EP0076905B2 - Flat-bed printing method - Google Patents

Description

The invention relates to a planographic printing process in which aqueous printing inks are transferred to the printing material without the use of a dampening unit using lithographic printing plates, the printing surfaces of which are hydrophilic and the non-printing surfaces are hydrophobic.

A well-known printing process is offset printing. In recent years, offset printing has gained considerable economic importance. In offset printing, the incompatibility of water and oil is used to divide a homogeneous oil-based ink film on a printing plate into the corresponding information fields.

The offset printing plates have the properties of being water-friendly in the non-information-carrying areas and being oil-friendly in the information-carrying areas. In the course of the printing process, the plate is completely wetted with a water film (wash water), then it is colored with the oil-based printing ink. The hydrophilic areas wetted by water cannot be wetted by the hydrophobic oil paint. In this way, a colored print image is created on the printing plate, which is transferred to the printing material via a roller mechanism.

The main advantages of offset printing are the simple, cheap and very quick to produce print carrier, the possibility to create printing plates directly with the help of electronic data processing and the possibility of four-color wet-on-wet printing.

The highly viscous printing inks used in offset printing require repeated rolling out to achieve an absolutely uniform color film across the entire width of the printing press. Inking units of offset printing presses have up to 30 rollers, between which the pasty printing inks are rolled out. Because of this long and intensive rolling, low-boiling solvents cannot be used, as is usual in gravure printing, but only high-boiling solvents, since otherwise the printing ink dries on the inking rollers and the ink flow would be disturbed.

In practice, high-boiling mineral oils and vegetable oils that are hydrophobic and water-insoluble are used as solvents.

The use of high-boiling mineral oils as a solvent in offset printing inks has considerable disadvantages. The drying either takes too long because the mineral oils take a long time to penetrate the paper surface (knocking away) and are hampered by the water in the paper, or it increases to an economically sensible level (150 to 160 ° C) when the drying temperature is increased ) drying out and curling of the paper, so that the subsequent processing steps such as cutting, collating and binding are considerably more difficult. Also used for drying e.g. consumed a considerable amount of energy in web offset.

During printing, offset printing and mixing water are mixed, i.e. an emulsion is formed. This process is advantageous in itself because the resulting consistency of the printing ink only enables economical four-color wet-on-wet printing. As is known to the person skilled in the art, the pre-printed inks are prevented from being lifted off from the printing material by the reprinting printing units by emulsifying water. On the other hand, this process has the disadvantage that it takes a certain time when starting up a printing press to adjust the ink-water balance. It is therefore quite possible that a total of 5 to 10% of a print run will be produced as waste when starting up a printing machine and when it is switched off and on again.

To avoid this disadvantage, attempts have been made to emulsify the water into the printing ink as early as during manufacture. Despite intensive propagation, this has not been able to prevail since these printing inks are often unstable. There is a separation of oil and water, and there is no need for a separate dampening unit on the printing press, since the amount of water is also not adjustable and cannot be adapted to the paper or the printed object. It should be noted that conventional printing inks can absorb about 10 to 30% water on the printing press in the printing unit.

Attempts have also been made to overcome the disadvantages of conventional offset printing with the aid of the so-called “reverse offset printing method” (compare, for example, US Pat. No. 3,356,030). The printing plates used here are coated with silicone rubber in the area of their non-printing surfaces, while their information-bearing surfaces are metal surfaces.

This process uses aqueous printing inks and uses an organic, hydrophobic wiping liquid instead of the wiping water used in the conventional offset printing process.

• wie beim konventionellen Offsetdruck ein Farb-Wischflüssigkeits-Gleichgewicht aufgebaut wird und damit auch die vom klassischen Offsetdruck her bekannten Probleme auftreten
• bei Verwendung relativ niedrig siedender Wischflüssigkeiten grössere Mengen an organischen Lösungsmitteln verdampfen und der Einsatz von hochsiedenden Wischflüssigkeiten neben unerwünschten Verunreinigungen auf den farbfreien Stellen des Bedruckstoffes auch eine auf die Emulsionsbildung zurückzuführende Verschlechterung im Trocknungsverhalten der wässrigen Druckfarben zur Folge hat
• beim Mehrfarbendruck nachfolgende Farben von den im ersten Durchlauf nicht bedruckten und daher mit der hydrophobierenden Wischflüssigkeit in Kontakt gekommenden Flächen des Bedruckstoffes abgestossen werden können.

The replacement of the aqueous wiping liquid with an organic hydrophobic wiping liquid has more disadvantages than advantages, however

• How a color-wiping liquid balance is built up in conventional offset printing and thus the problems known from classic offset printing also occur
• When using relatively low-boiling wiping liquids, large amounts of organic solvents evaporate and the use of high-boiling wiping liquids not only results in undesired impurities on the ink-free areas of the printing material, but also in the drying behavior of the aqueous printing inks due to the formation of emulsions
• in multi-color printing, subsequent colors can be repelled from the surfaces of the printing material that were not printed in the first pass and therefore come into contact with the hydrophobic wiping liquid.

US-A-2 288 073 discloses a planographic printing process in which printing is carried out with aqueous printing inks without the use of a dampening unit. The printing plates used in the process described in US-A-2 288 073 can only be used for copying typewriter letters, but not for printing full areas and halftone dots; they would not be able to withstand the stresses of modern offset printing machines, in which printing is carried out with rapid colors and at high speed, for long. Finally, no waterfast prints are obtained with the inks used in the process disclosed in US-A-2,288,073.

In addition to the flat printing processes described above, so-called waterless offset printing is also known. In this process, attempts are made to use special printing plates in which the ink-bearing layers are hydrophobic and the non-ink-bearing layers have been made extremely repulsive by means of silicone coating, without printing with conventional offset printing inks. Despite intensive processing, this printing process has not been able to gain widespread acceptance on the market, since four-color wet-on-wet printing poses problems similar to those of conventional letterpress printing. This applies in particular to pluck-sensitive papers.

The object of the present invention is to provide a planographic printing process in which the disadvantages of the prior art described above do not occur while retaining all the advantages of the conventional offset printing process.

This object is achieved by a planographic printing process of the type mentioned at the beginning, in which aqueous printing inks are used using lithographic printing plates, the non-printing surfaces of which are hydrophobic, are transferred to the printing material, in which printing plates are used, the non-printing surfaces of which are coated with silicone rubber and their information-bearing surfaces are metal surfaces or hydrophilic plastic surfaces and printing inks which predominantly contain water and correspond to the aqueous printing inks of gravure or flexographic printing and a viscosity of 10 to 250 Pa.s, preferably 20 to 50 Pa.s, measured at 23 ° C. , and the transfer of the printing inks from the inking unit with anilox roller, from the printing plate via an intermediate support to the printing substrate without the use of a dampening unit, without the use of the usual mixing of water and grease in offset printing (organic hydrophobic wiping liquid ) he follows.

It was surprising that good results can be achieved without using grease and water repulsion when using aqueous inks.

The printing inks to be used according to the invention contain color pigments and / or soluble dyes, binders which fix the pigments on the printing material surface, other auxiliaries such as waxes, defoamers, disinfectants and thickeners which are used in other fields, for example aqueous printing inks for rotogravure or flexographic printing or are known from the field of aqueous paints and coatings. Both dispersions and water-soluble binders can be used as binders.

The printing inks mainly contain water as a solvent and / or dispersant. In addition to water, they can contain a minor amount of organic solvents that are completely or partially miscible with water. Small quantities of petroleum fractions can only be used as aids. It should be noted here that the printing ink retains its watery character.

Printing inks which correspond to the aqueous printing inks of gravure or flexographic printing but which have a higher viscosity than these can advantageously be used.

As with other printing inks, natural and synthetic waxes, polyethylene etc. are suitable as auxiliaries for improving the lubricity. In some cases, partial fluorination of the auxiliaries improves the abrasion resistance.

Inorganic or organic substances, in particular soluble or swellable substances, are used as thickeners. Among others, the following are known: alginates, starch, cellulose and their derivatives, e.g. Cellulose ester or cellulose ether. Mineral bodies, which swell with polar solvents, act in the same way.

All known coloring components, for example organic and inorganic color pigments, preferably directly from water pastes, and dyes soluble in water or in solvents are suitable for coloring. These products are known from other areas of printing and painting technology.

The printing inks to be used according to the invention are produced by known processes. So you can e.g. Incorporate pigments, pigment preparations or predispersed pigments into the aqueous binder solution or mixture using stirrers and dispersing units.

To carry out the method according to the invention, printing plates are advantageously used, the non-printing surfaces of which are coated with silicone rubber and the information-carrying surfaces are metal surfaces or hydrophilic plastic surfaces. For example, printing plates such as are used for so-called waterless offset printing are suitable.

For better printability, i.e. to improve the color repellency of non-printing areas e.g. on printing plates coated with silicone rubber, additives of additives containing silicone groups are useful. These can be dispersed silicone oils or water-soluble silicone derivatives.

The transfer of the printing inks from the inking unit of the printing press to the printing material can take place directly or via an intermediate carrier. A blanket cylinder known from offset printing can serve as the intermediate carrier. In general, a known one can be advantageous Offset printing machine can be used whose dampening system has been removed or deactivated. If necessary, the printing unit's inking unit is reduced, ie all rollers that are not necessary for inking are removed. This counteracts the faster evaporation of water compared to high-boiling mineral oils. It is also possible to encapsulate the inking unit for this purpose.

In an advantageous embodiment of the method according to the invention, the printing inks are transferred to a printing plate cylinder and from there via a blanket cylinder to the printing material via an anilox roller with a doctor blade, via an ink transfer roller arranged downstream of the anilox roller.

The anilox roller advantageously works as an immersion roller, i.e. it protrudes into an ink tray.

Instead of a single ink transfer roller, two or more ink transfer rollers working in parallel can also be used. It is essential, however, that the ink transfer rollers each work directly with the anilox roller and with the printing plate cylinder and not several rollers are connected in series.

In an advantageous embodiment of the method according to the invention, the anilox roller works as an immersion roller, i.e. it protrudes into the ink supply. The printing inks can of course also be applied to the doctor roller in another known manner, for example by brushing.

In the method according to the invention, the disadvantages of the mechanical sensitivity of a rubber film forming an ink film are avoided by using a rastered metal roller. It is also more resistant to bending and can be precisely scraped off by using grid bars produced by etching or engraving as support elements.

The uniform ink-absorbing depressions over the entire width of the anilox roller ensure uniform coloring, regardless of the different ink consumption at different locations on the print motif. This also complies with the standardization in offset printing. With the previous inking units, the coloring cannot be kept constant during the printing process.

Special requirements for higher or lower ink layer thicknesses, e.g. for rougher papers, can be met by replacing anilox rollers that are etched deeper or shallower.

By tempering the anilox roller, even better ink guidance can advantageously be achieved, since the ink viscosity no longer changes due to temperature influences. In conventional long inking units, the inking unit heats up due to internal friction and through many splitting processes of the inks, which causes the inks to become less viscous during the print run. This changes the ink transfer and the ink layer thickness in the printed image. Waste can only be avoided by carefully regulating the color scheme by the printer.

To further reduce or completely avoid stenciling, the printing plate cylinder and the ink transfer roller advantageously have the same diameter. It is particularly advantageous if the anilox roller and the blanket cylinder also have the same diameter as the printing plate cylinder and the ink transfer roller.

The following advantages are achieved by the invention: A short inking unit with only a few splitting points for the ink considerably reduces the evaporation rate of the volatile components compared to conventional inking units with their many rollers. This is why colors can be used that contain the quickly evaporating water that breaks away into the paper. All physical drying processes, i.e. Throwing away into the paper, evaporation of the water in the web offset heatset or by IR emitters in the sheetfed offset are thus much faster than with the slowly kicking away or evaporating mineral oils of conventional inks.

A major advantage of the invention is the reduction in stenciling due to the short inking unit and the continuous renewal of the ink film by the anilox roller.

The temperature in the web offset heatset drying ovens can be significantly reduced, depending on the machine speed, substrate and ink layer thickness. This enables considerable energy savings. The papers dry out less and thus remain dimensionally stable, i.e. they do not absorb any water from the environment after drying and no longer "grow" after processing or storage. Under favorable conditions, these inks can even be dried in web offset with IR emitters. These limit cases depend on paper, printing speed and ink layer thickness. In contrast to non-polar conventional oil-based offset inks, the water-based inks also respond to high-frequency dryers.

With the planographic printing method according to the invention, the disadvantages of the prior art described in the introduction to the description _ in particular the problems associated with the setting of a printing ink / wiper liquid equilibrium _ are overcome while maintaining the advantages of conventional offset printing.

The invention is explained in more detail below on the basis of exemplary embodiments. The figure shows a section through a planographic printing machine.

The planographic printing machine has an inking unit (13), a printing plate cylinder (3), a blanket cylinder (11) and an impression cylinder (9). The printing material (10) runs between the rubber cylinder (11) and the impression cylinder (9). The inking unit (13) consists of an anilox roller (1) which bears a doctor blade (2) and an ink transfer roller (4) which is arranged between the anilox roller (1) and the printing plate cylinder (3). The anilox roller (1) protrudes into the ink tray (8). As the figure shows, all rollers or cylinders have the same diameter.

Example:

40 parts by weight of an aqueous emulsion of an alkali-soluble acrylate polymer having a solids content of 40% by weight, 20 parts by weight of propylene glycol, 4 parts by weight of triethanolamine, 4 parts by weight of a 35% strength methyl silicone oil emulsion, 4 parts by weight. Parts of mineral oil with a boiling range of 190-250 ° C and 8 parts by weight of starch ether mixed together. 20 parts by weight of an aqueous pigment dough containing 45% by weight of copper phthalocyanine pigment were added to this mixture. The mass was dispersed on a three-roller to form a homogeneous printing ink.

An offset printing machine according to the figure was used as the printing machine. A commercially available silicone-coated printing plate was used as the printing plate. It was printed on coated offset paper. The result was prints of excellent quality.

Claims (7)

1. A planographic printing process in which an aqueous printing ink is transferred onto the material to be printed by means of a lithographic printing plate whose printing areas are hydrophilic and whose non-printing areas are hydrophobic and are coated with a silicone rubber, the information-carrying areas being metal surfaces or hydrophilic plastic surfaces, and the printing ink, which predominantly contains water and corresponds to the aqueous printing inks of intaglio or flexographic printing, having a viscosity of from 10 to 250 Pa.s, preferably from 20 to 50 Pa.s, measured at 23°C, and the transfer of the printing ink from the inking unit taking place by means of an engraved roller and from the printing plate to the material to be printed via an intermediate carrier without using a damping system and without using the water/oil mixture (organic hydrophobic damping solution) which is customary in offset printing.

2. A process as claimed in claim 1, wherein the aqueous printing ink used contains an additive of silicone oil or a water-soluble silicone derivative.

3. A process as claimed in claim 1 or 2, wherein a conventional offset printing machine is used whose damping system has been removed or disabled.

4. A process as claimed in claim 1 or 2 or 3, wherein the printing ink is transferred via an engraved roller (1), against which a doctor blade (2) abuts, via an ink-transfer roller (4) arranged after the engraved roller, onto a printing plate cylinder (3), and from the latter via a rubber blanket cylinder (11) onto the material (10) to be printed.

5. A process as claimed in claim 4, wherein the engraved roller (1) projects, as a duct roller, into the ink trough (8).

6. A process as claimed in claim 4 or 5, wherein the printing plate cylinder (3) and the ink-transfer roller (4) have the same diameter.

7. A process as claimed in claim 4 or 5 or 6, wherein the engraved roller (1) and the rubber blanket cylinder (11) have the same diameter as the printing plate cylinder (3) and the ink-transfer roller (4).

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