JP7592985B2 - Image forming device - Google Patents

Description

This invention relates to an image forming device such as a copier, printer, facsimile, or a multifunction device that has multiple functions among these.

Image forming devices like the one above generally have a color mode in which multiple colors are layered to create an image, and a monochrome mode in which images are created using only black toner. In the color mode, multiple layers (multiple colors) of toner formed on the image carrier are transferred to the paper, so the transfer voltage is set higher than in the monochrome mode, which uses a single layer (only black toner).

However, even in color mode, the black parts of an image only have a single layer of black toner attached to them, so the transfer voltage in color mode causes excessive transfer and leaves it on the surface of the image carrier, resulting in a problem of low density in the black parts of the image transferred to the paper. This problem is not limited to black toner, but occurs in the thin toner layer in the toner image formed on the image carrier.

Patent document 1 discloses a technology for controlling an image forming device that forms a white toner image on a superimposed toner image formed on a transparent recording medium, in which the image forming means is controlled so as to change the amount of toner adhesion of the white toner image formed on the superimposed toner image according to the amount of toner adhesion of the superimposed toner image for each pixel.

Patent document 2 discloses an image forming apparatus that changes image forming conditions other than the most downstream color (K) depending on the amount of retransfer from the intermediate transfer pair to the image carrier in the primary transfer section.

Patent document 3 discloses an image forming device that controls the transfer efficiency so that the toner weight per unit area of each color component toner image in a black toner image formed by color-overlaying a plurality of color component toner images is transferred to approximately the same level.

JP 2015-64401 A JP 2001-42593 A Japanese Patent Application Publication No. 08-95337

However, the technologies described in these patent documents do not mention the deterioration of transferability of a thin toner layer (e.g., a portion where only a single layer of toner adheres) in a toner image formed on an image carrier in color mode, and do not provide a solution that can prevent such deterioration of transferability.

This invention was made in consideration of this technical background, and aims to provide an image forming device that can suppress deterioration of the transferability of the thin toner layer in the toner image formed on the image carrier in color mode.

The above object can be achieved by the following means:
(1) An image forming apparatus comprising: an image carrier; a toner image forming means for forming a toner image having a toner layer of different thicknesses on the image carrier using toners of multiple colors within a single image; a transfer means for transferring the toner image formed on the image carrier to paper; and a detection means for detecting a thin toner layer having a small thickness in the toner image formed on the image carrier, wherein the toner image is a non-foaming toner image not including an foaming toner image, the toner image forming means supplies toner of a different color from the thin toner layer among the multiple colors toners to a portion below the thin toner layer detected by the detection means, and a transfer voltage applied by the transfer means is set to match a toner layer that is thicker than the thin toner layer.
(2) An image forming apparatus comprising: an image carrier; a toner image forming means for forming a toner image having a toner layer of different thicknesses on the image carrier using toners of multiple colors within a single image; and a transfer means for transferring the toner image formed on the image carrier to paper, wherein the toner image forming means supplies toner of a color different from that of the thin toner layer to the underside of the thin toner layer in the toner image formed on the image carrier, and the toner supplied to the underside of the thin toner layer is a toner of a color other than that of the thin toner layer and having a relatively high charge amount.
(3) An image forming apparatus comprising: an image carrier; a toner image forming means for forming a toner image having a toner layer of different thicknesses within a single image on the image carrier using toners of multiple colors; and a transfer means for transferring the toner image formed on the image carrier to paper, wherein the toner image forming means supplies toner of a different color from the thin toner layer to the underside of the thin toner layer in the toner image formed on the image carrier, the toner image forming means having a screen consisting of multiple image dot patterns for each color, and when the thin toner layer is formed by the screen, the toner is supplied to the underside of the thin toner layer using the same screen as the thin toner layer.
(4) The image forming apparatus according to any one of (1) to (3) above, wherein the thin toner layer is a monochromatic toner layer.
(5) The image forming apparatus according to the above item 4, wherein the single color is black.
(6) The image forming apparatus according to any one of the above items 1 to 5, wherein the thickness of the thin toner layer is less than the thickness of a toner layer forming an image in one color.
(7) The image forming apparatus according to any one of (1) to (6) above, wherein the amount of toner replenished to the lower side of the thin toner layer is an amount that remains untransferred at 70 to 100% of the amount of the toner.
(8) An image forming apparatus according to any one of items 1 to 7, wherein the amount of toner replenished below the thin toner layer is controlled based on at least one of the environment, the durability of the developing unit, the color of the replenished toner, and the type of paper.
(9) The image forming apparatus according to any one of the preceding paragraphs 1 to 3, wherein the toner supplied to the lower side of the thin toner layer is a toner having a relatively high charge amount, among toners of colors other than those of the thin toner layer.
(10) The image forming apparatus according to any one of (1) to (9), wherein the color of the toner supplied to the lower side of the thin toner layer is yellow.
(11) The image forming apparatus according to any one of (1) to (10) above, wherein the toner supplied to the lower side of the thin toner layer is a toner supplied from a developing unit having the least durability.
(12) The image forming apparatus according to any one of (1) to (10) above, wherein the toner supplied to the lower side of the thin toner layer is supplied from a plurality of developing devices.
(13) The image forming apparatus according to any one of (1) to (12) above, further comprising a selection means for allowing a user to select between a monochrome mode and a color mode.
(14) The image forming apparatus according to any one of (1) to (13) above, further comprising a selection means for allowing a user to select between a mode in which toner is replenished to the underside of the thin toner layer and a mode in which toner is not replenished.
(15) The image forming apparatus according to the above item 1 or 2, further comprising a screen having a plurality of image dot patterns for each color.
(16) The image forming apparatus according to the above paragraph 15, wherein when the thin toner layer is formed by the screen, the toner is replenished to the underside of the thin toner layer by using the same screen as the screen of the thin toner layer.

According to the invention described in the preceding paragraph (1), in a toner image formed on an image carrier using a plurality of color toners, in which the toner layer has a different thickness within one image and is a non-foaming toner image that does not include a foaming toner image , a toner of a color different from the thin toner layer among the plurality of color toners is supplied to the underside of the thin toner layer having a smaller thickness detected by the detection means. Therefore, even if the transfer voltage by the transfer means is set to match a toner layer thicker than the thin toner layer, when the toner image formed on the image carrier is transferred to paper, the toner supplied to the underside of the thin toner layer remains on the surface of the image carrier, and the thin toner layer is transferred to the paper, so that the problem of the density of the thin toner layer becoming thin in the image transferred to the paper can be solved. In other words, the deterioration of the transferability of the thin toner layer can be suppressed, resulting in an image forming apparatus with good image quality.

According to the invention described in the preceding paragraph ( 4 ), since the thin toner layer is a monochromatic toner layer, the problem of the density of the monochromatic portion becoming thin in the image transferred to the paper can be eliminated.

According to the invention described in the preceding paragraph ( 5 ), the problem of the density of black parts becoming light in the image transferred to the paper can be solved.

According to the invention described in the preceding paragraph ( 6 ), when the thickness of the thin toner layer is less than the thickness of the toner layer forming an image in one color, the problem that the density of the thin toner layer becomes low in the image transferred to the paper can be more effectively solved.

According to the invention described in the preceding paragraph ( 7 ), the amount of toner supplied to the underside of the thin toner layer is set to an amount that leaves 70 to 100% of the toner amount as a transfer residue, so that the transfer of the supplied toner to the paper can be suppressed and color change due to toner mixing on the paper after transfer can be prevented.

According to the invention described in the preceding paragraph ( 8 ), the amount of toner replenished to the underside of the thin toner layer can be controlled to a desired value based on at least one of the environment, the durability of the developing device, the color of the replenished toner, and the type of paper.

According to the invention described in the preceding paragraph ( 9 ), the toner supplied to the underside of the thin toner layer is a toner having a relatively high charge amount among toners of colors other than the thin toner layer, so that the amount of toner supplied can be reduced and toner consumption can be suppressed.

According to the invention described in the preceding paragraph ( 10 ), the color of the toner supplied to the underside of the thin toner layer is yellow, which is inconspicuous even if transferred to paper, so that deterioration of image quality can be prevented.

According to the invention described in the preceding paragraph ( 11 ), the toner supplied to the lower side of the thin toner layer is the toner supplied by the developing unit with the least durability, so that it is possible to reduce the deviation in the life span of each developing unit, and to reduce the number of visits by service personnel and the number of times developing units need to be purchased.

According to the invention described in the preceding paragraph ( 12 ), the toner supplied to the lower side of the thin toner layer is toner supplied from a plurality of developing devices, so that the consumption of the supplied toner can be averaged, and unevenness in the life span of the developing devices due to uneven toner consumption can be prevented.

According to the invention described in the preceding paragraph ( 13 ), the user can select between the monochrome mode and the color mode.

According to the invention described in the preceding paragraph ( 14 ), the user can select a mode in which toner is replenished below the thin toner layer or a mode in which toner is not replenished, and the user's intention can be respected.

According to the invention described in the preceding paragraph ( 15 ), since a screen consisting of a plurality of image dot patterns is provided for each color, toner can be replenished under the thin toner layer of intermediate tones formed by the dot patterns by using a screen of a dot pattern of a different color.

According to the invention described in the preceding paragraph ( 16 ), the supply of toner to the underside of the thin toner layer is performed using the same screen as the screen for the thin toner layer, so that the dot patterns of the thin toner layer and the toner supplied to the underside thereof can be superimposed. This makes it possible to prevent the thin toner layer from remaining on the image carrier during paper transfer, and allows the thin toner layer to be efficiently transferred to paper.

1 is a configuration diagram of an image forming apparatus according to an embodiment of the present invention. 1 is a graph showing the relationship between transfer voltage and transfer efficiency for each toner layer thickness. FIG. 1 is a cross-sectional view for explaining a conventional problem, showing a thin toner layer formed on an image carrier before secondary transfer to paper. 4 is a perspective view showing the state of the image carrier and the toner image on the paper after the thin toner layer in FIG. 3 has been secondarily transferred; FIG. FIG. 2 is a cross-sectional view showing a thin toner layer formed on an image carrier before secondary transfer to paper in an embodiment of the present invention. 6 is a perspective view showing the state of the image carrier and the toner image on the paper after the thin toner layer in FIG. 5 has been secondarily transferred; FIG. 11 is a flowchart showing a process for determining whether toner for replenishing is to be applied to a lower side of a thin toner layer. 11 is a table showing a specific method of correcting the amount of replenishment toner based on each parameter. 11 is a table showing an example of the thickness of a replenishment toner layer relative to the thickness of a thin toner layer. 13A and 13B are diagrams for explaining a method of replenishing toner to the underside of a thin toner layer when the thin toner layer is formed to have a half-tone density by using a screen consisting of a dot pattern.

The following describes an embodiment of the invention with reference to the drawings.

Figure 1 is a diagram showing the configuration of an image forming apparatus according to one embodiment of the present invention.

In FIG. 1, reference numeral 4 denotes an intermediate transfer belt that serves as an image carrier and is placed horizontally in the center of the main body, stretched across a drive roller 15 and a driven roller 16, and the intermediate transfer belt 4 runs in the direction of the arrow.

Below the intermediate transfer belt 4, imaging units 20Y, 20M, 20C, and 20K are arranged from the upstream side to the downstream side of the intermediate transfer belt 4, respectively, to form toner layers of the respective colors of yellow (Y), magenta (M), cyan (C), and black (K) on the intermediate transfer belt 4.

Each of the imaging units 20Y, 20M, 20C, and 20K has a rotatable photoconductor 1, and around the photoconductor 1 are arranged a charging device 2, an exposure device 8, a developing device 3, a primary transfer roller (not shown), and a cleaning unit 9 for cleaning the photoconductor 1. After the photoconductor 1 is charged by the charging device 2, a latent image is generated by the exposure device 8, and the latent image is developed and visualized by toner supplied to the developing device 3 from each toner bottle 13. The visualized toner image is transferred to the intermediate image carrier 4 by the primary transfer roller.

In color mode, the toner images of each color formed on the four imaging units 20Y, 20M, 20C, and 20K are transferred in a superimposed state onto the intermediate transfer belt 4 to form one color toner image consisting of four toner layers on the intermediate transfer belt 4. In monochrome mode, a toner image consisting of monochrome toner layers is formed by the black imaging unit 20K.

The toner image formed on the intermediate transfer belt 4 is then transferred to a transfer material (paper) by the secondary transfer roller 5, after which the paper is separated from the intermediate transfer belt 4 by the separation section 14, fixed by the fixing unit 6, and discharged outside the device.

In FIG. 1, reference numeral 7 denotes a cleaning device that cleans the intermediate transfer belt 4, reference numeral 10 denotes a sensor that reads the density of the toner image on the intermediate transfer belt 4, reference numeral 11 denotes a sensor that reads the paper transport position, and reference numeral 12 denotes a scanner that reads the original document.

Furthermore, the configuration of the image forming apparatus and the configuration, arrangement, and operation of the imaging units 20Y, 20M, 20C, and 20K are not limited to the above example. For example, in an image forming apparatus with one imaging unit, the single imaging unit may form toner images of each color, and these may be superimposed on the intermediate transfer body to form one color image. Alternatively, the image forming apparatus may be of a cleaning-less type that does not have a cleaning means.

Normally, when copying or printing in color mode, the thickness of the toner layers (toner thickness) that are layered on one image varies depending on the image (color). In particular, the parts of the image that are only black will be less than a single layer thick, and the difference in toner thickness between these and the color parts, where multiple colors are layered, will be large.

When transferring a toner image onto paper, the thicker the toner, the higher the voltage required for the transfer output. Therefore, in color mode, it is common to output a transfer voltage that matches the part of the paper where the toner is thickest. This is explained using the graph in Figure 2.

Figure 2 is a graph showing the relationship between transfer voltage and transfer efficiency for each toner layer thickness. In Figure 2, A1 indicates a toner thickness of 0.5 layers, A2 indicates a toner thickness of 1 layer, A3 indicates a toner thickness when another toner layer of 0.7 layers is formed below the 0.5 layer, and A4 indicates a toner thickness of 2 layers. It shows that the transfer efficiency improves by setting a higher transfer voltage as the toner thickness increases. In the example of Figure 2, a transfer voltage that matches the thickest toner layer, 2 layers (marked as the 2-layer determination voltage in Figure 2) is output.

However, if a transfer voltage is output that matches the area where the toner thickness is the thickest, the transfer output will be excessive in thin toner layers such as black toner, which has a thin toner thickness, causing reverse transfer, which will result in poor transfer efficiency, lower density after transfer to the paper, and a faint image.

This phenomenon will be explained using Figures 3 and 4, which show the state after secondary transfer onto paper. Figure 3 is a cross-sectional view for explaining the conventional problem, showing a thin toner layer 30 formed on the intermediate transfer belt 4 before secondary transfer onto paper P. In the example of Figure 3, it is assumed that a thin toner layer 30 made of a single color of black K and having a thin toner thickness is formed as part of a color toner image.

When the color toner image of FIG. 3 formed on the intermediate transfer belt 4 is secondarily transferred to paper P, as shown in FIG. 4, much of the thin black toner layer 30 of the color toner image remains on the intermediate transfer belt 4 even after the secondary transfer, and the amount of black toner on the paper P after transfer decreases, resulting in a lower density and a lighter image. Note that FIG. 4 also shows the state of the two toner layers of magenta M and cyan C after the secondary transfer, and although part of the lower layer magenta M remains on the intermediate transfer belt 4, the remaining amount is less than the remaining amount of the thin black toner layer 30.

Therefore, in this embodiment, the following measures are taken to solve the problem that much of the toner in the thin toner layer 30 remains on the intermediate transfer belt 4 even after the secondary transfer, resulting in a decrease in density on the paper P after transfer.

Figure 5 is a cross-sectional view showing the thin toner layer 30 formed on the intermediate transfer belt 4 before secondary transfer to the paper P in this embodiment, and corresponds to Figure 3. In the example of Figure 5, toner of a color (e.g., yellow Y) different from the black color K of the thin toner layer 30 is replenished below the thin toner layer 30, i.e., between the thin toner layer 30 and the intermediate transfer belt 4, and the thin toner layer 30 is formed on top of the layer of replenished toner (replenishment toner layer) 40.

In this way, by supplying a replenishment toner of a color different from that of the thin toner layer 30 to the lower side of the thin toner layer 30, the total thickness of the thin toner layer 30 and the replenishment toner layer 40 becomes thick. When the toner image is secondarily transferred to the paper P in this state, as shown in FIG. 6, the toner of toner color Y of the lower replenishment toner layer 40 remains on the intermediate transfer belt 4, and the toner of toner color K of the upper thin toner layer 30 is efficiently transferred to the paper P. In this way, since the good transferability of the original thin toner layer 30 to the paper P can be maintained, even if the toner layer is thin, a decrease in density on the paper P after transfer can be prevented, and high image quality can be achieved. Note that, as shown in FIG. 6, a part of the toner of the replenishment toner layer 40 is transferred to the paper P together with the toner of the thin toner layer 30.

The color of the replenishment toner is one of the colors (Y in the examples of Figures 5 and 6) used in the imaging units 20Y, 20M, and 20C that are upstream of the color of the thin toner layer 30 (black K in this example) in the rotation direction of the intermediate transfer belt 4.

The thin toner layer 30 may be formed of two or more layers (two colors), but since a single-color toner layer is often thin, it is preferable to use a single-color toner layer. It is particularly effective to apply this to a thin black (K) toner layer 30, in which density reduction is easily noticeable in a single layer.

In addition, regardless of the color of the thin toner layer 30, when the thickness of the thin toner layer 30 is less than the thickness of a toner layer for forming an image in one color (for example, 0.5 layers), the transfer voltage required for a toner layer formed in two colors (two layers) is significantly different, so the replenishing effect of the replenishing toner is large. In addition, replenishing with the replenishing toner can also save the amount of toner in the thin toner layer 30.

Furthermore, it is desirable to set the amount of replenishment toner to an amount that leaves 70-100% of the amount of replenishment toner remaining after transfer. This makes it possible to minimize the transfer of replenishment toner to the paper P, and to prevent color changes due to the mixture of transferred toner.

Figure 7 is a flowchart showing the process for determining whether to supply replenishment toner to the lower side of the thin toner layer 30.

In step S01, printing control is started, and in step S02, the position and thickness of the thin toner layer 30 in the image to be formed are detected. In step S03, the amount of replenishment toner to be replenished to the lower side is determined based on the detected thin toner layer 30, and then printing is started in step S04, and the process ends in step S05.

In this embodiment, the amount of replenishment toner determined in step S03 is desirably set or corrected to a desired value based on at least one of the following parameters: the environment (temperature and/or humidity), the durability of the developing device 3 (developer life, etc.), the color of the replenished toner (developing device 3 used), the type of paper, the life of the transfer roller, etc., so that most of the replenishment toner is left on the intermediate transfer belt 4 by reverse transfer. This ensures good transferability of the thin toner layer 30 and prevents color changes due to color mixing after transfer.

The specific method of correcting the amount of replenishment toner based on each parameter is shown in the table in Figure 8. In other words, the higher the temperature and humidity, the more replenishment toner should be used, and the more pages the replenishment toner is required to be used for development.

It is also a good idea to use a color of replenishment toner with a high charge. It is known that toner with a high charge requires a higher voltage to transfer than toner with a low charge, just as a thicker layer of toner requires a higher transfer voltage. Therefore, by selecting toner with a high charge, it is possible to reduce the amount of replenishment toner, making it possible to conserve consumption of replenishment toner.

Also, yellow (Y color) toner is not very noticeable when mixed, and is especially difficult to notice when the image color is black, so it is desirable to use it as a replenishment toner. In addition, a replenishment toner layer 40 with multiple layers of multiple colors may be formed using toner from multiple developers 3 in multiple imaging units located upstream of the imaging unit corresponding to the color of the thin toner layer 30. For example, when the color of the thin toner layer 30 is K, at least two of the developers 3 of the imaging units 20Y, 20M, and 20C upstream of the imaging unit 20K are used. By using multiple developers 3, the consumption of replenishment toner can be averaged, and uneven life of the developers 3 due to uneven toner consumption can be prevented.

In addition, the thinner the thin toner layer 30, the more replenishment toner should be used, and when transferring to a type of paper P with low electrical resistance, the more replenishment toner should be used.

Figure 9 shows an example of the thickness of the replenishment toner layer 40 relative to the thickness of the thin toner layer 30. The thinner the thin toner layer 30, the thicker the replenishment toner layer 40 is set to be. For example, if the thin toner layer 30 is 0.2 layers, the replenishment toner layer 40 should be set to 0.4 to 0.7 layers.

Furthermore, since the ratio of toner used for each color generally varies depending on the customer's usage method, the toner supplied to the lower side of the thin toner layer 30 may be the toner supplied by the least durable (used) developing unit 3. In this case, the deviation in the life span of each developing unit 3 can be reduced, and the number of visits by service personnel and the number of times developing units 3 need to be purchased can be reduced.

By the way, by using a screen 1 having a dot pattern as shown in the upper part of Figure 10 (A) (B), the thin toner layer 30 may be formed with a half-tone density. In the screen 1, multiple dots are arranged in a vertical and horizontal alignment.

In this case, it is desirable to use a screen 2 with the same dot pattern as the thin toner layer 30 for the replenishment toner layer 40 that is replenished below the thin toner layer 30, as shown in the middle of the figure (B). This ensures good transferability of the thin toner layer 30 and prevents color changes due to color mixing after transfer, as shown in the cross-sectional view in the lower part of the figure (B) whereby the toner dots of the thin toner layer 30 and the replenishment toner layer 40 overlap, resulting in a state in which the toner dots of the replenishment toner layer 40 are present below the toner dots of the thin toner layer 30.

In contrast, when using a screen 2 with a dot pattern in which the dots are arranged so that they are shifted horizontally every other row, as shown in the middle of FIG. 10A, there is no overlap between the toner dots in the thin toner layer 30 and the toner dots in the replenishment toner layer 40 in every other row, as shown in the second row of the cross-sectional view in the bottom row of FIG. 10A, and the effect of improving the transferability of the thin toner layer 30 is halved.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. For example, the thin toner layer 30 has been mainly described as being black, but the thin toner layer 30 may be a color other than black.

In addition, when a user copies or prints, the user can select either the color mode or the monochrome mode using a selection means such as a hard key or a soft key on the operation screen, and if the color mode is selected, replenishment toner can be supplied to the lower side of the thin toner layer 30.

Furthermore, the user can select whether or not to supply replenishment toner to the bottom of the thin toner layer 30 using a selection means such as a hard key or a soft key on the operation screen, and if it is selected to supply replenishment toner, the replenishment toner can be supplied to the bottom of the thin toner layer 30.

1 Photoconductor 2 Charging roller 3 Developing unit 4 Intermediate transfer belt (image carrier)
5 Secondary transfer roller 6 Fixing unit 7 Intermediate transfer belt cleaning device 8 Exposure device 9 Photoconductor cleaning unit 20Y, 20M, 20C, 20K Imaging unit (toner image forming means)
30 Thin toner layer 40 Replenishment toner layer P Paper

Claims (16)

An image carrier;
a toner image forming means for forming a toner image having a different toner layer thickness within one image on the image carrier using a plurality of color toners;
a transfer means for transferring the toner image formed on the image carrier onto a sheet of paper;
a detection means for detecting a thin toner layer in a toner image formed on the image carrier;
Equipped with
the toner image is a non-foaming toner image not including a foaming toner image,
the toner image forming means supplies a toner of a color different from the thin toner layer among the plurality of colors toners to a portion below the thin toner layer detected by the detection means, and
13. An image forming apparatus according to claim 12, wherein a transfer voltage applied by said transfer means is set in accordance with a toner layer having a thickness greater than that of said thin toner layer. An image carrier;
a toner image forming means for forming a toner image having a different toner layer thickness within one image on the image carrier using a plurality of color toners;
a transfer means for transferring the toner image formed on the image carrier onto a sheet of paper;
Equipped with
the toner image forming means supplies a toner of a color different from that of the thin toner layer to a portion below the thin toner layer in the toner image formed on the image carrier;
The image forming apparatus according to claim 1, wherein the toner supplied to the lower side of the thin toner layer is a toner having a relatively high charge amount among toners of colors other than the thin toner layer. An image carrier;
a toner image forming means for forming a toner image having a different toner layer thickness within one image on the image carrier using a plurality of color toners;
a transfer means for transferring the toner image formed on the image carrier onto a sheet of paper;
Equipped with
the toner image forming means supplies a toner of a different color from the thin toner layer to a lower side of the thin toner layer in the toner image formed on the image carrier, the toner image being formed of a different color from the thin toner layer;
A screen is provided for each color, the screen being made up of a plurality of image dot patterns;
an image forming apparatus comprising: a screen for supplying toner to a portion below the thin toner layer; a screen for supplying toner to the portion below the thin toner layer; The image forming apparatus according to any one of claims 1 to 3, wherein the thin toner layer is a monochromatic toner layer. The image forming device according to claim 4, wherein the single color is black. An image forming apparatus according to any one of claims 1 to 5, wherein the thickness of the thin toner layer is less than the thickness of a toner layer forming an image in one color. The image forming apparatus according to any one of claims 1 to 6, wherein the amount of toner supplied to the lower side of the thin toner layer is an amount in which 70 to 100% of the toner amount remains as a transfer residue. An image forming device according to any one of claims 1 to 7, in which the amount of toner supplied to the lower side of the thin toner layer is controlled based on at least one of the environment, the durability of the developing device, the color of the toner being supplied, and the type of paper. The image forming apparatus according to claim 1 or 3, wherein the toner supplied to the lower side of the thin toner layer is a toner having a relatively high charge amount among toners of colors other than the thin toner layer. An image forming apparatus according to any one of claims 1 to 9, wherein the color of the toner supplied to the lower side of the thin toner layer is yellow. The image forming apparatus according to any one of claims 1 to 10, wherein the toner supplied to the lower side of the thin toner layer is the toner supplied by the least durable developing unit. The image forming apparatus according to any one of claims 1 to 10, wherein the toner supplied to the lower side of the thin toner layer is supplied by a plurality of developing devices. The image forming device according to any one of claims 1 to 12, which is provided with a selection means that allows a user to select between monochrome mode and color mode. The image forming apparatus according to any one of claims 1 to 13, further comprising a selection means that allows the user to select between a mode in which toner is replenished below the thin toner layer and a mode in which toner is not replenished. The image forming device according to claim 1 or 2, which has a screen consisting of multiple image dot patterns for each color. 16. The image forming apparatus according to claim 15, wherein when the thin toner layer is formed by the screen, the toner is replenished to the lower side of the thin toner layer by using the same screen as the screen of the thin toner layer.

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