Polyoxymethylene (POM) moulding compounds, designated under ISO 9988-1, are semi-crystalline acetal resins whose high crystallinity drives both useful stiffness and the defect cluster of Polyoxymethylene Moulding Defects: Warp, Sink and the Formaldehyde Smell. The defects are not independent. Warp arises from differential volumetric shrinkage, sink arises when local shrinkage cannot be supplied by packing flow, and formaldehyde odour arises from thermal depolymerisation of the acetal backbone. Unfilled POM homopolymer typically shows moulding shrinkage of 0.018–0.025 mm/mm parallel to flow and 0.016–0.023 mm/mm perpendicular to flow when measured according to ISO 294-4. Because the crystalline melting peak of homopolymer is near 175 °C and that of copolymer near 165 °C by ISO 11357-3, the processing window is narrow, and barrel-temperature overshoots are directly connected to formaldehyde generation.

Warp Is Driven by Differential Shrinkage, Not by Absolute Shrinkage

Warpage in POM is assessed as out-of-plane deviation on a calibrated surface plate. For a 100 mm square plaque, bow height above 0.5 mm is commonly rejected in precision applications. Differential shrinkage of 0.002–0.005 mm/mm between flow and cross-flow directions is sufficient to produce visible curvature because the part behaves as an anisotropic solidifying sheet. The differential develops from cavity-to-cavity temperature imbalance, wall-thickness-induced cooling gradients, and flow-induced molecular orientation that relaxes anisotropically during crystallisation. In a production-scale 1200 kN hydraulic toggle clamp moulding machine running a single-cavity 150 mm × 100 mm × 3 mm plaque, a temperature difference of 10 °C between fixed and moving cavity halves was associated with measurable curvature after 48 h of ambient conditioning. Mould temperature should be held at 80–100 °C for dimensional stability; when cavity-surface temperature falls below 60 °C, the skin freezes before the core crystallises, trapping residual stress that drives post-demoulding warp.

Post-moulding crystallisation continues for several days. A part measured immediately after ejection will understate the eventual dimension by 0.1–0.3 % depending on mould temperature and wall thickness. Warp assessment should therefore occur after conditioning at 23 ± 2 °C and 50 ± 10 % RH for at least 24 h, as specified by ISO 291. The use of a hot mould, although slower, reduces the magnitude of post-moulding shrinkage and stabilises the part against later flatness drift.

Sink marks in POM are surface depressions that occur over thick ribs, bosses, or screw bosses where the local section is more than 40 % thicker than the nominal wall. The local core remains molten after the surrounding skin has solidified, and as the POM crystallises, density increases from the melt to the solid by approximately 0.28–0.31 g/cm³. That volumetric contraction pulls the frozen surface inward if the gate has sealed before packing is complete. Sink visibility on textured surfaces is often placed at 0.02 mm; on high-gloss surfaces, depressions of 0.01 mm can be visible under harsh lighting.

For a 2 mm nominal wall and a 1.0 mm gate diameter, gate freeze time in POM typically falls between 2 s and 4 s because of the rapid crystallisation rate. Hold time must extend beyond gate freeze-off; a hold time of 1 s or less is frequently associated with sink over un-cored bosses. Hold pressure should be maintained at 80–100 MPa hydraulic pressure, and back pressure should remain low at 0.5–1.0 MPa to avoid excessive shear heating and degradation. Where sink persists, the melt-temperature setting should be lowered within the recommended range because a hotter melt increases volumetric contraction on cooling and extends the time during which the core can collapse. Part design rules for sink avoidance in POM include limiting wall-thickness variation to 25 %, maintaining rib thickness at 50–70 % of the adjacent wall, and coring bosses to maintain uniform melt-solidification balance.

Why Does POM Generate a Pungent Formaldehyde Odour During Moulding?

Formaldehyde odour is not an additive failure; it is the volatile product of thermal unzipping of the polyoxymethylene chain. POM degrades by acid-catalysed hydrolysis and by thermal depolymerisation from chain ends. In homopolymer, the end-capped chains can unzip when melt temperature exceeds 230 °C or when sustained melt residence time exceeds 10 min. Copolymer resins contain oxyethylene comonomer units that interrupt unzipping and reduce the rate of formaldehyde formation at equivalent temperature. Both resin families release formaldehyde when barrel temperature is excessive, when hot spots exist in the nozzle, or when screw speed is set so high that shear heating raises local melt temperature above the set point.

The odour threshold for formaldehyde is below 1 ppm, meaning the smell can be detected before occupational exposure limits are reached. The European binding occupational exposure limit for formaldehyde is 0.3 ppm as an 8-hour time-weighted average and 0.6 ppm as a short-term value under Directive (EU) 2019/983. For automotive interior parts, VDA 275 is commonly used to determine formaldehyde emission from moulded components; several automotive specifications set a limit of 10 mg/kg or lower for POM interior parts. Formaldehyde generation is therefore both a moulding defect and a regulatory compliance issue.

When Barrel Temperature or Residence Time Exceeds the Degradation Threshold, Formaldehyde Control Fails

Processing control for low-odour POM begins with the barrel temperature profile. For copolymer, a rear zone of 170–180 °C, centre zone of 185–195 °C, front zone of 195–205 °C, and nozzle at 195–210 °C are typical. For homopolymer, the nozzle should not exceed 215 °C. If melt temperature exceeds 220 °C for copolymer or 215 °C for homopolymer, formaldehyde generation accelerates. Residence time should be limited to 5 min when melt temperature is above 200 °C; below 200 °C, a maximum of 15 min is commonly specified. Injection speed should be moderate to high to avoid premature freeze-off, but sustained shear rates above 50,000 s⁻¹ can add local temperature spikes and should be avoided.

Venting must be adequate to allow formaldehyde gas and decomposition vapours to escape from the cavity rather than remain trapped and condense on the part surface. Vent depths of 0.01–0.02 mm and vent lands of 1.0–1.5 mm are used for POM. Pre-drying is not always required because POM has low equilibrium moisture uptake, but surface moisture can promote acid-catalysed backbone hydrolysis in the melt when relative humidity exceeds 60 %. Drying at 80 °C for 2–4 h in a desiccant dryer is applied for high-gloss or emission-critical parts. A mould-area extraction system should maintain airborne formaldehyde below the applicable occupational exposure limit; monitoring is by calibrated detector tubes or photoionisation detection.

In hot-runner moulds, dead spots in the manifold create long-residence melt volumes that are difficult to purge. These volumes are a common production-scale source of intermittent formaldehyde odour even when barrel settings are correct. The runner system should be designed without sharp flow restrictions, and the hot-runner nozzle temperature should be profiled no higher than the front barrel zone unless a validated low-emission POM grade with additional acid-scavenger stabilisation is used. Where published data for a specific formulation or mould configuration is limited, process verification should rely on VDA 275 measurements and direct part-odour evaluation after conditioning, rather than on barrel set-point readings alone.