Image forming apparatus with plural developing devices

In an image forming apparatus having two developers, toner deposition parameters in the second developing device are larger than the first developing device. Here, the toner deposition parameter is represented by an amount of electric charge of the toner particles, an amount of developer particles constituting a foggy background, an amount of carrier particles deposited on the image bearing member, or any combination of these parameters. The amount of the fog developer particles is determined first by a ratio of the total area occupied by the fog developer particles to the entire background area, second by a ratio of the fog developer particles having large particle sizes to the entire fog developer particles, third by a particle size at which a number of the fog developer particles accumulated in the large size side of a number/particle size distribution graph reaches a predetermined percentage to the entie number of fog developer particles, and fourth by a number of fog developer particles per unit area. By this arrangement of the developing devices, failure of transfer material separation from the image bearing member and retransfer of the image can be avoided when the toner image formed by the second developer is transferred.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to an image forming apparatus provided with a 
plurality of developing devices, wherein a process is executed plural 
times to one transfer material, including an image transferring operation 
for transferring toner images formed by plural developing devices onto the 
transfer material, a separating operation for separating the transfer 
material from an image bearing member particularly by a discharge 
separation method and an image fixing operation for fixing the toner image 
on the transfer material. 
Referring to FIG. 1, there is shown an example of a conventional apparatus 
of such a type, wherein two image forming operations using two developing 
devices are performed to one transfer material. The device depicted will 
produce superimposed images, although those skilled in the art will 
recognize that the device may be adapted to produce duplex images. The 
image forming process is as follows. A surface of an image bearing member 
10 such as a photosensitive drum or a dielectric member is uniformly 
charged by a primary charger 11, and then is exposed to image light 12 so 
that an electrostatic latent image is formed on the surface. The 
electrostatic latent image thus formed is developed by one of the 
developing devices 13 and 14 into a toner image, to which a transfer 
material 18 is closely contacted in an image transfer station, where an 
image transfer device 15 applies electric charge having the polarity 
opposite to that of the toner to the backside of the transfer material 18, 
whereby the toner image is transferred onto the transfer material 18 due 
to the electrostatic attraction force. Immediately after the image 
transfer process, a separation device 16 applies AC corona or the like to 
the backside of transfer material 18 to electrically discharge the 
transfer material 18 to permit the transfer material 18 to be separated 
from the image bearing member 10. The transfer material 18 which has been 
subjected to the first transfer and separation steps is transported to an 
image fixing device 19, where the toner image is fixed, where after the 
transfer material 18 is conveyed in the direction indicated by an arrow a 
and, in dependence on the position of pawl 20, is accommodated in a 
refeeding device 21. Meanwhile, image bearing member 10 is cleared of 
residual toner at cleaning station 17, and, after a number, preset by an 
operator, of the transfer materials have been subjected to the first image 
formation and have been stacked on the refeeding device 21, the transfer 
material 18 is fed out in the direction indicated by an arrow c by the 
refeeding device 21 one by one. The refed transfer material is subjected 
to the second toner image transfer operation and the second separating 
operation from the image bearing member 10. The second toner image is the 
one developed by the other developing device, that is, the developing 
device not used in the first image formation. The transfer material 18 is 
then fed from the apparatus as indicated at arrow b. 
By the first image fixing step, the transfer material 18 is more or less 
curled as shown in FIG. 2. As is known, in a discharge type separating 
method wherein the transfer material is separated from image bearing 
member using the elasticity of the transfer material, the separating 
action is largely dependent on the presence or absence or degree of the 
curling of the transfer material. For example, when the leading portion of 
the transfer material 18 is upwardly curled to tend to follow the curved 
surface of the image bearing member 10, the resilient force for separating 
the transfer material 18 from the image bearing member 10 surface is not 
sufficiently large. This results in inconveniences in the conveyance of 
the transfer material 18 and in the image quality, more particularly, the 
unsatisfactory separation and retransfer or the like. The "retransfer" is 
a phenomenon in which a toner image transferred from the image bearing 
member onto the transfer material 18 is transferred back to the image 
bearing member, or in which the toner image thus transferred back is again 
transferred to the transfer material 18 at a position different from the 
original position. Particularly in the discharge separation method, it has 
been recognized that the developed image is disturbed, in addition to the 
inconvenience of unsatisfactory separation of the transfer material. 
It has been proposed in order to solve this problem that a curl correcting 
roller is used for removing the curl itself. However, it has been found 
that this roller is not effective when the amount of curl varies greatly. 
In any event, when a transfer material is subjected to an image fixing 
operation, and thereafter is subjected to another image transfer and 
separating operation (discharge separation or electrostatic separation) as 
in the case of superimposing or duplex image formation, the separation 
becomes extremely unsatisfactory, resulting in improper separation or 
retransfer occurring frequently, and therefore, the conveyance of the 
transfer material and the image quality are extremely deteriorated. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the present invention to provide a 
solution to the above described problems. 
It is another object of the present invention to provide an image forming 
apparatus such as a printer, a copying machine or a facsimile machine, 
provided with plural developing devices, wherein good images can be stably 
provided. 
It is a further object of the present invention to provide an image forming 
apparatus wherein the developed image can be correctly transferred onto a 
transfer material with stabilized conveyance of the transfer material. 
The inventor of this application has made various investigations and 
experiments to provide a solution to the above-noted problems, and has 
found that although the problems are with image transfer or separation, 
they can be solved by making the developing condition or conditions by the 
second developing device (the developing device for forming a developed 
image which is to be transferred onto a transfer material which has been 
at least once subjected to the image fixing operation) different from that 
of the first developing device. 
More particularly, the inventor has particularly noted "a developer 
deposition parameter" which relates to a tendency of the developer 
depositing onto the image area or non-image area (background) of an image 
bearing member. The developer deposition parameter varies, when the 
developer mainly consists of toner, an amount of charge of the toner or a 
degree of magnetization of the toner to be deposited onto the image 
bearing member; and, when the developer mainly consists of toner and 
carrier, an amount of deposition of the carrier onto the image bearing 
member, in addition to the toner. The deposition parameter contains an 
amount of fog developer which constitutes a foggy background of the image. 
Firstly, the inventor has found that the transfer material separation 
property is greatly dependent on the state of deposition of the developer 
in the background area (fog), that is the developer deposited on the white 
area of the image bearing member surface during the developing process. 
Here, the white area includes the white area of the original to be copied 
or a white area formed by an erasing exposure step or the like. In the 
case of a reverse development as in the case of a laser beam printer, the 
white area is the area where the toner is not supposed to be deposited. 
Further investigation has revealed that there exists the following 
relation between the state of deposition of the fog developer in the white 
background area on the image bearing member surface and the separating 
performance of the discharge separation method. 
More particularly, when a first developing device for forming a first image 
is compared with a second developing device for forming a second image to 
be formed after the first image is fixed, the conventional apparatus or 
the apparatus involving the above described problem, is provided with the 
second developing device in which the fog is hardly formed or is less than 
the first developing device. In such an apparatus, unsatisfactory 
separation and/or retransfer have occurred. It has been found, however, 
that by making the second developing device easier to produce fog than the 
first developing device, the unsatisfactory separation is prevented, and 
the retransfer is greatly reduced. 
The state of deposition of the fog developer in the background area is 
different if the structure of the developing device and/or the developer 
is different. Therefore, when different types of developing devices are 
employed, the separation properties in the discharge separation method are 
greatly different. With a decrease in the deposition of the fog developer, 
unsatisfactory separation and retransfer increases. 
The present invention is based on the finding described above. 
According to an embodiment of the present invention, there is provided an 
image forming apparatus having a first and second developing devices 
wherein a toner image formed on an image bearing member by the first 
developing device is electrostatically transferred onto a transfer 
material; the transfer material is separated from the image bearing member 
by a discharge separation method; the toner image is fixed on the transfer 
material by an image fixing device, a second image formed on the image 
bearing member by the second developing device is electrostatically 
transferred onto the same transfer material; the transfer material is 
separated from the image bearing member again by the discharge separation 
method; and then the second toner image is fixed by the fixing, machine, 
characterized in that the amount of fog developer by the second developing 
device is larger than the amount by the first developing device. 
The amount of the fog developer is quite stabilized in the developing 
devices. 
The amounts of developer which cause fog, i.e. by fog developer, are 
compared with respect to ratios of respective total areas occupied by the 
fog developer particles to the entire background areas; ratios of fog 
developer particles having large particle sizes to the entire fog 
developer particles in the respective background areas; particle sizes at 
which respective numbers of the fog developer particles accumulated in the 
large size side of a number/particle size distribution graph reaches a 
predetermined percentage (50% or 10%, for example) to the entire number of 
fog developer particles; or numbers of fog developer particles per unit 
areas, respectively. 
According to this embodiment of the present invention, the transfer 
material separation can be performed satisfactorily, and the retransfer 
can be prevented, so that the transfer material can be stably conveyed, 
and the image quality is assured. 
According to another embodiment of the present invention, the deposition 
parameter is an amount of carrier particles deposited on the image bearing 
member. In this embodiment, the inventor has particularly noted the 
carrier particles, not the toner, slightly deposited on the white 
background area of the image bearing member during the developing process. 
Here, the white background area includes the white background of an 
original and a white area formed by erasing exposure or the like. The 
inventor has found that the amount of the deposited carrier particles is 
greatly influential to the transfer material separation or retransfer of 
the image. This embodiment provides an image forming apparatus including 
first and second developing devices, wherein a toner image formed by the 
first developing device is transferred onto a transfer material, which is 
then separated from the image bearing member and is subjected to an image 
fixing operation; and then a second toner image formed by the second 
developing device is transferred onto the same transfer material, which is 
then separated from the image bearing member, characterized in that the 
amount of carrier particles deposited on the surface of the image bearing 
member when the image is developed by the second developing device is 
larger than that of the amount of carrier particles by the first 
developing device. 
The first developing device may be of a type using two component developer 
or a type using one component developer. 
According to a further embodiment of the present invention, the deposition 
parameter is the amount of charge of the toner particles. In this 
embodiment, the inventor has particularly noted the relationship between 
the order of operations of the first and second developing devices and the 
amounts of charge of the toner particles of the respective developing 
devices. This embodiment provides an image forming apparatus including 
first and second developing devices wherein a toner image formed by the 
first developing device is transferred onto a transfer material, which is 
then separated from the image bearing member and is subjected to an image 
fixing operation; and then, a second toner image formed by the second 
developing device is further transferred onto the same transfer material, 
which is then separated from the image bearing member and is subjected to 
an image, fixing operation, characterized in that the amount of charge of 
the toner particles in the second developing device is larger than that in 
the first developing device. 
This embodiment is based on the finding that the frequency of occurrence of 
the above described retransfer is greatly dependent on the difference in 
the amounts of charge of toners in the first and second developing 
devices. It has been revealed that in the conventional machines, the 
amount of toner in the second developing device is smaller than that in 
the first developing device, and therefore, the retransfer easily occurs 
at the time of the transfer material being separated. Further, if the 
transfer material 18 is upwardly curled, the retransfer readily occurs 
depending on the change in the ambient condition or the amount of moisture 
of the transfer material. 
According to this embodiment, the inconveniences have been eliminated. 
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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is shown an image forming apparatus according to 
an embodiment of the present invention. The apparatus includes a 
developing device 13 (second developing device) of a magnetic brush 
developing type using a two component developer containing carrier 
particles or the like, and another developing device 14 first developing 
device) of a non-contact developing type using a one component developer 
not containing carrier particles. Since those developing devices are 
different in the developing process, the state of deposition of a small 
amount of developer deposited in the white area of a surface of an image 
bearing member is different when the toner image is formed. The deposition 
of the toner constitutes a foggy background. In this embodiment, the 
number of fog developer particles per unit area formed by the developing 
device 14 is smaller than the number of fog developer particles per unit 
area formed by the developing device 13. 
The toner image formed on the image bearing member 10 surface by the 
developing device 13 was observed by a microscope, and the number of fog 
developer particles per unit area was counted to be 390/mm.sup.2 ; in the 
developing device 14, it was 165/mm.sup.2. It is apparent that the number 
in the developing device 14 is smaller than the number in the developing 
device 13. 
In the image forming apparatus according to this embodiment, when the toner 
image formed by the developing device 13 and the toner image formed by the 
developing device 14 are transferred onto one and the same transfer 
material 18, the image forming operation is performed in the following 
sequence. After a first electrostatic latent image formed through a known 
process, the latent image is developed necessarily by the developing 
device 14 by which the number of fog developer particles per unit area is 
smaller. Thereafter, the toner image is transferred onto the transfer 
material 18 and is separated from the image bearing member, and then is 
subjected to an image fixing operation. The transfer material 18 is 
stacked on a refeeding device 21. The image transfer here is an 
electrostatic image transfer, and the separation is performed by a 
discharge separation method, in this embodiment. Of course, the first 
electrostatic latent image is the one which the operator designates as to 
be developed by the developer contained in the developing device 14. A 
number, preset by the operator, of the transfer materials 18 having 
received the first images, are stacked on the refeeding device 21, the 
transfer material 18 is fed out of the refeeding device 21 one by one in 
the direction indicated by an arrow c. The transfer material 18 receives a 
second toner image and is separated from the image bearing member 10. The 
second toner image is necessarily formed by the developing device 13 in 
which the number of fog developer particles per unit area is larger. In 
the first separating step, the transfer material 18 has never been 
subjected to the image fixing treatment, and therefore, it does not 
involve such a curling that adversely affects the separation. So, even if 
the toner image is formed by the developing device 14 by which the number 
of fog developer particles per unit area is smaller, the transfer material 
18 can be easily separated from the image bearing member without trouble 
such as improper separation or image retransfer. In the second separating 
step, the transfer material 18 is relatively largely curled as a result of 
the first image fixing treatment (see FIG. 3). However, the toner image is 
formed by the developing device 13 by which the number of fog developer 
particles per unit area is large, and therefore, the transfer material 18 
can be easily separated without improper separation or image retransfer. 
Table 1 shows experimental results regarding frequencies of occurrences of 
improper separation and image retransfer in a conventional apparatus 
wherein the toner image is first formed by a developing device by which 
the number of fog developer particles per unit area is larger and an 
apparatus according to the present embodiment. 
TABLE 1 
______________________________________ 
frequency of 
frequency of 
improper separation 
retransfer 
occurrences occurrences 
______________________________________ 
Embodiment 1 0% 0% 
Conventional 0.5% 45% 
______________________________________ 
The results prove that the effects of the present invention are 
significant. 
In this embodiment, the tendency of the fog occurrence is discriminated on 
the basis of the number density of the fog developer particles. However, 
this is only one method of discrimination. Other methods will be 
explained. 
The developing operation starts from a developing apparatus by which a 
ratio of fog developer particles having large particle sizes to the entire 
fog developer particles is smaller. The ratio is, for example, a ratio of 
fog developer particles having particle sizes larger than 10 microns to 
the entire fog developer particles in the white area. If necessary, the 
limit of 10 microns may be replaced with 15 microns, 20 microns or larger. 
Table 2 shows the results of experiments regarding frequencies of 
occurrences of improper separation and image retransfer in a conventional 
apparatus wherein the toner image is first formed by a developing device 
by which the ratio of the fog developer particles having particle sizes 
larger than 10 microns to the entire fog developer particles in the white 
area is larger, and an apparatus according to the present embodiment 
wherein the order is reversed. In this case, the numbers of the fog 
developer particles per unit area in the two apparatuses are comparable, 
more particularly, one of the numbers is not less than 0.5 but not more 
than 1.5 times the other number. 
TABLE 2 
______________________________________ 
frequency of 
frequency of 
improper separation 
retransfer 
occurrences occurrences 
______________________________________ 
Embodiment 2 0% 0% 
Conventional 0.85% 52% 
______________________________________ 
The results prove that the effects of the present embodiment is 
significant. 
The ratios of the fog developer particles having particle sizes larger than 
10 microns to the entire fog developer particles in the developing devices 
13 and 14, were 46% and 35%, respectively. 
Another method of discrimination will be described. In this method, a 
number/particle size distribution is first determined. The distribution 
may be a histogram as shown in FIG. 6 in which the abscissa indicates the 
particle size of the toner and the ordinate indicates the number of toner 
particles having a particular particle size. For example, a toner contains 
u particles of size A microns, or w particles of size C microns, etc. The 
particle size at which the number of the fog developer particles 
accumulated in the large size side of a number/particle size distribution 
graph sides reaches a predetermined percentage (50% in this embodiment) to 
the number of whole fog developer particles, is then determined. With 
reference to FIG. 6, if x+y is 50% of u+v+w+x+y, then particle size D is 
the "particle size at which the number of the fog developer particles 
accumulated in the large size side of a number/particle size distribution 
graph reaches a predetermined percentage". The developing operation starts 
from the developing device by which the thus determined particle size is 
smaller. The value of 50% is not limiting, and if necessary, it may be 
30%, 20% or 10%. 
Table 3 shows the results of experiments regarding frequencies of 
occurrences of improper separation and image retransfer in a conventional 
apparatus wherein the toner image is first formed by a developing device 
by which the particle size at which the number of the fog developer 
particles accumulated in the large size side of a number/particle size 
distribution graph reaches 10% to the number of whole fog developer 
particles was 15 microns, and then by the second developer wherein the 
particle size thus determined is 11 microns, and an apparatus according to 
this embodiment wherein the order is reversed. 
TABLE 3 
______________________________________ 
frequency of 
frequency of 
improper separation 
retransfer 
occurrences occurrences 
______________________________________ 
Embodiment 3 0% 0% 
Conventional 0.6% 48% 
______________________________________ 
The results prove that the effects of the present embodiment is 
significant. 
The fourth method is dependent on the area occupied by the fog developer 
particles. More particularly, the developing operation starts from a 
developing device wherein a ratio of a total area occupied by the fog 
developer particles to the entire white area is smaller. 
Table 4 shows the results of experiments regarding frequencies of 
occurrences of improper separation and image retransfer in a conventional 
apparatus wherein the toner image is first formed by a developing device 
13 by which the ratio of the total area occupied by the fog developer 
particles to the entire white area is 2.5%, and then the second image is 
formed by a developing device 14 by which the ratio is 0.8%, and an 
apparatus according to the present embodiment wherein the order of 
developments is reversed. 
TABLE 4 
______________________________________ 
frequency of 
frequency of 
improper separation 
retransfer 
occurrences occurrences 
______________________________________ 
Embodiment 4 0% 0% 
Conventional 0.8% 62% 
______________________________________ 
The results prove that the effects of the present embodiment is 
significant. 
As described in the foregoing, according to those embodiments of the 
present invention, the state of deposition of the fog developer particles 
is determined for each of the developing devices with respect to the 
number of fog developer particles, the ratio of large size toner 
particles, the number of larger toner particles or an area occupied by the 
toner, and the toner image formation is performed in the order from the 
less fog developing device. Although this is related to the developing 
operation, the effects appear in the transfer or separation operation 
wherein frequencies of the occurrences of improper separation and image 
retransfer can be reduced. 
According to the present invention the frequencies of occurrences of 
improper separation and image retransfer can be greatly reduced if at 
least one of the above described parameters is satisfied. 
However, it is preferable that two or more parameters are simultaneously 
satisfied in the developing devices. For example, the effects when the 
ambient conditions are varied are different between when the number 
relation is satisfied, and also the ratio of the large particle size 
developer is satisfied. More particularly, when the conditions are 
satisfied in plural discriminations, the stability against ambience 
variation is increased, as shown in Table 5. 
TABLE 5 
__________________________________________________________________________ 
32.5.degree. C., 90% 
23.degree. C., 50% 
Frequency of Frequency of 
improper 
Frequency of 
improper 
Frequency of 
Ambient temp. 
separation 
retransfer 
separation 
retransfer 
& humidity 
occurrences 
occurrences 
occurrences 
occurrences 
__________________________________________________________________________ 
1 0.5% 24% 0% 0% 
2 0% 0% 0% 0% 
3 2.1% 74% 0.6% 42% 
__________________________________________________________________________ 
1: Number only is satisfied. 
2: Number & large particle ratio are satisfied. 
3: Neither of 2 is satisfied. 
In the above embodiments, a magnetic brush type developing method using a 
two component developer and a non-contact type developing method using one 
component developer are used, but this is not limiting. The effects of 
those embodiments are not dependent on the arrangements, methods, 
developer and structure of the developing device, and the order of the 
developments is determined by the amount of the fog developer particles in 
the white area of the surface of the image bearing member as described 
above. 
In the foregoing description, the exemplary apparatus is provided with only 
two developing devices, but the present invention is applicable to the 
apparatus having three or more developing devices under the same concept. 
In summary, according to those embodiments, the second developing device 
satisfies the following as compared with the first developing device: 
1. The number of the fog developer particles per unit area is larger 
(preferably more than 1.5 times): 
2. The ratio of the fog developer particles having large particle sizes to 
the entire fog developer particles is larger (particularly, the ratio of 
the fog developer particles having the particle sizes not less than 10 
microns to the entire fog developer particles; further, the ratio of the 
fog developer particles having the particle sizes not less than 15 microns 
to the entire fog developer particles since the ratio greatly affects the 
separation property when they ambient conditions vary) (preferably more 
than 1.25 times): 
3. The particle size at which the number of the fog developer particles 
accumulated in the large size side of a number/particle size distribution 
graph reaches 50% to the number of whole fog developer particles is 
larger; the particle size at which the number of the fog developer 
particles accumulated in the large size side of a number/particle size 
distribution graph reaches 10% to the number of whole fog developer 
particles is larger (this greatly affects a separation property when the 
ambient conditions vary) (preferably more than 1.5 times): 
4. The ratio of the total area occupied by the fog developer particles to 
the entire white area is larger (preferably more than 1.5 times). 
Of these, the top priority is given to Item 4, and the order of priority of 
the rest is Item 2, Item 3 and Item 1. However, if one of Items 1-4 is 
satisfied, the advantageous effects of the present invention can be 
provided. 
According to those embodiments, the slight amount of fog developer 
particles deposited in the white area of the image bearing member when the 
toner image is formed by the second developing device is made larger than 
that of the first developing device, and therefore, when an image is to be 
added to the transfer material which has once been subjected to the image 
transfer, the improper separation and the image retransfer can be 
eliminated by a simple structure so that a conveyance of the transfer 
material can be assured, and the image quality can be improved. 
Fifth embodiment will be described. 
The developing device 13 is a magnetic brush type developing device using 
two component developer containing carrier particles, whereas a developing 
device 14 is a non-contact type developing device using a one component 
developer not containing carrier particles. The particle size distribution 
of the developer used with the developing device 13 is as shown in FIG. 4, 
whereas that of the developing device 14 is as shown in FIG. 5. Since the 
developer in the developing device 13 contain carrier particles, the 
particle size distribution has two peaks, wherein one is a peak f for the 
toner particles, and the other is a peak g for the carrier particles. The 
maximum particle size of the developer is approximately 140 microns. On 
the other hand, the developer of the developing device 14 does not contain 
carrier particles having a large particle size, and therefore, the maximum 
particle size is at most 25 microns. In the image forming apparatus 
according to this embodiment, a toner image formed by the developing 
device 13 and the toner image formed by the developing device 14 are 
transferred onto one and the same transfer material through one copy cycle 
in the following sequence. A first electrostatic latent image is formed. 
Thereafter, a toner image is formed necessarily by the developing device 
14 containing the developer having a smaller maximum particle size. After 
the first image transfer, separation and image fixing are completed, the 
second toner image is formed by the developing device 13 containing the 
developer having a larger maximum particle size, and then the second image 
transfer, separation and the image fixing are performed. In the first 
image transfer and the transfer material separating process, the transfer 
material 18 is not greatly curled, and therefore, the transfer material 18 
can be easily separated without problem even if the toner image has been 
formed thereon by the developing device 14 containing only a relatively 
small particle size component. The problems of improper separation and/or 
the image retransfer do not occur. The formation of the second toner image 
after the transfer material 18 has been subjected to the first image 
fixing process which would result in a relatively large curl, is effected 
by the second developing device 13 containing carrier particles having a 
relatively large particle size. Therefore, even if the transfer material 
18 is under the condition which is disadvantageous to the image transfer 
and the transfer material separation, can be easily separated without 
improper separation or retransfer of the image. The reason why the second 
separation is easy is that the white area of the toner image formed by the 
developing device 13 contains a small amount of carrier particles which 
have a very large particle size. 
The developing device 14 which is the first developing device is not 
limited to the one component developer type, but may be a two component 
developer type. In this case, the amount of carrier particles deposited on 
the image bearing member by the first developing device 14 is smaller than 
that by the second developing device, i.e., the developing device 13. By 
this, the carrier deposition amount by the second developing device is 
relatively larger than that of the first developing device, whereby the 
separation can be stably performed, and the image retransfer can be 
prevented, even if the transfer material is curled. 
In the foregoing example, one of the two developing devices is a magnetic 
brush type using a two component developer, and the other is a non-contact 
development type using a one component developer. However, both may be one 
of those types, or other type or types of developing method can be 
employed if the toner image is formed later by the developing device 
containing the developer having a larger maximum particle size. The 
foregoing explanation has been made with respect to the case of two 
developing devices being used. However, three or more developing devices 
may be employed under the concept of the present invention. 
Further preferably, the developer of the second developing device has a 
larger carrier particle size than that of the first developing device, in 
addition to the above requirement of the deposition of the carrier 
particles. 
As described in the foregoing, due to the above described difference in the 
amount of the carrier deposition, the transfer material separation is 
proper, and the image retransfer does not occur even if the transfer 
material is curled by the previous image fixing operation. 
Referring back to FIG. 1, a further embodiment using the developing devices 
13 and 14 and wherein an amount of triboelectric charge of toner is taken 
as the deposition parameter. 
In this embodiment, the developing device 13 is a magnetic brush developing 
method utilizing two component developer containing a toner a carrier or 
the like, whereas the developing device 14 is a non-contact developing 
method utilizing one component developer not containing a carrier. An 
amount of the triboelectric charge of the toner contained in the 
developing device 13 is larger than the amount of the triboelectric charge 
of the toner contained in the developing device 14. The amount of the 
triboelectric charge of the toner on a developing roller 25 was measured, 
and that of the developing device 13 was 14 micro-C/gr, whereas that of 
the developing device 14 was 5 micro-C/gr. When in this apparatus a toner 
image formed by the developing device 13 and a toner image formed by the 
developing device 14 are to be transferred onto one and the same transfer 
material through one copying operation (superimposing or duplex mode), the 
image forming operation is executed in the following sequence. After the 
first latent image is formed, the first toner image is formed necessarily 
by the developing device 14 containing toner of which the electric charge 
is smaller; then, the toner image is transferred onto the transfer 
material, and the transfer material is separated from the image bearing 
member and is subjected to an image fixing operation; the second toner 
image is formed by the developing device 13 containing the toner of which 
the amount of charge is relatively large; then, the second image transfer, 
the second transfer material separation and the second image fixing are 
performed. In the first image transfer and transfer material supporting 
process, the transfer material 18 is not curled very much, and therefore, 
the image retransfer does not occur even if the toner image is formed by 
the second developing device 14 containing toner on which the amount of 
the electric charge is relatively small. Further, although the second 
image transfer and the transfer material separation process has to be 
performed to the transfer material 18 which has been relatively greatly 
curled by the first image fixing device, the second toner image is formed 
by the toner of which the amount of charge is relatively large, so that 
the image retransfer does not occur in the separation process even if the 
state of curling is disadvantageous to the separation process. 
It has been found that an amount of charge of the toner particles supplied 
by the developing device 13 (the second developing device) for effecting 
superimposing or duplex recording to the transfer material which has been 
subjected to an image fixing operation is preferably larger than that of 
the toner of the first developing device and is not less than 10 micro-C/g 
from the standpoint of further preventing the image retransfer and improve 
the separating property 
In the foregoing description, the two developing devices are of magnetic 
brush developing type using a two component developer and non-contact 
developing type using a one component developer, respectively. However, 
this is not limiting, and the same advantageous effects are provided if 
the order of toner image formations is determined on the basis of the 
amounts of charge of the toner particles used with the developing devices. 
Also, in this embodiment, the structure wherein two developing devices are 
used is taken, but the number is not limited to two, and the same 
advantageous effects can be provided in the case of three or more 
developing devices used, if the order is determined under the above 
described concept. 
The amount of charge of the toner particles is dependent on the actual 
structure of the developing device, so it is difficult to determine, but 
in order to increase the amount of the charge to satisfy the above 
described requirement, a blade for triboelectrification to the toner may 
be used; an external charge may be applied; an amount of charge assisting 
agent contained in the toner particles may be increased. Those methods are 
known, and therefore, one skilled in the art can make adjustment without 
difficulty. 
As described in the foregoing, according to this embodiment, when the toner 
images formed by the two or more developing devices on one and the same 
transfer material are transferred, and the transfer material is separated 
and is subjected to the image fixing operation, the first toner image 
formation is effected by the developing device wherein the amount of 
electric charge of the toner is relatively small. Therefore, the image 
transfer and the transfer material separating process for the second and 
subsequent images can be performed without image retransfer despite the 
existence of the curling of the transfer material by the previous image 
fixing step. Those advantages are provided by a simple structure, that is, 
the order of the developing operations. Further, by making the two 
deposition parameters (amount of charge of the toner and amount of 
deposition of the carrier particles) larger in the developing device 13 
than in the first developing device 14, the above described advantageous 
effects of the present invention can be stably maintained for a long 
period of time. 
A further embodiment of the present invention will be described wherein two 
deposition parameters are selected to satisfy the respective requirements 
in the first and second developing devices. More particularly, in this 
embodiment, the first toner image is formed by a developing device by 
which the amount of fog developer particles deposited in the white area of 
the surface of the image bearing member is smaller and in which the amount 
of charge of the toner per unit weight is smaller. By doing so, the 
prevention of improper separation and image retransfer can be further 
stabilized. 
The toner image formed on the image bearing member 10 surface by the 
developing device 13 (the second developing means) was observed by a 
microscope, and the fog developer particles per unit area in the white 
area was 390/mm.sup.2. As for the developing device (the first developing 
means), it was 165/mm.sup.2. It is apparent that the number of the fog 
developer particles is smaller in the first developing device 14 than in 
the second developing device 13. In addition, the electric charge of the 
toner per unit weight is smaller in the first developing device 14 than in 
the second developing device 13. 
Table 6 shows the results of experiments regarding frequencies of 
occurrences of improper separation and image retransfer in a conventional 
apparatus wherein the toner image is first formed by the second developing 
device and an apparatus according to this embodiment (embodiment A) 
wherein the order is reversed. 
TABLE 6 
______________________________________ 
frequency of 
frequency of 
improper separation 
retransfer 
occurrences occurrences 
______________________________________ 
Embodiment A 0% 0% 
Conventional 0.6% 51% 
______________________________________ 
The results prove that the effects of the present invention is enhanced 
when two or more deposition parameters are satisfied. 
The amount of charge of the toner per unit weight was measured by the 
Faraday gauge method. In this method, an external cylinder made of metal 
and grounded is contacted to the surface of the image bearing member 10 
having a toner image thereon to attract all the toner particles in a 
defined area on the image bearing member 10 surface, and the toner 
particles are collected by an internal cylindrical filter The weight of 
the toner collected is determined by the increase of the weight of the 
filter. Simultaneously, the amount of the charge accumulated on the 
internal cylinder made of a metal electrostatically shielded by the 
external cylinder is measured, so that the amount of charge of the toner 
attracted from the surface of the image bearing member 10 and corrected in 
the filter can be determined. The principle of this method is disclosed 
in, for example, DENSHISHASHIN GAKKAISHI, Vol. 11, No. 1. 
A further embodiment will be described. The developing operation starts 
from a developing apparatus by which a ratio of fog developer particles 
having large particle sizes to the entire fog developer particles is 
smaller. The ratio is, for example, a ratio of fog developer particles 
having particle sizes larger than 10 microns to the entire fog developer 
particles in the white area. If necessary, the limit of 10 microns may be 
replaced with 15 microns, 20 microns or larger. 
Table 7 shows the results of experiments regarding frequencies of 
occurrences of improper separation and image retransfer in a conventional 
apparatus wherein the toner image is first formed by a developing device 
by which the ratio of the fog developer particles having particle sizes 
larger than 10 microns to the entire fog developer particles in the white 
area is larger and in which the amount of the charge of the toner per unit 
weight is larger, and an apparatus according to the present embodiment 
wherein the order is reversed. In this case, the numbers of the fog 
developer particles per unit areas in the two apparatuses are comparable, 
more particularly, one of the numbers is not less than 0.5 but not more 
than 1.5 times the other number. 
The ratios of the fog developer particles having particle sizes not less 
than 10 microns to the entire fog developer particles are 46% in the 
developing device 13 and 31% in the developing device 14. 
TABLE 7 
______________________________________ 
frequency of 
frequency of 
improper separation 
retransfer 
occurrences occurrences 
______________________________________ 
Embodiment B 0% 0% 
Conventional 1.1% 62% 
______________________________________ 
The results prove that the effect of the present embodiment is significant. 
Another method of discrimination will be described. In this method, a 
number/particle size distributions are first determined. And the particle 
size at which the number of the fog developer particles accumulated in the 
number/particle size distribution from the large size sides reaches a 
predetermined percentage (50% in this embodiment) to the number of whole 
fog developer particles, is determined. The developing operation starts 
from the developing device by which the thus determined particle size is 
smaller, and in which the amount of the charge of the toner per unit 
weight is smaller. The value of 50% is not limiting, and if necessary, it 
may be 30%, 20% or 10%. 
Table 8 shows the results of experiments regarding frequencies of 
occurrences of improper separation and image retransfer in a conventional 
apparatus wherein the toner image is first formed by a developing device 
by which the particle size at which the number of the fog developer 
particles accumulated in the large size side of a number/particle size 
distribution graph reaches 10% to the number of whole fog developer 
particles was 15 microns and in which the amount of the charge of the 
toner per unit weight is larger, and then by the second developer wherein 
the particle size thus determined is 11 microns, and an apparatus 
according to this embodiment wherein the order is reversed. 
TABLE 8 
______________________________________ 
frequency of 
frequency of 
improper separation 
retransfer 
occurrences occurrences 
______________________________________ 
Embodiment C 0% 0% 
Conventional 0.9% 60% 
______________________________________ 
The results prove that the effects of the present embodiment is 
significant. 
The similar effects can be provided if the first developing operation is 
performed by the developing device in which the ratio of the total area 
occupied by the fog developer particles to the entire white area is 
smaller and wherein the amount of charge of the toner per unit weight is 
smaller. 
In those embodiment, the magnetic brush type developing device using a two 
component developer and the non-contact type developing device using a one 
component developer are used. However, this is not limiting, and any 
developing method and developers can be used if the order of developing 
operation is determined on the basis of the amount of the fog developer 
particles deposited on the white area on the image bearing member surface 
and the amount of charge of the toner image per unit weight, the same 
effects can be provided. In those embodiments, two developing devices are 
employed. However, three or more developing devices may be used under the 
same concept with the same effects. 
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.