JP4475210B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus including a belt fixing device that heats and pressurizes unfixed toner on a recording paper, and in particular, the recording paper separation performance after the recording paper passes through the nip portion of the belt fixing device. The present invention relates to an image forming apparatus with improved image quality.

  2. Description of the Related Art In an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction machine having these functions, a latent image corresponding to an original is formed on a photosensitive member, and toner is applied to the latent image. Thus, the visualized toner image is transferred onto the recording paper, and then the toner image transferred onto the recording paper is fixed and discharged.

  When a color image is formed, Y, M, C, and K latent images corresponding to the original color are formed on four photosensitive drums, and the visualized four color toner images are endless. After primary transfer to an intermediate transfer member composed of a belt, secondary transfer is performed on recording paper, and the toner image transferred onto the recording paper is fixed and discharged.

  As a fixing device for fixing a toner image in this manner, a recording sheet on which a toner image is transferred is sandwiched between a nip portion formed by a heating roller incorporating a halogen heater and the like and a pressure roller that presses the heating roller. There is a fixing device of a heat roller fixing system that heats and pressurizes while conveying, and such a fixing device is widely used because of its simple configuration.

  In addition, an endless fixing belt is stretched between a heating roller incorporating a halogen heater or the like and a fixing roller, and has a pressure roller that presses the fixing roller through the fixing belt. There is a belt-fixing type fixing device that heats and pressurizes while sandwiching and transporting the recording paper onto which the toner image is transferred at the formed nip portion, and such a fixing device has a small heat capacity of the heating roller of the fixing belt. Therefore, there is an advantage that the warm-up time is shortened and energy is saved.

  However, in the fixing device, since the toner of the toner image on the recording paper is heated when passing through the nip portion, the toner acts as an adhesive, and the recording paper passing through the nip portion does not separate and the fixing belt. There is a risk of wrapping around the fixing belt while adhering to the surface of the toner and causing jamming. In particular, when a small amount of paper (thin paper) and a small amount of coated paper for printing are used as the recording paper, the separation performance is further deteriorated.

  In the case of a full-color image forming apparatus, it is desirable that the image has gloss. For this reason, it is conceivable to increase the nip width and lengthen the heating time. In order to increase the nip width, the rubber layer of the fixing roller may be thickened or the diameter of the fixing roller may be increased.

  When a heat roller fixing device is used, since the fixing roller incorporates a heat source such as a halogen heater, it is difficult to increase the thickness of the rubber layer having low heat conductivity. Further, when the diameter of the fixing roller is increased, the recording paper separation performance is deteriorated.

  On the other hand, in the belt fixing device, since the fixing roller does not have a heat source, the rubber layer can be thickened, and the nip width can be increased even if the fixing roller has a small diameter. Therefore, the belt fixing device is more advantageous than the heat roller fixing device in terms of recording paper separation performance.

  Particularly in the case of a high-speed machine, a belt fixing device that can increase the thickness of the rubber layer is advantageous.

  However, even if a belt fixing device is used, it is difficult to reliably separate the recording paper that has passed through the nip portion.

  Therefore, there is a type in which a separation claw is brought into contact with the fixing belt to separate the recording paper from the fixing belt on the downstream side in the recording paper conveyance direction of the fixing device. However, in this configuration, since the separation claw is always in contact with the fixing belt, the fixing belt may be worn out and deteriorated.

  In addition, a number of patent publications aimed at improving the separation performance of the recording paper in the fixing device are disclosed.

  For example, there is a belt fixing device in which a fixing belt is stretched by a heating roller, a fixing roller, and a fixing member having a large radius of curvature, and the fixing member is pressed against the pressure roller from the inside of the fixing belt via the fixing belt. . According to this fixing device, since the fixing belt also has a large radius of curvature due to the fixing member having a large radius of curvature, the separation performance of the recording paper is improved without providing a separation claw (see Patent Document 1).

  In addition, there is a belt fixing device in which a plurality of separation rotating bodies that are driven to rotate with a plurality of claws are provided on the exit side of the nip portion, and the recording paper is separated from the surface of the fixing belt by the separation rotating body ( Patent Document 2).

  In addition, there is a belt fixing device in which a separating unit made of a stainless plate or the like is disposed in a non-contact state with the fixing belt on the exit side of the nip portion, and prevents the recording paper from being wound around the fixing belt by the separating unit (Patent Document) 3).

Further, a small-diameter auxiliary roller is pressed against the fixing roller via the fixing belt on the exit side of the nip portion, so that a curved portion having a large curvature is formed between the nip portion and the pressing portion of the auxiliary roller. There is a belt fixing device in which the recording paper that has passed through the nip portion is peeled off from the fixing belt at the curved portion (see Patent Document 4).
Japanese Patent Laid-Open No. 2003-5565 JP 2003-122172 A JP 2003-330309 A JP 2003-330310 A

  However, the above patent document has the following problems.

  In Patent Document 1, the fixing belt is stretched by a heating roller, a fixing roller, and a fixing member having a large curvature radius, and the fixing member is pressed against the pressure roller from the inside of the fixing belt via the fixing belt. . Accordingly, since the back surface of the fixing belt is always in sliding contact with the fixing member having a small radius and not rotating, there is a problem that the fixing belt is easily worn and sufficient durability performance cannot be expected.

  In Patent Document 2, there is a problem that the fixed image is easily damaged by the claw provided on the separation rotating body.

  In Patent Document 3, there is a problem that the fixed image is easily damaged by the edge of the separating means.

  In Patent Document 4, there is a problem that uneven glossiness is likely to occur because the toner image on the recording paper and the fixing belt are unstable and uneven on the downstream side of the nip portion.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus including a belt fixing device that has a simple configuration and reliably separates even when a recording sheet to be passed is a thin sheet. To do.

The above object is achieved by the invention described below.
(1) The endless fixing belt is stretched around at least two rollers, and has a pressure roller that presses the fixing roller, which is one roller, through the fixing belt, and is heated by a predetermined heating unit. In an image forming apparatus including a fixing device that fixes a toner image on a recording sheet at a nip portion formed between a fixing belt and the pressure roller.
Tension changing means for changing the tension of the fixing belt;
Recording paper discriminating means for discriminating the type of recording paper to be fed;
When the recording paper discriminating unit determines that the recording paper is thin paper, the tension of the fixing belt is set to a predetermined range smaller than that of the thick paper, and the curvature of the fixing belt near the exit of the nip portion is set. Control means for controlling the tension changing means to operate so as to be larger than that during thick paper ;
An image forming apparatus comprising:
(2) The endless fixing belt is stretched around at least two rollers, and has a pressure roller that presses the fixing roller that is one of the rollers through the fixing belt, and is heated by a predetermined heating unit. In an image forming apparatus including a fixing device that fixes a toner image on a recording sheet at a nip portion formed between a fixing belt and the pressure roller.
Tension changing means for changing the tension of the fixing belt;
Separation priority mode setting means,
When the separation priority mode is set by the separation priority mode setting means, the tension changing means is operated to set the tension of the fixing belt to a predetermined range smaller than that during thick paper, and the vicinity of the exit of the nip portion Control means for controlling the curvature of the fixing belt to be larger than that of thick paper ;
An image forming apparatus comprising:

According to the image forming apparatus of the first aspect, since the tension of the fixing belt of the belt fixing device is changed according to the recording paper to be passed, even if the recording paper such as thin paper is passed, the fixing device passes through the fixing device. It can be reliably separated later.

According to the image forming apparatus of the second aspect , since the tension of the fixing belt of the belt fixing device is changed according to the selection of the separation priority mode, an image pattern such as a photographic image that has a large amount of toner and is difficult to separate. Even if the recording paper to be passed is passed, it can be reliably separated after passing through the fixing device.

  Embodiments of the image forming apparatus according to the present invention will be described below with reference to the drawings.

  First, an example of an image forming apparatus will be described based on the configuration diagram of FIG.

  The image forming apparatus includes an image forming apparatus main body GH and an image reading apparatus YS.

  The image forming apparatus main body GH is called a tandem type color image forming apparatus, and a plurality of sets of image forming units 10Y, 10M, 10C, and 10K, a belt-like intermediate transfer member 6, a sheet feeding and conveying unit, a fixing device 9, and the like. Consists of.

  An image reading device YS including an automatic document feeder 201 and a document image scanning exposure device 202 is installed on the upper part of the image forming apparatus main body GH. The document d placed on the document table of the automatic document feeder 201 is transported by a transport unit, and an image on one or both sides of the document is scanned and exposed by the optical system of the document image scanning exposure device 202, and the line image sensor CCD is scanned. Is read.

  A signal formed by photoelectric conversion by the line image sensor CCD is subjected to analog processing, A / D conversion, shading correction, image compression processing, and the like in an image processing unit, and then exposure means 3Y, 3M, 3C, 3K. Sent to.

  In the image forming unit 10Y that forms a yellow (Y) image, a charging unit 2Y, an exposure unit 3Y, a developing device 4Y, and a cleaning unit 8Y are arranged around the photosensitive drum 1Y. In the image forming unit 10M that forms a magenta (M) color image, a charging unit 2M, an exposure unit 3M, a developing device 4M, and a cleaning unit 8M are arranged around the photosensitive drum 1M. In the image forming unit 10C that forms a cyan (C) color image, a charging unit 2C, an exposure unit 3C, a developing device 4C, and a cleaning unit 8C are arranged around the photosensitive drum 1C. In the image forming unit 10K that forms a black (K) image, a charging unit 2K, an exposure unit 3K, a developing device 4K, and a cleaning unit 8K are arranged around the photosensitive drum 1K. The charging unit 2Y and the exposure unit 3Y, the charging unit 2M and the exposure unit 3M, the charging unit 2C and the exposure device 3C, and the charging unit 2K and the exposure device 3K constitute a latent image forming unit.

  The developing devices 4Y, 4M, 4C, and 4K include a two-component developer including yellow (Y), magenta (M), cyan (C), and black (K) toner having a small particle diameter and a carrier.

  The intermediate transfer body 6 is wound around a plurality of rollers and is rotatably supported.

  The fixing device 9 includes a pressure roller 94 that stretches an endless fixing belt 91 between a heating roller 92 and a fixing roller 93 and presses the fixing roller 93 via the fixing belt 91. The toner image on the recording paper (transfer material) P is heated and pressed at a nip formed between the pressure roller 94 and fixed.

  Thus, each color image formed by the image forming units 10Y, 10M, 10C, and 10K is sequentially transferred onto the rotating intermediate transfer body 6 by the transfer means 7Y, 7M, 7C, and 7K (primary transfer). A toner image synthesized with a color image is formed. The recording paper P stored in the paper feeding cassette 20 is fed by the paper feeding means 21 and is conveyed to the transfer means 7A via the paper feeding rollers 22A, 22B, 22C, 22D, the registration rollers 23, etc. A color image is transferred onto P (secondary transfer). The recording paper P to which the color image has been transferred is heated and pressurized by the fixing device 9 and the color toner image on the recording paper P is fixed. Thereafter, the sheet is sandwiched between the sheet discharge rollers 24 and placed on the sheet discharge tray 25 outside the apparatus.

  On the other hand, after the color image is transferred to the recording paper P by the transfer means 7A, the residual toner is removed by the cleaning means 8A from the intermediate transfer body 6 from which the recording paper P is separated by curvature.

  Although the above is an image forming apparatus that forms a color image, an image forming apparatus that forms a monochrome image may be used.

  Next, the fixing device according to the present invention will be described with reference to the sectional view of FIG.

  The fixing belt 91 is formed in an endless shape. For example, a heat-resistant resin belt made of PI (polyimide) having a thickness of 70 μm is used as a base, and the outer peripheral surface of the base is covered with a heat-resistant silicon rubber having a thickness of 200 μm. Furthermore, it is covered with a PFA (perfluoroalkoxy) tube having a thickness of 30 μm as a release layer. The outer diameter is 80 mm, for example, and the width is 340 mm, for example.

  The heating roller 92 incorporates a halogen lamp 92A as a heating means for heating the fixing belt 91. For example, the outer peripheral surface of a cylindrical hollow rotating body 92B having a thickness of 2 mm formed of aluminum or the like is formed on a heat-resistant PFA. It is covered with a tube 92C and configured as a hard roller. The outer diameter is 47 mm, for example.

  The fixing roller 93 is formed by, for example, covering a solid core metal 93A formed of a metal such as iron with a heat-resistant silicon rubber 93B having a thickness of 4.5 mm, and further using a sponge 93C having a thickness of 2.5 mm. Covered and configured as a soft roller. The outer diameter is 40 mm.

  The pressure roller 94 incorporates a halogen lamp 94A. For example, the outer peripheral surface of a cylindrical hollow rotating body 94B having a thickness of 3 mm formed of aluminum or the like is covered with a heat-resistant silicon rubber 94C having a thickness of 3 mm. Further, it is covered with a PFA (perfluoroalkoxy) tube 94D having a thickness of 30 μm as a release layer, and configured as a soft roller. In addition, an outer diameter dimension is 50 mm, for example.

  Note that any heating means may be used as a heating means for heating the fixing belt 91, for example, an induction heating heating element using an excitation coil may be used. Further, the heating means is not necessarily arranged in the heating roller 92 or the like, and may be arranged anywhere.

  Further, a support shaft (not shown) of the heating roller 92 can move in the vertical direction, and a tension spring 95 is hung on the heating roller 92, and the heating roller 92 is biased upward. Accordingly, when the urging force of the tension spring 95 is changed, the tension of the fixing belt 91 is changed.

  Here, a solenoid or a motor can be used as a drive source for changing the urging force of the tension spring 95.

  Although it is possible to change the tension of the fixing belt 91 even if a tension roller is provided, the configuration becomes complicated and heat is taken away by the tension roller, which is not preferable.

  Further, a temperature sensor S <b> 1 for detecting the temperature of the heating roller 92 and a temperature sensor S <b> 2 for detecting the temperature of the pressure roller 94 are provided via the fixing belt 91.

  In the above configuration, when the fixing roller 93 is rotated clockwise by a driving unit (not shown), the fixing belt 91 and the heating roller 92 are rotated clockwise, and the pressure roller 94 is rotated counterclockwise. Further, the fixing belt 91 in contact with the heating roller 92 is heated by the halogen lamp 92A, and the pressure roller 94 is also heated by the halogen lamp 94A. Since the pressure roller 94 is urged in the direction of the fixing roller 93 by an urging means (not shown), the nip portion N between the fixing belt 91 wound around the fixing roller 93 and the pressure roller 94. Thus, the fed recording paper P is heated and pressurized, and the toner image on the recording paper P is fixed.

Next, an experiment conducted by changing the tension of the fixing belt using the image forming apparatus provided with the above fixing device will be described.
-Experiment contents The tension of the fixing belt and the temperature of the heating roller were changed, and four types of recording papers were passed to confirm separation and fixing properties.

Control temperature: set so that the temperature of the pressure roller is 20 ° C. lower than the temperature of the heating roller. Paper feeding speed: 300 mm / s
Toner adhesion amount: 10 ± 0.3 g / m ²
Recording paper:
Plain paper A Weight 56g / m ²
Plain paper B 斤 amount 80g / m ²
Coated paper A Weight 80g / m ²
Coated paper B Weight of 100 g / m ²
Experimental results Table 1 shows separability data and Table 2 shows fixing data.

  The separability was evaluated as follows.

○: Separated Δ: Separated with a feeling of winding × ˜Δ: Though slightly wound, but separated temporarily ×: Not separated In addition, the fixing property was evaluated as follows.

After fixing the image of the predetermined pattern, the image was rubbed with a cloth to measure the density change. When the density was not less than the predetermined density, “Good” indicates that the fixing property is good, and “Other” indicates that the fixing is poor.
-Consideration First, the separability shown in Table 1 tends to be improved when the tension of the fixing belt is lowered. In this experiment, when the tension is set to 15 N, the temperature of the heating roller can be changed. All the recording sheets were separated. However, when the tension of the fixing belt was further reduced and the tension was set to 8N, almost no separation occurred.

  This is presumed that when the tension of the fixing belt is lowered, the pressure roller bites by the fixing roller via the fixing belt, and the nip width is widened, so that the separation performance is improved. However, if the tension of the fixing belt is excessively reduced, it is estimated that the fixing belt bends before and after the nip portion, the separation curvature near the nip portion exit becomes small, and the separation performance deteriorates.

  Moreover, almost all of the plain paper B and the coated paper B, which are thick papers, were separated even when the temperature of the heating roller was changed within the range of the tension of 15N to 33N. When the temperature was changed, all separated only when the tension was 15N.

  Further, when the tension was increased to 46 N, the separation performance of each paper decreased.

  On the other hand, the fixability shown in Table 2 tends to be worse when the tension of the fixing belt is lowered. In this experiment, when the tension is set to 46N and 33N, the temperature of the heating roller can be changed. The fixability of all the recording papers was good, but when the tension is set to 20 N or less, the fixability gradually deteriorates. This particularly affects the paper quality. The plain paper A and the coated paper A, which are thin papers, did not deteriorate the fixing property even when the tension was reduced to 15 N, but the plain paper B and the coated paper B that are thick papers. When the tension was set to 20 N, fixing failure occurred when the temperature of the heating roller was 180 ° C. or 170 ° C.

  This is because if the tension is set low, the contact between the fixing belt and the heating roller becomes insufficient, and as a result, the heat supply to the fixing belt decreases. In particular, when the paper is thick, the heat supply is further insufficient, resulting in poor fixing. Moreover, even if the amount of heat supplied is smaller than that of the thick paper, the thin paper can secure a sufficient fixing temperature, so that no problem occurs. Therefore, it is desirable that the tension is large from the viewpoint of fixing properties.

  From these results, it can be seen that fixing the tension is a problem in satisfying the separation performance and the fixing property at the same time.

  Here, the temperature of the heating roller is often set to about 190 ° C. Considering the variation in the setting temperature and the temperature drop due to paper passing immediately after warming up, the temperature of the heating roller is separated in the range of 170 ° C. to 200 ° C. Need to be guaranteed. In consideration of this point, Tables 1 and 2 are summarized as ◯ when the temperature of the heating roller is all in the range of 170 ° C. to 200 ° C., and when there is even one ×. Is shown in Table 3. In addition, the right column of Table 3 shows the overall result of combining the separability and the fixability.

  According to Table 3, if the tension of the fixing belt is set to 33N for the thick paper and the tension of the fixing belt is set to 15N for the thin paper regardless of the plain paper and the coated paper, the separability and the fixing are achieved. Both of the properties are improved.

  As described above, by changing the tension of the fixing belt in accordance with the thickness of the recording paper, the separability can be improved without deteriorating the fixing property of the recording paper. It is not an absolute value, and changes if the size and material of each member of the fixing device are different.

  Further, when the toner image is formed on the entire surface of the recording paper, for example, the separation performance is deteriorated even if the photograph and the presentation material are thick paper. Therefore, when feeding recording paper that is difficult to separate as described above, the fixing performance is lowered, but it is desirable to provide the image forming apparatus with a separation priority mode that is set by reducing the tension of the fixing belt to 20 N, for example.

  A configuration in which the tension of the fixing belt is changed in accordance with the type of recording paper fed in the belt fixing apparatus as described above is shown in FIG. 3 as a block diagram.

  In FIG. 3, a recording paper discriminating means 101 discriminates the type of recording paper to be passed, and discriminates whether the paper is plain paper or coated paper, and the amount of recording paper. Specifically, the user selects and inputs a recording sheet to be used on the operation board. Further, a sensor for measuring the reflectance, thickness, specific gravity, etc. of the recording paper may be arranged inside the image forming apparatus so that the type of the recording paper is automatically discriminated.

  Information output from the recording paper discriminating means 101 is input to the control means 102, and the tension changing means 103 is operated in accordance with the type and amount of the recording paper. More specifically, the information on the type and amount of input recording paper is compared with predetermined conditions to determine whether the recording paper corresponds to a thin paper. If it is determined that the paper is not thin, the tension is set to 33N, for example. If it is determined that the paper is thin, the tension is reduced and set to 15N, for example. The tension changing means 103 is a means for changing the tension of the fixing belt in accordance with the set tension, and a solenoid or a motor can be used.

  Further, when outputting an image pattern that is difficult to separate, if the user operates the separation priority mode setting means 104 on the operation board to set the separation priority mode, the control means 102 drives the tension changing means 103 to remove the fixing belt. Reduce to a predetermined tension.

1 is a configuration diagram of an image forming apparatus. FIG. 3 is a cross-sectional view of a fixing device. It is a block diagram concerning the present invention.

Explanation of symbols

9 Fixing Device 91 Fixing Belt 92 Heating Roller 93 Fixing Roller 94 Pressure Roller 95 Tension Spring

Claims (2)

An endless fixing belt is stretched between at least two rollers, and has a pressure roller that presses the fixing roller, which is one of the rollers, through the fixing belt, and the fixing belt heated by a predetermined heating unit; In an image forming apparatus provided with a fixing device for fixing a toner image on a recording sheet at a nip portion formed between the pressure roller,
Tension changing means for changing the tension of the fixing belt;
Recording paper discriminating means for discriminating the type of recording paper to be fed;
When the recording paper discriminating unit determines that the recording paper is thin paper, the tension of the fixing belt is set to a predetermined range smaller than that of the thick paper, and the curvature of the fixing belt near the exit of the nip portion is set. Control means for controlling the tension changing means to operate so as to be larger than that during thick paper ;
An image forming apparatus comprising: An endless fixing belt is stretched between at least two rollers, and has a pressure roller that presses the fixing roller, which is one of the rollers, through the fixing belt, and the fixing belt heated by a predetermined heating unit; In an image forming apparatus provided with a fixing device for fixing a toner image on a recording sheet at a nip portion formed between the pressure roller,
Tension changing means for changing the tension of the fixing belt;
Separation priority mode setting means,
When the separation priority mode is set by the separation priority mode setting means, the tension changing means is operated to set the tension of the fixing belt to a predetermined range smaller than that during thick paper, and the vicinity of the exit of the nip portion Control means for controlling the curvature of the fixing belt to be larger than that of thick paper ;
An image forming apparatus comprising:

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