The invention concerns a method particularly for decorating ceramic tiles and the relative silk-screen printing machine, namely, a process for propelling and positioning tiles in all conceivable materials, be they ceramic, vitreous, wooden plastic or other, in whatever state and format, from a few centimetres a side to a metre or more, including those not square; a machine suitable for implementing the method according to the invention being also subject matter of the present invention. The prior art involves silk-screen printing machines whereby the tiles are propelled by a chain fitted with cleats or by means of traction exerted on the edges of the tiles by a pair of horizontally-opposed endless belts,- this, in silk- screen printing processes involving glaze in both liquid and powder form.

In the case of a chain-drive, much time is wasted when changing the tile format, for the chain has to be changed, as do the cogwheels.

Moreover, when - in order to increase the production rate - two or more tiles may have to be placed consecutively in contact one with the other, so that each tile is propelling the one in front, the impact between the edges, if not parallel, can cause cracking, splintering and even breakages, an unacceptable form of waste.

In the case of propulsion by traction exerted on the edges of the tiles by a pair of belts, the length of the active section of each belt must be greater than the longitudinal length of the tile to be printed, i. e. taken along a direction parallel to the direction of motion of the ceramic tiles, thereby causing a markedly low production rate for tile formats smaller than the maximum envisaged for a given machine. In order to satisfy the need for formats ranging from the smallest to the largest, at least three silk screen printing machines are required, for

example: one for formats, in centimetres, 10x10 to 30x30, another for 15x15 to 44x44 and a third for 30x30 to 70x70 or more. The tile manufacturer thus being obliged to instal a range of machines at increased cost, not only in terms of initial outlay but also for spare parts: each machine in the range being substantially advantageous only for the maximum format envisaged for it.

Finally, difficulty has been encountered in inverting the direction of feed of the tiles, particularly in the case of chain-driven machines, insofar as the drive gearing must always be downstream of the silk screen printing station,- in the case of the machine driven by a pair of horizontally-opposed endless belts, insofar as the relative drive-motor must similarly be downstream (for reasons of precision) , while there must be a longer active-section of the belt above the silk screen printing station in order to ensure the expulsion of the tiles downstream.

In the case of a chain-driven machine, while it is possible to decorate two or more tiles simultaneously with the same pattern, or different ones, but with the same colour, the need nevertheless arises, given that the tiles are not identical in size, with differences of the order of 0.5% and more, the reference guides have to be positioned at a distance equal to a multiple of the maximum longitudinal dimension, with consequent errors in the print-setting when the dimensions of the tiles comprising the group are less than the maximum. There is also loss of accuracy in the print-setting along the transverse edge, since the reference guides have to be adjusted to the maximum transverse dimension, with the consequence that the positioning of the smaller tiles is not precise.

Again, in the case of the machine driven by a pair of horizontally-opposed endless belts, it is possible to decorate two or more tiles simultaneously with the same

pattern, or different ones, but with the same colour,- however, since the tiles are not identical in size, with differences of the order of 0.5% and more, when the tiles are placed in contact with one another, taking the rear transverse edge of that upstream as reference, print- setting errors consequently occur in those downstream, their being affected by the differences in size; the production rate of the belt-driven machine is found to be excessively low, owing to the fact that the endless belts have to travel more slowly than the feeder belt to avoid separating the group of consecutive tiles previously formed by an appropriate positioning device upstream. Such prior art may be subject to further improvements with a view to eliminating the above- outlined drawbacks. From the foregoing emerges the need to resolve the technical problem of inventing a method, with relative machine suitable for implementing the same, for the silk- screen-printing of ceramic tiles whereby it is possible to improve the flexibility of production and to increase the rate of production.

The method and the machine must allow simultaneous decoration of several elements grouped longitudinally, advantageously covering the whole range of possible formats, or a large part of it, while avoiding damaging impact between the tiles comprising each group. A further aspect of the technical problem consist in providing a method and a machine having in particular to ensure high-precision silk screen printing with tiles whose longitudinal dimensions may not be perfectly constant and may actually be very diverse; the machine also being capable of putting into effect methods other than that according to the present invention,- the whole being compact and cost-effective.

A further aspect of the technical problem to be solved is to provide a method and a machine for silk-screen printing of ceramic tiles whereby it is made possible to decorate

ceramic tiles having a longitudinal dimension greater than the length of the active portion of the side belts. The invention resolves the said technical problem by adopting a method of decoration whereby the ceramic tiles are propelled by pressure on their edges so that each tile can be positioned in a single silk-screen printing station so as to form a group of tiles that are in alignment yet separate one from another; at least one positioning device for each tile of a given group being provided so as to make accurate positioning possible, regardless of the variations in the dimensions of the tiles. According to a preferred embodiment, each tile to be printed is taken by a positioning device when it is still engaged by the preceding positioning device. A machine suitable for the implementation of said method comprising a conveyor device for feeding the tiles to a silk-screen printing means and at least two positioning devices having consecutive pairs of endless belts acting on the edges of the tiles in a plane comprising, or laying substantially parallel to, the surface of the tile to be printed and straddling the conveyor device,- each pair of endless belts being so designed as to clamp a tile, or only a part thereof, and to transfer it in a predetermined position under the silk-screen printing means. According to a preferrred embodiment, the minimum distance between a positioning device and the subsequent positioning device is sustantially smaller than the dimension of a tile taken along a direction parallel to the direction of motion of the tile, or of the group of tiles to be printed.

The advantages offered by the invention are: enhanced production rate over the whole range of tile formats,- elimination of rejects due to impact between tiles,- reduced capital outlay relative to that required to purchase several different machines to cover a given range of formats; possibility of screen-printing different

patterns and colours contemporaneously,- possibility of performing two or more consecutive serigraphs at the same station.

According to a further preferred embodiment, the belts of each positioning device comprise an active section co¬ operating with the sides of the tile for clamping and positioning it under the silk-screen printing means and a neutral section,- the active section of corresponding belts of subsequent positioning devices having a symmetrical configuration with respect to a trasverse plane passing through the intermediate zone between a positioning device and the subsequent one at substantially the same distance from the internal ends of the relevant pair of belts. This embodiment has the further advantage of easy inversion of the direction of the feed of the tiles without the need for skilled labour and with less loss of time.

Some embodiments of the invention are illustrated, by way of example, in the seventeen drawing tables attached, in which:

Figure l is the plan view of the positioning station of a silk screen printing machine according to the invention in the version with the motors of the two pairs of horizontal drive-belts mounted in opposition, namely, at the outside extremities of each pair of belts, in the case of silk- screen printing on one tile at a time: it to be noted that such a solution, which is symmetrical to a transverse medium plane, is particularly advantageous should it be possible to instal the machine so that the tiles can be introduced from either end by inserting the photoelectric cell at the effective point of entry and verifying the direction of rotation of the motors and the positioning of the tensioners;

Figure 2 is a view as in Figure 1, but with the motors mounted on the same side, namely, at the corresponding extremities of the pairs of belts, and when two tiles are

being screen-printed simultaneously: it to be noted that this solution, which is less advantageous than the first in terms of bulk and reversibility of feed, is nevertheless preferable on account of the shorter length of the side under tension of the first pair of belts, which enables the tile to be positioned even more accurately since there is less deformation in each belt; Figure 3 is a schematic front elevation of a silk screen printing machine according to the invention, with the carriage of the spatulas, or squegees, in the ready position;

Figure 4 is a view as in Figure 3, but partial and relating to a machine for silk screen printing with powdered glaze, still according to the invention,- Figure 5 is the vertical centre-line cross section V-V of Figure 3,-

Figure 6 is the plan view of an oscillating lever constituting a pressure device for making the active section of each pair of drive-belts bear firmly on the tile,-

Figure 7 is the section VII-VII of Figure 6; Figures 8, 10, 12, 14, 16, 18, 20 and 22 are the simplified schematic plan views of the machine as in Figure 1, in the case of simultaneous silk screen printing on two tiles during phases a, b, c, d, e, f, g and h, respectively, described below, four tiles A, B, C and D being represented, of which A and B have already been printed, while C and D have yet to be printed; the variation of speed in relation to time for the single phases of positioning of tiles C and D being represented in the corresponding Figures 9, 11, 13, 15, 17, 19, 21 and 23, each consisting of four simplified diagrams (relating in order to the first pair of belts, the_ second pair, the spatula group during positioning in the first pair of belts, i.e. farther upstream, and that in the second pair of belts, i.e. farther downstream, respectively) .

The sequence obtained considering the first, second and fourth diagram of Figures 9, 11, 13, 15, 17, 19, 21 and 23 corresponds to a cycle of silk screen printing whereby the pattern of each station is printed on all the tiles,- the sequence obtained considering, in succession, sequences consisting of the first, second and third diagram and sequences consisting of the first, second and fourth diagram for the same Figures corresponds to a cycle of silk screen printing whereby a single pattern is reproduced on each tile: in this case too the pattern can be the same for all the tiles or there may be as many patterns as there are silk screen printing stations. According to a further embodiment, not shown, in which three stations are provided, four positioning cycles can be adopted followed by a single cycle of silk screen printing in order to obtain a plain tile.

The positioning cycle may involve one or more of the following steps: a) beginning of the positioning cycle of tile C in the first pair of belts, with consequent acceleration from speed zero to that of the feeder-belts and consequent beginning of expulsion of tile A, which has already been printed (Figures 8, 9) ; b) insertion of tile C between the first pair of belts and continuing advancement of tiles A and B (Figures 10, 11) c) constant speed of the first pair of belts and, for a brief stretch, equal to the speed of the pair of feeder- belts in order both to reduce wear on the first pair of drive-belts as well as on the edges of the tiles and to ensure safe expulsion of tile A: it being envisaged that the second set of belts reaches working speed directly without risk of wear of belts and tiles, given that there is no sliding contact (Figures 12, 13) ; d) acceleration of the first pair of belts and of tiles B and C to working speed, expulsion of tile A continuing at working speed, while tile B reaches working speed on

meeting the second set of belts (Figures 14, 15) ; e) coincidence of the rear edge of tile B with the axis of the photoelectric cell with opposed lateral transmitter and receiver, in turn coinciding with the transverse centre-line axis Q (Figure 9) of the silk-screen printing station (Figures 16, 17) ; f) arrest, following deceleration, of the second pair of drive-belts in order to bring tile B to a standstill; coincidence, at working speed, of the rear edge of tile C with the transverse axis of the first photoelectric cell; tile A is expelled (Figures 18, 19) ,- g) advance of tile C at working speed to the point where deceleration starts prior to its being positioned in the silk screen printing station (Figures 20. 21) ; h) arrival of tile C at the silk screen printing station (Figures 22, 23) .

While waiting for tile D to be positioned, the spatulas remain idle in the back position, as in Figure 3. When the leading edge of tile D coincides with the transverse axis of the photoelectric cell at the entrance to the first pair of belts it triggers the same cycle of phases a) to h) as tile C went through, with consequent setting in motion of the spatulas for the spreading of the glaze and subsequent silk screen printing (see the fourth diagram of each of the Figures 9, 11, 13, 15, 17, 19, 21, 23) .

In order to enable a single tile E, which is longer than the active section of a pair of belts, to be expelled when it emerges from the active section of the first pair of belts, the speed of the second pair of belts is increased slightly (Figures 24, 25) , while the following tile F advances at working speed through the active section of the first pair; subsequently, the gap. between the two tiles increases (Figures 26, 27) , while the glaze continues to be spread over the screen,- finally (Figures 28, 29) , tile F enters the second active section and takes

up a position halfway between the two sections, while tile

E has already been expelled.

In this context, that of screen-printing a single long tile, it is to be noted that Figures 25, 27, 29 consist of only three diagrams, insofar as only one tile can be printed during each printing cycle.

Figure 30 is a vertical section V-V of Figure 3 showing a baffle plate S mounted on the transverse centre line of the screen and dividing the screen into two areas for the separation of glazes of different colours: the two spatulas being indented accordingly.

The positioning cycle in the case of Figures 24 to 29 may proceed in accordance with one or more of the following steps: al) beginning of the positioning cycle of tile F in the first pair of belts, both pairs of belts accelerating from speed zero to that of the feeder belts; bl) insertion of tile F between the first pair of belts with tile E continuing to advance; cl) constant speed of both pairs of belts equal, for a brief stretch, to the speed of the pair of feeder belts,- dl) acceleration of the first and second pair of belts and of tiles E and F to an intermediate rate of advancement; el) acceleration of the second pair of belts to working speed to expel tile E once the rear edge of said tile clears the first pair of belts (Figure 25) ,- fl) maintenance of working speed on the part of the second pair of belts for the complete expulsion of tile E with simultaneous acceleration of the first pair of belts to working speed for the positioning of tile F engaging between both pairs of belts without slippage (Figure 27) ,- gl) contemporaneous deceleration of both pairs of belts for positioning in the silk screen printing station

(Figure 29) ; hi) arrival of tile F at the silk screen printing station.

In the case of silk screen printing with powdered glaze

(Figure 4) the cycle does not include the phase in which the glaze is spread but only the actual silk screen printing phase, which occurs when the tiles are already in position as in phases a) to h) .

The figures show: 1, a pair of feeder belts moving with a speed of up to 30 or more metres per minute, said feeder belts defining a conveyor device for feeding the tiles to the silk-screen printing machine according to the invention,- 2 and 3 respectively, active sections of the two pairs of horizontal endless belts 4 and 5 that advance and position tiles A, B, C, D, ... , by bearing against their sides, with the possibility of reaching or exceeding working feed velocities of up to 100-120 or more metres per minute: each active section being defined between a first end-roller 10a and a second end-roller 10b; 6 and 7, two pairs of electric motors to drive the pairs of belts 4 and 5 and for the automatic positioning of tiles A, B, C, D, ... , for example, of the so-called step-motor type or direct-current motors with encoder and tachometric dynamo; 8, pneumatic jacks with levers 8a to keep the active sections of the two pairs of belts 2 and 3 in contact with the tiles on one side,- 9, tensioning pneumatic jacks,- 10, reaction rollers on the opposite side of the active sections 2 and 3 of the belts,- 12, a first vertical photoelectric cell, advantageously positioned across the path of the tiles, installed just before the point of contact of the tile with the first pair or first stretch of belts 2, that are excited with the passage of the front edge of the tile and de-energized with the passage the rear edge,- 13, a second photoelectric cell, for example, of the transmitting-receiving type, advantageously horizontal, having opposing elements positioned in line with the transverse central axis between the two pairs of belts 2 and 3, that are excited with the passage of the front edge of the tile before entering the second stretch and de-energized with the passage of the rear edge,- 13a, a

third photoelectric cell to indicate the arrival of a tile and to anticipate the start of the silk screen printing cycle,- 14, a device, for example, a lead screw device, operated manually with a handwheel, or operated automatically, to move closer together or further apart a pair of support carriages 15, depending on the transverse format of the tiles,- 16, the subframe of the positioning station that is part of the machine as described; 17 (Figure 2) , a subframe which is necessarily larger than that in Figure 1 because of the different position of motors 6 and 7; 18 (Figure 3), the spatulas for spreading the paint and the silk screen printing by means of screen 18a, controlled by crank and tie-rod mechanism 19 and driven by motor 20; 21 (Figure 4), an alternative belt- driven device to achieve a constant velocity of the spatulas 18 useful for screen printing using powdered paint supplied from a hopper which is not shown, it should be noted that there is never any contact between screen 18a and the tile,- 22 (Figures 6 and 7) , a pulley supported on central bearings 23 of lever 8a that pushes on the inside of the active sections of the pair of belts 2 and 3 from the elastic side of them,- 24, a roller to prevent buckling of the active section of the belt, positioned on the external extremity of lever 8a; 25, the fulcrum of lever 8a, mounted on support 15; 26, an arm of lever 8a to which the stem of pneumatic jack 8 is attached by means of a hinge coupling.

It is to be noted that, in order to invert the direction of entry of the tiles, it is enough to move the photoelectric cell 12 to a symmetrical position with respect to the transverse axis and the pneumatic tensioners 9 from their previous positions to the non- tensioned sections in the new configuration (compare Figures 1 and 8) .

The spatulas 27 (Figure ^' 30) , or squegees, are provided with indent 28 for baffle plate S which separates the

different colours on the screen: the provision of a single screen with an intermediate baffle separating different colours is to be regarded as particularly advantageous in view of the possibility of different pattern being printed on the tiles within a single machine.

As regards the longitudinal format of the tiles to be printed, it is to be noted that the maximum longitudinal format may correspond approximately to twice the length of the active section of each station increased by the distance between centres of the last pair of pulleys of the active sections of one station and the first pair of pulleys of the active section the following station,- the minimum longitudinal format corresponds approximately the distance between centres as above increased by 30%. In practice, the materials, dimensions and details of execution may be different from but technically equivalent to those described without departing from the juridical domain of the present invention.