GB2081178A - Sheet-guiding Foil as a Dressing for Back Pressure Cylinders - Google Patents

Abstract

A sheet-guiding foil suitable for use as a dressing for back pressure cylinders on rotary offset perfecting presses, of which one surface is smooth but the opposite surface is provided with substantially part- spherical inverted-cup-shaped embossments of equal height and of statistically uniform distribution is characterised in that it comprises a substrate 6 which has a textured surface, and is chemically resistant, non-wearing and rigid, the substrate having the requisite ink transfer properties, and in that a chromium layer 7 which evens out the microroughness is applied to the textured surface. <IMAGE>

Description

SPECIFICATION Sheet-guiding Foil as a Dressing for Back Pressure Cylinders The invention concerns a sheet-guiding foil as a dressing for back pressure cylinders on rotary offset perfecting presses, one surface of which is smooth while the opposite surface is provided with spherical cups of equal height and of statistically uniform distribution.

DE-PS 12 58 873 describes the roughening of an aluminium sheet or the convex surface of perfecting or sheet-guiding cylinders, for example, by means of sandblasting and subsequent coating with a thin chromium layer. The supporting areas of this surface are irregular in height and differ in size. Relatively pointed support areas are, of course, worn down more quickly by the paper than flat areas. The substrate, e.g. steel or aluminium, shows through at the worn points.

The ink transfer properties of these exposed areas of the substrate are so poor that the entire convex surface is no longer suitable for guiding freshly printed sheets during the perfecting process.

Furthermore, the chemical agents used in the offset process penetrate into these severely worn areas, causing corrosion.

DE-OS 28 20 549 describes a further metallic sheet-guiding foil which has at least two layers.

An aluminium or copper substrate is roughened on one side by sandblasting and is nickel-plated.

This second nickel layer can be provided with a further thin layer of solid nickel.

The roughening of the substrate by means of sandblasting produces supporting areas which are uneven in height and differ in size. The ink taken off by the freshly printed underside of the sheet during the printing process is, therefore, irregularly re-transferred when leaving the printing gap. This results in ink re-distribution and thus in a deterioration of the printed image.

Furthermore, on account of the excessive size of and excessive disparity between the valleys, such a sheet-guiding surface is an inadequate support for the sheet during the printing of the even page reverse.

A three-layer sheet-guiding surface, moreover, has the disadvantage that the texture of the roughened surface fades. The valleys become narrower and the supporting plateaus become larger. This results in a deterioration of the ink transfer properties of a three-layer sheet-guiding foil. Furthermore, it is difficult to bend the ends of three-layer sheet-guiding foils for the purpose of clamping them in position since the solid nickel layer tends to crack.

The object of the invention is to extend the service life of sheet-guiding foils, to improve their ink transfer properties and above all to keep their ink transfer properties more or less constant over the entire service life of the foil.

This aim is achieved in that applied to the textured surface of a chemically resistant, nonwearing and rigid substrate with good ink transfer properties is a thin chromium layer which evens out the microroughness.

The ink transfer properties of a surface are dependent on its texture and material. The design of the supporting surfaces in the form of spherical cups promotes the removal of the ink which has been accepted. Materials, such as chrome, nickel, chrome nickel steel or certain plastics, additionally promote ink transfer.

The substrate may, therefore, consist, for example, of a nickel foil produced by means of electroforming, a chrome nickel steel sheet processed by etching or embossing or of compression-moulded plastic foils with a high modulus of elasticity, e.g. rigid polyamide foils.

The chromium layer applied may, for example, have a thickness of 0.01 to 0.03 mm. It extensively evens out the microroughness of a nickel, chrome nickel steel or plastic layer, thereby providing a smoother surface than the substrate.

This means that a chromium-plated, sheetguiding foil exhibits optimum ink transfer properties already in the start-up phase and is quicker to wash than the initially somewhat rougher substrate.

Above all, however, the service life of the sheet-guiding foil is extended by the life of the very hard chromium layer. The ink transfer properties of such a chromium-plated, sheetguiding foil remain virtually constant over the entire service life of the foil, because, after the very thin chromium layer has worn down at the sheet-supporting areas, the substrate which shows through likewise has good ink transfer properties.

It is particularly advantageous to use pure nickel as a substrate, because the chemical resistances of chrome and nickel to the chemicals used in offset printing complement each other.

Thus, for example, the thin chromium layer may be damaged by the application of certain cleaning agents. Nickel, however, is resistant to such cleaning agents. Conversely, certain dampening solution additives may, in conjunction with local tap water, attack nickel. Chrome is resistant to such chemicals. The thin chromium layer therefore prevents premature wear on a nickel foil if use is made of unfavourable dampening solution additives.

Since the thin chromium layer is not only hard, but also brittle, it would very easily flake off when folding the clamping ends of a sheet-guiding foil.

For this reason, it is advantageous to fold the clamping ends of the substrate before the chromium layer is applied.

The invention is described below in greater detail on the basis of a drawing of a specimen embodiment.

Fig. 1 shows a section of the textured surface of a sheet-supporting foil according to the invention.

Fig. 2 shows section A-B through this foil section.

Fig. 3 shows section C-D through the same foil section.

As can be seen from Fig. 1, the textured surface 1 of the sheet-guiding foil 2 is covered by support areas in the form of spherical cups 3.

These spherical cups 3 have statistically uniform distribution, i.e. their distribution is not symmetrical, but is almost symmetrical.

Statistically uniform also means that there are approximately the same number of spherical cups 3 on one unit of area. This guarantees that the interspaces, the valleys 4, do not essentially differ from one another with regard to their size and form. Basically, therefore, there is also the guarantee of a uniform base for a sheet which is to be printed.

As can be seen from Figs. 2 and 3, the spherical cups 3 are all equal in height. The tops of the spherical cups 3 thus form a support plane 5. The cup shape of the support areas and the formation of the support plane 5 guarantee excellent support for the sheet which is to be printed on the other side. This arrangement of the spherical cups 3 also prevents the premature wearing down of higher support areas, as is the case with known, sheet-supporting foils.

As is shown in Fig. 2, the sheet-guiding foil 2 consists of two layers, namely the substrate 6 made of nickel and the top layer 7 made of chrome. The substrate 6 may, for example, be produced by means of electroforming, whereby, however, the supporting tops still have a relatively great microroughness. The thin chromium layer 7 may be applied in a different bath. Its surface is considerably smoother than the substrate 6. In this way, such a sheet-supporting foil has equally good ink transfer properties from the beginning of its use, whereas, when using the substrate alone, optimum ink transfer properties are not obtained until after a certain smoothing of the microroughness. It is also easier to wash the chromium-plated surface than the slightly rougher surface of the substrate 6.

After the production of the substrate 6 by means of electroforming, it is necessary first of all to fold the clamping edges 8 before the top layer 7 is applied in the chrome bath.

The specimen embodiment may be modified to the extent that, instead of the nickel substrate 6 produced by electroforming, use is made of an embossed or etched chrome nickel steel sheet or an embossed or etched plastic foil, e.g. a rigid thermoplast made of PVC, polyester, polyamide or glass which is suitable with regard to good ink transfer properties. The thermoplast must, however, exhibit a high modulus of elasticity, becausae it would otherwise give in the printing gap due to the flexing process and would cause, an increase in printing width.

Claims (7)

Claims

1. Sheet-guiding foil suitable for use as a dressing for back pressure cylinders on rotary offset perfecting presses, of which one surface is smooth but the opposite surface is provided with substantially part-spherical invertedcup-shaped embossments of equal height and of statistically uniform distribution, characterised in that it comprises a substrate which has a textured surface, and is chemically resistant, non-wearing and rigid, the substrate having the requisite ink transfer properties, and in that a chromium layer which evens out the microroughness is applied to the textured surface.

2. Foil according to claim 1, characterised in that the chromium layer has a thickness of 0.01 to 0.03 mm.

3. Foil according to claim 1 or 2, characterised in that the substrate is of nickel or chrome nickel steel.

4. Foil according to claim 1 or 2, characterised in that the substrate is of a plastics material with the requisite high modulus of elasticity.

5. Foil according to claim 4, characterised in that the substrate is of polyamide or polyvinyl chloride.

6. Foil according to any of claims 1 to 5, characterised in that the substrate has been folded at its clamping ends before the application of the chromium layer.

7. Foil according to claim 1, substantially as described with reference to the accompanying drawing.