JP2008076947A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus having a developing device capable of discharging deteriorated toner and stably forming images. <P>SOLUTION: The image forming apparatus includes; an image carrier; an exposure means for writing an electrostatic latent image on the image carrier based on an image signal; a developing means for forming a toner image by supplying toner onto the image carrier; a transfer means for transferring the toner image on the image carrier to a recording medium; and a cleaning means for cleaning the image carrier. The developing means has a cylindrical developing roller, a plurality of magnetic poles disposed in the developing roller, and a restricting member for restricting a quantity of conveyance of developer by the developing roller. When a pattern image for discharging toner is formed before or after a regular image area on the image carrier, the angle of at least one of the plurality of magnetic poles is switched to the angle assigned to the formation of a pattern image from that assigned to the formation of a regular image. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that uses an electrostatic transfer process such as an electrophotographic copying machine or a laser printer, and more particularly, to an image forming apparatus having a developing device that has been treated to discard deteriorated (fatigue) toner to improve image quality. Relates to the device.

  If the developer stays in the developing device for a long time, the developer (toner) is fatigued, and the charging characteristics of the toner become unstable, causing image quality defects. Also, toner with low charging characteristics is likely to scatter and cause toner contamination.

  A normal toner has a fluorine-based or silica-based release agent coating on the outer surface of a resin particle. However, the toner stays in the developing device for a long time, and when it is stirred, the coating is worn or An embedding hole (destructive hole) is generated in the toner and becomes a deteriorated toner. A part of the toner is consumed, but most of it stays in the developing device. Such deteriorated toner has a large physical adhesion.

  This phenomenon is likely to occur when image formation with a low printing rate is continued.

  In order to prevent this, as shown in FIG. 3, a pattern image PA or the like for ejection is formed for each predetermined number of prints in the D area before and after the normal image area on the image carrier. It is cleaned by cleaning means and discarded as discharged toner.

  FIG. 3 is a diagram showing a pattern image of the discharged toner formed before and after the normal image region.

As a conventional example, there is a method of detecting the density of a toner image, forming an internal pattern image (a pattern image for discharge) based on the density of the toner image, forcibly discharging the toner in the developing device, and replacing the deteriorated toner. It has been proposed (see, for example, Patent Document 1).
JP-A-9-62083

  However, conventionally, development was performed with a developer conveyance amount under the same conditions as those for normal image formation when creating a pattern image for discharging deteriorated toner. For this reason, at the time of pattern image formation, simultaneously with the deteriorated toner, toner with a low degree of deterioration is also discarded at the same ratio as during normal image formation. Therefore, in order to discard the desired deteriorated toner, there is a disadvantage that the entire discard (discharge) amount must be set.

  An object of the present invention is to provide an image forming apparatus having a developing device that efficiently discharges deteriorated toner and enables stable image formation.

  The above object is also achieved by the following configurations.

  An image carrier, an exposure unit for writing an electrostatic image on the image carrier based on an image signal, a developing unit for supplying a toner to the image carrier to form a toner image, and the image carrier In the image forming apparatus including a transfer unit that transfers the toner image to a recording medium and a cleaning unit that cleans the image carrier, the developing unit is disposed inside the developing roller and a cylindrical developing roller. A plurality of magnetic poles and a regulating member that regulates the conveyance amount of the developer conveyed by the developing roller, and a pattern image for discharging the toner is formed before or after the normal image area on the image carrier. In forming the image, the magnetic angle of at least one of the plurality of magnetic poles is switched from an angle at the time of normal image formation to an angle at the time of pattern image formation.

  When developing a pattern image for toner discharge, the magnetic angle (set angle) of the magnetic pole of the developing roller with respect to the image carrier is switched to a direction in which the conveyance amount increases, so that the rubbing force in the developing unit increases and the developing unit consumes (discards). The ratio of deteriorated toner in the toner to be used increases.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings.

  The description in the present invention does not limit the technical scope of the claims and the meaning of the terms.

  FIG. 1 is a schematic cross-sectional view of an image forming apparatus including a printer according to the present embodiment.

  The printer according to the present embodiment includes a paper feed cassette 21 that stores transfer paper P, a paper feed roller 22 that feeds transfer material P in the paper feed cassette 21, a conveyance path 24, and a photosensitive material that is a transfer material P and an image carrier. A timing roller 23 that synchronizes with the toner image on the body drum 31, an entry guide plate 25 that guides the transfer material P to the transfer region S, and a transfer material P separated from the photosensitive drum 31 after the transfer is conveyed to the fixing device 37. It consists of a conveyor belt 39 and the like.

  The photosensitive drum 31 is rotationally driven in the direction indicated by an arrow by obtaining a rotational force from a drive unit (not shown) according to a command from the control unit B1 serving as control means. Around the photosensitive drum 31, in order from the upstream side, a charging unit 32, an exposure unit 33, a developing unit 34 as a developing unit, a transfer unit 35 as a transfer unit, a separation unit 36, and a cleaning unit as a cleaning unit. A device 38 is provided.

  On the photosensitive drum 31, charging by the charging unit 32 and image exposure by the exposure unit 33 are performed (latent image formation), and a toner image is formed by developing processing by the developing unit 34 (visual image formation). Then, the toner image is transferred onto the transfer material P by the transfer means 35 to which a voltage having a polarity opposite to that of the toner is applied in the transfer region S.

  Thereafter, the transfer material P is separated from the photosensitive drum 31 by the separating means 36. The separated transfer material P is transported to a fixing device 37 by a transport belt 39 provided with an air suction box 391 inside. Thereafter, the transfer material P is heated and pressed by a pair of rollers (a pressure roller 37A and a heating roller 37B) of the fixing device 37, whereby the toner image is fixed on the transfer material P. On the other hand, the photosensitive drum 31 after the transfer is cleaned by the cleaning device 38 and is prepared for the next image formation. The operations of the respective units 31 to 38 constituting these image forming units are controlled by the control unit B1.

  Hereinafter, an embodiment of the developing device according to the present invention will be described with reference to FIG.

  As described above, if the developer stays in the developing device for a long time, the developer (toner) is fatigued due to stress caused by the developing roller 40 or the like, and the charging characteristics of the toner become unstable, causing image quality defects. In particular, when image formation with a low printing rate is continued, the amount of toner discharged is small, and unstable deteriorated toner with poor charging efficiency tends to remain in the developing device. Conventionally, a pattern image for ejection is simply formed before and after the normal image area, but there is a problem that toner with a low degree of deterioration is discarded at the same ratio as in the normal image formation.

  Therefore, if a developing device that selectively consumes toner having a high degree of deterioration is employed, this image defect can be prevented.

  FIG. 2 is an enlarged view of the developing device shown in FIG.

  The developing device employs a two-component porcelain brush method using a developer in which toner and a carrier are mixed. The developing roller 40 rotates with the developer attached to the outer peripheral side, and conveys the developer to the developing unit T that is a portion facing the photosensitive drum 31. A developing electric field as shown in FIG. 6 is formed between the developing roller 40 and the photosensitive drum 31 to control the amount of toner transferred to the photosensitive drum 31 in the developing portion T.

  FIG. 6 is a waveform showing the developing bias applied to the image carrier and the developing roller during normal image formation and pattern image formation.

  In FIG. 6, the development bias is obtained by superimposing a rectangular wave AC component (1000 V {PP}, frequency 5 kHz, duty 50%) on a DC component (400 V). The DC component is determined by the image stabilization control, and a standard value is a DC voltage at which the development potential difference from the image portion potential is about 350V.

  In order to form the developing electric field, a developing bias is applied to the developing roller 40 from the high voltage power supply HV1.

  Further, in the present embodiment, the developer conveyance amount can be controlled by switching the magnetic angle of the fixed magnets arranged on the inner peripheral side of the developing roller 40.

  FIG. 4 shows the magnetic angle setting A of the magnetic poles during normal image formation.

  FIG. 5 shows the magnetic angle setting B of the magnetic pole at the time of pattern image formation.

  4 and 5, the magnetic poles S1, N1, N2, S2, and N3 during normal image formation are set at an angle θ (θ = 10 ° in the present embodiment) upstream from the pattern image formation. )) Is out of phase.

  FIG. 7 is a diagram showing the configuration of the developing roller and the magnetic angle switching mechanism of the magnetic pole.

  In FIG. 7A, the developing roller 40 includes a nonmagnetic metallic rotating sleeve 401 such as aluminum or brass, sleeve support members 402 and 403 fitted on both sides of the rotating sleeve 401, and the rotating sleeve 401. The drive gear 404 and the rotating sleeve 401 are configured by a magnet fixing member 406 as a magnetic pole and the like. The sleeve support members 402 and 403 are supported by a frame A1 (see FIG. 2) of the developing device 34 via a bearing 405, and are rotated by the drive gear 404 integrated with the sleeve support member 402.

  On the other hand, the magnet fixing member 406 is supported by the sleeve supporting members 402 and 403 via a bearing 407, and the magnetic angle switching mechanism 50 is disposed on one end side of the magnet fixing member 406. The magnetic angle switching mechanism 50 has a function of shifting the phase by the angle θ of the magnet fixing member 406.

  The configuration of the magnetic angle switching mechanism 50 will be described below.

  FIG.7 (b) is the figure seen from the X direction of Fig.7 (a).

  In FIG. 7B, the operation plate 501 is attached to the magnet fixing member 406 via an integral hub 506. The operation plate 501 is pulled by a spring 503, and the edge portion e abuts against the stopper 505. The magnetic pole S1 on the magnet fixing member 406 is at the position indicated by the solid line. Further, an operating pin 504 is attached to the operating plate 501, and an eccentric cam 502 that engages with the operating pin 504 is in contact therewith. The eccentric cam 502 is rotated by a predetermined angle via a drive unit (not shown) in response to a command from the CPU in the control unit B1, and when the operation plate 501 is in a state indicated by a dotted line, The phase of the magnetic pole is shifted by an angle θ, and the magnetic angle at the time of normal image formation is switched to the magnetic angle at the time of pattern image formation.

  The magnetic angle switching determination process is performed based on the command route shown in FIG.

  FIG. 8 shows an algorithm for switching the magnetic angle of the magnetic pole.

  In FIG. 8, in step S1, the CPU in the control unit B1 issues an image formation instruction. In step S2, it is determined whether the object to be printed is normal image formation or pattern image formation. In step S3, in the case of normal image formation, the magnetic angle setting A is set via the magnetic angle switching mechanism. In step S2, in the case of the pattern image for ejection, the magnetic angle setting B is set through the magnetic angle switching mechanism. In step 5, an image is formed with a common developing bias.

  Next, a change in the developer conveyance amount by switching the magnetic angle according to the algorithm will be described with reference to FIG.

  FIG. 9 is a diagram illustrating magnetic lines of force and a magnetic brush in the vicinity of the developing unit and the regulating member.

  FIG. 9A shows a state of magnetic lines and magnetic brushes in the vicinity of the developing unit and the regulating member during normal image formation, and FIG. 9B shows a state of magnetic lines and magnetic brushes in the vicinity of the developing unit and the regulating member during pattern image formation. Shows the state.

  In FIG. 9, the developer forms a magnetic brush along the magnetic field lines of the magnetic field on the rotating sleeve 401 and is conveyed.

  During pattern image formation, the entire fixed magnet is rotated in the same direction (counterclockwise) with respect to the developer conveyance direction. By switching the magnetic angle, the magnetic pole S1 closest to the photosensitive drum 31 is closer to the photosensitive drum 31 than during normal image formation. Therefore, the developer is easily rubbed with the photosensitive drum 31 in the developing unit T. As a result, the deteriorated toner having a high physical adhesion force is difficult to separate from the carrier, but the adhesion force to the photosensitive drum 31 is also increased, so the contact time with the photosensitive drum 31 is reduced. By taking a long time, it is easy to move to the photosensitive drum 31, that is, to develop easily. That is, as a result of being easily rubbed, the deteriorated toner is easily developed.

  Generally, a so-called sorting phenomenon occurs in which toner is sorted and developed from a toner having a small physical adhesion.

  Since toner that has deteriorated from the surroundings has a high physical adhesion, it is difficult to separate from the carrier during development and there is a high probability that it will return to the developing device again without being developed. These toners are further subjected to mechanical stress, and further deteriorate and become difficult to be developed.

  In the present invention, it has been found by experiments described later that the sorting phenomenon due to the difference in physical adhesion force is improved by increasing the rubbing state of the developer (with respect to the image carrier) in the developing device.

  As can be seen from FIG. 9B, the magnetic pole S2, which is closest to the regulating member 341 during pattern image formation, is further away from the regulating member 341 than during normal image formation. The magnetic brush acts as a repulsive force when the developer on the upstream side is pushed by the rotation of the developing roller 340.

Accordingly, the magnetic pole S2 moves away from the regulating member 341, the magnetic field lines are weakened, and the repulsive force by the magnetic brush is also weakened. That is, the magnetic brush near the regulating member 341 lies in the tangential direction on the rotating sleeve 401. Therefore, the developer is densely packed and passes through the regulating member 341, and more developer is transported to the developing unit T than in normal image formation (in this embodiment, the transport amount is normal). (300 g / m ² during image formation and 450 g / m ² during pattern image development).

  As the transport amount increases, the density of the developer in the developing section T where the photosensitive drum 31 and the rotating sleeve 401 face each other increases, and as described above, the rubbing force with the image carrier (photosensitive drum) also increases. . Therefore, the deteriorated toner having a high physical adhesive force is difficult to separate from the carrier, but the toner having a higher degree of deterioration has an increased adhesive force with the image carrier, By making the contact time long, it is easy to move to the image carrier, that is, to develop easily.

  That is, as a result of increasing the conveyance amount and facilitating the rubbing, it is possible to easily develop the toner having a high degree of deterioration selectively.

  Note that when the developer transport amount is the same, a sorting phenomenon occurs in the developing section T, in which sorting is performed from those having a small physical adhesion force. Therefore, when the pattern image is formed, the magnetic pole S2 is moved away from the regulating member 341 in order to reduce the magnetic lines of force of the magnetic pole S2 in the vicinity of the regulating member 341.

  Specifically, in the vicinity of the regulating member 341, the magnetic density at the time of forming the pattern image PA can be made lower than that at the time of normal image formation to increase the transport amount of the developer. As a result, the magnetic density in the vicinity of the developing portion T increases and the rubbing force increases.

  At the time of pattern image formation, the effect is large when the conveyance amount is increased 1.1 to 2.0 times that at the time of normal image formation. If it is twice or more, the rubbing force becomes too large, and problems such as fogging and carrier detachment tend to occur.

  As a result, toner with a high degree of deterioration is mainly consumed.

  Hereinafter, the durability test of the comparative example (without magnetic angle switching) and the example (with magnetic angle switching) was performed at a low printing rate using the developing device of the present embodiment.

  The results are shown in Table 1.

Comparative Example 1
When the endurance test was performed under the conditions of normal image formation equivalent to 1% printing rate with the setting not to perform pattern image formation before or after the normal image area (D area), 10K (10,000) sheets At that time, the image arrangement in the halftone portion was outside the allowable range.

  When the surface of the toner in the developing device was observed with an electron microscope, there was a lot of toner that was released from the surface or embedded in the toner, and this caused the fluidity of the toner to decrease. It is considered that the toner having lowered fluidity has a high physical adhesion and is difficult to separate from the carrier and developability is deteriorated. Further, it is considered that the transferability to a transfer material is deteriorated when the physical adhesion is increased. When an image with a low printing rate is formed, the toner stays in the developing device for a long time, and the amount of toner that is subjected to mechanical stress for a long time increases. Further, since the amount of discharged toner is only equivalent to the residual transfer, it is considerably small at 60 g.

Comparative Example 2
In order to promote the replacement of the toner in the developing device, an endurance test was performed under the condition that a pattern image corresponding to a printing rate of 3% was formed in the D area. The potential difference formed between the developing roller and the regulating member when developing the pattern image was set to 50 V (see FIG. 6), which is the same as the image portion. The image quality level was maintained even at the time of 50K sheets and there was almost no change in the charge amount distribution, but the amount of toner consumed by the pattern image increased and the amount of toner discharged was very high at 1620 g. This method is suitable for practical use. Not found out.

Comparative Example 3
As a result of the endurance test under the condition that the toner amount of the pattern image created in the D area is set to be equivalent to a printing rate of 1%, the amount of discharged toner is 680 g, which can be reduced by nearly 1000 g from the comparative example 2 and is practical. However, although the deterioration of the image quality level became milder than that of Comparative Example 1, it did not reach the lifetime of 50K and became a level outside the allowable range at 30K.

  Further, the toner fluidity decreased as the number of image formations progressed.

  Next, an embodiment in which the magnetic angle of the fixed magnet in the developing roller according to the present invention is switched will be described.

Example 1
When developing a pattern image under the conditions of Comparative Example 3, the same durability test was performed while switching the magnetic angle of the fixed magnet inside the developing roller by the algorithm shown in FIG.

  As a result, as shown in Table 1, by switching the magnetic angle of the fixed magnet inside the developing roller, the image quality level could be maintained up to 100K even with a pattern image corresponding to a printing rate of 1%. That is, it can be seen that the toner consumption is suppressed and the deteriorated toner is discharged efficiently.

  In this embodiment, the control is performed by switching the magnetic angle so that two measures can be taken simultaneously: the magnetic pole near the developing portion T is brought close to the photosensitive drum, and the magnetic pole near the regulating member is moved away from the regulating member. However, the same effect can be obtained with either of the two measures. However, if it is rubbed too much, problems such as fogging and carrier detachment tend to occur. For example, when the carry amount is increased to 1.1 to 2.0 times that in normal image formation, the effect is large, and 2.0 times or more is not preferable.

  The same effect can be obtained with any of monochrome / color PPC, printer, fax, and the like as the image forming apparatus used in this embodiment.

1 is a schematic cross-sectional view of an image forming apparatus including a printer according to an embodiment. FIG. 2 is an enlarged view of the developing device shown in FIG. 1. FIG. 3 is a diagram illustrating a pattern image of discharged toner formed before and after a normal image region. An arrangement of magnetic angle setting A of magnetic poles during normal image formation is shown. An arrangement of magnetic angle setting B of the magnetic pole at the time of pattern image formation is shown. 6 is a waveform showing a developing bias applied to the image carrier and the developing roller during normal image formation and pattern image formation. It is a figure which shows the structure of a developing roller, and the magnetic angle switching mechanism of a magnetic pole. This is an algorithm for switching the magnetic angle of the magnetic pole. It is a figure which shows the magnetic force line and magnetic brush in the vicinity of the image development part and a control member.

Explanation of symbols

31 Photosensitive drum 34 Developing device 38 Cleaning device 341 Restricting member 342 Conveying stirring screw 40 Developing roller 406 Magnet fixing member 50 Angle switching mechanism 501 Acting plate 502 Eccentric cam 503 Spring HV1, HV2 High voltage power source PA pattern image

Claims (5)

An image carrier, an exposure unit for writing an electrostatic image on the image carrier based on an image signal, a developing unit for supplying a toner to the image carrier to form a toner image, and the image carrier In the image forming apparatus including a transfer unit that transfers the toner image to a recording medium and a cleaning unit that cleans the image carrier, the developing unit is disposed inside the developing roller and a cylindrical developing roller. A plurality of magnetic poles and a regulating member that regulates the conveyance amount of the developer conveyed by the developing roller, and a pattern image for discharging the toner is formed before or after the normal image area on the image carrier. In forming the image, the magnetic angle of at least one of the plurality of magnetic poles is switched from an angle at the time of normal image formation to an angle at the time of pattern image formation. The image forming apparatus according to claim 1, wherein a magnet closest to the image carrier among the plurality of magnetic poles is switched in a direction approaching the image carrier during the pattern image formation. The image forming apparatus according to claim 1, wherein the direction in which the angle is switched during the pattern image formation is a direction in which a developer conveyance amount increases. The image forming apparatus according to claim 3, wherein when the pattern image is formed, the amount of developer transported on the developing roller is 1.1 to 2.0 times that during normal image formation. The image forming apparatus according to claim 1, wherein a magnetic pole closest to the restriction member among the plurality of magnetic poles is switched in a direction away from the restriction member during the pattern image formation.

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