Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C1/00—Forme preparation
  • B41C1/14—Forme preparation for stencil-printing or silk-screen printing

Definitions

• This invention is concerned with a method of producing fine-apertured sheet materials and sheets produced by the method. It is in particular concerned with the production of rotary screen printing cylinders and will be described principally by reference to such cylinders. It is however to be understood that the invention has a wider applicability, for example in the production of flat fine-apertured sheets for other purposes, e.g. filtration or sieving.
• a cylinder of a fine-apertured sheet material is first engraved with the desired pattern to be printed by forming on the screen areas of blocked apertures forming a negative image of the pattern. There are thus areas on the screen whose apertures are unblocked and the print medium can be forced through these apertures to apply a positive image of the pattern onto the fabric.
• wire cloth rotary screen printing cylinders is described for example in British Patent Specifications 756315, 830506, 1050649 and 1208109. The problems and advantages associated with these screens can be summarized as follows.
• Woven wire mesh cloths which are sufficiently fine to give reasonably good definition in engraving and printing on textile fabrics (for example 60, 80 or 100 mesh per inch, usually woven from phosphor bronze wire or occasionally from Monel or stainless steel wire) invariably have too wide apertures and deposit too much print paste on the fabric for the printing conditions under which they have to work. In their normal loom state they also have low dimensional stability which can cause distortion and damage in printing as well as bad pattern registration in multi-colour printing; this is due to the different weights of print paste that each screen may contain at any given moment.
• the apertures can be reduced to a suitable size by electrodeposition of copper and/or nickel, but as the amount of copper or nickel is increased the wire mesh becomes increasingly brittle and very easily damaged in printing or handling. Additionally, towards the end of the electroplating process, and as the apertures become smaller, it becomes more difficult to control accurately the termination of the process and to achieve high standards consistently.
• the screen cylinder is formed by electrodeposition of metal on a mandrel.
• the so-called electroforms produced by this method can have very small aperture sizes and a high aperture density (e.g. up to about 2000 per cm 2 ) ; however, the mandrels required are expensive and the electroforms are brittle and require very careful handling.
• the plastic coating has the advantage of increasing the strength of the substrate and reducing its brittleness.
• the method can be operated with or without reduction in the aperture size, and if desired some of the apertures can be closed completely.
• the plastic can be coated onto the substrate quickly, evenly and accurately, and the deposition can be accurately terminated.
• the deposition is thus easily controllable and allows consistent standards to be achieved at low cost.
• the method is primarily applicable to the treatment of fine-apertured sheet for rotary screen printing cylinders, and is a fast and inexpensive way of making such cylinders.
• the method is operated to reduce the aperture size.
• This is particularly useful when the substrate apertures are relatively coarse (e.g. 100-500 ⁇ m), as for example in wire mesh rotary printing screens.
• a sheet can be made with a very small aperture size and a good aperture density that is sufficiently strong for continuous use without being brittle.
• the aperture size can be reduced in 1 to 3 minutes to such a degree that the fine-mesh fabric cover is unnecessary.
• Printing cylinders produced by this method are of particular advantage, on account of their strength, for use on wide textile printing machines and carpet printing machines, in place of the electroformed nickel screens normally used.
• the fine-apertured substrate which can be coated by this method may be any suitable electrically conduc tive material. It may for example be either metallic or non-metallic, and it may be woven or non-woven.
• the substrate is preferably made of metal, such as phosphor bronze, which may be lightly electroplated with for example nickel or copper, or it may be made of stainless or non-stainless steel, copper, aluminium, nickel, brass or Monel. These metallic substrate sheets are preferably used in the form of a wire mesh.
• a fine-apertured non-metallic substrate may be used, for example of a plastics material.
• Substrates of this kind are not naturally electrically conductive and they therefore have to be coated or otherwise treated with an electrically conductive material to enable them to be used in the electrodeposiion coating step.
• Plastics may for example be rendered conductive by coating with graphite or an electrolyte, as is well known in the electroplating and electrodeposition arts.
• Electroplated plastic e.g. nickel-plated polyester
• Suitable plastics for the substrate are synthetic or natural polymeric materials, e.g. polyesters, polyamides, polyolefins such as polypropylene, or regenerated cellulosic materials.
• the aperture size of the sheet substrate may for example be from 100 to 500 microns, usually 150 to 300 microns, and the size may be reduced by the electrophoretic coating to, for example, 40 to 200 microns. However, sheets having smaller aperture sizes (e.g. down to 35 ⁇ m) can also be used.
• My electrodeposition method can also be used to increase the strength of the substrate material with little or no reduction in aperture size. This is of particular value in connection with electroforms for rotary screen printing, which have a satisfactory combination of aperture size and aperture density in their original state but lack strength and structural stability.
• the plastic material is electrodeposited onto one side only of the electrically conductive fine-apertured sheet substrate.
• the substrate sheet may again be in the form of a cylinder, e.g. a rotary screen printing cylinder, and is preferably an electroformed rotary printing cylinder. If desired however the substrate may be a flat sheet, which can either be formed into a cylinder after being coated by the method of the invention or be left flat for other uses, e.g. as a filter or sieve.
• one side of the substrate may be temporarily protected by a coating or film of a non-conductive material.
• the non-conductive coating can then be removed after electrodeposition of the plastic material.
• the non- conductive coating material should be water-insoluble and easily removable after electrodeposition, for example with an organic solvent; examples of suitable materials are esters of poly (methylvinyl ether/maleic acid) such as the monobutyl ester sold under the trade name Gantrez ES 435 (GAF), bitumen or ultra-violet hardened polyvinyl alcohol. Gantrez ES 435 can for example be removed with an organic solvent such as isopropanol. Hardened polyvinyl alcohol is removable with sodium hypochlorite solution. Another material which may be used is "Pro-peel" (a polyvinyl chloride solution made by TAK Chemicals Ltd.) which can be peeled off after the electrodeposition process.
• Electroforms are produced by electrodeposition of metal (usually nickel) on a mandrel, and the plastic material can simply be electrodeposited into the outer surface of the electroform whilst it is on the mandrel.
• the mandrel can be transferred to the tank for electrodeposition of the plastic material; the electroform remains on the mandrel and the mandrel prevents coating of the inner surface of the electroform.
• a particular advantage of this method is that reduction of the aperture size is prevented by the resist already on the mandrel, the electrodeposited plastic material acting only to strengthen the cylinder.
• Electroform printing cylinders are of very light construction and normally have for example a metal thickness of 60 - 200 ⁇ m, aperture diameters of 50 - 400 ⁇ m and from 250 - 1800 apertures per cm 2 .
• the thinner cylinders generally have the larger aperture densities and smaller aperture sizes, and they are easily damaged in handling both on and off the printing machine. Extra strength and durability can be obtained by increasing the metal thickness, but the apertures then become coarser and much design detail is lost in engraving and printing.
• a cylinder 80 ⁇ m thick may have about 1800 apertures/cm 2 and aperture diameters of about 60 ⁇ m; this is a good combina tion of aperture sizes and diameters for detailed printing, but the screens are very delicate.
• screens 90 to 110 ⁇ m thick (having aperture diameters of 120 or 150 ⁇ m and aperture densities of about 1000 or 600) are noticeably stronger, but are less suitable for detailed work.
• the coating of one side of the sheet enables the strength of the thin electroform cylinders to be increased without reducing the aperture size or density.
• the electrodeposition method can for example be used to deposit a coat of plastic 5 - 40 ⁇ m thick on the electroform, and this considerably increases the strength and resistance to tearing and creasing.
• One-side coating can also be applied to coarser electroforms (e.g. those 90 - 200 ⁇ m thick), and here some reduction in aperture size can be allowed to occur and in some circumstances is positively desirable. This is achieved by applying an extra thick coating of plastic. This not only reduces the amount of colour which can pass through the screen but gives a smoother-edged aperture as compared to the irregular edged apertures frequently present on such screens. This allows for better registration of multicoloured designs, as the screens are again stronger, less flexible and less brittle than uncoated cylinders.
• the reduction in aperture diameter can for example be from 10 - 20 or 30 ⁇ m, depending on circumstances.
• the one-side coating method can also be applied to the relatively coarse substrates described above, of both the metallic and non-metallic kind. It can for example be applied to wire meshes for rotary printing cylinders, which generally have aperture diameters of 150 - 300 ⁇ m. Some aperture size reduction normally occurs during electrodeposition with such substrates, but as indicated above this is desirable.
• the coating of the sheet substrate in my method may be performed by known electrodeposition techniques. Such methods are for example described in British Patent Specifications 482548, 972169, 933175, 970506, 998937, 1003238, 1419607 & 1382512, U.S. Patent 3200057, and Dutch Patent Specifications 6407426, 6407427, 6407428 and 6407429.
• the clean substrate may be connected to an electrical supply and immersed in a tank containing an aqueous dispersion, emulsion or solution of the plastic material which is to form the coating.
• the substrate will usually be connected as the anode, but it can also be use as a cathode with cathodically-depositable plastics.
• a coating of the plastic is rapidly and evenly built up on the substrate.
• Currents of for example 2 - 20 amps/ft 2 (2-20 mA/cm 2 ) at 30 to 150 volts may be used at temperatures of 20 to 45oC, using coating times of 0.25 - 3 minutes.
• the coating operation can be accurately terminated by appropriate choice of coating time, voltage and current. In some cases, the process can be self-terminating as the coating itself is non-conductive.
• a cylindrical substrate is preferably rotated during the coating operation to ensure uniform coating and the current may be reversed when coating is complete to allow the sheet to be lifted out cleanly from the tank.
• the fluid in the tank is preferably continuously agitated, and may be continuously circulated and filtered to remove undesirably large particles.
• the coating tank is also preferably thermostatically controlled. On completion of the coating process, the substrate may be rinsed and air-dried to remove excess water.
• the substrate sheet is preferably first formed into a cylinder and thencoated.
• the method can however be also applied to flat substrate sheets on either a continuous or discontinuous basis; the coated sheet can then be formed into cylinders or left flat for other uses, e.g. as filters or sieves.
• the plastics material used for the coating may in general be any type of synthetic or natural polymeric material which can be used in electrodeposition methods, for example epoxy, acrylic, polyester, polyurethane or alkyd resins, and cross-linkable vinyl polymers and non-hardenable resins and polymers.
• a thermosetting plastic is preferably used, particularly if the coated sheet is subsequently to be subjected to baking (e.g.
• Non-hardenable thermoplastics may however also be used.
• the choice of resin may be varied according to particular circumstances; for example, modified alkyd resins give more flexible films, whereas butadiene and acrylic resins give harder films which are more resistant to abrasion.
• the electrodeposition bath may also contain additives (such as pigments, dyes or extenders), as in conventional practice.
• One advantage of the electrodeposition method is that the coating can easily be removed if desired, before baking. Stripping can be effected with ammonia solution, amines such as diethylamine, polyvinylpyrrolidone, paint stripping solutions or caustic soda. This can be useful if for some reason the coating is imperfect.
• the sheet may be engraved for use in screen printing by conventional screen engra ving techniques to give the desired pattern of permeable and impermeable areas.
• This can for example be done using photographic techniques by means of the photosensitive resin emulsions normally available for screen engraving, using in this case a positive film for the light exposure process.
• the cylindrical screen can be engraved before electrodeposition so that only the open permeable parts of the screen have the apertures reduced in size whilst the apertures in the non-permeable parts are completely blocked by light hardened resin emulsion.
• the wire mesh cylinder can be engraved photographically with a negative film of the design by coating the cylinder with light-sensitised polyvinyl alcohol so that the image formed acts as a resist during electrodeposition which is conducted to give complete closure of apertures. The light-hardened polyvinyl alcohol is then removed by stripping agents such as sodium hypochlorite to leave the permeable parts of the screen with completely open apertures.
• FIG. 1 of the drawings shows in elevation an electrodeposition tank suitable for coating rotary printing cylinders and Figure 2 is a plan of the same tank.
• a cylindrical wire cloth screen (1) is placed on a mild steel shaft (2) each end of which rests on two brass V-block contactors (3) which are connected to the positive terminal of a DC power source (4).
• the cylindrical screen (1) is fixed at each end to mild steel rings (5) by means of either steel screws, or by a Jubilee Clip firmly clamping the ends of the screen to the rings.
• the two rings (5) are in turn firmly bolted to the steel shaft (2) thereby completing contact to the positive terminal of the power source (4) so that the cylinder for the purpose of the coating process is the anode.
• the cathode can be either the sides of the mild steel coating tank (6) providing that it has not been lined with an insulating material, or preferably, the mild steel plates (7) corresponding to the length and diameter of the wirecloth cylinder.
• the mild steel plates are supported opposite each side of the cylinder, e.g. about 3 cms or more away, as may be desired.
• the coating solution (8) is pumped into the inner section of coating tank (6) and the level maintained by pumping the solution continuously over the weirs (9) from an adjacent overflow tank (not shown).
• a thermostatically controlled heater is fitted to maintain the desired solution temperature, normally for convenience 20°C, and a filter unit is placed between the coating tank and the pump to ensure solution cleanlines.
• the cylindrical screen (1) is rotated on the steel shaft (2) by means of spur gears fitted to the steel shaft and the driving motor (10).
• a direct current is then passed between the cylindrical anode and the cathode steel plates. Under the influence of the electrical field, negatively charged particles come into contact with the positively charged cylinder. The particles then lose their charge and deposit as a coating on the cylinder.
• the cylinder is removed from the coating tank and any undeposited coating solution is washed from the cylinder by a spray of cold water.
• the process is completed by drying, e.g. by first drying at low temperature and then at 120 - 200°C.
• the following Examples illustrate the invention. In each case the apparatus shown in the drawings was used.
• Substrate Phosphor bronze plain weave wirecloth cylinder (lightly nickel plated) with 60 apertures per25.4 mm, aperture size of 250 microns and wire diameter of 170 microns.
• Coating solution A melamine-modified alkyd resin (Code X6126; Macpherson Industrial Coatings Limited, Lancashire, England) which is a water soluble polymer neutralisedwith an organic base containing small amounts of coupling solvent (specifically butyl alcohol)or alternatively butyl glycols, plus a small amount of phthalocyanine blue pigment dispersion in low concentration added as a sighting agent.
• Solids content : 5%
• Substrate Phosphor bronze plain weave wirecloth cylinder (lightly nickel plated) with 80 apertures per
• Substrate Phosphor bronze twill weave wire cloth cylinder (lightly nickel plated) .with 66 apertures per
• Substrate Phosphor bronze twill weave wirecloth cylinder with 66x50 apertures per 25.4 mm,aperture size of 145x230 microns and wire diameters of 240 and 280 microns.
• Substrate Phosphor bronze twill weave wirecloth sheet (lightly nickel plated) with 66 apertures per 25.4 mm, aperture size of 145 microns and wire diameter of 240 microns.
• Coating solution A butadiene base resin (Macphersons Industrial Coatings Limited) Code 11180 neutralized with an organic base and containing small amounts of coupling solvents plus phthalocyanine blue pigment dispersion in low concentration as a sighting agent. Solids content : 5.4% pH : 8.8
• Substrate Electroformed fine apertured sheet with

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• Engineering & Computer Science (AREA)
• Manufacturing & Machinery (AREA)
• Printing Plates And Materials Therefor (AREA)
• Manufacture Or Reproduction Of Printing Formes (AREA)

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Citations (13)

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• 1979
  • 1979-09-21 JP JP50149379A patent/JPS55500670A/ja active Pending
  • 1979-09-21 US US06/192,512 patent/US4384945A/en not_active Expired - Lifetime
  • 1979-09-21 DE DE7979901142T patent/DE2965624D1/de not_active Expired
  • 1979-09-21 WO PCT/GB1979/000155 patent/WO1980000677A1/en unknown
• 1980
  • 1980-04-22 EP EP79901142A patent/EP0020409B1/en not_active Expired

Patent Citations (13)

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