JP3318820B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus for forming an image while supplying an appropriate transfer current according to a printing rate of a formed image for each image carrier.

[0002]

2. Description of the Related Art Conventionally, there is an electrophotographic image forming apparatus. The image forming apparatus forms a latent image on an image carrier by optical writing, converts the latent image into a toner image, and transfers and fixes the toner image on paper. When the above toner image is transferred to paper, a corona discharger is used as a transfer unit so that sufficient transferability can be obtained even when the system load fluctuates due to environmental fluctuations. The method requires a relatively large current, which is not only uneconomical in cost, but also increases the amount of ozone generated by corona discharge in order to increase the processing speed, thereby polluting the working environment.
In order to remove the ozone, a processing apparatus is required, which causes a problem that the apparatus is increased in size and cost is increased. For this reason, in recent years, various small-sized image forming apparatuses using a transfer device that does not rely on corona discharge are being put into practical use.

FIGS. 9A and 9B show only an essential part of an example of such an image forming apparatus. The image forming apparatus shown in FIG. 1A includes a photosensitive drum 1, and a charging brush 2, a writing head 3, a developing device 4, a transfer brush 5, and an initialization charging brush 2 along a circumferential surface of the photosensitive drum 1. The cleaner 6 is provided.

The photoconductor drum 1 is usually formed by uniformly applying a photoconductor to the surface of a conductive metal roller, and the metal roller portion is grounded. Then, the initialization charging brush 2
Applies a negative high voltage to the photosensitive drum 1 by the high voltage power supply 2-1. By this application, the photosensitive layer on the peripheral surface of the photosensitive drum 1 is uniformly charged to a negative high potential of, for example, "-650 V (volt)". The write head 3 includes a laser light source or an LED light source, and selectively exposes the peripheral surface of the photosensitive drum 1 charged to a negative high potential according to image information. Due to this exposure, a negative low potential portion whose potential is attenuated to approximately "-70 V" is formed on the surface of the photosensitive drum 1, and the low potential portion of "-70V" and the previously charged "-650V" are substantially maintained. An electrostatic latent image composed of the high potential portion is formed.

The developing device 4 has a non-magnetic toner 4-1 inside.
And the lower opening supports the developing roller 4-2. The developing roller 4-2 is, for example, “−2” from the power supply 4-3.
50V "developing bias is applied.
The developing roller 4 is charged to a weak negative potential by friction by a regulating member (not shown), and is regulated to a constant layer thickness.
2 adheres to the surface. The toner 4-1 is applied to the developing roller 4
-2 is conveyed to the portion facing the photosensitive drum 1 while rotating. At the opposing portion between the developing roller 4-2 and the photosensitive drum 1, a potential difference of "-180 V" is formed between the low potential portion of the electrostatic latent image of "-70 V" and the developing roller 4-2. . That is, the low potential portion of the electrostatic latent image forms a positive polarity potential relative to the developing roll 4-1. Due to the electric field due to the potential difference, the non-magnetic toner 4-1 charged to the negative polarity is transferred to the low potential portion of the electrostatic latent image of the positive polarity of the photosensitive drum 1 to form a toner image. The toner image is conveyed to the opposing portion between the photosensitive drum 1 and the transfer brush 5 by the rotation of the photosensitive drum 1.

The transfer brush 5 is formed of a conductive brush-like member, and is connected to a positive constant current power supply. The transfer brush 5 forms a transfer portion that is in sliding contact with the photosensitive drum 1. The transfer brush 5 applies a positive transfer bias to the sheet 8 conveyed to the transfer section by the convey roller pair 7. The negative toner image on the photosensitive drum 1 is transferred to the sheet 8 which has become a positive potential by the application of the voltage.

The method of directly transporting the paper 8 to the transfer section by the transport roll pair 7 depends on the shape of the transport path. However, the paper 8 may have an unstable state such as vertical fluttering. It has a drawback that it easily occurs and the image formation becomes unstable due to this effect.

FIG. 1B is a diagram showing the configuration of a belt-conveying image forming unit that is being put to practical use to maintain a stable conveying posture of a sheet. Figure (b) shows the same
The same components as those shown in (a) are indicated by the same reference numerals as in FIG. In FIG. 2B, a transport belt 9 is provided in place of the transport roll pair 7 to transport the paper 8. The transport belt 9 is formed of a film-like member having a specific volume resistance and a thickness of about one hundred and several tens μm. The transport transfer belt 9 is arranged in a loop in the horizontal direction, and its upper surface slidably contacts the photosensitive drum 1 and circulates in a counterclockwise direction indicated by an arrow A in the figure. Then, the sheet 8 is electrostatically attracted to the outer peripheral surface of the upper circulating section and is conveyed to the sliding contact portion with the photosensitive drum 1. The transfer brush 5 is in pressure contact with the rear surface of the transfer belt 9 in sliding contact with the photosensitive drum 1. The transfer brush 5 applies a positive charge to the paper 8 via the transport belt 9. Also in this case, the negative toner image on the photosensitive drum 1 is transferred to the sheet 8 which has become a positive potential by this application.

By the way, between the transfer brush 5 and the photosensitive drum 1, the conveyor belt 9, the paper 8, the toner 4-1 and the photoconductor of the photosensitive drum 1 are substantially non-conductive at a normal voltage. However, the voltage applied from the transfer brush 5 has a property that the electric charge is partially conducted by the discharge. As a power source connected to the transfer brush 5, a constant current source is generally used to supply a stable amount of transfer charge to the transfer portion in order to obtain a stable image. On the other hand, the printing form of an image is various, and for example, a portion having a high printing rate (a portion having a large amount of toner transfer) and a portion having a low printing rate (a portion having a small toner transfer amount) have an indeterminate position or an indefinite period. appear.

Alternatively, if the image has a different shape, for example, the printing rate is generally low in the case of printing characters, and the printing rate is high in the case of printing graphics. In each of these image patterns, there is an appropriate current range for obtaining an electric field required for transfer in the transfer section.

FIGS. 10 (a) and 10 (b) show a model of a member configuration system of a transfer unit on the left side, respectively, for easy understanding.
The equivalent circuit is shown on the right. FIG. 7A shows a case where a black portion where the toner transfer amount is the largest is printed as an example of an extreme case of a high printing ratio. And the same figure
(b) shows an example of an extreme case of a low printing rate in which a print process is performed on a plain white portion where the toner transfer amount is the smallest.

The power supply 5-2 will be described below with reference to FIGS. 1A and 1B as a normal power supply which is not a constant current source. Same figure
As shown in the equivalent circuit of (a), between the positive electrode side of the power supply 5-2 and the ground on the photosensitive drum 1 side, the internal resistance R of the transfer brush 5 and the static The capacitance CB, the capacitance CP of the sheet 8, the capacitance CT of the toner 4-1, and the capacitance C of the photoconductor of the photosensitive drum 1.
D forms a series circuit. The current I1 due to the above-described discharge flows through this circuit. On the other hand, as shown in the equivalent circuit of FIG.
No toner 8 exists between the positive electrode side and the ground on the photosensitive drum 1 side. Therefore, the internal resistance R of the transfer brush 5, the capacitance CB of the transport belt 9, and the capacitance C of the paper 8
A series circuit is constituted by P and the capacitance CD of the photoconductor of the photosensitive drum 1. The current I2 due to the discharge also flows in this circuit. As can be seen from the equivalent circuits of FIGS. 7A and 7B, the potential difference between the parts in FIG. 7B is larger than that in FIG. Therefore, I1 <I2 because a larger current flows. In other words, in an image formed by mixing normal white portions and black portions, the printing of a portion having a high printing ratio with a high gradation and the printing of a portion having a low printing ratio with a low gradation flow to the transfer portion. The current flow is different.

FIG. 1C shows the relationship between the transfer current and the toner transfer degree in the high print ratio image and the low print ratio image for transferring the toner 4-1 obtained experimentally by the above model in the same state. FIG. 6 is a graph showing transfer efficiency characteristics. In the figure, the vertical axis represents the transfer efficiency, and the horizontal axis represents the transfer current. As shown in the figure, the appropriate current value at high printing ratio is +2 μm.
A to +4 μA, and the appropriate current value at a low printing rate is +
3 μA or more.

However, as described above, the power supply 5-2 is
Actually, it is constituted by the constant current source 5-1. Therefore, if the transfer current value of the constant current source 5-1 is adjusted to the appropriate range of the high printing rate image (for example, +2 μA of the median value), it is impossible to cope with the appropriate range of the low printing rate image of +3 μA or more. On the other hand, if the transfer current value is adjusted to an appropriate range of the low printing ratio image (for example, +4 μA), printing of the high gradation portion of the high printing ratio image can hardly be performed.

Therefore, conventionally, as a method for simultaneously satisfying the cases where the printing rates are different, one of the methods is to irradiate an appropriate amount of light to the surface of the photosensitive drum before transfer to lower the potential of the non-image portion. The difference between the non-image portion potential and the image portion potential is reduced. Two methods have been adopted in which the resistance of the transfer brush is made high enough to absorb the change in the potential difference between the high printing rate image and the low printing rate image.

[0016]

However, as a method for simultaneously satisfying the cases where the respective printing rates are different, a method in which an appropriate amount of light is applied to the surface of the photosensitive drum before transfer in order to reduce the potential of the non-image portion. Will not only increase the product price by installing a new exposure equipment,
There has been a problem that the size of the apparatus has become large and it is not possible to meet the recent demand for downsizing OA equipment. In addition, the method of increasing the resistance of the transfer brush requires a high-voltage power supply in proportion to the increase in the resistance, which also raises the product price, and also requires the use of materials to cope with the high-voltage power supply. There is a problem that the degree of freedom of the design to be selected is restricted.

An object of the present invention is to provide an image forming apparatus which controls the current supplied from a constant current source to a transfer unit to an appropriate value regardless of the printing ratio.

[0018]

The construction of the image forming apparatus according to the present invention will be described below. First, a first aspect of the present invention, a plurality of image bearing members arranged in parallel, a plurality of toner image forming means for respectively forming a toner image on said plurality of image bearing member, said plurality of toner image forming means husband and the transfer material conveying means for conveying to contacting the transfer material to the plurality of image bearing members formed toner images, the transfer material conveyed by the transfer material conveying means, to the plurality of image bearing members by s Press
A plurality of contact type transferring device for transferring the toner image on the transfer material, and a constant current supply means for supplying the respective current to the plurality of contact transfer unit, the current output of the constant current supply means and assumptions and a plurality of transfer current control means for grounding connected to the apparatus body via a resistance element having a predetermined resistance value is branched from between the connection circuit between the plurality of contact transfer unit, the configuration of Ru with a
The plurality of transfer current control means are provided in the transfer material transport direction.
As going from upstream to downstream, the resistance value of each resistance element becomes
It is arranged to be high .

Further, for example, as set forth in claim 2, the transfer material transport means is provided with a transport belt having a resistance value of 10 ¹¹ Ωcm to 10 ¹³ Ωcm which adsorbs the transfer material on the outer peripheral surface and circulates. is, the plurality of contact transfer unit is respectively arranged on the inner side of the conveyor belt, configured to respectively pressed by the plurality of image bearing members via the conveying belt.

Next, the invention according to claim 3 is arranged side by side.
A plurality of image carriers, and a toner image on the plurality of image carriers;
A plurality of toner image forming means for individually forming and a transfer material on the outer peripheral surface
Adsorbs the toner and circulates to the toner image forming means.
The above-described transfer to the plurality of image carriers on which a toner image has been formed.
A transport belt for transporting the copying material to contact the transport belt;
Is held at a transferable position so that the transfer material is always brought into contact with the image carrier at the end of the plurality of image carriers in the side-by-side direction in the transfer material transport direction and in the same orientation. transfer and a belt moving means for selectively moving into the transfer disabling position which is separated from the image bearing member so as to the non-contact transfer material and the transfer-enabled position to be contacted with the transfer material to the other image bearing member Material conveying means and the conveying belt
Arranged inside and conveyed by the transfer material conveying means
The transfer material is transferred to the plurality of image bearing members via the transport belt.
The toner images are transferred to the transfer material by pressing them respectively onto the holding members.
A plurality of contact transfer devices, and a plurality of contact transfer devices, respectively.
Constant current supply means for supplying current, and the constant current supply means
Connection between the current output unit of the means and the plurality of contact type transfer units.
Via a resistive element that branches off from the road and has a predetermined resistance value
A plurality of transfer current control means connected to the apparatus main body by grounding;
The image carrier at the end in the juxtaposition direction
The corresponding transfer current control means is provided between the connection circuit and the transfer circuit.
Further comprising means for varying a resistance value between the device body and
Be composed.

[0021] Then, for example, as in claim 4, wherein the toner image formation is toner image forming means corresponding to the image bearing member of the juxtaposed direction end portion to form a black toner image, corresponding to the other image bearing member The means is configured to form a chromatic toner image .

The image carrier comprises, for example, a photosensitive drum, and the toner image forming means comprises, for example, an initialization charger, a writing head, a developing device, and the like. Or a transfer belt, the contact-type transfer device includes, for example, a transfer roll or a transfer brush, the constant current supply unit includes, for example, a constant current source, and the transfer current control unit includes, for example, a predetermined current source. , A switch, a branch circuit, a ground circuit, and the like.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 makes the invention easier to understand.
FIG. 2 is a diagram schematically illustrating a configuration of an image forming unit in order to perform the operation . As shown in FIG. 1, the image forming unit 10 includes a photosensitive drum 11, and an initialization charging brush 12, a writing head 13, and a developing device 1 along a circumferential surface of the photosensitive drum 11.
4, a transfer brush 15 and a cleaner 16.

The photosensitive drum 11 is made of, for example, an organic light guide such as OPC, which uniformly covers the surface of the metal roller, or an inorganic photoconductor such as Se or a-Si.
The metal roller part is grounded.

The initialization charging brush 12 includes a high-voltage power supply 12-
1 applies a negative high voltage to the photosensitive drum 11. By this application, the photosensitive layer on the peripheral surface of the photosensitive drum 11 is uniformly charged to a negative high potential of about "-650 V".

The writing head 13 comprises a laser head or an LED head, and selectively exposes the peripheral surface of the photosensitive drum 11 charged to a negative high potential in accordance with image information. By this exposure, a negative low potential portion attenuated to approximately “-50 V” is formed, and an electrostatic latent image is formed by the low potential portion of “−50 V” and the high potential portion of “−650 V”.

The developing device 14 has a non-magnetic toner 14 inside.
-1 is accommodated, and the developing roller 14-2 is supported at the lower opening. The developing roller 14-2 is supplied with a developing bias of about “−250 V” from the power supply 14-3,
-1 is charged to a weak negative potential due to friction by a regulating member (not shown), is regulated to a constant layer thickness, and is
4-2. The toner 14-1 is conveyed to a portion facing the photosensitive drum 11 while the developing roller 14-2 rotates. Developing roller 14-2 and photosensitive drum 11
A potential difference of “−200 V” is formed between the low potential portion of “−50 V” of the electrostatic latent image and the developing roller 14-2, and the low potential portion of the electrostatic latent image is A potential having a positive polarity is formed relatively to 14-1. The nonmagnetic toner 14-1 charged to the negative polarity is transferred to the low potential portion of the positive electrostatic latent image of the photoconductor drum 11 by the electric field due to the potential difference to form (develop) a toner image. The developed toner image is conveyed to the opposing portion between the photosensitive drum 11 and the transfer brush 15 by the rotation of the photosensitive drum 11.

The photosensitive drum 11 and the transfer brush 15
A transport belt 17 is stretched on the opposite portion of the transport belt 17. Transport
The belt 17 is made of a fluororesin (tetrafluoroethylene
Polymer (ETFE)) with carbon black
By anti-adjustment, 10 ¹¹-10¹⁴Ωcm volume solid
Approximately 150μm thick film with resistance
It is composed of wood. The transport belt 17 is biased in the horizontal direction.
The driving roller 18 and the driven roller are arranged in a flat loop.
19 in the counterclockwise direction indicated by arrow B in the figure.
Circular moves. The transport belt 17 is a driven roller 19
And an auxiliary roller 21 pressed against the roller via the transport belt 17.
To pinch the paper 22 fed from a paper feeding unit (not shown).
The paper 22 on the belt surface, and
It is transported so as to contact the optical drum 11.

The transfer brush 15 is used to transfer the transfer belt 1
7 is arranged so as to sandwich the transfer belt 17 at a position in contact with the inside of the upper circulation portion and facing the photosensitive drum 11 to form a transfer portion. The transfer brush 15
It is constructed by forming a woven fabric woven in piles using conductive rayon, nylon, acrylic or the like as a fiber material and attaching it to a metal or plastic substrate.
The transfer brush 15 is connected to a transfer power supply 15-1 that outputs a constant current of positive polarity, and a resistor 15-2 having a predetermined resistance value branched from this connection portion is grounded to the apparatus main body. I have. The transfer brush 15 applies a positive charge to the transport belt 17, and the charge is applied to the sheet 22 by the dielectric effect of the transport belt 17. Due to the electric field generated by the electric charges, the negative toner image on the photosensitive drum 11 is transferred to the sheet 22 in contact with the photosensitive drum 11.

The sheet 22 to which the toner image has been transferred is subsequently conveyed to the left in the figure by the conveyor belt 17, where the toner image is heat-fixed in a fixing unit (not shown), and discharged outside the machine.
In the transfer of the toner image, as shown in FIG. 10C, the transfer current is small in the image portion having a high printing ratio, and large in the image portion having a low printing ratio. In the present embodiment, the current value of the constant current source is set to correspond to image formation with a low printing rate. Then, when forming an image with a high printing rate, an excess current flows from the branch circuit to the ground circuit side via the resistance element 15-2 by an amount corresponding to the increase in the potential difference.

FIG. 2A shows a model of a member configuration system of the transfer section, and FIG. 2B shows an equivalent circuit thereof. Same figure
As shown in (b), the constant current source 15-1 outputs a constant current IT at a voltage V. The output portion of the constant current source 15-1 branches, and one branch path includes the internal resistance R of the transfer brush 15, the capacitance CB of the transport belt 17, and the capacitance CP of the paper 22.
, The electrostatic capacity CT of the toner 14-1 and the electrostatic capacity CD of the photoconductor of the photosensitive drum 11 are connected to the main body of the apparatus via a series circuit, and a transfer current I3 flows through this circuit. The other branch is the resistance R of the resistance element 15-2.
A grounded connection to the device body via E
The discharge current I4) flows. And IT = I
3 + I4.

The low printing rate image portion of the photosensitive drum 11 is present at the transfer portion shown in FIG. 3A, and the electrostatic circuit of the toner 14-1 is represented by the equivalent circuit of FIG. The capacitance CT is almost "0". At this time, the transfer current I3 is large and the charge elimination current I4 is small. Then, as shown by the arrow C in FIG. 3A, the rotation conveyance in the clockwise direction by the photosensitive drum 11 proceeds, and the high printing in which the toner 14-1 is transferred to the low potential portion of the electrostatic latent image. When the ratio image portion is conveyed to the transfer portion, the capacitance CT of the toner 14-1 shown by the equivalent circuit in FIG. At this time, much of the transfer current I3 is difficult to flow, and accordingly, the charge elimination current I4 flows correspondingly to IT = I3 + I4.

FIG. 3A shows a document image (text printing) portion A as an example of a low printing ratio image portion and a black solid image portion B often used for illustrations as an example of a high printing ratio image portion. 4 shows the relationship between the printing rate and the charge elimination current I4 during image transfer. As shown in FIG. 7A, in the image portion A having a low printing rate, the potential difference of the circuit becomes large, so that a large amount of the transfer current I3 flows. next,
In the image portion B having a high printing rate, the potential difference of the circuit becomes small,
The transfer current I3 is small, and the charge removal current I4 increases accordingly. When the image portion A has a low printing rate again, the transfer current I3 increases, and the charge removal current I4 decreases. As described above, the transfer current based on the output of the constant current source increases or decreases depending on the printing rate of image formation. The appropriate value of the transfer current I3 can be determined by the current value of the constant current source 15-1 and the resistance value of the resistance element 15-2.

FIG. 3B is a table showing combinations of the current value of the constant current source 15-1 and the resistance value of the resistance element 15-2 for the appropriate transfer current I3 obtained experimentally. . FIG. 6B shows two resistance values of 400 MΩ and 500 MΩ of the resistance element 15-2 in the left end frame. In the next column, the set current value of the constant current IT of the constant current source 15-1 is set to three values of 4 μA, 6 μA, and 7 μA for the above-described resistance value of 400 MΩ, and 5 μA and 6 μA for the resistance value of 500 MΩ. ,
And 7 μA. And the next 2
In the column frame, the current value of the transfer current I3 at the time of forming an image with a high printing rate corresponding to the above-mentioned "two kinds of resistance values" x "three kinds of constant current values" = "six kinds of setting environments". 6 μA,
2.9 μA and 3.8 μA and 2.6 μA, 3.5 μA
A and 4.2 μA and the evaluation of the results “△, ○,
○ ”and“ ○, △, Δ ”. Further, in the next two columns of frames, the current value of the transfer current I3 at the time of image formation with a low printing rate corresponding to the above six setting environments is 2.3 μA,
3.7 μA and 4.3 μA and 3.3 μA, 4.1 μA
A and 4.9 μA and the evaluation of the results “×, ○,
○ ”and“ ○, 、, ○ ”.

The transport belt 17 is, as described above,
Although it is configured to have a volume resistivity of 0 ¹¹ to 10 ¹⁴ Ω · cm, if the resistance is higher than this, as is apparent from the circuit diagram of FIG. Must be increased, which is not practical. Further, if the resistance is low, the degree of the influence of the manufacturing variation of the transport belt 17 increases, which is not practical.

As described above, based on the optimum volume resistivity of the transport belt 17, the resistance value RE of the resistance element 15-2 is set to 4
When the value of the constant current IT of the constant current source 15-1 corresponding to this is set to 4 .mu.A, the setting of FIG.
As shown in the experimental values in the chart of (b), good results were not obtained at both high and low print ratios (evaluation was not "O"). When the value of the constant current IT is 6 .mu.A or 7 .mu.A, good results can be obtained both at a high printing rate and at a low printing rate. That is, a stable printing result with good gradation is obtained.

The resistance value RE of the resistance element 15-2 is set to 5
When the value of the constant current IT of the constant current source 15-1 set to 00Ω is set to 5 μA or 6 μA, in this case, a good result is obtained in both the high printing rate and the low printing rate. can get. That is, a stable printing result with good gradation is obtained. However, when the value of the constant current IT is 7 .mu.A, good results are obtained at a low printing rate but good results are not obtained at a high printing rate.

From these facts, when the resistance value RE of the resistance element 15-2 is set to 400Ω, the constant current source 15-
1 is preferably set to 6 μA or 7 μA, and the resistance value RE of the resistance element 15-2 is set to 500Ω.
It can be seen that the value of the constant current IT of the constant current source 15-1 should be set to 5 μA or 6 μA.

Further, from these facts, the appropriate transfer current I3 at the time of a high printing rate is 2.9 μA, 3.
It turns out to be 8 μA, 2.6 μA and 3.5 μA, ie approximately in the range of 2.6 μA to 3.8 μA.
Similarly, when the printing rate is low, the appropriate transfer current I3 is 3.7 μA, 4.3 μA, 3.3 μA and 4.3 μA.
It turns out to be 1 μA, ie approximately in the range of 3.3 μA to 4.3 μA. Therefore, the appropriate range of the transfer current I3 corresponding to both the high printing rate and the low printing rate is as follows.
It can be seen that the range is from 2.6 μA to 4.3 μA.

As described above, in order to transfer a high quality image,
A relatively wide range of current change of 2.6 μA to 4.3 μA is required. In the present embodiment, this current change is determined by providing an appropriate resistor in parallel on the output side of the constant current source. Has been realized. That is, the transfer reference current IT can be maintained by the constant current source irrespective of environmental changes, and the transfer current I3 can be appropriately increased or decreased according to the printing rate.

In the above description , an image forming apparatus provided with one photosensitive drum 11 and forming a black toner image has been described. However, the image forming apparatus is limited to an image forming apparatus dedicated to black printing (black printing). Do not mean. The present invention is suitable for a full-color image forming apparatus having a multi-stage photosensitive drum.
It can be applied to

FIG. 4 shows a full-color image forming apparatus of the present invention.
Shows in side sectional view a. The full-color image forming apparatus 25 shown in the figure has an opening / closing tray 26 on the front surface (right side in the figure) of the apparatus main body, and a paper cassette 27 detachably provided on the lower part. A paper discharge tray 29 is formed on an upper surface of the upper cover 28, and sheets on which an image has been formed and discharged from the upper paper discharge port 31 are stacked. Its upper lid 2
A power switch, a liquid crystal display, a plurality of input keys, etc., which are not visible in the drawing, are provided on the front side of the front panel 8.

In the main body of the apparatus, a transport belt 34 circulating and held by a driving rotary roller 32 and a driven rotary roller 33 is provided substantially at the center, and a multi-stage in the paper transport direction by contacting the upper circulating portion of the transport belt 34. The four photosensitive drums 35 (35a, 35b, 35c, 35d) arranged side by side in a formula and the contact portions between the transport belt 34 and the respective photosensitive drums 35 are pressed against the rear surface of the transport belt 34 and arranged. And an image transfer unit including the transfer brush 36 (36a, 36b, 36c, 36d).

The photosensitive drum 35 is mounted on an image forming unit 37 (37a, 37b, 37c, 37d), and the upper cover 2 is placed directly above the photosensitive drum 35.
8 write head 38 (38a, 3
8b, 38c, 38d) are inserted and arranged by closing the upper lid 28. The upper cover 28 opens and closes around a support shaft 39 at the rear of the apparatus main body 25.

A pair of standby rolls 41 and a paper detection sensor 42 are disposed on the upstream side (right side in the drawing) of the belt 34 in the paper transport direction. A feed roller 43, a separating member 44, and the above-described opening / closing tray 26 are provided in the lateral direction. In addition, a transport path 45 formed from two guide plates is provided below,
Upstream thereof, the above-mentioned paper cassette 27 stores a large number of sheets P
The house is located. A paper feed roller 46 is provided above the paper feed end of the paper cassette 27. And belt 3
4, a separation claw 47, a fixing device 48,
A discharge roller 49 and a switching lever 51 are provided. The fixing device 48 includes a pressure contact roller, a heat generating roller, a peripheral surface cleaner, an oil application roller, a thermistor, and the like assembled in a heat insulating housing, and thermally fixes the toner image transferred onto the paper to the paper surface. When the switch lever 51 is at the lower position as shown in FIG.
When the paper is rotated upward, the paper is guided to a paper discharge port 53 opened on the rear surface of the apparatus. The downstream side of the discharge path 52 communicates with the upper discharge port 31 via a discharge roll pair 54.

Further, a cleaner bottle 55 is detachably disposed between the belt 34 and the paper cassette 27. this,
A blade scraper 56 is provided above the cleaner bottle 55.
Is attached, and its tip is in contact with the surface of the lower circulation portion of the transfer belt 34. Blade scraper 56
Removes the toner remaining on the surface of the transfer belt 34 to clean the transfer belt 34, and stores the scraped unnecessary toner in the cleaner bottle 55.

FIG. 5A shows the image forming unit 37 described above.
FIG. 2B is an enlarged perspective view of the image transfer unit together with a side sectional view of the internal structure.
Note that the image forming unit 37 (37a to 37a) shown in FIG.
d) has the same configuration except that the color of the toner contained therein is different. As shown in FIGS. 5 (a) and 5 (b),
The image forming unit 37 includes a photosensitive drum 35 and a cleaner 5 that surrounds the photosensitive drum 35 along a peripheral surface thereof.
7, an initializing charger 58 and a developing device 59. Developing device 5
Reference numeral 9 also serves as a toner container. The lower portion of the opening 9 rotatably holds the developing roller 61, and contains a toner 62 therein. The developer 59 of each image forming unit 37 shown in FIG. 4 contains Y (yellow), M (magenta), C (cyan) or Bk (black) toner, respectively.

A toner stirring member 63 is disposed below the inside of the developing device 59. The toner stirring member 63 rotates as shown by a two-dot chain line in FIG. The toner 62 is fed to the toner 64. The supply roller 64 is made of a sponge member,
The toner 62 is pressed against the developing roller 61 and is supplied to the peripheral surface of the developing roller 61 so as to rub the toner 62 sent from the stirring member 63. A blade spring 65 in the form of a leaf spring is in contact with the circumferential surface of the developing roller 61 in the direction of rotation. Reduce to thickness.

The various devices incorporated in the image forming unit 37 have gears or electrodes (not shown), and each gear is internally engaged. When the image forming unit 37 is mounted on the apparatus main body 25, the photosensitive drum 3
5 and the support shaft 61-1 of the developing roller 61
And each electrode is driven by engaging with the drive mechanism of the apparatus main body 25 and the power supply electrode.

The writing head 38 shown in FIG. 4 disposed on the upper cover 28 between the initialization charger 58 and the developing roller 61 is closed as shown by an arrow D in FIG. Is drawn down in an arc and fixed at the image forming position. When the upper head 28 is opened and the write head 38 is pulled up, the image forming units 37 can be individually taken out of the machine. At this time, the protective cover 66 slides counterclockwise to cover and protect the exposed lower surface of the photosensitive drum 35.

Then, the conveyor belt 3 is
The transfer brush 36 that is pressed in opposition to the transfer brush 4 via
The output of the constant current source 67 is connected, and the output is branched and connected to the device main body 25 via the resistance element 68 (68a, 68b, 68c, 68d). Also in this case, each transfer unit has the equivalent circuit shown in FIG. 2B. Then, even if the printing ratio changes and the capacity of the transfer portion changes, the current flowing to the resistance element 68 changes accordingly, so that good image transferability can be obtained.

In the full-color image forming apparatus 25 shown in FIG. 4 having such a configuration, the power is turned on and the number of sheets,
When full-color printing (image formation) and other designations are input by a key, the paper feed roller 46 transfers the paper P stored in the paper cassette 27 to the standby roll pair 4 via the transport path 45.
Feed to 1. Alternatively, the paper feed roller 43 is
Is fed to the standby roll pair 41. The fed sheet P is detected by the sheet detection sensor 42. The standby roll pair 41 stops rotating, and makes the leading end of the paper P abut on the holding portion to make it stand by.

The drive rotary roller 32 starts rotating in the counterclockwise direction, and the driven rotary roller 33 is driven to start rotating in the same counterclockwise direction. Thereby, the belt 34
The upper circulating portion abuts on the four photosensitive drums 35 and circulates counterclockwise as a whole.

At the same time, the image forming unit 37 is sequentially driven in accordance with the printing timing, and the photosensitive drum 35 is sequentially rotated clockwise in accordance with the drive. Then, the write heads 38 corresponding to the respective image forming units 37 are sequentially driven. First, an initialization charger (brush charger) 58 (see FIG. 5) applies a uniform charge to the peripheral surface of the photosensitive drum 35, and the writing head 38 applies an image to the peripheral surface of the photosensitive drum. Exposure is performed according to a signal to form an electrostatic latent image on the peripheral surface of the photosensitive drum.
The developing roller 61 transfers (transfers) the toner 62 to a low potential portion of the electrostatic latent image to form (develop) a toner image on the peripheral surface of the photosensitive drum.

At the timing when the leading end of the toner image on the peripheral surface of the photosensitive drum 35a at the most upstream position is rotated and conveyed to the point facing the belt 34, the printing start position of the paper P coincides with the point. Then, the standby roll pair 41 starts rotating, and feeds the sheet P to the sheet entrance of the image forming unit. The driven rotation roller 33 and the pressing roller 69 convey the sheet P while nipping the fed sheet P together with the conveyance belt 34. Paper P
Even after the pinching by the driven rotary roller 33 and the pressing roller 69 is released, the toner is attracted to the transport belt 34 and is transported as it is, and the current applied between the photosensitive drums 35 and the transfer brushes 36 causes the toner to remain on the paper. The toner images are sequentially transferred.

The paper P on which the toner images of four colors of Y (yellow), M (magenta), C (cyan) and Bk (black) are transferred is separated from the conveyor belt 34 by a separation claw 47, and a fixing device 48 It is carried in. After the toner image is thermally fixed on the sheet P by the fixing unit 48, the toner image is turned up from the rear sheet outlet 53 or the toner image is turned down from the upper sheet outlet 31 by the sheet discharging roller 49. Emitted outside the machine.

Incidentally, even if the transport belt 34 is a semiconductor belt or a dielectric belt as described above, first, the uppermost stream Y
(Yellow) transfer portion is applied with charge, then M (magenta) transfer portion is applied with charge, and further C (cyan) transfer portion and Bk (black) transfer portion are sequentially charged. Therefore, the charge amount of the transport belt 34 gradually increases toward the downstream. Therefore, the transfer current I3 from the transfer brush 56 is repelled and hardly flows. For this reason, in the above embodiment, the resistance elements 68a, 68b, 68c and 6
The resistance value of 8d is configured to increase sequentially with reference to 400Ω or 500Ω. This makes it more difficult for the static elimination current I4 to flow from upstream to downstream,
Accordingly, the transfer current I3 increases in reverse from the upstream to the downstream, and can maintain a more appropriate potential beyond the potential due to the charging of the conveyor belt 34. As a result, the appropriate transfer current in each transfer section can be maintained. I3 flows into the equivalent circuit.

Next, another embodiment of the multi-stage full-color image forming apparatus will be described. Described below full-color image forming apparatus has two printing modes printing Furka La over printing and black only.

FIG. 6 shows the structure of the main part.
The state of each transfer unit in the full color print mode is shown. FIG. 7 shows a state change of each transfer portion in a black-only print mode in the configuration of the main portion. In both figures, the same components as those in FIGS. 4 and 5 are denoted by the same reference numerals as those in FIGS. 4 and 5.

Referring to FIG. 6, in the full-color image forming apparatus, the tension roller 71 is disposed near the driving rotary roller 32 in contact with the inner peripheral side of the lower circulation portion of the conveyor belt 34. The tension roller 71 is rotatably supported at the tip of a rotatable arm (not shown), and is urged downward by an urging member (not shown) to convey the conveyance belt 34 regardless of a state change of the conveyance belt 34 described later. The rotation of the driving rotary roller 32 with a predetermined tension is performed by absorbing the deflection of the belt
And the driven rotation roller 33.

A swing arm 72 is disposed between the upper circulation part and the lower circulation part of the conveyor belt 34 and extends in parallel with the conveyance direction. The swing arm 72 has a fixed support roller 73 at one end and a movable support roller 74 at the other end. The swing arm 72 is fixedly supported by a fixed roller 73 by a cam (not shown) which slides in contact with the lower portion.
The other end side rotates up and down about the support shaft of.

The fixed support roller 73 is provided on the photosensitive drum 35.
d and a transfer brush 36d, and a transfer unit which is provided near the intermediate downstream of a transfer unit immediately upstream of the photosensitive drum 35c and the transfer brush 36c, is rotatably fixed to a frame of the apparatus main body, and is conveyed. The lowermost photosensitive drum 3 contacts the inner peripheral surface of the belt 34 and supports it from below.
The conveyor belt 34 is always in contact with 5d.

The movable support roller 74 is supported by the rotating free end of the swing arm 72, and has an end of the transport belt 34 supported by the driven rotary roller 33, the most upstream photosensitive drum 35 a and the transfer brush It is located at an intermediate position with the transfer portion 36a. When the swing arm 72 is rotating upward, the movable support roller 74 contacts the inner peripheral surface of the transport belt 34 to support the transport belt 34 from below, as shown in FIG. Each photoconductor drum 35a, 35b,
The transport belt 34 is brought into contact with all of 35c and 35d.

The lowermost transfer brush 36 d is fixed to the frame of the apparatus main body and is in contact with the transport belt 34.
On the other hand, the other transfer brushes 36a to 36c
2, when the swing arm 72 is rotating upward, as shown in FIG.
As a result, transfer portions are formed corresponding to the four photosensitive drums 35 from the upstream to the downstream, and a full-color print mode of four colors is set.

A belt support member 75 is disposed on the two transfer brushes 36b and 36c located at the middle of the four transfer brushes. Since the transport belt 34 is long in the front and rear direction, no matter how the deflection is absorbed by the tension roller 71 as described above, the central portion of the upper circulating portion bends and an appropriate contact state with the photosensitive drum 35 is obtained. May not be obtained. This bending is transferred to the transfer brushes 36b and 3
The support by 6c hastens the deterioration of the transfer brushes 36b and 36c having weak waist. The above-described belt support member 75 is
The conveyance belt 34 is slidably contacted with the conveyance belt 34 to support the conveyance belt 34 from below, and the conveyance belt 34 maintains appropriate contact with the photosensitive drum 35.

As in the case of FIG. 4, the transfer brush 36 has a constant current source 67 and a resistance element 68 for static elimination in parallel with the constant current source 67.
(68a, 68b, 68c, 68d) are provided. However, in this embodiment, the static elimination circuit connected to the transfer brush 36d of the lowermost transfer portion for forming a black toner image includes another resistance element 76 for further adjustment in parallel with the resistance element 68d. Are connected via a switch 76. The switch 77 is open when the full-color printing mode shown in FIG. 6 is set. Therefore, the transfer current of each transfer section is the same as that of the transfer current I3 described in FIGS. 4 and 5B. To work.

Next, the monochrome print mode for printing only black will be described. In the monochrome printing mode, the swing arm 72 rotates downward as shown in FIG. As a result, the movable support roller 74 moves downward and separates from the transport belt 34. The transport belt 34 is supported between the driving rotary roller 32 and the fixed support roller 73 as before, and maintains a state of contact with the photosensitive drum 35d.
On the other hand, between the fixed support roller 73 and the driven rotation roller 33,
The movable support roller 74 is inclined obliquely downward toward the upstream side due to the separation of the movable support roller 74, and is therefore separated from the three photosensitive drums 35 a, 35 b, and 35 c that form the color toner image together with the transfer brush 36. I have.

By the rotation of the swing arm, a monochrome printing mode in which only a black toner image is formed by the transfer unit including the photosensitive drum 35d and the transfer brush 36d at the most downstream is set. At this time, the switch 77 connected to the transfer brush 36d is closed, and the adjusting resistance element 76 is connected to the static elimination circuit. In the monochrome printing mode, there is only one transfer portion, and the transfer belt 34 is not charged in another transfer portion. Therefore, it is not necessary to increase the transfer current as described above. Therefore, in order to alleviate the strong resistance of the static elimination resistor 68d, static elimination is further performed by the adjusting resistance element 76 having an appropriately weaker resistance value, and the transfer current flowing through the transfer brush 36d is adjusted to an appropriate current. It is controlled to become.

In the above-described embodiments, all the embodiments using the transport belt have been described.
The configuration of the image forming unit is not limited to this.
As schematically shown in (a) and (b), a method in which a sheet is directly conveyed to a transfer unit by a pair of standby rolls may be used.
FIG. 7A shows a configuration in which a guide plate 78 for guiding paper and a pair of standby rolls 79 for transporting paper are arranged in place of the transport belt 17 described in FIG. b)
Indicates a state in which the sheet P has been transported to the transfer section. All other components are the same as those described with reference to FIG. 1, and are not particularly indicated by numbers. Also in this case, the configurations of the constant current source and the static elimination circuit are the same as those of the previous embodiment, and the equivalent circuit thereof is a configuration in which the capacitance CB of the transport belt is removed from the equivalent circuit of FIG. . Therefore, it may be smaller than the resistance value of the resistance element used.

[0070]

As described above, according to the present invention,
A constant current source that supplies the transfer current to the transfer unit is provided in parallel with a resistance element for static elimination, and the transfer current is adjusted according to the level of printing rate, so that the constant current source that maintains the reference current regardless of environmental changes The output can be fluctuated appropriately in accordance with the printing ratio, and therefore, even if the image has a large change in the printing ratio, it is possible to perform a proper transfer of any gradation portion and print a high-quality image. Become. In the present invention, since increasing the resistance of the resistor toward the downstream from the upstream in the case of transfer printing unit of multi-stage, a proper transfer current by correcting the charging of the conveyor belt in any of the transfer unit Thus, it is possible to print a high-quality color image by appropriate transfer for each transfer unit.

[Brief description of the drawings]

1 is a diagram schematically showing a configuration of an image forming portion which is a main component of the images forming apparatus.

FIG. 2A is a diagram illustrating a model of a member configuration system of a transfer unit of an image forming unit, and FIG. 2B is a diagram illustrating an equivalent circuit thereof.

3A is a diagram showing a relationship between a printing ratio and a static elimination current at the time of image transfer of a low printing ratio image portion A and a high printing ratio image portion B, and FIG. 5 is a chart showing a combination of a current value of a constant current source for a current and a resistance value of a resistance element.

FIG. 4 is a side sectional view of the full-color image forming apparatus of the present invention.

5A is an external perspective view of an image forming unit, and FIG. 5B is an enlarged view showing a configuration of an image transfer unit together with a side sectional view of an internal configuration thereof.

FIG. 6 is a diagram illustrating a state of each transfer unit in a full-color print mode of the color image forming apparatus.

FIG. 7 is a diagram illustrating a state change of each transfer unit in a monochrome print mode of the color image forming apparatus.

FIG. 8 is a diagram illustrating a configuration of an image forming unit according to another embodiment, in which a guide plate that guides a sheet and a pair of standby rolls that convey the sheet are arranged instead of a conveyance belt. FIG. 3B is a diagram illustrating a configuration, and FIG. 3B is a diagram illustrating a state in which a sheet is conveyed to the transfer unit.

9A and 9B are diagrams illustrating a configuration of an image forming unit of a conventional image forming apparatus, wherein FIG. 9A illustrates a configuration of a direct transport type, and FIG. 9B illustrates a configuration of a belt transport type.

FIGS. 10A and 10B are diagrams showing a model of a member configuration system of a transfer unit and an equivalent circuit thereof, and FIG. 10C is a transfer efficiency characteristic diagram for high and low print ratio images.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 photoconductor drum 2 initialization charging brush 2-1 high-voltage power supply 3 writing head 4 developing device 4-1 nonmagnetic toner 4-2 developing roller 4-3 power supply 5 transfer brush 5-1 constant current source 5-2 power supply 6 cleaner 7 Conveying Roll Pair 8 Paper 9 Conveying Belt 10 Image Forming Unit 11 Photosensitive Drum 12 Initializing Charging Brush 12-1 High Voltage Power Supply 13 Writing Head 14 Developing Device 14-1 Toner 14-2 Developing Roller 14-3 Power Supply 15 Transfer Brush 15 -1 Transfer power supply 15-2 Resistance element 16 Cleaner 17 Conveyor belt 18 Drive roller 19 Follower roller 21 Auxiliary roller 22 Paper 25 Full-color image forming device 26 Opening / closing tray 27 Paper cassette 28 Top lid 29 Discharge tray 31 Upper discharge port 32 Drive Rotating roller 33 driven rotating roller 34 transport belt 35 (35a, 35 , 35c, 35d) Photoreceptor drum 35-1 Support shaft 36 (36a, 36b, 36c, 36d) Transfer brush 37 (37a, 37b, 37c, 37d) Image forming unit 38 (38a, 38b, 38c, 38d) Write head 39 support shaft 41 standby roll pair 42 detection sensor 43 paper feed roller 44 separating member 45 transport path 46 paper feed roller 47 separation claw 48 fixing device 49 paper discharge roller 51 switching lever 52 discharge path 53 paper discharge port 54 paper discharge roll pair 55 Cleaner bottle 56 Blade scraper 57 Cleaner 58 Initializing charger 59 Developing device 61 Developing roller 61-1 Support shaft 62 Toner 63 Toner stirring member 64 Supply roller 65 Doctor blade 66 Protective lid 67 Constant current source 68 (68a, 68b, 68c, 68d) Resistance element for static elimination 69 Presser foot La 71 tension roller 72 swing arm 73 fixed support Russia one la 74 movable supporting rollers 75 the belt support member 76 adjusting resistor element 77 switch 78 guide plate 79 stand roll pair

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-20731 (JP, A) JP-A-7-5773 (JP, A) JP-A-6-102776 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G03G 15/16 G03G 15/01 114

Claims (4)

(57) [Claims] 1. A juxtaposed a plurality of image bearing member, and a plurality of bets <br/> toner image forming means for respectively forming a toner image on said plurality of image bearing member, said plurality of toner image forming means Transfer material transporting means for transporting the transfer material to contact the plurality of image carriers on which the toner images are formed, and transferring the transfer material transported by the transfer material transport means to the plurality of image carriers.
A plurality of contact type transferring device for transferring the toner image on the transfer material to respectively push the plurality of image bearing members, and a constant current supply means for supplying the respective current to the plurality of contact transfer unit, the A plurality of transfer current control means which branch from a connection circuit between the current output unit of the constant current supply means and the plurality of contact type transfer devices and are grounded to the apparatus main body via a resistance element having a predetermined resistance value; wherein the plurality of transfer current control means, upstream of the transfer material conveying direction
The resistance value of each resistance element increases as going downstream from
Is arranged such that the image forming apparatus according to claim Rukoto. 2. The transfer material transfer means includes a transfer belt having a resistance value of 10 ¹¹ Ωcm to 10 ¹³ Ωcm, which circulates the transfer material by adsorbing the transfer material on the outer peripheral surface, and wherein the plurality of contact transfer devices are
Said conveyor belt being inside in the respective arrangement of the image forming apparatus according to claim 1, wherein the respectively pressed against the plurality of image bearing members via the conveying belt. 3. A plurality of image carriers arranged side by side and a plurality of toner carriers each forming a toner image on the plurality of image carriers.
A toner image forming means, a transfer material is adsorbed on an outer peripheral surface and circulates,
The plurality of images on which toner images have been formed by image forming means;
A transport bell that transports the transfer material in contact with a carrier
And the transport belt is moved downstream in the transfer material transport direction and
For the image carriers at the ends in the direction of juxtaposition in the plurality of image carriers,
Transfer is possible to keep the transfer material in the same posture at all times
Position and contact the transfer material with other image carriers
The transferable position to be transferred and the transfer material
And selectively move it to a non-transferable position
A transfer material transporting means provided with a belt moving means, and the transfer material transporting means disposed inside the transport belt and
The transfer material conveyed by the
And pressing the plurality of image carriers respectively onto the transfer material.
A plurality of contact type transfer units for transferring a toner image, and a constant current supply for supplying a current to each of the plurality of contact type transfer units
A flow supply means, said plurality of contact transfer the current output section of the constant current supply means
Resistor with a predetermined resistance value that branches off from the connection circuit with the heater
Multiple transfer current controls that are grounded to the device via the device
Comprising a control means, wherein the transfer current control means corresponding to the image bearing member of the arrangement direction end portion further comprise a means for varying the resistance value between the connection circuit between the previous SL device body An image forming apparatus comprising: 4. A toner image forming means corresponding to the image carrier at the end in the juxtaposition direction forms a black toner image, and a toner image forming means corresponding to another image carrier forms a chromatic color toner image. 4. The image forming apparatus according to claim 3, wherein: