EP1500504B1 - Apparatus for printing flat articles produced from wood-fibre material - Google Patents

Description

The invention relates to a device for printing flat workpieces according to the preamble of patent claim 1.

Accordingly, such a device comprises an applicator roll, which rolls on the workpiece surface to be printed and here directly or indirectly, depending on whether the application roller itself is designed as a printing cylinder or as a transfer roller cooperates with such, applies a printed image on the workpiece surface, and a counter-roller which, in cooperation with the applicator roll, forms a nip through which the workpiece passes during the printing process. In addition, a transport device for feeding and discharging the workpiece into and out of the pressure gap formed by the applicator roll and the counter roll is present.

In particular, to give wood-based panels the appearance of real wood, due to the cost advantages over a real wood veneer or film coatings increasingly printed images of decors and grains are applied directly to the wood-based panels.

For a high quality impression, however, a simple single-color printing is not enough. Rather, it is desirable to apply the grain or the decor in multi-color printing on the material plate. This is certainly not the case only with wood-based panels; also applications with other materials that can be qualitatively improved by a surface pressure, such as stone or artificial leather, can be upgraded by a multi-color printing.

However, especially for high-quality multi-color printing, it is essential that the print image on the workpiece is positioned within very narrow tolerances typically in the range of ± 0.1 mm. Only in this way can an optical quality comparable to the conventional film coating be achieved.

To improve the positioning of the printed image, as seen in the transport direction of the workpiece, a detection device for detecting the position of a workpiece leading edge or a picture initial mark can be present in front of the printing nip, wherein this is designed cooperatively with the transport device and / or the printing cylinder, that the transport of the workpiece still accelerated or decelerated before reaching the printing nip and / or the angular position of the printing cylinder can be changed by accelerating or decelerating the rotational movement, to bring the beginning of the printed image with the leading edge of the workpiece or the intended repeat length in line.

In this case, by means of the detection device, an unambiguous position determination of the leading edge of the workpiece or a corresponding image start mark is possible relative to the rotational angle position of the printing cylinder known in the control of the device, so that, taking into account the fixed distance between the detection location and the printing nip, the position of the print image start on the workpiece exactly calculated and, where appropriate, by brief braking or accelerating the transport movement of the workpiece, or by brief braking or acceleration of the printing cylinder can be corrected. Of course, a combination of these two measures is possible.

The measure, the front edge of a conveyed workpiece by means of a sensor which is arranged upstream of an applicator roll, and to synchronize the print image start on the applicator roll with the workpiece leading edge optionally by accelerating or decelerating the applicator roll, is already out of the DE-A-197 29 513 , of the EP-A-0 689 915 and the EP-A-0 616 886 known. These documents relate to printing machines for printing on tiles, which are passed on conveyor belts under the applicator roll or, if used with a counter-roll, for printing of yielding, non-rigid workpieces such as cardboard and the like.

In contrast, the present invention is concerned with the printing of rigid workpieces in a printing nip, which need not be precisely printed not only with regard to their repeat position, but also in which image distortions due to varying image lengths must also be avoided.

However, these requirements are much harder to comply with the existing sheet-like workpieces, as in machines for paper or films. Because the present sheet-like workpieces do not run endlessly through the printing press, and printing on the model of paper-sheet printing machines is not possible because of the intransigence of the relevant flat workpieces.

The present invention is therefore based on the object to improve a device for printing unyielding of sheet-like workpieces of the type mentioned so that image distortions due to varying image lengths can be avoided.

This object is achieved by a device having the features of patent claim 1.

Advantageous embodiments and further developments of the invention are laid down in the claims 2 to 23.

According to the invention, the applicator roll is thus provided with a drive which can be controlled to generate or eliminate slippage between the applicator roll and the workpiece and / or optionally between the impression cylinder and the applicator roll formed as a transfer roll. Because the invention Position detection of the workpiece leading edge or a start of the image mark relative to the position of the print image initially only affects the exact position of the print image start on the workpiece surface. By contrast, the printed image length and thus the position of the image end is dependent on the relative speed between the applicator roll and the workpiece surface or, in the case of indirect printing, also on the relative speed between the impression cylinder and the transfer roller. Only if these relative velocities are exactly zero, ie there is no slippage, is the print image transferred on a scale of 1: 1 to the workpiece surface.

Slip and thus inaccuracies in the printed image are produced by different parameters, such as the pre-print setting of the applicator roll, the workpiece thickness tolerance or the inhomogeneity of the applicator roll surface. A targeted generation of a "counter slip" between the applicator roll and the workpiece or between the impression cylinder and an optional transfer roller can eliminate this disturbance and keep the print image length almost at the level of a 1: 1 mapping.

Unlike the adjustment of the print image start on the workpiece surface, the generation of slippage between the applicator roll and the workpiece or between the impression cylinder and an application roller designed as a transfer roller to compensate for errors in the print image length only affects the subsequent workpieces; it is therefore a tendency correction.

Particular advantages are achieved by the invention when a plurality of applicator rollers, each with a counter-roller and a transport device are arranged in line one behind the other, which is the rule in multi-color printing. The workpiece then passes through all the pressure gaps formed between the applicator rollers and the counter-rollers in a single operation and is printed in each printing gap with a different color. This continuous line process requires synchronous transport through the plant through a higher-level controller. Nevertheless, each printing gap is preceded by its own Detecting means for detecting the position of the workpiece leading edge or the initial image mark provided, and each printing cylinder or this directly associated transport device can be briefly accelerated or delayed due to the detected sensor values to synchronize the beginning of the image with the workpiece and optionally correct lying outside the predetermined tolerances shifts. The messages of the detection devices then trigger a corrective intervention in selected drives of the superordinate synchronized network.

This modular construction of a multi-color printing line ensures that the achievable positional accuracy of the printed image on the workpiece surface does not depend on the number of printing units connected in series. The modular design also allows individual printing units to be extended across the print line to perform a color or roll change. If at the same time a simple distance is moved into the pressure line, the production does not have to be significantly interrupted.

In a printing line with a plurality of printing units according to the invention, it brings particular advantages if a feed sensor for detecting the position of the workpiece leading edge or a top mark and a cooperating with this acceleration or deceleration distance for coarse alignment of the workpiece is provided to the print image position of the first printing cylinder before the first application roller. The modular correction option before each additional nip must then only take over the fine adjustment and the fine correction.

In front of the printing line arranged flow can be provided a longer distance for accelerating or decelerating the workpiece, which is naturally suitable for the coarse alignment of the workpiece. In order to ensure defined ratios in the coarse alignment in each case, the acceleration section can essentially be formed by a calender which clamps the workpiece in a defined manner and thus can transmit the position correction provided by the control without further friction to the workpiece. In the present technique of printing flat workpieces, in particular wood-based panels, these generally first pass through several other processing stations in line before they arrive at the flow sensor for coarse alignment in the inventive device. This makes it difficult to synchronize the abandonment of the individual workpieces at the very beginning of the processing line with the printing line, in particular in the necessary accuracy. This on the one hand, because the place of delivery for the workpieces is usually far away in space and otherwise does not have to meet precision requirements. On the other hand, the upstream of the printing line processing stations with respect to the position and orientation of the flat workpieces permanent sources of error. A very uniform sequence of workpieces on the transport device when approaching the printing line is thereby almost impossible.

A pressure line consisting of several printing units, each with a control circuit according to the invention for the modular correction of the print image beginning, forms an oscillatory system on account of the control processes which repeatedly occur with each arriving workpiece. The preferably provided coarse alignment of the workpieces before reaching the actual pressure line keeps the vibration amplitudes small and prevents a possible rocking. The individual control loops of the pressure column then have to make only minor corrections.

The coarse alignment also prevents two workpieces from following each other too closely, in order to be able to finely adjust the individual pressure gaps separately from each other. Normally, the coarse alignment ensures that the distance between the individual workpieces is a full-line multiple of the intended image length.

The preferably present coarse alignment of the workpieces by the flow sensor and the acceleration section thus decouples the pressure line from the normally preceding workstations. Particularly preferred works the coarse alignment so that it basically accelerates the workpieces and does not slow down; Delaying would create a risk of backlog in the production line.

Both the applicator roll and the counter roll can each be provided with a drive, as has been customary so far. To increase the accuracy of the image application to the workpiece surface and to avoid unwanted image distortion, it is advantageous if the drive of the counter-roller delivers a lower torque than that of the applicator roll. Thus, if necessary, it can be ensured that the applicator roll rolls on the workpiece surface without slippage. Alternatively it can be provided that the drive of the counter roller can be controlled depending on possibly existing image distortions to selectively create or eliminate slippage between the applicator roll and the workpiece or to support the order roller drive in this respect.

The image length in the printing is directly related to the slippage of the applicator roll on the workpiece or also with the slip between the applicator roll and the transfer roller. The goal in printing is therefore to avoid slippage; but even more important is the positional superimposition of the different colors when successively printing in successive printing columns. According to a particularly preferred embodiment of the invention, therefore, a drive of a roll of a printing nip, expediently the drive of the applicator roll, is selected as the master drive and its torque is taken as the reference value. All other drives of a pressure nip cooperating with this drive are then controlled in such a way that the values of their torques remain in all circumstances their sign, ie no zero passes of the torque values occur. This can most easily be achieved by a kind of torque cascade, that is to say by a deliberate descending selection of the torques of all drives with respect to the master torque. Preventing zero crossings ensures that all drive trains of the drives run without play, as they are permanently held in "train". Oscillations in the drive trains, for example due to backlash, Clutch play, torsion, etc., which would result in image distortions are thereby excluded in the simplest way, without great control effort or absolute value measurements. Also, fluctuations in the effective radius of the rollers involved, which are present due to the resilience of the roll surfaces and the thickness tolerances of the workpieces, can not lead to image length distortion, and in particular not to different image length distortion in different colors by this measure of the torque control.

In order to comply with the tight tolerances for the pressure in the lateral direction, a lateral guide for the workpieces may be present, wherein the workpieces are transported to this guide fitting through the device. As a result, the workpieces are aligned laterally exactly.

This lateral guide may consist of a simple ruler, but better a roller conveyor or a vertical conveyor belt. At the same time, it offers advantages if there are means for producing a transport force component oriented perpendicular to the transport direction and to the lateral guide for the workpiece. Such means could be, for example, a helical roller track, a small entanglement of mating roll and applicator roll in the horizontal axis position to each other or a slight misalignment relative to the passage direction.

For fine adjustment of the lateral position of the workpieces, the pressure cylinder may be formed axially adjustable. This transverse adjustment of the printing cylinder can also take place automatically via the feedback of an image recognition system in that the image recognition system detects a need for correction on the printed image transferred to the workpiece surface and reports this back to the axial adjustment of the printing cylinder.

The recognition of printed image length errors after the printing has taken place and the corresponding correction by creating or eliminating a slip can be done by eye and by hand. It is advantageous, however, if the Pressure gap means for detecting image length tolerances are connected downstream, which preferably consist essentially of a digital camera. If there is still an image processing device which operates on the control of the applicator roll on the basis of the digital image obtained by the camera to compensate for image length tolerances, an automatic control system is provided which minimizes the image length tolerances.

As already mentioned above, the digital camera and the associated image processing device can also recognize the tolerance compliance in the lateral direction and correct by an axial adjustment of the printing cylinder.

In particular wood-based workpieces have thickness tolerances that usually do not allow direct pressure with a printing cylinder. Therefore, preferably the applicator roll is not formed as a printing cylinder, but there is a separate printing cylinder, which transmits the printed image on the applicator roll, after which this transmits the printed image on the workpiece surface. The applicator roll is then designed as a transfer roller. The printing cylinder is preferably a gravure roll having recesses which fill in the inking unit with ink, while they subsequently deliver this to the transfer roller. The transfer roller is expediently designed as a rubberized steel roller, the rubber outer surface was ground smooth.

The application roller thus formed as a transfer roller is then in simultaneous contact with the printing cylinder and the workpiece surface, while it takes over the printed image from the printing cylinder and transmits to the workpiece surface.

As already mentioned above, as an alternative or in addition to generating the slip between the application roller or transfer roller and the workpiece surface, a slip between the impression cylinder and the application roller or transfer roller can also be generated for the print image length adjustment. Because an existing, accidental slip between the Druckzy-Linder and the transfer roller results in slight print image distortions, ultimately leading to errors in the print image length.

Insofar as a digital camera for image recognition and an image processing device are present, the latter can act on the regulation of the printing cylinder and / or the applicator roll to compensate for image length tolerances. This measure leads to an automatically controlled error correction method with regard to the print image length.

In order to further increase the quality feel of the printed surface, it may be advantageous to coat the same with a melamine resin and to supply the thus printed and coated workpiece to a downstream embossing device, for example an embossing roll or press. This imprints a structure on the surface, which however must correspond exactly to the previously applied printed image in order to achieve the intended effect.

If at least two detection devices spaced apart from one another in the transport direction of the workpiece are provided in a device according to the invention, these two can be used not only for position detection but also for measuring the current transport speed of the workpiece. If a device according to the invention contains not only one printing nip but several printing nips or printing units arranged one behind the other, in which each printing unit is provided with its own detection device according to the invention, two such actually individually arranged detection devices can cooperate for measuring the transport speed of the workpiece.

Figur 1

eine schematische seitliche Darstellung einer erfindungsgemäßen Vorrichtung mit drei Druckwerken;

Figur 2

eine ebensolche Darstellung des Vorlaufs der Vorrichtung aus Figur 1.

An embodiment of the invention will be described below with reference to the accompanying drawings and explained in more detail. Show it:

FIG. 1

a schematic side view of a device according to the invention with three printing units;

FIG. 2

a similar representation of the flow of the device FIG. 1 ,

FIG. 1 shows a total of three trained as a transfer roller 1 for the indirect pressure application rollers, each cooperating with a counter-roller 2 and form a pressure nip 3, which is traversed by a workpiece 4. The print image is transferred in each case from a printing cylinder 5 to the transfer roller 1 and from there to the workpiece surface 6.

The workpiece 4 runs on a roller conveyor 7, wherein the transfer roller 1, the counter roller 2 and the impression cylinder 5 are each driven synchronously and thereby also produce the transport movement of the workpiece 4.

Each printing nip 3 is preceded by at least one detection device 8 designed as an optical sensor for detecting the position of the front edge 9 of the workpiece 4, wherein an approximately necessary corrective engagement to perform the position of the leading edge 9 relative to the angular position of the printing cylinder 5, by briefly accelerating or decelerating the printing cylinder 5 and the transfer roller 1 is made. To also change the current transport speed of the FIG. 1 In each case, two detection devices 8 for detecting the position of the front edge 9 of the workpiece 4 are present here in each case.

The printing cylinders 5 are each provided with a registration mark 10, by means of which a marking arranged next to the printed image can be applied to the workpiece surface 6 in a manner known per se. This registration mark, usually a color cross or a color traffic light, allows a very fast detection of aberrations, which is particularly important for the print image length correction according to the invention. Registration marks and any adjacent prints can also be easily captured by a digital camera and automatically evaluated. Preferably, register marks are set both at the beginning and at the end of the image, wherein the former the tolerances of the print image start and the latter the tolerances of the print image length can be checked for compliance.

FIG. 2 finally shows the lead of in FIG. 1 shown device, in which case a device for coarse alignment of the workpiece 4 is present. In addition, it becomes clear that the first printing unit is preceded by a calender 11 in order to perform the acceleration or deceleration of the workpiece 4 according to the invention in a targeted and reproducible manner.

The device for coarse alignment also has a calender 12, which defines the deceleration or acceleration of the workpiece 4 and causes no friction.

The calender 11 acts for the first printing unit assigned to it as an acceleration or deceleration path, so that the drives of the application roller or here the transfer roller 1, the impression cylinder 5 and the counter roller 2 need not necessarily be actuated for correction purposes.

Finally, it should be noted that, in particular in the case of wood-based materials but also other non-flexible workpieces, it will be expedient to use the principle of indirect pressure, as described with reference to the embodiments illustrated in the figures; It is here the print image from the printing cylinder 6 is transferred to the transfer roller 1 and from this then on the workpiece surface 6. Nevertheless, the invention can also be used in a direct pressure; the transfer roller 1 would be omitted in the case and the impression cylinder 5 would be used directly as an applicator roll and roll on the workpiece surface 6.

LIST OF REFERENCE NUMBERS

1

transfer roller

2

backing roll

3

nip

4

workpiece

5

pressure cylinder

6

Workpiece surface

7

roller conveyor

8th

detection device

9

Workpiece front edge

10

registration mark

11

calender

12

calender

Claims (23)

1. Apparatus for printing inflexible flat workpieces, comprising an application roller which rolls on the workpiece surface (6) to be printed and thereby applies a printed image to the workpiece surface (6), wherein the application roller is a printing cylinder (5) or a transfer roller (1) cooperating with a printing cylinder (5), with a counter-roller (2) which in cooperation with the application roller forms a printing gap (3) through which the workpiece (4) moves during the printing process, and with a transport device (7) for feeding the workpiece (4) into and removing it from the printing gap (3) formed by the application roller and the counter-roller (2), wherein a detector device (8) for recognising the position of a workpiece front edge (9) or an_image start mark is provided before the printing gap (3) when viewed in the transport direction of the workpiece (4), the detector device being designed to co-operate with the transport device (7, 11) and/or the printing cylinder (5) in such a way that, before reaching the printing gap (3), the transport of the workpiece (4) can be accelerated or retarded and/or the angular position of the printing cylinder (5) can be changed by accelerating or retarding the rotational movement in order to bring the start of the printed image into agreement with the workpiece front edge (9) or into the intended relative position, characterised in that the application roller is provided with a drive which is controllable in order to create or eliminate slippage between the application roller and the workpiece (4), wherein the control of the drive is configured so that image length defects on the workpiece surface (6) recognised after completed printing are by creation or elimination of a slippage between the application roller and the following workpieces (4).
2. Apparatus as claimed in Claim 1, characterised in that a plurality of application rollers each having a respective counter-roller (2) and a transport device (7, 11) are disposed in line one behind the other, wherein a discrete detector device (8) is assigned to each printing gap (3) for recognising the position of the workpiece front edge (9) or an image start mark, whilst the transport devices (7, 11) are matched to one another so that the workpiece (4) runs successively through all printing gaps (3) formed between the application rollers and the counter-rollers (2) in one operation.
3. Apparatus as claimed in Claim 2, characterised in that before the first printing gap (3) a feed sensor (8) is provided for recognising the position of the workpiece front edge (9) or an image start mark and an acceleration or retarding path (12) is provided which co-operates with the feed sensor for rough alignment of the workpiece (4) with the printed image position of the first printing cylinder (6).
4. Apparatus as claimed in Claim 3, characterised in that the acceleration path is substantially formed by a calender (12).
5. Apparatus as claimed in any one of Claims 3 or 4, characterised in that the feed sensor (8) controls a delivery device which delivers the workpieces (4) to the transport device (7).
6. Apparatus as claimed in any one of Claims 1 to 5, characterised in that the application roller and the counter-roller (2) are each provided with a drive, wherein the drive of the counter-roller (2) produces a lower torque than that of the application roller.
7. Apparatus as claimed in any one of Claims 1 to 5, characterised in that the application roller and the counter-roller (2) and also optionally further rollers of a printing gap (3), wherein the drives are controlled so that one drive predetermines a torque value and the torques of all other drives are controlled so that no change of sign of the torque value takes place during the printing process.
8. Apparatus as claimed in Claim 7, characterised in that the drives controlled according to the predetermined value of the torque of the master drive are each acted on by a torque which in each case is at least slightly below the master torque.
9. Apparatus as claimed in any one of Claims 1 to 5, characterised in that the application roller and the counter-roller (2) are each provided with a drive, wherein the drive of the counter-roller (2) is controllable for producing or eliminating a slippage between the application roller and the workpiece (4).
10. Apparatus as claimed in any one of Claims 1 to 9, characterised in that a lateral guide is provided for the workpiece (4).
11. Apparatus as claimed in Claim 10, characterised in that the guide is formed by a straightedge, a roller track or a vertical conveyor belt.
12. Apparatus as claimed in any one of Claims 10 to 11, characterised in that means for generating a transport force component perpendicular to the transport direction and to the lateral guide are provided for the workpiece (4).
13. Apparatus as claimed in any one of Claims 1 to 12, characterised in that the printing cylinder (5) is designed to be axially displaceable for transverse adjustment of the printed image.
14. Apparatus as claimed in any one of Claims 1 to 13, characterised in that means for recognising image length tolerances are disposed downstream of the printing gap (3).
15. Apparatus as claimed in Claim 14, characterised in that the means for recognising image length tolerances comprise a digital camera.
16. Apparatus as claimed in Claim 15, characterised in that an image processing device is provided which on the basis of the digital image obtained from the camera acts on the control for driving the application roller and/or the counter-roller (2) in order to compensate for image length tolerances.
17. Apparatus as claimed in Claims 13 and 16, characterised in that the image processing device also acts on the axial displacement of the printing cylinder (5).
18. Apparatus as claimed in any one of Claims 1 to 17, characterised in that the printing cylinder (5) is an engraved roller.
19. Apparatus as claimed in any one of Claims 1 to 18, characterised in that the application roller is designed as a transfer roller (1) and is provided with a drive, that the printing cylinder (5) is provided with a discrete drive and that the drive for the transfer roller (1) and/or the drive for the printing cylinder (5) is adjustable in order to create or eliminate a slippage between the printing cylinder (5) and the transfer roller (1).
20. Apparatus as claimed in Claims 15 and 19, characterised in that on the basis of the digital image obtained from the camera the image processing device acts on the control for the printing cylinder (5) and/or the transfer roller (1) in order to compensate for image length tolerances.
21. Apparatus as claimed in any one of Claims 1 to 20, characterised in that an embossing device is disposed downstream of the printing gap (3).
22. Apparatus as claimed in any one of Claims 1 to 21, characterised in that the apparatus is suitable for printing workpieces which consist of wood or a wood-based material.
23. Apparatus as claimed in any one of Claims 1 to 22, characterised in that at least two detector devices (8) are provided which are spaced apart fromone another in the transport direction of the workpiece (4) and are arranged to co-operate so that they measure the transport speed of the workpiece (4).

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