Meaning
Solid contaminant build-up on the wire mesh filters of an extruder restricts the flow of molten polymer and increases barrel pressure. Uncontrolled screen pack clogging disrupts the stability of the extrusion process, leading to variation in output rate and melt temperature.
Pressure Rise
As contaminants accumulate on the screen pack, the effective open area of the filter decreases. This restriction forces the extruder motor to work harder to maintain the desired output, causing a steady rise in head pressure. When the pressure exceeds a pre-set safety limit, the extruder must be stopped to change the screens, or an automatic screen changer must be triggered to index a fresh filter.
Excessive pressure can also lead to polymer leakage or rupture of the rupture disk. Monitoring this pressure differential across the filter is the standard method for determining when to change the screen.
Contaminant Source
The debris that causes this restriction includes unmelted resin gel, carbonized polymer, metal fragments, and foreign material introduced with regrind. High loadings of recycled content or mineral fillers accelerate the rate of accumulation. In recycled plastic processing, paper fibers and aluminum foil represent the primary causes of rapid filter blockage.
Operational Control
Using multi-stage screen packs with varying mesh sizes can distribute the contaminant load and extend the running time between screen changes. In high-volume recycling lines, continuous back-flush screen changers automatically clean the filters using a small portion of the melt stream, minimizing downtime.
