Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1998-11-09
2002-07-09
Gordon, Raquel Yvette (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06416171
ABSTRACT:
GLOSSARY OF ABBREVIATIONS AND SYMBOLS
CTC Charged Toner Conveyor
f
a
acceleration limited frequency
TWTT Traveling Wave Toner Transport
C
a
numerical coefficient for f
a
PP Pixel Packet
E
o
electric field amplitude of traveling wave
XJ XeroJet (present invention)
V
o
voltage amplitude of traveling wave
DPP Digital Packet Printer (prior art)
k wave number (=&pgr;/&lgr;)
DP Digital Packet
&lgr; wavelength of traveling wave
CMYK cyan, magenta, yellow, black
f frequency of traveling wave
dpi dots per inch
q/m tribo, or charge to mass ratio of toner
v process speed of printer in cm/sec
Q/wav toner charge per unit length of wave front
ppm pages per minute
C
m
numerical coefficient for Q/wav
&tgr; period of traveling wave
M/wav toner mass per unit length of wave front
BACKGROUND OF THE INVENTION
Electrostatic deposition of dry powder inks (charged toner) directly onto paper, broadly identified as direct powder printing, can be classified according to whether or not the process includes the use of control apertures to modulate the quantity of toner deposited on the paper. Examples of processes that include control apertures are Direct Electrostatic Printing (DEP), invented by Schmidlin, U.S. Pat. Nos. 4,814,796, 4,755,837 and 4,876,561, and TonerJet™, invented by Larson, U.S. Pat. Nos. 5,774,159 and 5,036,341. This type of process is sensitive to wrong sign toner and requires the use of a cleaning process to clean the control apertures following every printed page. Direct powder printing processes which do not include control apertures have been disclosed by Rezanka, U.S. Pat. No. 5,148,204, Hays, U.S. Pat. No. 5,136,311, and Salmon, U.S. Pat. Nos. 5,153,617, 5,287,127 and 5,400,062.
The Salmon Patents disclose a process similar to the present invention to the extent that it utilizes a toner conveyor process. However, the toner conveyors in the Salmon Patents are very different from the Charged Toner Conveyor (CTC) (U.S. Pat. No. 4,647,179, invented by Schmidlin) in several important ways that are fully explained later on. The Salmon Patents disclose a “digital pumping” apparatus for moving discrete packets of toner, called “Digital Packets” (DPs), along an array of column conveyors from a toner source at one end of the column conveyors to a receiver sheet at the other end of the column conveyors. One column conveyor is used for each pixel site to be printed across the width of a page. Each column conveyor is an independently controlled linear array of narrow electrodes, optimally five microns wide, to accommodate single rows of toner that extend the length of the electrodes. Such rows of toner are called Digital Packets (DPs). One DP consists of two to five toner particles depending on the toner size. Discrete levels of gray are printed at each pixel site on the receiver sheet by counting out the number of DPs to be deposited on that site. For example, for a 600 dpi resolution printer, 16 DPs are deposited at a single pixel site to print black, or a saturated reflection density. White or gray pixels are then formed with 0 to 15 DPs.
Transport, or “digital pumping”, of DPs in the Salmon method is achieved with three-phase digital pulses. An end view of a trapezoidal potential well is illustrated in FIG.
1
. This figure depicts a moment in time when the digital voltage level of phase b is low and the voltage level of phases a and c are high. This produces a trapezoidal potential well whose spatial depth is effectively comparable to the combined width of one electrode and space. The size of the electrodes are claimed by Salmon to be optimally 5 microns so that the trapezoidal well will hold a single toner particle in the process direction (left to right in FIG.
1
). The ordinate in
FIG. 1
represents both voltage and distance above the conveyor surface, with their scales chosen to illustrate the effective depth of the potential well in relation to the size of the toner. The end view of a single DP is shown in
FIG. 1
to illustrate this important sizing feature. “Digital Pumping” moves a DP along the conveyor by cycling the low phase through the sequence b, c, a, b etc., with proper timing (c lowered slightly in advance of raising b, etc.). In this manner the trapezoidal potential well is stepped along the conveyor, carrying the DP with it. Because the potential wells are small (comparable to the size of a toner particle), the toner must move in sliding or rolling contact with the conveyor surface. Otherwise, any perturbing influence during the digital stepping process will cause a trapezoidal potential well to lose control of toner particles in a DP.
It is appropriate to recall here that movement of charged toner particles in sliding/rolling contact with a stationary solid boundary was an objective of my original CTC invention. Early experiments with CTCs, however, revealed that sliding or rolling contact of toner particles with the conveyor surface could not be achieved (cf., Fred Schmidlin, “A New Nonlevitated Mode of Traveling Wave Toner Transport”, IEEE Transactions on Industry Applications, Vol. 27, No. 3, May/June 1991). Instead, the toner particles were discovered to move in an aerosol state as tiny linear clouds, with one such cloud confined in the potential trough of each wave. This mode of Traveling Wave Toner Transport (TWTT), illustrated in
FIG. 2
, was called the “Surfing Mode” because toner particles are pushed by a traveling electrostatic sine wave in much the same way a surf rider is pushed by a water wave. The wavelength of the traveling wave required for this mode of transport must be at least six to eight times the particle diameter. Each particle needs room on the stable part of a wave (the concave upward portion of the wave following the wave minimum) to recover its equilibrium position on a wave after being scattered by the conveyor surface or other mutually repulsive toner.
Because toner scattering is difficult to avoid on a conveyor at particle speeds of practical interest for printing applications (greater than one meter per second), it is predicted that practical implementation of the Salmon invention, called Digital Packet Printing (DPP), is not feasible or severely limited. Although DPs can be moved with toner-sized, digitally-driven “square wells” at slow speeds (as demonstrated with miniature models by Salmon), the reliability required for quality printing at practical transport speeds has not been demonstrated and is claimed to be unreliable or impractical.
Another problem with DPP, as described in the aforementioned Salmon Patents, is that the mutual repulsion of same polarity toner will also cause particles to hop uncontrollably between contiguous channel conveyors. Salmon has recently addressed this problem by incorporating barrier electrodes, or “guide rails”, between adjacent conveyor channels. But this feature does not prevent toner particles from skipping or slipping between DPs in the process (or propagation) direction.
Another problem with DPP is the inclusion of “packet step” and “packet hold” processes wherein toner movement is stopped for periods of time. During this time, toner adhesion to the conveyor surface tends to grow with time, making it difficult to start the toner moving again. Indeed, experience has shown that toner inertia plays an important role in TWTT and collisions with other moving toner particles are generally required to get toner stalled on a conveyor moving again. Therefore, “packet hold” processes are undesirable and should be avoided.
Another problem with DPP is its complexity. The proposed DPP architectures include multiple toner conveyors and “writing heads”. Accurate registration and alignment of the writing heads is required for page width printing applications.
Another problem, or undesirable limitation, of DPP is its ability to print discrete density levels only. Forty-eight clock steps, or 16 “waves”, are required to print one of 16 density levels (including white), at one pixel site. Therefore, the usual half-toning process commonly used in the printing industry must be used to print more than 1
Basch Duane C.
Gordon Raquel Yvette
Greenwald & Basch LLP
Technology Innovations LLC
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