Capillary Rheometry Limits in Detecting Third Pass Regrind Fractions below Ten Percent
Capillary rheometry fails below 10% third-pass regrind because high shear rates mask molecular weight drop beneath equipment noise floors.
Third Pass Regrind
Meaning
A specific material classification defines the state of a plastic resin that has undergone three discrete cycles of thermal processing and mechanical granulation. Third pass regrind describes a feedstock that retains historical thermal history from its prior lives in the melt stream, which alters its rheological response during injection moulding.
Thermal Degradation
Chain scission occurs within the molecular structure as chains break down under repeated exposure to heat and shear forces during each cycle. Molecular weight decreases as a consequence, shifting the melt flow index upward compared to virgin material. Reduced intrinsic viscosity often creates shorter polymer chains that lack the entanglement density required for structural applications.
Tensile strength drops predictably while brittleness increases in the finished part.
Economic Value
Procurement managers view this feedstock as a low cost alternative for non structural components where physical properties remain secondary to raw material savings. Market pricing fluctuates according to the scarcity of high quality reclaim and the specific polymer family involved. Processors balance the lower cost against the risk of reduced output speed or potential rejects.
Manufacturers typically limit the inclusion rate to maintain acceptable mechanical thresholds.
Processing Impact
Tooling engineers adjust cycle times to account for the faster flow rate characteristic of these modified resins. Cooling requirements fluctuate as the heat transfer profile changes across the cavity. Cavity pressure settings require careful calibration because the material exhibits higher sensitivity to temperature variances during the plastication phase.
Uniform mixing prevents pockets of inconsistent viscosity from creating soft spots in the molded parts. This material behavior dictates the specific limit of how many times a particular resin batch travels through the machine.