WEB PRINTING METHOD AND APPARATUS

BACKGROUND

[0001] InkJet printing is widely used for printing of billboards, banners and point of sale displays. The ink-jet printing process involves manipulation of drops of ink ejected from an orifice or a number of orifices of a print head onto an adjacent print substrate. Paper, vinyl, textiles, fabrics, and others are examples of print substrates. Relative movement between the substrate and the print head enables substrate coverage and image creation. A majority of billboards and banners having relatively large dimensions are printed on flexible substrates. Such substrates represent rolls of flexible material that are up to five meters wide. Roll-to-Roll (R2R) printing machines are typically used for printing on flexible substrates.

[0002] In R2R printers transportation of the substrate is generally carried out by a number of rolls forming a so called substrate feed path, some of which induce the substrate movement and others change the substrate direction or form nips enabling substrate transportation. In the context of the present disclosure the term "nip" means the gap or width of the gap between two parallel rolls. The nip may have a desired width or the rolls may be in contact

having a nip width equal to zero. The print head typically reciprocates over the recording substrate at a printing zone or region, printing a section of image at each pass. After each reciprocating movement or pass the substrate is further transported to a position where the next section of a desired image may be printed on it. Frequently a solid flat or curved surface is located between the rolls. The surface supports the printing substrate and is supposed to keep it flat, at least in the printing zone or region of the substrate feed path.

[0003] The recording substrate is tensioned (back tension) on one of its sides so as to reduce undulations or wrinkles when the substrate spans over a flat or curved support surface of the printing zone or region. In the context of the present application the term "back tension" means the force that keeps the substrate tensioned with respect to the drive roll. A difference in rotational speed between two or more rolls in the substrate path typically generates the back tension. Despite this back tension undulations or wrinkles are sometimes formed before a nip and close to a location of one of the substrate feed path rolls, usually a tensioning roll. Small undulations are sometimes pulled into the nip between different rolls and reach a printing zone or region of the printer, degrading the quality of printed images. In some cases larger undulations may be pulled into the nip and irreparably damage the substrate or even disrupt the printing process. One approach to these issues is to aim to reduce the number and size of the undulations by improving the accuracy of the

printer, and/or using highly stable and relatively stiff substrates, which complicates substrate transportation and adds to the cost of both the printer and the substrate.

[0004] According to an embodiment of the invention there is provided a method of removing undulations in a continuously transported web of flexible substrate, said method comprising:

a) providing a web substrate feeding path comprising spaced apart drive and back tension rolls, a pressure roll associated with the back tension roll and forming with said tension roll a back tension nip, and a curved support surface;

b) pulling said substrate by rotating said drive roll at first speed;

c) imparting on said substrate back tension by rotating said back pressure roll at a second speed;

d) supporting said substrate between the drive and back tension rolls by the support surface;

e) removing said web substrate undulations by periodically changing the width of said nip such that said width change releases and applies different back tension to said web substrate.

[0005] According to a further embodiment of the invention there is provided a method of removing undulations in a transported web of flexible substrate, said method comprising:

a) providing a web substrate feeding path comprising a drive and back tension rolls, a pressure roll associated with the back tension roll and forming with the tension roll a back tension nip, and a curved surface;

b) threading said web into said substrate feed path such that said drive roll pulls said substrate and back tension roll tensions the substrate;

c) supporting said web between the drive and the back tension rolls by said curved surface;

d) removing said web substrate undulations by periodically varying said nip width between said back tension and pressure rolls.

[0006] According to another embodiment of the invention there is provided a method of web printing comprising applying a back tension to a web substrate being printed upon at a printing zone, and relaxing the back tension from time to time so as to allow undulations in the web downstream of a printing zone to relax out of the web prior to the web being wound onto a collection roller.

[0007] The printer may comprise a print head and at least one UV curing source. The UV curing sources may be operated in one of a continuous or flash operating modes.

[0008] The UV curing sources may be of hot or cold type. The UV curing sources may be one of UV lamps or LEDs.

BRIEF LIST OF DRAWINGS

[0009] Objects, features and advantages of embodiments of the method and embodiments of the printer will be apparent from the more particular description of the exemplary embodiment of the method and of the printer, as illustrated in the accompanying drawings in which like reference numbers refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the method.

[0010] Figures IA and IB are schematic illustrations of the frontal and top view respectively of a wide format printer according to one exemplary embodiment of the invention;

[0011] Figure 2 is a schematic cross section of the wide format printer of Figures IA and IB and illustrates the substrate feed path of the embodiment of Figures IA and IB;

[0012] Figure 3 is a schematic 3D illustration of the paper path of the wide format printer of Figures IA and IB;

[0013] Figure 4A is a schematic illustration of how in one embodiment of the invention a substrate pulling force and a back tension force vary with time;

[0014] Figure 4B is a schematic illustration of the variation of back tension nip size with time, for the embodiment of Figures IA and IB; and

[0015] Figures 5A and 5B are schematic illustrations of a back pressure nip width variation of the embodiment of Figures IA and IB.

DETAILED DESCRIPTION

[0016] The principles and execution of a method, and the operation, structure, and properties of a printer, in this example an ink jet printing apparatus, may be understood with reference to the drawings and the accompanying description of the non-limiting, exemplary embodiments, shown in the Figures.

[0017] Reference is now made to Figure 1, which is a schematic illustration of the frontal view of a wide format printer 100. Printer

100 has a rigid frame 104 on which a print head 108, is movable on linear guides 112. Drive means (not shown) are mounted on the carriage 107 for moving it with reciprocating movement on the guides 112. UV ink curing radiation sources 116 are also mounted on the carriage 107 on both sides of print head 108 and move with the same reciprocating movement as does the print head 108. Suitable UV ink curing sources include UV lamps with hot or cold mirrors, LEDs or any other suitable radiation sources known in the art. Depending on the UV curing radiation source and printing speed, only one source mounted on an appropriate part of the carriage 107 may be provided, or used. The curing radiation source may be operated in a continuous or flash mode of operation.

[0018] Mounted on frame 104 are components of a substrate feed path including a substrate supply roll 120, a back tension roll 124 and associated with it a first, or back tension, pressure roll 128 which is supported by a series of clamping rollers 140 (in this example a series of eight pairs of clamping rollers 140). The number of clamping rollers depends on the width of the printer. Back tension roll 124 and its associated first pressure roll 128 are generally parallel to each other and span at least the width of the substrate on which printing is to be performed. For example, the substrate may be

5 meters (5,000 mm) wide and the rolls 120, 124 and 128 will be of a similar length. The distance between the surfaces of back tension roll 124 and its associated first pressure roll 128 comprises a back tension nip 130 (see Figures 2 and 3). The width of the nip 130 may

vary from zero (contact position) to a desired width, which may be equal to or larger than the substrate thickness. Each of rollers 140 (Figure 2) is mounted on an intermediate clamping bracket 144, which in use applies force towards the pressure roll 128 through a rolling contact directly against the surface of the pressure roll 128. Each clamping bracket 144 is pivotally mounted and carries a pair of clamping rollers 140.

[0019] Figure 2 is a schematic cross section of the wide format printer 100 of Figures IA and IB and illustrates additional details of the substrate feed path. Spaced apart from back tension roll 124 is a drive roll 160 and associated with it, a second pressure roll 164 supported by a series of clamping rollers 140 (eight pairs of clamping rollers in this embodiment). Drive roll 160 and an associated second pressure roll 164 span at least the width of the substrate on which printing is to be performed. The distance between the surfaces of the drive roll 160 and its associated second pressure roll 164, which are generally parallel to each other, form a drive nip 170. Another group of clamping rollers 140 is mounted on an intermediate clamping bracket 144, and supports second pressure roll 164 by applying force directly to the surface of the second pressure roll 164 through rolling contact. Each clamping bracket 144 is pivotally-mounted and carries a pair of clamping rollers 140. Also shown in Figure 2 are the guides 112 along which print head 108 and UV curing sources 116 move back and forth. Also shown is a support surface 136 which is a curved unitary surface, or is assembled from separate segments, and

over which printing takes place. Support surface 136 is located in the space between the back tension roll 124 and the drive roll 160.

[0020] A control unit 150 controls the operation of printer 100, the rotation speed of the drive and back tension rolls, 160 and 124, the width of nips 130 and 170, the operation of the UV radiation sources 116, and the operation of the printing process itself.

[0021] Pistons 180 (in this embodiment hydraulic or pneumatic pistons, but they could be electromechanical or other powered movement — causing devices, such as pistons) can distance .the clamping rollers 140 relative from the back tension and drive rolls 124 and 160. Variation in the position of the different sets of clamping rollers 140 changes the width of nips 130 and 170. Variation in the position of clamping rollers 140 will apply different pressure to the pressure rolls 128 and 164 respectively and will vary the width of nips 130 and 170, and the substrate back tension will vary/change (the substrate back tension being the tension in the web of substrate upstream of the nip in question).

[0022] Substrate web 200 is threaded in the substrate feed path from a substrate supply roll 120 that stores substrate 200, through back tension nip 130 formed by back tension roll 124 and the associated first or back tension pressure roll 128, over support surface 136 where the printing takes place, over and past drive roll 160 and its associated second pressure roll 164, and through the drive nip 170

formed by them. Generally, printed substrate 200 may be collected on a collection roll 210, or collected as a free-fall substrate. The free- fall substrate satisfies the "print-on-demand" printing mode operation requiring immediate delivery of the printed substrate to the client.

[0023] In use, the drive roll 160 is caused to rotate at a first speed and continuously pulls (actually peels off) substrate 200 off roll 120. Back tension roll 124 rotates at a second, different, speed. The first rotation speed is higher than the second rotation speed and the difference in the speeds creates a back tension in a section of a web of substrate 200 located between spaced apart drive roll 160 and back tension roll 124. The web of substrate is pulled past the back tension roll 124 and slides relative to the roll, the relative movement between the web of substrate and the roll causing lateral tension in the web of substrate. The support surface 136 located between drive roll 160 and back tension roll 124 supports in this state the tensioned web of substrate 200 in the printing zone or region. Nips 130 and 170 between the back tension and drive rolls, 120 and 160, controllable by moving the pressure rolls, enable grasping of the substrate and its transportation.

[0024] In this embodiment, the release of tension in the web is for about half a second. The frequency of the release of the tension in the web is about every 10 seconds. In other embodiments, the release of tension in the web is for between 100 ms and 2 seconds. In

other embodiments, the time between releases of the tension in the web can be about every 10 to 20 seconds. In other embodiments, the time between the releases depends on the printing mode or printing resolution or other relevant factors or any combination of these.

[0025] It will be appreciated that in another embodiment the rolls 124 and 160 could have substantially different diameters, and so could have the same angular velocity, but different circumferential speeds at the point of contact with the web,- and cause a back tension even if they rotate at the same velocity.

[0026] Continuous web substrate transport through the printer and a fixed back tension cause the formation of undulations or wrinkles 190 on substrate 200, the undulations 190 being schematically shown as lines 190 in Figure 3 before the nip 130. Small undulations may pass through the nip 130 and get into the printing zone or region where they adversely affect the print quality. Larger undulations collecting before the nip may be pressed into the substrate, and may disrupt the printing process or irreparably damage the substrate. Some of these effects may be reduced by using high quality dimensionally stable substrates. This however, implies higher printing costs.

[0027] We have realised that the undesired undulations in the substrate 200 may be removed by varying and in some cases completely relieving the back tension of substrate 200 from time to

time (for example periodically or occasionally). This temporary release of substrate 200 in back tension nip 130 reduces the back tension force and in some cases even cancels it or reduces it to zero or substantially zero, allowing the undulations 190 to smoothen. This may reduce the size and/or number of undulations 190 in the continuously pulled/transported substrate 20O ₇ or even eliminate them. The release of tension in the web of substrate does not interrupt the continuous transport of the substrate - the transport is continuous whether the nip is open or closed.

[0028] In some embodiments, including the embodiment of the Figures, the web is pulled to advance it to bring an area of the web to the printing zone. Turn the web is stationary and is printed upon at the printing zone. Turn the is advanced to move a new part of the web to the printing zone. The web may advance in intermittent steps, and during its advancement (movement) the back tension is momentarily released prior to printing on the web. Optionally, the back tension is released whilst the web is still moving (e.g. being pulled forward by the drive roller).

[0029] In an embodiment where a collection roll 210 is used to collect the printed substrate, the width of the nip 170 downstream of the printing zone or region may be varied in order to release the tension in the web of substrate 200 prior to being collected on the collection roll 210. The width of the nip 170 may be changed by the effect of the piston 180 downstream of the printing zone or region. The

piston 180 may release pressure from the downstream pressure roll 164, such that it moves away from the drive roll 160 and partially or fully releases the substrate. This can avoid or reduce the presence of undulations or wrinkles in the web substrate between the nip 170 and the collection roll 210.

[0030] Figures 4A and 4B are schematic illustrations of how the substrate 200 pulling force and the back tension force vary with time, as they are artificially applied in the course of printing. The temporary release of substrate 200 in back tension nip 130 (or change of the back tension force), may be achieved by variation of the nip width, as shown in Figure 4B.

[0031] Figures 5A and 5B are schematic illustrations of the back pressure nip 130 width variation. The pistons 180 can apply pressure to the pressure roll 128 via the clamping rollers 140. The pressure or movement applied from the pistons 180 changes the width of the nip

130 between the pressure roll 128 and the back tension roll 124.

When the nip 130 is increased in width, the tension on the substrate 200 is released, as shown in Figure 5B. In this embodiment the tension is fully released so that clamping rolls 140 do not urge the pressure roll 128 to force the web into pressured contact with the back tension roll 124. In some embodiments the distance between the back tension roll 124 and the pressure roll 128 is significantly wider than the thickness of the web substrate so as to allow

relatively free movement of the web substrate over the back tension roll 124.

[0032] The method of printing disclosed, supports printing on an undulation free (or greatly reduced undulation content) low cost continuously pulled flexible substrate. It places less stringent mechanical accuracy requirements on the printer architecture and simplifies the machine design.

[0033] It will be appreciated that the force applied to the web by the drive roll could be any suitable force: large enough to move the web at a fast enough speed and to cause tension in it, but not so large as to tear the web. Similarly, the back tension value could be any suitable value: large enough to keep the web flat and taut, but not so great as to tear the w _s eb (and lower than the drive roll force, so that the web does move forwards with back tension).

[0034] The above disclosure is intended as merely exemplary, and not to limit the scope of the method and printer, which is to be determined by reference to the appended claims.