Skipping Polyoxymethylene Regrind Control: Colour and Strength Drift presents as a measurable batch-to-batch shift in moulded acetal parts when granulated sprues, runners, and rejected components are returned to the feed stream without closed-loop ratio verification. Polyoxymethylene homopolymer and copolymer grades are semi-crystalline engineering thermoplastics specified for gears, fuel system clips, conveyor chain guides, and snap-arm assemblies because of their low friction, high fatigue resistance, and dimensional reproducibility. These properties are governed by molecular weight, crystallinity, and stabilizer package integrity; each of these is degraded by repeated melt processing. Uncontrolled regrind addition therefore changes melt rheology, colour coordinates, and tensile impact response before dimensional failures become obvious. Standards including ISO 9988-2 for designation and ISO 1133-1:2022 for melt flow testing provide a baseline for incoming material control, but production regrind loops require tighter limits because the material has already accumulated one or more heat histories.

Thermal and Oxidative Chain Scission in Acetal Regrind Under Extended Barrel Residence

Degradation in POM begins at the chain ends when terminal hydroxyl or acetate groups are exposed to elevated temperature. The polymer unzips by depolymerization, releasing formaldehyde. In the presence of oxygen, formaldehyde oxidizes to formic acid, which accelerates acid-catalysed chain cleavage. Regrind is more vulnerable than virgin resin because the first moulding pass consumes part of the antioxidant and acid-scavenger package, and the grinding step increases surface area for oxygen ingress. At barrel set temperatures above 220°C for copolymer or 230°C for homopolymer, the rate of chain scission increases sharply; residence times longer than 20 min at these temperatures can shift melt volume-flow rate by more than 20% relative to virgin pellets. The observable result is a lowering of melt viscosity, with melt volume-flow rate measured according to ISO 1133-1:2022 at 190°C and 2.16 kg load. A medium-viscosity POM copolymer that begins at 8 cm³/10min can drift to 12–14 cm³/10min after three closed-loop regrind cycles. Colour shift often appears at this stage because the degradation products include chromophoric oxidation species and the residual stabilizer can no longer protect the melt.

Thermal gravimetric analysis of stabilized POM typically shows onset of mass loss near 260°C under nitrogen, but oxidative onset can be lower. In injection moulding, the screw plasticating zone, check ring, hot runner manifold, and nozzle are all potential stagnation points. If the hot runner is set above 220°C for copolymer, local material can generate formaldehyde pressure, leading to surface splay, porosity, and brown streaks in the regrind fraction. The degradation pathway is autocatalytic: acidic species generated in the first pass migrate into virgin melt when regrind is mixed unevenly, so an uncontrolled 30 wt% regrind stream can damage the full shot rather than only the regrind particles. Control begins with barrel temperature profiling and residence-time logging, not with colour measurement alone.

In production colour drift evaluations, the moulded plaque is conditioned for 24 h at 23°C and 50% relative humidity and measured with a d/8 sphere spectrophotometer using D65 illumination, 10° observer, and specular component included. Yellowness index is calculated to ASTM E313-20, and total colour difference is reported as ΔE*ab according to ASTM D2244-23. A virgin natural POM copolymer typically shows a yellowness index below 1.0. When regrind is introduced without separation of fines or removal of degraded first-pass material, yellowness index can increase by 1.5–3.0 units while tensile strength still appears acceptable. This is because the colour bodies are low-concentration oxidation products that do not directly indicate molecular weight loss. A ΔE*ab threshold of 2.0 is often used for internal visual parts; automotive and appliance specifications may reject above 1.0 CIELAB colour difference from the approved masterbatch or natural resin target. Spectrophotometric data are most useful when combined with melt flow and impact testing, because regrind colour drift correlates with stabilizer depletion rather than with chain length in a linear manner.

Colour drift sources include granulator blade wear producing metallic fines, cross-contamination from previous materials in shared grinders, dust accumulation on pellets, and oxidation of acetal surfaces during hot-air drying. Drying POM at temperatures above 100°C for extended periods can cause yellowing even without regrind. Regrind hoppers that are not purged between jobs transfer colourant residues, and regrind with particles below 200 µm oxidizes faster because of high specific surface area. The production audit therefore includes sieve analysis of regrind, granulator screen condition, and hopper-magnet inspection. Skipping these steps allows colour drift to appear even when the nominal regrind ratio is unchanged.

What Happens to Notched Impact Strength When Regrind Fraction Exceeds 30 wt% Without Instrumental Verification?

The first mechanical property to drift is usually elongation at break, followed by notched impact strength. Tensile modulus and yield stress can remain within 5% of virgin values until degradation is advanced, because POM modulus is controlled more by crystallinity than by molecular weight. Test specimens machined or moulded according to ISO 527-2:2012 type 1A show a drop in nominal tensile strain at break from typical virgin values of 15–25% for copolymer to below 8% when regrind content reaches 50 wt%. Notched Charpy impact strength to ISO 179-1:2010 using type 1eA specimens at 23°C often declines from a datasheet range of 6–7 kJ/m² to below 4 kJ/m² under the same conditions. Published data for this specific configuration is limited because commercial datasheets report virgin pellet values, not multi-pass regrind mixtures. On production lines, the practical failure signature is cracking at thin ribs, boss outer edges, or snap-fit hinges, especially when parts are moulded at the lower end of the melt temperature window to preserve cycle time. Notched Izod testing to ASTM D256-23 can be substituted for fast incoming audits, but it is less sensitive than Charpy for POM because of differences in notch radius and stress state.

Melt flow testing alone cannot detect this mechanical drift. An MVR increase of 20% may correspond to a notched Charpy loss of 30% or more in talc-free POM, but the relationship is non-linear and depends on the stabilizer formulation. The standard test suite for regrind acceptance should include melt volume-flow rate to ISO 1133-1:2022, tensile stress at yield and strain at break to ISO 527-2:2012, notched Charpy impact strength to ISO 179-1:2010, and yellowness index to ASTM E313-20. Colour measurement cannot replace mechanical testing because colour shift can plateau while impact strength continues to fall.

Regrind Ratio Control, Dosing Accuracy, and Process Set-Point Windows

The primary defence against drift is gravimetric ratio control. Volumetric additive feeders can vary by ±5% or more when regrind bulk density changes, whereas a gravimetric blender with weigh hopper resolution of 1 g or better can hold virgin/regrind ratio within ±0.5% of setpoint. Regrind should be screened to remove particles below 200 µm or above 6 mm. Fines above 5 wt% of the regrind stream cause feeding instability, localized overheating, and splay. The recommended closed-loop regrind ratio is 10–20 wt% for tight-tolerance functional parts; non-critical applications can run up to 30 wt% if the melt flow shift is verified by lot testing. At 50 wt% regrind, even well-controlled systems often show statistically significant drift in notched impact and colour, and the injection window narrows.

For POM copolymer, melt temperature should be kept at 190–210°C for standard grades; homopolymer requires 200–220°C. Mould temperature should remain at 80–120°C to control crystallinity and surface gloss. Barrel residence time should not exceed 15 min when regrind is present, and the shot size should be between 30% and 70% of machine capacity to avoid long residence and loss of cushion. Back pressure is typically limited to 0.3–0.7 MPa because higher back pressure increases shear heating and formaldehyde generation. Injection speed and pack pressure must be balanced to avoid jetting and sink marks; excessive hold pressure on thick sections can increase internal stress and reduce impact strength. These parameters are documented in the batch record so that any colour or strength drift can be traced to a residence-time excursion, a feeder calibration error, or a regrind ratio override.

On a production injection moulding line operating a 1300 kN clamp force machine with a 40 mm general-purpose screw, regrind drift is often first detected not in the laboratory but as increased shot-to-shot variation in cushion position and occasional nozzle drool. When cushion instability exceeds 2 mm, the melt is degrading or the regrind particle size distribution has shifted. Operators may compensate by raising screw speed or back pressure, which accelerates chain scission and increases colour shift. The correct response is to isolate the regrind lot, measure MVR and yellowness index, and verify granulator screen condition before resetting the feeder. If the MVR shift exceeds 20% and the yellowness index increase exceeds 1.5 units, the regrind is either re-blended at a lower ratio or diverted to non-functional parts. Published data for this specific configuration is limited, but the acceptance matrix above reflects common industrial practice for acetal regrind control.

In closed-loop regrind operations, the granulator is cleaned after each production order to prevent polyamide or polyolefin contamination, because POM is incompatible with many thermoplastics and can form heterogeneous domains with severe notch sensitivity. A dedicated granulator with screen size 6 mm, rotor speed below 400 min⁻¹, and blade clearance checked every 50 h reduces fines and dust loading. The regrind is not dried at high temperature after grinding; if moisture is suspected from high-humidity storage, dehumidified air at 80°C for 2–3 h is used rather than hot-air drying above 100°C. Regrind with yellowness index above 3.0 or with a burnt odour should not be reintroduced because it contains acidic degradation products that will catalyse further depolymerization in virgin melt. The material batch record therefore includes granulator campaign hours, sieve analysis, MVR shift, CIELAB ΔE*ab, and notched Charpy impact strength before a regrind lot is approved for return to the feed stream.