Image forming apparatus

An image forming apparatus allowing a main assembly thereof to be interchangeably loaded with a first unit and a second unit. The first unit cooperates with a mechanism disposed in the main assembly to copy an image of a given document by the electrophotographic copying process. The second unit cooperates with a mechanism disposed in the main assembly to reproduce an image on a paper by the thermal transfer printing process. When image data from a facsimile or a word processor are transmitted to a thermal head in the second unit, an image can be reproduced on the paper based on the image data.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an image forming apparatus and more particularly 
to an image forming apparatus provided interchageably with a first unit 
for analogously forming an image by the electrophotographic copying 
process and a second unit for digitally forming an image by the thermal 
transfer printing process. 
2. Description of the Related Arts 
The concept of using a casing integrally incorporating therein a 
photosensitive member, a developing device, etc. as a unit and detachably 
fitting this unit inside a copying machine for the purpose of facilitating 
and conveniencing the maintenance, management, and repair of the copying 
machine has found recognition (for example, U.S. Pat. No. 4,627,701). 
Electrophotographic copying machines adopting this concept are now in 
popular use. 
Today, in consequence of the growth of electronic devices, the practice of 
filing image data in the form of digital signals has been steadily 
disseminating and has come to urge the necessity for a printer which is 
capable of producing an image in response to a digital signal. When a 
printer capable of converting digital signals into images and an image 
reader capable of optically reading an original and generating a digital 
signal in response thereto are combined, the product of this combination 
offers the convenience that the printer can be additionally used as a 
copying machine. In cases where an image of a given document to be copied 
must be converted into digital signals in advance of the copying 
treatment, it is difficult to attain faithful reproduction of the image of 
the document. An attempt at attaining the faithful reproduction turns out 
to be a task in need of exceptionally high technique and immense cost. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide an image forming apparatus which 
combines the analog copying function of the electrophotographic copying 
process excelling in reproducibility and the digital printing function by 
the thermal transfer printing process advantageous in terms of a running 
cost. 
Another object of this invention is to provide an image forming apparatus 
which allows a first unit possessing the analog copying function and a 
second unit possessing the digital printing function to be interchangeably 
installed in a common main assembly. 
Yet another object of this invention is to provide an image forming 
apparatus which is possessed of the second unit for reproducing an image 
on a paper of the thermal transfer printing process, based on the image 
data received from an external device such as word processor, facsimile, 
or video deck. 
Yet further object of this invention is to provide an image forming 
apparatus which is possessed of a moving member for moving to and fixing 
at prescribed positions, depending on the kinds of the unit installed in 
the apparatus, a transfer charger capable of transferring onto plain paper 
a toner image adhering to a photosensitive member in the first unit in 
cooperation with the photosensitive member and a platen roller for 
depressing a thermal heat in the second unit. 
Still another object of this invention is to provide an image forming 
apparatus which is configured so that a heater used in a fixing device 
incorporated in the main assembly remains inactive while the second unit 
is set in an operating position in the apparatus. 
Another object of this invention is to provide an image forming apparatus 
which is provided with a first paper transport path for guiding a paper 
via a fixing device to a paper discharge tray while the first unit is set 
in an operating position and a second paper transport path for guiding a 
paper directly to the paper discharge tray by detouring the fixing device 
while the second unit is set in an operating position. 
In accordance with one aspect of the present invention, there is provided 
an image forming apparatus, comprising: a main assembly incorporating 
therein a paper transport mechanism for transporting a paper, a first unit 
and a second unit interchangeably set in place in the main assembly, the 
first unit including a photosensitive member and a processing device for 
copying an image on the paper by the electrophotographic copying process 
and the second unit including a thermal head and an ink ribbon transport 
device for transporting an ink ribbon carrying an ink thereon, wherein the 
formation of an image on the paper is effected by the first unit using the 
electrophotographic copying process or by the second unit using thermal 
transfer printing process resorting to the thermal head and the ink ribbon 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 to FIG. 3 are diagrams illustrating the whole structure of a typical 
image forming apparatus as one embodiment of this invention. The main body 
of the apparatus, namely a main assembly 26 comprises a lower body 26a and 
an upper body 26b adapted to be rotated about the terminal part of the 
lower body. The lower central part of the upper body 26b is adapted to 
accommodate therein a first unit 25 in a detachable manner. This first 
unit 25 is set in place by being pushed in the direction prependicular to 
the surface of the drawing and removed by being pulled out. 
The image forming apparatus in the state having the first unit 25 set in an 
operating position is enabled to effect analog reproduction of an image 
recorded in a given document on a paper by the electrophotographic copying 
process. To support the document, a document stand 27 made of glass is 
mounted bilaterally movably on the upper body 26b. A cover 28 for covering 
the document stand is attached to the document stand in such a manner as 
to be opened and closed relative thereto. A light source 29 for 
illuminating the document laid on the document stand 27 and a lens array 
30 for converging and transmitting the light reflected from the document 
toward the first unit 25 are installed inside the upper body 26b. 
Inside a unit case 25a of the first unit 25, a photosensitive drum 31 is 
rotatably set in place. This photosensitive drum 31 is so adapted that the 
light from the lens array 30 is projected on the outer peripheral surface 
to form an electrostatic latent image. An electric charger 32 for 
uniformly charging the surface of the photosensitive drum 31 before the 
projection of the light from the lens array 30 is disposed inside the unit 
case 25a. Further, a developing device 33 for depositing a toner on the 
electrostatic latent image and a cleaner 34 for removing the residual 
toner from the outer peripheral surface of the photosensitive drum 31 
after the transfer of the toner image to the paper are disposed inside the 
unit case 25a. 
The lower body 26a is provided at the position thereof opposed to the 
photosensitive drum 31 with a transfer charger 35 for transferring the 
toner adhering to the photosensitive drum 31 to the paper. To the right 
terminal part of the lower body 26a, a paper feed tray 36 for supporting a 
plurality of papers P in a pile is detachably attached. The papers on the 
paper feed tray 36 are sent out one by one by a supply roller 37. A timing 
roller 38 for conveying the paper to the gap between the photosensitive 
drum 31 and the transfer charger 35 as synchronized with the rotation of 
the photosensitive drum 31 is attached to the lower body 26b. A fixing 
device 40 for causing the toner image transferred onto the paper to be 
thermally fused on the paper is disposed inside the lower body 26a on the 
downstream side on the transfer charger 35 in the direction of the 
conveyance of the paper and a pair of discharge rollers 41 for discharging 
the paper on which the toner image has been fixed are disposed also inside 
the lower body 26a on the downstream side of the fixing device. A 
discharge tray 42 for receiving the discharged paper is detachably 
attached to the left terminal part of the lower body 26a. 
A guide plate 43 for guiding the paper P yet to have an image formed 
thereon is interposed between the supply roller 37 and the transfer 
charger 35. Another guide plate 44 for guiding the paper having a toner 
image already transferred thereto is interposed between the transfer 
charger 35 and the discharge tray 42. These guide plates 43, 44 jointly 
form a paper transfer path 45. 
In the left terminal part of the interior of the upper body 26b, an exhaust 
fan 46 is disposed above the fixing device 40. When this exhaust fan 46 is 
rotated by a motor (not shown), the waste heat and ozone generated by the 
fixing device 40 and other parts incorporated inside the body 26 are 
released through a blower outlet 47 into the ambient air. 
When the image of the document mounted on the glass document stand 27 is 
copied on the paper by the use of the apparatus in the state having the 
first unit 25 in the operating position as illustrated in FIG. 1, the 
surface of the photosensitive drum 31 is uniformly charged by the electric 
charger 32 in consequence of the rotation of the photosensitive drum 31. 
Then, on the surface of the photosensitive drum 31, the light from the 
lens array 30 forms an electtrostatic latent image corresponding to the 
image of the document. This electrostatic latent image is converted into a 
toner image by the developing device 33. After this toner is transferred 
onto the paper P supplied from the paper supply tray 36, it is thermally 
fixed by the fixing device 40. The paper on which the image has been 
formed as described above is fed out onto the discharge tray 42. 
In the image forming apparatus of this embodiment, since the unit 25 is 
adapted to be detachably set in place inside the body 26 of the apparatus, 
the color of the image to be formed on the paper can be changed by 
replacing the first unit 25 using the toner in black color with another 
unit using the toner in a color other than black. Optionally this change 
of the color of the image may be attained by replacing the developing 
device 33 with a developing device 33 using such other color as mentioned 
above instead of replacing the unit wholly. 
FIG. 2 illustrates the image forming apparatus having a second unit 
incorporated in the main body 26 thereof in the place of the first unit 25 
illustrated in FIG. 1. Thus, the main assembly or the main body 26 of the 
image forming apparatus is constructed in such a manner as to permit 
interchangeable accommodation therein of the first unit 25 and the second 
unit 50. The second unit 50 is a thermal transfer unit for digitally 
forming an image by the thermal transfer printing process. It has 
incorporated in a unit case 50a thereof an ink supply reel 52 having an 
ink ribbon 51, i.e. a strip of ink film coated with thermally fusible ink, 
wound round thereon in the form of a roll and a takeup reel 53 for 
rewinding the ink ribbon thereon. A thermal head 54 is set in place inside 
the unit case 50a as interposed between the reels 52, 53. A platen roller 
55 to be depressed against the thermal head 54 is attached to the unit 
case 50a. Further, the unit case 50a is provided therein with an image 
reader 56 which is disposed below the lens array 30. This image reader 56 
is formed of such light-receiving elements as CCD (charge coupled device). 
The light-receiving surface of the image reader 56 coincides with the 
focal piont of the lens array 30. The unit case 50a is provided therein 
with a control part 58 for controlling the operation of each of the 
members installed in the unit case. For the purpose of guiding the ink 
ribbon 51 paid out of the supply reel 52 to the takeup reel 53, the second 
unit 50 is provided therein with a guide roll 59. 
To reproduce the image of the document mounted on the document stand 27 by 
the use of the image forming apparatus loaded with the second unit 50, the 
image reader 56 reads the light from the lens array 30, transmits digital 
signals modulated in conformity with the image to the thermal head 54 
through the medium of the control part 58, and effects transfer of the ink 
of the ink ribbon 51 onto the paper from the supply tray 36. During this 
transfer of the image, the paper is passed between the platen roller 55 
and the ink ribbon 51. 
FIG. 4 is a diagram illustrating the bottom side of the second unit 50. The 
platen roller 55 mentioned above is rotatably supported on brackets 
projected from the lower side of the unit case 50a. This platen roller 55 
protrudes from the unit case 50a. To prevent the platen roller 55 and the 
transfer charger 35 from interfering with each other during the 
installation of the second unit 50 inside the main body 26 of the image 
forming apparatus, the transfer charger 35 is adapted to be moved freely 
in the vertical direction. A spring member (not shown) exerts a constant 
upward resilient force on this transfer charger 35. When the second unit 
case 50 is set in place within the main body 26 of the image forming 
apparatus, therefore, the platen roller 55 depresses the transfer charger 
35 to a retracted position. 
FIG. 5 illustrates the rear side of the second unit 50 and FIG. 6 the front 
side of the second unit 50 and part of the upper body 26b. Inside the unit 
case 50a, a rotary member 61 connected to the takeup reel 53 is rotatably 
installed as illustrated in FIG. 5. On this rotary member 61, a shaft hole 
62 and a plurality of pin receptacles 63 are formed in the rotary member 
61 as exposed on the outside of the case 50a. In the upper body 26b, a rod 
65 for insertion into the shaft hole 62 mentioned above and a pin 66 for 
insertion into othe pin receptacle 63 are projected from a toothed wheel 
64 as illustrated in FIG. 6. A toothed wheel 67 on the driving side 
connected to a motor not shown) is meshed with the toothed wheel 64. In 
the image forming apparatus having the second unit set in place in the 
main body thereof, this motor imparts a rotation to the takeup reel 53 
through the medium of the toothed wheels 67, 64 and the rotary member 61. 
A guide rail 69 for supporting projected parts 68 formed one each at the 
bilateral terminal parts of the second unit case 50a during the 
installation of the second unit 50 in the upper body 26b is disposed 
inside the upper body 26b. 
The upper body 26b, as illustrated in FIG. 6 incorporates therein two 
microswitches 71a, 71b for discerning whether the upper body 26b is loaded 
with the first unit 25 or with the second unit 50. On the rear side of the 
second unit case 50a, projections 72a, 72b for actuating the microswitches 
71a, 71b are optionally fixed at the positions opposite these 
microswitches 71a, 71b. By the fact that either of the two microswitches 
is actuated or both of them are actuated, a decision can be made as to 
which of two units 25, 50 is installed in the main assembly. 
To the rear side of the unit case 50 is attached a connector 73 provided 
with a plurality of electroconductive terminals. Inside the upper body 
26b, a connector 74 provided with a plurality of connection sockets is 
attached at the position destined to confront the connector 73. It is via 
these connectors that an image signal from an external device can be 
transferred to the thermal head 54 where the second unit 50 is installed 
or the control signal is transmitted to the electric charger 32, the 
developing device, etc. where the first unit 25 is installed. The 
aforementioned rotary member 61 and the connector 73 are incorporated also 
on the rear side of the first unit 25. 
The fixing device 40 mentioned above is provided, as illustrated in FIG. 1, 
with an upper roller 76 and a lower roller 77. Where the first unit 25 is 
used and the photosensitive drum 31 is relied on to form the image on the 
paper, the heat of the heater lamp incorporated in the upper roller 76 
fixes the toner image on the paper and the pressing force required to be 
applied to the paper for the purpose of the fixation of the image is 
furnished by the two rollers 76, 77. In contrast, where the second unit 50 
is used and the thermal head 54 is relied on to effect thermal transfer of 
the image onto the paper, it is necessary that the paper should be 
prevented from being heated and, at the same time, the pressing force 
generated by the two rollers 76, 77 should be decreased or nullified. 
FIG. 7 and FIG. 8 illustrate a mechanism for selectively effecting the 
adjustment of the pressing force exerted by the two rollers 76, 77 upon 
the paper, depending on the choice between the first unit 25 and the 
second unit 50 to be incorporated in the main assembly. 
The lower roller 77 is attached rotatably and movably in the vertical 
direction indicated by the arrow to a frame 79 fixed inside the lower body 
26a. A compressive coil spring 80 attached to the frame 79 exerts a 
resilient force tending upwardly upon the lower roller 77. To a supporting 
pin 81 fixed inside the lower body 26a, a frame 82 is attached swingably 
around the supporting pin 81. To this frame 82, the aforementioned upper 
roller 76 is rotatably attached. 
To the upper body 26b, two levers 86 are attached slidably in the 
horizontal direction above the fixing device 40 by means of guide pins 85 
fixed to the upper body 26b. The levers 86 are allowed to produce a 
sliding motion within the length of oblong holes 87 which are formed 
therein and in which the guide pins 85 are kept in engagement. A tensile 
coil spring 88 generates a resilient force tending toward the 
aforementioned units 25, 50 to be exerted upon the levers 86. The second 
unit 50 is provided in the case 50a thereof, as illustrated in FIG. 8, 
with cams 89 adapted to be meshed with the leading ends of the levers 86. 
When the second unit 50 is set in place within the upper body 26b, the 
levers 86 are retracted to the position indicated by an imaginary line in 
FIG. 7. The levers 86 are provided at the rear end parts thereof with 
projecting parts 90 directed downwardly. These projecting parts 90 do not 
contact the frame 82 when the main assembly is loaded with the second unit 
50. In contrast, since the first unit 25 is not provided with the 
aforementioned cams 89, when the main assembly is loaded with the first 
unit 25, the projecting parts 90 of these levers 86 contact the frame 82 
as indicated by a solid line in FIG. 7. 
When the upper body 26b with the first unit 25 is in a closed state, in 
which the leading end of a lock lever 83 swingably disposed on the upper 
body 26b is brought into engagment with lock hole 84 formed in the lower 
body 26a, therefore, the frame 82 and the upper roller 76 are lowered by 
the projecting parts 90 to the position indicated by the solid line. 
Depending on this position, the lower roller 77 is also lowered and the 
coil spring 80 is contracted. As the result, the two rollers 76, 77 exert 
a pressing force necessary for the fixation of the toner, generally in the 
range of 6 to 20 kg., upon the paper. 
The upper body 26b is provided with a projecting part 91 which has a lower 
end face at a position deviating upwardly from the position of the lower 
end face of the projecting part 90 of the lever 86. When the main assembly 
is loaded with the second unit 50, the levers 86 are retracted by the cams 
89 and the projecting parts 90 of the rear end parts are retracted from 
above the frame 82. When the upper body 26b with the second unit 50 is in 
the closed state, the frame 82 comes into contact with the projecting 
parts 91 and fixed at the position indicated by an imaginary line in FIG. 
7. The pressing force exerted jointly by the rollers 76, 77 on the paper 
may be smaller than the pressing force necessary for the fixation of the 
toner image and must be enough for the conveyance of the paper. Generally, 
it is set in the neighborhood of 100 g. Where the main assembly is loaded 
with the second unit 50 and these rollers 76, 77 are not relied on to 
convey the paper, the upper roller 76 may be separated from the lower 
roller 77 by rotating the frame 82 clockwise. 
When the main assembly is loaded with the second unit 50, since the fixing 
device is not required to heat the paper, a microswitch 92 actuated by the 
lever 86 is disposed in the upper body 26b for the purpose of turning off 
the heater lamp 78 which is incorporated in the upper roller 76. As the 
result, when the main assembly is loaded with the second unit 50, the 
lever 86 is moved to the position indicated by an imaginary line in FIG. 7 
and the microswitch 92 is actuated and the heater lamp 78 is automatically 
turned off. 
FIG. 9 is a block diagram illustrating a control circuit for the image 
forming apparatus. To a main CPU 93 installed in the main body 26 of the 
image forming apparatus, the discrimination signal issued by the 
microswitches 71a and 71b to indicate that whether the main body 26 is 
loaded with the first unit 25 for reproducing an image by the 
electrophotographic copying process or the second unit 50 for printing an 
image with a thermal head based on a digital image signal on the paper is 
transmitted. When the main body is loaded with the first unit 25, the CPU 
93 issues a drive signal to the electric charger 32 and other members 
installed inside the unit 25. When the main body is loaded with the second 
unit 50, the CPU 93 issues a motor drive signal for controlling a motor 
(not shown) which rotates the takeup reel 53 inside the unit 50 and a 
permitting signal for permitting the thermal transfer operation. 
Further to the CPU 93, the input data such as the number of copies to be 
produced which the operator has selected for the operation of the image 
forming apparatus and introduced through the key input part 84 installed 
on the operation board are forwarded. Such input data as the number of 
copies which have been introduced in consequence cf the procedure 
mentioned above are exhibited on a display part 95. The CPU 93 issues 
control signals to the light source 29, the lens array 30, the timing 
roller 38, the exhaust fan 46, the fixing device 30, and other members 
incorporated in the main body 26. For the detection of the temperature of 
the fixing device 40, a thermistor 96 is installed as a temperature sensor 
as illustrate in FIG. 9. In the illustrated embodiment, the control part 
58 installed within the second unit 50 is depicted as discharging the role 
of controlling the printing operation. Optionally, the CPU 93 may be 
configured so as to effect &:his control additionally. 
The CPU 93 has an interface for permitting connection of external devices. 
Thus, image data signals from these external devices may be utilized for 
forming images on the paper by the thermal transfer printing process. The 
external devices include a word processor 97a, a facsimile 97b, and a 
video deck 97c, for example, as illustrated in FIG. 9. It is further 
permissible to transmit image data from the image reader 56 in the second 
unit to the printer of the word processor 97a or the facsimile 97b and 
enable the printer to effect the reproduction cf the image. 
Though the illustrated second unit 50 of this embodiment is provided with 
the image reader 56 for reading a image of a given document mounted on the 
document stand 27, the second unit 50 may have no image reader. In this 
case, an image corresponding to a digital image signal from each of the 
external devices is reproduced on the paper by the thermal transfer 
printing process. 
FIG. 10 to FIG. 12 are flow charts indicating the procedures of control 
contemplated by this embodiment. 
When a main power source switch installed on the main body of the apparatus 
is turned ON, the initial setting is effected at the Step 101 as 
illustrated in FIG. 10. By this initial setting, a standard mode is set 
for the control of the parts used in common for all types of units, a 
decision is made as to which type unit has been installed in the main 
assembly, and the members ir the main body of the apparatus including 
those in the unit are set in an operative state. Then, at the Step 102, 
the internal timer of the CPU 93 is set at the time for one routine 
required for various controls. At the Step 103, a decision is made as to 
whether or not the apparatus is capable of forming an image. When this 
decision is for an affirmative answer, the key input treatment is executed 
at the Step 104. Then the treatment of data for the display part 95, the 
control for the formation of an image, and other treatments are executed 
at the Step 105. When the termination of internal timer is detected at the 
Step 106, the processing is returned to the Step 102. When the decision 
made at the Step 103 draws a negative answer, the display of abnormality, 
etc. are executed at the Step 107. The state cf abnormality is exhibited 
in the display part 95 at the Step 105. 
FIG. 11 illustrates in detail the initial setting subroutine at the Step 
101. First, at the Step 111, the initial value of the CPU 93 is reset and 
the flags F1, F2 for discerning which of the first unit 25 and the second 
unit 50 has been installed in the main assembly are reset to "0." 
In the illustrated embodiment, two microswitches 71a, 71b are installed as 
illustrated in FIG. 6 and they are turned ON or OFF to designate the kind 
among three kinds of unit. One of them is the first unit possessed of the 
electrophotographic copying function illustrated in FIG. 1, another of 
them is the second unit provided with the image reader (IR) 56 illustrated 
in FIG. 2 and possessed of the thermal transfer printing function, and the 
remainder in the second unit 50 of modified type not possessed of the 
image reader. 
Either of the flags F1, F2 is set at the Step 112, depending on the kind of 
unit installed in the main body 26. 
When the flag F1 is set at "1" and the flag F2 is kept reset at "0" this 
combination signifies that the first unit 25 has been installed. When the 
flag F1 is kept reset at "0" and the flag F2 is set at "1," this 
combination signifies that the second unit 50 of the modified type, namely 
the unit possessed of no image reader 56, has been installed. When the two 
flags F1, F2 are each set at "1," this combination signifies that the 
second unit illustrated in FIG. 2, namely the second unit 50 possessed of 
the image reader 56, has been installed. 
Next, at the steps 113 to 129, operation of each element of the apparatus 
is permitted or prohibited in accordance with the type of the installed 
unit as shown in the following table. In the table, "ON" means that the 
indicated operation is permitted and "OFF" means that the operation is 
prohibited. Since the members in the main body 26 of the apparatus are 
adapted to be controlled as described above, when there is carried out any 
opreation other than the copying operation by the electrophotographic 
method, any action unnecessary for that particular operation is prohibited 
and the otherwise possible waste of electric power is precluded. 
The Step 130 is meant for the initial setting in the other members 
mentioned above. When this step is completed, the processing returns to 
the main routine illustrated in FIG. 10. 
TABLE 
__________________________________________________________________________ 
TYPE OF CHARGERS 
FIXING LIGHT SCANNING 
EXHAUST 
UNIT FLAG 
32, 35 DEVICE 40 
SOURCE 29 
ORIGINAL 
FAN 46 
__________________________________________________________________________ 
FIRST UNIT 
F1 = 1 
ON ON ON ON ON 
F2 = 0 
SECOND UNIT 
F1 = 0 
OFF OFF OFF OFF OFF 
F2 = 0 
SECOND UNIT 
F1 = 1 
OFF OFF ON ON OFF 
WITH IMAGE 
F2 = 1 
READER 
__________________________________________________________________________ 
There are times when the main body of the apparatus is loaded with the 
second unit 50 immediately after the copying operation by the 
electrophotographic process is completed with the apparatus loaded with 
the first unit 25. Since the fixing device 40 is still at an elevated 
temperature immediately after the completion of the copying operation, 
there is a possibility that the thermally fusible ink transferred from the 
ink ribbon 51 to the paper by the thermal head 54 will be melted out. When 
the ink is melted out, the molten ink is suffered to adhere to the roller 
76 of the fixing device 40. For the purpose of preventing this trouble, 
the decision to be made as to the readiness of the apparatus for image 
formation at the Step 103 in FIG. 10 includes the control illustrated in 
FIG. 12 so that the operation for image formation by the use of the second 
unit 50 is prohibited, when the temperature of the fixing device 40 which 
is exposed to the image surface of the paper exceeds the melting point of 
the ink which is generally in the neighborhood of 70.degree. C. 
(158.degree. F.) and this fact is indicated on the display part 95 on the 
operation panel. 
First at the Step 131, a decision is made as to whether or not the printing 
operation by the use of the thermal head is to be performed as illustrated 
in FIG. 12. When the main body of the apparatus is loaded with the second 
unit 50 or its modified type and the printing operation is performed by 
the use of the thermal head 54, the decision as to the temperature of the 
fixing device 40 is made at the Step 132. Generally, it is on the upper 
roller 76 that the fixing device 40 comes into contact with the 
transferred image surface of the paper. The decision mentioned above, 
therefore, is accomplished by detecting the surface temperature of the 
upper roller 76 with the thermistor 96 illustrated in FIG. 9 and 
determining whether or not the surface temperature is less than 70.degree. 
C. which is the melting point of the ink. 
The flag indicating a permit for the thermal transfer operation is set at 
the Step 133 when the temperature of the fixing device 40 is less than the 
melting point of the ink. When the temperature of the fixing device 40 is 
found to exceed 70.degree. C., the thermal transfer operation is 
prohibited and a warning such as "WAIT" is displayed at the Step 134. The 
processing is returned to the main routine and the operations mentioned 
above are repeated and the apparatus is kept waiting for the temperature 
to fall below 70.degree. C. When the temperature falls below the level 
mentioned above, the flag of permit is set and the displayed warning is 
turned off at the Step 133. Since the melting point of the ink is variable 
with the kind of the ink, the temperature as the level for the 
aforementioned decision can be freely changed as required. 
FIG. 13 to FIG. 16 illustrate typical image forming apparatus as other 
embodiments of the present invention. In these diagrams, the members which 
are the same as in the embodiments described above are denoted by same 
symbols. 
In the embodiments illustrated in FIG. 1 to FIG. 5, the platen roller 55 is 
depicted as installed in the second unit 50. In the present embodiment, 
the platen roller 55 is adapted to be attached to the lower body 26a and 
the platen roller 55 and the transfer charger 35 are moved to their 
operating positions, depending on the kind of unit to be installed. 
A pair of swinging arms 141 are swingably attached in their middle parts to 
pins fixed on the lower body 26a as illustrated in FIG. 15 and FIG. 16. 
The platen roller 55 is attached rotatably to one terminal part each of 
the swinging arms 141. Supporting brackets 142 for supporting the opposite 
end parts of the transfer charger 35 are attached to the other terminal 
parts of the swinging arms 141. One of the supporting brackets 142 is 
provided with a voltage applying terminal 144 which is connected to a 
power source card 143. The platen roller 55 is provided at one terminal 
part thereof with a toothed wheel 145 as illustrated in FIG. 15 and FIG. 
16. A toothed wheel 146 meshed with the toothed wheel 145 is attached to 
the swinging arm 141 as disposed coaxially with the pin 140. By a motor 
(not shown), the toothed wheel 146 is driven to impart a rotation to the 
platen roller 55 through the medium of the toothed wheel 145. 
Inside the lower body 26a, a compressive coil spring 147 adapted to exert a 
resilient force tending to push the supporting brackets 142 upwardly is 
set in place. When the main body 26 of the apparatus is loaded with the 
first unit 25 as illustrated in FIG. 13, the spring 147 rotates the 
swinging arm 141 counterclockwise, approximates the transfer charger 35 to 
the photosensitive drum 31, and moves the platen roller 55 downwardly. As 
the result, the platen roller 55 functions as a conveyor roller for 
transporting the paper onto which the image has been transferred. 
The swinging arms 141 are each provided at the aforementioned other 
terminal part thereof with a part 118 projecting upwardly. The second unit 
50 is provided with a projection 149 adapted to collide with the part 148. 
When the upper body 26b of the apparatus in a state loaded with the second 
unit 50 is closed as illustrated in FIG. 14, the part 148 is depressed by 
the projection 149 and the swinging arm 141 is rotated clockwise. As the 
result, the platen roller 55 is pressed against the thermal head 54 and 
the transfer charger 35 is separated from the photosesitive drum 31 as 
illustrated in FIG. 14. FIG. 15 illustrates the guide plate 44 disposed on 
the downstream side of the platen roller 55 and the guide roller 150 
disposed on the upstream side of the transfer charger 35. Also in the 
present embodiment, the apparatus is controlled in the procedure 
illustrated in FIG. 10 to FIG. 12. 
FIG. 17 to FIG. 19 illustrate a typical image forming apparatus as a 
further embodiment of this invention. In these diagrams, the members which 
are the same as in the embodiments described above are denoted by same 
symbols. 
In the image forming apparatus of each of the embodiments described above, 
when the apparatus is loaded with the second unit 50 and is operated to 
print the image by the thermal transfer printing process, the heater lamp 
78 of the fixing device 40 is turned OFF and the pressing force of the two 
rollers 76, 77 is decreased or nullified. In the present embodiment, the 
paper is not passed through the fixing device 40 but is caused to detour 
it. 
In the lower body 26a, a guide plate 151 for guiding the paper in the 
direction of the fixing device 40 is disposed on the downstream side of 
the guide plate 44 which is located on the downstream side of the transfer 
charger 35. This guide plate 151 and the aforementioned guide plate 44 
jointly form a first paper transport path 45a for guiding the paper to the 
fixing device 40. On the downstream side of the guide plate 44, a guide 
plate 152 inclined downwardly and extended to the paper discharge tray 42 
is disposed. A guide plate 153 is above the guide plate 152 as laid 
parallelly thereto. These guide plates 152, 153 give rise to a bypass 
paper transport path branching off the first paper transport path 45a and 
caused to detour the fixing device 40, namely a second paper transport 
path 45b. This second paper transport path 45b is provided with a pair of 
paper discharge rollers 154 for advancing the papers received therein to 
the paper discharge tray 42. 
To the branching part, a switching plate 155 is swingably attached to 
permit choice between the first paper transport path 45a and the second 
paper transport path 45b for guiding the paper, depending on the kind of 
unit to be, installed in the main body of the apparatus. This switching 
plate 155 is swung to the first position for closing the second paper 
transport path 45b and guiding the paper to the fixing device 40 as 
illustrated in FIG. 17 or to the second position for opening the second 
paper transport path 45b as illustrated in FIG. 18. 
In order for the switching plate 155 to be swung, this switching plate 155 
is attached to a shaft 157 rotatably supported on a bearing 156 as 
illustrated in FIG. 19, a link 158 is fixed at one end of the shaft 157, 
and a link 160 adapted to be reciprocated by a solenoid 159 is connected 
to the link 158. When the main body is loaded with the second unit 50, 
therefore the switching plate 155 is swung upwardly as illustrated in FIG. 
18 and the paper P is guided to the second paper transport path 45b by the 
actuation of the solenoid 159. To the terminal part of the upper roller 76 
of the fixing device 40, a toothed wheel 161 adapted to be meshed with the 
toothed wheel 145 disposed at the terminal part of the lower roller 77 is 
attached. 
This image forming appratus has no use for the control illustrated in FIG. 
12. The heater lamp 78 incorporated in the upper roller 76 of the fixing 
device 40 is not energized when the main body is loaded with the second 
unit 50 similarly to the embodiments described above. 
The image forming apparatus of this invention described above is enabled to 
serve as a printer when the second unit 50 possessing the function of a 
thermal transfer printer is incorporated in the place of the first unit 25 
possessing the fucntion of a copying machine. This apparatus permits free 
selection between the function of an analog copying machine enjoying high 
resolution and the function of a digital thermal transfer printer of low 
running cost. Since various members such as the mechanism for paper 
transport paths can be used in common for the first and second units, this 
invention permits inexpensive production of an apparatus combining these 
two functions.