Printing by modulation of ink viscosity

In a liquid ink printing system, ink is supplied to a reservoir at a constant pressure, and viscosity of ink in orifices in the reservoir wall is decreased by electrically heating the ink in the orifices. By this means the amount of ink deposited on a paper sheet moving past the orifices can be varied.

This invention relates to printing by modulation of ink viscosity, and in 
particular is concerned with the variation in flow rate of ink through an 
orifice by modulation of the viscosity. 
Various techniques exist for facsimile and other printing, such as impact, 
thermal and ink ejection. 
Impact techniques require the mechanical displacement of a hammer which 
transfers ink from a ribbon to the paper to record the desired 
information. The main problems of these techniques are limited life and 
reliability of moving parts, noise, low speed, high power consumption and 
cost. With the present invention, there are no moving parts for the 
printing head and high speed, low noise and improved power consumption are 
obtained. 
Thermal printing consists in localized heating of a pre-coated heat 
sensitive paper. Heat is usually supplied by an electric current through 
thin or thick-film resistors in contact with paper. With the present 
invention there is no need for precoated paper. Moreover, inks of 
different colours can be handled. 
Ink jet printing comprises the ejection from an ink reservoir and 
subsequent deflection of ink droplets. The undeflected drops strike a 
paper sheet and form the desired pattern. Most droplets are however 
deflected to a gutter from which ink is returned to the reservoir through 
a recirculating and filtering system. This technique is bulky and complex 
owing to the hydraulic recirculating system, and hardly reliable because 
of the presence of high pressure ink containers and ink fog generated at 
the impact of ink with paper. With the present invention there is no 
continuous ink-jet, so that the recirculation system is not required and 
there is no high pressure impact of ink with paper. The system is more 
compact, and the projection of ink fog is avoided. 
Broadly, the invention provides for the variation in viscosity of the ink 
by heating. The ink continuously flows but the amount of ink flowing from 
an orifice into contact with the paper will vary with the ink viscosity. 
In one example the amount is varied from a minimum to a maximum so that a 
line will be printed at all times, the width of the line varying with the 
viscosity: the lower the viscosity the wider the line. In an alternative 
arrangement a bye-pass is provided through which all the ink flows at high 
viscosities, the ink only flowing through the orifice as the viscosity 
decreases. Again the amount flowing through the orifice will increase as 
the viscosity decreases, printing a wider line.

As illustrated in FIG. 1, ink, indicated at 10, is contained in a 
reservoir, the wall of which is indicated at 11. In the wall 11 is an 
orifice 12 and around the orifice is a resistor heating element 13. In 
practice a plurality of orifices are provided, as in FIG. 2. Spacers 15 
position the paper 16 relative to the surface of the wall 11. The orifices 
can be of varying shapes, for example circular or rectangular. The ink 10 
is under a constant hydrostatic pressure. 
Ink flows continuously through the orifice 12 to the paper under the effect 
of the hydrostatic pressure, the amount of ink depending upon its 
viscosity. If the paper is moved in one direction while ink is ejected, a 
continuous line will be printed on the paper. A series of parallel lines 
can be printed if there is a linear array of such slots, as illustrated in 
FIG. 2. By varying the ink viscosity, so is varied the ink flux through 
each slot and thus the line width along each line. In this manner, 
alphanumerics or greytone patterns can be printed by line-width 
modulation. Electrical connections to the resistor heating elements can 
readily be provided by conductors, deposited or otherwise formed, on the 
outer surface of the wall 11. Ink is supplied to the reservoir via a pump 
18 and inlet 19. 
More precisely, the ink flux through a cylindrical orifice is given by the 
following formula: 
##EQU1## 
where V is the volume transported per unit time: 
p is the pressure; 
.eta. is the viscosity coefficient; 
.lambda. is the length of the orifice; 
r is the radius of the orifice. 
The value of .eta. decreases when the ink is heated thus increasing the ink 
flow and widening the printed line. Some liquids, such as castor oil and 
glycerol, show a decrease in the value of .eta. by a factor as large as 50 
when heated from ambient temperature to 100.degree. C. Therefore, the 
width of the continuous line, and consequently the reflectivity of printed 
paper, can vary significantly when heat is applied, making it possible to 
print alphanumerics or continuous tone images by line-width modulation, 
obtained by variation of the current supplied to the resistor heating 
element 13. The hydrostatic pressure is dropped when the machine is not 
printing. As a typical example, a castor oil-based or glycerol-based ink, 
with a p.about.0.1 atm; .lambda..about.200 .mu.m and r.about.50 .mu.m, can 
provide a maximum ink flux high enough to print about 500 spots/sec. 
However various combinations of parameter values can be selected and if a 
less viscous ink is chosen, the value of r should be reduced or the 
pressure p decreased. 
FIGS. 3, 4, 5 and 6 illustrate modifications of the basic arrangement or 
form of FIGS. 1 and 2. In FIG. 3, a thick film resistor 20 is positioned 
on the surface 17 of the reservoir wall 11. Electric current can be 
supplied to the resistor by, for example, thin film conductors 21 
previously formed or deposited on the surface 17. A small orifice 22 is 
drilled through the resistor, for example by a laser, the orifice 22 
aligned with the orifice 12 in the wall 11, ink flowing through the 
orifice 22. The ink flow is primarily controlled by the orifice 22 because 
of its smaller size relative to orifice 12, and heating efficiency is 
good. 
In FIGS. 4 and 5, the orifice 12 is pyramidal in form, which can be 
obtained by preferentially etching a (100) oriented silicon wafer along 
its (111), (111) etc. planes. Conductors 23 can be formed on the inner 
surface of the wall 11. The walls 24 of the orifice 12 can be doped to 
decrease the driving voltage and concentrate the electric current near the 
surface in contact with the ink. 
In FIG. 6, instead of a series of orifices as in FIGS. 4 and 5, which will 
produce dots with a space between, a tapered slot 30 is formed in the wall 
11 and electrodes 31 are formed on the inner surface of the wall 11 and 
down into the slot. The electric current flows laterally, that is parallel 
to the slot 30, between two adjacent electrodes on both sides of the slot. 
This can reduce or even prevent spaces in the printed line, if desired, 
and is also easier to manufacture. 
FIG. 7 illustrates a form of the invention in which the ink flow is 
bye-passed to prevent any ink flowing out of the orifice when the 
viscosity is above a predetermined value. In FIG. 7, the resistor heating 
element 13 is at the inner end of the orifice 12. Partway along the 
orifice 12 is a bye-pass duct 35. Ink flows up through the lower part of 
the orifice but is withdrawn through the bye-pass duct 35 at high 
viscosities. A suction is applied to the duct 35 and the ink can be 
returned to the reservoir. As the viscosity of the ink is decreased, by 
heating via the heating element 13, the flow rate of the ink will increase 
and some ink will flow through the upper part of the orifice 12 and issue 
into contact with the paper, as indicated by dotted lines 36.