JP4854071B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus provided with a sheet separating mechanism used in an image forming apparatus such as a copying machine, a printer, or a facsimile machine.

  The toner image formed on the photosensitive drum is electrostatically attracted by the electric charge applied to the sheet and transferred onto the sheet. The electric charge applied to the sheet not only attracts the toner image to the sheet but also causes an adsorption action between the sheet itself and the photosensitive drum.

Various methods have been proposed for releasing the adsorption state between the sheet and the photosensitive drum.
Patent Document 1 discloses an example of means for canceling such an adsorption state.

FIG. 8 shows an apparatus disclosed in Patent Document 1.
FIG. 8 shows a registration roller (R) composed of a pair of rollers, a photosensitive drum (D) disposed downstream of the registration roller (R), and a downstream of the photosensitive drum (D) and a pair of rollers. A fixing roller (B) is shown.
The sheet (S) is sent to the photosensitive drum (D) by the registration roller (R). As described above, the sheet (S) is attracted to the photosensitive drum (D) by the electric charge applied to the sheet (S), and is formed on the circumferential surface of the photosensitive drum (D) on the sheet (S). The transferred toner image is transferred.
A transfer roller (T) is disposed so as to press the sheet (S) on the photosensitive drum (D). A transfer bias having a polarity opposite to that of the toner image formed on the photosensitive drum (D) is applied to the peripheral surface of the transfer roller (T), and the transfer roller (T) is a toner image on the sheet (S). Encourage transcription.

FIG. 8 further shows a separation claw (N). The tip of the separation claw (N) abuts on the circumferential surface of the photosensitive drum (D), and promotes separation of the sheet (S) from the photosensitive drum (D). A separation electrode (E) is arranged to facilitate separation of the front end portion of the sheet (S) from the photosensitive drum (D). The separation electrode (E) supplies a charge having a polarity opposite to the charging bias applied to the transfer roller (T) to the sheet (S).
As a result, the adsorption force of the front end portion of the sheet (S) to the photosensitive drum (D) is reduced, and the separation of the front end portion of the sheet (S) from the photosensitive drum (D) is promoted.

The charge supplied from the separation electrode (E) has an effect of promoting separation of the sheet (S) from the photosensitive drum (D), but a part of the supplied charge travels downstream through the sheet (S). As a result, the electric charge is also supplied to the fixing roller (B) disposed in the printer.
For this reason, the fixing roller (B) is charged, and the charge on the fixing roller (B) reduces the adhesion between the sheet (S) and the toner that forms the toner image transferred on the sheet (S). It will be.
As a result, a part of the toner on the sheet (S) adheres to the fixing roller (B), and the toner attached to the fixing roller (B) moves to the next sheet (S) to be conveyed. An “offset” phenomenon will occur.

  In order to prevent such a phenomenon, in Patent Document 1, there is provided means for stopping voltage application from the separation electrode (E) immediately before the front side end of the sheet (S) is sandwiched between the fixing rollers (B). Adopted.

JP 2004-354777 A

As described above, by stopping the voltage application immediately before the front end of the sheet (S) is sandwiched by the fixing roller (B), the offset phenomenon is preferably prevented, but as a result, the sheet (S) The charge for separating the sheet (S) from the photosensitive drum (D) is not supplied to the rear end.
For this reason, the rear side end of the sheet (S) is pulled downstream while maintaining a high suction force with the photosensitive drum (D). As a result, since a high tension is generated in the sheet (S), the rear end of the sheet (S) jumps up when the rear end of the sheet (S) is separated from the photosensitive drum (D). Then, the toner image transferred on the sheet (S) is rubbed.

  The present invention has been made in view of the above circumstances, and provides an image forming apparatus capable of suitably separating a sheet and a photosensitive drum and causing no offset phenomenon. Objective.

According to the first aspect of the present invention, an image carrier that conveys a toner image and transfers the toner image to a sheet, and an image carrier that is disposed adjacent to the image carrier and has a polarity opposite to that of the image carrier. A voltage having a polarity opposite to the charging polarity of the transfer body is applied to the back surface of the sheet after the toner image is transferred between the transfer body having a charging polarity and the image carrier and the transfer body. A separation electrode that facilitates separation of the image carrier and the sheet; a fixing unit that is disposed on the downstream side of the image carrier and thermocompression-bonds the toner image on the sheet; and timing of charge supply of the separation electrode The separation electrode control means controls the separation electrode so that the front end on the side of the sheet in the transport direction is separated from the image carrier. It performs charge supply, the front end Immediately before being pinched by the fixing means, the supply of electric charge to the separation electrode is stopped and voltage application of the separation electrode is started immediately before the rear end of the sheet is separated from the image carrier. In the image forming apparatus, a separation unit that maintains a separation distance between the separation electrode and the back surface of the sheet is provided between the fixing units.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the separating means is a plate-like body that is rotatably supported.
A third aspect of the present invention is the image forming apparatus according to the second aspect, wherein the plate-like body is formed in a polygonal shape and makes point contact with the sheet when the plate-like body rotates.
According to a fourth aspect of the present invention, in the image forming apparatus according to the first aspect, the separating means is made of an insulating member.
According to a fifth aspect of the present invention, in the image forming apparatus according to the first aspect, a volume resistance value of a constituent material of the separating means is 10 ¹² Ω / cm ³ or more.
A sixth aspect of the present invention is the image forming apparatus according to the first aspect, wherein the separating means is made of an uncharged material.

A seventh aspect of the present invention is the image forming apparatus according to the first aspect, wherein the image carrier is composed of an amorphous silicon photoconductor.
The invention according to claim 8 is the image forming apparatus according to claim 1, wherein the toner constituting the toner image is a one-component developer.

A ninth aspect of the present invention is the image forming apparatus according to the first aspect, wherein the separation electrode is composed of a plurality of needle-like electrodes aligned in the sheet width direction.
The invention according to claim 10 further includes a plurality of separation claws, wherein the separation claws are aligned in the width direction of the image carrier, and a tip portion of the separation claws abuts on the image carrier. An image forming apparatus according to claim 9.
According to an eleventh aspect of the present invention, in the image forming apparatus according to the first aspect, the separation unit can change a distance between the separation electrode and the back surface of the sheet.

According to the first aspect of the present invention, since a voltage is applied to the rear edge of the sheet from the separation electrode, the adhesion force between the sheet and the image carrier can be reduced, and the phenomenon of jumping up the rear edge of the sheet can be prevented. It becomes possible. Further, the separation means can prevent excessive approach to the separation electrode at the rear end portion of the sheet, can suppress the generation of current passing through the sheet, can prevent charging of the fixing means, and can prevent the offset phenomenon.
According to the second aspect of the present invention, since the separating unit is a plate-like body that is rotatably supported, the separating unit does not resist the conveyance of the sheet, and the toner transferred onto the sheet is scattered. Can be prevented.
According to the third aspect of the present invention, since the separation means makes point contact with the sheet, it is possible to prevent the separation means from being charged.
According to the fourth and fifth aspects of the present invention, it is possible to prevent the current from the separation electrode from flowing to the separating means.
According to the sixth aspect of the present invention, it is possible to prevent the charge of the separating means from being accumulated.

According to the seventh aspect of the invention, it is possible to construct a long-life image forming apparatus.
According to the invention described in claim 8, the image quality can be improved.
According to the tenth aspect of the present invention, the sheet can be suitably separated from the image carrier.
According to the eleventh aspect, the distance between the sheet and the separation electrode can be adjusted according to the material of the sheet.

The image forming apparatus according to the present invention will be described below with reference to the drawings.
FIG. 1 shows an example of an image forming apparatus according to the present invention. In this specification, the terms “upstream side” and “downstream side” refer to the side on which the sheet (S) flows first as “upstream” on the basis of the conveyance direction of the sheet (S), and the sheet ( The side through which S) flows is referred to as “downstream”. Further, regarding the terms “front” and “rear” used for the seat (S), when the seat (S) passes through one process, the side that passes first is referred to as “front” and passes later. The side is referred to as “rear”.

The image forming apparatus shown in FIG. 1 is a copying machine, but the present invention is not limited to this, and can be applied to other types of image forming apparatuses such as facsimile machines and printers.
The copier (100) includes an image reading unit (120) disposed on the copier (100) and an image forming unit (140) that forms a toner image on a sheet such as paper.

The image reading unit (120) includes a document pressing plate (121) and a transparent document table (122) disposed below the document pressing plate (121). The document pressing plate (121) can be rotated upward about the inner edge in FIG.
In the example shown in FIG. 1, the document (D1) is placed on the upper surface of the document table (122). The document pressing plate (121) presses the document (D1) placed on the document table (122) and is brought into close contact with the upper surface of the document table (122).

The image reading unit (120) further includes a scanning unit (123). In the example shown in FIG. 1, the scanning unit (123) is movable from the left side to the right side of the copying machine (100). A light source (124) and a first reflection mirror (125) are connected to the scanning unit (123), and the light source (124) and the first reflection mirror (125) move following the movement of the scanning unit (123). .
A second reflection mirror (126) and a third reflection mirror (127) are arranged on the right side of the image reading unit (120).

  When the scanning unit (123) moves from the left to the right, the image surface of the document (D1) in close contact with the upper surface of the document table (122) is scanned by the light source (124). Then, the image of the image plane reflected on the first reflection mirror (125) is sent to the second reflection mirror (126). The second reflecting mirror (126) sends the image sent by the first reflecting mirror (125) to the third reflecting mirror (127).

The image reading unit (120) further includes a lens array (128) and a CCD (129).
The third reflecting mirror (127) further reflects the image sent from the second reflecting mirror (126) and sends it to the lens array (128). The lens array (128) forms an image sent from the third reflection mirror (127), and sends the formed image to the CCD (129).

The copying machine (100) shown in FIG. 1 can read an image using a method other than moving the scanning unit (123) to read an image.
The image reading unit (120) of the copier (100) shown in FIG. 1 is disposed above the document pressing plate (121) and has a sheet feeding table (130) on which the document (D2) is placed, A paper feed roller (131) made up of a pair of rollers for sending the document (D2) on the table (130) to the left, and a pair of rollers for delivering the document (D2) sent from the paper feed roller (131) A paper discharge roller (132), and a paper discharge tray (133) for receiving the document (D2) sent by the paper discharge roller (132).

  The scanning unit (125) has an intermediate position between the paper feed roller (131) and the paper discharge roller (132) as a stop position below the paper feed roller (131) and the paper discharge roller (132). The light source (124) emits light toward the image surface of the document (D2) sent from the paper feed roller (131) to the paper discharge roller (132), and the first reflection mirror (125) is the document (D2). ) Is sent to the second reflecting mirror (126). The second reflecting mirror (126) sends the image sent by the first reflecting mirror (125) to the third reflecting mirror (127) in the same manner as described above. The third reflecting mirror (127) further reflects the image sent from the second reflecting mirror (126) and sends it to the lens array (128). The lens array (128) forms an image sent from the third reflection mirror (127), and sends the formed image to the CCD (129).

  The image forming unit (140) includes an image processing unit (not shown). The CCD (129) sends the sent image data to the image processing unit. The image processing unit appropriately performs necessary processing such as A / D conversion, density correction, and shading correction on the transmitted image data.

The image forming unit (140) includes an exposure unit (141) and a photosensitive drum (142). The image processing unit sends the processed image data signal to the exposure unit (141). The exposure unit (141) includes a laser oscillator, a polygon mirror, and the like (all not shown).
The laser oscillator irradiates the photosensitive drum (142) with a laser beam via a polygon mirror or the like. This laser light is controlled to be turned on / off based on a signal of image data sent from the image processing unit to the exposure unit (141).

  The photosensitive drum (142) includes a conductive drum base such as aluminum and a photosensitive layer such as OPC (organic photo semiconductor) or amorphous silicon disposed so as to cover the outer periphery of the drum base. The photosensitive drum (142) is rotatably provided in the copying machine (100) main body, and is rotated at a predetermined speed by a driving device (not shown).

The image forming unit (140) includes a charger (143). The charger (143) charges the photosensitive drum (142). As the charger (143), a scorotron type corona charger is used. The charger (143) can uniformly charge the peripheral surface of the photosensitive drum (142) to a predetermined polarity and potential when a charging bias is applied by a charging bias application power source (not shown). It is.
As described above, the charge is removed from the peripheral surface portion of the photosensitive drum (142) exposed by the laser light irradiation from the laser oscillator. As a result, an electrostatic latent image is formed.

The image forming unit (140) includes a developing unit (144). The developing device (144) includes a developing container (145) that stores a developer, and a developing roller (146) that is rotatably supported by the developing container (145). A developing bias is applied to the developing roller (146) by a developing bias applying power source (not shown), and the toner contained in the developing container (145) adheres to the peripheral surface of the developing roller (146).
The circumferential surface of the developing roller (146) is in contact with the circumferential surface of the photosensitive drum (142) and rotates together with the developing roller (146). The toner adhering to the developing roller (146) transfers to the electrostatic latent image formed on the peripheral surface of the photosensitive drum (142), and a toner image is formed on the photosensitive drum (142).

The image forming unit (140) includes a sheet feeding cassette (150) that accommodates sheets (S), a sheet feeding roller (151) that sends out sheets (S) from the sheet feeding cassette (150), and a sheet feeding roller (151). ) Is provided with a conveying roller (152) for sending the sheet (S) sent further downstream, and a registration roller (153) comprising a pair of rollers.
The registration roller (153) repeats the stopping and rotating operations to adjust the position of the sheet (S) in the flow direction to the position of the toner image formed on the photosensitive drum (142).

The image forming unit (140) includes a transfer roller (154). The transfer roller (154) is disposed adjacent to the photosensitive drum (142), and rotates at a peripheral surface moving speed equal to or higher than the moving speed of the peripheral surface of the photosensitive drum (142). Thereby, a transfer process will be performed suitably.
A transfer bias having a polarity opposite to that of the toner image on the photosensitive drum 142 is applied to the transfer roller 154 by a transfer bias application power source (not shown).

  The sheet (S) whose position has been adjusted by the registration roller (153) is supplied between the photosensitive drum (142) and the transfer roller (154). When the sheet (S) is pressed between the transfer roller (154) and the photosensitive drum (142), the toner image formed on the photosensitive drum (142) is transferred onto the sheet (S).

  The image forming unit (140) includes a fixing roller (155). The fixing roller (155) includes a heat roller (156) having a heat source therein and a pressure roller (157). The sheet (S) sent from the photosensitive drum (142) is disposed between the heat roller (156) and the pressure roller (157), and is pressed by the heat roller (156) and the pressure roller (157). The As a result, the toner image transferred onto the sheet (S) is thermocompression bonded and fixed onto the sheet (S).

The image forming unit (140) includes a paper discharge roller (158) and a paper discharge tray (159) that are a pair of rollers.
The sheet (S) is sent from the fixing roller (155) to the paper discharge roller (158). Thereafter, the sheet discharge roller (158) sends the sheet (S) to the sheet discharge tray (159).

FIG. 2 is an enlarged view of the path of the sheet (S) from the registration roller (153) to the fixing roller (155).
As shown in FIG. 2, the cleaning device (200) is disposed above the photosensitive drum (142). The cleaning blade provided in the cleaning device (200) contacts the circumferential surface of the photosensitive drum (142). Thereby, after the photosensitive drum (142) transfers the toner image to the sheet (S), the toner remaining on the peripheral surface of the photosensitive drum (142) is removed by the cleaning device (200).

A separation claw (201) is disposed below the cleaning device (200). The tip of the separation claw (201) is formed at an acute angle and comes into contact with the peripheral surface of the photosensitive drum (142). Thereby, the tip of the separation claw (201) mechanically separates the sheet (S) from the photosensitive drum (142). The separation claw (201) is preferably formed of an insulating synthetic resin or ceramic. More preferably, the volume resistance value of the separation claw (201) is preferably 10 ¹² Ω / cm ³ or more.
A separation electrode (1) is disposed above the registration roller (154) disposed to face the photosensitive drum (142). The tip of the separation electrode (1) is located away from the peripheral surface of the photosensitive drum (142).

FIG. 3 shows the bias relationship of the applied voltage to the photosensitive drum (142), the transfer roller (154), and the separation electrode (1).
In this embodiment, the peripheral surface of the photosensitive drum (142) is formed of amorphous silicon. Further, it is assumed that the peripheral surface of the photosensitive drum (142) is positively charged. At this time, the toner forming the toner image on the photosensitive drum (142) is positively charged.
At this time, the transfer roller (154) is biased to cause negative charging. When the sheet (S) passes between the transfer roller (154) and the photosensitive drum (142), the toner showing positive charge is attracted by the negative charge and moves onto the sheet (S). . As a result, the toner image on the photosensitive drum (142) is transferred onto the sheet (S).

  When the sheet (S) passes between the transfer roller (154) and the photosensitive drum (142), the transfer surface side of the sheet (S) (the surface on which the toner image on the photosensitive drum (142) is transferred). A positive charge is attached to. On the other hand, negative charges adhere to the back side of the sheet (S) (the surface opposite to the transfer surface of the sheet (S)). As a result, the negative charge attached to the back side of the sheet (S) is attracted to the positive charge on the photosensitive drum (142), and the sheet (S) is attracted to the photosensitive drum (142). Become.

  Here, by applying a positive voltage from the separation electrode (1), the negative charge on the back side of the sheet (S) and the positive charge blown from the separation electrode (1) cancel each other, and the sheet (S) The adsorption to the photosensitive drum (142) is eliminated.

FIG. 4 is a diagram showing the positional relationship between the separation claw (201) and the separation electrode (1), and is a view of the separation electrode (1) as viewed from above. In FIG. 4, the sheet (S) is not shown for clarity.
The separation electrode (1) is a plate-like electrode, and the tip is sharpened like a needle. As a whole, the tip of the separation electrode (1) has a saw blade shape. The separation electrode (1) extends in the width direction of the photosensitive drum (142).
In FIG. 4, four separation claws (201) are shown on the photoreceptor drum (142). The separation claws (201) are arranged at a predetermined interval from each other. The arrangement interval of the separation claws (201) is appropriately determined according to the width of the sheet (S) conveyed by the peripheral surface of the photosensitive drum (142).

A recess (11) is formed at the tip of the separation electrode (1) corresponding to the position where the separation claw (201) is disposed at the position in the width direction of the photosensitive drum (142). By forming the recess (11), voltage application from the recess (11) is suppressed.
The arm part (21) is placed on the upper surface of the electrode holder (12) that holds the base end of the separation electrode (1), and the arm part (21) extends in the distal direction of the separation electrode (1). At the position in the width direction of the separation electrode (1), the arm portion (21) is disposed at a position corresponding to the concave portion (11).
A spacing plate piece (22) is rotatably supported at the tip of the arm portion (21). The separation plate piece (22) protrudes from the tip of the separation electrode (1) and is located closer to the photosensitive drum (142) than the tip of the separation electrode (1).
The spacing plate piece (22) is preferably formed of an insulating synthetic resin or ceramic. More preferably, the volume resistance value of the separating plate piece (22) is preferably 10 ¹² Ω / cm ³ or more. Further, it is preferably made of a non-chargeable material such as acrylonitrile butadiene styrene copolymer resin.
The shape of the spacing plate piece (22) is not particularly limited, but it is preferably formed in a polygonal shape in which convex portions and concave portions such as a star shape are repeated. Moreover, it is preferable that the tip of this convex part is formed at an acute angle. As a result, the separation plate piece (22) makes point contact with the back surface of the sheet (S), the force against the movement of the sheet (S) can be minimized, and scattering of the toner transferred to the sheet (S) can be reduced. It becomes possible to prevent.
The arm portion (21) and the separating plate piece (22) constitute a separating means (2).

Please refer to FIG. 3 again.
When the rear end of the sheet (S) passes between the photosensitive drum (142) and the transfer roller (154), a positive voltage is applied from the separation electrode (1) to the rear end of the sheet (S), The adsorption of the photosensitive drum (142) at the rear end of the sheet (S) is released. As a result, the rear end of the sheet (S) passes through the transfer roller (154), then leaves the photosensitive drum (142) and approaches the separation electrode (1).
When the sheet (S) is excessively close to the separation electrode (1), the positive charge is excessively applied to the sheet (S), and the adhesion force of the toner image transferred to the sheet (S) is more than necessary. It will weaken. As a result, in the downstream process, when the rear end of the sheet (S) passes through the fixing roller (155), the toner of the toner image located at the rear end of the sheet (S) adheres to the fixing roller (155). The toner adhering to the fixing roller (155) adheres to the next flowing sheet (S) and causes an “offset phenomenon”.

Since the spacing plate piece (22) is positioned closer to the photosensitive drum (1) than the leading end of the separation electrode (1), the trailing end of the sheet (S) exceeds the position of the leading end portion of the spacing plate piece (22) and the separation electrode. (1) Never approach the tip.
Thereby, excessive voltage application to the sheet (S) is prevented.
In addition, although the attachment form of an arm part (21) is not specifically limited, For example, an arm part (21) base end part is fixed by the cylinder distribute | arranged on the electrode holder (12), and expansion / contraction of a cylinder rod is possible. The amount of protrusion of the separation plate piece (22) from the tip of the separation electrode (1) can be increased or decreased.

FIG. 5 shows a state in which the front end portion of the sheet (S) passes between the photosensitive drum (142) and the transfer roller (154).
In this state, the separation electrode (1) applies a voltage to release the adsorbed state between the front end of the sheet (S) and the peripheral surface of the photosensitive drum (142). The front end portion of the sheet (S) is mechanically peeled off from the peripheral surface of the photosensitive drum (142) by the separation claw (201).

FIG. 6 shows a state in which the front side end of the sheet (S) has reached just before the fixing roller (155).
While the sheet (S) is being conveyed from the state of FIG. 5 to the state of FIG. 6, the separation electrode (1) continues to apply a voltage to the back surface of the sheet (S). Immediately before the front end of the sheet (S) reaches the fixing roller (155), the application of voltage from the separation electrode (1) is stopped.
In addition, the timing of stopping the voltage application may be advanced so that the voltage application from the separation electrode (1) may be performed only on the front end portion of the sheet (S). Thereafter, the sheet (S) may be peeled off from the peripheral surface of the photosensitive drum (142) only by the peeling action by the separation claw (201). As a result, the sheet (S) can be conveyed to the fixing roller (155) while maintaining a high adhesive force between the sheet (S) and the toner image transferred to the sheet (S).

FIG. 7 shows a state immediately after the rear end of the sheet (S) passes between the transfer roller (154) and the photosensitive drum (142).
In this state, a voltage is applied from the separation electrode (1) to the sheet (S). Thereby, the adsorption state between the sheet (S) and the peripheral surface of the photosensitive drum (142) is released. Thereafter, the rear end of the sheet (S) moves in the direction of the separation electrode (1), but the movement is hindered by the spacing plate piece (22).
This prevents excessive voltage application from the separation electrode (1) as described above.

  The start and stop of voltage application to the separation electrode (1) is performed by separation electrode control means (such as a microprocessor) electrically connected to the separation electrode (1). The microprocessor receives a signal of a sensor (not shown) arranged immediately before the registration roller (153) and an output signal of a controller (not shown) for controlling the driving of the registration roller (153), thereby receiving the sheet (S ) Is sent to the separation electrode (1) to start and stop the voltage application of the separation electrode (1), and the separation electrode (1) applies the voltage application based on the signal from the separation electrode control means. Start and stop.

  Table 1 shown below shows the relationship between the protruding amount of the separation plate piece (22) from the tip of the separation electrode (1) and the occurrence of the offset phenomenon.

  As shown in Table 1, it is clear that the occurrence of the offset phenomenon is reduced as the protruding amount of the separating plate piece (22) from the separation electrode (1) increases.

In the above-described embodiments, the photosensitive drum (142) is used as the image carrier, but other types of image carriers (for example, when an intermediate transfer member is used) can be appropriately employed in the present invention. is there.
Furthermore, in the above example, the transfer roller has been described as the transfer body. However, other types of transfer bodies (for example, a transfer charger system) can be appropriately employed in the present invention.
In addition, in the above-described example, the fixing roller (155) including a pair of rollers has been described as the fixing unit. However, a unit for fixing a toner image using a belt or the like can be appropriately employed in the present invention. .

  The present invention is suitably applied to an image forming apparatus such as a copying machine, a facsimile machine, or a printer.

1 is a schematic view of an image forming apparatus according to the present invention. 1 is a diagram illustrating a main part of an image forming apparatus according to the present invention. It is a figure which shows the separation mechanism of the image forming apparatus which concerns on this invention. FIG. 3 is a diagram illustrating an arrangement of separation electrodes and separation means of the image forming apparatus according to the present invention. FIG. 6 is a diagram illustrating a state immediately after a sheet front side end portion passes between a photosensitive drum and a transfer roller. FIG. 6 is a diagram illustrating a state in which a sheet front side end has reached just before a fixing roller. FIG. 6 is a diagram illustrating a state immediately after a sheet rear side end portion passes between a photosensitive drum and a transfer roller. It is a figure which shows the separation mechanism of the conventional photoconductor drum and a sheet | seat.

Explanation of symbols

S ... Sheet 1 ... Separation electrode 142 ... Photosensitive drum 154 ... ... Transfer roller 155 ... Fixing roller 2 ... Separation means 22 ... ... Separating plate 201 ...

Claims (11)

An image carrier that conveys the toner image and transfers the toner image to a sheet; a transfer member that is disposed adjacent to the image carrier and has a charging polarity opposite to the charging polarity of the image carrier; A voltage having a polarity opposite to the charging polarity of the transfer member is applied to the back surface of the sheet after passing between the image carrier and the transfer member and the toner image is transferred to the image carrier and the sheet. A separation electrode that promotes separation of the toner image, a fixing unit that is arranged on the downstream side of the image carrier and that thermocompression-bonds the toner image on the sheet, and a separation electrode control unit that controls the timing of charge supply to the separation electrode And the separation electrode control means supplies the charge to the separation electrode so that a front end on the side of the sheet passing direction is separated from the image carrier, and the front side end is held on the fixing means Immediately before the supply of electric charge to the separation electrode is stopped, the voltage application of the separation electrode is started immediately before the rear end of the sheet is separated from the image carrier, and the separation electrode and the fixing unit are The image forming apparatus includes a separation unit that maintains a separation distance between the separation electrode and the back surface of the sheet. The image forming apparatus according to claim 1, wherein the separation unit is a plate-like body that is rotatably supported. The image forming apparatus according to claim 2, wherein the plate-like body is formed in a polygonal shape and makes point contact with the sheet when the plate-like body rotates.   The image forming apparatus according to claim 1, wherein the separation unit is made of an insulating member. The image forming apparatus according to claim 1, wherein a volume resistance value of a constituent material of the separating unit is 10 ¹² Ω / cm ³ or more.   The image forming apparatus according to claim 1, wherein the separation unit is made of an uncharged material.   2. The image forming apparatus according to claim 1, wherein the image carrier is composed of an amorphous silicon photoconductor.   The image forming apparatus according to claim 1, wherein the toner constituting the toner image is a one-component developer.   The image forming apparatus according to claim 1, wherein the separation electrode includes a plurality of needle-like electrodes aligned in the sheet width direction. A plurality of separating claws;
The separation claw is aligned in the width direction of the image carrier,
The image forming apparatus according to claim 9, wherein a leading end portion of the separation claw contacts the image carrier.   The image forming apparatus according to claim 1, wherein the separation unit varies a distance between the separation electrode and the back surface of the sheet.

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