JPH1152642A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable to feed a paper sheet of which length in a carrying direction is shorter than an interval between a transfer part and a fixing part, and form a carrying passage of a recessed form as necessary. SOLUTION: A carrying means 30 to carry a paper sheet, and a guide 40 to guide the sheet are provided between a transfer part 10 and a fixing part 20, the carring means 30 is capable of taking a first position for carrying the sheet in a recessed form from the transfer part 10 to the fixing part 20 in cooperation with the guide 40, and a second position to carry the sheet straight, and it takes the first position in the case where length of the sheet in a carrying direction is longer than a recessed carrying passage S1, and the second position in the case where the length of the sheet in the carrying direction is shorter than an interval between the transfer part 10 and the fixing part 20. The carrying means 30 is composed of a carrying belt having irregularities in the surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus using an electrophotographic technique, such as a printer, a facsimile, and a copying machine. More specifically, the present invention relates to an image forming apparatus for transferring and fixing a toner image on a sheet (a sheet-shaped recording medium including an envelope), and more particularly to a technique for conveying a sheet from a transfer section to a fixing section.

[0002]

2. Description of the Related Art Generally, an image forming apparatus such as a printer, a facsimile, a copying machine or the like using an electrophotographic technique forms an electrostatic latent image on the surface of a photoreceptor, and a toner as a developer is formed on the electrostatic latent image. To form a visible image (toner image), and the toner image is transferred directly to the paper at the transfer unit or via an intermediate transfer medium, and then the toner image is fixed to the paper at the fixing unit and discharged. ing.

[0003] In this type of image forming apparatus, for example, as shown in FIG.
a, 1b, and 2a, 2b, in which the transfer unit 1 and the fixing unit 2 convey the sheet S (apply a conveying force to the sheet). In such a configuration, it is ideal that the transport speed V1 of the sheet S in the transfer unit 1 and the transport speed V2 in the fixing unit 2 are completely equal. However, there is a limit in the accuracy of the parts in practice. , It is impossible to make them completely equal.

If it is assumed that V1> V2 and the paper S is linearly guided from the transfer unit 1 to the fixing unit 2 by a guide (not shown) as shown by a dashed line S in FIG. Bites into the roller pair 2a, 2b of the fixing section 2, an impact is temporarily applied to the sheet S.
That is, usually, the leading end Sa of the paper S is the roller 2a.
Alternatively, the sheet S is guided to the pressing portion of both rollers after hitting the peripheral surface of the roller 2b. At this time, if V1> V2, the paper S has a speed higher than the speed at which the leading end Sa is wound by the roller pair 2a, 2b. Since the sheet is fed by the transfer unit 1 at a high speed, the leading end Sa is a pair of rollers 2a and 2b of the fixing unit.
, And this is an impact. This impact propagates through the sheet S and reaches the transfer section 1, so that the transfer state becomes unstable, and the image (toner image transferred to the sheet)
Will be disturbed.

On the other hand, if V1 <V2, and the paper S is linearly guided from the transfer unit 1 to the fixing unit 2 by a guide (not shown) as shown by a dashed line S in FIG.
Although the above-described impact does not occur, this time, the leading end Sa of the paper S bites into the roller pair 2a, 2b of the fixing unit 2, and
When the sheet S is subjected to the conveyance force, the sheet S is constantly subjected to a pulling force until the rear end Sb of the sheet S comes off the transfer unit 1. Since such a pulling force acts on the transfer portion (a portion where the rollers 1a and 1b are sandwiched), the transfer state becomes unstable, and the image is disturbed.

Therefore, conventionally, the paper S
Compared to the transport speed V1 of the fixing unit 2
Is slightly increased (that is, slightly V1
<V2), as shown in FIG. 4, the sheet S is moved from the transfer unit 1 to the fixing unit 2 in a direction perpendicular to its transport direction as indicated by a two-dot chain line S ′ (perpendicular to the plane of FIG. 4). (In the direction in which the guide 3 moves), the guide 3 for guiding in a concave shape is provided.

With such a configuration, since V1 <V2, the occurrence of the impact can be prevented, and the leading end Sa of the sheet S is fixed to the roller pair 2a, 2b of the fixing unit 2.
Even if the sheet S is in a state of being conveyed by the fixing unit 2, the sheet S is in a state of being slackened in a concave shape by the guide 3.
No pulling force acts on. Therefore,
By configuring the guide 3 so that the slack does not disappear before the trailing end Sb of the paper S comes off the transfer unit 1, it is possible to prevent the image in the transfer unit 1 from being disturbed.

[0008]

In the above-described image forming apparatus, there is a demand for forming an image (including characters and the like) on an envelope as a sheet.

However, in general, the fixing unit 2 includes a relatively hard heating roller (for example, a metal roller) 2a having a heat source therein, and a relatively soft pressure roller (for example, a rubber roller) 2b which is pressed and driven by the heating roller. Therefore, when an envelope is passed through the pair of rollers 2a and 2b, the following problem occurs.

FIG. 5 is a schematic diagram showing a state in which the envelope 4 is being conveyed by the pair of rollers 2a and 2b of the fixing unit.

As shown in FIG. 1, when the angular velocity of the heating roller 2a is ω, the radius is r, and the thickness of a sheet (for example, paper) constituting the envelope 4 by stacking two sheets is t, the envelope 4
Sheet 4a on the upper side (heating roller 2a side)
Transport speed Va, sheet 4 on the lower side (pressure roller 2b side)
The transport speed Vb of b is Va = (r + t / 2) ω Vb = (r + 3t / 2) ω, and a transport speed difference of tω occurs between the upper sheet 4a and the lower sheet 4b. The reason is that, as shown in the figure, the pressure contact portion 2 between the heating roller 2a and the pressure roller 2b
In n, the soft pressure roller 2b is driven and rotated while deforming along the outer periphery of the hard heating roller 2a, and the upper sheet 4a and the lower sheet 4b constituting the envelope 4 are This is because the upper sheet 4a and the lower sheet 4b are conveyed while sliding relative to each other because they are not bonded.

For this reason, a bulge 4c is formed near the end of the envelope 4 due to the accumulation of the difference in conveyance speed between the upper sheet 4a and the lower sheet 4b.
When passing between a and 2b, there is a problem that it is crushed and becomes wrinkled. This problem occurs not only when the sheet is an envelope, but also when two or more sheets are stacked and their ends are joined.

In order to solve such a problem, the following measures (1) and (2) can be adopted.

(1) The heating roller 2a is not made of a hard material, but is made of a material (for example, a rubber roller) having substantially the same elasticity as the pressure roller 2b. Then, both are deformed into a substantially planar shape to bring the press contact portion 2n into a substantially planar state.

(2) The pressing force between the heating roller 2a and the pressing roller 2b is reduced to reduce the amount of deformation of the pressing roller 2b at the pressing portion 2n.

According to the above-mentioned measure (1), the above-mentioned difference in the conveying speed can be eliminated, and according to the measure (2), the above-mentioned difference in the conveying speed can be reduced.

However, when the measure (1) is adopted, the heating roller is made of an elastic layer having high heat insulation (for example, a rubber layer).
It is necessary to coat thickly. With such a configuration,
Since the thermal conductivity is extremely poor, the surface temperature of the heating roller 2a decreases during the passage of the sheet (during the fixing operation), and the fixing strength unevenness and the toner gloss unevenness easily occur even in one sheet. Become. Therefore, it is difficult to perform a high-speed fixing operation. In order to solve this, the temperature of the heater disposed at the center of the heating roller must be increased. However, the temperature of the interface between the elastic layer and the center increases abnormally, and the elastic layer is easily peeled. For example, the mechanical durability is reduced.

That is, according to the measure (1), it is difficult to perform high-speed fixing while maintaining high image quality and mechanical durability.

When the measure (2) is adopted, the amount of deformation of the pressure roller 2b at the pressure contact portion 2n is reduced, so that the length of the pressure contact portion 2n required to obtain desired fixing characteristics (paper transport) In order to obtain the length in the direction, the diameters of the heating roller 2a and the pressure roller 2b must be increased, which results in an increase in the size of the apparatus and an increase in power consumption.

The present inventors have proposed a third measure to solve the above-mentioned problem, namely, the problem that when the envelope is passed through the roller pair of the fixing unit, the vicinity of the end of the envelope becomes wrinkled. As shown in FIG. 6, it has been proposed to send the envelope 4 in the short direction.

When the envelope 4 is fed in the short direction as indicated by an arrow a in FIG. 6, the conveyance of the upper sheet 4a (see FIG. 5) and the lower sheet 4b is more difficult than in the case where the envelope 4 is sent in the long direction. Accumulation of the relative displacement generated between the two sheets due to the speed difference is reduced, so that the bulge 4c generated near the end of the envelope 4 is also reduced, and as a result, it is possible to suppress the generation of wrinkles.

However, as described above with reference to FIG. 4, in the conventional apparatus, the sheet S is guided in a concave shape from the transfer section 1 to the fixing section 2 as shown by a two-dot chain line S '. A guide 3 is provided. The guide 3 must be capable of smoothly guiding the sheet and capable of forming the above-described desired slack of the sheet. It must be loose and have the desired length. For this reason, the interval L1 between the transfer unit 1 and the fixing unit 2 (the interval between the pressure contact portions of the roller pairs 1a, 1b, 2a, 2b) must be relatively large.

In such a situation, when the envelope 4 is to be fed in the short direction, as shown in FIG. 6, the envelope 4 is shorter than the distance L1 between the transfer unit 1 and the fixing unit 2 in a normal apparatus. The direction length L2 is shorter, and as a result,
A situation occurs in which the envelope 4 cannot be sent in the short direction. Such a situation occurs not only when the envelope is fed in the short direction, but also when the length of the sheet in the transport direction is shorter than the interval between the transfer unit and the fixing unit.

[0024] An object of the present invention is to solve the above-mentioned problems, and to feed a sheet whose length in the transport direction is shorter than the interval between the transfer section and the fixing section, and to send the sheet as needed. To provide an image forming apparatus capable of forming the concave-shaped transport path.

[0025]

According to another aspect of the present invention, there is provided an image forming apparatus, comprising: a transfer section for transferring a toner image onto a sheet; and a transfer section on which the toner image is transferred by the transfer section. A fixing unit for fixing the toner image to the image forming apparatus, wherein a conveying unit for conveying the sheet and a guide for guiding the sheet are provided between the transfer unit and the fixing unit. A transport unit configured to transport the sheet in a concave shape from the transfer unit toward the fixing unit in cooperation with the guide, as viewed from a direction orthogonal to the sheet transport direction; A second position for transporting the sheet linearly compared to the path, and the first position when the length of the sheet in the transport direction is longer than the length of the concave transport path; Of the transfer unit and the fixing unit If shorter than the interval, characterized in that taking the second position.

The image forming apparatus according to the second aspect is the first aspect.
In the above-described image forming apparatus, the transporting unit may include a transport belt having a surface with irregularities.

[0027]

According to the image forming apparatus of the present invention, there is provided a transfer section for transferring a toner image onto a sheet, and a fixing section for fixing the toner image on the sheet on which the toner image has been transferred by the transfer section. Having a transport unit for transporting the paper and a guide for guiding the paper between the transfer unit and the fixing unit, and the transport unit, when viewed from a direction orthogonal to the transport direction of the paper, A first position for conveying the sheet in a concave form from the transfer section toward the fixing section in cooperation with the guide, and a second position for conveying the sheet linearly compared to the conveyance path of the concave sheet. The first position is taken when the length of the paper in the transport direction is longer than the length of the concave transport path, and the length of the paper in the transport direction is larger than the distance between the transfer unit and the fixing unit. When the length is short, the second position is adopted. Action effects can be obtained.

That is, when the length of the sheet in the transport direction is longer than the length of the concave transport path, the transport unit takes the first position. As a result, the sheet is conveyed by the conveying unit and guided by the guide, and is formed into a concave shape and conveyed from the transfer unit to the fixing unit.

Therefore, disturbance of the image in the transfer portion is prevented.

If the length of the sheet in the transport direction is shorter than the distance between the transfer section and the fixing section, the transport means adopts the second position. As a result, the sheet is conveyed linearly from the transfer unit to the fixing unit as compared with the concave sheet conveyance path.

Therefore, the envelope and the like can be transported in the short direction.

In this case, the paper transport path does not have the concave shape, but the length of the paper is shorter than the interval between the transfer unit and the fixing unit. Therefore, when the leading edge of the paper reaches the fixing unit, The trailing edge of the sheet has passed through the transfer section. Therefore, the above-mentioned problem, that is, the impact when the leading edge of the sheet reaches the fixing section is transmitted to the sheet and reaches the transfer section, so that the transfer state becomes unstable, or the conveyance force by the fixing section causes a pulling force on the sheet. This pulling force acts on the transfer portion, and the image is not disturbed due to the unstable transfer state.

Since the envelope and the like can be conveyed in the short direction, the above-mentioned wrinkles can be suppressed.

Further, in this case, since the sheet is transported more linearly than the transport path of the concave sheet, the arrival time from the transfer section to the fixing section is shortened. Therefore,
It is also possible to improve the image forming speed.

According to the image forming apparatus of the second aspect, in the image forming apparatus of the first aspect, the conveying means includes:
Since the transport belt is formed of a transport belt having a surface with irregularities, the paper can be transported reliably.

[0036]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a side view showing a main part of an embodiment of an image forming apparatus according to the present invention.

In FIG. 1, reference numeral 10 denotes a transfer unit, 20 denotes a fixing unit, 30 denotes a conveying unit, and 40 denotes a guide.

The transfer section 10 of this embodiment includes an intermediate transfer belt 11, a backup roller 12 that supports the intermediate transfer belt 11 from the inside, and an intermediate transfer belt 11 with respect to the backup roller 12. And a transfer roller 13 pressed against the transfer roller 13.

The intermediate transfer belt 11 is an endless image carrier belt, although only a part thereof is illustrated in the figure.
A toner image is carried on the outer peripheral surface of the intermediate transfer belt 11. More specifically, a photoreceptor (not shown) in contact with the intermediate transfer belt 11 is arranged on the outer periphery of the intermediate transfer belt 11, and an electrostatic latent image formed on the surface of the photoreceptor is developed with toner by developing means (not shown). The toner image is transferred and carried on the intermediate transfer belt 11 in a temporary transfer section (not shown). That is, the transfer unit 10
This is the secondary transfer unit. The intermediate transfer belt 11 is wound around a plurality of rollers other than the backup roller 12 and is driven to circulate in the direction indicated by the arrow in the figure. A cleaning unit 15 is disposed on the roller 14 via the intermediate transfer belt 11 so as to be in pressure contact with the roller 14.

On the intermediate transfer belt 11, toner images of four colors (for example, yellow, cyan, magenta, and black) are sequentially transferred at a primary transfer portion (not shown). Until the transfer is completed, the transfer roller 13 and the cleaning unit 15 are separated from the intermediate transfer belt 11. When the transfer of the four color toner images is completed, the paper S is supplied from the paper supply unit (not shown) to the transfer unit 10 and the transfer roller 1
3 and the cleaning means 15 are pressed against the intermediate transfer belt 11 to transfer the toner image (color image composed of four colors) on the intermediate transfer belt 11 to the sheet S in the transfer unit 10 and, after the transfer, to the intermediate transfer belt 11. The toner remaining on the top is removed by the cleaning unit 15.

The fixing section 20 has a heating roller 21 and a pressure roller 22.

The heating roller 21 is a relatively hard roller as a whole having a heat source 21a at the center, is rotatably supported by a frame 23, and is driven to rotate in a direction indicated by an arrow by a driving means (not shown). ing. The peripheral speed of the heating roller 21 is set slightly higher than the peripheral speed of the intermediate transfer belt 11.

The pressure roller 22 includes a metal shaft 22a,
And a roller portion 22b made of a relatively soft material (for example, rubber) fixed thereto, and both ends of the shaft 22a are rotatably supported by levers 24, 24 (only one is shown in FIG. 1). . One end of the lever 24 is rotatably supported on the frame 23 by a shaft 24a. Between the other end 24b and the frame 23, a tension spring 25 as a pressing means is provided.
Is provided. Since the shaft 22a of the pressure roller 22 is supported by an intermediate portion of the lever 24, the tension spring 2
5 functions as a pressurizing means. Therefore, the pressure roller 22 is pressed against the heating roller 21 by the action of the tension spring 25, and rotates following the heating roller 21.

On the outer peripheral surfaces of the heating roller 21 and the pressure roller 22, peeling claws 26 and 27 for preventing the paper from being wound around these rollers are arranged.

The transport means 30 comprises a transport belt 33 wound around a driving pulley 31 and a driven pulley 32.

The drive pulley 31 is fixed to a drive shaft 31a, and the driven pulley 32 is rotatably supported by a shaft 32a.

The driving pulley 31, the driven pulley 32,
As shown in FIG. 2, a plurality of sets of the transport belts 33 are provided at intervals.

The drive shaft 31a is rotatably supported by a pair of side frames 34, 34 as shown in FIG. One end of a pair of arms 35, 35 is rotatably supported by the drive shaft 31a.
A shaft 32a of the driven pulley 32 is rotatably supported at the other end of the shaft 5.

Therefore, the driven pulley 32 and its shaft 32
a and the conveyor belt 33 are rotatable with the pair of arms 35 around the drive shaft 31a.

That is, the transporting means 30 includes the drive shaft 3
It is rotatable around 1a, and can take a first position indicated by a solid line and a second position indicated by a virtual line in FIG.

A drive gear 3 is provided at one end of the drive shaft 31a.
1b, and a driving unit (not shown) is connected to the gear 31b, so that the conveying belt 33 is provided regardless of whether the conveying unit 30 is in the first or second position.
Are driven to circulate in the direction of the arrow in FIG. 1 at substantially the same peripheral speed as the peripheral speed of the heating roller 21. In addition, as shown in FIG. 3, it is desirable that the transport belt 33 be formed of a belt having a surface with irregularities 33a. Also, it is desirable to configure with a timing belt.

When the transport means 30 is at the first position (the position indicated by the solid line), the transport means 30 is positioned between the guide 40 and the direction perpendicular to the transport direction of the sheet (see S1) (the direction perpendicular to the plane of FIG. 1). In cooperation, the paper is conveyed from the transfer unit 10 to the fixing unit 20 in a concave shape as indicated by a virtual line S1. When the sheet S2 is at the second position (position of the virtual line), the sheet S2 is transported more linearly than the concave sheet transport path (see S1).

The switching means for switching between the first position and the second position can be appropriately configured. For example,
Eccentric cams 36, 36 are provided, and their cam surfaces are
A configuration in which the lower surface 35 is in contact with the lower surface may be employed. According to this configuration, when the eccentric cam 36 is at the position shown by the imaginary line in FIG. 1, the transport means 30 takes the first position, and the eccentric cam 36 is shifted by 180 ° from the position shown by the imaginary line in FIG. When rotated, the transport means 30 will assume the second position. Further, a unit for controlling the operation of the switching unit (for example, the eccentric cam 36) can be appropriately configured. For example,
A paper detector is provided at the preceding stage (upstream side in the paper transport direction) of the transfer unit 10, and the length of the paper (length in the transport direction) is detected by ON / OFF of the paper detector, and the transport path having a concave shape is provided. If the length is longer than the length (see S1), the switching unit is operated so that the transport unit 30 takes the first position, and the length of the sheet in the transport direction is the distance between the transfer unit 10 and the fixing unit 20. If it is shorter than the length of the straight path S2 when the transport means takes the second position, the switching means is operated so that the transport means 30 takes the second position.
Alternatively, a unit for detecting the length of the sheet is disposed in a sheet feeding unit (not shown), and the switching unit is operated in the same manner as above.

Reference numeral 37 denotes a guide member provided between the drive shaft 31a and the shaft 32a between the transport belts 33, and a tip 37a of the guide member protrudes toward the transfer unit 10.

The guide 40 is connected to the frame 23 of the fixing unit 20.
It is provided in. The guide 40 has a comb-like shape in plan view, and its tip 41 enters between the transport belts 33 as shown in FIG.

As shown in FIG. 1, a paper discharge path 50 for discharging the paper that has passed through the fixing unit 20 is formed downstream of the fixing unit 20 (downstream in the paper transport direction).
In this paper discharge path 50, paper discharge roller pairs 51 and 52 are arranged.

The above-described image forming apparatus operates as follows.

<When the length of the sheet is longer than the length of the concave transport path S1> In this case, the transport means 30 takes the first position (solid line position).

Accordingly, the sheet (S1) on which the toner image has been transferred by the transfer unit 10 is first conveyed while being conveyed from the transfer unit 10 while being guided by the conveying means 30 in a substantially horizontal state. Next, the toner image is fixed upward by being guided upward by the guide 40 to the fixing unit 20 and passing through a pressure contact portion 20n between the heating roller 21 and the pressure roller 22, whereby the paper discharge path 50 and the paper discharge roller It is discharged outside the machine via the pair 51,52.

Since the peripheral speed of the heating roller 21 is slightly higher than the peripheral speed of the intermediate transfer belt 11, when the sheet is conveyed by the heating roller 21 and the pressure roller 22, the concave shape is formed. The slack of the sheet formed by the transport path S1 is gradually reduced.
No. 1 is formed such that a slight slack still remains even when the trailing edge of the sheet leaves the transfer unit 10.

<Case where the length of the sheet is shorter than the interval between the transfer unit 10 and the fixing unit 20> In this case, the transporting unit 30 takes the second position (virtual line position).

Therefore, the sheet (S 2) on which the toner image has been transferred by the transfer unit 10 continues to receive the transfer force from the transfer unit 10 until the rear end of the sheet (S 2) passes through the transfer unit 10. And is transported substantially linearly toward the fixing unit 20. Next, when the trailing edge of the sheet passes through the transfer unit 10, the sheet (S2) is conveyed substantially linearly toward the fixing unit 20 by the conveyance unit 30 alone without contacting the guide 40. Thereafter, when the leading end of the paper reaches the fixing unit 20 and reaches the pressure contact portion 20n between the heating roller 21 and the pressure roller 22, the paper passes through the pressure contact portion 20n while being conveyed by the heating roller 21 and the pressure roller 22. As a result, the toner image is fixed, and is discharged out of the apparatus via the paper discharge path 50 and the paper discharge roller pair 51.

According to the above-described image forming apparatus, the transfer unit 10 for transferring the toner image onto the paper, and the transfer unit 10
And a fixing unit 20 for fixing the toner image on the sheet on which the toner image has been transferred, and a conveying unit 30 for conveying the sheet and a guide 40 for guiding the sheet between the transfer unit 10 and the fixing unit 20. And the transport means 30
When viewed from a direction perpendicular to the paper transport direction, the guide 4
0, the sheet is conveyed linearly as compared with the first position (solid line position) where the sheet is conveyed concavely from the transfer unit 10 toward the fixing unit 20 (solid line position). A second position (virtual line position), and the first position is taken when the length of the sheet in the transport direction is longer than the length of the concave transport path (S1), and the length of the sheet in the transport direction is taken. Is smaller than the distance between the transfer unit 10 and the fixing unit 20, the second
Therefore, the following operation and effect can be obtained.

(A) If the length of the paper in the transport direction is longer than the length of the concave transport path (S1), the transport means 30 adopts the first position. And is guided by the guide 40, and is conveyed from the transfer unit 10 to the fixing unit 20 in a concave shape.

Therefore, disturbance of the image in the transfer section 10 is prevented.

(B) If the length of the sheet in the transport direction is shorter than the distance between the transfer unit 10 and the fixing unit 20, the transport unit 30
Takes the second position, the paper is transferred from the transfer unit 10 to the fixing unit 20 linearly compared to the concave paper conveyance path (S1).
Transported to

Therefore, an envelope or the like (see a solid line S in FIG. 1)
Can be transported in the short direction.

In this case, the paper transport path (S 1) does not have the concave shape, but the length of the paper is shorter than the distance between the transfer unit 10 and the fixing unit 20.
When the number reaches 0, the rear end of the sheet has passed through the transfer unit 10. Therefore, the above-mentioned problem, that is, the impact when the leading edge of the sheet reaches the fixing section is transmitted to the sheet and reaches the transfer section, so that the transfer state becomes unstable, or the conveyance force by the fixing section causes a pulling force on the sheet. This pulling force acts on the transfer portion, and the image is not disturbed due to the unstable transfer state.

Since the envelope and the like can be conveyed in the short direction, the above-mentioned wrinkles can be suppressed. That is, the occurrence of wrinkles in the envelope can be suppressed without employing the above-described measures (1) and (2). As a result, (i) the heating roller 21 is formed of an elastic layer (for example, a rubber layer) having a high heat insulating property It is not necessary to apply a thick coating, and the thermal conductivity can be improved, so that a high-speed fixing operation without fixing unevenness can be performed. (Ii) When obtaining the length (the length in the paper transport direction) of the press contact portion 20n necessary to obtain desired fixing characteristics, the pressing roller 22 is made of a relatively soft material, and the pressing force is increased. The heating roller 21 and the pressing roller 2
It is not necessary to increase the diameter of 2, and therefore, it is possible to reduce the size of the device and reduce the power consumption.

(C) If the length of the sheet in the transport direction is shorter than the distance between the transfer unit 10 and the fixing unit 20, the transport unit 30
Takes the second position, and the sheet is conveyed linearly compared to the concave sheet conveyance path (S1).
, The time required to reach the fixing unit 20 is shortened. Therefore,
It is also possible to improve the image forming speed.

(D) As shown in FIG. 3, the transporting means 30 is constituted by a transport belt having a surface provided with irregularities 33a, so that the paper can be transported reliably. With such a configuration, the paper S can be transported by pressing the paper S by engaging the rear end of the paper S with the irregularities 33 a, so that the front end of the paper S is pressed against the heating roller 21 and the pressure roller 22. The part 20n can be surely eaten. When the sheet S is an envelope, the flap 4f (see FIGS. 1 and 6) is moved downward (to the conveyor belt 33).
Side), the trailing edge of the flap 4f and the unevenness 33a are engaged with each other, so that the sheet can be transported more reliably.

The embodiment of the present invention has been described above. However, the present invention is not limited to the above-described embodiment, and can be appropriately modified within the scope of the present invention.

For example, the conveying means may be constituted by a plurality of conveying rollers instead of the conveying belt.

[0075]

According to any of the image forming apparatuses of the first and second aspects, it is possible to feed a sheet whose length in the transport direction is shorter than the distance between the transfer section and the fixing section, and to form a concave as required. Can be formed.

Further, according to the image forming apparatus of the second aspect, the sheet can be transported reliably.

[0077]

[Brief description of the drawings]

FIG. 1 is a side view showing a main part of an embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a partially omitted plan view mainly showing a transport unit 30;

FIG. 3 is an enlarged view of a part of a conveyor belt, with parts omitted.

FIG. 4 is an explanatory diagram of a conventional technique.

FIG. 5 is a schematic view showing a state of transporting an envelope.

FIG. 6 is a plan view of an envelope.

[Explanation of symbols]

 S paper 10 transfer unit 20 fixing unit 30 conveyance means 33 conveyance belt 33a unevenness 40 guide

Claims (2)

[Claims] A transfer unit for transferring a toner image onto a sheet of paper;
A fixing unit for fixing the toner image on a sheet on which the toner image has been transferred by the transfer unit; and a conveying unit for conveying the sheet between the transfer unit and the fixing unit. A guide for guiding the sheet is provided, and the conveying unit conveys the sheet in a concave shape from the transfer unit toward the fixing unit in cooperation with the guide when viewed from a direction orthogonal to the sheet conveying direction. A first position and a second position for transporting the sheet linearly compared to the concave paper transport path can be taken, and the length of the paper in the transport direction is greater than the length of the concave transport path. The image forming apparatus according to claim 1, wherein the first position is taken when the sheet is long, and the second position is taken when the length of the sheet in the conveying direction is shorter than the interval between the transfer section and the fixing section. 2. An image forming apparatus according to claim 1, wherein said conveying means is constituted by a conveying belt having an uneven surface.