Meaning
Post-consumer or post-industrial high density polyethylene recyclate supplies injection moulding and extrusion operations with reprocessed polymer pellets derived from rigid packaging waste. Melt flow index variation between lots determines processing parameters and clamping force adjustments on the moulding machine. Contaminant degradation occurs when moisture levels exceed standard thresholds during the drying phase, leading to surface bubbling and structural weakness in finished mouldings.
Virgin resin economics rely on predictable molecular weight distribution, whereas reprocessed material introduces variable chain lengths that alter shrinkage rates in the mould cavity. A material specification defines intrinsic viscosity and density bounds, while a part specification establishes mechanical load tolerances for the final component under operating conditions.
Melt Behaviour
Rheological stability dictates whether high density polyethylene recyclate flows uniformly through the hot runner manifold during high-speed injection cycles. Screw rotation speed and back pressure must compensate for reduced shear viscosity caused by thermal history accumulated during previous processing lives. Viscosity drops trigger flash along the parting line if injection pressure remains unadjusted for the lower melt resistance.
Thermal degradation accelerates when barrel temperatures exceed recommended set points, producing volatile gases that create voids inside thick-walled sections. Shear heating inside the plasticising unit modifies the effective melt temperature beyond the external heater band readings, requiring constant monitoring by the machine operator.
Contaminant Tolerance
Gel count and foreign polymer cross-contamination govern the ultimate tensile strength of parts moulded from high density polyethylene recyclate. Melt filtration screens placed before the pelletising die remove paper fibres and metal fragments, but degraded polymer cross-links pass through the mesh and form visible black specks on the moulded surface. Trace polypropylene fractions lower the impact resistance at sub-zero temperatures by disrupting crystalline domains during cooling.
Part specifications fail qualification tests when residual additives from previous packaging applications reduce stress crack resistance below regulatory minimums. Extrusion blow moulding setups demand tighter contaminant thresholds than compression moulding presses because parison sag increases with inconsistent melt strength.
Shrinkage Control
Volumetric contraction during solidification forces toolmakers to cut cavities oversized to accommodate the specific crystallinity of high density polyethylene recyclate batches. Mould temperature regulation prevents differential cooling rates that cause warpage and dimensional drift across complex geometries. Moulders measure shrinkage along both flow and transverse directions to establish reliable process windows for automated production runs.
Part dimensions deviate from CAD models when regrind ratios fluctuate within the feed hopper, altering the specific gravity of the shot entering the mould. Dimensional stability relies on maintaining uniform packing pressure until the gate freezes off completely, counteracting the broader melting range inherent in recycled polymer streams.
