JP2020073975A - Fixing device and image forming apparatus - Google Patents

Abstract

To provide a fixing device that prevents the occurrence of offset and occurrence of tailing at the tip and can reduce an inter-sheet time as much as possible, and an image forming apparatus including the fixing device.SOLUTION: A fixing device, when a time within an inter-sheet time from when a bias is switched to a bias having the same polarity as that of a toner until when a recording material enters a nip part is taken as a switching time, records in advance different switching times corresponding to the lengths of the inter-sheet time, changes the switching time according to the inter-sheet time of an actual recording material on the basis of the correspondence relationship between the stored inter-sheet time and switching time, and controls a timing to switch bias application means.SELECTED DRAWING: Figure 7

Description

  The present invention relates to a fixing device for heating and fixing a toner image formed on a recording material, and an image forming apparatus such as an electrophotographic printer or a copying machine using the fixing device.

As a conventional fixing device of this type of image forming apparatus, for example, a film heating type fixing device as described in Patent Document 1 is known.
That is, it has a tubular fixing film heated by a heater and a pressure roller forming a nip portion with the heater via the fixing film, and heats the recording material carrying a toner image at the nip portion while transporting the recording material. Then, the toner image is fixed on the recording material.
In this fixing device, an image defect called "tip trailing" may occur. Tailing is a phenomenon in which, when a recording material having a large amount of moisture absorption is heated and fixed, water vapor ejected from the recording material causes a toner image to scatter in a direction opposite to the conveying direction of the recording material.
As a measure against this tailing, Patent Document 1 discloses a method of applying a bias having the same polarity as the toner to the fixing film to form an electric field in the direction in which the toner is attracted to the recording material. The toner is pressed against the recording material by this electric field, and as a result, scattering of the toner image due to water vapor is reduced.

On the other hand, in the fixing device, it is also known that an image defect called "offset" occurs in addition to the tailing defect. Offset is a phenomenon that occurs when the surface of the pressure roller is gradually charged up in the process of conveying the recording material in the nip portion, and as a result, unfixed toner on the recording material is peeled off.
As a countermeasure against this offset, in Japanese Patent Application Laid-Open No. 2004-242242, a bias having a polarity opposite to that of the toner is applied to the fixing film or the pressure roller during the non-passing period in which the recording material does not pass through the nip portion, and the pressure roller is charged up. A method of preventing is disclosed.

JP, 2000-131974, A JP, 2010-128474, A

However, in recent years, the printing speed has been increased for image forming apparatuses, and the non-passing period in which the recording material does not pass through the nip portion has become shorter. It is difficult to achieve both solutions. The reason will be described below.
The “offset” is caused by the fact that the pressure roller is charged up to the same polarity as the toner as the recording material P passes, and the force of the electric field that attracts the toner to the fixing film is generated. In an image forming apparatus in which it is difficult to secure a sufficient paper interval time, it is not possible to take sufficient time to apply a bias having a polarity opposite to that of the toner between paper sheets.
As a result, sufficient pressure cannot be applied to the pressure roller, the effect of preventing charge-up is reduced, and "offset" may occur.
On the other hand, even when the paper interval time is short, it is possible to set a long time for applying a bias having a polarity opposite to that of the toner to the fixing film between the papers in order to apply sufficient charge having a polarity opposite to the toner to the pressure roller. it can. However, in this case, after switching so as to apply the bias having the same polarity as the toner to be applied when the succeeding recording material P passes through the nip portion N, until the succeeding recording material P passes through the fixing nip. The switching time is shortened, which may result in "tip tailing".

  The present invention has been made to solve the above problems, and an object thereof is to solve both the occurrence of offset and the occurrence of trailing edge, and a fixing device capable of shortening the paper interval time as much as possible. An object is to provide an image forming apparatus.

In order to achieve the above object, the fixing device of the present invention,
A recording material having a cylindrical fixing film heated by a heating body and a pressing member that contacts the fixing film to form a nip portion, and conveys a recording material carrying an unfixed toner image in the nip portion. While heating, the toner image is fixed on the recording material.
A bias applying unit that applies a bias having the same polarity as the toner to the fixing film and a bias having the opposite polarity to the toner to the fixing film or the pressure member,
The bias applying unit applies a bias having the same polarity as the toner to the fixing film when the recording material passes through the nip portion, and the preceding recording material passes through the nip portion, and then the succeeding recording material. In a fixing device that applies a bias having a polarity opposite to that of the toner to the fixing film within a paper interval time until the toner rushes into the nip portion,
When the switching time is defined as the time from the switching to the bias having the same polarity as the toner until the recording material enters the nip portion within the paper interval time,
Pre-store different switching times corresponding to the length of the paper interval time,
It is characterized in that the switching time is changed according to the actual recording material interval time based on the stored correspondence relationship between the interval time and the switching time, and the switching timing of the bias applying means is controlled.
An image forming apparatus of the present invention includes an image forming unit that forms an unfixed toner image on a recording material, and the above-described fixing device of the present invention for fixing the unfixed toner image. Characterize.

  According to the present invention, it is possible to solve both the occurrence of the offset and the occurrence of the trailing edge and shorten the paper interval time as much as possible.

3 is a cross-sectional view of the image forming apparatus according to the first exemplary embodiment. FIG. FIG. 3 is a cross-sectional view of the fixing device according to the first exemplary embodiment. 3 is a sectional view of the fixing film according to Example 1. FIG. 6 is a graph showing the change over time of the surface potential of the fixing film according to Comparative Example 1. 6 is a graph showing the change over time of the surface potential of the fixing film according to Comparative Example 2. 9 is a graph showing changes in the number of printed sheets of the surface potential of the pressure roller according to Comparative Examples 1 and 2. 6 is a graph showing changes over time in the surface potential of the fixing film and changes in the surface potential of the pressure roller with the number of printed sheets according to Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the present invention will be illustratively described in detail below based on the embodiments with reference to the drawings.
[Example 1]
1 is a sectional view of an image forming apparatus according to a first embodiment of the present invention.
(Schematic configuration of image forming apparatus)
This image forming apparatus heats and fixes a toner image on a recording material to the recording material, an image forming portion 100A that forms a toner image on the recording material, a recording material feeding portion 100B that feeds the recording material to the image forming portion 100A. It has a fixing device 100C.
The image forming unit 100A has a drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 1 as an image bearing member. The photosensitive drum 1 is rotatably supported by an image forming apparatus main body (hereinafter, referred to as an apparatus main body) M that constitutes a housing of the image forming apparatus. Around the outer peripheral surface of the photosensitive drum 1, a charging roller 2, a laser scanner 3, a developing device 4, a transfer roller 5, and a cleaning device 6 are arranged in this order along the rotation direction.
The recording material feeding unit 100B has a feeding roller 11. The feed roller 11 is rotated at a predetermined timing in the arrow direction by a feed drive motor (not shown), and the feed roller 8 and the top sensor 9 are sequentially arranged along the feed path of the recording materials P stacked and stored in the cassette 7. A transport guide 10, a fixing unit (referred to as a fixing device) C, a transport roller 12, a discharge roller 13, and a discharge tray 14 are provided.

The image forming apparatus according to the present exemplary embodiment includes a control unit 31 that controls the image forming unit 100A, the recording material feeding unit 100B, the fixing device 100C, and the like. The control unit 31 is composed of a CPU and a memory such as a ROM and a RAM, and various programs necessary for image formation are stored in the memory.
The control unit 31 takes in a print signal from an external device such as a host computer and executes a predetermined image formation control sequence based on the print signal. As a result, the drum motor is rotationally driven, and the photosensitive drum 1 rotates in the arrow direction at a predetermined peripheral speed (process speed). The surface of the rotated photosensitive drum 1 is uniformly charged by the charging roller 2 to a predetermined potential having the same polarity as the toner (negative polarity here). The laser scanner 3 scans the charged surface of the surface of the photosensitive drum 1 with a laser beam L based on image information to expose the surface of the photosensitive drum 1. By this exposure, the charge of the exposed portion is removed, and an electrostatic latent image is formed on the surface of the photosensitive drum 1.

The developing device 4 includes a developing roller 41, a toner container 42 that stores toner, and the like. The toner is rubbed by a member such as a urethane blade (not shown) and charged to a predetermined polarity. In the first embodiment, the toner charged in the negative polarity is used. In the developing device 4, a negative bias is applied to the developing roller 41 by a developing bias power source (not shown), so that toner is attached to the electrostatic latent image on the surface of the photosensitive drum 1 by using a potential difference, and electrostatic charges are generated. The latent image is developed as an unfixed toner image T. The toner image T formed on the surface of the photosensitive drum 1 is transferred onto the recording material P by utilizing a potential difference due to the transfer bias by applying a positive bias having a polarity opposite to that of the toner to the transfer roller 5.
Further, the conveyance drive motor provided on the recording feeding member B is rotationally driven, and the feeding roller 11 feeds the recording material P from the cassette 7 to the feeding roller 8. The recording material P is transported by the transport roller 8, passes through the top sensor 9, and is transported to the transfer nip portion between the surface of the photosensitive drum 1 and the outer peripheral surface of the transfer roller 5. At this time, the leading edge position of the recording material P is detected by the top sensor 9 as a detecting means.
The recording material P on which the toner image T formed on the surface of the photosensitive drum 1 is transferred is conveyed to the fixing device 100C along the conveying guide 10, and the fixing device 100C heats the toner image T on the recording material P. The recording material P is pressurized and heat-fixed.

The recording material P on which the toner image T has been heated and fixed is conveyed in the order of the conveying roller 12 and the discharging roller 13, and is discharged to the discharging tray 14 on the upper surface of the apparatus main body M.
The transfer residual toner remaining on the surface of the photosensitive drum 1 after the toner image is transferred to the recording material P is removed by the cleaning blade 61 of the cleaning device 6 and accumulated in the cleaning device 6. By repeating the above operation, printing is sequentially performed. In the case of the A4 size image forming apparatus of the present embodiment, it is possible to print at a print speed of 60 sheets / minute.

(Structure of fixing device)
FIG. 2 is a cross-sectional side view of the fixing device of the image forming apparatus according to the first embodiment.
The fixing device 100C of the first embodiment includes a cylindrical fixing film 25 heated by a ceramic heater 20 as a heating body, and a pressure roller 26 as a pressure member that contacts the fixing film 25 to form a nip portion. And have. In this nip portion, the recording material carrying the unfixed toner image is heated while being conveyed to fix the toner image on the recording material.
In addition, as a characteristic configuration of the first embodiment, a bias applying device 50 that applies a bias to the fixing film 25 is included. That is, a bias having the same polarity as the toner is applied to the fixing film 25, and a bias having the opposite polarity to the toner is applied to the fixing film 25.

(heater)
The ceramic heater 20 has a heat resistant heater substrate 21 made of aluminum nitride, alumina or the like. On the surface of the heater substrate 21, a resistor pattern 22 as an energization heating resistance layer that generates heat when energized is formed, for example, by printing along the longitudinal direction of the heater substrate 21. The surface of the resistor pattern 22 is covered with a glass layer 23 as a protective layer. A thermistor 24 as a temperature detecting member for detecting the temperature of the ceramic heater 20 is provided on the back surface of the heater substrate 21 (the surface opposite to the nip portion N). The film guide member 29 acts as a support member for supporting the ceramic heater 20 and also as a guide member for guiding the rotation of the fixing film 25. As a material of the film guide member 29, a heat resistant resin such as liquid crystal polymer, phenol resin, PPS, PEEK or the like is used.

(Pressure roller)
The pressure roller 26 has an elastic layer 262 on the outer circumference of the core shaft portion 261 and a surface layer 263 on the outer circumference of the elastic layer 262. The outer diameter of the pressure roller 26 is about 30 mm. For the core shaft portion 261, a metal material such as aluminum and iron is used in solid or hollow. In Example 1, solid aluminum is used as the core metal material. The elastic layer 262 is made of a heat insulating silicone rubber, and is made conductive by adding an electrically conductive material such as carbon. In Example 1, the elastic layer 262 is made of silicone rubber in which an appropriate amount of carbon is added and the volume resistivity is adjusted to about 1 × 10 ⁵ (Ω · cm), and the thickness thereof is 3 mm.
The surface layer 263 is a releasable tube having a thickness of 10 to 80 μm and made of a fluororesin such as PFA, PTFE and FEP. Here, PFA is an abbreviation for tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer, PTFE is polytetrafluoroethylene (tetrafluoroethylene), and FEP is an abbreviation for tetrafluoroethylene / hexafluoropropylene copolymer (4,6 fluoro). Is.
In Example 1, the surface layer 263 of the pressure roller 26 is made of insulating fluororesin (pure PFA tube having a thickness of 30 μm). Therefore, the pressure roller 26 of Example 1 has a high volume resistivity of 1 × 10 ¹⁴ (Ω · cm) or more. The reason for using pure fluororesin is that when carbon or the like which lowers the resistance is added to the surface layer 263, the smoothness of the surface may be deteriorated and a phenomenon (contamination) of contamination with toner or paper dust may occur.

(Fixing film)
The endless belt-shaped heat resistant film (fixing film) 25 has a cylindrical shape with a diameter of 30 mm. The fixing film 25 has flexibility and is loosely fitted onto the semi-circular film guide member 29. The layer structure of the fixing film 25 is composed of multiple layers in which a base layer 251, an elastic layer 252, and a surface layer 253 are provided from the inside as shown in the circle in FIG.
As a material for the base layer 251, a thin metal such as SUS or Ni is used in order to improve thermal conductivity and durability. Further, as another material, a heat resistant resin material having a low heat capacity such as polyimide, polyamideimide, PEEK, PES can be used. The base layer 251 needs to have a small heat capacity to satisfy the quick start property and at the same time satisfy the mechanical strength. Therefore, the thickness is preferably 15 μm or more and 50 μm or less. The base layer 251 of Example 1 was a cylindrical stainless (SUS) tube having a thickness of 35 μm. The elastic layer 252 is made of silicone rubber. By providing this elastic layer 252, it becomes possible to wrap the toner image T and uniformly apply heat, so that it is possible to obtain a high-quality image with high gloss and no unevenness. Further, since the elastic layer 252 has a low thermal conductivity as a simple substance of silicone rubber, a thermally conductive filler is added. The thermal conductivity of the elastic layer 252 is preferably 1.2 W / mk or more.

Alumina, metallic silicon, silicon carbide, zinc oxide and the like are listed as candidates for the heat conductive filler. In Example 1, the elastic layer 252 contained 400 parts by weight of metal silicon as a heat conductive filler with respect to 100 parts by weight of dimethylpolysiloxane as a rubber material, and its thermal conductivity was 1.2 W / mk. There is. The thickness of the elastic layer 252 is 210 μm.
As the release layer, the surface layer 253 is required to have high abrasion resistance and high releasability to toner. As the material, the above-mentioned fluororesin such as PFA, PTFE and FEP is used. Then, an organic phosphorus compound, an ion conductive agent such as a lithium salt, or an electronic conductive material such as antimony pentoxide, titanium oxide, carbon black, carbon nanofibers is added to the fluororesin to adjust the resistance value. Further, the thickness is preferably about 10 μm to 50 μm, and the tube may be coated or the surface may be coated with a paint.
The surface layer 253 of Example 1 uses PFA as the fluororesin. 7 parts by weight of Hishicolin PX-2B (manufactured by Nippon Kagaku Kogyo Co., Ltd.), which is an organic phosphorus compound represented by (C ₂ H ₅ ) ₄ P.BR, is mixed with the PFA. The thickness was 15 μm, and the coating layer was formed.

(Bias applying means)
Next, the bias applying device 50 having the characteristic configuration of the first embodiment will be described.
The bias applied to the fixing film 25 is applied from the bias power source 53 to the conductive base layer 251 which is partially exposed on the surface of the fixing film 25, via the power supply means 51 such as a conductive brush. Further, the pressure roller 26 is grounded from the core shaft portion 261 via a power feeding member 55 such as a carbon chip.
In the first exemplary embodiment, a bias having the same polarity as the toner (negative bias) and a bias having the opposite polarity to the toner (positive bias) can be switched and applied to the fixing film 25.
Regarding the magnitude of the bias, the positive bias is + 800V and the negative bias is -500V, and for example, it is applied by the bias power source 53 composed of a transformer and a resistor. The bias is switched by the bias controller 54 by means of a relay or the like.

A negative bias is applied when the recording material P passes through the nip portion N (hereinafter referred to as sheet passing), and a positive bias is applied when the recording material P does not pass through the nip portion N (hereinafter referred to as sheet interval). Is being applied. The purpose is shown below.
The purpose of applying a bias having the same polarity as that of the toner during paper passing is to improve the tailing. Here, the tailing means that when the recording material P passes through the fixing nip, the toner of the toner image T is scattered by the water vapor ejected from the recording material P in the direction opposite to the conveying direction of the recording material P. Is a problem that is disturbed. By applying a negative bias to the fixing film, the force of the electric field for holding the toner of the unfixed toner image T on the recording material P works, so that the occurrence of the tailing can be prevented.
The purpose of applying a bias having a polarity opposite to that of the toner between the sheets is to improve electrostatic offset. Here, the electrostatic offset means that the toner of the unfixed toner image T carried by the recording material P at the nip portion N is peeled off from the recording material P by electrostatic force and adheres to the fixing film 25. However, it is a phenomenon that appears as an image in the next orbit.

This electrostatic offset is a phenomenon that tends to occur after a large amount of recording material P is passed. The reason is shown below. The surface layer of the pressure roller 26 has a high resistance of 1 × 10 ¹⁴ (Ω · cm) or more as described above. Further, PFA, which is a material forming the surface layer 263 of the pressure roller 26, is characterized in that it is easily negatively charged by rubbing against the recording material P. Therefore, when the recording material P is passed, a negative charge is accumulated on the surface layer 263 of the pressure roller 26 due to the rubbing, and the charge up occurs. When the sheet passing amount of the recording material P is increased, the surface potential of the pressure roller 26 becomes negatively larger than the surface potential of the fixing film 25 (reversal of potential). As a result, the toner of the unfixed toner image T is peeled off from the recording material P by the electrostatic force and adheres to the fixing film, so that the electrostatic offset occurs.

As a means for improving the electrostatic offset, it is effective to apply a + bias to the fixing film 25 between the sheets to give a positive charge to the surface layer 263 of the pressure roller 26 via the surface layer 253 of the fixing film 25. Is. The reason is that by canceling the negative charge given at the time of sheet passing by the positive charge received from the surface layer of the fixing film between the sheets, the surface layer 263 of the pressure roller 26 is prevented from being charged up to the same polarity as the toner. This is because it is possible. As a result, an electric field sufficient to hold the toner of the unfixed toner image T on the recording material P can be formed between the surface of the fixing film and the surface of the pressure roller, and offset does not occur.
It is also possible to apply a positive bias to the core shaft portion 261 of the pressure roller 26 as a means for applying a positive charge to the surface of the pressure roller. However, when the electric resistance value of the surface layer 263 of the pressure roller 26 is high as in Example 1, it is much more efficient to apply the electric charge through the surface of the fixing film 25 having a lower electric resistance value.

(Bias switching timing)
As described above, in the image forming apparatus of the first embodiment, −500 V is applied to the fixing film 25 when the paper is passed and +800 V is applied between the paper. At this time, the timing of switching from the positive bias between sheets to the negative bias during sheet passing is important.
As described above, in order to suppress the trailing, it is desirable that a bias of −500 V is applied to the fixing film when the leading end of the recording material P enters the fixing nip. For this reason, it is necessary to set to switch to the negative bias in consideration of the time required for rising when the fixing bias is switched.
The time required for the fixing bias to rise is affected by the time constant of the high voltage circuit, the degree of charge-up of the fixing film and the pressure roller, the moisture content (temperature and humidity) of the surrounding environment, and the like. On the other hand, in order to suppress the electrostatic offset, it is desirable that the time for applying the positive bias between the sheets is as long as possible.
The reason is that in order to cancel the negative charge on the surface of the pressure roller 26 as described above, it is desired to give as much positive charge as possible. In consideration of the conditions for suppressing the trailing and the electrostatic offset, the control is performed such that the recording material P is switched to the negative bias a predetermined time before entering the nip. (Hereinafter, t will be referred to as a switching time.) In the conventional technique, a constant switching time t is set regardless of the sheet interval length d.
However, with the recent trend toward higher speeds, there is an urgent need to shorten the paper interval time d. Then, when the paper interval time d is not sufficiently larger than the above t, there is a problem that the electrostatic offset and the problem of trailing at the leading end of the recording material P cannot be compatible. Specifically, there is a problem in that electrostatic offset occurs during continuous sheet feeding when the sheet interval is short, and leading end tailing occurs when the sheet interval is long.

Therefore, in the present invention, the bias applying device 50 applies a bias having the same polarity as the toner to the fixing film 25 when the recording material P passes through the nip portion N so that the recording material P passes through the nip portion N. When the nip does not pass between the unfed sheets, a bias having a polarity opposite to that of the toner is applied to the fixing film 25. That is, after the preceding recording material P passes through the nip portion N, a bias having a polarity opposite to that of the toner is applied to the fixing film 25 within the paper interval time until the succeeding recording material P enters the nip portion N.
Then, when the time from when the bias having the same polarity as that of the toner is switched to when the recording material P enters the nip portion N within the paper interval time is a switching time t, a difference corresponding to the length of the paper interval time is obtained. Correspondences of switching times are stored in advance. Based on the stored correspondence relationship between the sheet interval time and the switching time, the switching time is changed according to the actual sheet interval time of the recording material P, and the switching timing of the bias applying device 50 is controlled. ..
The different switching times corresponding to the length of the paper interval time are as follows.
That is, the surface potential of the fixing film when a bias having the same polarity as the toner is applied when the recording material P passes through the nip is set to V0. Further, among the surface potentials of the fixing film 25 within the paper interval time of the recording material P (when the nip is not passed), the surface potential having the largest value on the side opposite to the polarity of the toner is set to v. At this time, it is a time required for the surface potential of the fixing film 25 to reach V0 from v in accordance with the magnitude of | v−V0 |, and is a time at which tip tailing does not occur.

Here, the fixing bias control of Comparative Example 1 and Comparative Example 2 will be described with reference to FIGS. 4, 5, and 6. The fixing bias control of Comparative Example 1 is an example of control when the switching time t is long (t1 = 90 msec), and Comparative Example 2 is the control when the switching time t is short (t2 = 50 msec).
FIG. 4 shows the time change of the surface potential of the fixing film when the paper interval time d is long (dl = 160 msec) and when the paper interval time d is short (ds = 100 msec), in comparison with the fixing bias control of Comparative Example 1. It is a graph shown.
Further, FIG. 5 shows the time variation of the surface potential of the fixing film when the paper interval time d is long (dl = 160 msec) and when the paper interval time d is short (ds = 100 msec) in comparison with the fixing bias control of Comparative Example 2. It is a graph shown.
Further, FIG. 6 shows the surface potential of the pressure roller and the number of prints in the image forming apparatus of Comparative Example 1 and Comparative Example 2 when the paper interval time is set as short as ds = 100 msec and 300 continuous prints are performed. It is a graph showing a relationship.

Comparative Example 1
First, Comparative Example 1 will be described.
In Comparative Example 1, when the sheet interval time is dl = 160 msec, the switching time t is set to 90 msec, so that the fixing film surface potential profile shown in FIG. 4A is obtained. That is, in Comparative Example 1, the fixing bias is switched from positive to negative at the timing “a-t1”. As a result, at the timing "a" at which the leading edge of the recording material P enters the nip portion N, the fixing film surface rises to -500 V, so that the trailing edge of the leading edge does not occur.

On the other hand, in Comparative Example 1, even when the paper interval time is ds = 100 msec, the switching time t = 90 msec is set, and therefore the profile of the fixing film surface potential shown in FIG. 4B is obtained. That is, in Comparative Example 1, the fixing bias is similarly switched from the positive to the negative at the timing “a-t1”, so that the time for applying the positive bias between the sheets is short and a sufficient positive charge cannot be applied. As a result, it is not possible to prevent the charge roller surface from being charged up by the negative charge received from the recording material P.
FIG. 6 shows the relationship between the number of prints and the surface potential of the pressure roller when continuous printing is performed with the paper interval time ds = 100 msec in the image forming apparatus of Comparative Example 1.
At the start of sheet feeding, the surface potential of the pressure roller that has not been charged up reaches −500V when about 100 sheets have been continuously fed, and becomes −650V after 250 sheets have been continuously fed. As a result, the potential becomes negatively larger than the surface potential of the fixing film (−500 V), and an electric field is generated in the direction of attracting the unfixed toner on the recording material P and the fixing film, so that an offset occurs.

Comparative example 2
Next, Comparative Example 2 will be described. In Comparative Example 2, when the paper interval time is ds = 100 msec, the switching time t is set to 50 msec, so that the fixing film surface potential profile shown in FIG. 5B is obtained.
That is, in Comparative Example 2, the fixing bias is switched from positive to negative at the timing “at2”. As a result, the positive bias between the sheets does not rise to the control potential of +800 V and is +600 V, and even if the switching time t is shortened to t2 = 50 msec, the leading edge of the succeeding recording material P reaches −500 V, so No tailing of the part occurs.
Further, FIG. 6 shows the relationship between the pressure roller surface potential and the number of prints in the case of continuous printing in Comparative Example 2. In Comparative Example 2, even when the paper interval time is as short as ds = 100 msec, a sufficient positive charge is applied to the surface of the pressure roller during the paper interval. Therefore, even when continuous 250 sheets are printed, the pressure roller The surface potential reaches only -300V. As a result, no offset occurs.
On the other hand, in Comparative Example 2, when the paper interval time is dl = 160 msec, the switching time t is set to 50 msec, so that the fixing film surface potential profile shown in FIG. 5A is obtained. In this case, since the positive bias between the sheets reaches + 800V, when the switching time t is shortened to t2, -500V is not reached at the leading edge of the succeeding recording material P (area D in the drawing). .. As a result, there is a problem that trailing occurs at the leading end of the succeeding recording material P.

(Fixing bias control of Embodiment 1)
In the first embodiment, the relationship between the paper interval time d and the appropriate switching time t is obtained in advance and stored in the control table, and the relationship between the paper interval time d and the switching time t stored in the control table is calculated. Based on this, the corresponding switching time is derived from the actual paper interval time d, and the switching timing by the bias applying device 50 is set. The switching time t is set to increase as the sheet interval time d increases.
Table 1 is a control table for determining the fixing bias switching time t in the first embodiment. In the first embodiment, the sheet interval time d is estimated from the sheet feeding operation interval of the feed roller 11. In the first embodiment, this is done for all page prints.

7 (1) and 7 (2) show the relationship between the surface potential of the fixing film and the switching time when the paper interval time d is set to dl = 160 msec and ds = 100 msec in the image forming apparatus of the first embodiment. Show.
Further, FIG. 7 (3) shows the pressure roller surface potential and the number of prints in the image forming apparatus of Conventional Examples 1 and 2 when the paper interval time is set as short as ds = 100 msec and 300 continuous prints are performed. It is a graph showing a relationship.
In the first embodiment, the printing operation is performed with two types of paper interval times, dl = 160 msec and ds = 100 msec. The former is a mode that emphasizes the fixability so that the fixing performance can be satisfied even with a paper type having a large basis weight and a paper type with a rough surface, and the latter is a mode that emphasizes the printing speed.
Here, in Example 1, the maximum value v in the positive bias direction of the fixing film surface potential between the sheets can be predicted from the sheet-to-sheet time d. When the paper interval time d is long, v increases in the positive direction, and when d is short, v decreases in the positive direction. For example, when the paper interval time d = dl = 160 msec, v = v ₁ = + 800 V is reached, while when the paper interval time d = ds = 100 msec, only v = v ₂ = + 600 V is reached.

Next, the bias control of the fixing film of Example 1 will be described.
In the first embodiment, when the paper interval time is long as dl = 160 msec, t = t1 = 90 msec is set according to the table shown in Table 1. The relationship between the surface potential of the fixing film and time at that time is shown in FIG. 7 (1).
In this case, according to the graph of FIG. 7A, the surface potential of the fixing film 25 reaches a maximum of v ₁ = + 800 V in the positive bias direction between the sheets. At this time, the bias switching time for the subsequent recording material P needs to be set to t ₁ ≧ 90 msec, but in the first embodiment, it is set to 90 msec. As a result, since the fixing film surface potential reaches the desired potential (−500 V) at the leading end of the succeeding recording material P, trailing of the leading end does not occur.
When the paper interval time is as short as ds = 100 msec, the switching time t is set to t2 = 50 msec according to the control table shown in Table 1. The relationship between the surface potential of the fixing film 25 and time at that time is shown in FIG. 7 (2), and the relationship between the surface potential of the pressure roller 26 and the number of printed sheets when 250 sheets are continuously printed are shown in FIG. 7 (3). ing.
From the graph of FIG. 7 (2), the surface potential of the fixing film 25 reaches a maximum of v ₂ = + 600 V in the positive bias direction between the sheets. Therefore, even if the bias switching time for the subsequent recording material P is set to a short value of t ₂ = 50 msec, the surface potential of the fixing film 25 at the leading edge of the subsequent recording material P is the desired potential (V0 = -500V), and no tailing occurs.

Further, in Example 1, even when the paper interval time is as short as ds = 100 msec, as shown in FIG. 7B, a sufficient positive charge is applied to the surface of the pressure roller 26 during the paper interval. As a result, as shown in FIG. 7C, even when continuous 250 sheets are printed, the surface potential of the pressure roller 26 reaches only −300 V and no offset occurs.
As described above, by using the control table of the switching time t of the fixing bias of the first embodiment, it is possible to set the optimum switching time t at various paper interval times d. Therefore, the electrostatic offset and the tail end tail can be set. The occurrence of pulling can be suppressed.

(Experimental results of Example 1 and Comparative Example)
In order to explain the function and effect of the image forming apparatus of Example 1, a comparative experiment with the image forming apparatus of Comparative Example was conducted.
The image forming apparatus according to the first embodiment determines the fixing bias switching time t according to the table shown in Table 1. On the other hand, in the image forming apparatuses of Comparative Examples 1 and 2, the switching time t was set to a fixed value irrespective of the paper interval time d, that is, Comparative Example 1 was 50 msec and Comparative Example 2 was 90 msec. The other configuration is the same as that of the first embodiment, and thus the description is omitted.

First, the conditions of this experiment will be described.
The conveyance speed of the recording material of the image forming apparatus used in this experiment is 350 mm / sec. Regarding the paper interval time d, the operation interval of the feed roller 11 was adjusted, and the paper was passed at both 100 msec and 160 msec.
In the fixing device, the fixing film 25 is pressed against the pressure roller 26 by 186.2 N (19 kgf), and the nip width thereof is 9 mm. The environment in which the test was conducted was a temperature of 23 ° C. and a humidity of 50%, and the evaluation paper used was Canon Inc.'s CS-680 (A4 size, 68 g / cm ² ). Under these conditions, each image forming apparatus passed 500 sheets in the single-sided continuous sheet passing mode, and evaluated the electrostatic offset and the trailing level of the leading end.

  Table 2 shows the tip end tailing and electrostatic offset levels in Example 1 and Comparative Examples 1 and 2. In the table, ◯ means that the end tailing and electrostatic offset did not occur, which was good. The symbol x in the table indicates that there is a problem in practical use because the tailing of the tip portion and the entire surface offset occur at a bad level.

According to the results of Table 2, in the image forming apparatus of Example 1, a good image was obtained without causing the trailing of the leading end portion and the electrostatic offset when the paper interval time was 100 msec or 160 msec. The reason is that the optimum switching time t can be set even when the paper interval time d changes.
On the other hand, the image forming apparatuses of Comparative Examples 1 and 2 had a problem. In Comparative Example 1, the trailing edge of the tip portion occurred when the paper interval time d was 160 msec, and in Comparative Example 2, electrostatic offset occurred when the paper interval time was 100 msec.
As described above, by using the image forming apparatus according to the first embodiment, it is possible to obtain a good image that does not have the effects of the first and second comparative examples, that is, the trailing of the tip portion and the electrostatic offset.
In the first embodiment, the pressure roller 26 is configured to be grounded via the core shaft portion, but the same effect can be obtained even if a bias is applied. In this case, it is desirable to apply a bias having a polarity opposite to that of the toner to the pressure roller 26 during the passage of the sheet and between the sheets.

Further, in the first embodiment, the potential of the fixing film between the sheets is estimated from the length of the sheet interval, but a means for directly measuring the surface potential of the fixing film may be used. In this case, more precise control becomes possible.
Further, in the first embodiment, the case in which a relay or the like is used as the bias control unit 54 that switches the bias has been described, but it is also possible to use a switching unit that uses a Zener diode or the like. In this case, the time constant of the voltage change tends to be larger than that of the relay method, so that the action and effect of the first embodiment becomes large.
Further, in the first embodiment, the paper interval time d is estimated from the operation interval of the delivery roller 11, but it is also possible to directly detect it by using a sensor or the like. In that case, it is effective to use the detection result of the top sensor 9 to determine the sheet interval d.

[Example 2]
Next, a second embodiment of the present invention will be described.
The difference between the second embodiment and the first embodiment is only the items relating to the application timing control of the fixing bias, and the other configurations are the same as those of the first embodiment, and the description of the same configurations will be omitted.
The second embodiment has a configuration suitable for an image forming apparatus that uses a multi-stage cassette and interweaves a plurality of cassettes to feed a recording material. In this case, the conveyance path from the sheet feeding to the fixing nip differs depending on the sheet feeding port, and the actual measurement can simplify the fixing bias application timing control rather than predicting the sheet interval using the sheet feeding timing. ..
Therefore, the difference between the fixing bias application timing control of the second embodiment and that of the first embodiment is that the paper interval time d is estimated from the detection result of the top sensor 9 as the detection unit that detects the leading edge of the recording material P. That is the point. The control table is also changed according to the actual detection result.

Table 3 shows the control table of the second embodiment.
In the second embodiment, the time from when the top sensor 9 detects the trailing edge of the preceding sheet to the leading edge of the following sheet is defined as the sheet interval time d. Then, based on the obtained paper interval time d, the switching time t of the fixing bias in the second embodiment is determined according to Table 3. In the second embodiment, this is done for all page prints.

このように、実施例２の画像形成装置を用いれば、紙間時間ｄに対して、最適な切り替え時間ｔを設定することができるため、先端尾引きおよび静電オフセットの発生を抑制し、良質な画像を得ることができる。

As described above, when the image forming apparatus according to the second embodiment is used, the optimum switching time t can be set for the paper interval time d, so that the occurrence of the trailing edge and the electrostatic offset can be suppressed, and the image quality can be improved. It is possible to obtain a perfect image.

[Example 3]
Next, a third embodiment of the present invention will be described.
The difference between the third embodiment and the first embodiment is only that a humidity sensor Th that detects the environmental humidity is provided as a detection unit that detects the amount of water in the surrounding environment, and that the control table for the fixing bias application timing control is changed. Since the other configurations are the same as those of the first embodiment, the description of the same configurations will be omitted.
The third embodiment has a humidity sensor Th that detects the ambient environmental humidity, and the switching time is corrected according to the detection result of this humidity sensor Th. Specifically, the fixing bias control table is changed.
The reason for this change is that the responsiveness of the surface potential of the fixing film 25 is affected by the amount of water in the surrounding environment. The lower the moisture content, that is, the lower the humidity environment, the lower the responsiveness, and the higher the humidity environment, the higher the responsiveness.

Table 4 shows a fixing bias control table according to the third embodiment.
In the third embodiment, a plurality of types of control tables that store the relationship between the paper interval time d and the switching time t are provided. Specifically, it is divided into three types of "environmental humidity A: less than 25%", "environmental humidity B: 25% or more and less than 60%", and "environmental humidity C: 60% or more". Then, the control table used for control is changed based on the detection result of the humidity sensor Th.

As described above, when the image forming apparatus according to the third embodiment is used, the optimum switching time t can be set according to the environmental humidity, so that the generation of the trailing edge and the electrostatic offset can be suppressed, and a high-quality image can be obtained. Can be obtained.
In the third embodiment, the ambient humidity is used as the means for detecting the moisture content in the surroundings, but it is also possible to use the means for directly detecting the moisture content. In addition, in an environment such as an office that is generally used, there is a correlation between the ambient temperature and the moisture content of the ambient, and therefore the ambient temperature is detected from the detection result of the temperature sensor as a detection unit that detects the ambient temperature. It is also possible to estimate the water content and reflect it in the control.

100C Fixing device 20 Ceramic heater (heating body)
25 fixing film 26 pressure roller (pressure member)
50 Bias applying device (bias applying means)
N Nip portion P Recording material T Toner image Th Humidity sensor (detection means)
d Paper interval time t Switching time v Maximum value of surface potential of fixing film (when nip is not passed)
V0 Surface potential of fixing film (when passing through nip)

In order to achieve the above object, the image forming apparatus of the present invention,
An image forming unit for forming a toner image on the recording material,
It has a cylindrical fixing film and a roller that contacts the outer surface of the fixing film and forms a nip portion between the fixing film, and nip and convey the recording material on which the toner image is formed at the nip portion. While fixing the toner image on the recording material,
A function of applying a bias voltage having the same polarity as the toner charging polarity to the fixing film, and a function of applying a bias voltage having a polarity opposite to the toner charging polarity to the fixing film or the roller to apply the toner to the surface of the roller. A bias applying unit having a function of applying an electric charge having a polarity opposite to the charging polarity;
Have
A first print in which a paper interval time is a first time between the preceding recording material and the succeeding recording material in the nip portion when a toner image is continuously formed on a plurality of recording materials It is possible to set a mode and a second print mode in which the paper interval time is shorter than the first time,
When the recording material passes through the nip portion, the bias applying means applies the bias voltage of the same polarity to the fixing film when the recording material passes through the nip portion, and when the nip portion is a sheet interval, the bias applying means applies the bias voltage to the fixing film. In an image forming apparatus that applies a bias voltage of reverse polarity,
From the timing when the bias applying unit switches from the state in which the bias voltage of the opposite polarity is applied to the state in which the bias voltage of the same polarity is applied during the period when the nip portion is in the space between sheets, the succeeding recording material is the nip. It is characterized in that the switching time until the timing to enter the set is set shorter when the image is formed in the second print mode than when the image is formed in the first print mode.

Claims (11)

A recording material having a cylindrical fixing film heated by a heating body and a pressing member that contacts the fixing film to form a nip portion, and conveys a recording material carrying an unfixed toner image in the nip portion. While heating, the toner image is fixed on the recording material.
A bias applying unit that applies a bias having the same polarity as the toner to the fixing film and a bias having the opposite polarity to the toner to the fixing film or the pressure member,
The bias applying unit applies a bias having the same polarity as the toner to the fixing film when the recording material passes through the nip portion, and the preceding recording material passes through the nip portion, and then the succeeding recording material. In a fixing device that applies a bias having a polarity opposite to that of the toner to the fixing film within a paper interval time until the toner rushes into the nip portion,
When the switching time is defined as the time from the switching to the bias having the same polarity as the toner until the recording material enters the nip portion within the paper interval time,
Pre-store different switching times corresponding to the length of the paper interval time,
The fixing device is characterized in that the switching time is changed according to the actual paper interval time of the recording material based on the stored correspondence relationship between the paper interval time and the switch time, and the switching timing of the bias applying means is controlled. ..   The fixing device according to claim 1, wherein the fixing time is set to increase as the sheet interval time increases. The switching time stored in advance corresponding to the length of the paper interval time is
The surface potential of the fixing film when a bias of the same polarity as that of the toner is applied when the recording material passes through the nip is V0, and the surface potential of the fixing film when the recording material does not pass through the nip is the largest on the side opposite to the polarity of the toner. The surface potential of the fixing film is the time required for the surface potential of the fixing film to reach V0 from V depending on the magnitude of | v−V0 | Fixing device.   The fixing device according to claim 3, wherein the larger the value of | v−V0 |, the larger the switching time is set.   5. The fixing device according to claim 3, further comprising a detection unit that detects a leading end position of the recording material, and obtains the sheet interval time from the detection result of the detection unit.   The electric resistance value in the thickness direction of the fixing film is smaller than the electric resistance value in the thickness direction of the pressing member, and when the recording material does not pass through the nip, the fixing member is pressed through the fixing film. The fixing device according to any one of claims 1 to 5, wherein a charge having a polarity opposite to that of the toner is applied to the surface. Has a detection means for detecting the amount of water in the surrounding environment,
The fixing device according to claim 1, wherein the switching time is corrected according to the detection result of the detection unit.   The fixing according to claim 7, wherein a plurality of types of control tables that store the correspondence relationship between the paper interval time and the switching time are provided according to the amount of moisture in the surrounding environment, and the control table is changed based on the detection result of the detection means. apparatus.   The fixing device according to claim 7, wherein the detection unit is a humidity sensor that detects the humidity of the surrounding environment. The fixing device according to claim 7, wherein the detection unit is a temperature sensor that detects the temperature of the surrounding environment that has a correlation with the moisture content of the surrounding environment. An image forming apparatus including: an image forming unit that forms an unfixed toner image on a recording material; and a fixing device that fixes the unfixed toner image of the recording material formed by the image forming unit. hand,
The image forming apparatus, wherein the fixing device is the fixing device according to any one of claims 1 to 10.

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