Patent Grant
8655237
The present invention generally relates to a development device that uses two-component developer consisting essentially of toner and carrier, and an image forming apparatus, such as a copier, a facsimile machine, a printer, or multifunction machine capable of at least two of these functions, that includes the development device.
In image forming apparatuses such as electrophotographic copiers, electrostatic recording devises, or magnetic recording devices, two-component type development devices using two-component developer are widely used for developing electrostatic latent images formed on latent image bearers.
Such two-component development devices typically include a developer bearer rotatable relative to a casing of the development device, a stationary magnetic field generator provided inside the developer bearer, and a developer regulator disposed across a gap (regulation gap) from the surface of the developer bearer, upstream in the direction of rotation of the developer bearer from a development range facing a latent image bearer. The magnetic field generator has multiple magnetic poles and may be constructed of multiple magnets. The magnetic field generator includes an attraction pole or pump-up pole for generating a magnetic force to attract the developer (i.e., developer particles) to the surface of the developer bearer (hereinafter “attraction magnetic force”) and a development pole for generating a magnetic force to cause the developer to stand on end on the developer bearer in the development range.
With the magnetic force generated by the magnetic field generator, the developer is carried on the surface of the developer bearer and transported to the development range. In the development range, the developer standing on end on the developer bearer forms a magnetic brush, which slidingly contacts the surface of the latent image bearer. Then, toner in the developer adheres to the electrostatic latent image formed on the latent image bearer, thus developing it into a toner image (development process).
For example, JP-2008-256813-A proposes a two-component development device in which a developer supply compartment and a developer collection compartment are formed by the casing and interior wall therein, and conveyance screws (i.e., developer supply screw and developer collecting screw) are provided therein. The developer supply compartment is positioned adjacent to the developer bearer, and a side wall of the developer supply compartment or a partition divides, at least partially, the developer supply compartment from the portion where the developer bearer is provided. The developer supply screw supplies the developer from the developer supply compartment to the developer bearer while transporting the developer in the axial direction of the developer bearer. The developer in the developer supply compartment overstrides the side wall and is carried on the surface of the developer bearer due to the attraction magnetic force.
As the developer bearer rotates, the developer reaches the regulation gap, which is a gap between the surface of the developer bearer and the developer regulator. Only the developer adjacent to the surface of the developer bearer can pass through the regulation gap, and the developer positioned away from the surface of the developer bearer is blocked by the developer regulator. Thus, with the regulation gap, the amount of developer transported to the development range can be adjusted, and the developer removed by the developer regulator from the developer bearer is returned to the supply compartment and is again supplied to the developer bearer. Thus, the developer is circulated inside the development device.
The amount of developer transported to the regulation gap, however, fluctuates when the properties of the developer, such as fluidity, change due to the degradation of the developer over time or changes in the environment. In this case, the development ability becomes unstable.
In view of the foregoing, several approaches have been tried. For example, the magnetic field generator may be configured to have another magnetic pole for generating a magnetic force to cause the developer to stand on end on the developer bearer (hereinafter “regulation magnetic force”) when the developer passes through the regulation gap to alleviate the fluctuation in the amount of developer supplied to the development range.
Although this approach is effective to a certain extent, the regulation magnetic force can also act on the developer blocked by the developer regulator, retaining such developer (hereinafter “retained developer”) in a portion downstream from the developer regulator in the direction of rotation of the developer bearer (hereinafter “retaining portion”). In the retaining portion, the retained developer is circulated in the direction opposite the direction of rotation of the developer bearer. While thus retained by the regulation magnetic force and circulating in the retaining portion, the retained developer is further electrically changed by sliding contact. Accordingly, the amount of charge of the toner in the retained developer is higher than that of the other developer circulated in the development device, and thus the development ability, that is, the amount per unit area of toner adhering to the electrostatic latent image during the development process, is different therebetween.
Although unevenness in image density can be limited as long as such developers having different levels of development ability are mixed well, the unevenness in image density is visible if they are mixed insufficiently, degrading the image quality. In conventional development devices, it may be difficult to sufficiently mix developers having different levels of development ability. Consequently, unevenness in image density can occur, and accordingly the image quality can be degraded.
In view of the foregoing, in one illustrative embodiment of the present invention, a development device includes a cylindrical developer bearer to carry by rotation two-component developer to a development range where the developer bearer faces a latent image bearer, a magnetic field generator disposed inside the developer bearer for generating magnetic force to keep the developer on a circumferential surface of the developer bearer, a developer regulator disposed upstream from the development range and facing the circumferential surface of the developer bearer across a regulation gap for adjusting an amount of the developer carried by the developer bearer to the development range, a supply compartment disposed adjacent to the developer bearer, from which the developer is supplied to the developer bearer and in which the developer removed from the developer bearer by the developer regulator is collected, and a developer agitator provided in the supply compartment for transporting the developer in an axial direction of the developer bearer. A side wall partially separates the supply compartment from a portion where the developer bearer is provided, and a blocker is provided facing an upper end of the side wall of the supply compartment across a supply gap through which the developer moves from the supply compartment toward the developer bearer. The supply gap extends at least over the entire development range in the axial direction of the developer bearer. The blocker prevents the developer blocked by the developer regulator from moving along a magnetic force line of the regulation magnetic force toward the circumferential surface of the developer bearer. The magnetic field generator includes an attraction magnetic pole for generating an attraction magnetic force to attract the developer from the supply compartment over the upper end of the side wall of the supply compartment to the circumferential surface of the developer bearer as well as a regulation magnetic pole for generating a regulation magnetic force to cause the developer passing through the regulation gap to stand on end on the circumferential surface of the developer bearer.
In another illustrative embodiment, the attraction magnetic pole and the regulation magnetic pole of the magnetic field generator are adjacent to each other and have the opposite polarities.
In another illustrative embodiment, an image forming apparatus includes a latent image bearer on which an electrostatic latent image is formed and the above-described development device.
A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result.
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views thereof, and particularly to
An image forming apparatus according to one embodiment of the present invention, which may be a multicolor laser printer, is described below.
The image forming apparatus 100 includes four image forming stations 1M, 1C, 1Y, and 1K for forming magenta, cyan, yellow, and black toner images. The image forming stations 1M, 1C, 1Y, and 1K are arranged vertically in
The image forming stations 1M, 1C, 1Y, and 1K have a similar configuration except the color of toner used therein. Therefore, only the image forming station 1M is described below, and descriptions of other image forming stations 1C, 1Y, and 1K are omitted. The image forming station 1M includes a process unit 2M, an optical writing unit 10M, and a development device 20M.
The process unit 2M for magenta includes a drum-shaped photoreceptor 3M that rotates counterclockwise in
The charging unit 4M uniformly charges a surface of the photoreceptor 3M that rotates counterclockwise in
The optical writing unit 10M includes a light source such as a laser diode, a polygon minor that is a regular hexahedron, a polygon motor to rotate the polygon minor, an f-θ lens, lenses, and reflection mirrors. The light source is driven according to image data transmitted from, for example, computers and emits a laser beam L. As the polygon mirror rotates, the laser beam L is reflected on the faces of the polygon minor, thus deflected, and reaches the photoreceptor 3M. While the surface of the photoreceptor 3M is thus scanned optically, an electrostatic latent image is formed thereon.
The development device 20M includes a casing in which an opening is formed and a development roller 21M that is exposed partially through the opening. The casing of the development device 20M contains magenta developer constituting essentially of magnetic carrier and magenta toner charged to a negative potential. Referring to
The development device 20M further includes two conveyance screws (developer agitators), namely, a supply screw 32 and a collecting screw 35, to transport the magenta developer while agitating it to facilitate triboelectric charging thereof, and the magenta toner is adsorbed on a surface of the rotating development sleeve 22 of the development roller 21M by the magnetic force exerted by the magnet roller 23. The amount of the developer carried on the development sleeve 22 is adjusted by a doctor blade 25M as the rotating development sleeve 22 passes by the doctor blade 25M, after which the developer is carried to a development range facing the photoreceptor 3M.
A power source applies a development bias of negative polarity to the development sleeve 22, and, in the development range, a development potential acts between the development sleeve 22 and the electrostatic latent image formed on the photoreceptor 3M to transfer the magenta toner of negative polarity electrostatically from the development sleeve 22 to the latent image. By contrast, a non-development potential acts between the development sleeve 22 and the uniformly charged portions (background portion) of the photoreceptor 3M to transfer the magenta toner of negative polarity electrostatically from the photoreceptor 3M to the development sleeve 22. Thus, the magenta toner in the magenta developer carried on the development sleeve 22 is transferred by the effects of the development potential to the electrostatic latent image on the photoreceptor 3M, and the electrostatic latent image is developed into a magenta toner image. After the magenta toner therein is thus consumed, the magenta developer is returned from the development sleeve 22 inside the casing as the development sleeve 22 rotates.
The development device 20M further includes a toner concentration detector that in the present embodiment is a magnetic permeability sensor. The toner concentration detector outputs a voltage corresponding to the magnetic permeability of the magenta developer contained in a developer collection compartment 28, which is described later, provided in the development device 20M. Since the magnetic permeability of developer has a good correlation with the concentration of toner in the developer, the toner concentration detector outputs a voltage corresponding to the toner concentration. The value of the output voltage is transmitted to a toner supply controller. The toner supply controller includes a storage unit such as a random access memory (RAM) and stores target values Vtref for the output voltages from the toner concentration detectors respectively provided in the development devices 20M, 20C, 20Y, and 20K in the storage unit. For supplying magenta toner, the toner supply controller compares the voltage output from the magenta toner concentration detector with the target value Vtref for magenta and drives a magenta toner supply device for a time period corresponding to the comparison result. With this operation, fresh magenta toner is supplied to the developer collection compartment 28 in the development device 20M. By controlling the driving of the magenta toner supply device, toner is supplied as required to the magenta developer in which the toner concentration is decreased as the toner is consumed in image development, and the concentration of magenta toner in the magenta developer can be kept within a predetermined range. Similar toner supply control is performed in the development devices 20C, 20Y, and 20K.
Referring to
After the transfer process, the drum cleaning unit 5M removes any toner remaining on the surface of the photoreceptor 3M. Subsequently, the discharge lamp 6M removes the electrical potential remaining on the photoreceptor 3M, after which the charging unit 4M charges the surface of the photoreceptor 3M uniformly.
It is to be noted that, although the image forming station 1M for magenta is described above, also in other image forming stations 1C, 1Y, and 1K, cyan, yellow, and black toner images are respectively formed on the photoreceptors 3C, 3Y, and 3K through similar processes.
The transfer unit 50, positioned on the right of the vertically arranged image forming stations 1M, 1C, 1Y, and 1K in
Transfer chargers 55M, 55C, 55Y, and 55K are provided inside the loop of the intermediate transfer belt 51 in addition to the above-described three rollers. The transfer chargers 55M, 55C, 55Y, and 55K are positioned on the backsides of the respective primary-transfer nips and apply electrical charges to the back surface of the intermediate transfer belt 51. The electric charges thus applied to the intermediate transfer belt 51 generate transfer electric fields in the respective primary-transfer nips to transfer the toner electrostatically from the photoreceptors 3M, 3C, 3Y, and 3K to the front side of the intermediate transfer belt 51. It is to be noted that, instead of the corona charging transfer chargers, transfer rollers to which a transfer bias is applied may be used.
In the respective primary-transfer nips, the magenta, cyan, yellow, and black toner images are transferred primarily from the respective photoreceptors 3M, 3C, 3Y, and 3K and superimposed one on another on the front side of the intermediate transfer belt 51 due to the nip pressure and effects of the transfer electric field (primary transfer process). Thus, a superimposed four-color toner image is formed on the intermediate transfer belt 51.
Additionally, a secondary-transfer bias roller 56 is provided in contact with the front side of a portion of the intermediate transfer belt 51 winding around the driving roller 52, thus forming a secondary-transfer nip therebetween. A voltage application unit that includes a power source and wiring applies a secondary-transfer bias to the secondary-transfer bias roller 56, and thus a secondary-transfer electric field is generated between the secondary-transfer bias roller 56 and the driving roller 52 that is grounded. The four-color toner image formed on the intermediate transfer belt 51 is transported to the secondary-transfer nip as the intermediate transfer belt 51 rotates.
Additionally, the image forming apparatus 100 includes a sheet cassette for containing a bundle of recording sheets P. The recording sheets P contained in the sheet cassette are fed to a paper feeding path from the top at a predetermined timing. A pair of registration rollers 60 pressing against each other is provided downstream form the sheet cassette in a direction in which the recording sheet P is transported (hereinafter “sheet conveyance direction”), and the recording sheet P gets stuck in a nip between the registration rollers 60.
Although the pair of registration rollers 60 rotates to catch the recording sheet P in the nip, both rollers stop rotating immediately after catching a leading end of the recording sheet P. The recording sheet P is then transported to the secondary-transfer nip, timed to coincide with the four-color toner image formed on the intermediate transfer belt 51. In the secondary-transfer nip, the four-color toner image is transferred secondarily from the intermediate transfer belt 51 onto the recording sheet P at a time. Then, the four-color toner image becomes a full color toner image (hereinafter “multicolor toner image”) on the while recording sheet P. Subsequently, the recording sheet P carrying the multicolor toner image is transported to a fixing device, where the multicolor toner image is fixed on the recording sheet P.
A belt cleaning unit 57 is provided downstream from the secondary-transfer nip in the sheet conveyance direction and presses against the driven roller 54 via the intermediate transfer belt 51 to remove any toner remaining on the intermediate transfer belt 51 after the secondary transfer process.
It is to be noted that the suffixes M, C, Y, and K attached to each reference numeral indicate only that components indicated thereby are used for forming magenta, cyan, yellow, and black images, respectively, and hereinafter may be omitted when color discrimination is not necessary.
In
The development roller 21 is positioned with the circumferential surface of the development sleeve 22 partly exposed through the opening formed in the casing on the side facing the photoreceptor 3. On the side opposite the photoreceptor 3, the development sleeve 22 faces the supply compartment 27 as well as the collection compartment 28 over the substantially entire axial length of the development sleeve 22. The collection compartment 28 is positioned beneath the development roller 21. In the present embodiment, the supply compartment 27 is positioned on the side of the development roller 21, slightly lower than the development roller 21 in
The supply screw 32 provided inside the supply compartment 27 is formed of a nonmagnetic material such as resin and extends horizontally similarly to the photoreceptor 3 and the development roller 21. The supply screw 32 includes a rotary shaft 33, which may be nonmagnetic resin or nonmagnetic metal, and spiral-shaped screw blade 34 projecting from the circumferential surface of the rotary shaft 33. The rotary shaft 33 and the screw blade 34 integrally rotate counterclockwise in
The collecting screw 35 provided inside the collection compartment 28 extends horizontally as well, similarly to the photoreceptor 3 and the development roller 21. The collecting screw 35 includes a rotary shaft 36 and spiral-shaped screw blade 37 formed of a nonmagnetic material such as resin, projecting from the circumferential surface of the rotary shaft 36. The rotary shaft 36 and the screw blade 37 integrally rotate counterclockwise in
Although partially separated by a partition 43 forming a side wall of the supply compartment 27 on the side of the development roller 21, the supply compartment 27 and the collection compartment 28 can communicate with each other through openings formed in either end portion of the partition 43 in the axial direction of the development roller 21. It is to be noted that reference numeral 44 shown in
In the supply compartment 27, the developer carried inside the screw blade 34 of the supply screw 32 (hereinafter “developer G1”) is transported from the front to the back in the direction perpendicular to the surface of the paper on which
As the development sleeve 22 rotates, the developer carried thereon (hereinafter “developer G2”) is transported to the development range and is used in image development. Subsequently, the developer G2 is transported to a position facing the collection compartment 28 as the development sleeve 22 rotates. Then, separated from the surface of the development sleeve 22 by a repulsive magnetic field generated by the magnet roller 23, the developer G2 falls to the collection compartment 28 as indicated by broken arrow B shown in
In the collection compartment 28, the developer G2 carried inside the screw blade 37 of the collecting screw 35 is transported from the back side to the front side of the paper on which
In the present embodiment, the magnet roller 23 includes five magnetic poles N1, S1, N2, S2, and S3 arranged in that order in the direction opposite the direction in which the development sleeve 22 rotates as shown in
The magnetic pole N2 serves as a regulation pole to generate a regulation magnetic force for causing the developer to stand on end on the development sleeve 22 when the developer passes through a regulation gap, which is a gap between the surface of the development sleeve 22 and the doctor blade 25 serving as a developer regulator. The magnetic pole S2 serves as an attraction pole or pump-up pole to generate a magnetic force for pumping up the developer onto the surface of the development sleeve 22. The magnetic pole S3 cooperates with the magnetic pole S2 to generate the repulsive magnetic field for separating the developer from the development sleeve 22 and collecting it in the collection compartment 28.
In the above-described image forming apparatus 100 according to the present embodiment, the four photoreceptors 3M, 3C, 3Y, and 3K serve as the latent image bearers to rotate and carry the latent image formed on the their surfaces. The optical writing units 10M, 10C, 10Y, and 10K serve as latent image forming units to form latent images on the respective photoreceptors 3 charged uniformly. Further, the development devices 20M, 20C, 20Y, and 20K develop the latent images formed on the photoreceptors 3M, 3C, 3Y, and 3K.
Next, a comparative development device is described below with reference to
The development device 120 is different from the development device 20 according to the first embodiment in that an upper end of a partition 143 is positioned higher than that of the partition 43 in the development device 20 and that the shielding wall 44 (blocker) is not provided. Components of the development device 120 similar to those of the development device 20 shown in
In the comparative development device 120, the regulation magnetic force exerted by the regulation pole N2 acts on the developer G3 that has been prevented from passing through the regulation gap and retains the developer G3 in a retaining portion adjacent to and upstream from the doctor blade 25Z in the direction of rotation of the development sleeve 22Z. As the development sleeve 22Z rotates, the developer G3 retained in the retaining portion (hereinafter “retained developer G3”) is circulated in the retaining portion in the direction opposite the direction of rotation of the development sleeve 22Z as indicated by broken arrow Y1. It is to be noted that it is possible that the retained developer G3 includes the developer G1 flipped up by the supply screw 32Z.
While retained by the regulation magnetic force and circulated in the retaining portion, the retained developer G3 is further electrically changed by sliding contact. As a result, the amount of charge of the toner (hereinafter “toner charge amount”) in the retained developer G3 is remarkably higher than that of the developer G1 in the supply compartment 27Z. This causes a difference in development ability between the retained developer G3 and the developer G1 in the supply compartment 27Z. Even if the development ability is different, visible unevenness in image density is not caused as long as the developer G1 and the retained developer G3 are dispersed uniformly and mixed. The unevenness in image density, however, becomes visible if mixing of the developers G1 and G3 are insufficient, degrading the image quality.
In the comparative development device 120, the developer G3 escaped the restraint by the regulation magnetic force while being circulated is collected in the supply compartment 27Z. The developer G3 collected in the supply compartment 27Z can be sufficiently mixed with the developer G1 before pumped up to the development sleeve 22Z again, and thus the above-described degradation in image quality be prevented. However, the attraction pole S2 having the reverse polarity to that of the regulation pole N2 is positioned adjacent to and upstream from the regulation pole N2. Consequently, in the comparative development device 120, a magnetic field in which the magnetic force lines extending from the regulation pole N2 pass through the retaining portion and are curved toward the attraction pole S2 is formed. In such a magnetic field, a portion of the retained developer G3 closest to the attraction pole S2 (close to the upper end of the partition 143) moves to the attraction pole S2 along the magnetic force lines and then is attracted to the development sleeve 22Z. As a result, a part of the retained developer G3 is not collected in the supply compartment 27Z but is transported directly to the surface of the development sleeve 22Z.
At that time, when the amount of the developer G1 pumped up onto the development sleeve 22Z from the supply compartment 27Z is sufficient, the developer G3 attracted by the attraction magnetic force overlays the developer G1. In this case, because the developer G3 is positioned at the uppermost position, away from the surface of the development sleeve 22Z, the developer G3 is blocked by the doctor blade 25Z and does not pass through the regulation gap. Accordingly, the developer layer transported by the development range can contain the developer G1 only. Consequently, unevenness in image density and the degradation in image quality can be prevented or inhibited.
However, in the comparative development device 120 shown in
In particular, the comparative development device 120 shown in
Therefore, as shown in
The shielding wall 44 can prevent the retained developer G3 attracted by the attraction magnetic force from hindering pumping up the developer G1 from the supply compartment 27. Therefore, local shortage of the developer G1 pumped up from the supply compartment 27 can be prevented or restricted. Accordingly, the developer G3 attracted by the attraction magnetic force is less likely to pass through the regulation gap and be held in the portion adjacent to the surface of the development sleeve 22. Accordingly, the above-described developer layer in which the developer G3 including the excessively charged toner and the developer G1 including the normally charged toner are mixed insufficiently is not conveyed to the development range, thus restricting unevenness in the image density and the degradation of image quality.
Additionally, the shielding wall 44 is positioned across a slit 45 (supply gap) from the upper end of the partition 43 for allowing the developer G1 to move from the supply compartment 27 toward the development sleeve 22. More specifically, the slit 45 extends at least over the entire length of the development range in the axial direction of the development sleeve 22. Therefore, even in the configuration that includes the shielding wall 44, pumping up the developer G1 from the supply compartment 27 to the development sleeve 22 is not hindered. In particular, in the first embodiment, the slit 45 is positioned such that a straight line La (shown in
Additionally, as shown in
The opening width, which is the length of the slit 45 in the rotational direction of the development sleeve 22 or the distance between the lower face of the shielding wall 44 to the upper face of the partition 43 in
Further, the regulation gap between the doctor blade 25 and the surface of the development sleeve 22 has a sufficient size for reliable supply of the predetermined amount of developer to the development range because fluctuations in the amount of developer supplied to the development range can affect the development ability significantly. If a shielding wall gap, meaning a distance between the surface of the development sleeve 22 and a portion of the shielding wall 44 closest to the development sleeve 22, is smaller than the regulation gap, the amount of developer carried on the development sleeve 22 and transported through the shielding wall gap is reduced from the amount of developer transported through the regulation gap. In such a case, even if the developer transported through the shielding wall gap includes only the developer G1 pumped up from the supply compartment 27 without the retained developer G3, the developer layer that passes through the regulation gap can include the retained developer G3 overlying the developer G1. If the retained developer G3 is dispersed uniformly in the developer layer that passes through the regulation gap, a uniform image density can be maintained, keeping a satisfactory image quality, even in this case. However, a desired image density cannot be attained when the ratio of the retained developer G3 having excessively charged toner particles is higher in the developer layer that contributes to image development in the development range.
In view of the foregoing, in the present embodiment, the shielding wall gap between the shielding wall 44 and the surface of the development sleeve 22 is similar or wider than the regulation gap in a portion where the shielding wall 44 is closest to the surface of the development sleeve 22. With this configuration, the developer layer that has passed through the shielding wall gap can pass through the regulation gap as is. That is, the developer layer that passes through the regulation gap can include only the developer G1 pumped up from the supply compartment 27, having normally charged toner particles. Therefore, the unevenness in the image density can be resolved or restricted.
Referring to
In view of the foregoing, referring to
The first specific magnetic carrier C1 positioned at the edge 44a does not enter the slit 45 unless an external force toward the slit 45 (in the present embodiment, the external force inclined downward from the horizontal plane) acts on the first specific magnetic carrier C1. In the external force present at the edge 44a, the magnetic force F1M and the gravity F1G are dominant. In the present embodiment, as shown in
It is to be noted that, in practice, developer is present in the development device 20, and adjacent magnetic carrier particles are connected together with the magnetic force along the magnetic force line, forming chains of magnetic carrier particles. Therefore, the magnetic force component acting on the single magnetic carrier particle is stronger when the development device contains developer, and the magnetic force F1M increases relative to the gravity F1G. Consequently, the resultant F1 is inclined upward further from the horizontal plane, further away from the slit 45. Therefore, in such a configuration that can prevent the first specific magnetic carrier C1 from entering the slit 45 when developer is not present in the development device 20, magnetic carrier particles present at the identical or similar position to that of the first specific magnetic carrier C1 do not enter the slit 45 in practice, that is, when developer is present in the development device 20. Further, in such a configuration that can inhibit the magnetic carrier particles present at the edge 44a from entering the slit 45, the retained developer G3 can be prevent from entering the slit 45 reliably.
Therefore, in the present embodiment, the retained developer G3 can be prevented effectively from going around the shielding wall 44 and entering the slit 45. Accordingly, the above-described developer layer in which the developer G3 including the excessively charged toner and the developer G1 including the normally charged toner are mixed insufficiently is not conveyed to the development range, thus restricting unevenness in the image density and the degradation of image quality.
It is to be noted that, to prevent the retained developer G3 from entering the slit 45 effectively, the lower end portion of the shielding wall 44 facing the slit 45 should be positioned upstream from a straight line L2 (shown in
In addition, in the first embodiment, as shown in
Consequently, in the present embodiment, the retained developer G3 can be prevented effectively from passing through the slit 45. Accordingly, the above-described developer layer in which the developer G3 including the excessively charged toner and the developer G1 including the normally charged toner are mixed insufficiently is not conveyed to the development range, thus restricting unevenness in the image density and the degradation of image quality.
It is to be noted that the above-described effect can be also attained by changing the shape of the shielding wall 44 so that the shielding wall 44 itself can function as the wall constituted of the developer G4 standing on end on the shielding wall 44. In this case, however, the shielding wall 44 increases in size, making it difficult to dispose the shielding wall 44 in the limited space between the development sleeve 22 and the supply screw 32. Accordingly, a higher degree of accuracy is required in the dimension of the components and assembling, thus increasing the cost. Therefore, causing the developer G4 standing on end on the shielding wall 44 to form the wall is advantageous in terms of cost.
Further, typically, in the downstream end portion of the supply compartment 27 in the developer conveyance direction, the amount of the developer G1 tends to be smaller, and the amount of developer supplied from the supply compartment 27 to the development sleeve 22 tends to be insufficient as described above. In the present embodiment, to address this inconvenience, the height of the partition 43 is reduced from that in the comparative development device 120 shown in
The attraction magnetic force, however, might fail to catch the developer G1 that has overstridden the upper end of the partition 43, letting the developer to fall, if the height of the partition 43 is excessively low. This is described in further detail below with reference to
Referring to
In view of the foregoing, referring to
In addition, because a stronger attraction magnetic force F2M is exerted on magnetic carrier particles positioned closer to the surface of the development sleeve 22 than the edge 43a, that is, positioned between the edge 43a and the development sleeve 22, than the magnetic force exerted on the magnetic carrier positioned at the edge 43a, such magnetic particles do not drop under such conditions that can prevent the second specific magnetic carrier C2 from dropping. Moreover, because magnetic carrier particles positioned vertically above the edge 43a are closer to a peak point of the magnetic flux density of the attraction pole S2 in a normal line direction than the magnetic carrier positioned at the edge 43a, a stronger attraction magnetic force F2M is exerted on such magnetic carrier particles than that exerted on the magnetic carrier at the edge 43a. Accordingly, other magnetic carrier particles can be prevented from dropping under conditions that can prevent the second specific magnetic carrier C2 at the edge 43a from dropping. It is to be noted that magnetic carrier particles positioned above the peak point of the attraction pole S2 in the normal line direction are supported by the magnetic carrier particles positioned under them and can be prevented from dropping.
Therefore, the developer G1 that has overstridden the upper end of the partition 43 can be prevented from escaping the attraction magnetic force and dropping in the development device 20 according to the present embodiment configured so that, in a state in which no developer is present in the development device 20, the resultant F2 of the magnetic force F2M and gravity F2G acting on the second specific magnetic carrier C2 positioned at the edge 43a, which faces the development sleeve 22, is inclined upward or in parallel to the horizontal direction.
Further, referring to
It is to be noted that, in the first embodiment, the attraction pole S2 and the regulation pole N2 are adjacent to each other in the rotational direction of the development sleeve 22. In other words, no magnetic pole is present between the attraction pole S2 and the regulation pole N2. With this configuration, the developer particles carried on the development sleeve 22 between the attraction pole S2 and the regulation pole N2 do not stand on end but lie thereon along the magnetic force lines extending from the attraction pole S2 and the regulation pole N2. Lying developer particles can be densely carried on the development sleeve 22. Accordingly, even if attracted to the surface of the development sleeve 22 strongly between the attraction pole S2 and the regulation pole N2, the retained developer G3 just overlies the developer G1 supplied from the supply compartment 27 and does not push away the developer G1 to approach the surface of the development sleeve 22. Therefore, mixing the retained developer G3 in the developer that passes through the regulation gap can be restricted, and only the developer G1 having a standard toner charge amount can be used in image development.
Herein, referring to
Moreover, such development devices as shown in
Use of the retained developer G3′, however, is not preferable because the toner charge amount of the retained developer G3′ is higher compared with that of the toner contained in the developer G1 pumped up from the supply compartment 27. The development device 20 according to the present embodiment is advantageous over the configuration shown in
A development device according to a second embodiment is described below. For example, the electrophotographic image forming apparatus in which the development device is incorporated is a printer.
Although the development device 20 in the above-described first embodiment includes only a single developer bearer (i.e., development sleeve 22), the present invention can adapt to multistage development devices that include multiple developer bearers disposed facing the photoreceptor 3 for developing the electrostatic latent image formed on the photoreceptor 3 in multiple steps.
It is to be noted that only the differences from the above-described first embodiment are described below, and descriptions of similar portions are omitted.
The development device 220 according to the second embodiment is multistage development type and includes first and second developer bearers, namely, first and second development rollers 221A and 221B. The development rollers 221A and 221B are positioned adjacent to and facing the circumferential surface of the photoreceptor 3, and the portion where the development rollers 221A and 221B face the photoreceptor 3 serves as the development range. In the development range, the developer particles standing on end on the development rollers 221A and 221B form magnetic brushes and contact the surface of the photoreceptor 3. The development device 220 contains two-component developer including toner particles T and carrier particles C. The development device 220 develops the electrostatic latent image formed on the photoreceptor 3 into a toner image.
The development device 220 in the present embodiment is premix development type, and fresh developer G is supplied from a developer cartridge as required, and degraded developer (i.e., waste developer) is discharged outside the development device 220 to a waste developer container. The developer cartridge contains premixed developer including toner (toner particles) T and carrier (carrier particles) C to be supplied to the development device 220. The development device 220 includes a magnetic detector to detect the concentration of toner in the developer G in the development device 220, and the developer G is supplied from the developer cartridge to the development device 220 in response to the toner concentration detected by the magnetic sensor detector. The ratio of the toner T to the carrier C in the developer G contained in the developer container is relatively high.
The casing and interior of the development device 220 form three developer conveyance compartments 227, 228, and 229 (also “a supply compartment 227, a collection compartment 228, and an agitation compartment 229”). Conveyance screws 232, 235, and 238 are provided in the developer conveyance compartments 227, 228, and 228, respectively. Each of the conveyance screws 232, 235, and 238 includes a shaft and a screw blade projecting from the screw shaft and transports the developer G contained inside the development device 220 in its axial direction, that is, the longitudinal direction of the development device 220. While being transported in the longitudinal direction by the conveyance screw 232 (hereinafter also “supply screw 232”), the developer G in the supply compartment 227 is sequentially pumped up to the surface of the first development roller 221A. The conveyance screw 235 (hereinafter also “collecting screw 235”) is positioned vertically beneath the supply screw 232. After used in image development, the developer G carried on the second development roller 221B is separated therefrom, collected in the collection compartment 228, and transported therein by the collecting screw 235 in the longitudinal direction. The supply screw 232 and the collecting screw 235 are positioned with their axes of rotation in parallel to those of the development rollers 221A and 221B.
The conveyance screw 238 (also “agitation screw 238”) is positioned oblique to the supply screw 232 as well as the collecting screw 235 to linearly connect a downstream end portion of the collection compartment 228 and an upstream end portion of the supply compartment 227 in the developer conveyance direction. The agitation screw 238 transports the developer G transported from the collecting screw 235 to the upstream portion of the supply compartment 227 in the developer conveyance direction. In addition, the developer G transported by the supply screw 232 to the downstream end portion of the supply compartment 227 falls to the collection compartment 228 and then is returned to the upstream end portion of the supply compartment 227 by the agitation screw 238.
Next, image formation performed on the photoreceptor 3 in the second embodiment is described below.
As the photoreceptor 3 is rotated counterclockwise in
More specifically, a doctor blade 225 adjusts the amount of the developer G pumped up to the first development roller 221A, which is positioned in an upper portion in the development device 220. Then the developer G is transported to a first development range facing the photoreceptor 3. In the first development range, the developer G stands on end on the first development roller 221A due to the magnetic force exerted by a development magnetic pole of the first development roller 221A. Then the magnetic brush thus formed slides on the surface of the photoreceptor 3. At that time, the toner T in the developer G selectively adheres to only an image portion on the photoreceptor 3 due to a development magnetic field generated by a predetermined development bias applied to the first development roller 221A from a power source, and thus a toner image is formed.
After passing through the first development range, the developer G carried on the first development roller 221A is further transported to a developer receiving area facing a developer receiving magnetic pole inside the second development roller 221B as the first development roller 221A rotates. In the developer receiving area, the developer G is partly or entirely transferred from the first development roller 221A to the second development roller 221B and carried thereon due to the magnetic force exerted by the developer receiving magnetic pole. The developer G carried on the second development roller 221B is transported to a second development range facing the photoreceptor 3.
In the second development range, the developer G stands on end on the second development roller 221B due to the magnetic force exerted by a development magnetic pole of the second development roller 221B. Then the magnetic brush thus formed slides on the surface of the photoreceptor 3. At that time, the toner T in the developer G selectively adheres to only the image portion on the photoreceptor 3 due to a development magnetic field generated by a predetermined development bias applied to the second development roller 221B from the power source, and thus complementing the toner image. After passing through the second development range, the developer G carried on the second development roller 221B is separated from the second development roller 221B and is returned inside the development device 220.
The image portion on the surface of the photoreceptor 3 on which the toner image is formed in the first and second development ranges is subjected to image forming processes similar to those in the above-described first embodiment.
The development device 220 according to the second embodiment further includes the shielding wall (i.e., a shielding wall 244) serving as the blocker similarly. The shielding wall 244 is positioned to prevent the retained developer G3 that has been blocked by the doctor blade 225 from moving toward the first development roller 221A along the magnetic force lines of the regulation magnetic force. Therefore, similarly to the above-described first embodiment, the shielding wall 244 can prevent the retained developer G3 attracted by the attraction magnetic force from hindering pumping up the developer G1 from the supply compartment 227. Therefore, the local shortage of the developer G1 pumped up from the supply compartment 227 can be prevented or restricted. Accordingly, the developer G3 attracted by the attraction magnetic force is less likely to pass through the regulation gap and be held in the portion adjacent to the surface of the first development roller 221A. Accordingly, the above-described developer layer in which the developer G3 including the excessively charged toner and the developer G1 including the normally charged toner are mixed insufficiently is not conveyed to the development range, thus restricting the unevenness in image density and the degradation of image quality.
A development device according to a third embodiment is described below. For example, the image forming apparatus in which in the development device according to the present embodiment is incorporated is a printer.
Although the description below concerns a multistage development device 320 including multiple developer bearers disposed facing the photoreceptor 3 for developing the electrostatic latent image formed on the photoreceptor 3 in multiple steps similarly to the second embodiment, features of the third embodiment can adapt to development devices including only a single developer bearer (i.e., development sleeve 22) as in the above-described first embodiment.
It is to be noted that only the differences from the above-described second embodiment are described below, and descriptions of similar portions are omitted.
The development device 320 according to the third embodiment is multistage development type and includes first and second developer bearers, namely, first and second development rollers 321A and 321B. The casing and interior of the development device 320 form three developer conveyance compartments 327, 328, and 329 (also “a supply compartment 327, a collection compartment 328, and an agitation compartment 329”) similarly to the second embodiment. Conveyance screws 332, 335, and 338 are provided in the developer conveyance compartments 327, 328, and 329, respectively. Each of the conveyance screws 332, 335, and 338 includes a shaft and a screw blade projecting from the screw shaft and transports the developer G contained inside the development device 320 in its axial direction.
The conveyance screw 332 (hereinafter also “supply screw 332”) provided in the supply compartment 327 transports the developer G in the supply compartment 327 in the longitudinal direction of the development device 320. As shown in
The development device 320 according to the third embodiment further includes the shielding wall (i.e., a shielding wall 344) serving as the blocker similarly. The shielding wall 344, however, is different from those in the above-described first and second embodiments in that the shielding wall 344 is configured not only to prevent the retained developer that has been blocked by the doctor blade 325 from moving toward the first development roller 321A along the magnetic force lines of the regulation magnetic force but also to guide the retained developer to the portion away from the first development roller 321A (downstream portion) on the developer surface in the supply compartment 327 in the rotational direction of the supply screw 332. For example, the distance between the first development roller 321A and the shielding wall 344 are designed so that the blocked developer can be carried on the shielding wall 344, and the shielding wall 344 is inclined from the horizontal plane so that the developer thereon can slip down or be pushed out to the portion of the supply compartment 327 away from the first development roller 321A in the rotational direction of the supply screw 332.
Therefore, the shielding wall 344 is capable of returning the retained developer to the downstream portion of the supply compartment 327 in the rotational direction of the supply screw 332 in addition to preventing the retained developer G3 attracted by the attraction magnetic force from hindering pumping up the developer G1 from the supply compartment 327 similarly to the above-described second embodiment.
If the retained developer is returned in the portion of the supply compartment 327 close to the first development roller 321A in the rotational direction of the supply screw 332, it is possible that the retained developer is supplied to the first development roller 321A before sufficiently mixed with the other developer present in the supply compartment 327. The insufficient mixing of the developer can make the image density uneven, degrading the image quality.
By contrast, when the retained developer is returned to the portion of the supply compartment 327 away from the first development roller 321A in the rotational direction of the supply screw 332 as in the present embodiment, the retained developer can move in the rotational direction of the supply screw 332 around the shaft of the supply screw 332 while moving along the shaft of the supply screw 332. Therefore, the retained developer returned to the supply compartment 327 can go down inside the supply compartment 327, pass below the shaft of the supply screw 322, and then reach the upstream portion of the supply compartment 327 in the rotational direction of the supply screw 332, after which the retained developer is supplied to the first development roller 321A. Thus, the retained developer can be mixed with the other developer in the supply compartment 327 sufficiently before supplied to the first development roller 321A. Therefore, the above-described unevenness in the image density can be restricted.
(First Variation)
Next, a variation of the development device according to the above-described first embodiment is described below (hereinafter “first variation”) with reference to
It is to be noted that, features of this variation can adapt to the second and third embodiments as well.
In the above-described development device 20, the upper end of the partition 43 (i.e., the sidewall of the supply compartment 27) is positioned lower than the rotary axis of the development roller 21 as shown in
In view of the foregoing, referring to
In the first variation, the developer G transported through the slit 45 is supplied to a portion of the development sleeve 22 higher than the rotary axis of the development sleeve 22. At that time, the resultant of the magnetic force F exerted by the attraction pole S2 and the weight W of the developer acts on a single developer particle, that is, a single magnetic carrier coated with toner. The weight W of the developer can be decomposed into a tangential component Wt in the direction tangential to the surface of the development sleeve 22 and a normal component Wn in the direction normal to the surface of the development sleeve 22. The normal component Wn of the weight W of the developer functions as an external force to move the developer G1 that has traveled through the slit 45, overstridden the upper end of the partition 43-1, toward the surface of the development sleeve 22. Therefore, the developer can be supplied reliably to the development sleeve 22 even when the magnetic force exerted by the attraction pole S2 is smaller compared with a configuration in which the developer is brought up against the gravity.
In addition, when the developer is supplied to the surface of the development sleeve 22, it is necessary that the friction force between the developer and the surface of the development sleeve 22 is sufficient for the developer carried thereon to follow the rotation of the development sleeve 22. In the configuration in which the developer is brought up against the gravity, the developer is supplied to a portion of the development sleeve 22 facing downward, and a normal component Wn′ of the weight W of such developer is in the direction away from that portion in the surface of the development sleeve 22. Therefore, to generate a vertical drag force N required for the friction force between the surface portion of the development sleeve 22 facing downward and the developer carried thereon, it is necessary that the attraction pole S2 generates a magnetic force F having a normal component Fn equal to the sum of the vertical drag force N and the normal component Wn′ of the weight W of such developer (Fn=N+Wn′).
By contrast, in the first variation, the developer G1 is supplied to the surface portion of the development sleeve 22 facing upward. Accordingly, the normal component Wn of the weight W of the developer G1 is in the direction toward the surface of the development sleeve 22, and the required vertical drag force can be attained with a magnetic force F having a normal component Fn calculated by deducting the normal component Wn from the vertical drag force N (Fn=N−Wn). Therefore, the developer can follow the rotation of the development sleeve 22 even when the magnetic force exerted by the attraction pole S2 is smaller compared with the configuration in which the developer is brought up against the gravity. Accordingly, the developer can be supplied reliably to the development sleeve 22 even when the magnetic force exerted by the attraction pole S2 is smaller compared with the configuration in which the developer is brought up against the gravity. Consequently, the resultant of the magnetic force exerted by the attraction pole S2 and that by the regulation pole N2 can be reduced, thus reducing the stress to the developer.
As can be known from the graph shown in
(Second Variation)
Next, another variation of the development device according to the above-described first embodiment is described below (hereinafter “second variation”) with reference to
It is to be noted that, features of this variation can adapt to the second and third embodiments as well.
In the above-described development device 20 according to the first embodiment, the developer G3 removed from the development sleeve 22 by the doctor blade 25 is retained by magnetic force from the regulation pole N2 in the retaining portion, which is upstream from the doctor blade 25 in the rotational direction of the development sleeve 22. As the amount of the retained developer G3 increases, the amount of developer circulating in the development device 20 decreases relatively. Accordingly, in supply-collection separation type development devices in which the developer that has passed through the development range is collected in the collection compartment 28 separate from the supply compartment 27 as in the above-described first embodiment, it is possible that the amount of developer supplied to the development sleeve 22 is insufficient on the downstream side of the supply compartment 27 in the developer conveyance direction. As described above, if the amount of developer pumped up to the development sleeve 22 is insufficient, the retained developer G3 attracted by the attraction magnetic force compensates for the shortage, and then the retained developer G3 is used in image development in the development range.
In the above-described first embodiment, the amount by which the developer G3 retained in the retaining portion is replaced (hereinafter “replacement amount of retained developer”) is relatively small and identical developer particles tend to remain long in the retaining portion. As a result, the toner charge amount of the retained developer G3 can be remarkably high. Therefore, if the retained developer G3 passes through the regulation gap and is used in the image development, the image density becomes uneven.
As shown in
In the second variation, the developer G1 pumped up to the development sleeve 22 by the attraction pole S2 passes by the magnetic pole N3 as well as the magnetic pole S4 before reaching the regulation gap.
With this configuration, the developer G1 having normally charged toner, pumped up by the attraction pole S2 changes its state sequentially at the positions facing the multiple magnetic poles before reaching the regulation gap. That is, while being transported from the attraction position to the regulation position, the developer lies, stands when passing by the magnetic pole N3, lies, stands when passing by the magnetic pole S4, and again lies. While the developer G1 repeatedly lies and stands on end on the development sleeve 22, the retained developer G3 is mixed in the developer G1, and simultaneously the developer G1 is partly retained in the retaining portion and mixed with the retained developer G3. Thus, replacement of the retained developer G3 is facilitated, developer particles can be inhibited from remaining long in the retaining portion. Consequently, the excessive rise in the charge amount of toner in the retained developer can be prevented, and unevenness in the image density can be restricted even when the shortage of the developer G1 supplied to the development sleeve 22 is compensated by the retained developer G3.
As shown in
By contrast, in the second variation in which two stationary magnetic poles are provided in the range from the attraction position to the regulation position, the difference between the toner charge amount of the retained developer G3 and that of the developer contributing to the image development can be limited. Therefore, even if these developers are used to develop an identical image, significant unevenness in the image density does not occur.
It is to be noted that, although the above-described configuration concerns providing at least two stationary magnetic poles between the attraction position to the regulation position, alternatively, the magnet roller may be configured so that multiple magnetic poles move in the rotational direction of the development sleeve 22 to cause the developer to stand on end on the development sleeve 22 at least twice from the attraction position to the regulation position.
(Third Variation)
Next, yet another variation of the development device according to the above-described first embodiment is described below (hereinafter “third variation”) with reference to
It is to be noted that, features of this variation can adapt to the second and third embodiments as well.
In the above-described first embodiment, the slit 45, which is provided between the partition 43 and the shielding wall 44, extends relatively long in the axial direction of the development sleeve 22. The partition 43 defining the lower end of the slit 45 can be integrated with the casing of the development device 20 forming the sidewall of the supply compartment 27 over the entire length in the axial direction of the development sleeve 22. Therefore, the partition 43 does not deform even when pushed toward the development sleeve 22 by the developer passing through the slit 45.
By contrast, the upper side of the shielding wall 44 is not supported because the retaining portion is provided above it, and the lower side of the shielding wall 44 is not supported because the slit 45 is provided under it. Thus, the shielding wall 44 is supported only in the end portions in the axial direction of the development sleeve 22. Accordingly, the shielding wall 44 deforms with the axial end portions as fulcrums when pushed toward the development sleeve 22 by the developer passing through the slit 45. If the amount by which the shielding wall 44 deforms is large, the shielding wall 44 contacts the development sleeve 22, resulting in production of substandard images, abnormal noise, or unintended products.
The shielding wall 44 may be connected to the partition 43 at one position or greater multiple positions in the axial direction of the development sleeve 22. As shown in
An image quality evaluation was executed to examine the relation between image quality and the width (i.e., the length in the axial direction of the development sleeve 22) of each rib 46. Table 1 shows the results of evaluation. In the evaluation, the output images were visually observed, and the image quality was classified as level 1 when no substandard images are produced, level 2 when the image density is lower but is acceptable, and level 3 when the image density is excessively low and cannot be accepted.
When the width of each rib 46 was not greater than 1 mm, acceptable images of image quality level 1 or 2 were output. However, when the width of the ribs 46 was greater than 1.5 mm, the image quality ranked level 3, and thus the output images were not acceptable. In this example, the ribs 46 blocked the supply of developer from the supply compartment 27 to the development sleeve 22, and the retained developer was carried on the development sleeve 22 at that portions. Consequently, the retained developer having the higher toner charge amount was used to develop the image, decreasing the image density.
As described above, when the length (width) of each rib 46 in the axial direction of the development sleeve 22 is not greater than 1 mm, the strength of the shielding wall 44 can be enhanced so as to prevent the shielding wall 44 from deforming and contacting the development sleeve 22 without degrading the quality of output images.
The ribs 46A shown in
(Fourth Variation)
Next, yet another variation of the development device according to the above-described first embodiment is described below (hereinafter “fourth variation”).
It is to be noted that, features of this variation can adapt to the second and third embodiments as well.
The force exerted by the screw blade 34 of the supply screw 32 for conveying the developer to the development sleeve 22 is not uniform in the axial direction, and there are portions where the force for forwarding the developer to the development sleeve 22 is weaker. In such portions, it is possible that the retained developer G3 attracted by the attraction magnetic force hinders pumping up the developer G1 from the supply compartment 27, resulting in the shortage of the developer pumped up. As a result, the retained developer G3 attracted by the attraction magnetic force is carried in the area closer to the surface of the development sleeve 22 and transported through the regulation gap to the development range. Consequently, the image density becomes uneven.
The following approaches may be adopted to restrict the unevenness in image density caused by the axial unevenness in the force exerted by the screw blade 34 of the supply screw 32 for conveying the developer to the development sleeve 22.
As a first approach, the pitch of the screw blade 34 in the axial direction may be reduced. Although this approach can reduce the axial unevenness in the force of the screw blade 34 of the supply screw 32 for forwarding the developer and the unevenness in image density, simply reducing the pitch of the screw blade 34 decreases the velocity at which the supply screw 32 transports the developer (i.e., the amount of developer conveyed per unit time). Therefore, to secure the necessary conveyance mount of developer per unit time, it is necessary to increase the rotational frequency of the supply screw 32 or the external diameter of the screw blade 34. As a result, the stress to the developer increases, and the developer may coagulate or deteriorate due to a rise in the temperature of the developer. In addition, the development device may increase in size.
As a second approach, the number of threads of the screw blades 34 may be increased. Although this approach can reduce the axial unevenness in the force of the screw blade 34 forwarding the developer and the unevenness in image density, as the number of the threads thereof increases, the screw blade 34 occupies more of the space in the supply compartment 27, reducing the capacity for containing the developer. Accordingly, the amount of developer conveyed (hereinafter “developer conveyance mount”) per unit time is reduced, thus inviting the inconvenience similar to that in the first approach.
In view of the foregoing, in the fourth variation, the rotary shaft 33 of the supply screw 32 is constructed of nonmagnetic metal and does not include resin. Typically, developer conveyance screws are made of resin entirely or constituted of a resin screw blade and double layered rotary shaft including a metal base and a resin overlying the metal base. By contrast, in the fourth variation, because the rotary shaft 33 of the supply screw 32 is made of nonmagnetic metal having a strength greater than that of the resin, and the diameter of the rotary shaft 33 can be reduced from conventional rotary shafts that are made of resin entirely. In addition, even compared with conventional double layered rotary shafts constructed of a metal base coated with resin, the diameter of the supply screw 32 according to the fourth variation can be reduced for the amount of the resin coat. The reduction in diameter of the rotary shaft 33 can reduce the occupancy of the supply screw 32 in the developer conveyance compartments, thus increasing the capacity for containing the developer. Accordingly, the developer conveyance amount per unit time can be increased.
In the fourth variation, even when the above-described first or second approach is adopted to reduce the unevenness in image density, the increase in the rotational frequency of the supply screw 32 or that in the external diameter of the screw blade 34 can be unnecessary or minimized. Therefore, the unevenness in image density can be alleviated without an increase in the stress to the developer, which causes a rise in the temperature of the developer. Thus, coagulation or degradation of the developer as well as increases in size of the development device can be restricted.
Similarly, the collecting screw 35 may include a nonmagnetic metal rotary shaft to reduce the diameter compared with resin rotary shafts or double layered rotary shafts including the metal base coated with resin. Thus, the developer conveyance amount can be increased with the above-described inconvenience alleviated.
It is to be noted that, in the above-described first through third embodiment as well as the variations thereof, the voltage applied to the development roller 21, 221A, or 321A is preferably an alternating current (AC) voltage. This voltage may be either a symmetric AC voltage in which the positive and negative peak voltages have the same value or an asymmetric AC voltage in which direct-current (DC) voltage is superimposed on such an AC voltage. The peak-to-peak voltage is preferably within a range of from 300 V to 3,000 V, and the frequency is preferably within a range of from 200 Hz to 10,000 Hz. The peak-to-peak voltage and the frequency are set within these ranges depending on the development process. For example, the waveshape of the voltage can be triangular, rectangular, or a shape with the duty ratio changed. Such an AC voltage can enhance the development efficiency, and satisfactory images can be attained even when the amount of developer supplied to the development range is smaller compared with DC voltage. Therefore, the regulation gap between the between the development roller (21, 221A or 321A) and the doctor blade (25, 225, or 325) can be reduced, and thus only the developer positioned close to the surface of the development roller 21 can pass through the regulation gap. Therefore, the retained developer G3 can be further inhibited from passing through the regulation gap, and effects of the shielding wall (44, 244, or 344) for preventing degradation in image quality can be increased.
Additionally, in the above-described first through third embodiments and the respective variations, making the magnetic force generated by the attraction pole S2 smaller than that generated by the regulation pole N2 can generate a magnetic field that exerts a magnetic force for conveying the developer carried on the development sleeve 22 downstream in the rotational direction of the development sleeve 22. Such magnetic force can act on the developer carried on the development sleeve 22 in the direction identical or similar to the rotational direction of the development sleeve 22, thus facilitating conveyance of developer by the development sleeve 22. Further, the reduction in the magnetic force of the attraction pole S2 can alleviate the degradation of developer.
In addition, when the amount of developer supplied to the development range is smaller, the required amount of developer contained in the supply compartment (27, 227, 327) can be reduced. Therefore, the pitch of the screw blade of the supply screw (32, 232, 332) in the axial direction or the number of threads of the screw blade can be increased to restrict the axial unevenness in the force for forwarding the developer to the development roller (21, 221A, or 321A). With this effects in addition to that of the shielding wall (44, 244, or 344), the degradation in image quality resulting from the uneven image density can be alleviated better. Moreover, when the required amount of developer contained in the supply compartment is smaller, the external diameter of the screw blade or the rotational frequency of the supply screw can be reduced, which is advantageous in preventing the developer from deteriorating or coagulating as well as keeping the development device compact.
As described above, the image forming apparatus according to the above-described first through third embodiments includes the photoreceptor 3 serving as the latent image bearer; the charging unit 4 and the optical writing unit 10 together forming the latent image forming unit; and the development device 20, 220, or 320 for developing the latent image formed on the photoreceptor 3 with the developer including the toner and the carrier. The image forming apparatus transfers the toner image from the photoreceptor 3 to the recording sheet P (recording medium), thus forming an output image. The development device 20, 220, or 320 includes the development roller (21, 221A, or 321A) including the development sleeve (22, 221A, or 321A) serving as the developer bearer for transporting the developer by rotation to the development range facing the photoreceptor 3 as well as the magnet roller (23) provided inside the development sleeve for generating the magnetic force for carrying the developer on the surface of the development sleeve, the doctor blade (25, 225, or 325) positioned across the regulation gap from the surface of the development sleeve for adjusting the amount of developer transported to the development range, and the developer conveyance compartments.
The developer conveyance compartments includes the supply compartment (27, 227, or 327) positioned adjacent to the development sleeve, and the developer is supplied by the supply screw (32, 232, or 332) from the supply compartment to the development sleeve while conveyed in the axial direction of the development sleeve. The developer blocked by the doctor blade is also collected in the supply compartment. The magnet roller includes at least the attraction pole S2 to generate the attraction magnetic force for attracting the developer to the development sleeve from the supply compartment beyond the upper end of the partition (43, 243, or 343) forming the sidewall of the supply compartment and the regulation pole N2 to generate the regulation magnetic force for causing the developer to stand on end on the development sleeve when the developer passes through the regulation gap. The attraction pole S2 and the regulation pole N2 have the reverse polarities. Further, the attraction pole S2 and the regulation pole N2 are adjacent to each other in the rotational direction of the development sleeve (except the second variation shown in
Further, the shielding wall (44, 244, or 344) are provided for inhibiting the retained developer G3 that has been blocked by the doctor blade from moving toward the surface of the development sleeve along the magnetic force lines of the regulation magnetic force. The shielding wall is positioned across slit 45 from the partition, and the slit 45 extends at least over the maximum image forming range in the axial direction of the development sleeve for allowing the developer to move from the supply compartment to the development sleeve. The shielding wall can inhibit the retained developer G3 attracted by the attraction magnetic force from moving toward the development sleeve. Further, the shielding wall are provided across the slit 45 for inhibiting the retained developer G3 that has been blocked by the doctor blade from moving toward the surface of the development sleeve along the magnetic force lines of the regulation magnetic force. The slit 45 extends at least over the maximum image forming range in the axial direction of the development sleeve for allowing the developer to move from the supply compartment to the development sleeve. The shielding wall can inhibit the retained developer G3 attracted by the attraction magnetic force from moving toward the development sleeve and from hindering pumping up the developer G1 from the supply compartment. Therefore, the local shortage of the developer G1 pumped up from the supply compartment can be prevented or restricted, and the developer G3 is less likely to be held in the portion adjacent to the surface of the development sleeve, capable of passing through the regulation gap. Accordingly, the above-described developer layer in which the developer G3 including the excessively charged toner and the developer G1 including the normally charged toner are mixed insufficiently is not conveyed to the development range, thus restricting unevenness in the image density and the degradation of image quality.
In addition, the supply screw (32, 232, or 332) includes the screw-shaped blade 34 provided on the rotary shaft and transports the developer in the supply compartment (27, 227, or 327) in the direction of its rotary axis. The rotary shaft 33 of the supply screw may be constructed of nonmagnetic metal only and does not include resin, and the screw-shaped blade 34 may be constructed of resin.
In addition, the development devices 20, 220, and 320 are supply-collection separation type and include the collection compartment (28, 228, or 328) positioned adjacent to the surface of the development sleeve (22, 221A, or 321A) separated from the supply compartment (27, 227, or 327). The developer G2 that has passed through the development range is collected in the collection compartment, and the collecting screw (35, 235, or 335) transports the developer G2 in the axial direction of the development sleeve. According to the above-described first through third embodiments, satisfactory images can be obtained even in such supply-collection separation type development devices.
In addition, the magnet roller generates the release magnetic force for separating the developer G2 that has passed through the development range from the development sleeve and guiding it to the collection compartment, and the upper end of the partition (43, 243, or 343) is disposed downstream in the rotational direction of the development sleeve from the release portion in which the releaser magnetic force acts. With this configuration, the developer G2 separated from the development sleeve is blocked by the partition and does not move to the supply compartment beyond the partition. Accordingly, the developer G2 having a reduced toner concentration can be prevented from being carried over to the development range.
In addition, the relative positions of the supply compartment and the development sleeve are determined sot that the developer G1 that has overstridden the upper end of the partition can move in a direction inclined upward from a horizontal plane due to the attraction magnetic force. Although, in such a configuration in which the developer is pumped up against the gravity, the attraction magnetic force is stronger and accordingly the force attracting the developer G3 to the attraction pole S2 is stronger, satisfactory images can be obtained with the effects of the above-described first through third embodiments.
In addition, the development devices according to the above-described first through third embodiments are configured so that the level of the developer G1 in the supply compartment during image formation is higher than the upper end of the partition at least over the entire length of the image formation range in the axial direction of the development sleeve. For example, the change in the amount of developer can be estimated preliminarily, and the height of the partition may be designed so that the level of the developer G1 in the supply compartment during image formation is higher than the upper end of the partition at least over the entire image formation range in the axial direction. Further, since the supply screw rotates upward in the portion where the supply screw faces the development sleeve, the level of the developer can be higher on the side close to the development sleeve than the side away from the development sleeve.
This configuration can make it easy for the developer G in the supply compartment to overstride the upper end of the partition, and accordingly the developer G1 can move to the development sleeve smoothly through the slit. Therefore, the image density can be kept uniform, and the image quality is not degraded.
In addition, the development devices according to the above-described first and second embodiments are configured so that the attraction magnetic force can cause the developer to stand on end on the surface of the shielding wall (44 or 244) facing the supply compartment (27 or 227). The developer G4 standing on end on the shielding wall can form the wall to block the movement of the retained developer G3 attracted by the attraction magnetic force toward the slit. Therefore, the retained developer G3 can be better prevented from going through the slit.
In addition, the both end portions of the slit (45) in the axial direction of the development sleeve are positioned outside the maximum image forming range in that direction, and thus a sufficient amount of developer can be supplied to the axial end portions of the image forming range. Therefore, the retained developer G3 is not carried on the axial end portions of the development sleeve and is not used in image development. Therefore, unevenness in the image density can be restricted.
In addition, in the above-described first through third embodiments, it is preferable that the shielding wall (44, 244, or 344) be constructed of a nonmagnetic material not to affects the magnetic fields generated by the magnet roller.
In addition, the shielding wall are preferably constructed of a metal material to attain a necessary rigidity at a lower cost. It is to be noted that, when the shielding wall is electrically charged by the friction with the developer to such an extent that the difference in electrical potential between the shielding wall and the development sleeve is equal to or greater than the electric discharge starting voltage, an electric discharge occurs, degrading the quality of the image portion corresponding to the electric discharge. Therefore, the shielding wall preferably have an electrical potential identical or similar to that of the development sleeve. More specifically, for example, the doctor blade (25, 225, or 325) is electrically connected to the development sleeve to have the same or similar potential, and the shielding wall is electrically connected to such a doctor blade. Alternatively, the shielding wall may be connected to the development sleeve directly. The former is more preferable in that the occurrence of electrical discharge between the doctor blade and the shielding wall can be prevented as well.
Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-190373 | Aug 2010 | JP | national |
| 2010-234104 | Oct 2010 | JP | national |
| 2011-121747 | May 2011 | JP | national |
This patent application is a continuation of U.S. application Ser. No. 13/137,194, filed on Jul. 27, 2011, which is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos. 2010-190373 filed on Aug. 27, 2010, 2010-234104 filed on Oct. 19, 2010, and 2011-121747 filed on May 31, 2011, in the Japan Patent Office, the entire disclosure of which is hereby incorporated herein by reference.
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| Number | Date | Country | |
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| Child | 14019991 | US |
