Patent Publication Number: US-2019184641-A1

Title: Three-dimensional printer with feeders

Description:
BACKGROUND 
     Three-dimensional (3D) printing may produce a 3D object. In particular, a 3D printer may add successive layers of build material, such as powder, to a build platform. The 3D printer may selectively solidify portions of each layer under computer control to produce the 3D object. The material may be powder, or powder-like material, including metal, plastic, composite material, and other powders. The objects formed 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, heat sintering, electron beam melting, thermal fusion, and so on. The model and automated control may facilitate the layered manufacturing and additive fabrication. The 3D printed objects may be intermediate or end-use products, as well as prototypes. Product applications may include aerospace parts, machine parts, medical devices (e.g., implants), automobile parts, fashion products, structural and conductive metals, ceramics, and other applications. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Certain examples are described in the following detailed description and in reference to the drawings, in which: 
         FIG. 1  is a block diagram of a 3D printer in accordance with examples of the present techniques; 
         FIG. 2  is a block diagram of a 3D printer in accordance with examples of the present techniques; 
         FIG. 3  is a block diagram of a 3D printer in accordance with examples of the present techniques; 
         FIG. 4  is a block diagram of a material supply system of a 3D printer in accordance with examples of the present techniques; 
         FIG. 5  is a block flow diagram of a method of operating a 3D printer in accordance with examples of the present techniques; 
         FIG. 6  is a block flow diagram of a method of operating a 3D printer in accordance with examples of the present techniques; 
         FIG. 7  is a block flow diagram of a computing device in accordance with examples of the present techniques; 
         FIG. 8  is a block diagram of a computer-readable medium that may contain code to execute operation of a 3D printer in accordance with examples of the present techniques; 
         FIG. 9  is a perspective view of a rotary feeder of a 3D printer in accordance with examples of the present techniques; 
         FIG. 10  is a diagram of a 3D printer in accordance with examples of the present techniques; 
         FIG. 11  is a diagram of a 3D printer in accordance with examples of the present techniques; 
         FIG. 12  is a diagram of a 3D printer in accordance with examples of the present techniques; 
         FIG. 13  is a diagram of a 3D printer in accordance with examples of the present techniques; 
         FIG. 14  is a perspective view of a 3D printer in accordance with examples of the present techniques; and 
         FIG. 15  is a perspective view of a 3D printer in accordance with examples of the present techniques. 
     
    
    
     DETAILED DESCRIPTION 
     Three dimensional printers may form 3D objects from build material such as powder. The cost of a 3D printer producing 3D objects may be related to the cost of the build material. Thus, there may be a desire for 3D printers to utilize recycle material as build material. Yet, for some applications, there may be benefit in utilizing new material because of reasons such as product purity, strength, and finish in certain instances. To mix recycle material and new material as build material for some 3D printers, a user may employ extra floor space and equipment external to the 3D printer. A user may also rely on peripheral resources in the extraction of printed 3D objects from a 3D printer. Increased costs may result from dedicated resources external to the printer for mixing of build material and for extraction. Further, manual handling of build material in mixing, addition, and extraction may result in cross-contamination of build material with the environment. 
     Certain examples of the present techniques provide a 3D printer having internal or integrated handling of the build material and, therefore, reduce manual handling of build material and associated cross-contamination of the build material with the environment. Indeed, examples herein may include 3D printers that provide contained handling to mix recycle material and new material as the build material. Example 3D printers herein may also provide for contained handling in the recovery of excess or unfused build material in the extraction of the printed 3D object, and so on. The printer integrated conveying system(s) internal within the printer may include, for instance, a closed-loop or substantially closed-loop material handling system for transporting material internally within the 3D printer. Certain examples may generally not employ external dedicated resources, extensive floor space separate from the printer, or external equipment to mix powder or extract 3D objects from unfused powder. The technique may include mixing of fresh and recycle build material during pneumatic transport. 
     An example includes a 3D printer having a conveying system to transport build material, such as powder, for the printer to form a 3D object from the build material. The conveying system may be internal within the 3D printer. The build material may include a first material and a second material at a specified ratio. In one example, the conveying system is a dilute-phase pneumatic conveying system that promotes mixing of the first material and the second material in-line via the relatively high velocity transport. In examples, the first material is new material and the second material is recycled or recycle material. The 3D printer may include a first material vessel that stores or holds first material. Likewise, the 3D printer may include a second material vessel that stores or holds second material. The first material vessel and the second material vessel may be internal within the 3D printer. 
     In operation, the first material vessel discharges first material through a first feeder to the conveying system. The second material vessel discharges second material through a second feeder to the conveying system. At least one sensor provides indication of material discharge rate from the first material vessel or the second material vessel, or both. The 3D printer may have a controller to adjust, in response to the indication of material discharge rate, a first operating parameter of the first feeder and/or a second operating parameter of the second feeder to maintain the specified ratio of the first material to the second material. The first operating parameter and the second operating parameter may each be, for example, revolutions per minute (rpm) or other operating parameter of the feeder. Moreover, as indicated, multiple sensors may be employed. For example, a first sensor and a second sensor may provide the indication of material discharge rate from the first material vessel and the second material vessel, respectively. Further, a sensor associated with the conveying system may provide the indication, for example, of the combined material discharge rate from the first material vessel and the second material vessel. 
     In addition, the 3D printer may include an internal dispense vessel to supply build material for the 3D printer to form the 3D object. In some instances, the internal conveying system may provide the first material and second material, respectively, from the first material vessel and the second material vessel to the dispense vessel. In certain instances, the conveying system may transport and mix in-line the first material and the second material as build material to the dispense vessel. In one instance, the conveying system transports the first material and the second material to the dispense vessel during the print job, e.g., during formation of the 3D object. 
     Thus, examples may provide for internal mixing of new powder and recycle powder during the print job. In some examples, mixing may occur in the dilute-phase pneumatic transport of powder. The mix ratios may be checked with sensors such as load cells on material vessels or hoppers. Feeders at the discharge of material vessels may regulate, modulate, or meter the build material powder. Again, the ratio of first material to second material, and the overall rate of first material and second material, may be checked by, for example, the vessel load-cell feedback. Moreover, mixing uniformity via the conveying system may be with a scale of scrutiny of approximately 10 cubic centimeters (cc) or greater. In certain examples, a micro-aerator (or “puffer”) at the base of a dispense vessel or hopper that receives the first material and the second material may further assist mixing. In some examples, no extra process step or mechanism in addition to the in-line transport and micro-aerator is employed for mixing the two powders. In other examples, additional mixing equipment or actions may be employed. 
       FIG. 1  is a diagram showing a portion of 3D printer  100  that forms a 3D object from build material. The 3D printing may include selective layer sintering (SLS), selective heat sintering (SHS), electron beam melting (EBM), thermal fusion and fusing agent, 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. Also, the printer  300  components depicted in  FIG. 3  may be integrated internal within the printer  300 , such as within a housing  358  of the printer  300 . 
     The printer  100  in this example has an internal first material vessel  102  to discharge first material  104  through a first feeder  106  as build material. The printer  100  has an internal second material vessel  108  to discharge second material  110  through a second feeder  112  as build material. The first material may be different than the second material. For example, the first material and the second material may be the same or similar build material but with the first material as new material and the second material as recycle material. In other examples, the first material and the second material may be different types of build material. 
     The material vessels  102 ,  106  may be hoppers, containers, or other types of vessels. Moreover, as discussed below, in one example, the material vessels  102 ,  106  may receive material from a removable first material cartridge and a removable second material cartridge, respectively, inserted into the 3D printer  100  by a user. The feeders  106 ,  112  may be a rotary feeder, rotary valve, auger, screw feeder, on-off valve, pulsating valve, piston valve, reciprocating valve, reciprocating-piston valve, and so on. 
     The printer  100  includes a controller  114  that adjusts operation of the first feeder  106  and the second feeder  112  to maintain the build material as having a specified ratio of the first material  104  to the second material  112 . The controller  114  may adjust the operation of the first feeder  106  and the second feeder  112  based on indication of material discharge rate of the first material vessel  104  and the second material vessel  108  through the respective feeders  106 ,  112 . The controller  114  may be communicatively coupled with the feeders  106 ,  112  or with control devices associated with the feeders  106 ,  112 , as indicated by the dashed line  116 . 
     In some examples, the controller  114  may be a computing device having a processor and memory storing code executed by the processor to adjust operation of the feeders  106 ,  112  to maintain set point or the specified ratio. In certain examples, the controller  114  may be a component of the printer  100  computer system. The controller  114  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. 
     In some examples, a first sensor associated with the first material vessel  102  provides the indication of material discharge rate of first material  104  from the first material vessel  102 . A second sensor associated with the second material vessel  108  may provide the indication of material discharge rate of second material  110  from the second material vessel  108 . Alternatively, or in addition, a sensor associated with an internal conveying system may provide the indication of material discharge rate of the first material vessel  106  and the second material vessel  108 . 
     The example 3D printer  100  may include the internal conveying system to transport build material including the first material  104  and the second material  110 . In some examples, the conveying system may receive the first material  104  and the second material  110  discharged through the respective feeders  106 ,  112 . Indeed, in certain examples, the first material vessel  102  and the second material vessel  108  may discharge the first material  104  and the second material  110  through the feeders  106 ,  112 , respectively, into the conveying system. If so, as mentioned, a sensor associated with the internal conveying system may provide the indication of material discharge rate of the first material vessel  102  and the second material vessel  108 . 
     In one example, the first sensor and second sensor are weight sensors (e.g., load cells) and are a primary feedback of discharge rates from the vessels  102 ,  108 . In that example, level sensors on the vessels  102 ,  108 , if employed, may be a secondary feedback of the discharge rates. Further, in that example, for a pneumatic conveying system, a pressure sensor or other sensor in the conveying system, such as on a conveying conduit or at the motive component (blower), may provide secondary or tertiary feedback in which the combined discharge rate from the vessels  102 ,  108  can at least be inferred. For instance, the amount of build material powder being transported and entrained in conveying air in a conveying conduit may be calculated or estimated based on the pressure or pressure profile in the pneumatic conveying conduit. 
     In certain examples, the conveying system as a dilute-phase pneumatic conveying system mixes the first material and the second material in-line in the conveying gas during transport. The conveying gas may be air, nitrogen, filtered air, ambient air, and so forth. In one example for dilute phase, the conveying gas (e.g., air) flow rate through a conveying conduit is in the range of 4 cubic feet per minute (cfm) to 10 cfm, or in a range of 5 cfm to 9 cfm, or in the range of 6 cfm to 8 cfm, and the like. The units of these volumetric flow rates of conveying gas or air may be actual cfm or acfm. The velocity of the conveying gas may be in the range of 10 meters per second (m/s) to 25 m/s. or in the range of 13 m/s to 22 m/s, or in the range of 15 m/s to 19 m/s, and so on. The nominal inner diameter of a conveying conduit may be in the range of 0.375 inch to 1.5 inch, 0.375 inch to 1.0 inch, 0.5 inch to 1.0 inch, and so on. In some examples, the nominal diameter of a conveying conduit is 0.5 inch, 0.625 inch, or 0.75 inch. Conduits having a nominal inner diameter outside of these ranges or values are applicable. Further, for the dilute-phase conveying, the ratio of build material or powder mass to air mass may be less than 6, less than 5, less than 4, less than 3, less than 2, less than 1, and so forth. The mass rate of the build material or powder can be in a range of 2 grams per second (g/s) to 10 g/s, such as 5 g/s. Other mass rate ranges are applicable. 
     The 3D printer  100  may include a feed vessel or dispense vessel to make available build material for the 3D printer  100  to form the 3D object. If so, the internal conveying system may transport build material including the first material and the second material to the dispense vessel contemporaneous with the 3D printer forming the 3D object. 
     As indicated, the first material vessel  102  and the second material vessel  108  may be internal within the 3D printer  100 , such as within a housing  118  of the printer  100 . The housing  118  may also be the same housing around the build enclosure and the build platform on which the 3D object is formed. In some examples, the housing  118  may be an outer housing or casing of the 3D printer  100 . 
       FIG. 2  is a 3D printer  200  having a first material vessel  202  to store first material and provide the first material as build material (e.g., powder) through a first feeder  204  to a conveying system  206 . The 3D printer  200  has a second material vessel  208  to store second material and provide the second material as build material through a second feeder  210  to the conveying system  206 . In one example, the first material is fresh or new material, and the second material is recycle material. The recycle material, which may also be labeled as recycled material, may be unfused or excess build material recovered from the 3D printing within the printer  200 , such as from a previous print job. 
     A first sensor  212  may measure an operating condition of the first material vessel  202  to provide indication of the material discharge rate of the first material from the first material vessel. Likewise, a second sensor  214  may measure an operating condition of the second material vessel  208  to provide an indication of the material discharge rate of the second material from the second material vessel  208 . The sensors  212 ,  214  may be weight sensors such as load cells. The sensors  212 ,  214  may instead or additionally be level sensors (e.g., capacitive, optical, ultrasonic, etc.) or other types of sensors. Furthermore, one or more sensors  216  may measure an operating condition of the conveying system  206  to provide an indication of the material discharge rate from the first material vessel  202 , from the second material vessel  208 , or the combined material discharge rate from both the first material vessel  202  and the second material vessel  208 . 
     As discussed, the 3D printer  200  may include a controller  218  that receives the measurement or indication from the sensors  212 ,  214 ,  216  and adjusts operation of the feeders  204 ,  210  in response, as indicated by the dashed line  220 . The adjustment may be to maintain the desired material discharge rates through the feeders  204 ,  210 . In certain examples, the controller  218  may adjust an operating parameter of each feeder  204 ,  210  to maintain a specified ratio of first material to second material, as discharged to the conveying system  206 . In one example, the feeders  204 ,  210  are rotary feeders, augers, screw feeders, or rotary valves, and the operating parameter is rpm of the feeder. In one example, the feeders  204 ,  210  are rotary feeders and the rpm is in the range of 2 rpm to 20 rpm. The specified ratio of first material to second material may be based on weight or volume. Again, the conveying system  206  may transport both first material and second material as build material for the 3D printing. The conveying system  206  may provide build material having the specified ratio of first material to second material. The ratio may range from zero, e.g., no first material, all second material, to 1.0, e.g., all first material, no second material. The ratio may be a weight ratio, volume ratio, or other type of ratio. The ratio as a weight ratio (mass ratio) or volume ratio may range from 0.01 to 0.99, 0.05 to 0.95, 0.1 to 0.9, 0.15 to 0.85, 0.2 to 0.8, 0.25 to 0.75, 0.3 to 0.7, etc. In a particular example, the feed build material may be 20% first material (e.g., new material) by weight and 80% second material (e.g., recycle material by weight), yielding a weight ratio of 0.25. In another example, the feed build material has 20% new material by volume and 80% recycle material by volume, yielding a volume ratio of 0.25. 
     The controllers  206 ,  208  may include firmware or code  214 ,  216 , e.g., instructions, logic, etc., stored in memory and executed by a processor to implement their respective control functions discussed herein. If firmware is employed, the firmware may provide for interaction between code and hardware, including to interpret commands and provide for control generally. The firmware configuration may be unique to the printer  200  or given controllers  206 ,  208 , incorporating properties of the printer  200  and respective controller. In general, as mentioned, the controller may be a computing device or control card and may include a CPU, memory storing executable code, an integrated circuit, a PCB, a printer control card, a PCA or PCBA, an ASIC, a PLC, a DCS component, a FPGA, and so on. Moreover, as indicated, the printer  200  components depicted in  FIG. 2  may be internal within the printer  200 , such as within a housing  222  of the printer  200 . 
     Lastly, as noted, the build material including the first material and the second material may be powder. As used herein, the term “powder” as build material can, for example, refer to a powdered, or powder-like, material which may be layered and sintered via an energy source or fused via a fusing agent and energy source in a 3D printing job. The powdered material can be, for example, a powdered semi-crystalline thermoplastic material, a powdered metal material, a powdered plastic material, a powdered composite material, a powdered ceramic material, a powdered glass material, a powdered resin material, or a powdered polymer material, among other types of powdered material. 
       FIG. 3  is a 3D printer  300  having a first material vessel  302  to discharge first material through a first feeder  304  into a conveying system  306 . The printer  300  includes a second material vessel  308  to discharge a second material through a second feeder  310  to the conveying system  308 . The printer  300  components depicted in  FIG. 3  may be integrated internal within the printer  300 , such as within a housing  358  of the printer  300 . In the illustrated example, the first material and the second material may discharge into a conveying stream in a conduit of the conveying system  306 . Moreover, as depicted, the first material vessel  302  and the second material vessel  308  may be in series. 
     Additionally, in this example, the 3D printer  300  has a third material vessel  312  that discharges a third material through a third feeder  314  into the conveying system  306 . As depicted in examples of subsequent figures, the third material vessel  312  in certain cases may receive excess or unfused build material recovered from within the printer  300 . In some examples, the third material may be classified as first material or second material. Thus, in those examples, the third material may be incorporated as additional first material or additional second material. In one example, the first material is new material and the second material is recycle material. 
     In the illustrated example, the conveying system  306  is a pneumatic conveying system having a gas inlet  316  to receive a conveying gas. For instance, the gas inlet  316  may be an air inlet and the conveying gas may be air. In one example, the gas inlet  316  is a conduit (e.g., tubing, piping, etc.) with an open end to receive ambient air or filtered air as the conveying gas. In some examples, the conveying system  306  pulls in filtered air through the gas inlet  316  within the printer  300 . For instance, the open end of the conduit may be disposed in an internal volume region of the printer  300  to receive the filtered air. In certain examples, the internal volume region may be called a lung of the conveying system  306  or the printer  300 . In a particular example, the received gas may be ambient air having humidity, vapor, or moisture that has been filtered and resides as filtered air inside the printer  300 . 
     The conveying system  306  may transport the feed build material  318  via the conveying gas. The feed build material  318  may include the first material from the first material vessel  302  and the second material from the second material vessel  308 . In some instances, with the third material vessel  312  in operation discharging third material, the feed build material  318  may also include the third material discharged from the third material vessel  312 . In examples, the first material vessel  302  has a weight sensor  320 , the second material vessel  308  has a weight sensor  322 , and the third material vessel  312  has a weight sensor  324 . In some examples, the vessels  302 ,  308 ,  312  may be disposed on their respect weight sensors  320 ,  322 ,  324 . In certain examples, the weight sensors  320 ,  322 ,  324  are weight scales or load cells. The material vessels  320 ,  322 ,  324  may have additional sensors such as a level sensor, pressure sensor, temperature sensor, and so on. The weight sensors  320 ,  322 ,  324  may measure the weight of the material vessels  302 ,  308 ,  312 , respectively. Therefore, in examples, the respective material discharge rate from the material vessels  302 ,  308 ,  312  through the feeders  304 ,  310 ,  314  may be determined based on the change in the measured weight or mass over time. The determined material discharge amounts or rates may be in units of mass per time. Also, with known or estimated density of the material, or via level sensors, the material discharge amounts or rates may be determined in units of volume or volume per time. 
     In some examples, a controller  326  of the 3D printer  300  may receive the weight measurements from the sensors  320 ,  322 ,  324  and determine the material discharge rates of the respective vessels  302 ,  308 ,  312 . In other examples, the controller  326  may receive the values of the material discharge rates as determined by another computing component of the printer  300  based on the weight measurements or other measurements. Thus, the controller  326  may determine or receive the values of the material discharge rates. The controller  326  may adjust operation of the feeders  304 ,  310 ,  314  to give or maintain a desired amount or rate of feed build material  318  and a specified ratio of first material to second material in the feed build material  318 . 
     In cases with the third material different than the first material and the second material, the controller  326  may adjust operation of the feeders  304 ,  310 ,  314  to give or maintain, for example, specified ratios of first material to second material to third material. For instance, the controller  326  may maintain a first ratio of first material to second material, a second ratio of first material or second material to the third material, the specified amount of feed build material  318  with any constraints, and so forth. Other ratios may be applicable. 
     Table 1 below gives an example control-scheme basis that the controller  326  may employ to adjust an operating parameter of the feeders  304 ,  310  based on the weight or mass change in the material vessels. The first column of Table 1 is time over the incremented time interval in which the controller  326  may determine and make adjustments. The second column is the mass change of the material in the first vessel  302  over the time interval indicated by the vessel weight sensor  320 . Likewise, the third column is the mass change of the material in the second vessel  308  over the time interval indicated by the vessel weight sensor  322 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Example Control Calculations 
               
            
           
           
               
               
               
               
               
            
               
                   
                 First 
                 Second 
                 Actual Ratio 
                 Actual Sum 
               
               
                 Time 
                 Vessel 
                 Vessel 
                 (Target = X) 
                 (Target = Y) 
               
               
                   
               
               
                 t = t1 = 
                 Δm 1, t1   
                 Δm 2, t1   
                 Ratio = Δm 1, t1 / 
                 Sum = Δm 1, t1  + 
               
               
                 t0 + Δt 
                   
                   
                 Δm 2, t1   
                 Δm 2, t1   
               
               
                   
                   
                   
                 Error R  = X-Ratio 
                 Error S  = Y-Sum 
               
               
                 t = t2 = 
                 Δm 1, t2   
                 Δm 2, t2   
                 Ratio = Δm 1, t2 / 
                 Sum = Δm 1, t2  + 
               
               
                 t1 + Δt 
                   
                   
                 Δm 2, t2   
                 Δm 2, t2   
               
               
                   
                   
                   
                 Error R  = X-Ratio 
                 Error S  = Y-Sum 
               
               
                   
               
            
           
         
       
     
     The fourth column in the above Table 1 is the calculated actual mass ratio based on the mass change in the vessels  302 ,  308  over the time interval. For a given density of the material, the actual volume ratio may be calculated. As for error, the calculated actual ratio is compared to the specified or target ratio (X) to determine a ratio error (Error R ) for the controller to base adjustments of operation of the feeders  304 ,  310 . In one example, the target ratio (X) as a specified ratio is 0.25. However, as discussed above, this target ratio (X) as a specified ratio may be other values and fall within differing ranges. The fifth column of Table 1 is the calculated actual mass sum based on the mass change in vessels  302 ,  308  over the time interval. The sum may be the combined mass discharge over the time interval from the vessels  302 ,  308 . A combined material discharge rate (mass per time) may be calculated by dividing the sum by the time interval. In the third column, the calculated actual sum is compared to the specified or target sum (Y) to determine a sum error (Error S ) for the controller to base adjustments of operation the feeders  304 ,  310 . In one example, the target sum (Y) is at least 240 grams per minute. 
     In response to the Error R  and/or Error S , the controller  326  may adjust an operating parameter (e.g., rpm, etc.) of one or both of the feeders  304 ,  306  to alter the material discharge rate (mass or volume) from one or both of the vessels  302 ,  308 . The printer  300  control scheme may include constraints to guide the controller  326  adjustments of the feeders based on the error. The control scheme may also include, for example, tuning constants such as gain, proportional, integral, derivative, and so forth, for the controller  326  to determine adjustments of the feeder operation in response to the error. Lastly, the control routine may incorporate the third material vessel  312  and third feeder  314 , and additional material vessels and feeders. 
     Furthermore, the feeders  304 ,  310 ,  314  may each have a sensor, such as a Hall effect sensor, to synchronize the provision or drops (e.g., pocket-drops) of material through the feeders. A Hall effect sensor may be a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors generally may be employed for proximity switching, positioning, speed detection, and current sensing applications. In one form, the sensor operates as an analog transducer, directly returning a voltage. 
     The internal conveying system  306  may transport the feed build material  318  to a dispense vessel  328  (e.g., hopper) for the 3D printing. The 3D printing may be the 3D printer forming a 3D object  332  from build material on a build platform  330 . As for product applications, the 3D printer  300  may fabricate objects  332  as prototypes or products for aerospace (e.g., aircraft), machine parts, medical devices (e.g., implants), automobile parts, fashion products, structural and conductive metals, ceramics, and so forth. In one example, the 3D objects  332  formed by the 3D printer  300  are mechanical parts which may be metal or plastic, and which may be equivalent or similar to mechanical parts produced, for example, via injection molding. 
     In the transport, the feed build material  318  may enter a separator  334  that separates conveying gas  336  and discharges the build material  338  with little or no conveying gas to the dispense vessel  328 . In one example, the separator  334  is a centrifugal separator or cyclone. Moreover, the separator  334  may be integrated physically with the dispense vessel  338  or separate from the dispense vessel  334 . A motive component  338  of the conveying system  306  may pull the separated conveying gas  336  discharged by the separator  334 . The motive component  338  may discharge the conveying gas  336 , as indicated by arrow  340 , to a volume within the printer  300 , to ambient external to the printer  300 , or to other equipment for additional processing, and so on. The motive component  338  may be a blower, a fan, a centrifugal blower, axial blower, and the like, in examples of the conveying system  306  as a pneumatic conveying system. 
     In the illustrated example, the motive component  338  is disposed at an end portion of the conveying system  306  and, therefore, the conveying system  318  may be a negative-pressure pneumatic conveying system. In other examples, the motive component  338  may instead be disposed at a front or upstream portion of the conveying system  306  with the motive component  338  “pushing” the conveying gas instead of pulling the conveying gas. Thus, in those examples, the conveying system  318  may be a positive-pressure pneumatic conveying system. Moreover, the conveying system  306  may generally include the conveying conduit(s), the motive component  338 , and other equipment, as well as valves, instrumentation, conduit fittings, and so forth. In some examples, the separator  334  may be characterized as a component of the conveying system  306 . 
     In examples, the separator  334  may discharge the conveying gas  336  through a filter  342  and a meter or meter element  344  in route to the motive component  338 . In some examples, the filter  342  may remove residual build material or solids from the conveying gas  336 . In one example, the meter is a differential-pressure flow meter and the meter element  344  is a venturi or a section of conduit having a narrowing inner diameter. The meter element  344  may include a first pressure tap on the conduit upstream of the venturi and a second pressure tap downstream of the venturi to provide for measurement of pressure and therefore the determination of differential pressure across the venturi. The flow meter may determine the flow rate of conveying gas  336  based on the differential pressure. Other meters for measuring flow rate or pressure of the conveying gas  344  are applicable. In other examples, an orifice plate or other flow element may be employed as the meter element  344 . 
     The conveying system  306  may include a sensor  346  associated with the motive component  338 . For example, with the motive component  338  having a motor, the sensor  346  may be coupled with the motor or an encoder to indicate speed, amps, voltage, pressure, or other variables of the motive component  338  and its motor. In one example, blower voltage may indicate total amount of build material  318  being conveyed with closed-looped control on air flow rate. 
     The sensor  346  can also include a pressure sensor at the suction or discharge of the motive component  338  to indicate pressure. Such pressure indication may provide for an estimation of the upstream material  318  flow rate in some examples. The conveying system  306  may also include pressure sensors at other locations to estimate material mass flow rate. For example, a pressure sensor may be disposed on the conveying line or at the separator  334 , and so forth. Measurement of the pressure change, including if the blower or motor voltage is generally held constant, can provide in particular examples an indication or estimation of the total mass flow rate of the material  318 . 
     In certain examples, the controller  326  may construe, determine, or calculate an estimated amount of feed build material  318  flowing through the conveying system  306  based on measurements provide via the sensor  346  or flow element  344 , or via other sensors in the conveying system  306 . Therefore, the controller  326  in some examples may determine the combined material discharge rate from the material vessels  302 ,  308 ,  312 . The controller  326  may adjust operation of one or more of the feeders  304 ,  310 ,  314  in response to the combined material discharge rate as determined. The weight sensors  320 ,  322 ,  324  can provide feedback on proportions of build material powder delivered from the respective feeders  304 ,  310 ,  314 . 
     As mentioned, the separator  334  may discharge separated build material  338  to the dispense vessel  328  for the 3D printing. In the illustrated example, the dispense vessel  328  may discharge build material  338  through a feeder  348  to a feed handling system  350 . The feed handling system  350  may provide successive layers of build material  338  for the build platform  330 , as indicated by arrow  352 . The feeding handling system  350  may include a volume-delivery component, a provision vessel or dosing container, a supply surface or deck, a build-material applicator or powder spreader, and so forth. Furthermore, the dispense vessel  328  may have a level sensor  354  or weight sensor. Moreover, the controller  326  may provide for each layer of build material  338  on the build platform  330  to have a specified ratio of first material to second material. The specified ratio may different for respective layer or layers of build material  338  on the build platform  330  for the same 3D object  332  being formed. 
     Further, in some examples, the dispense vessel  328  has an aeration device  356  to promote mixing of the first material and the second material in the dispense vessel  328 , including in a lower portion of the dispense vessel  328 . In addition to the mixing in the upstream conveying line of the conveying system  306 , the aeration device  356  may provide for mixing to promote a relatively uniform mixture of the first material and the second material at the specified ratio. In certain examples, the aeration device  356  may provide for targeted blasts of gas or air to a base portion of the dispense vessel. 
     The blasts may be relatively small-volume to aerate and mix the powdered build material without significantly interfering with performance of separator  324 . Such “micro-aeration” may avoid upward air amounts and velocities that could meaningfully disturb cyclone separator  324  performance. In one example, the aeration device  356  includes a relatively small pump and, for instance, a solenoid valve. Thus, in that example, the aeration may be performed via targeted blasts of air provided by the pump at an adjustable interval and volume via the solenoid valve. 
     As indicated, the mixing of the first material and the second material may also be advanced in the conveying of the build material  318 . For example, for a dilute-phase pneumatic conveying system, the dilute conditions and the relatively high velocity (e.g., in the range of 10 to 20 meters per second) of the conveying gas and build material  318  may promote mixing. The mixing uniformity via the conveying system  306  may be at a scale of scrutiny of at least 10 cc of transported build material  318 . For example, a sample of 10 cc or greater of build material collected from the dispense vessel  328  may have the specified ratio. 
     Moreover, the transport and mixing in-line of the first material and the second material as the build material  318  may be contemporaneous with the print job. This may be in contrast to a batch system in which a dispense vessel or feed vessel is filled prior to the print job and build material is not transported during the print job. In the illustrated example, the continuous or semi-continuous transport of build material  318  during the forming of the 3D object  322  may accommodate a relatively smaller dispense vessel  328 . In some examples, the dispense vessel  328  does not have volume capacity to hold or store an adequate amount of build material for the print job. Therefore, the dispense vessel  328  may receive build material during the print job. In one example, the dispense vessel  328  is less than two liters in size. 
     To supply the build material  318 ,  338 ,  352  for the generating of the 3D object  332 , and with the feeders  304 ,  310 ,  314 ,  348  as rotary feeders, the rotation of the feeders may be generally continuous. The rotation may also be intermittent. In one example cycle, the rotation of the two feeders  304 ,  310  may be continuous for at least 25 seconds and off for less than 10 seconds. The rotation may be similar for the feeder  314  if in operation. In an example cycle for the feeder  348  at the dispense vessel  328 , the rotation timing cycle is continuous for less than 2 seconds and off for more than 3 seconds. Other timing cycles are applicable. Rotation of the feeders  304 ,  310  may be stopped to the dispense vessel  328  having an adequate amount of build material. Rotation of the feeder  348  may be stopped when the feed handling system  350  has an adequate amount of build material. 
     In summary for examples, the motive component  338  (e.g., blower) may pull air through a conduit (a “pneumatic line”) at a velocity, for example, in a range of 10 to 20 meters per second (m/s). In some examples, the feeders  304 ,  310  provide the build material powder into the pneumatic line. A user may set, via the controller  326  or other control element, the ratio of first material (e.g., fresh powder) to second material (e.g., recycle powder), including setting of operating parameters (e.g., rpm) of the feeders  304 ,  310 . The printer  300  or controller  326  may check the ratio via the sensors  320 ,  322  which may be weight sensors, load cells, etc. The material (powder) may be dropped from the feeders  304 ,  310  in such a way that the first material and second material powder dollops overlap. 
     In one example, the conduit or tube length between the feeder  310  and the separator  334  is less than 3 meters. In that example, the transit time of the build material  318  through the conduit or tube to the separator  334  and dispense vessel  328  is less than one second. The separator  334  (e.g., cyclone) may separate the build material from the air-stream. The build material  338  may then fall into the dispense vessel  328  (e.g., hopper). The first material and the second material may mix in the upstream pneumatic conveying line and further be mixed in the dispense vessel  328  by air-blasts administered by puffer or aerator  356 . The aerator  356  may be a micro-aerator. The aerator  356  may be at a base portion of the dispense vessel  328 . The aerator  356  may be near and upstream of the feeder  348 . 
     Moreover, as discussed, the 3D printer  300  may include a third material vessel  312  which may provide material for the mixing scheme. The third material vessel  312  may have a feeder  314  and weight sensor  324 . In general, the examples of the techniques may be extended from 2 to n material vessels. Lastly, a parallel arrangement of material vessels  302 ,  308  is depicted in  FIG. 4 . 
       FIG. 4  is a material supply system  400  of a 3D printer. The material supply system  400  includes a pneumatic conveying system  402  having parallel air inlets  404  and  406 . The conveying system  402  may have a motive component such as a blower to pull-in air via the air inlets  404 ,  406  for conveying gas. The air may be ambient air from the environment external to the printer or may be air (e.g., filtered air) internal to the printer. In some examples, the air inlets  404 ,  406  are each a conduit with an open end disposed in an internal volume region of the printer to receive air. 
     The material supply system  400  includes a first material vessel  302  that discharges first material through a feeder  304  into a first conduit of the conveying system  402 . The first conduit may be associated with the first air inlet  404 . The material supply system  302  includes a second material vessel  308  that discharges second material into a second conduit of the conveying system  402 . The second conduit may be associated with the second air inlet  406 . The conveying system  402  may combine the first material and the second material as feed build material  318  in a conduit that merges the first conduit and the second conduit. The conveying system  400  may transport the first material and second material via conveying air and a motive component such as a blower. The blower may provide a motive force on the conveying air. 
     The motive component of the conveying system  402  may be disposed near or at a terminal end of the conveying system  402  to pull, via the conveying gas, the build material  318  including the first material and the second material. If so, the conveying system  402  may be a negative-pressure pneumatic conveying system. On the other hand, the conveying system  402  may include a first motive component disposed near the first air inlet  404  and a second motive component disposed near the second air inlet  406 . In that case, those two motive components may push the build material  318  via the conveying air. If so, the conveying system  402  may be a positive-pressure pneumatic conveying system. In general, the conveying system  402  may apply, via the motive component(s), a motive force to the conveying air and the flowing build material  318 . 
     The conveying system  402  may transport the build material  318  for the 3D printing such as for a dispense vessel or build platform in which the printer forms a 3D object on the build platform. The material supply system  400  may include sensors to measure operating parameters of the material vessels  302 ,  308  to indicate the respective material discharge rate or amount from the vessels  302 ,  308 . In one example, the sensors include weight sensors  320 ,  322 . A controller of the 3D printer may receive indications from the sensors  320 ,  322  and adjust operation of the feeders  304 ,  310  based on the indications to maintain a specified ratio of first material to second material in the build material  318 , and to maintain a set point of total combined material discharge amount or rate from the vessels  302 ,  308 . 
       FIG. 5  is a method  500  of operating a 3D printer. At block  502 , the method includes forming a 3D object from build material. For example, the printer may form the 3D object layer-by-layer from build material on a build platform. At block  504 , the method includes discharging first material from a first material vessel through a first feeder into a conveying system. In examples, a controller may modulate via the first feeder the amount or rate of first material discharging from the first material vessel. At block  506 , the method includes discharging second material from a second material vessel through a second feeder into a conveying system. In examples, the controller may modulate via the second feeder the amount or rate of second material discharging from the second material vessel. Moreover, the first material vessel, the second material vessel, and the conveying system may be internal within the 3D printer. At block  508 , the method includes transporting build material including the first material and the second material during the forming of the 3D object. The transporting may be via the conveying system. The build material may have a specified ratio of first material to second material. The ratio may be a weight ratio or a volume ratio. Lastly, a controller may adjust operation of the first feeder and the second feeder to maintain the specified ratio and the total amount of build material. 
       FIG. 6  is a method  600  of operating a 3D printer and be implemented in view of the method  500  of  FIG. 5 . At block  602 , the method includes determining or measuring, via a first sensor associated with the first material vessel, a first discharge amount or rate of the first material from the first material vessel. 
     At block  604 , the method includes determining or measuring, via a second sensor associated with the second material vessel, a second discharge amount or rate of second material from the second material vessel. The first and second sensors may be level sensors, weight sensors, load cells, solids flow meters, and so forth. The determining or calculating of the discharge amounts or rates may be by a computing device or controller of the printer. 
     At block  606 , the transporting of the build material may include transporting, via the conveying system, the build material to a dispense vessel or feed vessel. The transporting may include mixing in-line the first material and the second material in the conveying system. For instance, the conveying system may be a dilute-phase pneumatic-conveying system and in which the dilute conditions and relatively high transport velocity promotes mixing. The mixing uniformity via the conveying system may be at a scale of scrutiny of at least 10 cc of transported build material. For example, a sample of 10 cc or greater of build material collected from the dispense vessel may have the specified ratio, or substantially close to the specified ratio. 
     At block  608 , the method includes determining, via a conveying system sensor, the first discharge rate, the second discharge rate, or the combined amount or combined rate of the first discharge rate and the second discharge rate. For example, the sensor may be a pressure sensor in the conveying system, or a sensor associated with a motor of the motive component (e.g., blower) of the conveying system, and so forth. The determining or calculating of the discharge amounts or rates via the conveying system sensor may be by a computing device or controller of the printer. 
     At block  610 , the method includes adjusting, via a controller, operation of the first feeder and the second feeder to maintain the specified ratio in response to the first discharge rate or the second discharge rate, or both. For example, the controller may change the set point of an operating parameter of the first feeder or the second feeder, or both. In one example, the controller changes the rpm of the first feeder and/or the second feeder to maintain the specified ratio. Also, the controller may change the operation of the feeders, such as set point of the operating parameter or the rpm, to maintain a desired total amount of build material or the desired total amount of first material and second material discharged from the vessels, respectively. 
     At block  612 , the method includes supplying build material from the feed vessel for the forming of the 3D object. For example, the feed vessel may discharge build material to a feed handling system that provides build material for a build platform on which the 3D object is formed. The feed vessel and the feed handling system may provide for successive layers of build material on the build platform. In one example, the printer feeders, conveying system, and feed vessel may provide for changing or different specified ratios for respective successive layers of build material on the build platform. 
       FIG. 7  is a computing device  700  of a 3D printer. The computing device  700  may have a hardware processor  702  and memory  704 . The hardware processor  702  may be a microprocessor, central processing unit (CPU), an ASIC or other circuitry, printer control card(s), and the like. The processor  702  may be one or more processors, and may include one or more cores. The memory  704  may include volatile memory such as random access memory (RAM), cache, and the like. The memory  704  may include non-volatile memory such as a hard drive, read only memory (ROM), and so forth. The computer system  700  may include code, e.g., instructions, logic, etc., stored in the memory  700  and executed by the processor to direct operation of the printer and to facilitate various techniques discussed herein with respect to control, and so on. In one example, the code includes an adjuster  706  executed by the processor  702  to adjust operation of feeders, as discussed above, to maintain a specified ratio of materials and a total amount of the materials, and so on. Firmware may be employed. 
       FIG. 8  is a computer-readable medium  800  that may contain code for execution to implement techniques described herein with respect to printer control. The medium may be a non-transitory computer-readable medium  800  that stores code that can be accessed by a processor  802  such as via a bus  804 . The computer-readable medium  800  may be a volatile or non-volatile data storage device. The medium  800  may also be a logic unit, such as an ASIC, FPGA, or an arrangement of logic gates implemented in one or more integrated circuits. Again, the medium  500  may store code, e.g., instructions, logic, firmware, etc., executable to facilitate the techniques described herein. For example, an adjuster code  506  may facilitate a controller of a 3D printer to adjust operation of feeders to maintain a specified ratio of a first material to a second material. An example of a non-transitory, computer readable medium for a 3D printer includes machine-readable instructions, that when executed, direct a processor to measure or determine, via a first sensor associated with a first material vessel, a first discharge rate of the first material from the first material vessel; measures or determines, via a second sensor associated with the second material vessel, a second discharge rate of second material from the second material vessel; and adjust operation of the first feeder and the second feeder to maintain the specified ratio in response to the first discharge rate or the second discharge rate, or both. 
       FIG. 9  is a rotary feeder  900  of a 3D printer. In certain examples, the rotary feeder  900  may be analogous to the feeders  106 ,  112 ,  204 ,  210 ,  304 ,  310  discussed with respect to the preceding figures. However, other types of feeders may be employed for the aforementioned feeders  106 ,  112 ,  204 ,  210 ,  304 ,  310 . In this example, the rotary feeder  900  has vanes (or spokes)  902  meeting an inside surface  904  of a housing  906  of the feeder  900 . The vanes  902  rotate within the inside surface  904 . The vanes  902  may be secured within the feeder  900  via a coupling element  908 , such as a bolt. In the illustrated example, the vanes  902  may rotate around the coupling element  908 . The feeder has drive or rotating elements  910 , such as gears, to rotate the vanes  902 . In operation, build material  912 , such as powder from a material vessel, may flow into a cavity or pocket  914  between two vanes  902 . In one example, the build material  912  may fall by gravity from the material vessel. 
     The feeder  900  is depicted with three vanes  902  and three pockets  914  and, therefore, three pockets  914  per revolution. However, the feeder  900  may have more or less than three pockets  914  per revolution. As for discharge, the feeder  900  may have an opening  916  in the bottom portion of the feeder  900  to discharge the build material  912 , such as into a conduit of a conveying system. The amount of build material  912  discharged may be a function of the rpm of the vanes  902  around the coupling element  908 . In some examples, each pocket  914  volume may be in the range of 2 cc to 15 cc, or 3 cc to 12 cc, or 4 cc to 10 cc, and the like. The number of pockets may be in the range of 3 to 10 pockets, 3 to 8 pockets, 3 to 6 pockets, 3 to 5 pockets, and so on. 
     A motor (e.g., a direct current or DC motor) may be employed under, for example, encoder control to control the rpm of the rotary feeder  900 . In some examples, the rpm of the feeder  900  in operation is in the range of 2 rpm to 20 rpm. There may be a gear train between the motor and the feeder wheel and, therefore, the motor speed may be greater (e.g., significantly greater) than the feeder rpm or pocket-wheel rpm. In operation in the 3D printer, the rotary feeder  900  rotation may be generally continuous. The rotation may also be intermittent. In one example cycle, the rotation is continuous for at least 25 seconds and off for less than 10 seconds. In another example, the rotation timing cycle is continuous for less than 2 seconds and off for more than 3 seconds. Other timing cycles are applicable. Rotation of a feeder  900  may be stopped because a downstream receiver has an adequate amount of build material. 
     Moreover, the 3D printer may have a sensor, such as a Hall effect sensor, on the feeder  900  to better synchronize the pocket-drops of material. Lastly, as mentioned, the aforementioned feeders may instead be an auger or other device. 
       FIG. 10  is a 3D printer  100  having an internal material supply system  1002  including a first material vessel  1004  that discharges first material as build material through a first feeder  1006  to an internal conveying system  1008  of the 3D printer  1000 . The material supply system  1002  includes a second material vessel  1010  that discharges second material as build material through a second feeder  1012  to the conveying system  1008 . The conveying system  1008  may transport the first material and the second material as build material to an internal dispense vessel  1014  or other feed apparatus. The 3D printer  1000  includes a controller to adjust operation of the feeders  1006 ,  1008  to maintain a total amount of build material conveyed, as well as a specified ratio of first material to second material in the build material. Sensors associated with the material vessels  1004 ,  1010  and the conveying system  1008  may provide feedback to the controller of the material discharge rate from the material vessels  1004 ,  1010 . 
     The material supply system  1002  may include a cartridge receiver to hold an operationally-removable material cartridge. For example, a user may insert a material cartridge into the cartridge receiver. The material cartridge may be a container, canister, or cylinder having build material. The cartridge receiver may be a receptacle, slot, cavity, or the like. The printer  1000  may make available build material from the material cartridge held in the cartridge receiver for the material vessels  1006 ,  1012 . Lastly, in some examples, the material vessels  1006 ,  1012  may be operationally removable. 
     The conveying system  1008  that transports build material from the material supply system  1002  to the dispense vessel  1014  may be a pneumatic conveying system, a mechanical conveying system, a screw or auger feeding system, vibrational conveying system, a belt conveying system, or any combinations thereof. Portions of the conveying system  1008  may rely on gravity. If pneumatic conveying is employed, the conveying system  1008  generally include conduits, fittings, and valves to transport the build material. The pneumatic conveyance system may be dilute phase or dense phase. If dilute phase is employed, the pneumatic conveyance system may be negative-pressure system or a positive-pressure system. The pneumatic conveyance system may include a motive component, such as a blower, to provide a motive force for conveying air through the transport conduits. Lastly, the pneumatic conveying system, if employed as the conveying system  1008 , can include separators, such as cyclone, filters, and the like, to separate build material or other solids from conveying air. For example, a cyclone or centrifugal separator may be disposed above the dispense vessel  1014  to separate conveying air from the build material, and discharge the build material (e.g., minus conveying air) to the dispense vessel  1014 . A pneumatic conveying system  1008  may be inside the housing  1038  and not an external system. 
     In the illustrated example, the 3D printer  1000  has a build platform  1016  on which the 3D printer  1000  forms the 3D object  1018 . The printer  1000  may have a build enclosure  1020  associated with or at least partially containing the build platform  1016 . The conveying system  1008  may transport the first material and the second material as build material to the dispense vessel  1014  for the 3D printer  1000  to print (form or generate) the 3D object  1018  from the build material. The conveying system  1008  may mix in-line the first material and the second material in the conveying to the dispense vessel  1014 . Moreover, the conveying system  1008  may transport the build material to the dispense vessel  1014  during the generation of the 3D object  1018 . The 3D printer  1000  may sinter, melt, or fuse build material to form the 3D object  1018 . For example, the 3D printer  100  may employ SLS, SHS, EBM, thermal fusion, or other AM technique to form the 3D object  1018 . The build material may be made from one or more of metal, plastic, polymer, glass, ceramic, or other material. 
     In operation, the dispense vessel  1014  may discharge build material through a feeder  1022  to a feed handling system  1024 . The feeder  1022  may be a metering component including a rotary feeder, rotary valve, screw feeder, auger, and the like. The feed handling system  1024  may include a volume or apparatus to specify and regulate an amount of build material for the build platform  1016 . In certain examples, the feed handling system  1024  may provide for a dosed amount in volume or weight of build material for the build platform  1016 . In a particular example, the feed handling system  1024  may apply or discharge a layer  1026  of build material onto a supply surface  1028  of the 3D printer  1000 . A build-material applicator, such as a powder spreader  1030 , may move as indicated by arrow  1032  across the supply surface  1028  to displace and spread the layer  1026  of build material to across the build platform  1016 . Such provisioning of build material to the supply surface  1028  and spreading of build material from the supply surface  1028  to across the build platform  1016  may be repeated for each layer of build material on the build platform  1016  in the forming of the 3D object  1018 . In other examples, the feed handling system  1024  may apply a layer of build material directly to the build platform  1016  with use of a supply surface  1028 . The supply surface  1028 , if employed, may be a dosing surface, a supply deck, a source platform, and the like. The build-material applicator or powder spreader  1030  may include a mechanical arm, a roller, or other feature to push or pull the layer  1026  of build material to the build platform  1016 . The 3D printer  100  may form the 3D object  1018  layer-by-layer from successive layers of build material on the build platform  1016 . 
     In examples, the build platform  1016 , which receives build material to form the 3D object  1018 , may reside on a piston or other elevating apparatus. The 3D printer  1000 , including a computing system or controller, may lower the build platform  1016  incrementally as each layer of the 3D object  1018  is formed. For example, each increment (e.g., 60 microns, 80 microns, 100 microns, etc.) may be in the range of 50 microns to 150 microns as the height amount in which the build platform  1016  is lowered for each layer of build material and for the associated formed layer of the 3D object  1016 . The increment amount may be less than 50 microns or greater than 150 microns. As depicted, the 3D object  1018  may be formed within the outer housing or housing  1038  of the 3D printer  1500 . 
     In one example, the build platform  1016  is removable and the 3D printer  1000  may be manufactured and sold without the build platform inserted in the 3D printer  1000 . As mentioned, the 3D printer  1000  may include a build enclosure  1020  associated with the build platform  1016 . The build enclosure  1020  may be a build bucket, build chamber, build container, build housing, and the like. In some examples, the build enclosure  1020  may at least partially contain the build platform  1016 . Moreover, the build enclosure  1020  and the build platform  1016  may be components of a build unit of the printer  100 . In particular examples, the build unit may be operationally removable from the 3D printer  1000 . In other examples, the build unit is not intended to be operationally removable. Moreover, the printer  1000  may also include a build unit processing module to separate printed objects from unfused material. In addition, the 3D printer  1000  may include a 3D printed object recovery zone from which separated 3D objects may be recovered after unfused material extraction. The 3D printer  100  may also include additional internal conveying systems, such as a vacuum system, for recovering excess or unused build material from the build enclosure  1020 . 
     The 3D printer  1000  may include an energy source  1034  to apply energy to build material on the build platform  1016  to form the 3D object  1016  from the build material. The energy source  1034  may be a laser source for SLS, an electron beam source for EBM, a thermal printhead for SHS, a heat source or light source for thermal fusion, and so on. The 3D printer  1000  under computer control may sinter, melt, or fuse selected portions of successive layers of build material on the build platform  1016  to solidify those selected portions to form the 3D object  1018  layer-by-layer. The computer control and the portions selected on each successive layer of build material on the build platform  1016  may be per a 3D model or other electronic data source. 
     The 3D printer  100  may include a movement device  1036 , such as carriage or other drive system, to move or position the energy source  1034  over the build material on the build platform  1016 . Indeed, the energy source  1034  may reside on the movement device  1036 . The movement device  1036  may include or be associated with a motor, belts, rails, wheels, etc. to provide for movement and positioning of the movement device  1036 . In certain examples, the movement device  1036  may have a rest position away from the build platform  1016 . Moreover, the powder spreader  1030  may also reside on the movement device  1036 . The powder spreader  1030  may share the movement device  1036  with the energy source  1034  or instead have a separate dedicated movement device  1036 . Indeed, the printer  1000  may have more than one movement device  1036 . The powder spreader  1030  and the energy source  1034  may reside on a movement device  1036 , such as on a support, platform, or frame of the movement device  1036 . A movement device  1036  may have a frame to hold and support the powder spreader  1030  and/or the energy source  1034 . 
       FIG. 11  is a 3D printer  1100  similar to the printer  1000  of  FIG. 10  but having a print assembly  1102  for ejecting print liquid onto the build material on the build platform  1016 . For instances of the 3D printer  1100  employing print liquid in the solidification of build material into the 3D object, the solidification may involve fusion, binding, curing, and so on, of the build material on the build platform  1016 . For example, the fusion may be thermal fusion with the print liquid as a fusing agent or other printing agent. For thermal fusion, the build material may be different materials including polymers, plastics, metals, ceramics, and so on. In one example with thermal fusion, the build material includes polyamide or Nylon. In some examples, the fusing agent accelerates or increases the absorption of energy from an energy source into the build material. In other examples, the fusing agent may react with the build material. As for binding of build material to form the 3D object, the build material may include, for example, gypsum powder, calcium sulfate dihydrate, or similar materials. Thus, the print liquid may include, for instance, a printing agent to bind the gypsum powder or similar powder to generate the 3D object on the build platform. Examples of curing as the solidification may include, for example, UV curing of selected portions of each layer of the build material applied to the build platform. 
     In all, the 3D printer  1100  may sinter, melt, fuse, bind, or cure build material to form the 3D object  1018 . For each successive layer of build material, the print assembly  1102  may eject print liquid onto selected portions of the build material. The print liquid may include a fusing agent, a curing agent, a binding agent, a detailing agent, a coloring agent, a fusing coloring agent, or any combinations thereof. In some examples, the print assembly  1102  may reside on or interface with one of the movement devices  1036 . Indeed, in certain examples, a movement device  1036  under computer control may position the print assembly  1102  over the build material on the build platform  1016 . Such that, for instance, the print assembly  1102  can eject print liquid onto selected portions of each successive layer of build material on the build platform  1016 . 
     The print assembly  1102  may include nozzles  1104  to eject the print liquid. The print assembly  1102  may include a printbar or printheads, or other type of print assembly. The print assembly may be a printbar having the print nozzles  1104  to eject the print liquid. The nozzles  1104  may be disposed on dies or printheads, or on other substructures, of the printbar. The number of print nozzles  1104  can be up to hundreds or thousands, or more. The diameter of each nozzle  1104  can be in the tens or hundreds of microns. The ejection of the print liquid through the nozzles  1104  may be via pressure differential, a pump, thermal or heat, heating elements, thermal bubble or bubble jet, piezoelectric, and so on. As mentioned, the print assembly  1102  may eject print liquid onto successive layers of build material applied to the build platform  1016 . The print assembly  1102  may eject the print liquid onto selected portions of each layer of the build material under computer control to generate respective layers of the 3D object  1018  being formed. The computer control may be per a model, e.g., 3D model, of the 3D object  1018  to be generated. 
     The 3D printer  1100  includes an energy source  1034  to apply energy to the build material on the build platform  1016  to form the 3D object  1018  from the build material. The presence of the print liquid ejected onto the selected portions of the build material may increase energy transfer into those portions of the build material such that those portions of build material are selectively solidified or fused. The energy source  1034  may include a light source, infrared light source, near-infrared light source, radiation source, heat source, heat lamps, ultraviolet (UV) light source, and so on. The energy source  1034  in conjunction with the print assembly  1102  may print the 3D object  1018  layer-by-layer from build material on the build platform  1016 . 
     Thus, in some examples, the 3D printer  1100  forms the 3D object  1018  layer-by-layer via thermal fusion of the build material on the build platform  1016 . In one example, the build material is a plastic or polymer, such as polyamide. In that example, the formed 3D object  1018  may be a prototype, or a product or product component. In another example, the build material is metal, such as stainless steel. In that example, the formed 3D object  1018  may be a prototype, or a machine part or other metal component that may also be formed, for example, by injection molding. Many other examples are applicable. 
     As indicated, the print liquid may be printing agents such as fusing agents to promote thermal fusion, detailing agents (e.g., water, etc.) to inhibit fusion, coloring agents, and other compounds. In operation, an internal print-liquid supply system  1106  may receive print liquid from the print liquid cartridge held within the printer  1000 . The 3D printer  100  may include the print-liquid supply system  1106  to provide print liquid to the print assembly  1102 . Again, the print liquid may include printing agents or other compounds. In certain examples, the print-liquid supply system  1106  may include at least one pump  1108  to provide a motive force for supply of the print liquid to the print assembly  1102 . In other examples, a pump  1108  is not employed but instead gravity or other motive force is employed to deliver print liquid to the print assembly  1102 . 
     The print-liquid supply system  1106  may include at least one liquid cartridge receiver  1110  to hold an operationally-removable print-liquid cartridge. The print liquid cartridge may be a container that stores print liquid and is inserted by a user into the liquid cartridge receiver  1110 . The liquid cartridge receiver  1110  may be a slot, receptacle, or cavity to receive and secure the print liquid cartridge. The print-liquid supply system  1606  may have multiple liquid-cartridge receivers  1110 , such as for respective different print liquids or redundant same print liquids. 
     In operation, the print-liquid supply system  1106  may receive print liquid from the print liquid cartridge held by the liquid cartridge receiver  1110 . Again, the supply system  1106  may deliver the print liquid to the print assembly  1102 . Lastly, the print-liquid supply system  1106  may include a reservoir vessel  1112  to hold and facilitate delivery of print liquid. The supply system  1106  may include multiple reservoir vessels  1112 . The supply system  1106  may employ a reservoir vessel  1112  in conjunction with a pump  1108  to facilitate delivery of print liquid to the print assembly  1102 . 
     In summary, the printer  1100  may include the print assembly  1102  to eject print liquid onto selected portions of the build material on the build platform  1016  to form the 3D object  1018  layer-by-layer from the build material. The print assembly  1102  may apply print liquid to selected portions of layers of build material applied to the build platform  1016  to form associated layers of the 3D object  1018 . The print assembly  1102  may eject print liquid onto selected portions of successive applications or layers of build material on the build platform  1016  to form successive layers of the 3D object  1018 . In operation, the 3D printer  1100  may lower the build platform  1016  incrementally as each layer of the 3D object  1018  is formed. As indicated, the print liquid may include fusing agent, detailing agent, coloring agent, ink, colorant, pigment, carrier, dye, thermoplastic, binding agent, curing agent, and so on. 
     As discussed, the 3D printer  1100  may also have a build enclosure  1020  which may at least partially contain or otherwise be associated with the build platform  1016  on which the 3D printer  1100  forms the 3D object  1018 . Moreover, the build enclosure  1020  and the associated build platform  1016  together may constitute a build unit. In certain examples, the build unit may be operationally removable. Indeed, while  FIG. 11  depicts a build platform  1016 , the printer  1100  may be manufactured and sold without the build platform  1016  (or the build enclosure  1020 ) in examples with a removable build unit. In other examples, the build unit is not operationally removable. 
     Furthermore, a build unit processing module may include or involve a build unit including the build enclosure  1020  and the build platform  1016 . The build platform  1016  may have holes to allow unsolidified powder to flow through the build platform  1016 . In addition, the build unit processing module may include sieves, vibration sources such as a motor with an eccentric or off-center mass, air flow devices, and other components to remove excess build material, e.g., unsolidified powder, from the build platform  1016 . The 3D object  1018  disposed on the build platform  1016  may cool at an accelerated rate after the excess material or powder is removed from the build enclosure  1510 . In other words, the 3D object  1018  may cool faster with surrounding excess build material removed. In this fashion, the build unit processing module may manage the cooling, e.g., by removing the excess build material. The build unit processing module may provide for discharge of excess material from the build enclosure  1020 . 
     At the conclusion of a print job and after most or all of the excess or unsolidified material or powder is removed from the build enclosure  1020 , the build enclosure  1020  may include a 3D object  1018  with partially-solidified powder caked on the outside of the 3D object  1018 . In certain examples, this partially-solidified powder may be removed by a bead blaster, a brush, or other tools that may be part of the build unit processing module. Partially-solidified powder may be removed from the build enclosure  1020 . Partially-solidified powder may be removed from the 3D object  1018  in the build enclosure  1020  or after the 3D object  1018  has been removed from the build enclosure  1020 . 
     Furthermore, in some examples, the printer  1100  may have a 3D-printed-object recovery zone. Indeed, once some or most of the unsolidified powder has been removed from the 3D object  1018  (and from the build enclosure  1020 ), the 3D object  1018  may be recovered via the 3D-printed-object recovery zone in those examples. In operation, the build platform  1016  may be manually or automatically lifted, e.g., via an underlying piston, towards the top of the build enclosure  1020  to the recovery zone so that a user may recover the 3D object  1018 . In an example, this 3D-printed-object recovery zone may be accessed by a user or machine through a top or side opening of the 3D printer  1100 . The opening may be through an outer housing or casing of the 3D printer  1100 . In some examples, the zone may be accessed by lifting a lid or a removable top of the 3D printer  1100 . In other examples, a door(s) of the 3D printer may be opened to access the zone. 
     The recovery zone may include tools to remove any remaining free build material or powder from the 3D object  1018  and to clean the build platform  1016 . The 3D-printed-object recovery zone may also include containers to store printed 3D objects, a light source to illuminate the zone, and devices to provide air flow to prevent or reduce excess build material from exiting the 3D printer  1100  during recovery of the printed 3D object, and so on. 
     Lastly, while various example 3D printers and printer components have been discussed with respect to  FIG. 10  and  FIG. 11 , it should be emphasized that the material supply system  1002  and associated controller may be employed in other types of 3D printers. Indeed, the material vessels, feeders, conveying system, and controller to adjust operation of the feeders, applies to other example 3D printers. 
       FIG. 12  is a 3D printer  1200  including a new material vessel  1202  that discharges new material  1204  through a feeder  1206  as build material to a pneumatic conveying system  1208 . The pneumatic conveying system  1208  may be dense phase or dilute phase. If dilute phase, the pneumatic conveying system  1208  may be a negative-pressure system or a positive-pressure system. Further, the printer  1200  includes a recycle material vessel  1210  that discharges recycle material  1212  through a feeder  1214  as build material into the pneumatic conveying system  1208 . In the illustrated example, the conveying system  1208  has an air inlet  1216  to intake air as conveying air for transport of the new material  1204  and the recycle material  1212  as build material. The conveying system  1208  may mix in-line the first material  1204  and the recycle material  1212 . The printer  1200  may include a reclaim material vessel  1218  that discharges reclaim material  1219  through a feeder  1220  into the pneumatic conveying system  1208 . Depending on the particular operation, the reclaim material  1219  may be recycle material or classified as new material, or other material. 
     The build material  1222  flowing through the conveying system  1208  may be a combination of the new material  1204 , recycle material  1212 , and reclaim material  1219 . The 3D printer  1200  may have a computing device, control system, or controller to facilitate the feed build material  1222  composition having a specified ratio of new material to recycle material. The control system may facilitate delivery of a specified ratio, for example, by accommodating metering or regulating of the mass, weight, or volume of material dispensed from the new material vessel  1202  and recycle material vessel  1210 . Sensors may be associated with the material vessels  1202 ,  1210 ,  1218  and the conveying system  1208  that provide for determinations of material discharge rates from the vessels  1202 ,  1210 ,  1218 . The computing device or controller may adjust, in response to the determinations, operation of the feeders  1204 ,  1212 ,  1220  to maintain a desired amount of build material  1222  and a specified ratio of new material to recycle material in the build material  1222 . 
     The printer  1200  may include a new cartridge receiver  1224  to hold a removable new material cartridge to supply new material to the new material vessel  1202 . Likewise, the printer  1200  may have a recycle cartridge receiver  1226  to hold a removable recycle material cartridge to supply recycle material to the recycle vessel  1210 . Further, the conveying of build material  1222  may be diverted, as indicated by reference numeral  1228 , to the recycle material cartridge or to the recycle material vessel  1210 . Such may be implemented, for example, with the material  1222  as reclaim material  1219  and with the reclaim material  1219  as recycle material. During the print job, the conveying system  1208  may transport build material  1222  including new material  1204  and recycle material  1212  (and in some instances, reclaim material  1219 ) to a separator  1230 . Again, the conveying system  1208  may mix in-line the new material  1204  and the recycle material  1212 . The separator  1230  (e.g., cyclone, centrifugal separator, etc.) may separate air  1232  from the build material  1222  and discharge the air  1232  through a motive component (e.g., blower) of the conveying system  1208 . The separator  1230  may discharge the build material  1222  to a dispense vessel  1238 . The dispense vessel  1238  may discharge the build material  1222  through a feeder  1238  and feed handling system for a selective solidification module  1240 , build enclosure  1242 , and build platform  1244 . The feed handling system may include, for example, a feed dosing apparatus for a build-material applicator or powder spreader to distribute the feed build material  1222  across the build platform  1244 . 
     The selective solidification module  1240  may include an energy source to apply energy to the build material on the build platform  1244  to form a 3D object. In examples in which a print liquid is employed, such as for binding, curing, or thermal fusion, the selective solidification module  1240  may include a print assembly (e.g., having a printbar with nozzles) to eject print liquid onto selected portions of build material on the build platform  1244 . The 3D printer  1200  may include an internal print-liquid supply system  1246  to provide print liquid to the print assembly, if employed. In some examples, the print liquid may include fusing agents to encourage fusing of the build material. In one example, a fusing agent absorbs near infrared (IR) light to promote melting or fusing of the build material. The fusing agents may be tailored to absorb energy such as light to promote heating and fusing of the build material on the build platform  1244 . The fusing agents may include a vehicle or carrier to hold particles that absorb light or radiation. The print liquid may also include detailing agents which inhibit fusing of the build material on the build platform  1244 . The print liquid may include coloring agents including for colors such as black, cyan, magenta, yellow, and so forth. The print liquid as coloring agents may be applied for cosmetic reasons and other reasons. The print liquid may also be generally clear or low tint. The print liquid may include pigmented inks, specially-formulated inks, and so on. The print liquid may also be binding agents or curing agents, and the like. Again, however, the 3D printer  1200  may not employ print liquid but instead the printer  1200  or selective solidification module  1240  may sinter the build material via a laser, melt the build material via an electron beam, fuse the build material via a thermal printhead, and so on. 
     The printer  1200  may include a second conveying system  1248  (e.g., vacuum, pneumatic, etc.) having a manifold  1250  to withdraw excess build material or excess powder, e.g., powder not becoming part of a 3D object, from the build enclosure  1242  as recovered material  1252 . In examples, such is performed after generation of the 3D object is complete. In another example, such withdrawal of excess build material is performed both during the print job and after completion of the print job. The recovered material  1252  may enter a separation system  1254  in which conveying air  1254  is withdrawn through a motive component  1258  (e.g., a blower, vacuum pump, etc.), as indicated by reference numeral  1260 . The separation system  1254  may also include a filter, sieve, screen, and the like, to remove larger or partially-agglomerated particles from the recovered material  1252 . The separation system  1254  may discharge the treated recovered material  1252  as reclaim material  1219  into the reclaim vessel  1218 . In certain examples, the recovered material  1252  may bypass the reclaim vessel  1218 , as indicated by reference numeral  1262 . If so, a feeder may be employed on the bypass  1262 . 
       FIG. 13  is a 3D printer  1300  having a new material vessel  1302  that discharges new material through a feeder  1304 , such as a rotary feeder, auger, or screw feeder, into a first conveying system  1306  which is a pneumatic conveying system. The feeder  1304  may drop the new material into a conduit of the conveying system  1306 . The feeder  606  may meter or regulate material discharge or otherwise facilitate dispensing of the desired amount of new material from the new material vessel  1302  into the first conveying system  1306 . In addition, the printer  1300  includes a recycle material vessel  1308  that discharges recycle material through a feeder  1310  into the first conveying system  1306 . 
     The new material vessel  1302  may have a weight sensor  1312  and a level sensor  1314 . Likewise, the recycle material vessel  1308  may have a weight sensor  1316  and a level sensor  1318 . A controller of the printer  1300  may adjust operation of the feeders  1304 ,  1310  in response to indications of material discharge amount or rate provided via the weight sensors  1312 ,  1316 . The controller may adjust operation (e.g., rpm) of the feeders  1304 ,  1310  to maintain a desired ratio of new material to recycle material. 
     The 3D printer  1300  may include new cartridge receiver  1320  to hold a removable new material cartridge. The new cartridge receiver  1320  may make available new material from the new material cartridge as build material. The new material vessel  1302  may receive new material from the new material cartridge held by the new cartridge receiver  1320 . The 3D printer  1300  may include a recycle cartridge receiver  1322  to hold a removable recycle material cartridge. The recycle material cartridge, when installed or inserted into the receiver  1322 , may receive excess build material as recycle material and make recycle material available as build material. The recycle material vessel  1308  may receive recycle material from the recycle material cartridge held by the recycle material receiver  1322 . 
     The printer  1300  may include a reclaim material vessel  1324  which discharges reclaim material  1328  through a feeder  1326  into the first conveying system  1306 . The reclaim vessel  1328  may have a weight sensor  1330  and a level sensor  1332 . The build material  1334  may include reclaim material from the reclaim material vessel  1328  in addition to the recycle material from the recycle material vessel  1308  and new material from the new material vessel  1302 . 
     Conveying air may flow through the first conveying system  1306 . An air intake such as a filtered manifold or an open conduit as a “lung” may receive, pull in, and/or filter air (e.g., ambient air) as conveying air for the first conveying system  1306 , and also for the second conveying system discussed below. The first conveying system  1306  may transport the build material  1334 , e.g., a mix of new material and recycle material from the vessels  1302  and  1308 , respectively. In some instances, the build material  1334  may also include reclaim material  1328 . In the illustrated example, the first conveying system  1306  may convey the build material  1334  to a separator  1336  associated with a dispense vessel  1338 . The dispense vessel  1338  may be a feed hopper. The separator  1336  may include a cyclone, filter, and so forth. The separator  1336  may separate conveying air  1340  from the build material  1334 . The build material  1334  minus most or all of the conveying fluid  1346  may flow into the dispense vessel  1338 . A feeder  1342  may receive build material from the feed or dispense vessel  1338  and discharge the build material to a powder handling system  1344  for the 3D printing. The dispense vessel  1338  may have a level sensor  1346  and other sensors. The level sensor  1346  may measure and indicate the level or height of build material in the dispense vessel  1338 . 
     The first conveying system  1306  may divert build material  1334  via a diverter valve  1348 , as indicated by reference number  1350 , to an alternate vessel  1352  or hopper through a separator  1354  such as cyclone, filter, etc. The alternate vessel  1352  may discharge the build material  1334  through a feeder  1356  and diverter valve  1358  to either the recycle material cartage or the recycle material vessel  1308 . This diversion of build material  1334  as recycle material  1360  may occur, for instance, when the build material  1334  is primarily recycle material or reclaim material  614  and with the desire to fill a recycle material cartridge or the recycle material vessel  1308 . Moreover, as with other material vessels, the alternate vessel  1352  may have a level sensor  1362 . 
     The separator  1354  associated with the alternate vessel  1352  may remove conveying air  1364  from the build material  1334 . The build material  1334  minus most or all of the conveying air  1364  may discharge from the separator  1354  into the alternate vessel  1352 . In the illustrated example, the conveying air  1364  from the separator  1354  may flow to a Y-fitting  1366 , where the conveying air  1364  is combined with the conveying air  1340  from the separator  1336  associated with the dispense vessel  1338 . A Y-fitting  1366  may be a conduit fitting having two inlets and one outlet. The combined conveying air  1368  may be pulled from the Y-fitting  1366  by a motive component  1370  of the first conveying system  1306  and discharged  1372  to the environment or to additional equipment for further processing. In some examples, the combined conveying air  1364  may flow through a filter  1374  in route to the motive component  1370 . 
     The motive component  1370  applies motive force for the conveying air in the first conveying system  1306  to transport build material. The motive component  1370  may be an air blower, eductor, ejector, vacuum pump, or other motive component. Because the first conveying system  608  is generally a pneumatic conveying system, the motive component may typically include a blower such as a centrifugal blower, fan, axial blower, and the like. 
     As for the 3D printing, as mentioned, the dispense vessel  1338  may discharge the build material  1334  through a feeder  1342  to a powder handling system  1344 . The feeder  1342  and the powder handling system  1344  may provide a desired amount and layers of build material  1334  across the build platform  1376 . 
     The powder handling system  1344  may include a feed apparatus, dosing device, build-material applicator, or powder spreader, and the like, to apply the build material to the build platform  1376  at the build enclosure  1378 . The printer  1300  may form a 3D object from build material  1334  on the build platform  1376 . 
     After the 3D object is complete or substantially complete on the build platform  1376 , a second conveying system  1380  having a vacuum manifold  1382  may remove excess build material from the build enclosure  1378  as recovered material  1384 . Alternatively, the second conveying system  1380  is not so employed. For example, the excess build material may be off-loaded with the 3D object or removed by a stand-alone vacuum. 
     If employed, the second conveying system  1380  may convey the recovered material  1384  through a cyclone or filter  1386  to separate the recovered material  1384  from the conveying air  1388 . The conveying air  1388  is discharged through a motive component  1390  of the second conveying system  1380 . The motive component  1390  may be a blower, fan, eductor, ejector, vacuum pump, or other type of motive component. In this example, the recovered material  1384  may discharge from the fitter  1386  and enter a sieve  1392  where larger particles (e.g., solidified build material not incorporated into the 3D object) may be removed. The sieve  1392  may have a level sensor  1394  which monitors the level or height of solid material in the sieve  1392 . The recovered build material  1384  minus the larger particles may enter the reclaim vessel  1324  as reclaim material  1328 . Moreover, in certain instances, the recovered material  1384  may bypass the separator, sieve, and reclaim material vessel  1324  and flow into a conduit of the first conveying system  1306 , as indicated by the dashed line  1396 . Lastly, the vessels, conveying systems, and associated equipment of the 3D printer  1300  may include instrumentation such as pressure sensors and temperature sensors, and so forth. 
       FIG. 14  is a 3D printer  1400  having an internal material vessel  1402  that discharges build material through a feeder  1404  into an internal conveying system  1406 . The printer  1400  may include another internal material vessel  1408  to discharge build material through a feeder  1410  to the conveying system  1406 . The build material in the first vessel  1402  may be different than the build material in the second vessel  1408 . The printer  1400  may have a controller to adjust operation of the feeders  1404 ,  1406  to maintain a composition and discharge rate of the build material for the 3D printing. Further, the printer  1400  may include an additional internal material vessel  1412  to discharge through a feeder  1414  recovered material  1416  as build material into the conveying system  1406 . The conveying system  1406  may transport the build material to a dispense vessel  1418  which may supply build material for 3D printing. In the illustrated example, the dispense vessel  1418  is disposed in an upper portion of the printer  1400 . Moreover, while the flow arrow of the conveying system  1406  for the feed build material is depicted outside of the 3D printer for clarity, the conveying system  1406  is internal within the housing of the printer  1400 . 
     The 3D printer  1400  may form a 3D object from the build material on a build platform  1420  associated with a build enclosure  1422 . Unfused or excess build material  1423  may be recovered from the build enclosure  1422 . The excess build material may be treated to give the recovered material  1416 . Further, the printer  1400  may include material cartridge receivers  1424  to hold a removable material cartridge inserted by a user. The receiver  1424  may make available build material from the material cartridge for the 3D printing. The internal material vessels  1402 ,  1408  may receive build material from the material cartridge in the receiver  1424 . Moreover, the material vessels  1402 ,  1408  may be operationally removable. Lastly, in the illustrated example, the 3D printer  1800  has doors or access panels  1426  and a top or lid  1428 . 
       FIG. 15  is a 3D printer  1500  that generates a 3D object from build material on a build platform. The build material may be powder. The printer  1500  has covers or panels over compartments  1502  for respective internal material vessels that supply build material. The material vessels may discharge build material through feeders into an internal conveying system for the 3D printing. The printer  1500  may have a controller to adjust operation of the feeders to maintain a desired composition of build material including a specified ratio of materials in the build material. Moreover, in certain examples, the internal material vessels may be operationally removable via user-access to the compartments  1502 . The printer  1500  may generally have a housing and with components internal to the housing for handling of build material. The printer  1500  has a top surface  1504 , a lid  1506 , and doors or access panels  1508 . The printer  1500  may include a compartment  1510  for an additional internal material vessel such as a reclaim material vessel that recovers unfused or excess build material from a build enclosure of the printer  1500 . The 3D printer  1500  may include a material cartridge receiver  1512  to hold a removable material cartridge  1514 . The printer  1500  may have more than one material cartridge receiver  1512 . The material cartridge receiver  1512  may make material available from the material cartridge  1514  as build material for the material vessels in compartments  1502 . In some examples, the material cartridge receiver  1512  may accept build material into the material cartridge  1514  from the 3D printing, such as from the build enclosure or from the internal reclaim material vessel. 
     For examples of a 3D printer  1500  that employ a print liquid, the printer  1500  may include print-liquid supply system  1516  to receive and supply print liquid for the 3D printing. The supply system  1516  includes a cartridge receiver assembly  1518  to receive and secure removable print-liquid cartridges  1520 . The supply system  1516  may include a reservoir assembly  1522  having multiple vessels or reservoirs for holding print liquid collected from the print liquid cartridges  1520  inserted into the cartridge receiver assembly  1518 . The vessels may provide feed capacity or surge capacity of the print liquid. The print liquid may be provided from the vessels or reservoirs to the 3D printing such as to a print assembly or printbar above a build enclosure and build platform. 
     Lastly, in the illustrated example, the printer  1500  includes a user control panel or interface  1524  associated with a computing system or controller of the printer  1500 . The control interface  1524  and computing system or controller may provide for control functions of the printer  1500 . Moreover, the fabrication of the 3D object in the 3D printing may be under computer control. A model and automated control may facilitate the layered manufacturing and additive fabrication. The model may be, for example, a computer aided design (CAD) model, a similar model, or other electronic data source. The computer system may have a hardware processor and memory. The hardware processor may be a microprocessor, CPU, ASIC, printer control card, or other circuitry. The memory may include volatile memory and non-volatile memory. The computer system or controller may include firmware or code, e.g., instructions, logic, etc., stored in the memory and executed by the processor to direct operation of the printer  1500  and to facilitate various techniques discussed herein. 
     In summary, an example includes a 3D printer that forms a 3D object from build material such as powder. The printer has an internal first material vessel to discharge first material through a first feeder as build material; an internal second material vessel to discharge second material through a second feeder as build material; and a controller that adjusts operation of the first feeder and the second feeder to maintain the build material as having a specified ratio of the first material to the second material. The controller may adjust the operation of the first feeder and the second feeder in response to, or based on, indication of material discharge rate of the first material vessel and the second material vessel. In some examples, a first sensor associated with the first material vessel provides the indication of material discharge rate from the first material vessel. A second sensor associated with the second material vessel may provide the indication of material discharge rate from the second material vessel. The example 3D printer may include an internal conveying system to transport build material including the first material and the second material. The first material vessel and the second material vessel may discharge the first material and the second material, respectively, into the conveying system. If so, a sensor associated with the internal conveying system may provide the indication of material discharge rate of the first material vessel and the second material vessel. In certain examples, the conveying system mixes in-line the first material and the second material. Furthermore, the 3D printer may include a feed vessel or dispense vessel to make available build material for the 3D printer to form the 3D object. If so, the internal conveying system to transport build material, to the dispense vessel contemporaneous with the 3D printer forming the 3D object. 
     While the present techniques may be susceptible to various modifications and alternative forms, the examples discussed above have been shown by way of example. It is to be understood that the techniques are not intended to be limited to the particular examples disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the scope of the present techniques.