Liquid purification or separation – Filter – Movable medium
Reexamination Certificate
1999-09-20
2002-04-23
Upton, Christopher (Department: 1724)
Liquid purification or separation
Filter
Movable medium
C210S400000, C210S387000
Reexamination Certificate
active
06375013
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to apparatus for purifying viscous materials, in particular to rid a plastic melt of solid contaminants.
The European patent document A1 615 825 discloses apparatus allowing to separate solid contaminants, for instance metal particles or the like, from a viscous thermoplastic melt. The known apparatus comprises a tubular-cylindrical sifting device fitted with a plurality of narrow holes in its casing. The sifting device is seated in a support pipe containing a plurality of circumferentially arrayed axial ducts to evacuate the purified material and merging at the inside surface of the support pipe into circumferential channels open toward the apertures of the sifting device. The circumferential channels may be formed either into the sifting device or into the support pipe. To carry out purification, the plastic melt is forced by an extruder into the cavity of the filter and is evacuated in purified form from the outside of the filter through the ducts of the support pipe. A rotating scraper shaft coaxially mounted in the filter scrapes the filtered contaminants from the filter. To prevent clogging the filter apertures of the filter, the hole diameter may not drop below a minimum size, entailing the drawback of degrading the purity of the purified plastic.
It is further known from the German patent document C1 35 35 491 and the Swiss patent document 469,547 to press the plastic melt to be purified by an extruder through a narrow-mesh wire cloth filter band resting at the exit side of the purified plastic melt on a support fitted with a plurality of comparatively large passages. The sifting band retains the solid contaminants to be separated and is moved by a hydraulic conveyor or by hydrostatic forces exerted by the melt along the supporting surface of the support in order to move the solid contaminants out of the flow path of the plastic. In the known apparatus the support surface is planar and the sifting band also is moved along a plane. Overall, however, the area of the of the sifting belt cross the flow path of the plastic is comparatively small and the purification output of the apparatus will not meet high requirements.
The molten plastic to be purified is highly viscous and accordingly requires high feed pressure which the narrow-mesh, preferably metal-woven filter band, cannot withstand per se at substantially large surface. The ratio of support surfaces where the filter can be supported on one hand to aperture cross-sections of the passages issuing into the support surface on the other hand must be selected in relation to the particular strength of the filter band.
Conventional supports are fitted with a plurality of boreholes arranged in a grid and passing many times through the lamellar support. However supports of this design cannot meet the higher requirements of filter performance because excessively small boreholes tend to clog and excessively wide boreholes cannot adequately brace the filter band. Moreover the borehole size affects the mechanical strength of the support and limits its load-bearing area to comparatively small values if an excessively bulky and hence heavy design of the purifying apparatus must be avoided.
The Japanese patent document A 61-175 016 (Patent abstracts of Japan M-548, 24 Dec. 1986, vol. 10/#385) and also U.S. Pat. No. 4,080,297 disclose that the filter band is guided over part of the circumference of a circular-cylindrical, hollow support drum fitted at its circumference with a plurality of radial slits. The plastic melt to be purified is fed radially from the outside to the filter band and is evacuated in purified form from the inside of the support drum. The wrap angle of the filter belt may amount to 180° and more, and accordingly a comparatively large effective filter surface is possible. Similar filter systems are described in U.S. Pat. Nos. 3,645,399 and 3,856,674. However as regards purifying apparatus with a rotating support drum, not only is it necessary to provide sufficient sealing of the intake and outlet ducts of the filter band relative to the housing on account of the high feed pressure of the plastic melt, but furthermore the support drum must be sealed in the zone of these ducts. Wedge-shaped cutters are integrated into the housing in the vicinities of the feed side and the evacuation side of the filter band and penetrate at the feed side or at the discharge side between the filter band and the circumferential surface of the support drum. Because such cutters must be of a minimum thickness on account of the high prevailing pressures, the filter band is strongly deformed especially when passing from the support drum to the evacuation-side cutter, and thereby this filter band is susceptible to damage.
Compared with rotating support drums, the filter belt used with stationary supports is exposed to very high motional drag already stopping the filter band from being advanced at a comparatively small effective filtering area as long as it will be exposed to the high pressure of the plastic melt. The high pressure of the plastic melt forces the filter belt into the plurality of passages of the stationary support and the filter then snags on the support. In order to nevertheless move the filter band, the pressure of the plastic melt is lowered in conventional purification apparatus on the feed side. Illustratively it is known from the German patent document C1 35 35 491 to enlarge the volume of the feed chamber to further lower the pressure. The European patent document A2 0,275,462 discloses alternatingly feeding the plastic melt to be purified through a three-way valve to two purifying apparatus. The particular purifying apparatus not in the purifying mode therefore is unpressurized and its filter band thus can be moved. In spite of the decrease in feed pressure of the plastic melt, the motional drag of the filter belt remains relatively high as before, and on that account the effective filtering area must be kept comparatively small when using a stationary support.
SUMMARY OF THE INVENTION
The object of the invention is to create apparatus to purify viscous materials, in particular to rid a plastic melt of solid contaminants, such apparatus on one hand being compact and on the other hand allowing purifying the material to high purity and with comparatively high purification outputs.
The apparatus of the invention for purifying a viscous material, in particular to rid a plastic melt of solid contaminants, comprises:
a housing having a feed duct for the material to be purified and an evacuation duct for the purified material,
a filter band/belt, hereafter generally called merely “filter belt”, displaceable in its longitudinal direction, to separate the solid contaminants and passing through the material's flow path in sealed manner between the feed duct and the evacuation duct,
a stationary support structure in the flow path on the side of the filter belt facing the evacuation duct and comprising a support surface supporting the filter belt of which the contour follows the filter belt over a curved segment of the support surface wrapping the filter belt over a wrap angle large than 90°, said segment being substantially in the form of a cylindrical surface, in particular a circular cylindrical surface and being substantially planar in the support surfaces which adjoin tangentially and tightly the curved support surface, fitted with a plurality of mutually parallel channels running next to each other in the direction of displacement of the filter belt and open toward the support surface at least within the curved support surface, and fitted with a plurality of passage ducts starting from the channels to move the purified material to the evacuation duct,
a conveyor to advance the filter belt in continuous or step-wise manner.
The purification apparatus is very compact because of the curvature of the support structure, even when the effective area of the filter belt is comparatively large. Moreover such a support structure better withstands the high feed pressure of th
Arent Fox Kintner & Plotkin & Kahn, PLLC
Upton Christopher
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