Abstract:
A fixing structure comprises a rotary drum having an outer peripheral surface, around which a printing plate is wound, and an inner peripheral surface portion, the drum including through-slots formed in the outer surface at predetermined intervals in the peripheral direction, each of which communicates the outer surface with the inner surface portion; and a chuck detachably mountable to the drum, for clamping one end of the printing plate onto the drum, the chuck including a support having opposite ends, a clamp plate provided on one support end, and a base section provided on the other support end, wherein when the chuck is mounted on the drum, the base section is inserted and passed through one associated through-slot and then hooked over the inner surface portion.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for detachably fixing a sheet member onto a rotor such as a rotary drum, and to an image forming apparatus incorporating the former. 
     2. Description of the Related Art 
     Generally, a photosensitive printing plate (hereinafter, “printing plate”), comprising a sheet-like support (e.g., a thin aluminum plate) having disposed thereon a photosensitive layer, is used for printing. Printing plates having different vertical and horizontal dimensions are used depending upon the size of the item to be printed. 
     An image exposure apparatus that image-exposes the printing plate includes a device for winding the printing plate around a rotary drum and irradiating the printing plate with a light beam corresponding to image data while the printing plate is rotated integrally with the rotary drum, whereby the printing plate is scan-exposed. 
     When the printing plate is wound around the rotary drum, the printing plate is nipped and fixed to the rotary drum by holding devices (chucks). The holding devices clamp ends of the printing plate along the circumferential direction of the rotary drum to thereby fix the ends between the chucks and the peripheral surface of the rotary drum. 
     Namely, a holding device corresponding to one end (e.g., a leading edge in the direction in which the printing plate is wound) of the printing plate along the circumferential direction of the rotary drum is attached at a predetermined position on the rotary drum. After the printing plate has been wound around the rotary drum, a holding device corresponding to the other end (i.e., a trailing edge) of the printing plate is attached at a position according to the size of the printing plate. 
     The holding device for attaching the printing plate to the rotary drum at the position according to the size has a fixing barrel (stanchion) which can be inserted into an arbitrary position of an attachment groove that is formed along the peripheral direction in the peripheral surface of the rotary drum. The fixing barrel is freely movable relative to the attachment groove, whereby the ends of printing plates having various sizes can be nipped in suitable positions. The attachment groove for housing the fixing barrel has a transverse cross section that can be quite complicated. 
     However, this type of rotary drum necessarily holds considerably large, solid portions in which such grooves can be formed. This means that the rotary drum inevitably becomes large and heavy, which poses an obstacle to high-speed rotation of the rotary drum. 
     SUMMARY OF THE INVENTION 
     In light of the above-mentioned fact, according to one aspect of the present invention, there is provided a fixing structure for detachably fixing a sheet member, comprising: a rotor having an outer peripheral surface, around which the sheet member is wound, and an inner peripheral surface portion, the rotor including a plurality of through-slots formed in the outer peripheral surface of the rotor at predetermined intervals in the peripheral direction, with each of the through-slots being elongated in the peripheral direction and communicating the outer peripheral surface with the inner peripheral surface portion; and a chuck detachably mountable to the rotor, for clamping one end of the sheet member onto the rotor, the chuck including a support having opposite ends, with a clamp plate being disposed at one end of the support and a base section being disposed at the other end of the support, wherein when the chuck is mounted on the rotor, the base section is inserted and passed through one associated through-slot and then hooked over the inner peripheral surface portion. 
     In accordance with another aspect of the present invention, there is provided an apparatus for forming an image on a printing plate, comprising: a rotary drum having an outer peripheral surface, around which the printing plate is wound, and an inner peripheral surface portion, the rotary drum including a plurality of through-slots formed in the outer peripheral surface of the rotary drum at predetermined intervals in the peripheral direction, with each of the through-slots being elongated in the peripheral direction and communicating the outer peripheral surface with the inner peripheral surface portion; a chuck detachably mountable to the rotary drum, for clamping one end of the printing plate onto the rotary drum, the chuck including a support having opposite ends, with a clamp plate being disposed at one end of the support and a base section being disposed at the other end of the support, wherein when the chuck is mounted on the rotary drum, the base section is inserted and passed through one associated through-slot and then hooked over the inner peripheral surface portion; and a recording head disposed for recording an image on the printing plate wound on the rotary drum. 
     The foregoing and other objects, features and advantages of the present invention will be apparent from the following description of a preferred embodiment of the invention, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general view showing a schematic structure of an image exposure apparatus according to an embodiment of the present invention. 
         FIG. 2  is a general view showing a schematic structure of a recording section of the image exposure apparatus. 
         FIG. 3  is a view showing disposal of leading edge chucks and trailing edge chucks with respect to a rotary drum. 
         FIG. 4  is a perspective view showing a chuck according to the embodiment, as seen from outside. 
         FIG. 5  is a perspective view showing the chuck according to the embodiment, as seen from inside. 
         FIG. 6  is a side view showing the chuck according to the embodiment. 
         FIG. 7  is a partial perspective view showing the inside of the rotary drum. 
         FIG. 8A  is an exploded view of the rotary drum. 
         FIG. 8B  is a side cross-sectional view showing the rotary drum of  FIG. 8A . 
         FIG. 9  is an enlarged plan view of a through-slot. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the accompanying drawings, an embodiment of the present invention will be described below.  FIG. 1  illustrates a schematic structure of an image exposure apparatus  10  according to the present embodiment. The image exposure apparatus  10  irradiates a sheet member (hereinafter, “printing plate  12 ”) with a light beam modulated on the basis of image data to thereby scan-expose the printing plate  12 . The printing plate  12  is a photosensitive planographic printing plate comprising a thin, rectangular plate-like support (e.g., aluminum) having disposed thereon a photosensitive layer. After the printing plate  12  is image-exposed in the image exposure apparatus  10 , the printing plate  12  is developed and processed by an automatic developing apparatus (not illustrated). 
     The image exposure apparatus  10  is disposed with a machine casing  14  having therein a cassette loading section  18 , a plate conveying section  20 , a recording section  22 , and a discharge buffer section  24 . The cassette loading section  18  is disposed at the lower right side of the machine casing  14  in  FIG. 1 . Cassettes  16 , which each house a number of the printing plates  12 , are disposed within the cassette loading section  18  and inclined at a predetermined angle. 
     The image exposure apparatus  10  can process different sizes (i.e., having different lengths and different widths) of the printing plates  12 . Each printing plate  12  is accommodated within the cassettes  16  so that the photosensitive layer faces upward and an end of the printing plate  12  is disposed at a predetermined position. The cassettes  16  are loaded at predetermined intervals in the cassette loading section  18  so that upper ends of the printing plates  12  housed in each cassette  16  reach substantially the same height. 
     The plate conveying section  20  is disposed above the cassette loading section  18 , and the recording section  22  is disposed at a lower, central region of the apparatus, adjacent to the cassette loading section  18 . The plate conveying section  20  is disposed with a pair of side plates  26  (only one is shown in  FIG. 1 ), and a reversal unit  28  and a sheet feeding unit  30  are mounted to the side plates  26 . 
     The reversal unit  28  includes a reverse roller  32  having a predetermined outer diameter, and a plurality of small rollers (e.g., four small rollers  34 A,  34 B,  34 C, and  34 D in the present embodiment) is disposed around the periphery of the reverse roller  32 . The small rollers  34 A to  34 D are positioned above the reverse roller  32 , from the cassette loading section  18  side to the recording section  22  side. An endless conveyor belt  36  is entrained around the small rollers  34 A to  34 D, with the conveyor belt  36  extending to roughly half the circumference of the reverse roller  32  between the small roller  34 A and the small roller  34 D. 
     The sheet-feeding unit  30  has a plurality of suction cups  38  that suck upper ends of the printing plates  12  in the cassettes  16 . The suction cups  38  are moved downward to oppose and suck the upper end of the printing plate  12 , whereby the printing plate  12  is pulled out from the cassette  16  and the leading edge of the extracted printing plate  12  is inserted between the reverse roller  32  and the conveyor belt  36 . In  FIG. 1 , two-dotted chain lines schematically show the positions to which the suction cups  38  move. 
     The reverse roller  32  and the conveyor belt  36  rotate in the direction that the printing plate  12  is pulled out from the cassette  16  (i.e., the direction of arrow A in  FIG. 1 ). The printing plate  12  is therefore nipped between the reverse roller  32  and the conveyor belt  36 , pulled out from the cassette  16 , wound around the periphery of the reverse roller  32  and conveyed and reversed while being curved. The radius of the reverse roller  32  is of a value (e.g., no less than 100 mm) such that the printing plate  12  is not folded or creased when the printing plate  12  is curved. 
     As shown by solid lines and two-dotted chain lines in  FIG. 1 , the side plates  26  move horizontally in accordance with the position of the cassette  16  from which the printing plate  12  is extracted. The suction cups  38  of the sheet feeding unit  30  face the printing plate  12  in the selected cassette  16 . 
     The side plate  26  is disposed with a guide  40  below the small roller  34 D. The printing plate  12  reversed by the reverse roller  32  is fed toward the guide  40  out from between the reverse roller  32  and the conveyor belt  36  at the small roller  34 D side. A conveyer  42  is disposed above the recording section  22 , and the printing plate  12  fed from the reversal unit  28  is guided by the guide  40  to the conveyer  42 . 
     The guide  40  swings in accompaniment with the movement of the side plate  26  so that the printing plate  12  is always guided toward the conveyer  42 . The small roller  34 D moves in accompaniment with the movement of the side plates  26 , to thereby change the direction in which the printing plate  12  is fed from the reversal unit  28 . The small roller  34 C moves so that a substantially constant tension is imparted to the conveyor belt  36  when the small roller  34 D moves. Accordingly, the printing plate  12  fed from the reversal unit  28  is gently curved by the guide  40 . 
     In the conveyer  42 , a conveyor belt  48  is entrained between a roller  44 , which is disposed adjacent to the lower part of the plate conveying section  20 , and a roller  46 , which is disposed adjacent to the upper part of the recording section  22 . The conveyor  42  is slanted such that the roller  46  is disposed lower than the roller  44 . 
     As shown in  FIGS. 1 and 2 , a roller  50  is disposed opposite the roller  46  in the conveyer  42 . The printing plate  12  sent on the conveyer  42  is conveyed on the conveyor belt  48  and nipped by the rollers  46  and  50 . In the recording section  22 , a rotary drum  54  and a recording head  56  are mounted on a base  52 . A puncher  58  is disposed above the rotary drum  54 . 
     As shown in  FIG. 2 , the puncher  58  is disposed with an opening  60 . When the conveyor  42  inserts the leading edge of the printing plate  12  into the opening  60  and the printing plate  12  is held by the rollers  46  and  50 , the puncher  58  creates a positioning notch at a predetermined position in the leading edge of the printing plate  12 . 
     When the notch has been formed in the printing plate  12 , the conveyer  42  drives the rollers  46  and  50  in reverse together with the conveyor belt  48 , and pulls out the leading edge of the printing plate  12  from the opening  60 . The conveyer  42  is disposed with swinging means (not illustrated). Using the roller  44  as a pivot, the swinging means swings the conveyor  42  downward so that the roller  46  approaches the rotary drum  54  (this swinging motion is shown by two-dotted chain lines in  FIGS. 1 and 2 ). Accordingly, the printing plate  12  is conveyed along the conveyor belt  48  toward the rotary drum  54 , with the leading edge of the printing plate  12  being directed toward a predetermined position on the peripheral surface of the rotary drum  54 . 
     The rotary drum  54  is rotated by driving means (not illustrated) in the direction in which the printing plate  12  is attached to the rotary drum  54  and exposed (i.e., the direction of arrow B in  FIGS. 1 and 2 ) and in the direction in which the printing plate  12  is detached from the rotary drum  54  (i.e., the direction of arrow C in  FIGS. 1 and 2 ). 
     As shown in  FIG. 2 , a leading edge chuck  62  is attached at a predetermined position on the peripheral surface of the rotary drum  54 . When the printing plate  12  is attached to the rotary drum  54 , the rotary drum  54  is first stopped so that the leading edge chuck  62  is positioned opposite to the leading edge of the printing plate  12  (printing plate attachment position) conveyed by the conveyer  42 . 
     The recording section  22  is disposed with an attachment cam  64  opposite to the leading edge chuck  62  at the printing plate attachment position. The attachment cam  64  is swung to press one end of the leading edge chuck  62 , whereby the printing plate  12  can be inserted between the other end of the leading edge chuck  62  and the peripheral surface of the rotary drum  54 . When the printing plate  12  has been inserted in this manner, the attachment cam  64  is returned to its original position so that the end of the leading edge chuck  62  is no longer pressed, and the leading edge of the printing plate  12  is nipped by and held between the leading edge chuck  62  and the peripheral surface of the rotary drum  54 . At this time, a positioning pin (not illustrated) that protrudes from a predetermined position on the peripheral surface of the rotary drum  54  enters the notch formed by the puncher  58 , to thereby position the printing plate  12  on the rotary drum  54 . 
     When the leading edge of the printing plate  12  has been fixed to the rotary drum  54 , the rotary drum  54  is rotated in the direction of arrow B shown in  FIGS. 1 and 2 , and the printing plate  12  is wound around the peripheral surface of the rotary drum  54 . 
     The size (length) of the printing plates  12  that may be used in the present embodiment can be generally classified into the following four types. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 LENGTH (IN THE DIRECTION IN WHICH THE 
               
               
                   
                   
                 PRINTING PLATE IS WOUND AROUND THE 
               
               
                   
                 TYPE 
                 ROTARY DRUM) (mm) 
               
               
                   
                   
               
             
             
               
                   
                 #1 
                 550~600 
               
               
                   
                 #2 
                 650~700 
               
               
                   
                 #3 
                 750~800 
               
               
                   
                 #4 
                 900~950 
               
               
                   
                   
               
             
          
         
       
     
     A squeeze roller  66  is disposed near the peripheral surface of the rotary drum  54  and downstream from the position at which the printing plate  12  is attached to the rotary drum  54 . The squeeze roller  66  is moved toward the rotary drum  54 , to thereby press and closely adhere the printing plate  12  to the peripheral surface of the rotary drum  54 . 
     A trailing edge chuck attachment/detachment unit  68  is disposed near the rotary drum  54  and upstream from the squeeze roller  66 , and a detachment cam  70  is disposed downstream from the squeeze roller  66 . The trailing edge chuck attachment/detachment unit  68  includes a shaft  72 , which projects toward the rotary drum  54  and has a tip at which a trailing edge chuck  74  is disposed. 
     When the trailing edge of the printing plate  12  wound around the rotary drum  54  opposes the trailing edge chuck attachment/detachment unit  68 , the shaft  72  is moved towards the rotary drum  54  to attach the trailing edge chuck  74  at a predetermined position on the rotary drum  54 . The trailing edge of the printing plate  12  is thereby nipped between the trailing edge chuck  74  and the rotary drum  54  and held. 
     When the leading and trailing edges of the printing plate  12  are held at the rotary drum  54 , the squeeze roller  66  is moved away from the rotary drum  54 . Thereafter, while the rotary drum  54  is rotated at a predetermined high speed, a light beam modulated on the basis of image data is emitted from the recording head  56  synchronously with the rotation of the rotary drum  54 . The printing plate  12  is thereby scan-exposed on the basis of the image data. 
     After scan-exposure of the printing plate  12  is completed, when the trailing edge chuck  74  holding the trailing edge of the printing plate  12  is positioned opposite the trailing edge chuck attachment/detachment unit  68 , the rotary drum  54  stops rotating. The squeeze roller  66  moves toward the rotary drum  54  and presses the printing plate  12  against the rotary drum  54 . The trailing edge chuck attachment/detachment unit  68  receives the trailing edge chuck  74  and pulls the trailing edge chuck  74  away from the rotary drum  54 . As a result, the trailing edge of the printing plate  12  is released. 
     After the trailing edge chuck  74  has been detached from the rotary drum  54 , the rotary drum  54  rotates in the direction of arrow C shown in  FIGS. 1 and 2 , whereby the printing plate  12  is sent out from between the squeeze roller  66  and the rotary drum  54 . 
     As shown in  FIG. 1 , the discharge buffer section  24  is disposed above the squeeze roller  66 . The rotary drum  54  rotates in the direction of arrow C to send the trailing edge of the printing plate  12  toward the discharge buffer section  24 . The rotary drum  54  stops at the position at which the printing plate is detached (i.e., where the leading edge chuck  62  opposes the detachment cam  70 ). By swinging the detachment cam  70  at this position, the leading edge chuck  62  is pressed, whereby the leading edge of the printing plate  12  is no longer nipped between the leading edge chuck  62  and the rotary drum  54 . As a result, the printing plate  12  is detached from the rotary drum  54 . 
     The discharge buffer section  24  includes an ejection roller  78  disposed on the inner side of an exhaust port  76  formed in the machine casing  14 . A plurality of small rollers (e.g., small rollers  80 A,  80 B,  80 C,  80 D and  80 E) is disposed around the ejection roller  78  and an endless conveyor belt  82  is entrained around the small rollers  80 A to  80 E. The conveyor belt  82  is thus entrained around the small rollers  80 A through  80 E in a range of between about ½ to about ¾ the circumference of the discharge roller  78 . 
     The small roller  80 A projects toward the squeeze roller  66  in the recording section  22 , and an idle roller  84  is disposed to oppose the small roller  80 A. The printing plate  12  sent out from the recording section  22  is guided to between the small roller  80 A and the idle roller  84  and is nipped thereby. 
     By rotating the ejection roller  78  in the direction in which the printing plate  12  is pulled out (ie., the direction of arrow D in  FIG. 1 ), the printing plate  12  nipped between the small roller  80 A and the idle roller  84  is pulled out from the recording section  22 , guided to between the ejection roller  78  and the conveyor belt  82 , nipped by the same, and thereby wound around the ejection roller  78 . By nipping the leading edge (i.e., the trailing edge when the printing plate  12  is sent out from the recording section  22 ) of the printing plate  12  between the small roller  80 A and the idle roller  84 , the printing plate  12  wound around the ejection roller  78  is temporarily held. 
     As shown by two-dotted chain lines in  FIG. 1 , in the discharge buffer section  24 , the small roller  80 A and the idle roller  84  move to a position where they oppose the discharge opening  76 . The small roller  80 A and the idle roller  84  rotate integrally, thereby directing the leading edge of the printing plate  12  to the discharge opening  76 . The small roller  80 B above the small roller  80 A moves in accompaniment with the movement of the small roller  80 A to impart a predetermined tension to the conveyor belt  82 . 
     When the leading edge of the printing plate  12  is directed to the discharge opening  76 , the ejection roller  78  is rotated in the direction that the printing plate  12  is fed out (i.e., the direction opposite to the direction of arrow D) at a rotational speed that corresponds to the speed at which the printing plate  12  is conveyed at processing apparatuses, such as an automatic developing apparatus, disposed adjacent to the discharge opening  76 . In this manner, the printing plate  12  is sent out from the discharge opening  76 . 
     As described above, the leading edge chuck  62  for fixing the leading edge of the printing plate  12  at the predetermined position on the peripheral surface of the rotary drum  54  is fixedly secured to the rotary drum  54  when the printing plate is wound around the rotary drum  54 . The trailing edge chuck  74  for fixing the trailing edge of the printing plate  12  is mounted to the rotary drum  54 . 
     As shown in  FIG. 3 , the leading edge chuck  62  is disposed with elongated plates  150  of a predetermined length. The plates  150  are arranged along an axial direction of the rotary drum  54  at predetermined intervals. Similarly, the trailing edge chuck  74  is disposed with elongated plates  150  of a predetermined length, and these plates  150  are also arranged on and mounted to the peripheral surface of the rotary drum  54  at predetermined intervals. 
     The leading edge chuck  62  and the trailing edge chuck  74  have approximately the same structure, but respective orientations of the plates  150  are different. In the present embodiment, the leading edge chuck  62  is always fixedly secured to the rotary drum  54 , whereas the trailing edge chuck  74  is attachable to and detachable from the rotary drum  54 . 
     With reference to  FIGS. 4 through 6 , the structures of the leading edge chuck  62  and the trailing edge chuck  74  will be now described. 
     The plate  150  of the trailing edge chuck  74  is disposed with a through-hole  152  formed at two predetermined locations along the lengthwise direction (of the plate  150 ), with each through-hole  152  being substantially centered in the widthwise direction (of the plate  150 ). The plate  150  has two supports  154 , each of which is inserted into the corresponding through-hole  152 . Each of the supports  154  comprises a rectangular, block-like base section  154 A and a pillar-shaped supporting section  154 B disposed on an upper surface of the base section  154 A 
     As shown in  FIGS. 3 ,  7 , and  8 , elongated through-slots  90  are formed in the peripheral surface of the rotary drum  54 . The through-slots  90  are arranged along imaginary lines K (two-dotted chain lines in  FIG. 8 ) with a fixed, regular interval being disposed each adjacent imaginary line K. The through-slots  90  are disposed along each line K according to the following rule. The trailing edge chuck  74  is attached to the rotary drum  54  in such a manner that the base sections  154 A of the supports  154  are inserted through the through-slots  90 , whereby the supports  154  are pivotable, in a direction orthogonal to the axial direction of the rotary drum  54 , about a point of contact between the through-slot  90  and the support  154 . 
     The widthwise and lengthwise dimensions of the through-slots  90  are determined in accordance with the supports  154 . The supports  154  are effectively movable in and along the through-slots  90  by a distance M (e.g., approximately 50 mm) as seen in  FIG. 9 . 
       FIG. 8A  is a schematic illustration of the rotary drum  54  developed on a flat surface for best illustrating the through-slots  90  in the peripheral surface of the rotary drum  54 .  FIG. 8B  is a longitudinal section taken along the line K of  FIG. 8A . 
     As shown in  FIG. 8A , a distance between the center of the through-slot  90 ( 1 ) for fixing the leading edge chuck  62  (the center being the cardinal or base point of measurement) and the center of one of the through-slots  90 ( 2 ),  90 ( 3 ),  90 ( 4 ), and  90 ( 5 ) is set as follows. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                 CORRESPONDING 
               
               
                   
                 POSITIONAL 
                 ACTUAL 
                 TYPE OF PRINTING 
               
               
                   
                 DISTANCE 
                 SIZE (mm) 
                 PLATE 
               
               
                   
                   
               
             
             
               
                   
                 P = 90 (1) to 90 (2) 
                 575 
                 #1 
               
               
                   
                 Q = 90 (1) to 90 (3) 
                 675 
                 #2 
               
               
                   
                 R = 90 (1) to 90 (4) 
                 775 
                 #3 
               
               
                   
                 S = 90 (1) to 90 (5) 
                 925 
                 #4 
               
               
                   
                   
               
             
          
         
       
     
     When the support  154  is inserted into the through-slot  90  formed in the rotary drum  54 , a narrow side of the support  154  corresponds to a widthwise direction of the through-slot  90 , so that it is possible to insert the narrow side of the support  154  into the through-slot  90 . Once inserted, the support  154  is rotated approximately 90°, whereby the base section  154 A engages with (abuts against) an inner peripheral surface of the rotary drum  54  so as to be prevented from slipping out. 
     It should be noted that it is not necessary for the leading edge chuck  62  to have the same structure as the trailing edge chuck  74 . For example, the leading edge chuck  62  may be fixed by bolts. 
     In a state in which all constituent members of the plate  150  have been assembled, the center of gravity (shown as a solid black circle  155  in  FIG. 6 ) of the plate  150  lies to the right of the axis of the support  154  in  FIG. 6 . The numerical values in the same figure for weight and dimensions of the trailing edge chuck  74  (the leading edge chuck  62 ) are given only as an example. 
     A clamp  160  is formed at one end (the left end in  FIG. 6 ) in a widthwise direction of the plate  150  of the trailing edge chuck  74 . The clamp  160  extends at a substantial right angle from the plate  150  toward the rotary drum  54 , and a rubber sheet  161  is stuck to a distal end surface of the clamp  160 . The rubber sheet  161  directly contacts the printing plate  12  and is an important element for determining coefficient of friction when the printing plate  12  is nipped by the clamp  160  to the peripheral surface of the rotary drum  54 . 
     One end of a plate spring  180  bent in a substantial L shape is fixed to the under surface of the right end (in  FIG. 4 ) of the plate  150 . The direction in which the plate spring  180  is bent approaches the rotary drum  54  and an anchor  182  is attached to the other end of the plate spring  180 . A surface at the distal-most end of the anchor  182  is substantially arc-shaped. 
     When the weight  182  with the plate spring  180  is attached to the rotary drum  54 , the weight  182  contacts the peripheral surface of the rotary drum  54  (at peripheral edges of the through-slot  90 ). Specifically, when the trailing edge chuck  74  approaches the rotary drum  54 , the weight  182  first abuts the peripheral surface of the rotary drum  54 . As the trailing edge chuck  74  further approaches the rotary drum  54 , the plate spring  180  is elastically (resiliently) deformed and the base section  154 A of the support  154  is inserted into the through-slot  90 . After the base section  154 A has passed through the through-slot  90 , the support  154  is rotated approximately 90° in order for the base section  154 A to engage with the inner peripheral surface of the rotary drum  54  to prevent the support from slipping out from the through-slot  90 . An urging force generated by the elastic deformation of the plate spring  180  causes the plate  150  to pivot on the support  154 . As a result, a pressing force at the clamp  160  is generated. 
     Description will now be given of the operation of the present embodiment. 
     In the image exposure apparatus  10 , image data to be exposed on the printing plate  12  is inputted, and the size and the number of printing plates  12  on which an image is to be formed by exposure are set. When a command to initiate image exposure is given, image exposure processing of the printing plate  12  begins. The command may be given by disposing an operation panel on the image exposure apparatus  10  and operating switches on the operation panel, or by a signal from an image processor or the equivalent for outputting image data to the image exposure apparatus  10 . 
     When the command to initiate processing is given, the sheet feeding unit  30  is moved together with the reversal unit  28  to a position corresponding to the cassette  16  housing the printing plate  12  of the designated size. The printing plate  12  in the corresponding cassette  16  is then sucked and extracted by the suction cup  38  and sent to between the reverse roller  32  of the reversal unit  28  and the conveyor belt  36 . The printing plate  12  is nipped between and carried by the reverse roller  32  and the conveyor belt  36 , and sent to the conveyer  42 . 
     In the conveyer  42 , the leading edge of the printing plate  12  is first inserted into the insertion opening  60  of the puncher  58 . The puncher  58  punches a positioning notch at a predetermined position in the printing plate  12 . After the notch has been formed in the printing plate  12 , the conveyer  42  pulls the printing plate  12  out from the insertion opening  60  of the puncher  58 , and the printing plate  12  is sent along a direction tangential to the rotary drum  54  toward the peripheral surface of the rotary drum  54 . 
     When the leading edge of the printing plate  12  is held on the rotary drum  54  by the leading edge chuck  62 , the printing plate  12  is wound around the rotary drum  54  while being squeezed by the squeeze roller  66 . Subsequently, the trailing edge of the printing plate  12  is held on the rotary drum  54  by the trailing edge chuck  74 . 
     Thereafter, in the recording section  22 , while the rotary drum  54  is being rotated at high speed, the light beam based on the image data is emitted from the recording head section  56  to the printing plate  12  to scan-expose the printing plate  12 . While the rotary drum  54  is rotated at high speed, the force by which the printing plate  12  is nipped by the leading edge chuck  62  and the trailing edge chuck  74  and the force by which the printing plate  12  is tensioned (stretched; to prevent the printing plate  12  from coming off of the rotary drum  54 ) are enhanced by the action of the centrifugal force of the rotary drum  54 . 
     After scan-exposure of the printing plate  12  is completed, the trailing edge chuck  74  is detached (separated from) the rotary drum and the printing plate  12  is sent to the discharge buffer section  24 . 
     In the discharge buffer unit  24 , the printing plate  12  is nipped between and conveyed by the small roller  80 A and the idle roller  84  so as to be wound around the discharge roller  78 . Thereafter, the small roller  80 A and the idle roller  84  are moved opposite to the discharge opening  76  and the printing plate  12  is sent from the discharge opening  76  at a predetermined conveying speed. 
     The nipping, tensioning, and holding of the printing plate  12  on the rotary drum  54  by the leading edge chuck  62  and the trailing edge chuck  74  will now be described in detail. Firstly, when the trailing edge chuck  74  is mounted on the rotary drum  54 , short sides of the base section  154 A of the support  154  are oriented so as to correspond to an opening widthwise direction of the through-slot  90 . Then, the base section  154 A is smoothly inserted into the through-slot  90  until it passes therethrough. Thereafter, the whole support  154  is rotated approximately 90°, whereby the long sides of the base section  154 A are set along the opening widthwise direction of the through-slot  90  such that they engage with (abut against) the inner peripheral surface of the rotary drum  54 , to thereby prevent the support  154  from slipping out of the through-slot  90 . 
     In order to detach the trailing edge chuck  74  from the rotary drum  54 , the base section  154 A can be rotated roughly another 90° and pulled out. 
     When the ends of the printing plate  12  come between the cylindrical body  210  of the guiding member  204  and the plate  150 , the cam  64  is released so that the urging force of the plate spring  180  causes the plate  150  to pivot around the support  154 . Because the clamp  160  is moved toward the peripheral surface of the rotary drum  54  by this pivoting, the printing plate  12  is nipped between the clamp  160  and the peripheral surface of the rotary drum  54 . 
     When the printing plate  12  reaches the predetermined position, the trailing edge chuck  74  is attached to the rotary drum  54 . Consequently, the urging force of the plate spring  180  works in accompaniment with the movement for attaching the trailing edge chuck  74  (the weight  182  at the distal end of the plate spring  180  firstly reaches the periphery of the through-slot  90 ), the plate  150  is gradually swung by using the support  154  as a fulcrum, and the positioned printing plate  12  is nipped between the trailing edge chuck  74  and the peripheral surface of the rotary drum  54 . 
     Once the printing plate  12  is held by the leading edge chuck  62  and the trailing edge chuck  74 , the rotary drum  54  starts to rotate at high speed in order to record an image. 
     As a result of this rotation, the plate  150  is pivoted on the support  154  by centrifugal force. Since the center of gravity of the plate  150  is on the side opposite to the clamp  160 , with the support  154  being situated therebetween, the centrifugal force acts on the center of gravity of the plate  150  in the same direction as that of the urging force of the plate spring  180 . Therefore, the force by which the printing plate  12  is nipped is increased when the rotary drum  54  rotates at high speed (i.e., during image recording). 
     In this respect, for the centrifugal force due to the high-speed rotation of the rotary drum  54 , the distance L 1  from the support  154  of the leading edge chuck  62  or trailing edge chuck  74  to the clamp  160 , the distance L 2  from the support  154  to the center of gravity  155 , and the position of the center of gravity  155  are important factors. 
     In the present embodiment, the rotary drum  54  comprises a hollow, cylindrical body having an inner peripheral surface in which a plurality of through-slots is formed. When the leading edge chuck  62  and the trailing edge chuck  74  are secured onto the rotary drum  54 , their supports  154  can be held by the inner peripheral surface of the rotary drum  54 . Namely, there is no need to provide a substantially inverted T-shaped groove that is typically used to prevent the chucks from slipping off or coming out of the rotary drum  54 . 
     In the present embodiment, printing plates having a high frequency of use are pre-classified by size (see Table 1). A pitch distance (P,Q,R,S) from the through-slot  90 ( 1 ) to one of the through-slots  90 ( 2 ),  90 ( 3 ),  90 ( 4 ), and  90 ( 5 ) is set such that it may correspond to one of the types of the printing plates (see Table 2). Therefore, printing plates of virtually any size can be used. 
     As described above, because the through-slots  90  for fixing the leading edge chuck  62  and trailing edge chuck  74  are disposed at the rotary drum  54 , the radial (wall) thickness of the rotary drum can be reduced in comparison with a case where inverted T-shaped grooves are formed in the rotary drum. Thus, a smaller and lighter rotary drum  54 , in which rotational load at the time the rotary drum  54  is rotated can be reduced and the rotary drum  54  can be rotated more stably at high speed, is achieved.