Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2002-02-19
2004-09-28
Gilliam, Barbara (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S309000, C430S330000, C430S348000, C430S413000, C101S463100
Reexamination Certificate
active
06797454
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention pertains to a process and apparatus for processing a photosensitive element using a thermal process without the use of solvents to form a relief pattern and, in particular, to a process and apparatus for thermal development of the photosensitive element to form a flexographic printing plate.
Burg et al. in U.S. Pat. No. 3,060,023 describe a process for transferring images from photopolymerized image-bearing elements to a receptor surface using a dry thermal process. A film support with a photopolymerizable stratum is exposed to an image to provide exposed areas that do not melt and “underexposed” areas that can melt and transfer to an image receptive element. In numerous examples, a paper image receptive element is placed in contact with the stratum surface on a film support and heat is applied by a hot flat heating element pressing against the film support to heat the assemblage. In some examples, an unheated assemblage is passed between two rollers, one of which is heated. The paper is stripped from the stratum while still warm, and the transfer of the unexposed material of the stratum to the paper element is effected. Multiple copies of the image on the paper can be obtained by repeating the thermal transfer process. In a few examples, the thermal transfer is accomplished by heating the paper and pressing it into intimate contact with the stratum. It is suggested without further explanation, that “. . . heat can be applied at any stage of the process prior to the separation step to either or both elements provided the transfer temperatures correspond to at least the softening temperature of the photopolymerizable stratum. Heat can be applied by means well known in the art, e.g., rollers, flat or curved heating surfaces or platens, radiant sources, e.g., heating lamps, etc.”. Heating times varied in the examples from 0.5 seconds to 10 seconds. Typical film thicknesses are 1 mil to 4 mils and stratum thicknesses are 0.5 mil to 4 mils. In a few examples, it is disclosed that the film support, with a relief image in the stratum, could be further treated with actinic light and used for printing in a rotary press to make copies.
U.S. Pat. No. 5,175,072 issued to Martens describes a thermal process for manufacturing a flexographic printing plate by providing a radiation hardenable composition as a layer on a flexible substrate, imagewise irradiating the composition to harden the composition in irradiated areas, heating the composition layer to a temperature of between 40° C. and 200° C. to soften the unirradiated area, contacting the composition layer with an absorbent layer which can absorb the softened unirradiated area which is able to flow into the absorbent layer, and removing the absorbent layer, thereby removing the absorbed flowable composition from the unirradiated areas of the composition on the flexible substrate. This leaves behind on the substrate a raised relief structure of irradiated, hardened composition that represents the irradiated image. This relief area becomes the ink receptive surface that both receives ink from an inking roll in a printing process and transfers the ink to the printing substrate during the printing operation. The process can also utilize a predevelopment step that can improve the adhesion of the irradiated, hardened composition to the flexible substrate by first developing a “floor” of hardened composition on the substrate. This is accomplished by transmitting ionizing radiation through the flexible substrate to polymerize a layer of composition adjacent the flexible substrate. This layer is not removed during the imagewise development and removal of the unirradiated composition.
During the heating process, a laminate comprising the flexible substrate, the composition layer, and the absorbent layer are heated by placing them on a heated platen with the flexible substrate, a polyester film, in contact with the platen. The absorbent layer comprises a non-woven nylon web. After a few seconds of warm-up time to allow the laminate to equilibrate with the platen temperature of about 135° C., the laminate is passed between two heated, rubber covered, nip rolls moving in a counter-rotating speed at about 30 cm/minute. The rolls are gapped apart so as to lightly compress the laminate as it is introduced into the gap. As the laminate exits the nip rolls, the absorbent layer is lifted from the heated composition surface with steady tension. The unirradiated areas of the composition are removed via absorption into the non-woven web. The heating and pressing steps are repeated several times until at least 75% of the unirradiated composition is removed from the unirradiated areas. It is desireable that the flexible substrate and the irradiated, hardened composition is stable at the elevated temperatures required to remove the unirradiated composition so that the flexible substrate and hardened composition are not distorted by more than 2% in any surface dimension.
U.S. Pat. No. 5,279,697 issued to Peterson et al. describes an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element. The element comprising the flexible film substrate and composition layer, with irradiated and unirradiated areas, is mounted on a preheating drum that is heated with an electrically heated blanket mounted on an inner surface of a main wall of the drum. The heat must travel through the wall of the drum and through the flexible substrate to preheat the composition layer to a temperature near the melt point of the unirradiated area.
A continuous sheet of absorbent layer of non-woven nylon web is pulled from a supply roll and passed over a hot roll to heat the web. The hot roll is urged toward the preheating drum, thereby pressing the heated web against the preheated composition layer of the printing element on the preheating drum. The heat in the absorbent web is transferred to the printing element upon contact so the temperature of the flexible film is raised to a temperature sufficient to enable the unirradiated portions of the composition layer to liquefy and be absorbed into the absorbent layer of non-woven web. The hot roll may be heated by an electrical core heater or by other means that might use steam, oil, hot air, or the like to provide a temperature sufficient to melt a portion of the composition on the flexible film. As the preheating drum and hot roll rotate in contact together, the web is pressed against the printing element to absorb the liquefied unirradiated composition and is then pulled away from the printing element, thereby separating the absorbed composition from the printing element. The web is transported away from the hot roll and is rewound for waste removal or recycling. Several cycles of passing the printing element past the hot roll are repeated to progressively remove most of the unirradiated composition from the printing element. When the unirradiated composition is removed, a printing plate with a raised relief of hardened irradiated area results that is suitable for the printing process. In some cases a final irradiation of the printing element is carried out to harden any residual unirradiated composition.
In the methods and apparatuses disclosed above to Burg et al., Martens, and Peterson et al., the common practice is to heat the film substrate to a temperature near the melting point of the unirradiated composition. However, heating the film substrate can undesirably distort the substrate to the extent that it may affect the quality of the relief area of the printing plate that becomes apparent during printing. It is common that the temperature of the film substrate gradually increases with each repeat of the heating and absorbing cycle, so by the last cycle, the film substrate is almost at the melt point of the unirradiated composition. The problem presented by substrate distortion is that for a three or four color printing process, three or four plates must be made with images t
Belfiore David Anthony
Brown Robert Lee
Cushner Stephen
Drury Robert Finley
Hackler Mark A.
E. I. du Pont de Nemours and Company
Gilliam Barbara
Magee Thomas H.
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