate freeze. Wall-thickness transitions greater than 25 % can create sink, voids, or warpage. Hot-runner manifolds are maintained at 200–220 °C, with valve-gate or open-pipe drops preferred. Drying is usually not required for material from sealed containers, but if pellets are exposed to relative humidity above 60 %, a dehumidifying dryer at 80 °C for 2–4 h and a dew point of −30 °C or lower is recommended. Drying above 100 °C can yellow the pellet surface. Acetal should not be processed after PVC or fluoropolymers without thorough purging because residual acid compounds catalyze depolymerization.

POM melts are shear-thinning. Capillary rheometry at 190 °C shows apparent melt viscosity of approximately 300–500 Pa·s at 100 s⁻¹ and 80–150 Pa·s at 1000 s⁻¹ for general-purpose injection grades. This behavior permits thin-wall filling but produces steep pressure loss in narrow gates. Barrel residence time should not exceed 25 min at 210 °C for homopolymer POM; at 220 °C, the safe residence time falls to 10–15 min. If the machine interrupts, the barrel should be cooled to 150 °C or purged within 5 min to limit formaldehyde generation. Polyethylene or commercial POM-compatible purge compounds can be used; halogenated purge materials are not recommended.

Compounding of filled POM is typically performed on co-rotating twin-screw extruders with 32:1–44:1 L/D ratios. Glass fiber at 20–30 wt% raises tensile modulus to approximately 7500–9000 MPa but reduces flow and weld-line strength; fiber is usually side-fed after polymer melt to limit fiber breakage. PTFE powder at 15–20 wt% or silicone oil at 2–4 wt% lowers coefficient of friction. Barrel temperatures above 210 °C require vacuum venting to remove residual formaldehyde. Fiber-reinforced POM often shows anisotropic mold shrinkage of 0.6–1.2 % along flow and 1.4–1.8 % transverse.

When Crystallization Kinetics Control Post-Molding Shrinkage

During cooling from the melt, POM crystallizes into spherulites with high linear growth rates. Homopolymer POM typically shows crystallization onset near 140–150 °C, and copolymer POM near 130–140 °C, depending on cooling rate and nucleation. The crystalline phase occupies less volume than the melt; unfilled POM therefore exhibits mold shrinkage of 1.8–2.2 % for homopolymer and 1.9–2.3 % for copolymer in the flow direction. Transverse shrinkage is often 0.1–0.3 % higher. Differential scanning calorimetry at a cooling rate of 10 °C/min records crystallization enthalpy of 130–150 J/g for homopolymer POM and 100–120 J/g for copolymer POM. Nucleated grades containing boron nitride or talc at 0.1–0.3 wt% reduce spherulite size and improve dimensional repeatability, though notched impact may decrease. Annealing at 140–150 °C in nitrogen or mineral oil for 30–60 min increases crystallinity by 3–5 % and stabilizes dimensions. In injection molding, hold pressure must be maintained until gate freeze; for a gate of 0.5–1.0 mm and wall thickness 2.5 mm, gate freeze usually occurs within 2–4 s. Premature hold-pressure release before gate freeze creates internal microvoids and higher post-mold shrinkage.

Under dry sliding against hardened steel, unfilled POM shows a dynamic coefficient of friction of 0.20–0.35 when measured by a thrust washer test according to ASTM D3702. Wear factors for unfilled POM are typically 1–3 × 10⁻⁶ mm³/N·m at PV values below 0.07 MPa·m/s; above this, surface heating causes local melting and rapid wear. For unfilled copolymer POM bearings, a limiting PV near 0.10 MPa·m/s at 40 °C is reported, falling to 0.04 MPa·m/s at 100 °C. Filled grades use PTFE at 15–20 wt% or silicone oil at 2–4 wt% to reduce friction and wear. Under tensile stress of 20 MPa at 23 °C, unfilled POM shows tensile creep modulus of 1200–1500 MPa after 1000 h according to ISO 899-1; at 60 °C, the creep modulus declines to 600–900 MPa under the same stress. Design calculations for POM gears and cams should use creep-modulus values at the service temperature rather than short-term tensile modulus. Published data for high-load continuous POM gearing is limited; tooth-surface temperature and flank wear dominate the allowable torque, and prototype testing under the intended duty cycle is required.

Automotive fuel system components often specify copolymer POM for resistance to gasoline, diesel, methanol blends up to M15, and ethanol blends up to E10 at service temperatures below 60 °C. Exposure to sour gasoline above 80 °C can cause embrittlement; for aggressive fuels, testing under SAE J1681 or OEM-specific fuel immersion protocols is required. Water pump impellers use POM because moisture absorption is low, but continuous exposure to hot water above 80 °C favors copolymer POM over homopolymer. In industrial fluid handling, POM is used for pump vanes, valve bodies, and spray nozzles. The polymer resists aliphatic hydrocarbons, glycols, and many neutral aqueous solutions up to 60 °C. Continuous contact with strong acids, oxidizing acids, free chlorine, or hydrogen peroxide is not recommended. Stress-cracking can occur in molded threaded connections exposed to zinc chloride or sodium hypochlorite solutions above 0.5 %; threads and press-fits should be annealed or designed with lower hoop stress.

In medical inhalers and injection devices, POM provides low friction and dimensional stability; however, steam sterilization at 121 °C and gamma irradiation above 25 kGy can cause surface cracking and loss of tensile strength. Ethylene oxide sterilization is generally compatible.

Compliance Matrix for Food Contact, Medical, and Electrical Use

Regulatory compliance for POM depends on chemistry, grade, colorant, and end-use. The following matrix summarizes typical pathways for unfilled grades; specific finished-part validation remains required.

In a medical inhaler component produced from unfilled acetal homopolymer, ethylene oxide sterilization is commonly used because it avoids the chain scission observed with high-energy radiation. The part supplier should verify that the specific grade complies with USP Class VI or ISO 10993-1 testing when skin or mucosal contact is required. For radiation-sterilized POM components, the supplier should confirm the specific grade under the intended sterilization dose because published comparative data across all dose ranges is limited.