Conversion Machine

Abstract

The invention relates to a method for aligning a plurality of printed motifs on a sheet (3) in a converting machine (10), wherein an actual position (Pa_1) of a first sensor is defined as an initial reference position (P_ref1) relative to the sheet, and at least one displacement error (Δd2) is calculated and compared to a first threshold (T1). If the displacement error is lower than the first threshold, the angular position (α) of a printing cylinder (42) in a second flexographic printing unit (16b) is adjusted. However, if the error is higher than the first threshold, at least a part of the displacement error (Δd_2) is corrected by modifying the speed (V) of a vacuum transport unit positioned between the first and second printing units.

Description

[Technical Field]

[0001] The present invention relates to a converting machine such as a rotary die-cutting machine or a folding-gluing machine suitable for producing flat-pack boxes or folding boxes. [Background technology]

[0002] Converting machines in the form of rotary die-cutting machines can be fed with sheets that are printed in a printing unit and then cut and notched to form flat-pack boxes that are then designed to be folded either manually or automatically in a folding-gluing machine. Converting machines configured as flexo folder-gluers are similar to rotary die-cutting machines, but additionally include a folding and gluing module that automatically glues and folds the blanks to form the folding boxes.

[0003] Finished boxes often need to be provided with printed motifs or patterns. To provide high and consistent quality, it is important that each sheet is correctly positioned within the printing unit relative to the angular position of the printing plate on the printing cylinder. However, some sheets arrive at the printing cylinder too early or too late due to variations in sheet transport. This causes color mismatch problems.

[0004] To avoid printing misalignment, existing register control systems are often used and function by controlling the transport of the sheet so that the position of the sheet can be adjusted before it reaches the print cylinder, so that the sheet is advanced or delayed, as needed, before it reaches the print cylinder. Summary of the Invention [Problem to be solved by the invention]

[0005] These types of register control systems are configured to detect and apply displacement corrections to each sheet. To perform the displacement corrections for each sheet, the transport system within the converting machine must constantly change the speed of the vacuum transport unit, and often must accelerate with large decelerations. This has an adverse effect on wear on the drive mechanisms, such as the belts and rollers within the vacuum transport unit. [Means for solving the problem]

[0006] In view of the above mentioned problems, it is an object of the present invention to provide a register control system which limits wear on the converting machine. The object of the present invention is solved by a method as defined in claim 1 and a converting machine as defined in claim 13.

[0007] According to a first aspect, there is provided a method of registering a plurality of printed motifs on a sheet in a converting machine, the converting machine comprising a flexographic printing module having at least a first printing unit configured to print a first motif and a second printing unit configured to print a second motif, the first and second flexographic printing units being arranged successively along a conveying direction of the sheet, the converting machine further comprising a register correction system having a first sensor arranged at a first detection position at a distance upstream of the first printing unit and a second sensor arranged at a second detection position at a distance upstream of the second printing unit, the method comprising: - detecting passage of a front leading edge of the sheet with a first sensor; - determining the actual position of the leading edge (5) at the first detection position from the detection time of the first sensor; - defining the actual position as an initial reference position for the seat; - calculating a second reference position at a second sensor location located downstream by adding a predetermined distance between the first and second detection locations to the initial reference position; - detecting the passage of the leading edge with a second sensor and determining the actual position of the leading edge at the second sensor location from the detection time provided by the second sensor; - calculating an individual displacement error at the second detected position by determining the difference between the detected actual position and a second reference position; - comparing each displacement error with a first threshold; - providing angular position corrections for the printing cylinder in the second flexographic printing unit when the individual displacement errors are below a first threshold; - providing a first correction in the form of an angular position correction to the printing cylinder and a second correction for changing the position of the sheet when the error is higher than a first threshold, said second correction being achieved by modifying the transport speed of a vacuum transport unit positioned between the second sensor position and the second printing unit, so that the sum of the first and second corrections is equal to the individual displacement errors; Includes:

[0008] The present invention is based on the recognition that the correction can be distributed over the print cylinder and the vacuum transport, which consequently reduces the correction required for the vacuum transport.

[0009] The second reference position is preferably calculated by adding a predetermined distance between the first and second detected positions to the initial reference position.

[0010] Rotational displacement correction aligns the print cylinder with respect to the position of the sheet.

[0011] The angular position correction corresponds to the displacement error if the displacement error is below a first threshold value, and therefore the error is only corrected using the angular correction of the printing cylinder.

[0012] In one embodiment, the angular position correction corresponds to a fixed angular length limit of the print cylinder, and the remaining portion of the displacement error is corrected by changes in velocity in the vacuum transport unit.

[0013] The change in speed is provided as an acceleration or deceleration of the transport speed in the vacuum transfer unit.

[0014] The fixed angular length limit may be a constant correction applied to each sheet having a displacement error above a first threshold.

[0015] The angular length limit of the printing cylinder can be between 0.5 mm and 1.5 mm, preferably about 1 mm. The angular length limit is the circumferential segment length of the printing cylinder. The angular length limit can also be defined as the arc length of the printing cylinder corresponding to the angle correction.

[0016] In one embodiment, the converting machine includes at least four flexographic printing units, and the sensor and vacuum transport unit are located upstream of each flexographic printing unit.

[0017] In one embodiment, each sensor positioned downstream of the first sensor is configured to detect the passage of the leading edge of the sheet, and a control unit of the register control system is configured to determine the actual position of the leading edge at each sensor location and provide angular position corrections for each downstream positioned print cylinder, and vary the speed of each vacuum transport unit positioned downstream of each sensor to correct the position of the sheet in the transport direction.

[0018] In one advantageous embodiment, the method comprises the steps of: - detecting the passage of the front leading edge of the sheet with a sensor located at a position downstream of the first sensor; - comparing the detected position of the front leading edge of the sheet with a reference position and determining the difference therebetween as a trend displacement error; - comparing the trend deviation error with a second threshold; - applying a trend speed correction to each controllable vacuum transport unit located downstream of the second sensor in the flexographic printing module when the displacement error exceeds a second threshold; Further includes:

[0019] Individual displacement errors are also found to have commonalities related to sheet quality. By analyzing the commonalities of these types, some displacements can be predicted and accommodated by trend analysis and correction. In one embodiment, the trend displacement error may be determined at a sensor location between the third and fourth flexographic printing units.

[0020] The method may further include determining the displacement error before applying the trend correction, which is accomplished by calculating the average displacement error for the number of sheets in a sample. The sample may contain, for example, between 5 and 10 sheets. The trend calculation and correction is preferably repeated after each sample.

[0021] In one embodiment, the acceleration and deceleration of the vacuum transport is adjusted relative to the sheet transport speed, preferably over the entire distance between the sensor location and the print cylinder.

[0022] The present invention also relates to a converting machine including a register correction system configured to at least partially carry out the method for registering a plurality of printed motifs on a sheet according to the first aspect, the converting machine comprising: a flexographic printing module having at least a first printing unit configured to print a first motif on a sheet and a second printing unit configured to print a second motif on the sheet, the first and second flexographic printing units being arranged successively along the conveying direction of the sheet; a registration system including a first sensor located at a distance upstream of a first flexographic printing unit and a second sensor located at a distance upstream of a second flexographic printing unit; Includes:

[0023] The invention will now be described, by way of example only, with reference to the embodiments illustrated in the enclosed drawings, in which like reference numerals are used for similar elements and in which: [Brief explanation of the drawings]

[0024] [Figure 1a] FIG. 1 shows an example of a folding box obtainable from a conversion machine. [Figure 1b] FIG. 1 illustrates an example of a flat-pack box that can be obtained from a converting machine. [Figure 1c] FIG. 1 shows an example of a folding carton obtainable from a conversion machine. [Figure 2] 1 is a schematic perspective view of a converting machine in the form of a rotary die cutter and known in the prior art; [Figure 3] 1 is a schematic diagram of a converting machine in the form of a flexo folder-gluer and known in the prior art; [Figure 4] FIG. 2 is a schematic perspective view of a flexographic printing assembly of the flexographic printing unit. [Figure 5] FIG. 1 is a schematic diagram of a vacuum transfer unit of a converting machine. [Figure 6] FIG. 1 is a schematic diagram of a vacuum transport unit and sheet transport in a flexographic printing assembly. [Figure 7] FIG. 1 is a schematic diagram illustrating sheet displacement errors in a flexographic printing module. [Figure 8] 1 is a schematic diagram of a register control system according to one embodiment of the present invention; [Figure 9] 1 is a schematic cross-sectional view of a register control system in a flexographic printing module according to one embodiment of the present invention; [Figure 10a] 4 is a schematic graph illustrating acceleration and deceleration profiles according to an embodiment of the present invention. [Figure 10b] 4 is a schematic graph illustrating acceleration and deceleration profiles according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] Reference is now made to Figures 1a and 1b, which show examples of a folding box 1' and a flat-pack box 1"'. The folding box 1' can be obtained, for example, from a flexo folding-gluing machine, and the flat-pack box can be obtained from a rotary die-cutting machine. Common to both types of converting machine, a sheet substrate in the form of square or rectangular corrugated or paperboard is placed in a feeder of the converting machine. The folding box 1' or flat-pack box 1" is typically provided with a motif 7 printed on at least one of its inner and outer surfaces.

[0026] As can be seen in Figure 1b, the flat-pack box 1'' is provided in the form of a sheet and includes a shaped peripheral edge 2, cutouts 4, for example for a handle, and crease lines 6 to allow folding.

[0027] A converting machine 10 in the form of a rotary die-cutter is shown in Figure 2 and includes several different modules. From an inlet of the converting machine 10 downstream along a conveying direction T, the converting machine 10 includes a pre-feeder module 12, a feeder module 14, a printing module 15, a die-cutting module 18, a stacking module 20, and a palletizer-breaker 22. A main operator interface 11 may also be provided near the converting machine 10. The converting machine 10 may further include an ejector module located downstream of the printing module 15 or downstream of the die-cutting module 18.

[0028] The flexographic folding and gluing machine 10 is shown diagrammatically in Figure 3 and includes, in order from upstream to downstream in a conveying direction T, a loader 30 for automatically loading sheet substrates 3, a feeder 31, a flexographic printing module 15 having a plurality of flexographic printing units 16a-16N, at least one slotting assembly 33 and at least one cutting unit 34, a scraping unit and optional vibrating unit 35, and a folding and gluing unit 36. The flexographic folding and gluing unit 10 may also include further optional modules 37 (partially represented by dashed lines in Figure 3), such as a counting and discharging unit, a bundler, a palletizer, etc.

[0029] The flexographic printing modules 15 of the rotary die-cutter and flexographic folding-gluing machine 10 may be configured in a similar manner and include a plurality of flexographic printing units 16. Depending on the number of colors required, the converting machine 10 may be provided with a corresponding number of flexographic printing units 16. For example, there may be four flexographic printing modules 16a-16d that may enable printing in a CMYK color code. Each flexographic printing unit 16 includes a flexographic printing assembly 40.

[0030] The configuration of a flexographic printing assembly 40 is shown in Figure 4. The flexographic printing assembly 40 includes a printing cylinder 42 provided with a mounting bracket 44 onto which a printing plate 46 can be mounted, and an anvil 43. The printing plate 46 is provided with a printing die for printing a particular motif 7 onto the sheet 3. An anilox cylinder 48 is positioned adjacent to the printing cylinder 42 and is configured to absorb and transfer ink from a liquid supply device, such as a doctor blade chamber 49, to the printing plate 46.

[0031] As best seen in FIGS. 5 and 6 , the converting machine 10 further includes a transport system 50 configured to transport the sheet 3 through the converting machine 10 in a transport direction T. The transport direction T is defined from the entrance to the exit of the converting machine 10. The transport system 50 may include multiple separate transport segments 52, referred to as transfer units 52. In particular, the transport system 50 may include a series of transfer units 52 configured as vacuum transporters 52. The vacuum transporters 52 include a transport surface 56 and drive elements 58, such as an endless belt conveyor and rollers 58, to transport the sheet 3 through the converting machine 10. Vacuum openings 60 are disposed around the rollers 58 to ensure that the sheet 3 adheres to the rollers 58. The transport speed V of the sheet 3 corresponds to the tangential speed of the print cylinder 42, which typically also corresponds to the transport speed provided by the vacuum transporter 52.

[0032] The predetermined register setting of the converting machine 10 can be calibrated in a teaching cycle during start-up or installation of the converting machine 10. The calibration defines the angular position of the flexographic printing cylinder relative to the position of the front leading edge 5 of the sheet 3. This is called the calibrated printing register setting of the converting machine 10.

[0033] As best seen in FIG. 7 , each individual sheet 3 typically undergoes an individual displacement distance Δd as the sheet 3 is transported by the transport system 50 from the first flexographic printing unit 16 a through the flexographic printing module 15 along the transport direction T. The individual displacement distance is therefore an individual displacement error Δd, so that the sheet 3 does not arrive perfectly in accordance with the calibrated print register position on each printing cylinder 42. The individual displacement error Δd can also be synonymously referred to in the context of the present application as an “individual longitudinal displacement error Δd.” The individual displacement error Δd is determined in the transport direction T.

[0034] These individual displacement errors Δd will result in different color mismatches of the motifs printed from the different flexographic printing units 16a-16N. The same sheet 3 may arrive too early for the angular position of one printing cylinder 42, while arriving too late for another printing cylinder 42.

[0035] 8 and 9, the transport system 50 is connected to a register control system 60, which is configured to detect and calculate individual displacement errors Δd at a plurality of detection positions P1 to PN (which may also be called "sensor positions") along the transport direction T of the sheet 3 through the flexographic printing module 15. The register control system 60 is further configured to correct the position of the sheet 3.

[0036] The register control system 60 includes a control unit 63, a memory 65, and a number of sensors 62, 64. The control unit 63 can be a central control unit. Alternatively, there are multiple control units 63, with each sensor 62, 64 connected to a dedicated control unit 63 to allow simultaneous calculations.

[0037] As best seen in FIG. 9 , the sensors 62, 64 are arranged at predetermined longitudinal positions P1-PN along the transport direction T of the sheet 3. Each sensor 64 is positioned at a predetermined distance D2-DN from the position P1 of the first sensor 62. The first sensor 62 may be a feed sensor 62. The first sensor 62 is positioned a distance L1 upstream of the flexographic printing cylinder 42 of the first flexographic printing unit 16a. A plurality of sensors 64 may be positioned downstream of the first feed sensor 62 and at a distance L1 upstream of the second and subsequent flexographic printing cylinders 42. The sensors 64 are preferably configured as transport sensors 64.

[0038] The distance L1 typically corresponds to the time required for the control unit 63 to determine the individual displacement error Δd. The distance L1 also provides a sufficiently large travel distance for the vacuum transporter 52 positioned between the sensor 64 and the downstream flexographic printing station 16 to allow for correcting the position of the sheet 3 before it arrives at the print cylinder 42 or achieving an angular position correction Δα of the print cylinder 42. In one embodiment, the distance L1 between the sensor and the print cylinder 42 is in the range of 200 mm to 600 mm, preferably about 400 mm.

[0039] 7, sensors 62, 64 are configured to detect the passage of the front leading edge 5 of each sheet 3. The sensors 62, 64 may be in the form of optical detectors and are arranged along the conveying direction T of the sheets 3. The sensors 62, 64 may be laser detectors with background suppression or other types of optical sensors configured to detect the passage of the front leading edge 5.

[0040] The speed and acceleration of the vacuum transporter 52 can be controlled and varied by a register control system 60 so that the position of the sheet 3 is adjusted before the sheet 3 reaches the downstream positioned print cylinder 42. As shown in Figure 6, the register control system 60 can also be configured to provide an angular position correction Δα to the print cylinder 42 to control the position at which the motif 7 is transferred onto the sheet 3.

[0041] As best seen in FIGS. 7 and 9, a plurality of transport sensors 64 are positioned at predetermined longitudinal positions P2 to PN from the feed sensor 62.

[0042] Initial displacement of the sheet 3 often occurs in the feeders 14, 31 when the sheet 3 is advanced too far or too little relative to the feeder eject signal indicated by the central control system of the converting machine 10.

[0043] The actual position Pa_1 of the front leading edge 5 of each sheet 3 at the first sensor 62 can be set as an initial reference position P_ref1 of the sheet 3 of the flexographic printing units 16b to 16N positioned further downstream. This means that no correction of the position of the sheet 3 or the angular position correction Δα of the printing cylinder 42 is performed inside or before the first flexographic printing unit 16a. The initial reference position P_ref1 is predetermined as a position at a specific time point. The predetermined time point may be related to a feeder discharge signal. The initial reference position P_ref1 is different for each sheet 3 coming from the feeders 14, 31 and is determined for each sheet 3.

[0044] The central control system of the converting machine 10 is configured to determine the actual positions Pa_1 to Pa_N of the front leading edge 5 of the sheet 3 from the detection times of the sensors 62, 64. The actual positions Pa_1 to Pa_N are determined at the respective sensor positions P1 to PN. The calculations to determine the actual positions are performed by the central control unit and integrated counters of the converting machine 10 by comparing the data captured from the sensors 62, 64 with calibrated master settings and machine sensor inputs (i.e., sensors indicating the relative positions of machine parts).

[0045] Alternatively, the actual position Pa_1 at the feed sensor 62 can be calculated by the product of the conveying speed V and the detection time at the feed sensor 62. Similarly, the actual position Pa_2 at the second sensor 64 can be calculated by the control unit 63 by retrieving the conveying speed V of the sheet 3 and multiplying it by the elapsed time of detection at the second sensor 64. The detection time starts counting when the comprehensive counter emits the discharge signal.

[0046] In order to align the printing motifs from different flexographic printing units 16b to 16N positioned further downstream of the first flexographic printing unit 16a, the actual position Pa_2 to Pa_N of the front leading edge 5 of each individual sheet 3 is detected by respective sensors 64 positioned upstream of each flexographic printing unit 16b to 16N.

[0047] As shown in FIG. 7, the actual position Pa_2 to Pa_N of each sheet 3 at each sensor position is detected and compared with the corresponding calculated reference position P_ref2 to P_refN for each particular sensor position P2 to PN.

[0048] The reference position P_ref2 at the second sensor position P2 can be calculated by adding a predetermined distance D2 (see FIG. 9) between the first sensor position P1 and the second sensor position P2 to the initial reference position P_ref1.

[0049] The reference positions P_ref2 to P_refN of the front leading edge 5 of the sheet 3 downstream of the first sensor position P1 can all be calculated in the same way, i.e., by adding the distance D2 to DN from the respective sensor positions P2 to PN to the first reference position P_ref1. The actual positions Pa_1 to Pa_N and the reference positions P_ref1 to P_refN are predetermined by their longitudinal coordinates in the conveying direction T. The control unit 63 determines an individual displacement error Δd for each sheet 3 at each sensor position P2 to PN downstream of the feed sensor 62.

[0050] 7 and 9, the individual displacement error Δd is therefore the difference in longitudinal distance (in the conveying direction T) between the actual position Pa detected at each sensor position and the reference position P_ref. The individual displacement errors at each respective transfer sensor position P2 to PN can be referred to as Δd2 to ΔdN.

[0051] The resulting displacement error Δd can be calculated from the actual position minus the reference position. Thus, the following relationship applies: Δd=Pa-P_ref At the second sensor position, the displacement error is equal to: Δd_2=Pa_2-P_ref2

[0052] The reference position P_ref2 at the second sensor position P2 depends on the first reference position P_ref1, and the distance between the first sensor position P1 and the second sensor position P2 is fixed. As a result, the following relationship applies: Δd_2=Pa_2-D2-P_ref1 … Δd_N=Pa_N-DN-P_ref1

[0053] The individual displacement errors Δd are corrected by the register control system 60. The individual displacement errors Δd at each of the sensor positions P2 to PN following the first flexographic printing unit 16a are compared with a first threshold value T1.

[0054] Correction is performed only by the angular position correction Δα of the printing cylinder 42 positioned most downstream if the individual displacement error Δd is less than a first threshold value T1. That is, the registration control system 60 provides rotational displacement correction to the printing cylinder 42 so that the printing plate 46 is aligned with the actual position Pa of the sheet 3. The first threshold value T1 can be between 0.5 and 1.5 mm, preferably about 1 mm. This means that the angular segment length of the printing cylinder 42 can be corrected up to an angular length limit Lα. The angular length limit Lα can be between 0.5 and 1.5 mm, preferably about 1 mm.

[0055] The angular position of the printing cylinder 42 can be corrected by a motorized system 55 of the converting machine 10. The motorized system 55 can accept the required angular position correction Δα from a register control system 60.

[0056] Managing large angular position corrections Δα on a heavy print cylinder 42 can be difficult due to its inertia. However, for small individual displacement errors Δd, adjusting the angular position of the print cylinder 42 is a stable and durable method of correction. This avoids the problems of excessive use of the vacuum transport and belt vibration on the vacuum transport mentioned above.

[0057] However, if the individual displacement error Δd is greater than a first threshold T1, correction is achieved by a combination of an angular position correction Δα of the print cylinders 42 and a velocity correction Δv in the form of a change in the velocity of the vacuum transporter 52 positioned between the transport sensor 64 and the downstream-most print cylinder 42. This is shown in Figures 10a and 10b.

[0058] A correction is therefore applied to both the vacuum transport unit 52 and the printing cylinder 42 if the resulting individual displacement error Δd is greater than the angular length limit Lα. The registration correction system 60 is therefore configured to make a first correction c1 in the form of an angular position correction Δα for the printing cylinder 42. Furthermore, a second correction c2 is made by modifying the transport speed V of the vacuum transport unit 52, which is positioned between the transport sensor positions P2-PN and the most downstream positioned printing cylinder 42. By changing the speed V, the position of the sheet 3 in the transport direction T can be corrected.

[0059] The sum of the first correction c1 and the second correction c2 is equal to the longitudinal displacement error Δd_2, where the first correction c1 preferably corresponds to the angular length limit Lα and the second correction c2 corresponds to the total displacement error Δd subtracted by the angular length limit Lα.

[0060] This is shown diagrammatically in Figures 10a and 10b and will be explained in more detail later. Figures 10a and 10b show a situation where the front leading edge 5 of the sheet 3 has not advanced far enough in the transport direction T. In other words, this means that the sheet 3, without correction, arrives at the print cylinder 42 too late. As can be seen in Figures 10a and 10b, the sheet 3 has an initial speed V1 when it arrives at the sensor 64. After the sensor 64, the sheet 3 is accelerated to a second speed V3 and then decelerated again to return to the initial speed V1. The initial speed V1 can be defined as the standard operating speed.

[0061] However, if the sheet 3 is accelerated too much in the transport direction, i.e., if the sheet 3 without correction arrives at the print cylinder 42 too early, there will first be a deceleration of the initial velocity V1 to reach the second velocity V3 based on the input from the sensor 64, and then an acceleration back to the initial velocity V1.

[0062] This correction of each individual sheet 3 in or before each flexographic printing unit 16b to 16N is called individual sheet correction.

[0063] The detected individual displacement error Δd is preferably corrected before the sheet 3 reaches the nearest subsequent print cylinder 42. However, for large displacement distances, such as greater than 2 mm, it may not be possible for the vacuum transporter 52 and print cylinder 42 to fully correct the individual displacement error Δd between the sensor 64 and the nearest downstream print cylinder 42. In such cases, the sheet 3 can be tagged and tracked by the register control system 60 for discharge at the discharge module. Optionally, a third tolerance threshold T3 can be provided, and the sheet 3 can be tagged for discharge only if the third tolerance threshold T3 is exceeded. The third tolerance threshold T3 may therefore depend on the quality requirements of the finished folding box 1′ or flat-pack box 1″.

[0064] There are variations in material properties between different piles of sheets 3 placed in the feeders 14, 31 of the converting machine 10. This variation is related to the quality of the sheets, such as the presence of camber, surface irregularities, variations in air permeability and stiffness, because some piles of sheets 3 may have been produced in different batches at different times in the creasing machine.

[0065] The individual displacement errors Δd of the sheets 3 may vary at different sensor positions P2-PN. However, as shown in Figure 7, a consistent total displacement error Δd_total can be determined at the final sensor position PN at the end of the flexographic printing module 15. Thus, the total displacement error Δd_total corresponds to the displacement error Δd_N at printing unit number N. A consistent displacement error means that the total displacement error Δd_total at the end of the flexographic printing module 15 among the multiple sheets 3 shows less variation than the individual displacement errors at each flexographic printing unit 16a-16N and for each sheet 3.

[0066] It has been found advantageous to determine the average trend error Δd_total_avg by analyzing a sample S of sheets 3 from a pile placed on the feeder 14, 31. The sample S may include a number of sheets 3 from the same pile, for example between 5 and 10 sheets 3. To calculate the average trend error Δd_total_avg, the sample S of sheets 3 is transported through the converting machine 10 and the total displacement error Δd_total of each sheet 3 is calculated at the final sensor position PN at the end of the flexographic printing module 15 by determining the total displacement error Δd_total of the front leading edge 5 of the sheet 3 relative to a reference position P_refN. Thus, the trend displacement error of each sheet can be calculated as follows: Δd_total=Pa_N-DN-P_ref1

[0067] The control unit 63 is then configured to calculate the average displacement error Δd_total_avg of the sheet sample S, thus: JPEG0007724365000001.jpg12150where ΣΔd_total_avg is the sum of the average displacement errors of all sheets in sample S, and s is the number of sheets in the sample.

[0068] Alternatively, the trend can be determined at a sensor position located downstream of the second sensor position P2. In one embodiment, the average trend displacement error Δd_total_avg can be detected at the fourth flexographic printing unit 16d relative to the initial reference position P_ref1. It has therefore been found that the trend can be calculated with sufficient accuracy at the fourth sensor position P4 at the fourth flexographic printing unit 16d.

[0069] A trend correction in the form of a change in speed, i.e., a trend speed correction Δvt, is applied such that the initial speed V1 of the vacuum transporter 52 is changed for the plurality of vacuum transporters 52. Preferably, all vacuum transporters 52 in the flexographic printing module 15 are provided with a speed change Δvt. Preferably, each vacuum transporter 52 is provided with an equal speed correction Δvt.

[0070] Therefore, any of the following formulas can be used to determine the new conveying speed: JPEG0007724365000002.jpg18150 JPEG0007724365000003.jpg15150

[0071] The trend correction is then calculated as follows: velocity correction Δvt=V2−V1. where: Δd_total = trend displacement error Δd_total_avg=average trend displacement error V1 = Initial operating speed V2 = new actuation speed D_total = distance between sensor positions (i.e., between sensor positions P2 and PN)

[0072] To initiate the trend velocity correction Δvt, a second error threshold T2 can be applied to initiate the trend correction. The register control system 60 can be configured to continuously analyze samples of the sheet 3 and initiate a new trend correction when a new average trend error Δd_total_avg is ascertained from a predetermined sample S of the sheet 3. The new average trend error Δd_total_avg and trend correction may therefore differ from the previous trend correction. This is advantageously performed continuously during machine operation (i.e., during box production).

[0073] For example, the threshold T2 may be defined by an average trend error Δd_total_avg of 0.5 mm for a sample of consecutive sheets 3. The threshold T2 can initiate or restart trend correction. Trend correction is therefore initiated or re-evaluated only for sufficiently large displacement errors Δd_total_avg that exceed the second threshold T2.

[0074] The trend correction is performed before the individual sheet corrections, thereby reducing the individual displacement error Δd of the sheets 3. This trend correction limits large corrections in terms of excessive acceleration / deceleration of the vacuum transporter 52.

[0075] The trend correction and the individual sheet correction are preferably performed simultaneously. This means that, despite the trend correction, each individual sheet 3 is still controlled and corrected individually. However, the individual sheet correction is reduced because part of the individual displacement error Δd is predicted and corrected by the trend correction. Optionally, the individual sheet correction can be always enabled, while the trend correction can be disabled.

[0076] 10a and 10b, the acceleration and deceleration of the vacuum transporter 52 can be adjusted in relation to the transport speed V of the sheet 3. At low speeds, a large acceleration or deceleration of the vacuum transporter 52 can cause vibrations in the drive belt driving the rollers 58, making the sheet transport unstable. To solve this problem, it has been found that the acceleration and deceleration can be adapted as a function of the transport speed V of the sheet 3. For example, at high speeds the acceleration and deceleration are higher than at low speeds.

[0077] The conveying speed V of the vacuum conveyor 52 can be changed, with a high speed preferably being up to 5 m / s and a low speed being around 1 m / s. The second correction c2 of the displacement error Δd provided by the vacuum conveyor 52 corresponds to Δd-Lα. The correction distance L1, preferably about 400 mm, represents the distance over which the remaining displacement error Δd-Lα should be corrected by the vacuum conveyor 52. At low speeds, there is a longer time to achieve acceleration or deceleration to correct the displacement error.

[0078] The acceleration profile is selected to be symmetrical over the distance L1, i.e., the sheet 3 is accelerated over half the distance L1 and decelerated over the other half of the distance L1, which ensures that acceleration is always kept to a minimum, further reducing tension and wear on the drive belt of the vacuum transport 52.

Claims

1. 1. A method for registering a plurality of printed motifs on a sheet (3) in a converting machine (10) including a flexographic printing module (15) having at least a first flexographic printing unit (16a) configured to print a first motif and a second flexographic printing unit (16b) configured to print a second motif, the first and second flexographic printing units (16a, 16b) being arranged successively along a conveying direction (T) of the sheet (3), the method further comprising a register control system (60) having a first sensor (62) arranged at a first detection position (P1) at a distance (L1) upstream of the first flexographic printing unit (16a) and a second sensor (64) arranged at a second detection position (P2) at a distance (L1) upstream of the second flexographic printing unit (16b), detecting the passage of the front leading edge (5) of the sheet (3) with the first sensor (62); determining an actual position (Pa_1) of the front leading edge (5) at the first detection position (P1) from the detection time (t1) of the first sensor (62); defining said actual position (Pa_1) as an initial reference position (P_ref1) relative to said seat (3); calculating a second reference position (P_ref2) at a second sensor position (P2) located downstream; detecting the passage of the front leading edge with the second sensor and determining the actual position (Pa_2) of the front leading edge (5) at the second sensor position from the detection time (t2) provided by the second sensor (64); calculating an individual displacement error (Δd_2) at the second detected position (P2) by determining the difference between the detected actual position (Pa_2) and the second reference position (P_ref2); comparing the individual displacement error (Δd_2) with a first threshold (T1); providing an angular position correction (Δα) for a printing cylinder (42) in the second flexographic printing unit (16b) if the individual displacement error is below the first threshold; providing, if the error is higher than the first threshold, a first correction (c1) in the form of an angular position correction (Δα) relative to the printing cylinder (42) and a second correction (c2) for changing the position of the sheet, the second correction being achieved by modifying the conveying speed (V) of a vacuum transport unit (52) positioned between the second sensor position (P2) and the second flexographic printing unit (16b), so that the sum of the first and second corrections is equal to the individual displacement error (Δd_2); A method comprising:

2. the angular position correction (Δα) corresponds to a fixed angular length limit (Lα) of the print cylinder; The remaining portion of the displacement error is corrected by a change in velocity in the vacuum transfer unit (52), the change in velocity being an acceleration or deceleration. The method of claim 1.

3. the angular length limit (Lα) of the printing cylinder (42) is between 0.5 mm and 1.5 mm, preferably about 1 mm; The method of claim 2.

4. said converting machine comprising at least four flexographic printing units (16a to 16d); A sensor (62, 64) and a vacuum transfer unit are located upstream of each flexographic printing unit; The method of claim 1.

5. each second sensor (64) positioned downstream of the first sensor (62) is configured to detect the passage of the front leading edge (5) of the sheet (3); a control unit (63) of the register control system (60) configured to determine the actual position of the front leading edge (5) at each sensor position (P1 to PN) and to provide an angular position correction (Δα) for each downstream positioned printing cylinder, the control unit being further configured to modify the speed of each vacuum transport unit positioned downstream of each sensor in order to correct the position of the sheet in the transport direction (T). The method of claim 4.

6. detecting the passage of the front leading edge (5) of the sheet (3) with the second sensor (64) located at a position (PN) downstream of the first sensor (62); comparing the detected position (Pa_N) of the front leading edge (5) of the seat (3) with a reference position (P_refN) and determining the difference therebetween as a tendency displacement error (Δd_total); comparing the trend deviation error to a second threshold (T2); and applying a trend velocity correction (Δvt) to each controllable vacuum transport unit (52) positioned downstream of the second sensor in the flexographic printing module when the trend displacement error (Δd_total) exceeds the second threshold (T2). The method of claim 1.

7. the tilt displacement error is determined at a sensor position (P4) between the third and fourth flexographic printing units; The method of claim 6.

8. further comprising verifying the displacement error before applying the trend velocity correction; The step of determining the displacement error is accomplished by calculating an average displacement error (Δd_total_avg) for a number of sheets in a sample (S). The method of claim 6.

9. The sample (S) contains between 5 and 10 sheets. The method of claim 8.

10. the calculation of the average displacement error and the application of the trend velocity correction are repeated after each sample; 10. The method of claim 9.

11. The acceleration and deceleration of the vacuum transport unit (52) are adjusted in relation to the sheet transport speed (V). The method of claim 10.

12. the acceleration and deceleration are performed over the entire distance (L1) between the sensor position (PN) and the printing cylinder (42); The method of claim 11.

13. a flexographic printing module (15) having at least a first flexographic printing unit (16a) configured to print a first motif on a sheet (3) and a second flexographic printing unit (16b) configured to print a second motif on the sheet (3), the first and second flexographic printing units (16a, 16b) being arranged successively along a conveying direction (T) of the sheet (3); the register control system (60) including a first sensor (62) arranged at a first detection position (P1) at a distance (L1) upstream of the first flexographic printing unit (16a) and a second sensor (64) arranged at a second detection position (P2) at a distance (L1) upstream of the second flexographic printing unit (16b), the register control system (60) being configured to at least partially perform the method for registering a plurality of printed motifs on the sheet (3) according to any one of claims 1 to 12; A transformation machine (10) comprising:

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