JP2000275993A - Fixing device and image forming device provided with fixing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the electrostatic offsetting of toner and the sticking of calcium carbonate to a fixing roller. SOLUTION: Relating to a fixing device negative bias voltage is applied to the core bar 4 of the fixing roller by a bias power source 11 so that the unfixed toner 13 electrified so as to be negative on the surface of a transfer material P is prevented from being offset on the fixing roller 1 at a fixing time. The calcium carbonate electrified so as to be positive in the transfer material P is stuck to the fixing roller 1 when the absolute value of bias voltage is large, and the offsetting of negative toner on the transfer material P passing afterward to the fixing roller 1 is caused, and the negative toner offset is offset on a succeeding transfer material P again, so that an image is soiled. Then, the temperature of the fixing roller 1 is detected by a thermistor 12 so that the sticking of the calcium carbonate is prevented, an the absolute value of the bias voltage applied to the fixing roller 1 is reduced in the case that electrostatic offsetting is hardly caused because the temperature is too high.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device used for a copying machine, a laser printer, and the like, and an image forming apparatus provided with the fixing device.

[0002]

2. Description of the Related Art Conventionally, as a fixing device in an image forming apparatus (for example, a copying machine or a laser printer) using toner, a heat and pressure fixing device is mainly used.

This heat and pressure fixing device includes a fixing roller and a pressure roller which is pressed against the fixing roller to form a fixing nip portion, and a heater is provided in the fixing roller.
The transfer material, on which the unfixed toner image has been transferred to the surface by the image forming unit, is heated and pressed while being nipped and conveyed in the fixing nip, so that the toner image is fixed on the surface as a permanent image. In this fixing device, since the fixing roller directly contacts the toner on the transfer material, a phenomenon (offset) in which the toner adheres to the fixing roller is likely to occur. Therefore, in order to prevent toner offset to the fixing roller,
The surface layer of the fixing roller is covered with a fluororesin-based tube having high releasability, or coated with a fluororesin-based tube.

However, this is not enough, and heat is not sufficiently transmitted to the fixing roller. For example, when the power is turned on in the morning or when the transfer material is continuously passed,
The fixing roller surface temperature decreases during paper passing. For this reason, a phenomenon called low-temperature offset occurs in which heat is not sufficiently transmitted to the toner on the transfer material and the toner is offset to the fixing roller. Further, the fluororesin system has a high electric insulation property and is easily charged. For this reason, when the transfer material carrying the unfixed toner passes between the fixing roller and the pressure roller, a phenomenon in which a part of the unfixed toner is offset from the transfer material on the surface of the fixing roller (hereinafter, “electrostatic offset”) ").

[0005] When these low-temperature offsets or electrostatic offsets occur, unfixed toner adheres to a transfer material to be sent next, and contaminates the transfer material. This is a major problem due to the recent increase in the speed of copying machines.

As a means for solving this problem, the fixing roller is cleaned by pressing a surface of the fixing roller with a felt or an oil applying member impregnated with silicone oil. However, in recent years, there has been a tendency not to use these cleaning members in view of advances in material technology and cost reduction of products.

Therefore, in a fixing device that does not use a cleaning member, for example, in JP-A-6-110350, the uppermost layer of the pressure roller is coated with a conductive layer. As shown in FIG.
The core metal 7 is grounded via a diode 14, and a negative bias voltage is applied to the core metal 4 of the fixing roller 1 by a bias power supply 11. As a result, a repulsive electric field that attracts the negative toner to the transfer material P is generated, and the toner is prevented from being electrostatically offset to the fixing roller 1.

In FIG. 1, 2 is an insulating adhesive layer, 3 is a release layer, 5 is a heater, 8 is an elastic layer, 9 is a release layer, and 13 is an unfixed toner.

[0009]

However, as described above, when a negative bias voltage is applied to the metal core 4 of the fixing roller 1, the surface layer of the fixing roller 41 is negatively charged. Therefore, when a type containing calcium carbonate having the property of being positively charged as a filler is used in the transfer material P, a large amount of calcium carbonate in the transfer material P adheres to the surface layer of the negatively charged fixing roller 1. Resulting in. It is known that the amount of calcium carbonate attached increases as the surface potential of the fixing roller 1 becomes more negative as shown in FIG. When this calcium carbonate adheres to the surface layer of the fixing roller 1, the surface potential of the fixing roller 1 is increased, and the toner releasability of the surface layer of the fixing roller 1 is greatly impaired.
The negative toner on the transfer material P that passes later is offset to the fixing roller 1, and the offset negative toner is again offset to the subsequent transfer material P to stain the image.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances. Even when a transfer material containing a filler having a polarity opposite to that of a toner is passed through the fixing device in the fixing device,
A fixing device capable of preventing offset of a toner to a fixing roller and adhesion of a filler to the fixing roller;
And an image forming apparatus provided with the same.

[0011]

According to a first aspect of the present invention, there is provided an image forming apparatus comprising: a fixing roller having a heater; and a pressing roller pressed against a fixing roller having a heater to form a fixing nip portion. In a fixing device, a transfer material is inserted into the fixing nip portion such that a surface side carrying a toner image is in contact with the fixing roller, and the toner image is fixed on the transfer material surface by applying heat and pressure. And a voltage for applying a bias voltage between the fixing roller and the pressure roller to prevent an unfixed toner image on the transfer material from being offset to the surface of the fixing roller. Control means for controlling a bias voltage applied between the fixing roller and the pressure roller by the voltage applying means based on an output of the temperature detecting means; Comprises, characterized in that.

According to a second aspect of the present invention, in the fixing device of the first aspect, a bias voltage having the same polarity as that of the unfixed toner on the transfer material is applied to the fixing roller.

According to a third aspect of the present invention, in the fixing device of the second aspect, the control unit increases the absolute value of the bias voltage as the temperature of the fixing roller detected by the temperature detecting unit decreases. The bias power supply is controlled as described above.

According to a fourth aspect of the present invention, there is provided an image forming apparatus for forming an image on a transfer material using toner, an image carrier, and a latent image forming means for forming an electrostatic latent image on the image carrier. Developing means for attaching a toner having a charge to the electrostatic latent image to develop the toner image as a toner image; transferring means for transferring the toner image on the image carrier to a transfer material surface; The fixing unit according to claim 1, 2 or 3, which fixes the unfixed toner image.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the outline of the present invention will be described.

In copying (image formation) by an image forming apparatus, intermittent copying of several sheets is performed more frequently than continuous copying of hundreds of sheets. For this reason, attention was paid to the fact that the surface temperature of the fixing roller hardly dropped and the low-temperature offset and the electrostatic offset were small except for the continuous copying in the first morning. The electrostatic offset hardly occurs when the surface temperature of the fixing roller is high, and the electrostatic offset amount increases as the surface temperature decreases. Further, the amount of calcium carbonate adhering to the fixing roller increases as the negative polarity of the surface potential of the fixing roller increases.

Accordingly, in the present invention, a fixing roller having an insulating layer and a fluororesin tube layer provided on the insulating layer, a conductive rubber elastic layer, and a fluororesin tube layer provided on the conductive rubber elastic layer are provided. In a fixing device composed of a pressure roller having the following, the electrostatic offset and the adhesion of calcium carbonate are solved by gradually increasing the negative bias voltage applied as the temperature of the fixing roller surface decreases. did.

That is, for example, in an image forming apparatus using a negative toner, in the fixing device of the present invention, as the surface temperature of the fixing roller decreases, a negative bias voltage suitable for the surface temperature is applied to the fixing roller. You. When the surface temperature of the fixing roller is high, low-temperature offset and electrostatic offset are extremely small, and toner offset hardly occurs even if the absolute value of the fixing roller surface potential is not increased. Since the fixing roller is not significantly negatively charged,
Almost no calcium carbonate adheres.

Next, an embodiment of the present invention will be described with reference to the drawings.

Embodiment 1 FIG. 1 shows an example of a fixing device according to the present invention. FIG. 1 is a longitudinal sectional view showing a schematic configuration of the fixing device.

The fixing device shown in FIG. 1 includes a fixing roller 1. The fixing roller 1 is composed of a hollow cored bar 4 having an insulating adhesive layer 2 and a release layer 3 on the surface. In the present embodiment, the release layer 3 is formed of an insulating PFA tube, and has a thickness of 30 μm. The metal core 4 is a cylindrical member made of aluminum, and its thickness is 12 mm. A heater 5 is disposed inside the fixing roller 1. A thermistor (temperature detecting means) 12 for detecting the surface temperature of the fixing roller is pressed against the surface of the fixing roller. The thermistor 12 is connected to a detection circuit 16, and the detection circuit 16 is connected to a control circuit (control means) 10. A bias power supply 11 is connected to the control circuit 10. The fixing roller 1 is driven to rotate in a direction indicated by an arrow R1 by a driving unit (not shown).

A pressure roller 6 is pressed against the fixing roller 1 from below, and a fixing nip portion T is formed between the two rollers. The pressure roller 6 has a core metal 7 made of aluminum covered with an elastic layer 8 made of conductive silicone rubber. The surface layer of the elastic layer 8 is provided with a release layer 9 to enhance the release property.
It is covered with. The release layer 9 is formed of a low-resistance PFA tube, and has a thickness of 50 μm. The pressure roller 6 is driven to rotate in the direction of the arrow R6 with the rotation of the fixing roller 1 in the direction of the arrow R1. A cleaning roller 15 is in contact with the surface of the pressure roller 6 as a countermeasure against offset in the event of a jam. Cleaning roller 15
Is for rubbing the surface of the pressure roller 6 by rotating the pressure roller 6 in the direction of arrow R6, thereby removing the toner on the surface of the pressure roller 6.

In this embodiment, the core 4 of the fixing roller 1
, A DC bias voltage 11 is applied via a control circuit 10. On the other hand, the core of the pressure roller 6 is grounded. The control circuit 10 is connected to a thermistor 12 via a detection circuit 16.

Hereinafter, the operation of the fixing device having the above-described configuration will be described. In the present embodiment, a case where the fixing device is incorporated in an image forming apparatus such as a copying machine or a laser printer will be described as an example.

In the image forming section of the image forming apparatus, the surface of a photosensitive drum as an image carrier is uniformly charged to a predetermined polarity and a predetermined potential by a primary charger, and then subjected to exposure by an exposure device to be statically charged. An electrostatic latent image is formed. The electrostatic latent image is developed with a toner by a developing device as a toner image. The toner image on the photosensitive drum is transferred onto the surface of the transfer material P by a transfer charger.

The transfer material P having the unfixed toner image formed on the surface in this manner is conveyed to and passed through a fixing nip portion T of a fixing device. The transfer material P is heated and pressurized in the fixing nip T to fix the unfixed toner image 13 on the surface, and then discharged outside the image forming unit main body.

During the passage of the transfer material P, the temperature of the surface of the fixing roller 1 is detected by the thermistor 12 in contact with the fixing roller 1, and the temperature of the heater 5 is controlled. At the same time as the temperature control of the heater 5, data of the detected surface temperature of the fixing roller 1 is sent to the control circuit 10.
The control circuit 10 applies a negative bias voltage to the metal core 4 of the fixing roller 1 from the bias power supply 11 in accordance with the graph shown in FIG. FIG. 4 shows the relationship between the fixing roller surface potential and the toner offset amount. As can be seen from FIG.
When the surface temperature is 180 ° C. and the surface potential of the fixing roller 1 is −800 V, and when the surface temperature is 200 ° C. and the surface potential is −100 V, the surface temperature of the latter is 200 ° C. Even when the surface potential is lower at this time, the toner offset amount to the fixing roller 1 can be suppressed almost as much as at 180 ° C.

Therefore, the surface temperature of the fixing roller 1 is set to 200
When the surface temperature of the fixing roller 1 is lower than 180 ° C. due to continuous paper feeding, the fixing roller 1 is not biased. The voltage is controlled by the control circuit 10 so as to apply up to about -800V.

By controlling in this manner, the fixing roller 1
While the surface temperature is maintained at a high level, the negative toner on the transferred transfer material P hardly causes low-temperature offset. In general, electrostatic offset is a phenomenon that often occurs in toner that is not sufficiently supplied with heat. Therefore, if the surface temperature of the fixing roller 1 is sufficient to provide sufficient heat to the toner, electrostatic offset also occurs. Absent.

At this time, since the surface potential of the fixing roller 1 is almost 0 V, the transfer material P containing a large amount of calcium carbonate is used.
Is used, calcium carbonate contained in the transfer material P does not adhere to the fixing roller 1 and the pressure roller 6.

When the surface temperature of the fixing roller 1 decreases due to continuous paper feeding or the like, heat cannot be sufficiently supplied to the toner, and the electrostatic offset amount increases. Therefore, according to FIG. 3, the bias voltage of the negative polarity is increased by the bias power supply 11. As a result, the amount of electrostatic offset can be reduced and the fixing roller 1 can be prevented from being stained.

[0032]

As described above, according to the present invention,
In the fixing device, even when a transfer material containing a filler having a polarity opposite to that of the toner (for example, calcium carbonate having a positive charge polarity for the negative toner) is passed through the fixing device, the toner is offset and fixed to the fixing roller. Adhesion of the filler to the roller can be prevented.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a schematic configuration of a fixing device.

FIG. 2 is a diagram showing a relationship between a fixing roller surface potential and a calcium carbonate adhesion amount.

FIG. 3 is a diagram illustrating a relationship between a fixing roller surface temperature and a bias voltage.

FIG. 4 is a diagram illustrating a relationship between a fixing roller surface potential and a toner offset amount when the temperature is 180 ° C. and 200 ° C.

FIG. 5 is a longitudinal sectional view showing a schematic configuration of a conventional fixing device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fixing roller 2 Insulating adhesive layer 3 Release layer 4 Core 5 Heater 6 Pressure roller 7 Core 8 Elastic layer 9 Release layer 10 Control means (control circuit) 11 Bias power supply 12 Temperature detection means (thermistor) 13 Unfixed Toner image 14 Diode 15 Cleaning roller P Transfer material

Claims (4)

[Claims] An image forming apparatus according to claim 1, wherein a pressure roller is pressed against a fixing roller having a heater to form a fixing nip portion. A fixing device that inserts the toner image onto the surface of the transfer material by applying heat and pressure through a fixing nip portion; and a temperature detecting unit that detects a temperature of the fixing roller; and between the fixing roller and the pressure roller. Voltage applying means for applying a bias voltage for preventing an unfixed toner image on the transfer material from being offset to the surface of the fixing roller; and between the fixing roller and the pressure roller by the voltage applying means. Controlling means for controlling the applied bias voltage based on the output of the temperature detecting means. 2. The fixing device according to claim 1, wherein a bias voltage having the same polarity as that of the unfixed toner on the transfer material is applied to the fixing roller. 3. The control unit controls the bias power supply such that the lower the temperature of the fixing roller detected by the temperature detector is, the larger the absolute value of the bias voltage is. Item 3. The fixing device according to Item 2. 4. An image forming apparatus for forming an image on a transfer material using toner, an image carrier, a latent image forming means for forming an electrostatic latent image on the image carrier, and Developing means for applying a toner having a charge to develop the toner image as a toner image; transferring means for transferring the toner image on the image carrier to a transfer material surface; and fixing the unfixed toner image transferred to the transfer material surface. A fixing unit according to claim 1, 2 or 3,
An image forming apparatus comprising: