Meaning
Recycled polyolefin covers recovered polyethylene and polypropylene streams derived from post-consumer or post-industrial waste, processed through washing, separation, and pelletization to supply injection moulders and extrusion lines. Melt flow rate consistency and contaminant loads govern the operational window for these reprocessed polymers, setting boundaries for scrap rates in thin-wall packaging and automotive components. Material specifications define virgin-equivalent melt indices and ash content ceilings, whereas part specifications dictate final mechanical load limits and elongation at break under service conditions.
Virgin resin pricing reflects primary petrochemical feedstocks and dedicated polymerization capacity, while regrind economics depend on collection logistics, sorting technology efficiency, and cleaning overheads. Datasheet values measured on compression-moulded plaques rarely match the actual processing window a moulder holds across a continuous production run due to batch variability in polymer chain length and residual moisture.
Thermal Degradation
Processing history degrades polymer chains through successive heating cycles, reducing molecular weight and shifting the melting point downward compared to virgin equivalents. Shear heating in compounding extruders accelerates chain scission when barrel temperatures exceed the stability threshold of the specific recycled grade. Viscosity drops during continuous production runs if volatile organic contaminants remain trapped within the pellet matrix, causing flash around parting lines and sink marks on thick sections.
Additive packages deplete during initial use, requiring stabilizer injection during reprocessing to prevent cross-linking or excessive chain breakdown in secondary moulding equipment.
Rheological Variance
Melt flow rate fluctuation across different supplier lots alters cavity fill times and packing behaviour in high-speed injection tools. Pressure spikes occur when incoming viscosity exceeds the nominal window established for the mould geometry, leading to short shots or incomplete clamp tonnage lockup. Shear rate sensitivity rises as molecular weight distribution broadens from mixed polymer inputs, demanding closed-loop pressure control on the injection unit to maintain part weight uniformity.
Melt fracture appears at lower extrusion speeds when degraded fractions segregate from high-molecular-weight chains during profile drawing.
Contamination Control
Foreign polymers, cellulosic fibres, and metallic particles migrate through the plasticating screw and lodge behind hot runner valve gates, causing localized burn marks and structural weakness. Filtration screens placed at the extruder head trap particulate matter down to specific micron thresholds, but excessive backpressure from fouled screens degrades throughput stability. Gel counts measured via optical scanning on cast film lines quantify cross-linked polymer lumps that cause localized tear initiation points during subsequent stretching operations.
Proper melt filtration prevents nozzle blockage and reduces scrap generation during long unattended moulding shifts.
