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(11) | EP 0 168 970 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Multi-cylinder stencil duplicator |
| (57) A multi-cylinder stencil duplicator has a coating member (8) defining a nip with
one (2) of the cylinders, and has means (38) for applying ink to one side of the nip
to form a bead (37) of ink at the nip. A roller (51) immersed in the bead or wedge
of ink is preferably driven for rotation in the same direction as the bead and is
preferably positioned at the centre of rotation of the bead. |
[0001] The present invention relates to an inker for a multi-cylinder stencil duplicator and in particular to one having an inker which applies a uniform layer of ink axially along a roller surface without the need for the ink coating to be re-distributed after application in order to achieve adequate uniformity of the ink layer which is important to the printing process.
[0002] Our GB-A-2,099,368 discloses a stencil duplicator incorporating an inker which applies a uniform layer of ink to the bottom cylinder of a multi-cylinder duplicator and does so by means of a coating member which holds back a bead of ink on a part of the surface of the bottom cylinder where it is not contacted by the stencil, so that this bead is doctored through the nip between the coating member and the cylinder, to emerge from the other side of the nip as an ink layer having the desired uniformity. Ink can be automatically fed to that bead, by means of the ink detecting system disclosed in our said GB-A-2,099,368.
[0003] The inker illustrated in our said GB-A-2,099,368 requires a carefully formulated ink which has not only the properties of a conventional stencil duplicating ink in being able to pass through a relatively fine mesh stencil and therefore being expected to be of relatively low viscosity, but should also be sufficiently "stiff" to be able to form a rolling bead which can be held back with the desired doctoring action, by means of the coating member.
[0004] The "stiffness" of the ink which allows it to form the appropriate rolling bead can be referred to as its "internal cohesion" which needs to be quite high for use with the type of inker disclosed in GB-A-2,099,368, but inks having such a high "internal cohesion" tend to be too thick or too stiff to pass through a fine mesh stencil.
[0005] Such "high internal cohesion" inks have in the past been used in offset printing presses where the operator is able to "tune" the ink by adding various thinning and thickening additives, but for a stencil duplicator the use of such inks requires too great a level of operator skill to be commensurate with the inherent simplicity of the stencil process, and furthermore the use of such inks will require much more frequent cleaning of the machine than is normally expected with a specially designed stencil ink having a lower viscosity.
[0006] It is an object of the present invention to provide a stencil duplicator in which the conflicting requirements of the ability of the ink to form a rolling bead at the coating member and the ability of the ink to pass through a fine mesh stencil can be compromised without the need for an unstable blend of thinning and thickening additives, and without the need for the operator to maintain special control on the ink used.
[0007] We are aware of the use of a nip roll bar between an ink applicator roller and a metering roller, in US-A-4,041,864, but this is in the context of an offset printer where, as indicated above, the rheology of the ink is adjusted in use by a skilled operator.
[0008] In accordance with the present invention there is provided a multi-cylinder stencil duplicator comprising a cylinder on which a uniform layer of ink is applied and on which an ink-pervious stencil is rolled during printing; a coating member alongside that cylinder and defining therewith a nip through which ink can pass to form a uniform layer on this cylinder during rotation of the cylinder; means for applying ink to one side of said nip to form a wedge-like bead of ink extending into the nip; and a roller positioned at the nip on said one side thereof, and arranged to be immersed in the bead of ink in use of the duplicator.
[0009] Preferably the roller is driven for rotation in the same direction as the direction of rotation of the ink bead entrained by rotation of said cylinder.
[0010] In order that the present invention may more readily be understood the following description is given merely by way of example, with reference to the accompanying drawing in which the sole figure is a side elevation showing the cylinder, the coating member and the roller immersed in the bead at the nip between the coating member and cylinder.
[0012] As shown in the drawing, the geometry of the inker on the bottom cylinder 2 of the duplicator is very similar to that shown in Figure 1 of GB-A-2,099,368, except that there is an additional roller 51 immersed in the bead 37 of ink and preferably driven for rotation in a clockwise direction, namely the direction of rotation of the bead of ink.
[0013] In this case the presence of the additional roller 51 allows the entire duplicator, which is intended to incorporate automatic inking, to be operated satisfactorily with much greater latitude as regards the rheology of the ink.
[0014] The use of a roller 51 rotating at the entry side of the nip where the bead 37 of ink occurs provides a support for the bead, thereby generating the necessary rolling bead of ink without requiring the ink itself to have the conventional high "internal cohesion" to give it the property of generating a rolling bead. Surprisingly, it has been found that the inker disclosed in GB-A-2,099,368 can be used with a conventional stencil printing ink, particularly one which has been designed for use with a multi-cylinder stencil duplicator and is therefore able to migrate through the fine mesh stencil without the need for a high pressure differential to achieve that migration.
[0015] As indicated above, the roller 51 is driven in the clockwise direction and the best results are obtained if the speed of rotation of the roller 51 is identical to the speed of rotation of the ink bead 37 itself. Furthermore it is desirable for the roller 51 to be positioned symmetrically within the nip between the coating member 8 and the cylinder 2 so that it coincides with the centre of rotation of the bead 37 of ink held back by the coating member 8.
[0016] As disclosed in our said GB-A-2,099,368, the coating member 8 may be stationary or may itself be a driven cylindrical member, provided it is able to hold back a bead 37 of ink and to allow this to pass through the nip in controlled manner to form the uniform layer 45 on the cylinder 2.
[0017] Although the drawing shows the electrode plate 9 of a capacitive ink detector, and the other plate may be either the coating member 8, or the roller 51 within the bead it is of course conceivable for some other form of ink detector system to be provided as capacitive detection is not essential to this invention.
[0018] The drawing also shows the ink distributor bar 38 and remnants 39 of ink left on the surface of the cylinder 2 after the cylinder and the ink screen have parted company.
[0019] In order to illustrate the ability of the stencil duplicator in accordance with the present invention to operate satisfactorily with a coating member 8 and without the need for any separate distributing means downstream of the coating nip between the member 8 and the cylinder 2, a series of tests was carried out using inks which are specially formulated to give optimum results in stencil duplicators, but which in the present case are expected to behave in a totally abnormal way for a stencil duplicating ink in settling into a rolling bead ahead of the coating nip. The inks used are identified in terms of their brand names and viscosity ratings, and the ability of those inks to be distributed along the cylinder as indicated in the table below
EXAMPLE 1
[0020] A stencil bearing an image having a width of 19 cm was printed onto A4 paper using the duplicator disclosed in our said GB-a-2,099,368, but with a conventional ink intended for use on a multi-cylinder stencil duplicator having axially reciprocating ink-distributing rollers. The ink was a black Gestetner AMI ink having a viscosity of 2.5 poise and was applied over the central region of the bottom cylinder with the intention of having it distributed over the rest of the cylinder by the rolling bead action disclosed in GB-A-2,099,368.
[0021] When conditions stabilised, the inking was found to be uniform over the central 12 cm of the image on the copy sheets, but laterally beyond that area the image was faint, and in places badly broken through under inking.
[0022] With the roller 51 in place and driven for rotation in the same sense as the bead 37 the inking was uniform over the entire width of the image on the copy sheets.
[0023] The viscosity measurements for Example 1, and for Example 2 (below), were made using a Ferranti Shirley Cone and Plate Viscometer and the values were measured over the shear rate range-200 to 1500 sec." at a temperature of 25°C.
EXAMPLE 2
[0024] Using the same duplicator as for Example 1, a series of tests was carried out on different proprietory inks, and the specially formulated AMI ink to see what effect the presence of the ink bead supporting roller 51 had on the ability of the inks to self-distribute along the "trough" formed by the coating roller 8 and the bottom cylinder 2 when the ink is dropped onto the bottom cylinder 2 at a location midway between its ends.
[0025] The results are given in Table 1. The lefthand column identifies the ink, the second column quotes its viscosity in Poise, the third column quotes the shear stress in dynes/cm2, the fourth column states the maximum speed of the duplicator (in copies/min.) at which the ink in question was able to form a rolling bead without the assistance of the driven bead-supporting roller 51, and the last column quotes the corresponding maximum speed when the roller 51 is present.
[0026] The Gestetner AMI ink mentioned above in both Example 1 and Table 1 was a special formulation developed for use with the stencil duplicator disclosed in our said GB-A-2,099,368 in that the ink has a very low viscosity to allow it to be applied and distributed without the conventional axially reciprocating distributor rollers, and the yield stress is not too high. Quite clearly, the presence of the ink bead-supporting roller 51 has a dramatic affect on the capacity of this ink to form the necessary rolling bead and to be distributed along the length of the bottom cylinder 2, without additional means to move it along the cylinder.
[0027] The Gestetner A and Gestetner B inks, as well as the Roneo Alcatel stencil ink and the AB Dick 3400 ink are all proprietory stencil inks, and in the case of the Roneo and AB Dick inks it was found that the presence of the roller gave a slight improvement in the ability of the ink to distribute along the bottom cylinder.
[0028] The Letterpress ink is not intended for use with a stencil duplicator and, as can be seen, it has a relatively low viscosity and a very low yield stress and is capable of a dramatic improvement when the roller 51 is present.
[0029] The water-based ink is a specially formulated stencil ink of a non-emulsion variety specially designed for use with the duplicator of our said GB-A-2,099,368. This ink has its performance dramatically improved by the presence of the roller 51.
[0030] As comparative examples, further tests were carried out using two offset inks which are formulated for their ability to roll, and the results of these are given below in Table 2.
[0031] In Table 2, the values of viscosity quoted were again measured using the Ferranti Shirley Cone and Plate Viscometer, but measured over a different shear rate range so the viscosity values are not directly comparable with those given in Table 1. However, it is quite clear that the offset ink in the first line of Table 2 has a much higher viscosity than those quoted in Table 1.
[0033] The aluminium soap-based ink is a special formulation of an aluminium soap-based mineral oil gel which has a very tacky consistency and is very elastic like an offset ink, but it has been found not to print when used with a stencil duplicator. It has been incorporated in Table 2 in order to show that the highly advantageous results obtained for stencil inks with the ink bead-supporting roller 51 can in any case be achieved with an offset ink without the roller, but the offset ink is too stiff to pass through the pores of the stencil and are therefore not capable of printing by the stencil duplicating process.
1. A multi-cylinder stencil duplicator comprising a cylinder (2) on which a uniform
layer of ink is applied and on which an ink pervious stencil is rolled during printing;
a coating member (8) alongside that cylinder and defining therewith a nip through
which ink can pass to form a uniform layer on this cylinder during rotation of the
cylinder; and means (38) for applying ink to one side of said nip to form a wedge-like
bead (37) of ink extending into the nip; characterised by a roller (51) positioned
at the nip on said one side thereof, and arranged to be immersed in the bead of ink
in use of the duplicator.
2. A duplicator according to claim 1 characterised in that the said roller (51) is
driven for rotation.
3. A duplicator according to claim 2, characterised in that the direction of rotation
of the roller and the direction of ink bead rotation induced by rotation of said cylinder
are the same.
4. A duplicator according to claim 3, characterised in that the speed of rotation
of the roller and the speed of rotation of the bead of ink are equal.
5. A duplicator according to any one of claims 2 to 4, characterised in that the roller
is positioned at the location of the centre of rotation of the bead of ink.
6. A duplicator according to any one of the preceding claims, characterised in that
the means for applying ink to form the bead comprises means (38) for applying ink
to the surface of said cylinder upstream of the nip (2,8) and the roller (51).
7. A duplicator according to any one of the preceding claims, further characterised
by means (9,51) for detecting the volume of ink forming said bead.
8. A duplicator according to claim 7, characterised in that said detector is a capacitive
ink detector and said roller (51) immersed in the bead (37) constitutes one of the
electrodes (9,51) of said capacitive ink detector.