Patent Publication Number: US-2021178688-A1

Title: Three-dimensional printer with pneumatic conveyance

Description:
BACKGROUND 
     Additive manufacturing (AM) may include three-dimensional (3D) printing to generate 3D objects. In some AM processes, successive layers of material are formed under computer control to fabricate the object. The material may be powder, or powder-like materials, including metal, plastic, ceramic, composite material, and other powders. The objects can be various shapes and geometries, and produced via a model such as a 3D model or other electronic data source. The fabrication may involve laser melting, laser sintering, electron beam melting, thermal fusion, and so on. The model and automated control may facilitate the layered manufacturing and additive fabrication. As for applications, AM may fabricate intermediate and end-use products, as well as prototypes, for aerospace (e.g., aircraft), machine parts, medical devices (e.g., implants), automobile parts, fashion products, structural and conductive metals, ceramics, conductive adhesives, semiconductor devices, and other applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain examples are described in the following detailed description and in reference to the drawings, in which: 
         FIGS. 1A-C  are diagrams of three respective implementations of a 3D printer in accordance with examples; 
         FIG. 2  is an illustration of the pneumatic conveyance system of the 3D printer of  FIG. 1C  in accordance with examples; 
         FIG. 3  is an illustration of the centrifugal separator and vessel used in the 3D printer of  FIG. 1C  in accordance with examples; 
         FIG. 4  is an illustration of two types of air leakage that are overcome by the 3D printer of  FIG. 1C  in accordance with examples; 
         FIG. 5  is an illustration of the feeder of the 3D printer of  FIGS. 1A-C  in accordance with examples; 
         FIG. 6  is an illustration of the feeder used in the 3D printer of  FIG. 1C  in accordance with examples; 
         FIG. 7  is a flow diagram of the sealing control mechanism used by the 3D printer of  FIGS. 1A-C  in accordance with examples; and 
         FIG. 8  is a detailed block diagram of the 3D printer of  FIG. 1C  in accordance with examples. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EXAMPLES 
     The techniques illustrated herein are directed to a three-dimensional (3D) printer having a pneumatic conveyance system (PCS) to transport build material, such as powder, enabling a 3D object to be generated. A feeder inside the 3D printer dispenses build material below the feeder. A chamber within the feeder is operated using a sealing control mechanism to control air upflow and downflow. In other words, as discussed below, the feeder has chambers that reduce air upflow from the feeder and air downflow from the feeder. This sealing control may provide that air upflow from the feeder does not cause the powder to become too aerated and agitated, which could impede powder downflow. The sealing control also substantially prevents air leakage (into the feeder) as upward airflow driven by a pressure gradient contrary to powder flow. The sealing control thus enables the feeder to isolate the pressure upstream of the feeder from that downstream of the feeder. This facilitates the 3D printer to contemporaneously transport build material and initiate a 3D print job to generate the 3D object for at least the reason that air upflow does not significantly interfere with powder flow or operation of a centrifugal separator disposed above the vessel. Moreover, because the conveyance of build material occurs during the print job, the cycle time to complete a 3D print job in the novel 3D printer is thus reduced, relative to those in which conveyance of build material is completed prior to generating the 3D object. 
       FIGS. 1A, 1B, and 1C  are examples of a 3D printer  100 A,  100 B, and  100 C, respectively, that may form a 3D object from build material such as on a build platform. Referring first to  FIG. 1A , the 3D printer  100 A includes a pneumatic conveyance system (PCS)  50 A for transporting build material (e.g., powder)  20  to generate a 3D object  90 . The PCS  50 A includes a feeder  40 , for dispensing the build material  20 . The 3D printer  100 A also includes a sealing control mechanism  30  for operating the feeder. The sealing control method  30  includes a DC motor and an encoder, as discussed in more detail, below. 
       FIGS. 1B and 1C  present further examples of the 3D printer  100 , indicated as  100 B and  100 C, respectively (collectively, “3D printer  100 ” or “3D printers  100 ”), and having PCS  50 B and  50 C, respectively (collectively, “PCS  50 ” or “PCSs  50 ”). In  FIG. 1B , 3D printer  100 E includes PCS  50 B and sealing control mechanism  30 , but further includes a build material supply  80  for dispensing build material  20  to the PCS  50 B. The PCS  50 B includes a vessel  60  disposed above the feeder  40  for receiving build material  20 , such as powder, from the build material supply  80 . In the 3D printer  100 C, the build material  20  is fed into a centrifugal separator  70 , then the vessel  60 , before being received into the feeder  40 . Coupled with the vessel  60 , the centrifugal separator  70 , known also as a powder trap or cyclone, separates the build material  20  from the conveying air more efficiently. 
     The 3D printing performed by the 3D printers  100  may include selective layer sintering (SLS), selective heat sintering (SHS), electron beam melting (EBM), thermal fusion, or other 3D printing and AM technologies to generate the 3D object from the build material. The build material may be powder, powder-like, or in powder form. The build material may be different materials including polymers, plastics, metals, ceramics, and so on. In operation, the 3D printer  100  employs additive manufacturing of the build material  20  to generate the 3D object  90 . 
     The sealing control mechanism  30  is illustrated as being outside the PCS  50 . However, the sealing control  30  may be part of the PCS  50  and with sealing control  30  elements as components of the feeder  40 . As described in more detail below, in conjunction with the sealing control mechanism  30 , the feeder  40  disallows air leakage driven by a pressure gradient contrary to powder flow. Further, during operation of the 3D printer  100 , there is a first pressure upstream of the feeder (upstream pressure) and a different pressure downstream of the feeder (downstream pressure), where upstream and downstream refer to the flow of build material  20  in the PCS. As illustrated in more detail herein, the sealing control  30  helps the feeder  40  to isolate the upstream pressure from the downstream pressure. This enables the conveyance of build material  20  within the 3D printer  100  to take place as the 3D object  90  is being printed. 
     Pneumatic conveyance is a mechanism by which particles, in this case, build material, are suspended in conveying air. The particles are obtained from one or more source locations, transported via conduits (e.g., piping, tubing, etc.), and received at one or more destination locations. Pneumatic conveyance may convey air using positive pressure or negative gage pressure. Dilute phase pneumatic conveyance is generally characterized as being a relatively high-speed with a low ratio of build material (powder) to gas (air) (e.g., a powder to air mass ratio of less than 15). Dense phase pneumatic conveyance involves a low volume of gas at high pressure (positive pressure) or at high vacuum (negative pressure), with the ratio of conveying material to the gas being relatively high. In one implementation, the PCS  50  employs negative (vacuum) pressure to transport build material  20  through the 3D printer  100 . In a second implementation, the PCS  50  employs negative pressure and dilute phase transmission to transport the build material  20 . In one example, the typical airflow rate through the PCS  50  is around 6-8 cubic feet per minute of air. This corresponds to air velocities of between 15 and 19 m/sec in a ⅝″ inside diameter tube. In another example, when moving the build material or powder, the ratio of powder mass to air mass is less than 2, which allows movement of the powder through the conduits  66  at up to 5 g/sec. 
     The feeder  40  of the 3D printer  100  receives the build material  20  from the PCS  50  and dispenses the build material such that the 3D object  90  may be generated. The feeder  40  is described in more detail in conjunction with  FIG. 6 , below. 
     The PCS  50  of the 3D printer  100  receives the build material  20  from the build material supply  80 . The build material supply  80  may be a vessel such as a hopper, a bin, or a cartridge. In one example, the build material supply  80  is a removable cartridge. This provides for the build material supply  80  to be removed from the 3D printer when empty and replaced with a second (full) cartridge. In another example, the build material supply  80  includes reclaimed (or recycled) build material from a prior 3D print job. In another example, the build material supply  80  includes new build material combined with reclaimed (and/or recycled) build material. In yet another example, the build material supply  80  has a volume that is less than the volume needed to generate the 3D object  90 . 
     In the 3D printer  100 , the build material  20  conveyed by the PCS  50  is fed into the centrifugal separator  70 , then the vessel  60 , before being received into the feeder  40 . The centrifugal separator  70 , known also as a powder trap or cyclone, is designed to separate the build material  20  from the conveying air more efficiently. As with the build material supply  80 , the vessel  60  may be a hopper, a bin, or a cartridge. The vessel  60  may have a conical or rectangular cross-section, with sloped walls, so that powder flows through it without adhering to the walls, and the build material  20  separates from the conveying air using gravity. 
       FIG. 2  illustrates the PCS  50  in more detail. The PCS  50  is made up of conduits  66 A-H (collectively, “conduits  66 ”) (e.g., tubes, pipes, lines), the centrifugal separator  70 , the vessel  60 , the feeder  40 , an air intake or lung  24 , a filter  18 , and a blower  86 . A driving force behind the PCS  50  is the blower  86  that provides motive force for the air disposed inside the conduits  66 . Indeed, once the blower  86  is operational, air at a negative pressure of air flows inside the conduits  66 , conveying build material  20  to flow along the connected conduits in the direction shown by the arrows. Respective conduits making up the PCS  50  may meet or couple, such as via conduit tees or other fittings. In an example, the conduits  66  are disposed within the housing of the 3D printer  100 . 
     The conduits  66  of the PCS  50  are fed build material  20  from the build material supply  80 , by way of a second feeder  40 B. The sealing control mechanism  30  controls both the feeder  40  and the feeder  40 B. The feeder  40 B controls the mass of powder relative to the mass of air in the conduit  66 B, and thus helps to maintain the mass within certain limits. The build material  20  is dispensed via the feeder  40 B to the conduit portion  668 , where it is conveyed via air at negative pressure toward the centrifugal separator or cyclone  70 , in this case, via conduits  66 B,  66 C, and  66 D. Arrows in  FIG. 2  indicate the direction of air flow. At the cyclone  70 , the air is separated from the build material  20  and the air is pulled by negative pressure through conduits  66 E,  66 F,  66 G, and  66 H. 
     In one implementation, the PCS  50  is a negative pressure system. Air flow in the PCS  50  is established by the blower  86 , located at the downstream end of the pneumatic line, setting up a negative pressure through the pneumatic line. When the blower  86  is activated, a negative pressure is created in the PCS  50 , such that air from the air intake (lung)  24  flows through conduit  66 A, conduit  66 B (which also has build material  20 ), conduit  660 , and conduit  66 D, where the build material is received into the centrifugal separator  70 . There, the build material  20  is separated from the air before being received into the vessel  60  that discharges the build material  20  through feeder  40 . In the centrifugal separator  70 , the separated air flows upward into conduit  66 E, conduit  66 F, conduit  66 G, and conduit  66 H, pulled by the negative pressure in the PCS  50 . The air may be filtered before leaving the 3D printer  100  in some examples. 
     In one example, the blower  86  generates an airflow which gives velocities sufficient to convey the build material  20 . Negative pressures through the PCS  50  provide that, if leaks happen, the build material  20  leaks inside the 3D printer  100 , so build material is not leaked from the printer. In one implementation, an airflow rate of build material of up to 5 grams/second (g/sec) can be maintained in the PCS  50 . 
     The PCS  50  can thus be characterized as having at least two general conduit sections, an input conveyance (conduits  66 A,  66 B,  66 C, and  66 D) and an output conveyance (conduits  66 E,  66 F,  66 G, and  66 H). The output conveyance, which should be air free or substantially free of build material  20 , may not actually leave the PCS  50  or the printer  100 , but may be used, for example, to fill the air intake  24  for subsequent operations. Where build material does leak into the output conveyance, the filter  18  disposed along the output conveyance may catch any stray particles. 
     The PCS  50 , made up of the conduits  66 , the air intake  24 , the blower  86 , the filter  18 , the cyclone  70 , the vessel  60 , and the feeder  40 , thus forms a system through which air flow moves the build material  20 . The PCS  50  may not be an entirely closed system, some leakage can be tolerated. Leakage below the cyclone  70 , however, can be problematic. For instance, if air velocity in the cyclone  70 , flowing upward through the feeder  40 , exceeds some rate, powder separation of the cyclone may be disturbed, and the separation efficiency of the cyclone may be lost. This principle is described in more detail below. 
     The feeder  40  is disposed below (or downstream of) the vessel  60 . The feeder  40  opens to receive build material  20  from upstream and to dispense the build material further downstream. The cyclone  70 , the vessel  60 , and the feeder  40  are connected, and are also coupled to the PCS  50 . This means that, when open, the feeder  40  may reduce the cyclone separation efficiency and thus compromise the efficiency of the PCS  50 . Because the PCS  50  conveys build material by applying air pressure, the opening of the feeder  40  compromises the operation of the PCS. 
     In one implementation of the 3D printer  100 , the mean air velocity in the build material-carrying conduits  66  of the PCS  50  is between 10 and 20 meters/second (m/sec). For example, for a build material such as polyamide 12 (PA12, a type of nylon), if the air flow velocity is less than 6 m/sec, the build material may settle in the horizontal conduit sections (see conduits  66 B and  66 D, for example). 
     Cyclonic separation may remove particles from air via vortex separation. A centrifugal separator, often known as a cyclone, performs this cyclonic separation to separate a received material into two portions, one of which is generally less dense than the other. Recall from  FIG. 1C  that the 3D printer  100  may include a centrifugal separator or cyclone  70  disposed above the vessel  60 . Once the build material  20  is fed into the air stream of the PCS  50 , the cyclone  70  is used to separate the build material from the conveying air before the build material is received into the vessel  60 . 
       FIG. 3  is one example of a possible configuration of the centrifugal separator  70  and vessel  60  of the 3D printer  100 . The centrifugal separator or cyclone  70  is disposed above the vessel  60  so that denser material, in this case, build material  20 , is separated from the conveying air and received into the vessel  60 . 
     The cyclone  70  is made up of an inner portion  76  and an outer portion  78 . The air combined with the build material  20  coming from the PCS  50  is received into the PCS intake  82 . The shape of the inner portion  76  creates a vortex in the middle of the cyclone  70  that causes the lighter air to flow upward (see air path arrow  72 ) while the heavier build material  20  flows downward and spreads centrifugally toward the walls of the separator (see build material path  74 ). This causes the build material  20  to drop into the vessel  60  while the air flows upward and out of the cyclone via the air outflow  84 . 
     In one implementation, the 3D printer  100  has a single cyclone. In another implementation, multiple cyclones are disposed in parallel to one another in the 3D printer  100  to perform the separation operations described above. The efficiency of cyclone separation may be governed by the size of the build material particles, their density, the speed of the conveying air, geometrical factors, and static cling, to name a few factors. 
     In one implementation, the cyclone  70  of the 3D printer  100  is capable of separating  99 . 95 % or more of build material in the 60-80 micron size range, 99.9% or more of build material in the 45-60 micron size range, and 99.5% of build material in the 10-20 micron size range. For build material smaller than 10 microns (known as fines), the cyclone  70  of the 3D printer  10  is designed to minimize or reduce the fines leaving the air outflow  84 . Moreover, other separating percentages and associated particle size ranges are applicable. 
     Air leakage below the cyclone  70  can disturb cyclone efficiency by causing an updraft inside the cyclone. Such a leak may undesirably carry build material  20  through the air outflow  84  and back into the “clean” part of the PCS  50  (e.g., the output conveyance,  66 E,  66 F,  66 G, and  66 H in  FIG. 2 ). The sealing control mechanism  30  of the novel feeder  40  in the 3D printer  100  is designed to prevent or reduce air leakage from entering the cyclone  70 . 
       FIG. 4  is a diagram showing relative positions of components of a 3D printer  200  that may have a leakage problem. The 3D printer  200  includes a cyclone  270 , a vessel  260 , and a feeder  240 . A downward spiraling arrow indicates the movement of build material  220  from the cyclone  270  into the vessel  260 , with build material  220  indicating how full the vessel is. A PCS  250  transports build material  220  to the cyclone  270 . An upward arrow indicates the flow of air back into the PCS  250 , similar to the air flow of the cyclone  70  described in conjunction with  FIG. 3 , above. The feeder  240  is coupled to and below the vessel  260 . External air may be pulled in as air leaks into the feeder  240 , such as through gaps in the feeder  240  housing. Two possible leakages of air from the feeder  240  are indicated, a first leakage (type 1) with an arrow going upstream toward the cyclone  270 , and a second leakage (type 2) with an arrow going downstream from the feeder  240 . 
     The type 1 leakage of air sends the unwanted air from the feeder  240  upstream, such as with the leakage air having a differential pressure counter to the flow direction of the build material  220 . This causes the unwanted air to move upward through the vessel  260 . If the unwanted air leak makes its way into the cyclone  270  above the vessel  260 , the separation efficiency of the cyclone may be compromised. As an example, if the unwanted air leak rate creates an air velocity through the cyclone cone of approximately 0.1 m/sec or more, cyclone separation efficiency is compromised. Thus, type 1 leakage is to be avoided because it can disturb powder flow through the cyclone  270 . 
     The type 2 leakage of air sends unwanted air from the feeder  240  downstream. Again, air in the environment around or external to the feeder  240  may enter the feeder through gaps in the feeder housing. The unwanted air may move down through the feeder  240 . This downstream transmission of the unwanted air may adversely impact the downstream conveyance and handling of build material. 
     Thus, there are at least two distinct types of leakage that may impact the PCS  50  and other solids handling of the printer. A given feeder may deal with one of these types of leaks at a given moment. For cyclone efficiency, type 1 leakage is of concern. For downstream feeders such as feeders  440 B,  440 C, and  440 D ( FIG. 7 , below), type 2 leakage may be a concern. A design to eliminate type 1 leakage does not necessarily eliminate type 2 leakage, and vice-versa. Both type 1 and type 2 leakage of unwanted air illustrated in  FIG. 4  may be solved by the sealing control mechanism  30  of the feeder  40 , as described in  FIGS. 5 and 6 , below. 
     Returning to  FIG. 2 , recall that the PCS  50  of the 3D printer  100  is a negative gauge pressure system. Whatever the PCS  50  is connected to is sealed off so that the negative pressure of the PCS operates effectively and efficiently. Thus, the other components of the PCS, such as the cyclone  70 , the vessel  60 , and the feeder  40 , thus form a larger system that is impacted by the negative pressure. While a leak in some parts of the PCS  50  may be tolerated, a leak below the cyclone  70  is of particular concern, and may affect the cyclone&#39;s efficiency. 
       FIG. 5  is a detailed diagram of the feeder  40  of the 3D printer  100  to transfer build material  20  from an upstream location to a downstream one. The feeder  40  includes an upper shoe  34 A, a lower shoe  34 B (collectively, “shoes  34 ”), and a housing  46  that is sandwiched orthogonally between the shoes. Inside the housing  46 , a chamber  42  is disposed below the upper shoe  34 A and above the lower shoe  348 . The chamber  42  is made up of a circular rim  46  and spokes or ribs  48 , which form distinct pockets  44 . The number of spokes  48  forming a like number of pockets  44  may vary. In one implementation, the chamber includes six spokes and six pockets of equal width. In a second implementation, the chamber includes at least three spokes and three pockets. In a third implementation, the number of pockets is great enough to give a smaller volume of each pocket such that air upflow velocity from an empty pockets is below a threshold (e.g., 0.1 m/sec) that would cause cyclone problems. In a fourth implementation, the volume of each feeder pockets  44  is between 4 and 10 cubic centimeters. 
     Surrounding the feeder  40  is a feeder wheel  94 , which is adjacent to a gear train  92 . The feeder wheel  94  has a number of teeth that may be engaged by adjacent teeth in the gear train  92 . A gear train is a mechanical system formed by mounting gears in such a way that teeth of the gears engage with one another. In  FIG. 6 , the gear train has multiple gears (four in this example), which are strategically spaced to smoothly transition rotation from one gear to the next. As illustrated in  FIG. 6 , the gear train  92 , which drives rotation of the feeder wheel  94 , causing the feeder  40  to rotate, is controlled by the sealing control  30 . 
     In one implementation, shown in  FIG. 6 , the sealing control mechanism  30  utilizes a DC motor  96 , which is activated and controlled by an encoder  98 . By digitally controlling the on and off states of the DC motor  96 , the encoder  98  provides for control of the rotations per minute (RPM) of the feeder wheel  94  to be strictly controlled. Due to the gear train  92  between the feeder wheel  94  and the motor  96 , the motor is run at faster speed than the desired RPM of the feeder wheel  94 . In one example, the feeder wheel  94  is moved at between 2 and 20 RPM. In another example, the rotation of the feeder wheel  94  may operate continuously for a period before being turned off. Further, the sealing control mechanism  30  may control more than one feeder in the 3D printer  100 . The operation of the sealing control mechanism  30  for multiple feeders is described in more detail below in  FIG. 8 , below. 
     An inlet (e.g., opening, slot, aperture, etc.)  32  is found in the upper shoe  34 A while an outlet (e.g., opening, slot, aperture, etc.)  38  is found in the lower shoe  34 B. The inlet  32  and outlet  38  may each be a hinged door, a size-varying aperture, a sliding slot, and so on. In one implementation, the inlet  32  and outlet  38  are typically open. In this example, the feeder  40  is cylindrical. In operation, the upper shoe  34 A is sealed against the top surface of the rim  46  of the chamber while the lower shoe  34 B is sealed against the bottom surface of the rim so that the chamber  42  and pockets  44  of the chamber are substantially sealed. Although the feeder  40  is depicted as being substantially cylindrical in shape, the feeder may be shapes other than as depicted in the illustration. 
     Build material  20  flows to the feeder  40  from an upstream location, such as a vessel, hopper, or build material supply. Through the inlet  32 , a dollop (e.g., a volume or portion) of the build material  20  drops, by way of gravity, into one of the pockets  44  of the chamber  42 , the one directly below the inlet  32 . In one example, a dollop is about 5 grams. Each pocket  44  will generally contain something, either air, build material  20 , or a combination of build material and air. Thus, the drop operation is an exchange. When the dollop of build material  20  drops into the designated pocket  44 , the air inside the pocket is displaced upward out of the pocket. 
     Under control of the sealing control mechanism  30 , the feeder  40  then moves rotationally such that the pocket  44  is no longer disposed directly below the inlet  32 . For each rotation of the feeder  40  below the upper shoe  34 A, the inlet  32  is disposed over an adjacent pocket  44 . In one example, the width of the inlet is smaller than the width of each pocket. At this point, the pressure upstream of the feeder  40  is isolated from the pressure downstream of the feeder. Once the pocket  44  is disposed over the outlet  38 , the dollop of build material drops (e.g., via aid of gravity) from the chamber  42  and moves downstream from the feeder  40 . Again, the drop is an exchange in which, this time, the dollop of build material in the pocket  44  is replaced with air. Because air pressure in the pocket  44  is fluidically isolated from the upstream channels, particularly the cyclone  70 , the incoming air generally will not travel upward and cause a problem. Instead, the sealing control  30  of the feeder  40  prevents or reduces backflow of air from the entry and exit points of the feeder mechanism, which, in turn, enables the negative pressure of the PCS  50  to move build material  20  within the conduits  66 . In the example of  FIG. 5 , the inlet  32  and the outlet  38  are similarly shaped, but these openings may be dissimilar in shape and size. In another example, the chamber  42  is rotated at least twice between receiving build material  20  from the inlet  32  and depositing build material through the outlet  38 . Put another way, there is at least one spoke-to-shoe seal, or sealing spoke, between the inlet  32  and the outlet  38  at any given position of the feeder  40 ; otherwise, there would be a direct leak path for the build material  20  to drop through the feeder. 
     Further, build material may become unintentionally disposed between the upper shoe  34 A and the wail  46 , between the wall and the lower shoe  348 , and between other components making up the feeder  40 . The volume of air leaking in these circumstances is generally small enough to not affect operation of the PCS. Thus, the escape of air from the feeder, whether upstream or downstream, may unintentionally occur. 
     In some examples, the sealing control mechanism  30  and encoder  98  may include or be associated with a computing device having a processor and memory storing code executed by the processor to adjust operation of the feeders. The computing device may be a controller. The controller may include a processor, microprocessor, central processing unit (CPU), memory storing code executed by the processor, an integrated circuit, a printed circuit board (PCB), a printer control card, a printed circuit assembly (PCA) or printed circuit board assembly (PCBA), an application-specific integrated circuit (ASIC), a programmable logic controller (PLC), a component of a distributed control system (DCS), a field-programmable gate array (FPGA), or other types of circuitry. Firmware may be employed. In some cases, firmware if employed may be code embedded on the controller such as programmed into, for example, read-only memory (ROM) or flash memory. Firmware may be instructions or logic for the controller hardware and may facilitate control, monitoring, data manipulation, and so on, by the controller. 
       FIG. 7  is a flow diagram illustrating the operations of the sealing control  30  in the feeder  40  of the 3D printer  100 . The operation of the feeder  40  follows the conveyance of build material  20  from the PCS  50  to the cyclone  70 , where the build material is separated from the conveying air. The build material  20  flows downstream to the vessel  60 , where it is next received at the feeder  40 . At this point, the operations of  FIG. 7  commence. 
     A volume or dose of the build material  20  is dispensed through the inlet  42  located on the upper shoe  34 A covering the chamber  42  of the feeder  40 . where it is received into one of the pockets  44  (block  302 ). At this point, air inside the pocket  44  is displaced by the incoming build material  20  and flows upward through the inlet  32 . By keeping the size of the pockets small, the 3D printer  100  manages the upward air flow without avoiding it entirely. In one example, the volume of air displaced in the pocket  44 , though flowing upward toward the cyclone  70 , is small enough to not negatively affect the cyclone performance. Put another way, the resultant air upflow rate would not exceed the threshold that causes a problem, such that powder downflow through the cyclone exit to the vessel would not be disturbed. 
     Next, the feeder wheel  94  is rotated so that the inlet  32  is no longer above the pocket containing build material (block  304 ). In one example, the upstream pressure is entirely isolated from the downstream pressure at this point. The feeder chamber wheel  94  is rotated again until the pocket  44  is disposed over the outlet  38  in the lower shoe  34 B (block  306 ). In one example, the feeder wheel is turned at a revolution rate that is proportional to the g/sec transfer rate of the build material. Once so disposed, the volume or dose of build material  20  is dispensed from the pocket  44  through the outlet  38  and fed downstream (block  308 ). Thus, the operations of the sealing function  30  are complete. 
       FIG. 8  is a diagram of a 3D printer  400  implementing the sealer function  30  described in  FIGS. 5 and 6  above. In the 3D printer  400 , there are five feeders  440 A,  440 B,  440 C,  440 D, and  440 E (collectively, “feeders  440 ”), one or more of which may benefit using the sealing control  30 . In one implementation, the sealing control mechanism  30  controls all five feeders  440 . In addition to the feeders  440  and sealing function  430 , the PCS  450  includes conduits  466 , an air intake or lung  424 , a blower  486 , and a filter  418  as before, but, in this example, also features a Venturi  422 , located at the end of the output conveyance of the PCS, just before the blower  486 . Recall that the output conveyance of the PCS should contain air, not build material. However, some build material may find its way in the output conveyance of the PCS. The filter  418  captures this errant build material. In one example, the filter  418  is accessible to a user of the 3D printer  400  and may be removed and replaced, such as following a recommended number of 3D print jobs. The venturi is a passive device used to measure a differential pressure used to discern the volumetric flow rate of air in the conduit  66 . 
     Once the blower  486  is activated, a negative pressure vacuum sucks air from the lung  424 , which sends air through the input conveyance of the PCS  450 . The 3D printer  400  includes two hoppers or vessels containing build material  420 , a build material vessel  480  and a recycle material vessel  416 . Each of these include a feeder  440 C and  440 D, respectively, which dispense build material  420  to the conduits  466 . The build material vessel  480  may include fresh or “new” build material while the recycle material vessel  416  contains recycled or “reclaimed” build material. The 3D printer  400  may accept new build material, recycled build material, or a combination of the two, into the PCS  450  for generating the next 3D object. 
     In one implementation, a build material cartridge  412  is connected to the build material vessel  480 . The build material cartridge  412  may be removable by a user and replaced with a new cartridge. Similarly, a recycle material cartridge  414  is coupled to the recycle material vessel  416 , allowing a user to remove and replace the cartridge as needed. 
     The cyclone  470  is connected downstream to a vessel or hopper  460 , and the feeder  440 A, which is controlled by the sealing control  430 . The feeder  440 A dispenses build material  420  to a powder handling system  402 . The build material  420  is then dispensed to the build chamber  404 . A 3D object is generated in the build bucket  406 . A feeder  440 E, also controlled by the sealing control mechanism  430 , is disposed below the build bucket  406  to orderly transport unused build material downstream. Operations of the powder handling system  402 , the build chamber  404 , and the build bucket  406  are beyond the scope of this disclosure. 
     A PCS diverter valve  424  allows the build material  420  to be diverted to a second cyclone and vessel  408  coupled to a second feeder  440 B. Like the feeder  440 A, the feeder  440 B may be controlled by the sealing control  430 . The isolation of pressure obtained by the sealing control  430 , prevents or reduces unwanted air from adversely impacting the efficiency of the cyclone  408 , and from negatively impacting the flow of build material downstream. In an example, the build material  420  received into the feeder  440 B flows either to the recycle material cartridge  414  or to the recycle material vessel  416 . 
     In an example, the 3D printer  400  includes two additional feeders  440 C and  440 D, one to dispense fresh build material from the build material supply  480  and another to dispense recycle build material from the recycle material vessel  416 . Both feeders  440 C and  440 D may be controlled by the sealing control  30 , to ensure that pressure between upstream devices and the feeder are isolated and pressure between the feeder and the downstream PCS  450  is isolated. 
     Feeders  440 A and  440 B are disposed below cyclones  470  and  408 . respectively. Both feeders will benefit from utilizing the sealing control mechanism  430 , because the mechanism prevents type 1 leaks from impacting the operation of the respective cyclones. For feeders  440 C,  440 D, and  440 E, the concern is to avoid type 2 leakage. The sealing mechanism  430  also will prevent type 2 leaks from adversely impacting the flow of powder downstream. The sealing control mechanism  430  is thus capable of mitigating the effects of both type 1 and type 2 leakages. 
     In an example, the sealing control  430  may establish continuous rotation of the lower feeders  440 C and  440 D for a time period (e.g., 25 seconds), then the lower feeders are stopped for a second time period (e.g., 10 seconds). The vessels of the 3D printer  400 , such as the upper vessel  460 , for example, may include a sensor that indicates how full the upper vessel is. This information may be used by the sealing control mechanism  430  to turn feeders on and off. This gives time for the PCS  50  to deliver powder to the upper feeders  440 A and  440 B. A feeder may be stopped because the receiving unit downstream of the feeder has received a sufficient supply of build material. 
     The above examples illustrate use of the feeder  40  and sealing control mechanism  30  in a 3D printer. The feeder  40  and sealing control  30  may also be used in a powder management station to maintain a desired flow of powder. 
     While the present techniques may be susceptible to various new modifications and alternative forms, the examples discussed above have been shown by way of example. It is to be understood that the technique is not intended to be limited to the particular examples disclosed herein. Indeed, the present techniques include alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.