An assembly for preventing the detachment of a substrate from a revolving cylinder

Abstract

 ASn assembly is provided which prevents the detachment of a substrate from a revolving cylinder (4).
This is achieved by maintaining or intermittently providing a zone of pressurised gas over the path of the substrate around the cylinder, forcing the substrate against the cylinder and preventing its detachment.
In one aspect, the assembly including an arcuate perforated plate (12) backed by a canopy (16) into which pressurised air is pumped. The pressurised air escapes through the perforations (14) which direct it against the surface of the cylinder.

Description

[0001] The present invention relates to an assembly for preventing the detachment of a substrate from a revolving cylinder, and in particular, but not exclusively from a revolving cylinder in a printing press.

[0002] A typical printing press comprises a succession of colour units each of which coats a different coloured coating on a substrate passing through the press. Each colour unit comprises a blanket cylinder for applying the coating e.g. ink, varnish or the like to the substrate and an impression cylinder on which the substrate is carried into contact with the blanket cylinder. Each pair of impression and blanket cylinders has a nip between them through which the substrate passes during printing.

[0003] In such an arrangement, it is common for the substrate e.g. paper or cardboard, to become detached from the surface of the impression cylinder as it emerges from one of these nips. This is particularly true of the tail-end of the substrate because of the tendency of the substrate to regain its usual planar shape. This particular problem is known as tail-end fliok-up, and is particularly acute if another body such as a drying unit is present adjacent to the downstream side of the nip as the detached substrate may rub against this causing the smudging of the freshly applied coating.

[0004] Drying units normally include a U-V lamp together with a focussing arrangement mounted adjacent to the face of the impression cylinder after the nip between the cylinders. The radiation from the lamp is directed towards the cylinder to dry the coating on the substrate as it passes beneath the unit. If the tail-end of the substrate flicks up as it leaves the nip between the blanket and the impression cylinders, then it is likely to drag along the- undersurface of the drying unit resulting in the coating being smudged or smeared.

[0005] The use of drying units also leads to the additional problem of overheating the cylinders at which they are directed, resulting in substrate distortion and an increase in the risk of press damage. To overcome this problem one proposal has been to extract the air from between the U-V lamp and the impression cylinder. Unfortunately this increases the problem of tail-end flick-up as it also has the effect of drawing the substrate away from the surface of the impression cylinder because of the partial vacuum produced.

[0006] In one aspect, the present invention aims to overcome the problem of tail-end flick-up by providing an assembly which helps to retain the substrate in place on the impression cylinder.

[0007] According to one aspect, the present invention provides an assembly for preventing the detachment of a substrate from a revolving cylinder comprising means for maintaining or intermittently providing a zone of pressurised gas over at least a part of the path of the substrate on the cylinder.

[0008] Preferably the assembly further comprises a U-V lamp for drying the substrate as it passes under the assembly. A quartz window may be mounted in front of the lamp to prevent cold air from being blown onto it, and to help retain the air pressure built-up between the assembly and the cylinder.

[0009] In another aspect, the present invention provides a printing press comprising a plurality of colour units, at least one of said units having a blanket cylinder and an impression cylinder including a nip therebetween, and an assembly as hereinbefore described mounted adjacent to the surface of the impression cylinder downstream of the nip for preventing the detachment of a substrate as it emerges from the nip.

[0010] In the drawings:

Figure 1 is a schematic sectional view of part of a printing press according to one aspect of the present invention.

Figure 2 is a schematic sectional view of one form of assembly for preventing the detachment of a substrate according to the invention.

Figure 3 is a schematic sectional view of a second form of assembly according to the invention incorporating a drying lamp.

Figure 4 is a detailed schematic sectional view of part of the assembly illustrated in Figure 3.

[0011] The present invention will now be more particularly described with reference to the accompanying drawings.

[0012] Turning firstly to Figure 1, this schematically illustrates part of a printing press which apart from the inclusion of the assembly of the invention is of typical design. Consequently only those parts needed for an understanding of the invention will be described in detail. The press comprises a series of colour units 2 each of which in turn comprises a blanket cylinder 3 for applying the coating (e.g. ink, varnish or the like) to the substrate (e.g. paper, cardboard or the like) and an impression cylinder 4 on which the substrate is carried into contact with the blanket cylinder. It is preferred that the coatings applied to the substrate are those which may be dried by the radiation from a U-V lamp, such as U-V inks or specially formulated varnishs.

[0013] In order to transfer substrates from one colour unit to the next, transfer cylinders 5 are provided between them. The substrate is fed from one colour unit to the next and hence through the printing press by the rotation of the cylinders i.e. from cylinder B to cylinder C to cylinder D etc.

[0014] As discussed above, the arrangement of cylinders 3 and 4 often results in the substrate detaching itself from the impression cylinder 4 as it leaves the nip between them. It is particularly common for the tail-end of the substrate to become detached as it emerges from the nip, resulting in the previously described problem of tail-end flick-up. In order to alleviate this problem, the invention provides, in one aspect, an anti-detachment assembly 10 located just downstream of the nip. As will be appreciated from Figure 1, access to the path of the substrate 6 upon the impression cylinder 4 is limited, and therfore the anti-detachment assembly needs to be of compact design particularly where it faces the cylinder.

[0015] Turning to Figure 2, this illustrates an anti-detachment assembly 10 according to one aspect of the invention. The assembly comprises an elongate plate 12 whose major axis runs parallel to that of the adjacent cylinder 4. The plate has a substantially arcuate cross- section, having a cylindrical, or substantially cylindrical e.g. faceted, form. The radius of the arc is selected to be slightly greater than the radius of the cylinder with which it is to be used so that there is a substantially constant separation between them.

[0016] Typically the assembly 10 will be mounted so that the separation between the cylinder and plate is between about 5-15 mm, and more typically between about 6-8 mm.

[0017] The plate has a plurality of perforations through its thickness in the form of orifices 14 formed throughout its body. A substantially air-tight canopy 16 having an inlet 15 is formed over the plate to define a space 17 therebetween. A source of compressed gas 18, for instance a compressor, is connected to the inlet to supply gas to the space 17. Usually the gas supplied will be air, though other gases can be used if the situation demands.

[0018] In use, the substrate emerges from the nip 8 attached to cylinder 4. The compressor supplies air under pressure to the inlet 15 of the canopy which is then expelled as indicated by arrows A through the orifices 14. These streams of air press the substrate against the cylinder 4, and thus prevent its detachment. In particular, the assembly prevents tail-end flick-up.

[0019] The air may be continuously supplied from the compressor 18 so that, according to one embodiment of the invention, air is continuously expelled from the orifices 14.

[0020] In another embodiment, the printing press housing the assembly includes sensors which sense the position of the substrate or substrates within it. These sensors can be used to control, amongst other things,.the compressor 18, and to actuate it only when a substrate, or the tail end of a substrate is about to emerge from the nip 8.

[0021] Turning now to Figures 3 and 4, these illustrate an assembly according to a further aspect of the invention for preventing the detachment of a substrate from a cylinder which further includes an elongate U-V lamp 22 for drying the coating on the substrate as it passes under the assembly.

[0022] The U-V lamp 22 is located within the canopy 16, and is backed by an elongate aluminium reflector 24 which focuses the radiation from the lamp towards the surface of cylinder 4 through an elongate slit 26 formed in the plate 12. The axis of the lamp and slit both extend parallel to the axis of the cylinder. The assembly 10 extends along the whole length of cylinder 4 so that it is able to dry substrates having a width of up to the whole length of the cylinder.

[0023] A quartz window may be mounted across the slit 26. This prevents both the air which is released from orifices 14 from cooling the UV lamp and also air escaping from between the plate and the cylinder.

[0024] When the press is in use, the substrate 20 emerges from the nip between cylinders 3 and 4 in contact with the impression cylinder 4. The substrate then passes under the UV lamp 22 and the coating which has been applied to its surface by the blanket cylinder 3 is dried by radiation from the U-V lamp.

[0025] The air which is expelled from the orifices also helps to cool the surface of the revolving cylinder preventing it from overheating. This effect can be reinforced by pre-cooling or refrigerating the air prior to it being expelled from the orifices. This can be achieved by using a modified compressor 18 which also acts to cool the air it compresses.

[0026] In Figure 4, the arrows A indicate the flow of air within the canopy and through the orifices 14 in the plate 12. The pressurised air is introduced into the canopy through an inlet 15 which extends substantially along the length of the assembly.

[0027] Because of the heat generated by the UV lamp, the aluminium reflector requires cooling. This can be achieved by using any of the conventional methods presently in use. For example as illustrated in Figure 4, cooling water may be circulated through the reflector by means of inlet and outlet pipes 32 through hollows 30 formed within it.

[0028] As is well known, UV lamps produce ozone. This gas has to be extracted from the assembly and safely disposed of due to its toxicity. This again can be done by any of the conventional methods presently in use. For example, as illustrated in Figure 4, an additional vent 34 may be formed integrally with the canopy. The vent is open to the atmosphere at one end of the canopy and has a negative pressure applied to its other end resulting in the ozone being extracted from the canopy and into the vent, through a slot 32 as indicated by arrows 0. The ozone is then circulated to disposal means on reaching the end of the vent.

Claims

1. An assembly for preventing the detachment of a substrate from a revolving cylinder (4) comprising means (10) for maintaining or intermittently providing a zone of pressurised gas over at least a part of the path (6) of the substrate on the cylinder.

2. An assembly according to claim 1 wherein the means comprises an elongate plate (12) having an axis parallel to that of the cylinder, the plate being positioned around the surface of the cylinder and spaced therefrom and having perforations (14) formed through its thickness for directing the gas towards the cylinder.

3. An assembly according to claim 2 wherein the plate is of substantially arcuate cross section.

4. An assembly according to claim 2 or claim 3 wherein the means further comprises a canopy (16) formed over the plate to define a substantially enclosed space (17) therebetween and means (18) for supplying pressurised gas to said space.

5. An assembly according to claim 4 wherein said gas supply means is a compressor.

6. An assembly according to claim 5 wherein the gas is pre-cooled or refrigerated by the compressor prior to its supply to the space.

7. An assembly according to claim 5 or claim 6 wherein the gas is supplied continuously by the compressor to said space, said gas being air.

8. An assembly according to any preceding claim further comprising an elongate U-V lamp (22) mounted above an elongate slit (26) in the plate for drying the substrate as it passes under the plate.

9. An assembly according to claim 8 further comprising a quartz window mounted across the slit.

10. A printing press comprising a plurality of colour units (2), at least one of said units having a blanket cylinder (3) and an impression cylinder (4) including a nip (8) therebetween, and an assembly (10) according to any preceding claim mounted adjacent to the surface of the impression cylinder downstream of the nip for preventing the detachment of a substrate as it emerges from the nip.

Drawing