Patent Publication Number: US-2019168458-A1

Title: 3d printing heat sinks

Description:
BACKGROUND 
     In three-dimensional (3D) printing, an additive printing process may be used to make three-dimensional solid parts from a digital model. 3D printing may be 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. In 3D printing, the building material may be cured or fused, which for some materials may be performed using heat-assisted extrusion, melting, or sintering, and for other materials, may be performed using digital light projection technology. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of the present disclosure are illustrated by way of example and not limited in the following figure(s), in which like numerals indicate like elements, in which: 
         FIG. 1A  shows a simplified isometric view of an example three-dimensional (3D) printer for generating, building, or printing three-dimensional parts; 
         FIG. 1B  shows a simplified top view of an example plurality of parts in the process of being formed of build materials located in preselected areas of a layer of the build materials; 
         FIG. 1C  shows an example graph of a temperature distribution across a portion of the first part shown in  FIG. 1B  following application of fusing radiation onto the first part; 
         FIG. 2  shows a simplified block diagram of an example computing apparatus that may be implemented in the 3D printer depicted in  FIG. 1A ; and 
         FIGS. 3 and 4 , respectively, depict methods for controlling delivery of liquid droplets to include distributed gaps that are to form heat sinks. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the present disclosure is described by referring mainly to an example thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be readily apparent however, that the present disclosure may be practiced without limitation to these specific details. In other instances, some methods and structures have not been described in detail so as not to unnecessarily obscure the present disclosure. As used herein, the terms “a” and “an” are intended to denote at least one of a particular element, the term “includes” means includes but not limited to, the term “including” means including but not limited to, and the term “based on” means based at least in part on. 
     Disclosed herein are a 3D printer, methods for implementing the 3D printer, and a computer readable medium on which is stored instructions corresponding to the methods. In the methods, a controller may determine a distribution at which gaps are to be formed in the delivery of liquid droplets by a delivery device onto a layer of build materials, in which the gaps are to form heat sinks in the build materials that are to prevent heat spikes from occurring across the build materials. For instance, in forming a part of a 3D object from build materials in a layer, the delivery device may deposit the liquid droplets onto the portions of the build materials that are to be fused together to form the part, in which the liquid droplets may increase absorption of energy by the build materials upon which the liquid droplets have been deposited. In some instances, such as when a relatively large amount of liquid droplets are deposited to form a part, the temperatures of the build materials that are to form the part may not be uniform across the part. That is, there may be areas at which the build materials have temperatures that spike above other areas of the build materials, for instance, due to the larger concentrations of the liquid droplets in those areas and thus, higher heat absorption rates at those areas. 
     According to an example, a distribution at which gaps are formed in the delivery of the liquid droplets may be determined such that the heat spikes may be avoided and that the build materials forming the part may be in thermal uniformity with each other. That is, the gaps may form heat sinks that may absorb heat through thermal bleeding from the build materials upon which the liquid droplets have been deposited. In addition, the build materials in the gaps may also become fused together through absorption of the heat. In one regard, by selectively forming the heat sinks, excess heat from some of the build materials may be dissipated to other build materials that are to form a part to thus enable heat to be more evenly distributed across those build materials. The formation of the heat sinks as disclosed herein may enable build materials to be more evenly fused together to form the part regardless of the part geometry or size. In addition, formation of the heat sinks as disclosed herein may also enable for smaller parts to be formed next to or in relatively close proximity to larger parts as the amount of thermal bleed from the larger parts may be reduced. 
     According to an example, a delivery device may be controlled to deliver the liquid droplets based upon print data, in which the print data may be modified to include the gaps. For instance, a mask that identifies when the gaps are to be formed during the delivery of the liquid droplets may be generated and implemented during application of the liquid droplets onto a preselected area of a layer of build materials to form a part from the build materials in the preselected area. In a further example, another delivery device may be controlled to deliver second liquid droplets into the gaps formed between the liquid droplets. In this example, the second liquid droplets may be detailing agent droplets and the liquid droplets may be fusing agent droplets. In addition, an opposite mask to the one discussed above may be used to identify when the second liquid droplets are to be deposited such that the second liquid droplets may be provided in the gaps. 
     With reference first to  FIG. 1 , there is shown a simplified isometric view of an example three-dimensional (3D) printer  100  for generating, building, or printing three-dimensional parts. It should be understood that the 3D printer  100  depicted in  FIG. 1  may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the 3D printer  100  disclosed herein. 
     The 3D printer  100  is depicted as including a build area platform  102 , a build material supply  104  containing build materials  106 , and a recoater  108 . The build material supply  104  may be a container or surface that is to position build materials  106  between the recoater  108  and the build area platform  102 . The build material supply  106  may be a hopper or a surface upon which the build materials  106  may be supplied, for instance, from a build material source (not shown) located above the build material supply  104 . Additionally, or alternatively, the build material supply  104  may include a mechanism to provide, e.g., move, the build materials  106  from a storage location to a position to be spread onto the build area platform  102  or a previously formed layer of build materials  106 . For instance, the build material supply  104  may include a hopper, an auger conveyer, or the like. Generally speaking, 3D objects or parts are to be generated from the build materials  106  and the build materials  106  may be formed of any suitable material including, but not limited to, polymers, metals, and ceramics. In addition, the build materials  106  may be in the form of a powder. 
     The recoater  108  may move in a direction as denoted by the arrow  110 , e.g., along the y-axis, over the build material supply  104  and across the build area platform  102  to spread the build materials  106  into a layer  114  over a surface of the build area platform  102 . The layer  114  may be formed to a substantially uniform thickness across the build area platform  102 . In an example, the thickness of the layer  114  may range from about 90 μm to about 110 μm, although thinner or thicker layers may also be used. For example, the thickness of the layer  114  may range from about 20 μm to about 200 μm, or from about 50 μm to about 200 μm. The recoater  108  may also be returned to a position adjacent the build material supply  104  following the spreading of the build materials  106 . In addition, or alternatively, a second build material supply (not shown) may be provided on an opposite side of the build area platform  102  and the recoater  108  may be positioned over the second build material supply after forming the layer of build materials  106 . The recoater  108  may be a doctor blade, roller, a counter rotating roller or any other device suitable for spreading the build materials  106  over the build area platform  102 . 
     The 3D printer  100  is also depicted as including a plurality of warming devices  120  arranged in an array above the build area platform  102 . Each of the warming devices  120  may be a lamp or other heat source that is to apply heat onto spread layers of the build materials  106 , for instance, to maintain the build materials  106  within a predetermined temperature range. The warming devices  120  may maintain the temperatures of the build materials  106  at a relatively high temperature that facilitates the selective fusing of the build materials  106 . That is, the warming devices  120  may maintain the build materials  106  at a sufficiently high temperature that enables the build materials  106  upon which fusing agent droplets are provided to fuse together upon receipt of fusing radiation without causing the build materials  106  to otherwise fuse together. The warming devices  120  may be activated in a non-continuous manner such that the build materials  106  may be kept within a predetermined temperature range as various processes, including application of fusing radiation, are performed on the build materials  106 . 
     The 3D printer  100  is further depicted as including a first delivery device  122  and a second delivery device  124 , which may both be scanned across the layer  114  on the build area platform  102  in both of the directions indicated by the arrow  126 , e.g., along the x-axis. For instance, the first delivery device  122  may deposit first liquid droplets as the first delivery device  122  is scanned in a positive x direction  126  and the second delivery device  124  may deposit second liquid droplets as the second delivery device  124  is scanned in a negative x direction  126 . The first delivery device  122  and the second delivery device  124  may be thermal inkjet printheads, piezoelectric printheads, or the like, and may extend a width of the build area platform  102 . The first delivery device  122  and the second delivery device  124  may each include a printhead or multiple printheads available from the Hewlett Packard Company of Palo Alto, Calif. Although the first delivery device  122  and the second delivery device  124  have each been depicted in  FIG. 1  as being formed of separate devices, it should be understood that each of the first delivery device  122  and the second delivery device  124  may be included on the same printhead. For instance, the first delivery device  122  may include a first set of actuators and nozzles in a printhead and the second delivery device  124  may include a second set of actuators and nozzles in the printhead. 
     In other examples in which the first delivery device  122  and the second delivery device  124  do not extend the width of the build area platform  102 , the first delivery device  122  and the second delivery device  124  may also be scanned along the y-axis to thus enable the first delivery device  122  and the second delivery device  124  to be positioned over a majority of the area above the build area platform  102 . The first delivery device  122  and the second delivery device  124  may thus be attached to a moving XY stage or a translational carriage (neither of which is shown) that is to move the first delivery device  122  and the second delivery device  124  adjacent to the build area platform  102  in order to deposit respective liquids in predetermined areas of the layer  114  of the build materials  106 . 
     Although not shown, the first delivery device  122  and the second delivery device  124  may each include a plurality of nozzles through which the respective liquid droplets are to be ejected onto the layer  114 . The first delivery device  122  may deposit a first liquid and the second delivery device  124  may deposit a second liquid. The first liquid and the second liquid may both be fusing agents, may both be detailing agents, or one may be a fusing agent and the other may be detailing agent. A fusing agent may be a liquid that is to absorb fusing radiation (e.g., in the form of light and/or heat) to cause the build materials  106  upon which the fusing agent has been deposited to fuse together when the fusing radiation is applied. A detailing agent may be a liquid that may absorb significantly less of the fusing radiation as compared with the fusing agent. In one example, the detailing agent may prevent or significantly reduce the fusing together of the build materials  106  upon which the detailing agent has been deposited. In other examples, the detailing agent may be implemented to provide coloring to exterior portions of the build materials  106  that have been fused together. 
     The first liquid and the second liquid may also include various additives and/or catalysts that either enhance or reduce radiation absorption. For instance, the first liquid may include a radiation absorbing agent, i.e., an active material, metal nanoparticles, or the like. The first liquid and the second liquid may also include any of a co-solvent, a surfactant, a biocide, an anti-kogation agent, a dispersant, and/or combinations thereof. 
     Although not shown, the 3D printer  100  may include additional delivery devices, e.g., printheads, that may deposit multiple liquids having different radiation absorption properties with respect to each other. By way of example, the multiple liquids may have different colors with respect to each other, may have different chemical compositions (e.g., different reactants and/or catalysts) with respect to each other, or the like. In the example in which the 3D printer  100  may deposit multiple liquids, the 3D printer  100  may include multiple printheads, in which each of the multiple printheads may deposit a liquid having a different radiation absorption property with respect to the other liquids. 
     The first delivery device  122  may be controlled to selectively deliver first liquid droplets onto build materials  106  in a layer  114  of the build materials  106 . The first liquid droplets may be delivered onto preselected areas of the layer  114 , for instance, the areas containing build materials  106  that are to be fused together to form a part of a 3D object. However, instead of delivering the first liquid droplets throughout the preselected areas, according to an example, gaps may be provided in the delivery of the liquid droplets. The gaps may be provided to form heat sinks in the build materials  106 . That is, the build materials  106  within the preselected area that do not receive liquid droplets from the first delivery device  122  may absorb heat from neighboring build materials  106  upon which the liquid droplets have been delivered, for instance, through thermal bleeding from those neighboring build materials  106 . In this regard, as fusing radiation is applied to the build materials  106 , the build materials  106  upon which the liquid droplets have been delivered may become heated through absorption of energy through the liquid droplets and the build materials  106  positioned in the gaps between the delivered liquid droplets may become heated through absorption of heat from the neighboring heated build materials  106 . 
     According to an example, the gaps may be distributed within the preselected area over which the liquid droplets are to be delivered to prevent heat spikes from occurring across the build materials  106  in the preselected area. That is, a determination may be made that the temperatures of the build materials  106  in any number of locations within the preselected area either exceed or may exceed a predetermined threshold temperature either prior to or during application of the fusing radiation. In order to reduce or remove the possibility of the temperatures exceeding the predetermined threshold, gaps may be distributed at the various locations to more evenly distribute heat among the build materials  106  in those locations. That is, for instance, a greater distribution of gaps may be provided in locations that have or are predicted to have higher temperatures to thus distribute heat over a greater area and thus result in thermal uniformity across the build materials  106  in the preselected area. Various manners in which the distribution of the gaps may be determined for various locations within a preselected area of build materials  106  are described in detail below. 
     The second delivery device  124  may be controlled in similar respects to the first delivery device  122  to provide distributions of gaps in a preselected area upon which second liquid droplets are to be delivered by the second delivery device  124 . In addition, in instances in which the 3D printer  100  includes additional delivery devices, similar control features may also be applied to those additional delivery devices. However, in other examples in which the second delivery device  124  is to deliver detailing agent droplets, the second delivery device  124  may be controlled to deposit detailing agent droplets into various gaps formed between the deposited first liquid droplets. In one regard, the detailing agent droplets may facilitate greater heat removal from the various gaps. 
     Following deposition of the first liquid droplets and/or the second liquid droplets onto selected areas of the layer  114  of the build materials  106 , a first radiation generator  130  and/or a second radiation generator  132  may be implemented to apply fusing radiation onto the build materials  106  in the layer  114 . Particularly, the radiation generator(s)  130 ,  132  may be activated and moved across the layer  114 , for instance, along the directions indicated by the arrow  126  to apply fusing radiation in the form of light and/or heat onto the build materials  106 . Examples of the radiation generators  130 ,  132  may include UV, IR or near-IR curing lamps, IR or near-IR light emitting diodes (LED), halogen lamps emitting in the visible and near-IR range, or lasers with desirable electromagnetic wavelengths. The types of radiation generators  130 ,  132  may depend, at least in part, on the type of active material used in the liquid(s). According to an example, the first delivery device  122 , the second delivery device  124 , the first fusing radiation generator  130 , and the second fusing radiation generator  132  may be supported on a carriage (not shown) that may be scanned over the build area platform  102  in the directions denoted by the arrow  126 . 
     Following application of liquid droplets during the multiple passes and following application of the radiation to fuse selected sections of the build materials  106  together, the build area platform  102  may be lowered as denoted by the arrow  112 , e.g., along the z-axis. In addition, the recoater  108  may be moved across the build area platform  102  to form a new layer of build materials  106  on top of the previously formed layer  114 . Moreover, the first delivery device  122  may deposit first liquid droplets and the second delivery device  124  may deposit second liquid droplets onto respective selected areas of the new layer of build materials  106  in single and/or multiple passes as discussed above. The above-described process may be repeated until parts of the 3D object have been formed in a predetermined number of layers to fabricate the 3D object. 
     Additionally, following a liquid deposition operation across a build material layer or following multiple liquid deposition operations across multiple build material layers, the first delivery device  122  and the second delivery device  124  may be positioned adjacent to a wiping mechanism  134 . The wiping mechanism  134  may wipe the nozzles of the first delivery device  122  and the second delivery device  124 , as well as the nozzles of additional delivery devices if included in the 3D printer  100 . The wiping mechanism  134  may be moved to a position in which a surface, such as a cleaning web (not shown), of the wiping mechanism  134  is in contact with the exterior surfaces of the nozzles. The wiping mechanism  134  may be moved in the z-direction as noted by the arrow  136  to remove debris such as, build materials  106 , liquid, dust, etc., that may be in contact with the exterior surfaces of the first delivery device  122  and the second delivery device  124 , to maintain the delivery devices  122 ,  124  at or above desired performance levels. 
     As further shown in  FIG. 1 , the 3D printer  100  may include a controller  140  that may control operations of the build area platform  102 , the build material supply  104 , the recoater  108 , the warming devices  120 , the first delivery device  122 , the second delivery device  124 , the radiation generators  130 ,  132 , and the wiping mechanism  134 . Particularly, for instance, the controller  140  may control actuators (not shown) to control various operations of the 3D printer  100  components. The controller  140  may be a computing device, a semiconductor-based microprocessor, a central processing unit (CPU), an application specific integrated circuit (ASIC), and/or other hardware device. Although not shown, the controller  140  may be connected to the 3D printer  100  components via communication lines. 
     The controller  140  is also depicted as being in communication with a data store  142 . The data store  142  may include data pertaining to a 3D object to be printed by the 3D printer  100 . For instance, the data may include the locations in each build material layer that the first delivery device  122  is to deposit a first liquid and that the second delivery device  124  is to deposit a second liquid to form the 3D object. In one example, the controller  140  may use the data to control the locations on each of the build material layers that the first delivery device  122  and the second delivery device  124  respectively deposit droplets of the first and second liquids. 
     With reference now to  FIG. 1B , there is shown a simplified top view of a plurality of parts  160 ,  162  in the process of being formed of build materials  106  located in preselected areas of a layer  114  of the build materials  106 , according to an example. As shown, a first part  160  may include a relatively larger size than the smaller parts  162 . In addition, the first part  160  and the second parts  162  may be formed through application of liquid droplets  164 , for instance, fusing agent droplets, by the first delivery device  122  onto preselected areas of the layer  114  of build materials  114 .  FIG. 1B  may thus depict a state in which the first delivery device  122  has delivered liquid droplets  164  onto the preselected areas and prior to application of fusing energy onto those areas. 
     The first part  160  is also depicted as including gaps  166  distributed among the liquid droplets  164 . Particularly, the gaps  166  are depicted as being more highly concentrated along a central portion  168  of the first part  160  as compared with the outer and inner portions  170 ,  172  of the first part  160 . In one regard, the gaps  166  may be formed in this manner because the central portion  168  of the first part  160  may absorb greater amounts of energy as compared to the outer and inner portions  170 ,  172  during application of fusing radiation because there may normally be a larger amount of liquid droplets  164  around the central portion  168  than the outer and inner portions  170 . 
     For similar reasons, the liquid droplets  164  delivered to form the second parts  162  may not include gaps because the build materials  106  located in the areas upon which the second parts  162  are to be formed may not exceed a predetermined threshold temperature because of the lower amounts of liquid droplets  164  applied to those build materials  106 . However, in other examples, gaps  166  may be provided between liquid droplets  164  that have been delivered onto build materials  106  to form parts of any size. 
     With reference now to  FIG. 10 , there is shown an example graph  180  of a temperature distribution across a portion of the first part  160  taken along section  174  in  FIG. 1B  following application of fusing radiation onto the first part  160 . That is, the graph  180  shows that the temperatures across the section  174  remain fairly constant or, in other words, the build materials  106  in the first part  160  are in thermal uniformity with respect to each other. For instance, thermal uniformity may be deemed to exist when there is a relatively small temperature difference, e.g., on the range of between about 1° C. to about 10° C., at the different positions across the section  174 . In other examples, thermal uniformity may be deemed to exist when the temperature difference is, for instance, between about 2° C. to about 5° C. In one regard, the gaps  166  may enable excess heat in the build materials  106  that have received the liquid droplets  164  to be dissipated onto the build materials  106  that have not received the liquid droplets  164  to thus cause an even heat distribution across the build materials  106  in the first part  160 . In addition, the build materials  106  that have not received the liquid droplets  164  may still be fused together through absorption of the excess heat while also preventing heat spikes or peaks in some of the build materials  106 . 
     By forming the gaps  166  in selected areas of the part  160 , the build materials  106  forming the part  160  in the layer  114  may be in thermal uniformity with respect to each other such that heat spikes, e.g., temperature differences that exceed greater than a predefined temperature from a low temperature, are prevented or minimized. By maintaining thermal uniformity throughout the build materials  106  forming the part  160 , for instance, the amount of thermal bleeding onto other build materials  106  around those build materials  106  may be significantly reduced. In addition, the amount of thermal bleeding onto build materials  106  in a subsequently deposited layer of build materials  106  may also be significantly reduced. That is, the amount of undesired transfer of heat from one set of build materials  106  to another set of build materials may significantly be reduced to thus enable more accurate formation of parts in multiple layers  114  of build materials  106 . 
     Although particular reference has been made to a single delivery device  122  as delivering liquid droplets  164  onto the build materials  106 , in other examples,  FIG. 1B  may be construed as depicting a state in which multiple delivery devices  122 ,  124  have delivered respective liquid droplets  164  with controlled gaps  166  between the delivered liquid droplets onto the preselected areas of the layer  114  to form the first part  160 . 
     Turning now to  FIG. 2 , there is shown a simplified block diagram of an example computing apparatus  200 . According to an example, the computing apparatus  200  may be implemented as part of the 3D printer  100 . For instance, the computing apparatus  200  may be a command module or other control system of the 3D printer  100 . In another example, the computing apparatus  200  may be separate from the 3D printer  100  and may be, for instance, a personal computer, a laptop computer, a server computer, or the like. It should be understood that the computing apparatus  200  depicted in  FIG. 2  may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the computing apparatus  200  disclosed herein. 
     The computing apparatus  200  is shown as including a controller  140  and a data store  142 , which may be the same as the controller  140  and the data store  142  depicted in and described above with respect to  FIG. 1 . As such, the controller  140  and the data store  142  depicted in  FIG. 2  are not described in detail and instead, the descriptions of the controller  140  and the data store  142  provided above with respect to the 3D printer  100  are intended to also describe these components with respect to the computing apparatus  200 . 
     The computing apparatus  200  may also include a computer readable storage medium  210  on which is stored machine readable instructions  212 - 228  that the controller  140  may execute. More particularly, the controller  140  may fetch, decode, and execute the instructions  212 - 228  to access data pertaining to a 3D object to be printed  212 , access print data for part to be formed by a delivery device  214 , identify properties of the part to be formed/thermal properties of build materials  216 , determine distributions at which gaps are to be formed in the delivery of liquid droplets  218 , update print data  220 , control a delivery device or multiple delivery devices  222 , control a radiation generator or multiple radiation generators  224 , control a build area platform  226 , and control a recoater  228 . As an alternative or in addition to retrieving and executing instructions, the controller  140  may include any number of electronic circuits that include components for performing the functionalities of the instructions  212 - 228 . In any regard, and as discussed above, the controller  140  may communicate instruction signals to the various components of the 3D printer  100  via communication lines such that the components may operate in the manners described herein. 
     The computer readable storage medium  210  may be any electronic, magnetic, optical, or other physical storage device that contains or stores executable instructions. Thus, the computer readable storage medium  210  may be, for example, Random Access Memory (RAM), an Electrically Erasable Programmable Read-Only Memory (EEPROM), a storage device, an optical disc, and the like. The computer readable storage medium  210  may be a non-transitory machine-readable storage medium, where the term “non-transitory” does not encompass transitory propagating signals. 
     Various manners in which the computing apparatus  200  may be implemented are discussed in greater detail with respect to the methods  300  and  400  respectively depicted in  FIGS. 3 and 4 . Particularly,  FIGS. 3 and 4 , respectively, depict example methods  300  and  400  for controlling delivery of liquid droplets to include distributed gaps that are to form heat sinks, in which the heat sinks cause build materials  106  in a preselected area to be thermally uniform with respect to each other. It should be apparent to those of ordinary skill in the art that the methods  300  and  400  may represent generalized illustrations and that other operations may be added or existing operations may be removed, modified, or rearranged without departing from the scopes of the methods  300  and  400 . 
     The descriptions of the methods  300  and  400  are made with reference to the 3D printer  100  illustrated in  FIG. 1A , the illustration depicted in  FIG. 1B , and the computing apparatus  200  illustrated in  FIG. 2  for purposes of illustration. It should, however, be clearly understood that 3D printers and computing apparatuses having other configurations may be implemented to perform either or both of the methods  300  and  400  without departing from the scopes of the methods  300  and  400 . 
     Prior to execution of either of the methods  300  and  400  or as parts of the methods  300  and  400 , the controller  140  may execute instructions  212  stored on the computer-readable medium  210  to access data pertaining to a 3D object that is to be printed. By way of example, the controller  140  may access data stored in the data store  142  pertaining to a 3D object that is to be printed. The controller  140  may determine the number of layers  114  of build material  106  that are to be formed and the locations at which first liquid droplets and/or second liquid droplets are to be deposited on each of the respective layers of build material  106  in order to print the 3D object. The controller  140  may further determine when each of the recoater  108 , the first delivery device  122 , the second delivery device  124 , the first fusing radiation generator  130 , and the second fusing radiation generator  132  are to be moved across the build area platform  102  during each layer processing operation. In other examples, however, a processing device (not shown) outside of the 3D printer  100  may execute instructions to access the 3D object data and to make these determinations. In these examples, the processing device may communicate this information to the controller  140  and the controller  140  may implement this information in executing either or both of the methods  300  and  400 . 
     With reference first to  FIG. 3 , at block  302 , print data that includes instructions for a delivery device  122  to deliver liquid droplets  164  onto a preselected area on a layer  114  of build materials  106  may be accessed. The controller  140  may access the print data, which may include instructions for the delivery of the liquid droplets  164  onto the preselected area to form a part  160  in the layer  114 , from the data store  142  or from an external source (not shown). The controller  140  may also access print data for the delivery of liquid droplets  164  on additional layers  114 . 
     According to an example, the print data may indicate where the liquid droplets are to be deposited with respect to the layer  114  to form the part  160 . In this regard, the print data may include instructions for controlling the timings at which the liquid droplets  164  are to be ejected from nozzles in the first delivery device  122  as the first delivery device  122  is scanned across the layer  114  such that the liquid droplets  164  are delivered onto the build materials  106  in the preselected area of the layer  114 . 
     At block  304 , distributions at which gaps  166  are to be formed in the delivery of the liquid droplets  164  within the preselected area may be determined based upon the print data. The determination of the distributions at which gaps  166  are to be formed may include a determination of the sizes of the gaps  166 , a number of gaps  166  to be included, locations at which the gaps  166  are to be provided, and the like. As discussed above, the gaps  166  may form heat sinks in the build materials  106  that are to result in thermal uniformity across the build materials  106  in the preselected area. For instance, the controller  140  may execute the instructions  218  to determine the distributions at which the gaps  166  are to be formed in the delivery of the liquid droplets  164  such that the heat sinks formed from the gaps  166  result in the thermal uniformity across the build materials  106  in the preselected area. According to an example, the controller  140  may determine the distributions based upon the print data, which may include information such as the size and/or the geometry of the preselected area over which the liquid droplets  164  are to be deposited to form a part  160 . 
     By way of example, the controller  140  may determine whether the part  160  includes sections that are larger than a predefined minimum size and may determine that liquid droplets  164  are to be delivered with gaps  166  to those sections of the part  160  having sizes that are larger than the predefined minimum size. In addition, the controller  140  may determine the distributions of the gaps  166  to correspond to the sizes of the sections. That is, the controller  140  may determine that larger sections are to receive a larger distribution of the gaps  166  and that smaller sections are to receive a smaller distribution of the gaps  166 . In addition, the controller  140  may determine the placements of the gaps  166  based upon the sizes and/or geometries of the sections. That is, for instance, the controller  140  may determine that larger concentrations of gaps  166  are to be placed in locations of the sections at which the temperatures are likely to be the highest. The locations may include, for instance, the central locations of the sections. In this example, a larger concentration of gaps  166  may be formed near the centers of the sections as compared with the edges of the sections. 
     According to an example, the predefined minimum size as well as the distributions of the gaps  166  may be determined based upon various characteristics of the build materials  106  and/or the 3D printer  100  components. For instance, the predefined minimum size as well as the distributions of the gaps  166  may vary for build materials  106  formed of different types of materials as well as for 3D printers having various types of fusing radiation generators. By way of particular example, the predefined minimum size and the distributions of the gaps  166  may be determined through testing of various combinations of build materials and 3D printer components. The results of the testing may be stored in the data store  142  and the controller  140  may access this information in determining the distributions at which gaps  166  are to be formed in the delivery of liquid droplets  164  for the formation of a particular part by a particular 3D printer. In other examples, the controller  140  may determine distributions of the gaps  166  in other manners, such as through calculations of predicted temperatures and correlations with the predicted temperatures and gap sizes and distributions. 
     At block  306 , the delivery device  122  may be controlled to deliver the liquid droplets  164  across the preselected area while forming the gaps  166  at the determined distributions. The controller  140  may execute the instructions  222  to control the delivery device  122  to deliver the liquid droplets  164  while forming the gaps  166 . As discussed in greater detail herein below, the controller  140  may modify the print data such that the print data includes instructions to prevent delivery of the liquid droplets  164  onto areas where the gaps  166  are to be formed and may implement the modified print data in controlling the delivery device  122  to deliver the liquid droplets  164 . According to an example, the controller  140  may generate a mask that identifies the locations at which the gaps  166  are to be formed, e.g., the timings at which the nozzles are to be prevented from firing as the delivery device  122  is scanned across the layer  114  to form the gaps  166 . The controller  140  may apply the mask during delivery of the liquid droplets to thus control the formation of the gaps  166 . In addition, the controller  140  may generate a second mask that identifies the locations at which a second delivery device  124  is to deliver detailing agent droplets, in which the locations may be the locations at which the gaps  166  have been formed. The controller  140  may apply the second mask during delivery of second liquid droplets to control deposition of detailing agent droplets into selected ones of the gaps  166 . 
     With reference now to  FIG. 4 , at block  402 , the controller  140  may access print data that includes instructions for a delivery device  122  to deliver liquid droplets  164  onto a preselected area on a layer  114  of build materials  106 . The controller  140  may execute the instructions  214  to access the print data of a part  160  to be formed from the build materials  106  in the preselected area as discussed above with respect to block  302  in  FIG. 3 . 
     At block  404 , various properties may be identified. For instance, the controller  140  may execute the instructions  216  to identify properties of the part  160  to be formed such as the sizes and geometries of sections of the part  160  as discussed above. The controller  140  may also execute the instructions  216  to identify various thermal properties of the build materials  106  such as the current temperatures of the build materials  106  located at various areas of the layer  114  (e.g., a heat profile of the build materials), temperatures at which the build materials  106  in the layer  114  are predicted to reach during application of fusing radiation onto the build materials  106 , a historic heat map that indicates temperatures across a previously processed layer  114  of build materials  106 , etc. The controller  140  may identify the thermal property information from data stored in the data store  142 , for instance, by a temperature sensor. 
     At block  406 , distributions at which gaps  166  are to be formed in the delivery of the liquid droplets  164  within a first location of the preselected area may be determined based upon the identified properties. For instance, the controller  140  may execute the instructions  218  to determine the distributions at which gaps  166  are to be formed based upon the print data and/or the identified thermal properties to maintain thermal uniformity across the build materials  106  that are to form the part  160 . In addition, the controller  140  may determine the distribution of gaps  166  for a particular location in the preselected area, for instance, a central location of the preselected area. By way of example, the controller  140  may determine that a larger distribution of gaps  166  is to be formed in a location having a higher temperature and a smaller distribution of gaps  166  is to be formed in a location having a lower temperature. The controller  140  may make similar determinations based upon predicted temperatures at the various locations. 
     At block  408 , the print data may be updated to include the determined distributions at which gaps  166  are to be formed. The controller  140  may execute the instructions  220  to update the print data. 
     At block  410 , the controller  140  may determine whether a distribution of gaps  166  for an additional location of the preselected area is to be made. In response to a determination that the distribution of gaps  166  for an additional location of the preselected area is to be made, the controller  140  may determine an additional distribution of gaps  166  for the additional location. For instance, the controller  140  may determine that the additional location does not require as many heat sinks and may thus determine a smaller distribution of gaps  166  for the additional location. In addition, the controller  140  may update the print data with the additional distribution of gaps  166 . 
     The controller  140  may repeat blocks  406 - 410  until distributions of gaps  166  for each of a plurality of locations in the preselected area are determined and the print data has been updated with the distributions of the gaps. Following the “no” condition at block  410 , the delivery device  122  may be controlled to deliver the liquid droplets  164  across the preselected area while forming the gaps  166  at the determined distributions, as indicated at block  412 . The controller  140  may execute the instructions  222  to control the delivery device  122  to deliver the liquid droplets  164  based upon the updated print data to thus cause the selective formation of gaps  166  during the delivery of the liquid droplets  164 . The controller  140  may also execute the instructions  222  to control the second delivery device  124  to deliver fusing agent droplets into various ones of the gaps  166 . 
     At block  414 , the controller  140  may execute the instructions  224  to control either or both of the fusing radiation generators  130 ,  132  to provide fusing radiation onto the build materials  106  in the layer  114 . The fusing radiation may cause the build materials  106  upon which the liquid droplets  164  have been deposited to be fused together, while also causing some of the build materials  106  positioned in the gaps  166  to absorb heat through thermal bleeding from the other build materials  106 , which may also cause those build materials  106  to fuse together. 
     At block  416 , the controller  140  may determine whether an additional layer  114  of build materials  106  is to be formed. The controller  140  may make this determination, for instance, based upon accessed information regarding the 3D part to be printed. In response to a determination that an additional layer is to be formed, a next layer of build materials  106  may be spread on top of the previous layer  114 , as indicated at block  416 . For instance, the controller  140  may execute the instructions  226  to control the build area platform  102  to be moved downward and may execute the instructions  228  to control the recoater  108  to spread additional build materials  106  across the previous layer  114 . In addition, blocks  402 - 418  may be repeated until no additional layers are to be formed, at which point the method  400  may end, as indicated at block  420 . 
     Although particular reference is made herein to a single delivery device  122 , it should be understood that additional delivery devices  124  may be implemented to deliver respective liquid droplets while forming gaps in the delivered liquid droplets without departing from a scope of the present disclosure. 
     Some or all of the operations set forth in the methods  300  and  400  may be contained as utilities, programs, or subprograms, in any desired computer accessible medium. In addition, the methods  300  and  400  may be embodied by computer programs, which may exist in a variety of forms both active and inactive. For example, they may exist as machine readable instructions, including source code, object code, executable code or other formats. Any of the above may be embodied on a non-transitory computer readable storage medium. Examples of non-transitory computer readable storage media include computer system RAM, ROM, EPROM, EEPROM, and magnetic or optical disks or tapes. It is therefore to be understood that any electronic device capable of executing the above-described functions may perform those functions enumerated above. 
     Although described specifically throughout the entirety of the instant disclosure, representative examples of the present disclosure have utility over a wide range of applications, and the above discussion is not intended and should not be construed to be limiting, but is offered as an illustrative discussion of aspects of the disclosure. What has been described and illustrated herein is an example of the disclosure along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the spirit and scope of the disclosure, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.