ALUMINUM STRIP FOR LITHOGRAPHIC PRINTING PLATE CARRIERS AND THE PRODUCTION THEREOF

Abstract

A method for producing aluminum strips for lithographic printing plate supports, wherein the aluminum strip is produced from a rolling ingot, which after optional homogenizing is hot-rolled to a thickness of 2 mm to 7 mm and cold-rolled to a final thickness of 0.15 mm to 0.5 mm provides for an aluminum strip having a thickness of 0.15 mm to 0.5 mm and a printing plate support produced from the aluminum strip.

Claims

1. Aluminium strip for producing lithographic printing plate supports, having a thickness of 0.15 mm to 0.5 mm, characterised in that the aluminium strip consists of an aluminium alloy having the following alloying constituents in weight percent: 0.3%≦Fe≦0.4%, 0.2%≦Mg≦1.0%, 0.05%≦Si≦0.25%, Mn≦0.1%, optionally Mn≦0.05%, Cu≦0.04%, with the remainder Al and unavoidable impurities, individually max. 0.05%, in total max. 0.15%, and has a reversed bending fatigue strength transverse to the rolling direction of at least 2320 cycles in a hard-as-rolled state in a reversed bending test.

2. Aluminium strip according to claim 1, characterised in that the aluminium strip has a tensile strength of up to 200 MPa in a hard-as-rolled state longitudinal to the rolling direction and a tensile strength of at least 145 MPa after a burning-in process longitudinal or transverse to the rolling direction.

3. Aluminium strip according to claim 1, characterised in that the aluminium alloy has a Mg content of 0.25 wt. % to 0.6 wt. %.

4. Aluminium strip according to claim 1, characterised in that the aluminium alloy has a Mg content of 0.4 wt. % to 1.0 wt. %.

5. Aluminium strip according to claim 1, characterised in that the aluminium alloy has a Ti content of max. 0.05 wt. %, a Zn content of max. 0.05 wt. % and a Cr content of less than 50 ppm.

6. Aluminium strip according to claim 1, characterised in that the aluminium alloy has a thickness of 0.25 to 0.5 mm.

7. Printing plate support produced from an aluminium strip according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1a) shows in a schematic sectional view the configuration of the reversed bending test apparatus used.

[0032] FIG. 1b) shows in a schematic cross-sectional view the different bending states of the reverse bending test.

DESCRIPTION

[0033] A comparison between a conventional aluminum strip for producing lithographic printing plate supports and two aluminum strips according to the invention and a comparison aluminum strip, which are also suitable for producing lithographic printing plate supports, is presented in the following. The alloying constituents of the different, tested aluminum strips are shown in Table 1.

TABLE-US-00001 TABLE 1 Alloy No. Fe Mn Mg Si Cu wt. % Vref 0.32 — 0.17 0.12 7 ppm Prior art VF583 0.3 0.0291 0.97 0.11 0.0233 Invention V582 0.36 0.0034 0.3 0.09 3 ppm Invention V581 0.36 0.018 0.2 0.08 5 ppm Invention V580 0.4 0.10 0.11 0.08 <10 ppm Comparison

[0034] Table 1 only shows the essential alloying constituents of the aluminum strips tested and furthermore the different test alloys had a Ti content of less than 0.015 wt. %, a Zn content of less than 0.05 wt. % and a Cr content of less than 100 ppm. The rolling ingots cast from the different aluminum alloys were subjected to homogenizing prior to rolling, wherein the rolling ingots were annealed to a temperature of about 580° C. for more than four hours. Subsequently, hot-rolling was carried out at temperatures of 250° C. to 550° C., wherein the hot-rolling final temperature was between 280° C. and 350° C. The aluminum hot strip consisting of the Vref alloy was subjected to an intermediate annealing during cold-rolling at a thickness of 2 to 2.4 mm, wherein the cold-rolled strip was exposed to a temperature of 300 to 450° C. for one to two hours. For the V581, V582 and VF583 aluminum strips the intermediate annealing thickness was only 0.9 to 1.2 mm at the same intermediate annealing temperatures, as can also be seen from Table 2. The aluminum strip consisting of the V580 alloy was, in contrast, not subjected to intermediate annealing. Since the intermediately annealed strips were cold-rolled further to a final thickness, without final annealing taking place, these were coiled in the hard-as-rolled state.

TABLE-US-00002 TABLE 2 Hot strip Intermediate final annealing Final Alloy No. thickness thickness thickness State Vref 3-4 mm   2-2.4 mm 0.29 mm hard-as-rolled VF583 3-4 mm 0.9-1.2 mm 0.28 mm hard-as-rolled V582 3-4 mm 0.9-1.2 mm 0.28 mm hard-as-rolled V581 3-4 mm 0.9-1.2 mm 0.28 mm hard-as-rolled V580 3-4 mm — 0.28 mm hard-as-rolled

[0035] The correspondingly produced aluminum strips for lithographic printing plate supports or lithostrips were subjected to further tests. All five aluminum strips are characterized by very good roughening characteristics. Furthermore, the tensile strength was tested in the hard-as-rolled state. In order to test the practical handling of the printing plates, particularly with outsized lithographic printing plates, tensile strengths were also measured after a burning-in process of 240° C. for 10 minutes. In addition, a reversed bending test was carried out, in which the test arrangement illustrated schematically in FIG. 1 was used.

[0036] FIG. 1a) shows in a schematic sectional view the configuration of the reversed bending test apparatus 1 used, which was employed to test the reversed bending fatigue strength of the aluminum strips according to the invention. Samples 2 from the aluminum strips for lithographic printing plate supports produced are attached to a movable segment 3 and to a fixed segment 4 on the reversed bending test apparatus 1. In the reversed bending test, the segment is moved back and forth on the fixed 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. FIG. 1b) shows the different bending states. The samples 2 were cut out of the aluminum strips for lithographic printing plate supports produced either longitudinal or transverse to the rolling direction. The radius of the segments 3, 4 was 30 mm.

[0037] The tensile strengths were measured in accordance with DIN. The results of the tensile strength measurements in the hard-as-rolled state or after a burning-in process, as well as the reversed bending test results, are illustrated in Tables 3a and 3b.

TABLE-US-00003 TABLE 3a Tensile strength (MPa) Tensile strength (MPa) 240 hard- as-rolled ° C./10 min Alloy No. longitud. transverse longitud. transverse Vref 198 201 154 154 VF583 212 223 179 185 V582 184 201 153 161 V581 177 192 145 155 V580 218 228 157 169

TABLE-US-00004 TABLE 3b Reversed bending test after 260 Reversed bending ° C./4 min test hard-as-rolled Number of Number of cycles Alloy No. longitud. transverse longitud. transverse Vref 3400 1500 3030 1930 VF583 4150 3430 3760 2950 V582 4570 2670 4070 2320 V581 4230 2150 4100 2000 V580 3190 2090 2840 2200

[0038] It was revealed that the conventional aluminum strip indeed had sufficient tensile strength for correcting the coil set before the burning-in process and for handling the lithographic printing plate support after the burning-in process, and sufficient reversed bending fatigue strength longitudinal to the rolling direction. Transverse to the rolling direction, the conventionally produced aluminum strip (Vref) only achieved 1500 bending cycles. The V582 and V581 aluminum strips according to the invention, on the other hand, exhibit very good tensile strengths in relation to a coil set correction and the handling of the printing plate after a burning-in process and very high reversed bending fatigue strength. An up to 78% higher number of bending cycles was achieved, cf. V582 alloy. Compared to this, the V580 comparison aluminum strip also, in fact, exhibited good values with regard to reversed bending fatigue strength. The very high tensile strengths of 218 and 228 MPa, longitudinal and transverse, respectively, to the rolling direction, make correction of the coil set difficult before burning-in the photo layer of the lithographic printing plate supports.

[0039] The aluminum strips consisting of the VF583 aluminum alloy also exhibited increased tensile strength values of 212 MPa and 223 MPa longitudinal and transverse, respectively, to the rolling direction. The increase in the reversed bending fatigue strength, however, is very distinct with a factor of about 2.47 compared to the reference material transverse to the rolling direction after the burning-in process. An increase in the reversed bending fatigue strength by a factor of 1.27 still arises anyway longitudinal to the rolling direction after a burning-in process. Coupled with unproblematic roughenability, this produces an outstanding suitability of the VF583 aluminum alloy for outsized printing plate supports clamped transverse to the rolling direction. It is assumed that the improved reversed bending fatigue strength properties are brought about by the increased Mg proportion of 0.97 wt. % in the VF583 alloy. The tensile strength values of the VF583 alloy can, however, be reduced still further by a further reduction in the intermediate annealing thickness, for example to between 0.9 mm and less than 1.1 mm, without the reversed bending fatigue strength properties being impaired.

[0040] In the hard-as-rolled state, which is used for negative printing plates, a distinct improvement in the reversed bending fatigue strength arose particularly longitudinal to the rolling direction. The values likewise increased transverse to the rolling direction. This in particular also applies for the VF583 aluminum alloy which allowed a maximum number of bending cycles transverse to the rolling direction even in the hard-as-rolled state.

[0041] It was revealed that selecting an aluminum alloy specifically matched to the requirements of large lithographic printing plate supports, in combination with selected method parameters, enables distinctly improved lithographic printing plate supports to be produced which even when using outsized ones, i.e. when these are clamped transverse to the rolling direction, can be easily handled and yet are resistant to plate ruptures.