JP2024131325A - Image forming apparatus and method for controlling the image forming apparatus - Google Patents

Abstract

[Problem] In an image forming apparatus that forms an image on an image recording medium such as paper by electrophotography, toner stains on the trailing edge of the image recording medium are prevented by a lower cost and less expensive configuration. [Solution] According to the image forming apparatus (10) of the present disclosure, a print rate Rx is derived based on image data provided for image formation processing. Then, a photoconductor drum motor (282) and a fixing motor (390) are controlled so that a speed difference ΔV, which is the difference between a transfer speed Vt, which is the surface speed of the photoconductor drum 28, and a fixing speed Vf, which is the surface speed of the pressure roller 388, has a magnitude corresponding to the print rate Rx. As a result, regardless of the print rate Rx, the deflection of the paper in the section from the transfer nip Nt to the fixing nip Nf becomes constant, and toner stains on the trailing edge of the paper are prevented. [Selected Figure] Figure 3

Description

This disclosure relates to an image forming apparatus and a method for controlling the image forming apparatus, and in particular to an image forming apparatus and a method for controlling the image forming apparatus that form an image on a sheet-shaped image recording medium by an electrophotographic method.

In this type of image forming apparatus, a toner image formed on the surface of an image carrier is transferred to an image recording medium by a transfer means. The transfer means has a transfer member that is rotatably arranged with its surface in contact with the surface of the image carrier, and transfers the toner image on the image carrier to the image recording medium by sandwiching and conveying the image recording medium at a transfer nip portion, which is the contact portion between the surface of the transfer member and the surface of the image carrier. The toner image transferred to the image recording medium is fixed (thermally fixed) to the image recording medium by a fixing means. The fixing means has a fixing member that is heated to a predetermined temperature and rotatably arranged, and a pressure member that is rotatably arranged with its surface in contact with the surface of the fixing member, and fixes the toner image on the image recording medium to the image recording medium by sandwiching and conveying the image recording medium at a fixing nip portion, which is the contact portion between the surface of the fixing member and the surface of the pressure member.

Here, the transfer speed, which is generally the surface speed of the image carrier or transfer member (in other words, the ideal transport speed of the image recording medium in the transfer nip), is set to be constant. In contrast, the fixing speed, which is the surface speed of the fixing member or pressure member (in other words, the ideal transport speed of the image recording medium in the fixing nip), is set to be slightly lower than the transfer speed. By setting it in this way, an appropriate deflection (slack) is given to the image recording medium transported in this section from the transfer nip to the fixing nip. This prevents the image recording medium in the transfer nip from being pulled toward the fixing nip, and the toner image is transferred appropriately in the transfer nip. In addition, when the leading edge of the image recording medium after the toner image is transferred enters the fixing nip, the image recording medium is given an appropriate deflection, and in particular the image recording medium is deflected toward the pressure member, preventing image defects caused by the image recording medium coming into contact with the fixing member before the fixing nip.

However, if the image recording medium is excessively warped, after the rear end of the image recording medium passes through the transfer nip, the rear end of the image recording medium comes into contact with the image carrier or transfer member and is conveyed while rubbing the residual toner adhering to the surface of the image carrier or transfer member, resulting in the inconvenience that the rear end of the image recording medium becomes soiled with the residual toner. One example of a technique for avoiding this inconvenience is disclosed in Patent Document 1. According to the technique disclosed in Patent Document 1, the degree of sagging of the image recording medium is detected by a sagging detection unit. Then, for example, the fixing speed is controlled so that the degree of sagging when the rear end of the image recording medium passes through the transfer nip is suppressed to be less than the degree of sagging before the rear end of the image recording medium passes through the transfer nip. This prevents toner contamination of the rear end of the image recording medium.

JP 2018-163197 A

However, the technology disclosed in Patent Document 1 requires the provision of a sensor called a slack detection unit, which increases the cost of the entire device and complicates its configuration.

The present disclosure therefore aims to provide a novel image forming apparatus and a method for controlling the image forming apparatus that can prevent toner stains on the trailing edge of an image recording medium with a lower cost and simpler configuration.

To achieve this objective, the present disclosure includes a first disclosure relating to an image forming apparatus and a second disclosure relating to a control method for an image forming apparatus.

The first disclosure of the image forming apparatus includes an image carrier, a first driving source, a transfer means, a fixing means, a second driving source, a printing rate deriving means, and a control means. The image carrier is rotatably provided, and a toner image is formed on the surface of the image carrier. The first driving source rotates the image carrier. The transfer means has a transfer member. The transfer member is rotatably provided with its surface in contact with the surface of the image carrier. The transfer means transfers the toner image on the image carrier to the image recording medium by sandwiching and transporting the sheet-shaped image recording medium in a transfer nip portion, which is the contact portion between the surface of the image carrier and the surface of the transfer member. The fixing means has a fixing member and a pressure member. The fixing member is heated to a predetermined temperature and rotatably provided. The pressure member is rotatably provided with its surface in contact with the surface of the fixing member. The fixing unit fixes the toner image on the image recording medium to the image recording medium by nipping and conveying the image recording medium at a fixing nip portion, which is a contact portion between the surface of the fixing member and the surface of the pressure member. The second driving source rotates and drives the fixing member or the pressure member. Furthermore, the printing rate deriving unit derives the printing rate of the toner image (the ratio of the area occupied by the toner image to the printable area on the image recording medium) based on image data provided for the toner image formation process on the image carrier. The control unit imparts a difference between the transfer speed, which is the surface speed of the image carrier or the transfer member, and the fixing speed, which is the surface speed of the fixing member or the pressure member, and controls the first driving source and the second driving source so that the speed difference, which is the difference between the transfer speed and the fixing speed (in other words, the speed ratio, which is the relative ratio between the transfer speed and the fixing speed), is a magnitude corresponding to the printing rate derived by the printing rate deriving unit.

Specifically, the control means applies a speed difference between the fixing speed and the transfer speed so that the fixing speed is lower than the transfer speed. In addition, the control means controls the first drive source and the second drive source so that the speed difference becomes smaller as the printing rate becomes lower.

The control means also controls the first drive source and the second drive source so that the fixing speed corresponds to the printing rate while keeping the transfer speed constant.

In the first disclosure, a storage means may further be provided. This storage means stores length information representing the length dimension of the image recording medium in the transport direction of the image recording medium, and thickness information representing the thickness dimension (thickness) of the image recording medium. Then, the control means may control the first drive source and the second drive source so that the aforementioned speed difference is a magnitude corresponding to the print rate when one or both of the length dimension of the image recording medium based on the length information and the thickness dimension of the image recording medium based on the thickness information meet a predetermined control execution condition.

In addition, in this first disclosure, a detection means may be further provided. This detection means detects that the downstream end of the image recording medium in the transport direction, i.e., the leading edge, has passed upstream of the fixing nip portion in the transport direction of the image recording medium. The control means may then control the first drive source and the second drive source so as to further reduce the aforementioned speed difference when a predetermined time has elapsed since the leading edge of the image recording medium passed through the fixing nip portion. The predetermined time here corresponds to the time from the leading edge of the image recording medium passing through the fixing nip portion to the time when a predetermined position on the rear end side of the leading edge of the image recording medium passes through the fixing nip portion. The time when the leading edge of the image recording medium passes through the fixing nip portion is also predicted based on the detection result by the detection means.

The fixing means may be of the pad type. The pad type fixing means has an endless fixing belt as the fixing member, a heating means for heating the fixing belt to a predetermined temperature, an opposing member as the pad, and a pressure roller as the pressure member. In particular, the opposing member is provided at a position facing the pressure member on the inside of the fixing belt. The opposing member then clamps the fixing belt between itself and the surface of the pressure roller as the pressure member, thereby forming a fixing nip between the outer surface of the fixing belt and the surface of the pressure roller.

A second disclosure of the present disclosure, which relates to a control method for an image forming apparatus, includes a printing rate deriving step and a control step. Here, the image forming apparatus includes an image carrier, a first driving source, a transfer means, a fixing means, and a second driving source. The image carrier is rotatably provided, and a toner image is formed on the surface of the image carrier. The first driving source rotates and drives the image carrier. The transfer means has a transfer member. The transfer member is rotatably provided with its surface in contact with the surface of the image carrier. The transfer means transfers the toner image on the image carrier to the image recording medium by sandwiching and transporting the sheet-shaped image recording medium in a transfer nip portion, which is the contact portion between the surface of the image carrier and the surface of the transfer member. The fixing means has a fixing member and a pressure member. The fixing member is heated to a predetermined temperature and rotatably provided. The pressure member is rotatably provided with its surface in contact with the surface of the fixing member. The fixing unit fixes the toner image on the image recording medium to the image recording medium by nipping and conveying the image recording medium in a fixing nip portion, which is a contact portion between the surface of the fixing member and the surface of the pressure member. The second driving source rotates and drives the fixing member or the pressure member. Then, in the printing rate derivation step, the printing rate of the toner image is derived based on image data provided for the toner image formation process on the image carrier. Then, in the control step, a difference is provided between the transfer speed, which is the surface speed of the image carrier or the transfer member, and the fixing speed, which is the surface speed of the fixing member or the pressure member, and the first driving source and the second driving source are controlled so that the speed difference, which is the difference between the transfer speed and the fixing speed, corresponds to the printing rate derived in the printing rate derivation step.

According to the present disclosure, toner stains on the trailing edge of the image recording medium can be prevented with a lower cost and simpler configuration.

FIG. 1 is a diagram illustrating a schematic internal configuration of an image forming apparatus according to a first embodiment of the present disclosure. FIG. 2 is an enlarged view of a part of the fixing device in the first embodiment. FIG. 3 is a diagram showing an example of a state in which a sheet is transported in the section from the transfer nip portion to the fixing nip portion in the first embodiment. FIG. 4 is a diagram showing another example of the state of transport of the paper in the section from the transfer nip portion to the fixing nip portion in the first embodiment. FIG. 5 is a diagram showing the relationship between the printing rate Rx and the correction coefficient β in the first embodiment. FIG. 6 is a diagram conceptually showing the configuration of the correction coefficient table in the first embodiment. FIG. 7 is a block diagram showing the electrical configuration of the image forming apparatus according to the first embodiment. FIG. 8 is a flow diagram showing the flow of a print control task in the first embodiment. FIG. 9 is a flow diagram showing the flow of a portion of the print control task in the second embodiment of the present disclosure. FIG. 10 is a flow diagram showing the flow of the remaining part of the print control task in the second embodiment. FIG. 11 is a flow diagram showing the flow of a portion of the print control task in the third embodiment of the present disclosure. FIG. 12 is a diagram for explaining the difference between a pad type fixing device and a roller type fixing device.

[First embodiment]
The first embodiment of the present disclosure will be described by taking as an example a monochrome image forming apparatus 10 shown in FIG.

The image forming device 10 according to the first embodiment is a so-called multifunction peripheral (MFP) having multiple functions such as a copy function, a printer function, an image scanner function, and a fax function. FIG. 1 is a diagram showing the internal configuration of the image forming device 10 when installed in a usable state, as viewed from the front side of the image forming device 10. That is, the up-down direction in FIG. 1 corresponds to the up-down direction of the image forming device 10. And the left-right direction in FIG. 1 corresponds to the left-right direction of the image forming device 10. Furthermore, the front side of the paper surface of FIG. 1 corresponds to the front of the image forming device 10. And the back side of the paper surface of FIG. 1 corresponds to the rear of the image forming device 10.

An image reading unit 12 is provided at the top of the image forming apparatus 10 as an image reading means. The image reading unit 12 performs image reading processing by reading an image of an original (not shown) and outputting two-dimensional read image data corresponding to the image of the original. For this purpose, the image reading unit 12 has an original table 14 on which an original is placed. The original table 14 is formed of a transparent material such as a roughly rectangular flat plate-shaped glass, and is provided so that both main surfaces are aligned in the horizontal direction. An image reading unit 16 is provided below the original table 14. Although detailed explanations are omitted, the image reading unit 16 has a light source, a mirror, a lens, a line sensor, etc., and forms a linear image reading position Pr extending along the front-rear direction of the image forming apparatus 10 on the upper surface of the original table 14. In addition, a drive mechanism (not shown) for moving (scanning) the image reading position Pr of the image reading unit 16 along the left-right direction of the image forming apparatus 10 is provided below the original table 14. That is, when a document is placed on the document table 14, the image reading position Pr of the image reading unit 16 is moved by a drive mechanism, and the image of the document is read by a so-called fixed reading method. The front-to-rear direction of the image forming device 10 is called the main scanning direction. And the left-to-right direction of the image forming device 10 is called the sub-scanning direction.

Also, above the document table 14, an automatic document feeder (ADF) 18 is provided, which also serves as a document pressing cover for pressing down a document placed on the document table 14. The automatic document feeder 18 is provided so as to be able to transition between a state in which the top surface of the document table 14 is exposed to the outside and a state in which the top surface of the document table 14 is covered. For this reason, the automatic document feeder 18 is connected to the main body (housing) of the image forming apparatus 10 via an appropriate movable support member such as a hinge (not shown). Note that FIG. 1 shows a state in which the automatic document feeder 18 covers the top surface of the document table 14. Also, when the automatic document feeder 18 is in a state in which it covers the top surface of the document table 14 as shown in FIG. 1, it performs its original function as described below.

That is, the automatic document feeder 18 has a document placement tray 20. Documents, strictly speaking, sheet-like documents, can be placed on this document placement tray 20, and in particular, multiple documents can be placed in a stack. Although detailed explanation is omitted, the automatic document feeder 18 takes in the documents placed on the document placement tray 20 one by one (one by one) and transports them along a document transport path 22 within the automatic document feeder 18. On the way, the document passes through the image reading position Pr, and strictly speaking, the image reading position Pr is in a fixed state. This causes the image of the document to be read, and is read in a so-called skimming method. The document is then discharged to the document discharge tray 24.

Below the image reading unit 12, an image forming unit 26 is provided as an image forming means. This image forming unit 26 is responsible for image formation processing that forms an image based on appropriate image data such as the above-mentioned read image data on a sheet-like image recording medium (not shown), such as paper, i.e., it is responsible for printing. This printing is performed by a known electrophotographic method.

Specifically, the image forming unit 26 has a photosensitive drum 28 as an image carrier, a charging device 30 as a charging means, an exposure device 32 as an exposure means, a developing device 34 as a developing means, a transfer device 36 as a transfer means, a fixing device 38 as a fixing means, a cleaning device 40 as a cleaning means, a static eliminator (not shown) as a static eliminator, and a toner supply device 42 as a toner supply means.

The photoconductor drum 28 has a base, which is a cylindrical conductive member made of a conductive material such as aluminum. This base is connected to a frame (not shown) of the image forming apparatus 10, i.e., it is grounded. A photosensitive layer is formed on the surface (outer periphery) of the base. The photosensitive layer is conductive in the area irradiated with light, and is insulating in the area not irradiated with light. The photoconductor drum 28 rotates by receiving a driving force from a photoconductor drum motor 282 (see FIG. 7) described later as a first driving source, for example, rotating counterclockwise in FIG. 1.

The charging device 30 applies static electricity to the surface of the photoconductor drum 28, charging the surface of the photoconductor drum 28 to a predetermined potential, for example, a negative potential based on the ground potential. The surface of the photoconductor drum 28 thus charged is exposed by the exposure device 32. The exposure device 32 is, for example, a laser scanning unit (LSU), and irradiates the surface of the photoconductor drum 28 with laser light in a pattern based on the image data to be printed, more specifically, irradiates a position downstream of the charging position by the charging device 30 in the rotation direction of the photoconductor drum 28. As a result, an electrostatic latent image based on the image data to be printed is formed on the surface of the photoconductor drum 28. Note that the exposure device 32 is not limited to a laser scanning unit, and may be, for example, an LED unit having an LED array in which LEDs are arranged as a light source.

The surface of the photoconductor drum 28 on which the electrostatic latent image is formed in this way is developed by the developing device 34. That is, the developing device 34 visualizes the electrostatic latent image formed on the surface of the photoconductor drum 28 by attaching toner (not shown) charged to the same polarity as the surface potential of the photoconductor drum 28, that is, charged to a negative potential with respect to the ground potential, to the electrostatic latent image formed on the surface of the photoconductor drum 28. For this reason, although a detailed explanation is omitted, the developing device 34 has a transport screw that charges the developer containing the toner in the developing device 34 by transporting the developer while stirring it, and a developing roller that transports the charged developer near the surface of the photoconductor drum 28. The developer is a two-component developer that contains a carrier in addition to the toner, but it may be a one-component developer consisting of only toner.

The toner image formed on the surface of the photosensitive drum 28 by the development by the developing device 34 is transferred to a sheet by the transfer device 36 at a position downstream of the development position by the developing device 34 in the rotation direction of the photosensitive drum 28. The transfer device 36 has a transfer roller 362 as a transfer member. The transfer roller 362 is provided so that its surface abuts against the surface of the photosensitive drum 28, and rotates by receiving the rotational driving force of the photosensitive drum 28, that is, rotates clockwise in FIG. 1. In addition, a transfer current that is a positive direct current based on the ground potential is supplied to the transfer roller 362 from a transfer bias power supply that is a constant current source of direct current (not shown). As a result, a transfer electric field for attracting the toner image on the surface of the photosensitive drum 28 toward the transfer roller 362 is formed in the transfer nip portion Nt, which is the abutment portion between the surface of the photosensitive drum 28 and the surface of the transfer roller 362. When a sheet of paper passes through this transfer nip Nt as described below, the toner image on the surface of the photoconductor drum 28 is transferred to the sheet of paper.

After the toner image has been transferred in this manner, the surface of the photosensitive drum 28 is cleaned by the cleaning device 40 downstream of the transfer position by the transfer device 36 in the rotation direction of the photosensitive drum 28. Although a detailed explanation is omitted, the cleaning device 40 has a cleaning blade that removes toner (residual toner) remaining on the surface of the photosensitive drum 28, and a transport member that transports the toner removed by the cleaning blade to a waste toner box (not shown).

After cleaning by the cleaning device 40, the surface of the photoconductor drum 28 is neutralized by a neutralization device (not shown) at a position downstream of the cleaning position by the cleaning device 40 in the rotation direction of the photoconductor drum 28. The neutralization device neutralizes the surface of the photoconductor drum 28 by irradiating the surface of the photoconductor drum 28 with light, that is, erases the latent image potential on the surface of the photoconductor drum 28.

After this, the same process is repeated: charging by the charging device 30, exposure by the exposure device 32, development by the development device 34, transfer by the transfer device 36, cleaning by the cleaning device 40, and de-electrification by the de-electrification device.

Furthermore, as a result of development by the developing device 34, toner within the developing device 34 is consumed. To compensate for this toner consumption, toner is replenished to the developing device 34 by the toner replenishing device 42. In this case, a print rate (the ratio of the area occupied by the toner image to the printable area on the paper) Rx is derived based on the image data to be printed, more precisely based on the pixel data contained in the image data. Then, based on the print rate Rx, the amount of toner consumed is estimated, in other words, the amount of toner required for replenishment is calculated. This required amount of toner is replenished to the developing device 34 by the toner replenishing device 42. Note that the print rate Rx is calculated in units (increments) of 1%, for example, but is not limited to this.

Further, in the image forming apparatus 10, a paper transport path 50 is provided from the paper feed section 44 to the paper discharge outlet 48 of the paper discharge tray 46, which will be described later, via the transfer nip Nt. At appropriate positions on the paper transport path 50, a plurality of transport rollers (strictly speaking, roller pairs) 52, 52, ... are provided for transporting the paper from the paper feed section 44 to the paper discharge outlet 48. That is, the paper supplied from the paper feed section 44 to the paper transport path 50 is transported toward the paper discharge outlet 48 by being sandwiched and transported by the respective transport rollers 52, 52, ..., as will be described later, and is discharged to the paper discharge tray 46 through the paper discharge outlet 48. On the way thereto, the paper is sandwiched and transported at the transfer nip Nt, that is, it passes through the transfer nip Nt, and at that time, the toner image on the surface of the photosensitive drum 28 is transferred to the paper as described above. Also, the paper that has passed through the transfer nip Nt, that is, the paper to which the toner image has been transferred, passes through the fixing nip Nf, which will be described later.

Of the transport rollers 52, 52, ..., the transport roller 52 located upstream of the transfer nip Nt in the paper transport direction on the paper transport path 50 and closest to the transfer nip Nt is a registration roller 52a for timing the entry of the paper into the transfer nip Nt. Also, of the transport rollers 52, 52, ..., the transport roller 52 located on the most downstream side of the paper transport direction on the paper transport path 50, that is, near the paper discharge port 48, is a paper discharge roller 52b for discharging the paper to the paper discharge tray 46 via the paper discharge port 48.

In addition, a fixing device 38 is provided at a position between the transfer nip portion Nt and the paper discharge port 48 on the paper transport path 50, in other words, at a position downstream of the transfer nip portion Nt in the paper transport direction on the paper transport path 50. This fixing device 38 is, for example, of a pad type.

Referring also to FIG. 2, the fixing device 38 has a fixing belt 382 as a fixing member, a heat source 384 as a heating means, a fixing pad 386 as an opposing member, and a pressure roller 388 as a pressure member. The fixing belt 382 is an endless belt-like body formed in a roughly cylindrical shape, and is made of a base material made of a synthetic resin such as polyimide or a metal such as nickel, on the surface of which a release layer such as Teflon (registered trademark) is provided. The fixing belt 382 is provided so as to be rotatable around its (roughly cylindrical) axis, and is supported by an appropriate frame (guide) (not shown) so as to be rotatable. The inner diameter of the fixing belt 382 is, for example, about 30 mm.

The heat source 384 is disposed inside the fixing belt 382 so as to extend along the axial direction of the fixing belt 382, and heats the fixing belt 382 to a predetermined fixing temperature. Note that, for example, a lamp heater such as a halogen lamp is used as the heat source 384.

The fixing pad 386 is a member having a slightly thick, approximately elongated (straight ruler) shape, and has a roughly flat surface portion 386a. The fixing pad 386 is provided at a position facing the pressure roller 388 on the inside of the fixing belt 382, and by sandwiching the fixing belt 382 between the surface of the pressure roller 388 and the fixing pad 386, a fixing nip portion Nf is formed between the outer surface of the fixing belt 382 and the surface of the pressure roller 388. In addition, the fixing pad 386 extends along the axial direction of the fixing belt 382, and is fixed by a fixing member (not shown) so that the flat surface portion 386a is in sliding contact with the inner surface of the fixing belt 382. The fixing pad 386 is formed of a heat-resistant resin such as metal or polyimide. In addition, the flat surface portion 386a of the fixing pad 386 is coated with sliding oil to reduce the frictional force between the fixing belt 382 and the fixing belt 382, or a sliding sheet (not shown) is provided. The width dimension of this flat portion 386a (the dimension in the direction perpendicular to the extension direction of the fixing pad 386), or in other words the pad width Lp, is, for example, about 15 mm, which is longer than the fixing nip width Lf, which is the width dimension of the fixing nip portion Nf described below (the dimension in the same direction as the pad width Lp).

The pressure roller 388 presses the fixing belt 382 between the flat surface 386a of the fixing pad 386 and the fixing belt 382 to form a fixing nip Nf between the pressure roller 388 and the outer surface of the fixing belt 382. The pressure roller 388 rotates by receiving a driving force from a fixing motor 390 (see FIG. 7) as a second driving source, which will be described later, and rotates, for example, clockwise in FIG. 1 and FIG. 2. Accordingly, the fixing belt 382 rotates, that is, it is driven to rotate counterclockwise in FIG. 1 and FIG. 2. Although not shown in detail, the pressure roller 388 has a cylindrical core material, an elastic layer provided to cover the outer peripheral surface of the core material, and a release layer provided to cover the outer peripheral surface of the elastic layer. The core material is made of a metal such as aluminum, and the elastic layer is made of an elastic material such as silicone rubber. The release layer is made of a material with a relatively low friction coefficient, such as a fluorine-based resin. As mentioned above, the fixing nip width Lf, which is the width dimension of the fixing nip portion Nf, is smaller than the pad width Lp, and is, for example, about 5 mm to 10 mm. Furthermore, the fixing belt 382, instead of the pressure roller 388, may also be configured to rotate by receiving driving force from the fixing motor 390.

That is, as described above, the paper that has passed through the transfer nip Nt, i.e., the paper onto which the toner image has been transferred, passes through the fixing nip Nf of the fixing device 38, and more specifically, is sandwiched and transported by the fixing nip Nf. This causes the toner image on the paper to be fixed onto the paper. This completes the series of image formation processes by the image forming unit 26.

In addition, with the pad-type fixing device 38, the fixing nip width Lf is larger than with a roller-type fixing device in which the fixing nip is formed by a fixing roller and a pressure roller (the fixing nip width Lf in a roller-type fixing device is about 2 mm), so the time it takes for the heat of the fixing belt 382 to be transferred to the toner is longer, and the toner image can be melted at a lower temperature. In other words, with the pad-type fixing device 38, fixing is possible at a lower temperature than with a roller-type fixing device, and the time it takes to heat up the toner can be shortened. Furthermore, the amount of heat dissipated is suppressed and the heat capacity is small, making it excellent in energy conservation.

After a series of image forming processes, including fixing by the fixing device 37, the paper (printed material) is discharged to the discharge tray 46 through the discharge port 48. The discharge tray 46 is provided between the image forming unit 26 and the image reading unit 12, and is provided in the so-called internal space of the image forming device 10. Alternatively, the discharge tray 46 may be provided on the outside of the image forming device 10.

Furthermore, a paper feed unit 44 is provided at the bottom of the image forming apparatus 10 as a paper feed means. The paper feed unit 44 has a paper feed cassette 44a, which can store multiple sheets of paper in a stacked state. The paper feed unit 44 also has a pickup roller 44b and a paper feed roller (strictly speaking, a roller pair) 44c. The paper feed unit 44 picks up the paper stored in the paper feed cassette 44a one sheet at a time using the pickup roller 44b, and supplies the picked up paper to the paper transport path 50 using the paper feed roller 44c.

In addition, a double-sided printing conveying path 54 is provided within the image forming apparatus 10. This double-sided printing conveying path 54 is a conveying path for taking in paper that has once passed through the fixing nip portion Nf, i.e., paper after printing, and providing the paper for printing again. That is, the paper taken into the double-sided printing conveying path 54 is again supplied to the paper conveying path 50, more specifically, to the upstream side of the registration rollers 52a. At that time, the paper is turned over. As a result, printing is performed on the back side of the paper, that is, double-sided printing is achieved. In addition, a plurality of conveying rollers (strictly speaking, roller pairs) 56, 56, ... are provided at appropriate positions on the double-sided printing conveying path 54 for conveying the paper along the double-sided printing conveying path 54.

Now, if we look at the state of the paper being transported in the section from the transfer nip Nt to the fixing nip Nf, as shown by the thick solid line 100 in FIG. 3, the paper is given an appropriate deflection. For this purpose, the transfer speed Vt, which is the surface speed of the photosensitive drum 28 (or the transfer roller 362), is kept constant, and the fixing speed Vf, which is the surface speed of the pressure roller 388 (or the fixing belt 382), is set slightly lower (slower) than the transfer speed Vt, for example, by a little more than 1%. In other words, a difference is given between the fixing speed Vf and the transfer speed Vt so that this occurs, that is, a speed difference ΔV (= |Vt-Vf|) is given. This prevents transfer misalignment (image loss) caused by the paper being pulled toward the fixing nip Nf at the transfer nip Nt, which occurs due to the reduction in the amount of deflection of the paper during transport. In other words, the toner image is transferred appropriately to the paper at the transfer nip Nt.

The relative positions of the transfer nip Nt and the fixing nip Nf and the arrangement of the pressure roller 388 are determined so that the paper being transported bends toward the pressure roller 388. In other words, the leading edge of the paper to which the toner image has been transferred first comes into contact with the surface of the pressure roller 388 before being guided to the fixing nip Nf. This prevents the paper from touching the fixing belt 382, which is heated before the fixing nip Nf, when the leading edge of the paper to which the toner image has been transferred enters the fixing nip Nf, causing the toner image to melt excessively, resulting in a defective image (the toner image on the paper becomes faint as the toner image in the part that touches the fixing belt 382 melts excessively and remains on the fixing belt 382 side).

However, when the printing rate Rx is high, especially when the printing rate Rx is 100% (solid image), slippage between the paper and the transfer nip Nt (reduced gripping force) is likely to occur. As a result, the paper feed speed (paper feed speed) at the transfer nip Nt is slightly lower than the transfer speed Vt, which is the surface speed of the photosensitive drum 28. In other words, the higher the printing rate Rx, the lower the paper feed speed at the transfer nip Nt is compared to the transfer speed Vt. On the other hand, in the fixing nip Nf, the pressing force acting on the paper is greater than that of the transfer nip Nt (the total load acting on the fixing nip Nf is set to, for example, 20 kgf or more because it is necessary to bring the heated toner into close contact with the paper without any gaps, whereas the total load acting on the transfer nip Nt is set to, for example, 2 kgf or less because it is necessary not to break the toner image transferred to the paper), so slippage between the paper and the fixing nip Nf (almost no) occurs. For this reason, the relationship between the fixing speed Vf and the transfer speed Vt, in which the fixing speed Vf is set to be slightly more than 1% lower than the transfer speed Vt, in other words, the value of slightly more than 1% is determined based on the case where the print rate Rx is 100%.

However, when the printing rate Rx is low, especially the lower the printing rate Rx, the less likely slippage occurs between the paper and the transfer nip Nt (the gripping force increases), and the paper feed speed at the transfer nip Nt approaches the transfer speed Vt. When the printing rate Rx is 0%, the paper feed speed at the transfer nip Nt is almost the same as the transfer speed Vt. When the printing rate Rx is lower (than 100%), if the same fixing speed Vf is set as when the printing rate Rx is 100%, the paper will bend more than when the printing rate Rx is 100%, as shown by the thick dashed line 100a in Figure 4. If the paper bends excessively, the rear end of the paper will come into contact with the photoconductor drum 28 or the transfer roller 362 after passing through the transfer nip Nt, and will rub the residual toner adhering to the surface of the photoconductor drum 28 or the transfer roller 362 as it is transported, causing the rear end of the paper to become soiled with the residual toner. To avoid this inconvenience, the following measures are taken in the first embodiment.

That is, the fixing speed Vf is set based on the following formula 1.

Formula 1
Vf=Vt×α×β

In this formula 1, α is a relative coefficient that represents the ratio of the fixing speed Vf to the transfer speed Vt when the printing rate Rx is 100%, and corresponds to the value of just over 1% mentioned above, for example, 1.5% here, that is, 98.5% (=100%-1.5%=0.985). The value of 1.5% here is the reference value of the speed difference ΔV, or more precisely, the reference value of the speed difference ΔV when expressed as a ratio to the transfer speed Vt (=ΔV/Vt). β is a correction coefficient that changes depending on the printing rate Rx. The relationship between this correction coefficient β and the printing rate Rx is shown in Figure 5.

As shown in FIG. 5, the correction coefficient β takes a value of 100% (=1) when the printing rate Rx is 100%. In other words, when the printing rate Rx is 100%, the speed difference ΔV is expressed as a ratio to the transfer speed Vt, which is the reference value of 1.5%. And, the lower the printing rate Rx, the larger the correction coefficient β takes, that is, the value becomes 100% or more, and the speed difference ΔV becomes smaller than the reference value. For example, when the printing rate Rx is 10%, the correction coefficient β takes a value of 100.7% (=1.007). Also, for example, when the printing rate Rx is 0%, the correction coefficient β takes a value of 101.2% (=1.012). The relationship between the correction coefficient β and the printing rate Rx shown in Figure 5 was determined by experimentation, and more specifically, the correction coefficient β for each of the various printing rates Rx was determined by experimentation so that the deflection (amount of deflection) of the paper was constant for each of the various printing rates Rx. Note that even when the printing rate Rx is 0%, that is, even when the correction coefficient β is at its maximum value of 101.2%, the fixing speed Vf is maintained in a state where it is lower than the transfer speed Vt (Vf<Vt), and therefore the state in which the paper is deflected is maintained.

Then, the relationship between the correction coefficient β and the printing rate Rx shown in FIG. 5 is compiled into a correction coefficient table 200 as shown in FIG. 6. In this correction coefficient table 200, the correction coefficient β corresponding to each printing rate Rx is stored. This correction coefficient table 200 is incorporated into a control program described below, and in particular into a print control program. Note that in the correction coefficient table 200 shown in FIG. 6, the printing rate Rx is divided in 1% increments from 0% to 100%, but the number of divisions of this printing rate Rx is not limited to this.

When printing is performed, the printing rate Rx is derived based on the image data to be printed as described above, and the printing rate Rx is compared with the correction coefficient table 200 to determine the correction coefficient β. The determined correction coefficient β is then applied to formula 1 to determine the fixing speed Vf, which is then set.

In this way, according to the first embodiment, the fixing speed Vf is controlled based on the printing rate Rx while the transfer speed Vt is kept constant. For example, when the printing rate Rx is 100%, the fixing speed Vf is set 1.5% lower than the transfer speed Vt, which is set to the reference value of the fixing speed Vf. The lower the printing rate Rx, the higher the fixing speed Vf is set to be above the reference value, in other words, the speed difference ΔV between the transfer speed Vt and the fixing speed Vf is controlled to be smaller. This makes the deflection of the paper constant regardless of the printing rate Rx, and prevents problems caused by excessive deflection of the paper, which means that toner stains on the rear end of the paper are prevented.

Figure 7 is a block diagram showing the electrical configuration of the image forming device 10. As shown in this figure, the image forming device 10 has a control unit 60. The image reading unit 12, the automatic document feeder 18, the image forming unit 26, and the paper feed unit 44 are connected to the control unit 60 via a bus 62. In addition, the control unit 60 is connected to an operation unit 64, an auxiliary storage unit 66, a communication unit 68, and the like via the bus 62. The image forming device 10 also has various other elements, but illustrations and descriptions of elements that are not directly related to the gist of this disclosure will be omitted here. The image reading unit 12, the automatic document feeder 18, the image forming unit 26, and the paper feed unit 44 are as described above. In particular, the image forming unit 26 has a photoconductor drum motor 282 and a fixing motor 390. For example, a DC motor or a stepping motor is used for the photoconductor drum motor 282 and the fixing motor 390.

The control unit 60 is a control means that controls the entire image forming apparatus 10. For this reason, the control unit 60 has a computer, such as a CPU 60a, as a control execution means. In addition, the control unit 60 has a main memory unit 60b as a main memory means that can be directly accessed by the CPU 60a. The main memory unit 60b includes, for example, a ROM and a RAM (not shown). Of these, the ROM stores a control program (firmware) for controlling the operation of the CPU 60a. This control program includes a print control program. In addition, the correction coefficient table 200 described above (Figure 6) is incorporated into the print control program. The RAM forms a working area and a buffer area when the CPU 60a executes processing according to the control program.

The operation unit 64 has a display with a touch panel (not shown). The display with a touch panel is a component that integrally combines a touch panel as an example of an operation reception means capable of receiving operations by a user (not shown) and a display as an example of a display means that displays various information. In addition to the display with a touch panel, the operation unit 64 also has an appropriate light-emitting means such as an LED (not shown) and an appropriate hardware switch such as a push button (not shown).

The auxiliary memory unit 66 is an example of an auxiliary storage means. Various data such as the above-mentioned scanned image data and print jobs described below are appropriately stored in this auxiliary memory unit 66. This auxiliary memory unit 66 has, for example, a hard disk drive (not shown). Additionally, the auxiliary memory unit 66 may have a rewritable non-volatile memory such as a flash memory.

The communication unit 68 is an example of a communication means. That is, the communication unit 68 is responsible for two-way communication processing via a LAN line (not shown). The communication unit 68 may be connected to the LAN line by wire or wirelessly. Various external devices such as a personal computer and a server (not shown) are connected to the LAN line. The communication unit 68 is also responsible for two-way communication processing via a public switched telephone network (not shown).

As described above, according to the first embodiment, the printing rate Rx is derived based on the image data to be printed. Then, with the transfer speed Vt constant, the fixing speed Vf is controlled based on the printing rate Rx. In other words, the speed difference ΔV between the transfer speed Vt and the fixing speed Vf is controlled, and strictly speaking, the photoconductor drum motor 282 and the fixing motor 390 are controlled. In order to realize the printing control including the control of the speed difference ΔV in this manner, the CPU 60a executes the printing control task according to the printing control program described above. The flow of this printing control task is shown in FIG. 8. The printing control task in FIG. 8 is executed in response to the acceptance of a print job that instructs printing by the printer function of the image forming device 10. FIG. 8 also shows an example in which the number of copies to be printed is one.

According to this print control task, the CPU 60a first analyzes the print job in step S1 to recognize the number of pages of the document and the size of the paper to be printed on. The CPU 60a then advances the process to step S3.

In step S3, the CPU 60a sets the variable n, which represents the number of the page to be printed, to its initial value of 1. The CPU 60a then proceeds to step S5.

In step S5, the CPU 60a expands the image data of the nth page of the document. The CPU 60a then advances the process to step S7.

In step S7, the CPU 60a derives the printing rate Rx based on the image data developed in step S5. The CPU 60a then advances the process to step S9. The printing rate Rx derived in step S7 is also used to calculate the amount of toner required for toner supply to the developing device 34 by the toner supply device 42 described above.

In step S9, the CPU 60a compares the printing rate Rx derived in step S7 with the aforementioned correction coefficient table 200 to identify the correction coefficient β that corresponds to the printing rate Rx. The CPU 60a then advances the process to step S11.

In step S11, the CPU 60a applies the correction coefficient β determined in step S9 to the above-mentioned formula 1 to obtain the fixing speed Vf and set the fixing speed Vf. The CPU 60a then advances the process to step S13.

In step S13, the CPU 60a starts printing the nth page. At this time, the fixing motor 390 of the fixing device 38 drives at a speed according to the fixing speed Vf set in step S11. In addition, the photoconductor drum motor 282 drives at a constant speed according to the transfer speed Vt. The CPU 60a then advances the process to step S15.

In step S15, the CPU 60a waits for printing of the nth page to finish (S15: NO). Then, when printing of the nth page finishes (S15: YES), the CPU 60a advances the process to step S17.

In step S17, the CPU 60a determines whether all printing according to the print job has been completed, i.e., whether there is a next page to be printed. If all printing according to the print job has been completed (S17: YES), the CPU 60a ends the print control task. On the other hand, if all printing according to the print job has not yet been completed (S17: NO), the CPU 60a advances the process to step S19.

In step S19, the CPU 60a increments the value of the variable n, which represents the number of the page to be printed. The CPU 60a then returns the process to step S5.

In this way, according to the first embodiment, the printing rate Rx is derived based on the image data to be printed. Then, with the transfer speed Vt constant, the fixing speed Vf is controlled based on the printing rate Rx, in other words, the speed difference ΔV between the transfer speed Vt and the fixing speed Vf is controlled, and more specifically, the speed difference ΔV is controlled so that the deflection of the paper in the section from the transfer nip Nt to the fixing nip Nf is constant. This avoids the inconvenience of excessive deflection of the paper, that is, prevents toner staining of the rear end of the paper. In other words, compared to the technology disclosed in the aforementioned Patent Document 1, which requires a sensor called a slack detection unit, toner staining of the rear end of the paper can be prevented with a lower cost and simpler configuration.

The CPU 60a that executes step S7 of the print control task, or more precisely, the control unit 60 that includes the CPU 60a, is an example of a print rate derivation means according to the present disclosure. The control unit 60 is also an example of a control means according to the present disclosure.

[Second embodiment]
Next, a second embodiment of the present disclosure will be described.

In this second embodiment, when the state of the paper meets the specified control execution conditions, the speed difference ΔV (fixing speed Vf) is controlled based on the print rate Rx in the same manner as in the first embodiment. On the other hand, when the state of the paper does not meet the control execution conditions, the speed difference ΔV is kept constant, that is, the speed difference ΔV is not controlled based on the print rate Rx. In other words, when the state of the paper does not meet the control execution conditions, the fixing speed Vf is controlled so that the speed difference ΔV becomes its reference value (for example, 1.5% as a ratio) regardless of the print rate Rx.

The paper aspect referred to here includes the length dimension La of the paper in the conveying direction. When the length dimension La of the paper is equal to or greater than a predetermined length threshold Lb (La≧Lb), the length dimension La of the paper is deemed to conform to the control execution condition. In addition, the paper aspect includes the thickness dimension ta of the paper, and more specifically, includes a physical quantity related to the thickness dimension ta of the paper, such as the basis weight Ga (the mass per 1 ^m2 , and the larger this value, the greater the correlation with the thickness dimension of the paper). When the basis weight Ga of the paper is less than a predetermined basis weight threshold Gb (Ga<Gb), in other words, when the thickness dimension ta of the paper is less than a predetermined thickness threshold tb (ta<tb), the basis weight Ga (thickness dimension ta) of the paper is deemed to conform to the control execution condition. That is, when the length dimension La of the paper is equal to or greater than a predetermined length threshold Lb, or when the basis weight Ga of the paper is less than a predetermined basis weight threshold Gb, the paper aspect is deemed to conform to the control execution condition.

Here, the larger (longer) the length dimension La of the paper, the more the paper accumulates warping as it is transported through the section from the transfer nip Nt to the fixing nip Nf, and the greater the warping near the rear end of the paper, making it more likely that toner stains will occur at the rear end of the paper. In other words, if the length dimension La of the paper is relatively small (short), toner stains will be less likely to occur at the rear end of the paper. Also, the smaller the basis weight Ga of the paper, that is, the smaller (thinner) the thickness dimension ta of the paper, the more likely the paper will warp, and therefore the more likely that toner stains will occur at the rear end of the paper. In other words, if the basis weight Ga of the paper is relatively large, that is, if the thickness dimension ta of the paper is relatively large (thick), toner stains will be less likely to occur at the rear end of the paper.

In light of these points, in this second embodiment, if the length dimension La of the paper is equal to or greater than a predetermined length threshold Lb, or if the basis weight Ga of the paper is less than a predetermined basis weight threshold Gb, the state of the paper is deemed to meet the control execution conditions, and since the paper is in a state in which toner stains at the rear end are likely to occur, the speed difference ΔV is controlled based on the print rate Rx in the same manner as in the first embodiment. If the state of the paper does not meet the control execution conditions, that is, if the paper is in a state in which toner stains at the rear end are unlikely to occur, the speed difference ΔV is not controlled based on the print rate Rx, and the speed difference ΔV is kept constant; more specifically, the fixing speed Vf is controlled so that the speed difference ΔV is its reference value regardless of the print rate Rx.

The length threshold Lb is, for example, 300 mm, but is not limited to this and may be set to any value. The basis weight threshold Gb is, for example, 70 g/ ^m2 , but is not limited to this and may be set to any value.

Furthermore, the auxiliary memory unit 66 described above (FIG. 7) stores length information representing the length dimension La of the paper, more precisely, the paper size information consisting of the length dimension La of the paper and the width dimension, which is the dimension in the direction perpendicular to the conveying direction of the paper. In addition, the auxiliary memory unit 66 stores thickness information representing the thickness dimension ta of the paper, more precisely, grammage information representing the grammage Ga of the paper. These length information and grammage information are stored in the auxiliary memory unit 66 in a state associated with the paper feed cassette 44a of the paper feed unit 44, for example, by operating the operation unit 64, and in particular, in a state associated with each paper feed cassette 44a when multiple paper feed cassettes 44a are provided. Therefore, when an arbitrary paper feed cassette 44a is specified as the paper feed source during printing, the length dimension La and grammage Ga of the paper used for the printing are recognized based on the paper length information and grammage information associated with the paper feed cassette 44a specified as the paper feed source. The auxiliary memory unit 66, in which the length information and basis weight information of this paper is stored, is an example of a storage means according to the present disclosure.

In this second embodiment, the print control task is also executed by the CPU 60a, but the print control task in this second embodiment is executed according to the flow shown in Figures 9 and 10.

Referring to these Figures 9 and 10, according to the print control task in this second embodiment, the CPU 60a first analyzes the print job in step S102, similar to step S1 of the print control task in the first embodiment, and recognizes the number of pages of the document and the size of the paper to be printed on, and further recognizes the length dimension La and basis weight Ga of the paper. The CPU 60a then proceeds to step S103.

In step S103, the CPU 60a determines whether the state of the paper satisfies the control execution conditions, that is, whether the length dimension La of the paper is equal to or greater than the length threshold Lb, and whether the basis weight Ga of the paper is less than the basis weight threshold Gb. If the state of the paper satisfies the control execution conditions, that is, if the length dimension La of the paper is equal to or greater than the length threshold Lb, or if the basis weight Ga of the paper is less than the basis weight threshold Gb (S103: YES), the CPU 60a advances the process to step S105. On the other hand, if the state of the paper does not satisfy the control execution conditions, that is, if the length dimension La of the paper is less than the length threshold Lb (La<Lb) and the basis weight Ga of the paper is equal to or greater than the basis weight threshold Gb (Ga≧Gb) (S103: NO), the CPU 60a advances the process to step S123, which will be described later.

In step S105, the CPU 60a sets the variable n, which indicates the number of the page to be printed, to its initial value of 1, in the same way as in step S3 of the print control task in the first embodiment. After this, in steps S107 to S121, the CPU 60a performs the same processing as in steps S5 to S19 of the print control task in the first embodiment. A description of the processing in steps S107 to S121 will be omitted.

In contrast, when the CPU 60a advances the process from step S103 to step S123, it sets the fixing speed Vf in step S123, specifically, sets the fixing speed Vf based on the following formula 2. Note that formula 2 is obtained by removing the correction coefficient β from formula 1, in other words, by applying a value of 100% as the correction coefficient β in formula 1. After executing step S123, the CPU 60a advances the process to step S125.

"Formula 2"
Vf=Vt×α

In step S125, the CPU 60a sets the variable n, which represents the number of the page to be printed, to its initial value of 1. The CPU 60a then proceeds to step S127.

In step S127, the CPU 60a expands the image data of the nth page of the document. The CPU 60a then advances the process to step S129.

In step S129, the CPU 60a derives the printing rate Rx based on the image data developed in step S127. The derived printing rate Rx is used to calculate the amount of toner required for the toner supply device 42 to supply toner to the developing device 34. The CPU 60a then advances the process to step S131.

In step S131, the CPU 60a starts printing the nth page. At this time, the fixing motor 390 of the fixing device 38 is driven at a speed according to the fixing speed Vf set in step S123. The CPU 60a then advances the process to step S133.

In step S133, the CPU 60a waits for printing of the nth page to finish (S133: NO). Then, when printing of the nth page finishes (S133: YES), the CPU 60a advances the process to step S135.

In step S135, the CPU 60a determines whether all printing according to the print job has been completed, i.e., whether there is a next page to be printed. If all printing according to the print job has been completed (S135: YES), the CPU 60a ends the print control task. On the other hand, if all printing according to the print job has not yet been completed (S135: NO), the CPU 60a advances the process to step S137.

In step S137, the CPU 60a increments the value of the variable n, which represents the number of the page to be printed. The CPU 60a then returns the process to step S127.

In this way, according to the second embodiment, the speed difference ΔV based on the print rate Rx is controlled in the same manner as in the first embodiment only when the situation is such that toner stains on the rear edge of the paper are likely to occur. This is beneficial in reducing the burden on the control unit 60, including the CPU 60a.

In the second embodiment, the state of the paper is deemed to conform to the control execution condition when the length dimension La of the paper is equal to or greater than the predetermined length threshold Lb, or when the basis weight Ga of the paper is less than the predetermined basis weight threshold Gb, but this is not limited to the above. In other words, the state of the paper may be deemed to conform to the control execution condition when the length dimension La of the paper is equal to or greater than the predetermined length threshold Lb and the basis weight Ga of the paper is less than the predetermined basis weight threshold Gb. In addition, the basis weight Ga of the paper is used as an index of the thickness dimension La of the paper, but a physical quantity other than the basis weight Ga, such as the ream weight of the paper, may be used. Furthermore, a suitable means for detecting the thickness dimension ta of the paper itself may be provided, and a determination of whether the state of the paper conforms to the control execution condition may be made based on whether the thickness dimension ta of the paper detected by the means is less than the thickness threshold tb.

[Third Example]
Next, a third embodiment of the present disclosure will be described.

This third embodiment is based on the first embodiment. In addition, in this third embodiment, the fixing speed Vf is changed so that the speed difference ΔV becomes smaller from the point in time when a predetermined position on the paper, which is a distance Lc away from the leading edge of the paper toward the trailing edge, reaches the fixing nip portion Nf, i.e., such control is added.

That is, as shown in FIG. 3 (and FIG. 4), if we look at a sheet of paper (at a certain moment) being transported through the section from the transfer nip Nt to the fixing nip Nf, the amount of paper fed into this section at the transfer speed Vt is greater than the amount of paper fed out from that section at the fixing speed Vf, and the paper bends to absorb this, and this bending continues to increase until the rear end of the paper passes through the transfer nip Nt. In other words, the length of the paper in the section from the transfer nip Nt to the fixing nip Nf increases at a pace corresponding to the speed difference ΔV, and the bending increases to absorb this. Then, the bending reaches its maximum when the rear end of the paper passes through the transfer nip Nt. Also, the bending of the paper increases as the length dimension La of the paper increases. For example, when A4 size (297 mm x 210 mm) paper is transported along its short side (when the length dimension La of the paper in the transport direction is 210 mm) and A3 size (420 mm x 297 mm) paper is transported along its long side (when the length dimension La of the paper in the transport direction is 420 mm), A3 size paper will bend more during transport than A4 size paper. Therefore, the larger the length dimension La of the paper in the transport direction, the more likely it is that toner stains will occur at the rear edge of the paper.

Therefore, in this third embodiment, the fixing speed Vf is changed so that the speed difference ΔV becomes smaller from the time when the predetermined position, which is a position that is a predetermined distance Lc away from the leading edge of the paper toward the trailing edge, reaches the fixing nip Nf. In other words, for a portion of the paper that is an area from the predetermined position to the trailing edge of the paper, the paper is transported at a fixing speed Vf that makes the speed difference ΔV smaller at the fixing nip Nf. This suppresses an increase in deflection in a portion of the paper that is on the trailing edge side of the predetermined position, and as a result, toner stains on the trailing edge of the paper are more reliably prevented. Note that the predetermined distance Lc is, for example, 300 mm, but the value of this predetermined distance Lc may be other than this, or may be arbitrarily set. Also, even if the fixing speed Vf is changed, the relationship (state) that the fixing speed Vf is lower than the transfer speed Vt is maintained.

In this third embodiment, the print control task is also executed by the CPU 60a, but in this third embodiment, steps S201 and S203 shown in FIG. 11 are further provided in the print control task in the first embodiment (FIG. 8), specifically between steps S13 and S15.

That is, according to the print control task in this third embodiment, the CPU 60a starts printing the nth page in step S13, and then proceeds to step S201. Then, in step S201, the CPU 60a waits for the timing to change the fixing speed Vf, that is, for a predetermined position that is a predetermined distance Lc away from the leading edge of the paper toward the trailing edge to reach the fixing nip Nf (S201: NO). Then, when the predetermined position of the paper reaches the transfer nip Nt (S201: YES), the CPU 60a proceeds to step S203.

Whether the predetermined position of the paper has reached the fixing nip portion Nf is determined based on the timing when the registration roller 52a starts to rotate, the fixing speed Vf, and the predetermined distance Lc. Specifically, a detection means is provided at an appropriate position upstream of the fixing nip portion Nf in the paper transport direction to detect that the leading edge of the paper has passed through. Then, based on the detection result by this detection means, the time when the leading edge of the paper has passed through the fixing nip portion Nf is predicted. In this third embodiment, the registration roller 52a functions as the detection means. Furthermore, since the distance from the registration roller 52a to the fixing nip portion Nf is known, the time from when the registration roller 52a starts to rotate until the leading edge of the paper reaches the fixing nip portion Nf can be calculated from the distance from the registration roller 52a to the fixing nip portion Nf and the transfer speed Vt. In addition, since the fixing speed Vf is also known, the time from when the leading edge of the paper reaches the fixing nip Nf until the predetermined position of the paper passes through the fixing nip Nf can be calculated based on the fixing speed Vf and the predetermined distance Lc. In this way, whether the predetermined position of the paper has reached the fixing nip Nf is determined based on the timing when the registration roller 52a starts to rotate, the fixing speed Vf, and the predetermined distance Lc.

In step S203, the CPU 60a changes the fixing speed Vf so that the speed difference ΔV becomes smaller, and more specifically sets the fixing speed Vf based on the following formula 3. Note that formula 3 is obtained by multiplying the above-mentioned formula 1 by a modification coefficient γ. This modification coefficient γ is a value greater than 100% (=1), for example, a value of approximately 100.2% to 100.3%. As a result, the fixing speed Vf is modified to an extent according to the modification coefficient γ, that is, the speed difference ΔV becomes smaller by approximately 0.2% to 0.3%. After executing this step S203, the CPU 60a advances the process to step S15.

"Formula 3"
Vf=Vt×α×β×γ

In this way, according to the third embodiment, the fixing speed Vf is changed so that the speed difference ΔV becomes smaller from the point when a predetermined position of the paper, which is a predetermined distance Lc from the leading edge of the paper toward the trailing edge, reaches the fixing nip portion Nf. This reduces the deflection near the trailing edge of the paper, and more reliably prevents toner stains on the trailing edge of the paper.

Note that this third embodiment is based on the first embodiment, but may also be based on the second embodiment. In this case, steps S201 and S203 shown in FIG. 11 are provided between steps S115 and S117 of the print control task (FIG. 9) in the second embodiment.

[Other application examples]
The above-described embodiments are specific examples of the present disclosure and do not limit the technical scope of the present disclosure. The present disclosure can be applied to aspects other than the embodiments.

For example, the print control task shown in FIG. 8 shows an example in which the number of copies is one, but even if the number of copies is multiple, the print rate Rx is derived appropriately, and the correction coefficient β is determined based on the print rate Rx, and the fixing speed Vf is controlled, that is, the speed difference ΔV is controlled. For this reason, the print control task is configured appropriately.

In addition, the correction coefficient β is determined by comparing the print rate Rx with the correction coefficient table 200, but this is not limiting. For example, the correction coefficient β may be determined (calculated) based on the following formula 4, which uses the print rate Rx as a variable. This formula 4 is a mathematical expression of the relationship between the correction coefficient β and the print rate Rx shown in FIG. 5.

"Formula 4"
β = f(Rx)

Note that the relationship between the correction coefficient β and the printing rate Rx shown in FIG. 5 is an example, and the relationship between the correction coefficient β and the printing rate Rx varies depending on the specifications of the image forming device 10. In other words, the relationship between the correction coefficient β and the printing rate Rx is determined by experiment for each specification of the image forming device 10.

Furthermore, while the print control tasks shown in FIG. 8 and the like relate to the printer function of the image forming device 10, similar print control can also be applied to the copy function and fax function (fax receiving function involving printing) of the image forming device 10.

In addition, the fixing device 38 is not limited to the pad type, and may be, for example, a roller type that employs a fixing roller as a fixing member. However, in the pad type fixing device 38, the entrance side 400 of the fixing nip portion Nf is narrower than that of the roller type, as shown in FIG. 12. The dashed line 500 in FIG. 12 indicates the fixing roller (outer circumference) when the fixing device 38 is a roller type. Therefore, when the leading edge of the paper enters the fixing nip portion Nf, the fixing device 38 of the pad type is more likely to contact the fixing belt 382 before the fixing nip portion Nf, which means that image defects are more likely to occur. Therefore, the fixing device 38 of the pad type is set to have a larger deflection of the paper in the section from the transfer nip portion Nt to the fixing nip portion Nf than that of the roller type, and as a result, the rear end of the paper is more likely to be stained with toner. Therefore, this disclosure is particularly useful for configurations that use a pad-type fixing device 38, which is prone to toner stains on the trailing edge of paper.

Incidentally, if the fixing device 38 is a roller type, the above-mentioned relative coefficient α is set to a value corresponding to just under 1%, that is, a value that is just under 1% smaller than 100% is set. In other words, when the printing rate Rx is 100%, the fixing speed Vf is set to just under 1% smaller than the transfer speed Vt.

In addition, in each of the above-described embodiments, a monochrome image forming device 10 is given as an example, but the present disclosure can also be applied to a color image forming device, particularly a tandem type image forming device having an intermediate transfer device.

The image forming device 10 in each embodiment is a multifunction device, but the present disclosure can also be applied to image forming devices other than multifunction devices, such as dedicated printers, copiers, and fax machines.

Furthermore, the present disclosure is not limited to being provided in the form of an image forming apparatus, but can also be provided in the form of a method for controlling an image forming apparatus.

REFERENCE SIGNS LIST 10 image forming apparatus 26 image forming section 28 photoconductor drum 36 transfer device 38 fixing device 60 control section 60a CPU
60b: Main memory unit 362: Transfer roller 382: Fixing belt 384: Heat source 386: Fixing pad 388: Pressure roller 390: Fixing motor Nf: Fixing nip portion Nt: Transfer nip portion

Claims (7)

an image carrier that is rotatably provided and has a surface on which a toner image is formed;
A first drive source that rotates the image carrier;
a transfer means having a transfer member rotatably provided with its surface in contact with the surface of the image carrier, and transferring the toner image onto the image recording medium by nipping and conveying a sheet-shaped image recording medium in a transfer nip portion, which is a contact portion between the surface of the transfer member and the surface of the image carrier;
a fixing means including a fixing member that is heated to a predetermined temperature and rotatably provided, and a pressure member that is rotatably provided with its surface in contact with the surface of the fixing member, and that fixes the toner image onto the image recording medium by nipping and conveying the image recording medium in a fixing nip portion that is a contact portion between the surface of the fixing member and the surface of the pressure member;
a second drive source that rotates the fixing member or the pressure member;
a printing rate deriving unit that derives a printing rate of the toner image based on image data provided for a process of forming the toner image on the image carrier; and
an image forming apparatus including a control means for controlling the first drive source and the second drive source so that a difference is provided between a transfer speed, which is a surface speed of the image carrier or the transfer member, and a fixing speed, which is a surface speed of the fixing member or the pressure member, and a speed difference, which is the difference between the transfer speed and the fixing speed, has a magnitude corresponding to the printing rate derived by the printing rate derivation means. The image forming apparatus according to claim 1, wherein the control means controls the first drive source and the second drive source to provide the speed difference such that the fixing speed is lower than the transfer speed, and to reduce the speed difference as the printing rate decreases. The image forming apparatus according to claim 1, wherein the control means controls the first drive source and the second drive source so that the fixing speed corresponds to the printing rate while the transfer speed is kept constant. The image recording medium is further provided with a storage means for storing length information representing a length dimension of the image recording medium in a transport direction of the image recording medium and thickness information representing a thickness dimension of the image recording medium,
2. The image forming apparatus according to claim 1, wherein the control means controls the first drive source and the second drive source so that the speed difference is a magnitude corresponding to the printing rate when one or both of the length dimension based on the length information and the thickness dimension based on the thickness information meet predetermined control execution conditions. a detection unit configured to detect that a downstream end portion of the image recording medium in the transport direction has passed upstream of the fixing nip portion in the transport direction of the image recording medium,
2. The image forming apparatus according to claim 1, wherein the control means controls the first drive source and the second drive source so as to reduce the speed difference at a predetermined time after the downstream end of the image recording medium in the transport direction, predicted based on the detection result by the detection means, passes through the fixing nip portion. The fixing means is
an endless belt-like fixing belt as the fixing member;
a heating means for heating the fixing belt to the predetermined temperature;
an opposing member that is provided at a position facing the pressure member on the inside of the fixing belt, and that sandwiches the fixing belt between the opposing member and a surface of the pressure member to form the fixing nip portion between an outer surface of the fixing belt and the surface of the pressure member; and
6. The image forming apparatus according to claim 1, further comprising a pressure roller as the pressure member. A control method for an image forming apparatus, comprising:
The image forming apparatus includes:
an image carrier that is rotatably provided and has a surface on which a toner image is formed;
a first drive source that rotates the image carrier;
a transfer means having a transfer member rotatably provided with its surface in contact with the surface of the image carrier, and transferring the toner image onto the image recording medium by nipping and conveying a sheet-shaped image recording medium in a transfer nip portion, which is a contact portion between the surface of the transfer member and the surface of the image carrier;
a fixing unit including a fixing member that is heated to a predetermined temperature and rotatably provided, and a pressure member that is rotatably provided with its surface in contact with the surface of the fixing member, and that fixes the toner image onto the image recording medium by nipping and conveying the image recording medium in a fixing nip portion that is the contact portion between the surface of the fixing member and the surface of the pressure member;
a second drive source that rotates the fixing member or the pressure member,
a printing rate deriving step of deriving a printing rate of the toner image based on image data provided for a process of forming the toner image on the image carrier; and
a control step of controlling the first drive source and the second drive source so that a speed difference between a transfer speed, which is a surface speed of the image carrier or the transfer member, and a fixing speed, which is a surface speed of the fixing member or the pressure member, is set to a value corresponding to the printing rate derived by the printing rate derivation step, and the first drive source and the second drive source are controlled so that a speed difference between the transfer speed and the fixing speed is set to a value corresponding to the printing rate derived by the printing rate derivation step.

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