GB2244456A

GB2244456A - Correcting skewed print images

Info

Publication number: GB2244456A
Application number: GB9108787A
Authority: GB
Inventors: Ingo Kobler
Original assignee: MAN Roland Druckmaschinen AG
Current assignee: Manroland AG
Priority date: 1990-04-24
Filing date: 1991-04-24
Publication date: 1991-12-04
Anticipated expiration: 2011-04-24
Also published as: GB2244456B; DE4013003C2; GB9108787D0; DE4013003C3; JP2809893B2; DE4013003A1; JPH04224946A

Abstract

The position of sheets on sheet-conveying drums (4) disposed between impression cylinders (3) of an offset press is acted on by skew register adjusters (6, 7, 8). The same are connected to control facilities (11 - 13) which obtain their data from memories (14, 15) for the position of the partial images and from the immediately preceding control facility (11, 12). They act on register adjusters (5) controlling the peripheral and lateral register of plate cylinders (1). The sheet position from the immediately preceding printing unit is taken into account in the control facilities (11 - 13) for adjustment for the next printing unit. More particularly, a skewed position of a sheet arising from the previous printing is restored for the next printing in the light of the skewing necessary therein. The sheet position is automatically adapted to the requirements of each individual printing unit. Corrections of the peripheral and lateral register are also taken into consideration automatically. <IMAGE>

Description

CORRECTING SKEWED PRINT IMAGES This invention relates to correcting skewed

print images.

When multicolour images are being printed on a printed sheet, it is of prime importance that the vdrious partial images or sub-images of the discrete colours be brought into proper association with one another. The shift of the sub-images peripherally and axially of the printing unit cylinders is used for this purpose as auxiliary correction means. However, special problems are posed by mispositioning due to one or more of the sub-images being rotated in their plane from their ideal position. Various attempts have been made to eliminate mispositioning of this kind by skewing relatively to one another selected printing unit cylinders concerned in conveyance of the print images. In the case of offset printing, attempts have been made in this directon by skewing the plate cylinder. However, all thhat happens as a result of this is that the printing plate is developed relatively to the blanket cylinder in a kind of helix, the front edge and rear edge being straight but the rear edge being laterally displaced. Attempts have therefore been made to rotate the printing plates on the 2 - plate cylinder in the plane of their extent. This can be done manually, by means of the existing plate-clamping means, and automatically. However, in this case, the mounting and control of the plate-clamping means become very elaborate. More particularly, the performance of the various functions is complex since, for skewing, the plate must first be unclamped, skewed, then correctly clamped in the new position. To automate the procedure, therefore, substantial installations must be provided for power supply and for adjusting the plate-clamping means on the plate cylinder. The whole system therefore becomes too expensive and is too prone to defects for satisfactory continuous operation.

A completely different approach to this problem is pursued in the method disclosed in German Patent Specification 465 246 for achieving satisfactory register in presses having a number of printing units. In this case, a number of printing units are disposed in consecutive relationship in a gravure press. In gravure printing, the print images are etched into the plate cylinder. There is therefore no possibility of the print image shifting relatively to the plate cylinder. Nevertheless the print image may become skewed from its ideal position. It is therefore suggested that the position for the sheet to be printed at this place be adapted to the position of the print image on the forme cylinder. To this end, the axis of the feeding sheetconveying drum is skewed relatively to the impression cylinder on which it is required to carry out the printing. The skewing can be done vertiically and horizontally. The effect of the skewing is that the relative position between the sheet and the image is harmonised for a single printing unit so that the sub- 3 - image can be printed out in its plane in the correct position relatively to the rotated position. A disadvantage of the method is that there are bound to be variations as regards the peripheral position and the lateral position. The method disclosed takes no account of these factors. A far more serious disadvantage is that the sheet moves through the complete press in this skewed position so that, with random exceptions, a defect in print-out is bound to be present in the next printing unit. Nothing is stated in this publication about the possibility of action to counter this mis-register produced by the method itself.

Also published German Patent Specification 34 00 652 Al discloses a sheet-processing press wherein three sheetguiding cylinders are disposed in, or substantially in, one plane along the path of sheet conveyance. To correct skewing of a print image, the central cylinder, called the correcting cylinder, is raised or lowered at one end. Conveyed sheets are therefore skewed between the first and third cylinders by twice the amount of cylinder end displacement. The first and third cylinders each cooperate with a printing unit, the first cylinder taking over the sheet from the first printing unit and the third cylinder transferring the sheet to the second printing unit with the required amount of skew.

A disadvantage of this press is the substantial mechanical outlay needed to skew the sheets. Each additional sheet transfer - and in the case under discssion, four such transfers instead of merely one are necessary impairs the efficiency of the press. Also, this publication is silent about the procedure to be followed in a press having more than two printing units.

q, 4 - The solution described is feasible only for two-colour presses.

It is clearly desirable to ensure that the position of all the print images of a multicolour press can be taken into consideration during skew registration even when such press has more than two printing units.

According to a first feature of the present invention, there is provided a method of correcting angular deviations in the printing of sub-images in a multicolour sheet-fed press by skewing the printed sheets before their passage through a printing unit and in the transfer is from one sheet-conveying drum to an adjacent sheetguiding cylinder by skewing such drum relatively to such cylinder, wherein, after their passage through each printer unit, the printed sheets are either skewed straight or are skewed by an amount derived from the skewed position of the sheet and the angular deviation of the sub-image in the next printing unit.

The invention also provides apparatus for effecting this method, in a multi-colour sheet-fed press having at least three printing units and, disposed therebetween, sheet- conveying drums having one end pivotable by means of a skewing device and adjusters for peripheral and lateral register on a plate cylinder of the printing units, the apparatus comprising interconnected control facilities for operating one adjusting motor each on skew register adjusters, a memory connected to the control facilities to record the position data of the particular printing plate or print image concerned, and a correcting unit interconnecting each of the register adjusters for peripheral and lateral register to the control facilities.

Such a method and apparatus enable register adjustment to occur in such a fashion that not only the effects of skew register adjustment on peripheral and lateral register, but also the position relatively to one another of the various sub-images in the consecutive printing units can be taken into consideration.

Very advantageously, after the relative position of the print images has been ascertained in the manner known from many published devices, sheet position during conveyance through the complete press is so adapted that all the print images are applied in the correct position.

It is not necessary to consider each individual print image on its own. Also, in the case of effects of the printing process on the paper which lead to partial variations in the relative position of print images, an individual correction can be made anywhere in the press without causing errors in subsequent printings. Also, skew register adjustment does not affect peripheral and lateral register.

The invention will be described in greater detail hereinafter with reference to a specific embodiment and by way of example, illustrated in the accompanying drawings in which:

Figure 1 shows a press in which the method according to the invention is used; Figure 2 is a diagram showing displacement of the sheet in the case of skewing on drums; Figure 3 shows diagrams showing the displacement associated with skew register adjustment in a multicolour press; Figure 4 is a block circuit diagram of the device, and Figure 5 shows a mechanical transmission.

A printing press, for example, a sheet-fed offset press, comprises, as shown in Fig. 1, a number of printing units each comprising a plate cylinder 1, a blanket cylinder 2 and an impression cylinder 3. The printing units are interconnected in each case by way of a sheetconveying drum 4 disposed between two impression cylinders 3. As a printed sheet passes through, the print images are transferred in accordance with their relative position on the plate cylinders 1 by way of the blanket cylinders 2 to the printed sheet present on the associated impression cylinder 3. To ensure correct register adjustment - i.e. in order to associate the print images with one another until they register accurately with one another register adjusters 5 for peripheral and lateral register are provided. Also, skew register adjustments can be provided on the drums 4 to ensure that the sheet is taken over and transferred skewed during its conveyance. To set up the press for continuous printing, the print images of the various printing units must be adapted to one another. For this purpose, in the initial state of the press, a test print is made, on which the position of the various print images relatively to one another can be ascertained. According to known methods, the position of the printing plates in the printing units can be ascertained and their relative position discovered by means of appropriate data processing facilities.

However, the effects of the paper in the printing process are not considered. When the data on the positions of the print images relatively to one another have been found, the printing plates or the plate cylinders 1 of the various printing units are given appropriate lateral and peripheral adjustment.

The angular position in which the various print images are disposed must also be known if it is to be possible to adjust the printed sheet appropriately for each printing unit by means of skew register adjusters 6 - 8.

This is effected for the first printing unit by known adjusters 9 on front marks of a sheet feed guide 10.

required new skewed posiition is ascertained for the subsequent printing units by reference to the actual position of the printed sheet, such position resulting from the skewing of the sheet-conveying drum 3 and of the print images on the plate cylinders 1. The necessary skewing for the measured position data of the printing plate are combined with the actual position of the sheet from the previous printing unit. In the skewing, adjustment is made by displacement of a drum 3, the fulcrum is disposed near the front edge of the printed sheet. There are, therefore, effects on the peripheral and lateral register of the printing. These influences are calculated from the geometry of the printing unit and output to correct peripheral and lateral register.

The interconnection of the facilities necessary for skew register adjustment will be described with reference to Figure 1. The skew register adjusters 6 - 8 for each printing unit are disposed on the shheet-conveying drums 4 and are connected to control facilities 11 - 13 respectively. Associated with the control facilities 11 - 13 is a memory 14 containing the position data formed from ascertaining the skewing of the various printing plates. The facilities 11 - 13 are interconnected and coupled with the memory 14 and with a memory 15, the latter determining the position of a front lay skew adjustment. Also, there are connections between a control facility 11 - 13 and one of the register adjusters 5.

Figure 2 shows how the skewing of the sheet on the sheetguiding cylinders or drums is effected. An axis T of the sheet-conveying drum 4 is normally disposed below a plane E interconnecting axes A of impression cylinders 3. 1, 3.2. For the sake of clarity, the sections through the impression cylinders 3.1, 3.2 and the drum 4 are shown in Figure 2 in a horizontal plane. A printed sheet B on the first impression cylinder 3.1 arrives at the transfer position to the drum 4. The normal'position of the latter at one end is shown in chain lines. The adjustment plane V is represented by a double arrow. When the axis T shifts down by an amount x into the position TV for skewed transfer, the takeover of the front edge of the sheet B is displaced to the rear by the amount x from the normal position on the drum 4. This is indicated by a horizontal chain-dotted transfer line L below the plane E. Consequently, as the drum 4 rotates, the sheet B is also conveyed onwards with a rearward displacement x. However, since after rotation through 180 the drum is offset from the normal transfer position on the other side too by the amount x, the amount of displacement on the adjustment side for the position on the second impression cylinder 3.2 is 2x. Hence for correcting a skewed position of, for example, 2x, the drum 4 needs to be shifted only by the amount x. Of course, these particulars all refer to displacement on just one side of the drum 4 and apply to corrections in both directions of the plane V in the sense that an upwards adjustment produces skewing correction on the adjustment side in the direction of sheet movement and a downwards adjustment produces skewing correction on the adjustment side against the direction of sheet movement.

is Figures 3a and 3b show, by way of example, an adjustment of a press having four printing units; the position of the sheet B to be supplied is shown in solid line, the position of an associated print image is shown in chain lines. The sheet B can be fed straight in the first printing unit since the print image P is unskewed in the first printing unit. It can be seen at the point I that the sheet has a skew position requiring a correction of 2x. To this end, the drum 4 must be shifted up by the amount x. Consequently, in the next printing unit, a matching correction 2x in the opposite direction must be made to the sheet, and the drum 4 is correspondingly adjusted by the amount x to restore the sheet to its correct position. However, since in the case indicated by the reference II, a correction of 2y in the other direction is necessary, the restoration has superimposed on it an additional adjustment by an amount y leading to the final adjustment Of the drun 4 by a total amount z = x + y. At position III, i.e. in the third printing unit, the sheet needs no displacement since the image is correct. However, that means that here only the adjustment of 2y referred to the previous printing unit must be cancelled. The drum 4 must therefore be raised by the amount y.

91 After this adjustment, all the print images are in register on the printing plate, taking into consideration a sheet displacement with the printing plate skewed or with the print image skewed. If, during continuous printing, the relative position of the images alters, it is readily possible to intervene in the printing process and make an on-line correction.

The control facility 11 of the skew register adjuster 6 is shown in detail in Figure 4. The facilities 12, 13 are of corresponding construction.

The facility 11 has an actual value input 16 and a set value input 17. The actual value input 16 is connected to a previous control facility or to a resetting facility, or, as in this case, to the memory 4 for skew adjustment on the front lays. The set value input 17 is connected to the store 14 for the position data of the print image at the particular printing unit concerned. Disposed after the set value input 17 is a converting unit 18 which ascertains a necessary skewing from the position data of the print image. These skewing data are transferred in the facility 11 to a summator 19 connected to the actual input value 16 of the control facility 11. The summator 19 forms the instructions necessary for the correcting value necessary for the printing unit. These instructions are evaluated correspondingly in a second converter unit 20 and transferred to the actual value output 21 whence these data are transferred both to the skew adjuster 6 of the first sheet-conveying drum 4 and to a correcting unit 22 for the register adjuster 5 and to the following control facility 12 of the next printing unit, where the data are given appropriate further processing.

9 Following on from the block schematic diagrams of Figure 1 and Figure 4, the adjuster can therefore take the form of an eccentric having an adjusting drive on the journal of a sheet-conveying drum 4, a motor control and, associated therewith, position value memories for set values and actual values of the register adjustment. The set values and actual values are coupled by evaluating electronics to convert and add up the adjusting values.

The complete arrangement is of relatively simple construction and can readily be devised and, if desired, retro-fitted to an existing press.

By way of illustration, Figure 5 shows a mechanical transmission enabling such a sequential control to be is effected. A first eccentric 24 and a second eccentric 25 disposed one in another are disposed on a journal 23 of each drum 4. The first eccentric 24 has a drive lever 26 with which a motor or manual drive 27 is coupled. The drive 27 is rigidly secured to the frame in the first printing unit but connected to an output lever 28 of the second eccentric 25 in the subsequent printing units.

The second eccentric 25 has a drive lever 30 whose drive 31 bears on the drive lever 26 of the first eccentric 24.

Adjustment of the first eccentric 24 causes adjustment of the second eccentric 25. Correction in accordance with the method of the invention can be effected by adjusting the drive 31 by way of the second eccentric 25.

Cancellation of the correcting value for the next printing unit is produced automatically by the opposite movement of the drive lever 26 and output lever 28 since the output lever 28 acts on the coupled drive lever 26 of the next printing unit to produce the same adjustment in the opposite direction.

Claims

1. A method of correcting angular deviations in the printing of subimages in a multicolour sheet-fed press by skewing the printed sheets before their passage through a printing unit and in the transfer from one sheet-conveying drum to an adjacent sheet-guiding cylinder by skewing such drum relatively to such cylinder, wherein, after their passage through each printer unit, the printed sheets are either skewed straight or are skewed by an amount derived from the skewed position of the sheet and the angular deviation of the sub-image in the next printing unit.

is

2. A method according to Claim 1 in which, for the correction, the sheet- conveying drum is skewed relatively to an impression cylinder, and wherein the position of the printing plates or print images relative to one another in relation to the rotational position is ascertained in the plane of their extent and, starting from a zero position, the position data are converted into adjusting values as correcting values for the skewing of the sheet-conveying drums from the new position of the printed sheet, such positioning being defined by a first correcting value, referred to a particular printing unit, the resulting skewing of the sheet-conveying drum in the immediately following printing unit is reascertained in the light of a second correcting value necessary in the latter unit, and from the correcting value known from the instantaneous printing unit is re-ascertained as restoring value and output as adjusting value for skewing the sheet-conveying drum to skew register adjusters and wherein the peripheral and lateral shifts resulting from the skewing are ascertained and output as correcting values to a register adjuster for peripheral and lateral register.

3. A method of correcting angular deviations in the printing of subimages in a multicolour sheet-fed press substantially as hereinbefore described with reference to the accompanying drawings.

4. Apparatus for carrying out a method according to any one of Claims 1 to 3 in a a multi-colour sheet-fed press having at least three printing units and, disposed therebetween, sheet-conveying drums having one end pivotable by means of a skewing device and adjusters for peripheral and lateral register on a plate cylinder of the printing units, the apparatus comprising interconnected control facilities for operating one adjusting motor each on skew register adjusters, a memory connected to the control facilities to record the position data of the particular printing plate or print image concerned, and a correcting unit interconnecting each of the register adjusters for peripheral and lateral register to the control facilities.

5. Apparatus according to Claim 4, wherein each control facility has a set-value input for position data, an actual value input for adjusting data of the previous printing unit and an actual value output for adjusting data of the particular printing unit concerned, a converting unit associated with the set value input, a summating unit associated with the output of the converting unit and with the actual value input, and a further converter unit connecting the output of the summating unit to the actual value output, the set value input being connected to the position data memory and the actual value input being connected to the actual value output of the previous control facility or to a zero resetting facility and the actual value output being connected to the actual value input of the control facility of the immediately following printing unit, to the skew register adjuster and to the correction unit of the register adjusters.

6. Apparatus for carrying out a method according to Claim 1 in a multicolour sheet-fed press having at least three printing cylinders and, disposed therebetween, sheet-conveying drums having one end pivotable by a skewing device and adjusters for peripheral and lateral register on a plate cylinder, wherein each skewing device is a mechanical power division transmission, the input, thereof controls a first skewing element for pivoting the sheet-conveying drum, a second skewing element for further pivoting of such drum is disposed in supported relationship in such transmission on the first skewing element, and an output for producing a drive motion for the first skewing element of the subsequent printing unit is present if required on the second skewing element.

7. Apparatus for correcting skewed print images on a multicolour sheet-fed printing press substantially as hereinbefore described with reference to the accompanying drawings.

8. A sheet-fed multicolour printing press including apparatus according to any one of Claims 4 to 7.

Published 1991 at The =tinoff House. Cardiff Road. Newport. Gwent NP9 IRH. Further copies may be obtained fmm Sales Branch. Unit 6. Nine Mile 0 t. h. Cross Keys. Newport. NP1 7HZ. PnntLd bY Multiplex techniques ltd. St Mary Cray. Kent.