WO2002030678A2

WO2002030678A2 - Method and apparatus for reduction of undesirable printing artifacts

Info

Publication number: WO2002030678A2
Application number: PCT/CA2001/001426
Authority: WO
Inventors: Bradley J. F. Palmer
Original assignee: Creo Srl
Priority date: 2000-10-12
Filing date: 2001-10-12
Publication date: 2002-04-18
Also published as: WO2002030678A3

Abstract

In accordance with the present invention, the surface of a positive working printing plate is rendered water retentive in the area where the plate is to be mechanically held or deformed during imaging. This reduces the incidence of printing artifacts.

Description

METHOD AND APPARATUS FOR

REDUCTION OF UNDESIRABLE

PRINTING ARTIFACTS

Reference to Related Applications

This application claims the benefit of the filing date of United States application No. 09/687,521 filed on 12 October, 2000.

Technical Field

The invention pertains to the field of printing and in particular to the manufacture and processing of printing plates.

Background Art

Irradiation and processing of printing plates is central to offset printing. Given the nature of economics in the printing industry it is important that all available exposable surface of a printing plate be used as efficiently as possible in rendering images on a printed medium.

The advent of technology for imaging plates on-press has also led to very particular requirements as regards the nature and distribution of imageable material on the printing plates. Unexposed plates are loaded onto plate- setters and printing presses with a variety of specific mechanical arrangements for securing the plate. Blaser in US 5,359,933 provides one example of a device for clamping a printing plate. The irradiation and processing of plates may be done via either a positive or a negative working plate. The term "irradiation" is used in what follows to denote the use of a form of radiation that will cause the imageable material on the printing plate to change so that processing will either cause the irradiated material to be removed and the un-irradiated to remain, or cause the un-irradiated to be removed and the irradiated to remain. Different kinds of printing plates respond in this way to different forms of radiation.

The term "negative" is used here to denote a situation where the areas of imageable material on the plate that are irradiated by the irradiation system, will ultimately remain on the plate. The rest of the imageable material will be removed through either ablation during the irradiation process, or through subsequent chemical processing. This results in a finished printing plate on which the areas where imageable material has been removed to expose an hydrophilic base (such as aluminum) will retain water and will therefore not be inked. The areas of the plate that are not exposed in this way retain their hydrophobic imageable material and will be inked, the ink not wetting areas that retain water. The result is that areas of plate that are exposed will be printed. In what follows we shall refer to "negative working printing plates" and "negative working imageable material" when denoting this "negative" process.

Conversely, the term "positive" is used here to denote situations where the imageable material is removed in the areas that are irradiated by the irradiation system. This process option results in the unexposed areas of the plate being inked. In this case, areas of the plate that are not exposed will be printed. In what follows we shall refer to "positive working printing plates" and "positive working imageable material" when denoting this latter "positive" process. Sheriff in US 6,063,544 provides an example of a positive working printing plate.

Consider now a situation where a positive working printing plate, covered completely with positive working imageable material, is held in position on a press or plate-setter by a mechanical mechanism that requires a small part of the plate surface near its edge to be covered by that mechanism. The radiation will not be able to interact with the imageable material in that particular area, the imageable material will remain substantially unexposed, and that particular area of the plate will ultimately be inked. Because ink will build up in these areas, this will result in undesirable artifacts being printed at the periphery of the printed image on the surface being printed upon, corresponding to the location of the mechanical mechanism originally holding the printing plate in position on the machine during the imaging of the plate.

This issue is not restricted only to the imaging source being obscured by mechanical mechanisms. In present day plate-imaging systems thermal irradiation laser heads are extensively employed, as given for example by Sheriff in US 6,063,544. These units have a very narrow depth of focus and therefore the distance between laser imaging head and imaging material on the plate must be quite accurately maintained. Near the mechanical mechanisms securing the plate, the plate will be slightly deformed away from the cylindrical shape ideally required for the plate imaging process. The focus of the laser head will be adversely affected and irradiation will be less than optimal, again leading to areas at the periphery of the image on the plate being inked and consequently to corresponding undesirable artifacts at the periphery of the printed image. In the case of on-press imaging of plates, the phenomenon is particularly debilitating as the requirement for "touching up" the plate detracts from the very purpose of the on-press imaging approach, that of obviating the separate imaging of plates before loading them on the printing press.

The issue described here also pertains when the printing plate is processed on a plate-setter in portrait mode, but employed in landscape mode on the printing press. The edge or border regions of the printing plate that are distorted by clamping in the plate-setter become free edges on the press and will retain ink and cause printing artifacts on the periphery of the image being printed.

It is the object of the present invention to present a method to avoid undesirable printing artifacts resulting from the mechanical attachment of positive working printing plates when they are being imaged.

Summary of the Invention

In accordance with the present invention, the surface of a positive working printing plate is rendered water retentive in the area where the plate is to be mechanically held or deformed during imaging.

Brief Description of the Drawings

FIG.l shows a printing plate mounted for imaging. FIG.2 shows a printing plate with imageable material absent from its border area.

FIG.3 shows a positive working printing plate with a border covered in hydrophilic material.

FIGS. 4A, 4B and 4C depict the process for irradiating a positive working plate to create a hydrophilic border.

Modes for Carrying Out the Invention

FIG.1 Illustrates the essence of the preferred embodiment of a method of the invention. A positive working printing plate 1, with thickness exaggerated for the sake of clarity, is mounted by means of mechanical fixtures 3 and 4 on the drum 2 of the machine on which it is to be imaged. Printing plate 1 is slotted in under the mechanical fixtures 3 and 4 to a certain depth. Furthermore, because of stresses induced by the mechanical fixtures 3 and 4, the printing plate 1 has mechanical deformations 5 and 6. It is to be noted that we depict here just one possible means of mounting a printing plate. There are various other mechanical arrangements by which plates are, or could be, held in position during imaging, all sharing the basic issue of parts of the plate being obscured and parts being deformed. In what follows the terms "imaged" and "imaging" shall be used to denote that process by which an image intended to be printed is committed to the printing plate. In the preferred embodiment, the printing plate 1 has a border 7 of extent at least equal to the depth to which the printing plate 1 is obscured by the mechanical fixtures 3 and 4 from any imaging beam intended to irradiate it. Border 7 preferably extends to the additional extent of the deformations 5 and 6. Since the plate may be rotated by 90 degrees between being imaged and being used on a press, the border preferably extends around the entire circumference of the printing plate. It is not always necessary for the border to extend all of the way around the printing plate. The border may be limited to areas in which imaging is affected by fixture and/or mechanical deformation. Such areas may be only on two sides of the plate or on portions of sides of the plate.

In the case of printing plates with imageable material on both faces, the border is preferably created on both faces of the printing plate. In the preferred embodiment the width of this border is selectable. In what follows we shall refer to the border, as described here, as "a selectable width of border". In what follows the term "face" will be used with reference to printing plates to denote the flat surface of the printing plate. A plate with imageable material on both flat surfaces will therefore be referred to as having two "faces".

In the preferred embodiment, now depicted in FIG.2 with the printing plate removed from the drum for the sake of clarity, the bulk of the printing plate is coated with a positive working imageable layer 8 and the border is not coated, leaving the bare hydrophilic aluminum base 9 as the exposed surface in the border. During imaging of the plate there is no imageable material in the border area and thus there will be no imageable material in that area to be inked and printed. This particular embodiment ensures that the problem of undesirable artifacts on the periphery of the final printed image will be avoided altogether. In what follows the artifacts described here will be referred to as "printing process-induced artifacts".

In another embodiment of the invention the entire plate is coated with a positive working imageable layer 8 during manufacture. Before the printing plate is imaged, this layer is removed in the border area, by either mechanical means or chemical means, to physically expose the hydrophilic base 9.

In a further embodiment, depicted in FIG.3, the entire printing plate is coated with positive working imageable layer 8. In this case the border is coated with a hydrophilic material layer 10. An example of such material is the imaging material employed in negative working printing plates.

Figures 4A, 4B and 4C depict yet a further embodiment. As shown in FIG.4 A the entire printing plate comprising a hydrophilic base 9 (which may comprise an aluminum base layer), is covered in a coating of positive working imageable material 8 during manufacture. As depicted in FIG.4B, the border area of the positive working printing plate is irradiated prior to imaging. As depicted in FIG.4C, the irradiated material is then removed either through ablation during the irradiation or by a subsequent processing step. This renders the border area hydrophilic by physically exposing the underlying aluminum base 9 before the plate is placed on the machine on which it is to be imaged. The irradiation may be performed by means of a contact frame and an illumination source in the case of photosensitive imageable materials, or by means of a powerful heat source in the case of heat mode, or thermally activated, imageable materials. The heat source may be applied by bringing it into direct contact with the surface to be thermally treated. This may be done, for example, using a heated frame. Heating may also be achieved without direct contact. For example, achieved with laser irradiation. The terms "heat mode imageable material", "thermally activated imageable material" and "thermal resist" all refer to materials that may be imaged using heat rather than light. In either case, the plate may also have its border irradiated on an additional off-line plate-setter.

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof.

Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.

Claims

What is claimed is

1. A method for printing with a positive-working printing plate, said method comprising two separate steps of a) changing the affinity for ink of a selectable width of border along at least one edge of said positive-working printing plate and b) imaging said positive-working printing plate.

2. A method for printing with a positive-working printing plate, said method comprising two separate steps of a) increasing the affinity for water of a selectable width of border along at least one edge of said printing plate and b) imaging said positive-working printing plate.

3. A method for printing with a positive-working printing plate, said method comprising covering a selectable width of border along at least one edge of said printing plate with a negative working imageable material.

4. A method as in claim 2, wherein said increase of the affinity for water of said selectable width of border comprises covering said selectable width of border with a hydrophilic material.

5. A method as in claim 4, wherein said covering of said selectable width of border with said hydrophilic material is performed after said positive working printing plate has been mounted on a printing press.

6. A method as in claim 2, wherein said increase of the affinity for water of said selectable width of border comprises the removal of any material covering said base hydrophilic aluminum of said selectable width of border.

7. A method as in claim 6, wherein said removal is by chemical means.

8. A method as in claim 6, wherein said removal is by mechanical means.

9. A method as in claim 2, wherein said change of the affinity for water of said selectable width of border comprises irradiating the positive working imageable material over the area of said selectable width of border prior to said positive-working printing plate being imaged.

10. A method as in claim 9 wherein said irradiating of positive working imageable material is done using a contact frame in conjunction with an irradiation source.

11. A method as in claim 9 wherein said irradiating of positive working imageable material is done using a heat source.

12. A method as in claim 11 wherein the heat source comprises a heated frame.

13. A method as in claim 1 wherein said selectable width of border is left uncoated as bare hydrophilic aluminum when said positive-working printing plate is coated with imageable material during the manufacture of said positive working printing plate.

14. A method as in claim 2 wherein said selectable width of border is left uncoated as bare hydrophilic aluminum when said positive-working printing plate is coated with imageable material during the manufacture of said positive working printing plate.

15. A method as in claim 3, wherein said covering is performed after said positive working printing plate has been mounted on a printing press.

16. A printing plate comprising a hydrophilic substrate and an imaging medium on a face of the substrate, the hydrophilic substrate being exposed in a border extending around an edge of the printing plate.

17. The printing plate of claim 16 wherein the imaging medium fills a rectangular area which completely covers the face of the printing plate except in the border

18. The printing plate of claim 17 wherein the substrate comprises an aluminum sheet.