Radiation imagery chemistry: process – composition – or product th – Effecting frontal radiation modification during exposure,... – Involving motion during exposure – e.g. – dodging – etc.
Reexamination Certificate
1999-06-23
2001-01-30
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Effecting frontal radiation modification during exposure,...
Involving motion during exposure, e.g., dodging, etc.
C430S396000, C430S005000, C430S935000, C430S300000, C430S306000, C430S273100, C427S425000, C427S510000, C427S512000, C427S521000, C427S385500
Reexamination Certificate
active
06180325
ABSTRACT:
TECHNICAL FIELD
This invention relates to printing, more specifically to methods and apparatus for exposing photosensitive flexographic printing plates. The methods and apparatus allow the creation of exposed printing plates without the need to handle a separate mask and without the disadvantages inherent in flexographic printing plates which are supplied with integral masks.
BACKGROUND
Photosensitive flexographic printing plates have a layer of a photo-polymer which undergoes a chemical reaction when exposed to actinic radiation. Typically ultraviolet light is used to expose the photo-polymer in flexographic printing plates. Upon exposure to ultraviolet light the photo-polymer undergoes a cross-linking reaction. The printing plates are exposed through a mask which blocks light from reaching certain portions of the photo-polymer. The mask is patterned with a pattern of opaque and non-opaque areas which form an image to be recreated on the printing plate. After exposure the printing plate can be developed. After development the pattern of exposed and unexposed areas on the printing plate provide a textured surface which may be used to print an image.
Some types of printing plate are used in conjunction with a separate mask. Typically the mask is made from a half-tone film, such as a silver-halide film, which can be placed on top of the plate before the plate is exposed. A problem with this method is that it is necessary to keep the film in intimate contact with and in accurate alignment with the plate during exposure. If the mask is misaligned with the printing plate or if the mask is not kept in intimate contact with the printing plate during exposure then the results obtained will be less then optimum. Even when a separate mask is applied carefully to a printing plate the results obtained are often not optimal.
Another method for exposing flexographic plates involves providing a photosensitive plate which has a mask formed integrally with the plate. Such plates include a layer of a photosensitive material such as a photo-polymer gel underlying a layer of mask material. Because the mask is integral with the printing plate, this approach provides a mask which is always in intimate contact with the photo-polymer layer and which is always properly aligned with the printing plate. A problem with this method is that the material from which the mask layer is made can diffuse into the photo-polymer gel. Over time this can degrade the gel. As a result, printing plates which include an integral mask may have a low shelf life.
The problem of low shelf-life is exacerbated because there are a large number of different types of photo-polymers used in printing plates. All have different characteristics and are used in different applications. If each of the different types of printing plates has a short shelf life then it is often not practical for a user to keep on hand a stock of printing plates of any types which will not be quickly used up.
A further problem with printing plates which have an integral masking layer is that the masking layer on existing plates is typically fragile. A scratch in any portion of the masking layer can make the entire plate unusable.
U.S. Pat. No. 5,262,275 discloses a flexographic printing element which incorporates a barrier layer which separates an infrared ablatable masking layer and a photo-polymerizable layer. The barrier layer prevents migration of material from the masking layer into the photo-polymerizable layer. While this construction improves the shelf life of a printing plate it does not avoid the requirement to maintain a number of different types of printing plate in stock which have different types of photo-polymer layer. It also does not address the problem that the masking layer of a printing plate may become damaged, thereby making the plate unusable.
There is a need for a method for imaging flexographic printing plates which avoids or reduces at least some of the difficulties which are inherent in the above-noted existing technologies.
SUMMARY OF INVENTION
This invention provides methods for exposing photosensitive printing plates which address some of the limitations of the prior art. In a preferred embodiment the method comprises mounting a printing plate to a holder; applying a patterned coating to an outer surface of the printing plate by moving a coating applicator relative to the holder; and, exposing the printing plate through the patterned coating to actinic radiation without removing the printing plate from the holder. The invention permits performing coating and exposure steps in a single apparatus. This avoids handling-induced damage.
Since the plate does not need to be handled between the application of the mask and the exposure to actinic radiation, the mask material can be selected to have excellent optical opacity. It is not necessary to select a mask material which is also capable of withstanding handling. Masks used in systems where plates must be handled before they are exposed must have acceptable shelf-life, scratch-resistance, flexibility (to avoid cracking when a plate is bent around a drum) and more. Selecting a mask material having acceptable handling characteristics could require one to sacrifice optimal optical opacity. Where the mask is not disturbed between application and exposure, as is possible with this invention, the one property of the mask which is the essence of the mask, its optical transmittance characteristics, can be optimized.
The invention also permits the use of printing plates which are supplied and stored in an uncoated form. Such printing plates tend to have longer shelf lives than printing plates which are pre-coated. Further, uncoated printing plates lack delicate coatings which can be damaged during storage.
In preferred embodiments of the invention the holder comprises a rotatable drum and the patterned coating is applied while rotating the drum. The coating may be applied by spraying a coating onto the printing plate.
After the coating has been applied selected portions of the sprayed coating may be removed by laser ablation. In the alternative, the coating may comprise a coating of a thermally sensitive material and patterning the coating may comprise selectively changing an optical transmission of the thermally sensitive material by locally heating the thermally sensitive material with a laser. In the further alternative a patterned coating may be applied by selectively applying an opaque coating to a surface of the printing plate with an ink jet printing head.
It is advantageous to simultaneously apply the coating to some parts of the printing plate, pattern the coating in other parts of the printing plate and expose the coating to actinic radiation in yet other parts of the printing plate. This may be done, for example, by providing a sprayer and a laser and simultaneously moving the sprayer and laser axially along the printing plate with the laser trailing the sprayer.
Another embodiment of the invention provides a method for exposing photosensitive printing plates. The method comprises mounting a printing plate to a rotatable drum, while rotating the drum, applying a patterned coating to an outer surface of the printing plate by providing a coating applicator and moving the coating applicator axially relative to the drum while applying a coating to the surface of the printing plate and moving a coating patterner axially relative to the drum trailing the coating applicator, the coating patterner either selectively removing the applied coating or selectively changing optical properties of the applied coating to create a mask layer on the printing plate. The patterned printing plate may subsequently be exposed to actinic radiation.
Yet another embodiment of the invention provides a method for exposing photosensitive printing plates. The method comprises: providing a printing plate mounted in a holder, the printing plate having a surface coated with a coating; patterning the coating by moving a coating patterner relative to the holder over the coated surface of the printing plate; and, e
Baxter Janet
Creo SRL
Lee Sin J.
Oyen Wiggs Green & Mutala
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