EP0821074A1 - Process for producing a strip of an aluminium alloy for lithographic printing plates - Google Patents

Abstract

Manufacturing aluminium or aluminium alloy strip for electrolytic textured lithographic plates, comprises continuous strip casting, and rolling the cast strip to finished size with a minimum reduction of 90 % and without further heating. Also claimed is a lithographic plate manufactured by the above method.

Description

The invention relates to a method for producing a Band made of aluminum or an aluminum alloy for electrolytic roughened, lithographic printing plates, whereby the alloy is continuously cast into a ribbon and the cast strip is then rolled to its final thickness.

• Eine gleichmässige Oberfläche, die sich gut zur mechanischen, chemischen und elektrochemischen Aufrauhung eignet.
• Eine harte Oberfläche nach dem Anodisieren, was eine grosse Zahl von Abdrucken ermöglicht.
• Leichtes Gewicht.
• Niedrige Gestehungskosten.

Lithographic printing plates made of aluminum, which are typically about 0.3 mm thick, have advantages over plates made of other materials, only a few of which are mentioned here:

• A uniform surface that is well suited for mechanical, chemical and electrochemical roughening.
• A hard surface after anodizing, which enables a large number of impressions.
• Lightweight.
• Low production costs.

The article ALUMINUM ALLOYS AS SUBSTRATES FOR LITHOGRAPHIC PLATES by F. Wehner and R.J. Dean, 8th International Light Metal Conference Leoben-Vienna 1987, gives an overview of the production and properties of ribbons for lithographic Printing plates.

For the production of lithographic printing plates today mainly aluminum strips used by hot and Cold rolling - with the activation of an intermediate annealing - off Continuous cast ingots are manufactured. In recent years various attempts have been made strip cast aluminum materials to lithographic Process printing plates, while during the rolling of the cast strip to final thickness at least one intermediate anneal was switched on.

To achieve a uniform roughening in the electrolytic or electrochemical etching of the final thickness rolled strip is its near-surface microstructure vital.

So far it has not been possible to start from a cast tape a lithographic printing plate with one compared to conventional Improved printing plates manufactured by continuous casting Manufacture etching structure.

The invention is therefore based on the object of a method to create the type mentioned, in which the End-thick rolled strip for electrochemical attack has optimal microstructure.

The solution to the problem according to the invention is that Rolling to final thickness with a rolling degree of at least 90% and is carried out without further heat supply.

"Without further heat supply" here means that the cast Belt after leaving the casting roll nip until it is finished No heat is applied from outside to the final thickness becomes. If the cast tape, which after the exit from the casting roll gap during a certain period Time has a relatively high temperature, rolled to final thickness shortly after its manufacture, so the starting temperature during rolling, in particular with large strip thicknesses, be increased. With small strip thicknesses the processing to the final thickness corresponds to cold rolling without intermediate annealing.

The thickness of the cast tape is preferably max. 5 mm, in particular max. 4 mm. An ideal microstructure is achieved if the cast strip max. 3 mm, especially about 2.5 to 2.8 mm.

Basically, can be used to manufacture the cast tape Every strip casting process can be used, ideally rapid solidification with simultaneous hot forming in the casting roll gap is desired. The latter two The casting and rolling process, for example, has properties in which the alloy in the casting roll gap between chilled rolls is cast to the belt. By further processing of the cast strip remains by cold rolling the advantageous structure of the structure near the surface Areas preserved due to rapid solidification.

The continuous casting process enables at the same time high solidification speeds and very fine grain sizes in areas near the surface through dynamic recovery immediately after the cast tape emerges from the Casting nip.

The further processing of the cast strip is carried out by Reeling the cast tape into a desired bundle Size. In the subsequent processing step, that is Band in a suitable for the production of litho sheets Cold rolling mill to the desired final thickness of 150 - 300 µm rolled.

The one that solidifies in the casting roll gap and is partially hot-formed Tape is preferred to prevent grain coarsening no further heat added. Is the thickness of the cast tape but much more than 3mm, e.g. 7mm, so it may be necessary that on the cast tape before rolling to final thickness immediately after exiting a hot rolling pass is carried out in the casting roll gap. For Achieve an optimal structure and at the same time Minimization of processing steps that cause costs however, should be as thin as possible be cast that dispenses with a hot rolling pass can be.

Cold rolling without intermediate annealing leads to a strong cold deformed substructure with high dislocation density and thus the preferred microstructure that has a uniform electrochemical attack guaranteed during etching.

In addition to the advantage of a uniform attack, a tape produced according to the invention has excellent mechanical properties Properties, e.g. high strength, which also during the baking of a photosensitive varnish only insignificant in the production of lithographic printing plates decreases.

The tape produced according to the invention is equally suitable for etching in HCL and HNO ₃ electrolytes, the advantages of the microstructure obtained being particularly apparent when etching in an HNO ₃ electrolyte.

As an alloy for the production of the tape according to the invention can basically all for the production of lithographic Printing plates commonly used aluminum materials be used. Are particularly preferred Alloys from the AA 1xxx, AA 3xxx or AA series 8xxx.

After electrolytic etching in an HNO ₃ electrolyte, lithographic printing plates made from a tape according to the invention have an etching structure which is improved in comparison with conventionally produced printing plates and at the same time has lower energy consumption.

The advantage of a lithographic produced according to the invention Pressure plate compared to a conventionally manufactured one Plate also shows up in that after baking a photosensitive varnish, for example during 10 min at 250 ° C, the produced according to the invention Printing plate has a higher strength.

The above-mentioned advantageous microstructure in the area of the strip near the surface essentially results from the rapid solidification on the surface. As a result of the high solidification rate, the secondary phase particles are separated out in the microstructure in a very fine form and in high density. These particles act as the first points of attack for the etching, especially when the electrochemical roughening is carried out in an HNO ₃ electrolyte. When the band solidifies rapidly on the surface, the particles mentioned have an average distance of less than 5 μm and thereby form a coherent network of uniform surface attack points. Starting from these first points of attack, distributed uniformly and in high density over the entire strip surface, the growth of the actual three-dimensional roughness pattern begins. The small size of the intermetallic phases mentioned has the further advantage that they considerably shorten the time span of the electrochemical dissolution in the initial phase of the etching, as a result of which electrical energy can be saved. Since the rapid solidification according to the invention preferably causes imbalance phases to occur in the surface areas of the strip, the dissolution rate of the fine particles mentioned is likewise higher than the dissolution rate of the coarse intermetallic phases with an equilibrium composition as are present in conventionally processed materials.

Another essential microstructural feature of the The tape produced according to the invention is the small grain size, which are near the surface during strip casting Areas results. The high density of the puncture points the grain boundaries on the surface along with a high defect density in the grains themselves too chemical active targets for continuous education new dimples.

• Gleichmässige Aetzstruktur als Folge einer hohen Dichte möglicher Aetzangriffspunkte an der Oberfläche
• Aetzen in einem HNO₃-Elektrolyten unter kritischen, elektrochemischen Prozessbedingungen
• Ausdehnung der Aetzparameter in den Bereich niedriger Ladungsdichte und damit Einsparung von elektrischer Energie
• Verhinderung von Aetzfehlern in HNO₃-Elektrolyten als Folge unerwünschter Passivierungsreaktionen
• Bildung eines dichten Netzes von Rissen in der Oxidschicht im Passivitätsbereich des anodischen Potentials durch eine hohe Dichte kleiner intermetallischer Partikel mit Ungleichgewichtsstrukturen
• Bildung eines dichten Netzes von Fehlstellen in der natürlichen Oxidschicht im Passivitätsbereich des anodischen Potentials als Folge einer geringen Korngrösse mit vielen Korngrenzendurchstosspunkten in der Oxidschicht.

The above-described microstructure in the strip surface leads to a substantial improvement in the electrochemical etching process in order to form the uniform roughness pattern required in lithographic printing plates. The advantages that result when using the tape produced according to the invention are the following:

• Uniform etching structure as a result of a high density of possible etching points on the surface
• Etching in an HNO ₃ electrolyte under critical, electrochemical process conditions
• Expansion of the etching parameters in the area of low charge density and thus saving of electrical energy
• Prevention of etching errors in HNO ₃ electrolytes as a result of undesired passivation reactions
• Formation of a dense network of cracks in the oxide layer in the passivity range of the anodic potential due to a high density of small intermetallic particles with imbalance structures
• Formation of a dense network of defects in the natural oxide layer in the passivity range of the anodic potential as a result of a small grain size with many grain boundary penetration points in the oxide layer.

Fig. 1 und 2

die Aetzstruktur konventionell hergestellter Druckplatten;

Fig. 3

die Aetzstruktur einer erfindungsgemäss hergestellten Druckplatte.

The advantageousness of a strip material produced according to the invention compared to a strip material from conventional production results from the test results summarized below on the surface condition of the strips, which - as mentioned above - decisively influences the etching behavior. The etching behavior of printing plates produced according to the invention compared to conventional printing plates is explained using two examples and documented by SEM images; it show at 1000x magnification

1 and 2

the etching structure of conventionally produced printing plates;

Fig. 3

the etching structure of a printing plate produced according to the invention.

The served as material for the comparative experiments Alloy AA 1050 (Al99.5). The conventional manufacturing a tape with a final thickness of 0.3 mm was made over conventional Continuous casting with an intermediate annealing at a Thickness of 2.5 mm and then cold rolling to final thickness.

For the production of a tape of 0.3 mm according to the invention Thickness was initially between the casting rolls of a belt casting machine cast a tape with a thickness of 2.5 mm. The strip cast in this way was then subjected to no intermediate annealing Cold rolled to final thickness.

The following number of particles of intermetallic phases per unit area was determined in the surface of the sheets with the final thickness of 0.3 mm: Band casting material: 6250 particles / mm ² Continuous casting material: 3400 particles / mm ²

The same measurement in the near-surface cross section led to the following result: Band casting material: 74000 particles / mm ² Continuous casting material: 17500 particles / mm ²

In both cases, the phases were AlFeSi-containing, their size and distribution during solidification in the areas close to the surface due to very different Solidification speed is given. The higher areal density the cross-sectional particle results from the Flattening the grains.

The second essential structural parameter, the grain size, was determined at the intermediate thickness of 2.5 mm. It should be noted here that the strip casting material is actually in a slightly deformed casting state, whereas the continuous casting material is in this thickness after soft annealing in the recrystallized state. In this way, two representative grain sizes are compared with each other, since the same rolling degrees to the final state are made from this thickness. The measurement of the number of grains per unit area on the belt surface or in the near-surface cross section led to the following results: surface cross-section Band casting material: 20000 grains / mm ² 48000 grains / mm ² Continuous casting material: 250 grains / mm ² 520 grains / mm ²

The fine grains of the tape casting material are mostly attributable to subgrain formation, their average size is about 5 µm, while the recrystallization grain after Coil annealing in conventional production is a medium one 70 µm in size.

As mentioned above, the continuous casting strip and the strip cast according to the invention are further processed by cold rolling to the desired final thickness of the litho strip in the thickness range from 0.2 to 0.3 mm. An essential property of the litho tape now results from the subsequent process step, the electrochemical roughening, which should produce the most uniform etching structure on the surface. On the one hand, electrolytes from dilute hydrochloric acid (HCL) and on the other hand electrolytes from dilute nitric acid (HNO ₃ ) are used.

If an etching is now carried out in the nitric acid electrolyte, practice shows that a uniform etching structure can only be achieved with certain etching parameters is. For example, for economic reasons too little charge (coulomb) is used, so the result is irregular etching with mostly stripe-like Places where there was no attack. Will be under etched under these critical conditions, all fine come Differences in the structure of the substrate (litho tape) to the fore and you can get a classification of the used Determine litho materials.

The reason for the sensitivity of the HNO ₃ electrolyte to the electrochemical etching behavior of aluminum lies in its anodic passive area (passive oxide) and the associated difficult nucleation of etching pits. Only at an anodic breakdown potential of +1.65 V (SCE) will this passive area be overcome by the formation of etching pits, while the pit formation in HCL already starts at the corrosion potential of -0.65 V (SCE). With anodic current load in the HNO ₃ electrolyte, this means that the intermetallic phases present in the structure are first dissolved in the potential range -0.5 to -0.3 V (SCE) before the aluminum matrix is attacked and "pitting" arises. The distribution of this intermetallic phase now forms a first network of pits on the etched surface and it is therefore important in which area density these phases are present on the surface.

The structure improved according to the invention is now obviously because of the high surface density of intermetallic Phases many initial points of attack on the still passive aluminum surface.

In the same sense, the second structural improvement is the fine grain. Grain boundaries are always weak points in the natural oxide film of aluminum. The finer the grain, the more imperfections are in the overlying Produces oxide film and the higher the nucleation rate for the emergence of dimples.

The example according to the invention will now be improved using two examples Etching behavior demonstrated:

example 1

Electrolyte: 20 g / l ENT ₃ 1 g / l Al Room temperature Substrate material: AA 1050, identical composition in both cases Counter electrode: Sample material

Conventionally produced litho sheet required a charge quantity of at least 480 coulomb / dm ² measured at constant voltage, corresponding to an etching time of 60 sec with an initial current density of 20 A / dm ² , in order to produce a uniform etching structure.

In contrast, the litho sheet produced according to the invention only required a charge of 360 coulombs / dm ² for the formation of a homogeneous etching structure. The initial current density was 17A / dm ² and the etching time was 55 seconds.

Example 2

The etching structures produced in the same electrolyte and under conditions as in Example 1 showed the following behavior as a function of the amount of charge used, documented by the images in FIGS. 1 to 3: 1: 450 coulombs / dm ² , conventionally produced litho sheet 2: 410 coulomb / dm ² , conventionally produced litho sheet 3: 380 coulombs / dm ² , Litho sheet produced according to the invention

Claims (10)

Process for producing a strip from aluminum or an aluminum alloy for electrolytically roughened, lithographic printing plates, the alloy being continuously cast into a strip and the cast strip subsequently being rolled to final thickness,
characterized in that
rolling to final thickness with a rolling degree of at least 90% and without further heat is carried out. A method according to claim 1, characterized in that the cast strip to final thickness without intermediate annealing is cold rolled. A method according to claim 1 or 2, characterized in that that the thickness of the cast tape is max. 5 mm, preferably max. 4 mm. A method according to claim 3, characterized in that the thickness of the cast tape is max. 3 mm, preferably about 2.5 to 2.8 mm. Method according to one of claims 1 to 4, characterized in that that the alloy in the casting roll gap between cooled rolls of a strip casting machine cast on the strip and processed by cold rolling is, so that the advantageous structure in the areas near the surface due to rapid solidification preserved. A method according to claim 5, characterized in that partially solidified in the casting roll gap thermoformed tape to avoid grain coarsening no further heat is added. A method according to claim 5, characterized in that on the cast strip before rolling to final thickness immediately after leaving the casting roll gap Hot rolling pass is carried out. Method according to one of claims 1 to 7, characterized in that that an alloy from the series AA 1xxx, AA 3xxx or AA 8xxx is cast to the tape. Lithographic printing plate with an electrolytically roughened surface, characterized in that it is made from a tape produced by the process according to one of claims 1 to 8 and, after electrolytic etching in an HNO ₃ electrolyte, an etching structure which is improved in comparison with conventionally produced printing plates and at the same time is less Has energy consumption. Lithographic printing plate with electrolytically roughened Surface, characterized in that it from a with the method according to one of claims 1 up to 8 manufactured tape is made and after baking of a photosensitive varnish in comparison to conventionally produced printing plates higher Has strength.

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