External drum imaging system

The present invention provides a media handling system for an imaging system, such as an external drum platesetter. Separate input and output trays are provided, thereby allowing pre-staging of a second printing plate while a first printing plate is being imaged. The input tray is positioned over the output tray to reduce the floor space (i.e., footprint) of the imaging system.

 

What is claimed is:

1. An external drum platesetter, comprising:

an exteral drum cylinder having a cylindrical media support surface for supporting a printing during recording of an image thereon;

a leading edge clamp and a trailing edge clamp for holding a leading edge and a trailing edge of the printing plate onto the cylindrical media support surface during rotation of the external drum clinder;

a stationary ironing roller system for flattening the printing plate against the cylindrical media support surface as the external drum rotates past the stationary ironing roller system;

an input tray for supporting the printing plate in a staging position prior to mounting the printing plate onto the external drum cylinder; and,

a pair of input nip rollers positioned between the input tray and the leading edge clamp for holding the leading edge of the printing plate while the printing plate is in the staging position and for delivering the leading edge of the printing plate from the staging position of the leading edge clamp.

2. The external drum platesetter according to claim 1 wherein each nip roller of the pair of input nip rollers has a common tangent and wherein the common tangent is tangent with the cylindrical media support surface.

3. An external drum platesetter according to claim 1 further comprising a scanning system for imaging data onto the printing plate during rotation of the external drum.

4. An external drum platesetter according to claim 1 further comprising a first drive system for controlling the rotation of the external drum cylinder.

5. An external drum platesetter according to claim 4 further comprising a second drive system for controlling the rotation of at least one nip roller of the pair of input nip rollers to advance the leading edge of the printing plate from the staging position to the leading edge clamp of the external drum.

6. An external drum platesetter according to claim 5 wherein the first drive system and the second drive system are synchronized to simultaneously rotate the external drum and the at least one top roller for advancing the printing plate form the staging position to the cylindrical media support surface.

7. The external drum platesetter of claim 1 wherein the position of the leading edge clamp is fixed with respect to the cylindrical media support surface and further wherein the position of the trailing edge clam is movable around the outer diameter of the external drum with respect to the leading edge clamp.

8. The external drum platesetter of claim 1, wherein the leading edge clamp further comprises:

a plurality of registration pins for registering the leading edge of the printing plate on the media support surface of the external drum.

9. The external drum platesetter of claim 8, wherein the registration pins align the leading edge of the printing plate along an axis substantially parallel to a longitudinal axis of the external drum.

10. The external drum platesetter of claim 1 wherein the leading edge clamp further comprises:

a mounting portion which is attached to the external drum; and,

a clamping portion, which is pivotally attached to the mounting portion and biased to provide a clamping force for clamping the leading edge of the plate with enough force to prevent the plate from moving during rotation of the external drum.

11. The external drum platesetter of claim 10, wherein a distal end of the clamping portion comprises sufficient mass such that a center of gravity of the clamping portion is located between the biasing system and the distal end.

12. The external drum platesetter of claim 10, wherein the clamping portion comprises a center of gravity configured to cause a clamping force of the clamping portion to increase as a rotational speed of the external drum increases.

13. The external drum platesetter of claim 1, wherein the leading edge clamp comprises a single clamping portion, which extends along a substantial length of the external drum.

14. The external drum platesetter of claim 1, wherein the leading edge clamp comprises a clamping portion which is divided into a plurality of individual sections.

15. The external drum platesetter of claim 1, wherein the stationary ironing roller system further comprises:

an ironing roller assembly; and

an actuating system for selectively extending the ironing roller assembly against the media support surface of the external drum.

16. The external drum platesetter of claim 1, wherein the printing plate comprises aluminum.

17. The external drum platesetter of claim 1, further comprising:

an output tray for receiving the printing plate from the external drum after imaging and wherein the output tray is different than the input tray.

18. The external drum platesetter of claim 17, further comprising a pair of output nip rollers wherein at least one of the pair of output nip rollers is a driven roller, the pair of output nip rollers being positioned to receive the plate from the media support surface and deliver the plate to the output tray.

19. An external drum platesetter, comprising:

an external drum cylinder having a cylindrical media support surface for supporting a printing plate during recording of an image thereon;

a leading edge clamp and a trailing edge clamp for holding a leading edge and a trailing edge of the printing plate onto the cylindrical media support surface during rotation of the external drum cylinder;

an input tray for supporting the printing plate in a staging position prior to mounting the printing late onto the external drum cylinder; and,

a pair of input nip rollers positioned between the input tray and the leading edge clamp for holding the leading edge of the printing plate while the printing plate is in the staging position and for delivering the leading edge of the printing plate from the staging position to the leading edge clamp;

wherein the trailing edge clamp employs a vacuum to hold the trailing edge of the printing plate onto the cylindrical media support surface.

20. An external drum platesetter according to claim 19 wherein the trailing edge clamp comprises:

a bar that extends past both ends of the external drum and wherein the bar in combination with a gasket forms at least one vacuum chamber extending across the external drum;

a first disc mounted coaxially to a first end of the external drum and a second disc mounted coaxially to a second opposing end of the external drum, said first and second discs supporting the bar above the media support surface for moving the bar around the outer diameter of the external drum;

a first guided support post mounted between the first disk and the bar and a second guided support post mounted between the second disc and the bar each guided support post providing a movable support of the bar with respect to the first and second discs for providing radial movement of the bar with respect to the media support surface;

a stationary actuator system for selectively forcing the bar against the media support surface and the plate trailing edge to create a seal between the gasket and the media support surface and the plate trailing edge; and

a vacuum source connected to the at least one vacuum chamber for drawing air therefrom.

21. An external drum platesetter according to claim 20 further comprising a drive system for rotating at least one of the first and the second discs with respect to the external drum for positioning the bar over the plate trailing edge.

22. An external drum platesetter, comprising:

an external drum cylinder having a cylindrical media support surface for supporting a printing plate during recording of an image thereon;

a leading edge clamp and a trailing edge clamp for holding a leading edge and a trailing edge of the printing plate onto the cylindrical media support surface during rotation of the external drum cylinder;

an input tray for supporting the printing plate in a staging position prior to mounting the printing plate onto the external drum cylinder;

a pair of input nip rollers positioned between the input tray and the leading edge clamp for holding the leading edge of the printing plate while the printing plate is in the staging position and for delivering the leading edge of the printing plate from the staging position to the leading edge clamp;

a first actuator for selectively opening a clamping portion of the leading edge clamp;

a second actuator for selectively forcing a clamping bar of the trailing edge clamp toward the external drum; and

a third actuator for selectively extending an ironing system toward the external drum.

23. The external drum platesetter of claim 22, wherein the first, second, and third actuators are fixed in position relative to the external drum.

24. An external drum platesetter, comprising:

an external drum cylinder having a cylindrical media support surface for supporting a printing plate drum recording of an image thereon;

a leading edge clamp and a trailing edge clamp for holding a leading edge and a trailing edge of the printing plate onto the cylindrical media support surface during rotation of the external drum cylinder;

an input tray for supporting the printing plate in a staging position prior to mounting the printing plate onto the external drum cylinder;

a pair of input nip rollers positioned between the input tray and the leading edge clamp for holding leading edge of the printing plate while the printing plate is in the staging position and for delivering the leading edge of the printing plate from the staging position to the leading edge clamp;

an output tray for receiving the printing plate from the external drum cylinder after imaging; and

a guide platen mounted to a frame member of the platesetter, for guide platen mounted to a frame member of the platesetter, for guide the printing plate form the media support surface to the output tray.

25. The external drum platesetter of claim 24 wherein the guide platen is moveable between an input guide position for guiding the printing plate from the input tray toward the leading edge clamp; and, an output position for guiding the printing plate from the media support surface toward the output tray.

26. A method for loading a printing plate onto an external drum of a platesetter, comprising:

manually loading a printing plate onto an input tray, the input tray being oriented to guide a downward facing lading edge of the printer plate to a nip of a pair of input nip rollers;

support the downward facing leading edge of the printing plate in the nip of the pair of input rollers;

advancing the downward facing leading edge into a leading edge clamping mechanism located on a cylindrical medial support surface of an external drum used for supporting the printing plate during image recording thereon; and

positioning an ironing system against the printing plate to flatten the printing plate against the media support surface while the printing plate is being advanced form the input tray to the media support surface.

27. The method of claim 26 wherein the platesetter further comprises a first drive system for rotating the external drum and a second drive system for rotating at least one of the pair of input nip roller further comprising the steps of:

positioning the external drum to located the leading edge clamping mechanism in a plate receiving position;

rotating the pair of input nip roller to advance the downward facing leading edge of the printing plate to the leading edge clamping mechanism; and,

synchronously rotating the pair of input nip rollers and the external drum until an upward facing trailing edge of the plate is advancing beyond the input nip rollers.

28. The method of claim 27 further comprising the steps of:

rotating the external drum to position a trailing edge of the printing plate under a trailing edge clamping mechanism; and,

securing the trailing edge of the printing plate against the external drum using the trailing edge clamping mechanism.

29. The method of claim 28, further comprising the step of:

determining when the trailing edge of the printing plate is positioned under an actuator of the trailing edge clamping mechanism.

30. The method of claim 29, wherein the determining step further comprises:

establishing a location of the trailing edge using a predetermined plate wrap dimension of the printing plate.

31. A method for loading a printing plate onto an external drum of a platesetter, comprising:

manually loading a printing plate onto an input tray, the input tray being oriented to guide a downward facing leading edge of the printing plate to a nip of a pair of input nip rollers;

supporting the downward facing leading edge of the printing plate in the nip of the pair of input rollers;

advancing the downward facing leading edge into a leading edge clamping mechanism located on a cylindrical media support surface of the external drum, the leading edge clamping mechanism including a plurality of registration pins for providing a registration axis; and

registering the leading edge of the printing plate against the registration axis.

32. A method for loading a printing plate onto an external drum of a platesetter, comprising:

manually loading a printing plate onto an input tray, the input tray being oriented to guide a downward facing leading edge of the printing plate to a nip of a pair of input nip rollers;

supporting the downward facing leading edge of the printing plate in the nip of the pair of input rollers;

advancing the downward facing leading edge into a leading edge clamping mechanism located on a sylindrical media support surface of the external drum; and

image data onto the printing plate.

33. A method for unloading a printing plate from an external drum of a platesetter, comprising:

positioning a clamping bar under an actuator of a trailing edge clamping mechanism, the clamping bar securing a trailing edge of the printing plate against the external drum;

turning off a vacuum under the clamping bar; and

automatically biasing the clamping bar radially away from the external drum, thereby releasing the trailing edge of the printing plate.

34. The method of claim 33, further comprising:

extending an ironing system against the printing plate.

35. The method of claim 33, further comprising:

rotating the clamping bar about an axis of the external drum to displace the clamping bar out of an output path of the printing plate.

36. The method of claim 33, further comprising:

rotating the external drum to direct the trailing edge of the printing plate toward an output tray.

37. The method of claim 36, further comprising:

directing the trailing edge of the printing plate between a pair of output nip rollers, the output nip rollers advancing the printing plate onto the output tray.

38. The method of claim 36, further comprising:

detecting when the trailing edge of the printing plate is located adjacent the output nip roller assemblies.

39. The method of claim 36, further comprising:

providing a guide member for guiding the printing plate toward the output tray.

40. The method of claim 32, further comprising:

rotating the external drum to position a leading edge clamping mechanism under an actuator, the leading edge clamping mechanism securing a leading edge of the printing plate against the external drum.

41. The method of claim 40, further comprising:

opening the leading edge clamping mechanism using the actuator to release the leading edge of the printing plate.

42. An external drum platesetter, comprising:

an external drum cylinder having a cylindrical media support surface for supporting a printing plate during recording of an image thereon;

a leading edge clamp for holding a leading edge of the printing plate onto the cylindrical media support surface during rotation of the external drum cylinder;

a trailing edge clamp for holding a trailing edge of the printing plate onto the cylindrical media support surface during rotation of the external drum cylinder; and,

wherein said trailing edge clamp employs a vacuum to hold the trailing edge of the printing plate onto the cylindrical media support surface.

43. The external drum platesetter of claim 42, further comprising a landing zone on which at least a portion of the printing plate may be rested by an operator while the operator is loading the printing plate onto the input tray.

44. The external drum platesetter of claim 42, further comprising means for registering a leading edge of the printing plate with a longitudinal axis of the external drum.

45. An external drum imaging system, comprising:

an external drum having a cylindrical media support surface for supporting a printing plate during imaging;

a leading edge clamp and a trailing edge clamp for holding a leading edge and a trailing edge of the printing plate onto the cylindrical media support surface during rotation of the external drum;

a stationary ironing roller system for flattening the printing plate against the cylindrical media support surface as the external drum rotates past the stationary ironing roller system; and

an input tray for supporting the printing plate in a staging position prior to mounting the printing plate onto the external drum.

46. An external drum imaging system, comprising:

an external drum having a cylindrical media support surface for supporting a printing plate during imaging;

a leading edge clamp and a trailing edge clamp for holding a leading edge and a trailing edge of the printing plate onto the cylindrical media support surface during rotation of the external drum; and

an input tray for supporting the printing plate in a staging position prior to mounting the printing plate onto the external drum;

wherein the trailing edge clamp employs a vacuum to hold the trailing edge of the printing plate onto the cylindrical media support surface.

47. An external drum imaging system, comprising:

an external drum having a cylindrical media support surface for supporting a printing plate during imaging;

a leading edge clamp and a trailing edge clamp for holding a leading edge and a trailing edge of the printing plate onto the cylindrical media support surface during rotation of the external drum;

a first actuator for selectively opening a clamping portion of the leading edge clamp;

a second actuator for selectively forcing a clamping bar of the trailing edge clamp toward the external drum; and

a third actuator for selectively extending an ironing system toward the external drum.

48. The external drum imaging system of claim 47, wherein the first, second, and third actuators are fixed in position relative to the external drum.

49. An external drum imaging system, comprising:

an external drum having a cylindrical media support surface for supporting a printing plate during imaging;

a leading edge clamp and a trailing edge clamp for holding a leading edge and a trailing edge of the printing plate onto the cylindrical media support surface during rotation of the external drum;

an input tray for supporting the printing plate in a staging position prior to mounting the printing plate onto the external drum;

an output tray for receiving the printing plate from the external drum after imaging; and

a guide platen for selectively guiding the printing plate from the input tray to the media support surface and from the media support surface to the output tray.

50. A method for loading a printing plate onto an external drum of an imaging system, comprising:

loading a printing plate onto a tray, the input tray being oriented to guide a leading edge of the printing plate to a nip of an input nip roller assembly;

supporting the leading edge of the printing plate in the nip;

advancing the leading edge of the printing plate from the input tray into a leading edge clamping mechanism located on a cylindrical media support surface of an external drum; and

positioning an ironing system against the printing plate to flatten the printing plate against the media support surface while the printing plate is being advanced from the input tray to the media support surface.

51. The method of claim 50, further comprising:

positioning the external drum to locate the leading edge clamping mechanism in a plate receiving position;

rotating at least one roller of the input nip roller assembly to advance the leading edge of the printing plate to the leading edge clamping mechanism; and,

synchronously rotating each rotating roller and the external drum until a trailing edge of the printing plate is advanced beyond the input nip roller assembly.

52. The method of claim 51 further comprising:

rotating the external drum to position a trailing edge of the printing plate under a trailing edge clamping mechanism; and,

securing the trailing edge of the printing plate against the external drum using the trailing edge clamping mechanism.

53. The method of claim 52, further comprising:

determining when the trailing edge of he printing plate is positioned under an actuator of the trailing edge clamping mechanism.

FIELD OF THE INVENTION

The present invention is in the field of imaging systems. More particularly, the present invention provides a media handling system for an external drum image recording apparatus for recording images onto sheets of recording material such as printing plates.

BACKGROUND OF THE INVENTION

In external drum image recording devices, a movable optical carriage is used to displace an image exposing or recording source in a slow scan direction while a cylindrical drum supporting recording material on an external surface thereof is rotated with respect to the image exposing source. The drum rotation causes the recording material to advance past the exposing source along a direction which is substantially perpendicular to the slow scan direction. The recording material is therefore advanced past the exposing source by the rotating drum in a fast scan direction.

An image exposing source may include an optical system for scanning one or more exposing or recording beams. Each recording beam may be separately modulated according to a digital information signal representing data corresponding to the image to be recorded.

The recording media to be imaged by an external drum imaging system is commonly supplied in discrete sheets and may comprise a plurality of plates, hereinafter collectively referred to as "plates" or "printing plates." Each plate may comprise one or more layers supported by a support substrate, which for many printing plates is a plano-graphic aluminum sheet. Other layers may include one or more image recording (i.e., "imageable") layers such as a photosensitive, radiation sensitive, or thermally sensitive layer, or other chemically or physically alterable layers. Printing plates which are supported by a polyester support are also known and can be used in the present invention. Printing plates are available in a wide variety of sizes, typically ranging, e.g., from 9".times.12", or smaller, to 58".times.80", or larger. The printing plate may additionally comprise a flexographic printing plate.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention will best be understood from a detailed description of the invention and embodiments thereof selected for the purpose of illustration and shown in the accompanying drawings in which:

FIG. 1 illustrates an external drum imaging system for recording images onto a supply of recording media such as a printing plate;

FIG. 2 illustrates an example of an imaging system including a movable optical carriage and scanning system, usable in the external drum imaging system of FIG. 1;

FIG. 3 is a perspective view of the external drum of the imaging system of FIG. 1, in accordance with an embodiment of the present invention

FIG. 4 is an end view of the external drum of FIG. 3;

FIG. 5 is a plan view of the external drum of FIG. 3;

FIG. 6 illustrates an external drum platesetter of the present invention;

FIG. 7 illustrates the media handling system of an external drum platesetter in accordance with the present invention;

FIG. 8 provides an end view of a leading edge clamping mechanism in an open orientation;

FIG. 9 provides an end view of the leading edge clamping mechanism of FIG. 8 in a closed orientation;

FIG. 10 is a plan view of the leading edge clamping mechanism of FIG. 8.

FIG. 11 illustrates the location of the registration pins within the leading edge clamping mechanism of FIG. 8;

FIG. 12 is a partial perspective view of the trailing edge clamping mechanism;

FIG. 13A is a cross-sectional view of the clamping bar of the trailing edge clamping mechanism;

FIG. 13B is a cross-sectional view illustrating the operation of the clamping bar of FIG. 13A;

FIG. 14 is a cross-sectional view of a slidable support post and biasing system for supporting the clamping bar above the surface of the external drum;

FIG. 15 illustrates a friction wheel system for rotating the discs that carry the clamping bar of the trailing edge clamping mechanism;

FIGS. 16-21 illustrate the operation and media input flow of the media handling system of the external drum platesetter of FIG. 7;

FIGS. 22-25 illustrate the operation and media output flow of the media handling system of the external drum platesetter of FIG. 7;

FIGS. 26A and 26B illustrate the general input/output flow of the external drum platesetter of FIG. 7;

FIG. 27 illustrates an external drum platesetter in accordance with an alternate embodiment of the present invention;

FIG. 28 is a cutaway perspective view of the external drum platesetter of FIG. 27;

FIG. 29 is a perspective view of the external drum platesetter of FIG. 27, coupled to an on-line processor;

FIGS. 30A and 30B illustrate the general input/output flow of the external drum platesetter of FIG. 27;

FIGS. 31-44 illustrate the operation and media input/output process of the media handling system of the external drum platesetter of FIG. 27;

FIG. 45 illustrates an additional embodiment of the registration pins in accordance with the present invention;

FIG. 46 is a plan view of a registration pin arrangement in accordance with the present invention;

FIG. 47 illustrates the use of the registration pin arrangement of FIG. 46;

FIG. 48 illustrates a control system for controlling the media input/output operation; and

FIGS. 49 and 50 illustrate the operation of the edge sensors.

DETAILED DESCRIPTION OF THE INVENTION

The features of the present invention are illustrated in detail in the accompanying drawings, wherein like reference numerals refer to like elements throughout the drawings. Although the drawings are intended to illustrate the present invention, the drawings are not necessarily drawn to scale.

An example of an imaging system 10 employing an external drum image recording system is illustrated in FIG. 1. In this example, the imaging system 10 comprises an external drum platesetter configured to record digital data onto a printing plate. Although described below with regard to an external drum platesetter, many aspects of the present invention may be used in conjunction with a wide variety of other types of external drum, internal drum, or flatbed imaging systems, including imagesetters and the like, without departing from the intended scope of the present invention.

The imaging system 10 generally includes a front end computer or workstation 12 for the design, layout, editing, and/or processing of digital files representing pages to be printed, a raster image processor (RIP) 14 for further processing the digital pages to provide rasterized page data (e.g., rasterized digital files) for driving an image recorder, and an image recorder, such as an external drum platesetter 16, for recording the rasterized digital files onto a printing plate or other recording media. The external drum platesetter 16 records the digital data (i.e., "job") provided by the RIP 14 onto a supply of photosensitive, radiation sensitive, thermally sensitive, or other type of suitable printing plate 18. In the present invention, the printing plate 18 is manually loaded onto a staging area of the external drum platesetter 16 by an operator. Alternately, or in addition to manual loading, the printing plate may be provided and loaded onto the external drum platesetter 16 by a media supply or autoloading system 60. The media supply system 60 may accept a plurality of the same size printing plates 18, and/or may accept a plurality of different size printing plates 18.

The external drum platesetter 16 includes an external drum 20 having a cylindrical media support surface 22 for supporting the printing plate 18 during imaging. The external drum platesetter 16 further includes a scanning system 24, coupled to a movable carriage 26, for recording digital data onto the imaging surface 21 of the printing plate 18 using a single or multiple imaging beams 28. An example of a scanning system 24 is illustrated in FIG. 2. In particular, the scanning system 24 is displaced by the movable carriage 26 in a slow scan axial direction (directional arrow A) along the length of the rotating external drum 20 to expose the printing plate 18 in a line-wise manner when a single beam is used or in a section-wise manner for multiple beams. Other types of imaging systems may also be used in the present invention.

The external drum 20 is rotated by a drive system 36 in a clockwise or counterclockwise direction as indicated by directional arrow B in FIG. 1. Typically, the drive system 36 rotates the external drum 20 at a rate of about 100-1000 rpm. In the present invention, the printing plate 18 is loaded onto the external drum 20 while rotating the drum in a first direction. The printing plate 18 is then imaged while the drum is rotated in the first, or in a second, opposite direction. Finally, the printing plate 18 is unloaded from the external drum 20 while rotating the drum in the second direction. The details of the loading, imaging, and unloading operations are discussed in further detail below.

As further illustrated in FIG. 2, the scanning system 24 typically includes a system 30 for generating the imaging beam or beams 28. The system 30 comprises a light or radiation source 32 for producing the imaging beam or beams 28 (illustrated for simplicity as a single beam), and an optical system 34 positioned between the radiation source 32 and the media support surface 22 for focusing the imaging beam or beams 28 onto the printing plate 18. It should be noted, however, that the system 30 described above is only one of many possible different types of scanning systems that may be used to record image data on the printing plate 18. In the present invention, the system 30 comprises a multiple address grating light valve (GLV) or functionally similar modulator based system, or a multiple beam fiber optic coupled laser system.

In the external drum imaging system 10 shown in FIG. 1, the leading edge 38 of the printing plate 18 is held in position against the media support surface 22 by a leading edge clamping mechanism 40. Similarly, the trailing edge 42 of the printing plate 18 is held in position against the media support surface 22 by a trailing edge clamping mechanism 44. Both the trailing edge clamping mechanism 44 and the leading edge clamping mechanism 40 provide a tangential friction force between the printing plate 18 and the external drum 20 sufficient to resist the tendency of the edges of the printing plate 18 to pull out of the clamping mechanisms 40, 44, at a high drum rotational speed. In accordance with the present invention, only a small section (e.g., 6 mm) of the leading and trailing edges 38, 42, is held against the external drum 20 by the leading and trailing edge clamping mechanisms 40, 44, thereby increasing the available imaging area of the printing plate 18.

A stationary ironing roller system 46 flattens the printing plate 18 against the media support surface 22 of the external drum 20 as the external drum 20 rotates past the ironing roller 46 during the loading of the printing plate 18. Alternately, or in addition, a vacuum source 45 may be used to draw a vacuum through an arrangement of ports and vacuum grooves 47 (see, e.g., FIG. 2) formed in the media support surface 22 to hold the printing plate 18 against the media support surface 22. A registration system, comprising, for example, a set of registration pins or stops on the external drum 20, and a plate edge detection system, may be used to accurately and repeatably position and locate the printing plate 18 on the external drum 20. The plate edge detection system, as described infra, may comprise, for example, a plurality of sensors and/or the scanning system 24.

A perspective view of the external drum 20 in accordance with the present invention is illustrated in FIG. 3. An end view and a plan view of the external drum 20 are illustrated in FIGS. 4 and 5, respectively. As shown, the external drum 20 comprises an outer wall 48 that includes the media support surface 22, a hollow cylindrical hub 50, and a plurality of radial spokes 52 extending between the cylindrical hub 50 and the outer wall 48. The external drum 20 is rotated by the drive system 36 (FIG. 1) via shafts 53 coupled to the ends of the hub 50.

The external drum 20 is formed in a single piece using an extrusion process from a lightweight and strong material such as an aluminum alloy. Suitable aluminum alloys may include, for example, aluminum alloy 6063-T5. Other aluminum alloys, or alloys formed of metals other than aluminum, that can be suitably extruded, may also be used to form the external drum 20. In other embodiments of the present invention, however, the external drum 20 may be formed of a material such as steel or other ferromagnetic alloy using other processes. Such a material may be required if the leading edge clamping mechanism 40 and/or trailing edge clamping mechanism 44 utilize magnetic or electromagnetic clamping components.

The present invention provides a stiff external drum 20 having low rotational inertia. This allows the external drum 20 to be accelerated and decelerated more rapidly than other currently available drums, using smaller and less expensive motors, power supplies, etc., thereby further increasing the throughput of the imaging system 10 of the present invention.

The outer wall 48 of the external drum 20 further includes a section 54 containing a groove 56 that provides an interface for the leading edge clamping mechanism 40. The leading edge clamping mechanism 40 is attached within the groove 56 by inserting and securing suitable mounting hardware (e.g., bolts, etc.) through the leading edge clamping mechanism 40 and corresponding apertures 58 formed in the bottom of the groove 56. In the present invention, the groove 56 is disposed above one of the radial spokes 52. The relative thickness of the outer wall 48 is increased below the groove 56 to maintain minimum wall thickness requirements, and to offset any change in drum balance as a result of removing material to form the groove 56. By forming the groove 56 in this location, the stiffness and strength of the external drum 20 are not compromised. The groove 56 may be formed as part of the extrusion process, and/or may be machined into the external drum 20 after extrusion.

To compensate for the weight of leading edge clamping mechanism 40, and other adjacent system components, thereby balancing the external drum 20 during rotation, the section 60 of the external drum 20 opposite the groove 56 is provided with extra material (i.e., extra mass). This is achieved by increasing the extruded thickness of the outer wall 48 opposite the groove 56. Thus, the present invention nominally and inexpensively balances the external drum 20 and leading edge clamping mechanism 40 by adding extruded material opposite the clamping mechanism. Proper balancing of the external drum 20 helps to prevent the introduction of vibration-induced artifacts into the images recorded on the printing plate 18 by the imaging system 10.

The basic structure of the media handling system 70 of an external drum platesetter 16 in accordance with the present invention is illustrated in FIG. 7. The external drum platesetter 16 includes an external drum 20 (see, e.g., FIGS. 3-5) having a cylindrical media support surface 22 for supporting a printing plate 18 during imaging. The external drum 20 is supported by a frame 72. A drive system 36 rotates the external drum 20 during imaging. A scanning system 24, carried by a movable carriage 26, travels axially along the rotating external drum 20 to record digital data onto the imaging surface of the printing plate (see, e.g., FIG. 2). The external drum 20 and scanning system 24 are positioned on a base 74. The base 74 is formed of heavy material, such as a polymer-concrete mixture, granite, or the like, to vibrationally isolate the external drum 20 and scanning system 24 from external vibrations, thereby reducing artifacts in the recorded image.

In order to load and hold the printing plate 18 in intimate contact with the cylindrical media support surface 22 of the external drum 20 while the external drum 20 is rotated and an image is recorded onto the printing plate 18, a leading edge clamping mechanism 40 is provided to hold a leading edge of a printing plate 18 in position against the media support surface 22. The clamping system of the present invention, comprising the leading edge clamping mechanism 40 and the trailing edge clamping mechanism 44, is capable of holding a variety of different printing plate widths either left, right, or center justified on the external drum 20. An actuating system 75, including an actuator 76 (e.g., a pneumatic actuator, solenoid, etc.), selectively opens and closes the leading edge clamping mechanism 40 to receive, capture, and release the leading edge 38 of the printing plate 18. The actuating system 75 of the leading edge clamping mechanism 40 is mounted to a frame member (not shown) of the external drum platesetter 16 such that the actuating system 75 is positioned above the media support surface 22 of the external drum 20.

The leading edge clamping mechanism 40 is fixed in position on the external drum 20. The leading edge clamping mechanism 40 is positioned within a groove 56 (see, e.g., FIGS. 3-5) formed in the external drum 20. A set of registration pins or stops 78 (hereafter referred to as "registration pins") are incorporated into the leading edge clamping mechanism 40 to accurately and repeatably position, or "register," the leading edge 38 of a printing plate 18 at a predetermined location on the external drum 20, and to align the leading edge 38 of the printing plate 18 along an axis which is substantially parallel to the longitudinal axis of the external drum 20. The registration pins 78 may also be incorporated into an electrical or other mechanical structure for other purposes, such as to electronically or mechanically detect the presence of the leading edge 38 of the printing plate 18 within the leading edge clamping mechanism 40.

An embodiment of the leading edge clamping mechanism 40 is illustrated in greater detail in FIGS. 8-11. FIGS. 8 and 9 provide end views of the leading edge clamping mechanism 40 in open and closed positions, respectively. FIGS. 8 and 9 additionally illustrate the operation of the actuator 76 and the function of the registration pins 78. FIG. 10 provides a plan view of the leading edge clamping mechanism 40. FIG. 11 illustrates the set of registration pins 78 and the mounting portion 80 of the leading edge clamping mechanism 40.

As shown in FIGS. 8 and 9, the leading edge clamping mechanism 40 includes a mounting portion 80 and a clamping portion 82. The mounting portion 80 is used to secure the leading edge clamping mechanism 40 within the groove 56 of the external drum 20. As described with reference to FIG. 5, the leading edge clamping mechanism 40 may be attached within the groove 56 by inserting and securing suitable mounting hardware (e.g., bolts, etc.) through the mounting portion 80 and corresponding apertures 58 formed in the bottom of the groove 56.

The clamping portion 82 of the leading edge clamping mechanism 40 is attached to the mounting portion 80 by a biasing system 84. The biasing system 84, which may comprise a spring system including one or more springs, biases the clamping portion 82 of the leading edge clamping mechanism 40 closed against the mounting portion 80 with sufficient force to prevent the printing plate 18 from moving during rotation of the external drum 20. The actuator 76 is used to selectively open the leading edge clamping mechanism 40 to receive or release the leading edge 38 of the printing plate 18. In particular, as shown in FIG. 8, the actuator 76 includes an extendable member 86 that is configured to selectively engage and press against the clamping portion 82, thereby counteracting the biasing force of the biasing system 84 and opening the leading edge clamping mechanism 40. When the clamping portion 82 of the leading edge clamping mechanism 40 is in an open position as illustrated in FIG. 8, a printing plate 18 may be loaded against the registration pins 78 until two points of the leading edge 38 of the printing plate 18 are in contact with two registration pins 78.

As further illustrated in FIG. 8, during the loading of a printing plate 18 onto the external drum 20, the leading edge 38 of the printing plate 18 is accurately and repeatably positioned and aligned on the external drum 20 using the registration pins 78. As will be further illustrated below, only two axially spaced registration pins 78 contact the leading edge 38 of the printing plate 18 to ensure that the leading edge 38 is correctly positioned along an axis which is substantially parallel to the longitudinal axis of the external drum 20. A plate edge detection system (not shown), comprising, for example, an optical sensor, a mechanical sensor, etc., is used to electronically and/or mechanically sense or detect a perpendicular edge of the printing plate 18 (i.e., an edge perpendicular to the axis of the leading edge 38) to determine the axial position of the printing plate 18 on the external drum 20. Once the exact position of the printing plate 18 is determined on the external drum 20, the scanning system 24 (see, e.g., FIGS. 1, 2, and 7) can be accurately positioned by the movable carriage 26 to record image data in predetermined locations on the printing plate 18 with respect to the leading and perpendicular edges thereof.

As illustrated in FIG. 9, the leading edge 38 of the printing plate 18 is secured in position on the external drum 20 by closing the leading edge clamping mechanism 40. In particular, to close the leading edge clamping mechanism 40, the actuator 76 retracts the member 86 away from the clamping portion 82. This removes the force that previously counteracted the biasing force applied against the clamping portion 82 by the biasing system 84. Accordingly, the clamping portion 82 is now forced toward the external drum 20 by the biasing system 84, thereby securing the leading edge 38 of the printing plate 18 against the external drum 20.

After the leading edge 38 of the printing plate 18 has been properly positioned against the registration pins 78 and secured to the external drum 20 by the leading edge clamping mechanism 40, subsequent media handling operations may then be performed to completely load the printing plate 18 onto the external drum 20. Thereafter, image data may be recorded on the printing plate 18 by the scanning system 24 as the printing plate 18 is rotated on the external drum 20. Upon completion of the imaging process, the printing plate 18 is unloaded from the external drum 20 as will be described supra with reference to FIGS. 22-25. A layer of rubber or other nonabrasive material 83 may be applied to the clamping portion 82 to prevent damage to the imaging surface of the printing plate 18, to take-up the tolerances of the location of the clamping surface of the clamping portion 82, and to increase friction between the clamping portion 82 and the printing plate 18.

A distal end 88 of the clamping portion 82 of the leading edge clamping mechanism 40 is weighted such that the center of gravity of the clamping portion 82 is located between the biasing system 84 and the distal end 88. In FIGS. 8 and 9, for example, the center of gravity is to the "right" of the biasing system 84. By forming the clamping portion 82 in this manner, the clamping force applied by the clamping portion 82 against the external drum 20 and printing plate 18 increases as the rotational speed of the external drum 20 increases. This helps to prevent the clamping portion 82 from inadvertently releasing the leading edge 38 of the printing plate 18 during high speed rotation (e.g., 1000 rpm) of the external drum 20 during imaging.

A plan view of the leading edge clamping mechanism 40 is illustrated in FIG. 10. As shown, the leading edge clamping mechanism 40 may include a single clamping portion 82 that extends the length of the external drum 20, which may be opened and closed by an actuating system comprising a single actuator 76. Alternately, an actuating system comprising a plurality of actuators 76 (shown in phantom) may be positioned (e.g., distributed along) the external drum 20 to open and close the single clamping portion 82. The clamping portion 82 may also comprise a plurality of discrete sections 90 (shown in phantom), collectively operated by an actuating system including a single actuator 76, or a plurality of actuators 76.

A plan view of the mounting portion 80 of the leading edge clamping mechanism 40 is illustrated in FIG. 11. The mounting portion 80 includes a channel 92 through which the registration pins 78 extend away from the external drum 20, and along which the registration pins 78 may be selectively positioned and secured according the size or sizes of the printing plates 18 to be imaged. The channel 92 extends along the entire length of the external drum 20 to allow the registration pins 78 to be positioned at any axial location along the external drum 20. Generally, the registration pins 78 are positioned on the external drum 20 such that the leading edge 38 of a printing plate 18 contacts two of the registration pins 78. The registration pins 78 may be fixed in position along the channel 92 in any suitable manner. Each registration pin 78 includes a base 94 having a width W1 larger than the width W2 of the channel 92.

Referring again to FIG. 7, a stationary ironing roller system 46 is used to flatten the printing plate 18 against the media support surface 22 of the external drum 20 as the external drum 20 rotates past the ironing roller system 46 during the loading of the printing plate 18. In particular, the stationary ironing roller system 46 applies a force that keeps the printing plate 18 in contact against the media support surface 22 of the external drum 20 as the external drum 20 is rotated and the printing plate 18 is applied. The stationary ironing roller system 46 comprises an ironing roller assembl