Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SYSTEM FOR CONTROLLING INK TEMPERATURE
USING A HEATED UMBILICAL
Technical Field
The present invention relates to
continuous ink jet printers and more particularly to
reducing heater watt density in an ink jet printer
without degrading the ink.
Background Art
In continuous ink jet printing, ink is
supplied under pressure to a manifold that
distributes the ink to a plurality of orifices,
typically arranged in linear array(s). The ink is
expelled from the orifices in jets which break up
due to surface tension in the ink into droplet
streams. Ink jet printing is accomplished with
these droplet streams by selectively charging and
deflecting some droplets from their normal
trajectories. The deflected or undeflected droplets
are caught and re-circulated and the others are
allowed to impinge on a printing surface.
During the start up cycle of ink jet
printers it is advantageous to clean the charge
plate face by means of means of water which
condenses on the charge plate face. This process
requires the ink to be heated rapidly above the
temperature of the charge plate face.
In one ink jet printing product line of
Scitex Digital Printing, Inc., of Dayton, Ohio,
there is an ink heater that is energized for about
90 seconds during startup. This heater is a
"cartridge" style, with approximately 5 square
inches of area emitting 500 watts. Unfortunately,
at this watt-density (100 watts per square inch) the
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surface temperature of the heater can get high
enough to cause the ink dye to come out of solution
and form massive globs on the heating element.
These globs can break off and clog filters or
orifices. Furthermore this agglomerated ink will
reduce the efficiency of the heater.
In other products the ink is heated to a
minimum temperature of 85°F during operation of the
printer, This is done to reduce the limit the range
of the temperature dependent fluid properties. In
this way the stimulation operating point can be
stabilized over a wide range of ambient
temperatures.
A need has therefore been recognized for
a system which can incorporate a lower watt-density
heater without degrading the ink used in the system.
Summary of the Invention
It is the object of the present
invention to provide a means to heat the ink while
keeping the surface of the heater cool enough to
prevent degradation of the ink.
In accordance with one aspect of the
present invention an improvement in the heating of
ink in an ink jet printer having a printhead and a
fluid system for supplying ink to the printhead via
at least one umbilical, is provided. The improved
ink heating technique proposes a fluid tube through
which the ink flows and a heater in close thermal
contact with the fluid tube for supplying heat to a
significant length of the fluid tubing. Thermal
insulation can enclose the fluid tubing and the
heater, and a protective covering can enclose the
thermal insulation. An elastomeric layer can be
situated between the fluid tubing and the heater,
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while maintaining close thermal contact between the
fluid tubing and the heater.
Other objects and advantages of the
invention will be apparent from the following
description and the appended claims.
Brief Description of the Drawing
Fig. 1 illustrates an ink jet printer
system including a heated umbilical in accordance
with the present invention;
Fig. 2 is a circuit diagram of the
heated umbilical arrangement; and
Fig. 3 is a cutaway view of the
umbilical constructed in accordance with the present
invention.
Detailed Description of the Invention
The present invention describes a means
of reducing heater watt density, by incorporating a
heated umbilical, a temperature sensor, associated
solid state controls for energizing the heater, and
associated software.
In accordance with the present
invention, reducing the heater watt density is
achieved by replacing the small cartridge style
heaters with ink heaters having much higher surface
area. One convenient means to do this is to apply
heat to the entire length of fluid tubing by means
of an extended heater element. One particularly
convenient means to do this is to heat the printhead
ink supply tubing of the printhead umbilical.
Consider the use of a 40 foot umbilical which is
commonly used to allow the print head to be placed
remotely on a bindery line, roll-to-roll printer, or
other printing equipment. With a 40 foot long
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umbilical the effective surface area of the heater
is 282 inz. This is an increase in surface area of a
factor of 56 relative to the typical cartridge style
heaters. In this way, the power density levels are
kept low enough to avoid degrading the ink.
Referring now to the drawings, Fig. 1
shows an ink jet printer system incorporating the
heated umbilical of the present invention, and Fig.
2 illustrates a circuit diagram 30. The circuit 30
comprises a temperature sensor or thermistor 32 at
print head 34, a heated umbilical 36 comprised of
heater unit 48 and umbilical 50, two solid state
relays 38, and two thermostats 40. A fluid system
35 supplies ink to the printhead 34 via at least one
umbilical 50. A signal from thermistors 32 is
transmitted between the printhead 34 and the fluid
system 35, as indicated by representative signal
line 37 in Fig. 1.
In an ink jet printer it is desirable to
prevent ink from draining out of the umbilical when
the printhead is removed. For this reason, the
fluid fittings which couple to the printhead normal
incorporate check valves. If the umbilical was to
heat the ink, and flow of ink from the umbilical was
blocked by the check valves, the umbilical could
become a potential hazard. Due to the thermal
expansion of the fluid in the sealed umbilical,
pressures could rise to a few thousand psi in a 40
foot umbilical by heating the ink by 120°C. For this
reason the heated umbilical must include several
safeguards.
One of these safeguards is a metal braid
46 around the plastic core 42 of the umbilical.
With a TFE tubing having a wall thickness of 0.030
inch reinforced by stainless steel braid, the tubing
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is rated for use up 450°F and 3000 PSI. The rated
burst strength is 13,000 PSI. The umbilical tubing
is therefore capable of withstanding not only normal
operating conditions with ink flowing, but also
abnormal operation, where there may be no ink flow
yet heat is still being applied.
A second safeguard is the use of
thermostats 40 which open, preventing current flow
to the heater element 48, when the umbilical gets
too hot (120°C). The thermostats 40 for each
umbilical 50 are arranged so that one thermostat is
on each leg of the input power. Similarly, the two
solid state relays 38 are also arranged so as to
have one on each leg of the input power. The
thermostats 40 will open up the circuit in the event
of an unsafe operating condition.
Third, the heated umbilical is provided
with pressure relief valve with a relief pressure of
75 psi. This prevents the pressure in the umbilical
from rising higher than 75 psi. The fluid vented
through the pressure relief valve goes to a waste
ink container.
In Fig. 3, umbilical 50 is illustrated
in detail. An inner core 42 of the umbilical 50
comprises plastic tubing 44 surrounded by a metal
braid 46 to provide strength. The tubing 44 is
preferably capable of withstanding not only normal
operating conditions with ink flowing, but also
abnormal operation, where there may be no ink flow,
yet heat is still being applied. Under abnormal
conditions, steam can be created, with its
inherently high temperatures and pressures. The
braided tubing structure of the heated umbilical of
the present invention is constructed to be capable
of withstanding the high pressures and temperatures
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that may occur.
Continuing with Fig. 3, those skilled in
the art will recognize that heating element 48 can
be placed around the tubing 44 in any of a variety
of suitable configurations. For example, heat tape
can be wrapped directly around the tubing 44.
Alternatively, an intermediate layer of flexible
elastomer can be placed around the tubing 44, and
then the heating element 48 can be wrapped around
the intermediate layer. Yet another configuration
proposes a heating wire, protected by a flexible
braid, wrapped around the tubing.
In a preferred embodiment of the present
invention, thermostats 40 of Fig. 2 are placed as
close as possible to the tubing 44, with the heating
element 48 wrapped around the tubing and the
thermostat, as best illustrated in Fig. 3.
Surrounding the heating element 48 is an insulation
layer 52, in the form of a flexible tape, fiberglass
felt, or other appropriate material. Preferably,
there is enough insulation 52 so that the exterior
surfaces are not hot to the touch. Surrounding the
insulation 52 is a layer of protective covering 54,
such as a flexible conduit or sleeve. The conduit
can be smooth or convoluted, of the appropriate size
and material. A protective sleeve is the preferred
method, since the bend radius is minimized.
The temperature control is accomplished
by reading the temperature of the ink in the
printhead 34. This is compared to the desired
temperature by a suitable temperature control
system, which energizes the solid state relays 38
for the umbilical as required.
When the system is initially turned on,
after preliminary self-diagnostic tests, the ink
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begins to flow and the heat is turned on. Once the
desired temperature is reached, the print head
startup process can begin. The entire "warm-up" and
"startup" process takes less than 30 minutes from a
"cold" condition. Once the system is warmed up, the
startup process takes approximately 8 to 12 minutes.
During startup, the heater is energized
to produce "condensation cleaning" of the charge
plate. To achieve good condensation, the ink must
be heated quickly, so that the catcher and charge
plate are still "cold" in relation to the fluid.
The fluid temperature has to rise by approximately 7
degrees C in 75 seconds for the process to work
well. With 800 ml/min of ink flow, this dictates
the wattage requirements for the umbilical.
The input power varies from 180 volts to
253 volts. The ambient can be cool, for example as
low as 15 degrees C. Given these two factors, a
heater resistance for a preferred embodiment gives
enough wattage for the "low-volts, low-temp"
condition, which is more than enough for other
conditions. In the preferred embodiment, therefore,
the resistance is 27 ohms. Thus, the instantaneous
heater power varies from 1200 to 2370 watts. The
temperature control simply changes the duty cycle of
the supplied power appropriately to maintain the
desired temperature.
This description has only referred to a single
umbilical connecting the fluid system and printhead.
It must be recognized however that the number of
umbilical is not limited to one. For example, in
keeping with the spirit of the invention, a supply
line to the printhead could be heated, while one or
more additional umbilical might contain the ink
return lines to the fluid system and the electrical
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connections.
Industrial Applicability and
Advantages
The present invention is useful in the
field of ink jet printing, and has the advantage of
reducing heater watt density. The reduction in
heater watt density, as proposed by the present
invention, has the additional advantage of having
power levels that are low enough to avoid degrading
the ink.
The invention has been described in
detail with particular reference to certain
preferred embodiments thereof, but it will be
understood that modifications and variations can be
effected within the spirit and scope of the
invention.
