Image forming apparatus

An image forming apparatus has a photosensitive member for carrying an image, an exposure device for exposing the photosensitive member to form a latent image on the same, a development device for developing the latent image on the photosensitive member with a toner to form a toner image, a transfer member carrier for supporting and conveying a transfer member to a transfer position on the phtosensitive member, a first conveyance mechanism for conveying the transfer member to the transfer carrier member, and a second conveyance mechanism for conveying the transfer member to the first conveyance mechanism. A cancellation mechanism is provided to cancel a transfer member conveying force of the first conveyance mechanism after a leading end of the transfer member conveyed by the first conveyance mechanism has been supported on the transfer member carrier. A trailing end of the transfer member passes through a nip of the second conveyance mechanism during a period of time when the transfer member conveying force of the first conveyance mechanism is canceled by the cancellation mechanism and when the exposure device is not operating. In this arrangement, vibration generated when the transfer member is conveyed to the transfer member carrier by the first and second conveyance mechanisms is prevented from affecting the latent image formation made by the exposure device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus such as a 
printer, a facsimile machine or a copying machine and, more particularly, 
to a multiple image forming apparatus in which an image is formed on a 
transfer member carried by a transfer member carrier. 
2. Description of the Related Art 
Conventionally, image forming apparatuses have been used in which a toner 
image formed on a photosensitive drum is transferred to a transfer member 
carried and transported by a transfer drum to complete image formation. 
FIG. 14 shows an example of such conventional image forming apparatuses. 
The process of the recording operation of the image forming apparatus shown 
in FIG. 14 will be described in detail. When the apparatus starts 
operating, one of recording paper sheets is fed from a paper feeder unit 
and is conveyed to register rollers 102 by pre-conveyance rollers. At this 
time, the register rollers 102 are stopped. The conveyance operation of 
the pre-conveyance rollers is continued for a short time after the moment 
at which the leading end of the recording paper sheet is brought into 
abutment against the register rollers 102. When the recording paper sheet 
forms a predetermined loop (hereinafter referred to as "pre-register 
loop"), the pre-conveyance rollers are stopped. 
Thereafter, the register rollers 102 and the pre-conveyance rollers are 
rotated in synchronization with an image recording start signal to feed 
the recording paper sheet onto a transfer drum 101. This operation is 
performed to enable an image to be formed at a predetermined position on 
the recording paper sheet. 
On the other hand, a latent image is formed on a photosensitive drum 100 by 
performing image exposure with laser light E on the basis of an image 
signal. This latent image is developed with a toner. 
The recording paper sheet is conveyed to an image transfer position T while 
being supported on the transfer drum 101, and the toner image on the 
photosensitive drum 100 is recorded by being transferred onto the surface 
of the recording paper sheet. The recording paper sheet is supported on 
the transfer drum 101 by one of several possible methods, i.e., an 
electrostatic attraction method, an air suction method and the like. 
If in the thus-arranged apparatus a difference occurs between the speeds of 
the photosensitive drum 100 and the transfer drum 101, a deterioration is 
caused in the quality of the resulting image, that is, the transferred 
image is expanded or contracted or is blurred by rubbing. Ordinarily, to 
avoid this problem, the two drums are driven by the same drive source and, 
more preferably, the two drums are linked and the driving force is 
directly transmitted to them so that the speed difference therebetween is 
minimized. 
If there is a difference between the recording paper conveying speed of the 
register rollers 102 and the peripheral speed of the transfer drum 101, a 
problem described below arises. If the recording paper conveying speed of 
the register rollers 102 is lower than the peripheral speed of the 
transfer drum 101, the recording paper sheet pulls the transfer drum 101 
to change the speed of this drum after being supported thereon. It is also 
possible that, in the worst case, the recording paper sheet peels off the 
transfer drum 101. 
Conversely, if the recording paper conveying speed of the register rollers 
102 is higher than the peripheral speed of the transfer drum 101, the 
recording paper sheet starts forming a loop between the register roller 
102 and the transfer drum 101 to cause and increase a force of pressing 
the transfer drum 101. When this force exceeds a certain magnitude, the 
speed of the transfer drum 101 is changed. This change causes a change in 
the speed of the photosensitive drum 100 linked to the transfer drum 101, 
resulting in expansion or contraction of the latent image formed during 
sheet conveyance. 
To cope with this problem, a method has generally been practiced in which 
the recording paper conveying speed of the register rollers 102 is set 
slightly higher than the peripheral speed of the transfer drum 101 at the 
stage of designing so that there is no possibility of the recording paper 
conveying speed of the register roller 102 being lower than the peripheral 
speed of the transfer drum 101 with respect to any dispersion in certain 
part accuracy ranges. This is because only a small relative reduction in 
the recording paper conveying speed of the register rollers results in a 
considerable deterioration in image quality while a relative increase in 
the recording paper conveying speed in certain allowance is negligible. 
A mechanism for setting the recording paper sheet free by separating a pair 
of register rollers 102 from each other has also been provided. This is 
intended to separate the register rollers 102 before a loop of the 
recording paper sheet between the register rollers 102 and the transfer 
drum 101 (hereinafter referred to as "post-register loop") formed by 
earlier registration pushes the transfer drum 101 (this separating 
operation hereinafter referred to as "register release") in order to 
increase the looping space so that the force of the recording paper sheet 
in the looping state is reduced. 
However, the recording paper conveying speeds of the pre-register rollers 
and the register roller 102 are ordinarily set to the same speed because 
the pre-register rollers and the register rollers 102 simultaneously 
convey the same sheet for a certain period of time. Accordingly, the 
recording paper conveying speed of the pre-conveyance rollers is slightly 
higher than the peripheral speed of the transfer drum 101 and, therefore, 
a loop is also formed and increased between the pre-conveyance rollers and 
the transfer drum 101. This loop ceases to exist instantaneously when the 
trailing end of the recording paper sheet passes the pre-conveyance 
rollers. Then, the force acting on the transfer drum 101 changes abruptly 
to cause a change in the speed of the transfer drum 101. The shock of such 
a change may also be transmitted to the photosensitive drum 100 to affect 
the latent image during the formation on the photosensitive drum 100. 
By considering this problem, a method may be used in which the pre-register 
or post-register looping space is maximized so that the curvature of the 
loop is reduced if the amount of loop is constant. However, it is 
difficult to provide such a large looping space and to limit the increase 
in the overall size of the apparatus. On the other hand, there is a 
limitation upon simply reducing the amount of loop since a certain effect 
of use of the loop must be achieved. 
Recently, apparatuses having more complicated sheet feeder units have 
generally been developed. Such apparatuses require an elongated and 
complicated sheet path from the feeder unit to the register rollers. Under 
such circumstances, variation in sheet conveyance speed tends to increase 
and a certain amount of loop is indispensable also for absorbing variation 
in sheet conveyance speed. 
A mechanism for removing the recording paper conveying speed may be 
provided for the pre-register rollers as well as for the register rollers 
to prevent occurrence of vibration when the trailing end of the recording 
paper sheet passes the pre-register rollers, as disclosed in EP 0 480 454 
A2. Use of such a mechanism, however, necessitates an increase in 
manufacturing cost and is also disadvantageous in terms of space factor. 
SUMMARY OF THE INVENTION 
In view of these problems, an object of the present invention is to provide 
an image forming apparatus capable of performing image formation always 
suitably without being affected by vibration generated when a transfer 
member is conveyed to a transfer member carrier. 
Another object of the present invention is to provide an image forming 
apparatus in which transmission of vibration of a sensitive member is 
prevented during latent image formation to ensure good performance of 
latent image formation. 
Still another object of the present invention is to achieve the above 
objects without providing any special mechanism for transfer member 
conveyance means. 
These and other objects and features of the present invention will become 
apparent from the following detailed description of the invention made 
with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An image forming apparatus in accordance with the present invention will be 
described in detail with reference to the accompanying drawings. 
Embodiment 1 
The image forming apparatus in accordance with first embodiment of the 
present invention will be described with reference to FIGS. 1 through 9. 
The image forming apparatus described below has an electrophotographic 
type image forming system and is arranged as a multicolor image forming 
apparatus which is assumed to be one in which the present invention can be 
realized most effectively. 
The color image forming apparatus shown in FIG. 1 has an upper section 
constructed as a digital color image reader and a lower section 
constructed as a printer. 
In the reader section, the operation is started by an image recording start 
signal to expose and scan an original 30 on an original table glass 31 
with an exposure lamp 32. Reflected image light from the original 30 is 
condensed to a full-color sensor 34 by a lens 33 to obtain a color 
decomposed image signal. This signal is amplified by an amplifier (not 
shown), undergoes various processing in a video processing unit (not 
shown), and is sent to the printer section. 
The printer section is constructed as described below. A photosensitive 
drum provided as an image carrier is axially supported at its center and 
is driven by a motor (not shown) to rotate in the direction of the arrow. 
An exposure optical system 3 formed of a pre-exposure lamp 11, a primary 
charging device 2, a laser device and other components, a potential sensor 
12, a development device 4, an on-drum light quantity detection means 13, 
a transfer drum 5 and a cleaning device 4 are arranged in this order along 
the direction of rotation of the photosensitive drum 1 so as to face the 
outer circumferential surface of the photosensitive drum 1. 
The surface of the photosensitive drum 1 is uniformly charged by the 
exposure lamp 11 and the primary charging device 2. Thereafter, the 
surface of the photosensitive drum 1 is exposed to light E, e.g., a laser 
beam, which is modulated with a recording image signal and scanned by a 
polygon mirror 3A, a lens 3B and other components of the optical system 3, 
thereby forming an electrostatic latent image on the photosensitive drum 
surface. The electrostatic latent image is developed by the development 
device 4. In the development device 4, four units 4Y, 4C, 4M, and 4Bk 
containing developers, i.e., toners for four colors, yellow, cyan, magenta 
and black, respectively, are arranged. The development device 4 is 
constructed so that each of these units can be selectively brought close 
to the photosensitive drum 1 by the operation of eccentric cams 24Y, 24C, 
24M, and 24Bk. 
The image formed by these toners is transferred to a recording paper sheet 
in a recording region. Residual toner on the photosensitive drum 1 after 
transfer to the recording paper sheet is scraped off by the cleaning 
device 6 on the downstream side of the image transfer region. Image 
formation with toners is performed by the above-described process. 
This image forming apparatus has three feeder cassettes, i.e., upper, 
middle and lower feeder cassettes 41A, 41B, and 41C, and one manual 
insertion cassette 41T. The three feeder cassettes 41A, 41B, and 41C have 
the same construction. In the following, only the middle feeder cassette 
41B will be described as a representative example. 
A sheet feeder unit 40 is formed of the middle feeder cassette 41B, a 
pickup roller 42 for feeding recording paper sheets P one by one out of 
the cassette 41B, and means for conveying each recording paper sheet P fed 
out by the pickup roller 42 to a transfer and conveyance unit 50, i.e., 
feed rollers 43A and 43B, conveyance rollers 45A, 45B, and 45C, sheet 
guides and other components. The manual insertion unit is formed of the 
manual insertion tray 41T, a feed roller 42M, a separating pad (not shown) 
and other components. 
The transfer and conveyance unit 50 has a conveyance section 50A and the 
transfer drum 5 as two essential portions. The conveyance section 50A will 
first be described with reference to FIG. 2. The conveyance section 50A 
has guides 48A, 48B, 48C, 48D, and 48E, conveyance rollers 46A, 46B, and 
46C, and a pair of pre-register conveyance rollers 47 for guiding 
recording paper sheet P from the sheet feeder unit 40, and register means 
having a pair of register rollers 44 for feeding recording paper sheet P 
to the transfer drum 5 in accordance with the timing of image formation in 
an image forming section, and attraction guides 48F and 48G for guiding 
recording paper sheet P in association with the register rollers 44. 
The conveyance rollers 46A, 46B, and 46C have a curling function for 
enabling the recording paper sheet to be easily wrapped around the 
transfer drum 5. Each of the guides 48B and 48D has a rounded shape such 
as to change the direction in which the recording paper sheet advances and 
is supported so as to be swingable in the direction of the arrow indicated 
in FIG. 2 (these guides hereinafter referred to as swingable guides) in 
order to provide a sufficient space for looping of recording paper sheet P 
caused when the leading end of the recording paper sheet is temporarily 
stopped by the register rollers 44. 
The construction of a mechanism including the register rollers 44 will be 
described with reference to FIGS. 3 and 4. Referring to FIG. 3, the pair 
of register rollers 44 consist of a metallic roller 44A and a rubber 
roller 44B disposed parallel to each other, and these two rollers are 
pressed against each other by springs (not shown). The rubber roller 44B 
is supported at its opposite ends by bearings in elongated holes formed in 
side plates 44C (the side plate 44C and the bearing on the foreground side 
not illustrated in FIG. 2) so as to be separable from the metallic roller 
44A. A cam shaft 44E is supported on the side plates 44C parallel to the 
metallic roller 44A and the rubber roller 44B, and two release cams 44D 
are fixed on the cam shaft 44E at a certain distance from each other. A 
gear 44F and a spring clutch 44G for transmitting a driving torque from a 
unit body to the cam shaft 44E are fixed on one end of the cam shaft 44E. 
Swingable arms 44J are provided in correspondence with the release cams 
44D. One end of each swingable arm 44J contacts the corresponding one of 
the release cams 44D while the other end is inserted into the spacing 
between end portions of the metallic rollers 44A and the rubber roller 
44B. 
In the thus-constructed mechanism, the gear 44F and the spring clutch 44G 
(and an unillustrated plunger) are operated in association with each other 
to selectively rotate each of the release cam 44D and to stop the same 
after rotation through 180.degree.. During rotation, each release cam 44D 
presses the end of the corresponding swingable arm 44J to cause the 
swingable arm 44J to swing on an arm center shaft 44H. With this swinging 
motion of the swingable arm 44J, the other end of the swingable arm 44J is 
brought into contact with the rubber roller 44B to displace the same 
against the force of the press-contact spring. The rubber roller 44B is 
thereby moved apart from the metallic roller 44A. That is, the position at 
which the two rollers 44A and 44B are pressed against each other and the 
position at which the two rollers 44A and 44B are moved apart from each 
other are set in accordance with the phase of the stoppage of the release 
cams 44D. This mechanism enables the register rollers 44 to effect 
register release by a timing described later. It is not always necessary 
to effect register release by separating the rollers 44A and 44B, and 
register release may be effected by reducing the pressure of contact 
between the rollers 44A and 44B to zero while the rollers 44A and 44B are 
maintained in contact with each other. 
The transfer drum 5 will next be described with reference to FIG. 5. As 
illustrated, the transfer drum 5 has a frame 55 formed of two annular 
members 55A and a connecting member 55B which connects these annular 
members 55A. A recording paper supporting sheet 55S is cylindrically 
wrapped around the frame 55F. The recording paper supporting sheet 55S is 
made of, for example, polyethylene telephthalate (PET) or polyvinylidene 
fluoride (PVF). 
Referring again to FIG. 2, an attraction roller 5G is supported so as to be 
able to contact and move apart from the transfer drum 5 at a position 
where recording paper sheet P fed from the register rollers 44 is brought 
into abutment against the transfer drum 5. Also, an attraction charging 
device 5C including a backup member for resisting a pressing force of the 
attraction roller 5G is disposed on the side of the recording paper 
supporting sheet 55S opposite from the attraction roller 5G. 
As also shown in FIG. 1, a transfer charging device 5B is also disposed on 
the reverse side of the recording paper supporting sheet 55S in an image 
region where the photosensitive drum 1 and the transfer drum 5 are face 
each other. On the downstream side of the transfer charging device 5B, 
separating members for separating the recording paper sheet, more 
specifically a charge-removing charging device 5H, a separating claw 8A 
and a sheet push-up roller are disposed. Subsequently, charging devices 5D 
and 5E for removing charge from the recording paper supporting sheet are 
disposed with the recording paper supporting sheet interposed 
therebetween. Further, on the downstream side of these charging devices 
are disposed a brush roller 14 and a corona discharging device or a brush 
type charge removing device 15 for cleaning the recording paper supporting 
surface of the supporting sheet by removing toner, paper particle and the 
like attached to the recording paper supporting surface. It is possible 
that oil used in a fixation unit 9 is attached to the recording paper 
supporting sheet surface through the recording paper sheet during 
later-mentioned both side recording. An oil removing roller 16 and a 
backup brush for cleaning the recording paper supporting sheet by removing 
attached oil are also provided. 
The transfer drum 5 can be brought into contact with or moved apart from 
the photosensitive drum 1. When the ordinary operation of the apparatus is 
not performed, the transfer drum 5 is maintained apart from the 
photosensitive drum 1 in order to prevent the transfer drum 5 and the 
photosensitive drum 1 from contacting each other for a long time as well 
as to prevent damage to the surface of the photosensitive drum 1 caused by 
contact with the recording sheet or any other matter during jam removal 
operation. The transfer drum S and the photosensitive drum 1 have gear 
flanges at their corresponding ends, which mesh with each other to 
transmit a torque of the photosensitive drum 1 to the transfer drum 5 when 
these drums contact each other. 
Further, in this apparatus, the transfer and conveyance unit 50 is 
constructed so as to be forwardly drawable out of the main body in order 
to perform jam removal or maintenance operation reliably and easily. 
The fixation unit 9 has a fixation roller 9A having an internal heat source 
such as a halogen heater, a pressing roller 9B which is pressed against 
the fixation roller 9A (which may also has a heat source), a conveying 
belt 9C and an entrance guide 9D for leading the recording paper sheet to 
the nip between the rollers 9A and 9B, and a sheet discharge roller 9E for 
leading the recording paper sheet discharged from the rollers 9A and 9B to 
the outside of the apparatus. 
A mechanism for automatically recording images on the two surfaces of the 
recording paper sheet will now be described. A conveyance path changeover 
guide 19 is disposed immediately downstream of the fixation unit 9. After 
fixation, one of a recording sheet path for discharging the recording 
paper sheet out of the apparatus body and another recording sheet path for 
both side recording is automatically selected by the conveyance path 
changeover guide 19. The both side recording sheet path is formed by a 
vertical conveyance path 20, an inverting path 21A, an inverting roller 
21B, and an intermediate tray 22. The recording paper sheet having an 
image recording on its one surface is led to temporarily enter the 
inverting path 21A and is conveyed out of the inverting path 21A in the 
direction opposite to the entering direction by the reverse rotation of 
the inverting roller 21B to be received by the intermediate tray 22 with 
its trailing end at the time of introduction reversely set as leading end. 
Thereafter, the recording paper sheet is fed again out of the intermediate 
tray 22 and an image is formed on the other surface of the recording paper 
sheet by the above-described image forming process. A plurality of guides 
23 are arranged above the intermediate tray 22 so as to be selectively 
movable downwardly or upwardly according to the recording paper size, 
thereby enabling recording paper sheets of any size to be accommodated and 
stacked in the intermediate tray 22 with the sheet ends aligned with a 
head position of the intermediate tray 22. 
A control unit (not shown) is provided with a control circuit board and a 
motor drive circuit board for controlling the operation of the 
above-described mechanism in each unit. 
The operation of the image forming apparatus constructed as described above 
will be described with respect to a case where paper sheets are fed from 
the intermediate cassette 41B. 
Referring to FIG. 1, when the image formation start signal is generated, a 
cam 71 rotates through 180.degree.. By this cam rotation, the transfer 
drum 5 moves swingingly to an operating position at which the gears on the 
flanges of the transfer drum 5 and the photosensitive drum 1 mesh with 
each other, and the transfer drum 5 rotates in synchronization with the 
photosensitive drum 1. On the other hand, recording paper sheets P in the 
intermediate cassette 41B are fed out one by one by the pickup roller 42. 
Each recording paper sheet P from the cassette 41B is conveyed between the 
feed guides to the conveyance section 50A by the feed rollers 43A and 43B 
and conveyance rollers 45B and 45C. 
In the conveyance section 50A, as shown in FIG. 2, the recording paper 
sheet P curls by a predetermined amount in a direction such as to be 
wrapped around the transfer drum 5 when it passes the conveyance rollers 
46A, 46B, and 46C which serve as curling rollers. Thereafter, the 
recording paper sheet P is conveyed to the register rollers 44 while the 
direction of conveyance is changed by the functions of the pre-register 
conveyance rollers 47. During this conveyance, the register rollers 44 are 
stopped and the leading end of the recording paper sheet is brought into 
abutment against the nip between the register rollers 44. 
The pre-register conveyance rollers 47 are driven for a certain period of 
time after the abutment, and are then stopped. A pre-register loop of the 
recording paper sheet is thereby formed. The space necessary for forming 
this loop is provided by the swinging motion of the swingable guides 48B 
and 48D in the feed guides 48A to 48E. This swinging motion is 
automatically made in such a manner that an intermediate portion of the 
recording paper sheet presses and outwardly displaces the guides as 
indicated at R in FIG. 2. 
In this embodiment, the amount of pre-register loop is set to 8 mm. This 
value represents the amount of feeding further made after the leading end 
of the recording paper sheet has been brought into abutment against the 
register rollers 44. This feeding is required to ensure desired stability 
of feeding from the lower cassette 41C (FIG. 1) in which a largest 
variation may be exhibited in the time taken for the recording paper sheet 
to reach the register section. 
Thereafter, at a certain time determined on the basis of the time at which 
image forming section starts image formation, the register rollers 44 and 
the pre-register conveyance rollers 47 start rotating. This rotating 
timing is set so that the recording paper sheet and the toner image on the 
photosensitive drum 1 coincide with each other at the image transfer 
region. 
When the recording paper sheet P is brought into abutment against the 
transfer drum 5, it is electrostatically attracted onto the recording 
paper supporting sheet 55S by corona discharge from the attraction 
charging device 5C and by the operation of the attraction roller 5G. The 
attraction roller 5G is normally set apart from the transfer drum 5 to 
reduce the load upon the transfer drum 5, and is pressed against the 
transfer drum only when the recording paper reaches the transfer drum 5. 
The transfer drum 5 is rotated in synchronization with the photosensitive 
drum 1, and the recording paper sheet is conveyed to the image transfer 
region while being supported on the transfer drum 5. The toner image 
formed on the photosensitive drum 1 by the above-described process is 
transferred onto the surface of the transfer drum 5 by the transfer 
charging device 5B, and the recording paper sheet is thereafter conveyed 
to the separation section. 
The image in one color is thus transferred. Then, if the image to be 
completed is a monochromic image, the recording sheet is separated by the 
operation described below. If a multicolor image is to be formed, the 
separating operation is not performed and the recording sheet is again 
conveyed to the transfer region by making one revolution while being 
supported on the transfer drum. The next toner image is then transferred 
by being superposed on the preceding toner image. These steps are repeated 
a necessary number of times. The brush roller 14, etc., for cleaning, 
which are supported so as to be able to contact and move apart from the 
transfer drum 5 surface, are set apart from the transfer drum 5 surface at 
least during the above-described multi-transfer process. 
As illustrated in FIG. 1, when the above-described process is completed, 
the attraction force between the recording paper sheet and the recording 
paper supporting sheet 55S is reduced by the operation of the 
charge-removing charging device 5H in the separation section, and the 
recording paper sheet P is thereafter separated from the transfer drum 5 
by the operation of the separating claw 8A, etc. 
The recording paper sheet separated from the transfer drum 5 is conveyed to 
the fixation roller section by the conveying belt 9C and is accurately 
guided along the entrance guide 9D to the nip between the rollers. The 
toner images are fixed on the paper surface by the heat from the fixation 
roller 9A. Thereafter, the recording paper sheet is conveyed by the 
discharge roller 9E to be discharged out of the apparatus. Finally, the 
transfer drum 5 is moved apart from the photosensitive drum 1 to a 
receding position and the operation of the apparatus is stopped. 
The operation of the image forming apparatus for both side recording will 
next be described. Referring to FIG. 1, when the changeover guide 19 is 
operated, recording paper sheet P having a recorded image fixed on one of 
its two surfaces is led to the vertical conveyance path 20, and is 
thereafter conveyed in the opposite direction through the inverting path 
21A by the inverting roller 21B to be accommodated in the intermediate 
tray 22. After a necessary number of one-side-recorded recording paper 
sheets P' have been stacked in the intermediate tray 22, one-side-recorded 
sheets P' are fed out of the intermediate tray 22 one by one by the pickup 
roller 25, and each recording sheet P' is conveyed to the conveyance 
section 50A by the feed rollers 26A, 26B, etc. The same process as that 
described above is thereafter performed to record an image on the other 
surface of the recording paper sheet. Finally, the discharge operation is 
performed without operating the changeover guide to discharge the 
recording sheet having recorded images fixed on its two surfaces out of 
the apparatus. 
The operation from the step of conveying each recording paper sheet from 
the pre-register conveyance rollers 47 to the step of transferring the 
image onto the recording paper sheet will be described in further detail 
with reference to FIGS. 6(A) to 6(C), which show three stages of this 
process; FIG. 6(A) shows a state immediately after the time when the 
register rollers 44 starts rotating after the formation of the 
pre-register loop, FIG. 6(B) shows a state immediately after the time when 
the register rollers 44 are released after the attraction of the recording 
paper sheet to the transfer drum 5, and FIG. 6(C) shows a state 
immediately after the time when the recording paper sheet passes the 
pre-register conveyance rollers 47. In the state (C), the loop has 
disappeared. 
Referring to FIGS. 6(A), 6(B), if the distance between a latent image 
formation position E to an image transfer position T along the 
circumferential surface of the photosensitive drum 1 is L.sub.L, and if 
the paper path length from the nip between the pre-register conveyance 
rollers 47 and the image transfer position T along the recording paper 
sheet is L.sub.P, then L.sub.L &gt;L.sub.P. If the distance between latent 
images in the case of successively making latent image formation by 
inputting another image formation start signal (this distance 
substantially equal to the interval between fed recording paper sheets in 
the case of successively making image formation on a plurality of 
recording paper sheets) is L.sub.S, then L.sub.P &gt;(L.sub.L -L.sub.S) is 
established. 
The operation in such a case will further be described with reference to 
the timing chart of FIG. 7. First, the surface of the photosensitive drum 
1 is exposed to laser light to continue latent image formation for a time 
period L.sub.G /V [A]. V represents the sensitive drum speed of the 
transfer drum speed. 
On the other hand, the leading end of the recording paper sheet is brought 
into abutment against the nip between the pre-register conveyance rollers 
47 with a small delay from the start of latent image formation, about 
(L.sub.L -L.sub.P)/V, and the conveying rotation is continued for a time 
period corresponding to the recording sheet length. Actually, the rotation 
is stopped for a short time after the formation of the pre-register loop 
during this period. However, this stop time is very short and negligible 
in the timing chart [D]. 
The recording paper sheet reaches the register rollers 44 with a further 
delay from the moment at which the leading end of the recording paper 
sheet is brought into abutment against the pre-register conveyance rollers 
47. After the formation of the pre-register loop, the operation of driving 
the register rollers 44 is started. At a time when the recording sheet 
leading end advances several ten millimeters after the moment at which it 
passing the attraction roller 5G (point Q) , register release is effected 
(point R) and the rotation is simultaneously stopped [C] 
Thereafter, the image and the recording sheet coincide with each other at 
the transfer position T (point T), and image transfer is effected [B]. 
The operation of this apparatus will now be described with respect to a 
feature of the present invention with reference to changes with time in 
the force of the recording paper sheet acting on the transfer drum 5 shown 
in FIG. 8. A value F1 in FIG. 8 represents a lower limit of the influence 
upon image formation of the force acting on the transfer drum 5. 
As shown in FIG. 8, latent image formation is not made at the moment when 
the trailing end of the recording paper sheet passes the nip between the 
pre-register conveyance rollers 47 (point S). That is, even if the 
magnitude of the shock thereby caused exceeds F1, there is no influence 
upon the latent image formation. During the period of time between the 
recording paper sheet attraction to the transfer drum 5 (point Q) and the 
register release (point R), the loop is increased between the register 
rollers 44 and the transfer drum 5 so that the force acting on the 
transfer drum 5 is gradually increased. However, register release has been 
effected at the time T.sub.O when F1 is exceeded (point R), and there is, 
therefore, substantially no influence upon the image. 
After the register release (point R), the loop between the pre-register 
conveyance rollers 47 and the transfer drum 5 is gradually increased. With 
respect to this state, a method will suffice in which an optimal loop 
space is maintained or the sheet conveying speed of the rollers is 
selected so that the loop does not cause a force exceeding F1 in a time 
period t.sub.r through which latent image formation is continued after 
register release. It is not difficult to achieve such an effect. For 
example, it is possible to use a method described below. In this 
embodiment, the pre-register conveyance rollers 47 are temporarily stopped 
after the formation of the pre-register loop, and the register rollers 44 
and the pre-register conveyance rollers 47 are thereafter driven 
simultaneously. The pre-register loop is not reduced by such driving. In 
contrast, if the rotation of the pre-register conveyance rollers 47 is 
started a certain time period after the moment at which the rotation of 
the register rollers 44 is stopped, then the pre-register loop is reduced 
by an amount corresponding to the time period through which the 
pre-register conveyance rollers 47 are stopped, so that the force of the 
recording paper sheet acting on the transfer drum 5 after register release 
can be reduced, as indicated by the double-dot-dash line in FIG. 8. There 
is no problem in terms of sheet conveyance performance since the 
pre-register loop is necessary only before the register rollers 44 start 
rotating. 
The image forming apparatus arranged as described above have advantages 
described below. At a moment when the trailing end of the recording paper 
sheet passes the nip between the pre-register conveyance rollers 47 in the 
register released state, a certain shock is transmitted to the transfer 
drum 5 and the photosensitive drum contacting the transfer drum 5. This 
shock, however, does not affect the image formation since latent image 
formation is not made at that moment. 
In this embodiment, a latent image can be prevented from being disturbed 
while no special mechanism for the pre-register conveyance rollers 47 is 
provided. It is also possible to reduce the force of the recording paper 
sheet acting on the transfer drum 5 and to reduce the necessary loop space 
by differentiating the time at which driving of the register rollers 44 is 
started and the time at which driving of the pre-register conveyance 
rollers 47 is started. 
Specific examples of the path lengths of the components of this embodiment 
will be described. In the type of apparatus having a plurality of 
development devices fixed in combination around the photosensitive drum as 
described above, the latent image formation position on the photosensitive 
drum is naturally set in a certain range if the various mechanisms 
necessary for image formation are arranged so that the overall size of the 
apparatus is reduced. For example, in this embodiment, the 
exposure-transfer path length L.sub.L =320 mm. Also, the recording sheet 
interval is determined by the specifications of the apparatus, and its 
value is L.sub.S =110 mm. Then, it is necessary that the path length 
L.sub.P between the pre-register conveyance rollers 47 and the transfer 
position T satisfies 320&gt;L.sub.P &gt;210 mm. In this embodiment, however, 
L.sub.P =250 mm. 
For limitation of the overall size of the apparatus, it is preferred that 
the point Q at which the recording paper sheet is attracted to the 
transfer drum 1 surface is brought close to the transfer position T. 
However, a certain path length is required to stabilize the effect of 
attracting the paper sheet. It has been experimentally found that the path 
length between the attraction position and the transfer position T has an 
optimal value of 90 mm. Accordingly, the paper path length between the 
pre-register conveyance rollers 47 and the attraction position is 160 mm. 
The selection of an optimal position of the register rollers 44 when the 
paper path length between the pre-register conveyance rollers 47 and the 
attraction positions is 160 mm as mentioned above will be described below. 
In determining the position of the register rollers 44, it is important to 
minimize the influence of the pre-register and post-register loops upon 
the transfer drum 5. It is necessary to set the amount of pre-register 
loop to 5 to 8 mm because of the need to correct biasing of the recording 
paper sheet as mentioned above. Conversely, the post-register loop is 
gradually formed due to a small difference between the speeds of the 
register rollers 44 and the transfer drum 5, and its amount is smaller 
than 1 mm at the maximum. 
By considering these conditions, it can be said that enlargement of the 
space for the pre-register loop is more effective in reducing the 
influence upon the transfer drum 5. Generally, for enlargement of the 
pre-register loop, it is preferred that path length between the register 
roller 44 position and the attraction position is smaller than that 
between the register roller 44 position and the pre-register conveyance 
rollers 47, although the suitable the register roller 44 position varies 
to some extent depending upon the guide configuration. In this embodiment, 
the ratio of the path length between the pre-register conveyance rollers 
47 and the register rollers 44 and the path length between the register 
rollers 44 and the attraction position is determined as 3:2, that is, the 
former length is set to 96 mm while the latter length is set to 64 mm. 
The forces acting on the transfer drum 5 before and after register release 
are well balanced thereby, as shown in FIG. 9. As a result, the force can 
be reduced at least through the entire period during latent image 
formation. 
However, the ratio of these path lengths is not limited to 3:2. For 
example, in this apparatus, the path length between the register rollers 
44 and the attraction position is changed between 74 mm and 54 mm, or the 
path length between the pre-register conveyance rollers 47 and the 
register rollers 4 is changed between 86 mm and 106 mm. That is, an 
experiment was made by disposing the register rollers 44 at two positions 
of 64.+-.10 mm to obtain the following result. 
As shown in FIG. 9, in either case, F1 is not exceeded at least during 
latent image formation, and there is therefore no influence upon the image 
formation. These two positions correspond to path length ratios of 7:6 and 
2:1. The value of the path length between the pre-register conveyance 
rollers 47 and the register rollers 44 ranging from 86 to 106 mm was 
determined by setting the path length between the pre-register conveyance 
rollers 47 and the attraction position to 160 mm. However, this setting 
was selected by considering the reduction in the overall size of the 
apparatus. If it is not necessary to limit the apparatus size, the 
probability of occurrence of deterioration in image quality or the like is 
rather reduced if the path length is longer. Consequently, the above value 
may be set to 86 mm or longer. 
Embodiment 2 
The first embodiment has been described by assuming the condition for 
eliminating the influence upon image formation of a shock caused at the 
time of register release is satisfied. However, it is possible that a 
shocked non-uniformity occurs in the resulting image depending upon the 
construction of the image forming apparatus to which the present invention 
is applied. The second embodiment of the present invention, which is 
intended to avoid such a problem, will be described with reference to 
FIGS. 10 to 12. 
This embodiment differs from the first embodiment in that the larger space 
provided between guides 48H and 48F for guiding the recording paper sheet 
to the attraction roller 5G is larger than that in the first embodiment. 
For ease of understanding, the guide of the first embodiment is indicated 
by the double-dot-dash line. In this arrangement, the shape of the 
post-register loop formed is moderated so that the rate at the force 
acting the transfer drum 5 is increased is reduced in comparison with the 
first embodiment. 
More specifically, referring to FIG. 12, the time t.sub.0 at which the 
force F1 acting on the transfer drum is reached is set after the 
completion of latent image formation. 
Further, in this embodiment, the time at which register release is effected 
(point R) is changed to a time after the completion of latent image 
formation, as shown in FIGS. 11 and 12, thereby eliminating the 
possibility of application of a force equal to or larger than F1 to the 
transfer drum at least during latent image formation. As a result, there 
is no influence upon the image formation even if the shock at the time of 
register release is large, and it is therefore possible to obtain a good 
image free from shocked non-uniformity. 
With respect to the manner of increasing the space formed by the guides 48F 
and 48H, it is desirable to limit the increase in the gap on the 
attraction roller 5 side. If the distance between the guides 48F and 48H 
on the attraction roller 5 side is excessively increased, the recording 
paper sheet flutters when it enters the attraction section, thereby 
increasing the probability of a reduction in attraction position accuracy. 
Since the time t.sub.0 at which the force F1 acting on the transfer drum 5 
is reached is after the completion of latent image formation as mentioned 
above, there is substantially no problem even if the paper conveyance is 
performed without register release. However, as can be understood from the 
broken line in [C] of FIG. 11, the next latent image formation is usually 
started at the moment when the trailing end of the recording paper sheet 
passes the register rollers 44. Conversely, if the apparatus is arranged 
so that the period for the operation of each of the pair of pre-register 
conveyance rollers 47 and the pair of register rollers 44 is set 
completely out of the period for latent image formation, the paper path 
length must be elongated, resulting in a deterioration in specified 
performance or characteristics of the apparatus. If the shock at such a 
moment is considerably large, register release is still necessary. 
Embodiment 3 
An example of an application of the present invention to an image forming 
apparatus having a rotary development unit and an intermediate transfer 
belt will next be described with reference to FIG. 13. This embodiment 
will be described only with respect to points of difference from the first 
and second embodiments. 
The surface of a photosensitive drum 1A is exposed at E to form a latent 
image. The latent image is developed first with respect to a first color 
image with a toner of, for example, a development device 4Y in a rotary 
development unit 4Z having development devices 4Y, 4C, 4M, and 4Bk. This 
image is transferred to a surface of an intermediate transfer belt 1B 
wrapped around a plurality of rollers at a first transfer position T.sub.1 
opposed to the belt 1B. This image again arrives at T.sub.1 by revolving 
while being retained on the intermediate transfer belt 1B. During this 
revolution, the development device is changed and a second color image is 
formed on the photosensitive drum 1A by, for example, the development 
device 4C. This image is transferred by being superposed on the first 
color image on the surface of the intermediate transfer belt 1B. 
Subsequently, the same operation is repeated with respect to the 
development devices 4M and 4Bk to superpose third and fourth color images. 
On the other hand, a recording paper sheet is fed in accordance with the 
time at which the four superposed color images arrive at a second transfer 
position T.sub.2. The recording sheet is conveyed to the position T.sub.2 
while being supported on the transfer drum 5. At the position T.sub.2, the 
four color images are simultaneously transferred onto the recording sheet. 
The recording sheet is thereafter separated from the transfer drum 5 to be 
sent to a fixation unit. The intermediate transfer belt 1B is driven by a 
driving roller 1C, and a driving torque is transmitted from the driving 
roller 1C to the transfer drum 5 through a gear (not shown). 
In this embodiment, if the path length between the first and second 
transfer positions on the intermediate transfer belt 1B is L.sub.L, the 
length of recording sheet path from the pre-register conveyance rollers 47 
to the second transfer position T.sub.2 is L.sub.P, and the recording 
sheet interval in this apparatus is L.sub.S, then the components are 
disposed so that L.sub.L &gt;L.sub.P &gt;(L.sub.L -L.sub.S). 
In this arrangement, the recording sheet passes the pre-register conveyance 
rollers 47 during the period time for transfer of the next first color 
image as well as during the period of time when the fourth color image is 
previously transferred from the photosensitive drum 1A onto the 
intermediate transfer belt 1B at the first transfer position T.sub.1. 
Therefore, even if a shock caused by the passage of the recording sheet is 
transmitted from the transfer drum 5 to the intermediate transfer belt 1B, 
it does not affect the image at T.sub.1. 
This arrangement of this embodiment is also advantageous in that the actual 
recording sheet path length can be reduced since there is no need to 
rotate the recording sheet a number of times while attracting the sheet to 
the transfer drum 5, and that it is therefore possible to reduce the 
probability of occurrence of sheet conveyance failure, and bending and 
contamination of the recording sheet. 
The effect of this embodiment is not lost even in the case of an image 
forming process described below. A transfer system may be adopted in which 
each color image on the intermediate transfer belt is separately 
transferred onto the recording sheet on the transfer drum, that is, the 
recording sheet is fed in correspondence with the first color image, the 
first color image is transferred at the second transfer position T2, the 
recording sheet revolves a number of times while being supported on the 
transfer drum 5, and the next color image is superposed each time the 
recording sheet makes one revolution. Also in this system, there is no 
possibility of the recording sheet passing the pre-register conveyance 
rollers 47 during image transfer at the first transfer position T1, thus 
ensuring formation of a good image. 
In this case, there is substantially no influence upon the latent image 
formation on the photosensitive drum 1A as long as the energy of a shock 
is not so large, since the force of the recording sheet is transmitted 
through two units, i.e., the transfer drum 5 and the intermediate transfer 
belt 1B. In this embodiment, therefore, there is no problem even if the 
recording sheet passage time is within the period for latent image 
formation on the photosensitive drum 1A. 
The present invention has been described with respect to an arrangement 
using a laser as a light source for the exposure unit. However, the 
present invention is also effective when applied to arrangements using any 
other light sources, e.g., a halogen lamp and a light emitting diode. In 
the above-described embodiments, the recording sheet is retained on the 
transfer drum by electrostatic attraction, but this attraction means is 
not exclusively used for the present invention. For example, a mechanical 
retaining means such as a clipper may be used. Also, the intermediate 
transfer member is not limited to a belt and may by be a drum-like member. 
The means for forming an image on the intermediate transfer member is not 
limited to electrophotographic means, and any other image forming means, 
e.g., a thermal transfer system, may be used as long as the recording 
apparatus is arranged so as to require a recording sheet conveyance 
mechanism.