IMPROVED GRAVURE PRINTING ROLLERS

Abstract: A gravure printing roller comprises a body or shell (15) moulded from syntactic compositions comprising microbeads of a plastic, glass or ceramic material in an e.g. polyurethane matrix. The shell (15) is machined to the required size and then a copper coating (17) is deposited on the machined surface, and chrome treated. An electrically conductive coating is first deposited on the machined surface which coating comprises a conductive agent such as finely divided metal.

Description: IMPROVED GRAVURE PRINTING ROLLERS This invention relates to improvements in gravure printing rollers. At present, gravure printing rollers are usually machined to the required size from stainless steel. Stainless steel rollers are however extremely heavy which is a major disadvantage, both from the point of view of transportation, and also in changing rollers in a printing machine. Accordingly, it has been long considered desirable to be able to use rollers of a lower weight material, such as a composite including a relatively low density synthetic material which retains a durable printing face such as chrome treated copper, yet is light to ease shipping and handling. It has been found that standard foam material is unsuitable for this purpose as the moulded foam core requires sealing with a filler to close the pores to enable a pore and void-free copper coating to be deposited and subsequently chrome treated as a print face. Even if a skinning foam is used to produce a roller shell, the great diversity of gravure roller dimeters requires that a machining operation is used to reduce the roller shell diameter to the designed value, which exposes foam pores which require filling. Further, the equipment required to produce a foam core is complicated because it is necessary to use clamping force to hold in the foam during the moulding cycle. According to the broadest aspect of the invention a lightweight shell for a gravure roller is made from a syntactic moulding composition. The invention also provides a gravure roller comprising a shell; formed from a syntactic moulding composition, having a non-porous machined surface with a printing face of chrome treated copper deposited on the non-porous surface. A syntactic moulding composition is defined for the purpose of this specification as being a synthetic matrix including preformed beads providing discrete bubbles. This composition is capable of providing a uniform cellular construction of homogenous and consistent density which can be machined without leaving a porous or void containing surface. The syntactic moulding composition preferably comprises a polyurethane composition formed into preformed plastics beads each comprising an individual shell surrounding an interior void and bonded by a polyurethane composition. The beads may be in the size range of 20 to 100 microns (weight average diameter) and may alternatively comprise other polymers, glass, or ceramic beads. The invention also includes a method of manufacturing a gravure roller, comprising moulding a roller shell from a syntactic moulding composition. The shell may then be machined to provide a non-porous void-free surface of the required dimensions, a printing face of copper deposited thereon, and the copper surface subsequently chrome treated. The shell may advantageously be mechanically reinforced, to prevent flexure of the roller, by means of a rigid tubular core upon which the shell is moulded. The core may be for example of glass reinforced plastics, or a concrete composition. The polyurethane used in the moulding composition can be manufactured from a range of ingredients. For example, polyether or polyester polyols may be used, with any of the common isocyanates; including methylene diphenyl diisocyanate; toluene diisocyanate; naphthalene diisocyanate; tetramethyl xylidene diisocyanate; isophorone diisocyanate; phenylene diisocyanate; cyclohexyl diisocyanate; xylidene diisocyanate; hexamethylene diisocyanate; as well as biuret, allophanate and prepolymeric adducts of these. Diol, triol, polyoi or polyamine crosslinkers may be used, with an amine or metal catalyst which is preferably of a type which negates blowing reaction in the moulding step following bead expansion. The isocyanate component may be pre-polymerised and contain the expansion agent so that the isocyanate component is a low density liquid whilst the polyoi is a standard polyoi. A suitable composition may be as follow:- Polyoi component:- (by weight ) Caster Oil - 46.8 Polyether hexol - 46.8 Antifoam - 0.19 Dessicant - 2.4 Catalysts - 0.25 Expancel 551 DE - 3.59 Isocyanate:- Polymeric methylene diphenyl diisocyanate (MDI) - 100% Mix ratio: 60 parts polyoi to 50 parts of isocyanate Gel time at 25°C - 10 to 15 minutes Hardness - 70 shore D Density - 0.72 Viscosity polyoi - 400 poise at 25 °C The moulding is carried out at 60°C in a low pressure two component polyurethane dispenser-2KM machine. In accordance with this invention, the surface of the syntactic moulding polyurethane composition is, after machining, preferably treated with a conductive coating to provide a host surface for the copper plating. This has been found to be necessary as the preferred syntactic polyurethane composition keys poorly to the cooper plating so that there is low adhesion between the machined roller surface and the cooper layer. The conductive coating is preferably an organic solvent based composition containing one or more conductive agents. The latter may comprise additions of conductive materials such as finely divided metals, or additives which adjust the ionic state of the coating to render it conductive. The rollers may be manufactured entirely from the syntactic moulding composition ,whilst in other possible embodiments, the roller shells are fitted with steel or aluminum end plates and/or shafts or trunnions. The rollers according to the invention may be used not only for gravure printing, but also for flexo printing. The invention will now be further described by way of example, with reference to the accompanying drawings, wherein:- Figure 1 is a simplified cross-sectional view of a gravure roller according to the invention; Figure 2 is an enlarged sectional view of part of the roller shell; Figure 3 is a much enlarged sectional view showing the results obtained by working a conventional urethane foam; and Figure 4 is a similar enlarged sectional view of the structure of a surface produced by working a syntactic foam on a roller according to the invention. In Figurel , a gravure roller 1 is shown which comprises a roller body or shell 15. In accordance with the invention the roller shell body 15 is made by moulding a syntactic polyurethane foam, which is then machined to achieve the desired diameter and produce a substantially non-porous and void-free outer surface 16, on which a layer of copper 17 is deposited after treatment with a conductive coating which provides adhesion for the copper, and then treated with chrome and processed as required to act as a gravure printing roller. The roller also includes a tubular core 12, which acts as a support and reinforcing member to prevent or minimise flexure of the roller, with end caps 13, 14 formed with trunnions 1 1 so that the roller 1 can be mounted in a machine. The core is of a suitably strong material, such as glass reinforced plastics, or a concrete composition, particularly a resin bonded concrete. The conductive composition is an organic based coating including a conductive agent such as powdered metal or ionic adjusting additives. A portion of the roller shell 15 with the copper layer 17 thereon is shown in Figure 2. Figure 4 shows, much magnified, the surface 16 of the roller shell 15, wherein the bonded beads or bubbles 18 of polyurethane or other plastics foam are shown in a polyurethane matrix 19 and providing a substantially non-porous surface 16 free of major voids and with only micros-pits caused by the opening or removal of individual beads 18. The beads may alternatively be of glass or ceramic material. By contrast, Figure 3 shows a machined surface 1 16 of a layer 1 15 of a normal open-celled foam which has, for example, been poured into the mould as liquid and allowed to cure and foam simultaneously in the mould. As shown, the foam, after planarising , has major voids 1 17 and pores 1 18 which, by linking of adjacent foam cell, often penetrate well in to the foam body so that the surface 1 16 is by no means non-porous or free voids. The roller shell 15 is made by moulding a syntactic polyurethane foam. This is then machined to the required size, a coat of a special paint applied over the surface, copper deposited and the shell is then assembled into the roller structure for subsequent chromium plating, etching and use in printing. The roller may weigh about 5 to 7 kg instead of the weight of 100 kg when steel is used for the roller shell. The roller can be refurbished by recoating with copper and re- chromium plating and etching. The relatively light weight ensures that transportation and handling, including mounting and dismounting the roller is easy and imposes no handling problems. A roller formerly requiring use of a 7.5 tonne truck for delivery of a set of rollers can now be delivered using a light weight van. Because of the reduced weight of the shell, the other parts of the roller, including the end plates, shaft and trunnions, can be made of reduced weight materials, e. g. lightweight metals such as aluminium alloys, or synthetic materials such as suitable rigid plastic materials including forming all or part of the roller from the syntactic foam material. A preferred syntactic foam composition used in manufacture of the shell is preferably as set out above. Alternatively, moulding may be carried out with a polyisocyanate composition, using a prepolymer with a polyester or polyether polyoi, a friability modifier and an isocyanate catalyst such as DABCO TMR5. Other urethane derivatives can be polyoxazolidone produced by means of epoxy/isocyanate addition compositions. As an alternative to polyurethane, the syntactic material can be based on other polymers such as unsaturated polyesters, vinyl esters or polyacrylic systems such as Modar (Trade Mark of ICI) can be used.

Claims: Claims 1. A gravure roller comprising a lightweight shell made from a syntactic moulding composition. 2. A gravure roller according to claim 1 , further comprising a non- porous machined surface with a printing face of chrome treated copper deposited on the non-porous surface. 3. A gravure roller according to claim 2 further comprising a conductive coating provided on the non-porous machined surface. 4. A gravure roller according to any proceeding claim, wherein the syntactic moulding composition comprises a matrix including preformed beads providing discrete bubbles. 5. A gravure roller according to claim 4, wherein said beads each comprise a polyurethane or other plastics composition or glass, or ceramic shell surrounding an interior void. 6. A gravure roller according to claim 3, wherein the conductive coating comprises an organic solvent based composition containing one or more conductive agents. 7. A gravure roller according to claim 6 wherein the conductive agent or agents comprises an additive or one or more conductive finely divided metals. 8. A gravure roller according to claim 6, wherein the conductive agent comprises an additive which adjusts the ionic state of the coating to render it conductive. 9. A gravure roller according to claim 4 wherein the beads have a size range of from 20 to 100 microns. 10. A gravure roller according to any proceeding claim wherein the shell is formed on a reinforcing core. 1 1 . A method of manufacturing a gravure roller, comprising moulding a roller shell from a syntactic moulding composition . 12. A method according to claim 1 1 , comprising machining the peripheral surface of said roller shell to produce a non-porous machined surface. 13. A method according to claim 12, comprising coating the non- porous machined surface with a conductive coating. 14. A method according to claim 12 or 13 comprising depositing a layer of chrome treated copper on the non-porous machined surface to provide a printing face.

Inventors: ARMFIELD SIDNEY ROBERT, CAMPION JOHN WILLIAM, MOSS ROBERT MALCOLM, ARMFIELD, SIDNEY, ROBERT, CAMPION, JOHN, WILLIAM, MOSS, ROBERT, MALCOLM
Applicants: ARMFIELD SIDNEY ROBERT, CAMPION JOHN WILLIAM, HYPERLAST LTD, LIBRA GRAVURE CYLINDERS LTD, MOSS ROBERT MALCOLM, ARMFIELD, SIDNEY, ROBERT, CAMPION, JOHN, WILLIAM, HYPERLAST LIMITED, LIBRA GRAVURE CYLINDERS (PLASTICS DIVISION) LIMITED, MOSS, ROBERT, MALCOLM
Application year: 2000
Main classification: B41C1
IPC codes: B41C1, B41F13, B41N7, C25D5, C25D7, F16C13