Recycled HDPE Melt Flow Variability: Stabilising Production

Recycled high-density polyethylene (HDPE) is not a single feedstock but a compounding input whose melt mass-flow rate (MFR) can shift within a single production shift. The processing consequence is not simply a higher or lower flow number; it is loss of viscosity stability at the die, progressive volumetric underfeeding in screw channels, and dimensional drift in moulded parts. Melt flow variability in recycled HDPE is measured under ISO 1133-1:2022 at 190 °C with a 2.16 kg load for standard MFR, and under 190 °C with a 21.6 kg load for high-load MFR where shear response and molecular weight distribution effects are amplified.

Stabilising production therefore requires control of three coupled variables: feedstock composition, thermomechanical degradation during recompounding, and melt pump or die pressure interaction. A single-point MFR value on a recyclate batch is insufficient without accompanying flow-rate ratio data, contaminant data, and melt filtration records.

Sources of melt flow variability in post-consumer HDPE bale material

Post-consumer HDPE bale material typically includes blow-moulded bottles, injection-moulded caps, extruded pipe, and occasional film. The MFR ranges of these input streams are discontinuous. Blow-moulding resin commercial grades commonly fall between 0.3 g/10 min and 0.7 g/10 min at 190 °C/2.16 kg, while injection moulding grades fall between 4.0 g/10 min and 12.0 g/10 min. Pipe grades are typically between 0.2 g/10 min and 0.6 g/10 min. Mixed bale input can therefore shift MFR by several grams per ten minutes when cap fractions change. In addition, polypropylene contamination from caps and labels creates a heterogeneous melt because the PP melting endotherm occurs near 165 °C, and PP domains remain distinct in HDPE at standard MFR test conditions. Residual label adhesives, ethylene vinyl alcohol barrier fragments, fillers, and oxidative degradation products contribute further viscosity variation.

A comparative MFR and molecular weight distribution profile of typical HDPE fractions is shown below. Values are presented as industrial technical bulletin ranges, not as a single lot release.

Polydispersity index in this comparison is measured by gel permeation chromatography or size-exclusion chromatography according to ISO 16014-2:2019. The value ranges reflect the broadened molecular weight distribution of regrind streams compared with single-grade virgin resin.

Why does washed recyclate still show MFR drift across batches?

Washing and flotation remove surface contaminants but do not eliminate molecular-level variability. The first unavoidable source is the distribution of chain lengths in the incoming regrind; repeated processing histories generate both chain scission and branching. Chain scission increases MFR by shortening the weight-average molecular weight, while long-chain branching and crosslinked gel particles reduce MFR and increase melt pressure. The direction of MFR drift therefore depends on the predominance of scission or branching in a particular lot. A second source is residual polypropylene. Because PP is molten at 190 °C but phase-separates from HDPE at standard test flow rates, small amounts of PP can either plasticise the flow front or form higher-shear domains depending on particle size and dispersion. A third source is the recyclate drying history; volatile carryover and moisture depress melt viscosity during processing but are not measured by a dry MFR test.

Independent of plant control, the characterisation laboratory should generate a three-point viscosity data set rather than a single MFR value. The MFR at 190 °C/2.16 kg is compared with the melt flow ratio obtained from 190 °C/21.6 kg high-load MFR. A ratio above the virgin control indicates broader molecular weight distribution or long-chain branching. Differential scanning calorimetry per ISO 11357-3:2018 identifies a PP melting endotherm near 165 °C and detects oxidative exotherms if stabiliser depletion has occurred. Thermogravimetric analysis under nitrogen separates polymer decomposition from filler or pigment residue. None of these tests replaces a production-scale extrusion trial, but they reduce the probability of assigning a process deviation to feeder error when the recyclate lot itself has shifted.

Stabilisation begins with fixed let-down ratios rather than variable line-side corrections. Gravimetric loss-in-weight feeders on the virgin and recycled streams maintain a mass ratio, typically 15 wt% to 30 wt% recycled flake for blow-moulding operations and up to 50 wt% for injection moulding when cap-grade recyclate is sorted. The recycled flake is metered into a side feeder or tumble blend immediately before the extruder throat. The additive package is adjusted according to the recyclate fraction. A stabiliser system containing 0.03 wt% to 0.08 wt% hindered phenolic primary antioxidant and 0.05 wt% to 0.10 wt% hydrolytically stable phosphite secondary antioxidant is common; 0.05 wt% zinc stearate or equivalent acid scavenger is added when recyclate contains residual acidic catalyst residues. All percentages are based on total polymer mass and should be verified by thermogravimetric analysis or extraction testing where food-contact status is claimed under FDA 21 CFR 177.1520.

Melt filtration is not optional. A screen changer with a 120 µm to 150 µm wire mesh pack removes unmelted PP domains, gel particles, and label residues. However, screen packs alone do not stabilise MFR; they protect downstream tooling and prevent valve clogging. The more relevant stabilising unit is the melt pump. A gear pump placed after the screen changer decouples die pressure from extruder screw fluctuations and maintains volumetric output within tight limits. The pump inlet pressure setpoint is typically maintained between 20 bar and 50 bar, though manufacturer-specific values vary. This prevents cavitation and reduces shear-induced temperature rise in the pump.

When twin-screw compounding imposes thermomechanical degradation

If the plant uses a co-rotating twin-screw extruder for recompounding recycled HDPE, specific mechanical energy input becomes the dominant rheological variable. Screw speed, throughput, and temperature profile interact; high shear zones can generate local melt temperatures well above the barrel setpoint. The melt temperature measured in the die adapter may exceed the setpoint by 10–20 °C when screw speed is raised without increasing feed rate. At these conditions, chain scission accelerates and MFR can rise by 0.2–0.5 g/10 min within a single pass for a starting MFR near 1.0 g/10 min. Published data for this specific configuration is limited, but the effect is routinely observed in production-scale twin-screw lines with 40:1 L/D ratios.

To stabilise output, the line should be operated at a moderate screw speed, typically 250–400 rpm for a 40 mm machine, with the barrel profile set from 160 °C near the feed throat to 210 °C at the die, and with vacuum devolatilisation at −0.8 bar or deeper to remove volatiles. Melt temperature and head pressure are recorded continuously; MFR is measured at least once per shift using ISO 1133-1:2022 or ASTM D1238-23 to detect drift. High-shear dispersion of the phosphite stabiliser is required to avoid local antioxidant depletion at the screw-flight surface.

Controlling extruder head pressure and melt pump suction stability

In single-screw extrusion with recycled HDPE, screw recovery time in injection moulding or die pressure in continuous extrusion is used as a real-time viscosity indicator. A declining die pressure at constant screw speed indicates lower melt viscosity and rising MFR; a climbing pressure indicates gel formation, crosslinking, or excessive fines accumulation. The control response should not be a large barrel temperature adjustment because temperature changes lag the process. Instead, the gravimetric feeder setpoint is trimmed. If the recycled HDPE MFR is above the target, the recycle let-down ratio is reduced by 2–5 wt% steps and virgin HDPE is increased accordingly. If MFR is below target, either the barrel temperature is raised in 5 °C increments within the resin supplier’s maximum, or the recycle fraction is increased.

Melt pump suction pressure and die pressure sensors are positioned after the breaker plate. A stable line typically holds die pressure variation within ±2% of setpoint for film and pipe, but short-term surging may exceed this when recycled flake bulk density varies. The use of a grooved feed throat improves intake consistency with flake and fines, but it also raises motor load and can reduce tolerance for high-MFR material.

Injection moulding lines using post-consumer HDPE regrind encounter MFR variability as shot-weight drift and flash or short-shot alternation. A moulding machine with a three-zone general-purpose screw of 20:1 L/D to 22:1 L/D is common, but a barrier screw or a low-compression screw is preferred for a feedstock containing flake and fines. Shot weight is monitored for 10–20 consecutive cycles before a production run. If the average shot weight shifts by more than 0.3–0.5% without a change in back pressure, the material feed is not rheologically uniform. Back pressure is set between 5 bar and 15 bar melt pressure to densify the melt and reduce unmelted flakes, but excessive back pressure raises melt temperature and promotes chain scission. Clamp force requirements should be verified against cavity pressure data; a reduction in melt viscosity can reduce peak cavity pressure and cause sink marks or part weight variation. Tensile yield strength and elongation at break are periodically tested according to ISO 527-2:2012 or ASTM D638-22 to ensure that the mechanical properties have not fallen below the moulded part specification.

The following production checks stabilise recycled HDPE MFR variability without requiring full molecular characterisation on every lot.

Operational boundaries exist at both ends of the melt-flow range. Recycled HDPE with MFR below 0.2 g/10 min at 190 °C/2.16 kg can produce excessive melt temperatures and screw torque on lines designed for 0.5–1.0 g/10 min material. Recycled HDPE with MFR above 4.0 g/10 min may not hold the die bubble in blown film and can cause parison sag in blow moulding. Materials containing unabsorbed moisture above 0.1 wt% should be pre-dried at 80 °C for 2–4 h or processed with deep vacuum devolatilisation; pre-drying is also required when ambient relative humidity exceeds 60%. Avoid combining recycled HDPE with amine-based antifog additives or certain primary antioxidants unless compatibility has been confirmed, as some amine chemistries can discolour or interfere with long-term thermal stability. The processing window is narrowest when very low-MFR pipe regrind is blended with high-MFR cap regrind; in that configuration the target MFR should be controlled by pre-blending into a more uniform intermediate lot before production rather than by line-side adjustments.