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.
Power Law Fluid
Meaning
Non-Newtonian flow behavior characterizes materials where the shear stress relates to the shear rate through a consistent mathematical power index. A power law fluid exhibits a viscosity that changes according to the applied rate of deformation. This relationship remains valid within a specific range of shear rates before the polymer chains reach a limit of orientation or degradation.
Flow Dynamics
Shear thinning occurs when the index remains below unity, allowing the melt to travel through injection nozzles with reduced resistance. High shear rates at the gate induce molecular alignment that lowers the internal friction of the molten resin. Moulders monitor the consistency index to predict pressure drops across complex cavity geometries.
Proper calculation of this variable ensures that the clamping unit maintains sufficient force to prevent flash during the injection phase. Production shifts from virgin material to regrind often alter the consistency index due to chain scission or molecular weight distribution changes. Standard datasheet values provide a baseline for cold material properties, yet the actual performance inside a hot barrel depends on the specific shear history experienced by the melt.
Thermal Consistency
Temperature fluctuations modify the flow index significantly because the thermal energy changes the mobility of long chain polymers. Excessive heat reduces the index value until the resin loses structural integrity under standard injection pressures. Operators adjust the barrel zones to stabilize the flow index when ambient conditions change the cooling rate of the tool.
Inconsistent heat management leads to part weight variation and dimensions that drift outside the tolerance window defined for the product.
Economic Optimization
Part specifications define the geometric limits, while material specifications govern the rheological behavior required to fill the mould cavity without defects. Waste increases when the power law fluid parameters deviate from the established processing window because the machine cannot compensate for erratic viscosity changes. Profitability relies on keeping these rheological properties within a narrow range to minimize scrap rates during high volume production runs.
Precise control of the shear index minimizes the energy requirement for maintaining constant flow rates.