JP2007322716A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus equipped with a hybrid developing type developing means, which yields a high-quality image with suppressed developing ghost over a long term. <P>SOLUTION: The image forming apparatus equipped with the hybrid developing type developing means has a control mechanism for reducing a difference between the density of an image formed in the first rotation of a toner carrier and that of an image formed in the second rotation thereof, and the control mechanism controls a difference between DC voltage applied to a developer carrier and that applied to the toner carrier. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine using an electrophotographic method, and particularly uses a two-component developer that charges a nonmagnetic toner using a magnetic carrier, The present invention relates to an image forming apparatus and an image forming method in which only a charged toner is held on a developing roller to fly to an electrostatic latent image and the latent image is developed.

Conventionally, as a developing method using a dry toner in an electrophotographic image forming apparatus, a one-component developing method and a two-component developing method are known.
Since the one-component development method does not include a carrier, an electrostatic latent image of an electrostatic latent image carrier (hereinafter also referred to as a photosensitive member) is not disturbed by a magnetic brush formed from a carrier and toner. Suitable for high image quality. However, in the one-component development method, it is difficult to stably maintain the charge amount of the toner. In the case of a color toner, since transparency is required, it must be a non-magnetic toner. For this reason, full-color image forming apparatuses often employ a two-component development system that uses a carrier as a medium for charging and transporting toner.
The two-component development method is suitable for extending the life because a stable charge amount can be obtained over a long period of time. However, the two-component development method is disadvantageous in terms of image quality because of the influence of the magnetic brush described above.

In recent years, in order to take advantage of the advantages of each of these development methods, hybrid development has been adopted in which the toner charging area adopts a two-component development system in consideration of extending the life and the development area adopts a one-component development system in order to improve image quality. The method is drawing attention. In particular, in a full-color image forming apparatus in which high image quality and long life are important, the characteristics of this development method are sufficiently exhibited.
In the hybrid development system, a two-component developer containing toner and carrier is carried on the surface of a developer carrier (hereinafter also referred to as a magnetic roller) to form a magnetic brush, and only toner is carried from the magnetic brush. The toner layer is transferred to the surface of the body (hereinafter also referred to as a developing roller) to form a toner layer, and the toner is ejected from the toner layer onto the surface of the photoreceptor on which the electrostatic latent image is formed. This is a method of developing as a toner image.

However, the hybrid development method has the following problems.
In other words, in the hybrid development method, it is necessary to supply toner from the magnetic brush to the developing roller at the same time while removing the residual toner on the developing roller with the magnetic brush of the magnetic roller. However, the residual toner is not sufficiently removed. If the toner is insufficiently supplied to the developing roller, a so-called development ghost in which an afterimage after the toner jumps from the developing roller to the photosensitive member appears on the second round is likely to occur.

Accordingly, various methods have been proposed as countermeasures against development ghosts in the hybrid development system. For example, in a two-component developer to be used, a method for eliminating toner development ghost by increasing the density of the magnetic brush and increasing the effect of peeling off the toner on the developing roller by making the carrier low magnetization is disclosed. (See Patent Document 1). However, in Patent Document 1, if the density and uniformity of the magnetic brush are impaired due to carrier deterioration, the stripping performance is also reduced and a development ghost is generated. Therefore, it is difficult to eliminate the development ghost over a long period of time. It is.
In addition, a method for promoting toner replacement on the developing roller by applying an AC voltage having an opposite phase to the AC component of the developing roller to the magnetic roller is disclosed (see Patent Document 2). However, since complicated control is required to cope with changes in the generation level of development ghosts and development leaks due to deterioration of the developer, it is not easy to completely eliminate them.
JP 2003-295613 A JP-A-2005-242281

  An object of the present invention is to provide an image forming apparatus and an image forming method capable of obtaining a high-quality image with suppressed development ghosts over a long period of time in an image forming apparatus having a hybrid developing type developing unit. is there.

As a result of intensive studies to solve the above problems, the inventor of the present invention has as the first invention the density of the image (first patch) formed on the first round of the toner carrier and the second round. It has been found that development ghosts can be suppressed by having a control mechanism (control means) that reduces the difference from the density of the image (second patch) formed in step (i), and the present invention has been completed.
That is, the image forming apparatus of the present invention has the following configuration.

(1) A two-component developer containing a toner and a carrier is supported on the surface of the developer carrier, and only the toner is transferred from the two-component developer on the surface of the developer carrier to the surface of the toner carrier. Developing means for forming a toner layer on the surface of the carrier, and causing the toner to fly from the toner layer to the surface of the electrostatic latent image carrier on which the electrostatic latent image is formed to develop the electrostatic latent image as a toner image An image forming apparatus comprising: a control mechanism that reduces a difference between a density of an image formed on the first circumference of the toner carrier and a density of an image formed on the second circumference. Forming equipment.
(2) The image forming apparatus according to (1), wherein the control mechanism controls a difference in DC voltage applied to the developer carrier and the toner carrier.

According to a second aspect of the present invention, after the solid density image (first patch) formed on the first circumference of the toner carrier is developed, the toner is subsequently produced on the second and third circumferences of the toner carrier. The second patch and the first patch of the third patch obtained by developing the second patch and the third patch of the half density image at a position including the same position as the first patch on the circumferential surface of the carrier. It has been found that development ghosts can be suppressed by having a control mechanism (control means) for reducing the density difference between half-density images at the same position, and the present invention has been completed.
That is, the image forming apparatus and the image forming method of the present invention have the following configurations.

(3) A two-component developer containing toner and carrier is carried on the surface of the developer carrying member, and only the toner is transferred from the two-component developer on the surface of the developer carrying member to the surface of the toner carrying member. Developing means for forming a toner layer on the surface of the carrier, and causing the toner to fly from the toner layer to the surface of the electrostatic latent image carrier on which the electrostatic latent image is formed to develop the electrostatic latent image as a toner image After the solid density image (first patch) formed on the first circumference of the toner carrier is developed at a predetermined position, and then the second circumference of the toner carrier and An image forming apparatus having a control mechanism for reducing a density difference between a second patch and a third patch of a half density image including the same position as the first patch on a toner carrier circumferential surface formed on a third circumference.
(4) A two-component developer containing toner and carrier is carried on the surface of the developer carrying member, and only the toner is transferred from the two-component developer on the surface of the developer carrying member to the surface of the toner carrying member. A developing process in which a toner layer is formed on the surface of the carrier, and toner is ejected from the toner layer onto the surface of the electrostatic latent image carrier on which the electrostatic latent image is formed to develop the electrostatic latent image as a toner image. And developing a solid density image (first patch) formed on the first circumference of the toner carrier at a predetermined position, followed by the second circumference of the toner carrier and Developing the second patch and the third patch of the half density image at a position including the same position as the first patch on the peripheral surface of the toner carrier on the third circumference, and the second patch and the third patch Half density at the same position as the first patch A step 2 for detecting an image density, a step 3 for controlling a difference in DC voltage applied to the developer carrier and the toner carrier based on a difference in image density between the second patch and the third patch; An image forming method comprising: setting the voltage difference when the image density difference between the second patch and the third patch is equal to or less than a predetermined value by repeating the steps 1 to 3. .

According to the present invention, by having a control mechanism for reducing the difference between the density of the image formed on the first circumference of the toner carrier and the density of the image (solid image) formed on the second circumference, Since the development ghost that is likely to occur in the hybrid development system is detected in advance by the control mechanism, the occurrence is prevented, so that a stable high-quality image can be obtained over a long period of time.
Further, according to the second invention, similar to the first invention, it has a control mechanism for reducing the density difference between the half density images formed on the second and third rounds. Since the development ghost that is likely to occur is detected in advance by the control mechanism, the development ghost is prevented, so that a stable high-quality image can be obtained over a long period of time.
Particularly in the second invention, since the density difference of the half density image is compared and controlled, it is possible to detect a slight difference compared to the case of detecting the density difference of the solid portion. .

<Image forming apparatus>
In the present invention, a two-component developer is carried on the surface of a developer carrier, and only the toner is transferred from the two-component developer on the surface of the developer carrier to the surface of the toner carrier. Development of a so-called hybrid development system in which a toner layer is formed and toner is ejected from the toner layer onto the surface of the electrostatic latent image carrier on which the electrostatic latent image is formed to develop the electrostatic latent image as a toner image. An image forming apparatus having a control mechanism for reducing the difference between the density of the image formed on the first circumference of the toner carrier and the density of the image formed on the second circumference of the toner carrier. It is.

  As the image forming apparatus of the present invention, for example, as shown in FIG. 1, a drum-shaped electrostatic latent image carrier (photoconductor) 12, a charging unit 14 for charging the surface of the photoconductor 12, and the photoconductor 12. Exposure means 16 for exposing the surface of the toner to form an electrostatic latent image; developing means 18 for developing the electrostatic latent image as a toner image by attaching toner to the electrostatic latent image; The image forming apparatus 10 includes a transfer unit 22 that transfers to a transfer target (not shown) that moves on the endless belt 20 and a cleaning unit 24 that cleans the surface of the photoreceptor.

Examples of the photoreceptor 12 include inorganic photoreceptors such as selenium and amorphous silicon, and organic photoreceptors in which a single-layer or multilayer photosensitive layer containing a charge generating agent, a charge transport agent, a binder resin, and the like is formed on a conductive substrate. Etc.
Examples of the charging unit 14 include a scorotron system, a charging roller, and a charging brush.
As the exposure unit 16, the transfer unit 22, and the cleaning unit 24, known ones may be used.
The developing means 18 includes a sleeve-like developer carrier (magnetic roller) 26 having a plurality of fixed magnets disposed therein and rotatable around the fixed magnets, and a magnetic brush formed on the magnetic roller 26. A toner carrier (developing roller) 28 on which a toner layer (not shown) is formed by (not shown), a power supply 30 for applying a direct current (DC) bias to the magnetic roller 26, and a direct current (DC) to the developing roller 28 ) A power source 32 for applying a bias, a power source 34 for applying an alternating current (AC) bias to the developing roller 28, a regulating blade 36 for keeping the height of the magnetic brush formed on the magnetic roller 26 constant, and toner A toner container 38 containing toner, a stirring mixer 40 that stirs and charges the toner supplied from the toner container 38 together with a carrier, and a mixing plate 42 through a partition plate 42. Paddle mixer 44 that supplies the two-component developer supplied from -40 to the magnetic roller 26 while stirring, and a frame 46 in which the magnetic roller 26, the developing roller 28, the stirring mixer 40, and the paddle mixer 44 are housed. To do.

<Image forming method>
An image forming method using the image forming apparatus 10 will be described below.
First, the surface of the photoreceptor 12 is charged by the charging unit 14 (charging process). Next, the surface of the photoreceptor 12 is exposed by the exposure means 16 to form an electrostatic latent image (exposure process).
On the other hand, in the developing means 18, a two-component developer is carried on the surface of the magnetic roller 26 to form a magnetic brush, and only toner is transferred from the magnetic brush on the surface of the magnetic roller 26 to the surface of the developing roller 28 for development. A toner layer is formed on the surface of the roller 28. Next, the toner is ejected from the toner layer of the developing roller 28, and the electrostatic latent image is developed as a toner image by attaching the toner to the electrostatic latent image on the photoreceptor 12 (developing process).
Next, the toner image is transferred from the photosensitive member 12 to the transfer target by the transfer means 22 (transfer process). Next, the surface of the photoreceptor 12 is cleaned using the cleaning unit 24. The above steps are repeated.
Examples of the image forming apparatus include a copying machine, a facsimile, a laser beam printer, and a complex machine of these.

<Development ghost>
The development ghost generation mechanism will be described with reference to FIG. In FIG. 2, after a solid image (first patch) is formed on the first turn using the developing roller 28, an image (second patch) formed on the second turn of the developing roller 28 is a desired image. This shows a phenomenon that the concentration becomes lower than the concentration. This is because the toner layer on the developing roller 28 is missing due to the image printing, but if the toner can be removed sufficiently by the magnetic brush carried by the magnetic roller 26 so that the image does not remain, the second round will occur. Development ghosts never occur. However, the development ghost occurs when the toner on the developing roller 28 cannot be sufficiently peeled off by the magnetic brush and the toner cannot be supplied enough to fill the missing part at the time of image formation on the first round. Become. As a result, when a solid image having the same size is formed at the same position, the density of the solid image on the second round is lowered when such a ghost occurs.

Next, the development ghost is more easily detected by using a halftone image (25% dot pattern) as the second patch as shown in FIG. After a solid image (first patch) is formed on the first round using the developing roller 28, a halftone image (second patch) including the same position as the first patch is formed on the second round of the developing roller 28. Then, a halftone image (third patch) is formed again using the same position as the second patch on the third round.
In this case, when a development ghost is generated by the ghost generation mechanism described with reference to FIG. 2, the density is lower on the second patch as the halftone image at a position corresponding to the first patch than the halftone image. A ghost occurs. Then, no ghost appears on the third patch on which the supply of new toner catches up on the developing roller 28.

  Here, the amount of toner transferred from the magnetic roller 26 to the developing roller 28 is determined by a difference in direct current (DC) bias between the magnetic roller 26 and the developing roller 28, a so-called potential difference. That is, if a sufficient amount of toner is supplied from the magnetic roller 26 to the developing roller 28, a development ghost will not occur. Therefore, the larger the potential difference of the direct current (DC) bias between the magnetic roller 26 and the developing roller 28, the more the development. Ghost is unlikely to occur. Further, even when the amount of toner supplied from the magnetic roller 26 to the developing roller 28 is small, the development ghost does not occur if the amount can be sufficiently peeled off by the magnetic brush. Therefore, there is a toner layer thickness at which development ghost is likely to occur, and in order to avoid this, it is necessary to avoid the potential difference that becomes the toner layer thickness with respect to the potential difference described above.

However, the toner supply performance from the magnetic roller 26 to the developing roller 28 varies depending on the toner concentration in the two-component developer, the toner charge amount, etc., even if the above-described potential difference is constant.
Further, regarding the ability to peel off the toner on the developing roller 28, the distance between the developing roller 28 and the magnetic roller 26, the layout of the magnetic pole, the height of the ears of the magnetic brush (the gap between the ear cutting blade 36 and the magnetic roller 26), the carrier Depends on the saturation magnetization and the like. Further, there are cases where the toner is hardly peeled off from the developing roller 28 due to the charge amount of the toner or the adhesive force, and is an element that is very likely to fluctuate through durability.
Therefore, it is appropriate to consider that the potential difference at which development ghosts are likely to occur fluctuates each time. The present invention is characterized in that a potential difference in which a development ghost is generated is detected in advance and a bias setting that avoids the potential difference is set.

<Development ghost detection method>
Next, the development ghost detection method will be described in the case of using a halftone image.
For the detection of the image density, it is possible to use a method of calculating the toner amount from the reflected light with respect to the background surface and the reflected light of the toner layer surface by using a reflection type density sensor and converting it to the image density. Since the image density detection timing needs to be performed before the end of the primary transfer and the start of the secondary transfer, the density sensor is installed between the primary transfer and the secondary transfer.
The flow from the detection of the image density to the development condition setting is as follows. First, the first patch is developed with a solid image at a certain timing of the rotating developing roller 28 (first round), and then a halftone image including the same position as that of the first patch is subsequently applied to the second image of the developing roller 28. A second patch is formed at the circumference, and an image of the third patch is formed at the third circumference of the developing roller 28 at the same location as the halftone image. It is desirable to make the difference in toner amount between the second patch and the third patch as close as possible. In this case, the first patch is preferably a solid image in which an afterimage becomes noticeable after toner flying to the photoreceptor 12. Here, if there is a difference of 0.1 or more as the image density, it is sufficiently recognized as the density difference, so that the development ghost becomes clear. Therefore, development ghost can be avoided by setting the potential difference between the magnetic roller 26 and the development roller 28 so that the image density difference between the second patch and the third patch is less than 0.1.

Specifically, it is implemented as shown in FIG. That is, as described above, after the first patch is developed with a solid image, if the difference in image density between the subsequent second patch and the third patch is detected and is 0.1 or more, the direct current applied to the developing roller 28 is applied. The voltage is fixed, the DC voltage applied to the magnetic roller 26 is increased, the second patch and the third patch are printed again, and the image density difference is detected. By repeating this and setting the applied DC voltage of the developing roller 28 and the magnetic roller 26 when the image density difference is less than 0.1 as the developing condition, it is possible to avoid the development ghost.
Here, when detecting a difference in image density, if a density sensor is used in which the density is detected with high accuracy of a half image rather than a solid image, the first image of the developing roller 28 is not developed without developing the solid image. A solid image is printed on the circumference, half images are printed on the second and third circumferences of the developing roller 28 at the same location, and the difference between the image densities is used as a criterion for determination.
As a method for detecting the development ghost, the first patch and the second patch can be detected as solid images by using these two patch formations without using the halftone image as described above. In this case, as shown in FIG. 5, after the first patch is formed, the image density is first detected, then after the second patch is formed, the image density is detected again, and the difference between the two image densities is used as a criterion. What is necessary is just to determine an electrical potential difference.

  It is important to adjust the development conditions using the above-described density sensor before printing an image on a paper surface so as not to cause a development ghost in an actual printed matter. Of course, it is ideally performed before every printing, but it causes a decrease in printing efficiency and an increase in toner consumption. Therefore, it is sufficient to carry out it once every fixed driving time of the developer, for example, every 30 minutes. . In addition, it is preferable to adjust the development conditions because the image quality is highly likely to change even when the printing environment changes significantly or immediately after being left undriven for a long time.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, this invention is not limited only to a following example.

As an image evaluation machine, FS-C5016N (Kyocera Mita, printer) was used.
As an evaluation method, a normal solid image 50 and a sample image (development ghost pattern) having a half portion 52 following the head solid image 50 as shown in FIG. Ghost and image density differences were evaluated.
A DC bias applied to the magnetic roller 26 and a DC bias applied to the developing roller 28 were applied in the combinations shown in Table 1, and the development ghost of the obtained image was visually measured. The measurement results are shown in Table 1. Judgment criteria are as follows.

○: No afterimage in the half part. (Judgment: Pass)
Δ: There is an afterimage in the half, but it is slight. (Judgment: Pass)
X: There is an afterimage in the half portion. (Judgment: Fail)

Further, the image density of the second patch and the third patch when the DC bias applied to the magnetic roller 26 and the developing roller 28 is combined is detected using a Macbeth reflection densitometer (RD914) as a density sensor, and the image density is detected. The difference was determined. The results are shown in Table 2.

As a result of the above, it can be seen from Table 1 that when the potential difference (hereinafter abbreviated as ΔV) of the DC voltage applied between the magnetic roller 26 and the developing roller 28 is 250 V, there is a remarkable development ghost region. Similarly, from Table 2, the image density difference between the second patch and the third patch detected by the density sensor is the largest when ΔV is 250 V, and exceeds 0.1.
From the obtained results, it can be seen that ΔV needs to be set to a developing condition that does not become 250V. However, when ΔV is set to less than 250 V, the potential difference is small and the toner layer is reduced, which may cause a decrease in solid density. Therefore, in order to avoid ΔV in which a development ghost occurs, it is desirable to set ΔV to 250V or more. Specifically, in the case of Table 2, when the DC voltage applied to the developing roller 28 is 100V, the DC voltage applied to the magnetic roller 26 needs to be set up to 450V.

1 is a schematic configuration diagram illustrating an example of a hybrid development type image forming apparatus according to the present invention. It is a figure for demonstrating an example of the generation | occurrence | production situation of a development ghost. It is a figure for demonstrating another example of the generation | occurrence | production state of a development ghost. It is a figure which shows an example of the flowchart for controlling the image density concerning this invention. It is a figure which shows another example of the flowchart for controlling the image density concerning this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 12 Electrostatic latent image carrier (photosensitive body)
DESCRIPTION OF SYMBOLS 14 Charging device 16 Exposure device 18 Developing device 22 Transfer device 26 Developer carrier (magnetic roller)
28 Toner carrier (developing roller)
30, 32 DC power supply 34 AC power supply

Claims (4)

A two-component developer containing a toner and a carrier is supported on the surface of the developer carrier, and only the toner is transferred from the two-component developer on the surface of the developer carrier to the surface of the toner carrier. A developing unit that forms a toner layer on the surface, and causes the toner to fly from the toner layer to the surface of the electrostatic latent image carrier on which the electrostatic latent image is formed to develop the electrostatic latent image as a toner image; An image forming apparatus,
An image having a control mechanism for reducing the difference between the density of the image (first patch) formed on the first circumference of the toner carrier and the density of the image (second patch) formed on the second circumference. Forming equipment.   The image forming apparatus according to claim 1, wherein the control mechanism controls a difference in DC voltage applied to the developer carrying member and the toner carrying member. A two-component developer containing a toner and a carrier is supported on the surface of the developer carrier, and only the toner is transferred from the two-component developer on the surface of the developer carrier to the surface of the toner carrier. An image having a developing means for forming a toner layer on the surface and developing the electrostatic latent image as a toner image by flying toner from the toner layer onto the surface of the electrostatic latent image carrier on which the electrostatic latent image is formed. A forming device,
After the solid density image (first patch) formed on the first circumference of the toner carrier is developed at a predetermined position, the toner subsequently formed on the second and third circumferences of the toner carrier An image forming apparatus having a control mechanism for reducing a density difference between a second patch and a third patch of a half density image including the same position as the first patch on the circumferential surface of the carrier. A two-component developer containing a toner and a carrier is supported on the surface of the developer carrier, and only the toner is transferred from the two-component developer on the surface of the developer carrier to the surface of the toner carrier. An image having a developing process in which a toner layer is formed on the surface, and toner is ejected from the toner layer onto the surface of the electrostatic latent image carrier on which the electrostatic latent image is formed to develop the electrostatic latent image as a toner image. A forming method comprising:
After the solid density image (first patch) formed on the first circumference of the toner carrier is developed at a predetermined position, the toner carrier circumference is subsequently developed on the second and third circumferences of the toner carrier. Developing a second patch and a third patch of a half-density image at a position including the same position as the first patch on the surface;
Detecting a density of a half density image at the same position as the first patch of the second patch and the third patch;
Controlling a difference in DC voltage applied to the developer carrier and the toner carrier based on a difference in image density between the second patch and the third patch;
Step 4 for setting the voltage difference when the image density difference between the second patch and the third patch is equal to or less than a predetermined value by repeating Steps 1 to 3;
An image forming method comprising: