WO2013035073A1

WO2013035073A1 - Apparatus and method for printing a web of paper

Info

Publication number: WO2013035073A1
Application number: PCT/IB2012/054630
Authority: WO
Inventors: Marco FARETRA; Giovanni Barbanti; Lia Grandi; Daniele Dall'olio
Original assignee: Barbalab S.R.L.; Innovando S.R.L.
Priority date: 2011-09-08
Filing date: 2012-09-07
Publication date: 2013-03-14
Also published as: ITBO20110514A1

Abstract

An apparatus (1) for printing a web (2) of paper comprises: manipulating means configured for operating on the web in an infeed zone (3) and in an outfeed zone (4) in order to keep the web taut in the stretch between the two zones, the web (2) being fed continuously along a longitudinal direction (5) between the infeed zone and the outfeed zone, and printing means interposed between the infeed zone and the outfeed zone; the printing means comprise a digital inkjet printing device (7) having a plurality of ink delivery nozzles (8) and a control unit (9) connected to the nozzles (8) in order to drive them.

Description

APPARATUS AND METHOD FOR PRINTING A WEB OF PAPER

Technical Field

This invention relates to an apparatus and a method for printing a web of paper.

More specifically, the invention relates to an apparatus and a method for printing a web of multi-ply paper.

Background Art

The technical sector of this invention is that of the continuous production of paper, normally referred to as "tissue", in the form a web which is then wound into rolls.

The web usually consists of multi-ply paper and, for the sake of simplicity, this description refers to a web of multi-ply paper. It should be noted, however, that this invention also applies to the printing of webs of one-ply paper.

Typically, the web of multi-ply paper can be divided into tear-off lengths.

Examples of this type of web are toilet paper and kitchen paper.

These rolls of web are made by very complex machine installations.

An installation of this kind includes a system of rollers connected to drive motors which constitute means for feeding the web from one station to another of a plurality of stations in the installation.

In effect, the installation comprises several stations, as listed below (by way of a non-exhaustive example):

- one or more stations for feeding paper from bulk reels;

- in the case of - multi-ply web, an assembly station where a plurality of sheets of paper are combined to form the web of multi-ply paper;

- a web embossing station;

- a web printing station;

- a station where lines of weakness are made transversely on the web in order to facilitate dividing the web into tear-off lengths;

- a station for winding the web onto rolls;

- a station where the rolls are cut into smaller rolls of suitable length. In these installations, the web moves at very high speeds, in the order of hundreds of metres per minute (up to 800 metres per minute and more).

Typically, the webs are from approximately 2 metres wide to approximately 5 metres wide or even wider.

This poses a problem regarding feeding and handling of the web.

As regards printing, in prior art installations, a system of rollers is used to print the web. The web is made to pass in contact with a roller which deposits ink on one face of the web continuously.

Thus, the diameter of the roller which deposits ink on the web determines the maximum length of the web print, after which the print is necessarily repeated identically.

Indeed, the continuous rotation of the roller which deposits ink on the web defines a repeated printed pattern.

This constitutes a limitation because it is impossible, with the prior art technology described above, to reproduce on the web a print which is never repeated for the entire length of the web or which is not repeated for any desired length.

Moreover, in these paper treatment installations, owing to the high speeds involved, the diameter of the rollers (in particular of the roller which deposits ink on the web) is subject to limitations due to the masses, dimensions and inertias and to the path which the paper has to follow around the roller itself.

In practice, prior art systems make it possible to print without repetitions portions of web not longer than a limited number of web lengths: for example, seven lengths. That means, in this example, that the eighth length must necessarily be printed with the same design (that is, the same print) as one reproduced in a preceding length of the web.

Prior art systems for printing a web of paper also have the disadvantage of requiring particularly complex changeover procedures to change the size of the paper being processed and the printing rollers.

In effect, in the printing station, the web must be made to pass through a complicated system of rollers.

For quality printing on sheets of paper, the use of digital inkjet printers is also known.

It should be noted that normally, digital inkjet printers are configured to make high resolution prints, for example 300 dpi (dots per inch) or more (typically on single sheets of paper or, in an industrial context, on webs of paper). In contrast, there is no need, in digital inkjet printers, to move the sheets at particularly high speeds such as those required by a continuous production line.

In light of this, digital inkjet printers are not in use suitable for printing a web of paper (multi-ply) fed continuously at high speed (the speed being determined by the production installation) because the maximum activation frequency of the ink dispensing nozzles is too low.

In practice, after a nozzle has fired its ink, the time needed for it to be recharged so it can fire ink again is too long, or rather it is incompatible with the speed at which the web is moving, to obtain the required longitudinal resolution.

In this regard, it should be noted that print heads are known which incorporate a plurality of nozzles (that is, an array of nozzles) arranged in rows spaced from each other along one direction (longitudinal) and slightly offset from each other along the transversal direction.

In these print heads, successive rows in the longitudinal direction are activated with a suitable time-phase resolution in such a way that the nozzles of all the rows deliver the ink (in a succession of deliveries) on points on the sheet of paper which are aligned along the same transversal axis.

Such a configuration is adopted in order to obtain a particularly high transversal print resolution where the spacing between two adjacent points printed on the paper is less than the minimum spacing at which two nozzles can be positioned (due to mechanical space limitations).

Thus, the first row of nozzles fires ink successively on a corresponding row of points. Next, the paper is fed longitudinally until that row of points is aligned along a transversal axis with the next row of nozzles, which fire ink on a corresponding row of points on the paper which, relative to the points made in the immediately preceding delivery, are located on the same transversal axis but transversely offset from each other.

This solution therefore makes it possible to increase resolution but is not suitable for printing a web which is fed continuously at a very high speed (since the firing repetition frequency of the nozzles is too low to guarantee the required resolution also in the longitudinal direction).

Thus, in digital inkjet printing systems, the feed speed of the paper is adapted to the optimum frequency for nozzle operation relative to a given frequency. Also known are print nozzles which are configured to operate at particularly high frequencies.

These nozzles, however, which are called "bubble-jet" nozzles, have the disadvantage of printing very small dots. Moreover, these nozzles are used to produce faint, barely visible print, such as for example, "best before" dates on food product labels.

These bubble-jet nozzles are therefore unsuitable for paper (especially multi-ply paper) intended for rolls because the print would be hardly visible. Aim of the Invention

This invention has for an aim to provide an apparatus for printing a web of paper (which can be divided into tear-off lengths) which overcomes the abovementioned drawbacks of the prior art.

More specifically, the aim of this invention is to provide an apparatus for printing a web of paper (which can be divided into tear-off lengths) which guarantees wide freedom of printing, with the possibility of printing web portions of any length even without having to repeat the same print (or the same design reproduced on the web by printing).

A further aim of the invention is to provide an apparatus for printing a web of paper (which can be divided into tear-off lengths) which allows paper size changeover to be effected in a particularly quick and easy manner.

These aims are fully achieved by the apparatus according to the invention as characterized in the appended claims.

More specifically, the apparatus of the invention is an apparatus for printing a web of paper, comprising:

- manipulating means configured for operating on the web in an infeed zone and in an outfeed zone in order to keep the web taut in the stretch between the two zones, the web being fed continuously along a longitudinal direction between the infeed zone and the outfeed zone;

- printing means interposed between the infeed zone and the outfeed zone.

According to the invention, the printing means comprise a digital printing device having a plurality of nozzles and a control unit connected to the nozzles in order to drive them.

The term "nozzle" is used to denote, generally speaking, an element which is configured to leave a mark on the paper and which can be driven (for example, by way of an electrical or optical signal) by a digitally programmable controller.

In the preferred embodiment described below, the digital printing device is a digital inkjet printing device and the nozzles are ink delivery nozzles.

According to another aspect of the invention, however, a digital printing system of a different type (other than inkjet) might be used, based for example on nozzles configured to generate a voltage arc and to burn the surface of, or perforate_s the paper.

Whatever the case, the nozzle acts on the paper in such a way as to leave a visually perceptible mark, independently of the technology used to make such a mark.

In light of this, all that pertains to the control unit and its operation, that is to say, the mode of controlling and driving the nozzles (as described below) also applies to such nozzles of a different type (that is, non-inkjet).

Having said that, this description will continue with specific reference to a digital inkjet printing system.

The use of a digital printing system (in particular, an inkjet system) makes it possible to determine the print pattern via software, without any need for the pattern to be repeated.

In principle, and by way of an example, such a system might be used to print the text of a novel on a roll of paper in such a way that the text reads along the length of the web.

The apparatus also preferably comprises at least a first and a second nozzle of the plurality which are longitudinally spaced by a predetermined quantity (and substantially aligned along a longitudinal axis).

In this context, the control unit is programmed to drive the at least first and second nozzles in such a way that in immediately successive deliveries they deposit ink on a first and a second point on the web, respectively, adjacent to one another, at a predetermined distance along the longitudinal axis and substantially aligned along the longitudinal axis.

This advantageously makes it possible to control a plurality of nozzles

(each of which has a limited maximum activation frequency) as a single nozzle having an activation frequency which is equal to the sum of the maximum activation frequencies of the individual nozzles.

As regards the expression "substantially aligned", it should be noted that the at least first and second nozzles in particular are spaced, relative to a transversal direction (perpendicular to the longitudinal axis) by a distance which is less than the minimum spacing between two adjacent printed points on the web, this minimum spacing being a function of the print resolution of the printing device.

Still more preferably, the at least first and second nozzles are exactly aligned along a longitudinal axis (longitudinal being the direction of web feed).

More specifically, the control unit is programmed to delay (that is, temporally displace) ink delivery from the second nozzle relative to ink delivery from the first nozzle by a predetermined length of time, as a function of web feed speed and nozzle spacing, the second nozzle being located after the first in the direction of web feed.

This makes it possible to provide a desired longitudinal print resolution, at a high speed of paper feed, independently of the spacing between the nozzles which are aligned and longitudinally spaced to act as a single nozzle with a higher frequency.

Preferably, the nozzles are arranged in a plurality of rows and columns defining an array.

One row of nozzles is arranged along a direction transversal, that is, perpendicular, to the longitudinal direction of web feed, or inclined to the latter at a small angle (for example, less than 20 degrees).

Other rows of nozzles are arranged alongside the first and spaced (by a known quantity) along the longitudinal direction in such a way as to define columns of nozzles equal in number to the number of nozzles making up the rows.

Each column comprises a number of nozzles (equal to the number of rows side by side) substantially aligned along the longitudinal direction.

The nozzles of each column are controlled by the control unit in synchronized manner (if necessary by assigning a delay time to the nozzles following them).

It should be noted that the control unit is configured to assign the delivery delay, if any, of the nozzles following the ones preceding them in such a way that the distance travelled by the web during such delay time is different from (typically greater than) the distance which separates the nozzles (longitudinally). The reason for that is to prevent corresponding successive nozzles (aligned and longitudinally spaced) from firing ink on the same point, that is, on points which are aligned along a transversal axis (even if these were slightly offset transversely, because these nozzles are not exactly aligned but substantially aligned).

The number of nozzles making up the rows (and the length of the rows) is such as to cover the width of the web or the (entire) portion of web to be printed.

By setting a transversal spacing between at least a first and a second nozzle which is small enough for them to be considered substantially aligned along the longitudinal axis, the delivery from a first nozzle and that from a second nozzle can be visually interpreted as a single delivery.

Preferably, the set of longitudinally aligned nozzles is organized in a plurality of groups of nozzles where each group comprises a predetermined number of nozzles.

Preferably, the nozzles of each group are activated in succession by the control unit in such a way as to deliver ink to points which are close together (longitudinally spaced by a first predetermined distance) located within the same zone of the paper (which will be perceived by an observer as one inked area).

Instead, the nozzles of one group, relative to the nozzles of the group preceding it (in the direction of paper feed) are activated by the control unit with a predetermined delay (that is, they are activated in succession, one after the other, in groups) in such a way that the nozzles of the following group deliver ink to a second zone of the paper which is spaced longitudinally from the first zone by a second predetermined distance (greater than the first predetermined distance). That way, the observer will perceive a sequence of inked areas which are longitudinally spaced from each other.

Thus, preferably, two or more groups of nozzles are located along a longitudinal axis (each group comprising a predetermined number of nozzles), the nozzles of each group being spaced longitudinally from each other by

and the groups being spaced from each other by predetermined quantities. The control unit is programmed to drive the nozzles in synchronized manner as a function of the speed of web feed and nozzle spacing, in such a way that nozzles in the same group fire ink in succession at points on the paper which are longitudinally spaced by a first predetermined distance, to produce on the web a first inked area, and (in such a way that) nozzles in different groups fire ink in succession at points spaced longitudinally at least by a second predetermined distance greater than the first distance, to produce on the web a second inked area which is longitudinally spaced from the first inked area. That way, these nozzles (that is, the nozzles in the set) correspond, for the purposes of the printing on the web, to a single nozzle which has a maximum activation frequency equal to the maximum activation frequency of the individual nozzles multiplied by the number of groups and which is designed, at each delivery, to fire a quantity of ink equal to the quantity deliverable by a single nozzle multiplied by the number of nozzles making up the group.

According to an alternative mode of driving the nozzles of the set, divided into groups of nozzles, the control unit is configured to activate the nozzles of one group simultaneously. In that case, the first distance (which separates the inked points within one area) is equal to the spacing which separates the nozzles of the group.

If two groups of nozzles are activated simultaneously, the distance between two successive inked areas is equal to the spacing between two consecutive groups of nozzles.

In a preferred embodiment, however, the control unit is configured to drive the nozzles (of the same group and of the various groups) in succession with predetermined delay times, with the possibility of producing inked points in the same area spaced at a first distance which is less than the spacing between the nozzles of the group, and with the possibility of producing consecutive inked areas spaced at a second distance which is less than the spacing between the two consecutive groups of nozzles.

This invention also provides a method for printing a web of paper (multiply)-

The method comprises the following steps:

- manipulating the web in an infeed zone and in an outfeed zone in order to keep the web taut in the stretch between the two zones;

- feeding the web continuously along a longitudinal direction between the infeed zone and the outfeed zone (at a constant speed determined by a production line in which the web is being processed);

- printing the web in the stretch between the infeed zone and the outfeed zone.

According to the invention, the printing step comprises driving a plurality of nozzles of a digital inkjet printing device through the agency of a control unit.

Preferably, the control unit drives at least a first and a second nozzle forming part of the plurality and being spaced longitudinally by a predetermined quantity and substantially aligned along a longitudinal axis, in such a way that they deliver the ink to a first and a second point on the web, located respectively at a predetermined distance along the longitudinal axis and substantially aligned along the longitudinal axis.

More generally speaking, in the context of the method for making the web and the roll, the printing step may be carried out before or after the embossing step.

Moreover, printing may be performed simultaneously on two sides of the web (by digital printing devices mounted on both sides of the web) in a particularly efficient manner (for example, minimizing dimensions).

Brief Description of the Drawings

These and other features of the invention will become more apparent from the following detailed description of a preferred, non-limiting embodiment of it, with reference to the accompanying drawings, in which:

- Figure 1 is a schematic side view of an apparatus for printing a web of paper;

- Figures 2a, 2b and 2c schematically illustrate a portion of the apparatus of Figure 1 , in corresponding steps of an operating sequence;

- Figure 3 is a top view showing the apparatus of Figure 1.

Detailed Description of the Preferred Embodiments of the Invention

With reference to the drawings, the numeral 1 denotes an apparatus for printing a web 2 of paper.

The apparatus 1 comprises an infeed zone 3, where the web is received, and an outfeed zone 4, where the web is delivered after being printed.

The web is fed continuously between the infeed zone 3 and the outfeed zone 4.

Between the infeed zone 3 and the outfeed zone 4, the web is fed at a substantially constant speed, determined by the operation of the installation in which the apparatus 1 is installed. More specifically, the installation is an installation for making rolls of paper divided into tear-off lengths.

The web of paper is, in particular, a web of multi-ply paper.

The apparatus comprises manipulating means configured for operating on the web 2 in the infeed zone 3 and in the outfeed zone 4 in order to keep the web taut in the stretch between the two zones. It should be noted that the web 2 is fed continuously along a longitudinal direction 5 between the infeed zone 3 and the outfeed zone 4.

The apparatus 1 comprises a digital inkjet printing device 7 interposed between the infeed zone 3 and the outfeed zone 4 of the apparatus 1.

The digital inkjet printing device 7 comprises has a plurality of ink delivery nozzles 8 and a control unit 9 connected to the nozzles 8 in order to drive them.

The nozzles 8 are, for example, piezoelectric nozzles (of a per se known type).

It should be noted that the manipulating means comprise, for example, a plurality of rollers 6. The web 2 is operatively coupled to the rollers 6 both upstream and downstream of the printing means, in the infeed and outfeed zones.

Preferably, at least a first and a second nozzle 8 of the plurality are longitudinally spaced by a predetermined quantity and are substantially aligned along a longitudinal axis.

The control unit 9 is programmed to drive the at least first and second nozzles 8 in such a way that in immediately successive deliveries they deposit ink on a first and a second point on the web, respectively, adjacent to one another, at a predetermined distance along the longitudinal axis and substantially aligned along the longitudinal axis.

Preferably, the control unit 9 is programmed to delay ink delivery from the second nozzle relative to ink delivery from the first nozzle by a predetermined length of time, as a function of the feed speed of the web and the spacing between the nozzles (that is, between consecutive aligned and longitudinally spaced nozzles), the second nozzle being located after the first in the direction of web feed.

In the example illustrated, there are four nozzles (but there may be any predetermined number of nozzles greater than two, depending on requirements, that is, on required printing resolution and on the- speed of the web in the installation for which the apparatus is made) located along a longitudinal axis and spaced from each other by predetermined quantities.

The control unit 9 is programmed to drive the four nozzles in synchronized manner as a function of web feed speed and nozzle spacing in such a way that, for the purposes of the printing on the web 2, the four nozzles correspond to a single nozzle having a maximum activation frequency which is equal to the sum of the maximum activation frequencies of the nozzles considered individually. Preferably, the apparatus 1 comprises:

- a row of nozzles 8 aligned along a transversal direction, perpendicular (or substantially perpendicular) to the direction of web feed;

- for each nozzle 8 in the row, at least one corresponding nozzle 8 substantially aligned and spaced relative to it along a longitudinal axis by a predetermined quantity.

In the example illustrated, the row of nozzles 8 comprises twelve nozzles, each of which has three corresponding nozzles longitudinally aligned relative to it. Thus, the apparatus illustrated comprises a total of four rows of nozzles 8, the rows being longitudinally spaced and arranged in such a way as to form twelve columns of nozzles 8 which are longitudinally aligned with each other.

Obviously, the number of rows and the number of nozzles per row may be varied as required.

More specifically, the choice of the number of nozzles 8 making up a row depends on the width of the web 2 and on the required printing resolution. The number of rows, on the other hand, depends on the maximum activation frequency of the individual nozzles 8 and on the feed speed of the web 2 (as well as on the required printing resolution).

The control unit 9 is programmed to drive each nozzle 8 in the (first) row in a manner synchronized with the at least one corresponding nozzle in such a way that (in immediately successive deliveries, or deliveries separated by a length of time less than the time needed for the nozzle 8 to be recharged) they deliver ink to a first and a second point on the web which are, respectively, adjacently spaced by a predetermined quantity along the longitudinal axis and substantially aligned relative to the longitudinal axis.

Preferably, the nozzles 8 of the row (preferably of each row) are uniformly spaced along the transversal direction.

It should be noted that a plurality of nozzles 8, in particular the at least first and second nozzles 8 (longitudinally aligned and spaced from each other) are contained in a single print head.

The print head might contain nozzles 8 arranged in rows which are slightly offset from each other transversely. In that case, the nozzles 8 of each column would not be exactly aligned along respective longitudinal axes but would in any case be substantially aligned, that is, transversely offset by a quantity which is less (smaller than) the minimum distance required between two adjacent points on the paper, as a function of printing resolution (it should be noted that the printing resolution for this type of application is, typically, approximately 5 dpi).

According to another aspect, the apparatus 1 might comprise a further digital printing device 7 located on the opposite side of the web 2 so as to print the web 2 on both sides simultaneously.

It should be noted that the invention also relates to an installation for making rolls of (multi-ply) paper divided into tear-off lengths.

The installation comprises:

- an assembly station where a plurality of sheets of paper are combined to form a web of (multi-ply) paper;

- a web embossing station;

- a web printing station;

- means for continuously feeding the web between the stations.

According to the invention, the printing station comprises an apparatus as described above.

Below is a description, with reference to Figure 2, of the operation of the apparatus 1.

The web 2 is fed in the direction indicated by the arrow 5 at a speed V along the longitudinal direction.

There are at least two nozzles 8 (four in the drawings) aligned longitudinally along a longitudinal axis and spaced along that axis.

The first nozzle 8 (in the direction 5 of web 2 feed) fires ink on a portion (point) a of the web 2 (Figure 2a), at an instant Ta.

Based on the printing resolution to be obtained, two adjacent points on the paper (that is, two adjacent pixels) must be spaced along the longitudinal direction bya predetermined quantity. Let "d" be the predetermined quantity (distance).

Let "s" be the distance between two consecutive nozzles 8 along the longitudinal axis.

The control unit 9 activates the second nozzle 8 after a time T (delay relative to the instant Ta) given by the following formula: "T = (d + s)/V".

Thus, the second nozzle fires ink on a portion (point) b of the web 2 (Figure 2b) located at the distance d from the portion a along the longitudinal axis.

Figure 2c illustrates the activation of the third nozzle.

Thus, it is sufficient for the first nozzle to be recharged within a time "Tr" equal to "4d/V", that is, the time taken by the web 2 to move by a quantity equal to d multiplied by the number of longitudinally aligned nozzles 8.

This also makes it possible to use a traditional print head comprising an array of nozzles 8, driving the longitudinally aligned nozzles 8 in such a way as to fire ink in sequence on a plurality of adjacent points longitudinally aligned on the paper in an interval of time less than or equal to the time for the nozzles 8 to be recharged.

That way, it is possible to use nozzles 8 having a relatively low activation frequency to ink adjacent points on the web which follow each other longitudinally, despite the fact that the belt is moving at a very high speed. Obviously, the printing resolution thus obtained is lower than that achievable with the same print head driven by a traditional control unit.

It should be noted that if the longitudinally aligned nozzles 8 were longitudinally spaced exactly by the quantity "d" (which the points on the paper are required to be spaced at), an alternative strategy of driving the nozzles 8 might be adopted, that is to say, all the nozzles might be made to fire simultaneously, at intervals of "4d/V".

Whatever the case, the nozzles can be driven in such a way that subsets of nozzles fire simultaneously, relative to a set of nozzles which are longitudinally spaced and aligned and which are driven in synchronized manner by the control unit 9.

Further modes and specific strategies for driving the nozzles are imaginable, using the concept taught by this invention, that is to say, driving a plurality of longitudinally aligned nozzles to fire ink on adjacent points in longitudinal sequence on the paper. That way, the plurality of nozzles are driven as if they were a single nozzle whose activation frequency is equal to the activation frequency of the individual nozzles.

More specifically, the invention provides the following mode of driving the nozzles 8, which are distributed to form a printing array.

The device comprises a set of nozzles 8 which are substantially aligned longitudinally and spaced longitudinally by predetermined quantities, the set defining two or more groups of nozzles, where each group comprises a predetermined number of nozzles.

In light of this, the control unit 9 is programmed to drive the nozzles 8 in synchronized manner as a function of web feed speed, in such a way that the nozzles in the same group fire ink on points on the paper which are longitudinally spaced by a first predetermined distance and nozzles in different groups fire ink on points on the paper which are longitudinally spaced by at least a second predetermined distance which is greater than the first distance.

Thus, the control unit 9 is configured to drive the nozzles of the set in such a way that, for the purposes of the printing on the web, they correspond to a single nozzle 8 having a maximum activation frequency which is equal to the maximum activation frequency of the individual nozzles, multiplied by the number of the groups, and designed to fire, at each delivery, a quantity of ink equal to the quantity deliverable by a single nozzle, multiplied by the number of nozzles making up the group.

This allows the print pattern to be created using particularly wide points, that is, using inked areas where each inked area is formed by a plurality of points made by individual nozzles.

Thus, the nozzles in the same group act in conjunction to form a single inked area.

One inked area is longitudinally spaced from the next (adjacent) inked area by the second predetermined distance.

The second predetermined distance is determined by the number of groups of nozzles.

The size of the individual inked areas, on the other hand, depends on the number of nozzles forming each group.

The points fired by the nozzles of the same group to form a single inked area may be at a small longitudinal distance, as desired, (even much smaller than the distance "d" which, in this driving mode, corresponds to the distance between the inked areas), by suitably displacing temporally the activation of the nozzles in the same group, so that they fire in rapid succession as the paper passes.

It should be noted that the fact that printing is divided into inked areas of relatively large size is particularly advantageous for printing multi-ply webs moving continuously at high speed, because it produces a clearly visible pattern, although a low printing resolution is used (considering that each inked area corresponds to one pixel).

The total number of longitudinally aligned nozzles in the context of the device 7 is given by the number of nozzles 8 forming each group, multiplied by the number of groups (where, for simplicity, it is assumed that each group has the same number of nozzles 8, so that all the inked areas are the same size, although this is not essential).

Preferably, the device 7 comprises a guide (of per se known type, not illustrated in the drawings) arranged transversely to the longitudinal direction of web feed in the print area.

The nozzles 8 of the plurality of nozzles (forming an array of nozzles) are preferably associated with carriers (of per se known type, not illustrated in the drawings, as for example, carriages) which are slidably coupled to the guide in order to be moved transversely.

The device 7 might comprise a plurality of substantially parallel, longitudinally spaced guides, each associated with a plurality of movable carriers.

This allows the number of nozzles 8 to be minimized, with the possibility of adapting the position of the nozzles along the transversal extension of the web, to cover the areas to be actually printed or to adapt to a different size of the web 2.

In light of this, the nozzles of the at least one row are preferably associated with the plurality of transversely movable carriers.

Furthermore, the apparatus 1 preferably comprises, in addition to the printing device 7 described above, at least one further digital printing device (of the same type) located on the same side of the web and spaced longitudinally from the first, each of the printing devices being configured to print the web with a specific colour.

This makes it possible to make colour prints.

This invention therefore also provides a method for printing a web 2 of paper (multi-ply).

The method comprises the following steps:

- manipulating the web 2 in an infeed zone 3 and in an outfeed zone 4 in order to keep the web taut in the stretch between the two zones;

- feeding the web 2 continuously along a longitudinal direction 5 between the infeed zone and the outfeed zone;

- printing the web 2 in the stretch between the infeed zone and the outfeed zone.

According to the invention, the printing step comprises driving a plurality of nozzles 8 of a digital inkjet printing device 7 through the agency of a control unit 9.

Preferably, the control unit 9 drives at least a first and a second nozzle 8 forming part of the plurality and being spaced longitudinally by a predetermined quantity and substantially aligned along a longitudinal axis 5, in such a way that they deliver the ink to a first and a second point on the web 2, located respectively at a predetermined distance along the longitudinal axis and substantially aligned along the longitudinal axis.

Preferably, the control unit 9 delays ink delivery from the second nozzle relative to ink delivery from the first nozzle by a predetermined length of time, as a function of the feed speed of the web and the spacing between the nozzles 8, the second nozzle being located after the first in the direction 5 of web feed.

It should be noted that the invention also provides a method for making rolls of (multi-ply) paper divided into tear-off lengths.

The method comprises the following steps:

- assembling a plurality of sheets of paper to form a web of paper (multi- ply);

- embossing the web;

- printing the web;

- feeding the web continuously during the aforestated steps.

According to the invention, the printing step comprises a method for printing a web of paper (multi-ply) as described above.

The main advantages of this invention are the following.

The invention allows printing on a web of paper (multi-ply) moving at a particularly high speed, varying the print pattern for any stretch of the web, if necessary even along the full length of the web, without having to repeat the pattern at predetermined periodic intervals.

The invention makes changeover to a different web size particularly easy in a paper production and roll rewinding installation.

Claims

1. An apparatus (1) for printing a web (2) of multi-ply paper, comprising:

- manipulating means configured for operating on the web (2) in an infeed zone

(3) and in an outfeed zone (4) in order to keep the web taut in the stretch between the two zones, the web being fed continuously along a longitudinal direction (5) between the infeed zone (3) and the outfeed zone (4);

- printing means interposed between the infeed zone (3) and the outfeed zone

(4) ,

characterized in that the printing means comprise a digital inkjet printing device

(7) having a plurality of ink delivery nozzles (8) and a control unit (9) connected to the nozzles (8) in order to drive them.

2. The apparatus according to claim 1 , wherein at least a first and a second nozzle

(8) forming part of the plurality are spaced longitudinally by a predetermined quantity and substantially aligned along a longitudinal axis, the control unit (9) being programmed to drive the at least one first and one second nozzle (8) in such a way that in immediately successive deliveries they fire the ink at a first and a second point on the web, located respectively at a predetermined distance along the longitudinal axis and substantially aligned along the longitudinal axis.

3. The apparatus according to claim 2, wherein the control unit (9) is programmed to temporally displace ink delivery from the second nozzle (8) relative to ink delivery from the first nozzle (8) by a predetermined length of time, as a function of web (2) feed speed and nozzle (8) spacing, the second nozzle (8) being located -after-the-first-in-the-directiOn of web (2) feed.

4. The apparatus according to claim 2 or 3, wherein two or more nozzles (8) in a predetermined number are spaced longitudinally from each other by predetermined quantities along a longitudinal axis, the control unit (9) being programmed to drive the nozzles (8) in synchronized manner as a function of the speed of web (2) feed and nozzle (8) spacing, in such a way that for the purposes of printing the web the predetermined number of nozzles (8) corresponds to a single nozzle having a maximum activation frequency which is equal to the sum of the maximum activation frequencies of the nozzles (8) considered individually.

5. The apparatus according to any of the preceding claims, wherein two or more groups of nozzles (8) are located along a longitudinal axis, each group comprising a predetermined number of nozzles, the nozzles (8) of each group being spaced longitudinally from each other by predetermined quantities and the groups being spaced from each other by predetermined quantities, and wherein the control unit (9) is programmed to drive the nozzles (8) in synchronized manner as a function of the speed of web (2) feed and nozzle (8) spacing, in such a way that nozzles (8) in the same group fire ink in succession at points on the paper which are longitudinally spaced by a first predetermined distance, to produce on the web (2) a first inked area, and nozzles (8) in different groups fire ink in succession at points spaced longitudinally at least by a second predetermined distance greater than the first distance, to produce on the web (2) a second inked area which is longitudinally spaced from the first inked area.

6. The apparatus according to any of the claims from 2 to 5, wherein the plurality of nozzles (8) comprises:

at least one row of nozzles spaced along a transversal direction perpendicular to the direction (5) of web (2) feed;

- for each nozzle (8) in the row, at least one corresponding nozzle (8) spaced from it along a longitudinal axis by a predetermined quantity and substantially aligned along that longitudinal axis,

the control unit (9) being programmed to drive each nozzle (8) in the row in a manner synchronized with the at least one corresponding nozzle (8) in such a way that they deliver ink to a first and a second point on the web (2) which are, respectively, spaced by a predetermined quantity along the longitudinal axis and substantially aligned relative to the longitudinal axis.

7. The apparatus according to claim 6, wherein the nozzles (8) in the at least one row are associated with a plurality of transversally movable carriers.

8. The apparatus according to any of the preceding claims, comprising a further digital printing device (7) located on the opposite side of the web (2) so as to print the web on both sides simultaneously.

9. The apparatus according to any of the preceding claims, comprising at least one further digital printing device (7) located on the same side of the web (2) and spaced longitudinally from the first, each of the printing devices (7) being configured to print the web with a specific colour.

10. An installation for making rolls of multi-ply paper divided into tear-off lengths, comprising:

- an assembly station where a plurality of sheets of paper are combined to form a web (2) of multi-ply paper;

- a web embossing station;

- a web printing station;

- means for continuously feeding the web between the stations,

characterized in that the printing station comprises a printing apparatus (7) according to any of the preceding claims.

1 1. A method for printing a web (2) of multi-ply paper, comprising the following steps:

- manipulating the web (2) in an infeed zone (3) and in an outfeed zone (4) in order to keep the web taut in the stretch between the two zones;

- feeding the web (2) continuously along a longitudinal direction (5) between the infeed zone (3) and the outfeed zone (4);

- printing the web (2) in the stretch between the infeed zone (3) and the outfeed zone (4),

characterized in that the printing step comprises driving a plurality of nozzles (8) of a digital inkjet printing device (7) through the agency of a control unit (9).

12. The method according to claim 11, wherein the control unit (9) drives at least a first and a second nozzle (8) forming part of the plurality and being spaced longitudinally by a predetermined quantity and substantially aligned along a longitudinal axis, in such a way that they deliver the ink to a first and a second point on the web (2), located respectively at a predetermined distance along the longitudinal axis and substantially aligned along the longitudinal axis.

13. The method according to claim 12, wherein the control unit (9) displaces ink delivery from the second nozzle (8) relative to ink delivery from the first nozzle (8) temporally by a predetermined length of time, as a function of web (2) feed speed and nozzle (8) spacing, the second nozzle (8) being located after the first in the direction of web (2) feed.

14. A method for making rolls of multi-ply paper divided into tear-off lengths, comprising the following steps:

- assembling a plurality of sheets of paper to form a web (2) of multi-ply paper;

- embossing the web (2);

- printing the web (2);

- feeding the web (2) continuously during the aforestated steps,

characterized in that the printing step comprises a method for printing a web (8) of multi-ply paper according to any of the claims from 11 to 13.