BACKGROUND

[0001 ] Three-dimensional printing, also known as“3D printing,” involves processes by which a printer transforms materials into a three-dimensional physical object. Methods of 3D printing have continued to develop over the last few decades and include, but are not limited to, powder or particle bed and liquid printing, selective laser sintering, selective laser melting, electron beam melting, as well as others. The area of powder or particle bed and liquid printing is growing and evolving. The demand for new techniques and printing methodologies in this area has increased as the area continues to expand and evolve.

BREIF DESCRIPTION OF DRAWINGS

[0002] FIG. 1 schematically illustrates an example 3D printing system that utilizes focused microwave energy in accordance with the present disclosure;

[0003] FIG. 2 is a flow diagram illustrating an example method of 3D printing in accordance with the present disclosure;

[0004] FIG. 3 is a flow diagram illustrating another example method of 3D printing in accordance with the present disclosure;

[0005] FIG. 4 schematically illustrates a facial view of example fluid ejectors and an example focused microwave emitter array in accordance with the present disclosure;

[0006] FIG. 5 schematically illustrates a cross-sectional view of single focused microwave emitter that can be included in the focused microwave emitter array shown in FIG. 4 in accordance with the present disclosure; [0007] FIG. 6 illustrates example polymer particle fusion in the presence of fusing agent with exposure to focused microwave energy in accordance with the present disclosure.

DETAILED DESCRIPTION

[0008] The present disclosure is drawn to three-dimensional (3D) printing systems and methods. More particularly, the systems and methods can be used with multi-jet fusion (MJF) where polymeric build material (with polymer particles or powder) is spread on a build platform on a layer by layer basis. The various layers are contacted with jetting fluid(s), and include a fusing agent that includes a fusing compound that is a good microwave energy absorber. The fusing compound can be carried by a liquid vehicle, for example. The fusing agent can include colorant. Other jetting fluid(s) can also be included, such as a colorant-containing fluid, a detailing agent, or other types of fluids. The jetting fluid(s) can be ejected from a print head, such as a fluid ejector similar to that used in some inkjet printing applications, for example, and then layers can be exposed to microwave energy. The presence of the fusing agent can cause the microwave energy to interact with the polymer particulates of the build material, and thus, form a fused 3D object on a layer-by-layer basis. Thus, a layer of polymeric build material can be exposed to a focused microwave energy field, and in areas where there is fusing agent, the 3D object can be fused. In areas where there is not fusing agent (where there may be nothing else present or a detailing agent, for example), the polymer particles can be transparent or nearly transparent to the microwave energy and not generate heat sufficiently to cause the polymer particles to become fused.

[0009] Thus, the present disclosure is drawn to systems and methods of 3D printing. In one example, a method of 3D printing can include iteratively applying individual build material layers, wherein the build material layers are of polymeric build material including from about 80 wt% to 100 wt% polymer particles based on a total weight of the polymeric build material. The method can also include selectively applying a fusing agent to individual build material layers to define individually patterned layers of a 3D object. The fusing agent can include an organic solvent fusing compound having a boiling point from about 20 °C to about 200 °C greater than the melting temperature of the polymeric build particles. In further detail, the method can also include iteratively applying a focused microwave energy field to the individual build material layers after or while selectively applying the fusing agent to an uppermost build material layer so that the focused microwave energy field fuses polymer particles at locations in contact with the fusing agent at the uppermost build material layer and at an n-1 build material layer without causing refusion of an n-2 build material layer. In one example, the fusing compound can be a C2 to C9 polyol, and the polymer particles are a polyamide. In further detail, when applying the focused microwave energy field, this can include emitting the focused microwave energy field from an emitter at from about 10 Watts to about 100 Watts relative to the uppermost build material layer. The focused microwave energy can be emitted, for example, from an array of multiple focusing microwave energy emitters. In another example, the method can include selectively applying a detailing agent (which includes the detailing compound). The detailing agent can be applied to the polymeric build material at a region immediately adjacent to a plurality of the individually patterned layers of the 3D object. The detailing agent can have a boiling point that is lower than the melting temperature of the polymeric build material.

[0010] In another example, a method of 3D printing can include iteratively applying individual build material layers, wherein the build material layers can be of polymeric build material including from about 80 wt% to 100 wt% polymer particles based on a total weight of the polymeric build material. The polymer particles can have an average particle size from about 20 pm to about 150 pm and a melting temperature from about 90 °C to about 350 °C. In further detail, based on a 3D object model, the method can include selectively applying a fusing agent to individual build material layers to define individually patterned layers of a 3D object. The fusing agent can include from about 1 wt% to 100 wt% fusing compound based on a total weight of the fusing agent.

In still further detail, the method can include iteratively applying a focused microwave energy field to the individual build material layers after or while selectively applying the fusing agent so that an uppermost build material layer of polymeric build material receives the focused microwave energy field to cause polymer particle fusion to occur where the fusing agent is in contact with the polymeric build material, wherein polymer particle fusion does not occur in the absence of fusing agent applied to the polymeric build material. Applying the focused microwave energy field can provide heating and allow for cooling of the uppermost build material layer and an n-1 build material layer within a maximum temperature range of about 15 °C to about 40 °C. In one example, when iteratively applying focused microwave energy field, this can include applying the focused microwave energy field from an array of microwave energy emitters that individually emit from about 20 Watts to about 50 Watts toward the uppermost build material layer. The fusing agent can be a liquid suspension, with the fusing compound including fusing particles selected from carbon black pigments, metal nanoparticles, ferrites, ferromagnetic metals, non-stoichiometrically reduced titania, ceramics, silicon carbide, copper oxide, manganese oxide, alumina, sodium metasilicate, talc, kaolin, metal salts, metal nitrates, or a combination thereof.

[0011 ] In another example, a 3D printing system can include a polymeric build material including from about 80 wt% to 100 wt% polymer particles based on the total weight of the polymeric build material. The polymer particles can have a melting temperature from about 90 °C to about 350 °C and an average particle size to allow for spreading of sequential build material layers on a build platform. The 3D printing system can further include a fusing compound having a boiling point from about 20 °C to about 200 °C greater than the melting temperature of the polymeric build particles. The 3D printing system can also include a focusing microwave energy emitter directed to deliver a focused microwave energy field toward an uppermost build material layer as well as to an n-1 build material layer. The focused microwave energy field can be sufficient to fuse polymer particles of the uppermost layer to polymer particles of the n-1 build material layer in the presence of the fusing agent, but insufficient to melt the polymeric particle of a previously fused n-2 build material layer even in the presence of the fusing agent. The polymeric build material not in contact with the fusing agent in this example can exhibit from about 90% to 100% transmittance when exposed to the focused microwave energy field. In one example, the focusing microwave energy emitter can be an individual microwave energy emitter of an array multiple focusing microwave energy emitters, and other individual focusing microwave energy emitters of the array individually emit from about 10 Watts to 100 Watts of the focused microwave energy field. In another example, the 3D printing system can also include a detailing agent including a detailing compound having a boiling point from about 50 °C to about 300 °C. The boiling point of the detailing compound can be less than the melting temperature of the polymeric build material, and wherein the detailing agent is devoid of the fusing compound. Example detailing compounds can include water, iso-butanol, n-butanol, tert-butanol, iso propanol, n-propanol, chlorobenzene, chloroform, cyclohexane, diglyme,

dimethylformamide, dioxane, ethyl acetate, heptane, n-hexane, tetrahydrofuran, toluene, xylene, or a combination thereof. The fusing compound, in one example, can be an organic solvent fusing compound having a boiling point from about 20 °C to about 200 °C greater than the melting temperature of the polymeric build particles. The fusing compound can likewise be a plasticizer for the polymer particles. In further detail, the fusing agent can be a liquid suspension, and the fusing compound can include fusing particles selected from carbon black pigments, metal nanoparticles, ferrites,

ferromagnetic metals, non-stoichiometrically reduced titania, ceramics, silicon carbide, copper oxide, manganese oxide, alumina, sodium metasilicate, talc, kaolin, metal salts, metal nitrates, or a combination thereof.

3D Printing Systems

[0012] Reference is now made to FIG. 1 , which depicts a 3D printing system 100, which can include a polymeric build material 106 (sometimes referred to as a powder bed material), a build platform 108 (typically with side walls to hold the polymeric build material therein), a fluid ejector 120, in some instances a second fluid ejector 130, and a focusing microwave energy emitter 140 that may be part of a focused microwave emitter array shown generally at 150. The focusing microwave energy emitters can include, as shown, a tip 144 to contribute to the focused application of microwave energy. A controller 160 may be present to control the focusing microwave energy emitters(s), the fluid ejector, and/or the second fluid ejector. The controller may be a single integrated circuit, or a system of interconnected or inter-operational circuits that work together to cause the 3D printing system to operate in appropriate sequence, e.g., apply fusing agent 122 with simultaneous or subsequent application of the focused microwave energy field 142. In some instances, a second fluid 132 can be ejected from the second fluid ejector, which can be a colored fluid or ink, a detailing agent, a second fusing agent, or the like. The particulate build material can be supplied by a build material source 118, for example, on a layer-by-layer basis. As shown in FIG. 1 , the “uppermost build material layer” is the layer to which fusing agent is applied to build the 3D object, and is shown at“n” in this FIG. The most immediate previously applied layer of particulate build material (with some portions being previously fused with fusing agent and microwave energy) can be referred to herein as the“n minus 1” or“n-1” build material layer, as shown. The layer applied just previously to the n-1 build material layer can be referred to herein as the“n minus 2” or“n-2” build material layer, also shown, and so forth. The focused microwave energy field in this FIG. is shown as penetrating the n build material layer for purposes of fusing the 3D object where the fusing agent is applied. In this example, the focused microwave energy field also penetrates the n-1 build material layer, causing the n build material layer to become fused to the n-1 build material layer, but does not penetrate the n-2 build material layer sufficiently to cause further fusing of the n-2 build material layer (or thereunder). By tuning the focusing microwave energy emitters in this manner, while forming the 3D printed object, the system can guard against polymer over-fusing that may result from thermal runaway, for example. As an example, as the n layer is typically intended to be the hottest layer during fabrication, if lower layers (n-1 layer or n-2 layer and therebeneath) get reheated multiple times when new layers are applied, there can be areas of the n layer that are currently being printed that can begin to melt (from overheated lower layers) before any fusing agent is applied. This can cause issues with respect to 3D object boundaries no longer being cleanly defined to those areas intended to be bused, e.g., where the fusing agent and microwave energy has been applied. Rather, the 3D object being formed can be further defined by heat transfer to polymer particles at these“runaway” locations during the build. By more carefully heating on a layer by layer basis, and for example, by also heating the n-1 layer to provide fusion between the n layer and the n-1 layer, and by allowing minimal at a upper level insufficient to melt the n-2 layer or other layers therebeneath, thermal runaway can be more easily controlled and avoided. In

accordance with this, the n-2 layer does not undergo“refusion” of an n-2 build material layer, in that the n-2 layer does not melt and further fuse while the n-1 layer and the uppermost (n) layer is being fused.

[0013] In further detail, in the absence of the fusing agent 120 ejected into the n build material layer particulate build material 106, the polymer particles can be used so that they are effectively transparent to the focused microwave energy field 142. For example, the polymeric build material can be selected or formulated relative to the microwave energy wavelength so that they exhibit from about 90% to 100%

transmittance.“Transmittance” is defined herein to be the ratio, expressed as a percentage, of microwave energy that is transmitted through the polymeric build material. Thus, in this example, from 0% to about 10% of the microwave energy may interact with polymeric build material in areas where there is no fusing agent present. With such high transmittance, there may not typically be enough energy input to the polymer particles to cause their fusion to adjacent particles or an adjacent build material layer, for example.

[0014] As can be seen, a portion of the polymeric build material 106 can thus be fused, and areas outside of where the fusing agent 122 is applied can remain free flowing or essentially free flowing (e.g., they do not become part of the three- dimensional object or part being fabricated). The build platform 108 can thus receive the polymeric build material, the polymeric build material can then be flattened by a roller or other mechanical device (not shown), and then after printing, can be printed upon and fused with the focused microwave energy field, which can be done sequentially until a 3D object is formed. Heat can be applied by the build platform or an overhead heat source or both to bring the temperature of polymer particles to a temperature near fusing or melting temperature so that the microwave energy applied to the fusing agent in the polymeric build material can be reduced. The fluid ejectors 120 and 130 can be operable to selectively deposit the various jettable fluid(s) onto the polymeric build material. As mentioned, the three-dimensional printing system can include multiple fluid ejectors, such as when there are multiple fluids to be applied to the polymeric build material. The fluid ejector(s) can be any type of printing apparatus capable of selectively applying the jettable fluid(s). For example, the fluid ejector(s) can be an inkjet applicator (thermal, piezo, etc.), a sprayer, etc.

[0015] In accordance with examples of the present disclosure, there can be the ability to control the melt of the polymer particles by using selective heating and selective cooling. For example, selecting an energy level and dwell time, e.g., Watts, and penetration depth of focused microwave energy field coupled with the selected polymeric build material and fusing compound can provide for selective heating and selective cooling within a reasonably narrow temperature range to provide thoroughly fused parts that are neither under-fused nor over-fused. In one specific example, the temperature fluctuation (heating to cooling to heating, etc.) can be in the range of about 15 °C to about 40 °C or from about 20 °C to about 30 °C. This range can straddle the melting temperature of the polymer particles, for example. In other words, focused microwave penetration depth, microwave energy level, and dwell time can be used to provide heating fluctuation that is within a relatively narrow range to provide acceptable heating for fusing, while not introducing overheating or overcooling during the build. As mentioned previously, the penetration depth can be through the uppermost build material layer and into the n-1 build material layer, e.g. from 10% to 100% into the n-1 build material layer. This can provide good cohesion or fusing between the uppermost build material layer and the n-1 build material layer. As the n-1 build material layer has already been fused to the n-2 build material layer (previously), there may be no reason to further heat the n-2 build material layer, and thus microwave penetration into the n-2 build material layer may not be beneficial, and in some instances, could lead to overheating of the n-2 build material layer in the form of thermal runaway. That stated, there may be instances where penetration into the n-2 build material layer can be beneficial, depending on the heating/cooling profiles and materials selected. When referring to penetration depth, in one example, this can be described in terms of a numeric value relative to the number of layers that the microwave energy penetrates to generate polymer particle fusion. It may penetrate further without polymer particle fusion, but at a weaker level. Penetration depths in accordance with examples of the present disclosure can be from about 1.1 to about 2, from about 1.25 to about 2, from about 1.25 to about 1.9, from about 1.5 to about 1.9, etc. As an example, a penetration of about 1.1 to about 2 indicates that the polymer particles in the presence of the fusing agent can be fused using the selected microwave energy profile (energy level and dwell time) through the uppermost layer and into about 10% of the n-1 build material layer as a minimum depth, and through the uppermost layer and into about 100% of the n-1 build material layer as a maximum depth. Thus, the focused microwave energy field can be used to fuse the uppermost polymer particles together while reheating and fusing a portion or all of the n-1 build material layer, but not heating the n-2 build material layer sufficiently to cause further fusing of the n-2 build material layer. Example wattage levels for the focused microwave energy (as it leaves the tip) can be from about 10 Watts to about 100 Watts, from about 15 Watts to about 75 Watts, or from about 20 Watts to about 50 Watts. As watts include both an energy level and time of exposure component (J/s), it is understood that wattage output can be changed by modifying either the microwave energy output and/or the dwell time. That stated, example dwell times (the time a focused microwave energy field footprint is over a build material layer) can be from about 0.1 second to about 10 seconds, from about 0.2 second to about 5 seconds, or from about 0.5 second to about 4 seconds. As further described herein in some examples, if additional cooling would be beneficial, particularly at the edge of the 3D object being manufactured, a detailing agent can be used.

3D Printing Methods

[0016] Turning now to FIG. 2, a method of 3D printing 200 can include iteratively applying 210 individual build material layers, wherein the build material layers are of polymeric build material including from about 80 wt% to 100 wt% polymer particles based on a total weight of the polymeric build material. The method can also include selectively applying 220 a fusing agent to individual build material layers to define individually patterned layers of a 3D object. The fusing agent can include an organic solvent fusing compound having a boiling point from about 20 °C to about 200 °C greater than the melting temperature of the polymeric build particles. In further detail, the method can also include iteratively applying 230 a focused microwave energy field to the individual build material layers after or while selectively applying the fusing agent to an uppermost build material layer so that the focused microwave energy field fuses polymer particles at locations in contact with the fusing agent at the uppermost build material layer and at an n-1 build material layer without causing refusion of an n-2 build material layer. In one example, the method can further include applying a detailing agent adjacent to locations where the fusing agent is applied or will be applied to promote cooling of the polymeric build material around boundaries of the 3D object. In further detail, when applying the focused microwave energy field, this can include emitting the focused microwave energy field from an emitter at from about 10 Watts to about 100 Watts. The focused microwave energy can be emitted, for example, from an array of multiple focusing microwave energy emitters. In another example, when selectively applying a detailing agent (which includes the detailing compound), this can be applied to the polymeric build material at a region immediately adjacent to a plurality of the individually patterned layers of the 3D object. The detailing agent can have a boiling point that is lower than the melting temperature of the polymeric build material. In further detail, the fusing compound can be an organic solvent fusing compound, such as a C2 to C9 polyol, and/or the fusing compound can be in a fusing agent liquid

suspension that includes fusing particles as the fusing compound. The polymer particles can be a polyamide. Other fusing compounds (including liquid suspensions with fusing particles) can be used, as well as other polymer particles, as described herein.

[0017] In another example, as shown in FIG. 3, a method of 3D printing 300 can include iteratively applying individual build material layers, wherein the build material layers can be of polymeric build material including from about 80 wt% to 100 wt% polymer particles based on a total weight of the polymeric build material. The polymer particles can have an average particle size from about 20 pm to about 150 pm and a melting temperature from about 90 °C to about 350 °C. In further detail, based on a 3D object model, the method can include selectively applying a fusing agent to individual build material layers to define individually patterned layers of a 3D object. The fusing agent can include from about 1 wt% to 100 wt% fusing compound based on a total weight of the fusing agent. In still further detail, the method can include iteratively applying a focused microwave energy field to the individual build material layers after or while selectively applying the fusing agent so that an uppermost build material layer of polymeric build material receives the focused microwave energy field to cause polymer particle fusion to occur where the fusing agent is in contact with the polymeric build material, wherein polymer particle fusion does not occur in the absence of fusing agent applied to the polymeric build material. Applying the focused microwave energy field can provide heating and allow for cooling of the uppermost build material layer and an n-1 build material layer within a maximum temperature range of about 15 °C to about 40 °C. In one example, when iteratively applying a focused microwave energy field, this can include applying the focused microwave energy field from an array of microwave energy emitters that individually emit from about 20 Watts to about 50 Watts toward the uppermost build material layer. The fusing agent can be a liquid suspension, with the fusing compound including fusing particles selected from carbon black pigments, metal nanoparticles, ferrites, ferromagnetic metals, non-stoichiometrically reduced titania, ceramics, silicon carbide, copper oxide, manganese oxide, alumina, sodium

metasilicate, talc, kaolin, metal salts, metal nitrates, or a combination thereof. In further detail, the fusing compound can be an organic solvent fusing compound, such as a C2 to C9 polyol. The fusing agent can include both fusing particles as well as organic solvent as the fusing compounds. The polymer particles can be a polyamide. Other fusing compounds (including liquid suspensions with fusing particles) can be used, as well as other polymer particles, as described herein.

[0018] The methods described herein can benefit from and utilize many of the 3D printing system components and compositions described herein. Thus, it is noted that when discussing the three-dimensional printing systems and/or methods of the present disclosure, these discussions can be considered applicable to other examples, whether they are explicitly discussed in the context of that example. For example, in discussing a polymeric build material related to the 3D printing systems, such disclosure is also relevant to and directly supported in context of the 3D printing methods, and vice versa.

Microwave Fusion

[0019] FIGS. 4-6 further detail aspects of microwave fusion of polymeric build material (or polymer particles thereof) using a focused microwave energy field and fusing agent. The term“focused” when referring to microwave energy or microwave energy emitters refers to directional microwave energy that is delivered in a cone-like or columnar manner from a microwave energy emitter. This type of microwave energy can be visualized to cylindrical concentrated microwave energy, similar to that of laser light but at a longer wavelength or frequency (in the microwave range). The footprint of the microwave energy emitted can be smaller than the wavelength of the microwave energy due to its directionality and due to the proximity between the focusing microwave energy emitter and the particle bed of particulate build material. If the spot emitted at the particle bed is not circular, the diameter can be determined by normalizing the total area of the spot size and calculating the diameter based on the area of the corresponding circular area.

[0020] To illustrate by way of example, in FIG. 4, a facial view of example fluid ejectors 120, 130 are shown along with a facial view of a focused microwave emitter array 150. It is noted that in this example the fluid ejectors and/or focused microwave emitter array may include additional components and some of the components described herein may be removed and/or modified. Regarding the fluid ejectors, in one example, fluid ejector 120 may be for ejecting a fusing agent onto a particulate build material (not shown in this FIG., but shown in FIG. 1 ), and fluid ejector 130 may be for ejecting a second fluid that is usable with 3D printing systems and methods shown and describe herein, such as a detailing agent, a colored agent or ink, etc. The fluid ejectors shown in this example include an array of printheads 124, 134. That stated, the fluid ejectors and printheads could be arranged in any of several arrangements. This particular arrangement can be for bidirectional single-axis printing across a particle bed or build platform but could alternatively be in a static arrangement or in the form of a carriage scanning system that moves along multiple axes. In this example, the individual printheads are arranged in offset columns to provide for delivering fusing agent (or other fluid) across a large swath of the particle bed during printing when scanning perpendicular to bidirectional movement shown by the arrow in FIG. 4. The fluid ejectors can be individually controllable and may have relatively high resolutions, e.g., 200 dpi, 600 dpi, 1200 dpi, or the like. By way of example, fluid ejectors may be operable as thermal inkjet printheads, piezoelectric printheads, or the like.

[0021 ] Also shown in FIG. 4 is the focused microwave emitter array 150 can include a plurality of focusing microwave energy emitters 140, with tips 122 thereof showing in this FIG. In this example, the individual emitters are arranged in a plurality of columns. Though other arrangements can be used, in this example, the columns may be arranged in a direction that is perpendicular to a scan direction of the array that is shown by the bidirectional arrow in FIG. 4. The columns may also be, for example, arranged in offset columns, as shown. This arrangement can provide for a large swath of the polymeric bed to be irradiated with the focused microwave energy field. In further detail, by using focusing microwave energy emitters as shown, in one example, individually emitters can be independently controllable for any of several reasons, including to save energy, to prevent over-fusing or thermal runaway, etc.

[0022] A more detailed view of the focusing microwave energy emitters 140 is shown by way of example in FIG. 5. In this example, individual emitters may be electrically coupled with a microwave energy source 170 and in some examples where applicable, a power splitter 172. A controller 160 is shown in this example that can be used to a control focusing microwave energy emitter(s). The controller may also control the fluid ejectors and/or other components or can be dedicated to controlling the emitter(s). In further detail, by way of example, the diameter of the focusing microwave energy emitter(s) may be from about 2 mm to about 6 mm at a distal end (relative to the bulk of the emitter) of the tip 144, and in some examples, may be arranged in the focused microwave emitter array (150 in FIG. 4) to have a periodicity from about 3 mm to about 12 mm, or from about 4 mm to about 10 mm, for example. [0023] In further detail regarding FIG. 5, the focusing microwave energy emitter(s) 140 can include a feed 148, which may be a coax feed connected to the microwave energy source 170 through the power splitter 172. In one example, the microwave energy source 202 may include three magnetron tubes for the array of microwave energy emitters 120 and the power splitter 204 may provide equal amounts of power to the various focusing microwave energy emitter of the focused microwave emitter array 150 shown in FIG. 4. The focusing microwave energy emitter(s) can also include a resonator 146, which may equivalently be termed a coax resonator. Both structures are shown as being housed within a protective layer 149. In this example, a gap 147 may be provided between an emitting end of the feed and a receiving end of the resonator. A portion of the protective layer may be positioned in the gap, in one example, although in other examples, a different type of dielectric material may be provided in the gap. The gap can be used to provide for the resonator to be capacitively coupled to the feed. Furthermore, the resonator may be coupled to the impedance of the coax with the impedance of the end of the tip having a minimum reflection of energy. As a result, energy emitted from the tip may be in the form of a focused microwave energy field and can be used for heating the fusing compound of the fusing agent that has been applied to a layer of the particulate build material rather than being reflected back to the microwave energy source and dissipated as heat at the microwave energy source. In accordance with a few examples, the feed, the resonator, and the tip may be formed of the same type of electrically conductive material or different types of materials with respect to each other, provided they can propagate and emit a focused microwave energy field. By way of example, the material for these and potentially other structures cooperating in generating a focused microwave energy field directed from the tip can include solid copper, stranded copper, copper plated steel wire, or the like.

[0024] In operation, the tip 144 may be positioned sufficiently close to an uppermost particulate build material layer (shown at“n” in FIG. 1 ) to propagate a focused microwave energy field therein, and furthermore in some examples, also into the n-1 build material layer. In still another example, the focused microwave energy field produced may be tuned so that it is insufficient to generate additional fusing at the n-2 build material layer. As mentioned, the tip can be sized to accomplish this, having a diameter at its most distal end (relative to the bulk of the focusing microwave energy emitter) of about 1 mm to about 6 mm, or from about 2 mm to about 4 mm. In addition, the tip 122 may have a relatively small diameter, e.g., between about 2 mm and about 4 mm, to focus the microwave energy 124. Furthermore, the microwave energy that can be used can have a frequency from about 1 GHz and about 300 GHz, from about 2 GHz to about 60 GHz, or from about 2 GHz to about 25 GHz. Some bands that may be used include S Band microwave energy, e.g., 2 GHz to 4 GHz, C Band microwave energy, e.g., 4 GHz to 8 GHz, or X Band microwave energy, e.g., 8 GHz to 12 GHz.

[0025] FIG. 6 provides a simple illustration showing how polymer particles 106A, 106B of polymeric build material, in the presence of the focused microwave energy field 142 and fusing agent 122, can be brought to a temperature typically above its melting temperature to cause adjacent polymer particles to become fused or coalesce together. For example, the adjacent polymer particles shown prior to heat fusing can coalesce together to form a fused polymer 106C, as shown after heat fusing. The fused polymer formed can be a portion of a larger fused 3D object as a result of surface energy reduction occurring as a result of increased surface temperature of the polymer particles. In this example, as shown after heat fusion, the dotted lines represent the original size of the polymer particles and the solid lines represent an example size of the fused polymer which includes polymer from the polymer particles with a bridging portion 107 therebetween resulting from microwave- and fusing agent-induced coalescence or fusion. In accordance with this, the terms“melt” or“melting” with respect to the polymer particles can be defined as the temperature at which individual particles transition from a semi-crystalline structure to an amorphous structure as a result of heating. Sometimes melting temperatures fall within a range, as they can be difficult to pinpoint. Thus, the “melting” temperature as defined herein can include temperatures at which a surface of the particles of adjacent particles transition sufficiently to fuse together, such as that shown in FIG. 6, even if adjacent particles retain some of their original form. The terms “fuse,”“fusing,”“fused,” etc., can be used to describe the coalescence of adjacent particles which become partially or fully joined together after a thermal event. Thus, when cooled, previously separate particles are joined as a single monolithic 3D object or portion thereof.

[0026] In some examples, coalescence or fusing may be minimal prior reaching polymer particle melting temperatures; however, when the melting point of a polymer is reached or exceeded, the viscosity tends to drop, leading to further coalescence or fusion. In some instances, where the fusing compound selected is also a plasticizer for the polymer particles, the fusing compound can also assist with decrease the viscosity of the polymer particles of the polymeric build material, and can thus in some cases, decrease the melting point of the polymer. Both the melting temperature and the plasticizing effect of the fusing compound (or if a separate plasticizer is included) can work to improve selectivity of printed voxels (pixels in three-dimensional space with z- axis defined by the build material layer depth), thereby increasing the fusing and density of the 3D parts, in some instances. Thus, according to some examples, the fusing agent which carries a fusing compound can be used to enhance absorption of microwave energy emitted from the plurality of focusing microwave energy emitters to heat the particulate build material to a temperature that is sufficient to cause the build material to melt at the surface and become fused together.

[0027] In specific examples, the 3D printing process described herein can be implemented using controlled thermal processes with relatively small thermal processing tolerance windows in order to produce consistent, high-density, high quality parts. Without such controls, 3D object inhomogeneities can be introduced due over fusing and/or thermal runaway. Thus, by controlling the focused microwave energy field applied, e.g., energy delivered at the tip (using Watts, accounting for energy emitted and dwell time), and controlling the penetration depth into the particulate build material with respect to energy levels and dwell time to cause polymer particle fusion, greater part densities and part consistency can result. When designing a system or carrying out a method as described herein, when selecting materials, things that can be considered include the boiling point of the fusing compound relative to the melting point of the polymer particles, the rate of conduction when both are combined, heat capacity of the materials, the depth of the build material layers to be used, the average size of the polymer particles, the microwave energy output at the emitter tip, the dwell time of the microwave energy, e.g., as the tips pass over, and the like.“Dwell time” can essentially be described as exposure time. This can be considered with respect to energy output and the dwell time, considering that the focusing microwave energy tips described herein can be highly directional to delivery to a relatively small area or spot of the particulate build material, e.g., smaller than the wavelength of the microwave energy. This type of microwave energy can be visualized to cylindrical concentrated microwave energy, similar to that of laser light but at a different frequency.

Fusing Agent

[0028] In accordance with examples of the present disclosure, the 3D printing systems and methods can utilize a fusing agent to provide a medium for focused microwave energy interaction, to thus generate enough heat to cause polymer particle fusion. The fusing agent can include a fusing compound at from 1 wt% to 100 wt% based on the total content of the fusing agent. Thus, fusing agent can be 100 wt% fusing agent(s), or alternatively, can be carried by or admixed with a liquid vehicle that includes components that are not fusing agent, e.g., water, polymer binder, surfactant, solvents less volatile than water, biocide, colorant, etc. Regardless of whether there is a liquid vehicle carrier or not, the fusing agent can include multiple fusing compounds, such as multiple organic solvent fusing compounds, multiple particulate fusing compounds, an organic solvent fusing compound and a particulate fusing compound, etc. As a point of clarification, a liquid vehicle can be defined to include organic solvents, water, other liquids, etc. However, in the context of the present disclosure, the liquid vehicle described herein, if used, excludes the content of any organic solvent fusing compound, as the organic solvent fusing compound is the active ingredient that us used to fuse the polymer particles in the presence of the focused microwave energy field. In accordance with this, if there is organic solvent fusing compound present in a fusing agent, the total fusing compound content can be from about 1 wt% to 100 wt% of the fusing agent. In further detail, the organic solvent fusing compound can be present in the fusing agent at from about 10 wt% to 100 wt%, from about 20 wt% to about 100 wt%, from about 30 wt% to about 100 wt%, from about 40 wt% to about 100 wt%, from about 50 wt% to about 100 wt%, from about 10 wt% to about 99 wt%, from about 10 wt% to about 95 wt%, from about 20 wt% to about 99 wt%, from about 30 wt% to about 95 wt%, from about 2 wt% to about 20 wt%, from about 5 wt% to about 30 wt%, etc. In some examples, when applying to the polymeric build material, the organic solvent fusing compound (within the fusing agent) can be applied at a fusing compound to polymer particle weight ratio from about 1 :50 to about 1 :1 , from about 1 :40 to about 1 :2, from about 1 :30 to about 1 :3, from about 1 :20 to about 1 :3, or from about 1 : 10 to about 1 :1 , for example. As a note, when using thermal jetting technology in particular, polar solvents can be used to dissolve or admix them with water. As polar solvents also tend to also absorb microwave energy, there may be many organic solvents that can work for both purposes, e.g., as a fusing compound and as an organic solvent to mix with water to provide good thermal jettability. To control fusing temperatures with some of these organics solvents, loading of organic solvents generally (which may also be organic solvent fusing agents) can be kept at relatively low concentration ratios within the particulate build material in some instances. Furthermore, by using organic solvents having a volatility that may evaporate partially off prior to sweeping with microwave energy can ameliorate some overheating in some instances. Additionally, lower boiling point solvents can be used that may also evaporation off upon exposure to the microwave energy can also reduce some overheating issues. That stated, in some other instances, these types of organic solvents can be used to provide appropriate heating within some systems as may be useful for a particular application.

[0029] If the fusing agent selected for use are in the form of solid particles, and there is not organic solvent fusing compound present in the fusing agent, then the fusing compound can be present at from about 1 wt% up to the weight percentage of solids that can be suspended and successfully ejected from jetting architecture, e.g., from about 1 wt% to about 30 wt%, from about 1 wt% to about 20 wt%, from about 1 wt% to about 10 wt%, from about 5 wt% to about 30 wt%, etc. With piezo ejectors, the ranges can have wider latitude than with thermal ejectors, for example. Thus, if the fusing compound is in the form of dispersed particles, application of fusing compound (within the fusing agent) can be applied to the particulate build material at a fusing compound to polymer particle weight ratio of about 1 :200 to about 1 :5, from about 1 : 150 to about 1 :10, from about 1 : 100 to about 1 :20, from about 1 :200 to about 1 :20, or from about 1 :200 to about 1 :40, for example. Carbon black is an example of a fusing compound in the form of a liquid suspension with fusing particles as the fusing compound carried by a liquid vehicle for ejection onto a particulate build material. Other particulate fusing compounds that can be used include metal nanoparticles, ferrites, and ferromagnetic metals, such as Fe, Ni, Cu, Al, Zn, Ag, Au, Co, alloys thereof, or mixtures thereof. In some examples, the fusing compound can be a non-stoichiometrically reduced titania, e.g., TiOx where x is from 1.92 to 1.99. With this compound, some of the stoichiometric oxygen is removed making the titania an oxygen receptor. Other particulate fusing compounds that can be used include ceramics including oxide or non oxide ceramics, silicon carbide, copper oxide, manganese oxide, alumina, sodium metasilicate, talc, kaolin, various metal salts, metal nitrates such as alkali metal nitrates (Li, Na, K, etc.) and alkaline earth metal nitrates (Ca, Mn, etc.), or any of the

aforementioned in combinations thereof.

[0030] As a note, there can be fusing agents that include both organic solvent fusing compound and particulate fusing compound. These can respectively be present at the concentrations described previously, and can be applied at weight ratios to the particulate build material also described previously. In some instances, concentrations at the lower end of the ranges may be usable to achieve good heating profile when both types of fusing compounds are present, but not in every instances.

[0031 ] The organic solvent fusing compound that is used can have a higher boiling temperature than the melting temperature of the polymer particles of the polymeric build material. This can be particularly useful if you want to introduce colorless fusing agent, e.g., to provide parts the color or near color of the polymeric build material or to provide a white or near white palate to add colorant thereto. With microwave energy, heating profiles can be more normalized across range of colors compared to other forms of energy, such as IR energy. For example, some colorants absorb more IR energy than others, and this can cause over-fusing of some colors over others. By using organic solvent fusing compounds as described herein, there can be the ability to decouple color from fusion in a manner with more flexibility than with other energy sources. As an example, C2 to C9 polyols, and in some examples, C5 to C9 diols can be particularly useful with many different types of polymer particles, as they cause fusion and do not brown significantly if at all.

[0032] As mentioned, the fusing compound can be ejected alone (such as with piezo ejectors), or there can be water and other components included in the fusing agent, e.g., fluid fusing compound or dispersed particle fusing compounds. For example, the fusing agent can include colorant, e.g., dye and/or pigment to impart color to the fabricated part, surfactant, organic co-solvent, anti-kogation agent, buffer, biocide, sequestering agent, viscosity modifies, polymeric binder, or the like. In other words, liquid vehicle formulations can be prepared which include other ingredients, such as other organic co-solvents that may not be a plasticizer for the polymeric build material, but which is added in for a different purpose, e.g., jettability, jetting reliability, decap performance, viscosity modification, etc. It is noted that these or other additional components may provide or enhance microwave heating or may be invisible or have high transmittance relative to microwave energy. The formulation as a whole can be considered when determining heating efficiency to establish processing properties such as energy input, dwell time, avoiding over-fusing and thermal runaway, etc.

[0033] With respect to the liquid vehicle, if used in a fusing agent to carry the fusing compound, there can be water and organic co-solvents present (other than organic solvents that act as fusing agents, which are defined above and calculated separately with respect to total content in the fusing agent). Example organic co solvents that can be used include aliphatic alcohols, aromatic alcohols, various diols less volatile than C2-C9 diol (as described elsewhere herein as fusing compounds), glycol ethers, polyglycol ethers, caprolactams, formamides, acetamides, and long chain alcohols. Examples of such compounds include ethylene glycol alkyl ethers, propylene glycol alkyl ethers, higher homologs (C6-C12) of polyethylene glycol alkyl ethers, N-alkyl caprolactams, unsubstituted caprolactams, both substituted and unsubstituted

formamides, both substituted and unsubstituted acetamides, and the like. For further clarity, it is noted that there are organic solvents listed herein and some not listed herein that can absorb microwave energy and have a high enough boiling, including many polar solvents, to be used as fusing compounds as defined herein. Thus, formulation can be carried out with these considerations in mind. Some specific organic solvent fusing compounds can include C2-C9 polyols, e.g., diols, triols, etc., such as 1 ,3- propanediol, 1 ,2-butanediol, glycerol, 1 ,5-pentanediol, 1 -6-hexanediol, tripropylene glycol, etc. 1 -hydroxyethyl-2-pyrrolidone is another example of an organic solvent fusing agent, and though not a polyol, it includes multiple oxygens, including a hydroxyl group and an oxygen with a double bond attached the 5-membered ring structure. There are still others that have a boiling point too low relative the melting temperature of the polymer particles to act as a fusing compound. These latter organic co-solvents may be useful for formulation or for jettability purposes (or for some other purpose), but may be more likely to mostly leave the finished 3D part after process (over time), and thus are not good for use as a fusing compound perse, though they can be present in the fusing agent formulation. In further detail, organic solvent fusing compounds and/or other organic co-solvents that may not be fusing compounds can be included for their plasticizing effect on the polymer particles of the polymeric build material.

[0034] As mentioned, one or more non-ionic, cationic, and/or anionic surfactant can be included, ranging from 0.01 wt% to 20 wt%, if present. Examples include alkyl polyethylene oxides, alkyl phenyl polyethylene oxides, polyethylene oxide block copolymers, acetylenic polyethylene oxides, polyethylene oxide (di)esters, polyethylene oxide amines, protonated polyethylene oxide amines, protonated polyethylene oxide amides, dimethicone copolyols, substituted amine oxides, or the like. The amount of surfactant added to the formulation of this disclosure may range from 0.01 wt% to 20 wt%. Suitable surfactants can include, but are not limited to, liponic esters such as Tergitol™ 15-S-12, Tergitol™ 15-S-7 available from Dow Chemical Company, LEG-1 and LEG-7; Triton™ X-100; Triton™ X-405 available from Dow Chemical Company; and sodium dodecylsulfate.

[0035] Consistent with the formulation of this disclosure, various other additives can be employed to enhance properties of the jettable fluids for specific applications. Examples of these additives are those added to inhibit the growth of harmful microorganisms. These additives may be biocides, fungicides, and other microbial agents, which are routinely used in ink formulations. Examples of suitable microbial agents include, but are not limited to, NUOSEPT® (Nudex, Inc.), UCARCIDE™ (Union carbide Corp.), VANCIDE® (R.T. Vanderbilt Co.), PROXEL® (ICI America), and combinations thereof. Sequestering agents, such as EDTA (ethylene diamine tetra acetic acid), may be included to eliminate the deleterious effects of heavy metal impurities, and buffer solutions may be used to control the pH of the ink. From 0.01 wt% to 2 wt%, for example, can be used. Viscosity modifiers and buffers may also be present, as well as other additives to modify properties of the ink as desired. Such additives can be present at from 0.01 wt% to 20 wt%. The liquid vehicle can also include dispersants if there are solids, such as pigment, that should be dispersed in the jettable fluid for purposes of retaining solid suspension, jettability, etc. In one example, the liquid vehicle can be predominantly water.

Detailing Agent

[0036] A detailing agent may be applied on the build material layers in some examples to assist with cooling during the formation of the portions of the 3D object. More specifically, the detailing agent may, for example, be applied at boundaries along where the fusing agent is applied to provide cooling at the boundary. As with the fusing agent, the detailing agent may be ejected from a fluid ejector, such as piezo or a thermal inkjet ejector. The detailing agent, if included in the systems or used in the methods herein, can thus include a detailing compound that is selected to have a boiling point from about 50 °C to about 300 °C. Additionally, the detailing agent can have a boiling point less than the melting temperature of the polymeric build material to generated cooling adjacent to the 3D object that is being formed. Thus, the detailing agent can be, in some examples, be devoid of the fusing compound or other compound that would cause the polymeric build material to fuse when exposed to the focused microwave energy field as described herein. If fusing compound (such as an organic solvent fusing compound) is included for some purpose other than promoting fusion of the polymer particles, it can be included at a low enough concentration that does not cause polymer particle fusing when exposed to the focused microwave energy field described herein.

[0037] Example detailing compounds that can be used include water, iso-butanol, n-butanol, tert-butanol, iso-propanol, n-propanol, chlorobenzene, chloroform,

cyclohexane, diglyme, dimethylformamide, dioxane, ethyl acetate, heptane, n-hexane, tetrahydrofuran, toluene, xylene, or a combination thereof. The detailing agent can include, for example, from 1 wt% to 100 wt% of the detailing compound. For example, the detailing compound can be 100 wt% water if the melting temperature of the polymer is from about 50 °C to about 300 °C above 100 °C (the boiling point of water at sea level). The detailing agent can likewise be an aqueous formulation of water and the detailing compound, for example. In some examples, both water and the added detailing compound can both be detailing compounds relative to the polymer particles of the polymeric build material, depending on the temperature differential. There can also be other components in the detailing agent, including those previously described with respect to the fusing agent, such as colorant, e.g., dye and/or pigment to impart color to the fabricated part, surfactant, organic co-solvent, anti-kogation agent, buffer, biocide, sequestering agent, viscosity modifies, polymeric binder, or the like.

[0038] In some examples, when applying the detailing agent to the polymeric build material layer, the weight ratio of detailing compound to polymer particles can be from about 1 : 15 to about 1 :1 , from about 1 : 10 to from about 1 :1 , from about 1 :5 to about 1 :2, from about1 :5 to about 1 :1 , from about 3:1 to about 1 :1 , or from about 5:1 to about 2:1 , or from about 3: 1 to about 1 : 1 , for example.

[0039] Polymeric Build Material

[0040] The polymeric material can include any polymer particles that that can exhibit enough microwave energy transmittance that to resist reaching melting temperatures in the absence of added fusing agent to a selected portion of the polymeric build material intended for undergoing polymer particle fusing. In some specific examples, the polymer particles can be polymerized or otherwise formulated or selected to have a high transmittance to the microwave energy or to the specific range of microwave energy that is to be emitted as a focused microwave energy field from the focusing microwave energy emitter(s). The term“high transmittance” can be defined herein to be from about 90% transmittance to 100% transmittance, or in some examples, from about 92.5% to 100%, from about 95% 100%, from about 90% to about 97.5%, or from about 92.5% to about 97.5%. Transmittance outside of these ranges can likewise be used provided the microwave energy used to fuse the polymeric build material in the presence of fusing agent is insufficient to fuse the polymer particles when exposed to the focused microwave energy field.

[0041 ] Examples of polymer particles that can be included in the polymeric build material one or multiple species of the following classes of polymers, namely

polyamides, polyethylenes, polypropylenes, thermoplastic polyurethanes (TPU), polytetrafluroethylenes, polystyrenes, polyvinyl chlorides, polyacetals, poly(lactic acid)s, polycarbonates, polyacrylates, methyl acrylates, polybutadienes, polyvinyl difluorides, polyethers, poly(C2-C4 terephthalates)s, e.g., polyethylene terephthalate (PET), polyether ether ketones (PEEK), polyesters, acrylonitrile butadiene styrenes, polyether ketoneketones (PEKK), polyacrylamides, polyacrylonitrile, poly(phenyl sulfide)s, or a combination thereof. The selection of the polymer can take into consideration the focused microwave energy field frequency, energy concentration, and/or dwell time in selecting the polymer particles for use. In some examples, the polymer particles can be semi-crystalline thermoplastic materials with a relatively wide temperature differential between the melting point and re-crystallization, e.g., greater than 5°C. Some more specific examples of the polymeric build material in the form of powders or particulates can include polyamides (PAs or nylons), such as nylon 6 (PA 6), nylon 8 (PA 8), nylon 9 (PA 9), nylon 11 (PA 11 ), nylon 12 (PA 12), nylon 66 (PA 66), nylon 612 (PA 612), nylon 812 (PA 812), and other polyamides. Core shell polymer particles of these materials may also be used. In one specific example, the particulate polymer can be a polyamide, such as nylon 12, which can have a melting point from about 175°C to about 200°C. With this particular type of polymer particle, e.g., polyamides, C2 to C9 polyols can work well as the fusing compound (of the fusing agent), and in particular, C5 to C9 diols can be used with good coalescence/fusion with little browning of the otherwise white particles.

[0042] More generally, the polymeric build material can have a melting point ranging from about 90 °C to about 350 °C, from about 100 °C to about 300 °C, from about 125 °C to about 275 °C, from about 150 °C to about 250 °C, etc. As examples, the polymeric build material can be a polyamide having a melting point of about 170 °C to about 190 °C, or a thermal plastic polyurethane having a melting point ranging from about 100°C to about 165°C. In further detail, the polymeric build material can be made up of similarly sized particles that are relatively homogenous in size or can have a wider particle distribution profile. The term“size” or“average particle size” is used herein to describe diameter or average diameter, which may vary, depending upon the

morphology of the individual particle. In an example, the respective particle can have a morphology that is substantially spherical. A substantially spherical particle (e.g., spherical or near-spherical) has a sphericity of >0.84. Thus, any individual particles having a sphericity of <0.84 are considered non-spherical (irregularly shaped). The particle size of the substantially spherical particle may be provided by its - diameter, and the particle size of a non-spherical particle may be provided by its average diameter (i.e. , the average of multiple dimensions across the particle) or by an effective diameter, which is the diameter of a sphere with the same mass and density as the non-spherical particle. In accordance with this, the average particle size can be from about 20 pm to about 150 pm, from about 50 pm to about 125 pm, or from about 60 pm to about 100 pm.

[0043] It is to be understood that the polymeric build material may include, in addition to the polymer particles, up to 20 wt% of other components, such as a charging agent, a flow aid, etc., or combinations thereof. However, in some examples, the polymeric build material can be at about 100 wt% polymer particles. Thus, the polymer particles can be present in the polymeric build material at from about 80 wt% to 100 wt%, from about 85 wt% to 100 wt%, from about 90 wt% to 100 wt%, from about 95 wt% to 100 wt%, from about 80 wt% to about 99 wt%, and so forth. Charging agent(s), for example, may be added to suppress tribo-charging. Examples of suitable charging agent(s) include aliphatic amines (which may be ethoxylated), aliphatic amides, quaternary ammonium salts (e.g., behentrimonium chloride or cocamidopropyl betaine), esters of phosphoric acid, polyethylene glycol esters, 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 from Clariant Int. Ltd. In an example, if charging agent(s) is added, it can be added in an amount ranging from greater than 0 wt% to about 10 wt% based upon the total wt% of the polymeric build material. Flow aid(s) may be added to improve the coating flowability of the polymeric build material. Flow aid(s) may be particularly desirable when the particles of the polymeric build material are on the smaller end of the particle size range. The flow aid can improve the flowability of the polymeric build material 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, if flow aid is included, it can be added in an amount ranging from greater than 0 wt% to less than about 10 wt%, based upon the total wt% of the polymeric build material.

Definitions

[0044] It is noted that, as used in this specification and the appended claims, the singular forms of”a,”“an,” and“the” include plural referents unless the context clearly dictates otherwise.

[0045] As used herein,“liquid vehicle” refers to a liquid carrier that can be used to carry a fusing compound in a fusing agent, colorant in an ink, a detailing compound in a detailing agent, etc. Liquid vehicles are described herein in detailing with respect to the fusing agent but can be used in a detailing agent (carrying detailing compound) or in a separate ink composition carrying colorant. A wide variety of liquid vehicles may be used with the systems and methods of the present disclosure, including, surfactants, solvents, co-solvents, anti-kogation agents, buffers, biocides, sequestering agents, viscosity modifiers, surface- active agents, water, etc.

[0046] As used herein,“ejector,”“ejecting,”“ejected,” or the like refers to digital jetting or ejection of various compositional agents described herein. Jetting architecture can include thermal or piezo architecture with printheads with printing orifices or openings suitable for ejection of small droplets of fluid. In some examples, the fluid droplet size can be from about 2 picoliters to about 100 picoliters, from about 2 picoliters to about 50 picoliters from about 2 picoliters to about 40 picoliters, from about 2 picoliters to about 30 picoliters, from about 2 picoliters to about 20 picoliters, from about 2 picoliters to about 10 picoliters, from about 3 picoliters to about 20 picoliters, or from about 3 picoliters to about 10 picoliters, or from about 3 picoliters to about 8 picoliters, etc.

[0047] As used herein, the term“about” is used to provide flexibility to a numerical range endpoint by providing that a given value may be“a little above” or“a little below” the endpoint. The degree of flexibility of this term can be dictated by a particular variable and determined based on experience and the associated description herein.

[0048] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience.

However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary.

[0049] Concentrations, amounts, and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. As an illustration, a numerical range of“about 1 wt% to about 5 wt%” should be interpreted to include not only the explicitly recited values of about 1 wt% to about 5 wt%, but also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3.5, and 4 and sub-ranges such as from 1 -3, from 2-4, and from 3-5, etc. This same principle applies to ranges reciting only one numerical value. Furthermore, such an interpretation should apply regardless of the breadth of the range or the characteristics being described.

EXAMPLES

[0050] The following illustrates several examples of the present disclosure.

However, it is to be understood that the following are only illustrative of the application of the principles of the present disclosure. Numerous modifications and alternative compositions, methods, and systems may be devised without departing from the spirit and scope of the present disclosure. The appended claims are intended to cover such modifications and arrangements.

Example 1 - Evaluation of Nylon-12 vs. Various Fusing Compounds Exposed to

Microwave Energy

[0051 ] Nylon-12 particulates having an average particle size of in the range of about 30 pm to about 70 pm was evaluated for fusing properties when exposed to microwave energy. The Nylon-12 particles were also evaluated for fusing properties when various organic solvent fusing compounds were applied thereto. In this evaluation, a reactor autoclave filled with water and the water. An Ethos UP microwave oven was used to heat the water, which was set up with a feedback loop thermocouple

temperature control. The power of the microwave oven was cycled on and off to support a prescribed temperature ramp for the water within the reactor. When in the“on” mode, the energy ranged from about 200 Watts to about 1000 Watts. The on and off cycles and/or the range of microwave energy inputs were used to provide an even ramp up of water temperature from room temperature to about 220 °C (under pressurized conditions), taking about 5 minutes. After reaching 220 °C, the temperature was held constant for 1 full minute. 20 ml_ vials of various samples of Nylon-12 powder (with and without organic solvent fusing compound) were exposed to the water bath in the reactor during ramp up and throughout the 1 -minute temperature hold period. The temperature of the nylon-12 powder was not otherwise controlled, but rather increased in

temperature as influenced by the water temperature and dwell time. The temperature of the nylon-12 powder (with and without fusing compound) generally reached

temperatures above about 185 °C during the experiment. Notably, the vials themselves were not sealed, allowing for any fusing compound solvents present to freely evaporate.

[0052] The fusing compounds used in this study were prepared using an acoustic mixer (60 g acceleration for 4 minutes). Colorants in some instances were added to confirm the uniformity of mixing, but once uniformity of mixing techniques were confirmed, colorant was not used in every instance. These studies were not conducted simultaneously, but rather in three separate evaluations at different times. The protocol was conducted the same for the various samples. Table 1 , as follows, shows the contents of the various samples in the various vials.

Table 1 - Fusion of Nylon-12 Polymer Particles

[0053] Note that in Table 1 above, the measured temperatures for the

compositions present in the various vials were at about 5 minutes after removing. All samples had 3 grams of Nylon-12 powder and variable amounts of fusing compound (and water in some instances), as outlined in Table 1. Water did not cause fusion.

[0054] In the case of 1 ,3-propanediol, the presence of 50 wt% water disable the 1 ,3-propane diol from causing fusion under these conditions, perhaps due the latent heat effect. However, weight ratios of 1 : 1 , 2:1 , and 3:1 of nylon-12 to fusing compound without added water all caused fusion to occur.

[0055] In the case of the 100 wt% glycerol, as after sonic mixing, the sample were held at 60 °C overnight to evaporate water that was present. The 100 wt% glycerol and the 50 wt% glycerol (in 50 wt% water) caused polymer particle fusion. The 20 wt% glycerol and the 5 wt% glycerol did not generate polymer particle fusion.

[0056] Browning characterization was established for some of the samples. In particular, 1 ,5-pentanediol as evaluated to have generated very good fusion properties as well as very slight browning. Hydroxyethyl-2-pyrrolidone had good fusion properties, also as noted in the Table, but had significant browning. The tripropylene glycol had only moderate browning, but only partial fusing. Others had various levels of browning, but the 1 ,5-pentanediol, which is one of the C5 to C9 diols described herein, exhibited the least amount of browning. Example 2 - Preparation of Colored Fused Polymeric Mass

[0057] A colored fused polymeric mass was prepared in a manner like that described in Example 1 using nylon-12 polymer particles (which were white) and 1 ,5- pentanediol (as the fusing compound). The polymer particles of the polymeric build material and the fusing compound of the fusing agent were admixed at a 1 : 1 w/w ratio.

1 ,5-pentanediol was selected for use because of the very slight browning that occurred in the testing conducted in Example 1. In this example, the polymeric build material included the nylon-12 polymer particles, but also included about 3 wt% of titanium dioxide (T1O2) particles as a whitening agent. The fusing agent included the fusing compound, but also included about 0.1 wt% of a tinting agent. In this example, using the heating protocol described in Example 1 , the polymer particulates with titanium dioxide dispersed therein admixed with the fusing agent including the fusing compound and colorant formed a thoroughly fused monolithic polymeric mass having a pink color.

Example 3 - Fusion of Polymeric Build Material using Fusing Agent and Focused

Microwave Energy Field

[0058] A pre-heater plate was used as a build platform to support and provide heat to polymeric build material with nylon-12 polymer particles. The pre-heater plate in this example is used to raise the temperature of the polymeric build material to a temperature within about 15 °C of the melting temperature of the nylon-12 powder, so that when the focused microwave energy field is applied, the temperature can be raised only a small amount to reach or exceed the melting temperature of the polymer particles. Temperature differentials between pre-heating and fusing temperatures can be greater than or less than this this, e.g., a smaller temperature differential can be used so that less energy input can bring polymer particles to an appropriate fusing temperature.

[0059] Furthermore, a fusing tip was prepared similar to that shown in FIG. 5. Specifically, the fusing tip included a coaxial cable with the following specifications: length 1 meter; total diameter 20 mm; copper central conductor diameter 2 mm, cylindrical outer conductor diameter 20 mm. At the end of the coaxial cable, the central conductor included a gap between the copper central conductor and a resonator. The resonator portion of the conductor was included with a similar coaxial structure. The resonator terminated at a tip configured so that as microwave energy was coaxially supplied to the resonator, the energy was released therefrom in a focused manner, applying a focused microwave energy field footprint of about a 12 mm diameter energy. The shape of the microwave energy field can be described as a narrow or focused “bagel” of microwave energy that is a few mm wide. In operation, the power to the resonator was relatively large, e.g., from about 100 Watts. The components described above were enclosed in a metal box in operation. The ability to regulate the focused microwave energy field supplied to the polymeric build material is further controllable in several ways, including modification of the current supplied to the coaxial cable, the distance between the tip and the particulate build material, through scanning speed, e.g., dwell time) of the tip over the polymeric build materials, etc.

[0060] This equipment was used to test nylon-12 powder with and without the presence of a fusing compound to determine if fusing would occur or how completely fusing would occur. More specifically, nylon-12 (PA2200 powder from EOS GmbH Electro Optical Systems) without any fusing agent was evaluated for fusing, and additionally, the same nylon-12 powder was admixed at about a 1 :1 w/w ratio with 1 ,5- pentande diol. The 1 ,5-pentanediol was prepared to include 2 w% AR52 dye as a tinting agent additive. Thus, the formulation of that was irradiated with the focused microwave energy field was about 50 wt% nylon-12 (polymer particles), 49 wt% 1 ,5-pentanediol (fusing compound), and 1 wt% AR52 dye (colorant or tinting additive). These two different compositions were spread at about 1 mm in thickness and pre-heated to about 90 °C using the pre-heater plate, and then the microwave energy was applied thereto as prescribed. The first sample that included no fusing agent did not fuse under these conditions. The second sample that included the fusing agent was thoroughly fused and had a pink tint.

[0061 ] It is to be understood that this disclosure is not limited to particular process steps and materials disclosed herein because such process steps and materials may vary somewhat. It is also to be understood that the terminology used herein is used for the purpose of describing particular examples. The terms are not intended to be limiting because the scope of the present disclosure is intended to be limited only by the appended claims and equivalents thereof.