EP0199520A2

EP0199520A2 - Gravure printing press and method of manufacturing the same

Info

Publication number: EP0199520A2
Application number: EP86302754A
Authority: EP
Inventors: Douglas Cliffe Ledgard; James Cox
Original assignee: Observer Ltd; Vickers PLC
Current assignee: Observer Ltd; Vinters Ltd
Priority date: 1985-04-19
Filing date: 1986-04-14
Publication date: 1986-10-29
Also published as: EP0199520A3

Abstract

A printing press comprises a rotatable cylindrical surface (6) carrying a gravure printing image and ink is applied to the surface by an inking unit comprising a pressurised ink duct. The ink duct is bounded by the surface (6), a doctor blade (12) and a sealing member (11). ink is supplied to the duct under pressure from a source and excess ink is returned to the source. The image may be on a plate mounted on a plate cylinder (6) in a readily removable manner. The press may be produced by conversion of a letterpress printing machine for newspaper production.

Description

This invention relates to printing and is concerned with a printing press for effecting gravure printing.
In gravure printing, a surface carrying a gravure printing image is used wherein the printing areas comprise a series of individual cells separated from one another by lands constituting the non-printing area. In use, ink is applied to the surface carrying the gravure printing image and the excess ink over and above that which enters the cells is removed by a doctor blade supported by the lands. Thereafter, the material to be printed is pressed against the inked image and ink is transferred from the cells onto the material. The cells vary in size. Thus they may be of constant area and varying depth, of varying area and constant depth, or a combination of the two. Hence there are variations in the quantity of ink held in the cells and these quantity variations enable tonal reproductions to be obtained on the material being printed.
Various methods have been devised for applying ink to the surface carrying the gravure printing image in gravure printing and it is an object of the present invention to provide an improved and more economic inking system for this purpose.
One form of conventional rotary letterpress printing machine for newspaper production includes a plate cylinder which carries a curved metal printing plate cast by stereotyping and including a relief image. In another form the plate cylinder carries a printing plate having a relief image formed by image-wise exposure and development of a photopolymerisable layer. Such printing plates place a limitation on the quality of letterpress newspaper production, and they cannot match the quality achieved by gravure or web-offset printing. The reason for this is that, in printing, pictures are reproduced as a series of dots formed by a screen. Generally, the coarser the screen, the less detailed is the picture. Conventional rotary letterpress machines are in the main restricted to a screen of 65 dots per linear inch. Web-offset and gravure printing allows the use of more than 120 dots per linear inch. Also, a conventional rotary letterpress machine is limited to using a mainly oil based ink which is formulated so as to remain 'open' (i.e. non-drying) on the machine and, as a consequence, it does not dry completely on the newspaper. This is a source of constant complaint by readers of the newspaper. Furthermore, conventional letterpress machines are expensive to operate because of their high energy requirements. This is due in part to the large roller trains which are required to ensure even ink application to the plate cylinder. Proposals have been made for replacing these ink trains with a single engraved drum and, while this does lower the power requirement and optimise the print quality_/from the printing plates, the overall result is still inferior to gravure or offset systems.
It is another object of the present invention to provide a high speed printing press for newspaper production which avoids the foregoing disadvantages.
Letterpress newspaper printing machines are extremely expensive and a press may have a working life of 25 years or more. Because of the high cost of re- equipping, newspaper production cannot economically take advantage of new printing technology.
It is a further object of the present invention to enable an existing letterpress machine for newspaper production to be converted to a gravure printing machine.
Newspaper production operates to very tight time schedules and requires frequent changes of the printing plates during the printing run to keep the newspaper up to date. It is essential that these changes are carried out as rapidly as possible. A recent development allows gravure plates to be made by a photo-polymerization process in which the image is formed in a polymer layer applied over a steel base sheet. Such plates can be made rapidly and give a high quality image.
It is a still further object of the present invention to provide a high-speed rotary printing machine for the production of newspapers or for other commercial printing purposes which utilises a gravure plate produced by such a photo-polymerisation process.
According to the present invention there is provided a gravure printing press comprising:

(i) a cylindrical surface carrying a gravure printing image,
(ii) a means of rotating said surface about its cylindrical axis,
(iii) an inking unit for applying ink to said surface comprising:-
- (a) a substantially closed axially extending ink duct for containing ink under pressure, said ink duct being bounded by said surface, an axially extending doctor blade in contact with said surface, and an axially extending sealing member,
- (b) a means of supplying ink from an ink source to said duct and thence into contact with said surface, and
- (c) a means of returning to said source ink in excess of that retained by said surface, and
(iv) a means of contacting said surface with material to be printed so as to transfer ink from said surface to the material.

The axially extending sealing member will generally be spaced from the cylindrical surface by as small a distance as possible so that the ink duct is, to all intents and purposes, substantially closed. Typically the gap between the sealing member and the cylindrical surface will be no more than 2.0 mm and is preferably less than 0.5 mm. However, in some circumstances it may be desirable for the axially extending sealing member to be in contact with the cylindrical surface.
The inking unit may be a single unit extending along the entire axial length of the cylindrical surface and sealed at the ends of said surface so that it has a width corresponding to the maximum width of the material to be printed. Alternatively, the press may include a plurality of narrower inking units closely arranged together side by side along the axial length of the cylindrical surface with each unit being individually sealed at its ends against the cylindrical surface. This enables the printing of a material narrower than the normal maximum width simply by shutting off the ink supply to the superfluous unit(s) or by removing the superfluous unit(s). In this way, the number of inking units to be used in a given case can be selected in dependence upon the width of the material being printed. Moreover, a particularly useful advantage of this embodiment is that the inking units may be mounted on the printing press in such a way that they can be readily detached and be replaced by another inking unit containing ink of a different colour so that different colour printing across the full width of the material is readily facilitated. Moreover, it is particularly preferred for these detachable inking units to be interchangeable, one with the other, along the axial length of the cylindrical surface to facilitate printing in different colours. Alternatively this can be achieved by feeding different coloured inks to the inking units without detaching and replacing the units.
The cylindrical surface carrying the gravure printing image may be in the form of a gravure cylinder. Such may be produced by a photomechanical process in a conventional manner by exposing carbon tissue or other suitable radiation sensitive material to a gravure screen to provide the image to be printed. The material is hardened to a variable degree by the exposure depending upon the amount of light it receives. The exposed material is then transferred to the surface of a copper cylinder and developed so as to selectively remove the radiation sensitive material depending upon the degree to which this material has become hardened during the exposure step. Thereafter, the surface of the copper cylinder is etched to produce the desired cells. The photomechanical resist constituted by the developed material is of differing hardness and thickness and the etchant, e.g. aqueous ferric chloride, breaks through the resist gradually, depending upon the amount of hardening and the thickness, and etches the surface of the cylinder. Those areas of the resist most exposed to light are the most resistant to etching. Thus the copper surface beneath the completely exposed areas, corresponding to the screen, does not etch at all and hence this part of the copper surface forms the lands for the doctor blade to run on. The remainder of the copper surface is etched out in inverse proportion to the amount of light received by the overlying resist and forms the cells of differing depth and/or size.
Alternatively, the surface carrying the gravure printing image may be constituted by a gravure printing plate which has been fixed to a plate cylinder. Such plates may be produced from a radiation sensitive plate comprising a radiation sensitive material coated onto a suitable substrate such as a synthetic resin or a metal sheet. The radiation sensitive material may be a photopolymer in which case the radiation sensitive plate may be produced from the photopolymer by moulding. Any suitable photopolymer may be used and it is particularly preferred to use, as the photopolymer, photosensitive linear polyamides such as those disclosed in British patent specifications No.767,912, No.795,961, No.862,276 and No.875,378. In the use of such photopolymer plates, the photopolymer is image-wise exposed to a continuous tone positive and a half-tone screen and the light entering the photopolymer insolubilises it. In those areas which receive most light, there is substantially complete photo insolubilisation and in those areas which receive the least amount of light there remains the greatest proportion of non-photo insolubilised polymer. Thereafter the plate is developed by applying a solvent for the photosensitive polymer so that the remaining photosensitive areas are selectively washed away leaving the insolubilised areas. There is thus obtained a gravure image comprising cells of differing volumes which are capable of receiving different quantities of ink to produce tonal reproductions on printing.
In the case where a gravure printing plate is used, it will be detachably mounted on the plate cylinder by a locking means. The locking means and the printing plate will preferably be adapted to provide for rapid attachment and detachment of said plate to and from said cylinder whilst providing a smooth and substantially uninterrupted surface for contact with the material to be printed.
In one embodiment, the gravure printing plate is mounted on the plate cylinder conventionally used for the aforementioned stereotype or photopolymer relief plates. In this case, a shim plate, which encloses the cylinder and which is preferably formed in two or more sections, is interposed between the gravure printing plate and the plate cylinder to compensate for the difference in thickness between the conventional relief plate and the gravure plate. Alternatively, the conventional plate cylinder may be replaced by another plate cylinder having a larger diameter so that when this other plate cylinder is carrying a thin gravure printing plate the overall diameter is the same as the overall diameter of the conventional plate cylinder when carrying a conventional stereotype or photopolymer relief printing plate. In this way, the difference in plate thickness can be compensated for without the need to use a shim plate.
The locking means may comprise a modified version of the mechanical plate locking arrangement used to secure the conventional cast stereotypes and photopolymer plates to a plate cylinder. Alternatively, in the case where a shim plate is used, the locking.device may be incorporated into the shim plate.
Where a letterpress printing press is converted to a gravure printing press in accordance with the invention, the ink train associated with the printing press is discarded and the required inking unit is fitted adjacent to the plate cylinder.
In one preferred embodiment of the present invention, which can be applied to a conversion or to a new press, the locking means comprises a printing plate supporting shim plate fitted with a plurality of magnetic inserts, and the backing element of the printing plate is made from magnetisable material such as sheet steel. In an alternative embodiment, magnetic inserts may be provided in the surface of the plate cylinder itself. In either case, the length of the printing plate may be greater than the overall circumferential length of the plate cylinder, together with its shim plate if needed, and the ends of the printing plate are overlapped when it is fitted in position. The plate cylinder, or shim plate if fitted, may be provided with an axial channel in its cylindrical surface having a depth substantially equal to the thickness of the printing plate into which a recessed portion at one end of the printing plate may be received so that when the two ends of the plate are overlapped, the circular profile of the plate cylinder is maintained. A rapid-setting plastic filler material may be used to fill any gap left at the free end of the plate, this filler material being cut away when the plate is changed.
The arrangement whereby a printing press is converted to accept a gravure printing plate has in practice substantial advantages. Firstly, the use of a gravure plate produced by the photo-polymerisation process provides higher print quality than the conventional letterpress printing plates. Secondly, the proposed direct inking system eliminates the ink trains used in conventional systems and thereby substantially lowers the energy requirements. Thirdly, the use of a gravure plate will more readily permit the use of water and emulsion-based inks that in turn provide rapid drying, reducing 'rub-off' of the ink and 'strike-through', (penetration of ink through the paper). Both ink mist and noise are substantially reduced, and the requirement for either ink column controls or engraved ink drums is eliminated.
For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:-

Figure 1 is a side view of a part of a printing press in accordance with one embodiment of the present invention,
Figure 2 is a view of apart of the printing press of Figure 1 in the direction A on a reduced scale,
Figure 3 is a diagrammatic side view of the printing press of Figures 1 and 2,
Figure 4 is a diagrammatic side view of the plate cylinder and impression cylinder of the printing press of Figures 1,2 and 3,
Figure 5 is a diagrammatic side view of a part of a conventional rotary letterpress printing machine for newspaper production,
Figure 6 is a diagrammatic side view of part of the plate cylinder of another embodiment of the present invention,
Figure 7 shows a part of the plate cylinder of Figure 6 on an increased scale, and
Figure 8 is a diagrammatic section through part of the plate cylinder of another embodiment of the present invention.

Referring first to Figure 2, this shows one side only of the press. The other side corresponds and hence has been omitted in the interests of clarity.
The printing press shown in Figures to 4 comprises a pair of side frames 1 (one side frame only being shown) with a rigid cast iron stretcher 2 pivotally mounted between the two. The press includes a printing unit comprising a plate cylinder 4 mounted for rotation about its cylindrical axis 5 in journals 3 carried by the side frames and an impression cylinder 29 similarly mounted for rotation in journals carried by the side frames and located so as to define, with the plate cylinder 4, a nip through which a web 28 of paper to be printed is passed. Cylinders 4 and 29 carry gears 35 and 36 respectively which are meshed together. The press includes a main drive motor 37 which rotates cylinders 4 and 29 via gears 38 and 39. The press may include a plurality of such printing units (not shown) in which case they will all be driven by a suitable drive mechanism in conventional manner by the motor 37.
A printing plate 6 having a gravure printing image is wrapped around the plate cylinder 4 and fixed thereto. The gravure printing image of the printing plate 6 was produced by exposing a photosensitive polyamide coating on a steel substrate to a continuous tone positive and to a half tone screen and then developing the exposed coating.
A plurality of inking units is included in the printing press to apply ink to the printing image. Two of these are denoted by references B and C (Figure 2). Each printing unit is secured to the stretcher 2 by means of retaining plates 8 clamped to the stretcher 2 by bolts 9. Up to four such inking units are provided and they are mounted on the printing press side by side along the axial length of the plate cylinder 4 or, alternatively, one full width inking unit may be present.
Each inking unit comprises a body portion 10 carrying an axially extending sealing member in the form of a plastics baffle 11 arranged to fit in close proximity to but spaced from the cylindrical surface of the plate 6 and mounted normally with respect to the surface of the plate 6. The body portion 10 also carries an axially extending doctor blade 12 which contacts the cylindrical surface of the plate 6. This blade 12 is flexible and formed, for example, of steel and is displaceably mounted on the body portion 10 so that., it can be moved towards or away from the cylindrical surface of the printing plate 6 by suitably rotating threaded adjusting bolt 13. In the embodiment shown in the drawings, the doctor blade 12 is shown contacting the cylindrical surface in the reverse angle position. However, in another embodiment, the doctor blade 12 may contact the cylindrical surface in a trailing position. Seals 7 bearing against the plate cylinder are provided at each end of the inking unit. The baffle 11 and blade 12, together with the body portion 10 and the surface of the printing plate 6, define an essentially closed ink duct extending along the width of the inking unit. The body portion 10 also includes a protruding edge 14 extending along the width of the duct, the edge 14 being slightly spaced from the cylindrical surface of the plate 6. Ordinarily, the gap between the protruding edge 14 and the cylindrical surface of the plate 6 will be no more than 2.0 mm and preferably it is less than 0.5 mm. The protruding edge 14 divides the ink duct into a first zone 15 and a second zone 16. The body portion 10 incorporates an ink feed conduit 17 which terminates in an inlet port 18 in the first zone 15 of the ink duct between the baffle 11 and the protruding edge 14. The inlet port 18 is positioned midway between the ends of the ink duct. The body portion 10 also incorporates an ink discharge conduit 19 communicating with branch conduits 20 which terminate in outlet ports 21 located at each end of the second zone 16 of the ink duct. This arrangement enables ink to egress from the second zone 16 of the ink duct.
The free end of the ink feed conduit 17 terminates in a connector 31 whereby an ink feed pipe 26 (see Figure 3) may be detachably connected thereto so that ink can be fed to the duct from a reservoir 22 via a pump 23 and a filter 24. The free end of the ink discharge conduit 19 terminates in a connector 32 whereby an ink discharge pipe 27 (see Figure 3) may be detachably connected thereto so that ink can be conducted back to the reservoir 22. A valve 30 is incorporated in the ink discharge pipe 27.
In use ink, which may be a quick drying ink containing water, is pumped from the reservoir 22 under pressure through the filter 24 and into the first zone 15 of the ink duct via ink feed pipe 26, conduit 17 and port 18. The ink flows between the protruding edge 14 and the surface of the plate 6 and into the second zone 16 of the ink duct and thence returns via ports 21, conduits 20 and 19, and ink discharge pipe 27 back to the reservoir 22. The plate cylinder 4 rotates in the direction indicated by the arrow, i.e. in the same direction as the ink flow. The flow of ink through the constriction defined by the edge 14 causes a pressure rise in the second zone 16 and ensures that the cells of the gravure printing image of the printing plate 6 are de-aerated and filled with ink. The pressure in this zone can be controlled by suitably adjusting the valve 30. The baffle 11 prevents contaminants entering the duct and also confines the ink within the duct. Excess ink is removed from the image by the doctor blade 12 and is returned to the reservoir 22. Any unwanted ink escaping past the end of seals 7 of the units is removed by an auxiliary-scraper blade 25. The ink remaining in the cells is transferred to the web 28 which passes between the plate cylinder 4 and the impression cylinder 29 so that the desired image is printed onto the web. Access to the plate cylinder 4, for example when fitting the printing plate 6, is facilitated by tripping the inking units away from the plate cylinder by pivoting the stretcher 2 about the side frames 1.
In the embodiment shown, four inking units are provided across the width of the printing press and the press is such that it can receive a web which has a width corresponding to four pages, each page being associated with one of the inking units. In this way, four pages can be printed simultaneously. If, however, it is desired to use a narrower web and print less than four pages simultaneously, the appropriate number of inking units can be taken out of service merely by having their pumps 23 switched off or by being removed altogether after slackening bolts 9. In this way, only the number of inking units appropriate to the width of the web being printed are used. If desired, each inking unit may contain ink of a different colour so that, for example, four colours can be applied to a four page width web simultaneously. The inking units are interchangeable so that they may readily be moved axially along the plate cylinder from one position to another as desired to facilitate printing in different colours. Printing in different colours can also be effected by disconnecting the ink feed and ink discharge pipes of a given inking unit and connecting the unit to ink feed and ink discharge pipes associated with a reservoir containing ink of a different colour.
The press is intended for printing newspapers and typically has been converted from a letterpress machine. The printing plate and the plate cylinder are shown in more detail in Figure 4. In the embodiment shown, a printing plate supporting shim plate 115 is fitted semi-permanently to the plate cylinder 4 of the letterpress machine to compensate for the thinner gravure printing plate. The shim plate incorporates magnetic inserts 117 around its periphery. The printing plate 6 is fitted to the outer periphery of the shimmed plate cylinder. The printing plate, which is formed as a flat sheet, comprises a thin sheet steel substrate 119 coated with a printing layer 120 of a polyamide material. The printing plate 6 is secured to the plate cylinder 4 by the magnetic inserts 117 and by a clamping device generally indicated at 125. This clamping device comprises a longitudinally extending clamping head 126 which is wedged-shaped towards the axis of the plate cylinder 4 and which is radially displaceable in both directions by means of a drive device 128 accessible from one end of the plate cylinder 4. Clamping head 126 is received in a corresponding longitudinally extending slot 130 formed in the periphery of plate cylinder 4. Clamping head 126 extends over the full length of the plate cylinder 4, and a plurality of printing plates 6 may be secured side-by-side to the periphery of the plate cylinder.
Printing plate 6 originates as a thin, flat sheet having a length which is substantially identical to the circumference of plate cylinder 4. In order to secure the printing plate 6 to the plate cylinder, the clamping head 126 is first displaced radially outwardly beyond the periphery of cylinder 4 by means of drive device 128. The printing plate 6 is then formed into a circle and the free ends 136, 137 are tucked between the clamping head 126 and the inclined faces of slot 130. The drive device 128 is then used to draw the clamping head radially inwardly into slot 130 to trap the free ends of the plate between the sides of the clamping head 126 and slot 130, thereby clamping plate 6 in position. In the retracted position, the clamping head 126 may be disposed radially inwardly of the outer printing surface of plate cylinder 4 and a corresponding blanket slot 146 on impression cylinder 29 ensures that printing does not take place in this area, which suitably corresponds to the margins in the finished newspaper. Releasing the plate is the reverse of this procedure, and it will be appreciated that the radial clamping device 125 enables rapid attachment and detachment of printing plate 6 to plate cylinder 4, enabling a printing plate to be changed rapidly during a printing run. In a variation of this apparatus, the clamping edges of the device 125 are interrupted e.g. dog-toothed, and the free ends of the printing plate are correspondingly profiled.
In an alterantive embodiment, the letterpress machine is provided with a new larger plate cylinder fitted with magnetic inserts corresponding to inserts 117. In this way the difference in thickness between the conventional stereotype or photopolymer relief printing plates can be compensated for without the need to use a shim plate.
The press, prior to conversion, is shown in Figure 5 where parts corresponding to parts of Figures 1 to.4 are denoted by like reference numerals. More particularly, the plate cylinder 4 mounted in side frames 1 is provided with a thick steroeotype or photopolymer letterpress printing plate and receives ink from ink source 22 by means of a train of ink rollers comprising a duct roller 42, rubber rollers 43, oscillating drums 44 and forme rollers 45.
It will be appreciated that, on conversion, the major components of the press, such as the side frames 1, the impression cylinder 29, the press drive, and optionally the plate cylinder 4, are retained and that the ink train is removed and replaced by the inking unit in accordance with the present invention. Also, in the case where the conventional plate cylinder 4 is retained, shims are fitted to compensate for the difference in thickness between the gravure plates used in accordance with the invention and the cast stereotype or photopolymer plates conventionally used and the clamping device conventionally used for securing the cast stereotype or photopolymer plates to the plate cylinder 4 is modified to receive the gravure plates for example as shown in Figure 4. Such a conversion provides a gravure system suitable for a high-speed newspaper press with lower operational costs than a conventional letterpress system and at only a fraction of the replacement cost normally associated-with a change in printing technology.
Figures 6 and 7 illustrate an alternative arrangement for holding the printing plate 6 onto the plate cylinder 4. In these Figures, parts corresponding to parts of Figures 1 to 4 are denoted by like reference numerals. The arrangement shown in Figures 6 and 7 may be applied to a converted letterpress printing press, or to a new press designed specifically for this application.
In the case where the plate cylinder of.the letterpress printing press is utilised, two semicircular printing plate supporting shim plates 152 and 153 are locked to the outer surface 155 of the plate cylinder. The shim plates 152 and 153 are provided at intervals with magnetic inserts 158. Nose 159 of shim plate 153 is formed with an end surface 160 of reduced diameter compared with the remainder of the plate.
This stepped portion 160 forms a shallow channel 162 extending in shim plate 153 in an axial direction and having a depth which is substantially equal to the thickness of printing plate 6.
As will be seen from the enlarged section in Figure 7, the printing plate 6 comprises steel substrate 119 and polyamide printing layer 120. The steel substrate is approximately 0.010 inches (0. 024 mm) thick and thus the plate is very thin and flexible. Prior to applying the plate to curved outer surface 167 of shim plates 152 and 153, the plate is provided with a gravure image in the manner previously described and is then cut to a length which is slightly longer than the circumference of the shim plates. At the same time, a first end 165 of plate 6 is bent inwardly to form a recessed portion conforming to the profile of channel 162 in shim plate 153 and also to conform with the angular leading edge of nose 159. The plate is then fitted onto the plate cylinder 4 by rotating the cylinder and feeding the plate into the nip formed between the cylinder 4 and an ironing roller 168. Ironing roller 168 is pre-loaded to press plate 6 onto surface 164 and, as plate 6 becomes attached to surface 164 by the magnetic inserts 158, rotation of the ironing roller 168 and plate cylinder 4 presses the plate against the cylinder and ensures the plate is lying absolutely uniformly on the surface 167 with the free second end 170 of plate 6 overlapping the end 165 and lying in the recessed portion as shown in order to maintain the correct circular shape for cylinder 4. As will be seen from the enlarged section in Figure 7, a small gap is left between end 170 of the printing plate 6 and the portion of end 165 adjacent the recessed portion and this gap is then filled with an adhesive filler material 172 which can be a heat or UV-settable plastics material or any other suitable rapidly-setting mastic which is applied either in liquid, semi-solid, or solid form. The surface of the filler material 172 can be smoothed with a smoothing tool which may itself be heated to assist in the setting process.
The overlapping ends of the printing plate provide a tolerance for the length of the printing plate and mean that the adhesive setting material 172 does not touch the outer surface of shim plates 152 and 153, and that the material may be cut open when the plate is removed without damage to the shim plate or to the cylinder. The use of a stepped portion 160 which is substantially the same depth as the printing plate thickness ensures that the outer surface of plate cylinder 4 is smooth and substantially uninterrupted. This is an essential requirement for gravure printing where a doctor blade is applied against the printing plate. The bent-over end of the printing plate around nose 159 assists in maintaining the plate in exact register with cylinder 4.
In the case where a new press is being provided, the plate cylinder will have a larger diameter to compensate for the thinner gravure printing plates without the need to use shim plates. In this case, the plate cylinder will include magnetic inserts corresponding to inserts 158 and will be provided with an axial channel corresponding to channel 162.
Referring now to Figure 8, parts corresponding to parts of Figures 1 to 7 are denoted by like reference numerals.
As in the case of the previous embodiments, the plate 6 is a gravure printing plate including a gravure printing image formed in a photopolymer layer coated onto a thin steel sheet substrate. The plate cylinder 4' has a larger diameter than the plate cylinder used for letterpress printing and thus shim plates are not needed to compensate for the thinner gravure plate. The cylinder includes an axially extending surface slot 50 including a pair of register pins 51. Also located in the slot are magnets 52 which have end surfaces 53 which terminate below the periphery of the plate cylinder 4' so as to form a shallow channel extending axially of the plate cylinder 4'. This channel has a depth which is substantially equal to the thickness of printing plate 6. The surface of the plate cylinder 4' is provided with magnetic inserts 54 at intervals.
The first end 165 of the printing plate 6 includes a pair of register holes corresponding to the register pins 51 and is bent inwardly to form a recessed portion conforming to the profile of the channel defined by end surfaces 53. The plate is affixed to the plate cylinder by engaging the register pins 51 in the register holes and rotating the plate cylinder 4' so as to feed the plate 6 into the nip between the plate cylinder 4' and an ironing roller as in the case of Figures 6 and 7. In this way, the plate becomes attached to the plate cylinder 4' by the magnetic inserts 54 and the free second end 170 of the plate 6 overlaps the end 165 and lies in the recessed portion. As before, an adhesive filler material is introduced into the small gap between the end 170 of the printing plate 6 and the portion of end 165 adjacent to the recessed portion.
The register pins 51 ensure that a series of identical printing plates can be secured in succession to the plate cylinder in exactly the same position. Moreover the mounting arrangement is such that the plate cylinder can be rotated in either the normal direction or the reverse direction. The printing plate 6 may have a length slightly greater than the circumference of the plate cylinder 4' as described. Alternatively, the plate cylinder can include two such surface slots 50 diametrically opposed to each other whereby a pair of plates having a length slightly greater than one half of the circumference of plate cylinder 4' may be fitted.
In the case where the plate cylinder has a width which is a multiple of one page, the slot(s) 50 will include a pair of register pins 51 for each page width.
It will be apparent from the foregoing that the invention offers the following advantages over known techniques;

i) it enables letterpress printing presses to be converted to gravure techniques;
ii) it provides the capability of applying ink to detachable gravure plates in one or two-page-wide increments, thus enabling economic printing by gravure of partial width webs, i.e. webs less than the normal full width of four pages;
iii) it provides a printing plate mounting and sealing arrangement which facilitates a full 360 degree printing plate surface;
iv) the printing system it proposes has the capability of using oil-based, oil-emulsion, water- emulsion or water-based printing inks; and
v) it provides a gravure printing system which has the ability to meet the timescales imposed by normal newspaper deadlines.

Although the above described embodiments are primarily presses which have been produced by converting letterpress machines used for newspaper production, it will be apparent that the press of the present invention may be a completely new press which has not been produced by converting an existing press and that the press of the present invention may be used for printing purposes other than for newspaper production.

Claims

1. A gravure printing press comprising:

(i) a cylindrical surface (6) carrying a gravure printing image,

(ii) a means (35,37,38,39) of rotating said surface about its cylindrical axis,

(iii) a means of applying ink to said surface, and

(iv) a means (29) of contacting said surface with material (28) to be printed so as to transfer ink from said surface to said material characterised in that said means of applying ink to said surface is in the form of an inking unit comprising:-

(a) a substantially closed axially extending ink duct for containing ink under pressure, said ink duct being bounded by said surface, an axially extending doctor blade (12) in contact with said surface, and an axially extending sealing member (11),

(b) a means (23,26) of supplying ink from an ink source (22) to said duct and thence into contact with said surface, and

(c) a means (27) of returning to said source ink in excess of that retained by said surface.

2. A press as claimed in claim 1 wherein said axially extending sealing member is spaced from said surface.

3. A press as claimed in claim 1 or 2 wherein the ink duct includes a protruding edge (14) extending towards but spaced from said surface so as to divide the ink duct into a first zone (15) including an ink inlet (18) to receive ink from the ink supplying means and a second zone (16) including an ink outlet (21) to discharge ink to the ink returning means whereby during printing ink flows between said protruding edge and said surface and is subjected to a pressure increase in said second zone due to the rotation of said surface about its axis.

4. A press as claimed in any one of claims 1 to 3 wherein the ink duct has a width corresponding to the axial length of the cylindrical surface.

5. A press as claimed in any one of claims 1 to 3 wherein a plurality of said inking units is provided, the ink ducts of each unit being arranged side by side along the axial length of the cylindrical surface.

6. A press as claimed in any one of the preceding claims wherein said cylindrical surface carrying the gravure image is the surface of a gravure printing plate (6) mounted on a plate cylinder (4).

7. A press as claimed in claim 6 wherein the printing plate is held on the plate cylinder by magnets (117)(158).

8. A press as claimed in claim 6 or 7 wherein a first end (165) of the printing plate is provided with a recessed portion and the length of the printing plate is such that, when mounted on the opiate cylinder, the second end (170) overlaps the first end and lies in the recessed portion.

9. A press as claimed in claim 8 wherein a gap is left between the portion of the printing plate adjacent the recessed portion and the second end overlapping said recessed portion and.said gap is filled with filler material (172).

10. A method of manufacturing a gravure printing press characterised in that it comprises

(i) providing a letter press printing machine comprising

(a) side frames (1),

(b) a plate cylinder (4) mounted for rotation in the side frames about its cylindrical axis and carrying a thick stereotype or photopolymer printing plate,

(c) a train of ink rollers (42,43,44,45) for transferring ink from a source (22) to the printing plate and

(d) an impression cylinder (29) mounted for rotation in the side frames about its cylindrical axis and located so as to contact material (28) to be printed with the printing plate so as to transfer ink to said material from the printing plate,

(ii) removing the thick stereotype or photopolymer printing plate from the plate cylinder and replacing the same with a thin gravure printing plate

(6) with the interposition of shim plates (115)(152,153) to compensate for the difference in printing plate thickness, and

(iii) removing the train of ink rollers and replacing the same with an inking unit comprising:

(a) a substantially closed axially extending ink duct for containing ink under pressure, said ink duct being bounded by the gravure printing plate, an axially extending doctor blade (12) in contact with the gravure printing plate, and an axially extending sealing member (11),

(b) a means (23,26) of supplying ink from the ink source to said duct and thence into contact with said gravure printing plate, and

(c) a means (27) of returning to the source ink in excess of that retained by the gravure printing plate.

11. A method of manufacturing a gravure printing press characterised in that it comprises

(i) providing a letter press printing machine comprising

(a) side frames (1),

(ii) removing the thick stereotype or photopolymer printing plate and the plate cylinder and replacing the same with a larger plate cylinder carrying a thin gravure printing plate (6) and having the same overall diameter, and