JPH08292673A - Thermal fixing device - Google Patents

Abstract

PURPOSE: To provide a thermal fixing device high in safety by preventing the generation of a leaking current by providing a heating means with a cylindrical heating element of which surface is coated with an insulating layer constituted of a heat shrinkable tube by the heat generation of the heating element itself. CONSTITUTION: A heat roller 5a whose external peripheral surface temperature is enough for fixing toner, has positive resistance characteristic in temperature dependency. For example, the roller 5a is constituted of a cylindrical heating resistor 10 whose resistant value is rapidly increased when the temperature rises to 180 deg.C, and a first and a second electrodes 11 and 12 respectively formed on external and internal peripheral surfaces in order to make a current flow in the thickness direction of the resistor 10. The insulating layer 13 consisting of the heat shrinkable tube is applied on the external peripheral surface of the first electrode 11 with which a recording paper to be fixed is brought into contact by the heating of the heating resistor 10 itself in order to prevent the current from leaking from the surface. Thus, a pin hole is prevented from occurring on the layer 13.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal fixing device for thermally fixing a toner image held on a recording medium such as recording paper.

[0002]

2. Description of the Related Art In a so-called image forming apparatus utilizing an electrophotographic process such as an electrophotographic copying machine, a laser beam printer, an LED printer, etc., a toner image visualized on the surface of a photoconductor is transferred to a recording medium. A thermal fixing device that thermally fixes the toner image held on the recording medium is provided. This kind of heat fixing device is disclosed in, for example, Japanese Utility Model Publication No. 3-45248.
As described in the publication, a halogen lamp as a heater is arranged along the axis of a metal roller, and the metal roller is indirectly heated by infrared rays emitted from the lamp.

Further, the applicant of the present application constituted the heat roller itself of the heat fixing device with a cylindrical heating resistor whose resistance value rapidly increases within a temperature range in which a toner image can be fixed, and the heat roller itself has a temperature control function. Japanese Patent Application No. 4-121827 (Japanese Patent Application Laid-Open No. 5-3135)
No. 32).

As described above, when the heat fixing device is composed of the heating resistor, it is necessary to provide an insulating layer on the outer surface thereof.

[0005]

The characteristic required for this insulating layer is that it has a small heat capacity and does not delay the temperature rise time of the heat fixing device. Therefore, P
When an insulating layer was formed by applying a heat-resistant resin of TFE (polytetrafluoroethylene) and baking it, pinholes were generated, and there was a problem that current leaked from the pinholes to the surface.

The present invention has been made for the purpose of solving the problem of safety due to the generation of such leak current.

[0007]

SUMMARY OF THE INVENTION The present invention is a thermal fixing device for fixing a toner image to a recording medium by contacting the recording medium holding a toner image to be fixed by a heat generating means.
In order to achieve the above object, the heat generating means includes a cylindrical heat generating element and an insulating layer including a heat-shrinkable tube coated on the surface of the heat generating element by its own heat generation.

The heating element includes a cylindrical heating resistor whose resistance value rapidly increases within the temperature range in which the toner image can be fixed, and the thickness of the insulating layer is set to 100 μm or less.

[0009]

As described above, the heat generating means is provided with the cylindrical heating element and the insulating layer made of a heat-shrinkable tube coated on the surface of the heating element by its own heat generation. No holes will be generated.

Further, the heating element includes a cylindrical heating resistor whose resistance value rapidly increases within the temperature range in which the toner image can be fixed, so that when the temperature reaches a predetermined temperature, the power supply to the heating element becomes a resistance value. The temperature rise time is not substantially delayed because the thickness of the insulating layer is set to 100 μm or less.

[0011]

FIG. 1 shows a schematic structure of an electrophotographic copying machine as an image forming apparatus incorporating a heat fixing device according to the present invention, in which an original placed on an original table 1 is not shown as optical means. And the light image is guided to the photosensitive drum 2 to form an electrostatic latent image. This latent image is visualized by toner in the developing device 3 sequentially by the rotation of the photosensitive drum 2, and the toner image is supplied from the paper feeding unit 4 toward the transfer device TC on the bottom surface of the photosensitive drum 2. And is transferred onto a recording sheet S as a recording medium. Then, the recording sheet S on which the toner image is transferred
Is a temperature sufficient to fix the toner on the outer peripheral surface (for example, 1
The heat roller 5a as a heat generating means having a temperature of 80 ° C. and a pressure roller 5b that comes into contact with the outer peripheral surface of the heat roller 5a at a predetermined pressure in the coaxial direction are conveyed to the heat fixing device 5. The toner image is thermally fixed between the heat roller 5a and the pressure roller 5b. When this fixing is completed, the recording paper S on which the original image is copied is ejected from the main body of the copying machine, and a series of copying steps is completed.

FIG. 2 shows a heat roller 5a of the thermal fixing device 5 whose outer peripheral surface has a temperature sufficient to fix toner. The heat roller 5a has a resistance characteristic of positive temperature dependence, and a cylindrical heating resistor 10 whose resistance value rapidly increases at a temperature sufficient to fix toner, for example, 180 ° C., The heat generating resistor 10 is composed of first and second electrodes 11 and 12 respectively formed on the outer peripheral surface and the inner peripheral surface so as to pass an electric current in the thickness direction, and contacts the recording paper S to be fixed. An insulating layer 13 made of a heat-shrinkable tube is coated on the outer peripheral surface of the one electrode 11 by the heat generated by the heat-generating resistor 10 itself so as to prevent current leakage from the surface. By using a fluorine-based tube such as a Teflon-based tube as the insulating layer 13, the releasability from the toner is improved.

Electric power is supplied to the heat roller 5a having the above structure.
In consideration of the fact that it rotates, it is performed by the slider method. That is, on the inner peripheral surface of the heating resistor 10, the first and second sliders 1 are attached to the conductive terminals 14a and 15a that are electrically coupled to the first and second electrode extension portions 11a and 12a in advance.
The coils 4 and 15 come into contact with each other by the axial biasing of the coil springs 16 and 17, and the first and second sliders 14 and 15 are energized via the lead wires 18 and 19. Then, in order to rotatably hold the heat roller 5a, a driven gear 20 that receives a driving force from a driving gear is coupled to the outer peripheral surface of the end of the heating resistor 10 on the side of the second slider, and both ends are connected. Ball bearings 21 and 22 are provided in the portion.

The heating resistor whose resistance value changes abruptly as described above is made of oxide ceramics mainly containing barium titanate and has a PTC (Positive Temperature C
commercialized as a heater. The heat roller 5a utilizing such temperature dependence rapidly increases when the resistance value of the heat roller 5a reaches a specific temperature within a temperature range sufficient for fixing the toner, and the current value is automatically increased by the increase of the resistance value. Since the amount is regulated, a temperature detecting element and a control circuit for controlling the temperature of the heat roller 5a as in the conventional case are unnecessary. Further, since the temperature detecting element itself is not used, accurate temperature control can be performed without foreign matter such as paper dust and paper pieces intervening in the contact gap between the temperature detecting element and the heat roller as in the conventional case.

Therefore, the abrupt change (increase) in the resistance value in the present invention means an increase to the extent that the amount of current is self-regulated by the change in the resistance value.

Thus, a heating resistor 10 having a diameter of 14 mm on which the first and second electrodes 11 and 12 are respectively formed is prepared, and the outer surface thereof is made of a product name "Flubon Tube" manufactured by Kurabe Co., Ltd. A heat-shrinkable tube is placed and the heating resistor 1
0 itself is heated at 180 degrees for 3 minutes and withstand voltage is 1kv / 50
A μm insulating layer 13 was coated and fixed. Then, the recording sheet S
When the temperature rise time of the surface in contact with was measured, a delay of 1 second occurred when the thickness was 100 μm, as compared with the case without the insulating layer 13.

However, in consideration of the fact that an insulating layer must be provided on the outer surface of the heating resistor 10, this one second is so small that it cannot be recognized as a substantial delay in the temperature rise time. It was late. On the other hand, the withstand voltage at this time is 2 kv.

Therefore, when such a heat-shrinkable tube is used, the insulation layer 13 has a thickness of 50 to 100 μm in order to prevent a dielectric breakdown voltage of 1 kv or more and a substantial delay in the temperature rise time. It has been found that the range is preferred.

Further, this thickness depends on the withstand voltage of the heat-shrinkable tube used, but is 100 μm for other tubes.
If so, sufficient insulating characteristics can be obtained, and if a material having an excellent withstand voltage is used, the insulating layer 13 can be made thinner.
Although the heating resistor 10 itself forms the raw pipe of the heat roller 5a in the above-mentioned embodiment, a raw pipe of a conductive material, for example, an aluminum raw pipe is prepared in place of it, and the heating resistor material is provided on the outer peripheral surface thereof. May be coated in a film form by a plasma spraying technique, in which case the aluminum tube serves as an inner peripheral surface electrode for the heating resistor.

[0020]

According to the present invention, since no pinhole is generated in the insulating layer, current does not leak from the surface of the insulating layer, and a highly safe device can be provided.

Further, when the predetermined temperature is reached, the power supply to the heating element is automatically limited based on the increase in the resistance value, so that the predetermined fixing temperature is reached very quickly, while the predetermined fixing temperature is reached. Can be accurately self-holding.

Further, the temperature rise time is not substantially delayed despite suppressing the current leakage.

[Brief description of drawings]

FIG. 1 is a schematic view showing a typical electrophotographic copying machine in which a heat fixing device of the present invention is incorporated.

FIG. 2 is a cross-sectional view showing a configuration of a heat roller which is a main part of the heat fixing device of the present invention.

[Explanation of symbols]

 2: Photoconductor drum 3: Developing device 5: Thermal fixing device 5a: Heat roller 10: Heating element 13: Insulating layer

Claims (4)

[Claims] 1. A thermal fixing device for contacting a recording medium holding a toner image to be fixed by the heating means to fix the toner image on the recording medium, wherein the heating means comprises a cylindrical heating element, A heat fixing device comprising an insulating layer made of a heat-shrinkable tube coated on the surface of the heating element by its own heat generation. 2. The heat fixing device according to claim 1, wherein the heating element includes a cylindrical heating resistor whose resistance value rapidly increases within a temperature range in which a toner image can be fixed. 3. The heat fixing device according to claim 2, wherein the insulating layer has a thickness of 100 μm or less. 4. A thermal fixing device for contacting a recording medium holding a toner image to be fixed by the heating means and fixing the toner image on the recording medium, wherein the heating means comprises a cylindrical heating element, A heat fixing device comprising an insulating layer made of a heat-shrinkable tube having a thickness of 100 μm or less on the surface of the heating element.