Image forming apparatus with delayed discharge by discharge means

An image forming apparatus includes an image bearing member for bearing an image, the image bearing member being rotatable at a first speed and a second speed which is different from the first speed, wherein during image formation on the image bearing member, the first speed is selected, and after completion of image formation on the image bearing member, a speed of rotation of the image bearing member is switchable from the first speed to the second speed; a discharger for electrically discharging the image bearing member at a discharging position, the discharger discharging the image bearing member at least one turn after the trailing edge of the image on the image bearing member passes through the discharging position and before the speed of the image bearing member is switched from the first speed to the second speed.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to an image forming apparatus, more 
particularly to such an apparatus in which a speed of an image bearing 
member or a recording material carrying member is changeable, further 
particularly to a color image forming apparatus in which images are 
superposedly transferred onto a recording material from an image bearing 
member. 
A multi-color image forming apparatus is known as disclosed in German 
Patent Publication No. 2607727 or Japanese Laid-Open Patent Application 
No. 50-50935. When a color image is formed on a transparent resin sheet 
for an overhead projector (OHP sheet) in the conventional multi-color 
image forming apparatus, the light transmission is required for the print. 
This is because, in the case of the overhead projector, the light 
transmitted through the print is projected, and therefore, if the 
transparency is poor, the projected image is dark (blackish) even to the 
worst extent that the image is projected as black and white image. 
In order to assure the transparency, it is desired that in the fixing step 
after the transfer of the toner onto the recording material from the image 
bearing member, a sufficient amount of sheet is applied to the developer 
to smooth the surface of the developer after the image fixing. However, it 
is difficult to rise the fixing temperature because of the sheet 
durability and service life of the image fixing apparatus. Therefore, in 
the apparatus used recently, the distance from a position where the 
transfer material is separated from a transfer drum for carrying the 
recording material or transfer material to the fixing position is made 
longer than the length of the transfer material, and when the sheet for 
the overhead projector (OHP) sheet is used, the transfer material feeding 
speed is reduced in the path after the separation of the transfer material 
from the transfer drum to the fixing station, without changing the image 
formation speed excepts for the fixing station, and the sufficient heat is 
applied during the fixing operation. 
According to this method, the sufficient heat can be applied during the 
fixing operation without changing the process speed except for the sheet 
feeding and the fixing operation after the sheet separation, that is, 
without influence to the latent image formation and the developing process 
involving the photosensitive member. Therefore, an OHP print having good 
transparency can be provided without influence to the image bearing member 
(photosensitive member) or the developing operation therefor. In this 
example, the normal process speed is 100 mm/sec, and it is 40 mm/sec for 
the OHP sheet. 
To accomplish this, the conventional method is such that the distance Ld 
from the separating position to the fixing position is made longer than 
the length of the transfer material, by which the fixing operation starts 
after the image transfer operation is completed. This however, results in 
bulkiness of the apparatus. 
The recent demand for the downsizing of the office equipment extends to 
multi-color image forming machines, and therefore, the efforts have been 
made to reduce the distance from the transfer position to the fixing 
position. In order to produce satisfactory OHP sheet print with reduced 
distance between the transfer position to the fixing position, the speed 
at the fixing position should be reduced. In this case, the leading edge 
of the transfer material enters the fixing position before the completion 
of the separation of the transfer material from the photosensitive drum. 
To avoid the problem, Japanese Laid-Open Patent Application No. 4.303858 
or EPA 577490 disclose that after the termination of the transfer step, 
the transfer drum is rotated additionally through one full-turn while 
maintaining the transfer material on the transfer drum, and before the 
transfer material is separated, the speed is reduced; and thereafter, the 
separation and fixing operations are carried out. By doing so, the size of 
the apparatus can be reduced, and a high quality prints can be provided on 
the OHP sheet. 
However, when the image forming operation is executed through this method, 
and when the speed is simply reduced, the charging condition and the 
exposing condition are not proper for the image bearing member with the 
result of charging memory, exposure memory or the like, and therefore, the 
image is remarkably damaged. 
In order to avoid the inconveniences, it would be possible to provide, 
downstream of the primary charger for the photosensitive drum, for 
example, a discharging device which operates only when the process speed 
is reduced. However, this would result in increase of the size of the 
photosensitive drum, against the demand for the downsizing. 
In addition, upon the completion of the image formation, the photosensitive 
drum is also discharged. In this case, the discharging condition changes 
by the reduction of the process speed, and therefore, the discharging is 
Not satisfactory. Then, no good image can be produced upon the next image 
forming operation. 
SUMMARY OF THE INVENTION 
It is a principal object of the present invention to provide an image 
forming apparatus in which memory in an image bearing member is prevented 
irrespective of the moving speed change of the image bearing member. 
It is another object of the present invention to provide an image forming 
apparatus in which an image bearing member and/or a recording material 
carrying member are properly discharged irrespective of the change of the 
moving speed of the image bearing member and/or the recording material 
carrying member. 
It is a further object of the present invention to provide an image forming 
apparatus having a small size. 
These and other objects, features and advantages of the present invention 
will become more apparent upon a consideration of the following 
description of the preferred embodiments of the present invention taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the accompanying drawings, the embodiments of the present 
invention will be described in detail. 
Referring to FIG. 1, there is shown a color laser beam printer, in 
cross-section, as an exemplary image forming apparatus. 
As shown in this Figure, the apparatus comprises a photosensitive drum 101 
as an image bearing member, a roller charger 2. At the left side of the 
photosensitive drum, there is a developing apparatus including a plurality 
of developing devices 3a, 3b, 3c and 3d which are supported on a rotatable 
support 4 in the manner that developing openings 5a, 5b, 5c and 5d of the 
developing devices 3a, 3b, 3c and 3d are disposed on the same 
circumference with the center thereof being the rotational axis of the 
support. 
The developing devices 3a, 3b, 3c and 3d contain yellow toner, magenta 
toner, cyan toner and black toner. As shown in FIG. 4, there are provided 
application rollers 6 and toner regulating members 7. With rotation of the 
developing roller 8, the toner is applied on the developing roller 8 by 
the application roller 6, and required triboelectric charge is applied to 
the toner by toner regulating member 7. The material Of the toner 
regulating member is preferably nylon or the like when the toner is to be 
charged to the negative polarity. On the contrary, it is to be charged to 
the positive polarity, the silicone rubber or the like are preferred. In 
any case, the material charged to the polarity opposite from that of the 
toner is preferable. Additionally, the peripheral speed of the developing 
roller 8 is preferably 1.0-2.0 times the peripheral speed of the 
photosensitive drum 101. The developing devices 3a, 3b, 3c and 3d on the 
support 4, as shown in FIG. 1, is such that the developing openings 5a, 
5b, 5c and 5d of the developing devices 3a, 3b, 3c and 3d are faced always 
to the photosensitive drum 101. The driving means may be the one disclosed 
in Japanese Laid-Open Patent Application No. 50-93437. 
At the right side of the photosensitive drum 101, there is a transfer drum 
9 (recording material carrying member) for carrying the recording material 
or the transfer material and for transferring the image from the 
photosensitive drum 101 onto the transfer material. 
The photosensitive drum 101 is rotated in the direction indicated by an 
arrow at the peripheral speed of 100 mm/sec by driving means. The 
photosensitive drum 101 comprises an aluminum cylinder having a diameter 
of 80 mm and an organic photoconductor (OPC) thereon. The organic 
photoconductor may be replaced with A-Si, CdS, Se or the like. 
At the upper position of the main assembly of the apparatus, there is an 
exposure device comprising a laser diode, a polygonal mirror rotated by 
high speed motor, an optical unit 10 including lens and folding mirror 11. 
The charging roller 2 is supplied with a DC based AC voltage including a DC 
component of -700 V and an AC component having a frequency of 1000 Hz and 
a peak-to-peak voltage Vpp of 1500 V, so that the surface of the 
photosensitive drum is uniformly charged to approx. -700 V. 
When the laser diode receives the signal indicative of the yellow image 
pattern, the laser beam is incident on the photosensitive drum 101 through 
the optical path 12, so that the potential of the portion exposed to the 
laser beam becomes approx. -100 V. When the photosensitive drum 101 
further rotates in the direction indicated by an arrow, the latent image 
is visualized by the developing devices 3a, 3b, 3c and 3d. 
The description will be made as to the image transfer step. The transfer 
drum 9 comprises a metal cylinder 13 having a diameter of 156 mm, an 
elastic layer 14 of foamed urethane having a thickness of 2 mm wrapped 
therearound, and an upper layer of 100 .mu.m in thickness and of 
dielectric sheet (polyvinylidene fluoride (PVDF)) 15, wrapped therearound. 
A transfer material is fed out by a pick-up roller not shown, from a 
transfer material cassette 16. It is retained on the transfer drum by a 
gripper 18, and is electrostatically attracted on the transfer drum by the 
attraction roller 19 supplied with the voltage. 
The toner image is transferred from the photosensitive drum 101 onto the 
transfer material attracted on the transfer drum 9 by the voltage applied 
on the transfer drum 9 by the voltage source not shown in the drawing. 
The above-described steps are repeated for the magenta, cyan and black 
colors, by which a multi-color toner image is superimposedly formed on the 
transfer material. The transfer material is separated from the transfer 
drum 9 by separation claws, and the toner image is fused and fixed on the 
transfer material by known fixing device 21 which heats and presses the 
toner image. The distance D.sub.max from the separation position of the 
transfer drum 9 to the fixing device is smaller than the maximum length of 
the transfer material measured in the direction of the transfer material 
feeding. 
The residual toner remaining on the photosensitive drum 101 is removed from 
the photosensitive drum 101 by known fur brush, blade means or another 
cleaning device 22. Additionally, the photosensitive drum 101 is 
electrically discharged and initialized. In the case of FIG. 1, the 
discharging means for the photosensitive drum 101 is in the form of a 
charging roller 2. In order to discharge the photosensitive drum 101, the 
DC component of the alternating voltage applied is substantially 0 V, 
while the AC component remains unchanged. 
It is preferable that the toner on the transfer drum 9 is removed by a 
transfer drum cleaning device 23 such as a fur brush, web or the like. 
The transfer drum 9 is electrically discharged and initialized by a 
discharging roller 24. 
The fixing step will be described. In a multi-color image forming 
apparatus, yellow, magenta, cyan toners are fused and mixed to produce 
natural color, or basic seven colors including red, green and blue. 
Therefore, the toner is given a sharp-melt property for the better mixing. 
Additionally, in order to supply sufficient heat, a relatively large nip 
is formed, and sufficient pressure is imparted to promote the color 
mixing. More particularly, in the case of monochromatic image formation 
5-10 kg pressure is applied, whereas in the color image forming apparatus 
in this embodiment, a high pressure such as 30-50 kg is imparted. 
In FIG. 2, the peripheral length of the photosensitive drum 201 is L2, and 
the transfer drum 109 has a peripheral length L1. In addition, the OHP 
sheet (transparent resin sheet) has a length L3. 
In this embodiment, the length of the feeding path from a point where the 
transfer material is separated from the transfer drum 109 to the fixing 
device 21 is shorter than the length of the transfer material, thus 
reducing the size of the apparatus. In addition, the circumferential 
length L2 of the photosensitive drum 201, the circumferential length L1 of 
the transfer drum and the length L3 of the OHP sheet satisfy L1-L3&gt;L2. 
More particularly, the diameter of the transfer drum 109 is 160 mm 
(peripheral length L1 thereof is 502.7 mm), the diameter of the 
photosensitive drum 201 is 40 mm (the circumferential length L2 thereof is 
125.7 mm). The OHP sheet size is A4 (297 mm) (longitudinal feeding), or a 
letter size (279 mm) (longitudinal feeding). The circumferential length of 
the transfer drum is preferably an integer multiple of the peripheral 
length or circumferential length of the photosensitive drum. 
Referring to FIG. 5, the description will be made as to driving means for 
the transfer drum 109 and the photosensitive drum 101. 
In this embodiment an output shaft of a motor 41 for driving the transfer 
drum 109 and the photosensitive drum 101 is provided with a gear 35, which 
is in meshing engagement with a large gear 36a of the stepped gear 36. The 
small one 36b of the two-step gear 36 is in meshing engagement with gears 
37 and 39. Here, the gear 39 is mounted to a flange of the photosensitive 
drum 101 to rotate integrally with the photosensitive drum 101. The gear 
37 is mounted to a flange of the transfer drum 109 and is in meshing 
engagement with a gear 38 integrally rotatable with the transfer drum 109. 
The gear 37 is rotatably mounted to a shaft 37a secured to an end of a 
rotatable disk 40. THe rotatable disk 40 is rotatably mounted around a 
rotational shaft 36c of the step gear 39, at the other end. The rotatable 
disk 40 is urged in the clockwise direction in FIG. 9 by a torsion coil 
spring 42. Therefore, the gear 30 is normally urged toward the gear 38. 
The coil spring 42 is mounted to the shaft 36c, and one arm thereof is 
hooked on a pin 43, and the other arm is hooked on the rotatable disk 40. 
With this structure, the rotational force of the motor 41 is transmitted to 
the gears 35 and 36 (36a and 36b) to rotate the photosensitive drum 101, 
and simultaneously, the rotational force of the motor 41 is transmitted to 
the gears 35 and 36 (36a and 36b), 37 and 38 to rotate the transfer drum 
109. 
The apparatus of this embodiment is operable in a plain paper mode for 
plain paper as the transfer material, a special sheet mode for an OHP 
sheet as the transfer material. The modes are selectable by a switch on an 
operation panel. 
The transfer material fed out by the pick-up roller 31 is stopped by a 
registration roller 32 for the purpose of timed relation with the gripper 
18 of the transfer drum. The transfer material is refed in synchronism 
with the gripper 18 and is gripped thereby. Then, it is electrostatically 
attracted on the transfer drum to permit transfer operation. Then, three 
color images are transferred, and then the fourth color transfer is 
executed. In the plain paper mode, the leading edge of the transfer 
material is separated by separation claws 20 in the fourth color transfer 
operation. Thus, the leading edge of the transfer material is separated 
and fed to the fixing device, during the fourth color transfer operation. 
However, in the special sheet mode, the leading edge of the transfer 
material is not separated during the fourth color transfer operation, and 
the transfer drum 109 is rotated further through one full-turn while 
attracting thereon the transfer material. 
At this time, at the point of completion of the transfer action of the 
fourth color image trailing end onto the transfer material, the DC voltage 
is changed from -700 V to approx. 0 V, while the alternating voltage 
component remains unchanged, for the charging roller 2, by which the 
photosensitive drum 101 is discharged to substantially 0 V. When the 
photosensitive drum discharging is effected for at least one full-turn and 
before the leading edge of the transfer material re-enter the transfer 
region, the charger and discharger for the drum (alternating voltage 
applied to the charging roller) are rendered off, thereafter, the speeds 
of the photosensitive drum 101 and the transfer drum 109 are reduced to 
the speed with which the image can be sufficiently fixed on the OHP sheet. 
More particularly, when the normal process speed is 100 mm/sec, the fixing 
speed for the OHP sheet is 40 mm/sec. 
In this case, the circumferential length L1 of the transfer drum 109, and 
the length of the transfer material L3 is 297 mm, and therefore, 205.7 mm 
region remains without the transfer material wrapped. On the other hand, 
the circumferential length L2 of the photosensitive drum 101 is 125.7 mm, 
and therefore, L1-L3&gt;L2 is satisfied, by which the photosensitive drum can 
be discharged through more than one-full-turn before the leading edge of 
the transfer material enters the transfer position after the completion of 
the fourth color image transfer operation. For this reason, the speed of 
the photosensitive drum can be reduced, while the charging and discharging 
of the photosensitive drum 101 is switched. 
By doing so, L1/L2&gt;3 is satisfied. In other words, the photosensitive drum 
is sufficiently downsized, without inconveniences such as charge memory or 
transfer memory, while the apparatus is downsized. Thus, OHP sheet print 
can be provided with high transparency. 
Embodiment 2 
Referring to FIG. 2, the description will be made as to a multi-color image 
forming apparatus according to a second embodiment of the present 
invention, wherein the photosensitive drum 201 has a circumferential 
length L6, and the transfer drum 109 has a circumferential length L5, and 
the OHP transfer sheet has a length L3. The distance from the position of 
the discharger 2 for the photosensitive drum 201 to the transfer position 
in the direction of the movement of the circumference of the drum 201 is 
L4. 
In this embodiment, the length of the feeding path from a point where the 
transfer material is separated from the transfer drum 109 to the fixing 
device 21 is shorter than the length of the transfer material, thus 
reducing the size of the apparatus. In addition, the circumferential 
length L6 of the photosensitive drum 201, the circumferential length L5 of 
the transfer drum, the length L3 of the OHP sheet and the distance L5 
between the charging device 2 and the transfer position satisfy 
L5-L3+L4&gt;L6. More particularly, the diameter of the transfer drum 109 is 
120 mm (peripheral length L1 thereof is 376.8 mm), the diameter of the 
photosensitive drum 201 is 30 mm (the circumferential length L2 thereof is 
94.2 mm). The distance between the transfer position and the 
photosensitive drum discharger position, which is a charging roller 2 
having charging and discharging functions. The OHP sheet size is A4 (297 
mm) (longitudinal feeding), or a letter size (279 mm) (longitudinal 
feeding). 
The transfer material fed out by the pick-up roller 31 is stopped by a 
registration roller 32 for the purpose of timed relation with the gripper 
18 of the transfer drum. The transfer material is refed in synchronism 
with the gripper 18 and is gripped thereby. Then, it is electrostatically 
attracted on the transfer drum to permit transfer operation. Then, three 
color images are transferred, and then the fourth color transfer is 
executed. In the plain paper mode, the leading edge of the transfer 
material is separated by separation claws 20 in the fourth color transfer 
operation. Thus, the leading edge of the transfer material is separated 
and fed to the fixing device, during the fourth color transfer operation. 
However, in the special sheet mode, the leading edge of the transfer 
material is not separated during the fourth color transfer operation, and 
the transfer drum 109 is rotated further through one full-turn while 
attracting thereon the transfer material. 
When such a position on the photosensitive drum 201 as is going to 
correspond to a trailing edge of the force color image passes by the 
charging roller 2, the DC component of the voltage applied to the charging 
roller 2 is changed from -700 V to approx. 0 V while maintaining the 
alternating component, by which the photosensitive drum 201 is discharged 
to approx. 0 V. The charger and discharger for the photosensitive drum is 
rendered off (the alternating component is rendered off) when the 
photosensitive drum 201 is discharged by one full-turn or more and before 
the leading edge of the transfer material enters the transfer zone after 
the transfer. Thereafter, the speeds of the photosensitive drum 201 and 
the transfer drum 109 are reduced down to the speed for sufficient image 
fixing on the OHP sheet. 
FIG. 6 is a timing chart in a special mode in this embodiment. After the 
position which is going to correspond to the trailing edge of the fourth 
color image passes through the primary charging position, the DC voltage 
for the charging roller is stopped. The time period from the stoppage of 
the DC voltage of the charging roller to the stoppage of the AC voltage 
for the charging roller is longer than the time required for one full-turn 
of the photosensitive drum 201. After the AC voltage for the charging 
roller is rendered off, the speeds of the photosensitive drum 201 and the 
transfer drum 109 are switched from the normal process speed to the speed 
capable of fixing the image on the OHP sheet. The speed reduction is 
carried out after the completion of the transfer of the fourth color image 
onto the OHP sheet carried on the transfer drum but before the OHP sheet 
reaches the fixing position. As described hereinbefore, the completion of 
the discharging operation of the photosensitive drum 201 by the charging 
roller 2 is prior to the speed reduction, and at least prior to the 
arrival of the OHP sheet at the fixing position. 
In this case, the circumferential length L5 of the transfer drum 109 is 
376.8 mm, and the length L3 of the transfer material is 297 mm, and 
therefore, the region (L5-L3) in which the transfer material is not 
wrapped around the photosensitive drum is 79.8 mm. On the other hand, the 
circumferential length L6 of the photosensitive drum 201 is 94.2 mm, and 
therefore L5-L3&lt;L6 is satisfied. Before the leading edge of the transfer 
material re-enter the transfer position after the fourth color transfer is 
completed, the photosensitive drum 201 is not discharged more than 
full-turn. However, if the discharging of the photosensitive drum is 
started at the timing when the portion which is going to correspond to the 
trailing edge of the fourth color image passes by the charging roller 2, 
the photosensitive drum 201 is discharged more than one-full-turn before 
the leading edge of the transfer material enters the transfer position. 
Further, the charging-discharging relation is rendered off, and the speed 
can be reduced. Since the length from the charging roller 2 to the 
transfer position L4 is 55 mm, L5-L3+L4&gt;L6, and L5/L6.gtoreq.3. By 
starting the drum discharging at the point of time when a position 
corresponding to the trailing edge of the fourth color image passes by the 
charging position, an OHP sheet print exhibiting good transparency can be 
produced without inconveniences such as charge memory, transfer memory or 
the like, while the diameter of the transfer drum is further reduced. 
In addition by the sufficient reduction of the diameter of the 
photosensitive drum, that is, by employing the relationship of 
L5/L6.gtoreq.3, the photosensitive drum and the cleaner or the like are 
unified for easy maintenance by the users, thus improving the operativity 
of the apparatus. 
Embodiment 3 
Referring to FIG. 3, the third embodiment of the multi-color image forming 
apparatus according to the present invention will be described. In the 
first and second embodiment, the charging roller 2 is supplied with an 
alternating voltage to provide charging and discharging functions of the 
charging roller 2. In this embodiment, the charging roller 2 is supplied 
with a DC voltage only and therefore functions only as a charger. There is 
provided a pre-exposure device prior to the charging device (pre-exposure 
device) 303, by which the charging and discharging functions are 
separated. The charging roller 2 is supplied with a DC voltage of -1250 V, 
so that the photosensitive drum 201 is uniformly charged to -700 V. By 
doing so, the vibration and the resultant noise of the photosensitive drum 
which is produced by application of an alternating electric field between 
the charging roller 2 and the drum, can be avoided, thus providing the 
user with better ambience. When the photosensitive drum 201 is discharged, 
the voltage applied to the exposure device 303 is not changed, and the 
quantity of the light of the exposure device is substantially constant. 
Similarly to the second embodiment, the photosensitive drum 201 has a 
circumferential length L6, and the transfer drum 209 has a circumferential 
length L5. The OHP sheet has a length L3. The distance between the 
pre-exposure device 303 and the transfer position as measured on the 
photosensitive drum 201 is L7. 
In this embodiment, the length of the feeding path from a point where the 
transfer material is separated from the transfer drum 209 to the fixing 
device 21 is shorter than the length of the transfer material, thus 
reducing the size of the apparatus. In addition, the circumferential 
length L6 of the photosensitive drum 201, the circumferential length L5 of 
the transfer drum and the length L3 of the OHP sheet and the distance L7 
between the pre-exposure position 303 and the transfer position, satisfy 
L5-L3+L7&gt;L6. More particularly, the diameter of the transfer drum 209 is 
120 mm (peripheral length L1 thereof is 376.8 mm), the diameter of the 
photosensitive drum 201 is 30 mm (the circumferential length L2 thereof is 
94.2 mm). The distance between the discharger for the photosensitive drum 
201 which is the pre-exposure 303 in this embodiment and the transfer 
position is 60 mm. The OHP sheet size is A4 (297 mm) (longitudinal 
feeding), or a letter size (279 mm) (longitudinal feeding). 
The transfer material fed out by the pick-up roller 31 is stopped by a 
registration roller 32 for the purpose of timed relation with the gripper 
18 of the transfer drum. The transfer material is refed in synchronism 
with the gripper 18 and is gripped thereby. Then, it is electrostatically 
attracted on the transfer drum to permit transfer operation. Then, three 
color images are transferred, and then the fourth color transfer is 
executed. In the plain paper mode, the leading edge of the transfer 
material is separated by separation claws 20 in the fourth color transfer 
operation. Thus, the leading edge of the transfer material is separated 
and fed to the fixing device, during the fourth color transfer operation. 
However, in the special sheet mode, the leading edge of the transfer 
material is not separated during the fourth color transfer operation, and 
the transfer drum 209 is rotated further through one full-turn while 
attracting thereon the transfer material. 
When such a position on the photosensitive drum as is going to correspond 
to the trailing edge of the fourth color image passes by the charging 
roller 2, the DC voltage applied to the charging roller 2 is stopped, and 
the photosensitive drum 201 is electrically discharged by the pre-exposure 
device alone. In this case, the discharging as already started at the 
pre-exposure position upstream of the charging roller 2. The charger and 
discharger for the photosensitive drum are rendered off when the 
photosensitive drum 201 is electrically discharged more than one full-turn 
and before the leading edge of the transfer material re-enter the transfer 
region. Thereafter, the speed of the photosensitive drum 201 and the 
transfer drum 209 are reduced down to the speed for sufficient fixing. 
The circumferential length L5 of the transfer drum 209 is 376.8 mm, and the 
length of the transfer material L3 is 297 mm, and therefore, the region on 
the transfer drum without the transfer material is 79.3 mm. On the other 
hand, the circumferential length L6 of the photosensitive drum 201 is 94.2 
mm, and therefore, L5-L3&lt;L6 is satisfied. Before the re-entering of the 
leading edge of the transfer material after the completion of the fourth 
color transfer, it is not possible to discharge the photosensitive drum 
201 more than one full-turn. However, if the DC voltage applied to the 
charging roller 2 is rendered off at the point of time when the position 
which is going to correspond to the trailing edge of the fourth color 
image passes by the charging roller 2, and by discharging the 
photosensitive drum 201 by the pre-exposure device 303 alone, the 
photosensitive drum 201 is discharged more than one full-turn before the 
leading edge of the transfer material enters the transfer position, the 
speeds can be further reduced. Since the length L7 from the pre-exposure 
position 303 is 60 mm, L5-L3+L7&gt;L6, and L5/L6.gtoreq.3. THerefore, an OHP 
sheet print having good transparency can be produced without 
inconveniences of charging memory, transfer memory or the like, while the 
diameter of the transfer drum is reduced, in this manner. 
In the foregoing embodiment, the use has been made with a charging roller 
as the charging means for the photosensitive drum, but corona charger, 
brush charger or another charging means is usable in place thereof. 
Embodiment 4 
Referring to FIG. 3, a multi-color image forming apparatus according to a 
fourth embodiment of the present invention will be described. In this 
embodiment, the diameter of the transfer drum 209 is 160 mm, and that of 
the photosensitive drum 201 is 40 mm. As for the OHP transfer sheet, the 
size thereof is A4 and is longitudinally fed (297 mm in the longitudinal 
direction), or a letter size and is longitudinally fed (279 mm). 
The transfer material fed out by the pick-up roller 31 is stopped by a 
registration roller 32 for the purpose of timed relation with the gripper 
18 of the transfer drum. The transfer material is refed in synchronism 
with the gripper 18 and is gripped thereby. Then, it is electrostatically 
attracted on the transfer drum to permit transfer operation. Then, three 
color images are transferred, and then the fourth color transfer is 
executed. In the plain paper mode, the leading edge of the transfer 
material is separated by separation claws 20 in the fourth color transfer 
operation. Thus, the leading edge of the transfer material is separated 
and fed to the fixing device, during the fourth color transfer operation. 
However, in the special sheet mode, the leading edge of the transfer 
material is not separated during the fourth color transfer operation, and 
the transfer drum 209 is rotated further through one full-turn while 
attracting thereon the transfer material. 
Subsequently, before the leading edge of the transfer material re-enter the 
transfer region, the chargers and dischargers for the photosensitive drum 
201 are rendered off, and thereafter, the speeds of the photosensitive 
drum 201 and the transfer drum 209 are reduced to such an extent that the 
OHP sheet is sufficiently fixed. More particularly, the normal process 
speed 100 mm/sec is reduced to 40 mm/sec. At this time, the DC voltage 
applied to the charging roller 2 and the pre-exposure device 303 are 
stopped. 
The OHP transfer material is separated at the separation point, and enters 
the fixing apparatus having been controlled to meet 40 mm/sec. 
After the trailing edge of the transfer material passes through the fixing 
device, the process speed returns to 100 mm/sec, and the pre-exposure 
device 303 is actuated to discharge the photosensitive drum 201 by one 
full-turn to substantially 0 V. Also, the transfer drum 209 is discharged 
by one full-turn to substantially 0 V by the discharging roller 24. 
Thereafter, the apparatus is stopped. 
FIG. 7 is a timing chart of the operation in this embodiment. 
By doing so, when the process speed is low, the charging, discharging and 
exposure operations are not carried out, but all the operations are 
carried out when the process speed is normal. In addition, the discharging 
is carried out immediately before stop of the apparatus, and therefore, 
the good images can be produced without charge memory, transfer memory or 
improper discharge of the transfer drum, while the size of the apparatus 
is reduced. 
Embodiment 5 
Referring back to FIG. 3, a multi-color image forming apparatus according 
to the fifth embodiment of the present invention will be described. 
In this embodiment, the driving source for the fixing device 21 is separate 
from that for the photosensitive drum 101 and the transfer drum 209. 
The transfer material fed out by the pick-up roller 31 is stopped by a 
registration roller 32 for the purpose of timed relation with the gripper 
18 of the transfer drum 209. The transfer material is refed in synchronism 
with the gripper 18 and is gripped thereby. Then, it is electrostatically 
attracted on the transfer drum 209 to permit transfer operation. Then, 
three color images are transferred, and then the fourth color transfer is 
executed. In the plain paper mode, the leading edge of the transfer 
material is separated by separation claws 20 in the fourth color transfer 
operation. Thus, the leading edge of the transfer material is separated 
and fed to the fixing device, during the fourth color transfer operation. 
However, in the special sheet mode, the leading edge of the transfer 
material is not separated during the fourth color transfer operation, and 
the transfer drum 209 is rotated further through one full-turn while 
attracting thereon the transfer material. 
Before the leading edge of the transfer material re-enter the transfer 
region, the charging and discharging devices for the photosensitive drum 
are stopped, and thereafter, the speeds of the photosensitive drum 201 and 
the transfer drum 209 are reduced to the level permitting sufficient 
fixing on the OHP sheet. More particularly, the normal process speed 100 
mm/sec is reduced to 400 mm/sec. At this time, the DC voltage applied to 
the charging roller 2 and the pre-exposure device 25 are stopped. 
The transfer material (OHP sheet) is separated at the next separation 
point, and is introduced into the fixing device 21 having been controlled 
to meet 40 mm/sec. 
Immediately after separation of the trailing edge of the transfer material 
from the transfer drum, the process speeds of the photosensitive drum 201 
and the transfer drum 209 are returned to the original 100 mm/sec. At this 
time, the transfer material is still in the fixing device, but the 
photosensitive drum 201 and the transfer drum 209 are driven by a device 
separate from the fixing device 21, and the speed of the fixing device 21 
remains unchanged until the transfer material is passed through, that is, 
40 mm/sec is maintained. 
Immediately after the speeds of the photosensitive drum 201 and the 
transfer drum 209 are returned to 100 mm/sec, the photosensitive drum 201 
is discharged by one full-turn, and the transfer drum 209 is discharged by 
discharging roller 24 by one full-turn, and thereafter, the apparatus is 
stopped. 
By doing so, the discharging for the photosensitive drum 201 and the 
transfer drum 209 can be started earlier, thus permitting reduction of the 
number of rotations until the stoppage. Therefore, the service lives of 
the photosensitive drum and the transfer drum can be expanded while the 
advantageous effects of Embodiment 1 are retained. 
Embodiment 6 
Referring to FIG. 1, a multi-color image forming apparatus according to the 
sixth embodiment will be described. In the fourth and fifth embodiments, 
the charging roller 2 is supplied with only a DC voltage and therefore 
functions only as a charger. A pre-exposure device 303 is provided so that 
the charging and discharging functions are separated. In the sixth 
embodiment, the pre-exposure device 303 is omitted, and the charging 
roller 2 is supplied with a DC voltage of -700 V biased with an AC voltage 
having a frequency of 1000 Hz and a peak-to-peak voltage Vpp of 2000 V. 
The photosensitive drum 1 is uniformly charged to -700 V. When the 
photosensitive drum 101 is discharged, the applied alternating voltage is 
maintained, and the DC voltage is set to approx. 0 V. 
With this structure, the pre-exposure device can be omitted so that the 
cost reduction is possible. 
The transfer material fed out by the pick-up roller 31 is stopped by a 
registration roller 32 for the purpose of timed relation with the gripper 
18 of the transfer drum. The transfer material is refed in synchronism 
with the gripper 18 and is gripped thereby. Then, it is electrostatically 
attracted on the transfer drum to permit transfer operation. Then, three 
color images are transferred, and then the fourth color transfer is 
executed. In the plain paper mode, the leading edge of the transfer 
material is separated by separation claws 20 in the fourth color transfer 
operation. Thus, the leading edge of the transfer material is separated 
and fed to the fixing device, during the fourth color transfer operation. 
However, in the special sheet mode, the leading edge of the transfer 
material is not separated during the fourth color transfer operation, and 
the transfer drum 109 is rotated further through one full-turn while 
attracting thereon the transfer material. 
Before the leading edge of the transfer material re-enter the transfer 
region, the charger and dischargers for the photosensitive drum are 
rendered off, and thereafter, the speeds of the photosensitive drum 101 
and the transfer drum 9 are reduced to such an extent that the fixing 
operation is sufficient on the OHP sheet. More particularly, the process 
speed is reduced from normal 100 mm/sec to 40 mm/sec. At this time, the DC 
voltage applied to the charging roller 2 and the pre-exposure device are 
stopped. 
The OHP sheet is separated at the next separation point, and is introduced 
to the fixing device already set to 40 mm/sec. 
After the trailing edge of the transfer material has passed through the 
fixing device, the process speed is returned to original 100 mm/sec, and 
the DC component of the bias voltage applied to the charging roller 2 is 
changed to approx. 0 V, by which the photosensitive drum 101 is discharged 
at least one full-turn, and the transfer drum 9 is also discharged by 
discharging roller 24 by at least one turn. Thereafter, the apparatus is 
stopped. 
By doing so, the charging and discharging operation are prevented from 
being carried out when the process speed is low, and such operations are 
all carried out when the process speed is normal. Immediately before the 
stoppage of the apparatus, the discharging operation is carried out. 
Therefore, good images can be provided without charge memory, improper 
discharge of the photosensitive drum or another inconveniences, while 
reducing the size and cost of the apparatus. 
According to 4-6 embodiments, the corona charger, brush charger or other 
charging means is usable in place of the charging roller. 
In the foregoing embodiments, the distance between the separation position 
of the transfer drum 9 and the fixing position can be set smaller than the 
special sheet such as OHP sheet or the like, as measured in the direction 
of the feeding of the transfer material. 
As another example of the special speed, there is a thick sheet of paper 
having a larger basis weight than usual sheets. 
While the invention has been described with reference to the structures 
disclosed herein, it is not confined to the details set forth and this 
application is intended to cover such modifications or changes as may come 
within the purposes of the improvements or the scope of the following 
claims.