TECHNOLOGY | MELT FILTRATION
Right: Cut-away view showing distribution of polymer melt among FlexDisc filters and removal of contaminants
packs. As a result, there is two 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 version. The actual increase is dependent on machine size. Screen changers equipped with Nord-
son’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 greater filtration efficiency of the new FlexDisc reduces the frequency of backflushing and thus the amount of specific backflush volume. As melt enters the filter stack from the upstream
side, it is evenly distributed among the FlexDiscs, which are rheologically optimised to provide efficient filtration with minimal shear stress. At the same time, the structural integrity of the FlexDisc design enables it to withstand pressure differentials up to 130 bar (1,900 psi) during the backflush procedure. “The substantially increased filtration efficiency
of the FlexDisc can enable the processor or recycler to save on investment cost by purchasing a smaller machine without sacrificing throughput,” says Christian Schröder, Global Product Manager at Nordson. “There are also significant operational savings possible with the reduction of specific backflush volume.” New screen packs for BKG screen changers from
Nordson effectively withstand the pressures of extrusion, providing maximum filtration and optimum melt flow while ensuring a longer working life and greater reliability than low-end screen packs on the market. One key to the enhanced performance of the new screen packs is a robust multi-layer structure that prevents failures caused by the pressure differential – up to 200 bar – encountered during filtration. One such failure is “screen dimpling”, in which the mechanical stress forces screen layers into the
Above: BKG FlexDisc filter stack from Nordson
holes of the breaker plate that supports the screen in the cavity. This distorts the screen, breaking the peripheral seal and causing contaminants to bypass the screen and become part of the end product. Nordson says that it tailors the size of the screen
pack precisely to the dimensions of the cavity, eliminating the common problem 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-
26 PLASTICS RECYCLING WORLD | March/April 2019
tains structural integrity while efficiently filtering out contaminants and ensuring optimal melt perme- ability,” says Schroeder. “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.” Depending on customer requirements, Nordson supplies the screen packs with different layer combinations. One common structure, for example, has four layers, including two coarse square-weave layers that serve as the outer components and a finer internal square-weave layer. These provide support and drainage functions and withstand 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 permeabil- ity. Filtration takes place crosswise with respect to the direction of melt flow, enabling dirt particles to be separated from the melt efficiently. “A key factor for efficient filtration is permeabil-
ity,” says Stefan Woestmann, 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 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.”
CLICK ON THE LINKS FOR MORE INFORMATION: �
www.erema.com �
www.gneuss.com �
www.ettlinger.com �
www.fimic.it �
www.britas.de �
www.adgs.net �
www.nordsonpolymerprocessing.com
www.plasticsrecyclingworld.com
PHOTO: NORDSON
PHOTO: NORDSON
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