Abstract:
A fused filament fabrication three dimensional printing system includes a build platform, an extruder for one or more deposition materials, the extruder including at least one nozzle movable relative to the build platform, and a controller configured to control the relative movement between the build platform and the nozzle, and to cause material to be extruded out of the nozzle to form a 3D object on the build platform. The build platform includes a first plate on which the 3D object is formed, a second plate that is positioned vertically below the first plate and defines at least one gap between the first and second plates, and a heating element that is configured to heat the second plate. The first plate defines at least one opening that is configured to allow passage of material extruded from the nozzle into the at least one gap between the first and second plates.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/275,706, filed on Jan. 6, 2016. The disclosure of the prior application is considered part of and is incorporated by reference in the disclosure of this application. 
     
    
     BACKGROUND 
       [0002]    This specification relates to a build plate for three dimensional (3D) printers, such as Fused Filament Fabrication (FFF) 3D printers. 
         [0003]    3D printers employ additive manufacturing techniques, where a product can be built by the addition of materials. Various types of additive manufacturing techniques can be employed, including granular techniques (e.g., Selective Laser Sintering (SLS) and Direct Metal Laser Sintering (DMLS)) and extrusion techniques (e.g., FFF). In addition, various types of 3D printer structures are employed for 3D printing. For example, FFF 3D printers include both Cartesian (xyz) type 3D printers and delta type 3D printers. In typical Cartesian (xyz) type 3D printers, a carriage for a hot end for an extruder, and/or a build platform, is connected with rails that extend in the three different dimensions of printing (x, y &amp; z). In contrast, in typical delta type 3D printers, a carriage for a hot end for an extruder is connected by arms with three rails that extend in only the z direction, and the carriage is moved in three dimensions by independently adjusting the positions of end points of the arms along the three rails. 
       SUMMARY 
       [0004]    This specification relates to 3D printing or additive manufacturing, such as FFF. According to one aspect, a FFF 3D printing system includes a build platform, an extruder for one or more deposition materials, the extruder including at least one nozzle that is movable relative to the build platform, and a controller configured to control the relative movement between the build platform and the nozzle, and to cause material to be extruded out of the nozzle to form a 3D object on the build platform. The build platform includes a first plate on which the 3D object is formed, second plate that is positioned vertically below the first plate and defines at least one gap between the first and second plates, and a heating element that is configured to heat the second plate. The first plate defines at least one opening that is configured and arranged to allow passage of material extruded from the nozzle into the at least one gap between the first and second plates. 
         [0005]    Implementations according to this aspect can include one or more of the following features. For example, the controller can be configured to cause extruded material to flow through one or more of the plurality of openings in the first plate into the gap to thereby form a rivet with a shaft portion that expands within the gap to form a head portion and couple the 3D object on a top surface of the first plate to the head portion of the rivet abutting a bottom surface of the first plate, and to activate the heating element to soften the head portion of the rivet to thereby allow removal of the coupling between the 3D object and the first plate. The controller can be configured to activate the heating element to facilitate softening of the shaft portion and the head portion of the rivet by heating the second plate to above a threshold temperature while the first plate remains below the target threshold temperature. The controller can be configured to activate, during rivet formation, the heating element to heat the second plate to an activation temperature that is below the target threshold temperature to thereby facilitate expansion of the head portion within the gap during rivet formation. In some cases, the heating element can be attached to a bottom surface of the second plate. 
         [0006]    In some implementations, a plurality of spacers can be disposed between the first plate and the second plate to maintain the vertical separation between the first and second plates. The plurality of spacers can be made from a thermally insulating material. Additionally, the first plate can be made from a first material and the second plate is made from a second material, the first material having a thermal conductivity that is lower than that of the second material. In some cases, the first plate and the second plate can be made from a same material, the first plate having a shape that leads to lower thermal conductivity compared to the second plate. The second plate can define two or more gaps between the first and second plates, and the at least one opening of the first plate can be configured and arranged to allow passage of material extruded from the nozzle into the two or more gaps. The second plate can define two or more gaps between the first and second plates, and the first plate defines two or more openings that are configured and arranged to allow passage of material extruded from the nozzle into the two or more gaps. In some cases, the two or more gaps defined between the first and second plates may not be in fluidic connection with one another. 
         [0007]    According to another aspect, a non-transitory computer-readable medium stores instructions that cause one or more computers to perform operations for controlling a 3D printer, the 3D printer comprising a build platform, the build platform including a first plate on which a 3D object is formed and a second plate that is positioned vertically below and separated from the first plate such that a gap is defined between the first and second plates, and an extruder for one or more deposition materials, the extruder including at least one nozzle that is movable relative to the build platform. The operations include causing at least some of the extruded material to flow through one or more openings that are defined in the first plate into the gap to thereby form a rivet with a head portion that expands within the gap to couple the 3D object to the first plate, causing material to be extruded out of the nozzle while controlling the relative movement between the build platform and the nozzle to thereby form the 3D object above the one or more rivets, and heating the second plate to a threshold temperature to facilitate plastic deformation of the head portion of the rivet to thereby allow removal of the coupling between the 3D object and the first plate. 
         [0008]    Implementations according to this aspect can include one or more of the following features. For example, the operations can further include heating, during rivet formation, the second plate to an activation temperature that is lower than the threshold temperature. In some cases, the operations can further include causing material to be extruded out of the nozzle while controlling the relative movement between the build platform and the nozzle to thereby form a raft on which the 3D object is to be built, the raft being attached to the one or more rivets. Forming the rivets can include filling all the openings that are positioned beneath the 3D object to be formed. Additionally, forming the rivets can include filling a select number of the openings that are positioned beneath the 3D object to be formed. Filling the select number of the openings can include filling only the openings that are positioned beneath a perimeter of the 3D object to be formed. In some cases, forming the rivet can include dipping the nozzle into the opening into which the extruded material is to flow. Forming the rivet can further include extruding the material while moving the nozzle out of the opening in a helical path. 
         [0009]    The details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the invention will become apparent from the description, the drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1A  is an isometric view showing an example of a Cartesian (xyz) type 3D FFF printer. 
           [0011]      FIG. 1B  is a schematic diagram showing an example of a 3D printer motion system. 
           [0012]      FIG. 1C  is an isometric view showing an example of a delta type 3D FFF printer. 
           [0013]      FIG. 2  is an exploded isometric view showing an example of a build platform. 
           [0014]      FIG. 3A-3D  are cross-sectional schematic drawings showing an example of a process for creating a 3D object using rivets. 
           [0015]      FIG. 4  is a block diagram showing an example of a rivet formation process. 
           [0016]      FIG. 5  is a cross-sectional view of the example of a build platform of  FIG. 2 . 
           [0017]      FIGS. 6A-6C  are cross-sectional schematic drawings showing an example of a process for creating a 3D object on top of a raft. 
       
    
    
       [0018]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0019]    Referring to  FIG. 1A , an example of a Cartesian (xyz) type 3D FFF printer  100  is shown. The 3D printer  100  includes an extruder assembly, which includes an extrusion motor  102  to drive filament to a hot end  104  that connects to a carriage  106 . The hot end  104  can include a nozzle having a generally conical shape as shown, but many different types of hot ends with differently shaped nozzles can be used in various implementations. The carriage  106  is moveable in a first direction (x) along a fixed arm of the 3D printer  100 , and the additional two directions of movement are achieved by a build platform or plate  108  being moveable in two additional directions (y &amp; z). Furthermore, the build platform or plate  108  includes openings  110 , which are described in further detail below. 
         [0020]    As will be appreciated, there are many types of motion system structures that can be used to create a Cartesian (xyz) type 3D FFF printer, and the hot end  104  and the build platform  108  can be used with all such variations in the 3D printer, including different systems in which the hot end is not considered part of the extruder. In general, an extruder is a group of parts that handles feeding and extruding the build material. The extruder can be viewed as including two assemblies: (1) a cold end that pulls/feeds the filament (e.g., thermoplastic from a spool), and (2) a hot end, or nozzle, that melts and extrudes the filament (e.g., thermoplastic melted at temperatures of up to 240° C., or in some cases at even higher temperatures). As will be appreciated, the hot end  104  needs to be constructed so as to withstand the high temperatures used to melt the build material. In addition, the hot end  104  needs various systems (e.g., a thermocouple or thermistor, a heater, and an actively controlled fan) to control the temperature of the hot end  104  with precision. As shown in  FIG. 1 , the hot end  104  has been constructed so as to keep these upstream components within a narrow cone extending from the nozzle tip back upstream (i.e., opposite the filament feed direction). This narrow profile for the hot end  104  can provide significant advantages in operation of a 3D printer. 
         [0021]    Referring also to  FIG. 1B , the 3D printer  100  can include a 3D printer motion system  120 . The 3D printer motion system  120  operates to move the carriage (and thus the hot end) in relation to the 3D build volume. Thus, the 3D printer motion system  120  can include at least one controller and an input/output (I/O) subsystem. 
         [0022]    In some implementations, the 3D printer motion system  120  includes at least one processor and medium encoding instruction  122  (e.g., a microprocessor with embedded firmware), one or more mechanical systems  124  (e.g., to physically move the carriage, the build plate, or both), and one or more sensor(s), amplifier(s), and actuator(s)  126 . Thus, the 3D printer motion system  120  can be a mechatronic system, which monitors the build environment and/or the 3D printer using sensors, and processes the sensor information in order to change the behavior of the system so as to react to changes in the build environment and/or the 3D printer itself. In this case, the encoded instructions (e.g., software) has become an integral element of the 3D printer, allowing the 3D printer to identify and react to situational changes that can occur during 3D printing. 
         [0023]    In some implementations, the 3D printer motion system  120  includes one or more portions of the extruder. For example, the system  120  can include the extrusion motor, filament drive mechanism, or both. In any case, the 3D printer motion system  120  causes movement of the hot end in relation to the build volume, and so these motions are coordinated with the melting and extruding done at the hot end. 
         [0024]      FIG. 1C  is an isometric view showing an example of another type of a 3D printer, namely a delta type 3D FFF printer  160 . The delta printer  160  includes FFF drive motor  162 , which drives filament material to a narrow hot end  164 . The narrow hot end  164  can be a narrow nozzle having a generally conical shape. As shown, however, the volume that contains the hot end components need not be a cone. In this case, a faceted cone has been used. Other types of volumes can also be used, as well as different types of hot ends with differently shaped nozzles, depending on the specific 3D printing application. Further, as shown, a carriage  166  that holds the hot end  164  can also be contained within the volume defined by the total included angle with respect to the target nozzle point. This can facilitate further “plunge” capability in the delta 3D printer  160  when printing an object on, for example, a round build platform  168 . In other implementations, such narrow angle hot ends are not used. 
         [0025]    Additionally, the delta 3D printer  160  includes a motion system, which includes a controller  170 , a rail motor assembly  172 , a rail cart  174 , and arms  176 . The controller  170  can be computer hardware programmed with software, or dedicated circuitry, such as an ASIC (application specific integrated circuit), that performs 3D printing in accordance with the systems and techniques described in this specification. Note that other parts of the motion system can be included within the defined volume of the hot end  164 , including mechanical structures that move the hot end and/or its carriage and mechanical structures that feed FFF material (e.g., FFF drive motor  162  can be included on the carriage  166  and be within the defined volume). 
         [0026]    Referring again to  FIGS. 1A and 1C , the build platforms used in 3D printer applications can include a plurality of openings  110 . The plurality of openings  110  can be defined in the build platform  108 , for example, to help provide mechanical coupling between the build platform  108  and a 3D object being built thereon. In this way, both the tensile as well as shear strength of the attachment between the object from the platform during the build can be increased. 
         [0027]    Referring further to  FIG. 2 , an exploded view of the build platform  108  of  FIG. 1A  is shown. The build platform  108  includes a first plate  202  and a second plate  204  that is positioned vertically below the first plate  202 . The second plate  204  is separated from the first plate  202  via, for example, spacers  208  such that a gap is defined between the two plates. In some cases, the spacers  208  can be washer-like shims that help maintain a precise gap between the first and second plates  202 ,  204 . To adjust the size of the gap between the two plates, spacers  208  having different thicknesses can be used. Alternatively, or additionally, two or more spacers  208  can be stacked to increase the size of the gap. 
         [0028]    In some cases, in order to promote thermal insulation between the first plate  202  and the second plate  204 , the spacers  208  can be made from thermally non-conductive materials. For example, the spacers  208  can be ceramic. In some cases, the spacers  208  can be made from mica, fabric, or other materials with low thermal conductivity. Thus, transfer of heat from the second plate  204  to the first plate  202  can be minimized. 
         [0029]    Alternatively, or additionally, transfer of heat from the second plate  204  to the first plate  202  can be minimized by minimizing the path of conduction therebetween, that is, minimizing the amount of physical connection between the two plates. For example, the contact surface between the plates and the spacers  208  may be minimized (e.g. by reducing the diameter of the spacers  208 ). 
         [0030]    In some implementations, structural features other than the spacers  208  can be used to achieve spacing between the first and second plates  202 ,  204 . For example, protrusions and/or depressions formed on the inner surface of one or both of the first and second plates  202 ,  204  be used to create the gap between the plates. In some cases, fasteners used to connect the first and second plates  202 ,  204  to each other can include a spacing mechanism to maintain separation between the plates. 
         [0031]    In some implementations, a plurality of gaps can be defined between the first plate  202  and the second plate  204 . For example, multiple, discrete gaps may be defined between the first and second plates  202 ,  204 . Such separated gaps may or may not allow fluidic connection between them. In some cases, a gap may be defined locally for each opening or a partial group of openings in the first plate  202 . In other cases, a single, fluidically connected gap may be provided for all openings in the first plate  202 . 
         [0032]    A heating element  206  is provided with the second plate  204  and can be used to heat the second plate  204 . The heating element  206  can include a ceramic heater, a coil heater, or the like. In some cases, the heating element  206  can be made from materials including, but not limited to, Kanthal, nichrome, cupronickel, molybdenum, disilicide, PTC ceramic, PTC rubber, etc. In some implementations, rapid heating of the second plate  204  may be desirable. For example, the second plate  204  may be heated to reach a desired temperature in one second or less. 
         [0033]    In some implementations, the heating element  206  can be attached to a bottom surface of the second plate  204  using, for example, an adhesive. In some cases, the heating element  206  can be clamped, welded, or bonded to the second plate  204 . Alternatively, or additionally, the heating element  206  can be integrated into the second plate  204 . In some cases, multiple heating elements may be provided to the second plate  204  to promote, for instance, uniform temperature distribution across the second plate  204 . In some cases, the heating element  206  can heat the second plate  204  via convection, for example in cases where there is little to no direct contact between the heating element and the second plate. 
         [0034]    The second plate  204  can be made from a material that as high thermal conductivity, such as aluminum. Other thermally conductive materials, such as copper and silver just to name a few, can also be used either alone or in combination with other materials. As such, the second plate  204  can be heated and cooled relatively quickly. 
         [0035]    The first plate  202  can be made from same or similar materials as the second plate  204 . For example, the first plate  202  can also be made from aluminum. Making one or both of the plates  202 ,  204  from aluminum can be advantageous since the extruded material, such as polyactic acid (PLA), may not stick well to aluminum, thus being useful during the object removal process. In some cases, the first plate  202  can be made from materials having a lower thermal conductivity relative to the second plate  204 . For example, the first plate  202  can be made from steel or titanium, among others. In some cases, the first plate  202  and/or the second plate  204  can include non-metallic materials, such as ceramic, glass, FR-4 glass epoxy, and high temperature plastics, just to name a few. In some cases, the first plate  202  can have a shape and/or aspect ratio that allows the first plate  202  to have a lower thermal conductivity than the second plate  204  even when the two plates are made from the same material. Just as one example, the first plate  202  may achieve reduced thermal conductivity by having a higher porosity than the second plate  204 . 
         [0036]    Because the first plate  202  is separated from the second plate  204  by a gap, indirect heating of the first plate  202  due to the heating element  206 , which is provided to the second plate  204 , may be minimized. And as noted above, use of spacers  208  that are thermally insulating can further prevent unwanted heat from reaching the first plate  202 . 
         [0037]    In some cases, the first plate  202  can include a heat sink as well as other passive or active cooling systems in order to keep the first plate  202  at a lower temperature than the second plate  204 . This can help, for example, minimize heat transfer to the 3D object built on the first plate  202 . In some cases, the second plate  204  can also include similar cooling systems in order to promote faster cooling after the end of a heating cycle. 
         [0038]    As mentioned above, the plurality of openings  110  can help provide mechanical coupling between a platform and an object being built on said platform. Specifically, the material being extruded from the nozzle can be injected one at a time into one or more of the plurality of openings  110  that are defined in the first plate  202 . As the extruded material, which can include PLA and the like, flows into the opening, a rivet-like structure can be formed that helps anchor the object being built on the first plate  202 . 
         [0039]    In more detail, referring further to  FIGS. 3A and 3B , the rivet-like structure, hereinafter referred to as a rivet  302 , can be formed when the extruded material flows through the opening  110  to form a shaft portion  304 . As extruded material continues to flow into the opening  110 , a portion of the shaft portion  304  expands within a gap  306 , which is defined between the first plate  202  and the second plate  204 , to form a head portion  308 . In some cases, the second plate  204  can be heated during the rivet forming step to help facilitate the formation of the head portion  308  within the gap  306 . Because the head portion  308  expands to a diameter larger than that of the opening  110 , the rivet  302  can be secured within the opening  110 . 
         [0040]    In some implementations, all of the openings  110  that are positioned immediately below the 3D object to be built on the first plate  202  can be filled with rivets  302  using the process described above. Alternatively, only a select number of the openings  110  positioned below the 3D object to be built may be filled with rivets  302 . Under this alternative scenario, the controller can determine, either automatically or based on user input, which of the openings  110  should be filled, for example, based on the 3D shape of the object, the footprint of the object, and/or properties of the extruded material, among other parameters. The number and position of rivets  302  required to provide the desired mechanical coupling between the object and the build platform  108  may be determined experimentally. In some cases, only the openings  110  that are positioned immediately below a perimeter of the 3D object may be filled with rivets. 
         [0041]    An example of a process for building a 3D object  310  on top of the rivets  302  is shown in  FIGS. 3C and 3D . Referring to  FIG. 3C , extruded material is deposited on the first plate  202  to form the object  310 . The lower surface of the object  310  that correspond to locations of filled openings  110  fuse with the rivets  302  to create a mechanical coupling to the build platform  108 . As such, relative movement between the object  310  and the build platform  108  can be prevented or mitigated. That is, even when the build platform  108  is rapidly moved along the x/y/z-axes and/or tilted with respect to the horizontal plane, the object  310  may remain securely coupled to the build platform  108 . 
         [0042]    Referring now to  FIG. 3D , an example of a process for removing the completed 3D object  310  from the build platform  108  is shown. Specifically, upon completion of the 3D object  310 , the heating element  206  can be activated to heat the second plate  204 . Here, the second plate  204  can be heated to a target threshold temperature at which just the rivet portion  302  becomes softened while the structural integrity of the object  310  remains intact. For example, when PLA is used as the extruded material, the heating element  206  can be activated to heat the second plate  204  to a temperature of around 85° C. In some cases, rapidly heating the second plate  204  can help minimize unwanted heating of the object  310 . For example, the second plate  204  can be heated at 25° C./sec., 50° C./sec., or higher. 
         [0043]    Once the rivets  302  are sufficiently heated and softened, the object  310  can be pulled away from the first plate  202  either by hand or machine. During this process, the softened rivets  302 , in particular the head portions  308 , are plastically deformed so as to allow each rivet  302  to be pulled out of the opening  110 . Because the first plate  202  can be thermally isolated from the second plate  204 , through platform design and material selection as well as rapid heating of the second plate  204 , the head portion  308  can be softened, and thus made deformable, without otherwise affecting the structural integrity of the object  310  or other portions of the rivet  302 . Once the object  310  has been pulled off from the build platform  108 , portions of the rivet  302  that remain attached to the underside of the object  310  can be removed by hand or a cutting tool. Moreover, in some implementations, rather than 3D printing the object  310  directly on the rivets  302 , a raft technique (e.g., as described in U.S. Patent Publication No. 2014/0371895, which is hereby incorporated by reference) can be used, where a raft is printed on the rivets  302 , and the part  310  is printed on the raft, such as is described in further detail below. 
         [0044]    Referring further to  FIG. 4 , an example of a process flow  400  for forming the rivets  302  is shown. In step  402 , the second plate  204  can be heated to a rivet formation, or activation, temperature of, for example, around 45° C. Heating the second plate  204  during rivet formation can help facilitate the formation of the head portion  308  by allowing it to expand outward within the gap  306 . In step  404 , the nozzle is moved and centered with respect to the first opening in which the rivet  302  is to be formed. 
         [0045]    In step  406 , the nozzle can be dipped into the opening. In step  408 , the opening can be filled with the extruded material to form the rivet  302 . For example, around 80% to 90% of the hole volume can be filled with the extruded material. In order to form the head portion  308  that expands into the gap  306 , an extra 20% to 30% (of the hole volume) of the extruded material, for example, can be further injected into the opening. 
         [0046]    In step  410 , the nozzle can be moved out of the opening while continuing to extrude the filament material. In some cases, a spiral extrusion method may be employed. By spiral extruding, for example, the volume of the opening may be filled more thoroughly and evenly by extruding along a helical path. Other extrusion techniques based on different movement patterns may be used to help spread and distribute the extruded filament within the opening more evenly. 
         [0047]    Once the first rivet has been formed, in step  412 , the nozzle can be moved to the next opening where the next rivet is to be formed. Steps  406 - 412  can be repeated until all of the desired rivets have been created. 
         [0048]    In some implementations, in step  412 , the nozzle can continue to extrude material on the build platform  108  as it is repositioned to the next opening such that each new rivet is connected to an immediately preceding rivet. The material that is extruded in this manner may be part of the 3D object, or may be part of a base on top of which the 3D object will later be built. 
         [0049]    In some cases, all the rivets may be formed first without being connected to each other. In this case, subsequently building the object on top of the rivets can connect them. In some implementations, the rivets may extend farther vertically above the surface of the build platform  108  such that a bridging technique can be used to print the 3D object on top of the rivets without actually making contact with the platform  108 . 
         [0050]    In step  414 , once all the rivets have been made, the second plate  204  can be cooled back to below the rivet formation temperature of around 45° C. For example, the second plate  204  can be cooled to room temperature. 
         [0051]    In some implementations, the rivets  302  may be formed together with the object  310 . That is, instead of first forming all the rivets  302 , as illustrated in  FIGS. 3A-3D , the rivet forming process can be performed one at a time during the 3D object formation process. For example, when the nozzle, while extruding material for the 3D object, becomes positioned immediately above one of the openings  110 , a rivet forming procedure may be performed. Once the rivet has been formed, the nozzle can revert back to extruding material for the 3D object. This process of interrupting object formation to create rivets may be performed each time the nozzle becomes positioned immediately above one of the openings  110  (or selected ones of the openings  110 ) during the 3D object formation process. 
         [0052]    A schematic drawing of the build platform  108  is shown in  FIG. 5 . Each of the openings  110 , which can be circular in shape, can have a width W 1  of approximately 0.06 inches. In some cases, the openings  110  may have a tapered cross-section in which the width W 1  decreases as it approaches the gap  306 . Other shapes or sizes may also be used depending on extrusion conditions. The openings  110  can be arranged in a geometric pattern across the first plate  202 . The required quantity and positions of openings  110  can be determined experimentally and may vary depending on extrusion conditions. 
         [0053]    Sizes of the openings  110  may be varied depending on the specific material extruded and/or aperture size of the extruder. That is, larger apertures may require larger openings. For example, the width W 1  of each opening  110  may range from around 0.005 inches for small aperture sizes and up to 5 inches in diameter for very large aperture sizes. Similarly, shapes of the openings  110  may be varied depending on specific extrusion conditions, but openings having rounded features may be preferred. 
         [0054]    As for the quantity and positions of openings  110 , which as noted above may depend on specific extrusion conditions, the first plate  202  can have as few as one opening or up to tens of thousands. Some plates may be designed for specific prints used in manufacturing and, therefore, may contain only holes in strategic locations to benefit those specific prints. 
         [0055]    The height of each opening  110  corresponds to the thickness h 1  of the first plate  202  and can be approximately 0.06 inches. The gap  306  can have a thickness h 2  of approximately 0.01 inches. Thicknesses h 1  and h 2 , of course, may be varied depending on specific extrusion conditions to be used. 
         [0056]    For example, extruders with larger apertures may require gaps with larger thickness h 2 , for example up to 1 inch or more. Conversely, extruders with smaller apertures, such as in 3D printers for nano-scaled printing applications, may require gaps with smaller thickness h 2 . 
         [0057]    Referring now to  FIGS. 6A, 6B, and 6C , an example process of forming an object on top of a raft is shown. Specifically, after the rivets  302  have been formed in the desired openings  110 , as shown in  FIG. 3B , a raft  600  can be formed on top of the rivets  302 . Here, the raft  600  refers to a structural support layer that is disposed between the rivets  302  and the object  310 . The raft  600  can be formed using the same extrusion process used to form the rivets  302  and the object  310 . For example, after the rivets  302  have been formed, the extruder can deposit one or more planar layers on top of the rivets  302  to form the raft  600 . Subsequently, the object  310  can be formed on top of the raft  600 , essentially using the raft  600  as a build platform. After softening the rivets  302  and pulling out the object  310  along with the raft  600  and the rivets  302 , as shown in  FIG. 6C , the object  310  can be removed by hand from the raft  600  or cut off using a tool. 
         [0058]    Implementations of the subject matter described in this specification can be implemented in combination with digital electronic circuitry, or computer software, firmware, or hardware. Implementations of the subject matter described in this specification can be implemented in an additive manufacturing system that uses one or more modules of computer program instructions encoded on a computer-readable medium for execution by, or to control the operation of, data processing apparatus. The computer-readable medium can be a manufactured product, such as hard drive in a computer system or an optical disc sold through retail channels, or an embedded system. The computer-readable medium can be acquired separately and later encoded with the one or more modules of computer program instructions, such as by delivery of the one or more modules of computer program instructions over a wired or wireless network. The computer-readable medium can be a machine-readable storage device, a machine-readable storage substrate, a memory device, or a combination of one or more of them. 
         [0059]    The term “data processing apparatus” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a runtime environment, or a combination of one or more of them. In addition, the apparatus can employ various different computing model infrastructures, such as web services, distributed computing and grid computing infrastructures. 
         [0060]    A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub-programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
         [0061]    Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device (e.g., a universal serial bus (USB) flash drive), to name just a few. Devices suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
         [0062]    To provide for interaction with a user, implementations of the subject matter described in this specification can be implemented using a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. 
         [0063]    While this specification contains many implementation details, these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. For example, while additive manufacturing techniques were used in the illustrative examples to form the 3D object, subtractive machining operations and/or pick and place operations may further be used. 
         [0064]    Moreover, certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
         [0065]    Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments. 
         [0066]    Thus, particular embodiments of the invention have been described. Other embodiments are within the scope of the following claims. In addition, the actions recited in the claims can be performed in a different order and still achieve desirable results.