JP4440004B2 - Development device - Google Patents

Description

  The present invention relates to a developing device used in an image forming apparatus such as a copying machine, a printer, and a facsimile machine.

  Conventionally, an electrophotographic image forming apparatus that optically scans a document image portion, which is an outer peripheral surface of a uniformly charged photoreceptor, to form an electrostatic latent image, and visualizes this with a toner that is a colored resin, Since high-speed image formation is possible, it is widely used in digital printers and copiers.

In recent years, there has been a particularly strong demand for colorization, and in electrophotographic image forming apparatuses, full-color image forming apparatuses composed of toner images of four colors of cyan, magenta, yellow, and black have been realized. The adoption of the tandem method, which is advantageous, has become very popular and is becoming mainstream.
This tandem system has the disadvantage that the apparatus is increased in size because four color image forming units are arranged in parallel. However, the non-magnetic one-component development system in which the developer is only toner. Japanese Patent Application Laid-Open No. 2004-228688 discloses a configuration in which the arrangement pitch of the photosensitive drums is shortened to reduce the size of the apparatus. In this configuration, a developing roller that develops an electrostatic latent image by attaching toner to the peripheral surface of the photosensitive drum is disposed above the cleaning member of the adjacent upstream photosensitive drum to shorten the pitch between the photosensitive drums. By doing so, a narrow pitch image forming system is realized.

On the other hand, as a development method, in addition to the non-magnetic one-component development method, a two-component development method using a developer composed of a magnetic carrier and a toner is widely adopted, and it can realize high image quality and low running cost. Has features. In this method, the developer is rubbed against the surface of the photosensitive drum in the form of a magnetic brush by a developing roller having a magnet inside the developing sleeve carrying the developer, and only the toner is transferred to the surface of the photosensitive drum. The electrostatic latent image is developed.
In order to satisfy all these requirements in response to the ever-increasing demands for miniaturization, higher image quality, and lower running costs in recent years, the inventors have formed an image forming system that incorporates a two-component development system in a narrow pitch image forming system. A device was developed.
In the two-component development system, a process of removing the developer attached to the surface of the developing sleeve after development is important. If the developer peeling after the development is insufficient, the toner is consumed. Since the toner density of the developer is different in a portion where it is not consumed, density unevenness called so-called ghost (or memory) occurs. Normally, the developer is peeled off by using an odd number of magnets in the developing sleeve, and providing a pair of magnets of the same polarity below the rotation axis of the developing sleeve to create a peeling region where the magnetic force is almost zero. Separation is performed by allowing the developer after development to drop naturally using gravity in the region. Thereafter, the peeled developer is transported by a transport screw installed in the vicinity of the stripping region, and is readjusted to a predetermined toner concentration by circulating in the developing device. However, in a narrow-pitch image forming system in which the developing sleeve is disposed above the adjacent upstream photosensitive drum cleaning member, the adjacent upstream photosensitive drum cleaning member is close to the area below the developing sleeve. Since the developer peeling area where the magnet pairs of the same polarity are naturally placed is also above the rotation axis of the developing sleeve, peeling using gravity cannot be performed, resulting in density irregularities such as ghosts. End up. To deal with this problem, for example, Patent Document 2 describes a configuration in which a scooping roll having a magnet is disposed in the vicinity of a peeling region on the developing sleeve, and the developer is peeled off with the magnetic force. . The peeled developer is further pumped up by another scooping roll and then conveyed to a developer agitating chamber having a screw to readjust the toner concentration and charge the toner.
JP 2001-356545 A Japanese Patent Laid-Open No. 11-65247

However, in the configuration in which the developer peeling region where the magnet pair of the same polarity is arranged is located above the rotation axis of the developing sleeve, the developer after development is fed by a scooping roller having a magnetic pole inside as in Patent Document 2. When the developer is peeled off, it is necessary to provide a developer stirring chamber or the like for stirring the developer, which causes a problem that the developing device becomes complicated and large. Further, since a magnet is required inside the drawing roller, the cost is increased and the weight of the apparatus is increased.

  The present invention has been made to solve the above-described problems, and a peeling region in which a pair of homopolar magnets such as a narrow-pitch image forming system employing a two-component development system is disposed is above the rotation axis of the developing sleeve. The present invention has an object to provide a developing device with high image quality, small size, light weight, and low cost that can peel off a developer with a simple configuration without using a scooping roller having a magnet. To do.

In order to solve this problem, the invention described in claim 1 includes a latent image carrier that forms an electrostatic latent image on the surface, and a developing sleeve that supplies a developer composed of toner and carrier to the latent image carrier. The developing sleeve includes at least one pair of magnet pairs having the same polarity, and a midpoint on the developing sleeve connecting the same poles of the magnet pair is in the developing device, and the developing sleeve A developer carrying member at a position higher than the rotation axis of the first developer carrying path, one of the two developer carrying paths arranged in parallel with the developer carrying body, and the first developer. A first developer conveying means that is installed in the conveying path and conveys the developer in a first direction; and the other of the developer carrying members positioned closer to the first developer conveying path than the first developer conveying path. Two developer transport paths and the second developer transport path The rotating shaft is at a position higher than the rotating shaft of the developing sleeve, and while rotating, conveys the developer in a second direction opposite to the first direction, and A second developer conveying means for supplying and peeling the developer, and a rotation radius r of the second developer conveying means, a rotation radius R of the developing sleeve, and a distance d between the respective rotation axes. The following formula (Equation 1) is satisfied, the developer transport amount at the middle point on the developing sleeve connecting the same poles of the magnet pair is 1 g / cm ² / s or more, and the toner is The toner surface shape factor SF value is 100 or more and 140 or less, and at least one metal soap of zinc stearate, calcium stearate, aluminum stearate, magnesium stearate as a lubricant in the toner. Seen, the carrier, the shape factor SF value is 100 or more 110 or less, a fluorine-modified silicone resin or fluorine-modified acrylic resin is coated on the surface, 2 kV / volume resistance in an electric field of cm is 1 × ¹⁰ 10 Ω · cm In the following, there is provided a developing device characterized in that a thermosetting resin is used as a binder and a resin carrier in which a magnetic material is dispersed is used .

According to the present invention, a pair of magnets having the same polarity as in a narrow pitch image forming system (an image forming system having a plurality of developing devices and a narrow interval between the developing devices) adopting a two-component developing system is disposed. Even if the peeled area is above the rotation axis of the developing sleeve, the developer after the development is sufficiently peeled off with a simple developing device configuration without using a scooping roller having a magnet inside, and ghosting Occurrence can be prevented.
Further, even when the distance between the conveying screw and the developing sleeve is shortened, uneven developer pressure caused by rotation of the conveying screw is reduced, and occurrence of uneven image density due to the eyes of the conveying screw can be prevented.
The effects of the invention of claim 1 will be described in detail below.
In a configuration where the developer can be peeled off by normal gravity, it is not necessary to bring the conveying screw close to the developing sleeve. Rather, from the viewpoint of preventing density unevenness due to the screw eyes, the distance between the conveying screw and the developing sleeve should be set as long as possible. ing.

  In contrast, the inventors conversely set the distance between the second conveying screw and the developing sleeve within the above-described range to ensure that the developer is taken into the screw and enhance the removal conveying effect to release the developing sleeve. The approach of improving sex was taken. The screw-like density unevenness that would newly occur was avoided by utilizing the high fluidity of the carrier as described below.

  Generally, when the developer fluidity is poor, the developer is likely to stay or aggregate, and the developer pressure is uneven depending on the location due to the shape of the object to which the transport pressure is applied, such as a transport screw. (Pressure unevenness). When the distance between the conveying screw and the developing sleeve is short, the developing sleeve and its pressure unevenness are transmitted onto the developing sleeve, resulting in uneven developer layer thickness and appear as image density unevenness corresponding to the shape of the conveying screw. However, even when the flowability of the developer is poor, if the distance between the conveying screw and the developing sleeve is sufficiently large, the pressure unevenness is relieved before reaching the developing sleeve, and the density unevenness hardly occurs.

  However, when the carrier surface is coated with a fluorine-modified silicone resin or fluorine-modified acrylic resin, which features low surface energy and a low coefficient of friction, the fluidity is drastically improved when the shape factor SF value of the carrier falls below 110. And have fluid properties. When the characteristics as a fluid become strong, the so-called Pascal principle allows a uniform developer pressure to propagate, and even when the distance between the conveying screw and the developing sleeve is short, pressure unevenness due to the shape of the conveying screw does not occur. A uniform image can be obtained.

  Further, by using such a carrier, the fluidity and releasability of the developer are improved, and the peelability of the carrier from the developing sleeve itself is also improved.

  As described above, the improvement of ghost and the avoidance of density unevenness due to screw eyes are not a short distance between the conveying screw and the developing sleeve, and the synergistic effect of both, not the single action of each of the high fluidity and high releasability carriers. Is achieved for the first time.

Further, it is possible to form a strong flow of current image agent near peeled area, since the developer in the vicinity of the peeling region can be performed circulation of good developer without staying, it is once peeled off at the peeling area It is possible to prevent the developer after development from reattaching to the developing sleeve. Further, by incorporating the developer having a reduced magnetic binding force with respect to the developing sleeve in the vicinity of the peeling region into the developer flow described above, the removal conveyance effect is improved, and a strong developer peelability can be obtained.

In addition , it is possible to prevent the charge generated during frictional charging with the toner from accumulating in the carrier, and as a result, the mirror image force of the carrier on the developing sleeve is reduced, and further excellent releasability can be obtained. Further, the mirror image force of the carrier with respect to each member in the developing device other than the developing sleeve such as the conveying screw and the developing device inner wall can be reduced, and the developer can be prevented from staying in the vicinity of each member, so that the developer can be smoothly circulated. It can be performed. Further, since the fluidity as the developer is improved, the pressure unevenness of the developer by the second conveying screw is further reduced.
Further , the sphericity of the carrier and the smoothness of the surface are improved, and the fluidity and releasability are further improved, the adhesive force to the developing sleeve is reduced, and good peelability can be obtained. In addition, by improving the fluidity, the developer pressure unevenness due to the second conveying screw is further reduced.

Further , since the sphericity of the toner is increased, the fluidity of the toner is improved, and the fluidity as a developer is further improved in combination with the high fluidity of the carrier. As a result, the peelability from the developing sleeve is further improved. Further, the developer pressure unevenness due to the second conveying screw is further reduced. In addition, the high fluidity of the toner can prevent the surface coating agent of the carrier from being worn and maintain the high fluidity of the developer over a long period of time.

In addition , the toner functions as a lubricant, the developer fluidity is further improved, and the peelability from the developing sleeve is further improved. Further, density unevenness due to the second conveying screw is also reduced. In addition, since the soft contact with the carrier can be realized due to the nature of the lubricant, the wear of the surface coating agent of the carrier can be prevented, and the high fluidity of the developer can be maintained over a long period of time.

  As described above, it is possible to realize a developing device with high image quality, small size, light weight, and low cost.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail based on specific examples shown in the accompanying drawings. FIG. 1 is a cross-sectional view of an essential part showing an outline of an image forming apparatus according to Embodiment 1 of the present invention, FIG. 2 is a cross-sectional view of an essential part showing an outline of an image forming unit, and FIG. FIG. 4 is a cross-sectional view of a main part of the developing device shown in FIG.

(Embodiment 1)
As shown in FIG. 1, the image forming apparatus according to the first embodiment of the present invention is held between the photosensitive drums 1Y and 1M by the support 2Y and comes into contact with the peripheral surface of the upstream photosensitive drum 1Y. A cleaning blade 3Y for removing the toner remaining on the surface of the body drum 1Y is disposed, and the developing roller 4M for adhering the toner to the peripheral surface of the adjacent photosensitive drum 1M has an axial center above the support 2Y. Further, they are arranged so as to be positioned on a line segment inclined toward the support 2Y side from the axis of the adjacent upstream photosensitive drum 1Y, and the members of other colors are also arranged in the same manner. With this arrangement, the pitch between the photosensitive drums 1Y, 1M, 1C, and 1K is shortened, and the apparatus is downsized.
Hereinafter, the configuration thereof will be described using the magenta developing device 8M, but the same applies to the developing devices 8Y, 8C, and 8K of other colors, and the description thereof will be omitted. The photoconductive drum 1M is a laminated organic photoconductor having an outer diameter of 24 mm, and rotates at a peripheral speed of 100 mm / s. The photosensitive drum 1M has a charging roller 5M for charging the photosensitive drum 1M while being driven to rotate by the photosensitive drum 1M, and a toner image formed on the photosensitive drum 1M for transferring to the intermediate transfer belt 11. A first transfer roller 6M is disposed. The charging roller 5M uses an outer diameter of 10 mm and a metal shaft formed with epichlorohydrin rubber, and the first transfer roller 6M uses an outer diameter of 12 mm and a metal shaft formed with a conductive urethane sponge. The intermediate transfer belt 11 was a polycarbonate sheet having an electric resistance of 1 × 10 ⁹ Ω · cm.

  An electrostatic latent image is formed by irradiating the surface of the photosensitive drum 1M uniformly charged by the charging roller 5M with a laser beam (not shown) according to image information, and then the developing area ( The developer magnetic brush conveyed between the developing roller 4M and the photosensitive drum 1M is rubbed against the electrostatic latent image, and only the toner moves to the surface of the photosensitive drum 1M to form a toner image. The laser power was 280 μW, a DC voltage of 1.2 kV was applied to the charging roller 5M, and the charged potential V0 and the post-exposure potential VL of the photosensitive drum 1M were measured to be −650V and −100V, respectively. . A bias voltage in which a rectangular wave AC voltage having a frequency of 3 kHz and a peak-to-peak voltage of 1.5 kV is superimposed on a DC voltage of −500 V is applied to the developing roller 4M.

  The toner image formed on the photosensitive drum 1M is transferred to the surface of the intermediate transfer belt 11 by the first transfer roller 6M to which a voltage of +600 V is applied. The above operation is performed for the image forming unit including the developing devices 8Y, 8M, 8C, and 8K for yellow, magenta, cyan, and black and the photosensitive drums 1Y, 1M, 1C, and 1K, and the four colors are combined on the intermediate transfer belt 11. A toner image is formed. Thereafter, the composite toner image is collectively transferred by the second transfer roller 7 onto the recording paper 10 conveyed from the recording paper tray 9, and heat, pressure, etc. are provided by the fixing device 12 provided on the discharge path of the recording paper 10. Is fixed on the surface of the recording paper 10 by the known means. On the other hand, the toner remaining on the surface of the photoconductive drum 1M is removed from the surface of each photoconductive drum 1M after the transfer of the toner image is completed by a cleaning blade 3M in which urethane rubber is formed into a sheet shape. One cycle of formation is complete.

  Hereinafter, the developing device according to Embodiment 1 of the present invention will be described in more detail using the developing device 8M with reference to FIGS. The same applies to the developing devices 8Y, 8C, and 8K for other colors.

  In the toner, 86% by weight of a polyester resin, 5% by weight of a pigment of each color, 6% by weight of a release agent carnauba wax, and 3% by weight of a charge control agent composed of zinc salicylate are premixed, melt-kneaded, and coarsely mixed. After pulverization, fine pulverization and classification were performed to obtain non-magnetic toner base particles having a volume average particle diameter of 7.1 μm. The toner base particles 98.5% by weight were externally added (mixed) with 1.0% by weight of hydrophobic silica and 0.5% by weight of hydrophobic titania as external additives. The toner shape factor SF value calculated in (Equation 2) described later was 148.

  The carrier will be described in detail later, including the experimental results.

  As shown in FIG. 2, the developing device according to the first embodiment is divided into two developer transport paths by a partition wall 13M, and is located above the partition wall 13M on the far side from the developing roller 4M. A first developer transport path 14M is provided, and a second developer transport path 15M is provided on the lower side of the partition wall 13M and closer to the developing roller 4M. An outward projecting portion 16M is formed in the second developer conveying path 15M, and a developing roller 4M projects from the projecting portion 16M. Here, the developing roller 4M is composed of seven magnets fixedly arranged in a rotatable aluminum developing sleeve 26M having a surface roughness Rz of 5 μm. These seven magnets are arranged so that a magnetic force peak is formed in the developing region where the developing roller 4M and the photosensitive drum 1M are close to each other, and a magnetic force valley is formed around the panning shaft 25M. In the present embodiment, the N pole is disposed in the development region, the main pole magnetic force is 95 mT, and the trimming shaft 25M is sandwiched between the S pole and the N pole. Further, an area where the magnetic force is almost zero is provided in the area where the second developer conveyance path 15M is close so that the south poles are adjacent to each other so that the developer after the development can be effectively peeled off. In order to make the magnetic force in this region as close to zero as possible, it is important to make the distance between magnets of the same polarity as large as possible. The magnetic force in this peeling region was measured, and it was confirmed that the magnetic force was 5 mT or less.

  The outer diameter of the developing roller 4M is 14 mm, the developing sleeve 26M rotates in the width direction with respect to the rotating direction of the photosensitive drum 1M at a peripheral speed of 114 mm / s, and has a circumference of 1.14 with respect to the photosensitive drum 1M. Has a speed ratio. The developing roller 4M and the photosensitive drum 1M are arranged so as to face each other, and the gap between the developing roller 4M and the photosensitive drum 1M can be adjusted by changing the diameters of the rollers 18M provided on both sides of the developing roller 4M. In the first embodiment of the present invention, the gap between the developing roller 4M and the photosensitive drum 1M in the developing region is adjusted to 0.4 mm.

  As shown in FIGS. 2 to 4, a second transport screw 17M extending along the axial direction is provided in the second developer transport path 15M located below the developing device 8M.

Since the developing sleeve 26M is arranged above the cleaning blade 3M of the adjacent upstream photosensitive drum 1M, the cleaning blade 3M of the adjacent upstream photosensitive drum 1M is close to the area below the developing sleeve 26M. In this position, the second conveying screw 17M cannot be installed, and is arranged above the rotation shaft of the developing sleeve 26M. From this, the separation region (around the middle point P connecting the south poles) where the south pole magnet pair for separating the developed developer from the development sleeve 26M is also higher than the rotation axis of the development sleeve 26M. Position.
Furthermore, a first conveying screw 19M that extends along the axial direction of the developing roller 4M is provided in the first developer conveying path 14M of the developing device 8M. As shown in FIG. 4, the second conveying screw 17M in the second developer conveying path 15M and the first conveying screw 19M in the first developer conveying path 14M are along the axial direction of the developing roller 4M. The blades or the rotation direction is set so as to convey the developer in opposite directions. For example, the first conveying screw 19M performs the agitating and conveying of the developer in the direction of the arrow X in FIG. The two conveying screws 17M are set so as to convey the developer in the direction of arrow Y in FIG.

  Further, as shown in FIG. 4, communication ports 20M and 21M that connect the first developer transport path 14M and the second developer transport path 15M are formed at both ends in the longitudinal direction of the developing device 8M. In the communication port 20M located at the tip of the developer conveying direction (X direction) of the first conveying screw 19M, an opening of an appropriate size is formed in the partition wall 13M, and the first The developer that has been stirred and conveyed by the conveying screw 19M falls from the first developer conveying path 14M into the second developer conveying path 15M due to gravity. In the embodiment of the present invention, the outer diameter, the shaft diameter, the screw pitch, and the number of rotations of the first conveying screw 19M and the second conveying screw 17M are set so that the toner is circulated satisfactorily by the two sets of conveying screws. , 12 mm, 5 mm, 25 mm, and 150 rpm, respectively.

  On the other hand, a magnet roller 22M is installed above the communication port 21M located at the tip of the second conveying screw 17M in the developer conveying direction (Y direction) so as to be connected to the end of the first conveying screw 19M. Yes. The magnet roller 22M is coupled so that its axis is coaxial with the rotation axis of the first conveying screw 19M, and its outer diameter is substantially matched with the outer diameter of the first conveying screw 19M. Then, as shown in FIG. 4, the magnet roller 22M is rotated in the same direction as the first conveying screw 19M, and the developer in the second developer conveying path 15M is conveyed in the form of a magnetic brush to the first developer. It is pumping up in the road 14M. The magnet roller 22M in the present embodiment is a rubber in which ferrite or magnetic powder-containing plastic is molded into a roller shape, and N poles and S poles are alternately magnetized in two poles (approximately every 90 °). A magnet is used. Also, magnetic members (not shown) such as SUS400 are pasted on both end faces, and the developer adheres to both end faces of the magnet roller 22M and prevents the developer from clogging and staying. is doing.

Further, a separation plate (not shown) is formed integrally with the first developer conveyance path 14M in the vicinity of the magnet roller 22M with respect to the rotation direction of the magnet roller 22M. Further, the separation plate is formed so that its separation surface is inclined with respect to the rotation axis direction of the first conveying screw 19M. Thereby, the developer separated from the magnet roller 22M by the separation surface is smoothly moved in the direction of the first conveying screw 19M. With the configuration described above, the developer is circulated between the second developer transport path 15M located on the lower side and the first developer transport path 14M located on the upper side.
Further, a toner concentration sensor 23M that detects the toner concentration in the developer from the magnetic permeability of the developer is disposed in the first developer conveyance path 14M, and when the toner concentration decreases due to printing, the toner supply port 24M. The controller is controlled so that the toner concentration is always 6% by supplying more toner.
The developer in the second developer conveyance path 15M adheres to the surface of the rotating developing sleeve 26M by a magnetic field from the developing roller 4M to form a magnetic brush, and an aluminum shaft having an outer diameter of 5 mm while rolling on the surface of the developing sleeve 26M. It is transported to a place where the ear cutting shaft 25M is installed. The magnetic brush reaches the developing area after being adjusted to a length of about 1 mm when passing through the panning shaft 25M, and the toner is transferred to the photosensitive drum 1M in accordance with the electrostatic latent image formed on the photosensitive drum 1M. Moves and development is performed. Note that the length of the magnetic brush can be changed by adjusting the inter-axis distance between the developing roller 4M and the ear cutting shaft 25M.

  The developer whose toner density is reduced due to toner consumption by development is transported to the peeling region P in which a pair of magnets of S poles is arranged along with the rotation of the developing sleeve 26M, and is released from the magnetic restraining force on the developing sleeve 26M. In addition, after being removed and conveyed by the second conveying screw 17M, the toner density is adjusted again to 6% while circulating in the first developer conveying path 14M and the second developer conveying path 15M.

  Using the developing device as described above, the removal conveyance effect of the second conveyance screw 17M close to the separation region P is used without using a drawing roller having a magnet or the like to separate the developer after printing. The ghost was evaluated using the distance d between the second conveying screw 17M and the developing sleeve 26M, the carrier shape factor SF value and the coating material as parameters. Also. The density unevenness due to the screw eyes that normally occurs when the distance d between the second conveying screw 17M and the developing sleeve 26M is shortened was also evaluated at the same time.

The experiment was conducted by changing the distance d between the rotary shaft of the second conveying screw 17M and the rotary shaft of the developing sleeve 26M to five levels of 15, 20, 25, 30, 35 mm. The carrier is a ferrite carrier having a volume average particle diameter of 50 μm, and the magnetization in a magnetic field with a specific gravity of 1 × 10 ¹⁴ Ω · cm and 79.58 kA / m in an electric field of 5 g / cc and 2 kV / cm, Under the condition of 65 Am ² / kg, the shape factor SF value was changed to five levels of 102, 110, 120, 130, and 140 to prepare carrier core particles, which were respectively modified with fluorine-modified silicone resin, fluorine-modified acrylic resin, and dimethyl A total of 20 types of carrier samples coated with silicone resin and acrylic resin were prepared. The shape factor SF value was calculated by the following (Equation 2) from the projected area (A) and the maximum length (ML) of each toner particle by an optical microscope using an image analyzer. Actually, the average value was obtained after measuring 500 pieces.

The evaluation of the ghost is based on a 25% duty halftone image immediately after printing a 100% duty solid patch of 14 × 3.14 / 1.14 = 39 mm, whose length in the sub-scanning direction corresponds to one rotation of the developing sleeve 26M. The difference was between the image density and the image density of a 25% duty halftone image when nothing was printed. When the above-mentioned image density difference ΔID is 0.03 or less with a Macbeth reflection densitometer RD914, there is no problem in image quality. Regarding the screw-like density unevenness, a 25% duty halftone image was printed on the entire surface of the paper, and the pass / fail judgment was made visually. Considering the difference in the characteristics of each color toner, ghost and screw-like density unevenness were evaluated for all colors of cyan, magenta, yellow and black.

  The evaluation results are shown in (Table 1) to (Table 4). (Table 1) is a case where the carrier coating agent is a fluorine-modified silicone resin, (Table 2) is a case where the coating agent is a fluorine-modified acrylic resin, (Table 3) is a case where the coating is a dimethyl silicone resin, and (Table 4) is a case where the acrylic resin is used. In the case where the image density difference ΔID used for the evaluation of the ghost is 0.03 or less, it is indicated by ◯, and when it is larger than 0.03, it is indicated by ×. The screw-like density unevenness is indicated by ◯ when the screw-like density unevenness is not confirmed by visual evaluation, and is indicated by × when it is confirmed.

  From the above results, a carrier coated with a fluorine-modified silicone resin or a fluorine-modified acrylic resin with a distance d between the rotation shaft of the second conveying screw 17M and the rotation shaft of the developing sleeve 26M within 25 mm and a shape factor SF value of 110 or less. When it was used, it was found that a uniform image without ghosting was obtained, and no screw-like density unevenness occurred.

  The above results can be explained as follows. As for the improvement of ghost, the effect of removing and transporting the developer in the vicinity of the peeling area has been improved by bringing the conveying screw close to the developing sleeve, and low surface energy such as fluorine-modified silicone resin or fluorine-modified acrylic resin. When the carrier surface is coated with a material having a low friction coefficient, the fluidity is drastically improved when the shape factor SF value is 110 or less. This is due to the two points that the peelability as a single carrier is improved. It is believed that there is.

  Regarding avoidance of screw-like density unevenness, when the shape factor SF value becomes 110 or less and the fluidity of the developer is improved and the characteristics as a fluid are strengthened, the pressure of the developer is uniformly propagated. Even when the distance between the conveying screw and the developing sleeve is reduced as described above, it is considered that pressure nonuniformity did not occur due to the shape of the conveying screw.

  Further, in a state where the developer fluidity is poor, there is a phenomenon in which developer clogging occurs between the blades of the conveying screw when the conveying screw rotates, and the conveying force of the developer decreases. Thus, in the state where the flowability of the developer is good, such a phenomenon does not occur, and it is possible to realize a stable developer conveying force and, in turn, a stable developer peelability. As a result of actually observing the state of the conveyance screw after the experiment, when the carrier has a shape factor SF value of 110 or less and a carrier coated with fluorine-modified silicone resin or fluorine-modified acrylic resin is used, it adheres between the blades of the screw. The amount of the developed developer was extremely small.

  As for the distance d between the rotating shaft of the second conveying screw 17M and the rotating shaft of the developing sleeve 26M, further experiments were conducted. As a result, the distance d-R-r at the position where the second conveying screw 17M and the developing sleeve 26M are closest to each other It was found that the relationship of the outer diameter r of the second conveying screw 17M is related to the developer peelability (ghosting). Here, R is the rotation radius of the developing sleeve 26M.

  FIG. 5 is a graph showing the relationship between the distance d-R-r and the ghost (the above-mentioned image density difference ΔID) where the second conveying screw 17M and the developing sleeve 26M are closest to each other. Thus, dRr is not more than twice the outer diameter r of the second conveying screw 17M (dRr ≦ 2r), that is, the rotational axis of the second conveying screw 17M and the rotational axis of the developing sleeve 26M. It can be seen that when the distance d is 3r + R or less (d ≦ 3r + R), the image density difference is 0.03 or less, and the ghost is improved.

  From the above results, the distance d between the rotation shaft of the second conveying screw 17M and the rotation shaft of the developing sleeve 26M is 25 mm, the carrier has a shape factor SF value of 105, and the surface is coated with a fluorine-modified silicone resin. .

  In the present invention, the carrier surface coating agent is a fluorine-modified silicone resin. However, according to experiments by the inventors, the same effect was obtained even with a fluorine-modified acrylic resin.

  Further, a tandem (one-pass) color image forming apparatus in which four color image forming units are arranged in parallel has been taken as an example. However, a peeling region in which a rotating shaft of a developer conveying screw and a magnet pair of the same polarity are arranged. May be a four-pass color image forming apparatus or a monochrome image forming apparatus in which four color developing devices are arranged on one photosensitive member, with the configuration being located above the rotation axis of the developing sleeve.

  Further, in the present invention, two sets of developer conveying screws are arranged vertically, and the developer is circulated in the vertical direction in the developing device. However, the two sets of developer conveying screws are arranged horizontally. The developer may be circulated horizontally.

  Further, in the present invention, an organic laminated type photoconductor having a negatively charged polarity is used, but a structure using a single layer type organic photoconductor or an a-Si material in which the charge transport layer and the charge generation layer are the same layer. It may be a photoconductor or the like, and the charge polarity of the photoconductor itself may be either negative charge or positive charge. The shape may be a belt shape instead of a drum shape.

In the present invention, the DC voltage is applied to the charging roller made of epichlorohydrin rubber, but a sine wave or rectangular wave AC voltage may be applied. Urethane rubber, silicone rubber, NBR rubber, acrylic rubber, fluorine rubber, or the like can be used as the charging roller, and surface treatment such as surface coating can be performed as necessary. Further, a scorotron method using a wire and a grid that have been conventionally used or a method using a solid charging element may be used.
As for the bias voltage applied to the developing roller, it is desirable to set the DC voltage to 50 to 650 V while adjusting the image density or the like as necessary. Further, the toner movement is further facilitated by applying a rectangular wave or sinusoidal AC voltage having a frequency of 1 to 6 kHz and an amplitude of about 0.2 to 10 kV in order to promote toner movement. By applying the AC voltage, it is possible to secure the necessary image density, prevent toner adhesion to the non-image area, and improve the fine dot reproducibility.
In the present invention, the number of magnets inside the developing roller is seven, but a plurality of other magnets may be used. The developing sleeve is also made of aluminum and has a surface roughness Rz of 5 μm, but other nonmagnetic materials may be used. The surface roughness Rz is preferably in the range of 3 to 10 μm from the viewpoint of image uniformity and developer transportability.
Further, in the present invention, the rotation direction of the developing sleeve is the width direction with respect to the rotation of the photosensitive drum, but it may be the rotation in the counter direction, and the peripheral speed ratio may not be 1.14. The developing roller, the first conveying screw, the outer diameter of the second conveying screw, the screw pitch of each conveying screw, and the number of rotations are not limited to the described values.

  The true specific gravity of the carrier is not limited to the above value, but from the viewpoint of obtaining high fluidity, the true specific gravity should be as small as possible, and is preferably 4 g / cc or less. The volume average particle diameter of the carrier need not be the above value, and is preferably 40 μm or less from the viewpoint of improving the image quality. The saturation magnetization of the carrier is also preferably as small as possible from the viewpoint of good releasability from the developing sleeve and the realization of a soft magnetic brush, but must be determined in consideration of carrier jump to the photosensitive drum.

(Embodiment 2)
FIG. 6 is a diagram showing the relationship between the distance from the rotating shaft of the screw, the unit cross-sectional area of the developer, and the transport amount per unit time in the range outside the screw. A carrier having a shape factor SF value of 105 and a surface coated with a fluorosilicone resin was used. As can be seen from FIG. 6, the developer conveyance amount decreases with increasing distance from the rotation axis of the conveyance screw, and is also related to the rotation radius of the conveyance screw.

FIG. 7 is a graph showing the relationship between the developer conveyance amount in the vicinity of the peeled area and the ghost (the aforementioned image density difference ΔID). From this result, it can be seen that the ghost is further improved when the developer transport amount in the peeling region is 1 g / cm ² / s or more. This is because a developer flow of 1 g / cm ² / s or more is necessary to completely prevent the developer once peeled off from re-adhering to the developing sleeve by the magnetic force of the magnet downstream of the peeling region. Means.
Therefore, in the first embodiment, for example, by making the distance d between the rotation shaft of the second conveying screw 17M and the rotation shaft of the developing sleeve 26M as short as 16 mm, it is possible to obtain a uniform image without further ghosting. . In addition, even when the rotation shaft of the second conveying screw 17M and the rotation shaft of the developing sleeve 26M are close to each other, since the high fluidity carrier is used, screw-like density unevenness does not occur.

(Embodiment 3)
The volume resistance resistance value in the electric field of 2 kV / cm of the carrier of Embodiment 1 is set to 1 × 10 ⁶ , 1 × 10 ⁸ , 1 × 10 ¹⁰ , 1 × 10 ¹² , 1 × 10 ¹⁴ , 1 × 10 ¹⁶ Ω · FIG. 8 shows the result of ghost evaluation with 5 levels changed from cm. It was found that when the volume resistance was 1 × 10 ¹⁰ Ω · cm or less, the above-described image density difference ΔID was further reduced, and ΔID was almost zero in the measured value of the reflection densitometer, and a uniform image was obtained. Further, there was no occurrence of screw-like density unevenness.

  This is a result that the image generated by frictional charging with the toner does not accumulate in the carrier, so that the mirror image force of the carrier with respect to the developing sleeve is reduced and the peelability is improved. In addition, the mirror image force of the carrier on each member in the developing device such as a conveying screw is also reduced, and the developer can be prevented from aggregating and staying around each member. Thus, the developer can be removed and transported with ease. In addition, pressure unevenness due to the conveying screw is less likely to occur.

(Embodiment 4)
In the configuration of Embodiment 1, 12% by weight of thermosetting phenol resin is used as a binder, 88% by weight of magnetite having an average particle size of 0.2 μm is dispersed, the average particle size is set to 35 μm, and the carrier surface is modified with fluorine. Evaluation was performed using a resin carrier coated with a silicone resin. When the volume resistivity of this carrier in an electric field of ² kV / cm was measured, it was 4 × 10 ⁸ Ω · cm, the true specific gravity was 3.7 g / cc, and the magnetization at a magnetic field of 79.58 kA / m was 60 Am ² / kg. It was. Further, when the shape factor SF value was measured, it was found that the shape was very close to a true sphere of 101, and the surface of the carrier was observed with an electron microscope.

  When the ghost was evaluated using this carrier, the above-mentioned image density difference ΔID was further reduced to 0.01 or less, and an image with extremely good image density uniformity was obtained. Further, screw-like density unevenness did not occur.

  This is because the SF value of the carrier is 101, which is very close to a true sphere, and the surface is also very smooth, so that the developer fluidity and releasability are good, and the peelability from the developing sleeve is very good. By becoming. Further, due to the high fluidity of the developer, screw-like density unevenness does not occur.

  As the thermosetting binder of the resin carrier, urea resin, melamine resin, polyester resin, epoxy resin and the like can be used in addition to the phenol resin. Since the thermosetting resin is superior in durability, impact resistance, and heat resistance as compared with the thermoplastic resin, a resin carrier composed of a magnetic body and a thermosetting resin taking advantage of these advantages is desired.

  The ferromagnetic iron compound particle powder used in the present invention contains one or more kinds of ferromagnetic iron oxide particle powders such as magnetite and maghemite, and metals other than iron (Mn, Ni, Zn, Mg, Cu, etc.). Spinel ferrite particle powder, magnetoplumbite type ferrite particle powder such as barium ferrite, and iron or iron alloy fine particle powder having an oxide film on its surface can be used. Ferromagnetic iron oxide particles such as magnetite are preferred. The ferromagnetic iron compound particles preferably have a particle size of 0.02 to 5 μm. The shape may be any of granular, spherical and acicular. Furthermore, the addition amount of the magnetic substance in the thermosetting resin is desirably 50 to 90% by weight. If it exceeds 90% by weight, dispersion in the resin becomes difficult and the magnetic substance is detached. On the other hand, when it is less than 50% by weight, the magnetic force of the carrier becomes small, and there arises a problem that the carrier easily adheres to the toner holding member.

  Moreover, a fluorine-modified acrylic resin can also be used for the coating agent on the carrier surface.

(Embodiment 5)
In the first embodiment, in order to investigate the relationship between the toner shape factor SF value, the ghost and the screw-like density unevenness, the toner base having the shape factor SF value 160 after the pulverization and classification process is converted into a sufusing device ( (Nippon Pneumatic) was used, and the spheroidization treatment was performed using heat at 350 degrees, 290 degrees, 230 degrees, 200 degrees, and no treatment. The shape factor SF values of the toner spheroidized under the respective conditions are 102, 120, 139, 148, 162, and experiments were performed using these toners. The carrier used is a fluorine-modified silicone resin coated on the surface. FIG. 9 shows the relationship between the toner shape factor SF value and the ghost occurrence state (the aforementioned image density difference ΔID). When the shape factor SF value is 140 or less, ΔID becomes almost zero, and a more uniform image is obtained. I understand that Further, there was no occurrence of screw-like density unevenness.

  Further, in the case of toner having a shape factor SF value of 120, as in the first embodiment, the distance d between the rotation shaft of the second conveying screw 17M and the rotation shaft of the developing sleeve 26M, and the shape factor SF value of the carrier are parameters. As shown in (Table 5), the tolerance of the ghost and screw-like density unevenness was increased.

  The above shows that by setting the toner shape factor SF value to 140 or less, the toner approaches the true sphere, the developer fluidity is further improved, and the peelability from the developing sleeve is improved. In addition, due to the high fluidity of the developer, pressure unevenness due to the conveying screw is reduced, and the tolerance of screw-like density unevenness is increased.

  Further, when the time-dependent characteristics of the ghost occurrence state are examined, when the shape factor SF value of the toner is 148 as shown in FIG. 10, the image density difference ΔID reaches 0.04 after printing 60 k sheets, and the ghost is NG. On the other hand, when the shape factor SF value is 120, up to 80k uniform images with no ghosting were obtained.

  This is because the clogging of the developer between the blades of the conveying screw does not occur due to the high fluidity of the developer, so that good removal and conveyance in the peeling area can be maintained and the shape of the toner is true. This is because, since it is close to a sphere, there is little stress on the carrier at the time of stirring, and the coating agent of the carrier does not peel off, and high fluidity and high releasability due to the action of the coating agent can be maintained. Actually, when the surface of the carrier after printing 80k sheets was observed with an electron microscope, the coating agent peeled off when the SF value was 148, whereas the coating agent peeled off when the SF value was 120. Was hardly seen.

  In the present invention, the heat treatment method is used as a method for increasing the sphericity of the toner. However, a chemical polymerization method such as a suspension polymerization method or an emulsion polymerization method may be used as a method for preparing the toner base.

(Embodiment 6)
As an external additive, 1.5% by weight of zinc stearate was added to the toner in the first embodiment, and ghost and screw-like density irregularities were evaluated in the same manner as in the first embodiment. In order to make a comparison with the first embodiment, a carrier having a surface coated with a fluorine-modified silicone resin is used, and the shape factor SF value of the toner is 148. The results are shown in (Table 6).

  Further, FIG. 11 shows the ghost characteristics over time when zinc stearate is not added to the toner (Embodiment 1) and when it is added (Embodiment 6). When zinc stearate was not added, the image density difference ΔID reached 0.04 after printing 60k sheets, and the ghost became NG, whereas when zinc stearate was added, 80k sheets Also, ΔID was 0.02, and a uniform image without ghost was obtained.

  The above results indicate that by adding zinc stearate to the toner, the toner functions as a lubricant, and the developer fluidity is improved as in the case of the shape factor SF value of the toner. This is considered to be because the peelability from the surface is improved. In addition, by improving the fluidity of the developer, the pressure unevenness due to the conveying screw is reduced, and the tolerance of the screw-like density unevenness is increased.

  The developer's peelability (ghosting resistance) with time stability is due to the improvement in developer fluidity, which prevents the developer from clogging with the conveying screw, and the function of the toner as a lubricant. This is achieved by the fact that high fluidity of the developer can be maintained over a long period of time by preventing peeling.

  In the present invention, zinc stearate is added as a method for enhancing the function of the toner as a lubricant. In addition, at least one metal soap of calcium stearate, aluminum stearate, and magnesium stearate is included in the toner. It does not matter as a configuration.

  The vertical circulation developing device of the present invention can be applied to electrophotographic image forming apparatuses such as copying machines, facsimiles, and laser printers.

FIG. 1 is a cross-sectional view of a main part showing an outline of an image forming apparatus according to Embodiment 1 of the present invention. FIG. 2 is a cross-sectional view of the main part showing an outline of the image forming unit in Embodiment 1 of the present invention. 1 is a perspective view schematically showing a developing device according to Embodiment 1 of the present invention. FIG. 2 is a cross-sectional view of the main part of the developing device shown in FIG. The figure which shows the relationship between dRr and an image density difference Diagram showing the relationship between the distance from the screw rotation axis and the developer transport amount Diagram showing the relationship between developer transport amount and image density difference The figure which shows the relationship between the carrier resistance value and image density difference in Embodiment 3 of this invention The figure which shows the relationship between the shape factor SF value of toner and image density difference in Embodiment 5 of this invention. The figure which shows the time-dependent characteristic of the image density difference in Embodiment 5 of this invention. The figure which shows the time-dependent characteristic of the image density difference in Embodiment 6 of this invention.

Explanation of symbols

1M photosensitive drum 4M developing roller 8M developing device 13M partition wall 14M first developer transport path 15M second developer transport path 16M overhang 17M second transport screw 19M first transport screw 23M toner concentration sensor 24M toner supply port 25M ear Cutting shaft 26M Development sleeve

Claims (1)

A latent image carrier for forming an electrostatic latent image on the surface;
A developing sleeve for supplying a developer comprising toner and a carrier to the latent image carrier; and at least one pair of magnets having the same polarity provided in the developing sleeve; A developer carrying member in which a midpoint on the developing sleeve that connects is in a developing device and is higher than a rotation axis of the developing sleeve;
One first developer transport path of two developer transport paths arranged in parallel with the developer carrier;
A first developer conveying means installed in the first developer conveying path for conveying the developer in a first direction;
The other second developer transport path located closer to the developer carrier than the first developer transport path,
It is installed in the second developer conveyance path, and the rotation shaft is at a position higher than the rotation shaft of the developing sleeve, and conveys the developer in a second direction opposite to the first direction while rotating. And a second developer conveying means for supplying and peeling the developer with respect to the developer carrying member,
The rotation radius r of the second developer conveying means, the rotation radius R of the developing sleeve, and the distance d between the respective rotation axes satisfy the following relational expression (Equation 1):
The developer conveyance amount at the middle point on the developing sleeve connecting the same poles of the magnet pair is 1 g / cm ² / s or more,
The toner has a toner surface shape factor SF value of 100 or more and 140 or less, and contains at least one metal soap of zinc stearate, calcium stearate, aluminum stearate, magnesium stearate as a lubricant in the toner,
The carrier has a shape factor SF value of 100 or more and 110 or less, is coated with a fluorine-modified silicone resin or a fluorine-modified acrylic resin on the surface, and has a volume resistance of 1 × 10 ¹⁰ Ω · cm or less in an electric field of 2 kV / cm. , Using a resin carrier in which a thermosetting resin is used as a binder and a magnetic material is dispersed inside ,
A developing device.

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