Ink transfer medium for toner, ink transfer process and re-inking process for the same

An ink transfer medium for toner is disclosed, comprising the following layers, in this order: a heat-generating resistive layer heated by input of electrical signal, a conductive layer, and an ink release layer having a toner layer on it, an adhesion from 0.2 g/mm to 40 g/mm as measured by the 90.degree. peeling off method and consisting of a viscoelastomer having a critical surface tension of 35 dyne/cm or less. It can be re-inked by attaching toner to the portions of the ink transfer medium from which the ink has been removed, using toner held in a toner holding means. An ink transfer process for toner is also disclosed, wherein an ink transfer medium having toner layer formed on the above described ink release layer is used and the toner layer contacts the recording medium under pressure, while a recording head contacts the anisotropic conductive layer of the ink transfer medium, so that an electrical signal is input and then toner fused by heat generated by the heat-generating resistive layer is transferred to the recording medium.

FIELD OF THE INVENTION 
This invention relates to an ink transfer medium for toner, an ink transfer 
process and a re-inking process for the same. 
BACKGROUND OF THE INVENTION 
Conventionally, various proposals have been made for current-conducting 
thermal transfer recording method in which ink is transferred to a 
recording medium by inputting an electrical signal corresponding to an 
image signal to an ink transfer medium having an anisotropic conductive 
layer and a heat-generating resistive layer. For example, Japanese 
unexamined patent publications Sho 56-10479 (1981), Sho 60-259485 (1985) 
and Hei 1-113276 (1989) describe a printing process using an ink transfer 
medium having a toner layer adhering to it, which is first fused and then 
mechanically leveled. Japanese unexamined patent publication Sho 63-297084 
(1988) describes a printing process in which toner is attached uniformly 
to an ink transfer medium by electrostatic force and then ink is 
transferred to a recording medium in a pattern corresponding to an image 
signal. 
In the first of the above described processes in which toner is held to an 
ink transfer medium by heat, however, because of the use of heat, there 
are problems of high power consumption and in that the recording apparatus 
is inevitably bulky. Although toner is fused to the ink transfer medium by 
heat and mechanically leveled to form a thin layer, inevitably there are 
fine irregularities in the ink layer itself. Because of these 
irregularities, and also because of irregularities remaining where ink has 
been removed in the transfer process, and further because material 
properties such as electrical conductivity, dielectric constant change 
when the toner is fused to form a solid layer, the re-inking of the ink 
transfer medium is not stable. The second of the above described processes 
in which toner is held to the ink transfer medium by electrostatic force 
has problems that toner is not retained well because the retention is done 
only by electrostatic force. The toner tends to fall off because of 
vibration and the like, and this leads to problems such as background 
fogging. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an ink transfer medium 
and an ink transfer process for toner free of the defects found in the 
above described conventional processes. 
Accordingly, it is an object of the present invention to provide an ink 
transfer medium for toner capable of retaining toner uniformly and 
reliably. 
It is a further object of the present invention to provide an ink transfer 
process for toner capable of good image printing with the use of the above 
described ink transfer medium. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows and in part will be apparent to a 
person with ordinary skill in the art from the description, or may be 
learned by practice of the invention. 
This invention relates to an ink transfer medium for toner comprising the 
following layers, in this order: a heat-generating resistive layer heated 
by input of an electrical signal, a conductive layer, and an ink release 
layer having a toner layer on it, having an adhesion from 0.2 g/mm to 40 
g/mm as measured by the 90.degree. peeling off method and consisting of a 
viscoelastomer having a critical surface tension of 35 dyne/cm or less. 
This invention also relates to an ink transfer process for toner in which 
an ink transfer medium having a toner layer formed on the above described 
ink release layer is used and the toner layer contacts the recording 
medium under pressure, while a recording head contacts the anisotropic 
conductive layer of the ink transfer medium so that an electrical signal 
is input and then toner fused by heat generated by the heat-generating 
resistive layer is transferred to the recording medium. 
The above described ink transfer medium for toner can be re-inked by 
attaching toner to the portions of the ink transfer medium from which the 
ink has been removed, using toner held in a toner holding means.

DETAILED DESCRIPTION OF THE INVENTION 
The following are explanations of each component of the ink transfer medium 
of this invention. 
In FIG. 1, the ink transfer medium 1 comprises the following layers, in 
this order: an anisotropic conductive layer 3, a heat-generating resistive 
layer 2, a conductive layer 4, and an ink release layer 5. 
The anisotropic conductive layer which reduces the contact resistance 
between the recording head and the ink transfer medium in the thickness 
direction and which also reduces heat generation and concomitant damage 
caused by contact friction with the recording head consists of a fine 
pattern of isolated electrodes distributed uniformly over the 
heat-generating resistive layer 2 which acts as a support. Metals such as 
nickel, copper, chrome, tin, tantalum, titanium, zinc, gold, silver, 
aluminum, platinum, conductive ceramics such as ruthenium oxide, silicon 
carbide, tungsten carbide, silicon molybdenum, titanium carbide, or 
polymers with conductive materials dispersed in them can be used for 
forming the isolated electrodes. The shape and size of the electrodes can 
be selected appropriately. 
The heat-generating resistive layer conducts the current from the 
anisotropic conductive layer to the conductive layer and produces heat 
ohmically, which then fuses the toner of the toner layer. For example, the 
layer may consist of a conductive material made up of a heat resistant 
resin with conductive materials such as carbon or metal powder dispersed 
in it (polyimide resins, polyimideamide resins, silicone resins, 
fluorine-based resins, epoxy resins, ceramics ) and having a thickness 
from 0.05 .mu.m to 50 .mu.m, preferably from 0.1 .mu.m to 30 .mu.m. 
The conductive layer acts as a return path for the current coming into the 
heat-generating resistive layer, and it comprises a material whose volume 
resistivity is from 10.sup.3 to 10.sup.-1 .OMEGA..multidot.cm. The film 
thickness is generably from 50 nm to 50 .mu.m, preferably from 0.1 .mu.m 
to 50 .mu.m. 
The ink release layer keeps the toner as a uniform thin layer, and it is 
made of materials capable of releasing the toner when they are fused. The 
ink release layer of this invention should have a critical surface tension 
of 35 dyne/cm or less, and an adhesion from 0.2 g/mm to 40 g/mm as 
measured by the 90.degree. peeling off method. 
Conventionally, adhesion is defined by JIS Z 0109 6007 as the force 
resulting from contact between the adhesive side of an adhesive tape or an 
adhesive sheet and an object, but for the purpose of this specification, 
it is regarded as the value of adhesion evaluated by the 90.degree. 
peeling off method described in JIS Z 0237 as the value of the adhesion 
between a standard adhesive tape (Nitoflon number 903 available from Nitto 
Denko Corp.) and an ink release layer. 
If the critical surface tension of the ink release layer exceeds 35 
dyne/cm, the affinity between the fused toner and the ink release layer is 
excessive and the release property of the toner declines, and perfect 
transfer of ink to the recording medium cannot be made. Since the fused 
toner remain as residual ink in the next process, this causes background 
fogging caused by new toner overlying old one. 
When the adhesion reaches 40 g/mm or more as measured by the 90.degree. 
peeling off method, as the adhesion of the ink release layer is excessive 
and toner cannot be released in the interface between the ink release 
layer and the toner layer, release occurs in the interface between the 
recording medium and the ink release layer, or as coagulation breakdown 
occurs in the recording medium and a part of the recording medium is 
peeled off, the peeled off part adheres to the ink transfer medium. In the 
worst case, the recording medium may cause a jam instead of being 
released. 
When the value of an adhesion as measured by the 90.degree. peeling off 
method becomes less than 0.2 g/mm, as the force keeping toner on the ink 
release layer becomes too low, toner may fall off because of vibration and 
the like, or background fogging may occur in printing. 
Viscoelastomers are used for forming the above described ink release layer 
of this invention. For example, they may be silicone rubber, liquid type 
silicone, modified silicone, fluorosilicone rubber, silicone resin, 
silicone coating agents and the like. The thickness of the ink release 
layer is from 0.3 .mu.m to 10 .mu.m. 
As toner adhering to the ink release layer of the above described ink 
transfer medium, any known toner can be used if it consists of particles 
having colorants in a thermally-melting resin and whose average particle 
diameter is from 2 .mu.m to 20 .mu.m. 
An ink transfer process for toner using the above described ink transfer 
medium is now described, referring to the attached Figures. FIG. 2 is a 
schematic of an example of a printing apparatus used in the ink transfer 
process for toner of this invention. In it, 10 is an endless belt type ink 
transfer medium having toner thereon of the structure shown in FIG. 1, and 
it is mounted on a carrying roller 11, facing roller 12, and tension 
roller 13 with a certain tension. 
20 is a re-inking apparatus. Toner holding means 23 is positioned in hopper 
21, and toner 22 is also kept in hopper 21. Toner holding means 23 is 
positioned such that it faces facing roller 12 with a specified spacing, 
and a bias voltage is applied to the toner holding means by a bias 
electrical source 25. 24 is a toner trimming means. 
30 is a recording section, and a recording head 31 which applies an 
electrical signal corresponding to an image to the heat-generating 
resistive layer of the ink transfer medium is positioned with the 
anisotropic conductive layer of the ink transfer medium contacting it. 
Backing pressure roller 33 is positioned facing the recording head 31 and 
pressure means 39 is positioned near the recording head. Recording medium 
34 is positioned such that it is carried by recording medium carrying 
rollers 35 to 38. 32 is a return-circuit contact roller which is arranged 
to be in contact with the electrically conductive portion of the 
print-recording medium exposed at the side of the former. 
When printing is done using the above described ink recording apparatus for 
toner, the ink transfer medium is carried in the direction of the arrow by 
driving carrying roller 11. Toner 22 is supplied to the ink release layer 
of the ink transfer medium and the ink transfer medium with toner on it is 
carried to the recording section. In the recording section, the recording 
head applies an electrical signal corresponding to an image to the ink 
transfer medium with the anisotropic conductive layer contacting it under 
pressure, and it makes the resistive layer of the ink transfer medium 
generate heat corresponding to the electrical signal, and then fuse the 
toner. As a toner layer on the ink transfer medium is carried with the 
recording medium contacting it under pressure, fused toner is transferred 
to the recording medium and the transferred image is formed on it. 
Sufficient contact pressure is necessary, because if the ink transfer 
medium having toner layer does not contact the recording medium under 
pressure, the fused toner layer is thinner than non-fused one, and it does 
not contact the recording medium, which leads to transfer failure. 
In this invention, as the ink release layer has the above described 
structure, when toner is supplied in a re-inking apparatus, the toner 
adheres to the exposed portions of the ink release layer where toner has 
been removed by the transfer operation. Since higher layers of toner would 
not contact the ink release layer, the toner forms a layer with 
substantially the thickness of a single layer, in another words, with a 
thickness corresponding to the diameter of the toner particles. In this 
invention, it is not necessary to level mechanically the toner adhering to 
the ink release layer by applying heat treatment, and it can form a stable 
single layer. 
FIG. 3 is a schematic of another example of a printing apparatus used in 
the ink transfer process for toner of this invention. In FIG. 3, a spring 
blade means having silicone rubber thereon 26 contacts the toner holding 
means of the re-inking apparatus under pressure, and it controls the 
supply of toner carried by toner holding means 23, together with giving a 
required amount of electrical charge to the toner. 
FIG. 4 is a schematic of another example of a printing apparatus used in 
the ink transfer process for toner of this invention. In the re-inking 
apparatus of FIG. 4, toner 22 is mixed with electrophotographic carriers 
28 by a known stirring apparatus not shown there, is given required amount 
of electrical charge and then is carried on toner holding means 23 by 
carriers 28. 
FIG. 5 is a schematic of another example of a printing apparatus used in 
the ink transfer process for toner of this invention. In FIG. 5, toner 
holding means 23 of the re-inking apparatus consists of a spongy roller 
with a spongy layer such as polyurethane foam formed on a metal shaft, and 
in this re-inking apparatus, toner is supplied with the spongy roller 
contacting the ink transfer medium. 26 is a spring blade means having 
silicone rubber thereon and it controls the toner supply carried on the 
toner holding means 23. 27 is a flexible blade, and it removes excess 
toner adhering to the ink transfer medium. 
EMBODIMENT 1 
Printing was done using the below described ink transfer medium and the ink 
recording apparatus for toner shown in FIG. 2. 
As an ink transfer medium, chromium having a thickness of 0.6 .mu.m was 
deposited on one side of a conductive polyimide film having a surface 
resistance value of 550 .OMEGA./.quadrature., and a thickness of 30 .mu.m 
by the high frequency sputtering method. A photoresist was formed on this 
chromium layer, and a resist film having a film thickness of 1.2 .mu.m was 
formed after being subject to prebaking at 90.degree. C. for eight 
minutes. Next, a residual resist film was hardened in a nitrogen 
atmosphere at 120.degree. C. for eight minutes after optically exposing a 
circular dot pattern having a spacing of 20 .mu.m and dot diameter of 16 
.mu.m on the resist film, developing and rinsing it. Next, an anisotropic 
conductive layer consisting of a circular dot pattern having a spacing of 
20 .mu.m and dot diameter of 16 .mu.m was formed after etching parts of 
the chromium layer having no resist film on them with hydrochloric acid 
and removing the resist film by ultrasonic vibration in acetone bath. 
Next, an ink release layer having a critical surface tension of 25 dyne/cm 
and a thickness of 2 .mu.m was formed after depositing aluminum on the 
other side of this conductive polyimide film with a thickness of 0.3 .mu.m 
by the high frequency sputtering method and further, applying a 
thermohardening silicone resin having a volume resistivity of 10.sup.14 
.OMEGA..multidot.cm(KR2706 available from Shin-Etsu Chemical Industry Co. 
Ltd.) over the film except both edges and hardening the film at 
150.degree. C. for 30 minutes. When adhesion was measured for this ink 
release layer using a standard adhesive tape (Nitoflon number 903 
available from Nitto Denko Corp.) and by the 90.degree. peeling off method 
described in JIS Z 0237, it was 4 g/mm. Next, an endless belt type ink 
transfer medium was obtained by joining the ends of the polyimide film 
with the anisotropic conductive layer inside. 
A toner layer was formed on this ink transfer medium as follows. 
Particles having a volume average particle diameter of 10 .mu.m were 
obtained by mixing 60 parts by weight of polyester resin and 40 parts by 
weight of magnetic powder as toner, applying heat and kneading, 
pulverizing by jet mill and classifying them, as is well-known in the art. 
The toner was prepared by mixing 97 parts by weight of these particles and 
3 parts by weight of carbon black using a Henschel mixer. When 10 g of 
this toner was molded by pressure forming into a disk having a diameter of 
5 cm and a thickness of 2.5 mm, its volume resistivity was found to be 
10.sup.3 .OMEGA..multidot.cm. 
An electrophotographic magnet roller was used as a toner holding means in 
the re-inking apparatus and it was positioned with a spacing of 0.4 mm 
from the above described ink transfer medium. Further, a toner layer 
consisting of substantially a single particle layer was formed by rotating 
the toner holding means in the forward direction of the ink transfer 
medium at a linear velocity of 250 mm/s, applying a bias voltage of 50 V 
to the toner holding means, and supplying the above described toner to the 
ink transfer medium. Printing was done by rotating an ink transfer medium 
with toner adhering to it as described above at a linear velocity of 100 
mm/s, making it contact the recording medium under a backing pressure 
roller, making the recording head having a resolution of 400 spot per inch 
contact the anisotropic conductive layer of the ink transfer medium under 
pressure, applying a pulse of 20 mA to the recording head for 600 .mu.s, 
fusing the toner, having the ink transfer medium with the recording medium 
adhering to it pass a pressure means consisting of a pair of rubber 
rollers and releasing them from the ink transfer medium. The toner 
transfer rate was 98% and the printing image optical density was measured 
as 1.6. On the other hand, fogging which was considered to be a practical 
problem did not occur on the non-printed part. 
After printing, re-inking was done by attaching toner to the places where 
ink had been transferred on the above described ink transfer medium in the 
same manner as described above. The toner transfer rate and the printing 
image optical density were measured after repeating the above described 
process 100 times. When measured, the toner transfer rate was 96% and the 
printing image optical density was 1.5. Fogging which was considered to be 
a practical problem did not occur on the non-printed part. 
EMBODIMENT 2 
An ink transfer medium was prepared in the same manner as Embodiment 1 
except that the ink release layer had a critical surface tension of 30 
dyne/cm, a thickness of 3 .mu.m and an adhesion of 28 g/mm after applying 
a thermohardening silicone low-adhesion agent having a volume resistivity 
of 10.sup.14 .OMEGA..multidot.cm or more instead of a thermohardening 
silicone resin having a volume resistivity of 10.sup.14 
.OMEGA..multidot.cm or more as an ink release layer, thermally hardening 
it at 100.degree. C. for 3 minutes. Toner adhered to the ink transfer 
medium and printing was done in the same manner as in Embodiment 1 except 
using this ink transfer medium, making the bias voltage of the toner 
holding means 70 V and applying a pulse of 25 mA to it. When measured, the 
toner transfer rate was 95% and the printing image optical density was 
1.4. Fogging which was considered to be a practical problem did not occur 
on the non-printed part. After printing, re-inking was done by attaching 
toner to the places where ink had been transferred on the above described 
ink transfer medium in the same manner as described above. The toner 
transfer rate and the printing image optical density were measured after 
repeating the above described process 100 times. The toner transfer rate 
was 94% and the printing image optical density was 1.3. Fogging which was 
considered to be a practical problem did not occur on the non-printed 
part. 
EMBODIMENT 3 
Printing was done using the ink recording apparatus for toner shown in FIG. 
3. In more detail, it was done by using the same toner and the same 
electrophotographic magnet roller as toner holding means as in Embodiment 
1 except that 0.8 parts by weight of silica particles were mixed with the 
toner instead of carbon black, pressing a spring blade means composed of 
SUS303 having silicone rubber thereon and having a thickness of 0.1 mm 
against the toner holding means surface with a linear pressure of 100 g/cm 
such that the amount of toner supplied to the toner holding means surface 
was controlled and an electrical charge of -5 .mu.C/g was added to the 
toner. 
A toner layer consisting of substantially a single particle layer was 
formed by positioning this toner holding means and the same ink transfer 
medium as in Embodiment 1 at a spacing of 0.3 mm, rotating the toner 
holding means in the forward direction of the ink transfer medium at a 
linear velocity of 200 mm/s, applying an AC voltage with a DC voltage of 
+400 V superimposed on it and having an amplitude of 1000 V and a 
frequency of 2 kH.sub.z as a bias voltage, supplying and attaching the 
above described toner to the ink transfer medium. 
When the toner transfer rate and the printing image optical density were 
measured after printing was done in the same manner as in Embodiment 1, 
using an ink transfer medium with toner adhering to it as described above, 
the toner transfer rate was 97% and the printing image optical density was 
1.7. On the other hand, fogging which was considered to be a practical 
problem did not occur on the non-printed part. 
After printing, re-inking was done by attaching toner to the places where 
ink had been transferred on the above described ink transfer medium in the 
same manner as described above. The toner transfer rate and the printing 
image optical density were measured after repeating the above described 
process 100 times. The toner transfer rate was 96% and the printing image 
optical density was 1.6. Fogging which was considered to be a practical 
problem did not occur on the non-printed part. 
EMBODIMENT 4 
Printing was done using the ink recording apparatus for toner shown in FIG. 
4. In more detail, particles having a volume average particle diameter of 
7 .mu.m were obtained after mixing 93 parts by weight of polyester resin 
and 7 parts by weight of carbon black as toner, and applying processes of 
fusing and kneading, pulverizing by jet mill and classifying, as is 
well-known in the art. Further, toner was prepared after mixing 99.2 parts 
by weight of this particle and 0.8 parts by weight of silica particles 
using a Henschel mixer. A developer having a triboelectrification value of 
-12 .mu.C/g was prepared by mixing 92 parts by weight of 
electrophotographic carrier particles having a volume average particle 
diameter of 50 .mu.m and 8 parts by weight of the above described toner. A 
toner layer consisting of substantially a single particle layer was formed 
by supplying the above described toner agent to the same ink transfer 
medium and attaching only toner to it in the same manner as in Embodiment 
1 except applying an AC voltage with a DC voltage of +500 V superimposed 
on it and having an amplitude of 1000 V and a frequency of 2 kH.sub.z as a 
bias voltage. 
When the toner transfer rate and the printing image optical density were 
measured after printing was done in the same manner as in Embodiment 1, 
using an ink transfer medium with toner adhering to it as described above, 
the toner transfer rate was 98% and the printing image optical density was 
1.7. On the other hand, fogging which was considered to be a practical 
problem did not occur on the non-printed part. 
After printing, re-inking was done by attaching toner to the places where 
ink had been transferred on above described ink transfer medium in the 
same manner as described above. The toner transfer rate and the printing 
image optical density were measured after repeating the above described 
process 100 times. The toner transfer rate was 96% and the printing image 
optical density was 1.6. Fogging which was considered to be a practical 
problem did not occur on the non-printed part. 
EMBODIMENT 5 
Printing was done using the same ink transfer medium as in Embodiment 1 and 
the ink recording apparatus for toner shown in FIG. 5. In more detail, 
particles having a volume average particle diameter of 10 .mu.m were 
obtained after mixing 95 parts by weight of polyester resin and 5 parts by 
weight of cyan pigment as toner, applying processes of fusing and 
kneading, pulverizing by jet mill and classifying them, as is well-known 
in the art, using the same ink transfer medium as in Embodiment 1. 
Further, toner was prepared after mixing 94 parts by weight of these 
particles and 6 parts by weight of titanium oxide powder using a Henschel 
mixer. When 5 g of these toner particles were molded by pressure forming 
into a disk having a diameter of 5 cm and a thickness of 2.5 mm, its 
volume resistivity was found to be 10.sup.5 .OMEGA..multidot.cm. 
A spongy roller with a polyurethane foam layer having an apparent specific 
gravity of 3.times.10.sup.-2 g/cm.sup.3 and a volume resistivity of 
10.sup.8 .OMEGA..multidot.cm formed on it was used as a toner holding 
means in a re-inking apparatus. Toner supplied to the toner holding means 
surface was controlled by pressing a spring blade means composed of SUS303 
having silicone rubber thereon and having a thickness of 0.1 mm to this 
toner holding means with a linear pressure of 50 g/cm. The above described 
toner was supplied and attached to the ink transfer medium by making this 
toner holding means contact the above described ink transfer medium, 
rotating the toner holding means in the forward direction of the ink 
transfer medium at a linear velocity of 250 mm/s, applying a bias voltage 
of 150 V to the toner holding means, and supplying and adhering the above 
described toner to the ink transfer medium. Further, a toner layer 
consisting of substantially a single particle layer was formed by removing 
excessive toner by a flexible blade. 
Printing was done by using the ink transfer medium with toner adhering to 
it as described above in the same manner as in Embodiment 1. The toner 
transfer rate was 98% and the printing image optical density was measured 
as 1.4. On the other hand, fogging which was considered to be a practical 
problem did not occur on the non-printed part. After printing, re-inking 
was done by attaching toner to the places where ink had been transferred 
on the above described ink transfer medium in the same manner as described 
above. The toner transfer rate and the printing image optical density were 
measured after repeating the above described process 100 times. The toner 
transfer rate was 97% and the printing image optical density was 1.4. 
Fogging which was considered to be a practical problem did not occur on 
the non-printed part. 
COMATIVE EMBODIMENT 1 
An ink transfer medium was prepared in the same manner as Embodiment 1 
except forming an ink release layer having a critical surface tension of 
25 dyne/cm, a thickness of 2 .mu.m, an adhesion of 0.12 g/mm by applying a 
thermohardening silicone resin having a volume resistivity of 10.sup.14 
.OMEGA..multidot.cm or more (KS-779H available from Shin-Etsu Chemical 
Industry Co. Ltd.) and thermally hardening it at 150.degree. C. for a 
minute. Printing was done using this ink transfer medium and attaching 
toner to it in the same manner as in Embodiment 1. When measured, the 
toner transfer rate was 97% and the printing image optical density was 
1.5. Fogging which was considered to be a practical problem occurred on 
the non-printed part. 
COMATIVE EMBODIMENT 2 
An ink transfer medium was prepared in the same manner as Embodiment 1 
except forming an ink release layer having a critical surface tension of 
38 dyne/cm, a thickness of 3 .mu.m, an adhesion of 32 g/mm by applying a 
thermohardening silicone resin and thermally hardening it at 150.degree. 
C. for three minutes. Printing was done in the same manner as Embodiment 1 
except using this ink transfer medium, making the bias voltage of the 
toner holding means 70 V and applying a pulse of 25 mA to it. When 
measured, the toner transfer rate was 68% and the printing image optical 
density was 1.0, and fused toner remained on the ink transfer medium. 
Re-inking and printing were done again in the same manner as in Embodiment 
1, and fogging which was considered to be a practical problem occurred in 
the places where there had been a printed image in the previous cycle. 
COMATIVE EMBODIMENT 3 
An ink transfer medium was prepared in the same manner as Embodiment 1 
except forming an ink release layer having a critical surface tension of 
30 dyne/cm, a thickness of 3 .mu.m, an adhesion of 6 g/mm by applying a 
thermohardening silicone rubber having a volume resistivity of 10.sup.14 
.OMEGA..multidot.cm or more (KE1820 available from Shin-Etsu Chemical 
Industry Co., Ltd.) and thermally hardening it at 150.degree. C. for three 
minutes. Attaching toner to the ink transfer medium and printing were done 
in the same manner as Embodiment 1 except using this ink transfer medium, 
making the bias voltage of the toner holding means 70 V and applying a 
pulse of 25 mA to it. Releasing of the recording medium from the ink 
transfer medium, however, could not be done, and as jams occurred, the 
experiment was abandoned.