Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
1998-06-11
2001-05-29
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C315S111810, C347S128000, C438S020000
Reexamination Certificate
active
06239823
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the generation of charged particles in air, and more particularly to the generation of charged particle images for electrographic imaging.
Charged particles for use in electrographic imaging can be generated in a wide variety of ways. Common techniques include the use of air-gap breakdown, corona discharges and spark discharges. Other techniques employ triboelectricity, radiation, and microwave breakdown. When utilized for the formation of latent electrostatic images, all of the above techniques suffer certain limitations in charged particle output currents and charge image integrity.
A further approach, which offers significant advantages in this regard, is described in Fotland, U.S. Pat. No. 4,155,093 (May 19, 1979) and the improvement disclosed in Carrish, U.S. Pat. No. 4,160,257 (Jul. 3, 1979). These patents disclose method and apparatus for generating charged particles in air involving what the inventors' term “silent electric discharge”. The prior art general view of
FIG. 1
shows a charge image generator
8
capable of forming an electrostatic latent image on electrostatic latent image receptor
25
. Charge image generator
8
is supplied with a high voltage alternating potential from generator
10
. This potential is applied between two electrodes, a generator electrode
12
and a control electrode
14
. Electrode
14
contains a plurality of circular or slotted apertures opposing generator electrode
12
. Solid dielectric member
16
is sandwiched between these electrodes. Generator electrode
12
is shown encapsulated by dielectric member
18
. As disclosed in U.S. Pat. No. 4,155,093, the alternating potential causes the formation of a pool or plasma of positive and negative charged particles in the air region adjacent dielectric
16
and defined by the apertures in discharge electrodes
14
. These charged particles may be extracted to form a latent electrostatic charge image.
The alternating potential supplied by generator
10
creates a fringing field between electrode
12
and electrode
14
. When the electrical stress exceeds the dielectric strength of air, a discharge occurs in the fringing field air gap. Charge built up on the surface of dielectric
16
reduces the electric field in the air gap thus quenching the discharge. Such silent electric discharges produce a faint blue glow. In order that no discharge occur in the region between adjacent control electrodes in space
15
, this region must be filled with a solid dielectric.
U.S. Pat. No. 4,160,257 teaches the use of isolation or screen electrode,
20
, separated from control electrode
14
by spacer layer
22
. Electrode
20
serves to screen the extraction electric fields in the region bounded by electrodes
14
and
20
from the external fields associated with the latent charge image formed on the surface of dielectric receptor
26
. In addition, aperture
24
in electrode
20
provides an electrostatic lensing action. Passage of charged particles through isolation aperture
24
to the surface of image receptor dielectric
26
is controlled by electrical potentials applied control electrodes
14
. The electrical potential of isolation electrode
20
is kept constant with time. The receptor dielectric is contiguous with conducting substrate
28
. The edge of a second control electrode
17
is also shown in FIG.
1
. The space electrically isolating control electrodes must be filled with a solid dielectric
15
to prevent air gap breakdown in this region.
The use of negative charges (electrons and negative ions) is preferred since higher negative output currents are obtained than when potentials are reversed to extract positive charges. Biasing power supply
34
provides a constant high-voltage accelerating field between dielectric receptor substrate
28
and isolation electrode
20
. Negative charges are extracted from the discharge when print selector switch
36
is in position Y. In this case, a charge extraction field, provided by power supply
30
, is present between electrodes
14
and
20
. When switch
36
is in position X, a retarding field is applied by supply
32
and the retarding field prevents charge from escaping aperture
24
.
The requirement that a high frequency voltage and an extraction voltage be simultaneously present to generate charge output provides the means for coincident selection thus enabling the multiplexing of charge output. The prior art view of
FIG. 3
illustrates how the charged particle generator
57
may be multiplexed. An array of control electrodes
58
-
1
through
58
-
6
contains apertures
62
at crossover regions opposing generator electrodes
60
-
1
through
60
-
4
. Dielectric layer
64
isolates generator and control electrodes. Isolation electrode
66
is contiguous with dielectric layer
64
. Generator electrodes are sequentially excited by a high frequency high voltage burst of several cycles. Any location in the matrix may be printed by timing a data, or control, pulse to the selected control electrode simultaneous with excitation of the appropriate generator line.
Two methods of fabricating charge image generators are described in the patent literature. One method involves first forming a laminate consisting of discharge dielectric
16
sandwiched between metal foils which are subsequently chemically etched to form generator electrodes
12
and control electrodes
14
. After etching, the generator electrode side of the laminate is bonded to dielectric
18
which, in turn, is bonded to a metal heat sink not shown in FIG.
1
. The photo-etched laminate is then laminated, on the control electrode side, with a photo-etchable dry film soldermask or dry film photoresist. Next, openings are formed in spacer layer
22
to expose the apertures previously etched into the control electrodes. Finally, a previously etched isolation, or screen, electrode
20
is bonded to the spacer layer. Briere U.S. Pat. Nos. 4,381,327 and 4,628,227 and Fotland et al, U.S. Pat. No. 4,408,214, incorporated herein by reference, describes this method in detail.
A second fabrication method involves building up the layers starting with generator electrode
12
that is formed on insulating support
18
. Layers are subsequently fabricated sequentially on this generator electrode structure. This technique is described in detail in the following U.S. Pat. Nos.: McCallum et al. 4,679,060; 4,745,421; 4,958,172; 5,030,975 and Kubelik 5,315,324 which are also incorporated herein by reference.
Both fabrication approaches employ spacer layers
22
between about 50 microns and about 150 microns in thickness. Since bathtub shaped apertures must be formed in the spacer layer, this layer is formed of either a dry film photomask or a dry film photoimagable solder mask material. Two layers are required for thicker spacing. Alternately, this spacer layer may be formed using screen printing of the appropriate thickness curable resin.
The space between adjacent control electrodes must be filled with solid dielectric
15
in order to prevent air-gap breakdown in the fringing fields adjacent the edges of the control electrodes. Air gap breakdown in this region increases the power required to drive the charge image generator and eventually results in arcing and catastrophic failure as the insulation is eroded in the highly oxidizing environment created by the discharge. U.S. Pat. Nos. 4,679,060 and 4,745,421 show a method of reducing the magnitude of the control electrode edge sealing problem by including the extra step of coating the control electrodes and spaces between these electrodes with a 25 micron layer of liquid solder mask. The cured solder mask effectively seals space
15
. A thicker solder mask film is then laminated to the cured solder mask and the finishing steps carried out.
When a separate and distinct sealing operation is not employed, the dry film solder mask must be laminated to the control electrode and surrounding dielectric using a vacuum laminator arranged to provide sufficient heat and pressure so that the semi-molten
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