JPH09288396A - Multicolor image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multicolor image forming device capable of reducing kinds of toner melting points and preventing the occurrence of an offset. SOLUTION: The image forming part of a color image forming device 20 is constituted of four process units 30a, 30b, 30c and 30d, a carrying belt 23 circulated in contact with the photoreceptor drums 33a, 33b, 33c and 33d of the process units, four transfer brushes 46a, 46b, 46c and 46d coming into press- contact with the rear surfaces of the contact parts from below, etc. The process units 30a, 30b, 30c and 30d house toners of magenta, cyan, yellow and black respectively. The melting point of the black toner arranged on the most downstream side is set highest. For the melting points for magenta, cyan and yellow, two kinds of melting points lower than that of the black toner are set and the lower one of two kinds of melting points is set for yellow.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multicolor image forming apparatus for transferring a plurality of types of color toner images onto a transfer paper in an overlapping manner and pressing and fixing them in a batch.

[0002]

2. Description of the Related Art Conventionally, there has been an electrophotographic multicolor image forming apparatus. This multicolor image forming apparatus forms a latent image on a photoconductor by optical writing and forms the latent image into a toner image (development).
Then, the developed toner image is transferred to paper and fixed.

[0003] Such a multicolor image forming apparatus is roughly classified into a single drum type and a multi-stage drum type (tandem type). The single-drum type uses three subtractive primary colors of Y (yellow: yellow) toner, M (magenta: red dye) toner, and C (cyan: greenish blue) toner for one page of paper. In order to transfer a total of four types of toner, a toner and a Bk (black: black) toner dedicated to printing (printing and image formation) of characters and black portions of an image, a printing process is performed individually for each toner. . For this reason, the printing process is repeated four times for one page of paper, and it takes a long time for the printing process. On the other hand, in the tandem type, four kinds of toners are sequentially superimposed on a sheet and transferred in one process, or are sequentially superimposed and transferred on an intermediate transfer body and then re-transferred to a sheet at a time. Therefore, almost 4 times compared to the single drum type
It has twice the processing speed. For this reason, a tandem type multicolor image forming apparatus has recently been put to practical use.

FIG. 10 is a side sectional view schematically showing an example of a tandem type multicolor image forming apparatus. In the multi-color image forming apparatus 1, a paper cassette 2 is detachably mounted on the lower right side of the main body. The paper P in the paper cassette 2 is taken out one by one by the paper feed roller 3 and is fed to the image forming unit by the pair of standby rolls 4. In the image forming unit, a belt 5 that conveys the paper P is stretched flat in the paper conveyance direction (from right to left in the drawing) by a plurality of rotating rollers, and four photoconductors are provided facing the belt conveyance surface. Drum 6
Are provided to form an image forming unit. Each of the photosensitive drums 6 is disposed so as to sequentially surround the vicinity of the peripheral surface thereof and sandwich the transport surface of the transport belt 5 with the initialization charger 7, the developing roller 8-1 disposed in the developing device 8. Transfer charger 9,
And a cleaner 11. Each of the developing units 8 described above
Bk (black) toner, C (cyan) toner, M (magenta) toner, and Y (yellow) toner are accommodated from the upstream side (right side in the figure) of the housing. A predetermined gap is provided between the initializing charger 7 and the developing unit 8, and digital exposure radiation is performed from the laser head 12 to the peripheral surface of the photosensitive drum 6 through this gap. The laser head 12 includes a light source, a rotating mirror 12-1, a focusing lens 12-2, a reflecting mirror 12-3, an irradiation port 12-4 and the like.

In this multicolor image forming apparatus, when the belt 5 circulates and moves in the counterclockwise direction in the drawing to convey the paper P, each initialization charger 7 causes the corresponding peripheral surface of the photosensitive drum 6 to move. The laser head 12 is charged uniformly with a predetermined charge.
Exposes the charged peripheral surface according to the image information to form an electrostatic latent image, and the developing roller 8-1 of the developing device 8 transfers the toner to the low potential portion of the electrostatic latent image. To form a toner image (development). The toner image formed (developed) on the peripheral surface of each photosensitive drum 6 is transferred to a corresponding transfer charger 9.
By the corona discharge, the charges generated on the conveyed sheet P are attracted and transferred to the surface of the sheet P. The paper P on which the toner image has been transferred onto the paper surface is heat-fixed to the paper surface by a fixing unit 13 including a pressure roller and a heat roller, and is discharged outside the apparatus by a pair of unloading rollers 14.

By the way, in such a large-sized multicolor image forming apparatus, the fixing section 13 is usually provided with a large-size offset preventing liquid applying mechanism (for a large-sized copying machine or the like, such a mechanism is conventionally used). Has been). This is because a so-called offset type toner having a low melting point is used in order to obtain a color toner image having high transparency at a high speed in the case of forming an OHP image. That is, since the offset toner has high transparency but high adhesiveness, as shown by the arrow in the figure, oil is poured from above onto the heat-generating roller that is in direct contact with the toner image on the paper surface of the fixing unit 13. The excess oil is received by the lower oil sump tank 16 so that a thick oil layer is formed on the peripheral surface of the heat generating roller so that the toner image pressed against the peripheral surface does not transfer to the heat generating roller side. However, the multi-color image forming apparatus using the offset toner as described above has a problem in that the entire main body device becomes large because a large-size offset preventing liquid applying mechanism must be provided.

Therefore, a non-offset type toner is used in order to reduce the size of the main body device, and the one in which the arrangement of a complicated offset preventing liquid applying mechanism is eliminated is put into practical use. The above-mentioned offset toner has a narrow molecular weight distribution, high transparency after melting and solidification, and a low melting point, but the non-offset toner used for downsizing is a polyester-based toner. Resin toner,
Many have a high molecular weight, a wide molecular weight distribution, a cross-linked structure, and a relatively high melting point.

However, the tandem type has two layers for two colors,
Since it is a system in which three layers of three colors and unfixed toner images are sequentially transferred and transferred onto the paper, even in the case of non-offset toner, this is applied to the tandem type multicolor image forming apparatus as described above. When it is used, a part of the toner image adheres to the fixing roller and is peeled off from the paper (OFF) at the portion where the toners of each color are overlaid and transferred, and then the fixing roller rotates to the other portion of the paper again. An offset phenomenon of sticking (set) occurs.

FIGS. 11A and 11B are views for explaining the offset phenomenon due to non-offset toner in such a multicolor image forming apparatus. FIG. 1A shows the heating roller 17 in the fixing unit of the small tandem type multicolor image forming apparatus.
The pressure roller 18, the paper P, and 2 transferred onto the paper P
The toner images T1 and T2 of the types (two colors) are shown. The heating roller 17 shown in FIG. 1 (a) has a halogen lamp 1 inside a hollow roller 17a made of metal such as aluminum as a base.
7b, and a heat-resistant release resin layer 17c such as tetrafluoroethylene resin having a thickness of 20 to 100 μm on the surface of the hollow roller 17a.
It is provided in m. On the other hand, in the pressure roller 18, an elastic layer 18b having a relatively good mold release property and heat resistance, such as silicon rubber, having a thickness of 2 to 2 is formed on the surface of a metal roller 18a as a base.
It is provided at about 10 mm. The heating roller 17 is rotated clockwise by a driving force from a driving motor (not shown), and the pressure roller 18 pressed against the heating roller 17 is driven by the heating roller 17. It rotates counterclockwise as indicated by arrow B in the figure. In this way, the unfixed toner image T1 on the paper P,
Heat fix T2, ... Usually, a resin with the same thermal characteristics is used for the particle base material used for each color toner (in many cases, cyan, magenta, yellow, and black), and the color is changed by appropriately changing the pigment that is the color forming base material. Configure another toner. Therefore, the fixing temperature setting of the multicolor transfer toner image in the multicolor image forming apparatus is almost the same as the fixing temperature setting of the single color toner image.

In this fixing, the unfixed toners T1 and T2 on the paper are superposed and transferred onto the toner T1 of the first color even though the heating roller 17 is held in a temperature range where it can be sufficiently fixed. If the toner T2 of the second color slightly adheres to the heating roller 17 and is peeled off from the paper, it becomes the peeled toner T2 ', which causes an offset phenomenon, as shown in FIG.

That is, as shown in FIG. 1B, it is known that the toner T1 and T2 causes cold offset at a temperature lower than the lower limit of about 160.degree. It is known that hot offset occurs at a temperature higher than about 200 ° C. That is, the toners T1 and T2 alone have a fixing temperature range of 160 ° C. to 200 ° C. as a proper fixing temperature range. However, when these two types of toner T1 and T2 are piled up (“T1 +
T2 "), as described above, the unfixed toner image" T1 + T2 "color-superimposed on the transfer paper is, although the heating roller 17 is held in the temperature range in which the toner can be sufficiently fixed. An offset (stripping offset) occurs in which only T2 of the toner upper layer is peeled off. Hot offset occurs even with a single color 20
Above 0 ° C 2 at temperatures above 205 ° C-210 ° C
Hot offset occurs in which color toner images are integrated.

The causes of the above problems are as follows. As is generally known, as one of the modes of anti-offset by the non-offset type toner, the force FR acting between the toner T1 and the fixing roll (heating roller) as shown in FIG. Cohesive force F
1. The force FP acting between the toner T1 and the transfer paper (paper) is FR <F1 <FP. In addition, the electrostatic force or the like acting between the respective members also works, but since it does not directly relate to the present invention, it will be omitted in the following description.

As shown in FIG. 1A, one color toner (1
In the case of the layer), it can be considered in the same manner as in the case of a normal monochrome printing device, and there is no problem. However, two colors (2
When three layers (three layers) or three layers (three layers) of color toners are superposed, a problem occurs because there is a boundary area between the first color toner T1 and the second color toner T2. In the case of the toner having a narrow temperature range from the start of melting to the complete melting and having no viscoelasticity in a molten state (so-called sharp melt type toner), even if two or three colors are used, It may be considered in the same manner as the toner.

To explain further, as shown in FIG.
In the case of two colors (two layers), the phase of the heat transfer system includes a fixing roll (heating roller), a boundary between the heating roller and the second color toner layer, and a second color toner layer (a relatively dense layer formed by development). ) Boundary between the second color toner layer and the first color toner layer (relatively coarse portion formed by transfer), first color toner layer (relatively dense layer formed by development), first color toner layer and transfer A boundary with the paper and each phase of the transfer paper are formed. Therefore, when the toners T1 and T2 are melted during fixing, the force applied between these phases is the force FR acting between the heating roller and the second color toner, the cohesive force F2 of the second color toner, the first color toner and the second color toner mixed. The boundary cohesive force F12 of the boundary layer, the cohesive force F1 of the first color toner, and the force FP acting between the first color toner and the transfer paper. The boundary layer is a relatively coarse toner layer formed by development and in a relatively dense state, in which two toners that are in contact with each other vertically are mixed when two or three layers are superposed by transfer. It can be said that the toner layer is in such a state.

FIG. 13A schematically shows a state before fixing when the two toner layers are superposed. As shown in FIG. 6A, the toner T12 layer in which the two toners are mixed is simply superposed.
A boundary phase with many gaps is formed. When pressure P and heat sufficient to melt the toners T1 and T2 are applied to this state from above and below, the toner layer T1 and the mixed layer T of each phase described above are added.
12 and the toner layer T2 are melted. However, as described above, in the case of the non-offset type toner that maintains the viscoelasticity in the melted state, as shown in FIG. 7B, the mixed layer T12 does not have the pressure P applied during fixing. Still retains many large voids.

Then, when released from pressurization and heating,
The toner layers T1 and T2 are aggregated (actually, a mixture of melted and aggregated), and the cohesive force F2 and the force FR for attracting this layer and the fixing roller and the cohesive force F1 and the force for attracting this layer and the transfer paper are used. FP (see FIG. 12 (b) described above) is large, and the toner cohesive force F12 of the boundary layer where the first color toner and the second color toner are mixed is extremely weak due to many voids, which causes the transfer paper to be heated. When it is separated from the sheet, the toner layers T1 and T2 are peeled off toward the heating roll side and the transfer sheet side, respectively, as shown in FIG. 13 (c).

In general, non-offset type toner does not always have a uniform amount of resin particles as a toner binder which generates a cohesive force for maintaining the non-offset property. Therefore, the cohesive force is weakened particularly in the portion where the amount of resin is small due to the gap between the two toner boundary layers. Therefore, although each toner alone has a sufficient offset resistance,
When separated from the heating roller, it was peeled off in two at the toner boundary layer portion and printed on the upper layer (overcoated)
A stripping offset occurs in which the toner portion is offset.

This phenomenon is likely to occur in a high temperature region where the toner is instantly melted in each toner fixing possible temperature range, so that it is as if hot offset occurs, but the offset portion is mainly the second area. It is a color, and the degree of offset that occurs once hardly changes in each toner fixable temperature range,
It is clear that even if two toners are mixed and two colors are superimposed on the transfer paper, one color is developed with the same amount of toner as the image and a toner layer is formed and the toner is fixed in the same manner. Offset is different in meaning.

[0019]

By the way, as a means for solving such a problem, in a color image forming apparatus using three color toners, a softening point of a toner (a glass transition point at which a phase changes from a solid phase) is used. It has been proposed that the intermediate point between the melting point and the melting point that changes to the fluid phase) is set to increase sequentially from the upstream side to the downstream side of the image forming section (Japanese Patent Publication No. 62-61147). For details, the image forming section, which will be described later, makes a supplementary test using a color image forming apparatus of a pressure transfer type, and it is sure that the softening point of the toner gradually increases from the upstream side to the downstream side of the image forming section as it goes downstream. When set to, the proper fixing temperature is 160 ℃ ~ 200
It is found that offset does not occur within the temperature range of ° C. For example, in the developing device of the upstream image forming unit, the melting point 124
Cyan toner, a middle-stream image forming unit developing device is loaded with a melting point of 126 ° C. magenta toner, and a downstream image forming unit developing unit is loaded with a melting point of 128 ° C. yellow toner.
When an image is formed, the fixing temperature is 160 ° C to 200 ° C.
Offset does not occur within the temperature range of.

However, a color image can be formed by using at least three color toners of cyan, magenta and yellow, but the black portion of the image is actually black even if these three colors are transferred in an overlapping manner. There is no sense of stability. Therefore, it is general to further dispose black toner to form a color image. There may be various arrangement orders of the toners of the four colors in the image forming unit arranged in a multi-stage manner, and the black toner may be arranged at the beginning (upstream) or at the end (downstream). In any case, at present, it can be said that in a color image forming apparatus, it is indispensable to add black toner to the three primary colors to form a four-color structure.

In the color image forming apparatus having such a four-color structure, according to the method of gradually increasing the softening point of each toner as described above, in other words, the softening of each toner. According to the method of sequentially lowering the points toward the upstream, the softening point of the reference most downstream toner has an upper limit, so that the softening point of the most upstream toner becomes extremely low. Especially when forming an image on OHP paper,
Since it is preferable that the toner has a lower softening point as a whole,
If the softening point of the most downstream toner is adapted to this, the softening point of the most upstream toner becomes lower. As described above, when the softening point of the toner becomes low, there is a problem that a blocking (consolidation) phenomenon is likely to occur in the toner during storage of the toner or suspension of the main body device. Not only that, but there is also a problem that managing toners having four different softening points involves extremely complicated and expensive work.

In view of the above-mentioned conventional circumstances, the object of the present invention is to
It is an object of the present invention to provide a multicolor image forming apparatus in which the types of toner melting points are small and offset does not occur.

[0023]

The construction of a multicolor image forming apparatus according to the present invention will be described below. According to the present invention, a plurality of image bearing members arranged in parallel, a plurality of electrostatic latent image forming means for respectively forming electrostatic latent images on the plurality of image bearing members, and a plurality of image bearing members formed on the surface of the image bearing members. A plurality of developing means for developing the electrostatic latent image into a corresponding color toner image by using a predetermined color toner, and a transfer material conveying means for conveying the transfer material to bring the transfer material into contact with the plurality of image carriers. A plurality of transfer means for transferring the color toner images to a transfer material which is arranged so as to correspond to the plurality of image carriers and constitutes a transfer portion, and a plurality of transfer means for transferring the color toner images to the transfer material in contact with the image carriers. A heat roller contacting the color toner image side for fixing the color toner image transferred and transferred onto the transfer material via the transfer means to the transfer material; and a pressure contact roller arranged to be in pressure contact with the heat roller. And a fixing roller pair for nipping and carrying the above-mentioned transfer material to be carried. It assumes the color image forming apparatus.

In the multicolor image forming apparatus of the present invention, the image carrier is composed of four, and the fourth developing means corresponding to the fourth image carrier from the upstream side in the transfer material conveying direction is provided. The melting point of the color toner used is set higher than the melting points of the color toners used in the third, second, and first developing units that are sequentially adjacent to the upstream side of the fourth developing unit, and The melting point of the color toner used in the third developing means is set lower than the melting point of the color toner used in either the second developing means or the first developing means.

Then, for example, as described in claim 2, the melting point of the color toner used in the fourth developing means corresponding to the fourth image carrier from the upstream side in the transfer material conveying direction is set to:
The melting point is set higher than the melting point of each color toner used in the third, second, and first developing means that are sequentially adjacent to the upstream side of the fourth developing means, and the third, second, and first developing means are set. Any two of the melting points of the color toner used for the above are the same melting points, and the melting point of the color toner used for the third developing means is the color used for either the second developing means or the first developing means. It is configured to be set lower than the melting point of the toner.

The four transfer means corresponding to the four image carriers are, for example, contact type transfer means provided for bringing the transfer material into pressure contact with the corresponding image carriers, respectively. Is configured to be.

[0027]

Embodiments of the present invention will be described below with reference to the drawings. Incidentally, in the present embodiment,
The above-mentioned image carrier is, for example, a photosensitive drum, the electrostatic latent image forming means is, for example, an optical writing head, the developing means is, for example, a process unit having a developing roller, and the fourth and third The second and first developing means are, for example, the most downstream process unit, two process units sequentially adjacent to the upstream side, the most upstream process unit, and the like. Further, the transfer material conveying means is composed of, for example, a conveying belt or a semi-conductive drum of a paper winding type,
The transfer means is composed of, for example, a contact type transfer brush or a transfer roller which comes into contact with the transfer material conveying means in order to press the transfer material to the corresponding image carrier.

FIG. 1 is a schematic side sectional view showing the structure of a tandem type color image forming apparatus according to one embodiment, and FIG. 2 shows a process unit detachably attached to the color image forming apparatus. It is a sectional side view. As shown in FIG. 1, the color image forming apparatus (main body apparatus) 20 includes an upper lid 21 on the upper surface. Although not shown in the figure, a power switch, a liquid crystal display device, a plurality of input keys and the like are arranged on the front side of the upper lid 21. A paper cassette 22 is removably provided at the bottom of the apparatus body. A large number of sheets are placed and accommodated in the sheet cassette 22.

A conveyor belt 23 is arranged in the main body device 20 substantially in the center thereof in the shape of a flat loop in the horizontal direction. The driving roller 24 and the driven roller 25 hold both ends of the loop in the horizontal direction. The conveyor belt 23 is driven by its driving roller 24 and driven roller 25, and circulates counterclockwise in the figure. Four auxiliary rollers 26 are arranged so as to be pressed against the back surface of the upper circulation portion of the conveyor belt 23. The auxiliary roller 26 prevents the conveyance belt 23 from loosening due to its own weight. Further, a tension roller 27 is arranged so as to be pressed against the back surface of the lower circulation portion of the conveyor belt 23. The tension roller 27 is urged downward by an urging member (not shown) to stretch the conveyor belt 23.

At the upstream side of the conveyor belt 23 (on the right side of the figure),
The suction roller 28 is in pressure contact with the driven roller 25 via the transport belt 23, and forms a sheet carry-in section here. The suction roller 28 presses the conveyance belt 23 while applying a suction bias to the sheet carried in the sheet carry-in portion, and electrostatically attracts the sheet to the conveyance belt 23. Four process units 3 are provided above the upper circulation portion of the conveyor belt 23.
0 (30a, 30b, 30c, 30d) are arranged side by side in a multi-stage manner in the paper transport direction (right to left in the figure).

As shown in FIG. 2, these process units 30 are composed of a drum setting section 31 and a developing setting section 32. The drum set portion 31 includes a photosensitive drum 33, a drum cover 34, a cleaner 35, and an initialization charging brush 36, and a groove 3 for mounting a recording head on the upper portion.
7 are formed. The drum cover 34 is indicated by a two-dot chain line in the figure in order to protect the exposed lower portion of the photosensitive drum 33 when the process unit 30 is detached from the main body apparatus (image forming apparatus) and is independent. When the process unit 30 is attached to the main body device 20 by moving to the right position and shielding the lower part of the photosensitive drum 33 from the outside, the process unit 30 moves to the left position shown by the solid line in the figure. Also,
The recording head 38 shown in FIG. 1 is inserted into the slot 37 and is arranged right above the photosensitive drum 33. Recording head 38
Is disposed on the back surface of the upper lid 21 and descends into the groove hole 37 when the upper lid 21 is closed.
Positioned within 7. The upper lid 21 opens and closes around a support shaft behind the main body device 20.

On the other hand, the developing set section 32 rotatably holds the developing roller 39 in the lower opening of a vertically long casing, and contains the toner 41 therein. A stirring member 42, a supply roller 43, and a doctor blade 44 are disposed below the toner 41 so as to be buried in the toner 41. Stirring member 42
Rotates as shown by the broken line in the figure to feed the toner 41 to the supply roller 43 while stirring the toner 41.
Is pressed against the developing roller 39 and rubs the toner 41 sent from the stirring member 42 so that the developing roller 39
Supply to the peripheral surface. The doctor blade 44, which abuts on the circumferential surface of the developing roller 39 in the rotational direction, imparts a triboelectric charge to the toner 41 to assist the toner 41 to adhere to the developing roller 39, and regulates the adhered toner layer to a constant thickness.

When the four process units 30 are mounted on the main body device 20 as shown in FIG.
The lower part of 3 (33a, 33b, 33c, 33d) contacts the upper circulation part of the conveyor belt 23. The transfer brush 46 (46a, 46b, 46) is provided on the lower surface of the contact portion of the conveyor belt 23.
c, 46d) are provided to form a transfer portion.

The developing set portion 30d of the process unit 30 closest to the drive rotation roller 24 contains Bk (black) toner dedicated to printing of a black portion of a character or an image. The other three process units 30a, 30b, and 30c contain M (magenta) toner, C (cyan) toner, and Y (yellow) toner, respectively.

A standby roll pair 47 is disposed on the upstream side of the transport belt 23 in the transport direction (on the right side in the drawing), and a transport path 48 formed of two guide plates is disposed below the upstream side thereof. Then, the sheet feeding end of the above-described sheet cassette 22 is disposed below the sheet feeding unit. A paper feed roller 49 having a semicircular cross section is arranged above the paper feed end of the paper cassette 22.

On the other hand, a fixing device 51 is arranged downstream of the conveying belt 23 in the conveying direction (on the left side of the drawing). Fixing device 51
Is a pressure contact roller 51-assembled in a heat insulating casing.
1, heating roller 51-2, oil application roller 51-3,
Separation claw 51-4, peripheral cleaning device 51-5, temperature measuring device 51
-6 etc. On the downstream side of the fixing device 51,
A pair of carry-out rolls 52, a carry-out guide 53 above the pair of carry-out rolls, and a pair of paper discharge rolls 54 are arranged so as to be reversed from the front of the apparatus.
A paper discharge tray 55 including a rear portion of the upper surface of the main body device 20 and a rear surface of the upper lid 21 is formed in front of the paper discharge roll pair 54.

The conveyor belt 23 and the paper cassette 22
An electrical component section 56 is provided between which a predetermined number of circuit boards can be mounted. A control device (control means) composed of a plurality of electronic components is mounted on the circuit board arranged in the electrical component section 56. A fan 57 is provided behind the electrical component unit 56 to discharge heat dissipated from the electrical component unit 56 and heat leaked from the fixing unit 51 to the outside of the machine.

With the above arrangement, the operation of the color image forming apparatus 20 in this embodiment will be described again with reference to FIGS.
This will be described with reference to FIG. First, after the color image forming apparatus 20 is powered on, the number of sheets, the sheet size, and other designations are input by key input or as a signal from a connected host device, initialization processing is performed, and printing start is instructed. Then, each unit shown in FIG. 1 is driven by a drive mechanism (not shown).

By this drive, the paper feed roller 49 makes one rotation, the papers stored in the paper cassette 22 are taken out one by one from the uppermost part, and are fed to the standby roll pair 47 via the conveyance path 48. To send. The standby roll pair 47 stops and waits while holding the leading edge of the fed sheet.

At the same time, on the one hand, the drive roller 24
Rotates counterclockwise, and the driven roller 25 is driven to rotate counterclockwise as well. As a result, the upper circulation portion of the conveyor belt 23 comes into contact with the photosensitive drum 33, and the entire conveyor belt 23 circulates counterclockwise. On the other hand, the photoconductor drum 33 rotates clockwise, and the initialization charging brush 36 applies a uniform charge to the peripheral surface of the photoconductor drum 33 to charge it to a high negative potential. Then, the recording head 38 performs exposure according to the image signal to form an electrostatic latent image. The developing roller 39 transfers the toner 41 to the low potential portion of the electrostatic latent image on the peripheral surface of the photoconductor drum 33 to form a toner image (reverse development).

At the timing when the leading edge of the toner image formed (developed) on the peripheral surface of the photosensitive drum 33 is rotatably conveyed to the opposed point with the conveyor belt 23, the print start position of the paper is set at the opposed point. The standby roll pair 47 starts to rotate so that the leading ends coincide with each other, and the sheet is fed to the sheet carry-in section where the suction roller 28, the transport belt 23, and the driven roller 25 are in pressure contact.

The driven roller 25 and the suction roller 28 nip the fed sheet together with the conveying belt 23 and rotate to convey the sheet. The sheet is conveyed while being pressed against each photoconductor drum 33, and the toner images on the photoconductor drums 33 are sequentially transferred by the transfer brush 46. The sheet on which the toner image has been transferred is conveyed in the downstream direction of the conveyor belt 23 and is carried into the fixing device 51. Heater roller 51-2 of fixing device 51
The press roller 51-1 and the press roller 51-1 carry out the conveyance of the sheet while thermally fixing the toner image onto the sheet and carry it out to the rear. Further, the carry-out roll pair 52 passes the paper sheet through the carry-out guide 53 and the paper discharge roll pair 54 with the toner image downward and the paper discharge tray 5
5. Discharge on top.

In the color image forming apparatus according to the present embodiment which operates as described above, the mode of manifestation of strip offset (hereinafter, simply referred to as “offset”) when a plurality of types of toners having different melting points are used was investigated by experiments. . Hereinafter, this will be described.

FIG. 3 is a chart showing the types of toner used in the following experiments. In the figure, toner colors and toners having different melting points for each color are numbered 1, 2, 3, and 4 in order from the lowest melting point, magenta 1, magenta 2 ,. ., Cyan 1, ..., Yellow 3, and yellow 4 are shown. The glass transition point Tg corresponding to those toners is shown in the middle row. The glass transition points Tg are the same at 60 ° C. The lower part shows melting points Tm corresponding to magenta 1, magenta 2, ..., Cyan 1, ..., Yellow 3, and yellow 4 of the toner. For each color, No. 1 has a melting point Tm of 124 ° C., No. 2 has a melting point Tm of 126 ° C., No. 3 has a melting point Tm of 128 ° C., and No. 4 has a melting point Tm.
Is 130 ° C., and the higher the number, the higher the melting point.

In order to form an image by combining various toners of these three colors, the most upstream process unit 30 among the four multi-stage process units 30 shown in FIG.
a was removed (or stopped), and the image formation described below was performed using the three process units 30b, 30c, and 30d from the next downstream.

FIGS. 4A, 4B, and 4C are charts showing the experimental results of the image formation. In the charts (a), (b) and (c) of the same figure, the first color toner shown in the leftmost item is the toner contained in the process unit 30b, and the second color toner is the process unit 30c. The toner stored in the process unit 30d and the toner of the third color represent the toner stored in the process unit 30d. The items below them show the temperature of the heating roll, 160 ° C, 170 ° C,
180 ° C, 190 ° C and 200 ° C are set. The heating roll temperature is the surface temperature of the heat generating roller 51-2 of the fixing device 51 shown in FIG.

In FIG. 3A, the numbers 1, 2 and 3 of the toners of the respective colors are used as a precaution, and only one of these is used as the three process units 30b, 30c and 3 described above.
It is a chart showing a result when an image is formed by being housed in any of 0d. To explain further, the three process units 30a, 30b, 30c shown in FIG.
From the left side of the figure,
When cyan 1 is put in the process unit 30b and nothing is put in the other process units 30c and 30d, when magenta 1 is put in the process unit 30c and nothing is put in the other process units 30b and 30d, Then, yellow 1 is put into the process unit 30d and the other process units 30b and 30c are inserted.
It is the case that nothing is put in. Further, next, when only cyan 2, magenta 2 or yellow 2 is put in the process unit 30b, 30c or 30d in the same manner as described above, and only cyan 3, magenta 3 or yellow 3 is processed in the process unit 30b, 30c in the same manner as described above. Or 30d, and shows the result of image formation in each state. The toner arrangement state of the same chart and the above-mentioned heating roll temperatures of 160 ° C, 170 ° C, 180 ° C, 19
The circles shown in the frames corresponding to 0 ° C. and 200 ° C. indicate the results of image formation with this combination, that is, the determination as to whether or not offset has occurred.
As shown in the judgment of FIG. 9A, when one color toner is used, the offset does not appear regardless of the melting point of the toner and the heating roll temperature.

Next, FIG. 2B shows the process unit 30.
Cyan 1, Cyan 2 or Cyan 3 is put in b and nothing is put in the process unit 30c.
In d, the results of the case where yellow 1, yellow 2 or yellow 3 are accommodated and image formation is performed in each of the above cases are shown. In this case, the three intermediate process units 30c perform idle transfer (the photoconductor drum 33c is pressed directly against the paper on the conveyor belt 23 and rotates without performing the print process during the image forming process. ) Is done. As shown in the judgment of FIG. 6B, the intermediate process unit 30 is emptied and the upstream and downstream process units 30 are
When toner is used in the heating roll, the heating roll temperature is 1
When the temperature is 90 ° C. or higher, the offset appears depending on the combination of the two types of toner. Observing this in detail, when cyan 1 is arranged upstream and yellow 2 is arranged downstream, when cyan 1 is arranged upstream and yellow 3 is arranged downstream, cyan 2 is arranged upstream and yellow 2 is arranged downstream. In the case of arranging No. 3, no offset was developed. That is, also in this case, if the softening point of the toner (which is replaced by the melting point in the above case) is set to increase sequentially from the upstream side to the downstream side of the image forming portion, the proper fixing temperature 160 ° C. to 200 ° C. Offset does not occur within the temperature range. Then, in other combinations of toners, that is, when the melting point of the toner arranged downstream is equal to or lower than the melting point of the toner on the upstream side, an offset appears in a high temperature range of 190 ° C to 200 ° C. ing.

Therefore, as a further precaution, as shown in FIG. 7C, it is the same as the above case that cyan 1, cyan 2 or cyan 3 is put in the upstream process unit 30b, but in combination with these. When changing the toner color to be matched to magenta, the process unit 30 in which it is inserted is also changed, and this time, the intermediate process unit 3
0c accommodates magenta 1, magenta 2, or magenta 3 corresponding to the above cases, and the downstream process unit 30d is emptied. Even in this case, the downstream process unit 30d, which is empty of toner, performs idle transfer during the image forming process. Looking at the results of image formation in this way, there are many different combinations of softening points (melting points) that are appropriate for downstream and upstream, as shown in Fig. 6 (c). Despite this, the proper fixing temperature is 160 ° C to 2
No offset appeared in all within the temperature range of 00 ° C. Obviously, in this image forming process, a principle different from the previously conceived principle is working.

Considering this, in the color image forming apparatus according to the present embodiment, a contact type transfer brush is used in the image forming section, and the sheet conveyed to the image forming section is pressed against the photosensitive drum ( This also applies to the transfer roller). In the case of FIG. 4B, the photosensitive drum 33c of the process unit 30c that performs the idle transfer simply presses the toner layer of the first color against the paper surface and is irrelevant to the mode of the toner layer of the second color. In the case of FIG. 4 (c), the photoconductor drum 33d of the process unit 30d performing the idle transfer is pressed against the paper surface in a state where the first-color toner layer and the second-color toner layer that have been superposed and transferred are superposed. are doing.

FIGS. 5 (a), (b), and (c) schematically show the mode. That is, first, the upstream process unit 30b transfers the toner layer T1 of the first color, and then the intermediate process unit 30c transfers the toner layer T2 of the second color with the transfer pressure P as shown in FIG. Has been
At the boundary between the two color toner layers, two color toners are mixed to form a boundary T12 with many gaps. Then, by performing the idle transfer at the transfer pressure P in the downstream process unit 30d where the toner is empty, the gap of the boundary T12 is crushed by the pressing of the idle transfer, as shown in FIG. The mixture of the two color toners mixed together becomes a stub, and is heated and pressed by the fixing device 51 to be melted with a small gap as shown in FIG. It is considered that the toner becomes large and the peeling of the toner in the upper layer is suppressed.

Considering in this way, even in the case of not performing the idle transfer as described above, that is, even in the case of the image formation using the three color toners or the image formation using the four color toners, the former stage (upstream) is used. It can be inferred that since the transfer pressure is applied to the formed two-color toner layers by the transfer in the subsequent stage (downstream), the lower toner layers may be prevented from peeling at the time of fixing regardless of the melting point. Since only the uppermost toner layer is subjected to the transfer pressure only once, it is considered that there is a gap at the boundary with the next layer and the releasability remains. It can be considered that the occurrence of offset can be prevented as a whole by making the melting point of the toner arranged in the above position higher than the melting point of the toner arranged in the upstream side. That is, in an image forming apparatus having a multi-stage contact type transfer section, it is inferred that the setting of the melting point of the toner arranged in the image forming section does not necessarily have to be successively increased from the upstream side to the downstream side. .

FIGS. 6 to 9 are charts showing the results of experiments conducted based on the above inference. This experiment is shown in Figure 3.
In addition to the total 12 kinds of color toners having 4 kinds of melting points for each color shown in the above, 1 kind of black toner having the highest melting point as compared with these color toners is added, and 13 kinds of toners are used. I was broken. The reason why one kind of the black toner is used and the melting point thereof is set to the maximum value is that the black toner is accommodated in the most downstream process unit 30d in the present embodiment. That is, the experiment is performed by setting the melting point of the toner arranged on the most downstream side to be the highest.

In the experiments shown in FIGS. 6 to 9, in each of the charts, the first color toner of the toner section shown in the leftmost item is the toner contained in the process unit 30a.
In this example, magenta 1 (M1), magenta 2 (M2), magenta 3 (M3), or magenta 4 (M4) were accommodated.
The second color toner is the toner contained in the process unit 30b, and in this example, cyan 1 (C1) and cyan 2
(C2), Cyan 3 (C3) or Cyan 4 (C4) were contained. The third color toner is used in the process unit 30.
The toner stored in c is shown as yellow 1 (Y
1), Yellow 2 (Y2), Yellow 3 (Y3) or Yellow 4 (Y4). The fourth color toner indicates the toner contained in the process unit 30d,
In this example, as described above, in all cases, only one kind of black toner having the highest melting point of 130 ° C. is stored in common. Also, in this case, the items of the heating roll temperature in the lower part of the chart are 160 ° C. and 170 °
C, 180 C, 190 C and 200 C are set.

First, FIG. 6 will be described. Here, as shown in the figure, the process unit 30 is used as the first color toner.
Magenta 1 (M1) was contained in a and 16 kinds of experiments were conducted. That is, as shown in order from the left side of the figure,
Cyan 1 (C1) is stored in the process unit 30b as the second color toner, and yellow 1 (Y1) and yellow 2 (Y2) are stored in the process unit 30c as the third color toner.
), Yellow 3 (Y3) or yellow 4 (Y4) are stored, cyan 2 (C2) is stored as the second color toner, and yellow 1 (Y1), yellow 2 (Y2) are stored as the third color toner, Yellow 3 (Y3) or Yellow 4
When (Y4) is stored, cyan 3 (C3) is stored as the second color toner, and yellow 1 (Y1), yellow 2 (Y2), yellow 3 (Y3) are stored as the third color toner.
Or if it accommodates Yellow 4 (Y4), then the second
Cyan 4 (C4) is stored as the color toner, and yellow 1 (Y1) and yellow 2 (Y2) are stored as the color toner 3.
), Yellow 3 (Y3) or Yellow 4 (Y4). In the figure, a combination of these 16 types of toner arrangements and heating roll temperatures of 160 ° C., 170 ° C.,
In the column corresponding to 180 ° C, 190 ° C and 200 ° C,
Each of the combinations of the toner arrangements and the quality of the result of the image forming process performed at the heating roll temperature is marked with a circle or a cross. Then, there is a pass / fail judgment item in the bottom row, and the pass / fail judgment is indicated by a symbol “/”, “⊚” or “Δ” corresponding to the above result. The symbol “/” is not acceptable, the symbol “⊚” is good, and the symbol “Δ” is good but somewhat unsatisfactory.

Table row 6 showing the judgment of the goodness of the symbol "◎"
The combination of the toner arrangements 1 and 62 (the black toner is constant, so the black toner will be omitted in the description of the determination below), focusing only on the melting point, the numbers 1 and 2 are sequentially arranged from the upstream side. And number 1 (see table row 61) or number 1, number 3 and number 1 (see table row 62).

Experiments were repeated as shown in FIG. 7 in order to further understand the tendency of the experimental results. FIG. 7 shows a case in which the magenta 2 (M
2) is accommodated, the remaining two colors are combined, and 16
A variety of experiments were conducted. That is, the process unit 30
Magenta 1 (M1) in the experiment of FIG.
To magenta 2 (M2) and all the combinations of the remaining two colors, that is, 16 types of combinations as in the case of FIG. 6, are associated with the new magenta 2 (M2) process units 30b and 30c. The experiment was carried out in a house.

Also in this case, as shown in FIG. 7, a combination of 16 types of toner arrangement and a heating roll temperature of 160 ° C., 1
In the columns corresponding to 70 ° C., 180 ° C., 190 ° C., and 200 ° C., the quality of the result of the image forming process performed at the combination of the toner arrangement and the heating roll temperature is marked with a circle or a cross. ing. Then, the pass / fail judgment is indicated by the symbol “/”, “⊚” or “Δ” in the column of the result of the pass / fail judgment at the bottom and the corresponding column of the above result. In this case also, the combination of the toner arrangements in the table rows 63, 64 and 65 in which the mark “⊚” is shown as good is again focused on only the melting point, and the number 2, the number 1 and the number 1 (in order from the upstream. Table column 63), number 2, number 2 and number 1 (see table column 64) or number 2, number 3
And No. 2 (see table row 65).

Further, in order to know the tendency of such experimental results, experiments were repeated as shown in FIG. FIG. 8 shows the process unit 30a as the first color toner with magenta 3
(M3) is accommodated, and 16 kinds of experiments, which are combinations of the remaining two colors, are performed. That is, the toner of the process unit 30a is changed from magenta 2 (M2) in the experiment of FIG. 7 described above to magenta 3 (M3), and the remaining 2
For all combinations by color, the experiments were carried out in the same manner as in FIGS. 6 and 7 by accommodating the new magenta 3 (M3) in the process units 30b and 30c.

Also in this case, as shown in FIG. 8, a combination of 16 types of toner arrangement and a heating roll temperature of 160 ° C., 1
In the columns corresponding to 70 ° C., 180 ° C., 190 ° C., and 200 ° C., the quality of the result of the image forming process performed at the combination of the toner arrangement and the heating roll temperature is marked with a circle or a cross. ing. Then, the pass / fail judgment is indicated by the symbol “/”, “⊚” or “Δ” in the column of the result of the pass / fail judgment at the bottom and the corresponding column of the above result. Also in this case, the combination of toner arrangements in the table rows 66, 67, 68 and 69 in which the judgment of goodness of the symbol “⊚” is shown, in this case as well, paying attention only to the melting point, number 3, number 1 and number in order from the upstream. 1 (see table column 66), number 3, number 2 and number 2 (see table column 67), number 3, number 3 and number 1 (see table column 68) or number 3, number 3 and number 2 (table column 69) See).

Subsequently, experiments were repeated as shown in FIG. 9 using a combination of toners, which is the final combination of toner arrangements. FIG. 9 shows the process unit 3 as the first color toner.
Magenta 4 (M4) is housed in 0a, and 16 kinds of experiments are carried out, which is a combination of the remaining two colors.
That is, the toner of the process unit 30a is changed from magenta 3 (M3) to magenta 4 (M4) in the experiment of FIG.
For all combinations of the remaining two colors, the process units 30b and 3 are made to correspond to the new magenta 4 (M4) in the same manner as in FIGS. 6, 7 and 8.
The experiment was carried out by storing in 0c.

Also in this case, as shown in FIG. 9, a combination of 16 types of toner arrangement and a heating roll temperature of 160 ° C., 1
In the columns corresponding to 70 ° C., 180 ° C., 190 ° C., and 200 ° C., the quality of the result of the image forming process performed at the combination of the toner arrangement and the heating roll temperature is marked with a circle or a cross. ing. Then, the pass / fail determination is shown in the column of the pass / fail determination in the bottom row and the corresponding column of the above result. In this case, the symbol "/" is used for all determinations.

By summing up the above, the following conclusion can be obtained. That is, when a combination of toner arrangements indicated by the symbol “⊚” is indicated by melting point numbers from the upstream side, “1, 2, 1”, “1, 3, 1”, “2” are extracted. 1,
1 ”,“ 2, 2, 1 ”,“ 2, 3, 2 ”,“ 3, 1, ”
1 ”,“ 3, 2, 2 ”,“ 3, 3, 1 ”,“ 3, 3, ”
2 ". When the melting point number "4" of the black toner at the most downstream is added to these, that is, the toner from the toner to be stored in the process unit 30a corresponding to the first photosensitive drum 33a from the upstream side is the fourth photosensitive member. The respective melting points up to the toner to be stored in the process unit 30d corresponding to the drum 33d are "1, 2, 1, 4", "1, 3,
"1, 4", "2, 1, 1, 4", "2, 2, 1, 4",
"2, 3, 2, 4", "3, 1, 1, 4", "3, 2,
It can be seen that the combination of "2, 4", "3, 3, 1, 4", "3, 3, 2, 4" is the most appropriate. From this, first, it is found that the melting point of the toner does not necessarily have to be sequentially increased from upstream to downstream. next,
It has been found that when four color toners are combined, three types of melting points are sufficient. Furthermore, it turns out that the melting point of the toner arranged on the most downstream side must be higher than the melting point of any toner arranged on the upstream side. are doing). Further, of the two types of melting points set for the three color toners on the upstream side, the third melting point of the toner needs to be the lower melting point of the two types of melting points. It turns out.

Then, an arbitrary combination is selected from the above eight kinds of combinations of toner arrangements obtained in this way, and the first to fourth process units 30a to 30 are selected.
When the image is formed by being housed in d, the offset does not occur.

[0065]

As described above, according to the present invention,
In a color image forming apparatus that forms an image of four colors, three types of toner melting points can be configured to eliminate the offset phenomenon, thus reducing the number of toner management steps. Similarly, since the four melting points of the toners of the four colors can be composed of three types, a relatively high melting point can be set even for the toner of the lowest melting point among the four colors, and therefore, the toner of It is possible to eliminate the solidification phenomenon of the toner. Similarly, since it is not necessary to use a toner having a low melting point, which is likely to cause the problem of solidification even with four colors, it becomes easy to obtain a four-color configuration in which black toner is added to the three primary color toners. An image can be formed on the black portion with black toner, and therefore, a high-quality image with good definition can be formed.

[Brief description of drawings]

FIG. 1 is a schematic side sectional view illustrating a configuration of a tandem type color image forming apparatus according to an embodiment.

FIG. 2 is a side sectional view of a process unit detachably attached to the color image forming apparatus according to the embodiment.

FIG. 3 is a chart showing types of toner used in an experiment of image forming processing by the color image forming apparatus according to the embodiment.

4A, 4B, and 4C are charts showing experimental results of image forming processing by the color image forming apparatus according to the embodiment.

5 (a), (b), and (c) are schematic views showing an embodiment in which a first-color toner and a second-color toner are overtransferred and then idle-transferred in a downstream transfer unit in one embodiment. FIG.

FIG. 6 is a chart (No. 1) showing a result of an image forming experiment using a combination of toners according to various melting points, which is performed using the color image forming apparatus according to the embodiment.

FIG. 7 is a chart (No. 2) showing the results of an image forming experiment using a combination of various melting point-based toners performed by using the color image forming apparatus according to the embodiment.

FIG. 8 is a chart (No. 3) showing a result of an image forming experiment using a combination of toners according to various melting points, which is performed using the color image forming apparatus according to the embodiment.

FIG. 9 is a chart (No. 4) showing a result of an image forming experiment using a combination of toners according to various melting points, which is performed using the color image forming apparatus according to the embodiment.

FIG. 10 is a side sectional view schematically showing an example of a conventional tandem type multicolor image forming apparatus.

11A and 11B are diagrams illustrating an offset phenomenon due to non-offset toner in a conventional small-sized multicolor image forming apparatus.

FIG. 12A is a fixing roller in an offset phenomenon,
FIG. 3B is a diagram for explaining the forces acting on the boundaries between the one-color toner layer and the transfer paper, and FIG. 6B is a diagram illustrating the forces acting on the boundaries between the fixing roller, the second-color toner layer, the first-color toner layer, and the transfer paper. It is a figure.

FIG. 13A is a diagram schematically showing a state before fixing when two toner layers are overlapped in the offset phenomenon, and FIG. 13B is also showing a large void in the mixed layer at the time of fixing. FIG. 6C is a diagram showing that the two toner layers are peeled off to the heating roll side and the transfer sheet side, respectively.

[Explanation of symbols]

1 Multicolor Image Forming Device 2 Paper Cassette 3 Paper Feed Roller 4 Standby Roll Pair P Paper 5 Belt 6 Photosensitive Drum 7 Initializing Charger 8 Developing Device 8-1 Developing Roller 9 Transfer Charger 11 Cleaner 12 Laser Head 12-1 Rotating mirror 12-2 Focusing lens 12-3 Reflecting mirror 12-4 Irradiation port 13 Fixing section 14 Outgoing roller pair 16 Oil sump tank 17 Heating roller 18 Pressure roller 20 Color image forming device (main unit) 21 Upper lid 22 Paper cassette 23 Conveyor belt 24 Drive roller 25 Driven roller 26 Auxiliary roller 27 Tension roller 28 Adsorption roller 30 (30a, 30b, 30c, 30d) Process unit 31 Drum setting section 32 Development setting section 33 (33a, 33b, 33c, 33d) Photosensitive drum 34 Drum Cover 35 Cleaner 36 Initial Charging brush 37 Groove hole 38 Recording head 39 Developing roller 41 Toner 42 Stirring member 43 Supply roller 44 Doctor blade 46 (46a, 46b, 46c, 46d) Transfer brush 47 Standby roll pair 48 Conveying path 49 Paper feeding roller 51 Fixing device 51- 1 Pressure Contact Roller 51-2 Heat Generation Roller 51-3 Oil Application Roller 51-4 Separation Claw 51-5 Peripheral Surface Cleaner 51-6 Temperature Measuring Device 52 Carry-out Roll Pair 53 Carry-out Guide 54 Discharge Roll Pair 55 Discharge Tray 56 Electrical Equipment Part 57 fan

Front page continuation (72) Inventor Osamu Ryukou 2-229 Sakuragaoka, Higashiyamato, Tokyo Casio Electronics Co., Ltd. (72) Inventor Takeo Hirono 2-229 Sakuragaoka, Higashiyamato, Tokyo Casio Electronics Co., Ltd. Within

Claims (3)

[Claims] 1. A plurality of image carriers arranged in parallel, a plurality of electrostatic latent image forming means for respectively forming electrostatic latent images on the plurality of image carriers, and a plurality of image carriers formed on the surface of the image carriers. A plurality of developing means for developing the formed electrostatic latent image into a corresponding color toner image by using a predetermined color toner; and a transfer material transport for transporting the transfer material to bring the transfer material into contact with the plurality of image carriers. A plurality of transfer means for transferring the color toner image to a transfer material which is arranged to correspond to the plurality of image carriers and constitutes a transfer portion, and which transfers the color toner images to a transfer material which is in contact with the image carrier. A heat roller contacting the color toner image side in order to fix the color toner image superposed and transferred onto the transfer material via the transfer means, and a pressure contact roller arranged to be in pressure contact with the heat roller. And a pair of fixing rollers for nipping and conveying the transferred transfer material, In the multi-color image forming apparatus, the color toner is composed of four image carriers and is used for the fourth developing unit corresponding to the fourth image carrier from the upstream side in the transfer material conveyance direction. The melting point of the third developing unit, which is sequentially adjacent to the upstream side of the fourth developing unit,
The melting point of each color toner used in the second and first developing means is set higher than that of the color toner used in the third developing means. A multicolor image forming apparatus characterized in that it is set lower than the melting point of the color toner used for. 2. The melting point of the color toner used for the fourth developing means corresponding to the fourth image carrier from the upstream side in the transfer material conveying direction is sequentially adjacent to the upstream side of the fourth developing means. Is set higher than the melting point of each color toner used in the third, second, and first developing means, and any two of the melting points of the color toners used in the third, second, and first developing means are set. And the melting point of the color toner used for the third developing means is set lower than the melting point of the color toner used for either the second developing means or the first developing means. The multicolor image forming apparatus according to claim 1. 3. The four transfer means corresponding to the four image carriers are contact-type transfer means provided to press the transfer material to the corresponding image carriers, respectively. The multicolor image forming apparatus according to claim 1.