BACKGROUND ART Research in the ceramic sector is currently aimed at obtaining products imitating natural stone, such as marble and granite. These products are characterised by the presence of continuous veining of random pattern extending through the entire thickness of the slab.

Such ceramic tiles or slabs are produced by compacting, by hydraulic presses, semi-dry atomized, ground or re-granulated powders variously mixed together in prearranged or random manner.

Specifically, the powder mixtures are deposited by suitable means into the forming cavities of rigid steel moulds with which the presses are provided, and are then pressed to obtain the product.

The same applicant has already conceived a tile forming method which is described in Italian patent application RE2001A000129.

This method consists of depositing in the hopper in a prearranged and/or random manner a mixture of powders of different characteristics in such a manner as to create in the hopper a mass of powders presenting veining variously disposed within the mass, the hopper having a discharge mouth of dimensions equal to the dimensions of the cavity of the mould in the

forming press, both in the translation direction of the carriage and in the direction perpendicular thereto; withdrawing, in succession from said powder mass, portions having, in the carriage translation direction, a dimension equal to a fraction of the dimension of the hopper mouth, and in the direction perpendicular thereto the same dimension as the hopper mouth, such as to withdraw an entire layer of powders from said mass; depositing said layer in an orderly manner in the interior of said at least one cavity by means of the carriage, and pressing the powders.

The described method is implemented by a plant comprising a fixed hopper for containing a mixture of powders presenting bulk veining in imitation of a natural stone. With the lower mouth of the hopper, which has the same plan dimensions as the cavity of the forming press, there are associated movable means, such as a usual loading carriage, arranged to deposit in the cavity a succession of powder portions withdrawn from the hopper, until the cavity is completely filled.

Although they operate well, the aforedescribed method and plant are little versatile.

In this respect, if a requirement exists for tiles of variable dimensions, or of large surface dimensions, they present drawbacks and disadvantages.

This is because the perimeter of the lower mouth of the hopper, which coincides with the surface dimension of the mould cavity, is fixed.

Current technology is increasingly aimed at obtaining ceramic slabs or tiles having variable dimensions in accordance with market requirements.

There is therefore a strongly felt need for a more versatile plant and method which enable the tile or slab format to be changed rapidly while

avoiding those lengthy interruptions during production which are typical of the known art, within the framework of a simple and rational solution.

The object of the present invention is to provide a method for forming ceramic tiles or slabs, particularly but not exclusively in a press of continuous type.

DISCLOSURE OF THE INVENTION This object is attained by a method in accordance with claim 1.

According to a further aspect, the present invention provides a plant for forming ceramic tiles or slabs having structural and functional characteristics such as to satisfy the aforesaid requirements while at the same time obviating the stated drawbacks of the known art, in accordance with claim 3.

The dependent claims define preferred and particularly advantageous embodiments of the method and plant of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Further characteristics and advantages of the invention will be apparent on reading the ensuing description provided by way of non-limiting example, with the aid of the figures shown in the accompanying drawings, in which: Figure 1 is a schematic side view of a first embodiment of the plant according to the invention; Figure 2 is a front view of the plant of Figure 1 with the hopper shown in section; Figure 3 shows a second embodiment of the present invention;

Figure 4 shows one method of using the plant of Figure 1; Figures 5 and 6 show two applications of an alternative method of using the plant of Figure 1; Figure 7 shows a third embodiment of the present invention; Figure 8 shows a fourth embodiment of the present invention; Figures 9A-9E show alternative configurations of the discharge aperture of the hopper of the present invention; Figure 10 shows a further embodiment of the plant of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION With reference to said figures, the reference numeral 1 indicates overall a plant for forming ceramic tiles or slabs in accordance with the present invention.

According to one embodiment of the present invention, said plant 1 comprises a fall-on surface, which in the example is a conveyor belt 2, above which there are positioned means, indicated overall by 3, for feeding a mixture of powders 33 to create a continuous or discontinuous powder strip 100 on the belt 2, a suction scraping system 4, a continuous pressing station 5 and a cutting station 6.

Said means for feeding a mixture of powders 33 comprise, in the example of Figure 1, three conduits 32 for feeding powders 33 of different characteristics and/or colours, a collection surface 31 for the powders and a pusher 30.

The three conduits 32 are fed with atomized powders 33 by a feed system, not shown, each conduit 32 being fed with powder 33 of a

different colour from that of the remaining conduits. Alternatively, several conduits 32 can be fed with powder of the same colour, or fed with a preformed mixture of powders of different colour, provided the overall conduits contain powders of at least two different colours.

Although the feed conduits 32 are three in number in the illustrated example, they can be of larger or smaller number if desired, The conduits 32 are individually slidably supported on guides 320 perpendicular to the axis of the conveyor belt 2.

The powders 33 are deposited by the feed conduits 32 in prearranged or random manner on the collection surface 31.

The manner in which the powders 33 are distributed on the collection surface 31 can be controlled by modifying the sequences and times of opening of the various distribution conduits 32, which are provided with shut-off gate valves, not shown.

For example, the veins typical of a slab of natural stone can be reproduced by suitable horizontal paths which the conduits 32 are made to traverse above the collection surface 31. It is however possible to use fixed powder feed conduits, although the use of movable conduits is preferable.

The collection surface 31 is disposed substantially horizontal and is preferably provided with opposing vertical side walls 36 for retaining the powders 33.

The pusher 30, shown in section in the figures, is positioned transverse to the two side walls 36, above the collection surface 31.

The bottom of the pusher 30 is in contact with the powder fall-on surface of the collection surface 31 and is movable between a withdrawn position

and an advanced position, operated by means which are known and therefore not shown.

The pusher 30 moves along the direction of movement of the underlying belt 2.

The pusher 30 can be replaced by a movable tray 301 (Figure 7) which pushes the powders 33 lying on the collection surface 31.

At the free edge of the collection surface 31, i. e. at the edge opposite that occupied by the pusher 30 when in its withdrawn position, there is a hopper 34 which receives the powders 33 urged by the pusher 30 in moving from its withdrawn position to its advanced position.

The hopper 34 is located vertically on the movable fall-on surface, which in the example is the belt 2, with the loading aperture 37 positioned adjacent to the free edge of the collection surface 31 and the discharge aperture 38 positioned above the belt 2 in proximity to it, however avoiding direct contact with the surface of the belt 2.

Advantageously, said hopper 34 presents a prismatic shape having two opposing faces substantially perpendicular to the direction of advancement of the belt 2, said faces being positioned at a distance"B" apart substantially equal to the thickness"S"of the powder strip 100 to be compacted.

Preferably, as shown, the hopper 34 has the shape of a rectangular parallelepiped having the two larger faces, i. e. the front face 34a and the rear face 34b, normal to the direction of advancement of the belt 2 and positioned at a distance"B"apart equal to the thickness of the continuous powder strip 100 which is to be obtained and then compacted.

The height of the hopper 34 (Figure 1), which is just less than the distance between the collection surface 31 and the advancement plane of the belt 2, is chosen on the basis of the dimensions of the plant 1 such as to ensure that powder 33 is present along the entire width"L"of the discharge aperture 38 (Figure 2).

The distance"L"between those hopper faces parallel to the advancement direction is equal to the width of the required continuous powder strip 100, the height dimensions H being generally chosen on the basis of the hopper width L, they varying from 0.2L to 2L, preferably from 0.5L to 1. 5L.

According to a preferred alternative embodiment of the present invention, the lower end of the rear face 34b of the hopper 34 can be positioned at a distance"h1"from the surface of the belt 2 of between 0 and S (generally between 0 and 3 mm), where S is the required thickness of the powder strip 100 to be compacted, the front face 34a being positioned at a distance equal to the thickness S of the powder strip 100 to be compacted (Figure 7).

To obtain with the same plant 1 any thickness S of the powder strip 100 to be compacted, the front face 34a can be made movable along the powder advancement direction to enable the distance B between the front face 34a and rear face 34b to be varied.

Alternatively, and equivalently, the rear face 34b or both faces can be made movable.

In a further embodiment the front face 34a and/or the rear face 34b is inclined away from a position perpendicular to the fall-on surface to obtain a lower discharge aperture 38 wider than the upper loading aperture 37 (Figures 9A-9D).

In this manner, the distance between the front face 34a and the rear face 34b of the hopper 34, measured at the discharge aperture 38 (equal to "B+d"), is greater than that measured at the loading aperture 37 (equal to "B"), however the distance"B+d"at the discharge aperture 38 should be less than double the distance between the discharge aperture 38 and the fall-on surface.

Preferably, the difference"d"between the distance between the two faces 34a, 34b at the discharge aperture 37 and that at the loading aperture 37 should be between 0 and 5 mm, more preferably between 1 and 3 mm.

As an alternative to the use of the hopper 34 having its vertical front face 34a and rear face 34b totally flat, its faces can have their lower ends, which define the discharge aperture 38, shaped with radii of curvature Ri, Re (Figure 9A) such as to facilitate emergence of the powders 33 from the hopper 34 in the advancement direction, so preventing any outflow velocity differences which could be encountered with a hopper 34 with flat faces.

Indicatively, the radius of curvature Ri of the front face 34a can vary from 0 to 2 times the thickness of the powder strip 100 to be compacted, and the radius of curvature Re of the rear face 34b from 2 to 4 times the thickness of the powder strip 100 to be compacted.

Other configurations are possible, such as that shown in Figures 9B, 9C, in which the curvatures of inner radius of curvature Ri and outer radius of curvature Re are"rearwards"of that shown in Figure 9A, imposing a slowing down of the flow on the front face 34a of the hopper 34 and favouring rotation, without excessive distortion, of the powders 33 leaving the hopper 34.

In a further alternative, improved over the previously described solution in which the ends of the front face 34a and rear face 34b are both shaped with radii of curvature, the curved portion of the front face 34a is replaced by a roller 41 rotating in the same direction as the outflow of powders 33, and having a radius Ri (Figure 9F).

Said rotary roller 41 extends axially along the entire width L of the front face 34a, bounding the discharge aperture 38, and is positioned such that its revolving surface is tangential to the inner surface of said front face 34a, and is spaced from the surface of the belt 2 by a distance equal to the thickness S of the required powder strip 100.

The distance"B"at the discharge aperture 38 should be substantially equal to said thickness"S"of the powder strip 100 so that, by virtue of the roller 41, the powders 33 can be advantageously transferred onto the belt 2 while maintaining substantially unaltered the arrangement which they possessed within the hopper 34; this advantage is particularly useful when said arrangement is able to give the powder strip 100 a predetermined desired graphical appearance, for example the typical veining of natural stone.

The rotary roller 41 can be mounted idly such that its rotation is induced by friction with the powders 33 as these pass from the hopper 34 to the belt 2.

Alternatively, said roller 41 can be motorized and rotated such that its peripheral speed is equal to and in the same direction as the advancement speed of the belt 2; this can be achieved by connecting the roller 41 to an independent gearmotor, or connecting it via a suitable transmission system to the drive members for the belt 2.

Again, the rotary roller 41 can be braked, i. e. associated with a braking device to slow it down relative to the outflow speed of the powder 33 within the hopper 34, and hence relative to the speed of the belt 2.

Finally, in a further embodiment of the invention, the central plane of the hopper 34 can be inclined with a predefined inclination"a"to the advancement direction of the powder strip 100 to be compacted (Figure 9D).

Particularly advantageous is the embodiment in which said central plane is inclined to the conveyor belt 2 from the top downwards in the advancement direction of the powder strip 100, i. e. with a negative inclination"oc" (Figure 9E).

In this case the actual weight of the powders 33 in the hopper 34 acts substantially only on the rear face 34b, to advantageously reduce mixing of the powder layers lying at the front face 34a, which are those defining the upper visible part of the leaving powder strip 100.

Consequently, when said powder layers are disposed in a prearranged arrangement within the hopper 34 to give the strip 100 a required graphic appearance (e. g. natural stone veining), this embodiment enables them to be transferred onto the belt 2 while preventing their excessive distortion.

In operation, with reference to Figure 1, the suitably manipulated feed conduits for the powders 33 deposit by gravity the powders 33 contained in them onto the collection surface 31, to reproduce the desired veining.

When the powders 33 have been deposited onto the surface 31, the pusher 30 is driven from its withdrawn position to its advanced position, to urge the mass of powders 33 towards the hopper 34, positioned vertically

over the belt 2, to then return to its withdrawn position and enable a new loading cycle for the powders 33.

The mass of powders 33, on arriving in the hopper 34 by gravity, settles gradually from the bottom thereof upwards towards the discharge aperture 38.

As the distance between the discharge aperture 38 and the surface of the conveyor belt 2 is equal to the thickness"S"of the powder strip 100 to be deposited, that portion of the mass of powders 33 which initially falls deposits directly onto the belt 2, which in this case represents the fall-on surface for the powders 33.

When the hopper 34 is full, the belt 2 is made to advance along the advancement direction, indicated by the arrow in the figures, so emptying the hopper 34.

The aforedescribed operations can be conducted either discontinuously (Figures 4,6) or continuously (Figures 1,5, 7), this latter mode increasing the productivity of the plant 1.

In other words, the mass of powders 33 deposited continuously in the hopper 34 reproduces throughout its bulk a slab of natural stone, lying vertically in a direction perpendicular to the belt 2. This slab, represented in practice by the continuous powder strip 100, deposits on the belt 2 by gravity during the belt advancement, while maintaining the previously obtained arrangement of the powders 33 within the hopper 34.

Preferably, downstream of the hopper 34 in the belt advancement direction, there is positioned the suction scraping system 4 comprising a usual device 40 for scraping the upper surface of the powder strip 100.

Said suction system 4, illustrated schematically but not described in detail as it is of known type, enables a thin surface layer of powders to be removed from the strip 100 such as to highlight the arrangement and contrast of the different coloured powders present in the powder strip 100.

The powder strip 100, guided by the belt 2, then reaches the continuous pressing station 5, also of known type. Said pressing station 5 comprises a compactor belt 50 positioned above the conveyor belt 2 to compact the powder strip 100 and obtain a continuous compacted article.

Essentially, compacting takes place between the surface of the conveyor belt 2 and the surface of the overlying parallel compactor belt 50.

The powder strip 100 is laterally retained on the belt 2 by laterally providing, parallel to the advancement direction of the belt 2, containment means which in the example are two lateral containing walls 35, only one of which is visible in the figures. Said containment walls 35 preferably extend as far as the termination of compaction.

The compaction effected with the compactor belt 50 is sufficient to obtain a compacted article which requires no further compaction, however in certain cases compaction forms a manipulable pre-compacted article intended for subsequent final compaction.

On termination of compaction, the article is divided into slabs of predefined dimensions in the cutting station 6 by usual cutting devices.

In the illustrated example, said cutting station 6 comprises two rotary blades 60 for cutting in the direction perpendicular to the advancement direction of the belt 2, and at least a further two rotary blades 61, only one of which is visible in the figures, for cutting the compacted article in the advancement direction of the belt 2.

Preferably, the described plant 1 is managed by a processor, not shown, which controls its operation.

According to an alternative embodiment, the plant 1 can present a double system for feeding the hopper 34 (Figure 3).

Essentially, to the side of the loading aperture 37 of the hopper 34 there is a further collection surface 31', also provided with feed conduits 32'for the powders 33'and a pusher 30', which are identical to those already described and are located in a position symmetrical thereto about the axis of the hopper 34.

With the double feed means 3 and 3'the operation of the two means can be synchronized to create a continuous powder strip 100 on the belt 2, hence improving continuity of powder discharge from the hopper 34.

According to a further alternative preferred embodiment, shown in Figure 7, the feed means 3 comprise, in addition to those already shown in Figure 1, a powder loading hopper 322 and a feed conveyor belt 321.

In practice, both the basic coloured powders from the powder loading hopper 322 and the powders of different nature and/or colour originating from the conduits 32 fall onto the generally horizontally disposed feed conveyor belt 321, with prearranged paths controlled by suitable electronic control systems, not shown.

The feed belt 321 discharges the powders reloaded in this manner onto the collection surface 31 for the powders 33.

The feed belt 321 can be fixed in one position, or be synchronized (or not synchronized) with the stroke of the pusher 30.

Preferably, the plant 1 is provided with a second feed system 3'totally similar to the preceding and disposed symmetrical therewith about a vertical plane of symmetry through the hopper 34 (Figure 7).

In addition a loading head 400 with multiple tubes is positioned exactly above the loading aperture 37 of the hopper 34, as visible in Figure 7.

This head 400 directly discharges into the hopper 34 without passing via the feed belt 321 or pusher 30 (or tray 301), achieving further effects, in particular layers and veins which are very thin and pronounced compared with the powder mass.

The method and plant for forming ceramic tiles and slabs of the present invention can also be used to deposit a powder layer 100'on the surface of a continuous monochromatic base slab 200 (Figure 5) previously obtained using means of the known art, indicated overall by 7 and 8.

Said means of the known art comprise, for example, a discharge conduit 7, a hopper 70, a levelling rod 72 for regulating the thickness of said base slab 200, and a possible station 80 arranged to provide slight compacting of the material, to avoid possible"remixing"of the base slab 200 with the powder layer 100'.

In this manner the veining effect obtainable with the method and plant 1 of the present invention does not extend through the entire mass of the final slab, but only through the surface part, obtaining a saving of coloured powders, which are more costly than the materials of the base slab.

Essentially, the mixture of powders 33 leaving the hopper discharge mouth 38 arrives on the base slab 200, which in this second case represents the fall-on surface for the powders 33.

A further alternative is that shown in Figure 6, in which the powders present in the hopper are deposited discontinuously onto a tile 201 previously formed by a first discontinuous pressing system 300. After applying the surface powder layer 100', the tile 201 is conveyed by rollers 303 to a second pressing system, for example a discontinuous pressing system 302 (Figure 6).

In this case, as in the others of discontinuous operation, it is expedient to provide the hopper 34 with a gate valve, not shown, which can be closed when necessary, to prevent outflow of powders 33 in the absence of the tile below the discharge aperture 38.

In the example shown in Figure 10, a loading tray 250 is used as the fall- on surface, associated with a carriage sliding with reciprocating movement along a surface 340 to deposit within a cavity 320 a layer 120 of powders withdrawn from the hopper 34, to be compacted by usual means, such as a punch 350.

The dimensions of the carriage 250 are such as to allow loading of a powder layer 120 having a thickness S equal to the distance B between the faces 34a, b of the hopper 34.

In this case the use of a suitable gate valve can be avoided by using instead a closure plate 330 extending from the rear of the carriage 250 such as to close the discharge aperture 38 even when in its advanced position for depositing the powder layer 120 into the cavity 320.

According to the invention, as shown in Figure 8, one of the faces (front 34a or rear 34b) of the hopper 34 can consist of a vertical portion of an endless movable belt 39 passing about three rollers, to accompany the material discharged onto it in a manner similar to that already described.

Preferably, the peripheral speed of the belt 39 is substantially equal to that of the belt 2.

Alternatively, the belt 39 can convey agglomerated materials into the hopper 34, so that they deposit on the surface of the powders 100 to be compacted.

From the aforegoing description it will be apparent that the method and plant of the present invention enable the requirements to be satisfied while overcoming those drawbacks stated in the introduction to the present description with reference to the known art.

In this respect, the method and plant of the present invention enable ceramic slabs of the desired thickness to be continuously formed with bulk motifs reproducing the veining of natural stone.

Moreover said method and relative plant enable the veining formed in the powder mixture present in the hopper to be quickly and efficiently transferred onto the ceramic slab.

In addition, the powder mixture distribution within the hopper is preserved for forming the powder strip to be compacted.

In order to satisfy contingent specific requirements, an expert of the art can apply numerous modifications and variants to the aforedescribed method and plant, all of which lie within the scope of protection of the invention, as defined by the following claims.