Patent Grant
7024139
1. Field of the Invention
The present invention relates to a multiple color image forming apparatus and method wherein a multicolor image is formed on a substrate using a liquid developer.
2. Discussion of the Background
Image forming methods using a liquid developer, such as an electro-photographic recording method and an electrostatic recording method, have certain advantages that a dry toner developing apparatus cannot achieve. In particular, because the liquid developer contains fine toner particles of a sub-micron size dispersed in a carrier solvent, methods using liquid developers can accomplish high image quality. Also, because the liquid developer image forming methods can obtain a sufficient image density using even a small amount of toner particles, they are economical and can accomplish a fine texture equivalent to that of printing, such as offset printing.
Various electro-photographic recording methods for forming a multiple color image are known.
A first known method uses four latent image retaining bodies, and simultaneously performs formation of latent images on respective latent image retaining bodies and development of each of the latent images for these latent image retaining bodies, and thereafter transfers sequentially developed visible images to a transfer body to form a superposed multicolor image on the transfer body.
A second known method uses only one latent image retaining body, and performs image formation by forming a latent image, developing the latent image, and transferring the developed image to a transfer body. The steps are sequentially repeated for each of several color images and a superposed multicolor image is formed on the transfer body.
A third known method uses serial sets of latent image formation and development devices to form serially stacked color images onto one latent image retaining body, whereby stacked multiple color image laminates defining a multiple color visible image are formed on the surface of the latent image retaining body and collectively transferred to a transfer body. This method, called the “Image on Image Process (IOI process),” is preferable from the standpoint of achieving a reduction in the size of the apparatus and precision of color superposition.
Nevertheless, the IOI process has a disadvantage caused by a peel off of a part of the toner particles of one color developed on a latent image retaining body from the latent image retaining body at a next color development station. The peel off may cause a degradation of image quality.
An object of the present invention is to provide a multiple color image forming apparatus and method in which peel off of toner particles of a previous color image from a latent image retaining body is suppressed, whereby a stable fine texture may be obtained.
Accordingly, according to a first aspect of the present invention, there is provided a multiple color image forming apparatus including a latent image retaining body, a first developing device, and a second developing device. The first developing device faces the latent image retaining body at a first developing station and is configured to develop a first latent image on the latent image retaining body using a first color liquid developer. The first color liquid developer contains a first solvent and first toner particles. The second developing device faces the latent image retaining body at a second developing station. The latent image retaining body at the second developing station retains a second latent image and supports a first color image developed by the first developing device. The second liquid developing device is configured to develop the second latent image using a second color liquid developer, and a reversible electrophoretic transfer efficiency of the toner particles of the first color image in the second liquid developer is 60% or less.
According to a second aspect of the present invention, there is provided a multiple color image forming apparatus including a latent image retaining body, a first developing device, a second developing device, and means for adjusting reversible electrophoretic transfer efficiency. The first developing device faces the latent image retaining body at a first developing station and is configured to develop a first latent image on the latent image retaining body using a first liquid developer. The first liquid developer contains a solvent and toner particles. The second developing device faces the latent image retaining body at a second developing station, and the latent image retaining body at the second developing station retains a second latent image and supports a first color image developed by the first developing device. The second developing device is configured to develop the second latent image using a second liquid developer. The reversible electrophoretic transfer efficiency of the toner particles of the first color image in the second liquid developer is set to be 60% or less by the means for adjusting.
According to a third aspect of the present invention, there is provided a multiple color image forming method including steps of forming a latent image on a latent image retaining body, which supports a first color image, and developing the latent image using a liquid developer. The first color image includes toner particles that have reversible electrophoretic transfer efficiency of 60% or less in the liquid developer.
A more complete appreciation of the invention and many of the attendant advantages thereof is readily obtained as the state becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
In various aspects, the present invention relates to a multicolor image forming apparatus and a multicolor image forming method using liquid developers.
The apparatus of
The latent image retaining body 1 rotates in the direction represented by an arrow A in
The charger 2-1 charges the surface of the latent image retaining body 1 from about +500 V to about +1,000 V, and the exposing device applies the light beam 3-1 to predetermined area of the surface of the latent image retaining body 1 so that the surface potential of the image area of the latent image retaining body 1, which is supposed to have a maximum concentration, is attenuated to about 0 to about +300 V.
A first developing device 4-1 in
The first developing device 4-1 includes a container storing a first liquid developer. The first developing device 4-1 also has a roller-shaped developing electrode (hereinafter called a “developing roller”) which is arranged to face the latent image retaining body 1 at a first developing station and has applied thereto a developing voltage. When the developing roller rotates, the liquid developer is brought from the container to the developing station to fill up a developing gap between the latent image retaining body 1 and the developing roller.
The toner particles having a positive electrical potential receive an electrophoretic force directed to an area of the latent image retaining body 1 where the electrical potential is attenuated or discharged by the exposing light 3-1, whereby the first color image is formed. This developing process is called a discharged area development.
The present embodiment can also be applied to a charged area development wherein negatively charged toner particles develop an image of a charged area which has a higher electrical potential than other areas.
The developing voltage applied to the developing electrode may be set to a potential between the electrical potentials of the image and non-image areas of the latent image retaining body 1. The developing voltage may be set to form an electrical potential difference from an image area of a maximum density of about 200 to about 500 V.
The developing gap between the surface of the developing roller and the surface of the latent image retaining body 1 is generally set to about 10 to about 200 μm.
The liquid developer includes a non-polar carrier liquid such as ISOPAR L® (a product of Exxon Mobil Co.) and toner particles containing a colorant of a predetermined color and dispersed in the carrier liquid. The toner particle may be a mixture of a colorant and a resin.
A squeeze roller may be juxtaposed with the developing roller in the developing device 4-1, disposed to face the latent image retaining body 1, and rotated in a direction so as to remove the excessive carrier liquid from the surface of the latent image retaining body 1.
A second charger 2—2 of
The charging and exposure conditions established by the charger 2-2 and the exposing device for the second electrostatic latent image may be the same as or changed from those of the first color image.
A second developing device 4-2, which may have the same structure as that of the first developing device, develops the second electrostatic latent image. The device 4-2 may also have other structures that are known in the art and different from the first developing device. The second developing device 4-2 stores a second liquid developer using toner particles of a different color from that of the first color image.
A heater 9-2 in
The second developing device 4-2 forms a second color toner image on the first color image on the surface of the latent image retaining body 1.
By performing similar steps through a plurality of times, a multicolor image is formed on the latent image retaining body and then may be brought into a substantial dry state (to the state where the carrier liquid is substantially removed from the developed image and only toner particles remain) by a suction valve 5 of
The multicolor image formed on the surface of the latent image retaining body is transferred to a transfer medium 6 of
The multicolor image on the surface of the latent image retaining body 1 may be transferred to the intermediate transfer medium 6 by a transfer method known in the art, for example the pressure transfer method. To transfer the multicolor image by pressure transfer, the latent image retaining body 1 and the intermediate transfer medium 6 may be brought into pressure contact at a first transfer station while an adhesion force of the multicolor image to the intermediate transfer medium 8 is arranged to be higher than an adhesion force to the latent image retaining body 1 by appropriate selection of surface materials of the medium 8 and the body 1.
In order to allow the toner particles to sufficiently exhibit their adhesion force at the first transfer station, it is preferred to heat the toner particles at or before the first transfer station by providing a heater at or near the first transfer station. The heater may be provided inside or outside of the transfer medium 6, and may be disposed between the suction valve 5 and the intermediate transfer medium 6 to face the latent image retaining body 1.
In place of the pressure transfer or with the pressure transfer, electrostatic or electrophoretic transfer may be used. A predetermined electric potential may be supplied to the intermediate transfer medium so that an electrostatic or electrophoretic force moves the toner particles towards the intermediate transfer medium 6 from the latent image retaining body 1.
The toner particles according to an example of the present invention have low reversible electrophoretic transfer efficiency as described below. Therefore, a transfer utilizing the pressure transfer may have a higher transfer efficiency than a transfer that utilizes electrostatic or electrophoretic transfer alone, and only small amounts of toner particles may remain on the surface of the latent image retaining body 1 by the method utilizing the pressure transfer.
The multicolor image transferred to the intermediate transfer medium 6 is then transferred to a recording medium such as a paper sheet 8 at a second transfer station where the intermediate transfer medium 6 faces a back-up roller 7. The sheet 8 is kept in pressure contact with the intermediate transfer medium 6 by the back-up roller (a pressure roller) 7, and pressure contact is employed to transfer the multicolor image on the surface of the intermediate transfer medium 6 to the sheet 8. The second transfer to this sheet 8 can be executed in combination or in substitution with the electrostatic transfer.
A direct transfer of the multicolor image from the latent image retaining body 1 to the recording medium 8 can be performed without using the intermediate transfer medium 6. The image may be transferred directly from the latent image retaining body 1 to the recording medium 8 by using the back-up roller 7 to provide a pressure or an electrostatic force to the multicolor image on the latent image retaining body 1. Applying the pressure from the back-up roller 7 to the latent image retaining body 1 during the direct transfer without using pressure may be preferable because of the low reversible electrophoretic transfer efficiency of the toner particles.
An electrostatic force is provided on the visible image formed on the surface of the latent image retaining body 1 during the second development. The second charger 2-2 charges the surface of the latent image retaining body 1 having the visible image of the first color to an electrical potential Vc, and the second exposing device selectively attenuates the potential of the latent image retaining body 1 by the laser beam 3-2 to form the electrostatic latent image of the second color. The second developing device 4-2 has a developing roller to which an electrical potential Vd (Vc≧Vd>0) is supplied and which develops the electrostatic latent image.
When the toner particles of the positive charge of the first toner image exist in an area not exposed by the second laser beam 3-2, the electrostatic force acts on the toner particles of the first toner image in the non-exposed area in a direction toward the developing electrode because of the electric field generated between the area of latent image retaining body 1 (potential Vc) and the developing electrode (potential Vd). Therefore, a peel off of the toner particles of the first color image tends to happen and the toner particles tend to be removed from the latent image retaining body 1. The removed toner particles may then be brought into the second developing device 4-2 and mixed with the second color developer in the developing device 4-2. The mixture may cause deteriorations of the successive development of the second color image.
Through repeated experiments on the relationship between peel off of the toner images of the first color during the development of the second color and the electrophoretic characteristics of the liquid developer, according to the present invention, a threshold condition has been found where the toner particles of the first color do not peel off while satisfying the relation Vc≧Vd.
In other words, according to the present invention, it has been found that when toner particles that have a reversible electrophoretic transfer efficiency in the second liquid developer of not greater than about 60% are used as the toner particles of the first color, peel off of the toner particles of the first color image does not occur during development of the second or other successive color image(s), and an appropriate development can be performed at the area having the electrical potential Vc (Vc≧Vd).
The term “reversible electrophoretic transfer efficiency” of the toner particles at the second developing station or other successive developing stations represents a ratio of the toner particles that undergo electrophoresis and peel off from an electro-deposited film formed on one electrode towards the other electrode. The electro-deposited film is formed by providing the liquid developer (the carrier liquid and the toner particles) between the pair of electrodes which are maintained at a certain electrical potential difference across the pair of electrodes. The electro-deposited film is formed through an electrophoresis movement of the toner particles in the liquid developer towards one of the pair of electrodes. After the toner particles are sufficiently aggregated to form the electro-deposited film, an inverse voltage is applied across the pair of electrodes. Part of the toner particles that peel off from the electro-deposited film and undergo electrophoresis towards the other electrode is measured and the ratio of the toner particles peeled off among all toner particles is calculated. The pair of electrodes correspond to the latent image retaining body 1 having the electrical potential and the developing electrode of the developing surface of the second /or other successive developing device. The electro-deposited film corresponds to the developed color image on the latent image retaining body 1.
Reversible electrophoretic transfer efficiency of the toner particles can be adjusted by, for example, controlling a resin composition in the toner particles. This reversible electrophoretic transfer efficiency can also be lowered by, for example, increasing a ratio of a resin having a low glass transition point, or by increasing a ratio of a resin having high solubility in the carrier liquid.
Because the reversible electrophoretic transfer efficiency decreases with an elevated temperature, it can be adjusted by carrying out the second toner image forming step at a higher temperature, for example at a heated condition or by keeping the latent image retaining body under the heated state by heating the second liquid developer.
The following example provides further explanation of the relationship between the reversible electrophoretic transfer efficiency of the liquid developer and peel off of a previously developed toner image, such as the first color image.
1) The Liquid Developer
ISOPAR-L® (a product of Exxon Mobil Co.) was used as a carrier solvent of the first liquid developer. “Cyanine blue-KRO,” a product of Sanyo Pigment Co., was used as a pigment (colorant) of the toner particles. Several kinds of acrylic ester resins were prepared by selecting and combining arbitrary monomers from acrylic acid, vinyl acetate, styrene, lauryl acrylate, lauryl methacrylate, butyl acrylate, butyl methacrylate, ethyl acrylate, ethyl methacrylate, methyl acrylate and methyl methacrylate. The weight ratio of these resins to the pigment was 4:1 or 7:3.
These resins, pigment and dispersant were mixed and dispersed with the carrier liquid in the presence of glass beads inside a paint shaker to obtain a concentrated developer. The concentrated developer was diluted so that the concentration of the toner particle component becomes l part by weight. Further, 10 parts by weight of zirconium naphthenate, a product of Dai-Nippon Ink Co., relative to the amount of the toner particle component, was added.
In this way, five kinds of first liquid developers (liquid developers a to e) each having a different toner particle composition were prepared.
To confirm peel off of the first color image, the second liquid developer includes only ISOPAR-L® without containing the toner particles in this experiment.
2) Evaluation of Characteristics of Liquid Developer
The reversible electrophoretic transfer efficiency of the toner particles in each of the resulting five liquid developers was measured in the following way.
First, a pair of ITO (indium tin oxide) transparent electrodes were arranged to face each other while sandwiching a 300 μm-thick Teflon sheet-like spacer between them to prepare a parallel flat sheet electrode cell.
The liquid developer (a) was charged between the electrodes of the parallel flat sheet electrode cell, and a DC voltage of about 200 V applied across the pair of ITO transparent electrodes for 10 seconds. The toner particles charged to a positive charge underwent electrophoresis, migrated towards a negative plate of the pair of electrodes, and formed an electro-deposited film.
An inverse voltage of about 200 V was then applied to the parallel flat sheet electrode cell for 10 seconds. At this time, a part of the toner particles remained adhered to the ITO transparent electrode and formed the electro-deposited film, and another part of the toner particles repeatedly undergoing electrophoresis adhered to the other ITO electrode. As a result, the toner particles adhered to both of the ITO transparent electrodes.
Thereafter, the cell was decomposed, the carrier liquid dried and the respective electrodes having the toner particles deposited thereto heated at 150° C. for 10 minutes to melt the toner particles. Optical transmission factors of these two electrodes with the melted toner particles was measured.
The reversible electrophoretic transfer efficiency of each of the liquid developers (a) to (e) used as the first liquid developer was measured, and the transfer efficiency for the liquid developer (a) was 10%, that of the liquid developer (b) was 50%, that of the liquid developer (c) was 60%, that of the liquid developer (d) was 70%, and that of the liquid developer (e) was 100%.
3) Condition of Liquid developer Electrophotographic Apparatus
The latent image retaining body 1 was a photosensitive drum having an organic photosensitive layer formed on the surface of an about 5 mm-thick aluminum drum, and an about 1 μm-thick silicone hard coat layer on the photosensitive layer.
The chargers 2-1 and 2-2 were a Scorotron charger and the surface of the latent image retaining body 1 was set so that the non-image area is charged to 800 V (Vc was 800 V).
Each of the first and second developing devices 4-1 and 4-2 had a developing roller of a diameter of about 17 mmφ and a squeeze roller of a diameter of about 17 mmφ. Each of the developing rollers was arranged to have a developing gap of about 150 μm and each of the squeeze rollers was arranged to have a squeezing gap of about 50 μm.
An electrical potential of 600 V was supplied to the developing roller and the squeeze roller of the first developing device 4-1. A variable power source was used for the developing roller and the squeeze roller of the developing device 4-2 so that an electrical potential Vb supplied to the respective rollers could be varied to an arbitrary value.
The first exposing device exposed an image area of the photosensitive drum, attenuated the potential of the exposed area to about 400 V by the laser beam 3-1, and exposed an area of 30 mm×30 mm square on the surface of the latent image retaining body 1 where the toner particles of the first liquid developer adhered.
The second exposing device did not oscillate the laser beam so as not to form a second color image.
The second developing device 4-2 had the same construction as that of the first developing device 4-1. An arbitrary electrical potential Vd was applied to the developing electrode and to the squeeze roller of the second developing device. The electrical potential of the developing electrode and the squeezing roller may be different from each other.
In other words, as the electrical potential Vd supplied to the developing roller of the second developing device 2-2 was varied to an arbitrary value, the relation between a threshold of a potential difference (Vc−Vd) and occurrence rates of peel off of the first toner image during the second development was examined.
The visible image formed on the latent image retaining body 1 was pressure-transferred to the intermediate transfer medium 6 that was arranged so as to keep pressure contact with the latent image retaining body 1. The intermediate transfer medium 6 may include a drum having an about 1 mm-thick urethane rubber layer on its surface. The intermediate transfer drum 6 was brought into pressure contact with the latent image retaining body 1 at a weight of about 50 kg per a width of A4 sheet size. The latent image retaining body 1 was kept at room temperature and the intermediate transfer drum 6 was kept at about 100° C.
The image on the intermediate transfer drum 6 was pressure-transferred and fixed to the sheet 8 conveyed between the intermediate transfer drum 6 and the pressure roller 7 which was disposed to keep a pressure contact with the intermediate transfer drum 6 to form a final image on the sheet 8. The pressure roller 7 was brought into pressure contact with the intermediate transfer drum 6 at about 50 kg per the width of A4 sheet size and was heated and held at about 100° C.
4) Evaluation
4-1) Preparation of Reference Sample
The visible image obtained by the first image forming step was as such transferred to the intermediate transfer medium 6 without forming a second color image and was further transferred from this intermediate transfer medium 6 to the sheet 8. The optical density of each image obtained by using the liquid developers (a) to (e) as the first liquid developer at the first developing station was measured with a Macbeth densitometer RD-914 (a product of Process Measurements Inc.) and was used as a reference density.
4-2) Peel off Evaluation of Visible Image
An image was formed on the sheet 8 by using the liquid developer (a) as the first liquid developer in the liquid developer electrophotographic apparatus. The threshold value of (Vc−Vd) where the optical density of the image formed on the sheet decreases was examined with respect to the reference density by changing the electrical potential Vd supplied to the developing roller of the second developing device 4-2.
In other words, the threshold value of (Vc−Vd), where peel off of the first color image occurred by an effect of the second developing device, was examined.
The threshold value of the drop of the density relative to the reference density using each of the liquid developers (b) to (e) was examined.
The result is shown in Table 1 and the relation between the image density and (Vc−Vd) obtained by this experiment is shown in the graph of
Table 1 and
For further study, the latent image retaining body 1 and the intermediate transfer body 6 were brought into mutual contact without substantial pressure/weight between each other, and an image output was conducted in the same way as in the above described example with an exception that the electrical potential of the intermediate transfer body 6 was kept at 0 V whereby electrostatic transfer was conducted. As a result, the multiple color image transferred by the pressure transfer was completely transferred to the intermediate transfer body 6. The multiple color image transferred by the electrostatic transfer was not good as obtained by pressure transfer but nevertheless had good transfer efficiency. An electrostatic transfer by applying an electrical potential of −1500 V to the intermediate transfer body 6 and the image transfer was conducted much more than the image by the electrical potential of 0 V applied to the intermediate transfer body 6. Therefore the transfer method may preferably be selected from those methods in accordance with the expected results. The electrical potential less than −1500 V may provide a discharge or a deterioration of the intermediate transfer body 6.
As described above, the present invention can prevent peel off of the toner particles forming the visible image of the previously developed color image formed on the surface of the latent image retaining body during the successive developing step developing color image(s) on the latent image retaining body.
Although the present invention has been particularly shown and described with reference to an embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made therein without departing from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2000-356100 | Nov 2000 | JP | national |
This application is a continuation of U.S. patent application Ser. No. 10/425,829, filed Apr. 30, 2003, now U.S. Pat. No. 6,792,233, and claims the benefit of priority under 35 USC §119 of Japanese patent Application No. 2000-356100, filed on Nov. 22, 2000, and under 35 USC §120 of U.S. Ser. No. 09/989,406, filed Nov. 21, 2001, now U.S. Pat. No. 6,600,891, the entire contents of which are incorporated herein by reference.
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|---|---|---|---|
| 4728983 | Zwadlo et al. | Mar 1988 | A |
| 5981134 | Miyamoto et al. | Nov 1999 | A |
| 6205312 | Kim | Mar 2001 | B1 |
| 6269233 | Yamaguchi | Jul 2001 | B1 |
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| 6792233 | Oooka et al. | Sep 2004 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20040259014 A1 | Dec 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09989406 | Nov 2001 | US |
| Child | 10425829 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10425829 | Apr 2003 | US |
| Child | 10892151 | US |
