In the field of screen printing, it is known to use a gripper assembly to transfer a substrate (eg a sheet of paper) between various stations in a screen printing device.

The basic arrangement and operation of the prior art screen printing device and of the gripper assembly is shown schematically in Figure 1 and is designated generally by reference numeral 1. The device has a print station comprising a print head 2 and a vacuum table 5 which are capable of relative vertical movement during a sheet transfer cycle. An unprinted sheet of paper 3 is located adjacent to the entry end of the print station against a sheet registration point (or lay position) 4 prior to printing.

The print head 2 contains the mechanism for applying the print to the paper 3. The vacuum table 5 holds the paper during printing. The gripper assembly 8 comprises grippers G1 and G2 which are laterally spaced apart at different heights and mounted on, connected to or run upon a gripper carriage 6. A static gripper safety guard 7 may be used. The gripper assembly 8 is capable of running substantially parallel to the vacuum table 5 between a gripping position and a retracted position.

Figure 2 illustrates schematically a single sheet print cycle (with the gripper safety guard 7 of Figure 1 omitted for clarity). In step 1, an unprinted sheet 3 is positioned against lay point 4 prior to printing. In this particular case, the grippers Gl and G2 are connected by a framework but it is known to provide a gripper assembly 8 in which Gl and G2 are independently driven (thereby leading to a different action to the one illustrated). In step 2, the vacuum table 5 and/or the print head 2 may be vertically displaced to permit the gripper assembly 8 to advance to the gripping position in which gripper Gl grips sheet 3. In step 3, the gripper assembly 8 retracts to the retracted position where the unprinted sheet is held during printing. The print head 2 lowers and the vacuum table 5 rises to allow printing after which the printed sheet is released on to the vacuum table 5.

In step 4, the print head 2 is raised and the vacuum table 5 is lowered to enable the gripper assembly 8 to advance to the gripping position so that gripper G2 grips the same edge of printed sheet 3. In step 5, the gripper assembly 8 retracts to the retracted position and the gripper G2 releases the printed paper. In the final step (step 6), the printed paper is transferred elsewhere to remote processing stations (eg drying or curing) in a conventional unit (eg an air dryer).

For this purpose, it is typically released onto and transferred by a conveyor system. As will be apparent, the gripper assembly 8 reaches the stage at which it can restart the cycle.

The cycle of the known single sheet printing device illustrated schematically in Figure 2 is readily converted into a continuous multi-sheet print cycle in which gripper G1 is arranged to draw a second unprinted sheet from the lay position 4 into the print station during steps 4 and 5 (ie the steps in which gripper G2 moves from the gripping position where it grips the first printed sheet to the retracted position). Printed sheets are therefore continuously released onto and transferred by the conveyor to the remote ink dryer or curing unit.

Single station colour screen printing devices using the basic transfer mechanism are known, for example the Printmaster (Svecia, Sweden) which is an automatic gripper machine and devices available from machine manufacturers such as Thieme (Germany) and Sakurai (Japan).

Existing multi-station colour screen printing devices generally have two different types of transfer arrangement.

The first colour screen printing device adopts a multi- gripper arrangement in which a plurality of grippers are continuously driven forward to the print stations using a system such as a tooth or chain driven belt so as to bring the paper into alignment with consecutive single colour printing stations and curing stations. This device may be disadvantageously complex. In the second colour screen printing device, the paper is transferred between consecutive single colour print stations by means of a conveyor (eg a vacuum conveyor). One disadvantage of the latter known multi-station colour screen printing device is that the printing sheet may be misaligned during transfer. This could have disastrous consequences where a high density of printing dots demands an accurate registration at each station.

The present invention seeks to improve existing single colour screen printing devices by introducing to the exit end of a print station a static holding station in which drying or curing of a"wet"print may be carried out.

Thus viewed from one aspect the present invention provides a screen printing device for printing on a substrate comprising: a print station having an entry end for receiving the substrate and an exit end from which the substrate departs, said print station comprising a print table and a print head in vertical spaced apart, substantially parallel relationship; a gripper assembly capable of advancing through the exit end of the print station to a gripping position and of retracting to a retracted position, said gripper assembly comprising at least two laterally spaced apart grippers for gripping the substrate at the gripping position and releasing the substrate at the retracted position; a holding station positioned at (eg attached to) the exit end of the print station capable of receiving the gripper assembly in the retracted position, said holding station comprising a substrate platform vertically spaced apart from a curing or drying means.

Preferably the print table and the substrate platform are capable of cyclical vertical displacement so as (1) to accommodate the gripper assembly in (and during transfer between) the gripping and retracted positions and (2) to print the substrate and (3) to cure the substrate. The print table may be linked to or independently driven from the substrate platform.

Preferably the print table and substrate platform are vertically staggered with the print table higher than the substrate platform throughout the cycle of vertical displacement. Preferably the lowest position of the print table in the cycle of vertical displacement is substantially equal to or higher than the highest vertical position of the substrate platform in the cycle of vertical displacement (ie the curing and drying position). In the gripping position, the print table may dip by a small amount (eg lmm) to facilitate the gripping action.

Preferably the cyclical vertical displacement of the print table and of the substrate platform are synchronous.

In providing a holding station at the exit end of a print station, the versatility of the present invention is such that one or more additional print stations and holding stations may be added at the free or exit end of the holding station. It may be possible to add up to 8-10 consecutive stations to produce a multi-station colour screen printing device with a higher registration and potential for greater speed than conventionally available multi-station colour screen printing devices.

Thus in a preferred embodiment of the device, the holding station comprises a means for registering the substrate on the substrate platform, said device further comprising: a second print station adjacent or near to the holding station having an entry end for receiving the substrate from the substrate platform and an exit end from which the substrate departs, said second print station comprising a second print table and a second print head in vertical spaced apart, substantially parallel relationship; and a second gripper assembly capable of advancing through the exit end of the second print station to a gripping position and of retracting to a retracted position, wherein said second gripper assembly comprises at least two laterally spaced apart grippers for gripping the substrate at the gripping position and releasing the substrate at the retracted position, wherein the second print table and the substrate platform are capable of relative vertical displacement to an essentially common height so as to assist the second gripper assembly to transfer the substrate from the substrate platform to the second print table.

In the gripping position, the second print table may dip by a small amount (eg lmm) to facilitate the gripping action. The substrate platform may dip synchronously by the same amount.

In a particularly preferred embodiment, the second print station is horizontally spaced apart from the holding station and the substrate platform is capable of cyclical horizontal displacement to and from the entry end of the second print station (eg to the registration point of the second print station). The spacing apart of the holding station and the second print station advantageously permits the first and second gripper assemblies to have sufficient clearance.

The cyclical vertical and horizontal displacements of the substrate platform may be sequential (ie stepwise displacement) or combined (ie single step displacement). Thus the substrate platform may be linearly (eg horizontally, diagonally or vertically) or non-linearly (eg arcwise) driveable. Preferably the vertical and horizontal displacements of the substrate platform are sequential. The driveability may be controlled electrically, pneumatically, electronically or mechanically.

Particularly preferably the substrate platform is capable of cyclical vertical displacement at least between the height of the second print table and a height for curing or drying.

Preferably the substrate platform is a vacuum table.

Viewed from a further aspect the present invention provides a holding station capable of being operatively positioned at the exit end of a print station as hereinbefore defined, said holding station comprising: a substrate platform vertically spaced apart from a curing or drying means, wherein the substrate platform is capable of cyclical vertical displacement between a first height for receiving the printed substrate from the print table, a second height for accommodating a gripper assembly in the retracted position, a third height for delivering the printed and cured substrate to a remote location and a fourth height for curing or drying the substrate.

Preferably the first and fourth heights are the same.

Preferably the second and third heights are the same.

Preferably the holding station is capable of being operatively positioned at or near to a second print station such that the substrate platform is capable of delivering the printed and cured substrate to a second print table of a second print station.

Preferably the cyclical vertical displacement includes a fifth height which is a small amount lower than the third height to facilitate transfer of a substrate to the second print table.

The holding station of the invention may be free- standing (ie independent) or may be integral to the screen printing device of the invention.

Preferably the substrate platform is capable of cyclical horizontal displacement to and from the entry end of a second print station (eg to the registration point of the second print station) to enable the second print station to be spaced apart from the holding station. The spacing apart of the holding station and the second print station advantageously permits the first and second gripper assemblies to have sufficient clearance. The substrate platform may be linearly (eg horizontally, diagonally or vertically) or non-linearly (eg arcwise) driveable. It may be driveable in a single step or stepwise.

It will be appreciated that any number of holding stations and print stations may be added consecutively thereby allowing multi-station colour screen printing devices to be constructed where consecutive print stations are capable of printing different colours. This may be advantageously achieved by retrofitting the holding station and additional print stations te-existing single colour print stations or manufacturing complete colour lines of two or more.

Viewed from a yet further aspect the present invention provides a multi-station colour screen printing device comprising one or more holding stations as hereinbefore defined.

Preferably, the drive or control of each print station/holding station is independent to allow different prints, sheet delivery, handling and drying/curing speeds.

Where the substrate platform is not driveable to the entry end of a spaced apart second print station, the movement of consecutive gripper assemblies may have to be sequenced to avoid contact (eg by appropriately raising or lowering the substrate platform).

Whilst the multi-station device would normally be expected to be stepped downwards in height from the first print station, a flat print line device may be developed.

The movement of the components of any embodiment of the present invention may be achieved electrically, electronically, pneumatically or mechanically in accordance with conventional practices.

In a preferred embodiment, the (or each) printing station comprises a print table (eg a vacuum table) and a print head in vertical spaced apart, substantially parallel relationship. Either or both of the print head and the print table (eg vacuum table) is vertically moveable to accommodate a gripper assembly in the gripping position within the print station or to assist printing. Preferably, the print table (eg vacuum table) is capable of vertical displacement between a first height which permits the unprinted substrate at the entry end of the print station to be gripped by the gripper assembly in its advanced position and permits the printed substrate on the print table (eg vacuum table) to be gripped by the gripper assembly in its advanced position and a second height which permits printing.

The print head may raise or lower to assist the transfer and/or print cycle.

The drying means of the holding station may comprise any conventional dryer/curer such as a W or jet air drying unit. A W drying unit is preferred.

The screen printing device may be hand fed or an automatic feeder may be located adjacent to the entry end of the first print station. The screen printing device may comprise a substrate registration point (eg a lay point) at or near to the entry end of the print station. An automatic stacker may be added if desired.

In the preferred embodiment, the (or each) gripper assembly preferably comprises a first gripper capable of gripping the substrate (eg from the lay position) in the gripping position. Particularly preferably, the (or each) gripper assembly comprises a second gripper capable of gripping the substrate in the print station in the gripping position. Preferably the first and second gripper are in parallel spaced apart relationship and are connected by a planar framework (although they may be independently driven).

Particularly preferably the first gripper is above the second gripper. The grippers may comprise fingers or plates operable in a jaw-like fashion. The framework is capable of advancing through or retracting from the exit end of the print station in a plane substantially parallel to the horizontal plane of the print table.

Where the gripper assembly comprises two or more (eg three) grippers, the grippers may be individually driven rather than mechanically connected as described above and the precise transfer mechanism may vary slightly.

Preferably the horizontal advancement/retraction of the gripper assembly is synchronous with the cyclical horizontal displacement of the substrate platform.

The substrate may of course be a sheet of paper, card or board of varying thicknesses.

Viewed from a still further aspect the present invention provides a kit of parts comprising: a substrate platform (eg a vacuum table) and a curing or drying means, wherein said substrate platform and said curing or drying means are operatively positionable at an exit end of a print station as hereinbefore defined wherein the substrate platform is capable of so as to accommodate the gripper assembly in the retracted position and to receive the substrate from the gripper assembly for drying or curing.

Preferably the substrate platform is capable of delivering the substrate to a remote location (eg a second print station).

In a preferred embodiment of the kit of the invention the substrate platform is positionable in vertical spaced apart relationship with the curing or drying means.

The present invention will now be described in a non- limitative with reference to the accompanying Figures in which: Figure 1 and Figure 2 (described hereinbefore) illustrate schematically transfer mechanisms in prior art screen printing devices; Figure 3 illustrates schematically a preferred embodiment of the screen printing device of the invention; Figure 4a and 4b illustrate schematically and in detail a further preferred embodiment of the screen printing device of the invention; Figure 5 illustrates schematically a continuous feed single station printing device of the invention; and Figure 6 illustrates schematically a continuous feed multi- station screen printing device of the invention.

Figure 3 is intended to show schematically the vertical displacements and resting positions of the components in an embodiment of the invention. In Figure 3, position A represents the pre-printing position in which the unprinted sheet 3 is in registry against a registration point 4. The print station 13 comprises vacuum table 5 and print head 2 in vertical spaced apart, substantially parallel relationship.

The holding station 11 attached to the exit end of the print station 13 comprises substrate platform 12 in vertical spaced apart parallel relationship to a means for drying and curing 14 such as a W drier. The gripper assembly 6 comprises grippers G1 and G2 which are laterally spaced apart in a planer framework at different heights and capable of horizontal movement between the print station 13 and the holding station 11.

The print head 2 and the vacuum table 5 are vertically displaced apart and the substrate platform 12 is moved downwardly (clear of G2) to permit the gripper assembly 6 to move into the print station where gripper G1 grips the unprinted sheet 3 in position B. The gripper assembly is- retracted and the sheet is drawn onto the vacuum table 5 (position C) whilst the vacuum table 5, substrate platform 12 and print head 2 return to their original position. Printing is conducted in a conventional manner and the printed sheet is released. The vacuum table 5 holding the sheet accurately in position is vertically lowered and the substrate platform 12 falls to allow the gripper assembly to move over the substrate platform 12 and print table 5 so as to permit gripper G2 to grip the opposite edge of the printed sheet (position D). The gripper assembly 6 is retracted and the sheet held on the substrate platform 12 where it is dried and cured in the holding station 11 before it is released (position E).

Figure 4a retains the numbering used in Figure 3 and is intended to show step-by-step in more detail the movements of the components in an embodiment of the invention in which the substrate platform 12 is also horizontally driveable.

The pre-printing position is shown in position A and essentially corresponds to position A described above in relation to Figure 3. In step B, print head 2 raises, vacuum table 5 lowers and the substrate platform 12 lowers clear of G2. The gripper assembly 6 moves horizontally across the substrate platform 12 and the vacuum table 5 and at the same time the substrate platform is driven horizontally forward (step C). In position D, the gripper Gl at the entry end of the print station 13 grips the sheet 3 at the lay position 4 whilst the substrate platform 12 reaches its final position.

In step E, the gripper assembly 6 retracts to locate the sheet 3 on the vacuum table 5 and at the same time the substrate platform 12 is driven horizontally backwards. The print head 2 lowers, the vacuum table 5 raises and the substrate platform 12 raises in step F. In the final position (position G), the print head 2 prints onto the paper 3.

After printing is complete (Figure 4b), the print head 2 raises, the vacuum table 5 lowers and the substrate platform 12 lowers clear of G2 in step H. The gripper assembly 6 moves horizontally across the substrate platform 12 and the vacuum table 5 and at the same time the substrate platform is driven horizontally forward (step I). In position J, the gripper G2 at the exit end of the print station 13 grips the opposite edge of the printed sheet 3 whilst the substrate platform 12 reaches its final position. To facilitate the gripping action, the print table dips typically by lmm. In step K, the gripper assembly 6 retracts to locate the printed sheet 3 on the substrate platform 12.

The print head 2 lowers, the vacuum table 5 raises (step L) and the substrate platform 12 raises to adopt the final position M where the printed sheet 3 can be cured and released.

Figure 5 illustrates schematically the use of the device of Figure 4 as a continuous feed single station printing device. In step J2 (corresponding to step J above), gripper Gl grips a second sheet 20 at the lay position 4. The retraction of the gripper assembly 6 in step K2 (as described above for step K) permits the unprinted sheet 20 to be drawn onto the vacuum table 5. Step L2 and position M2 in Figure 5 correspond to step L and position M described above and printing on sheet 20 is carried out (in a manner identical to that described above for the first sheet 3) at the same time as sheet 3 is cured in the holding station. In this manner, a continuous feed of printed and cured sheets may be obtained in the static holding station and removed (if desired) in any suitable manner.

Figure 6 illustrates the use of the device of Figure 5 as a continuous feed, multi-station printing. device. From the printing and curing position M2 described in detail above, the cycle is restarted in step M3 ie the print head 2 raises, vacuum table 5 lowers and the substrate platform 12 lowers clear of G2. The gripper assembly 6 moves horizontally across the substrate platform 12 and the vacuum table 5 and at the same time the substrate platform 12 is driven horizontally forward (step M4) towards a second print station 60 (at a lower height than the first print station). At position M5, the substrate platform 12 at its most forwardly point delivers the cured sheet 3 to the lay position 64 of the second print station 60 from where it is captured by a gripper 61 of a gripper assembly 66. To facilitate the gripping action of gripper 61, the second print table 64 and substrate platform 12 dip synchronously by a small amount (lmm). A second print cycle in the second print station 60 is then carried out in steps M6 and M7 to bring the sheet 20 into the printing position in a manner identical to that described above for the first sheet 3 (see steps E and F in Figure 4). It will be appreciated that the versatility of the holding station of the invention is such that any number of additional print stations and holding stations may be added in this manner.