Method and apparatus for making an offset printing plate

The present invention concerns a plate making method of forming an image on a heat sensitive blank plate by a multichannel method using a plate making apparatus of an outer surface cylinder scanning type. A blank plate (400) is secured to the outer circumferential surface of a hollow cylinder (131). A laser beam (800) is irradiated from an optical head (150) onto the blank plate (400). The laser beam (800) is a beam group consisting of a plurality of infrared laser beams arranged in a line.

TECHNICAL FIELD
 The present invention concerns a method of making a heat sensitive type
 offset printing plate and a manufacturing apparatus capable of easily
 practicing the method.
 BACKGROUND ART
 An apparatus adapted to irradiate a laser beam selectively on a sensitive
 material based on an image recording signal thereby forming an image has
 been known so far as a film plotter or an image setter. For example,
 Japanese Patent Unexamined Publication No. 60-203071 discloses a laser
 plate making apparatus of forming an image by a plurality of laser beams.
 On the other hand, along with popularization of computers or development of
 network techniques typically represented by internets in recent years, a
 CTP (computer to plate) system of directly making a plate for offset
 printing from digital image data edited on a computer not by way of a
 negative film or a positive film has been enabled. Then, the CTP system
 has attracted attention as a substitute for a PS (pre-sensitized) plate
 system using a film, which is predominant in offset printing at present.
 A system already put to practical use as a plate making system of an offset
 printing plate used for the CTP system is a print making system using
 photosensitive materials such as an OPC (organic photo-semiconductor), a
 silver salt, a hybrid material of a silver salt and a photopolymer and a
 highly sensitive photopolymer as a blank plate. However, it is necessary
 for the print making system described above that the blank plate has to be
 handled in a dark room like that existent PS print systems. Further, the
 plate making systems described above requires a developing step after the
 image drawing step to the blank plate like that the existent PS plate
 system and, therefore, involves a problem for discarding treatment of a
 liquid developer or the like.
 On the contrary, a CTP plate making system using a heat sensitive type
 blank plate having a response region in an infrared region, the blank
 plate can be handled in a light room. Further, a great amount of heat
 energy is charged in an image forming step by a laser beam in this system,
 thereby an image is formed by thermally converting a portion, to which an
 image is formed, of a heat sensitive layer from hydrophilic to oleophilic
 property, so it requires no developing step. Accordingly, such a heat
 sensitive type CTP system has been noted as a CTP system in the next
 generation.
 Generally, plate making apparatus used for the CTP system are broadly
 classified, depending on the difference of scanning system, into three
 types of an outer surface cylinder scanning system, an inner surface
 cylinder scanning system and a planer scanning system. A laser plate
 making apparatus of the outer surface scanning system is disclosed, for
 example, in Japanese Patent Examined Publication No. 51-46138.
 As the plate making apparatus used for photosensitive blank plates, a plate
 making apparatus of an inner surface cylinder scanning system of securing
 a blank plate to a cylinder inner surface and scanning a laser beam by a
 rotational end face mirror has been utilized generally, since this can
 conduct a high speed scanning and also easily cope with different sizes of
 blank plates. However, the inner surface cylinder scanning type plate
 making apparatus is not suitable as a plate making apparatus for heat
 sensitive type blank plates with the reason described below.
 That is, since a heat sensitive type blank plate generally has a
 sensitivity lower by about three digits compared with a photosensitive
 blank plate, when the inner surface cylinder scanning system is adopted,
 it requires an expensive solid laser of excellent beam characteristics,
 for example, an Nd-YAG laser capable of providing an extremely high output
 energy and having a long focal distance. However, since the sensitive
 wavelength region of usable blank plates is restricted to 1064 nm as an
 emitting wavelength of an Nd-YAG laser, the degree of freedom for the
 design of the blank plate is lowered in the inner surface cylinder
 scanning type plate making apparatus using the Nd-YAG laser as an image
 forming laser.
 On the contrary, a semiconductor laser having a central light emission
 wavelength region near 750-880 nm is inexpensive compared with the Nd-YAG
 laser. Accordingly, use of the semiconductor layer for the image forming
 laser is preferred in order to reduce the apparatus cost of the heat
 sensitive type CTP system. However, since no long focal distance can be
 available in the semiconductor laser in view of beam characteristics, it
 is difficult to adopt the inner surface cylinder scanning system in the
 plate making apparatus using the semiconductor laser.
 Accordingly, in the plate making apparatus using the semiconductor laser,
 an outer surface cylinder scanning system, that is, a system of winding a
 blank plate around the outer surface of a cylinder, and irradiating a
 laser beam to the blank plate from an optical head disposed near the
 cylinder outer surface is adopted. The plate making apparatus of this type
 is adapted, for example, such that a laser beam irradiated from a
 semiconductor laser is transmitted through an optical fiber and introduced
 to the optical system of an optical head disposed near the cylinder outer
 surface and a laser beam focused by an objective lens at the top end of
 the optical system to the blank plate at the cylinder outer surface.
 In the plate making apparatus of the outer surface cylinder scanning system
 described above, with an aim of increasing the plate making speed, an
 image is formed by a so-called multi-channel system of using a plurality
 of semiconductor lasers to increase the number of scanning lines per one
 rotation of the cylinder.
 Then, in a general multi channel system plate making apparatus, a plurality
 of laser beams are arranged each at an equal interval in line and the
 beams are formed into a group of beams parallel with each other and the
 beam group is introduced to a set of optical systems.
 However, when an image is formed by a plurality of infrared laser beams
 arranged in line, heat of infrared rays is absorbed in the heat sensitive
 layer, as well as a great amount of heat is also generated by chemical
 reaction in the heat sensitive layer. Then, the heat diffuses to the
 periphery by heat conduction while elevating the temperature of the blank
 plate. Accordingly, the temperature of the image area of the blank plate
 formed with an image by a beam situated at the center of the line is
 higher compared with that in the image region of the blank plate formed
 with an image by a beam situated on the end of the line.
 As described above, when an image is formed by a plurality of infrared
 laser beams disposed in line to the heat sensitive blank plate to be
 formed with images by thermal reaction, since a temperature distribution
 is caused to the blank plate upon image formation, it is difficult to form
 an image uniformly over the entire image formation region. That is, there
 is a room for the improvement of the image quality of the printing plate
 obtained by this method.
 On the other hand, in the process color printing, a color image is
 separated into that of four colors, namely, Y (yellow), M (magenta), C
 (cyan) and K (black), and a plate for each color is made, and each of
 images is printed with an ink of a corresponding color by using the four
 plates. Then, color printed matters of good quality can be obtained by
 overlapping images printed by the four plates with inks of different
 colors on an exact position of paper. Positional alignment for each of the
 plates in a printing machine is carried out by disposing one side as a
 reference to each of the plates and aligning the sides to each other.
 Accordingly, also in the plate making, an image has to be formed at an
 accurate position with the side being as a reference.
 However, the plate making apparatus of the outer surface cylinder scanning
 system in the prior art still has a room for the improvement in view of
 convenient and accurate positioning upon attaching the blank plate to the
 cylinder.
 Further, Japanese Patent Unexamined Publication No. 7-1849 discloses a
 material for forming a heat sensitive layer constituting a heat sensitive
 type blank plate, which contains microcapsules containing an oleophilic
 ingredient in the inside and destroyed by heat, hydrophilic binder polymer
 having functional groups capable of three dimensional cross linking and
 functional groups capable of reacting with the oleophilic ingredient, and
 photoreaction initiator for initiating three dimensional cross linking
 reaction of the hydrophilic binder polymer. However, the printing plate
 made by the existent method using the heat sensitive blank plate having
 the material as a heat sensitive layer is insufficient in the printing
 resistance for an image area and still leaves a room for improvement in
 the printing quality of the obtained printing plate.
 The present invention has been accomplished taking notice on the problems
 in the prior art described above, and it is a subject thereof to
 remarkably improve the quality of the image to be formed and the printing
 quality in the image area, upon making the heat sensitive blank plate into
 a printing plate by the outer surface cylinder scanning system plate
 making apparatus and, further, enable to accurately position images of
 four colors by a convenient method in a short period of time upon process
 color printing.
 DISCLOSURE OF THE INVENTION
 In order to solve the foregoing subject, the present invention provides a
 method of making an offset printing plate comprising a blank plate
 attaching step of winding a plate-shaped blank plate having a heat
 sensitive layer to which an image is formed thermally on a support around
 the outer circumferential surface of a cylinder with the heat sensitive
 layer being directed outward, thereby making the blank plate rotatable
 integrally with the cylinder, and an image forming step of irradiating a
 group of beams comprising a plurality of infrared laser beams arranged in
 line to the blank plate on the outer circumferential surface of the
 cylinder based on an image forming signal while rotating the cylinder,
 thereby forming an image in accordance with the image forming signal to
 the heat sensitive layer of the blank plate, wherein irradiation
 conditions for a plurality of infrared laser beams constituting the group
 of beams are set such that the temperature of the blank plate is uniform
 in a region in which an image is formed at once by the group of beams in
 line in the image forming step.
 According to this method, since the temperature of the blank plate is made
 uniform in the region in which an image is formed at once by the group of
 beams in line, the temperature of the blank plate is made uniform over the
 entire region in which the image is formed in one rotation of the
 cylinder. Accordingly, image formation by a uniform heat sensitive
 reaction is conducted for the entire surface of the heat sensitive layer
 of the blank plate, for example, by repeating the movement of the group of
 beams in line in the direction of the rotational axis of the cylinder on
 every one rotation of the cylinder. This can outstandingly improve the
 image quality of the obtained printing plate.
 The blank plate attaching step in the method according to the present
 invention preferably has a step of securing the top end of the blank plate
 to the circumferential surface of the cylinder by a clamp mechanism, in
 which positioning is conducted by utilizing one side at the top end of the
 blank plate upon securing by the clamp mechanism and the blank plate is
 attached while keeping the positioned state.
 According to this method, since the blank plate has been positioned by
 utilizing one side at the top end of the blank plate before the image
 forming step, an image is formed at an accurate position relative to one
 side as a reference of the blank plate in the image forming step. This
 enables to conduct positioning also in the process color printing by a
 convenient operation and accurately.
 After the image forming step in the method of the present invention, a post
 treating step of irradiating UV-rays at a wavelength of 200 to 400 nm to
 the heat sensitive layer of the blank plate is preferably conducted. In
 this method, printing quality such as printing resistance of an image area
 can be improved outstandingly by conducting the post treatment step of
 UV-ray irradiation.
 The present invention further provides a method of making an offset
 printing plate, comprising a blank plate attaching step of winding a
 plate-shaped blank plate having a heat sensitive layer to which an image
 is formed thermally on a support around the outer circumferential surface
 of a cylinder with the heat sensitive layer being directed outward,
 thereby making the blank plate rotatable integrally with the cylinder, and
 an image forming step of irradiating infrared laser beams to the blank
 plate on the outer circumferential surface of the cylinder based on an
 image forming signal while rotating the cylinder, thereby forming an image
 in accordance with the image forming signal to the heat sensitive layer of
 the blank plate, wherein a post treating step of irradiating UV-rays at a
 wavelength of 200 to 400 nm to the heat sensitive layer of the blank plate
 is conducted after the image forming step.
 According to this method, printing quality such as printing resistance of
 an image area is outstandingly improved by applying the post treating step
 of UV ray irradiation.
 In a case where the heat sensitive layer contains microcapsules containing
 an oleophilic ingredient in the inside and thermally destroyed,
 hydrophilic binder polymer having functional groups capable of three
 dimensional cross linking and functional groups capable of reacting with
 the oleophilic ingredient, and photoreaction initiator for initiating the
 three dimensional cross-linking reaction of the hydrophilic binder polymer
 as described in Japanese Published Unexamined Patent Application Hei
 7-1849, the hydrophilic binder polymer can be three dimensionally cross
 linked by the post treating step. This can modify the surface of the blank
 plate just after the image forming step to remarkably improve the printing
 quality such as ink receptibility and transferability, reproducibility of
 fine lines or mesh dots, or printing resistance.
 Further, the present invention provides an apparatus for making an offset
 printing plate, comprising a cylinder having a rotational mechanism, a
 blank plate attaching mechanism for winding and securing a plate-shaped
 heat sensitive type blank plate (having a heat sensitive layer on a
 support) to the outer circumferential surface of the cylinder, a cassette
 for keeping a plurality of blank plates, a blank plate supply mechanism of
 taking out blank plates from the cassette and directing them to the
 cylinder, a laser generation device for generating a plurality of infrared
 laser beams in line, an irradiation condition setting device for setting
 irradiation condition (intensity or irradiation time) on each of infrared
 laser beams based on an image forming signal and the position in the line,
 a laser irradiation head (hereinafter also referred to as "optical head")
 having an optical system for focusing a plurality of laser beams
 irradiated from the laser generation device to the blank plate wound
 around the outer circumferential surface of the cylinder, and a head
 moving mechanism for linearly moving the laser irradiation head along a
 line opposing in parallel with the rotational axis of the cylinder at a
 position spaced apart by a predetermined distance from the cylinder.
 The group of the laser beams in line to be generated from the laser
 generation device may be laser beams disposed only by one in the lateral
 direction of the line, or it may be disposed in plurality. Accordingly,
 the laser generation device can be obtained, for example, by providing a
 plurality of optical fibers coupled to semiconductor lasers and arranging
 each of the optical fibers in one direction at an equal distance, or
 arranging them both in the longitudinal direction and the lateral
 direction of the line each by a predetermined number at an equal distance.
 In this plate making apparatus, the plate-shaped heat sensitive blank plate
 is wound and secured to the outer circumferential surface of the cylinder
 with a heat sensitive layer being directed outward, the cylinder is
 rotated in this state and the laser generation device is operated, and a
 laser beam is irradiated over the entire surface of the blank plate of the
 outer circumferential surface of the cylinder by repeating movement of the
 irradiation head each by a predetermined amount by the head moving
 mechanism, on every one rotation of the cylinder for example. Further, by
 the setting of the irradiation condition setting device, an image in
 accordance with the image forming signal is formed to the heat sensitive
 layer of the blank plate.
 Particularly, when the irradiation condition of each of the infrared laser
 beams is set, for example, such that the irradiation energy is low for the
 laser beam at the center of the line and the irradiation energy is high
 for the laser beam on the ends of the line based on the position in the
 line, the temperature of the blank plate can be made uniform within a
 region in which an image is formed at once by a group of laser beams
 arranged in line.
 In the plate making apparatus according to the present invention,
 preferably the blank plate supply mechanism has a conveying device for
 conveying a blank plate from the laterally direction to the cylinder, the
 blank plate attaching mechanism has a clamp mechanism for securing the top
 end of the blank plate conveyed by the conveying device to the
 circumferential surface of the cylinder, and the clamp mechanism has a
 positioning surface for being touched by the top end face of the blank
 plate. With such a constitution, positioning can be conducted easily by
 utilizing one side at the top end of the blank plate upon securing the top
 end of the blank plate by the clamp mechanism.
 The plate making apparatus according to the present invention preferably
 has a UV-ray irradiation device for irradiating UV-rays at a wavelength of
 200 to 400 nm to the heat sensitive layer of the blank plate and a blank
 plate moving mechanism for detaching the blank plate from the cylinder and
 directing the same to the UV-ray irradiation device.
 Furthermore, the present invention provides an apparatus for making an
 offset printing plate, comprising a cylinder of a structure capable of
 winding and securing a plate-shaped blank plate to the outer
 circumferential surface thereof, a rotational mechanism for the cylinder,
 a laser generation device for generating a laser beam in an infrared
 region based on an image forming signal, a laser irradiation head having
 an optical system for focusing the laser beam from the laser generation
 device to the blank plate on the outer circumferential surface of the
 cylinder, a head moving mechanism for moving the irradiation head along a
 line opposing in parallel with the rotational axis of the cylinder at a
 position spaced apart by a predetermined distance from the cylinder, a
 UV-ray irradiation device for irradiating UV-rays at a wavelength of 200
 to 400 nm to the heat sensitive layer of the blank plate and a blank plate
 moving mechanism for detaching the blank plate from the cylinder and
 directing the same to the UV-ray irradiation device.
 According to this apparatus, the laser beam is irradiated over the entire
 surface of the blank plate on the outer circumferential surface of the
 cylinder after winding and securing the plate-shaped blank plate having a
 heat sensitive layer on a support to the outer circumferential surface of
 the cylinder with the heat sensitive layer being directed outward, and by
 rotating the cylinder in this state and operating the laser generation
 device, and repeating movement of the irradiation head by a predetermined
 amount by the head moving mechanism, on one rotation of the cylinder, for
 example. This can form an image in accordance with the image forming
 signal to the heat sensitive layer of the blank plate. Subsequently, the
 blank plate is detached by the blank plate moving mechanism from the
 cylinder and directed to the UV-ray irradiation device, and the heat
 sensitive layer thereof is irradiated with UV-rays at a wavelength of 200
 to 400 nm.
 In the plate making apparatus according to the present invention, the
 apparatus having the UV-ray irradiation device and the blank plate moving
 mechanism is suitable to a case in which the heat sensitive layer contains
 microcapsules containing an oleophilic ingredient in the inside and
 destroyed thermally, hydrophilic binder polymer having functional groups
 capable of three dimensional cross linking and functional groups capable
 of reacting with the oleophilic ingredient, and photoreaction initiator
 for initiating three dimensional cross linking reaction of the hydrophilic
 binder polymer. Further, the print making apparatus preferably has a blank
 plate attaching mechanism of winding a plate-shaped blank plate to the
 outer circumferential surface of the cylinder and capable of rotating the
 same integrally therewith.
 As a light source of the post treating device, a fluorescent lamp having
 wavelength peaks in emission wavelength regions of 300 to 400 nm and 360
 to 370 nm (chemical lamp) or a fluorescent lamp having wavelength peaks in
 emission wavelength regions of 200 to 300 nm and 250 to 255 nm
 (sterilizing lamp) can be used. Further, the chemical lamp and the
 sterilizing lamp can be used together.
 As the light source for the post-treating device, a high pressure mercury
 lamp having an emission wavelength region of 200 to 500 nm, superhigh
 pressure mercury lamp, or metal halide lamp can be used.
 When the high pressure mercury lamp, superhigh pressure mercury lamp, or
 metal halide lamp is used as the light source for the post-treating
 device, a cold mirror or a heat ray absorption glass is preferably
 disposed each alone or in combination. Further, if a blank plate is
 deteriorated by UV-rays in a specific wavelength region, a filter for
 cutting UV-rays in such a wavelength region is preferably disposed.
 When the high pressure mercury lamp, superhigh pressure mercury lamp or
 metal halide lamp is used as the light source for the post-treating
 device, the light source is preferably inserted in a water-cooled blue
 filter jacket tube for cutting a wavelength at 450 nm or higher.
 As the light source for the post-treating device, a UV ray laser having an
 oscillation wavelength in an ultraviolet region such as an He-Cd may also
 be used.
 Further, the post treating device is preferably constituted such that
 UV-rays can be irradiated to the blank plate in a state wound around the
 cylinder without attaching the blank plate from the cylinder. The
 constitution for this purpose can include, for example, an arrangement of
 disposing the light source to the periphery of the cylinder or
 transmission of UV-rays through optical fibers from the UV-ray generation
 device to the outer circumference of the cylinder.
 In a case of using optical fibers for the irradiation of UV-rays, it is
 preferably constituted such that the top ends of the optical fibers for
 irradiation of UV-rays are disposed together on a moving stage for
 attaching an optical head that irradiates infrared beams for image
 formation, the top ends of the optical fibers for irradiation of UV-rays
 are arranged at a position behind the optical head along the moving
 direction of the stage upon forming the image, so that UV-rays can be
 irradiated to the surface of the blank plate simultaneously with image
 formation by the infrared beams.
 In the print making apparatus according to the present invention, the image
 forming width of the laser beam by the optical head is determined
 depending on the number of the laser beams and resolution of the image
 formed to the blank plate, and the moving amount of the optical head is
 set in accordance with the image forming width.
 Further, it is preferably constituted such that the size of the blank plate
 in circumferential direction is made smaller than the cylinder
 circumference (up to about 70 to 80% of the cylinder circumference), to
 provide a marginal portion not mounted with the blank plate to the outer
 circumferential surface of the cylinder and the optical head is moved
 while it is opposed to the marginal portion.
 As an image forming signal used for a CTP system, a digital image recording
 signal (bit map data) formed, for example, by applying an RIP (Raster
 Image Processor) process to an image data edited by a DTP (Desk Top
 Publishing) of a computer or an electronic composing machine is utilized.
 The bit map data is, for example, compressed optionally in an RIP section,
 received by a control computer and stored in a main memory, and the
 compressed bit map data is optionally restored into an original data, and
 sent to a line memory of electronic control device. Further, a rotary
 encoder is disposed on the axis of the cylinder and the data of the
 rotational angle measured by the rotary encoder are sequentially taken
 into the electronic control device.
 Then, the coordinate for the start position of the laser irradiation to the
 blank plate wound around the cylinder is calculated on real time and, at
 the same time, a coordinate for the completion position of the laser
 irradiation is calculated from an optimal irradiation time on every laser
 within a range of a maximum laser irradiation time induced from the
 inter-pixel pitch determined depending on a desired resolution and the
 rotational circumferential speed of the cylinder. Then, the coordinate for
 the start position of the laser irradiation and the coordinate for the
 completion position of irradiation are superimposed on the image signal of
 the line memory to prepare a control signal and the laser generation
 device is controlled by the control signal.
 Further, an infrared ray intensity measuring sensor is disposed on an
 optical path of the semiconductor laser beam to sample a laser intensity
 upon actuation of the plate making apparatus or at an appropriate timing
 and the laser intensity data is taken into the control computer. Further,
 the data is calculated in comparison with a previously registered set
 value on each lasers and a driving input current for the semiconductor
 laser is controlled in accordance with the input current and the output
 intensity characteristic of the semiconductor laser to keep the intensity
 of each laser beam irradiated to the blank plate always at a predetermined
 value.
 Alternatively, a photosensor is disposed near the opposing side of the
 semiconductor oscillator on the side of the emitter (the laser beam
 emitting port) and the laser intensity is sampled on real time upon
 oscillation of the semiconductor laser. Then, the intensity data is taken
 into the control computer and the same calculation as described above is
 conducted by an automatic calculation function to control the input
 current for driving the semiconductor laser to keep the intensity of each
 laser beam irradiated to the blank plate always at a predetermined value.
 Since the focal position of the laser beam is displaced subtly from the
 surface of the blank plate on the outer circumferential surface of the
 cylinder depending on the difference of the thickness of the blank plate,
 circularity of the outer surface of the cylinder, deflection of the
 cylinder during rotation, or thermal expansion or thermal shrinkage of the
 cylinder or the like caused by the change of the atmospheric temperature
 in the plate making apparatus, the optical system preferably comprises an
 automatic focus correction mechanism adapted to move an objective lens in
 a direction vertical to the blank plate to always focus the laser beam at
 the surface of the blank plate.
 The infrared laser constituting the laser generation device is preferably a
 semiconductor laser emitting an infrared rays at an emission wavelength of
 750 to 880 nm and at the maximum power of 100 mW to 20 W, and the
 semiconductor laser is preferably used under PWM (Pulse Width Modulation)
 by directly controlling the input current at a modulation speed within a
 range from 0.1 to 10 Mbit/sec.
 The laser beam from the laser generation device has preferably a
 constitution to be transmitted through optical fibers to the optical head.
 The optical system is preferably incorporated with a zoom mechanism capable
 of automatically changing the optical magnification factor in accordance
 with a desired resolution. Further, the optical system is preferably
 constituted such that the beam spot diameter focused to the blank plate on
 the outer circumferential surface of the cylinder is from 5 to 50 .mu.m.
 An air blow and a vacuum suction mechanism are preferably disposed near the
 top end of the optical head with an aim of removing mists evaporated and
 scattered from the surface of the blank plate by thermal reaction in the
 course of image formation by the irradiation of the laser beams to the
 blank plate wound around the cylinder.
 The plate making apparatus is preferably constituted to blow cleaning air
 into the plate making apparatus to keep the inside of the apparatus in a
 pressurized state by the provision of the air blower and the air filter.
 Further, the rotational speed of the cylinder is preferably from 50 to 3000
 rpm.

BEST MODE FOR PRACTICING THE INVENTION
 First Embodiment
 A first embodiment of the plate making apparatus according to the present
 invention is to be explained with reference to FIGS. 1 to 6.
 As shown in FIG. 1 and FIG. 2, a plate making apparatus 100 comprises a
 hollow cylinder 131 having a rotational mechanism, a cassette 121 for
 keeping a plurality of blank plates 400, a blank plate supply mechanism
 120, a laser generation device 140, an optical head (laser irradiation
 head) 150, a linear stage (head moving mechanism) 160, a plate discharge
 mechanism 170, a plate discharge conveyor 180, a plate receiving tray 19,
 a control computer 200, an electronic control device (irradiation
 condition setting device) 210, and an RIP server 220 (computer connected
 to a network for exclusively conducting RIP process). Further, the plate
 making apparatus 100 has a blank plate attaching mechanism 130 shown in
 FIG. 5 and FIG. 6. Reference numeral 900 in FIG. 1 indicates a vibration
 proof rubber.
 The blank plate 400 is a heat sensitive type offset blank plate and the
 blank plate used herein comprises a hydrophilic layer as a heat sensitive
 layer formed on a support made of a thin aluminum sheet, the hydrophilic
 sensitive layer comprising a material that contains microcapsules
 containing an oleophilic ingredient in the inside and destroyed thermally,
 hydrophilic binder polymer having functional groups capable of
 three-dimensional crosslinking and functional groups capable of reacting
 with the oleophilic ingredient, and photoreaction initiator for initiating
 three-dimensional crosslinking reaction of the hydrophilic binder polymer.
 Such a blank plate is formed, for example, by a method described in
 Japanese Patent Unexamined Publication No. 7-1849.
 The cassette 121 has a structure capable of keeping about 100 sheets of
 blank plates in stack with the heat sensitive layer being faced upward,
 and supplement of the blank plate is informed by a photosensor for
 detecting the absence or presence of the blank plate 400.
 The blank plate supply mechanism 120, as shown in FIG. 5, has a vacuum
 suction pad 122 for sucking under vacuum the upper surface of the blank
 plate 400 to take out the blank plate 400 from the cassette 121, and a
 group of rolls 123 for transporting the blank plate 400 toward the hollow
 cylinder 131 while receiving the lower surface of the blank plate 400 and
 preventing sagging of the lower end thereof. Thus, the blank plate 400 is
 conveyed to the hollow cylinder 131 from the lateral direction.
 The blank plate attaching mechanism 130, as shown in FIGS. 5 and 6, has a
 top end clamp mechanism 300, a rear end clamp mechanism 301, a squeeze
 roll 325 and a vacuum suction mechanism 320.
 The top end clamp mechanism 300 is attached at a predetermined position of
 the hollow cylinder 131 for seizing the top end of the blank plate 400
 and, has a seizing surface opposing to the circumferential surface of the
 hollow cylinder 131 and a positioning surface 300A opposing to the top end
 surface of the blank plate 400 being conveyed toward the hollow cylinder
 131. The rear end clamp mechanism 301 is attached at a predetermined
 position of the hollow cylinder 131 for seizing the rear end of the blank
 plate 400 and the structure thereof is identical with that of the top end
 of the clamp mechanism 300.
 Accordingly, the top end of the blank plate 400 being conveyed by the blank
 plate supply mechanism 120 in the lateral direction to the hollow cylinder
 131 is inserted in a gap (several millimeters) between the top end
 mechanism 300 and the cylinder surface, and touched against a positioning
 surface 300A with a weak force. Since positioning is thus conducted by
 utilizing one side at the top end of the blank plate 400, image
 positioning for four plates in the subsequent process color printing step
 can be conducted easily.
 The blank plate supply mechanism 120 has a mechanism of finely correcting
 the conveying speed of the blank plate 400 such that the top end surface
 of the blank plate 400 touches the positioning surface 300A of the top end
 clamp mechanism 300 uniformly over the entire surface without causing
 twisting or the like at the top end of the blank plate 400.
 After the positioning, the opposing surface of the top end clamp mechanism
 300 to the circumferential surface of the cylinder moves toward the
 circumferential surface of the cylinder 131, thereby the top end of the
 blank plate 400 is put and held between the top end clamp mechanism 300
 and the circumferential surface of the hollow cylinder 131 while being
 kept in the positioned state. In this state, the hollow cylinder 131 is
 rotated and, at the same time, the squeeze roll 325 is pushed against the
 blank plate 400. Thus, the blank plate 400 is wound around the hollow
 cylinder 131 and the rear end thereof is seized by the rear end clamp
 mechanism 301. In this way, the blank plate 400 conveyed from the blank
 plate supply mechanism 120 is wound around the circumferential surface of
 the hollow cylinder 131 while being kept in the positioned state.
 The vacuum suction mechanism 320 is used for firmly holding the blank plate
 400 wound around the circumferential surface of the hollow cylinder 131 to
 the hollow cylinder 131, so that the attaching position does not change
 even if the hollow cylinder 131 is rotated at a high speed.
 As shown in FIG. 6, the vacuum suction mechanism 320 comprises vacuum
 suction holes 321 (fine through holes of about 1 to 3 mm diameter) formed
 to the outer circumferential surface of the hollow cylinder 131, an
 evacuation/air supply source 323 for discharging air from a cavity in the
 hollow cylinder 131, and a pipeline 322 connecting the inside of the
 hollow cylinder 131 with the evacuation/air supply source 323. The
 pipeline 322 is disposed passing through the inside of the shaft 133 and
 the end thereof on the side of the hollow cylinder 131 is disposed in the
 cavity of the hollow cylinder 131. Further, the shaft 133 and the pipeline
 322 are connected with a rotatable rotary joint 324.
 Accordingly, after winding the blank plate 400 to the hollow cylinder 131
 as described above, and then evacuating air in the hollow cylinder 131 by
 the evacuation/air supply source 323, thereby air in the gap between the
 hollow cylinder 131 and the blank plate 400 is compulsorily discharged
 through the vacuum suction holes 321. As a result, the blank plate 400 is
 firmly secured by vacuum suction.
 The hollow cylinder 131 is installed horizontally on a rack base 110. The
 rotational mechanism of the hollow cylinder 131 comprises shafts 132 and
 133 protruded from both ends, bearings 134 for rotatably supporting the
 shafts 132 and 133, a rotation motor 136 connected to the end of the shaft
 132 with a coupling 135, and a rotary encoder 137 disposed to the end of
 the shaft 133 for measuring the rotational angle of the hollow cylinder
 131.
 The rotation motor 136 having a power of rotating the hollow cylinder 131
 at a rotational speed of 50 to 3000 rpm is used. When the blank plate has
 a large size, the outer diameter of the hollow cylinder 131 is, for
 example, from 250 to 500 mm. When highly fine image data exceeding 1000
 dpi (dot/inch) are formed as an image by using such a large hollow
 cylinder 131, it is practically preferred to keep the rotational speed of
 the hollow cylinder 131 to about 1000 rpm or lower in view of the
 restriction for the performance of a general optical rotary encoder
 measuring system. A high performance optical rotary encoder having high
 resolution is easily available from "HEIDENHAIN Co." or "Canon Co.".
 The laser generation device 140 is used for generating a laser beam 800 in
 an infrared region to be irradiated to the blank plate 400. As shown in
 FIG. 3, it comprises a plurality of semiconductor layers 141, a heat sink
 base 142 having cooling Peltier devices mounted thereon, a laser driving
 device 143, and a fiber bundle 144. The plurality of semiconductor lasers
 141 are fiber-coupled and disposed on the heat sink base 142.
 As the semiconductor laser 141, those generating infrared laser at an
 oscillation wavelength of 750 to 880 nm are used and it is preferred to
 select those having an optimal oscillation wavelength in accordance with
 the absorption spectrum of an infrared absorbent added to the heat
 sensitive layer of the blank plate 400. Further, it is most preferred to
 use a semiconductor layer having an oscillation wavelength at 810 to 850
 nm in view of the overall performance as the device such as size, cost and
 working life.
 In a case of forming a highly fine image with resolution exceeding 1000
 dpi, the core diameter of the optical fiber coupled to the semiconductor
 laser 141 is preferably 100 .mu.m or less, and numeral aperture (NA) is
 generally from 0.12 to 0.15. Such a fiber-coupled semiconductor laser is
 available easily from "SDL Co." or "OPTOPOWER Co.".
 As the fiber bundle 144, those comprising bundled fibers having the same
 shape and function as the optical fibers used for the fiber-coupled
 semiconductor laser 141 are used. Each of the optical fibers of the fiber
 bundle 144 is connected with the semiconductor laser 141 by a connector or
 fusion splicing.
 In the sheath at the top end of the fiber bundle 144, optical fibers are
 arranged laterally each at an equal distance with the pitch of several
 hundreds .mu.m and aligned and fixed such that laser beam from each of the
 optical fibers is in parallel with each other. Thus, a group of laser
 beams arranged in line are generated from the sheath at the top end of the
 fiber bundle 144.
 Further, if the length of the fiber bundle 144 is as long several meters,
 it is preferred to insert the fiber bundle 144 into a flexible pipe made
 of plastic or metal for protection.
 If the semiconductor laser 141 is a laser that generates an output energy
 of about 1 W, a voltage at about 2-3 V is applied as a DC power source
 from the laser driving device 143 to the semiconductor laser 141. It is
 preferred that a current of about 500 to 2000 mA at the maximum is
 supplied to the semiconductor laser 141 upon image formation, while it is
 preferred to supply a bias current of 20 to 100 mA which is a current
 giving no thermal effects on the surface of the blank plate 400 when the
 image is not formed such that the semiconductor laser 141 instantaneously
 reaches the maximum power intensity.
 The top end sheath of the fiber bundle 144 is held, as shown in FIG. 4, by
 a fiber bundle securing portion 151 of an optical head 150.
 The optical head 150 comprises a lens cylinder 152, a group of condensing
 lenses 153, a prism 154, a group of zoom lenses 155, a zoom mechanism 156,
 a zoom motor 157, a group of objective lenses 158, an objective
 lens-actuator 159 and an astigmatism sensing mechanism 500.
 The infrared laser beam irradiated from the semiconductor laser 141 is
 transmitted through the optical fibers and, finally, emitted from the
 final end of the sheath of the fiber bundle 144 as the group of laser
 beams arranged in line to the outside. The group of condensing lenses 153
 condense the laser beams into parallel light, and the infrared laser beams
 converted into the parallel light are focused on the surface of the blank
 plate 400 wound around the hollow cylinder 131 into a beam spot diameter
 of several to several tens .mu.m through the prism 154, the group of zoom
 lenses 155, and the group of objective lens 158.
 The beam spot diameter to be focused on the surface of the blank plate 400
 can be optionally determined by varying the optical reduction factor of
 the group of zoom lens 155 and the group of objective lens 158.
 Practically, the lenses having a maximum reduction factor of about 5 are
 selected with the reason, for example, of intending to ensure a distance
 of (working distance) several .mu.m or more from the top end of the
 optical head 150 to the surface of the blank plate 400 and intending to
 minimize the intensity loss of the laser beams without enlarging the size
 of the optical system such as the lens or the lens cylinder 152 extremely.
 For this purpose, if the fiber core diameter used for the fiber bundle 144
 is 50 .mu.m, a beam spot diameter of about 10 .mu.m at the minimum can be
 obtained on the surface of the blank plate 400. A further smaller beam
 spot diameter can of course be obtained by making the fiber core diameter
 of the fiber bundle 144 smaller. Further, while smaller beam spot diameter
 can also be obtained by choosing a lens with a further maximum reduction
 factor, the intensity loss of the laser beam is increased.
 Further, the zoom lens group 155 is adapted to change the relative position
 in accordance with the movement of the zoom mechanism 156. Since the zoom
 mechanism 156 advances or retracts and the relative position in the zoom
 lens group 155 is also changed together by the rotation of the zoom motor
 157 that is gear-coupled with the zoom mechanism 156, the optical
 reduction factor is changed in accordance therewith. Then, if the zoom
 lens is chosen such that the zoom factor can be varied within range from 1
 to 5 times, the beam spot diameter focused on the surface of the blank
 plate 400 can be changed optically within a range from 10 to 50 .mu.m.
 The astigmatism sensing mechanism 500 comprises a visible light
 semiconductor laser 501 having a wavelength region of 600 to 700 nm and a
 maximum power energy of about several tens mW, a beam shaping mechanism
 502, a prism group 503, an automatic power control mechanism 504 and a
 4-divisional photodetector 505. A visible light laser beam irradiated from
 the visible light semiconductor laser 501 is shaped by the beam shaping
 mechanism 502 into parallel light and separated partially at the prism
 503. The separated beam is detected by the photodiode of the automatic
 power control mechanism 504. The current supplied to the visible light
 semiconductor laser 501 is controlled by the output signal of the
 photodiode to keep the output power of the laser constant.
 The visible light laser beam other than the beam transmitting the prism 503
 is reflected at a diagonal plane of the prism 154, superimposed with the
 image forming infrared laser beam 800 and entered to the blank plate 400.
 Most of the visible light laser beam is reflected on the surface of the
 blank plate 400 and entered again in the plasmas 154 and 503 and
 reflected. The reflected light is given with astigmatism by a cylindrical
 lens on the optical path and finally fed back to the 4-divisional
 photodetector 505.
 In this mechanism, output signals of the 4-divisional photodetector 505 are
 added diagonally to each other and further subtracted diagonally from each
 other, and these values are inputted as focus error signals to a
 focus-servo control circuit and an objective lens-actuator 159 is operated
 by the output signal from the focus-servo control circuit. By the
 mechanism, the objective lens group 158 suspended by a leaf spring from
 the objective lens-actuator 159 moves forward and backward. Thus, the
 image forming infrared laser beam 800 is focused together with the visible
 light laser beam on the surface of the blank plate.
 On the other hand, the optical head 150 is placed on the linear stage 160
 as a movable support means, and can be moved linearly by the linear stage
 160 in the longitudinal direction of the axis of the hollow cylinder 131.
 The linear stage 160 comprises a linear motor guide 161 disposed in
 parallel with the hollow cylinder 131, a linear motor 162, a linear scale
 163 and a support table 164 used for the optical head connected with the
 linear motor 162.
 Image formation by the optical head 150 (irradiation of the laser beam) is
 conducted over the entire surface of the blank plate 400 by the movement
 of the linear stage 160 having the optical head mounted thereon and the
 rotation of the hollow cylinder 131. That is, image formation from the
 optical head 150 to the blank plate 400 is conducted for a predetermined
 width in the direction of the cylinder axis during one rotation of the
 hollow cylinder 131, and the optical head 150 moves by a predetermined
 amount in the direction of the cylinder axis on every one rotation of the
 hollow cylinder 131. The process is repeated in entire axial direction of
 the cylinder.
 Then, the size of the blank plate 400 in the circumferential direction of
 the hollow cylinder 131 is made smaller than the circumference of the
 hollow cylinder 131 (up to about 70 to 80% of the circumference) to
 provide a marginal portion where the blank plate is not attached to the
 outer circumferential surface of the hollow cylinder 131. Then, the
 operation of the linear stage 160 is controlled such that the optical head
 150 is not moved while the optical head 150 opposes to the blank plate
 attaching surface of the hollow cylinder 131, and the optical head 150 is
 moved by a predetermined amount in the direction of the rotational axis of
 the hollow cylinder 131 while the optical head 150 is opposed to the
 marginal portion of the hollow cylinder 131.
 Thus, when an image is formed to the entire surface of the blank plate 400,
 it is no more necessary to stop the rotation of the hollow cylinder or
 form the image once per two rotations of the hollow cylinder 131 (image is
 formed during first rotation and the linear stage is moved during the
 succeeding rotation), so that the image can be formed efficiently over the
 entire surface of the blank plate 400 with no additional useless rotation.
 The moving amount of the optical head 150 is defined as a distance obtained
 by multiplying the beam pitch corresponding to resolution of the image
 data to be formed to an image by the number of laser beams.
 An RIP server 220 receives image data made by DTP or an electronic
 composing machine by a communication protocol such as TCP/IP or Apple Talk
 by way of a network line (Ethernet, etc.) and makes bit map data by
 applying RIP process to the received image data. Subsequently, the bit mat
 data is compressed by an algorithm such as a run length method to decrease
 the capacity of the bit map data.
 The control computer 200 receives the compressed bit map data from the RIP
 server 220 by way of the interface line, (for example, SCSI) and stores
 the data in the main memory (RAM) in the control computer 200. The control
 computer 200 properly defreezes the compressed bit mat data stored in the
 main memory and restores the data into the original bit map data and then
 transfers the restored bit map data by way of a control bus (Compact PCI
 or VME bus) to the line memory of the electronic control device 210.
 The electronic control device 210 has two sets of line memory of functions
 referred to as A bank/B bank and forms an image with the bit map data
 contained in one of the line memories (A bank) while transferring the bit
 map data for the next line to another empty line memory (B bank). It is
 adapted to complete transferring of the bit mat data in parallel while
 forming an image within a period for one rotation of the hollow cylinder
 131 by alternately switching image formation and relocation.
 Further, the electronic control device 210 has a receiving counter for the
 data of rotational angle sent from the rotary encoder 137 and calculates
 the basic number of pulses between pixels based on the outer diameter of
 the blank plate 400, resolution angle per one pulse from the rotary
 encoder 137 and setting resolution of the image. Further, the position for
 starting image formation to the blank plate 400 is calculated based on the
 rotational position information of the hollow cylinder 131 formed on real
 time in accordance with rotation of the hollow cylinder 131, to determine
 the position for completing image formation on every laser based on the
 rotational circumferential speed of the hollow cylinder 131 and the laser
 irradiation time previously determined individually on every laser.
 Then, the electronic control device 210 superimposes the thus determined
 position for completing image formation on every laser and a logic signal
 of the bit map data, and outputs the superimposed control signal to the
 laser driving device 143 of the laser generation device 140. Thus, the
 laser driving device 143 controls the image formation time on very laser
 independently.
 In this case, the set value for the irradiation time for each of the lasers
 is previously calculated based on the material and the thickness of the
 heat sensitive layer of the blank plate 400 to be used and the beam
 position at which the group of laser beams arranged in line are emitted
 finally. With respect to the beam position, the irradiation time is set
 shorter at the center of the line, while the irradiation time is set
 longer toward the ends of the line. This can make the temperature of the
 blank plate 400 uniform within a region in which an image is formed at
 once by a group of laser beams arranged in line.
 Accordingly, in the plate making apparatus, the temperature of the blank
 plate 400 is made uniform over the entire region in which the image is
 formed in one rotation of the hollow cylinder 131 and the group of beams
 800 arranged in line are moved repeated in the direction of the axis of
 rotation on every one rotation of the hollow cylinder 131, so that the
 image is formed by uniform heat sensitive reaction over the entire surface
 of the heat sensitive layer of the blank plate 400. This can remarkably
 improve the image quality of the obtained printing plate.
 Further, the plate making apparatus 100 has an infrared intensity sensor
 801 having a photo-receiving surface at the focusing position of the
 image-forming infrared laser beam 800 beside the hollow cylinder 131, so
 as to move the linear stage 160 to a position at which the image forming
 infrared resin beam 800 is detected by the infrared intensity sensor 801
 upon actuation of the plate making apparatus or at an appropriate timing.
 In this constitution, one laser is turned on by the laser driving device
 143 for several seconds, the measured intensity data is taken into the
 control computer 200 to control the laser driving current of the laser
 generation device 140, and the laser beam is irradiated at a predetermined
 laser intensity to the blank plate 400. Then, by repeating the process
 successively for the number of the laser beams, the laser intensity is set
 on every laser independently.
 Further, it may be adapted such that the window of the semiconductor laser
 141 opposing to an oscillator emitter window is made as a half-mirror
 structure, a portion of the laser beam generated in the oscillator is
 taken out and detected by the photodiode to control the laser intensity
 like that in the means described above.
 Further, a plate discharge mechanism 170 is disposed above the hollow
 cylinder 131 of the plate making apparatus 100. A vacuum suction pad is
 disposed to the plate discharge mechanism 170, and the blank plate 400
 after completing the image formation is sucked under vacuum by the vacuum
 suction pad, detached out of the hollow cylinder 131 and transported to
 the plate discharge conveyor 180. The blank plate 400 transported to the
 plate discharge conveyor 180 is received by the plate receiving tray 19.
 Second Embodiment
 A second embodiment of the plate making apparatus according to the present
 invention is to be explained with reference to FIGS. 7 to 9. As can be
 seen from comparison between FIG. 1 and FIG. 7 and comparison between FIG.
 2 and FIG. 8, the plate making apparatus 100 is different from the first
 embodiment, in that a UV-ray irradiation device 190 for irradiating
 UV-rays to a blank plate transported to the plate discharge conveyor 180
 is disposed but is identical with the first embodiment in other
 constitutions.
 As shown in FIG. 9, a blank plate 410 on the plate discharge conveyor 180
 is put to a post treatment by irradiation of UV-rays from the UV-ray
 irradiation device 190 along with movement of the plate discharge conveyor
 180. By the post treatment, the printing resistance and the printing
 quality for the obtained image portion of the plate are improved
 remarkably.
 A metal hydride lamp is used for the lamp 192 of the UV-ray irradiation
 device 190 and an inverter power source is used as a control power source
 for the metal halide lamp, and the lamp intensity is optionally variable
 within a range from 25 to 100%. Further, the lamp is air-cooled by a air
 cooling exhaust blower 195 and an exhaust duct 194. Further, the lamp 192
 is attached to a housing 191 capable of rotating by 180.degree. and an
 aluminum reflection plate 193 is disposed at a position of the housing 191
 for the back of the lamp 192.
 In this embodiment, since a long metal halide lamp can not be turned on
 instantaneously, it is lighted up in a stand-by state with a weak lamp
 intensity of about 25%, and a portion between the lamp 192 and a plate
 discharge conveyor 180 is shielded by the housing 191 so as not to leak
 UV-rays onto the plate discharge conveyor 180 by rotating the housing for
 180.degree..
 Then, the blank plate 410 is detached from the hollow cylinder 131 by a
 plate take-out pad 170 and transported to a plate discharge conveyor 180
 and, at the same time, the plate discharge conveyor 180 is driven and the
 housing 191 rotates by 180.degree. to return to the position above the
 lamp 192, and the power of the metal halide lamp 192 is increased to 100%
 lamp intensity.
 Further, when the blank plate 410 passes below the UV-ray irradiation
 device 190, the housing 191 rotates by 180.degree. and returns to the
 stand-by position, and the power of the metal halide lamp 192 is lowered
 to a weak lamp intensity.
 It is necessary to increase or decrease the amount of irradiation energy of
 UV-rays in accordance with the amount of irradiation energy of UV-rays
 required for the blank plate, and this can be increased or decreased by
 increasing or decreasing the lamp intensity of the metal halide lamp 192
 with inverter power supply. In addition to the above, since a speed
 variable mechanism is attached to the plate discharge conveyor 180, the
 amount of irradiation energy of UV-rays can be easily increased or
 decreased by changing the speed of the plate discharge conveyor 180.
 In this embodiment, an air-cooled type metal halide lamp is used as the
 lamp 192 of the UV-ray irradiation device 190, and same effect can also be
 expected by using a high pressure mercury lamp, super-high pressure
 mercury lamp or a chemical lamp or sterilizing lamp providing that the
 emission wavelength is within a ultra-violet region of 200 to 400 nm.
 Accordingly, the lamp to be used can be selected properly depending on the
 irradiation energy requiring for the blank plate.
 Further, if temperature elevation is undesired for the blank plate, it is
 preferred to make the reflection plate with a cold mirror allowing only
 the heat rays to permeate therethrough selectively instead of the aluminum
 reflection plate, or additionally dispose heat ray absorbing glass just
 below the lamp. For shielding heat rays more effectively, it is preferred
 to adopt a water-cooled type metal halide lamp of inserting a lamp in a
 water cooled blow filter jacket tube capable of cutting off visible rays
 at 450 nm or higher or heat rays by nearly about 100%.
 As shown in FIG. 4, it is preferred in the plate making apparatus according
 to the present invention to provide a vacuum suction mechanism 600 between
 an optical lens head 150 and a hollow cylinder 131, to prevent mists that
 are evaporated and scattered by thermal reaction from the surface of the
 blank plate during image formation to the blank plate 400 from depositing
 on the lens surface of the objective lens group 158. The vacuum suction
 mechanism 600 comprises a dust collecting hood 601, a vacuum pump 603, a
 filter and an exhaust duct 602.
 In this embodiment, the dust collecting hood 601 of the vacuum suction
 mechanism 600 is disposed on the support table 164 and the vacuum suction
 mechanism 600 is controlled to be moved together with the linear stage
 160, for example, by the control computer 200.
 Further, when the plate making apparatus according to the present invention
 is constituted as a tightly closed structure in which a cover is attached
 to the frame of the apparatus, clean air generated from a clean air supply
 mechanism 700 constituted with an air blower and an air filter (refer to
 FIG. 1 and FIG. 7) is sent into the apparatus to keep a pressurized state
 thereby keeping the inside of the apparatus clean, undesired effect of
 dusts or dirts in the atmosphere of the room can be eliminated, so that an
 offset printing plate of more excellent printing quality can be
 manufactured.
 Industrial Applicability
 As has been described above, the method of the present invention is a plate
 making method of forming an image to a heatsensitive type blank plate by
 an outer surface cylinder scanning system plate making apparatus.
 Then, according to the method of the present invention, since image
 formation with a uniform heat sensitive reaction is conducted for the
 entire surface of the heat sensitive layer of the blank plate in an image
 forming step by a multi-channel system, the image quality of the obtained
 printing plate can be improved outstandingly. Further, by conducting
 positioning utilizing one side at the top end of the blank plate, accurate
 positioning for blank plates of four colors can be conducted conveniently
 in a short time upon process color printing using the thus obtained
 printing plate. Further, the printing quality of the obtained printing
 plate can be improved outstandingly by conducting the post treating step.
 In view of the above, according to the method of the present invention, a
 practical heat sensitive type offset printing plate can be obtained at a
 commercial level.
 Further, according to the apparatus of the present invention, the method of
 the present invention can be practiced with ease.