Image forming apparatus

A developing portion forms a toner image, which corresponds to a recording image, with a toner which has been electrically charged to a predetermined electrical potential. A transfer portion, to which an electric potential, different from the electric potential of the toner image, is applied, transfers the toner image onto a recording medium. A first transfer-electric-potential applying portion applies a transfer electric potential to the transfer portion. A carrying portion carries the recording medium so as to cause the recording medium to pass by the transfer portion. A second transfer-electric-potential applying portion sets the recording medium and the carrying portion to cause the recording medium and the carrying portion to have a predetermined electric potential corresponding to the transfer electric potential of the transfer portion.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus, and, in 
particular, to an image forming apparatus for forming an image 
electrostatically. 
In an image forming apparatus using the electrophotographic recording 
method for performing color printing, toners having a plurality of colors 
such as yellow, cyan, magenta and black are transferred onto a recording 
medium so as to be overlaid on each other so that color printing is 
performed. At this time, the toners are powder and may scatter so as to 
stain recording paper and/or the apparatus as transfer dust. Therefore, it 
is necessary to reduce scattering of the transfer dust. 
2. Descriptions of the Related Art 
FIG. 1 shows a general arrangement of one example of the related art. 
A color printer 100 using the electrophotographic recording method includes 
electrostatic recording units 102-1 through 102-4 for four colors: yellow 
(Y), magenta (M), cyan (C) and black (K), for electrostatically recording 
a toner image, a fixing unit 103 for fixing a color image, recorded onto a 
recording paper 101, recorded by the electrostatic recording units for the 
four colors, on the recording paper 101, and a carrying mechanism 104 for 
carrying the recording paper 101. 
The recording paper 101 is drawn out from a hopper 105 by the carrying 
mechanism 104, and is carried to the recording units 102-1 through 102-4 
for the four colors. The electrostatic recording units 102-1 through 102-4 
for the four colors are disposed in the direction (the direction of the 
arrow C) in which the recording paper 101 is carried tandem, and transfer 
the toners of the four colors onto the recording paper 101 so as to be 
overlaid on each other sequentially. 
The recording paper 101 is carried in the direction of the arrow C by the 
carrying mechanism 104, and, is supplied to the fixing unit 103 after the 
toners of the four colors are transferred onto the recording paper 101 in 
the order of yellow (Y), magenta (M), cyan (C) and black (K) by the 
respective electrostatic recording units 102-1 through 102-4. 
The fixing unit 103 fixes the toners, transferred onto the recording paper 
101 by the electrostatic recording units 102-1 through 102-4 for the four 
colors, by means of heating and pressing. The recording paper 101, on 
which the toners have been fixed by the fixing unit 103, is further 
carried by the carrying mechanism 104 and is stacked on a stacker 106. 
FIG.2 shows a general arrangement of the electrostatic recording unit in 
one example of the related art. 
Each of the electrostatic recording units 102-1 through 102-4 for the four 
colors includes a photosensitive drum 107 on which an electrostatic latent 
image corresponding to a recording image is formed, an electric charger 
108 for electrically charging the photosensitive drum 107 uniformly, an 
LED array 109 for irradiating the photosensitive drum 107, which has been 
electrically charged uniformly, in accordance with the recording image, a 
developer 110 for developing the electrostatic latent image formed on the 
photosensitive drum 107 using the toner, and a transfer roller 111 for 
transferring the toner image developed by the developer 110 on the 
photosensitive drum 107 into the recording paper 101. 
At this time, in the electrostatic recording units 102-1 through 102-4 in 
the related art, in order to improve the toner transfer efficiency for 
transferring the developed image onto the recording paper, the polarity of 
the electric potential of the transfer roller 111 is set to be reverse of 
the polarity of the electric potential of the toners. 
Further, the electric potential of the transfer roller 111 is set to be the 
same between the electrostatic recording units 102-1 through 102-4. 
Thereby, for example, when the toner is transferred so as to be overlaid on 
the previously transferred toner on the recording paper by the 
electrostatic recording unit, the tone of the currently transferred toner 
is lowered due to the influence of the previously transferred toner. 
Further, because a distance occurs between the photosensitive drum 107 and 
the recording paper 101, unnecessary toner is transferred onto the 
recording paper 101, that is, the transfer dust occurs. Thereby, the 
printing quality is degraded. 
SUMMARY OF THE INVENTION 
The present invention has been devised in consideration of the 
above-mentioned problems, and, an object of the present invention is to 
provide an image forming apparatus in which the transfer dust is reduced, 
unevenness in the tone for each color is prevented from occurring, and 
thereby, the printing quality can be improved. 
An image forming apparatus, according to the present invention, comprises: 
developing means for forming a toner image, which corresponds to a 
recording image, with a toner which has been electrically charged to a 
predetermined electrical potential; 
transfer means, to which an electric potential, different from the electric 
potential of the toner image, is applied, for transferring the toner image 
onto a recording medium; 
first transfer-electric-potential applying means for applying a transfer 
electric potential to the transfer means; 
carrying means for carrying the recording medium so as to cause the 
recording medium to pass by the transfer means; and 
a second transfer-electric-potential applying means for setting the 
recording medium and the carrying means to cause the recording medium and 
the carrying means to have a predetermined electric potential 
corresponding to the transfer electric potential of the transfer means. 
In this arrangement, because the general transfer voltage is determined by 
the first and second transfer-electric-potential applying means, it is 
possible to set the transfer electric potential of the transfer means to a 
low value. Thereby, occurrence of electric-current leakage, generation of 
ozone, or the like can be prevented. 
The first transfer-electric-potential A applying means may set the transfer 
electric potential of a polarity the same as the polarity of the toner. 
In this arrangement, it is possible to set the transfer electric potential 
of the transfer means to a low value. Thereby, occurrence of 
electric-current leakage, generation of ozone, or the like, which may 
occur when the transfer electric potential of the transfer means is large, 
can be prevented. 
The forming apparatus may comprises electric potential control means for 
controlling, in accordance with the resistance of the recording medium, 
the transfer electric potential which is applied to the transfer means by 
the first transfer-electric-potential applying means and the predetermined 
electric potential which is applied to the recording medium and the 
carrying means by the second transfer-electric-potential applying means. 
When the type of the recording medium is different, the electrically 
charged electric potential of the carrying means after the transfer is 
different. In this arrangement, in a case where printing is repeated, 
different transfer electric potentials are used for various types of 
recording media. As a result, by changing the transfer electric potential 
through the transfer-electric-potential control means, it is possible to 
use the electric potential to be applied to the transfer means suitable 
for each type of a recording medium. 
The second transfer-electric-potential applying means may comprise: 
electric-charge removing means for removing the electric charges from the 
carrying means; 
electric charging means for electrically charging the carrying means from 
which the electric charges have been removed by the electric-charge 
removing means, and electrically charging the recording medium; and 
electric-charging control means for controlling the electric-charge removal 
electric potential of the electric-charge removing means and the 
electric-charging electric potential of the electric charging means. 
In this arrangement, by controlling the electric-charging electric 
potential of the electric charging means and the electric-charge removal 
electric potential of the electric-charge removing means, it is possible 
to set the electrically charged electric potential of the recording medium 
and the carrying means. 
The electric charging control means may cause the electric potential of the 
carrying means to have a different electric potential in accordance with 
whether the volume resistivity of the recording medium is lower than 
10.sup.14 (.OMEGA.) or is equal to or higher than 10.sup.14 (.OMEGA.). 
In this arrangement, in accordance with whether the volume resistivity of 
the recording medium is lower than 10.sup.14 (.OMEGA.) or is equal to or 
higher than 10.sup.14 (.OMEGA.), that is, whether the recording medium is 
ordinary paper or a film for an OHP, the transfer electric potential is 
controlled. Thereby, it is possible to set the transfer electric 
potentials suitable for ordinary paper and a film for an OHP, 
respectively. As a result, it is possible to improve the quality of a 
transferred image. 
The electric charging control means may cause the carrying means to be 
electrically charged so that the surface electric-charge density thereof 
is equal to or higher than 620 (.mu.C/m.sup.2) when the volume resistivity 
of the recording medium is lower than 10.sup.14 (.OMEGA.), and the 
electric charging control means may cause the carrying means to be 
electrically charged so that the surface electric-charge density thereof 
is equal to or higher than 1178 (.mu.C/m.sup.2) when the volume 
resistivity of the recording medium is equal to or higher than 10.sup.14 
(.OMEGA.). 
In this arrangement, when the volume resistivity of the recording medium is 
lower than 10.sup.14 .OMEGA., that is, when the recording medium is 
ordinary paper, the carrying means is electrically charged to have the 
surface electric-charge density equal to or higher than 620 .mu.C/m.sup.2. 
When the volume resistivity of the recording medium is equal to or higher 
than 10.sup.14 .OMEGA., that is, when the recording medium is a film for 
an OHP, the carrying means is electrically charged to have the surface 
electric-charge density equal to or higher than 1178 .mu.C/m.sup.2. 
Thereby, it is possible to set the transfer electric potentials suitable 
for ordinary paper and a film for an OHP, respectively. As a result, it is 
possible to improve the quality of a transferred image. 
An image forming apparatus, according to another aspect of the present 
invention, comprises: 
a plurality of recording units, each comprising: 
developing means for forming a toner image corresponding to a recording 
image with a toner charged to have a predetermined electric potential; and 
transfer means, which faces the developing means via a recording medium and 
to which an electric potential different from the electric potential of 
the toner image is applied, for transferring the toner image onto the 
recording medium, 
the plurality of recording units transferring the plurality of toner images 
onto the recording medium so as that the plurality of toner images are 
overlaid on each other; 
fixing means for fixing the plurality of toner images transferred onto the 
recording medium so that the plurality of toner images are overlaid on 
each other; and 
transfer-electric-potential applying means in which the electric potentials 
to be applied to the transfer means of the plurality of recording units 
are set such that the difference between the electric potential of the 
transfer means and the electric potential of the toner increases 
sequentially in the order of the arrangement of the plurality of recording 
units. 
In this arrangement, the difference between the electric potential of the 
transfer means and the electric potential of the toner is larger in the 
recording unit which performs the transfer later. Thereby, it is possible 
to perform the transfer of the toner without being subject to the 
influence of the previously transferred toner. As a result, it is possible 
to surely transfer the toner on the previously transferred toner. 
Consequently, the quality of the thus-formed image can be improved. 
The transfer-electric-potential applying means may comprise: 
first transfer-electric-potential applying means for applying a transfer 
electric potential to the transfer means; and 
second transfer-electric-potential applying means for setting the recording 
medium and the carrying means so as to cause the recording medium and 
carrying means to have a predetermined electric potential suitable for the 
transfer electric potential of the transfer means. 
In this arrangement, the general transfer electric potential is determined 
by the first and second transfer-electric-potential applying means. As a 
result, it is possible to set the transfer electric potential of each 
transfer means to be low. Thereby, it is not necessary to set the transfer 
electric potential of the transfer means of the recording unit which 
performs the transfer later to be very high. As a result, electric current 
leakage, ozone generation or the like, which occurs due to a very high 
electric potential of the transfer voltage, can be prevented. 
The image forming apparatus may further comprise electric potential control 
means for controlling, in accordance with the resistance of the recording 
medium, the transfer electric potential which is applied to the transfer 
means by the first transfer-electric-potential applying means and the 
predetermined electric potential which is applied to the recording medium 
and the carrying means by the second transfer-electric-potential applying 
means. 
When the type of the recording medium is different, the electrically 
charged electric potential of the carrying means after the transfer is 
different. In the above-described arrangement, different transfer electric 
potentials are used for various types of recording media. As a result, 
when the printing is repeated, by appropriately changing the transfer 
electric potential through the transfer-electric-potential control means, 
it is possible to use the electric potential to be applied to the transfer 
means suitable for each type of a recording medium. 
The second transfer-electric-potential applying means may comprise: 
electric-charge removing means for removing the electric charges from the 
carrying means; 
electric charging means for charging the carrying means for electrically 
charging,the carrying means from which the electric charges have been 
removed by the electric-charge removing means, and electrically charging 
the recording medium; and 
electric charging control means for controlling the electric-charge removal 
electric potential of the electric-charge removing means and the electric 
charging electric potential of the electric charging means. 
In this arrangement, by controlling the electric-charging electric 
potential of the electric charging means and the electric-charge removal 
electric potential of the electric-charge removing means, it is possible 
to set the electrically charged electric potential of the recording medium 
and the carrying means. 
The electric charging control means may cause the electric potential of the 
carrying means to have a different electric potential in accordance with 
whether the volume resistivity of the recording medium is lower than 
10.sup.14 (.OMEGA.) or is equal to or higher than 10.sup.14 (.OMEGA.). 
In this arrangement, in accordance with whether the volume resistivity of 
the recording medium is lower than 10.sup.14 (.OMEGA.) or is equal to or 
higher than 10.sup.14 (.OMEGA.), that is, whether the recording medium is 
ordinary paper or a film for an OHP, the transfer electric potential is 
controlled. Thereby, it is possible to set the transfer electric 
potentials suitable for ordinary paper and a film for an OHP, 
respectively. As a result, it is possible to improve the quality of a 
transferred image. 
The electric charging control means may cause the carrying means to be 
electrically charged so that the surface electric-charge density thereof 
is equal to or higher than 620 (.mu.C/m.sup.2) when the volume resistivity 
of the recording medium is lower than 10.sup.14 (.OMEGA.), and the 
electric charging control means may cause the carrying means to be 
electrically charged so that the surface electric-charge density thereof 
is equal to or higher than 1178 (.mu.C/m.sup.2) when the volume 
resistivity of the recording medium is equal to or higher than 10.sup.14 
(.OMEGA.). 
In this arrangement, when the volume resistivity of the recording medium is 
lower than 10.sup.14 .OMEGA., that is, when the recording medium is 
ordinary paper, the carrying means is electrically charged to have the 
surface electric-charge density of equal to or higher than 620 
.mu.C/m.sup.2. When the volume resistivity of the recording medium is 
equal to or higher than 10.sup.14 .OMEGA., that is, when the recording 
medium is a film for an OHP, the carrying means is electrically charged to 
have the surface electric-charge density of equal to or higher than 1178 
.mu.C/m.sup.2. Thereby, it is possible to set the transfer electric 
potentials suitable for ordinary paper and a film for an OHP, 
respectively. As a result, it is possible to improve the quality of a 
transferred image. 
Other objects and further features of the present invention will become 
more apparent from the following detailed description when read in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENT 
A general arrangement of an image forming apparatus in one embodiment of 
the present invention will now be described. 
FIG. 3 shows the general arrangement of the embodiment of the present 
invention. 
In the image forming apparatus in the embodiment, a recording medium, for 
example, recording paper, is held in a paper tray 3. The recording paper 2 
is picked up from the paper tray 3 by a picking-up roller 4 disposed above 
the paper tray 3, sequentially. 
The recording paper 2 picked up by the picking-up roller 4 is supplied to a 
paper-feeding roller 6 via a guiding portion 5. The paper-feeding roller 6 
sends the recording paper 6 supplied via the guiding portion 5 onto an 
endless belt 7 which forms a predetermined carrying path. The recording 
paper 2 is carried by the endless belt 7 on the predetermined carrying 
path. 
The endless belt 7 forms an endless course by means of rollers 8-1 through 
8-4. The recording paper 2 is carried on the outside of the side of the 
endless course of the endless belt 7 formed by the rollers 8-1 and 8-2. An 
electric charging roller 9 is provided opposite to the roller 8-1, and the 
recording paper 2 and the endless belt 7 are sandwiched between the roller 
8 and the electric charging roller 9. 
The recording paper 2 and the endless belt 7 are electrically charged by 
the roller 8-1 and the electric charging roller 9. Thereby, the recording 
paper 2 is adhered to the endless belt 7 electrostatically. Thereby, the 
recording paper 2 moves with the endless belt 7 as the endless belt 7 
moves. 
The roller 8-2 is rotated in the direction of the arrow A by a motor, and 
moves the endless belt 7 in the direction of the arrow B. Thereby, the 
recording paper 2 moves in the direction of the arrow B together with the 
endless belt 7. 
On the outside of the side of the endless course of the endless belt 7 
formed by the rollers 8-1 and 8-2, electrostatic recording units 10-1 
through 10-4 are disposed sequentially. Each of the electrostatic 
recording units 10-1 through 10-4 contains a toner, and records a toner 
image corresponding to a recording image on the recording paper 2 
electrostatically. The electrostatic recording units 10-1, 10-2, 10-3 and 
10-4 contain toners of yellow, magenta, cyan and black, respectively, and 
transfer toner images of the respective colors to the recording paper 2 
when the recording paper 2 passes under the electrostatic recording units 
10-1 through 10-4, respectively. 
FIG. 4 shows a general arrangement of each of the electrostatic recording 
units 10-1 through 10-4 in the embodiment of the present invention. 
Each electrostatic recording unit includes a photosensitive drum 21 on 
which a toner image to be transferred to the recording paper 2 is formed, 
an electric charger 22 for electrically charging the photosensitive drum 
21, a laser diode array 23 for forming an electrostatic latent image 
corresponding to recording data (image data) on the photosensitive drum 
21, a developer 24 for supplying the toner to the photosensitive drum 21 
so as to form the toner image from the electrostatic latent image using 
the supplied toner, a transfer roller 25, which is disposed opposite to 
the photosensitive drum 21 via the recording paper 2 and the endless belt 
7, for transferring the toner image to the recording paper 2, a toner 
cleaner 26 for removing the residual toner from the photosensitive drum 21 
after the toner image on the photosensitive drum 21 is transferred to the 
recording paper 2, and a screw conveyer 27 for returning the residual 
toner removed from the photosensitive drum 21 to the developer 24. 
The developer 24 includes a toner container 28 for containing the toner and 
a toner supply roller 29 for supplying the toner contained in the toner 
container 28 to the photosensitive drum 21. 
When the toner image is transferred to the recording paper 2, the 
photosensitive drum 21 is rotated in the direction of the arrow C. The 
photosensitive drum 21 is uniformly electrically charged by the electric 
charger 22. The electric charger 22 comprises, for example, a corona 
electric charger, scorotoron electric charger, or the like. 
The photosensitive drum 21 electrically charged uniformly by the electric 
charger 22 is irradiated by laser light emitted from the laser diode array 
23 corresponding to the recording data. When being irradiated by the laser 
light, the electric charges at the positions at which the photosensitive 
drum 21 is irradiated are reduced, and, thereby, the electrostatic latent 
image is formed on the photosensitive drum 21. 
When the electrostatic latent image is formed by the laser light on the 
photosensitive drum 21, the developer 24 electrically charges the toner 
and supplies the electrically charged toner to the photosensitive drum 21. 
Thereby, the toner is adhered on the photosensitive drum 21 in accordance 
with the electric charges of the electrostatic latent image. Thus, the 
toner image is formed on the photosensitive drum 21. 
The photosensitive drum 21 on which the toner image is formed comes into 
contact with the recording paper 2. The recording paper 2 is electrically 
charged to the polarity reverse of the polarity of the toner of the toner 
image. As a result, the toner image formed on the photosensitive drum 21 
is transferred to the recording paper 2. 
With reference to FIG. 3, when passing under the electrostatic recording 
units 10-1 through 10-4, the toner images of the colors of the 
electrostatic recording units 10-1 through 10-4 are transferred to the 
recording paper 2 so as to be overlaid on each other. Then, finally, the 
full-color toner image is recorded on the recording paper. After that, the 
recording paper 2 having the full-color toner image formed thereon is 
supplied to the roller 8-2. 
The electric charges of the recording paper 2 and the endless belt 7 are 
removed by the roller 8-2. Thereby, the recording paper 2 
electrostatically adhered to the endless belt 7 is released from the 
endless belt 7. Thus, when the endless belt 7 moves downward by the roller 
8-2, the recording paper 2 is removed from the endless belt 7, and, then, 
is supplied to a fixing unit 11. 
The fixing unit 11 fixes the full-color toner image of to the recording 
paper 2 as a result of, for example, heating the recording paper 2 on 
which the full-color toner image has been formed. The recording paper 2, 
to which the full-color toner image has been fixed, is supplied to a 
stacker 12 which holds the recording paper 2 on which the recording image 
has been recorded. 
After the recording paper 2 is removed from the endless belt 7, the 
electric charges on the endless belt 7 are removed by an electric-charge 
removing brush 13, and the endless belt 2 is electrically charged again by 
the electric charging roller 9. The movement of the endless belt 7 is 
detected by a position sensor 14, and, the moved position of the endless 
belt 7 is detected by the position sensor 14. Thereby, the position of the 
recording paper 2 on the endless belt 7 is detected. Thereby, the timing 
of transfer of the toner images of the electrostatic recording units 10-1 
through 10-4 onto the recording paper 2 is controlled, and, thus, the 
toner images of yellow, magenta, cyan and black are transferred onto the 
recording paper so as to be overlaid on each other, at appropriate 
positions. Thus, the full-color toner image is formed on the recording 
paper 2. 
A hardware arrangement of the image forming apparatus 1 in the embodiment 
of the present invention will now be described. 
FIG. 5 shows a block diagram of the embodiment of the present invention. In 
the block diagram, the same reference numerals are given to the 
parts/components the same as those shown in FIG. 3, and the descriptions 
therefor will be omitted. 
The image forming apparatus 1 in the embodiment includes a controller 
portion 31, which performs predetermined processing in accordance with 
data provided from a personal computer 30, and an engine portion 32, which 
forms an image in accordance with a result of the processing performed by 
the controller portion 31. 
In the personal computer 30, a printer driver 33 for supplying, to the 
image forming apparatus 1, the recording data and various parameters such 
as a type and a size of a recording medium, a setting of a recording mode 
and so forth. The printer driver 33 is linked with various application 
programs 34, starts in accordance with instructions provided from one of 
the application programs 34, and supplies the recording data specified by 
the one of the application programs 34 to the image forming apparatus 1 
via a printer port 35. 
Operations of the printer driver 33 will now be described with reference 
FIG. 6. 
FIG. 6 shows an operation flowchart of the printer driver 33 in the 
embodiment of the present invention. 
After receiving instructions for printing from one of the application 
programs 34, the printer driver 33 is started (in steps S1-1, S1-2). 
When the printer driver 33 is started in the step S1-2, a selection picture 
is displayed on the display of the personal computer 3 (in step S1-3). 
This selection picture is used for an operator to set the type of the 
recording medium as to whether the recording medium used in the printing 
is ordinary paper or a film for an OHP (Over Head Projector), the size of 
the recording medium, the recording mode as to whether an image to be 
printed is a monochrome image or a color image, and so forth. 
In the step S1-3, an operator selects the type and size of the recording 
medium and the recording mode through an inputting device such as a 
keyboard, a mouse and/or the like. Then, as a result of the `Enter` key of 
the keyboard being pressed, the personal computer 30 determines that the 
selection has been completed (in a step S1-4). 
When the selection is completed in the step S1-4, the information of the 
selected type and size of the recording medium, the recording mode, and 
the recording data to be printed out are output via the printer port 35 
(in a step S1-5). 
With reference to FIG. 5, the printer port 35 of the personal computer 30 
is connected with a connector 36 provided on the control portion 31 of the 
image forming apparatus 1. The connector 36 is connected with an interface 
circuit 37 which is connected with an MPU 38 provided in the controller 
portion 31. The interface circuit 37 acts as an interface between the 
printer port 35 of the personal computer 30 and the MPU 38 of the 
controller portion 31. Thereby, data supplied from the personal computer 
30 is supplied to the MPU 38. 
The MPU 38 develops the recording data supplied from the personal computer 
30 in image memories 39-1 through 39-4 for the respective colors, yellow 
(Y), magenta (M), cyan (C) and black (K). At the same time, the MPU 38 
generates control data in accordance with the information of the selected 
type and size of the recording medium and the various parameters such as 
setting of the recording mode, and sends the control data to an interface 
circuit 40. 
With reference to FIG. 7, operations of the MPU 38 will now be described. 
FIG. 7 shows an operation flowchart for the MPU 38 of the controller 
portion 31 in the embodiment of the present invention. 
After receiving the information of the type and size of the recording 
medium, the various parameters such as the recording mode and so forth, 
and the recording data from the printer driver 33 of the personal computer 
30 (in a step S2-1), the MPU 38 performs processing such as smoothing and 
so forth on the recording data (in a step S2-2). Then, the MPU 38 develops 
the recording data in the image memories 39-1 through 39-4 for the 
respective colors (in a step S2-3) 
After completing the processing in the steps S2-2, S2-3 performed on the 
recording data, the MPU 38 transmits, to the engine portion 32 of the 
image forming apparatus 1, the information of the type and size of the 
recording medium, the various parameters such as the recording mode and so 
forth, and the recording data developed in the image memories 39-1 through 
39-4 (in a step S2-5). 
With reference to FIG. 5, the interface circuit 40 of the controller 
portion 31 is connected with a connector 42 of the engine portion 32 via a 
connector 41. The interface circuit 40 acts as an interface with the 
engine portion 32. Thereby, the information of the type and size of the 
recording medium, the various parameters such as the recording mode and so 
forth and the recording data developed for the respective colors are 
supplied to the engine portion 32. 
The connector 42 of the engine portion 32 is connected with a mechanical 
controller 43 of the engine portion 32. A power supply board 44 for 
generating a transfer voltage and an electric charging voltage, a carrying 
motor (not shown in FIG. 5) for carrying the recording paper 2, a motor 
driving circuit 45 for driving motors, which rotate the photosensitive 
drums 21 of the electrostatic recording units 10-1 through 10-4, 
respectively, and a laser control circuit 46 for controlling the laser 
diodes of the laser diode arrays 23 which form the electrostatic latent 
images on the photosensitive drums 21 in the electrostatic recording units 
10-1 through 10-4, respectively, are connected to the mechanical 
controller 43. 
The mechanical controller 43 will now be described. 
FIG. 8 shows a block diagram of the mechanical controller 43 in the 
embodiment of the present invention. 
The mechanical controller 43 includes an interface circuit 47 acting as an 
interface with the interface circuit 40, a CPU 48 for processing data 
supplied via the interface circuit 47, a RAM 49 acting as a work area of 
the CPU 48, a ROM 50 for storing various control programs to be executed 
by the CPU 48, an interface circuit 51 acting as an interface with the 
power supply board 44, the motor driving circuit 45 and the laser control 
circuit 46, an interface circuit 52 for inputting a result of detection 
performed by the position sensor 14. 
The mechanical controller 43 controls the power supply board 44 and the 
motor driving circuit 45 in accordance with the information of the type 
and size of the recording medium supplied from the controller portion 31, 
and controls the laser control circuit 46 in accordance with the recording 
data supplied from the controller portion 31. Thus, an image in accordance 
with the recording data supplied from the personal computer 30 is recorded 
on the recording paper 2. 
FIG. 9 shows an operation flowchart of the mechanical controller 43 in the 
embodiment of the present invention. 
When the information of the type and size of the recording medium, the 
various parameters such as setting of the recording mode and the recording 
data for each color are supplied from the controller portion 31 (in a step 
S3-1), the mechanical controller 43 analyzes the thus-supplied information 
and data (in a step S3-2). 
In a case where it is determined, as a result of the analyzing, that the 
recording mode is the color mode and that the type of the recording medium 
is a film for an OHP (in steps S3-3, S3-4), a first selection signal VTCS1 
to be supplied to the power supply board 44 is caused to be at a low level 
and a second selection signal VTCS2 to be supplied to the power supply 
board 44 is caused to be at a high level (in a step S3-5). 
In a case where it is determined, as a result of the analyzing, that the 
recording mode is the color mode and that the type of the recording medium 
is the obverse side of ordinary paper (in steps S3-3, S3-4, S3-6), the 
first selection signal VTCS1 is caused to be at the low level and the 
second selection signal VTCS2 is caused to be at the low level (in a step 
S3-7). 
In a case where it is determined, as a result of the analyzing, that the 
recording mode is the color mode and that the type of the recording medium 
is the reverse side of ordinary paper (in the steps S3-3, S3-4, S3-6), the 
first selection signal VTCS1 is caused to be at the high level and the 
second selection signal VTCS2 is caused to be at the low level (in a step 
S-38). 
In a case where it is determined, as a result of the analyzing, that the 
recording mode is the monochrome mode (in the step S3-3), the first 
selection signal VTCS1 is caused to be at the high level and the second 
selection signal VTCS2 is caused to be at the high level (in a step S3-9). 
FIG. 10 illustrates the selection signals in accordance with the type of 
the recording medium and the recording mode. 
As a result of the steps S3-3 through S3-9 being executed, the first and 
second selection signals VTCS1, VTCS2 having the levels shown in FIG. 10 
are generated and supplied to the power supply board 44. 
The power supply board 44 is controlled by the thus-generated first and 
second selection signals VTCS1, VTCS2. As a result, the voltages to be 
applied to the transfer rollers 25 of the electrostatic recording units 
10-1 through 10-4 are set, respectively, and the voltage to be applied to 
the electric charging roller S and the voltage to be applied to the 
electric-charge removing brush 13 are set. After that, the mechanical 
controller 43 controls the motor driving circuit 45. Thereby, the motor 
driving circuit 45 drives a belt motor for driving the endless belt 7, 
photosensitive-drum motors for driving the photosensitive drums 21, 
respectively, and so forth, and the recording paper 2 is drawn out from 
the paper tray 3 (in a step S3-10). 
When the endless belt 7 is driven as mentioned above, the position of the 
endless belt 7 is detected by the position sensor 14, and the recording 
paper 2 is drawn out from the paper tray 3, the timing of which is 
controlled in accordance with a result of the detection performed by the 
position sensor 14. 
When the recording paper 2 reaches the position of the electrostatic 
recording unit 10-1, the mechanical controller 43 causes a timing control 
signal *VTYON, to be supplied to the power supply board 44, to be at a low 
level, and, thereby, causes the power supply board 44 to apply a voltage 
to the transfer roller 25 of the electrostatic recording unit 10-1. 
Further, the mechanical controller 43 controls the laser control circuit 
46 in accordance with the recording data to be supplied to the 
electrostatic recording unit 10-1, that is, the recording data of yellow. 
Thereby, the mechanical controller 43 causes the laser diode array 23 of 
the electrostatic recording unit 10-1 to emit light in accordance with the 
recording data of yellow so as to cause the toner image of yellow to be 
formed on the photosensitive drum 21, the thus-formed toner image of 
yellow being then transferred onto the recording paper 2. Then, when the 
recording paper 2 reaches the position of the electrostatic recording unit 
10-2, the mechanical controller 43 causes a timing control signal *VTMON, 
to be supplied to the power supply board 44, to be at the low level, and, 
thereby, causes the power supply board 44 to apply a voltage to the 
transfer roller 25 of the electrostatic recording unit 10-2. Further, the 
mechanical controller 43 controls the laser control circuit 46 in 
accordance with the recording data to be supplied to the electrostatic 
recording unit 10-2, that is, the recording data of magenta. Thereby, the 
mechanical controller 43 causes the laser diode array 23 of the 
electrostatic recording unit 10-2 to emit light in accordance with the 
recording data of magenta so as to cause the toner image of magenta to be 
formed on the photosensitive drum 21, the thus-formed toner image of 
magenta being then transferred onto the recording paper 2. Then, when the 
recording paper 2 reaches the position of the electrostatic recording unit 
10-3, the mechanical controller 43 causes a timing control signal *VTCON, 
to be supplied to the power supply board 44, to be at the low level, and, 
thereby, causes the power supply board 44 to apply a voltage to the 
transfer roller 25 of the electrostatic recording unit 10-3. Further, the 
mechanical controller 43 controls the laser control circuit 46 in 
accordance with the recording data to be supplied to the electrostatic 
recording unit 10-3, that is, the recording data of cyan. Thereby, the 
mechanical controller 43 causes the laser diode array 23 of the 
electrostatic recording unit 10-3 to emit light in accordance with the 
recording data of cyan so as to cause the toner image of cyan to be formed 
on the photosensitive drum 21, the thus-formed toner image of cyan being 
then transferred onto the recording paper 2. Then, when the recording 
paper 2 reaches the position of the electrostatic recording unit 10-4, the 
mechanical controller 43 causes a timing control signal *VTKON, to be 
supplied to the power supply board 44, to be at the low level, and, 
thereby, causes the power supply board 44 to apply a voltage to the 
transfer roller 25 of the electrostatic recording unit 10-4. Further, the 
mechanical controller 43 controls the laser control circuit 46 in 
accordance with the recording data to be supplied to the electrostatic 
recording unit 10-4, that is, the recording data of black. Thereby, the 
mechanical controller 43 causes the laser diode array 23 of the 
electrostatic recording unit 10-4 to emit light in accordance with the 
recording data of black so as to cause the toner image of black to be 
formed on the photosensitive drum 21, the thus-formed toner image of black 
being then transferred onto the recording paper 2. 
Thus, the laser control circuit 46 is controlled in accordance with the 
recording data, the laser diode arrays 23 of the electrostatic recording 
units 10-1 through 10-4 are caused to emit light, and the toner images are 
transferred onto the recording paper 2, respectively (in a step S3-11). 
The processing of the mechanical controller 43 is finished when the 
recording paper 2 on which the toner images have been transferred is 
supplied to the fixing unit 11, the toner images are fixed to the 
recording paper 2, and the recording paper 2 is ejected to the stacker 12 
(in a step S3-12). 
The power supply board 44 will now be described. 
FIG. 11 shows a block diagram of the power supply board 44 in the 
embodiment of the present invention. 
The power supply board 44 includes a power source connector 61 for 
inputting a power source voltage, and a control connector 62 for inputting 
various signals, which are output from the mechanical controller 43 in 
accordance with the selected recording medium and the selected recording 
mode. The power supply board 44 further includes a first transfer voltage 
generating circuit 63-1 which generates the voltage, in accordance with 
the control signals to be supplied thereto via the control connector 62, 
to be applied to the transfer roller 25 of the electrostatic recording 
unit 10-1, a second transfer voltage generating circuit 63-2 which 
generates the voltage, in accordance with the control signals to be 
supplied thereto via the control connector 62, to be applied to the 
transfer roller 25 of the electrostatic recording unit 10-2, a third 
transfer voltage generating circuit 63-3 which generates the voltage, in 
accordance with the control signals to be supplied thereto via the control 
connector 62, to be applied to the transfer roller 25 of the electrostatic 
recording unit 10-3, and a fourth transfer voltage generating 63-4 which 
generates the voltage, in accordance with the control signals to be 
supplied thereto via the control connector 62, to be applied to the 
transfer roller 25 of the electrostatic recording unit 10-4. The power 
supply board 44 further includes a belt voltage generating circuit 64 for 
generating the voltage, in accordance with the control signals input 
thereto via the control connector 62, to be applied to the endless belt 7, 
and an electric-charge-removing-brush voltage generating circuit 65 for 
generating the voltage, in accordance with the control signals input 
thereto via the control connector 62, to be applied to the electric-charge 
removing brush 13. 
The first and second selection signals VTCS1, VTCS2, which are supplied by 
the mechanical controller 43 in accordance with the selected recording 
medium and the selected recording mode, are supplied to the control 
connector 62. Further, timing signals *VTYON, *VTMON, *VTCON, *VTKON, 
*VBTON and *VBJON for controlling operation timings of the first through 
fourth transfer voltage generating circuits 63-1 through 63-4, the belt 
voltage generating circuit 64 and the electric-charge-removing-brush 
voltage generating circuit 65, respectively. 
The first and second selection signals VTCS1, VTCS2, and the timing control 
signal *VTYON are supplied to the first transfer voltage generating 
circuit 63-1 via the control connector 62. The first and second selection 
signals VTCS1, VTCS2, and the timing control signal *VTMON are supplied to 
the second transfer voltage generating circuit 63-2 via the control 
connector 62. The first and second selection signals VTCS1, VTCS2, and the 
timing control signal *VTCON are supplied to the third transfer voltage 
generating circuit 63-3 via the control connector 62. The first and second 
selection signals VTCS1, VTCS2, and the timing control signal *VTKON are 
supplied to the fourth transfer voltage generating circuit 63-4 via the 
control connector 62. 
The first and second selection signals VTCS1, VTCS2, and the timing control 
signal *VBTON are supplied to the belt voltage generating circuit 64 via 
the control connector 62. The first and second selection signals VTCS1, 
VTCS2, and the timing control signal *VBJON are supplied to the 
electric-charge-removing-brush voltage generating circuit 65 via the 
control connector 62. 
The first transfer voltage generating circuit 63-1 generates first through 
third transfer voltages VTY1 through VTY3 in accordance with the first and 
second selection signals VTCS1, VTCS2. The second transfer voltage 
generating circuit 63-2 generates first through third transfer voltages 
VTM1 through VTM3 in accordance with the first and second selection 
signals VTCS1, VTCS2. The third transfer voltage generating circuit 63-3 
generates first through third transfer voltages VTC1 through VTC3 in 
accordance with the first and second selection signals VTCS1, VTCS2. The 
fourth transfer voltage generating circuit 63-4 generates first through 
fourth transfer voltages VTK1 through VTK4 in accordance with the first 
and second selection signals VTCS1, VTCS2. The belt voltage generating 
circuit 64 generates first through third electric charging voltages VBT1 
through VBT3 in accordance with the first and second selection signals 
VTCS1, VTCS2. The electric-charge-removing-brush voltage generating 
circuit 65 generates first through third electric-charge removing voltages 
VBJ1 through VBJ3 in accordance with the first and second selection 
signals VTCS1, VTCS2. 
FIG. 12 shows relationships between the output mode, the levels of the 
selection signals, and the output voltages in the embodiment of the 
present invention. 
In the case where the first selection voltage VTCS1 is at the low level and 
the second selection voltage VTCS2 is at the low level, that is, in the 
case where color printing is performed on the obverse side of ordinary 
paper, the first through fourth transfer voltage generating circuits 63-1 
through 63-4 generate the transfer voltages VTY1, VTM1, VTC1, VTK1, 
respectively, the belt voltage generating circuit 64 generates the 
electric charging voltage VBT1, and the electric-charge-removing-brush 
voltage generating circuit 65 generates the electric-charge removing 
voltage VBJ1. The thus-generated voltages are applied to the respective 
portions. In the case where the first selection voltage VTCS1 is at the 
low level and the second selection voltage VTCS2 is at the high level, 
that is, in the case where printing is performed on a film for an OHP, the 
first through fourth transfer voltage generating circuits 63-1 through 
63-4 generate the transfer voltages VTY2, VTM2, VTC2, VTK2, respectively, 
the belt voltage generating circuit 64 generates the electric charging 
voltage VBT2, and the electric-charge-removing-brush voltage generating 
circuit 65 generates the electric-charge removing voltage VBJ2. The 
thus-generated voltages are applied to the respective portions. In the 
case where the first selection voltage VTCS1 is at the high level and the 
second selection voltage VTCS2 is at the low level, that is, in the case 
where color printing is performed on the reverse side of ordinary paper, 
the first through fourth transfer voltage generating circuits 63-1 through 
63-4 generate the transfer voltages VTY3, VTM3, VTC3, VTK3, respectively, 
the belt voltage generating circuit 64 generates the electric charging 
voltage VBT1, and the electric-charge-removing-brush voltage generating 
circuit 65 generates the electric-charge removing voltage VBJ1. The 
thus-generated voltages are applied to the respective portions. In the 
case where the first selection voltage VTCS1 is at the high level and the 
second selection voltage VTCS2 is at the high level, that is, in the case 
where monochrome printing is performed, the fourth transfer voltage 
generating circuit 63-4 generates the transfer voltages VTK4, the belt 
voltage generating circuit 64 generates the electric charging voltage 
VBT3, and the electric-charge-removing-brush voltage generating circuit 65 
generates the electric-charge removing voltage VBJ3. The thus-generated 
voltages are applied to the respective portions. 
At this time, the transfer voltages VTY1 through VTY3, VTM1 through VTM3, 
VTC1 through VTC3, and VTK1 through VTK4 generated by the first through 
fourth transfer voltage generating circuits 63-1 through 63-4, 
respectively, increase in the order of the first through fourth transfer 
voltage generating circuits 63-1 through 63-4 in a manner to be described 
later. 
Thereby, when the electric potential of the recording paper 2 decreases as 
the recording paper 2 passes under the electrostatic recording units 10-1 
through 10-4, this decrease of the electric potential of the recording 
paper 2 is compensated by the above-mentioned increase of the electric 
potentials of the transfer rollers 25 of the electrostatic recording units 
10-1 through 10-4. As a result, it is possible to balance the printing 
tones between the respective electrostatic recording units 10-1 through 
10-4. 
Further, at this time, if the electric potential of the transfer roller 25 
of the electrostatic recording unit 10-1 under which the recording paper 2 
passes first is set to be large, the electric potential of the transfer 
roller 25 of the electrostatic recording unit 10-4 under which the 
recording paper 2 passes last is extremely large. Thereby, leakage of 
electric currents and/or generation of ozone may occur. 
In order to prevent such problems, at least the electric potential of the 
transfer roller 25 of the electrostatic recording unit 10-1 under which 
the recording paper 2 first passes is set to have a minus polarity, 
similar to the minus polarity of the electric potential of electrically 
charged toner. 
As a result of setting the transfer voltages VTY1 through VTY3 generated by 
the first transfer voltage generating circuit 63-1 to have the minus 
polarity the same as the minus polarity of the electric potential of the 
electrically charged toner, the electric potential of the transfer roller 
25 of the electrostatic recording unit 10-4 under which the recording 
paper 2 passes last is not very large. Leakage of electric currents and/or 
generation of ozone can be prevented from occurring. 
The electric potentials of the transfer rollers 25 and the electric 
potential of the electrically charged endless belt 7 at this time are 
determined as follows: 
First, a method for setting the transfer voltage to be applied to the 
transfer roller 25 of the electrostatic recording unit 10-1 under which 
the recording paper 1 passes first will now be described. 
For example, it is assumed that the volume resistivity of the endless belt 
7 is 10.sup.13 through 10.sup.15 .OMEGA., the surface resistivity of the 
endless belt 7 (obverse side) is 10.sup.15 through 10.sup.17 .OMEGA., the 
surface resistivity of the endless belt 7 (reverse side) is 10.sup.15 
through 10.sup.17 .OMEGA., the electrostatic capacity of the endless belt 
7 is 0.62 through 0.75 .mu.F/m.sup.2, the volume resistivity of the 
transfer roller 25 is 9.times.10.sup.3 .OMEGA., 3.times.10.sup.4 .OMEGA. 
and 1.times.10.sup.5 .OMEGA., the volume resistivity of the electric 
charging roller 9 is 2.times.10.sup.6 through 9.times.10.sup.6 .OMEGA., 
and the volume resistivity of the electric-charge removing brush 13 is 
1.times.10.sup.4 through 7.times.10.sup.6 .OMEGA.. Further, the toner is 
electrically charged to have a minus polarity. Further, as the recording 
medium, ordinary paper having the volume resistivity of 10.sup.7 through 
10.sup.9 .OMEGA., the surface resistivity of 10.sup.9 through 10.sup.11 
.OMEGA., the relative permittivity of 2 through 3.5, and a film for an OHP 
having the volume resistivity of 10.sup.15 through 10.sup.16 .OMEGA., the 
surface resistivity of 10.sup.9 through 10.sup.16 .OMEGA., and the 
relative permittivity of 2 through 3.5. The transfer efficiencies with 
respect to the electric potentials of the transfer roller 25 and the 
endless belt 7, assuming the above-described conditions, will now be 
described. 
FIGS. 13A and 13B show the toner transfer efficiencies when printing is 
performed on the ordinary paper with respect to the toner transfer 
electric potentials and the belt electric potentials. FIG. 13A shows the 
toner transfer efficiencies in a case where the belt electric potential of 
the endless belt 7 before the toner transfer is 1000 V and the toner 
transfer electric potential of the transfer roller 25 (VT) varies from 
-600 through +1400 V. FIG. 13B shows the toner transfer efficiencies in a 
case where the belt electric potential of the endless belt 7 before the 
transfer is 1700 V and the toner transfer electric potential of the 
transfer roller 25 varies from -1700 through +1100 V. 
In the case shown in FIG. 13B where the electric potential of the endless 
belt 7 is set to 1700 V, the toner transfer efficiency is maintained 
higher than 80% as the electric potential of the transfer roller 25 is 
decreased to around -1000 V. However, in the case shown in FIG. 13A where 
the electric potential of the endless belt 7 is set to 1000 V, the toner 
transfer efficiency is decreased to lower than 80% as the electric 
potential of the transfer roller 25 is decreased to have a minus value, 
and thereby, printing tone decreases. Therefore, in the case where the 
electric potential of the transfer roller 25 is set to have a minus value, 
it is necessary to set the electric potential of the endless belt 7 to be 
equal to or higher than 1000 V for the ordinary paper. 
FIGS. 14A and 14B show the toner transfer efficiencies when printing is 
performed on the film for an OHP with respect to the transfer electric 
potentials and the belt electric potentials. FIG. 14A shows the toner 
transfer efficiencies in a case where the belt electric potential of the 
endless belt 7 before the toner transfer is 1900 V and the transfer 
electric potential of the transfer roller 25 varies from -200 through 
+2600 V. FIG. 14B shows the toner transfer efficiencies in a case where 
the belt electric potential of the endless belt 7 before the toner 
transfer is 2500 V and the transfer electric potential of the transfer 
roller 25 varies from -2100 through +2000 V. 
In the case shown in FIG. 14B where the electric potential of the endless 
belt 7 is set to 2500 V, the toner transfer efficiency is maintained 
higher than 80% as the electric potential of the transfer roller 25 is 
decreased to around -2000 V. However, in the case shown in FIG. 14A where 
the electric potential of the endless belt 7 is set to 1900 V, the toner 
transfer efficiency is decreased to lower than 80% as the electric 
potential of the transfer roller 25 is decreased to have a minus value, 
and thereby, printing tone decreases. Therefore, in the case where the 
electric potential of the transfer roller 25 is set to have a minus value, 
it is necessary to set the electric potential of the endless belt 7 to be 
at least equal to or higher than 1900 V for the film for an OHP. 
The electric potential of the endless belt 7 before the toner transfer is 
determined by the electric charging voltage applied by the electric 
charging roller 9 and the electric potential of the endless belt 7 after 
the electric-charge removal is performed on the endless belt 7 by the 
electric-charge removing brush 13. 
Each of FIGS. 15 and 16 shows the characteristics of the electric potential 
of the endless belt 7 before the toner transfer is performed with respect 
to the electric potential of the endless belt 7 after the electric-charge 
removal is performed on the endless belt 7 by the electric-charge removing 
brush 13 and the electric charging voltage applied by the electric 
charging roller 9. 
As shown in FIGS. 15 and 16, it is possible to set the electric potential 
of the endless belt 7 before the toner transfer to be higher, as the 
electric potential of the endless belt 7 after the electric-charge removal 
performed thereon by the electric-charge removing brush 13 is increased, 
and, also, as the electric charging voltage applied to the endless belt 7 
by the electric charging roller 9 is increased. 
Further, when electric charges of the endless belt 7 are removed by the 
electric-charge removing brush 13, an AC voltage is added to a DC offset 
voltage. At this time, the stability of the electric potential of the 
endless belt 7 is determined in accordance with the peak-to-peak voltage 
of the AC voltage (ACp-p). 
FIG. 17 shows the characteristics of the electric potential of the endless 
belt 7 after the electric-charge removal is performed by the 
electric-charge removing brush 13 with respect to the ACp-p. In FIG. 17, 
.largecircle. shows the characteristics in the case where the DC offset 
voltage is 1500 V. .diamond-solid. shows the characteristics in the case 
where the DC offset voltage is 2500 V. 
As shown in FIG. 17, in the case where the ACp-p is approximately 0.75 V, 
the electric potential of the endless belt 7 after the electric-charge 
removal is performed by the electric-charge removing brush 13 is close to 
the set DC offset voltage and is stable, in each of the cases where the DC 
offset voltage is 1500 V and 2500 V. 
Accordingly, the ACp-p of the electric potential supplied to the 
electric-charge removing brush 13 is set to be 0.75 V. 
FIG. 18 shows the characteristics of the DC voltage used in the 
electric-charge removal, with respect to the electric potential of the 
endless belt 7 after the electric-charge removal is performed by the 
electric-charge removing brush 13. 
FIG. 18 shows the characteristics in the case where the ACp-p of the 
electric potential to be supplied to the electric-charge removing brush 13 
is set to 0.75 V at which the electric potential of the endless belt 7 
after the electric-charge removal is performed by the electric-charge 
removing brush 13 is stable. 
Using the characteristics shown in FIG. 18, it is possible to obtain the 
electric potential of the endless brush 7 after the electric-charge 
removal is performed by the electric-charge removing brush 13 to be set. 
Using the characteristics shown in FIGS. 15 through 18, it is possible to 
obtain the electric potentials to be applied to the electric-charge 
removing brush 13 and the electric charging roller 9. 
For example, a case where 1000 V as the electric potential of the endless 
belt 7 is obtained will now be described. (As described above, the 
electric potential of the endless belt 7 should be equal to or higher than 
1000 V in the case where the electric potential of the transfer roller 25 
of the electrostatic recording unit 10-1 can have a minus polarity when 
the printing is performed on ordinary paper.) 
With reference to FIG. 16, in order to obtain 1000 V as the electric 
potential of the endless belt 7 before the toner transfer is performed, 
for example, it can be seen that it is necessary to cause the electric 
potential of the endless belt 7 after the electric-charge removal is 
performed thereon to be equal to or higher than 1450 V and cause the 
electric charging voltage of the electric charging roller 9 to be equal to 
or higher than 0 V; to cause the electric potential of the endless belt 7 
after the electric-charge removal is performed thereon to be equal to or 
higher than 900 V and cause the electric charging voltage of the electric 
charging roller 9 to be equal to or higher than 1000 V; or cause the 
electric potential of the endless belt 7 after the electric-charge removal 
is performed thereon to be equal to or higher than 400 V and cause the 
electric charging voltage of the electric charging roller 9 to be equal to 
or higher than 1500 V. 
Further, for example, in order to cause the electric potential of the 
endless belt 7 after the electric-charge removal is performed thereon to 
be equal to or higher than 1450 V, inferring from the characteristics 
shown in FIG. 18, it is necessary to cause the DC offset voltage of the 
electric-charge removing brush 13 to be equal to or higher than 1650 V. In 
order to cause the electric potential of the endless belt 7 after the 
electric-charge removal is performed thereon to be equal to or higher than 
900 V, inferring from the characteristics shown in FIG. 18, it is 
necessary to cause the DC offset voltage of the electric-charge removing 
brush 13 to be equal to or higher than 1020 V. In order to cause the 
electric potential of the endless belt 7 after the electric-charge removal 
is performed thereon to be equal to or higher than 400, inferring from the 
characteristics shown in FIG. 18, it is necessary to cause the DC offset 
voltage of the electric-charge removing brush 13 to be equal to or higher 
than 440 V. 
Thus, in order to cause the electric potential of the endless belt 7 before 
the toner transfer to be 1000 V, which is the minimum value in the case 
where the electric potential of the transfer roller 25 of the 
electrostatic recording unit 10-1 can have a minus polarity when the 
printing is performed on ordinary paper, in the case where the ACp-p of 
the electric-charge removing brush 13 is 0.75 V and the DC offset voltage 
of the brush 13 is equal to or higher than 1650 V, the electric potential 
of the electric charging voltage of the electric charging roller 9 is to 
be equal to or higher than 0 V. 
That is, the belt voltage generating circuit 64 is to generate the voltage 
equal to or higher than 0 V as the first electric charging voltage VBT1, 
and the electric-charge-removing-brush voltage generating circuit 65 is to 
generate the voltage having the ACp-p of 0.75 V and the DC offset voltage 
equal to or higher than 1650 V as the first electric-charge removing 
voltage VBJ1. 
In order to cause the electric potential of the endless belt 7 to be 1000 
V, which is the minimum value in the case where the electric potential of 
the transfer roller 25 of the electrostatic recording unit 10-1 can have a 
minus polarity when the printing is performed on ordinary paper, in the 
case where the ACp-p of the electric-charge removing brush 13 is 0.75 V 
and the DC offset voltage of the brush 13 is equal to or higher than 1020 
V, the electric potential of the electric charging voltage of the electric 
charging roller 9 is to be equal to or higher than 1000 V. 
That is, the belt voltage generating circuit 64 is to generate the voltage 
equal to or higher than 1000 V as the first electric charging voltage 
VBT1, and the electric-charge-removing-brush voltage generating circuit 65 
is to generate the voltage having the ACp-p of 0.75 V and the DC offset 
voltage equal to or higher than 1020 V as the first electric-charge 
removing voltage VBJ1. 
In order to cause the electric potential of the endless belt 7 to be 1000 
V, which is the minimum value in the case where the electric potential of 
the transfer roller 25 of the electrostatic recording unit 10-1 can have a 
minus polarity when the printing is performed on ordinary paper, in the 
case where the ACp-p of the electric-charge removing brush 13 is 0.75 V 
and the DC offset voltage of the brush 13 is equal to or higher than 440 
V, the electric potential of the electric charging voltage of the electric 
charging roller 9 is to be equal to or higher than 1500 V. 
That is, the belt voltage generating circuit 64 is to generate the voltage 
equal to or higher than 1500 V as the first electric charging voltage 
VBT1, and the electric-charge-removing-brush voltage generating circuit 65 
is to generate the voltage having the ACp-p of 0.75 V and the DC offset 
voltage equal to or higher than 440 V as the first electric-charge 
removing voltage VBJ1. 
A case where 1900 V of the electric potential of the endless belt 7 is 
obtained will now be described. (As described above, it is necessary that 
the electric potential of the endless belt 7 is equal to or higher than 
1900 V in the case where the electric potential of the transfer roller 25 
of the electrostatic recording unit 10-1 can have a minus polarity when 
the printing is performed on a film for an OHP.) 
With reference to FIG. 16, in order to obtain 1900 V of the electric 
potential of the endless belt 7 before the toner transfer is performed, 
for example, it can be seen that it is necessary to cause the electric 
potential of the endless belt 7 after the electric-charge removal is 
performed thereon to be equal to or higher than 2500 V and cause the 
electric charging voltage of the electric charging roller 9 to be equal to 
or higher than 500 V; to cause the electric potential of the endless belt 
7 after the electric-charge removal is performed thereon to be equal to or 
higher than 2100 V and cause the electric charging voltage of the electric 
charging roller 9 to be equal to or higher than 1000 V; to cause the 
electric potential of the endless belt 7 after the electric-charge removal 
is performed thereon to be equal to or higher than 1380 V and cause the 
electric charging voltage of the electric charging roller 9 to be equal to 
or higher than 2000 V; or cause the electric potential of the endless belt 
7 after the electric-charge removal is performed thereon to be equal to or 
higher than 400 V and cause the electric charging voltage of the electric 
charging roller 9 to be equal to or higher than 3000 V. 
Further, for example, in order to cause the electric potential of the 
endless belt 7 after the electric-charge removal is performed thereon to 
be equal to or higher than 2500 V, inferring from the characteristics 
shown in FIG. 18, it is necessary to cause the DC offset voltage of the 
electric-charge removing brush 13 to be equal to or higher than 2860 V. In 
order to cause the electric potential of the endless belt 7 after the 
electric-charge removal is performed thereon to be equal to or higher than 
2100 V, inferring from the characteristics shown in FIG. 18, it is 
necessary to cause the DC offset voltage of the electric-charge removing 
brush 13 to be equal to or higher than 2400 V. In order to cause the 
electric potential of the endless belt 7 after the electric-charge removal 
is performed thereon to be equal to or higher than 1380 V, inferring from 
the characteristics shown in FIG. 18, it is necessary to cause the DC 
offset voltage of the electric-charge removing brush 13 to be equal to or 
higher than 1570 V. In order to cause the electric potential of the 
endless belt 7 after the electric-charge removal is performed thereon to 
be equal to or higher than 400 V, inferring from the characteristics shown 
in FIG. 18, it is necessary to cause the DC offset voltage of the 
electric-charge removing brush 13 to be equal to or higher than 440 V. 
Thus, in order to cause the electric potential of the endless belt 7 before 
the toner transfer to be 1900 V, which is the minimum value in the case 
where the electric potential of the transfer roller 25 of the 
electrostatic recording unit 10-1 can have a minus polarity when the 
printing is performed on a film for an OHP, in the case where the ACp-p of 
the electric-charge removing brush 13 is 0.75 V and the DC offset voltage 
of the brush 13 is equal to or higher than 2860 V, the electric potential 
of the electric charging voltage of the electric charging roller 9 is to 
be equal to or higher than 500 V. 
That is, the belt voltage generating circuit 64 is to generate the voltage 
equal to or higher than 500 V as the second electric charging voltage 
VBT2, and the electric-charge-removing-brush voltage generating circuit 65 
is to generate the voltage having the ACp-p of 0.75 V and the DC offset 
voltage equal to or higher than 2860 V as the second electric-charge 
removing voltage VBJ2. 
In order to cause the electric potential of the endless belt 7 to be 1900 
V, which is the minimum value in the case where the electric potential of 
the transfer roller 25 of the electrostatic recording unit 10-1 can have a 
minus polarity when the printing is performed on a film for an OHP, in the 
case where the ACp-p of the electric-charge removing brush 13 is 0.75 V 
and the DC offset voltage of the brush 13 is equal to or higher than 2400 
V, the electric potential of the electric charging voltage of the electric 
charging roller 9 is to be equal to or higher than 1000 V. 
That is, the belt voltage generating circuit 64 is to generate the voltage 
equal to or higher than 1000 V as the second electric charging voltage 
VBT2, and the electric-charge-removing-brush voltage generating circuit 65 
is to generate the voltage having the ACp-p of 0.75 V and the DC offset 
voltage equal to or higher than 2400 V as the second electric-charge 
removing voltage VBJ2. 
In order to cause the electric potential of the endless belt 7 to be 1900 
V, which is the minimum value in the case where the electric potential of 
the transfer roller 25 of the electrostatic recording unit 10-1 can have a 
minus polarity when the printing is performed on a film for an OHP, in the 
case where the ACp-p of the electric-charge removing brush 13 is 0.75 V 
and the DC offset voltage of the brush 13 is equal to or higher than 1570 
V, the electric potential of the electric charging voltage of the electric 
charging roller 9 is to be equal to or higher than 2000 V. 
That is, the belt voltage generating circuit 64 is to generate the voltage 
equal to or higher than 2000 V as the second electric charging voltage 
VBT2, and the electric-charge-removing-brush voltage generating circuit 65 
is to generate the voltage having the ACp-p of 0.75 V and the DC offset 
voltage equal to or higher than 1570 V as the second electric-charge 
removing voltage VBJ2. 
In order to cause the electric potential of the endless belt 7 to be 1900 
V, which is the minimum value in the case where the electric potential of 
the transfer roller 25 of the electrostatic recording unit 10-1 can have a 
minus polarity when the printing is performed on a film for an OHP, in the 
case where the ACp-p of the electric-charge removing brush 13 is 0.75 V 
and the DC offset voltage of the brush 13 is equal to or higher than 440 
V, the electric potential of the electric charging voltage of the electric 
charging roller 9 is to be equal to or higher than 3000 V. 
That is, the belt voltage generating circuit 64 is to generate the voltage 
equal to or higher than 3000 V as the second electric charging voltage 
VBT2, and the electric-charge-removing-brush voltage generating circuit 65 
is to generate the voltage having the ACp-p of 0.75 V and the DC offset 
voltage equal to or higher than 440 V as the second electric-charge 
removing voltage VBJ2. 
In the endless belt 7, the electric-charge density of the belt is 
determined as the product of the electrostatic capacity of the belt and 
the electric potential of the belt. That is, 
EQU (electric-charge density of the belt) (.mu./m.sup.2) =(electrostatic 
capacity of the belt) (.mu.F/m.sup.2) .times.(electric potential of the 
belt) (V) 
Accordingly, for example, when the electric potential of the endless belt 7 
is +1000 V, assuming that the electrostatic capacity of the belt is 0.62 
.mu.F/m.sup.2, the surface electric-charge density of the endless belt 7 
is 620 .mu.C/m.sup.2 obtained from the following equation: 
EQU 0.62.times.1000=620(.mu.C/m.sup.2) 
Thus, it is possible to express the electric potential of the endless belt 
7 by the surface electric-charge density thereof. As mentioned above, it 
is necessary that the electric potential of the endless belt 7 when the 
printing is performed on ordinary paper be equal to or higher than 1000 V. 
For a recording medium having the volume resistivity lower than 10.sup.14 
.OMEGA. such as ordinary paper, the surface electric-charge density should 
be equal to or higher than 620 .mu.C/m.sup.2. Further, it is necessary 
that the electric potential of the endless belt 7 when the printing is 
performed on a film for an OHP be equal to or higher than 1900 V. For a 
recording medium having the volume resistivity equal to or higher than 
10.sup.14 .OMEGA. such as a film for an OHP, the surface electric-charge 
density should be equal to or higher than 1178 .mu.C/m.sup.2 
(0.62.times.1900=1178). 
A method of setting the electric potential of the transfer roller 25 in 
each of the electrostatic recording units 10-1 through 10-4 will now be 
described. 
The setting is performed such that the transfer voltages VTY1 through VTY3, 
VTM1 through VTM3, VTC1 through VTC3, and VTK1 through VTK4 to be applied 
to the respective transfer rollers 25 increase in the order of the 
arrangement of the electrostatic recording units 10-1 through 10-4. 
For example, 
VTY1&lt;VTM1&lt;VTC1&lt;VTK1 
VTY2&lt;VTM2&lt;VTC2&lt;VTK2 
VTY3&lt;VTM3&lt;VTC3&lt;VTK3 
Thus, the transfer voltage to be applied to the transfer roller 25 is 
higher for the electrostatic recording unit which performs the toner 
transfer later. Thereby, it is possible to transfer the toner image 
without suffering influence of the toner image transferred precedingly. 
Thus, it is possible to surely transfer the toner image on the toner image 
transferred precedingly. As a result, it is possible to improve the 
quality of the printed image. 
Specifically, the electric potentials of the transfer rollers 25 of the 
electrostatic recording units 10-1 through 10-4 are determined as follows: 
For example, it is assumed that the volume resistivity of the endless belt 
7 is 10.sup.13 through 10.sup.15 .OMEGA., the surface resistivity of the 
belt 7 (obverse side) is 10.sup.15 through 10.sup.17 .OMEGA., the surface 
resistivity of the belt 7 (reverse side) is 10.sup.15 through 10.sup.17 
.OMEGA., and the electrostatic capacity of the belt 7 is 0.62 through 0.75 
.mu.F/m.sup.2 ; the volume resistivity of each transfer roller 25 is 
9.times.10.sup.3 .OMEGA., 3.times.10.sup.4 .OMEGA., 1.times.10.sup.5 
.OMEGA., and the volume resistivity of the electric charging roller 9 is 
2.times.10.sup.6 through 9.times.10.sup.6 .OMEGA.; the volume resistivity 
of the electric-charge removing brush 13 is 1.times.10.sup.4 through 
7.times.10.sup.6 .OMEGA.; each toner is charged to have a minus polarity 
of electric charges; and further, as recording media, ordinary paper 
having the volume resistivity of 10.sup.7 through 10.sup.9 .OMEGA., the 
surface resistivity of 10.sup.9 through 10.sup.11 .OMEGA., and the 
relative permittivity of 2 through 3.5; and a film for an OHP having the 
volume resistivity of 10.sup.15 through 10.sup.16 .OMEGA., the surface 
resistivity of 10.sup.9 through 10.sup.16 .OMEGA., and the relative 
permittivity of 2 through 3.5 are used. The transfer efficiencies with 
respect to the transfer electric potentials of the transfer rollers 25 of 
the respective electrostatic recording units 10-1 through 10-4 in the 
above-described conditions will now be described. 
FIGS. 19A, 19B and 19C show the characteristics of the toner transfer 
efficiencies with respect to the transfer voltages when the printing is 
performed on ordinary paper. FIG. 19A shows the characteristics of the 
toner transfer efficiencies with respect to the transfer voltages VTY 
applied to the transfer roller 25 of the electrostatic recording unit 
10-1. FIG. 19B shows the characteristics of the toner transfer 
efficiencies with respect to the transfer voltages VTM applied to the 
transfer roller 25 of the electrostatic recording unit 10-2 when -500 V is 
applied to the transfer roller 25 of the electrostatic recording unit 
10-1. FIG. 19C shows the characteristics of the toner transfer 
efficiencies with respect to the transfer voltages VTC applied to the 
transfer roller 25 of the electrostatic recording unit 10-3 when -500 V is 
applied to the transfer roller 25 of the electrostatic recording unit 
10-1, and +100 V is applied to the transfer roller 25 of the electrostatic 
recording unit 10-2. The characteristics shown in FIGS. 19A, 19B and 19C 
are those when 1700 V is applied to the electric charging roller 9 and 
2000 V is applied to the electric-charge removing brush 13. 
FIG. 19A shows the toner transfer efficiencies with respect to the transfer 
voltage in the electrostatic recording unit 10-1 when the toner of one 
color, yellow, is transferred. As shown in the figure, using the 
electrostatic recording unit 10-1, the toner transfer efficiency exceeds 
80% when the transfer voltage applied to the transfer roller 25 is 
approximately -1000 V. 
In FIG. 19B, .circle-solid. shows the toner transfer efficiencies with 
respect to the transfer voltage in the electrostatic recording unit 10-2 
when the toner of one color, magenta, is transferred, and .quadrature. 
shows the toner transfer efficiencies with respect to the transfer voltage 
in the electrostatic recording unit 10-2 when the toner of magenta is 
transferred to be overlaid on the toner of yellow. As shown in the figure, 
for the electrostatic recording unit 10-2, which is arranged next to the 
electrostatic recording unit 10-1, the transfer efficiency exceeds 80% 
when the transfer voltage applied to the transfer roller 25 is equal to or 
higher than approximately -500 V. 
In FIG. 19C, .circle-solid. shows the toner transfer efficiencies with 
respect to the transfer voltage in the electrostatic recording unit 10-3 
when the toner of one color, cyan, is transferred, .quadrature. shows the 
toner transfer efficiencies with respect to the transfer voltage in the 
electrostatic recording unit 10-3 when the toner of cyan is transferred so 
as to be overlaid on the toner of yellow, .tangle-solidup. shows the 
transfer efficiencies with respect to the transfer voltage in the 
electrostatic recording unit 10-3 when the toner of cyan is transferred so 
as to be overlaid on the toner of magenta, and x shows the transfer 
efficiencies with respect to the transfer voltage in the electrostatic 
recording unit 10-3 when the toner of cyan is transferred so as to be 
overlaid on the toner of yellow and toner of magenta, the latter having 
been overlaid on the former. By inferring from the characteristics shown 
in FIG. 19C, for the electrostatic recording unit 10-3, which is arranged 
next to the electrostatic recording units 10-1 and 10-2, the transfer 
efficiency exceeds 80% when the transfer voltage applied to the transfer 
roller 25 is equal to or higher than approximately -300 V. 
Therefore, when recording is performed in which two colors or three colors 
are overlaid on each other on ordinary paper, it is possible to cause the 
toner transfer efficiency to be equal to or higher than 80% as a result of 
the transfer voltage applied to the transfer roller 25 of the 
electrostatic recording unit 10-2 being set to be higher than the transfer 
voltage applied to the transfer roller 25 of the electrostatic recording 
unit 10-1, and the transfer voltage to be applied to the transfer roller 
25 of the electrostatic recording unit 10-3 being set to be higher than 
the transfer voltage applied to the transfer roller 25 of the 
electrostatic recording unit 10-2. 
For example, when the printing is performed on ordinary paper, the transfer 
voltage to be applied to the transfer roller 25 of the electrostatic 
recording unit 10-2 is set to be higher than the transfer voltage applied 
to the transfer roller 25 of the electrostatic recording unit 10-1 by 500 
V, and the transfer voltage applied to the transfer roller 25 of the 
electrostatic recording unit 10-3 is set to be higher than the transfer 
voltage applied to the transfer roller 25 of the electrostatic recording 
unit 10-2 by 200 V. As a result, a recording result of printing with high 
tone is obtained. 
FIGS. 20A, 20B and 20C show the characteristics of the transfer 
efficiencies with respect to the transfer voltages when the printing is 
performed on a film for an OHP. FIG. 20A shows the characteristics of the 
toner transfer efficiencies with respect to the transfer voltages VTY 
applied to the transfer roller 25 of the electrostatic recording unit 
10-1. FIG. 20B shows the characteristics of the toner transfer 
efficiencies with respect to the transfer voltages VTM applied to the 
transfer roller 25 of the electrostatic recording unit 10-2 when -500 V is 
applied to the transfer roller 25 of the electrostatic recording unit 
10-1. FIG. 20C shows the characteristics of the toner transfer 
efficiencies with respect to the transfer voltages VTC applied to the 
transfer roller 25 of the electrostatic recording unit 10-3 when -500 V is 
applied to the transfer roller 25 of the electrostatic recording unit 10-1 
and +500 V is applied to the transfer roller 25 of the electrostatic 
recording unit 10-2. The characteristics shown in FIGS. 20A, 20B and 20C 
are those when 2500 V is applied to the electric charging roller 9 and 
2700 V is applied to the electric-charge removing brush 13. 
FIG. 20A shows the toner transfer efficiencies with respect to the transfer 
voltage in the electrostatic recording unit 10-1 when the toner of one 
color, yellow, is transferred. As shown in the figure, for the 
electrostatic recording unit 10-1, the transfer efficiency exceeds 80% 
through a wide range of the transfer voltage. 
In FIG. 20B, .circle-solid. shows the toner transfer efficiencies with 
respect to the transfer voltage in the electrostatic recording unit 10-2 
when the toner of one color, magenta, is transferred, and .quadrature. 
shows the toner transfer efficiencies with respect to the transfer voltage 
in the electrostatic recording unit 10-2 when the toner of magenta is 
transferred so as to be overlaid on the toner of yellow. As shown in the 
figure, for the electrostatic recording unit 10-2, which is arranged next 
to the electrostatic recording unit 10-1, the toner transfer efficiency 
exceeds 80% when the transfer voltage applied to the transfer roller 25 is 
equal to or higher than approximately +100 V. 
In FIG. 20C, .circle-solid. shows the transfer efficiencies with respect to 
the transfer voltage in the electrostatic recording unit 10-3 when the 
toner of one color, cyan, is transferred, .quadrature. shows the toner 
transfer efficiencies with respect to the transfer voltage in the 
electrostatic recording unit 10-3 when the toner of cyan is transferred so 
as to be overlaid on the toner of yellow, .tangle-solidup. shows the toner 
transfer efficiencies with respect to the transfer voltage in the 
electrostatic recording unit 10-3 when the toner of cyan is transferred so 
as to be overlaid on the toner of magenta, and x shows the toner transfer 
efficiencies with respect to the transfer voltage in the electrostatic 
recording unit 10-3 when the toner of cyan is transferred so as to be 
overlaid on the toner of yellow and toner of magenta, the latter having 
been overlaid on the former. As shown in FIG. 20C, for the electrostatic 
recording unit 10-3, which is arranged next to the electrostatic recording 
units 10-1 and 10-2, the transfer efficiency exceeds 80% when the transfer 
voltage applied to the transfer roller 25 is equal to or higher than 
approximately +1000 V. 
Therefore, when recording is performed in which two colors or three colors 
are overlaid on each other on a film for OHP, it is possible to cause the 
toner transfer efficiency to be equal to or higher than 80% as a result of 
the transfer voltage applied to the transfer roller 25 of the 
electrostatic recording unit 10-2 being set to be higher than the transfer 
voltage to be applied to the transfer roller 25 of the electrostatic 
recording unit 10-1, and the transfer voltage to be applied to the 
transfer roller 25 of the electrostatic recording unit 10-3 being set to 
be higher than the transfer voltage to be applied to the transfer roller 
25 of the electrostatic recording unit 10-2. 
For example, when the printing is performed on a film for an OHP, the 
transfer voltage applied to the transfer roller 25 of the electrostatic 
recording unit 10-2 is set to be higher than the transfer voltage applied 
to the transfer roller 25 of the electrostatic recording unit 10-1 by 800 
V, and the transfer voltage applied to the transfer roller 25 of the 
electrostatic recording unit 10-3 is set to be higher than the transfer 
voltage applied to the transfer roller 25 of the electrostatic recording 
unit 10-2 by 600 V. Thereby, a recording result of printing with high tone 
is obtained. 
In the above-described embodiment, the electrostatic recording units 10-1 
through 10-4 are arranged so that the toner colors are arranged in the 
order of yellow, magenta, cyan and black. However, the setting of the 
transfer voltages are not limited to the above-mentioned color 
arrangement. 
Further, the present invention is not limited to the above-described 
embodiment, and variations and modifications may be made without departing 
from the scope of the present invention. 
The contents of the basic Japanese Patent Application No. 9-326810, filed 
on Nov. 27, 1997, are hereby incorporated by reference.