EP3256323B1 - Web holding and stabilizing unit for printhead and printer equipped therewith - Google Patents

Description

The present invention relates to a support and stabilization unit for a continuous strip (described in the patent US5056431 ). Such a support and stabilization unit is in particular used for a print head. Such a print head makes it possible to print information and / or patterns using a contactless technology on a strip of raw material which may be of variable thickness and in different types of material, in particular a plastic material. The invention also relates to a printing station equipped with at least one printhead and at least one support and stabilization unit. In particular, the present invention relates to the field of the manufacture of packaging, and in particular packaging from flat elements cut from a previously printed strip.

State of the art

In the field of packaging manufacturing, high rates are reached for the processing of webs or webs, in particular plastic webs. Thus, belt feed speeds have values of up to 600 m / min on a single machine which can continuously carry out several successive treatments such as one or more prints, the laying of a coating before and / or after printing, embossing or embossing, and this on several modules arranged in line. At the end of the line, either the strip is rewound on a roll, known under the name reel-to-reel, or it is followed by a module carrying out an operation of cutting and rejection of waste, this subsequent operation having to be performed in strict accordance with previous impressions.

There are different types of printing modules, and in particular depending on the printing technique used. For printing in small runs, digital printing is now increasingly used. However, this type of printing requires extreme precision between the print heads and the position of the strip, in each direction of the strip: the relative position of the strip relative to the print head in the direction of the length, width and height must be adjusted spatially and temporally in order to synchronize the printing and cutting processes. In addition, in the case of digital printing, there are several printheads for the different ink colors and also sometimes multiple printheads to cover the full width of the tape when the tape is wider than the printheads.

Furthermore, in the case of fast or very fast forward speeds, the strip is likely to vibrate and to undulate, all the more so since the support and drive means of the strip do not allow maintain sufficient support and tension of the belt.

Consequently, in the case of fast or very fast forward speeds, it is difficult to obtain a good spatial and temporal agreement between the print heads and the position of the strip.

Furthermore, to guide and stabilize the strip, due to the deposit of ink during its printing, the relevant face of the strip must not be touched or brought into contact with any element whatsoever, as long as the ink is not dry, namely during its passage through the printing module, and at least still during its passage through one or more modules which follow the printing module to allow drying without degrading the print quality.

Because of this impossibility of contact with the side to be printed, and a fortiori of compression of the strip, by pressing on the strip, the techniques proposed are essentially based on the use of a vacuum creating a vacuum on the opposite side on the printing side, namely generally on the underside of the strip. However, this suction of the strip from its underside adds friction between the strip and the suction means which is capable of disturbing the tension of the strip, in particular for strips formed of thin films. Furthermore, this depression also risks disturbing the air flow present at the level of the printhead (s) and therefore of deteriorating the print quality.

Summary of the invention

An object of the present invention is to provide a support and stabilization unit free from the limitations of known devices. A second objective of the invention is to propose a support and stabilization unit which makes it possible to have good accuracy of the relative position between the strip and the printing module or modules which cooperate with this support and stabilization unit. . Another objective is also to seek to carry out a tensioning of the strip in order to have a strip with a face to be printed which is both well positioned in all dimensions and well planar, and this in order to ensure a printing having a quality little or not degraded compared to the print quality offered by the modules and / or printheads on a fixed surface. Also, the invention aims to achieve the stabilization of the strip without contact with the print side of the strip, and at least all along the stabilization device in order to avoid disturbing the print side whether before printing, then during printing, then after printing while the tape is covered with ink during drying.

• au moins un rouleau de support rotatif apte à venir en contact avec un côté inférieur de la bande, et
• au moins un dispositif de mise en tension de la bande, comprenant
  • une face de guidage, tournée en direction du côté inférieur de la bande,
  • une cavité interne, débouchant à l'extérieur du dispositif par une ouverture de sortie dirigée vers la face de guidage, et
  • une arrivée d'air à l'intérieur de la cavité interne,
    de façon à générer sur la bande une force orientée en direction de la face de guidage.

According to the invention, these objectives are achieved in particular by means of a support and stabilization unit for a strip advancing in its longitudinal direction above the unit from upstream to downstream, comprising in line:

• at least one rotary support roll capable of coming into contact with a lower side of the strip, and
• at least one belt tensioning device, comprising
  • a guide face, facing towards the lower side of the strip,
  • an internal cavity, opening to the outside of the device by an outlet opening directed towards the guide face, and
  • an air intake inside the internal cavity,
    so as to generate a force on the strip oriented in the direction of the guide face.

This solution resides on the arrival of a high speed air flow between the strip and the guide face to allow the formation of a vacuum between them. This air flow will tend to suck the strip towards the guide face, in other words to keep the strip at the level of the guide face. This generates a stable position of the strip along the guide face, without undulation or other irregularity in shape of the strip which follows the contour of the guide face. Thus the strip remains flat, in the case where the guide face is flat.

Furthermore, this situation generates a tension on the strip in a direction parallel to the longitudinal direction of the strip, by the fact that the strip is in permanent motion in an advancing movement along its longitudinal direction. The tension is directed from the outlet opening towards the guide face.

This solution has the particular advantage of not creating friction between the advancing strip and its support, which is very significant with regard to the modules and / or printheads where the accuracy of the position of the strip at all times is paramount. Indeed, this precision is necessary to ensure both the quality of the impression that the correct positioning of the image and / or the text printed on the strip, in particular for a strict superposition of the printed patterns from one module or from one print head to the next. It is in fact most of the time necessary to have recourse to the use of several modules or printheads distributed in line along the longitudinal direction of the strip, in particular for printing with inks of different colors, and / or several printing modules staggered in a row or staggered fashion along the transverse direction of the web. This is the case when the length of the printing modules does not cover the entire width of the strip.

The configuration of the entire system and above all the positioning of the belt tensioning devices makes it possible to minimize any air disturbance linked to the air wire at the level of the module (s) and / or printheads, which allows optimal ejection. ink when it comes to digital printing.

Furthermore, this air flow can contribute to reducing the temperature of the substrate.

A connection face, for example convex, can be formed between the outlet opening and the guide face. This arrangement allows the air flow to be easily guided in the direction of the guide face and to engage between the guide face and the strip practically without disturbance, apart from the change of direction.

The highest point of the support roller can be placed above the highest point of the guide face. In this way, a better effect of holding the strip on the support roller is obtained due to the suction of the strip in the direction of the device for tensioning the strip by the air flow generated by the device. tensioning of the strip.

The angular orientation, the longitudinal position and the distance of the belt tensioning device from the band can be adjustable. This makes it possible to adapt the distance between the tensioning device and the strip, to form an air flow having the good speed and flow properties to maintain the strip in the direction of the support roller and obtain the adequate tension for good print quality.

The outlet opening can be located upstream or downstream of the guide face, depending on the orientation chosen for the belt tensioning device.

The support and stabilization unit may further comprise at least one rotary anti-vibration roller arranged between the support roller and the belt tensioning device. The role of this anti-vibration roller is to serve as a guide and to support the advancement of the belt, while helping to dampen the vibrations of this band advancing at high speed.

The highest point of the anti-vibration roller can be placed above the highest point of the guide face of the adjacent web tensioning device. And the highest point of the anti-vibration roller can be placed below the highest point of the adjacent support roller. Thus, the strip which is in contact with the support roller adjacent to the anti-vibration roller, is also in contact with the anti-vibration roller. In fact, this contact is ensured in particular by the tension generated by the tensioning device adjacent to this anti-vibration roller and by shifting the positions of the tops of the elements present in the unit. The offset is made between the support roller, then the anti-vibration roller, located at an intermediate distance, and finally the tensioning device.

The support and stabilization unit may further include two adjacent rotating support rollers. The support and stabilization unit may further comprise two devices for tensioning the strip.

The support and stabilization unit may further comprise two anti-vibration rollers situated on either side of the support rollers and the highest point of which is arranged at a lower height than the highest point of the rollers of support.

The tensioning device may include an air ramp defining the internal cavity. The external face of the air ramp may include the guide face, the ramp having a slot connecting the internal cavity to the guide face and delimiting the outlet opening. The ramp can extend over a distance capable of covering at least the width of the strip.

According to another aspect of the invention, a printing station is equipped with at least one printing head and at least one support and stabilization unit as described and claimed below. In the printing station, the printhead (s) are arranged above the support and stabilization unit, facing the support roll.

According to another aspect of the invention, a printing machine comprises a printing station as described and claimed below, each of the printing heads delivering ink of different color.

Brief description of the figures

• la Figure 1 illustre un module d'impression de l'art antérieur;
• la Figure 2 illustre un premier mode de réalisation d'une unité de support et de stabilisation selon l'invention;
• la Figure 3 est une vue agrandie de la section du dispositif de mise en tension avec le tronçon de bande correspondant; et
• les Figures 4 à 6 illustrent respectivement des deuxième, troisième, et quatrième modes de réalisation d'une unité de support et de stabilisation selon l'invention;
• la Figure 7 illustre un mode de réalisation d'une machine d'impression selon l'invention.

Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which:

• the Figure 1 illustrates a printing module of the prior art;
• the Figure 2 illustrates a first embodiment of a support and stabilization unit according to the invention;
• the Figure 3 is an enlarged view of the section of the tensioning device with the corresponding strip section; and
• the Figures 4 to 6 illustrate respectively second, third, and fourth embodiments of a support and stabilization unit according to the invention;
• the Figure 7 illustrates an embodiment of a printing machine according to the invention.

Examples of embodiment of the invention

In the following description, the terms upstream and downstream are defined with respect to the direction of advancement of the strip and with respect to a printhead. An upstream element is placed in the zone which precedes the print head with respect to the direction of advancement of the strip and a downstream element is located after the print head with respect to the direction of advancement of the strip.

A print head 10 is used to print the upper face of a strip 20 continuously scrolling along its longitudinal direction (arrow A in Fig. 1 ). To this end, a first printing module 41 is spaced by a distance D from a second printing module 42 in the longitudinal direction of the strip. The first printing module 41 and the second printing module 42 are arranged above the upper face of the strip 20, in order to carry out its printing while the strip 20 is running.

This print head 10 comprises, for example, two printing modules 41 and 42. By way of example, these two printing modules 41 and 42 deliver ink of a single color. By way of example, these two printing modules 41 and 42 extend over the entire width of the strip. According to another possible configuration, and in particular in the case where these two printing modules 41 and 42 deliver an ink, these two printing modules 41 and 42 are not wide enough to cover the entire width of the strip 20 and are located offset in the transverse direction of the strip 20. The printheads may include more printing modules staggered in two different transverse rows.

By way of support and stabilization, these are two rollers 31 and 32 used respectively opposite and directly above each printing module 41 and 42 respectively. These support rollers 31 and 32 are not motorized but rotate freely around a horizontal axis orthogonal to the longitudinal direction of the strip 20. Each of these rollers 31 and 32 is positioned so that its highest point is separated of the corresponding printing module 41, 42 disposed opposite a height e + d. The distance e is the thickness of the strip 20 which passes over the rollers 31 and 32 and the distance d is the vertical spacing between the strip 20 and each printing module 41 or 42. Respecting a vertical spacing d predetermined is essential for optimal print quality.

However, the mere presence of the rollers 31 and 32 which guide the strip in order to keep it at the correct distance from the printing modules 41 and 42 is insufficient to ensure the necessary maintenance of the strip 20. In fact, in this case, the strip 20 is subjected to vibrations, see undulations, irregular voltages, so that despite a certain allowable tolerance in the value of d, it is difficult to guarantee a position precise enough for the print quality to be acceptable in all circumstances.

It is for this reason that some of the printheads 10 of the prior art are furthermore equipped with a suction module 50 or with several suction modules 50 ( Fig. 1 ). This suction module 50 is for example placed between the rollers 31 and 32, under the strip 20. When the suction module 50 is activated, a vacuum is generated under the strip 20 which thus tends to be sucked down. , hence a tendency to create a downward deformation in the form of a ripple whose apex is turned towards the suction module 50. In addition, as indicated above, such a vacuum slows down the advancement of the strip 20 .

A print head 110 is equipped with a support and stabilization unit 200 according to a first embodiment (see Fig. 2 ). This support and stabilization unit 200 comprises two adjacent rotary support rollers 31 and 32, forming a downstream support roller 31 and an upstream support roller 32 in the direction of advancement of the strip (Arrow A). Each roller 31 and 32 is placed directly above a printing module 41 and 42 with the interposition of the strip 20 which rests on the rollers 31, 32 mounted free to rotate. The rollers 31 and 32 and the printing modules 41 and 42 are similar to those described above in relation to the Fig. 1 .

In this first embodiment, the two rollers 31 and 32 are completed by a tensioning device 121 located downstream of the downstream support roller 31, which generates an air flow 111 which advances between a guide face 121a of the device energized 121, located below the air flow 111, and the underside of the strip 20. This air flow 111 is therefore located in the downstream part of the support and stabilization unit 200. This air flow 111 advances in the same direction as strip 20. Preferably, this flow of air 111 has a speed greater than the speed of advance of the strip 20.

The two rollers 31 and 32 are completed by a downstream anti-vibration roller 131 disposed between the tensioning device 121 and the downstream support roller 31. This downstream anti-vibration roller 131 is free to rotate about a horizontal axis orthogonal to the longitudinal direction of the strip 20.

The vertical position of the downstream anti-vibration roller 131 is adjustable. The highest point of the support rollers 31 and 32 (see Fig. 2 ) is taken as a level reference R. Preferably the downstream anti-vibration roller 131 is placed so that its highest point is lower than the highest point of the support rollers 31 and 32. The anti-vibration roller downstream 131 is here at a value x with respect to the reference level R, below this reference level R. Also, the tensioning device 121 is preferably placed so that its highest point is lower than the highest point of the downstream anti-vibration roller 131. The highest point of the tensioning device 121 is preferably a portion of the connection face 121e. The tensioning device 121 is here at a value y relative to the reference level R.

For the downstream tensioning device 121, the Fig. 3 illustrates its section along a vertical plane parallel to the longitudinal direction of the strip 20. The guide face 121a in the upper part is turned upwards and therefore faces the strip 20. The tensioning device 121 comprises a air ramp defining an internal cavity 121b and the external face of which includes the guide face 121a. The ramp has a slot 121c connecting the internal cavity 121b to the guide face 121a and delimits an outlet opening 121d. In particular, between the outlet opening 121d and the guide face 121a is formed a convex connection face 121e which makes it possible to guide the air flow 111 along the external face of the ramp.

Preferably, the air ramp extends over a distance capable of covering at least the width of the strip 20. Alternatively, the tensioning device comprises several adjacent ramps, each covering a portion of the width of the strip 20. This single ramp or these multiple adjacent ramps are advantageously oriented transversely, namely in a direction orthogonal to the longitudinal direction of the strip 20.

The outlet opening 121d is located upstream of the guide face 121a ( Fig. 3 ). In this case, the air flow 111 leaving the internal cavity 121b, through the slot 121c and up to the opening 121d, then runs along the connection face 121e while gradually changing orientation to follow the guide face 121a. The air flow 111 leaves almost vertically to come almost horizontally in the position shown in Fig. 3 . In this configuration, the air flow formed is engulfed between the strip 20 and the guide face 121a, parallel to the strip 20 and in the same direction as the advancement of the strip 20 (Arrow A). The strip 20 is located above the air flow, the guide face 121a being below the air flow.

This results in a suction of the strip downwards, in the direction of the guide face, with interposition of the air flow which accompanies the advance movement of the strip 20. The air flow 111 generates this suction which applies on the strip a force F2 substantially vertical and directed downwards along the guide face 121a, and a driving force F1 substantially horizontal and directed downstream, above the slot 121c ( Fig. 3 ). According to a variant not shown, in the downstream tensioning device 121, the outlet opening 121d is located downstream of the guide face 121a.

Preferably, the air flow and pressure arriving in the internal cavity 121b must be sufficient to create the air flow 111 necessary for the conditions of use. Preferably, the speed of the air leaving through the outlet opening 121d of the tensioning device 121 must be sufficient to create the phenomenon of depression and therefore suck the strip 20 against the support roller or the anti-vibration roller.

To facilitate the stabilization of the strip 20, and allow the tensioning of this strip 20, namely to avoid loosening, or even undulations, the elements of the support and stabilization unit 200 are placed at offset heights, from the top downwards, from the support rollers 31, 32 to the tensioning device 121. Thus, for this purpose, the highest point of the anti-vibration roller 131 is placed above the highest point of the guide face 121a of the tensioning device 121 adjacent downstream, and below the highest point of the support roller 31 adjacent upstream.

According to a variant not shown of this first embodiment, and in particular for lower speeds of advancement of the strip 20, the anti-vibration roller 131 can be avoided. The support rollers 31, 32 are combined with the tensioning device 121. This tensioning device 121 acting in the downstream part of the support and stabilization unit 200 according to the first embodiment, it makes it possible to tension strip 20 downstream of the two rollers 31 and 32 and, in so doing, it makes it possible to maintain a stable position of the strip 20.

Also, according to another variant, not shown, of this first embodiment, the support and stabilization unit 200 only has a single rotary support roller 31. It is a variant with a single head. print 141.

A print head 110 is equipped with a support and stabilization unit 300 according to a second embodiment ( Fig. 4 ). This support and stabilization unit 300 additionally comprises an upstream anti-vibration roller 132, disposed upstream of the upstream support roller. The upstream anti-vibration roller 132 is located upstream of the support and stabilization unit 300 on the Fig. 4 .

Like the downstream anti-vibration roller 131, the vertical position of the upstream anti-vibration roller 132 is adjustable. Also, the upstream anti-vibration roller 132 is preferably placed so that its highest point is lower than the highest point of the support rollers 31 and 32. The upstream anti-vibration roller 132 is placed at a value x with respect to the reference level R, therefore below this reference level R. In this way, the strip 20 is also accompanied upstream of the support rollers 31 and 32 in a convex shape or others, this which contributes to improving the tensioning of the strip 20 upstream of the printing modules 41 and 42.

This support and stabilization unit 300 comprises two anti-vibration rollers 131, 132 located on either side of the support rollers 31, 32 and the highest point of which is disposed at a height lower than the most point top of the support rollers 31, 32, in this case with a deviation of a value x from the reference level R (see Fig. 4 ).

A print head 110 is equipped with a support and stabilization unit 400 according to a third embodiment ( Fig. 5 ). In addition, this support and stabilization unit 400 comprises a second tensioning device 122, arranged upstream of the upstream support roller and of the support and stabilization unit 400 ( Fig. 5 ). This upstream tensioning device 122 is similar to the downstream tensioning device 121 and creates an upstream air flow 112 between a guide face and the strip 20.

In the case of the support and stabilization unit 400 according to the third embodiment, the outlet opening of the upstream tensioning device 122 is located downstream of the guide face, so that the flow of upstream air 112 goes from downstream to upstream ( Fig. 5 ), contrary to the advancement of the strip 20. This upstream tensioning device 122 carries out a suction of the strip 20 towards the bottom, towards the guide face, with interposition of the moving air flow opposite to the advance movement of the strip 20. As in the case of the Fig. 3 , the air flow 112 leaving the upstream tensioning device 122 generates a suction which applies to the strip a force F2 substantially vertical and directed downwards along the guide face 122a, and a driving force substantially horizontal and directed upstream, opposite to the force F1 of the Fig. 3 , above the slot.

The resulting tension forces T and T 'exerted on the strip 20 by all the elements of the support and stabilization unit 400 are shown on the Fig. 3 , in addition to the forces F1 and F2 of the downstream tensioning device 121. The section of the strip is subjected downstream to a tension T which is horizontal and directed downstream, and upstream to a tension T 'which is horizontal and directed upstream. Thus, the voltages T and T 'compensate in whole or in part.

A print head 110 is equipped with a support and stabilization unit 500 according to a fourth embodiment ( Fig. 6 ). In this case, the only difference with the fourth embodiment is that, the outlet opening of the upstream tensioning device 122 'is located upstream of the guide face. In this case, the upstream tensioning device 122 'carries out a suction of the strip 20 downwards, in the direction of the guide face, with the interposition of the air flow which advances in a movement which accompanies the movement of advance of strip 20. As in the case of Fig. 3 , the air flow 112 'leaving the upstream tensioning device 122' generates a suction which applies to the strip a force F2 substantially vertical and directed downwards along the guide face 122a ', and a force d drive substantially horizontal and directed downstream, in the same direction as the force F1 of the Fig. 3 , above the slot.

According to a preferred arrangement, the support and stabilization unit according to the invention 400 ( Fig. 5 ) or 500 ( Fig. 6 ) comprises two tensioning devices 121 and 122. In general, these two tensioning devices 121 and 122 are located on either side of the support roller or rollers 31 and 32, in particular in the case where there is no anti-vibration roller 131, 132. In the presence of two anti-vibration rollers 131 and 132, the two tensioning devices 121 and 122 are arranged on either side of the anti-vibration rollers vibration 131 and 132, themselves arranged on either side of the support roller (s) 31 and / or 32.

In this way, a method of stabilizing the strip is implemented in which, on either side of the rotary support roller 31 and / or 32, an air flow having a speed greater than the speed of the strip is sent between the band 20 and a guide face located under the section of the strip. On either side of the rotary support roller, a suction air flow is therefore formed downward the section of the strip located opposite the guide face, so as to maintain the strip at body level .

Thus, depending on the intensity, the place of application, and the orientation of the forces F1 and F2 which must be applied to stabilize the strip 20, one or two tensioning devices 121 and / or 122 and / or 122 ' are used, for each tensioning device, the side where the slot through which the air flow exits is chosen, and consequently the direction of advance of the air flow, for each setting device tension its height and its angular orientation are adjusted.

In a printing machine 700 ( Fig. 7 ), a station 600 is equipped with four digital print heads 141, 142, 143 and 144, each delivering an ink of different color among cyan, magenta, yellow and black (CMYK). Each of the four digital print heads is fitted with a series of print modules distributed across the width of the web. Also, to each print head 141, 142, 143 and 144 corresponds a stabilization unit 400. Each of the print heads 141, 142, 143 and 144 of the station 600 is arranged above a stabilization unit . Each of the printing modules of a head is found opposite the support roll.

Claims (12)

1. Support and stabilisation unit (200, 300, 400, 500) for a web (20) being advanced in its longitudinal direction above the unit from upstream to downstream, comprising in series:

   - at least one rotating support roller (31, 32) that is configured to come into contact with an underside of the web (20),

   - at least one rotating anti-vibration roller (131, 132), and

   - at least one tensioning device (121, 122, 122') of the web (20) comprising:

   - a guiding face (121a, 122a) oriented towards the underside of the web (20),

   - an internal cavity, leading to the outside of the device via an outlet opening (121d) oriented towards the guiding face (121a, 122a) and

   - an inlet of air to the inside of the internal cavity (121b),
   for generating on the web (20) a force (F2) oriented in the direction of the guiding face (121a, 122a);
   the anti-vibration roller being arranged between the support roller (31, 32) and the tensioning device (121, 122, 122'),
   characterised in that
   the highest point of the anti-vibration roller (131, 132) is placed above the highest point of the guiding face (121a, 122a) of the adjacent tensioning device (121, 122, 122') and below the highest point of the adjacent support roller (31, 32).
2. Unit according to claim 1, wherein a convex connecting face (121e) is formed between the outlet opening (121d) and the guiding face (121a, 122a).
3. Unit according to claim 1 or 2, wherein the highest point of the support roller (31, 32) is positioned above the highest point of the guiding face (121a, 122a).
4. Unit according to any of the preceding claims, wherein the angular orientation of the tensioning device and the distance of the tensioning device (121, 122, 122') from the web (20) are adjustable.
5. Unit according to any of the preceding claims, wherein the outlet opening (121d) is located upstream or downstream of the guiding face (121a, 122a).
6. Unit according any of the preceding claims, comprising two adjacent rotating support rollers (31, 32).
7. Unit according to any of the preceding claims, comprising two tensioning devices (121, 122, 122').
8. Unit according to any of the preceding claims, comprising two anti-vibration rollers located on either side of the support rollers (31, 32) and the highest point of which is arranged at a lower elevation than the highest point of the support rollers (31, 32).
9. Unit according to any of the preceding claims, wherein the tensioning device (121, 122, 122') comprises an air manifold which delimits the internal cavity (121b) and the external face of which comprises the guiding face (121a, 122a), the manifold having a slot that connects the internal cavity (121b) to the guiding face (121a, 122a) and that delimits the outlet opening (121d).
10. Unit according to claim 9, wherein the manifold extends over a distance that is suitable for covering at least the width of the web (20).
11. Printing station equipped with at least one printing head (141, 142, 143, 144) and at least one support and stabilisation unit (200, 300, 400, 500) according to any of the preceding claims, wherein the printing head (141, 142) is arranged above the support and stabilisation unit, facing the support roller (31, 32).
12. Printing machine, comprising a station according to claim 11, wherein each of the printing heads (141, 142) is configured to deliver ink of a different colour.

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