Method and apparatus for developing printing plates in press

What is claimed is:

1. A printing apparatus for making a plate by recording and developing images thereon, and thereafter printing the images by feeding inks to the plate, said printing apparatus comprising:

a plate cylinder for supporting said plate mounted peripherally thereof;

a plate cylinder rotating mechanism for rotating said plate cylinder;

an image recorder for recording said images on said plate mounted on said plate cylinder; and

a developing device for developing said plate mounted on said plate cylinder, said developing device including;

a developing roller reciprocable between an applying position for contacting said plate to apply a developer to said plate mounted peripherally of said plate cylinder, and a retracted position spaced from said applying position; and

a squeeze roller reciprocable between a squeezing position for contacting said plate to squeeze said developer applied by said developing roller off said plate mounted peripherally of said plate cylinder, and a retracted position spaced from said squeezing position,

wherein said plate cylinder rotating mechanism includes a first plate cylinder rotating mechanism which rotates said plate cylinder when making a plate and a second plate cylinder rotating mechanism which rotates said Plate cylinder when printing,

wherein said first plate cylinder rotating mechanism is operable for rotating said plate cylinder such that a period of time from said developing roller contacting said plate mounted on said plate cylinder to apply said developer thereto to said squeeze roller contacting said plate after moving from said retracted position to said squeezing position substantially corresponds to a period of time required for developing said plate with said developer,

wherein said first plate cylinder rotating mechanism is operable for rotating said plate cylinder at a turning angle velocity .theta./T after said developing roller contacts said plate to apply said developer thereto, where .theta. is an angle through which said plate cylinder rotates to move a position of said plate contacted by said developing roller into contact with said squeeze roller, and T is said period of time required for developing said plate with said developer.

2. A printing apparatus as defined in claim 1, wherein said developing device further includes a developing roller rotating mechanism for rotating said developing roller in a direction in which said plate cylinder is rotated by said plate cylinder rotating mechanism, while said developing roller is maintained in contact with said plate mounted peripherally of said plate cylinder.

3. A printing apparatus as defined in claim 1, wherein said developing device further includes a fixing roller reciprocable between an applying position for contacting said plate to apply a fixer to said plate mounted peripherally of said plate cylinder, and a retracted position spaced from said applying position, said squeeze roller being operable for squeezing also said fixer applied by said fixing roller off said plate.

4. A printing apparatus as defined in claim 3, wherein said squeeze roller is disposed between said developing roller and said fixing roller.

5. A printing apparatus as defined in claim 1, further comprising a developing device moving mechanism for moving said developing device between a developing position opposed to said plate cylinder, and a retracted position spaced from said developing position.

6. A printing apparatus as defined in claim 1, wherein said developing device further includes a cleaning mechanism for cleaning said squeeze roller with cleaning solution.

7. A printing apparatus for making a plate by recording and developing images thereon, and thereafter printing the images by feeding inks to the plate, said printing apparatus comprising:

a plate cylinder for supporting said plate mounted peripherally thereof;

a plate cylinder rotating mechanism for rotating said plate cylinder, the plate cylinder rotating mechanism including a first plate cylinder rotating mechanism which rotates said plate cylinder when making a plate and a second plate cylinder rotating mechanism which rotates said plate cylinder when printing;

an image recorder for recording said images on said plate mounted on said plate cylinder;

a plate cylinder moving mechanism for moving said plate cylinder between an image recording position opposed to said image recorder and a printing position for printing with said plate mounted on said plate cylinder; and

a developing device operable in said image recording position for developing said plate mounted on said plate cylinder, the developing device including:

a developing roller reciprocable between an applying position for contacting said plate to apply a developer to said plate mounted peripherally of said plate cylinder, and a retracted position spaced from said applying position, and

a squeeze roller reciprocable between a squeezing position for contacting said plate to squeeze said developer applied by said developing roller off said plate mounted peripherally of said plate cylinder, and a retracted position spaced from said squeezing position,

wherein said first plate cylinder rotating mechanism is operable for rotating said plate cylinder such that a period of time from said developing roller contacting said plate mounted on said plate cylinder to apply said developer thereto to said squeeze roller contacting said plate after moving from said retracted position to said squeezing position substantially corresponds to a period of time required for developing said plate with said developer,

wherein said first plate cylinder rotating mechanism is operable for rotating said plate cylinder at a turning angle velocity .theta./T apply after said developing roller contacts said plate to apply said developer thereto, where .theta. is an angle through which said plate cylinder rotates to move a position of said plate contacted by said developing roller into contact with said squeeze roller, and T is said period of time required for developing said plate with said developer.

8. A printing apparatus as defined in claim 7, wherein said developing device further includes a developing roller rotating mechanism for rotating said developing roller independently of said plate cylinder.

9. A printing apparatus as defined in claim 7, further comprising a developing device moving mechanism for moving said developing device between a developing position opposed to said plate cylinder in said image recording position, and a retracted position spaced from said developing position.

10. A printing apparatus for making a plate by recording and developing images thereon, and thereafter printing the images by feeding inks to the plate, said printing apparatus comprising:

a plate cylinder for supporting said plate mounted peripherally thereof;

a plate cylinder rotating mechanism for rotating said plate cylinder;

an image recorder for recording said images on said plate mounted on said plate cylinder; and

a developing device for developing said plate mounted on said plate cylinder, wherein said developing device includes a developer feeding mechanism for feeding said developer only in time of a developing process;

a developing roller reciprocable between an applying position for contacting said plate to apply a developer to said plate mounted peripherally of said plate cylinder, and a retracted position spaced from said applying position, wherein said developer feeding mechanism includes:

a first developer tank for storing said developer;

a second developer tank for storing said developer, said second developer tank being disposed below said first developer tank;

a developer supply line for supplying said developer from said second developer tank to said first developer tank;

overflow means for maintaining said developer at a fixed level in said first developer tank;

a drain port formed in a bottom of said first developer tank;

a developer collecting line for allowing part of said developer overflowing said overflow means and part of said developer flowing out through said drain port to flow down to said second developer tank; and

a pump disposed on said developer supply line for pumping said developer through said developer supply line to said first developer tank at a flow rate higher than a flow rate of said developer flowing from said drain port to said second developer tank; and

a squeeze roller reciprocable between a squeezing position for contacting said plate to squeeze said developer applied by said developing roller off said plate mounted peripherally of said plate cylinder, and a retracted position spaced from said squeezing position.

11. A printing apparatus as defined in claim 10, wherein said flow rate of said developer pumped by said pump through said developer supply line to said first developer tank is lower than a maximum flow rate of said developer flowing from said drain port and said overflow means to said second developer tank.

12. A method of developing a plate for use in a printing apparatus in which a plate cylinder holding the plate at the periphery thereof is rotated by a first plate cylinder rotating mechanism and images are recorded and developed on the plate, and thereafter said plate cylinder is rotated by a second plate cylinder rotating mechanism to print the images by feeding inks to the plate, said method comprising:

a step of rotating a plate cylinder with said plate mounted peripherally thereof, by operating a first plate cylinder rotating mechanism;

an image recording step for recording said images on said plate rotating as mounted on said plate cylinder, by operating an image recorder;

a developer applying step for applying a developer to said plate rotating as mounted on said plate cylinder, by placing a developing roller in contact with said plate after said images are recorded on said plate by said image recorder; and

a developer removing step for removing said developer from said plate rotating as mounted on said plate cylinder, by placing a squeeze roller in contact with said plate upon lapse of a time required for developing said plate with said developer after said developer is applied to said plate by said developing roller, wherein, at said developer applying step and said developer removing step, said plate cylinder is rotated by said first plate cylinder rotating mechanism at a turning angle velocity .theta./T, where .theta. is an angle through which said plate cylinder rotates after said developing roller contacts said plate mounted on said plate cylinder to apply said developer thereto until said squeeze roller contacts said plate, and T is a period of time required for developing said plate with said developer.

13. A method of developing a plate for use in a printing apparatus as defined in claim 12, wherein, at said developer applying step, said developing roller is rotated in a direction in which said plate cylinder is rotated by said first plate cylinder rotating mechanism, while said developing roller is maintained in contact with said plate mounted peripherally of said plate cylinder.

14. A method of developing a plate for use in a printing apparatus as defined in claim 12, further comprising a fixer applying step, executed after said developer removing step, for applying a fixer to said plate rotating as mounted on said plate cylinder, by placing a fixing roller in contact with said plate.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printing apparatus for making plates by recording and developing images on the plates, and thereafter printing the images by feeding ink to the plates. The invention relates also to a method of developing plates for use in the printing apparatus.

2. Description of the Related Art

In an ordinary conventional printing apparatus, a prepress process is carried out first to make a plate by exposing the plate placed in contact with a film having a binarized black and white image recorded thereon. Then, the plate is loaded into the printing apparatus to carry out a printing process.

Recently, printing apparatus commonly called digital printers have been proposed, one such printer being capable of performing both the prepress process and printing process. The digital printers employ a "computer-to-plate" system for forming images on plates by directly scanning and exposing the plates with laser beams or the like modulated with image signals.

Such printing apparatus are described in Japanese Patent Publication (Unexamined) H9-141821 (1997), Patent Publication (Unexamined) H9-123402 (1997) and Patent Publication (Unexamined) H9-131855 (1997), for example. Each of the printing apparatus described in these publications has an image recorder for recording images on a plate mounted peripherally of a cylinder such as a plate cylinder, an impression cylinder or a plate-making cylinder. In parallel with the image recording process, a developing process is carried out by placing a developing roller with the plate for applying a developer to the plate.

Since such conventional printing apparatus perform the image recording and developing processes concurrently, the turning angle velocity of the cylinder is dependent on the processing speed of the image recorder. The developing time from application of the developer to the plate to removal of the developer therefrom is determined by the processing speed of image recorder.

Consequently, the plate developing time does not agree with the time required for the developer to develop the plate. This could result in a failure to develop the plate properly, i.e. an overdevelopment or underdevelopment of the plate.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to provide a printing apparatus, and a method of developing a plate for use in the printing apparatus, for properly developing the plate mounted peripherally of a cylinder.

The above object is fulfilled, according to the present invention, by a printing apparatus for making a plate by recording and developing images thereon, and thereafter printing the images by feeding inks to the plate, the printing apparatus comprising:

a cylinder for supporting the plate mounted peripherally thereof;

a cylinder rotating mechanism for rotating the cylinder;

an image recorder for recording the images on the plate mounted on the cylinder; and

a developing device for developing the plate mounted on the cylinder, the developing device including;

a developing roller reciprocable between an applying position for contacting the plate to apply a developer to the plate mounted peripherally of the cylinder, and a retracted position spaced from the applying position; and

a squeeze roller reciprocable between a squeezing position for contacting the plate to squeeze the developer applied by the developing roller off the plate mounted peripherally of the cylinder, and a retracted position spaced from the squeezing position.

In a preferred embodiment of the invention, the cylinder rotating mechanism is operable for rotating the cylinder such that a period of time from the developing roller contacting the plate mounted on the cylinder to apply the developer thereto to the squeeze roller contacting the plate substantially corresponds to a period of time required for developing the plate with the developer.

Preferably, the cylinder rotating mechanism is operable for rotating the cylinder at a turning angle velocity .theta./T after the developing roller contacts the plate to apply the developer thereto, where .theta. is an angle through which the cylinder rotates to move a position of the plate contacted by the developing roller into contact with the squeeze roller, and T is the period of time required for developing the plate with the developer.

With this printing apparatus, the plate developing time is in agreement with the time required for developing the plate with the developer. Consequently, the plate may be developed properly.

In a preferred embodiment of the invention, the developing device further includes a developing roller rotating mechanism for rotating the developing roller in a direction in which the cylinder is rotated by the cylinder rotating mechanism, while the developing roller is maintained in contact with the plate mounted peripherally of the cylinder.

In another preferred embodiment, the developing device further includes a fixing roller reciprocable between an applying position for contacting the plate to apply a fixer to the plate mounted peripherally of the cylinder, and a retracted position spaced from the applying position, the squeeze roller being operable for squeezing also the fixer applied by the fixing roller off the plate.

Preferably, the squeeze roller is disposed between the developing roller and the fixing roller.

In a further preferred embodiment, the printing apparatus comprises a developing device moving mechanism for moving the developing device between a developing position opposed to the cylinder, and a retracted position spaced from the developing position.

In a still further preferred embodiment, the developing device further includes a cleaning mechanism for cleaning the squeeze roller.

In yet another preferred embodiment, the developing device further includes a developer feeding mechanism for feeding the developer only in time of a developing process.

In another aspect of the present invention, a printing apparatus comprises:

a cylinder for supporting a plate mounted peripherally thereof;

a cylinder rotating mechanism for rotating the cylinder;

an image recorder for recording images on the plate mounted on the cylinder;

a cylinder moving mechanism for moving the cylinder between an image recording position opposed to the image recorder and a printing position for printing with the plate mounted on the cylinder; and

a developing device operable in the image recording position for developing the plate mounted on the cylinder, the developing device including;

a developing roller reciprocable between an applying position for contacting the plate to apply a developer to the plate mounted peripherally of the cylinder, and a retracted position spaced from the applying position; and

a squeeze roller reciprocable between a squeezing position for contacting the plate to squeeze the developer applied by the developing roller off the plate mounted peripherally of the cylinder, and a retracted position spaced from the squeezing position.

In a further aspect of the invention, there is provided a method of developing a plate for use in a printing apparatus which makes the plate by recording and developing images thereon, and thereafter prints the images by feeding inks to the plate. This method comprises:

a step of rotating a cylinder with the plate mounted peripherally thereof, by operating a cylinder rotating mechanism;

an image recording step for recording the plate rotating as mounted on the cylinder, by operating an image recorder;

a developer applying step for applying a developer to the plate rotating as mounted on the cylinder, by placing a developing roller in contact with the plate after the images are recorded on the plate by the image recorder; and

a developer removing step for removing the developer from the plate rotating as mounted on the cylinder, by placing a squeeze roller in contact with the plate upon lapse of a time required for developing the plate with the developer after the developer is applied to the plate by the developing roller.

Other features and advantages of the present invention will be apparent from the following detailed description of the embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there are shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangement and instrumentalities shown.

FIG. 1 is a schematic side view of a printing apparatus according to the present invention;

FIG. 2 is a front view of a first and a second plate cylinder moving mechanisms;

FIG. 3 is a sectional side view showing bearings and adjacent components of the first cylinder moving mechanism;

FIG. 4 is a schematic view showing a relationship between a detent attached to a first plate cylinder and a release member disposed in a first printing position;

FIG. 5 is another schematic view showing the relationship between the detent and release member;

FIG. 6 is yet another schematic view showing the relationship between the detent and release member;

FIG. 7 is a schematic view showing a relationship between the detent attached to the first plate cylinder and a release member disposed in an image recording position;

FIG. 8 is a front view of a plate cylinder rotating mechanism and adjacent components;

FIG. 9 is a sectional side view showing a principal portion of FIG. 8;

FIGS. 10A and 10B are explanatory views each showing an arrangement of image areas on a plate;

FIG. 11 is a schematic view of a contact mechanism for acting on a first blanket cylinder;

FIG. 12 is a block diagram showing a principal electrical structure of the printing apparatus;

FIG. 13 is a schematic side view of a developing device;

FIG. 14 is a schematic view of a mechanism for circulating a developer through developer tanks;

FIG. 15 is a side view of a developing device moving mechanism and a roller moving mechanism;

FIG. 16 is a plan view of the developing device moving mechanism;

FIG. 17 is a front view of a principal portion of the roller moving mechanism;

FIG. 18 is a side view of the principal portion of the roller moving mechanism;

FIG. 19 is a front view of a motor and adjacent components for rotating a brush roller;

FIG. 20 is an explanatory view showing pivotal movements of arms caused by a cam;

FIG. 21 is another explanatory view showing pivotal movements of the arms caused by the cam;

FIG. 22 is a further explanatory view showing pivotal movements of the arms caused by the cam;

FIG. 23 is a still further explanatory view showing pivotal movements of the arms caused by the cam;

FIG. 24 is a still further explanatory view showing pivotal movements of the arms caused by the cam;

FIG. 25 is an explanatory view showing a developer applying operation of a developing roller;

FIG. 26 is an explanatory view showing an angle between the developing roller and a squeeze roller;

FIG. 27 is a flow chart showing an outline of prepress and printing operations of the printing apparatus: and

FIG. 28 is a flow chart of a prepress process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described hereinafter with reference to the drawings. FIG. 1 is a schematic side view of a printing apparatus according to the present invention.

This printing apparatus makes plates by recording and developing images on blank plates mounted on first and second plate cylinders 11 and 12, feeds inks to the plates having the images recorded thereon, and transfers the inks from the plates through first and second blanket cylinders 13 and 14 to printing paper held on an impression cylinder 15, thereby printing the images on the printing paper.

The first plate cylinder 11 is movable between a first printing position shown in a solid line and an image recording position shown in a two-dot chain line in FIG. 1. The second plate cylinder 12 is movable between a second printing position shown in a solid line in FIG. 1 and the same image recording position.

Around the first plate cylinder 11 in the first printing position are an ink feeder 20a for feeding an ink of black (K), for example, to the plate, an ink feeder 20b for feeding an ink of magenta (M), for example, to the plate, and dampening water feeders 21a and 21b for feeding dampening water to the plate. Around the second plate cylinder 12 in the second printing position are an ink feeder 20c for feeding an ink of cyan (C), for example, to the plate, an ink feeder 20d for feeding an ink of yellow (Y), for example, to the plate, and dampening water feeders 21c and 21d for feeding dampening water to the plate. Further, around the first or second plate cylinder 11 or 12 in the image recording position are a plate feeder 23, a plate remover 24, an image recorder 25 and a developing device 26.

According to this printing apparatus, the image recorder 25 and developing device 26 used in a prepress process, and the ink feeders 20 and dampening water feeders 21 used in a printing process may be arranged in separate positions never to interfere with each other, to provide increased freedom for arrangement. Further, images may be recorded on the plates mounted on the first and second plate cylinders 11 and 12 by using the single image recorder 25. This feature contributes toward simplification and low cost of the entire apparatus.

The first blanket cylinder 13 is contactable with the first plate cylinder 11, while the second blanket cylinder 14 is contactable with the second plate cylinder 12. The impression cylinder 15 is contactable with the first and second blanket cylinders 13 and 14 in different positions. The apparatus further includes a paper feed cylinder 16 for transferring printing paper supplied from a paper storage 27 to the impression cylinder 15, a paper discharge cylinder 17 with chains 19 wound thereon for discharging printed paper from the impression cylinder 15 to a paper discharge station 28, and a blanket cleaning unit 29.

The first and second plate cylinders 11 and 12 are coupled respectively to first and second plate cylinder moving mechanisms 31 and 32 described hereinafter. Each of the plate cylinders 11 and 12 is driven by the moving mechanism 31 or 32 to reciprocate between the first or second printing position and the image recording position.

As described hereinafter, each of the first and second plate cylinders 11 and 12 has a gear 51 disposed laterally thereof and coaxially therewith. Each of the first and second blanket cylinders 13 and 14 has a gear 52 disposed laterally thereof and coaxially therewith. The gear 51 of the first plate cylinder 11 is meshed with the gear 52 of the first blanket cylinder 13 when the first plate cylinder 11 is in the first printing position. Similarly, the gear 51 of the second plate cylinder 12 is meshed with the gear 52 of the second blanket cylinder 14 when the second plate cylinder 14 is in the second printing position.

Thus, in the first printing position, the first plate cylinder 11 is rotatable synchronously with the first blanket cylinder 13. In the second printing position, the second plate cylinder 12 is rotatable synchronously with the second blanket cylinder 14.

Adjacent the image recording position is a traction type plate cylinder rotating mechanism 30, described hereinafter, for rotating the first or second plate cylinder 11 or 12 whichever is in the image recording position.

FIG. 2 is a front view of the first and second plate cylinder moving mechanisms 31 and 32. FIG. 3 is a sectional side view showing bearings and adjacent components of the first cylinder moving mechanism 31. The first and second plate cylinder moving mechanisms 31 and 32 have similar structures symmetrical to each other. Common components of the first and second plate cylinder moving mechanisms 31 and 32 are affixed with the same reference numerals.

Each of the first and second plate cylinder moving mechanisms 31 and 32 has groove slots 38 formed in side plates 37 (FIG. 2 shows only the groove slots 38, and omits the side plates 37), in order to move a bearing assembly 33 including a pair of bearings 34 supporting a shaft 36 of the first or second plate cylinder 11 or 12. The bearing assembly 33 is connected to a slide holder 41 movable along a guide 39. The slide holder 41 includes a nut 42. The nut 42 is meshed with a ball screw 44 coupled to a drive shaft of a motor 35.

Thus, the first plate cylinder 11 is driven by the motor 35 to move with the slide holder 41. The first plate cylinder 11 is movable along the guide 39 and ball screw 44 between the first printing position shown in the solid line in FIG. 1 and in a two-dot chain line in FIG. 2, and the image recording position shown in the two-dot chain line in FIG. 1 and in a solid line in FIG. 2. Similarly, the second plate cylinder 12 is movable along the guide 39 and ball screw 44 between the second printing position shown in solid lines in FIGS. 1 and 2, and the image recording position shown in the two-dot chain line in FIG. 1 and in the solid line in FIG. 2.

A fixing member 43 is attached to the side plate 37 adjacent the first or second printing position for fixing the bearing assembly 33. The first plate cylinder 11 is fixed to the first printing position by the fixing member 43, while the second plate cylinder 12 is fixed to the second printing position by the fixing member 43.

Each of the first and second plate cylinders 11 and 12 has an antirotation mechanism, described hereinafter, for preventing rotation of the plate cylinder 11 or 12 during movement by the first or second plate cylinder moving mechanism 31 or 32.

The first and second plate cylinders 11 and 12 are held against rotation by using the antirotation mechanisms for the following reason; A displacement

in a relationship in rotational position between the first or second plate cylinder 11 or 12 and the first or second blanket cylinder 13 or 14 or other components would give rise to a problem of misregistration of printing positions or collision between components. It is therefore necessary to maintain a constant positional relationship between the cylinders 1 and 13 or 12 and 14. When the image recorder 25, described hereinafter, records images on a plate mounted on the first or second plate cylinder 11 or 12, a photocoupler 87, described hereinafter, or a rotary encoder monitors a rotational position of the plate cylinder 11 or 12. However, the first or second plate cylinder 11 or 12 could rotate inadvertently during the movement from the image recording position to the first or second printing position, resulting in a displacement in the relationship in rotational position between the first or second plate cylinder 11 or 12 and the first or second blanket cylinder 13 or 14. To avoid such an inconvenience, the antirotation mechanism is used to prevent rotation of the first or second plate cylinder 11 or 12 during movement.

Each antirotation mechanism includes a rotatable disk 46 connected to the shaft 36 of the first or second plate cylinder 11 or 12 and having a cutout formed peripherally of the disk 46, a detent 47 engageable with the cutout to stop rotation of the rotatable disk 46, a release member 48 disposed adjacent the first or second printing position for disengaging the detent 47 from the cutout formed in the rotatable disk 46, and a release member 49 disposed adjacent the image recording position for disengaging the detent 47 from the cutout formed in the rotatable disk 46.

FIG. 4 is a schematic view showing a relationship between the detent 47 attached to the first plate cylinder 11 and the release member 48 disposed in the first printing position. The detent 47 includes a lever 54 pivotable about an axis 53 attached to the bearing assembly 33, an engaging pin 55 projecting from a front surface of the lever 54, a guide pin 56 projecting from a rear surface of the lever 54 opposite from the engaging pin 55, a press pin 57 opposed to the guide pin 56 across the axis 53, and a spring 58 for biasing the lever 54 in a direction to move the engaging pin 55 toward the rotatable disk 46. The release member 48 is in the form of a cam defining a ramp 59 for guiding the guide pin 56.

The detent 47 of the second plate cylinder 12 has a construction similar to the above, and so does the release member 48 disposed in the second printing position.

FIG. 7 is a schematic view showing a relationship between the detent 47 and the release member 49 disposed in the image recording position. The release member 49 includes a presser plate 62 reciprocable by an air cylinder 61 for depressing the press pin 57 of the detent 47 of the first plate cylinder 11 having moved to the image recording position.

When the second plate cylinder 12 is in the image recording position, the press pin 57 of the detent 47 attached to the second plate cylinder 12 is depressed by the presser plate 62 of the release member 49.

FIG. 8 is a front view of the above plate cylinder rotating mechanism 30 and adjacent components. FIG. 9 is a sectional side view of a principal portion thereof. The plate cylinder rotating mechanism 30 is operable to rotate the first or second plate cylinder 11 or 12 in the image recording position. The plate cylinder rotating mechanism 30 is opposed to the antirotation mechanism across the first or second plate cylinder 11 or 12. It is to be noted that a transmission roller 78 and its moving mechanism are omitted from FIG. 9.

The plate cylinder rotating mechanism 30 includes a drive roller 76 rotatable by a drive motor 75, first and second driven rollers 77 connected to the shafts 36 of the first plate cylinder 11 and second plate cylinder 12 to be coaxial with the plate cylinders 11 and 12, respectively, and the transmission roller 78 for transmitting drive from the drive roller 76 to one of the driven rollers 77 mounted coaxially with the first plate cylinder 11 and second plate cylinder 12.

The drive roller 76 has a lower portion thereof immersed in oil stored in an oil pan 79. As noted above, each driven roller 77 is mounted coaxially with the first or second plate cylinder 11 or 12 through the shaft 36 of the plate cylinder 11 or 12, to be rotatable synchronously therewith. Further, the transmission roller 78 is rotatably supported at the distal end of a cylinder rod of the air cylinder 80 having a proximal end thereof oscillatably coupled to a support member 91 through an axis 92. The transmission roller 78 is movable by the air cylinder 80 between a drive transmitting position in pressure contact with both of the drive roller 76 and driven roller 77 and a retracted position away from the drive roller 76 and driven roller 77.

When, with the transmission roller 78 moved to the drive transmitting position, the drive motor 75 rotates the drive roller 76 at a fixed speed, the rotation of the drive roller 76 is transmitted to the driven roller 77 through the transmission roller 78, whereby the first or second plate cylinder 11 or 12 rotates at a fixed speed. At this time, the drive of the motor 75 is transmitted to the first or second plate cylinder 11 or 12. The traction type drive transmission mechanism utilizing friction among the drive roller 76, driven roller 77 and transmission roller 78. Thus, the first or second plate cylinder 11 or 12 is rotated accurately without any irregularities.

Particularly, in the foregoing embodiment, the transmission roller 78 is movable between the drive transmitting position in pressure contact with both of the drive roller 76 and driven roller 77 and the retracted position away from the drive and driven rollers 76 and 77. After the first or second plate cylinder 11 or 12 is moved to the image recording position, the transmission roller 78 is placed in pressure contact with the drive roller 76 and driven roller 77. Thus, the first or second plate cylinder 11 or 12 is rotated accurately with no slippage among the transmission roller 78, drive roller 76 and driven roller 77.

Consequently, even where, as in the printing apparatus in this embodiment, the driven rollers 77 move in different directions as the first and second plate cylinders 11 and 12 move between the image recording position and the first and second printing positions, the transmission roller 78 may be pressed not only on the drive roller 76 but on one of the driven rollers 77 having moved to the image recording position. This is achieved by moving the transmission roller 78 to the drive transmitting position in pressure contact with both of the drive roller 76 and driven roller 77 after the first or second plate cylinder 11 or 12 has moved to the image recording position. Thus, the first or second plate cylinder 11 or 12 is rotated accurately with no slippage among the transmission roller 78, drive roller 76 and driven roller 77.

Each driven roller 77 has four positioning members 81a, 81b, 81c and 81d in the form of V-blocks for positioning and fixing the first or second plate cylinder 11 or 12 to a predetermined angular position. The positioning members 81a, 81b, 81c and 81d are engageable with an engaging pin 85 disposed at a distal end of a lever 84 pivotable about an axis 83 by an air cylinder 82. In this way, the positioning members 81a, 81b, 81c and 81d are used to position and fix the first or second plate cylinder 11 or 12 to a predetermined angular position.

Each driven roller 77 further includes a detection plate 86 erected thereon for use in detecting an angular position of the driven roller 77. Each side plate 37 has a photocoupler 87 for detecting the detection plate 86. The angular position of the driven roller 77, i.e. that of the first or second plate cylinder 11 or 12, is monitored based on a detection signal of the photocoupler 87 detecting the detection plate 86, and a pulse signal generated, in response to rotation of the first or second plate cylinder 11 or 12, by a rotary encoder, not shown, connected to the shaft 36 of the first or second plate cylinder 11 or 12.

Further, a light-shielding plate 88 is mounted on the shaft 36 of each of the first and second plate cylinders 11 and 12.

An operation of the above antirotation mechanism for preventing rotation of the first or second plate cylinder 11 or 12, and an operation of the traction type plate cylinder rotating mechanism 30 for rotating the first or second plate cylinder 11 or 12, will be described next. In the following description, a rotation preventing operation and a rotating operation are effected for the first plate cylinder 11. It will be appreciated that similar steps are taken in a rotation preventing operation and a rotating operation for the second plate cylinder 12.

When the first plate cylinder 11 is in the first printing position as shown in FIG. 4, the gear 51 disposed coaxially with the first plate cylinder 11 is engaged with the gear 52 disposed coaxially with the first blanket cylinder 13.

When the first plate cylinder moving mechanism 31 starts moving the first plate cylinder 11 from the first printing position to the image recording position as shown in FIG. 5, the gear 51 disposed coaxially with the first plate cylinder 11 gradually moves out of engagement with the gear 52 disposed coaxially with the first blanket cylinder 13. At this time, the guide pin 56 of the detent 47 slides along the ramp 59 of the release member 48, causing the lever 54 to pivot about the axis 53. As a result, the engaging pin 55 starts moving into the cutout of the rotatable disk 46.

At this time, the gear 51 disposed coaxially with the first plate cylinder 11 is not completely out of engagement with the gear 52 disposed coaxially with the first blanket cylinder 13. The rotation of the first plate cylinder 11 is not yet prevented by the detent 47. Thus, in the state shown in FIG. 5, the first plate cylinder 11 moves toward the image recording position while rotating counterclockwise. Consequently, the gears 51 and 52 are prevented from damage due to a collision with each other.

As the first plate cylinder 11 moves further toward the image recording position, the gear 51 disposed coaxially with the first plate cylinder 11 becomes disengaged from the gear 52 disposed coaxially with the first blanket cylinder 13 as shown in FIG. 6. In this state, the guide pin 56 of the detent 47 is completely disengaged from the ramp 59 of the release member 48, and the engaging pin 55 is completely fitted in the cutout of the rotatable disk 46. Consequently, the first plate cylinder 11 i