GB2215665A - Printing plate registration - Google Patents

Abstract

In addition to images 2, flexographic printing plates are provided with relief datum structures 4, possibly in the form of chevrons along a marginal edge. These datum structures can be provided photographically from original artwork at the same time as the image areas; in muiti-colour work, the artwork may be common to all the plates to ensure that the datum structures are positioned identically. The datum structures may mate with complementary indented formations on the base plate of a hole-drilling tool, or with such formations provided on the underside of a registration bar associated with a printing press cylinder. Thus, although holes may be drilled through both the flexographic surface sheet 1 and the dimensionally stable film backing 3, (Fig 2), to serve as registration devices for the mounting of the plates on the press cylinders, these are not essential when the registration bar is itself provided with the complementary formations, (Fig 10). <IMAGE>

Description

Printing Plate Registratlon The invention relates to flexographic printing and is particularly concerned with the provision of common datum points upon f lexographic image carriers for use In facilitating positioning the Image carrier on printing press rollers.

Fiexographic printing Is a printing process by which an image is transferred using a flexible Image carrier having a three dimensional format. Flexographic printing is widely used, in particular in the packaging industry for printing on product packaging substrates and has been practiced for a large number of years.

Flexographic printing Is an effective method of printing which has many advantages. The materials used for making flexographic image carriers are principally natural or synthetic rubbers or photopolymers. These rubber materials, however, suffer from the disadvantage that they are dImensionally unstable under stresses experienced in printing, a flexographic Image carrier yielding and suffering displacement during printing so as to distort the image Imparted to the substrate to be prInted.This Is particularly undesirable since In practice the substrate will be printed in a plurality of colours using an equal plurallty of Identical replicated image carriers, the printing process at a first printing statlon printing those parts of the image having a first colour and at subsequent statlons applying different colours to other parts of the Image using a different image carrier. It will be recognized that distortions in the Image carrier will give rlse to print imperfections such as adJacent areas of different colours which elther overlap or are spaced apart leaving small areas of unprinted substrate between them.

The same problems as Just described also arlse for Image carrlers In all materials In practice if one Image carrier is fixed to the print roller (eg cylinder) In a position which differs even slightly from the roller position occupied by another Image carrier at either a preceding or subsequent printing station through which the same substrate passes in a multi-colour print run.

Slight differences in positioning readily occur in practice unless special precautions are taken. This is indeed an area where much effort has been expended to ensure consistent positioning of image carriers upon print rollers.

Prlnt problems having the former cause are avoided In practice by the provision of a stabilizing layer of polyester film. Bonding of polyester film to rubber substrates has been difficult to accomplish in the past but by use of approprlate bonding agents coated onto the polyester film, polyester film - rubber bonding is now practical and Is becoming widely practiced (together with the bonding of polyester and similar backing films to photopolymer flexographic Image carriers which has been practiced for some time).

Those print problems having the latter of the two causes mentloned can largely be overcome by providing each Image carrier wlth a serles of holes along a marginal edge or by optical registration techniques applied at the print drum. By the first of these methods, a registration plate assoclated with the print roller and having a constant fixed position has a complementary row of pins or lugs. The image carrier is laid upon the print roller and Its holes registered with the pins of the registratlon plate. This positions the Image carrier in a position upon the plate which is reproducible at other Identical print statlons by ensuring that the holes in all the image carrlers needed for a print run are provided in the same places.Once placed in position in the above way, the image plate is fixed to the print roller by use of double-sided adhesive tape between the plate and the roller or alternatively by applying transparent single-sided adhesive tape to non-image plate areas and the stereo roller, the holes raised from the registration plate pins to allow the roller to be rotated, and printing performed. Optical techniques are effective, but are time-consuming and require trained skilled operatives and significant capital equipment. It Is also I nconvenlent to employ optical referencing equlpment at a print station.

Cruclal to the efficacy of the above technique using registration holes is the need to ensure the making of the holes in the image carrier In pre-determi ned locatlons. One practice in the art has been to prepare a plurallty of film negatives representing the respective images to be prlnted one at each of an equal plurality of print stations. These are placed in interfacial relationship with thelr edges aligned and then drilled in a stack also contalning an equal number of sheets of photopolymer precursor typically having a polyester film backing (such as Melanex). A single sheet is then disposed beneath a single selected negative and the photosensitive sheet exposed to UV light.This is repeated for each film to produce an equal number of photopolymer rellef plates, a metal plate provided wlth dowels receiving the palr of sheets on each occasion so as to ensure that the film-photosensitive sheet relationship Is identical for each exposure. The resulting f lexographic image carriers each having registration holes thereln can then be mounted to the print drums at each of a sequence of print stations using a registration plate which references the holes and a multi-colour print run commenced. Boring of the plates Is not practical post-Image formation.The procedures descrIbed above cannot be applled to rubber flexographic plates pre-lmage formatlon as vulcanization is a robust treatment which would result in displacement of hole locatlons in an unpredictable manner.

Another practice in the art has been to determine correct positioning of registratlon holes optically using a prism-based apparatus by reference to datum points provided on the Image carrier, usually datum points formed at the same tlme and in the same way as the image itself. This practice Is widely recognized as effective and Is now in the nature of a standard practice in the flexographic printing art. It does, however, suffer from two problems.

The flrs and most significant of these two problems is the fact that optical referencing equipment is complex both in mechanical and optical terms. This, of course, means that necessarily there Is more potential for equipment failure, a need for care in use, and malntalnance costs. More importantly, however, the equlpment is expensive In capital terms and in particular its use requires skills which need to be taught to relatively expensive skilled staff. Even though practiced using skilled operatives, the use of optical referencing equipment means there is a need to expend significant operator time in setting up and using the equipment.

The second problem, which is In fact associated wlth both the conventlonal registratlon techniques described above, is the rev lance on the use of registration holes formed in the image carrier. This involves drilling using drill bits. Provision of a dlmenslonal-stabillzing layer of polyester film, now a conventional practice, tends to make drilling difficult or impossible to achieve in a satisfactory fashion. The drilling unit which has been available from GMS Ltd under the trade mark HSTW II, for example, uses so-called "tube drills".This equlpment is designed for drilling soft polymeric materlals -and experiences difficulty In drilling a composite image carrier having a hard laminated polyester film layer.

"Forclng" the drill bit results In drilling imperfections and sheet distortlon (and possibly serlous damage to either the plates or drill). No really satisfactory method of drilling holes in polyester film-backed flexographic image carriers has emerged despite the problem Just outlined.

Registration of such composite carrlers is therefore usually accomplished by the optical registration methods referred to earlier.

According to one aspect of the invention, there Is provided a method of forming a flexographic printing plate which method comprises forming a flexographic image carrier having one or more datum structures provided at predetermined positlon(s) on a surface thereof, securing treatment means in a fixed position relative to the carrier, a datum structure of said Image carrier and a cooperative structure of sald treatment means being mated together to determine the positions of the image carrier and the treatment means relative to one another, actuating said treatment means whereby material of said Image carrier Is removed locally from a region of a surface thereof to form a perforation or depression in said image carrier and removing the treatment means from the Image carrier.

The image carrier may be made of a natural or synthetic rubber or of a photopolymer.

Suitable synthetic rubbers Include butadieneacrylonitrile rubber, chloroprene, butadiene-styrene rubber, butyl rubber, ethylpropylene rubber, silastomer and urethane rubber.

A flexographic image carrier of moulded rubber may be formed by first producing a metallic photoengraving or photopo lymer original by image-wise exposure of a photosensitive precursor thereof and subsequent image development. This is followed by then moulding an uncured polymer, typically a thermosetting resin, upon said original to form a matrix sheet carrying an image.

Unvulcanized natural or synthetic rubber is then moulded into the matrix sheet. Vulcanization of the rubber is then effected. The metallic photoengraving or photopolymer original is convenlently image-wise exposed using artwork which Includes a representation responsible for originating the or each datum structure.

The Image carrier may alternatively be formed by engraving an Image upon a pre-vulcanized natural or synthetic rubber substrate. The Image may be hand cut or formed by laser engraving. in the latter case, the image is convenlently laser engraved In accordance wlth artwork which determines the path of the laser beam and which Includes a representation responsible for originating the or each datum structure.

Sultable photopolymerizable materlals for producing a photopolymer for the Image carrier are: Manufacturer Trade Mark Product Type Du Pont Cyrel Sheet Asahi Chemical A.P.R. Liquid Resin or Sheet W R Grace Letterflex Liquid Resin B.A.S.F. Nyloflex Sheet T.O.K. Elason Sheet TOK. Rigilon Sheet A photopolymer Image carrier is conveniently made by image-wise exposure of a photopolymerizable plate using artwork which Includes a representation responsible for originating the or each datum structure.

The Image carrier Is preferably a multi-layer member comprising an image-carrying layer and a film backing such as a polyester film layer. The latter provides dimensional stability to resist distortion under the forces experienced during f lexographic printing.

Suitable films are avallable under the trade marks Mylar and Melanex from, respectively, DuPont and ICI.

The polyester film will preferably have a bonding agent coated or lacquered on its surface when bonding to a rubber flexographic sheet Is desired.

Convenlently, the or each datum structure is a relief formatlon upon a surface of the image carrier.

The relief formatlon may comprise formations having the configuratlon of chevrons, circles or one or more linguistic Indlcia.

The image carrier is conveniently rectangular in plan and the or each datum structure is provided in a marginal edge portion of the Image carrier adjacent to one (or perhaps more than one) side thereof.

The treatment means referred to will normally be a handhold power tool and In any event the treatment means comprises power means, mounting means for mounting a materials removal tool bit (eg a borlng tool bit) to the treatment means and means for coupling the power means to the mounting means.

The cooperative structure of the treatment means conveniently comprlses a member which is apertured to receive therethrough a materials removal tool bit mounted to driven mounting means of the treatment means and wherein the apertured member and tool mounting means are displaceable to provide for such movement of the tool bit through the aperture as is required to plunge the tool bit to a desired range of depths In the image carrier.

The tool bit convenlently comprises an elongage member having a tip provided with an abrasive cutting surface.

The abrasive surface Is conveniently provided by exposed surface acicular tungsten carbide particles. The acicular particles are preferably sintered to the material of the tool bit underlying them. The acicular particles are preferably fixed In orientations in which their axes extend from the tool bit.

The acicular particles are In the latter case best sintered in place whilst orlented by the Influence of a magnetic field.

The tool bit conveniently comprises a member having generally the configuration of a hollow right circular cyllnder, the member having an abrasive tip portion and a shank portion generally In that same configuration and an Intermediate portlon having a cross-section in the form of that part of an annulus as is defined between two concentric arcs connected by a common chord, the smaller arc being concentric wlth the circumference of the cylindrical member and the longer arc being concentric or coincident wlth that circumference.

The exterior arcuate surface of the Intermediate portion is usually an abrasive surface.

The materials removal tool bit conveniently has a tip which has the cross-sectional configuration of an annulus, the annul us having an abrasive surface such that on actuation of the treatment means the plane of the annulus is brought to bear upon the Image carrier and material is removed therefrom by abrasive cutting action effected in rotating the tool bit on an axis perpendlcular to the plane of the annulus.

The cooperative structure of the treatment means comprises a member mounted thereto and having a surface configured wlth a formation which is the reverse of the datum structure(s) so that the or each datum structure functions wlth the said formation In the manner of a key and socket.

The datum structure(s) are preferably relief structures and the treatment means Includes a surface recessed In the configuration of the relief structures. However, of course, the datum structure(s) could be recesses and the treatment means provided with relief structures in the configuration of the recesses.

The invention will now be descrlbed, by way of example only, reference being made to the accompanying drawings In which: Figure 1 shows a moulded vulcanized rubber flexographic printing plate made in accordance wlth the inventlon; Figure 2 shows the multl-layer structure of a prevulcanized rubber flexographic printing plate made in accordance wlth the Invention; Figure 3 shows a power tool for use as a materials removal implement I n the method according to the Invention, the tool bit of the power tool being shown in Its retracted (non-use) position; Figure 4 Is a view similar to Figure 1 but showing the power tool with a tool bit In Its operative position;; Figure 5 is a vlew of the underside of the power tool of Flgures 3 and 4 shown from the directlon of the arrow V of Figure 3; Figure 6 shows the tool bit of Figures 4 and 5 on an enlarged scale; Figure 7 shows the tool bit of Figure 6 as a crosssectlon taken along the line VII-VII of Figure 6; Figure 8 Is a perspective view of the tool bit; Figure 9 shows an end view of a print drum equipped wlth a registration assembly for use in securlng a flexographic printing plate in position on the drum; Figure 10 shows a similar view of a print drum equipped wlth a modified registration assembly in which a printing plate datum structure mates wlth a phenol it resin datum strip;; Figure 11 shows diagrammatically a cross-sectlon through part of the combinatlon of a flexographic printing plate datum structure with a phenolic resin datum strip; and Figure 12 illustrates production of such datum strip.

Moulded Rubber Fiexographic Prlnting Plates A. Structure The moulded rubber flexographle printing plate shown in Figure 1 comprlses a sheet 1 made either of a synthetic elastomer or from a natural rubber of vegetable or mineral origin. Sheet 1 may have a thickness typically in the range from 2 to 5 millimetres, a 3-dimensional relief Image, part of which is designated in the figure by reference numeral 2, being provided as part of this thickness. To avoid the tendency of rubber sheet material to distort under such physical pressure as that experlenced durlng flexographic printing, the rubber is bonded to a polyester film backing 3.The difficulties ordinarily encountered in bonding polyester film to rubber materials are overcome by the provision of a bonding agent upon the rubber-contacting surface of the polyester film.

As will be seen from Figure 1 of the drawings, the sheet 1 is formed with two series of chevron-shaped rellef structures 4 adJacent one of the longitudinal edges of the sheet. Within each series of relief structures 4, a bore 5 Is optionally provided and, when provided, this perforates the sheet 1 from its top surface through the underlying polyester film backing 3 to the sheet undersurface.

B. Production The above-descrlbed flexographic printing plate is typically made by first producing artwork representative of the desired rellef image 2 and relief structures 4 to be formed on the flexographic printing plate. From original artwork, a reverse image film is produced. A metallic plate coated with a photosensitive emulsion or an uncured photopolymer plate Is then Imagewlse exposed to UV radiation through the reverse Image film to produce a metallic photoengraving or photopolymer original. A thermosetting polymer such as phenol I c resin (eg a phenol it resin filled with cut cellulose fibres) is then used to mould a reverse image matrix from the metallic photoengraving or photopolymer original.The resulting matrix is used to produce a flexographic image carrier by moulding the desired rubber composition for the flexographic Image carrier into the matrix sheet, effecting vulcanization of the rubber by application of heat and pressure and then separating the two. A coated polyester film Is then Interposed agalnst the non-image surface of the rubber and force applied to brlng the polyester film into close contact therewith. A bond between the two Is effected by applying heat at a temperature of more then 1300C. The polyester film of the flexographic Image carrier is then ground by conventlonal methods to provide the image carrier in the form of a finished composite sheet. The ground surface provides a very suitable fixture for bonding of the polyester film, by use of adhesive, to a print drum or roller.

Subsequent to grinding, holes 5, if desired, are formed by use of tool 10 in a manner described in greater detail herelnafter.

Pre-vulcanized Rubber Flexographic Printing Plates A. Structure The pre-vulcanized sheet designated generally by the reference numeral 1a in Figure 2 is a multi-layer structure comprising a polyester film backing 3a made of the same material as referred to in connection with the flexographic Image carrier shown in Figure 1. Overlying the polyester film 3a is a layer of vulcanized rubber 6 having a hardness In the range from 20 IRHD to 95 IRHD.

The Interfacing surface of the layer 6 is a ground surface provided to ensure accuracy of thickness desired for the plate. Overlying layer 6 Is an interface layer 7 made of woven natural fibre sheet material, cotton duck being a typical natural fibre used. Laminated to the upwardly facing surface of layer 7 Is a pre-vulcanized rubber layer 8 having a Shore hardness in the range 20 IRHD to 90 IRHD.

B. Productlon Sheet material comprising each of layers 6, 7 and 8 referred to Is widely available commercially in various forms.

A 3-dimensional relief Image and structure(s) constituting a datum structure (not shown) are formed upon the upper surface of layer 8 by hand cutting or laser engraving. A row of holes 9 is optionally formed adJacent one edge of the sheet la, each hole 9, If provided, penetrating the full thickness of the sheet la (includlng the polyester film backing layer 3a), by use of tool 10 In a manner described In detail hereinafter.

Power Treatment Tool The power tool shown generally at 10 In Flgures 3 and 4 comprlses an outer housing 1 1 I n whose cavity are provided various motive power components whose arrangement and construction are entirely conventional and do not need exposition herein. The power tool will typically be one of the several plunge routers available on the market, for example a plunge router such as sold under the trade mark Elu MOF 96. Externally, however, It will be seen from Figures 3 and 4 that electrical power is supplied through electrical power lead 12. Two handles or knobs 13 are secured near the base of the housing 11 at opposed sides thereof. Houslng 11 Is mounted upon a pair of pistons 14 which can be received to varying extents wlthin cylinders provided within the housing 11.Rod 15 and assoclated clamp 16 determine the upper limit of displacement of the housing 11 upon pistons 14. Biassing means (not shown) within the housing 11 exert a force to ensure that the normal position of the housing is as shown in Figure 3, manual force applied to knobs 13 being required to displace the housing downwardly In the direction of the arrow B shown In Figure 3.

Plstons 14 have mounted at their lower extremities a plate 17. As shown In Figure 5, the underside of plate 17 Is formed with a central recess In which Is received a plate 18 provided with a series of three chevron-shaped depressions 19.

An opening 20 is formed centrally through the thickness of the plate 17.

Depending from housing 11 Is a cowelling 21 and a chuck 22. Chuck 22 mounts a tool bit 23 having the cylindrical form best seen from Figures 6, 7 and 8 of the drawings.

Chuck 22 is formed with a 3-part collet which grips the circumference of the shank of tool bit 23.

As will be seen from Figure 6 to 8 of the drawings, the tool bit 23 is in the form of a hollow right circular cylinder over most of Its length. However, cavity 24 of the cylinder is exposed at the lower part 25 of the tool bit. The portion 25 of tool bit 23 has a surface 26 which Is rendered abrasive by provision of sintered acicular tungsten carbide particles whose axes extend outwardly from the surface of the tool bit. Surface 26 extends over the cylindrical surface of the part of the tool bit 23 between its tip and the portion having an exposed cavity 24 as well as over the remainder of the lower part 25 of the tool bit and the tip annulus Itself.

The cut-away configuration of portion 25, as best seen from Figures 7 and 8 ensures that material removed from a substrate by the cutting actlon of surface 26 Is removed from the vicinity of the tool bit during working.

Use of Power Tool The optional holes 5 in the sheet 1 shown in Figure 1 and the optional holes 9 formed in the sheet la shown In Figure 2 are, as already mentloned, formed In those sheets by use of the hand-hold power tool 10 shown in Flgures 3 and 4. In use, the tool 10 is offered to the surface of the sheet and disposed over the chevron-shaped rellef structures 4. The relief structures 4 are then brought Into register wlth the depresslons 19 formed In the plate 18 of the tool 10 and the plate and sheet then united. Pressure is then exerted on the knobs 13 of tool 10 so that the housing 11 descends in the direction of the arrow B shown in Figure 3 until the tool bit 23 penetrates the opening 20 in plate 17, as shown in Figure 4 of the drawings.At this point, the tip of tool bit 23 is In engagement with the surface of the sheet material 1 or la. Rotatlon of the chuck 22 Is started, Just prior to contact between the tool tip and the sheet material, by actuation of an electrical swltch (not shown) provided for this purpose. The rotating annular cutting tip of tool bit 23 cuts the surface of the sheet 1 or 1a and removes material therefrom progressively. As the tool bit 23 penetrates into the sheet material to produce a bore, the cylindrical portions of the cutting surface 26 act to remove any imperfections in the side walls of the bore as may be produced by the annular working tip.

Particulate material removed from the sheet is swept clear of the bore Itself by the cutaway portion 25 of the tool bit 23.

Fiexographic Printing Plate Registration Assemblles A registration assembly is utilized by mounting the assembly closely adjacent to a print drum and then mounting a prepared flexographic printing plate to the registration assembly. Whilst mounted to the registration assembly, the flexographic printing plate is disposed with the bulk of Its surface area upon the print drum and Is secured In that position by an adhesive or, more typically, by double-sided adhesive tape. The flexographic printing plate Is then disassembled from the registration assembly to allow rotation of the print drum so that printing may commence.

The registration assembly shown in Figure 9 of the drawings is designated generally by the reference numeral 100. The assembly 100 comprises an elongate aluminium or aluminium alloy extrusion 101 having broadly the configuration of an inverted U. The length of extrusion 101 is slightly greater than the axial length of the print drum with which It is intended to be used. An elongate rectangular cross-sectioned aluminium or aluminium alloy mounting bar 102 runs the length of the assembly 100 and is secured at three polnts along Its length by a mounting arm 103 bolted or otherwise fixed to the flange 104 of the extrusion 101. Each arm 103 Is made of machined aluminium or aluminium alloy casting provided with a through-bore fitted with a plastics material bush (not shown) ensuring that the mounting bar 102 is securely retained.A palr of cam members 105 are mounted to the mounting bar 102 and are slidable thereon so as In use to be movable to positlons adjacent to the axial ends of the print drum. The cam members 105 rest upon the cylindrical surface of the print drum to support the registration assembly thereon. A relatively short flange 106 of the extrusion 101 has an inclined -end surface 107 giving the end of the flange 106 a triangular configuration. This trlangular configuration is recelvable within an elongate recess of trlangular crosssection formed In the flexographic plate mounting block 108.The latter, in common wlth the cam members 105, Is made of a rigid wear-resistant plastics material such as polyacetal (eg that available under the Trade Mark DELRlN) which is moulded and machined to the configuration represented In the drawings. A dowel 109 is formed on the upwardly facing surface of the flexographic printing plate block 108. A plurality of such mounting blocks 108 are secured to the flange 106 of the extrusion 101, such mounting blocks 108 having a number equal to the number of holes 5 or 9 formed in the flexographic printing plate to be mounted to the print drum and the mounting blocks 108 being spaced apart in a desired manner which will be determined by the operator as applicable to any particular set of operational circumstances.Each block 108 is secured in position by a f I xture assembly compr I s I ng a knur led nut 1 10 threadedly recelvable over an externally screw threaded stud 111 mounted on the surface 132 of the extrusion 101.

Nut 110 has a flange 112 engagable with a flat surface 130 of the mounting block 108. Surface 132 is provided with length gradations along the longitudinal extent of the elongate extrusion 101, typically in the form of a ruler overlay fixed to surface 132 and forming a background to surfaces 130 of the mounting blocks 108 when fixed In position as described earller.

A polyacetal block 133 similar to blocks 108 but configured without a dowel 109 is disposed at each end of the flange 106. Each block 133 has an undersurface having the configuration of an inwardly facing Inverted "L", that part of such surface which faces downwardly resting upon the surface of the print drum adJacent an axial end thereof and that part of such surface which is vertical and faces inwardly engaging the flat axial print drum end. It will be appreciated that each such block 133 can be secured in position, in similar fashion to the blocks 108, by cooperatlon of a nut 110 engaging a surface of blocks 133 corresponding to surface 130 of blocks 108.In the interests of clarity, the position of one only of blocks 133 is shown in Figure 9, this being the one of blocks 133 which is in actualIty concealed In the vlew represented in the Figure.

The registration assembly shown In Figure 10 comprises similar components to those which constitute the registration assembly shown In Figure 9 and I i ke parts are designated by like reference numerals. in the arrangement shown in Figure 10, the flange 106 of extrusion 101 has mounted thereto an elongate extruded aluminium or aluminium alloy plate 131. Upstanding from plate 131 are a plurality of equally spaced studs 113 having at least a portion of their external cylindrical surfaces screw threaded. Typically, three such studs will be upstanding from the plate 131. Plate 131 Is secured In position upon the flange 106 of extrusion 101 in similar fashion to the block 108 descrlbed with reference to Figure 9 of the drawings.A further elongate plate 114 comprises an extruded aluminium or aluminium alloy plate or strip 115 apertured to receive studs 113, nuts 116 being provided to clamp the plate 114 adjacent plate 131. In Figure 10 of the drawings, a flexographic printing plate is represented by reference numeral 117 and it will be seen from the Figure that the marginal edge 118 of the flexographic printing plate 117 is formed with a datum structure 4 and received upon the upwardly facing surface 120 of plate 131. Figure 10 is an exploded vlew and it will be appreclated that plate 114 can be secured so as to clamp the marginal edge 118 of flexographic printing plate 117.The lower face of plate 114 carries a datum strip 121 of cured thermaset matrix material comprlsing phenolic resin filled wlth cut cellulose fibres, the strip of matrix material carrying a datum structure in the form of a recessed image matable with the relief Image datum structures 4 of the flexographic printing plate and having been produced and instal led by procedures as described hereinafter in Example 1. It will be appreciated from Figure 10 that when the marginal edge 118 of flexographic printing plate 117 Is clamped agalnst the surface 120, the datum structures can be mated together by approprlated positional adJustment of the plate 117.

As will be seen diagrammatically from Figure 11, plate 115 having datum strip 121 on Its underside defines a recess 121a having sides subtending an angle with the horizontal ( " 720). As will be recalled, the datum strip 121 is formed from a relief matrix made from a photographically produced precursor. Irradiation of the latter for curing purposes has decreased curing effect with increasing depth in the precursor material with the result that the bases of recessed image areas are relatively narrow. This results in a matrix with relatively narrow rellef Image peaks and thus relatively narrow recessed Image bases In the datum strip. These same considerations apply to the datum structures 4 upon the printing plate 117. However, in this case, irradiation practice is so selected wlth regard to the materials used, essentially on a trial and error basis, that the sides of each rellef datum structure component 4 subtend a more acute angle with the horizontal ( 600). The difference In configuration as between the two matable components ensures that structures 4 are receivable In recesses 121a despite the former being slightly oversized at the relief structure bases, the oversizing providing a firm engagement between printing plate 117 and datum strip 121.

It will be recognized from the foregolng that, by producing each of a series of flexographic printing plates needed for a multi-colour printing operation from the same artwork, relief structures 4 are located at the same chosen polnt on each flexographlc printing plate.

By locating the depresslons 19 of plate 18 of tool 10 over each of rellef structures 4, a registratlon hole can be provided In each of an unlimited number of such flexographic printing plates In a position which is the same on each one. Witch holes thus provided, each printing plate can be placed upon a print roller in precisely the same position by registration of the holes wlth a dowel or similar complementary datum structure on a registration plate associated wlth the print roller.

The need for relief structures 4 is anticipated In production of original artwork so that such relief structures are formed upon the flexographic Image carrier with formatlon of the desired relief Image for printing.

Drilling of the holes 5 can be accomplished using tool 10 not only in an accurately reproducible manner but with much more speed than is assoclated wlth conventional equipment where registration and setting up first needs to be carrled out optically. Tool 10 has been found to drill through polyester film backing material wlthout difficulty or corrupting accuracy of hole positioning.

Thus, the invention enables holes to be formed In a polyester film-backed flexographic image carrier by relatively simple mechanical means, an accomplishment not possible according to conventlonal practice In the art.

Not only does the above-described procedure have the maJor advantage of time saving but it can be performed wlth comparatively little skill whereas conventional techniques require skilled and experlenced personnel.

Surprisingly, despite the fact that the invention relies on what may be regarded as relatively unsophisticated mechanical methods of registration, the accuracy with which datum polnts can be provided on a flexographic Image carrier has been found to be of a very high order.

Indeed, wlth care, a series of flexographic printing plates can be produced whose positionability upon a print roller approaches identicallty more closely than achievable using plates for which current optical techniques are employed to determine the position at which registration holes should be provided.

The invention is even simpler to perform by registering the printing plates, wlthout use of holes penetrating the plates, using matable datum structures formed on the one hand on the printing plate, and on the other hand on surface of the registration assembly, as Illustrated in Figure 10 of the drawings and descrlbed herein above.

Reproducible positioning of printing plates to an extremely high level of accuracy can be achieved by this means wlth greater speed and less need for skill than in the case of registratlon using holes formed in printing plates using the tool 10.

The following Examples are intended further to illustrate the Invention:- Example 1 A datum structure representation Is produced by computer typesetting upon photographic paper, the design being In black on a white background. The image is photographed and a negative film obtalned to complete Stage 1 shown In Figure 12.

The negative film Is then superimposed upon a sheet of hard photopolymeric material. The assembly is exposed to ultra violet radiation using any of the various equipment well-known for such purposes. The exposed plate is then processed.

The processed photopolymer plate (called an 11original") Is used to produce a relief phenollc-based matrix using a moulding press designed specifically for the accurate vuicanisation of rubber stereos and their matrices. The two (top and bottom) platens of the press are maintained at temperatures of 150 - 1550C. Unvuicanised phenolic base matrix material in the form of Wilsolite, a thermosetting sheet form material impregnated with phenolic resin and available from Wilsolite Inc, Buffalo, NY, was disposed with the original inverted above. The combination was placed upon a press serving tray and entered into the press. The press Is firmly closed for a cure perlod of ten minutes.Upon completlon of cure, the assembly is removed and the original Is separated from the formed matrix reference strip 121 which faithfully reproduces the original design In reverse and accords with Figure 11.

The matrix reference strip (called a "reverse") is then reduced accurately to predetermined dimensions. The reference strip Is then fastened to the plate 115 using cyanoacry late adhesive.

Example 2 Artwork representative of a desired rellef image was first produced and from this a reverse image film on polyester negative film was made. A photopolymerizable plate of Rigilon was exposed to UV-radlation through the film to effect cure. The exposed plate was then developed by washing away unexposed and therefore unpolymerized areas of monomer material. Wilsolite, a thermosetting sheet-form material Impregnated with phenolic resin and available from Wllsolite Inc, was moulded Into the developed relief surface and thermoaset by application of heat and pressure.The resulting matrix was removed from the photopolymer plate and an unvulcanized natural rubber moulded Into the rel lef surface of the matrix. The rubber was vulcanized by application of heat and pressure using a vulcanizing press (temperature 1550C) in a conventional manner for four minutes. The assembly was removed from the press and bond-coated polyester film placed on the reverse rubber surface. The assembly then was replaced into the press and further pressure applied for three minutes at 1550C. The assembly Is then again removed from the press and the polyester film backing ground In a conventional manner.

The original artwork contalned datum structure representations computer typeset so as to be identical to those typeset for the matrix strip produced according to Example 1. This resulted in the formation of chevronshaped relief structures located adJacent one edge of the flexographic image carrier and conforming to the configuratlon Illustrated dlagrammatically in Figure 11.

A hand-hold power tool of the kind shown in Flgures 3 and 4 of the accompanying drawings was provided with a central base plate 18 formed with depressions in the configuration of the above relief structures. The tool was offered to the flexographic Image carrier and the depressions in its base plate 18 brought into register with the chevron-shaped relief structures on the surface of the flexographic Image carrier. The tool was then operated to drill a hole through the image carrler.

The flexographic Image carrier had been formed wlth a number of arrays of chevron-shaped relief structures of the kind Just mentloned. In the same manner as described above, the tool 10 was brought into register successively wlth the further rellef structures and further holes drilled through the flexographic image carrier. In this way, a longitudinal edge of the image carrier was provided with a row of holes penetrating through the thickness of the flexographic material.

In order to enable printing of a five colour design, eg upon packaging, four further flexographic Image carriers were produced In the manner described above and rows of holes provided in the same way.

Each flexographic printing plate was mounted to a print roller of a Carrare printing press, cooperation between a registration assembly 100 (Figure 9) associated with the print roller and the holes drilled in the manner already described providing accuracy of placement upon the roller. A single registration assembly moved from one roller to another may be used. Once accurate positioning had been established, double-sided adhesive tape previously applied to the surface of the rollers was adhered to the underlying polyester film surface of the flexographic printing plate to fix the plates to the stereo print cylinders. Flve printing stations, each for applying to cardboard packaging a different colour ink to the other, were set up using a different one of the five flexographic printing plates made In accordance with the above procedures.Print runs were then made.

It was found that the flexographic printing plates resisted distortion during printing. This and the identity of the positioning of each plate resulted in a high quality printed packaging in five colours.

Example 3 The above procedure was repeated except that the holes produced by the power tool were omitted. The resulting plate was registered using a registration assembly, as shown In Figure 10, which, It wi I I be appreciated, Incorporates a matrix reference strip 121 produced according to Example 1 fastened to plate 115 by cyanoacry late adhesive. Equally satisfactory results were obtalned but with greater speed and less effort in terms of mounting the flexographic printing plates to the print drums at each of the flve printing stations.

The invention as descrlbed earlier without reference to the drawings may Include any one or more features of the invention as descrlbed wlth reference to the drawings.

Claims (50)

Claims

1. A method of printing an Image upon a substrate which method comprises printing a first part of the image upon sald substrate by means of a first flexographic printing plate mounted to a print drum at a first prlnt-lng station, conveying the so-printed substrate to a second sequential printing statlon and there printing thereon a second part of sald Image by means of a second flexographic printing plate mounted to a further print drum, characterized In that:: (I) the first flexographic printing plate is formed photographically with both a printing image and a datum structure and the second flexographic printing plate is formed photographically with both a printing image and an equivalent datum structure; (ill) mounting of the first printing plate to the first printing station print drum is effected by assembling said first printing plate and a registratlon means together In predetermined positlonal relationship between sald registration means and the printing image of said first printing plate, said datum structure of sald first printing plate being used as a reference to determine the positional relationship between said registration means and the Image of sald first printing plate, securlng the registration means in adJacent relationship with respect to the first printing statlon print drum before or after assembling together the registration means and the first printing plate, securing the first printing plate to the first printing station print drum whilst said plate is fixed relative to the drum by said registration means and di sassemb I I ng the first pr I nt I ng plate and said registration means from each other; and (III) mounting of the second printing plate to the second printing station print drum is effected by assemblIng sald second plate and a registration means together in predetermined positional relationship between sald registration means and the printing Image of said second printing plate, said datum structure of said second printing plate being used as a reference to provide that the positional relationship between said registration means and the Image of said second printing plate Is the same as that between the first printing plate Image and registration means, securing the registration means, before or after assemblIng together the registration means and the second printing plate, In the same adJacent relationship with respect to the second printing statlon print drum as the adJacent relationship aforesaid between the first printing station print drum and registration means, securing the second printing plate to the second printing station print drum whilst sald second plate Is fixed relative thereto by sald registration means and disassembling the second printing plate and said registration means from each other.

2. A method as claimed In Claim 1 wherein the f lexographic printing plates are made of a material selected from the group consisting of natural and synthetic rubber.

3. A method as clalmed in Clalm 2 wherein said rubber Is selected from the group consisting of butadiene acrylonitri le rubbers, chloroprene, butadlene-styrene rubbers, butyl rubbers, ethylpropylene rubbers, silastomers and urethane rubbers.

4. A method as claimed in any one of Claims 1 to 3 wherein the flexographic printing plates are each formed by producing an original selected from the group consisting of a metallic photoengraving and a photopolymer original by Image-wlse exposure of a photosensitive precursor thereof and subsequent image development, moulding a polymer upon said original to form a matrix sheet, and forming a reverse Image flexographic image carrier selected from the group consisting of natural and synthetic rubber flexographic Image carriers by moulding natural or synthetic rubber upon said matrix sheet and effecting vulcanization of the rubber.

5. A method as claimed in Claim 4 wherein the precursor of the original is image-wlse exposed using artwork which Includes a representation responsible for originating the datum structure.

6. A method as clalmed in elther Claim 2 or Claim 3 wherein the Image carrier is formed by engraving an image upon a substrate selected from the group consisting of pre-vulcanized natural and synthetic rubber substrates.

7. A method as claimed In Claim 6 wherein the Image Is formed by an operation selected from the group consisting of hand cutting and laser engraving.

8. A method as clalmed in Claim 7 wherein the image is laser engraved in accordance wlth artwork which controls the path of the laser beam and which includes a representation responsible for originating the datum structure.

9. A method as clalmed In Claim 1 wherein the flexographic printing plates are made of photopolymer.

10. A method as clalmed in Clalm 9 wherein the flexographic printing plates are made by Image-wlse exposure of a photopolymerizable plate using artwork which Includes a representation responsible for originating the datum structure.

11. A method as claimed In any preceding claim wherein the flexographic printing plates each comprise a multllayer member comprising an image-carrying layer and a polyester film layer.

12. A method as claimed in Claim 11 wherein the polyester film has a bonding agent coated on Its surface.

13. A method as claimed In any preceding clalm wherein the datum structures are relief formations upon a surface of the flexographic printing plates.

14. A method as clalmed In Clalm 13 wherein the rellef formatlons comprise formatlons having a configuration selected from the group consisting of chevrons, circles and at least one linguistic indicium.

15. A method as claimed In any preceding claim wherein the flexographic printing plates are rectangular in plan and the datum structures are provided in a marginal edge portion of each of the flexographic printing plates adJacent to one side thereof.

16. A method as claimed In any preceding claim wherein the first flexographic printing plate and second flexographic printing plate each have treatment means secured In a fixed position relative to the flexographic printing plate, a datum structure of the respective plates and a cooperative structure of said treatment means being mated together to determine the positions of the image of said plate and the treatment means relative to one another and said treatment means being actuated whereby material of the respective plates is removed locally from a reglon of a surface thereof to form a recessed structure selected from the group consisting of perforations and depresslons In each sald plate, and wherein the recessed structure Is located with a cooperative element of registration means to assemble the registration means and the respective plates together such that the positional relationship between the Image of the first flexographic printing plate and reglstratlon means Is the same as that between the image of the second flexographic printing plate and registration means.

17. A method as claimed in Claim 16 wherein the treatment means comprlses power means, mounting means for mounting a materials removal tool bit to the treatment means and means for coupling the power means to the mounting means.

18. A method as claimed In Clalm 16 or Claim 17 wherein the cooperative structure of the treatment means comprlses a member which is apertured to receive therethrough a materials removal tool bit mounted to driven mounting means of the treatment means and wherein the apertured member and tool mounting means are displaceable to provide for such movement of the tool bit through the aperture as Is required to plunge the tool bit to a desired range of depths In the flexographic printing plates.

19. A method as clalmed in Claim 17 or Claim 18 wherein the tool bit comprises an elongage member having a tip provided wlth a cutting surface.

20. A method as clalmed In Clalm 19 wherein the cutting surface is an abrasive cutting surface which is provided by exposed surface acicular tungsten carbide particles.

21. A method as clalmed in Claim 20 wherein the acicular particles are sintered to the material of the tool bit underlying them.

22. A method as claimed in Claim 20 or Clalm 21 wherein the acicular particles are fixed in orientations In which thelr axes extend from the tool bit.

23. A method as claimed in Claim 22 wherein the acicular particles are sintered in place whilst oriented by the influence of a magnetic field.

24. A method as claimed In any one of Claims 17 to 23 wherein the tool bit comprises a member having generally the configuration of a hollow right circular cylinder, the member having a cutting tip portlon and a shank portion generally In that same configuration and an Intermediate portlon having a cross-section in the form of that part of an annulus as is defined between two concentric arcs connected by a common chord, the smaller arc being concentric with the circumference of the cylindrical member and the longer arc being concentric or coincident wlth that circumference.

25. A method as claimed In Claim 24 wherein the exterior arcuate surface of said intermediate portion Is an abrasive surface.

26. A method as claimed in any one of Claims 17 to 25 wherein the materials removal tool bit has a tip which has the cross-sectional configuration of an annulus, the annulus defining a cutting surface such that on actuation of the treatment means the plane of the annulus is brought to bear upon an associated flexographic printing and material is removed therefrom by cutting action effected In rotating the tool bit on an axis perpendicular to the plane of the annulus.

27. A method as claimed in any one of Claims 16 to 26 wherein the cooperative structure of said treatment means comprlses a member mounted thereto and having a surface configured with a formation which is the reverse of the datum structures so that each datum structure functions wlth the said formation In the manner of a key and socket.

28. A method as claimed in any one of Clalms 16 to 27 wherein the datum structures are relief structures and the treatment means includes a surface recessed and having the configuration of the relief structures.

29. A method as claimed in any one of Claims 16 to 28 wherein the treatment means is a hand-hold power tool.

30. A method as clalmed in any one of Claims 16 to 29 wherein the treatment means comprlses a rotating tool bit having a tip provided with an abrasive cutting surface for actlng upon said flexographic printing plates, said cutting surface of sald tool bit being a grlnding face defined by an annulus of said tool bit.

31. A method as claimed in Claim 30 wherein said annulus has an abrasive rotating plane perpendicular to the axis of rotation of said tool bit.

32. A method as claimed in Claim 31 wherlen said abrasive plane is rotated essentlally perpendicularly to the plane of the image carrier.

33. A method as claimed in any one of Claims 1 to 15 wherein the first flexographic printing plate is mounted to the first printing station print drum by locating the datum structure of sald first flexographic printing plate with a matable datum structure of sald registration means so that sald registration means and first printing plate are assembled to each other, the registration means being secured in adJacent relationship with respect to the print drum of the first printing station before or after assembly together of the registratlon means and the first printing plate, securing the first plate to the first printing station print drum whilst the plate is fixed relative to the drum by sald registration means and disassembling the first flexographic printing plate and the registration means from each other and wherein the second flexographic printing plate is mounted to the second printing station print drum by locating the datum structure of said second flexographic printing plate with a matable datum structure of said registration means so that said registration means and second printing plate are assembled to each other and so that the positional relatlonship between the Image of said second printing plate and registration means Is the same as that between the image of sald first printing plate and registration means, the registration means being secured in adJacent relationship with respect to the print drum of the second printing statlon before or after assembly together of the registration means and the second printing plate, securing the second plate to the second printing station print drum whilst the plate is fixed relative to the drum by said registration means and disassembilng the second flexographic printing plate and the registration means from each other.

34. A method as claimed in Claim 33 wherein the registration means comprises an elongate member having means for securing the registration means adJacent a print drum such that said elongate member is secured against Inadvertent dislodgement, an elongate plate formed wlth sald matable datum structure and means for securing said elongate plate to a flexographic printing plate having a datum structure thereof located with sald matable datum structure.

35. A method as claimed in Clalm 34 wherein the elongate member has a mounting member at or adjacent each end, each said mounting member being formed with a surface which In use engages the axial face of the print drum associated with said registration means and a surface which In use engages the cylindrical surface of sald drum.

36. A method as claimed in Claim 34 or Claim 35 wherein sald elongate plate cooperates wlth a further plate to clamp a marginal edge of a flexographic printing plate therebetween, said marginal edge carrying said datum structure and sald datum structure of sald printing plate and the matable datum structure of said elongate plate being located together.

37. A method as claimed in Claim 36 wherein sald elongate plate and said further plate are securable together by clamping means.

38. A method as clalmed in Clalm 37 wherein sald clamping means comprises a stud on sald further plate, an orifice through said elongate plate and a fixture member securable to said stud.

39. A method as claimed in any one of Claims 33 to 38 wherein said matable datum structure is formed photographically.

40. A method as claimed In Claim 39 wherein the matable datum structure is formed photographically by a procedure comprising imagewlse exposure of a photosensitive substrate to a representation of sald matable datum structure.

41. A method as claimed in Claim 40 wherein said representation is a representation of the datum structure carried by a flexographic printing plate.

42. A method as claimed in any one of Claims 33 to 41 wherein the flexographic printing plate and matable datum structures originate from Identical representatlons.

43. A method as claimed in any preceding claim wherein the flexographic printing plates are made by a procedure including imagewise exposure of a photosensitive precursor to the same artwork, said artwork including representatlons of the print Image and representations of the datum structures of said flexographic printing plates, the plates having the same print image-to-datum structure positional relationship.

44. A method of printing an Image upon a substrate which method comprises printing a first part of the Image upon said substrate by means of a first flexographic printing plate secured to a print drum located at a first printing statlon, conveying the partlally substrate to a second sequential printing statlon and there printing on the partlally prlnted substrate a second part of said image by means of a second flexographic printing plate secured to a print drum located at sald second printing station, characterized in that:: (l) the first and second flexographic printing plates are formed photographically from Identical artwork wlth both a printing image and a datum structure; (ill) the first fiexographic printing plate Is secured to the first printing station print drum by locating the datum structure of said first flexographic printing plate wlth a matable datum structure of registration means secured in adjacent relationship with respect to the print drum of the first printing station, whereby said registratlon means and first printing plate are assembled to each other, securlng the first flexographic printing plate to the first printing station print drum by adhesive means whilst the plate Is fixed relative to the drum by said reglstratlon means and disassemblIng the first flexographic printing plate and the registration means from each other;; (I ii) the second flexographic printing plate Is secured to the second printing station print drum by locating the datum structure of said second flexographic printing plate wlth a matable datum structure of registration means secured In adjacent relationship with respect to the print drum of the second printing statlon whereby sald registration means and second printing plate are assembled to each other and so that the positional relationship between the Image of said second printing plate and sald registration means Is the same as the first printing plate Image-to-registrati on means relationship , securlng the second flexographic printing plate to the second printing station print drum by adhesive means whilst the plate Is fixed relative to the drum by said registration means and disassembling the second flexographic printing plate and the registration means from each other.

45. A method as claimed in Claim 44 wherein each registration means is formed wlth means for locating the registration means wlth a print drum in predetermined positlon relative to the print drum.

46. A method as claimed In Claim 45 wherein said means for locating the registratlon means comprises one or more members mounted, optlonally movably, to the registration means and formed wlth a surface for abutting a planar axial end face of the print drum.

47. In a method of printing an Image upon a substrate which method comprises printing a first part of the image upon said substrate by means of a first flexographic printing plate secured to a print drum located at a first printing station, conveying the partially substrate to a second sequentlal printing station and there printing on the partially printed substrate a second part of said image by means of a second flexographic printing plate secured to a print drum located at sald second printing station, the improvement wherein (l) the first and second flexographic printing plates are formed photographically from Identical artwork with both a printing Image and a datum structure;; (ill) the first flexographic printing plate Is secured to the first printing station print drum by locating the datum structure of said first flexographic printing plate with a matable datum structure of registration means whereby said registratlon means and first printing plate are assembled to each other, securing the registration means in adJacent relationship with respect to the print drum of the first printing station, securing the first flexographic printing plate to the first printing station print drum by adhesive means whilst the plate Is fixed relative to the drum by sald registration means and disassembling the first flexographic printing plate and the registration means from each other;; (III) the second flexographic printing plate Is secured to the second printing station print drum by locating the datum structure of said second flexographic printing plate with a matable datum structure of registration means secured in adJacent relationship with respect to the print drum of the second printing station whereby said registration means and second printing plate .are assembled to each other and so that the positional relationship between the Image of said second printing plate and said registration means Is the same as the first printing plate image-to-registration means relationship, securing the second flexographic printing plate to the second printing statlon print drum by adhesive means whilst the plate is fixed relative to the drum by said registration means and disassembling the second flexographic printing plate and the registration means from each other.

48. A method as claimed In Claim 47 wherein each registratlon means Is formed wlth means for locating the registratlon means with a print drum in predetermined position relative to the print drum.

49. A method as claimed in Claim 47 or Claim 48 wherein sald means for locating the registration means comprlses one or more members mounted, optlonally movably, to the registration means and formed with a surface fro abutting a planar axial end face of the print drum.

50. A method of forming a flexographic printing plate which method comprlses forming a f lexograph Ic image carrier having one or more datum structures provided at predetermined positlon(s) on a surface thereof, securlng treatment means in a fixed position relative to the carrier, a datum structure of said Image carrier and a cooperative structure of sald treatment means being mated together to determine the positlons of the Image carrier and -the treatment means relative to one another, actuating said treatment means whereby material of said Image carrier Is removed locally from a reglon of a surface thereof to form a perforatlon or depression in sald image carrier and removing the treatment means from the image carrier.