Patent Application
20210263455
The present disclosure relates to a fixing unit for fixing an image on a recording material, and an image forming apparatus for forming an image on a recording material.
An example of a fixing unit that adopts a heat-fixing system installed in a printer or a copying machine of an electrophotographic system is equipped with a heater having a heating resistor provided on a substrate formed of ceramics or the like, a fixing film that moves while being in contact with the heater, and a pressure roller arranged to oppose to the heater with the fixing film interposed therebetween. The recording material that bears an unfixed toner image is heated while being nipped and conveyed by a nip portion, i.e., fixing nip portion, formed between the fixing film and the pressure roller, by which the toner image borne on the recording material is heated and fixed to the recording material.
In the above-described fixing unit, especially when a recording material that is dry and has a high electric resistance is passed through, there may occur a case where a surface of a pressure roller is charged-up by friction between the recording material and the pressure roller, causing image defects called “electrostatic offset” in which unfixed images on the recording material are peeled away. Japanese Patent Application Laid-Open Publication No. H06-202509 discloses a technique in which a conducting plane being exposed at a portion of a fixing film and a conductive elastic member provided on a core metal of a pressure roller are in contact with one another at a fixing nip portion to realize conduction, while having the fixing film and the pressure roller grounded to prevent charge-up.
However, in recent years, the speed of print output by the image forming apparatus has advanced, and the amount of charge-up of the pressure roller caused by the friction between the recording material and the pressure roller tends to be increased. However, an attempt to reduce electrostatic offset by providing a conductive layer to the fixing film and applying a bias voltage for cancelling out charge-up of the pressure roller may result in electrical breakdown of a part of a surface layer of the fixing film.
The present disclosure provides a fixing unit and an image forming apparatus that can reduce occurrence of electrostatic offset which reducing electrical breakdown.
According to one aspect of the invention, a fixing unit includes a film with a tubular shape, a nip forming unit including a heater and configured to be in sliding contact with an inner surface of the film. The heater includes a substrate made of metal, an insulating layer formed on the substrate, a heating element arranged on the insulating layer and configured to generate heat when electric current flows therethrough, and a power supply electrode electrically connected to the heating element. The fixing unit further includes a power supply connector electrically connected to the power supply electrode so that the power supply connector is connected to the heating element at a first end portion of the substrate in a longitudinal direction of the heater, a pressing member opposed to the nip forming unit with the film interposed therebetween and configured to form a nip portion with the film A recording material is nipped and conveyed at the nip portion while an image formed by toner on the recording material is heated and fixed to the recording material. The fixing unit further includes a voltage application circuit configured to apply a voltage of a same polarity as a normal charging polarity of the toner to the substrate, the voltage application circuit being connected to the substrate at a second end portion of the substrate opposite to the first end portion in the longitudinal direction.
According to another aspect of the invention, a fixing unit includes a film with a tubular shape, a nip forming unit including a heater and configured to be in sliding contact with an inner surface of the film. The heater includes a substrate made of metal, an insulating layer formed on the substrate, and a heating element arranged on the insulating layer and configured to generate heat when electric current flows therethrough. The substrate is electrically grounded. The fixing unit further includes a pressing member opposed to the nip forming unit with the film interposed therebetween and configured to form a nip portion with the film, wherein a recording material is nipped and conveyed at the nip portion while an image formed by toner on the recording material is heated and fixed to the recording material, and a voltage application circuit configured to apply a voltage of an opposite polarity as a normal charging polarity of the toner to the pressing member.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Exemplary embodiments according to the present disclosure will be described with reference to the drawings.
When a print command is received by the printer 100, a scanner unit 3 emits laser light L according to image information to a photosensitive member 1 serving as an image bearing member. The photosensitive member 1 charged to predetermined polarity by a charge roller 2 is scanned by laser light L, and an electrostatic latent image according to image information is thereby formed on a surface of the photosensitive member 1. Thereafter, toner is supplied to the photosensitive member 1 from a developing unit 4, and a toner image corresponding to the image information is formed on the photosensitive member 1. The toner image having reached a transfer portion, i.e., transfer nip portion, that has been formed between the photosensitive member 1 and a transfer roller 5 serving as a transfer unit along with the rotation of the photosensitive member 1 in the direction of arrow R1 is transferred onto a recording material P fed from a cassette 6 by a pickup roller 7. The surface of the photosensitive member 1 having passed the transfer nip portion is cleaned by a cleaner 8. The recording material P to which toner image t (
Thereafter, the recording material P is discharged onto a tray 11 by a sheet discharge roller 10. Various types of sheets of different sizes and materials may be used as the recording material P, such as paper including normal paper and thick paper, plastic films, cloth, coated paper and other sheet materials subjected to surface treatment, and sheets of special shapes such as envelopes and index paper. The present example is illustrated based on a system where toner image is directly transferred from the photosensitive member 1 to the recording material P, but it is also possible to apply the technique illustrated hereafter to an image forming apparatus that adopts a system where toner image formed on the photosensitive member is transferred to the recording material via an intermediate transfer member such as an intermediate transfer belt.
The fixing unit 9 will now be described. The fixing unit 9 is a tensionless-type film heating system. That is, the fixing unit 9 uses a fixing film in the form of an endless belt, or a round tubular shape, having flexibility as a heat resistant film, and adopts a configuration where at least a part of the circumference of the fixing film is constantly tensionless and the fixing film rotates by rotational driving force of the pressing member.
Hereafter, the fixing unit 9 of the film heating system according to the present embodiment will be described in detail.
The fixing unit 9 according to the present embodiment includes, as illustrated in
The heater holder 21 is a molded component formed of heat-resistant resin such as PPS (polyphenylene sulfide) or liquid crystal polymer. The heater 22 includes a substrate mainly composed of a pure metal or an alloy and having an elongated plate shape, i.e., metal substrate, a resistance heating element, i.e., heating element, that generates heat by electric power conduction, an insulating layer for insulating the resistance heating element and the substrate, and a glass coat layer for protecting the heating element. The details of the heater 22 will be described later.
A thermistor 25 serving as a temperature detecting element is abutted against the heater 22 at an opposite side, that is, upper side in the drawing, from an abutting surface against the fixing film 23. By controlling the electric power conduction to the heating element in accordance with the detection temperature of the thermistor 25, the temperature of the fixing nip portion Nf is maintained at a set temperature suitable for fixing the image.
The thickness of the fixing film 23 is preferably between 20 μm and 100 μm to ensure good thermal conductivity. A single-layer film formed of a material such as PTFE (polytetrafluoroethylene), PFA (tetrafluoroethylene—perfluoro alkyl vinyl ether copolymer) or PPS is suitable as the fixing film 23. Further, a composite layer film in which a surface of a base layer formed of a material such as PI (polyimide), PAI (polyamide imide), PEEK (polyether ether ketone) or PES (polyethersulfone) is coated with a material such as PTFE, PFA or FEP (tetrafluoroethylene—hexafluoropropylene copolymer) as a release layer, i.e., surface layer, is also suitable as the fixing film 23. Even further, it is also suitable to use a pure metal or an alloy having high thermal conductivity as the base layer, and to apply the aforementioned coating treatment and coating of a fluororesin tube to the release layer. The pure metal may be Al, Ni, Cu or Zn, and the alloy may be a stainless steel or an alloy of Al, Ni, Cu and/or Zn.
According to the present embodiment, PI having a thickness of 60 μm was used as the base layer of the fixing film 23, and coating of PFA having a thickness of 12 μm was provided as the release layer, considering both wear of the release layer by passing of sheets and thermal conductivity.
The pressure roller 30 serving as a pressing member, i.e., pressurizing rotary member, includes a core metal 30a formed of a material such as iron or aluminum, an elastic layer 30b formed of a material such as silicone rubber, and a release layer 30c formed of a material such as PFA (
The configuration of the fixing unit will now be described with reference to the cross-sectional view of
Next, the present embodiment is described by referring to the perspective view of
Both end portions of the reinforcement member 24 in the longitudinal direction of the heater 22 are projected portions that protrude from both ends of the fixing film 23, having flange members 26 and 26 respectively fit thereto. The fixing film 23, the heater 22, the heater holder 21, the reinforcement member 24 and the flange members 26 and 26 are assembled together as the film assembly 20.
A power feed terminal of the heater 22 is also protruded from one side (i.e., first end portion) in the longitudinal direction of the heater 22 with respect to the fixing film 23, and a power supply connector 27 is fit to the power feed terminal. The power supply connector 27 is in contact with an electrode portion of the heater 22 with a certain contact pressure and constitutes a power supply path for supplying power fed from a commercial power supply to the heater 22.
A heater clip 28 is attached to the other side (i.e., second end portion opposite to the first end portion, that is, the side opposite to the power feed terminal) of the heater 22 in the longitudinal direction of the heater 22. The heater clip 28 is a metal plate that is bent in a U shape and has a spring property that enables the end portion of the heater 22 to be held on the heater holder 21.
Next, the present embodiment is described with reference to the front view of
Further, the length of the pressure roller 30 in the longitudinal direction of the heater 22 is set approximately 10 mm shorter than the fixing film 23. This arrangement is adopted to prevent grease from leaking from ends of the fixing film 23 and adhering to the pressure roller 30, causing the pressure roller 30 to lose its gripping force on the recording material and slip.
The film assembly 20 is arranged to oppose to the pressure roller 30 and supported on a top-side casing 41 of the fixing unit 9 in a state where movement in the longitudinal direction of the heater 22, i.e., right-left directions in the drawing, is restricted and movement in the vertical direction is enabled. A pressurizing spring 45 serving as a pressurizing member is attached in a compressed manner to the top-side casing 41. The pressing force of the pressurizing spring 45 is received by the projected portion of the reinforcement member 24, and by having the reinforcement member 24 press against the pressure roller 30, the whole film assembly 20 is pressed against the pressure roller 30.
A bearing member 31 is provided to bear the core metal of the pressure roller 30 (refer also to
The bottom-side casing 43 and the top-side casing 41 constitute a casing, i.e., frame member, of the fixing unit 9 together with frame side panels 42 and 42 that are provided on both sides of the film assembly 20 in the longitudinal direction of the heater 22 and extend upward and downward.
Next, materials constituting the heater 22 according to the present embodiment and a method for manufacturing the same will be described with reference to
Materials such as stainless steel, nickel, copper and aluminum or an alloy mainly composed of these metals are suitably used as the material for the substrate 22a. Among these materials, stainless steel is most preferable from the viewpoint of strength, heat resistance and corrosion. The type of stainless steel is not specifically limited, and any type can be selected as required considering necessary mechanical strength, linear expansion coefficient corresponding to the shape of the insulating layer and the heating element described in the next section, availability of the plate material in the market, and so on.
As an example, a martensitic- or ferritic-type chromium-containing stainless steel has a relatively low linear expansion coefficient among stainless steels and is suitable since it can be easily applied to forming an insulating layer and a heating element.
The thickness of the substrate 22a is determined considering strength, heat capacity and radiation performance. A thin substrate 22a is advantageous for realizing a quick-start performance, that is, shortness of time from starting of electric power conduction to reaching a target temperature of the heater 22, since it has a small heat capacity, but if it is too thin, a problem such as distortion of the heating resistor during sintering (firing) treatment tends to occur. In contrast, a thick substrate 22a is advantageous from the viewpoint of preventing distortion of the heating resistor during sintering treatment, but excessive thickness increases the heat capacity and is disadvantageous in realizing a quick start. Preferable thickness of the substrate 22a, considering the balance of mass productivity, cost and performance, is between 0.3 mm and 2.0 mm.
The material of the insulating layers 22b and 22e is not specifically limited, but it is necessary to select an insulating material having heat resistance in view of the actual temperature during use. The material of the insulating layers 22b and 22e is preferably glass or PI (polyimide) from the viewpoint of heat resistance, and in the case of glass, the actual powder material to be used is selected within a range not deteriorating the characteristics of the present embodiment. A heat-conductive filler having an insulating property may be mixed as needed. There is no problem in using the same material or different materials for the insulating layers 22b and 22e. Similarly, the thickness may be the same within the insulating layers 22b and 22e or varied as needed.
In general, the heater 22 to be used in the image forming apparatus preferably has a dielectric voltage of approximately 1.5 kV. Therefore, the thickness of the insulating layer 22b is determined according to the material to realize a dielectric voltage performance of 1.5 kV between the heating element 22c and the substrate 22a.
The method for forming the insulating layers 22b and 22e is not specifically limited, but as an example, the insulating layers 22b and 22e can be formed smoothly by adopting screen printing. When forming an insulating layer of glass or PI (polyimide) on the substrate 22a, it is necessary to adjust the linear expansion coefficients of the substrate and the insulating layer material as required so that cracking and peeling do not occur in the insulating layer by the difference between linear expansion coefficients of the materials.
The heating element 22c is formed by printing a heating resistor paste having mixed (A) conductive component, (B) glass component and (C) organic binder component onto the insulating layer 22b, and then sintering the same. When the heating resistor paste is sintered, the (C) organic binder component is burnt out and only components (A) and (B) are left, so that the heating element 22c containing the conductive component and the glass component is formed.
In the embodiment, materials such as silver-palladium (Ag—Pd) and ruthenium oxide (RuO2) are used alone or in combination as the conductive component (A), and a sheet resistance of 0.1 [Ω/□] to 100 [KΩ/□] is preferable. Materials other than those mentioned above in (A) to (C) can also be contained as long as the amount is subtle enough so as not to deteriorate the characteristics of the present embodiment.
A power supply electrode 22f and a conductive pattern 22g illustrated in
Note that, it is desirable to select a material that softens and melts at a temperature lower than a melting point of the substrate 22a and a material that has sufficient heat resistance in consideration of the temperature during actual use as the aforementioned heating resistor paste and the paste for forming the power supply electrode and conductive pattern.
As illustrated in
In the present embodiment, a ferritic stainless-steel substrate (18 Cr stainless-steel) having a width of 10 mm, a length of 300 mm and a thickness of 0.5 mm was prepared as the substrate 22a.
Next, the glass paste for forming the insulating layer was applied on the aforementioned stainless-steel substrate by screen printing, and then dried at 180° C. and sintered at 850° C. to form the insulating layers 22b and 22e. The thickness of the insulating layers 22b and 22e after sintering was each 60 μm on both sides of the stainless-steel substrate.
Thereafter, a heating resistor paste and a paste for forming a power supply electrode and a conductive pattern were prepared. The heating resistor paste contains silver-palladium (Ag—Pd) as the conductive component, with a glass component and an organic binder component mixed thereto. The paste for forming the power supply electrode and the conductive pattern contains silver as the conductive component, with a glass component and an organic binder component mixed thereto. The respective pastes were applied to the stainless-steel substrate by screen printing, and then dried at 180° C. and sintered at 850° C. to form the heating element 22c, the power supply electrode 22f and the conductive pattern 22g. After sintering, the thickness of the heating element 22c was 15 μm, the length was 220 mm and the width was 1.1 mm.
Next, the glass paste for the cover layer was prepared, and the glass paste for the cover layer was applied on the heating element 22c and the conductive pattern 22g by screen printing, and then dried at 180° C. and sintered at 850° C. to form the cover layer 22d. The thickness of the cover layer 22d after sintering was 60 μm.
As illustrated in
The power supply DC1 applies a voltage of a same polarity as a normal charging polarity, which according to the present embodiment is negative polarity, of toner used for forming images through the bundle wire 29 and the heater clip 28 to the substrate 22a. In other words, the power supply DC1, the bundle wire 29 and the heater clip 28 constitute a voltage application circuit that applies voltage to the substrate 22a. A value of the voltage applied to the substrate 22a is adjusted as required to reduce electrostatic offset considering surface potential of the pressure roller 30 during fixing operation.
Moreover, a resistance value of the release layer 30c of the pressure roller 30 is adjusted to form an electric field between the substrate 22a and the pressure roller 30. A range of preferable resistance value is, for example, 1.0×108 [Ω/□] to 1.0×1013 [Ω/□] (sheet resistance (surface resistance) when a voltage of 500 V is applied: measured using ADVANTEST R8340A, a product of Advantest Corp.). As illustrated in
According to the present embodiment, by applying voltage to the substrate 22a, an electric field having a desired strength is formed in the fixing nip portion Nf and electrostatic offset is thereby reduced, without supplying current between the fixing film 23 and the pressure roller 30.
The effects of the present embodiment will be described through comparison with a comparative example. At first, a configuration where voltage is applied to a base layer 23a of the fixing film 23 as illustrated in
In order to verify the effects of the present embodiment, the voltage applied to the substrate 22a was varied, and the occurrence of electrostatic offset and the occurrence of electrical breakdown of a release layer 23b of the fixing film 23 were confirmed. The confirmation was performed with the voltage applied to the substrate 22a ranging between −100V and −1000 V.
The electrostatic offset was tested under a low temperature and low humidity (temperature: 15° C., humidity: 10%) environment. A Xerox Vitality Multipurpose Paper (Xerox Corp., letter size, 75 g/m2 grammage: hereinafter referred to as “Xerox paper”) and a Neenah Bond Writing Paper (Neenah Inc., letter size, 60 g/m2 grammage, cotton content of 25%: hereinafter referred to as “Neenah paper”), both having been left for two days under the above-mentioned low temperature and low humidity environment, were used. Neenah paper had higher paper surface resistance than Xerox paper, and the condition of Neenah paper is disadvantageous from the viewpoint of electrostatic offset. A halftone image with isolated dots of 600 dpi in which electrostatic offset is likely to occur are printed in the area of 5 mm to 20 mm from a leading edge of the sheet was printed as an evaluation image. Evaluation was performed by continuously printing images to 100 sheets and confirming whether soiling by offset toner has occurred to a solid white paper surface on a trailing side of 20 mm or farther from the leading edge of the paper.
Electrical breakdown of the release layer of the fixing film 23 was confirmed by the following method. As illustrated in
Table 1 shows the results of examination of the electrostatic offset and electrical breakdown tests of Neenah paper and Xerox paper according to the present embodiment and the comparative example. According to the configuration of the present embodiment, the current applied to the substrate 22a was interrupted by the insulating layer 22b, so that no occurrence of electrical breakdown was confirmed at the release layer 23b of the fixing film 23 even when a voltage as high as −1000 V was applied. Therefore, it was confirmed that electrostatic offset could be suppressed even by using a type of paper such as the Neenah paper that has a high resistance and is disadvantageous from the viewpoint of electrostatic offset. Meanwhile, based on the configuration of the comparative example, electrical breakdown of the release layer 23b had occurred at a point of time when a voltage of −750 V had been applied. According to the test, it was not possible to apply a voltage of −1000 V which is a voltage necessary to eliminate electrostatic offset of the Neenah paper, and so it was confirmed that the comparative example could only provide measures to cope with a recording material having a resistance equivalent to Xerox paper at most.
As described, according to the present embodiment, an electric field is formed in the fixing nip portion Nf by applying voltage to the substrate 22a, without supplying current between the fixing film 23 and the pressure roller 30. Thereby, the occurrence of electrostatic offset can be suppressed while preventing electrical breakdown of the release layer 23b, even if a high resistance paper such as Neenah paper was used.
According to the present embodiment, voltage was applied to the substrate 22a using an external power supply, but voltage can also be applied to the substrate 22a through a bypass from a charging bias, i.e., charging voltage, or a developing bias, i.e., developing voltage, having a same polarity as normal charging characteristics of toner. In that case, the high-voltage circuit board installed in the printer 100 and outputting these voltages constitutes the voltage application circuit, instead of the power supply DC1 according to the present embodiment.
Further according to the present embodiment, the resistance of the release layer 30c of the pressure roller 30 was set within the range of 1.0×108 [Ω/□] to 1.0×1013 [Ω/□], but the present disclosure is not limited to the above-mentioned range. A configuration can be adopted where electric conductivity is applied to the elastic layer 30b of the pressure roller 30 itself to provide grounding through the core metal 30a.
If the pressure roller 30 is designed to have an insulating property, a similar effect as the present embodiment can be achieved by adopting a configuration where the sheet discharge roller 10 arranged downstream of the fixing unit 9 in the conveyance direction of the recording material has conductivity to provide grounding.
A configuration of grounding the substrate 22a of the heater 22 and applying voltage to the pressure roller 30 to suppress electrostatic offset is illustrated as a second embodiment. A configuration similar to the first embodiment is adopted except for the configuration related to grounding of the substrate 22a and voltage application to the pressure roller 30, so that similar elements are denoted with the same reference numbers as the first embodiment and descriptions thereof are omitted.
As illustrated in
Table 2 shows the results of examination of the electrostatic offset and electrical breakdown tests of Neenah paper and Xerox paper according to the present embodiment. The outline of the tests is the same as the first embodiment, and descriptions thereof are omitted.
According to the configuration of the present embodiment, if voltage is applied to the release layer 30c of the pressure roller 30, approximately no current is supplied to the substrate 22a owing to the presence of the insulating layer 22b, the occurrence of electrical breakdown of the release layer 23b was not confirmed even if a voltage as high as +1000 V was applied. Therefore, it was confirmed that electrostatic offset could be suppressed even by using a type of paper such as the Neenah paper that has a high resistance and is disadvantageous from the viewpoint of electrostatic offset.
According to the present embodiment, voltage was applied to the substrate 22a using an external power supply, but voltage can also be applied to the release layer 30c of the pressure roller 30 through a bypass from a transfer bias and the like having a different polarity as the charging characteristics of toner.
Further according to the present embodiment, the release layer 30c of the pressure roller 30 has conductivity, but a configuration can be adopted where the elastic layer 30b of the pressure roller 30 has conductivity and voltage is applied to the elastic layer 30b through the core metal 30a.
According further to the fixing unit of the respective embodiments described above, the heater 22 is directly in contact with the inner surface of the film, but it is also possible to arrange a sheet having a high thermal conductivity, such as a sheet formed of a material such as ferrous alloy or aluminum, between the heater and the inner surface of the film. In other words, a nip forming unit with a configuration where the heater heats the film through a sheet can be adopted.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2020-026803, filed on Feb. 20, 2020, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-026803 | Feb 2020 | JP | national |
