The present invention relates to a method and plant for forming ceramic tiles and slabs.
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.
A forming method according to EP-A-1321260 which comprises the following operative steps:
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the forming press, there are associated movable means arranged to deposit in the cavity a succession of powder portions withdrawn from said hopper, until the cavity is completely filled.
Although it operates well, the described plant is little versatile in terms of format change, this now being an essential characteristic in a modern plant. In this respect, the technology is increasingly aimed at obtaining ceramic slabs or tile having variable dimensions in accordance with market requirements. There is therefore the need for a more versatile plant which enables the tile or slab format to be changed rapidly without the need for lengthy interruptions in production.
WO-A-96/15888 discloses an apparatus using a conveyor belt for loading a press for forming ceramic tiles or slabs according to the preamble of claim 4 and a method for forming ceramic tiles or slabs in a continuous press, comprising the following operative steps:
The object of the invention is to satisfy said requirement within the framework of a simple and rational solution.
The invention attains said objects by virtue of the characteristics defined in the claims.
Specifically the invention provides a method for forming ceramic tiles or slabs in a continuous press arranged to form a slab of indefinite length comprising the following operative steps:
According to the invention, said portions are withdrawn by moving the hopper with reciprocating movement in the belt advancement direction above suitable distribution means.
Said method is implemented by a tile forming plant defined in claim 4. Such a plant comprises a conveyor belt, above which there is positioned a hopper movable with to-and-fro movement in the direction of the belt axis and containing a mixture of powders presenting bulk veining in imitation of a natural stone. With said hopper there are associated powder distribution means arranged to enable a succession of powder portions to fall by gravity onto said belt, to create a continuous powder strip which said belt feeds to a continuous pressing station.
The hopper is driven by usual known means.
The powder distribution means comprise a fixed plate provided centrally with a slot perpendicular to the direction of translation of said belt, said plate having a dimension in the belt advancement direction equal to at least double the corresponding dimension of the lower mouth of said hopper, and a dimension in the direction perpendicular to said belt advancement direction equal to the corresponding dimension of the lower mouth of the hopper.
The slot with which the plate is provided presents, in the belt advancement direction, a dimension which is a function of the graphic resolution to be obtained on the finished tile. The slot dimension in the belt advancement direction varies between 10 and 50 mm, and preferably between 20 and 40 mm.
According to a first embodiment of the invention, said continuous pressing station is arranged to compact the powder strip created on the belt to a pressure sufficient to obtain a continuous coherent product intended to undergo a second final pressing after being divided into blanks of predefined dimensions by a usual cutting station, before feeding the blanks to firing.
In this first case, the pressure exerted on said powder strip reaches a value not exceeding 100 bar.
Alternatively, said continuous pressing station compacts the powder strip to a maximum pressure close to 250-300 bar. The thus compacted product does not require a subsequent second pressing, and can hence be divided into blanks by a usual cutting station, to be fed directly to firing. In both embodiments the plant can be provided with scraping devices to remove a thin layer of powders in order to display the veining present in said strip, and also with further devices for the controlled deposition of further decorative powders on said strip.
The operation of the method of the invention and the constructional characteristics and merits of the relative means for its implementation will be better understood from the ensuing description given with reference to the figures of the accompanying drawings which illustrate a particular preferred embodiment of said means and of some constructional variants thereof by way of non-limiting example.
Figure 1 is a schematic side view of the plant according to the invention.
Figure 2 is an enlarged view of the section II-II shown in Figure 1.
Figures 3 and 4 schematically show the operation of the invention.
Said figures show the plant 1, which comprises a conveyor belt 2, above which there are positioned means 3 for creating a continuous powder strip 100 on the belt 2, a suction scraping system 4, a continuous pressing station 5 and a cutting station 6.
The said means for creating the continuous powder strip 100 comprise a hopper 7 driven with reciprocating motion, in which the powders are deposited by an overlying movable feeder 70 also driven along two mutually perpendicular directions in a plane parallel to the plane of the belt 2, and caused by known means, not shown, to slide above the upper mouth of the hopper 7.
The purpose of the feeder 70 is to deposit by gravity at least two powders of different characteristics into the hopper 7 such that they together form a mass presenting variously disposed veining reproducing that present in a natural stone. In other words this mass can be said to reproduce a block of natural stone, which is then cut into layers which are deposited on the belt 2 by the distribution means 8 associated with the hopper and described hereinafter.
In the illustrated embodiment, to limit slippage of the powders during their discharge onto the belt 2, the internal volume of the hopper 7 is divided by baffles 71 (Figure 2) positioned at the hopper exit mouth. A second series of baffles 72 is associated with the upper mouth of the hopper, to mix the powders fed into the hopper 7 in order to create the veining and the aesthetic effects as randomly as possible in imitation of the veining appearing in a block of natural stone.
In other embodiments of the invention, only the baffles 71 or only the baffles 72 could be provided, or alternatively none at all.
The hopper 7 is associated with a drive system, not shown, which causes it to translate with reciprocating motion above the powder distribution means 8.
In the illustrated embodiment, said distribution means 8 are associated with the lower mouth of the hopper 7 and comprise a fixed plate 9 provided with a slot 10, disposed perpendicular to the translation direction of the plate 9.
The plate 9 has a dimension in the direction perpendicular to the belt advancement direction which is greater than the corresponding overlying dimension of the hopper 7, and a dimension in the belt advancement direction equal to at least double the corresponding dimension of the hopper 7. The slot 10 has a dimension in the direction perpendicular to the belt advancement direction which is equal to the corresponding dimension of the lower mouth of the hopper 7, and a dimension in the belt advancement direction which depends on the graphic resolution to be obtained on the finished tile.
The slot 10 is provided lowerly with a perimetral surround 101 which delimits the thickness of the powder strip deposited on the belt 2, to prevent the strip sliding against the lower surface of the plate 9.
The height of the surround 101 varies between 1 and 300 mm, depending on the actual embodiment of the invention.
Downstream of the hopper 7b there is positioned a usual device 40 for scraping the upper surface of the powder strip 100. Said device 40 is connected to a suction system, not shown being of known type, which enables a thin surface layer of powders to be removed from the strip 100 in order to exalt the graphics and the contrast between the different coloured powders of the strip 100.
The belt 2 feeds the powder strip to a continuous pressing station 5, also of known type, comprising an upper compacting band 50 provided to compact the powder strip on the lower belt 2 to obtain a continuous compacted product.
Means for laterally retaining the powder strip on the belt 2 are associated with said compacting band 50. These latter lateral retention means are not shown as they are known to the expert of the art.
According to the present embodiment, the compaction effected by the compacting band 50 is sufficient to obtain a compacted product which does not require further compacting. In this case the maximum compacting pressure reaches values close to 300 bar.
Once compacted, the product is divided into blanks of predefined dimensions by a usual cutting device 6. In the illustrated embodiment, said device essentially comprises a rotary blade 60 for making the cut in the direction perpendicular to the belt advancement direction, and at least two further rotary blades 61, only one of which is visible in the figures, for cutting the compacted product in the advancement direction of the belt 2. The described plant is controlled by a processor, not shown, which controls its operation in the following manner.
While the belt 2 advances (Figure 4), the hopper 7 translates reciprocatingly on the plate 9 in such a manner as to deposit adjacent powder portions on the belt 2, to create thereon a continuous powder strip which is firstly scraped by the system 4, and then compacted in the compacting station 5. Downstream of the compacting station the rotary blades 60 and 61 cut the strip into blanks of predefined dimensions.
It should be noted that when the hopper movement is reversed (Figure 5) the manner of depositing the powders on the belt 2 varies, to the advantage of their random distribution, resulting in a greater naturalness of the different coloured veining present in the powder strip.
The manner of distributing the powders can be further varied by changing the hopper translation speed, in addition to the movement of the overlying feeder 70.