Image forming apparatus with recording material loop forming and control means

An image forming apparatus in which a recording material having an unfixed image formed thereon by an image forming device is conveyed to a fixing device having a pair of rotatable members by a conveyor and the recording material is nipped and conveyed by the pair of rotatable members so that the unfixed image is fixed on the recording material. The pair of rotatable members is selectively rotatable at a first peripheral speed lower than the speed at which the conveyor conveys the recording material and a second peripheral speed higher than the first peripheral speed. The pair of rotatable members are rotated at the first peripheral speed to convey the recording material and form a loop of recording material rearwardly of the pair of rotatable members with respect to the direction of conveyance of the recording material, whereafter the peripheral speed of the pair of rotatable members is changed over from the first peripheral speed to the second peripheral speed to suppress the growth of the loop of the recording material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an image forming apparatus such as a copying 
machine, a facsimile apparatus or a printer utilizing the 
electrophotographic system, the electrostatic recording system, the heat 
transfer system or the like, to form images on recording materials and 
thereby obtain hard copies. 
2. Related Background Art 
An image forming apparatus such as an electrophotographic apparatus usually 
has, as the conveyance path of a recording material, a series of 
conveyance paths for feeding, transferring, fixing and discharging of the 
recording material having an image formed thereon out of the apparatus. In 
this case, any difference between the conveyance speeds of elements 
constituting the conveyance path has sometimes reduced the quality of the 
image formed. For example, in an electrophotographic apparatus as shown in 
FIG. 5 of the accompanying drawings, a toner image formed on a 
photosensitive drum 102 as an image bearing member is transferred to a 
recording material fed from a cassette 106, at a transfer station 104, by 
the action of a charger 103. By the corona discharge produced from the 
charger 103 during the transfer, coupled with the movement of the 
photosensitive drum 102, the conveyance force by the photosensitive drum 
for the recording material is created. This conveyance force results from 
electrostatic attraction and is very small as compared with the conveyance 
force provided in an apparatus of a construction in which a plurality of 
rollers such as a pair of conveying rollers 110 and a pair of fixing 
rollers 101 are urged against each other and a recording material is 
conveyed while being nipped between those rollers. In such case, with 
regard to the pair of conveying rollers 110, the drive force of the pair 
of conveying rollers 110 is released by the use, for example, of a clutch 
or the like at a point of time whereat the leading edge of the recording 
material arrives at the transfer station 104, whereby the influence 
thereof can be eliminated. However, the pair of fixing rollers 101 are 
generally such that a pair of rollers are opposed and urged against each 
other and a recording material is conveyed while being nipped between the 
pair of rollers to thereby accomplish the fixation of an unfixed toner 
image and therefore, the conveyance force of the pair of fixing rollers 
101 is usually very great as compared with the conveyance force of the 
photosensitive drum 102. Thus, where the same recording material is caught 
by both the transfer station 104 and the pair of fixing rollers 101, that 
is, where transfer is still effected in the trailing end portion of the 
recording material and the peripheral speed of the pair of fixing rollers 
101 is higher than the peripheral speed of the photosensitive drum 102 
when fixation is being effected on the recording material as the latter is 
nipped and conveyed between the pair of fixing rollers 101, the recording 
material is pulled by the pair of fixing rollers 101 and slips in the 
transfer station 104 and thus, image transfer misregistration occurs on 
the recording material. 
FIG. 6 of the accompanying drawings shows a full color image forming 
apparatus using a plurality of image bearing members. In this apparatus, a 
recording material fed from a cassette 106 is electrostatically attracted 
to a conveyor belt 105 and passes photosensitive drums 120, 130 and 140 in 
succession, whereby cyan, magenta and yellow toner images are successively 
transferred from the respective drums onto the recording material to form 
an unfixed full color image thereon, whereafter the unfixed image is fixed 
by a pair of fixing rollers 101 and the recording material having a 
completed image output thereon is discharged out of the apparatus. Also, 
in this apparatus, if the peripheral speed of the pair of fixing rollers 
101 becomes higher than the speed of the conveyor belt 105 during the 
image transfer at the transfer station 150 of the photosensitive drum 140 
and the recording material is pulled by the pair of fixing rollers 101, 
the attraction of the recording material to the conveyor belt 105 will be 
weakened and the recording material will slip in this portion. Therefore, 
transfer misregistration will occur to the image transferred from the 
photosensitive drum 140 onto the recording material, and this has led to 
color misregistration on the images which means clear degradation of the 
quality of image. 
In the above-described examples of the prior art, when the peripheral speed 
of the pair of fixing rollers 101 is higher than the feeding speed of the 
recording material at the transfer station 104 of the photosensitive drum 
102 or the feeding speed of the recording material at the transfer station 
150 of the photosensitive drum 140 and moreover the recording material 
lies between the pair of fixing rollers 101 and the transfer station 104 
or 150, the recording material is pulled by the pair of fixing rollers 101 
and slips at the transfer station 104 or 150, whereby transfer 
misregistration occurs on the recording material and causes the quality of 
image to be degraded. 
As a means for solving this problem, there is the measure of making the 
distance between the transfer station 104 or 150 and the pair of fixing 
rollers 101 greater than the length of the recording material used with 
respect to the direction of conveyance. According to such measure, the 
distance between the transfer station 104 or 150 and the pair of fixing 
rollers 101 is made longer than necessary and undesirably, this leads to 
an increased dead space occupied by the entire apparatus. Also, the 
direction of conveyance of the recording material is often the widthwise 
direction of the apparatus and accordingly, making said distance long has 
led to a problem that the apparatus becomes bulky and the degree of 
freedom of the space in which the apparatus is installed is remarkably 
decreased. 
Another means for solution is disclosed in Japanese Patent Application No. 
186671/1985 filed by the same assignee. This means comprises stopping the 
pair of fixing rollers when the leading edge of the recording material 
enters the nip between the pair of fixing rollers, forming a loop in the 
recording material, thereafter rotating the pair of fixing rollers and 
thereby absorbing the difference between the speed of rotation of the pair 
of fixing rollers and the feeding speed of the recording material at the 
transfer station. However, according to this method, the pair of fixing 
rollers are stopped and this requires a clutch, for example, and the 
mechanical rising and falling times of this clutch affects the inter-paper 
spacing, and this provides a hindrance when a number of recording 
materials are conveyed continuously. Also, when a recording material 
enters the nip between the fixing rollers stopped, the resulting shock 
disturbs the unfixed toner image on the recording material and degrades 
the quality of the image and moreover, in the case of a full color image, 
said shock has resulted in the problem of color misregistration. Further, 
where the bus line of the photosensitive drum in the transfer station and 
the bus lines of the fixing rollers are not parallel to each other, there 
has also arisen a problem that the recording material is distorted to 
disturb the image thereon. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the above-described 
background art and a primary object thereof is to provide an image forming 
apparatus which can obtain accurate and clear-cut images. 
Another object of the present invention is to provide an image forming 
apparatus which can ensure appropriate inter-paper spacing even during 
continuous feeding of paper sheets. 
Still another object of the present invention is to provide an image 
forming apparatus which is compact. 
Yet still another object of the present invention is to provide an image 
forming apparatus which can obtain color images free of color 
misregistration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Some embodiments of the present invention will hereinafter be described 
with reference to the drawings. FIG. 1 shows a full color image forming 
apparatus as an image forming apparatus to which the present invention is 
applied. A recording material as an image carrier is fed from a cassette 6 
to a pair of register rollers 21 by a pick-up roller 7. The recording 
material fed to the pair of register rollers 21 is further fed to a 
conveyor belt 5 with the timing of its feeding being measured by the pair 
of register rollers 21, and is electrostatically adsorbed to the conveyor 
belt 5 by an adsorbing charger 41. The recording material thus adsorbed 
passes through transfer stations 42, 43 and 44 comprising photosensitive 
drums 22, 23 and 24 as image bearing members and chargers 32, 33 and 34, 
whereby for example, unfixed magenta, cyan and yellow images are 
successively transferred to the recording material. Latent images are 
formed on the photosensitive drums 22, 23 and 24 by latent image forming 
means 50, 51 and 52, and magenta, cyan and yellow developers are supplied 
to these latent images from developing means 53, 54 and 55, respectively, 
whereby magenta, cyan and yellow developed images are pre-formed on the 
photosensitive drums 22, 23 and 24, respectively. The timing of the 
formation of these images is such that the leading edge of the image on 
each photosensitive drum is coincident with the leading edge of the 
recording material when the recording material arrives at each transfer 
station 42, 43 and 44. The method of forming the latent images may be a 
method whereby the photosensitive drums precharged by chargers are exposed 
to optical images color-resolved correspondingly to the image of an 
original and latent images corresponding to the respective color 
components are formed on the respective photosensitive drums, or a method 
whereby lights such as laser beams are applied to pre-charged 
photosensitive drums in accordance with image signals color-resolved 
correspondingly to image information and latent images corresponding to 
the respective color components are formed on the respective 
photosensitive drums. 
On the other hand, the recording material to which the unfixed images have 
been transferred from the respective photosensitive drums is conveyed to a 
pair of fixing rollers 11 and passes through the nip between the pair of 
fixing rollers 11, whereby the unfixed images on the recording material 
are fixed. After the fixation, the recording material is discharged onto a 
tray 9 by paper discharge rollers 8 and thus, an image output in full 
color is obtained. In the present embodiment, a heating-fixing system 
using heat and pressure is adopted as the fixing system but alternatively, 
a pressure fixing system using only pressure may be adopted. 
In FIG. 1, reference numeral 10 designates a sensor as detecting means for 
detecting the leading edge of the recording material. The sensor 10 may 
comprise, for example, a light source and a phototransistor, an ultrasonic 
wave sensor or the like. Reference numeral 12 denotes a guide plate for 
directing the recording material conveyed by the conveyor belt 5 to the 
nip between the pair of fixing rollers 11. 
In the above-described construction, the pair of fixing rollers 11 are 
designed to be rotatively driven selectively at a first peripheral speed 
Vr.sub.1 and a second peripheral speed Vr.sub.2 higher than the first 
peripheral speed Vr.sub.1. The first peripheral speed Vr.sub.1 is set so 
that Vr.sub.1 &lt;Vb, where Vb is the conveyance speed of the conveyor belt 
5. In the present embodiment, the pair of fixing rollers 11 when in their 
steady state are adapted to be driven at the second peripheral speed 
Vr.sub.2. 
The speed change-over of the pair of fixing rollers will now be described 
with reference to FIG. 2. 
The recording material S conveyed by the conveyor belt 5 has its leading 
edge detected by the sensor 10. By the detection signal, the peripheral 
speed of the pair of fixing rollers 11 is changed over and controlled to 
Vr.sub.1 at a point of time whereat at least the leading edge of the 
recording material S arrives at the nip between the pair of fixing rollers 
11. 
On the other hand, in the apparatus of the present embodiment, the distance 
along the recording material conveyance path between the nip between the 
pair of fixing rollers 11 and the transfer station 44 is shorter than the 
length of the recording material S used (with respect to the direction of 
conveyance) and therefore, the trailing end portion of the recording 
material S is being subjected at least to the image transfer at the 
transfer station 44 when the leading end portion of the recording material 
S is nipped between the pair of fixing rollers 11. Since the peripheral 
speed of the pair of fixing rollers 11 is controlled to Vr.sub.1, the 
recording material S present between the nip between the pair of fixing 
rollers 11 and the transfer station 44 forms a loop b rearwardly of the 
pair of fixing rollers, as shown in FIG. 2. The image being transferred is 
prevented from being disturbed by the formation of such loop even if the 
speed of the pair of fixing rollers varies more or less, and therefore the 
drive control of the fixing rollers can be accomplished roughly. However, 
if at this time, the loop b grows too large, the unfixed images will come 
into contact with a portion of the interior of the apparatus body, and the 
increased curvature of the recording material S will disturb the unfixed 
images, thus preventing an accurate copy image from being obtained. So, it 
is necessary that after the time during which a predetermined amount of 
deformation of the loop b is obtained has elapsed, the pair of fixing 
rollers 11 be driven at a peripheral speed higher than the first 
peripheral speed Vr.sub.1 to thereby prevent the loop of the recording 
material from increasing beyond a predetermined amount. For this purpose, 
after the lapse of a predetermined time t, the peripheral speed of the 
pair of fixing rollers 11 is restored to the aforementioned second 
peripheral speed Vr.sub.2 and the loop b of the recording material S shown 
in FIG. 2 is controlled to a predetermined amount of deformation. By the 
value of the second peripheral speed Vr.sub.2 being varied, the amount of 
deformation of the loop b can be controlled so as not to exceed a 
predetermined amount even when it exhibits a maximum value, or the amount 
of deformation of the loop b can be decreased below the predetermined 
amount. 
The value of the peripheral speed Vr.sub.2 may preferably be Vr.sub.2 
.gtoreq.Vb in order that the size of the loop of the recording material 
may be suppressed early. 
The control system of the present embodiment will now be described with 
reference to FIG. 3. When the sensor 10 detects the leading edge of the 
recording material S, the detection signal is received by speed control 
means 60, whereby drive means 70 connected to the pair of fixing rollers 
11 is controlled and the peripheral speed of the pair of fixing rollers 11 
is changed over from the second peripheral speed Vr.sub.2 to the first 
peripheral speed Vr.sub.1. Further, after the lapse of a predetermined 
time, a signal is delivered to the speed control means 60 by a timer 50 so 
that the peripheral speed of the pair of fixing rollers 11 restores the 
second peripheral speed Vr.sub.2. 
The operation in the process of the formation of the loop b of the 
recording material S as described above will now be explained with 
reference to the flow chart of FIG. 4. When an image formation start 
signal is first sent out at step .circle.1, the pair of fixing rollers 11 
are rotated at the second peripheral speed Vr.sub.2 (step 2), and then by 
steps .circle.3, .circle.4 and .circle.5, the recording material passes 
through the transfer stations 42, 43 and 44 comprising the photosensitive 
drums 22, 23 and 24 and the chargers 32, 33 and 34, whereby the unfixed 
images of respective colors are transferred onto the recording material in 
the order of magenta, cyan and yellow, and when at step .circle.6, the 
sensor 10 detects the leading edge of the recording material S, the 
peripheral speed of the pair of fixing rollers 11 changes over to the 
first peripheral speed Vr.sub.1 (step .circle.7). If at this time, the 
sensor 10 does not detect the leading edge of the recording material S, it 
re-detects the leading edge. Further, at step .circle.8, whether the 
predetermined time t has passed is judged, and if the predetermined time t 
has passed, the peripheral speed of the pair of fixing rollers restores 
the second peripheral speed Vr.sub.2. If in this case, the predetermined 
time t has not passed, the program returns to step .circle.7. When the 
formation of images on a set number of recording materials is completed at 
step .circle.10, the apparatus is stopped from operating at step 
.circle.11. On the other hand, if the formation of images on the set 
number of recording materials is not completed at step .circle.10, the 
program returns to step .circle.3, whereby an operation similar to what 
has been previously described is performed. 
In the above-described embodiment, the peripheral speed of the pair of 
fixing rollers 11 in their steady state has been the second peripheral 
speed Vr.sub.2, but alternatively, the control may be such that the 
peripheral speed of the pair of fixing rollers in their state is the first 
peripheral speed Vr.sub.1 and in a predetermined time after the recording 
material S has entered the nip between the pair of fixing rollers 11, the 
peripheral speed of the pair of fixing rollers 11 is changed over to the 
second peripheral speed Vr.sub.2, and in a further predetermined time 
thereafter, the peripheral speed of the pair of fixing rollers 11 restored 
to the first peripheral speed Vr.sub.1. 
Also, where the peripheral speed of the pair of fixing rollers 11 in their 
steady state is Vr.sub.2 as in the above-described embodiment, the point 
of time at which the peripheral speed is changed to Vr.sub.1 has been the 
point of time at which the leading edge of the recording material is 
detected, in the above-described embodiment, whereas this is not 
restrictive, but for example, the peripheral speed may be changed from 
Vr.sub.2 to Vr.sub.1 in a predetermined time after the recording material 
has been fed from the cassette. 
FIG. 7 shows another embodiment of the present invention. In FIG. 7, 
reference numeral 13 designates a sensor for detecting the magnitude of 
the amount of deformation of the formed loop of the recording material. 
The sensor 13 is disposed between the photosensitive drum 24 and the pair 
of fixing rollers 11, and comprises, for example, a light source and a 
phototransistor, an ultrasonic wave sensor or the like. 
Description will now be made of a method of detecting the size of the loop 
of the recording material by the use of the sensor 13 and controlling the 
speed of the pair of fixing rollers. FIGS. 8A and 8B show the state of the 
recording material S before it enters the nip between the pair of fixing 
rollers 11. (FIG. 8A is a view of the recording material as seen from the 
direction of conveyance thereof, and FIG. 8B is a view of the recording 
material as seen from a direction perpendicular to the direction of 
conveyance thereof). The peripheral speed of the pair of fixing rollers 11 
at this time is Vr.sub.1 in the present embodiment. When the recording 
material S thereafter enters the nip between the pair of fixing rollers 
11, a loop b is formed in the recording material S due to the difference 
between the speed Vb of the conveyor belt 5 and the peripheral speed 
Vr.sub.1 of the pair of fixing rollers 11. As the fixation progresses, the 
size of the loop b increases and reaches the state shown in FIGS. 9A and 
9B, whereupon the sensor 13 detects the loop b. In correspondence to the 
detection signal of the sensor 13, the peripheral speed of the pair of 
fixing rollers 11 is changed over from Vr.sub.1 to Vr.sub.2 higher than 
Vr.sub.1. By the peripheral speed of the pair of fixing rollers 11 being 
thus changed over, the difference between the peripheral speed Vr.sub.2 of 
the pair of fixing rollers 11 and the speed Vb of the conveyor belt 
becomes smaller than the difference between the peripheral speed Vr.sub.1 
of the pair of fixing rollers 11 and the speed Vb of the conveyor belt and 
therefore, the size of the formed loop b decreases. When the size of the 
loop b decreases and the sensor 13 no longer detects the loop b, the 
peripheral speed of the pair of fixing rollers 11 restores Vr.sub.1 in 
correspondence thereto and thus, the size of the loop b increases again. 
In this manner, in the present embodiment, the actually formed loop of the 
recording material is detected to control the peripheral speed of the pair 
of fixing rollers 11 so as to maintain the amount of formed loop of the 
recording material at a predetermined amount. Again in the present 
embodiment, as in the previous embodiment, it is preferable in decreasing 
the size of the loop of the recording material early that a value which 
satisfies the relation that Vr.sub.2 .gtoreq.Vb be used as the peripheral 
speed Vr.sub.2. 
The control system of the present embodiment will now be described with 
reference to FIG. 10. When the sensor 13 detects the loop b, the detection 
signal thereof is received by speed control means 60, whereby drive means 
70 connected to the pair of fixing rollers 11 is controlled to change the 
peripheral speed of the pair of fixing rollers 11 to Vr2. 
On the other hand, when the size of the loop b decreases and the sensor 13 
no longer detects the loop b, the drive means 70 is again controlled by 
the speed control means 60 to change the peripheral speed of the pair of 
fixing rollers 11 to Vr1 which is lower than the speed Vb of the conveyor 
belt 5. That is, the peripheral speed of the pair of fixing rollers at 
this time is made equal to the peripheral speed of the pair of fixing 
rollers before the recording material enters the nip between the pair of 
fixing rollers 11. 
FIGS. 11A, 11B and 11C show another example of the detecting means for 
detecting the loop of the recording material. In FIG. 11A, the recording 
material S is shown in its state before it enters the nip between the pair 
of fixing rollers 11, and the then peripheral speed of the pair of fixing 
rollers 11 is Vr1 which is lower than the speed Vb of the conveyor belt 5. 
When from this state, the recording material S enters the nip between the 
pair of fixing rollers 11, a loop is formed in the recording material S as 
shown in FIG. 11B. In this state of the recording material, the sensor 13a 
detects the bend b of the loop, but the peripheral speed of the pair of 
fixing rollers 11 remains to be Vr1. As the fixation further progresses, 
the size of the bend b increases as shown in FIG. 11C. When the peripheral 
speed of the pair of fixing rollers 11 is changed to Vr2 at this time as 
the sensor 13b detects the bend b, the size of the bend b decreases again 
as shown in FIG. 11B. In this state, the bend b is detected by only the 
sensor 13a, but if the peripheral speed of the pair of fixing rollers 11 
is restored to Vr1 at a point of time whereat the sensor 13a no longer 
detects the bend b, the size of the bend b increases again as shown in 
FIG. 11C. By such speed change-over, the size of the loop of the recording 
material is maintained at a predetermined size with the height of the bend 
held between h.sub.1 and h.sub.2 as shown in FIG. 12. 
Now, in the present embodiment, the peripheral speed of the pair of fixing 
rollers in their steady state has been Vr1, but alternatively, said 
peripheral speed may be Vr2 and further detecting means for detecting the 
leading edge of the recording material as in the previously described 
embodiment may be provided discretely and in accordance with the detection 
signal produced upon detection of the leading edge of the recording 
material, the peripheral speed of the pair of fixing rollers may be 
changed over from Vr2 to Vr1. Again in this case, after the recording 
material is nipped between the pair of fixing rollers, the peripheral 
speed of the pair of fixing rollers may be controlled in conformity with 
the detection signal of the sensor 13 or the sensors 13a and 13b. 
As described above, according to the present invention, the peripheral 
speed of the pair of fixing rollers is suitably changed over and 
controlled, whereby the recording material still present at the position 
whereat the unfixed image is formed on the recording material (for 
example, the transfer position) is prevented from being pulled by the pair 
of fixing rollers and therefore, in a full color image forming apparatus, 
color misregistration of images is eliminated, and in a monochromatic 
image forming apparatus, stretch of image is eliminated and thus, an 
accurate and clear-cut image output can be obtained. 
Also, the loop of the recording material formed rearwardly of the pair of 
fixing rollers is maintained at a predetermined size, whereby the behavior 
of the recording material changed little and the image on the recording 
material is not disturbed. In addition, the loop of the recording material 
can be prevented from becoming too large and therefore, spring-back occurs 
due to the resiliency of the loop-like recording material itself whereby 
even the problem that the trailing end portion of the recording material 
recedes to cause image misregistration can be solved. 
Further, according to the present invention, the pair of fixing rollers 
need not be rotated from their stopped condition each time the leading 
edge of the recording material arrives at the nip between the pair of 
fixing rollers and therefore, a drive interrupting mechanism such as a 
clutch need not be frequently used and accordingly, the problem of 
mechanical rising and falling is eliminated and accurate inter-sheet 
spacing during continuous feeding of recording sheets is secured. Also, 
the shock with which the recording material enters the nip between the 
pair of fixing rollers is small and therefore, the image on the recording 
material is not disturbed. 
Also, the distance between the position at which an unfixed image is formed 
on the recording material and the nip between the pair of fixing rollers 
may be shorter than the length of the recording material used and may be 
enough to permit the loop of the recording material to be formed, and this 
leads to the possibility of making the apparatus compact. 
The above embodiments of the present invention have been described with 
respect to the electrophotographic type in which a photosensitive member 
is used as an image bearing member and is subjected to charging and 
exposure to form a latent image, but adoption may also be made of the 
electrostatic recording type in which a dielectric member is used as an 
image bearing member and a latent image corresponding to image information 
is directly formed thereon. 
Also, in the previously described embodiments of the present invention, the 
formation of unfixed images on the recording material has been shown with 
respect to a case where the developed images formed on the image bearing 
members are transferred to the recording material, whereas this is not 
restrictive, but the present invention also covers a case where unfixed 
images are formed directly on the recording material. 
Further, in the present invention, where a heating-fixing roller device is 
employed as fixing means, when the peripheral speed of the pair of fixing 
rollers is made high, the amount of heat imparted to the recording 
material at the nip portion per unit time becomes smaller than when the 
peripheral speed of the pair of fixing rollers is low. Therefore, where a 
problem arises in respect of fixative property, the set temperature of 
heating means for heating the fixing rollers may be increased or auxiliary 
heating means may be operated as the peripheral speed of the pair of 
fixing rollers is accelerated, whereby the amount of heat imparted to the 
recording material at the nip portion may be compensated for.