THE PRODUCTION OF ROTARY SCREEN PRINTING CYLINDERS AND OTHER FINE-APERTURED SHEET MATERIALS

Abstract: A method of producing a rotary screen printing cylinder or other fine-apertured sheet material which comprises electrodeposition of a plastic material onto an electrically conductive fine-apertured cylinder or other sheet substrate. When applied to wire mesh cylinders, the method is a fast and effective way of reducing the aperture size. The method can also be operated so that plastic is deposited on one side only of the substrate to increase the substrate strength with little or no aperture size reduction.

Description: THE PRODUCTION OF ROTARY SCREEN PRINTING CYLINDERS AND OTHER FINE-APERTURED SHEET MATERIALS 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. In rotary screen printing, 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. The development of wire cloth ' rotary screen printing cylinders is described for example in British Patent -BUREΛ∑Γ OMPI 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. In certain cases 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. A major advance in overcoming these difficulties was brought about by the introduction of a two-ply wire mesh/fabric screen as described in British Patent Specification 1050649. In this method, greatly improved strength and dimensional stability is obtained by first making a very strong cylinder from heavy gauge phosphor bronze wire and then giving this cylinder a lightly electroplated coat of nickel. This makes the cylinder chemically resistant to print-paste constituents and gives it extra dimensional stability. By covering this cylinder with either flat or tubular screen fabrics, (e.g. of polyester or polyatnide) possessing small aperture sizes (e.g. between 50 and 150 microns), strong printing screens can be obtained with good dimensional stability. These screens are suitable for engraving and printing on textile fabrics to give well defined patterns. This type of screen has been used, with advantages in many cases over electroformed nickel perforated screens, since 1965. Some disadvantages of two-ply wire mesh/fabric screens include: the high cost of fitting flat or tubular fabric onto a wire mesh cylinder; the possibility of the tubular fabric not being stuck firmly to the wire mesh cylinder; the possibility of damage to the tubular screen fabric, and the engraved detail, where it is worn by the fabric selvedge, or damaged by adhesive tape, which may be used to temporarily "mask out" and narrow the pattern width; and the necessity of using either special engraving techniques, or when using conventional photo-sensitive resin emulsions for engraving, of being unable to bake at high temperature to obtain maximum durability, on account of the poor heat stability and heat resisting qualities of the fabric material. A similar method, in which a relatively coarse screen cylinder is covered with a tubular fine-mesh "BU EAU OMPI 4. ,. wipo . fabric, is also described in Swiss Patent Specifications 545692 and 7205/72. In another method which is widely used, 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 en") ; however, the mandrels required are expensive and the electroforms are brittle and require very careful handling. I have now found a new method of producing rine- apertured sheet materials, e.g. having aperture sizes ranging from 50 to 500 μm and particularly those for use in rotary screen-printing, which involves forming a coating of plastic material on a fine-aper¬ tured sheet substrate by electrodeposition of the plastic material. - This method has many advantages. 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 appli¬ cable to the treatment of fine-apertured sheet for rotary screen printing cylinders, and is a fast and inexpensive way of making such cylinders. As indicated above, in a principle embodiment of the invention, the method is operated to reduce the aperture size. This is particularly useful when the substrate OMP ^i?NA P apertures are relatively coarse (e.g. 100-500 μm) , 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. For example, if a nickel-plated phosphor bronze wire mesh cylinder (of the type normally intended to be covered with a tubular screen fabric) is coated by my method, the aperture size can be reduced in 1 to 3 minutes to such a degree that the fine-mesh fabric cover is unnecessary. Not only does the coating operation take less time to per¬ form than it takes to fit a screen cover over a cylin¬ der, but the expensive covers can be dispensed with altogether. Moreover, a range of printing screen cylinders having different aperture sizes can easily be produced for use with different print pastes and fabrics from one quality of wire mesh, thus eliminating the time and expense of fitting a selected quality of fabric cover over the same cylinder according to the particu- lar paste being used and fabric being printed. Printing cylinders produced by this method are of particular advantage, on account of their strength, for use on wide textile printing machines and carpet print¬ ing machines, in place of the electroformed nickel screens normally used. In comparison with the alternative • technique of reducing the aperture size by electrodeposition of metal, this method is again much quicker and cheaper. Plastics are less expensive materials than nickel or copper, and the deposition time of 1 to 3 minutes com¬ paresvery favourably with times of 0.5 to 3 hours required for electroplating. The method is also more accurate and, unlike electroplating, does not give a brittle product. The fine-apertured substrate which can be coated by this method may be any suitable electrically cond ce - B UREAU 0 PI 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 phos¬ phor 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. Alternatively, a fine-apertured non-metallic sub- strate 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 electrodeposi- ion coating step. Plastics may for example be rendered conductive by coating with graphite or an electrolyte, as is well known in the electroplating and electrodepo¬ sition arts. Electroplated plastic (e.g. nickel-plated polyester) may also be used, as described in British Patent Specification 1332046. Suitable plastics for the substrate are synthetic or natural polymeric materials, e.g. polyesters, polyamides, polyolefins such as polypropylene, or regenerated cellulosic materials. These materials are again preferably used in mesh form. 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 electro- phoretic coating to, for example, 40 to 200 microns. However, sheets having smaller aperture sizes (e.g. down to 35 μ,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 com¬ bination of aperture size and aperture density in their original state but lack strength and structural stabili¬ ty. In this embodiment of the method, 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. A number of different techniques can be adopted to- ensure that only one side of the substrate is coated. For example, one side of the substrate (the side which is not to be coated by electrodeposition) 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-insol¬ uble and easily removable after electrodeposition, for example with an organic solvent; examples of suitable materials are esters of poly (me hylvinyl ether/π-aleic acid) such as the monobutyl ester sold under the trade name Gantrez ES 435 (GAF), bitumen or ultra-violet hardened poly¬ vinyl 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-pccl" (a polyvinyl chloride solution made by TΛK Chemicals Ltd.) which can be peeled off after the electrodeposition process. When a cylindrical substrate is used, the inside of the cylinder can be protected during electrodeposition by _J.nflati.ng a bag or tube (e.g. of rubber) inside the cylinder. The bag or tube can be simply deflated and removed after electrodeposition, leaving the inner surfac of the cylinder uncoated. Another alternative technique can be used when apply ing my method to an electroform. Electroforms are produced by electrodeposition of metal (usually nickel) o a mandrel, and the plastic material can simply be electro deposited into the outer surface of the electroform whils it is on the mandrel. Thus, after deposition of the met onto the mandrel and washing with water, the mandrel can transferred to the tank for electrodeposition of the plas 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 t 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 thickn of 60 - 200 jum, aperture diameters of 50 - 400 /iπi and fro 2 250 - 1800 apertures per cm . 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 an durability can be obtained by increasing the metal thick¬ ness, but the apertures then become coarser and much desi detail is lost in engraving and printing. For example, 2 cylinder 80 jam thick may have about 1800 apertures/cm an aperture diameters of about 60 jam; this is a good combin 'BU E OMPI s- W1P0 tion of aperture sizes and diameters for detailed printing, but the screens are very delicate. On the other hand, screens 90 to 110 urn thick (having aperture diameters of 120 or 150 /im 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 pi thick on the electro¬ form, 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 μm 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 registra¬ tion 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 |jm, depending on circumstan¬ ces. 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 - 300μm. 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. Thus in general the clean substrate may be connected to an electrical supply and immersed in a tank containing an aqueous dispersion, emulsion or solution of the plasti material which is to form the coating. The substrate wi usually be connected as the anode, but it can also be use as a cathode with cathodically-depositable plastics. Wh current is passed through the bath, a coating of the plas is rapidly and evenly built up on the substrate. Curre 2 2 of for example 2 - 20 amps/ft (2-20 mA/cm ) at 30 to 15 volts may be used at temperatures of 20 to 45 C, using coating times of 0.25 - 3 minutes. The coating operatio 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 coating operation to ensure uniform coating and the curre may be reversed when coating is complete to allow the she to be lifted out cleanly from the tank. The fluid in th tank is preferably continuously agitated, and may be continuously circulated and filtered to remove undesirabl large particles. The coating tank is also preferably thermostatically controlled. On completion of the coati process, the substrate may be rinsed and air-dried to remove excess water. When the coated sheet is to be used in rotary screen printing, 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. at 120 to 200°C) during an engraving process. Non-hardenable thermoplastics may however also be used. The choice of resin may be varied according to particular circumstan¬ ces; for example, modified alkyd resins give more flex¬ ible 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. After electrodeposition, 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 photo¬ sensitive resin emulsions normally available for screen engraving, using in this case a positive film for the light exposure process. Alternatively, 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. To obtain the reverse effect when engraving before electrodeposition, the wire mesh cylinder can be engrave 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. The electrodeposition is described in further detail below, with reference to the drawings. Figure 1 of the drawings shows in elevation an electrodeposition tank suitable for coating rotary print- ing 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 positi 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 coatingtank (6) and the level maintained by pumping the solution continuously over the weirs (9) from an adjacent overflow tank (not shown) . A thermo¬ statically 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. Before commencing the coating process the cylindri¬ cal 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 cylin¬ drical 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. After completion of the required coating time, the cylinder is removed from the coating tank and any unde- posited 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. BUREAtf OMP! ^SNATIO ' The following Examples illustrate the invention. In each case the apparatus shown in the drawings was used. Example 1 Substrate : Phosphor bronze plain weave wirecloth cylinder (lightly nickel plated) with 60 apertures per 25.4 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 alter¬ natively butyl glycols, plus a small amount of phthalo- cyanine blue pigment dispersion in low concentration added as a sighting agent. Solids content : 5% Conductivity : between 500 and 5000 Microsiemens but specifically 2500 microsiemens. p_H : 7.9 Temperature : between 15-30 C (but specifically 20°C) Voltage : 30-150 volts (but specifically 50 volts) . 2 Current Density : 10 amps/square ' foot (10 mA/cm ) Coating Time : 1 minutes After coating the 250 micron square apertures had changed to 210 micron circular apertures. ft OM .^U- wi p - 15 - Example 2 Substrate : Phosphor bronze plain weave wirecloth cylinder (lightly nickel plated) with 80 apertures per 25.4 mm, aperture size of 186 Microns and wire diameter of 132 microns. Coating solution : Resin Code X6126 Solids content : 5% pH : 7.9 Conductivity : 2500 Microsiemens Temperature : 20 °C 2 Current Density : 10 Amps/square foot (10 mA/cm ) Voltage : 50 Coating time : 1 Minute After coating the 186 micron square apertures had changed to 140 micron round apertures. Example 3 Substrate : Phosphor bronze twill weave wire cloth cylinder (lightly nickel plated).with 66 apertures per 25.4 mm, aperture size of 145 microns and wire diameter of 240 microns. Coating solution : resin Code X6126 Solids content : 5% p_H : 7.9 Conductivity ' : 2500 microsiemens Temperature : 20°C 2 Current density : 9 amps/square foot (9 mA/cm ) Voltage : 50 volts Coating time : 1 minutes After coating the 145 μm square apertures changed to 100 μm rounded apertures. Example 4 __ • ^ Substrat____e___ : Phosphor bronze twill weave wirecloth cylinder, with 66x50 apertures per 25.4 mm aperture • size of 145x230 microns and wire diameters of 240 and 5 280 microns. Coating solution : Resin Code X6126 Solids content : 5% 2H : 7.9 Conductivity : 2500 Microsiemens 10 Temperature : 20°C Current density : 10 amps/square foot (10 mA/cm ) Voltage : 50 Coating time : 3 minutes After coating the rectangular apertures had changed 15 to elliptical apertures size 110x190 microns. Example 5 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 20 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 25 low concentration as a sighting agent. Solids content : 5.4% pH : 8.8 Conductivity : 2050 microsiemens Temperature : 20° 30 Current density : 10 amps/ft 2 . (10 mA/cm ) Voltage : 60 Coating time : 2 minutes After coating the aperture size had been reduced from 145 microns to 110 microns. Example 6 Substrate : Electroformed fine apertured sheet with 60 apertures per 25.4 mm, aperture size of 140 microns, half of one side being coated with the product Pro- peel (TAK Chemical Industries Limited, England) to act as a resist to the coating process. Coating solution : resin code X 6126 Solids content : 5% pH : 9 Conductivity : 2500 Microsiemens Temperature : 20°C Voltage : 30 Coating Time : 3 Minutes 2 ' Current density : 6.5 amps/square foot (6.5 mA/cm ) After coating the area covered by Pro-peel remained clear and the " opposite side of the electroformed sheet was coated with resin. The apertures in this part re¬ tained their original size. In the .part not coated with Pro-peel the electroform was coated on both sides with resin and the apertures. reduced in size to 120 microns. "BUREAU OMPI yfy WIPO > >

Claims: AMENDED CLAIMS (received by the International Bureau on 22 February 1980 (22.02.80) 1. • A method of coating a rotary screen printing cylinder with a plastic material, said cylinder being electrically conductive and having apertures in its surface of a size ranging from 35 to 500 am, which meth comprises the electrodeposition of the plastic materia uniformly over one or both surfaces of the cylinder to strengthen the cylinder and to reduce the aperture size uniformly or to strengthen the cylinder without apertur size reduction, or (where the cylinder has previously been engraved photographically with a film of the design to be printed) to close the apertures completely the non-engraved areas; 2. A method as claimed in claim 1 wherein the elect deposition of the plastic material reduces the aperture size. 3. A method as claimed in claim 3 wherein the cylin is a wire mesh rotary screen printing cylinder. 4. A method as claimed in claim 3 wherein the initi aperture sizes are from 100 to 500 am- and are reduced t 40 to 200 fiτa. by the electrodeposition of the plastic material. 5. A method as claimed in claim 1 wherein the plast material is only deposited on one surface of the cylind 6-. A method as claimed in claim 5 wherein the cylin is an electroformed rotary screen printing cylinder. 7. A method as claimed in claim 5 wherein the surfa of the cylinder which is not to be coated is protected during the electrodeposition process by (a) a coating o film of a non-conductive material, (b) by inflating a b or tube inside the cylinder, or (c) when the substrate an electroformed rotary screen printing cylinder formed electrodeposition of a metal on a mandrel, effecting th electrodeposition of the plastic material onto the cyli on the mandrel. O PI 8. A method as claimed in claim 1 wherein the plastic material is an epoxy, acrylic, polyester, polyurethane or. alkyd resin or a cross-linkable vinyl polymer. 9. A method as claimed in claim 1 wherein the electro- deposition is effected by immersing the cylinder in a tank containing an aqueous dispersion, emulsion or solution of the plastic material and then depositing the plastic using 2 a current density of 2-20•mA/cm at 50 to 150 volts at a temperature of 20 to 45 C for 0.25 to 3 minutes, the cylinder being connected as either the anode or cathode. O PI

Inventors: SWORD W
Applicants: SWORD W
Application year: 1979
Main classification: B41C1
IPC codes: B41C1