GLASS”

Description

The present invention relates to a method and a machine for the digital printing of flat glass sheets.

The invention can be usefully, but not exclusively, applied in the sector of manufacturing windows for motor vehicles.

In particular, the invention relates to the decoration of glass sheets with vitrifiable inks in the step prior to hardening. Such inks stand out due to the formation of a vitreous layer, with similar technical features to the surface to be decorated.

During the hardening step the decorated glass can remain flat or be subjected to a forming step in a mould for obtaining a three-dimensional shape. The forming step and the vitrification step of the ink applied are always simultaneous as both take place during the hardening of the glass.

The glass sheets are preferably decorated with completely covering bands, which sometimes affect the entire surface of the glass, and/or by affixing logos and/or various writing. Such dark and matt bands, as well as having aesthetic purposes, also have a technical function which depends on the field of application. For example, in the automotive sector, the bands must shield the solar radiation to protect the glue of the window onto the chassis of the motor vehicle from solar radiation.

Other examples of glass sheets are decorative glass items or glass sheets for hobs or glass sheets in general where digital printing needs to be performed with a certain decoration or drawing.

As is known, in the state of the art, the decoration of flat glass sheets mainly takes place through screen printing or alternatively through ink jet printing.

Screen printing is characterised by low installation costs and good continuous production speed. On the other hand, the starting of the screen printing line is time consuming, hence implying lost efficiency in small productions, and requires specialised personnel with long training times. Furthermore, the use of screen printing sheets does not allow production flexibility as, to change the decoration, the screen printing sheets need to be replaced.

Ink jet printing offers the advantage of high production flexibility, as it enables the decoration applied to the sheets to be changed as preferred. On the other hand, the current printing process is rather slow. The reason is connected with the non-absorbent nature of the medium to be printed and the chemical/physical characteristics of the inks used in digital printing. For example, the density and viscosity of the inks for digital printing are substantially lower with respect to screen printing inks. To obtain prints with sufficient definition and sufficient opaqueness, it is necessary to perform various printing steps, to deposit a sufficient thickness of ink on the sheets. Furthermore, despite it being applied in various steps, the decoration is often unstable and tends to be deformed after application, compromising the definition and overall quality of the final result.

In particular, this is more significant in cases in which, due to technical and/or aesthetic requirements, a large quantity of ink needs to be applied. The object of the present invention is to offer a method and a machine for the digital printing of glass sheets which enables obviating the limits of the current technologies.

Characteristics and advantages of the present invention will more fully emerge from the following detailed description of an embodiment of the invention, as illustrated in a non-limiting example in the accompanying figures, in which:

figure 1 shows an isometric view of a machine according to the present invention;

figure 2 shows the machine of figure 1 in a lateral view, with some covering panels removed to show the internal parts of the machine;

figure 3 is a sectional view of a component of the machine according to the present invention;

The method according to the present invention envisages, prior to the application of ink, a heating step of the glass sheets. The heating step takes place while the sheet advances on a transport surface, which will be better described below. Preferably, the heating is performed through a static source. This allows a source of power and considerable dimensions to be provided.

The heating step is extremely useful and advantageous, as it allows the subsequent application of a layer of ink with a consistent thickness, a thickness that, with current technologies, can only be applied through the subsequent superposition of various layers of ink.

In fact, in current technologies, it is not possible to apply a layer of ink whose thickness exceeds a certain limit. This is because, straight after application, it tends to spread outside the applied pattern, compromising the structure of the pattern itself. In current technologies it is therefore necessary to apply layers with a low thickness, superposed in succession with each other, to allow each layer to quickly consolidate prior to the application of the subsequent layer. As already mentioned, this implies a significant extension in the time scales for the completion of the decoration.

In the method according to the present invention, the heating of the sheet that precedes the application of the layer of ink allows a layer of ink to be applied with a significantly higher thickness with respect to what is allowed by current technologies.

In fact, the heating of the sheet determines quick consolidation at least of the lower area of the layer of ink, i.e. of the area in contact with the surface of the sheet. Such lower area quickly takes on a greater density and a greater consistency, and allows the layer of ink to be kept stable which, unlike what happens in current technologies, does not expand and does not spread outside the outlines envisaged for the decoration. In other words, the structure and graphics of the decoration applied remain faithful to the envisaged pattern, also for a high thickness of the layer of ink applied. In a possible embodiment of the method, the heating of the glass sheets, prior to the application of the ink, is performed by exposing the sheets to the radiation of one or more IR lamps. Such lamps are positioned so as to irradiate the sheets. For example, the lamps may be positioned above the sheets in a static position. Obviously, other solutions for heating the sheets are possible, e.g. using hot air or heaters of another kind, generally known to a person skilled in the art.

Preferably, the heating brings the sheets to a temperature comprised between 50 and 80 degrees centigrade.

Advantageously, the heating of the sheets is performed while the latter are transported in advancement on a mobile plane (2). Such mobile plane (2) transports the sheets to a printhead (4), e.g. an inkjet one.

Also the application of the layer of ink takes place while the sheets are transported in advancement on the mobile plane (2). In fact, as already underlined, the heating of the sheets produces quick consolidation of the ink as soon as it has been applied, which can therefore be applied in a single step, also for consistent thicknesses. The sheets are taken to the printhead (4) continuously, without needing to stop.

After application, the layer of ink is subjected to a drying step. Such drying step has the aim of further consolidating the layer of ink applied on the glass sheets.

The drying of the ink can be performed, for example, by exposing the glass sheets to the radiation of one or more UV lamps and/or one or more IR lamps. Alternatively, a heating means of another kind, known in the sector, can be used.

Advantageously, the drying of the ink can be performed, in combination or alternatively to the lamps mentioned above, by exposing the glass sheet to an air current directed downwards. The use of an air current, possibly heated, allows the drying of the ink to be made quicker. Furthermore, the downwards direction of the air current prevents the layer of ink being transversally strained. The drying of the layer of ink is performed while the sheet is transported in advancement on the mobile plane (2).

In another particularly advantageous embodiment, illustrated in figure 3 in a sectional view on a vertical plane parallel to the transport direction (X), the mobile plane (2) comprises a plurality of rollers (20) each of which has a cylindrical outer surface (21 ), provided to allow the support of a sheet to be transported. The rollers (20) are activated in rotation by a motor (23), connected by a transmission element (24). The transmission element (24) is placed in contact with the outer surface (21 ) of the rollers (20), or in contact with a portion of the rollers (20) which has the same diameter as the outer surface (21 ).

In the preferred embodiment, the transmission element (24) is placed in contact with the outer surface (21 ) of the rollers (20) so as to transfer the motion to the outer surface (21 ) itself. In other words, the contact between the transmission element (24) and the outer surface (21 ) takes place on a portion of the outer surface (21 ) tangential to the transport plane (a). This means that the outer surfaces (21 ) of the various rollers (20) necessarily have a peripheral speed equal to the speed of the transmission element (24), and therefore have the same peripheral speed as one another. Such equal peripheral speed condition between the outer surfaces (21 ) also occurs in the event in which the outer surfaces (21 ) do not have exactly the same diameter, or even have a different diameter.

The solution of placing the transmission element (24) in contact with the outer surface (21 ) of the rollers (20), or however of making the transmission element (24) interact with a portion of the rollers (20) that has the same diameter as the outer surface (21 ), therefore allows a movement to be performed such that all the rollers (20) have the same peripheral speed at the outer surface (21 ). This enables the objects to be transported in advancement with great precision, without producing undesired vibrations or rotations.

In the embodiment represented, which is preferred but not exclusive, the transmission element (24) comprises a belt. Preferably, but not necessarily, the mobile plane (2) comprises a plurality of tensioning rollers (25), interposed between the rollers (20) and placed in contact with the transmission element (24), on the opposite side with respect to the rollers (20). In substance, the tensioning rollers (25) are arranged so as to stretch the transmission element (24) in contact with the rollers (20) so as to press the transmission element (24) in contact with the outer surfaces (21 ). Thanks to the presence of the tensioning rollers (25) the winding angle of the transmission element (24) around the outer surfaces (21 ) increases.

Considering the advancement direction of the sheets along the transport direction (X), the machine for the actuation of the method comprises, proceeding from an inlet zone, a heating device (3), provided to heat the glass sheets on the mobile plane (2). As already indicated, in a possible embodiment, the heating device (3) comprises one or more IR lamps, facing towards the mobile plane (2) so as to irradiate the mobile plane (2) itself. Preferably, the lamps are connected above the mobile plane (2).

Downstream of the heating device (3) an inkjet printhead (4) is located. Also the printhead (4) is located above the mobile plane (2). An example of a particularly suitable printhead for the purpose is described in patent applications 202017000147594 of 20/12/2017 and 202018000001943 of 20/02/2018, filed in the name of the same Applicant.

The printhead indicated above comprises two or more printheads, each of which comprises two or more rows of nozzles that are parallel to a first alignment direction. The rows of nozzles are staggered with respect to a second direction perpendicular to the first alignment direction. The first alignment direction is arranged perpendicular with respect to the transport direction (X) of the sheets to be decorated. Alternatively, the alignment direction can be inclined diagonally with respect to a transport direction (X).

As is known, the projection of all the nozzles belonging to the various heads on a straight line perpendicular to the transport direction (X) defines a print front. Thanks to the staggered arrangement of the rows of nozzles with respect to the second parallel direction to the alignment direction, the printhead allows a print head to be obtained with greater definition, i.e. with a higher number of nozzles per unit of length, with respect to a printhead of the known type, with printheads oriented perpendicularly to the transport direction (X). Furthermore, it is possible to distribute the nozzles uniformly, also in the transition zones between one head and another, without needing to resort to a staggered arrangement of the heads.

Downstream of the printhead (4), the machine comprises the drying device (5), provided to dry the layer of ink applied by the printhead (4) onto the glass sheets.

The drying device (5) comprises one or more UV or IR lamps, facing towards the mobile plane (2) so as to irradiate the mobile plane (2) itself. The lamps are preferably arranged above the mobile plane (2). Alternatively or in combination with the lamps, the drying device (5) comprises a fan means (52), provided to send towards the mobile plane (2) an air current directed downwards. The fan means (52) may be provided with a heater for heating the air current and speeding up the drying of the ink.

The heating device (3), the printhead (4) and the drying device (5) operate on the mobile plane (2), while the sheets advance on the mobile plane (2). The production cycle is therefore continuous, i.e. the sheets can advance continuously along the mobile plane (2) without needing to stop.