GB2182889A - Proofing presses - Google Patents

Abstract

A proof-press for web material comprises:- (a) a printing station 3 at which a printing nip may be established (b) pay-off roll means 1 upstream of the printing station and associated with adjustable tension control means (e.g. a torque control motor) for maintaining a substantially constant web tension between the pay-off roll and the nip; and (c) take-up roll means 5 downstream of the printing station and associated with tensioning means for maintaining web tension between the nip and the take-up means. Register control for the second and subsequent print runs is achieved by measuring the print repeat length on the web, measuring the circumference of the printing cylinder to be used, and then adjusting the tension control means associated with pay-off roll 1 so that the web 2 is stretched or relaxed to ensure in-register printing. As described, the tension between the pay-off roll and the nip may be of the same order as that between the nip and the take-up roll,whereby transport of the web is effected by driving at the nip. <IMAGE>

Description

SPECIFICATION Proof press The present invention relates to a proof press, i.e. a press used primarily for printing comparatively short lengths of material (usually paper) for sampling or approval priorto undertaking a much longer production run on a conventional prints.

A proof press generally comprises a single printing station atwhich the same sample of material is printed several times (using different printing cylinders and different coloured inks each time) to produce the final print.

The conventional proof-press is a so-called drum type proof press which comprises a comparatively large diameter rotatable drum and a rotatable printing cylinder. The paper to be printed extends circumferentially around the drum and its ends are secured thereon. The printing cylinder is axially parallel to the drum and is juxtaposed thereto so that it contacts the paper. To printthe sample the drum and the printing cylinder are rotated about their respective axes whereby a first print is laid on the paper. The operation is then repeated using a different printing cylinder and different coloured ink to lay the second print on the paper. This process is repeated as many times as required to build up the final print.

The drum type proof press does however suffer from a number of disadvantages. Firstly, the length of paperwhich can be printed is limited bythe distance between the points at which the paper ends are secured on the drum (this length generally being a little less than the circumference of the drum) whereas with some prints (e.g. for wallpapers) it may be desirable to have much longer lengths so that the full effect to the print may be appreciated. Secondly the comparatively large diameter ofthe drum relative to the printing cylinder meansth at there is a comparatively large area of contact between the two and this may give rise to a reduction of print definition due to slightly less ink being drawn from the cells ofthe engraved roller.

Thirdly, register control between successive prints is generally not particularly good because no account is taken ofthefactthatthe different print cylinders will probably have slightly different circumferences (and therefore a slightly different print repeat length) and also that the paper may expand or contract after being printed with wet ink.

It is an object of the present invention to obviate or mitigatetheabovementioned disadvantages.

According to a first aspect of the present invention there is provided a proof-press comprising: (a) a printing station at which a printing nip may be established (b) pay-off roll means upstream ofthe printing station and associated with adjustable tension control means for maintaining a substantially constant web tension between the pay-off roll and the nip; and (c) take-up roll means downstream ofthe printing station and associated with tensioning means for maintaining web tension between the nip and the take-up means.

According to a second aspect of the present invention there is provided a method of printing a sample of material comprising: (a) feeding the sample from a pay-off roll to a take-up roll via a printing nip at which a print is applied with a printing cylinder, said material being maintained at a substantially constant, predetermined tension between the pay-off roll and the nip, and being maintained in tension between the nip andthetake-up roll; (b) determining the print repeat length on the sample; (c) determining the print repeat length of a further printing cylinderto be used in the method; (d) determining from (b) and (c) the difference (if any) between the two print lengths;; (e) preparing for a further printing operation using said further cylinder by adjusting (if necessary) thetension between the pay-off roll and the nip to compensatethe print repeat length onthesamplefor any difference detected in (d), and adjusting the position of said further cyl inder and/or the sample so that printing may begin in register; (f) effecting said further printing operation by the method of step (a); and (g) optionally repeating steps (b)-(f) for any further printing cylinders.

The invention will be further described byway of example only with reference to the accompanying drawings which illustrates one embodiment of proof-press in accordance with the invention.

The illustrated proof-press comprises a pay-off roll 1 (on which paper 2 to be printed is wound), a printing station 3 and a take-up roll 4. The printing station 3 itself comprises a back-up cylinder 5, a printing cylinder 6, cylinder register detection means 7, and an elongate doctor-blade assembly 8. Printing cylinder 6 is moveable towards back-up cylinder to establish a printing nip and is moveable away there from forthe purpose of changing the printing cylinder.

The press is completed by paper register detection means 9, a heater 10, and idler rolls 11 and 12.

Pay-off roll 1 is associated with adjustable tension control means (e.g. a torque motor) to ensurethata constant, predetermined tension may be imported to the paper 2 between the roll 1 and the printing nip.

Take-up roll 2 is associated with tensioning means ensuring that the paper 2 is maintained in tension between between the nip and the take-up roll.

Additionally, the tensioning means associated with take-up roll 2 is such that, when the press is in the condition illustrated in the drawing (i.e. printing cylinder 6 is away from back-up cylinder 5), the system is in equilibrium for positioning the web.

Tension for register will be imported to the web when the printcylinderand back-up cylinder establish a printing nip when the print cylinder drive will take overthe transport of the web.

In operation ofthe press, the cylinder 5 and the first printing cylinder 6 are moved toward each other to establish a printing nip.Awedge shaped space is thus defined between cylinder 6 and the doctor blade 7. The ends of this space may be closed by any suitable means (e.g. cotton wool) and the required ink may be poured into the reservoir thus defined.

The paper 2 is printed in a first pass through the press using the first cylinder 6, and the print is dried by heater 10. Inkforthe printing operation is fed from the reservoir which will hold sufficient inkforthe printing of the web in its pass through the apparatus.

During the first pass, the paper2 is maintained ata convenient, substantially constanttension between pay-offroll 1 and the printing nip. This tension may, forexample, be 50% ofthatwhich may betaken up by the paper.

Afterthisfirst printing operation the reservoir may be drained (and cleaned). Cylinder 6 is moved away from back-up cylinder 5 and replaced by a further printing cylinder, herein designated 6'. Additionally paper 2 is rewound from take-up roll 4 onto pay-off roll 1.

Although printing cylinders 6 and 6' will notionally be the same diameter, there will nevertheless probably be differences in manufacturing tolerances which will meanthat,foragiventension in paper2 and speed of pass through the printing nip, the print lengths laid down by cylinders 6 and 6' will be slightly different. Additionally, the application ofthe wet ink atthe printing nip and the subsequent drying ofthe paper at the heater 10may also effectthe repeat length ofthe pring laid down by cylinder 6.

Asthe nextstep,the circumference of print cylinder 6' is determined. This is then compared with the print repeat length on web 2 (measured atthe same tension as used in the previous print run between roll 1 and the nip. This print repeat length (which will be the distance between register marks printed by cylinder 6) may simply be measured with a ruler although other measuring means may be used. Any difference between these two measurements is compensated by adjusting the tension applied by the adjustable tension control means associated with roll 1 so that paper 2 extends or relaxes (as the case may be) to eliminate the difference.

Finally, cylinder 6' is rotated so that a register mark thereon (not shown) is at a pre-determined position (as detected by the optical detection means 7) and the paper is positioned such that a registered mark printed thereof by the previous cylinder 6 is also at a predetermined position, as determined by the paper register detection means 9.

The second printing operation may now commence, and will probably use a different coloured inkfrom the first print run. The positioning ofthe register marks ensures thatthe printing is commenced in register, and the adjustments (if any) which have been madeto web tension between pay-off roll 1 and the nip ensure that printing continues in register. It will thus be appreciated that there is no continuous monitoring and correction of register during the second (and subsequent passes) through the proof pass, all necessary adjustments to ensure register being made before commencement ofthe pass ofthewebthroughthe nip.

The advantages of the illustrated proof-press are severalfold. Firstly, any desired length of paper up to a particularlindit(eg 50m) may be printed. Secondly, there is good register between the successive prints.

Thirdly, the print is sharp due to the use of a printing nip. Fourthly, the press may be of open construction allowing easy accessforchanging printing cylinders and inks as required.

Claims (8)

1. A proof-press comprising: (a) a printing station at which a printing nip may be established (b) pay-off roll means upstream of the printing station and associated with adjustabletension control meansformaintaining a substantially constantwebtension between the pay-off roll and the nip; and (c) take-up roll means downstream of the printing station and associated with tensioning means for maintaining web tension between the nip and the take-up means.

2. A proof-press as claimed in claim 1 wherein the adjustable tension control means is a torque motor which controls the pay-off roll means.

3. Aproof-pressasclaimedinclaim 1 or2 wherein the printing station comprises a printing cylinder and a back-up cylinder relatively movable towards and away from each other.

4. A proof-press as claimed in claim 3 having an ink reservoir defined by an elongate element extending substantially parallel to the rotational axis of the printing cylinder and having a longitudinal edge in close proximity to the surface of the printing cylinder when a printing nip is defined.

5. A proof-press as claimed in any one of claims 1 to 4 having cylinder registerdetection means associated with the printing cylinderforviewing the position of a register markthereon.

6. A proof-press as claimed in any one of claims 1 to 5 having web register detection meansforviewing the position of a printed register mark on the sample.

7. A proof-press as claimed in any one of claims 1 to 6 wherein a heater is provided between the take-up roll and the printing nip.

8. A method of printing a sample of material comprising: (a) feeding the sample from a pay-off roll to a take-up roll via a printing nip at which a print is applied with a printing cylinder, said material being maintained at a substantially constant, predetermined tension between the pay-off roll and the nip, and being maintained in tension between the nip andthetake-up roll; (b) determining the print repeat length on the sample; (c) determining the print repeat length of a further printing cylinder to be used in the method; (d) determining from (b) and (c) the difference (if any) between the two print lengths; ; (e) preparing for a further printing operation using said further cylinder by adjusting (if necessary) the tension between the pay-off roll and the nip to compensate the print repeat length on the samplefor any difference detected in (d), and adjusting the position of said further cylinder and/orthesampleso that printing may begin in register; (f) effecting said further printing operation bythe method of step (a); and (g) optionally repeating steps (b)-(f) for any further printing cylinders.