JPH09190055A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a latent-image ghost by means of pre-exposure without shortening the life of a laser diode and to obtain a more satisfactory image. SOLUTION: Prior to exposure, the laser diode is turned on to irradiate only an irradiation area on a photoreceptor 12 which consists of an irradiation area E consisting of an image formation area I and margins M provided before and after the image formation area I, and a synchronous detection signal generation area S for laser beam detection by a synchronous detector.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention forms an electrostatic latent image by scanning a laser beam in accordance with the paper size of a print command while the photoconductor is uniformly charged. The present invention relates to an image forming apparatus that forms an image by transferring a visualized image onto a recording medium.

[0002]

2. Description of the Related Art Conventionally, in a laser beam printer or a copying machine, a uniformly exposed photosensitive member (for example, a drum-shaped photosensitive member) is scanned with a laser beam according to an image signal by a laser exposure device. A so-called electrophotographic method is adopted in which an electrostatic latent image of a digital image is formed, and this is visualized (developed) and transferred and fixed on a recording medium.

A laser diode is generally used as a light source of a laser exposure device that forms this digital image on a photosensitive member. A laser beam corresponding to the image to be formed is modulated and output from the light source. In order to obtain the writing start timing of the image to be modulated and output, a light receiving element is provided on the deflection scanning optical path, and the laser beam passing through this light receiving element is detected to achieve horizontal synchronization. The laser beam is deflected by a polygon mirror (rotary polygonal mirror), the beam diameter and the like are corrected by an optical system, and the laser beam is imaged on the photoconductor while being horizontally synchronized by the light receiving element. The output image of is obtained.

Therefore, the area where an image is actually formed is
Since the light receiving element, which is an area smaller than the laser beam scannable area, generally overlaps with the photosensitive area of the photoconductor, the area where an image is actually formed is the result of the photoconductor. The area is smaller than the photosensitive area.

As shown in FIG. 11, in such a digital image forming apparatus, a so-called solid black image A is formed in a white background in the first cycle after startup and a uniform halftone image B in the second cycle. When the image is formed, the photosensitive member is not uniformly charged by the charging after the first cycle, and even if it is intended to print the halftone image B due to the difference in the sensitivity of the surface of the photosensitive member to the laser beam light, only the solid black portion in the first cycle A thin image remains and a latent image ghost G appears. This is because the sensitivity of the photoconductor surface to light is very high at startup (see photoconductor surface potential V _A ), and in the first cycle of printing, only the solid black portion of the photoconductor, that is, only the exposed area The recovery force is reduced, and the sensitivity remains high in areas other than solid black (photoconductor surface potential V _B
(See) happen.

In order to prevent this, JP-A-48-5494
In JP-A-6, there is a technique in which a pre-exposure lamp is provided in addition to an exposure device for image formation in order to make the potential of the photo conductor uniform, and the photoconductor is uniformly discharged (preexposure). .

However, providing a lamp for pre-exposure in addition to the one for image formation as in Japanese Patent Laid-Open No. 48-54946 makes the apparatus structure complicated and requires a space for the lamp. As a result, the device itself becomes large, and there are problems such as a problem due to heat generation of the lamp and a reduction in static elimination efficiency due to a difference in wavelength from the light source for image writing.

Therefore, in Japanese Patent Laid-Open No. 64-25183, there is a technique in which pre-exposure between pages is performed by exposing the surface of the photoconductor to a laser beam for image formation.

[0009]

However, in Japanese Unexamined Patent Publication No. 64-25183, since the entire surface of the photoconductor is exposed by the laser diode for image writing, the cumulative light emission time of the laser diode increases accordingly. As a result, there is a problem that the life of the laser diode is shortened.

In consideration of the above facts, the present invention provides a control method that prevents latent image ghosting without shortening the life of the laser diode by pre-exposure and that minimizes the reduction of the recovery force of the photoconductor. The object is to obtain an image forming apparatus capable of obtaining a better image.

[0011]

According to another aspect of the present invention, there is provided an image forming apparatus, wherein an electrostatic latent image is formed corresponding to a charging unit for uniformly charging a photoconductor and a paper size of a print command on the photoconductor. In the image forming apparatus, there is provided a laser beam scanning unit, a developing unit that visualizes the electrostatic latent image, and a transfer unit that transfers the visualized image onto a recording medium. A pre-exposure control means for controlling the laser beam scanning means so as to irradiate a necessary minimum area including an electrostatic latent image forming area corresponding to the paper size of the photoconductor before charging is provided. There is.

According to the first aspect of the present invention, the laser beam scanning means is controlled by the pre-exposure control means to form an electrostatic latent image in accordance with the size of the sheet on which the formed electrostatic latent image is transferred. Irradiating a minimum area including the area eliminates unnecessary irradiation of the laser beam. As a result, in the pre-exposure, the life of the laser diode is not shortened as compared with the whole surface exposure, and the latent image ghost is effectively prevented from occurring.
Good images can be obtained.

An image forming apparatus according to a second aspect is the image forming apparatus according to the first aspect.
In the above, the pre-exposure control means sets the necessary minimum area according to the sheet size of the next print command and irradiates the area.

According to the second aspect of the present invention, since the minimum necessary area is set in advance based on the print command, the image forming area is determined accurately each time, and unnecessary irradiation is eliminated to be efficient. Pre-exposure processing can be performed.

According to a third aspect of the present invention, there is provided the image forming apparatus according to the first aspect.
Alternatively, in the pre-exposure control means, the beam diameter is made larger than that at the time of forming the electrostatic latent image by the laser beam scanning means so that a part of the adjacent beams in the main scanning direction overlap each other, and It is characterized by irradiating every fixed dot.

According to the third aspect of the present invention, in the pre-exposure, the beam diameter is increased and a part of the beam is overlapped in the main scanning direction to irradiate every fixed dot. The irradiation time of the laser beam can be shortened to prevent deterioration and the pre-exposure time can be shortened as compared with the case of irradiation with. Further, since a part of them overlaps in the main scanning direction, even if the laser diode is turned on every fixed dot, it is possible to obtain the same effect of the pre-exposure process as when turning on all dots. Therefore, only by changing the beam diameter, it is possible to efficiently perform the pre-exposure and prevent the deterioration of the laser diode.

An image forming apparatus according to a fourth aspect is the image forming apparatus according to the first aspect.
Alternatively, in the pre-exposure control means, the beam diameter is made larger than that at the time of forming the electrostatic latent image by the laser beam scanning means so that some of the adjacent beams in the sub-scanning direction overlap each other, and It is characterized by irradiating every fixed line.

According to the fourth aspect of the present invention, the beam diameter is increased so that the beam is overlapped in the sub-scanning direction and is irradiated at every constant line, so that the required number of lines is made smaller than that at the time of electrostatic latent image formation. The pre-exposure time can be shortened to prevent the latent image ghost from occurring. Moreover, since a part of them overlaps in the sub-scanning direction, even if the laser diodes are turned on every fixed line, the same effect of pre-exposure processing as when turning on all lines can be obtained. Therefore, the pre-exposure can be efficiently performed and the life of the laser diode can be extended.

An image forming apparatus according to a fifth aspect is the image forming apparatus according to the first aspect.
Alternatively, in the pre-exposure control means, the beam diameter is made larger than that at the time of forming the electrostatic latent image by the laser beam scanning means, and some of the adjacent beams in the main scanning direction and the sub scanning direction overlap each other. In this way, the irradiation is performed for each fixed dot and each fixed line.

According to the fifth aspect of the invention, since the beam diameter is increased and the beams are overlapped in both the main scanning direction and the sub-scanning direction, irradiation is performed at a predetermined interval for each dot and each line, and the laser diode of the laser diode is irradiated. Pre-exposure can be performed by shortening the irradiation time. In addition, since a part is overlapped in the main scanning direction and the sub-scanning direction, the pre-exposure process can be performed without irradiation unevenness even if the laser diode is turned on every fixed line and every fixed dot. Therefore, it is possible to prevent the appearance of a latent image ghost and perform efficient pre-exposure, reduce the irradiation time of the laser diode to prevent deterioration, and extend the life as compared with the case of full-surface exposure.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The present embodiment will be described below in detail with reference to the drawings.

A block diagram of the image forming apparatus 10 is shown in FIG. The image forming apparatus 10 includes a drum-shaped photoconductor 12, and the photoconductor 12 is provided around the photoconductor 12.
A charging device 14 and a developing device 16 are arranged along the rotational direction of the above, and a cleaner 20 for removing toner and the like remaining on the photoconductor 12 after transfer is provided at the rearmost end in the rotational direction.

The charger 14 is usually a corotron charger,
A conductive brush charger or the like is used, and ions or electrons are attached to the surface of the photoconductor 12 to uniformly charge the photoconductor 12 at a constant potential.

A laser diode 30 as a light source (see FIG. 2) is located above the charger 14 and the developer 16 in FIG.
Further, an exposure device 28 provided with a polygon mirror 38 as a scanning means and the like is provided. A mirror 44 is provided at the other end of the exposure device 28, and the laser light changes its direction by the mirror 44 and is emitted from the exposure device 28 to be exposed at a predetermined exposure position between the charger 14 and the developing device 16. The body 12 is exposed to remove the charged electric charge other than the image portion, and an electrostatic latent image with the electric charge left in the image portion is formed.

The developing device 16 is filled with toner charged to a polarity opposite to that of the electrostatic latent image, and the electrostatic latent image formed on the surface of the photoconductor 12 is electrostatically charged with toner that is colored charged fine particles. To form a visible image.

A recording paper 26 is disposed below the photosensitive member 12 in FIG. 1, one end of which is sandwiched by rollers (not shown) and can be conveyed from one side to the other side by driving the rollers. At the position where the photoconductor 12 and the recording paper 26 face each other, a corona transfer device,
A transfer device 22 including a bias roller transfer device is provided. As a result, the image visualized by the developing device 16 is superposed on the recording paper 26 to apply an electric charge to the recording paper 26, and the toner image is transferred to the recording paper 26 by electrostatic force. The toner image is fused to the recording paper 26 by applying heat or pressure in the fixing device 24 arranged on the downstream side of the recording paper 26 to which the image is transferred in the conveying direction.

Therefore, a desired image can be formed on the recording paper 26 by rotating the photoconductor 12 once with the charger 14 as a starting point.

As shown in FIG. 2, the exposure device 28 is provided with a laser diode 30 as a light source.

On the optical axis X of the laser light emitted from the laser diode 30, a lens portion composed of a collimator lens 32 and a cylindrical lens 34 is arranged. The lens portion is configured to collimate the laser light of the laser diode 30 into parallel rays and to collect the laser light into a predetermined beam diameter, whereby the laser light passes as the collected light rays.

A beam diameter adjuster 36 which can be separated from the optical axis X is arranged between the lens portion and the polygon mirror 38. The beam diameter adjuster 36 is the laser diode 3
It is provided with a slit that can adjust the beam diameter of the laser light emitted from 0 to a small size. Beam diameter adjuster 3
6, a beam diameter control unit 56 in an exposure control unit 56, which will be described later, is connected via a drive unit (not shown),
The beam diameter can be adjusted by separating from or inserting into the optical axis X based on an instruction from the beam diameter control unit 56.

The polygon mirror 38 has six reflecting surfaces and is rotatably attached to a motor drive board 42 by a shaft 40. A motor (not shown) is built in the motor drive board 42, and the motor is the exposure controller 5
6 is connected to the scan control unit 54 (see FIG. 3). As a result, the polygon mirror is moved to the axis 4 by driving the motor based on the instruction from the scanning control unit 54.
It rotates at high speed around 0, and the incident angle of the light beam on each reflecting surface is continuously changed and deflected. The polygon mirror 38 is configured to deflect the light beams and scan the light beams in the main scanning direction to send the light beams to the mirror 44 side.

The light beam main-scanned by the polygon mirror 38 is composed of a plano-convex lens and a plano-concave lens.
It reaches the mirror 44 via the θ lens 46,
It is reflected by and reaches the exposure position. The mirror 44 is tilted at a predetermined angle so that the reflected laser light is reflected vertically downward.

A reflection mirror 48 is arranged at the head of the scanning line of the laser light, and a synchronization detector 50 is provided below the reflection light axis of the reflection mirror 48. Sync detector 50
Is equipped with a sensor unit that detects laser light,
When the laser light is not sensed, a low level signal (hereinafter, L
When a laser beam is sensed, a high level signal (hereinafter referred to as H signal) is output.
The synchronization detector 50 is connected to a pre-exposure control unit 58 described later, and the H signal and the L signal from the synchronization detector 50 are output to the pre-exposure control unit 58.

As a result, the laser light emitted from the laser diode 30 scans the photosensitive member 12 by the polygon mirror 38, is reflected by the reflection mirror 48 provided at the head of the scanning line, and enters the synchronization detector 50. Then,
The L signal is output to the pre-exposure control unit 58 in the synchronization detector 50.

The image forming apparatus 10 is provided with an exposure control section 52 which monitors and controls each component of the exposure device 28. The exposure control unit 52 includes a CPU, a RAM, and an R (not shown).
It is composed of an OM, an input / output controller, and the like.

Next, the exposure controller 52 will be described with reference to FIG. As shown in FIG. 3, the exposure control unit 52 is connected to the motor drive substrate 42 of the polygon mirror 38, controls the rotation direction and rotation speed of the polygon mirror 38, and controls the rotation of the polygon mirror 38 to a predetermined value. A scan control unit 54 is provided for executing the main scan of the laser light. This makes it possible to control the scanning of the laser light by rotating the polygon mirror 38.

Further, the exposure control section 52 is provided with a beam diameter control section 56 for controlling the beam light from the laser diode 30 so as to have a predetermined beam diameter and reach the exposure position. The beam diameter controller 56 includes the beam diameter adjuster 3
6, the beam diameter is changed based on the pre-exposure processing by the pre-exposure control unit 58.
As a result, the beam diameter can be adjusted to a desired value.

The pre-exposure control section 58 is provided with a sync detection signal input section 60 to which the sync detection signal from the sync detector 50 is input. As described above, the signal from the synchronization detector 50 is detected at the head of the main scanning line by the polygon mirror 38 that rotates based on the instruction from the scanning controller 54. As a result, the pre-exposure control unit 58 can know that the laser beam is at the head of the main scanning line, and can give an instruction for the main scanning this time. The synchronization detection signal input unit 60 is connected to a pre-exposure irradiation instruction unit 66 that adjusts the timing of performing pre-exposure.

Further, the pre-exposure control section 58 is provided with a paper size input section 62. The size (main scanning direction and sub scanning direction) of the recording paper 26 which is the transfer target of the electrostatic latent image in the exposure after the pre-exposure is input to the paper size input unit 62. The size of the recording paper 26 is input by an input means provided outside (not shown). As the input means, for example, a sensor provided on the paper feed tray can be cited.

An irradiation area calculation section 64 is connected to the paper size input section 62, and the area of the photoconductor 12 irradiated by pre-exposure is calculated from the input size of the recording paper 26. . The irradiation area of the photoconductor 12 required for the pre-exposure may be about the size of the recording paper 26 to which the electrostatic latent image formed during the exposure is transferred, and does not need to be the entire surface of the photoconductor 12. Absent. Therefore, the minimum irradiation area E corresponding to a part of the surface of the photoconductor 12 is designated. This will be described later.

The irradiation area calculation unit 64, the synchronization detection signal input unit 60 and the scanning control unit 54 are connected to the pre-exposure irradiation instruction unit 66. The pre-exposure irradiation instruction unit 66 integrates each input data, that is, the specified minimum irradiation area E, the synchronization detection signal and the rotation speed of the polygon mirror 38, and sets them on the photoconductor 12 by the main scanning laser light. The timing for irradiating the predetermined minimum irradiation area E and the number of sub-scanning lines for irradiating the size of the minimum irradiation area E in the sub-scanning direction are calculated. The laser diode 30 is connected to the pre-exposure irradiation instruction unit 66, and the pre-exposure irradiation instruction unit 6 is connected in accordance with the calculated timing.
Irradiation is directed from 6 to the laser diode 30.

Therefore, the pre-exposure irradiation area is designated based on the size of the recording paper 26, and the laser light emitted from the laser diode 30 is detected by the synchronization detector 50 provided at the head of the main scanning line to detect synchronization. When the signal is input to the pre-exposure control unit 58, the pre-exposure irradiation instruction unit 66 gives an irradiation instruction to the laser diode 30 at the irradiation timing calculated from the rotation speed of the polygon mirror 38 controlled by the scanning control unit 54. Pre-exposure is performed.

Next, the pre-exposure irradiation will be described with reference to FIGS. 4 and 5. FIG. 4 shows the minimum irradiation area E irradiated in the pre-exposure process and the area S for generating the synchronization detection signal.
It is shown.

The minimum irradiation area E is composed of an image forming area I and a margin M provided before and after the image forming area I. The image forming area I is an area equal to the size of the recording paper 26 onto which the formed image is transferred. An area other than the image forming area I does not need to be irradiated with laser light because an image is not formed. The margin M is an area within a range expected for stabilizing the rotation of the motor that rotates the polygon mirror 38 and the variations in the optical system and its assembly and adjustment.
Therefore, by turning off the laser diode in areas other than the minimum irradiation area E, the cumulative lighting time of the laser diode 30 can be reduced.
These areas are time (count) based on the main scanning speed
Can be defined as

The synchronization detection signal generating area S is an irradiation area for laser light detection by the synchronization detector 50. When the laser diode 30 is turned on during pre-exposure and the synchronization detector 50 detects laser light. The laser diode 30 is turned off for a predetermined time until the synchronization detection signal (L signal) is transmitted and the margin M of the minimum irradiation area E is reached.

The size of the minimum irradiation area E in the sub-scanning direction is
The size of the recording paper 26 is specified as the total size of the size in the sub-scanning direction and the margin for misalignment.
By setting the size in the sub-scanning direction, the number of sub-scanning lines is set based on the rotation speed of the photoconductor 12, the pre-exposure density, the beam diameter, and the like.

FIG. 5 illustrates the timing of pre-exposure irradiation. The lighting of the laser diode 30 corresponding to the minimum irradiation area E is performed by the pre-exposure control unit 58 at the timing calculated based on the scanning speed of the laser light. As for the irradiation instruction signal, when the lighting signal (L signal) for the laser diode 30 is given (dA part), the synchronization detection signal from the synchronization detector 50 is H signal by irradiating the synchronization detector 50 with laser light. When the signal changes from the L signal to the L signal, the irradiation instruction signal switches from the L signal (lighting instruction signal) to the H signal (extinguishing instruction signal), and gives the laser diode 30 an instruction to turn off the light (dB portion). By receiving this turn-off instruction signal (H signal), counting of the time until the irradiation of the minimum irradiation region E is started. The count number up to the tip of the minimum irradiation area E is determined based on the rotation speed (main scanning speed) of the polygon mirror 38. As a result, the lighting instruction signal (L signal) is transmitted at the designated predetermined time (t ₁ in FIG. 4), the laser diode 30 is turned on, and the minimum irradiation area E is irradiated (dC).
Department). On the other hand, the turning-off of the laser diode 30 is executed by giving the turning-off instruction signal (H signal) after a predetermined turning-on time of the laser diode 30 (t ₂ in FIG. 4) ( dD section).

After the irradiation of the minimum irradiation area E, the polygon mirror 38 is rotated and one scanning is completed without irradiation of the laser beam, and immediately after that, the next scanning is started. For this reason,
After a predetermined period of time (t ₃ in FIG. 4) after the irradiation of the minimum irradiation region E is completed, the lighting instruction signal (L signal) is given to the laser diode 30 again, the laser diode 30 is turned on, and the scanning line starts. It is possible to illuminate the synchronization detector 50 in the section (dA 'section).

Therefore, when the synchronization detection signal from the synchronization detector 50 is switched from the H signal to the L signal, the pre-exposure process based on the irradiation instruction signal is started, and the minimum irradiation area E corresponding to the size of the recording paper 26 is started. Irradiation is performed.

Next, the operation of this embodiment will be described. In the exposure processing in the exposure device 28, when the laser light is emitted from the laser diode 30, the collimator lens 32 and the cylindrical lens 34 provided on the optical path.
And the beam diameter adjuster 36 adjusts the beam diameter to a predetermined value. In the present embodiment, the beam diameter is adjusted to be the same as that at the time of image formation. The laser light passing through these lenses is reflected by the reflecting surface of the polygon mirror 38 that rotates at a predetermined speed, passes through the fθ lens system 46, and then is deflected by the mirror 44 to irradiate the surface of the photoconductor 12.

Further, since the polygon mirror 38 rotates in a predetermined direction at a substantially constant angular velocity, the traveling direction of the light beam reflected by the reflecting surface changes, and accordingly, the surface of the photoconductor 12 is irradiated with the light beam. The position of the light spot is also changed. At this time, the light spot is scanned on the surface of the photosensitive drum 15 in the direction of arrow A (main scanning direction) at a substantially constant speed in line units by the action of the fθ lens system.

Since the photoconductor 12 rotates at a predetermined constant rotation speed, the rotation of the photoconductor drum 15 at a constant rotation speed and the movement of the light spot in the arrow B direction at a substantially constant speed. Thus, the surface of the photoconductor drum 15 is scanned (sub scanning). The modulation of the image signal in each line unit is performed by SOS (Scan On Star) not shown.
t) It starts after a lapse of a predetermined time from when the sensor detects the SOS beam, whereby an image is formed.

Next, the pre-exposure process will be described with reference to FIG. When the execution of the pre-exposure process is instructed prior to the exposure process in the exposure device 28, the size of the recording paper 26 is input in step 100. The size of the recording paper 26 may be input by an operator through an external input unit, or may be read by another size reading device.

When the size of the recording paper 26 is input, in step 102, the minimum irradiation area E is determined from the input size of the recording paper 26. The minimum irradiation area E is determined by
This is performed by adding a preset margin M to the size of the recording paper 26.

When the minimum irradiation area E is determined, in step 104, the irradiation timing values t ₁ , t ₂ , t ₃ based on the main scanning speed and the number of lines SL based on the sub-scanning density are determined. t ₁ is the number of counts from the input of the synchronization detection signal (L signal) to the start of irradiation, t ₂
Is the count number from the start of irradiation to the end of irradiation of the minimum irradiation area E, and t ₃ is from the end of irradiation of the minimum irradiation area E to the laser lighting for inputting the synchronous detection signal (L signal). It is defined as the number of counts. The number of lines is set based on the sub-scanning density and the beam diameter. In this embodiment, the scanning density and the beam diameter are set to be the same as those used for image formation.

When each of the irradiation timing values is determined, it is determined in step 106 whether or not the pre-exposure preparation is completed. The pre-exposure preparation includes start of rotation of the polygon mirror 38, preparation for oscillation of the laser diode 30, and the like,
The decision is denied until preparation is complete.

When the pre-exposure preparation is completed and the determination is affirmative, the number of scanning lines sl based on the dimension of the recording paper 26 in the sub-scanning direction is reset in step 107.

When the number of scanning lines sl is reset, the count t for the irradiation timing is reset in step 108, and in step 110, the laser diode 30 is instructed to turn on and main scanning is started.

When the laser diode 30 is instructed to be turned on, it is determined in step 112 whether or not the sync detector 50 inputs the sync detection signal (L signal). When the light from the laser diode 30 starts scanning by the rotationally driven polygon mirror 38, the light enters the synchronization detector 50 arranged at the head of the scanning line, and the synchronization detection signal in the synchronization detector 50 changes from the H signal to the L signal. Become. If this is detected, the decision is affirmed and step 1
Move to 14.

When timing counting is started in step 114, the laser diode 30 is instructed to turn off in step 116.

When the laser diode 30 is instructed to be turned off, it is judged in step 118 whether the count has reached t _1, and if it is not t ₁ , the judgment is denied and the routine proceeds to step 116.

On the other hand, when the count value becomes t ₁ , the determination is affirmative, and the routine proceeds to step 120, where the laser diode 30 which has been turned off is instructed to be turned on. When the laser diode 30 is turned on, irradiation of the minimum irradiation area E on the photoconductor 12 is started.

[0063] When lighting the laser diode 30 is instructed, in step 122, the count is determined whether or not becomes t _2, determination if not t _2, the process proceeds denied in step 120 The lighting is continued and the irradiation to the minimum irradiation area E is executed.

When the count value reaches t ₂ , the determination is affirmative and the routine proceeds to step 124, where the laser diode 30 is instructed to turn off, and the routine proceeds to step 126.

In step 126, the number of lines sl in the sub-scanning direction is incremented to complete the pre-exposure irradiation for one sub-scanning line, and the process proceeds to step 128 to determine whether the number of lines sl reaches the set number of lines SL. It is determined whether or not. If the number of sub-scanning lines SL is not set, the determination is negative and the process proceeds to step 130 because the dimension of the photoconductor 12 in the minimum irradiation area E is not irradiated.

At step 130, it is judged whether or not the count reaches t ₃ , and the judgment is denied until t ₃ . When t ₃ is reached, the determination is affirmative and the routine proceeds to step 108, where the count is reset and the pre-exposure is continued in accordance with the next scan line.

On the other hand, in step 128, when the number of lines sl reaches the set number of sub-scanning lines SL,
The irradiation of the minimum irradiation region E in the sub-scanning direction is completed, the determination is affirmative, and the pre-exposure irradiation is completed.

Upon completion of the pre-exposure irradiation, step 132
At, exposure processing is performed. Here, in the exposure process, since an electrostatic latent image is formed on the minimum irradiation area E in which the pre-exposure is completed, it is possible to form an image on the surface of the photoconductor 12 in which a latent image ghost does not appear. it can.

When the exposure process is completed, it is determined in step 134 whether or not to continue the process. If the process is to be continued, the determination is affirmative and the process returns to step 100 to start the pre-exposure process. On the other hand, when the processing is to be ended, the determination is denied and the series of processing is completed.

As a result, the cumulative lighting time of the laser diode 30 is shortened and pre-exposure is performed, so that the life of the laser diode 30 is extended and the problem of heat generation is eliminated, and the image forming area I on the surface of the photoconductor 12 is eliminated. The minimum necessary portion of the image is pre-exposed to prevent the appearance of latent image ghosts and a clear image can be obtained.

Further, the cost increase and the enlargement of the apparatus due to the use of the pre-exposure lamp for exclusive use of the pre-exposure are suppressed, and the effective charge removal efficiency is not reduced by using the same light source as that for image formation. Exposure can be performed.

Therefore, as shown in FIG. 7, for example,
By the print paper size designation signal at the start of printing,
In the case of B4 printing, the laser beam is turned on during the pre-exposure by the width of the B4 sheet + the length of the margin M (the minimum irradiation area E _B4 ,
7A), in the case of A4 printing, the laser beam during pre-exposure is turned on for the A4 sheet width + the length of the margin M (minimum irradiation area E _A4 , see FIG. 7B). As a result, B
It is possible to shorten the lighting time (2 × S _T ) of the laser diode by the width of the paper for A4 printing rather than 4 printing.

In the present embodiment, the minimum irradiation area E is set according to the size of the recording paper 26, but the irradiation area may be set according to the maximum size of the recording paper 26. According to this, it is possible to perform the processing more quickly without sequentially checking and resetting the size, and performing pre-exposure to the image forming area to prevent the occurrence of a latent image ghost. it can.

In this embodiment, the pre-exposure is performed by scanning the laser beam as in the case of forming the image. However, the pre-exposure uniformly exposes the image-forming area to the extent that the latent image ghost does not occur and the charge is eliminated. Therefore, it is not necessary to perform the scanning of the laser light in the exposure with high precision optical scanning at the image writing position, as in the image formation.

For example, as shown in FIG. 8, the image forming area of the photoconductor 12 can be scanned every fixed line. By scanning one scan line starting in the irradiation area S for the synchronization detection signal for each line, the irradiation time can be half that in the case of scanning all the lines. This makes it possible to further shorten the lighting time of the laser diode 30 and perform pre-exposure. The same effect can be obtained by scanning every fixed dot, and by irradiating every fixed dot every fixed line, the cumulative lighting time of the laser diode 30 can be further shortened and pre-exposure can be performed. .

Further, in the present embodiment, the case where the beam diameter is set to be approximately the same as that at the time of image formation has been described as an example, but by increasing the beam diameter, pre-exposure can be performed more efficiently. .

FIG. 9 shows a case where the beam diameter is increased and irradiation is performed for each dot per line. As shown in the figure, by setting the beam irradiation area so that the beam irradiation areas can overlap for each line, pre-exposure can be surely performed in the sub-scanning direction even if the cumulative lighting time of the laser diode 30 is shortened. it can.

Further, as shown in FIG. 10A, even if scanning unevenness occurs in the sub-scanning direction by increasing the beam diameter, the pre-exposure can be surely performed. As shown, it is possible to prevent the pre-exposure unevenness that has been conventionally caused by the scanning unevenness. When increasing the beam diameter in this way, the pre-exposure control unit 5
It is possible to adjust the irradiation interval of the irradiation instruction from the pre-exposure irradiation instruction unit 66 in FIG.

In the present embodiment, the beam diameter is adjusted by the beam diameter adjusting means provided with the slit, but the present invention is not limited to this. For example, the beam diameter may be adjusted by providing a mirror unit or the like and adjusting the optical path length, and all other known means capable of adjusting the beam diameter can be used.

Further, in the present embodiment, the synchronization detector 36 is provided on the leading side of the scanning line for pre-exposure irradiation. However, synchronization is performed by using an SOS sensor (not shown) for timing the writing of an image during image formation. The detection signal may be detected. In addition, EOS (En
It is also possible to give a similar effect to a d of Scan) sensor (not shown).

[0081]

According to the present invention, since the irradiation area is set according to the image forming area, the cumulative lighting time of the laser diode is shortened and unnecessary irradiation of the laser beam is eliminated. A latent image ghost can be prevented and a good image can be obtained without shortening the life of the laser diode.

Further, in the pre-exposure, since the beam diameter is increased and a part of the beam is overlapped in the main scanning direction and / or the sub-scanning direction to irradiate every fixed dot, irradiation is performed with a pixel density required for electrostatic latent image formation. It is possible to shorten the lighting time of the laser beam and efficiently perform the pre-exposure time as compared with the case of performing.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an image forming apparatus according to the present invention.

FIG. 2 is a schematic configuration diagram of an exposure device in the image forming apparatus.

FIG. 3 is a block diagram of an exposure control unit in the image forming apparatus.

FIG. 4 is a conceptual diagram showing a minimum irradiation area in pre-exposure irradiation.

FIG. 5 is a conceptual diagram illustrating control of pre-exposure processing in the image forming apparatus.

FIG. 6 is a flowchart illustrating a pre-exposure process.

FIG. 7A is a diagram showing a minimum irradiation area on a B4 size recording paper, and FIG. 7B is a diagram showing a minimum irradiation area on an A4 size recording paper.

FIG. 8 is a conceptual diagram of another pre-exposure process in the image forming apparatus.

FIG. 9 is a conceptual diagram of another pre-exposure process in the image forming apparatus.

10A is a conceptual diagram of a pre-exposure process when the beam diameter is increased, and FIG. 10B is a conceptual diagram of a conventional pre-exposure process.

FIG. 11 is a diagram for explaining the principle of latent image ghost generation.

[Explanation of symbols]

 10 image forming apparatus 12 photoconductor 28 exposure unit 30 laser diode 38 polygon mirror 50 synchronization detector 52 exposure control unit 58 pre-exposure control unit

Claims (5)

[Claims] 1. A charging means for uniformly charging a photoconductor, a laser beam scanning means for forming an electrostatic latent image corresponding to a paper size of a print command on the photoconductor, and a latent image for the electrostatic latent image. In an image forming apparatus having a developing unit for forming an image and a transferring unit for transferring the visualized image on a recording medium, the image forming apparatus corresponds to the sheet size of the photoconductor before charging by the charging unit. An image forming apparatus comprising pre-exposure control means for controlling the laser beam scanning means so as to irradiate a necessary minimum area including an electrostatic latent image forming area. 2. The image forming apparatus according to claim 1, wherein the pre-exposure control unit sets and irradiates the required minimum area according to the sheet size of the next print command. 3. The pre-exposure control means has a beam diameter larger than that when an electrostatic latent image is formed by the laser beam scanning means, so that adjacent beams in the main scanning direction partially overlap each other. The image forming apparatus according to claim 1 or 2, wherein irradiation is performed for each fixed dot. 4. The pre-exposure control means has a beam diameter larger than that when an electrostatic latent image is formed by the laser beam scanning means, so that adjacent beams in the sub-scanning direction partially overlap each other. The image forming apparatus according to claim 1 or 2, wherein the irradiation is performed every fixed line. 5. The pre-exposure control means has a beam diameter larger than that when the electrostatic latent image is formed by the laser beam scanning means so that a part of adjacent beams overlap in the main scanning direction and the sub scanning direction. The image forming apparatus according to claim 1 or 2, wherein the irradiation is performed for each fixed dot and for each fixed line.