具体实施方式：
[0005]In a method of 3D printing a 3D printed metal object, a binding agent may be selectively applied to a layer of build material comprising metal particles. When the build material is subsequently heated, the selectively applied binding agent may bind the build material to form a layer of the 3D printed metal object. Further build material may be applied and the process repeated layer-by-layer until the 3D printed object is formed. The 3D printed object may then be removed from the 3D printer and sintered to form the final metal object. Unbound build material may be re-used in the 3D printing process.
[0006]It has been found that, in some instances, recycled build material can differ in properties from, for example, fresh build material e.g. from the same source. For example, in some instances, the ease with which the build material spreads to form a layer may be compromised. Furthermore, in some cases, the mechanical properties of the final sintered object may be compromised.
[0007]The present disclosure relates to a kit for 3D printing a 3D printed metal object. The kit comprises build material comprising metal particles; and a binding agent. The binding agent comprises a hydrated metal salt having a dehydration temperature of from 100 to about 250° C., and water. The binding agent may be either free from organic solvent and surfactant, or the binding agent comprises organic solvent and/or surfactant and the total amount of organic solvent and/or surfactant is less than 3 weight % based on the total weight of the binding agent.
[0008]The present disclosure also relates to a method of 3D printing a 3D printed metal object. The method comprises selectively applying a binding agent as described in the present disclosure onto build material comprising metal particles, and heating the build material and selectively applied binding agent to bind a layer of the 3D printed metal object. In some examples, the method also comprises (i) applying a layer of build material to a previously bound layer of the 3D printed object, (ii) selectively applying binding agent to the applied layer of build material, and (iii) heating the layer of build material to bind a further layer of the 3D printed metal object. Steps (i) to (iii) may be repeated, for example, in sequence a plurality of times. In some examples, the method can also comprise sintering bound layer(s) of the 3D printed metal object to form a sintered 3D printed metal object.
[0009]It has been found that, when binding agents comprising hydrated metal salts are employed, organic solvent and/or surfactant can be either omitted, or restricted to amounts of less than 3 weight % based on the total weight of the binding agent. The resulting binding agents can be used in 3D printing methods, in which build material comprising metal particles can be effectively recycled.
[0010]During printing, build material may be treated by selectively applying binding agent to the build material. When the build material is heated, liquid from the binding agent can evaporate, binding the treated build material to form a layer of the 3D printed metal object.
[0011]During evaporation, it has been found that some of the components in the liquid vehicle of the binding agent less volatile than water may come into contact and contaminate the untreated portions of the build material. In examples of the present disclosure, the risk of contaminating untreated portions of build material can be reduced as the concentration of organic compounds in the evaporated liquid is reduced or eliminated. This can reduce the risk of contaminating the untreated build material with excessive organic residue, promoting the build material's recyclability. The concentration of organic residue in the sintered 3D printed metal object can also be reduced. Furthermore, it has been found that the anti-kogation properties and/or jettability of the binding agent can be maintained despite the absence or low levels of organic solvent and/or surfactant. In addition, reducing or removing less volatile constituents from the binding agent facilitates evaporation of the ink vehicle, thereby reducing the energy and/or time needed to remove liquids from the binder.
[0012]In some examples, the binding agent may be free from organic compounds, or the binding agent may comprise organic compounds in an amount of less than 3 weight % based on the total weight of the binding agent.
[0013]In some examples, the binding agent may comprise organic solvent in an amount of less than about 1 weight % based on the total weight of the binding agent.
[0014]In some examples, the binding agent may comprise surfactant in an amount of less than about 0.5 weight % based on the total weight of the binding agent.
[0015]In some examples, the binding agent may comprise organic solvent and/or surfactant and the total amount of organic solvent and/or surfactant is less than about 1 weight % based on the total weight of the binding agent.
[0016]In some examples, the binding agent may consist essentially of hydrated metal salt and water.
[0017]In some examples, the amount of hydrated metal salt in the binding agent may be about 20 to about 65 weight % based on the total weight of the binding agent.
[0018]In some examples, a metal present in the build material may be the same as a metal present in the hydrated metal salt.
[0019]In some examples, the hydrated metal salt in the binding agent may comprise a metal cation selected from at least one of aluminium, magnesium, copper, zinc, iron, nickel, manganese, cobalt, molybdenum, chromium, tin and vanadium, and an anion selected from at least one of hydroxide, carbonate, sulphate, nitrate, acetate, formate, borate, chloride and bromide.
[0020]In some examples, the hydrated metal salt may comprise a metal cation selected from copper.
[0021]In some examples, the hydrated metal salt in the binding agent may be hydrated copper nitrate.
[0022]In some examples, the binding agent may comprise 35 to 58 weight % hydrated copper nitrate and 42 to 65 weight % water.
[0023]In some examples, the build material may comprise copper. For instance, the build material may comprise particles of copper metal or copper alloy.
Build Material
[0024]The build material employed in the present disclosure comprises metal particles. The metal particles may comprise copper, for instance, in the form of copper metal or copper alloy. In some examples, the build material may comprise copper, for instance, copper metal or copper alloy.
[0025]The build material may be metallic. In some examples, the build material may comprise a single phase metallic material composed of one element. Alternatively, the build material may comprise two or more elements, which, for example, may be in the form of a single phase metallic alloy or a multiple phase metallic alloy. For some single phase metallic alloys, melting begins just above the solidus temperature (where melting is initiated) and is not complete until the liquidus temperature (temperature at which all the solid has melted) is exceeded. For other single phase metallic alloys, melting begins just above the peritectic temperature. The peritectic temperature is defined by the point where a single phase solid transforms into a two phase solid plus liquid mixture, where the solid above the peritectic temperature is of a different phase than the solid below the peritectic temperature. When the metallic build material is composed of two or more phases (e.g., a multiphase alloy made of two or more elements), melting generally begins when the eutectic or peritectic temperature is exceeded. The eutectic temperature is defined by the temperature at which a single phase liquid completely solidifies into a two phase solid. Generally, melting of the single phase metallic alloy or the multiple phase metallic alloy begins just above the solidus, eutectic, or peritectic temperature and is not complete until the liquidus temperature is exceeded. In some examples, sintering can occur below the solidus temperature, the peritectic temperature, or the eutectic temperature. In other examples, sintering occurs above the solidus temperature, the peritectic temperature, or the eutectic temperature. Sintering above the solidus temperature is known as super solidus sintering, and this technique may be useful when utilizing larger build material particles and/or to achieve high density. It is to be understood that the sintering temperature may be high enough to offer sufficient energy to allow atom mobility between adjacent particles.
[0026]Some examples of the build material include steels, stainless steel, bronzes, brasses, titanium (Ti) and alloys thereof, aluminum (Al) and alloys thereof, nickel (Ni) and alloys thereof, cobalt (Co) and alloys thereof, iron (Fe) and alloys thereof, gold (Au) and alloys thereof, silver (Ag) and alloys thereof, platinum (Pt) and alloys thereof, and copper (Cu) and alloys thereof. Some specific examples include AlSM OMg, 2xxx series aluminum, 4xxx series aluminum, CoCr MPI, CoCr SP2, MaragingSteel MS1, Hastelloy C, Hastelloy X, NickelAlloy HX, Inconel IN625, Inconel IN718, SS GP1, SS 17-4PH, SS 316L, Ti6Al4V, and Ti-6Al-4V EL7. While several example alloys have been described, it is to be understood that other alloy build materials may be used, such as refractory metals.
[0027]In some examples, the build material comprises copper particles. By “copper particles”, it is meant particles comprising copper metal or copper alloy. Examples of copper alloys include copper-tin (bronze), copper-zinc (brass) and copper-nickel (Monel) alloys. Copper may also be present in the copper particles as a minor constituent of the alloy. For example, the copper-containing alloy may be a copper-containing steel or an aluminium alloy that comprises copper. For instance, 17-4PH steel has 3-5 wt % Cu and Cu content in 2xxx aluminum ranges from 2-6 wt %.
[0028]Where the build material comprises a copper alloy, the amount of copper may be at least about 1 weight %, at least about 2 weight %, at least about 3 weight % of the weight of the alloy. In some examples, the amount of copper may be up to 100% by weight of the total weight of the alloy, for example, up to about 99 weight %, up to about 98 weight %, up to about 95 weight % of the total weight of the alloy.
[0029]The build material may be made up of similarly sized particles or differently sized particles. In some examples, the build material has an average particle size of from about 5 to about 20 microns.
[0030]The term “size”, as used herein with regard to the build material, refers to the diameter of a particle, for example, a substantially spherical particle (i.e., a spherical or near-spherical particle having a sphericity of >0.84), or the average diameter of a non-spherical particle (i.e., the average of multiple diameters across the particle).
[0031]In some examples, particles of a particle size of from about 5 microns to about 20 microns have good flowability and can be spread relatively easily. As an example, the average particle size of the particles of the build material may range from about 1 microns to about 200 microns. As another example, the average size of the particles of the build material ranges from about 10 microns to about 100 microns. As another example, the average size of the particles of the build material ranges from about 2 microns to about 20 microns. As still another example, the average size of the particles of the metallic build material ranges from 15 microns to about 50 microns.
Binding Agent
[0032]As mentioned above, the binding agent comprises a hydrated metal salt having a dehydration temperature of from 100 to about 250° C., and water. In some examples, dehydration temperature as used in this disclosure may be the temperature by which all, or nearly all, of the water molecules in the hydrated metal salt have either been removed by evaporation or reacted to form other compounds. Dehydration may be progressive. Dehydration may occur in multiple discrete steps as the hydrated metal salt is heated. The binding agent may be either free from organic solvent and surfactant, or the binding agent comprises organic solvent and/or surfactant and the total amount of organic solvent and/or surfactant is less than 3 weight % based on the total weight of the binding agent.
[0033]In some examples, the dehydration temperature may be more than about 100° C., or more than about 110° C., or more than about 120° C., or more than about 130° C., or more than about 140° C., or more than about 150° C., or more than about 160° C., or more than about 170° C., or more than about 180° C., or more than about 190° C., or more than about 200° C., or more than about 210° C., or more than about 220° C., or more than about 230° C., or more than about 240° C., or less than about 250° C., or less than about 240° C.
[0034]In some examples, the dehydration temperature may be less than about 230° C., or less than about 220° C., or less than about 210° C., or less than about 200° C., or less than about 190° C., or less than about 180° C., less than about 170° C., or less than about 160° C., or less than about 150° C., or less than about 140° C., or less than about 130° C., or less than about 120° C., or less than about 110° C.
[0035]In some examples, the dehydration temperature may be from about 100° C. to about 240° C., or from about 100° C. to about 230° C., or from about 100° C. to about 220° C., or from about 100° C. to about 210° C., or from about 100° C. to about 200° C., or from about 100° C. to about 190° C., or from about 100° C. to about 180° C., or from about 100° C. to about 170° C., or from about 100° C. to about 160° C., or from about 100° C. to about 150° C., or from about 100° C. to about 140° C., or from about 100° C. to about 130° C., or from about 100° C. to about 120° C., or from about 100° C. to about 110° C.
[0036]In some examples, the hydrated metal salt comprises at least one metal cation selected from the group consisting of aluminum, magnesium, copper, zinc, iron, nickel, manganese, cobalt, molybdenum, chromium, tin, vanadium, and combinations thereof. The hydrated metal salt may comprise at least one anion selected from the group consisting of hydroxide, carbonate, sulfate, nitrate, acetate, formate, borate, chloride, bromide, and combinations thereof.
[0037]In some examples, the hydrated metal salt may comprise copper cations.
[0038]In some examples, the hydrated metal salt may comprise nitrate anions.
[0039]In some examples, the hydrated metal salt may be selected from the group consisting of hydrated copper nitrate, hydrated iron nitrate, hydrated nickel nitrate, hydrated manganese nitrate, hydrated cobalt nitrate, hydrated iron acetate, and combinations thereof.
[0040]In some examples, the hydrated metal salt may be hydrated copper nitrate. For instance, the hydrated metal salt may be hydrated copper (II) nitrate. In some examples, the hydrated metal salt may be copper (11) nitrate trihydrate.
[0041]In some examples, the build material may comprise a metal (e.g. at least one metal) that is the same as the metal cation in the hydrated metal salt. For example, the build material may comprise copper particles (e.g. copper metal or copper alloy particles) and the metal cation may be copper (e.g. copper (II)).
[0042]In some examples, the hydrated metal salt may be present in the binding agent in an amount below the saturation limit of the hydrated metal salt at 25 degrees C.
[0043]In some examples, the hydrated metal salt may be present in the binding agent at about 10 weight % to up to 100 weight % of the saturation concentration of the hydrated metal salt in water at 25 degrees C. For example, the hydrated metal salt may be present in the binding agent at about 30 weight % to up to 99 weight % of the saturation concentration, for example, at about 25 weight % up to 95 weight % of the saturation concentration. In some examples, the hydrated metal salt may be present in the binding agent in an amount less than 95% of the saturation limit of the hydrated metal salt at 25 degrees C.
[0044]In some examples, the hydrated metal salt may be present in an amount of at least about 5 weight % of the total weight of the binding agent, for example, at least about 10 weight %, at least about 15 weight %, at least about 20 weight %, at least about 25 weight %, at least about 30 weight %, at least about 35 weight % or at least about 40 weight %.
[0045]In some examples, the hydrated metal salt may be present in an amount of at most about 70 weight % of the total weight of the binding agent, for example, at most about 65 weight % or at most about 60 weight %.
[0046]In some examples, the hydrated metal salt may be present in an amount of from about 5 wt % to about 70 wt % based on the total weight of the binding agent, or from about 10 wt % to about 65 wt % based on the total weight of the binding agent, or from about 15 wt % to about 60 wt % based on the total weight of the binding agent, or from about 20 wt % to about 55 wt % based on the total weight of the binding agent, or from about 25 wt % to about 55 wt % based on the total weight of the binding agent, or from about 30 wt % to about 50 wt % based on the total weight of the binding agent, or from about 35 wt % to about 50 wt % based on the total weight of the binding agent, or from about 40 wt % to about 50 wt % based on the total weight of the binding agent, or from about 45 wt % to about 50 wt % based on the total weight of the binding agent.
[0047]In some examples, the hydrated metal salt may be hydrated copper nitrate (e.g. hydrated copper (II) nitrate or copper (II) nitrate trihydrate). The amount of hydrated copper nitrate in the binding agent may be at least about 30 weight %, for example, at least about 40 weight % or at least 45 weight %. The amount of hydrated copper nitrate in the binding agent may be at most about 58 weight %, for example, at most about 56 weight %. In some examples, the amount of hydrated copper nitrate may be from about 35 to about 58 weight %, for instance, from about 40 to about 58 weight %.
[0048]As mentioned above, the binding agent also comprises water. Water may be the liquid vehicle of the binding agent. Water may be present in an amount of at least about 40 weight %, for example, at least about 42 weight %, at least about 45 weight %, at least about 50 weight % or at least about 55 weight %.
[0049]In some examples, water may be present in an amount of about 40 to about 70 weight %, for instance, about 42 to about 67 weight %, about 45 to about 65 weight %, about 50 to about 65 weight % or about 55 to about 60 weight %.
[0050]In some examples, the binding agent consists essentially of water and hydrated metal salt. The hydrated metal salt may be completely dissolved in water. For example, the binding agent may consist essentially of water and a hydrated copper salt, for instance, hydrated copper nitrate (e.g. copper (II) nitrate trihydrate).
[0051]The binding agent may consist essentially of about 30 to 60 weight % hydrated copper nitrate and 40 to 70 weight % water. In one example, the binding agent may consist essentially of about 40 to about 58 weight % hydrated copper nitrate and from about 42 to about 60 weight % water.
[0052]The binding agent may be free from organic solvent and/or surfactant. Alternatively, the binding agent may comprise organic solvent and/or surfactant but the total amount of organic solvent and/or surfactant may be less than 3 weight % based on the total weight of the binding agent. The total amount of organic solvent and/or surfactant may be less than 2 weight %, for instance, less than 1 weight % or less than 0.8 weight % based on the total weight of the binding agent. The total amount of organic solvent and/or surfactant may be less than 0.6 weight %, for instance, less than 0.5 weight % or less than 0.4 weight % based on the total weight of the binding agent.
[0053]Organic solvents that may be present include aliphatic alcohols, aromatic alcohols, diols, glycol ethers, polyglycol ethers, 2-pyrrolidones, caprolactams, formamides, acetamides, glycols, and long chain alcohols. Specific examples include primary aliphatic alcohols, secondary aliphatic alcohols, 1,2-alcohols, 1,3-alcohols, 1,5-alcohols, ethylene glycol alkyl ethers, propylene glycol alkyl ethers, higher homologs (C6-C12) of polyethylene glycol alkyl ethers, N-alkyl caprolactams, unsubstituted caprolactams, both substituted and unsubstituted formamides, and both substituted and unsubstituted acetamides.
[0054]Other organic solvents include water-soluble high-boiling point solvents (i.e., humectants), which have a boiling point of at least 120° C., or higher. Some examples of such high-boiling point solvents include 2-pyrrolidone (boiling point of about 245° C.), 2-methyl-1,3-propanediol (boiling point of about 212° C.), and combinations thereof.
[0055]Where an organic solvent is present, the organic solvent may be present in an amount of less than about 3 weight %, for example, less than about 2 weight % or less than about 1 weight %. In some examples, the organic solvent may be present in an amount of less than about 0.5 weight %. In some examples, the total amount of organic solvent may be less than about 3 weight %, for example, less than about 2 weight %, less than about 1 weight %, less than 0.5 weight %, less than 0.2 weight, or less than 0.1 weight %.
[0056]In some examples, the binding agent is substantially free from 2-pyrrolidone. Alternatively, 2-pyrrolidone may be present but in an amount of less than about 3 weight %, for example, less than about 2 weight %, less than about 1 weight %, less than 0.5 weight %, less than 0.2 weight, or less than 0.1 weight %.
[0057]Where present, suitable surfactants include non-ionic surfactants. Examples of suitable surfactants include surfactants based on acetylenic diol chemistry (e.g., SURFYNOL® SEF from Air Products and Chemicals, Inc.), fluorosurfactants (e.g., CAPSTONE® fluorosurfactants from DuPont, previously known as ZONYL FSO), and combinations thereof. In other examples, the surfactant may be an ethoxylated low-foam wetting agent (e.g., SURFYNOL® 440 or SURFYNOL® CT-111 from Air Products and Chemical Inc.) or an ethoxylated wetting agent and molecular defoamer (e.g., SURFYNOL® 420 from Air Products and Chemical Inc.). Still other suitable surfactants include non-ionic wetting agents and molecular defoamers (e.g., SURFYNOL® 104E from Air Products and Chemical Inc.) or water-soluble, non-ionic surfactants (e.g., TERGITOL™ TMN-6 or TERGITOL™ 15-S-7 from The Dow Chemical Company). In other examples, the surfactant may be a sulfonated surfactant, for example, a disulfonated surfactant, such as an alkyldiphenyloxide disulfonate (e.g. DOWFAX™ 2A1). In some examples, it may be useful to utilize a surfactant having a hydrophilic-lipophilic balance (HLB) less than 10.
[0058]When a surfactant is used, the total amount of surfactant(s) in the binding agent may range from about 0 to about 3 weight % based on the total weight of the binding agent. The total amount of surfactant(s) in the binding agent may be less than about 3 weight %, for example, less than about 2 weight %, less than about 1 weight %, less than about 0.5 weight %, less than about 0.2 weight % or less than 0.1 weight % based on the total weight of the binding agent.
[0059]In some examples, the binding agent may be free from fluorosurfactants. Alternatively, the binding agent may contain fluorosurfactant in an amount of less than about 1 weight %, for example, less than about 0.5 weight %, less than about 0.1 weight %, less than about 0.08 weight %, less than about 0.06 weight %, less than about 0.04 weight %, less than about 0.02 weight % or less than about 0.01 weight %.
[0060]In some examples, the binding agent may be free from sulfonated surfactants. Alternatively, the binding agent may contain sulfonated surfactant in an amount of less than about 1 weight %, for example, less than about 0.5 weight %, less than about 0.3 weight %, less than about 0.1 weight % or less than 0.05 weight %.
[0061]In some examples, the binding agent may be free from sulfonated surfactants and fluorinated surfactants. Alternatively, the total amount of sulfonated surfactant and fluorinated surfactant may be less than about 1 weight %, for example, less than about 0.5 weight %, less than about 0.3 weight %, less than about 0.1 weight % or less than 0.05 weight %.
[0062]In some examples, the binding agent may be free from organic compounds, or the binding agent may comprise organic compounds in an amount of less than 3 weight % based on the total weight of the binding agent.
[0063]Where the binding agent may comprise organic compounds, such organic compounds may be present in an amount of less than about 2 weight %, for example, less than about 1.5 weight % or less than about 1 weight % based on the total weight of the binding agent. In some examples, the total amount of organic compounds may be less than about 0.8 weight %, less than about 0.5 weight %, less than about 0.3 weight %, less than about 0.2 weight %, less than about 0.1 weight % or less than about 0.05 weight %.
[0064]Where organic compounds are present, the organic compounds may be organic solvent and/or surfactant. Examples of organic solvent and/or surfactant that may be present are described above. Examples of other organic compounds include antimicrobial agent(s), anti-kogation agent(s), viscosity modifier(s), pH adjuster(s) and/or sequestering agent(s).
[0065]Suitable antimicrobial agents include biocides and fungicides. Example antimicrobial agents may include the NUOSEPT™ (Troy Corp.), UCARCIDE™ (Dow Chemical Co.), ACTICIDE® M20 (Thor), and combinations thereof. Examples of suitable biocides include an aqueous solution of 1,2-benzisothiazolin-3-one (e.g., PROXEL® GXL from Arch Chemicals, Inc.), quaternary ammonium compounds (e.g., Bardac® 2250 and 2280, Barquat® 50-65B, and Carboquat® 250-T, all from Lonza Ltd. Corp.), and an aqueous solution of methylisothiazolone (e.g., Kordek® MLX from Dow Chemical Co.). The biocide or antimicrobial may be added in any amount ranging from about 0.05 wt % to about 0.5 wt % (as indicated by regulatory usage levels) with respect to the total weight of the binding agent.
[0066]In some examples, the biocide and/or antimicrobial component may be present in an amount of less than 0.1 weight %, for example, less than about 0.08 weight %, or less than 0.005 weight %. In some examples, the binding agent is devoid of biocide and/or antimicrobial agent.
[0067]As mentioned above, an anti-kogation agent may be included in the binding agent. Kogation refers to the deposit of dried ink (e.g., binding agent) on a heating element of a thermal inkjet printhead. Anti-kogation agent(s) is/are included to assist in preventing the buildup of kogation. Examples of suitable anti-kogation agents include oleth-3-phosphate (e.g., commercially available as CRODAFOS™ O3A or CRODAFOS™ N-3 acid from Croda), or a combination of oleth-3-phosphate and a low molecular weight (e.g., <5,000) polyacrylic acid polymer (e.g., commercially available as CARBOSPERSE™ K-7028 Polyacrylate from Lubrizol). Whether a single anti-kogation agent is used or a combination of anti-kogation agents is used, the total amount of anti-kogation agent(s) in the binding agent may range from greater than 0.2 wt % to about 0.8 wt % based on the total weight of the binding agent. In an example, the oleth-3-phosphate is included in an amount ranging from about 0.2 wt % to about 0.6 wt %, and the low molecular weight polyacrylic acid polymer is included in an amount ranging from about 0.005 wt % to about 0.03 wt %.
[0068]In some examples, the anti-kogation agent may be present in an amount of less than 0.2 weight %, for example, less than about 0.1 weight %, or less than 0.05 weight %. In some examples, the binding agent may be devoid of anti-kogation agent.
[0069]Sequestering agents, such as EDTA (ethylene diamine tetra acetic acid), may be included to eliminate the deleterious effects of heavy metal impurities, and buffer solutions may be used to control the pH of the binding agent. From 0.01 wt % to 2 wt % of each of these components, for example, can be used.
[0070]In some examples, the sequestering agent may be present in an amount of less than 2 weight %, for example, less than about 0.2 weight %, or less than 0.1 weight %. In some examples, the binding agent may be devoid of sequestering agent.
[0071]As mentioned above, in some examples, the binding agent may be free from organic compounds, or the binding agent may comprise organic compounds in an amount of less than 3 weight % based on the total weight of the binding agent. In some examples, the binding agent may be free from organic compounds, or comprise less than about 2.5 weight %, for example, less than about 2.0 weight %, less than about 1.5 weight %, less than about 1.0 weight %, less than about 0.8 weight %, less than about 0.6 weight %, less than about 0.5 weight %, less than about 0.2 weight % organic compounds.
3D Printing
[0072]In 3D printing, a layer of build material may be applied to a print platform. A binding agent may then be selectively jetted onto at least a portion of the layer of build material. A further layer of build material may then be applied, and a binding agent may then be selectively jetted onto a portion of the newly applied layer. The process may be repeated one or more times.
[0073]By selectively applying (e.g. by jetting) the binding agent onto the build material, the build material becomes patterned. The binding agent may be applied (e.g. jetted) by thermal inkjet or piezoelectric inkjet. The patterned build material may then be bound to form a layer. Binding may be carried out e.g. by applying heat to the patterned build material. For example, heating may cause at least some of the liquid in the binding agent to evaporate. This evaporation may result in some densification, for example, through capillary action of the layer. Alternatively or additionally, heating may cause physical and/or chemical changes in the binder that cause the build material to be stabilised.
[0074]Binding may be performed after a single pass of the binding agent or after a few passes of binding agent have been applied. Alternatively or additionally, binding may be performed to a patterned 3D printed object to affect the binding of multiple layers.
[0075]The procedure used to bind the build material may depend, for example, the nature of the build material and the binding agent. In some examples, binding may be performed by heating to a binding temperature of, for instance, about 80 to about 300 degrees C. Binding together of the printed build material particles can be accomplished by heating the printed layer to evaporate liquids in the binding agent and partially dehydrate the hydrated metal salt. In a 3D printing system thermal energy can be supplied from overhead energy sources or from energy sources that heat the build bed from the sides or bottom. Overhead energy sources can heat the surface of the powder in a pulsed fashion, consistent with layer-by-layer processing, whereas build bed heaters can heat the entire build bed volume and are suited for maintaining the temperature of the build bed at a desired value. Both types of heaters (overhead or build bed) may be employed to establish the binding temperature needed to create the printed metal object.
[0076]In some examples, the binding temperature may be from about 100° C. to about 280° C., or from about 100° C. to about 250° C., or from about 100° C. to about 240° C., or from about 100° C. to about 230°. In some examples, the binding temperature may be from about 130° C. to about 280° C., or from about 140° C. to about 250° C., or from about 150° C. to about 240° C., or from about 160° C. to about 230° C.
[0077]Where the build material comprises copper and/or the binding agent comprises a hydrated copper salt (e.g. hydrated copper nitrate), the binding temperature may be from about 110° C. to about 180° C.
[0078]In some examples, after binding, the build material (e.g. patterned with the binding agent) may be sintered. Sintering temperatures may be dependent on the metal alloy being printed and typically range from about 70% to 97% of the metal alloy melting point measured in degrees Kelvin. For example, suitable sintering temperatures for aluminum and aluminum