Patent Publication Number: US-2020282652-A1

Title: Flow control in a pneumatic build material transport system

Description:
BACKGROUND 
     Additive manufacturing machines produce 3D (three-dimensional) objects by building up layers of material. Some additive manufacturing machines are commonly referred to as “3D printers.” 3D printers and other additive manufacturing machines make it possible to convert a CAD (computer aided design) model or other digital representation of an object into the physical object. The model data may be processed into slices each defining that part of a layer or layers of build material to be formed into the object. 
    
    
     
       DRAWINGS 
         FIG. 1  illustrates one example of a pneumatic transport system to transport build material in an additive manufacturing machine. 
         FIG. 2  is a block diagram illustrating one example of a controller in the pneumatic transport system shown in  FIG. 1 . 
         FIG. 3  is a flow diagram illustrating one example of a process to control the flow of build material in a pneumatic transport system such as that shown in  FIG. 1 . 
         FIG. 4  illustrates another example of a pneumatic transport system to transport build material in an additive manufacturing machine. 
         FIG. 5  is a section illustrating one example of a centrifugal separator in a pneumatic transport system such as that shown in  FIG. 4 . 
         FIG. 6  is an exploded isometric illustrating one example of a feed control mechanism in a pneumatic transport system such as that shown in  FIG. 4 . 
         FIG. 7  is a block diagram of a control system with a motor and motor controller such as might be used to control the rotational speed and position the example feed control mechanism shown in  FIG. 6 . 
     
    
    
     The same part numbers designate the same or similar parts throughout the figures. 
     DESCRIPTION 
     In some additive manufacturing processes, powdered build materials are used to form a solid object. Particles in each of many successive layers of build material powder are fused in a desired pattern to form the object. Build material powder may be transported pneumatically to the build chamber in a stream of air. One of the challenges transporting powdered build material pneumatically is accurately controlling the rate of mass transfer to transport the desired quantity of powder to the build chamber, particularly when multiple build material powders are mixed together in the air stream during transport. If the rate of air flow is too slow, then build material powder introduced into the flow may settle out, reducing the rate of mass transfer and possibly clogging the flow conduit. 
     A new technique has been developed to help control the flow of powdered and other forms of build material in a pneumatic transport system in an additive manufacturing machine. In one example, a flow control process includes generating a stream of air with a blower or other source of air pressure, introducing a build material into the stream of air, separating the build material from the stream of air (to supply a build chamber), and measuring the rate of flow of the stream of air at a location downstream from where build material is separated from the air. If the flow rate falls below a threshold, then the rate of air flow is increased by reducing the amount of build material introduced into the stream of air and/or by increasing power to the blower. Also, it may be desirable in some circumstances to slow the rate of air flow based on measurements taken downstream from the separator, for example to increase the concentration of build material in the air stream. The rate of air flow may be slowed by increasing the amount of build material introduced into the stream of air and/or by decreasing power to the blower. Flow rates are measured downstream from the separator to help prevent inaccuracies or even fouling that may be caused by build material in the air flow upstream from the separator. 
     Examples are not limited to powdered build materials but may be used to help control the flow of other forms of pneumatically transported build materials including, for example, fibers and powder/fiber composites. The examples described herein illustrate but do not limit the scope of the patent, which is defined in the Claims following this Description. 
     As used in this document, “and/or” means one or more of the connected things; and a “memory” means any non-transitory tangible medium that can embody, contain, store, or maintain information and instructions for use by a processor and may include, for example, circuits, integrated circuits, ASICs (application specific integrated circuits), hard drives, random access memory (RAM), read-only memory (ROM), and flash memory. 
       FIG. 1  illustrates one example of a pneumatic transport system  10  to transport build material in an additive manufacturing machine. Referring to  FIG. 1 , transport system  10  includes a blower or other source of air pressure  12  to pull or push a single stream of air  14  through a conduit  16 . System  10  also includes multiple sources of build material  18 ,  20 ,  22 , a separator  24  to remove build material from air stream  14 , and a flow meter  26 . The presence of build material in system  10  during operation is indicated by stippling  28  in  FIG. 1 . 
     Each build material source  18 - 22  is configured to introduce a build material into conduit  16  independent of the other sources. While three build material sources  18 - 22  are shown, any number of sources may be used, as indicated by the designation PS 1 , PS 2  . . . PS n . Build materials mix in air stream  14  as they are carried to separator  24 . Separator  24  removes build material from the air stream and discharges it to the build chamber or an intermediate component, as indicated by stippled arrow  30  (labeled PS D ). Flow meter  26  measures the flow rate of air stream  14  in conduit  16 . Flow meter  26  is positioned downstream from separator  24  to prevent inaccuracies or even fouling that may be caused by build material  28  in conduit  16  upstream from separator  24 . While any suitable flow meter may be used, it is expected that a venturi flow meter will be desirable in pneumatic transport systems for build materials in additive manufacturing because they produce little pressure loss, they hold calibration well, and they can be designed and “printed” (manufactured with a 3D printer) faster than other types of measuring devices. 
     A controller  32  is operatively connected to flow meter  26 , air source  12 , and build material sources  18 - 22 . Controller  32  represents the programming, processing and associated memory resources, and the other electronic circuitry and components to control the transfer of build material  28  through conduit  16 . In particular, controller  32  includes programming to adjust the rate of flow of air stream  14  in conduit  16  based on measurements from flow meter  24 . 
     Referring to  FIG. 2 , flow control programming may be implemented, for example, through instructions  34  residing on a controller memory  36  for execution by a processor  38 . Controller  32  may be implemented as a local controller for the flow control elements of transport system  10 , as shown in  FIG. 1 , or as part of a larger system or machine controller. In one example, where the flow rate is controlled with the rate at which a build material is introduced into air stream  14 , controller  32  may be implemented as a local device controller for one or more of the build material sources  18 - 22 . In another example, where the flow rate is controlled with air pressure, controller  32  may be implemented as a local device controller for air source  12 . 
       FIG. 3  is a flow diagram illustrating one example of a process  100  to control the flow of build material in transport system  10 , for example by processor  38  in controller  32  executing flow control instructions  34 . Part numbers in the description of process  100  refer to  FIG. 1 . Referring to  FIG. 3 , process  100  includes generating a stream of air (block  102 ), for example with an air pressure source  12  pulling air through a conduit  16 , and introducing build material into the air stream (block  104 ), for example with one or more build material sources  18 - 22 . Build material is removed from the air stream (block  106 ), for example with a separator  24 , and the rate of air flow is measured downstream from where build material is removed from the air stream (block  108 ), for example with a flow meter  26 . The rate of air flow is adjusted based on the measured rate of air flow (block  110 ), for example by adjusting the rate at which build material is introduced into the air stream at one or more supplies  18 - 22  and/or by adjusting the speed of a blower  12  to change the magnitude of the force pulling air through conduit  16 . 
     Air flow rate is measured by flow meter  26  and build material feed rate is controlled at each source PS 1  through PS n . If the rate of air flow in stream  14  is too slow, build material will settle out of the air stream in horizontal runs of conduit  16 . Thus, the rate of air flow may be monitored at meter  26  as build material is introduced into conduit  16  at one or more sources PS 1  through PS n  and, if the rate of air flow falls to a threshold, then the rate of air flow may be increased by reducing the amount of build material introduced into the stream at one or more sources PS 1  through PS n  and/or by increasing power to blower  12 . Also, it may be desirable in some circumstances to slow the rate of air flow based on measurements from meter  26 , for example to increase the concentration of build material in air stream  14 . The rate of air flow may be slowed by increasing the amount of build material introduced into stream  14  at one or more sources PS 1  through PS n  and/or by decreasing power to blower  12 . 
     Referring again to  FIG. 1 , air enters conduit  16  at an intake  40 , as indicated by arrow Q in . Air leaves conduit  16  at a discharge  42 , the exhaust of blower  12  in this example, as indicated by arrow Q out . Conduit  16  in  FIG. 1  represents the one or more conduits carrying air through system  10 . In one example, a blower  12  pulls air through conduit  16 . A negative pressure pulling air through conduit  16  may be desirable for transporting build material for additive manufacturing to help reduce the risk of build material leaking from transport system  10 . 
       FIG. 4  illustrates another example of a pneumatic transport system  10  to transport build material in an additive manufacturing machine. Referring to  FIG. 4 , transport system  10  includes a blower  12  to pull a single stream of air  14  through a conduit  16 , and a source of new build material  18 , recycled build material  20 , and reclaimed build material  22  to feed build material into air stream  14  in conduit  16 . Arrows indicate the direction of air flow in  FIG. 4 . A feed control mechanism  45  may be used with each build material source to control the rate at which build material is introduced into conduit  16 . Build material is removed from conduit  16  at a separator  24  and fed to a build chamber  44 , for example through a feed control mechanism  45 . Objects are formed on a platform  46  in build chamber  44 . The presence of build material in conduit  16  during operation is indicated by a heavier line weight in  FIG. 4 . 
     Reclaimed build material source  22  is part of a reclamation subsystem  47  that includes a source of air pressure  48  to draw air and thus unused build material from the perimeter of build chamber  44  through a manifold  50  and conduit  52 , as indicated by arrows  54 , and from the bottom of build chamber  44  through a conduit  56 . Reclaimed build material source  22  may be implemented, for example, as a separator to remove build material from conduits  52 ,  54  for feeding to conduit  16 . 
     A filter  58  may be used ahead of flow meter  26  to remove any residual build material from air stream  14 . 
       FIG. 5  illustrates one example of a centrifugal separator  60  such as might be used for separator  24  and separator  22  in a transport system  10  shown in  FIG. 4 . Referring to  FIG. 5 , separator  60  includes an inner portion  62  and an outer portion  64 . The air and build material mix enters separator  60  at intake  66 . The shape of inner portion  62  creates a vortex in the middle of the separator that causes the lighter air to flow upward (see air path arrow  68 ) while the heavier build material flows downward and spreads centrifugally toward the walls of the separator (see build material path  70 ). This causes build material  28  to drop down and out of the separator, into a feed control mechanism  45  ( FIG. 4 ) or a container intermediate to the feed control mechanism, while the air flows up and out of the separator at outflow  71 . 
     Each separator  24 ,  22  in  FIG. 4  may be implemented as a single centrifugal separator  60  or multiple separators  60  arranged in parallel. The efficiency of centrifugal separation may vary based on the size and density of the particles or fibers in the build material, the speed of the conveying air stream, geometrical factors, and static cling. Centrifugal separation with one or more separators  60 , for example, may be capable of separating at least 99.95% of build material powder from the incoming air stream for particle size 60-80 microns, at least 99.9% for particle size 45-60 microns, and 99.5% for particle size 10-20 microns. For build material powder particles smaller than 10 microns (known as “fines”), separator  60  is designed to minimize or reduce the fines in the air outflow stream  68 . 
       FIG. 6  illustrates one example of a feeder  72  such as might be used for each feed control mechanism  45  in a transport system  10  shown in  FIG. 4 . Referring to  FIG. 6 , feeder  40  includes an upper shoe  74 , a lower shoe  76 , and a housing  78  sandwiched orthogonally between shoes  74 ,  76 . A chamber  80  inside housing  78  is made up of a circular rim  82  and spokes  84 , which form distinct pockets  86 . In the example shown in  FIG. 6 , chamber  80  includes six spokes  84  and six pockets  86  of equal size. In one example, chamber  80  includes at least three spokes  84  and three pockets  86 . In one example, the number of pockets  86  is great enough and thus the volume of each pocket small enough to keep air upflow from an empty pocket below a performance inhibiting threshold, 0.1 m/sec for example. In one example, the volume of each pocket  86  is 4-10 cubic centimeters. 
     A circular wheel gear  88  surrounding pockets  86  is operatively connected to a drive motor ( FIG. 7 ) through a gear train  90  to selectively rotate chamber  80 . Build material powder may enter feeder  72  through an inlet  92  in upper shoe  74  and leave through an outlet  94  in lower shoe  76 . Upper shoe  74  is sealed against the top surface of rim  82  and spokes  84  and lower shoe  76  is sealed against the bottom surface of rim  82  and spokes  84 , to seal chamber  80  and pockets  86  except at inlet  92  and outlet  94 . Inlet  92  and outlet  94  are diametrically opposed or otherwise arranged on shoes  74 ,  76 , respectively, so that the same pocket  86  is not open to both inlet  92  and outlet  94  at the same time and so that there is at least one spoke-to-shoe seal between inlet  92  and outlet  94 . Thus, air pressure upstream of feeder  72  is isolated from air pressure downstream of feeder  72 . Build material  28  drops into a pocket  86  as it is rotated into position under inlet  92  and drops out of a pocket  86  as it is rotated into position over outlet  94 . Feeder  72  inhibits air backflow into separator  24 ,  22  and conduit  16  by fluidically isolating downstream air entering a pocket  86  through outlet  94  during a build material drop from upstream air at inlet  92 . 
     In the example shown in  FIG. 6 , inlet  92  and outlet  94  are the same size and shape, but these openings may be dissimilar in shape and size. 
     As shown in the block diagram of  FIG. 7 , a control system  96  with a motor  97  and motor controller  98  may be used to control the rotational speed and position of pockets  86  in feeder  72  to alternately fill and empty each pocket  86  at the desired rate. Motor controller  98  represents the programming, processing and associated memory resources, and the other electronic circuitry and components to control motor  97  to achieve the desired feed rate of build material through feeder  72 . For example, chamber  80  may be rotated faster to increase the feed rate or slower to decrease the feed rate, for example in response to air flow measurements as described above. Controller  98  may be implemented as a local controller for feeder motor  97 , as shown in  FIG. 7 , or as part of a transport system or additive manufacturing machine controller. 
     As noted at the beginning of this Description, the examples shown in the figures and described above illustrate but do not limit the scope of the patent. Other examples are possible. Therefore, the foregoing description should not be construed to limit the scope of the patent, which is defined in the following Claims. 
     “A” and “an” as used in the Claims means one or more.