Melt filter for purifying plastic melts

A melt filter for cleaning a plastic melt issuing from an extruder has a wheel rotatable about an axis and having an outer rim and a plurality of spokes forming an annular array of axially open spaces, a pair of housing plates axially sandwiching and completely covering the wheel and forming offset from the axis a melt passage extending axially through the wheel at the spaces, and removable filter elements braced axially against the wheel at the spaces between the spokes and angularly movable with the wheel with the melt flowing through the filter elements when same are aligned with the passage. One of the plates being formed with an edge cutout. A part on the one housing plate that can cover and close the cutout during normal operation of the melt filter can open and uncover the cutout for changing the filter element aligned axially with the cutout.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US national stage of PCT application PCT/EP2004/014093, filed 10 Dec. 2004, published 23 Jun. 2005 as WO2005/056273, and claiming the priority of German patent application 10358672.5 itself filed 12 Dec. 2003, whose entire disclosures are herewith incorporated by reference.

FIELD OF THE INVENTION

The invention concerns a melt filter for purifying especially plastic melts discharged by extruders, with a filter disk installed between two plates that form a housing equipped with a filter-element-changing station, that can be rotationally driven by a power-driven ratchet drive, and that has spaces separated by spokes and arranged in a circularly annular array for holding exchangeable filter elements supported by perforated disks against the forces that arise due to the pressure drop that occurs in them in the direction of flow of the plastic melt, and with a melt passage that passes through the plates in the area of the circular array, feeds the melt to the filter elements, and widens toward the filter elements.

Prior-art melt filters are described, for example, in EP 0 114 651. However, the previously known melt filter has a very large and thus expensive filter disk, whose filter-element-changing station is also very large, but the surface of the filter disk against which the melt flows is very small, so that extremely poor efficiency results. EP 0 569 866 (U.S. Pat. No. 5,336,223) has already proposed that the melt passage be widened toward the filter elements, so that the melt to be filtered can flow against two filter elements at the same time, but here again, there is no flow against large areas of the filter elements, so that similarly poor efficiency is obtained.

In addition, the teeth on the filter disk have a relatively large pitch, so that, when the filter disk is rotationally stepped, large surfaces of the dirty filter elements are exchanged for correspondingly large surfaces of clean filter elements. This results in pressure differences in the cleaned melt which are unacceptable during further processing of the melt, so that additional pumps are often needed to guarantee constant pressure of the cleaned melt.

It has already been proposed that gear drives that produce smaller steps be used instead of the sturdy, inexpensive ratchet drive in order to exchange only small filter disk areas with dirty filter elements for filter disk areas with clean filters at any given time and thus to ensure constant pressure However, expensive rotational drives of this type constitute an immense cost factor.

DE 42 12 928 (U.S. Pat. No. 5,449,458) has already disclosed a large-surface cover for a filter disk, but large areas of the filter disk are still exposed to ambient air when the filter disk is rotated, so that undesired changes in the plastic can occur.

OBJECT OF THE INVENTION

The object of the invention is to provide a melt filter that is as small as possible, with which hardly any changes occur in the plastic during operation, which guarantees filter element exchange at more or less constant pressure, even at high pressures, and which is nevertheless inexpensive to manufacture.

SUMMARY OF THE INVENTION

To this end, it is proposed that the plates completely cover the filter disk, with at least one of the plates being cut away at the filter-element-changing station, and that the filter-element-changing station be designed larger than one filter element and smaller than or the same size as two filter elements. This results in a housing that encloses, if possible, the whole filter disk and is better able to withstand the high pressures that are required.

It has been found to be effective for at least one of the plates to have a reversibly movable region that covers the filter-element-changing station, for the filter disk to be completely covered during the operation of the filter and closed snugly toward the filter disk, and for the filter-element-changing station to be uncovered for the filter element change to be carried out during the operation of the filter. On the one hand, the complete covering of the filter disk makes it possible to employ higher pressures, and, on the other hand, it is guaranteed that during the operation no plastic melt adhering to the filter disk comes into contact with the ambient air.

It is advantageous that the angular spacing between filter elements against which the melt is flowing and the filter-element-changing station be larger than or the same size as the width of one filter element and a spoke, and smaller than the width of two filter elements and a spoke. This guarantees that melt flows against the largest possible surface area of the filter disk with the smallest possible filter changing station without it being possible for melt to be pressed out of the filter-element-changing station.

Due to these optimum relationships between the size of the filter-element-changing station and the surface of the filter disk against which melt is flowing, the filter disk can be made more compact than the prior-art filter disks and yet make a larger effective filter surface available.

It is advantageous if the ratio of the spoke area against which the melt is flowing to the area of the filter disk through which melt is flowing is less than 18% and greater than 12%. This guarantees that the spokes are provided with dimensions that still enable them to withstand the high pressures but oppose the melt to be filtered with the least possible surface areas that cannot be used for filtration, so that the greatest possible filter surface area can be effectively realized. In this connection, it has been found to be effective if the ratio of the spoke area against which the melt flows, to the area through which the melt flows, is 15±1%.

To be able to guarantee constant pressure during the filter change, it is advantageous that, for each stroke of the ratchet drive, a maximum of 10% of the area of the filter disk against which the plastic melt flows can be exchanged for corresponding filter disk areas with unused filter elements. In this connection, it has been found to be effective if 6±1% of the filter area is exchanged per stroke of the ratchet drive. The exchange of a maximum of 10% of the area of the filter disk, i.e., of the filter elements and the spokes, against which the plastic melt flows guarantees that approximately constant pressure is present in the filtered melt, and this allows trouble-free further processing of the melt in the downstream machines.

SPECIFIC DESCRIPTION

The drawing shows a melt filter1that consists of a housing2, a filter disk3, and a ratchet drive4. The housing2is formed by a plate5is connected by fastening devices6with another plate5′, whose outline is indicated by a broken line. The plates5,5′ enclose the filter disk3between them and the filter disk3is braced against a perforated support disk15. A filter-element-changing station7having a cover14is indicated in the plate5. It is essentially the same size as a single filter element8. In addition, the plate5has a melt passage9that widens toward the filter disk3in the form of an annular segment10.

In addition to the filter elements8, the filter disk3has spokes11. Teeth12, which interact with the ratchet drive4, are provided on the periphery of the filter disk3. The spokes11are connected to a wheel rim13.

Due to the fact that the filter-element-changing station7is selected to be as small as possible, most of the filter disk3is enclosed by the plates5. This makes it possible to work with the largest possible pressure in the melt passage without the filter disk3jamming in the housing2. The spokes11and the peripheral wheel rim13of the filter disk3are supported on the plates5,5′ and seal the melt passage9and the annular segment10toward the outside.

The annular segment10spans the spokes11′ to11″″′ and the filter elements8′ to8″″″. In this regard, as a result of the relationships, in accordance with the invention, between the size of the surface against which the melt is flowing, the distance to the filter-element-changing station, and the size of the filter-element-changing station, an optimum condition is achieved, so that even with a small filter disk3at high pressures, the melt can be optimally filtered with a good constant pressure.