Meaning
Regrind contamination is polymer degradation and particulate impurity introduced into virgin feedstock when reclaimed scrap plastic mixes into a production stream. Mechanical recycling processes subject post-industrial or post-consumer material to thermal and shear histories that reduce molecular weight, leaving the recovered polymer vulnerable to further chain scission during subsequent moulding cycles. Injection moulding operations govern melt flow index stability and thermal history through this parameter, which is set at the material blending station before resin enters the barrel.
Part failure and inconsistent mechanical strength appear when degraded fractions alter viscosity gradients across the cavity. A material specification defines virgin resin property limits for a single grade, whereas a part specification establishes structural and dimensional tolerances for the finished moulded article. Virgin economics rely on predictable processing windows and known additive packages, while regrind economics balance lower material costs against scrap generation risks caused by variable viscosity.
Datasheet values describe resin performance under laboratory conditions, but the value a moulder actually holds across a production run depends heavily on strict management of reclaimed content ratios.
Thermal Degradation
Polymer chains experience thermal scission and oxidative breakdown during every heating cycle, accumulating heat history that accelerates yellowing and reduces tensile strength in finished components. Extruder barrel heaters and hot runner systems impart cumulative heat energy, causing low molecular weight fractions to volatilize or crosslink prematurely. Viscosity drops as chain lengths shorten, allowing high-shear zones to accelerate degradation rates in thick-walled sections.
Temperature controllers must compensate for reduced melt viscosity by lowering barrel set points to prevent material scorching.
Particulate Inclusion
Foreign debris enters reclaimed streams during grinding and handling operations, introducing metallic flakes, paper fragments, and cross-contaminated polymer types into the feed hopper. These insoluble particles block gate lands in multi-cavity moulds, causing short shots and catastrophic localized stress concentrations under load. Melt filtration systems capture coarse particles at the nozzle tip, but fine particulates pass through standard screens and compromise dielectric strength in electrical enclosures.
Regulated feed rates and optical sorting prior to granulation minimize foreign matter before compounding begins.
Process Variation
Viscosity fluctuations across a moulding run force operators to adjust injection pressure and holding times constantly to avoid flash or sink marks. Inconsistent melt density alters volumetric shrinkage inside the mould cavity, shifting final part dimensions outside acceptable tolerances. Pressure transducers monitor injection resistance to detect batch-to-batch variations in reclaimed material quality before parts reach final inspection.
Closed-loop machine controls adjust screw recovery speeds automatically to stabilize shot-to-shot weight despite shifting feedstock characteristics.
