Abstract:
Method for forming large-dimension ceramic tiles comprising the following operative stages: feeding a determined quantity of powders into the mould cavity of a first pressing station; pressing said quantity of powders to obtain a slab of congruent material the thickness of which is reduced by a quantity between 20 and 40%; feeding said slab to at least one decorating station which deposits in a controlled manner at least a second layer of powders; feeding said decorated slab to a second pressing station; pressing the decorated slab.

Description:
[0001]     This is a division of copending parent application Ser. No. 09/991,905, filed Nov. 26, 2001, the contents of which are incoporated herein by reference. 
     
    
       [0002]     This invention relates to a method for forming tiles of ceramic material, and in particular tiles having dimensions exceeding 50×50 cm.  
         [0003]     Methods for forming small-dimension ceramic tiles, i.e. having maximum dimensions of 40×40 cm, are known; these comprise preparing a mass of powders, precompacting it to obtain a flat blank, depositing on said blank, in a controlled manner, at least a second layer of powders to form a surface decoration, and finally pressing the decorated blank to obtain the formed tile.  
         [0004]     All the known methods, which involve a material precompacting stage, solve the problem of deaerating the powder mass, however the percentage reduction in the thickness of the powder mass during material precompacting is insufficient to ensure surface stability of the precompacted blank.  
         [0005]     As a result, although the powders of said at least one second layer for forming the tile surface decoration are intimately bonded to the powders of the upper surface of the blank, they do not rest on a stable surface, with the consequence that mixing of the decoration powders occurs during transportation of the slab to the second pressing stage. Particularly in the case of elaborately decorated tiles, this causes decoration defects in the finished tile, which show mainly as lack of sharpness along the decoration edges.  
       SUMMARY  
       [0006]     An object of the present invention is to solve the problems of the known art within the framework of a simple and rational solution.  
         [0007]     A further object of the invention is to form large-dimension tiles, i.e. tiles having a size up to 180×120 cm and beyond.  
         [0008]     The invention attains said objects by virtue of the characteristics stated in the claims.  
         [0009]     In particular, with the method of the invention the precompacting pressure is sufficient to create a consistent powder slab presenting good surface stability while at the same time allowing the powders of said at least second layer to mix intimately with the powders of the surface layer of the slab. This ensures that during transport of the decorated slab and its subsequent pressing there is no movement of the decoration powders relative to the surface of the precompacted slab, to consequently obtain a perfect sharpness of the decoration edges.  
         [0010]     The invention also provides a forming plant, the special characteristics of which are defined in the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     To better clarify the method of the invention and the relative plant, a preferred embodiment thereof is described hereinafter by way of nonlimiting example and is illustrated in the accompanying drawings.  
         [0012]      FIG. 1  is a schematic front view of the forming plant according to the invention.  
         [0013]      FIG. 2  shows a detail of  FIG. 1 .  
         [0014]      FIG. 3  is a view taken in the direction  111 - 111  of  FIG. 2 .  
         [0015]      FIG. 4  is an enlarged view of a detail of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0016]     It is stated precisely that in the following description the wording “powder” comprises: 
        dry powders (having a moisture degree less than 2%), for instance regranulated and/or atomized glazes, or finely minced ceramic frits,     semi-dry powders (having a moisture degree between 2% and 6%), for example atomized or milled or micronized (finely grinded) ceramic mixes,     agglomerated materials, as flakes of ceramic mixtures, flakes of ceramic frits or glazes, and granules (obtained by wet or dry way), and     wet pastes (having a moisture degree more than 20%) of ceramic mixes (slips), or wet ceramic glazes, or silk screen printing pastes.        
 
         [0021]     The figures show the forming plant  1  for implementing the method of the invention.  
         [0022]     The plant  1  comprises a first press  2 , in which a consistent (or self-supporting) slab  3  is created from powders.  
         [0023]     The ceramic powders are fed into the mould of the press  2  by a usual loading carriage  4  provided with a bottomless slider  5  which is filled with powders by an overlying loading hopper  6 .  
         [0024]     The carriage is driven with reciprocating rectilinear movement and can translate between a retracted loading position, in which the slider  5  is filled with powders by the hopper  6 , and an advanced powder discharge position in which the slider  5  is positioned exactly above the mould cavity of the press  2 , to release the powders into the cavity.  
         [0025]     In a variant of the invention, the hopper  6  can also be driven with to-and-fro reciprocating movement, to hence load the slider  5  which remains at rest, as described by the Applicant in IT 1248243 (EP 519373).  
         [0026]     The carriage  2  is driven by usual means, not shown being of a known type, such as a geared motor.  
         [0027]     To the front edge of the carriage there is fixed a usual expeller  7  which, during the advancement of the carriage  4 , removes the slab formed by said first pressing station.  
         [0028]     Downstream of the first press  2  there is a conveyor  8 , the purpose of which is to feed the preformed slab  3  below a plurality of decorating stations  9 , each of which is arranged to deposit decorating powders on the exposed surface of the slab in accordance with a predetermined pattern.  
         [0029]     For example each of said decorating stations could comprise a plurality of hoppers, not shown, or any other device suitable for the purpose.  
         [0030]     To the side of the conveyor  8  there is positioned a device  10  for feeding the slab to a second press  18 , forming the second pressing station, and for making the loading rate of the press  18  independent of the decorating rate of said decorating stations  9 . Said device comprises a frame  11  provided with wheels  12  and supporting two roller tables  13  and  14 , each of which has its own operating unit  15  and  16 .  
         [0031]     With reference to  FIG. 2 , each unit comprises a geared motor  150  and  160 , to rotate the respective roller table  13  and  14  by means of a toothed belt  151  and  161 , which engages a series of pulleys  200 .  
         [0032]     Above the two roller tables there is positioned a carriage  17  which receives the decorated slab  3 , orients it in the correct position, and transports it above the mould cavity of the press.  
         [0033]     With reference to  FIGS. 2 and 3 , the carriage  17  comprises two longitudinal members  171  joined together by cross-members  172 , one of which is shown in  FIG. 3 . The carriage  17  is provided with wheels  178  which slide on guides  179  forming part of a structure ( FIG. 3 ) external to the roller tables  3  and  4 .  
         [0034]     The carriage  17  is driven by a geared motor  180  which rotates a toothed belt  181  to which one end of an element  182  is fixed, the other end of which is rigid with the carriage.  
         [0035]     To the front part of the longitudinal members  171  there are also fixed two movable walls  173  arranged to interact respectively with the front edge and rear edge of the slab  3  to both orient it in its correct advancement position and to feed it to the pressing station.  
         [0036]     As shown in  FIG. 4 , the walls  173  are hinged at their upper ends to the longitudinal members  171 , and are provided with a lug  175 , the free end of which is associated with the rod  176  of a cylinder-piston unit  177 .  
         [0037]     The cylinder-piston units  177  rotate the walls  173  to move them between a non-operative position, in which the decorated slab  3  is able to pass, driven by the action of the roller table  14 , and a lowered operative position, in which they rest against the edges of the decorated slab  3 , to lock it and orient it such that the longitudinal axis of the slab coincides perfectly with the longitudinal axis of the carriage.  
         [0038]     The operation of the carriage  17  is controlled by a processor, not shown, which also controls the entire forming plant of the invention.  
         [0039]     The method, which is apparent from the plant description, results in the creation, by the press  2 , of a large-dimension slab to be decorated by at least one decorating station which deposits coloured powders in a predefined pattern on the upper surface of the slab.  
         [0040]     For the upper surface of the slab to present good surface stability while enabling the coloured decorating powders to mix intimately with the powders of its upper surface, according to the invention the thickness reduction caused by the first press  2  must be between 20 and 40% of the thickness of the powders fed into the mould cavity. This is achieved by a pressing pressure between 50 and 100 kg/cm 2 .  
         [0041]     Once the slab has been decorated by the decorating stations  8 , it is fed to the second press  18 , which forms the decorated slab. According to the method of the invention the second pressing takes place at a pressure between 300 and 500 kg/cm 2 .