Printing Sleeves

This invention relates to methods and means for manufacturing and mounting printing sleeves.

Printing sleeves which are mountable on and demountable from printing rolls are known in several forms of printing particularly flexography. The manner in which they may be mounted and demounted on the roll cores has caused problems and their manufacture in a manner which is compatible with mounting and demounting has also created considerable difficulty.

Examples of some ways which have been explored for allowing mounting and demounting of various types of sleeves or rolls, are seen in U.S. Pat. Nos. 2,117,722, 2,450,727, 2,501,630 and 3,035,331. The last of these is of some interest in the present context since it shows the contraction of an outer surface of a printing roll core by the application of vacuum within it while an inextensible printing sleeve is fitted over it. When the vacuum is released, the core surface expands into the sleeve. Though this is conceptually elegant it does imply necessarily a compressible printing roll core, something which may be incompatible with good working results, and even if this problem were overcome the design of such a core from the point of view of allowing a sufficient vacuum passage volume would be a matter of great difficulty.

It is probably for these reasons that the application of positive gas pressure to expand the sleeve during the time that it is being fitted on or taken off a printing roll core was proposed in U.S. Pat. No. 3,146,709 which was published in 1964. There, the inventors had the general idea that air could be blown from the roll core outwardly so as to tend to expand the printing sleeve as it was fitted on it. However difficulty is encountered in the initial fitting of the sleeve since no expansion will take place until sufficient of the apertures through which air issues from the centre of the core have already been covered. Also, there will be a loss of efficiency in the process as a whole because of the loss of air through uncovered holes.

This same U.S. Pat. No. 3,146,709 discloses a way of making a printing sleeve which consists of prepreparing a cylindrical body of a sheet of polyester plastics, taping it into a cylindrical form by a helical winding of an adhesive tape and then helically winding on the adhesive tape a double-sided adhesive tape known in the trade as "sticky-back". Then, a layer of rubber or rubber-like material is lapped around the cylindrical body and held by adhesion on the sticky-back. Indeed the rubber layer is referred to in that patent as a rubber plate. It will be realised that when such a compound sleeve is expanded by the effect of gas underneath it there is a tendency for leakage through adjacent edges of the sheet which can cause a blister or bubble to form under the attached plates.

To overcome the problem involved in the initial fitting of these prior art sleeves while at the same time avoiding the special and complex design problems encountered if the core is to be compressed for fitting the sleeve, there is provided a method of fitting a printing sleeve to a printing roll which consists in preparing a roll core with an outer surface which is at least partly tapered and a printing sleeve with an inner surface designed to be an interference fit with the core at a designed working position, which consists of moving the sleeve from the end of the core which has a lesser diameter, with the end of the sleeve of greater diameter leading, until the sleeve and core touch around the inner circumference of the sleeve and the sleeve has covered all gas outlets in the core, applying gas under pressure inside the sleeve from the said gas outlets to expand the sleeve radially and moving the sleeve while so expanded to its designed working position on the core.

The axial length of the sleeve is the same as the axial length of the core which bears said sleeve, said length being designated as the working length. The inner surface of the sleeve is preferably of a taper configuration corresponding to that of the outer surface of the roll.

The method also comprises forming the printing sleeve by coating a shell of fibre-reinforced polyester or epoxy resin with an uncured rubber and curing the rubber in situ to provide a printing surface of the sleeve.

Also in the invention there is provided a printing sleeve which consists of a fibre reinforced polyester or epoxy resin shell, of which the radially inner surface may be of at least partly tapering configuration, on an outer and cylindrical surface of which sleeve is a printing surface. The sleeve may include a rubber coating, cured in situ on the shell. To prepare the coating for printing, known finishing and engraving processes may be used upon its rubber surface when, the rubber is already vulcanised onto the shell. When the inner surface of the shell is tapered the outer surface of the shell may be cylindrical so that the shell is of varying thickness, or the shell may be of uniform thickness so that the rubber layer, when its outer surface is trued is of varying thickness. A flexible printing plate or plates may be adhesively secured to a cylindrical outer surface of the coating or of the shell, to provide the printing surface.

The invention also provides a printing roll core on the outer surface of which a printing sleeve is to be demountably mounted, the radially outer surface of the core being a supporting surface for the sleeve and being at least partly of tapering configuration and having, only in regions remote from the axial ends of the roll, gas passages leading to the support surface, and means for bringing gas under pressure to those passages to pass radially outwardly through them. The taper is slight and a preferred taper is in the range of 5 parts in 10,000 to 5 parts in 20,000. The latter is most preferred. In such a taper, the diameter of the roll core changes by 0.00025 units per unit of axial length of the roll. If as is preferred the whole of the outer surface of the roll core has a conicity it can be seen that a properly prepared printing sleeve of which the shell has a corresponding internal conicity can be freely fitted by hand from one end of the roll core until it gets to a position at which the surfaces of the sleeve and roll touch. This is a certain distance from its designed working position. Since the gas outlets are grouped away from the axial ends of the roll core, in this position it will cover all such outlets and then gas pressure may be applied to within the sleeve from within the core to expand the sleeve and enable it to be moved further along the roll to its designed working position upon this core. To take a specific example, if the core is essentially 12 cm in diameter, the sleeve is undersize by 0.012 cm in its working position, and the taper of the core is 5 parts in 20,000, the position at which the sleeve and core touch will be approximately 48 cm from the working position. If the sleeve has been made as indicated above, the rubber of the sleeve has been bonded to the shell and has no seam so it will not be lifted or distorted by the application of this pressure, and there will be no gap through which radial leakage can occur.

A particular embodiment of the invention and a particular method according to the invention will now be described with reference to the accompanying drawings, wherein:

FIG. 1 is a diametrical section through the one embodiment of sleeve and core on the line I--I, FIG. 2,

FIG. 2 is a section on the line II--II, FIG. 1,

FIG. 3 is a section as FIG. 1, but showing the sleeve in an initial position, and

FIGS. 4 and 5 are sections like that of FIG. 1 but showing, respectively, alternative forms of sleeve.

Referring first to FIG. 1, a printing sleeve 1 consists of a radially inner shell 2 surrounded by a rubber layer 3, on an outer surface 4 of which relief may be formed for printing purposes. A radially inner surface 5 of the shell has a slight frusto-conical taper (much exaggerated in the drawing). A taper of the order of 0.00025 units (e.g. inch or cm) of change in diameter per 1 unit of axial length is suitable. The shell 2 is of constant radial thickness and its radially outer surface 16 has the same taper as the inner surface 5; in modifications the outer surface 16 could be cylindrical or could be tapered but of a taper different from that of the inner surface 5. The printing surface 4 is, of course, a true cylinder.

FIG. 1 shows the printing sleeve 1 in its working position (i.e. axially central along) a printing roll core 6. The core 6 is hollow, having an incompressible hollow metal tube supported at each end by axled roll ends 8,9. One core end 9 has a gas line connector 10 through which gas under pressure may be introduced to ducting 13 inside the volume enclosed by the tube 7 and ends 8,9. Gas can only escape from this radially through radial ports 18 in a block 17 which lead to outlets 2 circumferentially spaced apart around the core in a plane remote from both axial ends of the core, and preferably in the region of the middle of the axial length of the core 6. The region where the outlets 12 are has an outer diameter of d.sub.3. The radially outer surface 11 of the tube 7 is given a taper corresponding to that of the surface 5 of the shell 2 of the printing sleeve 1 so that one end has a diameter d.sub.1 and the other end has a diameter d.sub.4 ; d.sub.1 is greater than d.sub.3 and d.sub.3 is greater than d.sub.4.

To form the printing sleeve, a fibre reinforced resin such as a glass reinforced polyester or glass reinforced epoxy resin is laid-up on a former having a desired taper, to a depth of approx 1/16" (approx. 1.5 mm). The layer is of uniform thickness so that its outer surface has the same taper as its inner surface. It is allowed to harden to form the seamless shell 2 and a coating of uncured rubber (the term including, of course, synthetic rubbers and other suitable elastomers) is applied to the outer surface. The rubber is cured in situ and thereby is hardened as a complete, gapless, seamless, tube 3 simply bonded to the shell 2. When as in the preferred embodiments the taper is as low as 5 parts in 20,000, a uniform thickness shell may all the more readily be used since the difference in radial thickness in the rubber layer along the length of the core (the outer surface of the rubber layer being brought to a true cylinder, of course) is negligible in its effect. The outer surface of the rubber is then ground true and engraved or otherwise prepared for printing. Alternatively as indicated in FIG. 5, the outer surface 4' of the rubber layer may be ground to a true cylinder after curing, and used as a base to which a flexible printing sheet or sheets such as stereos 15 are secured by adhesion by the use, for example, of "sticky-back".

Although the preparation of a constant thickness shell 2 is preferred since its laying-up is a simple matter, it is possible to lay-up a shell of varying thickness, to yield either a shell with a taper at its outer surface 16 different from that of the inner surface, or one with a true cylindrical outer surface.

In FIG. 4 the latter is shown. A shell 2' is laid-up as before, but so as to give a cylindrical outer surface 16'. After curing of the GRP, this is ground true and flexible plate (s) such as stereos 15 may be adhered direct to the surface 16' by means of "sticky-back".

It is apparent that, at least as far as the steps of making the sleeve itself are concerned they are applicable also to the making of seamless sleeves with a cylindrical internal surface.

The following discussion of fitting procedures will for brevity be given using the reference numbers seen in FIGS. 1 and 3 but it is of completely equal applicability to the embodiments of sleeve seen in FIGS. 4 and 5.

The former on which the shell is laid-up is diametrically undersize relative to the printing roll 6 on which the sleeve is to be fitted, so that the sleeve when formed with have the greatest diameter greater than the least diameter of the core but less than its greatest diameter and will be able to be passed freely along the core in the direction of the arrow A, FIG. 3, only for part of the length of the core. A major part, say 3/4, is preferred. Then, the inner circumference of at least the leading end 14, of which the unexpanded inner diameter is d.sub.2 (d.sub.1 >d.sub.2 >d.sub.3) touches all round on the core and wedges against it. This condition is seen in full lines in FIG. 3. To bring the sleeve 1 to the working position it is expanded by means of gas passed through outlets 12 (all of which are at that stage covered by the sleeve) and pressed further in the direction of the arrow A to its working position 1' in FIG. 3, or as it is seen in full lines in FIG. 1. Because of the undersize of the sleeve 1 and the only very slight taper given to the surfaces 5 and 11, when the gas pressure is vented to the atmosphere the sleeve will hold itself firmly in stressed condition on the core in its working position, ready for use.

Because of the taper of the surfaces and the positioning of the gas outlets, gas pressure need not be applied during the initial positioning of the sleeve 1 on the core 6 but only after all outlets 12 have been covered by the sleeve 1. Although it is preferred that the surfaces 5, 11 shall be continuously and constantly tapered, at least some of the advantages of the invention will be obtained when only a portion of the surface 11 of the core is tapered, preferably a major portion at the end which is last covered by the sleeve during the fitting process.