Polyoxymethylene (POM) is specified as either a homopolymer, designated POM-H, or a copolymer, designated POM-C. The split changes the chain architecture, the crystalline fraction, the melt processing window, the centerline porosity risk, and the long-term response to hot water and alkaline cleaning. Polyoxymethylene Grades: What the POM-H vs POM-C Split Changes is therefore a material-selection question that must be resolved against the actual flow length, wall thickness, thermal load, and chemical exposure—not against tensile strength alone.

In semi-finished machining stock above 50 mm cross-section, the split appears as centerline porosity and residual stress. POM-H cast or extruded rod can present a porous core because the homopolymer crystallizes with higher exotherm and greater shrinkage than POM-C. POM-C extruded plate and rod generally provide lower void content at equivalent diameter because the comonomer interruption reduces crystal growth rate and spherulite size. For gear blanks, valve bodies, and fluid-handling manifolds, this single difference changes rejection rate and inspection interval before a single cut is made.

The datasheet gap in tensile modulus and yield strength does not capture the full split. Tensile stress at yield according to ISO 527-2 is commonly 70–74 MPa for POM-H and 60–66 MPa for POM-C. Flexural modulus under ISO 178 is correspondingly higher for the homopolymer. However, POM-C grades often provide higher heat deflection temperature under ISO 75-2/A and better resistance to hot-water and alkaline media, while sacrificing some dry mechanical strength and surface hardness.

What Does the Copolymer Sequence Interruption Alter in Thermal and Hydrolytic Stability?

The POM-C chain contains typically 1.5–3.5 mol% comonomer units such as ethylene oxide or 1,3-dioxolane. These units shorten the uninterrupted oxymethylene sequence. In POM-H, thermal or acid-catalyzed degradation can proceed by unzipping from chain ends; the process releases formaldehyde at measurable rates above 210 °C. In POM-C, the comonomer interruptions terminate the unzipping reaction, reducing gas evolution and improving melt stability. Differential scanning calorimetry according to ISO 11357-3 shows the melting endotherm of POM-H near 175–180 °C, while POM-C typically melts at 165–172 °C.

Hot-water aging data show a measurable split. POM-C retains a higher fraction of original tensile strength after prolonged contact with demineralized water at 80 °C; the homopolymer loses strength more rapidly because its longer methylene oxide sequences are more accessible to hydrolytic attack. Thermal degradation is not indicated by a single temperature. At a barrel setpoint of 210 °C, POM-H can generate formaldehyde gas at measurable levels. At 230 °C, POM-C also degrades, but the onset is more gradual. Vented barrels with vacuum in the range 20–40 mbar are used in molding cells where off-gassing destabilizes the process. In hot-runner systems, POM-H is more likely to form degraded material in dead spots at the gate tip, leading to short shots, gate burn, and surface splay.

When colored or impact-modified POM is compounded on a 25:1 L/D co-rotating twin-screw extruder, POM-C tolerates wider zone-temperature variation from 170 °C to 190 °C. POM-H requires tighter barrel mapping below 200 °C to avoid intrinsic viscosity loss and formaldehyde odor. This thermal stability gap explains why POM-C is frequently selected for multi-cavity tools with longer material residence time.

Centerline Porosity and Injection Molding Behavior in Production Cells

Injection molding cells with clamp force between 80 t and 120 t and general-purpose screws of 20:1 to 25:1 L/D observe the split as melt-temperature sensitivity. POM-H has a narrow processing window. Barrel setpoints above 215 °C can produce silver streaks and formaldehyde odor at the mold vent; excessive hold time at high temperature reduces molecular weight. POM-C processes more tolerantly with melt temperatures from 180 °C to 230 °C. Mold temperatures between 80 °C and 100 °C are typical for both grades to control crystallization and dimensional stability, but POM-H requires tighter holding pressure and injection speed control because the homopolymer crystallization front is sharper.

The density of unfilled POM-H measured under ISO 1183-1 is often 1.41–1.42 g/cm³, while POM-C is 1.40–1.41 g/cm³. This density gap is small, but the crystallization pattern is not. POM-H forms larger spherulites, which can produce anisotropic shrinkage; POM-C forms more uniform spherulites and often demonstrates better flatness in long flat parts. Mold designers compensate for POM-H shrinkage anisotropy with increased gate count and balanced cooling circuits.

For rod and plate, the difference is equally practical. POM-H solid stock above 50 mm diameter can present centerline porosity after machining; POM-C often avoids that defect at the same diameter. The comonomer in POM-C reduces spherulite size and crystallization enthalpy, which reduces exothermic temperature rise in thick sections. This is why machine shops may accept a 5–10 % lower tensile strength in POM-C to obtain a void-free core in a large-diameter seal blank.

Flow length and melt volume-flow rate do not track the POM-H/POM-C split directly. Melt volume-flow rate under ISO 1133-1:2022 at 190 °C and 2.16 kg typically ranges from 8 cm³/10 min to 23 cm³/10 min in both families, depending on polymerization control and additive package. The copolymeric nature of POM-C confers a wider processing window, but a high-flow POM-H and a low-flow POM-C may still have comparable MVR. The selection must therefore separate flow grade from polymer architecture.

In dry-running tribological service, the selection logic reverses. POM-H displays lower wear factor and higher allowable PV in thrust washer tests according to ASTM D3702, because the higher crystalline fraction increases surface hardness and reduces adhesive transfer. For unlubricated gears, bushings, and counter components with wall stock below 3 mm, POM-H remains the preferred grade where the molder holds melt temperature below 210 °C and the service environment is dry. Published wear-factor data are strongly dependent on mating-surface roughness and sliding speed; a universal wear advantage should not be inferred without paired-surface tests against the intended counter material.

When Hot-Water Contact and Alkaline Cleaning Agents Dominate Service Conditions

Food-processing, plumbing, and medical devices that contact 80–95 °C water or steam show the largest selection change. POM-H loses tensile strength faster in hot water and is less tolerant of alkaline cleaning solutions than POM-C. POM-C is therefore the standard for dishwasher spray arms, hot-water pump impellers, and shower valve cartridges. Regulatory status follows the grade, not the polymer class. Polyoxymethylene copolymer for repeat-use food-contact articles is covered by FDA 21 CFR 177.2470, subject to end-testing. EU food-contact use requires grade-specific verification under Regulation (EU) No 10/2011. Both grades require verification of residual formaldehyde, overall migration limits, and REACH SVHC status, particularly where prolonged food or water contact is intended.

Chemical compatibility remains the boundary condition. At 23 °C, POM-H and POM-C resist aliphatic hydrocarbons, alcohols, ketones, and weak acids in ISO 175 immersion testing. Strong acids, oxidizing acids, and chlorine-bearing water at elevated temperature attack the acetal linkage. POM-C provides better resistance to hot dilute alkaline solutions, but neither grade is compatible with strong acid exposure at processing temperatures or with phenolic solvents at molding temperature. For fuel-system clips, sensor housings, and pump components in warm aggressive media, POM-C is specified when chemical tolerance controls service life; POM-H is reserved for dry mechanical loads where chemical exposure is limited.

In high-speed machining centers producing bearing cages from bar stock, the split changes tool behavior and dimensional drift. POM-H machines with tight tolerance but can show stress relief after heavy cuts; POM-C often produces lower burr formation and more stable flatness in thin sections. For thin-walled gears, spring-loaded valves, or slide bearings, the selection should be based on the processing window, hot-water exposure, and alkaline cleaning requirement rather than only on the tensile strength shown on a datasheet.