Apparatus for forming large-dimension ceramic tiles

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.

This invention relates to a method for forming tiles of ceramic material, and in particular tiles having dimensions exceeding 50×50 cm.

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.

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.

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

An object of the present invention is to solve the problems of the known art within the framework of a simple and rational solution.

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.

The invention attains said objects by virtue of the characteristics stated in the claims.

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.

The invention also provides a forming plant, the special characteristics of which are defined in the claims.

DETAILED DESCRIPTION

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), andwet pastes (having a moisture degree more than 20%) of ceramic mixes (slips), or wet ceramic glazes, or silk screen printing pastes.

The figures show the forming plant1for implementing the method of the invention.

The plant1comprises a first press2, in which a consistent (or self-supporting) slab3is created from powders.

The ceramic powders are fed into the mould of the press2by a usual loading carriage4provided with a bottomless slider5which is filled with powders by an overlying loading hopper6.

The carriage is driven with reciprocating rectilinear movement and can translate between a retracted loading position, in which the slider5is filled with powders by the hopper6, and an advanced powder discharge position in which the slider5is positioned exactly above the mould cavity of the press2, to release the powders into the cavity.

In a variant of the invention, the hopper6can also be driven with to-and-fro reciprocating movement, to hence load the slider5which remains at rest, as described by the Applicant in IT 1248243 (EP 519373).

The carriage2is driven by usual means, not shown being of a known type, such as a geared motor.

To the front edge of the carriage there is fixed a usual expeller7which, during the advancement of the carriage4, removes the slab formed by said first pressing station.

Downstream of the first press2there is a conveyor8, the purpose of which is to feed the preformed slab3below a plurality of decorating stations9, each of which is arranged to deposit decorating powders on the exposed surface of the slab in accordance with a predetermined pattern.

For example each of said decorating stations could comprise a plurality of hoppers, not shown, or any other device suitable for the purpose.

To the side of the conveyor8there is positioned a device10for feeding the slab to a second press18, forming the second pressing station, and for making the loading rate of the press18independent of the decorating rate of said decorating stations9. Said device comprises a frame11provided with wheels12and supporting two roller tables13and14, each of which has its own operating unit15and16.

With reference toFIG. 2, each unit comprises a geared motor150and160, to rotate the respective roller table13and14by means of a toothed belt151and161, which engages a series of pulleys200.

Above the two roller tables there is positioned a carriage17which receives the decorated slab3, orients it in the correct position, and transports it above the mould cavity of the press.

With reference toFIGS. 2 and 3, the carriage17comprises two longitudinal members171joined together by cross-members172, one of which is shown inFIG. 3. The carriage17is provided with wheels178which slide on guides179forming part of a structure (FIG. 3) external to the roller tables3and4.

The carriage17is driven by a geared motor180which rotates a toothed belt181to which one end of an element182is fixed, the other end of which is rigid with the carriage.

To the front part of the longitudinal members171there are also fixed two movable walls173arranged to interact respectively with the front edge and rear edge of the slab3to both orient it in its correct advancement position and to feed it to the pressing station.

As shown inFIG. 4, the walls173are hinged at their upper ends to the longitudinal members171, and are provided with a lug175, the free end of which is associated with the rod176of a cylinder-piston unit177.

The cylinder-piston units177rotate the walls173to move them between a non-operative position, in which the decorated slab3is able to pass, driven by the action of the roller table14, and a lowered operative position, in which they rest against the edges of the decorated slab3, to lock it and orient it such that the longitudinal axis of the slab coincides perfectly with the longitudinal axis of the carriage.

The operation of the carriage17is controlled by a processor, not shown, which also controls the entire forming plant of the invention.

The method, which is apparent from the plant description, results in the creation, by the press2, 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.

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 press2must 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/cm2.

Once the slab has been decorated by the decorating stations8, it is fed to the second press18, 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/cm2.