Fixing unit and electrophotographic apparatus having the same

A fixing unit includes: a heating roller; a pressing roller that is capable of being in pressure contact with the heating roller; a compressor that generates compressed air; an air tank that reserves therein the compressed air; a first pipe through which the compressed air generated by the compressor is fed into the air tank; an injection nozzle that is formed by the heating roller and the pressing roller and that causes the compressed air to blow a printing medium fed from the fixing nip; a second pipe through which the compressed air reserved in the air tank is fed into the injection nozzle; and a compressed air injection solenoid valve that is provided in middle of the second pipe. A droplet separator for separating a droplet from the compressed air is provided to the first pipe or to the second pipe.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2010-188666 filed in Japan on Aug. 25, 2010.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fixing unit of an electrophotographic apparatus.

2. Description of the Related Art

FIG. 3is an outline configuration diagram of an electrophotographic apparatus having the compressed air supply device.

As illustrated in the drawing, the surface of a photosensitive element1that is rotated at a constant rotation speed in the direction indicated by the arrow is uniformly charged by a charging unit7, and a laser beam is emitted from an exposure unit8onto the surface of the charged photosensitive element1to form an electrostatic latent image corresponding to the image information to be recorded. The electrostatic latent image is visualized with toner supplied by a developing unit18to form a toner image5on the surface of the photosensitive element1.

When a sheet6fed at an appropriate timing passes through a gap between the photosensitive element1and a transfer unit2by being in contact with the photosensitive element1, the toner image5on the photosensitive element1is transferred to the sheet6. The sheet6to which the toner image5is transferred is electrostatically peeled off from the photosensitive element1by a peeling unit3and is conveyed to a fixing unit10by a conveying belt4. In general, the sheet6is conveyed to the fixing unit10by being sucked by the conveying belt4.

The toner image5on the sheet6fed to the fixing unit10is fixed to a surface of the sheet6by heat and pressure at a fixing nip9including a heating roller11and a pressing roller12.

Due to a problem that the sheet6having passed through the fixing nip9tightly sticks, via the melted toner, to the heating roller11and is therefore not conveyed, Japanese Patent Application Laid-open No. 2009-300704 proposes a technique, for example, by which the sheet6is peeled off from the heating roller11with compressed air A that blows a leading end of the sheet6.

InFIG. 3, an injection nozzle13spouts the compressed air A toward the sheet6; the compressed air A is supplied to the injection nozzle13through a second pipe14b; a compressed air injection solenoid valve15is provided in the middle of the second pipe14b; an upper guide plate20and a lower guide plate21guide the sheet6peeled off from the heating roller11to the direction of a discharging unit (not illustrated).

FIG. 4is an outline configuration diagram of a compressed air generator included in a fixing unit illustrated in Japanese Patent Application Laid-open No. 2009-300704.

The compressed air generated by a compressor17is allowed to pass through a first pipe14ato be reserved in an air tank16. The air tank16is connected with the injection nozzle13by the second pipe14bthrough the compressed air injection solenoid valve15. The solenoid valve15is a normally closed solenoid valve, and the pressure in the air tank16is maintained with the solenoid valve15being closed.

When a leading end of the sheet6reaches a predetermined position, the compressed air injection solenoid valve15is opened for a predetermined period of time, and the released compressed air A is spouted from the injection nozzle13to peel off the sheet6from the heating roller11, thus preventing the sheet from sticking to the heating roller11.

However, if the pressure of the compressed air is too weak, the sheet6cannot be reliably peeled off. Conversely, if the pressure of the compressed air is too strong, the sheet6is buckled or the posture of the sheet6is off-balanced, resulting in a trouble in conveying the sheet. Therefore, it is necessary to properly adjust and maintain the pressure, and accordingly, a pressure adjusting valve19is provided to the air tank16. The pressure adjusting valve19allows a surplus of the compressed air generated by the compressor17to be released to the atmosphere, so that the whole pressure of a compressed air circuit can be appropriately maintained.

It should be noted that a method and an apparatus for automatically discharging a droplet are described in, for example, Japanese Patent No. 3581960.

When the air is compressed, the dew-point temperature of the air increases. When the temperature of the compressed air falls below the dew-point temperature, water vapor contained in the air is condensed to turn into a droplet. In general, it has been known that when the air is continuously compressed and the droplets are not drained from the compressed air circuit, the droplets are reserved in the compressed air circuit to cause a trouble.

If the droplets are reserved in the compressed air circuit in the conventional fixing unit illustrated inFIG. 4, the droplets are spouted from the injection nozzle13together with the compressed air, and the sheet6gets wet, thus causing the print quality to be markedly deteriorated. Furthermore, the droplet washes away grease used in the compressed air injection solenoid valve15to shorten the durable life of the solenoid valve15.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a fixing unit that includes: a heating roller; a pressing roller that is capable of being in pressure contact with the heating roller; a compressor that generates compressed air; an air tank that reserves therein the compressed air; a first pipe through which the compressed air generated by the compressor is fed into the air tank; an injection nozzle that is provided at a downstream in a printing-medium conveying direction relative to a fixing nip formed by the heating roller and the pressing roller and that causes the compressed air to blow a printing medium fed from the fixing nip; a second pipe through which the compressed air reserved in the air tank is fed into the injection nozzle; a compressed air injection solenoid valve that is provided in middle of the second pipe. A droplet separator for separating a droplet from the compressed air is provided to either one of the first pipe between the compressor and the air tank and the second pipe between the air tank and the compressed air injection solenoid valve. A first droplet discharging pipe extends from the droplet separator and a second droplet discharging pipe extends from a bottom portion of the air tank, and the first and second discharging pipes join together with one droplet discharging solenoid valve arranged at a downstream of a joining portion in a droplet discharging direction.

According to another aspect of the present invention, there is provided an electrophotographic apparatus that includes: a photosensitive element; a charging unit that charges a surface of the photosensitive element; an exposure unit that forms an electrostatic latent image on the surface of the charged photosensitive element; a developing unit that make visible the electrostatic latent image with a supply of toner to form a toner image; a transfer unit that transfers the toner image on the photosensitive element to a conveyed printing medium; and a fixing unit that fixes the toner image transferred to the printing medium, wherein the fixing unit includes: a heating roller; a pressing roller that is capable of being pressed to be in contact with the heating roller; a compressor that generates compressed air; an air tank that reserves therein the compressed air; a first pipe through which the compressed air generated by the compressor is fed into the air tank; an injection nozzle that is provided at a downstream in a printing-medium conveying direction relative to a fixing nip including the heating roller and the pressing roller and that causes the compressed air to blow a printing medium fed from the fixing nip; a second pipe through which the compressed air reserved in the air tank is fed into the injection nozzle; and a compressed air injection solenoid valve that is provided in middle of the second pipe. A droplet separator for separating a droplet from the compressed air is provided to the first pipe between the compressor and the air tank or to the second pipe between the air tank and the compressed air injection solenoid valve, and a first droplet discharging pipe extending from the droplet separator and a second droplet discharging pipe extending from a bottom portion of the air tank are allowed to join together, of which joined part is provided with one droplet discharging solenoid valve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, embodiments of the present invention will be described with reference to the drawings.FIG. 1is an outline configuration diagram of a compressed air generator of a fixing unit according to a first embodiment of the present invention.

An electrophotographic apparatus according to the first embodiment also includes, similarly to the one illustrated inFIG. 3, a photosensitive element1that is rotated at a constant speed in a predetermined direction, a charging unit7that uniformly charges the surface of the photosensitive element1, an exposure unit8that emits a laser beam to the surface of the charged photosensitive element1so as to form an electrostatic latent image corresponding to image information to be recorded, a developing unit18that makes the electrostatic latent image visible with a supply of toner to form a toner image5, a transfer unit2that transfers the toner image5on the photosensitive element1to sheet6having been conveyed to a fixing unit10, and the fixing unit10that fixes, on the sheet6, the toner image5transferred to the sheet6.

In the fixing unit10, the toner image5is fixed to a surface of the sheet6by heat and pressure at a fixing nip9including a heating roller11and a pressing roller12that can be pressed to be in contact with the heating roller11.

In order to avoid a problem that the sheet6having passed through the fixing nip9tightly sticks to the heating roller11due to the melted toner and is not conveyed, an injection nozzle13is provided at the downstream in the sheet conveying direction, so that the sheet6is peeled off from the heating roller11with compressed air A blowing to a leading end of the sheet6.

As illustrated inFIG. 1, in the compressed air generator of the fixing unit according to the first embodiment, the compressed air generated by a compressor17passes through a first pipe14ato be reserved in an air tank16. The injection nozzle13and the air tank16are connected to each other through a second pipe14b, in the middle of which a compressed air injection solenoid valve15is provided. The solenoid valve15is a normally closed solenoid valve, and the pressure in the air tank16is maintained with the solenoid valve15being closed.

When a paper sensor (not illustrated) detects that a leading end of the sheet6reaches a predetermined position, the compressed air injection solenoid valve15is opened for a predetermined period of time on the basis of the detected signal. Then, the released compressed air A is spouted from the injection nozzle13to peel off the sheet6from the heating roller11, and prevents the sheet6from sticking to the heating roller11.

The presence or absence of injection of the compressed air A from the injection nozzle13, injection time, and the number of times of injections are predetermined by the degree of stiffness, which depends on the thickness, of the sheet6to be used. For example, a control unit (not illustrated) preliminarily installs therein computer programs that causes the injection nozzle13to spout the compressed air A twice in a short period of time to the sheet6with a small thickness and a low degree of stiffness, once with a slightly longer period of time than the above case to the sheet6with a normal thickness, and to stop to spout the compressed air A to the sheet6with a large thickness and a high degree of stiffness.

Accordingly, various types of sheet ranging from thin sheet to thick sheet can be prevented from being jammed, and the reliability in printing can be improved.

A pressure adjusting valve19is provided to the air tank16to discharge a surplus of the compressed air generated by the compressor17to the atmosphere and the entire pressure of the compressed air circuit is properly maintained.

An air filter22is provided in the middle of the first pipe14athrough which the compressor17and the air tank16are connected to each other. The air filter22has a function of collecting foreign substances such as dust and a function of collecting droplets.

In the first embodiment, A1019-2C-FL439853 manufactured by CKD Corporation is used as the air filter22. The air filter22is formed by inserting foam made of polypropylene into a zinc alloy die-casting, and the filtration degree thereof is about 5 μm.

A droplet reservoir22ais incorporated in a lower part of the air filter22. A first droplet discharging pipe23extends downward from the droplet reservoir22a. In addition, a droplet discharging solenoid valve25is connected to a leading end of the first droplet discharging pipe23.

Furthermore, a second droplet discharging pipe24is connected to a lower part of the air tank16. A leading end of the second droplet discharging pipe24is connected to and is joined together with the first droplet discharging pipe23provided between the air filter22and the droplet discharging solenoid valve25. An evaporating dish26used for natural evaporation of the droplets are arranged under an outlet of the droplet discharging solenoid valve25.

A backflow prevention valve27is provided in the middle of the second droplet discharging pipe24. The backflow prevention valve27blocks a flow of air blowing toward the air tank16and permits only an air flow blowing toward the droplet discharging solenoid valve25.

It may look sufficient to connect the first droplet discharging pipe23under the air filter22and the second droplet discharging pipe24under the air tank16without using the backflow prevention valve27. However, if the backflow prevention valve27is not provided, the air flows out to the air tank16from the lower part of the air filter22without passing through a filter inside the air filter22. This is because air resistance in a flowing route from the lower part of the air filter22to the air tank16not through the filter inside the air filter22is smaller than that in a flowing route to the air tank16through the filter inside the air filter22.

As a result, the air containing dust, foreign substances, and the like reaches the compressed air injection solenoid valve15to shorten the durable life of the solenoid valve15. Thus, it is necessary to provide the backflow prevention valve27that permits only a flow to discharge droplets.

The droplet discharging solenoid valve25can be provided at each of the leading ends of the first droplet discharging pipe23and the second droplet discharging pipe24instead of connecting the second droplet discharging pipe24to the first droplet discharging pipe23. However, providing the two units of the droplet discharging solenoid valve25results in an increase in cost. Furthermore, providing the two units of the droplet discharging solenoid valve25with a relatively short durable life in the fixing unit increases a chance of failure, resulting in deterioration in operational reliability as an electrophotographic apparatus (fixing unit).

In contrast, the backflow prevention valve27used in the present invention is a mechanical component, and costs less than to provide the two units of the droplet discharging solenoid valve25as described above. In addition, the backflow prevention valve27has a longer durable life than the droplet discharging solenoid valve25does, and the chance of failure is reduced as compared with the case in which the two units of the droplet discharging solenoid valve25are provided, leading to improvement of operational reliability as an electrophotographic apparatus (fixing unit).

In general, an electrophotographic apparatus requires, after the electric power is turned on, a few minutes of preparation time to prepare for printing in heating the fixing rollers (the heating roller and the pressing roller) and adjusting electronic conditions for respective constituent devices. Furthermore, it is necessary to interrupt the printing due to sheet jam, a refill of consumables and sheet, sheet removing work, and the like. In general, interruption time of printing due to the troubles listed above occurs at least a few times a day.

The operation of opening the droplet discharging solenoid valve25in a state where the compressed air is reserved by operating the compressor17during the preparation time and the interruption time is pre-installed in a control program of the electrophotographic apparatus, and a droplet discharging operation is executed for a few seconds and is completed. Furthermore, in the case where the electrophotographic apparatus continuously runs without a halt, the printing may be forcibly interrupted to execute the droplet discharging operation for a few seconds. Alternatively, this operation may be performed immediately after the printing is stopped.

If the droplet discharging operations are frequently performed, the amount of droplet discharged at a time is reduced in inversely proportional to the frequency. The operation period of time of the compressor17is at least about 3 seconds. Given that the opening period of time of the droplet discharging solenoid valve25is about 0.3 to 3 seconds and the inner diameters of the droplet discharging pipes23and24are, for example, 3 to 5 millimeters, several cubic centimeters (cc) to more than ten cc of water can be drained.

If a pressure of the pressure adjusting valve19is set at 0.05 MPa to 0.3 MPa and the discharging performance of the compressor17is 5 L/min (at 0.3 MPa) to 20 L/min (at 0.3 MPa), the amount of droplets collected in continuous operations for 24 hours a day is a dozen cc at the most, and the droplets can be discharged under the above-described conditions.

As long as the amount of discharged water is a dozen cc at the most, the droplet can be discharged to the evaporating dish26and left to natural evaporation. Specifically, neither providing the droplet discharging port outside the electrophotographic apparatus nor connecting the droplet discharging port to a sewer pipe for discharging droplets is necessary. Thus, a space for placing the electrophotographic apparatus can be made small. In general, a diaphragm compressor is preferable as the compressor17that meets the above-described conditions.

FIG. 2is an outline configuration diagram of a compressed air generator of a fixing unit according to a second embodiment of the present invention. The second embodiment is different from the first embodiment in that a cooler28as a second droplet separator is arranged between the compressor17and the air filter22.

As described above, if the cooler28is arranged before the air filter22(first droplet separator) and the compressed air to be fed to the air filter22is cooled by the cooler28to condense and separate, in advance, part of the vapor by forming droplets, the electrophotographic apparatus can be used under, for example, a high humidity condition. In addition, the amount of droplets generated can be reduced under a normal use conditions. As a refrigerant in the cooler28, normal water or organic liquid is used.

In the first embodiment, the air filter22is provided on the first pipe14abetween the compressor17and the air tank16. However, the air filter22may be provided on the second pipe14bbetween the air tank16and the compressed air injection solenoid valve15.

According to the present invention, sheet is used as a printing medium. However, the present invention can be applied to a case in which different printing media such as overhead projector (OHP) sheets are used.

Next, advantageous effects obtained by the claims of the present invention will be described as follows.

According to the present invention, only one droplet discharging solenoid valve is necessary, and the numbers of wirings, control-related units, and electric power units for actuating the solenoid valve can be reduced along with the reduction in the number of the droplet discharging solenoid valve. In addition, the chance of failure of the solenoid valve becomes low as compared to a case in which a plurality of droplet discharging solenoid valves are provided. Thus, it is possible to provide a fixing unit that is inexpensive, requires less space, and is high in operational reliability.

According to the present invention, downsizing and reducing cost of the fixing unit can be realized by using an air filter as a droplet separator.

According to the present invention, a backflow prevention unit that blocks a flow toward an air tank is provided to a second droplet discharging pipe, so that it is possible to solve the problem that the air containing dust, foreign substances, and the like reaches a compressed air injection solenoid valve to shorten the durable life of the solenoid valve.

According to the present invention, a unit for reserving therein droplets is provided on the outlet side of the droplet discharging solenoid valve. In addition, according to the present invention, the unit for reserving therein the droplets is an evaporating dish. Thus, it is not necessary to provide a droplet discharging port outside the main body of the electrophotographic apparatus, and the electrophotographic apparatus can be more easily placed.

According to the present invention, a compressed air cooling unit is provided between a compressor and the air filter, so that the compressed air to be fed to the air filter is cooled by the compressed air cooling unit and part of condensed vapor droplets can be removed in advance.

In the case where the fixing unit is used under stronger sticking strength to a heating roll (for example, in the case where toner having high melt viscosity is used, or in the case where the amount of toner sticking to sheet is increased to improve the print quality), it is necessary to enhance the performance of peeling off the sheet by increasing the number of injection nozzles, the air pressure, and the opening period of time of the solenoid valve. As a result, it is necessary to enhance the discharging performance of the compressor. Furthermore, if the printing speed of the electrophotographic apparatus becomes fast even under the same sticking strength to the heating roll, the number of times to open the solenoid valve is increased. In this case, too, it is necessary to enhance the discharging performance of the compressor. A piston-type compressor with a discharge performance of about 20 L/min (at 0.4 MPa) to 40 L/min (at 0.4 MPa) is used instead of a diaphragm compressor.

In the above-described case, much compressed air is fed to the inside of the pipe. Thus, the amount of droplets is increased to exceed the droplet separating capability of the air filter. Thus, it is advantageous to provide the compressed air cooling unit. Accordingly, it is possible to provide a fixing unit that can realize high-quality and high-speed printing.

According to the present invention, it is possible to provide an electrophotographic apparatus that is inexpensive, requires less space, and is high in operational reliability.

According to the present invention, the droplet can be discharged without having impacts on the printing operations of the electrophotographic apparatus.

The present invention is configured as described above, and can provide a highly reliable fixing unit and an electrophotographic apparatus having the same by which droplet generated in a compressed air circuit is discharged to prevent deterioration in the print quality caused by the droplets and to prevent a reduction in the durable life of a compressed air injection solenoid valve.