Liquid ink printing apparatus and system

A new printing system for drop on demand printing divides the printing process into two stages: a drop selection stage to select drops to be printed which generates a difference in meniscus position between selected drops and unselected drops of fluidized ink; and a drop separation stage, attracting the ink of the drops to a substrate, the attraction being insufficient to overcome the surface tension of drops in an unselected meniscus position but being sufficient to overcome the surface tension of drops in a selected meniscus position so as to cause movement of the drops to the substrate. The drop selection stage produces a difference in meniscus position in the absence of the drop separation stage. The separation of the drop selection stage from the drop separation stage significantly reduces the energy required to select which ink drops are to be printed. Only the drop selection stage must be driven by individual signals to each nozzle. The drop separation stage can be a field or condition applied simultaneously to all nozzles.

CROSS REFERENCE TO RELATED APPLICATIONS 
Reference is made to my commonly assigned, co-pending U.S. patent 
applications Ser. No. 08/701,021 entitled CMOS PROCESS COMPATIBLE 
FABRICATION OF PRINT HEADS filed Aug. 21, 1996; Ser. No. 08/733,711 
entitled CONSTRUCTION AND MANUFACTURING PROCESS FOR DROP ON DEMAND PRINT 
HEADS WITH NOZZLE HEATERS filed Oct. 17, 1996; Ser. No. 08/734,822 
entitled A MODULAR PRINT HEAD ASSEMBLY filed Oct. 22, 1996; Ser. No. 
08/736,537 entitled PRINT HEAD CONSTRUCTIONS FOR REDUCED ELECTROSTATIC 
INTERACTION BETWEEN PRINTED DROPLETS filed Oct. 24, 1996; Ser. No. 
08/750,320 entitled NOZZLE DUPLICATION FOR FAULT TOLERANCE IN INTEGRATED 
PRINTING HEADS and Ser. No. 08/750,312 entitled HIGH CAITY COMPRESSED 
DOCUMENT IMAGE STORAGE FOR DIGITAL COLOR PRINTERS both filed Nov. 26, 
1996; Ser. No. 08/753,718 entitled NOZZLE PLACEMENT IN MONOLITHIC 
DROP-ON-DEMAND PRINT HEADS and Ser. No. 08/750,606 entitled A COLOR VIDEO 
PRINTER AND A PHOTO CD SYSTEM WITH INTEGRATED PRINTER both filed on Nov. 
27, 1996; Ser. No. 08/750,599 entitled COINCIDENT DROP SELECTION, DROP 
SEATION PRINTING METHOD AND SYSTEM, Ser. No. 08/750,435 entitled 
MONOLITHIC PRINT HEAD STRUCTURE AND A MANUFACTURING PROCESS THEREFOR USING 
ANISTROPIC WET ETCHING, Ser. No. 08/750,436 entitled POWER SUPPLY 
CONNECTION FOR MONOLITHIC PRINT HEADS, Ser. No: 08/750,437 entitled 
MODULAR DIGITAL PRINTING, Ser. No. 08/750,439 entitled A HIGH SPEED 
DIGITAL FABRIC PRINTER, Ser. No. 08/750,763 entitled A COLOR PHOTOCOPIER 
USING A DROP ON DEMAND INK JET PRINTING SYSTEM, Ser. No. 08/765,756 
entitled PHOTOGRAPH PROCESSING AND COPYING SYSTEMS, Ser. No. 08/750,646 
entitled FAX MACHINE WITH CONCURRENT DROP SELECTION AND DROP SEATION 
INK JET PRINTING, Ser. No. 08/759,774 entitled FAULT TOLERANCE IN HIGH 
VOLUME PRINTING PRESSES, Ser. No. 08/750,429 entitled INTEGRATED DRIVE 
CIRCUITRY IN DROP ON DEMAND PRINT HEADS, Ser. No. 08/750,433 entitled 
HEATER POWER COMPENSATION FOR TEMPERATURE IN THERMAL PRINTING SYSTEMS, 
Ser. No. 08/750,640 entitled HEATER POWER COMPENSATION FOR THERMAL LAG IN 
THERMAL PRINTING SYSTEMS, Ser. No. 08/750,650 entitled DATA DISTRIBUTION 
IN MONOLITHIC PRINT HEADS, and Ser. No. 08/750,642 entitled PRESSURIZABLE 
LIQUID INK CARTRIDGE FOR COINCIDENT FORCES PRINTERS all filed Dec. 3, 
1996; Ser. No. 08/750,647 entitled MONOLITHIC PRINTING HEADS AND 
MANUFACTURING PROCESSES THEREFOR, Ser. No. 08/750,604 entitled INTEGRATED 
FOUR COLOR PRINT HEADS, Ser. No. 08/750,605 entitled A SELF-ALIGNED 
CONSTRUCTION AND MANUFACTURING PROCESS FOR MONOLITHIC PRINT HEADS, Ser. 
No. 08/682,603 entitled A COLOR PLOTTER USING CONCURRENT DROP SELECTION 
AND DROP SEATION INK JET PRINTING TECHNOLOGY, Ser. No. 08/750,603 
entitled A NOTEBOOK COMPUTER WITH INTEGRATED CONCURRENT DROP SELECTION AND 
DROP SEATION COLOR PRINTING SYSTEM, Ser. No. 08/765,130 entitled 
INTEGRATED FAULT TOLERANCE IN PRINTING MECHANISMS; Ser. No. 08/750,431 
entitled BLOCK FAULT TOLERANCE IN INTEGRATED PRINTING HEADS, Ser. No. 
08/750,607 entitled FOUR LEVEL INK SET FOR BI-LEVEL COLOR PRINTING, Ser. 
No. 08/750,430 entitled A NOZZLE CLEARING PROCEDURE FOR LIQUID INK 
PRINTING, Ser. No. 08/750,600 entitled METHOD AND APATUS FOR ACCURATE 
CONTROL OF TEMPERATURE PULSES IN PRINTING HEADS, Ser. No. 08/750,608 
entitled A PORTABLE PRINTER USING A CONCURRENT DROP SELECTION AND DROP 
SEATION PRINTING SYSTEM, and Ser. No. 08/750,602 entitled IMPROVEMENTS 
IN IMAGE HALFTONING all filed Dec. 4, 1996; Ser. No. 08/765,127 entitled 
PRINTING METHOD AND APATUS EMPLOYING ELECTROSTATIC DROP SEATION, 
Ser. No. 08/750,643 entitled COLOR OFFICE PRINTER WITH A HIGH CAITY 
DIGITAL PAGE IMAGE STORE, and Ser. No. 08/765,035 entitled HEATER POWER 
COMPENSATION FOR PRINTING LOAD IN THERMAL PRINTING SYSTEMS all filed Dec. 
5, 1996; Ser. No. 08/765,036 entitled APATUS FOR PRINTING MULTIPLE DROP 
SIZES AND FABRICATION THEREOF, Ser. No. 08/765,017 entitled HEATER 
STRUCTURE AND FABRICATION PROCESS FOR MONOLITHIC PRINT HEADS, Ser. No. 
08/750,772 entitled DETECTION OF FAULTY ACTUATORS IN PRINTING HEADS, Ser. 
No. 08/765,037 entitled PAGE IMAGE AND FAULT TOLERANCE CONTROL APATUS 
FOR PRINTING SYSTEMS all filed Dec. 9, 1996; and Ser. No. 08/765,038 
entitled CONSTRUCTIONS AND MANUFACTURING PROCESSES FOR THERMALLY ACTIVATED 
PRINT HEADS filed Dec. 10, 1996. 
FIELD OF THE INVENTION 
The present invention is in the field of computer controlled printing 
devices. In particular, the field is liquid ink drop on demand (DOD) 
printing systems. 
BACKGROUND OF THE INVENTION 
Many different types of digitally controlled printing systems have been 
invented, and many types are currently in production. These printing 
systems use a variety of actuation mechanisms, a variety of marking 
materials, and a variety of recording media. Examples of digital printing 
systems in current use include: laser electrophotographic printers; LED 
electrophotographic printers; dot matrix impact printers; thermal paper 
printers; film recorders; thermal wax printers; dye diffusion thermal 
transfer printers; and ink jet printers. However, at present, such 
electronic printing systems have not significantly replaced mechanical 
printing presses, even though this conventional method requires very 
expensive setup and is seldom commercially viable unless a few thousand 
copies of a particular page are to be printed. Thus, there is a need for 
improved digitally controlled printing systems, for example, being able to 
produce high quality color images at a high-speed and low cost, using 
standard paper. 
Inkjet printing has become recognized as a prominent contender in the 
digitally controlled, electronic printing arena because, e.g., of its 
non-impact, low-noise characteristics, its use of plain paper and its 
avoidance of toner transfers and fixing. 
Many types of ink jet printing mechanisms have been invented. These can be 
categorized as either continuous ink jet (CIJ) or drop on demand (DOD) ink 
jet. Continuous ink jet printing dates back to at least 1929: Hansell, 
U.S. Pat. No. 1,941,001. 
Sweet et al U.S. Pat. No. 3,373,437, 1967, discloses an array of continuous 
ink jet nozzles where ink drops to be printed are selectively charged and 
deflected towards the recording medium. This technique is known as binary 
deflection CIJ, and is used by several manufacturers, including Elmjet and 
Scitex. 
Hertz et al U.S. Pat. No. 3,416,153, 1966, discloses a method of achieving 
variable optical density of printed spots in CIJ printing using the 
electrostatic dispersion of a charged drop stream to modulate the number 
of droplets which pass through a small aperture. This technique is used in 
ink jet printers manufactured by Iris Graphics. 
Kyser et al U.S. Pat. No. 3,946,398, 1970, discloses a DOD ink jet printer 
which applies a high voltage to a piezoelectric crystal, causing the 
crystal to bend, applying pressure on an ink reservoir and jetting drops 
on demand. Many types of piezoelectric drop on demand printers have 
subsequently been invented, which utilize piezoelectric crystals in bend 
mode, push mode, shear mode, and squeeze mode. Piezoelectric DOD printers 
have achieved commercial success using hot melt inks (for example, 
Tektronix and Dataproducts printers), and at image resolutions up to 720 
dpi for home and office printers (Seiko Epson). Piezoelectric DOD printers 
have an advantage in being able to use a wide range of inks. However, 
piezoelectric printing mechanisms usually require complex high voltage 
drive circuitry and bulky piezoelectric crystal arrays, which are 
disadvantageous in regard to manufacturability and performance. 
Endo et al GB Pat. No. 2,007,162, 1979, discloses an electrothermal DOD ink 
jet printer which applies a power pulse to an electrothermal transducer 
(heater) which is in thermal contact with ink in a nozzle. The heater 
rapidly heats water based ink to a high temperature, whereupon a small 
quantity of ink rapidly evaporates, forming a bubble. The formation of 
these bubbles results in a pressure wave which cause drops of ink to be 
ejected from small apertures along the edge of the heater substrate. This 
technology is known as Bubblejet.TM. (trademark of Canon K.K. of Japan), 
and is used in a wide range of printing systems from Canon, Xerox, and 
other manufacturers. 
Vaught et al U.S. Pat. No. 4,490,728, 1982, discloses an electrothermal 
drop ejection system which also operates by bubble formation. In this 
system, drops are ejected in a direction normal to the plane of the heater 
substrate, through nozzles formed in an aperture plate positioned above 
the heater. This system is known as Thermal Ink Jet, and is manufactured 
by Hewlett-Packard. In this document, the term Thermal Ink Jet is used to 
refer to both the Hewlett-Packard system and systems commonly known as 
Bubblejet.TM.. 
Thermal Ink Jet printing typically requires approximately 20 .mu.J over a 
period of approximately 2 .mu.s to eject each drop. The 10 Watt active 
power consumption of each heater is disadvantageous in itself and also 
necessitates special inks, complicates the driver electronics and 
precipitates deterioration of heater elements. 
Other ink jet printing systems have also been described in technical 
literature, but are not currently used on a commercial basis. For example, 
U.S. Pat. No. 4,275,290 discloses a system wherein the coincident address 
of predetermined print head nozzles with heat pulses and hydrostatic 
pressure, allows ink to flow freely to spacer-separated paper, passing 
beneath the print head. U.S. Pat. Nos. 4,737,803; 4,737,803 and 4,748,458 
disclose ink jet recording systems wherein the coincident address of ink 
in print head nozzles with heat pulses and an electrostatically attractive 
field cause ejection of ink drops to a print sheet. 
Each of the above-described inkjet printing systems has advantages and 
disadvantages. However, there remains a widely recognized need for an 
improved ink jet printing approach, providing advantages for example, as 
to cost, speed, quality, reliability, power usage, simplicity of 
construction and operation, durability and consumables. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide liquid ink printing 
systems which afford significant advancements towards attaining the 
above-noted advantages. The invention provides a drop-on-demand printing 
mechanism wherein the means of selecting drops to be printed produces a 
difference in position between selected drops and drops which are not 
selected, but which is insufficient to cause the ink drops to overcome the 
ink surface tension and separate from the body of ink, and wherein an 
additional means is provided to cause separation of said selected drops 
from said body of ink. 
A preferred aspect of the invention is that the means of producing a 
difference in position between selected drops and unselected drops is 
electrothermal reduction of surface tension of pressurized ink. 
An alternative preferred aspect of the invention is that the means of 
producing a difference in position between selected drops and unselected 
drops is electrothermal ink vapor bubble generation, said ink vapor bubble 
being insufficient to cause the separation of said selected drops from the 
body of ink in said nozzle. 
A further alternative preferred aspect of the invention is that the means 
of producing a difference in position between selected drops and 
unselected drops is activation of a piezoelectric transducer which is in 
direct or indirect mechanical contact with said ink, and when activated 
causes a change to the volume of an ink cavity which communicates with ink 
in the printing nozzle, such volume change being insufficient to cause the 
separation of said selected drops from the body of ink in said nozzle. 
A further alternative preferred aspect of the invention is that the means 
of producing a difference in position between selected drops and 
unselected drops is electrostatic attraction of electrically conductive 
ink, such electrostatic attraction being insufficient to cause the 
separation of said selected drops from the body of ink in said nozzle. 
A further alternative preferred aspect of the invention is that the means 
of separating said selected drops from the body of ink comprises arranging 
the printing medium in such a manner so that selected drops contact said 
print medium, and so that drops which are not selected do not contact said 
printing medium. 
A preferred aspect of the invention where the means of separating said 
selected drops from the body of ink comprises arranging the printing 
medium in such a manner so that selected drops contact said print medium 
and that the rate that said selected drops soak into and/or wet the 
surface of said printing medium is greater than the rate of egress of ink 
from the printing nozzle. 
A further preferred aspect of the invention is that the ink pressure 
oscillates. 
A further preferred aspect of the invention is that the ink pressure 
oscillates at a frequency which is an integral multiple of the drop 
ejection frequency from the nozzle. 
An alternative preferred aspect of the invention is that the means of 
separating said selected drops from the body of ink comprises 
electrostatic attraction of electrically conducting ink towards the 
recording medium. 
A preferred aspect of the invention is that the electric field producing 
said electrostatic attraction is applied substantially evenly to all 
nozzles. 
A preferred aspect of the invention is that the difference in electric 
force experienced by selected drops and unselected drops is largely due to 
the difference in position between said selected drops and said unselected 
drops. 
An alternative preferred aspect of the invention is that the means of 
separating said selected drops from the body of ink comprises magnetic 
attraction of ink which contains magnetically active substances towards 
the recording medium, 
A preferred aspect of the invention is that the magnetic field producing 
said magnetic attraction is applied substantially evenly to all nozzles. 
A preferred aspect of the invention is that the difference in magnetic 
force experienced by selected drops and unselected drops is largely due to 
the difference in position between said selected drops and said unselected 
drops. 
In another particularly preferred embodiment, the present invention 
constitutes methods and apparatus employing an acoustic wave as a 
coincident force in drop selection. 
In another preferred embodiment, the present invention constitutes methods 
and apparatus for varying the distance between print heads of the 
invention and the print region to vary the drop size of printed ink.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
In one general aspect, the invention constitutes a drop-on-demand printing 
mechanism wherein the means of selecting drops to be printed produces a 
difference in position between selected drops and drops which are not 
selected, but which is insufficient to cause the ink drops to overcome the 
ink surface tension and separate from the body of ink, and wherein an 
alternative means is provided to cause separation of the selected drops 
from the body of ink. 
The separation of drop selection means from drop separation means 
significantly reduces the energy required to select which ink drops are to 
be printed. Only the drop selection means must be driven by individual 
signals to each nozzle. The drop separation means can be a field or 
condition applied simultaneously to all nozzles. 
The drop selection means may be chosen from, but is not limited to, the 
following list: 
1) Electrothermal reduction of surface tension of pressurized ink 
2) Electrothermal bubble generation, with insufficient bubble volume to 
cause drop ejection 
3) Piezoelectric, with insufficient volume change to cause drop ejection 
4) Electrostatic attraction with one electrode per nozzle. 
The drop separation means may be chosen from, but is not limited to, the 
following list: 
1) Proximity (recording medium in close proximity to print head) 
2) Proximity with oscillating ink pressure 
3) Electrostatic attraction 
4) Magnetic attraction 
The table "DOD printing technology targets" shows some desirable 
characteristics of drop on demand printing technology. The table also 
lists some methods by which some embodiments described herein, or in other 
of my related applications, provide improvements over the prior art. 
DOD printing technology targets 
______________________________________ 
Target Method of achieving improvement over prior art 
______________________________________ 
High speed 
Practical, low cost, page width printing heads with 
operation more than 10,000 nozzles. Monolithic A4 pagewidth 
print heads can be manufactured using standard 
300 mm (12") silicon wafers 
High image 
High resolution (800 dpi is sufficient for most 
quality applications), six color process to reduce image noise 
Full color 
Halftoned process color at 800 dpi using stochastic 
operation screening 
Ink flexibility 
Low operating ink temperature and no requirement for 
bubble formation 
Low power Low power operation results from drop selection 
requirements 
means not being required to fully eject drop 
Low cost Monolithic print head without aperture plate, high 
manufacturing yield, small number of electrical 
connections, use of modified existing CMOS 
manufacturing facilities 
High Integrated fault tolerance in printing head 
manufacturing 
yield 
High reliability 
Integrated fault tolerance in printing head. Elimination 
of cavitation and kogation. Reduction of thermal 
shock. 
Small number of 
Shift registers, control logic, and drive circuitry can be 
electrical 
integrated on a monolithic print head using standard 
connections 
CMOS processes 
Use of existing 
CMOS compatibility. This can be achieved because 
VLSI the heater drive power is less is than 1% of Thermal 
manufacturing 
Ink Jet heater drive power 
facilities 
Electronic 
A new page compression system which can achieve 
collation 100:1 compression with insignificant image 
degradation, resulting in a compressed data rate low 
enough to allow real-time printing of any combination 
of thousands of pages stored on a low cost magnetic 
disk drive. 
______________________________________ 
In thermal ink jet (TIJ) and piezoelectric ink jet systems, a drop velocity 
of approximately 10 meters per second is preferred to ensure that the 
selected ink drops overcome ink surface tension, separate from the body of 
the ink, and strike the recording medium. These systems have a very low 
efficiency of conversion of electrical energy into drop kinetic energy. 
The efficiency of TIJ systems is approximately 0.02%). This means that the 
drive circuits for TIJ print heads must switch high currents. The drive 
circuits for piezoelectric ink jet heads must either switch high voltages, 
or drive highly capacitive loads. The total power consumption of pagewidth 
TIJ printheads is also very high. An 800 dpi A4 full color pagewidth TIJ 
print head printing a four color black image in one second would consume 
approximately 6 kW of electrical power, most of which is converted to 
waste heat. The difficulties of removal of this amount of heat precludes 
the production of low cost, high speed, high resolution compact pagewidth 
TIJ systems. 
One important feature of embodiments of the invention is a means of 
significantly reducing the energy required to select which ink drops are 
to be printed. This is achieved by separating the means for selecting ink 
drops from the means for ensuring that selected drops separate from the 
body of ink and form dots on the recording medium. Only the drop selection 
means must be driven by individual signals to each nozzle. The drop 
separation means can be a field or condition applied simultaneously to all 
nozzles. 
The table "Drop selection means" shows some of the possible means for 
selecting drops in accordance with the invention. The drop selection means 
is only required to create sufficient change in the position of selected 
drops that the drop separation means can discriminate between selected and 
unselected drops. 
Drop selection means 
______________________________________ 
Method Advantage Limitation 
______________________________________ 
1. Electrothermal 
Low temperature 
Requires ink pressure 
reduction of 
increase and low drop 
regulating mechanism. Ink 
surface tension of 
selection energy. Can be 
surface tension must reduce 
pressurized ink 
used with many ink 
substantially as temperature 
types. Simple fabrication. 
increases 
CMOS drive circuits can 
be fabricated on same 
substrate 
2. Electrothermal 
Medium drop selection 
Requires ink pressure 
reduction of ink 
energy, suitable for hot 
oscillation mechanism. Ink 
viscosity, 
melt and oil based inks. 
must have a large decrease 
combined with 
Simple fabrication. 
in viscosity as temperature 
oscillating ink 
CMOS drive circuits can 
increases 
pressure be fabricated on same 
substrate 
3. Electrothermal 
Well known technology, 
High drop selection energy, 
bubble simple fabrication, 
requires water based ink, 
generation, with 
bipolar drive circuits can 
problems with kogation, 
insufficient 
be fabricated on same 
cavitation, thermal stress 
bubble volume to 
substrate 
cause drop 
ejection 
4. Piezoelectric, 
Many types of ink base 
High manufacturing cost, 
with insufficient 
can be used incompatible with 
volume change to integrated circuit processes, 
cause drop high drive voltage, 
ejection mechanical complexity, 
bulky 
5. Electrostatic 
Simple electrode 
Nozzle pitch must be 
attraction with 
fabrication relatively large. Crosstalk 
one electrode per between adjacent electric 
nozzle fields. Requires high 
voltage drive circuits 
______________________________________ 
Other drop selection means may also be used. 
The preferred drop selection means for water based inks is method 1: 
"Electrothermal reduction of surface tension of pressurized ink". This 
drop selection means provides many advantages over other systems, 
including; low power operation (approximately 1% of TIJ), compatibility 
with CMOS VLSI chip fabrication, low voltage operation (approx. 10 V), 
high nozzle density, low temperature operation, and wide range of suitable 
ink formulations. The ink must exhibit a reduction in surface tension with 
increasing temperature. 
The preferred drop selection means for hot melt or oil based inks is method 
2: "electrothermal reduction of ink viscosity, combined with oscillating 
ink pressure". This drop selection means is particularly suited for use 
with inks which exhibit a large reduction of viscosity with increasing 
temperature, but only a small reduction in surface tension. This occurs 
particularly with non-polar ink carriers with relatively high molecular 
weight. This is especially applicable to hot melt and oil based inks. 
The table "Drop separation means" shows some of the possible methods for 
separating selected drops from the body of ink, and ensuring that the 
selected drops form dots on the printing medium. The drop separation means 
discriminates between selected drops and unselected drops to ensure that 
unselected drops do not form dots on the printing medium. 
Drop separation means 
______________________________________ 
Means Advantage Limitation 
______________________________________ 
1. Electrostatic 
Can print on rough 
Requires high voltage 
attraction 
surfaces, simple 
power supply 
implementation 
2. AC electric 
Higher field strength is 
Requires high voltage AC 
field possible than power supply synchronized 
electrostatic, operating 
to drop ejection phase. 
margins can be Multiple drop phase. 
increased, ink pressure 
operation is difficult 
reduced, and dust 
accumulation is reduced 
3. Proximity 
Very small spot sizes can 
Requires print medium to 
(print head in 
be achieved. Very low 
be very close to print head 
close proximity 
power dissipation. High 
surface, not suitable for 
to, but not 
drop position accuracy 
rough print media, usually 
touching, requires transfer roller or 
recording belt 
medium) 
4. Transfer 
Very small spot sizes can 
Not compact due to size of 
Proximity (print 
be achieved, very low 
transfer roller or transfer 
head is in close 
power dissipation, high 
belt. 
proximity to a 
accuracy, can print on 
transfer roller or 
rough paper 
belt 
5. Proximity with 
Useful for hot melt inks 
Requires print medium to 
oscillating ink 
using viscosity reduction 
be very close to print head 
pressure drop selection method, 
surface, not suitable for 
reduces possibility of 
rough print media. Requires 
nozzle clogging, can use 
ink pressure oscillation 
pigments instead of dyes 
apparatus 
6. Magnetic 
Can print on rough 
Requires uniform high 
attraction 
surfaces. Low power if 
magnetic field strength, 
permanent magnets are 
requires magnetic ink 
used 
______________________________________ 
Other drop separation means may also be used. 
The preferred drop separation means depends upon the intended use. For most 
applications, method 1: "Electrostatic attraction", or method 2: "AC 
electric field" are most appropriate. For applications where smooth coated 
paper or film is used, and very high speed is not essential, method 3: 
"Proximity" may be appropriate. For high speed, high quality systems, 
method 4: "Transfer proximity" can be used. Method 6: "Magnetic 
attraction" is appropriate for portable printing systems where the print 
medium is too rough for proximity printing, and the high voltages required 
for electrostatic drop separation are undesirable. There is no clear 
`best` drop separation means which is applicable to all circumstances. 
Further details of various types of printing systems according to the 
present invention are described in the following Australian patent 
specifications filed on 12 Apr., 1995, the disclosure of which are hereby 
incorporated by reference: 
`A Liquid ink Fault Tolerant (LIFT) printing mechanism` (Filing no.: 
PN2308); 
`Electrothermal drop selection in LIFT printing` (Filing no.: PN2309); 
`Drop separation in LIFT printing by print media proximity` (Filing no.: 
PN2310); 
`Drop size adjustment in Proximity LIFT printing by varying head to media 
distance` (Filing no.: PN2311); 
`Augmenting Proximity LIFT printing with acoustic ink waves` (Filing no.: 
PN2312); 
`Electrostatic drop separation in LIFT printing` (Filing no.: PN2313); 
`Multiple simultaneous drop sizes in Proximity LIFT printing` (Filing no.: 
PN2321); 
`Self cooling operation in thermally activated print heads` (Filing no.: 
PN2322); and 
`Thermal Viscosity Reduction LIFT printing` (Filing no.: PN2323). 
A simplified schematic diagram of one preferred printing system according 
to the invention appears in FIG. 1(a). 
An image source 52 may be raster image data from a scanner or computer, or 
outline image data in the form of a page description language (PDL), or 
other forms of digital image representation. This image data is converted 
to a pixel-mapped page image by the image processing system 53. This may 
be a raster image processor (RIP) in the case of PDL image data, or may be 
pixel image manipulation in the case of raster image data. Continuous tone 
data produced by the image processing unit 53 is halftoned. Halftoning is 
performed by the Digital Halftoning unit 54. Halftoned bitmap image data 
is stored in the image memory 72. Depending upon the printer and system 
configuration, the image memory 72 may be a full page memory, or a band 
memory. Heater control circuits 71 read data from the image memory 72 and 
apply time-varying electrical pulses to the nozzle heaters (103 in FIG. 
1(b)) that are part of the print head 50. These pulses are applied at an 
appropriate time, and to the appropriate nozzle, so that selected drops 
will form spots on the recording medium 51 in the appropriate position 
designated by the data in the image memory 72. 
The recording medium 51 is moved relative to the head 50 by a paper 
transport system 65, which is electronically controlled by a paper 
transport control system 66, which in turn is controlled by a 
microcontroller 315. The paper transport system shown in FIG. 1(a) is 
schematic only, and many different mechanical configurations are possible. 
In the case of pagewidth print heads, it is most convenient to move the 
recording medium 51 past a stationary head 50. However, in the case of 
scanning print systems, it is usually most convenient to move the head 50 
along one axis (the sub-scanning direction) and the recording medium 51 
along the orthogonal axis (the main scanning direction), in a relative 
raster motion. The microcontroller 315 may also control the ink pressure 
regulator 63 and the heater control circuits 71. 
For printing using surface tension reduction, ink is contained in an ink 
reservoir 64 under pressure. In the quiescent state (with no ink drop 
ejected), the ink pressure is insufficient to overcome the ink surface 
tension and eject a drop. A constant ink pressure can be achieved by 
applying pressure to the ink reservoir 64 under the control of an ink 
pressure regulator 63. Alternatively, for larger printing systems, the ink 
pressure can be very accurately generated and controlled by situating the 
top surface of the ink in the reservoir 64 an appropriate distance above 
the head 50. This ink level can be regulated by a simple float valve (not 
shown). 
For printing using viscosity reduction, ink is contained in an ink 
reservoir 64 under pressure, and the ink pressure is caused to oscillate. 
The means of producing this oscillation may be a piezoelectric actuator 
mounted in the ink channels (not shown). 
When properly arranged with the drop separation means, selected drops 
proceed to form spots on the recording medium 51, while unselected drops 
remain part of the body of ink. 
The ink is distributed to the back surface of the head 50 by an ink channel 
device 75. The ink preferably flows through slots and/or holes etched 
through the silicon substrate of the head 50 to the front surface, where 
the nozzles and actuators are situated. In the case of thermal selection, 
the nozzle actuators are electrothermal heaters. 
In some types of printers according to the invention, an external field 74 
is required to ensure that the selected drop separates from the body of 
the ink and moves towards the recording medium 51. A convenient external 
field 74 is a constant electric field, as the ink is easily made to be 
electrically conductive. In this case, the paper guide or platen 67 can be 
made of electrically conductive material and used as one electrode 
generating the electric field. The other electrode can be the head 50 
itself. Another embodiment uses proximity of the print medium as a means 
of discriminating between selected drops and unselected drops. 
For small drop sizes gravitational force on the ink drop is very small; 
approximately 10.sup.-4 of the surface tension forces, so gravity can be 
ignored in most cases. This allows the print head 50 and recording medium 
51 to be oriented in any direction in relation to the local gravitational 
field. This is an important requirement for portable printers. 
FIG. 1(b) is a detail enlargement of a cross section of a single 
microscopic nozzle tip embodiment of the invention, fabricated using a 
modified CMOS process. The nozzle is etched in a substrate 101, which may 
be silicon, glass, metal, or any other suitable material. If substrates 
which are not semiconductor materials are used, a semiconducting material 
(such as amorphous silicon) may be deposited on the substrate, and 
integrated drive transistors and data distribution circuitry may be formed 
in the surface semiconducting layer. Single crystal silicon (SCS) 
substrates have several advantages, including: 
1) High performance drive transistors and other circuitry can be fabricated 
in SCS; 
2) Print heads can be fabricated in existing facilities (fabs) using 
standard VLSI processing equipment; 
3) SCS has high mechanical strength and rigidity; and 
4) SCS has a high thermal conductivity. 
In this example, the nozzle is of cylindrical form, with the heater 103 
forming an annulus. The nozzle tip 104 is formed from silicon dioxide 
layers 102 deposited during the fabrication of the CMOS drive circuitry. 
The nozzle tip is passivated with silicon nitride. The protruding nozzle 
tip controls the contact point of the pressurized ink 100 on the print 
head surface. The print head surface is also hydrophobized to prevent 
accidental spread of ink across the front of the print head. 
Many other configurations of nozzles are possible, and nozzle embodiments 
of the invention may vary in shape, dimensions, and materials used. 
Monolithic nozzles etched from the substrate upon which the heater and 
drive electronics are formed have the advantage of not requiring an 
orifice plate. The elimination of the orifice plate has significant cost 
savings in manufacture and assembly. Recent methods for eliminating 
orifice plates include the use of `vortex` actuators such as those 
described in Domoto et al U.S. Pat. No. 4,580,158, 1986, assigned to 
Xerox, and Miller et al U.S. Pat. No. 5,371,527, 1994 assigned to 
Hewlett-Packard. These, however are complex to actuate, and difficult to 
fabricate. The preferred method for elimination of orifice plates for 
print heads of the invention is incorporation of the orifice into the 
actuator substrate. 
This type of nozzle may be used for print heads using various techniques 
for drop separation. 
Operation with Electrostatic Drop Separation 
As a first example, operation using thermal reduction of surface tension 
and electrostatic drop separation is shown in FIG. 2. 
FIG. 2 shows the results of energy transport and fluid dynamic simulations 
performed using FIDAP, a commercial fluid dynamic simulation software 
package available from Fluid Dynamics Inc., of Illinois, U.S.A. This 
simulation is of a thermal drop selection nozzle embodiment with a 
diameter of 8 .mu.m, at an ambient temperature of 30.degree. C. The total 
energy applied to the heater is 276 nJ, applied as 69 pulses of 4 nJ each. 
The ink pressure is 10 kPa above ambient air pressure, and the ink 
viscosity at 30.degree. C. is 1.84 cPs. The ink is water based, and 
includes a sol of 0.1% palmitic acid to achieve an enhanced decrease in 
surface tension with increasing temperature. A cross section of the nozzle 
tip from the central axis of the nozzle to a radial distance of 40 .mu.m 
is shown. Heat flow in the various materials of the nozzle, including 
silicon, silicon nitride, amorphous silicon dioxide, crystalline silicon 
dioxide, and water based ink are simulated using the respective densities, 
heat capacities, and thermal conductivities of the materials. The time 
step of the simulation is 0.1 .mu.s. 
FIG. 2(a) shows a quiescent state, just before the heater is actuated. An 
equilibrium is created whereby no ink escapes the nozzle in the quiescent 
state by ensuring that the ink pressure plus external electrostatic field 
is insufficient to overcome the surface tension of the ink at the ambient 
temperature. In the quiescent state, the meniscus of the ink does not 
protrude significantly from the print head surface, so the electrostatic 
field is not significantly concentrated at the meniscus. 
FIG. 2(b) shows thermal contours at 5.degree. C. intervals 5 .mu.s after 
the start of the heater energizing pulse. When the heater is energized, 
the ink in contact with the nozzle tip is rapidly heated. The reduction in 
surface tension causes the heated portion of the meniscus to rapidly 
expand relative to the cool ink meniscus. This drives a convective flow 
which rapidly transports this heat over part of the free surface of the 
ink at the nozzle tip. It is necessary for the heat to be distributed over 
the ink surface, and not just where the ink is in contact with the heater. 
This is because viscous drag against the solid heater prevents the ink 
directly in contact with the heater from moving. 
FIG. 2(c) shows thermal contours at 5.degree. C. intervals 10 .mu.s after 
the start of the heater energizing pulse. The increase in temperature 
causes a decrease in surface tension, disturbing the equilibrium of 
forces. As the entire meniscus has been heated, the ink begins to flow. 
FIG. 2(d) shows thermal contours at 5.degree. C. intervals 20 .mu.s after 
the start of the heater energizing pulse. The ink pressure has caused the 
ink to flow to a new meniscus position, which protrudes from the print 
head. The electrostatic field becomes concentrated by the protruding 
conductive ink drop. 
FIG. 2(e) shows thermal contours at 5.degree. C. intervals 30 .mu.s after 
the start of the heater energizing pulse, which is also 6 .mu.s after the 
end of the heater pulse, as the heater pulse duration is 24 .mu.s. The 
nozzle tip has rapidly cooled due to conduction through the oxide layers, 
and conduction into the flowing ink. The nozzle tip is effectively `water 
cooled` by the ink. Electrostatic attraction causes the ink drop to begin 
to accelerate towards the recording medium. Were the heater pulse 
significantly shorter (less than 16 .mu.s in this case) the ink would not 
accelerate towards the print medium, but would instead return to the 
nozzle. 
FIG. 2(f) shows thermal contours at 5.degree. C. intervals 26 .mu.s after 
the end of the heater pulse. The temperature at the nozzle tip is now less 
than 5.degree. C. above ambient temperature. This causes an increase in 
surface tension around the nozzle tip. When the rate at which the ink is 
drawn from the nozzle exceeds the viscously limited rate of ink flow 
through the nozzle, the ink in the region of the nozzle tip `necks`, and 
the selected drop separates from the body of ink. The selected drop then 
travels to the recording medium under the influence of the external 
electrostatic field. The meniscus of the ink at the nozzle tip then 
returns to its quiescent position, ready for the next heat pulse to select 
the next ink drop. One ink drop is selected, separated and forms a spot on 
the recording medium for each heat pulse. As the heat pulses are 
electrically controlled, drop on demand ink jet operation can be achieved. 
FIG. 3(a) shows successive meniscus positions during the drop selection 
cycle at 5 .mu.s intervals, starting at the beginning of the heater 
energizing pulse. 
FIG. 3(b) is a graph of meniscus position versus time, showing the movement 
of the point at the centre of the meniscus. The heater pulse starts 10 
.mu.s into the simulation. 
FIG. 3(c) shows the resultant curve of temperature with respect to time at 
various points in the nozzle. The vertical axis of the graph is 
temperature, in units of 100.degree. C. The horizontal axis of the graph 
is time, in units of 10 .mu.s. The temperature curve shown in FIG. 3(b) 
was calculated by FIDAP, using 0.1 .mu.s time steps. The local ambient 
temperature is 30 degrees C. Temperature histories at three points are 
shown: 
A--Nozzle tip: This shows the temperature history at the circle of contact 
between the passivation layer, the ink, and air. 
B--Meniscus midpoint: This is at a circle on the ink meniscus midway 
between the nozzle tip and the centre of the meniscus. 
C--Chip surface: This is at a point on the print head surface 20 .mu.m from 
the centre of the nozzle. The temperature only rises a few degrees. This 
indicates that active circuitry can be located very close to the nozzles 
without experiencing performance or lifetime degradation due to elevated 
temperatures. 
FIG. 3(e) shows the power applied to the heater. Optimum operation requires 
a sharp rise in temperature at the start of the heater pulse, a 
maintenance of the temperature a little below the boiling point of the ink 
for the duration of the pulse, and a rapid fall in temperature at the end 
of the pulse. To achieve this, the average energy applied to the heater is 
varied over the duration of the pulse. In this case, the variation is 
achieved by pulse frequency modulation of 0.1 .mu.s sub-pulses, each with 
an energy of 4 nJ. The peak power applied to the heater is 40 mW, and the 
average power over the duration of the heater pulse is 11.5 mW. The 
sub-pulse frequency in this case is 5 Mhz. This can readily be varied 
without significantly affecting the operation of the print head. A higher 
sub-pulse frequency allows finer control over the power applied to the 
heater. A sub-pulse frequency of 13.5 Mhz is suitable, as this frequency 
is also suitable for minimizing the effect of radio frequency interference 
(RFI). 
Inks with a negative temperature coefficient of surface tension 
The requirement for the surface tension of the ink to decrease with 
increasing temperature is not a major restriction, as most pure liquids 
and many mixtures have this property. Exact equations relating surface 
tension to temperature for arbitrary liquids are not available. However, 
the following empirical equation derived by Ramsay and Shields is 
satisfactory for many liquids: 
##EQU1## 
Where .gamma..sub.T is the surface tension at temperature T, k is a 
constant, T.sub.c is the critical temperature of the liquid, M is the 
molar mass of the liquid, x is the degree of association of the liquid, 
and .rho. is the density of the liquid. This equation indicates that the 
surface tension of most liquids falls to zero as the temperature reaches 
the critical temperature of the liquid. For most liquids, the critical 
temperature is substantially above the boiling point at atmospheric 
pressure, so to achieve an ink with a large change in surface tension with 
a small change in temperature around a practical ejection temperature, the 
admixture of surfactants is recommended. 
The choice of surfactant is important. For example, water based ink for 
thermal ink jet printers often contains isopropyl alcohol (2-propanol) to 
reduce the surface tension and promote rapid drying. Isopropyl alcohol has 
a boiling point of 82.4.degree. C., lower than that of water. As the 
temperature rises, the alcohol evaporates faster than the water, 
decreasing the alcohol concentration and causing an increase in surface 
tension. A surfactant such as 1-Hexanol (b.p. 158.degree. C.) can be used 
to reverse this effect, and achieve a surface tension which decreases 
slightly with temperature. However, a relatively large decrease in surface 
tension with temperature is desirable to maximize operating latitude. A 
surface tension decrease of 20 mN/m over a 30.degree. C. temperature range 
is preferred to achieve large operating margins, while as little as 10 
mN/m can be used to achieve operation of the print head according to the 
present invention. 
Inks With Large -.DELTA..gamma..sub.T 
Several methods may be used to achieve a large negative change in surface 
tension with increasing temperature. Two such methods are: 
1) The ink may contain a low concentration sol of a surfactant which is 
solid at ambient temperatures, but melts at a threshold temperature. 
Particle sizes less than 1,000 .ANG. are desirable. Suitable surfactant 
melting points for a water based ink are between 50.degree. C. and 
90.degree. C., and preferably between 60.degree. C. and 80.degree. C. 
2) The ink may contain an oil/water microemulsion with a phase inversion 
temperature (PIT) which is above the maximum ambient temperature, but 
below the boiling point of the ink. For stability, the PIT of the 
microemulsion is preferably 20.degree. C. or more above the maximum 
non-operating temperature encountered by the ink. A PIT of approximately 
80.degree. C. is suitable. 
Inks with Surfactant Sols 
Inks can be prepared as a sol of small particles of a surfactant which 
melts in the desired operating temperature range. Examples of such 
surfactants include carboxylic acids with between 14 and 30 carbon atoms, 
such as: 
______________________________________ 
Name Formula m.p. Synonym 
______________________________________ 
Tetradecanoic acid 
CH.sub.3 (CH.sub.2).sub.12 COOH 
58.degree. C. 
Myristic acid 
Hexadecanoic acid 
CH.sub.3 (CH.sub.2).sub.14 COOH 
63.degree. C. 
Palmitic acid 
Octadecanoic acid 
CH.sub.3 (CH.sub.2).sub.15 COOH 
71.degree. C. 
Stearic acid 
Eicosanoic acid 
CH.sub.3 (CH.sub.2).sub.16 COOH 
77.degree. C. 
Arachidic acid 
Docosanoic acid 
CH.sub.3 (CH.sub.2).sub.20 COOH 
80.degree. C. 
Behenic acid 
______________________________________ 
As the melting point of sols with a small particle size is usually slightly 
less than of the bulk material, it is preferable to choose a carboxylic 
acid with a melting point slightly above the desired drop selection 
temperature. A good example is Arachidic acid. 
These carboxylic acids are available in high purity and at low cost. The 
amount of surfactant required is very small, so the cost of adding them to 
the ink is insignificant. A mixture of carboxylic acids with slightly 
varying chain lengths can be used to spread the melting points over a 
range of temperatures. Such mixtures will typically cost less than the 
pure acid. 
It is not necessary to restrict the choice of surfactant to simple 
unbranched carboxylic acids. Surfactants with branched chains or phenyl 
groups, or other hydrophobic moieties can be used. It is also not 
necessary to use a carboxylic acid. Many highly polar moieties are 
suitable for the hydrophilic end of the surfactant. It is desirable that 
the polar end be ionizable in water, so that the surface of the surfactant 
particles can be charged to aid dispersion and prevent flocculation. In 
the case of carboxylic acids, this can be achieved by adding an alkali 
such as sodium hydroxide or potassium hydroxide. 
Preparation of Inks with Surfactant Sols 
The surfactant sol can be prepared separately at high concentration, and 
added to the ink in the required concentration. 
An example process for creating the surfactant sol is as follows: 
1) Add the carboxylic acid to purified water in an oxygen free atmosphere. 
2) Heat the mixture to above the melting point of the carboxylic acid. The 
water can be brought to a boil. 
3) Ultrasonicate the mixture, until the typical size of the carboxylic acid 
droplets is between 100 .ANG. and 1,000 .ANG.. 
4) Allow the mixture to cool. 
5) Decant the larger particles from the top of the mixture. 
6) Add an alkali such as NaOH to ionize the carboxylic acid molecules on 
the surface of the particles. A pH of approximately 8 is suitable. This 
step is not absolutely necessary, but helps stabilize the sol. 
7) Centrifuge the sol. As the density of the carboxylic acid is lower than 
water, smaller particles will accumulate at the outside of the centrifuge, 
and larger particles in the centre. 
8) Filter the sol using a microporous filter to eliminate any particles 
above 5000 .ANG.. 
9) Add the surfactant sol to the ink preparation. The sol is required only 
in very dilute concentration. 
The ink preparation will also contain either dye(s) or pigment(s), 
bactericidal agents, agents to enhance the electrical conductivity of the 
ink if electrostatic drop separation is used, humectants, and other agents 
as required. 
Anti-foaming agents will generally not be required, as there is no bubble 
formation during the drop ejection process. 
Cationic surfactant sols 
Inks made with anionic surfactant sols are generally unsuitable for use 
with cationic dyes or pigments. This is because the cationic dye or 
pigment may precipitate or flocculate with the anionic surfactant. To 
allow the use of cationic dyes and pigments, a cationic surfactant sol is 
required. The family of alkylamines is suitable for this purpose. 
Various suitable alkylamines are shown in the following table: 
______________________________________ 
Name Formula Synonym 
______________________________________ 
Hexadecylamine 
CH.sub.3 (CH.sub.2).sub.14 CH.sub.2 NH.sub.2 
Palmityl amine 
Octadecylamine 
CH.sub.3 (CH.sub.2).sub.16 CH.sub.2 NH.sub.2 
Stearyl amine 
Eicosylamine CH.sub.3 (CH.sub.2).sub.18 CH.sub.2 NH.sub.2 
Arachidyl amine 
Docosylamine CH.sub.3 (CH.sub.2).sub.20 CH.sub.2 NH.sub.2 
Behenyl amine 
______________________________________ 
The method of preparation of cationic surfactant sols is essentially 
similar to that of anionic surfactant sols, except that an acid instead of 
an alkali is used to adjust the pH balance and increase the charge on the 
surfactant particles. A pH of 6 using HCl is suitable. 
Microemulsion Based Inks 
An alternative means of achieving a large reduction in surface tension as 
some temperature threshold is to base the ink on a microemulsion. A 
microemulsion is chosen with a phase inversion temperature (PIT) around 
the desired ejection threshold temperature. Below the PIT, the 
microemulsion is oil in water (O/W), and above the PIT the microemulsion 
is water in oil (W/O). At low temperatures, the surfactant forming the 
microemulsion prefers a high curvature surface around oil, and at 
temperatures significantly above the PIT, the surfactant prefers a high 
curvature surface around water. At temperatures close to the PIT, the 
microemulsion forms a continuous `sponge` of topologically connected water 
and oil. 
There are two mechanisms whereby this reduces the surface tension. Around 
the PIT, the surfactant prefers surfaces with very low curvature. As a 
result, surfactant molecules migrate to the ink/air interface, which has a 
curvature which is much less than the curvature of the oil emulsion. This 
lowers the surface tension of the water. Above the phase inversion 
temperature, the microemulsion changes from O/W to W/O, and therefore the 
ink/air interface changes from water/air to oil/air. The oil/air interface 
has a lower surface tension. 
There is a wide range of possibilities for the preparation of microemulsion 
based inks. 
For fast drop ejection, it is preferable to chose a low viscosity oil. 
In many instances, water is a suitable polar solvent. However, in some 
cases different polar solvents may be required. In these cases, polar 
solvents with a high surface tension should be chosen, so that a large 
decrease in surface tension is achievable. 
The surfactant can be chosen to result in a phase inversion temperature in 
the desired range. For example, surfactants of the group 
poly(oxyethylene)alkylphenyl ether (ethoxylated alkyl phenols, general 
formula: C.sub.n H.sub.2n+1 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.m 
OH) can be used. The hydrophilicity of the surfactant can be increased by 
increasing m, and the hydrophobicity can be increased by increasing n. 
Values of m of approximately 10, and n of approximately 8 are suitable. 
Low cost commercial preparations are the result of a polymerization of 
various molar ratios of ethylene oxide and alkyl phenols, and the exact 
number of oxyethylene groups varies around the chosen mean. These 
commercial preparations are adequate, and highly pure surfactants with a 
specific number of oxyethylene groups are not required. 
The formula for this surfactant is C.sub.8 H.sub.17 C.sub.4 H.sub.6 
(CH.sub.2 CH.sub.2 O).sub.n OH (average n=10). 
Synonyms include Octoxynol-10, PEG-10 octyl phenyl ether and POE (10) octyl 
phenyl ether 
The HLB is 13.6, the melting point is 7.degree. C., and the cloud point is 
65.degree. C. 
Commercial preparations of this surfactant are available under various 
brand names. Suppliers and brand names are listed in the following table: 
______________________________________ 
Trade name Supplier 
______________________________________ 
Akypordox OP100 
Chem-Y GmbH 
Alkasurf OP-10 Rhone-Poulenc Surfactants and Specialties 
Dehydrophen POP 10 
Pulcra SA 
Hyonic OP-10 Henkel Corp. 
Iconol OP-10 BASF Corp. 
Igepal O Rhone-Poulenc France 
Macol OP-10 PPG Industries 
Malorphen 810 Huls AG 
Nikkol OP-10 Nikko Chem. Co. Ltd. 
Renex 750 ICI Americas Inc. 
Rexol 45/10 Hart Chemical Ltd. 
Synperonic OP10 
ICI PLC 
Teric X10 ICI Australia 
______________________________________ 
These are available in large volumes at low cost (less than one dollar per 
pound in quantity), and so contribute less than 10 cents per liter to 
prepared microemulsion ink with a 5% surfactant concentration. 
Other suitable ethoxylated alkyl phenols include those listed in the 
following table: 
______________________________________ 
Trivial name 
Formula HLB Cloud point 
______________________________________ 
Nonoxynol-9 
C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.-9 
OH 13 54.degree. C. 
Nonoxynol-10 
C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.-10 
OH 13.2 62.degree. C. 
Nonoxynol-11 
C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.-11 
OH 13.8 72.degree. C. 
Nonoxynol-12 
C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.-12 
OH 14.5 81.degree. C. 
Octoxynol-9 
C.sub.8 H.sub.17 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.-9 
OH 12.1 61.degree. C. 
Octoxynol-10 
C.sub.8 H.sub.17 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.-10 
OH 13.6 65.degree. C. 
Octoxynol-12 
C.sub.8 H.sub.17 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.-12 
OH 14.6 88.degree. C. 
Dodoxynol-10 
C.sub.12 H.sub.25 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.-10 
OH 12.6 42.degree. C. 
Dodoxynol-11 
C.sub.12 H.sub.25 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.-11 
OH 13.5 56.degree. C. 
Dodoxynol-14 
C.sub.12 H.sub.25 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.-14 
OH 14.5 87.degree. C. 
______________________________________ 
Microemulsion based inks have advantages other than surface tension 
control: 
1) Microemulsions are thermodynamically stable, and will not separate. 
Therefore, the storage time can be very long. This is especially 
significant for office and portable printers, which may be used 
sporadically. 
2) The microemulsion will form spontaneously with a particular drop size, 
and does not require extensive stirring, centrifuging, or filtering to 
ensure a particular range of emulsified oil drop sizes. 
3) The amount of oil contained in the ink can be quite high, so dyes which 
are soluble in oil or soluble in water, or both, can be used. It is also 
possible to use a mixture of dyes, one soluble in water, and the other 
soluble in oil, to obtain specific colors. 
4) Oil miscible pigments are prevented from flocculating, as they are 
trapped in the oil microdroplets. 
5) The use of a microemulsion can reduce the mixing of different dye colors 
on the surface of the print medium. 
6) The viscosity of microemulsions is very low. 
7) The requirement for humectants can be reduced or eliminated. 
Dyes and pigments in microemulsion based inks 
Oil in water mixtures can have high oil contents--as high as 40% and still 
form O/W microemulsions. This allows a high dye or pigment loading. 
Mixtures of dyes and pigments can be used. An example of a microemulsion 
based ink mixture with both dye and pigment is as follows: 
1) 70% water 
2) 5% water soluble dye 
3) 5% surfactant 
4) 10% oil 
5) 10% oil miscible pigment 
The following table shows the nine basic combinations of colorants in the 
oil and water phases of the microemulsion that may be used. 
______________________________________ 
Combination 
Colorant in water phase 
Colorant in oil phase 
______________________________________ 
1 none oil miscible pigment 
2 none oil soluble dye 
3 water soluble dye 
none 
4 water soluble dye 
oil miscible pigment 
5 water soluble dye 
oil soluble dye 
6 pigment dispersed in water 
none 
7 pigment dispersed in water 
oil miscible pigment 
8 pigment dispersed in water 
oil soluble dye 
9 none none 
______________________________________ 
The ninth combination, with no colorants, is useful for printing 
transparent coatings, UV ink, and selective gloss highlights. 
As many dyes are amphiphilic, large quantities of dyes can also be 
solubilized in the oil-water boundary layer as this layer has a very large 
surface area. 
It is also possible to have multiple dyes or pigments in each phase, and to 
have a mixture of dyes and pigments in each phase. 
When using multiple dyes or pigments the absorption spectrum of the 
resultant ink will be the weighted average of the absorption spectra of 
the different colorants used. This presents two problems: 
1) The absorption spectrum will tend to become broader, as the absorption 
peaks of both colorants are averaged. This has a tendency to `muddy` the 
colors. To obtain brilliant color, careful choice of dyes and pigments 
based on their absorption spectra, not just their human-perceptible color, 
needs to be made. 
2) The color of the ink may be different on different substrates. If a dye 
and a pigment are used in combination, the color of the dye will tend to 
have a smaller contribution to the printed ink color on more absorptive 
papers, as the dye will be absorbed into the paper, while the pigment will 
tend to `sit on top` of the paper. This may be used as an advantage in 
some circumstances. 
Surfactants with a Krafft point in the drop selection temperature range 
For ionic surfactants there is a temperature (the Krafft point) below which 
the solubility is quite low, and the solution contains essentially no 
micelles. Above the Krafft temperature micelle formation becomes possible 
and there is a rapid increase in solubility of the surfactant. If the 
critical micelle concentration (CMC) exceeds the solubility of a 
surfactant at a particular temperature, then the minimum surface tension 
will be achieved at the point of maximum solubility, rather than at the 
CMC. Surfactants are usually much less effective below the Krafft point. 
This factor can be used to achieve an increased reduction in surface 
tension with increasing temperature. At ambient temperatures, only a 
portion of the surfactant is in solution. When the nozzle heater is turned 
on, the temperature rises, and more of the surfactant goes into solution, 
decreasing the surface tension. 
A surfactant should be chosen with a Krafft point which is near the top of 
the range of temperatures to which the ink is raised. This gives a maximum 
margin between the concentration of surfactant in solution at ambient 
temperatures, and the concentration of surfactant in solution at the drop 
selection temperature. 
The concentration of surfactant should be approximately equal to the CMC at 
the Krafft point. In this manner, the surface tension is reduced to the 
maximum amount at elevated temperatures, and is reduced to a minimum 
amount at ambient temperatures. 
The following table shows some commercially available surfactants with 
Krafft points in the desired range. 
______________________________________ 
Formula Krafft point 
______________________________________ 
C.sub.16 H.sub.33 SO.sub.3.sup.- Na.sup.+ 
57.degree. C. 
C.sub.18 H.sub.37 SO.sub.3.sup.- Na.sup.+ 
70.degree. C. 
C.sub.16 H.sub.33 SO.sub.4.sup.- Na.sup.+ 
45.degree. C. 
Na.sup.+- O.sub.4 S(CH.sub.2).sub.16 SO.sub.4.sup.- Na.sup.+ 
44.9.degree. C. 
K.sup.+- O.sub.4 S(CH.sub.2).sub.16 SO.sub.4.sup.- K.sup.+ 
55.degree. C. 
C.sub.16 H.sub.33 CH(CH.sub.3)C.sub.4 H.sub.6 SO.sub.3.sup.- Na.sup.+ 
60.8.degree. C. 
______________________________________ 
Surfactants with a cloud point in the drop selection temperature range 
Non-ionic surfactants using polyoxyethylene (POE) chains can be used to 
create an ink where the surface tension falls with increasing temperature. 
At low temperatures, the POE chain is hydrophilic, and maintains the 
surfactant in solution. As the temperature increases, the structured water 
around the POE section of the molecule is disrupted, and the POE section 
becomes hydrophobic. The surfactant is increasingly rejected by the water 
at higher temperatures, resulting in increasing concentration of 
surfactant at the air/ink interface, thereby lowering surface tension. The 
temperature at which the POE section of a nonionic surfactant becomes 
hydrophilic is related to the cloud point of that surfactant. POE chains 
by themselves are not particularly suitable, as the cloud point is 
generally above 100.degree. C. 
Polyoxypropylene (POP) can be combined with POE in POE/POP block copolymers 
to lower the cloud point of POE chains without introducing a strong 
hydrophobicity at low temperatures. 
Two main configurations of symmetrical POE/POP block copolymers are 
available. These are: 
1) Surfactants with POE segments at the ends of the molecules, and a POP 
segment in the centre, such as the poloxamer class of surfactants 
(generically CAS9003-11-6) 
2) Surfactants with POP segments at the ends of the molecules, and a POE 
segment in the centre, such as the meroxapol class of surfactants 
(generically also CAS 9003-11-6) 
Some commercially available varieties of poloxamer and meroxapol with a 
high surface tension at room temperature, combined with a cloud point 
above 40.degree. C. and below 100.degree. C. are shown in the following 
table: 
______________________________________ 
Surface 
BASF Trade Tension 
Cloud 
Trivial name 
name Formula (mN/m) 
point 
______________________________________ 
Meroxapol 
Pluronic HO(CHCH.sub.3 CH.sub.2 O).sub..about.7 -- 
50.9 69.degree. C. 
105 10R5 (CH.sub.2 CH.sub.2 O).sub..about.22 -- 
(CHCH.sub.3 CH.sub.2 O).sub..about.7 OH 
Meroxapol 
Pluronic HO(CHCH.sub.3 CH.sub.2 O).sub..about.7 -- 
54.1 99.degree. C. 
108 10R8 (CH.sub.2 CH.sub.2 O).sub..about.91 -- 
(CHCH.sub.3 CH.sub.2 O).sub..about.7 OH 
Meroxapol 
Pluronic HO(CHCH.sub.3 CH.sub.2 O).sub..about.12 -- 
47.3 81.degree. C. 
178 17R8 (CH.sub.2 CH.sub.2 O).sub..about.136 -- 
(CHCH.sub.3 CH.sub.2 O).sub..about.12 OH 
Meroxapol 
Pluronic HO(CHCH.sub.3 CH.sub.2 O).sub..about.18 -- 
46.1 80.degree. C. 
258 25R8 (CH.sub.2 CH.sub.2 O).sub..about.163 -- 
(CHCH.sub.3 CH.sub.2 O).sub..about.18 OH 
Poloxamer 
Pluronic L35 
HO(CH.sub.2 CH.sub.2 O).sub..about.11 -- 
48.8 77.degree. C. 
105 (CHCH.sub.3 CH.sub.2 O).sub..about.16 -- 
(CH.sub.2 CH.sub.2 O).sub..about.11 OH 
Poloxamer 
Pluronic L44 
HO(CH.sub.2 CH.sub.2 O).sub..about.11 -- 
45.3 65.degree. C. 
124 (CHCH.sub.3 CH.sub.2 O).sub..about.21 -- 
(CH.sub.2 CH.sub.2 O).sub..about.11 OH 
______________________________________ 
Other varieties of poloxamer and meroxapol can readily be synthesized using 
well known techniques. Desirable characteristics are a room temperature 
surface tension which is as high as possible, and a cloud point between 
40.degree. C. and 100.degree. C., and preferably between 60.degree. C. and 
80.degree. C. 
Meroxapol HO(CHCH.sub.3 CH.sub.2 O).sub.x (CH.sub.2 CH.sub.2 O).sub.y 
(CHCH.sub.3 CH.sub.2 O).sub.z OH! varieties where the average x and z are 
approximately 4, and the average y is approximately 15 may be suitable. 
If salts are used to increase the electrical conductivity of the ink, then 
the effect of this salt on the cloud point of the surfactant should be 
considered. 
The cloud point of POE surfactants is increased by ions that disrupt water 
structure (such as I.sup.-), as this makes more water molecules available 
to form hydrogen bonds with the POE oxygen lone pairs. The cloud point of 
POE surfactants is decreased by ions that form water structure (such as 
Cl.sup.-, OH.sup.-), as fewer water molecules are available to form 
hydrogen bonds. Bromide ions have relatively little effect. The ink 
composition can be `tuned` for a desired temperature range by altering the 
lengths of POE and POP chains in a block copolymer surfactant, and by 
changing the choice of salts (e.g Cl.sup.- to Br.sup.- to I.sup.-) that 
are added to increase electrical conductivity. NaCl is likely to be the 
best choice of salts to increase ink conductivity, due to low cost and 
non-toxicity. NaCl slightly lowers the cloud point of nonionic 
surfactants. 
Hot Melt Inks 
The ink need not be in a liquid state at room temperature. Solid `hot melt` 
inks can be used by heating the printing head and ink reservoir above the 
melting point of the ink. The hot melt ink must be formulated so that the 
surface tension of the molten ink decreases with temperature. A decrease 
of approximately 2 mN/m will be typical of many such preparations using 
waxes and other substances. However, a reduction in surface tension of 
approximately 20 mN/m is desirable in order to achieve good operating 
margins when relying on a reduction in surface tension rather than a 
reduction in viscosity. 
The temperature difference between quiescent temperature and drop selection 
temperature may be greater for a hot melt ink than for a water based ink, 
as water based inks are constrained by the boiling point of the water. 
The ink must be liquid at the quiescent temperature. The quiescent 
temperature should be higher than the highest ambient temperature likely 
to be encountered by the printed page. The quiescent temperature should 
also be as low as practical, to reduce the power needed to heat the print 
head, and to provide a maximum margin between the quiescent and the drop 
ejection temperatures. A quiescent temperature between 60.degree. C. and 
90.degree. C. is generally suitable, though other temperatures may be 
used. A drop ejection temperature of between 160.degree. C. and 
200.degree. C. is generally suitable. 
There are several methods of achieving an enhanced reduction in surface 
tension with increasing temperature. 
1) A dispersion of microfine particles of a surfactant with a melting point 
substantially above the quiescent temperature, but substantially below the 
drop ejection temperature, can be added to the hot melt ink while in the 
liquid phase. 
2) A polar/non-polar microemulsion with a PIT which is preferably at least 
20.degree. C. above the melting points of both the polar and non-polar 
compounds. 
To achieve a large reduction in surface tension with temperature, it is 
desirable that the hot melt ink carrier have a relatively large surface 
tension (above 30 mN/m) when at the quiescent temperature. This generally 
excludes alkanes such as waxes. Suitable materials will generally have a 
strong intermolecular attraction, which may be achieved by multiple 
hydrogen bonds, for example, polyols, such as Hexanetetrol, which has a 
melting point of 88.degree. C. 
Surface tension reduction of various solutions 
FIG. 3(d) shows the measured effect of temperature on the surface tension 
of various aqueous preparations containing the following additives: 
1) 0.1% sol of Stearic Acid 
2) 0.1% sol of Palmitic acid 
3) 0.1% solution of Pluronic 10R5 (trade mark of BASF) 
4) 0.1% solution of Pluronic L35 (trade mark of BASF) 
5) 0.1% solution of Pluronic L44 (trade mark of BASF) 
Inks suitable for printing systems of the present invention are described 
in the following Australian patent specifications, the disclosure of which 
are hereby incorporated by reference: 
`Ink composition based on a microemulsion` (Filing no.: PN5223, filed on 6 
Sep., 1995); 
`Ink composition containing surfactant sol` (Filing no.: PN5224, filed on 6 
Sep., 1995); 
`Ink composition for DOD printers with Krafft point near the drop selection 
temperature sol` (Filing no.: PN6240, filed on 30 Oct., 1995); and 
`Dye and pigment in a microemulsion based ink` (Filing no.: PN6241, filed 
on 30 Oct., 1995). 
Operation Using Reduction of Viscosity 
As a second example, operation of an embodiment using thermal reduction of 
viscosity and proximity drop separation, in combination with hot melt ink, 
is as follows. Prior to operation of the printer, solid ink is melted in 
the reservoir 64. The reservoir, ink passage to the print head, ink 
channels 75, and print head 50 are maintained at a temperature at which 
the ink 100 is liquid, but exhibits a relatively high viscosity (for 
example, approximately 100 cP). The Ink 100 is retained in the nozzle by 
the surface tension of the ink. The ink 100 is formulated so that the 
viscosity of the ink reduces with increasing temperature. The ink pressure 
oscillates at a frequency which is an integral multiple of the drop 
ejection frequency from the nozzle. The ink pressure oscillation causes 
oscillations of the ink meniscus at the nozzle tips, but this oscillation 
is small due to the high ink viscosity. At the normal operating 
temperature, these oscillations are of insufficient amplitude to result in 
drop separation. When the heater 103 is energized, the ink forming the 
selected drop is heated, causing a reduction in viscosity to a value which 
is preferably less than 5 cP. The reduced viscosity results in the ink 
meniscus moving further during the high pressure part of the ink pressure 
cycle. The recording medium 51 is arranged sufficiently close to the print 
head 50 so that the selected drops contact the recording medium 51, but 
sufficiently far away that the unselected drops do not contact the 
recording medium 51. Upon contact with the recording medium 51, part of 
the selected drop freezes, and attaches to the recording medium. As the 
ink pressure falls, ink begins to move back into the nozzle. The body of 
ink separates from the ink which is frozen onto the recording medium. The 
meniscus of the ink 100 at the nozzle tip then returns to low amplitude 
oscillation. The viscosity of the ink increases to its quiescent level as 
remaining heat is dissipated to the bulk ink and print head. One ink drop 
is selected, separated and forms a spot on the recording medium 51 for 
each heat pulse. As the heat pulses are electrically controlled, drop on 
demand ink jet operation can be achieved. 
Manufacturing of Print Heads 
Manufacturing processes for monolithic print heads in accordance with the 
present invention are described in the following Australian patent 
specifications filed on 12 Apr., 1995, the disclosure of which are hereby 
incorporated by reference: 
`A monolithic LIFT printing head` (Filing no.: PN2301); 
`A manufacturing process for monolithic LIFT printing heads` (Filing no.: 
PN2302); 
`A self-aligned heater design for LIFT print heads` (Filing no.: PN2303); 
`Integrated four color LIFT print heads` (Filing no.: PN2304); 
`Power requirement reduction in monolithic LIFT printing heads` (Filing 
no.: PN2305); 
`A manufacturing process for monolithic LIFT print heads using anisotropic 
wet etching` (Filing no.: PN2306); 
`Nozzle placement in monolithic drop-on-demand print heads` (Filing no.: 
PN2307); 
`Heater structure for monolithic LIFT print heads` (Filing no.: PN2346); 
`Power supply connection for monolithic LIFT print heads` (Filing no.: 
PN2347); 
`External connections for Proximity LIFT print heads` (Filing no.: PN2348); 
and 
`A self-aligned manufacturing process for monolithic LIFT print heads` 
(Filing no.: PN2349); and 
`CMOS process compatible fabrication of LIFT print heads` (Filing no.: 
PN5222, 6 Sep., 1995). 
`A manufacturing process for LIFT print heads with nozzle rim heaters` 
(Filing no.: PN6238, 30 Oct., 1995); 
`A modular LIFT print head` (Filing no.: PN6237, 30 Oct., 1995); 
`Method of increasing packing density of printing nozzles` (Filing no.: 
PN6236, 30 Oct., 1995); and 
`Nozzle dispersion for reduced electrostatic interaction between 
simultaneously printed droplets` (Filing no.: PN6239, 30 Oct., 1995). 
Control of Print Heads 
Means of providing page image data and controlling heater temperature in 
print heads of the present invention is described in the following 
Australian patent specifications filed on 12 Apr., 1995, the disclosure of 
which are hereby incorporated by reference: 
`Integrated drive circuitry in LIFT print heads` (Filing no.: PN2295); 
`A nozzle clearing procedure for Liquid Ink Fault Tolerant (LIFT) printing` 
(Filing no.: PN2294); 
`Heater power compensation for temperature in LIFT printing systems` 
(Filing no.: PN2314); 
`Heater power compensation for thermal lag in LIFT printing systems` 
(Filing no.: PN2315); 
`Heater power compensation for print density in LIFT printing systems` 
(Filing no.: PN2316); 
`Accurate control of temperature pulses in printing heads` (Filing no.: 
PN2317); 
`Data distribution in monolithic LIFT print heads` (Filing no.: PN2318); 
`Page image and fault tolerance routing device for LIFT printing systems` 
(Filing no.: PN2319); and 
`A removable pressurized liquid ink cartridge for LIFT printers` (Filing 
no.: PN2320). 
Image Processing for Print Heads 
An objective of printing systems according to the invention is to attain a 
print quality which is equal to that which people are accustomed to in 
quality color publications printed using offset printing. This can be 
achieved using a print resolution of approximately 1,600 dpi. However, 
1,600 dpi printing is difficult and expensive to achieve. Similar results 
can be achieved using 800 dpi printing, with 2 bits per pixel for cyan and 
magenta, and one bit per pixel for yellow and black. This color model is 
herein called CC'MM'YK. Where high quality monochrome image printing is 
also required, two bits per pixel can also be used for black This color 
model is herein called CC'MM'YKK'. Color models, halftoning, data 
compression, and real-time expansion systems suitable for use in systems 
of this invention and other printing systems are described in the 
following Australian patent specifications filed on 12 Apr., 1995, the 
disclosure of which are hereby incorporated by reference: 
`Four level ink set for bi-level color printing` (Filing no.: PN2339); 
`Compression system for page images` (Filing no.: PN2340); 
`Real-time expansion apparatus for compressed page images` (Filing no.: 
PN2341); and 
`High capacity compressed document image storage for digital color 
printers` (Filing no.: PN2342); 
`Improving JPEG compression in the presence of text` (Filing no.: PN2343); 
`An expansion and halftoning device for compressed page images` (Filing 
no.: PN2344); and 
`Improvements in image halftoning` (Filing no.: PN2345). 
Applications Using Print Heads According to this Invention 
Printing apparatus and methods of this invention are suitable for a wide 
range of applications, including (but not limited to) the following: color 
and monochrome office printing, short run digital printing, high speed 
digital printing, process color printing, spot color printing, offset 
press supplemental printing, low cost printers using scanning print heads, 
high speed printers using pagewidth print heads, portable color and 
monochrome printers, color and monochrome copiers, color and monochrome 
facsimile machines, combined printer, facsimile and copying machines, 
label printing, large format plotters, photographic duplication, printers 
for digital photographic processing, portable printers incorporated into 
digital `instant` cameras, video printing, printing of PhotoCD images, 
portable printers for `Personal Digital Assistants`, wallpaper printing, 
indoor sign printing, billboard printing, and fabric printing. 
Printing systems based on this invention are described in the following 
Australian patent specifications filed on 12 Apr., 1995, the disclosure of 
which are hereby incorporated by reference: 
`A high speed color office printer with a high capacity digital page image 
store` (Filing no.: PN2329); 
`A short run digital color printer with a high capacity digital page image 
store` (Filing no.: PN2330); 
`A digital color printing press using LIFT printing technology` (Filing 
no.: PN2331); 
`A modular digital printing press` (Filing no.: PN2332); 
`A high speed digital fabric printer` (Filing no.: PN2333); 
`A color photograph copying system` (Filing no.: PN2334); 
`A high speed color photocopier using a LIFT printing system` (Filing no.: 
PN2335); 
`A portable color photocopier using LIFT printing technology` (Filing no.: 
PN2336); 
`A photograph processing system using LIFT printing technology` (Filing 
no.: PN2337); 
`A plain paper facsimile machine using a LIFT printing system` (Filing no.: 
PN2338); 
`A PhotoCD system with integrated printer` (Filing no.: PN2293); 
`A color plotter using LIFT printing technology` (Filing no.: PN2291); 
`A notebook computer with integrated LIFT color printing system` (Filing 
no.: PN2292); 
`A portable printer using a LIFT printing system` (Filing no.: PN2300); 
`Fax machine with on-line database interrogation and customized magazine 
printing` (Filing no.: PN2299); 
`Miniature portable color printer` (Filing no.: PN2298); 
`A color video printer using a LIFT printing system` (Filing no.: PN2296); 
and 
`An integrated printer, copier, scanner, and facsimile using a LIFT 
printing system` (Filing no.: PN2297) 
Compensation of Print Heads for Environmental Conditions 
It is desirable that drop on demand printing systems have consistent and 
predictable ink drop size and position. Unwanted variation in ink drop 
size and position causes variations in the optical density of the 
resultant print, reducing the perceived print quality. These variations 
should be kept to a small proportion of the nominal ink drop volume and 
pixel spacing respectively. Many environmental variables can be 
compensated to reduce their effect to insignificant levels. Active 
compensation of some factors can be achieved by varying the power applied 
to the nozzle heaters. 
An optimum temperature profile for one print head embodiment involves an 
instantaneous raising of the active region of the nozzle tip to the 
ejection temperature, maintenance of this region at the ejection 
temperature for the duration of the pulse, and instantaneous cooling of 
the region to the ambient temperature. 
This optimum is not achievable due to the stored heat capacities and 
thermal conductivities of the various materials used in the fabrication of 
the nozzles in accordance with the invention. However, improved 
performance can be achieved by shaping the power pulse using curves which 
can be derived by iterative refinement of finite element simulation of the 
print head. The power applied to the heater can be varied in time by 
various techniques, including, but not limited to: 
1) Varying the voltage applied to the heater 
2) Modulating the width of a series of short pulses (PWM) 
3) Modulating the frequency of a series of short pulses (PFM) 
To obtain accurate results, a transient fluid dynamic simulation with free 
surface modeling is required, as convection in the ink, and ink flow, 
significantly affect on the temperature achieved with a specific power 
curve. 
By the incorporation of appropriate digital circuitry on the print head 
substrate, it is practical to individually control the power applied to 
each nozzle. One way to achieve this is by `broadcasting` a variety of 
different digital pulse trains across the print head chip, and selecting 
the appropriate pulse train for each nozzle using multiplexing circuits. 
An example of the environmental factors which may be compensated for is 
listed in the table "Compensation for environmental factors". This table 
identifies which environmental factors are best compensated globally (for 
the entire print head), per chip (for each chip in a composite multi-chip 
print head), and per nozzle. 
Compensation for environmental factors 
______________________________________ 
Factor Sensing or user 
Compensation 
compensated 
Scope control method 
mechanism 
______________________________________ 
Ambient Global Temperature sensor 
Power supply voltage 
Temperature mounted on print 
or global PFM patterns 
head 
Power supply 
Global Predictive active 
Power supply voltage 
voltage nozzle count based 
or global PFM patterns 
fluctuation on print data 
with number of 
active nozzles 
Local heat build- 
Per Predictive active 
Selection of 
up with nozzle nozzle count based 
appropriate PFM 
successive on print data 
pattern for each 
nozzle actuation printed drop 
Drop size control 
Per Image data Selection of 
for multiple bits 
nozzle appropriate PFM 
per pixel pattern for each 
printed drop 
Nozzle geometry 
Per Factory Global PFM patterns 
variations 
chip measurement, data- 
per print head chip 
between wafers file supplied 
with print head 
Heater resistivity 
Per Factory Global PFM patterns 
variations 
chip measurement, data- 
per print head chip 
between wafers file supplied 
with print head 
User image 
Global User selection 
Power supply voltage, 
intensity electrostatic 
adjustment acceleration voltage, 
or ink pressure 
Ink surface 
Global Ink cartridge sensor 
Global PFM patterns 
tension or user selection 
reduction method 
and threshold 
temperature 
Ink viscosity 
Global Ink cartridge sensor 
Global PFM patterns 
or user selection 
and/or clock rate 
Ink dye or 
Global Ink cartridge sensor 
Global PFM patterns 
pigment or user selection 
concentration 
Ink response time 
Global Ink cartridge sensor 
Global PFM patterns 
or user selection 
______________________________________ 
Most applications will not require compensation for all of these variables. 
Some variables have a minor effect, and compensation is only necessary 
where very high image quality is required. 
Print head drive circuits 
FIG. 4 is a block schematic diagram showing electronic operation of an 
example head driver circuit in accordance with this invention. This 
control circuit uses analog modulation of the power supply voltage applied 
to the print head to achieve heater power modulation, and does not have 
individual control of the power applied to each nozzle. FIG. 4 shows a 
block diagram for a system using an 800 dpi pagewidth print head which 
prints process color using the CC'MM'YK color model. The print head 50 has 
a total of 79,488 nozzles, with 39,744 main nozzles and 39,744 redundant 
nozzles. The main and redundant nozzles are divided into six colors, and 
each color is divided into 8 drive phases. Each drive phase has a shift 
register which converts the serial data from a head control ASIC 400 into 
parallel data for enabling heater drive circuits. There is a total of 96 
shift registers, each providing data for 828 nozzles. Each shift register 
is composed of 828 shift register stages 217, the outputs of which are 
logically anded with phase enable signal by anand gate 215. The output of 
the nand gate 215 drives an inverting buffer 216, which in turn controls 
the drive transistor 201. The drive transistor 201 actuates the 
electrothermal heater 200, which may be a heater 103 as shown in FIG. 
1(b). To maintain the shifted data valid during the enable pulse, the 
clock to the shift register is stopped the enable pulse is active by a 
clock stopper 218, which is shown as a single gate for clarity, but is 
preferably any of a range of well known glitch free clock control 
circuits. Stopping the clock of the shift register removes the requirement 
for a parallel data latch in the print head, but adds some complexity to 
the control circuits in the Head Control ASIC 400. Data is routed to 
either the main nozzles or the redundant nozzles by the data router 219 
depending on the state of the appropriate signal of the fault status bus. 
The print head shown in FIG. 4 is simplified, and does not show various 
means of improving manufacturing yield, such as block fault tolerance. 
Drive circuits for different configurations of print head can readily be 
derived from the apparatus disclosed herein. 
Digital information representing patterns of dots to be printed on the 
recording medium is stored in the Page or Band memory 1513, which may be 
the same as the Image memory 72 in FIG. 1(a). Data in 32 bit words 
representing dots of one color is read from the Page or Band memory 1513 
using addresses selected by the address mux 417 and control signals 
generated by the Memory Interface 418. These addresses are generated by 
Address generators 411, which forms part of the `Per color circuits` 410, 
for which there is one for each of the six color components. The addresses 
are generated based on the positions of the nozzles in relation to the 
print medium. As the relative position of the nozzles may be different for 
different print heads, the Address generators 411 are preferably made 
programmable. The Address generators 411 normally generate the address 
corresponding to the position of the main nozzles. However, when faulty 
nozzles are present, locations of blocks of nozzles containing faults can 
be marked in the Fault Map RAM 412. The Fault Map RAM 412 is read as the 
page is printed. If the memory indicates a fault in the block of nozzles, 
the address is altered so that the Address generators 411 generate the 
address corresponding to the position of the redundant nozzles. Data read 
from the Page or Band memory 1513 is latched by the latch 413 and 
converted to four sequential bytes by the multiplexer 414. Timing of these 
bytes is adjusted to match that of data representing other colors by the 
FIFO 415. This data is then buffered by the buffer 430 to form the 48 bit 
main data bus to the print head 50. The data is buffered as the print head 
may be located a relatively long distance from the head control ASIC. Data 
from the Fault Map RAM 412 also forms the input to the FIFO 416. The 
timing of this data is matched to the data output of the FIFO 415, and 
buffered by the buffer 431 to form the fault status bus. 
The programmable power supply 320 provides power for the head 50. The 
voltage of the power supply 320 is controlled by the DAC 313, which is 
part of a RAM and DAC combination (RAMDAC) 316. The RAMDAC 316 contains a 
dual port RAM 317. The contents of the dual port RAM 317 are programmed by 
the Microcontroller 315. Temperature is compensated by changing the 
contents of the dual port RAM 317. These values are calculated by the 
microcontroler 315 based on temperature sensed by a thermal sensor 300. 
The thermal sensor 300 signal connects to the Analog to Digital Converter 
(ADC) 311. The ADC 311 is preferably incorporated in the Microcontroler 
315. 
The Head Control ASIC 400 contains control circuits for thermal lag 
compensation and print density. Thermal lag compensation requires that the 
power supply voltage to the head 50 is a rapidly time-varying voltage 
which is synchronized with the enable pulse for the heater. This is 
achieved by programming the programmable power supply 320 to produce this 
voltage. An analog time varying programming voltage is produced by the DAC 
313 based upon data read from the dual port RAM 317. The data is read 
according to an address produced by the counter 403. The counter 403 
produces one complete cycle of addresses during the period of one enable 
pulse. This synchronization is ensured, as the counter 403 is clocked by 
the system clock 408, and the top count of the counter 403 is used to 
clock the enable counter 404. The count from the enable counter 404 is 
then decoded by the decoder 405 and buffered by the buffer 432 to produce 
the enable pulses for the head 50. The counter 403 may include a prescaler 
if the number of states in the count is less than the number of clock 
periods in one enable pulse. Sixteen voltage states are adequate to 
accurately compensate for the heater thermal lag. These sixteen states can 
be specified by using a four bit connection between the counter 403 and 
the dual port RAM 317. However, these sixteen states may not be linearly 
spaced in time. To allow non-linear timing of these states the counter 403 
may also include a ROM or other device which causes the counter 403 to 
count in a non-linear fashion. Alternatively, fewer than sixteen states 
may be used. 
For print density compensation, the printing density is detected by 
counting the number of pixels to which a drop is to be printed (`on` 
pixels) in each enable period. The `on` pixels are counted by the On pixel 
counters 402. There is one On pixel counter 402 for each of the eight 
enable phases. The number of enable phases in a print head in accordance 
with the invention depend upon the specific design. Four, eight, and 
sixteen are convenient numbers, though there is no requirement that the 
number of enable phases is a power of two. The On Pixel Counters 402 can 
be composed of combinatorial logic pixel counters 420 which determine how 
many bits in a nibble of data are on. This number is then accumulated by 
the adder 421 and accumulator 422. A latch 423 holds the accumulated value 
valid for the duration of the enable pulse. The multiplexer 401 selects 
the output of the latch 423 which corresponds to the current enable phase, 
as determined by the enable counter 404. The output of the multiplexer 401 
forms part of the address of the dual port RAM 317. An exact count of the 
number of `on` pixels is not necessary, and the most significant four bits 
of this count are adequate. 
Combining the four bits of thermal lag compensation address and the four 
bits of print density compensation address means that the dual port RAM 
317 has an 8 bit address. This means that the dual port RAM 317 contains 
256 numbers, which are in a two dimensional array. These two dimensions 
are time (for thermal lag compensation) and print density. A third 
dimension--temperature--can be included. As the ambient temperature of the 
head varies only slowly, the microcontroller 315 has sufficient time to 
calculate a matrix of 256 numbers compensating for thermal lag and print 
density at the current temperature. Periodically (for example, a few times 
a second), the microcontroller senses the current head temperature and 
calculates this matrix. 
The clock to the print head 50 is generated from the system clock 408 by 
the Head clock generator 407, and buffered by the buffer 406. To 
facilitate testing of the Head control ASIC, JTAG test circuits 499 may be 
included. 
Comparison with thermal ink jet technology 
The table "Comparison between Thermal ink jet and Present Invention" 
compares the aspects of printing in accordance with the present invention 
with thermal ink jet printing technology. 
A direct comparison is made between the present invention and thermal ink 
jet technology because both are drop on demand systems which operate using 
thermal actuators and liquid ink. Although they may appear similar, the 
two technologies operate on different principles. 
Thermal ink jet printers use the following fundamental operating principle. 
A thermal impulse caused by electrical resistance heating results in the 
explosive formation of a bubble in liquid ink. Rapid and consistent bubble 
formation can be achieved by superheating the ink, so that sufficient heat 
is transferred to the ink before bubble nucleation is complete. For water 
based ink, ink temperatures of approximately 280.degree. C. to 400.degree. 
C. are required. The bubble formation causes a pressure wave which forces 
a drop of ink from the aperture with high velocity. The bubble then 
collapses, drawing ink from the ink reservoir to re-fill the nozzle. 
Thermal ink jet printing has been highly successful commercially due to 
the high nozzle packing density and the use of well established integrated 
circuit manufacturing techniques. However, thermal ink jet printing 
technology faces significant technical problems including multi-part 
precision fabrication, device yield, image resolution, `pepper` noise, 
printing speed, drive transistor power, waste power dissipation, satellite 
drop formation, thermal stress, differential thermal expansion, kogation, 
cavitation, rectified diffusion, and difficulties in ink formulation. 
Printing in accordance with the present invention has many of the 
advantages of thermal ink jet printing, and completely or substantially 
eliminates many of the inherent problems of thermal ink jet technology. 
Comparison between Thermal ink jet and Present Invention 
______________________________________ 
Thermal Ink-Jet 
Present Invention 
______________________________________ 
Drop selection 
Drop ejected by pressure 
Choice of surface tension or 
mechanism wave caused by viscosity reduction 
thermally mechanisms 
induced bubble 
Drop separation 
Same as drop selection 
Choice of proximity, 
mechanism mechanism electrostatic, magnetic, and 
other methods 
Basic ink carrier 
Water Water, microemulsion, 
alcohol, glycol, or hot melt 
Head construction 
Precision assembly of 
Monolithic 
nozzle plate, ink channel, 
and substrate 
Per copy printing 
Very high due to limited 
Can be low due to 
cost print head life and 
permanent print heads and 
expensive inks wide range of possible inks 
Satellite drop 
Significant problem 
No satellite drop formation 
formation which 
degrades image quality 
Operating ink 
280.degree. C. to 400.degree. C. (high 
Approx. 70.degree. C. (depends 
temperature 
temperature limits dye 
upon ink formulation) 
use and ink formulation) 
Peak heater 
400.degree. C. to 1,000.degree. C. 
Approx. 130.degree. C. 
temperature 
(high temperature 
reduces device life) 
Cavitation (heater 
Serious problem limiting 
None (no bubbles are 
erosion by bubble 
head life formed) 
collapse) 
Kogation (coating 
Serious problem limiting 
None (water based ink 
of heater by ink 
head life and ink 
temperature does not 
ash) formulation exceed 100.degree. C.) 
Rectified Serious problem limiting 
Does not occur as the ink 
diffusion ink formulation 
pressure does not go 
(formation of ink negative 
bubbles due to 
pressure cycles) 
Resonance Serious problem limiting 
Very small effect as 
nozzle design and 
pressure waves are small 
repetition rate 
Practical Approx. 800 dpi max. 
Approx. 1,600 dpi max. 
resolution 
Self-cooling 
No (high energy 
Yes: printed ink carries 
operation required) away drop selection energy 
Drop ejection 
High (approx. 10 m/sec) 
Low (approx. 1 m/sec) 
velocity 
Crosstalk Serious problem 
Low velocities and 
requiring careful acoustic 
pressures associated with 
design, which limits 
drop ejection make 
nozzle refill rate. 
crosstalk very small. 
Operating thermal 
Serious problem limiting 
Low: maximum 
stress print-head life. 
temperature 
increase approx. 
90.degree. C. at centre of heater. 
Manufacturing 
Serious problem limiting 
Same as standard CMOS 
thermal stress 
print-head size. 
manufacturing process. 
Drop selection 
Approx. 20 .mu.J 
Approx. 270 nJ 
energy 
Heater pulse 
Approx. 2-3 .mu.s 
Approx. 15-30 .mu.s 
period 
Average heater 
Approx. 8 Watts per 
Approx. 12 mW per heater. 
pulse power 
heater. This is more than 500 times 
less than Thermal Ink-Jet. 
Heater pulse 
Typically approx. 40 V. 
Approx. 5 to 10 V. 
voltage 
Heater peak pulse 
Typically approx. 200 
Approx. 4 mA per heater. 
current mA per heater. This 
This allows the use of small 
requires bipolar or 
MOS drive transistors. 
very large 
MOS drive transistors. 
Fault tolerance 
Not implemented. Not 
Simple implementation 
practical for edge shooter 
results in better yield and 
type. reliability 
Constraints on ink 
Many constraints 
Temperature coefficient of 
composition 
including kogation, 
surface tension or viscosity 
nucleation, etc. 
must be negative. 
Ink pressure 
Atmospheric pressure or 
Approx. 1.1 atm 
less 
Integrated drive 
Bipolar circuitry usually 
CMOS, nMOS, or bipolar 
circuitry required due to high 
drive current 
Differential 
Significant problem for 
Monolithic construction 
thermal expansion 
large print heads 
reduces problem 
Pagewidth print 
Major problems with 
High yield, low cost and 
heads yield, cost, precision 
long life due to fault 
construction, head life, 
tolerance. Self cooling due 
and power dissipation 
to low power dissipation. 
______________________________________ 
Yield and Fault Tolerance 
In most cases, monolithic integrated circuits cannot be repaired if they 
are not completely functional when manufactured. The percentage of 
operational devices which are produced from a wafer run is known as the 
yield. Yield has a direct influence on manufacturing cost. A device with a 
yield of 5% is effectively ten times more expensive to manufacture than an 
identical device with a yield of 50%. 
There are three major yield measurements: 
1) Fab yield 
2) Wafer sort yield 
3) Final test yield 
For large die, it is typically the wafer sort yield which is the most 
serious limitation on total yield. Full pagewidth color heads in 
accordance with this invention are very large in comparison with typical 
VLSI circuits. Good wafer sort yield is critical to the cost-effective 
manufacture of such heads. 
FIG. 5 is a graph of wafer sort yield versus defect density for a 
monolithic full width color A4 head embodiment of the invention. The head 
is 215 mm long by 5 mm wide. The non fault tolerant yield 198 is 
calculated according to Murphy's method, which is a widely used yield 
prediction method. With a defect density of one defect per square cm, 
Murphy's method predicts a yield less than 1%. This means that more than 
99% of heads fabricated would have to be discarded. This low yield is 
highly undesirable, as the print head manufacturing cost becomes 
unacceptably high. 
Murphy's method approximates the effect of an uneven distribution of 
defects. FIG. 5 also includes a graph of non fault tolerant yield 197 
which explicitly models the clustering of defects by introducing a defect 
clustering factor. The defect clustering factor is not a controllable 
parameter in manufacturing, but is a characteristic of the manufacturing 
process. The defect clustering factor for manufacturing processes can be 
expected to be approximately 2, in which case yield projections closely 
match Murphy's method. 
A solution to the problem of low yield is to incorporate fault tolerance by 
including redundant functional units on the chip which are used to replace 
faulty functional units. 
In memory chips and most Wafer Scale Integration (WSI) devices, the 
physical location of redundant sub-units on the chip is not important. 
However, in printing heads the redundant sub-unit may contain one or more 
printing actuators. These must have a fixed spatial relationship to the 
page being printed. To be able to print a dot in the same position as a 
faulty actuator, redundant actuators must not be displaced in the non-scan 
direction. However, faulty actuators can be replaced with redundant 
actuators which are displaced in the scan direction. To ensure that the 
redundant actuator prints the dot in the same position as the faulty 
actuator, the data timing to the redundant actuator can be altered to 
compensate for the displacement in the scan direction. 
To allow replacement of all nozzles, there must be a complete set of spare 
nozzles, which results in 100% redundancy. The requirement for 100% 
redundancy would normally more than double the chip area, dramatically 
reducing the primary yield before substituting redundant units, and thus 
eliminating most of the advantages of fault tolerance. 
However, with print head embodiments according to this invention, the 
minimum physical dimensions of the head chip are determined by the width 
of the page being printed, the fragility of the head chip, and 
manufacturing constraints on fabrication of ink channels which supply ink 
to the back surface of the chip. The minimum practical size for a full 
width, full color head for printing A4 size paper is approximately 215 
mm.times.5 mm. This size allows the inclusion of 100% redundancy without 
significantly increasing chip area, when using 1.5 .mu.m CMOS fabrication 
technology. Therefore, a high level of fault tolerance can be included 
without significantly decreasing primary yield. 
When fault tolerance is included in a device, standard yield equations 
cannot be used. Instead, the mechanisms and degree of fault tolerance must 
be specifically analyzed and included in the yield equation. FIG. 5 shows 
the fault tolerant sort yield 199 for a full width color A4 head which 
includes various forms of fault tolerance, the modeling of which has been 
included in the yield equation. This graph shows projected yield as a 
function of both defect density and defect clustering. The yield 
projection shown in FIG. 5 indicates that thoroughly implemented fault 
tolerance can increase wafer sort yield from under 1% to more than 90% 
under identical manufacturing conditions. This can reduce the 
manufacturing cost by a factor of 100. 
Fault tolerance is highly recommended to improve yield and reliability of 
print heads containing thousands of printing nozzles, and thereby make 
pagewidth printing heads practical. However, fault tolerance is not to be 
taken as an essential part of the present invention. 
Fault tolerance in drop-on-demand printing systems is described in the 
following Australian patent specifications filed on 12 Apr., 1995, the 
disclosure of which are hereby incorporated by reference: 
`Integrated fault tolerance in printing mechanisms` (Filing no.: PN2324); 
`Block fault tolerance in integrated printing heads` (Filing no.: PN2325); 
`Nozzle duplication for fault tolerance in integrated printing heads` 
(Filing no.: PN2326); 
`Detection of faulty nozzles in printing heads` (Filing no.: PN2327); and 
`Fault tolerance in high volume printing presses` (Filing no.: PN2328). 
Printing System Embodiments 
A schematic diagram of a digital electronic printing system using a print 
head of this invention is shown in FIG. 6. This shows a monolithic 
printing head 50 printing an image 60 composed of a multitude of ink drops 
onto a recording medium 51. This medium will typically be paper, but can 
also be overhead transparency film, cloth, or many other substantially 
flat surfaces which will accept ink drops. The image to be printed is 
provided by an image source 52, which may be any image type which can be 
converted into a two dimensional array of pixels. Typical image sources 
are image scanners, digitally stored images, images encoded in a page 
description language (PDL) such as Adobe Postscript, Adobe Postscript 
level 2, or Hewlett-Packard PCL 5, page images generated by a 
procedure-call based rasterizer, such as Apple QuickDraw, Apple Quickdraw 
GX, or Microsoft GDI, or text in an electronic form such as ASCII. This 
image data is then converted by an image processing system 53 into a two 
dimensional array of pixels suitable for the particular printing system. 
This may be color or monochrome, and the data will typically have between 
1 and 32 bits per pixel, depending upon the image source and the 
specifications of the printing system. The image processing system may be 
a raster image processor (RIP) if the source image is a page description, 
or may be a two dimensional image processing system if the source image is 
from a scanner. 
If continuous tone images are required, then a halftoning system 54 is 
necessary. Suitable types of halftoning are based on dispersed dot ordered 
dither or error diffusion. Variations of these, commonly known as 
stochastic screening or frequency modulation screening are suitable. The 
halftoning system commonly used for offset printing--clustered dot ordered 
dither--is not recommended, as effective image resolution is unnecessarily 
wasted using this technique. The output of the halftoning system is a 
binary monochrome or color image at the resolution of the printing system 
according to the present invention. 
The binary image is processed by a data phasing circuit 55 (which may be 
incorporated in a Head Control ASIC 400 as shown in FIG. 4) which provides 
the pixel data in the correct sequence to the data shift registers 56. 
Data sequencing is required to compensate for the nozzle arrangement and 
the movement of the paper. When the data has been loaded into the shift 
registers 56, it is presented in parallel to the heater driver circuits 
57. At the correct time, the driver circuits 57 will electronically 
connect the corresponding heaters 58 with the voltage pulse generated by 
the pulse shaper circuit 61 and the voltage regulator 62. The heaters 58 
heat the tip of the nozzles 59, affecting the physical characteristics of 
the ink. Ink drops 60 escape from the nozzles in a pattern which 
corresponds to the digital impulses which have been applied to the heater 
driver circuits. The pressure of the ink in the ink reservoir 64 is 
regulated by the pressure regulator 63. Selected drops of ink drops 60 are 
separated from the body of ink by the chosen drop separation means, and 
contact the recording medium 51. During printing, the recording medium 51 
is continually moved relative to the print head 50 by the paper transport 
system 65. If the print head 50 is the full width of the print region of 
the recording medium 51, it is only necessary to move the recording medium 
51 in one direction, and the print head 50 can remain fixed. If a smaller 
print head 50 is used, it is necessary to implement a raster scan system. 
This is typically achieved by scanning the print head 50 along the short 
dimension of the recording medium 51, while moving the recording medium 51 
along its long dimension. 
Computer simulation of nozzle dynamics 
Details of the operation of print heads according to this invention have 
been extensively simulated by computer. FIGS. 8 to 18 are some results 
from an example simulation of a preferred nozzle embodiment's operation 
using electrothermal drop selection by reduction in surface tension, 
combined with electrostatic drop separation. 
Computer simulation is extremely useful in determining the characteristics 
of phenomena which are difficult to observe directly. Nozzle operation is 
difficult to observe experimentally for several reasons, including: 
1) Useful nozzles are microscopic, with important phenomena occurring at 
dimensions less than 1 .mu.m. 
2) The time scale of a drop ejection is a few microseconds, requiring very 
high speed observations. 
3) Important phenomena occur inside opaque solid materials, making direct 
observation impossible. 
4) Some important parameters, such as heat flow and fluid velocity vector 
fields are difficult to directly observe on any scale. 
5) The cost of fabrication of experimental nozzles is high. 
Computer simulation overcomes the above problems. A leading software 
package for fluid dynamics simulation is FIDAP, produced by Fluid Dynamics 
International Inc. of Illinois, U.S.A (FDI). FIDAP is a registered 
trademark of FDI. Other simulation programs are commercially available, 
but FIDAP was chosen for its high accuracy in transient fluid dynamic, 
energy transport, and surface tension calculations. The version of FIDAP 
used is FIDAP 7.51. 
The simulations combine energy transport and fluid dynamic aspects. 
Axi-symmetric simulation is used, as the example nozzle is cylindrical in 
form. There are four deviations from cylindrical form. These are the 
connections to the heater, the laminar air flow caused by paper movement, 
gravity (if the printhead is not vertical), and the presence of adjacent 
nozzles in the substrate. The effect of these factors on drop ejection is 
minor. 
To obtain convergence for transient free surface simulations with variable 
surface tension at micrometer scales with microsecond transients using 
FIDAP 7.51, it is necessary to nondimensionalize the simulation. 
Only the region in the tip of the nozzle is simulated, as most phenomena 
relevant to drop selection occur in this region. The simulation is from 
the axis of symmetry of the nozzle out to a distance of 40 .mu.m. 
A the beginning of the simulation, the entire nozzle and ink is at the 
device ambient temperature, which in this case is 30.degree. C. During 
operation, the device ambient temperature will be slightly higher than the 
air ambient temperature, as an equilibrium temperature based on printing 
density is reached over the period of many drop ejections. Most of the 
energy of each drop selection is carried away with the ink drop. The 
remaining heat in the nozzle becomes very evenly distributed between drop 
ejections, due to the high thermal conductivity of silicon, and due to 
convection in the ink. 
Geometry of the simulated nozzle 
FIG. 7 shows the geometry and dimensions of the a preferred nozzle 
embodiment modeled in this simulation. 
The nozzle is constructed on a single crystal silicon substrate 2020. The 
substrate has an epitaxial boron doped silicon layer 2018, which is used 
as an etch stop during nozzle fabrication. An epitaxial silicon layer 2019 
provides the active substrate for the fabrication of CMOS drive 
transistors and data distribution circuits. On this substrate are several 
layers deposited CMOS processing. These are a thermal oxide layer 2021, a 
first interlevel oxide layer 2022, first level metal 2023, second 
interlevel oxide layer 2024, second level metal 2025, and passivation 
oxide layer 2026. Subsequent processing of the wafers forms the nozzles 
and heaters. These structures include the active heater 2027(a), an ESD 
shield formed from `spare` heater material 2027(b), and a silicon nitride 
passivation layer 2028. 
The heater is atop a narrow `rim` etched from the various oxide layers. 
This is to reduce the `thermal mass` of the material around the heater, 
and to prevent the ink from spreading across the surface of the print 
head. 
The print head is filled with electrically conductive ink 2031. An electric 
field is applied to the print head, using an electrode which is in 
electrical contact with the ink, and another electrode which is behind the 
recording medium. 
The nozzle radius is 8 .mu.m, and the diagram is to scale. 
Theoretical basis of calculations 
The theoretical basis for fluid dynamic and energy transport calculations 
using the Finite Element Method, and the manner that this theoretical 
basis is applied to the FIDAP computer program, is described in detail in 
the FIDAP 7.0 Theory Manual (April 1993) published by FDI, the disclosure 
of which is hereby incorporated by reference. 
Material characteristics 
The table "Properties of materials used for FIDAP simulation" gives 
approximate physical properties of materials which may be used in the 
fabrication of the print head in accordance with this invention. 
The properties of `ink` used in this simulation are that of a water based 
ink with 25% pigment loading. The ink contains a suspension of fine 
particles of palmitic acid (hexadecanoic acid) to achieve a pronounced 
reduction in surface tension with temperature. The surface tensions were 
measured at various temperatures using a surface tensiometer. 
The values which have been used in the example simulation using the FIDAP 
program are shown in the table "Properties of materials used for FIDAP 
simulation". Most values are from direct measurement, or from the CRC 
Handbook of Chemistry and Physics, 72nd edition, or Lange's handbook of 
chemistry, 14th edition. 
Properties of materials used for FIDAP simulation 
______________________________________ 
Material 
or Tem- Dimensionless 
Property perature Physical value 
value 
______________________________________ 
Characteristic, length (L) 
All 1 .mu.m 1 
Characteristic velocity 
Ink 1 m/s 1 
(U) 
Characteristic time 
All 1 .mu.s 1 
Time Step All 0.1 .mu.s 0.25 
Ambient temperature 
All 30.degree. C. 
30 
Boiling point 
Ink 103.degree. C. 
103 
Viscosity (.eta.) 
At 20.degree. C. 
2.306 cP 3.530 
Viscosity (.eta.) 
At 30.degree. C. 
1.836 cP 2.810 
Viscosity (.eta.) 
At 40.degree. C. 
1.503 cP 2.301 
Viscosity (.eta.) 
At 50.degree. C. 
1.259 cP 1.927 
Viscosity (.eta.) 
At 60.degree. C. 
1.074 cP 1.643 
Viscosity (.eta.) 
At 70.degree. C. 
0.930 cP 1.423 
Viscosity (.eta.) 
At 80.degree. C. 
0.816 cP 1.249 
Viscosity (.eta.) 
At 90.degree. C. 
0.724 cP 1.108 
Viscosity (.eta.) 
At 0.648 cP 0.993 
100.degree. C. 
Surface Tension (.gamma.) 
28.degree. C. 
59.3 mN/m 90.742 
Surface Tension (.gamma.) 
33.degree. C. 
58.8 mN/m 89.977 
Surface Tension (.gamma.) 
38.degree. C. 
54.1 mN/m 82.785 
Surface Tension (.gamma.) 
43.degree. C. 
49.8 mN/m 76.205 
Surface Tension (.gamma.) 
47.degree. C. 
47.3 mN/m 72.379 
Surface Tension (.gamma.) 
53.degree. C. 
44.7 mN/m 68.401 
Surface Tension (.gamma.) 
58.degree. C. 
39.4 mN/m 60.291 
Surface Tension (.gamma.) 
63.degree. C. 
35.6 nN/m 54.476 
Surface Tension (.gamma.) 
68.degree. C. 
33.8 mN/m 51.721 
Surface Tension (.gamma.) 
73.degree. C. 
33.7 mN/m 51.568 
Pressure (p) Ink 10 kPa 15.3 
Thermal Conductivity (k) 
Ink 0.631 Wm.sup.-1 K.sup.-1 
1 
Thermal Conductivity (k) 
Silicon 148 Wm.sup.-1 K.sup.-1 
234.5 
Thermal Conductivity (k) 
SiO.sub.2 
1.5 Wm.sup.-1 K.sup.-1 
2.377 
Thermal Conductivity (k) 
Heater 23 Wm.sup.-1 K.sup.-1 
36.45 
Thermal Conductivity (k) 
Si.sub.3 N.sub.4 
19 Wm.sup.-1 K.sup.-1 
30.11 
Specific Heat (c.sub.p) 
Ink 3,727 Jkg.sup.-1 K.sup.-1 
3.8593 
Specific Heat (c.sub.p) 
Silicon 711 Jkg.sup.-1 K.sup.-1 
0.7362 
Specific Heat (c.sub.p) 
SiO.sub.2 
738 Jkg.sup.-1 K.sup.-1 
0.7642 
Specific Heat (c.sub.p) 
Heater 250 Jkg.sup.-1 K.sup.-1 
0.2589 
Specific Heat (c.sub.p) 
Si.sub.3 N.sub.4 
712 Jkg.sup.-1 K.sup.-1 
0.7373 
Density (p) Ink 1.036 gcm.sup.-1 
1.586 
Density (p) Silicon 2.320 gcm.sup.-1 
3.551 
Density (p) SiO.sub.2 
2.190 gcm.sup.-1 
3.352 
Density (p) Heater 10.50 gcm.sup.-1 
16.07 
Density (p) Si.sub.3 N.sub.4 
3.160 gcm.sup.-1 
4.836 
______________________________________ 
Fluid dynamic simulations 
FIG. 8(a) shows the power applied to the heater. The maximum power applied 
to the heater is 40 mW. This power is pulse frequency modulated to obtain 
a desirable temporal distribution of power to the heater. The power pulses 
are each of a duration of 0.1 .mu.s, each delivering 4 nJ of energy to the 
heater. The drop selection pulse is started 10 .mu.s into the simulation, 
to allow the meniscus to settle to its quiescent position. The total 
energy delivered to the heater during the drop selection pulse is 276 nJ 
FIG. 8(b) shows the temperature at various points in the nozzle during the 
simulation. 
Point A is at the contact point of the ink meniscus and the nozzle rim. For 
optimal operation, it is desirable that this point be raised as close as 
possible to the boiling point of the ink, without exceeding the boiling 
point, and maintained at this temperature for the duration of the drop 
selection pulse. The `spiky` temperature curve is due to the pulse 
frequency modulation of the power applied to the heater. This `spikiness` 
can be reduced by increasing the pulse frequency, and proportionally 
reducing the pulse energy. 
Point B is a point on the ink meniscus, approximately midway between the 
centre of the meniscus and the nozzle tip. 
Point C is a point on the surface of the silicon, 20 .mu.m from the centre 
of the nozzle. This shows that the temperature rise when a drop is 
selected is very small a short distance away from the nozzle. This allows 
active devices, such as drive transistors, to be placed very close to the 
nozzles. 
FIG. 9 shows the position versus time of a point at the centre of the 
meniscus. 
FIG. 10 shows the meniscus position and shape at various times during the 
drop selection pulse. The times shown are at the start of the drop 
selection pulse, (10 .mu.s into the simulation), and at 5 .mu.s intervals, 
until 60 .mu.s after the start of the heater pulse. 
FIG. 11 shows temperature contours in the nozzle just before the beginning 
of the drop selection pulse, 9 .mu.s into the simulation. The surface 
tension balances the combined effect of the ink pressure and the external 
constant electric field. 
FIG. 12 shows temperature contours in the nozzle 5 .mu.s after beginning of 
the drop selection pulse, 15 .mu.s into the simulation. The reduction in 
surface tension at the nozzle tip causes the surface at this point to 
expand, rapidly carrying the heat around the meniscus. The ink has begun 
to move, as the surface tension is no longer high enough to balance the 
combined effect of the ink pressure and the external constant electric 
field. The centre of the meniscus begins to move faster than the outside, 
due to viscous drag at the nozzle walls. In FIGS. 12 to 17 temperature 
contours are shown starting at 35.degree. C. and increasing in 5.degree. 
C. intervals. 
FIG. 13 shows temperature contours in the nozzle 10 .mu.s after beginning 
of the drop selection pulse, 20 .mu.s into the simulation. 
FIG. 14 shows temperature contours in the nozzle 20 .mu.s after beginning 
of the drop selection pulse, 30 .mu.s into the simulation. 
FIG. 15 shows temperature contours in the nozzle 30 .mu.s after beginning 
of the drop selection pulse, 40 .mu.s into the simulation. This is 6 .mu.s 
after the end of the drop selection pulse, and the nozzle has begun to 
cool down. 
FIG. 16 shows temperature contours in the nozzle 40 .mu.s after beginning 
of the drop selection pulse, 50 .mu.into the simulation. If is clear from 
this simulation that the vast majority of the energy of the drop selection 
pulse is carried away with the selected drop. 
FIG. 17 shows temperature contours in the nozzle 50 .mu.s after beginning 
of the drop selection pulse, 60 .mu.s into the simulation. At this time, 
the selected drop is beginning to `neck`, and the drop separation process 
is beginning. 
FIG. 18 shows streamlines in the nozzle at the same time as FIG. 17. 
The approximate duration of three consecutive phases in the drop ejection 
cycle are: 
1) 24 .mu.s heater energizing cycle 
2) 60 .mu.s to reach drop separation 
3) 40 .mu.s to return to the quiescent position 
The total of these times is 124 .mu.s, which results in a maximum drop 
repetition rate (drop frequency) of approximately 8 Khz. 
Proximity Drop Separation 
Drop separation of liquid ink by print media proximity preferably operates 
under the following conditions: 
1) The difference in meniscus positions between selected and unselected 
drops is greater than the surface roughness of the print medium. 
2) The surface roughness of the print medium is less than approximately 30% 
of the ink drop diameter. 
3) The rate at which the volume of the ink drop increases due to wetting 
the print medium surface and/or soaking into a porous print medium is 
greater than the rate of flow from the ink nozzle under the applied ink 
pressure. 
These conditions can be met over a wide range of nozzle radii, ink types, 
media, and print resolutions. 
For hot melt printing, the molten ink drop freezes when in contact with the 
print medium, and the characteristics of ink absorption into the print 
medium are not as important. 
The principle of operation of proximity separation printing is shown in 
FIG. 19(a) through FIG. 19(i). In this case, the drop is selected by 
electrothermal transducers, which heat the ink at the nozzle tip, causing 
an increase in temperature at the meniscus. The increased temperature 
causes a reduction of surface tension below a critical surface tension, 
resulting in ink egress from the nozzle tip. 
In FIGS. 19(a) to 19(i) 1 is the selected drop, 10 is the nozzle from which 
the selected drop 1 was produced, 11 is a nozzle in which the heater 103 
was not activated, and therefore no drop was selected, 5 is the direction 
of print medium movement, 51 is the print medium, 100 is the body of ink, 
101 is silicon, 102 is silicon dioxide, 103 is the electrothermal actuator 
(also referred to as "heater"), and 109 is the print head hydrophobic 
layer. 
FIG. 19(a) shows a cross section through two adjacent nozzles 10 and 11 in 
the quiescent state. The nozzles are in close proximity to the recording 
medium 51 which is moving relative to the nozzles in the direction 5. The 
cross section is at an angle of 45 degrees to the direction of media 
movement, through the plane of the diagram. The nozzles 10 and 11 
represent two staggered nozzles offset by one pixel width in the direction 
normal to the plane of the diagram. All surfaces of the nozzle have a 
hydrophobic surface layer 109, and the ink 100 is hydrophilic. The ink is 
under pressure, resulting in the ink meniscus bulging. 
FIG. 19(b) shows the ink in the two nozzles shortly after an energizing 
pulse has been applied to the heater 103 of nozzle 10, but not of nozzle 
11. The heat is conducted to the ink surface, where the resultant rise in 
temperature causes a local decrease in the surface tension of the ink The 
decrease in surface tension may be the result of the natural properties of 
the ink, but is preferably enhanced by the inclusion of an agent in the 
ink which causes a significant fall in surface tension at the temperature 
to which the ink is heated. This agent may be a surfactant which is in the 
form of a suspended solid particles at the quiescent temperature, but 
melts when the heaters are activated. When in solid form, the surfactant 
has little effect on surface tension. When molten, surfactant molecules 
rapidly migrate to the ink surface, causing a significant decrease in 
surface tension. In this case, the surfactant is 1-Hexadecanol, a 16 
carbon alcohol with a melting point of 50.degree. C. 
FIG. 19(c) shows the drop evolution a short time later. The selected drop 1 
takes on a substantially cylindrical form due to a surface tension 
gradient from the nozzle tip to the centre of the meniscus, and due to 
viscous drag slowing ink movement near the walls of the nozzle. In this 
case, there are no external electrostatic or magnetic fields applied, and 
gravity is not significant on this scale. 
FIG. 19(d) shows the selected drop 1 at the instant that it contacts the 
recording medium 51. The "tilt" of the selected drop is due to the laminar 
air flow between the print head and the recording medium 51, caused by the 
movement of the recording medium. In many practical situations the 
recording medium will be paper, which will typically have a surface which 
is rough on the scale of the distance between the nozzle and the recording 
medium. This roughness will cause variation in the time of contact between 
the drop 1 and the recording medium 51, and therefore cause variations in 
the printed dot area. This variation can be minimized by using coated 
paper and/or passing the recording medium through compression rollers 
immediately prior to printing. 
FIG. 19(e) shows the selected drop as it begins to "soak into" the 
recording medium 51. 
FIG. 19(f) shows the selected drop a short time later. The ink is absorbed 
by the recording medium at a rate approximately proportional to the 
gradient of the saturation. In many fibrous print media such as paper, the 
circle of contact between the print medium and the ink meniscus will not 
follow the lateral absorption of the ink into the print medium. This is 
because the surface fibers do not become fully wetted. 
Ink flow into the print medium is highly dependent upon print medium 
composition. In many circumstances ink can be made to flow more quickly 
into the printing medium 51 by wetting the medium before printing. This 
may be achieved by using a series of rollers. The technology for 
continuously applying an even coat of liquid using rollers is well known 
in the offset printing industry. Most offset printing systems use damping 
rollers to apply a thin coating of fount solution, and inking rollers to 
apply a thin coating of ink, to the printing plates. 
FIG. 19(g) shows the selected drop 1 immediately after it has separated 
from the body of ink 100. The ink will separate if the rate of ink flow 
into the porous recording medium 51 exceeds the flow rate of pressurized 
ink from the nozzle 10. This can be achieved for a wide range of inks, 
media, and nozzle radii. Non-porous media such as plastic or metal films 
can also be used. In this case, drop separation occurs when the rate of 
volume increase of a drop as it wets the non-porous medium exceeds the 
rate of ink flow from the nozzle 10. For some combinations of ink and 
non-porous media, the medium may need to be coated with an agent to 
promote wetting. 
FIG. 19(h) shows the selected drop 1 after it has mostly soaked into the 
recording medium. Momentum of the ink returning to the nozzle carries the 
meniscus at the nozzle 10 past the quiescent position. The degree of this 
"overshoot" is very small compared to conventional thermal ink jet or 
piezoelectric ink jet systems. 
FIG. 19(i) shows the nozzle 10 after the meniscus has returned to the 
quiescent position, and is ready for the next drop selection pulse. The 
selected drop 1 is shown fully absorbed into the print medium 51. The rate 
of absorption is highly dependent upon the print medium, and the selected 
drop 1 may not be completely absorbed by the time a drop of a different 
color is printed at the same location. In some circumstances this may 
degrade print quality, in which case a more absorptive print medium can be 
used, a different ink composition can be used, a print head with greater 
separation between colors can be used, the print medium can be heated to 
promote fast drying, or a combination of the above techniques can be used. 
Acoustic ink waves for proximity separation printing 
Correctly applied acoustic waves in the ink of proximity printing systems 
of the invention can achieve several benefits: 
1) Drop growth can proceed faster when the period of maximum forward ink 
velocity caused by the acoustic wave coincides with the drop growth 
period. 
2) The amount of ink delivered to the recording medium by a selected drop 
can be reduced when the drop separating time coincides with a period of 
reduced ink pressure, as less ink will flow out of the nozzle, and the 
drop will separate earlier. 
3) The degree of variation in the amount of ink delivered to the recording 
medium will be reduced, as both the contact time and separating time of 
the selected drop are influence by the acoustic wave, which can be created 
with a highly accurate and stable frequency and amplitude. 
4) The use of pigments instead of dyes is augmented, as the ink is 
constantly agitated by the acoustic waves, reducing one of the major 
problems of pigments, which is pigment setting in the ink. 
5) Blocking of nozzles with dried ink is reduced, as the constant motion of 
the ink meniscus stirs the ink in the vicinity of the meniscus, replacing 
drying ink with "fresh" ink. 
FIG. 20(a) shows the acoustic wave 820 applied to the ink. 
FIG. 20(b) is a space/time diagram showing the ink occupancy along the 
nozzle axis for both selected drops 821 and drops which have not been 
selected 822. The graph shows ink position versus time for a small region 
along the nozzle axis, ranging from a small distance inside the body of 
the ink 100 (at the bottom of the graph) to a small distance within the 
paper 51 (at the top of the graph) 
The two graphs are superimposed to allow direct comparison of the selected 
drop with the unselected drop. 
The graph of ink occupancy for unselected drops 822 shows a sinusoidal 
oscillation of the same frequency as the acoustic wave 820, but with a 
certain phase shift .DELTA..PHI.. The degree of phase shift depends upon 
the shape and dimensions of the ink nozzles and ink reservoirs, and the 
fluid characteristics of the ink. The phase shift will approach 90.degree. 
as the frequency of the acoustic wave approaches the resonant frequency of 
the ink in the nozzle. The phase shift is easily compensated by altering 
the phase of the drive voltage to the piezoelectric or other transducer 
which is used to create the acoustic wave. The wave shape for the 
unselected drop is shown as being sinusoidal. The actual shape will have 
substantial harmonic distortion, and depends upon the geometry of the 
nozzle tip and the fluid characteristics of the ink. 
FIG. 20(b) is specifically related to a head embodiment with eight drive 
phases and four ink colors (for example, CMYK colors). Only one ink color 
is shown. The phases of the acoustic waves in the other ink colors are 
90.degree., 180.degree., and 270.degree. out of phase with the phase of 
the acoustic wave 820. The eight drive phases in the drop ejection cycle 
extend over two periods of the acoustic wave 820. There are two drive 
phases per ink color in one drop ejection cycle. These are separated by 
360.degree. of the acoustic wave, and do not apply to the same nozzle, but 
to interleaved nozzles. The periods 829 and 831 are two successive heater 
drive periods of one nozzle (in this case, the nozzle with the selected 
drop 821. The period 830 is the period in which the heaters of the 
alternate nozzles of the same ink color may be enabled. 
At the time that the heater is turned on 823 the ink occupancy history of 
the selected drop 821 begins to diverge from the ink occupancy history of 
drops which are not selected 822. Ink flow from the nozzle is aided by 
being at a period of maximum forward velocity caused by the acoustic wave 
820. At the time 824 this divergence is irreversible, as the oscillating 
equilibrium between surface tension and oscillating ink pressure is 
broken. Ink continues to emerge from the nozzle until the ink contact the 
recording medium 51 at time 825. Ink wets the surface of the recording 
medium 51, and is absorbed into the medium, as is shown by the ink 
overlapping the recording medium in the space-time region 832. The 
selected drop 821 separates from the body of ink 100 when the rate of 
volume flow into and/or along the surface of the recording medium exceeds 
the rate of flow from the nozzle at the separation time and position 826. 
After the instant of separation at 826 the ink meniscus rapidly contracts 
for both the ink which remains in the recording medium 51 and for the body 
of ink 100. The separation is aided by occurring at a time of low ink 
pressure, when the ink for unselected drops is flowing back into the 
nozzle. 
Ink on the nozzle side of the separation point 826 rapidly moves back into 
the nozzle by ink surface tension. The ink meniscus undergoes a damped 
oscillation at the resonant frequency of the ink in the nozzle tip for a 
short period. This damped oscillation is superimposed on the oscillation 
caused by the acoustic wave. In most cases, it will be neither necessary 
nor practical to match the resonant frequency of the ink in the nozzle 
with the frequency of the acoustic wave. The example shown in FIG. 20(b) 
the heater on period is 18 .mu.s, and the drop ejection cycle is 144 
.mu.s. The period of the acoustic wave is 72 .mu.s, therefore the 
frequency of the acoustic wave is 13.8 KHz. The resonant frequency of the 
ink column is 25 Khz. 
At the time 827 that the acoustic wave is at the same phase as the start of 
heater energizing 823, the ink meniscus has not returned sufficiently to 
the quiescent oscillating state of drops which are not selected 822. 
However, the alternate nozzle of the same ink color are ready for heater 
energizing for the period 830 at the time 827. At the next time 828 the 
acoustic wave is at the same phase as the start of heater energizing 823 
the heater of the same nozzle can again be energized, as the meniscus has 
returned to the quiescent oscillating state with very minor error. 
The region of ink 832 which has been absorbed into the recording medium 51 
is shown first growing thicker, then thinner with progressing time. The 
actual ink region in the recording medium only gets thinner, slows, and 
stops at a certain thickness. The thinning of the ink region is because 
FIG. 20 is a space/time diagram of ink occupancy along the axis of the 
nozzle, and the recording medium 51 is moving relative to the nozzle axis. 
By the time 833 that the next selected drop for the nozzle has reached the 
recording medium the edge of the previous drop has been passed. The second 
ink spot flows back in the recording material to join with the first ink 
spot, thereby providing a continuous layer of ink when subsequent drops 
are selected. 
A simple means of achieving an acoustic wave in the ink is by placing a 
piezoelectric crystal in such a way that it displaces the ink in the ink 
channel behind the nozzles. The piezoelectric crystal should be the entire 
length of the row of nozzles to ensure that all nozzles receive a n 
acoustic wave of the same amplitude and phase. The amplitude of the 
voltage applied to the piezoelectric crystal depends upon the physical 
characteristics of the crystal, the dimensions of the nozzles, the shape, 
location and dimensions of the ink reservoir, the placement of the 
piezoelectric crystal in relation to the ink nozzles and ink reservoir, 
the fluid characteristics of the ink, and other factors. The simplicity 
and low cost of trying differing voltages, amplitudes and phases of drive 
voltage makes experimentation a more effective means of deriving the 
appropriate drive waveforms than calculation. 
In the example shown in FIG. 20, the frequency of the acoustic wave is 13.8 
KHz. This is within the normal audible range of humans, and may be 
perceived as an annoying high pitched hiss if significant amplitudes of 
the wave are transmitted to the air and escape the printer enclosure. The 
level of annoyance perceived is subjective, and highly variable from 
person to person. For example, only some people are annoyed by the 15.625 
KHz line frequency emitted by NTSC and television sets, while most 
people are unaware of the sound. There are several remedies to the problem 
of sound emission. One is to ensure that the acoustic wave frequency is 
above 20 KHz, the normal maximum audible frequency. Another solution is to 
encase the print head assembly with acoustic absorptive material. This 
need only absorb strongly at the fundamental frequency of the acoustic 
wave, as the second harmonic is above 20 KHz. Another solution is to 
minimize the acoustic coupling between the ink and the air (via the ink 
channel assembly and other components) at the appropriate frequency. 
Drop size adjustment in proximity separation printers 
FIG. 21(a) shows a cross section of a Proximity separation print head and 
platen assembly for a web fed printing system. 
The print head 50 prints six colors (CC'MM'YK) for high quality full color 
printing using digital halftoning. The head is approximately 8 mm wide and 
600 .mu.m thick. The print head is positioned a distance D.sub.HtoP away 
from the recording medium 51 which moves in a direction shown by the arrow 
5. The recording medium 51 is tensioned against a platen 67. The platen 67 
should have a highly polished and optically flat surface to reduce 
friction with the recording medium, and to maintain positioning accuracy 
across the entire print region. The platen may alternatively be formed by 
two or more rollers (not shown), to reduce friction further. The rollers 
may be surrounded by an band (not shown) to maintain positional accuracy 
of the recording medium 51. The platen 67 is fixed to a piezoelectric 
ceramic 31 which has an axis of polarization 33. The piezoelectric crystal 
is fixed to a plate 30 which is mechanically fixed in relation to the 
print head 50 during printing. Electrodes 32 are applied to the 
piezoelectric crystal 31. To adjust the distance D.sub.HtoP a voltage is 
applied to the electrodes 32. 
Ink 100 is supplied to the head by the ink channel assembly 75. The ink 
channel assembly 75 may also serve the function of holding the print head 
rigidly in place, and of correcting warp in the print head. Alternatively, 
these functions may be provided by alternative means. Power to actuate the 
thermal heaters is supplied by the two power connections 38 and 39. 
Because these connections can be manufactured from a conductive metal 
which can readily be several hundred microns thick, and because these 
connections may be the entire length of the print head, high currents can 
be supplied to the print head with a small voltage drop. This is 
important, as pagewidth color print heads may consume as much as 20 Amps 
when several thousand nozzles are actuated simultaneously. 
A paper guide lightly contacts the recording medium 51 under pressure 
provided by an elastically deformable material 35 acting against a fixed 
block 34. The guide 36 has two purposes: to tension the recording medium 
against the platen in conjunction with the paper transport roller 65, and 
to temporarily flatten any fibers which may protrude from a recording 
medium such as paper. It is desirable to flatten protruding fibers to 
improve print quality by reducing variations in the distance from the 
print head to the effective surface of the recording medium. Protruding 
fibers do not have as significant an affect on the printed dot size as may 
be implied by the reduced distance from the nozzle to the closed part of 
the recording medium. This is because the ink drop will not soak into or 
wick along the surface of a small protruding fibers as fast as it will 
soak into the bulk surface. Therefore, the time before ink drop 
separation, and thus the total amount of ink delivered, will not vary 
greatly. Depending upon the printing speed, the recording medium type, and 
other aspects of the printing system, the guide 36 may not be necessary, 
or may be replaced by tensioned rollers to reduce friction. 
FIG. 21(b) shows a small distance D.sub.HtoP between the print head and the 
recording medium 51. This results in a small volume of the selected drop 1 
at the instant of contact between the selected drop and the recording 
medium. This value of D.sub.HtoP is achieved by applying a voltage of 
V.sub.nom +.DELTA.V to the piezoelectric crystal. 
FIG. 21(c) shows a nominal distance D.sub.HtoP between the print head and 
the recording medium 51. This results in a nominal volume of the selected 
drop 1 at the instant of contact between the selected drop and the 
recording medium. This value of D.sub.HtoP is achieved by applying a 
voltage of V.sub.nom to the piezoelectric crystal where V.sub.nom is the 
nominal voltage. V.sub.nom may be zero, or may be biased so that the full 
range of required adjustment can be achieved with a unipolar adjustment 
voltage. .DELTA.V may be positive or negative, depending upon the crystal 
orientation and choice of electrodes. 
FIG. 21(d) shows a relatively large distance D.sub.HtoP between the print 
head and the recording medium 51. This results in a relatively large 
volume of the selected drop 1 at the instant of contact between the 
selected drop and the recording medium. This value of D.sub.HtoP is 
achieved by applying a voltage of V.sub.nom -.DELTA.V to the piezoelectric 
crystal. 
The volume of ink delivered to the recording medium is not equal to the 
volume of the selected drop at the instant of contact with the recording 
medium, as ink continues to flow from the nozzle while the selected drop 
is soaking into the recording medium. However, the volume of ink delivered 
to the recording medium will be approximately proportional to the volume 
of the selected drop at the instant of contact over an operating range 
determined by ink, recording medium, and nozzle characteristics. 
An alternative configuration of the apparatus is to use a piezoelectric 
crystal to alter the position of the print head in relation to a fixed 
platen, instead of vice versa This arrangement is equivalent in function, 
with no significant disadvantage over the preferred apparatus, except that 
in many cases it will be more difficult to manufacture. 
It is possible to derive many different arrangement of piezoelectric 
crystal, including arrangements where the crystal operates in shear mode, 
and arrangements which use multiple stacked layers of piezoelectric 
crystal to reduce the magnitude of the control voltage required. These 
variations are obvious to those skilled in the art, and are within the 
scope of the invention. 
The foregoing describes various general and preferred embodiments of the 
present invention. Modifications, obvious to those skilled in the art, can 
be made in regard to the general and particular embodiments without 
departing from the scope of the invention.