WO2001068378A1

WO2001068378A1 - Roll grained aluminium sheet

Info

Publication number: WO2001068378A1
Application number: PCT/US2000/006281
Authority: WO
Inventors: Sallie L. Blake; Bobby J. Cunningham; Michael A. Ringle
Original assignee: Alcoa Inc.
Priority date: 1998-03-10
Filing date: 2000-03-10
Publication date: 2001-09-20

Abstract

The method for producing lithosheet by providing at least one roll of a pair of rolls with a surface texture of microsopic size peaks of roll material and a pattern having substantially no directionality such that the roll surface is substantially uniformly rough. A metal sheet is directed between the rolls such that the rolls contact the sheet surfaces to provide at least one surface of the sheet with the non-directional roughness of the textured roll surface and extends the surface area of the sheet by an amount in the range of 0.05 to 10 % in the process of forming microscopic depressions in the sheet surface for receiving printing inks, for adhesion of hydrophobic coatings and/or photopolymers.

Description

ROLL GRAINED ALUMINIUM SHEET The present invention relates generally to a method of making aluminum substrates for lithographic plates, which is more commonly identified as lithosheet, and more particularly, to using a textured roll-to-roll aluminum sheet in a manner that "extends" the surface area of the sheet and substantially eliminates the directional patterns that traditional work rolls leave on the rolled sheet. The resulting sheet surface is non-directional, uniform in appearance and provides increased or extended surface area to hold suitable amounts of materials applied to the surface of the sheet such as photopolymer coatings or printing inks. Lithography is defined as the process of printing from a plane surface such as a metal plate on which an image to be printed is ink-receptive and blank areas are ink-repellent. The metal plate used in this process is referred to as "lithoplate". In some cases, the metal plate is a substrate for subsequent polymer coating for flexograpliic plates. For the purposes of this invention, the term "lithosheet" will refer in particular to aluminum sheet used to make lithoplates which is inclusive of flexograpliic plates. While "lithoplate" incorporates the word "plate", the substrate used to make lithoplate is better described as sheet or foil.

To make the image area, the lithosheet is coated with a hydrophobic (water repelling) light-sensitive material. In general, this material is resistant to attack or dissolution from acids until it is exposed to light and is commonly called a resist. An extended run of the lithosheet is coated and then cut into a lithoplate. The lithoplate is next overlaid with a negative of a desired image and exposed to light. In the non-image area, the light causes a reaction in the resist which makes it soluble in acid. Thus, after exposure the plate is washed with acid to remove the resist leaving the water retentive metal surface of the lithoplate in the non-image areas. The ink-receptive and ink-repellent areas on the lithoplate are developed in the printing process by subjecting the plate to contact with water. The inks used in the printing process are such that they will not stick or adhere to wet surfaces and, thus when the lithoplate is contacted with an ink-laden roller, ink is transferred only to the image area.

It is evident that the quality or suitability of a lithoplate for printing is directly related to the hydrophobic and hydrophilic characteristics of the image. It is known that uniform roughening of the surface by a "graining" process is advantageous to both adherence of the hydrophobic coating to the lithosheet and in enhancing the water retention character of the metal surface. The objective of graining is to increase or extend the surface area and obtain a uniform non- directional roughness necessary for quality production of printed images. The quality and extension of the grained surface in the present invention is best measured by the amount or percentage of "extension" above a normally flat, planar surface of a workpiece such that the area of the surface is increased by peaks and valleys formed in the sheet surface. Originally, graining lithosheet was accomplished mechanically by ball graining or brushing. In ball graining, a slurry of steel balls and abrasive material is agitated on the sheet with the extent of roughening controlled. In brush graining, brushes are rotated or oscillated over the surface covered with an abrasive slurry. These mechanical graining techniques require cleaning the sheet by immersion in caustic solution before further processing of the sheet. The uniformity and quality of the roughened surface is difficult to control with these methods and mechanical graining is generally slow and costly.

Because of difficulties in prior mechanical graining, the constant growth of lithographic printing, higher operating speeds of modern printing presses, the need for longer lithoplate life, etc., increasing attention has been given to electrochemical methods of graining. By these methods, the graining is accomplished by controlled etching of the surface with the use of chemicals or the combination of passing current through a chemical solution and the lithosheet. U.S. Patents 4,301,229, 4,377,447, and 4,600,482 are cited as examples of many patents directed to electrochemically grained lithoplate. To achieve a uniform non- directional roughness with this method requires extended etching of the surface and detailed control of the metal structure. This method also tends to be slow and costly and may result in hazardous chemical waste.

Whether mechanically grained or electrochemically grained, the graining process requires added cost. to the fabrication of lithoplates. If lithosheet were produced with non-directional, properly extended surfaces, the necessity of graining would be eliminated. In addition, both the electrochemical and mechanical graining with slurries have certain environmental consequences. What is therefore needed is the production of lithosheet having the necessary extended surface area. The present invention provides the above need by texturing a steel roll or rolls in such a manner that when the roll or rolls are employed to produce lithosheet, such as in the last stand of a rolling mill or are used as pinch rolls or skin pass rolls in some other process such as leveling, the rolls increase or extend the lithosheet surface to provide a lithosheet product with the required non- directional extended surface area. The roll surface is preferably provided with an appropriate texture by electron discharge texturing (EDT). EDT employs a plurality of arc generating electrodes spaced from the roll surface. The arcs provide a generally uniform roll surface of peaks and valleys of appropriate dimensions. The dimensions are controlled by settings on the machine such as the voltage and current of the arcs and pulse length and pulse delay times between arcs, rotational speed and traverse rates, etc., of the electrodes of the EDT machine relative to the roll surface. Electron discharge texturing is disclosed in such patents as U.S. Patents 3,619,881 and 4,789,447 to Bills et al and Ahmed et al, respectively.

Other texturing methods may be employed to achieve the required surface area and non-directional appearance on the roll surface. Examples of these other methods may be the well known sand blasting techniques, or as sophisticated as laser beam texturing and focused electron beam texturing, as discussed in U.S. Patent 5,025,547 to Sheu et al.

In the present invention, after the grind marks are removed from the roll surface and the appropriate surface applied by a method of texturing, the textured roll can be used to texture the sheet using one or more passes of the sheet in a rolling mill. Any reduction in sheet thickness is slight, i.e., in the range of zero to fifteen percent of the thickness such that there is substantially little or no elongation of the texture pattern transferred from the roll to the lithosheet surface. This same transfer can take place from the pressure of textured pinch rolls on the surface of the sheet being passed between them or from the minimal reduction taken in a skin pass mill. Both surfaces of the lithosheet may also be textured to provide a two-sided product; but if not, a relatively smooth roll surface is used on the bottom sheet surface to prevent the directional roll grind pattern of the bottom sheet surface from transferring to an upper sheet surface in a tightly wound coil of the lithosheet.

The invention, along with its advantages and objectives, will be better understood from consideration of the following detailed description and the accompanying drawings in which:

Figure 1 is a three-dimensional, computer generated plot of a surface portion of an aluminum alloy sheet showing minimal extended surface area and directional grind lines on the surface of the sheet;

Figure 2 is a three-dimensional, computer generated plot of a portion of an aluminum alloy sheet surface that has been electrochemically grained to increase surface area and reduce directional grind lines;

Figure 3 is a three-dimensional, computer generated plot of a portion of an aluminum alloy sheet surface rolled by an EDT roll, the plot showing pockets formed in the sheet surface by the roll surface; and Figure 4 is a diagrammatic partial sectional view of an aluminum alloy sheet provided with a surface suitable for lithoplate purposes, the surface being exaggerated to show peaks of surface material.

Referring now to the drawings, Figure 1 is a three-dimensional plot of a portion of the surface of an as-rolled aluminum alloy lithosheet, the surface having generally parallel lines formed thereon by a work roll in the final stand or pass of a rolling mill. (The surface views of Figures 1 to 3 were obtained by a Phase Shift interferometer viewing the surface of a 1050-H18 aluminum lithographic sheet. This instrument uses white light interference fringes [shifts in light phases] to measure relative peak heights of surface material at a magnification of ten times). Work rolls are ground to a certain finish before being installed and used in a rolling mill. This process involves a stone wheel rotated against the surface of the mill roll that is also rotated. The process leaves elongated fine scratches in the roll surface. These marks are transferred to the sheet surface physically contacting the roll surface, the grind marks appearing as parallel lines on the sheet surface extending in the rolling direction.

The buyer of the lithosheet product of Figure 1 grains the surface of the product, as explained above, to obtain increased surface area, a non-directional appearance and required roughness and peak count. The electrochemically grained surface effected by the buyer is depicted in Figure 2 of the drawings. As shown, the grained surface still has a somewhat directional pattern.

Figure 3 of the drawings shows a lithosheet product of the present invention as rolled using a roll textured by the EDT process, the roll surface having substantially no directional pattern for transferal to a sheet surface. Figure 3 shows, in addition, a generally isotropic pattern of pockets of the extended surface provided by the textured roll for holding water or hydrophobic coating material, the pockets providing an average surface area extension of the nominally flat sheet surface of 0.05 to 10%. The roll texture may not transfer perfectly to the sheet surface, as there can be some elongation of the sheet surface in the rolling process, depending upon any relative roll/sheet velocities that may occur. Multiple passes may be required to achieve the surface area extension. The surface area extension is an attribute to improve adhesion of both coatings for lithographic and flexograpliic plate-making and inks and water for printing.

In Figure 4 of the drawings, pockets of different depths are diagrammatically shown in a sectional view of an aluminum alloy sheet 10 of Figure 3, and are labeled 12 and 14. The pockets are surrounded by peaks of surface material of different heights, labeled by numerals 16 and 18. The peaks and pockets provide an increased or extended surface area for receiving, for example, a photosensitive hydrophobic coating and water to repel printing inks. The average surface elevation in Figure 4 is shown as phantom line 20. There is also shown two other phantom lines, labeled 22 and 24, which define equal distances above and below the average elevation 20. The distance between lines 22 and 24 is known as the "bandwidth" and therefore for a defined bandwidth of 20 microinches, line 22 is 10 microinches above line 20, and line 24 is 10 microinches below line 20. "Peak-count" is defined as the number of times the surface extends above line 22 and the number of times it extends below line 24. In Figure 4, both the pockets labeled 14 and the peaks labeled 18 are counted in the peak-count number while the pockets labeled 12 and the peaks labeled 16 are not counted in the peak-count number. Preferably, the peak-count for the finite area of the printing surface of sheet 10 lies in the range of 300 to 450 peaks per linear inch measured with a total bandwidth of 20 microinches. Such a peak count for the lithosheet product of the invention is provided by a textured roll having an appropriate count in the range of 350 to 500 peaks per linear inch extending above and below a total bandwidth of say 20 microinches. While in practice the peak-count perpendicular to the direction in which sheet 10 is rolled is slightly higher than the peak-count parallel to the rolling direction, this same peak-count range is applicable when measured in both directions and results in the non-directional performance of the lithosheet. Such surface heights and depths, along with peak-counts in both directions, provide the isotropic surface needed for quality printing and for other uses where retention of coating materials is necessary.

For reasons described above, it is required that the directional roll grind pattern (Figure 1) be significantly reduced in a graining process so that a non-directional relatively rough surface be available for the lithographic printing processes. By using a textured surface suitable for subsequent graining, a multigrained lithosheet product can be produced. The product of the invention can also be used with no graining at all, and will resemble a grained plate product because of the above "extension" of the sheet surface. This allows the lithoplate producer to eliminate an operation and its associated cost.

Claims

C L A I M S

1. A method of producing a lithosheet product comprising: providing at least one roll of a pair of rolls with a surface texture of microscopic size peaks of roll material and a pattern having substantially no directionality such that a surface of the one roll is substantially uniform in roughness; and directing an aluminum sheet between said pair of rolls one or more times such that the rolls contact the surface of the sheet and provide at least one surface of the sheet with the non-directional roughness of the textured roll surface and extends the surface area of the sheet by an amount in the range of 0.05 to 10% in forming numerous microscopic peaks and depressions in the surface of the sheet for the purpose of receiving and retaining printing inks for adhesion of hydrophobic coatings, and/or photopolymers.

2. The method of claim 1, in which the roll surface has an extended surface area of 0.05 to 10% and which provides the lithosheet surface with an extended surface area of 0.05 to 10%.

3. A lithosheet product comprising: an aluminum alloy sheet provided with a surface of uniform, non- directional roughness and microscopic depressions suitable for receiving and retaining printing inks for adhesion of hydrophobic coatings and/or photopolymers as generated by directing the sheet between a pair of rolls, with at least one of the rolls being provided with a texture of microscopic peaks and valleys and having substantially no directional pattern such that the rolls contact the sheet surfaces and provide at least one surface of the sheet with the non-directional roughness of the roll surface and extends the surface area of the sheet by an amount in the range of 0.05 to 10%.