Matching POM-H or POM-C to Your Part: Wall Thickness, Wear and Environment

Polyoxymethylene (POM) is available as homopolymer (POM-H) and copolymer (POM-C) resins; selection between these two acetal families for a machined or injection-molded part is governed by three interacting constraints: wall thickness, sliding wear regime, and chemical or hydrolytic environment. Both grades are semi-crystalline engineering thermoplastics with density near 1.41 g/cm³ under ISO 1183-1, tensile moduli above 2,500 MPa under ISO 527-2, and low equilibrium moisture absorption below 0.35% under ISO 62. The distinction is not compositional marketing; it alters solidification behavior, shrinkage tolerance, tribological response, and resistance to hot aqueous media. A part designed for POM-H may exhibit centerline porosity or formaldehyde evolution in a continuous 80 °C water-pump application where POM-C retains dimensional stability, while a POM-C gear may show slightly higher wear against a carbon steel shaft than a POM-H gear when measured by ASTM D3702 thrust-washer testing.

What Separates POM-H from POM-C Beyond Tensile Modulus?

In homopolymer acetal, the repeating –CH₂O– chain has no secondary alcohol sites; this minimizes chain transfer during thermal degradation but also creates rapid unzipping when degradation begins. Copolymer acetal contains 1–2 mol% comonomer units, typically oxyethylene, which interrupt the oxymethylene sequence and terminate unzipping. As a result, POM-H shows higher tensile yield stress values of 70–76 MPa under ISO 527-2/1A, while POM-C grades typically report 62–70 MPa. Flexural modulus for POM-H spans 3,000–3,600 MPa under ISO 178; POM-C spans 2,500–3,200 MPa. The notched Izod impact values under ISO 180/1A are 6–8 kJ/m² for POM-H and 5–7 kJ/m² for POM-C at 23 °C.

POM-H exhibits a melting peak around 175–183 °C by differential scanning calorimetry, and POM-C around 160–175 °C. The lower melt temperature of POM-C allows barrel settings of 180–210 °C in single-screw extruders with L/D ratios of 20:1 to 24:1, whereas POM-H often requires 190–220 °C. In thick-section extrusion, POM-H has a greater tendency toward centerline porosity because the high crystallinity drives a sharper crystallization front; this is observable in rods above 30 mm diameter as a frosted core on machined surfaces.

Wall thickness selection for unfilled POM begins with a nominal dimension of 1.5 mm to 6.0 mm for injection-molded parts. Below this range, filling pressure rises sharply; above this range, differential cooling across the section reduces internal soundness. The flow length-to-wall-thickness ratio for acetal at 190 °C melt temperature and 60 °C mold temperature is typically 150:1 to 250:1 in open channels. An abrupt change from a 2.0 mm wall to a 4.0 mm boss base introduces a sink-mark zone because the deep section remains molten after the gate freezes; design guidance limits wall variation to ±10%, with a transition length of at least three times the wall step.

POM-H grades with melt flow rates below 6 g/10 min under ISO 1133-1 often require injection pressures above 100 MPa in a 1.5 mm section. On a 120-ton hydraulic injection machine, a 1.2 mm-thick POM-C gear with a 45 mm flow length may require melt temperature 215 °C, mold temperature 80 °C, and holding pressure 80–100 MPa; the same tool in POM-H at equivalent melt flow rate may require a 10–15 °C higher melt temperature or a wider gate. Gate freeze time depends on the cube of wall thickness, so a 3.0 mm wall freezes in approximately nine times the time of a 1.0 mm wall. This nonlinearity forces homopolymer parts with thick bosses to use longer hold-pressure time or gas-assist processing.

Mold shrinkage under ASTM D955 for unfilled POM-H is 1.8–2.5% in flow and cross-flow directions; POM-C is 1.6–2.2%. The difference of 0.3–0.7 percentage points across the two materials is enough to shift hole-to-hole pitch by 0.15 mm on a 100 mm part. Therefore, tooling built for POM-H cannot simply be switched to POM-C without checking clearance and tolerance stack.

When Wall Thickness Falls Below 2 mm in Unfilled POM Injection Molding

Thin-wall molding in POM-C below 2 mm requires a minimum melt flow rate of 10 g/10 min to prevent premature solidification at the flow front. POM-H is less forgiving in this regime because its higher crystallinity and rapid solidification reduce the flow-to-cooling window. Mold temperature must be raised to 80–100 °C; below 70 °C, the frozen layer increases and the effective channel height decreases. This leads to a non-uniform cross-sectional morphology: a highly oriented skin with anisotropic shrinkage of 0.4–0.6% in the flow direction and 1.5–2.0% in the transverse direction under ASTM D955.

On a 150-ton clamp electric injection machine with a 25 mm screw, a 0.8 mm-thick POM-C snap fit using a 0.6 mm edge gate may require injection speed of 150–250 mm/s and decompression no greater than 2 mm. If the screw cushion is lost, cavitation in front of the screw causes unstable shot weight and weld-line tensile strength under ISO 527-2 drops from 55 MPa to below 35 MPa. For POM-H, the same configuration requires a higher melt temperature of 220 °C and a reduced residence time below 15 min; longer residence triggers formaldehyde evolution and visible splay.

Sliding Wear Response and PV Limits for Unmodified Acetal

Tribological selection between POM-H and POM-C is often evaluated by thrust-washer test ASTM D3702 against 1018 steel at 0.5 MPa and 1.0 m/s. POM-H typically shows a steady-state wear rate 10–20% lower than POM-C in unlubricated sliding, with a dynamic coefficient of friction between 0.20 and 0.35. The homopolymer's higher crystallinity increases surface hardness and shear resistance. However, below 0.2 MPa contact pressure in intermittent motion, the difference is not statistically meaningful; both materials develop a transfer film on the steel counterface.

Continuous unlubricated PV limits for unfilled acetal against steel are generally reported between 0.06 MPa·m/s and 0.12 MPa·m/s; above this range, frictional heat exceeds solidification temperature and the wear rate becomes nonlinear. At PV values above 0.15 MPa·m/s, localized surface melting produces a white, degraded ring on the test specimen. The test must maintain counterface roughness Ra below 0.2 µm; rougher surfaces increase wear by plowing and remove the transfer film. Published data for POM-H and POM-C under identical PV conditions in oscillating bearings is limited; direct substitution therefore requires coupon-level validation.

Hydrolytic and Chemical Environment Screening for POM-C and POM-H

POM-C is preferred for continuous exposure to hot water because the oxyethylene comonomer interrupts acid-catalyzed hydrolysis along the chain. POM-H loses surface gloss and develops hairline cracks earlier in boiling water immersion when tested under ISO 175. POM-C retains tensile strength above 80% of original after 1,000 h in 80 °C water in published immersion studies, whereas POM-H may fall below 70% in the same period. Neither grade is recommended for strong acids, oxidizing agents, or aqueous chlorine at concentrations above 1 ppm.

Fuel-system components machined from POM-C are used where continuous exposure to gasoline, diesel, and methanol blends does not exceed 60 °C; POM-H is serviceable in dry hydrocarbon environments but should not be specified where aqueous phase separation can occur. In water-meter internals, POM-C demonstrates lower swell and longer dimensional stability than POM-H when immersed in chlorinated potable water at 23–40 °C. In all cases, contact with zinc-plated inserts or zinc chloride-based fluxes produces rapid stress cracking; brass inserts with electroplated nickel or stainless steel are preferred. Continuous exposure to aqueous glycol mixtures at 90 °C is permissible for POM-C but not POM-H; published data for POM-H under these specific conditions is limited.