Polyoxymethylene (POM), a semi-crystalline engineering thermoplastic with 60–80% crystallinity and a melt-processing range of 180–230°C, is used in gears, fuel system components, and precision injection-moulded parts. In POM compounds, migrating additives—particularly colourants, lubricants and release agents—are low-molar-mass species that diffuse through the amorphous phase and accumulate at the part surface or on the mould steel. The driving force is solubility incompatibility: as the melt cools below the crystallization point, amorphous regions contract and reject additives whose molar volume, polarity, or hydrogen-bonding capacity differs from the polyoxymethylene chain. Because the glass transition of POM is below -60°C, segmental mobility persists at room temperature, allowing slow migration even during storage. The following sections address Migrating Additives in POM: Colourants, Lubricants and Release Agents, their processing thresholds, and the test methods used to quantify migration.

Why Do Colourants and Process Aids Migrate from Semi-Crystalline POM?

Migration in POM is not governed by bulk melting but by diffusion through amorphous tie-chain regions and spherulite boundaries. Low-molar-mass additives with molecular weight below 500 g/mol migrate at rates orders of magnitude faster than oligomeric additives above 2,000 g/mol. Dyes such as anthraquinone and azo types are therefore poor choices for POM contact applications because they remain mobile and can plate out. Pigments such as phthalocyanine blue, carbon black, titanium dioxide, and mixed-metal oxides are preferred because their primary particle size, typically 0.2–1.0 µm, limits diffusion. However, even pigment-based colourant systems can cause surface deposits when the carrier resin is a low-viscosity POM or when the concentrate is diluted below 1.0 wt%. The melt rheology of the final compound is typically controlled by measuring the melt flow rate per ISO 1133-1:2022 at 190°C and 2.16 kg; a shift of more than 15% from the base resin indicates additive interaction or degradation.

Temperature dependence follows an Arrhenius relationship; mould and service temperatures above 80°C increase migration rate. At a mould temperature of 90°C, the surface is hot enough to maintain additive solubility in the amorphous surface layer during cooling, but upon demoulding the surface cools rapidly and becomes supersaturated. The additive then blooms. On long production runs, this deposit accumulates on the mould parting line and venting channels. The condition is assessed by placing polished witness plates in the mould and measuring diffuse reflectance before and after 100 injection cycles using ISO 7724-2. A visible colour shift without mechanical wear indicates additive bloom rather than abrasion.

Quantifying Internal Lubricant Plate-Out and Demoulding Load

Internal lubricants are used in POM to reduce melt viscosity, improve screw recovery, and lower demoulding force. Common release agents include zinc stearate, calcium stearate, ethylene bis-stearamide (EBS), and silicone masterbatch. Zinc stearate at 0.1–0.3 wt% acts as an acid scavenger and external release agent. EBS is normally limited to 0.2–0.5 wt% because its melting point of 140–146°C is lower than the POM melt temperature, and it re-crystallizes at the mould surface. Silicone-based release agents, especially ultra-high-molecular-weight polydimethylsiloxane, are added at 0.5–2.0 wt% for high-cycle thin-wall parts but are thermodynamically incompatible with POM and migrate rapidly.

Compounding these additives requires a twin-screw extruder with L/D 40:1 and vacuum venting to remove volatile condensation products. In a typical trial on a 25 mm twin-screw line at 200 rpm and barrel temperatures of 180–220°C, addition of 0.5 wt% EBS to a POM copolymer with MFR 9 g/10 min reduces screw torque by 10–20%. Demoulding force is monitored with ejector-pin force transducers calibrated to ISO 7500-1. When the internal release agent loading is too high, the ejection force decreases further but the part surface becomes hazy after 24 h because additive blooms. Surface haze is quantified by gloss measurement at 60° per ISO 2813; a drop below 70 GU on a polished mould surface indicates deposit.

When a 0.3 wt% phthalocyanine blue concentrate is dry-blended with POM copolymer and moulded at a melt temperature of 215°C, the pigment disperses into particles below 1 µm if the concentrate carrier is a compatible POM. At a concentrate dilution of 1:25, unmelted pigment agglomerates can form because POM has low wetting for pigment surfaces. These agglomerates become visible as specks, and the associated low-molar-mass dispersants migrate. Dispersion quality is rated by pressure-rise filtration or by film analysis under transmitted light. Typical acceptance criteria require fewer than 10 visible specks per 100 cm² at 20× magnification. Colour migration is tested by contact with PVC or polycarbonate sheets under 5 kPa load at 80°C for 24 h; any transfer of colour above a delta E of 1.5 measured by ISO 7724-2 is considered unacceptable. This test simulates storage with dissimilar polymer parts. A more severe test for POM components is contact with sebum or synthetic perspiration per ISO 105-E04, because polar contact media extract mobile colorants from the amorphous surface.

When Silicone-Based Release Agents Interfere with Adhesive Bonding and Laser Welding

Silicone migration is particularly problematic when POM parts are bonded, painted, or joined by laser transmission welding. Because polydimethylsiloxane is incompatible with POM, it migrates to the surface within hours after moulding and forms a monolayer. The surface energy drops below 30 mN/m; untreated POM typically has a surface energy of 35–38 mN/m. Adhesive bonding using epoxy or polyurethane systems then fails at the interface because the adhesive cannot wet the surface. Paint adhesion is tested by cross-cut according to ISO 2409; a result of class 3 or worse is observed on silicone-rich surfaces. Plasma or corona pretreatment can temporarily remove the silicone layer, but migration continues from the bulk. For these applications, internal silicone release agents should be avoided, or the part must be washed with isopropanol and immediately bonded. In laser welding, silicone bloom scatters the beam and reduces weld strength; tensile weld strength is measured by ISO 527-2 after preparing lap joints. A reduction of more than 25% relative to unreinforced POM indicates unacceptable surface contamination.

The processing conflict is that removing silicone raises demoulding force and cycle time, but improves downstream adhesion. For parts requiring painting, a mould spray based on a waterborne external release agent is often used instead of internal silicone. The external release agent must be re-applied based on a fixed number of cycles; spray deposition is monitored gravimetrically to avoid build-up.

Regulatory Limits for Additive Migration in POM Food-Contact Articles

POM formulations intended for food-contact use are constrained by overall migration and specific migration limits. Under EU Regulation (EU) No 10/2011, plastics food-contact articles must demonstrate total migration below 10 mg/dm² or 60 mg/kg of food. The test method is EN 1186-1, with simulant selection based on the intended food type. POM is hydrophobic, so fatty food simulants such as ethanol or olive oil are relevant. Specific migration limits are specified for each approved additive; for example, calcium stearate and zinc stearate are covered by their lubricant listings, but the specific limit values are obtained from the EU positive list. In the United States, polyoxymethylene copolymer is referenced in FDA 21 CFR 177.2470, which includes end-use conditions and extraction requirements. Suppliers of POM food-contact grades provide compliance statements, but formulators must verify that all colourants and lubricants are cleared for the intended conditions of use.

In a dry-running gear pair moulded from POM copolymer with 0.2 wt% zinc stearate and 2.0 wt% carbon black, the internal lubricant blooms to the gear tooth surface during operation. The surface deposit reduces wear scar width measured by pin-on-disc testing per ASTM G99, but it also transfers to adjacent electrical contacts. On a production-scale 25 mm injection-moulding machine with clamp force 800 kN, part weight is held at 3.5 g with variation ±0.02 g. After 2,000 h at 80°C, the surface deposit is extracted and analysed by Fourier transform infrared spectroscopy. The deposit matches EBS or zinc stearate, not the base POM, confirming additive migration rather than polymer degradation. This distinction is critical because the corrective action is formulation modification rather than drying or thermal profiling. For applications where lubricant bloom cannot be tolerated, an external mould release agent is used only during start-up, and the insert is cleaned after a defined number of cycles to avoid transfer to downstream operations.