Image forming apparatus and its control method Number:7,187,878 from the United States Patent and Trademark Office (PTO) owispatent In continuous printing operation which repeatedly sets a state in which when a preceding printing material passes through the nip area of a thermal fixing unit, a succeeding printing material has been supplied to an image forming unit and image forming operation has been started, a first temperature indicating an ambient temperature near the thermal fixing unit is measured at an early time point in the continuous printing operation. A second temperature indicating the temperature of the printing material which has passed through the press contact nip portion at a predetermined period of time during the continuous printing operation is measured. The amount of heat to be supplied to the succeeding printing material is controlled on the basis of the measured first and second temperatures.<H apparatus, control, Image, forming, a, 7187878.html, Patent, pto, 7187878

Title: Image forming apparatus and its control method

Abstract: In continuous printing operation which repeatedly sets a state in which when a preceding printing material passes through the nip area of a thermal fixing unit, a succeeding printing material has been supplied to an image forming unit and image forming operation has been started, a first temperature indicating an ambient temperature near the thermal fixing unit is measured at an early time point in the continuous printing operation. A second temperature indicating the temperature of the printing material which has passed through the press contact nip portion at a predetermined period of time during the continuous printing operation is measured. The amount of heat to be supplied to the succeeding printing material is controlled on the basis of the measured first and second temperatures.

Patent Number: 7,187,878 Issued on 03/06/2007 to Izawa, et al.

Inventors: Izawa; Satoru (Sunto-gun, JP), Nihonyanagi; Koji (Susono, JP), Kubochi; Yutaka (Mishima, JP), Inui; Fumiki (Mishima, JP), Hashiguchi; Shinji (Mishima, JP)
Assignee: Canon Kabushiki Kaisha (Tokyo, JP)

Appl. No.: 11/060,678

Filed: February 18, 2005

Foreign Application Priority Data


Feb 27, 2004 [JP]			2004-054638

Current U.S. Class: 399/44 ; 399/69

Current International Class: G03G 15/00 (20060101)

Field of Search: 399/44,69,337

References Cited [Referenced By]

U.S. Patent Documents


5083168	January 1992	Kusaka et al.
5138379	August 1992	Kanazashi
5149941	September 1992	Hirabayashi et al.
5162634	November 1992	Kusaka et al.
5210579	May 1993	Setoriyama et al.
5221682	June 1993	Effland et al.
5262834	November 1993	Kusaka et al.
5300997	April 1994	Hirabayashi et al.
5343280	August 1994	Hirabayashi et al.
5450181	September 1995	Tsukida et al.
5525775	June 1996	Setoriyama et al.
5722026	February 1998	Goto et al.
5767484	June 1998	Hirabayashi et al.
5860051	January 1999	Goto et al.
5862435	January 1999	Suzuki et al.
5920757	July 1999	Izawa et al.
5960233	September 1999	Goto et al.
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6314252	November 2001	Matsumoto
6404998	June 2002	Tanaka et al.
6493521	December 2002	Miyamoto et al.
6519426	February 2003	Goto et al.
6671488	December 2003	Izawa et al.
6701102	March 2004	Hasegawa et al.
6713725	March 2004	Izawa et al.
6748192	June 2004	Izawa et al.
6763205	July 2004	Izawa et al.
6787737	September 2004	Uekawa et al.
6794611	September 2004	Kataoka et al.
6862416	March 2005	Hashiguchi et al.
2002/0159785	October 2002	Masuda et al.
2003/0206756	November 2003	Kanamori et al.
2004/0037579	February 2004	Hashiguchi et al.
2004/0042827	March 2004	Izawa et al.
2004/0091279	May 2004	Izawa et al.
2004/0120737	June 2004	Kataoka et al.
2004/0120740	June 2004	Uekawa et al.

Foreign Patent Documents


63-313182	Dec., 1988	JP
1-150185	Jun., 1989	JP
2-157878	Jun., 1990	JP
4-44075	Feb., 1992	JP
4-204980	Jul., 1992	JP
6-308854	Nov., 1994	JP
7-230231	Aug., 1995	JP
7230231	Aug., 1995	JP
2002-214961	Jul., 2002	JP

Primary Examiner: Gray; David M.
Assistant Examiner: Ready; Bryan
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper & Scinto

Claims

What is claimed is:

1. An image forming apparatus comprising: an image forming unit configured to form a toner image on a printing material; a thermal fixing unit configured to have a rotating member incorporating a heating element and a pressurizing member which comes into contact with the rotating member and pressurizes the rotating member, convey a printing material on which a toner image is formed to a nip area between the rotating member and the pressurizing member, and fuse the toner image on the printing material by making the printing material pass through the nip area while applying heat from the heating element to the printing material; a temperature sensor configured to detect a temperature of the printing material which has passed through the nip area, wherein said temperature sensor is capable of detecting a temperature near said thermal fixing unit when the printing material does not pass through said thermal fixing unit; and a control unit configured to, when continuous printing operation is performed in which a plurality of printing materials are printed continuously, control an amount of heat supplied to a succeeding printing material on the basis of a first temperature detected by said temperature near said thermal fixing unit sensor at a time point before a first printing material is conveyed to the nip area in the continuous printing operation and a second temperature of the printing material that has passed through the nip area, detected by said temperature sensor at a predetermined time point during the continuous printing operation.

2. The apparatus according to claim 1, wherein said control unit includes: a unit configured to set a threshold for the second temperature on the basis of the first temperature; and a unit configured to determine a controlled variable for an amount of heat supplied to the succeeding printing material in accordance with a comparison result between the set threshold and the second temperature.

3. The apparatus according to claim 2, wherein the controlled variable comprises at least one of a throughput indicating the number of sheets printed per unit time and a temperature of the heating element.

4. The apparatus according to claim 1, further comprising a paper delivery sensor unit configured to detect the presence/absence of a printing material passing through said thermal fixing unit, wherein said temperature sensor is provided in said paper delivery sensor unit.

5. The apparatus according to claim 4, wherein said paper delivery sensor unit includes a sensor lever which comes into contact with a conveyed printing material and is made to pivot as the printing material is conveyed, and said temperature sensor is mounted on the sensor lever.

6. The apparatus according to claim 5, wherein the sensor lever is provided on an unprinted surface side of a printing material.

7. The apparatus according to claim 1, further comprising an ambient temperature sensor configured to detect an ambient temperature near said thermal fixing unit, wherein said control unit detects the second temperature at the predetermined time point by using said ambient temperature sensor in place of said temperature sensor.

8. The apparatus according to claim 1, further comprising an outside air measuring unit configured to measure at least one of an outside air temperature and a humidity, wherein said control unit further controls the amount of heat supplied to the succeeding printing material on the basis of a measurement result obtained by said outside air measuring unit before the continuous printing operation.

9. The apparatus according to claim 8, wherein said control unit includes: a unit configured to set a threshold for the second temperature on the basis of a measurement result obtained by said outside air measuring unit and the first temperature; and a unit configured to determine a controlled variable for the amount of heat supplied to the succeeding printing material in accordance with a comparison result between the set threshold and the second temperature.

10. A control method for an image forming apparatus including an image forming unit configured to form a toner image on a printing material, and a thermal fixing unit configured to have a rotating member incorporating a heating element and a pressurizing member which comes into contact with the rotating member and pressurizes the rotating member, convey a printing material on which a toner image is formed to a nip area between the rotating member and the pressurizing member, and fuse the toner image on the printing material by making the printing material pass through the nip area while applying heat from the heating element to the printing material, comprising: a first measuring step of, when continuous printing operation in which a plurality of printing materials are printed continuously is performed, measuring a first temperature indicating an ambient temperature near the thermal fixing unit at a time point when the printing material does not pass through the thermal fixing unit in the continuous printing operation; a second measuring step of measuring a second temperature indicating a temperature of the printing material which has passed through the nip area at a predetermined time point during the continuous printing operation; and a control step of controlling in the continuous printing operation an amount of heat supplied to the succeeding printing material on the basis of the first temperature near the thermal fixing unit at a time point before a first printing material is conveyed to the nip area and the second temperature of the printing material that has passed through the nip area, respectively measured in the first measuring step and the second measuring step.

11. The method according to claim 10, wherein the control step includes: a step of setting a threshold for the second temperature on the basis of the first temperature; and a step of determining a controlled variable for the amount of heat supplied to the succeeding printing material in accordance with a comparison result between the set threshold and the second temperature.

12. The method according to claim 11, wherein the controlled variable comprises at least one of a throughput indicating the number of sheets printed per unit time and a temperature of the heating element.

13. The method according to claim 10, further comprising an outside air measuring step of measuring at least one of an outside air temperature and a humidity before the continuous printing operation, wherein in the control step, the amount of heat supplied to the succeeding printing material is controlled on the basis of a measurement result obtained in the outside air measuring step.

14. The method according to claim 13, wherein the control step includes: a step of setting a threshold for the second temperature on the basis of a measurement result obtained in outside air measuring step and the first temperature; and a step of determining a controlled variable for the amount of heat supplied to the succeeding printing material in accordance with a comparison result between the set threshold and the second temperature.

Description

FIELD OF THE INVENTION

The present invention relates to an image forming apparatus and its control method and, more particularly, an image forming apparatus having a thermal fixing unit that fixes a toner image formed on a printing material by heating it and a control method for the apparatus.

BACKGROUND OF THE INVENTION

Many copying machines, printers, and the like based on an electrophotographic method use, as thermal fixing methods, a heating roller fusing method of a contact heating type which exhibits high thermal efficiency and safety and a film heating method of an energy saving type.

As shown in FIG. 19, a thermal fixing device based on the heating roller fusing method is basically comprised of a heating roller (fixing roller) 40 serving as a heating rotating member containing a heater 41 such as a halogen heater and an elastic pressurizing roller 50 serving as a pressurizing rotating member which comes into contact with the heating roller 40 to pressurize it. This pair of rollers are rotated, and a printing material P (a transfer sheet, print sheet, electrostatic printing sheet, electrofax sheet, or the like) serving as a material to be heated on which an unfused toner image is formed/borne is guided to the nip area (fusing nip area) between the roller pair. The printing material is then passed through the nip area to thermal-fuse the unfused toner image as a permanent fixed image on the printing material surface by using heat from the heating roller 40 and pressurizing force at the nip area. The heating roller 40 is configured such that a separating layer 43 made of fluororesin or the like is formed on the outer surface of a hollow cored bar 42 made of iron or the like. The pressurizing roller 50 is configured such that an elastic layer 52 made of silicone rubber or the like is formed on the surface of a cored bar 51 made of iron or the like, and a separating layer 53 such as a fluororesin tube is formed on the outer surface of the elastic layer. The heating roller 40 is heated by energizing the heater 41. The surface temperature of the heating roller 40 is detected by a temperature detection element such as a thermistor to be maintained at a predetermined temperature, thereby heating the nip area.

Thermal fixing devices based on the film heating method (on-demand fusing devices) are disclosed in, for example, Japanese Patent Laid-Open Nos. 63-313182, 2-157878, 4-44075, and 4-204980. FIG. 20 shows a typical example of these devices. Referring to FIG. 20, reference numeral 60 denotes a film assembly, which is configured such that a heater 61 having an electro heat-producing resistance layer formed on a ceramic substrate made of alumina, aluminum nitride, or the like is fixed to a stay holder 62 made of a heatproof resin, and a heatproof thin film 63 (to be referred to as a fusing film hereinafter) made of a resin such as polyimide or a metal such as SUS is loosely fitted on the stay holder 62. The heater 61 of the film assembly 60 and a pressurizing roller 50 clamp and pressurize the fusing film 63 to form a fusing nip area.

The fusing film 63 is conveyed/moved in the direction indicated by the arrow, while being in tight contact with and slid on the heater 61 at the fusing nip area, by the rotating/driving force of the elastic pressurizing roller 50 in the direction indicated by the arrow. The elastic pressurizing roller 50 is obtained by forming an elastic layer 52 made of silicone rubber or the like and a separating layer 53 made of fluororesin or the like on the surface of a cored bar 51. The temperature of the heater 61 is detected by a temperature detection means 64 such as a thermistor placed on the back of the heater and fed back to an energization control unit (not shown) to adjust the temperature of the heater 61 to a predetermined constant temperature (fusing temperature).

Various types of image forming apparatuses such as printers and copying machines which use such a thermal fixing device based on the film heating method have many advantages over image processing apparatuses using a thermal fixing device based on the conventional heating roller method. For example, they can eliminate the necessity of pre-heating and shorten the wait time because of high heating efficiency and quick startup.

Currently, a wide variety of printing materials used for image formation, which greatly change in thickness and surface properties, have been on the market. It is known that in the above conventional thermal fixing device, the fusing properties of toner images on such printing materials are influenced by the thicknesses and surface properties of the printing materials. The fusing properties are considerably degraded on a type of paper with rough surface properties, in particular. This is because the contact area between the heating member and the printing material decreases in the fusing nip area, and a sufficient amount of heat is not supplied to the toner on the printing material. As a consequence, in order to obtain good fusing properties even on a type of paper with poor surface properties, it is necessary to increase the fusing pressurizing force or fusing temperature.

The method of increasing the fusing pressurizing force, however, leads to an increase in the driving torque of the fusing device and hence tends to increase the device cost. In the above thermal fixing device based on the film system, in particular, since a fusing film that is a heating rotating member is slid on the heater serving as a heating member at the fusing nip area, the rotational torque tends to be high. This makes it difficult to increase the pressurizing force; the total pressure is limited to about 196 N (20 kgf) at most, and the linear pressure in the fusing nip area is set to be relatively low. It is therefore inevitable to raise the fusing temperature in order to improve the fusing properties of a type of paper with poor surface properties.

If, however, the fusing temperature is simply raised, an excessive amount of heat is supplied to a thin sheet or a sheet with good surface properties. This may lead to adverse effects, e.g., the occurrence of hot offset and an increase in the amount of curl of a sheet.

In addition, not only a fusing temperature but also a fusing nip width is important parameters for contradictory phenomena such as the fusing properties of toner images on printing materials, the curls of printing materials, and the hot offset of toner. That is, as the fusing nip width is increased, the time during which heat is transferred to a printing material is prolonged even at a low fusing temperature, and hence good fusing properties may be realized. In contrast, this suppresses the occurrence of phenomena such as the curls of printing sheets and the hot offset of toner.

Although the fusing nip width mainly depends on the hardness of a pressurizing roller and the pressurizing force of a pressurized spring, they change to some extent. Different fusing devices therefore have different fusing nip widths. For this reason, if fusing temperature setting is made in consideration of variations in fusing nip width, it is very difficult to satisfy requirements for all the phenomena such as fusing properties, curl, and hot offset with respect to various types of printing materials described above by setting only one temperature.

As described above, it is difficult to set fusing conditions optimal for both a printing material with rough surface properties and a printing material with good smoothness. Conventionally, in order to cope with this problem, a user selects a fusing temperature in accordance with the printing material to be used. It is, however, difficult for the user to set a fusing mode in accordance with a parameter like surface roughness that is incomprehensible to the user. For this reason, it has been required to automatically set an optimal fusing temperature in accordance with the printing material to be used (surface roughness in particular).

From this viewpoint, a method of feeding back information for fusing control by detecting the temperature of a printing material delivered from a fusing nip is disclosed in, for example, Japanese Patent Laid-Open Nos. 1-150185, 6-308854, 7-230231, and 2002-214961.

FIG. 21 shows an example of a conventional thermal fixing device designed to perform temperature detection by using a contact type sensor. In this thermal fixing device, a temperature sensor 18 such as a temperature detection thermistor is placed downstream of the fusing nip, and a facing member such as a rubber roller is placed at a position to face the temperature sensor 18 to clamp a printing material between them and measure its temperature.

FIG. 22 shows an example of a conventional thermal fixing device designed to perform temperature detection by using a non-contact type sensor. In this thermal fixing device, a non-contact type sensor 20 such as an infrared sensor is placed downstream of the fusing nip to measure the temperature of a printing material in a non-contact manner.

There has been proposed a method of preventing curl and hot offset by lowering the fusing temperature for a thin sheet or smooth sheet, which is easily heated, by feeding back information for energization control on the heater of a heating member on the basis of the measurement result on the delivery temperature of such a printing material, while satisfying requirements for fusing properties by raising the fusing temperature for a printing material with rough surface properties or a thick printing material.

The following problems, however, have arisen in the conventional thermal fixing device described above.

A method of detecting the temperature of a printing material while clamping it between a temperature sensor and a facing member such as a roller as shown in FIG. 21 will be described first. In this method, since the facing member of the temperature sensor is always in contact with a printing material, heat in the printing material is taken away by the facing member. This makes it impossible to accurately detect the temperature of the printing material. In order to stably clamp and convey a printing material, a rubber roller serving as a facing member needs to be formed to have a certain size. The heat capacity of the roller as the facing member cannot be neglected, and hence it is difficult to make a noticeable difference in the temperature detection thermistor in accordance with the surface roughness or thickness of a printing material. In addition, if a rubber roller is used as a facing member, the amount of heat taken away from a recoding material by the rubber roller changes depending on the surface properties of the rubber roller. If the surface condition of the rubber roller has changed after the passage of sheets, this causes a variation in detected temperature.

In addition, when the temperature of a printing material delivered from the fusing nip, consideration must be given to the influence of the dissipation of heat from the printing material delivered from the fusing nip. This is because the state of dissipation of heat from the printing material changes due to the influences of convection in the device and the temperature outside of the device in the area where the printing material delivered from the fusing nip is conveyed. For this reason, temperature detection is influenced more easily with the lapse of time after a printing material is delivered from the fusing nip, and hence different detection results are obtained by the temperature detection element even if identical printing materials are conveyed. For this reason, if the temperature detection element is placed next to the fusing nip, the influences of convection in the device and the like can be reduced, and the accuracy of the discrimination of a printing material can be improved.

On the other hand, if a temperature detection element such as a thermistor is placed next to the fusing nip, since the temperature of an atmosphere in which the temperature detection element changes depending on the heated state of the fusing roller and the fusing member for a fusing film or a past thermal fixing history, the type of printing material must be properly discriminated in each case.

From the above viewpoint as well, when a member such as a rubber roller having a large heat capacity is placed next to the fusing nip to face and come into contact with the temperature detection element, the rubber roller having a large heat capacity is easily influenced by the ambient temperature next to the fusing nip, and the heated state of the rubber roller placed to face the temperature detection element greatly changes. As a consequence, the amount of heat taken away from a printing material delivered from the fusing nip by the rubber roller changes. This makes it difficult to accurately discriminate the type of printing material in every case.

According to the prior art, in particular, a printing material is discriminated with a predetermined value by the temperature detection element placed next to the fusing nip. If, however, the temperature detection element is actually placed next to the fusing nip, the influence of ambient temperature cannot be neglected. This makes it difficult to accurately discriminate the type of printing material and optimally control the thermal fixing device.

A problem in temperature detection using a non-contact type sensor like the one shown in FIG. 22 will be described next. When a printing material is heated/fused, since moisture contained in the printing material is also heated at the same time, steam is produced from the surface of the printing material. The surface of the non-contact sensor is then covered with the steam. As a consequence, the sensor cannot correctly detect the temperature of the printing material. As described above, the temperature of a printing material is preferably detected immediately after it is delivered from the fusing nip in terms of reducing the influence of dissipation of heat from the printing material immediately after it is delivered from the fusing nip. On the other hand, temperature detection is most susceptible to the influence of steam from the printing material immediately after it is delivered from the fusing nip.

In order to prevent this, it may be conceivable to prevent the surface of a non-contact type sensor from being covered with steam by forming an air path or the like using a fan. In this case, however, this air path influences the temperature of the surface of a printing material. In consideration of variations in wind velocity, it is difficult to discriminate the printing material.

As described above, in practice, no technique has been developed to discriminate the type of printing material by measuring the temperature of the printing material using a non-contact type sensor such as an infrared sensor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image forming apparatus and its control method which can prevent deformation of a printing material and obtain stable fusing performance by automatically switching optimal fusing conditions in accordance with the type of printing material and the state of a fusing device.

An image forming apparatus and its control method according to an aspect of the present invention perform continuous printing operation which repeatedly sets a state in which when a preceding printing material passes through the nip area of a thermal fixing unit, a succeeding printing material has been supplied to an image forming unit and image forming operation has been started. In this continuous printing operation, first of all, a first temperature indicating an ambient temperature near the thermal fixing unit is measured at an early time point in the continuous printing operation. A second temperature indicating the temperature of the printing material which has passed through the press contact nip area at a predetermined time point during the continuous printing operation is measured. The amount of heat to be supplied to the succeeding printing material is controlled on the basis of the measured first and second temperatures.

Other and further objects, features and advantages of the present invention will be apparent from the following descriptions taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the descriptions, serve to explain the principle of the invention.

FIG. 1 is a schematic sectional view of an image forming apparatus according to the first embodiment of the present invention;

FIGS. 2 and 3 are schematic sectional views of a thermal fixing device according to the first embodiment of the present invention;

FIG. 4 is a schematic sectional view showing the thermal fixing device according to the first embodiment of the present invention and explaining the arrangement of a fused paper delivery guide;

FIG. 5 is a schematic sectional view showing the thermal fixing device according to the first embodiment of the present invention and explaining the arrangement of a delivery sensor unit;

FIG. 6 is a perspective view showing the schematic arrangement of a portion near a delivery sensor lever according to the first embodiment of the present invention;

FIG. 7 is a perspective view showing the schematic arrangement of a portion near the delivery sensor lever according to the first embodiment of the present invention when viewed from the opposite direction to that of FIG. 6;

FIG. 8 is a perspective view of a portion near the delivery sensor lever of the fused paper delivery guide according to the first embodiment of the present invention;

FIG. 9 is a flowchart showing operation in continuous printing by the image forming apparatus according to the first embodiment of the present invention;

FIG. 10 is a graph showing the transition of delivered sheet temperatures of each printing material in continuous printing operation;

FIG. 11 is a graph showing the transition of delivered sheet temperatures for each initial temperature in continuous printing operation;

FIG. 12 is a graph showing the delivered sheet temperature of the 30th sheet of each printing material for each initial temperature in continuous printing operation;

FIG. 13 is a view showing an example of a table describing the relationship between the number of sheets printed and the throughput in the first embodiment of the present invention;

FIGS. 14A to 14C are views showing experimental results for the confirmation of the effects of the first embodiment of the present invention;

FIG. 15 is a schematic sectional view of an image forming apparatus according to the second embodiment of the present invention;

FIG. 16 is a flowchart showing operation in continuous printing by the image forming apparatus according to the second embodiment of the present invention;

FIG. 17 is a graph showing an example of temperature control performed on a heater in accordance with the environmental temperature;

FIG. 18A is a graph showing the delivered sheet temperature of the 30th sheet of each printing material for each initial temperature in continuous printing operation at an environmental temperature of 15.degree. C.;

FIG. 18B is a graph showing the delivered sheet temperature of the 30th sheet of each printing material for each initial temperature in continuous printing operation at an environmental temperature of 25.degree. C.;

FIG. 18C is a graph showing the delivered sheet temperature of the 30th sheet of each printing material for each initial temperature in continuous printing operation at an environmental temperature of 35.degree. C.;

FIG. 19 is a view showing the arrangement of a thermal fixing device based on a conventional heating roller fusing system;

FIG. 20 is a view showing the arrangement of a thermal fixing device based on a conventional film heating method;

FIG. 21 is a view showing a conventional thermal fixing device which performs temperature detection by using a contact type sensor; and

FIG. 22 is a view showing an example of a conventional thermal fixing device which performs temperature detection by using a non-contact type sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. Note that the present invention is not limited by the disclosure of the embodiments and all combinations of the features described in the embodiments are not always indispensable to solving means of the present invention.

<First Embodiment>

(Schematic Arrangement of Image Forming Apparatus)

FIG. 1 is a schematic sectional view of an image forming apparatus according to this embodiment. This image forming apparatus includes a paper feed device comprising a paper feed tray 1, sheet stacking plate 2, and feed roller 3. Printing materials P stacked on the sheet stacking plate 2 in the paper feed tray 1 are picked up one by one by the feed roller 3, starting from the uppermost printing material, and conveyed to a registration unit by convey rollers 4 and 5. After the conveying direction of the printing material P is adjusted by the registration unit comprised of registration rollers 6 and 7, the printing material P is fed to an image forming unit.

The image forming unit is comprised of a photoconductive drum 8, toner cartridge 9, laser scanner unit 10, transfer drum 12, and the like. Although the illustration of the arrangement of the toner cartridge 9 will be omitted, the toner cartridge 9 has an arrangement in which a charger which charges the photoconductive drum 8, a developing device which develops a latent image on the photoconductive drum 8 with toner as a developing agent, a cleaner which removes and holds residual toner on the photoconductive drum 8, and the like are integrated into a unit. The laser scanner unit 10 is formed by integrating a polyhedral mirror 11, a motor (not shown) for rotating the polyhedral mirror, a laser unit, and the like into a unit.

A laser beam L based on image information is emitted from the laser scanner unit 10, and the photoconductive drum 8 is exposed to the laser beam, thereby forming a latent image based on the image information on the photoconductive drum 8 by the electrophotographic method. This latent image is developed by the developing device with toner. The developed toner image is transferred onto the printing material P by the transfer roller 12.

The printing material P on which the transfer of the toner image has been complete is conveyed to a thermal fixing device including a heating unit 13 and pressurizing roller 24. The transferred toner image is heated/fused by this device. Thereafter, the printing material P is delivered onto a delivered paper tray 17 by a paper delivery unit comprised of intermediate paper delivery rollers 15, paper delivery rollers 16, and the like.

Reference numeral 14 denotes a control circuit, which controls the image forming operation of the image forming unit and performs fusing control in the thermal fixing device.

(Arrangement of Thermal Fixing Device)

The arrangement of the image forming apparatus according to this embodiment has been described in brief. The arrangement of the thermal fixing device which heats/fuses an unfused toner image as a permanent image on the printing material P will be described in detail next.

FIGS. 2 and 3 are schematic sectional views showing the arrangement of the thermal fixing device according to this embodiment. FIG. 2 shows a state wherein no printing material P is supplied to the thermal fixing device. FIG. 3 shows a state wherein the printing material P bearing an unfused toner image T is passing through the thermal fixing device. Note that this embodiment uses the thermal fixing device (on-demand fusing device) based on the film heating method, which heats/fuses a toner image on a printing material through a thin film. However, the present invention is not limited to this on-demand fusing device, but can be applied to a thermal fixing system such as a heating roller system which heats a printing material while clamping/conveying it by using a heating roller maintained at a predetermined temperature and a pressurizing roller which pressurizes the heating roller by coming into contact with the heating roller through an elastic layer.

As described above, the printing material is conveyed to the thermal fixing device after the toner image is developed and transferred by the image forming unit comprised of the photoconductive drum 8, transfer roller 12, and the like. The leading end of the printing material is guided through a fusing inlet guide 21 to a nip area N formed by a heater 23 and the pressurizing roller 24 with a thin fusing film 22 being clamped between them.

The thin film 22 is a thin, flexible fusing film in the form of an endless belt, which serves as a heating rotating member and is made of a heat-resistant resin such as polyimide, polyamide, or PEEK, or a metal such as SUS or Ni. As the surface layer of this fusing film, a fluororesin layer made of PFA, PTFE, or the like is formed as a separating layer. The heat capacity of the thin film 22 is reduced to improve its quick start property. To satisfy a requirement for mechanical strength, the thickness of the base material made of polyimide, SUS, or the like is set to 100 .mu.m or less, preferably 60 .mu.m or less and 20 .mu.m or more, and the fluororesin layer serving as a separating layer is formed on the outer surface of the film to a thickness of about 8 .mu.m to 16 .mu.m. The thin film 22 in the form of an endless belt is fitted on an arcuated film guide member 25 formed from a liquid crystal polymer, ferrite, or a heat-resistance resin such as PPS, with some perimeter margin being ensured.

The pressurizing roller 24 serves as a rotating member for pressurization and has an elastic layer made of silicone rubber, silicone sponge rubber formed by foaming silicone rubber, or the like, which is formed on a cored bar made of iron, aluminum, or the like. In addition, the elastic layer is coated with a fluororesin layer made of PFA, PTFE, or the like as a separating layer or covered with it in the form of a tube.

The thin film 22 is rotated/driven upon rotating/driving of the pressurizing roller 24 in the clockwise direction indicated by the arrow at least at the time of the execution of image fusing operation in tight contact with the surface of the heating element (heater 23), while being slid on the surface of the heating element, at a predetermined peripheral velocity, i.e., a peripheral velocity almost equal to the convey speed of the printing material P bearing the unfused toner image T, which is conveyed from the image forming unit side (not shown), without any creases.

The heater 23 is, for example, a ceramic heater, which includes an electro heat-producing element (resistive heat-producing element) and raises the temperature by making the electro heat-producing element generate heat. This heating element uses alumina (Al.sub.2O.sub.3) or aluminum nitride (AlN) for its substrate. A heating element pattern having a desired resistive value is formed on the substrate by printing a thick resistive element made of silver or palladium on the substrate by a screen printing method or the like. In addition, a glass layer serving as a sliding layer for the protective layer/fusing film is formed on the heating element. A thermistor 23a serving as a heat temperature detection element is bonded/fixed on the reverse side to the surface on which the heating element is formed, or is brought into contact with the surface with a predetermined pressure, thereby monitoring the temperature of the heater. The monitor temperature information is input to the control circuit 14. In order to maintain the heater temperature at a predetermined temperature, the control circuit 14 controls a driver to control the amount of electricity supplied from an AC power supply to the heating element of the heater by a phase control method, wave number control method, or the like.

Assume that the electro heat-producing element of the heater 23 is heated by supplying power to the heating element, and the fusing film 22 is rotated/driven. In this state, when the printing material P is guided to the nip area N (fusing nip area) formed between the pressurizing roller 24 and the heater 23 by elastic force generated by the deformation of the elastic layer of the pressurizing roller 24, the printing material P passes through the nip portion N while being in tight contact with the fusing film 22 and stacked thereon.

In the process of passing through the fusing nip portion, the printing material P receives heat energy and pressurizing force from the heater through the fusing film 22. As a consequence, the unfused toner image on the printing material P is heated and thermal-fused. Thereafter, the printing material P passes through the fusing nip portion N and separates from the fusing film 22. The printing material P is then delivered and conveyed to the paper delivery unit by the paper delivery rollers 26 and 27.

A fused paper delivery guide 28 which forms a printing material convey path is provided between the fusing nip portion N and the paper delivery roller nip portion (formed by the paper delivery rollers 26 and 27). The fused paper delivery guide 28 is made of a heatproof material such as PBT or PET. The convey surface of the fused paper delivery guide 28 is set below a line A connecting the fusing nip portion N and the paper delivery roller nip portion (see FIG. 4). In addition, the printing material convey speed at the paper delivery roller 26 is set to be higher than that at the fusing nip portion N. This prevents the printing material P from coming into direct contact with the convey surface of the fused paper delivery guide 28 when a printing material passes.

A paper delivery sensor unit which detects the presence/absence of a printing material passing through the thermal fixing device is placed on the printing material convey path extending from the thermal fixing device to the paper delivery unit. This paper delivery sensor unit is mainly comprised of a paper delivery sensor lever 29 and photointerrupter 30.

FIGS. 6 and 7 are perspective views showing the schematic arrangement of this paper delivery sensor unit. FIG. 7 is a view showing this unit when viewed from the opposite side to FIG. 6. The paper delivery sensor lever 29 is mainly formed from a plastic member with high slidability such as polyacetal, and a printing material passage portion at its distal end is placed as a home position at a position to shield the line A (FIG. 4) connecting the fusing nip portion N and the paper delivery roller nip portion. A shielding member 29a protrudes from the paper delivery sensor lever 29 in a direction perpendicular to the lever body. The paper delivery sensor lever 29 is configured to integrally pivot about a fulcrum S. When a printing material passes, the paper delivery sensor lever 29 falls in the printing material convey direction (see FIG. 3). Along with this movement, the shielding member 29a shields infrared light from the photointerrupter 30. When there is no printing material, the paper delivery sensor lever 29 returns to the home position, and the shielding member 29a comes to a position where it does not shield infrared light from the photointerrupter 30 (see FIG. 1). By moving the paper delivery sensor lever 29 in this manner, infrared light from the photointerrupter 30 is turned on/off to detect the presence/absence of a printing material.

As will be described below, in the image forming apparatus of this embodiment, the paper delivery sensor unit placed in the above manner is provided with a delivered sheet temperature detection unit comprised of a heat collecting plate and temperature detection sensor.

This delivery paper temperature detection unit is configured to detect the temperature of the unprinted surface of a printing material which has undergone image fusing and is delivered from the thermal fixing device. There are two merits in detecting the temperature of the unprinted surface of a printing material. First, in normal single-sided printing, a printing material comes into contact with the heat collecting plate at the surface opposite to the surface on which toner is fused, temperature detection is free from the influence of the adhesion of toner to the heat collecting plate. That is, there is no chance of degradation of temperature detection accuracy due to the adhesion of toner to the heat collecting plate. Second, since heat energy is mainly supplied from the heater 23 to the printed surface of a printing material through the fusing film 22, detecting the temperature of the unprinted surface makes it possible to predict the characteristics of each printing material from the detected temperature by using the differences in temperature gradient characteristics among printing materials which are based on heat conduction from the printing material printed surface side to the unprinted surface side. For example, the temperature on the unprinted surface side of a thin printing material is higher than that on the unprinted surface side of a thick printing material. A printing material with smooth surface properties can obtain higher adhesion properties with respect to the fusing film 22 at the fusing nip portion than a printing material with rough surface properties, and hence makes heat energy conduct more readily. As a consequence, the temperature on the unprinted surface side of the printing material with smooth surface properties becomes higher than that of the other.

FIG. 5 is a detailed view of a portion near the paper delivery sensor lever 29. At the printing material passage portion on the distal end of the paper delivery sensor lever 29, the heat collecting plate 31 formed from a thin plate which has a thickness of about 0.1 mm and is made of aluminum, stainless steel, or the like which has a small heat capacity is integrally formed with the paper delivery sensor lever 29 by outsert molding or the like. The heat collecting plate 31 is brought into contact with the unprinted surface of a printing material delivered from the thermal fixing device by a biasing means such as a spring.

The heat collecting plate 31 is placed above the line A connecting the fusing nip portion and the paper delivery roller nip portion (see FIG. 4). The leading end of a printing material which has passed through the fusing nip portion comes into contact with the plastic portion of the paper delivery sensor lever 29 first. When the printing material is further fed to the downstream side, the paper delivery sensor lever 29 pivots, and the heat collecting plate 31 comes into contact with the unprinted surface of the printing material. Reducing the heat capacity of the heat collecting plate 31 and actively bringing the heat collecting plate 31 into contact with a printing material in this manner can raise the temperature of the heat collecting plate 31 within a short period of time in accordance with the temperature of the printing material. In this case, in order to reduce the heat capacity of the heat collecting plate 31, it is preferable to minimize the size of the heat collecting plate 31 in the printing material convey direction and a direction which is perpendicular to the printing material convey direction and almost parallel to the width of a printing material.

In performing double-sided printing, when the second surface of a printing material passes through the fusing nip portion, the heat collecting plate 31 on the paper delivery sensor lever comes into contact with the first surface of the printing material, i.e., the printed surface. Therefore, toner may adhere to the surface of the heat collecting plate. In order to prevent this, the surface of the heat collecting plate 31 may be coated with fluororesin such as PFA or PTFE, or may be subjected to surface treatment such as UV coating within the range in which no influence is exerted on the heat conduction of the heat collecting plate 31. Alternatively, a PI (polyimide) coating or the like may be formed on the surface of the heat collecting plate 31.

A temperature sensor (delivered sheet temperature sensor) 32 with a fast response characteristic such as a thermistor is bonded to the rear surface of the heat collecting plate 31 at the distal end of the paper delivery sensor lever by adhesion or the like. When the printing material P on which an image is fused is conveyed from the thermal fixing device, the paper delivery sensor lever 29 pivots, and the heat collecting plate 31 comes into contact with the unprinted surface of the printing material P. As a consequence, the heat collecting plate 31 takes away heat from the printing material and conducts the heat to the delivered sheet temperature sensor 32 placed on the rear surface, thereby making the sensor detect a temperature dependent on the temperature of the printing material. When the paper delivery sensor lever 29 pivots, i.e., the paper delivery sensor lever 29 detects the presence of the printing material, since the delivered sheet temperature sensor 32 is placed immediately below the position where the printing material P and the heat collecting plate 31 come into contact with each other, the accuracy of the detected temperature dependent on the temperature of the printing material is improved by minimizing the influence of a temperature gradient in the heat collecting plate. In addition, using a metal member for a sliding portion which slides on a printing material makes it possible to prevent the wear of the sliding portion and improve the durability of the paper delivery sensor lever 29.

Providing the heat collecting plate 31 and the delivered sheet temperature sensor 32 such as a thermistor on the paper delivery sensor lever which detects the presence/absence of a printing material in this manner can accurately synchronize the position information and temperature information of the printing material and hence can accurately detect at which position on the printing material the temperature information output from the thermistor has been obtained. For example, temperature information from the thermistor indicates that the temperature of the trailing end portion of a printing material tends to be higher than that of the leading end portion. Therefore, by synchronizing the temperature information of the printing material with the position information, a temperature dependent on the temperature of the printing material can be detected more accurately.

A thermistor is an element which changes its resistance with a change in temperature and is sealed in glass while a Dumet wire 33 is fused to the electrode of a thermistor chip. The paper delivery sensor lever 29 is configured such that two electrodes 34 made of a metal such as stainless steel are integrally formed with a plastic portion by outsert molding or the like (see FIGS. 6 and 7). The Dumet wire 33 described above is welded to the two electrodes. These electrodes are further connected to the control circuit 14 to transfer the temperature information detected by the thermistor.

Each electrode 34 is formed from a thin sheet metal which is about 0.1 mm thick and made of stainless steel, and has both the function of transferring temperature information from the thermistor to the control circuit 14 and the function of biasing pivoting force to the paper delivery sensor lever 29. One end portion of each electrode 34 is integrally formed with the plastic portion of the paper delivery sensor lever 29 and welded to the Dumet wire 33 of the thermistor. The other end portion of each electrode 34 is connected to a terminal (not shown) fixed to the fused paper delivery guide 28. As the paper delivery sensor lever 29 pivots, the electrodes 34 move together with the paper delivery sensor lever 29 and are twisted from the fixed terminal connecting portions, thereby biasing pivoting force to the paper delivery sensor lever 29 in a direction to return it to the home position. Each electrode 34 is formed in the shape of a crank so as to bias proper pivoting force to the paper delivery sensor lever 29 and prevent itself from being permanently deformed or fractured upon receiving repetitive stress.

The distal end portion of the paper delivery sensor lever will be described in more detail. As described above, the heat collecting plate 31 made of a material with a small heat capacity is integrally formed with the paper delivery sensor lever 29 made of a plastic material with a low thermal conductivity at the distal end portion of the paper delivery sensor lever. In this case, the rea