WO2016060703A1

WO2016060703A1 - Additive manufacturing using heated and shaped gas beams

Info

Publication number: WO2016060703A1
Application number: PCT/US2014/068951
Authority: WO
Inventors: Bulent M. Basol
Original assignee: 3D Forms, Inc.
Priority date: 2014-10-14
Filing date: 2014-12-05
Publication date: 2016-04-21

Abstract

A first unprocessed layer with portions may be formed over a base, the unprocessed layer comprising particles. An orifice (26A) of a fusing tool (20A) may be brought in proximity to a first portion of the first unprocessed layer, thereby forming a first gap between the orifice of the fusing head and the first portion. A first heated gas beam (29A) may be directed from the orifice to the first portion through the first gap so as to heat the particles therein and convert the first portion of the first unprocessed layer into a first fused segment. To fuse other portions of the first unprocessed layer, the orifice may then be moved over the first unprocessed layer as the heated gas beam continues to heat and fuse the other portions.

Description

ADDITIVE MANUFACTURING USING HEATED AND SHAPED GAS BEAMS

Related Application Data

This application is related to U.S. Provisional Application No 62/122,115 filed on October 14, 2014, and U.S. Provisional Application No. XX/XXX,XXX, titled "Additive

Manufacturing Using Heated and Shaped Gas Beams," to Bulent M. Basol, filed on

November , 2014, the contents of which are hereby incorporated by reference for all purposes.

Field of the Invention

[0001] The present inventions relate to methods and apparatus for forming thin fused

(cured, sintered) layers on top of each other by melting, fusing, curing or sintering particulate or powder materials by a heated and shaped gas beam. The present inventions have applications in additive manufacturing wherein three-dimensional (3D) objects are manufactured by a layer-by-layer processing approach.

Background of the Invention

[0002] 3D printing is a general term often used to define the general class of additive manufacturing approaches that sometimes may also be called solid free-form manufacturing techniques, among other names. The terms "3D printing" and "additive manufacturing" are used interchangeably herein and they refer to techniques that form a solid 3D object from a digital model using 3D printers or materials printers. 3D printing technologies are used for prototyping and distributed manufacturing in a variety of fields such as automotive, health, military, aerospace, tools, food, art and fashion.

[0003] In additive manufacturing, virtual blueprints of designs are provided to the materials printers and digital cross sections of these designs are produced or printed successively in layer form using materials that may be in the form of liquid, powder, paper or sheet metal. As the printed cross sectional layers join and merge, the final 3D shape emerges. Depending on the material used and the nature of the 3D shape, the printing process may take several hours or even days.

[0004] Additive manufacturing techniques differ from each other in the ways the printed cross sectional layers are formed. Some methods that can process plastics may deposit polymeric materials in molten phase to form the cross sectional layers on top of each other. In a material extrusion approach, for example, a filament of a thermoplastic material is extruded onto a base through a heated orifice in a continuous manner or in the form of droplets. In material jetting processes an ink-jet printing head delivers the build material on a base in the form of small droplets. These build materials are often photopolymers that get cured and solidified by UV light right after deposition. In another technique called "vat

Photopolymerization", a liquid photopolymer is exposed to UV radiation in the shape of a cross sectional layer to be formed, and hardened.

[0005] In powder bed fusion processes thermal energy fuses, cures or sinters selective regions of a powder layer by at least partially melting the particles therein, and forms the desired cross section. There have been various sources of thermal energy proposed in prior art 3D printing processes. One source is a laser beam as described in US Patent No.

5,017,753. This is a method that is commonly used in 3D printers of today. Another source is electron beam as disclosed in US Patent No. 7,454,262 and this approach is also commercial today. One other method of providing heat to a selective region of a powder layer is described in US Patent No. 5,389,408 and US patent application publication 2012/0329659, wherein an alternating magnetic field is generated within an applicator tip and wherein the magnetic field is directed from the applicator tip onto the surface of a powder layer. It is stated that the alternating magnetic field inductively heats and fuses the particles within the powder layer, provided that the particles are conductive. In another approach, as described in US Patent No. 8,535,036, a polymeric powder layer may be masked and infrared (IR) radiation may be sent through the mask onto the powder layer, heating regions that are not covered by the light- absorbing mask. This technique may not be able to process high melting materials and it requires a mask for each cross sectional layer to be formed during the process. In yet another method given in US patent application publication 2012/0201960, a thermal printer head is used to selectively heat regions of a low temperature melting powder layer through a protective sheet which is in contact with the powder layer. Such an approach would not melt and fuse metallic powders for 3D printing. In US Patent No. 8,070,474 a lamination shaping apparatus is described comprising an optical unit, which irradiates a concentrated light beam onto a selected region of the powder bed. In US Patent No.

5,786,562 a method is described employing an energy-producing device, which comprises at least two electrodes detached from each other and connected to at least a voltage source. Figure 1 A reproduces a drawing of the prior art energy producing means 22 A of US Patent No. 5,786,562, brought in proximity to a powder layer 11, disposed on a base or substrate 12, to heat and melt the particles in a section of the powder layer 11 across from a burning point 50. A plasma-arc 51 is formed at the burning point 50 by applying a voltage to a first electrode 47 and a second electrode 46 using a voltage source 48. A gas 49 is flown in through a duct 49 and it passes through a porous spacing means 54, arriving at the burning point 50 where plasma-arc 51 is generated. This prior art method proposes to utilize the plasma-arc 51 formed at the burning point 50 to melt the particles close to the burning point 50. In other embodiments described in US Patent No. 5,786,562, an arc is formed between an electrode and a conductive powder layer to melt portions of the powder layer exposed to the arc. There are many problems associated with these prior art approaches. First of all, conductive powders such as metal powders have very high (typically > 1000 °C) melting points and thus they require the plasma-arc 51 of Figure 1 to be extremely close to the particles, such as 0.1-2 mm. Such close proximity is also necessary because of the

requirements of 3D printing process which needs to control the size of the fused sections of the powders to the tune of 0.05-0.2 mm. Arc is an electric discharge between two electrodes separated by an insulating gas. At a sufficiently high voltage between the two electrodes, the gas electrically breaks down forming an arc of low resistance and the heat from the arc helps ionize more gas molecules forming a plasma-arc. The temperature of a plasma-arc may reach 10,000-20,000 °C instantaneously, and the heat causes sudden expansion of the gas around and creates large amounts of pressure. When applied to 3D printing, which involves melting thin (such as 0.05-2 mm thick) layers of powder particles weighing a fraction of a milli-gram, such creation of an arc nearby disturbs the powder layer and scatters particles therein instead of melting and fusing them. Even the portions of the powder layer a few millimeters, even centimeters away from the arc get disturbed before getting exposed to the arc for melting and fusing. As shown by the data in reference (R.H. Lee, "Pressures developed by arcs", in IEEE Transactions on Industry Applications, vol. IA-23, No. 4, p.760, July/ August, 1987) a 500

Ampere arc can create about 30 gm/cm of pressure at a distance of 3 cm. Extrapolating this data to smaller distances show that the pressure generated by an arc can exceed 250 gm/cm at a distance of 3 mm, and can be much higher at even smaller distances even if the current is reduced. Considering the fact that in a 3D printing process with sufficient resolution the light weight powder particles may be 0-2 mm away from the arc, the pressure exerted on the particles weighing only a fraction of a milli-gram would be very high for not to disturb them.

[0006] As the prior art review above demonstrates, the thermal energy sources that cause melting and/or fusing of particles in "powder bed fusion" type 3D printing processes are light beams, electron beams and focused electromagnetic waves. Light beams that have been utilized include laser beams and small area or large area beams formed using sources such as high intensity infrared lamps. Direct heating by a thermal printer head has also been reported. However, the limited thermal energy provided by such a technique might be adequate to process only the low melting (<200 °C) polymeric substances. Also contacting the powder layer during processing may not be desirable since such physical contact may disturb the powder and it may also cause sticking of the molten material to the surface touching it. A prior art method discussed above disclosed the use of a plasma-arc or an arc or electric discharge as the energy producing means to heat and fuse powders. However, this method has the problem of having an exposed plasma-arc or arc in close proximity to the powder layer, which disturbs, even scatters around the particles of the powder layer and thus does not provide the accuracy and consistency that is required by 3D printing processes. Therefore, there is still a need to develop low cost and high throughput processing approaches and tools with flexibility to fabricate 3D shapes from polymeric, ceramic as well as metallic powders.

Brief Description of the Drawings

[0007] FIG. 1 shows a prior art technique utilizing a plasma-arc generator in close proximity to a powder layer.

[0008] FIGS. 2A through 2E show a process flow to fabricate a 3D object using a fusing head providing a heated gas beam.

[0009] FIGS. 3A-3E show various fusing tools comprising plasma cavities to heat a process gas.

[0010] FIG. 4 shows a fusing tool comprising a filament heater to heat a process gas.

[0011] FIG. 5 shows a fusing tool comprising multiple heads.

[0012] FIG. 6 shows a fusing tool comprising used gas removal lines.

Detailed Description of the Invention [0013] Present inventions provide methods and equipment for forming a 3D object in a layer-by-layer fashion, the methods comprising the steps of dispensing a source material over a base or substrate forming an unprocessed layer comprising particles, and then converting a selected region of the unprocessed layer into a fused layer section through a Heated Gas Beam Sintering (HGBS) process. It should be noted that in describing the present inventions "heated gas beam" refers, in general, to a shaped directional beam or column of a process gas heated to a process temperature by any means including by heat sources such as electrically heated filaments, electric discharges and plasmas. Electric discharges include capacitive plasmas, arcs and plasma-arcs that may be formed between two conductive electrodes by applying a voltage by a power supply between the two electrodes. The electrodes may be disposed in an enclosure or cavity and are separated by a gap. Plasmas formed by other energy sources such as lasers and microwave generators, or inductively coupled plasmas formed in cavities without electrodes may also be used to heat the process gas. The heated gas at the process temperature is shaped into a beam and directed towards the unprocessed layer by an orifice, the unprocessed layer being in a process environment kept at a background temperature, which is lower than the process temperature. A fused, cured or sintered layer refers to a film or layer that has been formed by melting, fusing, curing, or sintering of particles within an unprocessed layer, wherein the particles merge and join together to form a solid shape as a result of the melting, fusing, curing or sintering process. The source material may be in the form of a paste, ink or a dry powder comprising particles. The unprocessed layer may additionally comprise a binder, adhesive or glue that binds the particles therein to each other, to the base or substrate, or to both. Particles may be conductive, insulating, or semiconducting, and they may range in size from nanometers to millimeters, preferably from 10 nanometers to 500 micrometers, and they may be in a variety of shapes including, but not limited to, wires, whiskers, filaments, fibers, spheres, and flakes. [0014] Figures 2A through 2E show a process flow for manufacturing a 3D object in accordance with an embodiment of the present inventions. As shown in Figure 2A, during the first step of the manufacturing process, a fusing head 10 (fusing device, fusing tool) with an opening, hole or orifice 10A may be brought in close proximity to a first unprocessed layer 11 formed by dispensing a first source material over a surface of a substrate 12. A gap 13 may be formed between the orifice 10A and a top surface 1 IB of the first unprocessed layer 11. As shown in Figure 2B a heated gas column or beam 14 may be directed from the orifice 10A onto a portion 11A (see Figure 2A) of the first unprocessed layer 11 through the gap 13. As a result, the portion 11 A of the first unprocessed layer 11 may be heated selectively by the heated gas beam 14 and converted into a first fused segment or fragment 15. As shown in Figure 2B and Figure 2C, the fusing head 10 may then be moved over the first unprocessed layer 11 in directions controlled by a computer, such as a direction shown by the arrow 19 A, continuing to melt, fuse, cure or sinter more of the portions of the first unprocessed layer 11, eventually forming a first fused layer 16 with a predetermined shape, and leaving an excess portion 16A of the first unprocessed layer 11 unfused. The process may then be repeated by dispensing a second source material over the first fused layer 16 and the excess portion 16A of the first unprocessed layer 11, forming a second unprocessed layer 17, and by fusing a selected region of the second unprocessed layer 17 using the heated gas beam 14 as described before. As a result, as shown in Figure 2D, a second fused layer 18 may be formed over the first fused layer 16, wherein the first fused layer 16 and the second fused layer 18 may be bonded and fused together forming a three dimensional object 19 over the substrate 12, embedded in the excess portion 16A of the first unprocessed layer 1 land an excess section 17A of the second unprocessed layer 17. When the excess portion 16A, the excess section 17A and the substrate 12 are removed, the 3D object 19 may be obtained in its freestanding form as shown in Figure 2E. It should be noted that although the process is described using only two unprocessed layers for simplicity, in practice, considering the fact that a 3D object may be a few centimeters, or maybe meters high, many more unprocessed layers and process steps would be needed since a typical thickness of an unprocessed layer may be in the range of about 0.05-2 mm.

[0015] As can be seen from Figure 2A the first unprocessed layer 11 comprises particles. The first unprocessed layer 11 may additionally comprise a binder or glue that may attach or bind the particles to each other and possibly to the substrate 12. This way, no portion of the first unprocessed layer 11 may be disturbed by the heated gas beam 14, which may exert a gas pressure, especially on a section of the first unprocessed layer 11 across from the orifice 10A during the process. If the binder is present in the first unprocessed layer 11 then a flow rate of the heated gas beam 14 may be selected to be any convenient value. If the binder is not present in the first unprocessed layer 11, however, careful selection of the flow rate of the heated gas beam 14 would be necessary as will be discussed later.

[0016] Referring back to Figure 2A, although it is possible to bring the orifice 10A of the fusing head 10 in physical contact with the top surface 1 IB of the first unprocessed layer 11 during processing, it is preferable that the gap 13 is established between the orifice 10A and the top surface 1 IB so that the fusing head may not disturb and push around the particles within the first unprocessed layer 11, and so that any molten materials may not stick to the orifice 10A. The gap 13 may be less than 5 mm, preferably less than or equal to 3 mm, and most preferably less than or equal to 2mm. A small gap is preferred because the heated gas beam 14 (see Figure 2B) may travel through small gaps to the top surface 1 IB of the first unprocessed layer 1 lwithout loosing much heat to the environment and without loosing its shape, which is important for controlling the dimensional accuracy of the fused layer(s) to be formed. Also a flow rate of the gas within the heated gas beam 14 may be more easily minimized for small values of the gap 13, as will be explained later. The orifice 10A may have any cross sectional shape (taken in a plane substantially vertical to the heated gas flow direction) including a circular shape with a diameter in the range of about 0.05-5mm depending on the minimum width of an area of the first unprocessed layer 11 to be fused, smaller orifices yielding tighter control of the dimensions achievable. A preferred range of the largest dimension of a cross section of an orifice (taken in a plane substantially vertical to the heated gas flow direction) may be 0.05-2 mm, smaller dimensions being more preferable, for a preferred gap value of 0.2-3 mm. It is possible to provide suction (not shown) around the orifice 10A to remove an excess gas after it is used so that it does not overheat the process environment and the parts of an unprocessed layer that are not intended to be fused.

However, this excess heat may also be beneficially utilized to control a temperature of the unprocessed layer at a level that is lower than its melting temperature.

[0017] The heated gas beam 14 may comprise a variety of pure or mixed gasses including, but not limited to, argon, nitrogen, helium, neon, oxygen and air. A temperature of the heated gas beam may range from about 100 °C to over 2000 °C depending on the source material to be processed and the method of heating the gas to form the heated gas beam 14. For example, the temperature of the heated gas beam 14 may be in the range of 150-500 °C to process powder layers comprising polyamides (melting temperatures of 190-350 °C), polypropylene (melting temperature of about 130 °C), PEEK (melting temperature of about 343 °C) or TPU polyurethane (melting temperature of up to about 230 °C), whereas to process metallic or ceramic powders much higher temperatures may be needed.

[0018] Figures 3A-3E show various exemplary embodiments of fusing devices or tools, wherein heating of a process gas may be achieved through forming a thermal plasma in an enclosed cavity filled with the process gas and terminated with a process surface comprising a small hole, opening or orifice. It should be noted that a thermal plasma is different from a cold plasma that is often used in semiconductor processing. Whereas in cold plasmas the gas temperature is much lower than the electron temperature, which may be high, in thermal plasmas the gas temperature may be as high as the electron temperature. The fusing device 20 A of Figure 3 A may comprise a first electrode 21 A and a second electrode 22A. An insulator 23A may separate the first electrode 21A from the second electrode 22A except at a plasma cavity 24A or heating cavity where the first electrode 21 A and the second electrode 22A may face each other. The plasma cavity 24A acts as a plasma generator and it contains or encloses the plasma to prevent any arcs to reach an unprocessed layer being fused. The second electrode 22A comprises an orifice 26A. The first electrode 21A and the second electrode 22A may comprise highly conductive materials such as Cu, W, Mo, Ta and C and may both be cooled through a circulating liquid or gas either within or in proximity to them (not shown). At least one gas passage 25A may connect the plasma cavity 24A to the outside of the fusing device 20A so that a process gas 28A may be flown into the plasma cavity 24A through the at least one gas passage 25A. When a power supply (not shown) is connected between the first electrode 21 A and the second electrode 22A and a large enough voltage is applied, a thermal plasma may be formed within the plasma cavity 24A between the first electrode 21A and the second electrode 22A, heating the process gas 28A therein. It should be noted that depending on the type of gas, the voltage and current applied between the first electrode 21A and the second electrode 22A, temperatures as high as 20,000 °C can be generated in the thermal plasma and a heated process gas column 29A may be flown out of the orifice 26A at controlled temperatures in the range of 100-3000 °C, or even higher. It should be noted that the orifice 26A beneficially forms a shaped, heated gas column, which is free from arcs or plasma-arcs that would exert high pressures on an unprocessed layer and scatter around the particles therein, especially in cases where the unprocessed layer may not contain a binder or glue to bind such particles together. Therefore, a gap between the unprocessed layer and the outside opening of the orifice 26A can be kept small, such as in a range of 0.5-2mm.

[0019] Another version of an elongated fusing device 20B is shown in Fig. 3B. The elongated fusing device 20B may comprise a center electrode 2 IB and a peripheral electrode 22B. An insulator plug 23B may separate the center electrode 21B from the peripheral electrode 22B. A bottom plate 23BB, which acts as a process surface may comprise an orifice 26B that is the source of a heated process gas column 29B. The bottom plate 23BB may or may not be substantially perpendicular to a sidewall 22C of the elongated fusing device 20B and it may be insulating. At least one gas passage 25B may connect a plasma generation cavity 24B to the outside of the elongated fusing device 20B so that a process gas 28B may be flown into the plasma generation cavity 24B through the at least one gas passage 25B. It should be noted that in the elongated fusing device 20B of Figure 3B comprising an insulating bottom plate 23BB, a plasma or arcs may only form between the center electrode 2 IB and the peripheral electrode 22B. There would be, however, no arcs formed between the center electrode 2 IB and the bottom plate 23BB since the bottom plate 23BB is insulating. This way any arcs formed within the plasma generation cavity 24B are substantially contained within the cavity and they don't come out of the small orifice 26B. A parallel plate version of a fusing tool 20C, comprising two parallel plate electrodes 21C and 22C mounted between a top insulating block 23C and a bottom insulating block 23CC is shown in Figure 3C. A process gas 28C may enter a heating cavity 24C, where a plasma may be formed between the parallel plate electrodes 21C and 22C, and a heated gas 29C may be directed onto an unprocessed layer through an orifice or hole 26C in the bottom insulating block 23CC. It should be noted that in the designs of Figures 3A, 3B and 3C care is taken to keep any high pressure arcs or plasma-arcs that may be present, away from an unfused or unprocessed layer by substantially containing or enclosing them in a plasma cavity and by supplying an arc-free, heated and shaped gas beam through an orifice.

[0020] It is known that the universal voltage-current characteristics of a direct current

(DC) plasma, as well as a low frequency alternating current (AC) plasma may operate in a "dark discharge" at high voltage and low current values. After gas breakdown occurs at sufficiently high voltage, the current may increase and a "glow discharge" regime may initiate at a lower voltage value. At even higher currents, "arc discharge" occurs and voltage gets reduced further. It is in this "arc discharge" regime that thermal arcs are formed.

Thermal arcs or plasma-arcs are used in the industry to melt bulk materials and cut thick layers of metals because of the extremely high temperatures they provide. However, as explained before they are not suitable for 3D printing applications where a thin unprocessed layer comprising particles is exposed to such arcs, especially if there is no glue in the unprocessed layer holding the particles together. Figures 3A-3C show preferred

embodiments of the present inventions that have capability to contain plasma-arcs within enclosed cavities and use them just to heat the processes gasses, which may then be directed towards unprocessed layers in the form of arc-free heated gas beams.

[0021] In another embodiment of the present inventions, it is preferable that a plasma generated in a plasma cavity or a heating cavity of a fusing device or tool is a Radio

Frequency (RF) plasma with a frequency selected from the group of High Frequency (HF), Very High Frequency (VHF), Ultra High Frequency (UHF), Super High Frequency (SHF) and Extremely High Frequency (EHF). It is more preferable that the frequency is one of HF and VHF. It is most preferable that the frequency is VHF and it is higher than 50 MHz, such as 60-162 MHz. It should be noted that the frequency ranges in the HF, VHF, UHF, SHF and EHF regimes are 3-30 MHz, 30-300 MHz, 300 MHz-3GHz, 3-30GHz and 30-300GHz, respectively, and the RF plasmas can be generated using RF power supplies with the desired frequency output. RF plasmas with the frequency ranges given above are different from the DC or lower frequency AC plasmas. Using RF plasmas one can minimize, even eliminate arcs and therefore provide arc-free heated gas beams to the process. RF plasmas can be generated and sustained at higher pressures and at low gas flow rates, which are attractive for 3D printing application of the present inventions. It should be noted that use of a process environment at atmospheric pressure reduces cost, increases plasma density and the generated heat, which are important factors for 3D printing. RF plasma generation also delivers most of the heat energy to the ionized gas in the plasma rather than to the electrodes. In DC and low frequency AC plasmas electrodes may get excessively heated and get damaged since the heavy gas ions can follow the low frequency of the applied voltage and continually hit the surfaces of the electrodes. To reduce this detrimental effect, large flows of gas may be needed around the electrodes. Large gas flow rates, however, may not be compatible with the 3D printing processes as described herein. Therefore, use of an RF plasma has unique benefits for the processes of the present inventions, one of these benefits being the ability to utilize process gasses at very low flow rates.

[0022] The fusing tools or devices shown in Figures 3A-3C can be used to provide heated process gases to the 3D printing process described before, wherein the process gases may be heated by arc-free plasmas within the heating cavities or plasma cavities by applying RF power to the electrodes. It should be noted that in this case any or all of the various electrodes shown in Figs. 3A-3C (such as 21A, 22A, 21B, 22B, 21C and 22C) may have an insulating layer on their surface within the plasma or heating cavities, since RF power can be transmitted through insulators but arcs cannot form. Fusing tools without any electrodes may also be employed to heat a process gas if an RF power is used to generate the plasma. Figure 3D shows such an exemplary fusing tool 20D. The exemplary fusing tool 20D comprises a cavity 24D enclosed by an insulating envelope 20DD. An RF coil 20DDD is wrapped around the insulating envelope 20DD and an RF power from a RF power supply is provided to the RF coil 20DDD generating an arc-free RF plasma within the cavity 24D. A process gas inlet 25D brings a process gas 28D into the cavity 24D, and the arc-free heated process gas 29D flows out of the orifice or hole 26D towards an unprocessed layer to be processed.

[0023] A flow rate of a heated gas within a heated gas beam such as the heated process gas column 29B of Figure 3B may be selected so that the heated gas does not hit the unprocessed layer at high velocity and disturb or blow off the particles therein, especially if the unprocessed layer does not comprise a binder or glue to bind the particles to each other and possibly to a substrate that the unprocessed layer may be disposed on. This is very different from the use of plasma-arcs in bulk material processing such as metal cutting where extremely high gas velocities are needed to cool the electrodes and blow off the molten material from the metal sheet being cut. The velocity of the heated gas coming out of an orifice would be a function of multiple variables including the temperature of the gas, the pressure difference between the heating cavity and the process environment outside where the unprocessed layer is, and the size of the orifice. The preferred flow rate of a heated gas coming through an exemplary 1 mm diameter orifice onto an unprocessed layer may be < 0.5 liter/minute so that it does not disturb the particles within the unprocessed layer. The preferred flow rate of the heated gas may be scaled in proportion to the cross sectional area of the orifice, i.e. doubling the orifice diameter would increase the upper limit of the preferred heated gas flow rate by about 4 times (to < 2 liters/minute), and reducing the diameter to 0.5 mm would reduce the upper limit of the preferred heated gas flow rate by about 4 times (to < 0.125 liters/minute). This relationship may be approximately represented by the formula F< X/0.016, where F is the flow rate of the gas in liters/minute and X is the area of the orifice in units of cm . These small flow rates can be achieved, especially using the RF plasmas with the preferred frequency ranges indicated above. In DC or low frequency plasma-arcs, the plasma-arc formation voltage increases as the gas flow rate decreases and the temperature of the electrodes may also increase as described before. Therefore, at very small flow rates it becomes difficult or impossible to form and maintain plasmas at a voltage range of 100- 1000V, especially for diatomic process gases such as N₂, which are superior to mono-atomic gases such as Ar in terms of providing higher temperatures in their plasmas. The preferred RF plasmas with frequencies in the HF and VHF region can be formed at the low gas flow rates needed in the 3D processing approach of the present inventions, and they can be formed using diatomic gases at atmospheric pressure to obtain temperatures well above 3000 °C. For example, use of an RF power with a frequency of 50-162 MHz can start and maintain plasmas, even with N₂ gas, at around atmospheric pressure at the flow rates that are discussed above.

[0024] A fusing tool design comprising an auxiliary exhaust is shown in Figure 3E.

The fusing tool 20E of Figure 3E comprises an auxiliary exhaust line 25BB in addition to the orifice 26B. In this case the flow rate of a process gas coming in through the gas passage 25B can be selected to be able to maintain a plasma at a given frequency within the plasma generation cavity 24B, whereas by adjusting the relative diameters of the orifice 26B and the auxiliary exhaust line 25BB, only a desired low rate of the heated process gas may be flown out of the orifice 26B towards an unprocessed layer, the rest of the heated process gas leaving through the auxiliary exhaust line 25BB.

[0025] Figure 4 shows an embodiment of a fusing tool 30 wherein heat to a gas 31 is provided by a filament 32 placed in a heating cavity 33. As the gas 31 is flown into the heating cavity 33, electric power may be applied between a first terminal 34A and a second terminal 34B of the filament 32, heating it to elevated temperatures. The gas 31 may thus be heated and flown out of the heating cavity 33 through an orifice 35 in the form of a heated gas beam 36. This design may be used to process plastic powders with relatively low melting points.

[0026] When a heated gas beam contacts a portion of an unprocessed layer, it transfers at least some of its thermal energy to the portion. Thermal accommodation coefficient (TAC) is a constant that represents the extent to which the interchange of energy takes place by a stream of hot gas molecules striking a solid surface, a TAC value of 1 indicating complete transfer of energy and a TAC value of 0 indicating no energy transfer. Usually atomically clean smooth solid surfaces have small TAC values, and the TAC value may increase as the surface roughness of the solid increases and if the surface of the solid comprises adsorbed species such as oxides and other organic/inorganic materials. In the processes discussed above TAC may play an important role in determining the speed of the melting and fusing process by a heated gas beam. For increased speed, a high value of TAC would be desirable. Since the unprocessed layer comprises particles, its surface is very rough, which is good for obtaining higher TAC. Presence of a binder, adhesive or glue in an unprocessed layer may also be beneficial in increasing the TAC value since such binder creates adsorbed species on the surfaces of the particles within the unprocessed layer.

[0027] The temperature of a heated gas beam coming out of an orifice of a fusing device, tool or head such as those in Figures 3A, 3B, 3C, 3D and 4 depends on the magnitude of the electric power applied to the fusing head for heating the process gas, higher powers yielding higher temperatures. Therefore, if an array of fusing heads are used in high throughput processing, operation of individual heads within the array may be controlled by a computer, which may provide more or less power to any of the heads and thus cause the portions of a powder layer directly beneath the heads receiving high power to melt and fuse, and the portions that are beneath those receiving less power (or no power) to stay unfused. Another way of control is the flow rate of the process gas into a heating cavity of a fusing head. If very little gas or no gas is flown into a heating cavity of a fusing head, a properly heated gas beam that can melt and fuse particles may not be formed. Therefore, if an array of fusing heads are used in high throughput processing, operation of individual heads within the array may be controlled by a computer, which may provide more or less gas flow to any of the fusing heads and thus cause them to turn "on" or "off. For example, heads receiving a gas flow rate, which may be higher than a "critical gas flow rate" may turn "on" and heads receiving a gas flow rate, which may be less than or equal to the "critical gas flow rate" may turn "off. The critical gas flow rate may be zero. This way the portions of an unprocessed layer directly beneath the heads that are receiving adequate gas flow (heads that are "on") may melt and fuse, and the portions that are beneath those heads receiving less gas flow or no gas flow (heads that are "off) to stay unfused.

[0028] Figure 5 shows an integrated fusing tool 40 comprising a linear array of fusing heads. In Figure 5 there are six fusing heads labeled A, B, C, D, E and F. Each head has its own gas inlet labeled as 41A, 41B, 41C, 41D, 41E, 41F, orifice labeled as 43A, 43B, 43C, 43D, 43E, 43F and electrical connection labeled as 42A, 42B, 42C, 42D, 42E and 42F. As explained before, the electrical connections and the gas inlets can be used to individually turn each head on and off during a process when the integrated fusing tool 40 may be scanned over an unprocessed layer comprising particles. It should be noted that the individual heads of Figure 5 may have different size orifices to process wider or narrower regions on the unprocessed layer. Two or more linear arrays such as the one shown in Figure 5 may be attached to form two-dimensional arrays to increase the throughput of the process.

[0029] Figure 6 shows an exemplary fusing head array 50 wherein a process gas 58 is flown into a fusing head 54 through a supply line 59. At least one return line 51 is provided to collect a heated process gas 52 after it leaves an orifice 53 of the fusing head 54 and applies heat to an unprocessed layer 55. By collecting the used gas this way, the heat load on other parts of the powder layer 55 may be minimized. The return lines 51 may preferably be located in close proximity of the orifice 53. It should be noted that all the used gas does not have to be collected. In fact some or all of the used gas can be used to replenish a process environment within an enclosure that the exemplary fusing head array 50 may be placed in. The used gas may also be utilized to control a background temperature of the process environment and the unprocessed layer at a predetermined value, the predetermined value being less than a process temperature of the heated process gas and the melting temperature for the particles within the unprocessed layer.

[0030] Although the foregoing description has shown, illustrated and described various embodiments of the present invention, it will be apparent that, various substitutions, modifications and changes to the embodiments described may be made by those skilled in the art without departing from the spirit and scope of the present invention.

Claims

What is claimed:

1. A method of fabricating a three-dimensional object in a layer-by-layer fashion in a process environment, the method comprising:

forming a first unprocessed layer with portions over a base, the unprocessed layer comprising particles;

positioning a fusing tool over the first unprocessed layer, the fusing tool comprising one or more fusing heads;

placing an orifice of a fusing head in proximity to a first portion of the first unprocessed layer, thereby forming a first gap between the orifice of the fusing head and the first portion; and

directing a first heated gas beam from the orifice to the first portion through the first gap so as to heat the particles therein and convert the first portion of the first unprocessed layer into a first fused segment.

2. The method of claim 1 further comprising:

positioning the orifice of the fusing head in proximity to a second portion of the first unprocessed layer, thereby forming a second gap between the orifice of the fusing head and the second portion; and

applying a second heated gas beam from the orifice to the second portion through the second gap so as to heat the particles therein and convert the second portion of the first unprocessed layer into a second fused segment, wherein:

applying the second heated gas beam and converting the second portion of the first unprocessed layer into the second fused segment bonds the second fused segment to the first fused segment.

3. The method of claim 2 wherein the positioning and the applying are repeated to convert other portions of the first unprocessed layer into a plurality of fused segments forming a first fused layer.

4. The method of claim 3 wherein the base comprises a previously formed fused layer, and the positioning and the applying bond one or more of the plurality of fused segments to the previously formed fused layer.

5. The method of claim 4 wherein the first heated gas beam and the second heated gas beam are arc-free and they comprise at least one of Ar, He, N₂, Ne, air and H₂.

6. The method of claim 5 wherein the first gap and the second gap are each less than 5 mm.

7. The method of claim 6 wherein the largest dimension of a cross section of the orifice taken in a plane vertical to a flow direction of the first heated gas beam and the second heated gas beam, is in the range of 0.05-2 mm.

8. The method of claim 7 wherein a flow rate of the first heated gas beam and the second heated gas beam are each less than a value given by the formula X/0.016 where X is the area of the cross section of the orifice in units of centimeter- square.

9. The method of claim 7 wherein the fusing head comprises a heating cavity and wherein the formation of the first heated gas beam and the second heated gas beam comprise flowing a process gas into the heating cavity, heating the process gas to a process temperature and flowing it out of the heating cavity through the orifice.

10. The method of claim 9 wherein heating the process gas to the process temperature comprises establishing a plasma within the heating cavity, and wherein the heating cavity is configured to prevent arcs to go through the orifice.

11. The method of claim 9 wherein heating the process gas to the process temperature comprises establishing a RF plasma in the heating cavity using a RF power at a frequency range of 3 MHz to 300 GHz.

12. The method of claim 10 wherein the plasma is a RF plasma generated by a RF power at a frequency range of 3 MHz to 300 GHz.

13. The method of claim 11 wherein the RF plasma is established between a first electrode and a second electrode in the heating cavity by applying the RF power between the first electrode and the second electrode.

14. The method of claim 11 wherein the RF plasma is established by applying the RF power to a coil disposed around the heating cavity.

15. The method of claim 13, wherein the frequency of the RF power is in the range of 3-300 MHz and the process environment is at atmospheric pressure.

16. The method of claim 14, wherein the frequency of the RF power is in the range of 3-300 MHz and the process environment is at atmospheric pressure.

17. The method of claim 15 wherein the frequency of the RF power is higher than 50 MHz.

18. The method of claim 16 wherein the frequency of the RF power is higher than 50 MHz.

19. The method of claim 4 wherein the first unprocessed layer comprises a binder that binds the particles to each other and increases a thermal accommodation coefficient.

20. The method of claim 10 wherein the particles comprise at least one of a metal, and a ceramic material.

21. The method of claim 11 wherein the particles comprise at least one of a metal, and a ceramic material.

22. The method of claim 9 wherein heating the process gas to the process temperature comprises passing electrical current through a filament in the heating cavity, the electrical current heating the filament.

23. A method of fabricating a three-dimensional object in a layer-by-layer fashion in a process environment, the method comprising:

forming a first unprocessed layer with portions over a base, the unprocessed layer comprising particles; positioning a fusing tool over the first unprocessed layer, the fusing tool comprising a plurality of fusing heads each fusing head comprising a heating cavity, a gas inlet to flow a process gas into the heating cavity at a flow rate, an electrical connection to apply an electrical power to generate a plasma in the heating cavity and an orifice to direct a heated gas beam out of the heating cavity;

placing the orifices of the plurality of fusing heads in proximity to an exposed surface of the first unprocessed layer, thereby forming a gap between the orifices and the exposed surface of the first unprocessed layer;

moving the fusing tool over the exposed surface while turning one or more fusing heads on by adjusting the flow rate of the process gas flown into each heating cavity of the one or more fusing heads to a value more than a critical gas flow rate, and turning other fusing heads off by adjusting the flow rate of the process gas flown into each heating cavity of the other fusing heads to a value less than or equal to the critical gas flow rate; and

converting a portion of the unprocessed layer to a fused layer, the portion being in proximity to the orifices of the one or more fusing heads.

24. The method of claim 1 wherein the critical gas flow rate is 0 liters/minute.