JP3815066B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a developing device that develops an electrostatic latent image on an image carrier to form a visible image, and an image forming device such as a copying machine, a printer, and a facsimile machine that includes the developing device to form an image. .
[0002]
[Prior art]
Conventionally, a developer is carried on the surface of the developer carrying member, and the developer is transported to a developing area facing the image carrying member carrying an electrostatic latent image, and an AC voltage is applied to the developer carrying member. There is known an image forming apparatus that develops an electrostatic latent image to form a visible image while continuously superimposing and applying the image.
[0003]
Further, when developing an electrostatic latent image, contact development such as magnetic brush development for developing the developer layer carried on the developer carrying member by sliding on the image carrying member, and developing toner particles in the developing region Non-contact development is known in which an electrostatic latent image is developed by flying in an air layer from an agent carrier to an image carrier.
[0004]
[Problems to be solved by the invention]
FIG. 10 is an enlarged schematic diagram showing toner movement in the developing region where the image carrier and the developer carrier face each other.
[0005]
For example, when non-contact development is performed in the development region described above, the toner t near the edge portion on the opposite developer carrier (development sleeve) at the edge portion where the latent image potential on the image carrier has a gap, The toner is sucked into the high density part (potential difference large area), the toner adhesion amount near the boundary part is increased more than expected, and the image density is increased. In addition, a region where a large amount of toner t adheres is widened (hereinafter, this phenomenon is referred to as “suction”).
[0006]
On the contrary, as a result of the toner t adhering to the low density part (potential difference small area) adjacent to the high density part being sucked into the high density part as shown by the broken line in FIG. 10, the toner t is difficult to adhere to the low density part. In some cases, white spots are lost (hereinafter, this phenomenon is referred to as “white spots”).
[0007]
These problems of suction and white spots occur remarkably in the non-contact development method.
[0008]
The suction and white spot generation mechanism will be further described with reference to the schematic diagram of FIG.
[0009]
In the air layer until the toner moves away from the developer carrier (developing sleeve) and adheres to the image carrier (photoreceptor), electricity is transferred from the region (space) on the low density portion to the region (space) on the high density portion. Force lines (arrows shown) are generated. The electric lines of force act perpendicularly to the equipotential lines in the figure.
[0010]
Usually, the amount of toner attached to the electrostatic latent image on the image carrier is uniquely determined by the latent image potential. However, in actuality, the toner t on the developer carrying member facing the low density portion flies along the lines of electric force, so that the toner adhesion amount on the high density portion side of the edge portion is expected from the latent image potential. It is considered that the toner adhesion amount that is larger than the adhesion amount and on the low density portion side of the edge portion is smaller than the toner adhesion amount expected from the latent image potential.
[0011]
That is, when an AC continuous wave is applied to the developer carrier, the direction of the electric field due to the AC voltage changes every half cycle, so the direction in which the toner t moves changes each time, and thus the direction in which the toner t moves changes. The toner flying speed in the vicinity is attenuated, and the toner t tends to follow the lines of electric force shown in FIG. By this suction, the toner adhesion amount on the low density portion side of the edge portion is reduced, and white spots occur.
[0012]
Further, even if the toner t flies to the low density portion of the image carrier, it is sucked that the toner t will finally land on the high density portion when it repeatedly bounces on the image carrier. Conceivable.
[0013]
In non-contact development, in order to maintain the developer layer on the developer carrier and the image carrier surface in a non-contact state, the distance between the developer carrier surface and the image carrier surface is compared with that in contact development. Need to be set longer. With this distance setting, the bending of the electric field lines increased, and suction and white spots became conspicuous.
[0014]
In addition, the phenomenon of suction and white spots is not as noticeable as in non-contact development, but also occurs in contact development.
[0015]
In the voltage waveform in which a bias voltage in which a DC voltage is superimposed on an AC voltage is applied to the suction and white spots, a blank part that intermittently pauses the AC component is included. In recent years, it has been discovered that this can be solved by applying an applied voltage (blank pulse waveform) to the developer carrier.
[0016]
A known example of a developing method using a blank pulse waveform as a conventional developing bias voltage is shown below. JP-A-7-311497, JP-A-8-160725, JP-A-5-35063, JP-A-60-134262, JP-A-60-53968, JP-A-7-295373, JP-A-6-348117 JP-A-7-92786, etc.
[0017]
However, while the development methods shown in these known examples are effective against suction and white spots, the toner is separated from the surface of the developer carrier in the blank portion where the application of AC voltage is stopped. Since the applied AC voltage is not applied, the amount of toner detachment is reduced, and there is a problem in that developability (toner adhesion) is generally reduced as compared with a continuous waveform portion that is not a blank portion.
[0018]
SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that prevents the above-described suction and white spots and prevents a decrease in developability (toner adhesion).
[0019]
In recent years, a toner having a small particle diameter has been adopted in order to improve image quality. However, the toner having a small particle size is easily affected by the above-mentioned lines of electric force.
[0020]
Accordingly, the present invention provides an image forming apparatus that stably forms a high-quality image having high resolution and high gradation by preventing suction and white spots that occur remarkably when the toner particle size is small. The purpose is to do.
[0021]
[Means for Solving the Problems]
  The image forming apparatus of the present invention that achieves the above object forms an electrostatic latent image by exposing at least the surface of the image carrier around the image carrier and exposing the charged surface of the image carrier. A developing device having a developer carrying member for carrying the developer for developing the electrostatic latent image, and a transfer device for transferring the developed image developed by the developing device to a transfer material. The developing device has an asymmetrical waveform in one cycle of the AC voltage between the developer carrier and the image carrier, and an AC voltage and a DC voltage applied intermittently. In an image forming apparatus having bias voltage applying means for applying a superimposed developing bias voltage,The bias voltage applying means includes a duty ratio [T ₂ / (T ₁ + T ₂ )] Regardless of the development bias voltage effective voltage level,In each cycleDevelopment biasVoltage waveformTheThe peak and front of the AC voltageRewriteAbsolute value of difference from current voltageDifferent from the higher side and the lower side than the DC voltage,AppliedSaidHigher and lower than DC voltageSo that the sides are of equal constant area,To satisfy the following relational expressionSaidDC voltage andSaidAn AC voltage is applied.
  0.5 <[T₂/ (T₁+ T₂)] <0.9
  Where
  T₁[Sec]:markThe time for which the AC voltage is applied to the side where the developer moves from the developer carrier to the image carrier relative to the applied DC voltage
  T₂[Sec]:markThe time for applying the AC voltage to the side where the developer moves from the image carrier to the developer carrier relative to the applied DC voltage
  T₁+ T₂: 1 cycle time of AC voltage
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Prior to the description of the embodiment of the present invention, the configuration and operation of a color printer which is an example of the image forming apparatus of the present invention will be described with reference to the cross-sectional configuration diagram of FIG.
[0023]
In this color printer, the respective color toner images sequentially formed on the image carrier are superimposed, and then transferred onto the recording paper at one time by the transfer unit to form a color image, and then separated from the surface of the image carrier by the separating means. This is a color image forming apparatus that separates recording paper.
[0024]
In FIG. 1, reference numeral 1 denotes a photoconductive drum (hereinafter referred to as a photoconductor) which is an image carrier, which is formed by coating an organic photoconductor layer on a drum base, and rotates in the clockwise direction shown in the figure. Reference numeral 2 denotes a charging device (scorotron charger), which is a photosensitive member charging potential V having a high potential with respect to the peripheral surface of the photosensitive member 1._HThe uniform charging of the grid potential V_GIs applied by a corona discharge by a grid and a corona discharge wire held at a potential. Prior to charging by the charging device 2, in order to eliminate the history of the photoconductor 1 so far, exposure by a pre-charge neutralizer (PCL) 8 using a light emitting diode or the like is performed to neutralize the peripheral surface of the photoconductor 1. Keep it. The history on the photosensitive member 1 refers to an electrostatic latent image pattern remaining on the photosensitive member 1 formed by charging and image exposure at the time of preceding image formation.
[0025]
After the photoreceptor 1 is uniformly charged, the exposure device (image exposure means) 3 performs image exposure based on the image signal, and an electrostatic latent image is formed on the surface of the photoreceptor 1. The exposure apparatus 3 performs main scanning through a polygon mirror, an fθ lens, a cylindrical lens, and a reflection mirror that rotate using a laser diode (not shown) as a light source, and forms a latent image by rotating (sub-scanning) the photosensitive member 1. Is done. In the present embodiment, a portion to which toner is to be attached is exposed, and the latent image potential V of the image portion (exposure portion) on the photoreceptor 1 is exposed._LIs the photoreceptor charging potential V on the photoreceptor 1._HA reversal latent image having a potential lower than the absolute potential of_H｜ ＞ ｜ V_L|).
[0026]
A plurality of developing devices 4Y and 4M each incorporating a two-component developer composed of toner such as yellow (Y), magenta (M), cyan (C), and black (K) and a carrier are provided on the periphery of the photoreceptor 1. , 4C, 4K are provided.
[0027]
First, yellow of the first color is developed by a developer carrying member (hereinafter referred to as a developing sleeve) 41 that incorporates a magnetic field generating means 42 composed of a plurality of magnet bodies and rotates while holding the developer. The developer has a magnetite core and an insulating resin coating around it, a particle size of 30-80 μm, a polyester-based pigment, a charge control agent, silica, titanium oxide, etc. The developer is composed of toner having a thickness of 4 to 10 μm, and the developer is regulated to a developer layer thickness of 100 to 600 μm on the developing sleeve 41 by a developer layer regulating member 43 described later, and is conveyed to the development region.
[0028]
The gap between the developing sleeve 41 and the photosensitive member 1 in the developing region is 0.5 to 1.0 mm, which is larger than the developer layer thickness._ACAnd DC voltage V_DCAre superimposed and applied. DC voltage V_DCAnd photoreceptor charging potential V_HSince the toner is charged with the same polarity, the AC voltage V_ACThe toner given the opportunity to leave the carrier by the DC voltage V_DCPhotoconductor charging potential V with higher absolute potential_HDC voltage V_DCThe latent image potential V of the image portion (exposure portion) having a lower absolute potential_LAnd is visualized (reverse development).
[0029]
After the first color visualization is completed, the second color magenta image forming process is started, and uniform charging is performed again by the charging device 2, and a latent image based on the second color image data is formed by the exposure device 3. . At this time, the charge removal by the PCL 8 performed in the first color image forming process is not performed because the toner attached to the image portion of the first color is scattered due to a rapid decrease in the surrounding potential.
[0030]
Again, the charging potential V over the entire circumference of the photosensitive member 1_HA portion of the photosensitive surface charged with no image of the first color has a latent image similar to that of the first color and is developed, but a portion of the portion having the first color image is developed again. Then, the latent image potential V of the exposed portion of the first color is affected by the light shielding by the toner attached to the first color and the influence of the charge of the toner itself._LSlightly higher potential V_MLatent image is formed and the DC bias voltage V_DCAnd potential V_MDevelopment according to the potential difference is performed.
[0031]
For the third color cyan and the fourth color black, an image forming process similar to that of the second color magenta is performed, and four color toner images are formed on the circumferential surface of the photoreceptor.
[0032]
The developing devices 4Y, 4M, 4C, and 4K are supplied with new color toners from the toner cartridge 5 (Y, M, C, and K).
[0033]
On the other hand, a single recording medium (transfer paper or the like) P carried out from the paper feed cassette 20 via the half-moon roller 21 passes through the intermediate paper feed roller pair 22A, 22B in the vicinity of the position of the registration sensor (not shown). When the transfer is stopped and the transfer timing is ready, the paper is fed to the transfer area by the rotation of the registration roller pair 23 of the paper feed unit.
[0034]
In the transfer area, a transfer device 6 such as a transfer roller for applying a voltage for transferring a toner image formed on the peripheral surface of the photoconductor 1 to the recording medium P in synchronism with the transfer timing is pressed against the photoconductor 1. Then, the four color toner images are collectively transferred to the recording medium P with the fed recording medium P interposed therebetween.
[0035]
Next, the recording medium P is neutralized by the saw-tooth electrode 7, separated from the peripheral surface of the photoreceptor 1, and conveyed to the fixing device 24, where the heat roller (upper roller) 241 and the pressure roller (lower roller) 242 are heated. After the toner transferred by the pressure is welded, the toner is discharged onto a discharge tray 26 outside the apparatus through a pair of discharge rollers 25A, 25B, and 25C. The transfer device 6 is retracted away from the peripheral surface of the photoreceptor 1 after passing through the recording medium P to prepare for the formation of the next toner image.
[0036]
On the other hand, the photosensitive member 1 from which the recording medium P has been separated is subjected to removal and cleaning of residual toner by the pressure contact of the blade 91 of the cleaning device 9, and is subjected again to charge removal by the PCL 8 and charging by the scorotron charger 2, and the next image forming process. to go into. The blade 91 moves immediately after cleaning the surface of the photoconductor 1 and retracts from the peripheral surface of the photoconductor 1. The waste toner scraped off into the cleaning device 9 by the blade 91 is discharged by the screw 92 and then stored in a waste toner collection container (not shown).
[0037]
FIG. 2 is a sectional view of the developing device 4 comprising a plurality of sets of developing devices 4Y, 4M, 4C and 4K according to the present invention. In these developing units 4Y, 4M, 4C, and 4K, the developing sleeves 41Y, 41M, 41C, and 41K are arranged in parallel in the vertical direction with the photosensitive surface of the photoreceptor 1 facing each other.
[0038]
Since the plurality of sets of developing devices 4Y, 4M, 4C, and 4K have substantially the same structure, the developing device 4Y will be described below as a representative.
[0039]
FIG. 3 is a cross-sectional view of the developing device 4Y.
[0040]
In the figure, reference numeral 40 denotes a developer accommodating portion for accommodating a two-component developer composed of a toner and a carrier, 41 is a developer carrying member (hereinafter referred to as a developing sleeve) that carries and conveys the developer, and 42 is a developing sleeve. A magnetic field generating means (hereinafter referred to as a magnet roll) in which a plurality of fixed magnet bodies are arranged inside 41, 43 is a developer layer comprising a magnetic round bar that regulates the developer layer thickness on the developing sleeve 41 to a predetermined amount. It is a regulating member (a member that regulates the rising of the developer). 44 is a scraper that removes the developer adhered on the developing sleeve 41 after development, and 45 is a paddle wheel-shaped development that conveys the developer removed from the peripheral surface of the developing sleeve 41 by the scraper 44 to the developer agitating unit. A developer conveying roller 46 is a paddle wheel-shaped developer supply roller for supplying developer to the developing sleeve 41 from a developer stirring portion, and 47A and 47B are developer stirring screws for stirring the developer in the developer stirring portion. The illustrated arrows indicate the rotation directions of the rollers. Instead of the scraper 44 that is in pressure contact with the developing sleeve 41, a magnet may be arranged as a developer removing means in close proximity to the developing sleeve 41 without contacting the outer periphery.
[0041]
In the developing sleeve 41, a plurality of N poles, N1, N2, N3, and a magnet roll 42 in which a plurality of S poles, S1, S2 are alternately arranged are fixed (5-pole arrangement). Of the plurality of magnetic poles, the two magnetic poles N2 and N3 arranged adjacent to each other have the same polarity, and a repulsive magnetic field is formed by the adjacent magnetic poles N2 and N3 having the same polarity. The developer is removed. The magnetic pole S1 faces the developer layer regulating member 43. The tip of the scraper 44 is in pressure contact with the circumferential surface of the developing sleeve 41 that is intermediate between the magnetic poles N2 and N3 having the same polarity.
[0042]
The outer diameter of the developing sleeve 41 is preferably φ8 mm or more and φ60 mm or less. When the outer diameter is 8 mm or more, it is easy to form the magnet roll 42 having at least five magnetic poles necessary for image formation.
[0043]
Further, if the outer diameter of the developing sleeve 41 is 60 mm or less, the developing device can be easily downsized. In particular, in a color printer (see FIG. 1) having a plurality of sets of developing devices (developing devices 4Y, 4M, 4C, and 4K), if the developing device 4 is downsized, the outer diameter of the photoreceptor 1 can be reduced. After the transfer to the medium P and the charge removal, the recording medium P can be separated from the circumferential surface of the photosensitive member 1 by the curvature. Further, the image forming apparatus can be made compact by reducing the size of the developing device 4 and the photosensitive member 1.
[0044]
The developer layer regulating member 43 is formed of a round bar made of a magnetic member such as magnetic stainless steel (SUS).
[0045]
The developer agitating screw 47A and the developer agitating screw 47B are composed of a first agitating chamber 40b and a second agitating chamber 40b formed on both sides of a partition wall 40a standing upright from the bottom of the developer housing (developer accommodating portion) 40. They are arranged in parallel in the stirring chamber 40c and rotate in opposite directions. Upper portions of the first stirring chamber 40b and the second stirring chamber 40c are closed by a lid 40A.
[0046]
The toner replenished from the toner cartridge 5 is charged into the first stirring chamber 40b of the developing device housing 40 through a toner replenishing port (not shown) formed in the lid 40A and then replenished. In the agitating chamber 40 b and the second agitating chamber 40 c, the developer is mixed and agitated by the developer agitating screws 47 A and 47 B, and supplied to the developing sleeve 41 by the developer supplying roller 46. The developer supplied onto the rotatable developing sleeve 41 containing the magnet roll 42 is regulated in developer layer thickness by the developer layer regulating member 43 and conveyed to the developing area facing the photoreceptor 1 for non-contact. Development is performed.
[0047]
In FIG. 3, 48 is a bias voltage applying means comprising an AC power supply E1, a DC power supply E2, etc., and 49 is a waveform control circuit. The waveform control circuit 49 has a rest part (blank part) time T of an AC voltage waveform to be described later._B, Pulse part time T_P, Time ratio of pulse part to rest part (T_P/ (T_P+ T_B)), Pulse amplitude, pulse frequency (f), etc. are controlled.
[0048]
FIG. 4 is a diagram showing a waveform of a blank pulse. The waveform of the blank pulse is a voltage waveform in which a bias voltage obtained by superimposing a DC voltage on an AC voltage is applied, and has a blank part that intermittently pauses an AC component. In this blank part, only the DC component is applied. It is a waveform.
[0049]
In the problem to be solved by the invention, the suction and white spots due to the behavior of the toner have been described with reference to FIG. 10. Here, the behavior of the toner due to the difference between the continuous wave and the blank pulse will be described.
[0050]
In FIG. 4, when a continuous wave indicated by a solid line and a broken line is applied to the developing sleeve, the direction of the electric field changes every half cycle of the AC voltage. The toner flying speed near the point where the direction changes is attenuated, and the toner t tends to follow the lines of electric force shown in FIG. 10, the amount of toner adhering to the high density portion side of the edge portion increases, and suction occurs. By this suction, the toner adhesion amount on the low density portion side of the edge portion is reduced, and white spots occur.
[0051]
Further, when the wave is a continuous wave, the toner t frequently bounces on the image carrier, so that white spots are more likely to occur.
[0052]
When a blank pulse indicated by a solid line in FIG. 4 is applied to the developing sleeve, there is no change in the direction in which the toner t moves in the blank portion compared to the continuous wave, and the toner flying speed is less attenuated.
[0053]
As a result, the toner t tends to adhere to the low density portion, which is difficult to follow the bent electric field lines in the blank portion, and suction can be prevented. Further, since the bounce of the toner t on the image carrier is suppressed in the blank portion, white spots can be prevented.
[0054]
This suction and white spot are prominent when the toner particle size is 10 μm or less, and it is very effective if a blank pulse is applied to the developing sleeve in an image forming apparatus using such a toner particle size. The inventor has discovered that.
[0055]
When the toner diameter is large (for example, a particle diameter of 15 μm), the toner adheres to the latent image in the development region, so that the latent image potential of the image portion is likely to be saturated. When the latent image potential is saturated, the toner does not adhere any more, and therefore there is no difference in the amount of toner adhesion between the inside of the high density portion where the latent image potential is large and the edge portion. In addition, a predetermined amount of toner adheres to the edge portion of the low density portion where the latent image potential is small, and white spots are less likely to occur.
[0056]
In this way, it was found that by applying a blank pulse, suction and white spots can be prevented, but in the blank portion where the application of the AC voltage is paused, no AC voltage is applied that causes the toner to detach from the surface of the developing sleeve. Therefore, there is a problem in that the amount of toner detachment is reduced, and developability (toner adhesion) is generally reduced (decreased by 10 to 20%) as compared with a continuous waveform portion having no blank portion.
[0057]
Accordingly, the present inventor has examined measures for preventing the deterioration in developability of the blank pulse.
[0058]
FIG. 5 is a diagram showing a conventional developing bias voltage waveform and image defects.
[0059]
FIG. 5A shows an AC voltage V applied to the developing sleeve 41Y by the yellow toner Yt on the developing sleeve 41Y in the developing region._ACAnd DC voltage V_DCFIG. 6 is a schematic diagram showing that the light flies and adheres to the peripheral surface of the photoreceptor 1.
[0060]
The developing sleeve 41Y has an AC voltage V_ACAnd DC voltage V_DCAre superimposed and applied. AC voltage V_AC2800V, DC voltage V_DCIs −600 V, the superimposed applied voltage changes between +800 V [(+1400 V) + (− 600 V)] to −2000 V [(−1400 V) + (− 600 V)].
[0061]
Latent image potential V of the image area on the photoreceptor 1_LIs a high density of −50V, the yellow toner Yt has a DC voltage V of −600V during development by the developing device 4Y._DCThus, the latent image potential V of the image portion from the surface of the developing sleeve 41Y having the sleeve potential of −2000 V is obtained._LIt flies over the surface of the photosensitive member (−50V) (in the direction of the arrow in the figure).
[0062]
In order to increase the toner adhesion amount on the photosensitive member 1, the AC voltage value V while the AC bias voltage is applied is applied._ACIs increased from 2800 V to 3800 V to 4800 V, the force for peeling off the toner from the carrier is increased, and the toner adhesion amount on the developing sleeve 41 can be increased. However, when this is done, the potential difference between the developing sleeve 41 and the photosensitive member 1 is periodically increased, and the latent image potential V of the image portion is increased._LThe potential difference with respect to (−50V) increases, and discharge breakdown (lightning strike phenomenon) occurs between the developing sleeve 41Y and the photosensitive member 1, and a so-called ring mark that generates a crater in a ring shape on the photosensitive member 1 is generated. Therefore, the AC voltage value V_ACThere is a limit to increasing the toner adhesion amount by increasing the toner amount.
[0063]
FIG. 5B shows a case where the yellow toner Yt previously formed on the photoconductor 1 is peeled off from the photoconductor 1 in the developing region when a plurality of toner images are formed on the photoconductor 1 in an overlapping manner. FIG. 6 is a schematic diagram showing that the developing sleeve 41 is reattached.
[0064]
When a magenta image is developed by the developing device 4M, yellow toner Yt adheres on the photoreceptor 1 by a yellow image forming process including preceding charging, exposure, and development. When developing a magenta image by the developing device 4M, the AC voltage V is applied as described above._ACIs increased, the force to peel off the yellow toner Yt adhering to the recharged photoconductor 1 at the photoconductor potential (−750 V) from the photoconductor 1 also increases, and is applied to the developing sleeve 41M. AC voltage V_ACAnd DC voltage V_DCAs a result, the yellow toner Yt is peeled off from the developing sleeve 41Y and further flies toward the developing sleeve 41M having the sleeve potential (+ 800V), and the yellow toner Yt is mixed with the magenta toner Mt on the developing sleeve 41M. (Hereinafter, this phenomenon is called stripping).
[0065]
In summary, the AC voltage V_ACIs increased, the toner adhesion amount (developability) increases. However, when a certain voltage value is exceeded, (1) the discharge breakdown (lightning strike phenomenon) occurs and the photoreceptor 1 is damaged, or (2). In an image forming apparatus that superimposes a plurality of toner images on the photoreceptor 1, the peeling occurs and mixing of different color toners occurs. Therefore, in order to improve the toner adhesion amount (developability), it is necessary to devise measures for preventing discharge breakdown and suppressing the occurrence of peeling.
[0066]
Therefore, as a result of diligent study, the present inventor is able to prevent deterioration in developability in the blank pulse part by making the waveform of the application part of the alternating voltage of the blank pulse waveform asymmetrical in one cycle of the alternating voltage. I found
[0067]
  FIG. 6 is a diagram showing an astigmatic waveform of the developing bias voltage. In the figure, a broken line indicates a symmetric waveform, and a solid line indicates an astigmatic waveform according to the present invention. This waveform is a waveform having no symmetry point in one cycle of the AC voltage, that is, an astigmatic waveform. Preferably, this astigmatic waveform is an integral value on the developing drive side (the area of the hatched portion on the upper side in the drawing) where the toner moves from the developing sleeve 41 onto the photosensitive member 1, and the developing sleeve 41 from above the photosensitive member 1. The return side integral value (the area of the hatched portion on the lower side in the figure) on which the toner moves to the sideIs equal to constant value and none (constant effective voltage level),The absolute value of the difference between the peak value of the asymmetry waveform and the DC voltage value that is effectively applied is made different between the high side and the low side of the DC voltage value. The ratio between the applied voltage and the time is changed.
[0068]
T shown₁Is the application time on the development drive side, T₂Is the application time on the return side. The waveform shown in the figure is a duty ratio: T₂/ (T₁+ T₂) Is set to 0.6 (T₂> T₁). Further, this effective applied DC voltage V ′_DCIs actually applied to the DC potential difference V_DCIs desired to be equal to
[0069]
As shown in FIG. 6, by making the AC voltage of the development bias voltage an asymmetrical waveform, the absolute value of the applied voltage on the development drive side is increased compared to the symmetrical waveform, so that the developability that decreases in the blank portion is This can be compensated by the AC voltage application unit, and the amount of toner adhering from the developing sleeve 41 to the photosensitive member 1 is increased as a whole, and the developability is improved. In addition, since the absolute value of the applied voltage on the return side becomes small, the above-described peeling can be suppressed.
[0070]
A comparison of developability between a symmetric blank pulse waveform and an astigmatic blank pulse waveform is shown below. The charging potential V of the photoreceptor 1_H= -750 V, latent image potential V on the photoreceptor 1_L= -50V, DC voltage V_DC= −600 V, closest distance d between the photosensitive member 1 and the developing sleeve 41 = 0.57 mm, developer transport amount DWS of the developing sleeve 41 = 9 mg / cm²The frequency f was set to 6 kHz. In addition, a pulse portion corresponding to two wavelengths and a blank portion corresponding to two wavelengths were applied thereafter.
[0071]
(1) Symmetric blank pulse waveform development: At this time, the maximum AC voltage at which peeling does not occur is V_AC= 2.4 kV, toner adhesion amount (M / A) on photoconductor 1 is 0.56 mg / cm²Met.
[0072]
(2) Duty ratio: T₂/ (T₁+ T₂) Is set to 0.6 astigmatic blank pulse waveform development: At this time, the maximum AC voltage at which peeling does not occur is V_AC= 2.6 kV, and the toner adhesion amount on the photoreceptor 1 is 0.78 mg / cm.²Increased.
[0073]
By making the waveform of the pulse part of the blank pulse shown in FIG. 4 an asymmetrical waveform (asymmetrical blank pulse), it is possible to suppress the developability degradation that was a problem with the blank pulse, and to reduce the developability. By pressing, the potential of the high density portion is likely to be saturated, and there is an effect that the above-described suction and white spots are less likely to occur.
[0074]
FIG. 7 is a diagram illustrating an example of an astigmatic blank pulse waveform of the developing bias voltage. In the figure, a broken line shows the above-mentioned continuous waveform, and a solid line shows an astigmatic blank pulse waveform.
[0075]
The charging potential V_H, Latent image potential V_L, DC voltage V_DC, Distance dmm, developer transport amount DWS, and frequency f are the same as the above set values.
[0076]
By adopting the astigmatic blank pulse, the toner adhesion amount (M / A) on the photosensitive member 1 is increased by increasing the ratio of the application time of the AC voltage to the blank time. For example, when the time of the pulse part is set from 1 to 2 with respect to the time 1 of the blank part, the toner adhesion amount (M / A) is 0.7 mg / cm.²To 0.8mg / cm²To increase. The evaluation of the toner adhesion amount (M / A) on the photoconductor 1 is based on the unit area (cm) attached on the photoconductor 1.²), And the amount of toner (weight, mg) measured with a balance.
[0077]
Blank time is 1, pulse part T_PWhen the time was 2 to 3, the above-mentioned suction was less than one line (170 μm) of 150 LPI (line / inch).
[0078]
The developer transport amount DWS [mg / cm²] And the closest distance d [mm] between the developing sleeve 41 and the photosensitive member 1 desirably satisfy the following relationship.
[0079]
5 <(DWS / d) <40
The developer transport amount (DWS) refers to the transport amount of the developer transported to the development area by the developing sleeve 41, and the evaluation is performed by collecting the developer on the surface of the developing sleeve 41 by a unit area with a tape and using a balance. The weight per unit area of the developer is measured.
[0080]
Furthermore, the present inventor_ACThe maximum electric field Emax (= (1/2 · V) in the direction in which the developer (toner) moves (flies) from the developing sleeve 41 to the photosensitive member 1 when the angle is changed._AC+ | V_DC|-| V_L|) / 2) and pulse part time T due to asymmetry blank pulse_P, T_BPulse part ratio T when changing_P/ (T_P+ T_B) To form a plurality of images under various variable conditions combined with the change in the above), and evaluate these images to achieve an appropriate development area that can achieve an increase in the amount of toner adhesion and a reduction in the suction phenomenon due to the astigmatic blank pulse. Was confirmed experimentally.
[0081]
FIG. 8 shows the maximum electric field Emax [MV / m] and the pulse part ratio T when the frequency f of the pulse part is set to a predetermined value._P/ (T_P+ T_BFIG. 6 is a characteristic diagram showing a limit of a proper development area in correlation with ().
[0082]
The image forming apparatus according to the present invention uses a Konica KL-2010 color laser printer (manufactured by Konica Corporation) experimental machine, and the outer diameter of the photosensitive member 1 is φ100 mm, and the outer diameter of the developing sleeve 41 is φ18 mm. .
[0083]
In the experiment in FIG. 8, the closest distance d between the developing sleeve 41 and the photoreceptor 1 is d = 0.5 mm, and the DC voltage V_DC= -600V, photoreceptor charge potential V_H= -750 V, latent image potential V in the image area_L= -50V.
[0084]
DWS = 11.6 mg / cm²D = O. Since it is set to 5 mm, (DWS / d) is 23.2.
[0085]
First, the frequency f of the pulse part is set to 4 kHz, Duty, V_ACChanging Emax and T_P/ (T_P+ T_B) Are appropriately selected and developed, and the above-described suction generated in the main scanning direction, white spots generated in the sub-scanning direction, and toner adhesion amount were evaluated. As a result, as shown in FIG. It was all good in the region on the right side of the line segment A.
[0086]
For example, V_ACWhen = 2600 V and Duty = 0.6, the maximum electric field Emax is Emax = (2600 × 0.6 + 600−50) /0.5=4.22 [MV / m].
[0087]
V when the Emax is set to 2.6 [MV / m] and the duty is varied in the range of 0.5 to 0.9._ACThe changes are shown below.
[0088]
  When Duty: 0.5, V_AC= 1500V
  When Duty: 0.6, V_AC= 1250V
  When Duty: 0.7, V_AC= 1071V
  When Duty: 0.8, V_AC= 938V
  When Duty: 0.9, V_AC= 833V
  That is,DutyWhen V is 0.9, V_ACWhen E = 833V, Emax = 2.6 [MV / m]. When the duty is 0.5, V_AC= 1500V, Emax = 2.6 [MV / m].
[0089]
In the evaluation of the above suction, it was determined that an image was defective when one 150 LPI (line / inch) line was missing (170 μm) at the boundary between the exposure amount of 20% and 100% on the photoreceptor 1. .
[0090]
Similarly, in the case where the frequency f of the pulse part is 6 kHz, all of the regions in the right side of the line B as shown in FIG.
[0091]
In the case of the frequency f = 8 kHz of the pulse part, as shown in FIG.
[0092]
In the case of the frequency f = 8 kHz of the pulse part, all were good in the region on the right side of the line segment D as shown in FIG.
[0093]
Based on the above results, the appropriate development area where the increase in the toner adhesion amount and the reduction in the suction phenomenon due to the astigmatic blank pulse can be achieved is determined as the maximum electric field Emax and the pulse portion ratio T._P/ (T_P+ T_B), An approximate expression using the frequency f of the pulse part as a parameter, T_P/ (T_P+ T_B)> (− 4Emax) + 11.3 + 0.4 (f−4): It was found to be the formula (1).
[0094]
Similarly in FIG. 9, the maximum electric field Emax [MV / m] and the pulse portion ratio T when the pulse portion frequency f is set to a predetermined value._P/ (T_P+ T_BFIG. 6 is a characteristic diagram showing a limit of a proper development area in correlation with ().
[0095]
First, the frequency f of the pulse part is set to 4 kHz, and Emax and T_P/ (T_P+ T_B) Are appropriately selected and developed, and the above-described suction generated in the main scanning direction, white spots generated in the sub-scanning direction, and toner adhesion amount were evaluated. As a result, as shown in FIG. From the line segment E, it was all good in the region on the left side of the figure.
[0096]
In the evaluation of the above suction, it was determined that an image was defective when one 150 LPI line was missing at the boundary between the exposure amount on the photosensitive member 1 of 20% and 100%.
[0097]
For example, when the pulse ratio is 0.1, V_AC= 2000V and Duty = 0.9, the maximum electric field Emax is 4.70 [MV / m].
[0098]
V when the Emax is set to 4.70 [MV / m] and the duty is varied in the range of 0.5 to 0.9._ACThe changes are shown below.
[0099]
When Duty: 0.5, V_AC= 3600V
When Duty: 0.6, V_AC= 3000V
When Duty: 0.7, V_AC= 2571V
When Duty: 0.8, V_AC= 2250V
When Duty: 0.9, V_AC= 2000V
That is, when Duty is 0.9, V_ACWhen E = 2000V, Emax = 4.70 [MV / m]. When the duty is 0.5, V_AC= 3600V, Emax = 4.70 [MV / m].
[0100]
Similarly, in the case where the frequency f of the pulse part is 6 kHz, all the regions in the left side of the line segment F are good as shown in FIG. 9B.
[0101]
In the case of the frequency f = 8 kHz of the pulse part, as shown in FIG.
[0102]
In the case where the frequency f of the pulse part was 10 kHz, all the regions in the left side of the line H were better than the line H as shown in FIG.
[0103]
Based on the above results, the appropriate development area where the increase in the toner adhesion amount and the reduction in the suction phenomenon due to the astigmatic blank pulse can be achieved is determined as the maximum electric field Emax and the pulse portion ratio T._P/ (T_P+ T_B), An approximate expression using the frequency f of the pulse portion as a parameter, (−4Emax) + 18.9 + 0.4 (f−4)> T_P/ (T_P+ T_B): It turned out that it is (2) type | formula.
[0104]
In summary, the AC component (blank pulse) that pauses intermittently is formed asymptotically, and within an appropriate range that satisfies the equations (1) and (2), Emax, T_P, T_BBy setting this, it is effective for suction and white spotting, and it is possible to prevent deterioration in developability.
[0105]
The developing device of the present invention and the image forming apparatus including the developing device are not limited to the above-described embodiments, and include a color image forming apparatus including a belt photoreceptor and a transparent photoreceptor. The present invention can also be applied to a color image forming apparatus, a tandem color image forming apparatus, and other electrostatic color image forming apparatuses. Further, the developing device of the present invention is not limited to a two-component developer, and can be applied to a one-component developer.
[0106]
【The invention's effect】
The developing device and the image forming apparatus of the present invention prevent suction and white spots that occur in non-contact development, and in particular, prevent suction and white spots that occur remarkably when the toner particle size is small, and provide sufficient density. Thus, developability and development stability are improved, and a high-quality image can be stably obtained.
[Brief description of the drawings]
FIG. 1 is a cross-sectional configuration diagram of a color printer equipped with a plurality of sets of developing devices of the present invention.
FIG. 2 is a cross-sectional view of a developing device including a plurality of sets of developing devices according to the present invention.
FIG. 3 is a cross-sectional view of the developing device of the present invention.
FIG. 4 is a diagram showing a waveform of a blank pulse.
FIG. 5 is a diagram illustrating a development bias voltage waveform and an image defect.
FIG. 6 is a view showing an astigmatic waveform of a developing bias voltage according to the present invention.
FIG. 7 is a view showing an astigmatic blank pulse waveform of a developing bias voltage according to the present invention.
FIG. 8 is a characteristic diagram showing a relationship between a maximum electric field and a pulse part ratio.
FIG. 9 is a characteristic diagram showing a relationship between a maximum electric field and a pulse part ratio.
FIG. 10 is an enlarged schematic diagram showing lines of electric force and toner movement in a development region.
[Explanation of symbols]
1 Image carrier (photosensitive drum, photoconductor)
2 Charging device (Scorotron charger)
3 Exposure equipment
4 Development device
4Y, 4M, 4C, 4K Developer
6 Transfer device
40 Developer housing (developer container)
41 Developer carrier (developing sleeve)
42 Magnetic field generation means (magnet roll)
43 Developer layer regulating member
44 Scraper
45 Developer transport roller
46 Developer supply roller
47A, 47B Developer stirring screw
48 Bias voltage application means
49 Waveform control circuit
Emax Maximum electric field in the direction in which the toner moves from the developing sleeve to the photosensitive member
V_AC  AC voltage applied to developing sleeve
V_DC  DC voltage applied to developing sleeve
V_L  Latent image potential of the image area on the photoconductor (latent image potential of the exposed area)
V_H  Photoconductor charging potential
T_P  Pulse time of AC component applied to developing sleeve
T_B  Rest time of AC component applied to developing sleeve
f Pulse part frequency of AC component applied to developing sleeve
DWS Developer transport amount
d The closest distance between the developing sleeve and the photosensitive member

Claims (2)

A charging device for charging at least the surface of the image carrier around the image carrier, an exposure device for exposing the charged surface of the image carrier to form an electrostatic latent image, and developing the electrostatic latent image And a transfer device that transfers a developed image developed by the development device to a transfer material. The development device includes the developer carrier and Bias voltage applying means for applying a developing bias voltage which is an asymmetrical waveform in one cycle of an AC voltage between the image carrier and an alternating voltage and a DC voltage applied intermittently. In an image forming apparatus having
The bias voltage applying means, while the effective voltage level of the developing bias voltage regardless Duty ratio of the AC voltage _{[T 2 / (T 1 + T} 2) ] constant, the developing bias voltage in each one cycle waveform, the absolute value of the difference between the peak and the previous Kijika current voltage of the AC voltage varied in the high side and low side from the DC voltage, the side is equal to the higher side and lower than the DC voltage applied as a certain area, the image forming apparatus and applying the DC voltage and the AC voltage so as to satisfy the following relationship.
0.5 <[T ₂ / (T ₁ + T ₂ )] <0.9
Where
T ₁ [sec]: with respect to the DC voltage being marked pressurized, developer time to apply an AC voltage on the side moves toward the image bearing member from the developer carrying member T ₂ [sec]: are marked pressurized the relative DC voltage, time to apply an AC voltage to the side where the developer is moved from the image bearing member to the developer carrying member T ₁ + T _2: the one period of the AC voltage time The image forming apparatus according to claim 1, wherein the AC voltage is applied so that the bias voltage applying unit satisfies the following relational expression.
0.1 <[T _P / (T _P + T _B )] <0.9
  Where
T _P [Msec]: Time for applying AC voltage
T _B [Msec]: Time for stopping application of AC voltage

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