Photocopying – Projection printing and copying cameras – Detailed holder for photosensitive paper
Reexamination Certificate
2000-01-04
2001-07-24
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Detailed holder for photosensitive paper
C264S401000, C430S270100
Reexamination Certificate
active
06266134
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to apparatus and methods for making plates from liquid polymer by supporting a liquid polymer on a flat surface and then selectively curing the liquid polymer by exposure to actinic radiation. The invention is particularly useful for making photopolymer flexographic printing plates.
BACKGROUND OF THE INVENTION
In many industries solid plates (also referred to as sheets) of polymer are made by pouring a liquid polymer onto a flat horizontal surface, letting the liquid polymer level under the influence of gravity and curing it to form a solid. The liquid “polymer” may be a polymerizable material which is not itself a polymer according to the literal meaning of the word polymer. Typically the material may be selectively solidified, or may have its solubility in a solvent altered, by exposure to actinic radiation.
Some polymers can be cured by exposure to actinic radiation such as ultraviolet (UV) light. When the curing is done by UV light, the polymer is referred to as a photopolymer. Photopolymers can be selectively cured by placing an exposed and developed film under the liquid polymer layer shining a light on the underside of the polymer layer and using an image on the film to control the amounts of light reaching different parts of the polymer. Flexographic printing plates and letter press polymeric printing plates can be prepared in this manner.
Prior art systems provide an ultraviolet light source which has dimensions approximately equal to those of the largest printing plate to be cured. The light source evenly illuminates an under side of a large support plate, which is typically a thick sheet of glass. To make a plate, liquid polymer is poured on an upper surface of the support plate. The upper surface of the support place provides a flat base surface for the polymer.
Such prior art systems have two main disadvantages. First, providing a large light source capable of evenly illuminating each portion of the support plate is expensive and is wasteful in cases where the polymer plates to be made are significantly smaller than the support plate.
Secondly, support plates must generally be thick to reduce sagging. Any large glass plate which is supported around its edges tends to sag in its center. The sag of the glass support plate causes the finished polymer plate to be thicker in its middle area. This is because the top surface of the polymer is always flat due to the liquid levelling itself, while the bottom follows the glass support plate. To achieve a desired thickness uniformity of, for example, 25 microns (0.001″) over an area of a few square meters, special measures must be taken to support the weight of the glass. Thicknesses of 15 mm or more are common. A thick support plate tends to attenuate radiation which is passing through it. Therefore, a powerful illumination source is typically required for curing polymer printing plates.
An additional problem occurs when such prior systems are used to image photopolymer plates. Such systems typically offer little control over the cross sectional profiles of features in the imaged plates.
There is a need for apparatus and methods for curing photopolymer plates which are more efficient and less expensive than current methods and apparatus. There is also a need for a system which provides enhanced control over the cross sectional profiles of features in imaged photopolymer plates.
SUMMARY OF THE INVENTION
This invention provides apparatus for curing a photopolymer in which a light source is moved relative to the photopolymer to expose the photopolymer. The light source preferably emits collimated light. This allows the generation of sharper flexographic printing plates, as the collimated light will cast sharper shadows of an image on a masking film than would uncollimated light. In preferred embodiments of the invention, the collimation of the light source may be adjusted to control the form of the edges of imaged regions of the plates and the resulting cross sectional profiles of imaged features. If a small sheet of polymer is being made then the light source may be scanned over a region which is smaller than, and lies within, the maximum area over which the light source can be scanned. This improves efficiency.
While the invention may be used for the manufacturing of any solid polymer object from liquid polymer, the preferred embodiment relates to flexographic printing plates. An exposure system according to this invention may be used to expose plates made from liquid photopolymer or plates made from unexposed solid gel-like photopolymer (which are also known as “rigid plates”).
One aspect of the invention provides a method for exposing a photopolymer, such as a printing plate. The method comprises providing a light source; providing a layer of a photopolymer; and, operating the light source to generate ultraviolet light and exposing the photopolymer to the ultraviolet light while moving the light source in at least two dimensions to evenly expose the photopolymer to the ultraviolet light. The light source preferably comprises an arc lamp. Most preferably the light source has variable collimation.
In preferred embodiments the method comprises: providing first and second masks having areas substantially opaque to ultraviolet light and areas substantially transparent to ultraviolet light, the transparent areas in the first mask corresponding to and being slightly larger than the transparent areas in the second mask; inserting the first mask between the light source and the photopolymer layer, and exposing the photopolymer layer to ultraviolet light from the light source with the collimator set to provide light of a first degree of collimation incident on the mask; and, inserting the second mask between the light source and the photopolymer layer, and exposing the photopolymer layer to ultraviolet light from the light source with the collimator set to provide light of a second degree of collimation, less than the first degree of collimation, incident on the mask. The steps of inserting the first mask and inserting the second mask are performed in any order. This provides the ability to better control the cross sectional profiles of exposed features.
Another aspect of the invention provides apparatus for curing a photopolymer. The apparatus comprises: a light source which emits actinic radiation, such as ultraviolet light, when energized; a movable stage supporting the light source, the movable stage capable of moving the light source in at least two dimensions on a surface of motion; and, a support surface for supporting a photopolymer plate, the support surface parallel to the surface of motion of the light source. In some preferred embodiments the light source comprises an arc lamp and a reflector and the arc lamp is movable relative to the reflector to allow a collimation of light emitted by the light source to be adjusted. The movable stage is preferably controlled by a computer controller. The controller preferably permits a user to select an area over which the light source is scanned. The computer controller may scan the light source in a pattern including a plurality of parallel lines.
Further features and advantages of the invention are described below.
REFERENCES:
patent: 5096530 (1992-03-01), Cohen
patent: 5171490 (1992-12-01), Fudim
patent: 5772947 (1998-06-01), Hull et al.
Adams Russell
Creo Products Inc.
Kim Peter B.
Oyen Wiggs Green & Mutala
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