EP2067871B2 - Aluminium strip for lithographic pressure plate carriers and its manufacture - Google Patents

Description

The invention relates to a method for producing aluminum strips for lithographic printing plate supports, the aluminum strip being produced from a rolling ingot which, after optional homogenization, is hot-rolled to a thickness of 2 mm to 7 mm and to a final thickness of 0.15 mm to 0.5 mm is cold rolled. Very high demands are placed on the quality of aluminum strips for the production of lithographic printing plate supports. The aluminum strip for the production of lithographic printing plate supports is usually subjected to electrochemical roughening, which should result in roughening over the entire area and a structureless appearance without streaky effects. The roughened structure is important for applying a photosensitive layer, which is then exposed. The photo layer is baked at temperatures of 220 °C to 300 °C and annealing times of 3 to 10 minutes, with typical combinations of baking times representing, for example, 240 °C for 10 minutes, 260 °C for 6 minutes and 260 °C for 4 minutes. The printing plate support must lose as little strength as possible after baking, so that it can still be easily handled and easily clamped in a printing device. At the same time, the printing plate carrier and thus also the aluminum strip to be produced must have the highest possible reverse bending strength, so that plate tearing due to mechanical stress on the printing plate can be almost completely ruled out. So far, these requirements have been met with conventional aluminum coils be fulfilled. To increase productivity, however, printing presses are increasingly being used, which require that the printing plate supports be clamped in such a way that they are bent transversely to the rolling direction and are therefore also mechanically loaded transversely to the rolling direction. At the same time, large lithographic printing plate supports are becoming more difficult to handle as size and strength levels remain the same.

For example, from the applicant's European patent EP 1 065 071 B1 a band for the production of lithographic printing plate supports is known, which is characterized by good roughenability combined with high flexural fatigue strength and sufficient thermal stability after a baking process. Due to the increasing size of the printing machines and the resulting increase in the number of printing plate supports required, however, it has become necessary to further improve the properties of the known aluminum alloy and the lithographic printing plate supports produced from it. A simple increase in the tensile strength, which is possible, for example, by changing the aluminum alloy, did not lead to the desired success, since the correction of the coil set of the aluminum strip became more difficult with high tensile strength. This is usually carried out in the as-rolled state before the stoving process.

Proceeding from this, the present invention has for its object to provide a method for producing an aluminum strip for lithographic printing plate supports to make available, the coil set is easily corrected in the rolled condition and from which oversized printing plate carriers can be produced, which are easy to handle and show only a low tendency to plate tearing.

According to the present invention, the task outlined above is achieved in terms of the method in that the aluminum strip consists of an aluminum alloy with the following alloy components in percent by weight: 0.3% ≤ Fe ≤ 0.4%, 0.3% ≤ Mg ≤ 0.4%, 0.05% ≤ Si ≤ 0.25%, Mn ≤ 0.05%, Cu ≤ 0.04%,

remainder Al and unavoidable impurities, individually max. 0.05%, in total max. 0.15%; the hot rolling is carried out at a temperature of 250 °C to 550 °C, the hot strip finishing temperature being 280 °C to 350 °C, during the cold rolling an intermediate annealing is carried out at a thickness of 1.5 mm to 0.5 mm, during the intermediate annealing, the metal temperature is 200 °C to 450 °C and the aluminum strip is kept at the specified metal temperature for at least one to two hours, the aluminum strip is then cold-rolled to a final thickness of 0.15 mm to 0.5 mm and for further processing is wound up to form a lithographic printing plate support in the as-rolled condition.

The aluminum strip produced by the method according to the invention provides a moderate increase in strength together with a very high resistance to flexural fatigue and, at the same time, very good thermal stability. Coil set corrections are possible without difficulty due to the moderate increase in strength. At the same time, however, the handling of the printing plate, even in the burned-in state, is simple, for example when it is clamped in the printing press, since good thermal stability of the aluminum strip is obtained with the method according to the invention. When the aluminum strip is used for the manufacture of very large lithographic printing plate supports, it is preferable to cold-roll the aluminum strip to a final thickness of 0.25 to 0.5 mm after the intermediate anneal. The particular suitability of the aluminum strips produced by the process according to the invention for oversized lithographic printing plate supports results from the fact that, on the one hand, due to the low degree of rolling after the intermediate annealing, high elongation is made available and, on the other hand, the increased magnesium content provides higher strength through hardening, which simplify handling.

According to one embodiment of the present invention, the properties according to the invention can be achieved in a particularly process-reliable manner in that the aluminum alloy additionally has a titanium (Ti) content of max. 0.05% by weight, preferably max. 0.015% by weight, a zinc( Zn) content of max. 0.05% by weight and a chromium (Cr) content of less than 100 ppm, preferably a Cr content of max. 50 ppm. Titanium is commonly used for grain refinement in casting. An increased Ti content however, leads to casting problems. Zinc influences the roughenability, so that its content should not exceed 0.05% by weight. Typical problems arise with increased Zn content due to inhomogeneities when roughening the lithographic printing plate support. Chromium inhibits recrystallization and should therefore only be contained in the aluminum alloy in very small proportions of less than 100 ppm, preferably a maximum of 50 ppm.

The aluminum alloy for use in the method according to the present invention has a Mg content of 0.3 to 0.4% by weight. As a result, maximum strength with high flexural fatigue strength can be made available. Higher Mg contents enable a further reduction in the degree of rolling after intermediate annealing while at the same time maintaining or increasing the tensile strength values, especially transverse to the rolling direction.

By setting the hot rolling temperatures in the range from 250 °C to 550 °C, with the hot strip final temperature being 280 °C to 350 °C, a continuous recrystallization of the surface is achieved during hot rolling, which, for example, allows the wall surface to be easily roughened during the production of the lithographic Printing plate carrier guaranteed.

According to the invention, the metal temperature of the aluminum strip is 200° C. to 450° C. during the intermediate annealing. The aluminum strip is then held at metal temperature for at least one to two hours. This is usually done in batch ovens. Through the intermediate annealing in the temperature range mentioned, the Further processing of the aluminum coil can take place either in the recovered or recrystallized state or a combination of both. Recrystallization begins at temperatures of around 300 to 350 °C, depending on the production parameters, in particular the hardening that has been introduced. A recovery anneal at lower temperatures, on the other hand, can only reduce the hardening, so that very low degrees of rolling are possible after the recovery anneal. However, depending on the degree of rolling after intermediate annealing and the composition of the alloy, it may also be necessary to perform recrystallization annealing as an intermediate anneal.

An aluminum strip produced using the method according to the invention consists of an aluminum alloy with the following alloy components in % by weight: 0.3% ≤ Fe ≤ 0.4%, 0.3% ≤ Mg ≤ 0.4%, 0.05% ≤ Si ≤ 0.25%, Mn ≤ 0.05%, Cu ≤ 0.04%,

remainder Al and unavoidable impurities, individually max. 0.05%, in total max. 0.15%; In addition, the aluminum strip has a tensile strength of up to 200 MPa in the as-rolled condition along the rolling direction and after a baking process at a temperature of 240 °C and a duration of 10 minutes of at least 145 MPa as well as a flexural fatigue strength transverse to the rolling direction of at least 1850 cycles in the flexural fatigue test on.

In the reverse bending test, a strip is cut out of the aluminum strip and bent back and forth between two cylindrical segments with a radius of 30 mm. In contrast to the aluminum strips previously produced for lithographic printing plate supports, the aluminum strips according to the invention achieve reverse bending cycles of more than 1850 also transverse to the rolling direction after a baking process, which means an increase of over 70% compared to the standard alloys previously used. Due to the moderate increase in tensile strength to values of up to 200 MPa in the as-rolled state, measured along the rolling direction, the coil set of the aluminum strip according to the invention can also be corrected in a simple manner. Due to the good thermal stability, which is shown by a tensile strength of at least 14b MPa after a baking process in the longitudinal or transverse direction to the rolling direction, the handling of the lithographic printing plate base made from the aluminum strip is good even after a baking process. Even with very large lithographic printing plate supports, the increased strength after baking can make the printing plates easier to handle. In addition, the high number of possible reverse bending cycles greater than 1850 both in the as-rolled and in the baked state of the aluminum strip according to the invention shows that the tendency to plate tearing due to mechanical loads is low in lithographic printing plate supports clamped transversely or longitudinally to the rolling direction.

The aluminum strip produced by the process according to the invention with an Mg content of 0.3% by weight to 0.4% % by weight also enables particularly high elongation values in the as-rolled state with sufficiently high tensile strength values, since the necessary strength values are already achieved with low degrees of rolling after intermediate annealing.

The properties of the finished aluminum strip are reliably achieved in that the aluminum alloy has a Ti content of max. 0.05% by weight, preferably max. 0.015% by weight, a Zn content of max. 0.05% by weight. -% and a Cr content of less than 100 ppm, preferably of max. 10 ppm.

From aluminum strips produced according to the invention with a thickness of 0.25 to 0.5 mm, oversized printing plate supports can be produced particularly well and processed and handled in a simple manner.

There are now a large number of possibilities for further developing and designing the method according to the invention for the production of aluminum strips for lithographic printing plate supports. For this purpose, reference is made on the one hand to the patent claims subordinate to patent claim 1 and to the description of exemplary embodiments in connection with the drawing. In the drawing, the only figure shows a schematic representation of the flexural fatigue test for testing the flexural fatigue strength.

A comparison between a conventional aluminum tape for the production of lithographic printing plate supports and an aluminum tape produced according to the invention and two comparative aluminum tapes, which are also used for suitable for the manufacture of lithographic printing plate supports is set out below. The alloying components of the different aluminum coils tested are set out in Table 1. Tab.1 legs -No. feet Mn mg si Cu /wt -% Vref 0.32 - 0.17 0.12 7 ppm State of the art V582 0.36 0.0034 0.3 0.09 3 ppm invention V581 0.36 0.018 0.2 0.08 5 ppm comparison V580 0.4 0.10 0.11 0.08 < 10ppm comparison

Table 1 only shows the essential alloying components of the aluminum strips examined. In addition, the various test alloys had a Ti content of less than 0.015% by weight, a Zn content of less than 0.05% by weight and a Cr content of less than 100 ppm. The rolling ingots cast from the various aluminum alloys were subjected to homogenization before rolling, with the rolling ingots being annealed at a temperature of around 580°C for more than four hours. This was followed by hot rolling at temperatures of 250 °C to 550 °C, with the final hot strip temperature being between 280 °C and 350 °C. The aluminum hot strip of alloy VRef was subjected to an intermediate anneal during cold rolling at a thickness of 2 to 2.4 mm, the cold rolled strip being exposed to a temperature of 300 to 450 °C for one to two hours. At the same interanneal temperatures, the interanneal thickness for the other aluminum strips was only 0.9 to 1.2 mm, as can also be seen from Table 2. Since the intermediately annealed strips were further cold-rolled to the final gauge without a final final anneal, they were coiled in the as-rolled condition. Table 2 legs -No. hot strip thickness intermediate anneal thickness final thickness Condition Vref 3 - 4mm 2 - 2.4mm 0.29mm rolling hard V582 3 - 4mm 0.9 - 1.2mm 0.28mm rolling hard V581 3 - 4mm 0.9 - 1.2mm 0.28mm rolling hard V580 3 - 4mm 0.9 - 1.2mm 0.28mm rolling hard

The correspondingly produced aluminum ribbons for lithographic printing plate supports or litho ribbons were subjected to further tests. All four aluminum strips are characterized by very good roughening behavior. In addition, the tensile strength in the as-rolled condition was examined. In order to test the practical handling of the printing plates, particularly in the case of oversized lithographic printing plates, tensile strengths were also measured after a baking process at 240° C. for 10 minutes. In addition, reverse bending tests were carried out, in which the in 1 the test arrangement shown schematically was used.

Fig. 1a ) shows a schematic sectional view of the structure of the reverse bending test device 1 used to examine the resistance to reverse bending of the aluminum strips according to the invention. Samples 2 from the aluminum strips produced for lithographic printing plate supports are mounted on a movable segment 3 and a stationary segment 4 in the reverse bending test device 1 . During the reverse bending test, the segment is moved back and forth on the stationary segment 4 by means of a rolling movement, so that the sample 2 is exposed to bending perpendicular to the extent of the sample 2. The various bending states are shown schematically Fig. 1b ). Samples 2 were cut out either longitudinally or transversely to the rolling direction from the prepared aluminum webs for lithographic printing plate supports. The radius of the segments 3.4 was 30 mm.

The tensile strengths were measured according to DIN. The results of the tensile strength measurements in the as-rolled state or after a baking process and the reverse bending test results are shown in Tables 3a and 3b. Table 3a leg no. Tensile strength (MPa) as rolled Tensile Strength (MPA) 240°/10 min . along across along across Vref 198 201 154 154 V582 184 201 153 161 V581 177 192 145 155 V580 218 228 157 169 leg no. Alternating bending test after 260°/4 min. number of cycles Alternating bending test, rolled Number of cycles along across along across Vref 3400 1500 3030 1930 V582 4570. 2670 4070 2320 V581 4230 2150 4100 2000 V580 3190 2090 2840 2200

It was found that the conventional aluminum strip has sufficient tensile strength for correcting the coil set before the stoving process and for handling the lithographic printing plate support after the stoving process, as well as sufficient flexural fatigue strength along the rolling direction. However, the conventionally produced aluminum strip (VRef) achieved only 1500 bending cycles across the rolling direction. The aluminum strip V582 according to the invention, on the other hand, shows very good tensile strengths with regard to coil set correction and handling of the printing plate after a baking process, as well as very high flexural fatigue strength. Up to 78% more flex cycles were achieved, alloy V582. In comparison, the comparative aluminum strip V580 also showed good values in terms of flexural fatigue strength. However, the very high tensile strengths of 218 and 228 MPa longitudinally and transversely to the rolling direction make it difficult to correct the coil set before baking the photo layer of the lithographic printing plate base.

In the as-rolled condition, which is used for negative printing plates, there was a significant improvement in the flexural fatigue strength, particularly along the direction of rolling. The values also increased transversely to the rolling direction.

It has been shown that the selection of an aluminum alloy specially tailored to the needs of large lithographic printing plate supports, in combination with selected process parameters, enables the production of significantly improved lithographic printing plate supports, which also when oversizes are used, i.e. if they are clamped transversely to the rolling direction can be handled easily and yet are resistant to plate tearing.

Claims (2)

1. A method for producing aluminium strips for lithographic printing plate carriers, wherein the aluminium strip is produced from a rolling ingot, which after optional homogenization is hot-rolled to a thickness of 2 mm to 7 mm and the aluminium strip is cold-rolled to a final thickness of 0.15 mm to 0.5 mm by cold rolling the hot strip,
   characterized in that

   the aluminium strip is made from an aluminium alloy with the following alloy components in percent by weight: 0.3% ≤ Fe ≤ 0.4%, 0.3% ≤ Mg ≤ 0.4%, 0.05% ≤ Si ≤ 0.25%, Mn ≤ 0.05%, Cu ≤ 0.04%,

   the remainder being Al and unavoidable impurities, individually 0.05% at maximum, in total 0.15% at maximum; hot rolling is carried out at a temperature of 250 °C to 550 °C, wherein the final hot strip temperature is 280 °C to 350 °C, an intermediate annealing is carried out during cold rolling at a thickness of 1.5 mm to 0.5 mm, during the intermediate annealing the metal temperature is 200 °C to 450 °C and the aluminium strip is maintained at said metal temperature for at least one to two hours, the aluminium strip is then rolled to a final thickness of 0.15 mm to 0.5 mm by cold-rolling, and is coiled in the hard-as-rolled state for further processing into a lithographic printing plate carrier.
2. The method according to claim 1,
   characterized in that
   the aluminium alloy has a Ti content of 0.05% by weight at maximum, a Zn content of 0.05% by weight at maximum, and a Cr content of less than 100 ppm.

Images