JPH10105016A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device where unnecessary toner consumption at the time of starting copying operation is prevented, developer carrier attraction and developer drop or the like are surely prevented and also wasteful waiting time until starting the copying operation is solved and whose reliability is excellent. SOLUTION: At the time of starting the copying operation, developing bias voltage having a polarity reverse to a normal time is applied to a developing unit 32 until the last end of an unelectrified area on a photosensitive drum reaches the developing unit 32. On the other hand, the surface potential of the photosensitive drum is detected and the dark decay time (Tx) of the surface potential is detected, and when the power source of a copying machine is turned off in the middle of the copying operation, elapsed time (T) since then is measured, and the start of the copying operation is inhibited until the elapsed time measured (T) reaches the dark decay time (Tx) at the time of resupplying power to the copying machine afterward.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus employing a reversal developing system.

[0002]

2. Description of the Related Art In an image forming apparatus such as an electrophotographic digital copying machine or a printer, a reversal developing system is mainly used as a developing system. In this reversal development method, an image is formed on a photosensitive drum as an electrostatic latent image, and the electrostatic latent image on the photosensitive drum is visualized by attaching a developer from a developing device, and the developed image is transferred. The image is transferred to an image forming medium, for example, a sheet by a container.

An advantage of using the reversal developing system is that it is only necessary to expose a portion of the photosensitive drum on which toner development is to be performed, and it is not necessary to expose a white background, which occupies most of a normal document, and it is not necessary to expose the original. It is suitable for an image forming apparatus such as a digital copying machine using a laser diode or the like as a light source.

On the other hand, in the reversal developing system, there is a problem that toner adheres to a region on the photosensitive drum where no charge is placed, and the toner is unnecessarily consumed. That is, at the time of warm-up or pre-run of the apparatus, an uncharged portion (zero V potential) exists between the position corresponding to the charger and the position corresponding to the developing device on the photosensitive drum, and the toner is pulled and adhered to the portion, which is unnecessary. This leads to excessive toner consumption.

As a countermeasure for preventing such unnecessary consumption of toner, a developing bias voltage which is normally set at -300 V to -500 V at the start of rotation of the photosensitive drum is changed to a developing bias of the opposite polarity, for example, +150 V. Means for setting the voltage is adopted.

However, in this case, when the rotation of the photosensitive drum advances, the end of the uncharged portion on the photosensitive drum passes through the developing position, and the charged area on the photosensitive drum reaches the developing position. When the developing bias voltage having the opposite polarity of +150 V is still applied, the difference between the potential of the photosensitive drum (static charge) and the potential of the developing sleeve (+150 V) becomes considerably large. A serious problem such as developer drop occurs.

[0007] In order to prevent such developer carrier pulling and developer falling, the timing at which the end of the uncharged portion on the photosensitive drum reaches the developing device and the developing bias voltage are changed from the reverse polarity of +150 V to the normal polarity. -300V to -5
It is necessary to properly synchronize the timing of switching to the forward polarity of 00V. In practice, such timing control is performed.

However, a problem arises, for example, during a normal copying operation, when the power supply of the apparatus is cut off before the end of the operation, and immediately thereafter, the power supply of the apparatus is turned on again to return to the copy enabled state. . In this case, at the start of the copying operation, the electrostatic charge remains at the position corresponding to the developing device on the photosensitive drum, and a developing bias voltage having a reverse polarity of +150 V is applied to one developing unit. As a result, a situation is unavoidable in which the developer carrier pulling and the developer dropping described above are unavoidable.

Therefore, if the power supply of the apparatus is cut off before the end of the copying operation, the start of the copying operation after the power supply of the apparatus is turned on again, at least the surface potential (charge of electrostatic charge) of the photosensitive drum is darkly attenuated. It is necessary to prohibit until a situation occurs in which the developer carrier pulling or the developer drop does not occur.

In practice, a measure is taken to prohibit the start of the copy operation until a certain period of time elapses after the power supply to the apparatus is cut off before the end of the copy operation. The certain time is obtained by adding an arbitrary margin to a general time considered to be required for dark decay.

[0011]

The time required for dark decay of the surface potential of the photosensitive drum is susceptible to changes in characteristics of the photosensitive drum due to changes over time and environmental changes. Therefore, the fixed time for prohibiting the start of the copying operation as described above must be set to a time sufficiently including the time until the end of the dark decay, in consideration of the characteristic change of the photosensitive drum.

[0012] For this reason, the fixed time becomes longer, and a wasteful waiting time is generated that the copying operation cannot be started forever, even though the dark decay has already been completed. The user feels frustrated.

The present invention has been made in view of the above circumstances.
The purpose is to prevent unnecessary toner consumption at the start of image formation, to reliably prevent the developer carrier from being pulled or the developer from dropping, and to uselessly wait for the start of image formation. An object of the present invention is to provide a highly reliable image forming apparatus capable of solving the problem.

[0014]

According to a first aspect of the present invention, there is provided an image forming apparatus in which a charging unit, an exposing unit, and a developing unit are sequentially disposed around a photosensitive drum, and the charging unit charges the photosensitive drum. A reversal development method in which an electrostatic latent image is formed on the photosensitive drum by an exposure unit, and the electrostatic latent image is visualized by a developer while applying a developing bias voltage to a developing unit and transferred to a medium on which an image is to be formed. A voltage applying means for applying a bias voltage having a polarity opposite to that of the developing bias to the developing means until an end of an uncharged portion on the photosensitive drum reaches the developing means; and Potential detecting means for detecting the surface potential of the drum; detecting means for detecting a dark decay time required for the surface potential of the photosensitive drum to attenuate to a predetermined potential from the detected potential of the potential detecting means; means Based on the more the detected dark decay time, and control means for controlling the timing of applying a voltage to the developing means from said voltage applying means.

In the image forming apparatus according to a second aspect of the present invention, a charging unit, an exposing unit, and a developing unit are sequentially disposed around the photosensitive drum, and the charging unit charges the photosensitive drum. A reversal developing method of forming an electrostatic latent image by exposure means, visualizing the electrostatic latent image by a developer while applying a developing bias voltage to a developing means, and transferring the latent image to an image forming medium; At the start of image formation, a voltage applying means for applying a developing bias voltage having a polarity opposite to the developing bias voltage to the developing means until the end of the uncharged portion on the photosensitive drum reaches the developing means; A potential detecting means provided between the charging means and the developing means for detecting a surface potential of the photosensitive drum; and a surface potential of the photosensitive drum attenuated to a predetermined potential from a detection potential of the potential detecting means. Important Detecting means for detecting the dark decay time of the apparatus, time measuring means for measuring the elapsed time after the power supply of the apparatus is cut off during the image formation, and power supply for the apparatus after the power supply of the apparatus is cut off during the image formation. And control means for prohibiting the start of image formation until the time measured by the time measuring means reaches the dark decay time detected by the detecting means.

In the image forming apparatus according to a third aspect of the present invention, a charging unit, an exposing unit, and a developing unit are sequentially arranged around the photosensitive drum, and the charging unit charges the photosensitive drum. A reversal developing method of forming an electrostatic latent image by exposure means, visualizing the electrostatic latent image by a developer while applying a developing bias voltage to a developing means, and transferring the latent image to an image forming medium; At the start of image formation, a voltage application unit for applying a bias voltage having a polarity opposite to the developing bias to the developing unit until the end of the uncharged portion on the photosensitive drum reaches the developing unit; Potential detecting means for detecting the surface potential of the photosensitive drum, temperature detecting means for detecting the temperature of the photosensitive drum, detecting temperature of the temperature detecting means and detecting the potential hand Detecting means for detecting a dark decay time required for the surface potential of the photosensitive drum to attenuate to a predetermined potential according to the detected potential of the photosensitive drum; Time measuring means for measuring time, and when the apparatus power is turned on again after the apparatus power is cut off during the image formation, the image is not changed until the time measured by the time measuring means reaches the dark decay time detected by the detecting means. Control means for prohibiting the start of formation.

In an image forming apparatus according to a fourth aspect of the present invention, a charging unit, an exposing unit, and a developing unit are sequentially disposed around a photosensitive drum, and the charging unit charges the photosensitive drum. A reversal developing method of forming an electrostatic latent image by exposure means, visualizing the electrostatic latent image by a developer while applying a developing bias voltage to a developing means, and transferring the latent image to an image forming medium; At the start of image formation, a voltage applying means for applying a bias voltage having a polarity opposite to the developing bias voltage to the developing means until the end of the uncharged portion on the photosensitive drum reaches the developing means; Means for detecting the surface potential of the photosensitive drum, temperature detecting means for detecting the temperature of the photosensitive drum, and the charging means during non-image formation, provided between the means and the developing means. Photoconductor The ram is charged, the potential of the charged area is detected by the potential detecting means, and the dark decay time until the detected potential attenuates to a predetermined value is detected while taking the temperature detected by the temperature detecting means into consideration as a calibration factor. Dark decay detecting means, a time measuring means for measuring an elapsed time after the power supply of the apparatus is cut off during the image formation, and a time when the power supply of the apparatus is turned on again after the power supply of the apparatus is cut off during the image formation. And control means for prohibiting the start of image formation until the time measured by the time measuring means reaches the dark decay time detected by the dark decay detecting means.

In the image forming apparatus according to a fifth aspect of the present invention, a charging unit, an exposing unit, and a developing unit are sequentially arranged around the photosensitive drum, and the charging unit charges the photosensitive drum. A reversal developing method of forming an electrostatic latent image by exposure means, visualizing the electrostatic latent image by a developer while applying a developing bias voltage to a developing means, and transferring the latent image to an image forming medium; At the start of image formation, a voltage application unit for applying a bias voltage having a polarity opposite to the developing bias to the developing unit until the end of the uncharged portion on the photosensitive drum reaches the developing unit; A potential detecting means provided between the photosensitive drum and the developing means for detecting the surface potential of the photosensitive drum; and a potential required until the surface potential of the photosensitive drum attenuates to a predetermined potential from the detected potential of the potential detecting means. Darkening Detecting means for detecting the time, temperature detecting means for detecting the temperature of the photosensitive drum, and, when the apparatus power is turned on again after the apparatus power is cut off during image formation, the time after the apparatus power is cut off An estimating means for estimating the elapsed time based on the temperature detected by the temperature detecting means; and an estimating time of the estimating means detected by the detecting means when the apparatus power is turned on again after the apparatus power is cut off during image formation. And control means for prohibiting the start of image formation until the dark decay time reaches.

In an image forming apparatus according to a sixth aspect of the present invention, a charging unit, an exposing unit, and a developing unit are sequentially arranged around a photosensitive drum, and the photosensitive drum is charged with an electrostatic charge by the charging unit.
A reversal development method in which an electrostatic latent image is formed on the photosensitive drum by an exposure unit, and the electrostatic latent image is visualized by a developer while applying a developing bias voltage to a developing unit and transferred to a medium on which an image is to be formed. A voltage for applying a bias voltage having a polarity opposite to that of the developing bias voltage to the developing means until the end of the uncharged portion on the photosensitive drum reaches the developing means at the start of image formation. An applying unit, a potential detecting unit provided between the charging unit and the developing unit, and detecting a surface potential of the photosensitive drum,
Dark decay detecting means for charging the photosensitive drum by the charging means, detecting the potential of the charged area by the potential detecting means, and detecting a dark decay time until the detected potential attenuates to a predetermined value, A temperature detecting means for detecting the temperature of the photosensitive drum; and, when the apparatus power is turned on again after the apparatus power is cut off during image formation, the time elapsed since the apparatus power was cut off is referred to as the temperature detecting means. Estimating means for estimating based on the detected temperature, and when the apparatus power is turned on again after the apparatus power is cut off during image formation, the estimated time of the estimating means is equal to the dark decay time detected by the dark decay detecting means. And control means for prohibiting the start of image formation until the time has elapsed.

In an image forming apparatus according to a seventh aspect of the present invention, a charging unit, an exposing unit, and a developing unit are sequentially disposed around a photosensitive drum, and the charging unit charges the photosensitive drum. An electrostatic latent image is formed by an exposure unit, the electrostatic latent image is visualized by a developer while applying a developing bias voltage to a developing unit, and the developed image is transferred to an image forming medium by a transfer unit. The image forming apparatus is of a reversal developing type in which fixing is performed by a fixing unit. At the start of image formation, the developing bias is applied to the developing unit until the end of an uncharged portion on the photosensitive drum reaches the developing unit. A voltage applying means for applying a bias voltage having a reverse polarity, a potential detecting means provided between the charging means and the developing means for detecting a surface potential of the photosensitive drum, and The above photoconductor And a dark decay detecting means for detecting a dark decay time until the detected potential attenuates to a predetermined value, and a temperature of the fixing means. Temperature estimating means, and estimating means for estimating, based on the temperature detected by the temperature detecting means, an elapsed time since the power supply of the apparatus is turned off when the power supply of the apparatus is turned on after the power supply of the apparatus is turned off during the image formation. When the power supply of the apparatus is turned on again after the power supply of the apparatus is cut off during the image formation, the image forming is started until the estimated time of the estimating means reaches the dark decay time detected by the dark decay detecting means. Prohibiting control means.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. First, FIG. 2 shows a configuration of an image forming apparatus, for example, an electronic copying machine. A document table (glass plate) 2 for placing a document is provided on the upper portion of the main body 1, and a document table cover 3 is provided on the document table 2 so as to be freely opened and closed.
A document 4 is appropriately set on the document table 2.

An exposure system 10 is provided in the upper part of the main body 1. The exposure system 10 includes exposure means such as an exposure lamp 11, first to third reflection mirrors 12a, 12b, and 12c, a variable magnification lens block 13, and an image signal output provided on the lower surface of the document table 2. Means such as CCD
Type line sensor (hereinafter referred to as CCD sensor) 14
It consists of.

The exposure lamp 11 is provided on a carriage (CRG) 15, which is a moving means, and can freely reciprocate. Exposure scanning is performed over the entire surface of the document table 2. By this exposure scanning, a reflected light image of the document 4 placed on the document table 2 is obtained, and the reflected light image is projected on the CCD sensor 14 by the above-mentioned respective reflecting mirrors and the lens block 13 for zooming. The CCD sensor 14
An image signal of a level corresponding to the amount of received light is output. This image signal is sent to the laser exposure unit 20.

At substantially the center of the main body 1, there is provided a photosensitive drum 30 rotatable in the direction of the arrow. Around the photosensitive drum 30, a charging charger 31, the laser exposure unit 20, a surface potential sensor 51, a developing device 32, a transfer charger 33, a peeling charger 34, and a peeling nail 3
5, a PCC charger 36, a cleaner 37, and a static elimination lamp 38 are sequentially arranged.

The laser exposure unit 20 is a first lens that gives a convergence to a laser beam emitted from a semiconductor laser element (not shown) and converts it into a laser beam having a substantially circular cross section. A laser deflecting device 21 for deflecting the beam along the axial direction of the photosensitive drum 30; and a deflection angle of the laser beam for sequentially forming an image of the laser beam deflected by the laser deflecting device 21 on the photosensitive drum 30. An imaging lens 22 that matches the distance from the optical axis on the photosensitive drum 30 to the position where the beam is to be imaged, and the imaging lens 22
And a mirror 23 for guiding the laser beam having passed through the photoconductor drum 30 onto the photosensitive drum 30.

The surface potential sensor 51 is connected to the photosensitive drum 30
The surface potential of is detected. The charging charger 31 applies a high voltage supplied from a charging high voltage transformer 63 described later to the photosensitive drum 30 by a corona wire,
The surface of the photosensitive drum 30 is charged with an electrostatic charge. In addition,
The charged charger 31 applies an electrostatic charge to the photosensitive drum 3.
Charging grid 31a for uniformly charging the surface
Is provided.

An electrostatic latent image is formed on the photosensitive drum 30 by the charging and the imaging of the photosensitive drum 30 by the laser beam of the laser exposure unit 20. The developing device 32 contains a two-component developer D composed of a non-magnetic toner T and a magnetic carrier C, and the developer D
Roller 32a for supplying the photosensitive drum 30 to the photosensitive drum 30
Having.

The developing roller 32a causes only the negatively charged toner T to adhere to the electrostatic latent image on the photosensitive drum 30 while holding the developer D on the outer periphery. Due to this attachment, the electrostatic latent image on the photosensitive drum 30 is visualized. To perform this development, a predetermined level of a developing bias voltage is applied to the developing roller 32a and the developer D from a developing bias generating circuit 64 described later.

The developing roller 32a moves in the circumferential direction.
The magnet roller includes a pole and an N pole, and a non-magnetic sleeve that rotates around the magnet roller in the direction of the arrow. Ears (ears) of the carrier C are formed on the sleeve and along the lines of magnetic force of the magnet roller. The toner T adheres to the ears of the carrier C by a mirror image force, and the toner T
The developing roller 32a is moved to the photosensitive drum 30 side by an electric field formed by the developing bias voltage and the surface potential of the photosensitive drum 30 at the developing position where the developing roller 32a and the developing roller 32a face each other.

At the bottom of the main body 1, a plurality of paper feed cassettes 40 are provided as paper feed means. These paper cassettes 4
0 stores a large number of copy sheets of different sizes as image forming media. Operation unit 81 described later
In response to the ON operation of the copy key and the detection result of the document size sensor (not shown), the pickup roller (not shown) takes out one copy sheet from any one of the paper feed cassettes 40 one by one. Is done. The taken out copy paper is sent to the alignment roller 42 by the transport roller 41, where the rotation of the photosensitive drum 30 is waited.

The alignment roller 42 corrects the inclination of the copy sheet, aligns the leading end of the toner image on the photosensitive drum 30 with the leading end of the copy sheet, and has the same speed as the moving speed of the outer peripheral surface of the photosensitive drum 30. Feed copy paper at speed. The aligning roller 42
An aligning switch (not shown) for detecting the leading edge of the copy sheet is disposed in the vicinity of for controlling the operation timing of the aligning roller.

The transfer charger 33 applies a high voltage supplied from a transfer high voltage transformer 65 described later to the photosensitive drum 30.
The photosensitive drum 3 is applied to the copy paper fed from the alignment roller 42 in synchronization with the rotation of the photosensitive drum 3.
The visible image (toner T) on 0 is transferred to copy paper.

The peeling charger 34 applies a high voltage supplied from a peeling high voltage transformer 66, which will be described later, to the copy paper by a corona wire, thereby forming a photosensitive drum 30.
Peel the copy paper from.

The peeling claw 35 functions to assist the peeling of the copy sheet by the peeling charger 34. The PCC charger 36 is for removing the charge for detecting the dark decay on the photosensitive drum 30, and applies an AC voltage supplied from an AC power supply circuit 69 described later to the photosensitive drum 30 by a corona wire.

The cleaner 37 scrapes and collects the untransferred toner T remaining on the surface of the photosensitive drum 30. The collected toner T is collected in a toner collecting device (not shown), and is discarded every time a predetermined amount is reached.

The static elimination lamp 38 includes a light source driver 7 described later.
The light emitting operation is performed by the voltage supplied from 0, and the residual potential remaining on the photosensitive drum 30 is removed. Peeling charger 3
The copy sheet peeled at 4 is sent to a fixing device (heat roller) 44 by a transport belt 43. Fixing device 44
Is to fix the visible image transferred to the copy paper by heating / pressing it on the copy paper.

The copy paper on which the fixing has been completed is delivered to a paper discharge roller 45.
Is discharged to the paper discharge tray 46. Discharge roller 45
A paper discharge switch (not shown) is arranged in the vicinity of. The detection of the rear end of the copy paper by the paper discharge switch indicates that the image forming operation on the copy paper is completed.

FIG. 1 shows a main part of the control circuit. Control unit 60
A motor driver 61, a charging high voltage transformer 63, the surface potential sensor 51, a developing bias generating circuit 64, a transfer high voltage transformer 65, a peeling high voltage transformer 66, a solenoid driver 67, an AC power supply circuit 69, a light source driver 70,
Memory 71, timer 72, drum thermistor 52, heat roller thermistor 53, operation unit 73, and display unit 7
4 are connected.

The motor driver 61 is connected to the photosensitive drum 30
The main motor 61 for covering the rotation of the motor is driven.
The developing bias generating circuit 64 applies a developing bias voltage of, for example, -400 V to the developing device 32, and supplies a voltage having a polarity opposite to that of the developing bias voltage to the developing device 3 during image formation.
2

The solenoid driver 67 is provided with the release claws 35.
Is driven to move the solenoid 68. Memory 71
Is a non-volatile type and is prepared for storing the dark decay time of the photosensitive drum 30.

The timer 72 is provided for measuring the dark decay time of the photosensitive drum 30. The drum thermistor 52 senses the temperature of the photosensitive drum 30.

The heat roller thermistor 53 detects the temperature of the fixing device (heat roller) 44. The operation unit 73 and the display unit 74 are provided on the upper surface of the main body 1 and the like for setting and displaying operating conditions of the copying machine.

The control section 60 has the following [1] to [5] as main functional means. [1] An electrostatic charge is charged on the photosensitive drum 30 by the charging charger 31, an electrostatic latent image is formed on the photosensitive drum 30 by exposure of the laser exposure unit 20, and the electrostatic latent image is And the transferred image is transferred to copy paper by the transfer charger 33.
Control means for performing image formation (copy operation) of the reversal development system.

[2] At the start of image formation, the photosensitive drum 3
Until the end of the uncharged area on 0 reaches the developing device 32,
Normally (DC voltage -300 V to -50
0V) and a developing bias voltage having a polarity opposite to that of the DC voltage (DC voltage +15).
0 V).

[3] At the time of non-image formation (for example, when the power of the copying machine is turned on for the first time in a day), the photosensitive drum 30 is charged with an electrostatic charge by the charging charger 31 and the potential of the charged area is measured by the surface potential sensor 51. And a dark decay detecting means for detecting a dark decay time Tx required for the detected potential to decay to a predetermined value while taking the temperature detected by the drum thermistor 52 into consideration as a calibration factor. The detected dark decay time Tx is stored in the memory 71.

[4] Time-measuring means for measuring the elapsed time T after the power of the copying machine is turned off during image formation. [5] When the power of the copying machine is turned on again after the power of the copying machine is cut off during the image formation, the time T measured by the clock means reaches the dark decay time Tx detected by the dark decay detection means. Control means for inhibiting the start of image formation.

Next, the operation of the above configuration will be described. FIG.
As shown in the flowchart, the main motor 62 is driven by turning on the copy button of the operation unit 73. With this drive, the rotation of the photosensitive drum 30 starts. When the rotation of the photosensitive drum 30 is stabilized, charging by the charging charger 31 is started.

At the same time as the main motor 62 is driven, a developing motor (not shown) of the image unit 32 is driven, and a developing bias voltage is applied to the developing unit 32. in this case,
Until the end of the uncharged area on the photoreceptor drum 30 reaches the developing device 32, the developing device 32 operates normally (DC voltage -300
V to -500 V) and a developing bias voltage (DC voltage +150 V) having a polarity opposite to that of the developing bias voltage is applied.

By applying the developing bias voltage of the opposite polarity, an uncharged area (zero V potential) existing between the position corresponding to the charger and the position corresponding to the developing device on the photosensitive drum 30.
The adhesion of the toner to the toner is prevented. This prevents unnecessary consumption of toner.

When the end of the uncharged area on the photosensitive drum 30 reaches the developing device 32, the developing bias voltage applied to the developing device 32 changes from the opposite polarity to a normal voltage of -300 V to -5.
It is set to 00V. As a result, it is possible to avoid a situation where the difference between the potential (static charge) of the photosensitive drum 30 and the potential (+150 V) of the developing sleeve becomes large, and it is possible to prevent the developer carrier from being pulled or the developer from falling.

By the way, during the copying operation, the power of the copying machine may be cut off before the copying operation ends normally, and immediately thereafter, the power of the copying machine may be turned on again to start the copying operation again.

In this case, the electrostatic charge remains at the position corresponding to the developing device on the photosensitive drum 30. On the other hand, a developing bias voltage of +150 V having a reverse polarity is applied to the developing means. Or a situation in which the developer is inevitably dropped.

Then, based on the experimental data shown in FIGS. 4 and 5, the operation in the dark attenuation detection mode shown in the flowchart of FIG. 6 is executed. FIG. 4 shows the relationship between the surface potential of the photosensitive drum 30 and the output voltage of the surface potential sensor 51. FIG. 5 shows the dark decay characteristic of the surface potential of the photosensitive drum 30 using the temperature of the photosensitive drum 30 as a parameter. When the developing bias voltage has the opposite polarity of +150 V, the surface potential of the photosensitive drum 30 is reduced. It has been confirmed that, when the voltage is −400 V or less, the developer carrier pulling and the developer drop do not occur. The time required for the surface potential of the photoconductor drum 30 to darkly decay to −400 V is 20 seconds when the temperature of the photoconductor drum 30 is 10 ° C. and 1 when the temperature of the photoconductor drum 30 is 23 ° C.
5 seconds, 10 seconds at 35 ° C.

During a non-copy operation, for example, when the power of the copying machine is turned on for the first time of the day (morning, etc.), the temperature of the photosensitive drum 30 is first detected by the drum thermistor 52 as shown in the flowchart of FIG. Based on the detected temperature, a calibration value for the dark decay time of the detection target is calculated by the control unit 6.
It is obtained in advance from the memory table in 0 (step 10
1).

The calibration values take into account the fact that the dark decay characteristic varies depending on the temperature as shown in FIG. 5, and the temperature of the photosensitive drum 30, which is a general use condition, is 23 ° C.
Is provided to determine the dark decay time in the case of.

The rotation of the photosensitive drum 30 is started by driving the main motor 62, and the photosensitive drum 30 is charged with an electrostatic charge by the specific output of the charging charger 31 (step 102).

Simultaneously with the driving of the main motor 62, a developing bias voltage (DC voltage +150
V) is applied (step 103). This is for preventing toner from adhering to an uncharged area (zero V potential) existing between the charging start position on the photosensitive drum 30 and the position corresponding to the developing device.

When the charged area on the photosensitive drum 30 corresponds to the surface potential sensor 51, the rotation of the photosensitive drum 30 is stopped (step 104). Then, the time until the detection potential of the surface potential sensor 51 attenuates to -400 V (a state in which there is no fear of pulling the developer carrier or dropping of the developer) is measured by the timer 72 (step 105).

The time measured by the timer 72 is corrected by the obtained calibration value, and is stored in the memory 71 as the dark decay time Tx (step 106). Thereafter, the photosensitive drum 30 is rotated, and the static charge remaining on the photosensitive drum 30 for detecting dark attenuation is completely removed by the PCC charger 36 (step 107). Thus, the operation in the dark decay detection mode ends.

Thereafter, during the copying operation, the power of the copying machine is cut off before the copying operation ends normally, and immediately thereafter, the power of the copying machine is turned on again, and the control shown in the flowchart of FIG. 7 is executed. You.

That is, it is determined whether or not the previous copy operation performed a normal copy end operation (step 20).
1). If the processing is not abnormal termination (NO in step 202), the copying operation is immediately started, and the start of the copying operation is permitted (step 207).

However, in the case of abnormal termination (step 20
2 YES), the elapsed time T since the power of the copier was cut off
Is continuously measured by the timer 72 (step 203).
Further, the dark decay time Tx stored in the memory 71 is read (step 204), and is compared with the measured time T (step 205).

If the measured time T does not reach the dark decay time Tx (T <Tx), it is determined that the dark decay of the surface potential of the photoreceptor drum 30 is not sufficient, and the timer is not set to the copy enabled state. Continue counting. The copy is not enabled, and the start of the copy operation is prohibited (step 206).

When the measurement time T reaches the dark decay time Tx (T ≧ Tx), that is, when the detection potential of the surface potential sensor 51 attenuates to −400 V, a copy is enabled, and the start of the copy operation is permitted (step S1). 207).

As described above, since the start of the copying operation is not allowed until the surface potential of the photosensitive drum 30 has been attenuated to -400 V, it is possible to reliably prevent the developer carrier from being pulled or the developer from dropping. Thus, good image formation is always possible.

In particular, the dark decay time T of the photosensitive drum 30
Since x is detected every day in consideration of the drum temperature, even if the dark decay characteristic of the photosensitive drum 30 changes due to aging or the like, regardless of the change, the development is performed while eliminating the useless waiting time until the start of copying. It is possible to reliably prevent the agent carrier from being pulled or the developer from dropping.

In the above embodiment, the charging for dark decay detection is performed by the specific output of the charging charger 31.
In addition, while monitoring the detection result of the surface potential sensor 51 or the detection result of another potential detecting means, the output of the charging charger 31 is controlled to set the surface potential of the photosensitive drum 30 to a constant value. May be entered.

In the above embodiment, the operation in the dark decay detection mode is executed when the power of the copying machine is turned on for the first time in a day. If it is executed several times, the dark decay time Tx can be more appropriately captured.

In the above-described embodiment, the predetermined value for determining whether or not dark attenuation has occurred is -400 V. However, the present invention is not limited to -400 V. Toner, photoreceptor drum used,
Various settings can be made according to the environment to be used and other conditions.

In the above-described embodiment, the timer 72 measures the elapsed time T from when the power of the copying machine is cut off during the copying operation. However, after the power of the copying machine is turned on again, the drum thermistor 52 turns on the timer 72. The elapsed time T may be estimated based on the detected temperature of the photosensitive drum 30. Alternatively, the elapsed time T may be estimated based on the temperature of the fixing device (heat roller) 44 detected by the heat roller thermistor 53 after the power of the copying machine is turned on again. In addition, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the invention.

[0071]

As described above, according to the present invention, at the start of image formation, until the end of the uncharged portion on the photosensitive drum reaches the developing means, the developing means has a polarity opposite to that of the normal state. A developing bias voltage is applied, and the surface potential of the photosensitive drum is detected to detect a dark decay time required for the surface potential to attenuate to a predetermined potential. When the power supply is cut off, the elapsed time is measured, and when the power of the copying machine is turned on again, the start of image formation is prohibited until the measured time reaches the dark decay time. Providing a highly reliable image forming apparatus that can prevent excessive toner consumption, reliably prevent developer carrier pulling and developer dropping, and eliminate wasteful waiting time until image formation starts. it can

[Brief description of the drawings]

FIG. 1 is a block diagram of a control circuit according to one embodiment.

FIG. 2 is a sectional view showing the configuration of the embodiment.

FIG. 3 is a time chart for explaining the operation of the embodiment.

FIG. 4 is a view showing the relationship between the surface potential of a photosensitive drum and the output of a surface potential sensor in the embodiment.

FIG. 5 is a view showing dark attenuation characteristics of the photosensitive drum in the embodiment.

FIG. 6 is a flowchart for explaining the operation of the dark decay detection mode of the embodiment.

FIG. 7 is an exemplary flowchart for explaining the operation of the embodiment when the power of the copying machine is turned on again.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Body 2 ... Document table 20 ... Laser exposure unit 30 ... Photoconductor drum 31 ... Transfer charger 32 ... Developing device 33 ... Transfer charger 34 ... Peeling charger 36 ... PCC charger 37 ... Cleaner 38 ... Static electricity removal lamp 44 ... Fixing device 51 ... Surface potential sensor 52: drum thermistor 53: heat roller thermistor 60: control unit

Claims (7)

[Claims] 1. A charging device, an exposure device, and a developing device are sequentially arranged around a photoconductor drum, the photoconductor drum is charged by the charging device, and an electrostatic latent image is formed on the photoconductor drum by the exposure device. An image forming apparatus of a reversal developing type, which forms an electrostatic latent image, develops the electrostatic latent image with a developer while applying a developing bias voltage to a developing unit, and transfers the latent image to an image forming medium; Voltage applying means for applying a bias voltage having a polarity opposite to the developing bias to the developing means until the end of the portion reaches the developing means; potential detecting means for detecting the surface potential of the photosensitive drum; Detecting means for detecting a dark decay time required for the surface potential of the photosensitive drum to attenuate to a predetermined potential from a detection potential of the detection means; and detecting the dark decay time based on the dark decay time detected by the detection means. Te,
An image forming apparatus comprising: a control unit configured to control timing of applying a voltage from the voltage applying unit to the developing unit. 2. A charging device, an exposure device, and a developing device are sequentially arranged around a photosensitive drum, the charging device charges the photosensitive drum, and an electrostatic latent image is formed on the photosensitive drum by the exposure device. An image forming apparatus of a reversal developing system for forming an electrostatic latent image, developing the image by a developer while applying a developing bias voltage to a developing unit, and transferring the image to an image forming medium; Voltage applying means for applying a developing bias voltage having a polarity opposite to that of the developing bias voltage to the developing means until the end of the uncharged portion on the body drum reaches the developing means; and between the charging means and the developing means. A potential detecting means for detecting a surface potential of the photosensitive drum; and detecting a dark decay time required for the surface potential of the photosensitive drum to attenuate to a predetermined potential from the potential detected by the potential detecting means. Detecting means for outputting, when the apparatus power is cut off during the image formation, a time measuring means for measuring an elapsed time from the time, and when the apparatus power is turned on again after the apparatus power is cut off during the image formation, An image forming apparatus, comprising: a control unit that prohibits the start of image formation until the time measured by the clock unit reaches the dark decay time detected by the detection unit. 3. A charging unit, an exposing unit, and a developing unit are sequentially arranged around the photosensitive drum, the charging unit charges the photosensitive drum, and an electrostatic latent image is formed on the photosensitive drum by the exposing unit. An image forming apparatus of a reversal developing system for forming an electrostatic latent image, developing the image by a developer while applying a developing bias voltage to a developing unit, and transferring the image to an image forming medium; A voltage applying unit for applying a bias voltage having a polarity opposite to the developing bias to the developing unit until an end of an uncharged portion on the body drum reaches the developing unit; and a voltage applying unit provided between the charging unit and the developing unit. A potential detecting means for detecting a surface potential of the photosensitive drum; a temperature detecting means for detecting a temperature of the photosensitive drum; a detecting means for detecting a temperature of the temperature detecting means and a detecting potential of the potential detecting means. Detecting means for detecting a dark decay time required for the surface potential of the photosensitive drum to attenuate to a predetermined potential; and a timer for measuring an elapsed time from when the apparatus power supply is cut off during image formation. Means for prohibiting the start of image formation until the time measured by the time measuring means reaches the dark decay time detected by the detecting means when the apparatus power is turned on again after the apparatus power is cut off during image forming. An image forming apparatus comprising: 4. A charging device, an exposing device, and a developing device are sequentially arranged around a photoconductor drum, the photoconductor drum is charged by the charging device, and an electrostatic latent image is formed on the photoconductor drum by the exposure device. An image forming apparatus of a reversal developing system for forming an electrostatic latent image, developing the image by a developer while applying a developing bias voltage to a developing unit, and transferring the image to an image forming medium; Until the end of the uncharged portion on the body drum reaches the developing means, a voltage applying means for applying a bias voltage having a polarity opposite to the developing bias voltage to the developing means, and between the charging means and the developing means. A potential detecting means for detecting a surface potential of the photosensitive drum, a temperature detecting means for detecting a temperature of the photosensitive drum, and charging the photosensitive drum by the charging means during non-image formation. A dark decay detecting means for detecting the potential of the charged area by the potential detecting means and detecting a dark decay time until the detected potential attenuates to a predetermined value, taking into account the temperature detected by the temperature detecting means as a calibration factor. When the power supply of the apparatus is cut off during the image formation, a time measuring means for measuring an elapsed time from the time when the power supply of the apparatus is turned off after the power supply of the apparatus is cut off during the image formation. An image forming apparatus, comprising: control means for prohibiting the start of image formation until the measurement time reaches the dark decay time detected by the dark decay detection means. 5. A charging device, an exposing device, and a developing device are sequentially arranged around a photoconductor drum, the photoconductor drum is charged by the charging device, and an electrostatic latent image is formed on the photoconductor drum by the exposure device. An image forming apparatus of a reversal developing system for forming an electrostatic latent image, developing the image by a developer while applying a developing bias voltage to a developing unit, and transferring the image to an image forming medium; A voltage applying unit for applying a bias voltage having a polarity opposite to the developing bias to the developing unit until an end of an uncharged portion on the body drum reaches the developing unit; and a voltage applying unit provided between the charging unit and the developing unit. A potential detecting means for detecting a surface potential of the photosensitive drum; and a detecting means for detecting a dark decay time required until the surface potential of the photosensitive drum attenuates to a predetermined potential from the potential detected by the potential detecting means. Output means, a temperature detecting means for detecting the temperature of the photosensitive drum, and when the apparatus power is turned on again after the apparatus power is cut off during image formation, the time elapsed since the apparatus power was cut off is referred to as Estimating means for estimating based on the temperature detected by the temperature detecting means; and dark decay in which the estimating time of the estimating means is detected by the detecting means when the apparatus power is turned on again after the apparatus power is cut off during image formation. Control means for prohibiting the start of image formation until the time has elapsed. 6. A charging device, an exposing device, and a developing device are sequentially arranged around the photosensitive drum, and the photosensitive drum is charged with an electrostatic charge by the charging device. At the start of image formation, in a reversal development type image forming apparatus that forms a latent image, visualizes the electrostatic latent image with a developer while applying a developing bias voltage to a developing unit, and transfers the latent image to an image forming medium. A voltage application unit that applies a bias voltage having a polarity opposite to the developing bias voltage to the developing unit until the end of the uncharged portion on the photosensitive drum reaches the developing unit; and the charging unit and the developing unit. And a potential detecting means for detecting a surface potential of the photosensitive drum, and a charging means for charging the photosensitive drum at the time of non-image formation, and detecting a potential of the charged area by the potential detecting means. Dark decay detecting means for detecting and detecting a dark decay time until the detected potential is attenuated to a predetermined value; temperature detecting means for detecting the temperature of the photosensitive drum; Estimating means for estimating the elapsed time since the power of the apparatus was turned off based on the temperature detected by the temperature detecting means when the power of the apparatus was turned on again after the power was turned off, and the apparatus after the power of the apparatus was turned off during the image formation. Control means for prohibiting the start of image formation until the estimated time of the estimating means reaches the dark decay time detected by the dark decay detecting means when power is turned on again. apparatus. 7. A charging unit, an exposing unit, and a developing unit are sequentially arranged around a photosensitive drum, the charging unit charges the photosensitive drum, and an electrostatic latent image is formed on the photosensitive drum by the exposing unit. After the electrostatic latent image is formed, the electrostatic latent image is visualized by a developer while applying a developing bias voltage to a developing unit, and the developed image is transferred to an image forming medium by a transfer unit and then fixed by a fixing unit. In the developing type image forming apparatus, at the start of image formation, a bias voltage having a polarity opposite to the developing bias is applied to the developing unit until the end of the uncharged portion on the photosensitive drum reaches the developing unit. A voltage applying unit, a potential detecting unit provided between the charging unit and the developing unit, for detecting a surface potential of the photosensitive drum, and charging the photosensitive drum by the charging unit during non-image formation. , A dark decay detecting unit that detects a potential of the charged area by the potential detecting unit, and detects a dark decay time until the detected potential attenuates to a predetermined value; a temperature detecting unit that detects a temperature of the fixing unit; Estimating means for estimating an elapsed time since the power of the apparatus is turned off based on the temperature detected by the temperature detecting means when the power of the apparatus is turned on again after the power of the apparatus is turned off during the image forming; Control means for inhibiting the start of image formation until the estimated time of the estimating means reaches the dark decay time detected by the dark decay detecting means when the apparatus power is turned on again after the apparatus power is cut off, An image forming apparatus comprising: