Emulsion printing plates and evacuation channels

Abstract: A photo printing plate having an upper air evacuation channel and a lower air evacuation channel with the lower air evacuation channel formed in a emulsion layer and the upper air evacuation channels formed by spacers located partially on top of the opaque pattern that projects outward from the emulsion layer and partially on top of the nonopaque emulsion layer adjacent to to the opaque pattern to provide an air evacuation channel formed by the evacuation channels in the emulsion surface and the spacers so that the air can be rapidly evacuated between the photo printing plate and a layer of etchant resist while maintaining the layer of etchant resist less than 10 microns from the top surface of the photo printing plate.

Description: FIELD OF THE INVENTION This invention relates generally to photo printing plates and etchant resist covered metal webs and, more specifically, to air evacuation passages on the photo printing plates used to project etchant patterns in the etchant resist located on the surface of a metal web or the like and also to a method of accurately and precisely forming images in the etchant resist covering the metal web while providing rapid evacuation of air between a photo printing plate and an etchant resist covered metal web. BACKGROUND OF THE INVENTION In a colored cathode ray picture tube a shadow mask or aperture mask is located between the electron guns at the rear of the tube and the phosphor coated face plate at the front of the tube. Electron beams pass through tiny openings or apertures in the shadow mask and impinge upon suitable color producing phosphor dots, a triad one dot for each of the three primary colors. During operation of the picture tube the shadow mask openings are used as a guide for the electron beams. In order to manufacture the shadow masks for use in the cathode ray tubes that are used in color monitors it is necessary to proceed through a number of steps to etch a metal web into a shadow mask having the small precision openings of proper dimensions and proper location. In a typical shadow mask manufacturing process one has to first make a photographic printing plate with a glass base. The photographic printing plate is referred to as a photo printing plate and it contains a master pattern for use in projecting an etchant resist pattern in a layer of etchant resist such as fish glue resist which is located on the surface of a metal web. One of the methods used to to create a photo printing plate involves applying a layer of a light sensitive emulsion to a sheet of smooth, flat glass. Next, the emulsion layer is exposed to a light source to form a first pattern in the emulsion. The emulsion is then developed to produce a master pattern on the glass base. The photo printing plate is used for optically defining an etchant resist pattern in a layer of etchant resist such as fish glue resist which is located on a metal web or the like. Since precision etching of small openings usually requires selective etching from both sides of the metal web a second photo printing plate is made for projecting a second master pattern in a second layer of fish glue resist that is located on the opposite side of the metal web. Fish glue resist is a commonly used light sensitive etchant resist which is applied as a liquid and allowed to dry forming a layer of etchant resist that has a thickness of about 3 to 10 µm (microns). The two photo printing plates with the master patterns are placed in register on opposite sides of the metal web and in contact with the respective layers of fish glue resist on the metal web. Next, one draws the photo printing plates into intimate contact with their respective layers of fish glue resist by pumping out the air between the photo printing plates and their respective layers of fish glue resist on the metal web. The removal of the air brings the photo printing plates into close proximity of their respective layers of fish glue resist. Next, one selectively exposes the light sensitive fish glue resist by directing light through the photo printing plates. After exposing the light sensitive fish glue resist to a suitable light source one develops the fish glue resist. During developing, the soluble portions of the fish glue resist are removed leaving a pattern of metal exposed openings in the fish glue resist. The openings in the fish glue resist permit an operator to etch the metal web in the areas not protected by the fish glue resist. One of the problems in accurately etching openings in a metal web is the difficulty in accurately projecting the image on the photo printing plate onto the fish glue etchant resist on the metal web. In one type of photo printing plates a master pattern on a glass base is formed in an emulsion layer about 3 to 5 µm (microns) thick. The actual opaque pattern formed in the emulsion may project outward from the emulsion surface a distance of about one micron. One of the factors affecting the accuracy of the image formed in the fish glue etchant resist on the metal web is how close the top of the opaque master pattern on the photo printing plate is to the etchant resist when the master pattern is projected onto the fish resist glue. Since light scatters as it travels between the master pattern and the fish glue resist it is extremely important that the etchant resist master pattern on the photo printing plate is located in as close proximity as possible to the layer of fish glue resist on the metal web. Ideally the master pattern on the photo printing plate would be located in direct contact with the fish glue resist on the metal web. However, the closer to direct contact between the photo printing plate and the fish glue resist the longer it takes to evacuate the air between the two and consequently the slower and more costly process for forming the aperture mask. To bring the etchant resist master pattern on the glass base to as close proximity as possible to the fish glue resist on the metal web the air between the etchant resist master pattern on the glass base and the fish glue resist on the metal web must be pumped out through channels located between the projecting master pattern on the glass photo printing plate. One of the problems associated with accurately making the etchant resist pattern in the fish glue on the metal web is that in order to obtain rapid evacuation of the air between the printing plate and the fish glue etchant resist it is necessary to have air evacuation channels that have a minimum depth of about 8 µm (microns). If the depth of the air evacuation passages is less than about 8 µm (microns) the time required to evacuate the air increases considerably. On the other hand if the spacing between the fish glue and the photo printing plate is made larger than about 8 µm (microns) the accuracy of the final image in the fish glue is reduced. In order to space the fish glue and the master pattern about 8 µm (microns) from each other islands of rubbery material are placed on top of the master pattern on the photo printing plate. These islands or spacers form air evacuation passages and consequently the time necessary to evacuate the air between the fish glue resist and the master pattern. However, the addition of islands on top of the emulsion causes the master pattern on the photo printing plate to be further away from the fish glue resist when the image is projected onto the fish glue resist. Consequently, the sharpness of the image formed in the fish glue resist decreases as the height of the spacers increases. The present invention provides a method and evacuation channels that are formed partially in the emulsion layer and partially by islands located on the emulsion. The result is a set of composite air evacuation channels that extend contiguously from the glass surface of a photo printing plate to a fish glue resist layer located on the metal web. The present invention permits a user to obtain rapid evacuation while still maintaining the accuracy and preciseness of the image in the etchant resist. BRIEF DESCRIPTION OF THE DRAWINGS The sole figure shows an enlarged view of a photo printing plate with an emulsion coating and spacers sandwiched against a metal web. To illustrate the invention the magnification scales on the vertical and horizontal axis are of different magnification. SUMMARY OF THE INVENTION Briefly, the present invention comprises an emulsion coated printing plate according to claim 1. The present invention also comprises a method of precise etching of a metal web, using such an emulsion coated printing plate. This method is stated in claim 10. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1 reference numeral 10 identifies a an enlarged portion of conventional photo printing plate and a fish glue etchant resist covered metal web for forming a color monitor aperture mask. The drawing is not a full scale enlargement since the vertical axis is magnified on the order of about 100 times and the horizontal axis is magnified on the order of about 10 times. Photo printing plate 10 has a flat glass support surface 19 with a compressible emulsion layer 17 extending over portions of top surface 19. Located in and projecting outward from emulsion 17 is an opaque master pattern formed by a set of opaque regions 16 that are to be used to optically project opaque master pattern on a layer of etchant resist 13 on a metal web 11. Located on top of emulsion 17 and opaque regions 16 is a set of spacers 15 which have top surfaces 15a in contact with resist surface protrusions 14. To appreciate the relative size of the present invention typical dimensions of the thickness of the layers are indicated in microns on the left side of the drawing. The dimensions are included for the purpose of indicating the relative relationships that occur between a fish glue etchant resist 13 and the photo printing plate 20. It should be understood that during any run there will be variation in the thickness of the emulsion spacers and the fish glue etchant resist. It should also be understood that other emulsions or resist could be used with invention. In order to illustrate the effect of irregularities in the surface of the metal web on irregularities on the surface of the fish glue etchant resist 13 the metal web surface irregularities 12 are indicated as having a maximum amplitude of 1 µm (micron). The corresponding irregularities in the fish glue etchant resist 13 are still present but at an amplitude of about 50% of the irregularities of the metal surface. In some instances the irregularities 12 in the surface of the fish glue etchant resist 13 can be used to provide a portion of a top air evacuation channel which is identified by V c . Generally, it is preferable to have the metal surface irregularities small in comparison to the overall tolerances of the openings formed in the metal web. That is, the larger the irregularities in the surface of the metal web the greater the detrimental effect on the accuracy of the image in the etchant resist that can be formed and etched in the metal web. The distance that master pattern images must be projected to penetrate through the fish glue etchant resist is designated by P o . Typically, for high grade aperture masks this dimension should be kept to a projection distance of about 8 µm (microns) or less in order to minimize the effects of light scattering as the image of the master pattern is projected on and into the etchant resist 13. In order to rapidly evacuate the air from between the photo printing plate and the fish glue etchant resist the depth of the air channels should be about 8 µm (microns) or more. To achieve the projecting distance of about 8 µm (microns) or less and obtain air evacuation channels of 8 µm (microns) or more I have formed air evacuation channels that are located partially in the emulsion and partially between spacers located on top of the emulsion 17. The drawing shows the etchant resist protrusions 14 located in intimate contact with the top surfaces of spacers 15. The top surfaces of spacers 15 are brought into intimate contact with etchant resist 13 through the type of prior art printing frames as shown in US-A- 3,199,430 and US-A- 2,814,975. To obtain intimate contact between the printing plates and the layers of etchant resist the air is evacuated between the etchant resist layer and the photo printing plate to draw printing plate 20 into into intimate contact with the light sensitive etchant resist 13. The drawing shows that in my invention the channels or paths for evacuation of air between plate 20 and etchant resist 13 are provided for by a set of evacuation channels that are located in a stacked or contigous relationship to each other. That is the air evacuation channel of my invention includes, an upper air evacuation channel V b, , a lower air evacuation channel V a and if desired a smaller top air evacuation channel V c formed in the top surface irregularities of etchant resist 13. Typically, the width of the channel may be on the order of two to four times the width of the slots formed in the metal web. Thus the air evacuation channels are substantially wider than they are deep. In the present invention the transparent plate 20 is used for projecting light energy therethrough to project opaque images of members 16 onto resist covered metal web 11. Located on top surface 19 of transparent plate 20 is a transparent layer of compressible emulsion 17. Also located on top top surface 19 is a first opaque master pattern 16 which prevents light from projecting through transparent plate 20 and onto resist 13 in those regions covered by opaque master pattern 16. The drawing shows the transparent layer of compressible emulsion 17 projecting upward from top surface 19 of transparent plate 20 and defining a lower evacuation channel V a for evacuation of a portion of the air between top surface 19 and the layer of etchant resist 13. An upper evacuation chamber is formed by a plurality of spacers 15 located on first opaque master pattern 16 and emulsion 17. The spacers 15 space the first opaque master pattern 16 from the layer of etchant resist 13 to form an upper evacuation channel V b proximate and contigous the lower evacuation channel V a . The combination of the upper evacuation channel and the lower evacuation channel coact with the layer of etchant resist 13 to define a passage for rapid evacuation of air between plate 20 and the etchant resist 13. By utilizing a composite air evacuation channel with the composite air evacuation channel formed partially in the emulsion and partially by separate spacers with the spacing of the surface of the top surface of the transparent plate and the metal web being on the order of about 8 to 10 µm (microns) or less it enables a user to precisely project an image on the layer of etchant resist 13 and to evacuate the air between the top surface of the transparent plate and the etchant resist in less than 30 seconds. In the method of the present invention for etching aperture masks for cathode ray tubes the user forms a layer of compressible emulsion 17 on a support plate. Next one selectively develops the layer of compressible emulsion 17 to produce opaque images 16 that project outward from the layer of compressible emulsion 17. One can then selectively remove the layer of compressible emulsion to form a lower air evacuation channel V a in the layer of compressible emulsion 17. By placing spacers 15 on top of the compressible emulsion opaque pattern 16 and on top of a portion of the compressible emulsion 17 that does not have the opaque pattern one defines an upper air evacuation channel V b . After forming the composite channel one places the spacers against etchant resist 13 located over a material such as a metal web 11. One can then quickly evacuate the air from between the glass base plate 20 and the etchant resist 13 by removing the air through the contiguous upper air evacuation channel and the lower air evacuation channel. After removing the air one develops etchant resist 13 so that the material not covered by the etchant resist can be etched. In the final step one etches the material with an etchant such as ferric chloride to produce an aperture mask. The forming of channels in the emulsion is described in US-A- 3,897,251. Similarly, the formation of islands on top of a pattern is described in US-A 4,588,676, US-A-4,656,107 and US-A-4,664,676. EP-A- 0 402 616 describes the use of spacers located on top of the opaque regions of the layer of transparent emulsion in order to improve the evacuation of air from between the emulsion layer and the etchant resist. The present invention provides for the improved evacuation of air from between the emulsion layer and the etchant resist by forming composite channels made up of an air evacuation channel located in the emulsion and an air evacuation channel located between spacers on top of the emulsion layer. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: An emulsion coated printing plate for printing an etchant resist pattern on a resist coated material by evacuating the air from between said emulsion coated printing plate and said resist coated material to bring said emulsion coated printing plate and said resist coated material into close proximity to produce a sharp image in said resist coated material comprising: a transparent plate (20) having a top surface (19); a layer of transparent emulsion (17) having a first surface extending over portions of said top surface (19) of said transparent plate (20); an opaque master pattern (16) located in said layer of transparent emulsion (17) to provide opaque regions that extend upward from said first surface of said layer of transparent emulsion (17); and a plurality of spacers (15) located on top of said first surface of said layer of transparent emulsion (17) and on top of said opaque regions to form upper air evacuation channels (V b ), CHARACTERIZED IN THAT said layer of transparent emulsion (17) has lower air evacuation channels (V a ) formed therein so that when said transparent plate (20) having said layer of transparent emulsion (17) and said spacers (15) is placed against a second surface one can quickly remove the air from between said transparent plate (20) and said second surface by evacuating air through an air evacuation passage formed by the coacting of said upper air evacuation channel (V b ) and said lower air evacuation channel (V a ). The emulsion coated printing plate of claim 1 wherein said spacers (15) extend approximately 3 µm (3 microns) above said first surface of said layer of transparent emulsion (17) to thereby define said upper air evacuation channels (V b ) between said spacers (15). The emulsion coated printing plate of claims 1 or 2 wherein said layer of transparent emulsion (17) is about 3 µm (3 microns) thick. The emulsion coated printing plate of claim 1 wherein said spacers (15) project a minimum of 2 µm (2 microns) above said first surface of said layer of transparent emulsion (17). The emulsion coated printing plate of any of claims 1-4 in which said second surface comprises a metal web (11) having a surface (12) to be etched, covered on the side facing said transparent plate (20) with an etchant resist (13) in which a shadow mask pattern is to be laid out, where the spacing (P o ) between the top of said opaque master pattern (16) and said metal web (11) is of the order of 10 µm or less to thereby enable a user to precisely project an image on said layer of etchant resist (13) and evacuate the air between said top surface (19) of said transparent plate (20) and said layer of etchant resist (13) in less than 60 seconds. The emulsion coated printing plate of claim 5 wherein said metal web (11) having said surface (12) to be etched comprises a surface having surface irregularities of 1 µm (1 micron) or less. The emulsion coated printing plate of claims 5 or 6 wherein said opaque master pattern (16) comprises projections extending about one µm (one micron) above said layer of transparent emulsion (17) and wherein said layer of transparent emulsion (17) is compressible. The emulsion coated printing plate of one or more of claims 1-7 wherein said evacuation channels (V a , V b ) are spaced substantially uniformly over said transparent plate (20). The emulsion coated printing plate of one or more of claims 5-8 including a top air evacuation channel (V c ) located in said layer of etchant resist (13) on said metal web (11). A method of precise etching of a metal web (11) using the emulsion coated printing plate of any of claims 1-9, comprising the steps of: forming a layer of transparent emulsion (17) with a top surface on said transparent plate (20); selectively developing said layer of emulsion (17) to produce an opaque pattern (16) that projects about one µm (one micron) outward from the top surface of said layer of emulsion (17); placing one end of a plurality of spacers (15) on top of said opaque pattern (16) and on the top surface of said layer of emulsion (17) that does not have said opaque pattern (16) to partially define an upper air evacuation channel (V b ); removing a portion of said layer of emulsion (17) to form a lower air evacuation channel (V a ) ; placing the other end of said plurality of spacers (15) against a layer of etchant resist (13) located over a metal web (11) to be etched so as to define an air evacuation passage formed by the coacting of said upper air evacuation channel (V b ) and said lower air evacuation channel (V a ) ; evacuating the air from between said layer of emulsion (17) and said etchant resist (13) by removing the air through said upper air evacuation channel (V b ) and said lower evacuation channel (V b ); then developing said etchant resist (13) so that said metal web (11) not covered by said etchant resist (13) can be etched; and then etching said metal web (11) with said etchant resist (13) to produce an etched article.

Inventors: THOMS ROLAND H, THOMS, ROLAND H.
Applicants: BMC IND INC, BMC INDUSTRIES, INC.
Application year: 1992
Main classification: G03F1
IPC codes: H01J9, G03F1, C23F1