Patent Publication Number: US-2021170687-A1

Title: Additive manufacturing system leak control

Description:
BACKGROUND 
     Additive manufacturing systems are increasingly being used to fabricate three-dimensional physical objects for prototyping and/or production purposes. The physical object is constructed layer-by-layer. Some additive manufacturing systems utilize hermetically-sealed enclosures filled with inert gases, and/or are located and operated in specially-controlled environments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an additive manufacturing system in accordance with an example of the present disclosure. 
         FIG. 2  is a schematic representation of another additive manufacturing system in accordance with an example of the present disclosure. 
         FIG. 3  is a schematic side view representation of a build mechanism usable with the additive manufacturing system of  FIG. 2  in accordance with an example of the present disclosure. 
         FIG. 4  is a schematic representation of the additive manufacturing system of  FIG. 2  having an additional pressure-controlled zone in accordance with an example of the present disclosure. 
         FIG. 5  is a flowchart according to an example of the present disclosure of a method of operating an additive manufacturing system. 
     
    
    
     DETAILED DESCRIPTION 
     In additive manufacturing systems, a 3D digital representation (design) of the object to be fabricated may first be divided (“sliced”) into a series of thin, adjacent parallel planar slices. The 3D object may then be fabricated by printing the object layer-by-layer. Each slice of the representation corresponds to a layer of the physical object to be printed. During fabrication, the next layer is formed on top of the adjacent previous layer. In one example, each layer is about 0.1 millimeter in thickness. Such a fabrication process is often referred to as “additive manufacturing”: 
     Some additive manufacturing systems use a build material as the material from which each layer is fabricated. In one example, the build material is a fine powder, such as for example polyamide (nylon). In one example, the polyamide particles are in the range of 20 to 80 microns in size. In one example, the particles have an average size of 50 microns. During fabrication of each layer, the regions of the build material which correspond to the location of the object within the corresponding slice, are selectively fused together, while the other regions remain in unfused form. Once the object is completely fabricated, any unfused build material is removed, leaving the fabricated 3D object behind. In some examples, the unfused build material is removed within the additive manufacturing system, while in other examples the unfused build material is removed external to the additive manufacturing system. 
     In one example, the additive manufacturing system may have a build mechanism which uses a laser to selectively fuse the build material layer-by-layer. To do so, the laser is accurately positioned to irradiate the regions of the build material to be fused in each layer. Such a laser-based system with accurate position control for the fusing laser can be costly, however. 
     Another example additive manufacturing system may have a build mechanism that uses a simpler and less expensive heat source to fuse the build material in each layer, instead of a laser. The build material may be of a light color, which may be white. In one example, the build material is a light-colored powder. A print engine controllably ejects drops of a liquid fusing agent onto the regions of powder which correspond to the location of the object within the corresponding digital slice. In various examples, the fusing agent is a dark colored liquid such as for example black pigmented ink, a UV absorbent liquid or ink, and/or other liquid(s). A heat source, such as for example one or more infrared fusing lamps, is then passed over the entire print zone. The regions of the powder on which the fusing agent have been deposited absorb sufficient radiated energy from the heat source to melt the powder in those regions, fusing that powder together and to the previous layer underneath. However, the regions of the powder on which the dark colored liquid have not been deposited remain light in color, and as such do not absorb sufficient radiated energy to melt the powder, but rather reflect the radiated energy. As a result, the light-colored regions of the layer remain in unfused powdered form. To fabricate the next layer of the object, another layer of powder is deposited on top of the layer which has just been fabricated, and the printing and fusing processes are repeated for the next digital slice. This process continues until the object has been completely fabricated. 
     In such an additive manufacturing system, the 3D object may be built in a build chamber which houses various components of the build mechanism of the system, such as for example the print engine, fusing lamp(s), and the powder. To maintain proper operation of the system and high reliability, some system components, such as for example printheads of the print engine and the fusing lamp(s), are cooled during system operation. To do so, a clean supply of air, filtered to remove contaminants in the atmosphere external to the system, is flowed into the build chamber and into some of these components. In some examples, the clean supply of air is cooling air which is used to clean at least some of the system components. Some system components could be damaged if the air becomes contaminated. One source of air contamination is small particles of certain types of build material, such as for example powder, which become airborne and suspended as particulates, either inside or outside the system. Air contaminated with powder particulates should be prevented from escaping the additive manufacturing system into the air outside the system. One way to avoid these situations is to hermetically seal the interior of the additive manufacturing system to the external atmosphere. In addition, some hermetically sealed additive manufacturing systems may also fill the interior with an inert gas during 3D object fabrication. The hermetic seals prevent air from the atmosphere external to the system from entering the system, and also prevent the gases inside the system from escaping into the external atmosphere. However, hermetically sealing the entire additive manufacturing system enclosure can be quite expensive, both in parts and in manufacturing process costs. Access doors and their hermetic seals can become product reliability issues which increase service and/or warranty costs. And supplies of the inert gases are an on-going expense. 
     Referring now to the drawings, there is illustrated an example of an additive manufacturing system that uses clean air for certain purposes, which may in some examples include cooling. Various closed volumes of the system are not completely hermetically sealed during operation (a condition defined herein as “unsealed”), allowing leaks at which the ambient air outside the system and the air within the different closed volumes of the system could come into contact. As such, ambient air could enter the additive manufacturing system, and gases from the system could escape to the external atmosphere. To inhibit or prevent such situations, various regions of the system are maintained at different pressures in order to control the direction of air leaks, both within the system and between the system and its external environment. The chosen leakage directions inhibit or prevent leakage of ambient air into the system, and inhibit or prevent leakage of powder particulates from the system to the external atmosphere. The use of air within the system, rather than an inert gas, is more convenient and less costly, as it eliminates the need to supply these gases to the system. Also, the omission of hermetic seals reduces the parts count (no such seals are used); simplifies certain system subsystems and parts (e.g. sheet metal parts can have open corners); decreases the manufacturing cost of the system; and/or reduces the cost of ownership of the system. 
     Considering now an additive manufacturing system, and with reference to  FIG. 1 , the additive manufacturing system  100  includes an air supply enclosure  110  and a processing chamber  140 . An air pathway  170  connects the air supply enclosure  110  and the processing chamber  140 . 
     The system  100  is disposed in an ambient air environment  105 . The ambient air in the environment  105  is at an atmospheric pressure. 
     The air supply enclosure  110  houses a body of clean cooling air for the system  100  during operation. The air supply enclosure  110  is unsealed. Due to the lack of sealing, the enclosure  110  can be open to the ambient air environment  105  in at least one location. The enclosure  110  may also be open to the processing chamber  140  at another location  125   
     The processing chamber  140  houses the build mechanism of the system and the 3D object as it is being additively manufactured. The processing chamber  140  is unsealed. Due to the lack of sealing, the processing chamber  140  can be open to the ambient air environment  105  in at least one location  145  of the chamber  140 , in addition to being open to the air supply enclosure at location  125 . 
     The air pathway  170  provides an air passage for the clean air in the air supply enclosure  110  to flow  175  into the processing chamber  140 . In some examples, the air pathway  170  may include, may be coupled to, or may be formed within other components, such as for example an air duct, an air conduit, an air cooling circuit, a chamber, a fan, a functional subsystem of the additive manufacturing system, and/or other components. 
     The pressure in the air supply enclosure  110  is maintained at a first pressure greater than the ambient air pressure in the environment  105 . This pressure differential between the enclosure  110  and the ambient air environment  105  determines the direction of air leakage  117  that occurs through the location  115 . The pressure differential inhibits unfiltered ambient air from outside the system  100  from leaking into the air supply enclosure  110 . Instead, if leakage  117  through the location  115  does occurs, it is clean air from the enclosure  110  that leaks out of the enclosure  110  to the atmosphere of the ambient environment  105  external to the system  100 . 
     The pressure in the processing chamber  140  is maintained at a second pressure less than the ambient air pressure in the environment  105 . This pressure differential between the processing chamber  140  and the ambient air environment  105  determines the direction of air leakage  147  that occurs through the location  145 . The pressure differential inhibits air in the processing chamber  140 , which may have been contaminated with powder particulates, from leaking out of the processing chamber  140  to the atmosphere of the ambient environment  105  external to the system  100 . Instead, if leakage  147  through the location  145  does occurs, it is ambient air from the environment  105  external to the system  100  that leaks into the processing chamber  140 . 
     In addition, the pressure differential between the processing chamber  140  and the air supply enclosure  110  determines the direction of leakage  127  that occurs through the location  125 . The pressure differential inhibits the contaminated air in the processing chamber  140  from leaking into the air supply enclosure  110 . Instead, if leakage  127  through the location  125  does occur, it is clean air from the enclosure  110  that leaks into the processing chamber  140 . 
     Considering now another additive manufacturing system, and with reference to  FIG. 2 , the additive manufacturing system  200  includes an air supply enclosure  210 , a build chamber  240 , and air ducts  270 A- 270 F, which in some examples are the same as or similar to the air supply enclosure  110 , processing chamber  140 , and air duct  170  ( FIG. 1 ) respectively. The dashed lines in  FIG. 2  indicate the direction of air flow within the system  200 , as is described below. 
     The build chamber  240  includes a print engine  242  and at least one heat source  244 . In one example, a heat source  244  may be an infrared fusing lamp. The print engine  242  and heat source(s)  244  are mounted on a moveable carriage  246 . The carriage  246  is controllably movable in a direction  248 . In some examples, the print engine  242 , heat source(s)  244 , and carriage  246  are part of a build mechanism disposed in the build chamber  240 . The build chamber  240  is one example of a processing chamber  140  ( FIG. 1 ) of an additive manufacturing system. In various examples, an additive manufacturing system may include alternative and/or additional processing chambers. 
     Before discussing the system  200  further, consider with reference to  FIG. 3  one example build mechanism  300 . The build mechanism  300  is disposed in the build chamber  242  of some example systems  200 . A bucket  302  contains a supply of build material  304 , which may be light-colored powder, useable to fabricate a 3D object. A plate  310  is coupled to an elevating support mechanism  315  which can raise or lower the printing surface  310  in the direction  306 . A top surface of the plate  310  is a printing surface (or build surface)  312  of the build mechanism  300  for the first layer of a 3D object. During fabrication of a 3D object, the plate  310  is initially positioned so that the printing surface  312  is at its initial location to receive the first layer. The surface is then lowered, and a powder lifting mechanism (not shown) causes some of the powder  304  to move from a build material store below the plate  310  into a feed tray (not shown) at an edge of the plate. A spreading mechanism (not shown) then arranges the powder  304  into a powder layer of a desired thickness on the printing surface  312 . 
     The print engine  242  then controllably emits drops of a fusing agent in the direction  322  onto the regions of the powder  304  on the printing surface  312  which correspond to the slice of the 3D object to be printed. The print engine  242  may employ thermal or piezo printhead printing technology to emit the drops. In one example, printheads (not shown) in the print engine  242  collectively span the width of the printing surface  312  (i.e. the direction into and out of  FIG. 2 ). In one example, multiple printheads are disposed end-to-end in a staggered arrangement which spans the width of the printing surface  312 . The carriage  246  is controllably movable in the direction  248  by a positioning mechanism such that the printheads can print on any position across the length of the surface  312  (i.e. the span of the surface  312  in the horizontal direction of  FIG. 2 ). In one example, the positioning mechanism may include a slider bar  320  along which the carriage  246  is moved. In one example, the drops are emitted in a single pass of the print engine  242  over the printing surface  312 . 
     In some examples, the powder  304  on the printing surface  312  is preheated using a fixed-position heat source  330 , which may be an infrared lamp which evenly preheats all of the powder  304  on the printing surface  312 . In some examples the powder  304  may be preheated to a temperature near, but below, its melting point. After the fusing agent has been printed onto the powder  304 , the heat source(s)  244  are then passed over the entire printing surface  312 . The regions of the powder  304  on which the dark colored liquid have been deposited absorb sufficient radiated energy from the heat source(s)  244  to melt the powder  304  in those regions. The melted powder  304  fuses together, and fuses to any previously-fabricated layer underneath. However, the regions of the powder  304  on which the dark colored liquid have not been deposited do not absorb sufficient radiated energy from the heat source(s)  244  to melt the powder  304 ; instead these light-colored powder regions reflect at least some of the radiated energy. As a result, the light-colored regions remain in unfused form. In one example, the melting and fusing is accomplished during a single pass of the heat source(s)  244  over the print surface  312 . In one example, the carriage  246  moves in one direction to emit the drops from the print engine  242 , and then moves in the opposite direction to melt and fuse the powder  304 . 
     To fabricate the next layer of the object, the plate  310  is lowered further into the bucket  302  by the support mechanism  315  by an amount corresponding to the thickness of a layer, and powder  304  in the bucket  302  is formed into a new powder layer as explained above. The printing and fusing processes described above are then repeated to fabricate the next layer. This process continues until the desired 3D object has been completely fabricated. 
     In some examples, the carriage  246 , including the print engine  242  and heat source(s)  244 , is disposed substantially parallel to, and quite close to, the printing surface  312 . In one example, they are spaced apart by a distance  324  of about 2.3 millimeters, and the carriage  246  can move at a velocity of 20 inches per second. As a result, the movement of the carriage  246  during fabrication can cause some of the powder  304  to become airborne and contaminate the air by forming powder particulates in the air around the print engine  242 . Powder may also become airborne in other ways, such as when the liquid droplets impact the powder layer, and/or by the mechanism which forms the powder into a layer on the printing surface  312 . The powder particulates and contaminated air are undesirable, as discussed heretofore. 
     Returning to the additive manufacturing system  200 , and again with reference to  FIG. 2 , the air supply enclosure  210  and the build chamber  240  are unsealed. Leakage can occur between the ambient air environment  205  outside the system  200  and the air supply enclosure  210  at opening  212 ; between the ambient air environment  205  outside the system  200  and the build chamber  240  at opening  242 ; and/or between the air supply enclosure  210  and the build chamber  240  at opening  214 . The openings  212 ,  214 ,  242  are representative of locations at which the air supply enclosure  210  and the build chamber  240  are unsealed; there can be many such openings, at many different locations in the enclosure  210  and chamber  240 . 
     To avoid contaminated air in the build chamber  240  from entering either the ambient environment  205  and/or the air supply enclosure  210 , and to avoid unfiltered ambient air in the ambient environment  205  from entering the air supply enclosure  210 , the pressures in the air supply enclosure  210  and the build chamber  240  are controlled relative to each other and to the ambient environment  205 . As discussed hereinafter, the pressure in the air supply enclosure  210  is maintained above the atmospheric pressure in the ambient environment  205 , and the pressure in the build chamber  240  is maintained below the atmospheric pressure in the ambient environment  205 . By doing so, the direction of leakage at the openings  212 ,  214 ,  242  is controlled. Any leakage at location  212  will be leakage of clean air in the air supply enclosure  210  into the ambient environment  205 . Any leakage at location  214  will be leakage of clean air in the air supply enclosure  210  into the build chamber  240 . And any leakage at location  242  will be leakage of unfiltered ambient air in the ambient environment into the build chamber  240 . In one example, the ambient environment is at a pressure of 0 Pa (Pascals); the air supply enclosure  210  is maintained within a positive pressure range that is nominally a pressure of +15 Pa; and the build chamber  240  is maintained within a negative pressure range that is nominally a pressure of −15 Pa. The magnitudes of the specific nominal pressures and pressure ranges utilized may be large enough to ensure that there are no local pressure effects in the enclosure  210  or build chamber  240  that interfere with the desired direction of leakage, but small enough to minimize fan size and energy usage for their operation. 
     Clean air is generated by a fan  220  and a filter  218  coupled to an inlet  216  of the air supply enclosure  210 . In some examples the fan  220  and/or the filter  218  may be disposed at the inlet  216 , or they may be spaced away from the inlet  216  and connected to the inlet  216  by a duct. The fan  220  has a flow rate sufficient to pull sufficient air through the filter  218  and into the fan intake  219  to supply clean air to the build chamber  240  at a given total flow rate, to compensate for leakage through the openings  212 ,  214 , and to maintain the positive pressure in the enclosure  210 . The filter  218  is of an appropriate size and composition to prevent particles (particulates) above a predetermined size from entering the enclosure  210 . In some examples, the enclosure  210  also includes a baffle  222  at a surface of the enclosure  210 . The baffle  222  may include exit holes sized to allow a certain flow of clean air to escape from the enclosure  210  to the ambient environment  205  at a given pressure differential across the baffle  222 . In this way the baffle  222  can assist in maintaining the pressure in the enclosure  210  within the desired pressure range, for example when the amount of air flow from the enclosure  210  to the build chamber  240  is varied. 
     Contaminated air is removed from the build chamber  240  by a fan  250  and a filter  248  disposed at an outlet  246  of the build chamber  240 . In some examples the fan  250  and/or the filter  248  may be disposed at the outlet  246 , or they may be spaced away from the outlet  246  and connected to the outlet  246  by a duct. The fan  250  has a flow rate sufficient to expel sufficient contaminated air through the filter  248  to compensate for leakage through the openings  242 ,  214 , and to maintain the negative pressure in the build chamber  240 . The filter  248  is of an appropriate size and composition to prevent powder particulates in the contaminated air within the build chamber  240  from passing through the filter  248  and being expelled into the ambient environment  205  by the fan  250 . 
     Air ducts  270 A through  270 F receive cool cleaning air from the air supply enclosure  210 . In some examples, one of fans  272 A through  272 F supplies clean air from the air supply enclosure  210 , through the corresponding air duct  270 A through  270 F, which delivers it to a corresponding subsystem of the system  200 . Fan  272 C and duct  270 C deliver clean air to the print engine  242  disposed in the build chamber  240 . Fans  272 B,  272 D and ducts  270 B,  270 D deliver clean air to the two heat sources  244  disposed in the build chamber  240 . Flowing clean air onto or past the print engine  242  and heat sources  244  inhibits or reduces buildup of powder particulates on the print engine  242  and heat sources  244 . 
     In some examples, the system  200  also includes other subsystems which receive clean air during system operation but which are not disposed within the build chamber  240 . In some examples, the pressure in the subsystems  260 ,  270  is uncontrolled. However, the pressure differential in the subsystems  260 ,  270  between the air supply enclosure  210  at their intake and the build chamber  240  at their outlet ensures that air flows from these subsystems  260 ,  270  into the build chamber  240 , rather than in the opposite direction. Preventing contaminated air from the build chamber  240  from entering the subsystems  260 ,  270  maintains their cleanliness. 
     In some examples, the total aggregated working flow rate of fans  272 A- 272 F, plus the leakage rates of flows  213  and  215  at openings  212  and  214 , is lower than the maximum flow rate of fan  220 . In addition, the total aggregated working flow rates of fans  272 A- 272 F, plus the leakage flow rates of flows  243  and  215  at openings  242  and  214  is lower than the maximum flow rate of fan  250 . In one example, fans  220 ,  250  each have a maximum flow rate of 10 kilograms/minute; the total aggregated working flow rate of fans  272 A- 272 F is 6 kilograms/minute; and the rates of leakage flows  213 ,  215 ,  243  are each 0.500 kilograms/minute. This ensure that fans  220 ,  250  can maintain the desired pressures in air supply enclosure  210  and build chamber  240 . In a steady-state situation, the aggregated working flow rates plus the leakage flow rates are equal to the working flow rate of the corresponding fan  220 ,  250 . The term “working flow rate” denotes the flow rate of a fan operating at an intended fan speed. The term “leakage rate” of a flow denotes the flow rate through an opening. 
     In some examples, the fans  220 ,  250  operate at a fixed speed. In other examples, the speeds of the fans  220 ,  250  are dynamically controlled based on the measured pressure in the air supply enclosure  210  and build chamber  240  respectively. Dynamically controlling the fan speed can allow the pressure to be maintained within a narrower pressure range than fans which operate at a fixed speed. 
     In some examples, the system  200  includes a controller  280 . In various examples, some or all of the controller  280  may be implemented in hardware, firmware, software, or a combination of these. In some examples where the controller  280  is implemented in whole or in part in firmware or software, the controller  280  may include a memory  282  having the firmware or software instructions, including instructions which measure the pressure and control the speed of the fans  220 ,  250 . The controller  280  may also include a processor  284  which is communicatively coupled to the memory  282  to access and execute the instructions. 
     In examples that include dynamic fan speed control, a pressure sensor  224  is disposed in the air supply enclosure  210  to detect the air supply enclosure pressure, and a pressure sensor  294  is disposed in the air build chamber  240  to detect the build chamber pressure. The controller  280  measures the pressure sensor  224  to determine the pressure in the air supply enclosure  210 . The pressure detected by the pressure sensor  224  is sent via signal  225  from the sensor  224  to the controller  280 . The controller  280  is also coupled to a fan speed control  226 . The fan speed control  226  receives a signal  227  from the controller  280  indicating the desired speed of the fan  220 , and sends a corresponding signal to the fan  220  to set it to that desired speed. In operation, the controller  280  calculates the desired speed of the fan  220  based on the measured pressure, and commands the fan speed control  226  to set the fan  220  to the desired speed. If the pressure is too high, the fan speed is reduced, and if the pressure is too low, the fan speed is increased. By continuously repeating this process, closed-loop pressure control is implemented to maintain the pressure in the air supply enclosure  210  within a predetermined pressure range above the pressure in the ambient environment  205  of the system  200 . Doing so can compensate for variations in the system, such as for example the buildup of particulates removed from the ambient air in the filter  218  over time. This in turn can increase the time interval between filter replacements. 
     The controller  280  similarly measures the pressure sensor  294  to determine the pressure in the build chamber  240 . The pressure detected by the pressure sensor  294  is sent via signal  295  from the sensor  294  to the controller  280 . The controller  280  is also coupled to a fan speed control  296 . The fan speed control  296  receives a signal  297  from the controller  280  indicating the desired speed of the fan  250 , and sends a corresponding signal to the fan  250  to set it to that desired speed. In operation, the controller  280  calculates the desired speed of the fan  250  based on the measured pressure, and commands the fan speed control  296  to set the fan  250  to the desired speed. If the pressure is too high, the fan speed is reduced, and if the pressure is too low, the fan speed is increased. By continuously repeating this process, closed-loop pressure control is implemented to maintain the pressure in the build chamber  240  within a predetermined pressure range below the pressure in the ambient environment  205  of the system  200 . Doing so can compensate for variations in the system, such as for example the buildup of particulates removed from the contaminated build chamber air in the filter  248  over time. This in turn can increase the time interval between filter replacements. 
     In addition to controlling the fan speed, in some example the controller  280  can also control various other functions and operations of the system  200 . These can include the movement of the carriage  246 , the ejection of drops of the liquid from the print engine  242 , raising and lowering of the print surface  312  ( FIG. 3 ) within the bucket  302 , and operation of other functional subsystems of the additive manufacturing system. 
     As mentioned heretofore, the pressure in the subsystems  260 ,  270  of  FIG. 2  is uncontrolled. However, in other examples it can be desirable to have at least one subsystem in which a subsystem pressure is controlled in a specified manner. Considering now an additive manufacturing system having an additional pressure-controlled zone, and with reference to  FIG. 4 , a system  400  is similar to system  200  ( FIG. 2 ). For clarity of illustration, numerous features of the system  400  which are the same as or similar to the corresponding details of the system  200  are omitted from  FIG. 4 . 
     A subsystem  460  is disposed between, and external to, the air supply enclosure  210  and the build chamber  240 , in a similar manner to subsystems  260 ,  270  ( FIG. 2 ). A fan  472 A delivers clean air from the air supply enclosure  210  through air duct  470 A to the subsystem  460 . Air exits the subsystem  460  into the build chamber  240 . 
     A pressure sensor  464  is disposed in the subsystem  460  to measure the pressure therein. A fan speed control  476  is coupled to the fan  472 A to control the fan speed. The pressure sensor  464  is the same as or similar to pressure sensors  224 .  294 , and the fan speed control  476  is the same as or similar to fan speed controls  226 ,  296  ( FIG. 2 ). 
     In order to ensure that air flows between the air supply enclosure  210 , the subsystem  460 , and the build chamber  240  occur in the desired direction, the pressures in the air supply enclosure  210 , the subsystem  460 , and the build chamber  240  are maintained at specific levels relative to each other, and relative to the ambient air environment  205  external to the system  400 . The pressure in the subsystem  460  is maintained at a pressure below the ambient air pressure of the environment  205 , below the pressure in the air supply enclosure  210 , and above the pressure in the build chamber  240 . In one example, where the ambient environment is at a pressure of 0 Pa, the air supply enclosure  210  is maintained at a pressure of +15 Pa, the subsystem  460  is maintained at a pressure of −15 Pa, and the build chamber  240  is maintained at a pressure of −30 Pa. As a result of these pressure differentials, clean air flows from the air supply enclosure  210  into the subsystem  460 , and from the subsystem  460  into the build chamber  240 . Contaminated air from the build chamber  240  does not flow back into the subsystem  460 , and air from the subsystem  460  does not flow back into the air supply enclosure  210 . 
     Furthermore, in examples where the subsystem  460  is unsealed, the direction of air leakage between the subsystem  460  and the ambient air environment  205  at an opening  462  in the subsystem  460  can be controlled by the pressure maintained in the subsystem  460 . By maintaining the pressure in the subsystem  460  below the pressure of the ambient air environment  205 , any air leakage will be from the environment  205  into the subsystem  460 . In some examples, a subsystem  460  which produces contaminated air. For example, the subsystem  460  may be a service station that cleans fluid and/or powder from printheads, and maintaining the pressure in the subsystem  460  below the pressure of the ambient air environment  205  will inhibit or prevent particulates in the subsystem  460  from escaping through the opening  462  into the ambient air environment  205 . 
     In another example, if the pressure maintained in the subsystem  460  were to be changed from −15 Pa to +7 Pa, any air leakage would occur in the opposite direction, from the subsystem  460  to the ambient air environment  205 . 
     The system  400  includes a controller  480 , which may be the same as or similar to the controller  280  ( FIG. 2 ). In some examples where the controller  280  is implemented in whole or in part in firmware or software, the controller  480  may include a memory  482  having firmware or software instructions, including instructions which measure the pressure in the subsystem  460  and control the speed of the fan  472 A. The controller  480  may also include a processor  284  which is communicatively coupled to the memory  482  to access and execute the instructions. The controller  480  measures the pressure in the subsystem  460  using the sensor  464 , and controls the speed of the fan  472 A using the fan speed control  476 , in the same or similar manner as described heretofore for the controller  280  ( FIG. 2 ). 
     Considering now a method of operating an additive manufacturing system, and with reference to  FIG. 5 , a method  500  begins at  510  by pressurizing clean air in an unsealed air supply enclosure to a first pressure above ambient air pressure to inhibit unfiltered ambient air from leaking into the air supply enclosure  510 . In some examples, at  512 , ambient air from outside the printer is drawn into the unsealed air supply enclosure through a filter to generate the clean air. In some examples, at  514 , the first pressure is measured, and at  516  a pressure control arrangement is adjusted to maintain the first pressure within a first pressure range. In some examples, the pressure control arrangement may include a pressure sensor, a fan speed control module, and a controller. 
     At  520 , the clean air from the air supply enclosure is flowed to an unsealed build chamber. At  530 , a 3D part is fabricated in the build chamber. As a by-product of the fabrication process, the air in the build chamber becomes contaminated with particulates. The particulates may include airborne particles of a powder used to fabricate the 3D part. 
     At  540 , the build chamber is depressurized to a second pressure below the ambient air pressure to inhibit the particulates from leaking out of the build chamber to the atmosphere external to the printer. In some examples, at  542 , the contaminated air from the build chamber is expelled to the atmosphere outside the system through a filter which retains the powder particulates in the build chamber. In some examples, at  544 , the second pressure is measured, and at  546  a pressure control arrangement is adjusted to maintain the second pressure within a second pressure range. 
     From the foregoing it will be appreciated that the system, method, and medium provided by the present disclosure represent a significant advance in the art. Although several specific examples have been described and illustrated, the disclosure is not limited to the specific methods, forms, or arrangements of parts so described and illustrated. For example, examples of the disclosure are not limited to additive manufacturing systems, but may be any type of system having unsealed portions in which control of the direction of air leakage between unsealed portions and/or with the external environment id desired. 
     This description should be understood to include all combinations of elements described herein, and claims may be presented in this or a later application to any combination of these elements. The foregoing examples are illustrative, and different features or elements may be included in various combinations that may be claimed in this or a later application. Terms of orientation and relative position (such as “top,” “bottom,” “side,” and the like) are not intended to indicate a particular orientation of any element or assembly, and are used for convenience of illustration and description. Unless otherwise specified, operations of a method claim need not be performed in the order specified. Similarly, blocks in diagrams or numbers (such as (1), (2), etc.) should not be construed as operations that proceed in a particular order. Additional blocks/operations may be added, some blocks/operations removed, or the order of the blocks/operations altered and still be within the scope of the disclosed examples. Further, methods or operations discussed within different figures can be added to or exchanged with methods or operations in other figures. Further yet, specific numerical data values (such as specific quantities, numbers, categories, etc.) or other specific information should be interpreted as illustrative for discussing the examples. Such specific information is not provided to limit examples. The disclosure is not limited to the above-described implementations, but instead is defined by the appended claims in light of their full scope of equivalents. Where the claims recite “a” or “a first” element of the equivalent thereof, such claims should be understood to include incorporation of at least one such element, neither requiring nor excluding two or more such elements. Where the claims recite “having”, the term should be understood to mean “comprising”.