Patent Publication Number: US-11648784-B2

Title: Devices, systems, and methods for supplying makeup air through openings in carrier plates of printing system via an air guide structure

Description:
FIELD 
     Aspects of this disclosure relate generally to inkjet printing, and more specifically to inkjet printers having a media transport device utilizing vacuum suction to hold print media. Related devices, systems, and methods also are disclosed. 
     INTRODUCTION 
     In some applications, inkjet printing systems use an ink deposition assembly with one or more printheads, and a media transport device to move print media (e.g., a substrate such as sheets of paper, envelopes, or other substrate suitable for being printed with ink) through an ink deposition region of the ink deposition assembly (e.g., a region under the printheads). The inkjet printing system forms printed images on the print media by ejecting ink from the printheads onto the media as the media pass through the deposition region. In some inkjet printing systems, the media transport device utilizes vacuum suction to assist in holding the print media against a movable support surface (e.g., conveyor belt, rotating drum, etc.) of the transport device. Vacuum suction to hold the print media against the support surface can be achieved using a vacuum source (e.g., fans) and a vacuum plenum fluidically coupling the vacuum source to a side of the moving surface opposite from the side that supports the print medium. The vacuum source creates a vacuum state in the vacuum plenum, causing vacuum suction through holes in the movable support surface that are fluidically coupled to the vacuum plenum. When a print medium is introduced onto the movable support surface, the vacuum suction generates suction forces that hold the print medium against the movable support surface. The media transport device utilizing vacuum suction may advantageously allow print media to be securely held in place without slippage while being transported through the ink deposition region under the ink deposition assembly, thereby helping to ensure correct locating of the print media relative to the printheads and thus more accurate printed images. The vacuum suction may also advantageously allow print media to be held flat as it passes through the ink deposition region, which may also help to increase accuracy of printed images, as well as helping to prevent part of the print medium from rising up and striking part of the ink deposition assembly and potentially causing a jam or damage. 
     One problem that may arise in inkjet printing systems that include media transport device utilizing vacuum suction is unintended blurring of images resulting from air currents induced by the vacuum suction. In some systems, such blurring may occur in portions of the printed image that are near the edges of the print media, particularly those portions that are near the lead edge or trail edge in the transport direction of the print media. During a print job, the print media are spaced apart from one another on the movable support surface as they are transported through the deposition region of the ink deposition assembly, and therefore parts of the movable support surface between adjacent print media are not covered by any print media. Thus, adjacent to both the lead edge and the trail edge of each print medium there are uncovered holes in the movable support surface. Because these holes are uncovered, the vacuum of the vacuum plenum induces air to flow through those uncovered holes. This airflow may deflect ink droplets and cause blurring of the image. 
     In some cases, holes along inboard and/or outboard edges that are parallel to the transport direction of the print media may also be uncovered, for example due to accommodating different sizes of print media. Similar blurring problems may also occur on these edges of the print media for similar reasons. 
     A need exists to improve the accuracy of the placement of droplets in inkjet printing systems and to reduce the appearance of blur of the final printed media product. A need further exists to address the blurring issues in a reliable manner and while maintaining speeds of printing and transport to provide efficient inkjet printing systems. 
     SUMMARY 
     Exemplary embodiments of the present disclosure may solve one or more of the above-mentioned problems and/or may demonstrate one or more of the above-mentioned desirable features. Other features and/or advantages may become apparent from the description that follows. 
     In accordance with at least one embodiment of the present disclosure a printing system comprises a print fluid deposition assembly, a media transport device, and an air flow control system. The print fluid deposition assembly comprises a carrier plate and a printhead supported by the carrier plate, wherein the printhead is arranged to eject a print fluid through an opening of the carrier plate and to a deposition region of the print fluid deposition assembly. The media transport device comprises a movable support surface, the media transport device configured to hold a print medium against the movable support surface by vacuum suction and the movable support surface configured to transport the print medium along a process direction through the deposition region of the print fluid deposition assembly. The air flow control system comprises an air supply unit arranged to flow air through the opening of the carrier plate between the carrier plate and the printhead. The air flow control system is configured to control the air supply unit to selectively flow the air based on a location of a print medium transported by the media transport device relative to the printhead. The air supply unit comprises an air guide structure comprising: an air outlet portion extending into the opening of the carrier plate between the printhead and the carrier plate, the air outlet portion terminating in an end wall that is angled obliquely relative to the movable support surface towards a reverse-process direction. 
     In accordance with at least one embodiment of the present disclosure a printing system comprises a print fluid deposition assembly, a media transport device, and an air flow control system. The print fluid deposition assembly comprises a carrier plate and a printhead supported by the carrier plate, wherein the printhead is arranged to eject a print fluid through an opening of the carrier plate and to a deposition region of the print fluid deposition assembly. The media transport device comprises a movable support surface, the media transport device configured to hold a print medium against the movable support surface by vacuum suction and the movable support surface configured to transport the print medium along a process direction through the deposition region of the print fluid deposition assembly. The air flow control system comprises an air supply unit arranged to flow air through the opening of the carrier plate between the carrier plate and the printhead. The air flow control system is configured to control the air supply unit to selectively flow the air based on a location of a print medium transported by the media transport device relative to the printhead. The air supply unit comprises an air guide structure attached to the printhead. 
     In accordance with at least one embodiment of the present disclosure, a method of operating a printing system comprises transporting a print medium through a deposition region of a print fluid deposition assembly of the printing system, wherein the print medium is held against a moving support surface via vacuum suction during the transporting. The method further comprises ejecting print fluid from a printhead of the printing assembly through an opening in a carrier plate supporting the printhead to deposit the print fluid to the print medium in the deposition region. The method also comprises controlling an airflow control system to selectively flow air through the opening in the carrier plate between the carrier plate and the printhead and to the movable support surface, wherein the controlling is based on a location of the print medium relative to the printhead. The selectively flowing the air through the opening comprises flowing the air through an air guide structure comprising an air outlet portion extending into the opening of the carrier plate between the printhead and the carrier plate and terminating in an end wall that is angled obliquely relative to the movable support surface toward a direction opposite a direction of the transporting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure can be understood from the following detailed description, either alone or together with the accompanying drawings. The drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments of the present teachings and together with the description explain certain principles and operation. In the drawings: 
         FIG.  1 A- 1 I  schematically illustrate airflow patterns relative to a printhead assembly, transport device, and print media during differing stages of print media transport through an ink deposition region of a conventional inkjet printing system, and resulting blur effects in the printed media product. 
         FIG.  2    is a block diagram illustrating components of an embodiment of an inkjet printing system including an airflow control system. 
         FIGS.  3 A- 3 E  are schematic illustrations of components of an embodiment of an inkjet printing system including an airflow control system with various states of the airflow control system in use depicted. 
         FIG.  4    is a schematic illustration of an ink deposition assembly, media transport device, and airflow control airflow control system of the inkjet printing system of  FIG.  3   . 
         FIG.  5    is a plan view from above the printhead assemblies of one embodiment of an inkjet printing system including an airflow control system. 
         FIG.  6    is a cross-sectional view of the inkjet printing system including an airflow control system of  FIG.  5   , with the cross-section taken along An in  FIG.  5   . 
         FIG.  7    is a cross-sectional, schematic illustration of components of another embodiment of an inkjet printing system including an airflow control system. 
         FIG.  8    is a perspective view of yet another embodiment of the components of an inkjet printing system including an airflow control system. 
         FIG.  9    is a sectional view of another embodiment of the airflow control system, with the cross-section taken along the B in  FIG.  8   . 
         FIG.  10    is a cross-sectional, schematic illustration a yet another embodiment of components of an inkjet printing system including an airflow control system. 
         FIG.  11    is a schematic, plan view from above the printhead assemblies of another embodiment of an inkjet printing system including an airflow control system. 
         FIG.  12    is a cross-sectional view of the inkjet printing system of  FIG.  11   , with the cross-section taken along C in  FIG.  11   . 
         FIG.  13    is a plan view from above the printhead assemblies of one embodiment of an inkjet printing system including an airflow control system. 
         FIG.  14    is a workflow diagram of a method of operating an airflow control system of an inkjet printing system according to an embodiment. 
         FIG.  15    is a workflow diagram of a method for controlling airflow from an airflow control system according to an embodiment. 
         FIG.  16    is a workflow diagram of a method for controlling airflow from an airflow control system according to an embodiment. 
         FIG.  17    is a block diagram illustrating a control loop for controlling an airflow control system. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, in inkjet printing systems utilizing vacuum to suction the print media to the transport device, various airflow patterns can occur that lead to undesirable displacement of droplets ejected from the printheads, thereby resulting in blurring of printed images on the print media. To better illustrate some of the phenomenon occurring giving rise to the blurring issues, reference is made to  FIGS.  1 A- 1 I .  FIGS.  1 A,  1 D, and  1 G  illustrate schematically a printhead  10  printing on a print medium  5  near a trail edge TE, a lead edge LE, and a middle, respectively, of the print medium  5 .  FIGS.  1 B,  1 E, and  1 H  illustrate enlarged views of the regions A, B, and C, respectively.  FIGS.  1 C,  1 F, and  1 I  illustrate enlarged pictures of printed images, the printed images comprising lines printed near the trail edge TE, lead edge LE, and middle, respectively, of a sheet of paper. 
     As shown in  FIGS.  1 A,  1 D, and  1 G , the inkjet printing system comprises a printhead  10  to eject ink to a print medium  5   a  near a trail edge TE of the print medium  5   a , and a movable support surface  20  transports the print media  5  in a process direction P, which corresponds to a positive y-axis direction in the Figures. The movable support surface  20  slides along a top of a vacuum platen (not illustrated), and a vacuum environment is provided on a bottom side of the platen. The movable support surface  20  has holes  21  and the vacuum platen has platen holes, and the holes  21  and platen holes periodically align as the movable support surface  20  moves so as to expose the region above the movable support surface  20  to the vacuum below the platen. In regions where the print medium  5  covers the holes  21 , the vacuum suction through the aligned holes  21  and  27  generates a force that holds the print medium  5  against the movable support surface  20 . However, little or no air flows through these covered holes  21  and  27  since they are blocked by the print medium  5 . On the other hand, as shown in  FIGS.  1 A and  1 D , in the inter-media zone  22  the holes  21  and  27  are not covered by the print media  5 , and therefore the vacuum suction pulls air to flow down through the holes  21  and  27  in the inter-media zone  22 . This creates airflows, indicated by the dashed arrows in  FIGS.  1 A and  1 D , which flow from regions around the printhead  10  towards the uncovered holes  21  and  27  in the inter-media zone  22 , with some of the airflows passing under the printhead  10 . 
     In  FIG.  1 A , the print medium  5   a  is being printed on near its trail edge TE, and therefore the region where ink is currently being ejected (“ink-ejection region”) (e.g., region A in  FIG.  1 A ) is located downstream of the inter-media zone  22  (upstream and downstream being defined with respect to the process direction P). Accordingly, some of the air being sucked towards the inter-media zone  22  will flow upstream through the ink-ejection region. More specifically, the vacuum suction from the inter-media zone  22  lowers the pressure in the region immediately above the inter-media zone  22 , e.g., region R 1  in  FIG.  1 A , while the region downstream of the printhead  10 , e.g., region R 2  in  FIG.  1 A , remains at a higher pressure. This pressure gradient causes air to flow in an upstream direction from the region R 2  to the region R 1 , with the airflows crossing through the ink-ejection region (e.g., the circled region in  FIG.  1 A ) which is between the regions R 1  and R 2 . Airflows such as these, which cross through the ink-ejection region, are referred to herein as crossflows  15 . In  FIG.  1 A , the crossflows  15  flow upstream, but in other situations the crossflows  15  may flow in different directions. 
     As shown in the enlarged view in  FIG.  1 B , which comprises an enlarged view of the circled region in  FIG.  1 A , as ink is ejected from the printhead  10  towards the medium  5 , main drops  12  and satellite drops  13  are formed. The satellite drops  13  are much smaller than the main drops  12  and have less mass and momentum, and thus the upstream crossflows  15   a  tend to affect the satellite drops  13  more than the main drops  12 . Thus, while the main drops  12  may land on the print medium  5  near their intended deposition location  16  regardless of the crossflows  15 , the crossflows  15  may push the satellite drops  13  away from the intended trajectory so that they land at an unintended location  17  on the medium  5 , the unintended location  17  being displaced from the intended location  16 . This can be seen in the actual printed image in  FIG.  1 C , in which the denser/darker line-shaped portion is formed by the main drops  12  which were deposited predominantly at their intended locations  16 , whereas the smaller dots dispersed away from the line are formed by satellite drops  13  which were blown away from the intended locations  16  to land in unintended locations  17 , resulting in a blurred or smudged appearance for the printed line. Notably, the blurring in  FIG.  1 C  is asymmetrically biased towards the trail edge TE, due to the crossflows  15  near the trail edge TE blowing primarily in an upstream direction. The inter-media zone  22  may also induce other airflows flowing in other directions, such as downstream airflows from an upstream side of the printhead  10 , but these other airflows do not pass through the region where ink is currently being ejected in the illustrated scenario and thus do not contribute to image blur. Only those airflows that cross through the ink ejection region are referred to herein as crossflows. 
       FIGS.  1 D- 1 F  illustrate another example of such blurring occurring, but this time near the lead edge LE of the print medium  5   b . The cause of blurring near the lead edge LE as shown in  FIGS.  1 D and  1 F  is similar to that described above in relation to the trail edge TE, except that in the case of printing near the lead edge LE the ink-ejection region is now located upstream of the inter-media zone  22 . As a result, the crossflows  15  that are crossing through the ink-ejection region now originate from the upstream side of the printhead  10 , e.g., from region R 3 , and flow downstream. Thus, as shown in the enlarged view of  FIG.  1 E , which comprises an enlarged view of the circled region in  FIG.  1 D , in the case of printing near the lead edge LE, the satellite drops  13  are blown downstream towards the lead edge LE of the print medium  5   b  (positive y-axis direction). As shown in  FIG.  1 F , this results in asymmetric blurring that is biased towards the lead edge LE. 
     In contrast, as shown in  FIG.  1 G  and the enlarged view in  FIG.  1 H , which corresponds to an enlarged view of circled region in  FIG.  1   , farther from the edges of the print media  105  there may be little or no crossflows  15  because the inter-media zone  22  is too distant to induce much airflow. Because the crossflows  15  are absent or weak farther away from the edges of the print medium  5 , the satellite drops  13  in this region are not as likely to be blown off course. Thus, as shown in  FIGS.  1 H and  1 I , when printing farther from the edges of the print medium  5   b , the satellite drops land at locations  18  that are much closer to the intended locations  16  resulting in much less image blurring. The deposition locations  18  of the satellite drops may still vary somewhat from the intended locations  16 , due to other factors affecting the satellite drops  13 , but the deviation is smaller than it would be near the lead or trail edges. 
     As described above, when an inter-media zone is near or under a printhead, the uncovered holes in the inter-media zone can create crossflows that can blow satellite droplets off course and cause image blur (see  FIGS.  1 A-D ). Example technologies disclosed herein may, among other things, reduce or eliminate such image blur by utilizing an airflow control system that reduces or eliminates the crossflows. With the crossflows reduced or eliminated, the satellite droplets are more likely to land nearer their intended deposition locations, and therefore the amount of blur is reduced. 
       FIG.  2    is a block diagram schematically illustrating an embodiment of printing system  100  that utilizes such an airflow control system.  FIG.  3    also illustrates aspects of the printing system  100 . As shown in  FIG.  2   , the printing system  100  comprises an ink deposition assembly  101 , a media transport device  103 , an airflow control system  150 , and a control system  130 . The ink deposition assembly  101  (also referred to herein as a “print fluid deposition assembly”) is configured to eject a print fluid, such as ink, onto print media passing through an ink deposition region of the ink deposition assembly  101 . The media transport device  103  is configured to transport the print media through the ink deposition region. The airflow control system  150  is configured to provide make-up air  155  as described the above. The control system  130  comprises processing circuitry to control operations of the printing system  100 . 
     The ink deposition assembly  101  comprises one or more printhead modules  102 . One printhead module  102  is illustrated in  FIG.  2    for simplicity, but any number of printhead modules  102  may be included in the ink deposition assembly  101 . In some examples, each printhead module  102  may correspond to a specific ink color, such as cyan, magenta, yellow, and black. Each printhead module  102  comprises one or more printheads  110  configured to eject the ink onto the print media to form an image. One printhead  110  is illustrated in  FIG.  2    for simplicity, but any number of printheads  110  may be included per printhead module  102 . The printhead modules  102  may also include additional structures and devices to support and facilitate operation of the printheads  110 , such as carrier plates (e.g., carrier plates  511 ,  711 ,  1011 ,  1111  described further below), ink supply lines, ink reservoirs, electrical connections, and so on. 
     The media transport device  103  comprises a movable support surface  120 , a vacuum plenum  125 , and a vacuum source  128 . The movable support surface  120  is to transport the print media through a deposition region of the ink deposition assembly  101 . The vacuum plenum  125  may supply vacuum suction to one side of the movable support surface  120  (e.g., a bottom side), and print media may be supported on an opposite side of the movable support surface  120  (e.g., a top side). As shown in  FIGS.  3 A- 3 E , holes  121  through the movable support surface  120  communicate the vacuum suction through the surface  120 , such that the vacuum suction acts to hold down the print media  105  against the surface  120 . The movable support surface  120  may be movable relative to the ink deposition assembly  101 , and thus the print media held against the movable support surface  120  is transported relative to the ink deposition assembly  101  as the movable support surface  120  moves. The movable support surface  120  can comprise any structure that can be driven to move relative to the ink deposition assembly  101  and which has holes  121  to allow the vacuum suction to hold down the print media, such as a belt, a drum, etc. The vacuum plenum  125  may comprise baffles, walls, or any other structures arranged to enclose or define an environment in which a vacuum state (e.g., low pressure state) is maintained by the vacuum source  128 , with the plenum  125  fluidically coupling the vacuum source  128  to the movable support surface  120 . The vacuum plenum  125  may include one or more holes or openings near the movable support surface  120  that expose the movable support surface  120  to the vacuum within the vacuum plenum  125 . For example, in some embodiments a top surface of the vacuum plenum  125  is a vacuum platen having a number of holes which communicate the vacuum suction to the underside of the movable support surface  120 . As another example, in some embodiments the movable support surface  120  is itself part of the vacuum platen  126 , which comprises a rotating drum, as described in further detail below. The vacuum source  128  may be any device configured to remove air from the plenum  125 , such as a fan, a pump, etc. 
     The airflow control system  150  comprises two or more air supply units  155 . The air supply units  155  are configured to supply make-up air  114  to the movable support surface  120  at timings based on the locations of the lead edge LE and trail edge TE of print media so as to reduce or eliminate cross-flow air patterns tending to cause leading edge or trailing edge blur. The airflow control system  150  may reduce or eliminate the crossflows by providing makeup air  114  to the inter-media zone  122  at strategic timings to neutralize the pressure gradients that would otherwise cause the crossflows. 
     The air supply units  155  are arranged in pairs, with each pair corresponding to one of the printhead modules  102  or one of the individual printheads  110 . As illustrated in  FIGS.  2  and  3 A- 3 E , one of the air supply units  155  of each pair is arranged adjacent to and upstream of its corresponding printhead module  102  or printhead  110 , and may be referred to as an upstream air supply unit  155   u  in relation to that printhead module  102  or printhead  110 . The other one of the air supply units  155  in each pair is arranged adjacent to and downstream of its corresponding printhead module  102  or printhead  110 , and may be referred to as a downstream air supply unit  155   d  in relation to that printhead module  102  or printhead  110 . In some embodiments, the same air supply unit  155  may serve as the upstream supply unit  155   u  for one pair and also as the downstream air supply unit  155   d  of another pair. As shown in  FIGS.  3 A- 3 E , each air supply unit  155  comprises an air guide structure  156  coupled to an air source  157  (e.g., controllable valve, fan, pump, etc.) which is controlled to selectively provide the makeup air  114  at the desired timings. The guide structures  156  can include, but are not limited to, for example, any of baffles, nozzles, air knives, vents, ducts, or combinations thereof to direct and/or alter the pressure or flow rate of the make-up airflow as desired. 
     The timings and locations of supplying the makeup air  114  correspond generally to the timings when the inter-media zone  122  is near (e.g., passing under) the printhead  110 , for example when the inter-media zone  122  is located in a deposition region under the printheads  110 . In other words, the makeup air  114  is supplied while portions of the print media  105  that are near a trail edge TE are being printed and while portions of the print media  105  that are near the lead edge LE are being printed. The timings and effects of supplying makeup air  114  from air supply units  155  will be described below with reference to  FIGS.  3 A- 3 E . 
       FIGS.  3 A- 3 E  illustrate a sequence of events involving print media  105  passing through a deposition region of a given printhead  110  or printhead module  102  of the printer  100 , including the supplying of makeup air  114  from a given pair of air supply units  155   u  and  155   d  that correspond to the given printhead  110  or printhead module  102 . As noted above, the printing system  100  may include multiple printheads  110  and/or multiple printhead modules  102 , but  FIGS.  3 A- 3 E  illustrates the operations associated with just one printhead  110 /printhead module  102  to simplify the description. In systems in which additional printheads  110  or printhead modules  102  are included, the timings for supplying makeup air  114  from air supply units  155  associated with the additional printheads  110 /printhead modules  102  would be similar to those described in relation to  FIGS.  3 A- 3 E , except that the various locations and timings would be relative to the additional printhead  110  and/or printhead module  102 . 
     For example, as illustrated in  FIGS.  3 A and  3 B , as the trail edge TE of the print medium  105   a , and consequently the inter-media zone  122 , enters into and travels through the deposition region under the printhead  110 , the air supply unit  155   u  that is upstream of the printhead  110 /printhead module  102  may supply makeup air  114  while the air supply unit  155   d  does not supply makeup air. The positive makeup air  114  supplied from the upstream air supply unit  155   u  increases the pressure in the region R 1  between the printhead  110 /printhead module  102  and the inter-media zone  122 , and thus reduces or eliminates the pressure gradient that would otherwise exist between the region R 1 , where the uncovered holes  121  of the movable support surface  120  corresponding to the inter-media zone  122  are, and the region R 2  immediately downstream of the printhead  110 /printhead module  102 . Accordingly, air from the downstream side of the printhead  110 /printhead module  102  is no longer pulled (or is pulled less strongly) upstream under the printhead  110 /printhead module  102  toward the inter-media zone  122 , and thus the upstream crossflows  15  illustrated in  FIG.  1 A  are reduced or eliminated. Once the trail edge TE of the print media  105  has advanced to an end of the deposition region of the printhead  110 /printhead module  102 , the airflow control system can be controlled such that upstream air supply unit  155   u  ceases to supply makeup air  114 , as the issues associated with the trail edge blurred are no longer problematic for the print medium  105   a  past this point. 
     Conversely, as illustrated in  FIGS.  3 C and  3 D , when the lead edge LE of a print medium  105 , such as the subsequent print medium  105   b  that is being printed in a print job, is entering into and passing through the deposition region under the printhead  110 /printhead module  102 , the airflow control system can control the air supply unit  155   d  that is downstream of the printhead  110 /printhead module  102  to supply makeup air  114  while the air supply unit  155   u  does not supply makeup air. The positive makeup air  114  supplied from the downstream air supply unit  155   d  increases the pressure in the region R 1  above the inter-media zone  122  with the uncovered holes  121 , and thus reduces or eliminates the pressure gradient that would otherwise exist between the region R 1  and a region R 3  immediately upstream of the printhead  110 /printhead module  102 . Accordingly, air from the region R 3  upstream of the printhead  110 /printhead module  102  is no longer pulled (or is pulled less strongly) downstream under the printhead  110 /printhead module  102  toward the inter-media zone  122 , and thus the downstream crossflows  15  illustrated in  FIG.  1 C  are reduced or eliminated. Once the leading edge LE has advanced to an end of the deposition region under the printhead  110 /printhead module  102 , the risk of lead edge image blur is reduced because the uncovered holes  121  of the support surface  120  are relatively distant from the deposition region and thus are less likely to draw air through the deposition region. Accordingly, as depicted in  FIG.  3 E , the air control system can cease the supply of makeup air from the downstream air supply unit  155   d , with the upstream air supply unit  155   u  also not supplying makeup air, when the lead edge LE is at or beyond this point. 
     Thus, the upstream air supply unit  155   u  and the downstream air supply unit  155   d  may alternate when they supply makeup air  114 , with the supply of makeup air  114  being timed (at least in part) based on the location of the lead edges LE and/or trail edges TE of the print media  105  relative to the printhead  110 /printhead module  102 , or in other words based on the position of the inter-media zone  122  relative to the printhead  110 /printhead module  102 . The airflow control system of  FIGS.  2 A- 2 E  can thus supply makeup air  114  in a manner that reduces or eliminates the crossflows  15  induced by the uncovered air-holes  121  in the inter-media zone  122 , thus addressing the issues of blur caused by such crossflows  15  carrying satellite ink droplets to undesired locations on the print media. 
     One possible concern with supplying the makeup air  114  is that the makeup air  114  itself could create or contribute to crossflows that cause image blur. However, by controlling the timings and amounts at which the makeup air  114  is supplied, the risk of the makeup air  114  causing crossflows through the region where the ink is being ejected can be reduced. 
     For example, as illustrated in  FIG.  3 A , the supply of makeup air  114  from the upstream air supply unit  155   u  may begin when the inter-media zone  122  approaches the upstream side of the printhead  110 /printhead module  102 , i.e., when the inter-media zone  122  reaches a first position relative to the printhead  110 /printhead module  102 . In other words, the supply of makeup air  114  may begin when the trail edge TE of the print medium  105   a  approaches the upstream side of the printhead  110 /printhead module  102  and is entering or about to enter the deposition region. More specifically, in some embodiments, the supply of makeup air  114  from the upstream air supply unit  155   u  may begin when the inter-media zone  122  reaches a first position in which the trail edge TE of the downstream print medium  105   a  is near or aligned with one of the following features: the upstream edge of a carrier plate of a printhead module  102  (not illustrated in  FIGS.  2 - 3 E , but see the carrier plates  511 ,  711 ,  1011 , and  1111  as examples), the upstream edge of an opening in the carrier plate (not illustrated in  FIGS.  2 - 3 E , but see the openings  519 ,  710 ,  1019 , and  1119  as examples), an air outlet in the upstream air supply unit  155   u  (not illustrated in  FIGS.  2 - 3 E , but see the air outlets  558 ,  758 ,  858 ,  1065 , and  1158  as examples), the upstream edge of the printhead  110 , and the upstream edge of an ink ejection zone  112  of the printhead  110  or printhead module  102  (the ink ejection zone  112  corresponding to a region from which ink is ejected, such as a region containing ink ejection nozzles). 
     Furthermore, as illustrated in  FIG.  3 C , the supply of makeup air  114  from the upstream air supply unit  155   u  may cease when the inter-media zone  122  reaches a second position relative to the printhead  110  in which the trail edge TE of the print medium  105   an  is at, is approaching, or has passed a point on the downstream side of the printhead  110  and/or when the LE of the next print medium  105   b  is at, is approaching, or has passed a point on the upstream side of the printhead  110 . More specifically, in some embodiments, second position of the inter-media zone  122  at which the supply of makeup air  114  from the upstream air supply unit  155   u  ceases corresponds to the trail edge TE of print medium  105 , such as the first medium  105   a , being near or aligned with one of the following features: the downstream edge of a carrier plate, the downstream edge of an opening in the carrier plate, an air outlet in the downstream air supply unit  155   d , the downstream edge of the printhead  110 , and the downstream edge of an ink ejection zone  112  of the printhead  110  or printhead module  102 . In some embodiments, second position of the inter-media zone  122  at which the supply of makeup air  114  from the upstream air supply unit  155   u  ceases corresponds to the LE of a print medium  105 , such as the subsequent print medium  105   b , being near or aligned with one of the following features: the upstream edge of a carrier plate, the upstream edge of an opening in the carrier plate, an air outlet in the upstream air supply unit  155   u , the upstream edge of the printhead  110 , and the upstream edge of an ink ejection zone  112  of the printhead  110 . 
     As illustrated in  FIG.  3 C , the supply of makeup air  114  from the downstream air supply unit  155   d  may begin when the inter-media zone  122  reaches a third position relative to the printhead  110 /printhead module  102  in which the trail edge TE of the print medium  105   a  is at, is approaching, or has passed a point on the downstream side of the printhead  110 /printhead module  102  and/or when the LE of the next print medium  105   b  is at, is approaching, or has passed a point on the upstream side of the printhead  110 . In other words, the supply of makeup air  114  from the downstream unit  155   d  may begin when the lead edge LE of the next print medium  105   b  to be printed on approaches an upstream side of the printhead  110 , and/or when the trail edge TE of print medium  105 , such as the preceding print medium  105   a , approaches a downstream side of the printhead  110 . More specifically, in some embodiments, the third position of the inter-media zone  122  at which the supply of makeup air  114  from the downstream air supply unit  155   u  begins corresponds to the lead edge LE of a print medium  105 , such as the next print medium  105   b , being near or aligned with one of the following features: the upstream edge of a carrier plate, the upstream edge of an opening in a carrier plate, an air outlet in the upstream air supply unit  155   u , the upstream edge of the printhead  110 , and the upstream edge of an ink ejection zone  112  of the printhead  110 /printhead module  102 . In some examples, the third position of the inter-media zone  122  at which the supply of makeup air  114  from the downstream air supply unit  155   u  begins corresponds to the trail edge TE of the prior print medium  105   a  being near or aligned with one of the following features: the downstream edge of a carrier plate, the downstream edge of an opening in the carrier plate, an air outlet in the downstream air supply unit  155   u , the downstream edge of the printhead  110 , and the downstream edge of an ink ejection zone  112  of the printhead  110 . In some embodiments, including that of  FIG.  3 C , the second and third positions of the inter-media zone  122  are the same, and thus the timing when the downstream air supply unit  155   d  starts supplying makeup air  114  may coincide with the timing when the upstream air supply unit  155   u  ceases supplying makeup air  114 . 
     As illustrated in  FIG.  3 E , the supply of makeup air  114  from the downstream air supply unit  155   d  may cease when the inter-media zone  122  reaches a fourth position relative to the printhead  110 /printhead module  102 , e.g., a position in which the inter-media zone  122  is downstream of the printhead  110 /printhead module  102 . In other words, the supply of makeup air  114  from the downstream unit  155   d  may cease when the lead edge LE of the print medium  105   b  approaches the downstream side of the printhead  110 /printhead module  102 . More specifically, in some examples, the supply of makeup air  114  from the downstream air supply unit  155   d  may cease when the inter-media zone  122  reaches a fourth position in which the lead edge LE of the print medium  105   b  is near or aligned with one of the following features: the downstream edge of a carrier plate, the downstream edge of an opening in the carrier plate, an air outlet in the downstream air supply unit  155   u , the downstream edge of the printhead  110 , and the downstream edge of an ink ejection zone  112  of the printhead  110 /printhead module  102 . As also illustrated in  FIG.  2 E , the upstream air supply unit  155   u  continues to also be controlled to not supply any makeup air during this state of operation. 
     In various embodiments, as those having ordinary skill in the art would appreciate, the airflow control system  150  generally controls the upstream air supply unit  155   u  to not be supplying makeup air during the operational state in which the downstream air supply unit  155   d  is supplying makeup air and vice versa. However, in other embodiments, the upstream air supply unit  155   u  and the downstream air supply unit  155   d  may occasionally supply makeup air at the same time. For example, in some embodiments, when the inter-media zone  122  is relatively wide, the trail edge TE of the print medium  105   a  reaches the third position before the lead edge LE of the subsequent print medium  105   b  reaches the second position, in which case the upstream air supply unit  155   u  and the downstream air supply unit  155   d  may both supply makeup air during the time period between the timing when the trail edge TE reaches the third position and the timing when the lead edge LE reaches the second position. 
     Thus, throughout the period during which makeup air  114  is supplied (see  FIGS.  3 A- 3 E ), the inter-media zone  122  is located near the air supply unit  155   u  or  155   d  that is supplying the makeup air  114 , and therefore most or all of the supplied makeup air  114  tends to get sucked into the inter-media zone  122 . Further, the rate and/or direction at which makeup air  114  is supplied can be controlled by the airflow control system such that nearly all of the makeup air  114  ends up getting sucked down into inter-media zone  122 , with very little or none of the makeup air  114  being left over to flow to other locations so as to create undesirable flow patterns. Moreover, the side of the printhead  110  from which the makeup air  114  is supplied is controlled such that the makeup air  114  that gets sucked from the air supply unit  114  into the inter-media zone  122  does not pass through the portion of the deposition region in which ink droplets are being ejected from the printhead  110 . In other words, the region in which ink is being ejected is never located between the inter-media zone  122  and the air supply unit  155  that is currently supplying makeup air  114 . Thus, most or all of the makeup air  114  gets sucked into the inter-media zone  122  without passing through the region where ink is being deposited, and therefore the makeup air  114  does not create significant blur-inducing crossflows. 
     In the discussion above, specific examples for the timings and locations for when makeup air  114  is supplied are described. However, the precise locations of the lead edge LE and the trail edge TE with respect to the printhead  110 /printhead module  102  which are used to trigger the supply of makeup air  114  and the ceasing of supply of makeup air  114  may vary from system to system or even from time to time within the same system. The airflow control systems disclosed herein, including the airflow control system  150 , are not limited to any specific set of timings/trigger locations. Any desired timings/trigger locations for supplying or ceasing the makeup air may be used as long as the supply of makeup air is selectively turned on and off based on the location of the inter-media zone  122  (lead edge LE and trail edge TE). In some cases, the specific timings that are used may be programed into a control system that controls operations of the airflow control system  150  (e.g., control system  130 , described below). In some examples, timings that produce adequate blur reduction may be determined experimentally by iteratively printing test images, determining an amount of image blur, adjusting the timings based on the blur, and repeating the process until acceptable results are obtained. In some cases, the timings may be determined and adjusted automatically and dynamically by the printing system based on feedback obtained during actual usage. For example, the printing system may automatically scan the images it prints, detect an amount of image blur in the printed image, adjust the timings for starting/stopping supply of the makeup air  114  based on the amount of blur that is detected, and repeat this process for successive printed images until timings which produce acceptable amounts of image blur are converged upon. 
     Referring again to  FIG.  2   , the control system  130  comprises processing circuitry to control operations of the printing system. The processing circuitry may include one or more electronic circuits configured with logic for performing the various operations. The logic of the processing circuitry may comprise dedicated hardware to perform various operations, software instructions to perform various operations, or any combination thereof. In examples in which the logic comprises software instructions, the processing circuitry may include a processor to execute the software and a memory device that stores the software. The processor may comprise one or more processing devices capable of executing machine readable instructions, such as, for example, a processor, a processor core, a central processing unit (CPU), a controller, a microcontroller, a system-on-chip (SoC), a digital signal processor (DSP), a graphics processing unit (GPU), etc. In examples in which the processing circuitry includes dedicated hardware, in addition to or in lieu of the processor, the dedicated hardware may include any electronic device that is configured to perform specific operations, such as an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a Complex Programmable Logic Device (CPLD), discrete logic circuits, a hardware accelerator, a hardware encoder, etc. The processing circuitry may also include any combination of dedicated hardware and processor plus software. 
     Although the various components of the printing system  100  are illustrated and described separately for ease of understanding, it should be understood that in practice these components are not necessarily physically or logically distinct. For example, in some embodiments the air supply units  155  may be located within the printhead modules  102 , and thus the air supply units  155  could be considered as being part of the ink deposition assembly  101  from that perspective. As another example, the supply of makeup air  114  by the air supply units  155  may be controlled, in whole or in part, by components of the control system  130 , and therefore those component of the control system  130  may be considered as also being parts of the airflow control system  150  from that perspective. 
     As noted above, the timings at which makeup air  114  is supplied may be based on the position of the inter-media zones  122  between printed media  105  (i.e., locations of the lead edge LE and trail edge TE of the print media  105 ), or to put the same point differently, based on the location of the inter-media zone  122 . Thus, embodiments disclosed herein may utilize a location tracking system to track the location of the print media  105  as they are transported through the ink deposition assembly, and a controller of the printer may determine locations of the lead edge LE and trail edge TE of a print medium  105  based on tracked location information. As used herein, tracking the location of the print media  105  refers to the system having knowledge, whether direct or inferred, of where the print media is located at various points as it is transported through the ink deposition assembly. Direct knowledge of the location of the print media  105  may comprise information obtained by directly observing the print media, for example via a sensor (e.g., an edge detection sensor). Inferred knowledge of the location of the print media  105  may be obtained by inference from other known information, for example by calculating how far the print media  105  would have moved from a previously known location based on a known speed of the movable support surface. In some examples disclosed herein, the location tracking system may explicitly track a location of the lead edge LE and/or the trail edge TE. However, in other embodiments disclosed herein, the location tracking system may explicitly track the location of some other part of the print medium, in which case the locations of the lead edge LE and/or the trail edge TE may be inferred based on known dimensions of the print medium. 
     Most existing printing systems are already configured with print registration mechanisms to track the locations of the print media as they are transported through the ink deposition assembly, as knowledge of the locations of the print media may be helpful to ensure accurate image formation on the print media. Thus, various systems for tracking the location of print media are well known in the art. Because such location tracking systems are well known, they will not be described in detail herein. Any known location tracking system (or any new location tracking system) may be used in the embodiments disclosed herein to track the location of print media, and a controller may use this information to determine the locations of the lead edge LE and/or the trail edge TE (if not already known). 
     As noted above, it may be helpful in some circumstances for the flow rate of the makeup air  114  to be matched to the rate at which air is sucked into the inter-media zone. This flow rate may be determined experimentally, for example by printing test images with different flow rates for the makeup air and identifying the flow rate that produces the best results. Alternatively, the desired flow rate may be estimated by calculating an estimated rate of suction through the inter-media zone based on known dimensions of the inter-media zone and its air-holes and based on known characteristics of the vacuum source. In some examples, the size of the inter-media zone may vary depending on the size of the print media selected for printing, and therefore the printing system may be programmed with multiple different flow rates for the makeup air, each corresponding to a different type of print medium. 
     In some examples, the printing system may be configured to automatically and dynamically adjust the airflow rate of the makeup air based on feedback obtained during actual usage. For example, the printing system may scan the images it prints and detect an amount of image blur, adjust the flowrate based on the amount of blur, and repeat this process for successive printed images until a flowrate is converged upon which results in acceptable amounts of image blur. The printing system may continue to check the image blur periodically and adjust the flowrate as needed, which may help to account for changing conditions which could affect the desired flowrate. 
       FIG.  4    illustrates one example embodiment of a printing system, namely the printing system  400 . The printing system  400  can be used as the printing system  100 . As illustrated in  FIG.  4   , the printing system  400  compromises an ink deposition assembly  401 , media transport device  403 , and airflow control system  450 , which can be used as the ink deposition assembly  101 , media transport device  103 , and airflow control system  150 , respectively, which were described above. The printing system  400  may also comprise additional components not illustrated in  FIG.  4   , such as a control system (e.g., the control system  130 ). 
     The ink deposition assembly  401  includes four printheads  110  or four printhead modules  402 . The printheads  110 /printhead modules  402  are arranged in series along a process direction P above a media transport device  403 , such that the print media  405  is transported sequentially beneath each of the printheads  110 /printhead modules  402 . The media transport device  403  of  FIG.  4    comprises a flexible belt providing the movable support surface  420 . The movable support surface  420  is driven by rollers  429  to move along a looped path, with a portion of the path passing through the ink deposition region of the ink deposition assembly  401 . In this embodiment, the vacuum plenum  425  (such as vacuum plenum  125  of the printing system  100 ) comprises a vacuum platen  426 , which forms a top wall of the plenum  425  and supports the movable support surface  420 . The platen  426  comprises platen holes  427 , which allow fluidic communication between the interior of the plenum  425  and the underside of the movable support surface  420 . The platen holes  427  may include channels on a top side thereof, as seen in the expanded cutaway of  FIG.  4   , which may increase an area of the opening of the holes  427  on the top side thereof. The holes  421  of the movable support surface  420  are arranged to align with corresponding platen holes  427  as the movable support surface  420  slides across the platen  426 . When a hole  421  aligns with a platen holes  427 , the environment above the movable support surface  420  becomes fluidically coupled to the vacuum plenum  425 , and thus is exposed to the low pressure state of the vacuum plenum  425 . Accordingly, a bottom side of a print medium  405  located on the movable support surface  420  is exposed to the low pressure of the vacuum plenum  425  via the holes  426  and  427 , and a top side of the print medium  405  is exposed to a higher ambient pressure, and this pressure differential creates a force that holds the print medium  405  against the movable support surface  420 . 
     In another embodiment (not illustrated) of the media transport device  103  of  FIG.  2   , the movable support surface is a rigid cylindrical drum that is driven to rotate around an axis, with the print media being supported on an outer circumferential surface of the drum. In such an embodiment of the media transport device  103 , with which those have ordinary skill in the art are familiar, walls of the drum also serve as the vacuum plenum  125 , with the vacuum environment being located inside the drum. 
     As noted above,  FIG.  4    illustrates one embodiment of an airflow control system  450  that can be used as the airflow control system  150  of the printing system  100 , which was described above. The airflow control system  450  includes air supply units  455  arranged upstream and downstream of each printhead  410  or printhead module  402 . The air supply units  455  each comprise an air guide structure  456  in selective fluid communication with an air source  457 . The air guide structure  456  may comprise baffles, nozzles, air knives, tubes, ducts, plenums, or any other structures configured to receive airflows from the air source  457  and direct the airflow towards the movable support surface  420  of the media transport device  403 . The air source  457  may comprise a device configured to selectively provide the airflows to the air guide structure  456  at select timings. For example, the air source  457  may comprise a controllable valve that can be opened or closed to selectively provide airflows to the air guide structure  456 . In such an example, the controllable valve may receive the airflows from a fan, pump, high pressure chamber, or the like to which the controllable valve is coupled. For example, in  FIG.  4    each air source  457  comprises a controllable valve, and each of the controllable valves is fluidically coupled to a shared air chamber  459 . The shared air chamber  459  may be provided with pressured air, for example via one or more air moving devices such as fans, pumps, etc. In another embodiment, each air source  457  may comprise its own individual air moving device, such as a fan, pump, etc., which can be controlled to turn on and off at selected timings to selectively provide airflows to the air guide structure  456 . Each air guide structure  456  may be positioned adjacent the corresponding printhead module  402  or printhead  410  and near the movable support surface  420 , such that the makeup air  414  supplied therefrom flows under the printhead  410 /printhead module  402  toward an inter-media zone when the inter-media zone is under the printhead  410 /printhead module  402 . The timings of supplying makeup air  414  from the air supply units  455  are similar to the timings explained above with reference to  FIGS.  3 A- 3 E . 
     Turning now to  FIGS.  5 - 12   , various embodiments of airflow control systems that can be used for the airflow control system  150  or  450  will be described in greater detail below. 
       FIGS.  5  and  6    illustrate one embodiment of an airflow control system, namely the airflow control system  550 . The airflow control system  550  can be used as the airflow control system  150  or  450 . The airflow control system  550  can be used in a printing system, such as the printing systems  100  or  400 . In  FIGS.  5  and  6   , the airflow control system  550  is illustrated in the context of an embodiment of a printing system comprising a vacuum platen  526 , a movable support surface  520 , and one or more printhead modules  502 . The vacuum platen  526  can be used as part of the vacuum plenum  125  and/or as the vacuum platen  426 . The movable support surface  520 , can be used as any of the movable support surfaces  120  and  420 . The printhead module  502  can be used as one of the printhead modules  102  and  402 . The printhead module  502  comprises printheads  510 , which can be used as the printheads  110  or  410 .  FIG.  5    is a partial plan view of the printing system taken from above the printhead assembly and  FIG.  6    is a cross-section taken along the line An in  FIG.  5   . 
     In  FIGS.  5  and  6   , each printhead module  502  comprises three printheads  510  (i.e., printheads  510 _ 1 ,  510 _ 2 , and  520 _ 3 ) arranged in an offset pattern as illustrated in  FIG.  5   , but this embodiment is non-limiting, and one of ordinary skill in the art would appreciate the airflow control system  550  could be used in a printing system having differently arranged printhead modules  502 . Furthermore, in  FIGS.  5  and  6    only one printhead modules  502  is illustrated to simplify the description, but in practice there may be more printhead modules  502  present. 
     Like the airflow control systems  150  and  450 , the airflow control system  510  comprises two or more air supply units, namely the air supply units  555 . The air supply units  555  can be used as the air supply units  155 ,  355 , or  455 . In the embodiment of  FIGS.  5  and  6   , the air supply units  555  are provided on a per-printhead  510  basis. In other words, each printhead  510  has a dedicated pair of corresponding upstream and downstream air supply units  555 . Thus, for example, a first printhead  510 _ 1  has a corresponding upstream air supply unit  555   u _ 1  arranged adjacent to and upstream of the printhead  510 _ 1  and a corresponding downstream air supply unit  555   d _ 1  arranged adjacent to and downstream of the printhead  510 _ 1 . Similarly, a second printhead  510 _ 2  has a corresponding upstream air supply unit  555   u _ 2  and a corresponding downstream air supply unit  555   d _ 2 , and a third printhead  510 _ 3  has a corresponding upstream air supply unit  555   u _ 3  and a corresponding downstream air supply unit  555   d _ 3 . 
     In  FIGS.  5  and  6   , each printhead module  502  comprises a carrier plate  511  with openings  519  through the carrier plate  511 . The printheads  510 _ 1 ,  510 _ 2 , and  520 _ 3  are arranged to eject ink through respectively corresponding openings  519 _ 1 ,  519 _ 2 , and  519 _ 3  in a carrier plate  511  of the, with a nozzle end of each printhead  510  extending down partway into the corresponding opening  519  of the carrier plate  511 . The air supply units  515  are also arranged to blow the makeup air  514  down through these openings  519  in the carrier plate  511 . For example, as shown in  FIGS.  5  and  6   , there is a small gap around the perimeter of the printheads  510  between the printhead  510  and the edge of the corresponding opening  519 , and air outlets  558  of the air supply units  555  are positioned over this gap to blow down through the openings  519 . 
     Similar to the air supply units  155  and  455 , the air supply units  555  each comprise an air guide structure  556  in selective fluid communication with an air source  557 . The air guide structure  556  may comprise baffles, nozzles, air knives, tubes, ducts, plenums, or any other structures configured to receive airflows from the air source  557  and direct the airflow towards the movable support surface  520  of the media transport device  503 . The air source  557  may comprise a device configured to selectively provide the airflows to the air guide structure  556  at select timings. For example, the air source  557  may comprise a controllable valve that can be opened or closed to selectively provide airflows to the air guide structure  556 . In such an example, the controllable valve may receive the airflows from a fan, pump, high pressure chamber, or the like to which the controllable valve is coupled. As another example, each air source  557  may comprise its own individual air moving device, such as a fan, pump, etc., which can be controlled to turn on and off at selected timings to selectively provide airflows to the air guide structure  556 . Each air guide structure  556  may be positioned adjacent the corresponding printhead module  502  or printhead  510  and near the movable support surface  520 , such that the makeup air  515  supplied therefrom flows under the printhead  510 /printhead module  502  toward an inter-media zone when the inter-media zone is under the printhead  510 /printhead module  502 . The timings of supplying makeup air from the air supply units  555  are similar to the timings explained above with reference to  FIGS.  3 A- 3 E . 
       FIG.  7    illustrates another embodiment of an airflow control system, namely the airflow control system  750 . The airflow control system  750  can be used as one of the airflow control systems  150  and  450 . The airflow control system  750  can be used in a printing system such as the printing systems  100  or  400 . In  FIG.  7   , the airflow control system  750  is illustrated in the context of a printing system comprising a printhead module  702  with one or more printheads  710  and a movable support surface  720 . The printhead modules  702 , printhead  710 , and movable support surfaces  720  may be used as the printhead modules  102  or  402 , printheads  110  or  410 , and movable support surfaces  120  or  420 , respectively. For simplicity, only one printhead  710  and one printhead module  702  is shown, but this embodiment of the airflow control system  750  could be used with any ink deposition assembly having any number and/or arrangement of printheads or printhead modules. For example, the airflow control system  750  could be used with a printhead module such as the printhead module  502 . 
     The airflow control system  750  comprises pairs of air supply units  755  corresponding to each printhead, with an upstream air supply unit  755   u  arranged upstream of the corresponding  710  printhead and a downstream air supply unit  755   d  arranged downstream of the corresponding printhead  710 . Similar to the air supply units  155 ,  455 , and  555  the air supply units  755  each comprise an air guide structure  756  in selective fluid communication with an air source  757 . The air guide structure  756  may comprise baffles, nozzles, air knives, tubes, ducts, plenums, or any other structures configured to receive airflows from the air source  757  and direct the airflow towards the movable support surface  720  of the media transport device  703 . The air source  757  may comprise a device configured to selectively provide the airflows to the air guide structure  756  at select timings. For example, the air source  757  may comprise a controllable valve that can be opened or closed to selectively provide airflows to the air guide structure  756 . In such an example, the controllable valve may receive the airflows from a fan, pump, high pressure chamber, or the like to which the controllable valve is coupled. As another example, each air source  757  may comprise its own individual air moving device, such as a fan, pump, etc., which can be controlled to turn on and off at selected timings to selectively provide airflows to the air guide structure  756 . Each air guide structure  756  may be positioned adjacent the corresponding printhead module  702  or printhead  710  and near the movable support surface  720 , such that the makeup air  715  supplied therefrom flows under the printhead  710 /printhead module  702  toward an inter-media zone when the inter-media zone is under the printhead  710 /printhead module  702 . The timings of supplying makeup air from the air supply units  755  are similar to the timings explained above with reference to  FIGS.  3 A- 3 E . 
     In the embodiment of  FIG.  7   , the air supply units  755  are provided on a per-printhead  710  basis and are arranged to blow the makeup air  714  down through the openings  719  in a carrier plate  711 , similar to the embodiment of  FIGS.  5  and  6   . However, rather than having an air outlet sitting above the gap between the printhead  710  and the edge of the opening  519 , in this embodiment a portion  756   e  of the air guide structure  756  extends down into the opening  719  through the gap. In some examples, the portion  756   e  that extends down into the opening  719  may extend as far as the bottom surface of the printhead  710 . Thus, the air outlets  758  of the air supply unit  755  is brought closer to the movable support surface  720 , which may improve the effectiveness of the ability of the air supply units  755  to have the makeup air reach the intended locations in the inter-media zone  722  such that the make-up air is better sucked through the uncovered holes  721  in that area so as to address the issues associated with blur. 
       FIGS.  8  and  9    illustrate yet another embodiment of an airflow control system, namely the airflow control system  850 . The airflow control system  850  can be used as one of the airflow control systems  150 ,  450 , or  750 . The airflow control system  850  can be used in a printing system, such as the printing systems  100  or  400 . In  FIGS.  8  and  9   , the airflow control system  850  is illustrated in the context of an embodiment of a printing system comprising a vacuum platen  826 , a movable support surface  820 , and one or more printhead modules  802  comprising one or more printheads  810 . The vacuum platen  826  can be used as part of the vacuum plenum  125  and/or as the vacuum platen  426 . The movable support surface  820 , can be used as any of the movable support surfaces  120 ,  450 , or  720 . The printhead module  802  can be used as any of the printhead modules  102 ,  402 ,  502 , or  702 . The printheads  810 , can be used as the printheads  110 ,  410 ,  510 , or  710 . 
     The airflow control system  850  comprises pairs of air supply units  855  corresponding to each printhead  810 , with an upstream air supply unit  855   u  arranged upstream of the corresponding  810  printhead and a downstream air supply unit  855   d  arranged downstream of the corresponding printhead  810 . Similar to the air supply units  155 ,  455 , and  555  the air supply units  855  each comprise an air guide structure  856  in selective fluid communication with an air source  857 . The air guide structure  856  may comprise baffles, nozzles, air knives, tubes, ducts, plenums, or any other structures configured to receive airflows from the air source  857  and direct the airflow towards the movable support surface  820  of the media transport device  803 . The air source  857  may comprise a device configured to selectively provide the airflows to the air guide structure  856  at select timings. For example, the air source  857  may comprise a controllable valve that can be opened or closed to selectively provide airflows to the air guide structure  856 . In such an example, the controllable valve may receive the airflows from a fan, pump, high pressure chamber, or the like to which the controllable valve is coupled. As another example, each air source  857  may comprise its own individual air moving device, such as a fan, pump, etc., which can be controlled to turn on and off at selected timings to selectively provide airflows to the air guide structure  856 . Each air guide structure  856  may be positioned adjacent the corresponding printhead module  802  or printhead  810  and near the movable support surface  820 , such that the makeup air  815  supplied therefrom flows under the printhead  810 /printhead module  802  toward an inter-media zone when the inter-media zone is under the printhead  810 /printhead module  802 . The timings of supplying makeup air from the air supply units  855  are similar to the timings explained above with reference to  FIGS.  3 A- 3 E . 
     More specifically,  FIGS.  8  and  9    illustrate a specific implementation of the air guide structures  856  of air supply units  855 .  FIG.  9    illustrates a sectional view taken along the line B in  FIG.  8   . The carrier plate and housing of the printhead module  802  are omitted from the illustration to increase visibility. In this example, the air supply units  855  may be provided on a per-printhead  810  basis and may be arranged to blow the makeup air (not shown) down through the openings  819 , similar to the embodiments discussed above with respect to  FIGS.  4 - 7   , and a portion of the air guide structure  856  may extend down into the opening  819 , similar to the embodiment of  FIG.  7   . As shown in  FIG.  8   , the air guide structure  156  comprises an air inlet portion  861 , an air outlet portion  862 , and a transition portion  863 . The air inlet portion  861  is relatively narrow in a cross-process direction as compared to the air outlet portion  862 . The air outlet portion  1562  may span a width of the printhead  110  in the cross-process direction (the process direction being shown by P in  FIG.  8   ), and may extend down into the opening (such as opening  519  or  719  in  FIGS.  5 - 7   , but not shown in  FIG.  8   ) in the gap between the printhead  810  and the edge of the opening. The air guide structure  856  may gradually increase in width throughout the transition portion  863  going from the air inlet portion  861  to the air outlet portion  862 . The air inlet portion  861  may be fluidically coupled to an air supply source  857  via, for example, a tube, duct, baffle, pipe, etc. As shown in  FIG.  9   , air outlet portion  862  may comprise a bottom wall  863  which faces the movable support surface  820  and air outlets  858  may be provided in the bottom wall  863 . In  FIG.  9   , the air outlets  858  are a plurality of holes along a length of the bottom wall  863 . In other examples, the air outlets  858  may be one or more slots, nozzles, or any other type of opening. For example, the air outlet  858  may comprise a single slot spanning across the length of the bottom wall  863 . 
     As shown in  FIG.  9   , in some embodiments the bottom wall  863  may be angled or sloped relative to the movable support surface  820  such that the holes  858  of the bottom wall  863  face slightly toward a reverse-process direction (opposite to the process direction). The angle of the bottom wall  863  relative to the movable support surface  820  may be more than 0° and less than 90°, and in some embodiments it may range from 10° to 45°. For example, the angle may be 20°. Such angling of the bottom wall  863  may reduce the likelihood of a jam occurring in the event that a lead edge LE of a print medium  805  lifts off from the movable support surface  820  as the print medium  805  approaches the printhead  810 . In some embodiments, the air outlet portion  862  extends through the opening in the carrier plate such that it is very close to the movable support surface  820 , and therefore if a lead edge LE of a print medium  805  lifts up there is a chance that it will strike the air outlet portion  862 . By angling the bottom wall  863  of the air outlet portion  862  as shown and described above, then if the lead edge LE lifts up and strikes the air outlet portion  862 , the angled bottom wall  863  may deflect the lead edge LE back downward toward the movable support surface  820 , thus avoiding a jam. In contrast, if the bottom wall  863  is not angled and lies in a plane generally parallel to the support surface  820 , then the lead edge LE may strike the side wall of the air outlet portion  862 , and the relatively steep angle of the side wall may result in the lead edge LE being deflected upwards, resulting in a jam of the print medium. 
     In some embodiments, the air guide structure  856  is configured to snap or attach directly to the printhead  810 . In some embodiments, the air guide structure  856  is attached to a housing of the printhead  810  via clips or other snap-fitting attachment features (not illustrated). This capability of snapping or attaching directly to the printhead may allow screws or other such fasteners to be omitted and facilitate easier installation and removal of the air guide structures  856 , including easier field installation (e.g., when a printhead  810  needs to be replaced). In other embodiments, the air guide structure  856  may be attached to the printheads  810  by screws or other mechanical fasteners. In other embodiments, the air guide structure  856  may be attached to the housing of the printhead module  102 , for example by clips, screws, or any other mechanical fasteners. In some embodiments, the air guide structure  856  is configured to be attachable to existing printheads in already deployed printing systems and sized and shaped to fit through the existing gaps between the carrier plate openings and the printheads. This may facilitate the retrofitting of already deployed printing systems to add in an airflow control system post manufacture. In particular, this may allow for the retrofitting of systems that were not originally designed to have an airflow control system, without requiring new printhead modules, carrier plates, or printheads to also be installed in the printing system. 
       FIG.  10    illustrates yet another embodiment of an airflow control system, namely the airflow control system  1050 . The airflow control system  1050  can be used as the airflow control system  150  or  450 . The airflow control system  1050  can be used in a printing system, such as the printing systems  100  or  400 . In  FIG.  10   , the airflow control system  1050  is illustrated in the context of an embodiment of a printing system comprising a movable support surface  1020  and one or more printhead modules  1002 . The movable support surface  1020  can be used as the movable support surface  120  or  420 . The printhead module  1002  can be used as the printhead module  102  or  402 . The printhead module  1002  comprises printheads  1010 , which can be used as the printheads  110  or  410 . For simplicity, only one printhead  1010  and one printhead module  1002  are shown in  FIG.  10   , but this embodiment of the airflow control system  1050  could be used with any ink deposition assembly with any number of printheads  1010  and printhead modules  1002 . 
     The airflow control system  1050  comprises pairs of air supply units  1055  corresponding to each printhead, with an upstream air supply unit  1055   u  arranged upstream of the corresponding  1010  printhead and a downstream air supply unit  1055   d  arranged downstream of the corresponding printhead  1010 . In this example, the air supply units  1055  are provided on a per-printhead  1010  basis and may be arranged to blow the makeup air  1014  down through the openings  1019  (for simplicity makeup air  1014  is shown only being supplied from air supply unit  1055   u , but it can also be supplied from air supply unit  1055   d  as in other embodiments), similar to the embodiments of  FIGS.  4 - 9    described above. Similar to the air supply units  155 ,  455 , and  555 ,  755 , and  855  the air supply units  1055  each comprise an air guide structure  1056  in selective fluid communication with an air source  1057 . The air guide structure  1056  may comprise baffles, nozzles, air knives, tubes, ducts, plenums, or any other structures configured to receive airflows from the air source  1057  and direct the airflow towards the movable support surface  1020  of the media transport device  1003 . The air source  1057  may comprise a device configured to selectively provide the airflows to the air guide structure  1056  at select timings. For example, the air source  1057  may comprise a controllable valve that can be opened or closed to selectively provide airflows to the air guide structure  1056 . In such an example, the controllable valve may receive the airflows from a fan, pump, high pressure chamber, or the like to which the controllable valve is coupled. As another example, each air source  1057  may comprise its own individual air moving device, such as a fan, pump, etc., which can be controlled to turn on and off at selected timings to selectively provide airflows to the air guide structure  1056 . Each air guide structure  1056  may be positioned adjacent the corresponding printhead module  1002  or printhead  1010  and near the movable support surface  1020 , such that the makeup air  1015  supplied therefrom flows under the printhead  1010 /printhead module  1002  toward an inter-media zone when the inter-media zone is under the printhead  1010 /printhead module  1002 . The timings of supplying makeup air from the air supply units  1055  are similar to the timings explained above with reference to  FIGS.  3 A- 3 E . 
     A portion of the air guide structure  1056  may extend down into the opening  1019 , as in the embodiments of  FIGS.  7 - 9   . In this embodiment, the air guide structure  1056  may further comprise a directed air outlet portion  1065  that is configured to eject the makeup air  1014  in a direction that is angled under the printhead  1010 , rather than ejecting the makeup air  1014  straight downwards towards the movable support surface  1020 . Thus, an upstream air supply unit  1055   u  may guide the makeup air  1014  such that its initial direction is angled downstream under the printhead  1010 , while a downstream air supply unit  1055   d  may guide the makeup air  1014  such that its initial direction is angled upstream under the printhead  1010 . In some circumstances, this helps the makeup air  1014  to be able to flow further to reach uncovered holes  1021  that are relatively far from the air supply unit  1055 . Without such directing of the makeup air  1014 , in some circumstances the makeup air  114  may have a harder time reaching those distant holes  1021 . For example, in the situation illustrated in  FIG.  10   , the trail edge TE is near the downstream side of the printhead  1010 , and therefore hole  1021   a  is relatively distant from the air supply unit  1055   u . If the initial flow direction of the makeup air  114  after ejection were straight down, then more of the makeup air  114  would be sucked into the nearer holes  1021  and less of the makeup air  114  would make it all the way over to the distant hole  1021   a . Thus, the pressure near the distant hole  1021   a  may be lower than desired. In contrast, if the upstream air supply unit  1055   u  directs its makeup air to initially blow in a generally downstream direction as illustrated in  FIG.  10   , more of the makeup air  114  is able to reach the relatively distant hole  1021   a  than would have otherwise been the case. This may improve the blur reduction effect in some circumstances. 
       FIGS.  11  and  12    illustrate yet another embodiment of an airflow control system, namely airflow control system  1150 . The airflow control system  1150  can be used as the airflow control system  150  or  450 . The airflow control system  1150  can be used in a printing system, such as the printing system  100  or  400 . In  FIGS.  11  and  12   , the airflow control system  1150  is illustrated in the context of an embodiment of a printing system comprising a vacuum platen  1126 , a movable support surface  1120 , and one or more printhead modules  1102 . The vacuum platen  1126  can be used as part of the vacuum plenum  125  and/or as the vacuum platen  426 . The movable support surface  1120 , can be used as any of the movable support surfaces  120  and  420 . The printhead module  1102  can be used as one of the printhead modules  102  and  402 . The printhead module  1102  comprises printheads  1110 , which can be used as the printheads  110  or  410 .  FIG.  12    illustrates a cross-section taken along the line C in  FIG.  11   . 
     In this embodiment, the air supply units  1155  are provided on a per-printhead-module  1102  basis, rather than on a per-printhead  1110  basis. Thus, each printhead module  1102  has its own pair of corresponding upstream and downstream air supply units  1155 , and the printheads  1110  within the same module  1102  may share the air supply units  1155  of that module  1102 . Thus, for example, a first printhead module  1102 _ 1  has a corresponding upstream air supply unit  1155   u _ 1  arranged adjacent to and upstream of the printhead module  1102 _ 1  and a corresponding downstream air supply unit  1155   d _ 1  arranged adjacent to and downstream of the printhead module  1102 _ 1 . Similarly, a second printhead module  1102 _ 2  has a corresponding upstream air supply unit  1155   u _ 2  and a corresponding downstream air supply unit  1155   d _ 2 . In some embodiments, the same air supply unit  1155  may serve as both a downstream air supply unit  1155   d  with respect to one printhead module  1102  and an upstream air supply unit  1155   u  with respect to another printhead module  1102 —for example, the air supply unit labeled  1155   d _ 1 ,  1155   u _ 2  in  FIG.  11    is the downstream air supply unit  1155   d _ 1  of the first printhead module  1102 _ 1  and also the upstream air supply unit  1155   u _ 2  of the second printhead module  1102 _ 2 . In the embodiment of  FIGS.  11  and  12   , the air supply units  1155  may extend in a cross-process direction across a width of the deposition region of the ink deposition assembly  1101  (with the process direction labeled as P in  FIG.  11   ). In some circumstances, providing the air supply units  1155  on a per-printhead module  1102  basis may be beneficial in that the air supply units  1155  do not have to be arranged within a housing of the printhead modules  1102 . In some systems, there may not be sufficient space within the printhead modules  1102  for an air supply unit  1155 , while there may be sufficient space between printhead modules  1102 . 
     Similar to the air supply units  155 ,  455 ,  755 ,  855 , and  1055 , the air supply units  1155  each comprise an air guide structure  1156  in selective fluid communication with an air source  1157 . The air guide structure  1156  may comprise baffles, nozzles, air knives, tubes, ducts, plenums, or any other structures configured to receive airflows from the air source  1157  and direct the airflow towards the movable support surface  1120  of the media transport device  1103 . The air source  1157  may comprise a device configured to selectively provide the airflows to the air guide structure  1156  at select timings. For example, the air source  1157  may comprise a controllable valve that can be opened or closed to selectively provide airflows to the air guide structure  1156 . In such an example, the controllable valve may receive the airflows from a fan, pump, high pressure chamber, or the like to which the controllable valve is coupled. As another example, each air source  1157  may comprise its own individual air moving device, such as a fan, pump, etc., which can be controlled to turn on and off at selected timings to selectively provide airflows to the air guide structure  1156 . Each air guide structure  1156  may be positioned adjacent the corresponding printhead module  1102  and near the movable support surface  1120 , such that the makeup air  1114  supplied therefrom flows under the printhead module  1102  toward an inter-media zone when the inter-media zone is under the printhead module  1102 . The timings of supplying makeup air from the air supply units  1155  are similar to the timings explained above with reference to  FIGS.  3 A- 3 E . 
       FIG.  13    illustrates one embodiment of an airflow control system, namely the airflow control system  1350 . The airflow control system  1350  can be used as the airflow control system  150  or  450 . The airflow control system  1350  can be used in a printing system, such as the printing systems  100  or  400 . In  FIG.  13   , the airflow control system  550  is illustrated in the context of an embodiment of a printing system comprising a vacuum platen  1326 , a movable support surface  1320 , and one or more printhead modules  1302 . The vacuum platen  1326  can be used as part of the vacuum plenum  125  and/or as the vacuum platen  426 . The movable support surface  1320 , can be used as any of the movable support surfaces  120  and  420 . The printhead module  1302  can be used as one of the printhead modules  102  and  402 . The printhead module  1302  comprises printheads  1310 , which can be used as the printheads  110  or  410 .  FIG.  13    is a partial plan view of the printing system taken from above the printhead assembly. 
     In  FIG.  13   , each printhead module  1302  comprises three printheads  1310  (i.e., printheads  1310 _ 1 ,  1310 _ 2 , and  1320 _ 3 ) arranged in an offset pattern as illustrated in  FIG.  13   , but this embodiment is non-limiting, and one of ordinary skill in the art would appreciate the airflow control system  1350  could be used in a printing system having differently arranged printhead modules  1302 . Furthermore, in  FIG.  13    only one printhead modules  1302  is illustrated to simplify the description, but in practice there may be more printhead modules  1302  present. 
     In the embodiment of  FIG.  13   , each printhead module  1302  comprises a carrier plate  1311  and one or more ports  1391  are provided along a side of the carrier plate  1311 . The ports  1391  comprise holes or openings through the carrier plate  1311 . In  FIG.  13   , two ports  1391  configured as circular holes are illustrated, but in practice any number of ports  1391  could be provided and the ports  1391  could have any desired shape, such as a slot. In this embodiment, the airflow control system  1350  comprises one or more air supply units  1390  arranged to supply makeup air through the ports  1391  to neutralize the vacuum suction from uncovered holes  1321  along the side of the movable support surface  1320 . 
     Because the print media  1305  are registered to one side of the platen  1326 , the holes  1321  on the opposite side will be uncovered if the print medium  1305  is less wide than the largest print medium  1305  the system is designed to handle. For example, in  FIG.  13    four columns of holes  1321  on an inboard side (left side) of the movable support surface  1320  are uncovered. These uncovered holes  1321  may create crossflows which can contribute to blurring along a side edge of the print medium, for reasons similar to those described above with respect to the lead and trail edges. Thus, the ports  1391  are provided along a side of the carrier plate  1311  that is opposite from the side to which the print media  1305  are registered such that the makeup air provided through the ports  1301  can neutralize the vacuum suction through those uncovered holes  1321  in the vicinity of the printhead  1310 . In  FIG.  13    the print media  1305  are registered to an outboard side of the platen  1326  (right side in  FIG.  13   ), and thus in  FIG.  13    the ports  1390  are provided on the inboard side of the carrier plate  1311  (left side in  FIG.  13   ). By providing makeup air through the ports  1390 , the crossflows near the side edges of the print media  1305  may be reduced or eliminated, thus reducing or eliminating image blur along the side edges off the print media  1305 . 
     Unlike the other air supply units described herein, the air supply unit(s)  1390  are not controlled to supply makeup air based on the location of the inter-media zones. This is because the uncovered hole  1321  along the side edge of the print medium  1305  are present throughout the printing process regardless of the location of the inter-media zone. Thus, the air supply unit(s)  1390  are configured to supply makeup air through the ports  1390  whenever a print medium  1305  is being printed on by a printhead  1310  adjacent the ports  1390 , unless the print medium  1305  is wide enough to cover all of the holes  1321  in a cross-process direction. In some examples, the air supply unit(s)  1390  may supply the makeup air continuously during printing. 
     In some embodiments, the amount of makeup air supplied by an air supply unit  1390  is controlled based on the size of the print medium  1305  being printed, or in other words based on the number of columns of holes  1321  that are left uncovered by the print medium  1305 . The more holes  1321  left uncovered, the more makeup air may be supplied, so that the amount of makeup air supplied is sufficient to neutralize the vacuum suction near the printhead  1310  to reduce crossflows while not being too large and creating its own crossflows. The amounts of air to supply for each size of print media  1305  may be determined in advance experimentally, in the same manner as described above in relation to the air supply units  155 . The amounts of air to supply may also be learned and adjusted automatically by the printing system during operation, in the same manner as described above in relation to adjusting the timings of supplying makeup air from the air supply units  155 . 
     In some embodiments, the airflow control system  1350  also comprises air supply units  1355  arranged around the printheads  1310  or printhead modules  1302  and configured to supply makeup air based on the location of an intermedia zone. For example, in  FIG.  13    the printheads  1310 _ 1 ,  1310 _ 2 , and  1320 _ 3  are arranged to eject ink through respectively corresponding openings  1319 _ 1 ,  1319 _ 2 , and  1319 _ 3  in the carrier plate  1311 , and the air supply units  1315  are also arranged to blow makeup air down through these openings  1319  in the carrier plate  1311 , similar to the air supply units  155 ,  455 , and  555  described above. The timings of supplying makeup air from the air supply units  1355  are similar to the timings explained above with reference to  FIGS.  3 A- 3 E . 
       FIGS.  14 - 16    illustrate exemplary embodiments of methods  1400 ,  1500 , and  1600  of operating a printing system, respectively. The methods  1400 ,  1500 ,  1600  may be performed in an inkjet printing system comprising a media transport device that utilizes vacuum suction to hold print media against a movable support surface as the movable support surface transports the print media through a deposition region of an ink deposition assembly, such as any of the printing system  100  or  400  and any other embodiments of the printing systems described above. The printing system may have an airflow control system, such as the airflow control system  150 ,  450 ,  550 ,  750 ,  850 ,  1050 , or  1150 , which comprises air supply units associated with printheads or printhead modules, as described above. The method may be performed, for example, by a control system of the printing system. For example, a machine-readable medium may store machine readable instructions that, when executed, cause the control system to perform operations of one or more of the methods, for example by generating instructions or signals to control operations of the airflow control system and/or to control other components of the printing system. Although various other components of the printer may participate in the performance of the operations, the control system may be considered as performing the operations because the control system directs and controls the operations of those components. In addition, the methods may be performed, for example, by a user of the printer by virtue of the user placing the printer in an operational state in which the printer performs the operation. 
       FIG.  14    illustrates a method  1400  pertaining to controlling the supply of makeup air from a pair of air supply units associated with a printhead or printhead module. The method  1400  comprises operations illustrated in blocks  1401  through  1404  of  FIG.  14   , which are described in greater detail below. 
     Operations of block  1401  comprise, in response to an inter-media zone reaching a first position relative to a printhead or printhead module, beginning to supply makeup air to the movable support surface from an upstream air supply unit associated with the printhead or printhead module. Operations of block  1401  may include determining that the inter-media zone has reached the first position. In some embodiments, the first position of the inter-media zone is a position in which the trail edge of a print medium adjacent to and upstream of the inter-media zone is at a location on an upstream side of a printhead, such as a location near or aligned with an upstream edge of the printhead or printhead module. Determining the inter-media zone has reached the first position can include sensing, for example, by a location tracking system, a location of the print medium adjacent and upstream of the inter-media zone and determining, based on the sensed location, when the trailing edge of the print medium is at the location on the upstream side of the printhead. Sensing a location of the print medium may include sensing a lead edge or trail edge of the print medium using an edge sensor. 
     Operations of block  1402  comprise, in response to the inter-media zone reaching a second position relative to the printhead or printhead module, ceasing supply of the makeup air from the upstream air supply unit. Operations of block  1402  may also include determining that the inter-media zone has reached the second position. In some examples, the second position of the inter-media zone is a position in which the trail edge of the print medium adjacent to and upstream of the inter-media zone is at a location on a downstream side of a printhead, such as a location near or aligned with a downstream edge of the printhead or printhead module. In some examples, the second position of the inter-media zone is a position in which the lead edge of a second print medium adjacent the inter-media zone is at a location on an upstream side of a printhead, such as a location near or aligned with an upstream edge of the printhead or printhead module. The second location is different than, and downstream of, the first location. Determining the inter-media zone has reached the second position can include sensing, for example, by a location tracking system, locations of a print medium and determining, based on the sensed locations, when the trailing edge or lead edge of the print medium is at the corresponding location mentioned above. Sensing a location of the print medium may include sensing a lead edge or trail edge of the print medium using an edge sensor. 
     Operations of block  1403  comprise, in response to the inter-media zone reaching a third position relative to the printhead or printhead module, beginning to supply makeup air from a downstream air supply unit associated with the printhead or printhead module. Operations of block  1402  may also include determining that the inter-media zone has reached the third position. In some examples, the third position of the inter-media zone is a position in which the trail edge of the first print medium is at a location on a downstream side of a printhead, such as a location near or aligned with a downstream edge of the printhead or printhead module. In some examples, the third position of the inter-media zone is a position in which the lead edge of the second print medium is at a location on an upstream side of a printhead, such as a location near or aligned with an upstream edge of the printhead or printhead module. In some examples, the third position is the same as the second position, in which case operations of blocks  302  and  303  may be performed simultaneously. In other examples, the third location may be different than (either upstream or downstream of) the second position. Determining the inter-media zone has reached the third position can include sensing, for example, by a location tracking system, locations of a print medium and determining, based on the sensed locations, when the trailing edge or lead edge of the print medium is at the corresponding location mentioned above. Sensing a location of the print medium may include sensing a lead edge or trail edge of the print medium using an edge sensor. 
     Operations of block  1404  comprise, in response to the inter-media zone reaching a fourth position relative to the printhead or printhead module, cease supplying the makeup air from the downstream air supply unit. Operations of block  1402  may also include determining that the inter-media zone has reached the fourth position. In some examples, the fourth position of the inter-media zone is a position in which the lead edge of second print medium is at a location on a downstream side of a printhead, such as a location near or aligned with an upstream edge of the printhead or printhead module. Determining the inter-media zone has reached the fourth position can include sensing, for example, by a location tracking system, locations of a print medium and determining, based on the sensed locations, when the lead edge of the print medium is at the corresponding location mentioned above. Sensing a location of the print medium may include sensing a lead edge or trail edge of the print medium using an edge sensor. 
     In the operations of blocks  1401  and  1403 , beginning to supply the makeup air from one of the air supply units may comprise generating and supplying an airflow-on control signal and/or power supply signal to the relevant air supply unit, the airflow-on control signal and/or power supply signal being configured to turn on airflow of an air supply source of the air supply unit. In some examples, the air supply source may be a valve, and turning on the airflow of the air supply source may comprise moving the valve from a closed state to an open state. In some examples, the air supply source may be an air moving device (e.g., fan, pump, etc.), and turning on the airflow of the air supply source may comprise supplying motive power to a rotor of the air moving device. 
     Conversely, in the operations of blocks  1402  and  1404 , ceasing supplying the makeup air may comprise generating and supplying an airflow-off control signal and/or ceasing to supply a power supply signal to the relevant air supply unit, the airflow-off control signal being configured to turn off airflow of the air supply source of the air supply unit. In some examples, the air supply source may be a valve, and turning off the airflow of the air supply source may comprise moving the valve from an open state to a closed state. In some examples, the air supply source may be an air moving device (e.g., fan, pump, etc.), and turning off the airflow of the air supply source may comprise ceasing to supply motive power to a rotor of the air moving device. 
       FIG.  15    illustrates an embodiment of a method  1500  for determining an airflow rate to use for the makeup air of an air supply unit. In one embodiment, the method  1500  may be performed automatically by the control system of the printing system, and thus in some embodiments the airflow rate may be dynamically adjusted during printing. 
     Block  1501  comprises printing an image using a printing system comprising an airflow control system according to the various embodiments described herein. In one embodiment, the image may be a test image generated specifically for the airflow rate adjustment process. The test image may comprise a one or more printed features (e.g., one or more lines) have a predetermined pattern or shape. For example, the test image may comprise one or more lines extending in the cross-process direction, printed near one or both of the lead and trail edges. The line may be, for example, a few (e.g., two, three, four, five, etc.) pixels wide. In another embodiment, the image may not be specific to the airflow rate adjustment process—for example, the image may be part of a regular print job unrelated to the adjustment process. 
     Block  1502  comprises determining an amount of edge blur in the printed image. This may involve obtaining an electronic copy of the printed image, for example by scanning or photographing the printed image. An inline image capture system can be used to scan the printed images while they are still being transported through the printing system. The copied image may then be analyzed to determine an amount of blur in the image. Analyzing the copied image may include measuring the amount of ink that landed outside of an intended deposition area associated with a printed feature (e.g., a line) in the printed image, and this quantity may represent the amount of blur in the image. Determining the amount of ink that landed outside of the intended deposition area may involve identifying where the intended deposition area is located in the copied image. The location and shape of the intended inked area for a printed feature may be determined, for example, by edge detection or other image processing techniques and/or based on the master image file used to print the image. Once the boundaries of the intended deposition region are determined, the number of dots in the printed image that are beyond the edge of the intended deposition region may be counted, and this value may be used to characterize the extent of the edge blur, with more dots being indicative of more image blur. Experimentally, it has been determined that having less than 20 drops/mm 2  outside the intended inked region is acceptable with respect to image blur, in some circumstances. Alternatively, or in addition, the number of dark pixels that are outside of the intended deposition region in the copied image may be determined, and this value may be used to characterize the extent of the edge blur, with more dark pixels being indicative of more edge blur. Alternatively, or in addition, the average brightness value of the pixels in a given region in the copied image that is outside of the intended deposition region may be determined, and this value may be used to characterize the extent of the edge blur, with lower average brightness being indicative of more edge blur. 
     In examples that use edge detection to identify the boundary of the intended inked area, the boundary (edge) may be identified by analyzing the local density of inked dots in the printed image (local average darkness of pixels in the copied image). Ideally, the edges of the printed feature would transition in a sharp, binary fashion from inked (e.g., dark) to non-inked (e.g., white) and vice versa. In reality, due to manufacturing tolerances, environmental conditions, etc., the edges of a printed feature tend to transition from inked to non-inked over a finite distance. Accordingly, the edge of the intended inked region can be defined as the contour (e.g., line) where the localized average print density falls below a threshold. For example, if an ideal inked region of the print has a localized average greyscale value of 255 (8 bit grayscale) and the ideal non-inked region has a localized average greyscale value of 0 (8 bit grayscale), then the edge of the intended inked region could be determined to be the boundary where the localized average greyscale falls below 80. 
     Alternatively, the boundaries of the printed feature (e.g., line) may be inferred based on knowledge of the dimensions of the printed features. For example, if the printed feature is a line and it is known that the line is supposed to be four (4) pixels wide, then the system may identify a center of the printed line in the copied image and determine that the boundaries (edges) of the line are each located on opposite sides of and two pixels from this center. 
     Various other known image processing techniques, image quality analysis techniques, barcode quality analysis techniques, and blur detection techniques may also be used to quantify the extend of the image blur. As another example, the techniques for measuring blur disclosed in U.S. patent application Ser. No. 16/818,847, filed on Mar. 13, 2020, which is incorporated herein by reference in its entirety, may be used to determine the amount of edge blur. 
     Block  1503  comprises adjusting the flow rate of the makeup air supplied by an air supply unit based on the determined amount of edge blur. For example, the amount of edge blur may be used as feedback in a control loop, such as a proportional-integral-derivative (PID) control loop, with the airflow rate being the controlled variable. For example, the larger the amount of edge blur, the greater the amount by which the airflow rate is adjusted. The airflow rate may be adjusted by, for example, adjusting the airflow source of the air supply unit. For example, if the airflow source comprises a valve with variable settings for the size of its opening—i.e., the valve can be partially open to various degrees, as opposed to being just fully open or fully closed—then the flowrate can be adjusted by adjusting the opening size of the valve. As another example, if the airflow source comprises (or is coupled to) an air moving device (e.g. fan), the flowrate of the air moving device may be adjusted (e.g., the fan speed). As another example, a mass flow controller may be coupled to the airflow source, and the mass flow controller may be controlled to adjust the airflow rate. For example, a baffle in a flow path of the air may be moved to increase or decrease an area of an opening in the flow path, thereby adjusting the flowrate of the air through the path. Those having ordinary skill in the art would understand a combination of any of these mechanisms can be implemented to adjust the flow rate and would further appreciate other techniques for adjusting the flow rate. 
     In some embodiments, the airflow rate of all of the air supply units may be set to the same level and may be adjusted together. In other embodiments, the airflow rate of individual air supply units or of groups of air supply units may be adjusted independently. In such cases, portions of the method  1500  may be performed multiple times, for example, once for each air supply unit or group of air supply units. 
       FIG.  16    illustrates an embodiment of a method  1600  pertaining to determining timings at which the makeup air of an air supply unit is supplied. In one example, the method  1600  may be performed automatically by the control system of the printer, and thus in some examples the timings of the makeup air may be dynamically adjusted during usage. The method  1600  comprises the operations of blocks  1601 - 1603 . 
     The operations of block  1601  comprise printing an image. This may be a test image or any other image, similar to block  1502  as described above. 
     The operations of block  1602  comprise determining an amount of edge blur in the printed image. This may involve obtaining an electronic image of the printed image, for example by scanning or photographing the printed image, similar to block  1502  as described above. 
     Block  1603  comprises adjusting a timing associated with supplying makeup air by an air supply unit based on the determined amount of edge blur. For example, the amount of edge blur may be used as feedback in a control loop, such as a PID control loop, with the timing being the controlled variable. Each air supply unit may have two timings that need to be set: a timing of starting the supply of the makeup air and the timing that supply of the makeup air ceases. These timings may be determined separately by repeating the process  500 , once for the start timings and once for the end timings. It should be understood that the start and end timings are determined based on the location of the print media, as described above. Specifically, the start and end timings correspond to the timings when the relevant parts of the print media reach corresponding trigger locations. Thus, adjusting the start and end timings is accomplished by adjusting the associated trigger locations. 
     In other embodiments, the method  1600  may be performed individually for each air supply unit. Thus, in such examples, the method  1600  may be performed 2N times, where N is the number of air supply units (once for start timings and once for end timings, for each air supply unit). 
     In some embodiments, the timings of a group of similarly-situated air supply units may be set to the same levels, meaning the same trigger locations are used for each of the similarly-situated air supply units relative to their respectively corresponding printheads or printhead modules. For example, if the start timing of one air supply unit is set to a location 1 mm upstream of its printhead, the start timing of the other similarly-situated air supply units may also be set to locations 1 mm upstream of their respective printheads. Thus, in such examples, the method  1600  may be performed for one member of a group of similarly situated air supply units, but does not need to be performed for the other members of that group. In some examples, groups of similar situated air supply units may include a group comprising all upstream air supply units and a group comprising all downstream air supply units. In another example, air supply units that are arranged in a similar position within their print module (e.g., front inboard side) may be considered as being part of the same group of similarly situated air supply units. 
       FIG.  17    is a block diagram illustrating a control loop for a controller  730  controlling the amount of makeup air supplied and/or the timings at which the makeup air is supplied from an air supply unit  1755 . The air supply unit  1755  may be used as any of the air supply units described herein. The controller  1730  may be used as, or as part of, the control system  130 . The controller  1730  controls the amount of makeup air supplied by the air supply unit  1755  by sending a flow amount control signal to a mass flow controller  1781  associated with the air supply unit  1755 . The mass flow controller  1781  receives pressurized air from a pressure regulator  1728 , such as a fan, and adjusts a rate of airflow to the air supply unit  1755  based on the flow amount control signal. For example, the mass flow controller  1781  may comprise a device that changes an airflow impedance between the pressure regulator  1728  and the valve  1757 , thereby controlling a rate of airflow. The controller  1730  controls the timings at which makeup air are supplied by sending an open/close control signal to a valve  1757  of the air supply unit  1755 . In an open state, the valve  1757  allows air to flow from the mass flow controller  1781  to the air guide structure  1756 , which in turn supplies the air as makeup air. In a closed state, the valve  1757  prevents air from flowing to the air guide structure  1756 , thus preventing the supply of makeup air. The valve  1757  is configured to transition to the open state when an open command is received and to transition to the closed state when a close command is received. Thus, by varying the timings at which open and close commands are sent to the valve  1757 , the controller  1730  controls the timings at which makeup air is supplied. 
     As shown in  FIG.  17   , an ink deposition assembly  1701  prints an image, and the amount of edge blur is measured in the printed image and fed back to the controller  1730 . The measurement of the amount of image blur may be obtained in the manner described above. Based on the blur amount, the controller  1730  adjusts the amount of makeup air to be provided and/or the timing of providing the makeup air based on the blur feedback. As shown in  FIG.  17   , in some embodiments the measured amount of image blur is combined with (e.g., subtracted from) an edge blur specification. The edge blur specification is a parameter which indicates an amount of detected blur that would be considered acceptable by the system (since zero blur may not be feasible or desired in all circumstances). The edge blur specification may be a fixed value, or it may be a changeable parameter (e.g., user-selectable). The blur amount resultant from combining the measured edge blur and the edge blur specification is fed back into the controller  1730 , and the controller  1730  determines whether (and by how much) to adjust the airflow rate and/or timings based on the blur amount. If the measured edge blur exceeds the edge blur specification, then corrective action is taken by adjusting airflow rate and/or timings. If the measured edge blur does not exceed the edge blur specification, then the controller  1730  may abstain from further adjustments to the airflow rate and/or timings. In other embodiments, the measured blur amount is fed back directly to the controller  1730 . The controller  1730  may also receive additional inputs which are used to control the amounts and/or timings of the makeup air. For example, as illustrated in  FIG.  17   , the controller  1730  may receive an indication of a page size, a print speed, and a page sync timing, which the controller  1730  may use to determine the locations of the inter-media zones, and hence the timings for supplying makeup air. The controller  1730  may also receive information about the digital print content, which the controller  1730  may use as part of measuring the amount of edge blue, as already described above. 
     In some embodiments, the controller  1730  may also dynamically adjust the flow rate of the makeup air while the makeup air is being supplied based on the location of the inter-media zone. Specifically, the flow rate of a given air supply unit  1751  may be varied based on the proportion of the inter-media zone that is currently under the printhead (or in the deposition region of the printhead) corresponding to that given air supply unit  1751 . Thus, when a relatively small proportion of the inter-media zone is under the printhead, such as when the inter-media zone first arrives at the printhead (e.g., the state illustrated in  FIG.  3 A ), the controller  1730  may control the flow rate to be relatively low. As the inter-media zone continues to move downstream, more of the inter-media zone comes under the printhead (e.g., see the state illustrated in  FIG.  3 B ), and thus the controller  1730  progressively increases the flow rate of the makeup air. When the largest proportion of the inter-media zone is under the printhead (e.g., the state illustrated in  FIG.  3 C ), the controller  1730  may control the flow rate to a highest level. As the inter-media zone continues to move downstream, less of the inter-media zone will be under the printhead, and thus the controller  1730  will progressively decrease the flow rate. 
     Although in the description above the control of the flow rate is described as being based on the location of the inter-media zone, with the rate varying according to the proportion of the inter-media zone that is under the printhead, the location of the inter-media zone and the proportion thereof that is under the printhead are defined by the locations of the print media. Thus, the control described above may equivalently be described as the flow rate being controlled based on the location of the print media. Furthermore, the proportion of the inter-media zone that is located under the printhead (or in the deposition region) is inversely related to the surface area of the print medium that is under the printhead (or in the deposition region)—the more of the inter-media zone that is under the printhead, the smaller the area of the print medium that is under the printhead, and vice-versa. Thus, the varying of the flow rate based on the proportion of the inter-media zone that is under the printhead (or in the deposition region) can be equivalently described as varying the flow rate based on the surface area of the print medium that is under the printhead (or in the deposition region). Thus, controller  1730  may be configured to control an air supply unit  1751  to flow the air at a first flow rate when the print medium is at a first location relative to the printhead and to flow the air at a second flow rate, higher than the first flow rate, when the print medium is at a second location relative to the print head, wherein in a larger surface area of the print medium is in the deposition region in the first location than in the second location. 
     This description and the accompanying drawings that illustrate inventive aspects and embodiments should not be taken as limiting—the claims define the protected inventions, including equivalents. Various mechanical, compositional, structural, electrical, and operational changes may be made without departing from the spirit and scope of this description and the claims. In some instances, well-known circuits, structures, and techniques have not been shown or described in detail in order not to obscure the invention. Like numbers in two or more figures represent the same or similar elements. 
     Further, the terminology used herein to describe aspects of the invention, such as spatial and relational terms, is chosen to aid the reader in understanding example embodiments of the invention but is not intended to limit the invention. For example, spatial terms—such as “upstream”, “downstream”, “beneath”, “below”, “lower”, “above”, “upper”, “proximal”, “distal”, “up”, “down”, and the like—may be used herein to describe directions or one element&#39;s or feature&#39;s spatial relationship to another element or feature as illustrated in the figures. These spatial terms are used relative to the poses illustrated in the figures, and are not limited to a particular reference frame in the real world. Thus, for example, the direction “up” in the figures does not necessarily have to correspond to an “up” in a world reference frame (e.g., away from the Earth&#39;s surface). Furthermore, if a different reference frame is considered than the one illustrated in the figures, then the spatial terms used herein may need to be interpreted differently in that different reference frame. For example, the direction referred to as “up” in relation to one of the figures may correspond to a direction that is called “down” in relation to a different reference frame that is rotated 180 degrees from the figure&#39;s reference frame. As another example, if a device is turned over 180 degrees in a world reference frame as compared to how it was illustrated in the figures, then an item described herein as being “above” or “over” a second item in relation to the Figures would be “below” or “beneath” the second item in relation to the world reference frame. Thus, the same spatial relationship or direction can be described using different spatial terms depending on which reference frame is being considered. Moreover, the poses of items illustrated in the figure are chosen for convenience of illustration and description, but in an implementation in practice the items may be posed differently. 
     The terms “upstream” and “downstream” refer to relative locations along a path that print media takes as it is transported through an ink deposition assembly. The path begins where the print media is introduced onto the movable support surface and ends where the print media leaves the support surface. When “upstream” is used to describe something this means that the thing is closer to the beginning of the path as compared to another location or element. Conversely, when “downstream” is used to describe something this means that the element is closer to the end of the path as compared to another location or element. The other location or element to which the thing is compared may be explicitly stated (e.g., “an upstream side of a printhead”), or it may be inferred from the context. Specifically, the air supply units may be arranged in pairs, with an upstream air supply unit of the pair being disposed upstream relative to a downstream air supply unit of the pair. Moreover, a pair of air supply units may be associated with a printhead or printhead module, and the upstream air supply unit of the pair may be arranged upstream of the printhead or printhead module while the downstream air supply unit of the pair may be arranged downstream of the printhead or printhead module. 
     The terms “inboard” and “outboard” refer to opposite sides of the media transport device along a cross-process direction. “Outboard” refers to the side of the media transport device closest to a registration location to which the edges of the print media are registered. For example, in  FIGS.  5 ,  11 , and  13   , the outboard side of the media transport device corresponds to a right side of the device in the perspective of  FIGS.  5 ,  11 , and  13   . “Inboard” refers to the side of the media transport device opposite from the outboard side. For example, in  FIGS.  5 ,  11 , and  13   , the inboard side of the media transport device corresponds to a left side of the device in the perspective of  FIGS.  5 ,  11 , and  13   . The terms “inboard” and “outboard” are also used to refer to directions, with “inboard” referring to a cross-process direction that points from the outboard side to the inboard side (e.g., leftward in  FIGS.  5 ,  11 , and  13   ) and “outboard” referring to the cross-process direction that points from the inboard side to the outboard side (e.g., rightward in  FIGS.  5 ,  11 , and  13   ). The terms “inboard” and “outboard” are also used to refer to relative locations or positions, with inboard being used to refer to a position that is further inboard than some other reference location and outboard being used to refer to a position that is further outboard than some other reference location. Thus, for example, an inboard side of a carrier plate refers to a side of the carrier plate that is relatively closer to the inboard side of the media transport device as compared to another side of the carrier plate. 
     The term “vacuum” has various meanings in various contexts, ranging from a strict meaning of a space devoid of all matter to a more generic meaning of a relatively low pressure state. Herein, the term “vacuum” is used in the generic sense, and should be understood as referring broadly to a state or environment in which the air pressure is lower than that of some reference environment, such as ambient or atmospheric pressure. The amount by which the pressure of the vacuum environment should be lower than that of the reference environment to be considered a “vacuum” is not limited, and may be a small amount or a large amount. Thus, “vacuum” as used herein may include, but is not limited to, states that might be considered a “vacuum” under the more strict senses of the term. 
     The term “air” has various meanings in various contexts, ranging from a strict meaning of the atmosphere of the Earth (or a mixture of gases whose proportions is similar to that of the atmosphere of the Earth), to a more generic meaning of any mixture of gases. Herein, the term “air” is used in the generic sense, and should be understood as referring broadly to any gas or mixture of gases. This may include, but is not limited to, the atmosphere of the Earth, an inert gas or mixture such as a gas or mixture comprising one of the Noble gases (e.g., Helium, Neon, Argon, etc.), Nitrogen (N 2 ) gas, or any other desired gas. 
     In addition, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context indicates otherwise. And, the terms “comprises”, “comprising”, “includes”, and the like specify the presence of stated features, steps, operations, elements, and/or components but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups. Components described as coupled may be electrically or mechanically directly coupled, or they may be indirectly coupled via one or more intermediate components, unless specifically noted otherwise. Mathematical and geometric terms are not necessarily intended to be used in accordance with their strict definitions unless the context of the description indicates otherwise, because a person having ordinary skill in the art would understand that, for example, a substantially similar element that functions in a substantially similar way could easily fall within the scope of a descriptive term even though the term also has a strict definition. 
     Elements and their associated aspects that are described in detail with reference to one embodiment may, whenever practical, be included in other embodiments in which they are not specifically shown or described. For example, if an element is described in detail with reference to one embodiment and is not described with reference to a second embodiment, the element may nevertheless be claimed as included in the second embodiment.