Image formation device

Abstract

To suppress image defects in an image formation device providing a potential between a supply roller and a developing roller.SOLUTION: An image formation device comprises: an image carrier; a developer carrier; a supply member that supplies developer to the developer carrier; a restriction member that restricts a layer thickness of the developer carried in the developer carrier; an application unit that applies bias to the developer carrier, supply member and restriction member respectively; and a control unit that controls the application of the bias by the application unit. The control unit is configured to: in a first period, control the application of the bias so that electrostatic power in a direction toward the developer carrier from the supply member acts with respect to the developer charged to a normal polarity acts; and, in a second period, control the application of the bias so that electrostatic power in a direction toward the supply member from the developer carrier acts with respect to the developer charged to the normal polarity, in which a polarity of a restriction bias to be applied to the restriction member is the normal polarity, and an absolute value of the restriction bias is greater than that of the restriction bias to be applied to the restriction member in the first period.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] In an image forming apparatus in which an electrostatic latent image formed on a photosensitive drum is developed by a toner carried by a developing roller, residual toner may remain on the developing roller without being used for development. The residual toner is peeled off from the developing roller by mechanical rubbing with the supply roller at the contact portion with the supply roller, but fine toner of the residual toner may not return to the supply roller by mechanical rubbing and remain on the developing roller. In this case, the toner supplied from the supply roller is difficult to coat on the surface of the developing roller, and the amount of toner carried by the developing roller may become uneven or insufficient, resulting in a poor image with low density. As a countermeasure against this, it is considered to control the supply bias and development bias applied to the supply roller and the developing roller, respectively, so that an electrostatic force acts on the residual toner in a direction from the developing roller to the supply roller. Specifically, when the normal polarity of the toner is negative, the supply bias is made higher in potential than the development bias (the absolute value of the supply bias is made smaller than the absolute value of the development bias). By performing such bias control (called stripping bias control), the fine toner can be stripped off from the surface of the developing roller.

[0003] Patent document 1 describes that when forming images continuously on multiple recording materials, peeling bias control is performed during the period from the end of image formation on one recording material to the start of image formation on the next recording material (called the paper interval). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-175993 A Summary of the Invention [Problem to be solved by the invention]

[0005] There is an image forming apparatus equipped with a regulating blade that contacts the surface of a developing roller and regulates the thickness of the toner layer on the surface of the developing roller. The toner coat on the surface of the developing roller in the region downstream of the contact position with the supply roller and upstream of the contact position with the regulating blade in the rotation direction of the developing roller is called a precoat. When performing peeling bias control between sheets of paper in continuous image formation, the precoat may not be formed sufficiently. When the precoat is insufficient and toner charged with the opposite polarity (positive toner when the normal polarity is negative, also called positive toner) is generated on the surface of the developing roller due to toner deterioration or the like, the positive toner may melt to the tip of the regulating blade. When the toner melts to the tip of the regulating blade, the toner is scraped off from the surface of the developing roller, which may cause image defects in which streaks appear on the formed image (called development streaks).

[0006] The present invention has been made in view of this problem, and has an object to provide an image forming apparatus capable of suppressing image defects in a configuration in which a potential difference is provided between a supply roller and a developing roller. [Means for solving the problem]

[0007] The present invention provides an image forming apparatus for forming an image on a recording material by using a developer charged to a normal polarity, an image carrier on which an electrostatic latent image is formed; a developer carrier that develops the electrostatic latent image formed on the image carrier with the developer; a supply member for supplying the developer to the developer carrier; a regulating member that regulates a layer thickness of the developer carried on the developer carrier; an application unit that applies a bias to each of the developer carrier, the supply member, and the regulating member; A control unit that controls application of the bias by the application unit; Equipped with The control unit is during a first period, application of the bias is controlled so that an electrostatic force acts on the developer charged to the normal polarity in a direction from the supply member toward the developer carrier; The image forming device is characterized in that, during a second period, an electrostatic force acts on the developer charged to the normal polarity in a direction from the developer carrier to the supply member, and the polarity of a regulating bias applied to the regulating member is the normal polarity, and the application of the bias is controlled so that the absolute value of the regulating bias is greater than the absolute value of the regulating bias applied to the regulating member in the first period. Effect of the Invention

[0008] According to the image forming apparatus of the present invention, in a configuration in which a potential difference is provided between the supply roller and the developing roller, image defects can be suppressed. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment. [Diagram 2] FIG. 2 is a schematic cross-sectional view of a process cartridge in the embodiment. [Diagram 3] FIG. 2 is a schematic cross-sectional view of a developing device in the embodiment. [Figure 4] 5 is a diagram showing the toner charge distribution depending on each bias. [Diagram 5] 4 is a timing chart of bias control in the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] An embodiment of an image forming apparatus according to the present invention will be described in detail below with reference to the drawings. The following description is not intended to limit the scope of the present invention. Unless otherwise specified, the dimensions, materials, shapes, control values, etc. in the following description are shown as examples for implementing the present invention.

[0011] (Overall configuration and operation of the image forming apparatus) 1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment of the present invention. The image forming apparatus 100 of the first embodiment is a laser beam printer that forms images using an electrophotographic method. The image forming apparatus 100 employs a cartridge system, and a process cartridge 120 is detachably attached to an apparatus main body 110.

[0012] The image forming apparatus 100 is connected to an external host device such as a personal computer or an image reading device, receives image information from the host device, and forms and outputs (prints) an image corresponding to the image information on a recording material (recording medium, transfer material). A sheet material such as paper is preferably used as the recording material.

[0013] The image forming apparatus 100 has a photosensitive drum 1, which is a drum-type (cylindrical) electrophotographic photosensitive member (photoconductor) serving as an image carrier. Around the photosensitive drum 1, the following means are arranged in order along the direction of rotation. First, a charging roller 2, which is a roller-shaped charging member serving as a charging means, is arranged. Next, an exposure device (laser scanner unit) 3, which is an exposure means, is arranged. Next, a developing unit 4, which is a developing means, is arranged. Next, a transfer roller 5, which is a roller-shaped transfer member serving as a transfer means, is arranged. Next, a transfer roller 6, which is a transfer roller, which is a transfer roller, is arranged. A cleaning device 6 is provided as cleaning means.

[0014] When a print start signal is input to the image forming apparatus 100 and image formation is started, a rotational driving force is transmitted to the photosensitive drum 1 from a driving motor (not shown) as a driving means provided in the apparatus main body 110. As a result, the photosensitive drum 1 is rotationally driven at a predetermined peripheral speed (process speed) (for example, 300 mm / s) in the direction of the arrow X1 in the figure. In the first embodiment, the photosensitive drum 1 has an aluminum drum base and an OPC photosensitive layer provided on the drum base. The charging roller 2 is arranged in contact with the photosensitive drum 1 and rotates following the rotation of the photosensitive drum 1. The surface (outer circumferential surface) of the rotating photosensitive drum 1 is charged approximately uniformly to a predetermined potential of a predetermined polarity (negative polarity in the first embodiment) by the charging roller 2. At this time, a predetermined charging bias (charging voltage) is applied to the charging roller 2 from a charging power source (high voltage power source) (not shown) provided in the apparatus main body 110.

[0015] The charged surface of the photosensitive drum 1 is exposed to a laser beam L corresponding to image information from an exposure device 3. The exposure device 3 outputs, from a laser output section 3a, a laser beam (exposure beam) L modulated in response to a time-series electric digital image signal of image information input to a control section 19 from a personal computer or the like (not shown) external to the image forming apparatus 100. The laser beam L output from the exposure device 3 is incident on a process cartridge 120 and irradiated onto the surface of the photosensitive drum 1. The surface of the photosensitive drum 1, which is substantially uniformly charged, is scanned and exposed by the laser beam L, whereby an electrostatic latent image (electrostatic image) corresponding to the image information is formed on the surface of the photosensitive drum 1.

[0016] The electrostatic latent image formed on the surface of the photosensitive drum 1 is developed with toner as a developer by the developing unit 4. The developing unit 4 will be described in detail later.

[0017] On the other hand, at a predetermined control timing, a pickup roller 8 as a conveying means is driven, and recording materials P such as recording paper stacked and stored in a recording material tray 7 as a recording material storage section are separated and fed one by one. As a result, the recording materials P are conveyed to the transfer section N at a predetermined control timing. Also, the transfer roller 5 contacts the surface of the photosensitive drum 1 with a predetermined pressing force to form a transfer section (transfer nip) N. The recording material P is conveyed to the transfer section N via a transfer guide 9 as a guide member. Then, in the process in which the recording material P passes through the transfer section N by being sandwiched and conveyed between the photosensitive drum 1 and the transfer roller 5, the toner image on the surface of the photosensitive drum 1 is electrostatically transferred to the surface of the recording material P. At this time, a transfer bias (transfer voltage) which is a DC voltage of the opposite polarity to the charging polarity (negative polarity in Example 1) of the toner during development is applied to the transfer roller 5 from a transfer power source (high voltage power source) not shown in the figure provided in the apparatus main body 110.

[0018] The recording material P to which the toner image has been transferred is separated from the photosensitive drum 1 and conveyed to a fixing device 10 as a fixing means provided downstream of the transfer portion N in the conveying direction of the recording material P. The recording material P is heated and pressurized in the fixing device 10 to undergo a fixing process of the toner image. In the first embodiment, the fixing device 10 has a heating roller with a halogen heater inside and a pressure roller pressed against the heating roller. The fixing device 10 heats and presses the toner image transferred to the surface of the recording material P while nipping and conveying the recording material P in the fixing nip between the fixing roller and the pressure roller. As a result, the toner image is melted and fixed to the surface of the recording material P. The recording material P is then discharged to a discharge tray 11 provided at the top of the device main body 110.

[0019] After the recording material P is separated from the surface of the photosensitive drum 1, the surface of the photosensitive drum 1 is cleaned by the cleaning device 6 and is repeatedly used for the image forming process starting from the charging described above. The cleaning device 6 is a cleaning blade as a cleaning member arranged in contact with the photosensitive drum 1. The toner cartridge 61 removes residual toner and other adhering matter from the surface of the rotating photosensitive drum 1 and collects the toner in a toner recovery container 62 .

[0020] (Process cartridge) 2 is a schematic cross-sectional view of the process cartridge 120. In the first embodiment, the photosensitive drum 1, the charging roller 2 as a process means acting on the photosensitive drum 1, the developing unit 4, and the cleaning device 6 are integrated into a cartridge to constitute the process cartridge 120 that is detachably mountable to the main body 110 of the apparatus.

[0021] The process cartridge 120 is configured by connecting a cleaning unit 12 and a developing unit (developing device) 4 that is separate from the cleaning unit 12.

[0022] The cleaning unit 12 has a photosensitive drum 1, a charging roller 2, and a cleaning device 6. The cleaning unit 12 also has a cleaning frame 60 that forms a collected toner container 62 and supports the photosensitive drum 1, the charging roller 2, and a cleaning blade 61. The developing unit 4 will be described in detail later.

[0023] A process cartridge is generally a cartridge in which an image carrier such as a photoconductor and a process means acting on the image carrier are integrated into one cartridge, and the cartridge is detachable from the main body of an image forming apparatus. Examples of the process means include a charging means, a developing means, a cleaning means, and a toner charging means for charging the transfer residual toner. Here, the process cartridge is a cartridge in which at least a developer container or a developing device and an image carrier are integrated into one cartridge, and the cartridge is detachable from the main body of an image forming apparatus.

[0024] 3 is a schematic cross-sectional view of the developing unit 4 in Example 1. The developing unit 4 in Example 1 has a developing chamber 46a, a developer container 46b that contains a one-component developer (toner T) as a developer, and a developing frame 40 for supporting each element described below. The developer container 46b also has an agitating member 45 consisting of an agitating shaft 45a and an agitating sheet 45b, and the agitating shaft 45a rotates in the direction of the arrow X4 to transport the toner T to the developing chamber 46a.

[0025] In the developing chamber 46a, a developing roller 41, which is a cylindrical member serving as a developer carrier, is disposed. A part of the developing roller 41 is disposed so as to be able to come into contact with the photosensitive drum 1 through an opening 46c formed on the photosensitive drum 1 side. The developing roller 41 is rotatably supported at both ends in its longitudinal direction (rotation axis direction) by the developing frame 40. The developing roller 41 is disposed so as to come into contact with the photosensitive drum 1. A rotational driving force is transmitted to the developing roller 41 from a driving motor (not shown) provided in the apparatus main body 110, and the developing roller 41 is driven to rotate in the direction of the arrow X2 in the figure. A developing bias required for developing the electrostatic latent image into a toner image is applied to the developing roller 41 from a developing bias application unit 51.

[0026] A supply roller 43, which is a supply member that rotates in the X3 direction in contact with the developing roller 41 and supplies toner to the photosensitive drum 1, is disposed on the circumference of the developing roller 41. A supply bias is applied to the supply roller 43 from a supply bias application unit 52. This supply bias application unit 52 applies a voltage divided from a high-voltage power supply for the charging bias through a bipolar transistor, enabling high-speed switching of the supply bias.

[0027] In Example 1, a developing unit 4 is exemplified in which the surfaces of the developing roller 41 and the supply roller 43 move in opposite directions at their contact portions, but the present invention is also applicable to a developing device in which the surfaces of the developing roller and the supply roller move in the same direction at their contact portions.

[0028] In addition, the developing chamber 46a is provided with a developing roller 41. A developing blade 42 is disposed on the developing roller 41, which is a regulating member that regulates the thickness of the toner layer carried by the developing roller 41. The developing blade 42 is supported by the developing frame 40. A blade bias (regulating bias) is applied to the developing blade 42 from a blade bias application unit 53.

[0029] The developing bias application section 51, the supply bias application section 52, and the blade bias application section 53 constitute a power supply section 500, and the application of the developing bias, the supply bias, and the regulating bias by the power supply section 500 is controlled by the control section 19. The bias control by the control section 19 will be described later.

[0030] Next, the developing roller 41, the supply roller 43, and the developing blade 42 will be described. The developing roller 41 has a conductive core metal with a silicone rubber base layer and a urethane rubber surface layer. Roughening particles are present on the surface layer to optimize the amount of toner carried. The supply roller 43 is a conductive sponge roller with a foamed layer formed on a conductive core metal. The developing blade 42 is made of a SUS sheet metal, and the long side portion that contacts the developing roller 41 and is the free end is laminated with resin.

[0031] The developing roller 41 and the photosensitive drum 1 rotate such that their surfaces move in the same direction at the opposing portion (contact portion). Note that, although the developing roller 41 is disposed in contact with the photosensitive drum 1 in the first embodiment, the developing roller 41 may be configured to be disposed close to the photosensitive drum 1 with a predetermined gap therebetween.

[0032] The photosensitive drum 1 is electrically grounded, and an electric field is generated in the region between the photosensitive drum 1 and the developing roller 41 to which a developing bias is applied, according to the potential difference between the developing bias and the surface potential of the photosensitive drum 1. The toner T charged to the normal polarity and transported to the developing region is subjected to the action of electrostatic force by this electric field, and is transferred to the surface of the photosensitive drum 1 according to the electrostatic latent image on the surface of the photosensitive drum 1. As a result, the electrostatic latent image on the photosensitive drum 1 is developed by the toner T. In the first embodiment, the toner T charged to the same polarity (negative polarity) as the charging polarity of the photosensitive drum 1 adheres to the exposed portion (image portion) on the photosensitive drum 1, which is uniformly charged and then exposed to light, thereby attenuating the absolute value of the potential, thereby developing the electrostatic latent image (reverse development method).

[0033] In the first embodiment, an image forming apparatus using toner whose normal polarity is negative will be described as an example, but the present invention is also applicable to an image forming apparatus using toner whose normal polarity is positive.

[0034] (Toner movement in the developing unit) The toner T is conveyed by the stirring member 45 to the opening 46c as a toner supply port. The toner T conveyed from there to the developing chamber 46a is supplied to the developing roller 41 by the supply roller 43. The supply roller 43 and the developing roller 41 come into contact with each other to form a nip portion. In the first embodiment, the supply roller 43 and the developing roller 41 rotate in opposite directions at the nip portion. The supply roller 43 and the developing roller 41 may rotate in the same direction at the nip portion. An electrostatic force according to the potential relationship between the developing bias and the supply bias and the charging polarity of the toner acts on the toner held on the outer periphery of the supply roller 43. In the first embodiment, the normal charging polarity of the toner is negative (minus), and the developing bias and the supply bias are negative potentials. When the developing bias is at a higher potential than the supply bias (the absolute value of the developing bias is smaller than the absolute value of the supply bias), an electrostatic force acts on the toner charged to the normal polarity, that is, negative polarity, in the direction from the supply roller 43 to the developing roller 41. As a result, the toner held by the supply roller 43 is supplied onto the developing roller 41. Controlling the developing bias and the supply bias so that an electrostatic force acts on the toner from the supply roller 43 toward the developing roller 41 is called supply bias control. When the normal polarity of the toner is negative, the developing bias and the supply bias are set to a negative potential, and in the supply bias control, the absolute value of the supply bias is set to the developing bias. The application of the bias is controlled so that it is greater than the absolute value of the astigmatism. That is, the supply bias is controlled to a lower potential (a negative potential with a larger absolute value) than the development bias (for example, development bias -400V, supply bias -500V).

[0035] Thereafter, the toner coated on the surface of the developing roller 41 is charged by friction at the contact portion between the surface of the developing roller 41 and the tip of the developing blade 42, and is regulated to a predetermined layer thickness. This adjusts the amount of toner carried on the surface of the developing roller 41 to a constant amount. The toner whose layer thickness is regulated by the developing blade 42 is transferred from the surface of the developing roller 41 to the image portion of the photosensitive drum 1 to develop the electrostatic latent image, but the toner not used for development (residual toner) reaches the contact portion with the supply roller 43 by the rotation of the developing roller 41. The residual toner is scraped off from the developing roller 41 depending on the mechanical friction between the developing roller 41 and the supply roller 43, the charging characteristics of the residual toner, the potential difference between the developing roller 41 and the supply roller 43, etc. Or, it is mixed with the toner in the developing chamber 46a or the supply roller 43 on the surface of the developing roller 41.

[0036] (Charging characteristics of toner on developing roller) The toner coat on the surface of the developing roller 41 in the region downstream of the contact position with the supply roller 43 and upstream of the contact position with the developing blade 42 in the rotation direction of the developing roller 41 is called a precoat. That is, it is the toner on the developing roller 41 that is supplied from the supply roller 43 and regulated by the developing blade 42. The amount and charging characteristics of the precoat change according to the potential relationship between the supply bias and the developing bias. Also, the blade bias when the precoated toner passes through the developing blade 42 determines the charging characteristics of the toner on the developing blade 42 after it has passed through the developing blade 42.

[0037] (Low density due to residual toner adhering to the surface of the developing roller) Next, the low density caused by the adhesion of the residual toner to the surface of the developing roller 41 will be described. As described above, the residual toner is mixed with the toner in the developing chamber 46a on the surface of the developing roller 41 or scraped off from the surface of the developing roller 41 by the supply roller 43, so that it is usually difficult to remain on the developing roller 41. Here, in a state where the supply bias control is being performed (for example, a state where the developing bias is -400V and the supply bias is -500V), an electrostatic force acts on the negatively charged toner in a direction from the supply roller 43 to the developing roller 41. Due to this electrostatic force, when the fine toner on the surface of the developing roller 41 holds a high negative charge, the fine toner tends to maintain a state of adhering to the developing roller 41 side in a state where the supply bias control is being performed. If this state continues, the lower layer of the toner coat on the surface of the developing roller 41 will be covered with the charged up fine toner, and the upper layer will be less likely to be directly affected by the bias from the developing roller 41 and will be less likely to hold a charge. Therefore, when the negatively charged toner supplied from the supply roller 43 reaches the developing roller 41, it becomes difficult to hold a charge, and the negatively charged toner is sparsely present in the upper layer. After this, when the toner on the developing roller 41 is transferred to the latent image on the photosensitive drum 1, the lower layer is not easily used for development because the fine toner is strongly adhered thereto, and remains on the developing roller 41 as residual toner. On the other hand, the toner in the upper layer is mainly used for development. Therefore, especially in an all-black image, the amount of toner used for development is insufficient for the amount of toner required for image formation, and low density may occur. In this way, in the developing device, the fine toner that is easily charged among the residual toner continues to adhere to the lower layer of the surface of the developing roller 41, causing image defects such as low density.

[0038] (Strip bias control) As a countermeasure, by controlling the application of a bias so that an electrostatic force acts on the toner in a direction from the developing roller 41 to the supply roller 43, it is possible to peel off the minute toner particles from the developing roller 41 and prevent the minute toner particles from remaining on the developing roller 41. Controlling the developing bias and the supply bias so that an electrostatic force from the developing roller 41 to the supply roller 43 acts on the positively charged toner is called peeling bias control. When the normal polarity of the toner is negative, the developing bias and the supply bias are set to negative potentials, and in peeling bias control, the application of bias is controlled so that the absolute value of the supply bias is smaller than the absolute value of the developing bias. In other words, the supply bias is controlled to a higher potential (a negative potential with a smaller absolute value) than the developing bias (for example, developing bias -400V, supply bias -300V).

[0039] In addition, when minute toner is attached to the surface of the developing roller 41, there is a lot of negative charge on the surface of the developing roller 41, so the surface potential of the developing roller 41 becomes lower potential (negative potential with a large absolute value) than the applied developing bias. For example, when the developing bias is -400V, the surface potential of the developing roller 41 becomes -450V when a lot of minute toner is attached to the surface. For the stripping bias control, since it is only necessary that the supply bias has a higher potential than the surface potential of the developing roller 41, for example, the application of the bias may be controlled so that the absolute value of the supply bias and the absolute value of the developing bias are equal. In other words, the supply bias and the developing bias may be at the same potential (for example, the supply bias is -400V and the developing bias is -400V).

[0040] Furthermore, if the peeling bias control is performed from the pre-rotation to the time of paper passing, there is a possibility that the amount of normally charged toner required for image formation will not be supplied to the developing roller 41. Therefore, the period during which the peeling bias control is performed may be a period of about one rotation of the developing roller 41 between sheets. Here, the sheet interval refers to the period from the end of image formation on one recording material to the start of image formation on the next recording material when images are formed continuously on multiple recording materials P.

[0041] (development streaks) Next, the mechanism of development streaks will be explained. When the toner deteriorates due to the removal of external additives, etc., toner charged with the opposite polarity to the normal charging polarity is easily generated. If the amount of toner in the precoat is small, this oppositely charged toner enters the contact area between the development roller 41 and the development blade 42, and the oppositely charged toner fuses to the free end tip of the development blade 42. When fusion occurs, the fused area scrapes off the toner coat on the development roller 41, appearing as streaks on the image. This phenomenon is called development streaks. On the other hand, if the precoat is sufficiently secured, the intrusion of the oppositely charged toner into the blade contact area is suppressed, and development streaks are less likely to occur.

[0042] Thus, the occurrence of development streaks depends on the amount of precoat. The stronger the electrostatic force acting on the toner in the direction from the supply roller 43 to the development roller 41, the better the precoat is formed. In the supply bias control state, a large amount of toner is expelled from the supply roller 43, and the amount of precoat increases, so that development streaks can be suppressed. Conversely, in the stripping bias control state, an electrostatic force acts on the toner in the direction from the development roller 41 to the supply roller 43, and the amount of precoat decreases, so that development streaks are more likely to occur.

[0043] As described above, one effective measure to prevent low density caused by contamination of the surface of the developing roller 41 with fine toner particles is to carry out stripping bias control. However, when stripping bias control is carried out, development streaks are likely to occur.

[0044] (Strip Bias Control in the Example) Therefore, in the first embodiment, the blade bias during the stripping bias control is set to a lower potential (the absolute value of the negative blade bias is increased) than the blade bias during the supply bias control. Here, the polarity of the blade bias is the normal polarity of the toner. As a result, the developing blade 42 and the developing roller 41 The toner charged to the opposite polarity (positively charged toner in the case of the first embodiment, also called positive toner) that has entered the contact portion of the blade can be forcibly charged to the normal polarity (negative polarity in the case of the first embodiment). Therefore, by implementing the peeling bias control, it is possible to suppress the low density caused by the adhesion of minute toner particles to the developing roller 41, and also to suppress development streaks caused by the toner that has entered the blade contact portion being pinched and fused by the tip of the blade.

[0045] In addition, if the absolute value of the blade bias is maintained at a large value during image formation, this may lead to charging up of the entire toner and fusing of external additives etc. that are strongly positively charged, which may in turn make it easier for development stripes to occur. Therefore, it is desirable to increase the absolute value of the blade bias, especially when stripping bias control is being performed.

[0046] (Toner charge distribution after passing the developing blade) The details and effects of the control will be described below. Figure 4 shows the charge distribution of the toner before and after passing the developing blade 42. Figure 4(a) shows the case where the supply bias control is being performed, Figure 4(b) shows the case where the stripping bias control of the comparative example described below is being performed, and Figure 4(c) shows the case where the stripping bias control of the first embodiment described above is being performed. The solid line in the figure shows the charge distribution after passing the developing blade 42, and the dashed line shows the charge distribution of the precoat before passing the developing blade 42.

[0047] The developing bias is -400V in all of Figures 4(a) to 4(c). In the supply bias control of Figure 4(a), the supply bias is -500V and the blade bias is -500V. In the stripping bias control of the comparative example of Figure 4(b), the supply bias is -300V and the blade bias is -500V. In the stripping bias control of Example 1 of Figure 4(c), the supply bias is -300V and the blade bias is -600V. The values ​​of each bias are merely examples and are not limited to these.

[0048] When the supply bias control shown in FIG. 4(a) is performed, an electrostatic force acts on the negatively charged toner in the direction from the supply roller 43 to the developing roller 41, and the toner is discharged from the supply roller 43. As a result, the amount of toner on the precoat increases, and the median V1a of the charge distribution of the toner on the precoat before the development blade 42 passes becomes more negative. When the toner on the precoat enters the contact area between the development blade 42 and the development roller 41, the toner is charged by frictional charging and charge injection from the development blade 42. At this time, since the blade bias is more negative than the development bias, the toner regulated by the development blade 42 stays on the development roller 41 while mainly carrying a negative charge. As a result, the median V2a of the charge distribution of the toner on the development roller 41 after the development blade 42 passes becomes more negative than the median V1a of the charge distribution of the toner on the precoat. The period during which the supply bias control shown in FIG. 4(a) is performed is called the first period.

[0049] When the stripping bias control of the comparative example shown in FIG. 4(b) is performed, an electrostatic force acts on the negative toner charged to the normal polarity in the direction from the developing roller 41 to the supply roller 43, so that the toner is not supplied from the supply roller 43 to the developing roller 41. Therefore, the amount of toner in the precoat is reduced compared to the supply bias control of FIG. 4(a), and the median value V1b of the charge distribution is closer to the positive side. And, the blade bias is more negative than the developing bias, and negative charge is injected by the developing blade 42. However, the amount of negative toner in the precoat is reduced, and the charge distribution is closer to the positive side. Therefore, although the median value V2b of the charge distribution after passing the developing blade 42 is more negative than the median value V1b of the charge distribution of the toner in the precoat, there is still a lot of toner charged to the positive side.

[0050] When the stripping bias control of the first embodiment shown in FIG. 4C is executed, an electrostatic force acts on the negatively charged toner in the direction from the developing roller 41 to the supply roller 43. Therefore, toner is not easily supplied from the supply roller 43 to the developing roller 41. Therefore, the amount of toner in the precoat is less than that in the supply bias control of FIG. 4(a), and the median V1 of the charge distribution is closer to the positive side. The charge distribution of the precoat is similar to that in FIG. 4(b). In the stripping bias control of the first embodiment, the absolute value of the blade bias is greater than the absolute value of the blade bias in the first period (the blade bias in the supply bias control of FIG. 4(a)). Therefore, the potential difference between the developing bias and the blade bias in the stripping bias control of the first embodiment is (-600V)-(-400V)=-200V, which is greater than the potential difference (-500V)-(-400V)=-100V in the comparative example shown in FIG. 4(b). Therefore, the amount of charge injected from the developing blade 42 to the toner is greater, and the median V2 of the toner charge distribution after passing the developing blade 42 is more negative than the median V2b in the comparative example, and the amount of toner charged to the positive side is less than in the comparative example. The period during which the stripping bias control shown in FIG. 4(c) is performed is referred to as a second period.

[0051] Thus, according to the first embodiment, even if the amount of precoat is reduced by the stripping bias control, the potential difference between the development bias and the blade bias is large, so that the toner of the opposite polarity present on the development roller 41 is forcibly charged to the negative polarity when passing the development blade 42. Therefore, it is possible to prevent the positive toner from fusing to the tip of the development blade 42, and to prevent the occurrence of development streaks.

[0052] (Supply Bias and Blade Bias Control) The bias control for the developing roller 41, the supply roller 43, and the developing blade 42 in the embodiment 1 will be described with reference to Fig. 5. Fig. 5 is a timing chart showing the bias control when two sheets are printed consecutively in the embodiment 1, the comparative example 1, and the comparative example 2.

[0053] The timing and operation in this timing chart will be explained below. In the figure and in the following explanation, "developing drive start" indicates the timing when the developing roller 41 and the supply roller 43 start to rotate upon receiving a driving force from the driving means provided in the apparatus main body 110. "image formation start" indicates the timing when the image is written by sweeping the laser light in the sub-scanning direction. "image formation end" indicates the timing when the laser exposure swept in the sub-scanning direction ends. "developing drive end" indicates the timing when the driving force received from the driving means stops.

[0054] In the first embodiment, the developing bias applied to the developing roller 41 is a constant −400 V from the start to the end of the developing drive. That is, the developing bias in the first period and the developing bias in the second period are equal. Note that this control of the developing bias is an example, and the developing bias may be changed.

[0055] From the start of development drive to the start of image formation on the first sheet, and during the image formation operation on the first sheet (from the start of image formation to the end of image formation), the control unit 19 performs the supply bias control shown in Fig. 4(a). That is, the bias application is controlled so that an electrostatic force acts on the toner in a direction from the supply roller 43 to the development roller 41. In the first embodiment, the bias application is controlled so that the absolute value of the supply bias is greater than the absolute value of the development bias. Therefore, the period from the start of development drive to the start of image formation on the first sheet, and during the image formation operation on the first sheet (the period from the start of image formation to the end of image formation) are included in the first period.

[0056] During the period (paper interval) from the end of image formation on the first sheet to the start of image formation on the second sheet, which corresponds to one rotation of the developing roller 41, the control unit 19 performs the stripping bias control shown in FIG. 4(c). That is, the control unit 19 controls the bias application so that an electrostatic force acts on the toner in the direction from the developing roller 41 to the supply roller 43, and the absolute value of the blade bias is greater than the absolute value of the blade bias in the supply bias control in the first period. In the first embodiment, the bias application is controlled so that the absolute value of the supply bias is smaller than the absolute value of the developing bias. Therefore, at least a part of the period between sheets (in the first embodiment, the period for one revolution of the developing roller 41 (period for one rotation)) is included in the second period. The supply bias control shown in FIG. 4(a) is performed during the period between sheets other than the period during which the stripping bias control is performed. Therefore, the period between sheets other than the period during which the stripping bias control is performed (period between sheets not included in the second period) is included in the first period. Here, the timing of changing the blade bias in the stripping bias control may be simultaneous with the timing of changing the supply bias, or the timing of changing the blade bias may be delayed with respect to the timing of changing the supply bias. In the case of delaying, the delay time can be set based on the time until the contact part of the developing roller 41 with the supply roller 43 reaches the contact part with the developing blade 42. The delay time depends on the dimensions and the rotation speed setting of the developing roller 41, and is, for example, several ms. In addition, in the case of delaying the timing of changing the blade bias with respect to the timing of changing the supply bias, the start timing of the second period may be the timing of changing the supply bias or the timing of changing the blade bias. The end timing of the second period may be the timing at which the supply bias is returned to the control value of the supply bias control, or the timing at which the blade bias is returned to the control value of the supply bias control.

[0057] The stripping bias control may be performed during the entire period between sheets. In other words, the entire period between sheets may be included in the second period. However, depending on the period during which the stripping bias control is performed, the amount of stripping may become too large, resulting in a shortage of precoat toner that should be negatively charged by the blade bias, which may make development streaks more likely to occur. It is desirable to appropriately set the appropriate period for performing the stripping bias control depending on the environment, toner characteristics, etc.

[0058] Comparative Example 1 is an example in which the supply bias control of FIG. 4(a) is executed even between sheets. That is, in Comparative Example 1, the entire period from the start of development drive to the end of development drive is included in the first period. Comparative Example 2 is an example in which the stripping bias control of FIG. 4(b) is executed in a part of the sheet interval. That is, when performing stripping bias control between sheets, bias control that increases the absolute value of the blade bias is not performed.

[0059] (Evaluation experiment) The experiment conducted to confirm the effect of Example 1 will be described. In this experiment, 10,000 sheets of LETTER size paper were printed with 3dot297space horizontal lines in an environment of 32.5 degrees Celsius and 80% humidity, and then an all-black image and a halftone image were printed, and the density of the all-black image and development streaks were evaluated. The halftone image of Example 1 is an image pattern with a density of 0.6 in X-rite.

[0060] The evaluation of low density of all black images was carried out by measuring the density of the center of the leading edge and trailing edge of the image with X-Rite. The following rankings, A to C, were used as the evaluation criteria. If the density was 1.2 or higher with X-Rite, the image was deemed to be satisfactory. A: Full black central density 1.3 or more B: Full black central density 1.2 or more and less than 1.3 C: Full black central density less than 1.2

[0061] The evaluation of development streaks was carried out by visually counting the number of streaks that appeared on the halftone image from the leading edge to the trailing edge of the image, and classifying them into the following ranks A to C as the evaluation criteria. If no vertical streaks appeared on the halftone image, it was considered that there was no problem with the image. A: 0 vertical streaks on halftone B: Halftone vertical stripes 1 to 3 C: 3 or more vertical lines on halftone

[0062] The results are shown in FIG. 5. An evaluation experiment was conducted by applying the asterisk control to the printing of two consecutive full black images and a halftone image. The evaluation results are shown in Table 1. [Table 1]

[0063] When the bias control of Comparative Example 1 was performed, low density occurred during all black printing. This is thought to be because, since the peeling bias control was not performed, printing was repeated with the negatively charged fine toner adhering to the developing roller 41, and the lower layer of the toner coat on the surface of the developing roller 41 was gradually covered with the fine toner. As a result, even if the amount of toner coat on the developing roller 41 was appropriate, the toner in the lower layer remained attached and was not developed, and the toner in the upper layer was only sparsely charged, which is thought to have caused the low density.

[0064] Moreover, when the bias control of Comparative Example 2 was performed, the occurrence of low density was suppressed, but development stripes occurred. It is believed that this is because the fine toner on the developing roller 41 was peeled off from the developing roller 41 by performing the peeling bias control between sheets, and the coating with the fine toner was suppressed. On the other hand, since the control to increase the absolute value of the blade bias was not performed in the peeling bias control, it is believed that the toner that was positively charged due to deterioration and the toner that had aggregated and become foreign matter could not be forcibly charged to negative polarity, and thus fused to the developing blade 42.

[0065] The above-mentioned experiment was carried out in a high-temperature and high-humidity environment with a temperature of 32.5 degrees and a humidity of 80%, but if the usage environment of the image forming apparatus 100 changes, the charge distribution of the toner and the charge-up property of the micro toner will change. Even under usage environment conditions with different temperatures and humidity, by appropriately adjusting the bias settings and then executing the stripping bias control of Example 1, it was possible to similarly achieve both suppression of low black density and suppression of development streaks.

[0066] According to the first embodiment, even if the toner charged to the opposite polarity or aggregates are present in the developing chamber due to deterioration, it is possible to suppress both low density and development stripes. This makes it possible to provide an image forming apparatus with a long life and high image quality.

[0067] In the first embodiment, the normal charge polarity of the toner is negative and the applied bias is also negative, but the present invention is also applicable when the normal charge polarity of the toner is positive and the applied bias is positive.

[0068] The disclosure of this embodiment includes the following configuration. (Configuration 1) An image forming apparatus for forming an image on a recording material by using a developer charged to a normal polarity, an image carrier on which an electrostatic latent image is formed; a developer carrier that develops the electrostatic latent image formed on the image carrier with the developer; a supply member for supplying the developer to the developer carrier; a regulating member that regulates a layer thickness of the developer carried on the developer carrier; an application unit that applies a bias to each of the developer carrier, the supply member, and the regulating member; A control unit that controls application of the bias by the application unit; Equipped with The control unit is during a first period, application of the bias is controlled so that an electrostatic force acts on the developer charged to the normal polarity in a direction from the supply member toward the developer carrier; an image forming apparatus characterized in that, during a second period, an electrostatic force acts on the developer charged to the normal polarity in a direction from the developer carrier to the supply member, and the polarity of a regulating bias applied to the regulating member is the normal polarity, and the application of the bias is controlled so that the absolute value of the regulating bias is greater than the absolute value of the regulating bias applied to the regulating member during the first period. (Configuration 2) 2. The image forming apparatus according to claim 1, wherein, in the first period, an absolute value of a supply bias applied to the supply member is greater than an absolute value of a development bias applied to the developer carrier. (Configuration 3) 3. The image forming apparatus according to claim 1, wherein in the second period, an absolute value of the supply bias applied to the supply member is smaller than an absolute value of the development bias applied to the developer carrier. (Configuration 4) 3. The image forming apparatus according to claim 1, wherein in the second period, an absolute value of the supply bias applied to the supply member is equal to an absolute value of the development bias applied to the developer carrier. (Configuration 5) 5. The image forming apparatus according to any one of Configurations 1 to 4, wherein the developing bias applied to the developer carrier in the second period is equal to the developing bias applied to the developer carrier in the first period. (Configuration 6) 6. The image forming apparatus according to any one of configurations 1 to 5, wherein the first period includes a period during which an image is formed on one of the recording materials. (Configuration 7) The image forming apparatus according to any one of configurations 1 to 6, wherein the second period is at least a part of a period between a period in which image formation is performed on one of the recording materials and a period in which image formation is performed on the next recording material. (Configuration 8) The first period includes a period between a period in which an image is formed on one of the recording materials and a period in which an image is formed on the next recording material, the period not being included in the second period. (Configuration 9) 9. The image forming apparatus according to any one of configurations 1 to 8, wherein the second period is a period during which the developer carrier makes one rotation. [Explanation of symbols]

[0069] 1: photosensitive drum, 19: control unit, 41: developing roller, 42: developing blade, 43: supply roller, 51: developing bias application unit, 52: supply bias application unit, 53: blade bias application unit, 100: image forming apparatus

Claims

1. An image forming apparatus for forming an image on a recording material using a developer charged to a normal polarity, an image carrier on which an electrostatic latent image is formed; a developer carrier that develops the electrostatic latent image formed on the image carrier with the developer; a supply member for supplying the developer to the developer carrier; a regulating member that regulates the layer thickness of the developer carried on the developer carrier; an application unit that applies a bias to the developer carrier, the supply member, and the regulating member; a control unit that controls application of the bias by the application unit; Equipped with The control unit during a first period, the application of the bias is controlled so that an electrostatic force acts on the developer charged to the normal polarity in a direction from the supply member toward the developer carrier; an electrostatic force acting on the developer charged to the normal polarity in a direction from the developer carrier to the supply member during the second period; a regulating bias applied to the regulating member has the normal polarity; the absolute value of the regulating bias is greater than the absolute value of the regulating bias applied to the regulating member during the first period; and the application of the bias is controlled so that the difference between the developing bias applied to the developer carrier and the regulating bias is greater than the difference between the developing bias and the supply bias applied to the supply member.

2. 2. The image forming apparatus according to claim 1, wherein the absolute value of the supply bias applied to the supply member is greater than the absolute value of the development bias applied to the developer carrier during the first period.

3. 3. The image forming apparatus according to claim 1, wherein the absolute value of the supply bias applied to the supply member during the second period is smaller than the absolute value of the development bias applied to the developer carrier.

4. 3. The image forming apparatus according to claim 1, wherein the absolute value of the supply bias applied to the supply member during the second period is equal to the absolute value of the development bias applied to the developer carrier. Place.

5. 3. The image forming apparatus according to claim 1, wherein the developing bias applied to the developer carrier during the second period is equal to the developing bias applied to the developer carrier during the first period.

6. 3. The image forming apparatus according to claim 1, wherein the first period includes a period during which image formation is performed on one of the recording materials.

7. 3. The image forming apparatus according to claim 1, wherein the second period is at least a part of a period between a period in which an image is formed on one of the recording materials and a period in which an image is formed on the next of the recording materials.

8. 8. The image forming apparatus according to claim 7, wherein the first period includes a period between a period in which image formation is performed on one of the recording materials and a period in which image formation is performed on the next of the recording materials, the period not being included in the second period.

9. 3. The image forming apparatus according to claim 1, wherein the second period is a period during which the developer carrier makes one rotation.

10. An image forming apparatus as described in claim 1, wherein the polarity of the supply bias is the normal polarity.

Images