Regrind Fractions Nobody Writes into the Grade Specification
Undeclared regrind degrades melt stability and impact strength; verify batch history using melt flow rate delta and oxidative induction time limits.
Gravimetric Feeder Segregation
Meaning
Demixing of composite resin components during mechanical feeding results in uneven material distribution before the polymer enters the extruder. When gravimetric feeder segregation occurs, ingredients of different sizes, shapes, or densities separate within the hopper. It leads to continuous drifts in the formulation of the melt, producing parts with inconsistent properties.
Molders encounter defects like color variation or unstable mechanical performance when feed ratios fluctuate. This phenomenon occurs during the solid handling phase and is not a melt-state issue.
Separation Mechanism
Vibration and gravity cause smaller or heavier particles to settle through the gaps between larger pellets. In a blend of virgin resin and fine regrind, the fines migrate to the bottom of the hopper. This creates a high concentration of regrind at the start of the run and a depletion later on.
The physical geometry of the feed hopper influences this flow pattern.
Processing Trouble
Inconsistent feed ratios alter the viscosity of the polymer melt in the barrel. When the feeder delivers a batch rich in additives or fillers, the pressure profile shifts, causing dimensional variations in the molded parts. This variation can lead to part rejection during quality control.
It simplifies process stabilization.
Preventative Measure
Redesigning the hopper and minimizing feeder vibration prevent particle separation. Continuous mass-flow hoppers promote uniform discharge where all particles move downward at the same rate. This ensures a consistent blend reaches the screw.
Some systems use separate feeders for each ingredient.