BACKGROUND Screening apparatus of the type with which the invention is concerned is generally used for screening, grading or separating materials such as ores or other particulate or granular materials and comprises an array of screen panels removably fixed to a frame to provide a continuous screen deck. This deck is vibrated to move bulk materials over and through its screening surface. Each screen panel of the array is commonly formed in one piece by moulding a resilient plastics material and is typically flat and square or rectangular in plan view and includes an upper screening surface from which apertures extend through the panel for allowing passage of bulk material as determined by the sizing of the apertures.

For a screen deck frame to provide an efficient and accurate grading of bulk materials, it is necessary that the apertures of each screen panel be accurately sized and not become clogged. In order to meet these objectives it is known to provide continuous grooves across the screening surface of a panel in the direction of travel of the bulk material such that that material "tracks"across the screening surface over and along the aperture openings. It is also known to provide the apertures with an increasing cross-sectional area through the depth of a panel in the direction in which the material passes through the screen. However, in screen panels which include both continuous tracking grooves and apertures of increasing cross-section, the cross-sectional area of the apertures is expanded from the base of the grooves, i. e. from a

distance (which equates to the depth of the grooves) below the upper screening surface. The sides of the grooves which define this distance are parallel. In forming these panels by moulding, one die contains ribs for providing the parallel sided grooves and another die contains lugs for providing the apertures, and end surfaces of these ribs and lugs close against each other for moulding. However, if any flashing is formed because of inaccuracies in the dies at the locations where they are closed against each other, that flashing will be located at the entrances to the apertures below the screening surface, which makes it very difficult to remove. This flashing formation problem can add considerably to the cost of manufacture of the screen panels due to both the cost of removing the flashing and the high costs that are incurred for maintaining the moulding apparatus in excellent condition in an attempt to avoid the flashing being formed.

DISCLOSURE OF THE INVENTION The present invention provides a moulded screen panel structure, dies for moulding the panel and a method of manufacturing the panel in which the above-mentioned cost burdens associated with the formation of any flashing are reduced.

According to a first aspect of the invention, there is provided a moulded screen panel including a series of equally spaced parallel ribs having upper surfaces which define an upper screening surface of the panel and side surfaces extending to bottom surfaces of the ribs to define a depth for the ribs, a plurality of cross ribs extending substantially orthogonally of the series of parallel ribs and having upper surfaces located below the upper screening surface, wherein the series of parallel ribs and the plurality of cross ribs define screening apertures which pass through the panel, wherein the side surfaces of each of the series of parallel ribs taper towards each other towards said bottom surfaces such that the cross-sectional area of each aperture increases from the upper screening surface over the depth of the ribs, wherein the location of the upper surfaces of the cross ribs relative to the upper screening

surface and the spacing of the series of equally spaced parallel ribs provide continuous grooves extending across the panel.

The invention is described herein with reference to a screen panel in a normal use orientation wherein it is incorporated in an horizontal extending screen deck, and terms such as"above","downward","upper","lower"and "bottom"should be construed in the light of this orientation. However it is to be understood that other orientations may be equally possible and that consequential changes in terms such as those above may be required in the light of those orientations.

The screen panel is formed such that the sides of each continuous groove immediately over each cross rib are parallel. Thus each continuous groove is defined by a pair of opposing sides that taper away from each other, except for those portions over the width of a cross rib, which portions are substantially parallel and extend downwardly from the upper screening surface to the upper surface of a cross rib.

Preferably the panel includes strengthening ribs, some of which are parallel to said equally spaced parallel ribs and at least one other of which is parallel to the cross ribs, wherein the strengthening ribs have upper surfaces that are co-planar with the screening surface of the panel. Said at least one other strengthening rib may be a centrally located rib which extends for the length of the panel. Preferably the strengthening ribs that extend parallel to said equally spaced parallel ribs are offset from each other on either side of said at least one other strengthening rib to thereby divide the screening surface into panel segments. Preferably the continuous grooves are continuous over at least one panel segment. Thus the continuous grooves extending over one panel segment and which meet an offset strengthening rib do not continue in that rib; that is, the strengthening ribs that are parallel to said equally spaced parallel ribs do not contain continuous grooves. This segmentation of the screening surface of a screen panel using non-grooved offset strengthening ribs ensures that the bulk material being screened cannot travel over a screen deck along any non-grooved portions.

According to a second aspect of the invention, there is provided a die set for manufacturing a moulded screen panel according to the first aspect of the invention, wherein the die set comprises two dies, a first one of which includes parallel rows of upstanding tapered lugs for defining each aperture in the panel and wherein each lug has an end surface corresponding to the entrance opening of an aperture, the second die including a flat surface against which the end surfaces of the lugs of the first die close during a moulding operation, wherein the flat surface defines the screening surface of the panel and includes rows of upstanding parallel-sided lugs each of which has an end surface and each of which seats between two of the tapered lugs of the first die during a moulding operation, wherein the end surfaces of the parallel-sided lugs correspond to the upper surfaces of the plurality of cross ribs to thereby locate the upper surfaces of the cross ribs below the upper screening surface, wherein any flashing formed between the dies during a moulding operation and which may block or restrict the apertures is located at the upper surface of the panel and thereby easily removable.

This second aspect of the invention includes the provision, individually, of one or the other of the two dies of the die set.

According to a third aspect of the invention there is provided a method of manufacturing a screen panel according to the first aspect of the invention, wherein the panel is moulded from a resilient plastics material using a die set according to the second aspect of the invention, wherein after removal of a screen panel from the die set, the upper screening surface of the panel is treated to remove any flashing at that surface.

The treatment accorded the screening surface of the panel may comprise grinding or buffing or any other suitable method for removing flashing.

For a better understanding of the invention in all its aspects and to show how it may be put into effect, an embodiment thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES Fig. 1 is a top plan view showing a portion of a moulded screen panel according to an embodiment of the invention Fig. 2 is a view on section line II in Fig. 1 Fig. 3 is a view on section line III in Fig. 1 Fig. 4 is a view on section line IV in Fig. 1 Fig. 5 is a sketch of a perspective view of portion of a screen panel for illustrating some features of the invention.

Fig. 6 is a section view of portion of an embodiment of a die set according to the second embodiment of the invention, the dies being shown in a closed position, and Fig. 7 is a view similar to Fig. 6 but showing the dies in a separated position after a moulding operation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT A screen panel 10 as shown in Fig. 1 is rectangular in plan shape and is cast or moulded (in this specification including the claims, the terms"moulding" or"moulded"are to be understood as encompassing"casting"or"cast") from a plastics material, such as polyurethane. It includes an outer frame made up of opposed longitudinal sides 12 and opposed ends 14 (only one of which is shown) which may include one or more reinforcing elements such as metal rods (not shown). The bottom surface of sides 12 and ends 14 provides a bearing surface 16 that bears on support beams of a screen deck frame when the panel is mounted in such a frame with a plurality of other like panels to provide a screen deck. Ends 14 include connection elements in the form of ribs 18 for fixing the panel in a screen deck. There are various known ways for fixing a plurality of screen panels to a screen deck frame to form a screening deck and thus such fixation is not described herein. Reference may be made

to the applicant's Australian patent application No. 34352/95 if further description of an example fixation system is required.

Screen panel 10 has an upper screening surface 20 for receiving ore or other particulate or granular material to be screened by means of apertures 22 which extend through the panel 10. The apertures 22 are defined by a series of equally spaced parallel ribs 24 extending between longitudinal sides 12 and a plurality of cross ribs 26 extending substantially orthogonally to ribs 24 between opposed ends 14 (note that for clarity only the left hand side of Fig. 1 illustrates the ribs 24. The ribs 24, in fact, extend across the panel along its full length between ends 14).

The equally spaced parallel ribs 24 each have an upper surface 28, which surfaces define the upper screening surface 20, and side surfaces 30 extending to a bottom surface 32. The side surfaces 30 taper towards each other towards bottom surface 32. The taper may be, for example, about 4°.

The cross ribs 26 each have upper surfaces 34 which extend between each of the parallel ribs 24 and which are located below the upper screening surface 20. This location of the surfaces 34 of cross ribs 26 relative to parallel ribs 24 effectively provide continuous grooves 36 extending across the panel 10. That is, the spacing between two adjacent ribs 24 effectively acts as a continuous groove 36 given that the upper surfaces 34 of the cross ribs 26 are located below the upper screening surface 20. As best seen in Fig. 5 (which shows portion of two parallel ribs 24 and two cross ribs 26 defining an aperture 22), the sides of a groove 36 as defined by the sloping side surfaces 30 of adjacent parallel ribs taper away from each other except for those portions, referenced 38, over the width of a cross rib 26. These portions 38 of the side walls are parallel and each portion 38 extends from the upper screening surface 20 to an upper surface 34 of a cross rib 26.

Panel 10 also includes a central longitudinal strengthening rib 40 extending between opposed ends 14 and strengthening ribs 42, parallel to the equally spaced parallel ribs 24, extending between the opposed longitudinal sides 12 and central rib 40. As shown in fig. 1, the strengthening ribs 42 are

off-set from each other on either side of the central strengthening rib 40. The strengthening ribs 40 and 42 have a substantial width and depth compared to the parallel ribs 24 and cross ribs 26 and their upper surfaces are co-planar with the screening surface 20. The strengthening ribs 40 and 42 divide the screening surface 20 into panel segments and the continuous grooves 36 are continuous over at least one such panel segment. Thus the continuous grooves 36 that extend over one panel segment and which meet an offset strengthening rib 42 do not continue in that rib. This means that the strengthening ribs 42 do not contain any of the continuous grooves 36. This structure ensures that none of the material being screened can travel across the panels in a screen deck aiong any of the ribs 42 without eventually encountering a groove 36 and thus some of the apertures 22.

With reference to Fig. 5, it will be appreciated that each aperture 22 is defined by the oppositely sloping walls 30 of adjacent ribs 24 and has a length determined by the spacing of the facing side surfaces 44 of adjacent cross ribs 26. As shown in Fig. 4, the side surfaces 44 of each cross rib may taper towards each other similarly to the side walls 30 of ribs 24. Thus the cross- section area of each aperture increases from the upper screening surface 20 over the whole depth of each rib 24 to minimise clogging of the apertures by the material being screened. It is to be understood that according to the invention apertures having a shape other than that which is illustrated may be provided.

A die set for moulding a panel 10 as shown in Figs. 6 and 7 comprises two dies 50,52, a first one of which, 50, includes upstanding lugs 54 that have the same shape as an aperture 22. These lugs are arranged in rows and columns wherein the spacing between adjacent rows defines. the series of equally spaced parallel ribs 24 and the spacing between adjacent columns of the lugs defines the plurality of cross ribs 26, with appropriate spacing being provided to define the strengthening ribs 40 and 42. The second die 52 includes a flat surface 56 which defines the upper screening surface of a panel and from which rows and columns of parallel sided lugs 58 upstand with each

lug 58 corresponding in shape to the volume defined by opposed parallel wall portions 38 and the upper surface portion 34 of a cross rib 26.

During a moulding operation, because the end surfaces of the aperture defining lugs 54 on the first die 50 close on the flat"screening surface"defining surface 56 of the second die 52, any flashing that may occur and which could restrict or block the aperture openings 22 will occur at the screening surface 20 of a panel 10 and not below that surface at the level of the surfaces 34 of cross ribs 26 as in the prior art. Flashing which is located at surface 20 is easily removed by grinding, polishing or otherwise appropriately treating that surface.

Thus the invention provides an improved panel structure, and associated dies and a method of manufacture over the prior art.

The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the scope of the following claims.