Full-color image forming apparatus with two side capability and two-side full-color image forming method

A two-side image forming method apparatus and related method, the method including steps of: forming a multi-color image on one side of a recording material; heating the recording material, bearing the multi-color image on a side thereof, with a first amount of heat; forming a multi-color image on the other side of the recording material heated with the first amount of heat; and heating said recording material, bearing the multi-color images on both sides thereof with a second amount of heat larger than the first amount of heat.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus such as a 
copying machine or a laser beam printer and more particularly to an image 
forming apparatus capable of forming multi-color images on both sides of a 
recording material. 
2. Related Background Art 
In the image forming apparatus designed for forming a monochromatic (for 
example black-and-white) image, such as the copying machine, it is already 
widely conducted to form the images on both sides of the recording 
material, in view of saving the resources and the space for image output. 
In a two-side copying machine, an image is at first formed and fixed on 
one side of the recording material, and then another image is formed and 
fixed on the other side of the recording material. 
On the other hand, with the increase in complexity, diversity, and amount 
of information in recent years, the use of colored image is spreading and 
there is emerging a demand for forming full-color images on both sides of 
the recording material. However, such formation of full-color images on 
both sides of the recording material will result in perturbation of the 
image if the conventional method for monochromatic image is applied. 
More specifically, in the formation of a full-color image, the desired 
color is obtained by mixing, by fusion under heating with fixing means, 
powered toner images of plural colors provided on the recording material. 
If an image is formed with this method on the first side of the recording 
material and another image is then formed in a similar manner on the 
second side, the heat applied at the fixation of the image on the second 
side is transmitted to the first side, whereby the properly mixed image on 
the first side is excessively fused and becomes defective. 
Also, even if an adequate amount of heat is supplied to the second side 
from the fixing device, the multi-color image already formed on the first 
side takes away a part of said heat, so that the color mixing and fixation 
of the image on the second side cannot be conducted sufficiently, and a 
satisfactory image cannot therefore by formed also on the second side. 
SUMMARY OF THE INVENTION 
In consideration of the foregoing, an object of the present invention is to 
provide an apparatus and a method enabling the formation of full-color 
images on both sides of a recording material. 
Another object of the present invention is to provide an apparatus and a 
method, in the formation of full-color images on both sides of a recording 
material, for providing the recording material with a larger amount of 
heat at the image fixation on the second side than for the first side. 
Still another object of the present invention is to provide an apparatus 
and a method, in the formation of full-color images on both sides of a 
recording material, for providing the recording material with an amount of 
heat, for image fixation on the first side, smaller than that for fixation 
of a one-side image, and with an amount of heat, for image fixation on the 
second side, larger than that for fixation of a one-side image. 
Still other objects of the present invention will become fully apparent 
from the following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the discussion there will be explained a first embodiment of the present 
invention, with reference to FIGS. 1A to 4. 
FIG. 3 shows the arrangement of principal components of an image forming 
apparatus capable of forming multi-color images on both sides (hereinafter 
called "first side Pa" and "second side Pb") of a transfer or recording 
material P. A photosensitive drum 1 rotated in a predetermined direction 
constitutes an image bearing member, around which are provided a primary 
charger 2, developing units 3M, 3Y, 3C and 3K respectively for magenta, 
yellow, cyan and black colors, and a cleaning unit 4. 
At the side of said photosensitive drum, there is provided a transfer drum 
6, which supports the recording material P by a grip 6a and transfers the 
toner image formed on the photosensitive drum 1 onto said recording 
material P by the function of a transfer charger 5 provided inside said 
transfer drum 6. At the sheet discharge side of said transfer drum 6 there 
are provided a separating finger 7, a conveyor unit 8 and a fixing unit 9, 
while, at the sheet feeding side there are provided a sheet cassette 10, a 
sheet feeding roller 11, registration rollers 12 and first and second 
guide members 13, 14. Also at the sheet discharge side of the fixing unit 
9 there is provided an inverting guide 15 for forwarding the recording 
material P either to a sheet discharge tray 16 or to a transport tray 
composed of a third guide member 17, an intermediate tray 18 and a feeding 
roller 19. 
The fixing unit 9 is provided with a fixing roller 91 provided therein with 
a heater H and rotated in a predetermined direction, a pressure roller 91 
rotated in pressure contact with said fixing roller 90, and first and 
second applicator rollers 92, 93 which can be contacted with or separated 
from the rollers 90, 91 and can apply thereto a releasing agent, such as 
oil. The pressure roller 90 is not provided with a heater. 
In the main body of the image forming apparatus there are provided, as 
shown in FIG. 4, a main body input unit 30 for entering the kind of image 
to be formed (single or multiple colors) and whether the image forming 
unit forms the image on one or both sides of the recording material P; 
roller heating amount control means 31 for varying the amount of heating 
of the recording material P according to whether multi-color images are 
formed on both sides of the recording material P; and a roller speed 
control unit 32 for varying the rotating speed of the fixing and pressure 
rollers 90, 91, thereby varying the amount of heating of the recording 
material P in response to the signal from said roller heating amount 
control means 31. 
In case of forming multi-color images on both sides of the recording 
material P, the roller heating amount control means 31 sends the signal to 
the roller speed control unit 32 in such a manner that the amount of 
heating by the fixing roller 90 at the fixation of the image on the second 
face Pb of the recording material P becomes larger than that for the image 
on the first face Pa, and that the amount of heating by the fixing roller 
90 at the fixation of the image on the first face Pa is smaller than that 
in case of image formation on one side only. 
At first, there will be explained functions of the image forming apparatus 
in case of forming a full-color image on one side (first side Pa) only of 
the recording material P. Referring to FIG. 3, the photosensitive drum 1, 
uniformly charged with the primary charger 2, is exposed to the light L 
corresponding to the magenta information from a scanner unit (not shown), 
whereby an electrostatic latent image based on said magenta information is 
formed on said photosensitive drum 1 and is developed, by the magenta 
developing unit 3M containing magenta toner, into a visible magenta toner 
image. Said toner image is transferred onto the recording material P 
supported on the transfer drum 6, by the function of the transfer charger 
5. 
After the image transfer, the photosensitive drum 1 is cleaned by the 
cleaning device 4 and is again uniformly charged by the primary charger 2. 
The photosensitive drum 1 is exposed to the light corresponding to the 
yellow information from the scanner unit to form an electrostatic latent 
image. The latent image is developed, by the yellow developing unit 3Y 
containing yellow toner, into a visible yellow toner image, which is in 
overlapping manner transferred onto the magental toner image supported on 
the recording material P on the transfer drum 6. Similarly, toner images 
formed in succession on the photosensitive drum 1 by the cyan developing 
unit 3C containing cyan toner and the black developing unit 3K containing 
black toner are transferred in overlapping manner onto the recording 
material P on the transfer drum 6. 
On the other hand, the recording material P, advanced, one by one, by the 
feed roller 11 from the cassette 10, is transported through the first 
guide members 13 toward the registration rollers 12, and, after adjustment 
of timing by said registration rollers 12, advanced through the second 
guide members 14 toward the transfer drum 6. The recording material P is 
positioned on and supported by the grip 6a on the transfer drum 6, and is 
subjected to the multiple transfers of toner images from the 
photosensitive drum 1, along with the rotation of the transfer drum 6. 
After said image transfers, the recording material P is separated from the 
transfer drum 6 by the separating finger 7, then transported by the 
conveyor unit 8 to the fixing device 9 and is subjected to the fixation of 
toner image in said fixing device 9. 
When the recording material P is transported into the fixing device 9, the 
toner images on said recording material P are heated and pressed by the 
fixing and pressure rollers 90, 91, thereby being fused and mixed, and are 
fixed onto the recording material P in an appropriate color by cooling 
after passing through said rollers 90, 91. In this operation, the first 
applicator roller 92 is maintained in contact with the fixing roller 90 
for applying the releasing agent. After said toner image fixation, the 
recording material P is advanced through the inverting guide 15 and is 
stacked on the discharge tray 16. 
In the following discussion, there will be explained the functions of this 
image forming apparatus in the case of forming full-color images on both 
sides of the recording material P. The recording material P, bearing 
multi-color toner images on the first face Pa as explained above, is 
transported to the fixing device 9 for heat fixation of said toner images. 
In this case, however, a signal indicating the formation of multi-color 
images on both sides of the recording material 30 is entered in advance by 
the operator into the input unit 30, whereby the roller heating control 
means 31 controls the roller speed control means 32 so as to increase the 
rotating speed of the rollers 90, 91 compared with the speed the ordinary 
case. For this reason, the recording material P emerges from the fixing 
device 9 in a semi-fixed state in which the toner images are not 
sufficiently fused and mixed because of somewhat deficient heating. 
Said recording material P is then transported through the third guide 
members 17 and temporarily stacked on the intermediate tray 18. 
Subsequently said recording material P is advanced by the feed roller 19 
toward the registration rollers 12, then, after adjustment of timing by 
said registration rollers 12, advanced to the transfer drum 6 and 
supported thereon with the second side Pb without the image outside. After 
multiple transfers of the toner images in a similar manner on said second 
side Pb, the recording material P is transported to the fixing device 9 
for heat fixing of said toner images on the second side Pb. 
In this case, the roller heating control means 31 controls the roller speed 
control means 32 to somewhat decrease the rotating speed of the rollers 
90, 91 whereby the recording material P receives an amount of heat larger 
than in the normal situation. However, since the recording material P 
bears not only multi-color toner images on the second side Pb but also on 
the rear first side Pa, the excessive heat after fixation of the toner 
image on the second side Pb is directed to that on the first side Pa which 
has been in insufficiently fused and mixed state due to somewhat deficient 
heating, whereby said image on the first side Pa is sufficinetly mixed and 
developed into a proper color by said excessive heat. After such proper 
fixation of the toner images on the first and second sides Pa, Pb, the 
recording material P is advanced through the inverting guide 15 and 
stacked on the discharge tray 16. 
In the following discussion, there will be explained, with reference to 
FIGS. 1A to 2B, the mechanism of formation of proper multi-color images on 
both sides of the recoridng material P, through variations of the amount 
of heating by the fixing device 9. 
At first, reference is made to FIGS. 2A and 2B for explaining the situation 
in a fixing device 9 in a conventional image forming apparatus. In the 
image formation on the first side Pa of the recording material P, an 
unfixed toner image T1 on the recording material P is, as shown in FIG. 
2A, properly fused and mixed to form a fixed toner image T2 under the 
application of a normal heat amount by the ordinary rotating speed of the 
fixing roller 90. Then, if an unfixed toner image T1 is formed on the 
second side Pb of the recording material P which already has the fixed 
toner image T2 on the first side Pa and is subjected to the fixation in 
the fixing deivce 9 as shown in FIG. 2B, the amount of heat supplied to 
said unfixed toner image T1 on the second side Pb becomes deficient 
because of the enlarged total heat capacity of the recording material P 
due to the presence of the fixed toner image T2 on the first side Pa, 
whereby, after passing the fixing device 9, the toner image on the second 
side Pb becomes a semi-fixed toner image T3 with incomplete fusing. Also, 
the properly fixed toner image T2 on the first side Pa receives heat 
thereby undergoing excessive fixation, and becomes an excessive fixed 
toner image T4 showing excessively fusion and flow of toner. Therefore, a 
satisfactory image connot be obtained on either side of the recording 
material P. 
The function of the fixing device 9 of the image forming apparatus of the 
present invention is shown in FIGS. 1A, 1B and 1C. In the fixation of an 
unfixed toner image T1 formed on the first side Pa of the recording 
material, the rotating speed of the fixing roller 90 is somewhat increased 
to slightly reduce the amount of heat applied by the fixing device 9 as 
shown in FIG. 1A, whereby the unfixed toner image T1 becomes a semi-fixed 
toner image T3 with deficient fusion. Then, an unfixed toner image T1 is 
formed on the second side Pb of said recording material P, already having 
a semi-fixed toner image T3 on the first side Pa, as shown in FIG. 1B and 
is subjected to the fixation in the fixing device 9. This fixation is 
conducted with a somewhat lowered rotating speed of the fixing roller 90 
to increase the amount of heat supplied by the fixing device 9, whereby 
the unfixed toner image T1 on the second side Pb is properly heated to 
form a fixed toner image T2, and the semi-fixed toner image T3 on the 
first side Pa also receives heat thereby becoming a properly fixed toner 
image T2. 
Also in the fixing device 9 of the image forming apparatus of the present 
invention, in case of image formation only on one side, the unfixed toner 
image T1 on the recording material can naturally be properly fixed as the 
fixed toner image T2 as shown in FIG. 1C by rotating the fixing roller 90 
at the ordinary rotating speed, thereby applying the ordinary amount of 
heat from the fixing device 9. 
As explained in the foregoing, in the present invention, in case of forming 
multi-color images on both sides of the recording material P, the amount 
of heat supplied by the fixing device 9 is varied for the toner image on 
the first side Pa of said recording material P and for that on the second 
side Pb, whereby the toner images on the first and second sides can be 
properly heated and fixed. 
In the following, there will be explained a second embodiment of the 
present invention, with reference to FIGS. 5 to 7, wherein components 
equivalent in function to those in the first embodiment are represented by 
same numbers and will not be explained further. 
In the image forming apparatus of the present embodiment, along an endless 
transfer belt 21 rotatably supported between two rollers 20, there are 
provided four photosensitive drums 1, around which provided are chargers 
2, developing units 3M, 3Y, 3C and 3K of magenta, yellow, cyan and black 
colors, cleaning units 4, and transfer chargers 5. Toner images of 
magenta, yellow, cyan and black images are respectively formed on said 
photosensitive drums 1, and are transferred in overlapping manner onto the 
recording material P fixed on and moving with the transfer belt 21. 
Also in such image forming apparatus, in case of forming multi-color images 
on both sides of the recording material P, satisfactory image formations 
as in the first embodiment can be achieved by somewhat decreasing the 
amount of heating by the fixing device 9 for the toner image on the first 
side Pa and somewhat increasing the amount of heating for the toner image 
on the second side Pb. 
In the fixing device 9 of the present embodiment, the adequate variation in 
the rotating speed of the fixing roller 90 is difficult because of the 
limited distance between said fixing device 9 and the transfer belt 21. 
For this reason, the amount of heating by the fixing device 9 is regulated 
by the surface temperature of the fixing roller 90. When a main switch of 
the main body of the image forming apparatus is turned on, the surface 
temperature of the fixing roller 90 of the fixing device 9 is controlled 
at a temperature Tm suitable for ordinary one-side image fixation. 
However, if the two-side mode, for forming multi-color images on both 
sides, is entered into the input unit 30, the surface temperature of the 
fixing roller 90 is lowered to T1 at the fixation of the toner images on 
the first side Pa of the recording material P and is elevated to Th at the 
fixation of the toner images on the second side Pb. 
Thus, a signal from the input unit 30 is transmitted to the roller heating 
amount control means 31, which in turn send a signal to the roller 
temperature control means 33 thereby controlling the surface temperature 
of the fixing roller 90. 
In the foregoing first and second embodiments, the recording material P is 
assumed to be plain paper, but the present invention is naturally likewise 
applicable when said recording material P is composed of cardboard or a 
plastic film such as polyester film. 
The present invention is not limited by the foregoing embodiments but is 
subject to any and all modifications within the scope and spirit of the 
appended claims.