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Title:
THREE-DIMENSIONAL (3D) PRINTING
Document Type and Number:
WIPO Patent Application WO/2018/199955
Kind Code:
A1
Abstract:
In a 3D printing method example, a polymeric or polymeric composite build material is applied. A functional agent is selectively applied on at least a portion of the build material. The functional agent includes a functional additive selected from the group consisting of a thermochromic agent, a gasochromic agent, a potential hydrogen (pH) sensitive agent, and a combination thereof. Either a fusing agent is selectively applied on the at least the portion of the build material, and the build material is exposed to electromagnetic radiation to fuse the at least the portion of the build material in contact with the fusing agent to form a region of a layer; or the at least the portion of the build material is selectively laser sintered to form the region of the layer. The region of the layer exhibits a thermochromic property, a gasochromic property, a pH sensitive property, or a combination thereof.

Inventors:
TANDY JESISKA (US)
LEBRON HECTOR (US)
WOODRUFF SHANNON (US)
RUDISILL STEPHEN G (US)
WRIGHT JAKE (US)
KABALNOV ALEXEY S (US)
Application Number:
PCT/US2017/029856
Publication Date:
November 01, 2018
Filing Date:
April 27, 2017
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
HEWLETT PACKARD DEVELOPMENT CO (US)
International Classes:
B29C64/165; B29C64/20; B33Y10/00
Domestic Patent References:
WO2016080993A12016-05-26
WO2017019102A12017-02-02
WO2015014381A12015-02-05
WO2016068899A12016-05-06
WO2016072966A12016-05-12
Attorney, Agent or Firm:
LEMMON, Marcus et al. (US)
Download PDF:
Claims:
What is claimed is:

1 . A three-dimensional (3D) printing method, comprising:

applying a polymeric or polymeric composite build material;

selectively applying a functional agent on at least a portion of the polymeric or polymeric composite build material, the functional agent including a functional additive selected from the group consisting of a thermochromic agent, a

gasochromic agent, a potential hydrogen (pH) sensitive agent, and a combination thereof; and

one of:

selectively applying a fusing agent on the at least the portion of the polymeric or polymeric composite build material, and exposing the polymeric or polymeric composite build material to electromagnetic radiation to fuse the at least the portion of the polymeric or polymeric composite build material to form a region of a layer; or

selectively laser sintering the at least the portion of the polymeric or polymeric composite build material to fuse the at least the portion of the polymeric or polymeric composite build material to form the region of the layer;

wherein the region of the layer exhibits a thermochromic property, a gasochromic property, a pH sensitive property, or a combination thereof.

2. The 3D printing method as defined in claim 1 wherein:

the at least the portion of the polymeric or polymeric composite build material is less than all of the polymeric or polymeric composite build material; and

the method further comprises one of:

selectively applying the fusing agent on an other portion of the polymeric or polymeric composite build material, and the exposing of the polymeric or polymeric composite build material to electromagnetic radiation fuses the other portion of the polymeric or polymeric composite build material and forms a remaining region of the layer; or

selectively laser sintering the other portion of the polymeric or polymeric composite build material to fuse the other portion of the polymeric or polymeric composite build material and form the remaining region of the layer.

3. The 3D printing method as defined in claim 2 wherein the remaining region of the layer does not exhibit the thermochromic property, the gasochromic property, the pH sensitive property, or the combination thereof.

4. The 3D printing method as defined in claim 1 wherein the functional agent includes the functional additive in a liquid vehicle.

5. The 3D printing method as defined in claim 4 wherein the functional additive is selected from the group consisting of a triarylmethane dye, a leuco dye, an azo dye, an eurhodin dye, an indigo dye derivative, a thiazine dye, and a combination thereof.

6. The 3D printing method as defined in claim 4 wherein the functional additive is present in the functional agent in an amount ranging from about 0.1 wt% to about 8 wt% based on a total wt% of the functional agent. 7. The 3D printing method as defined in claim 1 wherein the polymeric or polymeric composite build material includes:

an antioxidant; and

a brightener. 8. The 3D printing method as defined in claim 1 wherein each of the fusing agent and the functional agent excludes a binder.

9. The 3D printing method as defined in claim 1 wherein the fusing agent is selectively applied and each of the selectively applying of the fusing agent and the selectively applying of the functional agent is accomplished by thermal inkjet printing or piezo electric inkjet printing.

10. A 3D printing method, comprising:

applying a polymeric or polymeric composite build material, the polymeric or polymeric composite build material including a functional additive selected from the group consisting of a thermochromic agent, a gasochromic agent, a potential hydrogen (pH) sensitive agent, and a combination thereof; and

one of:

selectively applying a fusing agent on at least a portion of the polymeric or polymeric composite build material, and exposing the polymeric or polymeric composite build material to electromagnetic radiation to fuse the at least the portion of the polymeric or polymeric composite build material to form a layer; or

selectively laser sintering the at least the portion of the polymeric or polymeric composite build material to fuse the at least the portion of the polymeric or polymeric composite build material to form the layer;

wherein the layer exhibits a thermochromic property, a gasochromic property, a pH sensitive property, or a combination thereof.

1 1 . The method as defined in claim 10 wherein the functional additive is selected from the group consisting of a triarylmethane dye, a leuco dye, an azo dye, an eurhodin dye, an indigo dye derivative, a thiazine dye, and a combination thereof.

12. The method as defined in claim 10 wherein the functional additive is present in the polymeric or polymeric composite build material in an amount ranging from about 0.05 wt% to about 4 wt% based on a total wt% of the polymeric or polymeric composite build material.

13. The method as defined in claim 10 wherein the polymeric or polymeric composite build material further includes:

an antioxidant; and

a brightener.

14. The method as defined in claim 10 wherein:

the fusing agent is selectively applied;

the fusing agent excludes a binder; and

the selectively applying of the fusing agent is accomplished by thermal inkjet printing or piezo electric inkjet printing.

15. A three-dimensional (3D) printing system, comprising:

a supply of polymeric or polymeric composite build material;

a build material distributor;

a supply of a fusing agent;

a first applicator for selectively dispensing the fusing agent;

a supply of a functional agent including a functional additive selected from the group consisting of a thermochromic agent, a gasochromic agent, a potential hydrogen (pH) sensitive agent, and a combination thereof;

a second applicator for selectively dispensing the functional agent;

a source of electromagnetic radiation;

a controller; and

a non-transitory computer readable medium having stored thereon computer executable instructions to cause the controller to:

utilize the build material distributor to dispense the polymeric or polymeric composite build material;

utilize the first applicator and the second applicator to respectively and selectively dispense the fusing agent and the functional agent on at least a portion of the polymeric or polymeric composite build material; and

utilize the source of electromagnetic radiation to expose the polymeric or polymeric composite build material to electromagnetic radiation to fuse the portion of the polymeric or polymeric composite build material to form a region of a layer, wherein the region of the layer exhibits a thermochromic property, a gasochromic property, potential hydrogen (pH) sensitive property, or a combination thereof.

Description:
THREE-DIMENSIONAL (3D) PRINTING

BACKGROUND

[0001 ] 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

[0002] 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.

[0003] Fig. 1 a simplified isometric and schematic view of an example of a 3D printing system disclosed herein;

[0004] Figs. 2A through 2E are schematic and partially cross-sectional views depicting the formation of a 3D part using examples of a 3D printing method disclosed herein; [0005] Fig. 3 is a flow diagram illustrating examples of a 3D printing method disclosed herein;

[0006] Fig. 4 is a flow diagram illustrating other examples of a 3D printing method disclosed herein;

[0007] Fig. 5 is a black and white representation of an originally colored photograph showing example 3D parts and comparative example 3D parts;

[0008] Figs. 6A and 6B are black and white representations of originally colored photographs showing example 3D parts after additional heating;

[0009] Figs. 7A and 7B are black and white representations of originally colored photographs showing example 3D parts after being dipped in a hydrochloric acid solution (7A) and after being washed with deionized water (7B); and

[0010] Figs. 8A and 8B are black and white representations of originally colored photographs showing comparative example 3D parts after being dipped in a hydrochloric acid solution (8A) and after being washed with deionized water (8B).

DETAILED DESCRIPTION

[001 1 ] Some examples of the three-dimensional (3D) printing method and the 3D printing system disclosed herein may utilize a fusing agent. During these examples, 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. The 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.

[0012] Other examples of the 3D printing method and the 3D printing system disclosed herein may utilize selective laser sintering (SLS) or selective laser melting (SLM). During selective laser sintering or melting, a laser beam is aimed at a selected region (generally less than the entire layer) of a layer of the build material. Heat from the laser beam causes the build material under the laser beam to sinter or melt. This causes the build material to fuse, bind, cure, etc. to form the layer of the 3D part.

[0013] As used herein, the terms "fuse," "fusing," or the like generally refer to the build material joining, binding, or blending to form a single entity. Fusing may be the result of sintering, melting, curing, or another chemical process which depends, in part, upon the build material and 3D print conditions utilized.

[0014] It has been found that including a functional additive selected from the group consisting of a thermochromic agent, a gasochromic agent, a potential hydrogen (pH) sensitive agent, and a combination thereof in a functional agent used during printing, and/or in a polymeric or polymeric composite build material used during printing causes the resulting 3D part to exhibit a thermochromic property, a gasochromic property, a potential hydrogen (pH) sensitive property, or a

combination thereof. The inclusion of the functional additive in the functional agent or the polymeric or polymeric composite build material forms a 3D printed and thermochromic, gasochromic, and/or pH sensitive part.

[0015] 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.

[0016] By "thermochromic," it is meant that the 3D part will change color when a designated temperature threshold (e.g., 165°C) is crossed. In any of the examples disclosed herein, the change in color may be a change in the shade (e.g., from dark blue to light blue) or a change in the hue (e.g., from blue to white or off white). In some examples, the 3D part may be maintained at or above the designated temperature threshold for a designated amount of time (e.g., 2 hours) for the color change to be observed. It is to be understood that a thermochromic 3D part may exhibit its thermochromic property globally (i.e., the entire part exhibits the thermochromic property) or locally (i.e., less than all of the part exhibits the thermochromic property).

[0017] By "gasochromic," it is meant that the 3D part will change color when in the presence of a designated gas (e.g., oxygen). In some examples, the 3D part may remain in the presence of the designated gas for a designated amount of time for the color change to be observed. It is to be understood that a gasochromic 3D part may exhibit its gasochromic property globally (i.e., the entire part exhibits the gasochromic property) or locally (i.e., less than all of the part exhibits the

gasochromic property).

[0018] By "pH sensitive," it is meant that the 3D part will change colors when a designated pH threshold is crossed. As previously mentioned, the change in color may be a change in the shade or a change in the hue. As an example, the color change of a pH sensitive part may go from blue to yellow and/or green. In some examples, the 3D part may cross the designated pH threshold and then may be maintained at a particular pH for a designated amount of time for the color change to be observed. It is to be understood that a pH sensitive 3D part may exhibit its pH sensitive property globally (i.e., the entire part exhibits the pH sensitive property) or locally (i.e., less than all of the part exhibits the pH sensitive property).

[0019] By "constant" or "consistent" color, it is meant the region of the 3D part will not change colors (e.g., remain white) regardless of the temperature(s), gas(es), and/or pH(s) to which the region is subjected.

[0020] As previously mentioned, by including the functional additive (i.e., the thermochromic agent, the gasochromic agent, the potential hydrogen (pH) sensitive agent, or combination(s) thereof) in the functional agent and/or the polymeric or polymeric composite build material used during 3D printing, at least a portion of the resulting 3D part exhibits the thermochromic property, the gasochromic property, the pH sensitive property, or the combination thereof.

[0021 ] In an example, the functional additive is selected from the group consisting of a triarylmethane dye, a leuco dye, an azo dye, an eurhodin dye, an indigo dye derivative, a thiazine dye, and a combination thereof. As examples of the combination, a leuco dye may also be a triarylmethane dye or a thiazine dye. Examples of triarylmethane dyes include acid blue 1 , acid blue 3, acid blue 5, acid blue 7, acid blue 9, methyl violet (crystal violet), brilliant green, malachite green, bromophenol blue, phenol red, bromocresol green, bromocresol purple,

bromothymol blue, phenolphthalein, thymolphthalein, thymol blue, cresol purple, xylenol blue, fuchsine, food green 3, acid green 15, etc. Examples of leuco dyes include acid blue 9, brilliant green, malachite green, phenolphthalein,

thymolphthalein, methylene blue, thionine, etc. Suitable leuco dyes may also be part of other dye classes, such as xanthene dyes and oxazine dyes. Examples of azo dyes include congo red, methyl orange, methyl yellow, methyl red, etc.

Examples of eurhodin dyes include neutral red, etc. An example of an indigo dye derivative is acid blue 74 (i.e., indigo carmine). Examples of thiazine dyes include thionine and methylene blue.

[0022] The functional additive that is included in the functional agent and/or the polymeric or polymeric composite build material may be selected based upon the thermochromic, gasochromic, and/or pH sensitive property to be integrated. For example, if the desired property is the thermochromic property, then a

thermochromic agent may be selected for the functional additive. Examples of the functional additive that act as thermochromic agents include acid blue 9, methyl violet (crystal violet), acid blue 74, thionine, brilliant green, malachite green, and methylene blue. As another example, if the desired property is the gasochromic property, then a gasochromic agent may be selected for the functional additive. Examples of the functional additive that act as gasochromic agents include acid blue 9, bromophenol blue, acid blue 1 , acid blue 3, acid blue 5, acid blue 7, food green 3, and acid green 15. As still another example, if the desired property is the pH sensitive property, then a pH sensitive agent may be selected for the functional additive. Examples of the functional additive that act as pH sensitive agents include acid blue 9, methyl violet (crystal violet), acid blue 74, malachite green, congo red, methyl orange, bromophenol blue, methyl yellow, methyl red, phenol red, neutral red, azolitmin, bromocresol green, bromocresol purple, bromothymol blue, phenolphthalein, thymolphthalein, thymol blue, cresol purple, xylenol blue, neutral red, fuchsine, etc.

[0023] As a specific example, if the desired thermochromic property is to cause a color of the 3D part to change at a temperature above 165°C; or if the desired gasochromic property is to cause a color of the 3D part to change in the presence of oxygen (or other oxidizing gases (e.g. , those that will not also oxidize the build material)); or if the desired pH sensitive property is to cause a color of the 3D part to change at a pH threshold of i) 0 (e.g. , in a strong acid with a pH below 0, the color is yellow, and in a strong acid with a pH from 0 to <4, the color is lime green), ii) 4 (e.g. , in a weaker acid, at which the color is green or aqua), or iii) 9 (at which the color is royal blue), acid blue 9 may be used as the functional additive. As another example, if the desired pH sensitive property is to cause a color of the 3D part to change at a pH threshold of 0.2 (below which the color is yellow), 1 .8 (above which the color is green), 1 1 .5 (below which the color is green), or 13.2 (above which the dye is colorless and, thus, the color of the 3D part is the color of the polymeric or polymeric composite build material, e.g., white), malachite green may be uses as the functional additive. As still another example, if the desired pH sensitive property is to cause a color of a blue 3D part to change to yellow at a pH ranging from 1 1 .5 to 14, acid blue 74 may be uses as the functional additive. As yet another example, if it desirable for the 3D part to go from green to colorless at about 27°C, brilliant green may be used as the functional additive.

[0024] In some examples of the system and method disclosed herein, the functional additive is included in the functional agent, which is selectively applied on at least a portion of the polymeric or polymeric composite build material. The functional additive may be included in the functional agent regardless of the ability of the functional additive to absorb electromagnetic radiation and/or regardless of whether all or less than all of the fused layer is to exhibit the thermochromic, gasochromic, and/or pH sensitive property.

[0025] In an example, the functional additive in the functional agent may absorb some of the electromagnetic radiation used during the 3D printing process, however, the amount absorbed is generally not enough to cause fusing, unless a fusing agent is applied along with the functional agent, or unless selective laser sintering/melting is utilized at areas where the functional agent is applied.

[0026] When all or less than all of the fused layer is to exhibit the thermochromic property, the gasochromic property, and/or the pH sensitive property, it may be desirable to include the functional additive in the functional agent. This is because the functional agent may be selectively applied wherever it is desirable to impart the thermochromic property, the gasochromic property, and/or the pH sensitive property.

[0027] When the functional additive is included in the functional agent, the thermochromic, gasochromic, and/or pH sensitive property may be exhibited globally or locally in the 3D part that is formed. For example, if the functional agent is applied on all of the polymeric or polymeric composite build material that is to become the fused layer in each layer of the 3D part, then the thermochromic, gasochromic, and/or pH sensitive property may be exhibited globally. The thermochromic, gasochromic, and/or pH sensitive property may also be exhibited globally across the external portions of the 3D part, if the functional agent is applied on the polymeric or polymeric composite build material that is to become the outer most edges (i.e., shell) of the 3D part. In this example, the functional agent may not be applied on the interior portions of the polymeric or polymeric composite build material. As another example, the thermochromic, gasochromic, and/or pH sensitive property may be exhibited locally when the functional agent is applied on some portions of the polymeric or polymeric composite build material and is not applied on other portions of the polymeric or polymeric composite build material that is to become the 3D part.

[0028] In other examples of the system and method disclosed herein, the functional additive is included in the build material.

[0029] In an example, the functional additive in the build material may absorb some of the electromagnetic radiation used during the 3D printing process, however, the amount absorbed is generally not enough to cause fusing, unless a fusing agent is selectively applied on the build material where it is desirable for fusing to take place, or unless selective laser sintering/melting is utilized. If the functional additive absorbs some of the electromagnetic radiation and is included in the polymeric or polymeric composite build material, then portions of the polymeric or polymeric composite build material that are not patterned with the fusing agent may be capable of at least partially fusing due to the at least partial absorption of the functional additive. In these instances, a detailing agent may be used to prevent fusing in the non-patterned area(s). A fusing agent and detailing agent are not utilized with selective laser sintering, because those areas of the build material exposed to the laser alone will sinter and exhibit the desired property of the functional additive in the fused build material.

[0030] When all of the fused layer is to exhibit the thermochromic property, the gasochromic property, and/or the pH sensitive property, it may be desirable to include the functional additive in the polymeric or polymeric composite build material. This is because the functional additive will be present wherever the build material is fused, and thus the desired property will be imparted wherever the build material is fused. When the functional additive is included in the polymeric or polymeric composite build material, the thermochromic, gasochromic, and/or pH sensitive property may be exhibited globally in the 3D part that is formed, depending upon where the fusing agent is applied or where selectively laser sintering/melting is applied.

[0031 ] When the functional agent is used, the functional agent may include the functional additive in a liquid vehicle. The amount of the functional additive present in the functional agent may depend, at least in part, on the functional additive used, the thermochromic, gasochromic, and/or pH sensitive property that is to be imparted, and/or the polymeric or polymeric composite build material used. In an example, the functional additive may be present in the functional agent in an amount ranging from about 0.1 wt% to about 8 wt% based on a total wt% of the functional agent. It is believed that these functional additive loadings provide a balance between the functional agent having jetting reliability and effectively imparting the thermochromic, gasochromic, and/or pH sensitive property.

[0032] As used herein, "functional agent vehicle" may refer to the liquid fluid in which the functional additive is placed to form the functional agent. A wide variety of functional agent vehicles, including aqueous and non-aqueous vehicles, may be used in the functional agent. In some instances, the functional agent vehicle may include water alone or a non-aqueous solvent alone. In other instances, the functional agent vehicle may further include co-solvent(s), surfactant(s),

antimicrobial agent(s), and/or anti-kogation agent(s).

[0033] When the functional agent vehicle is water-based, the aqueous nature of the functional agent enables the functional agent to penetrate, at least partially, into the layer of the polymeric or polymeric composite build material. The polymeric or polymeric composite build material may be hydrophobic, and the presence of a co- solvent and/or a surfactant in the functional agent may assist in obtaining a particular wetting behavior. Co-solvent(s) and surfactant(s) may also be used in the non-aqueous vehicle. [0034] Examples of suitable co-solvents include 2-pyrrolidinone, N- methylpyrrolidone, 1 -(2-hydroxyethyl)-2-pyrrolidinone, 1 ,6-hexanediol or other diols (e.g. , 1 ,5-pentanediol, 2-methyl-1 ,3-propanediol, etc.), triethylene glycol, tetraethylene glycol, tripropylene glycol methyl ether, or the like, or combinations thereof.

[0035] Whether used alone or in combination, the total amount of the co- solvents) ranges from about 1 wt% to about 80 wt% of the total wt% of the functional agent.

[0036] Examples of suitable surfactants include a self-emulsifiable, nonionic wetting agent based on acetylenic diol chemistry (e.g., SURFYNOL® SEF from Air Products and Chemicals, Inc.), a nonionic fluorosurfactant (e.g., CAPSTONE® fluorosurfactants from DuPont, previously known as ZONYL FSO), and

combinations thereof. In other examples, the surfactant is an ethoxylated low-foam wetting agent (e.g., SURFYNOL® 440 or SURFYNOL® CT-1 1 1 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, nonionic surfactants (e.g., TERGITOL™ TMN-6 from The Dow Chemical Company). In some examples, it may be desirable to utilize a surfactant having a hydrophilic- lipophilic balance (HLB) less than 10.

[0037] Whether a single surfactant is used or a combination of surfactants is used, the total amount of surfactant(s) in the functional agent may range from about 0.1 wt% to about 4 wt% based on the total wt% of the functional agent.

[0038] The functional agent 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 PROXEL® (Arch Chemicals) series,

ACTICIDE® M20 (Thor), and combinations thereof.

[0039] In an example, the functional agent may include a total amount of antimicrobial agents that ranges from about 0.1 wt% to about 1 wt%. In an example, the antimicrobial agent is a biocide and is present in the functional agent in an amount of about 0.27 wt% (based on the total wt% of the functional agent).

[0040] An anti-kogation agent may be included in the functional agent (e.g., when it is to be used with thermal inkjet printing). Kogation refers to the deposit of dried ink (e.g., functional 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™ 03A 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).

[0041 ] 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 functional agent may range from about 0.1 wt% to about 5 wt% based on the total wt% of the functional agent.

[0042] In some examples, the functional agent excludes a binder. By excluding a binder, the functional agent may be easily jetted from an inkjet applicator.

[0043] The balance of the functional agent is water or the non-aqueous solvent. As an example, deionized water may be used. As another example, dimethyl sulfoxide (DMSO), acetone, acetates, alcohols (e.g., ethanol), or the like may be used as the non-aqueous solvent. The balance of the functional agent may depend, in part upon the jetting technology that is to be used to dispense the functional agent. For example, if thermal inkjet is to be used, the balance may be water and/or ethanol. For another example, if piezoelectric inkjet is to be used, the balance may be a variety of solvents, such as methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, other ketones, acetates (e.g., methyl acetate), ethylene glycol ethers, propylene glycol ethers, diols (e.g., 1 ,3-propanediol), polyols (e.g., glycerol), etc.

[0044] When the functional agent is applied on at least some of the polymeric or polymeric composite build material, the functional agent may be dispensed at a contone level ranging from about 10 contone to about 255 contone (which refers to the number of drops, which is divided by 256, that will be placed on average onto each pixel). The functional agent may be applied at this contone level so that the desired property is imparted to the 3D part, and so that the color change that is a result of the thermochromic, gasochromic, and/or pH sensitive property may be easily observed. The contone level at which the functional agent may be dispensed may depend, at least in part, on the concentration of the functional additive in the functional agent. In an example, when acid blue 9 is included in the functional agent in a weight percentage at the higher end of the given range, the contone level may range from about 10 contone to about 32 contone. However, when acid blue 9 is included in the functional agent in a weight percentage at the lower end of the given range, the contone level may be higher than 32 contone. In another example, the amount of functional agent applied may be measured in terms of ng per pixel of build material. For example, when the functional additive is included in the functional agent in an amount ranging from about 0.1 wt% to about 8 wt% (based on the total wt% of the functional agent), the functional agent may be applied in an amount ranging from about 1 .4 ng to about 36 ng per pixel (i.e., 1 /600 inch by 1/600 inch).

[0045] In some examples, the system and method disclosed herein may include another or second functional agent. The other or second functional agent includes at least another or second functional additive, which may cause the 3D part to exhibit another or second thermochromic, gasochromic, and/or pH sensitive property. The other or second functional additive included in the other or second functional agent may be different than the functional additive included in the first functional agent. Utilizing different functional additives may allow one to 3D parts with different thermochromic, gasochromic, and/or pH sensitive properties in the same or different regions (e.g., in the x-y plane) or in different layers (e.g., in the z- direction).

[0046] The other or second functional agent may also include a liquid vehicle. The liquid vehicle used in the other or second functional agent may be any of the liquid vehicles described in reference to the first functional agent. In some examples, the other or second functional agent excludes a binder. By excluding a binder, the other or second functional agent may be easily jetted from an inkjet applicator. While one additional functional agent has been described, it is to be understood that examples of the system and method disclosed herein may include and/or utilize any desirable number of different functional agents.

[0047] In some examples of the system and method disclosed herein, the functional additive is included in the polymeric or polymeric composite build material. When the functional additive is included in the polymeric or polymeric composite build material and the 3D printing process involves a fusing agent, it may be desirable to select a functional additive that does not cause a temperature rise of more than 5°C at the area that is not to be fused (i.e., when exposed to the blanket electromagnetic radiation). Examples of these functional additives may include acid blue 9, bromothymol blue, methyl yellow, malachite green, etc. Because the fusing agent effectively absorbs the radiation to fuse the build material, significant temperature changes resulting from the functional additive may be undesirable so that the powder not exposed to the fusing agent does not fuse/sinter. When the functional additive is included in the polymeric or polymeric composite build material and the 3D printing process involves selective laser sintering/melting, any desirable functional additive may be used. The peak laser power may be adjusted so that any temperature changes resulting from the functional additive may be accommodated for.

[0048] When the functional additive is included in the polymeric or polymeric composite build material, the amount of the functional additive present in the polymeric or polymeric composite build material may depend, at least in part, on the functional additive used, the thermochromic, gasochromic, and/or pH sensitive property desired, and/or the polymeric or polymeric composite build material used. In an example, the functional additive may be present in the polymeric or polymeric composite build material in an amount ranging from about 0.05 wt% to about 4 wt% based on a total wt% of the polymeric or polymeric composite build material.

[0049] It is to be understood that the functional additive may be included in the functional agent(s) alone (i.e., without including the functional additive in the polymeric or polymeric composite build material), or in the polymeric or polymeric composite build material alone (i.e., without using a functional agent which includes the functional additive), or in both the functional agent(s) and the polymeric or polymeric composite build material. When the functional additive is included in both the functional agent(s) and the polymeric or polymeric composite build material, the functional additive(s) included in the functional agent(s) may be the same or different than the functional additive(s) included in the polymeric or polymeric composite build material. In an example, the functional agent and the polymeric or polymeric composite build material may include different functional additives that exhibit the respective thermochromic, gasochromic, or pH sensitive property at different ranges.

[0050] Moreover, the functional additive may be included in the functional agent or the build material with another non-functional dye. This additional dye may not exhibit a thermochromic, gasochromic, and/or pH sensitive property, but will also not deleteriously affect the thermochromic, gasochromic, and/or pH sensitive property of the functional additive. This additional non-functional dye may be used with the functional dye to provide a secondary color. The non-functional dye may be used in similar amounts as the functional additive.

[0051 ] 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.

[0052] In an example, the three-dimensional (3D) printing system 10 generally includes a supply 14 of polymeric or polymeric composite build material 16; a build material distributor 18; a supply of a fusing agent 26; a first applicator 24A for selectively dispensing the fusing agent 26; a supply of a functional agent 28 including a functional additive selected from the group consisting of a

thermochromic agent, a gasochromic agent, a potential hydrogen (pH) sensitive agent, and a combination thereof; a second applicator 24B for selectively dispensing the functional agent 28; a source 36, 36' of electromagnetic radiation 46 (see, e.g., Figs. 2C and 2D); a controller 32; and a non-transitory computer readable medium having stored thereon computer executable instructions to cause the controller 32 to utilize the build material distributor 18 to dispense the polymeric or polymeric composite build material 16; utilize the first applicator 24A and the second applicator 24B to respectively and selectively dispense the fusing agent 26 and the functional agent 28 on at least a portion 42 (see, e.g., Fig. 2C) of the polymeric or polymeric composite build material 16; and utilize the source 36, 36' of

electromagnetic radiation 46 to expose the polymeric or polymeric composite build material 16 to electromagnetic radiation 46 to fuse the portion 42 of the polymeric or polymeric composite build material 16 to form a region of a layer 48 (see, e.g. , Fig. 2D), wherein the region of the layer 48 exhibits a thermochromic property, a gasochromic property, potential hydrogen (pH) sensitive property, or a combination thereof.

[0053] When selective laser sintering/melting is utilized, the system 10 shown in Fig. 1 may not include the supply of a fusing agent 26 or the first applicator 24A for selectively dispensing the fusing agent 26, and the source 36, 36' of

electromagnetic radiation 46 will be a laser. In the SLS/SLM system, the computer executable instructions may cause the controller 32 to utilize the build material distributor 18 to dispense the polymeric or polymeric composite build material 16; utilize the first applicator 24A to selectively dispense the functional agent 28 on at least a portion 42 of the polymeric or polymeric composite build material 16; and utilize the source 36, 36' of electromagnetic radiation 46 to expose the portion(s) 42 to light in order to fuse the portion(s) 42 of the polymeric or polymeric composite build material 16 to form a region of a layer 48.

[0054] It is to be understood that in another example, the three-dimensional (3D) printing system 10 includes the supply 14 of polymeric or polymeric composite build material 16, where the build material 16 includes the functional additive selected from the group consisting of a thermochromic agent, a gasochromic agent, a potential hydrogen (pH) sensitive agent, and a combination thereof. In this example, the system 10 may further include the build material distributor 18; the supply of the fusing agent 26; the first applicator 24A for selectively dispensing the fusing agent 26; in some instances, a supply of a detailing agent (not shown), an applicator (such as second applicator 24B) for selectively dispensing the detailing agent; the source 36, 36' of electromagnetic radiation 46; the controller 32; and the non-transitory computer readable medium having stored thereon computer executable instructions to cause the controller 32 to utilize the build material distributor 18 to dispense the polymeric or polymeric composite build material 16; utilize the first applicator 24A and the second applicator 24B to respectively and selectively dispense the fusing agent 26 and the detailing agent on different portions of the polymeric or polymeric composite build material 16; and utilize the source 36, 36' of electromagnetic radiation 46 to expose the polymeric or polymeric composite build material 16 to electromagnetic radiation 46 to fuse the portion of the polymeric or polymeric composite build material 16 having the fusing agent 26 in contact therewith. In this example, the fused region/layer exhibits the thermochromic property, the gasochromic property, the potential hydrogen (pH) sensitive property, or the combination thereof. Also in this example, the polymeric or polymeric composite build material 16 having the detailing agent in contact therewith does not fuse.

[0055] When selective laser sintering/melting is utilized in this other example, the system 10 shown in Fig. 1 may not include the supply of a fusing agent 26 or the first applicator 24A for selectively dispensing the fusing agent 26, and the source 36, 36' of electromagnetic radiation 46 will be a laser. In this example of the SLS/SLM system, the computer executable instructions may cause the controller 32 to utilize the build material distributor 18 to dispense the polymeric or polymeric composite build material 16; and utilize the source 36, 36' of electromagnetic radiation 46 to expose the desired portion(s) of the polymeric composite build material 16 having the functional additive therein, to form a region of a layer 48.

[0056] As shown in Fig. 1 , the printing system 10 includes the build area platform 12, the build material supply 14 containing polymeric or polymeric composite build material particles 16, and the build material distributor 18.

[0057] 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.

[0058] 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 48 (see, e.g., Fig. 2E) 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 40 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.

[0059] 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 48 of the 3D part.

[0060] 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.

[0061 ] 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 to composite particles made up of polymer and ceramic. Any of the polymeric or polymeric composite build material particles 16 may be in powder form.

[0062] 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 1 1 / nylon 1 1 , 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.

[0063] 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 0 3 ), glass, silicon mononitride (SiN), silicon dioxide (Si0 2 ), zirconia (Zr0 2 ), titanium dioxide (ΤΊΟ2), 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 58 (see, e.g., Fig. 2F) 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. [0064] 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. Depending upon the composition of the composite, the melting or softening point may be higher or lower. As an example, the build material particles 16 may be a polyamide having a melting point of 180°C.

[0065] 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 2 μιη to about 200 μιη. In another example, the average size of the polymeric or polymeric composite build material particles 16 ranges from 20 μιη to about 90 μιη. In still another example, the average size of the polymeric or polymeric composite build material particles 16 is about 60 μιη.

[0066] It is to be understood that the polymeric or polymeric composite build material 16 may include, in addition to polymeric or polymeric composite particles, the functional additive, an antioxidant, a brightener, a charging agent, a flow aid, or combinations thereof. In an example, the polymeric or polymeric composite build material 16 includes the antioxidant and the brightener.

[0067] As mentioned above, the functional additive may be included in the polymeric or polymeric composite build material 16 when the entire fused layer 48 is to exhibit the thermochromic property, the gasochromic property, the pH sensitive property, or a combination thereof. As also mentioned above, the functional additive may be selected from the group consisting of a triarylmethane dye, a leuco dye, an azo dye, an eurhodin dye, an indigo dye derivative, a thiazine dye, and a combination thereof and/or may be present in the polymeric or polymeric composite build material 16 in an amount ranging from about 0.05 wt% to about 4 wt% based on a total wt% of the polymeric or polymeric composite build material 16.

[0068] Antioxidant(s) may be added to the polymeric or polymeric composite build material 16 to prevent the yellowing of the polymeric or polymeric composite build material 16 or to slow the yellowing of the polymeric or polymeric composite build material 16 by slowing the oxidation of the polymeric or polymeric composite build material 16. It may be desirable to at least reduce the yellowing of the polymeric or polymeric composite build material 16 to improve visibility so that other desired color changes may be more easily observed. Additionally, in some instances, antioxidant(s) may be added to the polymeric or polymeric composite build material 16 to facilitate or aid in the reduction of some of the functional additives (e.g., acid blue 9, malachite green, etc.) to their colorless form. The presence of the antioxidant may increase the rate of the color change that occurs at a temperature threshold, in the presence of a gas, and/or at a pH threshold (e.g., when the color change occurs as a result of a reduction reaction).

[0069] Examples of suitable antioxidants include a radical scavenger. In these examples, the antioxidant may include I RGANOX® 1098 (benzenepropanamide, N,N'-1 ,6-hexanediylbis[3,5-bis(1 ,1 -dimethylethyl)-4-hydroxy), IRGANOX® 254 (a mixture of 40% triethylene glycol bis(3-tert-butyl-4-hydroxy-5-methylphenyl), polyvinyl alcohol and deionized water), and/or other sterically hindered phenols. In other examples, the antioxidant may include a phosphite and/or an organic sulfide (e.g., a thioester).

[0070] In an example, the antioxidant may be included in the polymeric or polymeric composite build material 16 in an amount sufficient to prevent or slow the yellowing of the polymeric or polymeric composite build material 16. In another example, the amount of the antioxidant included in the polymeric or polymeric composite build material 16 may be less than the amount that would give the build material 16 a yellow color through the yellowing of the antioxidant. In still another example, the antioxidant may be included in the polymeric or polymeric composite build material 16 in an amount ranging from about 0.01 wt% to about 5 wt% based on the total wt% of the polymeric or polymeric composite build material 16.

[0071 ] Brighteners may be added to the polymeric or polymeric composite build material 16 to improve visibility so that color changes may be more easily observed. Examples of suitable brighteners include titanium dioxide (Ti0 2 ), zinc oxide (ZnO), calcium carbonate (CaC0 3 ), zirconium dioxide (Zr0 2 ), aluminum oxide (Al 2 0 3 ), silicon dioxide (Si0 2 ), and combinations thereof. In some examples, a stilbene derivative may be used as the brightener. In these examples, the temperature(s) of the 3D printing process may be below a threshold temperature above which the stilbene derivative may become unstable.

[0072] In an example, the brightener may be included in the polymeric or polymeric composite build material 16 in an amount ranging from about 0.01 wt% to about 10 wt% based on the total wt% of the polymeric or polymeric composite build material 16.

[0073] Charging agent(s) may be added to the polymeric or polymeric composite build material 16 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.

[0074] 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 μιη 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.

[0075] It is to be understood that the antioxidant, the brightener, the charging agent, and/or the flow aid may be added to the polymeric or polymeric composite build material 16 whether or not the functional additive is added to the polymeric or polymeric composite build material 16. The antioxidant, the brightener, the charging agent, and/or the flow aid may interact with the polymeric or polymeric composite build material 16 similarly, whether or not the functional additive is included in the polymeric or polymeric composite build material 16. Further, the antioxidant may interact with the functional additive similarly, whether the functional additive is included in the polymeric or polymeric composite build material 16 alone, the functional agent 28 alone, or both the polymeric or polymeric composite build material 16 and the functional agent 28.

[0076] As shown in Fig. 1 , the printing system 10 also includes the first applicator 24A, which may contain the fusing agent 26.

[0077] In one example, the fusing agent 26 is a low tint fusing agent, in that it imparts little or no color to the 3D part. This example fusing agent generally includes an aqueous or non-aqueous vehicle and a plasmonic resonance absorber dispersed therein. The plasmonic resonance absorber allows the fusing agent 26 to absorb radiation at wavelengths ranging from 800 nm to 4000 nm (e.g., at least 80% of radiation having wavelengths ranging from 800 nm to 4000 nm is absorbed), which enables the fusing agent 26 to convert enough radiation to thermal energy so that the polymeric or polymeric composite build material particles 16 fuse. The plasmonic resonance absorber also allows the fusing agent 26 to have transparency at wavelengths ranging from 400 nm to 780 nm (e.g., 20% or less of radiation having wavelengths ranging from 400 nm to 780 nm is absorbed), which enables the 3D part to be white or slightly colored, and also enables the desirable color change to be observed.

[0078] The absorption of the plasmonic resonance absorber is the result of the plasmonic resonance effects. Electrons associated with the atoms of the plasmonic resonance absorber may be collectively excited by electromagnetic radiation, which results in collective oscillation of the electrons. The wavelengths required to excite and oscillate these electrons collectively are dependent on the number of electrons present in the plasmonic resonance absorber particles, which in turn is dependent on the size of the plasmonic resonance absorber particles. The amount of energy required to collectively oscillate the particle's electrons is low enough that very small particles (e.g., 1 -100 nm) may absorb electromagnetic radiation with wavelengths several times (e.g., from 8 to 800 or more times) the size of the particles. The use of these particles allows the fusing agent 26 to be inkjet jettable as well as electromagnetically selective (e.g., having absorption at wavelengths ranging from 800 nm to 4000 nm and transparency at wavelengths ranging from 400 nm to 780 nm).

[0079] In an example, the plasmonic resonance absorber has an average particle diameter (e.g., volume-weighted mean diameter) ranging from greater than 0 nm to less than 220 nm. In another example the plasmonic resonance absorber has an average particle diameter ranging from greater than 0 nm to 120 nm. In a still another example, the plasmonic resonance absorber has an average particle diameter ranging from about 10 nm to about 200 nm.

[0080] In an example, the plasmonic resonance absorber is an inorganic pigment. Examples of suitable inorganic pigments include lanthanum hexaboride (LaB 6 ), tungsten bronzes (A x W0 3 ), indium tin oxide (ln 2 0 3 :Sn0 2 , ITO), aluminum zinc oxide (AZO), ruthenium oxide (Ru0 2 ), silver (Ag), gold (Au), platinum (Pt), iron pyroxenes (A x Fe y Si 2 0 6 wherein A is Ca or Mg, x = 1 .5-1 .9, and y = 0.1 -0.5), modified iron phosphates (A x Fe y P0 4 ), and modified copper pyrophosphates

(A x Cu y P 2 0 7 ). Tungsten bronzes may be alkali doped tungsten oxides. Examples of suitable alkali dopants (i.e., A in A x W0 3 ) may be cesium, sodium, potassium, or rubidium. In an example, the alkali doped tungsten oxide may be doped in an amount ranging from greater than 0 mol% to about 0.33 mol% based on the total mol% of the alkali doped tungsten oxide. Suitable modified iron phosphates

(A x Fe y P0 4 ) may include copper iron phosphate (A = Cu, x = 0.1 -0.5, and y = 0.5- 0.9), magnesium iron phosphate (A = Mg, x = 0.1 -0.5, and y = 0.5-0.9), and zinc iron phosphate (A = Zn, x = 0.1 -0.5, and y = 0.5-0.9). For the modified iron phosphates, it is to be understood that the number of phosphates may change based on the charge balance with the cations. Suitable modified copper pyrophosphates (A x Cu y P 2 0 7 ) include iron copper pyrophosphate (A = Fe, x = 0-2, and y = 0-2), magnesium copper pyrophosphate (A = Mg, x = 0-2, and y = 0-2), and zinc copper pyrophosphate (A = Zn, x = 0-2, and y = 0-2). Combinations of the inorganic pigments may also be used.

[0081 ] The amount of the plasmonic resonance absorber that is present in the fusing agent 26 ranges from about 1 wt% to about 20 wt% based on the total wt% of the fusing agent 26. In some examples, the amount of the plasmonic resonance absorber present in the fusing agent 26 ranges from about 1 wt% up to about 10 wt%. In other examples, the amount of the plasmonic resonance absorber present in the fusing agent 26 ranges from greater than 4 wt% up to about 15 wt%. It is believed that these plasmonic resonance absorber loadings provide a balance between the fusing agent 26 having jetting reliability and electromagnetic radiation 46 absorbance efficiency.

[0082] As used herein, "fusing agent vehicle" may refer to the liquid fluid in which the plasmonic resonance absorber is placed to form the fusing agent 26. A wide variety of fusing agent vehicles, including aqueous and non-aqueous vehicles, may be used with the plasmonic resonance absorber. In some instances, the fusing agent vehicle includes water alone or a non-aqueous solvent (e.g. dimethyl sulfoxide (DMSO), ethanol, etc.) alone. In other instances, the fusing agent vehicle may further include a dispersing additive, a surfactant, a co-solvent, an antimicrobial agent, an anti-kogation agent, a silane coupling agent, a chelating agent, or combinations thereof.

[0083] Similar to the functional agent vehicle, when the fusing agent vehicle is water-based, the aqueous nature of the fusing agent 26 enables the fusing agent 26 to penetrate, at least partially, into the layer 40 of the polymeric or polymeric composite build material particles 16. As mentioned above, the polymeric or polymeric composite build material particles 16 may be hydrophobic, and the presence of the co-solvent, the surfactant, and/or the dispersing additive in the fusing agent 26 when the fusing agent 26 is water-based or non-aqueous based may assist in obtaining a particular wetting behavior.

[0084] The fusing agent 26 may include any of the previously listed co- solvents), surfactant(s), antimicrobial agent(s), and/or anti-kogation agent(s) in the previously described amounts (except that the wt% is based on the total wt% of the fusing agent 26).

[0085] The plasmonic resonance absorber in the fusing agent 26 may, in some instances, be dispersed with a dispersing additive. As such, the dispersing additive helps to uniformly distribute the plasmonic resonance absorber throughout the fusing agent 26. As mentioned above, the dispersing additive may also aid in the wetting of the fusing agent 26 onto the polymeric or polymeric composite build material particles 16. Some examples of the dispersing additive include a water soluble acrylic acid polymer (e.g., CARBOSPERSE® K7028 available from

Lubrizol), a styrene-acrylic pigment dispersion resin (e.g., JONCRYL® 671 available from BASF Corp.), a high molecular weight block copolymer with pigment affinic groups (e.g., DISPERBYK®-190 available BYK Additives and Instruments), and combinations thereof.

[0086] Whether a single dispersing additive is used or a combination of dispersing additives is used, the total amount of dispersing additive(s) in the fusing agent 26 may range from about 10 wt% to about 200 wt% based on the wt% of the plasmonic resonance absorber in the fusing agent 26.

[0087] A silane coupling agent may be added to the fusing agent 26 to help bond the organic and inorganic materials. Examples of suitable silane coupling agents include the SILQUEST® A series manufactured by Momentive.

[0088] Whether a single silane coupling agent is used or a combination of silane coupling agents is used, the total amount of silane coupling agent(s) in the fusing agent 26 may range from about 0.1 wt% to about 50 wt% based on the wt% of the plasmonic resonance absorber in the fusing agent 26. In an example, the total amount of silane coupling agent(s) in the fusing agent 26 ranges from about 1 wt% to about 30 wt% based on the wt% of the plasmonic resonance absorber. In another example, the total amount of silane coupling agent(s) in the fusing agent 26 ranges from about 2.5 wt% to about 25 wt% based on the wt% of the plasmonic resonance absorber.

[0089] The fusing agent 26 may also include other additives, such as a chelating agent. 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.).

[0090] 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 26 may range from 0 wt% to about 2 wt% based on the total wt% of the fusing agent 26.

[0091 ] Still another suitable additive for the fusing agent 26 is a humectant and lubricant (e.g., LIPONIC® EG-1 (LEG-1 ) from Lipo Chemicals).

[0092] The balance of the fusing agent 26 is water or the non-aqueous solvent. As an example, deionized water may be used.

[0093] In an example, the fusing agent 26 may include CTO nanoparticles as the plasmonic resonance absorber, a zwitterionic stabilizer, and an aqueous vehicle. In one version of this example, the aqueous vehicle may include a surfactant and a balance of water. In another version of this example, the aqueous vehicle of the fusing agent 26 may include a co-solvent, a surfactant, and a balance of water. Any of the co-solvents and/or surfactants previously described may be used in this example of the fusing agent 26 in the respective amounts previously described. This example of the fusing agent 26 may also include a humectant and lubricant.

[0094] In this example, the CTO nanoparticles in the fusing agent 26 have a general formula of Cs x W0 3 , where 0<x<1 . The cesium tungsten oxide

nanoparticles may give the fusing agent 26 a light blue color. The strength of the color may depend, at least in part, on the amount of the CTO nanoparticles in the fusing agent 26. When it is desirable for the 3D part to be white, less of the CTO nanoparticles may be used in the fusing agent 26 in order to achieve the white color. In an example, the CTO nanoparticles may be present in the fusing agent 26 in an amount ranging from about 1 wt% to about 20 wt% (based on the total wt% of the fusing agent 26).

[0095] The average particle size (e.g., volume-weighted mean diameter) of the CTO nanoparticles may range from about 1 nm to about 40 nm. In some examples, the average particle size of the CTO nanoparticles may range from about 1 nm to about 15 nm or from about 1 nm to about 10 nm. The upper end of the particle size range (e.g., from about 30 nm to about 40 nm) may be less desirable, as these particles may be more difficult to stabilize. [0096] This example of the fusing agent 26 may also include the zwitterionic stabilizer. The zwitterionic stabilizer may improve the stabilization of the fusing agent 26. While the zwitterionic stabilizer has an overall neutral charge, at least one area of the molecule has a positive charge (e.g., amino groups) and at least one other area of the molecule has a negative charge. The CTO nanoparticles may have a slight negative charge. The zwitterionic stabilizer molecules may orient around the slightly negative CTO nanoparticles with the positive area of the zwitterionic stabilizer molecules closest to the CTO nanoparticles and the negative area of the zwitterionic stabilizer molecules furthest away from the CTO

nanoparticles. Then, the negative charge of the negative area of the zwitterionic stabilizer molecules may repel CTO nanoparticles from each other. The zwitterionic stabilizer molecules may form a protective layer around the CTO nanoparticles, and prevent them from coming into direct contact with each other and/or increase the distance between the particle surfaces (e.g., by a distance ranging from about 1 nm to about 2 nm). Thus, the zwitterionic stabilizer may prevent the CTO nanoparticles from agglomerating and/or settling in the fusing agent 26.

[0097] Examples of suitable zwitterionic stabilizers include C2 to C8 betaines, C2 to C8 aminocarboxylic acids having a solubility of at least 10 g in 100 g of water, taurine, and combinations thereof. Examples of the C2 to C8 aminocarboxylic acids include beta-alanine, gamma-aminobutyric acid, glycine, and combinations thereof.

[0098] The zwitterionic stabilizer may be present in the fusing agent 26 in an amount ranging from about 2 wt% to about 35 wt% (based on the total wt% of the fusing agent 26). When the zwitterionic stabilizer is the C2 to C8 betaine, the C2 to C8 betaine may be present in an amount ranging from about 8 wt% to about 35 wt% of a total wt% of fusing agent 26. When the zwitterionic stabilizer is the C2 to C8 aminocarboxylic acid, the C2 to C8 aminocarboxylic acid may be present in an amount ranging from about 2 wt% to about 20 wt% of a total wt% of fusing agent 26. When the zwitterionic stabilizer is taurine, taurine may be present in an amount ranging from about 2 wt% to about 35 wt% of a total wt% of fusing agent 26.

[0099] In this an example of the fusing agent 26, the weight ratio of the CTO nanoparticles to the zwitterionic stabilizer may range from 1 : 10 to 10: 1 . In another version of this example of the fusing agent 26, the weight ratio of the CTO nanoparticles to the zwitterionic stabilizer is 1 : 1 .

[0100] In another example, the fusing agent 26 is a darker fusing agent, in that it imparts grey or black to the 3D part. This example fusing agent generally includes the previously described aqueous or non-aqueous vehicle and a near infrared absorber. Any near-infrared colorants, e.g., those produced by Fabricolor, Eastman Kodak, or Yamamoto, may be used in the fusing agent 26. As one example, the fusing agent 26 may be a printing liquid formulation including carbon black as the active material. Examples of this printing liquid formulation are commercially known as CM997A, 516458, C18928, C93848, C93808, or the like, all of which are available from HP Inc. This darker fusing agent may be desirable for forming an interior and/or bottom of a 3D part, while the previously described low tint fusing agent may be desirable for forming the exterior or outermost layer(s) of a 3D part.

[0101 ] As depicted in Fig. 1 , some examples of the printing system 10 may include a second applicator 24B. In one example, the printing system 10 includes the second applicator 24B, which may contain the functional agent 28, in addition to the first applicator 24A.

[0102] As mentioned above, the functional agent 28 may include the functional additive and a liquid vehicle. As also mentioned above, the functional additive is selected from the group consisting of a triarylmethane dye, a leuco dye, an azo dye, an eurhodin dye, an indigo dye derivative, a thiazine dye, and a combination thereof, and may be present in the functional agent 28 in an amount ranging from about 0.1 wt% to about 8 wt% based on a total wt% of the functional agent 28.

[0103] In some examples, each of the fusing agent 26 and the functional agent 28 excludes a binder. By excluding a binder, the fusing agent 26 may be easily jetted from the first applicator 24A and the functional agent 28 may be easily jetted from the second applicator 24B.

[0104] As mentioned above, some examples of the system 10 and method 100 (see, e.g., Figs. 2A through 2E) disclosed herein may include another or second functional agent. In the examples in which the system 10 and method 100 include the other or second functional agent, another (e.g., third) applicator (not shown) may apply the other or second functional agent. The third applicator may be a separate cartridge (for dispensing the other or second functional agent) within the first applicator 24A or the second applicator 24B, or it may be a separate applicator.

[0105] In some examples, the plasmonic resonance absorber of the fusing agent 26 and the functional additive of the functional agent 28 may be compatible with the same liquid vehicle (i.e., able to be incorporated into the same vehicle and then successfully dispensed from the applicator 24A, 24B). When the plasmonic resonance absorber and the functional additive are compatible with the same liquid vehicle, the fusing agent 26 and the functional agent 28 may be combined into a single fusing/functional agent, in which both the plasmonic resonance absorber and the functional additive are dissolved or dispersed in the same liquid vehicle. In these examples, the amount of the single fusing/functional agent that is dispensed will control the absorption of the electromagnetic radiation 46 as well as the thermochromic, gasochromic, and/or pH sensitive property that is exhibited.

[0106] In the examples in which the fusing agent 26 and the functional agent 28 are a single agent, one applicator 24A or 24B may be used.

[0107] If it is desirable to decouple the electromagnetic radiation 46 absorption from the exhibition of the thermochromic, gasochromic, and/or pH sensitive property, a different fusing agent 26 and functional agent 28 may be used (even if the liquid vehicle in the two agents 26, 28 are the same). Additionally, it may be desirable for the fusing agent 26 to be separate and distinct from the functional agent 28 when less than all of the fused layer 48 is to exhibit the thermochromic, gasochromic, and/or pH sensitive property. When the fusing agent 26 is a separate and distinct agent from the functional agent 28, the liquid vehicle in the respective agents may be the same or different. As an example, the fusing agent 26 may be separate and distinct from the functional agent 28 when the plasmonic resonance absorber is not compatible in the liquid vehicle(s) in which the functional additive is compatible.

[0108] The applicator(s) 24A, 24B may be scanned across the build area platform 12 in the direction indicated by the arrow 30, e.g., along the y-axis. The applicator(s) 24A, 24B 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 each of the applicator(s) 24A, 24B is shown in Fig. 1 as a single applicator, it is to be understood that each of the applicator(s) 24A, 24B may include multiple inkjet applicators that span the width of the build area platform 12. Additionally, the applicator(s) 24A, 24B may be positioned in multiple printbars. The applicator(s) 24A, 24B may also be scanned along the x- axis, for instance, in configurations in which the applicator(s) 24A, 24B does/do not span the width of the build area platform 12 to enable the applicator(s) 24A, 24B to respectively deposit the fusing agent 26 and the functional agent 28 (respectively) over a large area of a layer of polymeric or polymeric composite build material particles 16. The applicator(s) 24A, 24B may thus be attached to a moving XY stage or a translational carriage (neither of which is shown) that moves the applicator(s) 24A, 24B adjacent to the build area platform 12 in order to deposit the fusing agent 26 and the functional agent 28 (respectively) 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(s) 24A, 24B may include a plurality of nozzles (not shown) through which the fusing agent 26 and the functional agent 28 (respectively) are to be ejected.

[0109] The inkjet applicators 24A, 24B may respectively deliver drops of the fusing agent 26 and the functional agent 28 at a resolution ranging from about 300 dots per inch (DPI) to about 1200 DPI. In other examples, the applicator(s) 24A, 24B may deliver drops of the respective fluids 26, 28 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 (pi) per drop, although it is contemplated that a higher or lower drop size may be used. In some examples, the applicators 24A, 24B are able to deliver variable size drops of the fluids 26, 28, respectively.

[01 10] Each of the previously described physical elements may be operatively connected to a controller 32 of the printing system 10. The controller 32 may process print data that is based on a 3D object model of the 3D object/part to be generated. In response to data processing, the controller 32 may control the operations of the build area platform 12, the build material supply 14, the build material distributor 18, and the applicator(s) 24A, 24B. As an example, the controller 32 may control actuators (not shown) to control various operations of the 3D printing system 10 components. The controller 32 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 32 may be connected to the 3D printing system 10 components via communication lines.

[01 1 1 ] The controller 32 manipulates and transforms data, which may be represented as physical (electronic) quantities within the printer's registers and memories, in order to control the physical elements to create the 3D part. As such, the controller 32 is depicted as being in communication with a data store 34. The data store 34 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, the functional agent 28, 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 polymeric composite build material particles 16 that the first applicator 24A is to deposit the fusing agent 26 and the locations that the second applicator 24B is to deposit the functional agent 28. In one example, the controller 32 may use the data to control the first applicator 24A to selectively apply the fusing agent 26. In another example, the controller 32 may use the data to control the second applicator 24B to selectively apply the functional agent 28. The data store 34 may also include machine readable instructions (stored on a non-transitory computer readable medium) that are to cause the controller 32 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(s) 24A, 24B, etc.

[01 12] As shown in Fig. 1 , the printing system 10 may also include a source 36, 36' of electromagnetic radiation 46. In some examples, the source 36, 36' of electromagnetic radiation 46 may be in a fixed position with respect to the build material platform 12. In other examples, the source 36, 36' of electromagnetic radiation 46 may be positioned to apply electromagnetic radiation 46 (see, e.g., Fig. 2C) to the layer 40 of polymeric or polymeric composite build material particles 16 immediately after the fusing agent 26 has been applied thereto. In the example shown in Fig. 1 , the source 36, 36' of electromagnetic radiation 46 is attached to the side of the applicators 24A, 24B which allows for patterning and heating/exposing to electromagnetic radiation 46 in a single pass.

[01 13] While not shown, in some examples the source 36, 36' of electromagnetic radiation 46 may be a laser that emits light through optical amplification based on the stimulated emission of electromagnetic radiation. The laser may emit light coherently (i.e., constant phase difference and frequency), which allows the electromagnetic radiation 46 to be emitted in the form of a laser beam that stays narrow over large distances and focuses on a small area. When the source 36, 36' of electromagnetic radiation 46 is the laser, the laser may be any laser that emits electromagnetic radiation. The laser may be a pulse laser (i.e., the optical power appears in pluses). Using a pulse laser allows energy to build between pluses, which enable the laser beam to have more energy. A single laser or multiple lasers may be used.

[01 14] The source 36, 36' of electromagnetic radiation 46 may emit

electromagnetic radiation 46 having wavelengths ranging from about 800 nm to about 1 mm. As one example, the electromagnetic radiation 46 may range from about 800 nm to about 2 μιη. As another example, the electromagnetic radiation 46 may be blackbody radiation with a maximum intensity at a wavelength of about 1 100 nm. The source 36, 36' of electromagnetic radiation 46 may be infrared (IR) or near-infrared light sources, such as I R or near-IR curing lamps, IR or near-IR light emitting diodes (LED), or lasers with the desirable IR or near-IR electromagnetic wavelengths.

[01 15] The source 36, 36' of electromagnetic radiation 46 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 38. The radiation system components 38 may operate together to control the source 36, 36' of electromagnetic radiation 46. 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 36, 36' of electromagnetic radiation 46 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 36, 36' of electromagnetic radiation 46. This is one example of the radiation system components 38, and it is to be understood that other radiation source control systems may be used. For example, the controller 32 may be configured to control the source 36, 36' of electromagnetic radiation 46.

[01 16] Referring now to Figs. 2A through 2E, an example of the 3D printing method 100 is depicted. This method 100 may be used to form 3D printed and thermochromic, gasochromic, and/or pH sensitive parts.

[01 17] It is to be understood that, in some examples of the method 100, the functional additive is included in the polymeric or polymeric composite build material 16 and not in the functional agent 28. In these examples, the method 100 does not include the functional agent 28. Further, it is to be understood that, in other examples of the method 100, the functional additive is included in the functional agent 28 and may or may not be included in the polymeric or polymeric composite build material 16. In these examples, the method 100 does include the functional agent 28.

[01 18] It is further to be understood that, in some examples of the method 100, the fusing agent 26 is selectively applied to the portion(s) 42 of the layer 40 of the polymeric or polymeric composite build material 16 that are to become the fused layer 48. In these examples, the entire layer 40 of the polymeric or polymeric composite build material 16 is exposed to the electromagnetic radiation 46, but the portion(s) 42 that are in contact with the fusing agent 26 will fuse and the remaining portion(s) 44 will not fuse. Further, it is to be understood that, in other examples of the method 100, the portion(s) 42 of the layer 40 of the polymeric or polymeric composite build material 16 that are to become the fused layer 48 are selectively exposed to the laser beam. In these examples, the method 100 does not include the fusing agent 26.

[01 19] Prior to execution of the method 100 or as part of the method 100, the controller 32 may access data stored in the data store 34 pertaining to a 3D part that is to be printed. The controller 32 may determine the number of layers of polymeric or polymeric composite build material 16 that are to be formed, the locations at which the fusing agent 26 from the first applicator 24A is to be deposited on each of the respective layers, and the locations (if any) at which the functional agent 28 from the second applicator 24B is to be deposited on each of the respective layers.

[0120] 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 32 (not shown in Figs. 2A and 2B) may process control build material supply data, and in response control the build material supply 14 to appropriately position the polymeric or polymeric composite build material particles 16, and may process control spreader data, and in response 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 40 of polymeric or polymeric composite build material particles 16 thereon. As shown in Fig. 2B, one layer 40 of the polymeric or polymeric composite build material particles 16 has been applied.

[0121 ] The layer 40 has a substantially uniform thickness across the build area platform 12. In an example, the thickness of the layer 40 is about 100 μιη. In another example, the thickness of the layer 40 ranges from about 50 μιη to about 300 μιη, although thinner or thicker layers may also be used. For example, the thickness of the layer 40 may range from about 20 μιη to about 500 μιη, or from about 30 μιη to about 300 μιη. The layer thickness may be about 2x (i.e., 2 times) the particle diameter (as shown in Fig. 2B) at a minimum for finer part definition. In some examples, the layer thickness may be about 1 .2x the particle diameter. [0122] Prior to further processing, the layer 40 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 250°C. In another example, the pre-heating temperature ranges from about 150°C to about 170°C.

[0123] Pre-heating the layer 40 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) integrated into the platform 12) or the electromagnetic radiation source 36, 36'.

[0124] Referring now to Fig. 2C, after the layer 40 is formed, and in some instances is pre-heated, the fusing agent 26 is selectively applied on at least a portion 42 of the polymeric or polymeric composite build material 16.

[0125] As also shown in Fig. 2C, some examples of the method 100 include selectively applying the functional agent 28 on at least a portion 42 of the polymeric or polymeric composite build material 16. The selective application of the functional agent 28 will be described below. It is to be understood that in the examples of the method 100 that do not include the functional agent 28, the functional agent 28 is not applied.

[0126] As illustrated in Fig. 2C, the fusing agent 26 may be dispensed from the first applicator 24A, and the functional agent 28 may be dispensed from the second applicator 24B. In an example, the fusing agent 26 may be dispensed onto the portion 42 first, and then the functional agent 28 may be dispensed onto the portion 42. In another example, the functional agent 28 may be dispensed onto the portion 42 first, and then the fusing agent 26 may be dispensed onto the portion 42. In still another example, the fusing agent 26 and the functional agent 28 may be dispensed at least substantially simultaneously (e.g., one immediately after the other in a single printing pass, or at the same time).

[0127] Although shown as separate applicators 24A, 24B, it is to be understood that a single applicator with individual cartridges for dispensing the respective fluids 26, 28 may be used. In still other examples, a single applicator 24A or 24B with a single cartridge may be used to dispense a fusing/functional agent. When the fusing/functional agent (which combines the fusing agent 26 and the functional agent 28) is used, the selective application is accomplished in a single step.

[0128] The applicators 24A and/or 24B may each be a thermal inkjet printhead, a piezoelectric printhead, etc., and each of the selectively applying of the fusing agent 26 and the selectively applying of the functional agent 28 may be accomplished by thermal inkjet printing, piezo electric inkjet printing, etc. The fusing agent 26 may be dispensed at a contone level ranging from about 10 contone to about 255 contone (which refers to the number of drops, which is divided by 256, that will be placed on average onto each pixel). As mentioned above, the functional agent 28 may be dispensed at a contone level ranging from about 10 contone to about 255 contone. As also mentioned above, when the functional additive is included in the functional agent 28 in an amount ranging from about 0.1 wt% to about 8 wt% (based on the total wt% of the functional agent 28), the functional agent 28 may be applied in an amount ranging from about 1 .4 ng to about 36 ng per pixel (i.e., 1 /600 inch by 1/600 inch).

[0129] The controller 32 may process data, and in response, control the first applicator 24A (e.g., in the directions indicated by the arrow 30) to deposit the fusing agent 26 onto predetermined portion(s) 42 of the polymeric or polymeric composite build material 16 that are to become part of the 3D part. The first applicator 24A may be programmed to receive commands from the controller 32 and to deposit the fusing agent 26 according to a pattern of a cross-section for the layer of the 3D part that is 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 first applicator 24A selectively applies the fusing agent 26 on those portion(s) 42 of the layer 40 that is/are 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 40 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 42 of the layer 40 and not on the portions 44.

[0130] The controller 32 may also process data, and in response, control the second applicator 24B (e.g., in the directions indicated by the arrow 30) to deposit the functional agent 28 onto predetermined portion(s) 42 of the polymeric or polymeric composite build material 16 that are to exhibit the thermochromic, gasochromic, and/or pH sensitive property. The second applicator 24B may be programmed to receive commands from the controller 32 and to deposit the functional agent 28 according to a pattern of a cross-section for the region (of the layer of the 3D part that is to be formed) that is to exhibit the thermochromic, gasochromic, and/or pH sensitive property. In the example shown in Fig. 2C, the second applicator 24B selectively applies the functional agent 28 on those portion(s) 42 of the layer 40 that are to exhibit the thermochromic, gasochromic, and/or pH sensitive property in the first layer of the 3D part. In the example shown in Fig. 2C, the functional agent 28 is deposited in a square pattern on the portion 42 of the layer 40 and not on the portions 44.

[0131 ] As mentioned above, the fusing agent 26 may include the plasmonic resonance absorber or other radiation absorber and the fusing agent vehicle. 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 42 may be sufficient to absorb and convert enough electromagnetic radiation 46 so that the polymeric or polymeric composite build material 16 in the patterned portion 42 will fuse. The volume of the fusing agent 26 that is applied per unit of the polymeric or polymeric composite build material 16 may depend, at least in part, on the plasmonic resonance absorber used, the plasmonic resonance absorber loading in the fusing agent 26, and the polymeric or polymeric composite build material 16 used.

[0132] When the functional agent 28 is selectively applied in the desired area(s) of the portion(s) 42, the functional additive (present in the functional agent 28) infiltrates the inter-particles spaces among the polymeric or polymeric composite build material 16. The volume of the functional agent 28 that is applied per unit of the polymeric or polymeric composite build material 16 in the patterned portion 42 may be sufficient to achieve a desired thermochromic, gasochromic, and/or pH sensitive property.

[0133] In some examples, such as the example shown in Fig. 2C, the fusing agent 26 and the functional agent 28 are applied in the same portion(s) (e.g., portion 42). In these examples, the region containing the functional additive and thus, exhibiting the thermochromic, gasochromic, and/or pH sensitive property is the entire layer 48 of the 3D part. In other examples, the darker fusing agent (without the functional agent 28) maybe applied to an interior portion of a layer and/or to interior layer(s) of a 3D part, and the low tint fusing agent and the functional agent 28 may be applied to exterior portion(s) of a layer and/or to exterior layer(s) of the 3D part. In the latter examples, the desired thermochromic, gasochromic, and/or pH sensitive property will be exhibited at the exterior of the part. Additionally in the latter examples, the outer layer(s) formed with the low tint fusing agent may be thick enough to mask the darker color or the interior of the 3D part.

[0134] While the portion 42 of the layer 40 is shown having both the fusing agent 26 and the functional agent 28 applied thereto, it is to be understood that in some examples of the method 100, some area(s) of the portion 42 may have the fusing agent 26 applied thereto, but may not have the functional agent 28 applied thereto. These area(s) of the portion 42 will become part of the 3D part that is formed, but will not exhibit the thermochromic, gasochromic, and/or pH sensitive property. As such, these area(s) do not become part of the region of the 3D part that exhibits the thermochromic, gasochromic, and/or pH sensitive property. Rather, these area(s) make up a portion of the 3D part that has a constant or consistent color (i.e., the color of the build material 16 of fusing agent 26). For example, the edges of the portion 42 (e.g., adjacent to portion(s) 44) may have the functional agent 28 and the fusing agent 26 applied thereto (and thus will form an edge region that exhibits the thermochromic, gasochromic, and/or pH sensitive property), while a center of the portion 42 may have the fusing agent 26 alone applied thereto (and thus will form a center region that exhibits a constant or consistent color). [0135] As such, in other examples, the method 100 may include applying the fusing agent 26 on a portion of the polymeric or polymeric build material 16 to which the functional agent 28 is not applied. For example, the functional agent 28 may be applied to a portion of the polymeric or polymeric build material 16 (and thus the portion is less than all of the polymeric or polymeric build material 16), and in this example, one of: i) the fusing agent 26 may be selectively applied on another portion of the polymeric or polymeric build material 16 (and thus the exposing of the polymeric or polymeric composite build material 16 to electromagnetic radiation 46 fuses the other portion of the polymeric or polymeric composite build material 16 and forms a remaining region of the layer 48), or ii) the other portion of the polymeric or polymeric build material 16 may be selectively laser sintered to fuse the other portion to form the remaining region of the layer 48. In these examples, a region (i.e., the remaining region) that does not exhibit the thermochromic property, the gasochromic property, the pH sensitive property, or the combination thereof is formed. The region without the thermochromic property, the gasochromic property, the pH sensitive property, or the combination thereof may be an entire layer of the 3D part or may be a remaining region of a layer that includes a region exhibiting the thermochromic, gasochromic, and/or pH sensitive property (i.e., part of a layer exhibits a constant or consistent color and another part of a layer exhibits the thermochromic, gasochromic, and/or pH sensitive property). When the region that does not exhibit the thermochromic property, the gasochromic property, the pH sensitive property, or the combination thereof is a remaining region of a layer that also exhibits the thermochromic, gasochromic, and/or pH sensitive property, the portion of the polymeric or polymeric composite build material 16 to which the functional agent 28 is applied is less than all of the polymeric or polymeric composite build material 16.

[0136] It is to be understood that a single fusing agent 26 may be selectively applied on the portion 42, or multiple fusing agents 26 may be selectively applied on the portion 42. When multiple fusing agents 26 are utilized, each is capable of absorbing enough electromagnetic radiation 46 so that the polymeric or polymeric composite build material 16 in the patterned portion 42 will fuse. As an example, multiple fusing agents 26 may be used when one fusing agent 26 is included with the functional agent 28, and another fusing agent 26 is applied on another portion of the polymeric or polymeric composite build material 16 to which the functional agent 28 is not applied.

[0137] While not shown, in some examples, the method 100 may further include selectively applying another or second functional agent including another or second functional additive on a portion of the polymeric or polymeric composite build material 16. The other or second functional agent may be used to introduce another or second functional additive, which may be different than the functional additive in the functional agent 28, to the layer 40. In this example, the fused layer 48 exhibits another thermochromic, gasochromic, and/or pH sensitive property, which may be different than the thermochromic, gasochromic, and/or pH sensitive property imparted by the functional additive in the functional agent 28.

[0138] The other or second functional agent may be applied to the same portion(s) (e.g., portion 42) as, or different portion(s) than, the portion(s) (e.g., portion 42) to which functional agent 28 is applied. For example, if the agents result in different color changes when exposed to the same or different conditions, it may be desirable to apply the agents in different area(s) or portion(s) 42. For another example, if the agents result in the same color change when exposed to different conditions, it may be desirable to apply the agents in the same area(s) or portion(s) 42.

[0139] The other or second functional agent may be applied to the polymeric or polymeric composite build material 16 with one of the applicators 24A, 24B (from a separate cartridge for dispensing the other or second functional agent) or with a third applicator (not shown) that may be similar to the applicators 24A, 24B.

[0140] After selectively applying the fusing agent 26 and/or selectively applying the functional agent 28, the polymeric or polymeric composite build material 16 is exposed to electromagnetic radiation 46. The electromagnetic radiation 46 may be applied with the source 36 of electromagnetic radiation 46 as shown in Fig. 2D or with the source 36' of electromagnetic radiation 46 as shown in Fig. 2C.

[0141 ] The fusing agent 26 enhances the absorption of the electromagnetic radiation 46, converts the absorbed electromagnetic radiation 46 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 40 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. The application of the electromagnetic radiation 46 forms the fused layer 48, as shown in Fig. 2D.

[0142] It is to be understood that portions 44 of the polymeric or polymeric composite build material 16 that do not have the fusing agent 26 applied thereto do not absorb enough radiation 46 to fuse. As such, these portions 44 do not become part of the 3D part that is ultimately formed. The polymeric or polymeric composite build material 16 in portions 44 may be reclaimed to be reused as build material in the printing of another 3D part.

[0143] As mentioned above, in some examples, the portion(s) 42 of the layer 40 of the polymeric or polymeric composite build material 16 that are to become the fused layer 48 are selectively exposed to the laser beam, and the fusing agent 26 is not used. In these examples, the fusing agent 26 is not applied at Fig. 2C, and the functional agent 28 alone may be applied at Fig. 2C. In examples when the functional additive is included in the polymeric or polymeric composite build material 16, Fig. 2C may be skipped and the method 100 may proceed directly from Fig. 2B to Fig. 2D. At Fig. 2D, rather than exposing the entire layer 40 to the

electromagnetic radiation 46, the portion 42 alone is exposed to the electromagnetic radiation 46 using the laser beam to form the fused layer 48.

[0144] 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. 2E illustrates the initial formation of a second layer of polymeric or polymeric composite build material particles 16 on the previously formed layer 48. In Fig. 2E, following the fusing of the predetermined portion(s) 42 of the layer 40 of polymeric or polymeric composite build material 16, the controller 32 may process data, and in response 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 40. In addition, following the lowering of the build area platform 12, the controller 32 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 pre-heated, patterned with the fusing agent 26, patterned with the functional agent 28, and then exposed to electromagnetic radiation 46 from the source 36, 36' of electromagnetic radiation 46 to form the additional fused layer.

[0145] As mentioned above, some examples of the method 100 may involve the selective application of the functional agent 28. Two examples of this method are shown at reference numerals 200 and 200' in Fig. 3. Since the examples of the method 200, 200' shown in Fig. 3 are similar to the example of the method 100 that involves the use of the functional agent 28, it is to be understood that Figs. 2A through 2E and the associated text may be applicable to the method 200, 200'.

[0146] The method 200 involves reference numerals 202, 204, 206, 208 and 212, while the method 200' involves reference numerals 202, 204, 210 and 212.

[0147] As shown at reference numeral 202, the methods 200, 200' include applying the polymeric or polymeric composite build material 16.

[0148] As shown at reference numeral 204, methods 200, 200' further include selectively applying the functional agent 28 on at least the portion 42 of the polymeric or polymeric composite build material 16, the functional agent 28 including a functional additive selected from the group consisting of a

thermochromic agent, a gasochromic agent, a potential hydrogen (pH) sensitive agent, and a combination thereof.

[0149] As shown in Fig. 3 and noted above, two variations of the method 200, 200' may take place after reference numeral 204. After applying the build material 16 and selectively applying the functional agent 28 as shown in reference numerals 202 and 204, the method 200, 200' further includes one of: selectively applying a fusing agent 26 on the at least the portion of the polymeric or polymeric composite build material 16, and exposing the polymeric or polymeric composite build material 16 to electromagnetic radiation to fuse the at least the portion 42 of the polymeric or polymeric composite build material 16 to form a region of a layer 48; or selectively laser sintering the at least the portion 42 of the polymeric or polymeric composite build material 16 to fuse the at least the portion of the polymeric or polymeric composite build material 16 to form the region of the layer 48.

[0150] More specifically with reference to Fig. 3, as shown at reference numeral 206, one example of the method 200 further includes selectively applying the fusing agent 26 on the at least the portion 42 of the polymeric or polymeric composite build material 16.

[0151 ] As shown at reference numeral 208, this example of the method 200 further includes exposing the polymeric or polymeric composite build material 16 to electromagnetic radiation 46 to fuse the at least the portion 42 of the polymeric or polymeric composite build material 16 to form a region of a layer 48.

[0152] As shown at reference numeral 210, another example of the method 200' further includes selectively laser sintering the at least the portion 42 of the polymeric or polymeric composite build material 16 to fuse the at least the portion 42 of the polymeric or polymeric composite build material 16 to form the region of the layer 48.

[0153] As shown at reference numeral 212, in both examples of the method 200, 200', the region of the layer 48 exhibits a thermochromic property, a gasochromic property, a pH sensitive property, or a combination thereof.

[0154] Also as mentioned above, some examples of the method 100 may involve the inclusion of the functional additive in the polymeric or polymeric composite build material 16. Two examples of this method are shown at reference numerals 300, 300' in Fig. 4. It is to be understood that Figs. 2A through 2E and the associated text may also be applicable to the method 300, 300'.

[0155] The method 300 involves reference numerals 302, 304, 306 and 310, while the method 300' involves reference numerals 302, 308 and 310.

[0156] As shown at reference numeral 302, the methods 300, 300' include applying the polymeric or polymeric composite build material 16, the polymeric or polymeric composite build material 16 including a functional additive selected from the group consisting of a thermochromic agent, a gasochromic agent, a potential hydrogen (pH) sensitive agent, and a combination thereof.

[0157] As shown in Fig. 4, two variations of the method 300, 300' may take place after reference numeral 302. After applying the build material 16 as shown in reference numeral 302, the method 300, 300' further includes one of: selectively applying a fusing agent 26 on at least a portion of the polymeric or polymeric composite build material 16, and exposing the polymeric or polymeric composite build material 16 to electromagnetic radiation 46 to fuse the at least the portion of the polymeric or polymeric composite build material 16 to form a layer 48; or selectively laser sintering the at least the portion 42 of the polymeric or polymeric composite build material 16 to fuse the at least the portion of the polymeric or polymeric composite build material 16 to form the layer 48.

[0158] More specifically with reference to Fig. 3, as shown at reference numeral 304, one example of the method 300 further includes selectively applying the fusing agent 26 on at least a portion 42 of the polymeric or polymeric composite build material 16.

[0159] As shown at reference numeral 306, this example of the method 300 further includes exposing the polymeric or polymeric composite build material 16 to electromagnetic radiation 46 to fuse the at least the portion 42 of the polymeric or polymeric composite build material 16 to form a layer 48.

[0160] As shown at reference numeral 308, another example of the method 300 further includes selectively laser sintering the least the portion 42 of the polymeric or polymeric composite build material 16 to fuse the at least the portion 42 of the polymeric or polymeric composite build material 16 to form the layer 48.

[0161 ] As shown at reference numeral 310, in both examples of the method 300, 300' the layer 48 exhibits a thermochromic property, a gasochromic property, a pH sensitive property, or a combination thereof.

[0162] When the functional additive in the build material 16 is capable of initiating an undesirable temperature rise during fusing, the method 300 may involve the selective application of the detailing agent on the portion(s) (e.g., portion(s) 44 as shown in Fig. 2C) of the build material 16 that are not supposed to be fused or to become part of the layer 48.

[0163] The detailing agent may be selectively applied using any suitable applicator, such as applicator 24A, 24B, etc.

[0164] The detailing agent may be water alone. The detailing agent may also include a surfactant, a co-solvent, and a balance of water. In some examples, the detailing agent consists of these components, and no other components. In some instances, the detailing agent further includes an anti-kogation agent, a biocide, or combinations thereof. The components of the detailing agent may be similar to the surfactants, co-solvents, anti-kogation agents, and biocide described above in reference to the functional agent 28.

[0165] When used, the detailing agent may be applied to actively cool portion(s) 44 of the build material 16 that do not have the fusing agent 26 applied thereto. The detailing agent may provide an evaporative cooling effect that reduces the temperature of the build material 16 in contact with the detailing agent during the radiation 46 exposure. As such, the detailing agent may prevent the build material 16, which is not exposed to the fusing agent 26 but is absorbing of at least some of the electromagnetic radiation 46 because of the functional additive therein, from fusing.

[0166] The thermochromic, gasochromic, and/or pH sensitive 3D printed parts formed by the examples of the method disclosed herein may be used in a variety of applications. For example, a 3D printed part formed with a gasochromic property may be an oxygen sensor. This part may be used for detecting oxygen or an oxidizing agent (e.g., when an inert environment is desired). For another example, a 3D printed part formed with a thermochromic property. This part may be desirable when the part is used in high temperature application. The color change of this part may indicate to a user that the melting temperature of the part is approaching, but has not yet been reached. For still another example, a toy may be made with the thermochromic, gasochromic, and/or pH sensitive property. As an example, a thermochromic bath toy may indicate to a user the temperature of the water. For yet another example, a pH sensitive 3D printed part may be used for pH detection, e.g., in fish tanks, pools, etc. [0167] The 3D parts formed by the examples of the method disclosed herein may also have custom and/or complex geometries to fit the desired use.

[0168] 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

[0169] Example 1

[0170] Three example parts (referred to as "example part 1 " or "EP1 ," "example part 2" or "EP2," and "example part 3" or "EP3") and four comparative parts

(referred to as "comparative part 1 " or "CP1 ," "comparative part 2" or "CP2," comparative part 3" or "CP3," and "comparative part 4" or "CP4") were printed. The example and comparative parts were printed using polyamide-12 (PA-12) as the build material and two fusing agents, one containing CTO nanoparticles (with a general formula of Cs x W0 3 , where 0<x<1 ) and the other containing carbon black. As each layer of powder was applied and patterned with the fusing agents and/or functional agent for each example and comparative part, it was exposed to IR radiation using an incandescent light source.

[0171 ] For the example parts, the exterior layers (e.g., the first/bottom two layers (layers 1 and 2) and the last/top two layers) did not have any fusing agent, and the example functional agent (including acid blue 9 dye) alone was applied to these layers. The dye at these exterior layers can fuse with the adjacent layers during printing to impart additional color and color changing property at the exterior of the part. The two layers (layers 3 and 4) adjacent to the first/bottom two layers had the CTO fusing agent applied with the example functional agent, and the next two layers (layers 5 and 6) had the CTO fusing agent applied across the layers and the example functional agent applied near the edges. Four to six layers adjacent to the last/top two layers had the CTO fusing agent applied with the example functional agent. At the innermost layers, the CTO fusing agent was applied near the edges of each layer and the carbon black fusing agent was applied near the center of each layer. The example functional agent was also applied to the edges of the innermost layers (i.e., with the CTO fusing agent). Each resulting part had a total number of layers ranging from 204 layers to 404 layers.

[0172] The edges of each innermost layer of example part 1 were patterned with the example functional agent at a contone level of 16. The edges of each innermost layer of example part 2 were patterned with the example functional agent at a contone level of 32, and the edges of each innermost layer of example part 3 were patterned with the example functional agent at a contone level of 64.

[0173] The comparative parts 1 and 2 were formed in a similar manner as examples parts 1 , 2 and 3, except that a comparative functional agent including cyan 854 (C854) dye in a first vehicle (referred to as "comparative agent 1 ") was used in place of the example functional agent. The edges of each innermost layer of comparative part 1 were patterned with comparative agent 1 at a contone level of 16, and the edges of each innermost layer of comparative part 2 were patterned with comparative agent 1 at a contone level of 32.

[0174] The comparative parts 3 and 4 were formed in a similar manner as examples parts 1 , 2 and 3, except that a comparative functional agent including cyan 854 (C854) dye in a second vehicle that was different from the first vehicle (referred to as "comparative agent 2") was used in place of the example functional agent. The edges of each innermost layer of comparative part 3 were patterned with comparative agent 2 at a contone level of 16, and the edges of each innermost layer of comparative part 4 were patterned with comparative agent 2 at a contone level of 32.

[0175] Acid blue 9 dye is a thermochromic dye, while cyan (C854) is not a thermochromic dye.

[0176] The exposure to the IR radiation heated the example and comparative parts initially to about 190°C. Then, the example and comparative parts cooled slightly to the temperature of the print bed, which was about 164°C. The parts were maintained at the 164°C during the 1 hour it took to print the parts. The 164°C temperature caused the example parts to fade from dark blue to light blue within the 1 hour of temperature exposure.

[0177] The example and comparative parts are shown in black and white in Fig. 5. The left most portion of each example and comparative part corresponds to the first/bottom layers printed, and the right most portion of each example and comparative part corresponds to the last/top layer printed. As such, the left most portion of each part was exposed to the 164°C temperature longer than the right most portion. As indicated by the lighter and darker spots in examples EP1 and EP2 in Fig. 5, the left most portion of the example parts EP1 , EP2 had faded to light blue and the right most portion of the example parts EP1 , EP2 remained a darker blue. As also shown in Fig. 5, the color change from dark blue to light blue was most easily observed in example part 1 , the edges of which were patterned with the example functional agent at a contone level of 16. Fig. 5 further shows that the comparative parts underwent no color change. Even with a vehicle change, the comparative parts did not undergo a color change. This indicates that the color change is due to the inclusion of a functional dye (which C854 is not) and not because of the vehicle.

[0178] Example 2

[0179] Two additional example parts (referred to as "example part 4" or ΈΡ4," and "example part 5" or "EP5") were printed. The example parts were printed using polyamide-12 (PA-12) as the build material, the two fusing agents, the one containing CTO nanoparticles (with a general formula of Cs x W0 3 , where 0<x<1 ) and the other containing carbon black, and the example functional agent (including acid blue 9 dye) at a contone level of 16. As each layer of powder was applied and patterned with the fusing agent(s) and/or functional agent (as described in Example 1 ) for both example parts, it was exposed to IR radiation using an incandescent light source. Both resulting parts had 204 layers.

[0180] During printing, the bottom most portion of each example part 4 and 5 was exposed to the 165°C temperature longer than the top most portion. Likely due to the prolonged temperature exposure, the color at the bottom of each part was lighter blue than the top of each part. As such, as a result of printing, each of parts 4 and 5 had a color gradient, with darker blue at the top fading to lighter blue at the bottom.

[0181 ] Both example parts 4 and 5 were heated in an oven at 162°C for about 20.5 hours. Example part 4 was heated for 20.8 hours in air, and example part 5 was heated for 20.4 hour in a vacuum (with an air pressure of about 200 torr).

Example part 4 regained the dark blue color (originally at the top most portion) across the entire part, and example part 5 remained the light blue color (originally at the bottom most portion) across the entire part.

[0182] Fig. 6A shows example part 4 after heating, and Fig. 6B shows example part 5 after heating. It is believed that example part 4 was able to regain the dark blue color because of the presence of oxygen during the reheating. This example illustrates both the thermochromic and gasochromic properties that can be imparted to a part using acid blue 9.

[0183] Example 3

[0184] Two additional example parts (referred to as "example part 6" or ΈΡ6," and "example part 7" or "EP7") and one additional comparative part (referred to as "comparative part 5" or "CP5") were printed. The example and comparative parts were printed using polyamide-12 (PA-12) as the build material and one or two fusing agents, one containing CTO nanoparticles (with a general formula of Cs x W0 3 , where 0<x<1 ) and/or the other containing carbon black.

[0185] Example parts 6 and 7 were formed in the same manner as example parts 1 , 2 and 3 of Example 1 using the same type of example functional agent (including acid blue 9 dye) applied at the edges at a contone level of 16.

[0186] Comparative part 5 was formed with one fusing agent, namely the CTO fusing agent, and without any functional agent (i.e., the functional agent was not applied to comparative part 5).

[0187] After printing, example parts 6 and 7 and comparative part 5 were each dipped in a strong hydrochloric acid solution (37%). Then, each of the parts was washed with deionized water. The hue angles of parts 6 and 7 were measured before dipping, after dipping in acid, and after dipping in water.

[0188] After printing, example parts 6 and 7 were dark blue in color (hue angle = 231 °). After being dipped in the hydrochloric acid solution, example parts 6 and 7 turned green (greenish yellow or lime green) in color (hue angle = 143° and 128°, respectively). Fig. 7A shows (in black and white) example parts 6 and 7 after being dipped in the hydrochloric acid solution. After being washed with deionized water, example parts 6 and 7 regained their dark blue color (hue angle = 228° and 229°, respectively). Fig. 7B shows (in black and white) example parts 6 and 7 after being washed with deionized water. These parts regained the original dark blue color.

[0189] After printing, comparative part 5 was light grey in color. After being dipped in the hydrochloric acid solution, comparative part 5 remained light grey in color. Fig. 8A shows comparative part 5 (in black and white) after being dipped in the hydrochloric acid solution. After being washed with deionized water,

comparative part 5 also remained light grey in color. Fig. 8B shows comparative part 5 (in black and white) after being washed with deionized water.

[0190] It is believed that example parts 6 and 7 were able to turn green in color due to the exposure to the low pH of the hydrochloric acid solution. It is further believed that example parts 6 and 7 were able to regain their dark blue color due to the exposure to the pH of 7 of the deionized water. The pH sensitivity was maintained even after the conditions of the 3D printing process. Comparative part 5 did not exhibit such pH sensitive properties.

[0191 ] It is to be understood that the previous examples are examples of how the 3D printing method can be performed, and that the 3D printing methods disclosed herein may be performed with other build materials, layer numbers, fusing agent combinations, order of applying the fusing agents and/or functional agent in the various layers, etc.

[0192] 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 50°C to about 400°C should be interpreted to include not only the explicitly recited limits of from about 50°C to about 400°C, but also to include individual values, such as 75.5°C, 255°C, 275°C, 380.85°C, etc. , and sub-ranges, such as from about 135°C to about 365°C, from about 220.5°C to about 270.7°C, from about 1 15°C to about 381 °C, etc. Furthermore, when "about" or the symbol "~" is utilized to describe a value, this is meant to encompass minor variations (up to +/- 15%) from the stated value. [0193] 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.

[0194] In describing and claiming the examples disclosed herein, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

[0195] 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.