GB2243330A - Screen printing apparatus - Google Patents

Abstract

A control system for use on a screen printing apparatus for assisting correct registration of subsequent images screen printed on a substrate at a screen printing station includes an electronic scanner (86) for locating the image of a print or marker printer on the substrate, and an encoding disk attached to a printing drum (16) with a further scanner to measure the rotational movement of the printing drum from which the expected position of the image of a print or marker can be determined. Processing means is provided for comparing the located image position with the expected image position and for effecting connectional movement of the printing screen (18) by means of a stepper motor (85) controlled by the processing means. The control system is employed in a four-colour screen printing apparatus which includes a plurality of printing stations (2, 4, 6, 8) in which the substrate is moved in a first direction through first (2) and second (4) printing stations, and in a direction substantially opposite thereto through the third (6) and fourth (8) printing stations. <IMAGE>

Description

SCREEN PRINTING APPARATUS The invention relates to a screen-printing apparatus.

A conventional multicolour screen printing apparatus has a plurality of printing stations at each of which a differently coloured image is printed onto a substrate by transfer from a printing screen. Each printing station includes a printing drum over which the substrate passes and is brought into contact with the printing screen for transfer of the image during simultaneous forward movement of the print screen and of the substrate. Generally the printing stations are arranged in series, the substrate being fed continuously through subsequent printing stations. Such an arrangement requires considerable floor space, particularly where four printing stations are required in a four-colour printing apparatus.

According to a first aspect of the present invention there is provided a multi-colour screen printing apparatus having a plurality of printing stations including at least one printing station arranged to move a substrate therethrough in a first direction, and at least one printing station arranged to move the substrate therethrough in substantially the opposite direction, turning means being situated between oppositely directed stations to change the direction of movement of the substrate therebetween.

Preferably, the multi-colour screenprinting apparatus has four printing stations, first and second printing station being arranged in a first line so that the substrate is moved therethrough in a first direction, and third and fourth printing stations being arranged in a second line so that the substrate is moved therethrough in the opposite direction, the turning means being situated between the second and third printing stations to change the direction of movement of the substrate through the apparatus through substantially 0 180 Preferably, the first and fourth printing stations are arranged substantially side by side, and the second and third printing stations are arranged substantially side by side.

The side by side arrangement of the printing stations provides a particularly compact arrangement for the printing apparatus. Furthermore, such an arrangement of printing stations also allows an operator to observe both the outfeed from the fourth printing station which shows the final printed image, as well as the infeed to the first printing station. Thus, an operator can both check the quality of the final printed image, and observe the infeed of substrate. The observation of infeed and out feed is generally not possible for a single operator utilising a conventional system in which all the printing stations are arranged in a common line.

Preferably, the first and fourth printing stations have a common drive unit. The second and third printing stations may have a common drive unit which may also be connected to the drive unit for the first and fourth print heads. Employing a common drive unit provides a particularly economic drive system.

When a substrate such as a web of material is to be printed with more than one colour it is necessary to ensure that each colour which is applied to the substrate is dry before subsequent colour is applied to the substrate. Conventionally a substrate is led over a plurality of rollers in a multiple loop dryer, the rollers effectively increasing the length of travel within the dryer. Particularly in respect of the first few rollers in the dryer, there is a risk that the freshly printed image may be disrupted by travel over the rollers.

Generally between the printing head where the image is printed and the first roller the substrate is left to lie in a loop, to give the image maximum time to dry before arriving at the first roller. However, use of a loop does not allow for any control or tensioning of the substrate According to a second aspect of the invention there is provided an air turn roller for turning a substrate bearing a substantially wet ink, comprising an annular roller member perforated with a plurality of holes therethrough, wherein, in use, the interior of the roller is fed by a supply of air which passes outwardly through the holes, to thereby provide a cushion of air between the substrate and roller member.

Preferably the roller member is rotatably mounted.

By providing an outflow of air from the roller the substrate can be moved over the roller with minimum contact with the roller, so that a drying image printed thereon will not be disrupted by movement over the roller. The roller assists in the control of the feeding of the substrate to the multiple loop dryer.

Preferably, a fixed semi-annular shield is provided within the annular roller at an upper part thereof, which shield prevents the outflow of air from the upper half of the roller. The roller member may be mounted between a pair of substantially upright supports, so as to be movable in a substantially vertical direction.

According to a third aspect of the invention, there is provided a drive assembly for a printing drum and screen, the drive assembly comprising a reciprocatably driven main rack which is connected by first gear means to a printing drum shaft, the printing drum shaft being connected by second gear means to a screen rack thereby to provide reciprocating movement of the screen rack, and wherein first clutch means is provided between the printing drum and drum shaft which clutch causes rotation of the printing drum with the drum shaft when the shaft rotates in one direction only.

Preferably, a second printing drum is connected to the drum shaft via second clutch means which causes unidirectional movement of the second printing drum.

Preferably, the screen rack is connected to a pair of screen frames.

Where a side by side arrangement of printing stations is utilised as referred to above this drive means readily allows two printing drums and two screens to be driven from a single main rack. Further gear means and a drum shaft can be connected to the main rack thereby to drive all four screens and printing drums of a four-colour printing apparatus.

The main rack may be reciprocatingly driven by a drive motor, which drive motor is connected via an eccentric and a drive arm to the main rack. The eccentric may be provided with stroke varying means to adjust the linear stroke of the main rack.

The stroke varying means may comprise a plate pivotably attached to the eccentric, to which the drive arm is pivotably attached, and stroke varying drive means to change the position of the pivotal attachment of the drive arm relative to the axis of rotation of the eccentric, thereby to change the stroke of the drive arm.

Preferably, the stroke varying drive means includes a stepper motor connected via a worm and worm wheel drive to the plate. An electromagnetic switch can be provided to lock the plate in a selected position, thereby to fix the stroke of the drive arm.

Particular problems arise in multi-colour printing in ensuring that subsequently printed images are accurately in register. A substrate in the form of a web may stretch or contract between printing stations resulting in the misalignment of subsequent images. It is now proposed to provide a system for registration control which can provide a correctional movement of the screen to facilitate correct registration.

According to a fourth aspect of the present invention there is provided a control system for assisting correct registration of subsequent images screenprinted on a substrate at a screenprinting station, including scanning means for scanning the substrate thereby to locate the image of a print or marker printed on the substrate, and means for determining the expected position of the image of a print or marker at that printing station, further means being provided for comparing the located image position with the expected image position and for effecting correctional movement of the printing screen if the located and expected image positions differ, thereby to ensure an image is printed in register with an existing image.

By employing correctional movement of the screen which is of lower mass than the printing drum, a simple correctional drive system can be utilised.

Preferably, the means determining the expected image position includes an encoding disk attached to a printing drum, and a further scanner to measure the rotational movement of the printing drum.

Preferably, the means comparing the located image position with the expected image position comprises electronic processing means, which control a stepper motor to provide correctional movement of the printing screen.

Such a control system can be used in a four-colour screenprinting apparatus, separate scanning means and a separate stepper motor being provided at each printing station. Where a single drive system is utilised to drive all four print stations of a four-colour printing apparatus as referred to above only a single encoding disk and scanner is required.

According to a fifth aspect of the invention, there is provided a print head assembly for attachment to a screen printing apparatus comprising a holder for a squeegee blade and a holder for a flood blade, and a holder support to which the holders are attached, which support is releasably attachable to the screen printing apparatus.

The detachability of the entire print head assembly allows a dirty assembly to be quickly replaced by a clean one, thus reducing the shut down time of the printing apparatus which is required to fit a clean print head.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a perspective view of a four-colour screen printing apparatus; Figure 2 is a perspective view of an air turn roller which is incorporated in the apparatus of Figure 1; Figure 3 shows part of a drive assembly for a printing drum and a printing screen which is incorporated in the apparatus of Figure 1; Figrue 4 shows a further part of a reciprocating drive for a printing drum and screen which is incorporated in the apparatus of Figure 1; Figure 5 shows a mechanism for providing correctional movement of a screen which is incorporated in the apparatus of Figure 1; Figure 6 shows a scanning device attached to a printing drum which is incorporated in the apparatus of Figure 1; and, Figure 7 shows a detachable print head assembly which is incorporated in the apparatus of Figure 1.

Turning firstly to Figure 1, the Figure shows an apparatus for multicolour printing onto a web of substrate material. The apparatus incorporates printing stations 2, 4, 6, 8 at each of which a respective image is printed onto a substrate 10. The substrate 10 is in a form of a web which, when the apparatus is in use, extends between an infeed reel 12 on which the substrate to be printed is stored and a collection reel 14 on which the substate is received after printing.

Each printing station includes a printing drum or cylinder 16 over which the substrate 10 passes, and a printing screen 18 which is brought into contact with the substrate 10 on the cylinder 16 for transfer of the image during a simultaneous forward movement of the screen 18 and the cylinder 16 which has the substrate 10 held thereon. Printing is effected by the relative movement between the screen and a sgueegy blade which forces ink through the mesh of the screen, and causes ink to pass from the screen to the surface of the substrate.

Drying stations 20,22,24,26 are situated downstream of respective printing stations 2,4,6 and 8. Each drying station comprises a conventional drying cabinet in which warm air is circulated or alternatively, in which an ultra violet drying technique is utilised, and a number of rollers 28 over which the substrate passes. The use of rollers effectively increases the length of travel of the web within the dryer, thereby to increase the residence of the substate in the drying station without increasing the floor space required for the apparatus. At the drying stations 20,22,24,26 the ink printed on the substrate at one printing station in dried before it reaches the next printing station, or before it is wound onto the collection reel 14 in the case of the fourth drying station 26.

As is readily appreciated from Figure 1, first and second printing stations 2,4 are arranged end to end in a line with a first drying station 20 therebetween, third and fourth printing stations 6 and 8 also being arranged in line, with the third drying station 24 therebetween.

The first and fourth printing stations 2,8 are arranged side by side, as are the second and third stations 4,6.

Side by side printing stations may be combined to give a single two-head print module. The direction of travel of substrate 10 through first and second printing stations is from right to left of Figure 1, and through third and fourth printing stations from left to right. Means (not shown) are provided between second and third printing stations 4,6 to reverse the direction of travel of the substrate therebetween. Conventional means for reversing the direction of travel of a web substrate comprise a pair of rollers angled at 450 to the longitudinal direction of the apparatus. Thus, in plan view the substrate describes a U-shape between infeed reel 12 and collection reel 14.As discussed in more detail below the side by side arrangement of the printing stations allows a single main drive to be readily provided to operate movement of each printing drum 16 and screen 18 at each printing station.

Where it is required to print only two colours, a two-colour print apparatus could be provided which has only two printing stations arranged side by side, that is, referring to Figure 1, printing stations 4 and 6 and drying stations 22 and 24 are omitted, with turning means being provided between the drying station 20 and the print station 8.

Subsequent to an image being printed at each printing station, substrate 10 is led to the first of the rollers 28 in the dryers via and under an air turn roller 30, shown in detail in Figure 2. This air turn roller 30 includes an outer annular roller 32 which is rotatably mounted and which is perforated with a plurality of holes 34. The air turn roller 30 is held between blocks 38 which are mounted on substantially upright supports 40. A semi-annular inner air shield 36 is fixed within the outer roller 32 at the upper part of the roller 32, extending along the length of the outer roller 32.

Linear ball bushings 42 are provided within blocks 40 so that the entire air turn roller 30 is freely movable up and down on the supports 40. Air is fed into the interior of the outer roller 32 by means of a compressed air feed line 44, the air passing outwardly through the holes 34.

The air shield 36 prevents air from exiting through the upper half of the outer roller 32. Web side guides 46 provided adjacent opposite ends of the roller assist in positioning the web correctly on the roller 46. These guides 46 are movable to allow use of the roller with different widths of web.

Thus, with air exiting from the outer roller 32 the substrate 10 is led from a printing station beneath the roller effectively on a cushion of air, and a substrate having a freshly printed image thereon is turned around the roller 30 substantially without contact of the substrate 10 on the roller 30. Since the roller 30 is free to move vertically, the weight of the roller and of the air flow provides a tensioning control of the web of substrate 10.

The drive for the movement of the printing screen 18 and printing cylinder 16 is provided by a motor connected thereto by a mechanical linkage incorporating a rack and pinion arrangement. A main rack 50 shown in Figure 3 is reciprocatingly driven by a motor via an eccentric 52, shown in Figure 4 and discussed in more detail below. The main rack 50 is connected by a first gear wheel 54 to a drum shaft 56. The drum shaft 56 is connected to screen frame drive 58 having a rack 60 on its underside, by means of a second gear wheel 62. The screen frame drive 58 is connected to at least one screen frame (not shown); thus, reciprocating movement of the main rack 50 produces simultaneous reciprocating movement of the screen frame.

As is apparent from Figure 3, the gear ratio between first and second gear wheels can be chosen so that a relatively small movement of main rack 50 causes a larger movement of screen frame drive 58. A typical gear ratio employed is 1:2.

Drum shaft 56 is connected to a printing cylinder which is in the form of a vacuum drum 64, by means of a sprag clutch 66, which causes rotation of the drum 64 with the drum shaft 56 when the shaft 56 rotates in one direction but not when the drum shaft rotates in the opposite direction. Thus, the drive assembly provides simultaneous forward movement of printing drum 64 and printing screen 18, the transfer of the image from the screen to the substrate occurring during this "printing stroke". On the reverse movement of main drive rack 50 the printing drum remains stationary (i.e. the non-printing stroke).

A further vacuum printing drum can be conencted to the shaft 56 on the opposite side of the gear means to the vacuum drum 64; thus, a single drum shaft 56 can be used to operate the printing drums at two printing stations which are arranged side by side.

The main drive rack 50 can also be used to drive a further assembly identical to that shown in Figure 3.

Thus, the reciprocating movement of main rack 50 can be drive printing drums and printing frames at all four printing stations 2,4,6 and 8.

Main drive rack 50 is connected to drive arm 68 which is itself connected to eccentric 52, as shown in Figure 4. The eccentric 52 is connected to shaft 70 which is driven by a motor (not shown). Stroke adjustment means 72 is provided on the eccentric 52, the drive arm 68 being pivotably attached at 75 to this plate 74. The plate 74 may be driven by a stepper motor (not shown) through a worm and worm wheel arrangement 76. Actuation of the motor causes the plate 74 to pivot about pivot point 78, thereby moving the pivotal attachment 75 of the drive arm 68 either towards or away from the axis of rotation of the eccentric 52, thereby reducing or increasing the stroke of the drive arm. An electromagnetic switch (not shown) locks the plate 74 in a particular position.By providing remote control of the stepper motor and switch the stroke of the drive arm (and therefore the print length) can be remotely adjusted.

Turning now to Figures 5 and 6, a mechanism is shown for providing automatic registration correction of subsequent printed images.

Figure 5 shows screen frame drive member 58 (the drive for this member being shown in Figures 3 and 5) to which a cross member 80 is attached. Screen 18 is held on a screen frame 82 which is itself attached to screen frame holder 84. Screen frame holder 84 is movable attached to cross member 80 and stepper motor 85 is provided to move the screen frame holder 84 relative to the cross member 80 in a direction parallel to the screen frame drive member 58. Cross member 80 is provided with linear ball bushings 81.

An encoding disk and drum scanner (not shown) is provided on the printing drum which measures the rotation of the drum. The encoding disk may comprise a graticulated disk attached to an end face of the drum which rotates beneath the scanner. Since a single drive system is used to drive all four printing drums only a single encoding disk is required.

An electronic scanner 86 is shown in Figure 6 which is attached adjacent printing drum 16 on an arm 87, the scanner comprising a photoelectric cell for locating the image of a print or registration marker printed on the substrate. Positioning screws 89 are provided to facilitate setting up of the scanner. The output from the scanner 86 is fed to a microprocessor or computer.

The rotation of the drum is detected by means of the encoding disk and drum scanner, the signal from the scanner being fed into the microprocessor or computer control system. Using the drum rotation information it is possible to predict when, assuming no stretch or slipping of the substrate, an image printed at a previous printing station, or a previously printed registration marker should be detected by the scanner 86. The actual position which is measured by the scanner is compared with the expected position. If the two positions are not the same, a signal is sent to the stepper motor to provide correctional of the screen position by movement of the frame holder 84 relative to cross member 80. The system allows both forward and backward correctional movement of the screen.

As will be apparent to the man skilled in the art, various microprocessors or electronic or computerised control systems could be utilised to process the signals from the scanner 86 and drum scanner, and to provide the necessary output to control the stepper motor 85.

A separate scanner 86 and stepper motor arrangement can be used at each printing station of the multi-colour screenprinting apparatus, to thereby provide separate registrational control at each printing station.

Turning now to Figure 7, a print head assembly 90 is shown which includes a holder 92 for a squeegy blade, and a holder 94 for a flood blade, the holders 92 and 94 being attached to a holder support 96. The holder support 96 is attached to the rest of the printing assembly by means of a pivoting pin 98 which can be released to enable the head assembly 90 to be detached from the printing assembly.

This detachability allows a dirty head assembly to be guickly and easily removed for cleaning, and a clean head quickly attached, thereby reducing the required shut down time of the printing apparatus.

A setting up jig could be used to facilitate positional settings of the squeegee, the flood heights and screen positions.

Claims (30)

1. A control system for use on a screen printing apparatus for assisting correct registration of subsequent images screen printed on a substrate at a screen printing station, including scanning means for scanning the substrate thereby to locate the image of a print or marker printed on the substrate, and means for determining the expected position of the image of a print or marker at that printing station, further means being provided for comparing the located image position with the expected image position and for effecting correctional movement of the printing screen if the located and expected image positions differ, thereby to ensure an image is printed in register with an existing image.

2. A control system according to claim 1 wherein the means determining the expected image position include an encoding disk attached to a printing drum, and a further scanner to measure the rotational movement of the printing drum.

3. A control system according to claim 1 or 2 wherein the scanning means comprises an electronic scanner arranged adjacent a printing drum.

4. A control system according to claim 3 wherein the means for comparing the located image position with the expected image position comprise electronic processing means connected to both the electronic scanner and the further scanner.

5. A control system according to claim 4 wherein a stepper motor is arranged to provide correctional movement of the printing screen, and is controlled by the electronic processing means.

6. A control system according to any preceding claim when utilised in a multi-colour screen printing apparatus further comprising additional scanning means, and indepentently operable stepper motors at each respective printing station.

7. A control system substantially as hereinbefore described with reference to Figures 5 and 6, optionally in consideration with Figure 1 of the accompanying drawings.

8. A multi-colour screen printing apparatus having a plurality of printing stations including at least one printing station arranged to move a substrate therethrough in a first direction, and at least one printing station arranged to move the substrate therethrough in substantially the opposite direction, turning means being situated between oppositely directed stations to change the direction of movement of the substrate therebetween.

9. A multi-colour screen printing apparatus according to claim 8, wherein the multi-colour screen printing apparatus has four printing stations, first and second printing station being arranged in a first line so that the substrate is moved therethrough in a first direction, and third and fourth printing stations being arranged in a second line so that the substrate is moved therethrough in the opposite direction, the turning means being situated between the second and third printing stations to change the direction of movement of the substrate through the apparatus through substantially 1800.

10. A multi-colour screen printing apparatus according to claim 9 wherein the first and fourth printing stations are arranged substantially side by side, and the second and third printing stations are arranged substantially side by side.

11. A multi-colour screen printing apparatus according to claim 9 or 10 wherein the first and fourth printing stations have a common drive unit.

12. A multi-colour screen printing apparatus according to any one of claims 9 to 11 wherein the second and third printing stations have a common drive unit.

13. A multi-colour screen printing apparatus according to any one of claims 9 to 12 wherein the first and fourth printing stations have a common drive unit connected to a common drive unit provided for second and third printing stations.

14. A multi-colour screen printing apparatus substantially as hereinbefore described with reference to and as shown in Figure 1 of the accompanying drawings.

15. An air turn roller for turning a substrate bearing a substantially wet ink, comprising an annular roller member perforated with a plurality of holes therethrough, wherein, in use, the interior of the roller is fed by a supply of air which passes outwardly through the holes, to thereby provide a cushion of air between the substrate and roller member.

16. An air-turn roller according to claim 15 wherein the roller member is rotatably mounted.

17. An air turn roller according to claim 15 or 16 wherein a fixed semi-annular shield is provided within the annular roller at an upper part thereof, which shield prevents the outflow of air from the upper half of the roller.

18. An air turn roller according to any one of claims 15 to 17 wherein the roller member is mounted between a pair of substantially upright supports, so as to be movable in a substantially vertical direction.

19. An air turn roller substantially as hereinbefore described with reference to and as shown in Figure 2 of the accompanying drawings.

20. A drive assembly for a printing drum and screen of a screen printing apparatus, the drive assembly comprising a reciprocatably driven main rack which is connected by first gear means to a printing drum shaft, the printing drum shaft being connected by second gear means to a screen rack thereby to provide reciprocating movement of the screen rack, and wherein first clutch means is provided between the printing drum and drum shaft which clutch causes rotation of the printing drum with the drum shaft when the shaft rotates in one direction only.

21. A drive assembly according to claim 20 wherein a second printing drum is connected to the drum shaft via second clutch means which causes unidirectional movement of the second printing drum.

22. A drive assembly according to claim 20 or 21 wherein the screen rack is connected to a pair of screen frames.

23. A drive assembly according to any one of claims 20 to 22 wherein the main rack is reciprocatingly driven by a drive motor, which drive motor is connected via an eccentric and a drive arm to the main rack.

24. A drive assembly according to claim 23 wherein the eccentric is provided with stroke varying means to adjust the linear stroke of the main rack.

25. A drive assembly according to claim 24 wherein the stroke varying means comprises a plate pivotably attached to the eccentric, to which the drive arm is pivotably attached, and stroke varying drive means to change the position of the pivotal attachment of the drive arm relative to the axis of rotation of the eccentric, thereby to change the stroke of the drive arm.

26. A drive assembly according to claim 25 wherein the stroke varying drive means includes a stepper motor connected via a worm and worm wheel drive to the plate.

27. A drive assembly according to claim 26 wherein an electromagnetic switch is provided to lock the plate in a selected position, thereby to fix the stroke of the drive arm.

28. A drive assembly substantially as hereinbefore described with reference to and as shown in Figures 3 and 4 of the accompanying drawings.

29. A print head assembly for attachment to a screen printing apparatus comprising a holder for a squeegee blade and a holder for a flood blade, and a holder support to which the holders are attached, which support is releasably attachable to the screen printing apparatus.

30. A print head assembly substantially as hereinbefore described with reference to and as shown in Figure 7 of the accompanying drawings.