Process for size separating toner particles

A process for classifying toner particles supplied through a supply nozzle into at least three fractions in a classifying chamber divided into at least three sections and placed under a reduced pressure under the action of the inertia force of the toner particles supplied together with a gas stream and the centrifugal force of the curved gas stream due to Coanda effect. A first gas introduction pipe and a second gas introduction pipe are disposed above the classifying chamber so as to provide a first inlet and a second inlet opening with the first inlet being disposed closer to the supply nozzle than the second inlet. The absolute values of the static pressures P.sub.1 and P.sub.2 in the first and second gas introduction pipes are controlled so as to satisfy the relations of: .vertline.P.sub.1 .vertline..gtoreq.150 mm.aq., .vertline.P.sub.2 .vertline..gtoreq.40 mm.aq. and .vertline.P.sub.1 .vertline.-.vertline.P.sub.2 .vertline..gtoreq.100 mm.aq.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to a process for producing a toner having a 
predetermined particle size for developing electrostatic images, by 
effectively classifying solid particles containing a binder resin. 
In image forming processes such as electrophotography, electrostatic 
photography and electrostatic printing, a toner is used to develop an 
electrostatic image. In order to produce a toner for developing 
electrostatic images, that is, a final product of fine particles, 
particles of a starting material after pulverization are classified to 
obtain the final product. Such a process involves melt-kneading starting 
materials such as a binder resin and a coloring agent (e.g., dye, pigment 
or magnetic material), cooling the kneaded mixture for solidification 
followed by pulverization. Solid particles obtained after pulverization 
are introduced into a classifier for removing fine particle fraction to 
obtain a product having a prescribed particle size range. 
The particle size used herein is expressed in terms of a weight-average 
particle size based on the results of measurement, e.g., by a Coulter 
counter available from Coulter Electronics, Inc. (U.S.A.). This is 
hereinafter simply referred to as "average particle size" or 
"weight-average particle size". 
For example, to provide a group of particles having a weight average 
particle size of 10 to 15 microns and containing 1% or less of particles 
having a particle size smaller than 5 microns, feed particles are 
subjected to classification by means of a gas stream classifier or a 
mechanical classifier to remove fine particles with a size below the 
prescribed value, whereby a product of desired size is obtained. 
Such a conventional process involves a problem that the residence time in a 
conventional classifier is so long as several minutes so that fine 
particles can be aggregated into larger particles which are difficult to 
remove as fine particles. As a result, the aggregates can be mixed into a 
final product so that it becomes difficult to obtain a product with an 
accurate particle size distribution. Further, such aggregates can be 
disintegrated during the use of the product toner to cause degradation in 
image quality. These problems are pronounced if a product with a smaller 
prescribed size is desired. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for producing 
a toner for developing electrostatic images, wherein the above mentioned 
various problems found in the prior art processes are overcome. A more 
specific object of the invention is to provide a process for effectively 
producing a toner for developing electrostatic images with an accurate 
particle size distribution. 
Another object of the invention is to provide a process for effectively 
producing a toner with good quality and a small particle size (e.g., 
weight-average particle size of about 2-8 .mu.m). 
Another object of the invention is to provide a process for producing a 
toner for developing electrostatic images with less aggregates of very 
fine particles. 
A further object of the invention is to provide a process for effectively 
producing a toner for developing electrostatic images which is capable of 
easily controlling a classification point. 
More specifically, the present invention relates to a process for producing 
a fine particle product (used as a toner) having an accurate and 
prescribed particle size distribution by effective classification in short 
time of solid particles obtained through melt-kneading, cooling and 
pulverization of a mixture of a binder resin, a colorant and various 
additives. 
The present invention further relates to a process for effectively 
classifying in short time of a polymerization toner produced by suspension 
polymerization. 
According to the present invention, there is provided a process for 
producing a toner for developing electrostatic latent images, comprising: 
generating a reduced pressure in a classifying chamber which is divided 
into at least three sections including a coarse powder section having a 
first outlet for withdrawing a coarse powder, a medium powder section 
having a second outlet for withdrawing a medium powder, and a fine powder 
section having a third outlet for withdrawing a fine powder, by sucking 
the classifying chamber through at least one of the first to third 
outlets; 
supplying to the classifying chamber a feed toner material comprising toner 
particles of 20 .mu.m or less in particle size in a proportion of 50% or 
more by number through a supply pipe having a supply nozzle opening into 
the classifying chamber at a velocity of 50 m/sec to 300 m/sec along with 
a gas stream flowing through the pipe; 
controlling the absolute value of a static pressure P.sub.1 to 150 mm.aq. 
or above in a first gas introduction pipe having a first gas inlet opening 
into the classifying chamber at a position upstream of the first gas inlet 
by a first gas introduction control means; 
controlling the absolute value of a static pressure P.sub.2 to 40 mm.aq. or 
above in a second gas introduction pipe having a second gas inlet opening 
into the classifying chamber at a position just upstream of the second gas 
inlet by a second gas introduction control means, the second gas inlet 
being disposed farther than the first gas inlet with respect to the supply 
nozzle; and 
distributing the feed toner material supplied to the classifying chamber 
into at least the coarse powder section, the medium powder section and the 
fine powder section under the action of the inertia force of the feed 
toner particles in the gas stream and the centrifugal force of the curved 
gas stream due to Coanda effect and under a condition where the absolute 
value .vertline.P.sub.1 .vertline. of the static pressure P.sub.1 and the 
absolute value .vertline.P.sub.2 .vertline. of the static pressure P.sub.2 
satisfy the relation of .vertline.P.sub.1 .vertline.-.vertline.P.sub.2 
.vertline..gtoreq.100 (mm.aq.). 
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 
In the process of the present invention, feed toner particles obtained 
through pulverization or polymerization are supplied to a multi-division 
classifying zone or chamber to be classified into at least three particle 
size fractions including a large particle size fraction (coarse powder 
comprising primarily coarse particles), a medium particle size fraction 
(medium powder comprising primarily particles having a particle size 
within a prescribed or defined range) and a small particle size fraction 
(fine powder comprising primarily particles having a paticle size smaller 
than the prescribed range), and each particle size fraction is taken out 
from the multi-division classifying zone through an appropriate takeout or 
withdrawal means. 
The particles of the medium particle size fraction thus taken out have a 
suitable particle size distribution and may be used as they are. On the 
other hand, it is possible to reuse the particles of the small particle 
size fraction by recycling them to the melt-kneading step. The particles 
of the large particle size fraction may be reused by recycling them to the 
pulverization step. 
An embodiment for providing such a multidivision classifying means may for 
example be a multidivision classifier as shown in FIG. 1 (sectional view) 
and FIG. 2 (perspective view). Referring to FIGS. 1 and 2, the classifier 
has side walls 22, 23 and 24, and a lower wall 25. The side wall 23 and 
the lower wall 25 are provided with knife edge-shaped classifying wedges 
17 and 18, respectively, whereby the classifying zone is divided into 
three sections. At a lower portion of the side wall 22, a feed material 
supply nozzle 16 opening into a classifying chamber is provided. A Coanda 
block 26 is disposed along the lower tangential line of the nozzle 16 so 
as to form a long elliptic arc shaped by folding the tangential line 
downwardly. The classifying chamber has an upper wall 27 provided with a 
knife edge-shaped gas-intake wedge 19 extending downwardly. Above the 
classifying chamber, gas-intake pipes 14 and 15 opening into the 
classifying chamber are provided. In the intake pipes 14 and 15, a first 
gas introduction control means 20 and a second gas introduction control 
means 21, respectively, comprising, e.g., a damper, are provided; and also 
static pressure gauges 28 and 29 are disposed communicatively with the 
pipes 14 and 15, respectively. The locations of the classifying wedges 17, 
18 and the gas-intake wafer 19 may vary depending on the kind of the feed 
material to be classified and the desired particle size. At the bottom of 
the classifying chamber, outlets 11, 12 and 13 are disposed corresponding 
to the respective classifying sections and opening into the chamber. The 
outlets 11, 12 and 13 can be respectively provided with shutter means like 
valve means. 
The feed material supply pipe 16 comprises a flat rectangular pipe section 
and a tapered rectangular pipe section, and it is preferred in order to 
obtain an appropriate introduction speed that the ratio between the 
internal size of the flat rectangular pipe section and the narrowest part 
of the tapered rectangular pipe section is 20:1 to 1:1, particularly 10:1 
to 2:1. 
A classifying operation is effected by using the above described 
multi-division classifying chamber or zone as follows. The classifying 
chamber is sucked or evacuated to a reduced pressure through at least one 
of the outlets 11, 12 and 13. A feed toner powder material is supplied to 
the classifying chamber through the feed supply nozzle 16 along with a gas 
stream flowing at a rate of 50-300 m/sec, preferably 70-200 m/sec. At that 
time, the first gas stream introduction control means 20 and the second 
gas stream introduction control means 21 are driven so that the absolute 
value of a static pressure (gauge pressure) P.sub.1 at a position in the 
intake pipe 14 upstream of the inlet (downstream end of the pipe) opening 
into the classifying chamber is 150 mm.aq. or above, preferably 200 mm.aq. 
or above, further preferably 210 to 1000 mm.aq.; the absolute value of a 
static pressure P.sub.2 (gauge pressure) at a position in the intake pipe 
15 upstream of the inlet opening into the classifying chamber is 40 mm.aq. 
or above, preferably 45 to 400 mm.aq., further preferably 45 to 70 
mm.aq.abs.; and the absolute values .vertline.P.sub.1 .vertline. and 
.vertline.P.sub.2 .vertline. satisfying the relation: .vertline.P.sub.1 
.vertline.-.vertline.P.sub.2 .vertline..gtoreq.100 (mm.aq.). The pressures 
are measured downstream of the gas stream control means 20 and 21. The 
absolute value of the static pressure P.sub.2 in the range of 45-70 mm.aq. 
is especially preferred because fine particles and coarse particles are 
more broadly distributed in the classifying zone so that the control of 
the classifying point becomes easier. Further preferably, the absolute 
values of the static pressures P.sub.1 and P.sub.2 satisfy the relations 
of 150.ltoreq..vertline.P.sub.1 .vertline.-.vertline.P.sub.2 
.vertline..ltoreq.700, and .vertline.P.sub.1 .vertline./ .vertline.P.sub.2 
.vertline.=2 to 10 (preferably 4 to 6). 
When .vertline.P.sub.1 .vertline.-.vertline.P.sub.2 .vertline.&lt;100 
(mm.aq.), the classification accuracy is lowered and it becomes impossible 
to accurately remove the fine powder fraction, so that the resultant 
classified product is caused to have a broad particle size distribution. 
When the feed toner powder material is supplied to the classifying chamber 
at a rate below 50 m/sec, the aggregation of the feed powder cannot be 
sufficiently disintegrated, thus lowering the classification yield and the 
classification accuracy. When the feed toner material is supplied to the 
classifying zone at a rate of above 300 m/sec, the toner particles can be 
pulverized because of collision therebetween to newly produce fine 
particles, thus tending to lower the classification accuracy. 
The feed toner particles thus supplied are caused to fall along curved 
lines 30 due to the Coanda effect given by the Coanda block 26 and the 
action of the streams of a gas such as air, so that larger particles 
(coarse particles) fall along an outward gas stream to form a fraction 
outside the classifying wedge 18, medium particles (particles having sizes 
in the prescribed range) form a fraction between the classifying wedges 18 
and 17, and small particles (particles having sizes below the prescribed 
range) form a fraction inward of the classifying wedge 17. Then, the large 
particles, the medium particles and the small particles are withdrawn 
through the outlets 11, 12 and 13, respectively. 
The above process may be generally operated by using a system in which the 
classifier is connected with other apparatus by communicating means such 
as pipes. A preferred embodiment of such an apparatus system is shown in 
FIG. 3. The apparatus system shown in FIG. 3 comprises a three-division 
classifier 1 as explained with reference to FIGS. 1 and 2, a metering 
feeder 2, a vibration feeder 3, a collecting cyclone 4, a collecting 
cyclone 5 and a collecting cyclone 6 connected through communication 
means. The supply of the feed toner material from the metering feeder 2 to 
the vibration feeder 3 is performed in an open system. 
More specifically, in the apparatus system, the feed toner material is 
supplied to the metering feeder 2 by appropriate means, and through the 
vibration feeder 3 and the feed supply nozzle 16, introduced into the 
three-division classifier at a velocity of 50-300 m/sec. As the size of 
the classifying zone or chamber in the classifier 1 is generally on the 
order of (10-50 cm).times.(10-50 cm), the feed toner particles can be 
generally classified into three or more particle size fractions in a short 
period of 0.1 sec to 0.01 sec or less. In the three-division classifier 1, 
the feed toner material is divided into the large particles (coarse 
particles), the medium particles (particles with sizes in the prescribed 
range) and the small particles (particles with sizes below the prescribed 
range). The large particles are then sent through an exhaust pipe 11 to 
the collecting cyclone 6 to be recovered. The medium particles are 
withdrawn out of the system through an exhaust pipe 12 and collected by 
the collecting cyclone 5 to be recovered as a toner product 51. The small 
particles are withdrawn out of the system through an exhaust pipe 13 and 
collected by the collecting cyclone 4 to be recovered as fine powder 41 
with sizes outside the prescribed range. The collecting cyclones 4, 5 and 
6 function as suction and reduced pressure-generation means for 
introducing the feed powder material through the nozzle 16 into the 
classifying chamber. A commercially available apparatus which may be 
suitably used in the present invention may includes the ELBOW 
JET.RTM.multi-division classifier available from Nittetsu Kogyo K.K. 
As described above, according to the process of the present invention, 
particles including toner particles obtained through pulverization or 
polymerization of a toner material are effectively and rapidly classified 
into a particle fraction comprising particles with sizes in a prescribed 
range and having an accurate particle size distribution. In a conventional 
classification system using a fixed wall-type classifier or a rotational 
classifier, aggregates of fine particles causing fog of developed images 
are liable to be formed. Further, when such aggregates are formed, it is 
difficult to separate them from the medium particle size fraction in the 
conventional classification system. According to the process of the 
invention, however, the aggregates, even if formed, are disintegrated due 
to the Coanda effect and/or high-speed movement into fine particles which 
are separated from the medium particles. Further, even if some aggregates 
are not disintegrated, they can be simultaneously separated as coarse 
particles, whereby the aggregates can be effectively removed as a whole to 
increase the classification yield. 
A toner for developing electrostatic images according to the pulverization 
process may be generally prepared by melt-kneading the starting materials 
including a binder resin such as a styrene resin, a styrene-acrylic resin 
or a polyester resin (ordinarily in an amount of 25 -90 wt. % of the 
toner); a colorant such as carbon black or phthalocyanine blue (ordinarily 
0.5-20 wt. % of the toner) and/or a magnetic material (ordinarily 10-70 
wt. % of the toner); an antioffset agent such as low-molecular weight 
polyethylene, low-molecular weight polypropylene or paraffin wax 
(ordinarily, 0.1-10 wt. % of the toner); and a positive or negative charge 
control agent (ordinarily, 0.1-10 wt. % of the toner), followed by 
cooling, pulverization and classification. In case of production of a 
toner through the pulverization process, it is difficult to obtain a 
uniform melt dispersion of the starting materials in the kneading step so 
that the pulverized particles can include particles which are not suitable 
as toner particles commingled therein, such as those free of a colorant or 
magnetic particle or comprising an individual particle of a single 
starting material. In the conventional process involving a long residence 
time in the classification stage such unsuitable particles are liable to 
aggregate with each other and it is difficult to remove the resultant 
aggregates, so that toner characteristics are remarkably impaired thereby. 
In contrast thereto, in the process of the invention, the feed particles 
are classified into three or more fractions so that such aggregates are 
not readily formed, and even if formed, they can be removed into the fine 
particle fraction or the coarse particle fraction. As a result, a toner 
product comprising particles of a uniform mixture and having an accurate 
particle size distribution is obtained. 
A polymerization toner is prepared by subjecting a monomer composition 
comprising at least a polymerizable monomer and a colorant to suspension 
polymerization in the presence of a polymerization initiator and a 
dispersion stabilizer. Even if the dispersion stabilizer particles are 
allowed to remain in the polymerization toner particles, the stabilizer 
particles can be effectively separated from the toner particles according 
to the classification process of the present invention. 
A toner produced by the process of the present invention has a stable 
triboelectric charge provided by friction between the toner particles, or 
between the toner and a toner carrying member such as a sleeve or carrier. 
Development fog and scattering of toner around the edge of a latent image, 
which have not been fully solved heretofore, are extremely reduced, and a 
high density of image is achieved, leading to a good reproducibility of 
half tone. Even in the continuous use of a developer including the toner 
over a long period, an initial performance can be maintained and high 
quality images can be provided over a long period. Further, even in the 
use of the toner under environmental conditions of a high temperature and 
a high humidity, the triboelectric charge of the developer is stable and 
little vary as compared with that when used under normal temperature and 
normal humidity, because the presence of extremely fine particles and the 
aggregate thereof are reduced. Threfore, the fog and decrease in density 
of image are reduced, enabling the development of images faithful to 
latent images. Moreover, the resulting toner images have an excellent 
transfer efficiency to a transfer material such as a paper. Even in the 
use of the toner under the conditions of a low temperature and a low 
humidity, a distribution of triboelectric charge is little different from 
that in the use at normal temperature and normal humidity, and because the 
extremely fine particle component having an extremely large charge has 
been removed, the toner produced by the process of the present invention 
has such characteristics that there occur little reduction in density of 
image and little fog, and roughening and scattering during transfer hardly 
occur. 
In producing a toner powder having a smaller particle size (e.g., an 
average particle size of 3 to 7.mu.), the process ff the present invention 
can be carried out more effectively than the prior art process is. 
The present invention will now be described in detail by way of Examples. 
EXAMPLE 1 
______________________________________ 
Styrene-acrylic acid ester resin 
100 wt. parts 
(weight ratio of styrene to the acrylic 
ester 7:3, weight-average molecular 
weight of about 300,000) 
Magnetite 60 wt. parts 
(particle size: 0.2.mu.) 
Low molecular weight polyethylene 
2 wt. parts 
(weight-average molecular weight of 
about 3,000) 
Negatively chargeable control agent 
2 wt. parts 
(Bontrone E81) 
______________________________________ 
A toner feed material of a mixture having the above prescription was 
melt-kneaded at 180.degree. C. for about 1.0 hour, and cooled for 
solidification. The resulting mixture was roughly pulverized into 
particles of 100 to 1,000 microns in a hammer mill and then moderately 
pulverized into a weight-average particle size of 100 .mu.m in ACM 
pulverizer available from Hosokawa Micron K.K. Then, the pulverized 
material was further pulverized by means of a hypersonic speed jet mill 
(PJM-I-10, mfd. by Nippon Pneumatic Kogyo K.K.) into a pulverized material 
having a weight-average particle size of 10.9 .mu.m (containing 11.1 wt. % 
of particles having sizes below 5.04 .mu.m and 4.1 wt. % of particles 
having sizes above 20.2 .mu.m). The pulverized material was classified in 
an apparatus system as shown in FIG. 3 including a multi-division 
classifier 1 as shown in FIGS. 1 and 2 ELBOW JET.RTM. EJ-45-3 model, 
available from Nittetsu Kogyo K.K.), into which the pulverized material 
was introduced at a rate of 2.0 kg/min to be classified into three 
fractions including a coarse powder, a medium powder and a fine powder 
under utilization of the Coanda effect. 
For effecting the introduction, the collecting cyclones 4, 5 and 6 
communicated with the outlets 11, 12 and 13 were operated to generate a 
reduced pressure in the classification chamber, by which the pulverized 
material was introduced at a velocity of about 100 m/sec through the 
supply nozzle 16. At this time, the static pressure P.sub.1 in the intake 
pipe 14 at a point upstream of the inlet to the chamber was controlled at 
-280 mm.aq., i.e. -280 mm H.sub.2 O (gauge), and the static pressure 
P.sub.2 in the intake pipe 15 was controlled at -60 mm.aq. The introduced 
particles were classified in an instant of 0.01 second or less. A medium 
powder suitable as a toner was collected in a yield of 83 wt. % in the 
collecting cyclone 5 for collecting the classified medium powder, and had 
a weight-average particle size of 11.5.mu. (containing 0.3 wt. % of 
particles having a particle size of below 5.04.mu. and 0.1 wt. % or less, 
i.e., a substantially negligible amount, of particles having a particle 
size of 20.2.mu. or more). As used herein, the term "yield" refers to a 
percentage of the amount of the medium powder finally obtained based on 
the total weight of the powdered material fed. Substantially no aggregate 
of about 5.mu. or larger resulting from the aggregation of extremely fine 
particles was found by the observation of the obtained medium powder 
through an electron microscope. 
The obtained medium powder showed a negative chargeability with respect to 
a sleeve of aluminum or stainless steel and was electrically insulating. 
The medium powder was used as a toner, and 0.3% by weight of hydrophobic 
silica was mixed with the toner to prepare a developer. The prepared 
developer was supplied to a copier NP-270 RE (available from Canon K.K.) 
to effect a copying test. The results showed that copied images having no 
fog and a good developing property for thin lines were provided. 
COMATIVE EXAMPLE 1 
A pulverized material having a weight-average particle size of 10.9 .mu.m 
produced in the same manner as in Example 1 was introduded at a rate of 
2.0 kg/min and classified in the same apparatus system used in Example 1. 
For effecting the introduction, the collecting cyclones 4, 5 and 6 
communicated with the outlets 11, 12 and 13 were operated to generate a 
reduced pressure in the classification chamber, by which the pulverized 
material was introduced at a velocity of about 80 m/sec through the supply 
nozzle 16. At this time, the static pressure P.sub.1 in the intake pipe 14 
was controlled at -70 mm.aq., and the static pressure P.sub.2 in the 
intake pipe 15 was controlled at -50 mm.aq. 
A medium powder as a toner was collected in a yield of 60 wt. % in the 
collecting cyclone 5 for collecting the classified medium powder, and had 
a weight-average particle size of 11.2 microns (containing 1.5 wt. % of 
particles having a particle size of below 5.04.mu. and 2.0 wt. % of 
particles having a particle size of 20.2.mu. or more). The observation of 
the medium powder through an electron microscope showed that aggregate of 
about 5.mu. or more was present in dots, resulting from the aggregation of 
the extremely fine particles. 
The resultant medium powder was used as a toner, and 0.3% by weight of 
hydrophobic silica was mixed with the toner to prepare a developer. The 
prepared developer was supplied to a copier NP-270RE to effect a copying 
test. The results showed that the duplicated images had increased fog as 
compared with those obtained in Example 1. 
EXAMPLE 2 
______________________________________ 
Styrene-acrylic acid ester resin 
100 wt. parts 
(weight ratio of styrene to the acrylic 
ester 7:3, weight-average molecular weight 
of about 300,000) 
Magnetite 60 wt. parts 
(particle size: 0.2.mu.) 
Low molecular weight polypropylene 
2 wt. parts 
(weight-average molecular weight of 
about 10,000) 
Negatively chargeable control agent 
2 wt. parts 
(Bontrone E81) 
______________________________________ 
A toner feed material of a mixture having the above prescription was 
melt-kneaded at 180.degree. C. for about 1.0 hour, and cooled for 
solidification. The resulting mixture was roughly pulverized into 
particles of 100 to 1000.mu. in a hammer mill and then moderately 
pulverized into a weight-average particle size of 50.mu.m pulverized into 
in ACM pulverizer available from Hosokawa Micron K.K. Then, the pulverized 
material was further pulverized by means of a hypersonic speed jet mill 
(PJM-I-10, mfd. by Nippon Pneumatic Kogyo K.K.) into a pulverized material 
having a weight-average particle size of 7.1 .mu.m (containing 12.0 wt. % 
of particles having sizes below 4.0 .mu.m and 4.0 wt. % of particles 
having sizes above 12.7 .mu.m). The pulverized material was classified in 
an apparatus system as shown in FIG. 3 including a multi-division 
classifier 1 as shown in FIGS. 1 and ELBOW JET.RTM. EJ-45-3 model, 
available from Nittetsu Kogyo K.K.), into which the pulverized material 
was introduced at a rate of 2.0 kg/min to be classified into three 
fractions including a coarse powder, a medium powder and a fine powder 
under utilization of the Coanda effect. 
For effecting the introduction, the collecting cyclones 4, 5 and 6 
communicated with the outlets 11, 12 and 13 were operated to generate a 
reduced pressure in the classification chamber, by which the pulverized 
material was introuuced at a velocity of about 110 m/sec through the 
supply nozzle 16. At this time, the static pressure P.sub.1 in the intake 
pipe 14 at a point upstream of the inlet to the chamber was controlled at 
-420 mm.aq., and the static pressure P.sub.2 in the intake pipe 15 was 
controlled at -70 mm.aq. The introduced particles were classified in an 
instant of 0.01 second or less. A medium powder suitable as a toner was 
collected in a yield of 84 wt. % in the collecting cyclone 5 for 
collecting the classified medium powder, and had a weight-average particle 
size of about 7.5.mu. (containing 2.5 wt. % of particles having a particle 
size of 4.0.mu. and 0.1 wt. % or less, i.e., a substantially negligible 
amount, of particles having a particle size of above 12.7.mu.). 
Substantially no aggregate of about 3.mu. or larger resulting from the 
aggregation of extremely fine particles was found by the observation of 
the obtained medium powder through an electron microscope. 
EXAMPLE 3 
______________________________________ 
Styrene-acrylic acid ester resin 
100 wt. parts 
(weight ratio of styrene to the acrylic 
ester 7:3, weight-average molecular weight 
of about 300,000) 
Magnetite 60 wt. parts 
(particle size: 0.2.mu.) 
Low molecular weight polypropylene 
2 wt. parts 
(weight-average molecular weight of 15,000) 
Negatively chargeable control agent 
2 wt. parts 
(Bontrone E81) 
______________________________________ 
A toner feed material of a mixture having the above prescription was 
melt-kneaded at 180.degree. C. for about 1.0 hour, and cooled for 
solidification. The resulting mixture was roughly pulverized into 
particles of 100 to 1000.mu. in a hammer mill and then moderately 
pulverized into a weight-average particle size of 30.mu.m in ACM 
pulverizer available from Hosokawa Micron K.K. Then, the pulverized 
material was further pulverized by means of a hypersonic speed jet mill 
(PJM-I-10, mfd. by Nippon Pneumatic Kogyo K.K.) into a pulverized material 
having a weight-average particle size of 5.8.mu.m (containing 13.0 wt. % 
of particles having sizes below 3.17 .mu.m and 3.9 wt. % of particles 
having sizes above 10.08 .mu.m). The pulverized material was classified in 
an apparatus system as shown in FIG. 3 including a multi-division 
classifier 1 as shown in FIGS. 1 and ELBOW JET.RTM. EJ-45-3 model, 
available from Nittetsu Kogyo K.K.), into which the pulverized material 
was introduced at a rate of 2.0 kg/min to be classified into three 
fractions including a coarse powder, a medium powder and a fine powder 
under utilization of the Coanda effect. 
For effecting the introduction, the collecting cyclones 4, 5 and 6 
communicated with the outlets 11, 12 and 13 were operated to generate a 
reduced pressure in the classification chamber, by which the pulverized 
material was introduced at a velocity of about 120 m/sec through the 
supply nozzle 16. At this time, the static pressure P.sub.1 in the intake 
pipe 14 at a point upstream of the inlet to the chamber was controlled at 
-600 mm.aq., and the static pressure P.sub.2 in the intake pipe 15 was 
controlled at -70 mm.aq. The introduced particles were classified in an 
instant of 0.01 second or less. A medium powder suitable as a toner was 
collected in a yield of 81 wt. % in the collecting cyclone 5 for 
collecting the classified medium powder, and had a weight-average particle 
size of about 6.2.mu. (containing 2.0 wt. % of particles having a particle 
size of below 3.17.mu. and 1.0 wt. % of particles having a particle size 
of above 10.08.mu.). Substantially no aggregate of about 3.mu. or larger 
resulting from the aggregation of extremely fine particles was found by 
the observation of the obtained medium powder through an electron 
microscope. 
COMATIVE EXAMPLE 2 
Example 1 was repeated except that the pulverized material was introduced 
at a rate of 65 m/sec, the static pressure P.sub.1 was changed to -200 
mm.aq., and the static pressure P.sub.2 was changed to -150 mm.aq. As a 
result, the stream of the fed pulverized material was biased toward the 
Coanda block 26 to cause an insufficient dispersion in the classifying 
zone, whereby the separation of the coarse powder, the medium powder and 
the fine powder was insufficient. 
The particles recovered as the medium powder fraction had an average 
particle size of 11.2 .mu.m, whereas they contained about 1 wt. % of 
particles having a particle size below 5.04 .mu.m and about 2 wt. % of 
particles having a particle size of above 20.2 .mu.m, thus showing a 
clearly broader particle size distribution compared with that of Example 
1.