Method Of Manufacturing Filter Elements

Abstract

A method of manufacturing a slotted filter element comprising the steps of forming opposite sides of the filter elements by respective electrolysis operations carried out to different degrees so that the slots in cross-section are more flared towards one side than the other, and swaging the filter element to give the slot cross-section the form of a single funnel.

Description

FIELD OF THE INVENTION

This invention relates to filter strips containing rows of slots of general oblong or rectangular form, from which strips filters of generally frusto-conical or cylindrical form may be made.

BACKGROUND OF THE INVENTION

Such filter strips may be made by punching a metallic strip or formed directly by electrodeposition, the rows of slots then being subjected to a swaging (i.e. pressure) operation with the help of a swaging tool so as to reduce the depth and width of the slots while forming, between said slots, linear projections on at least one of the faces of the strip according to applicant's prior U.S. Pat. No. 3,520,418.

To obtain well calibrated slots which, during a filtering or draining operation, permit solid particles of predetermined sizes to pass through the filter without these particles blocking said slots, it is desirable for the cross-section of the slots to include a short portion having substantially parallel walls which then diverge in the direction of flow to form a wide opening.

This configuration, having a funnel-shaped cross-section, is obtained prior to swaging, either during the punching operation by using punches having the desired cross-section, or after punching, by any appropriate means such as sand-blasting.

SUMMARY OF THE INVENTION

The present invention is more particularly concerned with the use of a swaging operation for reducing the width of the slots in the case of filter plates having parallel rows of slots and made by an electrolytic method, particularly such a method employing nickel. This method comprises coating a zinc plate with a light-sensitive electrically insulating coating, e.g. a varnish, in forming on the layer of varnish a negative image of the slotted plate that it is desired to obtain, the zones exposed to the light becoming insoluble, and in removing the unaffected zones of the coating of varnish by swilling with water. The face of the zinc plate, that then has insulating zones occupying the positions at which it is desired to form the slots of the filter, is then covered with a coating of electrically conducting material (e.g. a varnish) and then covered with nickel by electrolysis in a nickel-salt bath in which the plate is immersed, the zinc plate forming the cathode and the nickel being deposited over the entire surface except for the zones covered by the insulating varnish and corresponding to the slots. The slotted nickel foil thus obtained is then detached from the layer of electrically conducting varnish. The nickel foil thus obtained has, between the slots, a curved cross-section of general semi-circular form (as can be seen in FIG. 1 above line X--X), so that the slots have the required divergent form.

However, if it is required to obtain by this process a foil of adequate thickness and mechanical strength, prolongation of the operation of electrolysis, while increasing the thickness of the nickel deposit between the slots, also causes nickel to be deposited on the edges of the slots, this occurring in an irregular fashion so that the dimensional tolerance on the width of the slots obtained is too great for many applications, some of which require a tolerance not exceeding 0.01 mm; it is clear that a subsequent swaging operation, though enabling the mechanical properties of the plate to be improved, cannot eliminate the lack of uniformity in the original slots.

It is possible to increase the thickness of the nickel foil by a second operation of electrolysis by depositing nickel on its second planar face, that was attached to the electrically conducting varnish during the first operation of electrolysis, the nickel foil itself constituting the cathode in this second operation. However, the cross-section of the slots thereby acquires the general form of a cotton reel or a double funnel because of the curved form of the nickel deposit between the slots, and the swaging operation on the rows of slots does not then make it possible to obtain slots having a cross-section in the form of a short inlet portion with substantially parallel walls on one side of the strip which then diverges abruptly towards the outlet in the direction of flow in the form of a single funnel on the other side of the strip.

The present invention enables these drawbacks to be substantially obviated and is mainly characterized in that the filter strip is obtained by a first complete operation of electrolysis which, after the strip has been turned round, is followed by a second partial operation of electrolysis or vice versa, so that the cross-sections between the slots provide a general semi-circular form on the first side and a similar but much flattened form on the second side, these operations of electrolysis being followed by swaging of the rows of slots so as substantially to impart to them the required single-funnel form and to form linear projections on at least one of the faces of the strips between the rows of slots.

In a modified form, this swaging operation involves the use of a backing plate or die having at least one projecting portion between the slots in the strip, so as to facilitate the formation of a substantially parallel-walled section having relatively sharp edges preceding the divergent portion of each slot.

This modified form employing a backing plate or die having projecting portions can be used also when the perforated strips are formed by punching.

In these modified forms, the projecting portions on the backing plate or die form burrs at the marginal portions of the slots, which burrs can, if not required, for a particular application, be removed for example by buffing.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention reference will now be made to the accompanying drawings, provided merely by way of example, in which:

FIG. 1 is a cross-section on a very large scale through a filter strip obtained by electrolysis in accordance with the invention but prior to swaging,

FIG. 2 is a view corresponding to the FIG. 1, but after swaging,

FIG. 3 is a plan view of the strip shown in FIG. 2,

FIG. 4 is a cross-section on the line IV-IV of FIG. 3, and

FIG. 5 corresponds to FIG. 3, but shows the result of using a swaging arrangement employing a backing plate or die having projecting portions.

DESCRIPTION OF SPECIFIC EMBODIMENT

The filter strip 1, shown in FIG. 1, is produced by electrolysis in two successive operations. A first complete operation of electrolysis as described above enables the portion of the strip located above the line X--X to be obtained. After the strip has been turned round, a second electrolysis operation is carried out as already described, but in accordance with the invention this is arrested before its final stage, which enables the strip to be completed by addition of the portion below the line X-X.

It will be observed, that on either side of the line X--X in FIG. 1, the slots 2 have enlarged shapes 2A and 2B, but the portion 2B, located below the line X-X and corresponding to the incomplete operation of electrolysis, is much less accentuated. The two different degrees of electrolytic deposition can be carried out in the opposite order.

When swaging is carried out, this smaller widened portion 2B is practically eliminated so that it has substantially parallel walls as seen in particular in FIG. 2. The swaging operation, in which a swaging tool that is slightly wider than the length of the slots 2 is employed, enables projecting portions 3 to be formed between the rows of slots 2, these projecting portions strengthening the filter strips while permitting the entirety of the slots to be employed. Furthermore, this swaging operation on the filter strips obtained by electrolysis improves the quality of the metal, the granular structure of which is converted into a lamellar structure, which imparts greater rigidity to the strip while improving its elastic properties and its wear-resistance because of the work-hardening of the metal which imparts greater hardness to the surfaces.

To facilitate deformation and flow of the metal in the zone 2B, so as to provide the short substantially parallel-walled portion of the slot, a backing plate or die employed in the swaging operation can have slight projections located between the slots, which projections then impart to the filter strip 1 the form illustrated in FIG. 5 in which these projecting portions form slightly concave areas 4 with raised portions 5 at the slot edges. This modified form of the swaging operation can also be employed on filter strips obtained by punching as previously indicated.

When the filter is in use, these slightly concave areas 4 between the raised portions or burrs 5 permit a layer of the treated product to accumulate and this inhibits wear on the filter. In certain cases, and depending upon the nature of the products treated, it is desirable to prevent this accumulation; then, the burrs 5 can simply be removed, by a buffing operation for example.

The present invention also comprises filter plates or strips obtained by the above-described method, as well as cylindrical or frusto-conical filters particularly for centrifugal draining systems formed by assembling and bending said plates or strips.

Claims

I claim:

1. A method of producing a filter element, said filter element having a first and a second side and a plurality of parallel rows of slots therein, each slot extending through the filter element from the first side to the second side, the method comprising the steps of forming a first slotted portion of the filter element at the first side thereof by a first electrolysis operation, adding a second correspondingly slotted portion to said first portion forming a second side of the filter element by means of a second electrolysis operation, said first and second electrolysis operations being carried out to different degrees to produce on said first side slots separated from one another by parts having in cross-section a substantially semicircular convex form, and on said second side slots separated by parts having in cross-section a similar convex but considerably flattened form, and swaging the partly formed filter element so that the slots acquire a cross-section in the shape of substantially a single funnel diverging from said second side towards said first side and linear projecting portions are formed at least on the first side of the element between rows of slots.

2. A method of producing filter elements as claimed in claim 1, wherein the swaging operation involves the use of a backing member having projections located between the slots to facilitate the formation of a substantially parallel-walled section of each slot having relatively sharp edges at said second side of the filter element.

3. A method of producing filter elements as claimed in claim 2, wherein the projections on the backing member form burrs at the marginal portions of the slots on said second side.