NL2032263B1

NL2032263B1 - Flexographic printing assembly for same-sized plate rollers

Info

Publication number: NL2032263B1
Application number: NL2032263A
Authority: NL
Inventors: A Wind Anne
Original assignee: Canon Production Printing Holding Bv
Priority date: 2022-05-16
Filing date: 2022-06-23
Publication date: 2023-11-24

Abstract

100245NL01 18 ABSTRACT A simplified method of flexographic printing comprises the step of conveying sheets (30- 34) towards a flexographic printing station (10), wherein the sheets (30-34) are conveyed at a first speed (V0) and adjacent sheets (30-34) are spaced apart from one 5 another by a predetermined first inter-sheet distance (DO), and Wherein the flexographic printing station (10) comprises a plate roller (11)which transfers a marking material onto the sheets (30-34), the method being characterized by the flexographic printing station (10) first accelerating each sheet (30-34) to a speed greater than the first speed (V0) at the plate roller (11), thereby increasing the inter-sheet distance (D2) between said sheet 10 (30-34) and a subsequent sheet (30-34) above the first inter-sheet distance (DO) and then decelerating said sheet (30-34) back to the first speed (V0), thereby decreasing the inter-sheet distance (D1) with respect to an downstream sheet.

Description

100245NL01 1

Flexographic printing assembly for same-sized plate rollers

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to a method of flexographic printing and to a flexographic printing assembly. 2. Description of Background Art

Flexographic printing comprising the step conveying sheets towards a flexographic printing station, wherein the sheets are conveyed at a first speed and adjacent sheets are spaced apart from one another by a predetermined first inter-sheet distance, and wherein the flexographic printing station comprises a plate roller which transfers a marking material onto the sheets. Such a flexographic printing station is illustrated in

Fig. 2 and described in more details in the corresponding paragraphs below. In flexographic printing stations, the plate rollers, or specifically their outer layers (which are commonly referred to as ‘sleeves; ) are often exchanged in between different print jobs. The outer layer comprises on it a fixed pattern corresponding to an image. The outer layer has a circumference C, which is preferably equal or close to the length L of the sheets to be printed on. In case the circumference C of the outer layer of the plate roller is greater than the sheet length L, the inter-sheet distance DO between sheets is corrected preserve the synchronization between the plate roller and the sheet. However a large inter-sheet distance DO between sheets reduces the productivity (in terms of sheets printed per unit time). It is known to exchange the plate roller for another plate roller having a smaller diameter closer to the sheet length to maintain productivity. That however requires other components to be adjustable to this change in diameter, as well as the work required to perform those adjustments.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a simplified method for flexographic printing, specifically with regard to the operator workload when changing between print jobs which require different sheet lengths.

In accordance with the present invention, a process of flexographic printing according to claim 1 and a flexographic printing assembly according to claim 8 are provided.

This method comprises conveying the printed sheets towards a flexographic printing

100245NL01 2 station, wherein the sheets are conveyed at a first speed and adjacent sheets are spaced apart from one another by a predetermined first inter-sheet distance, and wherein the flexographic printing station comprises a plate roller which transfers a marking material onto the sheets, the method being characterized by the flexographic printing station first accelerating each sheet to a speed greater than the first speed at the plate roller, thereby increasing the inter-sheet distance between said sheet and a subsequent sheet above the first inter-sheet distance and then decelerating said sheet back to the first speed, thereby decreasing the inter-sheet distance with respect to an downstream sheet.

It is the insight of the inventor that the frequency of sheets passing through the flexographic printing station can be increased by a temporarily increasing the speed of each sheet at the plate roller. This allows the frequency of the flexographic printing station to be adjusted, such that it matches the frequency by which sheets are fed towards the flexographic printing station, even when the circumference of the plate roller is (much) greater than the sheet length. This allows plate rollers of a single diameter to be applied, while still allowing for a small inter-sheet distance for all sheet lengths. No changes or adjustments to the hardware components of the flexographic printing station are required when changing to a different sheet length, except a change of the outer layer, when a different printing pattern is desired. This simplifies construction of the flexographic printing assembly as well as its operation. Thereby the object of the present invention has been achieved.

More specific optional features of the invention are indicated in the dependent claims.

In an embodiment, when decreasing the inter-sheet distance with respect to the downstream sheet, it is decreased back to the first inter-sheet distance. In another embodiment, sheets leave the flexographic printing station at the first speed. The sheet is temporarily driven to a higher speed than the first speed by the plate roller. The distance between the accelerated sheet and the sheet directly downstream of it will increase. In most cases, the sheets will move downstream of the flexographic printing station at a speed comparable to the first speed. Thus, the distance between the accelerated sheet and a sheet directly downstream will decrease, in this case back to the first-sheet distance. Downstream and up of the flexographic printing station sheets are transported with similar speeds and inter-sheet distances. It will be appreciated that

100245NL01 3 in another embodiment, the speeds and inter-sheet distance downstream of the flexographic printing station may be different from the upstream side, as long as the frequency (i.e. the number of sheets passing a point per unit time) is the same on both sides of the flexographic printing station.

In an embodiment, the flexographic printing station first accelerates and decelerates the sheet if a length L of a sheet in its transport direction is substantially smaller than a circumference C of the plate roller. The acceleration-deceleration is preferably only applied when a significant difference between the sheet length L and the circumference ~~ C has been determined, either by the printing assembly’s controller or operator. For relatively small differences, the inter-sheet distance DO may be increased to allow the plate roller to rotate at a constant speed. When the difference is sufficiently great, more time is available for accelerating and decelerating the plate roller, thereby avoiding exposing the plate roller and its drive to high torques.

In an embodiment, the method further comprises the step of driving the plate roller with a non-constant speed per revolution of the plate roller, such that a displacement as defined by the circumferential velocity of the plate roller integrated over time is the same for each revolution of the plate roller. The temporary increase in velocity increases the distance between the sheet being accelerated and decelerated and the sheet downstream of it, as the downstream sheet still moves at the first speed. Downstream of the flexographic printing station the sheets may also move relatively slower in the transport direction as compared to the sheet being accelerated and decelerated. The distance between the sheet in the flexographic printing station and a sheet directly downstream thus decreases. When this relative displacement by the flexographic printing station is the same for every sheet, a constant and homogenous flow of sheets is output by the flexographic printing station. This constant flow is easy to process and one avoids collisions or overlapping between adjacent sheets.

In an embodiment, the plate roller is accelerated and decelerated per revolution, such that the average circumferential velocity of the plate roller per revolution is a constant value greater than the first speed. Effectively, a point on the outer surface of the plate roller on average moves faster than the first speed.

In an embodiment, the acceleration and deceleration of the plate roller are proportional

100245NL01 4 to a difference between on one hand a sum of the first inter-sheet distance DO and the length L of the sheets and on the other hand the circumference C of the plate roller.

Upstream of the flexographic printing station sheets pass through a fixed point at a frequency fO of: 0 = L+ DO 7

Since the frequency must be conserved to avoid local accumulation of sheets, downstream of the flexographic printing station, the frequency f2 is: 12 foo L+D2 where D2 and V2 are the inter-sheet distance and speed downstream of the flexographic printing station.

At the flexographic printing station the frequency is determined by the velocity profile, which observes:

T w(t)dt = 27 0 where (tf) is the angular velocity as a function of time, 7 the time period required for a single revolution of the plate roller, and { the time itself. The frequency at the flexographic printing station is 7/7, so the angular velocity profile may be selected, such that: 1 0 = L+D0 rn

Any suitable velocity profile may be applied given these constraints, though it will be appreciated that different routes for determining velocity profiles may also be applied, such as numerical or computer-based methods.

The present invention further relates to a flexographic printing assembly comprising: - a sheet feeding station configured to output printed sheets at a first speed with a

100245NL01 predetermined inter-sheet distance between adjacent sheets; - a flexographic printing station positioned downstream of the sheet feeding station, wherein the flexographic printing station comprises a plate roller positioned for transferring a marking material onto the sheets, wherein a controller is configured to 5 drive the plate roller to accelerate each sheet to a speed greater than the first speed, thereby increasing the inter-sheet distance with respect to a subsequent sheet above the first inter-sheet distance and then to decelerate each sheet, thereby decreasing the inter-sheet distance with respect to an downstream sheet.

The assembly is configured to perform the method described above. Preferably, the sheet feeder is part of a first printing station, such that printed sheets can supplied to the flexographic printing station for a final coating or printing.

In an embodiment, the controller is configured to compare a first parameter corresponding to a sheet length L to a second parameter corresponding to a circumference C of the plate roller to determine whether the flexographic printing assembly is to operate in a first or second printing mode, wherein in the first printing mode the plate roller is driven at a constant speed, while in the second printing mode the plate roller is accelerated and decelerated during every revolution of the plate roller.

The controller further synchronizes the movement of the plate roller with the motion of the sheet, such that the pattern is printed on each sheet at the same relative position.

The controller may therein continuously correct for deviations in e.g. the transport speed of the sheets and/or their inter-sheet distance by adjusting the velocity profile.

In an embodiment, in the first printing mode the controller is configured to adjust set inter-sheet distance between sheets upstream of the flexographic printing station, such that a sum of the sheet length L and the inter-sheet distance DO is equal to the circumference C of the plate roller. When sheet length L is close to the circumference C only a short time is available for accelerating and decelerating the plate roller. The acceleration and deceleration would then require very high torques, which are preferably avoided. The controller is set to avoid this by increasing the first inter-sheet distance DO of the sheets upstream of the flexographic printing station. Preferably, in the first printing mode the controller is configured to adjust set inter-sheet distance DO between sheets upstream of the flexographic printing station, such that a sum of the sheet length L and the inter-sheet distance DO is equal to the circumference C of the plate roller. The plate roller is then operated at a constant angular velocity. Productivity

100245NL01 6 is slightly decreased in this manner, but it protects against excessive wear on the plate roller, its bearings, and drive. Preferably, the controller has stored on its memory a predetermined margin of difference between the circumference C and the sheet length

L. As long as the difference is within the margin, the first printing mode is applied.

Outside the margin, the controller accelerates and decelerates the plate roller as described above.

In an embodiment, a first printing station is positioned between the first sheet feeding station and the flexographic printing station, which first printing station comprises a page-wide inkjet printhead array, along which sheets are transported in a continuous motion during printing. The flexographic printing station is provided in line with an inkjet sheet printer. The page-wide printhead array ensures a uninterrupted flow of sheets.

With the present invention the print speed or frequency of the flexographic printing station can easily match that of the inkjet printing station, resulting in a highly productive assembly.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the present invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

Fig. 1 is a schematic side view of a printing assembly comprising a flexographic printing station;

Fig. 2 a detailed, schematic side view of the flexographic printing station in Fig. 1;

Figs. 3A to G illustrate the flexographic printing station during various steps of the method according to the present invention;

Fig. 4 is a graph illustrating different velocity profiles for driving the

100245NL01 7 flexographic printing station in Fig. 2; and

Fig. 5 is block diagram illustrating the different steps of the the method according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described with reference to the accompanying drawings, wherein the same reference numerals have been used to identify the same or similar elements throughout the several views.

Fig. 1 shows a printing assembly 1. On input side (left in Fig. 1) a first printing station 3 is provided with a sheet input module 2. The sheet input module 2 may be configured to store a plurality of sheets in stack form or to cut sheets from an roll of web material, which sheets are transported to the first printing station 3. The first printing station 3 provides an image on one or both sides of the sheet. The first printing station 3 is preferably an inkjet printing station 3. In the example in Fig. 1, the first printing station 3 is an inkjet printer comprising a page-wide printhead array. Sheets pass the printhead array in a continuous motion while being printed on. This allows for highly productive sheet printing. The printed sheets are transported from the printing station 3 via a first intermediate module 4 to a flexographic printing station 10. The flexographic printing station 10 will be described in more detail further on with regard to Fig. 2. The flexographic printing station 10 is configured to apply one or more marking materials on the printed sheets, for example as a coating. From the flexographic printing station 10 the sheets are brought to a output station 8 via a second intermediate module 5. The output station 6 may for example be a stacker or folder.

Fig. 2 illustrates in detail the flexographic printing station 10. Flexographic printing stations as known in the art are commonly applied for the printing of covers and/or labels, for example for packaging materials like folded carton. The flexographic printing station 10 comprise a plate roller 11, which comprises an outer layer 12 {which is the so-called ‘sleeve’) configured to releasably hold marking material. The plate roller 11 is rotatable around its axis 13 by means of a drive. The outer layer 12 of the plate roller 11 has been processed to transfer the marking material in a specific pattern onto the sheet 30. The pattern may be a specific image or continuous layer in case of a full surface coating. The sheet 30 and the plate roller 11 move in a timed and synchronous motion, such that the pattern of marking material is transferred onto the sheet 30 in the desired

100245NL01 8 positions. The pattern is printed on each sheet at the same relative position. For printing different images generally a different outer layer 12 is required. Different plate rollers 11 may also be positioned in series to print different colours and/or different parts of an image. A single full surface coating can also be applied by a plate roller 11. A roller processing device 22 may be provided for cleaning or inspecting the outer layer 12.

Opposite the plate roller 11 a pressing roller 14 is provided rotatable around its axis 14.

The sheet 30 is pressed or pinched between the plate roller 11 and the pressing roller 14 to ensure reliable contact and transfer between the sheet 30 and the plate roller 11.

At the flexographic printing station 10 the sheet 30 is transported by being pinched between the plate roller 11 and the pressing roller 14. The angular velocity of the plate roller 11 determines the speed of the sheet 30 in the transport direction X. The sheet 30 may further be supported by one or more sheet supports 24, 25, for example in the form of plates, ridges, or rollers.

The marking material is transferred onto the plate roller 11 via the transfer roller 18. The transfer roller 16 is commonly referred to as the ‘Anilox’ roller. The transfer roller is configured to releasably hold marking material and to transfer it onto the plate roller 11 only in the places dictated by the pattern in the outer layer 12. The transfer roller 17 is rotatable around its axis 17. A knife 18 is provided along the periphery of the transfer roller 16 to thin the layer of marking material received from the feed roller 19. The feed roller 19 is contact with a source 21 of marking material, for example in the form of a bath. The feed roller 19 rotates around its axis 20 and transfers marking material from the source 21 onto the transfer roller 19.

Fig. 3A illustrates the first step i of the method in the diagram in Fig. 5. Sheets 30-33 are printed and output by the first printing station 3 at a constant rate or speed VO. The sheets 30-33 move at a substantially constant speed VO in the transport direction X and with a substantially constant inter-sheet distance DO towards the flexographic printing station 10. The controller 9 of the flexographic printing assembly 1 compares the length

L of the incoming sheets 30-33 to the circumference C of the plate roller 11 in step ii.

The sheet length L in the transport direction X is for example input through a user interface (not shown), determined from a pre-stored media catalogue upon selection of a specific print medium, and/or detected by means of a sensor (not shown). A parameter or setting corresponding to the circumference C is stored on the controller's

100245NL01 9 memory. The sheet length L is compared to the circumference C. When it has been determined that the sheet length L is similar to the circumference C, the controller 9 operates the flexographic printing station 10 in a first printing mode (step viii). It will be appreciated that the controller 9 may comprise a setting to determine a margin wherein the sheet length L is considered sufficiently close to the circumference C to apply the first printing mode. For example the sheet length L may be within £5% or less of the circumference C to trigger the first printing mode. In the first printing mode, the controller 9 controls the first printing station 3 and/or the first intermediate module 4, such that the sheets 30-33 are provided to the flexographic printing station 10 with the sum of the sheet length L and the inter-sheet distance DO being equal to the circumference C (step ix):

L+D0=C

The controller 9 then in the first printing mode controls the plate roller 11 to rotate at a constant angular velocity which matches the first speed VO of the incomings sheets 30- 33 (step x).

When the controller 9 determines that the sheet length L deviates sufficiently from the circumference C to lie outside the margin set for the first printing mode, a second printing mode is selected (step iii). In the second printing mode, the plate roller 11 is driven at a non-constant speed. Upstream of the flexographic printing station 10, the speed VO and the inter-sheet distance DO are determined by the first printing station 3.

Sheets 30-33 move in a continuous flow towards the plate roller 11 at the first speed VO and with a substantially constant inter-sheet distance DO. The applied velocity profile is determined in step iv by comparing the circumference C to the sheet length L, the inter- sheet distance DO, and the first velocity VO. Since the sheet length L is less than the circumference, a pattern of marking material is only present along a portion of the circumference of the plate roller 11. The velocity profile is further selected such that the pattern of marking material is positioned at the intended location of the sheet 30. The velocity profile (illustrated in Fig. 4) comprises an acceleration followed by a deceleration.

Fig. 3B illustrates the acceleration (step v) of the leading sheet 30 upon contact with the plate roller 11. At the initial contact between the plate roller 11 and the first sheet 30 in the row of sheets 30-33, the circumferential velocity of the plate roller 11 is preferably

100245NL01 10 equal to the first speed VO of the incoming sheet 30. When the sheet 30 has been sufficiently engaged by the plate roller 11 and the pressing roller 14, one or both rollers 11, 14 are driven to temporarily accelerate. In consequence, the distance D1 between the leading sheet 30 and its trailing sheet 31 becomes larges than the first inter-sheet distance DO. Consequently in step vi and shown in Fig. 3C a deceleration is applied to one or both of the rollers 11, 14 after the initial acceleration. Thereby, the circumferential velocity of the plate roller 11 is preferably brought back to the first velocity VO. The first sheet 30 leaves the flexographic printing station 10 at the first velocity VO in Fig. 3C.

Next, in Fig. 3D, the second sheet 31 arrives at the plate roller 11. Again, the angular velocity of the plate roller 11 is again increased, which increases the distance D2 between this second sheet 31 and the third sheet 32 in the row. The acceleration also decreases the distance D1 between the first sheet 30 and the second sheet 31, as shown in Fig. 3F. Preferably, as shown in Fig. 3G the final distance between the first and second sheets 30, 31 is restored to the first inter-sheet distance DO, as shown in

Fig 3G. The sheets 30, 31 thus move downstream of the flexographic printing station 10 with same first velocity VO and inter-sheet distance DO as upstream of the flexographic printing station 10. It will be appreciated that the downstream velocity and inter-sheet distances may also be selected to be different from upstream, as long as the frequency by which sheets pass a point along the transport path remains constant. For example, a relatively larger downstream velocity V2 will require a relatively larger inter-sheet distance D2, and vice versa. This configuration allows for productive flexographic printing with a plate roller 11 with a fixed diameter. Changing to different sheet lengths requires no hardware changes (aside from possibly changing the outer layer 12 on the plate roller 11 when a different pattern is also desired).

It will be appreciated that while the above has been described with regard to a single sheet passing the plate roller 11 every revolution, the present invention can also be applied to any integer of sheets passing the plate roller 11 per revolution.

Fig. 4 illustrates some examples of velocity profiles, which can be applied to the plate roller 11. The velocity profiles comprises a repeating section, wherein each section comprises an acceleration (upward slope} and deceleration (downward slope). Each repeating section has a width matching the frequency fO by which sheets 30-34 are supplied to the flexographic printing station 10. Fig. 4 plots the velocity profiles with the

100245NL01 11 angular velocity of the plate roller 11 w(t) on the vertical axis versus the time f on the horizontal axis. Any other suitable expression may be also applied along the axes to define the velocity profiles, such as the speed, frequency, circumferential speed, etc.

The velocity profile sin Fig. 4 are relative to the first speed VO and all comprise a single upwards ‘bump’ to facilitate the acceleration and deceleration. For each section, the acceleration precedes the deceleration, such that the average circumferential velocity of the plate roller per section is greater than the first speed VO. The velocity profiles may also be defined in terms of absolutes speeds with respect to a stationary point.

Although specific embodiments of the invention are illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations exist. It should be appreciated that the exemplary embodiment or exemplary embodiments are examples only and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents. Generally, this application is intended to cover any adaptations or variations of the specific embodiments discussed herein.

It will also be appreciated that in this document the terms "comprise", "comprising", “include”, "including", "contain", “containing”, "have", "having", and any variations thereof, are intended to be understood in an inclusive (i.e. non-exclusive) sense, such that the process, process, device, apparatus or system described herein is not limited to those features or parts or elements or steps recited but may include other elements, features, parts or steps not expressly listed or inherent to such process, process, article, or apparatus. Furthermore, the terms "a" and "an" used herein are intended to be understood as meaning one or more unless explicitly stated otherwise. Moreover, the terms “first”, "second", "third", etc. are used merely as labels, and are not intended to impose numerical requirements on or to establish a certain ranking of importance of their objects.

The present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the

100245NL01 12 spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

100245NL01 13

EMBODIMENTS

1. A method of flexographic printing comprises the step of conveying sheets (30-34) towards a flexographic printing station (10), wherein the sheets (30-34) are conveyed at a first speed (VO) and adjacent sheets (30-34) are spaced apart from one another by a predetermined first inter-sheet distance (DO), and wherein the flexographic printing station (10) comprises a plate roller (11) which transfers a marking material onto the sheets (30-34), the method being characterized by the flexographic printing station (10) first accelerating each sheet (30-34) to a speed greater than the first speed (VO) at the plate roller (11), thereby increasing the inter-sheet distance (D2) between said sheet (30-34) and a subsequent sheet (30-34) above the first inter-sheet distance (DO) and then decelerating said sheet (30-34) back to the first speed (V0), thereby decreasing the inter-sheet distance (D1) with respect to an downstream sheet (30-34). 2. The method according to claim 1, wherein when decreasing the inter-sheet distance (D1) with respect to the downstream sheet (30-34), it is decreased back to the first inter-sheet distance (DO). 3. The method according to any of the previous claims, wherein sheets (30-34) leave the flexographic printing station (10) at the first speed (V0). 4. The method according to any of the previous claims, wherein the flexographic printing station (10) accelerates and decelerates the sheet (30-34) if a length (L) of a sheet in its transport direction (X) is substantially smaller than a circumference (C) of the plate roller (11). 5. The method according to any of the previous claims, further comprising the step of driving the plate roller (11) with a non-constant speed per revolution of the plate roller (11), such that a displacement as defined by the circumferential velocity of the plate roller (11) integrated over time is substantially the same for each revolution of the plate roller (11). 6. The method according to claim 5, wherein the plate roller (11) is accelerated and decelerated per revolution, such that the average circumferential velocity (V2) of the plate roller (11) per revolution is a constant value greater than the first speed (V0).

100245NL01 14 7. The method according to claim 8, wherein the acceleration and deceleration of the plate roller (11) are proportional to a difference between on one hand a sum of the first inter-sheet distance (DO) and the length (L) of the sheets (30-34) and on the other hand the circumference (C) of the plate roller (11).

8. A flexographic printing assembly (1) comprising: - a first feeding station (2) configured to output sheets (30-34) at a first speed (VO); - a flexographic printing station (10) positioned downstream of the first sheet feeding station (3), wherein the flexographic printing station (10) a plate roller (11) positioned for transferring a marking material onto the sheets (30-34), characterized by a controller configured to drive the plate roller (11) to accelerate each sheet (30-34) to a speed greater than the first speed (VO), thereby increasing the inter- sheet distance (D2) with respect to a subsequent sheet (30-34) above the first inter- sheet distance (DQ) and then to decelerate said sheet (30-34), thereby decreasing the inter-sheet distance (D1) with respect to an downstream sheet (30-34). 9. The flexographic printing assembly (1) according to claim 8, wherein the controller is configured to compare a first parameter corresponding to a sheet length (L) to a second parameter corresponding to a circumference (C) of the plate roller (11) to determine whether the flexographic printing assembly (1) is to operate in a first or second printing mode, wherein in the first printing mode the plate roller (11) is driven at a constant speed, while in the second printing mode the plate roller (11) is accelerated and decelerated every revolution of the plate roller (11).

10. The flexographic printing assembly (1) according to claim 9, wherein the controller is configured in the second printing mode to accelerate and decelerate the plate roller (11), such that an average frequency by which sheets (30-34) pass the flexographic printing assembly (1) is equal to an average frequency by which sheets (30-34) leave the first feeding station (2).

Claims

100245EN01 15 CONCLUSIONS

A method of flexographic printing comprising the step of conveying sheets (30-34) to a flexographic printing station (10), wherein the sheets (30-34) are conveyed at a first speed (V0) and adjacent sheets (30- 34) are separated from each other by a predetermined first inter-sheet distance (DQ), and wherein the flexographic printing station (10) comprises a plate roller (11) which transfers a marking material to the sheets (30-34), the method being characterized is in that the flexographic printing station (10) first accelerates each sheet (30-34) on the plate roller (11) to a speed greater than the first speed (VO), creating a distance between sheets (D2) between the equalizing sheet (30 -34) and on a subsequent sheet (30-34) is increased to greater than the first distance between sheets (DO) and then slowing down each sheet (30-34) back to the first speed (V0), thus the distance between sheets (D1) relative to a downstream sheet (30-34) becomes smaller.

Method according to claim 1, wherein when reducing the distance between {D1) relative to the downstream sheet (30-34), said distance is reduced back to the first distance between sheets (D0).

A method according to any one of the preceding claims, wherein sheets (30-34) leave the flexographic printing station (10) at the first speed (V0).

A method according to any one of the preceding claims, wherein the flexographic printing station (10) accelerates and decelerates the sheet (30-34) if a length (L) of a sheet in its transport direction (X) is substantially less than a circumference ( C) of the plate roller (11).

A method according to any one of the preceding claims, further comprising the step of driving the plate roller (11) at a non-constant speed per revolution of the plate roller (11), such that a displacement determined as the peripheral speed of the plate roller (11) ) integrated over time is essentially the same for each revolution of the plate roller (11).

6. Method according to claim 5, wherein the plate roller (11) accelerates and decelerates per revolution such that the average peripheral speed (V2) of the

100245EN01 16 plate roller (11) per revolution is a constant value greater than the first speed (VO).

Method according to claim 6, wherein an acceleration and deceleration of the plate roller (11) are proportional to a difference between an addition of the first distance between sheets (DO) and the length (L) of the sheets (30-34) on the one hand. and on the other hand the circumference circumference (C) of the plate roller (11).

8. Flexographic printing assembly (1) comprising: - a first sheet feeding station (2) arranged to output sheets (30-34) at a first speed (VO) with a first inter-sheet distance (DO) between consecutive sheets (30-34); - a flexographic printing station (10) located downstream of the first sheet feeding station (3), the flexographic printing station (10) comprising a plate roller (11) provided for transferring a marking material onto the sheets (30-34), with the characterized in that a control unit is arranged to drive the plate roller (11) to accelerate each sheet (30-34) to a speed greater than the first speed (VO), whereby a distance between sheets (D2) with respect to a subsequent sheet (30-34) is increased to greater than the first sheet spacing (DO) and then decelerates the sheet (30-34) in question, increasing a sheet spacing (D1) relative to a downstream sheet ( 30-34) becomes smaller.

9. Flexographic printing assembly (1) according to claim 8, wherein the control unit is arranged to compare a first parameter corresponding to a sheet length (L) with a second parameter corresponding to a circumference (C) of the plate roller (11) to determine whether the flexographic printing assembly (1) will operate in a first or second print mode, wherein in the first print mode the plate roller (11) is driven at a constant speed, while in the second print mode the plate roller (11) accelerates each revolution of the plate roller (11) and is delayed.

10. Flexographic printing assembly (1) according to claim 9, wherein the control unit is designed to accelerate and decelerate the plate roller (11) in the second printing mode in such a way that an average frequency with which sheets (30-34) pass the plate roller

100245EN01 17 is equal to an average frequency with which sheets (30-34) leave the first sheet feeding station (2).