WO2005029198A1

WO2005029198A1 - Low mass impression cylinder

Info

Publication number: WO2005029198A1
Application number: PCT/IL2003/000772
Authority: WO
Inventors: Eli Grinberg
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2003-09-25
Filing date: 2003-09-25
Publication date: 2005-03-31
Also published as: AU2003272044A1

Abstract

An elongate cylindrical structure (332) suitable for rotation while applying a force against another surface in a printing apparatus, including: an outer cylinder (310) having an inner surface (312) and an outer pressure surface; an inner cylinder (320) internal to and radially spaced from the outer cylinder (310) and having an outer surface facing the inner surface (312) of the outer cylinder (310); and a plurality of supporting ribs (330) attached to the outer cylinder (310) and the inner cylinder (320) and running axially along the cylinders.

Description

LOW MASS IMPRESSION CYLINDER FIELD OF THE INVENTION The present invention relates to an impression cylinder on a web press and in particular, an improved impression cylinder with low moment of inertia. BACKGROUND OF THE INVENTION In one type of web imaging, multiple tandem printing stations, each of which produces a different color separation, are used. In some digital web printing systems, a single printing engine is used. Such systems generally have multiple development stations, which produce color separations which are transferred either serially or after accumulation, for example, on an intermediate transfer member, to the web. In general, the web is backed by an impression cylinder which provides intimate contact between the final substrate and a cylinder (e.g., the intermediate transfer member), from which the image is transferred thereto. The impression cylinder is usually not driven by a separate drive, but is a follower, which is driven by friction with the web. The impression cylinder presses against the paper with a strong force, for example a force of about 1000 Newtons is common in a liquid toner digital press. Excessive deflection in the impression cylinder as a result of this force contributes to uneven image application that may show up as areas of decreased image brightness. To reduce cylinder deflection, the cylinder is manufactured with a robust mass and with a relatively thick wall. For systems using a single print engine to print multiple color separations, the web is subject to starts and stops and even reversals. For example, when each separation is transferred to the final substrate separately, the substrate must be repositioned at a starting point for each separation. This entails a reversal of the motion of the web after each separation is transferred. Impression cylinders and imaging rollers of various designs are known in the art. For example, Kesahara et el., in US Patent 5,708,949, disclose an imaging cylinder having relatively thin walls and a cylindrical internal support band.

There is thus provided, in accordance with an exemplary embodiment of the invention, an elongate cylindrical structure suitable for rotation while applying a force against another surface in a printing apparatus, comprising: an outer cylinder having an inner surface and an outer pressure surface; an inner cylinder internal to and radially spaced from the outer cylinder and having an outer surface facing the inner surface of the outer cylinder; and a plurality of supporting ribs attached to the outer cylinder and the inner cylinder and running axially along the cylinders. Optionally, the outer cylinder has a thickness of between 0.5 and 3 mm, between 0.6 and

2 mm or between 0.8 and 1.2 mm in thickness. Optionally, the inner cylinder has a thickness of between 0.5 and 3 mm, between 0.6 and 2 mm or between 0.8 and 1.2 mm. Optionally, the outer surface of the inner cylinder and the inner surface of outer cylinder are spaced from each other by between 2 and 8 mm or between 4 and 8 mm. Optionally, the thickness of each of the ribs is between 0.5 and 1.5 mm or between 0.8 and 1.2 mm. Optionally, a circumferential distance between adjacent ribs at the inner surface of the outer cylinder is between 4 and 12 millimeters, or between 6 and 8 millimeters. Optionally, a connection between a rib and a cylinder is formed with a fillet to reduce stress. Optionally, the outer cylinder has a diameter of between 60-100 mm or 70-90 mm. In an embodiment of the invention, the ribs extend in a radial direction. In an embodiment of the invention, adjacent ribs form an oblique angle with respect to each other. Optionally adjacent ribs meet at at least one of the junctures of the ribs with a cylinder, to form a triangular opening, when viewed in an axial direction. Optionally, adjacent ribs meet at the junctures of the ribs with both the inner and outer cylinders to form a series of triangular openings when viewed along an axial direction. Optionally, the structure includes bearing supports for allowing the structure to rotate. Optionally, the structure includes air circulation slots that allow air to circulate between the ribs. h an embodiment of the invention, the cylindrical structure is formed of a metal. Optionally, the metal is aluminum. Optionally, the outer surface of the outer cylinder is anodized.

In an embodiment of the invention, the structure has an MOI at least 20% less than that of a single solid cylinder having the same deflection properties when an axial surface of the cylinders is pressed against another surface. Optionally, the reduction in MOI is at least 30% or at least 40%. There is further provided, in accordance with an embodiment of the invention, an elongate cylindrical structure suitable for rotation while applying a force against another surface in a printing apparatus, comprising: an outer cylinder having an inner surface and an outer pressure surface; an inner cylinder internal to and radially spaced from the outer cylinder and having an outer surface facing the inner surface of the outer cylinder; and a plurality of supporting ribs attached to the outer cylinder and the inner cylinder. There is further provided, in accordance with an exemplary embodiment of the invention, printing apparatus comprising: an image holding surface adapted to support a toner image; a cylindrical structure, according to the invention, juxtaposed with the surface, such that it can be selectively pressed toward the image holding surface or removed therefrom; and a web feed that feeds a web of printing stock between the cylindrical structure and cylindrical structure, to allow the transfer of images thereto. Optionally, the printing apparatus includes a controller operative to control the position of the cylindrical structure with respect to the image holding surface, wherein during transfer of an image from the image holding surface to the web, the controller causes the cylindrical structure to be pressed against the web, such that the web is pressed against the image surface. In an embodiment of the invention, the image surface is the surface of a drum and the drum rotates during the image transfer, feeding the web into the space between the drum and the cylindrical structure Optionally, feeding of the web during said transfer also drives the cylindrical structure. Optionally, the apparatus includes web drive, wherein the controller causes retraction of the cylindrical structure and controls the web drive to selectively move and stop the web when no image transfer is occurring. Optionally, when the cylindrical structure is retracted, the web is held against the cylindrical structure by tension in the web, friction caused by said tension being sufficient to drive and stop the cylindrical structure. Optionally, the motion of the web when image transfer is not occurring includes reversal of the direction of the web from its direction during image transfer. There is further provided, in accordance with an embodiment of the invention, a method of manufacturing a cylindrical structure according to the invention, comprising extruding the structure. SUMMARY OF THE INVENTION An aspect of some embodiments of the invention is related to providing rollers or cylmders with reduced MOI, especially for use in printing machines, in which the roller or cylinder is subject to high acceleration. Optimally, the reduced MOI should not result is substantially increased deflection of the cylinder, in its intended use. In an embodiment of the invention, the cylinder is an impression cylinder and the reduced MOI allows it to accelerate and decelerate without hesitation, for example when driven by a tensioned moving web. During parts of a printing process in a web digital printer, the rotational motion of the impression cylinder is driven by it^'s friction with the linearly moving web. Hesitation, meaning slippage in the motion of the impression cylinder, is often a result of a delay in rotational motion due to the moment of inertia (MOI) of the cylinder. Hesitation can occur during starting, stopping or reverse in direction of the impression cylinder. While hesitation is not generally a problem in simplex images, so long as image placement (which is not generally a function of the impression cylinder) is correct. In duplex web imaging, when the impression cylinder contacts a freshly printed image on the first side while pressing the second side of the paper against the new image. Any hesitancy in cylinder stopping, starting or direction reversal may result in a line of abrasion in the image on the first side or even smudging. An abrasion line may appear as a rubbed-out section of the image. An aspect of some embodiments of the present invention relates to an impression cylinder having a reduced MOI. In an exemplary embodiment of the invention, the MOI is reduced by redistributing the mass of the cylinder, by placing more of the mass toward the center of rotation. Alternatively or additionally, the MOI is reduced by reducing the mass of the cylinder. In an exemplary embodiment of the present invention, an impression cylinder comprises a relatively thin- walled outer cylinder, a relatively thin walled inner support cylinder and a series of ribs that interconnect the two cylinders. In an exemplary embodiment, the resultant impression cylinder has a reduced mass. Alternatively or additionally, the mass of the resultant impression cylinder mass is redistributed toward its center of the cylinder radius so the cylinder has a reduced MOI. Optimally, the inner cylinder and ribs that support the outer impression cylinder provide robust support so that at least the outer cylinder exhibits relatively low deflection during impact with the paper. BRIEF DESCRIPTION OF THE DRAWINGS Exemplary non-limiting embodiments of the invention will be described with reference to the following description of embodiments in conjunction with the figures. Identical structures elements or parts that appear in more than one figure are preferably labeled with a same or similar number in all the figures in which they appear. Fig. 1 is a schematic diagram of the impression portion of a simplex web press; in accordance with an exemplary embodiment of the invention; Fig. 2 is a cut-away view of an impression cylinder, in accordance with a prior art embodiment; and Fig. 3 is a cut-away view of an impression cylinder, in accordance with an exemplary embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS Fig. 1 is a schematic diagram of the imaging section of a web press 120. As shown, this drawing applies to both the prior art and the invention, since the invention is manifested in the impression cylinder whose details are not shown in Fig. 1. In general, the following discussion represents the simplified operation of a W3200 press, distributed by Hewlett Packard Company, Indigo Division, in which the -impression cylinder of the present invention will be substituted for the existing impression cylinder. However, the cylinder of the present invention is useful in practically any web press and especially in a press in which duplex printing is performed. In this press, a series of images are defined serially on a photoreceptor 122 by an image forming system shown generally at 124. As indicated above the details for the image formation are not important to the invention and the method of image formation is described in summary form. In the W3200 press, the images are liquid toner images formed by charging the photoreceptor with a scorotron (not shown), selectively discharging the charged photoreceptor utilizing a multi-beam laser (not shown) to form a latent image having image and background portions and then developing the image by transferring a partial thickness of a layer of concentrated liquid toner to image portions of the latent image. A resultant single color image separation is transferred an intermediate transfer member 126, for subsequent transfer to a web 128 of paper or plastic. A plurality of single color image separations are similarly sequentially produced and transferred. In some embodiments of the invention, the images are sequentially transferred to the web. In other embodiments the images are sequentially transferred to and accumulated on intermediate transfer member 126 and then transferred as a group to the web. All the processes described above are well known in the art and have been described in the patent literature. As indicated above, while the described process is that used in the above referenced press, the process of producing the images is not part of the invention and any method of producing a single color or multicolor image on a surface (such as intermediate transfer member 126 or on photoreceptor 122 for direct transfer to web 128) can be used in the practice of the invention. The image can be either a liquid toner or powder toner image. Prior to transfer of an image to web 128, a first side 130 of web 128 is held against an impression cylinder 132 by tension rollers 134. As image transfer begins, the area of web 128 to receive the image moves in direction 136 and is aligned with the image on intermediate transfer member 126. Impression cylinder 132 presses on web 128 to engage a second side 138 with intermediate transfer member 126 during image transfer. At each image transfer from intermediate transfer member, impression cylinder 132 presses web 128 against intermediate transfer member 126 with sufficient force to aid transfer of the image from the intermediate transfer member to the web by heat and pressure. Generally, the intermediate transfer member is heated to a temperature at which toner polymer solvates and is plasticized by carrier liquid in the image. Force between the web and the intermediate transfer member (about 1000 Newtons of force on a 35 cm long impression cylinder) aids transfer of the image from the intermediate transfer member to the web. Such transfer is well known and has been widely reported in the patent literature. Web 128 is driven by intermediate transfer member 126 and impression cylinder 132 is driven as a result of its friction against the moving web 128. As indicated above, the printer can operate in one of two modes, in each of which a low MOI impression roller can be useful. Consider first the mode of operation in which the individual separations are transferred separately to the web from the intermediate transfer member. After a first separation is transferred to web, the web 128 is disengaged from intermediate transfer member 126 by moving impression cylinder 132 away from intermediate transfer member 126. At this point the web direction is reversed by a separate drive (not shown) and returned to a starting position, ready to engage the intermediate transfer member for the transfer of the next separation. This process is repeated until all of the separations are transferred. In an exemplary embodiment of the invention, a low MOI roller allows for faster stopping and/or reversal of the impression cylinder for a same amount of friction force. When all of the separations have been transferred, the web is stopped shortly after the transfer of the last image, to await the transfer of the first separation of the next image in a next position on the web. In the second mode of operation, the separations are transferred one at a time to the intermediate transfer member, on which they are accumulated. When all of the separations have been accumulated on the intermediate transfer member, the web is engaged with the intermediate transfer member by pressing impression cylinder against the first side of the web as described above. After transfer of the accumulated images, the web is stopped to await transfer of the next image. As indicated above, during image transfer, web 128 and impression cylinder 132 are subject to stops and starts. During image transfer, impression cylinder 132 is pressed against web 128 with great force, such that there is no slippage between the two. However, when the web is reversed or stopped (in either direction), the pressure of the impression cylinder against the web is limited, by the tension applied to the web by tension rollers 134. Since this tension is limited in order not to tear or stretch the web, slippage may occur between the impression cylinder and the web, if the MOI of the impression cylinder and the acceleration are high. When nothing is printed on the back of the web, this slippage causes no problem. When the press prints at a slow speed, so that the stops and starts can be gradual, the problems of slippage are minimal. However, when an attempt is made to produce high speed digital presses, the stops ad starts are of necessity more sudden and damage to images on the back of the web can occur, especially is they are freshly printed. Reducing the MOI of the impression cylinder, by making the cylinder wall thinner is not a viable solution to the problem. Due to the high force applied, the portion of the surface of thin cylinder that is pressed against the web and which is supported at its ends will bow outwards from the intermediate transfer member. However, in order to reduce bowing to less than 50 micrometers, the cylinder wall is generally at least 4.5 mm thick (which results in about 35 micrometer deflection), depending on the length of the impression cylinder. Attempts to substantially reduce the thickness (and thus reduce the MOI and the slippage) result in excessive bending of the impression cylinder. It is noted that since the intermediate transfer member is resilient, the problem of bowing is different than for a hard surface such as a photoreceptor (for example for direct image transfer therefrom to the web), however, the problem exists for both direct and indirect transfer. Fig. 2 is a cut-away view of a half of a prior art impression cylinder 232 (corresponding to generic cylinder 132 of Fig. 1). Impression cylinder 232 is typically extruded from Aluminum 6061 3 A, a standard composition of aluminum for impression cylinders, and machined on its outer surface after which the outer surface is hard anodized and coated with a thin layer (20 micrometers) of teflon. Additionally, support bands 244 are extruded and welded against inner wall 212. Bearing bands 240 that interface with bearings (not shown) are separately extruded, welded against support band 244 and machined on an inner surface 252. Cylinder 232, in the described embodiment has a diameter of 79 mm, although other sizes are of course possible. As indicated above, the prior art impression cylinder has a wall thickness of 4.5 mm. Fig. 3 is a cut-away view of an impression cylinder 332 (corresponding to generic cylinder 132 of Fig. 1). Impression cylinder 332 comprises an outer cylinder 310, and an internal cylinder 320 with a space 340 between cylinder 310 and cylinder 320. In an exemplary embodiment, ribs 330 radially project from cylinder 320 to span space 340 and com ect to cylinder 310, thereby stiffening outer cylinder 310. Typically, outer cylinder 310, internal cylinder 320 and ribs 340 are simultaneously extruded in one extrusion operation. In an exemplary embodiment of the invention, the juncture of the ribs and the cylinders is formed with filets for reducing stress. Impression cylinder 332 is optionally machined along its internal surface 312 to form optional bearing supports 360 and optional air circulation slots 362. Circulation slots 362 allow air to circulate between ribs 340 as the cylinder rotates, helping to keep it cool so that it does not distort with excessive heat. Each wall of cylinder 332, outer cylinder 310, internal cylinder 320 and ribs 340 typically have a thickness of one millimeter and cylinder 320, typically has an inside diameter of 61 millimeters, though these dimensions can be varied as explained below. In an exemplary embodiment, space 340 has a radial dimension of five millimeters and a circumferential dimension of about 6-8 millimeters (depending on the radius). This results in 24 ribs along the circumference. The number of ribs (or rather the distance between them), their thickness and the thickness of the inner and outer cylinders defines the mass, MOI and the ability of the surface of the outer cylinder to resist bending. Space 340 results in a portion of the mass being distributed toward the center of cylinder 332, contributing to its lowered MOI over that of cylinder 232. It should be understood that while the dimensions given (for an impression cylinder having a length of 350 mm) has been chosen to give a low deflection of 35 micrometers and an MOI that is 42% lower than the prior art impression cylinder 232, having a similar deflection, other tradeoffs between deflection and MOI as well as between dimensions is possible.

Table 1 Table 1 is a table comparing impression cylinder 232 of Fig. 2 to impression cylinder 332 of Fig. 3. The reduced MOI of the new design allows for much shorter stops and starts of the web and impression cylinder, without slippage, even at speeds at least as high as 3.6 m/sec, without damage to an image on the reverse side of the web. This is a general result, and not specific to the particular choice of wall thicknesses and dimensions. Furthermore, while the stress in the aluminum is slightly higher in the new design, it is still well within safe limits for the material. In an exemplary embodiment of the invention, outer cylinder 310, internal cylinder 320 and ribs 340 are extruded in one extrusion operation and each have a thickness of one millimeter. The thickness of outer cylinder 310, internal cylinder 320 and ribs 340 may be increased to 1.2 millimeters for use in a web press that requires higher impact forces, for example on thick media. Alternatively, their thickness may be decrease to 0.8 millimeters for use in printing on soft media. In an embodiment of the invention the ribs are radially extending. Alternatively, every two ribs 340 may intersect as they join cylinder 320 and or cylinder 310, thereby forming a series of triangular supports under cylinder 310. Alternatively or additionally, the number of ribs 340 may vary greatly, for example dependent upon anticipated stress in the web press or thickness of rib walls 340. Changes in the dimensions and spacing of the ribs may be contemplated. Although increasing the spacing of the ribs and increasing their thickness results in a same overall stiffness, too large a spacing will result in weakness of the outer cylinder between the ribs. Similarly, the spacing and thickness could be decreased. However, this may result in a more expensive extrusion, due to the small spacing and wall thickness. Other dimensions of the ribs and inner cylinder may also be varied within the invention. The present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. While the cylindrical structure has been described as useful in an impression roller, it can also be used in other applications within a printer and is especially useful where substantially uniform pressure must be applied over a substantial length, such as the width of a sheet or web of printing material. Furthermore, while it is especially useful for use where the cylinder is not driven by its own drive, it can also be used where a separate drive for the cylinder is supplied. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art. For example, the method of manufacture can vary, for example, in relation to the steps and or order of extruding, machining and/or finishing. Furthermore, the terms "comprise," "include," "have" and their conjugates, shall mean, when used in the claims, "including but not necessarily limited to." It is noted that some of the above described embodiments may describe the best mode contemplated by the inventors and therefore may include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. For example, while ribs that extend axially along the cylinders are described, under some operating conditions, the ribs may be provided at an angle to the axis or even extending in a circumferential direction. Furthermore, while, a structure with inner cylinders is described, it may be possible under some circumstances to achieve satisfactory strength and MOI reduction without the inner cylinder. Structure and acts described herein are replaceable by equivalents that perform the same function, even if the structure or acts are different, as known in the art. Therefore, the scope of the invention is limited only by the elements and limitations as used in the claims.

Claims

1. An elongate cylindrical structure suitable for rotation while applying a force against another surface in a printing apparatus, comprising: an outer cylinder having an inner surface and an outer pressure surface; an inner cylinder internal to and radially spaced from the outer cylinder and having an outer surface facing the inner surface of the outer cylinder; and a plurality of supporting ribs attached to the outer cylinder and the inner cylinder and running axially along the cylinders.

2. A cylindrical structure according to claim 1 wherein the outer cylinder has a thickness of between 0.5 and 3 mm.

3. A cylindrical structure according to claim 2 wherein the outer cylinder has a thickness of between 0.6 and 2 mm.

4. A cylindrical structure according to claim 3, wherein the outer cylinder has a thickness of between 0.8 and 1.2 mm.

5. A cylindrical structure according to any of the preceding claims wherein the inner cylinder has a thickness of between 0.5 and 3 mm.

6. A cylindrical structure according to claim 5 wherein the inner cylinder has a thickness of between 0.6 and 2 mm.

7. A cylindrical structure according to claim 6, wherein the im er cylinder has a thickness of between 0.8 and 1.2 mm.

8. A cylindrical structure according to any of the preceding claims wherein the outer surface of the inner cylinder and the inner surface of outer cylinder are spaced from each other by between 2 and 8 mm.

9. A cylindrical structure according to claim 8 wherein the outer surface of the inner cylinder and the inner surface of outer cylinder are spaced from each other by between 4 and 8 mm.

10. A cylindrical structure according to any of the preceding claims, wherein the thickness of each of the ribs is between 0.5 and 1.5 mm.

11. A cylindrical structure according to claim 10 wherein the thickness of each of the ribs is between 0.8 and 1.2 mm.

12. A cylindrical structure according to any of the preceding claims, wherein a circumferential distance between adjacent ribs at the inner surface of the outer cylinder is between 4 and 12 millimeters.

13. A cylindrical structure according to claim 12, wherein a circumferential distance between adjacent ribs at the inner surface of the outer cylinder is between 6 and 8 millimeters.

14. A cylindrical structure according to any of the preceding claims wherein a connection between a rib and a cylinder is formed with a fillet to reduce stress.

15. A cylindrical structure according to any of the preceding claims wherein the outer surface of the outer cylinder has a diameter of between 60-100 mm.

16. A cylindrical structure according to claim 15 wherein the outer surface of the outer cylinder has a diameter of between 70-90 mm.

17. A cylindrical structure according to any of the preceding claims wherein the ribs extend in a radial direction.

18. A cylindrical structure according to any of claims 1-16, wherein adjacent ribs form an oblique angle with respect to each other.

19. A cylindrical structure according to claim 18 wherein adjacent ribs meet at at least one of the junctures of the ribs with a cylinder, to form a triangular opening, when viewed in an axial direction.

20. A cylindrical structure according to claim 19 wherein adjacent ribs meet at the junctures of the ribs with both the inner and outer cylinders to form a series of triangular openings when viewed along an axial direction.

21. A cylindrical structure according to any of the preceding claims, including bearing supports for allowing the structure to rotate.

22. A cylindrical structure according to any of the preceding claims, including air circulation slots that allow air to circulate between the ribs.

23. A cylindrical structure according to any of the preceding claims formed of a metal.

24. A cylindrical structure according to claim 23 wherein the metal is aluminum.

25. A cylindrical structure according to claim 24 wherein the outer surface of the outer cylinder is anodized.

26. A cylindrical structure according to any of the preceding claims and having an MOI at least 20% less than that of a single solid cylinder having the same deflection properties when an axial surface of the cylinders is pressed against another surface.

27. A cylindrical structure according to claim 26 wherein the reduction in MOI is at least 30%.

28. A cylindrical structure according to claim 27 wherein the reduction in MOI is at least 40%.

29. An elongate cylindrical structure suitable for rotation while applying a force against another surface in a printing apparatus, comprising: an outer cylinder having an inner surface and an outer pressure surface; an inner cylinder internal to and radially spaced from the outer cylinder and having an outer surface facing the inner surface of the outer cylinder; and a plurality of supporting ribs attached to the outer cylinder and the inner cylinder.

30. Printing apparatus comprising: an image holding surface adapted to support a toner image; a cylindrical structure, according to any of the preceding claims, juxtaposed with the surface, such that it can be selectively pressed toward the image holding surface or removed therefrom; and a web feed that feeds a web of printing stock between the cylindrical structure and cylindrical structure, to allow the transfer of images thereto.

31. Printing apparatus according to claim 30 and including a controller operative to control the position of the cylindrical structure with respect to the image holding surface, wherein during transfer of an image from the image holding surface to the web, the controller causes the cylindrical structure to be pressed against the web, such that the web is pressed against the image surface.

32. Printing apparatus according to claim 31 wherein the image surface is the surface of a drum and wherein the drum rotates during the image transfer, feeding the web into the space between the drum and the cylindrical structure

33. Printing apparatus according to claim 32 wherein feeding of the web during said transfer also drives the cylindrical structure.

34. Printing apparatus according to any of claims 31-33 and including a web drive, wherein the controller causes retraction of the cylindrical structure and controls the web drive to selectively move and stop the web when no image transfer is occurring.

35. Printing apparatus according to claim 34 wherein, when the cylindrical structure is retracted, the web is held against the cylindrical structure by tension in the web, friction caused by said tension being sufficient to drive and stop the cylindrical structure.

36. Printing apparatus according to any of claims 31-35 wherein the motion of the web when image transfer is not occurring includes reversal of the direction of the web from its direction during image transfer.

37. A method of manufacturing a cylindrical structure according to any of claims 1-30, comprising: extruding the structure.