CA2976782A1

CA2976782A1 - Metal 3d printing method and metallic 3d printing materials

Info

Publication number: CA2976782A1
Application number: CA2976782A
Authority: CA
Inventors: Chao Xu; Daniel Therriault; Louis LABERGE LEBEL; Gilles L'esperance; Arslane Bouchemit
Original assignee: Lesperance Gilles
Current assignee: Lesperance Gilles
Priority date: 2017-08-16
Filing date: 2017-08-16
Publication date: 2019-02-16

Abstract

A metallic ink for solvent-cast 3D printing, the ink comprising a solution or a gel of a polymer in a volatile solvent, and heat-sinterable metallic particles dispersed in the solution or gel, wherein the particles are present in a particles:polymer weight ratio of more than about 85:15, is provided. There is also provided a method of manufacturing this ink and a method of manufacturing a solvent-cast metallic 3D printed material using this ink.

Description

Metal 30 Printing Method and Metallic 3D Printed Materials CROSS REFERENCE TO RELATED APPLICATIONS
N/A
FIELD OF THE INVENTION
[0001]
The present invention relates to the 3D printing of metallic materials. More specifically, the present invention is concerned with solvent-cast 3D printing of metallic inks, followed by sintering.
BACKGROUND OF THE INVENTION

[0002]
Metallic structures fabricated by three-dimensional (3D) printing are progressively more used in medical (e.g., artificial bones), microwave (e.g., antennas), and microelectro-mechanical systems (MEMS) fields (e.g., sensors and micro-electrodes). These applications benefit from the high mechanical, electrical and electromagnetic properties of metals, and the design freedom, mass customization, and ease of use related to 3D printing. Selective laser sintering (SLS) and selective laser melting (SLM) are commonly used for the fabrication of 3D metallic structures of various sizes. The main techniques use a laser beam to deliver energy on the surface of a powder bed in order to activate bonding between the powder particles. The bonds can be obtained by metallic sintering, metallic melting and binder melting. Typical materials used are titanium alloys, while a few studies reported the SLS of steel.
For example, McAlea et al. reported SLS with polymer-coated steel powders (i.e., 1080 steel, 316 or 420 stainless steel particles coated with thermoplastic polymer blend) followed by a bronze infiltration process. The Young's modulus of manufactured part was 193 GPa.16 These high temperature fabrication processes require costly laser systems, a large amount of powder to form the powder bed, and several operator protection setups. Moreover, the numerous heat cycles due to repeated laser strikes change the mechanical and chemical properties of the loose powder neighboring the structure's span. As a result, some of the powders cannot be reused. Furthermore, the mechanical properties of SLS/SLM processed parts are usually limited by metallurgical defects such as porosities, cracking, oxide inclusions and loss of alloying elements.

[0003]
Solvent-cast 3D printing is an alternative approach that creates microstructures by depositing a liquid ink on a substrate, layer-by-layer, and even in freeform. Inks contain a volatile solvent that evaporates rapidly after extrusion from the deposition nozzle, leading to a rigid filamentary structure. Various ink solutions have been developed for solvent-cast 3D printing. Ink functions, such as conductivity and mechanical properties, are created by adding micro- or nano- fillers including carbon nanotubes (CNTs), nano-clays and metal microparticles. Woo Jin Hyun et al. reported the screen printing of 2D filaments with metallic ink loaded with silver nanoparticles synthesized from silver nitrate solution. The electrical conductivity of the as-printed ink filament was 1.8 x S/m.30 Eunji Hong et al. reported the solvent-cast printing of titanium ink filaments to build two-dimensional (2D) lattices. The as-printed 2D lattices were rolled into scrolls and heat-treated in a vacuum furnace. The compression yield strength of the scrolls after heat-treatments was 735 MPa.31 These two methods provide structures with good mechanical properties or electrical conductivity, but are mainly limited to thin 2D geometries.
Another work by Skylar-Scott et al. presented a laser-assisted sintering of an ink filament to create 3D freeform metallic structures.32 While this method is used to print complex wire-type geometries, it is limited to a few compatible materials, susceptible to oxidation and porosity, and is unable to fabricate mechanically strong and highly dense structures.
The low-cost 3D printing of highly dense metallic structures featuring complex geometries still represent a significant scientific and technological challenge.
SUMMARY OF THE INVENTION

[0004] In accordance with the present invention, there is provided:
1. A metallic ink for solvent-cast 3D printing, the ink comprising:
= a solution or a gel of a polymer in a volatile solvent, and = heat-sinterable metallic particles dispersed in the solution or gel, wherein the particles are present in a particles:polymer weight ratio of more than about 85:15.
2. The ink of item 1, wherein the particles are present in a carbon particles:polymer weight ratio of:
= about 86:14, about 87:13, about 88:12, about 89:11, about 90:10, about 91:9, about 92:8, about 93:7, about 94:6, or about 95:5 or more, and/or = about 99:1, about 98:2, about 97:3, about 96:4, about 95:5, about 94:6, about 93:7, about 92:8, about 91:9, or about 90:10, or less.
3. The ink of item 1 or 2, wherein the particles are present in a carbon particles:polymer weight ratio between about 90:10 to about 95:5.
4. The ink of any one of items 1 to 3, wherein the particles are present in a carbon particles:polymer weight ratio of about 95:5.

5. The ink of any one of items 1 to 4, wherein the heat-sinterable metallic particles are steel, cast iron, titanium, silver, copper, zinc, gold, platinum, aluminum, nickel, bronze, or brass particles.

6. The ink of any one of items 1 to 5, wherein the heat-sinterable metallic particles are steel particles.

7. The ink of any one of items 1 to 6, wherein the heat-sinterable metallic particles are microparticles.

8. The ink of any one of items 1 to 7, wherein the heat-sinterable metallic particles are between about 0.1 pm and about 100 pm in size.

9. The ink of any one of items 1 to 8, wherein the heat-sinterable metallic particles are between about 5 pm and about 50 pm in size.

10. The ink of any one of items 1 to 9, wherein the heat-sinterable metallic particles are spheroidal.

11. The ink of any one of items 1 to 10, wherein the heat-sinterable metallic particles are spherical.

12. The ink of any one of items 1 to 11, comprising between about 10 and about 50 w/w% of the solvent (based on the total weight of the ink).

13. The ink of any one of items 1 to 12, wherein the polymer is poly(lactic acid), polystyrene, poly(methyl acrylate), poly(methyl methacrylate), poly(n-butyl acrylate), poly(2-hydroxyethyl methacrylate), poly(glycidyl methacrylate), poly(acrylic acid), poly(N-N-dimethylacrylamide), poly( 1-vinyl anthracene), poly(2-vinyl pyridine), poly(4-vinyl pyridine), poly(N-vinyl carbazole), poly(N-vinyl carbazole), poly(N-vinyl imidazole), poly(vinyl benzyl chloride), poly(4-vinyl benzoic acid), poly(vinyl acetate), polycaprolactone, poly(1144-(4-butylphenylazo)phenoxyFundecyl methacrylate) (poly(AzoMA)), poly(ferrocenyldimethylsilane), polyisoprene, polybutadiene, polyisobutylene, poly propylene glycol, poly(ethylene glycol), or a polysaccharide, or a mixture thereof.

14. The ink of any one of items 1 to 13, wherein the polymer is poly(lactic acid).

15. The ink of any one of items 1 to 14, wherein the solvent is dichloromethane (DCM), chloroform (CHCI3), tetrahydrofuran (THF), acetone, methanol (Me0H), ethanol (Et0H), or water.

16. The ink of any one of items 1 to 15, wherein the solvent is dichloromethane, chloroform, tetrahydrofuran, acetone, methanol, or ethanol.

17. The ink of any one of items 1 to 16, wherein the solvent is dichloromethane.

18. The ink of any one of items 1 to 17, wherein the ink further comprises one or more additive.

19. A 3D printer ink cartridge, the cartridge comprising a container having an ink outlet, the container comprising the ink of any one of any one of items 1 to 18.

20. The cartridge of item 19, wherein the cartridge is adapted to be installed on a 3D printer.

21. The cartridge of item 19 or 20, wherein the cartridge is adapted to be fitted to a nozzle for delivering the ink, so that, for ink dispensing, the ink is extruded through the ink outlet and through the nozzle.

22. The cartridge of any one of items 19 to 21, wherein the cartridge is designed so that when a pressure is applied by a 3D printer, the ink is extruded through the ink outlet.

23. A method of method of manufacture of the ink of any one of items 1 to 18, the method comprising the steps of:
a) providing a solution or a gel of the polymer in the solvent, b) providing heat-sinterable metallic particles in a particles:polymer weight ratio of more than about 85:15, c) dispersing the particles in the solution or gel of the polymer, thereby producing the ink.

24. The method of item 23, wherein step a) comprises mixing the polymer in the solvent until the polymer is dissolved or the gel is formed.

25. The method of item 23 or 24, wherein the polymer concentration of the solution or gel is:
= about 2 wt%, about 3 wt%, about 4 wt%, about 5 wt%, about 6 wt%, about 7 wt%, about 8 wt%, about 9 wt%, or about 10 wt% or more and/or = about 30 wt%, about 35 wt%, about 20 wt%, about 15 wt%, or about 10 wt%
or less, based on the total weight of the solution or gel.

26. The method of any one of items 23 to 25, wherein the polymer concentration of the solution or gel is about 20wr/o, based on the total weight of the solution or gel.

27. The method of any one of items 23 to 26, wherein the particles are dispersed by ball milling in step c).

28. The method of any one of items 23 to 27, further comprising adding solvent, or removing part of the solvent.

29. The method of any one of items 23 to 28, further comprising adding one or more additives to:
= the solvent before it is used to form the solution or gel of the polymer, = the solution or gel of the polymer, and/or = the ink.

30. The method of item any one of items 23 to 29, further comprising the step of packaging the ink in a 3D printer ink cartridge.

31. A method of manufacturing a solvent-cast metallic 3D printed material, the method comprising the steps of:
a) providing the metallic ink for solvent-cast 3D printing of any one of items 1 to 18, b) using a 3D printer, extruding the ink through a nozzle into a controlled pattern;
c) allowing solvent evaporation, thereby producing a printed material;
d) removing the polymer from the printed material by heating the printed material to a polymer degradation temperature or above, thereby leaving the particles arranged into the controlled pattern;
and e) heat-sintering the particles, thereby producing the solvent-cast metallic 3D printed material.

32. The method of item 31, wherein step a) includes the method of any one of items 23 to 30.

33. The method of item 32 or 32, wherein step b) is carried out at about room temperature.

34. The method of any one of items 31 to 33, wherein step c) is partly or completely carried out at about room temperature.

35. The method of any one of items 31 to 34, wherein steps d) and e) are performed in a single heat treatment.

36. The method of item 35, wherein the heat treatment comprises increasing the temperature to a sintering temperature and then holding the temperature at the sintering temperature.

37. The method of item 35 or 36, wherein a heating rate up to a temperature T between:
= about the polymer degradation temperature and = up to about 100 C above the polymer degradation temperature, is lower than a heating rate from the temperature T to the sintering temperature.

38. The method of item 37, wherein the heating rate up to the temperature T
is from about 1 to about 5 C/min.

39. The method of item 37 or 38, wherein the heating rate from the temperature T to the sintering temperature is from about 7 to about 15 C/min.

40. The method of any one of items 31 to 39, wherein a sintering temperature in step e) is between:
= about 400 C below the melting point of the particles and = about 200 C below the melting point of the particles.

41. The method of any one of items 31 to 40, wherein a sintering time is from about 30 minutes to about 12 h.

42. The method of any one of items 31 to 41, wherein a sintering time is from about 6 h to about 12 h.

43. The method of any one of items 31 to 41, wherein a sintering time is from about 30 minutes to about 6 h.

44. The method of any one of items 31 to 43, wherein step d) and e) are carried out in an inert atmosphere.

45. The method of any one of items 31 to 44, further comprising the step f) of partly or completely filling the pores created in-between the particles by the removal of the polymer in step d) with a second metal or alloy, the second metal or alloy having a melting point lower than the melting point of the metal or alloy constituting the particles.

46. The method of item 45, wherein the second metal or alloy is steel, cast iron, titanium, silver, copper, zinc, gold, platinum, aluminum, nickel, bronze, or brass

47. The method of item 45 or 46, wherein the second metal or alloy is copper.

48. The method of any one of items 45 to 47, wherein step f) comprises contacting part of the solvent-cast metallic 3D printed material with the second metal or alloy, the second metal or alloy being in the molten state, and allowing the second metal or alloy to diffuse by capillarity into the pores.

49. The method of any one of items 45 to 48, wherein step f) comprises placing a piece of the second metal or alloy on top of the solvent-cast metallic 3D printed material and then heating at a temperature above the melting point of the second metal or alloy.

50. The method of any one of items 45 to 49, wherein an excess of the second metal or alloy is used.

51. The method of any one of items 45 to 50, wherein step f) is carried out in an inert atmosphere.

52. A solvent-cast metallic 3D printed material, shaped into a controlled pattern and made of particles of a metal or alloy peripherally attached to one another

53. The material of item 52, wherein the pores in-between the particles are completely or partially filled with a second metal or alloy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] In the appended drawings:
Fig. 1 is a schematic of the fabrication process of a 3D metallic scaffold combining (a) solvent-cast 3D printing: the metallic ink is extruded through a micronozzle and the solvent evaporates right after extrusion at room temperature, (b) sintering: the as-printed scaffold is heated to burn the polymer away and sinter the HAS microparticles, and (c) copper infiltration: the sintered scaffold is heated again with a piece of copper placed on top of it.
Fig 2 shows the temperature profiles used during sintering and copper infiltration.
Fig. 3 is a secondary electron micrograph of highly alloyed steel (HAS) powder particles ( 20 pm).
Fig. 4 show SEM images of as-printed 20-layers scaffolds of different concentrated inks (85, 90, 95, 98 wt.%) (first row from left to right) and their close-up view images (middle and bottom rows).
Fig. 5 show SEM images of sintered 20-layers scaffolds of different concentrated inks (85, 90, 95, 98 wt.%) (first row from left to right) and their close-up view images (middle and bottom rows).
Fig. 6 SEM images of copper infiltrated 20-layers scaffolds of different concentrated inks (90, 95, 98 wt.%) (first row from left to right) and their close-up view images (middle and bottom rows).
Fig. 7 shows optical images of structures printed with 95 wt.% HAS/PLA ink through 250pm inner diameter tapered nozzle, printing speed of 10 mm/s: (a) printing process of a tensile bar sample (inset: a tensile bar and five 20-layer scaffolds placed on a Canadian dollar coin), (b) top, side and oblique views of a 20-layer scaffold, (c) a planetary gear with a Canadian 5 cents coin, and (d) an as-printed replica of the Olympic stadium of Montreal with a photograph of the stadium for comparison.
Fig. 8 shows optical (left) and SEM images and their close-up views (right) of 20-layer scaffolds printed using 95 wt.% ink and 250 pm inner tapered nozzle, printing speed of 10 mm/s: (a) as-printed, (b) sintered, and (c) copper infiltrated. The optical image on the left shows the scaffolds on top of a Canadian 25-cent coin.
Fig. 9 shows the TGA results of 95 wt.% HAS/PLA scaffold. The temperature was raised from 20 C to 500 C at a rate of 1 C/min (the same heating rate as the sintering process). The degradation of PLA finished before 225 C.
Fig. 10 shows the porosity of the filament in the sintered and copper infiltrated 20-layer scaffolds for different ink concentrations, and optical microscope images of the polished cross sections at 0.5H (scale bar: 50 pm). Error bars indicate the standard deviations obtained from five samples.
Fig. 11 shows (a) the tensile mechanical response for as-printed, sintered and copper infiltrated 3D printed tensile bars and (b) to (e) the tensile fracture surfaces of sintered and copper infiltrated tensile bars at (b, c) low and (d, e) high magnification, respectively. Error bars indicate the standard deviations obtained from three samples.
DETAILED DESCRIPTION OF THE INVENTION
[0006] The present invention relates to solvent-cast 3D printing.
[0007] In solvent-cast 3D printing, an ink containing a volatile solvent is deposited in a controlled pattern using a 3D printer. A 3D printer is a computer-controlled robot that is able to create a 3D object, usually from a model designed by a computer aided design (CAD), by laying down successive thin layers of a 3D printing ink. To make a solvent-cast 3D printed structure, the ink is extruded through a moving micronozzle, thereby depositing the ink in the desired pattern. Usually, this pattern is multilayered. After extrusion, the solvent from the ink usually quickly evaporates (generally at room temperature) producing a solid 3D printed structure.
[0008] This 3D printed structure can be further modified via sintering followed or not by metal infiltration.
Metallic Ink for Solvent-Cast 3D Printing [0009] Turning now to the invention in more details, there is provided a metallic ink for solvent-cast 3D printing, the ink comprising:
= a solution or a gel of a polymer in a volatile solvent, and = heat-sinterable metallic particles dispersed in the solution or gel, wherein the particles are present in a particles:polymer weight ratio of more than about 85:15.
[0010] Herein, a "metallic ink for solvent-cast 3D printing" is an ink that is useful for manufacturing a 3D printed metallic material by solvent-cast 3D-printing.
[0011] As noted above, the ink comprises the particles and the polymer in a certain weight ratio range. For certainty, this weight ratio is expressed as follows: weight ratio = weight of metallic particles :
weight of polymer. A ratio of 85:15 thus means that the ink comprises 85 wt%
of the particles and 15 wt% of the polymer, both percentages being based of the total weight of the polymer and the particles (i.e. excluding the weight of the solvent and any other potential additives).
[0012] In embodiments, the particles are present in a carbon particles:polymer weight ratio of:
= about 86:14, about 87:13, about 88:12, about 89:11, about 90:10, about 91:9, about 92:8, about 93:7, about 94:6, or about 95:5 or more and/or = about 99:1, about 98:2, about 97:3, about 96:4, about 95:5, about 94:6, about 93:7, about 92:8, about 91:9, or about 90:10, or less.
In embodiments, the particles are present in a particles:polymer weight ratio of about 86:14, about 87:13, about 88:12, about 89:11, about 90:10, about 91:9, about 92:8, about 93:7, about 94:6, about 95:5, about 96:4, about 97:3, about 98:2, or about 99:1. In preferred embodiments, the particles are present in a particles:polymer weight ratio between about 90:10 to about 95:5, more preferably about 95:5.
[0013] Hererin, "heat-sinterable metallic particles" are particles made of one or more metal or alloy that are capable of being sintered by heat. Heat sintering is a process of compacting and forming a solid structure made of a material by heating without melting the material to the point of liquefaction.
Hence, sintering involves the heating of the material to a temperature near, but not reaching, its melting point for a time sufficient for the material to become compact and form a solid mass. When applied to the present heat-sinterable metallic particles, sintering results in the fusion of the particles via "necks" formed between the particles. In other words, the particles do not coalesce, but rather peripherally attach to one another.
[0014] Non-limiting examples of heat-sinterable metallic particles include particles of the metal of groups 3 (including the lanthanides and the actinides) to 16 of the periodic table and their alloys. Of note, the elements B, C, Si, Ge, N, P, As, Sb, 0, S, Se, Te, F, Cl, Br, and I, while being part of groups 12 to 16 of the periodic table are not metals and thus are not "metals of groups 3 to 16 of the periodic table". More specific examples of metals and alloys include steel, cast iron, titanium, silver, copper, zinc, gold, platinum, aluminum, nickel, bronze, and brass. Preferably, the heat-sinterable metallic particles are particles of steel or titanium, more preferably steel. A preferred steel is a high-alloy steel (HAS), i.e. a steel containing more than about 4 w/w% of alloyants other than carbon.
[0015] The heat-sinterable metallic particles are preferably microparticles. Herein, microparticles are particles between about 0.1 pm and about 200 pm and in size. Preferably, the heat-sinterable metallic particles have preferably a size between about 0.1 pm and about 100 pm, more preferably between about 5 and about 50 pm, and most preferably with a size of about 20 pm. Typically, smaller particles are preferred to minimize clogging in fine nozzles, however they may require higher extrusion pressures during printing.
Of note, when using larger particles, the nozzle used for 3D printing must have an inner diameter large enough to accommodate the particles and also the surface of the 3D printed material may be rougher.
[0016] The heat-sinterable metallic particles can be of any shape regular or irregular. Preferably, the particles are spheroidal, and more preferably spherical. Indeed, smooth spheroidal/spherical shapes tend to minimize the area/volume ratio, which reduces the friction between the particles and facilitates the extrusion. Herein, the terms "spheroidal" and "spherical" are not limited to perfectly spheroidal/spherical particles, but rather also encompass particles that present irregularities while being substantially spheroidal/spherical in shape.

[0017] The polymer is a polymer that is soluble or that forms a gel, preferably at room temperature, in the solvent.
In embodiments, the polymer is poly(lactic acid), polystyrene, poly(methyl acrylate), poly(methyl methacrylate), poly(n-butyl acrylate), poly(2-hydroxyethyl methacrylate), poly(glycidyl methacrylate), poly(acrylic acid), poly(N-N-dimethylacrylamide), poly(1-vinyl anthracene), poly(2-vinyl pyridine), poly(4-vinyl pyridine), poly(N-vinyl carbazole), poly(N-vinyl carbazole), poly(N-vinyl imidazole), poly(vinyl benzyl chloride), poly(4-vinyl benzoic acid), poly(vinyl acetate), polycaprolactone, poly(11-[4-(4-butylphenylazo)phenoxy]-undecyl methacrylate) (poly(AzoMA)), poly(ferrocenyldimethylsilane), polyisoprene, polybutadiene, polyisobutylene, poly propylene glycol, poly(ethylene glycol), or a polysaccharide, such as chitosan, or a mixture thereof.
[0018] Polysaccharides, and in particular chitosan, are typically in the form of a gel in the ink of the present invention, while the other polymers mentioned above are typically in the form of solutions.
[0019] In preferred embodiments, the polymer is poly(lactic acid). Herein, the term "poly(lactic acid)" refers to a poly(lactic acid) homopolymer or a mixture thereof. The poly(lactic acid) homopolymers include those derived from d-lactic acid, I-lactic acid, or a mixture thereof. Poly(lactic acid) is typically prepared by the catalyzed ring-opening polymerization of the dimeric cyclic ester of lactic acid, which is referred to as "lactide." Poly(lactic acid) may also be made by living organisms such as bacteria or isolated from plant matter that include corn, sweet potatoes, and the like. Poly(lactic acid) made by such living organisms may have higher molecular weights than those made synthetically. In preferred embodiment, the poly(lactic acid) is that sold under number PLA 40320 by Natureworks LLC. This polymer is preferably used in the form of a solution in the ink of the present invention.
[0020] In alternative preferred embodiments, the polymer is chitosan.
Chitosan is produced commercially by deacetylation of chitin, which is the structural element in the exoskeleton of crustaceans (such as crabs and shrimp) and cell walls of fungi. The degree of deacetylation (%DD) can vary and, in commercial chitosans, ranges from 60 to 100%. On average, the molecular weight of commercially produced chitosan ranges from a few thousand to several hundred thousand Daltons. Chitosan is preferably used in the form of a gel in the ink of the present invention.
[0021] The solvent may be any volatile solvent capable of dissolving the polymer or forming a gel of the polymer as well as being capable of dispersing particles without reacting with the particles, even at temperatures involved in sintering of the particles. In preferred embodiments, the solvent is dichloromethane (DCM), chloroform (CHCI3), tetrahydrofuran (THF), acetone, methanol (Me0H), ethanol (Et0H) or water.
[0022] In embodiments where the polymer (for example poly(lactic acid)) is used in the form of a solution, the solvent is preferably dichloromethane, chloroform, tetrahydrofuran, acetone, methanol, or ethanol, more preferably dichloromethane.
[0023] In alternative embodiments where the polymer (for example chitosan) is used in the form of a gel, the solvent is preferably water.
[0024] Higher solvent concentrations in the ink yield thinner inks, while lower concentrations yield thicker inks that dry more quickly. Thicker inks that dry more quickly tend to retain better their shape after 30 printing. Hence, a skilled person will adjust the solvent content to obtain an ink with the desired performances, i.e. and ink that is thin enough to be extruded through the nozzle of a 30 printer, while being think enough to hold its shape after extrusion.

In embodiments, the ink comprises between about 10 and about 50 w/w% solvent (based on the total weight of the ink).
[0025] In embodiments, the ink further comprises one or more additives. Non-limitative examples of such additives include:
= glycerol (with a view to conferring flexibility to the 3D printed structure), = pigments to change the color of the ink, = short carbon fibers, fiberglass, and/or boron nitride to change the mechanical properties of the ink, and/or = carbon black spheres, graphene, or metal nanowires such as silver, copper, and/or nickel nanowires to change the electrical properties of the ink.
[0026] Preferred additives are metal nanowires such as silver, copper, and/or nickel nanowires.
[0027] Other examples of additives include acids and bases, preferably acids. Preferably, the acids and bases are used when water is the solvent for the polymer (preferably chitosan) in the ink. In such cases, the acids and bases change the pH and/or the rheological properties (in particular, the viscosity) of the ink. In particular, acids decrease both the pH and the viscosity of chitosan hydrogels. The acids and bases are preferably weak acids and bases. Weak bases and acids are defined as bases and acids that do not ionize fully in an aqueous solution.
Typically, weak acids have a pKa between about -2 and about 12, preferably between about 2 and about 8, and more preferably between about 3 and about 6.5. Typically, weak bases have a pKb between about -2 and about 13, preferably between about -2 and about 2, and the base has more preferably a pKb of about 0.2 These acids and bases are preferably organic. These acids and bases are preferably non-toxic.
Non-limiting examples of acids include acetic acid, lactic acid, citric acid as well as mixtures thereof. A
preferred acid is acetic acid alone or together with one or more other acids such as lactic acid and/or citric acid.
Preferably, the total acid concentration ranges from about 40 to about 90 wt% (based on the total weigh of the solvent and the acid(s)). Preferably, the solvent for the ink is water and comprises 70 vol% acetic acid alone or together with 10 vol% lactic acid and 3 wt% citric acid, the vol`)/0 being based on the total volume of the water and acids and the wt%
being based on the total weight of the water and acids.
[0028] In particular embodiments, the ink comprises a gel of chisotan in water (hydrogel) containing one or more non-toxic acids, preferably 70 vol% acetic acid alone or together with 10 vol%
lactic acid and 3 wt% citric acid. In such cases, the ink could be used to print biomaterials and materials for biomedical applications as well as any other electrically conductive materials for which the use of toxic solvents is not allowed or is undesirable.
3D Printer Ink Cartridge [0029] In another aspect, the present invention provides a 3D printer ink cartridge, the cartridge comprising a container having an ink outlet, the container comprising the ink as described in the previous section.
[0030] In embodiments, the cartridge is adapted to be installed on a 3D
printer.
[0031] In embodiments, the cartridge is adapted to be fitted to a nozzle for delivering the ink, so that, for ink dispensing, the ink is extruded through the ink outlet and through the nozzle.

[0032] In embodiments, the cartridge is designed so that when a pressure is applied by a 3D printer, the ink is extruded through the ink outlet.
Method of Manufacture of an Ink for Solvent-Cast 3D Printing [0033] In another aspect, the present invention provides a method of manufacture of the above ink for solvent-cast 3D printing, the method comprising the steps of:
a) providing a solution or a gel of a polymer in a solvent, b) providing heat-sinterable metallic particles in a particles:polymer weight ratio of more than about 85:15, c) dispersing the particles in the solution or gel of the polymer, thereby producing the ink.
[0034] In this method, the ink, the polymer, the solvent, the solution or gel, the particles, their concentrations, their preferred embodiments, etc. are as described above.
[0035] As the polymer is soluble in the solvent or can form a gel with the solution, the solution or gel in step a) can be prepared simply by mixing the polymer in the solvent until the polymer is dissolved or the gel is formed. In embodiments, the polymer concentration of this solution or gel is:
= about 2 wt%, about 3 wt%, about 4 wt%, about 5 wt%, about 6 wt%, about 7 wt%, about 8 wt%, about 9 wt%, or about 10 wt% or more and/or = about 30 wt%, about 35 wt%, about 20 wt%, about 15 wt%, or about 10 wt%
or less, based on the total weight of the solution or gel. In preferred embodiments where the polymer is PLA, the polymer concentration of this solution or gel is about 20 wt%, based on the total weight of the solution or gel. In preferred embodiments where the polymer is chitosan, the polymer concentration of this solution or gel is about 4 wt%, based on the total weight of the solution or gel.
[0036] The dispersion of the particles in step c) can be effected by any dispersion technique known to the skilled person. No matter which dispersion technique is used, it should be carried out with sufficient energy and for sufficient time so that particles are dispersed in the solution or gel. In preferred embodiments, dispersion is achieved ball milling for example for a few minutes.
[0037] In embodiments, the method further comprises the step of adding solvent, or removing part (for example by partial evaporation) of the solvent. This allows adjusting the solvent content in the ink to achieve a desired ink viscosity. Preferred solvent contents are as noted in the previous section.
[0038] In embodiments, the method further comprises, the step of adding one or more additives to the solvent before it is used to form a solution or gel of the polymer, to the solution or gel of the polymer or to the ink. The step at which this additive is added and how it is mixed may vary depending on the additive. In preferred embodiment, the additive is added to the solution or gel of polymer before step c), and mixed in the ink during step c). In alternative embodiments, the additive is mixed into the ink after step c).
When the additive is a base or acid, it is preferably added to the solvent before it is used to produce the solution or gel of the polymer (i.e. prior to step a)).
[0039] In embodiments, the method further comprises, the step of packaging the ink in a 3D printer ink cartridge.

Method of Manufacturing a Solvent-Cast Metallic 30 Printed Material [0040] In another aspect, the present invention provides a method of manufacturing a solvent-cast metallic 3D
printed material, the method comprising the steps of:
a) providing the above described metallic ink for solvent-cast 3D printing containing a polymer and heat-sinterable metallic particles;
b) using a 3D printer, extruding the ink through a nozzle into a controlled pattern;
c) allowing solvent evaporation, thereby producing a printed material;
d) removing the polymer from the printed material by heating the printed material to a polymer degradation temperature or above, thereby leaving the particles arranged into the controlled pattern; and e) heat-sintering the particles, thereby producing the solvent-cast metallic 3D printed material.
[0041] Herein, a "controlled pattern" refers to a pattern with a controlled morphology, such as that obtained by 3D
printing from a model. Controlled patterns do not include random pattern such as those obtained by simple extrusion, electrospinning or other such methods. However, controlled patterns include porous patterns, fully-filled patterns, interlocked patterns and overhung patterns as well as patterns involving so-called freeform printing, i.e.
patterns including one or more structures printed in the vertical direction with no adjacent supporting layers (e.g. a column). The controlled pattern is typically a layered pattern.
[0042] In embodiments of this method, providing step a) includes the method of manufacture of a solvent-cast 3D
printing ink described in the previous section.
[0043] The speed of the extrusion (in step b)) depends on many interrelated ink- and printer-related factors.
These factors include the inner diameter of the nozzle, the applied pressure, the displacement speed of the nozzle, the volatility of the solvent, particle concentration, and the viscosity of the ink. For any given ink and desired nozzle diameter, the remaining printer-related factors are adjusted to allow successful deposition into the desired pattern.
Exemplary 3D printing conditions include:
= an applied pressure between about 0.2 and about 4.2 MPa, = a displacement speed of the nozzle ranging from about 0.3 to about 10 mm/sec, and/or = an inner diameter of the nozzle ranging from about 100 pm to about 410 pm.
[0044] In embodiments, step b) is advantageously carried out at about room temperature.
[0045] It is to be understood that solvent evaporation (in step c) typically begins as soon as the ink is extruded out of the nozzle in step b). In embodiments, the solvent has completely evaporated from 3D printed material before heating in step d), i.e. step c) is entirely carried out at about room temperature. In other embodiments, some residual solvent evaporates from the 3D printed material during the heating involved in step d), i.e. part of step c) is carried out at about room temperature and then the remaining of step c) is carried at a temperature above room temperature.
[0046] Step c) results in solid particles/polymer composite material disposed in the controlled pattern.
[0047] In step d), the polymer is removed from the 3D printed material by heating the material at or above the polymer degradation temperature. The polymer degradation temperature will vary depending on the nature of the polymer. The polymer degradation temperature of a polymer is a well-known to skilled persons and readily available or, failing that, can be easily determined by heating the polymer and observing the temperature at which it degrades.
For example, the polymer degradation of PLA is about 225 C.
[0048] In step e), the particles (resulting from step d)) are heat-sintered. In other words, the particles are heated to a sintering temperature for a time sufficient for the particles to peripherally become attached to one another thus forming a solid mass. The sintering temperature is a temperature at which sintering occurs. It will vary according to the nature of the particles. The sintering temperature of a metal or alloy is well-known to skilled persons and readily available or, failing that, can be easily determined by heating the metal or alloy and observing the temperature at which sintering is observed.
[0049] Typically, the sintering temperature is a temperature approaching, but not reaching, the melting point of the particles. For example, the sintering temperature may be as low as about 400 C below the melting point of the particles or as high as about 200 C below the melting point. The sintering time (i.e. the length of time at which the materiel is held at the sintering temperature) will be adjusted according to the desired properties for the solvent-cast metallic 3D printed material produced. If a more porous material is desired, especially in view of carrying out optional step f) below, the sintering time may be shorter, for example from about 30 minutes to about 6 h, preferably about lh.
If a more robust material is desired, the sintering time may be longer, for example from about 6 h to about 12 h, preferably about 6h.
[0050] It should be noted the polymer degradation temperature is lower than the sintering temperature. Also, if desired, steps d) and e) can advantageously be performed by a single heat treatment in which the temperature is increased to sintering temperature (thus passing the polymer degradation temperature) and then held at the sintering temperature. Typically, there is no need to stabilize the temperature and/or hold the temperature at or around the polymer degradation temperature. In embodiments, the heating rate will vary during the heat treatment, for example being slower up until about the polymer degradation temperature is reached and being faster afterwards until the sintering temperature is reached. For example, the heating rate up to about the polymer degradation temperature or above (e.g. up to about 100 C above the polymer degradation temperature) may be from about 1 to about 5 C/min, preferably about 1 C/min. The heating rate thereafter, up to the sintering temperature may be from about 7 to about 15 C/min, preferably about 10 C/min. Then, the sintering temperature may be held for the sintering time discussed above.
[0051] After step c), the printed material comprises the particles and the polymer arranged into the controlled pattern (potentially together with one or more additives as described above).
The removal of the polymer in step d) results a material comprises the particles arranged into the controlled pattern (potentially together with one or more of the additives that have not been removed by the heating). After step e), similarly to after step d), the material comprises the particles arranged into the controlled pattern (potentially together with one or more of the additives that have not been removed by the heating). However, the particles are now peripherally attached to one another thus forming a solid structure and thus yielding the desired solvent-cast metallic 3D printed material. As noted above, the particles do no coalesce. Therefore, pores created in-between the particles by the removal of the polymer will remain in the solvent-cast metallic 3D printed material resulting from step e).
[0052] Some shrinkage will typically be observed during step d) because the polymer is removed and the particles settle. However, this step nevertheless results in a material arranged in the same controlled pattern as steps b) and c) ¨ for example see Fig. 1. Typically, little shrinkage is observed during step e).
[0053] In embodiments, the method further comprises the step f) of partly or completely filling the pores created in-between the particles by the removal of the polymer with a second metal or alloy. As such, step f) results in a metal/metal composite. The second metal or alloy has melting point lower than the melting point of the metal or alloy of the particles. Non-limiting examples of the second metal or alloy include metal of groups 3 (including the lanthanides and the actinides) to 16 of the periodic table and their alloys.
Of note, the elements B, C, Si, Ge, N, P, As, Sb, 0, S, Se, Te, F, Cl, Br, and I, while being part of groups 12 to 16 of the periodic table are not metals and thus are not "metals of groups 3 to 16 of the periodic table". More specific examples of metals and alloys include steel, cast iron, titanium, silver, copper, zinc, gold, platinum, aluminum, nickel, bronze and brass, preferably copper. The pores created in-between the particles by the removal of the polymer can be filled with the second metal or alloy, for example, by contacting part of the solvent-cast metallic 3D printed material, for example an end, edge or side thereof, with the second metal or alloy, the second metal or alloy being in the molten state, and allowing the molten second metal or alloy to diffuse by capillarity into the pores of the material. Preferably, the molten second metal or alloy is placed on top of the solvent-cast metallic 3D printed material and the gravity force eases the diffusion. This can be achieved by placing a piece of the second metal or alloy on top of the solvent-cast metallic 3D printed material and then heating the assemblage at a temperature above the melting point of the second metal or alloy.
Preferably, the quantity of second metal or alloy contacted with the solvent-cast metallic 3D printed material can be calculated from the porosity and volume of the solvent-cast metallic 3D
printed material and the desired filling level.
When fully filling the pores, the quantity of second metal or alloy can be slightly in excess (for example about 10 vol%
in excess) of the calculated quantity of second metal or alloy needed to fully filled the pores.

[0054] Interestingly, the above diffusion method allows filling the pores of the above-mentioned pores without significantly filing the macroscopic voids within the solvent-cast metallic 3D
printed material. For example, if two filaments have been printed adjacent to one another, the pores in-between the particles within each filament will be partly or completely filed, but the space between the two filaments will not be significantly filed ¨ see Fig. 1.
However, sharp corners near interconnecting filaments will then to be filled.

[0055] As noted above, the pores may be completely or partially filled by the second metal or alloy. Pores that are more filled result in denser, less porous, metal/metal composite material.

[0056] Steps d), e) and f) can be carried out in air. In embodiments, steps d), e) and f) are preferably carried out in an atmosphere that is inert toward (i.e. does not react with) the heated materials. This can be advantageous when the metal or alloy when heated reacts with air, which may results in oxidation of the material or even explosions.
Alternatively, a reactive atmosphere could be used to alter the material (i.e.
the metal or alloy of the particles and/or the second metal or alloy) in a desirable manner.

Solvent-Cast Metallic 3D Printed Material

[0057] In another aspect, the present invention provides a solvent-cast metallic 3D printed material. This material has been manufactured by the above described method using the above described ink. Therefore, the above teachings regarding the metal(s), optional additive(s), etc., including preferred embodiments thereof, apply to the material described below.

[0058] This material is shaped into a controlled pattern (described above).

[0059] This material is made of particles of a metal or alloy (described above) peripherally attached to one another.

[0060] In embodiments, pores in-between the particles are completely or partially filled with a second metal or alloy (described above).
Potential Advantages of the Invention

[0061] In one or more embodiments or aspects, the present invention may present one or more of the following advantages.

[0062] The easy-to-implement method of the invention enables lower-cost 3D
printing (compared to SLS and SLM).

[0063] The invention allows 3D printing of highly dense metallic structures featuring complex geometries. It can be used to manufacture high-performance metallic parts.

[0064] The method is highly flexible and various complex 3D structures, including fully-filled, porous, interlocked and overhung structures, can be fabricated.

[0065] The method of the invention can be used with diverse metallic materials.

[0066] The method of the invention involves 3D printing at room temperature.

[0067] The method of the invention does not involve complex and expensive equipment. It is cheap and safe.

[0068] The method of the invention can be applied to fabricate small sized devices (e.g. the artificial bones for human) in the medical field, benefiting from its high resolution and ability to fabricate complex microstructures with desirable mechanical properties.

[0069] In addition, the method is also suitable to fabricate conductive porous microstructures serving as electromagnetic shields in the microwave field.
Definitions

[0070] The use of the terms "a" and "an" and "the" and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context.

[0071] The terms "comprising", "having', "including', and "containing" are to be construed as open-ended terms (i.e., meaning "including, but not limited to") unless otherwise noted.

[0072] Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All subsets of values within the ranges are also incorporated into the specification as if they were individually recited herein.

[0073] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.

[0074] The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed.

[0075] No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0076] Herein, the term "about' has its ordinary meaning. In embodiments, it may mean plus or minus 10% or plus or minus 5% of the numerical value qualified.

[0077] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0078] Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0079] The present invention is illustrated in further details by the following non-limiting examples.
Example 1 - Solvent-cast based Metal 3D Printing and Secondary Metallic Infiltration Summary

[0080] We developed a method to fabricate dense metallic structures by combining a room temperature 3D
printing and subsequent heat-treatments: sintering and secondary metallic infiltration. The high flexibility of this method enabled the fabrication of customized 3D structures, such as fully-filled, porous, interlocked and overhung structures. These geometries were printed using a highly concentrated metallic ink (metallic load up to 98 wt.%) consisting of highly alloyed steel (HAS) microparticles, polylactic acid (PLA) and dichloromethane (DCM). In order to improve the mechanical properties and the electrical conductivity, the as-printed structures were sintered and infiltrated by copper in a furnace protected by a mixture of H2 and Ar. The filament porosity of the copper infiltrated samples was as low as 0.2%. Mechanical testing and electrical conductivity measurement on the copper infiltrated structures reveal that the Young's modulus reached up to -195 GPa and the electrical conductivity was as high as 1.42 x 106 S/m.
Introduction

[0081] We describe herein the solvent-cast 3D printing of metallic inks at room temperature, followed by sintering and secondary infiltration (see Fig. 1). To demonstrate this concept, a metallic ink was prepared by mixing spherical highly alloyed steel (HAS) microparticles, PLA and DCM. In the printing process (Fig. la), an extrusion device was employed to extrude the metallic ink. This device included a pressure dispensing system, a micronozzle and a syringe barrel that contains the metallic ink. It was mounted on the moving head of a computer controlled 3-axis positioning stage, in order to deposit the extruded ink filament layer by layer on a substrate to create 3D structures.
Right after the ink filament was extruded, the solvent in the metallic ink evaporated rapidly at room temperature and the filament became solid. The filament layer was then used as a support for subsequent filament layers to create a HAS/PLA composite multi-layer scaffold. In the as-printed scaffold (Fig. 1b), PLA served as a binder holding the HAS
microparticles together. To directly connect the HAS particles, the polymer binder was removed by heating the printed scaffold in a furnace above the polymer degradation temperature. The temperature was then rapidly raised to slightly below the melting point of the HAS. At this temperature, sintering occurred by creating necking links between neighboring particles. As time elapsed, the necks growth effectively reduced the size and the number of the pores within the metallic continuum. Porosity can be reduced to obtain a strong and conductive filament structure.
However, the sintering process was halted at a porosity favorable for melted copper infiltration (Fig. 1c). For copper infiltration, the sintered scaffold was heated again with a piece of copper (Cubond IP C-437 infiltration copper) placed on top of it. The melted copper filled the pores within the filament driven mainly by capillary forces. Melted copper flowed through the pores within filaments to obtain a highly dense metal/metal composite.
Experimental Materials

[0082] 2g of PLA (4032D, Natureworks LLC, glass transition temperature Tg =50-60 C) was dissolved in 8g of DCM (Sigma-Aldrich, boiling point=39.6 C) to prepare polymer solutions. After resting for 24h, the solutions were sonicated in an ultrasonic bath (Ultrasonic cleaner 8891, Cole-Parmer) for 5 minutes. The metallic inks were prepared by mixing the polymer solution and HAS microparticles using a ball mill mixer (8000M Mixer/Mill, SPEX
SamplePrep) for 5 minutes.
3D printing

[0083] The inks were loaded into 3 cc syringes (EFD) attached to a smooth-flow tapered nozzle (exit inner diameter = 250 m, EFD). Loaded syringes were mounted on a pressure dispensing system (HP-7X, EFD), which was placed on a computer controlled 3-axis positioning stage (I&J2200-4, I&J
Fisnar). The ink was printed on a glass slide (PN 16004-422, VWR). The scaffolds, tensile bars and Olympic stadium were printed at a speed of 10 mm/s and under a pressure around 0.7 MPa using the 95 wt.% concentrated ink. These structures were designed by a computer aided design software (i.e. CATIA) and sliced into several layers by a slicing software (i.e. Cura). Each was filled by a filament path and the filament path was interpreted to G-code.
Then, the G-code was converted by a customized python program into a point-to-point program that can be recognized by the JR Points software to control the positioning moving stage.

Sintering and copper infiltration

[0084] The as-printed samples were sintered and copper infiltrated in a laboratory electric tubular furnace (59256-P-COM, Lindberg) using a ceramic substrate. A gas mixture of 2.5% H2/ 97.5% Ar (flow rate=5 ft,/h) was circulated inside the quartz tube to prevent the oxidation of the samples. The temperature profiles used during the sintering and copper infiltration are provided in Fig. 2. Debinding started from 25 C to 300 C with a heating rate of 60 C/h. Then the temperature was raised up to 1165 C with a heating rate of 600 C/h and held at 1165 C for 6h for sintering. For copper infiltration, the temperature was raised up to 1120 C and held for 0.5 h, then cooled down to the room temperature.
Porosity analysis

[0085] Each sintered and copper infiltrated scaffold was sealed in a resin (EpoFix Resin, Struers) block before polishing. The scaffold was polished until reaching 0.2H, 0.5H and 0.8H of the printed structure (H being the initial height of the scaffold). The polished cross sections were observed under an optical microscope (Zeiss Axioplan EL-Einsatz). For each cross section, five images were taken at different areas of the cross section and analyzed by an image analyzing software (Clemex, ST-2000). The porosity was calculated as the ratio of voids area over the filament area in the polished cross sections.
Tensile tests

[0086] The tensile bars were printed with 95 wt.% ink and 250 gm tapered nozzle, of which the average size of the cross section of the narrow part was -4.0 x 1.8 mm. The filaments were oriented 00/900 to the tensile direction, i.e. the filaments from same layer were parallel, while adjacent layers were orthogonal. The tensile tests were carried out on a MTS Insight machine with a 50 kN load cell (MTS 569332-01) at a crosshead speed of 1 mm/min and using an extensometer (MTS 632.26, C-20). Three specimens for each sample type were tested.
Electrical conductivity test

[0087] The test samples used for the electrical conductivity measurements are sintered and copper infiltrated rods having typical dimensions of 30x1x1 mm. They are printed with the 95 wt.%
ink and the 250 jim tapered nozzle.
The current values of 1 A, 2 A and 3 A is provided by an EMS 150-33-D-RSTL
power supply. A NI 6211 device is employed for the current and voltage acquisition. The control of the power supply and the logging of test data are carried out via a Lab VIEW program. Three specimens of each sample type were tested.
Results and discussion Metallic ink recipe

[0088] The HAS powder particles used in this work were fine (5 20 pm) and had a spherical shape (see Fig. 3).
The PLA solution served as lubrication to assist the extrusion of the HAS
microparticles through the micronozzle. The solvent was DCM as it efficiently dissolves PLA and rapidly evaporates at room temperature.

[0089] The HAS particle concentration within the ink affected the properties of the fabricated structures. To investigate its influence on as-printed, sintered and copper infiltrated structures, 20-layer scaffolds were printed using HAS/PLA inks with four different concentrations of HAS particles: 85, 90, 95 and 98 wt.%. The inks were named according to the metal particles weight percentage in the as-printed structure (solid state with no solvent). In the inks, the PLA/DCM weight ratio was 1:4, except for 98 wt.% ink, which was 1:9 (see Table 1 for detailed compositions).
Table 1. Ink formulations created for 3D printing Ink constituent 85 wt.% (47 vol. /0) 90 wt.% (59 vol. /0) 95 wt.% (75 vol.c/o) 98 wt.% (90 vol.%) HAS/PLA ink HAS/PLA ink HAS/PLA ink HAS/PLA ink _____________________ [g1 ________ [g] _______ [g] [gL

HAS microparticles 85 90 95 98

[0090] The 85 wt.% ink contained the highest amount of solvent, and thus took more time to evaporate. As a result, the ink transition from liquid to solid state was slower. This was detrimental to the shape retention of the extruded filament. The surface of the scaffold printed with the 98 wt.% ink was the roughest due to the low amount of PLA (see Fig. 4). The HAS microparticles were covered and bonded by the polymer. The filaments of all four scaffolds aligned well. However, as mentioned above the 85 wt.% scaffold distorted and the surface of the 98 wt.%
scaffold was rough.

[0091] During sintering, 85 wt.% ink printed scaffolds collapse while the higher concentrated ink successfully preserved their shape during the sintering (Figs. 5 and 6). After sintering, the polymer was burned away and the HAS
microparticles were sintered together. The filaments of 90, 95 and 98 wt.%
scaffold keep their shapes and aligned well, while the 85 wt.% ink printed scaffold collapsed during sintering and the surface of 98 wt.% scaffold was rough.
Melted copper infiltrated into the sintered filaments. Some excessed copper was left on the top of the scaffold.

[0092] Since low concentration inks contain more polymer, the structure had higher risk to flow and collapse during the heating and removing the polymer binder. Hence, the 95 wt.% ink was selected for further testing because it appeared to be the best compromise between having a highly concentrated ink for dense sintered structures while ensuring smooth extrusion and low deformation during the solvent evaporation printing.
3D printing of metallic inks

[0093] Fig. 7 shows various 3D structures fabricated using the above solvent-cast 3D printing method with the 95 wt.% ink at a printing speed of 10 mm/s, including tensile bars, 20-layer scaffolds, a planetary gear and a small-scale replica of Montreal's Olympic stadium. The high flexibility of the technique enabled the fabrication of customized structures, such as fully-filled (Fig. 7a), porous (Fig. 7b), interlocked (Fig. 7c), and overhung structures (Fig. 7d).

[0094] Fig. 7a displays the printing process of a tensile bar. The ink uniformly and continuously flowed through the micronozzle under a constant pressure. No significant shape distortion was observed. The inset of Fig. 7a shows several scaffolds and the tensile bar samples next to a Canadian dollar (diameter = 26.5 mm). The fabrication time for the 20-layer scaffold and the fully-filly tensile bar were -2 min and 30 min, respectively.

[0095] Fig. 7b shows optical images of a 20-layer scaffold from top, side, and oblique views, respectively. The center-to-center distance between neighboring filaments was 0.5 mm and the layer thickness was 0.2 mm. It was observed that the top layer of filaments perpendicularly stacked on the previous layer, by which all subsequent layers were completely overlapped (from top view). The creation of the complete overlap implied the accuracy of the printing. The side view optical image of the structure further shows good layer superposition. It illustrates, additionally, that the adjacent layers cohere together.

[0096] A planetary gear consisting of 1 sun gear, 1 ring gear and 4 planet gears was fabricated. The planetary gear is shown in Fig. 7c next to a Canadian 5-cent coin (diameter = 21.2 mm).
The diameter of the ring gear was 30 mm. All the parts of the planetary gear were simultaneously printed and the printed structure required no assembly.

[0097] There is an overhung structure in the replica of the Olympic stadium showed in Fig. 7d. The overhang part was printed without any additional supporting structures, and it did not bend or deform after printing (tower inclination of -45 ).
Sintering and secondary metallic infiltration

[0098] To improve the mechanical properties and electrical conductivity, the as-printed metal/polymer composite structures were converted to metal and metal composites by sintering and copper infiltration. These heat-treatments were carried out in a laboratory electrical furnace. The heating and cooling rates were adjusted according to the temperature profiles (shown in Fig. 2). To prevent oxidation of the samples during the heat-treatments, a mixture of H2 and Ar continuously flowed inside the quartz tube of the furnace.

[0099] Fig. 8 presents an optical and SEM images of the as-printed, sintered and copper infiltrated 20-layer scaffolds printed with 95 wt.% ink and 250 pm tapered nozzle. The three types of scaffolds were placed next to each other on a Canadian dollar. The as-printed scaffold had a dark gray color which turned to light gray after sintering.
The sintered scaffold shrunk to - 84.9% 0.6% of the initial size, because the PLA was removed and the HAS
particles were brought closer together. The copper infiltrated scaffold turned to brown red and its final dimension was reduced to - 88.6% 0.8% of the as-printed scaffold. The shrinkage observed for the copper infiltrated was slightly less compared to the sintered one probably due to either the shorter duration of the sintering or the extra thickness of copper. No oxidation was observed on the surface of the three scaffolds. No significant distortion happened to the scaffolds after sintering and copper infiltration. The SEM images of the as-printed filaments showed HAS particles were covered and linked together by the PLA binder (Fig. 8a and Fig. 1b).

[00100] Before sintering, a debinding step was required to remove the PLA
within the structure. The degradation temperature of the PLA in the samples was around 225 C, which was investigated by thermogravimetric analysis (TGA) on 95 wt.% HAS/PLA sample. The temperature was raised from 20 C to 500 C at a rate of 1 C/min (the same heating rate as the sintering process). As can be seen in Fig. 9, the degradation of PLA finished before at temperature of 225 C was reached. The debinding temperature was thus set at 300 C to ensure the complete degradation of PLA.

[00101] After debinding, the structure integrity was held by the friction forces between the HAS microparticles. The low heating rate (60 C/h) facilitated the PLA to fully degrade, and prevented the structures from collapsing due to the rapid disappearance of PLA binder. After debinding, the temperature was raised to 1165 C and maintained for 6h.

This sintering temperature was set slightly lower than the melting point of HAS for the particles to connect through the apparition of necks between them. The size and amount of the necks increased gradually, which densified the sintered structure and reduced the pores. A long duration of 6 hours was set to ensure adequate sintering of the HAS
particles and obtain a denser structure. In the sintered scaffold (Fig. lc and Fig. 8b), the PLA was completely gone and the HAS particles directly connected with each other through the necks.

[00102] If the structure was prepared for copper infiltration, the sintering duration at 1165 C was limited to 1 hour.
This shorter sintering duration left more pores for melted copper to flow through the filaments. A piece of copper was placed on top of the sintered structures inside the furnace (Fig. 1c). The furnace was heated at 1120 C (see Fig. 2).
This temperature was higher than the melting point of copper (1085 C), but lower than the previously used sintering temperature. The amount of the copper was calculated by the porosity and the volume of the filaments in the sintered scaffolds. 10 vol.% extra copper was added to ensure that all the pores within the filaments were filled. Mainly driven by capillary forces, the melted copper filled the pores within the filaments of the structure and the sharp corners near connecting filaments (Fig. 8c).
Porosity analysis of sintered and copper infiltrated samples

[00103] The filament porosity of sintered and copper infiltrated structures affects the mechanical properties of the structures. To investigate the porosity, each scaffold was polished until reaching three different height positions of 0.2H, 0.5H, and 0.8H (H being the initial .height of the scaffold). Fig. 10 shows a scheme of polishing positions, representative optical microscope images of the polished cross sections, and the average value of porosities of the sintered (6h) and copper infiltration scaffolds. The first three columns represent the porosities of 90, 95, and 98 wt.%
sintered scaffolds, which were 10.4% 4.4%, 12.1% 5.3% and 12.4% 2.0%, respectively. The porosities of 90, 95 and 98 wt.% copper infiltrated scaffolds were extremely low and ranged from 0.2% to 0.3%. These are presented as the three columns on the right. This data is also shown in Table 2.
Table 2. Porosity analysis results of 90, 95 and 98 wt.% sintered (6h) and copper infiltrated 20-layer scaffolds.
Porosity 90 wt.% 95 wt.% 98 wt.%
Sintered 10.4 % 4.4%
12.1 % 5.3% 12.4 % 2.0%
Copper infiltration 0.3 % 0.3% 0.2 % 0.1% 0.2 % 0.2%

[00104] The microscope image of the polished cross section for each type of scaffold is displayed above each of the column. In the sectional images of sintered scaffolds, we observe that the copper only filled the pores inside the filament, while no large amount copper was outside the filament. Thus, the infiltration flow was limited within the porous filaments rather than in the empty space between filaments (i.e.
interfilamentous pores). The pores had similar sizes and were uniformly distributed within the filament. It is noted that the pores in 98 wt.% sintered scaffold were greater in quantity but smaller in size compared to the others, which resulted in the smaller error bars. The difference of measured porosities among the three types of sintered scaffolds was lower than 2% despite their different metal to polymer ratios. This can be explained by the densification of the structure at the debinding stage.

The polymer was removed and the adjacent HAS particles were brought to similar distances before the sintering was completed. For the copper infiltrated scaffolds, it is observed from the microscope images that the melted copper almost completely filled the porous HAS filaments.
Electrical properties of sintered and copper infiltrated samples

[00105] To assess the electrical properties of fabricated structures, the conductivities of sintered and copper infiltrated rods were measured using the four-point probe technique. The conductivity of the sintered samples was (6.24 0.18) x 106 S/m, which is 45% of that of the bulk stainless steel (1.4 x 106 S/m 39. The relatively lower value is attributed to the pores in the sintered structures. The copper infiltrated sample had a conductivity of (1.42 0.32) x

106 S/m. As copper is more conductive than steel, the conductivity of the sample was further improved after infiltration with copper.
Mechanical characterization of as-printed, sintered and copper infiltrated samples [00106] Tensile tests were carried out on as-printed, sintered and copper infiltrated tensile bars to evaluate their mechanical properties. Fig. 11 shows the tensile curves, optical images of representative tested bars and SEM
observations of the tensile fracture surfaces. The Young's modulus E, Ultimate Tensile Strength (UTS) and Elongation (%) were determined from the tensile curves presented in Fig. 11a.
See Table 3 for more details on the tensile results.
Table 3. Tensile test results of 95 wt.% as-printed, sintered and copper infiltrated tensile bars compared with: (1) Wrought stainless stee138, (2) Nitrogen alloyed, high strength, medium elongation, sintered at 1290 C (2350 F) in dissociated ammonia39, (3) PM steel containing 0.8% carbon and 2% copper39, and (4) Copper infiltrated steel containing 0.8% carbon39.
UTS Elongation [GPal [MPa] [%]
As-printed sample 3.1 0.3 28.0 3.0 1.45 0.10 Sintered sample 196 16 485 70 0.47 0.06 Copper infiltrated sample 195 16 511 57 0.77 0.07 Wrought stainless steel: SS-316 (1) 193 515 30 PM steel stainless steel: SS-316N2-38 (2) 140 480 13 PM steel FC-0208-60 (3) 155 520 <1 Cu infiltrated PM steel: FX-2008-60 (4) 145 550 1

[00107] The E modulus increased (by -63 times) from 3.1 GPa for the as-printed bars to 196 GPa and 195 GPa after sintering and copper infiltration, respectively. The stiffness achieved is an indication that sintering and copper infiltration are effective and that strong metallic bonds between individual particles are created. The high modulus of the 6h sintered bars was attributed to the dense HAS microstructure. Besides, the copper infiltrated bars were only sintered for 1h creating a porous and more compliant HAS microstructure.
However, since the pores were subsequently filled with copper, the effective modulus raised to the same level as the sintered bars. The E modulus obtained after sintering and copper infiltration was similar to those of wrought and cast steels.39 The UTS also increased (by 17- 18 times) from 28 MPa for the as-printed bars to 485 MPa and 511 MPa after sintering and copper infiltration, respectively. These UTS values are similar to those obtained for carbon steels, stainless steels, tools steels and highly-alloyed cast irons.39 The rather low ductility of the sintered and copper infiltrated tensile bars was partly explained by the composition of the HAS material. The large volume fraction of carbides results in elongation at break lower than 1 %. Such elongation values are also typical of some tool steels, highly alloyed cast irons (elongation 1 ¨ 10%)39 and many Powder Metallurgy (PM) steel parts (0¨ 3y0)40.

[00108] The SEM images of tensile fracture surfaces (Fig. 11b, c) show the internal structure of the sintered and copper infiltrated tensile bars. The tensile fracture always occurred at the interface of the filaments oriented transverse to the tensile directions, as the stress was maximal at this point.
This also explains the lower ef value compared to bulk stainless steel (68.2%). The high magnification SEM images (Fig. 11d, e) reveal the details of tensile fracture surface. There were visible pores, under SEM, within the filaments of sintered tensile bars. In addition, the filament from adjacent layers are firmly bound, while some filaments from the same layer are detached.
This is the main reason why the ductility of the samples is lower compared to the bulk stainless steel.
Conclusions

[00109] In summary, we develop a method of fabrication of fully-dense 3D
metallic structures consisting of the solvent-cast 3D printing and the following heat-treatments. The Young's modulus of fabricated structures was up to 195 GPa and approached typical values for similar bulk material while the conductivity was 1.62 x 106 S/m, which is superior to the structures fabricated by most commercial 3D printing techniques. 41-43 Example 2 ¨ Inks containing Chitosan

[00110] In a manner similar to Example 1, we produced an ink containing chitosan (90% deacetylated, weight average molecular weight = 207 kDa, from Biolog in Germany) as the polymer instead of PLA. The chitosan (CHI) was provided as a gel in water comprising with 80 vol/vol% acetic acid (AA), the % being based on the total volume of the water.

[00111] The ink comprised 0.8g CHI per 10mL AA aqueous solution and comprised the HAS microparticles of Example in a HAS:CHI solution weight ratio of 6.5: 1.

[00112] These inks were successfully solvent-cast 3D printed with an applied pressure of 0.6 to 1.2 MPa, a platform speed of 5 mm/s using nozzles of 200 and 250 microns.
Example 3 ¨ Inks containing copper particles

[00113] In a manner similar to Example 1, we produced inks containing 20pm copper particles instead of HAS
particles.

[00114] The inks comprised 20 w/e/0 of PLA/solvent (DCM) and 90 w/w% of the copper particles.

[00115] These inks were successfully solvent-cast 3D printed using anapplied pressure of 0.7¨ 1.4 MPa, a speed of 10 mm/s, and a nozzle of 250 microns.

[00116] The polymer was then removed from the 3D printed material, which was then successfully sintered.

[00117] The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

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Claims

26

1. A metallic ink for solvent-cast 3D printing, the ink comprising:
.cndot. a solution or a gel of a polymer in a volatile solvent, and .cndot. heat-sinterable metallic particles dispersed in the solution or gel, wherein the particles are present in a particles:polymer weight ratio of more than about 85:15.

2. The ink of claim 1, wherein the particles are present in a carbon particles:polymer weight ratio of:
.cndot. about 86:14, about 87:13, about 88:12, about 89:11, about 90:10, about 91:9, about 92:8, about 93:7, about 94:6, or about 95:5 or more, and/or .cndot. about 99:1, about 98:2, about 97:3, about 96:4, about 95:5, about 94:6, about 93:7, about 92:8, about 91:9, or about 90:10, or less.

3. The ink of claim 1 or 2, wherein the particles are present in a carbon particles:polymer weight ratio between about 90:10 to about 95:5.

4. The ink of any one of claims 1 to 3, wherein the particles are present in a carbon particles:polymer weight ratio of about 95:5.

5. The ink of any one of claims 1 to 4, wherein the heat-sinterable metallic particles are steel, cast iron, titanium, silver, copper, zinc, gold, platinum, aluminum, nickel, bronze, or brass particles.

6. The ink of any one of claims 1 to 5, wherein the heat-sinterable metallic particles are steel particles.

7. The ink of any one of claims 1 to 6, wherein the heat-sinterable metallic particles are microparticles.

8. The ink of any one of claims 1 to 7, wherein the heat-sinterable metallic particles are between about 0.1 µm and about 100 µm in size.

9. The ink of any one of claims 1 to 8, wherein the heat-sinterable metallic particles are between about 5 µm and about 50 µm in size.

10. The ink of any one of claims 1 to 9, wherein the heat-sinterable metallic particles are spheroidal.

11. The ink of any one of claims 1 to 10, wherein the heat-sinterable metallic particles are spherical.

12. The ink of any one of claims 1 to 11, comprising between about 10 and about 50 w/w% of the solvent (based on the total weight of the ink).

13. The ink of any one of claims 1 to 12, wherein the polymer is poly(lactic acid), polystyrene, poly(methyl acrylate), poly(methyl methacrylate), poly(n-butyl acrylate), poly(2-hydroxyethyl methacrylate), poly(glycidyl methacrylate), poly(acrylic acid), poly(N-N-dimethylacrylamide), poly(1-vinyl anthracene), poly(2-vinyl pyridine), poly(4-vinyl pyridine), poly(N-vinyl carbazole), poly(N-vinyl carbazole), poly(N-vinyl imidazole), poly(vinyl benzyl chloride), poly(4-vinyl benzoic acid), poly(vinyl acetate), polycaprolactone, poly(11-[4-(4-butylphenylazo)phenoxy]-undecyl methacrylate) (poly(AzoMA)), poly(ferrocenyldimethylsilane), polyisoprene, polybutadiene, polyisobutylene, poly propylene glycol, poly(ethylene glycol), or a polysaccharide, or a mixture thereof.

14. The ink of any one of claims 1 to 13, wherein the polymer is poly(lactic acid).

15. The ink of any one of claims 1 to 14, wherein the solvent is dichloromethane (DCM), chloroform (CHCI3), tetrahydrofuran (THF), acetone, methanol (MeOH), ethanol (EtOH), or water.

16. The ink of any one of claims 1 to 15, wherein the solvent is dichloromethane, chloroform, tetrahydrofuran, acetone, methanol, or ethanol.

17. The ink of any one of claims 1 to 16, wherein the solvent is dichloromethane.

18. The ink of any one of claims 1 to 17, wherein the ink further comprises one or more additive.

19. A 3D printer ink cartridge, the cartridge comprising a container having an ink outlet, the container comprising the ink of any one of any one of claims 1 to 18.

20. The cartridge of claim 19, wherein the cartridge is adapted to be installed on a 3D printer.

21. The cartridge of claim 19 or 20, wherein the cartridge is adapted to be fitted to a nozzle for delivering the ink, so that, for ink dispensing, the ink is extruded through the ink outlet and through the nozzle.

22. The cartridge of any one of claims 19 to 21, wherein the cartridge is designed so that when a pressure is applied by a 3D printer, the ink is extruded through the ink outlet.

23. A method of method of manufacture of the ink of any one of claims 1 to 18, the method comprising the steps of:
d) providing a solution or a gel of the polymer in the solvent, e) providing heat-sinterable metallic particles in a particles:polymer weight ratio of more than about 85:15, f) dispersing the particles in the solution or gel of the polymer, thereby producing the ink.

24. The method of claim 23, wherein step a) comprises mixing the polymer in the solvent until the polymer is dissolved or the gel is formed.

25. The method of claim 23 or 24, wherein the polymer concentration of the solution or gel is:
.cndot. about 2 wt%, about 3 wt%, about 4 wt%, about 5 wt%, about 6 wt%, about 7 wt%, about 8 wt%, about 9 wt%, or about 10 wt% or more and/or .cndot. about 30 wt%, about 35 wt%, about 20 wt%, about 15 wt%, or about 10 wt% or less, based on the total weight of the solution or gel.

26. The method of any one of claims 23 to 25, wherein the polymer concentration of the solution or gel is about 20wt%, based on the total weight of the solution or gel.

27. The method of any one of claims 23 to 26, wherein the particles are dispersed by ball milling in step c).

28. The method of any one of claims 23 to 27, further comprising adding solvent, or removing part of the solvent.

29. The method of any one of claims 23 to 28, further comprising adding one or more additives to:
.cndot. the solvent before it is used to form the solution or gel of the polymer, .cndot. the solution or gel of the polymer, and/or .cndot. the ink.

30. The method of claim any one of claims 23 to 29, further comprising the step of packaging the ink in a 3D
printer ink cartridge.

31. A method of manufacturing a solvent-cast metallic 3D printed material, the method comprising the steps of:
f) providing the metallic ink for solvent-cast 3D printing of any one of claims 1 to 18, g) using a 3D printer, extruding the ink through a nozzle into a controlled pattern;
h) allowing solvent evaporation, thereby producing a printed material;
i) removing the polymer from the printed material by heating the printed material to a polymer degradation temperature or above, thereby leaving the particles arranged into the controlled pattern;
and j) heat-sintering the particles, thereby producing the solvent-cast metallic 3D printed material.

32. The method of claim 31, wherein step a) includes the method of any one of claims 23 to 30.

33. The method of claim 32 or 32, wherein step b) is carried out at about room temperature.

34. The method of any one of claims 31 to 33, wherein step c) is partly or completely carried out at about room temperature.

35. The method of any one of claims 31 to 34, wherein steps d) and e) are performed in a single heat treatment.

36. The method of claim 35, wherein the heat treatment comprises increasing the temperature to a sintering temperature and then holding the temperature at the sintering temperature.

37. The method of claim 35 or 36, wherein a heating rate up to a temperature T between:
.cndot. about the polymer degradation temperature and .cndot. up to about 100°C above the polymer degradation temperature, is lower than a heating rate from the temperature T to the sintering temperature.

38. The method of claim 37, wherein the heating rate up to the temperature T is from about 1 to about 5 °C/min.

39. The method of claim 37 or 38, wherein the heating rate from the temperature T to the sintering temperature is from about 7 to about 15 °C/min.

40. The method of any one of claims 31 to 39, wherein a sintering temperature in step e) is between:
.cndot. about 400 °C below the melting point of the particles and .cndot. about 200 °C below the melting point of the particles.

41. The method of any one of claims 31 to 40, wherein a sintering time is from about 30 minutes to about 12 h.

42. The method of any one of claims 31 to 41, wherein a sintering time is from about 6 h to about 12 h.

43. The method of any one of claims 31 to 41, wherein a sintering time is from about 30 minutes to about 6 h.

44. The method of any one of claims 31 to 43, wherein step d) and e) are carried out in an inert atmosphere.

45. The method of any one of claims 31 to 44, further comprising the step f) of partly or completely filling the pores created in-between the particles by the removal of the polymer in step d) with a second metal or alloy, the second metal or alloy having a melting point lower than the melting point of the metal or alloy constituting the particles.

46. The method of claim 45, wherein the second metal or alloy is steel, cast iron, titanium, silver, copper, zinc, gold, platinum, aluminum, nickel, bronze, or brass

47. The method of claim 45 or 46, wherein the second metal or alloy is copper.

48. The method of any one of claims 45 to 47, wherein step f) comprises contacting part of the solvent-cast metallic 3D printed material with the second metal or alloy, the second metal or alloy being in the molten state, and allowing the second metal or alloy to diffuse by capillarity into the pores.

49. The method of any one of claims 45 to 48, wherein step f) comprises placing a piece of the second metal or alloy on top of the solvent-cast metallic 3D printed material and then heating at a temperature above the melting point of the second metal or alloy.

50. The method of any one of claims 45 to 49, wherein an excess of the second metal or alloy is used.

51. The method of any one of claims 45 to 50, wherein step f) is carried out in an inert atmosphere.

53. The material of claim 52, wherein the pores in-between the particles are completely or partially filled with a second metal or alloy.