Method and apparatus for determining electrical charge characteristics of toner materials

The electrostatic quality of toner material is measured by introducing the toner material into a measuring cell so that it flows through an opening in the measuring cell into a receiving chamber that is substantially free of gas flows and collects on a charged electrode disposed within the receiving chamber. The quantity of toner material collected on the electrode is utilized as an index of the quality of the toner material with respect to its electrostatic charge capability. In addition, the dispersal profile of the toner material collected on the electrode may be utilized for analytic purposes.

The present invention relates to a method and an apparatus for determining 
electrical charge characteristics of toner materials. 
The analysis of the physical condition of toner materials, such as 
developers used in copiers, printers and the like, generally requires 
time, much technical expenditure and occasional limited modification of 
the equipment. The knowledge of changes in time and place in the condition 
of the developer in the development zone of a copier is, however, 
essential for understanding the proceedings with electrostatic picture 
reproductions. A basic knowledge of the developing process is most 
important for the construction of a copier or the like. 
To compare different toner materials and developers it is of great 
advantage to determine the physical parameters by use of test units which 
are easy to handle. The concept of suitable testing conditions suggests 
the development of standardized methods to make it possible to compare 
decisive criteria of different toner materials. This is particularly 
important for determining the electrostatic charge of developers. 
In order to obtain similar charging conditions of toner materials used in 
copiers, the parameters occurring in copiers have to be simulated, since 
the electrostatic charge of the toner material in electrographic copiers 
depends on the following conditions: 
1. Toner Material Parameter 
chemical composition 
one or multi-component toner materials of developers 
surface conditions 
electric conductivity 
2. Activation Parameter 
frequency of collisions between toner particles and with their surroundings 
intensity of collisions 
mixing speed in multi-component developers 
3. Outside Parameters 
electric field 
magnetic field 
interactions with ambient materials (walls, electrodes, climate 
conditions). 
In this context it is also essential, especially when determining the 
electrostatic qualities of relatively coarse toner starting materials, to 
grind or pulverize such material in order to obtain the most accurate 
production-oriented data possible. Under normal processing conditions such 
starting material consists initially of particles with average grain size 
ranging from 0.8 mm to approximately 2 mm which are then finely ground to 
a granular size of 1 to 50 .mu.m when being processed as toner for use in 
copiers. Special pulverizing methods have been developed for industrial 
purposes in which in particular so-called jet mills for pulverizing toner 
starting material are currently being employed. In this case pulverization 
of the material is achieved with the aid of jets of pressurized air. These 
are injected or discharged at a high velocity into a cylindrical grinding 
chamber, creating a spiral air flow, in which the blown or injected 
grinding material is pulverized through collision of the individual 
particles with one another. Further, in counter-jet mills, pulverization 
is achieved by introducing particles being pulverized into crossing air 
flows and causing them to collide against one another at a high velocity. 
For laboratory purposes it is not feasible to employ an industrial-type jet 
mill for pulverizing toner material. Nevertheless, it is desirable to 
simulate such pulverization on a laboratory scale, in order to achieve the 
most realistic pulverization of toner material by employing procedures 
that closely resemble the actual industrial processes used. 
DESCRIPTION OF RELATED ART 
By way of example, an industrially employed, fluidized bed type-counter jet 
mill is shown in German Patent No. DE PS 3,338,138 C2, which is also 
suited for, among other things, pulverizing toner material. 
From U.S. Pat. No. 4,375,673, an apparatus and a method for measuring the 
distribution of toner particles as a function of the size and charge 
thereof are known. The toner particles, which have been blown off from the 
carrier, are transported through a narrow guide pipe into a tube with 
laminar air flow (x-axis). By means of two electrodes an electric field, 
which is in a vertical position to the air flow (y-axis), deflects the 
toner particles proportional to the charge/diameter-value of a toner 
particle away from the original line of flow. The toner is collected as a 
toner-charge/diameter-spectrum vertical to the flow direction on a filter. 
R. H. Epping, M. Munz and M. Mehlin, in Electrophotography, Vol. 27, No. 4 
(1988), pages 528-532, describe electrical charge and conductivity 
measurements with modern mono-component developers. The blown-off toner 
material is transported from a measuring cell into a laminar air flow 
inside a tube, wherein the toner material at once gets the same speed as 
the laminar air flow. Cross-linked to the air flow, an electrical field is 
applied for moving the toner to a registration electrode, which collects 
the toner particles. Due to the dynamic equation of the toner particles, 
all particles with constant ratio q/d (charge/diameter) deposit as a toner 
spectrum line, depending on the deflection of the toner in direction of 
the laminar air flow. For good electrostatic charging conditions, the 
toners are activated by a rotating magnetic field generated by a pair of 
two cross-linked magnets, thus simulating a magnetic brush station of a 
copier. 
In the prior art apparatus and methods for measuring the distribution of 
toner particles as a function of the size and charge thereof, the 
measuring results depend greatly on the nature and method of treating the 
powder of the toner materials. The resultant values are influenced not 
only by the type of movement and contact conditions of the individual 
particles to each other, called "activation," but also by marginal 
conditions such as material and geometry of the developing space, 
frequency and polarization direction of the electromagnetic field, 
temperature and relative humidity. 
In addition, heretofore known methods and apparatus for determining 
electric charge characteristics of toner materials have proven to be 
relatively complex and correspondingly expensive, both in construction and 
in operation. 
SUMMARY OF THE INVENTION 
In accordance with the invention, the electrostatic quality of toner 
material useful for electrostatic copiers and printers which is submitted 
in sample quantities for testing, is determined by measurements related to 
accumulations on a charged electrode. Toner material introduced into a 
measuring cell flows through an opening in the measuring cell into a 
receiving chamber that is substantially free of gas flows and collects on 
a polarized electrode disposed within the chamber. The quantity of toner 
material collected on the electrode is utilized as an index of the quality 
of the toner material with respect to its electrostatic charge capability. 
In addition, the dispersal profile of the toner material collected on the 
electrode may be utilized for analytic purposes. Specifically, toner 
quality is determined by measurement using only one integral dimension 
with respect to the quantity ratio of both charges present in the toner 
material. A method according to a specific embodiment of the present 
invention for determining the electrical charge characteristics of any 
type of toner material, comprises the steps of: 
introducing the toner material into a measuring cell; 
allowing the toner material to flow from an opening in the measuring cell 
into a receiving chamber that is substantially free of gas flows; 
allowing the inflowing toner material to collect on an electrode disposed 
within the receiving chamber to which a voltage with a predetermined 
polarity is applied; 
measuring over a certain time frame the quantity of toner material that 
enters the receiving chamber and collects on the electrode; and 
defining the electrical charge characteristics of the toner material as a 
function of the measuring results. 
In a preferred embodiment of the present invention the measurement is 
duplicated, but with a voltage of an alternate polarity is applied to the 
electrode. Hence, by defining electrical properties as a function of the 
difference in quantities of toner material that collect on the electrode 
when voltages of opposite polarity are applied, an index for the quality 
of the toner material is obtained. 
An object of the present invention is therefore to develop a method for 
determining electrical charge characteristics of toner material which can 
be employed with greater facility and efficiency, but which also allows 
one to make sufficiently accurate and precise determinations about the 
quality of the toner material. 
A further object is to design an apparatus for determining electrical 
charge characteristics of toner material that is simple in construction 
and cost effective. Still further, the present invention enables one to 
analyze any type of toner material desired, including, for example, coarse 
grade toner starting materials as well as toner material with and without 
a carrier, i.e. so-called developers, with respect to their electrical 
charge properties. 
In contrast to the prior art, in which the electrostatic properties of 
toner materials are determined by measuring the q/d ratio, only one 
integral dimension is determined with respect to the quantity ratio of 
both charges present in the toner material. It has been found that only 
one property is sufficient for an initial determination of the quality of 
toner material with respect to its electrostatic charging capacity. 
Moreover, in contrast to prior art methods, the method of the present 
invention is simpler and substantially more cost effective. 
A further measuring variable for yielding information about additional 
characteristics regarding toner quality is obtained by recording and 
evaluating the distribution profiles of the toner material that has 
collected on the electrode. Based on such information it is even possible 
to draw tentative inferences about the q/d ratio of particles in the toner 
material. This allows further conclusions to be drawn about the electrical 
charging capacity of the toner material. 
To evaluate even coarser starting materials with respect to their 
electrostatic suitability the present invention provides a further step in 
which the toner material is pulverized to obtain average sized grains in 
the range of about 1 to 50 .mu.m, preferably 1 to 15, and which includes 
the further individual steps of: 
introducing the toner material into a grinding chamber; 
injecting at least two jets of gas at high velocity into the grinding 
chamber, thereby pulverizing the toner material present in the chamber; 
passing the pulverized toner material through a sieve; 
sifting and separating the sieved toner material in a centrifugal sifter 
apparatus, in particular a cyclone; 
removing the toner material from the centrifugal sifter apparatus. 
The apparatus of the present invention which is designed to determine the 
electrical charge characteristics of toner material comprises a measuring 
cell for receiving the toner material, together with an opening through 
which the toner material flows into a chamber substantially free of gas 
flows; further an electrode disposed within the chamber of the measuring 
cell, to which a voltage with a predetermined polarity is applied. 
In a preferred embodiment of the present invention at least a portion of 
the electrode is made of a translucent or transparent (diaphanous) 
material, which enables the quantity of toner material that has collected 
on the electrode to be detected and analyzed, e.g., by means of a 
photomicroscope. In addition, the present invention may also include, for 
purposes of measuring the properties of, in particular, coarse starting 
material, a jet mill pulverizing apparatus for grinding down the toner 
material into average grain sizes ranging from about 1 to 50 .mu.m, 
preferably 1 to 15 .mu.m. The apparatus comprises: 
a grinding chamber for receiving the toner material; 
at least two gas nozzles opening out into the chamber for generating jets 
of gas that are discharged into and criss-cross in the grinding chamber; 
a sieve mounted on the grinding chamber for separating the toner material 
ground in the grinding chamber into a residue and undersized material; 
further a centrifugal sifter, in particular, a cyclone for removing the 
sifted undersized material. 
Only minimal quantities of toner material are required for evaluating 
electrical charge characteristics, hence the level of efficiency of the 
jet mill-pulverizing apparatus is of only secondary concern. This 
simplifies substantially the design and construction of such a jet 
mill-pulverizing apparatus.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
The basic principle underlying a preferred embodiment of the present 
invention is discussed with reference to FIG. 1, which shows a measuring 
cell 2 into which the toner material 4 being measured is introduced. In 
the present example the toner material is composed of a developer with 
carrier particles 6 and the actual toner particles 8, i.e., a 
two-component-toner material. It is feasible, however, to test 
mono-component toner material (without a carrier), as well as any other 
type of toner material desired with respect to its electrostatic charge 
characteristics. 
Usually the introduced toner material 4 is at least partially charged, or 
the toner material 4 is charged by the particles colliding with one 
another within or by colliding against the walls of measuring cell 2. For 
reproducing the measurement it is advantageous to charge the toner 
material 4 evenly. This is achieved by a magnetic device 19 disposed 
outside measuring cell 2, which generates a variable magnetic field by 
means of rotating magnetic fields disposed in a criss-cross orientation. 
Throughout measuring procedure, toner material 4 is activated as a result 
of magnetic device 10, that is, the toner material 4 is homogenized with 
respect to its electrostatic charge. 
Measuring cell 2 is has an opening 12, the diameter of which is selected to 
allow at least toner particles 8 to flow through said opening 12. The 
toner material i.e. the toner particles 8 flow into a receiving chamber 14 
which is substantially free of gas flows and is located, for example, 
above the measuring cell 2. Flow-free chamber 14 may be constructed as an 
enclosed housing (indicated by the dashed lines) with side walls 16. 
FIG. 1 further shows an electrode device 18 consisting of a registration 
electrode 20 and a counter electrode 22 which are spaced a predetermined 
distance apart from one another and between which an electric field of 
approximately 20 to 200 kV/m is activated. In the embodiment shown in FIG. 
1 electrodes 20, 22 of electrode device 18 are also constructed as walls 
for flow-free chamber 14, in which counter electrode 22 serves as a cover 
for measuring cell 2 and has an opening 12 through which the toner 
material flows. Due to the electric field present in flow-free chamber 14, 
toner particles 8 are accelerated in direct proportion to their electrical 
charge, e.g. toward the registration electrode, where they collect after 
having travelled a certain path through the electric field. Based on 
current technical knowledge it is known that several factors affect the 
transport of the toner particles. Toner particles 8 that have just exited 
opening 12 briefly accelerate in the electric field, then enter into a 
dynamic equilibrium, due to Stokes friction which is caused by the gas 
(air) particles (not shown in greater detail) present in flow-free chamber 
14. The particles then travel at a relatively constant velocity toward 
registration electrode 20. The rate of critical velocity is largely 
determined by the quotient of the electrostatic charge and the diameter of 
the individual toner particles 8. Hence, toner particles 8 having a 
greater electrostatic charge but similar diameters traverse flow-free 
chamber 14 more quickly and arrive correspondingly sooner at registration 
electrode 20 than toner particles 6 which carry a weaker electrostatic 
charge. 
Equally charged toner particles that repel one another, result in 
additional components of motion with respect to the toner particles, for 
example, motion perpendicular to the direction of the electric field. The 
greater the repellant force, the more such components predominant, i.e. 
they are proportional to the level of electrostatic charge of the 
respective toner particle. However, as was stated above, toner particles 
carrying a greater electrostatic charge are subject to a greater velocity 
component in the direction of the electric field. On the whole, therefore, 
the wider the dispersal of charge values or charge/diameter-values present 
in the toner material 4, the greater the mean diameter d of the particle 
collected on registration electrode 20. Experimentation has shown that the 
dispersal range of the toner material which has collected on registration 
electrode 20 represents a further qualitative measure for the evaluation 
of electric charge characteristics of the toner material. 
In the embodiment of the present invention shown in FIG. 1 a positive 
voltage is initially applied to registration electrode 20, thereby 
attracting toner particles 8 with a negative charge which then collect on 
the registration electrode. It was found that the integral value relating 
to the quantity of toner particles which collected on registration 
electrode 20 over a predetermined time period is in itself sufficient for 
making certain determinations about the quality of toner material 4 with 
respect to electrical charge characteristics. For reproducing measurements 
it is essential that chamber 14 through which toner particles travel 
remain substantially free of gas or air currents. This eliminates any 
distorting effects during transport of the toner particles 8. 
When required, the outflow of toner material 4 through opening 12 may be 
augmented by injecting an auxiliary gas flow 24 into measuring cell 2. In 
such case, however, auxiliary gas flow 24 may not enter at such a rate 
that it disrupts the flow-free conditions prevailing inside flow-free 
chamber 14. Auxiliary gas flow 24 is provided in the form of, for example, 
an air flow injected in pulses, in which the pulse duration falls within 
the range of approximately a second and a volume flow of ca. 10 m/s is 
obtained. Further, it is possible to augment the outflow of toner material 
through opening 12 (not described in greater detail) by repeatedly 
knocking on the measuring cell 2 or by subjecting measuring cell 2 to 
mechanical impulses. 
Further information about the quality of toner material 4 with respect to 
its electrical charge characteristics can be determined when the 
measurement is repeated using the same toner material 4, but in which the 
direction of the electrical field within flow-free chamber 14 is reversed 
such that, e.g. a negative voltage is applied to registration electrode 
20. Thus, toner particles 8 having an opposite polarity, i.e., 
positively-charged particles, are attracted to and collect on electrode 
20. By comparing the quantity of collected toner material with the 
quantity collected previously when the polarity of registration code 20 
was reversed, it is possible to draw significant conclusions about the 
electrical charge characteristics of the toner material that are a 
function of the difference in polarities present in the toner material 4. 
It was determined that, essentially, toner material in which electrical 
charges of one polarity clearly predominate is of a higher quality than 
toner material in which the proportion of positive and negative charges is 
essentially the same. 
In a preferred embodiment of the present invention registration electrode 
20 is at least partially translucent or diaphanous. This enables the toner 
material 4 which has collected on registration electrode 20 to be easily 
detected and analyzed by a photomicroscope 26, which in turn is connected 
to a computing device for processing the image (not shown in greater 
detail). 
FIGS. 2 and 3 show a preferred embodiment of the present invention in 
greater detail, in which reference numerals in FIG. 1 refer to equivalent 
features in FIGS. 2 and 3. FIG. 2 clearly shows, in particular, a 
measuring cell 2 for receiving the toner material, over which a counter 
electrode 22 is mounted, a flow-free chamber 14 disposed immediately above 
the counter electrode, and further a registration electrode 20 with a 
transparent window 28. After a measurement is taken registration electrode 
20, which is constructed as a rotatable electrode plate and, as is clearly 
seen from the plan view in FIG. 3, is rotated around a center point M in a 
counterclockwise direction until window 28 comes to rest under a lens 30. 
This enables the quantity of toner material which has collected on window 
28 of registration electrode during measurement to be optically recorded. 
Once the material has undergone optical analysis and, if required, the 
distribution of particle sizes in the toner material that has collected on 
window 28 has been measured, the rotatable electrode plate with window 28 
is pivoted to a cleaning apparatus 32, where the collected toner material 
is removed or suctioned from window 28. This allows registration electrode 
20 to be utilized for further measurements. The cleaning apparatus has a 
suction device 36 actuated by a motor 34 and which ends in a slot shaped 
suction nozzle 38 immediately proximate to the position to which window 28 
is pivoted for cleaning registration electrode 20. It is clear that the 
process of cleaning the electrode is simplified substantially in the 
present invention as compared with prior known solutions. 
To investigate the suitability of both finished toner material as well as 
coarse-grained starting material with respect to their electrostatic 
charging capabilities, the latter composed initially of average sized 
particles ranging from approximately 0.8 to 2 mm, a pulverizing apparatus 
40, shown in FIGS. 4 and 5 is provided in accordance with the present 
invention. Pulverizing apparatus 40 consists of a semispherically shaped 
grinding chamber 42 into which the toner material 4 being pulverized is 
introduced. At least two jet nozzles 44 are mounted along the sides of and 
open out into grinding chamber 42. By means of nozzles 44 jets of gas are 
discharged into the chamber 42 at a pressure of 6 bars, respectively, and 
intersect inside the chamber at a point P. Pressurized jets of air blown 
at high velocity into grinding chamber 42 stir up the toner material 
contained in the chamber 42, causing the material to become pulverized 
through collision and deflection of the individual particles. 
Outside air is drawn via a ventilator (not shown in greater detail) through 
a filter 46 into a channel 48 into a sifting chamber 50 and past a 
separator 52. Channel 48 is disposed immediately above and connected to 
grinding chamber 42 via a sieve 54 which has a mesh size of approximately 
25 .mu.m to 50 .mu.m, This allows the sufficiently pulverized toner 
material to pass undersized through said sieve 54, from where it is 
introduced in an air flow passing through channel 48 to the sifting 
chamber 50 of a centrifugal-type sifter. In sifting chamber 50 the 
pulverized toner material is graded by utilizing the different rates at 
which the various solid particles of the toner material gravitate 
downward. Simultaneously, the toner material is cleaned of all dust 
impurities in a cyclone defined by the sifting chamber 50 and separator 
52. Toner material that has been pulverized, graded and cleaned 
accumulates in a receptacle 56 from which it can later be removed. 
The invention has been explained with reference to specific embodiments. 
Other embodiments will be apparent to those of ordinary skill in the art. 
It is therefore not intended that the invention be limited except as 
indicated by the appended claims.