EP0813957A2 - Process and apparatus for intaglio printing using an erasable form - Google Patents

Abstract

The device has a rotating photogravure rough form (11) with a basic screen set out for at least the maximum ink quantity and an application device (2) for ultraviolet printing ink, suitable for completely filling of the basic screen. An ultraviolet drier (3) can pivot to and fro and engages over the printing form width. A picture point transmission device (5) performs ablation on the surface of the photogravure form and an inking system (7) emits ultraviolet printing ink. For the photogravure rough form a ceramic screen roller with high line count is provided. For the application device and the inking system a chamber ductor is provided.

Description

The invention relates to a method and a device for gravure printing by means of an erasable and reusable gravure printing form, starting from a gravure printing form with a basic grid designed at least for the maximum amount of ink to be transferred.

Gravure is a printing process with printing elements that are set lower than the surface of the mold. After the printing form has been completely inked, the surface is freed from the printing ink. This only remains in the deepened positions. As a printing form z. B. copper-plated steel cylinders, hollow cylinders pushed onto clamping cores or in some cases copper sheets clamped onto cylinders.

The type of coloring and the squeegee of the mold surface do not allow a pure surface pressure. The entire drawing must be broken down into lines, dots or grid elements. Because of the different depth and size of the individual printing elements, they contain more or less printing ink, which means that the imprint has different color strength at the different image points.

Various working methods for the production of a gravure form are used today. The etching principle of the variable-depth processes is the slow diffusion of concentrated iron chloride solutions through a pigment gelatin layer. The pigment copy on the copper printing form consists of a hardened gelatin relief that corresponds to the tone gradations of the slides. The engraving processes are characterized by scanning image and text line by line with photocells and simultaneously engraving the printing form with engraving heads. Particularly noteworthy is the introduction of depressions into the copper layer of the printing form by means of a high-energy electron beam, which is directed onto the raw form in a vacuum and removes material there in an image. The printing form engraved in this way can be provided with depth and area-variable grids.

Recesses can also be made using a high-energy laser beam, whereby it must be noted that suitable measures must be taken to ensure the coupling of the laser energy into the substrate, since copper in particular largely reflects a laser beam without special pretreatment.

Furthermore, from DE-OS 27 48 062 a method for producing an engraved printing form is known, in which a gravure printing form is first provided by uniformly providing the smooth surface with depressions of the same depth and size, then the engraved surface with a light-sensitive mass is covered so that all the wells are filled. Thereupon the raw form with the desired image is photographically exposed so that the exposed areas polymerize, the unexposed portions can be washed out and this results in an image differentiation.

In general, it can be said that the following applies to all gravure printing processes: image areas of the printing form are lower than non-image areas. In the case of squeegee printing in particular, the raster network forms uniformly high webs which delimit image areas and form a contact surface for the squeegee. For each print job, a special set of printing form cylinders per printing ink (a printing form cylinder with a corresponding number of print pages) is required. Depending on the print format, these cylinders are manufactured in the required cylinder size. When setting up the rotogravure or rotary printing machine, the corresponding printing form cylinders must be replaced. Such a cylinder z. B. in a width of 200 cm today weighs about 800 kg. Since the previously described methods can only be carried out outside the printing press, this requires a high level of mechanical effort. In addition, each of these production processes includes steps such as electroplating or coating, exposing and developing, which rule out that the same printing form can be reused without extensive, in particular chemical treatment. In addition, chrome plating is usually carried out after the pictorial etching or engraving, i.e. removal of material, to increase the service life.

If the printing form for the repetition orders is to be stored, the space for the entire cylinder must generally be provided. The printing form production is also especially if galvanic steps are necessary, very complex and therefore expensive. The resulting toxic sludge is also ecologically unsafe.

In contrast, DE 38 37 941 C 2 discloses a method for producing an intaglio printing plate, by means of which the imaging can take place directly in the printing press, in addition the imaging of the intaglio printing plate can be deleted in the printing press and prepared again for new imaging. A gravure blank form is also produced with a basic grid designed at least for the maximum amount of ink to be transferred. In the printing press, a quantity of a thermoplastic substance, which is inversely proportional to the image information, is then introduced into the depression from a nozzle of the pixel transmission unit or by image-correlated ironing in order to reduce the scooping volume of the depressions. That is to say, in contrast to the other methods, material is applied imagewise for imaging a gravure blank. After the print job, the thermoplastic substance can then be liquefied in the printing press by means of a heat source and removed again from the printing form cylinder by means of a wiping and / or blowing or suction device.

The pictorial material application, however, poses problems for the positional accuracy of the illustration. It is not readily possible to completely bring material that is placed on the webs into the depressions. However, full insertion is necessary so that the entire transferred material also contributes in a desired manner to reducing the volume of the depressions.

For this reason, it has already been proposed in the earlier patent application P 195 03 951 to fill the depressions of the basic grid of the intaglio printing blank evenly by means of a liquefiable substance by means of an application device, then to remove material from the wells by means of a pixel transmission device, which means that the gravure printing form is screened in this way of an inking system and then printing in gravure printing, whereby the gravure blank form can be regenerated after the printing process and the depressions can be filled evenly again

The liquefiable substance for filling the basic grid is, for example, a thermoplastic, that is, thermoplastics (plastomers), such as, for example, polyolefins, vinyl polymers, polyamides, polyesters, polyacetates, Polycarbonates, partly also polyurethanes and ionomers, or hot melt (wax), lacquer or a cross-linkable polymer melt or solution are used.

Using a laser, preferably an NdYAG or NDYLF laser, the filled gravure printing plate is burned free of image.

Using a chambered doctor blade, inking is carried out with aqueous printing ink and preferably printed on a printing material in indirect gravure printing.

After the required edition has been printed, the gravure form is cleaned of ink residues by means of a regeneration device, preferably in the form of an ultrasound cleaning system, and the liquefiable substance is removed from the depressions in the basic grid, so that the filling, imaging ablation, coloring, printing and regeneration cycle start again can.

The image deletion (regeneration) can also be made possible by completely cleaning the basic shape with the help of a pressurized water jet from a high-pressure cleaner. For this purpose, an arrangement is used, as has already been disclosed, for example, by EP 9 310 798. Such an arrangement consists of a double-walled chamber which is open towards the gravure form and is sealed off from the surroundings by means of seals guided over the form. The inner cell contains nozzles through which the water is sprayed onto the surface of the gravure form at high pressure. It is suctioned off from the coated outer chamber area, so that the liquid is drawn off in particular from the already cleaned area and the gravure form is clean and dry after the treatment.

The high-pressure cleaner can operate at at least two different levels, one level with low liquid pressure and / or temperature essentially used to remove the remaining colors and the other levels with correspondingly higher liquid pressure and / or temperature used to partially or completely remove the filler material .

The above statements all relate to the implementation of the measures described within a gravure printing press, of course however, that the measures described can of course also be carried out outside of a printing press.

It should be noted that, as a rule, in such rotary printing processes, a solvent-containing printing ink (even water-based printing inks contain certain amounts of solvent) is transferred to the printing material and then the solvent in a drying section of the printing machine from the printing material and from the printing material lying thereon or partially on it penetrating layer of paint must be driven out. The high space requirement for the dryer and the high energy consumption for drying are disadvantageous. The release of solvents also pollutes the environment. In the case of water-based printing inks, the amines used for saponification or the ammonia also escape into the ambient air when the ink film dries. These released compounds are not only unpleasant to smell but also harmful to health.

Furthermore, especially when using a high-pressure cleaner to regenerate the basic form, at least three different process media, namely the extinguishing fluid (water), the printing ink (usually solvent-based printing ink) and the filler (liquefiable substance), have been involved in the process, so that one there is a certain risk of mixing process media during production.

A further development of the printing inks is directed to those with UV-curable binder systems which make use of the generally known principle of UV curing.

DE 43 07 766 C1 describes a method for producing a UV-curing flexographic printing ink and its use in flexographic printing machines.

The UV printing inks contain coloring substances and additives in addition to photoinitiators, which trigger UV polymerization of the binder component also contained. Such UV systems can, as is also described in DE 43 07 766 C1, polymerize according to a radical or a cationic mechanism. Solvents or water are not included in UV inks. Technical, health or environmentally relevant Disadvantages and problems caused by solvents or water in printing inks do not occur with UV printing inks.

Proceeding from this, it is the object of the present invention to provide a method and a device for gravure printing, whereby on the one hand evaporating solvents or harmful compounds can be avoided when using an extinguishable and reusable gravure form and on the other hand the generic process concept is simplified.

This object is achieved by the method steps of claim 1 and the device for carrying out the method according to claim 8.

As a result of the fact that in the filling process the depressions of the basic grid of the gravure printing blank are filled with a UV printing ink, this is hardened in the depressions, then the UV printing ink hardened in the imaging process by thermal ablation is removed, and then the image screen is again filled with liquid UV Ink is colored and after the printing process for a retrofit the gravure form goes through an erase process using UV ink, only one process medium is involved in the process, which fills all functions, namely the extinguishing fluid, the ink and the filler, apart from that that evaporating solvents or compounds harmful to health no longer occur, process media mixing is ruled out and, at the same time, increased process reliability is achieved.

The direct result of this advantage is that the overall process is simplified and fewer subsystems are required for a device for carrying out the process. Thus, in the present invention, the extinguishing, filling and chamber doctor blade systems required in the generic method can be realized by one unit.

The use of liquid UV printing inks in the process concept according to the invention results in further particular advantages.

The property of the ink that it does not dry has a favorable effect especially for indirect gravure printing and for automatic process sequences over a longer period of time.

The high color strength, in turn, is favorable for laser power requirements and, last but not least, the dryers can advantageously be made comparatively very small, as a result of which a compact construction of the device is possible.

Fig. 1

den prinzipiellen Ablauf der erfindungsgemäßen Verfahrensschritte,

Fig. 2

den prinzipiellen Aufbau einer Vorrichtung zur Durchführung des erfindungsgemäßen Verfahrens in einem Tiefdruckwerk,

Fig. 3

eine Druckvariante mittels des erfindungsgemäßen Verfahrens.

A preferred exemplary embodiment or variants of the invention are explained below with reference to the drawing. It shows a highly schematic:

Fig. 1

the basic sequence of the method steps according to the invention,

Fig. 2

the basic structure of a device for carrying out the method according to the invention in a gravure printing unit,

Fig. 3

a print variant using the inventive method.

As shown in FIG. 1, the process according to the invention can be described by four process steps:

Filling a gravure blank 1 with filler (A).
The depressions of the basic grid of the gravure blank 1 are evenly filled by means of a UV printing ink through an application device 2 in the form of a chambered doctor blade and a UV dryer 3, which is positioned directly above the blank 1, is activated. The printing ink fed through the chamber doctor blade 2 is thereby hardened in the depressions. The slight shrinkage that occurs when the ink solidifies is compensated for by turning it over several times during the filling process, which means that successive filling can produce an arbitrarily precise filling. In practice, 1 to 20, preferably 3 to 8 revolutions of the rotogravure blank 1 per filling process resulted in an optimal filling.

Imaging of the filled gravure blank 4 by thermal ablation of the filler (B).
The ink supply of the filling process (A) is interrupted, the doctor blade 2 is switched off and the imaging process started. The imaging is done using a pixel transmission device 5 using a laser by thermal ablation. A YAG laser is preferably used, which due to its emitted wavelength of 1.064 µm is very well absorbed by the pigment black of the black UV printing ink. Alternatively, IR diode lasers with a comparable effect can be used. When using this type of laser, the printing ink "black" is therefore preferably used as the filler. When using lasers whose absorption behavior is determined less by the pigment type of the printing ink and more by the binder of the printing ink, colored printing inks can also be used for filling. For example, the emitted light of the CO ₂ laser with a wavelength of approximately 10.6 µm is preferably absorbed by many organic binders.

Print (C)
The gravure printing plate 6, which is screened in terms of image, is inked by means of a inking system 7 in the form of a chamber doctor blade with UV printing ink, so that printing can take place on the printing material 8.
Indirect gravure, which permits an expanded range of substrates, is preferably used as the printing method. An intermediate carrier has a positive effect with regard to color management and color gradient when using UV printing ink. In principle, direct gravure printing is of course also possible with this method.

Deletion of gravure form 6 (D)
In the method according to the invention, the gravure form 6 is deleted primarily by refilling the ablated image areas. Nevertheless, different variants can be used for implementation.

The simplest case can be described as follows for the printing ink "black" (or, if a laser-sensitive binder material is used, also for "color printing ink"): The deletion process (D) is identical to the filling process (A), that is, after the printing process (C ) the UV dryer 3 is activated after the filling process (A), which is usually carried out again at a significantly lower speed, is set. The liquid ink, which was recorded at the image points, hardens, so that the gravure blank 1 is again filled uniformly by means of a hardened UV printing ink. This variant is the number of cylinder revolutions for the filling process (A) is preferably identical to that for the extinguishing process (D).

When using "black" as a filler and "color printing ink" as a printing ink, the cleaning process of the gravure printing plate 6 must preferably precede the deletion by neither filling.

A printing plate cleaning is not necessary in order to achieve a good bond between old and new filling (same type of binder results in good bond), but in order not to change the sensitivity of the filler at the contact points. In the presence of colored pigments, the absorber density (pigment black) is reduced. The result is a reduction in sensitivity, which can lead to color density errors in the proof of the subsequent order.

The cleaning process can be made very simple: after the chamber doctor blade 7 has been switched off with colored printing ink and a large part of the printing ink remaining in the depressions has been removed by the printing process over the printing material (paper) 8, the chamber doctor blade 2 is used for the filler (identical to the chamber doctor blade 7 for coloring the image areas). After a few revolutions of the gravure printing plate 6 under the activated filler chamber doctor blade 2, but with the UV dryer 3 turned off, the residual ink is flushed out of the depressions in the gravure printing plate 6. The contamination of the filler in the chambered doctor blade 2 with colored printing ink is in the range per mil and is therefore irrelevant.

The stability of the refill is comparable to the previous fill. Nevertheless, the accumulation of foreign substances in the filler due to contamination (from the room air and in the printing process due to paper dust, etc.) cannot be ruled out. This fact can be countered by a complete removal of the filler from the depressions of the gravure blank 1 by means of "laser erasing" (that is, the laser ablates the filler completely from the depressions of the gravure blank 1 and writes a solid image) after a definable number of conversion processes.

Fig. 2 shows a printing unit 10 for indirect gravure printing with an ink transfer cylinder 12 and an impression cylinder 13, the printing material 14 being carried out between the latter. At a gravure cylinder 11 is in Direction of rotation an application device 2 which emits UV printing ink and is suitable for completely filling a basic pattern of a raw form in the form of a chambered doctor blade. A pixel transmission device 5 for imagewise ablation in the form of a laser, in particular in the form of a high-power laser, which can be part of an exposure unit traversing via the gravure blank, but also a semiconductor laser arrangement composed of several semiconductor lasers, is positioned on the surface of the gravure cylinder 11. On the side of the gravure cylinder 11 opposite the application device 2 for the filler, an inking system 7 which emits UV printing ink is also arranged in the form of a chambered doctor blade. The chambered doctor blade 2 for the filler and the chambered doctor blade 7 for inking are preferably constructed identically and are positioned on and off the circumference of the gravure cylinder 11. Furthermore, at least one UV dryer 3 that can be swung in and out across the printing form width is provided on the gravure cylinder 11. By skillful positioning, in the present case in the direction of rotation of the gravure cylinder 11 after the inking system 7, a pivotable UV dryer, a UV dryer is sufficient for the filling process and for the printing process.

In a special case, the application device 2 and the inking system 7 can also be designed as a common unit in the form of a chamber doctor blade instead of two identical chamber doctor blades, so that the filling and inking process can be carried out with a single chamber doctor blade.

Due to the permanent presence of two UV printing inks in the printing unit ("black" and "color printing ink"), the rapid automatic changeover from color printing to b / w printing or vice versa is possible. For example, according to FIG. 3 it is possible to convert a four-color printing press to four single-color printing presses by skilfully handling the printing material.

A ceramic anilox roller with a high number of lines is preferably used as the gravure blank. Ceramic material shows better wetting behavior than chrome. The high number of lines improves the color transfer qualitatively and quantitatively.

The UV lamps with mercury vapor tubes available on the market are suitable as UV dryers. Ozone-reduced or ozone-free lamp types are preferably used.

UV printing inks are characterized by a high reaction rate when solidifying. Solidification in the ms range is state of the art. A highly cross-linked and therefore hardly soluble and infusible substance is formed during the solidification. In principle, any UV-curable, solvent-free liquid ink can be used as UV printing ink. The cathionically crosslinking color types are particularly suitable for this.

Claims (13)

Process for rotogravure using an erasable and reusable rotogravure form, starting from a rotogravure raw form with a basic grid designed at least for the maximum amount of ink to be transferred, in which a filling process and a subsequent imaging process is carried out to produce the printing form,
characterized in that
in the filling process (A), the depressions of the basic grid of the gravure blank (1) are filled uniformly by means of a UV printing ink through an application device (2), the evenly applied UV printing ink is hardened by means of a dryer (3) that can be positioned over the gravure blank, then in the Imaging process (B) is removed by thermal ablation by means of a pixel transfer device (5) hardened UV printing ink, the gravure printing plate (4), which is screened in the image, is colored with UV printing ink by means of a coloring system (7) and then the printing process (C) is carried out and the gravure form undergoes a deletion process (D) for conversion. The method of claim 1
characterized in that
the deletion process (D) is carried out using UV printing ink Method according to claim 1,
characterized in that
the printing process (C) is carried out in indirect gravure printing. Method according to claims 1 to 3,
characterized in that
the erasing process (D) comprises the steps of uniformly filling the gravure printing plate (4) with UV ink and solidifying the applied liquid UV printing ink by means of a UV dryer (3). Method according to claims 1 to 3,
characterized in that
the deletion process (D) comprises a cleaning process of the gravure printing plate (4) in the form of rinsing out the residual liquid ink from the printing form grid by liquid UV printing ink by means of the application device (2) provided for filling, and then uniform filling of the gravure printing plate (4) with UV Color and the solidification of the applied liquid UV printing ink is carried out by means of a UV dryer (3). Method according to one of the preceding claims,
characterized in that
after a definable number of changeover processes, a complete restoration of the basic grid of the intaglio printing blank (1) is carried out by completely removing the filling in the depressions. Method according to one of the preceding claims,
characterized in that
cationic or free radical curing UV printing inks are used at least for the filling process (A) and erasing process (D). Device for gravure printing for carrying out the method according to claim 1,
characterized in that
on a rotating rotogravure blank (11) with a basic grid laid out at least for the maximum amount of ink to be transferred in the circumferential direction, an application device (2) which emits UV printing ink and is suitable for completely filling the basic grid, at least one UV dryer which can be swung in and out across the printing form width (3), a pixel transmission device (5) for imagewise ablation on the surface of the gravure printing form and a coloring system (7) which emits UV printing ink can be positioned. Device according to claim 8,
characterized in that
a ceramic anilox roller with a high number of lines is provided for the gravure blank. Device according to claim 8,
characterized in that
an identical chamber doctor blade is provided for the application device (2) and the inking system (7) Apparatus according to claim 10,
characterized in that
the application device (2) and the inking system (7) are constructed as a common unit in the form of a single doctor blade. Device according to claim 8,
characterized in that
a traversing exposure unit in the form of a laser, in particular in the form of a high-power laser, is used for the pixel transmission unit (5). Device according to claim 12,
characterized in that
a semiconductor laser arrangement comprising a plurality of semiconductor lasers is provided for the laser (5).

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