Systems and method for monitoring three-dimensional printing

Abstract

This invention provides systems and method for monitoring three-dimensional printing of printing material. A system comprises two coplanar and electrically conductive electrodes and a substrate, which provides a printing surface. The proximate edges of the electrodes, which are on the surface, are separate by a distance ranging from 5 mm to 300 mm. Each electrode is smaller in area than the substrate. The system also comprises a plurality of layers, which are formed layer-by-layer by the printing, and are derived from the printing material. The electrodes are electrically oppositely charged, as enabled by an alternating electric current between the two electrodes. The current partly flows in the layers. The two electrodes exhibit between them a capacitance ranging from 0.1 pF to 10 nF.

Claims

1. A system for monitoring three-dimensional printing of printing material, said system comprising two coplanar electrodes, said electrodes being electrically conductive, said system also comprising substrate, said substrate providing a surface on which said printing occurs, said electrodes being positioned on said substrate, said electrodes exhibiting proximate edges, said edges being separate from one another by a distance, said distance ranging from 5 mm to 300 mm, each of said electrodes being smaller in area than said substrate, said system also comprising a plurality of layers, said layers being derived from said printing material, said layers being formed layer-by-layer on said substrate by said printing, said layers being positioned in a region, said region extending from the location of one said edge to the location of the other said edge, said two electrodes being electrically oppositely charged, said charge being enabled by an alternating electric current, said alternating electric current flowing between said two electrodes, said alternating electric current partly flowing in said layers, said two electrodes exhibiting a capacitance between them, said capacitance ranging from 0.1 pF to 10 nF.

2. The system of claim 1, wherein said layers comprise material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, ceramic, aluminum oxide, aluminum nitride, silicon carbide, silicon dioxide, ceramic-based material, ceramic-matrix composite, cement-based material, cement paste, cement mortar, concrete, polymer, polymer-based material, polymer-matrix composite, thermosetting polymer, thermoplastic polymer, biopolymer, photopolymer, organic-based material, and combinations thereof.

3. The system of claim 1, wherein said substrate comprises material selected from the group consisting of: ceramic, ceramic-based material, ceramic-matrix composite, aluminum oxide, aluminum nitride, silicon carbide, silicon dioxide, cement-based material, cement paste, cement mortar, concrete, polymer, polymer-based material, polymer-matrix composite, organic-based material, and combinations thereof.

4. The system of claim 1, wherein said electrodes comprise material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, electrically conductive polymer, and combinations thereof.

5. The system of claim 1, wherein said two electrodes are the same in composition.

6. The system of claim 1, wherein said two electrodes are electrically connected, said electrical connection being positioned on said substrate, and said electrical connection providing an exterior surface on which said printing occurs.

7. A system for monitoring three-dimensional printing of printing material, said system comprising two coplanar electrodes, said electrodes being electrically conductive, said system also comprising substrate, said substrate providing a surface on which said printing occurs, said electrodes being positioned on said substrate, said electrodes exhibiting proximate edges, said edges being separate from one another by a distance, each of said electrodes being smaller in area than said substrate, said system also comprising a plurality of layers, said layers being derived from said printing material, said layers being material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, and combinations thereof, said layers being formed layer-by-layer on said substrate by said printing, said layers being positioned in a region, said region extending from the location of one said edge to the location of the other said edge, said two electrodes being electrically oppositely charged, said charge being enabled by an alternating electric current, said alternating electric current flowing from one electrode to the other electrode, said alternating electric current partly flowing in said layers, said two electrodes exhibiting a capacitance between them, said capacitance ranging from 0.1 pF to 10 nF.

8. The system of claim 7, wherein said distance ranges from 5 mm to 300 mm.

9. The system of claim 7, wherein said two electrodes are electrically connected, said electrical connection being positioned on said substrate, said electrical connection providing an exterior surface on which said printing occurs, said electrical connection comprising material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, and combinations thereof, said electrical connection extending the distance between said two proximate edges.

10. The system of claim 7, wherein said two electrodes are electrically connected, said electrical connection comprising material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, and combinations thereof, said electrical connection extending the distance between said two proximate edges, a dielectric film being positioned between said electrical connection and at least one of said electrodes, said dielectric film exhibiting electrical resistance.

11. The system of claim 7, wherein said substrate comprises material selected from the group consisting of: ceramic, ceramic-based material, ceramic-matrix composite, aluminum oxide, aluminum nitride, silicon carbide, silicon dioxide, and combinations thereof.

12. The system of claim 7, wherein said electrodes comprise material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, and combinations thereof.

13. The system of claim 7, wherein said two electrodes are the same in composition.

14. A method of monitoring the three-dimensional printing of printing material, said method comprising (a) positioning two coplanar electrodes on a substrate, said electrodes being electrically conductive, said electrodes exhibiting proximate edges, said edges being separate from one another by a distance, said electrodes being smaller in area than said substrate, said substrate providing a surface on which said printing occurs, said printing involving layer-by-layer formation of a plurality of layers on said substrate, said layers being derived from said printing material, said layers being positioned in a region, said region extending from the location of one said edge to the location of the other said edge, said electrodes being electrically oppositely charged, said charge being enabled by an alternating electric current, said alternating electric current flowing from one electrode to the other electrode, said alternating electric current partly flowing in said layers, and (b) measuring the capacitance between said two electrodes.

15. The method of claim 14, wherein said distance ranges from 5 mm to 300 mm.

16. The method of claim 14, wherein said two electrodes are the same in composition.

17. The method of claim 14, wherein each of said electrodes comprises material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, electrically conductive polymer, and combinations thereof.

18. The method of claim 14, wherein said substrate comprises material selected from the group consisting of: ceramic, ceramic-based material, ceramic-matrix composite, aluminum oxide, aluminum nitride, silicon carbide, silicon dioxide, cement-based material, cement paste, cement mortar, concrete, polymer, polymer-based material, polymer-matrix composite, organic-based material, and combinations thereof.

19. The method of claim 14, wherein said layers comprise material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, ceramic, ceramic-based material, ceramic-matrix composite, aluminum oxide, aluminum nitride, silicon carbide, silicon dioxide, cement-based material, cement paste, cement mortar, concrete, polymer, polymer-based material, polymer-matrix composite, thermosetting polymer, thermoplastic polymer, biopolymer, photopolymer, organic-based material, and combinations thereof.

20. The method of claim 14, wherein said two electrodes are electrically connected, said electrical connection being positioned on said substrate, and said electrical connection providing an exterior surface on which said printing occurs.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the side view of the basic configuration for 3D printing monitoring. This configuration involves two electrodes (10 and 11) positioned on a substrate (12). The proximate edges (18 and 19) of the two electrodes are essentially parallel. The printing area (i.e., the area for positioning the layers 15 being printed) is located in the region that extends essentially from edge 18 to edge 19.

(2) FIG. 2 shows the top view of the basic configuration for 3D printing monitoring. This top view corresponds to the side view shown in FIG. 1. The layers 15 extend essentially along the length of the electrodes, such that their length can exceed the length of the electrodes.

(3) FIG. 3 shows the side view of an embodiment of the configuration for 3D printing monitoring. This configuration involves two electrodes (10 and 11) positioned on a substrate (12). The proximate edges (18 and 19) of the two electrodes are essentially parallel. The printing area (i.e., the area for positioning the layers 15 being printed) is located in the region that extends essentially from edge 18 to edge 19. The two electrodes (10 and 11) are electrically connected using an electrical connection (16) which extends essentially from electrode 10 to electrode 11. Positioned between each edge (18 and 19) and electrical connector 16 is a dielectric film 17.

(4) FIG. 4 shows the top view of the embodiment of the configuration for 3D printing monitoring. This top view corresponds to the side view shown in FIG. 3. The layers 15 extend essentially along the length of the electrodes, such that their length can exceed the length of the electrodes. The electrical connector 16 extends essentially along the length of the electrodes, such that its length can exceed the length of the electrodes. The layers 15 are completely on electrical connector 16. Dielectric film 17 extends essentially the complete length of the electrodes.

(5) FIG. 5 illustrates testing configuration I, which involves the printing of a polymer-based material on a polymer-based substrate, such that the substrate is not modified (having no slot cut into the substrate). The stack of the multiple layers that constitute the layers in the printed object is on the substrate and is positioned in the region between the two electrodes. The two essentially coplanar electrodes (copper in the laboratory simulation) are positioned on the two sides of the stack region and are on top of the substrate. An insulating (dielectric) film is present between the electrode and substrate in case of the monitoring of the printing of a metal-based material, but not for the monitoring of the printing of a polymer-based material that is substantially non-conductive electrically. In the laboratory simulation of monitoring the printing of a polymer-based material, cellulosic paper is used as both the polymer-based layer and the polymer-based substrate. All dimensions are in mm. The vertical dimensions are not to scale. (a) Top view. (b) Side view.

(6) FIG. 6 illustrates testing configuration II, which involves the printing of a polymer-based material on a polymer-based substrate, such that the substrate has been modified (having multiple slots cut into the substrate). The stack of the multiple layers that constitute the layers in the printed object is positioned in the region between the two electrodes. The two essentially coplanar electrodes (copper in the laboratory simulation) are positioned on the two sides of the stack region and are on top of the substrate. The insulating (dielectric) film is present between the electrode and substrate for the monitoring of the printing of a metal-based material or other materials that are electrically conductive, but not for the monitoring of the printing of a polymer-based material that is substantially non-conductive electrically. All dimensions are in mm. The vertical dimensions are not to scale. (a) Top view. (b) Side view.

(7) FIG. 7 illustrates testing configuration III (33, i.e., 3 squares by 3 squares of aluminum oxide to form the substrate, with each square of dimensions 1 inch1 inch, so that the substrate size is 3 inches3 inches), which involves the printing of a metal-based material on a ceramic substrate. In other words, the substrate is made up of 9 aluminum oxide squares (dimensions 1 inch1 inch each) arranged in a 33 setup. In the laboratory simulation, the metal is aluminum and the ceramic is aluminum oxide. The stack of the multiple layers that constitute the layers in the printed object is on the substrate. The illustration shows the stack positioned at location A, which is the 1 inch1 inch area between the two electrodes. The testing is performed separately with the stack at locations A, B and C. The two essentially coplanar electrodes (copper in the laboratory simulation) are positioned on the two sides of the stack region and are on top of the substrate. All dimensions are in mm. The vertical dimensions are not to scale. (a) Top view. (b) Side view.

(8) FIG. 8 illustrates testing configuration III (55, i.e., 5 squares by 5 squares of aluminum oxide to form the substrate, with each square of dimensions 1 inch1 inch, so that the substrate size is 5 inches5 inches), which involves the printing of a metal-based material on a ceramic substrate. In other words, the substrate is made up of 25 squares of aluminum oxide (dimensions 1 inch1 inch for each square) arranged in a 55 setup. In the laboratory simulation, the metal is aluminum and the ceramic is aluminum oxide. The illustration shows the stack positioned at location A, which is the 1 inch1 inch square at the center of the 5 inch5 inch substrate. The testing is performed separately with the stack at locations A, B, C, D, E, F, G and H. The two essentially coplanar electrodes (copper in the laboratory simulation) are positioned on the two sides away from the center of the substrate and are on top of the substrate. All dimensions are in mm. The vertical dimensions are not to scale. (a) Top view. (b) Side view.

(9) FIG. 9 illustrates the positions of the artificially made through-holes (through the thickness of a layer) that serve as defects for the demonstration of the ability to detect defects. Each hole is indicated by an open circle, which is much larger than the actual hole.

(10) FIG. 10 illustrates testing configuration IV (33), which involves the printing of a metal-based material on a ceramic substrate. In the laboratory simulation, the metal is aluminum and the ceramic is aluminum oxide (dimensions 3 inches3 inches). The substrate is made up of 9 squares of aluminum oxide (dimensions 1 inch1 inch for each square) arranged in a 33 setup (i.e., a set-up with 3 squares by 3 squares). The stack of the multiple layers that constitute the layers in the printed object is on the substrate, i.e., in the 1 inch1 inch area between the two electrodes. A continuous metal-based material film (aluminum foil in the laboratory simulation) lines both the substrate and the proximate vertical edges (i.e., the proximate edges that are essentially perpendicular to the plane of the substrate) of the two electrodes (copper in the laboratory simulation), such that a dielectric film (double-sided adhesive tape in the laboratory simulation) is positioned between the vertical edge of each electrode and the proximate vertical surface of the metal-based material film. The dielectric film serves to avoid electrical short-circuiting of the two electrodes.

(11) FIG. 11 shows the effect of the applied normal compressive stress on the capacitance measured between the two electrodes. (a) Configuration I, for which the stress for the capacitance to level off as the stress increases is 2.77 kPa at which the capacitance is 1.91 pF. (b) Configuration III, for which the stress for the capacitance to level off as the stress increases is 30.12 kPa, at which the capacitance is 15.55 pF.

(12) FIG. 12 shows the effect of the number of layers in the stack on the capacitance measured between the two electrodes in the case of the printing of a polymer-based material, with the polymer-based material simulated in the laboratory with cellulosic paper. (a) The capacitance. (b) The fractional change in capacitance relative to the case of the bare substrate (without the stack). Solid circles: configuration I. Open circles: configuration II.

(13) FIG. 13 shows the effect of the fraction of a layer on the capacitance measured between the two electrodes in the case of the printing of a polymer-based material, with the polymer-based material simulated in the laboratory with cellulosic paper. (a) The capacitance. (b) The fractional change in capacitance relative to the case of the bare substrate (without the stack). Solid circles: configuration I. Open circles: configuration II.

(14) FIG. 14 shows the effect of the number of layers in the stack on the capacitance measured between the two electrodes in the case of the printing of a metal-based material, such that the printing is simulated in the laboratory with different numbers of layers of aluminum being printed on aluminum oxide using configuration III (33) (i.e., a set-up with 3 squares by 3 squares) for location A. (a) The capacitance. (b) The fractional change in capacitance relative to the case of the bare substrate (without the stack).

(15) FIG. 15 shows the effect of the fraction of a layer on the capacitance measured between the two electrodes in the case of the printing of a metal-based material, such that the printing is simulated in the laboratory with fractions of a layer of aluminum being printed on aluminum oxide using configuration III (33) (i.e., a set-up with 3 squares by 3 squares) for location A. (a) The capacitance. (b) The fractional change in capacitance relative to the case of the bare substrate (without the stack).

(16) FIG. 16 shows the effect of the number of layers in the stack on the capacitance measured between the two electrodes in the case of the printing of a metal-based material, such that the printing is simulated in the laboratory using different numbers of layers of aluminum being printed on aluminum oxide using configuration III (33) (i.e., a set-up with 3 squares by 3 squares) for location B. (a) The capacitance. (b) The fractional change in capacitance relative to the case of the bare substrate (without the stack).

(17) FIG. 17 shows the effect of the number of layers in the stack on the capacitance measured between the two electrodes in the case of the printing of a metal-based material, such that the printing is simulated in the laboratory using different numbers of layers of aluminum being printed on aluminum oxide using configuration III (33) (i.e., a set-up with 3 squares by 3 squares) for location C. (a) The capacitance. (b) The fractional change in capacitance relative to the case of the bare substrate (without the stack).

(18) FIG. 18 shows the effect of the number of layers in the stack on the capacitance measured between the two electrodes in the case of the printing of a metal-based material, such that the printing is simulated in the laboratory using different numbers of layers of aluminum being printed on aluminum oxide using configuration III (55) (i.e., a set-up with 5 squares by 5 squares) for location A. (a) The capacitance. (b) The fractional change in capacitance relative to the case of the bare substrate (without the stack).

(19) FIG. 19 shows the effect of the number of layers in the stack on the capacitance measured between the two electrodes in the case of the printing of a metal-based material, such that the printing is simulated in the laboratory using different numbers of layers of aluminum being printed on aluminum oxide using configuration III (77) (i.e., a set-up with 7 squares by 7 squares) for location A. (a) The capacitance. (b) The fractional change in capacitance relative to the case of the bare substrate (without the stack).

(20) FIG. 20 shows the effect of the number of layers in the stack on the capacitance measured between the two electrodes in the case of the printing of a metal-based material, such that the printing is simulated in the laboratory by using different numbers of layers of aluminum being printed on aluminum oxide using configuration III (99) (i.e., a set-up with 9 squares by 9 squares) for location A. (a) The capacitance. (b) The fractional change in capacitance relative to the case of the bare substrate (without the stack).

(21) FIG. 21 shows the effect of defects in the form of through holes on the capacitance measured between the two electrodes in the case of the printing of a metal-based material, such that the printing is simulated in the laboratory by using a single layer of aluminum, which is on aluminum oxide. Solid circles: configuration III (33 substrate arrangement, i.e., a set-up with 3 squares by 3 squares), location A). Open circles: configuration IV. (a) The capacitance. (b) The fractional decrease in capacitance relative to the layer with no holes.

DETAILED DESCRIPTION OF THE INVENTION

(22) This invention provides systems for monitoring the three-dimensional printing, including a system for monitoring the three-dimensional printing of metal-based materials. The invention also provides a method of monitoring three-dimensional printing.

(23) This invention provides a system for monitoring the three-dimensional printing of printing material. This system (illustrated in FIGS. 1 and 2) comprises two essentially coplanar electrodes (10 and 11), which are electrically conductive. The system also comprises substrate (12), which provides a surface on which the printing occurs. The substrate is substantially electrically non-conductive. The electrodes are positioned on the substrate. The proximate edges (18 and 19) of the electrodes are essentially parallel. The edges are separate from one another by a distance, which ranges from 5 mm to 300 mm. Each of said electrodes is substantially smaller in area than said substrate. The system also comprises a plurality of layers (15), which are formed layer-by-layer on the substrate (12) by the printing. The layers are derived from the printing material. For example, the printing material is a liquid metal, whereas the layers derived from the printing material are the metal that has been solidified from the liquid metal. The layers are positioned in a region, which extends in the direction essentially perpendicular to the edges from the location of one edge (18) to the location of the other edge (19) essentially in the plane of the surface. The two electrodes (10 and 11) are electrically oppositely charged, as enabled by an alternating electric current, which flows from one electrode to the other electrode. The alternating electric current partly flows in the layers (15). The two electrodes (10 and 11) exhibit a capacitance between them. This capacitance ranges from 0.1 pF to 10 nF. The capacitance is essentially in a direction parallel to the surface and substantially perpendicular to the proximate edges.

(24) The layers (15) are preferably selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, ceramic, ceramic-based material, ceramic-matrix composite, aluminum oxide, aluminum nitride, silicon carbide, silicon dioxide, cement-based material, cement paste, cement mortar, concrete, polymer, polymer-based material, polymer-matrix composite, thermosetting polymer, thermoplastic polymer, biopolymer, photopolymer, organic-based material, and combinations thereof.

(25) The substrate (12) preferably comprises material selected from the group consisting of: ceramic, ceramic-based material, ceramic-matrix composite, aluminum oxide, aluminum nitride, silicon carbide, silicon dioxide, polymer, polymer-based material, polymer-matrix composite, organic-based material, cement-based material, cement paste, cement mortar, concrete, and combinations thereof. The two electrodes (10 and 11) are preferably essentially the same in composition and preferably comprise material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, electrically conductive polymer, and combinations thereof. The electrodes (10 and 11) are preferably electrically connected, the electrical connection (16) being positioned on the substrate (12), being in the plane of said substrate (12), and providing an exterior surface on which said printing occurs; the electrical connection (16) is preferably more electrically conductive than the substrate.

(26) This invention also provides a system for monitoring three-dimensional printing of printing material. The system (as illustrated in FIGS. 3 and 4) comprising two essentially coplanar electrodes (10 and 11), which are electrically conductive. This system also comprises substrate (12), which provides a surface on which the printing occurs. The electrodes are positioned on the substrate (12). The proximate edges (18 and 19) of the electrodes are essentially parallel. The edges (18 and 19) are separate from one another by a distance. Each of said electrodes is substantially smaller in area than said substrate (12). The system also comprises a plurality of layers (15), which are formed layer-by-layer on the substrate (12) by the printing, and are derived from the printing material. The layers (15) comprise material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, and combinations thereof. The layers (15) are positioned in a region, which extends in the direction essentially perpendicular to said edges from the location of one said edge (18) to the location of the other said edge (19) essentially in the plane of said surface. The substrate (12) is less electrically conductive than the metal layers (15). The two electrodes (10 and 11) are electrically connected. The electrical connection (16) is positioned on the substrate (12) and is in the plane of the substrate (12). The electrical connection provides an exterior surface on which the printing occurs. Furthermore, the electrical connection (16) is more electrically conductive than the substrate (12). The two electrodes (10 and 11) are electrically oppositely charged, as enabled by an alternating electric current, which flows from one electrode to the other electrode. The alternating electric current partly flows in the layers (15). The two electrodes (10 and 11) exhibit a capacitance between them. This capacitance ranges from 0.1 pF to 10 nF. The capacitance is essentially in a direction parallel to the surface and substantially perpendicular to the proximate edges.

(27) The distance mentioned in the last paragraph preferably ranges from 5 mm to 300 mm. The electrical connection (16) preferably comprises material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, and combinations thereof. The electrical connection extends essentially the distance between said two proximate edges (18 and 19); a dielectric film (18 and/or 19), which exhibits substantial electrical resistance, is preferably positioned between the electrical connection and at least one of the electrodes (10 and 11). The substrate (12) preferably comprises material selected from the group consisting of: ceramic, ceramic-based material, ceramic-matrix composite, aluminum oxide, aluminum nitride, silicon carbide, silicon dioxide, and combinations thereof. The two electrodes (10 and 11) preferably comprise material selected from the group consisting of: metal, metal-based material, metal alloy, metal-matrix composite, aluminum, copper, nickel, titanium, steel, and combinations thereof, and are preferably essentially the same in composition.

(28) The dielectric film (18 and/or 19) preferably has thickness less than 200 m. In case of printing that involves elevated temperatures (as in the case of typical metal printing), the dielectric film should be a material that can withstand the elevated temperatures. Examples of materials that can withstand the elevated temperatures are ceramic fiber mats, ceramic sheets, and ceramic fabric.

(29) This invention also provides a method of monitoring three-dimensional printing of printing material. This method comprises as the first step positioning two essentially coplanar electrodes (10 and 11) on a substrate (12). The electrodes (10 and 11) are electrically conductive. The proximate edges (18 and 19) of the electrodes (10 and 11) are essentially parallel and are separate from one another by a distance. Each of said electrodes (10 and 11) is substantially smaller in area than said substrate (12), which provides a surface on which the printing occurs. The substrate (12) is substantially electrically non-conductive. The printing involves layer-by-layer formation of a plurality of layers (15) on the substrate (12). The layers (15) are derived from the printing material, and are positioned in a region, which extends in the direction essentially perpendicular to said edges from the location of one said edge (18) to the location of the other said edge (19) essentially in the plane of the surface. The electrodes (10 and 11) are electrically oppositely charged, as enabled by an alternating electric current, which flows between said two electrodes (10 and 11). The alternating electric current partly flows in the layers (15). The method also comprises as the second step measuring the capacitance between the two electrodes (10 and 11).

(30) The distance mentioned in the last paragraph preferably ranges from 5 mm to 300 mm. The two electrodes (10 and 11) are preferably essentially the same in composition and preferably comprise material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, electrically conductive polymer, and combinations thereof. The substrate (12) preferably comprises material selected from the group consisting of: ceramic, ceramic-based material, ceramic-matrix composite, aluminum oxide, aluminum nitride, silicon carbide, silicon dioxide, cement-based material, cement paste, cement mortar, concrete, polymer, polymer-based material, polymer-matrix composite, organic-based material, and combinations thereof. The layers (15) preferably comprise material selected from the group consisting of: metal, metal alloy, metal-based material, metal-matrix composite, aluminum, copper, nickel, titanium, steel, ceramic, ceramic-based material, ceramic-matrix composite, aluminum oxide, aluminum nitride, silicon carbide, silicon dioxide, cement-based material, cement paste, cement mortar, concrete, polymer, polymer-based material, polymer-matrix composite, thermosetting polymer, thermoplastic polymer, biopolymer, photopolymer, organic-based material, and combinations thereof. The two electrodes (10 and 11) are preferably electrically connected, with the electrical connection (16) being positioned on the substrate (12), being in the plane of the substrate (12), and providing an exterior surface on which the printing occurs; this electrical connection (16) is preferably more electrically conductive than the substrate.

(31) This invention provides a novel technique of 3D printing monitoring. The technique involves electrical measurement in the form of capacitance measurement. The installation of devices in the printer is not necessary. Only electrodes need to be placed on the substrate (also known as the build plate).

(32) The electric field extending from one electrode to the other electrode spreads to the regions beyond the region between the two electrodes, thus enabling the monitoring of printed layers in the region between the electrodes as well as the regions in the vicinity of the electrodes. As illustrated in FIGS. 2 and 4, the printed layers can extend beyond the length of the electrodes, although they can be shorter than the length of the electrodes. The technique is effective for following the progress of layer-by-layer printing and for detecting defects in specific printed layers as the printing progresses.

(33) Relevant to this invention are a wide variety of 3D printing methods, which include Material Jetting, 3D Inkjet Printing (3DP), Polyjet Printing (MJP), Material Extrusion, Directed Energy Deposition (DED), Laser Metal Deposition (LMD), metal-wire-based Electron Beam Melting (EBM), and Fused Deposition Modelling (FDM).

EXAMPLES

Example 1

(34) The 3D printing monitoring is shown by laboratory simulation for up to 20 layers (consolidated by a pressure of 2.71 kPa in the laboratory simulation) and for fractional layers down to of a layer. The method involves placing two copper electrodes on the substrate used for printing and measuring the capacitance between the two electrodes using an LCR meter at 2.000 kHz. The AC voltage between the two electrodes is 1.000 V.

(35) The capacitance increases with increasing pressure applied on the electrodes and levels off as the pressure increases. Therefore, a chosen adequate pressure is applied on each copper electrode in order to achieve a leveled-off high value of the capacitance. The pressure is 2.77 kPa and 30.12 kPa for polymer-based material printing and metal-based material (aluminum) printing, respectively. The capacitance is measured both for the bare substrate and for the substrate with the printed layer(s) on it.

(36) In case of the printing of a metal-based material, the demonstration involves aluminum layers (16 m thick for each layer) on an aluminum oxide substrate. The monitoring is effective for electrode spacing up to 76 mm.

(37) In case of the printing of a polymer-based material, the demonstration involves cellulosic paper layers (99 m thick for each layer) on a cellulosic paper substrate. The presence of slots in the cellulosic paper substrate reduces the contribution of the substrate to the measured capacitance.

(38) The fractional change in capacitance per layer is up to 0.41 and 0.0140 for polymer printing and aluminum printing, respectively. For polymer printing on a slotted cellulosic paper substrate, the capacitance of the region between the two electrodes is 1.65 and 9.48 pF for 0 and 10 layers, respectively. For aluminum printing on aluminum oxide, the capacitance of the region between the two electrodes is 15.55 and 25.65 pF for 0 and 10 layers, respectively.

(39) The capacitance method is also effective for the detection of defects in the printed layer, as shown for aluminum printing on aluminum oxide. The capacitance decreases monotonically with increasing amount of defects. The sensitivity for defect detection is enhanced by adhering the ends of the aluminum layer closest to the substrate to the copper electrodes, so as to promote the current through this aluminum layer, and hence also promoting the current through the layers above this aluminum layer.

Example 2

(40) The laboratory simulation of 3D printing involves layer-by-layer stacking of a material in the form of a thin sheet, with the stack being built on the surface of a substrate, and measuring the in-plane capacitance (i.e., the capacitance in the plane of the surface) of the stack using essentially coplanar copper electrodes that are on the substrate away from the stack.

(41) Four configurations are used. Configuration I (FIG. 5) involves an unmodified cellulosic paper (commercial writing paper with conventional cellulosic fibers) substrate, whereas configuration II (FIG. 6) involves a modified cellulosic paper substrate. The substrate modification in configuration II involves the presence of 15 slots that are equally spaced at a distance of 2.0 mm and are all perpendicular to the surface of the substrate and are parallel to the proximate edges of the electrodes. The slots are made using an office paper cutter and serve to decrease the contribution of the substrate to the measured capacitance of the system consisting of the substrate and the stack. By decreasing the substrate contribution to the measured capacitance, the sensitivity of the monitoring of the stack built on the substrate is enhanced.

(42) The stack consists of various numbers of layers that range from 1 to 20. In case of a single layer, various fractions of a layer are used, such that each fractional layer is rectangular, centered between the two electrodes, and parallel to the proximate edges of the electrodes.

(43) In configurations I and II, ordinary commercial writing paper (cellulose fiber paper) of thickness 98.70.3 m (as obtained by measuring the thickness of a stack of 10 sheets of the paper and dividing this thickness by 10) constitutes the thin sheet (layer) used to form the stack for the simulation of layer-by-layer printing of a polymer-based material. The paper is used both as the substrate and the layers in the layer-by-layer printing (FIGS. 5 and 6).

(44) In configurations I and II, the stack (area=27.0925.42 mm.sup.2=688.63 mm.sup.2) is positioned and centered between the two electrodes, each of size 25.4225.42 mm (FIGS. 5 and 6). The distance d between the electrodes is the distance between the proximate edges of the two electrodes, as measured by using calipers; its value is 27.09 mm. In case of the stack consisting of a fraction of a layer, the fractional layer is centered in the area between the two electrodes.

(45) In configuration III, aluminum foil of thickness 16.40.1 m constitutes the thin sheet (layer) used to form the stack for the simulation of layer-by-layer metal printing. Aluminum oxide is used as the substrate (FIG. 7). The aluminum oxide is a commercial substrate material containing 96 wt. % Al.sub.2O.sub.3. The thickness is 6774 m. The edge length of each square piece is 25.500.06 mm. A close-packed 33 arrangement of 9 pieces makes up a square of dimensions 76.5 mm76.5 mm, as shown in FIG. 7(a), where locations A, B and C refer specifically to three of the nine pieces of the aluminum oxide (dimensions 25.5 mm25.5 mm, i.e., 1 inch1 inch for each piece) that make up the substrate with dimensions 76.5 mm76.5 mm.

(46) In configuration III (33), the stack (area=25.50 mm25.50 mm=650.25 mm.sup.2) is positioned to cover the entirety of region A, B or C, each of size 25.4225.42 mm (FIG. 7). The distance d between the electrodes is the distance of 25.50 mm between the proximate edges of the two electrodes. In case of the stack consisting of a fraction of a layer, the fractional layer is centered in the region A, B or C.

(47) Similarly, 55, 77 and 99 arrangements are made, as illustrated in FIG. 8 for the 55 substrate arrangement, for which locations A, B, C, D, E, F, G and H are comparatively studied. The study of the multiple locations is for evaluating the effectiveness of the printing monitoring for different locations of the stack relative to the positions of the electrodes. In addition, location A is studied for the 33, 55, 77 and 99 substrate arrangements in order to investigate the effect of d on the sensing effectiveness.

(48) In order to test the effectiveness of the monitoring method of this invention for the detection of defects in a 25 mm25 mm square specimen, an aluminum foil (a single layer) is punctured with through holes (made by manual penetration using a commercial thumbtack) placed at different selected locations on the specimen (FIG. 9). Initially, the capacitance is measured for the case without holes. Thereafter, capacitance measurements are taken with the addition of each new hole. These holes constitute the defects to be detected by capacitance measurement. The aluminum specimen is placed at location A of configuration III (substrate arrangement 33).

(49) Configuration IV, as illustrated in FIG. 10, is the same as configuration III, except that a single continuous layer of aluminum foil lines both the substrate (location A) and the proximate vertical edges of the two copper electrodes. Double-sided adhesive tape (commercial, three layers) is used to join the aluminum foil to the two electrodes. Configuration IV enables the current that flows from one electrode to the other electrode to flow in the aluminum foil layer to a greater degree than the case of configuration III. The adhesive tape also serves to provide an insulating (dielectric) film between the foil and each electrode, so that there is no short-circuiting of two electrodes. Short-circuiting means that the resistance is very small or negligible, and would make it not possible to measure the capacitance. By enhancing the current flow in the aluminum layer, the effectiveness of the technique to detect defects in the aluminum is enhanced.

(50) A known metal weight (electrically insulated) is placed on top of each copper electrode in order to provide an adequate electrical contact between the electrode and the specimen. This weight is determined by measuring the capacitance vs. pressure. The capacitance increases with increasing pressure, until it levels off, as shown in FIG. 11 in case of configurations I and III. The pressure applied to each electrode (with the weight of the copper included) is approximately the minimum pressure for the curve of capacitance vs. pressure to level off. The pressure used is 2.77 kPa (at which the capacitance is 1.91 pF) and 30.12 kPa (at which the capacitance is 15.55 pF) for polymer printing (configurations I and II) and metal printing (configuration respectively.

(51) In all four configurations (I, II, III and IV), by using a known weight, a pressure of 2.71 kPa is applied to the stack in the direction perpendicular to the layers in the stack in order to consolidate the stack. Only the part of the substrate directly beneath the stack receives this pressure. The weight applied on a fractional layer is proportionally reduced, so that the pressure is kept the same, regardless of the area of the fractional layer.

(52) The capacitance is measured using a precision LCR meter (Instek LCR-816 High Precision LCR Meter, 100 Hz-2 kHz). The frequency used is 2.000 kHz. The voltage is 1.000 V. The capacitance is that for the equivalent electrical circuit of a capacitor and resistor in series. Measurement of the capacitance is conducted firstly on the bare substrate and then conducted in the order of increasing number of layers in the stack and in the order of decreasing fraction of a layer in case of a single layer.

Example 3

(53) Four configurations (I, II, III and IV) are used in the laboratory simulation of the layer-by-layer monitoring of 3D printing. The four configurations are described in Example 2.

(54) In case of configuration I (with the substrate in the form of cellulosic paper without slots, as described in Example 2) and configuration II (with the substrate in the form of cellulosic paper with slots, as described in Example 2), the capacitance increases monotonically with increasing number of layers, including fractional numbers of layers down to , thus indicating the effectiveness of the layer-by-layer monitoring (FIGS. 12 and 13). Configuration II is more effective than configuration I.

(55) In case of whole numbers of layers, FIG. 12 shows that, for the same number of layers, the capacitance is essentially equal for configurations I and II, while the fractional change in capacitance relative to the bare substrate is higher for configuration II than configuration I. In case of fractions of a single layer, FIG. 13 shows that, for the same fraction of a layer, the capacitance is lower for configuration II than configuration I, while the fractional change in capacitance is higher for configuration II. The greater fractional change in capacitance for configuration II is attributed to the lower capacitance for the bare substrate (0 layer) for configuration II, as resulting from the slots in configuration II reducing the current that can flow in the substrate.

(56) In case of configuration III (printing aluminum on aluminum oxide, 33 (i.e., a set-up with 3 squares by 3 squares, as described in Example 2), the layer-by-layer monitoring is also effective; including that of fractions of a single layer down to (FIGS. 14 and 15). The fraction of is not included in the study concerning configuration III, but is included in the study concerning configurations I and II. This is because of the greater difficulty of cutting aluminum foil than cellulosic paper into narrow strips that can be handled.

(57) FIGS. 14, 16 and 17 show corresponding results for locations A, B and C for configuration III (substrate arrangement 33). As shown in FIG. 7(a), location A is entirely between the two electrodes, location B is directly above region A, so that it is not between the two electrodes; and location C is directly above one of the electrodes. The monitoring is comparably effective for locations A and B, but is less effective for location C. For location C, the increase of the capacitance with the number of layers does not follow a smooth curve; for locations A and B, the increase follows a smooth curve. As shown in Table 1, the capacitance and fractional change in capacitance are both similar for locations A and B, but are considerably smaller for location C. This means that the electric field lines spreading away from the region between the two electrodes have no difficulty reaching location B strongly, but has difficulty reaching location C strongly.

(58) TABLE-US-00001 TABLE 1 Results of laboratory simulation of capacitance-based layer-by-layer 3D printing monitoring. Regions A, B and C are defined in FIG. 7(a). Fractional change in Capacitance (pF) capacitance.sup. Correlation Configuration Location 0 layer 10 layers 1 layer 10 layers coefficient I* A 1.91 9.33 0.96 3.91 0.989 II.sup. A 1.65 9.48 1.37 4.75 0.978 III.sup. A 15.55 25.65 0.18 0.65 0.977 III.sup. B 15.55 26.01 0.14 0.67 0.994 III.sup. C 15.55 18.21 0.04 0.17 0.910 *Paper on paper without slots. .sup.Paper on paper with slots. .sup.Aluminum on aluminum oxide (3 3) (i.e., a set-up with 3 squares by 3 squares). .sup.Relative to the case of 0 (zero) layer.

(59) The correlation coefficient in the plot of capacitance (y value) vs. the number of layers (x value) is obtained by using the equation

(60) $\begin{array}{cc}\mathrm{Correlation}\mathrm{coefficient}=\frac{\left(x-x^{}\right)\left(y-y^{}\right)}{\sqrt{{\left(x-x^{}\right)}^2{\left(y-y^{}\right)}^2}},& \left(1\right)\end{array}$
where x and y are the averages of the x values and they values. The higher is the correlation coefficient, the less is the data scatter associated with the curve of capacitance vs. the number of layers.

(61) As shown in Table 1, the correlation coefficient is similarly high for location A of all three configurations (I, II and III). For configuration III, the coefficient is similarly high for locations A and B, but is lower for location C.

(62) Among configurations I, II and III, Table 1 shows that configuration II gives the highest fractional change in capacitance, whereas configuration III gives the lowest fractional change. On the other hand, configuration III gives the highest capacitance, whereas both configurations I and II give similarly low values of the capacitance. The high capacitance for configuration III is attributed to the conductivity of aluminum and the consequent substantial current that can flow through the aluminum. In contrast, the cellulosic paper in configurations I and II is essentially electrically non-conductive. The high fractional change in capacitance for configurations I and II is due to the low value of the capacitance for the bare substrate (0 layer). The low fractional change for configuration III is due to the high value of the capacitance for the bare substrate. In practice, both a high fractional change in capacitance and a high capacitance are desirable.

(63) Table 2 shows that the capacitance at 10 layers tends to be relatively low for location C of configuration II and III (substrate arrangement 33). For configuration III (substrate arrangement 55), the capacitance at 10 layers tends to decrease in the order from location A to location H. The fractional change in capacitance per layer, as obtained from the gradient of the best-fit straight line for the curve of fractional change in capacitance vs. the number of layers, is also shown in Table 2. This quantity shows the dependence on the location more clearly than the capacitance at 10 layers.

(64) TABLE-US-00002 TABLE 2 Results of testing conducted for the stack of layers positioned at different locations relative to the electrodes. The fractional change in capacitance per layer is obtained from the gradient of the best-fit line obtained for the specific set of data. Fractional Capacitance change in at 10 capacitance/ Correlation Configuration Location layers (pF) layer coefficient II A 4.750 0.3600 0.978 II B 9.090 0.4100 0.985 II C 2.630 0.0420 0.804 III (3 3) A 25.65 0.0590 0.977 III (3 3) B 26.01 0.0630 0.994 III (3 3) C 18.21 0.0160 0.910 III (5 5) A 9.190 0.0140 0.998 III (5 5) B 9.080 0.0130 0.998 III (5 5) C 8.640 0.0075 0.998 III (5 5) D 9.060 0.0130 0.999 III (5 5) E 8.920 0.0110 0.999 III (5 5) F 8.330 0.0036 0.958 III (5 5) G 8.240 0.0024 0.911 III (5 5) H 8.230 0.0022 0.919

(65) The correlation coefficient depends on the location, as shown in Table 2. For configurations II and III (substrate arrangement 33), the coefficient is high for locations A and B and is lower for location C. For configuration III (substrate arrangement 55), the coefficient is high for locations A, B, C, D and E, is lower for location F, and even lower for locations G and H. Nevertheless, the sensing is effective for all of the locations listed in Table 2.

(66) FIGS. 15, 18, 19 and 20 show the results for location and configuration III for substrate arrangement 33, 55, 77 and 99, respectively. Table 3 shows the results for the cases of 1 and 10 layers. The capacitance measured for location A decreases in the order: 33, 55, 77 and 99, whatever is the number of layers. The fractional increase in capacitance at location A due to the layers decreases from 33 to 55, whatever is the number of layers. The fractional increase in capacitance remains low for 77 and 99. The correlation coefficient is high for substrate arrangements 33 and 55, but is lower for 77 and even lower for 99. Thus, the maximum distance between the electrodes for reliable sensing corresponds to the case of substrate arrangement 55.

(67) TABLE-US-00003 TABLE 3 Results of testing conducted for location A for substrate arrangements 3 3, 5 5, 7 7 and 9 9 and configuration III. Fractional increase Substrate Capacitance (pF) in capacitance* Correlation arrangement 0 layer 10 layers 1 layer 10 layers coefficient 3 3 15.55 25.65 0.18 0.65 0.977 5 5 8.02 9.19 0.01 0.15 0.998 7 7 6.28 6.41 0.01 0.02 0.674 9 9 1.07 1.19 0.02 0.11 0.283 *Relative to the case of zero (0) layer.

(68) Concerning the detection of defects, FIG. 21 shows the relationship between the number of holes and (a) the capacitance and (b) the fractional decrease in capacitance. The fractional decrease is measured in relation to a layer with no holes. The capacitance decreases monotonically in very small steps as the number of holes is increased (FIG. 21(a)). Also, as the number of holes is increased, the fractional decrease in capacitance increases monotonically. This means that the defect detection is feasible. The capacitance decrease is attributed to the decrease in the effective electrical permittivity of the aluminum due to the holes and the fact that the relative permittivity of air is low (equal to 1).

(69) FIG. 21 shows that configuration IV is more effective than configuration III for sensing defects in the form of holes. Configuration IV gives larger fractional decrease in capacitance and greater linearity in the curve of the capacitance vs. number of holes. The superiority of configuration IV is expected, due to the enhanced current path through the aluminum in configuration IV.

(70) Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various additions, substitutions, modifications and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the claims which follow.