JP2671041B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Objects of the Invention (Industrial field of application) The present invention uses an image forming apparatus utilizing an electrostatic transfer process, such as an electrostatic copying machine and a printer, and particularly an amorphous silicon photoconductor. The present invention relates to a transfer material separating device suitable for use in a conventional image forming apparatus.

(Problems to be Solved by Prior Art) Toner on the image carrier side is obtained by bringing a transfer material such as paper into contact with a toner image electrostatically formed on the surface of the image carrier and applying a transfer bias by a transfer charger. In a known image forming apparatus including a step of transferring an image to a transfer material,
Since the transfer material tends to be electrostatically attracted to the image carrier by applying the transfer bias, a bias having a polarity opposite to that at the time of transfer is applied to the transfer material so that the charge acquired by the transfer material at the time of transfer is applied. It is well known that an image forming apparatus in which a separation charging device for neutralizing and discharging is disposed at a position after transfer has been widely used in the past.

This type of separation charger is usually configured to superimpose an AC voltage with a DC voltage of appropriate polarity and apply the voltage, and to eliminate the charge by corona discharge at this time. It depends on the peak-to-peak voltage value of the AC voltage to be generated, and it is known that the higher this value, the better the static elimination function.

Therefore, in view of the recent trend toward higher speeds of this type of apparatus, it is desirable to increase the peak-to-peak voltage value as much as possible in order to perform charge removal after transfer as quickly and reliably as possible. However, on the other hand, increasing this value cannot avoid the risk of abnormal discharge such as spark discharge and creeping discharge.

By the way, regarding the photosensitive layer used in this type of image forming apparatus, various materials such as selenium, other inorganic photoconductive materials, organic photoconductive materials (OPC), and amorphous silicon semiconductors are used as photosensitive layer materials. Although it is selected and used according to the intention, amorphous silicon has high sensitivity,
From the viewpoint of high durability, it is gradually being used.

Here, the photoconductor using amorphous silicon will be briefly described. This photoconductor, as shown in FIG. 6, has a surface of a conductive substrate 8 made of metal or the like, regardless of its charging polarity.
Amorphous silicon layer 10 as photosensitive layer (layer thickness about 25 μm)
And two charge injection blocking layers 9 and 11 (both having a layer thickness of 0.1
To 1.0 μm), and the blocking layer 11 blocks the charge applied to the surface of the photoconductor to charge the photoconductor and prevents the charge from entering the photoconductive layer. 9
Means that only charges having the same polarity as that of the photoconductor are allowed to pass therethrough and blocks charges of the opposite polarity from entering the photosensitive layer. Of course, the materials of these blocking layers differ depending on the charge polarity of the photoconductor.

In such an amorphous silicon photoconductor, for example, when a positive charge is applied to the surface of a positive charge type photoconductor by a separation charger, negative charges are induced at the boundary between the substrate and the charge injection blocking layer according to the amount of the positive charge. This negative electrode is attracted to the positive charge, but the presence of the charge injection blocking layer prevents the negative electrode from entering the photosensitive layer.

For this reason, positive and negative charges are attracted to each other, so that an electric field is generated in the photoconductor, but when an excessively large positive charge is momentarily applied to the surface of the photoconductor due to the occurrence of abnormal discharge, the strong electric field generated with this causes a strong electric field to be generated in the photoconductive layer. In some cases, dielectric breakdown occurs in the charge injection blocking layer and pinholes are generated.

Such pinholes are not preferable because they lead to deterioration of image quality and remarkably shortened life of the photoconductor.

When a negative charge is applied to the surface of the photoconductor, the positive charge induced in the substrate passes through the charge injection blocking layer and is neutralized with the negative charge on the surface, suppressing the generation of electrolysis and causing dielectric breakdown. Hard to happen.

That is, the positive charging type photoconductor is likely to cause dielectric breakdown with respect to the positive charge type charging, and similarly, the negative charging type photoconductor may cause dielectric breakdown with respect to the negative electrolytic charging.

By the way, conventionally, when the transfer material is separated from the photoconductor after the transfer by electrostatic separation, an AC bias is applied to the separation charger, but the AC waveform has a duty ratio of 1 when the separation bias is applied. It is common that the positive and negative peak voltage values of are equal, and when an amorphous silicon photoconductor is used, dielectric breakdown may occur especially at the time when a bias of the same polarity as the charging polarity is applied. Was great.

Furthermore, looking at the withstand voltage characteristics, the OPC photosensitive member is about 20 μm.
The layer thickness is 5 KV or more (250 V / μm), Se-Te and Se-As are about 50 μm and 3 KV or more (60 V / μm), whereas amorphous silicon is about 2 KV when the layer thickness is 25 μm. Is.

Therefore, if amorphous silicon is exposed to high-pressure corona for a long time like a high-speed machine and the maintenance interval is long and the charging line is easily contaminated, abnormal discharge is likely to occur, which causes the above problems. Will be invited.

In addition, the relative dielectric constant ε _{s of} amorphous silicon is OP
Compared to that of C and Se system, which is about 3.6, it is as large as about 10, and the amount of corona discharge for obtaining the same photoconductor potential becomes large, which necessitates higher corona discharge. Will also promote the occurrence of abnormal discharge.

Of course, it is possible to avoid the above problems by increasing the film thickness and raising the withstand voltage limit. However, amorphous silicon has poor film forming characteristics, so from the viewpoint of productivity and cost, Such a thing is not realistic.

The present invention has been made in view of the above situation, and in an image forming apparatus that uses an amorphous silicon photoconductor and performs electrostatic separation by a separation charger to separate a transfer material, a peak-to-peak voltage of an applied bias. It is an object of the present invention to provide a transfer material separating device that does not require an increase in the peak value and that can speed up the image forming apparatus without causing the occurrence of abnormal discharge.

(2) Configuration of the Invention (Technical Means for Solving the Problem and Its Action) In order to achieve the above object, the present invention provides an amorphous silicon photoconductor and a transfer charger for transferring an image from the photoconductor to a transfer material. And an image forming apparatus having a separation charger for removing charge from the transfer material in order to separate the transfer material from the photoconductor after image transfer, the AC voltage applied to the separation charger has a positive / negative peak value ratio of 1. With respect to the AC voltage having a duty ratio of 1, the duty ratio is set so that the areas of the positive and negative components exceeding the discharge start voltage are equalized and the peak value on the same polarity side as the charging polarity of the photoconductor becomes low. Are set to different values, or in the above (1), the waveform of the AC voltage is a rectangular wave. Or the above ( 1) The image forming apparatus (3) is characterized in that the waveform of the AC voltage is a sine wave.

By configuring in this way, while maintaining the charge removal function of the separation charger, it prevents the occurrence of abnormal discharge on the side of the same polarity as the charging polarity of the photoconductor, dielectric breakdown due to this,
It is possible to prevent deterioration of image quality.

(Explanation of Embodiments) FIG. 1 is a diagram showing an embodiment in which the present invention is applied to a copying machine having a cylindrical photoconductor having an axis line in a direction perpendicular to the plane of the drawing and rotating in the direction of arrow A. It is a side view which shows the separation site vicinity.

The photoconductor 1 is a positive charging type image carrier using amorphous silicon, and a transfer charger 2 and a separation charger 7 are arranged in the vicinity of the photoconductor 1 and formed on the surface of the photoconductor 1 by charged toner. When a toner image and a transfer material (not shown) that is conveyed by the conveying path 6 at the same timing as the toner image arrive at the transfer portion where the transfer charger 2 and the photoconductor 1 face each other, the transfer material is transferred together with the transfer material. A transfer bias having a polarity opposite to that of the toner is applied to the charger, and the toner image on the photoconductor side is transferred to the transfer material by the action of the electric field formed.

Since the transfer material tends to be attracted to the surface of the photoconductor due to the transfer bias during the transfer, the separation charging device 7 is disposed downstream of the transfer charging device 2 in the running direction of the transfer material, and the transfer after the transfer is completed. A separation bias of the opposite polarity to that used during transfer is applied to the material to neutralize and remove the charge that the transfer material acquired during transfer to separate it from the photoconductor, and then supply this to a fixing portion (not shown). And

Needless to say, a primary charging device, an image signal applying means, a developing device, a cleaner and other members necessary for image formation are provided around the photoconductor 1, but these are not directly related to the present invention. It is omitted because it is not relevant.

In the above device, in order to apply a separation bias to the separation charger, an AC power source 3 that outputs a rectangular wave to the separation charger 7 and can change the duty ratio thereof, a DC bias for controlling the corona current. A power supply 4 and a corona control circuit 5 for detecting the corona current and controlling the power supply 4 so that the current becomes a predetermined value are connected.

FIG. 2A shows a rectangular wave with a duty ratio of t ₁ / t ₂ = 1 which is a waveform when no means is provided for suppressing abnormal discharge, where V ₀ is the discharge start voltage, V ₁ and V ₂ are positive and negative peak values, S ₁ and S ₂ are areas of V ₀ or more, and the reference applied voltage is V ₁ = V ₂ = 6.0 KV.

In this arrangement, maintaining the S ₁ constant by varying the duty ratio t ₁ / t ₂ with lowering V _1, by keeping the S ₂ constant by increasing the peak value V ₂ of the negative side Positive and negative corona current can be kept constant.

The reference value as described above, that is, the duty ratio t ₁ / t
_When a rectangular wave of ₂ = 1 and V ₁ = V ₂ = 6.0 KV was applied for continuous paper feeding, abnormal discharge occurred at 70,000 sheets and pinholes were generated.

 The discharge starting voltage in this case is about 3.0 KV.

By setting the duty ratio to 1.67, V ₁ can be reduced to about 5.4 KV, and V ₂ at this time was 7.0 KV.

Applying this rectangular wave alternating current to the separation charger, continuous paper feeding
Although 100,000 sheets were printed, no abnormal discharge occurred.

Also, the duty ratio t ₁ / t ₂ = 2.0, V ₁ = 5.25KV, V ₂ =
After passing 100,000 sheets continuously at 7.5KV, abnormal discharge did not occur.

Next, the duty ratio t ₁ / t ₂ = 2.6, V ₁ = 5.1KV, V ₂
= 8.0 KV, abnormal discharge occurred between the charging wire and the shield plate of the charger.

In the charger, the distance between the charging wire and the shield plate is lm
It is empirically known that in the case of m, the withstand voltage between them is approximately lKV.

The distance between the charging wire and the shield plate is usually 7.5 to 11.0 mm
The withstand voltage is almost 7.5-11.0KV
You can think of it.

Therefore, in the above case, it is considered that abnormal discharge has occurred on the negative side, and it is necessary to set the peak voltage to this value or less as much as possible.

From the above results, in the case of a rectangular wave with V ₁ = V ₂ = 6.0KV, set the duty ratio t ₁ / t ₂ as 1.0 <t ₁ / t ₂ ≦ 2.0 and adjust the positive and negative peak voltages. This can reduce abnormal discharge on the positive side. A particularly suitable value is t ₁
/ t ₂ = 1.6.

Next, an embodiment in which the voltage applied to the separation charger is a sine wave will be described with reference to FIGS. 3A and 3B. The apparatus used in this case is similar to that shown in FIGS.

FIG. 3A shows a basic waveform in which the duty ratio t ₁ / t ₂ is 1, and the peak voltage V ₁ = V ₂ = 6.0 KV.

When the paper was continuously fed with this waveform, abnormal discharge occurred at 40,000 sheets, and pinholes were formed in the photosensitive layer.

t ₁ / t ₂ = 1.40, V ₁ = 5.6KV, V ₂ = 7.0KV and t ₁ / t ₂ =
_{1.60KV, V 1 = 5.4KV, V} 2 = none set to 7.5KV was performed 60,000-sheet continuous paper feed, there was no occurrence of abnormal discharge.

When the value of t ₁ / t _{2 was} further increased, abnormal discharge occurred on the negative side.

From the above results, by setting 1 <t ₁ / t ₂ ≦ 1.60, the occurrence of pinholes can be avoided, and a desirable value for the duty ratio t ₁ / t ₂ is about 1.4.

Next, an embodiment will be described in which negative charging type amorphous silicon is used as the photoconductor and the same device as that shown in FIG. 1 is used as the device.

Negative charging type amorphous silicon easily causes dielectric breakdown of the photosensitive layer when abnormal discharge occurs on the negative side.
It is desirable to keep V ₂ small.

The basic waveform of the applied voltage is the duty ratio t ₁ / t ₂ = 1 and V
A rectangular wave of ₁ = V ₂ = 6.0 KV was used, and when continuous paper feeding was performed by this, abnormal discharge occurred at 70,000 sheets and pinholes were generated.

Next, set t ₁ / t ₂ = 0.62, V ₁ = 7.0KV, V ₂ = 5.4KV, and t ₁ / t ₂ = 0.5, V ₁ = 7.5KV, V ₂ = 5.25KV and set 10
After passing 10,000 sheets continuously, no abnormal discharge occurred.

When the value of t ₁ / t _{2 was} further reduced, abnormal discharge occurred when a positive voltage was applied.

In conclusion, by setting 0.5 ≦ t ₁ / t ₂ <1.0, the occurrence of abnormal discharge on the negative side and the formation of pinholes can be reduced, and a particularly preferable value is about 0.6.

FIG. 4 and FIG. 5 are graphs showing the case of using a rectangular wave in the above-mentioned embodiments.

In Fig. 4, the horizontal axis is V ₁ (= V ₂ ) when t ₁ / t ₂ = 1
Therefore, it is appropriately set depending on the copying speed.

The vertical axis shows a value of a desirable duty ratio for preventing abnormal discharge and pinholes from occurring. The value is calculated as follows.

In the case of a positive charging type photoconductor, dielectric breakdown is more likely to occur than in the case of a negative type, so it is better to lower the peak value on the positive side than on the negative side as shown in FIG. 2B.

V (= V ₁ = V ₂ ) is the positive and negative peak voltage when t ₁ = t ₂ (Fig. 2A), V ₀ is the discharge start voltage, and t ₁ ′ and t ₂ ′ are the respective duty ratios. Positive and negative voltage application time when changed from 1 (Fig. 2B), v is when duty ratio is changed from 1 (Fig. 2B)
Peak voltage value of the opposite polarity (negative here) to the charging polarity of the photoconductor, x is when the duty ratio is changed from 1 (Fig. 2B)
The peak voltage values of S ₁ ′ and S ₂ ′ of the same polarity (positive here) as the charging polarity of the photoconductor are positive and negative of the voltage when each duty ratio is changed from 1 (Fig. 2B). , The area of the part exceeding V ₀ , is the condition to keep the positive current constant (S ₁ = S ₁ ′) and the condition to keep the negative current constant (S ₂ =
From S ₂ ′), (V−V ₀ ) t ₁ = (x−V ₀ ) t ₁ ′ (1) (V−V ₀ ) t ₂ = (v−V ₀ ) t ₂ ′ (2) Second A Since the duty ratio is 1 in the figure, t ₁ / t ₂ = 1 (3) Since the AC frequency is the same in Figures 2A and 2B, t ₁ + t ₂ = t ₁ ′ + t ₂ ′ (4) (3) (2), clearing the t ₂ are substituted into (4), (V-V 0) t 1 = (v-V 0) t 2 '(5) 2t 1 = t 1' + t 2 ′ (6) If t ₂ ′ is deleted from the above (5) and (6), t ₁ = (v−V ₀ ) t ₁ ′ / (2v−V−V ₀ ) (7) (7) becomes (1 ), X−V ₀ = (v−V ₀ ) (V−V ₀ ) / (2v−V−V ₀ ) (8) x = (vV + vV ₀ −2VV ₀ ) / (2v−V−V ₀ ) (9) From (1) ÷ (2), t ₁ ′ (x−V ₀ ) / {t ₂ ′ (v−V ₀ )} = t ₁ / t ₂ Substituting equation (3) into this _{t 1 '/ t 2' =} (v-V 0) / (x-V 0) This Wherein _{t 1 '/ t 2' =} (2v-V-V 0) / (V-V 0) is substituted into (8) (10) That is, when a positive charging type photosensitive material, preferably a duty ratio Is as shown in equation (10).

When a negative charge type photoconductor is used, the preferable duty ratio is x and v opposite to those in the case of FIG. 2B. Therefore, when the preferable duty ratio is similarly calculated, t ₁ ′ / t ₂ ′ = (V−V ₀ ) / (2v−V−V ₀ ).

In FIG. 4 (the same applies to FIG. 5 described later), V ₀ = 3.0KV, v
= 7.0KV.

If v is too large, abnormal discharge is likely to occur between the charging wire of the charger and the shield plate, and if v is too small, the peak voltage on the same polarity side as the charging polarity cannot be made sufficiently low, resulting in dielectric breakdown of the photoconductor. Does not fit the purpose of blocking.

In Fig. 5, the horizontal axis is the same as that in Fig. 4, and the vertical axis is t ₁ / t.
It is the peak voltage on the same polarity side as the charging polarity of the photoconductor when ₂ is set to a desired value.

 This value can be roughly expressed by the following equation.

When the photoconductor is of the positive charging type, it is the above formula (9), and x = (vV + vV ₀ −2VV ₀ ) / (2v−V−V ₀ ) When the photoconductor is of the negative charging type, the case of FIG. 2B and x Since v and v are opposite to each other, when calculated in the same manner as in the case of using the positive charging type photoconductor, x = (vV + vV ₀ −2VV ₀ ) / (2v−V−V ₀ ). Then, the transfer material is brought into contact with the toner image formed on the surface of the photoconductor, and a transfer bias is applied to transfer the toner image to the transfer material. In a separation device of an image forming house configured to apply a voltage to separate a transfer material from an image carrier, the area on the positive and negative sides of the portion of the AC waveform applied to the separation charger that is equal to or higher than the discharge start voltage is constant. By changing the duty ratio while maintaining
It is possible to reduce the peak value on the same polarity side as the charging polarity of the photoconductor, and it is possible to prevent abnormal discharge on the polarity side and damage to the photoconductor due to this.

(3) Effects of the Invention As described above, according to the present invention, in the separating device configured to electrostatically separate the transfer material from the photoconductor by the separating charger, abnormal discharge of the separating charger occurs. Based on this, it is possible to effectively prevent damage to the photoconductor, and it is possible to achieve stable and good separation action at all times, and it is easy to speed up the image forming apparatus.
In particular, the present invention is not limited to image forming apparatus using amorphous silicon photoconductors.
It has a very remarkable effect when used for.

[Brief description of the drawings]

FIG. 1 is a schematic side view of a main part of an image forming apparatus which is an embodiment of the present invention, FIGS. 2A and 2B are explanatory views showing waveforms of a bias voltage applied to a separation charger of the same apparatus, and FIG. 3A. , FIG. 3B is an explanatory view showing another waveform of the bias voltage applied to the separation charger of the same apparatus, FIG. 4 is a graph showing the set value of the duty ratio in FIGS. 2A and 2B, and FIG. 5 is a duty. FIG. 6 is a graph showing the charging polarity of the photoconductor and the peak voltage on the same polarity side when the ratio is set as in the previous figure, and FIG. 6 is an enlarged sectional view showing the configuration of the amorphous silicon photoconductor. 1 ... Photosensitive member, 2 ... Transfer charger, 3 ... AC power source, 4
...... DC power supply, 5 ...... corona current detection circuit, 7 ...... separation charger, 8 …… photoreceptor substrate, 9 …… charge injection blocking layer, 10
…… Photosensitive layer, 11 …… Charge injection blocking layer.

Claims (3)

(57) [Claims] 1. An amorphous silicon photoconductor, a transfer charger for transferring an image from the photoconductor to a transfer material, and a separation charger for destaticizing the transfer material to separate the transfer material from the photoconductor after image transfer. In the image forming apparatus having, the AC voltage applied to the separation charger has a positive and negative component in a portion exceeding a discharge start voltage with respect to an AC voltage having a positive and negative peak value ratio of 1 and a duty ratio of 1. An image forming apparatus, wherein the duty ratios are set to different values so that the peak value on the same polarity side as the charging polarity of the photoconductor becomes low while making the areas equal. 2. The image forming apparatus according to claim 1, wherein the waveform of the AC voltage is a rectangular wave. 3. The image forming apparatus according to claim 1, wherein the waveform of the AC voltage is a sine wave.