Paving slabs are made by press moulding a slurry of sand and cement to form generally rectangular blocks, the press serving to squeeze excess water from the mix, the blocks then being left to dry, cure and harden.

The press moulding apparatus conventionally uses press plates in a press machine, the press plates being perforated with drainage holes to allow the escape of water expressed from the block. Traditionally sheets of paper are used to prevent particles from entering the press machinery, and to provide a smooth flat forming surface for the paving slabs. Each set of sheets is used only once as they are discharged with the wet paving slab then peeled off and discarded.

In GB-A-2277536 and GB-A-2358593 it has recently been proposed to use a reusable, usually knitted, textile barrier between the cementitious slurry and the metal press plates. The knitted textile is mounted on a perforated sheet and secured to the press plates with adhesive strips.

It has been found however that the reusable textile barrier is quickly abraded at the edges and corners due to the harshness of the slurry and high mould pressures typically in the order of 2000 psi (1.4 x 104mm2). Such deterioration causes the paving slabs produced to have rounded uneven edges which cannot be used safely as part of a paved surface and do not meet the requirements of the relevant British, European or International standards.

It is an object of the invention to provide an improved filter for apparatus for dewatering of paving slabs during production in moulding presses.

The invention accordingly provides a filter for paving slab dewatering apparatus comprising a filter element comprising a sheet of filter medium having reinforced edge and corner regions.

The filter medium may comprise a sheet of fused polymeric particles, and may be formed by forming an even layer of thermoplastic particles having a size range of for example 40-150, um, preferably in a narrower size distribution between for example 70-130, um, and subjecting the layer of particles to heat and pressure to produce a more compacted layer of particles which are fused on their touching surfaces leaving a porous structure for passage of liquid phase medium therethrough. The porosity, void space, and mean pore diameter will depend upon the mean particle size, compression and depth of surface softening of the particles.

The coherence of the layer of thermoplastic particles may be enhanced by inclusion of a proportion of thermoplastic fibres amongst the particles.

These are preferably of a length sufficient to adhere to many particles and to other fibres where their touching surfaces are fused. The fibres may be randomly oriented in at least two dimensions and preferably three dimensions so that the particles and fibres are thoroughly mixed.

The layer of thermoplastic particles, after fusion under pressure may have a thickness of 3-5mm. The initially formed unfused layer will normally

be somewhat thicker. The air permeability of the fused sheet of thermoplastic particles is preferably from 100 to 300 litre dm-2 min-'and their shape and area will depend upon the size and shape of the paving slabs being produced, for example 500 x 500mm.

The reinforced edge and corner regions of the filter element may preferably be reinforced by toughening to prevent abrasion and to extend the life of the filter element. This toughening may be achieved by completely or substantially completely melting and fusing the particles in the edge and corner regions into an effectively non-porous strip of polymer, or alternatively by impregnating the edge and corner regions with a strip of adhesive or resin to thereby fill the void spaces of the filter material to form a non-porous encapsulated edge strip to the filter element. Reinforcement in this way is effective because it prevents erosion or flaking by removal of particles from the substrate as a result of friction.

The adhesive may have its abrasion resistance enhanced by incorporation of a hard filler material such as stainless steel powder or alumina powder.

The adhesive composition may be an epoxy or polyurethane system comprising for example a base and hardener, with 10-50% by weight of particulate material such as stainless steel, Hastelloy, alumina silica or zircon sand.

The polymer particles from which the filter element is made are preferably a polyolefin such as polypropylene or polyethylene.

The filter element sheet may be fastened to a perforated metal support plate by for example blind rivets secured into recesses on the filter element sheet with the same or a different adhesive. Alternatively, the back (downstream) side of the filter element sheet may be formed with elongate recesses or depressions into which adhesive tape can be located to fix the filter element sheet flush to the metal plate.

The filter element sheet may be formed with grooves or channels on the downstream (back) side of the sheet to aid the flow of the filtrate as it is forced through the filter element. This may be achieved by forming the sheet on a ribbed base plate or belt.

Preferred embodiments of a filter for paving slab moulding apparatus, dewatering arrangement and filter element according to the invention will now be described by way of example with reference to the accompanying drawings wherein : Figure 1 is a diagrammatic sectional view of a paving slab moulding apparatus/press; Figure 2 is a diagrammatic enlarged cross-sectional view of an edge region of a filter element in accordance with the invention; Figure 3 is a similar view of an edge region of an alternative filter element incorporating a first mounting means; Figure 4 sectional view of part of a filter element of the invention

incorporating two further variants of mounting means; and Figure 5 is a sectional view of part of a filter element of the invention showing tow further variants of mounting means; and Figure 6 is a fragmentary enlarged sectional view of the edge region of another embodiment of filter element of the invention.

As diagrammatically shown in Figure 1, a press for forming concrete paving slabs comprises a lower static plate 10, having apertures 11 for outflow of water expressed from the concrete mix, and an upper movable plate 12 also having apertures 13 for outflow of expressed water. The upper plate is moved by a ram 14 which operates to press down on a body of concrete mix 15 to shape it as required to form a paving slab, and express water from the mix, the water exiting via the apertures 11 and 13 on the lower and upper plates.

The upper plate 12 is then lifted clear to allow the moulded slab to be removed for drying, and a fresh body of concrete mix placed on the lower plate 10.

In order to prevent entry of sand and/or other particulate material from the concrete mix entering the machinery with the expressed water, it is customary to provide some form of filter barrier over the apertures on the plates. In the prior art this filter barrier may comprise sheets of paper, or a knitted fabric.

In the present invention, respective filter barriers 16 and 17 are mounted between the lower and upper plates 10 and 12 respectively. Each

of these filter barriers comprises a sheet formed by fusing a layer of polymeric particles into a continuous sheet using heat and pressure. This may be done by disposing a layer of the particles on a plate or conveyor and then compressing and heating the layer to achieve fusion of the particles on sheet touching surface, by for example a heated platen or heated roller.

The layer of fused particles produced typically is formed from particles of polypropylene or polyethylene having sizes in the range 40-150, um, preferably 70-130, um, and a final thickness from 3.0 to 5. 0mm. The initial thickness of the layer before heating and compression is of course greater.

The porosity and mean pore size depend upon the degree of heating and pressure applied to the layer of particles as well as upon the particle size.

The sheets used have air permeability of between 100 and 300 litre dm-2 min-'and are of a size and shape appropriate for the paving slabs being produced, such as 500 x 500mm.

The filter element 20 shown in Figure 2 is of the kind thus produced.

The edge regions 21 of the filter element 20 are toughened by impregnating the fused polymeric particle matrix with a resin or adhesive 22 preferably an epoxy or polyurethane two component system, comprising a base and a hardener and 10-50% by weight of particulate material such as stainless steel, Hastelloy (Trade Mark), alumina, silica or zircon sand.

The impregnation is achieved by painting the resin on as a 5-10mm wide strip around the whole outer edge of the filter element sheet, with extra at the corners. The resin then cures in the air at ambient temperature to

leave a toughened abrasion resistant edge.

Reinforcement of the edges thus hinders destruction of the edge regions by preventing"flaking"or removal of particles one by one from the substrate due to abrasion. Even if the melting process weakens the polymer and makes it relatively brittle, a smooth cohesive surface prevents friction or abrasion from acting on individual particles. A smooth surface is thus more important than the mechanical strength of the polymer itself.

Figure 3 shows a variant, wherein the filter element 30 includes randomly oriented thermoplastic fibres 31 mixed with the polymeric particles , to improve the coherence of the fused particle matrix, again edge region 32 is impregnated or'potted'with resin or adhesive by the same method using a similar composition as in Figure 2. However a longitudinal recess 33 is provided in the underside of the edge region of the sheet 30, and a strip 34 having adhesive 35,36 on its respective upper and lower surfaces is received in the recess 33. This enables the edges of the filter element 30 to be located and mounted by adhesion on the respective upper or lower plate of the slab press.

In Figure 4 is shown two alternative mounting systems, wherein a filter element sheet 40 which is similar to either of the filter elements described above, is secured to a plate 41 by either push in stud 42 received in a bore 43 through filter element, the stud having a slotted head 44 and an end flange 45. An alternative form of connection is shown alongside in a bore 46 which comprises a barbed push fastener having a barbed head 47

and an end flange 48.

Figure 5 shows two further alternative fasteners comprising a dowel 49 in a blind bore 50, and a push rivet 51 in a bore 52, having a flange 53 and bifurcated body comprising divergent resilient limbs 54.

All the connectors shown in Figs. 4 and 5 may be made of a resilient metal material, but are preferably of a resilient chemically inert plastics material such as a polyolefin. The connectors can be reused. Normally they retain the filter elements fixed to the press plate during operation and removal of the slab. However, if the filter element should need to be replaced, the filter element can be levered from the plate, and a new filter element can be attached reusing the same connectors.

Fig. 6 illustrates in an enlarged cross-sectional view, an embodiment of filter element 60, made of a layer of thermoplastic particles 61 which have been sintered under heat and moderate pressure to form a structure of particles which are fused or adhere on contacting surfaces, with pores or void space between the particles such as to allow fluid to pass through the layer. Instead of being impregnated with a sealant however the edge region 62 of the element is sealed by complete fusion of the particles in the end region 62 to form a non-porous impermeable edge to the filter element. This may be accomplished by heating the edge region to above the melting point of the particles and moulding the edge region. The thermoplastic material then sets into a coherent non-porous zone which protects the edge of the filter element against abrasion and erosion due to dislodging of the particles

as would occur if the edge of the sintered sheet is exposed without protection.

In any of the embodiments shown, the surface of the filter element may be formed with grooves which are e. g. moulded during manufacture of the element, to aid drainage.

The invention is not restricted to the details of the preferred embodiments illustrated and described, and other materials and mounting means within the scope of the invention may be used.