JP5336753B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to a scorotron charger for uniformly charging a surface of an electrostatic latent image carrier used in an electrophotographic image forming apparatus, and an image forming apparatus using the charger.

  In recent years, the printing speed of an image forming apparatus such as an electrophotographic system has been increased, and a charger that can handle a higher process speed, that is, a printing speed, is required.

  As a charger for charging the surface of the latent electrostatic image bearing member, a contact charging method in which charging is performed by a roller, a brush, or the like that comes into contact with the latent electrostatic image bearing member has been put into practical use, and is characterized by being small and inexpensive. Widely used in low-speed image forming apparatuses. However, the contact charging method is difficult to adapt to a high process speed, and at a high process speed (about 400 mm / s or more), a corotron charging method using a corona discharge or a charger using a scorotron charging method is still used. There are many.

  In particular, a charger for charging the photosensitive drum (electrostatic latent image carrier) uniformly (often referred to as a main charger, a primary charger, a main charger, etc.) has a grid electrode and has a charged potential. It is common to use a scorotron charging system charger (scorotron charger) that is excellent in stability, uniformity and controllability.

  When using a scorotron charger to handle a high process speed, in order to increase the charging time, it is necessary to increase the charging width as viewed in the direction of movement of the electrostatic latent image carrier and at the same time increase the corona discharge current. We often take measures such as increasing the number of wires. However, since there are exposure means, developing devices, cleaning means, etc. in addition to the charger around the electrostatic latent image carrier, there is a limit to increasing the size of the charger. There is a problem that the image forming apparatus is also enlarged.

  In addition, by reducing the grid aperture ratio without increasing the size of the charger, an average potential can be obtained, which may correspond to a high process speed, but the stability, uniformity, and control of the charging potential The properties are inferior, which is not preferable.

  As described above, there is a demand for a small and compact scorotron charger that can cope with a high process speed.

  To solve such a problem, as described in Patent Document 1, Patent Document 2, etc., the aperture ratio of the grid of the scorotron charger is changed as seen in the moving direction of the electrostatic latent image carrier, and the charger is made large-sized. There is a conventional technique in which the stability and uniformity of the charged potential is ensured and the high process speed can be accommodated without making it easy. The basic idea of this method is to supply a large amount of corona to the photosensitive drum at a portion with a large aperture ratio to cope with a high speed. On the other hand, at a portion with a small aperture ratio, that is, a portion with high charge controllability, The idea is to ensure the stability and uniformity of

  However, this conventional technique requires a mesh grid electrode with a changed aperture ratio, and the mesh grid electrode itself is expensive, and the mesh grid has a direction to be attached. There is a problem that there is difficulty in grid interchangeability. In addition, in image forming apparatuses, it is often performed to use a charger or the like for the purpose of sharing parts even between models having different process speeds. The optimum conditions differ depending on the characteristics of the photosensitive drum to be used, and it is necessary to prepare a different mesh grid for each model, resulting in poor compatibility.

  On the other hand, the scorotron charging method has the following problems.

  In the scorotron charging system, the stability and uniformity of the surface of the photosensitive drum to be charged are realized by grid electrodes that control the corona supplied to the surface of the photosensitive drum. Therefore, when it is desired to obtain a desired photosensitive drum charging potential (target potential), the voltage applied to the grid electrode (grid voltage) is also set to be substantially the same as the target potential (about 0.9 to 1.1 times). It is normal.

  The grid voltage is often applied to the grid electrode by using a high-voltage power supply (grid power supply) dedicated to the grid electrode or by grounding the grid electrode via a varistor. When a varistor is used, the varistor value of the varistor is often set to be substantially the same as the above target potential.

  In an electrophotographic image forming apparatus, it may be necessary to change the charged potential of the photosensitive drum in order to adjust the print density or white background stain. In some cases, the charging potential of the photosensitive drum is monitored by a surface potential sensor or the like, and feedback control is performed based on the sensor output so that the charging potential is always constant.

  When a grid power supply is used, there is an advantage that the charging potential can be changed by changing the output voltage of the grid power supply, and the charging potential can be changed relatively easily. However, a dedicated grid power source is required, and there are problems such as an increase in cost and an increase in the size of the device for the accommodation space. On the other hand, when using a varistor, there is a merit that a grid power supply is not necessary, but it is difficult to make the charging potential variable, for example, it is necessary to prepare multiple varistors with different varistor values in order to change the varistor value. There is.

Japanese Patent Laid-Open No. 10-274873 Japanese Patent Laid-Open No. 4-251876

Therefore, the problem to be solved by the present invention is that the image forming apparatus does not increase in size, and it is not necessary to use a mesh grid in which the aperture ratio is changed which is expensive and inferior in assemblability, interchangeability, and compatibility. Another object of the present invention is to provide an image forming apparatus having a small and compact charger capable of adapting to the process speed and easily changing the charging potential and having excellent controllability.

In order to solve the above-described problems, in the present invention, an electrophotographic image forming apparatus having a process speed of 400 (mm / s) or more , which includes only one discharge wire to which a high voltage is applied, the discharge wire A shield plate having an opening on the side facing the electrostatic latent image carrier, and a grid electrode disposed in the opening, at least a grid voltage applied to the grid electrode being Vg (V), When the target potential of the electrostatic latent image carrier is Vd (V), 1.2 <Vg / Vd <1.4, and the distance between the discharge wire and the surface of the electrostatic latent image carrier is Charger for uniformly charging the surface of the electrostatic latent image carrier , which is 9 mm or more, and surface potential measurement for measuring the surface potential of the surface of the electrostatic latent image carrier after charging by the charger Means, applying a high voltage to the discharge wire High-voltage power supply, based on the measurement result of the surface potential measuring means, wherein further comprising an image forming apparatus an output current control means for controlling the output current to the discharge wire high-voltage power supply is provided.

In a preferred aspect of the present invention, there is provided an image forming apparatus in which the grid electrode of the charger is grounded via a varistor using the grid voltage Vg as a varistor voltage .

In a further preferred aspect of the present invention, there is provided an image forming apparatus wherein the electrostatic latent image carrier is negatively charged and the charger is negatively discharged .

  As described above, in the scorotron charger, the stability, uniformity, and controllability of the charging potential are achieved by the grid electrode to which the grid voltage is applied. Therefore, in the prior art, the target potential grid potential of the photosensitive drum is set to be substantially the same.

  However, as a result of the study by the present inventors, even if the grid potential and the target potential are not necessarily the same, by optimizing the number of discharge wires and the distance from the photosensitive drum, the stability and uniformity of the charging potential, It became clear that controllability can be obtained without problems in practice.

According to the image forming apparatus of the present invention, when the grid voltage applied to the grid electrode is Vg (V) and the target potential of the electrostatic latent image carrier is Vd (V), 1.2 <Vg / Vd < By changing the distance between the discharge wire and the surface of the electrostatic latent image carrier to 9 (mm) or more, the aperture ratio which is expensive and inferior in assemblability, interchangeability and compatibility is changed. An image forming apparatus having a small and compact charger that can cope with a high process speed without using a mesh grid and that can easily change the charging potential and has excellent controllability can be obtained. it can.

  Hereinafter, an embodiment of the charger of the present invention and an image forming apparatus having the same will be described with reference to FIG.

  An electrophotographic printer 1 as an image forming apparatus includes a main charger 3, an LED print head 4, a developing unit 5, a transfer charger 6 as a transfer member, and a separation charger 7 around a photosensitive drum 2 as an electrostatic latent image carrier. A pre-cleaning charger 8, a carrier recovery unit 9, a cleaning unit 10 and a charge removal lamp 11 are arranged, and a fixing unit 20 as a fixing unit is arranged on the downstream side of the photosensitive drum 2 in the conveyance path through which the recording medium MRC is conveyed. . The main charger 3 uniformly charges the surface of the photosensitive drum 2.

  On the upstream side of the photosensitive drum 2 in the conveying path of the recording medium MRC, a guide plate 13 that is a conveying means for the recording medium MRC and a tractor 14 that is also a conveying means are opposed to the fixing unit 20 on the downstream side of the photosensitive drum 2. A transport unit 15 that is another transport means is provided at each position.

  The photosensitive drum 2 is rotationally driven in the direction of arrow A in the figure by a drive motor and a speed reduction mechanism (not shown). The peripheral speed of the photosensitive drum 2, that is, the process speed, is set to 423.3333 mm / s.

  Here, in the electrophotographic printer 1, the recording medium MRC uses continuous paper having feed holes (not shown) at both ends thereof. As shown in the figure, the recording medium MRC is introduced from the right inlet 1b of the electrophotographic printer 1 and discharged from the left outlet 1c. The tractor 14 is driven by a drive motor (not shown) and conveys the recording medium MRC in the direction of arrow B in the figure.

  The photosensitive drum 2 is formed by forming a photosensitive layer made of a negatively charged organic photosensitive material on the surface of an aluminum cylindrical drum having a diameter of 120 mm, and an electrostatic latent image is formed on the surface by light emitted from the LED print head 4. Is done. Prior to the formation of the latent image on the photosensitive drum 2 by the LED print head 4, the main charger 3 uniformly charges the surface of the photosensitive drum 2 with ions generated by discharge.

  A surface potential sensor 27 serving as a surface potential measuring unit is provided between the main charger 3 and the next LED print head 4 to measure the surface potential of the photosensitive drum 2 charged by the main charger 3.

  The LED print head 4 exposes the surface of the photosensitive drum 2 with a light beam to form an electrostatic latent image corresponding to the recorded information. The LED print head 4 has a resolution of 600 dpi (dot per inch, 1 inch = 2600 dots per 25.4 mm).

  The developing unit 5 is opposed to the photosensitive drum 2 and uses a two-component developer (not shown) in which toner and a carrier are mixed so that the toner density is 6% by weight. Develop the electrostatic latent image above. The toner is a minus toner for flash fixing mainly composed of a polyester resin having a weight average particle diameter of 9.0 μm, and the carrier is a silicone-coated ferrite carrier having a weight average particle diameter of 72.5 μm.

  The transfer charger 6 transfers the unfixed toner image developed on the photosensitive drum 2 to the recording medium MRC using ions generated by the discharge. The separation charger 7 is a charger that electrically separates the recording medium MRC from the photosensitive drum 2. For example, a corotron charger to which an alternating voltage is applied is used. The transfer charger 6 and the separation charger 7 are integrally configured with a common shield plate member S.

  A separation claw 12 for mechanically separating the recording medium MRC from the photosensitive drum 2 is provided between the separation charger 7 and the pre-cleaning charger 8 as viewed along the outer periphery of the photosensitive drum 2.

  In this embodiment, in addition to the main charger 3 and the transfer charger 6, the pre-cleaning charger 8 is charged. The pre-cleaning charger 8 charges the surface of the photosensitive drum 2 prior to cleaning, The toner particles remaining on the surface of the photosensitive drum 2 without being transferred to the recording medium MRC are used for the purpose of facilitating recovery by the cleaning unit 10.

  The carrier recovery unit 9 is provided between the transfer charger 6 and the cleaning unit 10 when viewed in the rotation direction of the photosensitive drum 2, and is not transferred to the recording medium MRC and remains on the surface of the photosensitive drum 2. The carrier scraped off from the surface of the photosensitive drum 2 due to the brush roller 10b described later is collected. The carrier recovery unit 9 has a recovery roller 9a, which is a magnetic roller, and recovers the carrier particles by magnetically attracting them.

  The cleaning unit 10 mainly removes toner particles remaining on the surface of the photosensitive drum 2 without being transferred to the recording medium MRC. A brush roller 10b and a recovery roller 10c are provided in the housing 10a. The brush roller 10b is applied with a bias voltage having a polarity opposite to that of the toner particles, scrapes the toner particles remaining on the surface of the photosensitive drum 2 with a brush, and electrostatically adsorbs the toner particles for cleaning. The recovery roller 10c is applied with a bias voltage having an absolute value greater than that of the brush roller 10b, and electrostatically recovers the toner particles adsorbed on the brush roller 10b. The collected toner particles are scraped off from the collection roller 10c by a blade (not shown) and transferred to a collection unit (not shown) provided in one direction on both sides of the paper.

  The static elimination lamp 11 is composed of a plurality of small light bulbs, and removes residual charges from the surface of the photosensitive drum 2 by light irradiation. A light emitting element such as an LED can be used instead of the light bulb.

  The transport unit 15 has a structure in which a transport belt 15b is stretched between two transport rollers 15a, and the transport roller 15a is rotationally driven in the direction of the arrow in the figure by a drive motor (not shown). The recording medium MRC placed on the is transported.

  A scarfola pair 16 exists behind the transport unit 15. Of the two rollers constituting the pair of scat flora 16, only the roller positioned below the recording medium MRC is rotationally driven by a drive motor (not shown). The other roller is urged downward by its own weight with the recording medium MRC interposed therebetween, and is driven to rotate.

  As shown in FIG. 1, in the electrophotographic printer 1, the toner image is fixed by the fixing unit 20 from the transfer position where the unfixed toner image developed on the photosensitive drum 2 is transferred by the transfer charger 6. The conveyance path through which the recording medium MRC is conveyed, including the period up to the fixing position, is configured to be substantially linear and substantially horizontal. In this way, if the conveyance path of the recording medium MRC is substantially linear, the recording medium is not bent when conveying the recording medium, and so-called special media such as cardboard, thin paper, synthetic paper, and resin film medium can be conveyed. It can be easy.

  In the fixing unit 20, two fixing lamps 21 serving as a radiant heat source, a reflection plate 22, and a heat radiation fin 23 for cooling the reflection plate 22 are provided in the housing 20 a, and a protection is provided on the front surface of the housing 20 a facing the conveyance unit 15. Glass 24 is arranged. A suction duct 26 for sucking air is provided on the downstream side of the fixing unit 20.

  For example, a xenon lamp or a halogen lamp is used as the fixing lamp 21. The reflecting plate 22 is formed by processing a metal such as aluminum having a high reflectance on the inner surface thereof. However, if the reflection plate 22 is a metal, it is difficult to be formed into an arbitrary shape because the metal plate has a low degree of freedom in shape and becomes heavy. For this reason, the reflection plate 22 may be formed of a heat-resistant resin such as polyimide deposited with metal. For example, a polyimide film having a thickness of about 0.1 to 0.5 mm is integrally formed into a desired reflector shape by vacuum forming, and a metal such as aluminum, silver, or gold is vapor-deposited on the inner surface. Good.

  The suction duct 26 is arranged in the front and back direction of the electrophotographic printer 1 in FIG. 1, has an opening on the transport unit 15 side, and is provided with suction means such as a filter for deodorization and smoke removal and a blower. Yes. The suction duct 26 sucks air around the fixing unit 20 and discharges it to the outside of the electrophotographic printer 1, so that unpleasant odors and smoke of a toner component mainly generated by fixing of an unfixed toner image in the fixing unit 20 are generated. Remove.

The electrophotographic printer 1 is configured as described above, and performs a desired print by fixing an unfixed toner image on the recording medium MRC by the electrophotographic method described below.
First, the surface of the photosensitive drum 2 that has been cleaned by the cleaning unit 10 and rotated at a constant speed in the direction of the arrow A is charged by the main charger 3 after the residual charge on the surface is removed by the charge eliminating lamp 11.

  Next, the LED print head 4 irradiates the photosensitive drum 2 with a light beam to form an electrostatic latent image corresponding to the recording information on the surface of the photosensitive drum 2. The electrostatic latent image is developed by the developing unit 5 by the rotation of the photosensitive drum 2 and becomes an unfixed toner image (visible image).

  The unfixed toner image developed on the photosensitive drum 2 is transferred by the transfer charger 6 onto the recording medium MRC conveyed along the conveyance path indicated by the arrow B. The recording medium MRC to which the unfixed toner image is transferred is separated from the photosensitive drum 2 by the separation charger 7 and conveyed to the fixing unit 20 by the conveyance unit 15. In the fixing unit 20, the fixing lamp 21 emits light, and the unfixed toner image is melted by the generated heat and fixed on the recording medium MRC.

  In the electrophotographic printer 1, as described above, an image corresponding to the recording information is formed on the recording medium MRC, charged by the pre-cleaning charger 8, the carrier recovery by the carrier recovery unit 9, and the cleaning unit 10. After the surface cleaning of the photosensitive drum 2, the residual latent image on the photosensitive drum 2 is erased by the charge eliminating lamp 11 and is used for the next subsequent image formation.

  Next, the main charger 3 will be described in detail with reference to FIG.

  FIG. 2 is a cross-sectional configuration diagram of the main charger 3. The discharge wire 31 is a gold-plated tungsten wire having a diameter of 80 μm, and is stretched between V grooves 35 provided in the end blocks 34 at both ends of the shield plate 32. The shield plate 32 is obtained by bending a stainless steel plate having a thickness of 0.8 mm, and the grid electrode 33 is a so-called mesh obtained by processing the opening pattern shown in FIG. 3 on a stainless sheet having a thickness of 0.1 mm by electrolytic etching. It is a grid. The grid electrode 33 is also supported and stretched on the end block 34.

  FIG. 4 is a schematic diagram showing the operation of the main charger 3 and the charging potential control method. The discharge wire 31 is connected to a high-voltage power supply 30 with variable output. The grid electrode 33 is grounded via a varistor 36 having a varistor voltage of 810V. Hereinafter, a varistor having a varistor voltage of 810 V is simply referred to as an 810 V varistor.

  A surface potential sensor 27, which is a surface potential measuring means, is provided on the downstream side of the main charger 3 as viewed in the rotation direction of the photosensitive drum 2, and measures the surface potential of the photosensitive drum 2. The output of the surface potential sensor 27 is fed back to the output control unit 42 (output current control means) of the high-voltage power supply 30 and the output current of the high-voltage power supply 30 is controlled according to the output of the surface potential sensor 27.

  The target potential of the photosensitive drum 2 is 680 V as a standard, and the basic operation of the output control unit 42 is to increase the output current of the high-voltage power supply 30 when the output of the surface potential sensor 27 is lower than the target potential. Conversely, when the voltage is higher than the target potential, the high-voltage power supply 30 is controlled so as to decrease the output current, and the surface potential of the photosensitive drum 2 is kept constant near the target potential. In this embodiment, the grid electrode 33 is grounded via the varistor 36. However, even if a 810V high-voltage power supply (grid power supply) is connected in place of the varistor 36, it functions as the main charger 3. The same is true.

  FIG. 5 is a diagram showing the discharge characteristics of the main charger 3. The horizontal axis represents the discharge voltage of the discharge wire, the vertical axis represents the discharge current (mA) on the left side, and the photosensitive drum surface potential (V) on the right side.

  FIG. 6 shows the discharge characteristics of the charger 3 when a DC high-voltage power supply set to 810 V is connected instead of the 810 V varistor, but no difference is seen in the discharge characteristics when using the 810 V varistor. From this result, in this specification, grounding the grid via a varistor having a varistor value of the grid voltage Vg is regarded as equivalent to applying the grid voltage Vg to the grid using a DC high-voltage power supply.

  The target potential is 680 V by default, but as described above, it is necessary to make the surface potential variable in the vicinity in order to adjust processes such as printing density and fogging.

  Next, FIG. 7 is a cross-sectional configuration diagram of the main charger 40 based on the prior art (conventional example). In addition, the same code | symbol is provided with respect to the same member as FIG. Compared to FIG. 2, the major differences are that there are two discharge wires 31 and that the distance between the discharge wires 31 and the surface of the photosensitive drum 2 is closer to 7 mm, which is not shown. Are the three points that the grid is grounded via a 680V varistor.

  FIG. 8 shows a discharge characteristic diagram of the main charger 40 according to the prior art. A point indicated by x in the figure indicates the occurrence of leakage, and indicates that operation at a discharge voltage higher than the discharge voltage (leak generation voltage) is impossible.

  The present invention will be further described while comparing FIG. 5 and FIG.

First, in the main charger 40 (FIG. 8) according to the prior art, the grid is grounded through a 680V varistor which is almost the same as the standard target potential 680V. For this reason, as shown in the discharge characteristic diagram of FIG. 8, the surface potential has almost no inclination with respect to the discharge voltage in the vicinity of 600 to 680V. This can be said to operate ideally as a scorotron charger, but the photosensitive drum surface potential cannot be made variable for process adjustment.
In addition, a target potential of 680 V cannot be realized because leakage has occurred at 680 V before that.

  Hereinafter, the process of completing the present invention will be further described along with the intermediate aspects thereof.

  First, in the configuration of the prior art shown in FIG. 7, the discharge characteristics when the varistor value is 810 V are shown in FIG. 9 (Comparative Example 1). It can be seen that the surface potential has a slope with respect to the discharge voltage (discharge current) near the target potential of 680 V, and the surface potential can be easily controlled by the discharge voltage (discharge current). This is because the grid voltage is discharged at a surface potential lower than that of 810 V, and thus the grid electrode is included, but a part of the discharged corona is considered to behave like a corotron charger.

  In FIG. 9, the surface potential can be controlled by the discharge voltage (discharge current), but the discharge uniformity is not acceptable. FIG. 10A shows a 20% halftone image printed at a resolution of 600 dpi, and shading (printing unevenness) can be confirmed at random intervals of several mm in the width direction of the process. This is because the potential controllability by the grid electrode 33 has been reduced, and potential unevenness has occurred in the surface potential after charging.

  In this embodiment, a reversal development method is used in which toner is developed on the exposed portion by the LED head 4. However, if there is a potential unevenness more than allowable in the surface potential, exposure of one dot constituting a halftone is caused by the potential unevenness. The post-potential and the post-exposure dot shape change slightly depending on the potential unevenness. For this reason, the dot shape and dot size after development change slightly depending on the potential unevenness, and are visually recognized as streak-like unevenness. Of course, when the potential is extremely low, the portion is developed and becomes a clear black stripe.

  Next, FIG. 11 (Comparative Example 2) is a further improvement from FIG. 9, and the discharge wire 31 is moved away from the surface of the photosensitive drum 2 (7 mm is changed to 10 mm). Note that the distance between the discharge wire 31 and the surface of the photosensitive drum 2 is that the photosensitive drum 2 is on a cylindrical shape having a diameter of 120 mm, but the charger is located immediately above the discharge wire 31 and the uppermost portion of the photosensitive drum 2. The vertical distance of

  In FIG. 11, compared with FIG. 9, the discharge voltage is shifted in the higher direction. Further, although the printing unevenness is still not at an allowable level, as shown in FIG. 10B, it is greatly improved as compared with the configuration of FIG.

  This is considered because the influence of the discharge unevenness from the discharge wire 31 is reduced because the discharge wire 31 is far from the photosensitive drum 2. This embodiment is a minus process using a negatively charged photosensitive drum 2, and the main charger is also negatively discharged. As described in the reference ("Basics and Applications of Electrophotography", edited by Electrophotographic Society, Corona, 1988, p49), it is generally said that negative discharge is inferior in discharge uniformity in the longitudinal direction of the discharge wire compared to positive discharge. ing.

  This is because, as described in the same document, in the negative discharge, uniform discharge (charge discharge) is not performed as viewed in the longitudinal direction of the discharge wire, and as shown in FIG. It is considered that bright spots arranged at regular intervals are generated, and discharge charges (corona) are emitted radially from the bright spots toward the surface of the photosensitive drum 2. For this reason, as conceptually shown in FIG. 12B, it is considered that when the position of the discharge wire is too close, the charge density decreases near the seam of the radial discharge charges and the surface potential decreases.

  The discharge unevenness interval on the print shown in FIG. 10 also substantially coincides with this bright spot interval. In the state of FIG. 9, the grid voltage is set to 810 V and the controllability of the grid electrode is lowered, so that it is considered that the grid cannot guarantee the discharge unevenness due to the bright spot discharge. This is considered to be an effect of reducing the potential unevenness on the surface of the photosensitive drum 2 as shown in FIG. 12B by moving the discharge wire 31 away from the surface of the photosensitive drum 2.

  Furthermore, in order to reduce discharge unevenness, the configuration of the main charger 3 (Example 1) in this embodiment is such that one discharge wire 31 is used. When FIG. 11 (Comparative Example 2) is compared with FIG. 5 (Example 1), the discharge voltage is shifted in a higher direction. Further attention should be paid to the amount of discharge current. The amount of discharge current near the standard target potential of 680 V is 3 to 3.5 mA for both, which is not much different. However, in FIG. 11, there are two discharge wires 31, and the discharge current amount per one is seen to be half (1.5 to 1.75 mA).

In other words, it is considered that the use of one discharge wire 31 increases the amount of discharge current per discharge wire and suppresses the aforementioned discharge unevenness. In general, in negative discharge, the discharge unevenness tends to decrease as the discharge voltage increases (that is, the discharge current increases).
In this configuration, as shown in FIG. 10C, the printing unevenness has no practical problem.

  FIG. 13 summarizes the printing unevenness and leakage occurrence state when only one discharge wire 31 is used. Underlined conditions in the figure were able to be printed experimentally, but they were difficult to actually use because of the occurrence of leaks. As a result, when the distance between the discharge wire 31 and the photosensitive drum 2 is 9 mm or more, the grid voltage Vg is in the range from 810 V to 950 V, that is, Vg / Vd is approximately 1.2 to 1.4, no leakage occurs and printing unevenness is also caused. It is possible to print at a satisfactory level.

  As a result of the above examination, even with a configuration using an inexpensive varistor, a main charger in which the surface potential of the photosensitive drum 2 can be easily changed and printing unevenness is within an allowable range can be realized.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to the present invention. It is a figure which shows the cross-sectional structure of the main charger by this invention. It is a figure which shows a mesh grid. It is a figure which shows schematic structure of the charging device and surface potential control in connection with this invention. It is a discharge characteristic figure of the main charger by the present invention. It is a discharge characteristic figure of the main charger by the present invention. It is a figure which shows the cross-sectional structure of the main charger by a prior art. It is a discharge characteristic figure of the main charger by a prior art. It is a discharge characteristic figure of the main charger of a comparative example. It is a figure of the halftone image by the discharge nonuniformity of a charger. It is a discharge characteristic figure of the main charger of a comparative example. It is a conceptual diagram of discharge unevenness It is the figure which summarized the printing nonuniformity and leak generation | occurrence | production state in one discharge wire.

Explanation of symbols

1. Electrophotographic printer 1b. Inlet 1c. Outlets 1e, 1f. Stay 1g, 1h. Shield plate 2. 2. Photosensitive drum Main charger 4. LED print head 5. Development unit 5c. Developing roll 5d. Magnet 6. Transfer charger 7. Separation charger 8. 8. Pre-cleaning charger Carrier recovery unit 10. Cleaning unit 10b. Brush roller 11. Static elimination lamp 12. Separation claw 13. Guide plate 14. Tractor 10b. Brush roller 14. Tractor 14a. Tractor pin 14b. Tractor belt 15. Transport unit 15a. Conveying roller 15b. Conveying belt 16. Scaffa flora vs. 20. Fixing unit 21. Flash lamp 22. Reflector 23. Radiating fin 24. Protective glass 26. Suction duct 27. Surface potential sensor 30. High voltage power supply 31. Discharge wire 32. Shield plate 33. Grid electrode 34. End block 35. V groove 36. Varistor 40. Main charger 41. End block 42. Output control unit

Claims (3)

An electrophotographic image forming apparatus having a process speed of 400 (mm / s) or more, wherein only one discharge wire to which a high voltage is applied, and the side surrounding the discharge wire and facing the electrostatic latent image carrier At least a grid electrode disposed in the opening, a grid voltage applied to the grid electrode is Vg (V), and a target potential of the electrostatic latent image carrier is Vd (V ) and was at the time, 1.2 <a Vg / Vd <1.4, and the distance between the discharge wire and the surface of the latent electrostatic image bearing member is the 9 (mm) or more, the electrostatic latent image A charger for uniformly charging the surface of the carrier, surface potential measuring means for measuring the surface potential of the surface of the electrostatic latent image carrier after charging by the charger, and a high voltage for applying a high voltage to the discharge wire Measurement result of power source, surface potential measuring means Based, the image forming apparatus characterized by further having an output current control means for controlling the output current to the discharge wire of the high voltage power supply.
2. The image forming apparatus according to claim 1, wherein the grid electrode of the charger is grounded via a varistor using the grid voltage Vg as a varistor voltage .
3. The image forming apparatus according to claim 1, wherein the electrostatic latent image carrier is negatively charged and the charger is negatively discharged . 4.

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