Meaning
Physical principles governing the rate of flow and the efficiency of particle removal as a polymer melt passes through a porous screen or metal mesh. Mathematical models for melt filtration kinetics define the relationship between the pressure drop across the filter, the viscosity of the resin, the temperature of the melt and the accumulation of contaminants.
Pressure Accumulation
Continuous deposition of degraded polymer or inorganic debris on the screen surface reduces the available flow area over time. These melt filtration kinetics dictate that as the cake layer thickens, the extruder must exert higher force to maintain a constant output.
Sieving Efficiency
High velocity through the mesh can force soft gel particles to deform and pass through the openings. The dynamics of melt filtration kinetics suggest that maintaining a controlled temperature prevents the sudden thinning of the resin which would otherwise allow impurities to bypass the barrier.
Filter Life
Monitoring the rate of pressure rise allows the operator to schedule screen changes before the mechanical limit of the breaker plate is reached. When the flow enters a non-linear decay phase, the risk of rupture or bypass increases and the quality of the extrudate begins to vary. This monitoring involves tracking the differential pressure across the screen pack and correlating it with the total throughput of the machine.
Frequent changes of the filtration media are necessary when processing lower grades of regrind that contain high levels of particulate matter.
