GB2034247A - Improvements in or Relating to Register Control - Google Patents

Abstract

A marking apparatus, for ensuring register of a marking being applied to a pre-marked web includes marking station 1 for applying the marking with nip pressure, first and second tension sources 13, 17 a tension, accumulator (dancer arm assembly) 2 for controlling one of said tension sources, register control means 9 for providing a correction signal to correct for detected register errors, and means for varying the rate of movement of web material through the nip in response to the correction signals. This last means may be dancer arm assembly 2 or means for adjusting the nip pressure. Each tension source is capable of maintaining the tension it applies unaffected by any change at the other tension source, and the tension sources are capable of maintaining a substantially constant ratio of in-feed to out-feed tension across the nip. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Register Control The present invention relates to register control.

Register control is important in printing, embossing and like marking operations during which it is necessary to ensure that a subsequent marking being applied to a pre-marked web is applied at the correct position, i.e. in register, with markings on the pre-marked web.

For example, in the printing of a web material with a multi-coloured pattern, the pattern is formed by using a plurality of printing cylinders each of which prints a pattern of one colour of ink onto the web material in a manner such that the overall desired multi-coloured pattern is successively built up. Such a multi-coloured pattern may be formed by a single pass of the web through a multi-station printing machine or alternatively by a plurality of passes through a single stage machine with the printing cylinder being changed after each pass.

In such printing operations it is essential to ensure that the second and subsequent cylinders each prints their pattern at exactly the right position on the web otherwise the pattern may take on a blurred appearance as a result of colours being out of position i.e. out of register.

It is in fact necessary to effect continuous control of the printing process to ensure that the correct register is achieved. This is due to the fact that there will generally be continuous changes in behaviour of the material at the printing cylinder nip due to such factors as creep, bodily slip, variation in nip pressure at the printing cylinders, and/or length changes caused by variations in material consistency and by physical factors (e.g.

temperature, humidity). Such length changes are in fact quite small but are sufficient to displace the register of the printed pattern.

Register control is generally achieved by sensing the register error and effecting appropriate corrective action. In the prior art such corrective action has involved altering the overall length of a web between two nips either by means of a linearly movable roller around which the web passes and which is positively moved to effect the length changes, or by varying the speed of a printing cylinder to adjust tension in a manner which compensates for the length change.

Alternatively it has been known to sense tension changes in the web, which changes give rise to length changes, and to effect appropriate corrective action in one or both of the above described ways.

However, such methods are difficult to operate to the required degree of accuracy and it is an object of the present invention to provide a method and apparatus which allow improved register control.

According to a first aspect of the present invention there is provided a marking apparatus for applying a marking, referred to as a subsequent marking, in register on a pre-marked web of material, the apparatus comprising a marking station at which the subsequent marking is applied with nip pressure, first and second tension sources respectively upstream and downstream of the marking station, each tension source being capable of maintaining the tension it applies unaffected by any change at the other tension source and said tension sources being capable of maintaining a substantially constant ratio of in-feed to out-feed tension source across the nip, a tension accumulator for controlling one of said tension sources, register control means for providing a correction signal to correct detected register errors, and means for varying the rate of movement of web material through the nip in response to correction signals provided by the register control means.

According to a second aspect of the present invention there is provided method of controlling the register of a marking, referred to as a subsequent marking being applied at a nip of a marking station to a pre-marked web of material, the method comprising passing the pre-marked web from a first tension source upstream of the nip to a second tension source downstream of the nip which tension sources remain unaffected by each other and which maintain a substantially constant ratio of in-feed to out-feed tension source across the nip throughout the marking operation, using a tension accumulator to control one of said tension sources sensing the register of the subsequent marking, and correcting any register error by varying the rate of movement of the wet material through the nip.

The present invention is particularly applicable to register control in printing, embossing and like operations.

The present invention has arisen from a consideration by the inventor of the behaviour of a web of material at and in the region of a printing nip in a rotary printing process. The inventor has established that the most important factors in controlling register at a printing nip are the ratio of the in-feed tension at the nip to the out-feed tension from the nip, and the nip pressure. If this tension ratio and pressure are constant then the printing cylinder will accurately repeat the print length it lays on the web. Any variation of this tension ratio will cause relative elongation or relaxation of the web (due to web elasticity) on the in-feed side with respect to the out-feed side which will affect the print. For example, a relative increase of in-feed tension as compared to outfeed tension will cause elongation of the web on the in-feed side of the nip.This will have a twofold effect on the print which, in relation to the invention, will be most easily understood by considering a pre-printed web being fed to the printing cylinder under consideration. Firstly, assuming the pre-printed web has a reference mark and that the second cylinder also has a reference mark which it will print onto the web, then elongation of the web in-feed will mean that the register mark on the pre-printed web will be pulled slightly upstream of the nip and the printing cylinder will thus print its reference mark in advance of the position that it would have been printed had there been no tension change.

Secondly, once the web has been relatively relaxed on the out-feed side of the nip it will have a shorter print length of pattern printed by the second cylinder as compared to when the in-feed tension was at the lower value.

Consequently, an increase of in-feed tension relative to out-feed tension will cause the printing cylinder to produce, for the relaxed web, a shorter print length, and this print will be advanced as compared to a pre-printed reference mark on the web. A decrease in in-feed tension relative to outfeed tension will give the opposite results.

It should at this point be noted that a variation of the tension ratio causes creep of the material at the nip in the direction of the side having the nip at which the tension has relatively increased in value. It is this creep which contributes to the variation in the print laid down by the second cylinder. Consequently, instead of increasing infeed tension to give an advanced print and shorter print length for the printing cylinder, it is possible to lower the out-feed tension. However, the transmission of a change in out-feed tension across the nip is to a certain extent blocked by the nip pressure and a large decrease in out-feed tension would be required to give the same effect as a slight increase in in-feed tension (which is not as dependent on nip pressure).

The movement of the web across the nip is dependent on nip pressure and the tension ratio across the nip. If the tension ratio is kept constant then the nip pressure will determine the rate at which material moves through the nip. The nip now only effects how quickly the web will move for change in the tension ratio and the nip pressure is directly attributed to how quickly the marks will move relative to one another, i.e.

response time.

From the foregoing description, it will be appreciated that register control in accordance with the invention may be effected by maintaining a constant nip pressure dependent on the material being marked and by initially maintaining a substantially constant ratio of in-feed tension to out-feed tension. Small register errors caused by the factors discussed above may be corrected merely by changing the in-feed tension, preferably directly rather than indirectly by a change in outfeed tension, to change the position and length of the print laid down by the cylinder.

If the constant tension ratio is 1:1 then the print length would be the same at any nip pressure, providing that the pressure does not cause permanent deformation of the web, i.e.

calendering. Consequently therefore, the method of the invention would ideally be operated with a.

1:1 ratio of in-feed to out-feed tension, with a constant nip pressure.

It will be seen from the foregoing description therefore, the ratio of in-feed and out-feed -tensions must be kept substantially constant, otherwise than when changed for register control, since any unwanted variation in the ratio will give an unwanted change in print length. It is therefore necessary to maintain accurate tension control, and in particular to ensure that a change in the tension at one side of the nip does not affect the tension at the other side of the nip. This is achieved by ensuring that each tension source maintains the tension it applied irrespective of any change at the other source. In this way, fluctuations of the tension ratio caused by tension transmission across the nip are prevented. The tension source could be for example a roll of web material, or feed rollers for feeding the web to or from the nip.

A change in the in-feed tension to correct a register error will of course change the ratio of infeed tension to out-feed tension. However, the changes which occurs in normal operation of the method will be very slight and consequently the ratio can be said to be maintained substantally constant.

In carrying out this embodiment of the invention, in which nip pressure is maintained constant, there are preferably two tension accumulators each of which controls a tension source. One accumulator will be such that, in response to correction signals provided by the register control means, it will vary the tension it applies so as to produce a small change in the rate of movement of the web through the nip to give register control. Preferably the tension accumulators are dancer rollers and preferably are such that the web passes over the accumulator in S-shaped fashion. Preferably also, it is the upstream accumulator which is adjusted for register control, the out-feed tension being maintained constant by a standard construction of tension accumulator downstream of the marking station.

The tension control means which effects the adjustment should be capable of responding rapidly to detected register errors to rapidly bring the print back to register.

As pointed out above, the tension accumulator is preferably a pivotal dancer arm assembly to which a variable force may be applied to effect the desired tension changes. Such a dancer arm assembly acts as a tension accumulator to maintain tension at a constant value until that value is changed to a new value by the variable force. The force may be applied mechanically, electricaily or pneumatically. Preferably however the dancer assembly has a rack and pinion unit on which a weight is mounted, and the dancer further has a motor mounted thereon for rotating said pinion.

An alternative way of effecting register control in accordance with the invention, is to maintain the' ratio of in-feed to out-feed tension at a constant imbalance, i.e. the ratio is a constant value other than 1:1. The nip pressure may then be varied by any suitable means in response to detected register errors, to change the rate at which material moves through the nip, thereby controlling register. In this case, the tension sources will be such that they maintain constant values for the in-feed and out-feed tensions.

Preferably tension accumulators will be provided between each tension source and the nip and it is these accumulators which will maintain constant tension at the respective source. The constant tension accumulators may be conventional dancer arm assemblies.

The means for detecting register error may in the case of a rotary printing process be a mark to mark or cylinder to mark registration system.

The invention is particularly applicable to rotary gravure printing in a single stage press.

The invention will be further described by way of example only with reference to the accompanying drawings, in which: Fig. 1 diagrammatically illustrates a single stage printing apparatus in accordance with the invention and showing also a web of material to be printed being fed to the machine; and Fig. 2 shows an exploded perspective view of a dancer arm assembly for use as a tension control device in the apparatus shown in Fig. 1.

Referring to Fig. 1 a single stage printing apparatus includes a rotary printing station 1, a dancer arm assembly 2 (one example of which is described more fully later) upstream of the station 1 and including a motor 3 for controlling a tensioning force applied to the dancer arm assembly 2 by a weight 4, and a register control system 5 which receives signals from photohead detectors 6, 7 and 8 and which feeds register error signals to a controller 9 for the motor 3. The apparatus also includes a manual control 10 into which an operator may pre-set a desired tensioning force to be applied to the dancer arm assembly 2 and there is a weight position feed back unit 11 the function of which is described more fully later.

As shown in Fig. 1 the apparatus is, in use, supplied in the direction of arrow A with a web 12 of material to be printed paid off from a reel 13 rotated by a motor 14. The reel 13 acts as a tension source. A dancer arm position sensor 1 5 is provided and feeds a signals corresponding to the position of the dancer arm assembly 2 to a control 1 6 for the motor 14, for the purpose to be described. Finally downstream of the photoheads 7 and 8 is a wind-on reel 17 (which acts as a tension source) and there is a conventional dancer arm 1 8 which controls this tension source in a manner akin to the control of the reel 13 by the dancer assembly 2.In the illustrated embodiment the tension control device is shown as a conventional dancer arm with a weight 1 8a but any other known means of controlling the tension source could be used. The tension applied by the tension control 1 8 is set by the control 10 by means not shown.

In place of the dancers 2 and 1 8 effecting control of the reels 13 and 1 7 respectively, it would be possible to have feed rollers acting as tension sources upstream of the nip and these rollers would be controlled by the dancers 2 and 18.

The printing station 1 is of conventional construction and comprises a driven printing cylinder 19 and a back up cylinder 20. The nip pressure between cylinders 1 9 and 20 is maintained constant by a standard nip pressure applicator 20a. The cylinder 1 9 is provided with a register mark 21 which is detected on each rotation of the cylinder 1 9 by the photohead 6 which acts as a phase detector. The photoheads 6, 7 and 8 together form a 'mark to mark' detection arrangement for register control.

The manner in which the illustrated printing apparatus is operated to achieve register control during printing will not be described.

A reel 13 of a blank web to be printed is provided as shown in Fig. 1 at the upstream end of the apparatus and the web 12 is fed successively around the dancer arm assembly 2, through the printing station 1, through the second tension control 1 8 and finally onto the wind-on reel 13.

For the first pass of the web 12 through the apparatus, the printing station 1 will be provided with an appropriate cylinder 19, for printing a single colour in a repeating pattern on the web. In the following description this printing cylinder used for printing the first colour is designated by reference numeral 19'. Additionally, the nip pressure applied by the applicator 20a is set at a constant value depending on the material to be printed, this value being one which will give the required response time for the dancer assembly 2 during its operation as a register control in the manner described later.

Obviously during the first pass of the web 12 through the apparatus no register control will be effected since register control is only required for the application of the second and subsequent colours. Consequently, during the first pass the photoheads 6, 7 and 8 and the register control system 5 will remain inoperative.

The first printing cylinder 19' will however have a register mark 21' (e.g. an I-bar) for printing a corresponding register mark onto the moving web. Such a register mark 21' will be provided on the cylinder 1 9 at a known circumferential distance from a reference mark (not shown) at a given position on that cylinder.

For register control during the printing of the second and subsequent colours, it is desired that the print repeat length of the first colour (i.e., the distance between successive register marks 21' printed on the web 12 by the first printing cylinder 19') should be fairly constant, but an exact repeat length is not required since the register control effected during subsequent printing will automatically provide for register, even with minor variations in first print length.

A substantially constant first print repeat length may be obtained by ensuring a constant ratio of in-feed to out-feed tension at the nip between the printing cylinder 1 9, and back up cylinder 20, as discussed more fully above. This constant ratio is achieved by operation of the dancer assembly 2 and tension control device 1 8 in the manner to be described.

The repeat length of the first print is, as pointed out above, dependent on the in-feed to out-feed tension ratio.

Prior to the first printing operation therefore, the operator will set a predetermined in-feed tension into the control 10 which will cause the motor 3 to rotate to move the weight 4 until the weight position feed back unit 11 indicates that the weight 4 is in the correct position for that tension. Simultaneously, a signal from the control 10 will set the tensioning force for the tension control device 1 8 at a value to give the desired in feed to out-feed tension ratio. This ratio will generally be 1:1 but other ratios could be used.

The print run may then be started, and after a few seconds the web 12 will settle down to a stable condition and will run at the pre-set in-feed and out-feed tensions.

The web will continue to run with the pre-set in-feed tension since any tendency for tension change, e.g. caused by extension of the web or variation in rotational speed of the printing cylinder 19' is automatically compensated for by pivotal movement of the dancer arm assembly 2 about its horizontal axis of rotation i.e. the dancer arm assembly 2 acts as a tension accumulator.

Any movement of the dancer arm assembly 2 about its horizontal pivot axis will be sensed by the sensor 1 5 which feeds a signal to the control 16. The control 15 will, in known manner, vary the speed of the motor 14 in a manner such that the web 12 is paid off the reel at a rate which allows the dancer assembly 2 to move back to a vertical position whilst still maintaining the constant in-feed tension.

Whilst the dancer assembly 2 is maintaining a substantially constant in-feed tension, the second tension control device 18 retains a constant out feed tension. There is thus produced a web having a substantially constant first print repeat length.

In preparation for printing the second colour, the web is rewound from the wind-on reel 1 7 to the feed reel 13. Additionally, the first printing cylinder 19' will be exchanged for a second printing cylinder 19" which will print the second colour onto the web 12. The printing cylinder will have an identically positioned reference (not shown) to that on the first cylinder 19', and will have a register mark, e.g. an I-bar, 21" which is circumferentially spaced from the reference mark by a known distance which is greater than the distance between the register mark 21' and reference mark of the first printing cylinder.

The two photoheads 7 and 8 are now spaced by a linear distance corresponding to the expected spacing between the register mark 21' printed on the web by the first cylinder 1 9' and the register mark 21 n on the web by the second cylinder 19". If, for example, the register mark 21' on the cylinder 19' was spaced from the reference mark on that cylinder by 10 mm and the corresponding spacing of the two marks on the second cylinder is 30 mm, the photoheads 7 and 8 will be spaced from each other by a distance of 20 mm. since this is the exact spacing of the printed register marks required for a registering print.

Register of the second print with the first print will of course be best achieved if the same ratio of in-feed to out-feed tension is used. This is most conveniently achieved by having the control 10 of such construction that, once the tensions have initially been pre-set, they may be re-established simply by pressing a button on the control 10. The same nip pressure as used for the first print will of course be used for the second print. Additionally register is facilitated by aligning a reference mark on the second cylinder 1 9" with a mark on the pre-printed web. By adopting these tension and setting procedures the second print run maybe started and register is achieved with minimum of web wastage.

During the second print run, the photohead 6 (i.e. the phase detector) detects the registration mark 21" on each revolution of the second printing cylinder 19" and, through the register control system 5, instructs the photohead 7 to look for the register mark printed by the first printing cylinder 19' and instructs the photohead 8 to look for the register mark printed by the second printing cylinder 1 9". If the second colour is being printed exactly in register with the first, then the photoheads 7 and 8 will read the respective register marks at exactly the same time, and there will be no need for any register correction.If however the second print is out of register with the first, then the register marks as printed by the first and second cylinders will be read by the photoheads 7 and 8 at different times, and this time difference is a measure of the extent to which the print is out of register. The register control 5 processes any time difference in the detection of the printed register marks and emits a correction signal representative of the magnitude of the register error to be corrected.

This correction signal instructs the motor controller 9 to operate the motor 3 so that the weight 4 is moved to the position required to give a change of in-feed tension necessary to bring the print back to register. The changes caused by a change of in-feed tension have been discussed more fully above.

With the first and second register marks being continuously monitored as described above, the weight 4 maybe continually moved so as to vary the in-feed tension and ensure that a correctly registering print is obtained.

Any movement of the weight 4 in the manner described above may of course cause the dancer assembly 2 to move out of its vertical orientation.

This movement will be sensed by the sensor 1 5 which will instruct the controller 1 6 to vary the speed of the motor 14 so that the web 12 is paid off the reel at a rate which allows the dancer arm assembly 2 to return to a vertical position with the new in-feed tension.

It should also be noted that the response time of the dancer arm assembly 2 to any required tension change is extremely rapid since the weight 4 is moved as soon as the motor 3 is rotated. The correction for small errors in register displacement is therefore extremely rapid, and this is an important advantage of the present invention over the prior art. In fact, due to the rapid and accurate register control which the apparatus provides, it is not necessary for the repeat print length of the first printed colour to be extremely accurate along the length of the web, since any minor variations automatically cause the in-feed tension to be adjusted to ensure that the second colour is printed in register with the first. A certain amount of latitude in the print length of the first colour is therefore permissible.

The change of the in-feed tension will produce a slight effect in the rate at which the web issues from the printing station 1. The tension control device 18 will automatically ensure that a constant out-feed tension is maintained with this new rate. It will of course be seen therefore that variation of in-feed tension produces a change in the ratio of in-feed to out-feed tension, but the change required for the correction of register errors is small so that the ratio may be considered to remain substantially constant.

It will of course be appreciated that during the printing of the second colour, any variation in the rotation speed cylinder or elongation of the web will be automatically compensed by movement of the dancer assembly 2 about its horizontal axis with consequent adjustment of the pay off rate of the web 12 from the reel 13, as described above for the first printing operation.

To print a third colour, the web is again rewound from the reel 1 7 onto the reel 1 3. The second printing cylinder 19" is replaced by a third cylinder 19"' for printing the third colour. This third cylinder 19"' has a third register mark 21"', e.g. on I-bar, and a reference mark (not shown) positioned identically to the reference mark on the first cylinder 19'. The register mark 21"' is circumferentially spaced from the reference mark by a known distance which is greater than the spacing between the register and reference marks on the second cylinder 19".

The method of printing the third colour is exactly the same as printing the second colour save that the photoheads 7 and 8 are spaced by a distance corresponding to the expected distance between the register mark printed by the first printing cylinder 19' and that printed by the third printing cylinder 19"'.

Similarly, fourth and subsequent colours can be printed by the above described procedure.

A modification (not shown) of the above described apparatus may be used for effecting register control by varying the nip pressure control whilst keeping a constant imbalance of in feed to out-feed tension.

In this modification, the register control system 5 feeds an appropriate correction signal to a means for varying the nip pressure applied by the nip pressure applicator 20a, and there is no connection between the register control system 5 and the motor controller 9. Otherwise the modified apparatus is substantially the same as that illustrated in Fig. 1.

It will be appreciated that in this modified apparatus the values of in-feed and out-feed tension will be set at the required imbalance (which will depend on the material being printed) prior to the beginning of a print run, and these values will be maintained throughout the run since the in-feed tension will not be varied by the register control system 5.

Any register error sensed by the photoheads will cause the pressure applied by the applicator 20a to vary, in accordance with a signal from the register control system 5, so as to change the rate of movement of the web through the nip and thereby effect register control. It is of course possible to control register error by a combination of variation of nip pressure and varying the tension applied by a dancer.

The invention is also applicable to multi-stage printing presses. In such a multi-stage press, there will be between each printing station a dancer assembly such as that represented by the reference numeral 2 above. The dancer will.

control a tension source, which will be a nip at an adjacent printing cylinder, in a manner similar to that shown above for the control of the reel 1 3 by the dancer 2, only in a multi-stage press it will be the speed of the printing cylinder which is changed. Additionally, assuming that nip pressure is maintained constant, the tensioning force applied by the dancer assembly will be varied to correct for register error.

A construction of dancer arm assembly 2 suitable for use in either the above described single or multi-stage press is shown in Fig. 2.

The assembly 2 comprises a pair of horizontal parallel rollers 22 and 23 which are mounted between and supported by, two roller supports 24 and 25, a motor 26 (equivalent to the motor 3 of Fig. 1), and a weight assembly 27 mounted between the roller supports 24 and 25.

The roller supports 24 and 25 are mounted on a frame (not shown) of the printing apparatus to provide for pivotal movement thereof about a horizontal axis midway between said rollers 24 and 25.

Mounted on the inside surface of each roller support 24 and 25 is a respective plate 28 or 29 which is provided with four peripherally Vgrooved rollers 30 (only one shown) located one at each corner of the plate 28 or 29. Additionally the plate 28 is fast on a hollow shaft 31 which is rotatable with respect to the roller support 24.

Associated with the plate 28 is a rack unit 32 which comprises upper and lower parallel racks 33 and 34, respectively mounted on a plate 35 having upper and lower pointed edges for location in the grooves of the rollers 30 to enable the plate 35 to be guided between the upper and lower pairs of said rollers 30. A similar rack unit 32a is mounted on the plate 29 save that the lower rack 34 is omitted for reasons which will be apparent.

A weight bar 36 (corresponding to the weight 4 of Fig. 1) is mounted between the plates 28 as shown and additionally between said plates there is a shaft 37 with terminal pinions 38 engaging the racks 33. Additionally, a connecting bar 39 is secured as shown between the rack units 32 and 32a.

The motor 26 has an output shaft 26a which is connected, via a slipping clutch 40 to a shaft 41 which passes through the hollow shaft 31 and which is provided with a pinion 42 engaging the lower rack 34 of the rack unit 31 associated with the roller support 24.

The hollow shaft 31 is secured to a flange 43 rotatable about the shaft 41. The flange 43 is secured to a further flange 44 on which the motor is mounted.

In use of the dancer arm assembly 2, a web of material will be passed around the rollers 22 and 23 in the manner illustrated in Fig. 1. The tension in the web will be dependent on the position of the weight bar 36 with respect to the rollers 22 and 23. The position of the weight bar 36 may be adjusted by energising the motor 26. When the motor 26 is rotated in response to a signal from a control system such as that shown in Fig. 1, the rotation is transmitted through the slipping clutch 40 and shaft 41 to pinion 42. This causes rack unit 32 to move and thus motion is also transmitted to rack unit 32a through the shaft 37 and connecting bar 39. The weight 26 is thus moved to alter the tensioning force applied by the dancer assembly 2.

In addition, any pivotal movement of the roller supports 24 and 25 is transmitted via the hollow shaft 31 and flanges 43 and 44 to the motor 26 which is correspondingly pivotally moved.

Consequently the dancer assembly 2 operates as an integral unit, and energisation of the motor 26 will immediately cause the position of the weight bar 36 to be moved.

The described dancer assembly provides a rapid response time which is highly desirable for effecting register control in accordance with the invention.

Claims (25)

Claims

1. A marking apparatus for applying a marking, referred to as a subsequent marking, in register on a pre-marked web of material, the apparatus comprising a marking station at which the subsequent marking is applied with nip pressure first and second tension sources respectively upstream and downstream of the marking station, each tension source being capable of maintaining the tension it applied unaffected by any change at the other tension source and said tension sources being capable of maintaining a substantially constant ratio of in-feed to out-feed tension source across the nip, a tension accumulator for controlling one of said tension sources, register control means for providing a correction signal to correct detected register errors, and means for varying the rate of movement of web material through the nip in response to correction signals provided by the register control means.

2. Apparatus as claimed in Claim 1 wherein the means for varying the rate of movement of the web material through the nip is provided by a tension accumulator, referred to as an adjustable tension accumulator, being adapted to vary the in-feed tension in response to correction signals provided by the register control means

3. Apparatus as claimed in Claim 2 wherein the adjustable tension accumulator is provided upstream of the marking station.

4. Apparatus as claimed in Claim 2 or 3 wherein the adjustable tension accumulator is the said tension accumulator which controls the tension source.

5. Apparatus as claimed in any one of Claims 2 to 4 wherein the adjustable tension accumulator is a dancer arm assembly.

6. Apparatus as claimed in Claim 5 wherein said dancer arm assembly is provided with a weight movable with respect to the rollers in response to register correction signals to vary the tension applied by the dancer arm assembly.

7. Apparatus as claimed in Claim 6 wherein said dancer arm assembly includes a motor which moves said weight by a rack and pinion.

8. Apparatus as claimed in any one of Claims 5 to 7 wherein the dancer arm has two radially spaced rollers over which the web will pass in an S-shape.

9. Apparatus as claimed in Claim 1 wherein the means for varying the rate of movement of the web through the nip comprises means for varying the nip pressure in response to correction signals from the register control means.

10. Apparatus as claimed in Claim 9 wherein the tension accumulator is a dancer arm assembly.

11. Apparatus as claimed in Claim 9 or 10 wherein said tension accumulator is provided upstream of the marking station and a further tension accumulator is provided downstream of said station.

12. Apparatus as claimed in any one of Claims 1 to 11 in which the tension source is controlled by the position of the tension accumulator.

13. Apparatus as claimed in any one of Claims 1 to 12 wherein the tension source is provided by a reel of web material.

14. Apparatus as claimed in any one of Claims 1 to 12 wherein the tension source is provided by feed rollers.

1 5. Apparatus as claimed in any one of Claims 1 to 14 provided with means for automatically setting the in-feed tension and out-feed tensions for the marking station prior to a marking operation and for re-establishing the pre-set tensions prior to subsequent marking operations.

1 6. Apparatus as claimed in any one of Claims 1 to 1 5 wherein detection of register error is by a mark to mark registration system.

1 7. Apparatus as claimed in any one of Claims 1 to 1 6 which is a rotary printing press.

1 8. Apparatus as claimed in Claim 17 which is for gravure printing.

1 9. Apparatus as claimed in Claim 1 6 or 1 7 which is a single stage press.

20. Method of controlling the register of a marking referred to as a subsequent marking being applied at a nip of a marking station to a pre-marked web of material, the method comprising passing the pre-marked web, from a first tension source upstream of the nip to a second tension source downstream of the nip which tension sources remain unaffected by each other and which maintain a substantially constant ratio of in-feed to out-feed tension source across the nip throughout the marking operation, using a tension accumulator to control one of said tension sources sensing the register of the subsequent marking, and correcting any register error by varying the rate of movement of the web material through the nip. ~~~~~~~~~~~~~~~~

21. Method as claimed in Claim 20 wherein the nip pressure is maintained constant and the rate of movement of the web material is varied by varying the in-feed tension.

22. Method as claimed in Claim 20 wherein the in-feed and out-feed tensions are maintained at a constant imbalance and the rate of movement of the web material is varied by varying the nip pressure.

23. Method as claimed in any one of Claims 20 to 22 wherein the pre-marking of the web has been effected by passing a blank web through the marking station with constant nip pressure and a constant ratio of in-feed to out-feed tension.

24. A single stage printing apparatus substantially as hereinbefore described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

25. A method of controlling the register of a marking substantially as hereinbefore described.