An image-forming method including the steps of forming a thin layer of a photosensitive toner charged to have a given polarity on a conductive drum, exposing the thin layer to light according to image information, and transferring a part of the photosensitive toner onto a transfer material. The transferring step includes contacting the transfer material with the thin layer after the exposing step as applying a pressure to the transfer material, and irradiating a corona ion from a back side of the transfer material to the thin layer by using a corona charger. Thus, the transfer material is maintained in pressure contact with the thin layer of the toner during the transferring step, thereby improving a transfer efficiency of the toner to the transfer material to obtain a satisfactory image having a high density and no void.

FIELD OF THE INVENTION 
The present invention relates to an improved electrophotographic 
image-forming method for forming an image by exposing a thin layer 
comprised of photosensitive toner to light to form an electrostatic latent 
image on the thin layer, and transferring the toner after exposed onto a 
transfer material. More particularly, the present invention relates to 
said image-forming method which provides a remarkable improvement in 
transfer efficiency. 
BACKGROUND OF THE INVENTION 
Conventionally, an image-forming method using a so-called photosensitive 
toner functioning as both a developer and a photoconductor is known. In 
such a known method, a thin layer of the toner having a photoconductivity 
is formed on a conductive drum, and the toner thin layer is then exposed 
to light to obtain a toner image. 
FIG. 2 shows an example of the prior art image-forming method. Referring to 
FIG. 2, a toner thin layer 23 of a photosensitive toner 22 charged to have 
a given polarity by triboelectric charging or the like is formed on a 
toner retaining member 21 having a conductivity (the photosensitive toner 
22 may be charged to have a given polarity by corona charging or the like 
after forming the toner thin layer). Then, exposure 24 according to an 
original image is applied to the toner thin layer 23. Accordingly, the 
toner is made conductive by the exposure 24, and the charge possessed by 
the toner is dissipated through the conductive toner retaining member 21 
which is grounded, or a charge having a reverse polarity is injected into 
the toner. Thus, an electrostatic latent image 25 is formed on the toner 
thin layer. On the other hand, a transfer material 26 is brought into 
contact with the toner thin layer on which the latent image 25 has been 
formed, and a corona ion is irradiated from a back side of the transfer 
material 26 by means of a transfer charger 27. As a result, the transfer 
material 26 is charged to have a polarity reversed to or the same as that 
of the toner not exposed, thereby transferring the toner onto the transfer 
material 26. In the former case where the transfer material is charged to 
have the reversed polarity, a positive image is formed, while in the 
latter case where the transfer material is charged to have the same 
polarity, a negative image is formed. 
However, the above-mentioned transfer step is different from a general 
transfer process for transferring a toner image only formed on a 
photosensitive body in a xerography system which is represented by a 
Carlson process. That is, the transfer step as shown in FIG. 2 includes 
contacting of the transfer material with the toner thin layer and 
separating of the toner (toner image) having a given polarity only from 
the toner thin layer. Generally, the adherence between the toner and the 
transfer material is lacking, and it is accordingly difficult to 
sufficiently contact the transfer material with the toner thin layer, thus 
making an improvement in transfer efficiency difficult. 
In the xerography system as mentioned above wherein a toner image only to 
be transferred is formed on the photosensitive body, there has been 
proposed that a presser means for pressing the transfer material is 
provided on an upstream side of a transfer region into which the transfer 
material is introduced or on a downstream side of the transfer region from 
which the transfer material is discharged, so as to improve the contact 
between the transfer material and the toner on the toner retaining member 
(e.g., a photosensitive drum). However, since the adherence between the 
transfer material and the toner is low, the presser means as mentioned 
above cannot yet provide a sufficient contact between the transfer 
material and the toner thin layer (toner image) at an opening portion of 
the transfer charger located at the transfer region. Therefore, the image 
density cannot be increased. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved 
electrophotographic image-forming method which can eliminate the 
above-mentioned problem in the conventional electrophotographic 
image-forming method, and can form a toner image having a desired high 
density steadily and stably. 
According to the present invention, there is provided an image-forming 
method comprising the steps of forming a thin layer of a photosensitive 
toner charged to have a given polarity on a conductive drum, exposing said 
thin layer to light according to image information, and transferring a 
part of said photosensitive toner onto a transfer material, said 
transferring step comprising contacting said transfer material with said 
thin layer after said exposing step while applying a pressure to said 
transfer material, and irradiating a corona ion from a back side of said 
transfer material to said thin layer by using a corona charger. 
The feature of the present invention is that a presser member for pressing 
the transfer material against the toner thin layer is provided at the 
opening portion of the transfer charger. 
According to the present invention, the presser member for maintaining the 
transfer material in pressure contact with the toner thin layer is 
provided at the opening region of the transfer charger where the corona 
ion is irradiated to carry out an actual transfer operation. With this 
arrangement, the contact between the toner thin layer and the transfer 
material can be maintained sufficient during the transfer operation, 
resulting in remarkable improvement in transfer efficiency.

DETAILED DESCRIPTION AND THE PREFERRED EMBODIMENTS OF THE INVENTION 
The electrophotographic image-forming method of the present invention is 
embodied by using a suitable electrophotographic image-forming system for 
forming an image by forming an electrostatic latent image with use of a 
photosensitive toner and transferring the latent image onto a transfer 
material such as a sheet of paper. 
A typical example of such an electrophotographic image-forming system is 
shown in FIG. 4. Referring to FIG. 4, a developing device 41 for uniformly 
forming a thin layer of a photosensitive toner on a conductive drum 43 is 
provided at a left upper position with respect to the conductive drum 43 
as viewed in FIG. 4. The developing device 41 includes a magnetic sleeve 
42 and an agitating roller 56. A mixture of a ferrite carrier and the 
photosensitive toner is uniformly agitated by the agitating roller 56, and 
is fed to the magnetic sleeve 42 while a triboelectric charge is imparted 
to the photosensitive toner, whereby a magnetic brush of the mixture of 
the photosensitive toner and the carrier is formed on the magnetic sleeve 
42. The magnetic brush formed on the magnetic sleeve 42 is rubbed on the 
conductive drum 43 to form the thin layer of the photosensitive toner on 
the conductive drum 43. In actually forming the thin layer, the conductive 
drum 43 is grounded, and a bias voltage is applied from a power source 60, 
which is grounded, to the magnetic sleeve 42. Accordingly, a developing 
bias is operated between the magnetic sleeve 42 and the conductive drum 43 
to carry out bias development with the operation of the triboelectric 
charge possessed by the photosensitive toner. 
The thin layer of the photosensitive toner formed on the conductive drum 43 
as mentioned above is fed to an exposure region by the rotation of the 
conductive drum 43. Exposure means 44 having a light source using a 
semiconductor laser is provided on a right side of the conductive drum 43 
as viewed in FIG. 4. A laser beam is irradiated from the exposure means 44 
to the thin layer of the photosensitive toner according to image 
information. The photosensitive toner exposed is improved in its 
conductivity, and as a result, the charge possessed by the toner is 
dissipated through the drum 43. 
Thereafter, the photosensitive toner is fed to a transfer device provided 
under the drum 43 as viewed in FIG. 4. The transfer device includes a 
transfer charger 45 of a corona discharge type. A sheet of paper 63 is fed 
to between the drum 43 and the transfer charger 45, and a charge generated 
by corona discharge is provided from a back surface of the paper 63, 
thereby transferring the photosensitive toner corresponding to the latent 
image onto the paper 63. 
Thereafter, the paper 63 on which the photosensitive toner has been 
transferred is fed to a fuser 47, and is fixed by a heat roller 65 and a 
pressure roller 66 in the fuser 47. The remaining photosensitive toner not 
transferred but left on the conductive drum 43 is squeezed off from the 
drum 43 by a cleaning blade 48 provided under the magnetic sleeve 42 of 
the developing device 41, and is then fed to the agitating roller 56 by a 
feeding roller 64. 
In such an image-forming device, the present invention improves the 
transfer charger 45 in order to well transfer the photosensitive toner. As 
shown in FIG. 1, reference numeral 11 designates a known transfer charger 
of a corona discharge type in itself. The transfer charger 11 is 
constructed of a sectional U-shaped shield case 10 (having opposed side 
walls 10a and 10b and a bottom wall 10c as shown in FIG. 3) and a charging 
wire 10' stretched in the shield case 10. A pair of presser members 13 for 
bringing a transfer material 33 into pressure contact with the thin layer 
of the photosensitive toner are fixedly mounted on the side walls 10a and 
10b of the transfer charger 11 at an upper opening 12 thereof. That is, 
the presser members 13 are formed of an insulating film having an 
elasticity, and they are mounted at two positions so as to retain opposite 
side portions of the transfer material passing over the presser members 13 
(i.e., at the opposite end portions of the transfer charger 11 at the 
upper opening 12). As shown in FIG. 3, in an actual transferring step, 
each presser member 13 is elastically deformed to form a downward recess 
14 which presses the transfer material 33 against a peripheral surface 31 
of the conductive drum through a toner thin layer 32 to thereby 
sufficiently bring the transfer material 33 into pressure contact with the 
toner thin layer 32, thus improving a transfer efficiency. The insulating 
film forming the presser members 13 is selected preferably from a material 
having a good slidability and mold-releaseability such as high-molecular 
polyethylene and polyester resin. 
While the pressure of the presser members 13 to be applied to the transfer 
material 33 depends upon a kind of the toner to be used, a peripheral 
speed of the conductive drum, a size of the upper opening of the transfer 
charger, etc., it is adjusted to be preferably in the range of from 30 to 
200 g/cm.sup.2, more preferably in the range of from 55 to 110 g/cm.sup.2. 
If the pressure is higher than this range, the toner corresponding to an 
non-image portion is unnecessarily transferred to cause the generation of 
fog. On the other hand, if the pressure is lower than this range, a 
desired effect of the present invention cannot be obtained. 
The construction of the presser members 13 may be modified as shown in 
FIGS. 6A and 6B. 
In the modified construction as shown in FIG. 6A, the presser members 13 
formed of an insulating film similar to that shown in FIG. 1 are fixed to 
one side wall 61 of the transfer charger at an upper opening thereof. As 
compared with the construction shown in FIG. 1 wherein the presser members 
31 are fixed to the opposed side walls of the transfer charger, the 
pressure to be applied to the transfer material can be made lower even in 
the case where the material of the insulating film is the same. 
In the modified construction as shown in FIG. 6B, the presser members 13 
formed of an insulating film are not fixed to the transfer charger but are 
fixed directly to any body independent of the transfer charger, e.g., a 
base body of the image-forming device. In this case, it is particularly 
advantageous in maintenance that the charger may be easily taken out for 
the purpose of cleaning, for example. 
In the image-forming method of the present invention, a voltage to be 
applied to the transfer charger is suitably set to 3.5-5.0 kV. 
It is preferable that the presser members may be movable according to the 
size of the transfer material. For example, the size of the transfer 
material is detected, and the presser members are slid on the transfer 
charger to the positions so as to press the opposite side portions of the 
transfer material according to the detected size of the transfer material. 
The photosensitive toner to be used in the present invention may be a known 
photosensitive toner in itself. For example, it may be selected from 
particles of composition formed by dispersing a photoconductive pigment in 
an electrical insulating resin medium. Examples of the photoconductive 
pigment may include an inorganic photoconductor such as zinc oxide and 
cadmium sulfide, and a photoconductive organic pigment such as perylene 
pigment, quinacridon pigment, pyranthrone pigment, phthalocyanine pigment, 
disazo pigment and trisazo pigment. The photoconductive pigment may be 
used in a proportion of preferably 3 to 600 parts by weight, more 
preferably 5 to 500 parts by weight versus 100 parts by weight of the 
fixing resin medium. 
The fixing resin medium may be selected from a known electrical insulating 
resin and a photoconductive resin such as polystyrene, styrene-acrylic 
copolymer, acrylic resin, polycarbonate, polyallylate, polyester and 
polyvinylcarbazole. Such a photoconductive resin may be used solely or in 
combination with the electrical insulating resin. Further, a known dye 
sensitizer or chemical sensitizer may be compounded in the fixing resin, 
so as to provide a sensitivity to a monochromatic light having a given 
wavelength range. In addition to the above essential components, a known 
auxiliary such as an offset prevention agent and a pressure fixation 
promoting agent such as wax can be used. 
It is desired that the particle size of the toner in median on the basis of 
a volume is preferably in the range of 6 to 12 .mu.m, more preferably 8 to 
10 .mu.m. Further, it is also desirable that a standard deviation 
(.sigma.) of distribution of the particle size on the basis of a volume is 
preferably 3.33 .mu.m or less, more preferably 2.24 .mu.m or less. If the 
particle size of the toner is larger than the above range, a charging 
quantity per unit weight is small. As a result, a contrast between an 
image portion and a non-image portion is reduced, and the toner 
corresponding to the non-image portion tends to be deposited onto the 
transfer material by the pressure applied at the transfer region. On the 
other hand, if the particle size of the toner is smaller than the above 
range, a light decay speed per particle of the toner is large, but it is 
difficult to obtain a desirable thickness of the toner thin layer. 
Further, if the standard deviation (.sigma.) of the particle size 
distribution is larger than the above range, the close contact between the 
transfer material and the toner thin layer becomes insufficient because a 
small-size particle portion and a large-size particle portion are present 
in the toner thin layer. As a result, a reduction in density and the 
fogging tend to occur. 
In the present invention, it is desired that the thickness of the toner 
thin layer is preferably in the range of 6 to 30 .mu.m, more preferably 10 
to 25 .mu.m. Accordingly, considering the above-mentioned range of the 
particle size of the toner, the number of the toner thin layer is 
preferably in the range of 1.5 to 2.5. In the above preferred embodiment 
as described with reference to FIG. 4, the toner thin layer is formed by 
forming a magnetic brush from the toner and a developer carrier such as 
ferrite which is generally used in this field, and rubbing the conductive 
drum with the magnetic brush. In another preferred embodiment, the toner 
may be supplied onto the conductive drum by a so-called non-magnetic 
monocomponent developing device as shown in FIG. 5 wherein an elastic 
metal blade 51 is pressed against a developing sleeve 52 so as to supply a 
toner 53. 
It is desired that the charging quantity of the toner for forming the thin 
layer is preferably in the range of .+-.5 to .+-.25 .mu.C/g, more 
preferably in the range of .+-.8 to .+-.10 .mu.C/g. In this range, the 
generation of fog can be prevented in the transfer region, and a 
high-density image can be obtained. 
In the following, some examples of the present invention will be described. 
However, it is to be noted that the present invention should not be 
limited to these examples. 
EXAMPLES 1 TO 4 
In accordance with the following recipe, a photosensitive toner having an 
average particle size of 10 .mu.m was obtained. 
______________________________________ 
Zinc Oxide 300 parts by weight 
(Hakusui Kagaku Grade #2) 
Styrene-Acrylic Resin 
100 parts by weight 
(Mitsui Toatsu 25) 
Cyanine Pigment 0.3 parts by weight 
(Nippon Kanko Shikiso NK1414) 
Black Perylene Pigment 
5 parts by weight 
(BASF L0086) 
______________________________________ 
The photosensitive toner obtained above was mixed with a ferrite carrier in 
the ratio of 5:95 to prepare a developer. The developer prepared above was 
applied to the image-forming device shown in FIG. 4 to carry out image 
formation. 
The development was conducted under the following conditions of; a brush 
cutting clearance (d1) of 0.9 mm, a drum-sleeve distance (d2) of 1.15 mm, 
a drum peripheral speed of 90 mm/sec, and a toner charging quantity of -9 
.mu.C/g. Under the above development conditions, a bias voltage of -300 V 
having the same polarity as that of the charge of the toner was applied to 
the magnetic sleeve 42 to form a toner thin layer constituted of two 
layers on the conductive drum 43. 
Then, the toner thin layer deposited on the conductive drum 43 was exposed 
by the semiconductor laser 44 according to image information to form an 
electrostatic latent image 49 which was in turn fed to the transfer 
device. 
In the transfer device, a voltage (-4.7 kV) having the same polarity as 
that of the charge of the toner was applied to the corona charger 45 by 
the power source 46. Accordingly, a back surface of a sheet of paper as 
the transfer material was negatively charged to form an electric field 
between the drum and the paper. As the photosensitive toner exposed is 
reduced in electric resistance, a positive charge is injected into the 
toner by the electric field. As a result, the toner having the positive 
charge was transferred onto the paper. 
The insulating films as the presser members 13 as shown in FIG. 1 were 
previously mounted at the upper opening of the transfer charger, so as to 
improve the close contact of the paper as the transfer material with the 
toner thin layer. 
In the transferring step, the pressure of the films 13 to be applied to the 
drum surface was variously adjusted to 30, 55, 110 and 200 g/cm.sup.2 by 
using four kinds of polyethylene terephthalate films having different 
thicknesses with the same size as follows: 
______________________________________ 
Applied Pressure (g/cm.sup.2) 
Film Thickness (.mu.m) 
______________________________________ 
33 100 (Example 1) 
55 130 (Example 2) 
110 170 (Example 3) 
200 220 (Example 4) 
______________________________________ 
After the transferring step, the transfer material was fed to the fuser 47 
to fix the toner image. On the other hand, the remaining toner not 
transferred was recovered into the developing device by the cleaning blade 
48 provided therein. 
COMATIVE EXAMPLE 
The image formation was carried out under the same conditions as those in 
Example 1, provided that the insulating films were not provided (i.e., no 
pressure was applied to the drum). 
EVALUATION 
A transfer rate at the image portion and the non-image portion according to 
Examples 1 to 4 and Comparison was evaluated to obtain the result shown in 
FIG. 7. As apparent from FIG. 7, the transfer rate at the image portion 
according to Examples 1 to 4 wherein the pressure was applied is 
remarkably improved. Further, the toner images obtained according to 
Examples 1 to 4 using the insulating films were satisfactory with no void. 
To the contrary, the toner image obtained according to Comparison using no 
insulating film was unsatisfactory such that an image density was low and 
a void was present. 
The density and the quality (void) of the images obtained according to 
Examples 1 to 4 and Comparative Example are shown in Table 1. 
TABLE 1 
______________________________________ 
Applied 
Pressure 
Image Non-image 
(g/cm.sup.2) 
Density Density Void 
______________________________________ 
Example 1 33 0.86 0.009 None 
Example 2 55 0.90 0.012 None 
Example 3 110 0.90 0.015 None 
Example 4 200 0.96 0.016 None 
Comparative 
0 0.36 0.008 Observed 
Example 
______________________________________ 
As apparent from the foregoing description, the image-forming method of the 
present invention using a photosensitive toner employs the presser members 
for bringing the transfer material into pressure contact with the toner 
thin layer at the opening portion of the corona charger (i.e., at an 
actually transferring region). Therefore, a transfer efficiency of the 
toner to the transfer material can be improved to thereby obtain a 
satisfactory image having a high density and no void. 
While the invention has been described with reference to specific 
embodiments, the description is illustrative and is not to be construed as 
limiting the scope of the invention. Various modifications and changes may 
occur to those skilled in the art without departing from the spirit and 
scope of the invention as defined by the appended claims.