Patent Grant
9423738
This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-129471 filed Jun. 29, 2015.
(i) Technical Field
The present invention relates to a heat generating unit, a fixing unit, and an image forming apparatus.
(ii) Related Art
In recent years, to achieve an energy-saving and convenient fixing unit and image forming apparatus that require short rise time, there is a demand for reduction in heat capacity of a heating source, such as a heater, and a member to be heated, such as a fixing belt, of a fixing unit and image forming apparatus in which a fixing belt is heated by a heater (heat generating unit) disposed inside an endless fixing belt, through heat conduction.
Such a fixing unit and image forming apparatus having reduced heat capacity tend to cause overheating due to the small heat capacity, so, there is also a demand for a mechanism for preventing fuming and smell due to overheating, occurring when the temperature control becomes defective.
According to an aspect of the invention, there is provided a heat generating unit including a substrate; a heat-generating element that is provided on the substrate and generates heat by receiving electric power; and a thermal destruction element provided on the substrate and connected in series to the heat-generating element, the thermal destruction element having a positive temperature coefficient and causing thermal destruction due to self-heating when heated to a temperature higher than a certain temperature by the heat of the heat-generating element. Note that “a certain temperature” as used herein is a temperature at which the maximum resistance can be obtained.
Exemplary embodiment of the present invention will be described in detail based on the following figures, wherein:
An exemplary embodiment of the present invention will be described below with reference to the drawings.
A printer 10 shown in
Sheet trays 21 are provided at the bottom of the printer 10. The sheet trays 21 each accommodate a stack of sheets P. The sheet trays 21 are configured such that they may be freely pulled out for supply of sheets P. The sheet trays 21 may accommodate OHP sheets, plastic paper, envelopes, etc., serving as recording media of the present invention, instead of the paper sheets P. Although the operation of the printer 10 will be described with reference to
A sheet P in one of the sheet trays 21 is sent to standby rollers 24 by a pickup roller 22 and separating rollers 23. At the standby rollers 24, the transportation timing of the sheet P is adjusted, and the sheet P is transported further on.
The printer 10 includes a cylindrical photoconductor 12 that rotates in a direction indicated by arrow A. A charger 13, an exposure unit 14, a developing unit 15, a transfer unit 16, and a photoconductor cleaner 17 are arranged around the photoconductor 12. The photoconductor 12, the charger 13, the exposure unit 14, the developing unit 15, and the transfer unit 16 are collectively an example of a forming unit of the present invention.
The charger 13 charges the surface of the photoconductor 12, and the exposure unit 14 exposes the surface of the photoconductor 12 according to an image signal transmitted from the controller 11, thus forming an electrostatic latent image. The electrostatic latent image is developed by the developing unit 15 into a toner image.
Herein, the standby rollers 24 send the sheet P such that the sheet P reaches a position facing the transfer unit 16, at the time when the toner image on the photoconductor 12 reaches the aforementioned position. Then, the toner image on the photoconductor 12 is transferred to the sheet P sent to the aforementioned position by the transfer unit 16. In this manner, an unfixed toner image is formed on the sheet P.
The sheet P having the unfixed toner image thereon moves further in an arrow B direction and is heated and pressed by a fixing unit 18. Thus, the toner image is fixed onto the sheet P. As a result, an image, formed of a fixed toner image, is formed on the sheet P. The fixing unit 18 corresponds to an exemplary embodiment of a fixing unit of the present invention.
The sheet P that has passed through the fixing unit 18 advances in an arrow C direction toward an output unit 19. The sheet P is further sent in an arrow D direction by the output unit 19 and is output onto a sheet output tray 20.
The fixing unit 18 includes a pressure roller 110 and a heating roller 120.
The pressure roller 110 is formed of a metal core and a rubber layer formed thereon. The pressure roller 110 rotates in an arrow E direction. The pressure roller 110 is an example of a pressure member of the present invention.
The heating roller 120 has an outer circumferential belt 121. A heater 122, a pressure pad 123, etc. are accommodated inside the outer circumferential belt 121. The outer circumferential belt 121 is an example of a revolving member of the present invention, and the heater 122 corresponds to an exemplary embodiment of a heat generating unit of the present invention.
The outer circumferential belt 121 of the heating roller 120 revolves in an arrow F direction while being heated by the heater 122 that makes surface contact with the inner circumferential surface of the outer circumferential belt 121. The outer circumferential belt 121 is urged against the pressure roller 110 by the pressure pad 123. Thus, force and heat are applied to a sheet P passing between the outer circumferential belt 121 and the pressure roller 110.
The heater 122 has an elongated shape extending in a depth direction of
The heater 122 has a structure in which multiple pairs of a heating resistor 132 and a PTC element 133, connected in series, are arranged side-by-side on a heater base 131.
Although the heater base 131 has an elongated shape extending in the left-right direction in
The heating resistors 132 are formed of a wiring pattern that is made of, for example, AgPb. Each heating resistor 132 is formed of a wire that forms a series of bends with a width of approximately 15 mm in the longitudinal direction (i.e., the left-right direction in
The PTC elements 133 are ceramic elements that are made of, for example, barium titanate mixed with lead. The PTC elements 133 are square flat plates having a thickness of approximately 0.2 mm and a length of each side of approximately 4 mm. The PTC elements 133 are elements having a positive temperature coefficient and are an example of a thermal destruction element of the present invention.
Multiple pairs of the heating resistor 132 and the PTC element 133 are arranged side-by-side in the longitudinal direction (i.e., the left-right direction in
In this exemplary embodiment, the PTC elements 133 suppress overheating of the heater 122. A detailed description will be given below.
In
A graph curve 150, which shows the PTC characteristics of the PTC elements 133 employed in this exemplary embodiment, steeply rises at a temperature exceeding a Curie temperature Tc. This shows that the resistance of the PTC elements 133 steeply increases when the temperature of the elements exceeds the Curie temperature Tc. As a result, the ratio of a minimum resistance Rmin at a temperature lower than the Curie temperature Tc to a maximum resistance Rmax at a temperature higher than or equal to the Curie temperature Tc typically exceeds 1:100, and sometimes it reaches 1:100000.
Such ceramic elements are used as the PTC elements 133 shown in
Because these PTC elements 133 are arranged as shown in
As has been described above, the PTC elements 133 have a flat plate shape, which efficiently causes thermal destruction. A critical temperature difference ΔTc that determines whether or not an infinitely spread flat plate is fractured by thermal shock is calculated from the following expression, on the basis of Young's modulus E, coefficient of linear expansion α, Poisson's ratio ν, fracture strength σmax, coefficient of heat transfer αM, characteristic length D, and thermal conductivity λ.
When a Young's modulus E of 1.15×1011 [N/m], a coefficient of linear expansion α of 12.5×1011 [K−1], a Poisson's ratio ν of 0.3, a fracture strength σmax of 70 [N/m2], a coefficient of heat transfer αM of 1×106 [W/m2K], a characteristic length D of 0.2 [mm], and a thermal conductivity λ of 6 [W/mK], serving as the values of the physical properties, are assigned to the above expression, the resulting critical temperature difference ΔTc is approximately 50K. The above-described maximum resistance Rmax is determined by conducting heat simulation or the like such that self-heating that generates an inside temperature difference of approximately 50K or more occurs, and, according to the thus-determined maximum resistance Rmax, the size of the PTC elements 133 is determined. By determining the maximum resistance Rmax in this way, thermal destruction of the PTC elements 133 is reliably caused, making it possible to reliably suppress overheating of the heating resistors 132. Furthermore, because the thus-determined maximum resistance Rmax is the resistance for causing self-heating, it is much smaller than the resistance for suppressing the current flow by increasing the resistance. Hence, the size and heat capacity of the PTC elements 133 are reduced, enabling thermal destruction to be caused immediately in response to overheating of the heating resistors 132.
In the above-described exemplary embodiment, although a ceramic element composed in large part of barium titanate has been shown as an example thermal destruction element of the present invention, the thermal destruction element of the present invention may be a ceramic element that is composed in large part of a material other than barium titanate or a non-ceramic element, as long as it causes thermal destruction.
Furthermore, in the above-described exemplary embodiment, the curved heater 122 that comes into contact with the inner circumference of the outer circumferential belt 121 has been shown as an exemplary embodiment of the heat generating unit of the present invention, the heat-generating member of the present invention may be one that has a flat-plate shape, one that comes into contact with the outer circumferential of the outer circumferential belt 121 for heating, one that heats a metal tube or the like other than the outer circumferential belt 121, or one that is used for heating in a unit other than the fixing unit 18.
Furthermore, although a monochrome printer has been shown as an example in the above-described exemplary embodiment, the present invention may be applied to a color printer, or it may be applied to a facsimile, a copier, or a multi-function apparatus.
Furthermore, although a device for forming a toner image using an electrophotographic system has been shown as an example in the above-described exemplary embodiment, the forming unit of the present invention may be one that forms a toner image on a recording medium by using a method other than the electrophotographic system.
The foregoing description of the exemplary embodiment of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiment was chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-129471 | Jun 2015 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 7203438 | Omata et al. | Apr 2007 | B2 |
| 8126383 | Kagawa | Feb 2012 | B2 |
| 9235166 | Shimura | Jan 2016 | B2 |
| Number | Date | Country |
|---|---|---|
| 2000260553 | Sep 2000 | JP |
| 2009244595 | Oct 2009 | JP |
