FIELD OF THE INVENTION

[0001] The present invention relates to the field of printing and, more particularly, to methods and apparatus for fastening printing plates onto vacuum drums, such as imaging cylinders.

BACKGROUND OF THE INVENTION

[0002] Computer-to-plate (CTP) is an imaging technology used in printing processes, in which Image data is output directly from a computer to a printing plate according to the image data. Exemplary printing plates include flexographic plates (sometimes referred to as "flexo plates" or flexible plates) comprising a photopolymer material disposed on a non- metal backing substrate. Other exemplary plates include letterpress plates, which comprise a metal backing substrate with a photopolymer layer. In an imaging step, a printing plate is positioned on an imaging cylinder (also called a drum) of an external drum imaging device. As the cylinder rotates around its axis, an image head moves in the axial direction and focuses one or more laser beams modulated with image data onto an ablatable layer of the plate — e.g. a laser ablatable mask (LAM) layer -- to ablate a pattern in the form of holes in the mask that subject the underlying photopolymer of the plate to actinic radiation (e.g. UV light) in an exposure step.

[0003] There are several different types of cylinder devices currently available for CTP imagers, dependent upon the types of printing plates used. For flexographic plates, a hollow cylinder is coupled to a vacuum drum system, or in the alternative, the hollow cylinder bears a piping system connected the vacuum drum system. The vacuum drum system is In communication with an internal volume formed by the hollow cylinder. The imaging cylinder or drum is typically comprised of a thickness of metal (such as aluminum) that is perforated such that the vacuum drawn through the thickness generates suction at the outer surface sufficient to releasably adhere a flexographic plate to the surface of the imaging cylinder, particularly as the imaging cylinder rotates around its axis. Additionally, the vacuum forces also operate to retain the flexographic plate in a fixed position relative to a position of the one or more laser beams. Further, the perforations, such as a pattern of holes or small grooves formed on the outer surface of the imaging cylinder, allow for the application of vacuum forces over a wide area of the flexographic plate.

[0004] For letterpress plates, the cylinder may comprise a ferromagnetic metal or may otherwise be rendered magnetically attractive such that the metal of the letterpress plate (e.g. steel) adheres to the drum by magnetic force. Hybrid systems, such as are described in U.S. Pat. No. 9,238,359, and incorporated herein by reference in its entirety, include a non-ferrous vacuum drum and a ferromagenetic foil that permit the same drum to be used for both flexographic plates without the foil and metal back letterpress plates with the foil. Various damping mechanisms may also be present to secure a first end of the plate (or foil for holding a plate) to the cylinder in a specific location.

[0005] Referring to the figures, FIGS. 1 and 2 depicts an exemplary drum apparatus 100 according to prior art. Drum apparatus 100 includes imaging cylinder 102 that is configured to rotate about axis 108. In operation, the exemplary Imaging cylinder 102 is coupled to a vacuum system 110. The outer surface 205 of imaging cylinder 202 is perforated (e.g. by a pattern of holes or grooves 202) such that the vacuum drawn through cylinder 102 generates suction at the surface of the imaging cylinder that releasably adheres a flexographic plate 204 on the outer surface of imaging cylinder 102. In one embodiment of the prior art, the flexographic plate 204 may further be attached to imaging cylinder 102 via a clamping mechanism 106.

[0006] The internal volume defined by the cylinder is in fluid communication with a plurality of vacuum grooves 202 and vacuum system 110. As the flexographic plate 204 is wound around the circumference of imaging cylinder 102, it is also retained in part by the suction drawn through vacuum grooves 202. Clamping mechanism 106 is operable to releasably clamp at least one end of a flexographic plate 204 to the outer surface of the imaging cylinder 102, such that the combination of damp and suction secures flexographic plate 204 to imaging cylinder 102. In use, flexographic plate 204 may be positioned via a loading table (not shown) of an imager, as is known by those of skill in the art, such that a leading edge of the plate is adjacent leading edge 206 of the damping mechanism in an open position. Upon closing the leading edge 206 of the clamp 106, the edge of flexographic plate 204 is secured. A trailing edge of the clamp 108 may then be opened and imaging cylinder 102 rotated 360 degrees about axis 108 to receive the flexo plate around its circumference. The remaining edge of flexographic plate 204 is then positioned under the trailing edge of clamp, and trailing edge is closed. In this way, the combination of damping mechanism and suction secures flexographic plate 204 to imaging cylinder 102.

[0007] One downside of the use of aluminum drums in laser-based ablation systems is that the laser energy used for ablating the mask may shine through the photopolymer and the non-metalllc plate backing substrate such that reflections of the laser energy from the shiny surface of the aluminum drum may impact the overall quality of the imaging.

Additionally, because the surface of the drum is perforated with holes or grooves, the surface of the drum does not have homogenous back-reflection properties, ranging from no back-reflection in portions where the light energy falls upon a hole/groove, to materially more back-reflection in the areas between the holes/grooves. In other embodiments, a chemical surface treatment is applied to the drum surface, including etching and anodization, in order to reduce the reflectivity of the drum surface to the laser radiation of the imaging beam. In all, the complexity of prior art vacuum drum systems, and the steps required for their manufacture, result in costly and time-consuming production.

[0008] Exemplary prior art solutions, such as are described in European Pat. No. 1920923B1, include a nickel mesh placed between the outer surface of the drum and the flexographic plate. The mesh is permeable such that the vacuum drawn through the mesh generates suction and provides a constant reflectivity for the infrared radiation of the one or more laser beams. However, the process of placing the mesh on the outer surface of the drum is complex and it is difficult to simultaneously place the flexographic plates and the mesh on the drum surface.

[0009] Other proposed solutions include increasing the absorption of the drum surface for the wavelength of the ablating radiation and reducing the depth of the grooves, such that the difference in reflectivity between the groove area and drum surface is negligible.

However, the former is an inadequate countermeasure because it is difficult to produce a constant absorption over the entirety of the drum surface and sometimes doing so results in a deinking of the drum surface. The latter is similarly an ineffective countermeasure because not enough of the incoming air volume can be removed through the relatively shallower groves. [0010] Thus, for various reasons, there remains a need in the art for improvements in vacuum drum systems.

SUMMARY OF THE INVENTION

[0011] One aspect of the invention relates to a vacuum drum of an imaging system. The vacuum drum comprises an outer wall of porous metal that defines a plate receiving outer surface radially spaced from a rotation axis of the cylinder and an interior volume bounded by the outer wall and a pair of opposite end covers. The porous metal is selected from the group consisting of aluminum, magnesium, iron, copper, and alloys or combinations thereof, and in particular, may comprise microporous aluminum, such as a product of the process of casting a composite comprising aluminum and salt, and subsequently washing away the salt. Embodiments may or may not include a clamping mechanism operable to releasably clamp at least one end of a printing plate to the vacuum drum.

[0012] In embodiments, the drum includes at least one peripheral area that Is porous and at least one peripheral portion that is non-porous. In one embodiment, the porous peripheral area and the at least one non-porous peripheral are both include the porous metal, wherein the at least one non-porous peripheral are comprises a sealant that plugs the pores of the porous metal. In another embodiment, the porous peripheral area and the at least one non-porous peripheral area both comprise the same metal, wherein the at least one non-porous peripheral area is fabricated without pores and the porous peripheral area is fabricated with pores. For example, the porous peripheral area may comprise microporous aluminum that is a product of a process comprising casting a mixture of molten aluminum and salt, cooling, and subsequently washing away the salt, wherein the at least one non- porous peripheral area comprises aluminum that is a product of casting the molten aluminum without the salt. In another embodiment, at least one non-porous peripheral area comprises a peripheral portion of the end cap having a periphery aligned with a periphery of the porous metal.

[0013] Another aspect of the invention relates to an imaging system comprising an imaging cylinder as described above. A vacuum system coupled to the interior volume of the vacuum drum is configured to apply a vacuum through the outer wall of the vacuum drum suitable to retain at least one printing plate on the outer surface of the vacuum drum. The suction may be sufficient to couple at least one flexographic printing plate directly to and in contact with the outer surface of the vacuum drum, including via a coupling force transmitted by the vacuum system suction through the outer surface in the vacuum drum. The suction may in some embodiments be sufficient to couple a ferromagnetic foil directly to and in contact with the outer surface of the vacuum drum including via the coupling force transmitted by the vacuum system suction through the outer surface in the vacuum drum, with at least one leterpress printing plate with a metal substrate coupled magnetically to the ferromagnetic foil. The system may further comprises a motor and a transmission connected to the vacuum drum configured to cause the vacuum drum to rotate in a predetermined direction.

[0014] Yet another aspect of the invention relates to a method of imaging a printing plate. The method includes providing the imaging system as described above, pulling a vacuum in the interior volume of the vacuum drum using the vacuum system, securing and retaining the at least one printing plate to an outer surface of the vacuum drum at least in part using suction drawn from the outer surface to the internal volume through the outer wall of the vacuum drum, and rotating the vacuum drum about the cylinder axis. The steps of securing and retaining may include or may omit the step of damping at least one end of the printing plate to the vacuum drum using a clamping mechanism. The printing plate may Include a flexographic printing plate secured directly to and in contact with the outer surface of the vacuum drum including via a coupling force transmitted by the vacuum system suction through the outer surface in the vacuum drum, or a letterpress printing plate magnetically secured to a ferromagnetic foil, wherein the ferromagnetic foil is secured directly to and in contact with the outer surface of the vacuum drum including via the coupling force transmitted by the vacuum system suction through the outer surface in the vacuum drum.

[0015] Yet another aspect of the invention relates to a vacuum drum for receiving a sheet on a sheet-receiving surface thereof, the sheet-receiving surface defined by an outer wall of microporous aluminum that defines at least in part an interior volume, the vacuum drum including a connector for connection to a source of vacuum operable to create a suction force at the sheet-receiving surface sufficient to removably secure the sheet to the sheet- receiving surface. In embodiments, the vacuum drum has a cylindrical geometry and is connected to means for rotating the vacuum drum about a longitudinal axis. The sheet may comprise a printing plate BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention may be understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, various features of the drawing may not be drawn to scale. On the contrary, the dimensions of the various features may be arbitrarily expanded or reduced for clarity. Moreover, In the drawing, common numerical references are used to represent like features. Included in the drawing are the following figures:

[0017] FIG. 1 is an isometric top view schematic diagram of an exemplary prior art drum apparatus;

[0018] FIG. 2 is an isometric side view schematic diagram of an exemplary prior art drum having a clamp mechanism;

[0019] FIG. 3 depicts a photograph of an exemplary drum according to an aspect of the Invention.

[0020] FIG. 4 depicts a cross-sectional schematic view of another exemplary drum according to an aspect of the invention.

[0021] FIG. 5 is a flowchart depicting an exemplary process for making a vacuum drum embodiment of one aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Aspects of the present invention include apparatus and methods for attaching flexible plates onto imaging cylinders, such as for use in digital imagers. An exemplary drum apparatus of an imaging device includes an imaging cylinder comprising microporous aluminum. The outer surface of the imaging cylinder is configured to receive at least one printing plate.

[0023] Embodiments of the invention will now be described in terms of exemplary methods and apparatus corresponding to an exemplary drum imager, in particular a Cyrel® Digital Imager (GDI), and more particularly, one of various CDI imagers made by Esko- Graphics Imaging GmbH of Itzehoe, Germany. Those in the art will understand, however, that the invention is not limited to use in connection with any particular imager embodiment.

[0024] FIGS. 3 and 4 depict imaging cylinder embodiments 300, 400, in which certain elements (e.g. vacuum source 408, motor 410) are depicted only in one figure to reduce clutter in the other, but should be understood to be present in both. The plate receiving substrate 302, 402 of the cylinder, configured for having a vacuum pulled therethrough, is radially spaced from the rotation axis A of the cylinder and defines an outer wall of the cylinder. The plate receiving substrate 302, 402 comprises a porous metal selected from the group consisting of aluminum, magnesium, copper, iron, and alloys and combinations thereof. The porous metal is permeable to pressurized gas, as well as negative pressure as applied by a vacuum. In a preferred embodiment, the plate receiving substrate 302, 402 comprises microporous aluminum, such as a section of microporous pipe or tube. Sealed end covers 304a, b, 404a,b cap the opposite ends of the section of pipe, thus defining an interior volume bounded by the microporous outer wall of the pipe and the end covers. One of the sealed end covers includes a connection 306, 406 to a source of vacuum 408 configured to evacuate the interior of the pipe, thus pulling a suction across the microporous pipe walls, A motor 410, attached to a transmission system (e.g. gearbox, shaft, etc.) is connected to shaft 411 and configured to rotate the imaging cylinder about axis A. Shaft 411 may be attached to flanges 313, 413a, b which may in turn be connected to respective end covers 304a, b, 404a, b. The system for driving the imaging cylinder may be similar to prior art systems, including various bearings, gears, shafts, and the like, without limitation. In one embodiment, to keep the manufacturing process relatively simple, the section of porous pipe may be attached to the end covers 304a, b, 404a, b, which are attached to the corresponding faces of flanges 313a, b, 413a, b, and are atached to the shaft of the drum by a press fit, adhesive, or a combination thereof. This and other constructions are discussed below in more detail.

[0025] Without limitation to any particular design, a suitable outer wall of microporous metal pipe comprise a wall thickness of up to 15 mm. In a preferred embodiment, a suitable microporous aluminum pipe may have a porosity in the range of 50% to 55%, thus reducing by 45% to 55% the mass relative to solid aluminum although not limited to any particular range of porosity. Pore sizes may be in the range of 0.2 mm to 0.65 mm, such as one preferred range of 0.30 to 0.50 mm, and in another preferred range of 0.20 to 0.35 mm, although not limited to any particular pore size or range thereof. Density of the porous aluminum may be in the range of 1.0- 1.2 gram/cm ³ , although not limited to any particular density or range of densities. Accordingly, the significant portion of the overall mass of the drum 300 is of the microporous metal pipe. This is especially advantageous because a low mass moment of inertia allows motor to operate the rotation of drum more efficiently, which reduces overall imaging time. In order to keep the mass moment of inertia low, the ratio of solid components versus the porous metal component of the drum is preferably kept as low as possible. Therefore, in a preferred embodiment, the porous peripheral area extends substantially across the entirety of the outer surface of the drum 400. More preferably, the porous peripheral area extends along a greater length than the maximum length of the one or more flexographic plates 430 releasably attached to the drum 400.

[0026] Additionally or optionally, portions 440 of the outer surface outside of the area intended to retain the one or more flexographic plates 430, may be filled with epoxy resin to reduce unnecessary leaks that may affect the efficiency of the suction force per surface unit generated by the vacuum drum 300. Preferably, the step of filling the exposed pores in areas not intended to be covered by flexographic plates attached to the drum surface is completed prior to the mechanical processing of the drum 400. In other embodiments, the non-porous portion of the drum may comprise portions 340 of end caps 304a, 304b aligned with the surface of the porous cylinder. In still other embodiments, the non-porous portion may comprise the same metal as the porous portion, but fabricated without pores.

[0027] According to aspects of this invention, the process for producing the imaging cylinder 300 is improved as compared to prior art processes for producing exemplary drum apparatus 100. First, the prior art step of forming perforations (e.g. a pattern of holes, grooves or channels for the application of vacuum forces over an area of the flexographic plate) is rendered moot by the invention. Furthermore, the prior art step of surface treatment for reducing the reflectivity of the drum surface in view of the laser radiation of the imaging beam is omitted. This is because the porous metal has a constant reflectivity within a certain range of tolerance that not only has less back- reflection than in the areas between the prior art perforations, but has a more consistent reflectivity over the surface of the metal. [0028] Assuming an approximately 0% effective reflectivity of the pores, reflectivity is proportional to the nonporous aluminum content of the porous drum times the reflectivity of the aluminum at the drum surface. For example, for a porosity of 50 % and an approximate reflectivity of aluminum of 80%, the reflectivity of porous aluminum is approximately 0.5 * 0.8 = 0.4 (40%). Accordingly, the reflectivity of the outer surface of the imaging cylinder formed of microporous metal may typically be in the range of 20% to 60%, without being limited to any particular value.

[0029] According to aspects of this invention, a suitable process of forming the porous metal component of the imaging cylinder 300 includes processes known to one skilled in the art, such as are described in U.S. Pat. No. 10,538,078, incorporated herein by reference, which is directed to permeable cylinders formed by way of sintering or foaming methods. Regarding the former method, a portion of a cylinder for a printing sleeve is made of sintered metal to allow compressed air to build an air cushion between the cylinder and the sleeve for mounting and unmounting the sleeve on the cylinder. Regarding the latter method, foaming introduces gases into the liquid aluminum melt, by way of (1) adding gas- producing agents to aluminum grains prior to melting or (2) pressing gas into the aluminum melt such that small gas bubbles are included in the aluminum after solidification. However, some sintering processes may introduce shrinkage as well non-homogenelty across different sections of a sintered object (especially of a larger size). Due to the fast oxidation of aluminum and the fact that the oxide layers on top of the aluminum grains have higher melting temperatures, some larger completed sintered parts may lack the mechanical stability required for application. Likewise, porous metal formed by some foaming processes may suffer from poor homogeneity, because the size of the gas bubbles produced may cause the ratio between metal (e.g. aluminum) and the space in between to be difficult to control.

[0030] In a preferred embodiment, a suitable process to form the porous metal component of the imaging cylinder 300 includes a process performed by Alumeco A/S of Odense, Denmark (to make its ALUPOR™ porous aluminum) or Exxentis AG, of Switzerland. Both companies create cast porous aluminum products using a mixture of metal and salt via a process depicted schematically in the flowchart of FIG. 5. Exemplary casting process 500 includes in step 510 creating a mold filled with crystal salt (with the size of the crystals selected according to the targeted pore size), impregnating the salt-filled mold with molten aluminum under pressure or vacuum in step 520, and cooling the salt-aluminum cast composite in step 530. The cast composite is then processed — e.g. machined by milling, grinding, or turning on a lathe in step 540. The salt is then washed away in step 550, leaving behind a porous metal body. A plurality of porous metal bodies (e.g. rings) may be bolted and glued together to build tubes of varying lengths. Other processes may include, rather than first filling the mold with salt in step 510 and then impregnating with aluminum in step 520, forming a molten combination of salt and aluminum in first step and then pouring the molten combination into the mold in a second step. The remaining steps may be as depicted in FIG. 5.

[0031] In one embodiment, to provide both porous 402 and non-porous 440 portions of the cylinder, a pair of end portions of the cylinder 440 are cast in solid material instead of the molten metal and salt mixture. Specifically, the first section of the casting form Is filled with solid metal to form the first end portion of the cylinder 440a. Then, the second section of the casting form is filled with the mixture of molten metal and salt to form the porous metal component of the cylinder 402. Finally, the third section of the casting form is filled with solid metal to form the second end portion of the cylinder 440b.

[0032] The porous substrate 302, 402 of the imaging cylinder may also be coupled to respective end covers 304a, b, 404a, b by a press fit and adhesive. Without limitation to any particular materials or constructions, the sealed end covers 304a, 304b may comprise a any type of material and may be attached to the porous metal pipe adhesively or using any technology known in the art for attaching prior art end covers to perforated/grooved metal cylinders. In one embodiment, the sealed end covers 304a, 304b are made of the same material as the pair of end portions of the cylinder 300, but the invention is not limited to any particular types of materials.

[0033] According to aspects of this invention, the vacuum drum 300 defining the plate receiving surface 302 comprising porous metal is improved as compared to prior art vacuum drums, because the flexographic plates can be releasably atached to the drum surface at any position along the drum circumference without compromising suction forces. This advantage is due to the homogenous porosity of all sections of the drum surface, the suction force per surface unit are evenly distributed over the entirety of plate receiving surface 302. Accordingly, embodiments of the vacuum drum 300 rely solely on suction forces le without use of a damping mechanism or other non-vacuum means (e.g. adhesive tape) i.e. systems that releasably retain the one or more flexographic plates to the drum 300. Advantageously, the vacuum drum 300 configured to rotate about its axis at relatively lower revolutions per minute (RPM) may retain partial flexographic plates that do not extend along the complete length and/or circumference of the drum 300. Further, a plurality of plates or plate patches may be releasably adhered to the drum 300, which may thereby allow processing of a plurality of plates or plate patches, preferably all of a same plate thickness, but potentially all having different lengths and widths, in a single imaging step. Thus, whereas prior art drums may be capable of holding plates up less than 2mm thickness using adhesive tape at up to 200 RPM on cylindrical drums having a circumference between 42 inch and 50 inch circumference, embodiments of the present invention are expected to provide sufficient retention forces using vacuum alone on porous aluminum drum of the same or similar circumferential dimensions operated at approximately the same or similar speeds.

[0034] According to aspects of this invention, the system for securing flexographic plate 204 to imaging cylinder 102 is improved as compared to prior art systems, because the microporous aluminum creates a more homogenous vacuum and a more homogenous, less reflective surface, thus leading to more homogenous imaging, and higher quality printing plates.

[0035] Retention ability of a vacuum drum improves with the strength of the vacuum as measured by negative pressure per unit area exerted at the outer surface of the drum. Energy costs can be minimized by increasing efficiency of the vacuum system, such as by minimizing losses across the thickness of the drum material, such that less vacuum inside the volume of the drum is required to pull the same amount of vacuum at the outer surface. Furthermore, the mass of the imaging drum directly affects how much energy is required to operate the drum, particularly with respect to acceleration and deceleration of the drum. Porous aluminum cylinders may have less mass and may have less pressure loss across the wall thickness than prior art perforated /grooved aluminum, thus allowing operation with less energy consumption by the motors configured for rotating the cylinders and less energy consumption by the connected vacuum system. [0036] The microporous aluminum thus facilitates a homogenous vacuum applied from the outer surface of the imaging cylinder 102 to the loaded flexo plate 204. Further, the improved suction may allow for easier and safer loading and unloading of flexo plates 204 in embodiments that may avoid use of a damping mechanism. In other embodiments, such a clamping mechanism may still be present, as described above with respect to the prior art systems of FIGS. 1 and 2, without limitation to any particular clamping mechanism design. Various damping mechanism designs are well known in the art and may be formed into the porous aluminum cylinder in the same way as such clamping mechanisms are formed in prior art perforated cylinders.

[0037] The homogeneity and efficiency of suction using microporous aluminum may permit the use of greater suction force per unit energy than prior art systems, or use of the same suction using less energy. In still other embodiments, the power provided by the vacuum pump may be lower for the porous drum in comparison to a conventional vacuum drum because a good part of the pumping volume in prior art systems is needed to compensate for losses that arise from the flow resistance of the extracted air in the small apertures that connect the inside drum volume with the grooves on the drum surface. The flow resistance is much lower with porous aluminum, whereas the minimum pressure required to hold a sheet is generally lower (at least in part because the pressure per unit area is spread over a greater area). This generally requires a vacuum source with relative higher displacement volume, but having a minimum pressure that does not have to be as far below atmospheric pressure to provide a suitable relative vacuum. In some cases, this may allow for the use of a less expensive vacuum source, such as using suction provided by a fan or a blower rather than a vacuum pump.

[0038] Although described herein with respect to imaging cylinders, it should be understood that invention as described herein may be applied to any type of vacuum drum apparatus constructed for retaining a sheet of any materials of construction on an outer surface of the drum using vacuum, including devices having geometries other than that of a right circular cylinder, as shown in the figures.

[0039] Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. In particular, the Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.