Patent Application
20090245903
This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-78754 filed Mar. 25, 2008.
1. Technical Field
The present invention relates to a gloss imparting method, an image forming method, a fixing apparatus and an image forming apparatus.
2. Related Art
In recent years, higher image quality high-gloss images have been demanded with the spread of full color copiers and printers.
The present invention provides a gloss-imparting method that includes (a) applying heat and pressure to one side of a recording material having an image of toner formed on both sides, and fixing the image to the one side of the recording material, (b) cooling the one side of the recording material on which the image was fixed in the step (a), (c) applying heat and pressure to the other side of the recording material cooled in the step (b), and fixing the image to the other side of the recording material, and (d) cooling the other side of the recording material on which the image was fixed in the step (c). In the step (a), the fixing is performed such that a value of exp(−T·tan δ0) will be approximately 0.85 or less, where tan δ0 is a loss tangent of the toner when measured by dynamic viscoelastic measurement at a frequency of approximately 1 rad/sec at approximately the same temperature as a fixing temperature of the step (a), and T is a fixing time in the step (a).
Exemplary embodiment(s) of the present invention will be described in detail based on the following figures, wherein:
The configuration of an image forming apparatus according to an exemplary embodiment will be described. This image forming apparatus is constituted by an image forming section that forms images and a secondary fixing section that imparts gloss to images formed by the image forming section. Here, the configuration of the image forming section will be described firstly, after which the configuration of the secondary fixing section will be described.
The controller 11 performs various processes as well as controlling the various components of the image forming section 1. The controller 11 includes a central processing unit (CPU) that executes processes in accordance with programs, a read only memory (ROM) that stores programs used by the CPU, a random access memory (RAM) that is used as a work area of the CPU, a communication interface that communicates with external computer devices (not shown), an image processing circuit that performs image processing on image data input from a scanner device or an external computer device (not shown), and an input/output interface that inputs and outputs signals from and to the display operating section 12. The display operating section 12 includes a touch panel, and outputs operating signals according to operations by an operator, as well as performing image display instructed by the controller 11. The paper feeding section 13 houses sheets of paper cut to a prescribed size such as A3 or A4, and feeds out the sheets one at a time at a timing instructed by the controller 11. Sheets fed out from the paper feeding section 13 are transported to the secondary transfer section 16.
The image forming units 14Y, 14M, 14C, 14K and 14T form toner images respectively using yellow, magenta, cyan, black and transparent toner, and transfer these toner images in layers to the intermediate transfer belt 15. These image forming units 14 are arranged along the travel direction of the intermediate transfer belt 15. The image forming units 14 each include a photosensitive drum that carries an image, a charging section that uniformly charges the photosensitive drum to a predetermined potential, an exposure section that forms a latent image by irradiating the photosensitive drum with light according to the image data of the corresponding color, a developing section that forms a toner image by developing the latent image with toner of the corresponding color, and a primary transfer roller that transfers the toner image to the intermediate transfer belt 15 using the potential difference with the photosensitive drum.
The image forming units 14Y, 14M, 14C and 14K each form a toner image according to image data input from a scanner device or an external computer device (not shown). The image forming unit 14T forms a layer of transparent (colorless) toner on the toner images formed by the other image forming units, so as to cover the entire surface of the sheet on which these toner images are formed. The transparent color toner (hereinafter, “transparent toner”), being colorless, is transparent on the sheet.
Here, the composition of the transparent toner will be described in detail. This transparent toner is generated by mixing a crystalline polyester resin with one or more amorphous polyester resins. That is, the uppermost toner to be layered on the sheet is configured to include a crystalline linear aliphatic polyester resin and one or more amorphous resins. The inclusion of a crystalline polyester resin enables fixing to be favorably performed at a relatively low temperature, and further improves toner blocking resistance (toner storage stability) and image preservability. Since charge characteristics deteriorate when a crystalline polyester resin is included, this could cause image defects, although since the transparent toner is transparent on the sheet as mentioned above, image defects will not be readily noticeable should they occur.
The intermediate transfer belt 15 is caused to travel around by drive rollers (not shown). Once the toner images have been sequentially transferred in layers by the image forming units 14Y, 14M, 14C, 14K and 14T, the transferred toner images are transported to the secondary transfer section 16. The secondary transfer section 16 includes a secondary transfer roller and an opposing roller. The secondary transfer section 16 transfers the toner images on the intermediate transfer belt 15 to the sheet transported from the paper feeding section 13, using the potential difference with the intermediate transfer belt 15. The primary fixing section 17 includes a fixing roller and a pressure roller. The primary fixing section 17 applies heat and pressure to the sheet transported from the secondary transfer section 16 and fixes the toner images to the sheet.
The sheet fixed by the primary fixing section 17 is transported directly to a paper exit port by a paper transport section if the controller 11 instructs to form a single-sided image. If the controller 11 instructs to form a double-sided image, the sheet is again transported to the secondary transfer section 16 after being reversed back to front by a reversal transport section (not shown). This sheet is then transported to the paper exit port after the toner images have this time been transferred to the backside thereof by the secondary transfer section 16 and fixed by the primary fixing section 17. That is, the image forming section 1 acts as an image forming part that forms images of toner of various colors in layers on both sides of a sheet. The user sets the sheet exited the paper exit port of the image forming section 1 in the secondary fixing section when he or she wants to finish the image in high gloss.
The fixing belt 23 is stretched over the fixing roller 24, the steering roller 26 and the separation roller 27, and travels around in the direction of arrow A in
The fixing roller 24 is rotated by a drive section (not shown) with the center of the fixing roller as the axis. The fixing roller 24 applies heat to the sheet transported by the paper transport section 22. The fixing roller 24 is formed with a release layer composed of PFA tubing or the like around a highly heat-conductive metal core, and is provided with a halogen lamp inside the core. The surface of the fixing roller 24 is heated to approximately 120° C. to 190° C. using heat generated by the halogen lamp.
The pressure roller 25 is provided so as to face the fixing roller 24 with the fixing belt 23 sandwiched therebetween by pressure, and applies pressure to the sheet transported by the paper transport section 22 while rotating with the rotation of the fixing roller 24. The pressure roller 25 has an elastic body layer composed of fluororubber with a rubber hardness (JIS-A) of about 60° coated around a highly heat-conductive metal core, or the like, with a release layer similar to the fixing roller 24 formed on the surface thereof. Note that while the exemplary embodiment is described using an example in which a halogen lamp is not provided inside the core of the pressure roller 25, a halogen lamp may be provided inside the core.
The steering roller 26 rotates with the circular movement of the fixing belt 23, and corrects for bias that occurs when the fixing belt 23 travels around continuously (phenomenon whereby the fixing belt 23 moves in the rotation axis direction of the steering roller 26). Specifically, the steering roller 26 changes the angle relative to the travel direction of the fixing belt 23 by rocking according to the bias of the fixing belt 23 and suppressing movement of the fixing belt 23 in the axis direction of the steering roller 26.
The separation roller 27 rotates with the circular movement of the fixing belt 23, and is configured such that the sheet transported in close contact with the fixing belt 23 is separated at the installation position of the separation roller. The outer diameter of the separation roller 27 and the wrap angle of the fixing belt 23 are determined according to the adherence between the paper and the fixing belt 23 and the stiffness of the paper.
The cooling section 28 is a so-called heat sink that is provided so as to contact the inner peripheral surface of the fixing belt 23 between the fixing roller 24 and the separation roller 27. The cooling section 28, which includes a casing formed with a highly heat-conductive metal and a fin (plate-like protruding member) formed with a highly heat-conductive metal and provided inside the casing, absorbs heat from the fixing belt 23 and the sheet heated by the fixing roller 24, and reduces the temperature of the sheet to approximately 60 to 80° C. The heat absorbed by the casing is released into the air inside the cooling section 28 by the fin, before exiting the secondary fixing section 2 by a rotation fan (not shown). By cooling the sheet in this way, the toner images on the sheet solidify with the smoothness of the surface of the fixing belt 23 transferred thereto and it is made easier to separate the sheet from the fixing belt 23.
The reversal transport section 29 transports the sheet separated from the fixing belt 23 at the position of the separation roller 27 to the nip area N between the fixing roller 24 and the pressure roller 25 with the sheet having been reversed front to back. The sheet transported to the nip area N by the paper transport section 22 thereby firstly has the front side thereof fixed and cooled, before again being transported to the nip area N by the reversal transport section 29 and this time having the backside thereof fixed and cooled. That is, the fixing roller 24 and the pressure roller 25 act as a first fixing part that adds heat and pressure to one side of a sheet having a toner image formed on both sides to fix the toner image, and as a second fixing part that adds heat and pressure to the other side of the sheet cooled by the cooling section 28 to fix the toner image. Also, the cooling section 28 acts as a first cooling part that cools the one side of the sheet on which the toner image was fixed by the fixing roller 24 and the pressure roller 25, and as a second cooling part that cools the other side of the sheet on which the toner image was fixed by the fixing roller 24 and the pressure roller 25.
Note that in the following description, the transporting of a sheet to the nip area N by the paper transport section 22 and the fixing and cooling of the front side thereof will be called a “first side fixing step”, while the reversing and transporting of a sheet that has undergone the first side fixing step to the nip area N and the fixing and cooling of the backside thereof will be called a “second side fixing step”.
Next, the operations of the image forming section 1 and the secondary fixing section 2 will be described. When image data is input from a scanner device or an external computer device (not shown) and double-sided image forming is instructed, the image forming section 1 forms images according to the image data on the front and back sides of a sheet, and outputs the sheet from the paper exit port. At this time, a layer of transparent toner is formed on the front and back sides of the sheet so as to cover the entire surface thereof. Once a sheet having images formed on both sides has exited the image forming section 1, the user sets this sheet in the paper feeding section 21 of the secondary fixing section 2. The sheet set in the paper feeding section 21 is transported to the nip area N between the fixing roller 24 and the pressure roller 25 by the paper transport section 22, with the front side thereof facing the fixing roller 24.
Having been transported to the nip area N, this sheet has heat and pressure applied thereto by the fixing roller 24 and the pressure roller 25 while in close contact with the fixing belt 23. The toner images on the front side of the sheet thereby melt and adhere to the surface of the fixing belt 23. As described above, the sheet adhered to the surface of the fixing belt 23 is transported to the cooling section 28 and the separation roller 27 in this state, since the fixing belt 23 travels around in the direction of arrow A in
The sheet that has thus undergone the first side fixing step is reversed back to front by the reversal transport section 29, and again transported to the nip area N between the fixing roller 24 and the pressure roller 25, with the backside facing the fixing roller 24. This sheet then undergoes fixing and cooling similarly to the first side fixing step, this time on the backside. The sheet that has thus undergone the second side fixing step exits the paper exit port. The images on both sides of the sheet are thereby finished in high gloss.
If stress remaining in the toner on the front of the sheet is high after undergoing the above first side fixing step, the gloss of the image on the front of the sheet will be less than the gloss of the image on the back of the sheet once the sheet has passed through the second side fixing step. This is because when the back of the sheet is heated in the second side fixing step, the toner images on the front of the sheet that have solidified in a smooth state are again melted by the heat, creating unevenness in the toner images due to the stress remaining in the toner on the front of the sheet being released, and resulting in the toner images on the front of the sheet losing their smoothness.
The stress in the toner on the front of the sheet after the first side fixing step can be derived by the following calculation.
γ=γ1+γ2 (1)
Stress σ can be represented by the following expression (2), where G is the elastic modulus of the spring component m1, and η is the viscous modulus of the dashpot component m2.
σ=G·γ1=η·dγ2/dt (2)
Solving the simultaneous equation of expression (2) and the above expression (1) gives the following expression (3). Here, the total deformation γ is constant, and τ is the relaxation time.
σ(t)=σ0·exp(−t/τ) (3)
The relaxation time τ can be represented by the following expression (4), where ω0 is the angular frequency, and tan δ0 is the loss tangent of the toner.
τ=η/G=1/(ω0·tan δ0) (4)
Substituting expression (4) into expression (3) gives the following expression (5). Here, the time t for imparting deformation is a fixing time T of the fixing roller 24, and the angular frequency ω0 is 1 rad/sec.
σ=exp(−T·tan δ0) (5)
As evident from expression (5), the stress σ of the toner on the front on the sheet after the first side fixing step can be represented by the relation between the first side fixing time T and the loss tangent tan δ0 of the toner. If the value of exp(−T·tan δ0) in expression (5) is large, the stress σ of the toner on the front on the sheet after the first side fixing step will be high, resulting in a large reduction in the gloss of the image on the front of the sheet. On the other hand, if the value of exp(−T·tan δ0) in expression (5) is small, the stress σ of the toner on the front on the sheet after the first side fixing step will be low, resulting in a small reduction in the gloss of the image on the front of the sheet.
The inventors, through testing, arrived at an exp(−T·tan δ0) value at which the gloss difference between the front and back of the sheet is lessened. Firstly, the content of this testing will be described. This testing involved investigating the gloss difference obtained by measuring the gloss level of the image on the front of the sheet and the gloss level of the image on the back of the sheet for four image samples A to D having different fixing times and loss tangents of the toner, and subtracting the gloss level of the image on the back of the sheet from the gloss level of the image on the front of the sheet. Gloss levels were measured using a BYK-Gardner micro-gloss gloss meter at a 20° angle of incidence.
All of the image samples A to D were formed by the image forming section 1 in which a secondary color patch image obtained by mixing yellow, magenta and cyan was formed on the front and back of the sheet, and a layer of transparent toner was layered on the secondary color patch images. Single-sided 256 gsm (grams per square meter) cast-coated paper was used for the image samples A to D, with the cast-coated surface being the front side. Additionally, all of the image samples A to D were cooled at a cooling temperature of 70° C. after being fixed at a fixing temperature of 120° C. and a fixing nip pressure of 0.86 MPa in the first side fixing step, and then fixed at a fixing temperature of 180° C. in the second side fixing step by the secondary fixing section 2. Note that other conditions such as the fixing nip pressure and the cooling temperature in the second side fixing step were similar to the first side fixing step.
Here, the fixing times and loss tangents of the toner for the image samples A to D will be described in detail.
While polyester obtained by a suspension polymerization method was a main component of both toners p and q, the Theological properties of the toners differed. Toner p had an elastic modulus G′ in a range of 3×103 to 4×103 Pa and a loss tangent tan δ in a range of 0.8 to 1.2, when measured by dynamic viscoelastic measurement at a frequency of 1 rad/sec in a temperature range of 110° C. to 130° C. On the other hand, toner q had an elastic modulus G′ in a range of 4×102 to 1×103 Pa and a loss tangent tan δ in a range of 2.5 to 3.0, when measured by dynamic viscoelastic measurement under the same conditions as above.
Substituting the fixing times T and loss tangents tan δ0 of the toner for the image samples A to D into the above expression (5) gives the exp(−T·tan δ0) values of the image samples A to D. With the examples shown in
Next, the test results will be described.
Note that setting 30% or less as the benchmark at which the gloss difference is regarded as small is based on the following test results.
According to the test results, the evaluation sample X having a 15% gloss difference with the reference sample was evaluated at “1” or “2” by all of the subjects I to X. The evaluation sample Y having a 30% gloss difference with the reference sample was evaluated at “3” by subject V, and “2” by the other subjects. On the other hand, the evaluation sample Z having a 40% gloss difference with the reference sample was evaluated at “2” by subjects VII and VIII, and “3” by the other eight subjects. In other words, nearly all of subjects evaluated the gloss reduction in the evaluation sample Z as being unacceptable. While the exp(−T·tan δ0) value needs to be reduced in order to lessen the amount of gloss reduction, the fixing time T has to be lengthened or the value of the loss tangent tan δ of the toner has to be increased to achieve this. Lengthening the fixing time T hinders device miniaturization, while increasing the value of the loss tangent tan δ0 of the toner makes offset more likely to occur during fixing. Consequently, in this exemplary embodiment, a gloss difference of up to 30% is established as an acceptable range for gloss reduction.
According to the exemplary embodiment described above, by setting the above exp(−T·tan δ0) value to approximately 0.85 or less, the gloss difference between the image on the front and the image on the back of the sheet can be reduced to 30% or less. That is, the gloss difference between the gloss of an image formed on one side of a recording material and the gloss of an image formed on the other side of the recording material can be lessened. In the case where sheets with images formed on both sides are bound as a booklet, for example, the backside of the previous sheet will be arranged as the left page and the front side of the next page will be arranged as the right page. In this case, the user will notice something strange if there is a large gloss difference between the right and left pages, since he or she views these pages together. According to the above exemplary embodiment, the user will feel little incongruity in such cases, because the gloss difference between the right and left pages will have been lessened.
While an exemplary embodiment has been described above, this exemplary embodiment can be modified as follows. The following modifications may be appropriately combined.
The above secondary fixing section 2 may be provided with an air blast cooling section that cools the front of the sheet by air blasting in the second side fixing step.
While the fixing times and loss tangents tan δ0 of the toner for the image samples were known in advance in the above exemplary embodiment, the inventors arrived, through testing, at a method for determining whether the above exp(−T·tan δ0) value is approximately 0.85 or less even with an image sample for which these values are not known. Here, this determination method will be described. Firstly, the gloss level of a given image sample in which a toner image having a gloss level of approximately 70% or greater measured at an approximately 20° angle of incidence is formed by an image forming apparatus having the above image forming section 1 and secondary fixing section 2 is measured at an approximately 20° angle of incidence, and the measured gloss level is set as a pre-heating gloss level. Next, one side of the image sample is heated for approximately 3 minutes in an approximately 85° C. oven, the gloss level of the image sample after heating is measured at an approximately 20° angle of incidence, and the measured gloss level is set as a post-heating gloss level. Next, the gloss difference between the pre-heating gloss level and the post-heating gloss level is computed, and it is determined whether the exp(−T·tan δ0) value is approximately 0.85 or less depending on whether the gloss difference is greater than a prescribed threshold. This is because if there is a large reduction in gloss level before and after heating, it can be said that stress σ remaining in the toner of the pre-heating image sample is high, whereas if there is a small reduction in gloss level before and after heating, it can be said that stress σ remaining in the toner of the pre-heating image sample is low, since the gloss level of the image sample after heating in the oven decreases according to the same principle by which the gloss level of the front of the sheet decreases in the above second side fixing step.
Next, the test results of investigating the gloss differences before and after heating in an oven with the above procedure for four image samples E to H having different fixing times and loss tangents of the toner will be described. In the testing, a Yamato Scientific oven DK400T was used as the oven for heating the image samples.
While the above exemplary embodiment was described using an example of a configuration in which the image forming section 1 and the secondary fixing section 2 are provided separately, the image forming section 1 and the secondary fixing section 2 may be provided within the same casing. Processing in the image forming section 1 and processing in the secondary fixing section 2 is thereby performed continuously without the user setting sheets exited the image forming section 1 in the secondary fixing section 2. Also, the secondary fixing section 2 may be used as a sole fixing apparatus.
In the above exemplary embodiment, the reversal transport section 29 is provided in the secondary fixing section 2, and sheets that have undergone the first side fixing step are again transported to the nip area N by the reversal transport section 29, although the method of performing the first side fixing step and the second side fixing step is not limited to this. For example, the secondary fixing section 2 shown in
The above exemplary embodiment was described using a method of controlling the transport speed of a sheet when passing through the nip area N between the fixing roller 24 and the pressure roller 25 as a method of changing the length of the fixing time T, although the present invention is not limited to this. For example, two fixing rollers 24 may be provided, one with a large roller diameter and one with a small roller diameter, and the length of the fixing time T may be changed by selectively using these rollers. Note that in the case where the length of the fixing time T is changed by controlling the transport speed of sheets when passing through the nip area N, as in the above exemplary embodiment, device miniaturization can be achieved in comparison to the case where two fixing rollers are used selectively.
The above transparent toner desirably has a storage elastic modulus G′ of approximately 2×103 Pa or less and a loss tangent tan δ0 of approximately 2 or more, when measured by dynamic viscoelastic measurement at a frequency of 1 rad/sec at approximately the same temperature as the fixing temperature in the first side fixing step. In this case, device miniaturization can be achieved in comparison to lengthening the fixing time T, since the fixing time T for reducing the exp(−T·tan δ0) value to approximately 0.85 or less can be set relatively short. Also, fixing properties can be improved by setting the storage elastic modulus G′ to approximately 2×103 Pa or less. Further, the yellow toner, the magenta toner and the cyan toner may also have a storage elastic modulus G′ of approximately 2×103 Pa or less and a loss tangent tan δ0 of approximately 2 or more, when measured by dynamic viscoelastic measurement at a frequency of 1 red/sec or approximately 1 rad/sec at approximately the same temperature as the fixing temperature in the first side fixing step. That is, it is sufficient if at least the uppermost toner layered on the sheet has a storage elastic modulus of approximately 2×103 Pa or less and a loss tangent tan δ0 of approximately 2 or more, when measured by dynamic viscoelastic measurement at a frequency of 1 rad/sec or approximately 1 rad/sec at approximately the same temperature as the fixing temperature of the first side fixing step.
In the above exemplary embodiment, only the transparent toner is generated by mixing a crystalline polyester resin with an amorphous polyester resin, although the present invention is not limited to this. The yellow toner, the magenta toner and the cyan toner may also be generated by mixing a crystalline polyester resin with an amorphous polyester resin. That is, it is sufficient if at least the uppermost toner to be layered on a sheet is configured to include a crystalline linear aliphatic polyester resin and one or more amorphous resins.
The foregoing description of the exemplary embodiments 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 embodiments were 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 |
|---|---|---|---|
| 2008-078754 | Mar 2008 | JP | national |
