EP1842666A2 - Method and device for printing plate-shaped workpieces - Google Patents

Abstract

The method involves measuring the thickness of each workpiece (7) before or during the transport to a printing gap (2), which is adjusted before the printing by parallel or slant angular changes of the gap of two press rollers (3) together. The thickness is measured at two points, where the line connecting the two measuring points, runs parallel to the axis of press rollers. The measurement is done in real time in the direction of transport across the entire length of the workpiece. The additional forces, which act upon the bearing of the press rollers, are measured during the printing. An independent claim is also included for a device for accomplishing the method for printing disc shaped workpieces in printing gap.

Description

The invention relates to a method and an apparatus for printing plate-shaped workpieces according to the preambles of claims 1 and 9. Accordingly, the workpieces are printed in a printing nip, which is formed by two spaced-apart adjustable pressure rollers. The plate-shaped workpieces are transported for this purpose by means of a transport device to the printing nip. The pressure rollers forming the printing nip can be printing cylinders for direct printing, application rollers for indirect printing and impression cylinders.

Such printing methods and corresponding devices have been known for a long time. Recently, by means of such a printing process, printed images of decors and grains are increasingly applied to wood-based panels and the like in order to give them an appearance of real wood. This is not only more cost effective compared to a real wood veneer, but also compared with the film coating of wood-based panels that has been in use for a long time.

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

However, especially for high quality multi-color printing, it is indispensable for the print image to be positioned on the workpiece within very close tolerances, typically with tolerances of about ± 0.1 mm to ± 0.25 mm. Only in this way is it possible to achieve an optical quality of the printed surface that is comparable to conventional film coating.

However, these requirements are much harder to comply with the presently processed plate-shaped workpieces, as in conventional printing machines for paper or film. Because the workpieces do not run endlessly through the printing press, and printing on the model of paper sheet printing machines is usually not possible because of the intransigence of the relevant plate-shaped workpieces. At the same time straight plate-shaped workpieces that are to be printed with wood grain and wood decors, up to several square meters in size, without the pressure to be maintained tolerances would change. The requirements for the exact alignment of the pressure rollers involved in the printing and their manufacturing tolerances are accordingly extremely high and hardly preset in realiter.

To make matters worse, that in printing presses of the present type, the printing gap forming pressure rollers are adjustable in their distance from each other to print plate-shaped workpieces of different thickness can. Again, completely different conditions are present, such as the printing of paper sheets and paper webs or films. The mutual adjustment of the pressure rollers is difficult to create in the required parallelism, which ensures the desired accuracy in printing even with large workpieces and correspondingly long pressure rollers. A height restriction of the pressure rollers, ie a non-parallelism in the plane spanned by the pressure roller axes level results in pressure by the flexibility of rolling on the workpiece surface pressure roller over the width not constant remaining effective radius. In the case of indirect printing and the printing rollers usually used there, however, a change in the printing roller effective radius by only 0.1 mm already results in a change in length of the printed image by about 0.6 mm per unwinding length of the printing cylinder. This would already far beyond the desired or required for a high-quality multi-color printing print tolerances.

The parallelism of the pressure rollers must therefore be readjusted by hand. For this purpose, a sheet of paper is introduced into the nip, optionally with a plate therein, and reciprocated in the nip. An experienced printer can then use the hand-held sheet of paper to sense whether or not the nip is parallel across its entire width. This tactile measurement and fine-tuning provides useful results in tenths to hundredths of a millimeter range.

Nevertheless, it can be observed, in particular in the case of large-area plate-shaped workpieces, that the multicolor printing looks blurred and the plates are sometimes even rotated minimally around the vertical axis when they pass through the printing gap.

The present invention is therefore based on the object to propose a method and an apparatus of the type mentioned, which allow to print plate-shaped workpieces with improved print image tolerances.

This object is achieved by a method having the features of claim 1 and by an apparatus having the features of claim 9.

Preferred embodiments of the method are set forth in claims 2 to 8; Preferred embodiments of the device according to the invention can be found in claims 10 to 16.

According to the invention has been recognized in the analysis of the problem of blurred prints or too large print tolerances despite accurate alignment of the pressure rollers by hand that the main problem lies in the thickness tolerances of the plate-shaped workpieces. In particular, large wood-based panels usually have a slightly wedge-shaped cross section, which corresponds to a height entanglement of the pressure rollers in the result. Because the thicker side of the wood-based panel leads to a reduction in parallel pressure rollers the effective radius of the pressure roller against the thinner side of the workpiece, which may well be 0.1 to 0.2 mm. However, as already mentioned above, this causes a change in length of the printed image per unwinding length of the pressure roller by about 0.65 to 1.3 mm. Even a slight ripple or wedge shape of the workpiece in the longitudinal direction affects the print quality and the image tolerances by a respective higher or lower pressure height, resulting on the one hand too high pressure forces and on the other hand to lower pressure forces with weak or blurred print image.

Based on these findings, the invention proposes to measure before the nip, the thickness of each workpiece, optionally - if present - together with the conveyor belt and its tolerant thickness, and the pressure nip forming pressure rollers by means of actuators at their bearings automatically to the measured thickness of the workpiece - if necessary with conveyor - set. Insofar as a thickness that is different across the width of the workpiece is measured, for example a wedge shape, the press nip is accordingly adjusted to this wedge shape by interleaving the two pressure rollers relative to one another, ie by obliquely changing the distance between the two pressure rollers, so that the effective radius of the pressure roller exceeds the entire width of the workpiece remains constant and so the desired minimum image tolerance values can be achieved.

The basic principle of the present invention is thus provided on both sides, alternately independent motorized pressure gap adjustment as a function of a previously measured in-line workpiece thickness signal.

It makes sense to measure the thickness of the workpiece at at least two measuring points whose connecting path runs essentially parallel to the pressure roller axes. This allows a very easy to produce correlation of the workpiece thickness can be achieved with the pressure gap adjustment, especially for minimal wedge-shaped plates.

The thickness measurement can take place in the transport direction over the entire workpiece length, so that a thickness profile is measured, by means of which the printing gap can then be adjusted in real time as the workpiece passes. This measure allows regardless of a change in thickness of the workpiece over the width of an adjustment of the pressure nip at changes in thickness of the workpiece over its length. Of course, it is ideal if both the thickness tolerances in the width and those in the length of the workpiece can be compensated for by adjusting the pressure gap to apply an exact pressure to the plate surface.

In the context of the present invention, particular advantages also result if the adjustment of the pressure nip is not only carried out on the basis of the thickness measurement carried out prior to the printing, but also the forces acting upon the pressure rollers and accordingly on their bearings are measured. For a measured difference in the bearing forces gives an indication that either the pressure roller surfaces are not parallel, so there is a height restriction, or that the currently printed workpiece has no parallel surfaces. In both cases, the existing on the bearings of a pressure roller servomotors are then operated until the difference in the bearing forces is approximately zero or optionally corresponds to a desired offset value.

Alternatively, or in addition to the direct or indirect control of the servomotors due to the measured bearing forces difference, such can also be used to correct the correlation between the thickness measured before the pressure and the adjustment of the pressure gap. For example, it may well be the case that there is an undesirable parallelism offset by a small height restriction of the two pressure rollers, which is not eliminated by an adjustment of the pressure gap due to the thickness measurements of the workpieces. Here, the measured difference of the bearing forces can generate a correction element.

However, the conditions in printing presses of the present type are complex, and an adjustment of the printing gap in real time in response to corresponding measured values generates a vibratory system, so that it is advantageous may be to average the bearing forces and their difference in time intervals and then perform a trend correction calculated by a suitable algorithm. A measured bearing force difference then does not directly affect the workpiece just measured, but tends to result in better and better results across several workpieces.

Insofar as not only the differential value of the two bearing forces is measured, but also their absolute value, conclusions can be obtained about the thickness profile in the longitudinal direction of the workpiece. For example, in a slight wedge shape of workpieces not only in width, but also in length, the course of the absolute values of the measured bearing forces is a measure of the wedge shape of the workpiece in the longitudinal direction, while the difference of the bearing forces is a measure of the wedge shape in width - Taking into account any existing height restriction of the two pressure rollers - is.

With the present invention for measuring the workpiece thickness before reaching the nip sensor sensor advantageously also the leading edge of the workpiece can be detected and accelerated or delayed with this information, the workpiece and / or a pressure roller involved in printing to ensure a positionally correct image beginning on the workpiece , This method is inherently out of the DE 103.33 626 A1 known.

If it allows the required print image tolerance, the two pressure rollers of the printing nip can be interlocked to a small extent as offset adjustment, so that a transversely directed to the transport direction of the workpiece force acts on this. This may be desirable in order to guide the workpiece reliably along an edge boundary, for example of a side ruler, and thus to ensure a precise pass through all printing gaps of a multicolor printing press in the case of rectangular plates.

The present invention thickness measuring device in front of the printing nip can be designed so that above and below the workpiece Distance sensors are arranged and the difference of the thickness of the workpiece is determined.

Alternatively, the thickness measuring device may also consist of two imaging sensors which laterally scan the two side edges of the workpiece and accordingly record a profile thereof.

As a further alternative, the thickness gauge may consist of a measuring calender provided with displacement sensors for determining the thickness of the workpiece. The great advantage of a measuring calender is that the values measured by it correspond exactly to those values which have to be set at the printing nip, even if the workpiece does not have an exact wedge shape, but a certain waviness, which can only be averaged out by the correction of the printing nip because the pressure rollers are cylindrically shaped. As a further, not to be underestimated advantage of a Meßkalander offers the possibility not only displacement sensors, but also to attach force sensors to its bearings in order to carry out the above-described force measurement or force difference measurement before the workpiece reaches the nip. This opens up the possibility of adjusting the pressure gap before the pressure by means of a force difference measurement.

As mentioned above, the present invention is particularly suitable for multi-color printing, so that preferably a plurality of pressure gaps are arranged in line one behind the other. In this case, due to the possibility of correction due to a force measurement at all printing nips, a very high accuracy results when printing on the different colors, while the workpiece-dependent correction requirement can be carried out by a single thickness measurement before reaching the first printing nip. In contrast, the force measurement at the bearings of the individual pressure rollers is specific for each pressure nip.

Figur 1

eine schematische Draufsicht auf ein Ausführungsbeispiel einer erfindungsgemäßen Druckvorrichtung;

Figur 2

ein Diagramm der Lagerkräfte einer Druckwalze aus Figur 1;

Figur 3

eine schematische Darstellung einer Dickenmesseinrichtung für die in Figur 1 gezeigte Vorrichtung;

Figur 4

eine schematische Darstellung eines anderen Ausführungsbeispiels einer Dickenmesseinrichtung.

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

FIG. 1

a schematic plan view of an embodiment of a printing device according to the invention;

FIG. 2

a diagram of the bearing forces of a pressure roller of Figure 1;

FIG. 3

a schematic representation of a thickness measuring device for the device shown in Figure 1;

FIG. 4

a schematic representation of another embodiment of a thickness measuring device.

In Figure 1, an embodiment of a device according to the invention is shown schematically in plan view. It is a printing machine for a simple direct printing or a first printing station for a multi-color printing machine, the other printing stations are not shown for simplicity. A conveyor belt 1 passes through a pressure nip 2, which is formed by a pressure roller formed as a gravure cylinder 3 and a non-visible in plan view, arranged below the conveyor belt 1 impression cylinder. By means of a left 4 and a right servomotor 5, the gravure cylinder 3 is height adjustable, wherein the two servomotors 4 and 5 can be operated independently. Lying on an upstream roller conveyor 6, a plate-shaped workpiece 7 is brought in a symbolized by the arrow 8 passage direction to the pressure nip 2, where it is transferred to the conveyor belt 1. Previously, it passes through a thickness measuring device 9, which is formed here from each of two above and below the workpiece 7 arranged right and left distance sensors 10. Accordingly, the workpiece 7 is measured both on its right and on its left side to detect differences in thickness, for example by a slight wedge shape. If such differences in thickness are present, the left positioning motor 4 and the right positioning motor 5 are correspondingly adjusted differently, so that the gravure cylinder 3 is slightly entangled against the impression cylinder in order to reproduce the wedge shape of the workpiece 7. Accordingly, the effective radius of the gravure cylinder 3 is then constant over the entire width of the workpiece 7 during printing, so that no image distortions occur. A lateral deflection of the workpiece 7 is then not to be feared in the nip 2.

The left and right servomotors 4, 5 shown only symbolically in FIG. 1 are also provided with force measuring sensors in order to measure, directly or indirectly, the bearing forces acting on the gravure cylinder during the printing of the workpiece 7. A difference of the left and right bearing forces means that the setting of gravure cylinder 3 and counter-pressure cylinder due to an offset or other effects, the thickness profile of the workpiece 7 does not exactly over the width and therefore undesirably a difference in the effective radius of the engraving cylinder 3 from the left to the right side of the workpiece 7 in spite of the adjusted due to the thickness gauge 9 servo motors 4 and 5 occurs. This can then be used for the tendency correction of the relationship between the values recorded in the thickness measuring device 9 and the adjustment of the servomotors 4 and 5 for the following workpieces 7.

FIG. 2 shows a diagram of the forces measured by the two force measuring sensors assigned to the servomotors 4 and 5. Above, the bearing force is recorded on the left and bottom, the bearing force on the right over time, whereby - due to the approximate rectangular shape of the forces occurring - is clearly visible, that in the time applied, a workpiece 7 has been transported through the printing nip 2. In the case shown here, the bearing force on the left was significantly higher than the bearing force on the right, which suggests that on average the thickness profile of the workpiece 7 does not harmonize with the setting of the engraving cylinder 3 and the counterpressure roller. The higher left bearing force indicates that there the effective radius of the engraving cylinder 3 is lower than on the right side.

Since, as shown in Figure 2, the force measuring sensors record the time course of the left and right bearing forces, as well as with the thickness gauge 9, not only the thickness difference of the workpiece 7 between the right and left sides are measured, but also its thickness profile in throughput direction. As a result, a height adjustment of the left and right servomotors 4 and 5 during the passage of the workpiece 7 through the nip 2 on the basis of the known thickness profile in real time possible, which further improves the print image tolerance.

Figure 3 and Figure 4 show two different embodiments of a thickness measuring device 9 of the device according to the invention. In FIG. 3, a total of four distance sensors 10 are provided for thickness measurement, seen above the workpiece in the direction of passage 8 on the left side 10a and on the right side 10b and below the workpiece on the left side 10c and on the right side 10d. By subtracting the two left-hand distance sensors 10a and 10c and the two right-hand distance sensors 10b and 10d respectively, the thickness of the workpiece 7 is measured on the right and left.

In addition, it can be seen in FIG. 3 that a front edge sensor 11 sets down a signal as soon as the leading edge 7 of the workpiece passes it. With the help of this signal can then - as is well known - the printed image are triggered or optionally the gravure cylinder 3 or arranged between the gravure cylinder and the workpiece 7 blanket cylinder so accelerated or decelerated that the print image start coincides with the front edge of the workpiece 7 and no Edge remains.

Another principle of a thickness measurement is shown in FIG. 4: viewed in the direction of passage 8, laterally next to the workpiece, two CCD cameras 12 are arranged, with a left CCD camera 12a scanning the left side edge of the workpiece 7, while a right CCD camera 12b monitors the right side edge of the workpiece 7. Based on the pixels recorded by the CCD cameras 12 and their gray values, the respective thickness of the workpiece 7 guided past the CCD cameras 12 can be determined.

Claims (16)

Method for printing plate-shaped workpieces in a printing nip, which is formed by two pressure rollers which are adjustable with respect to each other, the plate-shaped workpiece being transported on a transport device to the printing nip and printed on passing through the printing nip,
characterized,
in that the thickness of each workpiece is measured prior to or during transport to the printing nip, and that the printing nip is adjusted before printing by means of parallel or oblique changing of the distance of the two pressure rollers from one another according to the measured thickness of the workpiece. Method according to claim 1,
characterized,
that the thickness of the workpiece is measured at at least two measuring points, wherein the link of the two measurement points is substantially parallel to the pressure roller axes. Method according to one of claims 1 or 2,
characterized,
that the measurement of the thickness of the workpiece is carried out in the transport direction over the entire length of the workpiece, and thereby a thickness profile with the pressure gap during the passage of the workpiece corresponding to the measured thickness profile in real time is set is measured. Method according to at least one of claims 1 to 3,
characterized,
that during the pressure acting on the bearings of the pressure rollers bearing forces are measured and that the distance of the two pressure rollers is changed to one another at an oblique angle with a difference between the measured bearing forces, until the difference of the bearing forces in approximately zero or approximately equal to a desired offset value is. Method according to at least one of claims 1 to 4,
characterized,
in that the bearing forces acting on the bearings of the pressure rollers are measured during the printing, and that a difference of the measured bearing forces for the workpieces to be subsequently printed acts as a correction value in the adjustment of the pressure gap according to the measured thickness of the workpieces. Method according to one of claims 4 or 5,
characterized,
in that the variation of the spacing of the two pressure rollers relative to one another according to the measured bearing forces or the influence as a correction value in the adjustment of the pressure gap is carried out on the basis of differential values of the bearing forces determined at time intervals in order to achieve overall tendency correction. Method according to at least one of claims 1 to 6,
characterized,
that when measuring the thickness of the workpieces at the same time the leading edge is detected, and with this information, the workpiece and / or an imaging pressure roller is accelerated or decelerated to ensure a positionally accurate image on the workpiece. Method according to at least one of claims 1 to 7,
characterized,
that the two pressure rollers of the printing gap are offset from each other as an offset setting, so that a transversely directed to the transport direction Force acts on the workpieces to guide them along a perimeter boundary. Apparatus for carrying out the method according to at least one of claims 1 to 8, with two spaced apart adjustable pressure rollers (3) forming a pressure nip (2) for printing a plate-shaped workpiece (7), and a transport device (1) for transporting of the workpiece (7) to the printing nip (2),
characterized,
that connected upstream in front of or on the transport device (1), the printing gap (2), a thickness measuring means (9) for detecting the thickness of the transported workpieces (7) is arranged, and that on at least one of the two the printing gap (2) forming pressure rollers ( 3) on both sides in the region of their bearings servomotors (4, 5) for unilateral and / or two-sided changing the distance between the two pressure rollers (3) are mounted to each other, wherein the thickness gauge (9) and the servo motors (4, 5) with a control unit are linked, which is programmed according to the measured thickness of the workpiece (7) by means of one or two-sided adjustment of the servo motors (4, 5) the printing gap (2) before printing parallel or obliquely. Device according to claim 9,
characterized,
in that the thickness measuring device (9) comprises distance sensors (10) arranged above and below the workpiece (7). Device according to claim 9,
characterized,
that the thickness measuring means (9) at least two side edges of the workpiece (7) comprises scanning imaging sensors (12). Device according to claim 9,
characterized,
that the thickness measuring means (9) comprises a Messkalander with displacement and / or force measuring sensors. Device according to at least one of claims 9 to 12,
characterized,
that the transport device (1) essentially consists of a conveyor belt, which is guided through the pressure nip (2). Device according to at least one of claims 9 to 13,
characterized,
in that force measuring sensors for detecting the bearing forces are attached to the bearings of at least one pressure roller (3), the force measuring sensors being linked to the control unit and the control unit being programmed such that, given a difference in the bearing forces measured by the force measuring sensors, the distance of the two pressure rollers ( 3) is changed obliquely to each other until the difference of the bearing forces is approximately zero or approximately equal to a desired offset value. Device according to at least one of claims 9 to 14,
characterized,
in that a measured difference of the bearing forces for the workpieces (7) to be subsequently printed flows as correction value into the adjustment of the pressure gap (2) taking place according to the measured thickness of the workpieces (7). Device according to at least one of claims 9 to 15,
characterized,
that a plurality of pressure gaps (2) are arranged in line one behind the other.

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