Patent Publication Number: US-10781228-B2

Title: Fusing agent including a metal bis(dithiolene) complex

Description:
BACKGROUND 
     Three-dimensional (3D) printing may be an additive printing process used to make three-dimensional solid parts from a digital model. 3D printing is often used in rapid product prototyping, mold generation, mold master generation, and short run manufacturing. Some 3D printing techniques are considered additive processes because they involve the application of successive layers of material. This is unlike traditional machining processes, which often rely upon the removal of material to create the final part. 3D printing often requires curing or fusing of the building material, which for some materials may be accomplished using heat-assisted extrusion, melting, or sintering, and for other materials may be accomplished using digital light projection technology. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of examples of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear. 
         FIG. 1  is a simplified isometric and schematic view of an example of a 3D printing system disclosed herein; 
         FIGS. 2A through 2D  are schematic and partially cross-sectional views depicting the formation of a 3D part using examples of a 3D printing method disclosed herein; 
         FIG. 3  is a graph depicting absorbance (y-axis) versus wavelength (nm, x-axis) of two comparative fusing agents and an example of the fusing agent disclosed herein; 
         FIGS. 4A and 4B  are black and white representations of originally colored photographic images of example 3D printed parts, formed with examples of the fusing agent disclosed herein; and 
         FIG. 5  is a flow diagram illustrating an example of a method of making an example of the fusing agent disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     Examples of the three-dimensional (3D) printing method and the 3D printing system disclosed herein utilize Multi Jet Fusion (MJF). During MJF, an entire layer of a build material (also referred to as build material particles) is exposed to radiation, but a selected region (in some instances less than the entire layer) of the build material is fused and hardened to become a layer of a 3D part. A fusing agent is selectively deposited in contact with the selected region of the build material. The fusing agent(s) is capable of penetrating into the layer of the build material and spreading onto the exterior surface of the build material. This fusing agent is capable of absorbing radiation and converting the absorbed radiation to thermal energy, which in turn melts or sinters the build material that is in contact with the fusing agent. This causes the build material to fuse, bind, cure, etc. to form the layer of the 3D part. 
     As used herein, the terms “3D printed part,” “3D part,” or “part” may be a completed 3D printed part or a layer of a 3D printed part. 
     Some fusing agents used in MJF tend to have significant absorption (e.g., 80%) in the visible region (400 nm-780 nm). This absorption generates heat suitable for fusing during 3D printing, which leads to 3D parts having mechanical integrity and relatively uniform mechanical properties (e.g., strength, elongation at break, etc.). This absorption, however, results in strongly colored, e.g., black, 3D parts. In some instances, it may not be desirable to generate strongly colored parts. Rather, it may be desirable to generate a part that is white, off-white, or some color other than black. 
     Examples of the fusing agent disclosed herein, which may be utilized in examples of the method and system disclosed herein, contain a metal bis(dithiolene) complex, which has absorption at wavelengths ranging from 600 nm to 1600 nm. The metal bis(dithiolene) complex, and the fusing agent including the complex, is capable of absorbing at least 80% of radiation having wavelengths ranging from 600 nm to 1600 nm. Moreover, the absorption maximum of the metal bis(dithiolene) complex may undergo a bathochromic shift (e.g., further into the near-infrared region toward the medium infrared region) or a hypsochromic shift (e.g., in the near-infrared region toward the visible region) depending upon the chemistry of the complex and/or fusing agent. As examples, the shift may depend upon a polar aprotic solvent present in the fusing agent and/or upon the nature of the functional group(s) attached to the complex. Like the visible region absorbing fusing agents, the absorption of the fusing agents including the metal bis(dithiolene) complex generates heat suitable for fusing polymeric or polymeric composite build material in contact therewith during 3D printing, which leads to 3D parts having mechanical integrity and relatively uniform mechanical properties (e.g., strength, elongation at break, etc.). 
     It has been found that a combination of a polar aprotic solvent and a thiol surfactant renders the metal bis(dithiolene) complex i) more soluble in the polar aprotic solvent and ii) readily reducible at room temperature (e.g., from about 18° C. to about 25° C.). As such, an initial reduction of the metal bis(dithiolene) complex may be initiated prior to fusing (e.g., during fusing agent formulation), which results in the complex changing color. It is to be understood that this change in color is not a loss of color (i.e., is not discoloration as defined herein), and the initially reduced complex still readily absorbs the applied electromagnetic radiation, in the examples disclosed herein, further reduction of the metal bis(dithiolene) complex may take place during and/or after fusing. While not being bound to any theory, it is believed that the further reduction of the metal bis(dithiolene) complex may be due, at least in part, to the heat generated during fusing and/or on a build platform after fusing, the electromagnetic radiation used during fusing, the components of the fusing agent, the polymeric or polymeric composite build material, or a combination thereof. As the complex reduces, it may change color and ultimately undergo discoloration (i.e., become at least substantially colorless). By “at least substantially colorless,” it is meant that the original color of the complex changes or fades to a point that the formed part exhibits a color of the build material, a color of a colorant present in the fusing agent, or a color of a colorant subsequently applied to the build material. Thus, the fusing agent, containing the metal bis(dithiolene) complex, may be used to print white 3D parts, off-white 3D parts, or colored 3D parts. 
     The polar aprotic solvent may be selected to at least partially dissolve the metal bis(dithiolene) complex and to react with the complex by reducing it (e.g., via an electron transfer reaction). The reduction reaction may cause a shift in the absorption maximum of the metal bis(dithiolene) complex. In some instances, the shift in the absorption maximum of the complex may enable the complex to harvest energy from the electromagnetic radiation applied during 3D printing. For example, the energy from the electromagnetic radiation may be harvested when the absorption maximum wavelength of the complex matches that of the source of radiation. As such, it may be desirable to utilize a complex and polar aprotic solvent combination that will shift the absorption maximum of the complex to the wavelength or within the wavelength range of the source of radiation. The absorbed radiation and harvested energy may fuse the portion of the polymeric or polymeric composite build material in contact with the fusing agent, and may initiate or further discoloration of the complex, even after the application of the radiation ceases. 
     The thiol surfactant may be included in the fusing agent to stabilize the metal bis(dithiolene) complex. In particular, the thiol surfactant may render the complex more soluble in a vehicle of the fusing agent, thus reducing its tendency to precipitate out of the vehicle. Stabilizing the metal bis(dithiolene) complex with the thiol surfactant may also facilitate the reduction of the metal bis(dithiolene) complex by the polar aprotic solvent (i.e., may enable the reduction to occur at room temperature and within a few seconds) and/or improve the jettability of the fusing agent. 
     The fusing agent disclosed herein generally includes a liquid vehicle and the metal bis(dithiolene) complex. The metal bis(dithiolene) complex allows the fusing agent to absorb radiation at wavelengths ranging from 600 nm to 1600 nm, which enables the fusing agent to convert enough radiation to thermal energy so that the polymeric or polymeric composite build material particles in contact with the fusing agent fuse. 
     Examples of the metal bis(dithiolene) complex may have a general formula I: 
     
       
         
         
             
             
         
       
     
     Examples of M include nickel, zinc, platinum, palladium, and molybdenum. Examples of each of W, X, Y, and Z include a hydrogen (H), a phenyl group (Ph), a phenyl group bonded to an R group (i.e., PhR), wherein R is C n H 2n+1 , or OC n H 2n+1 , or N(CH 3 ) 2 , and a sulfur bonded to an R group (i.e., SR), wherein R is C n H 2n+1 , or OC n H 2n+1 , or N(CH 3 ) 2 . In these examples, n may be greater than or equal to 2 and less than or equal to 12 (i.e., 2≤n≤12). When the metal bis(dithiolene) complex has the general formula I, the strong NIR absorption of the metal bis(dithiolene) complex may be the result of the electron delocalization about the dithiolene ring and the interaction of the delocalized electrons with the empty d-orbitals of the metal center. 
     The amount of the metal bis(dithiolene) complex in the fusing agent may range from about 1 wt % to about 3 wt % based on the total wt % of the fusing agent. In an example, the amount of the metal bis(dithiolene) complex present in the fusing agent is about 1 wt % based on the total wt % of the fusing agent. It is believed that these metal bis(dithiolene) complex loadings provide a balance between the fusing agent having jetting reliability and electromagnetic radiation absorbance efficiency. 
     As mentioned above, the polar aprotic solvent may be included in the fusing agent to at least partially dissolve the metal bis(dithiolene) complex and to shift the absorption of the metal bis(dithiolene) complex. In some instances, the shift is further into the near-infrared (NIR) region (e.g., shifting from an absorption maximum of about 850 nm when the metal bis(dithiolene) complex is not reduced to an absorption maximum of about 940 nm when metal bis(dithiolene) complex is reduced (e.g., to its monoanionic form or to its dianionic form)). The polar aprotic solvent may shift the absorption maximum of the metal bis(dithiolene) complex by reducing the metal bis(dithiolene) complex to its monoanionic form or to its dianionic form according to equation II: 
                         
The reduction of the metal bis(dithiolene) complex to its monoanionic form or to its dianionic form (left side of equation II) may shift the absorption maximum of the metal bis(dithiolene) complex up to about 100 nm, depending, in part, upon the solvent that is used. For example, 1-methyl-2-pyrrolidone may shift the absorption maximum to about 925 nm, and a 94 nm shift in the absorption maximum may be observed when switching from toluene to DMF. When the metal bis(dithiolene) complex is reduced to its monoanionic form or to its dianionic form, the color of the metal bis(dithiolene) complex may change. For example, the initial reduction of a nickel bis(dithiolene) complex may result in the color changing from green to reddish brown. Other color changes may be observed with different metals in the complex. As noted above, the color changed complex can still absorb infrared radiation, and becomes at least substantially colorless during the application of the electromagnetic radiation.
 
     In some examples, the polar aprotic solvent is a polar aprotic solvent containing a tert-amide. In other examples, the polar aprotic solvent is a polar aprotic solvent containing a sec-amine or a tert-amine. In still other examples, the polar aprotic solvent is an organosulfur, a ketone, or an ether. Some specific examples of the polar aprotic solvent include 1-methyl-2-pyrrolidone (1M2P), 2-pyrrolidone, 1-(2-hydroxyethyl)-2-pyrrolidone, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and combinations thereof. 
     The polar aprotic solvent is present in the fusing agent in an amount sufficient to reduce the metal bis(dithiolene) complex to its monoanionic form or to its dianionic form. In an example, the amount of the polar aprotic solvent in the fusing agent may range from about 5 wt % to about 50 wt % based on the total wt % of the fusing agent. In another example, the amount of the polar aprotic solvent present in the fusing agent is about 40 wt % based on the total wt % of the fusing agent. In still another example, the amount of the polar aprotic solvent present in the fusing agent is about 50 wt % based on the total wt % of the fusing agent. 
     As also mentioned above, the thiol surfactant may be included in the fusing agent to stabilize the metal bis(dithiolene) complex. The thiol surfactant may facilitate the reduction of the metal bis(dithiolene) complex by the polar aprotic solvent. More specifically, the thiol surfactant may render the complex readily reducible and thus more soluble in the polar aprotic solvent. Without the thiol surfactant, the reduction of the metal bis(dithiolene) complex to its monoanionic form or to its dianionic form may require the mixture of the neutral, non-reduced metal bis(dithiolene) complex and the polar aprotic solvent to be heated to an elevated temperature (e.g., a temperature ranging from about 50° C. to about 200° C.) for an extended time period (e.g., a time period ranging from about 5 hours to about 48 hours). When the thiol surfactant is included in the mixture of the metal bis(dithiolene) complex and the polar aprotic solvent, the reduction of the metal bis(dithiolene) complex to its monoanionic form or to its dianionic form may be accomplished at room temperature (e.g., from about 18° C. to about 25° C.) and within a few seconds (e.g., less than 10 seconds). 
     The thiol surfactant may also improve the jettability of the fusing agent by stabilizing the metal bis(dithiolene) complex. Without the thiol surfactant, the metal bis(dithiolene) complex may precipitate out of solution when water or a liquid vehicle is added. When the thiol surfactant is included in the mixture of the metal bis(dithiolene) complex and the polar aprotic solvent, the reduced metal bis(dithiolene) complex can be easily formulated into (i.e., dissolved or dispersed rather than precipitated out of) a liquid vehicle. 
     An example of the thiol surfactant is dodecanethiol, 1-undecanethiol, 2-ethyihexanethiol, 1-octanethiol, 1-tetradecanethiol, and combinations thereof. 
     The thiol surfactant is present in the fusing agent in an amount sufficient to stabilize the metal bis(dithiolene) complex. In an example, the amount of the thiol surfactant in the fusing agent may range from about 1 wt % to about 5 wt % based on the total wt % of the fusing agent. 
     As used herein, “FA vehicle” may refer to the liquid fluid in which the metal bis(dithiolene) complex is placed to form the fusing agent. A wide variety of FA vehicles may be used with the fusing agent, system, and method of the present disclosure. The FA vehicle may include water, alone or in combination with a mixture of a variety of additional components. Examples of these additional components may include water soluble organic solvent(s), wetting agent(s), surface tension reduction agent(s), emulsifier(s), scale inhibitor(s), anti-deceleration agent(s), chelating agent(s), and/or antimicrobial agent(s). 
     One example FA vehicle includes water, the polar aprotic solvent, and the thiol surfactant. Another example FA vehicle consists of water, the polar aprotic solvent, and the thiol surfactant (without any other components). 
     The water in the FA vehicle may prevent (further) reduction of the metal bis(dithiolene) complex until the water is driven off as a result of the build material platform temperature and/or the temperature achieved during radiation exposure. After the water is driven off, the metal bis(dithiolene) complex is capable of being further reduced and becoming colorless/discolored, which enables the 3D part to exhibit a color of the build material (e.g., white or off-white) or to exhibit a color of a colorant present in the fusing agent. 
     The aqueous nature of the fusing agent enables the fusing agent to penetrate, at least partially, into the layer of the polymeric or polymeric composite build material particles. The build material particles may be hydrophobic, and the presence of the wetting agent(s) in the fusing agent may assist in obtaining a particular wetting behavior. 
     Examples of suitable wetting agents include non-ionic surfactants. Some specific examples include a self-emulsifiable, non-ionic wetting agent based on acetylenic diol chemistry (e.g., SURFYNOL® SEF from Air Products and Chemicals, Inc.), a non-ionic fluorosurfactant (e.g., CAPSTONE® fluorosurfactants from DuPont, previously known as ZONYL FSO), and combinations thereof. In other examples, the wetting agent is 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 wetting agents 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, TERGITOL™ 15S7, and TERGITOL™ 15S9 from The Dow Chemical Company). In some examples, an anionic surfactant may be used in combination with the non-ionic surfactant. In some examples, it may be desirable to utilize a wetting agent having a hydrophilic-lipophilic balance (HLB) less than 10. 
     The wetting agent(s) may be present in the fusing agent in an amount ranging from about 0.1 wt % to about 4 wt % of the total wt % of the fusing agent. In an example, the amount of the wetting agent(s) present in the fusing agent is about 0.1 wt % (based on the total wt % of the fusing agent). In another example, the amount of the wetting agent(s) present in the fusing agent is about 0.04 wt % (based on the total wt % of the fusing agent). 
     The FA vehicle may also include surface tension reduction agent(s). Any of the previously mentioned wetting agents/surfactants may be used to reduce the surface tension. As an example, the surface tension reduction agent may be the self-emulsifiable, non-ionic wetting agent based on acetylenic diol chemistry (e.g., SURFYNOL® SEF from Air Products and Chemicals, Inc.). 
     The surface tension reduction agent(s) may be present in the fusing agent in an amount ranging from about 0.1 wt % to about 4 wt % of the total wt % of the fusing agent. In an example, the amount of the surface tension reduction agent(s) present in the fusing agent is about 1.5 wt % (based on the total wt % of the fusing agent). In another example, the amount of the surface tension reduction agent(s) present in the fusing agent is about 0.6 wt % (based on the total wt % of the fusing agent). 
     When a surfactant is both a wetting agent and a surface tension reduction agent, any of the ranges presented herein for the wetting agent and the surface tension reduction agent may be used for the surfactant. 
     The FA vehicle may also include water soluble organic solvent(s). In some examples, the water soluble organic solvent(s) may be the same type of solvent as the polar aprotic solvent. In these examples, the water soluble organic solvent(s) may be 1-methyl-2-pyrrolidone (1M2P), 2-pyrrolidone, 1-(2-hydroxyethyl)-2-pyrrolidone, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or a combination thereof. In other examples, the water soluble organic solvent(s) may be different than the polar aprotic solvent. For example, two different polar aprotic solvents may be selected. For another example, the water soluble organic solvent(s) may be 1,5-pentanediol, triethylene glycol, tetraethylene glycol, 2-methyl-1,3-propanediol, 1,6-hexanediol, tripropylene glycol methyl ether, or a combination thereof. 
     The water soluble organic solvent(s) may be present in the fusing agent in an amount ranging from about 2 wt % to about 80 wt % of the total wt % of the fusing agent. In an example, the amount of the water soluble organic solvent(s) present in the fusing agent is about 40 wt % (based on the total wt % of the fusing agent). In another example, the amount of the water soluble organic solvent(s) present in the fusing agent is about 16 wt % (based on the total wt % of the fusing agent). 
     The FA vehicle may also include emulsifier(s). Examples of suitable emulsifiers include oleth-3-phosphate (commercially available as CRODAFOS™ O3A or CRODAFOS™ N-3 acid) or dextran 500k. Other suitable examples of the emulsifiers include CRODAFOS™ HCE (phosphate-ester from Croda Int.), CRODAFOS® N10 (oleth-10-phosphate from Croda Int.), etc. 
     The emulsifier(s) may be present in the fusing agent in an amount ranging from about 0.1 wt % to about 2 wt % of the total wt % of the fusing agent. In an example, the amount of the emulsifier(s) present in the fusing agent is about 1 wt % (based on the total wt % of the fusing agent). In another example, the amount of the emulsifier(s) present in the fusing agent is about 0.4 wt % (based on the total wt % of the fusing agent). 
     The fusing agent may include scale inhibitor(s) or anti-deceleration agent(s). One suitable scale inhibitor/anti-deceleration agent is an alkyldiphenyloxide disulfonate (e.g., DOWFAX™ 8390 and DOWFAX™ 2A1 from The Dow Chemical Company). 
     The scale inhibitor(s)/anti-deceleration agent(s) may be present in the fusing agent in an amount ranging from about 0.05 wt % to about 5 wt % of the total wt % of the fusing agent. In an example, the scale inhibitor(s)/anti-deceleration agent(s) is/are present in the fusing agent in an amount of about 0.25 wt % (based on the total wt % of the fusing agent). In another example, the scale inhibitor(s)/anti-deceleration agent(s) is/are present in the fusing agent in an amount of about 0.1 wt % (based on the total wt % of the fusing agent). 
     The fusing agent may also include chelating agent(s). The chelating agent may be included to eliminate the deleterious effects of heavy metal impurities. Examples of suitable chelating agents include disodium ethylenediaminetetraacetic acid (EDTA-Na), ethylene diamine tetra acetic acid (EDTA), and methylglycinediacetic acid (e.g., TRILON® M from BASF Corp.). 
     Whether a single chelating agent is used or a combination of chelating agents is used, the total amount of chelating agent(s) in the fusing agent may range from 0 wt % to about 2 wt % based on the total wt % of the fusing agent. In an example, the chelating agent is present in the fusing agent in an amount of about 0.08 wt % (based on the total wt % of the fusing agent). In another example, the chelating agent is present in the fusing agent in an amount of about 0.032 wt % (based on the total wt % of the fusing agent). 
     The FA vehicle may also include antimicrobial agent(s). Suitable antimicrobial agents include biocides and fungicides. Example antimicrobial agents may include the NUOSEPT® (Ashland Inc.), UCARCIDE™ or KORDEK™ (Dow Chemical Co.), and PROXELr (Arch Chemicals) series, ACTICIDE®@ M20 (Thor), and combinations thereof. 
     In an example, the fusing agent may include a total amount of antimicrobial agents that ranges from about 0.1 wt % to about 0.35 wt %. In an example, the antimicrobial agent is a biocide and is present in the fusing agent in an amount of about 0.32 wt % (based on the total wt % of the fusing agent). In another example, the antimicrobial agent is a biocide and is present in the fusing agent in an amount of about 0.128 wt % (based on the total wt % of the fusing agent). 
     The balance of the fusing agent is water. As an example, deionized water may be used. 
     In an example, the fusing agent includes from about 1 wt % to about 3 wt % of the metal bis(dithiolene) complex, from about 1 wt % to about 5 wt % of the thiol surfactant, from about 5 wt % to about 50 wt % of the polar aprotic solvent, and a balance of water (based on the total wt % of the fusing agent). 
     In some examples the fusing agent may include a colorant in addition to the metal bis(dithiolene) complex. While the metal bis(dithiolene) complex functions as an electromagnetic radiation absorber and becomes colorless after fusing the build material, the additional colorant may impart color to the fusing agent and the resulting 3D part. The amount of the colorant that may be present in the fusing agent ranges from about 1 wt % to about 10 wt % based on the total wt % of the fusing agent. The colorant may be a pigment and/or dye having any suitable color. Examples of the colors include cyan, magenta, yellow, etc. Examples of colorants include dyes, such as Acid Yellow 23 (AY 23), Acid Yellow 17 (AY 17), Acid Red 52 (AR 52), Acid Red 289 (AR 289), Reactive Red 180 (RR 180), Direct Blue 199 (DB 199), or pigments, such as Pigment Blue 15:3 (PB 15:3), Pigment Red 122 (PR 122), Pigment Yellow 155 (PY 155), and Pigment Yellow 74 (PY 74). 
     In some other examples, the fusing agent excludes a colorant other than the metal bis(dithiolene) complex. It may be desirable to exclude the colorant from the fusing agent when the 3D part to be created is to be the color of the polymeric or polymeric composite build material (e.g., white or off-white) or when a colored ink will be applied to the 3D part. 
     Also disclosed herein is a method of making the fusing agent. An example of this method  200  is shown in  FIG. 5 . As shown at reference numeral  202  of  FIG. 5 , the fusing agent may be prepared by exposing a metal bis(dithiolene) complex to a solution including a reducing agent and a thiol surfactant, thereby forming a reduced metal bis(dithiolene) complex and dissolving the reduced metal bis(dithiolene) complex in the solution. As shown at reference numeral  204  of  FIG. 5 , an example of the method  200  further includes incorporating the solution into a vehicle including a water soluble organic solvent and an additive selected from the group consisting of an emulsifier, a surface tension reduction agent, a wetting agent, a scale inhibitor, an anti-deceleration agent, a chelating agent, an antimicrobial agent, and a combination thereof. 
     As previously mentioned, the solution to which the metal bis(dithiolene) complex is exposed includes at least the reducing agent and the thiol surfactant. In some instances, this solution may also include water. The metal bis(dithiolene) complex may be reduced to its monoanionic form or to its dianionic form by the reducing agent in the solution. The reduced metal bis(dithiolene) complex may then dissolve in the solution. 
     In some examples, the metal bis(dithiolene) complex is exposed to the solution at room temperature (e.g., a temperature ranging from about 18° C. to about 25° C. The metal bis(dithiolene) complex may be reduced and dissolved in the solution within few seconds (e.g., less than 10 seconds). 
     In some examples, the reducing agent is the polar aprotic solvent that is included in the fusing agent. In these examples, the reducing agent may be 1-methyl-2-pyrrolidone (1M2P), 2-pyrrolidone, 1-(2-hydroxyethyl)-2-pyrrolidone, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or a combination thereof. In other examples, the reducing agent is different than the polar aprotic solvent that is included in the fusing agent. When the reducing agent is different than the polar aprotic solvent in the fusing agent, the reducing agent may be selected from reducing agents that are capable of reducing the metal bis(dithiolene) complex to its monoanionic form or to its dianionic form and that are compatible with the FA vehicle (i.e., able to be formulated into the FA vehicle). 
     As mentioned above, examples of the metal bis(dithiolene) complex have the general formula I: 
     
       
         
         
             
             
         
       
     
     Examples of M include nickel, zinc, platinum, palladium, and molybdenum. Examples of each of W, X, Y, and Z include a hydrogen, a phenyl group, a phenyl group bonded to C n H 2n+1 , or OC n H 2n+1 , or N(CH 3 ) 2 , and a sulfur bonded to C H 2n+1 , or OC n H 2n+1 , or N(CH 3 ) 2 . In these examples, n may be greater than or equal to 2 and less than or equal to 12. 
     As also mentioned above, the thiol surfactant may be dodecanethiol, 1-undecanethiol, 2-ethylhexanethiol, 1-octanethiol, 1-tetradecanethiol, or the like. 
     After the metal bis(dithiolene) complex is reduced and dissolved in the solution, the solution may be incorporated into the FA vehicle (reference numeral  204  of  FIG. 5 ). As mentioned above, the FA vehicle may include water, a water soluble organic solvent, and an additive selected from the group consisting of an emulsifier, a surface tension reduction agent, a wetting agent, a scale inhibitor, an anti-deceleration agent, a chelating agent, an antimicrobial agent, and a combination thereof. The water soluble organic solvent and the additive may be any of examples and may be included in any of the amounts described above. 
     Referring now to  FIG. 1 , an example of a 3D printing system  10  is schematically depicted. It is to be understood that the 3D printing system  10  may include additional components and that some of the components described herein may be removed and/or modified. Furthermore, components of the 3D printing system  10  depicted in  FIG. 1  may not be drawn to scale and thus, the 3D printing system  10  may have a different size and/or configuration other than as shown therein. 
     The printing system  10  includes a build area platform  12 , a build material supply  14  containing polymeric or polymeric composite build material particles  16 , and a build material distributor  18 . 
     The build area platform  12  receives the polymeric or polymeric composite build material  16  from the build material supply  14 . The build area platform  12  may be integrated with the printing system  10  or may be a component that is separately insertable into the printing system  10 . For example, the build area platform  12  may be a module that is available separately from the printing system  10 . The build material platform  12  that is shown is also one example, and could be replaced with another support member, such as a platen, a fabrication/print bed, a glass plate, or another build surface. 
     The build area platform  12  may be moved in a direction as denoted by the arrow  20 , e.g., along the z-axis, so that polymeric or polymeric composite build material  16  may be delivered to the platform  12  or to a previously formed layer of the 3D part. In an example, when the polymeric or polymeric composite build material particles  16  are to be delivered, the build area platform  12  may be programmed to advance (e.g., downward) enough so that the build material distributor  18  can push the polymeric or polymeric composite build material particles  16  onto the platform  12  to form a substantially uniform layer of the polymeric or polymeric composite build material  16  thereon (see, e.g.,  FIGS. 2A and 2B ). The build area platform  12  may also be returned to its original position, for example, when a new part is to be built. 
     The build material supply  14  may be a container, bed, or other surface that is to position the polymeric or polymeric composite build material particles  16  between the build material distributor  18  and the build area platform  12 . In some examples, the build material supply  14  may include a surface upon which the polymeric or polymeric composite build material particles  16  may be supplied, for instance, from a build material source (not shown) located above the build material supply  14 . Examples of the build material source may include a hopper, an auger conveyer, or the like. Additionally, or alternatively, the build material supply  14  may include a mechanism (e.g., a delivery piston) to provide, e.g., move, the polymeric or polymeric composite build material particles  16  from a storage location to a position to be spread onto the build area platform  12  or onto a previously formed layer of the 3D part. 
     The build material distributor  18  may be moved in a direction as denoted by the arrow  22 , e.g., along the y-axis, over the build material supply  14  and across the build area platform  12  to spread a layer of the polymeric or polymeric composite build material  16  over the build area platform  12 . The build material distributor  18  may also be returned to a position adjacent to the build material supply  14  following the spreading of the polymeric or polymeric composite build material particles  16 . The build material distributor  18  may be a blade (e.g., a doctor blade), a roller, a combination of a roller and a blade, and/or any other device capable of spreading the polymeric or polymeric composite build material  16  over the build area platform  12 . For instance, the build material distributor  18  may be a counter-rotating roller. 
     The polymeric or polymeric composite build material particles  16  may be a polymeric build material or a polymeric composite build material. As used herein, the term “polymeric build material” may refer to crystalline or semi-crystalline polymer particles. As used herein, the term “polymeric composite build material” may refer or composite particles made up of polymer and ceramic. Any of the polymeric or polymeric composite build material particles  16  may be in powder form. 
     Examples of semi-crystalline polymers include semi-crystalline thermoplastic materials with a wide processing window of greater than 5° C. (i.e., the temperature range between the melting point and the re-crystallization temperature). Some specific examples of the semi-crystalline thermoplastic materials include polyamides (PAs) (e.g., PA 11/nylon 11, PA 12/nylon 12, PA 6/nylon 6, PA 8/nylon 8, PA 9/nylon 9, PA 66/nylon 66, PA 612/nylon 612, PA 812/nylon 812, PA 912/nylon 912, etc.). Other examples of crystalline or semi-crystalline polymers suitable for use as the build material particles  16  include polyethylene, polypropylene, and polyoxomethylene (i.e., polyacetals). Still other examples of suitable build material particles  16  include polystyrene, polycarbonate, polyester, polyurethanes, other engineering plastics, and blends of any two or more of the polymers listed herein. 
     Any of the previously listed crystalline or semi-crystalline polymer particles may be combined with ceramic particles to form the polymeric composite build material particles  16 . Examples of suitable ceramic particles include metal oxides, inorganic glasses, carbides, nitrides, and borides. Some specific examples include alumina (Al 2 O 3 ), glass, silicon mononitride (SiN), silicon dioxide (SiOC 2 ), zirconia (ZrO 2 ), titanium dioxide (TiO 2 ), or combinations thereof. The amount of ceramic particles that may be combined with the crystalline or semi-crystalline polymer particles may depend on the materials used and the 3D part to be formed. In one example, the ceramic particles may be present in an amount ranging from about 1 wt % to about 20 wt % based on the total wt % of the polymeric composite build material particles  16 . 
     The polymeric or polymeric composite build material particles  16  may have a melting point or softening point ranging from about 50° C. to about 400° C. As an example, the build material particles  16  may be a polyamide having a melting point of 180° C. 
     The polymeric or polymeric composite build material particles  16  may be made up of similarly sized particles or differently sized particles. The term “size”, as used herein with regard to the polymeric or polymeric composite build material particles  16 , refers to the diameter of a spherical particle, or the average diameter of a non-spherical particle (i.e., the average of multiple diameters across the particle), or the volume-weighted mean diameter of a particle distribution. In an example, the average size of the polymeric or polymeric composite build material particles  16  ranges from 5 μm to about 200 μm. 
     It is to be understood that the polymeric or polymeric composite build material  16  may include, in addition to polymeric or polymeric composite particles, a charging agent, a flow aid, or combinations thereof. 
     Charging agent(s) may be added to suppress tribo-charging. Examples of suitable charging agent(s) include aliphatic amines (which may be ethoxylated), aliphatic amides, quaternary ammonium salts (e.g., behentrimonium chloride or cocamidopropyl betaine), esters of phosphoric acid, polyethylene glycolesters, or polyols. Some suitable commercially available charging agents include HOSTASTAT® FA 38 (natural based ethoxylated alkylamine), HOSTASTAT® FE2 (fatty acid ester), and HOSTASTAT® HS 1 (alkane sulfonate), each of which is available from Clariant Int. Ltd.). In an example, the charging agent is added in an amount ranging from greater than 0 wt % to less than 5 wt % based upon the total wt % of the polymeric or polymeric composite build material  16 . 
     Flow aid(s) may be added to improve the coating flowability of the polymeric or polymeric composite build material  16 . Flow aid(s) may be particularly beneficial when the particles of the polymeric or polymeric composite build material  16  are less than 25 μm in size. The flow aid improves the flowability of the polymeric or polymeric composite build material  16  by reducing the friction, the lateral drag, and the tribocharge buildup (by increasing the particle conductivity). Examples of suitable flow aids include tricalcium phosphate (E341), powdered cellulose (E460(ii)), magnesium stearate (E470b), sodium bicarbonate (E500), sodium ferrocyanide (E535), potassium ferrocyanide (E536), calcium ferrocyanide (E538), bone phosphate (E542), sodium silicate (E550), silicon dioxide (E551), calcium silicate (E552), magnesium trisilicate (E553a), talcum powder (E553b), sodium aluminosilicate (E554), potassium aluminum silicate (E555), calcium aluminosilicate (E556), bentonite (E558), aluminum silicate (E559), stearic acid (E570), or polydimethylsiloxane (E900). In an example, the flow aid is added in an amount ranging from greater than 0 wt % to less than 5 wt % based upon the total wt % of the polymeric or polymeric composite build material  16 . 
     As shown in  FIG. 1 , the printing system  10  also includes an applicator  24 , which may contain the fusing agent  26  disclosed herein. 
     As mentioned above, the fusing agent  26  may include the metal bis(dithiolene) complex and the FA vehicle. In an example, the fusing agent  26  includes the metal bis(dithiolene) complex, the thiol surfactant, the polar aprotic solvent, and a balance of water. In another example, the fusing agent  26  consists of these components and no other components. In still another example, the fusing agent  26  includes from about 1 wt % to about 3 wt % of the metal bis(dithiolene) complex, from about 1 wt % to about 5 wt % of the thiol surfactant, from about 5 wt % to about 50 wt % of the polar aprotic solvent, and a balance of water (based on the total wt % of the fusing agent  26 ). As also mentioned above, in some examples, the fusing agent  26  includes the colorant. In an example, the fusing agent  26  consists of the metal bis(dithiolene) complex, the thiol surfactant, the polar aprotic solvent, the colorant, and a balance of water. In still other examples, the fusing agent  26  excludes the colorant. 
     The applicator  24  may be scanned across the build area platform  12  in the direction indicated by the arrow  28 , e.g., along the y-axis. The applicator  24  may be, for instance, a thermal inkjet printhead, a piezoelectric printhead, a continuous inkjet printhead, etc., and may extend a width of the build area platform  12 . While the applicator  24  is shown in  FIG. 1  as a single applicator, it is to be understood that the applicator  24  may include multiple applicators that span the width of the build area platform  12 . Additionally, the applicators  24  may be positioned in multiple printbars. The applicator  24  may also be scanned along the x-axis, for instance, in configurations in which the applicator  24  does not span the width of the build area platform  12  to enable the applicator  24  to deposit the fusing agent  26  over a large area of a layer of polymeric or polymeric composite build material particles  16 . The applicator  24  may thus be attached to a moving XY stage or a translational carriage (neither of which is shown) that moves the applicator  24  adjacent to the build area platform  12  in order to deposit the fusing agent  26  in predetermined areas of a layer of the polymeric or polymeric composite build material particles  16  that has been formed on the build area platform  12  in accordance with the method(s) disclosed herein. The applicator  24  may include a plurality of nozzles (not shown) through which the fusing agent  26  is to be ejected. 
     The applicator  24  may deliver drops of the fusing agent  26  at a resolution ranging from about 300 dots per inch (DPI) to about 1200 DPI. In other examples, the applicator  24  may deliver drops of the fusing agent  26  at a higher or lower resolution. The drop velocity may range from about 5 m/s to about 24 m/s and the firing frequency may range from about 1 kHz to about 100 kHz. In one example, each drop may be in the order of about 10 picoliters (μl) per drop, although it is contemplated that a higher or lower drop size may be used. In some examples, applicator  24  is able to deliver variable size drops of the fusing agent  26 . 
     In some examples of the system  10  disclosed herein, another applicator (not shown) may be included that is capable of applying a colored ink to the 3D part that is formed. This applicator may be similar to or the same as applicator  24 . It may be desirable to include this applicator and the colored ink when the 3D part is the color of the polymeric or polymeric composite build material (e.g., white or off-white), and when it is desirable to apply color to the white or off-white 3D part. 
     An example of a pigment based colored ink may include from about 1 wt % to about 10 wt % of pigment(s), from about 10 wt % to about 30 wt % of co-solvent(s), from about 0.5 wt % to about 2 wt % of dispersant(s), from 0.01 wt % to about 1 wt % of anti-kogation agent(s), from about 0.1 wt % to about 5 wt % of binder(s), from about 0.05 wt % to about 0.1 wt % biocide(s), and a balance of water. An example of a dye based colored ink may include from about 1 wt % to about 7 wt % of dye(s), from about 10 wt % to about 30 wt % of co-solvent(s), from about 0.25 wt % to about 2 wt % of dispersant(s), from 0.05 wt % to about 0.1 wt % of chelating agent(s), from about 0.005 wt % to about 0.2 wt % of buffer(s), from about 0.05 wt % to about 0.1 wt % biocide(s), and a balance of water. Some specific examples of suitable colored inks include a set of cyan, magenta, and yellow inks, such as C1893A (cyan), C1984A (magenta), and C1985A (yellow); or C4801A (cyan), C4802A (magenta), and C4803A (yellow); all of which are available from HP Inc. Other commercially available colored inks include C9384A (printhead HP 72), C9383A (printhead HP 72), C4901A (printhead HP 940), and C4900A (printhead HP 940). 
     Each of the previously described physical elements may be operatively connected to a controller  30  of the prnting system  10 . The controller  30  may control the operations of the build area platform  12 , the build material supply  14 , the build material distributor  18 , and the applicator  24 . As an example, the controller  30  may control actuators (not shown) to control various operations of the 3D printing system  10  components. The controller  30  may be a computing device, a semiconductor-based microprocessor, a central processing unit (CPU), an application specific integrated circuit (ASIC), and/or another hardware device, Although not shown, the controller  30  may be connected to the 3D printing system  10  components via communication lines. 
     The controller  30  manipulates and transforms data, which may be represented as physical (electronic) quantities within the printer&#39;s registers and memories, in order to control the physical elements to create the 3D part. As such, the controller  30  is depicted as being in communication with a data store  32 . The data store  32  may include data pertaining to a 3D part to be printed by the 3D printing system  10 . The data for the selective delivery of the polymeric or polymeric composite build material particles  16 , the fusing agent  26 , etc. may be derived from a model of the 3D part to be formed. For instance, the data may include the locations on each layer of polymeric or polymer composite build material particles  16  that the applicator  24  is to deposit the fusing agent  26 . In one example, the controller  30  may use the data to control the applicator  24  to selectively apply the fusing agent  26 . The data store  32  may also include machine readable instructions (stored on a non-transitory computer readable medium) that are to cause the controller  30  to control the amount of polymeric or polymeric composite build material particles  16  that is supplied by the build material supply  14 , the movement of the build area platform  12 , the movement of the build material distributor  18 , the movement of the applicator  24 , etc. 
     As shown in  FIG. 1 , the printing system  10  may also include a source of electromagnetic radiation  34 ,  34 ′. In some examples, the source of electromagnetic radiation  34 ,  34 ′ may be in a fixed position with respect to the build material platform  12 . In other examples, the source of electromagnetic radiation  34 ,  34 ′ may be positioned to expose the layer of polymeric or polymeric composite build material particles  16  to electromagnetic radiation immediately after the fusing agent  26  has been applied thereto. In the example shown in  FIG. 1 , the source of electromagnetic radiation  34 ′ is attached to the side of the applicator  24  which allows for patterning and heating in a single pass. 
     The source of electromagnetic radiation  34 ,  34 ′ may emit electromagnetic radiation having wavelengths ranging from about 800 nm to about 1 mm. As one example, the electromagnetic radiation may range from about 800 nm to about 2 μm. As another example, the electromagnetic radiation may be blackbody radiation with a maximum intensity at a wavelength of about 1100 nm. The source of electromagnetic radiation  34 ,  34 ′ may be infrared (IR) or near-infrared light sources, such as IR or near-IR curing lamps, IR or near-IR light emitting diodes (LED), or lasers with the desirable IR or near-IR electromagnetic wavelengths. 
     The source of electromagnetic radiation  34 ,  34 ′ may be operatively connected to a lamp/laser driver, an input/output temperature controller, and temperature sensors, which are collectively shown as radiation system components 36. The radiation system components 36 may operate together to control the source of electromagnetic radiation  34 ,  34 ′. The temperature recipe (e.g., radiation exposure rate) may be submitted to the input/output temperature controller. During heating, the temperature sensors may sense the temperature of the polymeric or polymeric composite build material particles  16 , and the temperature measurements may be transmitted to the input/output temperature controller. For example, a thermometer associated with the heated area can provide temperature feedback. The input/output temperature controller may adjust the source of electromagnetic radiation  34 ,  34 ′ power set points based on any difference between the recipe and the real-time measurements. These power set points are sent to the lamp/laser drivers, which transmit appropriate lamp/laser voltages to the source of electromagnetic radiation  34 ,  34 ′. This is one example of the radiation system components 36, and it is to be understood that other radiation source control systems may be used. For example, the controller  30  may be configured to control the source of electromagnetic radiation  34 ,  34 ′. 
     Referring now to  FIGS. 2A through 2D , an example of the 3D printing method  100  is depicted. This method  100  may be used to form 3D printed parts having mechanical integrity (e.g., having an ultimate tensile strength ranging from about 40 MPa to about 55 MPa) and being white, off-white, or colored. In other examples, the ultimate tensile strength of the 3D printed part may range from about 40 MPa to about 51 MPa or from about 40 MPa to about 45 MPa. 
     Prior to execution of the method  100  or as part of the method  100 , the controller  30  may access data stored in the data store  32  pertaining to a 3D part that is to be printed. The controller  30  may determine the number of layers of polymeric or polymeric composite build material  16  that are to be formed, and the locations at which the fusing agent  26  from the applicator  24  is to be deposited on each of the respective layers. 
     As shown in  FIGS. 2A and 2B , the method  100  includes applying the polymeric or polymeric composite build material  16 . In  FIG. 2A , the build material supply  14  may supply the polymeric or polymeric composite build material particles  16  into a position so that they are ready to be spread onto the build area platform  12 . In  FIG. 2B , the build material distributor  18  may spread the supplied polymeric or polymeric composite build material particles  16  onto the build area platform  12 . The controller  30  may execute control build material supply instructions to control the build material supply  14  to appropriately position the polymeric or polymeric composite build material particles  16 , and may execute control spreader instructions to control the build material distributor  18  to spread the supplied polymeric or polymeric composite build material particles  16  over the build area platform  12  to form a layer  38  of polymeric or polymeric composite build material particles  16  thereon. As shown in  FIG. 2B , one layer  38  of the polymeric or polymeric composite build material particles  16  has been applied. 
     The layer  38  has a substantially uniform thickness across the build area platform  12 . In an example, the thickness of the layer  38  ranges from about 50 μm to about 300 μm, although thinner or thicker layers may also be used. For example, the thickness of the layer  38  may range from about 20 μm to about 500 μm, or from about 30 μm to about 300 μm. The layer thickness may be about 2× the particle diameter (as shown in  FIG. 2B ) at a minimum for finer part definition. 
     Prior to further processing, the layer  38  of the polymeric or polymeric composite build material particles  16  may be exposed to heating. Heating may be performed to pre-heat the polymeric or polymeric composite build material particles  16 , and thus the heating temperature may be below the melting point or softening point of the polymeric or polymeric composite build material particles  16 . As such, the temperature selected will depend upon the polymeric or polymeric composite build material particles  16  that are used. As examples, the pre-heating temperature may be from about 5° C. to about 50° C. below the melting point or softening point of the polymeric or polymeric composite build material particles  16 . In an example, the pre-heating temperature ranges from about 50° C. to about 350° C. In another example, the pre-heating temperature ranges from about 150° C. to about 170° C. 
     Pre-heating the layer  38  of the polymeric or polymeric composite build material particles  16  may be accomplished using any suitable heat source that exposes all of the polymeric or polymeric composite build material particles  16  on the build material surface  12  to the heat. Examples of the heat source include a thermal heat source (e.g., a heater (not shown) of the particles  16 ) or the electromagnetic radiation source  34 ,  34 ′. 
     Referring now to  FIG. 2C , after the layer  38  is formed, and in some instances is pre-heated, the fusing agent  26  is selectively applied on at least a portion  40  of the polymeric or polymeric composite build material  16 . 
     It is to be understood that a single fusing agent  26  may be selectively applied on the portion  40 , or multiple fusing agents  26  may be selectively applied on the portion  40 . As an example, multiple fusing agents  26  may be used when the colorant is included in at least one of the multiple fusing agents  26  to create a multi-colored part. 
     As illustrated in  FIG. 2C , the fusing agent  26  may be dispensed from the applicator  24 . The controller  32  may execute instructions to control the applicator  24  (e.g., in the directions indicated by the arrow  28 ) to deposit the fusing agent  26  onto predetermined portion(s)  40  of the polymeric or polymeric composite build material  16  that are to become part of the 3D part. The applicator  24  may be programmed to receive commands from the controller  30  and to deposit the fusing agent  26  according to a pattern of a cross-section for the layer of the 3D part to be formed. As used herein, the cross-section of the layer of the 3D part to be formed refers to the cross-section that is parallel to the surface of the build area platform  12 . In the example shown in  FIG. 2C , the applicator  24  selectively applies the fusing agent  26  on those portion(s)  40  of the layer  38  that are to be fused to become the first layer of the 3D part. As an example, if the 3D part that is to be formed is to be shaped like a cube or cylinder, the fusing agent  26  will be deposited in a square pattern or a circular pattern (from a top view), respectively, on at least a portion of the layer  38  of the polymeric or polymeric composite build material particles  16 . In the example shown in  FIG. 2C , the fusing agent  26  is deposited in a square pattern on the portion  40  of the layer  38  and not on the portions  42 . 
     As mentioned above, the fusing agent  26  may include the metal bis(dithiolene) complex and the FA vehicle. In an example, the fusing agent  26  includes the metal bis(dithiolene) complex, the thiol surfactant, the polar aprotic solvent, and a balance of water. In another example, the fusing agent  26  consists of these components and no other components. In still another example, the fusing agent  26  includes from about 1 wt % to about 3 wt % of the metal bis(dithiolene) complex, from about 1 wt % to about 5 wt % of the thiol surfactant, from about 5 wt % to about 50 wt % of the polar aprotic solvent, and a balance of water (based on the total wt % of the fusing agent  26 ). As also mentioned above, in some examples, the fusing agent  26  includes the colorant. In an example, the fusing agent  26  consists of the metal bis(dithiolene) complex, the thiol surfactant, the polar aprotic solvent, the colorant, and a balance of water. In still other examples, the fusing agent  26  excludes the colorant. 
     The volume of the fusing agent  26  that is applied per unit of the polymeric or polymeric composite build material  16  in the patterned portion  40  may be sufficient to absorb and convert enough electromagnetic radiation so that the polymeric or polymeric composite build material  16  in the patterned portion  40  will fuse. The volume of the fusing agent  26  that is applied per unit of the polymer or polymeric composite build material  16  may depend, at least in part, on the metal bis(dithiolene) complex used, the metal bis(dithiolene) complex loading in the fusing agent  26 , and the polymeric or polymeric composite build material  16  used. 
     As shown between  FIGS. 2C and 2D , after applying the fusing agent  26 , the entire layer  38  of the polymeric or polymeric composite build material  16  is exposed to electromagnetic radiation (shown as EMR Exposure between  FIGS. 2C and 2D ). 
     The electromagnetic radiation is emitted from the source of electromagnetic radiation  34 ,  34 ′ (shown in  FIG. 1 ). The length of time the electromagnetic radiation is applied for, or energy exposure time, may be dependent, for example, on one or more of: characteristics of the electromagnetic radiation  34 ,  34 ′; characteristics of the polymeric or polymeric composite build material particles  16 ; and/or characteristics of the fusing agent  26 . 
     The fusing agent  26  enhances the absorption of the radiation, converts the absorbed radiation to thermal energy, and promotes the transfer of the thermal heat to the polymeric or polymeric composite build material particles  16  in contact therewith. In an example, the fusing agent  26  sufficiently elevates the temperature of the polymeric or polymeric composite build material particles  16  in layer  38  above the melting or softening point of the particles  16 , allowing fusing (e.g., sintering, binding, curing, etc.) of the polymeric or polymeric composite build material particles  16  to take place. Exposure to electromagnetic radiation forms the fused layer  44 , as shown in  FIG. 4D . 
     It is to be understood that portions  42  of the polymeric or polymeric composite build material  16  that do not have the fusing agent  26  applied thereto may not absorb enough radiation to fuse. As such, these portions  42  may not become part of the 3D part that is ultimately formed. The polymeric or polymeric composite build material  16  in portions  42  may be reclaimed to be reused as build material in the printing of another 3D part. 
     The processes shown in  FIGS. 2A through 2D  may be repeated to iteratively build up several fused layers and to form the 3D printed part.  FIG. 20  illustrates the initial formation of a second layer of polymeric or polymeric composite build material particles  16  on the previously formed layer  44 . In  FIG. 2D , following the fusing of the predetermined portion(s)  40  of the layer  38  of polymeric or polymeric composite build material  16 , the controller  30  may execute instructions to cause the build area platform  12  to be moved a relatively small distance in the direction denoted by the arrow  20 . In other words, the build area platform  12  may be lowered to enable the next layer of polymeric or polymeric composite build material particles  16  to be formed. For example, the build material platform  12  may be lowered a distance that is equivalent to the height of the layer  38 . In addition, following the lowering of the build area platform  12 , the controller  30  may control the build material supply  14  to supply additional polymeric or polymeric composite build material particles  16  (e.g., through operation of an elevator, an auger, or the like) and the build material distributor  18  to form another layer of polymeric or polymeric composite build material particles  16  on top of the previously formed layer with the additional polymeric or polymeric composite build material  16 . The newly formed layer may be in some instances preheated, patterned with the fusing agent  26 , and then exposed to radiation from the source of electromagnetic radiation  34 ,  34 ′ to form the additional fused layer. 
     Since the metal bis(dithiolene) complex is further reduced and becomes colorless at least during fusing, the layer  44  (and the final 3D part) exhibits a color of the build material (e.g., white or off-white) or exhibits a color of a colorant present in the fusing agent  26 . In the latter example, if it is desirable to impart color to the layer  44 , the colored ink may be selectively applied to at least a portion of the layer  44 . 
     To further illustrate the present disclosure, examples are given herein. It is to be understood that these examples are provided for illustrative purposes and are not to be construed as limiting the scope of the present disclosure. 
     EXAMPLES 
     Example 1 
     An example of the metal bis(dithiolene) complex was prepared in an example of the polar aprotic solvent and an example of the thiol surfactant (referred to as “composition 1”). The metal bis(dithiolene) complex used was nickel dithiolene. The polar aprotic solvent used was 1-methyl-2-pyrrolidone (1M2P), and the thiol surfactant used was dodecanethiol. The general formulation of the composition is shown in Table 1, with the wt % of each component that was used. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Composition 1 
               
               
                   
                 Ingredient 
                 Special component 
                 (wt %) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Polar aprotic solvent 
                 1-methyl-2-pyrrolidone 
                 83 
               
               
                   
                 Thiol surfactant 
                 Dodecanethiol 
                 15 
               
               
                   
                   
                 (in water) 
                   
               
               
                   
                 Metal bis(dithiolene) 
                 Nickel dithiolene 
                 2 
               
               
                   
                 complex 
                   
                   
               
               
                   
                   
               
            
           
         
       
     
     The nickel dithiolene, in the presence of dodecanethiol, was readily reduced and dissolved in the 1-methyl-2-pyrrolidone within seconds at room temperature. The nickel dithiolene changed colors from green (before reduction) to reddish brown (after reduction). 
     Composition 1 was incorporated into a vehicle to form a fusing agent. The general formulation of the vehicle is shown in Table 2, with the wt % of each component that was used. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Ingredient 
                 Specific component 
                 Vehicle (wt %) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Co-solvent 
                 1-methyl-2-pyrrolidone 
                 40 
               
               
                   
                 Emulsifier 
                 CRODAFOS ® O3A 
                 1 
               
               
                   
                 Surface tension 
                 SURFYNOL ® SEF 
                 1.5 
               
               
                   
                 reduction agent 
                   
                   
               
               
                   
                 Wetting agent 
                 CAPSTONE ® FS-35 
                 0.10 
               
               
                   
                 Scale inhibitor/Anti- 
                 DOWFAX ™ 2A1 
                 0.20 
               
               
                   
                 deceleration agent 
                   
                   
               
               
                   
                 Chelating agent 
                 TRILON ® M 
                 0.08 
               
               
                   
                 Biocide 
                 PROXEL ® GXL 
                 0.36 
               
               
                   
                   
                 DI (deionized) Water 
                 Balance 
               
               
                   
                   
               
            
           
         
       
     
     The fusing agent included about 50% of composition 1, about 40% of the vehicle, and about 10% of additional deionized water (in addition to the water already present in the vehicle). 
     An additional composition (referred to as “composition 2”) containing an example of the metal bis(dithiolene) complex was prepared in a comparative solvent, namely chloroform. The metal bis(dithiolene) complex used in composition 2 was nickel dithiolene. However, composition 2 did not include the thiol surfactant. The general formulation of composition 2 is shown in Table 3, with the wt % of each component that was used. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                 Composition 2 
               
               
                   
                 Ingredient 
                 Specific component 
                 (wt %) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Solvent 
                 Chloroform 
                 98 
               
               
                   
                 Metal bis(dithiolene) 
                 Nickel dithiolene 
                 2 
               
               
                   
                 complex 
                   
                   
               
               
                   
                   
               
            
           
         
       
     
     Composition 2 was heated to 70° C. for 24 hours to reduce and dissolve the nickel dithiolene in the chloroform. Composition 2 was incorporated into the vehicle shown in Table 2 to form a comparative fusing agent. The comparative fusing agent included about 50% of composition 2, about 40% of the vehicle, and about 10% of additional deionized water (in addition to the water already present in the vehicle). 
     Composition 1 was also incorporated into the vehicle shown in Table 2 to form a second fusing agent. Composition 1, the vehicle, and the additional water were used in amounts that provided the second fusing agent with 0.005 wt. % of the nickel dithiolene. 
     The absorbance of each of the first and second fusing agents and the comparative fusing agent was measured. The results of the absorbance measurements are shown in  FIG. 3 . The absorbance values are shown along the Y axis and the wavelength values in nm are shown along the X axis. As shown in  FIG. 3 , the maximum absorbance of the first fusing agent shifted to about 940 nm, while the maximum absorbance of the comparative fusing agent was at about 850 nm. The example fusing agent (including the polar aprotic solvent, the thiol surfactant, and the metal dithiolene complex) had about a 90 nm shift in the absorbance maximum. 
     The absorbance of the second fusing agent illustrates that at low concentration (e.g., 0.005%), the dye completely reduces to the colorless state after 24 hours in a reducing solvent. This example demonstrates how the dye turns colorless. 
     Example 2 
     Several example 3D parts were printed. The build material used to print the example parts was polyamide-12 (PA-12). The fusing agent used to print the example parts was the first fusing agent from Example 1 (which contained nickel dithiolene, dodecanethiol, and 1-methyl-2-pyrrolidone (1M2P), as well as the vehicle and additional Dl water). 
     For each example part, the first fusing agent was thermal inkjet printed with a HP761 printhead (manufactured by Hewlett-Packard Company) in a pattern on a portion of the PA-12 in subsequent layers. Each layer was about 100 μm in thickness. New layers were spread onto the fabrication bed from a supply region using a roller. The temperature of the supply region was set at 115° C. The temperature of the printing region was set at 155° C. with a platen underneath it heated to 148° C. The example parts were printed at a contone level ranging from about 20 contone to about 128 contone (which refers to the number of drops, which is divided by 256, that will be placed on average onto each pixel). The example parts were then exposed to high-intensity light from a halogen lamp with a power ranging from about 500 watts to 750 watts and a color temperature ranging from about 2700 K to about 3400 K passing over the fabrication bed with a fusing speed ranging from about 20 inches per second (ips) to about 30 ips. After fusing, the nickel dithiolene was further reduced from its reddish brown form to its colorless form. After all layers were printed, the example parts were removed from the fabrication bed and sandblasted to remove excess powder. The example parts were not subjected to any further post treatment. 
       FIGS. 4A and 4B  show the example parts in black and white. The example parts as formed had vivid colors.  FIG. 4A  shows a child teeth model in black and white, and the original photographs showed bright white teeth and red gums. The teeth were sharp and strong.  FIG. 4B  shows four dog bones. From top to bottom, the dog bones (shown herein black and white) were bright pink, bright white, blue, and yellow. The dog bones had a tensile strength of about 40 MPa. 
     It is to be understood that the ranges provided herein include the stated range and any value or sub-range within the stated range. For example, a range from about 1 wt % to about 3 wt % should be interpreted to include not only the explicitly recited limits of from about 1 wt % to about 3 wt %, but also to include individual values, such as 1.35 wt %, 1.55 wt %, 2.5 wt %, 2.85 wt %, etc., and sub-ranges, such as from about 1.35 wt % to about 2.5 wt %, from about 1.5 wt % to about 2.7 wt %, etc. Furthermore, when “about” is utilized to describe a value, this is meant to encompass minor variations (up to +/−10%) from the stated value. 
     Reference throughout the specification to “one example”, “another example”, “an example”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the example is included in at least one example described herein, and may or may not be present in other examples. In addition, it is to be understood that the described elements for any example may be combined in any suitable manner in the various examples unless the context clearly dictates otherwise. 
     In describing and claiming the examples disclosed herein, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
     While several examples have been described in detail, it is to be understood that the disclosed examples may be modified. Therefore, the foregoing description is to be considered non-limiting.