GB2056371A - Ink unit for offset printer - Google Patents

Abstract

An ink unit for an offset printer includes a duct roller (9), and, in place of the usual ink trough, a surface (26) which is generally planar and parallel to a tangent to the duct roller (9) but closely spaced from the surface of the duct roller. Ink is supplied to the working gap between the duct roller (9) and the surface (26) by means of supply ports fed from an ink pump, in this case a can having a barrier member which is driven along the can towards a closed end thereof to expel ink through a central opening of the barrier member. <IMAGE>

Description

SPECIFICATION Ink unit for offset printer The present invention relates to an ink unit for an offset printer.

Hitherto, it has been conventional for an ink unit for an offset machine to comprise a duct roller with a doctor blade in very close contact with the duct roller and lying in a plane which is a secant to the cylindrical surface of the duct roller so that the duct roller and the doctor blade together define an open-topped, generally V-shaped ink trough. It is conventional to scoop, or otherwise to spread, ink into the trough, and to fill the trough to an extent which will ensure adequate supply of ink until the end of printing operations with that particular colour.

We now propose to provide an ink unit incorporating a duct roller without the conventional open-topped shallow trough which has hitherto been subject to several disadvantages, for example (a) the extensive interface between ambient air and the ink in the trough giving rise to contamination of the ink by materials such as paper dust, (b) the fact that ink in the trough tends to stagnate and needs constant agitation by the operator (particularly when relatively "stiff" inks are used), and (c) the fact that the operation of filling the trough gives considerable risk of dirtying the hands of the operator. The last disadvantage is particularly important in the case of an office offset machine where the offset printer may be a duplicator which should require the minimum operator intervention and should not involve the operator in any messy tasks.

All these advantages are to be overcome with the ink unit of the present invention.

In accordance with the present invention we provide an ink unit including a duct roller; an ink distributor member having a surface which generally conforms to a plane parallel to a local tangent to the cylindrical surface of the duct roller but is closely spaced from the duct roller so as to define with the said duct roller an area where high shear forces are exerted on printing ink in said area; and means for supplying ink to said area.

Preferably the ink supplying means includes ink supply ports in the ink distributor member opening into said surface thereof, and adapted to be fed with ink from an ink pump. More preferably there may be four such outlet ports regularly disposed along said ink distributor member to permit ink to be spread along the duct roller by the working action of the rotating duct roller and the nearby ink distributor member surface to provide uniform appiication of the ink to the duct roller.

In a preferred form of the present invention the ink pump comprises a first pressure plate for engaging an annular barrier member slidable within an open-ended can of ink, and a second pressure plate for engaging the closed end wall of said can, said first pressure plate including centrally thereof the mouth of an ink passage communicating with said supply ports; means being provided for urging said second pressure plate towards said first pressure plate for expelling ink from a can placed between said first and second pressure plates with the first pressure plate in contact with the slidable barrier member thereof.

Advantageously the ink passage from said first pressure plate to said ink supply ports comprises a primary ink passage branching into two secondary ink passages which each in turn branch into a respective one of two pairs of tertiary ink passages each said tertiary ink passage communicating with one of said ink supply ports.

In order that the present invention may more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawings, in which: FIGURE 1 is a longitudinal sectional view through an offset ink supply can; FIGURE 2 is a front elevation of the righthand side of an offset printing machine ink unit for use with the ink can shown in Figure 1, part of the side frame of the ink unit being shown in section; FIGURE 3 is an end elevation of the ink unit shown in Figure 1, but viewed from the lefthand end; FIGURE 4 is a top plan view of the lefthand end of the ink unit of Figures 2 and 3; FIGURE 5 is a sectional view through the duct roller and the ink distributing member of the ink unit of Figures 2 to 4;; FIGURE 6a is a transverse section of the duct roller and ink distributing member in an alternative form of the ink unit, the section being taken at a location where the ink thickness on the duct roller is adjusted to a maximum value; FIGURE 6b is a view corresponding to Figure 6a but with the section taken at a location where the ink layer thickness on the duct roller is adjusted to a minimum value; FIGURE 7 is a side elevational view, viewed from the right, of the ink unit shown in Figures 2 and 4 with the control knobs omitted; and FIGURE 8 is a front view showing in more detail the indexing screw mechanism of Figure 7 but with the control knobs attached.

As shown in Figure 1 the ink supply can 1, formed of any suitable material, in this case sheet aluminium, has a cylindrical side wall 2 and an end panel 3 forming the can body, and a peripheral bead 4 at the open end of the can body. The bead 4 is rolled inwardly so as to provide a stop to prevent a substantially planar annular barrier member 5, in this case of plastics material, from being detached from the can body. This barrier member 5 has at its outer edge a peripheral lip 6 turned towards the body of ink in the can, so as to execute a wiping seal action on the interior of the can body during movement of the barrier member 5 along the can body toward the closed end wall 3 thereof. The inner edge 7 of the annular barrier member is turned outwardly away from the body of the ink and the barrier member 5 thus serves to contain the mass of high viscosity offset ink in the can 1.As will be appreciated, the barrier member 5 is slidable along the can 1 so that movement of the barrier member 5 towards the enclosed end wall 3 of the can will expel ink through the control aperture bounded by the inner edge 7. The can 1 is intended to fit into an offset ink unit having a first abutment member in the form of a first pressure plate which bears against the barrier member 5 while simultaneously the closed end wall 3 of the can is driven leftwardly towards the barrier member 5 so as to expel ink through the aperture in the centre of the annular barrier member 5 and thereby to drive the ink through an inlet to an ink passageway in the first pressure plate (to be described later).

FIGURE 2 shows, in the form of a front elevation, the righthand end of an offset ink unit comprising a duct roller 9 rotatably mounted by means of the ends of its shaft 10, in the side frames 11 of the ink unit and co-operating with an ink distributing member 12 including passages generally designated 1 3 (Figure 3) which transmit the ink from the can 1 to a gap 14 (Figure 3) formed between the ink distributing member 12 and the duct roller 9.

FIGURE 2 furthermore shows the first pressure plate 1 7 in engagement with the annular barrier member 5 of the can 1 and also a second abutment member in the form of a second pressure plate 1 spa formed on a nut 1 5 which is threadedly engageable with a lead screw 16 rotated by a pawl and ratchet mechanism to be described latsr with reference to Figure 7. This lead screw 1 6 is, by virtue of the pawl and ratchet mechanism, driven for rotation in a manner which is dependent upon the indexing rotation of duct roller 9.

As the lead screw 1 6 rotates, the nut 1 5 and consequently the pressure pad 1 spa move leftwardly so as to press the bottom wall 3 of the can 1 in a leftward direction towards the first pressure plate 1 7 shown in Figure 4. This action drives the barrier member 5 down the can towards the end wall 3 and simultaneously expels ink through the central aperture of the barrier member 5 and into a countersunk inlet 18 of a first part 1 3a of a primary ink passage extending through the first pressure plate 17.

As further illustrated in Figure 4, part 13 of this primary ink passage communicates with a Ushaped further portion 1 3b of the primary ink passage, formed in an upstanding bracket 1 9 which is also illustrated in Figure 3. The bracket 1 9 is integral with, or may be separately formed but subsequently secured to, the ink distributing member 12 and thus its U-shaped passage portion 1 3b communicates with a third portion 1 3c of the primary ink passage formed in the ink distributing member 12 and communicating with two secondary ink passages 13d which extend rightwardly and leftwardly respectively, away from the end of third primary ink passage portion 1 3c to convey ink axially away from the centre of the ink distributing member 12.The outlet end of the lefthand secondary passage 1 3d is shown in Figure 4 as joining with the upstream parts 1 3e of two tertiary ink passages which split the ink flow into a righthand portion towards a righthand ink supply port 20b, and a lefthand portion towards an ink supply port 20a. Two further ink supply ports will be provided at the righthand end of the ink distributing member 12 such that the ink flow from the countersunk inlet 18 of the primary ink passage portion 1 3a eventually arrives at each of four ink supply ports 20 regularly distributed along the length of the end distributing member 12.

At the ends of the ink distributing member 12 are side forks 21 which, as viewed in Figure 3, each have an opening 22 to co-operate with a pair of diametrically opposed parallel flats 23 on the duct roller shaft 10 to facilitate attachment of the ink distributing member 12 to the ink unit and removal from the ink unit, when required. This removal will be required for the purposes of cleaning or maintenance of the machine, and for example for changing the colour of ink being used.

The forks 21 are each attached to the ink distributing member 12 by means of screws 24 shown in Figure 4.

In order to relieve the upstanding bracket 1 9 of bending moment which arises when the very viscous ink 8 in the can is squeezed between the end wall 3 and the barrier member 5, the bracket 1 9 has a leftwardly extending thrust portion 25 within which, at one end, is mounted a threaded shank 26 of the screw having a knurled head 27 and carrying a locking nut 28. The knurled head 27 provides an adjustable thrust member to engage the left-hand side frame 11 of the ink unit.

When the ink distributing member 12 is to be removed, the lock nut 28 is first slackened off sufficiently to allow the knurled head 27 to be rotated to screw the screw shank 26 inwardly of the thrust portion 25 of the bracket 19 so that the knurled head 27 will have been freed from abutment with the side frame 11 and will permit easy removal of the distributing member 12 from the duct roller shaft 10. Likewise, after the ink distributing member 12 has been replaced in position it will be necessary to rotate the knurled head 27 to bring it back into abutting engagement with the side frame 11 and then to fasten the lock nut 28 so the knurled thrust head 27 provides adequate thrust to resist the bending moment in the bracket 1 9 in use of the offset ink unit.

It is conventional in offset printers for the ink unit to include a shallow ink trough having a generally flattened V-shaped configuration with a doctor blade at the apex of the V and with one of the side walls of the V defined by the cylindrical surface of the duct roller so that movement of the doctor blade towards and away from the duct roller will allow adjustment of the thin layer of ink entrained by rotation of the duct roller. In such an arrangement the doctor blade conforms generally to a plane which is secant to the cylindrical surface of the duct roller and defines a very narrow gap through which the ink, adhering to the surface of the duct roller 9 in the trough, is able to pass onto a part of the duct roller outside the trough ready for subsequent transfer from the duct roller by a laterally oscillating transfer roller.

By contrast, the inker illustrated in Figures 5, 6a and 6b has a working gap 14 defined between the duct roller 9 and a chamfer 26 of the ink distributor member 1 2 such that the chamfer conforms generally to a plane which is parallel to a local tangent of the duct roller 9 but closely spaced from the surface of the duct roller. In this way the working gap provides a working area which occupies a considerable peripheral extent of the path of the surface of the duct roller 9 (as compared with the linear gap arising with the conventional doctor blade ink trough).In this gap 14 there is a very high shear force exerted on the ink, due to the high viscosity of the ink and the conflicting tendencies of the boundary layer of the ink in contact with the duct roller 9 to move with the roller while the boundary layer of ink on the surface of chamfer 26 tries to stay static on that surface. The high shear forces generated assist in promoting flow of the ink not only peripherally around the duct roller 9 but also longitudinally of the duct roller away from the locations of the four ink supply ports 20.In this way, the ink unit in accordance with the present invention has been found to operate satisfactorily with simply four outlets, whereas hitherto it has been customary for the conventional doctor blade type of ink trough to extend along the full length of the duct roller and to be provided with upwards of sixteen adjustment screws to provide for local variation of the gap between the doctor blade and the duct roller in order to ensure uniformity of the thickness of the ink layer applied to the duct roller.

In the preferred embodiment of ink unit illustrated in the drawings the gap 14 between the distributor chamfer surface 26 and the duct roller surface is 0.004" (0.01 mm), but it is envisaged that some variation in this gap could be tolerated without excessive loss of uniformity of ink distribution along the duct roller 9. For example the likely range of adjustment may be from 0.003" to 0.006" (from 0.075 mm to 0.15 mm). The gap is preferably 5 mm long.

As shown in Figures 6a and 6b, it is contemplated that the ink distributor member 12 may include, on its flat underside 33, a set of slidable L-shaped ink limiting plates 27 held in place by virtue of flat heads 28 of studs 29 and retaining springs 30 under the stud heads 28 to hold the L-shaped slide plate 27 in place. Each ink limiting plate 27 will be movable between an open position illustrated in Figure 6a, and a closed position illustrated in Figure 6b where a chamfer 31 on the L-shaped plate is virtually in contact with the duct roller 9 to shut down the flow of ink locally. Generally, for uniformly inked images, such as pages of typescript, it is envisaged that the slide plates 27, where provided, will all be in the open position shown in Figure 6a.Adjustment will only be needed where the image has strongly defined zones of heavy and light inking (or even nil inking), and it is desirable to shut down the application of ink to the lightly inked or uninked areas in order to avoid localised build-up of ink on the duct roller 9 and on the later rollers (not shown) in the ink train of the ink unit between the transfer roller and the forme roller.

As shown in Figures 6a and 6b, the L-shaped ink limiting plates 27 each have an elbow 32 which, in the closed position shown in Figure 6b or in a partly closed position, provides a means for holding the ink limiting plate in a pre-selected position. The elbow 32 is thrust into engagement with the flat underside 33 of the ink distributor member 12 by the action of the springs 30 under the stud heads 28.

As indicated above, when the ink limiting plates 32 are retracted to the Figure 6a position, the uniformity of distribution of ink along the duct roller 9 is adequate for most printing tasks and in any case the conventional axially moving oscillator roller in the ink train between the duct roller and the ink forme roller serves to enhance the spread of ink along the forme roller.

Figures 6 and 6b show that only a small roll of ink is retained upstream of the distributor member 12 (when considered in the direction of movement of the surface of the duct roller 9 past the working face 26 of the ink distributor). The indexing rotation of the lead screw 1 6 is selected such that the relationship between the incremental advance of the pressure pad 1 5a for each indexing rotation movement of the duct roller 9 will expel a quantity of ink from the can 1 into the working gap 14 by way of the outlets 20 sufficient to compensate for the amount of ink transferred from the duct roller 9 under normal operating conditions.Naturally, when the ink unit is switched off during rotation of the plate cylinder (the unusual condition) the transfer roller will not be moving into and out of contact with the duct roller 9, and similarly neither will the duct roller 9 itself be rotating. Thus, a substantially balanced condition will exist. When a more heavily inked image is present, the operator will in any case adjust the ink unit to give more ink to the forme roller, by increasing the amplitude of the indexing rotation of the duct roller 9 and this will in turn increase the rate of expulsion of ink from the can 1.

A limit switch will be positioned near the lefthand side of the ink unit (as viewed in Figure 4) in order to provide a signal for the operator when the can body or the pressure pad 1 5a, has arrived at an "end of travel" position indicating the need for replacement of the ink supply can 1.

The arrangement shown in Figure 5 illustrates that at each end of the ink distributor 12 there is a groove 34 flanked by two parallel planar shoulders 35. The groove 34 will normally be free from ink but with only a very slight quantity of ink at the lowest part of the inclined floor 36 of the groove near the point of merger with the working face 26.

If, for example during manual priming of the ink unit by the operator winding the lead screw 1 6 round by hand, too much ink exudes out of the ink supply ports 20a, 20h etc. too much ink is fed to the duct roller 9 to be passed up the ink train, the quantity of ink just upstream of the working gap 14 (Figure 3) will increase and thus the smear of ink near the bottom end of the floor 36 of the groove 34 will begin to spread back up the inclined floor 36 of the groove. On seeing this, the operator will take appropriate correcting action to remove some of the excess ink. In normal operation of the ink unit the rotation rate of the lead screw 16 and the indexing rotation of the duct roller 9 will be related to give the correct flow of ink to the gap 14.If desired, some form of ink sensor may be incorporated on the groove floor to signal to the operator when too much ink is present.

Figure 5, and also Figures 6a and 6b, show that under normal operating conditions there will be a small roll of ink which is located just upstream of the working gap 14 (Figure 3) and which is subject to extremely high shear stresses which both "work" the ink in order to ensure it has the optimum consistency for printing, and also assist in spreading the ink axially along the duct roller.

The applicants do not wish to be limited by any suggestion of the theory behind the surprisingly effective working action of the inker illustrated in the drawings; however, it is believed that the ease of transfer of the ink axially along the duct roller 9 may result from the intensity of the shear stresses in the narrow working gap causing it to flow more readily axially along the duct roller 9.

Another advantage of the "working gap" principle of the ink unit is that the quantity of ink in the "roll" just upstream of the working gap 14 is very small in comparison to the average quantity of ink present in the ink trough of a "doctor blade" type of ink unit. Thus, when it is desired to change colours, or to carry out periodic cleaning of the ink unit, it is a simple matter to remove the ink distributor 12 from the duct roller 9 (by rotating the distributor 12 through an angle of approximately 300 from the position shown in Figure 3 in order to bring the gap 22 at the end of the fork 21 into register with the flats 23 on the ends of the duct roller shaft 10, after which the distributor 12 can be removed from the ink unit).

Before this is carried out the knurled head 27 of the thrussscrew will, as described above, be released (by first releasing the lock nut 28) in order to allow the rotation and withdrawal of the ink distributor 12, and furthermore the lead screw 16 will have been backed off slightly in order to release the pressure of the pressure pad 1 5a from the back of the can to allow the can to be removed from the ink unit.

The various ink rollers will then be cleaned down using the appropriate solvent, such as blanket cleaning solvent.

A fresh ink distributor 12, for the new ink colour which it is desired to print, is then placed in position and allowed to rest on the support bar 37, and then a can of ink of the appropriate colour is inserted in front of the pressure pad 15a, and the lead screw 1 6 then advanced in order to take up the slack. Of course, the mention of inserting a fresh distributor 12 implies subsequent adjustment of the thrust screw thereof.

By having two ink distributors 12 available, it is possible to reduce the wastage of ink to an absolute minimum. With the conventional trough type ink unit it is necessary to remove the trough from the duct roller (a messy operation often involving wastage of ink falling from the trough floor as the trough is lifted off the duct roller) and in many cases it is safer to dispense with the ink remaining in that trough because there is the danger of dust and other contamination getting into the ink in the trough during the printing with the alternate colour. However, with the ink unit illustrated in Figures 2 to 7, the supply of ink is contained within the various ink passage portions 1 3a, 1 3b, 1 3c, 1 3d and 1 3e, and is thus substantially free from the possibility of contamination.

If the ink unit is to be cleaned down and reinked with a fresh colour then it would be sufficient for the ink distributor 12 to be removed before cleaning of the rollers of the ink train, and then for one distributor 12 to be simply wiped clean of an external traces of ink (for example to remove the ink residue on the working face 26) and for the face of the adjustable thrust member 17 to be cleaned. There is no need to remove all the ink from within the various passage portions 13a, 13b, 13c, 13dand 13e, as this ink is contained and, in any case, the normal pumping flow of ink will ensure that there is no area of stagnation of the ink flow and that the surfaces of the passages will be constantly subjected to a fresh flow of ink.However, if it should be necessary to clean the passage portions 1 3a, 1 3b, 1 3c, 1 3d, 1 3e, then the ink distributor 12 is suitably designed in that it comprises a main body 38 and a plate 39 forming a top cover thereof and having the primary ink passage portions 1 3c, the secondary ink passages 1 3d and part 1 3e of each tertiary ink passage formed therein as grooves which can be easily cleaned out. The final part 1 3f of each tertiary ink passage 1 3e (leading up to the ink supply port 20a,20b, etc.) is formed in the main body as a straight bore which can be easily cleaned out.

This construction using milled grooves for the passage portions in the cover plate 39 of the ink distributor 12 also facilitates removal of any obstruction from the ink flow path 1 3 c, 13d, 13e, 13f, in the unlikely event that this should be required.

Figure 7 shows an elevational view looking from the righthand side of the ink unit as viewed in Figure 4, and showing the lead screw shaft 1 6 and the duct roller shaft 10. A partially toothed cam wheel 40 mounted coaxially and freely rotatably on the end of the duct roller shaft 10 has teeth 41 occupying one quarter of its periphery and a stepped cam surface 42, 43, forming the remainder of its periphery such that there is a concentric cam surface 42 occupying one half of the periphery of wheel 40 and having a higher radius than a concentric portion 43 occupying the remainder of the wheel periphery. A pawl 44 carries a peg 45 which rides on the cam portion 42, 43, of the wheel 40.The pawl 44 has a tooth 46 which engages with ratchet teeth 47 on a ratchet wheel 48 coaxial with the duct roller shaft 10 and behind the cam wheel 40.

The pawl 44 serves to drive the duct roller shaft 10 for indexing rotation since the pawl 44 is pivotally mounted at 49 on a drive yoke 50 driven by a crank 51 to which it is pivotally secured at 52.

A spring biasing mechanism 53 drives a detent tooth 53a into engagement with the interdental spaces between teeth 51 to hold the cam wheel 40 in any desired position along a range occupying approximately one quarter of a turn.

Adjustment of the positon of the cam wheel 40 is achieved by operation of an integral control knob portion 54 shown in Figure 8. A cylindrical portion 55 mounted eccentrically on the shaft 10 freely rotatably supports a further pawl 56 having a tooth 57 engaging in the interdental spaces between teeth 58 on a ratchet wheel 59 carried by the end of lead screw 1 6. A helical tension spring 60 holds the tooth 57 of pawl 56 in engagement with the ratchet teeth 58. Frictional resistance to rotation of the lead screw 1 6 prevents reverse rotation as the pawl tooth 57 reverses over the ratchet teeth 58.

Turning now to Figure 8, showing a sectional front view, in the form of a section taken on the line VIll-VIll of Figure 7, it will be seen that the shaft of lead screw 1 6 has a control knob 61 fixed thereto by means of a screw 62, and that the duct roller shaft 10 has a hand knob 63 secured thereto by means of a screw 64.

The operation of the mechanism illustrated in Figures 7 and 8 is as follows:- During normal operation of the offset printing machine, the crank 51 will oscillate along the direction of the arrow 65 shown in Figure 7 and will thus cause the yoke 50 to oscillate about the central axis of the duct roller shaft 10 on which it is rotatably carried. This causes the pawl 44 to oscillate, and all the time the pin 45 is riding on the low lift concentric cam portion 43 of wheel 40 the pawl tooth 46 can engage in an interdental gap between two adjacent teeth 47 of the ratchet wheel 48 to drive the ratchet wheel 48 for clockwise rotation. Movement of the pawl in the reverse sense will cause the pawl tooth 46 to ride over the ratchet teeth 47.

When the pin 45 rides up on to the high lift concentric cam portion 42, the pawl tooth 46 will be lifted clear of the ratchet teeth 47 and oscillation of the yoke 50 and pawl 44 over this part of the travel of the yoke will be ineffective to drive the ratchet wheel 48. Thus, the position of the step between the low lift cam portion 43 and the high lift cam portion 42 dictates the amplitude of each indexing rotation of the duct roller shaft 10 and this step is adjustable in position by rotation of the control knob portion 54 integral with the cam wheel 40 allowing it to be adjusted to any one of the positions which will be held by engagement of the cam tooth 53a between the teeth 41 on the wheel 40.

As the duct roller drive shaft 10 rotates, the eccentric portion 55 thereof will orbit around the shaft axis and this will cause the tooth 57 to rise for one half of the complete rotation of the duct roller shaft 10 and to descend for the other complete half of the rotation. Although the amplitude of the indexing rotation of the shaft 10 is fully adjustable, the link provided by the pawl 56 between the duct roller shaft 10 and the lead screw 1 6 provides a constant ratio integrating system so that after one half of the rotation of the duct roller shaft 10 the lead screw will have rotated through a given fraction of a turn, and throughout the next half rotation of the duct roller shaft 10 the lead screw will be stationary.

During removal of installation of the ink supply can 1, the lead screw can be backed off and advanced using the manual control knob 61 (after releasing the pawl tooth from the ratchet teeth 58 before anticlockwise rotation of the lead screw control knob).

Similarly, during normal inking operation if it is desired to provide an increased flow of ink to the transfer roller, the duct roller 9 can be rotated by hand using the control knob 63.

Starting from a completely clean condition, the operation of the ink unit described above is as follows.

Initially the ink distributor member 12 is inserted (as described above) and the thrust screw head 27 adjusted to the appropriate position. Then the lead screw 1 6 is backed off to retract the nut 1 5 far enough to allow a can of ink to be placed between the two pressure plates 1 spa and 1 7 and then, when such a can is in place and the barrier member 5 of that can is held firmly in contact with the pressure plate 17, the lead screw 1 6 is advanced so as to bring the pressure plate 1 spa into engagement with the end wall 3 of the can body 1.

The ink unit is then set in operation with the ink supply selected to "heavy inking" in order to drive the can body 1 leftwardly to begin flow of ink firstly into the countersunk inlet 18 of the primary passage portion 13a, and then gradually through the various passage portions 1 3b, 1 3c, 1 3d, 1 3e and 1 3f, until ink exudes from the ink supply ports 20a, 20b, etc., and arrives on the surface of the duct roller 9.

Although it is conceivable that a hand-operated override mechanism may be provided in order to achieve this function, such a complication would not normally be necessary, as it would be convenient for the operator to leave the machine in operation to promote ink flow through the various ink passages 13a, 13b, 13c, 13d, and 1 3f while the operator is attending to another task, for example setting up the blanket wash system or the damping system. The operation of building up a quantity of ink in the various ink passage portions 1 3a to 1 3f may take a few minutes.

Once the ink begins to exude from the ink supply ports 20a, 20b etc., the ink unit is ready for use and ink will then build up on the surface of the duct roller 9, firstly as four separate bands, and then very quickly spreading along the duct roller to provide overall substantially uniform inking thereof. As indicated above, if the operator notices that the extent of the ink on the inclined floor 36 of the sighting grooves 34 is too high, in use, then it is possible to reduce the rate of indexing rotation of the lead screw 1 6 by adjustment of the pawl and ratchet mechanism. Alternatively, if the roll of ink is too small and there is no ink build-up on the floor 36 of the groove 34, then of course the indexing rotation of the lead screw 16 must be increased.Normally there will be ten separate detent positions of the adjuster in the pawl and ratchet mechanism covering 60 of rotation of the cam wheel 40.

Whereas with the conventional trough type of ink unit it is necessary for the viscous ink to be spread along the trough using a spatula, or a scoop, or at best by squeezing it from a flexible tube of ink, there is considerable risk of dirtying the hands of the operator and also constant risk of paper dust and other contaminants entering the ink supply in the trough. By contrast, the present system provides a completely enclosed arrangement in which the ink from the supply can is automatically pumped along the enclosed passage portions to the supply ports where the ink is allowed to build up as a very small roll (only a fraction in size relative to the quantity of ink normally found in a trough type of ink unit, at least when the trough is first filled), with that roll virtually concealed from ingress of paper dust and other contaminants.

The working gap arrangement in accordance with the present invention may be used with any suitable ink supply, not necessarily a can having a slidable barrier member. The ink may, for example, be contained in a flexible tube from which it is pumped either by squeezing of the tube or by use of an extractor pump which sucks the ink from the tube, or ink may come from any other suitable ink supply. Likewise, the particular configuration of the ink distributor 12 illustrated herein can be modified to whatever extent is desirable in order to ensure substantially uniform supply of ink to the outlet ports 20a, 20b, etc., at the working gap 14.

The important principle is that there should be a gap defined between the duct roller 9 and a surface 26 which is generally in a plane which is parallel to a local tangent to the duct roller 9 but does not intersect the cylindrical surface of the duct roller 9.

Claims (14)

1. An ink unit including a duct roller; an ink distributor member having a surface which generally conforms to a plane parallel to a local tangent to the cylindrical surface of the duct roller but is closely spaced from the duct roller so as to define with the said duct roller an area where high shear forces are exerted on printing ink in said area; and means for supplying ink to said area.

2. An ink unit according to claim 1, wherein said ink supplying means includes a plurality of ink supply ports opening in said ink distributor member adjacent or at said surface and communicating with an ink supply pump.

3. An ink unit according to claim 2, wherein said ink supply pump comprises a first pressure plate having centrally thereof an ink passageway communicating with each of the said supply ports, a second pressure plate mounted parallel to said first pressure plate and movable towards and away from said first pressure plate, and means for moving said first and/or second pressure plate towards the other of said first and second pressure plates at a progressive rate during normal operation of the ink unit.

4. An ink unit according to claim 3, wherein said means for moving said first and/or second pressure plate comprise a lead screw having a nut to which the movable said pressure plate is connected, and a mechanism linking said lead screw to the duct roller to cause said lead screw to index simultaneously with indexing of said duct roller.

5. An ink unit according to claim 4, wherein said linking mechanism comprises a pawl and ratchet mechanism connecting a drive shaft of said duct roller with said lead screw.

6. An ink unit according to claim 5, and including adjustable means for limiting the return stroke of a pawl of said pawl and ratchet mechanism for adjusting the relationship between indexing rotation of said lead screw and indexing rotation of said duct roller.

7. An ink unit according to any one of claims 2 to 6, wherein there are four of said ink supply ports.

8. An ink unit according to any one of claims 2 to 7, wherein said ink distributor member comprises a body member having a cover plate secured thereto, said cover plate having ink passageways machined therein whereby securing said cover plate in contact with said main body portion of the ink distributor member leaves said ink passageways open-ended but otherwise closed and connected to respective ones of said supply ports.

9. An ink unit according to claims 7 and 8 when taken together, wherein each of said four supply ports defines the end of a respective tertiary ink passage, said tertiary ink passages being linked in pairs to a respective one of two secondary ink passages, and said two secondary ink passages both being connected to a primary passage, the arrangement being such that the length of the flow path of the ink from the inlet and of said primary passage to each of said supply ports is the same in the case of ink flow to each of the four ports.

10. An ink unit according to any one of claims 2 to 9, wherein said ink distributor member includes grooves formed near the ends thereof, each groove having a shallowly inclined floor sloping upwards from said surface of the ink distributor member whereby excess ink supply to said duct roller will manifest itself as a wedge of ink on said inclined floor.

11. An ink unit according to any one of claims 2 to 10, wherein said ink distributor member includes end forks adapted to fit over the ends of the shaft of said duct roller to permit the ink distributor member to be installed parallel to and close to said duct roller, and removed therefrom, as desired.

12. An ink unit according to any one of claims 1 to 11, and including a plurality of ink limiting plates positioned slidably on the underside of said ink distributor member and arranged for sliding movement towards and away from the surface of said duct roller, each of said ink limiting plates being adjustable in position independently of the other plates and being capable of holding any one of a desired range of positions for the purpose of limiting the thickness of ink at any location on said duct roller in relation to the ink thickness at other axial locations along said duct roller.

13. An ink unit according to claim 12, wherein each of said ink limiting plates is substantially Lshaped in cross-section and has an elbow which is resiliently urged into contact with a planar underside of said ink distributor member for the purpose of ensuring that said ink limiting plate holds any position in which it is set.

14. An ink unit according to any one of the preceding claims, wherein the said surface of the ink distributor member is spaced from the duct roller surface by a distance of substantially 0.1 mm.

1 5. An ink unit for an offset printer, substantially as hereinbefore described with reference to, and as illustrated in the accompanying drawings.