U.S. patent application number 14/440613 was filed with the patent office on 2015-10-22 for system and method for direct inkjet printing of 3d objects.
The applicant listed for this patent is Yehoshua SHEINMAN. Invention is credited to Yehoshua SHEINMAN.
Application Number | 20150298394 14/440613 |
Document ID | / |
Family ID | 50626593 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150298394 |
Kind Code |
A1 |
SHEINMAN; Yehoshua |
October 22, 2015 |
SYSTEM AND METHOD FOR DIRECT INKJET PRINTING OF 3D OBJECTS
Abstract
A direct inkjet printing system for fabricating a part by an
additive manufacturing process includes an ink delivery system
operative to circulate the ink, a printhead associated with the ink
delivery system, the printhead operative to dispense ink from the
ink delivery system through a plurality of nozzles and based on a
defined pattern, a building table for receiving the dispensed ink
one layer at a time based on the defined pattern, wherein the part
is formed from a plurality of layers of the ink dispensed from the
printhead and a drying station operative to perform a drying
process on layers of the ink dispensed from the printhead on a per
layer basis.
Inventors: |
SHEINMAN; Yehoshua;
(RaAnana, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SHEINMAN; Yehoshua |
|
|
US |
|
|
Family ID: |
50626593 |
Appl. No.: |
14/440613 |
Filed: |
November 5, 2013 |
PCT Filed: |
November 5, 2013 |
PCT NO: |
PCT/IL2013/050908 |
371 Date: |
May 5, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61722433 |
Nov 5, 2012 |
|
|
|
Current U.S.
Class: |
427/402 ;
118/600 |
Current CPC
Class: |
B29C 64/364 20170801;
B29C 64/40 20170801; B29L 2009/00 20130101; B29C 33/448 20130101;
B33Y 30/00 20141201; B29C 64/112 20170801; B29C 64/295 20170801;
B33Y 10/00 20141201; B29C 64/209 20170801; B29C 64/188 20170801;
B29C 64/25 20170801; B29C 64/106 20170801; B29C 64/245
20170801 |
International
Class: |
B29C 67/00 20060101
B29C067/00 |
Claims
1. A direct inkjet printing system for fabricating a part by an
additive manufacturing process, the system comprising: a printhead
operative to dispense ink from an ink delivery system through a
plurality of nozzles and based on a defined pattern; a building
table for receiving the dispensed ink one layer at a time based on
the defined pattern, wherein the part is formed from a plurality of
layers of the ink dispensed from the printhead; and a drying
station operative to perform a drying process on layers of the ink
dispensed from the printhead on a per layer basis, wherein the
building table is operable to advance into the drying station on
the per layer basis.
2. The system according to claim 1, wherein the building table is
movable in the printing direction and in the cross printing
direction for scanning the plurality of layers while the printhead
dispenses the ink.
3. The system according to claim 1, wherein the building table
includes a building platform that is rotatable.
4-5. (canceled)
6. The system according to claim 1, comprising a mat positioned on
the building table, wherein the mat is operative to receive the
dispensed material, wherein the mat is adapted to provide a surface
tension that is higher than a surface tension of the ink dispensed
thereon.
7. (canceled)
8. The system according to claim 6, wherein the mat is inkjet
paper.
9. The system according to claim 1, wherein the drying station
comprises: a housing; a sliding door for receiving the building
table into the housing; a blower operative for circulating air in
the housing; and a heating unit operative to heat the air
circulated by the blower.
10. The system according to claim 9, wherein the drying station is
operative to impinge jets of hot air on the layers of the ink
dispensed on the building table, wherein the drying station
includes a nozzle plate through which the hot air is jetted.
11. (canceled)
12. The system according to claim 1, comprising two drying
stations.
13. The system according to claim 1, comprising a first printhead
for dispensing building material and a second printhead for
dispensing support material.
14. The system according to claim 1, wherein the ink that is
dispensed by the printing head has a viscosity of 10-20 cps at the
dispensing temperature.
15-16. (canceled)
17. The system according claim 1, wherein the ink that is dispensed
by the printing head is ink that is formulated with at least one of
ceramic powder, encapsulated metal micro-particles and soluble
polymer.
18-19. (canceled)
20. The system according to claim 1, wherein the ink that is
dispensed by the printhead is adapted for sintering.
21. The system according to claim 1, wherein the ink delivery
system is operative to circulate the ink with gravitation based
circulation.
22. The system according to claim comprising a roller operative to
flatten a plurality of layers at a time, wherein the roller is
operative to be lowered toward the dispensed ink on demand.
23. (canceled)
24. The system according to claim 1, comprising a maintenance
station operable to align with a position of the printhead during
idle periods of the system and to be displaced from the printhead
during printing, wherein the maintenance station comprises a nozzle
operative to spray cleaning fluid on the printhead and blotting
paper with a mechanism for raising the blotting paper toward the
printhead.
25. A method for direct inkjet printing by an additive
manufacturing process to fabricate a part, the method comprising:
dispensing droplets of ink in a layerwise manner according to a
pattern defined for fabricating the part, wherein the ink is
formulated with at least one of ceramic powder, encapsulated metal
micro-particles and soluble polymer; advancing the building; table
into a drying station on a per layer basis; and drying the droplets
of ink in the drying station on a per layer basis.
26. The method according to claim 25 comprising; printing a layer
in a two step process; and drying the droplets of ink in the drying
station after each of the irs and second steps, wherein the first
step of the two step process includes scanning a first array of the
droplets in a printing direction with one pixel gaps in a cross
printing direction and wherein the second step of the two step
process includes scanning a second array of the droplets in the
printing direction with one pixel gaps in a cross printing
direction, wherein the second array fills at least a portion of the
one pixel gaps formed by the first array.
27. (canceled)
28. The method according to claim 26, comprising dispensing
droplets of support material in a layerwise manner according to a
pattern defined for supporting the part during fabrication, wherein
a layer of droplets of support material is dispensed subsequent to
forming a corresponding layer of the droplets of ink for
fabricating the part.
29. The method according to claim 25, comprising rotating the layer
by 90 degrees prior to printing a subsequent layer.
30. The method according to claim 25, wherein the droplets of ink
are dispensed with a printhead including an array of nozzles and
wherein the printhead is displaced by a half a pixel distance in a
crossing printing direction prior to printing a subsequent
layer.
31. The method according to claim 25, comprising flattening each of
a plurality of layers at a time.
32. The method according to claim 25, comprising sintering the
part.
Description
FIELD AND BACKGROUND OF THE INVENTION
[0001] The present invention, in some embodiments thereof, relates
to additive manufacturing and/or 3D printing and, more
particularly, but not exclusively, to 3D printing for rapid
manufacturing.
[0002] A number of different processes for fabricating solid
objects by 3D printing are known. Typically, the processes
successively print layers of material in different shapes based on
a 3D model of the object. The different processes typically differ
in the way layers are deposited and in the materials that can be
used.
[0003] U.S. Pat. No. 6,322,728 entitled "Mass production of dental
restorations by solid free-form fabrication methods," the contents
of which is incorporated herein by reference, describes processes
for manufacturing dental restorations. One process includes ink-jet
printing a binder into selected areas of sequentially deposited
layers of powder. Each layer is created by spreading a thin layer
of powder over the surface of a powder bed. Instructions for each
layer may be derived directly from a CAD representation of the
restoration. While the layers become hardened or at least partially
hardened as each of the layers is laid down, once the desired final
shaped configuration is achieved and the layering process is
complete, in some applications it may be desirable that the form
and its contents be heated or cured at a suitably selected
temperature to further promote binding of the powder particles.
Another process includes providing a mixture of powder material
dispersed in a binder; and dispensing the mixture from a dispensing
apparatus onto a platform. A slurry or paste of a polymer or
ceramic powder is mixed with a carrier formed into a coping of a
multi-layered final shape of a dental restoration using a machine
similar to a fused deposition modeling machine. This machine emits
or sprays beads of slurries as opposed to a fused deposition
modeling machine which emits liquefied thermoplastic materials.
[0004] U.S. Pat. No. 7,896,639 entitled "Rapid Prototyping
Apparatus," the contents of which is incorporated herein by
reference, describes an inkjet type of apparatus for producing an
object by sequentially forming thin layers of a photopolymer
material one on top of the other responsive to data defining the
object. The apparatus includes a plurality of printing heads each
having a surface formed with a plurality of output orifices and
controllable to dispense the construction material through each
orifice independently of the other orifices and a shuttle to which
the printing heads are mounted. A controller moves the shuttle back
and forth over a support surface and as the shuttle moves controls
the printing heads to dispense the construction material responsive
to the data to form a first layer on the support surface and
thereafter, sequentially the other layers. Typically, UV radiation
is used to cure each of the layers of photopolymer material.
[0005] An article published in the Journal of the American Ceramic
Society, 85 [110 2514-20 (2001), entitled "Ink-Jet Printing of
Wax-Based Alumina Suspensions," the contents of which is
incorporated herein by reference, describes a method for producing
solid bodies that contain ceramic particles dispersed in
low-melting-point waxes. Suspensions of fine alumna powder in a
paraffin wax were formulated with viscosity values sufficiently low
to allow ink-jet printing using a commercial printer. Suspensions
with powder loading of up to 40 volume percent were passed through
the ink-jet printer head. A direct ink-jet printing process was
used to fabricate the solid bodies with the formulated suspensions
in wax.
[0006] Inkjet printers are also known to be used for printing
information on paper. One source of operation failure of inkjet
printers used for printing on paper can be caused by sedimentation
and evaporation of components in the ink being dispensed through
the nozzles and/or orifices. For example during periods of non-use,
the ink that is retained in the print head may deteriorate and lead
to sedimentation of solid particles. Deterioration of the ink may
also include evaporation of components contained in the ink. This
may lead to a change in viscosity of the ink in the vicinity of the
nozzle, having a negative effect on its jetting properties.
Sedimentation and evaporation may potentially lead to a nozzle fall
out or nozzle blockage.
[0007] U.S. Pat. No. 8,091,987 entitled "Ink jet print head with
improved reliability," the contents of which is incorporated herein
by reference, describes a method for refreshing ink that will be
used for ejecting through the nozzle of a print head. The method
includes creating an ink flow in excess of that required to
replenish drops ejected from the print head, and passing that flow
of ink along the inner end of the nozzle and through an ink path in
a nozzle plate of the print head. The ink flow refreshes the ink
that will be used for ejecting through the nozzle.
SUMMARY OF THE INVENTION
[0008] According to an aspect of some embodiments of the present
invention there is provided a system and method for manufacturing
ceramic, metal and/or high quality polymer parts using a direct
inkjet printing process. According to some embodiments of the
present invention, the ceramic, metal and/or high quality polymer
parts are fabricated with ink formulated with a high volume
percentage of high density pigment and/or particles such as ceramic
powder, encapsulated metal micro-particles, and/or soluble polymer.
Optionally the ink is one of water based ink or solvent based ink.
According to some embodiments of the present invention, the system
and method provides for direct inkjet printing with such ink that
typically has high viscosity, poor dispersion stability and/or high
solvent evaporation rate. According to some embodiments of the
present invention, ceramic and/or metal parts formed with the
direct inkjet printing process are adapted for sintering.
[0009] An aspect of some embodiments of the present invention
provides for a direct inkjet printing system for fabricating a part
by an additive manufacturing process, the system comprising: an ink
delivery system operative to circulate the ink; a printhead
associated with the ink delivery system, the printhead operative to
dispense ink from the ink delivery system through a plurality of
nozzles and based on a defined pattern; a building table for
receiving the dispensed ink one layer at a time based on the
defined pattern, wherein the part is formed from a plurality of
layers of the ink dispensed from the printhead; and a drying
station operative to perform a drying process on layers of the ink
dispensed from the printhead on a per layer basis.
[0010] Optionally, the building table is movable in the printing
direction and in the cross printing direction for scanning the
plurality of layers while the printhead dispenses the ink.
[0011] Optionally, the building table includes a building platform
that is rotatable.
[0012] Optionally, the building table is operable to advance into
the drying station on the per layer basis.
[0013] Optionally, the building table includes a removable tray
adapted for being placed in a sintering oven.
[0014] Optionally, the system includes a mat positioned on the
building table, wherein the mat is operative to receive the
dispensed material.
[0015] Optionally, the mat is adapted to provide a surface tension
that is higher than a surface tension of the ink dispensed
thereon.
[0016] Optionally, the mat is inkjet paper.
[0017] Optionally, the drying station comprises: a housing; a
sliding door for receiving the building table into the housing; a
blower operative for circulating air in the housing; and a heating
unit operative to heat the air circulated by the blower.
[0018] Optionally, the drying station is operative to impinge jets
of hot air on the layers of the ink dispensed on the building
table.
[0019] Optionally, the drying station includes a nozzle plate
through which the hot air is jetted.
[0020] Optionally, the system includes two drying stations.
[0021] Optionally, the system includes a first printhead for
dispensing building material and a second printhead for dispensing
support material.
[0022] Optionally, the ink that is dispensed by the printing head
has a viscosity of 10-20 cps.
[0023] Optionally, the ink that is dispensed by the printing head
ink that is formulated with at least one of ceramic powder,
encapsulated metal micro-particles and soluble polymer.
[0024] Optionally, the ink is water-based ink.
[0025] Optionally, the ink is solvent-based ink.
[0026] Optionally, the ink that is dispensed by the printhead is
adapted for sintering.
[0027] Optionally, the ink delivery system is operative to
circulate the ink with gravitation based circulation.
[0028] Optionally, the system includes a roller operative to
flatten a plurality of layers at a time.
[0029] Optionally, the roller is operative to be lowered toward the
dispensed ink on demand.
[0030] Optionally, the system includes a maintenance station
operable to align with a position of the printhead during idle
periods of the system and to be displaced from the printhead during
printing, wherein the maintenance station comprises a nozzle
operative to spray cleaning fluid on the printhead and blotting
paper with a mechanism for raising the blotting paper toward the
printhead.
[0031] An aspect of some embodiments of the present invention
provides for a method for direct inkjet printing by an additive
manufacturing process to fabricate a part, the method comprising:
dispensing droplets of ink in a layerwise manner according to a
pattern defined for fabricating the part, wherein the ink is
formulated with at least one of ceramic powder, encapsulated metal
micro-particles and soluble polymer; and drying the droplets of ink
in a drying station on a per layer basis.
[0032] Optionally, the method includes printing a layer in a two
step process, wherein the first step of the two step process
includes scanning a first array of the droplets in a printing
direction with one pixel gaps in a cross printing direction and
wherein the second step of the two step process includes scanning a
second array of the droplets in the printing direction with one
pixel gaps in a cross printing direction, wherein the second array
fills at least a portion of the one pixel gaps formed by the first
array.
[0033] Optionally, the method includes drying the droplets of ink
in the drying station after each of the first and second steps.
[0034] Optionally, the method includes dispensing droplets of
support material in a layerwise manner according to a pattern
defined for supporting the part during fabrication, wherein a layer
of droplets of support material is dispensed subsequent to forming
a corresponding layer of the droplets of ink for fabricating the
part.
[0035] Optionally, the method includes rotating the layer by 90
degrees prior to printing a subsequent layer.
[0036] Optionally, the droplets of ink are dispensed with a
printhead including an array of nozzles and wherein the printhead
is displaced by a half a pixel distance in a crossing printing
direction prior to printing a subsequent layer.
[0037] Optionally, the method includes flattening each of a
plurality of layers at a time.
[0038] Optionally, the method includes sintering the part.
[0039] Unless otherwise defined, all technical and/or scientific
terms used herein have the same meaning as commonly understood by
one of ordinary skill in the art to which the invention pertains.
Although methods and materials similar or equivalent to those
described herein can be used in the practice or testing of
embodiments of the invention, exemplary methods and/or materials
are described below. In case of conflict, the patent specification,
including definitions, will control. In addition, the materials,
methods, and examples are illustrative only and are not intended to
be necessarily limiting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] Some embodiments of the invention are herein described, by
way of example only, with reference to the accompanying drawings.
With specific reference now to the drawings in detail, it is
stressed that the particulars shown are by way of example and for
purposes of illustrative discussion of embodiments of the
invention. In this regard, the description taken with the drawings
makes apparent to those skilled in the art how embodiments of the
invention may be practiced.
[0041] In the drawings:
[0042] FIG. 1 is a simplified cross-sectional view of an exemplary
direct inkjet printing system in accordance with some embodiments
of the present invention;
[0043] FIG. 2 is a simplified cross-sectional view of an exemplary
direct inkjet printing system including two drying stations in
accordance with some embodiments of the present invention;
[0044] FIG. 3 is a simplified schematic drawing of an exemplary
drying station in accordance with some embodiments of the present
invention;
[0045] FIG. 4 is a simplified flow chart of an exemplary method for
drying layers of printed material in accordance with some
embodiments of the present invention;
[0046] FIG. 5 is a simplified schematic drawing of an exemplary Y
axis stage for supporting the printheads in accordance with some
embodiments of the present invention;
[0047] FIG. 6 is a simplified schematic drawing of an exemplary
rotatable building tray in accordance with some embodiments of the
present invention;
[0048] FIG. 7 is a simplified schematic drawing of an exemplary
roller for flattening printed layers in accordance with some
embodiments of the present invention;
[0049] FIGS. 8A and 8B are simplified schematic drawings of ink
droplets dispensed from odd nozzles of a printhead shown in a
perspective and front view respectively and in accordance with some
embodiments of the present invention;
[0050] FIG. 9 is a simplified schematic drawing of the ink droplets
dispensed from odd nozzles of a printhead after a drying process,
shown in a front view and in accordance with some embodiments of
the present invention;
[0051] FIGS. 10A and 10B are simplified schematic drawings of
additional ink droplets subsequently dispensed from even nozzles of
a printhead shown in a perspective and front view respectively and
in accordance with some embodiments of the present invention;
[0052] FIGS. 11A and 11B are simplified schematic drawings of an
exemplary complete printed layer after a drying process, shown in a
perspective and front view respectively and in accordance with some
embodiments of the present invention;
[0053] FIG. 12 is a simplified schematic drawing showing a
plurality of exemplary printed layers responsive to applying
lateral shifting of the printhead in accordance with some
embodiments of the present invention;
[0054] FIG. 13 is a simplified schematic drawing showing a
plurality of exemplary printed layers responsive to applying
rotation of the building platform in accordance with some
embodiments of the present invention;
[0055] FIG. 14 is a simplified schematic drawing of an exemplary
maintenance station in accordance with some embodiments of the
present invention;
[0056] FIG. 15 is a simplified flow chart of an exemplary method
for printing a 3D object in accordance with some embodiments of the
present invention;
[0057] FIG. 16 is a simplified schematic diagram of an ink
circulation unit with replaceable ink cassette in accordance with
some embodiments of the present invention;
[0058] FIG. 17 is a simplified schematic diagram of an ink
circulation unit with replaceable cleaning cassette in accordance
with some embodiments of the present invention; and
[0059] FIG. 18 is a simplified schematic diagram of a purging
device for cleaning a nozzle of the printhead in accordance with
some embodiments of the present invention.
DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION
[0060] The present invention, in some embodiments thereof, relates
to additive manufacturing and/or 3D printing and, more
particularly, but not exclusively, to 3D printing for rapid
manufacturing.
[0061] According to some exemplary embodiments there is provided a
system and method using a direct inkjet printing process for
fabricating 3D objects. As used herein, a direct inkjet printing
process for fabricating 3D objects refers to a process for
fabricating a 3D object by selectively dispensing droplets of ink
as the construction and/or building material, one layer at a time
based on a computed model of the 3D object. According to some
embodiments of the present invention, the ink has high viscous
properties, e.g. 10-20 cps due to its high volume percentage of
ceramic powder, and/or encapsulated metal micro-particles.
Alternatively, the construction material is a viscous ink that
includes a soluble polymer. Optionally, support material for
supporting a geometrical structure of the part during printing is
also dispensed based on the computed model of the 3D object. In
some exemplary embodiments, the support material is dispensed from
a dedicated print head.
[0062] The present inventors have found that current developments
in inkjet printhead technology that support circulation of ink with
slow flow around the nozzle plate may also support printing with
high viscosity inks as described herein. Typically, ceramic and/or
metal inks as described herein have poor dispersion stability due
to their high volume percentage of particles while polymer inks as
described herein have a high solvent evaporation rate that is
sensitive to heating. The present inventors have found that the
circulation can replace the typical process of heating the ink
during inkjet printing. Heating is typically accompanied by an
increased dispersion instability and solvent evaporation.
[0063] According to some embodiments of the present invention, the
system includes an ink delivery system for circulating the high
viscosity ink to maintain dispersion stability of the ink, avoid
sedimentation inside the printhead and/or prevent clogging of the
nozzle due to particle sinking. According to some embodiments of
the present invention, the ink delivery system uses gravitation for
circulation generation. Optionally, the ink delivery system
functions as a cartridge that can be replaced by a cleaning fluid
cartridge or an alternate cartridge including different color ink.
Optionally, the ink circulation per printhead provided by the ink
delivery system can reach 20 to 65 cc/min, e.g. 50 cc/min.
Typically, during circulation, air trapped in the ink is removed.
In some exemplary embodiments, when gravitation is used to generate
circulation, a purging device is included to provide pressure for
nozzle cleaning.
[0064] The present inventor has found that direct inkjet printing
as described herein may provide advantages over known methods for
fabrication by printing. For example in known processes that print
a binder into layers of powder, the surface quality of the object
is known to be relatively rough, while direct inkjet processing as
described herein may provide smooth glossy surfaces without need
for a polishing and/or buffing process. In some applications such
as applications for dental restoration, a highly polished finish is
typically required.
[0065] The present invention may also provide advantages over known
fabrication processes that use direct inkjet printing with low
viscosity construction material, e.g. ceramic powder suspension in
wax and then apply sintering. Inkjet printing processes that use
low viscosity construction material are typically associated with a
significant and/or an unpredictable degree of shrinkage during
sintering. Rather, the present inventor has found that when using
ink with high volume percentage of high density particles and/or
content as described herein, sintering may be applied with little
or no shrinkage of the part being built. The present inventor has
also found that based on the system and method described herein,
porosity resulting from binder removal may be minimal so that the
mechanical properties of the part may be maintained after binder
removal.
[0066] According to some embodiments of the present invention, the
direct inkjet printing process as described herein can also be used
to fabricate high quality plastic parts by using solvent soluble
polymer, e.g. polyamide, as opposed to photopolymers. The present
inventor has found that the quality, e.g. mechanical properties of
these plastic parts when printed by direct inkjet printing as
described herein may be comparable to like parts fabricated by
injection molding. For some applications, cost for manufacturing by
direct inkjet printing may be significantly lower than that for
injection molding.
[0067] According to some embodiments of the present invention, a
drying process is applied to each of the layers that are printed.
Typically, the drying process is applied to evaporate the water
and/or low boiling temperature solvents in the printed layer.
Optionally, the drying process is also applied to humectants, high
temperature boiling solvents and/or to activate the binder in the
ink. In some exemplary embodiments, the drying process also
provides for drying and/or solidifying supporting material that is
dispensed together with building material.
[0068] According to some embodiments of the present invention, the
system includes at least one drying station for performing the
drying process. Typically, a building tray supporting the dispensed
material is advanced into the drying station after each layer is
printed and/or scanned. In some exemplary embodiments, each layer
is printed in two steps including a first scanning step for
dispensing ink with even nozzles and then a second scanning step
for dispensing ink with odd nozzles. Optionally, scanning is
performed first in one direction and then in the opposite
direction. Optionally, drying is applied after each step.
Optionally, the system includes two drying stations at opposite
ends of the system and the building tray is alternately advanced
into each one of the drying stations.
[0069] According to some embodiments of the present invention,
flattening is applied for every plurality of layers, e.g. per
several tens of layers to compensate for accumulated error in the
model in the vertical direction, e.g. Z-axis direction. According
to some embodiments of the present invention, system includes a
flattening pressure roller for flattening the layers.
[0070] According to some embodiments of the present invention, a
wetting surface is used for absorbing the water or solvent in the
ink so that the structure of the droplets of ink is stable on the
building surface until the drying process is initiated. Typically,
the wetting surface is selected to have a surface tension higher
than the surface tension of the ink on which the part is
fabricated. In some exemplary embodiments, inkjet paper and/or
paper coated with receptive coating, e.g. polyvinylpyrrolidone
(PVP) coating is used as the wetting surface. Typically, the inkjet
paper or the like is positioned on the building tray of the system
and removed after building of the part is completed and/or burnt
away after sintering.
[0071] The present inventor has found that while printing layers of
ink droplets to form the part, ditches along the printing direction
can appear on the top surface of the printed layer(s) due to the
structure of the droplets. According to some embodiments of the
present invention, the system is operable to shift a printing head
of the system laterally by a half a pixel distance and/or half the
distance between odd and even lines prior to printing a subsequent
layer. Alternatively and/or additionally, the incremental printhead
motion can be applied to reduce the nozzle density, e.g. the number
of nozzles in the printhead and provide a tradeoff between number
of printing nozzles (price) and the model building rate
(throughput). For example, each layer may be printed in a two step
process; scanning the layer with all nozzles over a first step,
incrementally shifting the nozzles and then scanning the layer
again with all nozzles over a second step. The process can then be
repeated to form the next layer. Optionally, the printing head of
the system is subsequently shifted laterally by a half a pixel
distance to print the next layer.
[0072] According to some exemplary embodiments, the printing system
includes a rotatable building table that is operable to rotate 90
degrees around its center prior to printing one or a plurality of
subsequent layers so that the part is built in a crisscross
fashion. Optionally, building in a crisscross fashion provides for
avoiding ditch formation on the layer surface.
[0073] According to some embodiments of the present invention, the
printing system includes a maintenance station that is operable to
align with the printheads and engage and/or support maintenance
operations while the printheads are idle.
[0074] Before explaining at least one embodiment of the invention
in detail, it is to be understood that the invention is not
necessarily limited in its application to the details of
construction and the arrangement of the components and/or methods
set forth in the following description and/or illustrated in the
drawings and/or the Examples. The invention is capable of other
embodiments or of being practiced or carried out in various
ways.
[0075] Referring now to the drawings, FIG. 1 shows a simplified
cross-sectional view of an exemplary direct inkjet printing system
in accordance with some embodiments of the present invention.
According to some embodiments of the present invention, a direct
inkjet printing system 100 includes an ink print head 102 and a
support material printhead 104 that are stationary while printing.
Optionally, support material printhead 104 is not required and/or
is not included. In some exemplary embodiments, the system includes
a maintenance station 106 that is operable to align under ink
printhead 102 and/or under both ink print head 102 and a support
material printhead 104 during idle times and to move and/or slide
aside during printing procedures. According to some embodiments of
the present invention, system 100 includes a building table and/or
tray 108 that is movable in two direction, a printing and/or
scanning direction, e.g. along an X axis and a vertical and/or
layer building direction, e.g. along a Z axis. Alternatively, the
building table is stationary in one or more directions and
printheads 102 and 104 are operative to move in the scanning
direction and/or in the cross scan direction.
[0076] According to some embodiments of the present invention,
after a layer 50 is scanned, a building table 108 that supports the
dispensed material is moved toward a drying station 116 and is
inserted into drying station 116. Optionally, insertion is by
pushing a spring loaded drying tray 114 of the drying station 116
open. Typically, drying station 116 includes air circulation and
air heating devices as is described in detail herein.
[0077] Typically, building tray 108 is lowered in the layer
building direction for each layer printed and dried so that a
subsequent layer can be applied. Optionally, lowering occurs as the
building table 108 is removed from drying station 116. Optionally,
building tray 108 includes a rotating plate 110 that is operable to
rotate a surface of building tray 108 by 90 degrees after a layer
has been printed. Optionally, rotating plate 110 provides for
building in a crisscross fashion as is described in more detail
herein.
[0078] According to some embodiments of the present invention, ink
print head 102 and support material printhead 104 are mounted on a
Y axis stage 112. Optionally, each of ink print head 102 and
support material printhead 104 can be shifted and/or incremented in
the Y direction, e.g. along Y axis stage 112 when printing
different layers to avoid a ditch pattern forming on the
layers.
[0079] According to some embodiments of the present invention, a
pressure roller 118 is actuated once per several layers, e.g.
several tens of layers after drying in drying station 116.
Typically, pressure roller 118 is stationary in the X axis
direction but movable in the Z direction. Typically, building table
108 advances in X direction from drying station 116 toward roller
118 while roller 118 is lowered so that roller 118 engages a top
surface of layer(s) 50. Typically, as building table 108 continues
to advance in the scanning direction, ink print head 102 and
support material printhead 104 proceed to apply an additional
layer. Optionally, each of ink print head 102 and support material
printhead 104 are operated consecutively, e.g. first ink print head
102 is operated to scans the layer and then support material
printhead 104 is operated to scan the layer.
[0080] Typically, each of ink print head 102 and support material
printhead 104 include an array of nozzles. In some exemplary
embodiments, each layer is printed by alternatively scanning the
layer with the odd nozzles of each of the printheads, drying the
sub-layer and then repeating the printing and drying process with
the even nozzles of each of the printheads. Alternatively, the even
nozzles are used first and then followed by the odd nozzles. The
present inventor has found that a typical printing/drying cycle may
take around 2.5 sec. for a table size of 200 mm, so that a full
layer can be printed in about 5 sec. Optionally, a typical building
rate may be around 3.6 mm per hour. Optionally, the building rate
may vary between 2 mm/hour to 5 mm/hour depending table size and/or
other parameters.
[0081] Reference is now made to FIG. 2 showing a simplified
cross-sectional view of an exemplary direct inkjet printing system
including two drying stations in accordance with some embodiments
of the present invention. According to some embodiments of the
present invention, the building rate can be improved by including
two drying stations 116 in printing system 101, a first drying
station 116 on a right side of ink print head 102 and support
material printhead 104 along the scanning direction and a second
drying station 116 on a left side ink print head 102 and support
material printhead 104 along the scanning direction. Optionally,
building table 108 travels to the left as old nozzles of ink print
head 102 and support material printhead 104 are printing a first
portion of layer 50. Optionally, when the first portion is
completed, layer 50 with building table 108 is inserted into drying
station 116 on the left. According to some embodiments of the
present invention, after drying with the drying station 116 on the
left, building tray 108 travels toward the right in the scanning
direction as even nozzles of ink print head 102 and support
material printhead 104 print a the second portion of layer 50.
Optionally, when the second portion is completed, layer 50 with
building table 108 is inserted into drying station 116 on the
right. Typically, the layer is completed after the second portion
is printed and dried. This process is typically repeated to print
additional layers. It is noted that roller 118 is shown to be
positioned between printing heads 102 and 104 and drying station
116 on the right, since the complete layer is ready when exiting
drying station 116 on the right. Optionally, if the first portion
of the layer is printed while building table 108 moves toward the
right, roller 118 will be placed on the left. Alternatively, two
rollers 118 may be used, one next to each of the drying stations.
The present inventors have found that when using two drying
stations 116 as described herein, a typical cycle for a table size
of 200 mm may consume about 2.6 sec for a full layer. This
represents a building rate of about 6.9 mm per hour.
[0082] Reference is now made to FIG. 3 showing a simplified
schematic drawing of an exemplary drying station in accordance with
some embodiments of the present invention. According to some
embodiments of the present invention, drying station 116 provides
heat transfer to dry layers of the printed object by impinging jets
of hot air onto the layers. According to some embodiments of the
present invention, a blower 154 includes a blower inlet 156 for
sucking hot air from a housing 165 of drying station 116 and a
blower outlet 158 for blowing air into housing 165. Optionally,
blower 154 is capable of air transfer at a temperature of around
220.degree. C. as may typically be required for humectant
evaporation. In some exemplary embodiments, hot air is
substantially continuously circulated to reduce power consumption
of drying station 116 and also to contain the solvent vapor that is
formed within the drying station so that the vapor is not spread
into the surrounding environment of the printing system, e.g. the
room in which the printing system is stationed.
[0083] According to some embodiments of the present invention, air
that is blown into housing 165 through blower outlet 158 passes
through a fin based heat exchanger 160 which is electrically heated
by heating elements 162. According to some embodiments of the
present invention, hot air that leaves heat exchanger 160 is jetted
in high speed through an air nozzle plate 164 toward building table
108 when positioned in drying station 116. Optionally, nozzle plate
164 is equipped with an array of round nozzles, e.g. round nozzle
array and/or a slot array of nozzles.
[0084] According to some embodiments of the present invention, in
the absence of the building table 108, sliding cover and/or door
166 is closed and hot air blowing out through nozzle plate 164
cyclically impinges sliding cover 166 and is then sucked back to
blower inlet 156. According to some embodiments of the present
invention, as building table 108 starts to push sliding cover 166
open, the blower speed and/or flow rate is increased. In addition,
as sliding cover 166 is pushed open, fresh air penetrates into
housing 165 which increases the internal pressure of housing 165.
According to some embodiments of the present invention, the
increase in pressure causes air to flow through blower output 158
and out of louvers 170 to an air spreader 172. Typically loaded
louvers 170 are spring loaded and only open in response to pressure
applied by air flowing out of blower output 158. According to some
embodiments of the present invention, flow though louvers 170
releases a volume of humid air into the environment. Typically, the
air replacement provides for maintaining a steady dew point inside
drying station 116. In some exemplary embodiments, the volume of
humid air is vented through a dedicated valve and/or tubing to a
condenser to liquidize any accumulated vapors. In some exemplary
embodiments, the air in the drying station is also periodically
vented to avoid saturation of vapors accumulated in drying station
due to the drying process of the dispensed ink. Typically, vented
air is directed to a condenser to liquidize vapors so that the
vapors are not expelled into the surrounding environment of the
printing system.
[0085] According to some embodiments of the present invention, as
building table 108 (FIG. 1) advances toward drying station 116 and
pushes against sliding cover 166, sliding cover 166 slides open and
building table 108 enters drying station 116. Typically, as
building table 108 enters drying station 116 it begins to
decelerate, then stops and accelerates in the opposite direction to
exit from drying station 116. Typically, when building table 108 is
situated in station 116, a pair of springs 168 positioned on
opposite sides of sliding cover 166 slide cover 166 closed. In some
exemplary embodiments, the estimated drying time required and/or
period of time that building table 108 is positioned within drying
station 116 is typically in the range of 0.5 sec to 1 sec, e.g. 0.7
sec.
[0086] According to some embodiments of the present invention, the
layer that has been dried is maintained at a temperature between
50-70.degree. C. after the building table 108 exits drying station
108. Typically, due to the elevated temperature of the layer,
slight evaporation in a subsequent layer dispensed thereon is
achieved. Optionally, the elevated temperature additionally
provides for improving bonding with the subsequent layer. Typically
the temperature of the layer depends on the heat capacity of the
ink's particles. Optionally, external heat can be applied to
maintain the elevated temperature of the printed layer while the
subsequent layer is being dispensed.
[0087] Reference is now made to FIG. 4 showing a simplified flow
chart of an exemplary method for drying layers of printed material
in accordance with some embodiments of the present invention.
According to some embodiments of the present invention, over a
duration in which building table is outside of drying station 116,
a flow rate of blower 154 is maintained at a nominal rate (block
205) and heater 160 is maintained at a nominal temperature (block
210). According to some embodiments of the present invention, as
building table 108 is received (block 215) blower 154 accelerates
its flow rate (block 220) and fresh air is received. According to
some embodiments of the present invention, in response to the
increase in pressure due to accelerated flow rate and the incoming
fresh air, air exhaust is expelled through louvers 170 (block 225).
According to some embodiments of the present invention, blower 154
continues to operate at high speed over the duration that building
table 108 is inside drying station 116. According to some
embodiments of the present invention, once building table 108 exits
drying station 116 (block 230), the drying station waits to receive
sufficient air replacement (block 235) and then reduces the flow
rate of the blower to a nominal speed (205). Optionally, the heater
is maintained at constant temperature as long as the drying station
is operating.
[0088] Reference is now made to FIG. 5 showing a simplified
schematic drawing of an exemplary Y axis stage for supporting the
printheads in accordance with some embodiments of the present
invention. According to some embodiments of the present invention,
printheads 102 and 104 are mounted on Y axis stage 112 that
provides for shifting printheads 102 and 104 between a half pixel
position and a full pixel position. For example, for a printing
resolution of 260 dots per inch, Y axis stage 112 provides for
shifting the printheads by 35 micrometers intervals. Optionally, Y
axis stage 112 may provide for shifting of printheads 102 to more
than two positions per pixel. According to some embodiments of the
present invention, Y axis stage 112 includes rails 124, linear
bearings 126 and actuator 128. According to some embodiments of the
present invention, Y axis stage 112 operates to shift position of
printheads 102 and 104 back and forth by a half a pixel distance
for each subsequent layer printed. In some exemplary embodiments,
operation of Y axis stage 112 provides for smoothing out ditches
that typically form along the print lines. Optionally, operation of
Y axis stage 112 provides a tradeoff between number of printing
nozzles (price) and the model building rate (throughput).
Optionally, when number of nozzles per printhead is reduced by
half, a first half of the layer is printed by using all the
nozzles, after which the printheads are shifted by a half a pixel
distance and the second half of the layer is printed by using all
the nozzles. As such, a same pixel density may be achieved with
half the number of nozzles.
[0089] Reference is now made to FIG. 6 showing a simplified
schematic drawing of an exemplary rotatable building tray in
accordance with some embodiments of the present invention.
According to some embodiments of the present invention, a table
platform 132 is operable to rotate at 90 degree intervals about a
central axis 134 by a gear motor 136. According to some embodiments
of the present invention, building table 108 and/or table platform
132 include one or more stoppers 138 such that provide for aligning
edges of table platform 132. Optionally, differential screws are
used to obtain high accuracy in aligning the edges. According to
some embodiments of the present invention, table platform 132
includes a tray 140 on which an object 280 is built. Typically,
tray 140 is operable to be pulled out and transferred with object
280 to a sintering oven. Typically, tray 140 is made from a ceramic
material or other material that is suitable for placing in a
sintering oven. In some exemplary embodiments, tray 140 is
positioned on table platform 132 using slides 141. In some
exemplary embodiments, a plurality of spring loaded balls 142, e.g.
four balls are positioned on table platform 132 and near edges of
tray 140 and push tray 140 upwards so as to prevent any relative
motion of tray 140 during movement of building table 108 and/or
rotation of table platform 132. Optionally, balls 142 are
rotatable. Typically, the balls are made from ceramic material.
[0090] According to some embodiments of the present invention, tray
140 is covered with a mat 148 that is used as a primary hydrophilic
surface for absorbing water and/or solvent from the dispensed ink
so that the powder, particles and/or polymer is stabilized on mat
148 so that coalescence of the droplets can be avoided. Typically,
mat 148 is selected to provide a surface tension higher than a
surface tension of the ink. Optionally, mat 148 is inkjet paper
and/or a layer coated with receptive coating, e.g. PVP. Optionally,
mat 148 is separated from object 280 prior to sintering.
Optionally, mat 148 burns in temperatures of about 235.degree.
C.
[0091] Reference is now made to FIG. 7 showing a simplified
schematic drawing of an exemplary roller for flattening printed
layers in accordance with some embodiments of the present
invention. In some exemplary embodiments, roller unit 118 is fixed
on building table 108 and connected to frame 153 but includes a
roller 144 that is connected to a swing mechanism 146 so that
roller 144 can be pulled up by solenoid 151 and spring 149 while
not in use. Typically, roller 144 is mounted on building table 108
and extends over a width of tray 140. Typically, roller 144 is
lowered for flattening printed layers and is operated by passing
the printed layer below roller 144. In some exemplary embodiments,
while roller 144 is in use, two pre-aligned stoppers 150 mounted on
building table 108 support arms of swing mechanism 146 and maintain
roller 144 at a defined height above tray 140 (FIG. 6). Optionally,
roller 144 is operative to provide a relative large force while
avoiding nipping of the roller and/or surface responsive the
relative high speed of table while passing below roller 144.
Optionally, roller 144 provides a force of 50-300 N, e.g. 200 N for
flattening printed layers of an object positioned on a 200 mm
building table 108. Typically, the linear table speed is between
100 to 1000 mm/sec.
[0092] Reference is now made to FIGS. 8A and 8B showing simplified
schematic drawings of ink droplets dispensed from odd nozzles of a
printhead and/or with one pixel gaps shown in a perspective and
front view respectively and FIG. 9 showing the ink droplets
dispensed from odd nozzles after a drying process, shown in a front
view, all in accordance with some embodiments of the present
invention. According to some embodiments of the present invention,
printing heads 102 and 104 are operative to dispense lines of
contiguous droplets of ink 390 in successive manner along a
printing direction, e.g. along a direction of an X axis, e.g. with
a pixel separation between the droplets and to dispense droplets
390 in cross printing direction, e.g. along a direction of a Y axis
with a one pixel gap between droplets 390 and/or a two pixel
separation between droplets 390. In some exemplary embodiments, the
one pixel gap between droplets 390 is provided by printing with
either odd nozzles or even nozzles. Alternatively, the one pixel
gap is provided by printing with lower density array of nozzles.
According to some embodiments of the present invention, when
printheads are operative to print with one pixel gap between
droplets 390, a layer is completed in a two step process including
a first step of scanning with a one pixel gap in the cross scan
direction and then scanning again to fill in the gaps in the cross
scan direction.
[0093] Typically, size of droplets 390 is selected based on a
desired thickness of a printed layer. In some exemplary
embodiments, when a 5 micrometer layer is desired, droplets with a
volume of 80 pico-liter and a 53 micrometer diameter may be used.
Typically, size of the droplet 390 on the building surface, e.g.
tray 140 after drop landing may be around 85 micrometer, presenting
a dot ratio (the ratio between the dot and the drop diameters) of
around 1.6. Optionally, the height of the droplet will be around 14
micrometer. The present inventor has found that droplets 390
typically keep their surface dimensions, e.g. diameter in X-Y plane
due to the surface wetting and a loss of the volume due to drying
mainly influences the dimensions in the vertical axis, e.g. Z axis.
Typically, after the drying process 60% to 80% of the volume may be
evaporated due to water, humectant or solvent evaporation and a
height of a dried droplet 395 may be reduced by 3 to 5.7
micrometer, due to wetting and drying in drying station 116.
[0094] Reference is now made to FIGS. 10A and 10B are simplified
schematic drawings of additional ink droplets subsequently
dispensed from even nozzles of a printhead and/or with one pixel
gaps shown in a perspective and front view respectively and in
accordance with some embodiments of the present invention. The
present inventor has found that coalescence requirements between
adjacent or close droplets should be considered to avoid pattern
distortion and/or loss of detail in the object being printed. The
present inventor has also found that there is a difference in the
coalescence behavior of droplets dispensed in a successive manner
from a same nozzle to generate a line in a printing direction, e.g.
along a direction of an X axis and that of droplets dispensed in
successive passes of the printing heads in cross printing
direction, e.g. along a direction of a Y axis.
[0095] The present inventor has found that while the relatively
short interval time between successive dispensing of droplets in
the printing direction results in stabilized droplets, the
relatively long interval times between successive passes of the
printheads in the cross print direction can lead to distortion at
the seams between a line of droplets printed in a previous pass and
a new line of droplets. For example, a scan rate of about 10 KHz
results in an interval period of about 100 microseconds between
successive droplets in the printing direction, which is typically
sufficient for stabilizing successive droplets. Considering this
time frame, a new droplet is typically dispensed before a present
droplet coalesces with a previously dispensed droplet and the
presence of the new droplet bounds the mobility of the current
droplet. On the other hand, for different passes of the printing
head, the time interval between dispensing contiguous droplets can
be several milliseconds. In this case, the first set of droplets
may be fixed to their position due to the partial drying and the
next contiguous set of droplets dispensed along a border of the
previous pass may coalesce with the first set of droplets. If the
first set of droplets only appears on one side of the next
contiguous set, droplets along a border between passes will move
toward the fixed droplets and cause distortion. According to some
embodiments of the present invention, distortion is avoided by
printing the layers in a two step process. According to some
embodiments of the present invention, during a first step a first
set of droplets are dispensed with a pixel gap between the droplets
in the cross scan direction and dried to produce dried droplets
395. According to some embodiments of the present invention, during
a subsequent step, a second set of droplets 390 are printed within
the one pixel gaps established during the first step. The present
inventors have found that by printing the second set of droplets
between two dried droplets 395, the coalescence behavior of
droplets 390 is maintained symmetrical and the distortion is
avoided. According to some embodiments of the present invention,
the completed layer including both the first and second sets of
droplets is dried in the drying station before an additional layer
is printed. According to some embodiment of the present invention,
the layer is heated to 50-70.degree. C. during the drying process.
In some exemplary embodiments, heating of the layer is applied to
initiate a sufficient amount of evaporation and thereby reduce
mobility of droplets before the layer undergoes a drying process,
e.g. in drying station 116. The present inventor has found that
heating a layer to 50-70.degree. C. reduces mobility of droplets in
a successive layer without causing an adverse effect of significant
condensation on the nozzle plate of the printheads 102 and 104.
Optionally, an ambient temperature of the printing apparatus is
also heated to avoid destabilization of the droplets.
[0096] Reference is now made to FIGS. 11A and 11B showing
simplified schematic drawings of an exemplary complete printed
layer after a drying process, shown in a perspective and front view
respectively and in accordance with some embodiments of the present
invention. According to some embodiments of the present invention,
ditches and/or valleys 205 may form in a layer 210 between print
lines in the printing direction, e.g. X axis direction. Optionally,
for a 5 micrometer layer, a ditch with a depth of 1.5-2 micrometer
may form. Typically, when printing a successive layer, similar
ditches will occur and will be aligned with ditches of the current
layer. In addition, due to the non-uniformity of the surface of the
present layer, a subsequent layer may not completely fill ditches
in a current layer. The present inventor has found that this
irregularity in the building process can cause dimensional
instability, dimensional inaccuracy in Z axis and can also reduce
mechanical strength of the resulting object.
[0097] Reference is now made to FIG. 12 showing a simplified
schematic drawing of a plurality of exemplary printed layers
responsive to applying lateral shifting of the printhead in
accordance with some embodiments of the present invention.
According to some embodiments of the present invention, the
printheads 102 and 104 are shifted back and forth by a half a pixel
distance in the cross printing direction to reduce the effect of
ditch formation in the cross printing direction. Optionally, in a
second layer 220, ditches 205 are formed between ditches 205 formed
in a first layer 215. Shallower ditches 207 in a second layer for
example may also formed directly above ditches 205 in the first
layer. Typically, ditches 205 and 207 are less pronounced in layer
220 and subsequent even layers since these layers provide for
filling ditches 205 of the previous layer and are therefore
slightly thinner. In some exemplary embodiments, a half pixel shift
may cause distortion at the edge of the layer which may make the
effected edge of the printed object coarse and/or rough.
[0098] Reference is now made to FIG. 13 showing a simplified
schematic drawing of a plurality of exemplary printed layers
responsive to applying rotation of the building platform in
accordance with some embodiments of the present invention.
According to some embodiments of the present invention, an
accumulated ditch effect is avoided and/or reduced by rotating the
building platform in the X-Y plane by 90 degrees prior to printing
a subsequent layer. Optionally, rotation is applied after a
plurality of layers has been printed. According to some embodiments
of the present invention, rotation breaks the accumulated ditch
pattern and reduces the adverse effects along the Z axis, e.g.
adverse effect on the mechanical properties of the printed object.
Typically, rotation of the building platform by 90 degrees has the
advantage that it does not lead to coarse edges as discussed in
reference to FIG. 12.
[0099] According to some embodiments of the present invention, if a
nominal layer thickness of about 5.1 micrometer is obtained, after
printing for example 50 layers, roller 144 is applied to compress
height of the layers to 250 micrometer (average of 2% compression)
to reduce accumulated height error. Optionally a compression of
about 2% is applied. The present inventor has found that the
compression due to rolling may enhance bonding of the powder
particles. In the case of soluble polymer ink, the present inventor
has found that compression due to rolling may support better layer
adhesion. Optionally, for polymer ink, a lower compression during
rolling is desired, e.g. 1% compression. Optionally, printing
density used is 320 to 360 dpi for a full layer. Reference is now
made to FIG. 14 showing a simplified schematic drawing of an
exemplary maintenance station in accordance with some embodiments
of the present invention. According to some embodiments of the
present invention, a maintenance station 106 is positioned under
print heads 102 and 104 during idle periods of system 100.
According to some embodiments of the present invention, during idle
periods of system 100, positioning of maintenance station 106 is
controlled to align with one of a plurality of sub-stations of
maintenance station 106. In some exemplary embodiments, a first
substation is a spitting tray 176 that is operable to collect ink
that is dispensed during a drop spitting procedure and/or a
printhead purging procedure of the printhead(s). Typically, the
drop spitting procedure is required for slow-ink circulation
through the nozzle of the printhead to avoid nozzle drying,
crusting and clogging. In some exemplary embodiments, printhead
purging is initiated after long idle periods. Typically during a
printhead purging procedure, ink is pushed out through nozzles of
the printing head at high pressure to open nozzles that may be
blocked.
[0100] In some exemplary embodiments, a second sub-station 178 is a
misting station. According to some embodiments of the present
invention, during a misting procedure, a fine mist 180 of cleaning
fluid, e.g. water and detergents in case of water based ink, or
solvent in case of solvent base ink is sprayed onto the nozzle
plate of the printhead with a spraying nozzle 182. Optionally, the
nozzle plate is coated with hydrophobic coating. Typically, the
misting procedure is applied after a drop spitting procedure and/or
a printhead purging procedure while the nozzle plate is wet from
the dispensed ink.
[0101] In some exemplary embodiments, a third sub-station 184 is a
blotting and capping station. In some exemplary embodiments, this
sub-station extends toward the printing heads and engages blotting
paper 186 to the nozzle plate. Optionally blotting paper is firmly
attached to the nozzle plate and collects residual fluid on the
nozzle plate for cleaning the nozzle plate. Optionally, the nozzle
plate is capped with blotting paper during long idle periods in
which the ink is not removed from the system. Optionally, blotting
paper is replaced and/or replenished by rolling paper incrementally
from the supply roll 188 to the collecting roll 190 by the
gear-motor 192.
[0102] Reference is now made to FIG. 15 showing a simplified flow
chart of an exemplary method for printing a 3D object in accordance
with some embodiments of the present invention. According to some
embodiments of the present invention, a printing procedure is
activated after displacing the maintenance station 106 from the
scanning area of the building table (block 405). According to some
embodiments of the present invention, a new layer is printed (block
410) by first scanning the layer with odd nozzles of ink print head
102 (block 415). Alternatively, the layer is scanned with a
printhead including a sparse set of nozzles, e.g. nozzle set
reduced nozzle density that provide the one pixel gap and all the
nozzles are used. Optionally, more than one printhead is used for
printing and the printheads are operated in a consecutive manner.
Optionally, the layer, e.g. the incomplete layer is dried (block
420). Typically, drying is applied to dry water, humectants and/or
solvents in the printing layer and/or also to activate a binder if
present in the ink. According to some embodiments of the present
invention, the same layer is scanned again with even nozzles (block
425). Optionally, scanning with even nozzles is performed in the
opposite direction to the scanning with the odd nozzles.
Alternatively, the printhead(s) is shifted laterally by a pixel
length and gaps formed by missing pixels in the previous scanning
are filled. Typically, drying is applied again to dry the remaining
part of the layer (block 430). According to some embodiments of the
present invention, support material printhead 104 scans the layer
after the ink material has been dispensed and dried (block
432).
[0103] In some exemplary embodiments, before printing a subsequent
layer, the present layer is either rotated by 90 degrees and/or the
printhead(s) are shifted by a half a pixel distance to avoid the
ditch pattern (block 435). If a threshold number of layers have not
been completed, a new layer is applied (block 440). Alternatively,
when the threshold number of layers has been reached, the layers
are flattened with roller 144 prior to adding the next layer (block
445). Typically, when building with ceramic and/or metal ink, the
object is sintered as a final stage after all the layers have been
applied.
Sintering and Final Product
[0104] According to some embodiments of the present invention, as a
final step, an object that has been fabricated by the printing
process with ceramic and/or metal ink is inserted into a sintering
chamber. The present inventor has found that the contribution of
the binder to the shrinkage of the model, due to its burning during
the sintering process is .sup.3 (7%) or 1.9% or less. The present
inventor has found that the relatively low concentration of the
binder in the ink as described herein below in more detail allows
"particle to particle" engagement even while the binder is present.
The present inventor has also found that even though there is
little external pressure while building during occasional
flattening with the roller, the porosity resulting from binder
removal is minimal and may have little or no negligible effect on
the mechanical properties of the final object.
Ink Circulation System
[0105] Reference is now made to FIG. 16 showing a simplified
schematic diagram of an ink circulation unit with replaceable ink
cassette in accordance with some embodiments of the present
invention. According to some embodiments of the present invention,
the ink circulation in the printheads 102 and 104 can reach flow of
50 cc/min. The present inventor has found that this flow rate can
be served by gravitational controlled ink delivery as described
herein. According to some embodiments of the present invention, ink
delivery system 600 includes two ink tanks, an upper tank 198 and
lower tank 500. Typically, both tanks are connected by a delivery
line 502 that includes at least a tube 504 that is connected to a
peristaltic pump 505. Typically, both tanks are ventilated by air
ports 506 and the level is controlled by two float sets 508.
According to some embodiments of the present invention, the ink
circulation rate when assuming constant viscosity and surface
tension values is defined by the total pressure difference .DELTA.P
between the upper tank 198 and lower tank 500 and the local
pressure difference .delta.p between the upper print head and the
printhead 102 or 104. Typically, the total and local pressure
differences are constant for a specified cartridge design.
Typically, upper tank 198 and lower tank 500 together with delivery
line 502 and peristaltic pump 505 are housed in a replaceable
cartridge 550. Typically, cartridge 550 is fluidly connected to
printhead 102 or 104 via a plurality of quick connections 534 that
can be engaged and/or detached with a handle 536 of cartridge
550.
[0106] According to some embodiments of the present invention, a
feed line 510 travels from upper tank 198 via an inlet valve 512,
through a purging device 514, to printhead 102 or 104. The purging
device is described in detail herein for example in reference to
FIG. 18. According to some embodiments of the present invention, a
return line 516 leaves printhead 102 or 104 via an outlet valve 518
to lower tank 500. According to some embodiments of the present
invention, ink is transferred by a pump 522 from upper tank 198,
through a porous "lung" 524 and back to upper tank 198. In some
exemplary embodiments, dissolved air is removed by a vacuum pump
526 and released through a vent 528.
[0107] According to some embodiments of the present invention,
during cartridge replacement, this line passing through porous
"lung" 524 can be drained by changing ports of the universal 3/2
valve 530 from an "ink to ink" state to "air to ink" state using a
ventilation line 532. Typically during "air to ink" state, pump 522
sucks air (instead of ink) and pushes the ink back to upper tank
198.
[0108] Reference is now made to FIG. 17 showing a simplified
schematic diagram of an ink circulation unit with replaceable
cleaning cassette in accordance with some embodiments of the
present invention. According to some embodiments of the present
invention, cartridge 550 (FIG. 16) is operable to be replaced by a
cleaning fluid cartridge 650 during non-operational periods and/or
prior to replacing the existing cartridge 550 with another
cartridge, e.g. with different color shade and/or ink type.
Typically, the ink circulation with cleaning fluid cartridge 650 is
similar to that described in reference to FIG. 16. According to
some embodiments of the present invention, lower tank 500 for
cleaning fluid cartridge 650 is larger than that used for ink
cartridges and includes a port 546 for adding cleaning fluid as
required. According to some embodiments of the present invention, a
return line 116 is connected to a draining port and cleaning fluid
together with ink flowing to printheads 102 or 104 are drained to
clean the system.
[0109] Reference is now made to FIG. 18 showing a simplified
schematic diagram of a purging device for cleaning a nozzle of the
printhead in accordance with some embodiments of the present
invention. According to some embodiments of the present invention,
the ink circulation unit includes a dedicated purging device 514.
Typically, dedicated purging device 514 is required since
gravitation based circulation does not include a pump that can be
applied for purging. According to some embodiments of the present
invention, purging device includes a pair of rollers 542 that are
operable to squeeze a portion of line 510 made from tubing that can
typically be used with a peristaltic pump. According to some
embodiments of the present invention, during a purging procedure,
rollers 542 are driven in closed track along line 510 squeezing the
tube over a given length and thereby generating a purge process.
Typically, inlet valve 512 is maintained closed until rollers 542
substantially reach the given length at which point valve 512 is
opened to expel the ink and complete the purging procedure.
[0110] Typically, during printing, rollers 542 are not attached to
line 510. In some exemplary embodiments, purging device 514 is also
used for line priming when feed line 510 is empty. Optionally, in
this case purging device 514 is operated as a peristaltic pump for
initiate suction for line filling.
Ink
[0111] The present inventor has found that the ability to use water
based ink that contains high volume percentage of high density
pigments, particles and/or content such as ceramic and/or metal
particles provides an option to fabricate three dimensional models
using the described printing process. An exemplary ceramic ink,
that may be suitable for direct inkjet printing as described herein
is presented in Table 1.
TABLE-US-00001 TABLE 1 Typical composition of ceramic water based
ink The component % in volume % in weight The function Zirconia
powder, 29% 63% Building material 250 nm average particle size.
Propylene glycol 25% 12.5%.sup. humectants H2O 35% 10% Carrier
Binder of a polymeric 7% 3.5% Binding the dry resin and film.
neutralizing composition Poly-acrylic acid 0.7% dispersant
dispersant Non-Silicon surfactant 0.3% Surface tension
controller
[0112] According to some embodiments of the present invention, a
metal ink can be formulated by encapsulating metal micro particles
in a pH sensitive polymer matrix and replacing the Zirconia powder
with the encapsulated metal micro particles. Optionally,
encapsulating is achieved with a suspension containing the polymer.
Optionally, metal micro particles that are smaller than 1
micrometer are used.
[0113] According to some embodiments of the present invention, a
polymer ink is formulated with polyamide based polymer, e.g. Nylon
6 and/or Nylon 66. Optionally, water is used as a solvent.
Optionally, water is used together with co-solvent, e.g. ethanol or
with an additional solvent that has a high boiling point, or high
solvency power. Typically, the additional solvent is used to
increase the ink stability over time. An exemplary water based
polymer ink is presented in Table 2.
TABLE-US-00002 TABLE 2 Exemplary water based soluble polymer ink.
The component % in volume % in weight The function Nylon 6
flakes/water 18-20% .sup. 21% Building material soluble polymer
Propylene glycol 10% 12.5%.sup. humectants H2O 50% 50% Primary
solvent Ethanol 15% 12% Secondary solvent Furfuryl Alcohol 5-7%
4.5% stabilizer Non-Silicon surfactant 0.3% Surface tension
controller
[0114] According to some embodiments of the present invention, the
water based ink has relatively low boiling point. Optionally, when
using this ink the printhead temperature is maintained below
35.degree. C. and humectants are typically added. Typically, the
addition of humectants necessitates adding more polymer which
typically increases the viscosity of the ink. In some exemplary
embodiments, ink viscosity is limited to around 20 cps due to the
printhead jetting capabilities. Optionally, if the viscosity is too
high, an alternate solution can be used in place of water.
[0115] According to some embodiments of the present invention,
solvents, e.g. organic solvents with a relatively high boiling
point, e.g. greater than 150.degree. C. are used. Optionally, when
using solvents with higher boiling points, the printhead
temperature can be increased, e.g. increased to around 75.degree.
C. and the addition of humectants may not be required. As such the
viscosity of this polymer ink can typically be lower than water
based polymer inks. An example an organic solvent based polymer ink
is presented in Table 3.
TABLE-US-00003 TABLE 3 Organic solvent based soluble polymer ink
The component % in volume % in weight The function Nylon 6 flakes/
45% 50% Building material organic solvent soluble polymer NMP
(N-Methyl-2- 45% 40% Primary solvent pyrrolidone) Limonene 10% 10%
Secondary solvent Non-Silicon surfactant 0.3% Surface tension
controller
Support Material
[0116] According to some embodiments of the present invention,
support material is required to hold overhanging or negative angled
surfaces of the object e.g. surfaces that are not supported by the
building table or a previously formed layer. Due to external
forces, like gravity or the pressure roller, these surfaces may
collapse. According to some embodiments of the present invention,
support material is printed and/or dispensed in a layer wise manner
similar to the manner in which the building material is printed
and/or dispensed. Optionally, the support material can be applied
after a layer of building material is completed, e.g. after an odd
printing; first drying; even printing; second drying process.
Alternatively, the support material can be dispensed together with
the building material. Typically, the height of the support
material droplets is required to match the height of the building
material droplets. In some exemplary embodiments, wax is used as
the support material. Optionally, the wax can be applied using a
phase change material printhead similar to Xerox Phaser technology
provided by Xerox and as described for example in Canadian Patent
Application Publication No. CA2355533 entitled "Colorant compounds,
phase change ink composition and methods of printing," the contents
of which is incorporated herein by reference. In some exemplary
embodiments, a drop volume of about 30-45 pico-liter is used for a
5 micrometer layer. In some exemplary embodiments, roller 144 is
required to be used every layer to reduce the height of the support
material to the correct layer height.
[0117] In some alternative exemplary embodiments, the support
material is formulated with similar solvent composition and
substantially same volume of solids as the building material ink.
An exemplary support material ink, that may be suitable for direct
inkjet printing as described herein is presented in Table 4.
TABLE-US-00004 TABLE 4 support material ink having liquid "solids"
The component % in volume The function Glycerol/PEG300 36% Support
H2O 63% Carrier Poly-acrylic acid 0.7% Dispersant dispersant
Non-Silicon surfactant 0.3% Surface tension controller
[0118] Optionally, support ink composition for a solvent based ink
is configured based on a similar concept. In some exemplary
embodiment, printhead 104 scans the layer after printhead 102.
Typically the support material does not evaporate in the drying
process in drying station 116 and is maintained intact. Optionally,
when the support layer(s) is not sufficient for supporting,
additional supporting columns may be added. Optionally, cages are
built from the building material to support and/or avoid spilling
of the support material.
[0119] In some alternative embodiments, support material is
formulated from self crystallizing materials soluble in the water
and/or solvent, e.g. such as sugar based solutions. Typically, self
crystallizing materials can with withstand the fast drying process
applied in the drying station. In some exemplary embodiments,
binder is added to ink formulated from self crystalline materials
to avoid powder spreading while drying the next model layer.
[0120] It is appreciated that certain features of the invention,
which are, for clarity, described in the context of separate
embodiments, may also be provided in combination in a single
embodiment. Conversely, various features of the invention, which
are, for brevity, described in the context of a single embodiment,
may also be provided separately or in any suitable subcombination
or as suitable in any other described embodiment of the invention.
Certain features described in the context of various embodiments
are not to be considered essential features of those embodiments,
unless the embodiment is inoperative without those elements.
* * * * *