Image forming system

Abstract

An image recording apparatus for recording an image on a sheet by a main scanning with a laser beam and a sub-scanning of the sheet, the image recording apparatus having: a pair of upstream rollers, arranged upstream of an exposing position of the laser beam; a pair of downstream rollers, wherein the sub-scanning is conducted by nipping and conveying the sheet with the pairs of upstream rollers and downstream rollers; wherein the pair of downstream rollers includes: a first roller driven by a drive section; a second roller, having a total weight of 300 g or less, wherein a gap amount of a nip between the first and the second rollers is set smaller than a thickness of the sheet; and a roller holding section for holding both ends of the second roller so as to be capable of driven rotation.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an image recording apparatus for recording an image wherein a sheet-like object to be scanned is conveyed while being sandwiched between a pair of rollers.

Patent Document 1 discloses a method for recording an image on the whole surface of a sheet as a recording medium by conveying it with two sets of nip rollers. It is well known that an image defect (irregularities across the sheet feed direction) tends to be produced by the flapping of the sheet or irregular rotation of the drive system, when the leading edge of the sheet enters the second nip roller located downstream and the tailing edge of the sheet gets out of the upstream first nip roller, according to this method. To solve the problem caused when the tailing edge of the sheet gets out of the upstream first nip roller, it is a common practice to release the first nip roller before the sheet gets out of the roll, as described in the Patent Document 2.

To solve the problem caused when the leading edge of the sheet enters the second nip roller, a proposal has been made of a method of detaching the nip when the sheet enters the second nip roller, as described in the following Patent Document 3. According to this method, irregularities caused by entry of the film can be prevented. However, when the detached roller contacts the sheet, irregularities still tend to occur. To remove such irregularities, a proposal has been made of a method of keeping the roller rotating before it comes in contact with the sheet (Patent Document 3). However, this method involves a complicated mechanism for rotating and rocking the roller (concurrent revolving and rotating). And because of using heavy weight and large inertia rollers, this method is not sufficient from the point of reducing the irregularities at the time of contact. The timing of occurrence of the irregularities has merely shifted in the direction of conveyance.

The applicants of the present invention have proposed a method as described in the following Patent Document 4. According to this method, a pair of rollers is detached from each other with a predetermined distance prior to sheet entry to reduce the irregularities at the time of sheet entry. This method does not depend on the aforementioned the mechanism of concurrent revolution and rotation. Further, in the Japanese Patent Application TOKUGAN-2003-025863, the present applicants proposed a method wherein the nip pressure of the second roller can be switched in such a way that a decreased nip pressure is selected at the time of sheet entry and an increased nip pressure is selected subsequent to sheet entry, thereby reducing the irregularities at the time of sheet entry and ensuring stable sheet conveyance.

Incidentally, the laser imager (image recording apparatus) for medical use is required to produce a mammographic image output of a multiple size (8×10-inch sized through 14×17-inch sized recording medium). For the purpose of interpreting a printed film including micro-calcification, the mammographic image is required to provide higher image quality and stability than that of normal modality. This requires further reduction of the rotational irregularities of the drive system, and it is necessary to avoid the flapping of the film produced when the sheet enters the nip roller and gets out of it or variations in load due to irregular rotation of the drive system.

With downsizing of the apparatus, the curvature of the conveyance path after the second nip roller will be increased. The exposure section will be affected by the degree of the toughness (rigidity) of the film located at the position corresponding to this curvature, in response to the film size. Thus, the conveyance force of the nip roller will lose to an increased resistance of conveyance by the guide, with the result that a slip will occur during conveyance. There is concern about the possibility of such problems.

The prior art pressure contact roller (nip roller) comprises a 10 mm-diameter through-shaft made of a stainless steel or the like, for keeping rotation, capable of conveying a 14-inch wide film; and a 20 mm-diameter roller portion made of rubber, metal material or the like directly in contact with the film. The weight of the total nip roller assembly is about 500 through 600 g. Thus, according to the method of using a pair of rollers detached from each other with a predetermined distance prior to sheet entry, as described in the following Patent Document 4, the second pair of rollers must be rotated prior to film entry; otherwise, there will be increased irregularities at the time of film entry. Further, if foreign substances such as dust and dirt have deposited on the structural portion (hitting runner portion) for this preliminary rotation, the hitting runner will become loose by the thickness of foreign substances. When coming in contact with the film or drive roller again, the potential energy is released by the quantity corresponding to thickness of foreign substances, and disturbance will be applied to the film and drive system at each rotation of the hitting runner. This will result in irregularities appearing in the image. Further, the outer diameter of the roller is changed with the passage of time due to the difference in the materials of the conveyance section and hitting runner and in the coefficients of linear expansion and volatilization of the additives of the rubber roller for hardness adjustment. This will result in unstable conveyance. There is concern about the possibility of such problems.

[Patent Document 1] Official Gazette of Japanese Patent Tokkaisho 62-94068

[Patent Document 2] Official Gazette of Japanese Patent Tokkaihei 02-264563

[Patent Document 3] Official Gazette of Japanese Patent Tokkaisho 62-135064

[Patent Document 4] Official Gazette of Japanese Patent Tokkaihei 09-156797

SUMMARY OF THE INVENTION

In view of the prior art problems described above, it is an object of the present invention to provide an image recording apparatus capable of avoiding an adverse effect caused by the entry of foreign substances on the roller pair to convey the sheet-like object for sub-scanning transportation, whereby a high-quality image is achieved.

One aspect of the present invention for the solution of the aforementioned problems is concerned with an image recording apparatus for recording an image on a sheet-like object to be scanned, wherein the aforementioned sheet-like object is subjected to sub-scanning while being conveyed, sandwiched by a pair of upstream rollers arranged upstream from the position exposed to a laser beam and a pair of downstream rollers arranged downstream from that position, and is concurrently subjected to main scanning by application of the laser beam; the aforementioned pair of upstream rollers comprising:

• • a first roller driven by a drive section;
  • a second roller, having a total weight of 300 g or less, capable of carrying the sheet-like object to be scanned, by sandwiching it in collaboration with the first roller, wherein a nip gap with the first roller is set at a level smaller than that of the sheet-like object to be scanned; and
  • a holding section for holding both ends of the second roller in such a manner as to be driven.

Another aspect of the present invention is concerned with an image recording apparatus for recording an image on a sheet-like object to be scanned, wherein the aforementioned sheet-like object is subjected to sub-scanning while being conveyed, sandwiched by a pair of upstream rollers arranged upstream from the position exposed to a laser beam and a pair of downstream rollers arranged downstream from that position, and is concurrently subjected to main scanning by application of the laser beam; the aforementioned pair of upstream rollers comprising:

• • a first roller driven by a drive section;
  • a second roller, having a starting torque of 1.7 gf·cm or less, capable of carrying the sheet-like object to be scanned, by sandwiching it in collaboration with the first roller, wherein a nip gap with the first roller is set at a level smaller than that of the sheet-like object to be scanned; and
  • a holding section for holding both ends of the second roller in such a manner as to be driven.

According to the aforementioned image recording apparatus, in the aforementioned downstream roller pairs to convey the recording medium for sub-scanning, the second roller has a smaller weight and/or lower starting torque than that of the prior art, and such holding means as a bearing is configured so as to be driven. Accordingly, when a sheet-like object to be scanned has entered the nip gap, the second roller is easily driven and the nip gap expands rapidly according to the thickness. This arrangement reduces the irregularities when the leading edge of the sheet-like object to be scanned has been fed by the upstream roller pair. At the same time, this arrangement eliminates the need of preliminary rotation of the second roller. This makes it possible to eliminate the use of the prior art runner having a poor resistance to incoming foreign substances, and to avoid a bad effect of the incoming foreign substances upon the downstream roller, whereby a high-quality image can be obtained.

The aforementioned image recording apparatus is provided with an adjusting section for adjusting the nip gap, and the adjusting section has an energizing section working on the gravity component of the second roller working on the first roller side, in the direction opposite to that of gravity component. This energizing section allows the nip gap to be adjusted. Thus, the required gap can be obtained by simple procedure of adjusting the position of the energizing section (adjustment of energizing force).

It is preferred to include a nip pressure controller for changing the nip pressure between the first and second roller.

In this case, it is preferred that control be provided to increase the nip pressure a predetermined time after the leading edge of the sheet-like object to be scanned has entered the downstream roller. This increases the nip pressure of the downstream roller pair a predetermined time after the leading edge of the sheet-like object to be scanned has entered the downstream roller. This arrangement reduces the irregularities at the time of entry of the sheet-like object, and improves the conveying performance after the entry.

It is preferred that the nip pressure controller include a rotary solenoid, and the rotary solenoid be gradually moved under the pulse width modulation control.

The image recording apparatus of the present invention eliminates an adverse effect of the entry of foreign substances such as dust and dirt, upon the downstream roller for conveying the sheet-like object for sub-scanning, whereby a high-quality image is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of the major portions of an image recording apparatus as an embodiment of the present invention;

FIG. 2 is a schematic drawing of the optical beam scanner of the image recording apparatus of FIG. 1;

FIG. 3 is a perspective view showing the sub-scanning mechanism of an optical beam scanner 120 of FIGS. 1 and 2;

FIG. 4 is a side view of the major portions showing a first conveyance roller pair 170 of FIG. 3;

FIG. 5 is a side view of the major portions showing a second conveyance roller pair 180 of FIG. 3;

FIG. 6 is a front view schematically showing the second conveyance roller pair 180 of FIGS. 3 and 5;

FIG. 7 is a block diagram showing the control system of the sub-scanning mechanism of the optical beam scanner 120 of FIGS. 1 through 6;

FIG. 8 is a diagram showing the relationship between the weight of the driven roller of a second conveyance roller pair in the first embodiment and evaluation of image quality based on irregularities at the time of entry of the leading edge; and

FIG. 9 is a diagram showing the relationship between the starting torque of the driven roller of a second conveyance roller pair in the second embodiment and evaluation of image quality based on irregularities at the time of entry of the leading edge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following describes the best form of the embodiment of the present invention with reference to drawings: FIG. 1 is a front view of the major portions of an image recording apparatus as an embodiment of the present invention. FIG. 2 is a schematic drawing of the optical beam scanner of the image recording apparatus of FIG. 1.

As shown in FIG. 1, the image recording apparatus 100 comprises: a feed section 110 having first and second loading sections 11 and 12 for loading a package of a predetermined number of films as sheet-like thermal development photosensitive materials (recording media), and a supply section 90 for conveying and supplying each of the films for exposure and development; an optical beam scanner 120 for forming a latent image by scanning two-dimensionally while sub-scanning, and exposes the film fed from the feed section 110; a developing section 130 for thermally developing the film with the latent image formed thereon; a densitometer 200 for measuring the density of the developed film and getting the density information; a cooling conveyance section 150 for concurrently cooling and conveying the heated film; and an ejection tray 160 where the film is ejected.

Each sheet of film is conveyed from the first and, second loading sections 11 and 12 of the feed section 110 by means of the supply section 90 and a pair of conveyance rollers 139 and 141 in the arrow-marked direction (1). By controlling each of the supply sections 90, it is possible to switch and convey the sheets of film from the first and second loading sections 11 and 12.

Referring to FIG. 2, the following describes the optical beam scanner 120 of the image recording apparatus 100. As shown in FIG. 2, in the optical beam scanner 120, the rotary polygon mirror 113 deflects the laser beam L having a predetermined wavelength within the range from 780 through 860 nm with its intensity having been modified based on the image signal S of the diagnostic image information inputted into the input section 133. The optical beam scanner 120 scans the surface of the film F, and at the same time, provides sub-scanning with causing relative movement of the film F in the horizontal direction approximately at right angles to the main scanning direction with respect to the laser beam L by the sub-scanning mechanism composed of rollers 171 and 181. The optical beam scanner 120 uses the laser beam L to form the latent image on the film F. It should be noted that each driven roller of the first and second conveyance roller pairs 170 and 180 is not illustrated in FIG. 2.

In the optical beam scanner 120 of FIG. 2, when the image signal S as a digital signal inputted from the outside is inputted through the input section. 133, the image signal S is converted into the analog signal and is inputted into the modulator 123. Based on this analog signal, the modulator 123 controls the driver 124 of the laser beam source 110a, to emit the laser beam L modulated from the laser beam source 110a.

After having passed through the lens 112, the laser beam L emitted from the laser beam source 110a is converged only in the vertical direction by the cylindrical lens 115. In this case, the laser beam L enters to the polygon mirror 113 as a linear image perpendicular to the drive axis of the rotary polygon mirror 113 rotating in the arrow-marked direction A of FIG. 2. The rotary polygon mirror 113 reflects and deflects the laser beam L in the main scanning direction, and the deflected laser beam L passes through the fθ lens 114 including the cylindrical lens consisting of a combination of four lenses. After that, the laser beam L is reflected by the mirror 116 arranged in extension on the optical path in the main scanning direction, and provides main scanning repeatedly in the arrow-marked direction X on the scanned surface 117 of the film F which is being fed (sub-scanned) in the arrow-marked direction Y by the first and second conveyance roller pairs 170 and 180. This procedure allows the laser beam L to scan the entire surface of the scanned surface 117 on the film F.

The cylindrical lens of the fθ lens 114 allows the incoming laser beam L to be converged on the surface of the film F to be scanned only in the sub-scanning direction. The distance from the fθ lens 114 to the surface of the film F to be scanned is equal to the focal distance of the entire fθ lens 114. In this manner, the cylindrical lens 115 and fθ lens 114 including a cylindrical lens are arranged in the optical beam scanner 120. The laser beam L is once converged on the rotary polygon mirror 113 only in the sub-scanning direction. Accordingly, even if planar inclination or shaft vibration has occurred to the rotary polygon mirror 113, a scanning line of regular pitch can be formed, without the scanning position of the laser beam L deviating in the sub-scanning direction on the surface of the film F to be scanned. The rotary polygon mirror 113 has a better scanning stability than a galvanometer or other light deflectors. In this manner, a latent image in conformity to the image signal S is formed on the film F.

The following describes the developing section 130 of the image recording apparatus given in FIG. 1. As shown in FIG. 1, the developing section 130 comprises: a heating drum 14 capable of heating the film F while holding it on its outer periphery; and a plurality of opposing rollers 16, arranged face to face with the heating drum 14, for holding the film by sandwiching it with the heating drum 14. The film is heated while being sandwiched between the heating drum 14 and opposing rollers 16, and is conveyed by the rotation of the heating drum 14.

The heater (not illustrated) arranged inside the heating drum 14 is placed under power on/off control and maintains the temperature at a level equal to or greater than a predetermined thermal development temperature (for example, about 110 degrees Celsius) for a predetermined duration of time, whereby the film F is heated and subjected to thermal development. Thus, the latent image formed on the film F by the optical beam scanner 120 is formed into a visible image. Further, density range can be adjusted by changing the heater temperature by the power supply control.

The densitometer 200 of FIG. 1 contains a light emitting section 200a and a light receiving section 200b. When the developed film is conveyed between the light emitting section 200a and light receiving section 200b in the manner described above, the light emitted from the light emitting section 200a is received by the light receiving section 200b through the film, and the density is measured in conformity to the degree of the attenuation in the amount of received light. Based on the measured density information, the amount of the laser beam of the optical beam scanner 120 is fed back and controlled so that the film finishing accuracy will be constant.

The film conveyed in the arrow-marked direction (1) is then conveyed in the arrow-marked direction (2) and a latent image is formed as described above. Then the film is fed to the heating drum 14 of the developing section 130 in the arrow-marked direction (3) by the conveyance roller pair 142 and is subjected to thermal development and a latent image is visualized, as described above. After that, while being cooled, the film is conveyed by the conveyance roller pair 144a and 144, and its density is measured by the densitometer 200. The film is further fed in the arrow-marked direction (4) by the conveyance roller pair 144 and is ejected to the ejection tray 160.

Referring to FIGS. 3 through 7, the following describes the sub-scanning mechanism of the optical beam scanner 120.

FIG. 3 is a perspective view showing the sub-scanning mechanism of an optical beam scanner 120 of FIGS. 1 and 2. FIG. 4 is a side view of the major portions showing a first conveyance roller pair 170 of FIG. 3. FIG. 5 is a side view of the major portions-showing a second conveyance roller pair 180 of FIG. 3. FIG. 6 is a front view schematically showing the second conveyance roller pair 180 of FIGS. 3 and 5. FIG. 7 is a block diagram showing the control system of the sub-scanning mechanism of the optical beam scanner 120 of FIGS. 1 through 6.

As shown in FIGS. 1 and 3, the sub-scanning mechanism of the optical beam scanner 120 includes a first conveyance roller pair 170 arranged upstream and a second conveyance roller pair 180 arranged downstream. While laser beam L emitted from the laser beam source 110a is irradiated between the first conveyance roller pair 170 and second conveyance roller pair 180 as shown in FIG. 2, the film is conveyed by the conveyance roller pairs 170 and 180.

As shown in FIGS. 1 through 6, the first conveyance roller pair 170 located upstream in the sub-scanning direction Y contains a driving-roller 171 driven by a motor (not illustrated) in the direction of rotation R′ given in FIG. 4, and a driven roller 172 driven by the driving roller 171. The second conveyance roller pair 180 located downstream in the sub-scanning direction Y contains a driving roller 181 driven by a motor (not illustrated) in the direction of rotation R given in FIG. 4, and a driven roller 182 driven by the driving roller 181.

As shown in FIG. 6, in the second conveyance roller pair 180, the driven roller 182 has two roller sections 182a, of the same diameter on both sides with respect to the driving roller 181, and has a plurality of roller sections 182b having the same diameter as those of the roller sections 182a at the intermediate position. In the driven roller 182, a through-shaft 182c is held rotatably by the bearings 182d and 182e on both ends on the further outer side of the roller sections 182a, so that the driven roller 182 can be driven.

To ensure that a high friction coefficient will be applied to the surface in contact with the film, the roller sections 182a and 182b of the second conveyance roller pair 180 are made of rubber materials such as silicone rubber and EPDM, and are fitted or bonded onto the outer periphery of the through-shaft 182c. The through-shaft 182c is made of such light metal as aluminum or its alloy. The second conveyance roller pair 180 has a total mass of 300 gf or less, more preferably 200 gf or less, as a roller assembly. The driving roller 181 is made of steel such as stainless steel.

The second conveyance roller pair 180 can be moved integrally with the bearings 182d and 182e, with respect to the driving roller 181. As will be described later, a nip gap 183 is formed between the driving roller 181 and a plurality of roller sections 182a and 182b of the driven roller 182 in such a way that of the nip gap 183 does not exceed the thickness of the film.

Referring to FIGS. 3, 4, 5 and 7, the following describes the mechanism for switching the nip pressure of the first conveyance roller pair 170 and second conveyance roller pair 180 and the control system thereof.

As shown in FIGS. 3 and 4, the sub-scanning mechanism of the optical beam scanner 120 comprises: a first driving source 50 consisting of a rotary solenoid for switching the nip pressure of the first conveyance roller pair 170; a shaft 51 moved by the first driving source 50 in the horizontal direction m of FIG. 4 and in the opposite horizontal direction m′; a shaft member 52, connected to the shaft 51, extending in the same direction as that of the first conveyance roller pair 170 and second conveyance roller pair 180 (direction approximately orthogonal to the horizontal directions m and m′); a first lever member 54 for rotating the driven roller 172 relative to the drive roller 171 by rotating about the fulcrum shaft 6b in the rotating direction s and opposite rotating direction s′, with the movement of the shaft member 52 in the horizontal directions m and m′; and a biasing member 53, having its one end fixed to the securing section 100a on the casing side of the apparatus, for applying energy so as to drive the first lever member 54 in the rotating direction s′.

Further, as shown in FIGS. 3 and 5, the sub-scanning mechanism of the optical beam scanner 120 comprises: a second drive source 56 consisting of a rotary solenoid for switching the nip pressure of the second conveyance roller pair 180; a shaft 57 moved by the second drive source 56 in the horizontal direction n of FIG. 5 and in the opposite horizontal direction n′; a shaft 57a connected with the shaft 57 through the biasing member 58 so as to be energized in the horizontal direction n; a shaft member 63, connected to the shaft 57a, extending in the same direction as that of the first conveyance roller pair 170 and second conveyance roller pair 180; a second level member 59 for rotating the driven roller 182 relative to the drive roller 181 by rotating about the fulcrum shaft 61 in the rotating direction t and opposite rotating direction t′, with the movement of the shaft member 63 in the horizontal directions n and n′; and a biasing member 64, having its one end fixed to the securing section 100b on the casing side of the apparatus, for applying energy so as to drive the second level member 59 in the rotating direction t′. It should be noted that the rotary solenoid (second drive source 56) in FIG. 3 is not illustrated.

In the second conveyance roller pair 180 of FIG. 5 the driven roller 182 works on the side of the drive roller 181 under the gravity. However, the biasing member 64 works in the direction opposite to the drive roller 181 against the gravity of the driven roller 182, with the result that the driven roller 182 is energized in the direction of moving away from the drive roller 181 against the gravity (direction of decreasing the nip pressure). Then the balance of energizing force between the second drive source 56 and biasing member 58 is adjusted. Thus, when the rotary solenoid of the second drive source 56 is located at the neutral position, the nip gap 183 is formed between the drive roller 181 and a plurality of roller sections 182a and 182b of the driven roller 182, as shown in FIG. 6, in such a way that the thickness of the nip gap 183 does not exceed the film thickness.

The control system of the image recording apparatus 100 given in FIG. 7 contains: a reflection type optical sensor 149 arranged upstream from the first conveyance roller pair 170 given in FIG. 1; and a controller 148 including a central processor unit (CPU) for controlling the first driving source 50 and second drive source 56 given in FIGS. 4 and 5, based on the detection signal from the optical sensor 149, and switching the nip pressure between the first conveyance roller pair 170 and second conveyance roller pair 180.

A beam is applied to the film from the optical sensor 149 and the reflected beam thereof enters the light receiving section of the optical sensor 149, whereby the leading edge of the film fed by the conveyance roller pair 141 given in FIG. 1 is detected by the optical sensor 149 and a detection signal is issued. Based on the detection signal coming from the optical sensor 149, the controller 148 determines the timed intervals and controls the driving sources 50 and 56 so that the nip pressure is switched. Control is provided in such a way that the nip pressure is increased after a predetermined period of time from the leading edge of the film has entered the second conveyance roller pair 180.

The controller 148 of FIG. 7 controls the rotary solenoids of the first driving source 50 and second drive source 56 through pulse width modulation. To be more specific, the controller 148 allows the phase angle of the rotary solenoid to be changed by adjusting the on/off duty ratio. When the duty ratio is gradually changed and the phase angle is gradually adjusted, the nip pressures of the first conveyance roller pair 170 and second conveyance roller pair 180 can be gradually changed and switched, independently of each other.

The following describes the operation of the sub-scanning mechanism of the optical beam scanner in the image recording apparatus given in FIGS. 1 through 7. In the first place, when the film is fed in the direction (1) given in FIG. 1 from the first and second loading sections 11 and 12 by the supply section 90 and the conveyance roller pairs 139 and 141, the leading edge of the film is detected by the optical sensor 149 of FIG. 1, and the leading edge of the film enters the first conveyance roller pair 170. The controller 148 controls the first driving source 50 and rotates the rotary solenoid of the first driving source 50 of FIG. 4 so that the phase angle of the rotary shaft 50a will change. When the shaft 51 has moved in the horizontal direction m, the first lever member 54 is pushed through the shaft member 52, whereby the driven roller 172 is driven in the rotating direction s of FIG. 4. This procedure increases the nip pressure with respect to the drive roller 171 of the driven roller 172 in the first conveyance roller pair 170, and allows the first conveyance roller pair 170 to feed the film stably in the sub-scanning direction Y.

In the meantime, the nip gap 183 having a thickness smaller than that of the film shown in FIG. 6 is formed before the film enters the second conveyance roller pair 180. If the film has entered this nip gap 183, the driven roller 182 rotates easily since the driven roller 182 is held rotatably by the bearings 182d and 182e of FIG. 6. Further, light weight permits easy movement in such a way that the nip gap 183 will increase. This arrangement reduces obstacles when the leading edge of the film enters the nip gap 183 of FIG. 6. The driven roller 182 is set by the bearings 182d and 182e not to exceed the starting torque of 1.7 gf#cm is held so as to rotate easily when the film has entered. This arrangement eliminates the need of previously rotating the driven roller 182.

After the film has entered the second conveyance roller pair 180, the rotary solenoid of the first driving source 50 rotates the rotary shaft 50a under the control of the controller 148, and moves the shaft 51 in the horizontal direction m′ of FIG. 4. Then the first lever member 54 is pushed in the horizontal direction m′ by the energizing force of the biasing member 53, and the driven roller 172 is driven in the rotating direction s′, thereby reducing the nip pressure in the first conveyance roller pair 170 and releasing the pressure. This procedure reduces obstacles when the trailing edge of the film gets out of the first conveyance roller pair 170.

In the meantime, after a predetermined period of time from the film has entered the second conveyance roller pair 180, the controller 148 controls the second drive source 56 on the second conveyance roller pair 180, and drives the rotary solenoid of the second drive source 56 of FIG. 5 so that the phase angle of the rotary shaft 56a will change. When the shaft 57 has been moved in the horizontal direction n, the second level member 59 is driven in the rotating direction t against the energizing force of the biasing member 64, through the biasing member 58, shaft 57a and shaft member 63, whereby force is applied in the direction where the driven roller 182 moves toward the drive roller 181 (in the direction of decreasing the nip gap 183). Accordingly, in the second conveyance roller pair 180, the nip pressure with respect to the drive roller 181 of the driven roller 182 is increased. This procedure allows the second conveyance roller pair 180 to feed the film stably and improves film conveying performances.

In each of the aforementioned operations, the rotary shaft 50a and rotary shaft 56a of the rotary solenoid rotate gradually, instead of rapidly, and the rotation is accompanied by a small change in phase angle. The shaft 51 and shaft 57 gradually move in the directions m, n, m′ and n′. The nip pressures in the first conveyance roller pair 170 and second conveyance roller pair 180 change gradually, instead of rapidly. This arrangement ensures smooth switching of each nip pressure in the first conveyance roller pair 170 and second conveyance roller pair 180, and reduces vibration, without giving an adverse effect to the film conveyance for sub-scanning.

As described above, the sub-scanning mechanism of the optical beam scanner given in FIGS. 1 through 7 prevents a conveyance trouble from occurring and eliminates a bad effect upon the image formation during the conveyance of the film in the sub-scanning direction Y, when the leading edge of the film enters the first and second conveyance roller pairs 170 and the film gets out of the first conveyance roller pair 170.

Further, a predetermined period of time after the film has entered the second conveyance roller pair 180, the nip pressure of the second conveyance roller pair 180 is switched over to the higher pressure so that the conveyance power is increased. This ensures stable conveyance of the film in the sub-scanning direction, and hence improved image quality.

As described above, the present embodiment pays attention to the second conveyance roller pair 180 located downstream, in particular, to the structure and weight. It solves the problem of irregularities at the time of entry of the film and ensures stable film conveyance, by reducing the starting torque for rotation of the driven roller 182 and reducing the weight, through reviewing the structure and the weight, wherein the prior art of applying the hitting runner is not used.

To be more specific, the prior art conveyance roller pair as roller assemblies has a weight of 500 through 600 gf. This requires preliminary rotation prior to film entry. If foreign substances such as dust and dirt have deposited on the structural portion for preliminary rotation (hitting runner), then disturbance will be applied to the film and drive system at each rotation of the hitting runner. This will result in irregularities appearing in the image. By contrast, the present embodiment uses bearings to provide low starting torque, free rotation and light weight of the driven roller 182. This arrangement eliminates the need of preliminary rotation at the time of film entry and adoption of hitting runners, thereby solving the problem of irregularities at the time of entry of the film leading edge, and preventing possible adverse effect of the deposition of foreign substances such as dust and dirt upon image quality.

In FIGS. 5 and 6, the amount of gap of the nip gap 183 prior to the entry of the film can be set at an appropriate level by adjusting the balance in the energizing forces of the biasing member 58 and biasing member 64. If the amount of gap of the nip gap 183 is adjusted, it is possible to determine the nip pressure in the time duration from the entry of the film to the operation of the pressure controller by the controller 148 and second drive source 56.

Embodiment 1

The following gives more specific description of the present invention with reference to the first embodiment. The weight of the driven roller 182 of the second conveyance roller pair 180 in FIGS. 3 and 5 was changed to 100, 200, 300, 400 and 540 gf in that order, and the spring constant was changed, thereby adjusting the balance of the energizing force between the biasing member 58 and the biasing member 64. The gap amount of the nip gap 183 of FIG. 6 was changed to 100, 125, 150 and 190 μm in that order. Then the image was recorded and the disturbed image (irregularities in the image) at the time of entry of the film leading edge were observed, thereby evaluating the image quality. FIG. 8 shows the result of this experiment. In the image quality evaluation in FIG. 8, the image is not affected by irregularities at the time of film entry, if the evaluation score is “Excellent” (almost no disturbed image at the time of film entry) or better.

In the first embodiment, the driven roller 182 used is constructed in such a way that the through-shaft 182c is made of aluminum (cylindrical form) having a diameter of 16 mm, and silicone rubber having a thickness of 2 mm (JIS A hardness 70 through 75 deg.) is coated on a predetermined portion, where the outer diameter of the rubber layer is 20 mm. The drive roller 181 used is made of a stainless steel having a diameter of 20 mm. Model MF 106ZZS by NSK Ltd. is used as the bearings 182d and 182e of driven roller 182, and the film thickness is 200 μm.

FIG. 8 shows that the image quality is deteriorated by the irregularities at the time of entry of the film leading edge more seriously as the weight of the driven roller 182 is greater and the gap amount is smaller. When the gap amount is 100 through 190 μm, the image quality is hardly affected by irregularities at the time of film entry if the weight of the driven roller 182 does not exceed 300 gf, and the image quality is not affected by irregularities at the time of film entry if the weight of the driven roller 182 does not exceed 200 gf.

Emvodiment 2

The following gives more specific description of the present invention with reference to the second embodiment. The weight of the driven roller 182 of the second conveyance roller pair 180 in FIGS. 3 and 5 was changed to 100, 200, 300, 400 and 540 gf in that order, and the spring constant was changed, thereby adjusting the balance of the energizing force between the biasing member 58 and biasing member 64. The gap amount of the nip gap 183 of FIG. 6 was changed to 100, 125, 150 and 190 μm in that order. Then the image was recorded and the disturbed image (irregularity in the image) at the time of entry of the film leading edge were observed, thereby evaluating the image quality. FIG. 9 shows the result of this experiment. In the image quality evaluation in FIG. 9, the image is not affected by irregularities at the time of film entry, if the evaluation score is “Excellent” (almost no disturbed image at the time of film entry) or better.

In the second embodiment, the driven roller 182 used is constructed in such a way that the through-shaft 182c is made of aluminum (cylindrical form) having a diameter of 16 mm, and silicone rubber having a thickness of 2 mm (JIS A hardness 70 through 75 deg.) is coated on a predetermined portion, where the outer diameter of the rubber layer is 20 mm. The drive roller 181 used is made of a stainless steel having a diameter of 20 mm. The driven roller 182 used has a weight of 200 gf, wherein friction coefficient between the roller and film emulsion surface is 2.0. The film thickness is 200 μm.

Model MF106ZZSPS2-S, MF106ZZSPS-L, MF1060T12MC3 D4MA and MF106T12ZZ1MC3 NS7A by NSK Ltd. is used as the bearings 182d and 182e of driven roller 182. The starting torque of the driven roller 182 was changed for evaluation. FIG. 9 shows that the image quality is deteriorated by the irregularities at the time of entry of the film leading edge more seriously as the starting torque of the driven roller 182 is greater and the amount of gap is smaller. When the gap amount is 100 through 190 μm, the image quality is hardly affected by irregularities at the time of film entry if the starting torque of the driven roller 182 does not exceed 1.7 gf·cm, and the image quality is not affected by irregularities at the time of film entry if the weight of the driven roller 182 does not exceed 1.3 gf·cm.

The present invention has been described with reference to the embodiments and examples. It is to be expressly understood, however, that the present invention is not restricted thereto. The present invention can be embodied in a great number of variation, without departing from the technological spirit of the present invention. For example, timing of increasing the nip pressure of the second conveyance roller pair 180 can be set appropriately in conformity to the position of the guide member 190 downstream from the second conveyance roller pair 180 and the radius dimensions thereof.

Claims

1. An image recording apparatus for recording an image on a sheet by conducting a main scanning of the sheet with a laser beam while conducting a sub-scanning of the sheet, the image recording apparatus comprising:

a pair of upstream rollers, arranged upstream of an exposing position of the laser beam;

a pair of downstream rollers, arranged downstream of the exposing position of the laser beam, wherein the sub-scanning is conducted by nipping and conveying the sheet with the pair of upstream rollers and the pair of downstream rollers; wherein the pair of downstream rollers comprises:

a first roller driven by a drive section;

a second roller, having a total weight of 300 g or less, capable of carrying the sheet by nipping the sheet in cooperation with the first roller, wherein a gap amount of a nip between the first roller and the second roller is set smaller than a thickness of the sheet; and

a roller holding section for holding both ends of the second roller in such a manner as to be capable of driven rotation.

2. The image recording apparatus of claim 1, further comprising an adjuster for adjusting the gap amount of the nip, the adjuster comprising a biasing section for biasing to an opposite direction of a gravity component of the second roller, the gravity component working on the first roller side, wherein adjuster is capable to adjust the gap amount of the nip by using the biasing section.

3. The image recording apparatus of claim 1, further comprising a nip pressure controller for changing a nip pressure between the first roller and the second roller.

4. The image recording apparatus of claim 3, wherein the nip pressure controller controls to increase the nip pressure a predetermined time after the leading edge of the sheet has entered in the nip of the pair of downstream rollers.

5. The image recording apparatus of claim 3, wherein the nip pressure controller comprises a rotary solenoid, and the rotary solenoid is gradually moved under a pulse width modulation control.

6. An image recording apparatus for recording an image on a sheet by conducting a main scanning of the sheet with a laser beam while conducting a sub-scanning of the sheet, the image recording apparatus comprising:

a pair of upstream rollers, arranged upstream of an exposing position of the laser beam;

a pair of downstream rollers, arranged downstream of the exposing position of the laser beam, wherein the sub-scanning is conducted by nipping and conveying the sheet with the pair of upstream rollers and the pair of downstream rollers; wherein the pair of downstream rollers comprises:

a first roller driven by a drive section;

a second roller, having a starting torque of 1.7 gf·cm or less, capable of carrying the sheet by nipping the sheet in cooperation with the first roller, wherein a gap amount of a nip between the first roller and the second roller is set smaller than a thickness of the sheet; and

a roller holding section for holding both ends of the second roller in such a manner as to be capable of driven rotation.

7. The image recording apparatus of claim 6, further comprising an adjuster for adjusting the gap amount of the nip, the adjuster comprising a biasing section for biasing to an opposite direction of gravity component of the second roller, the gravity component working on the first roller side, wherein the adjuster is capable to adjust the gap amount of the nip by using the biasing section.

8. The image recording apparatus of claim 6, further comprising a nip pressure controller for changing a nip pressure between the first roller and the second roller.

9. The image recording apparatus of claim 8, wherein the nip pressure controller controls to increase the nip pressure a predetermined time after the leading edge of the sheet has entered in the nip of the pair of downstream rollers.

10. The image recording apparatus of claim 8, wherein the nip pressure controller comprises a rotary solenoid, and the rotary solenoid is gradually moved under a pulse width modulation control.