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Some techniques on electrostatic separation of particle size utilizing electrostatic traveling-wave field
ARTICLE IN PRESS
Journal of Electrostatics 66 (2008) 220–228 www.elsevier.com/locate/elstat
Some techniques on electrostatic separation of particle size utilizing electrostatic traveling-wave field H. Kawamoto Department of Mechanical Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan Received 2 February 2007; received in revised form 15 September 2007; accepted 7 January 2008 Available online 5 February 2008
Abstract The author has developed five kinds of techniques on electrostatic separation of particle size utilizing the balance of the electrostatic and gravitational force. The first is an inclined plate conveyer system. A plate conveyer consisted of parallel electrodes was constructed and four-phase electrostatic traveling wave was applied to the electrodes to transport particles on the conveyer. Particles were separated with size under the voltage application of appropriate frequency based on the feature that small particles were transported upward against the gravity but large particles were apt to fall down. The second technique is an inclined tube system. The principle is common with that of the inclined plate system. The third technique utilizes a circular electrostatic conveyer similar with the mass spectroscopy but utilizes the feature that small particles fly high altitude compared to that of the large particle. The forth technique, a vortex system, also utilizes the difference of flying locus of small and large particles. The last technique is the combination of the linear conveyer and an electrostatic separation roller located at the end of the conveyer. Small particles were attached onto the roller charged by a charger roller. Although the yield was reduced to realize the high separation performance with the former four techniques, relatively high yield was realized without reducing the separation performance with the roller system. This technique is expected to be utilized to the separation of toner and carrier particles used in electrophotography. r 2008 Elsevier B.V. All rights reserved. Keywords: Traveling wave; Electrostatic force; Electrophotography; Toner; Carrier
1. Introduction Electrostatic traveling-wave transport of particles has been studied and fundamental performances have been clarified by an experimental and numerical investigation, because it has a potential to realize a sophisticated particle supplier in electrophotography [1,2]. The technology will be applied not only for an electrophotographic developer [3–13] but also for electronic [14], chemical, biochemical [15,16], and space applications [17], because it has the advantage that the transport can be controlled through electrical parameters instead of mechanical means and it is almost free against acoustic noise, mechanical vibration, and contamination of impurities. In addition to these applications, the author is developing some techniques to separate particle size utilizing the Tel./fax: +81 3 5286 3914.
E-mail address: [email protected] 0304-3886/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2008.01.002
electrostatic traveling wave [1], because the distribution of particle size must be narrow to realize high-quality images in color laser printers [18–21]. Although conventional mechanical methods such as a sieve and a cyclone have been employed for the industrial separation of particle size, these have demerits that particles are damaged by the mechanical force and contaminated with impurities such as dust, metal fragments, and oil mist. With respect to the electrostatic method, the electrostatic separation has been widely used for the dry separation of small parts and fragments based on the electric force acting on charged mass in the electrostatic field [22,23]. However, the technology is not applied for the separation of particle size but for mineral processing, removal of foreign substances in food industry, composite refinement, and recycling of resources [24]. In this report we have introduced five separation methods utilizing the electrostatic traveling wave and studied feasibility of these methods. The effectiveness of
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these techniques have been evaluated with small carrier particles used in the magnetic brush development system of electrophotography [18–21] and some of these technologies have been expected to be utilized not only to the particle supplier but also to the separation of the particle size and a charge-to-mass ratio of toner and carrier particles used in electrophotography [18]. 2. Inclined linear system Fig. 2. Photograph of plate-type electrostatic conveyer.
2.1. Experimental set-up The first method utilizes a balance of the electrostatic force and the gravitational force. An electrostatic conveyer is inclined in y to the horizontal position. It is expected that small particles are transported upward by the electrostatic traveling field such that particles move up the incline, but large and heavy particles roll down at the condition that the electrostatic force applied to the particle is smaller than the gravitational force. A conveyer and a power supply used for the inclined linear separation system are shown in Fig. 1 [1,2]. The conveyer consists of 125 parallel copper electrodes, 1.0 mm width and 2.0 mm pitch, etched by photolithography on a plastic substrate, 120 mm width and 250 mm length, as shown in Fig. 2. The surface of the conveyer is covered with an insulating film made of acetate rayon (40 mm thickness, 1.3 relative permittivity, 3 M, 810-18D) to prevent from electrical breakdown between electrodes. A limiting voltage against the insulation breakdown was 800 V. Particles are tribocharged in contact with the film. Traveling-wave propagation is achieved utilizing four amplifiers (Matsusada Precision, HOPS-1B3) and five function generators (Iwatsu, SG-4105), one of which is used to control phase-differences of the other four generators. Rectangular voltage was applied to electrodes, because it is the most efficient for the particle transport [1]. Spherical particles made by the polymerization method (Toda Kogyo) were used for experiments [1,2,19–21]. applied voltage
insulating film traveling wave large particles
CH1 CH2 CH3
small particles
θ
CH4 time
function gernerators amplifiers CH1 CH2
electrode array (Cu) plastic substrate
z
y x
CH3 CH4
Fig. 1. Inclined plate-type electrostatic particle separation system and power supply.
Particles, originally manufactured for carriers of a twocomponent magnetic brush development system in electrophotography, were made of phenol resin. Distribution of the particle diameter, derived by an optical method of randomly selected each 3000 particles, is shown in Fig. 3, designated in dark curves and cited as ‘initial.’ An averaged diameter is 73 mm, but two peaks exist at 65 and 100 mm. The density is 3.62 103 kg/m3 and the resistivity is 3 109 O cm measured at 10 V applied voltage with an aggregated sample. Particles were placed on the conveyer without prior charging. It has been reported that particles were transported almost linearly with time, even when particles were placed on the conveyer without prior charging [2]. It has been clarified that particles were slightly charged when they were settled on the conveyer due to the static electrification. If the traveling field was applied, the Coulomb force and dielectrophoresis force were applied to particles and then particles were driven and collided with each other and with the conveyer. This increased charge and polarization with time. The Coulomb force was the predominant force to drive particles except when particles were on the conveyer. 2.2. Results and discussion Fig. 3 shows measured diameter distributions of initially settled particles on the lower end of the conveyer and particles reaching the top end of inclined plate conveyer. Particles were collected on a sheet of charta settled at the top end of the conveyer. The applied voltage, the inclination of the conveyer, and the frequency of the applied voltage were experimental parameters. The ordinate of the figure designated the abundance ratio defied as the percentage of particle numbers reached to the top end of the conveyer to the initially settled number of particles on the lower end of the conveyer. The particle separation is not clear, but in the case that the frequency was higher than 60 Hz, particles that reached to the top end of the conveyer contained almost no large particles larger than a certain diameter that is determined by the applied voltage and the inclination of the conveyer. That is, it is possible to eliminate large particles by this inclined linear system like a screener. On the other hand, a yield of the separated particles was low at the condition that the separation
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abundance ratio %
20
500 V inclination: 18°
15
60 Hz 80 Hz
10
5
abundance ratio %
20
500 V inclination: 22°
abundance ratio %
initial 40 Hz
15
60 Hz 80 Hz
to be applied not only in vertical direction but also in horizontal direction. That is, the basic motion equation is a 6-degree-of-freedom system of particle i: d2 xi dxi ¼ qi E þ F image þ F dipole þ mi g, þ 6pZR dt2 dt I i j€ i ¼ 0,
mi
E ¼ E 0 þ E q ¼ rf þ 5
400 V inclination: 35°
initial 40 Hz
15
60 Hz 80 Hz
F image ¼
20
400 V inclination: 50°
N 1 X qi qn 2 d , 4p0 n¼1 2d jdj
60 Hz
F dipole ¼ 4p0
80 Hz 10
5
0 20
40
60 80 100 particle diameter μm
120
(2)
(3)
where d is an effective distance to the electrode including the film thickness. The charge of the insulating film is neglected. The dielectrophoresis force Fdipole applied to the polarized particle in the non-uniform field is determined by [25]
initial 40 Hz
15
N 1 X r q , 4p0 nai n jrj3
where e0 is the permittivity of free space, f is the electric potential, N is a number of particles, and r ¼ (xixn, yiyn, zizn). The potential distribution f is calculated with the two-dimensional finite element method in a cyclic domain of two-pitch width. Parabolic elements were employed for the FEM calculation. The image force Fimage is calculated by
10
5
(1)
where m is a mass of the particle, Z is the viscosity of air, R is a the radius of the particle, q is the charge of the particle, g [ ¼ (g sin y,0,g cos y)] is the gravitational acceleration, x ¼ (x, y, z), j ¼ (jx, jy, jz) and I is an inertia of the particle. The electric field E in the term of the Coulomb force consists of the traveling field E0 generated by the power supplies and the electrostatic field Eq created by other charged particles:
10
20
abundance ratio %
initial 40 Hz
140
Fig. 3. Measured distributions of particle diameter reached to the top end of inclined plate conveyer.
performance was high. The yield is a trade-off of the separation performance. 2.3. Numerical simulation A numerical calculation was conducted to simulate these experimentally observed characteristics. The calculation method was almost the same with that reported in Ref. [2], but the gravitational force applied to particles was assumed
1 3 R E 0 rE 0 , þ2
(4)
where e is a relative permittivity of the particle. The particle is assumed to be a dipole and force from other polarized particles is neglected, because particles usually do not distribute dense except for the initial condition. Eq. (1) is calculated for 1000 particles at the same time with the Runge–Kutta method. During a time-step calculation, initial conditions of velocities are renewed when collisions between particle–particle and particle–conveyer take place. Linear and angular velocities after collision are calculated by a two-body impact equation that includes repulsive and frictional effects. Linear and rotational motions interact with each other at the collision. This calculation method is so-called a hard sphere model of distinct element method. Before conducting transport simulations in the electrostatic field particles distributed in 10 10 mm area were free-fallen to the level conveyer from the 10 mm height from the conveyer surface and stationary positions on the conveyer were determined as initial positions of the transport calculation to conform virtually to the experiment. Diameters and charges of
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particles were assigned randomly to each particle but these distributions were conformed to measurements. Charge distribution was measured by the separate experiment with a free-fallen method. Detailed procedure of the measurement was reported in Ref. [2]. The charge variation with time is neglected. Calculated results shown in Fig. 4 agreed qualitatively with the measured results. The calculation also deduces the reason why the yield is low under conditions of low applied voltage and high frequency. Because small particles are apt to fly over at a high position, several mm from the conveyer as shown in Section 3.2, the electrostatic force applied to the particle is small and the gravitational force is comparatively predominant for the airborne small particles at a high position. Therefore small particles are apt to fall down on backward of the inclined conveyer especially under the conditions of low applied voltage and the high frequency and thus the yield becomes low. On the other hand, in the case that the conveyer is in a horizontal position particles
abundance ratio %
600 V
initial 40 Hz-20° 40 Hz-40° 40 Hz-60° 60 Hz-20° 60 Hz-40° 60 Hz-60°
15
10
5
20 abundance ratio %
fall down to the forward direction and transported to the forward. In conclusion, the inclined linear system is excellent for the separation but the yield of separated particles is low. 3. Inclined tube system 3.1. Experimental set-up The second method utilizes an inclined tube conveyer. The system configuration is shown in Fig. 5. The tube conveyer consisted of alternately stacked metal rings and insulation rings, both 5 mm inner diameter and 0.8 mm thickness. Total length of the tube was 57 mm. The tube was inclined in y to the horizontal position and located above the linear flat-plate conveyer that is settled on the level. Common four-phase rectangular voltage was applied both to the linear and tube conveyers. The power supply and particles used for experiment were the same with those for the inclined linear system. 3.2. Results and discussion
20
500 V
initial 40 Hz-20° 40 Hz-40° 40 Hz-60° 60 Hz-20° 60 Hz-40° 60 Hz-60°
400 V
initial 40 Hz-20° 40 Hz-40° 40 Hz-60° 60 Hz-20° 60 Hz-40° 60 Hz-60°
15 10
5
20
abundance ratio %
223
15
10
Fig. 6 shows measured diameter distributions of particles run over the top end of the inclined tube conveyer. The inclination of the tube conveyer and the gap between the flat plate conveyer and the lower end of the tube conveyer were experimental parameters. Because the counted number of particles was very small, in particular for particle diameter less than 40 mm and larger than 80 mm in case of 5 mm gap, and therefore deduced abundance ratio was statistically unreliable, these data were omitted in the figure. Because of the same reason the ordinate of the figure designated the abundance ratio not to initially settled particles on the plate conveyer but to particles run over the top end of the tube conveyer; that is, the abundance ratio in Fig. 6 is defined as the number of designated diameter particles ran over the top end of the tube divided by the total number of particles ran over the top end of the tube. The high separation performance was realized for the large gap configuration but the inclination of the tube did not affect the separation performance. To investigate the reason why the separation performance highly depended on the air gap a numerical small particles plate conveyer
tube conveyer
5 θ
0 20
40
60 80 100 particle diameter μm
120
gap
140
Fig. 4. Calculated distributions of particle diameter reached to the top end of inclined conveyer (parameters: voltage, frequency, and inclination angle).
traveling wave Fig. 5. Inclined tube-type separation system.
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20
10 30° 45°
15 10
gap: 0 mm 5
23 μm 8 axial position (mm)
abundance ratio %
15°
6 4 2
20 30° 45°
15
10 106 μm
10
gap: 2.4 mm 5
20 abundance ratio %
15° 30° 45°
15
8 axail position (mm)
abundance ratio %
15°
6 4 2
10
gap: 5.0 mm
0 transport direction x (20 mm/div.)
5
Fig. 7. Calculated trajectories of small (23 mm) and large (106 mm) particle in traveling-wave field (800 V–10 Hz, 7 1014 C).
0 20
40
60 80 100 particle diameter μm
120
140
Fig. 6. Measured distributions of particle diameter reached to the top end of inclined tube conveyer (500 V–10 Hz).
calculation was conducted on trajectories of small and large particles. The numerical method was the same with that of Section 2.2 except that the flat linear conveyer was assumed to be settled in the level. Fig. 7 shows calculated typical trajectories of small (23 mm diameter) and large (106 mm diameter) particles. The diameters of particles are assumed uniform and identical in each case. It is clearly seen that small particles jump and fly at high altitude, more than 8 mm from the conveyer, but large particles roll on the conveyer and fly at less than 2 mm altitude. Although actual trajectories are not so simple as shown in Fig. 7 but complex due to mechanical and electrostatic interactions between mixed of small and large particles, the similar characteristics were observed experimentally by the high-speed camera. Based on these numerical and experimental results it is assumed that small particles at high altitude were introduced selectively in the tube that is placed on the conveyer with the large gap. Although the high separation performance was realized by this system, the yield of small particles reached at the top end of the tube conveyer was extremely low. Introduction of the small particles to the tube was rate controlling and it must be improved for the practical use.
4. Circular system 4.1. Experimental set-up A circular system was intended to apply a principle of the mass spectroscopy. When particles were introduced in circular traveling-wave field created by a circular conveyer shown in Fig. 8(b), particles would be separated with size because a locus of the particle depended on the weight of the particle due to the centrifugal force. The circular conveyer consisted of two or three segments of a quartersection electrode shown in Fig. 8(a), 360 mm outer diameter, 240 mm inner diameter nd therefore 60 mm effective width. The width and pitch of electrodes were 1.0 and 2.0 mm, respectively, at the inner circumference and 1.5 and 3.0 mm at the outer circumference. Particles used for experiment was the mixed of small (30 mm diameter) and large (107 mm diameter) particles. Particles of 0.05-g were settled initially on the end of the circular conveyer as shown in Fig. 8(b) and then four-phase electrostatic traveling wave was applied to the electrodes. 4.2. Results and discussion Fig. 9 shows the measured relative number of circulated particles that reached to the end of the conveyer, a portion marked ‘R’ in Fig. 8(b), at the optimum frequency, 30 Hz, which was determined by a preliminary experiment. We
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60
225
120
R
initial
Fig. 8. Centrifugal separation system. (a) 1/4 segment and (b) centrifugal conveyer (2701 system).
abundance ratio (%)
20 initial 180° 270°
15
10
φ
φ
5
0 20
40
60 80 100 particle diameter (μm)
120
140
Fig. 9. Measured distributions of particle diameter of circulated particles on centrifugal conveyer (800 V–30 Hz).
can see that large particles were circulated and reached to the end of the conveyer and small particles were flicked out of the conveyer, and thus the separation was achieved. High separation performance was realized with the threesegment conveyer (2701) compared to the two-segment conveyer (1801), but on the contrary the yield was low with the three-segment conveyer to the two-segment conveyer. The yield of this system is also a trade-off of the separation performance. In any case large particles were circulated and small particles were flicked out of the circular conveyer contrary to the initial expectation. This was also due to the difference of the trajectories of small and large particles. That is, because small particles fly at high altitude where electrostatic force is weak, particles are driven to the outside of the circular conveyer due to the centrifugal force. On the other hand, large particles are almost trapped to the conveyer by the electrostatic force, and therefore circulate on the circular conveyer. 5. Vortex system 5.1. Experimental set-up A vortex separation system shown in Fig. 10 was developed based on the finding that the flying altitude
Fig. 10. Photograph of spiral conveyer (outer diameter: 160 mm).
small particles sheet
gap
Fig. 11. Spiral separation system.
depends on particle diameter. The conveyer consisted of inner vortex electrodes and outer concentric circle electrodes. The traveling field was applied to the concentric electrodes from the outer to the inner direction to prevent the spill of particles out of the conveyer. A sheet was placed above the conveyer as shown in Fig. 11 to collect small particles that fly at high altitude. High yield was expected
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in this system, because particles circulated without running over the conveyer, and therefore many chances existed to capture large particles. Particles used for experiment was the same with those used for the inclined linear and tube systems. 5.2. Results and discussion
20 abundance ratio %
226
Fig. 12 shows the measured relative number of collected particles on the sheet after 10-s operation. The high
15
abundance ratio %
initial 5 mm 10 mm 15 mm 20 mm
initial 1 kV 750 V 500 V 250 V
gap: 2.0 mm
initial 1 kV 750 V 500 V 250 V
gap: 2.5 mm
initial 1 kV 750 V 500 V 250 V
gap: 3.5 mm
initial 1 kV 750 V 500 V 250 V
gap: 5.0 mm
initial 1 kV 750 V 500 V 250 V
gap: 10.0 mm
initial 1 kV 750 V 500 V 250 V
15 10 5
20 15 10 5
10 20 5
0 20
40
60 80 100 particle diameter μm
120
140
abundance ratio %
abundance ratio %
20
gap: 1.5 mm
Fig. 12. Measured distributions of particle diameter of trapped particles on spiral conveyer (700 V–30 Hz).
particles electrode array
small particles
10 5
20 abundance ratio %
charger roller separation roller
15
air gap
15 10 5
cleaner blade
large particles
Fig. 13. Roller separation system.
abundance ratio %
20 15 10 5
separation roller charger roller abundance ratio %
20 15 10 5 0
parallel electrodes
20
40
60 80 particle diameter μm
100
120
Fig. 15. Distributions of particle diameter of attached to the roller. Fig. 14. Photograph of roller separation system.
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Fig. 16. Photograph of particles before and after operation of roller separation system (1.0 kV). (a) Initial; (b) attached to roller and (c) non-attached.
separation performance was achieved without decreasing the yield by adjusting a proper gap, in this case 5 mm, that is larger than the altitude of large particles and smaller than that of small particles.
continuous separation of particle size was realized with this system. Particles used for experiment was the same with those used for the circular system, i.e., mixed of the small and large particles.
6. Roller system 6.2. Results and discussion 6.1. Experimental set-up The last is a roller system that utilizes the difference of flying altitude and the balance of the Coulomb force and the gravitational force. It is composed of two parts. One is a parallel electrode array that transports particles by virtue of the electrostatic traveling wave, the same as the linear conveyer, and another is a charged separation roller located on the upper side of the array as shown in Figs. 13 and 14. In this system, a biased charger roller is used to charge the separation roller [26,27]. The charger roller consists of a center shaft made of steel and electroresistive elastmer bulk rubber (outer diameter: 12 mm). The roller is in contact with the separation roller (outer diameter: 30 mm) and micro-discharge controls the charge of a surface insulation film on the separation roller after charge cancellation by attached particles, repeatedly. The surface charged voltage of the separation roller due to micro-discharge is determined by Paschen’s law. The thickness of the insulation film on the separation roller is 55 mm and the relative dielectric constant is 3.0. Particles mounted on the left side of the linear conveyer were charged in contact with the insulation film on the conveyer and transported to the right side where the separation roller was approximated. Low-frequency wave, 5 Hz, was applied to the linear conveyer, because transport of particles was almost synchronized with the traveling wave at this frequency [1,2]. Because large particles were apt to roll or fly at low altitude on the conveyer, these were not attached to the separation roller but run over the right end of the conveyer. Small particles, on the other hand, flew at a high altitude larger than the gap and attached to the roller by virtue of the Coulomb force. Then attached particles were scratched up by a cleaner blade made of urethane. It is widely used in laser printers to clean any excess toner particles on a photoconductor drum after the transfer process of electrophotography [28]. Thus
Fig. 15 shows experimental results. It was demonstrated that particles were well separated and high yield was realized. Fig. 16 shows photographs of initially settled particles, particles attached to the roller, and particles nonattached to the roller and run over from the right end of the linear conveyer, in case of 1.0 kV surface potential on the separation roller. We can confirm the separation of particle size. 7. Concluding remarks Five techniques have been developed on the electrostatic separation of particle size. They are inclined linear system, inclined tube system, circular system, vortex system, and roller system. Feasibility of these systems was evaluated with respects to separation and yield performance of carrier particles, several tens micrometer in diameter, used in two-component magnetic development system of electrophotography. All of five systems showed good separation performance but it is a trade-off of the yield performance. However, relatively high yield was realized above all systems with the roller system without reducing the separation performance. The system is possible to build in the developer of the electrophotography machine by replacing the separation roller to a photoconductor drum and it is expected to realize a continuous supply of classified particles. Acknowledgments The author would like to express his thanks to Yoji Okada, Tetsuya Kashima, and Shinjiro Umezu for their help of carrying out the experiment. This work is supported by Grant-in-aid for Scientific Research (B) of Japan Society for Promotion of Science and Nippon Sheet Glass Foundation for Materials Science and Engineering.
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References [1] H. Kawamoto, N. Hasegawa, Traveling wave transport of particles and particle size classification, J. Imaging Sci. Technol. 48 (2004) 404–411. [2] H. Kawamoto, K. Seki, N. Kuromiya, Mechanism on traveling-wave transport of particles, J. Phys. D 39 (2006) 1249–1256. [3] J.R. Melcher, E.P. Warren, R.H. Kotwal, Theory for finite-phase traveling-wave boundary-guided transport of triboelectrified particles, IEEE Trans. Ind. Appl. 25 (1989) 949–955. [4] J.R. Melcher, E.P. Warren, R.H. Kotwal, Traveling-wave delivery of single-component developer, IEEE Trans. Ind. Appl. 25 (1989) 956–961. [5] F.W. Schmidlin, A new nonlevitated mode of traveling wave toner transport, IEEE Trans. Ind. Appl. 27 (1991) 480–487. [6] F.W. Schmidlin, Modes of traveling wave particles transport and their applications, J. Electrostat. 34 (1995) 225–244. [7] M.D. Thompson, Y. Gartstein, J.T. LeStrange, Aspect of toner transport on a traveling wave device, IS&T’s NIP15, in: International Conference on Digital Printing Technologies, 1999, pp. 262–265. [8] F. Schmidlin, Advances in traveling wave toner transport, IS&T’s NIP15, in: International Conference on Digital Printing Technologies, 1999, pp. 302–305. [9] K. Taniguchi, et al., Improved driving characteristics for the toner transportation system, IS&T’s NIP16, in: International Conference on Digital Printing Technologies, 2000, pp. 740–742. [10] R. Kober, Traveling wave transport of conductive toner particles, IS&T’s NIP16, in: International Conference on Digital Printing Technologies, 2000, pp. 736–739. [11] R. Kober, Simulation of traveling wave toner transport, IS&T’s NIP18, in: International Conference on Digital Printing Technologies, 2002, pp. 453–457. [12] R. Kober, Transport geladener partikel in elektrischen wanderfeldern, Dissertation, Achen University Technology, 2003. [13] K. Taniguchi, et al., A new technique for measuring the distribution of charge-to-mass ration for toner particles with on-line use, J. Imaging Sci. Technol. 47 (2003) 224–228. [14] F.M. Moesner, T. Higuchi, Electrostatic devices for particle microhandling, IEEE Trans. Ind. Appl. 35 (1999) 530–536.
[15] S. Masuda, M. Washizu, M. Iwadare, Separation of small particles suspended in liquid by nonuniform traveling field, IEEE Trans. Ind. Appl. IA-23 (3) (1987) 474–480. [16] H. Kawamoto, S. Hayashi, Fundamental investigation on electrostatic traveling-wave transport of liquid drop, J. Phys. D 39 (2006) 418–423. [17] M.K. Mazumder, M. Zahn, R. Sharma, J. Zhang, C. Calle, C. Immer, N. Mardesich, Development of self-cleaning transparent dust shields using low-power electrodynamic fields for solar panels on mars, in: Proceedings of the ESA/IIII/SFE Joint Conference on Electrostatics, vol. 1, 2006, pp. 177–205. [18] L.B. Schein, Electrophotography and Development Physics, Laplacian Press, CA, 1996 (revised second ed.). [19] N. Nakayama, H. Kawamoto, M. Yamaguchi, Statics of magnetic bead chain in magnetic field, J. Imaging Sci. Technol. 46 (2002) 422–428. [20] N. Nakayama, H. Kawamoto, S. Yamada, Resonance frequency and stiffness of magnetic bead chain in magnetic field, J. Imaging Sci. Technol. 47 (2003) 408–417. [21] N. Nakayama, Y. Watanabe, Y. Watanabe, H. Kawamoto, Experimental and numerical study on the bead-carry-out in two-component development process in electrophotography, J. Imaging Sci. Technol. 49 (2005) 539–544. [22] D.K. Yanar, B.A. Kwetkus, Electrostatic separation of polymer powders, J. Electrostat. 35 (1995) 257–266. [23] Y. Matsushita, N. Mori, T. Sometani, Electrostatic separation of plastics by a friction mixer with rotary blades, IEEJ Trans. Ind. Appl. 117 (1997) 1449–1454. [24] M. Saeki, Electrostatic separation, J. Japan Soc. Appl. Electromagn. Mech. 11 (2003) 141–146. [25] T.B. Jones, Electromechanics of Particles, Cambridge University Press, New York, 1995. [26] H. Kawamoto, H. Satoh, Numerical simulation of the charging process using a contact charger roller, J. Imaging Sci. Technol. 38 (1994) 383–388. [27] H. Kawamoto, S. Umezu, Development of electrostatic paper separation and feed mechanism, J. Electrostat. 65 (2007) 438–444. [28] H. Kawamoto, Chatter vibration of a cleaner blade in electrophotography, J. Imaging Sci. Technol. 40 (1996) 8–13.
Journal of Electrostatics 66 (2008) 220–228 www.elsevier.com/locate/elstat
Some techniques on electrostatic separation of particle size utilizing electrostatic traveling-wave field H. Kawamoto Department of Mechanical Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan Received 2 February 2007; received in revised form 15 September 2007; accepted 7 January 2008 Available online 5 February 2008
Abstract The author has developed five kinds of techniques on electrostatic separation of particle size utilizing the balance of the electrostatic and gravitational force. The first is an inclined plate conveyer system. A plate conveyer consisted of parallel electrodes was constructed and four-phase electrostatic traveling wave was applied to the electrodes to transport particles on the conveyer. Particles were separated with size under the voltage application of appropriate frequency based on the feature that small particles were transported upward against the gravity but large particles were apt to fall down. The second technique is an inclined tube system. The principle is common with that of the inclined plate system. The third technique utilizes a circular electrostatic conveyer similar with the mass spectroscopy but utilizes the feature that small particles fly high altitude compared to that of the large particle. The forth technique, a vortex system, also utilizes the difference of flying locus of small and large particles. The last technique is the combination of the linear conveyer and an electrostatic separation roller located at the end of the conveyer. Small particles were attached onto the roller charged by a charger roller. Although the yield was reduced to realize the high separation performance with the former four techniques, relatively high yield was realized without reducing the separation performance with the roller system. This technique is expected to be utilized to the separation of toner and carrier particles used in electrophotography. r 2008 Elsevier B.V. All rights reserved. Keywords: Traveling wave; Electrostatic force; Electrophotography; Toner; Carrier
1. Introduction Electrostatic traveling-wave transport of particles has been studied and fundamental performances have been clarified by an experimental and numerical investigation, because it has a potential to realize a sophisticated particle supplier in electrophotography [1,2]. The technology will be applied not only for an electrophotographic developer [3–13] but also for electronic [14], chemical, biochemical [15,16], and space applications [17], because it has the advantage that the transport can be controlled through electrical parameters instead of mechanical means and it is almost free against acoustic noise, mechanical vibration, and contamination of impurities. In addition to these applications, the author is developing some techniques to separate particle size utilizing the Tel./fax: +81 3 5286 3914.
E-mail address: [email protected] 0304-3886/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2008.01.002
electrostatic traveling wave [1], because the distribution of particle size must be narrow to realize high-quality images in color laser printers [18–21]. Although conventional mechanical methods such as a sieve and a cyclone have been employed for the industrial separation of particle size, these have demerits that particles are damaged by the mechanical force and contaminated with impurities such as dust, metal fragments, and oil mist. With respect to the electrostatic method, the electrostatic separation has been widely used for the dry separation of small parts and fragments based on the electric force acting on charged mass in the electrostatic field [22,23]. However, the technology is not applied for the separation of particle size but for mineral processing, removal of foreign substances in food industry, composite refinement, and recycling of resources [24]. In this report we have introduced five separation methods utilizing the electrostatic traveling wave and studied feasibility of these methods. The effectiveness of
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these techniques have been evaluated with small carrier particles used in the magnetic brush development system of electrophotography [18–21] and some of these technologies have been expected to be utilized not only to the particle supplier but also to the separation of the particle size and a charge-to-mass ratio of toner and carrier particles used in electrophotography [18]. 2. Inclined linear system Fig. 2. Photograph of plate-type electrostatic conveyer.
2.1. Experimental set-up The first method utilizes a balance of the electrostatic force and the gravitational force. An electrostatic conveyer is inclined in y to the horizontal position. It is expected that small particles are transported upward by the electrostatic traveling field such that particles move up the incline, but large and heavy particles roll down at the condition that the electrostatic force applied to the particle is smaller than the gravitational force. A conveyer and a power supply used for the inclined linear separation system are shown in Fig. 1 [1,2]. The conveyer consists of 125 parallel copper electrodes, 1.0 mm width and 2.0 mm pitch, etched by photolithography on a plastic substrate, 120 mm width and 250 mm length, as shown in Fig. 2. The surface of the conveyer is covered with an insulating film made of acetate rayon (40 mm thickness, 1.3 relative permittivity, 3 M, 810-18D) to prevent from electrical breakdown between electrodes. A limiting voltage against the insulation breakdown was 800 V. Particles are tribocharged in contact with the film. Traveling-wave propagation is achieved utilizing four amplifiers (Matsusada Precision, HOPS-1B3) and five function generators (Iwatsu, SG-4105), one of which is used to control phase-differences of the other four generators. Rectangular voltage was applied to electrodes, because it is the most efficient for the particle transport [1]. Spherical particles made by the polymerization method (Toda Kogyo) were used for experiments [1,2,19–21]. applied voltage
insulating film traveling wave large particles
CH1 CH2 CH3
small particles
θ
CH4 time
function gernerators amplifiers CH1 CH2
electrode array (Cu) plastic substrate
z
y x
CH3 CH4
Fig. 1. Inclined plate-type electrostatic particle separation system and power supply.
Particles, originally manufactured for carriers of a twocomponent magnetic brush development system in electrophotography, were made of phenol resin. Distribution of the particle diameter, derived by an optical method of randomly selected each 3000 particles, is shown in Fig. 3, designated in dark curves and cited as ‘initial.’ An averaged diameter is 73 mm, but two peaks exist at 65 and 100 mm. The density is 3.62 103 kg/m3 and the resistivity is 3 109 O cm measured at 10 V applied voltage with an aggregated sample. Particles were placed on the conveyer without prior charging. It has been reported that particles were transported almost linearly with time, even when particles were placed on the conveyer without prior charging [2]. It has been clarified that particles were slightly charged when they were settled on the conveyer due to the static electrification. If the traveling field was applied, the Coulomb force and dielectrophoresis force were applied to particles and then particles were driven and collided with each other and with the conveyer. This increased charge and polarization with time. The Coulomb force was the predominant force to drive particles except when particles were on the conveyer. 2.2. Results and discussion Fig. 3 shows measured diameter distributions of initially settled particles on the lower end of the conveyer and particles reaching the top end of inclined plate conveyer. Particles were collected on a sheet of charta settled at the top end of the conveyer. The applied voltage, the inclination of the conveyer, and the frequency of the applied voltage were experimental parameters. The ordinate of the figure designated the abundance ratio defied as the percentage of particle numbers reached to the top end of the conveyer to the initially settled number of particles on the lower end of the conveyer. The particle separation is not clear, but in the case that the frequency was higher than 60 Hz, particles that reached to the top end of the conveyer contained almost no large particles larger than a certain diameter that is determined by the applied voltage and the inclination of the conveyer. That is, it is possible to eliminate large particles by this inclined linear system like a screener. On the other hand, a yield of the separated particles was low at the condition that the separation
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abundance ratio %
20
500 V inclination: 18°
15
60 Hz 80 Hz
10
5
abundance ratio %
20
500 V inclination: 22°
abundance ratio %
initial 40 Hz
15
60 Hz 80 Hz
to be applied not only in vertical direction but also in horizontal direction. That is, the basic motion equation is a 6-degree-of-freedom system of particle i: d2 xi dxi ¼ qi E þ F image þ F dipole þ mi g, þ 6pZR dt2 dt I i j€ i ¼ 0,
mi
E ¼ E 0 þ E q ¼ rf þ 5
400 V inclination: 35°
initial 40 Hz
15
60 Hz 80 Hz
F image ¼
20
400 V inclination: 50°
N 1 X qi qn 2 d , 4p0 n¼1 2d jdj
60 Hz
F dipole ¼ 4p0
80 Hz 10
5
0 20
40
60 80 100 particle diameter μm
120
(2)
(3)
where d is an effective distance to the electrode including the film thickness. The charge of the insulating film is neglected. The dielectrophoresis force Fdipole applied to the polarized particle in the non-uniform field is determined by [25]
initial 40 Hz
15
N 1 X r q , 4p0 nai n jrj3
where e0 is the permittivity of free space, f is the electric potential, N is a number of particles, and r ¼ (xixn, yiyn, zizn). The potential distribution f is calculated with the two-dimensional finite element method in a cyclic domain of two-pitch width. Parabolic elements were employed for the FEM calculation. The image force Fimage is calculated by
10
5
(1)
where m is a mass of the particle, Z is the viscosity of air, R is a the radius of the particle, q is the charge of the particle, g [ ¼ (g sin y,0,g cos y)] is the gravitational acceleration, x ¼ (x, y, z), j ¼ (jx, jy, jz) and I is an inertia of the particle. The electric field E in the term of the Coulomb force consists of the traveling field E0 generated by the power supplies and the electrostatic field Eq created by other charged particles:
10
20
abundance ratio %
initial 40 Hz
140
Fig. 3. Measured distributions of particle diameter reached to the top end of inclined plate conveyer.
performance was high. The yield is a trade-off of the separation performance. 2.3. Numerical simulation A numerical calculation was conducted to simulate these experimentally observed characteristics. The calculation method was almost the same with that reported in Ref. [2], but the gravitational force applied to particles was assumed
1 3 R E 0 rE 0 , þ2
(4)
where e is a relative permittivity of the particle. The particle is assumed to be a dipole and force from other polarized particles is neglected, because particles usually do not distribute dense except for the initial condition. Eq. (1) is calculated for 1000 particles at the same time with the Runge–Kutta method. During a time-step calculation, initial conditions of velocities are renewed when collisions between particle–particle and particle–conveyer take place. Linear and angular velocities after collision are calculated by a two-body impact equation that includes repulsive and frictional effects. Linear and rotational motions interact with each other at the collision. This calculation method is so-called a hard sphere model of distinct element method. Before conducting transport simulations in the electrostatic field particles distributed in 10 10 mm area were free-fallen to the level conveyer from the 10 mm height from the conveyer surface and stationary positions on the conveyer were determined as initial positions of the transport calculation to conform virtually to the experiment. Diameters and charges of
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particles were assigned randomly to each particle but these distributions were conformed to measurements. Charge distribution was measured by the separate experiment with a free-fallen method. Detailed procedure of the measurement was reported in Ref. [2]. The charge variation with time is neglected. Calculated results shown in Fig. 4 agreed qualitatively with the measured results. The calculation also deduces the reason why the yield is low under conditions of low applied voltage and high frequency. Because small particles are apt to fly over at a high position, several mm from the conveyer as shown in Section 3.2, the electrostatic force applied to the particle is small and the gravitational force is comparatively predominant for the airborne small particles at a high position. Therefore small particles are apt to fall down on backward of the inclined conveyer especially under the conditions of low applied voltage and the high frequency and thus the yield becomes low. On the other hand, in the case that the conveyer is in a horizontal position particles
abundance ratio %
600 V
initial 40 Hz-20° 40 Hz-40° 40 Hz-60° 60 Hz-20° 60 Hz-40° 60 Hz-60°
15
10
5
20 abundance ratio %
fall down to the forward direction and transported to the forward. In conclusion, the inclined linear system is excellent for the separation but the yield of separated particles is low. 3. Inclined tube system 3.1. Experimental set-up The second method utilizes an inclined tube conveyer. The system configuration is shown in Fig. 5. The tube conveyer consisted of alternately stacked metal rings and insulation rings, both 5 mm inner diameter and 0.8 mm thickness. Total length of the tube was 57 mm. The tube was inclined in y to the horizontal position and located above the linear flat-plate conveyer that is settled on the level. Common four-phase rectangular voltage was applied both to the linear and tube conveyers. The power supply and particles used for experiment were the same with those for the inclined linear system. 3.2. Results and discussion
20
500 V
initial 40 Hz-20° 40 Hz-40° 40 Hz-60° 60 Hz-20° 60 Hz-40° 60 Hz-60°
400 V
initial 40 Hz-20° 40 Hz-40° 40 Hz-60° 60 Hz-20° 60 Hz-40° 60 Hz-60°
15 10
5
20
abundance ratio %
223
15
10
Fig. 6 shows measured diameter distributions of particles run over the top end of the inclined tube conveyer. The inclination of the tube conveyer and the gap between the flat plate conveyer and the lower end of the tube conveyer were experimental parameters. Because the counted number of particles was very small, in particular for particle diameter less than 40 mm and larger than 80 mm in case of 5 mm gap, and therefore deduced abundance ratio was statistically unreliable, these data were omitted in the figure. Because of the same reason the ordinate of the figure designated the abundance ratio not to initially settled particles on the plate conveyer but to particles run over the top end of the tube conveyer; that is, the abundance ratio in Fig. 6 is defined as the number of designated diameter particles ran over the top end of the tube divided by the total number of particles ran over the top end of the tube. The high separation performance was realized for the large gap configuration but the inclination of the tube did not affect the separation performance. To investigate the reason why the separation performance highly depended on the air gap a numerical small particles plate conveyer
tube conveyer
5 θ
0 20
40
60 80 100 particle diameter μm
120
gap
140
Fig. 4. Calculated distributions of particle diameter reached to the top end of inclined conveyer (parameters: voltage, frequency, and inclination angle).
traveling wave Fig. 5. Inclined tube-type separation system.
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20
10 30° 45°
15 10
gap: 0 mm 5
23 μm 8 axial position (mm)
abundance ratio %
15°
6 4 2
20 30° 45°
15
10 106 μm
10
gap: 2.4 mm 5
20 abundance ratio %
15° 30° 45°
15
8 axail position (mm)
abundance ratio %
15°
6 4 2
10
gap: 5.0 mm
0 transport direction x (20 mm/div.)
5
Fig. 7. Calculated trajectories of small (23 mm) and large (106 mm) particle in traveling-wave field (800 V–10 Hz, 7 1014 C).
0 20
40
60 80 100 particle diameter μm
120
140
Fig. 6. Measured distributions of particle diameter reached to the top end of inclined tube conveyer (500 V–10 Hz).
calculation was conducted on trajectories of small and large particles. The numerical method was the same with that of Section 2.2 except that the flat linear conveyer was assumed to be settled in the level. Fig. 7 shows calculated typical trajectories of small (23 mm diameter) and large (106 mm diameter) particles. The diameters of particles are assumed uniform and identical in each case. It is clearly seen that small particles jump and fly at high altitude, more than 8 mm from the conveyer, but large particles roll on the conveyer and fly at less than 2 mm altitude. Although actual trajectories are not so simple as shown in Fig. 7 but complex due to mechanical and electrostatic interactions between mixed of small and large particles, the similar characteristics were observed experimentally by the high-speed camera. Based on these numerical and experimental results it is assumed that small particles at high altitude were introduced selectively in the tube that is placed on the conveyer with the large gap. Although the high separation performance was realized by this system, the yield of small particles reached at the top end of the tube conveyer was extremely low. Introduction of the small particles to the tube was rate controlling and it must be improved for the practical use.
4. Circular system 4.1. Experimental set-up A circular system was intended to apply a principle of the mass spectroscopy. When particles were introduced in circular traveling-wave field created by a circular conveyer shown in Fig. 8(b), particles would be separated with size because a locus of the particle depended on the weight of the particle due to the centrifugal force. The circular conveyer consisted of two or three segments of a quartersection electrode shown in Fig. 8(a), 360 mm outer diameter, 240 mm inner diameter nd therefore 60 mm effective width. The width and pitch of electrodes were 1.0 and 2.0 mm, respectively, at the inner circumference and 1.5 and 3.0 mm at the outer circumference. Particles used for experiment was the mixed of small (30 mm diameter) and large (107 mm diameter) particles. Particles of 0.05-g were settled initially on the end of the circular conveyer as shown in Fig. 8(b) and then four-phase electrostatic traveling wave was applied to the electrodes. 4.2. Results and discussion Fig. 9 shows the measured relative number of circulated particles that reached to the end of the conveyer, a portion marked ‘R’ in Fig. 8(b), at the optimum frequency, 30 Hz, which was determined by a preliminary experiment. We
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60
225
120
R
initial
Fig. 8. Centrifugal separation system. (a) 1/4 segment and (b) centrifugal conveyer (2701 system).
abundance ratio (%)
20 initial 180° 270°
15
10
φ
φ
5
0 20
40
60 80 100 particle diameter (μm)
120
140
Fig. 9. Measured distributions of particle diameter of circulated particles on centrifugal conveyer (800 V–30 Hz).
can see that large particles were circulated and reached to the end of the conveyer and small particles were flicked out of the conveyer, and thus the separation was achieved. High separation performance was realized with the threesegment conveyer (2701) compared to the two-segment conveyer (1801), but on the contrary the yield was low with the three-segment conveyer to the two-segment conveyer. The yield of this system is also a trade-off of the separation performance. In any case large particles were circulated and small particles were flicked out of the circular conveyer contrary to the initial expectation. This was also due to the difference of the trajectories of small and large particles. That is, because small particles fly at high altitude where electrostatic force is weak, particles are driven to the outside of the circular conveyer due to the centrifugal force. On the other hand, large particles are almost trapped to the conveyer by the electrostatic force, and therefore circulate on the circular conveyer. 5. Vortex system 5.1. Experimental set-up A vortex separation system shown in Fig. 10 was developed based on the finding that the flying altitude
Fig. 10. Photograph of spiral conveyer (outer diameter: 160 mm).
small particles sheet
gap
Fig. 11. Spiral separation system.
depends on particle diameter. The conveyer consisted of inner vortex electrodes and outer concentric circle electrodes. The traveling field was applied to the concentric electrodes from the outer to the inner direction to prevent the spill of particles out of the conveyer. A sheet was placed above the conveyer as shown in Fig. 11 to collect small particles that fly at high altitude. High yield was expected
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in this system, because particles circulated without running over the conveyer, and therefore many chances existed to capture large particles. Particles used for experiment was the same with those used for the inclined linear and tube systems. 5.2. Results and discussion
20 abundance ratio %
226
Fig. 12 shows the measured relative number of collected particles on the sheet after 10-s operation. The high
15
abundance ratio %
initial 5 mm 10 mm 15 mm 20 mm
initial 1 kV 750 V 500 V 250 V
gap: 2.0 mm
initial 1 kV 750 V 500 V 250 V
gap: 2.5 mm
initial 1 kV 750 V 500 V 250 V
gap: 3.5 mm
initial 1 kV 750 V 500 V 250 V
gap: 5.0 mm
initial 1 kV 750 V 500 V 250 V
gap: 10.0 mm
initial 1 kV 750 V 500 V 250 V
15 10 5
20 15 10 5
10 20 5
0 20
40
60 80 100 particle diameter μm
120
140
abundance ratio %
abundance ratio %
20
gap: 1.5 mm
Fig. 12. Measured distributions of particle diameter of trapped particles on spiral conveyer (700 V–30 Hz).
particles electrode array
small particles
10 5
20 abundance ratio %
charger roller separation roller
15
air gap
15 10 5
cleaner blade
large particles
Fig. 13. Roller separation system.
abundance ratio %
20 15 10 5
separation roller charger roller abundance ratio %
20 15 10 5 0
parallel electrodes
20
40
60 80 particle diameter μm
100
120
Fig. 15. Distributions of particle diameter of attached to the roller. Fig. 14. Photograph of roller separation system.
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Fig. 16. Photograph of particles before and after operation of roller separation system (1.0 kV). (a) Initial; (b) attached to roller and (c) non-attached.
separation performance was achieved without decreasing the yield by adjusting a proper gap, in this case 5 mm, that is larger than the altitude of large particles and smaller than that of small particles.
continuous separation of particle size was realized with this system. Particles used for experiment was the same with those used for the circular system, i.e., mixed of the small and large particles.
6. Roller system 6.2. Results and discussion 6.1. Experimental set-up The last is a roller system that utilizes the difference of flying altitude and the balance of the Coulomb force and the gravitational force. It is composed of two parts. One is a parallel electrode array that transports particles by virtue of the electrostatic traveling wave, the same as the linear conveyer, and another is a charged separation roller located on the upper side of the array as shown in Figs. 13 and 14. In this system, a biased charger roller is used to charge the separation roller [26,27]. The charger roller consists of a center shaft made of steel and electroresistive elastmer bulk rubber (outer diameter: 12 mm). The roller is in contact with the separation roller (outer diameter: 30 mm) and micro-discharge controls the charge of a surface insulation film on the separation roller after charge cancellation by attached particles, repeatedly. The surface charged voltage of the separation roller due to micro-discharge is determined by Paschen’s law. The thickness of the insulation film on the separation roller is 55 mm and the relative dielectric constant is 3.0. Particles mounted on the left side of the linear conveyer were charged in contact with the insulation film on the conveyer and transported to the right side where the separation roller was approximated. Low-frequency wave, 5 Hz, was applied to the linear conveyer, because transport of particles was almost synchronized with the traveling wave at this frequency [1,2]. Because large particles were apt to roll or fly at low altitude on the conveyer, these were not attached to the separation roller but run over the right end of the conveyer. Small particles, on the other hand, flew at a high altitude larger than the gap and attached to the roller by virtue of the Coulomb force. Then attached particles were scratched up by a cleaner blade made of urethane. It is widely used in laser printers to clean any excess toner particles on a photoconductor drum after the transfer process of electrophotography [28]. Thus
Fig. 15 shows experimental results. It was demonstrated that particles were well separated and high yield was realized. Fig. 16 shows photographs of initially settled particles, particles attached to the roller, and particles nonattached to the roller and run over from the right end of the linear conveyer, in case of 1.0 kV surface potential on the separation roller. We can confirm the separation of particle size. 7. Concluding remarks Five techniques have been developed on the electrostatic separation of particle size. They are inclined linear system, inclined tube system, circular system, vortex system, and roller system. Feasibility of these systems was evaluated with respects to separation and yield performance of carrier particles, several tens micrometer in diameter, used in two-component magnetic development system of electrophotography. All of five systems showed good separation performance but it is a trade-off of the yield performance. However, relatively high yield was realized above all systems with the roller system without reducing the separation performance. The system is possible to build in the developer of the electrophotography machine by replacing the separation roller to a photoconductor drum and it is expected to realize a continuous supply of classified particles. Acknowledgments The author would like to express his thanks to Yoji Okada, Tetsuya Kashima, and Shinjiro Umezu for their help of carrying out the experiment. This work is supported by Grant-in-aid for Scientific Research (B) of Japan Society for Promotion of Science and Nippon Sheet Glass Foundation for Materials Science and Engineering.
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