EP0061093A1 - Doctor blade for intaglio printing by means of layers of organic plastics - Google Patents

Abstract

1. The use of a doctor blade whose wiping surface has rounded edges and which exhibits adequate flexural elasticity and a surface hardness of at least 350 (Vickers hardness according to DIN 50,133) in intaglio printing processes in which plastic printing layers are employed.

Description

The invention relates to the use of doctor blades consisting essentially of steel in gravure printing processes in which a plastic printing layer is used.

Today, gravure printing generally uses a printing cylinder made of a steel core with a copper jacket, on the surface of which the so-called "Ballard" skin is applied as the actual printing layer. This Ballard skin is a chrome-plated copper-metal layer in which the recesses (cups) necessary for the color absorption are located. The printing takes place in such a way that the printing cylinder first passes through an ink trough and then past a knife-shaped squeegee, as a result of which the cells are filled with ink, but the surface color is scraped off again by the raised webs. The printing cylinder then runs over the material to be printed using a counter-pressure roller, the ink being drawn out of the cells.

Print quality and undisturbed printing largely depend on the quality and correct setting of the squeegee. The steel doctor blades used in today's gravure printing practice grind against the hard Ballard skin of the printing cylinder during the printing process. Attempts to increase the service life of these doctor blades by chrome-plating the knives were not without problems, since the effort and the effect achieved were not economically related to one another. Today, steel doctor blades with a stepped facet shape are predominantly used in gravure printing, in which the running surface of the doctor blade lying against the printing layer remains constant even when sanded. The number of copies in gravure printing is generally determined by the uniform slow abrasion of the chrome layer of the Ballard skin or, Rss / P 'by destroying this chrome layer by flaking off fine' scales and averages around 500,000 to 5,000,000 cylinder revolutions.

This conventional gravure printing combines a long service life and very good print quality and, in comparison to other printing processes, enables considerably better halftone reproduction. Due to the very complicated and complex production of impression cylinders (application of a copper layer to the steel cylinder, mechanical or chemical engraving of the copper layer, chrome plating of the engraved copper layer), the use of gravure printing is limited to those applications where high Print quality and large print runs are required. It is desirable to further develop the gravure printing process in use today in such a way that it can be used economically in a wider and more varied manner than before.

Accordingly, there is no lack of recommendations in the relevant literature to use plastics as a printing layer instead of Ballard skin to simplify the manufacture of printing cylinders. The cells can be formed, for example, by engraving or exposure of light-sensitive systems using a laser beam (cf., inter alia, US Pat. No. 3,506,779). When using photopolymer systems for the production of the printing layer, as described, for example, in DE-OS 20 61 287, the cells are - similar to the production of high-pressure and flexographic printing plates - by imagewise exposing the photopolymerizable layer and then washing it out of the unexposed areas.

The fact that such comparatively fast, simple and variable 'plastic printing layers to be produced in gravure printing have so far not found any practical application is essentially due to their low print run resistance when using the steel doctor blades previously used in practice.

The steel squeegees in the form of stepped facets do not grind to the setting angle set in the printing press without damaging the plastic printing layer. In addition, the previously used steel squeegees often cause scratch marks in the plastic printing layer due to the formation of sharp burrs, holes, tips, etc. and cause high abrasion of this layer, which limits the number of runs for quality gravure printing to approximately 5,000 to 50,000 cylinder revolutions becomes.

According to the teaching of DE-OS 27 52 500 ', these disadvantages of gravure printing with plastic printing layers, which occur due to the use of commercially available steel doctor blades, can be avoided by using doctor blades made of plastic. Although this allows the circulation to be increased, the specified circulation heights of approximately 50,000 cylinder revolutions are still unsatisfactory. The plastic doctor blades have to be about 1 mm thick due to their low flexural strength and be short. The considerably higher contact forces and pressures required as a result lead to increased wear. The frictional resistance between the plastic doctor blade and the plastic printing layer is also comparatively very high.

Accordingly, the object of the invention is to show an improvement for the gravure printing process which works with plastic printing layers, which makes it possible to largely avoid the disadvantages mentioned and to significantly to achieve much larger print runs than before, without having to accept losses in print quality.

It has now surprisingly been found that this object can be achieved by using a doctor blade with sufficient flexural elasticity in the rotogravure printing process using plastic printing layers, the bevel edges of which are rounded and which has a very hard surface.

The invention thus relates to the use of a doctor blade with sufficient bending elasticity, a surface hardness of at least 350 (Vickers hardness according to DIN 50 133) and rounded bevel edges in gravure printing processes in which a plastic printing layer is used.

According to a preferred embodiment of the invention, a doctor blade is used, the bevel of which is ground to the plastic printing layer in accordance with the angle of attack.

According to a further preferred embodiment of the invention, a doctor blade is used, which consists either of a hard, fine-grained spring steel or of a multi-phase steel core, the surface of which is coated with a correspondingly hard material.

It was by no means obvious to the person skilled in the art that the object could be achieved by using a doctor blade with the combination of shape and material properties according to the invention. If, in gravure printing with plastic printing layers, the commercially available doctor blades made of mostly multi-phase spring steel with a crystallite structure are used for gravure printing, even with rounding off the bevel edges and even grinding the bevel, only a small print run can be achieved depending on the angle of attack on the printing layer. By The comparatively strong interactions between the plastic print layer and the steel doctor blade cause parts to break out of the doctor blade. This process, which starts from parts of the chamfer, spreads out over a large area, so that holes, grooves and burrs are created, which lead to excessive wear of the plastic printing layer. If one tries to reduce the strong interaction between doctor blade and plastic printing layer and thus also its high abrasion while maintaining the bending strength and elasticity of the steel doctor blade, for example according to the teaching of DE-OS 27 52 500, by working with plastic-coated steel doctor blades, there is still an unsatisfactory result. It was therefore extremely surprising that when using a steel doctor blade with a very hard surface with rounded bevel edges, wear and abrasion of the plastic print layer are so low that print runs of around 500,000 cylinder revolutions can be achieved with good print quality.

In the drawing, a possible embodiment of the squeegees to be used according to the invention is shown schematically in section for the purpose of illustration and further explanation. The doctor blade has a stepped facet shape, the lamella (1) of the doctor blade being provided with a coating (2) made of a hard material. The bevel (3) of the squeegee is rounded off at the bevel edges (4, 5), the degree of rounding being given by the radius of curvature (r). The bevel angle (α) is advantageously based on the angle of attack of the doctor blade on the plastic printing layer. The doctor blade thickness is marked with (a), the lamella width with (b) and the lamella thickness with (c).

With regard to the shape of the doctor blades to be used according to the invention, it can be said that at least the bevel edges of the doctor blades must be rounded. The chamfer surface should be as flawless and smooth as possible and can advantageously also be completely rounded. Squeegees with rounded bevel edges, in which the bevel is ground to the plastic printing layer in accordance with the angle of attack, are particularly favorable. The squeegee angle that arises in the printing machine is influenced by the frictional resistance between the squeegee chamfer and the plastic printing surface and by the squeegee line pressure, the squeegee line pressure, which must be set to clean the doctoring of the ink, generally fluctuate between 2 and 5 N / cm due to the surface tolerance can. For these reasons, the doctor blade angle of attack cannot be kept constant in practice and the bevel angle of the doctor blade is preferably not ideally adapted to the conditions in the printing press. The most favorable chamfer grinding angle under the given conditions in a printing press is familiar to the person skilled in the art, or is easy to determine, whereby chamfer grinding angles of approximately 60 to 65 ° have often proven to be suitable. According to the invention, use is made in particular of such doctor blades whose bevel edges are rounded with radii of curvature between 10 and 70 _/ um, preferably between 20 and 40 _/ um. For the rest, the shape of the doctor blade is largely uncritical; for practical reasons, however, the usual step-faced doctor blades are generally preferred.

The doctor blades to be used according to the invention for gravure printing with a plastic printing layer should - in the same way as the conventional doctor blades for conventional gravure printing - have sufficient flexural elasticity, that is to say the doctor blades must be pliable enough to prevent surface thickness fluctuations (surface tolerances) in the plastic-- Compensate the print layer. The bending elasticity is determined both by the geometry factors of the doctor blade and by the modulus of elasticity for the manufacture of the Squeegee used materials determined. In addition to the doctor blade thickness and, in the case of step facets - doctor blades of the blade thickness and blade width, the geometry factors also include the clamping length of the doctor blade in the printing press. The geometry factors of the doctor blades which are used today in conventional gravure printing have also proven themselves for the doctor blades to be used according to the invention which essentially consist of steel. The modulus of elasticity of the doctor blade material is advantageously equal to or greater than approximately 2100 N / mm ² (measured according to DIN 50 145). In squeegees made of a multi-phase steel with a hard surface coating, the flexural elasticity of the squeegee should not be significantly influenced by the hard surface coating. For this reason, because of the brittleness of the materials to be used for the coating, the hard surface coating usually has a layer thickness in the range from 1 to 20 _μm , preferably in the range from 5 to 10 _μm . If, according to the invention, steel doctor blades in the form of stepped facets with a hard surface coating are used, it is advantageous and sufficient if only the lower part of the lamella is coated with the hard material in order to avoid extensive influencing of the bending elasticity.

The doctor blade should at least have a hardness (measured according to DIN 50 133) of at least 350 (Vickers hardness). Surprisingly, it has been found that the doctor blade damage to the plastic printing layer is less, the harder the surface of the doctor blade used. The required surface hardness of the doctor blades to be used according to the invention can be achieved, for example, by the doctor blade being produced uniformly from a suitably suitable hard material. However, it is also possible to use a doctor blade which consists of a softer core which is coated with a suitable hard material. The thickness of this hard surface layer is from practical reasons limited to small and large values. In order to ensure a sufficiently long service life of the doctor blade, the thickness of the coating should generally not be less than 1 μm. As already mentioned, the limitation to large values here is given by the brittleness of the coating materials and the necessary bending elasticity of the doctor blade. The upper limit for the thickness of the hard coating is generally about 20 _µm . It is particularly advantageous if the coating has a thickness of 5 to 10 _/ to.

The doctor blade to be used according to the invention or, in the case of the coated doctor blades, the doctor blade core preferably consists of a suitable steel. For the uncoated doctor blades, which consist of one material, the fine-grained hard steels with spring steel properties are particularly suitable. In the case of coated doctor blades, the steel core consists in particular of a multi-phase steel with a crystallite structure, as is used, for example, for the manufacture of the doctor blades used in today's gravure printing practice. In principle, any materials can be used for coating, provided that they meet the required hardness requirements, can be firmly adhered to the steel core of the doctor blade, without splintering or chipping under the stress given in the printing process, and becoming an error-free smooth surface, free of burrs, Have grooves, tips etc. processed. For example, hard metals such as nickel, chromium, manganese and others are suitable as hard coating materials. Hard metal alloys, hard carbides such as titanium or chromium carbide or ceramic materials can also be used for coating. The hard surface layer can be applied to the steel core of the doctor blade according to the generally known and customary methods. So the metal coating is advantageous through galvanic deposition of the metals. '

The use of coated squeegees has the advantage that the squeegees customary for conventional gravure printing - after rounding off the bevel edges and a corresponding coating with a hard surface material - can also be used for gravure printing with plastic printing layers.

The choice of the material from which the squeegee and / or the hard surface layer of the squeegee is made depends, among other things. also on the type of plastic from which the plastic print layer is built. Squeegees which have good sliding properties on the plastic printing layer are advantageously used, i.e. whose frictional resistance to the plastic is low.

According to the invention, the doctor blades with the rounded bevel edges, sufficient bending elasticity and a hard surface are used in the known gravure printing processes described in the introduction, in which a plastic printing layer is used. The plastics which are customary and customary for this application can be used as plastics for producing the plastics printing layer. The plastics must meet a number of requirements in a known manner: they must and should be chemically resistant to the inks used in gravure printing, in particular the solvents used for these inks, primarily toluene and petrol, but also water, alcohol, esters or ketones if possible, have a swelling of less than 5% by weight in these solvents when stored for several days; In order that the ink is not pressed out of the cells due to the deformation of the cell webs during doctoring, the deformation of the plastic fabric print layer be small compared to the depth of the cells. 'With common surface contact pressures of the doctor blade of about 25 kp / cm ² and a cup depth generally between 2 and 40 _/ um, it follows that the ball pressure hardness of the plastic printing layer (measured according to DIN 53 456) is generally greater than 10 N / mm ² should be. Depending on the printing inks used, the following, for example, have proven suitable for the production of the plastic printing layer: polyamides and photopolymerizable printing plates produced on a polyamide basis; polymerized unsaturated polyesters and photopolymer printing plates based on unsaturated polyester resins; linear saturated polyesters, such as polyethylene or polybutylene terephthalate; Polyformaldehyde; Polyimides and polyamideimides; Polyurethane paints, such as modified polyurethane or polyester paints; Melamine formaldehyde or phenol formaldehyde resins. In principle, less suitable plastics can also be used, provided they are provided with coatings made of, for example, siloxanes, polyimides or crosslinked polyurethanes to improve the chemical resistance and the sliding properties.

The use of the doctor blade according to the invention not only makes it possible to use a considerably larger number of plastics for the production of the plastic printing layer in gravure printing, but it can also have a 10 in comparison to the previous gravure printing processes which work with a plastic printing layer -fold or even greater improvement in the number of copies can be achieved without having to accept disadvantages in the printing properties. This makes it possible to use gravure printing, which uses plastic printing layers, economically wherever low print runs are required.

The invention is illustrated by the following examples:

Comparative experiment A

An intaglio printing plate was produced in a manner known per se from a photopolymer printing plate based on polyamide and printed with a printing machine from Albert, Frankenthal. As a doctor blade, a commercially available steel doctor blade in the form of stepped facets, as is customary for conventional gravure printing, was used without rounding off the bevel edges and without a hard metal coating. After 4000 revolutions of the cylinder, stripes were clearly visible in the printed image; after approximately 40,000 cylinder revolutions, a clear decrease in the tone density could be determined.

Comparative experiment B

A gravure form produced according to Example 1 was tested in an abrasion tester from Burda. Commercial steel doctor blades in stepped facet form (blade thickness 135 _/ µm, blade width 3 mm, blade thickness 225 _/ µm, chamfer grinding angle 60 to 65 ° or blade thickness 80 mm, blade width 1 mm, doctor blade thickness 165 _/ µm, chamfer grinding angle 60 to 65) also came as squeegees. without rounded bevel edges and without hard surface coating. After approximately 50,000 squeegee runs, the printing form showed deep scratches and a decrease in the volume of the cells.

Comparative experiment C

A gravure printing plate was produced from a polyformaldehyde (® Ultraform H 2320 from BASF) by engraving with a helioclischograph. Checking the Printing took place as in comparative test B. After approximately 50,000 squeegee passes, deep scratches and a decrease in the volume of the cells were observed.

example 1

The gravure printing form used in comparative test B was tested in the same abrasion tester from Burda, but this time the bevel edges of the squeegees were rounded and the lamellae of the squeegees were chrome-plated. After about 500,000 squeegee runs, the printing form was completely undamaged.

Example 2

A stepped facet blade of a multi-phase steel having crystallite structure was plated until a nickel layer of 8 _/ to was deposited. The nickel-plated doctor blade was carefully removed and rounded off and used in the abrasion tester from Burda. In this case too, the gravure form described in comparative experiments A and B was used. After about 500,000 squeegee runs, the printing form was undamaged except for one scratch.

Example 3

A gravure printing plate which had been produced in accordance with comparative test C was tested with a nickel-plated doctor blade as described in Example 2. After 500,000 squeegee passes, there was only a slight abrasion.

example

A stepped facet blade of a multi-phase steel having crystallite structure was plated electrodeposited hard, so that its surface about 5 _/ consisted of a to strong chromium layer. The chrome-plated doctor blade was pulled off and rounded off and inserted into the abrasion tester. A corresponding gravure form as described in comparative experiments A and B was used. After 500,000 doctor blade passes, the printing form was undamaged.

Example 5

A gravure form made from polyformaldehyde (comparative test C) was tested in an abrasion tester with the chrome-plated doctor blade described in Example 4. In this case too, the printing form was undamaged after 500,000 squeegee passes.

Claims (7)

1. Use of a doctor blade with rounded bevel edges, sufficient flexural elasticity and a surface hardness of at least 350 (Vickers hardness according to DIN 50 133) in gravure printing processes, in which plastic printing layers are used. 2. Use of a doctor blade according to claim 1, characterized in that the doctor blade has a rounded bevel. 3. Use of a doctor blade according to claim 1, characterized in that the doctor blade is ground according to the angle of attack on the plastic printing layer and the bevel edges are rounded with radii of curvature between 10 and 70 _/ um. 4. Use of a doctor blade according to claims 1 to 3, characterized in that the doctor blade consists essentially of steel. 5. Use of a doctor blade according to claim 4, characterized in that the doctor blade consists of a hard, fine-grained steel with spring steel properties. 6. Use of a doctor blade according to claim 4, characterized in that the doctor blade consists of a core made of a multi-phase steel and a hard, 1 to 20 _/ um thick surface layer. 7. Use of a doctor blade according to claim 6, characterized in that the surface layer of the doctor blade is formed from hard metals, hard metal alloys, hard carbides or hard ceramic materials.

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