Note: Descriptions are shown in the official language in which they were submitted.
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INKJET PRINTER WITH SELECTIVELY ISOLATABLE PUMP
FIELD OF THE INVENTION
The present invention relates to printers and in particular the fluidic
architecture of
inkjet printers.
BACKGROUND OF THE INVENTION
Inkjet printing is a popular and versatile form of print imaging. The Assignee
has
developed printers that eject ink through MEMS printhead IC's. These printhead
IC's
(integrated circuits) are formed using lithographic etching and deposition
techniques used for
semiconductor fabrication.
The micro-scale nozzle structures in MEMS printhead IC's allow a high nozzle
density
(nozzles per unit of IC surface area), high print resolutions, low power
consumption, self
cooling operation and therefore high print speeds. Such printheads are
described in detail in
US 6746105 filed June 4, 2002 and US 7246886 filed on December 8, 2003 to the
present
Assignee.
The small nozzle structures and high nozzle densities can create difficulties
with nozzle
clogging, de-priming, nozzle drying (decap), color mixing, nozzle flooding,
bubble
contamination in the ink stream and so on. Each of these issues can produce
artifacts that are
detrimental to the print quality. The component parts of the printer are
designed to minimize
the risk that these problems will occur. The optimum situation would be
printer components
whose inherent function is able to preclude these problem issues from arising.
In reality, the
many different types of operating conditions, and mishaps or unduly rough
handling during
transport or day to day operation, make it impossible to address the above
problems via the
`passive' control of component design, material selection and so on.
To address this, the Applicant has developed printers with active control of
the fluidic
systems. These active fluidic systems are described in US patent and
publication numbers
7645034; 7637602; 7645033; 7661803; 2007/0206070; 7771029; 2007/0206050;
7658482.
While these systems provide the user with the ability to actively manage the
static and
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dynamic fluid conditions throughout the printer, it has been found that the
active components
within a printer are responsible for a large proportion of the ink borne
contaminants. Pumps in
particular are prone to shedding particles into the ink flow which can be
detrimental to the
operation of the nozzles. The wear and friction of surfaces acting against
each other
eventually generate particle which are directly entrained in the ink flow.
Many of the above
referenced fluidic designs use peristaltic pumps which introduce additional
problems. The
flexible tubing within the pump can eventually crack and leak, the tubing
loses elasticity and
no longer returns to a fully open condition, and the pump has a high torque
requirement
because of the need to compress the tubing enough to form a seal. To meet the
torque
requirements, the pump needs to be relatively large which is counter a compact
form factor for
the printer as a whole.
Ink borne contaminants can be removed with a filter upstream of the printhead.
However, the particle size requires the filter pore size to be very small. To
maintain the ink
flow rate required by a high speed, pagewidth printhead, the filter surface
area needs to be
impractically large and precludes the compactness required by market
expectations.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides an inkjet printer comprising:
a printhead for printing onto a media substrate;
a reservoir for supplying ink to the printhead;
a pump for drawing ink from the reservoir and pumping ink into the printhead;
and,
a valve arrangement for selectively opening fluid communication between the
pump
and the printhead, and closing fluid communication between the pump and the
printhead while
opening fluid communication between the reservoir and the printhead.
The invention is predicated on the realization that the purging and priming
functions of
the pump can be performed while in fluid communication with the printhead and
then be
fluidically isolated from the printhead when printing. By removing the pump
from the direct
fluid line between the reservoir and the printhead, the valve arrangement
allows it to connect
to the reservoir or the printhead only when necessary. The purging and priming
operations
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have become a two-stage processes where the pump initially draws a charge of
ink from the
reservoir and then delivers it to the printhead. The ink from the pump can be
passed through a
fine filter to remove any particulate contaminants without then constricting
the ink flow from
the reservoir during normal printing operations.
Preferably, the printer further comprises a filter for ink flowing from the
pump to the
printhead, the filter being positioned such that it is not in fluid
communication with the
printhead when the valve arrangement is configured for fluid communication
between the
reservoir and the printhead.
Preferably, the valve arrangement is configured such that the pump is not in
fluid
communication with the reservoir whenever the pump is in fluid communication
with the
printhead.
Preferably, the valve arrangement is configured such that the pump is not in
fluid
communication with the reservoir whenever the reservoir is in fluid
communication with the
printhead.
Preferably, the printer further comprises an upstream line for establishing
fluid
communication between the reservoir and the printhead wherein the valve
arrangement is a
three-way valve in the upstream line and the pump connects to the three-way
valve via a
branch line, the three way valve having three settings; a first setting
connecting the upstream
line to the reservoir to the upstream line to the printhead while closing the
branch line, a
second setting connecting the branch line to the upstream line to the
reservoir while closing
the upstream line to the printhead, and a third setting connecting the branch
line to the
upstream line to the printhead while closing the upstream line to the
reservoir.
Preferably, the pump has a chamber with a reciprocating plunger. In another
embodiment, the pump is a bulb of elastomeric material for holding a volume of
ink and an
actuator for selectively compressing the bulb.
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Preferably, the reservoir has a pressure regulator for establishing a
predetermined
pressure in a headspace above the ink in the reservoir, the predetermined
pressure being less
than atmospheric pressure. In a further preferred form, the pump draws ink
from the reservoir
to reduce the pressure in the headspace to the predetermined pressure in
preparation for
printing.
Preferably, the ink drawn from the reservoir by the pump in preparation for
printing is
pumped into the printhead in preparation for printing. Preferably, the
printhead has a
distribution manifold and a plurality of printhead integrated circuits mounted
to the
distribution manifold such that priming the distribution manifold with ink
also primes the
printhead integrated circuits.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described by way of example
only, with reference to the accompanying drawings in which:
Figure 1 shows a schematic diagram of a printer fluidic system according to
the present
invention whilst in a standby mode;
Figure 2 shows the fluidic system of Figure 1 in a ink tank pressurization
mode;
Figure 3 shows the fluidic system of Figure 1 in a printhead priming/purging
mode;
Figure 4 shows the fluidic system of figure 1 in the printing mode; and,
Figure 5 schematically shows an alternative ink pump arrangement for the
fluidic system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figures 1 to 3, the printer fluidics system is shown
schematically for the
purposes of illustration. The fluidic architecture shown in the figures is for
a single ink line
for one color only. A color printer would have separate lines and ink tanks
for each ink color.
Most of the individual components within the system are shown and described in
much greater
detail in the Applicant's co-pending application US Publ. No. 2007/0206056,
filed on March
21, 2007. Components of the present system that are not shown in the cross
referenced
document, are commercially available.
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The fluidic system shown in Figures 1 to 3 has a printhead 2 supplied with ink
46 from
an ink tank 8 via an upstream ink line 20. The upstream ink line 20 has a
three-way valve 18
which connects to the pump 30 via a filter 32. A downstream line 24 connects
the printhead 2
5 to a sump 28 via a shut off valve 26. The printhead has a maintenance
station 22 for capping,
blotting and wiping the nozzles. A drain line 34 connects the maintenance
station 22 to the
sump 28.
The printhead 2 is an assembly of an ink distribution manifold 4 on which a
series of
printhead integrated circuits (ICs) 6 are mounted. The printhead ICs 6 define
the nozzle arrays
which eject the ink to the media substrate. The nozzles are MEMS devices which
can be
thermally actuated such as those described in US 7654645 filed on July 10,
2006 or
mechanically actuated such as those disclosed in US 6746105 filed June 4,
2002.
The ink distribution manifold 4 is an LCP molding with a system of large
channels
feeding a network of smaller channels to supply the ink to many points along
the length of
each printhead IC 6. An embodiment of the distribution manifold 4 and the
printhead ICs 6 is
disclosed in detail in US Publ. No. 2007/0206056 filed March 21, 2007
reference listed above.
This document also details the manner in which the printhead is primed with
ink or, if
necessary, purged of ink to correct any cross channel color contamination
and/or bubble
removal.
The ink tank 8 and the bubble point pressure regulator 10 are described in co-
pending
US Publ. No. 2009/0096853 filed October 16, 2007. However, for the purposes of
this
description, the regulator 10 is shown as a bubble outlet 76 in the tank
headspace 12 and
vented to atmosphere via microchannel 74 extending to an air inlet 78. Ink is
retained in the
microchannel 74 by capillary action. As the printhead IC's 6 consume ink, the
pressure in the
tank 8 drops until the pressure difference at the bubble outlet 76 sucks air
into the tank. The
air is drawn through an air filter 16 to remove contaminants that might clog
or obstruct the
microchannel 74. The filtered air forms a bubble in the ink within the
micrchannel 74 which
travels to the outlet 76. This pressure difference is the bubble point
pressure and will depend
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on the diameter (or smallest dimension) of the microchannel 74 and the Laplace
pressure of
the ink meniscus. This maintains a constant negative pressure in the headspace
12. The
hydrostatic pressure in the ink at the outlet to the tank 8 will vary as the
ink level drops. To
minimize this variation, the ink tanks 8 are dimensioned to be short and
squat.
Figure 1 shows the printer is a standby mode. The printhead 2 is fluidically
isolated
from the ink tank 8 by the three-way valve 18. This prevents any ink mixing
across the
nozzles of the printhead ICs 6 from diffusing up into the tank 8. The valve 18
connects the
tank 8 to the pump 30 via the branch line 36. The pump 30 is a piston 38 that
reciprocates in a
chamber 40. It is possible to use a peristaltic pump but these suffer the
problems discussed
above in the background to the invention. Namely, the potential for failure of
the tubes,
inaccuracy as the tubes no longer return to their original uncompressed shape
and high torque
requirements. A more suitable pump is shown in Figure 5. The branch line 36
feeds a bulb 30
of elastomeric material. Actuator 38 compresses or releases the bulb 30 to
pump ink toward
the printhead or draw ink from the tank.
During long periods of standby, the pressure in the headspace 12 can rise
above the
bubble point pressure. Outgassing of dissolved gases, and diurnal temperature
variation can
cause pressure increases. In the worst case, the headspace 12 pressure is no
longer negative
relative to atmosphere.
Figure 2 shows the printer coming out of standby and preparing for a print
job. The
piston 38 retracts in the chamber 40 to draw ink 46 out of the tank 8. The ink
displacement
lowers the air pressure in the headspace 12 until the bubble point regulator
10 allows air into
the tank 8. With the headspace at the bubble point pressure, the negative
hydrostatic pressure
of the ink is within the expected operating range required by the printhead 2.
Figure 3 shows the priming or purging of the printhead 2 in preparation for
printing.
The valve 18 is reconfigured to close fluid communication between the pump 30
and the tank
8, and open fluid communication between the pump 30 and the printhead 2. The
ink in the
pump is forced out of the chamber 40 by depressing the piston 38. The ink is
forced through
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the filter 32 to remove any particulate contaminants shed by the pump. To
prime the printhead
2, ink is forced into the main channels of the ink distribution manifold 4 and
from there,
capillary action primes the small conduits and the nozzles in each of the
printhead ICs 6. This
is done with the shut off valve 26 in the downstream line 24 open so that any
excess ink feeds
straight to the sump 28.
If the printhead 2 contains ink when it is brought out of standby, it may be
necessary to
remove air bubbles or mixed ink. The problem of ink mixing is discussed in
detail in the cross
referenced application US Publ. No. 2007/0206056 filed March 21, 2007, listed
above. Put
briefly, the ink from nozzles of one color can wick across the surface of the
printhead ICs 6,
and diffuse into ink in the nozzles and supply lines for another color. This
is corrected with a
printhead purge. The downstream shutoff valve 26 is closed and filtered ink
from the pump
30 is forced through the distribution manifold 4 to flood the printhead ICs 6.
The maintenance
station 22 cleans away the flooded ink.
Figure 4 shows the printer in printing mode. The three-way valve 18 is
configured to
fluidly connect the tank 8 to the printhead 2. The blind end 44 seals off the
branch line 36 to
the pump 30. The ejection of ink from the nozzles on the printhead ICs 6 draws
ink from the
ink tank 8. The upstream ink line 20 is not constricted by the filter 32 or
any of the structural
elements of the pump 30. In the embodiment shown in US 7654640, filed on March
21, 2007,
the printhead 2 is a pagewidth printhead that prints full color at
photographic quality
resolution at a rate greater than one page per second. This requires a high
ink supply flow rate
which is not throttled by unnecessary elements in the upstream ink line 20.
At the completion of the print job, the printer can return to standby mode as
shown in
Figure 1. The valve 18 moves the blind end 44 over the upstream ink line 20 to
seal the
printhead 2 from the tank 8. This prevents any ink mixing at the printhead
from reaching the
ink tank. Ink contamination in the tank would be irretrievable and has to be
replaced. As a
further safeguard against color mixing, the shut off valve 26 is held open
during standby. The
sump 28 is at a lower elevation relative to the printhead ICs 6. This allows
the column of ink
in the downstream ink line 24 to `hang' from the distribution manifold 4 to
create a negative
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hydrostatic pressure at the printhead ICs 6. A negative pressure at the
nozzles draws the ink
meniscus inwards and inhibits color mixing.
The maintenance station 22 that seals the nozzles during standby periods to
avoid
dehydration of the printhead ICs 6 and shield the nozzle plate from paper dust
and other
particulates. The maintenance station 22 is also configured to wipe the nozzle
plate to remove
dried ink and other contaminants. Dehydration of the printhead ICs 6 occurs
when the ink
solvent, typically water, evaporates and increases the viscosity of the ink.
If the ink viscosity
is too high, the ink ejection actuators fail to eject ink drops. Dehydrated
nozzles are typically
a problem when reactivating the printer after a power down or standby period.
The problems outlined above are not uncommon during the operative life of a
printer
and can be effectively corrected with the relatively simple fluidic
architecture shown in the
figures. It also allows the user to initially prime the printer, deprime the
printer prior to
moving it, or restore the printer to a known print ready state using simple
trouble-shooting
protocols. Several examples of these situations are described in detail in the
above referenced
US 7771029 filed on February 21, 2007.
The invention has been described by way of example only. Ordinary workers in
this
field will readily recognize any variations and modifications which do not
depart from the
spirit and scope of the broad inventive concept.
