Liquid purification or separation – Filter – Supported – shaped or superimposed formed mediums
Patent
1994-07-18
1996-12-24
Walker, W. L.
Liquid purification or separation
Filter
Supported, shaped or superimposed formed mediums
210499, 298966, 2989662, B01D 3910
Patent
active
055870775
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a method of manufacturing a screen product, and to a screen product manufactured according to the method. The invention relates in particular to manufacturing a screen or a filter plate or a drum by a new machining method by which a product is produced the flow properties of which are remarkably better than the ones of prior art apparatus. The screen plate or cylinder of the invention is especially well applicable in the screens, filters, thickeners, washers, etc. used in the wood processing industry, but of course it can be applied also in sorting and thickening purposes in any other industry.
There are known several different prior art methods of manufacturing the screen products mentioned above. Some of the oldest and still used methods are drilling to produce round holes, punching to form apertures of the desired configuration, and milling to produce narrow slots. U.S. Pat. Nos. 239,837, 1,467,758, and 1,928,216 may be mentioned as examples of the above methods of manufacturing a screen plate. The mechanical machining methods mentioned above have for a relatively long time been the only applicable methods even though they have their drawbacks. Punching is usually out of the question when producing screen plates for the wood processing industry because the slot size required in the screens is so small that punching does not produce it without difficulty. Drilling can produce holes which are just about small enough. Drilling small holes is effectively limited by the tendency of the drills to break. Therefore, when a plate is manufactured by drilling, it is necessary either to mill or drill a larger opening in the plate into which opening the relatively small apertures produced by drilling open. This procedure reduces the tendency of the drills to break. However, the drawbacks of the increased number of work phases and the burrs or fins left in the edge of the aperture machined last, which is typical of mechanical machining methods, still remain. The fibers of the fibre suspension are easily caught by these burrs which gradually causes clogging of the screen. It is often very difficult to remove the burrs because they in most cases are located at the bottom of very narrow grooves or apertures the diameter of which is very small.
The same problems are met with also in screen products made by milling where the burrs remain at the bottom of the so-called back grooves. Because of the strength requirements of the product the back grooves cannot be made much wider than the slots extending through the plate. In most cases there is one screening slot per one back groove. This results also in that the open surface of the plate is limited mostly by the size of the neck surfaces required for the strength reasons between the back grooves both in the lateral and in the longitudinal direction of the grooves. Further, in the slots, more precisely in the side surfaces of the slots made by machining there are, of course, because of the machining method, small ridges almost parallel with the longitudinal direction of the slot which remarkably impair the flow through the plate. It is very difficult to eliminate this kind of factors affecting the quality of the surface and the capacity of the screen. This is, however, tried by finishing the plate after the machining, the purpose of the finishing being to remove a thin surface layer from all over the plate, both from the surface of the plate and from the apertures. However, it has been proved that this kind of finishing is not adequate to remove the burrs, but the screen plate industry all over the world is continuously looking for new methods of removing the burrs from the bottom of the narrow grooves. Further, the finishing unavoidably results in an increase in the size of the apertures in the plate which cannot always be taken into account in advance when the plate is originally being machined. The sizes of the apertures in the screen and sieve plates usable in the pulp industry are the smallest and
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Aaltonen Frank
Alajaaski Timo
CAE Investments B.V.
Walker W. L.
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