LITHOGRAPHIC COATING

The present invention relates to a coating process for a lithographic printer for applying a transparent coating, for example of lacquer or varnish to a printed image and relates to a printing press for carrying out the process.

It is known to coat printed images, for example posters and book covers with an at least partially transparent coating of lacquer or varnish, which protects the image and provides it with a desired level of shine or reflectivity. In lithographic printing presses, coating units are provided, inline with printing units, typically located after the lithographic printing press as an add-on for applying such coatings. Such units generally coat a substrate with a transparent coating having a thickness in the range of 8 to 14 g/m ² in order to provide the desired level of reflectivity. However, these coatings dry relatively slowly and so, as they are in line with the remainder of the printing press, can have the disadvantage of slowing down the print speed of the printing press.

According to a first aspect of the present invention there is provided a lithographic transparent coating process carried out on or within a lithographic printing press, comprising the steps of: applying a layer of curable varnish to a substrate; pressing a finished surface of a film against the layer of varnish while the varnish is wet; curing the varnish layer while the film is pressed onto it; and after the varnish layer is cured separating the film from the varnish

layer, wherein the step of pressing is carried out on an intermediate lithographic printing or coating unit of the printing press.

The curing of the varnish layer while the film is pressed onto it enables a high quality high gloss or patterned finish to be applied to the substrate without requiring a thick layer of varnish to be applied. This and the curing of the varnish facilitates quick drying of the varnish and so enables high speed print runs having a transparent coating process carried out in-line on the printing press. The varnish is preferably a duct varnish.

The step of applying may be carried out on an upstream lithographic printing or coating unit of the printing press. Using a lithographic printing unit for applying the varnish layer facilitates the application of a thin and even layer of varnish to the substrate. The step of pressing may be carried out on an intermediate lithographic printing or coating unit of the printing press. The use of a lithographic or coating unit of the printing press allows a high pressure to be applied when the varnish is applied, leading to the possibility of considerably thinner varnish layers than those yielded by prior art methods. The intermediate unit may be downstream of the upstream unit.

The varnish may be radiation curable and the curing step may comprise exposing the varnish layer to a radiation source. In particular, the varnish may be curable using ultra violet radiation and the curing step may comprise exposing the varnish layer to ultra violet radiation.

In order to provide a smooth high gloss finish to a printed substrate, the finished surface of the film may be smooth. In order to provide a holographic effect, embossed effect or security protection, the finished surface may carry a relief pattern for transfer to the varnish layer.

The varnish layer may have a thickness, expressed as amount of uncured varnish applied, of less than 3 g/m ² , for example less than 2.6 g/m ² , or even less than 2.0 g/m ² .

According to a second aspect of the present invention, there is provided a lithographic printing press for applying a transparent coating to a substrate, comprising: an upstream lithographic printing or coating unit for applying a layer of curable, varnish to the substrate; an intermediate lithographic printing or coating unit, for pressing a finished surface of a film against the layer of varnish while the varnish is wet; a curing unit for curing the varnish layer while the film is pressed onto it; and web or sheet transport systems for the coated substrate and for the film, which systems are configured to separate the film from the varnish layer after the varnish is cured. The varnish is preferably a duct varnish.

The varnish may be radiation curable in which case the curing unit may comprise a radiation source. In particular, the varnish may be curable using

ultra violet radiation in which case the curing unit may comprise a source of ultra violet radiation.

The intermediate unit typically comprises a pair of rollers: such as an impression roller having an impression contact surface and an offset roller having an offset contact surface, and the varnished substrate and film may be passed through the rollers (for instance between the impression contact surface and the offset contact surface) so as to press the finished surface of the film against the layer of varnish while the varnish is wet. The finished surface of the film may be smooth or may carry a relief pattern which is transferred to the varnish layer. Because of the high pressures which can be applied between the contact surface and the transfer surface of the intermediate unit of the lithographic press, excellent results may be achieved from varnish films which are substantially thinner than varnish films provided by prior art methods. For instance the varnish layer may have a thickness (i.e. coating level) of up to 3 g/m ² , preferably up to 2.6 g/m ² or even as low as 2.0 g/m ² The varnish layer thickness is suitably at least 0.7g/m ² , preferably at least 1.0 g/m ² The use of such a thin varnish layer was not previously possible, and allows for a considerable reduction in the amount of varnish needed in order to obtain an excellent visual result. Furthermore, the thinness of the varnish layer allows for more rapid curing of the layer, giving scope to increase production rate.

Where the film coating is carried out on an intermediate unit of a lithographic printing press, this allows for the web or sheet transport system of the press to be used throughout. In prior art methods, an add-on unit for film coating will have its own web or sheet transport system, which will need to be synchronised to that of any upstream printing unit

The web or sheet transport system for the coated substrate may comprise the web or sheet transport system of the printing press for the substrate and the web transport system for the film may comprise a web tensioner module for supplying the film to the intermediate unit and for separating the film from the varnish layer. By "web transport system" for the substrate , the skilled person would understand sheet transport systems to be included when the substrate is in sheet rather than web form.

Typically, when the coated substrate is in the form of discrete substrate sheets rather than a web, the roller (corresponding to an impression roller of the intermediate unit of the lithographic press) will comprise a plurality of gripper finger assemblies arranged in one or more grooves along its length. These gripper finger assemblies are adapted to grip the front edge of each substrate sheet, holding the sheet onto the contact surface of the impression roller as the sheet has the film pressed against it by the contact surface of the offset roller of the intermediate unit of the lithographic press.

In a typical arrangement for a lithographic press, the offset roller will have half the diameter of the impression roller such that one full revolution of the offset roller will correspond to a half revolution of the impression roller. This means that two substrate sheets will be treated for each full revolution of the impression roller, requiring two rows of gripper finger assemblies, positioned diametrically opposite each other at the contact surface of the impression roller. The substrate sheets, coated with vanish, are pressed against the film between the contact surface of the impression roller and the contact surface of the offset roller.

Other arrangements would also be suitable. For instance, the transfer cylinder could be such that on full revolution of it corresponded to one third of a revolution of the impression cylinder, in which case three rows of gripper fingers would be provided on the impression cylinder.

The gripper fingers are adapted to grasp and to release the sheet at appropriate times as the impression cylinder contact surface rotates against the transfer cylinder's transfer surface. This is typically achieved by means of lever arrangements forming part of the substrate sheet transport system.

The gripper fingers necessarily stand proud of the impression cylinder contact surface and will thus prevent the film from contacting a region at the front edge of the substrate when the substrate is a sheet. This is because the film will be held at a tangent to the substrate sheet, held at the contact surface of

the impression cylinder, by the parts of the gripper fingers standing proud of the contact surface of the impression cylinder. Hence it is greatly advantageous for the invention for the gripper finger assemblies to be adapted to stand proud of the impression cylinder surface to as low a height as is possible while commensurate with their effective gripping of substrate sheets. This may be achieved by making the gripper fingers as thin as possible and by ensuring that the assemblies controlling the gripper finger assemblies are kept as low as possible relative to the impression cylinder surface.

Prior art lithographic units, when used for the method of the invention, were found to prevent the film from contacting the first 75mm of varnish layer at the leading edge of the substrate sheet. By suitable adaptation of the gripper finger assemblies, such as by milling the fingers to reduce their thickness and by modifying the gripper finger assemblies to reduce their height relative to the impression cylinder contact surface, it has been found that the film can contact all but the first 20mm of the varnish layer at the leading edge of the substrate sheet without significant loss in reliability of grip.

Hence, the gripper finger assemblies are preferably adapted to extend to a height above the contact surface of the impression cylinder whereby the film can contact all but a strip less than 50mm in width of the varnish layer at the leading edge of the substrate sheet, more preferably all but a strip less than 30 mm wide, even more preferably a strip less than 20mm wide.

According to a third aspect of the present invention, there is provided a lithographic transparent coating process carried out on the lithographic printing press according to the second aspect of the present invention, comprising the steps of: applying a layer of curable varnish to a substrate using the upstream unit; pressing a finished surface of a film against the layer of varnish while the varnish is wet using the intermediate unit; curing the varnish layer while the film is pressed onto it using the curing unit; and after the varnish layer is cured separating the film from the varnish layer using the web transport systems. The varnish is preferably a duct varnish.

The invention will now be described by way of example only and with reference to the accompanying schematic drawings, wherein:

Figure 1 shows a side on view of a portion of a lithographic printing press modified to carry out lithographic coating according to an embodiment of the present invention,

Figure 2 shows a flow diagram of a lithographic coating process according to an embodiment of the present invention,

Figure 3 shows a cross section through a gripper finger assembly according to the invention, and

Figure 4 shows a cross section through a gripper finger assembly according to the prior art.

Figure 1 shows a portion of a lithographic printing press (2) comprising an upstream printing unit (4), an intermediate printing unit (6) and a downstream printing unit (10 - only partially shown). Although printing units are shown in the figure, the upstream printing unit (4) is used as a varnish coating unit and the intermediate printing unit (6) is used to press film against the varnish layer.

A substrate (8), in the form of a sheet to be printed on is passed through the printing press (2) from right to left so as to pass through the upstream unit (4), the intermediate unit (6) and then the downstream unit (10), as is well known in the art. Additional printing units of the printing press (2) (not shown), further upstream of the upstream printing unit (4) may apply ink using standard or waterless lithographic printing so as to generate the image to be coated, as is well known in the art.

A standard lithographic printing press uses a planographic image plate on which the portions of the plate carrying the image to be printed obtains ink from ink rollers, while the non-print areas of the plate attract a film of a water based fountain solution which keep the non-print areas ink free. Waterless lithographic printing uses a waterless plate and avoids the use of a fountain solution. The coating method according to the present invention can be used in both standard and waterless lithography.

The upstream printing unit (4) comprises a normal lithographic ink train system (24) which is supplied with a source of ultra violet (UV) initiated duct varnish and functions to apply a thin layer of the varnish to a lithographic plate cylinder (26). The varnish carried on the plate cylinder (26) is then transferred to varnish application plate cylinder (28). From whence the varnish is printed onto the substrate (8) as the substrate passes between the varnish application cylinder (28) and an impression cylinder (30) [Step i, Figure 2]. Gripper fingers (19) hold the front edge of each substrate sheet (8) onto the surface of the impression cylinder (30).

The varnish or lacquer is preferably a duct varnish, so that a thin layer of varnish can be lithographically offset printed onto the substrate by the lithographic print unit (4). The varnish or lacquer is also curable, for example, heat or radiation curable, in particular curable using a source of ultra violet radiation.

A typical UV curable lithographic duct varnish might have the following composition:

Vegetable oil modified epoxy acrylate 20-25% Epoxy modified acrylate 22-26%

Chlorinated polyester acrylate 32-38%

Urethane modified acrylate 7-10%

Photoinitiator 7-10%

The photoinitiator blend might be made up of cyclic ketones and amines and might have the following composition:

Phospine oxide 15%

Thioxanthone 15% Phenyl ketone 10%

Amino benzoate 15%

Benzyl phenyl amino ketone 20%

Monofunctional monomer 25%

Alternative lithographic varnish compositions and alternative photoinitiator blends would be apparent to a person skilled in the art of ink rheology.

The intermediate unit (6) is shown in Figure 1 as a printing unit of a lithographic printer (2) but could equally be a coating unit. No ink is supplied via the ink train of the intermediate unit (6) and instead a web tensioner module (18) is mounted on the intermediate unit (6) and the adjacent downstream unit (10). The web tensioner (18) module comprises a roll off spool (32), a roll on spool (34), a plurality of guide rollers (36) and a tensioning arrangement (38). A strip of film (40), which generally has the same width as the width of the substrate passing through the printing press (2) is stored on the roll off spool (22). The film (40) is wound off the roll off spool (32) and passed, via the guide rollers (36) between an offset roller (42) and an impression roller (44) of the intermediate unit (6) so that it is pressed onto the still wet varnish applied to the substrate (8) at the printing unit (4). Gripper

fingers (19) hold the front edge of each substrate sheet (8) onto the surface of the impression roller (44).

The film (40) is adhered to the varnish, due to the tack of the varnish and due to the downstream guide roller (36a) maintaining the contact of the film with the varnish layer [Step ii, Figure 2]. In this way the substrate (8) coated with the varnish layer and with the film (40) pressed onto it are passed under a source of ultra violet radiation (50). The source (50) is located between the nip point of the rollers (42, 44) and the downstream guide roller (36a). The source of ultra violet radiation (50) acts to cure the layer of varnish applied to the substrate (8) at the printing unit (4) and may have an intensity of 360 watts per inch [Step iii, Figure 2]. The film (40) and substrate (8) are peeled apart after passing the downstream guide roller (36a), when the varnish layer has been cured. The film (40) is pulled upwards via the guide roller (36b) to the roll on spool (34) and the varnish layer coated substrate (8) is pulled downwards with the varnish layer already cured [Step iv, Figure 2]. The movement of the film (40) between the roll off and roll on spools (32, 34) is powered by the rollers (42, 44) due to the movement of the rollers. In response to the speed with which the film (40) is pulled through the rollers a pair of pneumatic dancer arms (38) appropriately tension the film (40), as is known in the art. After the film (40) has been completely wound onto the roll on spool (34) it can be re-used.

The lithographic printing of a layer of varnish onto a substrate (8) and the subsequent curing of the varnish in contact with the film (40) generates a high level of reflectivity for a thin layer of varnish. Conventional varnish coatings may require a thickness of the order of 8-14 g/m ² in order to generate the required reflectivity, whereas the method described above may require a thickness of varnish of the order of 0.7-2.6 or 3.0 g/m ² . Thick layers of varnish take a relatively long time to cure or otherwise dry whereas the method according to the present invention uses a thin layer of varnish which can be cured relatively quickly by exposure to ultra violet radiation. Accordingly, the transparent layer coating method according to the present invention enables an increase in the speed at which the printing press (2) with the lacquer coating stage can be used.

Where a high gloss smooth finish is required, the film (40) may be a polypropylene film having a thickness, for example of around 30μm, and having a smooth surface for contacting and pressing against the varnish layer so as to impart to the varnish layer the high gloss finish. The film (40) could be made of an alternative film material having a similar high dimensional stability to polypropylene.

Where a patterned finish is required, the film (40) may be micro-etched to have a relief pattern formed into it. The patterned side of the film (40) is then pressed against the varnish layer so as to imprint the reverse image of the relief pattern onto the varnish layer. The varnish layer is then cured with the

film (40) pressed into it and so carries the reverse image of the relief pattern. The film (40), which may be a polypropylene film is moulded against a micro etched drum so that the relief pattern is imprinted into the film during its manufacture. Again the micro-etched film could be made from an alternative film material having a similar high dimensional stability to polypropylene.

The micro-etched film (40) might be patterned in any way, for example, to imprint a security code onto the varnish layer, or a repeated array of a pattern or a company name or logo. The film (40) might be patterned to generate a varnish layer providing a holographic finish.

Figures 3 and 4 show cross-sectional views through gripper finger assemblies according to the invention and according to the prior art respectively. The various components are arranged and function in a similar way for each and will be detailed together.

The gripper finger housing (65) is located in a channel (61 ) in the surface (60) of the impression roller (44) and is pivotally mounted on an axle (64). The substrate sheet 8 is gripped between the gripper finger (62) and a replaceable strip (63) mounted at the edge of the roller surface (60) and the channel (61 ). The gripper finger (62) is held in place on the housing (65) by a bolt (66). The housing (65) may be adjusted with respect to the axle (64) by means of adjustment screws (not shown).

In use, the coated substrate (8) is gripped at its leading edge between the gripper finger (62) and the strip (63) and the film (40) meets it at an angle determined by the geometry of the lithographic press. Gripping and release of the substrate is achieved by rotating the gripper finger assembly (65) by means of the axle (64) to which it is attached. The gripping and release of the substrate is synchronised with the passage of the substrate through the press by the web/substrate transport system.

For prior art gripper finger assemblies, as shown in figure 4, the film (40) first meets the varnish coating of the substrate (8) at about 75mm from the leading edge of the substrate (indicated as the distance y in the figure). For the gripper finger assembly of the invention shown in figure 3, adaptations have been made, to ensure that the various components of the assembly, such as bolt (66), housing (65) and gripper finger (62), do not stand as proud of the surface (60) as for the prior art gripper finger assembly. For the same angle of contact between film (40) and coated substrate (8), this results in the film (40) first contacting the coated substrate at a position much closer to the leading edge of the substrate, indicated by the distance x in the figure, than for the prior art gripper finger assembly.