Printing – Planographic – Lithographic printing plates
Reexamination Certificate
2000-01-18
2001-12-04
Zimmerman, John J. (Department: 1775)
Printing
Planographic
Lithographic printing plates
C428S687000, C029S895320, C492S037000, C072S199000
Reexamination Certificate
active
06324978
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printing plate substrate made of an aluminum material with a specified surface topography and a method for making the printing plate substrate.
2. Description of the Related Art
Printing plate substrates are typically made of rolled aluminum strips. Substrate materials are pure aluminum (AA1050) as well as alloys of the type AlMn1 and AlMn1Mg (AA3003, AA3103, AA3005). Input stock for producing the strips are rectangular ingots manufactured by a direct chill casing process and having a thickness of up to 600 mm. After the casting skin is milled off, the ingots are rolled into thin strips by hot forming and cold forming; cold forming can be performed either without an intermediate annealing step, or with one or more intermediate annealing steps. The finished rolled strip has a final thickness of 0.1-0.3 mm and a standard so-called mill-finish surface, which is characterized by rolling striations extending parallels to the rolling direction. The striations are produced when the ground steel roller is pressed into the aluminum strip; the steel roller leaves on the strip a roughness Ra=0.1-0.3 &mgr;m in the rolling direction (∥RD) and Ra<0.15 &mgr;m perpendicular to the rolling direction (⊥RD).
The strip is processed into printing plate substrates by roughening the surface of the strip in a subsequent process step. Mechanical, chemical and electrochemical (EC) roughening processes and combinations of such processes are known in the art. The roughened structure is protected (through anodic oxidation) with a thin, hard oxide layer.
An offset printing plate is produced from the printing plate substrate by applying a light-sensitive/photoelectric layer. The surface of the printing plate substrate provides important functions for a.) the adhesion of the photo layer and b.) guiding the water in the subsequent printing process. The printing plates are then exposed, developed and installed on the printing press.
The roughened structure of the printing plate substrate is an important feature of the printing plate, since this structure determines, for example, the lifetime of the printing plate and thus the number of copies that can be printed. Electrochemically roughened plates have optimum properties for printing. Microscopically small indentations are etched in the surface with an acid based either on HCl or HNO
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, using alternating current. The roughening step is intended to eliminate the directional mill-finish surface and to produce a fine, nondirectional structure with Ra∥RD=Ra⊥RD in the region of Ra≧0.3-1.5 &mgr;m. The EC process is disadvantageously associated with high processing costs caused by a high power consumption, a need to reprocess the spent chemicals, and disposal of the wastewater and sludge. The conventional processes have so far not been able to achieve a sufficiently fine, nondirectional mechanical roughening. WO 97/31783 (Alcoa) describes a mechanical roughening process using textured rollers Bombardment of the steel roller with a sand-blasting jet, a laser beam, an electron beam or a spark discharge (EDT) creates holes/depressions which have a nondirectional pointed ridge-shaped structure; the roughness of the roller of Ra=0.56-0.76 &mgr;m (22-30 micro-inches) is transferred to the aluminum strip during the last roll-down pass reducing the thickness of the strip; the average roughness of the strip is then approximately Ra=0.33-0.43 &mgr;m (13-17 micro-inches). The Alcoa patent reports a reduction in thickness of 0-15%. This process, which represents a combination of rolling and embossing, produces a flatter and—with higher roll-down ratios—a directional structure on a microscopic scale; the quality of this surface structure for application as printing plates is comparable to that obtained by wet brushing, but is easier and less expensive to produce. However, this process is not suitable for more demanding requirements (i.e., high quality prints): it can not replace the electrochemical roughening process, but may provide a suitable alternative for a mechanical pre-roughening step in a combined mechanical/electrochemical roughening process.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a purely mechanical roughening process which advantageously combines the cost-effective texturing process using rollers with the superior surface characteristics of EC roughening, so that the resulting structure is at least equivalent to that of an EC-roughened printing plate substrate. The object is solved by the invention through the features recited in the claims.
Printing plate substrates are typically characterized by the average peak-to-valley height Ra (for mechanical tracing according to DIN 4768). The applicant's experiments have shown the unexpected result that the roughness values Ra and Rz do not represent the characteristic parameters which predict the quality of printing plate substrates for finely screened prints. Rather, the dominant characteristic parameter is the micro-roughness, which for high-quality printing plates is characterized by stochastically arranged, tightly packed, trough-shaped depressions having a diameter of <25 &mgr;um. Such a fine structure could until now only be produced by locally dissolving the metal in a chemical/ electrochemical etching step. When this structure is a traced in a linear direction, this structure with the trough-shaped depressions can be referred to as a pointed ridge.
It is possible to emboss the aluminum strip with a roller instead of treating the aluminum strip electrochemically while still attaining the topography of a commercial electrochemically roughened printing plate substrate, if the embossing roller has a profile of a “rounded ridge,” since the negative pattern is generally transferred during the rolling process. According to the invention, the surface of the steel rollers used for the embossing are provided with a coating having stochastically distributed, microscopically small, meniscus-shaped projections (spherical calottes), thereby producing the desired pointed ridge-shaped surface structure on the printing plate during the rolling process.
According to the invention, rounded ridge-shaped roller surfaces can be produced by sinter-fusing metal powder or by galvanic precipitation (ECD process), as described, for example, in the patents DE 4211881 and DE 4334112. Preferably, the rollers are coated to produce calottes with an average diameter of ≦20 &mgr;m with a density of at least 3000/mm
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, i.e., the calottes are tightly packed. The frequency distribution curve of the diameters of the calottes should correspond to a normal distribution. The roughness values of the coated roller are Ra≦1.5 &mgr;m and Rz≦8 &mgr;m, as measured with a mechanical sensing unit having a contact stylus instrument and an electrical wave filter.
It has been observed that when a roller which is coated in this manner is used for embossing, the thickness should be consistently reduced by at least 0.2% in order to transfer the trough-shaped structure to the strip. However, the thickness should not be reduced by more than 5% since the strain caused by a larger deformation may smear the structure.
The printing plate substrates which are embossed with this type of roller, show macroscopically a uniform satin-finish appearance. A scanning electron microscope (SEM) image shows a nondirectional structure with depressions having an average size of ≦25 &mgr;m and a ratio of the length of the depression (∥RD) to the widths of the depression (⊥RD) of ≦1.5 (when referring to aspect ration, the ration of length/width relative to the transport direction of the strip is meant). The depressions abut each other, but are not connected with each other, so that a pointed ridge-shaped structure is produced. The average peak-to-valley height is in the range Ra=0.5-1.5 &mgr;m; the values measured in the longitudinal direction are only slightly different from
Grzemba Barbara
Kaulen Jürgen
Schmiedel Helmut
von Asten Wolfgang
Darby & Darby
VAW Aluminum AG
Zimmerman John J.
LandOfFree
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