JPH0423786B2 - - Google Patents

Description

[Detailed description of the invention] 1 Industrial application fields The present invention relates to an image forming method.

2. Prior Art First, an image forming method in which development is repeated a plurality of times will be explained using a color copying machine shown in FIG. 1 as an example.

In the case of a color copying machine, a filter is required to separate the reflected light from the original and extract light of a single color, and this copying machine is equipped with a filter bag 2 containing three types of filters of different colors. ing. For example, first, the reflected light from the original is separated by a green filter, and this transmitted light forms an electrostatic latent image on the photoreceptor drum 1, which is an image carrier, which has already been charged by the charging electrode 9, and is developed. When the developing device 3B containing magenta toner in the device 3 develops the image, a magenta toner image is formed on the photoreceptor drum 1. This toner image is conveyed from a paper feed box 8 and is transferred onto copy paper 7, which is a transfer body, wrapped around a transfer drum 4, by a transfer pole 10. Thereafter, the photosensitive drum 1 is neutralized by the removing electrode 11, residual toner is removed by the cleaning device 5, and then charged again by the charging electrode 9.
This time, image exposure is performed by the blue filter's excellent light, and a yellow toner image is formed on the photoreceptor drum 1 by the developing device 3A. After being transferred, the copy paper 7 is separated from the transfer drum 4, fixed by the fixing device 6, and then discharged outside the machine. Although this image forming method is desirable because it can add color information compared to information obtained from only black and white images, it has the following problems.

(1) Each time the development of each color is completed, a transfer drum 4 must be provided to transfer the image onto copy paper, which increases the size of the machine and increases the time required for image formation.

(2) It is necessary to repeat the developing process and the transferring process several times, and it is necessary to guarantee the accuracy of positional deviation.

For these reasons, instead of transferring the toner image to the copy paper 7 for each development, by forming each toner image of all colors on the photoreceptor drum 1 and then transferring it to the copy paper 7 at once, Image forming methods have been proposed that solve the above-mentioned problems. According to this image forming method, a toner image of a different color is again formed in an image area where a toner image has already been formed on the photoreceptor drum 1. This may disturb the toner formed on the photoreceptor drum 1, or the toner that has already formed an image on the photoreceptor drum 1 may return to the developer conveying body in the developing device, and this may contain toner of a different color from the toner in the previous stage. There is a problem that it may invade the developing device in the subsequent stage, causing color mixing. For this purpose, except for the developing device that initially forms a toner image on the photoreceptor drum 1, the photoreceptor drum 1 and the developer layer of the developer transporting member that develops this electrostatic latent image are not in contact with each other, and the development bias is For example, Japanese Patent Application Laid-Open No. 144452/1983 discloses a method for superimposing alternating current components as follows.
There are problems in that sufficient image density cannot be obtained under developing conditions, and image disturbance and color mixture cannot be eliminated.

3. Purpose of the Invention The present invention has been made to solve the above problems, and an object of the present invention is to provide an image forming method that has a desirable density and records images without image disturbance or color mixture.

4. Structure of the Invention In other words, the present invention repeats the step of forming a latent image on an image carrier and the step of supplying the developer on the developer conveying member to the image carrier and developing the latent image multiple times. In the image forming method of forming a visible image on the image carrier, an electric field including an alternating current component is generated between the image carrier and the developer conveying member, and the frequency of the alternating current component is changed to develop the image. The present invention relates to an image forming method characterized in that the image forming method is made to gradually increase in value.

5 Embodiments Hereinafter, embodiments of the present invention will be described in detail using the drawings.

First, the process by which the present inventor came to make this invention will be explained. An image forming method in which a process of forming a latent image on an image carrier and a process of developing the latent image are repeated, and a visible toner image is sequentially superimposed on the image carrier. It is necessary to carry out development to an appropriate density without disturbing the toner image already formed on the image carrier. As a result of study, in order to satisfy this condition, the gap d (mm)) between the image bearing member and the developer transporting body in the developing area (hereinafter sometimes simply referred to as gap d), the amplitude VAC of the AC component of the developing bias ( It has become clear that v) and frequency (Hz) are parameters when forming a toner image in the subsequent stage at an appropriate density without disturbing the toner image formed in the previous stage.

Therefore, an experiment was conducted using a developing apparatus as shown in FIG. 2 while changing the frequency and voltage parameters of the AC component of the developing bias.

In the developer reservoir 19 of this developing device 13, there is a developer D composed of toner T and carrier.
As the stirring screw 18 rotates, the toner T rubs against the carrier and is sufficiently charged from positive to negative polarity.
Here, when the sleeve 20 and/or the magnetic roll 21 as a developer transport body rotate, the developer D
is conveyed along the circumferential surface of the sleeve 20. In FIG. 2, the magnetic roll 21 is in the direction of arrow A, and the sleeve 20 is
The rotation in the direction of the arrow B indicates that the developer D is being conveyed in the direction of the arrow B on the circumferential surface of the sleeve 20. The thickness of the developer D is regulated by the spike control plate 17, and the developer D forms a uniform layer toward the photoreceptor drum 1, which is an image carrier. When the developer D reaches the development area 24, the toner in the developer D is applied according to the potential of the electrostatic latent image formed on the photoreceptor drum 1 and the electric field generated by the AC and DC voltages as the development bias. An electrostatic force acts on the toner, causing the toner to fly from the sleeve 20 and adhere to the surface of the photoreceptor drum 1, thereby forming a toner image. The developer D on the circumferential surface of the sleeve 20 containing toner that has not been subjected to development is scraped off by the cleaning blade 26. The AC power supply 22 and the DC power supply 23 are power supplies for applying a developing bias to the sleeve 20, and R is a protection resistor. When the developer D is used for development and there is insufficient toner in the developer D in the developer reservoir 19, the toner T is replenished from the toner hopper 14 by rotating the replenishment roller 16.

The inventor previously formed a toner image on the photoreceptor drum 1 using the developing device described above, and developed the toner image by superimposing it on top of the toner image. At this time, we conducted experiments by changing the above-mentioned parameters in various ways, and measured the disturbance of the toner image that had been formed in advance and the image density of the overlapping toner images. As a result, we obtained the results shown in Figures 3 and 4. I was able to obtain. Note that a two-component developer in which the toner is positively charged was used as the developer.

In Figure 3, the gap d between the photoreceptor drum 1 and the sleeve 20 is 1.0 (mm), the developer layer thickness is 0.5 (mm), the charging potential of the photoreceptor drum 1 is 600 (V), and the DC component of the developing bias. is 500 (V), and the frequency of the AC component is 1 (K
The graph shows the relationship between the amplitude VAC of the AC component and the image density of the toner image developed on the exposed portion (value: 0 (V)) on the photoreceptor drum 1 when set to 0 (V). The amplitude E _AC of the alternating current electric field is the value obtained by dividing the amplitude (ie, 1/2 times the peak-to-peak value) VAC of the alternating current component of the developing bias by the gap d. Curve A shown in FIG.
For B and C, the average charge amount of the toner is 30 (μc/
These are the results obtained when using those whose charge was controlled to g), 20 (μc/g), and 15 (μc/g).
All three curves A, B, and C are the amplitude of the alternating current electric field.
E When _AC is 200 (V/mm) or more, the effect of the AC component appears, and when it is 2500 (V/mm) or more, the photoreceptor drum 1
It was observed that the toner image previously formed thereon was partially destroyed.

Figure 4 shows the frequency of the AC component of the developing bias.
2.5 KHz, and shows the change in image density when the amplitude E _AC of the alternating current electric field is changed under the same conditions as in the experiment shown in Fig. 3. According to this embodiment, when the amplitude E _AC of the alternating current electric field exceeds 500 (V/mm), the image density increases, and although not shown, the image density increases by 4 (KV/mm).
At this point, it was observed that a part of the toner image previously formed on the photoreceptor drum 1 was destroyed.

In FIGS. 3 and 4, the shapes of curves A, B, and C, respectively, are almost the same except that the image density increases as the average toner charge amount increases. This is considered to be related to the fact that toner particles having a high charge amount and having a distribution centered on the average value are preferentially developed.

Now, when we conducted experiments similar to those shown in Figures 3 and 4 while changing the conditions, we were able to sort out the relationship between the amplitude E _AC of the alternating current electric field and the frequency, and obtained the results shown in Figure 5. In FIG. 5, the area indicated by is an area where uneven development is likely to occur, the area indicated by is an area where the effect of the alternating current component does not appear and the density is insufficient, and the area indicated by is the area where toner has already adhered to the photoreceptor drum 1. from photoconductor drum 1 to sleeve 2
The region where the value easily returns to 0 is a region where the effect of the alternating current component appears, a high density is obtained, and the toner previously attached to the photoreceptor drum 1 does not return to the sleeve 20, and is a particularly preferable region. It is. These areas are divided by straight lines G, H, and I. Among these, H and I each indicate conditions that provide a special density. This result shows that in order to develop the next (later) toner image at an appropriate density without destroying the toner image that has already been formed on the photoreceptor drum 1, the amplitude of the alternating current electric field E _AC This shows that there is an appropriate range for that frequency.
The cause is considered to be due to the reasons described below.

In areas where the image density tends to increase with respect to the amplitude E _AC of the alternating current electric field, for example, in the density curve A in Figure 3, the area where the amplitude E _AC of the alternating current electric field is 0.2 to 1.2 (KV/mm) must be developed. Due to the alternating current component of the bias,
The force acting in the direction exceeding the threshold for flying from the sleeve 20 makes a large contribution to the toner, and as the alternating current component becomes stronger, toner with a smaller charge amount is successively used for development. Therefore, as the amplitude E _AC of the AC electric field increases, the image density increases.

On the other hand, in the region where the image density is saturated with respect to the amplitude E _AC of the AC electric field, curve A in Figure 3 shows the amplitude of the AC electric field.
For areas where E _AC is 1.2 (KV/mm) or more,
This development can be explained as follows.
That is, in this region, as the amplitude E _AC of the alternating current electric field increases, the toner vibrates strongly, the aggregated toner clusters are destroyed, and only the highly charged toner selectively adheres to the photoreceptor drum 1. do. In such a situation, since the amount of toner supplied to the development area in a certain period of time is finite, the image density is considered to be saturated with respect to the amplitude E _AC of the alternating current electric field. Furthermore, when the amplitude E _AC of the alternating current electric field increases, the charge on the surface of the photoreceptor drum 1 leaks, and a phenomenon occurs in which it becomes difficult to develop the toner. In reality, it is considered that these factors overlap to keep the image density constant as the amplitude E _AC of the alternating current electric field increases.

Furthermore, the amplitude E _AC of the alternating current electric field is increased, and for example, under the conditions for obtaining curve A in Fig. 3, the amplitude is increased to 2.5.
(KV·mm) or more, the toner image previously formed on the photoreceptor drum 1 is destroyed as described above, and it was found that the larger the alternating current component, the greater the degree of destruction. The cause of this is that the toner adhering to the photoreceptor drum 1 is caused by AC components, which cause the toner to stick to the sleeve 2.
This is thought to be because the value is pulled back to 0. When toner images of different colors are superimposed and developed on the photoreceptor drum 1 to form a color image, it is fatal to destroy the already formed toner images.

Based on the above experimental results, the inventor determined that the amplitude of the alternating current component of the developing bias was adjusted to V _AC during each development.
(V), frequency (Hz), and gap between photoreceptor drum 1 and sleeve 20 as d (mm), 0.2≦V _AC /(d・) {(V _AC /d)−1500}/≦1.0 It was concluded that if development is carried out under conditions that satisfy these conditions, subsequent development can be carried out at an appropriate density without destroying the toner image already formed on the photoreceptor drum 1. In order to obtain sufficient image density, it is desirable to satisfy the condition of 0.8≦V _AC /(d·) and further to satisfy the condition of 1.0≦V _AC /(d·).

Here, a method of setting the frequency of the alternating current component applied as the developing bias will be explained using FIG. As mentioned above, the regions , , , are divided by straight lines G, H, and I. Among these lines, G is a straight line indicating the boundary where the toner that has previously formed an image on the photoreceptor drum 1 begins to return to the sleeve 20 due to the alternating current electric field, and increases as the frequency increases. For example, the amplitude of the alternating electric field
E Point J, where _AC is 2.5 (KV・mm) and frequency is 1KHz, is
It is on the straight line G. Also, the amplitude E _AC of the AC electric field is
Point K, whose frequency is 2.5 (KV/mm) and 3KHz, is away from straight line G. As can be inferred from the above description, the further away from the area, the more difficult it is for the toner on the photosensitive drum 1 to return to the sleeve 20.

This means that when the frequency is high, the toner is released from the photoreceptor drum 1 more than when the frequency is low.
This shows that it is effective in preventing the film from returning to the sleeve 20.

Incidentally, as the frequency becomes higher, the amplitude range of the toner particles is narrowed and the amount of toner that adheres to the photoreceptor drum 1 decreases, but it is not always necessary that the same amount of toner of each color adheres to the photoreceptor drum 1 during each development. do not have. Color correction for color images depends on other development conditions,
For example, this can be done by adjusting the DC component of the developing bias, the potential of the latent image, the amount of toner supplied, and the like.

From the above, if the frequency of the AC component of the developing bias is increased sequentially each time development is performed, toner particles that have already been developed in the previous stage and adhered to the photoreceptor drum 1 can be prevented from returning to the sleeve 20, and the toner particles can be prevented from returning to the sleeve 20. They concluded that it was possible to perform the development. By doing the above, the subsequent development can be carried out without disturbing the already formed toner image or causing color mixing problems.

The frequency of the AC component applied as a developing bias is adjusted to prevent uneven development due to the AC component.
200 (Hz) or higher, and when the developer D is supplied to the photosensitive drum 1 by the rotation of the magnetic roll 21 shown in FIG. , 500
(Hz) or higher is desirable.

Furthermore, in order to develop successive toner images on the photoreceptor drum 1 without further destroying the toner image formed on the photoreceptor drum 1, the frequency should be increased sequentially and the toners with a larger charge amount should be sequentially developed. use.

The amplitude of the electric field due to the AC component of the developing bias is gradually decreased.

The amount of developer supplied to the development area is gradually increased.

It is preferable to use these methods alone or in any combination.

That is, toner particles with a larger amount of charge are more susceptible to the influence of the electric field. Therefore, if toner particles with a large amount of charge adhere to the photoreceptor drum 1 during initial development,
During the subsequent development, these toner particles are transferred to the sleeve 20.
may return to. Therefore, as described above, by using toner particles with a small amount of charge in the initial development, the toner particles are prevented from returning to the sleeve 20 during the subsequent development.
is the amplitude of the alternating current electric field E _AC
This is a method of preventing toner particles already attached to the photoreceptor drum 1 from returning by reducing the size of the toner particles. Specific methods for this include a method in which the voltage of the AC component is gradually lowered in later stages of development;
There is a method in which the gap d between the photosensitive drum 1 and the sleeve 20 is made wider as the developing stage progresses. on the other hand,
This method takes advantage of the fact that the larger the amount of toner supplied to the development area, the easier the development, and is a method in which a weaker external force is applied to later stages of development to prevent the toner from returning. The amount of toner supplied is determined by the conveyance speed of developer D,
It can be adjusted by adjusting the thickness of the developer layer on the sleeve 20, the mixing ratio of toner and carrier, stirring the developer D, etc. In addition to these methods, by adjusting the DC voltage of the developing bias for each development, it is possible to maintain appropriate image density or color balance.

A specific example implemented using the configuration described above will be described using FIG. 6.

Embodiment 1 FIG. 6 is a schematic diagram of main parts of a color image forming apparatus, in which a photosensitive drum 1 uniformly charged by a scorotron charger 30 is connected to a rotating polygon mirror from a He-Ne laser light source (not shown). 31, image exposure is performed by the light sent through the imaging lens 32, and an electrostatic latent image is formed. This electrostatic latent image is developed by the first developing device 13A, and a first toner image is formed on the photosensitive drum 1. Then, this toner image is charged again by the scorotron charger 30 without being transferred to the recording paper, and imagewise exposed in the same manner to form a second electrostatic latent image. Then, a second toner image is formed by the developing device 13B. Similar steps are repeated until a third toner image is formed. That is, the steps of charging (not necessarily required from the second time onwards), exposure, and development are repeated three times. Thereafter, the pre-transfer exposure lamp 33 irradiates the area of the photoreceptor drum 1 on which the toner image has been formed, and the recording paper (its path is indicated by a broken line 35) fed from a paper feeder (not shown) by the transfer device 34. This toner image is transferred to the paper (as shown). The recording paper is heated and fixed by a fixing device 36 including at least one heated roller, and then discharged outside the machine.

On the other hand, after the transfer has been completed, the photosensitive drum 1 is neutralized by the static eliminator 37, which was not used during the toner image formation, and the remaining toner on the surface is removed by the cleaning device, which was turned off during the toner image formation. 3
8 is removed. This color image forming apparatus repeats the above operation every time the operation button is operated.

Each developing device used in this color image forming apparatus has the same configuration as the developing device 13 shown in FIG. contains cyan toner. And the amplitude of the AC component of the developing bias applied during development.
V _AC is 1KV for each developing device, and the frequency is 1.2 (KHz), 1.5 (KHz), and 2.0 (KHz) depending on the order of development.
and set. These developing biases are applied only to the developing device that performs development, and the developing devices that do not participate in development are grounded. Furthermore, the amount of charge of each toner is 20 (μ/g), the charging potential of the photoreceptor drum 1 is 600 (V), and the gap d between the photoreceptor drum 1 and the sleeve 20 is 0.5 (mm) on the circumferential surface of the sleeve 20. The thickness of the developer layer was 0.3 (mm), and a DC voltage of 500 (V) was applied to each developing device as a developing bias.

When a multicolor image is formed using the above configuration, the toner image already formed on the photoreceptor drum 1 may be destroyed during subsequent development, or toner of another color may be mixed into each developing device. A visible image with sufficient density was obtained.

Example 2 In the same color image forming apparatus as in Example 1 shown in FIG. 6, the charging potential of the photoreceptor drum 1 was changed to 500 (V), 600 (V), and 700 (V) for each development, and the development bias was DC voltage is also 400 (V), 500 (V), 600
(V) to maintain the density balance of each color toner image, and when a multicolor image was formed under the same conditions as in Example 1, a multicolor image was already formed on the photoreceptor drum 1 during subsequent development. A visible image with sufficient density was obtained without destroying the toner image or contaminating each developing device with toner of another color.

Note that the present invention can be further modified based on its technical idea. In the above embodiments, a two-component developer is used as the developer, but the same effect as described above can be obtained even if a one-component developer is used. Further, the present invention can be applied not only to the case of forming a color image but also to the case of forming a single color image. In this case, it is also possible to form a toner image with gradation by using one developing device and increasing the frequency of the AC component of the development balance in sequence for each development.

Further, the present invention can be applied not only to electrophotographic recording methods but also to non-impact printers using electrostatic recording methods and magnetic recording methods.

6 Effects of the Invention According to the present invention, by sequentially increasing the frequency of the alternating current component of the developing bias applied between the image bearing member and the developer transporting member for each development, it is possible to increase the frequency of the alternating current component of the developing bias applied between the image carrier and the developer transporting member. The electrostatic force applied to the developer adhering to the image carrier during development can be reduced. Therefore, during the subsequent development, the subsequent development can be performed without disturbing the image formed on the image carrier by the previous development. As a result, even if an image is formed on the image carrier by repeating development multiple times, a clear image without any disturbance can be obtained.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a conventional color image forming apparatus. 2 to 6 show embodiments of the present invention, in which FIG. 2 is a cross-sectional view of a developing device and a photosensitive drum, and FIGS. 3 and 4 are cross-sectional views of a developing device and a photosensitive drum. A diagram showing the concentration change when
FIG. 5 is a diagram showing density characteristics when the amplitude and frequency of an alternating current electric field are changed, and FIG. 6 is a diagram showing essential parts of a color image forming apparatus equipped with a plurality of developing devices. Regarding the symbols used in the drawings, 1
...Photosensitive drum, 13, 13A, 13B, 13
C...Developing device, 14...Toner hopper, 17...
... Ear standing regulation plate, 20 ... Sleeve, 21 ... Magnetic roll, 22 ... AC power supply, 23 ... DC power supply,
D...developer, T...toner.

Claims (1)

[Claims] 1 Repeating the step of forming a latent image on the image carrier and the step of supplying the developer on the developer conveying member to the image carrier to develop the latent image multiple times to form a visible image on the image carrier. In an image forming method for forming an image,
An image forming method characterized in that an electric field including an alternating current component is generated between the image carrier and the developer transporting member, and the frequency of the alternating current component is increased sequentially for each development.