The image formation unit 208 transfers a toner image formed on a photosensitive member to a sheet. The image formation unit 208 serving as an image printing unit includes a developing device, a transfer unit, and a toner supply unit. The developing device uses toner charged to negative polarity. Four developing devices corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K) are provided. The developing device visualizes an electrostatic latent image on a photosensitive member using toner of a corresponding color. If an amount of toner in the developing device becomes insufficient due to the formation of toner images, toner is supplied by the toner supply unit to the developing device.

The transfer unit includes an intermediate transfer belt 118, and transfers a toner image to the intermediate transfer belt 118 from each photosensitive member. A primary transfer roller is provided at a position facing a photosensitive member across the intermediate transfer belt 118. By positive potential being applied to the primary transfer rollers, toner images are transferred to the intermediate transfer belt 118 in a superimposed manner from the four photosensitive members. A full-color toner image is thereby formed on the intermediate transfer belt 118. The full-color toner image formed on the intermediate transfer belt 118 is transferred from the intermediate transfer belt 118 to a sheet P by bias of the same polarity as toner being applied from a secondary transfer inner roller 119a in the intermediate transfer belt 118.

The fixing unit 209 fixes the transferred toner image to the sheet. The fixing unit 209 includes a heater and a roller pair. The fixing unit 209 melts and fixes the toner image to the sheet by heating and applying pressure to the toner image on the sheet using the heater and the roller pair. An image is thereby formed on the sheet. The sheet feeding unit 210 includes a conveyance roller and various sensors on a conveyance path, and controls a feeding operation of sheets. The conveyance unit 211 (conveyance control unit) controls sheet conveyance in the image forming apparatus 101. The conveyance unit 211 conveys a sheet fed by the sheet feeding unit 210 to a transfer unit in synchronization with a transfer timing of a toner image, and conveys the sheet bearing the transferred toner image to the fixing unit 209. The sheet to which the toner image is fixed by the fixing unit 209 is conveyed to a conveyance path selected from a conveyance path 125 and a reversing conveyance path 126 in accordance with information of input print data after passing through the fixing unit 209.

Based on an instruction of the CPU 203, the on-belt image reading unit 212 reads an image formed on the intermediate transfer belt 118 of the transfer unit. In a case where the adjustment of an image formation condition is performed, for example, the CPU 203 reads an image for adjustment of an image formation condition that is formed on the intermediate transfer belt 118, using the on-belt image reading unit 212. The operation unit 205 is an input device that receives the entry of various settings and operation instructions from the user. The operation unit 205 includes, for example, various entry keys and a touch panel. The display 206 serving as a display unit is an output device that displays setting information of the image forming apparatus 101 and a processing status of a print job (status information).

Next, a configuration of the charge removing apparatus 103 will be described. The charge removing apparatus 103 includes a communication I/F 221, a contact charge removing control unit 222, and a non-contact charge removing control unit 223. These components are connected via a system bus 225.

The communication I/F 221 is connected to the printing apparatus 102 via a communication cable 229, and controls communication between the printing apparatus 102 and the charge removing apparatus 103. The communication I/F 221 is connected also with the finisher 104 via a communication cable 239, and controls communication between the charge removing apparatus 103 and the finisher 104.

The contact charge removing control unit 222 and the non-contact charge removing control unit 223 perform various types of control based on control instructions issued by the CPU 203 and received via the communication cable 229. The contact charge removing control unit 222 includes a charge removing control switching unit 251 and a charge removing voltage adjustment unit 252, and controls charge removing voltage of a contact charge removing unit 129 to be described below. The charge removing control switching unit 251 acquires the state of a mode lever 141 illustrated in FIG. 3, and switches ON/OFF of the contact charge removing unit 129 based on the state of the mode lever 141. The charge removing voltage adjustment unit 252 acquires a setting value set in a dial 142 to be described below, and controls the magnitude (voltage value, high voltage value) of voltage to be applied to the contact charge removing unit 129, based on the setting value and the polarity of voltage to be applied to a charge removing roller 130b that is determined by a charge removing CPU 224. In the present exemplary embodiment, a setting value set in the dial 142 is an absolute value. The non-contact charge removing control unit 223 controls charge removing to be executed by a non-contact charge removing unit 131 to be described below. In the present exemplary embodiment, the charge removing apparatus 103 operates based on a control instruction issued by the CPU 203 included in the image forming apparatus 101, but an operation of the charge removing apparatus 103 is not limited to this, and a control instruction may be issued by a CPU included in the charge removing apparatus 103, for example. A drive unit 226 controls the driving of a motor for conveying sheets that is provided in the charge removing apparatus 103.

Next, a configuration of the finisher 104 will be described. The finisher 104 includes a communication I/F 231, a CPU 232, a memory 233, and a sheet discharge control unit 234. These components are connected via a system bus 235.

The communication I/F 231 is connected with the charge removing apparatus 103 via the communication cable 239, and controls communication between the charge removing apparatus 103 and the finisher 104. In accordance with control programs stored in the memory 233, the CPU 232 performs various types of control necessary for sheet discharge. The memory 233 is a storage device storing control programs. Based on an instruction from the CPU 232, the sheet discharge control unit 234 performs control of conveying a conveyed sheet to the discharge tray 137.

FIG. 4 is a cross-sectional view of the image forming apparatus 101. The display 206 is provided above a casing of the printing apparatus 102. The display 206 displays a printing status of the image forming apparatus 101 and information for settings.

The printing apparatus 102 includes sheet feeding decks 111 and 112, conveyance paths 113, 122, 124, and 125, the reversing conveyance path 126, a duplex printing conveyance path 127, and various rollers. Sheets of different types can be stored in the sheet feeding decks 111 and 112 serving as the sheet feeding unit 210. From the sheets stored in the sheet feeding decks 111 and 112, the uppermost one sheet is separated and fed to the conveyance path 113. The printing apparatus 102 includes, as the laser exposure unit 207, image forming units 114, 115, 116, and 117 for forming images, and forms a color image on a sheet. The image forming unit 114 forms a black (K) image (toner image). The image forming unit 115 forms a cyan (C) image. The image forming unit 116 forms a magenta (M) image. The image forming unit 117 forms a yellow (Y) image.

The printing apparatus 102 includes, as the image formation unit 208, the intermediate transfer belt 118 to which toner images are transferred from the image forming units 114, 115, 116, and 117, and a secondary transfer roller 119.

The intermediate transfer belt 118 rotates clockwise in FIG. 4, and toner images are transferred (primarily transferred) to the intermediate transfer belt 118 in a superimposed manner from the image forming unit 117, the image forming unit 116, the image forming unit 115, and the image forming unit 114 in this order. A full-color toner image is thereby formed on the intermediate transfer belt 118. By rotating, the intermediate transfer belt 118 conveys the toner image to the secondary transfer roller 119. In synchronization with a timing at which the toner image is conveyed to the secondary transfer roller 119, the conveyance unit 211 conveys a sheet into the conveyance path 113, and feeds the sheet to a secondary transfer portion 214. To transfer (secondarily transfer) the toner image on the intermediate transfer belt 118 to a conveyed sheet, high voltage with the same polarity as the polarity of toner is applied to the secondary transfer inner roller 119a. The toner on the intermediate transfer belt 118 repels from the intermediate transfer belt 118, and is transferred onto the sheet in a transfer nip. In the present exemplary embodiment, because toner has negative polarity, negative voltage is applied to the secondary transfer inner roller 119a. On the other hand, a secondary transfer outer roller 119b is electrically grounded. Alternatively, bias of reverse polarity to the polarity of toner may be applied to the secondary transfer outer roller 119b, and the secondary transfer inner roller 119a may be electrically grounded.

An on-belt image reading sensor 120 serving as the on-belt image reading unit 212 is provided near the intermediate transfer belt 118. The on-belt image reading sensor 120 is positioned on the downstream side of the image forming units 114, 115, 116, and 117 in a rotational direction of the intermediate transfer belt 118. The on-belt image reading sensor 120 reads an image transferred to the intermediate transfer belt 118 from the image forming units 114, 115, 116, and 117. The on-belt image reading sensor 120 is an optical sensor, for example, and reads an image on the intermediate transfer belt 118 by emitting light to the image and receiving reflected light. For example, the on-belt image reading sensor 120 reads an image for adjustment formed on the intermediate transfer belt 118 for adjusting an image formation condition. The CPU 203 analyzes a reading result of the image for adjustment that is obtained by the on-belt image reading sensor 120, and performs calibration by feeding the reading result back to the image formation condition.

The printing apparatus 102 includes a first fixing device 121 and a second fixing device 123 serving as the fixing unit 209. The first fixing device 121 and the second fixing device 123 have the same configuration, and fix a toner image to a sheet. To fix a toner image to a sheet, the first fixing device 121 and the second fixing device 123 each include a pressure roller and a heating roller. The sheet is heated and pressed by being passed through between the pressure roller and the heating roller, and the toner image is melt and fixed on the sheet with pressure. A sheet having passed through the second fixing device 123 is conveyed to the conveyance path 124. The second fixing device 123 is arranged on the downstream side of the first fixing device 121 in a sheet conveyance direction, and is used to add gloss to an image on a sheet that has been subjected to fixing processing in the first fixing device 121, and to ensure fixability. For this reason, the second fixing device 123 is not used in some cases depending on the type of a sheet or the type of a print job. The conveyance path 122 is provided to convey a sheet having been subjected to the fixing processing in the first fixing device 121, without passing through the second fixing device 123.

After the conveyance path 124 and the conveyance path 122 join together, the conveyance path 125 and the reversing conveyance path 126 are provided. In a case where an instruction is issued to execute duplex printing or to stack sheets on the discharge tray 137 with an image formation surface being set to a bottom surface (hereinafter, will be referred to as face-down sheet discharge), a sheet is conveyed to the reversing conveyance path 126. In a case where a duplex printing instruction is issued, the conveyance direction of the sheet conveyed to the reversing conveyance path 126 is reversed on the reversing conveyance path 126, and the sheet is conveyed to the duplex printing conveyance path 127. By the reversing conveyance path 126 and the duplex printing conveyance path 127, the surface (first surface) of the sheet on which an image is formed is reversed. The sheet is conveyed again to the conveyance path 113 by the duplex printing conveyance path 127, and by being passed through the secondary transfer portion 214 again, an image is formed on a second surface. In the case of the face-down sheet discharge, by switching back a sheet drawn to the reversing conveyance path 126, and conveying the sheet to a conveyance path 160, it becomes possible to discharge the sheet with an image formation surface oriented downward. In the case of simplex printing or in a case where images are formed on both sides in duplex printing, the sheet is conveyed to the charge removing apparatus 103 via the conveyance path 125 and the conveyance path 160. In the case of a simplex print job, an image is formed only on the first surface of the sheet, and in the case of a duplex print job, images are formed on both surfaces of the first surface and the second surface of the sheet. Such sheet conveyance control is performed by the conveyance unit 211 (conveyance control unit).

As described above in <Configuration of Image Forming Apparatus>, in the secondary transfer portion 214, at a transfer nip portion formed by the secondary transfer roller 119 and the intermediate transfer belt 118, a toner image on the intermediate transfer belt 118 is transferred (secondarily transferred) to a sheet. As illustrated in FIG. 5, a configuration in the present exemplary embodiment is a configuration of transferring a toner image by the secondary transfer inner roller 119a in the intermediate transfer belt 118 applying high voltage from a secondary transfer high voltage substrate 138. Thus, it is necessary to apply polarity in a direction in which the toner image is detached from the intermediate transfer belt 118, and high voltage with the same polarity as the polarity of toner is applied. In the present exemplary embodiment, because the polarity of toner has a property of being negatively charged, high voltage with negative polarity is applied to the secondary transfer inner roller 119a. The top surface of the sheet having passed through the secondary transfer portion 214 is negatively charged, and by dielectric polarization, the bottom surface of the sheet is positively charged.

The charge removing apparatus 103 includes a conveyance path 128, the contact charge removing unit 129, the non-contact charge removing unit 131, and a plurality of conveyance rollers 256. As described above, according to the present exemplary embodiment, after a sheet passes through the secondary transfer portion 214, the top surface of the sheet is negatively charged, and the bottom surface of the sheet is positively charged by dielectric polarization. For this reason, as illustrated in FIG. 6, if sheets are stacked on the discharge tray 137 without the execution of charge removing processing, polarities of contact surfaces of stacked sheets become opposite polarities, and the sheets might stick to each other by electrostatic force. The charge removing apparatus 103 in the present exemplary embodiment removes charges on sheet surfaces by the contact charge removing unit 129 and the non-contact charge removing unit 131 to prevent sheets from sticking to each other by electrostatic force. The non-contact charge removing unit 131 (non-contact charge removing unit) provided downstream of the contact charge removing unit 129 removes charges of a conveyed sheet in a state of not being in contact with the sheet (non-contact state). On a top surface 103a that constitutes an apparatus top surface on the exterior of the charge removing apparatus 103, a charge removing operation unit 132 for an operator to turn ON/OFF the contact charge removing unit 129 and making settings related to a voltage value (charge removing voltage) is provided. A sheet conveyed from the printing apparatus 102 to the charge removing apparatus 103 passes through the conveyance path 128, and is subjected to charge removing executed by the contact charge removing unit 129 and the non-contact charge removing unit 131. Then, the sheet having been subjected to charge removing processing is conveyed to the finisher 104.

The finisher 104 stacks sheets delivered from the printing apparatus 102. The finisher 104 includes a conveyance path 135 and the discharge tray 137 on which sheets are to be stacked. On the conveyance path 135, conveyance sensors 133, 134, and 136 are provided. A sheet conveyed from the printing apparatus 102 is discharged to the discharge tray 137 via the conveyance path 135. The conveyance sensors 133, 134, and 136 detect the passage of a sheet conveyed on the conveyance path 135. In a case where a leading edge or a trailing edge in a sheet conveyance direction is not detected by the conveyance sensors 133, 134, and 136 even when a predetermined time elapses since sheet conveyance has started, the CPU 232 determines that a conveyance jam (conveyance abnormality) has occurred in the finisher 104. In this case, the CPU 232 notifies the printing apparatus 102 that a conveyance jam has occurred.

FIG. 7 is a cross-sectional view illustrating a configuration of the contact charge removing unit 129. The contact charge removing unit 129 includes the charge removing counter roller 130a and a charge removing roller 130b that serve as a charge removing roller pair that comes into contact with the sheet P. The charge removing roller 130b includes an elastic layer of ion conductive foamed rubber and core metal, has an outside diameter of 20 to 25 mm and a resistance value of 1×10⁵to 1×10⁸Ω in a case where measurement is conducted by applying a voltage of 2 kV in environmental measurement at 23° C. and 50% RH, and is a member similar to the secondary transfer inner roller 119a. The charge removing counter roller 130a is made of stainless steel (SUS), and is electrically grounded (connected to the ground). By using a roller with an outside diameter of 20 to 25 mm, and arranging the roller in such a manner as to face the charge removing roller 130b, a charge removing nip is formed. The charge removing counter roller 130a rotates by receiving driving from a charge removing drive motor (not illustrated), and conveys a sheet nipped by the charge removing nip. Furthermore, a charge removing high voltage substrate 230 is provided in the contact charge removing unit 129. Specifically, the charge removing high voltage substrate 230 applies voltage to the charge removing roller 130b serving as a charge removing member. In the present exemplary embodiment, the charge removing high voltage substrate 230 is configured to be able to apply voltages with both polarities of positive polarity and negative polarity in such a manner that their absolute values becomes approximately the same level. In the present exemplary embodiment, the charge removing roller 130b is arranged on the bottom surface side of the sheet, but the charge removing roller 130b and the charge removing counter roller 130a may be in reverse arrangement. In the present exemplary embodiment, the charge removing high voltage substrate 230 can apply voltages with both polarities of positive polarity and negative polarity by one substrate, but two high voltage substrates, one for positive polarity and one for negative polarity may be included. The polarity of voltage to be applied by the charge removing high voltage substrate 230 to the charge removing member is determined in accordance with the polarity of the charge on the surface of the sheet conveyed to the contact charge removing unit 129. In the present exemplary embodiment, a first charge removing roller is the charge removing roller 130b, and a second charge removing roller is the charge removing counter roller 130a.

The contact charge removing unit 129 according to the present exemplary embodiment has a high charge removing effect because the contact charge removing unit 129 directly applies voltage by being in a contact with the sheet P. On the other hand, the contact charge removing unit 129 has such characteristics that a variation in surface potential of charge-removed sheets P is large, and charge removing tends to become non-uniform. In view of the foregoing, in the charge removing apparatus 103 according to the present exemplary embodiment, the non-contact charge removing unit 131 is provided on the downstream side of the contact charge removing unit 129 in the conveyance direction.

FIG. 8 is a cross-sectional view illustrating a configuration of the non-contact charge removing unit 131. FIG. 9 is a plan view of a conveyance guide 260. The non-contact charge removing unit 131 according to the present exemplary embodiment can uniformize the surface potentials of the sheets P that have been made non-uniform by the above-described charge removing processing executed by the contact charge removing unit 129. The non-contact charge removing unit 131 includes an ionizer 240 and the conveyance guide 260. Bar type ionizers IZS40 (SMC Corporation) are arranged above and below the sheet P to form an ionizer irradiation portion. As the conveyance guide 260 arranged at the ionizer irradiation portion, an insulating resin obtained by synthesizing polycarbonate (PC) and acrylonitrile butadiene styrene (ABS) is employed. A volume resistivity of the conveyance guide 260 according to the present exemplary embodiment is 1×10¹⁴Ω*cm. Furthermore, as illustrated in FIG. 9, to prevent ions generated from the ion irradiation portion from being physically blocked by a sheet conveyance member, holes are provided in the conveyance guide 260 arranged on the top surface side and the bottom surface side of the sheet. Alternating current (AC) bias is applied to the ionizer 240, and by corona discharge, ions of both of positive polarity and negative polarity are alternately emitted. For this reason, it is possible to remove remaining charges irrespective of the direction of the polarity of the remaining charges in the contact charge removing unit 129. A charge removing effect of the sheet P that is obtained by the non-contact charge removing unit 131 according to the present exemplary embodiment is smaller than the charge removing effect obtained by the contact charge removing unit 129, but a variation in surface potential of the sheets P that is caused after the charge removing processing is small. For this reason, the non-contact charge removing unit 131 can uniformize the surface potentials of the sheets P that have been made non-uniform by the contact charge removing unit 129. In the present exemplary embodiment, an ionizer is employed as the non-contact charge removing unit 131, but the non-contact charge removing unit 131 is not limited to this. For example, an AC Colotron method of applying high voltage to a wire may be used as the non-contact charge removing unit 131.

In the present exemplary embodiment, in the non-contact charge removing unit 131, ionizers are arranged on the top and bottom surface sides of the sheet, but the arrangement is not limited to this. For example, the non-contact charge removing unit 131 may be arranged only on one side of the top side or the bottom side of the sheet. The high voltage to be applied may be direct-current (DC) voltage instead of AC voltage.

As illustrated in FIG. 3, the charge removing apparatus 103 is provided with the charge removing operation unit 132 for setting an operation of the contact charge removing unit 129. The charge removing operation unit 132 is arranged on the top surface 103a (apparatus top surface) on the exterior of the charge removing apparatus 103. The charge removing operation unit 132 includes the mode lever 141 and the dial 142. The mode lever 141 is a selector switch for manually switching “ON” and “OFF” (enabled and disabled) of voltage application to the charge removing roller 130b by the charge removing high voltage substrate 230. The state of the mode lever 141 is acquired by the charge removing control switching unit 251. The charge removing control switching unit 251 switches ON/OFF of the contact charge removing unit 129 based on the state of the mode lever 141. The non-contact charge removing unit 131 is always in an ON state irrespective of the settings of the mode lever 141 and the dial 142. Even when the mode lever 141 is in an OFF state, a sheet is conveyed. The dial 142 includes a button 142a and a display unit 142b, and by the button 142a being pressed by an operator, a number displayed on the display unit 142b switches. A setting value set in the dial 142 is acquired by the charge removing voltage adjustment unit 252. In the present exemplary embodiment, a value obtained by multiplying a value displayed on the dial 142 by 0.1 kV becomes an absolute value of voltage to be applied to the contact charge removing unit 129. Nevertheless, instead of displaying upper two digits of an absolute value of voltage to be applied, an absolute value itself of voltage to be applied may be displayed, or a numerical value indicating the level of voltage to be applied, on a scale of one to ten, for example, may be displayed.

As described above, by the secondary transfer portion 214 according to the present exemplary embodiment, the top surface (image formation surface) of the sheet has charges with the negative polarity and the bottom surface (rear surface) of the sheet has charges with the positive polarity. The user selects any of face-up sheet discharge of simplex printing, face-down sheet discharge of simplex printing, and duplex printing sheet discharge as illustrated in FIG. 19 using the client PC 106 or the operation unit 205. The face-up sheet discharge of simplex printing and the face-down sheet discharge of simplex printing refer to discharge methods of sheets in a simplex print job. Based on the designation of printing information that is made by the user, the sheet P is conveyed in such a manner that a selected sheet discharge method is used, and discharged to the discharge tray 137. In the case of face-up sheet discharge of simplex printing, the sheet P is discharged to the discharge tray 137 in such a manner that a simplex-printed image formation surface becomes a top surface. In the case of face-down sheet discharge of simplex printing, the sheet P is discharged to the discharge tray 137 in such a manner that the image formation surface becomes a bottom surface. In the case of duplex printing, the sheet P is discharged to the discharge tray 137 in such a manner that a second surface on the rear side of a first surface becomes a top surface. The first surface of the sheet in a duplex print job refers to a surface on which an image is first formed out of both surfaces of the sheet, and the second surface refers to a surface on which an image is formed secondly out of both surfaces of the sheet. As illustrated in FIG. 4, in a case where the reversing conveyance path 126 is provided on the upstream side of the charge removing apparatus 103 in the conveyance direction, the polarity of charged surface of the sheet conveyed to the charge removing apparatus 103 varies depending on the setting of simplex printing/duplex printing or a front/rear setting of the sheet in sheet discharge. For this reason, the charge removing high voltage substrate 230 of the charge removing apparatus 103 switches polarity to appropriate polarity, and applies voltage to the charge removing roller 130b.

FIG. 10A is a diagram illustrating face-up sheet discharge of simplex printing, and FIG. 10B is a diagram illustrating face-down sheet discharge of simplex printing. As illustrated in FIG. 10, when face-up sheet discharge of simplex printing is designated, because a sheet is not reversed, the sheet is conveyed to the charge removing apparatus 103 with the top surface of the sheet being charged to negative polarity and the bottom surface of the sheet being charged to positive polarity. That is, the sheet is conveyed to the charge removing apparatus 103 while maintaining the polarities of the top and bottom surfaces of the sheet charged in the secondary transfer portion 214. When charges are removed from the sheet in this state, the charge removing roller 130b arranged on the bottom surface side of the sheet needs to apply negative charge to the sheet. For this reason, the charge removing high voltage substrate 230 applies negative voltage to the charge removing roller 130b.

On the other hand, when face-down sheet discharge of simplex printing is designated, the sheet is conveyed to the charge removing apparatus 103 via the reversing conveyance path 126. At this time, because the front and rear sides of the sheet are reversed by the reversing conveyance path 126, an image formation surface becomes a bottom side and a non-image formation surface becomes a top side. That is, because the front and rear sides of the sheet are reversed, the sheet is conveyed to the charge removing apparatus 103 in a state in which the top surface side of the sheet is positively charged and the bottom surface side of the sheet is negatively charged. For this reason, the charge removing roller 130b arranged on the bottom surface side of the sheet needs to apply positive charges to the sheet. For this reason, the charge removing high voltage substrate 230 applies positive voltage to the charge removing roller 130b.

FIG. 18 is a diagram illustrating duplex printing sheet discharge. As illustrated in FIG. 18, in the case of duplex printing, first of all, an image is formed on a first surface of a sheet by the secondary transfer portion 214. After that, the conveyance direction of the sheet is reversed on the reversing conveyance path 126, and the sheet is conveyed to the duplex printing conveyance path 127. At this time, because the front and rear sides of the sheet is reversed by the reversing conveyance path 126, when an image is formed on a second surface of the sheet, the sheet is conveyed to the secondary transfer portion 214 in a state in which the top surface of the sheet is positively charged, and the bottom surface of the sheet is negatively charged. At this time, by scraping against a grounded conveyance guide while the sheet is conveyed by the reversing conveyance path 126 and the duplex printing conveyance path 127, a charge amount of the sheet attenuates while the sheet is conveyed. Because a high-resistance sheet has smaller attenuation in a charge amount while the sheet is conveyed via the reversing conveyance path 126 and the duplex printing conveyance path 127, as compared with a low-resistance sheet, the charge of the sheet remains even at the time of transferring to the second surface. The magnitude of transfer current (current flowing from a secondary transfer roller to the sheet S) for obtaining good transfer performance is basically the same between a simplex printing timing, a secondary transfer timing of the first surface in duplex printing, and a secondary transfer timing of the second surface. Part of negative charges supplied to the surface of the sheet from the secondary transfer inner roller 119a at a transfer timing of the second surface in duplex printing are consumed by compensating for positive charges on the sheet surface that are generated at a secondary transfer timing of the first surface and remaining up to a transfer timing of the second surface. Consequently, a charge amount of the sheet immediately after secondary transfer to the second surface in duplex printing becomes smaller than a charge amount of the sheet immediately after secondary transfer in simplex printing.

That is, a charge amount obtained when a duplex-printed sheet is conveyed to the charge removing apparatus 103 becomes smaller than a charge amount obtained when a simplex-printed sheet is conveyed to the charge removing apparatus 103. That is, in the case of duplex printing, the sheet is conveyed to the charge removing apparatus 103 in a state in which the top surface of the sheet is negatively charged and the bottom surface of the sheet is positively charged. In this case, the charge removing roller 130b arranged on the bottom surface side of the sheet needs to apply a negative charge to the sheet. For this reason, the charge removing high voltage substrate 230 applies negative voltage to the charge removing roller 130b. Because a charge amount obtained when a duplex-printed sheet is conveyed to the charge removing apparatus 103 is smaller than a charge amount obtained when a simplex-printed sheet is conveyed to the charge removing apparatus 103, an absolute value of a charge removing voltage in duplex printing has a smaller value than an absolute value of a charge removing voltage value in simplex printing.

In the present exemplary embodiment, the charge removing high voltage substrate 230 can apply voltage by selecting either polarity of the both polarities. For this reason, at the time of face-up sheet discharge of simplex printing and at the time of duplex printing, the charge removing roller 130b can apply negative charges to the sheet, and at the time of face-down sheet discharge of simplex printing, the charge removing roller 130b can apply positive charges to the sheet. In other words, in the case of face-up sheet discharge of simplex printing and duplex printing, the charge removing CPU 224 determines negative polarity (first polarity) as the polarity of voltage to be applied by the charge removing high voltage substrate 230 to the charge removing roller 130b, and in the case of face-down sheet discharge of simplex printing, the charge removing CPU 224 determines positive polarity (second polarity) as the polarity of voltage to be applied by the charge removing high voltage substrate 230 to the charge removing roller 130b.

That is, in the case of face-up sheet discharge of simplex printing and duplex printing, it is sufficient that voltage is applied to the charge removing roller 130b in such a manner that the direction of an electric field becomes a direction from the charge removing counter roller 130a (second charge removing roller) to the charge removing roller 130b (first charge removing roller). Furthermore, at the time of face-down sheet discharge of simplex printing, it is sufficient that voltage is applied to the charge removing roller 130b in such a manner that the direction of an electric field becomes a direction from the charge removing roller 130b (first charge removing roller) to the charge removing counter roller 130a (second charge removing roller). At this time, because it is sufficient that voltage is applied in such a manner that the direction of an electric field becomes the above-described direction of the electric field, the charge removing high voltage substrate 230 needs not be able to apply voltages both polarities, and voltage of only either polarity may be applied. For example, negative high voltage power sources may be connected to the charge removing roller 130b and the charge removing counter roller 130a, and the connection with the negative high voltage power source may be switched depending on a sheet discharge method. Specifically, in a case where the direction of an electric field is set to a direction from the charge removing counter roller 130a to the charge removing roller 130b, the charge removing roller 130b is applied with negative voltage, and in a case where the direction of an electric field is set to a direction from the charge removing roller 130b to the charge removing counter roller 130a, the charge removing counter roller 130a is applied with negative voltage. Nevertheless, in the present exemplary embodiment, whether to remove charges of a sheet passing through the charge removing apparatus 103 is determined based on the above-described state of the mode lever 141, and the magnitude (absolute value) of voltage to be applied when the charges are removed is set using the dial 142. In the present exemplary embodiment, first simplex printing sheet discharge is face-up sheet discharge of simplex printing, and second simplex printing sheet discharge is face-down sheet discharge of simplex printing.

FIG. 11 is a block diagram illustrating an operation according to the present exemplary embodiment. Print data is input directly from the PC 106 connected to the outside of the printing apparatus 102, or via the external controller 105 (not illustrated in FIG. 11). As the print data, print data stored in an HDD in the printing apparatus 102 may be designated by the user operating the operation unit 205. The CPU 203 in the printing apparatus 102 that has received the print data determines a necessary image formation condition and a conveyance condition from the print data. At this time, the CPU 203 determines sheet discharge surface information of the sheet from information input from the operation unit 205 or the PC 106. The CPU 203 delivers sheet information, image information, and sheet discharge surface information designated in the print data, to the charge removing CPU 224. Based on the received sheet discharge surface information, the charge removing CPU 224 determines the polarity of voltage to be applied to a charge removing member to remove charges of the sheet. Whether to actually apply charge removing high voltage, and the magnitude of the charge removing high voltage are determined based on the settings of the charge removing control switching unit 251 and the charge removing voltage adjustment unit 252 connected to the contact charge removing control unit 222. Specifically, the charge removing voltage adjustment unit 252 acquires a setting value (absolute value) set in the dial 142, and the polarity determined by the charge removing CPU 224. Then, the charge removing voltage adjustment unit 252 sets the magnitude (voltage value, high voltage value) of voltage that is based on the acquired absolute value and the polarity, in the charge removing high voltage substrate 230. The charge removing high voltage substrate 230 applies voltage to the charge removing roller 130b with the set magnitude of voltage. The charge removing CPU 224 also outputs charge removing setting information to the non-contact charge removing control unit 223, and the non-contact charge removing control unit 223 controls voltage to drive the ionizer 240. The charge removing CPU 224 also outputs a drive command to a charge removing motor driver 253, and a driver drives various motors to convey sheets. By the driving of a charge removing roller motor 254, the charge removing counter roller 130a rotates, and by the driving of a conveyance roller motor 255, the plurality of conveyance rollers 256 rotates, whereby a sheet is conveyed.

FIG. 12 is a flowchart illustrating an operation according to the present exemplary embodiment. First of all, in step S001, the user transmits print data, and printing is started. In step S002, print data designated by the user is acquired, and the charge removing CPU 224 obtains designation information of a sheet discharge surface. In step S003, whether printing is simplex printing or duplex printing is checked. In a case where printing is simplex printing (YES in step S003), the processing proceeds to step S004. In step S004, it is determined whether sheet discharge surface designation of simplex printing is face-down sheet discharge. In a case where discharge surface designation is face-down sheet discharge of simplex printing (YES in step S004), the processing proceeds to step S005. In step S005, because the sheet is reversed, the polarity of applied voltage to the charge removing roller 130b is determined to be positive polarity. In a case where discharge surface designation is face-up sheet discharge of simplex printing or duplex printing (NO in step S004), the processing proceeds to step S013. In step S013, the polarity of applied voltage is determined to be negative polarity. Next, in step S006, the charge removing CPU 224 checks whether the mode lever 141 is turned ON. If the mode lever 141 is turned ON (YES in step S006), the processing proceeds to step S007. In step S007, the magnitude of a charge removing voltage is determined from the dial 142. If it is determined in step S006 that the mode lever 141 is turned OFF (NO in step S006), the processing proceeds to step S014. In step S014, a charge removing voltage is not applied. If the setting of charge removing is determined, in steps S008 and S015, a motor in the charge removing apparatus 103 is driven and rollers are rotated.

After that, in a case where charge removing is turned ON, in step S009, high voltage is applied to the charge removing roller 130b. If it is determined that an input print job is to be ended (YES in step S010 or S016), in a case where charge removing is turned ON, in step S011, high voltage to the charge removing roller 130b is turned OFF, in step S012, rollers in the charge removing apparatus 103 are stopped, and in step S017, printing ends.

By executing the above-described configuration, it is possible to execute charge removing with polarity appropriate for a charged sheet.

In the present exemplary embodiment, AC voltage is applied to the non-contact charge removing unit 131, and by corona discharge, ions of both of positive polarity and negative polarity are alternately emitted. For this reason, the non-contact charge removing unit 131 is always in the ON state irrespective of designated sheet discharge is face-up sheet discharge of simplex printing, face-down sheet discharge of simplex printing, or sheet discharge of duplex printing. Nevertheless, the contact charge removing unit 129 needs not always perform the switching of polarity of a charge removing unit depending on a sheet discharge method, and the non-contact charge removing unit 131 may perform the switching. In a case where the non-contact charge removing unit 131 is enabled to switch between positive and negative DC voltages, voltage is desirably applied with polarity suitable for the polarities of the charged top and bottom surfaces of the sheet conveyed to the charge removing apparatus