Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BIASED SERIAL INK JET PRINTING
SYSTEM FOR TEXTILES
Technical Field
The present invention relates to
continuous ink jet printing technology and, more
particularly, to applying ink jet printing
technology to textile printing.
Background Art
Continuous binary array ink jet
technology was first successfully commercialized by
Mead Corporation of Dayton, Ohio, in the mid-
nineteen-seventies. In this technology, a print
head defines one or more rows of orifices which
receive an electrically conductive recording fluid,
such as for instance a water base ink, from a
pressurized fluid supply manifold and ejects the
fluid in rows of parallel streams. Printers using
such print heads accomplish graphic reproduction by
selectively charging and deflecting the drops in
each of the streams and depositing at least some of
the drops on a print receiving medium, while others
of the drops strike a drop catcher device.
Textiles are traditionally printed by
gravure, rotary screens, or flat screen technologies
and are printed in continuous web form to enable in-
line steaming, washing or other processes. Color
patterns are generally limited to multicolor
applications using custom mixed ink colors at each
of a number of print stations along the web.
Changing colors requires changing the inks and
cleaning the screens or cylinders of each print
station. Changing designs requires changing of the
screens or gravure cylinders. Setup time is
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extensive and costly. This in turn drives the
industry to long print runs and costly inventorying
of large quantities of printed fabric. Ink jet can
generally print with dye or pigment based inks
similar to those currently used by textile printers.
Unlike electrostatic printing technologies such as
electrophotography (laser), ion deposition, or
magnatography there is generally no downstream
fusing or fixing step required. The two most
commonly used ink jet technologies are drop-on-
demand (DOD) and continuous ink jet (CIJ). DOD ink
jet printing heads form drops only when needed to
print. This intermittent drop formation is
generally limited to maximum drop frequencies of
10,000 to 20,000 hertz. DOD printheads have been
developed primarily for use in serial printers where
the printhead scans along the length of a roller
supporting the paper. Thus the array size of many
DOD printheads is relatively small (less than 2
cm.). These two factors (low frequency of drop
formation and small array size) limit the area of
substrate (paper or textile) that can be printed
within a set time. Since fabric printing involves a
much wider width than paper, often two meters or
more, DOD technology has been limited to low speed
proofing and sample printing rather than applied to
production printing of textiles. The achievement of
production speeds (greater than 1,000 square meters
per hour) would require an impractically large
number of DOD printheads.
Continuous ink jet printing, on the
other hand, is geared more toward production speeds
in industrial applications. This technology
continually produces drops enabling drop rates of
100,000 hertz or more. Drops not needed for
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printing are charged, deflected, captured and
recirculated. CIJ, as practiced, for example, by
Scitex Digital Printing, Inc., of Dayton, Ohio,
utilizes array lengths of 10-25 cm. However, the
current state of CIJ technology does not enable a
single fixed printhead to cover the entire width of
a textile web. While multiple fixed printheads
could do so, this number would need to be repeated
for each of the three or more process colors.
Although this would result in a very high speed
system (100-200 meters per minute), it would, like
DOD, require an impractically large number of
printheads.
It is seen then that there is a need for
a configuration that requires movement of both the
printheads and the textile to permit full array
coverage with a reasonable number of printheads.
Summary of the Invention
This need is met by the biased serial
ink jet printing system according to the present
invention. Ink jet is particularly well suited to
textile printing in that it is a non-contact
technology. The ink is projected across a small air
gap to the fabric which accommodates a wide variety
of fabric textures. Ink jet can generally print
with dye or pigment based inks similar to those
currently used by textile printers. Unlike
electrostatic printing technologies such as
electrophotography (laser), ion deposition, or
magnatography, there is generally no downstream
fusing or fixing step required.
In accordance with one aspect of the
present invention, a method is provided for using an
ink jet printer to print on a textile substrate.
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The one or more printheads are oriented to scan the
width of the textile web at a bias angle. A
different printhead can be used for each color ink,
whereby the ink is projected across an air gap to
the textile web during the scan. Arranging the
multiple printheads for parallel scanning allows
each printhead to scan a separate swath
simultaneously. Additional scan widths can be left
between printheads to allow for sufficient drying
time.
Accordingly, it is an object of the
present invention to provide a continuous ink jet
system for printing on textiles. It is a further
object of the present invention to combine movement
of printheads and the textile to achieve full array
coverage with a reasonable number of printheads. It
is an advantage of the present invention that the
biased serial ink jet printing system according to
the present invention maximizes textile printing
efficiency. It is a further advantage of the
present invention that reciprocating printhead
movement across the web width is utilized.
Other objects and advantages of the
invention will be apparent from the following
description, the accompanying drawings and the
appended claims.
Brief Description of the Drawinqs
Fig. 1 illustrates bi-directional
scanning of a printhead with intermittent web
motion;
Figs. 2a and 2b illustrate the period of
the scan that operates at maximum velocity;
Fig. 3 illustrates another longer scan
distance printing configuration embodiment in
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accordance with the present invention, achieved by
angling the scan direction across the width of the
web;
Fig. 4 is a vacuum bed configured to
hold the web flat under biased scanning printheads;
Fig. 5 illustrates a printhead array
parallel to the web edge;
Fig. 6 illustrates an angled printhead
array embodiment of the present invention which
enables a wider swath measured parallel to the web
edge;
Fig. 7 illustrates serially mounted
process color printheads; and
Fig. 8 illustrates an embodiment of the
present invention wherein the printheads are mounted
in a parallel configuration.
Detailed Description of the Preferred Embodiments
The present invention relates to the
development of a configuration that combines
movement of both the printheads and the textile to
achieve full array coverage with a reasonable number
of printheads. The present invention maximizes
printing efficiency by providing for a longer sweep
and, therefore, a greater percentage of maximum
velocity print time. Efficiency of a configuration
can be defined as the throughput actually achieved
versus the throughput calculated assuming all
printing is continuous at maximum speed.
Referring now to the drawings, one
obvious approach to maximizing textile printing
efficiency is to utilize the reciprocating printhead
movement across a web width 10, much like a desktop
serial printer. As illustrated in Fig. 1,
printhead(s) 12 print on both the forward scan and
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return directions, with the scan direction and
distance indicated by arrows 14a, 14b, 14c, 14d.
The web advances, in the direction indicated by
arrows 16, the width of one printing swath 18, as
the printhead 12 pauses at the end of each scan 14.
The printhead 12 will need to have completed
printing on a given pass prior to advancing the
substrate, or fabric 20, and the fabric will need to
stop its movement prior to the start of the next
pass of the printhead 12. One method for improving
efficiency of this approach is to accelerate and
decelerate the printhead during printing by
utilizing a true position encoder to control timing
of the print drops. Due to the high drop frequency
of continuous ink jet printing, it is necessary to
move the printhead with relatively high speed to
take full advantage of the printing capability.
The large mass of a typical continuous
ink jet printhead makes the ramp-up acceleration and
ramp-down deceleration of the printhead significant
factors in determining the configuration efficiency.
That is, the longer the scan distance 14, the
greater the percent of the scan that can be printed
at maximum printhead speed and thus the higher the
efficiency of the system. Figs. 2a and 2b
illustrate how a longer scan distance results in a
higher percent of scan period at maximum velocity.
With the scan direction perpendicular to the web
edge, as shown in Fig. 2a, the only way to improve
efficiency due to the acceleration and deceleration
ramps needed is to widen the web, as shown in Fig.
2b.
To further maximize efficiency, the
present invention proposes that the printhead scan
the width of the textile web at a bias angle to the
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standard cross web perpendicular, as illustrated in
Fig. 3. The angled configuration illustrated in
Fig. 3 enables a longer scan at full velocity with
the same width web. In Fig. 3, a longer scan
distance 14 is achieved, as compared to the scan of
Fig. 1, without increasing the web width 10. With
the textile configuration shown in Fig. 3, the ramp-
up acceleration and the ramp-down deceleration of
printhead 12 has less negative impact on overall
printing efficiency.
Due to the length of the array and the
close proximity required of the print array to the
fabric, it is not practical to wrap the web over a
roller as is done in the analogous serial printer.
Fabric 20 must be held flat over the area being
printed. It is also necessary to stabilize the
fabric over this area. In applications using flat
screen printing of multiple colors, this is
accomplished by means of adhesive bonding the
textile to a moving belt which, in turn, lies on a
flat bed. After the fabric advances beyond the
print station, it is stripped off the belt. Fig. 4
illustrates a vacuum bed 22 configured to hold the
web and belt flat under biased scanning printheads.
In accordance with the present invention, it is
possible to utilize the vacuum array on the flat bed
to hold the belt and, thus, the fabric 20, flat
during the printing scan. This flat bed 22 is
preferably shaped in the plan view to match the
biased orientation of the print scan. For example,
the flat bed 22 is shaped as a parallelogram in Fig.
4, to match the biased orientation of the print scan
of Fig. 3. A second encoder (not shown) can be used
to control the precise movement of the fabric and
belt between scans to assure registration of swath
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edges, as well as registration of the various color
nk scans.
If the printhead 12 were oriented such
that the array were parallel to belt 24 and fabric
20 motion, the printhead swath 18 would equal the
array length along the web direction, as illustrated
in Fig. 5. This increases the printed resolution
perpendicularly across the swath by a factor of
1/COS ~, where ~ is the bias angle of the swath to
the cross web perpendicular. If the jet resolution
of the array were sufficient, the printhead 12 could
be angled such that the array is perpendicular to
the biased scan direction, indicated by arrow 26,
and an even wider swath 18 would be created during
the scan, as is illustrated in Fig. 6. Thus, in the
preferred orientation of Fig. 6, increasing the bias
angle of the scan lengthens the scan, widens the
swath as measured along the web direction, and
increases the configuration efficiency. However,
the disadvantage of increasing the bias angle is
that it requires an even larger flat bed 22 to
maintain good quality printing. Also, as the bias
angle increases, an enlarging portion of printhead
travel occurs where a portion of the jet array is
not used at the edges of the fabric, subtracting
some of the efficiency gained from the biased
configuration. Hence, a practical compromise might
be a printhead angle in the range of 30 to 60
degrees.
To print process color images it is
desirable to use at least three printheads with
process color inks, which are cyan (c), magenta (m)
and yellow (y). A fourth printhead with black ink
(k) can also be included to improve image contrast.
Additional printheads with other colors may be added
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to broaden the color space achievable. Multiple
printheads can be aligned on a single carriage to
scan across the web serially. Serially mounting of
the printheads would minimize the width of the flat
bed controlling the belt and fabric, as illustrated
in Fig. 7. However, the total scan length 28 would
increase by Ua", as shown in Fig. 7, to enable each
of the three or more printheads 12 to cover the full
width of the textile. Also, this configuration
would provide insufficient time for each ink to
absorb into the textile or dry prior to the next
color ink striking the surface. This could cause
spatter of inks and reduce image quality. This
arrangement would also introduce color shift by
laying down the process colors in a different
sequence on alternate direction scans.
Parallel scanning, in accordance with
the present invention, allows each printhead 12 to
scan a separate swath simultaneously, as illustrated
in Fig. 8. The next color would be added at the
subsequent printhead after the fabric and belt are
advanced, providing time for the ink to penetrate or
dry. Parallel scanning also enables printing the
process colors in the same sequence with each scan.
As can be seen in Fig. 8, the angling of
the printheads 12 accommodates the parallel
configuration by allowing the printheads to overlap,
thereby minimizing the size of the flat bed 22. If
necessary, the printheads could be spaced to require
one or more unprinted scan widths between adjacent
printheads, to allow drying time before the next
color overlaps. Of course, this approach requires a
wider flat bed.
The biased scan approach creates a
rectilinear matrix at the angle ~ to the fabric.
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Although this might pose problems for printing text
on paper, it is not a significant factor for
printing patterns and designs on fabric. The angled
matrix minimizes interference patterns caused by
slight misalignment of the printing matrix with the
weave of the fabric, which could occur more easily
if the printhead scanned perpendicular to the web
along the weave pattern.
Although the preferred mode of
practicing the invention has been described with
reference to an ink jet print head for a continuous
ink jet printer printing on textile, the principle
of the present invention can also be applied to a -
wide variety of ink jet printers and substrates.
Industrial Applicability and
Advantaqes
The biased serial ink jet printing
system according to the present invention is useful
for printing colors and patterns on textiles,
because ink jet technology offers a way to print
digitally on textiles. Multiple printheads printing
a range of process colors can produce a color gamut
enabling thousands of colors and shades within a
single image. This digital printing technology can
instantly change colors and patterns with no change
of ink or screens. Thus, the economics of printing
a range of colors and patterns are not affected by
the length of the run. The textile printer can
print only the yardage needed to fill current orders
without concern for carrying large backup
inventories. Repeat orders can be printed ~on
demand~ with little concern for the size of the
order. This capability can provide textile
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consumers with a larger variety of colors and
patterns in a timely and cost effective manner.
This in turn can provide a larger number of "design
collections~ per year. Sending digital images
directly to the printing system without making
screens or mixing ink, digital printing of textiles
can provide quicker responses to customers' demands.
Having described the invention in detail
and by reference to the preferred embodiment
thereof, it will be apparent that other
modifications and variations are possible without
departing from the scope of the invention defined in
the appended claims.
