JPH01269960A - Multicolor image forming device - Google Patents

Abstract

PURPOSE:To prevent fogging as well as color mixture by setting a bias low while setting an AC bias voltage applied to a developing means following a 2nd stage to a low value so that a toner image which is formed at a front stage is not peeled off. CONSTITUTION:The developing means 7 following the 2nd stage of a device which forms a toner image by developing a latent image selectively by developing means 4 and 7 provided stepwise has a developer carrier 7a which carries a developer to an image carrier 1, and the developer which is carried by the developer carrier 7a does not contact the image carrier 1. Then an AC electric field is applied to the developer carrier 7a and set substantially weak enough not to peel part of the toner image formed on the image carrier in the front stage off the surface of the image carrier 1, and the developer carrying speed is set larger than the moving speed of the image carrier. Consequently, a multicolor image which has neither color mixture nor fogging is obtained.

Description

Detailed Description of the Invention - No. 1 The present invention relates to an electrophotographic image forming apparatus for forming a multicolor image of two or more colors. A multicolor image that forms images of multiple colors in one image forming cycle, in which a latent image is formed and a toner image is formed by selectively developing the latent image with a plurality of developing means provided in a step-like manner. The present invention relates to a forming device.

In recent years, for reasons such as recording images becoming clearer and information easier to understand, for example, the color of the format and the color of the calculated value or data value are expressed in different colors, and CA
It is desired that a part of the drawing outputted by D is outputted in another color, so that it can be identified in two colors or in multiple colors, and toners of colors other than black are increasingly being used. .

For example, two-color electrophotographic image formation? At position i, image formation was performed using an image forming process as shown in FIG. That is, the electrophotographic photoreceptor, which is an image carrier, is uniformly charged to, for example, -600V by the first Wf electric device (FIG. 5(a)).

A first latent image is formed on the photoconductor by the first exposure means, with the surface potential of the exposed part being, for example, -1oov (the fifth
Figure (b)), the first latent image is formed into an appropriate size by a first developing device having a two-component developer consisting of negatively charged red toner and magnetic particles such as ferrite. For example, 1 inversion development is performed by applying a bias of 1600 Hz, 1800 pp alternating current and -500 v direct current superimposed (5th
At this time, the red toner image potential visualized as a visible image increases by about -toov due to the toner charge.
The voltage will be approximately 00V.

Next, the second image signal is irradiated onto the photoreceptor by the t52'i@ optical means, and a second latent image is formed on the photoreceptor, with the surface potential of the exposed portion being, for example, -100V (fifth Figure (d)).

The second latent image formed in this manner is developed using a second developing device containing, for example, negatively charged black one-component magnetic toner, at an appropriate frequency, for example, 1600 Hz, 1300 Vp.
The second latent image is visualized by applying a bias consisting of p(7) alternating current and -500 V(7) direct current (FIG. 5(e)).

Next, the two-color image on the photoreceptor is generally transferred to a transfer material by a transfer charger, peeled off from the photoreceptor by a one-separation charger, fixed by a fixing means, and then sent out of the machine as a two-color color print. is discharged.

However, during the second development, since the first toner image potential is as low as around 1,200 V, a considerable amount of toner is developed in the first toner image area at the same time as in the second exposure area, resulting in so-called color mixing. In order to avoid such color mixture, the second
If the developing bias voltage is set low enough so that the first exposed area is not developed during development, a problem arises in that the second development becomes insufficient.

In order to solve this problem, the second development is performed after the first development.
It was proposed that a charger be installed to recharge the photoreceptor.

According to this method, as shown in FIG. 6 (image forming process up to forming the first toner image)
- (C) are the same as the image forming process (A) - (C) in Figure 5), and the first toner image potential is raised to, for example, -600V by recharging (Figure iR6 (D)). ,
When the second exposure is performed, the potential of the second exposed area is -
The second latent image formed in this way is, for example, a negatively charged black one.
The first toner image area is developed by applying a developing bias of an appropriate magnitude, for example, 1600 Hz, j300 VPP AC knee and 500 V DC superimposed, in a second developing device having component magnetic toner. The second latent image can be visualized without any interference (FIG. 5).

However, due to the electric field caused by the developing bias applied during the second development, a small amount of the first toner gradually mixes into the second developing device, and the problem of color mixture cannot be completely solved. The second development is performed by setting a gap between a non-contact developing means, for example, a photosensitive drum, and a toner carrier disposed in the developing device that supports toner and conveys the toner to the photosensitive drum from 10θ to 500 ILm. , the toner layer supported on the toner carrier at that time is 50 to 200 gm.
Even by using a non-contact developing device that performs development without the toner layer coming into contact with the photoreceptor drum, the problem of two-sided color mixing could not be completely solved.

The purpose of the present invention is to obtain a multicolor image without color mixture, and to
It is an object of the present invention to provide a multicolor image forming apparatus capable of obtaining a clear, high-quality multicolor image without fog and having sufficient density.

3. The above-mentioned object of the present invention is achieved by the multicolor image forming apparatus according to the present invention. In summary, the present invention forms a latent image on an image carrier that moves in a predetermined direction, and selectively develops the latent image using a plurality of stepwise developing means to form a toner image. In a multicolor image forming apparatus configured to obtain a color image, the second and subsequent stages of development have a developer carrier that carries developer and conveys it to the image carrier, and the developer carrier carries developer. A non-contact developing device is configured so that the developer to be transported does not come into contact with the image carrier, and an alternating current electric field is applied to the developer carrier. The electric field strength is set to be substantially weaker than the electric field strength that peels off a part of the toner image on the image carrier from the surface of the image carrier, and the developer transport speed of the developer carrier of the developing means is set to be lower than the electric field strength that peels off a part of the toner image on the image carrier. Preferably, the alternating current electric field applied to the developing means in the second and subsequent stages is set to be larger than the moving speed of Vac, and the direct current If the component is Ddc, the gap between the image carrier and the developer carrier is d, and the toner image potential when the toner image formed in the previous stage is developed is Vt, when the latent image potential is positive, { Vt-(Vdc-Vac/2) 't/d
i 2.3 (V/g) If the latent image potential is negative.

((Vdc+Vac/2)-Vt)/d5,2.
3 (V/am), and the developer transport speed of the developer carrier is 1.5 to 3 of the moving speed of the image carrier.
More preferably, the AC electric field applied to the second and subsequent developing means satisfies V a c /d≧2.0.

Support j Next, the multicolor image forming apparatus according to the present invention will be explained in more detail with reference to the drawings.

FIG. 1 schematically shows an embodiment of an electrophotographic apparatus capable of two-color recording, which embodies a multicolor image forming apparatus according to an uninvented invention. According to this embodiment, a drum-shaped image carrier That is, an electrophotographic photosensitive drum l having a photoconductive layer made of amorphous silicon or the like is supported rotatably in the direction of the arrow, and image forming means are arranged around it. To explain further, a first charger 2, a first image exposure means 3, a first developer 4, a second charger (recharger) 5, and a second charger 5 are arranged around the photoreceptor drum l. a two-image exposure means 6, a second developing device 7,
It has a transfer charger 8 and a cleaning means 11.

The first image exposure means 3 and the second image exposure means 6 each include a first semiconductor laser that emits a first laser beam modulated by the first image signal, a second semiconductor laser that emits a second laser beam, and a second semiconductor laser that emits a second laser beam. A second semiconductor laser 13 is driven by a motor 15 to polarize the first and second laser beams, and direct the first and second images onto the photoreceptor 1 through an imaging lens 16 and a folding mirror 17. It has a rotating polygon mirror 14 that performs rask scanning.

Since the operation of the two-color electrophotographic apparatus having the above configuration is the same as that described in connection with FIGS. 5 and 6, further explanation will be omitted.

According to the present invention, the number of developing devices other than the first developing device 4, that is, the number of developing devices arranged after the first developing device 4 is one.
The device is a non-contact developing device (jumping developing device M) described in the above issue, and has a developer carrying member, in this embodiment, a developing sleeve 7a, which carries developer (toner) and conveys it to the photosensitive drum 1. The developer conveyed by the developing sleeve 7a is configured so as not to come into contact with the photosensitive drum l.

That is, a gap of 100 to 5001 Lm is provided between the developing sleeve 7a and the surface of the photoreceptor drum l, and the number of toner layers supported on the developing sleeve 7a is one black mono-component magnetic toner layer in this embodiment. is 50 to 200 pm, and 50 ILm in this embodiment, and the latent image formed on the photoreceptor drum L is developed without the toner layer coming into contact with the photoreceptor drum. A developing bias is applied so that only the second exposed area is developed without developing the toner image area. FIG. 2 shows the relationship between the developing bias applied to the second developing device 7 and the latent image potential.

In FIG. 2, the first toner image potential Vt on the photosensitive drum l developed by the first developing device 4 is increased to about -600 V by the charge of the second charger 5. In FIG. Further, the second exposed portion potential Vl is attenuated to about -100V, forming a second electrostatic latent image.

The developing bias applied to the second developing device is a voltage in which a DC component Vdc is superimposed on an AC component vaC having a sinusoidal waveform.

By the way, during the second development, two types of forces act on the first toner image. Namely, since the toner forming the second toner image is negatively charged in this embodiment, the toner is The force that presses the toner against the drum is 1Va1-Vt+, and the force that pulls the toner away from the drum is 1Vt-Vazl. At the same time, a force IVaI-V for developing the second exposed area is applied to the second exposed area.
The force that separates the developed toner is 1VazV and 1V.
acts. Here, Val and Vaz are the minimum value and maximum value of the developing bias voltage.

The present inventors have discovered that if the electric field acting on the first toner image and trying to separate the toner is below a certain value, it is possible to prevent the toner of the first toner image from entering the second developing device. I found out.

Next, the effects of the present invention will be explained with reference to experimental examples.

Experimental Example 1 In the two-color electrophotographic apparatus shown in FIG.
The distance d between this and the developing sleeve 7a of the second developing device 7 is 300.
It was the end of the day.

In such a configuration, the results of evaluating the state of toner contamination with respect to the peak-to-peak voltage VPP of the AC component of the developing bias and the difference Δv (=1vt-Vdcl) between the DC component Vdc superimposed on the AC voltage and the toner image potential are shown. 0 Table 1 shown in 1
In (), the numerical value indicates (Vaz-Vt), and 1 frequency is 1
It was 600Hz. Also, the 0 mark indicates a state where there is no toner mixed in, and the Δ mark indicates a state where there is some toner mixed in, but it is acceptable.
The mark X means that there is an unacceptable level of contamination.

Table 1 From Table 1, the separation electric field acting on the first toner image is (Vaz-vt)/cl≦7007300=2.3・
・Q ・If (1), it can be seen that it is possible to effectively prevent toner from entering the second developing device 1F.

Also, since Vaz = Vdc + Vpp/2, formula (1)
When the latent image polarity is negative, (V d c + V p P / 2- V t )
/ d≦2.33 (2).

Experimental Example 2 In the two-color electrophotographic apparatus shown in FIG.
The distance d between the second developer 'JA7 and the developing sleeve 7a is 4.
00 JLm.

Table 2 shows the results of the same evaluation as in Experimental Example 1.

Table 2 From Table 2, the drawing electric field acting on the first toner image is (Vaz-Vt)/d≦900/400 = 2.
25, therefore, similarly when the latent image potential is negative, (V
d c + V p P / 2- V t ) /
d≦2.25... (3) Experimental Example 3 In the two-color electrophotographic apparatus shown in FIG.
The distance d between the developing sleeve 7a of the second developing device 7 is 20
It was set as 0 end m.

Table 3 shows the results of the same evaluation as in Experimental Example 1.

Table 3
2.25, therefore, similarly if the latent image potential is negative,
(Vdc+Vpp/2-Vt)
/ d≦2.25··Φ·(4).

From the above experiments, in general, when the latent image polarity is negative, (V d c + V pp / 2- V t )
/ d≦2.25...1(5) W! When the J actual potential is positive, (Vt - Vdc + Vpp/2)
/ d≦2.251 (6) It can be seen that it is necessary to set the value as follows.

Under the above conditions, for the first toner image potential Vt,
By setting the second developing bias, toner can be prevented from entering the developing device, and a high-quality two-color image without color mixture can be obtained.

In this embodiment, the AC component waveform of the second developing bias is a sine wave, but it is also possible to use a rectangular wave or the like or an asymmetric waveform.

However, under the setting conditions of the above-mentioned equations (5) and (6), the developing power 1 val - 1 x 1 shown in FIG. 2 also becomes weak, making it difficult to obtain a sufficient image density.

In order to prevent color mixing due to toner contamination, the inventors of the present invention have determined that the rotational speed of the developer carrier, that is, in this embodiment, the developing sleeve 7a, can be adjusted to reduce the rotational speed of the developer carrier, that is, the developing sleeve 7a in this embodiment.
That is, in this embodiment, the rotational speed of the photoreceptor drum l is 1.5~
It has been found that this can be compensated for by increasing the image density by three times, and that sufficient image density can be obtained.

FIG. 3 shows the measurement of image density (V-D curve) with respect to development contrast while changing the rotational speed Vs of the developing sleeve under the above-mentioned color mixing prevention +h condition (5).

At this time, the rotation speed Vd of the photosensitive drum serving as the image carrier was 87 mm/sec. Also, at this time, PP about 1 mm is vt=-ao.

(V), Vl=-100(V), development bias is 20th
It was 0Hz, 1ooOVpp.

From FIG. 3, it can be seen that sufficient density can be obtained by setting the rotational speed of the developing sleeve 7a to 1.5 times or more the rotational speed of the photoreceptor drum 1, that is, Vs≧1.5XVd.

However, the rotational speed of the developing sleeve 7a is more than three times the rotational speed of the photoreceptor drum l, that is, Vs≧3.0XV.
d, toner deterioration and toner scattering increase.
Therefore, it is necessary that the rotational speed of the developing sleeve be less than three times the speed of the photoreceptor drum.

In FIG. 4, the gap d between the developing sleeve 7a and the photoreceptor drum 1 is 3001 Lm, and the rotational speed of the developing sleeve 7a is three times the rotational speed of the photoreceptor drum l, that is, Vs=3.
．． The results of measuring V-D curves when Vac was changed at 0XVd are shown.

From FIG. 4, it is difficult to obtain a sufficient concentration when Vac<Boo, and in order to obtain a sufficient concentration, VaC must be Vac≧600v.

It can be seen that it is desirable that Vac/d≧2.0 (Vpp/xm).

According to the present invention, as described above, since the AC bias voltage applied to the second and subsequent stage developing devices is set weakly, it is possible to simultaneously obtain the advantage of being able to prevent the occurrence of background fog.

In other words, according to the present invention, since the flying force of the toner is weakened, it is possible to suppress the amount of toner adhering to the white background area to an extremely small amount.On the other hand, in the image area, although the flying force of the toner is weak, The speed ratio increases the amount of toner supplied and achieves the effect of obtaining sufficient image density, and at the same time solves the problem that background fog occurs when simply rotating the developing sleeve relatively quickly.

In the above embodiments, the present invention has been described as a two-color image forming apparatus, but is not limited thereto, and can be similarly applied to a multi-color image forming apparatus using three or more colors.

As described above, in the multicolor image forming apparatus according to the present invention, the AC bias voltage applied to the developing means in the second and subsequent stages is set to be weak so that the toner image formed in the previous stage is not peeled off. At the same time, the toner flying force, that is, the developing ability, which has decreased due to the weak bias setting, is compensated for by increasing the relative speed of the developer carrier to the image carrier by 1.5 to 3 times, thereby preventing color mixing. At the same time, it has the advantage of preventing fogging and producing high-quality images with clear image density.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an embodiment of a multicolor image forming apparatus according to the present invention. FIG. 2 is a diagram showing the relationship between the developing bias applied to the second H) voltage and the latent image potential in the apparatus of FIG. 1. FIG. 3 is a graph in which image density (VD curve) with respect to development contrast was measured while changing the rotational speed Vs of the developer carrier. FIG. 4 is a graph showing a V-D curve when it is assumed that the rotational speed of the developer carrier is three times that of the image carrier. FIGS. 5 and 6 are diagrams illustrating a two-color image forming process. l: Image carrier (photosensitive drum) 4: First developer 7: Second developer (non-contact developing device) 7a: Developer carrier (developing sleeve) 5: Recharger development contrast (Vdc-VJ) Development contrast (Vdc-VZ) (a) (d) (b)
(E) (C) (A) (B) (C) Diagram (D) (E) (F)

Claims (1)

[Scope of Claims] 1) Forming a latent image on an image bearing member moving in a predetermined direction, and selectively developing the latent image using a plurality of stepwise developing means to form a toner image; In a multicolor image forming apparatus configured to obtain a multicolor image, the second and subsequent stages of development have a developer carrier that carries developer and transports the developer to the image carrier. The developer transported by is a non-contact developing device configured so as not to come into contact with the image carrier, and an alternating current electric field is applied to the developer carrier,
The alternating current electric field is set to be substantially weaker than the electric field strength that peels off a part of the toner image on the image carrier formed in the previous stage from the surface of the image carrier, and is A multicolor image forming apparatus, wherein a developer conveying speed is set higher than a moving speed of the image carrier. 2) The alternating current electric field applied to the second and subsequent stages of development means has the peak-to-peak voltage of the alternating current component as Vac, the direct current component as Ddc, the gap between the image carrier and the developer carrier as d, and the voltage formed in the previous stage. If the toner image potential when the toner image is developed is Vt, then if the latent image potential is positive, {Vt-(Vdc-Vac/2)}/d≦2.3(V/
μm) If the latent image potential is negative, {(Vdc+Vac/2)-Vt}/d≦2.3(V/
2. The apparatus according to claim 1, wherein the developer transport speed of the developer carrier is 1.5 to 3 times the moving speed of the image carrier. 3) The alternating current electric field applied to the second and subsequent developing means is Vac.
The device according to claim 1 or 2, wherein /d≧2.0.