MACHINERY | MELT FILTRATION
life and greater reliability. The company says one key factor in the enhanced performance of the new screen packs is a robust multi-layer structure that prevents failure caused by the pressure differential – up to 200 bar – encoun-
tered during filtration. One typical pressure differen-
tial failure is ‘screen dimpling’, where the mechanical stress can force the
screen layers into the holes of the breaker
Above: The RSFgenius from Gneuss is a fully automated rotary screenchanger with backflush designed for demanding processing tasks
plate that supports the screen in the cavity. This distorts the screen, breaking the peripheral seal and allowing contaminants to bypass the screen and become part of the end product. Nordson says it precisely tailors the size of the
Right: This cut-away view of the BKG FlexDisc filter stack shows how the melt is distributed among the different FlexDisc filters
screen pack to the dimensions of the cavity, eliminating the risk of contaminant passing the screen pack at the edges. In addition, all screen pack components are ultrasonically pre-cleaned during manufacture to eliminate contamination from the oil and grease used in the weaving process. “The new Nordson screen pack design main- tains structural integrity while efficiently filtering out contaminants and ensuring optimal melt permeability,” says Christian Schröder, Global Product Manager for melt delivery systems. “As a result, our screen pack yields more output between screen changes than screen packs offered as low-cost alternatives, increasing the productivity of the extrusion line and enhancing profitability.” Nordson supplies the screen packs with differ- ent layer combinations optimised for specific customer applications. One common structure, for example, incorporates four layers, including two coarse square-weave layers that serve as the outer components and a finer internal square-weave layer. This combination provides support and drainage functions and withstands high tensile forces. The fourth layer is an optimised Dutch- weave wire mesh that serves as the fine filtration component. The small geometric pore size of the Dutch weave layer ensures high permeability. Filtration takes place crosswise with respect to the direction of melt flow, enabling dirt particles to be very efficiently separated from the melt. “A key factor for efficient filtration is
permeability,” says Stefan Woestmann, Nordson Application Engineer. “The Nordson screen pack provides the optimum ratio of solid material and free volume, so that the polymer melt can use as many flow paths as possible to make its
28 COMPOUNDING WORLD | July 2019
www.compoundingworld.com
way through the filtration medium. In processes with low-end screens, which are not flow-optimised in this way, polymer melt builds up and causes a significant increase in pressure. This leads to premature and costly screen changes and endangers the quality of the end product. If the polymer flow is not uniform, the end product cannot be either.”
Production critical Nordson has also introduced the BKG FlexDisc design for producers of critical films and PET recyclers. The company says the FlexDisc filter for piston-activated screen changers substantially enlarges available filtration area without the need to increase machine size, enabling processors and recyclers to achieve finer filtration, higher throughput, longer filter service life, and reduced specific backflush volume. Nordson recommends the BKG FlexDisc for bottle-to-bottle PET recycling, PET fibre recycling, and battery separator film applications. The function of the hydraulic pistons, which
many BKG screen changers are equipped with, is to insert the screen cavities into the melt stream for filtration and to remove them for cleaning or replacement. In systems where the new FlexDiscs are used, each cavity contains a filter stack comprised of two to four FlexDiscs, depending on machine size, and each FlexDisc is equipped with two screen packs. As a result, there is up to 4.5 times more filtration area available for each cavity with the new FlexDisc than with conventional standard round screens and 25% additional area (on average) when compared to the former FlexDisc model (actual increase is dependent on machine size). Screen changers equipped with Nordson’s backflush technology include the BKG V-Type 3G and BKG HiCon K-SWE-4K-75/RS (backflushing diverts the contaminant from the melt stream). The
PHOTO: NORDSON POLYMER PROCESSING SYSTEMS
PHOTO: GNEUSS
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