Process and device for printing substrates

Abstract

Method for printing substrates by injection of ink, which comprises the following steps: - placing a first substrate and a second discrete substrate in a device for conveying substrates, said substrates being arranged immediately consecutively in the conveyor device, there being a distance between the first and the second substrate in the direction of travel of the conveyor device, - supplying, consecutively, by means of said conveyor device, said first substrate and second substrate to a printing area, - consecutively printing said first substrate and said second substrate, said substrates being in movement during printing, which comprises the following step: - reducing said distance between the first substrate and the second substrate, prior to supplying said second substrate to the printing area, such that the distance between the first substrate and the second substrate during the printing step is less than the distance between the first substrate and the second substrate when they were placed in the conveyor device.

Description

[0001] The present invention relates to a method and a device for printing substrates. More specifically, the present invention relates to a device for printing substrates, preferably laminar substrates, such as in particular sheets, planks, plates, plaques, panels, etc., which are moved by a conveyor device, such as for example a conveyor belt or system of conveyor belts. The present invention is especially advantageous when it is applied to a system that uses an inkjet printer.

[0002] Currently, inkjet printers, especially "single pass" inkjet printers (in which the substrate passes below the print heads only once), require, in order to obtain printing without any errors in the position of the droplets of ink emitted by the printer (or to minimize error as much as possible), a good retention system that prevents the substrate to be printed from slipping with respect to the conveyor device which moves the substrates. The conveyor device passes the substrates to be printed below the print heads (injectors) for them to be printed with inks of different colours and thus be printed with the desired design before being brought to other parts of the production line. In order to obtain good definition in the printing, this is carried out with the substrate moving at a constant speed. It is known practice for the conveyor system to comprise a perforated belt which, in conjunction with an air suction system, generates a vacuum which holds the substrates in place through the holes which are covered by the substrates to be printed on the conveyor belt. This system has the advantage that it makes it possible to flatten, as they are being conveyed, laminar substrates which have some degree of curvature which could give rise to printing errors or, worse, to a collision. This is beneficial, since the injectors tend to be located at a distance of between 1 and 4.5 mm above the substrate to be printed, and the tendency is to reduce the distance, because the greater the resolution of the printing, and the smaller the ink drops, the smaller the distance between substrate and injector needs to be. In light of the scant and ever decreasing distance between substrate and injectors, a curve in the substrate could give rise to a collision between the substrate and an injector, with the consequent stoppage of the line and need for repair. To prevent collisions, it is known practice for printing systems to have a safety system with a sensor configured to detect the height of the substrate. The information from the sensor is sent to the control system, which stops the line if, despite the action of the suction system, there is the likelihood of a collision between the substrate and one of the injectors.

[0003] One problem associated with the suction mechanism is that it can give rise to air flows and / or turbulence around the edges of the substrate to be printed. Said turbulence and / or flows can interfere with the path of the ink drops, leading to printing errors in said edge areas, or can even, in extreme cases, "unflatten" the substrate to be printed, resulting in a collision with the injector.

[0004] For this reason, it is known practice for printing machines to have a mechanical adjustment system to make the width of the air suction match the width of the substrate(s) to be printed by means of adjustable walls that define the width of suction along the conveyor belt as the substrate passes through the printer. Thus, there are no air flows at the sides of the substrates while they are being conveyed along.

[0005] However, when the substrates are discrete or non-continuous, these systems leave gaps between a substrate to be printed and the subsequent substrate to be printed. Such a gap appears during the process of supplying the discrete substrates to the conveyor device. The abovementioned problem of undesired turbulence or flows arises at these gaps between every two substrates, since at the gaps between substrates holes appear through which air is suctioned. This suction air flow and / or the turbulence associated therewith gives rise to errors in the printing of the front and rear edges due to changes in the path of the ink droplets injected, as they are diverted from the place where they are supposed to go. This results in defects leading to blurring, etc.

[0006] Owing to the tendency to use injector heads with a higher resolution (meaning ever smaller drops), increasingly higher printing speeds and the increase in the variety of substrates that can be printed (in particular thick substrates), the applicant has noticed the emergence of similar phenomena in the vicinity of the gaps between substrates, even in the absence of suction, owing to the lower inertia of the drops of ink of smaller size and the small degree of turbulence associated with the higher speed and the greater thickness of the substrates. Specifically, the applicant has determined that the air flow resulting from the speed between the substrates and the air surrounding the substrates is disrupted owing to the existence of gaps between substrates, which can affect the path of the drops of ink in the areas close to said gaps, in particular for drops of smaller size.

[0007] Discrete substrates are moved on the production line with a gap between them since the way in which they are supplied creates said gaps in order to avoid collisions between the substrates which would lead to stoppages on the line and disastrous errors, which is why it is, in practice, very difficult to supply the substrates without a gap between them.

[0008] Two different solutions are currently known for overcoming or preventing the defects resulting from turbulence or even preventing air turbulence between pieces to be printed. Document EP3825135 discloses reducing the vacuum in the injection area. Document EP3825136 discloses a system of valves on the conveyor belt which closes off the passage for air on the belt where there is no substrate.

[0009] However, both of these systems have technical and / or financial drawbacks and, moreover, fail to solve the abovementioned problem in all cases, and it is therefore desirable to have another means for solving the abovementioned problem.

[0010] To solve or overcome the abovementioned problem, the present invention proposes bringing contiguous discrete substrates closer together as they are conveyed towards the printing injectors, reducing or eliminating the gap between them so as to reduce or eliminate the air flow, or eliminate possible slipstreams or turbulence in the gap between substrates. To this end, preferably, the gap between a first substrate to be printed and a second, immediately subsequent substrate to be printed is detected and / or measured, at a point on the production line prior to printing. Once the gap has been measured or detected, in order to reduce same, preferably, the second substrate undergoes acceleration, followed by deceleration. The operation of acceleration and deceleration allows the second substrate to return to the speed of translational movement required for printing. As stated above, it is important that printing be performed at a controlled speed, and therefore the acceleration and deceleration should be completed, preferably, before the immediately subsequent substrate reaches the printing area.

[0011] More specifically, the present invention discloses a method for printing substrates, preferably laminar substrates, by injection of ink and a device for printing substrates, preferably laminar substrates, by injection of ink, the features of which are advantageous for the purpose of implementing said method.

[0012] In particular, the present invention discloses a method for inkjet printing substrates, which comprises the following steps: placing a first substrate and a second discrete substrate in a device for conveying substrates, said substrates being arranged immediately consecutively in the conveyor device, there being a distance between the first and the second substrate in the direction of travel of the conveyor device, supplying, consecutively, by means of said conveyor device, said first substrate and second substrate to a printing area, printing said first substrate and said second substrate, said substrates being in movement during printing, which method involves: reducing said distance between the first substrate and the second substrate, prior to supplying said second substrate to the printing area, such that the distance between the first substrate and the second substrate during the printing step is less than the distance between the first substrate and the second substrate when they were placed in the conveyor device. Preferably, and if possible, said distance between the first and the second substrate is reduced to less than 3 mm, and more preferably to less than 2 mm. Even more preferably, said distance is eliminated, in other words the distance is reduced to zero.

[0013] By eliminating or reducing the distance or gap existing between two consecutive substrates to be printed, the problem of air flows between substrates is prevented or mitigated.

[0014] Preferably, the reduction of said distance is achieved by means of a modification of the relative speed of travel of the second substrate with respect to the first substrate. The modification of the relative speed of travel of the second substrate with respect to the first substrate may also include a modification of the relative speed of at least one third substrate immediately consecutive to the second substrate, with respect to the first substrate. The expression "at least one third substrate" may include all the substrates coming after the second substrate in the conveyor device. In general, it may be advantageous to modify the speed of all of the substrates coming after said second substrate, in order to lessen the accelerations and decelerations necessary to reduce the gaps between substrates. Advantageously, the modification of the relative speed is made up of an acceleration of the second substrate with respect to the first substrate and a subsequent deceleration of the second substrate with respect to the first substrate, until the second substrate reaches the speed of the first substrate. Preferably, said at least one third substrate is accelerated at the same time as the second substrate, with an acceleration equal to that of the second substrate. This makes it possible to lessen the accelerations necessary to reduce the gap. The deceleration of the at least one third substrate may, if desired, be implemented after the deceleration of the second substrate, which makes it possible to reduce the gap between the second substrate and the third substrate prior to the reduction or elimination of the gap between the second and third substrates.

[0015] Since the printing process, preferably, is carried out with said substrates being moved by the conveyor device at a constant speed, in a preferred embodiment, the first substrate is moved by the conveyor device at said constant speed during the process of reducing said distance.

[0016] Preferably, the distance between the first substrate and the second substrate is detected and measured, by means of a sensor system, before reducing said distance. This measurement makes it possible, by means of a control system, to send to the conveyor device precise orders which lead, in a controlled manner, to the reduction or elimination of said distance.

[0017] Preferably, during printing, said substrates are held in place on the conveyor device by means of suction.

[0018] Advantageously, said first and second substrates are held in place on the conveyor device by means of suction while they are being conveyed by the conveyor device to the printing area, and / or when the distance between substrates is reduced.

[0019] Several gap detection / measurement operations may be carried out while the substrates are being conveyed to the printing area.

[0020] The present invention also discloses printing devices which are especially suitable for carrying out the printing method according to the present invention, in any of its embodiments.

[0021] The present invention discloses a device for printing substrates by injection of ink, said device comprising: a printing area with ink injectors, a conveyor device adapted to convey substrates to be printed, preferably laminar substrates, from a supply area to the printing area, and a control element configured to reduce or eliminate, in the conveyor device, a distance between a first substrate and a second substrate which are immediately consecutive.

[0022] The control device may be configured to carry out a method according to the present invention.

[0023] Preferably, the printing device may comprise a sensor element adapted to detect said distance on the conveyor device between the first substrate to be printed and the second substrate to be printed, the control element being configured to reduce or eliminate said distance as a function of the reading from the sensor element.

[0024] Preferably, the control element is configured to reduce the distance between the first substrate and the second substrate by modifying the relative speed between the first substrate and the second substrate. More preferably, the control device is configured to accelerate and subsequently decelerate the second substrate in order to reduce or eliminate said distance. Alternatively, it would also be possible to implement, for example, a deceleration and subsequent acceleration of the first substrate. In this case, account must be taken of whether there are any substrates in front of (or downstream of) the first substrate, since the acceleration of the first substrate could affect the gap existing between them. To prevent this, said substrates in front of the first substrate could undergo a modification of speed equal in value to that of the first substrate, in such a way that the distance between them remains constant.

[0025] Preferably, the control element is configured to also modify the speed of at least one third substrate immediately consecutive to the second substrate, more preferably is configured to modify the relative speed of the third substrate with respect to the first substrate. By modifying the speed of the third and subsequent substrates it is possible to obtain a reduction in the accelerations necessary. For example, if the third substrate (and, preferably also subsequent substrates) is accelerated and decelerated, it is possible to maintain the distance between the second and the third substrate. Otherwise, when the speed of the second substrate is modified, the gap existing between the second and the third substrate could be increased.

[0026] Preferably, the conveyor device is a perforated conveyor device, and has, at least in the printing area, a conveyor surface for conveying substrates to be printed which has holes, the printing device comprising a suction device in communication with the holes in the conveyor device, in such a way that a suction flow generated by the suction device is adapted to pass through the holes in order to hold the substrate in place on the conveyor device.

[0027] Preferably, said conveyor device has at least two drive units for actuating the conveyor device, the speed of which may be controlled independently, the control device being configured to change the speed of the drive units so as to reduce said distance between the first substrate to be printed and the second substrate to be printed, preferably as a function of the reading from the sensor element.

[0028] Preferably, the control device may be configured to accelerate and subsequently decelerate the second substrate in order to reduce or eliminate said distance.

[0029] Since it is preferable for printing to be carried out at a constant speed, it is advantageous for the conveyor device to have independent speed controls for the printing area and for the conveying area from the supply area to the printing area. For similar reasons, it is preferable for the control device to be configured to reduce or eliminate the distance between the first substrate and the second substrate in said conveying area.

[0030] Preferably, the conveyor device may comprise, between the supply area and the printing area, at least one conveyor surface for conveying substrates to be printed which has holes in communication with a suction device for holding the substrate in place on the conveyor device in said area.

[0031] In a preferred embodiment, the sensor is a photocell.

[0032] For a clearer understanding of the present invention, drawings illustrating an embodiment of the subject matter of the invention are attached by way of explanatory but non-limiting example. Figure 1 schematically depicts an example of a printing device according to the present invention, seen from one side. Figure 2 shows a detail of the conveyor device shown in Figure 1. Figure 3 shows a cross section in the printing area of the conveyor device with suction. Figure 4 shows a speed profile in line with an embodiment of a method according to the present invention. Figure 5 corresponds to Figure 1, with the position of the substrates to be printed at different moments shown under the device. Figure 6 depicts various moments in a conveying process followed by the illustrated example of a printing device according to the present invention, showing the progressive changes in the distance between substrates when a method according to the present invention is followed.

[0033] Figure 1 shows an example of a device 1 for printing discrete substrates to be printed according to the present invention. The device of the example comprises a supply means 2 for supplying discrete substrates (for example, sheets of cardboard 100), a roller applicator 3 for applying a water-based primer, a means 4 for drying / curing the primer and an inkjet printer 5. The injectors 51 have been shown schematically in the printer 5. As is usual in inkjet printers, the printer has various heads or injectors 51 which contain inks of different colours. Known inkjet printers 5 usually have a sensor for detecting the start and / or end of a piece to be printed. Said sensor has not been shown in Figure 1.

[0034] A conveyor device 6 has the function of conveying the sheets between the various stations, and through the latter. As can be seen, there is a system of rollers 62 in the supply means 2, another system of rollers 63 in the applicator 3, another system of multiple aligned rollers 64 in the drying means 4, while in the printer 5, the conveying function is performed by a conveyor belt 65. In some sections, the conveyor device 6 comprises rollers facing one another, as in the case of the last two rollers of the system of rollers 62 of the supply means 2, right at the interface with the applicator 3, while in other sections it comprises a succession of conveyor belts. In the example shown, the conveyor device 6 is split up into the various stations, which are located one immediately after the other. However, it is possible for the stations to have a degree of separation between them, in which case there would be devices for conveying the substrates between stations. As can be seen, the conveyor device has various drive actions which may be controlled independently, allowing the speed to vary along the conveyor device.

[0035] In the example shown, all of the stations and / or conveyor devices have suction mechanisms 72, 73, 74, 75 such that the position of the sheets 100 is ensured at all times during the process. The most important holding mechanism is that of the printer (suction mechanism 75), since it is here that it is most important to ensure the position of the sheet 100 with respect to the conveyor belt 65 in order to ensure correct printing.

[0036] The device 1 comprises a sensor 8 having the purpose of determining the distance between two consecutive sheets 100 in order to reduce the gap between said sheets. Various types of sensor are suitable for said function, and a photocell, for example, may be used.

[0037] The supply means 2, by its very nature, supplies the sheets 100 with a gap between them and the gap is maintained in subsequent stations. It would therefore be possible to implement the method for reducing the gap between sheets without the need for the information provided by the sensor 8. However, the implementation and use of the sensor 8 is advantageous, since it prevents problems such as the existence of an actual gap that is variable and / or different from the theoretical, and allows greater flexibility of the printing device when it comes to adapting to the different substrates. For optimal operation of the printer 5, the speed of travel through the printing area must be kept constant, and therefore, in the example shown, the conveyor belt 65 of the printer 5 stays at a constant speed at all times during printing. Since the aim is for the gap between sheets to be reduced or eliminated in the printer 5, it is advantageous for the operation of reducing the gap between substrates to be completed before the subsequent substrate reaches the conveyor belt 65. In the example shown, it is the control device that is tasked with reducing the gap, by modifying the conveying speeds as a function of the point of the conveyor device. The control device may modify said speeds, for example, by modifying the speed of the various motors of the conveyor device, as a function of the point at which they are located. The implementation will depend on the type of motor used at each point by the conveyor device (for example, via encoder signal or via electrical shaft). Likewise, the control system may determine and order changes of speed, for example, by means of software or by means of logic blocks implemented in the hardware.

[0038] Figures 2 and 3 show details of the conveying system, in particular the part corresponding to the printer. As can be seen, the moving surface on which the substrates to be printed are located is perforated, having holes 651. The moving surface is driven by drive rollers 652 independently of the rest of the conveyor device 6. The holes 651 are connected to a suction mechanism 75 which sucks in air, and this holds in place, by means of a vacuum, the substrates on the conveyor belt 65. The suction mechanism 75 comprises an air suction box 90. The mechanical device which produces the vacuum has not been shown in the figures. The air suction box 90 has adjustable walls 91 which delimit the width of suction along the conveyor belt as the substrate travels through the printer. Adjusting the width makes it easier for the same printer to print substrates of different widths since, by virtue of the action of the adjustable walls 91, it prevents the production of a suction flow through the holes located at the sides of the substrate to be printed when the width of the substrate to be printed is less than the width across which the holes 651 extend.

[0039] However, uncovered holes 651 may remain in the gap between one substrate and the next. The suction air flows through said holes at a given speed. This air may give rise to turbulence in the printing areas, which can affect the paths of the drops of ink injected since, because the print heads are so close to the surface to be printed, the path of the droplets which are injected is modified, and they are diverted from the place where they are supposed to go. This leads to defects resulting in blurring, etc.

[0040] This problem is solved by reducing the gap between two successive substrates to be printed, which gap appears during the supply process. In the example shown, this reduction is achieved by causing an acceleration and then a deceleration of the subsequent substrate, in such a way as to shorten the distance with respect to the one in front. Shortening the gap in this way has the advantage of only requiring a controlled change in the speed at which the substrates are conveyed. In order to improve the adjustment and make the printing process more flexible, the gap is measured by means of a sensor 8. Ideally, the gap is reduced to zero, or made as small as possible, depending on the capabilities of the technology used.

[0041] The process of reducing the gap in the example is illustrated in Figures 4, 5 and 6.

[0042] When the gap between the first two sheets 100 passes by the sensor 8, said gap is read and the previous drive motors are accelerated, increasing their speed. Figure 4 shows a profile of the increase in speeds of the conveyor motors as a function of the space available to close the gap between the first and the second substrate. In said figure, the X-axis 904 represents the distance available for the reduction of the gap, while the Y-axis 903 represents the increase in speed applied to the substrate with respect to the "base" speed, which will normally be the printing speed. The speed applied to each substrate to be printed at each point is the speed of the corresponding motor / conveyor. As can be seen, there is an increase in the speed of all subsequent pieces (located at previous points of the system), depicted in Figure 4 with the upward slope 901. Said pieces then slow down until they reach the printing speed (the speed of the entire line in the absence of acceleration or deceleration). Deceleration corresponds to the downward slope 902. The fact that there is a mechanism for holding in place the substrates to be printed, not only in the printer, but also in other areas of the conveyor device (suction mechanism 72, 73, 74, 75) facilitates the processes of acceleration and especially deceleration of the substrates.

[0043] Figure 5 schematically depicts, under the printing device 1 shown, the position of various sheets to be printed at various moments 1000, 1001, 1002, 1003, 1004. It can be seen that, at the moment when a gap between a sheet 100 and an immediately subsequent sheet 100' passes by the sensor 8, the gap is detected and measured, and the system proceeds to accelerate and decelerate the subsequent sheet 100' in such a way that the gap is reduced or eliminated before the subsequent sheet 100' is completely on the belt 65 of the printer, which runs at a constant speed. Figure 6 schematically depicts two of said moments, from above.

[0044] When the subsequent substrate reaches the printing speed, the distance with respect to the substrate in front no longer changes, since the substrate in front is also travelling at said speed. Therefore, and because the subsequent substrate has been accelerated and decelerated, at said point at which the subsequent substrate reaches the printing speed, the gap has been reduced from the initial gap to the target gap. The best target gap, from a theoretical point of view, is zero. However, in accordance with the maximum uncertainty, which depends on the technology used, the target gap may be other than zero, and is preferably as small as possible.

[0045] For example, the target gap may depend on the response time of the device and on the speed of the line. For example, for a response time of 1 ms and a maximum line speed of 120 m / min (2 mm / ms) it is not appropriate to establish a target gap of less than 2 mm as there could be a collision between substrates or the pieces could be caused to slide on top of one another. However, with a response time of 1 microsecond, a target gap of up to 0.002 mm may be established for the same line speed. Therefore, it is advantageous to reduce the response time of the device as far as possible. Reductions in the response time may be achieved, for example, by switching from implementation of the method via software to implementation via functional blocks implemented directly in the electronics, servocontrols, etc. Logically, and to be able to take advantage of a reduced response time of the device, the sensor for reading the gap between substrates must have a suitable sampling frequency, preferably with a reading period which is less than the response time.

[0046] When implementing the method, it is preferable for the speed acceleration slope for the substrate and the deceleration slope to be as gentle as possible to avoid damaging the mechanics and also to avoid slipping of the pieces. Therefore, it may be advantageous to involve the highest number of conveyor mechanisms (62, 63, 64) in the implementation of the slopes.

[0047] Regarding the location of the sensor 8 for detecting the gap, this may be situated at various points of the device, all having different advantages and disadvantages. Obviously, a location of the sensor further upstream (closer to the supply means) increases the distance available for the reduction or elimination of the gap between substrates to be printed, whereas a location further downstream (closer to the printer) has the advantage of reducing the risk of the gap between substrates changing as they are being conveyed. It is also possible to place various sensors 8 at different locations along the conveyor device. This makes it possible to make various corrections at various locations, which may make it possible to lessen the acceleration profiles and improve accuracy. If there are multiple sensors 8, various algorithms may be applied to reduce the gaps between substrates. For example, it is possible to combine the actions corresponding to the measurement of each sensor at a given moment. In other words, each sensor detects a gap and the control device can determine or calculate a modification of speed for each second substrate whose gap with respect to a first substrate has been measured, detected or determined. Said modification of speed may be applied to all the substrates subsequent to each second substrate. Thus, for a substrate which is in the device subsequent to (i.e. upstream of) a given number of sensors, the modification of speed which is applied by the control device is the sum total of the modification of speed corresponding to the reduction of the gap measured by each one of the previous sensors (i.e. located upstream).

[0048] As stated above, in printers of known type there are usually detectors for detecting the start of a piece to be printed. It must also be borne in mind that, if the gap has been reduced, this could affect the detection of the start of a piece, depending on the sensitivity of said sensor for detecting the start of a piece. Therefore, in a device which implements a gap reduction method, it is advantageous for the sensor for detecting the start of a piece to be able to detect the start of a piece when the gap has been reduced.

[0049] Figures 5 and 6 show substrates travelling in a single row in the conveyor device. However, is possible for the substrates to be conveyed in the conveyor device in a double row, triple row, etc., with two, three or another number of substrates travelling in parallel. In this type of machine, usually, the conveyor device in the printing area is a single device, but the printer has sensors that detect the start of each one of the substrates to be printed, in such a way that the printing of each substrate travelling in parallel takes into account the exact position of each substrate, including whether any of the substrates in a row is in advance or delayed with respect to the others in the same row. Thus, the printer operates as if there were a number of printers equal to the number of rows of substrates. However, the distance between each one of the various substrates in a row and the corresponding substrate in the immediately consecutive row of substrates may be unequal. Therefore, in this case, it is advantageous to have a sensor for detecting / measuring each distance between each substrate in a row and the corresponding immediately subsequent substrate and also for the conveyor device between the supply area and the printing area to have a motorization with various columns that can be actuated independently, in such a way that it is possible to modify independently the speed of each one of the substrates in a row of substrates which are travelling, in principle, in parallel. Thus, for each column, the correction of the distance between a substrate and the immediately subsequent substrate is carried out independently, as if there were independent conveyor devices in parallel, something which is also possible.

[0050] As can be seen in Figures 5 and 6, the substrates subsequent to the second substrate 100' maintain the distance with respect to the second substrate during the process of acceleration and deceleration of the second substrate. This is because said substrates are accelerated at the same time as the second substrate, in other words undergo the same change of speed as the second substrate 100'. However, if desired, the subsequent substrates are not decelerated at the same time, which would make it possible to pre-reduce the gap before it is detected. However, to carry out said pre-reduction, it would be beneficial to have a multiplicity of sensors 8 arranged all along the conveyor device 6, in order to prevent collisions between immediately consecutive substrates, or prevent consecutive substrates from sliding on top of one another.

[0051] In the examples, the relative speed applied by the conveyor device 6 to reduce or eliminate the gap between substrates has been changed by means of the acceleration of the second substrate. However, alternatively, it would also be possible to decelerate the first substrate, in such a way that the relative speed between substrates changes in a manner similar to what occurs when the second substrate is accelerated. In this case, account must be taken of whether there are any substrates in front of, in other words downstream of, the first substrate, since the deceleration of the first substrate could result in the creation or widening of a gap between the first substrate and the substrate immediately in front. To prevent this, a modification of speed equal to that of the first substrate could be applied to said substrates in front. On the other hand, it would also be possible to reduce the distance by other means such as, for example, modifying the distance travelled by each substrate by means of a system of pit-lanes which offers alternative paths.

[0052] Although the invention has been presented and described with reference to embodiments thereof, it will be appreciated that these are non-limiting embodiments of the invention and therefore several different structural or other details could become obvious to a person skilled in the art after interpreting the subject matter disclosed in the present description, claims and drawings. Consequently, the present invention encompasses all variants and equivalents if they can be considered to fall within the broadest scope of the claims which follow.

Claims

1. Method for inkjet printing substrates, which comprises the following steps: - placing a first substrate and a second discrete substrate in a device for conveying substrates, said substrates being arranged immediately consecutively in the conveyor device, there being a distance between the first and the second substrate in the direction of travel of the conveyor device, - supplying, consecutively, by means of said conveyor device, said first substrate and second substrate to a printing area, - consecutively printing said first substrate and said second substrate, said substrates being in movement during printing, characterized in that it comprises the following step: - reducing said distance between the first substrate and the second substrate, prior to supplying said second substrate to the printing area, such that the distance between the first substrate and the second substrate during the printing step is less than the distance between the first substrate and the second substrate when they were placed in the conveyor device.

2. Method, according to the preceding claim, characterized in that the reduction of said distance is achieved by means of a modification of the relative speed of travel of the second substrate with respect to the first substrate.

3. Method, according to the preceding claim, characterized in that the modification of the relative speed comprises an acceleration of the second substrate with respect to the first substrate and a subsequent deceleration of the second substrate with respect to the first substrate.

4. Method, according to Claim 2 or 3, characterized in that a modification of the relative speed between the first substrate and at least one third substrate immediately consecutive to the second substrate is also carried out.

5. Method, according to the preceding claim, characterized in that the at least one third substrate immediately consecutive to the second substrate is accelerated at the same time as the second substrate, with an acceleration equal to that of the second substrate.

6. Method, according to any of the preceding claims, characterized in that during the step of printing said first and second substrates, they move at a constant speed.

7. Method, according to any of the preceding claims, characterized in that during said reduction of the distance between the first substrate and the second substrate, the first substrate is moved by the conveyor device at said constant speed.

8. Method, according to any of the preceding claims, characterized in that it comprises detecting and measuring, by means of a sensor system, the distance between the first substrate and the second substrate before reducing said distance.

9. Method, according to any of the preceding claims, characterized in that, during printing, said substrates are held in place on the conveyor device by means of suction.

10. Device for printing substrates by injection of ink, said device comprising: a printing area with ink injectors, a conveyor device adapted to convey substrates to be printed from a supply area to the printing area, characterized in that it comprises a control element configured to reduce or eliminate, in the conveyor device, a distance between a first substrate and a second substrate which are immediately consecutive.

11. Device, according to the preceding claim, characterized in that it comprises a sensor element adapted to detect said distance, on the conveyor device, the control element being configured to reduce or eliminate said distance as a function of the reading from the sensor element.

12. Device, according to Claim 10 or 11, characterized in that the control element is configured to reduce the distance between the first substrate and the second substrate by modifying the relative speed between the first substrate and the second substrate.

13. Device, according to the preceding claim, characterized in that the control device is configured to accelerate and subsequently decelerate the second substrate in order to reduce or eliminate said distance.

14. Device, according to Claim 12 or 13, characterized in that the control element is configured to also modify the relative speed with respect to the first substrate of at least one third substrate immediately consecutive to the second substrate.

15. Device, according to any of Claims 12 to 14, characterized in that said conveyor device has at least two drive units for actuating the conveyor device, the speed of which may be controlled independently, the control device being configured to change the speed of said drive units so as to reduce said distance between the first substrate to be printed and the second substrate to be printed.

16. Device, according to any of Claims 10 to 15, characterized in that the conveyor device is a perforated conveyor device, and has, at least in the printing area, a conveyor surface for conveying substrates to be printed which has holes, the printing device comprising a suction device in communication with the holes in the conveyor device, in such a way that a suction flow generated by the suction device is adapted to pass through the holes in order to hold the substrate in place on the conveyor device.

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