EP1961569A1 - Device and method of adjustment for a rotary printing machine - Google Patents

Abstract

The device has a control unit controlling movement of a printing unit from a predefined initial position to a predefined end position. A vision system captures images of formats (F), which are successively printed on a printing web (10) between the initial position and the end position. A calculation unit evaluates the images and determines a specific position of the printing unit after the initial position at which the printed images are identical. The calculation unit calculates a surface of each printed format. An independent claim is also included for a method for adjusting the position of a printing unit with respect to a printing web in a rotary printing machine.

Description

Technical area

The present invention relates to a device for adjusting the position of a printing unit with respect to a printing tape in a rotary printing machine comprising means for adjusting the position of said printing unit with respect to said printing tape. .

The invention also relates to a method of adjusting the position of a printing unit relative to a printing tape in a rotary printing machine.

Finally, the invention relates to a rotary printing machine comprising at least one printing unit capable of transferring ink onto a printing tape for printing formats and comprising means for adjusting the position of said printing unit with respect to said printing unit. print tape.

State of the art

A rotary printing machine generally comprises at least one printing unit corresponding to a back-pressure cylinder on which a printing tape is based. For rotary printing machines of the flexographic type, the printing unit is generally formed by an assembly comprising a screen roll (or anilox) and a forme cylinder (or impression cylinder). In this type of machine, the search for the print quality of the formats is done by optimizing the adjustment of the relative position of the various cylinders participating in the printing process. The image obtained by the complete revolution of the forme cylinder on the printing tape is called format.

Adjustment devices are known in which the position of a printing unit with respect to a printing tape is automatically adjusted by the closed-loop analysis of the images of the formats taken by a vision system, see for example the application patent DE 4413735 A .

In this known type of adjustment device, the image of the format printed on the printing tape is transferred to a control unit which compares in real time and continuously to a reference image (or target image). According to this state of the art, as long as the image of the printed format does not correspond to the reference image, a control unit modifies the position of the printing unit with respect to the printing tape. This setting is dynamic, that is, the position of the printer group can change automatically during production, depending on the images taken by the vision system. If such a device makes it possible to correct in real time and continuously any drift of printed formats during production, it is not at all adapted to the setting of the rotary printing machine before the launch of production, also called preset or prepositioning.

Indeed, before starting a print job, the rotary printing machine must be preset. This presetting consists in putting the printer unit in contact with the printing tape and advancing it in successive steps towards the backpressure cylinder while the tape is running. At the same time, printed formats are controlled by suitable means. When the print quality of the sizes reaches the required level, the printer group feed is stopped, the preset is completed. The printing unit is then in a position called optimal, denoted P _opt .

In an adjusting device according to the state of the art, the position of the printing unit is slaved to the vision system so that to go from a position P _i to a successive position P _{i + 1} , it is necessary to wait for the format printed by the printing unit in the position P _i passes in front of the vision system. The length of tape that then passed is equal to L + D, where L represents the length of a format and D the distance that separates the vision system from the printing group, L and D being expressed in meters. Thus to successively move from an initial position P _i to a final position P _{i + n} , where n represents the total number of steps in advance of the printing unit, n × (L + D) meters of tape must be scrolled in the printing machine. In other words, the length of tape that passes through the printing machine is proportional to the distance that separates the printing machine. vision system of the printing unit, on the one hand, and the length of the printed format, on the other hand.

When presetting a rotary printing machine, the tape running through the machine while the printing unit is advancing to the optimum position is lost tape, also known as a strike. The waste being a loss of material, it generates an additional cost of production. Thus, if a prior art adjusting device is used to pre-set a printing machine, the strike is proportional to the distance separating the vision system from the printing unit, on the one hand, and the length of the printed format, on the other hand, which is not satisfactory.

Presentation of the invention

An object of the present invention is to overcome the aforementioned drawbacks by providing a setting device that limits the waste during the presetting of a rotary printing machine.

For this purpose, the subject of the present invention is an adjusting device according to claim 1.

In an adjusting device according to the invention, the position of the printing unit is not slaved to the vision system. Thanks to this new design, the waste is not proportional to the distance between the vision system and the printing unit, but only to the length of the printed format, which considerably reduces the waste.

Other objects and advantages of the invention will appear more clearly in the description of embodiments, which description will be made with reference to the accompanying drawings.

Brief description of drawing figures

• The Figure 1 is a schematic and partial view of an in-line flexographic printing machine;
• The Figure 2 is a schematic and partial view of a flexographic printing machine with central drum;
• The Figure 3 is a graph showing the evolution of the surface of printed formats covered by ink as a function of the advance of the printing group.
• The Figure 4 is a schematic view of a printing tape;

Best way to realize the invention

The Figure 1 illustrates an online flexographic printing machine. The printing unit 6 comprises a screen cylinder (or anilox) 1 and a plate cylinder 2. The screen cylinder 1 is in contact with the plate cylinder 2 to ensure sufficient inking of the plate. The prepositioning of the screen roll 1 with respect to the forme cylinder 2 not forming part of the invention, it will not be described here.

The forme cylinder 2 of the printing unit 6 is in contact with a printing band 10 which bears against a backpressure cylinder 3. A camera 4 is placed downstream of the printing unit 6 in order to capture images. printed formats on the printing tape 10.

The contacting of the forme cylinder 2 with the printing tape 10 is a preliminary step to the adjusting operation according to the invention, this preliminary step is performed without rotating the printing machine, that is, without scroll the print band. The contact pressure of the forme cylinder 2 on the backpressure cylinder 3 is chosen sufficient so that ink is transferred onto the printing tape 10 but not excessive to avoid damage to the plate. Once this preliminary step has been completed, the operation of adjusting the position of the printing unit 6 with respect to the printing tape 10 can begin.

According to the invention, the presetting of the rotary printing machine is carried out in two phases. In a first phase, a control unit 7 controls the displacement of the printing unit 6 from a predefined initial position P _i to a final position P _f predefined by successive steps T, while the printing band 10 runs in the machine, the direction of travel is illustrated by the arrow 5. The initial position P _i corresponds to the position of the printing unit 6 at the end of the preliminary step. The final position P _f is defined by the choice of the pitch T, on the one hand, and by the total number of movement steps of the printing unit, on the other hand. In the example, the pitch is 10 μm and the total number of movements of the printing unit from position P _i is equal 9, thus, the final position P _f is at 90 μm from the initial position P _i . If only one format is printed for each position of the printer group, 10 formats will be printed on the print tape after the first phase (see Figure 3 ). For an N number of printed formats for each position of the printing group, it is 10 × N formats that will be printed on the print tape at the end of the first phase.

In a second phase, the camera 4 captures images of the 10 × N F formats printed on the printing tape 10 during the first phase and transmits them to a calculation unit 8 to be evaluated. The purpose of the evaluation is to determine the optimal position P _opt , that is to say the position of the printing unit 6 for which the print quality of the formats F reaches the required level. According to the invention, the position P _opt corresponds to the position from which the images of the printed formats F are identical.

In a first evaluation mode, the unit 8 calculates the surface of the 10 × N printed formats F covered by ink. To do this, the unit 8 calculates the number of pixels of each image whose gray level is different from the blank print tape. At the end of the second phase, the computing unit 8 stores the position of the image from which the number of pixels no longer varies, in other words, the position from which the surface covered by ink becomes constant, this position corresponds to the optimum position P _opt sought. The optimum position P _opt is also the first position for which the transfer of the ink reaches a maximum.

In a second evaluation mode, the unit 8 calculates the gray level of each pixel composing the 10 × N images. To do this, the unit 8 compares the gray level of each pixel of a given image with the gray level of the same pixel of the previous image. At the end of the second phase, the computing unit 8 stores the position of the image from which the gray level no longer varies, that is, the position from which the gray level becomes constant, this position corresponds to the optimal position P _opt sought.

Thanks to the invention, the first phase of the presetting of the printing machine is independent of the second phase. This characteristic of the invention makes it possible to carry out a pre-adjustment which is faster and with less waste than the known adjustment devices. Indeed, in an adjustment device according to the invention, the position of the printing unit 6 is not locked to the camera 4 so that to go from a position P _i to a successive position P _{i + 1} , it is It is not necessary to wait for the format F _i printed by the printing unit in the position P _{i to} pass in front of the camera 4. Thus, successively passing from an initial position P _i to a final position P _f and printing that a format for each position of the printing unit (N = 1), the length of tape that passes through the printing machine is equal to (nx L) + D meters, where n is the total number of steps in advance of the group printer 6, L the length of a format and D the distance between the camera 4 and the printer group 6. With reference to the figure 1 , the distance D is that which separates the points A and B.

Thanks to the invention, the length of the band that passes through the printing machine during the presetting is not proportional to the distance that separates the camera 4 from the printing unit 6 but only to the length of the printed format, which greatly reduces the waste.

The Figure 2 illustrates a flexographic printing machine with a central drum. The description made in relation to the Figure 1 also being valid for the Figure 2 it will not be repeated here. It will be noted that the distance D which separates the points A and B in the machine from the Figure 1 must be measured along a line while in the machine the Figure 2 it must be measured along a curve.

Advantageously, the optimum position P _opt memorized by the calculation unit 8 is transmitted to the control unit 7 to control the displacement of the printing unit 6 from the final position P _f to the optimum position P _opt at the end of the second phase. This variant embodiment is represented by a dotted arrow on the Figures 1 and 2 .

In the examples of printing machine illustrated in Figures 1 and 2 , only one printing group is represented, it goes without saying that these machines may have several printing units, the adjustment method according to the invention then applies for each group.

The Figure 3 is a graph which illustrates by an example, in connection with the first evaluation mode, the evolution of the surface of the printed formats covered by ink as a function of the advance of the printing unit. The abscissa axis is expressed in micrometer, the ordinate axis is expressed as a percentage of the area of the printed formats covered by ink relative to the total area of said printed formats. Except for flat areas, only a part of a given format is covered by ink, the other part remains blank, therefore, the total area of the printed formats is the sum of the areas of inked parts and blank parts .

It can be seen that between zero (initial position P _i ) and 80 μm the surface of the printed formats covered by ink increases continuously, then, from 80 μm, said surface becomes constant, the optimal position P _opt is therefore at 80 μm from the initial position P _i .

The Figure 4 shows a print band 10 at the end of the first phase. On this strip, 10 length formats L have been printed, one format per position of the printing unit 6. Note F _i the format printed by the printing unit in the position P _i , F _{i + 1} the format printed by the printing group in the next position P _{i + 1} and so on. In this example, we see that the formats F _{i + 8} and F _{i + 9} are identical, in other words P _opt = P _{i + 8} . By using an adjusting device according to the invention for the presetting of a printing machine, the length of the tape that must pass through the machine, the time to find the optimal position, is equal to (10 × L) + D . If we take, for example, L equal to 0.5 meters and D equal to 10 meters, the waste is then 15 meters.

By way of comparison, if a prior art setting device is used for presetting the previous printing machine, the length of the tape that must pass through the printing machine, the time to find the position optimal, is equal to 9 x (L + D). As a result, taking the previous numerical values, the waste is 94.5 meters.

Thus, thanks to the invention the waste is reduced by more than 80%. In addition, for a given bandwidth, the invention also makes it possible to reduce the presetting time by more than 80%.

Claims (7)

Device for adjusting the position of a printing unit (6) with respect to a printing tape (10) in a rotary printing machine, characterized in that it comprises a control unit (7) able to control the displacement of said printing unit (6) from a predefined initial position P _i to a final position P _f predefined by successive steps T, a vision system (4) able to capture images of the formats (F) printed successively on said strip printing (10) between said initial position P _i and said final position P _f , a calculation unit (8) able to evaluate said images and to determine the position P _{opt of} said printing unit from which said images are identical. Adjusting device according to claim 1, characterized in that said calculating unit (8) is able to calculate the area of said printed formats (F) covered by ink and to determine said position P _{opt of} said printing unit (6) from which said surface covered by ink is constant. Adjusting device according to claim 1, characterized in that said calculating unit (8) is able to calculate the gray level of each pixel composing said images and to determine said position P _{opt of} said printing unit (6) from which said gray level is constant. A method of adjusting the position of a printing unit (6) with respect to a printing tape (10) in a rotary printing machine, characterized in that said printing unit (6) moves from an initial position P _i predefined to a final position P _f predefined by successive steps T, and in that a vision system captures images of the formats (F) printed successively on said printing tape (10) between said initial position P _i and said final position P _f and transmits them to a computing unit (8) which evaluates said images and determines the position P _{opt of} said printing unit (6) from which said images are identical. An adjusting method according to claim 4, characterized in that said calculating unit (8) calculates the area of said ink-covered printed formats (F) and determines said position P _{opt of} said printing unit (6) from which said surface covered with ink is constant. Adjustment method according to claim 4, characterized in that said calculation unit (8) calculates the gray level of each pixel composing said images and determines said position P _{opt of} said printing unit (6) from which said gray level is constant. A flexographic printing machine comprising at least one printing unit (6) capable of transferring ink onto a printing tape (10) for printing formats and a device for adjusting the position of said printing unit (6) relative to said printing tape (10) defined in any one of claims 1 to 3.

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