DE19823790A1 - Multigrain aluminum or alloy strip used for making lithographic plate base and offset printing plate - Google Patents

Abstract

Aluminum (Al) or Al alloy lithographic strip, which is 0.1-0.5 mm thick, has a surface structure of unaligned fibrous hollows with a width/length ratio of at least 1/3. An Independent claim is also included for the production of the strip.

Description

The invention relates to a litho tape, printing plate support and a Process for the production of a litho tape, a printing plate carrier or an offset printing plate.

Carrier for offset printing plates (lithographic printing plates) who which are made from rolled aluminum strips as standard. Pure aluminum (AA1050) and alloys are used as materials of the type AlMn1 or AlMn1Mg used (AA3003, AA3103, AA3005) used. The raw material for the tapes is after continuous casting rectangular rolled bars manufactured up to 600 mm thick. After the casting skin has been milled off, the bars are heated and Cold formed into thin strips; cold rolling can without, with one or more intermediate anneals. The The finished strip has a final thickness of 0.1-0.5 mm and will referred to as litho tape.

Litho tape is processed into printing plate supports by the belt surface is roughened; they are mechanical, che mixing, electrochemical / EC roughening or combinations known of these procedures. Roughening has optimal properties structures with rough, fine and finest roughness overlay (multigrain); generated in a first step coarse structures are also called "pre-roughening". To increase the abrasion resistance, electro chemically roughened anodized litho bands, d. H. the Roughening structure is achieved by a thin, hard oxide layer protects. The oxide layer is often given a special after-treatment to improve the hydrophilic properties in printing  process. The roughened, anodized litho tape is used as a print plate carrier called.

By applying a photosensitive layer / photo layer becomes an offset printing plate from the printing plate carrier (Lithographic printing plate). The rough surface takes over of the printing plate carrier important functions: 1.) for the port processing of the photo layer and 2.) for the water flow in the following Printing process. The roughness determines u. a. the lifespan of Printing plate. - The printing plates are exposed, developed and used in the printing press.

The lithium tape used as the raw material for the printing plate carrier has a standard mill-finish surface. Mill-finish surfaces are characterized by rolling grooves parallel to the rolling direction, which are created by pressing the ground steel roller into the Al strip; a precise description of this surface is given in Comparative Example 1, cf. Fig. 2a. The roughness of mill-finish surfaces is in the range R _a = 0.10-0.30 µm (measurement with a mechanical measuring head, DIN 4768), cf. Table 1. Mill finish surfaces have a flat, directional structure.

The litho tape is further processed into a printing plate carrier by mechanical, chemical and / or electrochemical (EC) roughening of the tape surface. In order to achieve the "mul tigrain" structure described, the rough structure is first produced on the strip surface by mechanical roughening or "pre-roughening". Pre-roughening offers these advantages:

• - The directional mill-finish surface of the litho tape completely destroyed,
• - Due to the existing rough structure, the fol only as much electrical energy be used that fine and finest structures can form.

A common process for making pre-roughened lithobanks  that's wet brushing. This is done in a separate Manufacturing step after rolling.

Patent WO 97/31783 (Alcoa) describes a way of producing pre-roughened litho strips in the rolling process. According to this method, a roller - with EDT, laser beam, electron beam, etc. - is treated so that it receives a microscopically uniform, non-directional roughness of R _a = 0.56-0.76 µm (22-30 microinches). At the last roll pass, the topography of the roll is transferred to the Al strip surface; Thickness reductions of 0-15% are specified; the roughness on the aluminum strip is then R _a = 0.33-0.43 µm (13-17 microinches). The roughness of a pressure plate carrier produced by the Alcan process is comparatively flat with Ra <0.45 µm for pre-roughening. Another disadvantage of the Alcoa process is that an exact embossing can only take place with an uneconomically small thickness reduction of <1% - the person skilled in the art knows that a 100% transfer in combination with a rolling pass is technically not possible. If rolling and embossing are carried out in one work step, the top surface of the strip always extends in the rolling direction - even with low degrees of rolling <10%. According to this method, a relatively flat and directed structure is obtained on the printing plate carrier.

Another way to manufacture pre-roughened litho tapes in the rolling process describes the patent WO 95/08408 (Alcan International). The pre-roughening is achieved in that two aluminum strips lying on top of each other are rolled together become (double rolling process); this creates on the band-in a corrugated surface on the sides; after separating the tapes the inside / matt sides as pre-roughened litho tape used and electrochemically in a subsequent step roughened.

The pre-roughening on the inside of the strip is caused by the ribs / transverse folds created during double rolling perpendicular to the rolling direction; a detailed description of this surface is given in Comparative Example 2, cf. Fig. 3a. The roughness obtained by the double rolling process is specified in the document WO95 / 08408 with R _a = 0.78-0.80 µm (patent specification page 12, table 2).

The values were determined using a non-contact, optical measurement (patent specification page 9). Optical roughness measurement is a relatively new method of measurement that has not been established in practice in the production of litho tape and printing plate supports. Experience in round robin tests has shown that the values obtained in the optical roughness measurement are device-dependent - this is not the case with mechanical measurements. Standard measurements are therefore usually carried out with a mechanical measuring head according to DIN 4768. It should also be noted that optically and mechanically measured roughness values are not comparable: when using the mechanical measuring head, the values - with the same sample - are generally only about half as large as with an optical measurement. In relation to the information in the patent specification, we have determined that the mechanical roughness of a corrugated litho tape matt side is significantly below R _a = 0.5 µm. That is, the matt sides created in the double-rolling process have a flat, directional structure - similar to mill-finish surfaces.

From the magazine "Kobelco Technology Review", No. Oct 12 1991 pages 60-64, FIGS. 4 and 6 and Table 4 show that the surface roughness R _{a is} between 0.2 and 0.5 μm, mainly between 0.3 and 0.4 μm. In Fig. 3 as the inner surface is deformed in the last rolling pass is shown. Wavy corrugations are formed on the mat side, which run transverse to the rolling direction.

The object of the present invention is to perform an omnidirectional pre-roughening in a manufacturing step integrated in the cold rolling process and thus to provide a pre-roughened litho band and a printing plate support in a “multigrain structure” with a roughness level of R _a = 0.5-2 μm the subsequent EC roughening enables significant energy savings.

This object is achieved by the in the patent characteristics specified resolved. It has been shown that by embossing a litho tape with one made of three dimensions nalen, undirected, fibrous structures existing pre a special story by using an intermediate layer advantageous pre-roughening of the litho tape can be achieved, which according to the description according to claim 5e) in the last Cold rolling pass is generated.

The fiber direction shows a random distribution, so that the with their help, indentations created a random distribution have with respect to their longitudinal axes.

In contrast to the known method of double rolling shows the structure of the litho tape according to the invention no prev direction in relation to the rolling process. The elongated Depressions, which preferably have a length <100 μm, have a "statistical distribution".

The process for the production of the printing plate carrier consists in selecting an aluminum material of the type pure aluminum, AlMn, AlMnMg, AlMg, casting a rolled ingot from it and homogenizing it in the temperature range of 500-600 ° C, hot-rolling the bar and, if necessary, cold-rolling with intermediate annealing, that To split the strip before the last cold rolling pass, to lay it on top of each other and to provide it with an intermediate layer that has a granular or fibrous structure; during the last rolling pass, the inside of the superimposed strips in contact with the intermediate layer are embossed with an undirected structure and roughness values in the range R _a = 0.5-2 µm; after a subsequent EC roughening, the surface of the printing plate carrier has a multigrain structure.

With the integration of pre-roughening in the production of litho strips can be used in an economical process and after  following also printing plate supports with a high surface quality be produced.

Since the surface quality is essentially the same of the intermediate layer and not like the Alcan-Ver drive to the rolling pair required for double rolling meters, the method according to the invention is less susceptible against operational fluctuations. This allows the Fer production of large-scale products particularly economically and with consistent quality.

Execution and comparative examples

The invention is explained in more detail below with the aid of several exemplary embodiments and comparative examples. All examples are based on the same basic material, namely an Al99.5 strip, which was made from a continuous cast ingot by hot and cold rolling without intermediate annealing. The strips differ in the different handling of the last cold rolling pass, in which the following surfaces were created:

• - mill finish surface according to standard (example 2)
• - One manufactured according to the Alcan patent by double rolling belt (example 3)
• - A printing plate carrier manufactured according to the invention (in game 1) without intermediate annealing. The involvement of a Zwi ash annealing for recrystallization of the cold-rolled strip has proven to be useful in cases where Intermediate layers with pronounced coarse structures are used should be. Presumably the beneficial effect of the Re crystallization attributed to the figure of the fibers in an intermediate annealed band follows and also the finest fiber tips of each ver fiber structure used.

The strips were electrochemically roughened in the ENT ₃ bath with various charges (C / dm ² ) and the resulting surfaces were characterized in three states:

• 1. a.) Surface before EC roughening
• 2. b.) Surface after EC roughening in ENT ₃ with a charge of 800C / dm ²
• 3. c.) Surface after EC roughening in ENT ₃ with a charge of 1600C / dm ² .

The characterization was done by an image in the Rastermi microscope (REM / SE mode) and a mechanical roughness measurement (DIN 4768).

Inventive example 1: printing plate support with fibrous upper surface structure

Printing plate supports were produced by embossing a litho tape in the last cold rolling pass using a structured intermediate layer. Various types of paper, including waxed paper and SiC sandpaper (grit 1200) were used as an intermediate layer. The fibrous or granular structure of the intermediate layer is transferred to the Al substrate during rolling; Fig. 1a shows an example of the structure of a printing plate carrier, which is created using tissue paper at a rolling rate of 6%. The surface has a mixture of coarse and fine scars, the structure is obviously unoriented; the mechanically measured roughness is R _a = 1 µm; see. Table 1. Depending on the type of paper used, roughness can be set in the range R _a <0.5 µm-2 µm, cf. Table 2.

Surprisingly, the one produced according to the invention Litho tape found no rib or cross fold structure. This is probably due to the fact that the ver turned liner both in the rolling direction and in the Cross-roll direction has a minimum elongation of 5%. Thereby it appears possible to use the one used in the last cold rolling pass Liner exactly without wrinkling on the facing Show the top of the band by embossing.

If the printing plate carrier is roughened electrochemically in ENT ₃ so that a widespread attack occurs (charge approx. 800 C / dm ² ), the roughness remains at the initial level - in the present example at R _a = 1 µm: the undirected scar structure becomes fine Pickling troughs superimposed ( Fig. 1b), which themselves hardly contribute to the - mechanically measured - roughness. With an EC roughening with a high charge (1600 C / dm ² ), as expected, coarser, deeper pickling troughs occur ( Fig. 1c), the R _a value remains unchanged at 1 µm. In this case, in order to produce a roughness Ra <0.5 µm, a high charge is unnecessary during the electrochemical roughening, since the carrier brings with it the rough roughness - before the EC roughening; the structure can only be supplemented by a full surface roughening with little charge (800 C / dm ² ). - The relationship between the charge density during the EC roughening and the roughness achieved is shown in diagram D1.

The printing plate support according to the invention is characterized in that its roughness R _a <0.5 μm-2 μm remains unchanged in the range of the charges used in the roughening of the EC. A surface-covering fine / finest roughening structure can be superimposed on the printing plate support; the use of electricity for the production of a coarse pickling bowl structure is saved. The result is a topography with a mixture of coarse, fine and finest roughness (multigrain ").

Comparative Example 1: mill finish surface

Mill-finish surfaces are created during standard rolling and have characteristic grooves in the rolling direction, which are created by pressing the ground steel roller into the Al strip. As a further feature, there are fine transverse cracks (hydrodynamic lubrication pockets) on the rolling surface perpendicular to the rolling direction, the origin and number of which depends, among other things, on the rolling parameters and the nature of the rolling oil. The roughness of mill-finish surfaces is in the range R _a = 0.10-0.30 µm (measurement with a mechanical measuring head, mean value from 3 measuring directions 0 °, 45 °, 90 ° to the rolling direction)), cf. Table 1. The flat, directional structure of a mill-finish surface is shown in Fig. 2a.

If the mill-finish surface of a litho tape is roughened electrochemically in ENT ₃ so that a surface attack occurs (charge approx. 800 C / dm ² ), the roughness remains unchanged at a low level with R _a <0.5 µm: the Roll grooves and cross cracks are overlaid by hollows; the troughs themselves are so fine at this stage that they do not contribute to the - mechanically measurable - roughness; Fig. 2b. In order to produce a printing plate carrier with higher roughness, significantly more charge is required - for a roughness level around R _a = 0.5 µm, approximately 1600 C / dm ² . The roughness is achieved electrochemically by coarsening the Mul den structure, cf. Fig. 2c. The relationship between the charge density during EC roughening and the roughness achieved is shown in diagram D2.

Comparative Example 2: pre-roughened litho tape according to Alcan patent

The tape samples were produced according to the patent WO 95/08408 mentioned by the double-rolling process. Two directions can be seen on the corrugated inside of the strip: remnants of rolling grooves of the original mill-finish surface parallel to the rolling direction and the ribs / transverse folds created by double rolling perpendicular to the rolling direction, see p. Fig. 3a. The roughness is R _a = 0.15-0.20 µm (mechanical measuring head, mean value from 3 measuring directions 0 °, 45 °, 90 ° to the rolling direction), see. Table 1. That is. the matte sides created in the double rolling process have a similarly flat, aligned structure to mill-finish surfaces.

The tape also behaves similar to a conventional litho tape with a mill-finish surface when roughening the EC. With a comprehensive HNO ₃ roughening (charge approx. 800 C / dm ² ), the roughness remains unchanged at a low level with R _a <0.5 µm: the ribs / transverse folds are overlaid by a fine pickling tray structure, which only makes a small contribution to - mechanically measurable - roughness; Fig. 3b. In order to produce a printing plate carrier with higher roughness, significantly more charge is required - for a roughness level around R _a = 0.5 µm approximately 1600 C / dm ² ; the roughness is produced electrochemically by coarsening the pickling tray structure, see . 3c Fig. The relationship between the charge density with EC roughening and the roughness achieved is shown in Diagram D3.

In contrast to the Alcan process, in which transverse folding is vertical to the direction of rolling will occur when printing according to the invention a rough topography with an undirected structure generated. This is due to the use of an intermediate layer that have a fibrous and / or granular structure points, the roughness values of these coarse structures by one Order of magnitude above that according to Alcan patent WO 95/08408 generated roughness.

The advantage of the method according to the invention results from the Ver same as diagrams D1, D2, D3 (roughness depending on the Amount of charge) clearly. On saving electricity at the EC Roughening results in additional Ko when using the method Cost savings through lower consumption of etching chemicals lien and smaller amounts of waste.  

Table 2

Roughness values of aluminum rolling surfaces (mean values from 3 measuring directions 0 °, 45 °, 90 ° to the rolling direction)

Claims (14)

1. Litho tape made of aluminum or an aluminum alloy with a thickness of 0.1-0.5 mm, characterized by a surface structure of undirected, fibrous Ver recesses, the recesses having a width / length ratio of at least 1/3. 2. Litho tape according to claim 1, characterized in that the surface has an average roughness of R _a = 0.5-2 µm, measured according to DIN 4768 with a mechanical measuring head. 3. Litho tape according to one of the preceding claims, characterized in that the center roughness R _{a is} between 1 and 1.5 µm. 4. printing plate support for offset printing plates, produced using a litho tape according to one of the preceding claims, characterized in that the printing plate support has a surface with a multi-grain structure consisting of

• 1. fibrous depressions, the longitudinal axes of which are subject to a random distribution
• 2. a superimposed etch-well structure with a well diameter D = 0.5-15 µm.

5. A method for producing a litho tape, characterized in that

• a) an aluminum alloy of the type pure aluminum, AlMn, AlMnMg or AlMg is selected;
• b) the alloy is cast into a rolled ingot, homogenized at temperatures of 500-600 ° C and cooled to a temperature of 400-550 ° C at a minimum cooling rate of 25 ° C / h;
• c) the ingot is hot-rolled to form a hot strip of thickness 2-5 mm, the metal temperature during winding of the strip being at least 280 ° C;
• d) the hot strip is reduced to 1.5-1.1 times the final thickness by multiple cold rolling;
• e) before the last cold rolling pass, two or more strips manufactured according to a) to d) are placed on top of one another and provided with intermediate layers which have a granular and / or fibrous structure with an average roughness Ra <0.5 μm, and that when the end is reached In contact with the intermediate layer, a three-dimensional, non-directional embossing of the aluminum surfaces is created on the inside of the strips that are placed one above the other.

6. Process for producing a printing plate support under Use of a litho tape according to the previous An saying characterized, that following steps a) -e) in another Process step f) with an electrochemical roughening subsequent anodization and / or hydrophilization by to be led. 7. Process for making an offset printing plate under Use of a printing plate support according to one of the previously claims, characterized, that after performing steps a) to f) a coating tion with a light-sensitive layer. 8. A method for producing a printing plate according to claim 7, characterized, that a thermal coating takes place. 9. The method according to any one of the preceding claims, characterized, that in process step d) with an intermediate thickness of at least 2 times the final thickness of an intermediate annealing to the right Crystallization of the cold-rolled strip takes place.   10. The method according to any one of the preceding claims, characterized, that used in process steps d) and e) Work rolls have a roughness value Ra <0.5 µm. 11. The method according to any one of the preceding claims, characterized, that paper, textiles and / or solid par substrates containing particles can be used. 12. The method according to any one of the preceding claims, characterized, that the intermediate layer absorbs an elongation of 3-15%. 13. The method according to any one of the preceding claims, characterized, that the intermediate layer has a thickness of 0.05-0.2 mm. 14. The method according to any one of the preceding claims, characterized, that the liner fibrous three-dimensional structure with a width / length ratio of at least 1/3 having.