EP0313511A2 - Flexographic press - Google Patents

Abstract

The flexographic press, as is typical from a definition of flexographic printing, features a printing cylinder with a soft (flexible) printing plate and a hard impression cylinder. In accordance with the invention, the flexible printing plate is very much thinner than is the case in prior art plates. The thickness of the printing plate is less than 1 mm. A sleeve is pushed onto the printing cylinder, the sleeve being built around a tube 1 and featuring a highly compressible layer 2, on which a slightly or still essentially compressible barrier layer 7 is provided, which accepts the friction forces arising during printing and onto which the printing plate is secured. It is advantageous if the thickness of the printing plate is between 0.5 and 0.8 mm. Because of the basic reduction in thickness of the printing plate in combination with the sleeve, which is constructed in a force-absorbing manner, a significant improvement in quality is obtained. <IMAGE>

Description

The invention relates to a flexographic printing machine according to the preamble of claim 1.

Such flexographic printing presses work according to the high-pressure process, but not hard against soft as with the traditional high-pressure - letterpress - but soft against hard, i.e. the printing block or the cliché are soft. In the past, the cliché consisted essentially of vulcanized rubber, today it is predominantly made of photopolymers.

Approx. 98% of the photopolymer cliché plates are single-layer plates, i.e. A photopolymer layer that reacts to UV light is extruded on a carrier film of approx. 130 µm (e.g. polyester).

Depending on the cliché plate type, it can be washed out with different solvents after exposure, for example from a mixture of butanol or perchlorethylene or water.

In the case of flexographic printing machines for the metric size, the print heights for large-format quality printing are almost 99% 3.0 mm.

As a rule, photo-polymeric plates for the production of the clichés have a height of between 2.54 and 2.84 mm for quality screen printing. The difference of up to 3.0 mm is compensated for by an adhesive tape with which the plates are attached to the printing cylinder.

Due to the relatively high thickness of the plates, it expands in the direction of travel. Exactly true-to-size prints cannot be achieved due to the current state of the art. In order to be able to deliver approximately true-to-size prints and to reduce the print increase in the raster printing in the running direction, all films in the reproduction must be shortened by the factor of stretch. This means that each halftone print has to be distorted by this factor in order to later regain its previously calculated shape by stretching, i.e. if in the printed image e.g. If a circle is to be displayed, it must have an elliptical shape on the film and on the cliché plate that points transversely to the direction of travel.

The percentage elongation in turn depends on the size of the impression cylinder. In addition to the resulting increased manufacturing costs, only a theoretical thickness of the cliché plate can be assumed when determining the elongation factor, not the effective one, which will be washed out later.

According to the manufacturer, the thickness tolerance of the cliché plates is +/- 13 µm, so that the previously calculated gauge block is only an approximate value. By strength Tolerances of the plates and the plus-minus tolerance of the adhesive tape and the printing cylinder result in the current state of the art in 6-color printing from printing cylinder to printing cylinder different diameters measured via the cliché and thus shortening and lengthening in the individual colors of the print image in the direction of settlement. The result is register fluctuations of approx. 0.75 mm.

According to the current state of the art - in comparison to the conventional methods - an image matching all 6 colors can hardly be achieved.

These errors increase by a factor of 2 if instead of a printing block sandwich consisting of a sheet 2.84 mm thick and an adhesive tape 0.20 mm thick, a plate thickness of 2.54 mm is used and on a foamed tape of 0, 54 mm thickness is mounted. The tolerance of this band is +/- 0.050 mm.

Another handicap with the current state of the art is that the processing, i.e. the pressure dimension in the running direction, due to the stretching factor of the cliché, cannot be determined exactly.

With the same percentage of stretch, the cliché diminishes in strength. This reduces the prescribed pressure level, e.g. of 3.0 mm.

If the photopolymer plate is used as the starting point, the flexible layer bearing the printed image, including the adhesive tape, must absorb the entire tolerance forces. That means the own tolerances, that of the adhesive tape, that of the anilox roller and the printing cylinder as well as the printing material and the Back pressure cylinders can only be compensated for by increasing the pressure, ie the setting values between the cliché and the back pressure cylinder.

In parallel with the strength of the pressure, the ink-bearing layer of the cliché is deformed. Since both rubber and the photopolymer layer are not compressible, material is displaced within the cliché, i.e. Deformation of the color-bearing, screened picture elements.

With the current state of the art, additional values are sufficient when printing fine screen motifs by using photopolymer plates with a thickness of 2.84 mm mounted on a 0.20 mm thick adhesive tape with an average of 45 to 55 µm, if on a relatively homogeneous printing block (e.g. 50 µm polyethylene film) is printed.

If sheets with a thickness of 2.54 mm are used and mounted on a foamed tape, the additional values increase by a factor of approx. 2.5. At the start of printing, the adhesive tape can only absorb a small part of the deformation forces.

The elasticity of this foamed tape decreases with the running time and the heat emitted by the intermediate ink drying. At the same time, the pressure increase in the light area increases by a factor of 2, and the full surface coverage is reduced by approx. 10% points. The relatively expensive foamed tape is very difficult to assemble, disassemble and cannot be used again.

Even the smallest tolerance differences in the range between 5 and 10 µm have a negative impact on the image. Over tolerant zen between 10 to 20 µm, which can only be compensated for by increasing the pressure, are sometimes no longer justifiable optically and technically in the fine pitch range.

Homogeneous images with multi-use prints, e.g. two in the cylinder circumference and three in the working width cannot be achieved with the current state of the art.

In the known printing presses, the vibration frequency of the press is not completely absorbed by the flexible cliché of 2.84 mm. As a result, vibration stripes appear in the print image depending on the motif and speed.

The invention is now based on the object, while substantially avoiding the disadvantages described above, of creating a flexographic printing machine of the type mentioned at the outset which enables printing of a much higher quality, i.e. overcomes the existing technical and physical limits of flexographic printing.

This object is basically achieved by the characterizing part of claim 1.

Further advantageous refinements are the subject of claims 2 to 6.

According to the invention, it is possible to achieve the desired quality improvement by substantially reducing the thickness of the cliché in conjunction with the sleeve which is constructed to absorb the forces.

With this printing block sandwich, the printing block itself cannot and should not absorb the deformation forces due to the low material thickness of the printing block. You will be dated Sleeve structure resorbed. As a result, the ink-bearing layers of the cliché are essentially no longer deformed and the precision of the printing, the color accuracy and the reproducibility are increased by a factor of about 9 compared to the current state of the art. The compressibility increases by a factor of approx. 5.

By reducing the plate thickness to a height of preferably 0.51 to 0.76 mm, the force-absorbing, incompressible photopolymer layer of the plate is reduced by a factor of 9. For comparison: a plate of 2.84 mm has a polymer thickness of 2.17 mm, the plate of 0.5 mm and 0.292 mm.

The cliché must, can and should no longer absorb the tolerance forces. The deformation forces are absorbed by the substructure on the sleeve. The cliché takes over, e.g. with conventional letterpress printing, only the task of the color and image bearing layer. By separating cliché (picture) elements and printing elements, the quality is improved many times over.

Many of the disadvantages described above are avoided.

The cliché on the sleeve, i.e. The surface and frictional forces derived from the substructure, which increase with the increase in speed, are absorbed and neutralized by the different, partly opposing layers. As a result, there is almost no "beading" before and after the pressure zone.

In order to eliminate the radial forces acting on the center of the axis of the pressure cylinder, a combination of a little and a highly compressible layer located above can be built up on the outside of the fabric layer will. This layer, which acts like a severed grid, is able to absorb the surface forces. The structure achieves a high level of compressibility. The return to the original position, almost without deformation on the surface, is achieved by two different compressible layers. The structure of these layers is oriented in the opposite direction to the running direction. The surface and tensile forces can be neutralized by the vulcanization of the sandwich combination.

The last layer, i.e. The external barrier layer can be adjusted to the required fastness properties of the different solvents of the adhesive to be used, or the adhesive of the assembly tapes, or expanded to the required fastness properties of the colors, e.g. Resistance to esters, alcohols or ketones.

The external, slightly compressible barrier layer can be sanded. The tolerance of the entire sleeve structure is +/- 10 µm.

For the first time, the pressure height can be exactly to 3 mm, can be realized by mounting it on the cliché plate to be used, e.g. to 0.51 or 0.64 mm. In addition, the height of the substructure (sleeve) is increased by the factor of the expansion of the cliché plate. The substructure is also increased by the factor of compression (e.g. 50 µm). This makes it possible for the first time to determine the plate size for all printing cylinders beforehand based on their exact length and later on the printing material.

For the first time, the printing height can be realized precisely, for example to 3 mm, by building it on the cliché plate to be used in each case, for example to 0.51 or 0.64 mm. In addition, the height of the substructure (sleeve) is increased by the factor of the expansion of the cliché plate. This makes it possible for the first time to determine cliché plates for all printing cylinder sizes beforehand based on their exact length, which will later be realized on the printing material.

Due to the quantitative compensation of the tensile and compressive forces during the printing stage by the invention (structure of the sleeve), the flexo will for the first time be able to largely eliminate the high mechanical, adhesive tape and plate-specific tolerances without visually changing the print image or could be determined by measuring method.

In the current state of the art, pressure increases between the cliché and the impression cylinder are e.g. of plus approx. 20 - 25 µm above the standard provision is extremely visible, with an increase in the light area of up to 100% area coverage.

As a result of the substructure according to the invention, an increase in the additional values by 80 in order to neither be substantiated by measuring methods nor visually visible.

Only with an increase of 312 µm can an optical and measuring method change in the area coverage be documented, which corresponds approximately to the value that can be expected from a 2.84 mm thick cliché plate in the current state of the art with a pressure increase of 20 to 25 µm.

For this reason, it will be possible for the first time in the sleeve structure according to the invention to realize multi-use prints in the scope and in the working width with a homogeneous printout. With the provision of pressures of +80 µm and above Furthermore, it is not the cliché but the base that is deformed in the pressure zone. The "sleeve" absorbs the tensile and compressive forces.

Due to the qualitative and quantitative absorption of the deformation forces, the substructure of the sleeve according to the invention will make it possible for the first time to realize line and surface elements in combination with autotypical grids in a cliché plate without sacrificing quality in gradation and contrast for the grid section and coverage losses for the surface .

In addition to the high economic benefit, the flexo also achieves the reproducible printing qualities of the conventional processes.

Due to the variable position (7) of the substructure of the sleeve, it will be possible for the first time to reproducibly maintain the pressure level specified mechanically by the circumferential changes by the factor of the expansion of the cliché. This will e.g. the processing for all printing cylinders of the same size is optimized to the pressure level of e.g. 3.0 mm. This gives the gears an exact technical fit.

In the known technology, the engagement of the gearwheels already varies within the four colors of an image due to the different developments. Depending on the circumferential error, there is deeper intervention into the gear in the case of under-tolerances and further out of the gear in the case of over-tolerances, with the success of increased vibration readiness up to the gear wheel marking on the printed image.

Due to the exact handling when using a printing block sandwich on the base after the sleeve according to the invention, registration errors in the running direction are also eliminated.

By combining the different materials, layers and their different properties in their optimal directional response to the pressure, it is possible to neutralize the high forces. The alternation of vertically and horizontally oriented directional instability and directional stability creates a base block that enables a homogeneous print surface image.

With the use of the substructure according to the invention, it is possible for the first time ever to use photopolymer plates with a thickness of up to 0.51 mm. These plates also have the advantage that the reprotechnical assembly and dowel system can be completely taken over.

Due to the low thickness, the raw plate is punched on the same punch as the film material, which is required in the individual work phases during production, and locked and exposed with the film that bears the image motif via a dowel pin.

This makes it possible for the first time to complete the plate manufacturing and plate mounting of the Flexo with the same accuracy as in the repro area, as in the conventional processes.

According to the known prior art, the access time of a plate of 2.84 mm for the grid area is 12 Hours. When using the thin cliché plate in conjunction with the correspondingly constructed sleeve, this value is reduced to one hour and far below.

Due to the low material thickness of the cliché, the swelling factor is practically disregarded.

The invention makes it possible for the first time to also print frequency-modulated raster images in flexographic printing. The sleeve is reusable.

The invention is explained in more detail below with reference to the drawing using an example of the sleeve.

In the single figure, a sleeve that can be pushed onto a pressure cylinder is shown in cross section. This sleeve is built on a metal tube 1. There is a highly compressible layer 2 on the metal tube 1, on the outer circumference of which a dimensionally stable fabric 3 is provided. This dimensionally stable fabric 3 is in turn delimited by a comparative barrier layer 4.

The barrier layer 4 is followed by a layer 5 for absorbing the radially acting surface forces. A barrier layer 6 for absorbing the tensile forces is provided on the outside of the layer 5. The outside of the sleeve is delimited by a barrier layer 7 for absorbing the frictional forces arising during printing. The cliché is attached in a conventional manner, for example by means of an adhesive tape, to this layer 7, which can also be ground to size.

• Rohr 1 Wandung des Druckzylinders oder eines Sleeves vorzugsweise galvanisch erstellt oder eines Sleeves aus Kunststoff o.ä.
• Schicht 2 Eine leicht poröse flexible Schicht aus synthetischem Kautschuk o.ä.
  Diese Schicht dient einmal dazu, die Toleranz des Gewebes auszugleichen, zum anderen durch den kompressiblen Aufbau eine Flexibilität zwischen der starren Wandung und dem relativ unflexiblen Gewebe herzustellen.
• Gewebe 3 Diese Lage besteht aus einem mehrlagigen, aus Kunststoff-Fäden o.ä. aufgebauten Gewebe. Dieses horizontal-stabile Gewebe hat zum einen die Auf­gabe, gegenüber der vertikal instabilen Schicht 5 als Sperrzone zu dienen. D.h. die Einwirkungen der Radialkräfte bleiben auf eine vertikale Verformung beschränkt. Die zweite Aufgabe dieses Gewebes besteht neben der hohen partiellen Stabi­lität darin, auch eine hohe Flächenstabilität zu bekommen, z.B. über die gesamte Arbeitsbreite.
• Schicht 4 Sperrschicht als Trennung zwischen der hochkompressiblen und der verformbaren Schicht aus synthetischem Kautschuk oder einem ähnlichen Material.
• Schicht 5 Diese Lage ist hochkompressibel, z.B. in Form einer gitterähnlichen Struktur aus synthetischem Kautschuk o.ä., deren Zwischenräume mit einer stark porösen Mischung ausgefüllt sind. Diese Schicht ist aufgrund ihres Aufbaus vertikal verformbar. Ihre Kompressibilität liegt im Ver­gleich zu der des Klischees um den Faktor ca. 5 höher. Ihr Rückstelleffekt ist im Vergleich zum Klischee um den Faktor 6 höher. Die Hauptaufgabe dieser Schicht besteht darin, die auf die Achsen­mitte hinwirkenden Flächenkräfte aufzunehmen und zu neutralisieren.
• Schicht 6 Diese Lage übernimmt die Egalisierung der noch nachwirkenden Friktionskräfte zum einen durch ihre entgegengesetzt der Laufrichtung orientierte Schicht aus Kautschuk o.ä., zum anderen durch ihre um den Faktor ca. 0,5 (im Vergleich zu 7) höhere Flexibilität.
  Die beiden Schichten 6 und 7 haben durch ihre entgegengesetzt orientierten Lagen und durch ihre unterschiedlichen Shorehärten die Hauptfunktion darin, die durch die Druckbeistellung frei werdenden Friktionskräfte zu neutralisieren.
• Schicht 7 Diese entgegengesetzt der Drehrichtung des Druckzylinders orientierte geringflexible Schicht ist aus synthetischem Kautschuk o.ä. Ihre Shore­härte beträgt ca. z.B. 80 - 90°.
  Diese Schicht hat eine '4-fach-Funktion':
  • 1. Die Hauptaufgabe besteht darin, die durch die Friktion das Druckbild stark negativ verändernden Kräfte weitgehendst zu neutralisieren. Das wird zum einen durch ihre relativ unflexible Schicht erreicht (im Vergleich zur Shorehärte des Klischees ist diese Lage um den Faktor ca. 2 härter), zum anderen durch ihre entgegen der Laufrichtung orientierte Anordnung auf dem Zylinder. Dadurch wird eine Wulstbildung vor und nach der Druckzone vermieden.
  • 2. Diese Schicht kann den geforderten Resistenzen der jeweilig unterschiedlich eingesetzten Lösungsmittel angeglichen werden, z.B. resistent gegen Alkohol oder Ester oder Ketone etc.
  • 3. Diese Schicht wird in ihrer Stärke variabel gestaltet, d.h. sie erhöht sich um den Prozentsatz des Dehnungsfaktors des Klischees im Verhältnis zum Zylinderum­fang, dadurch wird die wird maschinenbau­technisch vorgegebene Druckhöhe (z.B. von 3,0 m/m exakt eingehalten, die wiederum garantiert zum einen den Passer in Laufrichtung, zum anderen einen exakten Eingriff des Zahnrades. Dadurch werden zu starke oder zu schwache Zahnradeingriffe - die wiederum Abwicklungsfehler zufolge haben - mit ihren 'Zahnrad'-Markierungen auf dem Druckbild vermieden.
  • 4. Diese Schicht ist aufgrund der hohen Shorehärte mit hoher Genauigkeit nachzubehandeln, vorzugsweise schleifbar.

In one embodiment, the layers were constructed as follows:

• Tube 1 wall of the pressure cylinder or a sleeve, preferably galvanically created or a sleeve made of plastic or the like.
• Layer 2 A slightly porous flexible layer made of synthetic rubber or the like.
  This layer serves on the one hand to compensate for the tolerance of the fabric, and on the other hand to create flexibility between the rigid wall and the relatively inflexible fabric due to the compressible structure.
• Fabric 3 This layer consists of a multilayer, made of plastic threads or the like. constructed tissue. On the one hand, this horizontally stable fabric has the task of serving as a barrier zone with respect to the vertically unstable layer 5. This means that the effects of the radial forces are limited to vertical deformation. In addition to the high partial stability, the second task of this fabric is to achieve high surface stability, for example over the entire working width.
• Layer 4 barrier layer as a separation between the highly compressible and the deformable layer made of synthetic rubber or a similar material.
• Layer 5 This layer is highly compressible, for example in the form of a lattice-like structure made of synthetic rubber or the like, the spaces between which are filled with a highly porous mixture. Due to its structure, this layer can be deformed vertically. Their compressibility is about 5 times higher than that of the cliché. Their reset effect is 6 times higher than the cliché. The main task This layer consists of absorbing and neutralizing the surface forces acting on the center of the axis.
• Layer 6 This position takes over the equalization of the after-effect friction forces on the one hand by their rubber-oriented layer or the like, which is oriented in the opposite direction, and on the other hand by their flexibility, which is about a factor of 0.5 (compared to 7).
  Due to their oppositely oriented layers and their different Shore hardness, the two layers 6 and 7 have the main function of neutralizing the friction forces released by the pressure provided.
• Layer 7 This slightly flexible layer, which is oriented counter to the direction of rotation of the printing cylinder, is made of synthetic rubber or the like. Their Shore hardness is approx. 80 - 90 °, for example.
  This layer has a '4-fold function':
  • 1. The main task is to largely neutralize the forces that have a very negative effect on the printed image due to the friction. This is achieved on the one hand by their relatively inflexible layer (compared to the Shore hardness of the cliché, this position is about 2 times harder), on the other hand by their arrangement on the cylinder which is oriented in the opposite direction. This prevents the formation of beads before and after the pressure zone.
  • 2. This layer can be adapted to the required resistances of the different solvents used, for example resistant to alcohol or esters or ketones etc.
  • 3. This layer is variable in its thickness, ie it increases by the percentage of the elongation factor of the cliché in relation to the cylinder circumference. This ensures that the pressure height specified by mechanical engineering (eg 3.0 m / m) is exactly adhered to, which in turn is guaranteed to one with the register in the running direction, the other with an exact engagement of the gearwheel, thus avoiding gear engagements that are too strong or too weak - which in turn have caused processing errors - with their 'gearwheel' markings on the printed image.
  • 4. Due to the high Shore hardness, this layer can be post-treated with high accuracy, preferably grindable.

Claims (6)

1. Flexographic printing machine with a printing cylinder for receiving the soft (flexible) cliché and a hard impression cylinder, characterized in that the flexible cliché has a thickness of less than 1 mm, and that a sleeve built onto a tube (1) is pushed onto the printing cylinder is, which has a highly compressible layer (2) and on this a low or non-compressible barrier layer (7) which absorbs the friction forces generated during printing and on which the cliché is attached. 2. Flexographic printing machine according to claim 1, characterized in that the thickness of the plate is between 0.5 and 0.8 mm, preferably between 0.51 and 0.76 mm. 3. Flexographic printing machine according to claim 1 or 2, characterized in that a dimensionally stable fabric layer (3) is arranged between the barrier layer (7) and the highly compressible layer (2). 4. Flexographic printing machine according to claim 1, 2 or 3, characterized in that a second barrier layer (6) for receiving tangentially acting tensile forces is arranged under the cliché bearing and absorbing the friction forces barrier layer (7). 5. Flexographic printing machine according to one or more of the preceding claims, characterized in that between the one or more barrier layers (6,7) and the highly compressible layer (2) and optionally the fabric layer (3) another Layer (5) is arranged to absorb radially acting surface forces. 6. Flexographic printing machine according to claim 5, characterized in that a comparative barrier layer (4) is arranged between the further layer (5) for absorbing the surface forces and the fabric layer (3).

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