JPH07120116B2 - Development device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a developing device capable of regular development and reversal development.

(B) Conventional Technology Conventionally, in order to perform regular development and reversal development, when a normal photosensitive drum, for example, a negatively charged type photosensitive drum is used, development in which positively charged toner is stored during regular development It is necessary to operate the apparatus and also the developing apparatus in which the negatively charged toner is stored at the time of reversal development. Therefore, each time the developing mode is changed, the developing device is attached to or detached from the printer body, or two types of developing devices are provided in advance in the printer body.
It was operated by selectively switching them.

Therefore, in the case of using a photosensitive drum (for example, an amorphous silicon photosensitive drum) that is charged with both positive and negative polarities and has photosensitivity, and using negatively charged toner, the photosensitive drum is used during regular development. A developing method has been proposed in which the body drum is positively charged and negatively charged at the time of reversal development (see Japanese Patent Application Laid-Open No. 58-107562), but a photosensitive drum capable of being charged in both polarities becomes expensive.

(C) Problems to be Solved by the Invention As described above, in the conventional technology, the developing device has to be attached to and detached from the main body of the printer or the like, and two types of developing devices have to be prepared. It has the disadvantage of being bad or becoming large.

Further, there is a problem in that it is difficult to manufacture a photosensitive drum that can be charged in both polarities, and it becomes expensive.

(D) Means for Solving the Problems In the present invention, a toner storage portion for storing magnetic toner that can be charged in both polarities, and a magnetic brush formed by the toner provided in the toner storage portion are formed. Toner supply roller, a positive bias means for applying a positive DC bias to the toner supply roller, a negative bias means for applying a negative DC bias to the toner supply roller, the positive bias means and a negative bias means. A switch means for selectively switching the bias means, and a developing roller made of a conductive material that receives the supply of the charged toner having a polarity corresponding to the polarity of the selected DC bias from the toner supply roller and is in contact with the photosensitive member. When,
The developing device is provided with an AC bias means for applying an AC bias to the developing roller.

(E) Operation In the present invention, a toner that can be charged in both polarities is used, and either the positively charged toner or the negatively charged toner is selected by switching the DC bias means to the toner supply roller. The toner is temporarily attached to the developing roller. Therefore, even if the charging polarity of the photoconductor is constant, the toner charged to one polarity adheres to the photoconductor during normal development, and the toner charged to the other polarity during reversal development. Will be attached to.

(F) Embodiment FIG. 1 is an illustrative view showing a main part of an embodiment of the present invention. The developing device 10 includes a developer chamber 14 as a toner storage portion having an opening arranged near a photoconductor drum 12 on which a photoconductive layer such as polyvinyl carbazole is formed. In the developer chamber 14, a toner supply roller having a width substantially equal to that of the toner hopper 15 and the photoconductor drum 12 and formed of a non-magnetic material such as aluminum, that is, a material capable of passing magnetic flux. 16 is rotatably provided (the drive mechanism is not shown). One terminal of a bias changeover switch 18 is connected to the sleeve 16. One terminal of the positive DC power supply 30 is connected to one switching terminal of the bias changeover switch 18, and the other terminal of the DC power supply 30 is grounded. Further, one terminal of a negative DC power supply 31 is connected to the other switching terminal of the bias changeover switch 18, and the other terminal of the DC power supply 31 is grounded.

Therefore, from the positive DC power supply 30, a DC bias of voltage V _{1 of} + 1000V is applied to the sleeve 16 via the bias changeover switch 18, and from the negative DC power supply 31 of voltage V _{2 of} −1000V, for example. A DC bias is applied via the bias changeover switch 18 as described above.

Inside the sleeve 16, a magnet 20 assembled in a cylindrical shape is provided. The magnet 20 is for forming a magnetic brush of the charged toner 34 on the sleeve 16 by a magnetic attraction force. A doctor blade 22 for adjusting the height of the formed magnetic brush is provided above the sleeve 16, and the doctor blade 22 is fixed to a part of the developer chamber 14. In this embodiment, for the reasons described below, the doctor blade 22
The gap a between the sleeve 16 and the sleeve 16 is set to 0.3 mm, for example.

In the vicinity of the sleeve 16 of the developer chamber 14, the toner 34, which is a magnetic one-component developer, is stored. The toner 34 is made of an insulating magnetic material having a resistivity of 10 ¹³ Ω · cm or more.

On the left side of the sleeve 16, that is, on the downstream side, the photosensitive drum 12
A developing roller 24 is provided which rotates in contact with the developing roller 24. An elastic layer 26 having elasticity, such as rubber, is provided on the surface of the developing roller 24. The thickness of the elastic layer 26 is set to, for example, 0.2 to 0.8 mm. The elastic layer 26 is provided on the developing roller 24 so that the developing roller 24 does not damage the surface of the photosensitive drum 12 even if the developing roller 24 comes into contact with the photosensitive drum 12 and rotates.

One terminal of an AC power supply 28 is further connected to the developing roller 24, and the other terminal of the AC power supply 28 is grounded. The AC voltage V ₃ of the AC power supply 28 has a frequency of, for example, 1.6 kHz, and its peak value is set to, for example, Vp-p 1000V.

Below the developing roller 24, there is provided a peeling roller 32 for rotating in contact with the developing roller 24 and scraping off the toner remaining without being adsorbed on the surface of the photosensitive drum 12.

In operation, the toner 34 is positively or negatively charged by friction between them or by contact with the sleeve 16 or the doctor blade 22, respectively. For that purpose, in this embodiment, the toner 34 is formed of an insulating material having a resistivity of 10 ¹³ Ω · cm or more and capable of being charged in both polarities. In this way, the charged toner 34 is collected on the sleeve 16 by the magnetic attraction force of the magnet 20, and is formed as a magnetic brush. This magnetic brush is transferred by a rotating sleeve 16 and is leveled by a doctor blade 22 on the way.

The magnetic brush whose height is adjusted by the doctor blade 22
It is further transferred by the rotation of the sleeve 16 and comes into contact with the elastic layer 26 of the conductive developing roller 24. When the tip of the magnetic brush comes into contact with the elastic layer 26, the toner is transferred onto the developing roller 24.
34 will be supplied. Now, as shown in FIG. 1, when the negative DC power source 31 is electrically connected to the sleeve 16, the sleeve 16 has a DC bias V ₂ = −1 as described above.
Since 000V is applied and the AC bias is applied to the developing roller 24, only the negatively charged toner 34n is supplied to the developing roller 24 from the magnetic brush (see FIG. 2). Further, when the positive DC power source 30 is electrically connected to the sleeve 16, a DC bias V ₁ = + 1000V is applied to the sleeve 16, and only the positively charged toner 34P is supplied to the developing roller 24. (See Figure 2).

Therefore, when a bias higher than the AC bias applied to the developing roller 24 is applied to the sleeve 16, only the positively charged toner 34P is transferred to the developing roller, so that the photosensitive drum 12 is negatively charged. For example, regular development is executed. When a bias lower than the AC bias is applied to the sleeve 16, only the negatively charged toner 34n is transferred, and reversal development is executed.

The toner 34 in this embodiment is a mixture of magnetite (iron or nickel) in styrene resin, and the styrene resin is charged positively and the magnetite is charged negatively. However, the charge amounts of the styrene resin and magnetite are in an imbalanced state, and each toner 34 particle is apparently charged either positively or negatively.

By the way, when only the toner 34P charged to one polarity (for example, positive) is consumed, only the toner 34n charged to the other polarity (in this case, negative) is likely to remain, but actually, The toner 34n that is initially charged negatively and is being stored is also charged positively while being stirred, so the toner 34n that is positively charged
The amounts of P and the toner 34n that are negatively charged always exist at a substantially constant rate.

Charged toner (34P or 34n) supplied to the developing roller 24
Are carried to the electrostatic latent image surface of the photoconductor drum 12 according to the rotation of the developing roller 24. When the electrostatic latent image surface comes into contact with the developing roller 24, the toner 34 on the elastic layer 26 is attracted to the electrostatic latent image surface to be developed.

As described above, on the developing roller 24, only the charged toner 34 of one polarity (for example, minus: this can be arbitrarily selected depending on whether the normal development is reversal development or the polarity of the latent image potential as described above). Supplied. Therefore,
In combination with the action of the doctor blade 22, the thickness of the toner layer becomes very thin, for example, about 30 μm to 100 μm. Further, the AC bias applied to the developing roller 24 causes the toner 34 to be balanced, so that the toner layer is formed as a more uniform thin layer. Therefore,
“Fog” due to excess toner hardly occurs.

Next, based on the experimental results of the inventors, the gap a between the sleeve 16 and the doctor blade 22, the sleeve 16 and the developing roller
The relationship between 24 and the gap b will be described.

FIG. 3 is a graph showing the gap ratio b / a on the horizontal axis and the thickness of the toner on the developing roller 24 on the vertical axis based on the experimental results. As is clear from this result, the gap ratio b / a is
When it is 0.8 or less, the electric field between the rollers 16 and 24 becomes strong and the toner layer becomes thick, so that good development cannot be performed.
That is, when the toner layer becomes thick, the toner 34 is excessively pressed against the photosensitive drum 12 by the elastic layer 26. Then, so-called "fog" due to toner accumulation occurs on the electrostatic latent image surface, and good development cannot be performed.

On the contrary, when the gap ratio b / a is 1.3 or more, there is no fear of toner accumulation, but the gap b becomes large, so that the electric field between the rollers 16 and 24 is weakened and the sleeve 16 and the developing roller 24 move.
The toner 34 is less supplied to the toner. Then, the development density becomes low, and good development cannot be performed.

From the above experimental results, the relationship between the gaps a and b is 0.8a ≦ b ≦ 1.
It has been found that good development can be achieved by setting 3a.

Next, referring to FIG. 4, the sleeve 16 and the developing roller
The relationship between the voltage applied to 24, that is, the DC bias voltage V ₁ (or V ₂ ), the average value V ₀ of the AC bias voltage, the peripheral speed ratio N, and the “fog” with respect to the gap b will be described.

FIG. 4 shows the results of experiments conducted by the inventors, with the horizontal axis representing | V ₁ -V _0.
Is a graph showing the degree of "fog" on the vertical axis. However, N represents the peripheral speed ratio between the sleeve 16 and the developing roller 24. As is clear from this result, | V ₁ -V ₀ | ・ N / b
When is less than A point, the "fog" in the white background increases. ｜ V ₁ -V ₀ ｜ ・ One of the reasons why N / b is below A point is
The absolute value | V ₁ −V ₀ | of the voltage difference applied to the sleeve 16 and the developing roller 24 may be small. When the absolute value of the voltage difference | V ₁ -V ₀ | becomes smaller, the developing roller 24
It is conceivable that the upper electric field is weakened and the action of selecting the polarity of the toner is lowered, so that the toner charged to the same polarity as the electrostatic latent image on the photoconductor drum 12 is mixed and supplied. The toner charged to the same polarity as the electrostatic latent image does not have an attractive force with the electrostatic latent image, but is mixed with the toner having the attractive force and adheres to the electrostatic latent image surface. Therefore, it is considered that an excessive amount of toner is supplied to cause so-called "fog".

On the other hand, when the absolute value | V ₁ -V ₀ | · N / b exceeds the point B, “fog” increases again. It is considered that this cause is caused by the toner layer on the developing roller 24 becoming too thick. That is, when the toner layer on the developing roller 24 becomes thick, the toners are frictionally charged by the rotation of the developing roller 24, and a small amount of toner charged to the same polarity as the electrostatic latent image surface is generated. It is thought that the "fog" will increase as it is supplied.

From the above results, the proper development area is limited between the points A and B, and it is necessary to satisfy the relationship of A ≦ | V ₁ −V ₀ | · N / b ≦ B. However, when the peripheral speed ratio N increases and the absolute value of the voltage difference | V ₁ −V ₀ | decreases, the electric field weakens and “fog” increases, so Vp−p ≦ | V ₀ | ≦ | V The relationship of ₁ | must also be satisfied. However, Vp-p is the peak value of the AC power supply 28. In this way, in order to obtain a good screen without "fog", if necessary,
| V ₁ -V ₀ | · N and b may be appropriately selected.

In the embodiment, the peripheral speed ratio between the developing roller 24 and the photosensitive drum 12 is set to 1.0 to 1.5, and the peripheral speed ratio N between the toner supply roller 16 and the developing roller 24 is set to 1.2 to 2.0.

(G) Effect of the Invention In the present invention, a magnetic toner capable of being charged in both polarities is used,
Normal development and reversal development can be selected as necessary by simply switching the DC bias applied to the toner supply roller.

Therefore, it is possible to realize an inexpensive and high-performance developing device without the need for two types of developing devices and the need for a photosensitive member that can be charged in both polarities.

[Brief description of drawings]

FIG. 1 is an illustrative view showing one embodiment of the present invention. FIG. 2 is a diagram for explaining the voltage applied to the sleeve and the developing roller in the embodiment of FIG. FIG. 3 is a graph showing the thickness of the toner on the developing roller with respect to the gap ratio b / a. Fig. 4 shows "fog" of the electrostatic latent image with respect to | V ₁ -V ₀ | ・ N / b.
It is a graph which shows the degree of. 12 …… photosensitive drum, 14 …… developer chamber (toner reservoir), 16 …… sleeve (toner supply roller), 18 …… bias changeover switch, 24 …… developing roller, 28 …… AC power supply, 30 ・31 …… DC power supply, 34 …… toner.

Claims (1)

[Claims] 1. A toner storage portion for storing magnetic toner that can be charged to both polarities, a toner supply roller provided in the toner storage portion for forming a magnetic brush with the toner, and the toner supply roller. Positive bias means for applying a positive DC bias to the toner supply, a negative bias means for applying a negative DC bias to the toner supply roller, and a switch for selectively switching between the positive bias means and the negative bias means. Means, a developing roller made of a conductive material that receives a supply of charged toner having a polarity corresponding to the polarity of the selected DC bias and is in contact with the photosensitive member, and an AC bias to the developing roller. A developing device provided with an AC bias means for applying.