GB2137561A - Printing apparatus - Google Patents

Abstract

Electrostatically assisted gravure printing apparatus comprises a gravure cylinder (1); an impression roller (2); and means for supplying an electrostatic charge to the nip between the gravure cylinder and the impression roller. The impression roller (2) comprise an outer semiconductive layer (5) and an inner highly conductive layer (4). The inner layer (4) is additionally provided with one or more insulating rubber annuli (not shown). The insulating annuli restrict axial flow of electrostatic charge in the impression roller (2). <IMAGE>

Description

SPECIFICATION Printing apparatus The invention relates to electrostatically assisted gravure printing.

Electrostatically assisted gravure printing apparatus comprises a gravure cylinder; an impression roller; and means for supplying an electrostatic charge to the nip between the gravure cylinder and the impression roller. Such apparatus is hereinafter referred to as of the kind described.

Apparatus of the kind described is already well known and is illustrated for example in British Patent Specification No. 1,159,923. In this apparatus, the transfer of gravure ink from the cells of the gravure cylinder to a web of material passing between the gravure cylinder and the impression roller is improved by inducing an electrostatic charge on the surface of the impression roller at the nip. Various methods for applying an electrostatic charge to the nip exist and some of these are disclosed in the British Patent Specification mentioned above.

One of the biggest problems on high speed machines is ensuring that electrostatic charge can be supplied to the nip as fast as it is being removed by leakage paper movement taking charge away, and losses due to rotation of the impression roller.

Generally, the impression roller has voltage applied to it in as safe a way as possible which then induces charge to flow into the relatively high capacitance of the nip. Commonly, the surface of the impression roller has a semiconductive layer to reduce sparks, reduce localised charge drain, and to help with safety requirements. This semiconductive layer slows down the charge progression into the nip and also allows the impression roller to remove charge by rotation, i.e. the charge distributes itself (limiting input current) and does not concentrate itself in the nip.

A further example of apparatus of the kind described is illustrated in British Patent Specification No.

1,293,302. This construction provides a complex impression cylinder which cannot cope with variations in the width of the web.

The usual method of alleviating these defects is to provide the roller with a rubber layer in such a way that a highly conductive layer is placed under the semiconductive layer, thus allowing current to flow to the nip.

Although this solution is generally acceptable, a further problem arises since printing webs commonly vary in width over quite a large range and any areas of the gravure cylinder not covered by the web directly short the charged impression roller to ground.

In the past, impression rollers have been selected in accordance with the width of the web to be printed so that very little area is left for charge to short circuit to ground via the gravure cylinder. This means that a large number of impression rollers must be stocked in order to cope with a full range of web widths.

Furthermore, on modern presses with deflection compensating impression rollers, reduction in impression roller rubber width is ceasing to be an option.

In accordance with the present invention, we provide apparatus of the kind described wherein the impression roller comprises an outer semiconductive layer and an inner highly conductive layer, the inner layer being additionally provided with one or more insulating annuli, whereby axial flow of electrostatic charge in the impression roller is restricted.

With this invention, although short circuiting may still arise, a far greater proportion of charge imparted to the impression roller is transferred to the nip since axial flow is restricted.

Preferably, the annuli comprise insulating rubber.

An example of electrostatically assisted gravure printing apparatus in accordance with the invention will now be described and contrasted with a prior art example with reference to the accompanying drawings, in which Figure 1 is a schematic side view of part of an example in accordance with the invention; Figure2 is a plan partly in section of part of an example according to the prior art illustrating motion of electrostatic charges; Figure 3 is a view similar to Figure 2 but of the example according to the invention; Figure 4 is an enlarged schematic view of a part of a conductive layer of an impression roller according to the prior art; and, Figure 5 is a view similar to Figure 4 but of an example in accordance with the invention.

In this example, the apparatus is substantially the same as conventional electrostatically assisted gravure printing apparatus except for the construction of the impression roller. Figure 1 illustrates schematically a portion of the apparatus which comprises a gravure cylinder 1, an impression roller 2, and a charging roller 3. The impression roller 2 comprises an inner, rubber, conductive layer 4 to be described in detail below, and an outer rubber, semi-conductive layer 5. A web of material 6, such as paper, is fed by means not shown between the gravure cylinder 1 and the impression roller 2. The impression roller 2 exerts pressure on the web of material 6 passing through the nip between the gravure cylinder 1 and the impression roller 2.

In use, electrostatic charge is imparted to the charge roller 3 by a means not shown, rotation of the roller 3 transferring the charge to the outer layer 5 of the impression roller 2. The charge flows through the outer layer 5 into the highly conductive layer 4 and thence to the nip. The flow of charge in conventional apparatus is indicated by the arrows in Figure 2. As may be seen in Figure 2, the width of the web of material 6 is smaller than that of the gravure cylinder 1 and the impression roller 2 so that there is a considerable degree of direct contact between the gravure cylinder 1 and the impression roller 2. Since the gravure cylinder 1 is grounded and the highly conductive layer 4 is homogenous, charge will flow preferentially towards the areas of direct contact between the cylinder and the impression roller.Thus, only a small amount of charge reaches the nip and transfer of ink from the gravure cylinder 1 to the web 6 is not greatly affected.

One way in which the problem can be quantified is to consider a portion of the conductive layer 4 unwrapped from the roller 2, as shown in Figure 4. The effect of flow of charge in the outer semi-conductive layer 5 can be ignored since typically the resistivity of the outer layer is between 2000 and4000 times as great as that of the highly conductive layer 4. Figure 4 illustrates half of the conductive layer 4 having a length Land a width W. In this case, the width of the web 6 is two thirds the width of the conductive layer 4.

Electrostatic charge is transferred from the charging roller 3 to the impression roller 2 at 7 and flows through the conductive layer 4 towards the web 6. If it is assumed that leakage of charge at the nip is small compared with the amount of charge short-circuiting from the impression roller 2 to the gravure cylinder 1,then there are two main current paths from the point 7 to those parts of the conductive layer 4 axially outward of the web 6. The effective resistance R1 of these two paths is given by R1 = 3 Lp 2WT where T is the thickness of the conductive layer 4, and p is the resistivity of the conductive layer 4.

The difference between the prior art example and the example in accordance with the invention may best be appreciated by comparing Figures 4 and 5. In the example according to the invention shown in Figure 5, the highly conductive layer 4 is broken into separate axially spaced sections by insulating rubber strips 8. In this case, charges imparted to the impression roller 2 at 7 will be unable to flow for any significant distance in an axial direction in view of the barriers provided by the insulating strips 8. The flow of charge is indicated by the arrows in Figure 3 where it will be seen that the restriction on axial flow causes a significant amount of the charge to flow to the nip unlike the situation shown in Figure 2. Thus, the amount of charge flowing directly into the gravure cylinder 1 is considerably reduced.

A similar calculation can be performed to determine the effective resistance R2 of the conductive layer 4 shown in Figure 5. This is given by R 3Lp 2 WT where p and T have the same meaning as before.

Comparison of R1 and R2 shows that the effective resistance of the conductive layer 4 in Figure 5 is double that of the conductive layer 4 shown in Figure 4. Thus, in practice, far more charge will be lost in the prior art construction than in the example according to the invention where the wastage of charge is at least halved.

Typically, the width of each insulating strip 8 may be 20 mm while the strips 8 may be spaced about 70 mm apart. These figures are based on the layer 4 being built up of conventional rubbers which become bonded together. As an alternative, we believe it may be possible to use other materials for the insulating strips such as polyethylene sheeting. This has the advantage that when constructing the layer 4, a continuous rubber sheet can be cut and the insulating strips inserted into the cuts.

Claims (3)

1. Electrostatically assisted gravure printing apparatus comprising a gravure cylinder; an impression roller; and means for supplying an electrostatic charge to the nip between the gravure cylinder and the impression roller, wherein the impression roller comprises an outer semiconductive layer and an inner highly conductive layer, the inner layer being additionally provided with one or more insulating annuli, whereby axial flow of electrostatic charge in the impression roller is restricted.

2. Apparatus according to claim 1, wherein the annuli comprise insulating rubber.

3. Electrostatically assisted gravure printing apparatus substantially as herein described with reference to Figures 1, 3, and 5 of the accompanying drawings.