WO1999021059A1

WO1999021059A1 - Method and apparatus for imaging a continuous web

Info

Publication number: WO1999021059A1
Application number: PCT/US1998/022180
Authority: WO
Inventors: Howard A. Fromson; William J. Rozell; Paul C. Schunk
Original assignee: Fromson H A
Priority date: 1997-10-21
Filing date: 1998-10-21
Publication date: 1999-04-29

Abstract

A novel method and apparatus for the imaging of a continuous web (12) in an imaging device (44). The web (12) is punched with registration holes (42) by a punching mechanism (40) and imaged in a flat platen (46) imager (44) or an internal drum type imager (44) prior to cutting the web in a cutting device (52) into individual imaged plates, particularly for use as printing plates. The web (12) with an imageable surface is punched, imaged, and sheeted with the punched holes (42) being utilized to assure registration and alignment in the imaging operation and in subsequent use of the plates on a printing press. The web (12) is advanced under tension and then tension and/or vacuum (47) may be used to hold the web (12) on the imaging platen (46). With the internal drum type imager, the web is advanced under tension and then stopped while a roller (62) on an arm (68) forces the web down into contact with the platen (63).

Description

Method and Apparatus for Imaging a Continuous Web

The present invention relates to a method and apparatus for imaging a continuous web, and subsequently cutting the web into discrete sheets. More particularly, it relates to a method and apparatus for punching registration holes and imaging a web which is subsequently cut into discrete lithographic printing plates. The imaging may be in a planar, flat bed imaging station or in an internal drum type imager. The present invention assures that the plates are imaged and cut with improved alignment and subsequent registration on the printing press.

Background of the Invention

In the typical process for the formation of lithographic printing plates, the sensitized web of lithographic plate stock is cut into individual plates and stacked prior to imaging. The individual plates are optionally separated by individual sheets of interleaving paper where needed to protect the integrity of the coating. To image the plates, each individual plate is picked up from the stack, the interleaving paper is removed when present, and the plate is transferred to the imaging device. The imaging device may be of the internal drum type, the external drum type or of the flat bed design. After positioning the plate in the imaging device, vacuum is typically employed to hold the plate securely in place. Once the image is formed, the vacuum is released and the plate is removed from the imaging device. United States Patent

No. 5,61 9,246 discloses an apparatus for loading and unloading of individual plates into an imaging device. Where punched plates are desired, the individual plates are optionally punched prior to or subsequent to the imaging process as a discrete, separate operation. Such prior art processes require considerable material handling equipment, precise alignment techniques and consume a lot of time. An imaged plate must be completely removed from the imaging station before the next plate can be loaded for imaging.

Summary of the Present Invention The present invention relates to a novel method and apparatus for imaging a continuous web of imageable material. Specifically, the invention is for the imaging of a web of imageable lithographic printing plate stock and the sheeting or cutting of the imaged web into individual imaged printing plates. An object of the invention is to punch the web with registration holes and image the web in a flat platen imager or an internal drum type imager prior to cutting the web into individual imaged plates. The web of material with the imageable surface is serially imaged and sheeted (cut) and preferably also serially punched in a continuous process. The web is advanced through the punching and imaging steps into a set of pull rolls and then passed to a sheeter where it is cut into discrete sheets. The punching and imaging may be performed at separate stations or at a single punching and imaging station. The punched holes are utilized to assure registration and alignment in the imaging and sheeting operations. The web is preferably advanced under tension and tension and/or a vacuum may be used to hold the web in position during the operations. In the case of the flat platen imager, the tension and vacuum hold the web flat on the platen. In the case of the internal drum imager, the web is advanced under tension in a path running through the imager which has an arcuate concave imaging platen. With the web held in position at one side of the drum, the web is moved to conform to and be held against the arcuate concave imaging platen and then imaged. In either case, the web is then pulled to the next imaging position and the steps repeated. After imaging, the imaged areas of the web are cut into individual plates. The present invention thus simplifies the process of obtaining finished, imaged sheets, by combining several discrete process steps into a continuous process. Further, the time required to image individual sheets is reduced since the present invention allows the imageable web to be loaded into the imaging device as the already imaged area of the web is being removed. Since all operations are controlled with improved precision, the subsequent alignment and registration of the finished plates is greatly improved.

Brief Description of the Drawings Figure 1 is an illustration of a process line illustrating the method and the equipment for practicing the present invention using a flat platen imager.

Figure 2 is a top view of straight grain punched web according to the invention. Figure 3 is another top view of a punched web showing a cross grain form of punching.

Figure 4 is an illustration of a process line illustrating an alternate embodiment of the present invention.

Figure 5 is an illustration of a process line illustrating the method and the equipment for practicing the present invention using an internal drum type imager.

Figure 6 is a detailed side view of the imaging station of Figure 5.

Figure 7 is a front view of the imaging station viewing from the right in Figure 6. Figures 8A to 8D are illustrations of the imaging station of Figures

5 and 6 and the various stages of positioning and holding the web on the arcuate concave platen.

Figure 9 is a detailed side view of an alternative embodiment of the imaging station. Figure 10 is a detailed side view similar to Figure 9 but ready for imaging.

Description of the Preferred Embodiment The present invention relates to the imaging of any suitable web with an imageable surface where the desired resultant article is discrete, imaged sheets. In the preferred embodiment, the web of material is suitable for the formation of printing plates. The web will typically be a grained, anodized aluminum stock having a sensitized coating which is imageable by exposure to actinic radiation in the infrared, visible or ultraviolet wavelength ranges in a manner well known to those skilled in the art. The invention is particularly suited for but not limited to lithographic printing plates.

Figure 1 illustrates a web of imageable printing plate stock 1 2 coming off the unwind reel 1 4. The unwind reel 1 4 is driven by a controlled power source such as motor 1 6 with regenerative capabilities as known in the art. Such a device is capable of operating in reverse or of applying negative torque. A diameter sensing device 1 8 is used to provide a signal to the control system 20 representing the current diameter of the web on the unwind reel. Any suitably accurate distance-measuring device can be used. This diameter is required by the control system to accurately calculate the proper revolutions per minute of the unwind reel and thus control the speed of the motor 1 6 to set the proper web speed. The control system 20 is merely a conventional type of timing sequencing and calculating device to control the various functions of the entire process as discussed later.

Web tension is created by the pull roll system 22 providing a forward pull on the web and the unwind reel 1 4 providing the back tension through the motor 1 6. Web tension is sensed by a linear variable-differential transformer (LVDT) transducer or a stain gauge or similar known tension sensing device 24 associated with the roll 26. For example, the pillow blocks for the journals of the roll 26 can be mounted on the transducers which will then produce a signal proportioned to the force of the web on the roll 26 and thus proportional to the web tension. Rolls 28 and 30 maintain a constant angle of wrap of the web around the roll 26. The tension feedback signal from 24 is utilized by control system 20 to determine the proper control parameters of speed and torque for motor 1 6 thereby insuring that the proper web tension is maintained. The primary control of the registration in the process line is the pull roll system 22. The pull roll system 22 consists of two feed rolls 32 and 34 that are ground to a precisely controlled diameter. An optical encoder 36 is mounted on the pull roll drive motor 38 or one of the pull rolls. The optical encoder 36 determines the length of the web as the web advances to the next position and will feedback the position to the control system 20. The feed rolls are conventional intermittently driven types of feed rolls which engage the web and intermittently move the web forward a predetermined distance equivalent to the size of one printing plate as set into the control system 20. Following the roll 30 is a punching mechanism 40 as known in the art for punching registration apertures in the web. Examples are the holes 41 and notches 42 as shown in Figures 2 and 3. These holes and notches may be of any desired configuration and they may be straight grain punches as shown in Figure 2 (across the web) or cross grain punches as shown in Figure 3 (lengthwise along the edges of the web).

The notches 42 are for registration on the printing press while the holes 41 are typically used for registration in the equipment for forming the bend in the ends of the plates for mounting on the press cylinder. Since the registration notches and the bending holes, if required for bending, are pre-registered to the imaging system, the invention provides a very accurate registration system on press.

From the punching mechanism 40, the punched web is fed to the imaging station generally designated 44. This comprises a platen 46 and an imager 48. The punched web 1 2 is fed onto the platen 46 and is stopped in the proper position by the position sensor 50, such as an optical sensor, which detects an appropriate punched hole 42. This then provides for the indexing of the web to each successive position. The sensor 50 is ideally located within the imaging station 44 thus insuring the highest degree of accuracy in the imaging process. The control signal generated by position sensor 50 is used by control system 24 to identify the web stopping position and insure the accurate position of the punched features 42 in the imaging station 44. With the web 1 2 at a stationary position on the platen 46, the web is imaged by the imaging device 48 which, for example, might be any conventional laser or other imager. Upon completion of the imaging, the controller 20 again triggers the web to move forward the required amount ready for the next image.

The web 1 2 is maintained flat on the platen 46 by the web tension or by a vacuum which is drawn on the interface between the web and the platen by the vacuum pump 47. For example, the surface of the platen may contain a series of small holes connected with the vacuum pump 47 by appropriate channels. Also, a combination of web tension and vacuum may be used. A unique feature is that the vacuum need not be cycled on and off since the web can be moved across the platen in the direction perpendicular to the pull of the vacuum without releasing the vacuum.

Following the pull roll system 22 is a shear device 52 which cuts the web into individual plates. The shear device 52 is also controlled by the controller 20 and is preferably located an exact distance from the position sensor 50 that is a multiple of the desired plate length. Since the accuracy of the shear cut-off length is determined by this distance, the cutter is preferably located as close to the feed rolls as practical to minimize any accumulative errors. After cutting, the plates can be sent to a washing station which is not shown.

The imageable surface of the web 1 2 is on the upper surface as depicted in Figure 1 . Since the imageable surface is often a coating which should be protected, the web may be rolled together with an interleaving web of paper 54 which is located between each layer of the web 1 2 on the reel and protects the coating from the adjacent back-side of the web 1 2. As can be seen in Figure 1 , this paper web 54 is separated from the printing plate web 1 2 as it is unreeled. The paper web can then merely be rolled onto the spool 56 for disposal or reuse. The rotation of the spool 56 is controlled by the control unit 20. Another embodiment of the present invention is shown in Figure

4 which illustrates several alternate features of the invention. In this embodiment, although the punching and imaging steps are performed serially, they are performed at a single station. In this arrangement, the imaging station which is again generally designated 44, comprises not only an imaging platen 46 and an imager 48 but also punching means

45. The punching means 45 is mounted in the imaging station at 48 so that it is adjustable in relationship to the platen and imager. In this way, the punching can be adjusted with respect to the image area on the web and then fixed in that position, the punching and imaging take place serially without moving the web. In fact, the punches of the punching means 45 may optionally be maintained in engagement through the holes or notches in the web while the imaging operation is performed. This holds the web in a fixed position so that the punched holes and images are in total registration with each other. After imaging, the punches are disengaged by the control system 20 and the web is advanced to the next punching and imaging area. Since the registration on the printing press uses the same registration means, the plates and images are perfectly registered on the press. Although the punching means 45 has been shown downstream from the imager, it could be upstream or on the sides of the imager.

In the Figure 4 embodiment, an arrangement is illustrated for adjusting the position of the cutter 52 with respect to the punching and imaging. In this generally illustrated arrangement, the imaging station is mounted on a base 54. The cutter 52 is adjustably mounted at 56 on the same base 54. By this adjustable mounting means, the exact plate cut off position can be obtained. Such an adjustable mounting arrangement can also be used in the Figure 1 embodiment.

As indicated, the present invention may also be practiced with an internal drum type imager. Figure 5 illustrates such an arrangement in which like numbered components are the same as or generally similar to the corresponding components in Figure 1 . The primary difference is that the imager in Figure 5 is an internal drum type imager. As shown in Figures 5 and 6, the imager 44 now comprises an entrance roll 60, arcuate concave imaging platen 63, exit roll 61 , positioning roll 62 mounted on arms 68 and a conventional laser imaging head 65. The rolls 60, 61 and 62 are of conventional design utilizing a steel core covered with a non-marking material such as a rubber compound. The arcuate concave imaging platen 63 has a series of pin holes 66 that are formed in the surface of the platen which connect to the vacuum chamber 67 and the vacuum pump 47.

The web 1 2 passes over the imaging chamber during the indexing cycle as illustrated in Figure 8A. The web is stopped at the proper position by the position sensor 50, such as an optical sensor, which detects an appropriate punched hole 42. This can then provide for the indexing of the web to each successive position. The sensor 50 is ideally located at or close to the imaging station 44 to ensure the highest degree of accuracy in the imaging process. The control signal generated by the sensor 50 is used by the control system 20 to identify the web stopping position and stop the feed rolls to ensure the accurate position of the punched holes 42 in the imaging station 44. As a variation, the sensor can determine the actual position of the press registration apertures 42 and transmit that position to the control system 20. The control system then calculates the difference between the actual position of the apertures and the preset theoretical position. The calculated difference generates a datum line offset that is used by the control system to initiate the starting point for the laser imaging system in a known manner.

Upon completion of the indexing cycle, the leading end of the web 1 2 is held in a fixed position by the feed rollers 32 and 34 and the positioning roll 62 on the arms 68 begins to rotate in an arc parallel to the inner surface of the platen 63 pulling the web 1 2 from the supply reel 14 and forcing it into contact with the platen as illustrated in Figure 8B. The roll 62 then continues in the arc as shown in Figures 8C and 8D until the web 1 2 is in contact with the entire platen. The movement of the positioning roll 62 is controlled by the control system 20. As the web comes in contact with the platen the vacuum is activated utilizing the vacuum pump 47 with sufficient vacuum pressure to secure the web in position on the platen.

The web is now in position and is imaged with the laser imager 65 or any other suitable means for imaging on a concave platen. Once imaged, the control system 20 causes the web to move forward to the next position and the process is repeated.

The embodiment of the invention described thus far and shown in Figures 5,6, 7 and 8 relates to an internal drum imager where the arc of the platen and the arc that is imaged is less than 1 80°. Since the imaging head 65 must be located at the center of curvature of the platen and since the web in this embodiment is pulled straight through the imaging station and since the web must be pulled through at a level between the imaging head and the platen, it is required that the web be pulled through at a level below the center of curvature. This results in the platen and the web on the platen being less than 1 80°.

The embodiment of the invention shown in Figure 9 has a platen 63 which extends through an arc of more than 1 80°. In order to pull the web through the imaging station and have the web located between the imaging head 65 and the platen 63, guide rollers 70 and 72 are provided. These guide rollers 70 and 72 are mounted for vertical movement such as by the hydraulic cylinders 74. In Figure 7, the guide rollers 70 and 72 are in the extended or lower position for pulling the web 1 2 through the imaging station 44. Once the web has been pulled to a position for imaging, the positioning roll 62 is activated to force the web 1 2 into contact with the platen 63 after which the vacuum is activated just as in the previous embodiment. Either before or after the web has been forced into contact with the platen, the guide rollers 70 and 72 are retracted or raised to the positions shown in Figure 8. The positioning roll 62 has also been returned to its starting position such that all of the rolls 62, 70 and 72 are out of the imaging field or path. When the imaging step has been completed, the guide rolls 70 and 72 are lowered into the positions shown in Figure 7 and the web is advanced to the next position. There are a number of advantages of the present invention over the prior art. The punching, imaging and cutting are performed as a continuous operation instead of as individual, discrete processing steps. This eliminates the time and equipment for the handling of individual plates between each step. The time for imaging is reduced over the prior art since the web to be imaged can be fed into the imager as the already imaged region is being removed. Also, where interleaving paper is used, the cumbersome handling of individual interleaving sheets is replaced by a simple rewind device for a continuous web of paper. Further, with the flat platen imager, the use of vacuum to hold the web during imaging is optional. In some applications, the web tension will be sufficient to secure the web in position on the flat surface of the platen. Where desirable, vacuum may be used, but since the web is transported along the imaging surface in a direction transverse to the pull of the vacuum, the vacuum need not be cycled on and off during the loading and unloading operations. Thus a time savings can be realized even where vacuum is employed.

An additional advantage lies in the fact that the punching, imaging and cutting are done in a way so as to assure alignment and registration with much greater precision than the prior art. By punching and indexing the process for the imaging and sheeting operations from the punched holes, the finished plates can be prepared with a greatly improved registration when mounted on a press, resulting in a reduction of losses on start-up of the press. This alleviates the need for the costly and time consuming optical bending as is currently often used in the printing industry for registration.

Claims

Claims:

1 . Apparatus for imaging an imageable continuous web material and subsequently cutting said web into individual sheets for the production of imaged separate plates of a desired plate length comprising: a. an imaging station including means for producing an image on a portion of said imageable web material located in said imaging station; b. means for periodically advancing said continuous web material in steps of a desired plate length through said imaging station including means for stopping and positioning said web material in said imaging station; and c. cutting means for cutting said imaged web material into separate imaged plates of said desired length.

2. Apparatus as recited in claim 1 wherein said imaging station includes a flat imaging platen.

3. Apparatus as recited in claim 2 and further including means for maintaining said web material under tension through said imaging station.

4. Apparatus as recited in claim 3 wherein said means for periodically advancing said web material and for maintaining said web material under tension includes a supply reel for said web material, feed rolls for pulling said web from said supply reel through said imaging station, drive means for said supply reel, web tension sensing means and means responsive to said web tension sensing means for controlling said drive means to maintain a desired web tension.

5. Apparatus as recited in claim 4 wherein said drive means includes means for detecting the diameter of said web material on said supply reel and for controlling the speed of said supply reel.

6. Apparatus as recited in claim 1 wherein said imaging station comprises: a. an arcuate concave imaging platen; b. guide means for guiding said web material in a straight path through said imaging station spaced from said arcuate concave imaging platen; c. means adapted to force said web material from said straight path to a position conforming to said arcuate concave imaging platen; d. vacuum means for holding said web material in said position conforming to said arcuate concave imaging platen; and e. means for producing an image on the portion of said web material conforming to said arcuate concave imaging platen.

7. Apparatus as recited in claim 6 wherein said means adapted to force said web material to a position conforming to said arcuate concave imaging platen comprises roller means mounted on a pivot arm and adapted to swing in an arcuate path over the surface of said arcuate concave imaging platen and thereby force said web material into contact with said arcuate concave imaging platen.

8. Apparatus as recited in any one of claims 1 to 7 wherein said web material contains registration holes and wherein said means for stopping and positioning said web material in said imaging station includes means for sensing said registration holes and control means for activating and deactivating said means for periodically advancing said web material in response to said means for sensing said registration holes.

9. Apparatus as recited in any one of claims 1 to 7 and further including punching means for punching registration holes in said web material at intervals corresponding to said desired plate length.

10. Apparatus as recited in claim 9 wherein said means for stopping and positioning said web material in said imaging station includes means for sensing said registration holes and control means for activating and deactivating said means for periodically advancing said web material in response to said means for sensing said registration holes.

1 1 . Apparatus as recited in any one of claims 1 , 2, 3, 6 and 7 and further including a supply reel for said web material and punching means for punching registration holes in said web material at intervals corresponding to said desired plate length, said punching means being located between said supply reel and said imaging station.

1 2. Apparatus as recited in claim 1 1 wherein said means for stopping and positioning said web material in said imaging station includes means for sensing said registration holes and control means for activating and deactivating said means for periodically advancing said web material in response to said means for sensing said registration holes.

1 3. Apparatus as recited in any one of claims 4 and 5 and further including punching means for punching registration holes in said web material at intervals corresponding to said desired plate length, said punching means being located between said supply reel and said imaging station.

1 4. A method of forming individual printing plate images onto a continuous web of flexible printing plate stock containing an imageable surface thereon comprising the steps of: a. intermittently passing said web of printing plate stock into an imaging station having an imaging platen; b. stopping said web of printing plate stock in said imaging station and holding said web of printing plate stock on said imaging platen; c. imaging said web of printing plate stock in said imaging station; d. intermittently pulling said web of imaged printing plate stock from said imaging platen; and e. cutting said web of imaged printing plate stock into individual printing plates.

1 5. A method of forming individual printing plate images onto continuous web of flexible printing plate stock containing an imageabl surface thereon comprising the steps of: a. punching registration holes at selected locations in said web or printing plate stock; b. intermittently passing said web of printing plate stock containing said registration holes into an imaging station having an imaging platen; c. stopping said web of printing plate stock in said imaging station and holding said web of printing plate stock on said imaging platen; d. imaging said web of printing plate stock in said imaging station; e. intermittently pulling said web of imaged printing plate stock from said imaging platen; and f. cutting said web of imaged printing plate stock into individual printing plates.

1 6. A method of forming individual printing plate images onto a continuous web of flexible printing plate stock containing an imageable surface thereon wherein said continuous web is wound on a supply reel comprising the steps of: a. intermittently unwinding the web of printing plate stock from said supply reel into a punching mechanism and punching registration holes at selected locations in said web of printing plate stock; b. intermittently passing said web of printing plate stock from said punching mechanism into an imaging station having an imaging platen; c. stopping said printing plate stock on said imaging platen and imaging said printing plate stock on said imaging platen with a desired image; d. intermittently pulling said web of imaged printing plate stock from said imaging platen; and e. cutting said web of imaged printing plate stock into individual printing plates.

1 7. A method as recited in any one of claims 14, 1 5 and 1 6 wherein said continuous web is wound on said supply reel together with an interleaving protective web and comprising the step of removing said protective web onto a protective web reel as said continuous web is unwinding from said supply reel.

1 8. A method as recited in claim 1 5 wherein said steps of imaging and punching are both performed in said imaging station while said web of printing plate stock is stopped.

1 9. A method as recited in any one of claims 14, 1 5 and 1 6 wherein said step of imaging comprises imaging on a flat platen.

20. A method as recited in claim 1 9 and including the step of maintaining tension on said web of printing plate stock on said flat platen.

21 . A method as recited in claim 20 including the step of sensing said tension on said web of printing plate stock and maintaining a desired web tension.

22. A method as recited in claim 21 and further including the step of applying a vacuum to hold said web of printing plate stock against said flat platen.

23. A method as recited in any one of claims 1 5 and 1 6 wherein said step of stopping said web of printing plate stock in said imaging station comprises the step of sensing said registration holes.

24. A method as recited in claim 1 4 wherein said web of printing plate stock contains registration holes and wherein said step of stopping said web of printing plate stock in said imaging station comprises the step of sensing said registration holes.

25. A method as recited in claim 23 wherein said step of imaging comprises imaging on an arcuate imaging platen.

26. A method as recited in any one of claims 14, 1 5 and 1 6 wherein said step of imaging comprises imaging on an arcuate concave imaging platen.

27. A method as recited in claim 26 including the steps passing said web of printing plate stock in a path through said imaging station and forcing said web of printing plate stock from said path to a position in contact with said platen.

28. A method as recited in claim 27 and further including the step of applying a vacuum to hold said web of printing plate stock against said platen.