Self-cleaning screen changer reduces need for frequent filter changes

A SELF-CLEANING screen changer is making the difference in significantly reducing time and effort in processing postconsumer plastics and boosting recycling output. A European company specializing in polyethylene recycling has found this approach eliminates the need for frequent changing of filter screens to ease the burden on the melt filtration equipment in recycling lines caused by contaminants in plastics.

The recycling company replaced its two 300mm slide-plate screen changers with 200mm “V-Type” screen changers from Nordson Kreyenborg. These were installed in two recycling lines that transform postconsumer polyethylene into pellets used in the blown-film manufacture of trash bags.

The automatic, self-cleaning system from Nordson Kreyenborg requires a screen change of only once daily, a significant departure from the conventional method that needed screen changes every eight minutes. The secret is in the “power backflush” technology, which automatically initiates a backflush sequence when the differential pressure across the screen changer reaches a preset level due to contaminant buildup. In this automatic backflush sequence, hydraulic pistons compress some of the already filtered molten polymer and discharge it reversely back through the screen, carrying away contaminants and removing them from the system.

The V-Type screen changer removes even heavy contaminant from the melt filter element without compromising extrusion throughput. It minimizes operator intervention and avoids downtime, thereby contributing to increased efficiency.

In the traditional system, the polymer flows through four cavities. Three cavities continue working while one is being changed. In the V-type screen changer, the melt flow from the extruder separates into four streams for filtration in two pairs of screen cavities. The streams rejoin without any significant changes in the melt flow. Each pair of cavities is mounted in a piston, positioned in such a way to filter their respective melt streams or removes one of them from the process to remove contaminant buildup by means of backflushing.

Once a preset number of backflushes has been reached, the process for changing screens is initiated. This involves triggering the outward movement of the screen-bearing piston to allow removal of the screen pack and the installation of a new filter element. Three of the cavities remain in the process while a new screen pack is installed in the fourth.