Three-Dimensional Printing System with Multi-Fluid Servicing Module

Abstract

A three-dimensional printing system includes a printhead having an ejection face with nozzles for ejecting different printing fluids, a movement mechanism coupled to the printhead, a fluid supply coupled to the printhead, a maintenance module, and a controller. The maintenance module includes a purge platform, a wiper, and a spittoon. The controller is configured to: (a) receive a new print job; (b) activate the movement mechanism to position the ejection face over the purge platform; (c) activate the fluid supply to at least partially fill a space between the ejection face and the purge platform with fluid; (d) activate the movement mechanism to translate the ejection face over the wiper to remove excess fluid; (e) activate the movement mechanism to position the ejection face over the spittoon; (f) activate the plurality of nozzles to eject drops into the spittoon until the received mixture is expelled from the nozzles.

Description

FIELD OF THE INVENTION

The present disclosure relates to a layer-by-layer fabrication of a three-dimensional article by a process that includes a use of a printhead. More particularly, the present disclosure concerns a maintenance method for the printhead.

BACKGROUND

Three-dimensional printing systems are in wide use for fabricating three-dimensional articles with metals, plastics, ceramics, composites, and other materials. One major type of three-dimensional printing technology utilizes a dry powder and a liquid binding agent. The basic process is a repeated layer-by-layer dispensing of a uniform powder layer followed by a selectively dispensed binding agent. The selective dispensing of the binding agent determines a cross-section of the article for a given layer.

The binding agent that is used typically provides a matrix that binds together the particles. In some systems, the binding agent can react with or partially dissolve the particles. Some of these binding agents are challenging to dispense with enough precision and reliably. For some systems, a “drop-on-demand”printhead such as a piezoelectric printhead is utilized to dispense the binding agents. Maintaining a piezoelectric printhead for dispensing binding agents is particularly challenging.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram schematic of a three-dimensional printing system.

FIG. 2 is an isometric drawing of a portion of an embodiment of a three-dimensional printing system.

FIG. 3 is an isometric view of an embodiment of a maintenance module in isolation.

FIG. 4 is a top view of an embodiment of a maintenance module in isolation.

FIG. 5 is a cutaway isometric view of an embodiment of an maintenance module.

FIG. 5A depicts cross-sectional detail of a portion of the maintenance module of FIG. 5 with particular emphasis on interengagement of peripheral portions of components and on a sensing system that includes a magnet and hall effect sensor.

FIG. 5B depicts cross-sectional detail of a portion of the maintenance module of FIG. 5 with emphasis on a seal between a motorized shaft and an effluent tray. The motorized shaft is used to raise and lower a maintenance tray.

FIG. 6A is an isometric drawing of a portion of an embodiment of a three-dimensional printing system with emphasis on a printhead purging operation. A vertical space is defined by a distance between an ejection face and an upper surface of a purge platform. During purging, the space becomes partially or completely filled with a mixture of printing fluids.

FIG. 6B is an isometric drawing of a portion of an embodiment of a three-dimensional printing system with emphasis on printhead wiping.

FIG. 7 is a flowchart depicting an embodiment of a method for manufacturing a three-dimensional article with emphasis on maintenance processes.

FIG. 8 is a flowchart depicting an embodiment of a method for manufacturing a three-dimensional article with emphasis on a maintenance process.

SUMMARY

In a first aspect of the disclosure, a three-dimensional printing system includes a printhead having an ejection face with a plurality of nozzles for ejecting a plurality of different printing fluids, a movement mechanism coupled to the printhead, a fluid supply containing the plurality of different printing fluids coupled to the printhead, a maintenance module, and a controller. The maintenance module includes a purge platform, a wiper, and a spittoon. The controller is configured to: (a) receive a new print job; (b) activate the movement mechanism to position the ejection face over the purge platform; (c) activate the fluid supply to at least partially fill a space between the ejection face and the purge platform with fluid, the nozzles receive a mixture of the different printing fluids; (d) activate the movement mechanism to translate the ejection face over the wiper to remove excess fluid; (e) activate the movement mechanism to position the ejection face over the spittoon; (f) activate the plurality of nozzles to eject drops into the spittoon until the received mixture is expelled from the nozzles.

In one implementation the printhead is a piezoelectric printhead having a plurality of groups of piezoelectric drop ejectors. The groups individually eject different printing fluids. The plurality of different printing fluids can include at least three different fluid types. The fluid types can have a difference based upon a colorant with the different colorants including at least two or more of black, white, cyan, light cyan, magenta, light magenta, yellow, blue, red, orange, green, violet, and a spot color.

In another implementation the purge platform is a raised platform with a non-porous surface. After step (c) but before step (d) the movement mechanism can laterally move the printhead around over the purge platform to fully wet the ejection face with the fluid mixture.

In yet another implementation the controller is further configured to perform the following steps after step (a) but before step (b): halt operation to allow a user to remove a portion of the maintenance module; receive an indication concerning a presence of a maintenance tray within the servicing module; and proceed to step (b). After halting the operation the controller can provide instructions to the user to remove and replace the maintenance tray from the maintenance module. The indication can concern one or more of removal of the maintenance tray, replacement of the maintenance tray, and confirmation of the presence of the maintenance tray. The system can include a sensor that provides the indication.

In a second aspect of the disclosure, a method of manufacturing a three-dimensional article includes (a) receiving a new print job; (b) activating a movement mechanism to position a printhead ejection face having a plurality of nozzles over a purge platform; a space is defined vertically between the ejection face and the purge platform; (c) activating a fluid supply to force printing fluid out of the printhead and into the space; (d) activating the movement mechanism to translate the ejection face over a wiper to remove residual printing fluid from the ejection face; (e) activating the movement mechanism to position the ejection face over a spittoon; and (f) activating the ejection nozzles to eject drops into the spittoon until any purged fluid received by the nozzles is removed.

In a third aspect of the disclosure, a non-transitory computer-readable storage medium stores computer-readable code portions for manufacturing a three-dimensional article. In response to execution by a processor the computer-readable code portions cause a three-dimensional printer to perform the steps comprising: (a) receive a new print job; (b) activate a movement mechanism to position a printhead ejection face having a plurality of nozzles over a purge platform; a space is defined vertically between the ejection face and the purge platform; (c) activate a fluid supply to force printing fluid out of the printhead and into the space; (d) activate the movement mechanism to translate the ejection face over a wiper to remove residual printing fluid from the ejection face; (e) activate the movement mechanism to position the ejection face over a spittoon; (f) activate the ejection nozzles to eject drops into the spittoon until any purged fluid received by the nozzles is removed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram schematic of a three-dimensional printing system 2 for printing a three-dimensional article 4. A build plate 6 is for supporting the three-dimensional article 4 within a build volume container 8. An elevator mechanism 10 is configured to controllably adjust a vertical position of the build plate 6 and also an upper surface 12 of either the build plate 6 or article 4 for dispensing and forming layers onto the article 4.

A supply of powder 14 is configured to provide the powder to a powder dispenser 16. The powder dispenser 16 is configured to controllably dispense layers of the powder onto the upper surface 12. The build volume container 8 includes an overflow chamber 17 for receiving excess powder during a powder dispensing operation.

A fluid supply 18 is configured to supply a printing fluid (e.g., a binding agent) to a printhead 20. The printhead 20 is a drop-on-demand fluid-jetting printhead 20. In an illustrative embodiment, the fluid jetting printhead 20 is a piezoelectric printhead with a plurality of drop ejectors or nozzles. A movement mechanism 22 is configured to impart a lateral motion along two axes between the printhead 20 and the build upper surface 12.

In an illustrative embodiment, the printhead includes an ejection face 24 having a plurality of nozzles. The fluid supply 18 provides a plurality of different printing fluids that are ejected by different nozzles. The printing fluids differ from one another in terms of chemical composition. They may be different from one another in terms of one or more of the chemistry and/or concentration of chemical components. In the illustrative embodiment, they differ in terms of colorant. In an illustrative embodiment, there are four different printing fluids with four different colorants including black, cyan, yellow, and magenta. In other embodiments, there can be more or less different colorants which can also include one or more of red, orange, green, blue, violet, white, and other primary colors. Also, the different printing fluids can vary in terms of colorant concentration such as full concentration cyan versus light cyan and magenta versus light magenta. In yet other embodiments, the colorants can include spot colors which have precisely controlled color coordinates for identity purposes. One use of spot colors is product branding which typically require exact color matches required by the trade dress of products.

A maintenance module 26 is for maintaining the printhead 20. An embodiment of maintenance module 26 will be discussed in more detail infra.

A controller is 28 is electrically or wirelessly coupled to various portions of the system 2 including the elevator mechanism 10, the powder supply 14, the powder dispenser 16, the fluid supply 18, the printhead 20, the movement mechanism 22, the maintenance module 26 and other portions of system 2. The controller includes a processor coupled to an information storage device. The information storage device includes a non-transient or non-volatile computer-readable storage medium storing software instructions or computer-readable code portions. When executed, the software instructions control the portions of the system 2 listed supra. The controller 28 can be a single computer integrated into system 2 or it can include more than one coupled computer including a host computer.

The controller 28 is configured to operate portions of the printing system 2 to manufacture the three-dimensional article 4 and to perform maintenance on the printhead 20. In an illustrative embodiment, printing-based manufacture includes the following steps: (1) operate the elevator mechanism 10 to vertically position the upper surface 12 for receiving a layer of powder; (2) operate the powder dispenser 16 to dispense a layer of powder onto the surface 12; (3A) operate the movement mechanism 22 to scan the printhead 20 over the powder layer and (3B) while scanning the printhead, operate the printhead 20 to selectively dispense printing fluids (binding agent) upon the dispensed powder layer; (4) repeat steps (1)-(3) to complete fabrication of the three-dimensional article 4.

Before, during, or after printing, the controller 28 can also control the fluid supply 18, the printhead 20, the movement mechanism 22, and the maintenance module 26 to maintain reliability of the printhead 20. The controller 28 can also operate these components to maintain the printhead 20 during idle (non-printing) periods of time.

FIG. 2 is an isometric drawing of a portion of the three-dimensional printing system 2. In describing system 2, mutually perpendicular axes X, Y, and Z will be used. Axes X and Y are lateral axes that are generally horizontal. Axis Z is generally vertical. By “generally” it is by design and to within manufacturing tolerances.

In the figure, printhead 20 is moved along X and Y by the movement mechanism 22. Movement mechanism 22 includes a main carriage 30 which is moved along a slider rod 32 by a belt mechanism 34. The slider rod 32 extends along the lateral X-axis. The printhead 20 is coupled to the main carriage 30 and configured to move along the carriage 30 along the Y-axis. The motion of printhead 20 with the carriage along X and along the carriage along Y allows the printhead access to powder layers for printing and to the maintenance module 26.

FIG. 3 is an isometric view of the maintenance module 26 in isolation. Maintenance module 26 includes a maintenance tray 36, effluent tray 38, and motor 40. The maintenance tray 36 is removably mounted over the effluent tray 38. “Removably mounted” in this context implies that a user of system 2 can remove the maintenance tray 36 for replacement or cleaning by grasping the maintenance tray 36 and pulling it straight up and off of the effluent tray 38. The motor 40 is configured to raise and lower the maintenance tray 36 along the vertical Z-axis.

The maintenance tray 36 has an upper side 42 that includes a storage cap 44, a wiper blade 46, a first spittoon 48, and a purge platform 50. A lateral extent of the maintenance tray 36 is bounded by a periphery 52. The lateral extent of the effluent tray 38 is bounded by a periphery 54. The effluent tray 38 has a downwardly extending conduit 56 for draining of accumulated fluid.

FIG. 4 is a top view of the maintenance module 26 that illustrates some additional details. The storage cap 44 includes an array of pillars 58 for absorbing ink droplets and a central opening 60. The array of pillars 58 allow the cap 44 to be utilized as a second spittoon 59. The central opening 60 relieves a pressure generated when the cap 44 sealingly engages the ejection face 24 of printhead 20 for storage. Unless otherwise specified, reference to a “spittoon” can refer to either the first spittoon 48 or second spittoon 59.

The first spittoon 48 includes an array of pillars 62 for absorbing printing fluid drops. Within or proximate to the first spittoon 48 is a fluid outlet 64 to allow printing fluid accumulating in the first spittoon 48 to drain to the effluent tray 38 and out the downwardly extending conduit 56. The purge platform 50 has a generally domed (convex upward) and nonporous upper surface 66. In some embodiments, the upper surface 66 is flat or with some other geometry.

FIG. 5 is a cutaway isometric view of the maintenance module 26 with the maintenance tray 36 mounted over the effluent tray 38. A cavity 68 is defined between a lower surface 70 of the maintenance tray 36 and an upper surface 72 of the effluent tray 38. In the illustrated embodiment, the fluid outlet 64 extends into the cavity 68 directly above the downwardly extending conduit 56. The downwardly extending conduit 56 extends to a waste receptacle 74.

FIG. 5A is a cross-sectional view that illustrates some additional details from FIG. 5. The periphery 52 of the maintenance tray 36 and the periphery 54 of the effluent tray 38 interengage when the maintenance tray 36 is removably mounted over the effluent tray 38. The periphery 52 of the maintenance tray 36 defines a downwardly facing recess 76. The periphery 54 of the effluent tray 38 defines an upwardly extending peripheral ridge 78. Interengagement occurs when the upwardly extending ridge 78 is received into the downwardly facing recess 76. This interengagement is important for containing waste material within the cavity 68.

The lower surface 70 of the maintenance tray 36 also defines a downwardly facing recess 80. Within the recess 80 is a magnet 82. The recess 80 is closed off by a cover 84. Mounted on a lower surface 86 of the effluent tray 38 is a hall effect sensor 88. The hall effect sensor 88 generates a signal indicative of a magnetic field generated by the magnet 82 to allow installation of the maintenance tray 36 onto the effluent tray 38 to be verified.

FIG. 5B is a cross-sectional view that illustrates further details from FIG. 5. The maintenance tray 36 is coupled to a motorized shaft 90. The motorized shaft 90 is coupled to motor 40 (FIG. 3) which is in turn controlled by the controller 28. The motorized shaft 90 passes through an opening 92 in the effluent tray 38 and is coupled to the lower side 70 of the maintenance tray 36. A seal 94 mounted within the opening 92 provides a sliding seal between the shaft 90 and effluent tray 38 to prevent waste fluid from leaking out of the opening 92.

FIGS. 6A and 6B illustrate certain maintenance operations. The printhead 20 has a downward facing ejection face 24 with an array of nozzles 25. The nozzles are divided up into groups, with each group for ejecting a different printing fluid.

FIG. 6A is an isometric drawing depicting the printhead 20 positioned above the purge platform 50. The view of FIG. 6A is generally looking upward and sideways toward the printhead 20 so that the downward facing ejection face 24 is in view. The ejection face 24 of printhead 20 is in facing relation with the upper surface 66 of the purge platform 50. A space 94 is defined between the ejection face 24 and the upper surface 66. In some illustrative embodiments, a vertical distance between the ejection face 24 and the upper surface 66 can be in a range of one to three millimeters.

In a purge operation, the fluid supply 18 supplies pressurized printing fluid to the printhead 20 until the space 94 is at least partially filled with printing fluid. In some illustrative embodiments an amount of fluid that is purged into the space 94 is about 1 gram to 10 grams by weight. In other illustrative embodiments an amount of fluid that is purged into the space 94 is about 1 cubic centimeter to 10 cubic centimeters by volume. The printing fluid may vertically span the space 94 but not entirely span the space 94 laterally. During the purge operation, the movement mechanism 22 can laterally move the printhead in X and Y until the ejection face 24 is fully wetted. During such an operation, the nozzles 25 may absorb the printing fluid by capillary action. Because a plurality of different printing fluids are employed, the absorbed fluid may be a mixture of the different printing fluids.

FIG. 6B illustrates the printhead 20 being translated in the −X direction as the ejection face 24 passes over the wiper blade 26 to remove bulk ink after the purge (from FIG. 6A) is complete.

FIG. 7 is a flowchart depicting an embodiment of a method 100 for fabricating a three-dimensional article 4. According to 102, the system has been idle for a time period T with the ejection face “parked” against the storage cap 44. According to 104, a new print is received by system 2.

According to 106, an initial servicing sequence is determined based upon the time T. According to 108, the initial servicing sequence determined in step 106 is executed.

According to 110, the powder dispenser 16 is operated to dispense a layer of powder onto the build plate 6. According to 112, printing fluid is selectively dispensed upon the powder to bind the powder. According to 114, the printhead 20 is passed over the first spittoon 48 and nozzles are “spit” into the spittoon 48. In step 114, emphasis is particularly on nozzles that are not used during the printing. As indicated by the return arrow, steps 110-114 are repeated until the article 4 is fully fabricated. Finally, according to 116, the ejection face 24 is “parked” against the storage cap 44.

The method 100 can vary from the illustrated sequence. For example, orders of the steps can vary. In one implementation, step 110 may occur during or before step 108 for the first layer of powder. In another implementation the system 2 can form articles 4 from UV curable printing fluids that do not require a powder. Then step 110 would not be included.

FIG. 8 is a flowchart that illustrates a particular embodiment of a method of maintaining the printhead 20 if system 2 has been idle for a time period T that is greater than some threshold. FIG. 8 corresponds to part of FIG. 7 as a particular example. For example, element 122 of FIG. 8 corresponds to element 102 of FIG. 7. Element 124 of FIG. 8 corresponds to element 104 of FIG. 7. Elements 126 to 136 of FIG. 8 corresponds to element 108 of FIG. 7.

According to 122, the system has been idle for a time T that exceeds a threshold. For example, the threshold can be 8 hours.

According to 124, a new print job is received. According to 126, the operation is halted and the user is instructed to remove and replace the maintenance tray 36. When the user removes tray 36, an indication is received of the removal of tray 36. The indication can be an absence of a signal from the sensor 88. Later, when the user replaces the maintenance tray 36 (with either the same tray 36 after cleaning or another clean tray 36) a signal is received indicating a new presence of the maintenance tray 36. Then the process moves to step 128.

According to 128, the movement mechanism 22 and motor 40 are activated to position the printhead 20 over the purge platform 50. There is then a controlled space between the ejection face 24 and the upper surface 66 of the purge platform.

According to 130, the fluid supply 18 is activated to purge printing fluid through the printhead 20 and out of nozzles of the ejection face 24. In an illustrative embodiment, the purged fluid vertically spans the space 94 (FIG. 6A discussion). In an illustrative embodiment, the printing fluid includes a plurality of different printing fluids which mix in the space 94 between the ejection face 24 and the upper surface 66 and are then drawn into the nozzles 25. As part of step 110, the movement mechanism can laterally translate the printhead in X and Y and the ejection face 24 can be fully wetted by the purged printing fluid.

According to 132, the movement mechanism 22 is activated to pass the ejection face 24 over the wiper blade 46 to wipe the mixed printing fluid off of the ejection face 24. According to 134, the movement mechanism is activated to move the ejection face over the first spittoon 48 or the second spittoon 59. According to 136 the printhead 20 is activated to eject drops from the nozzles until they are purged of fluid that they received during step 110.

After step 136 is completed, the process can proceed to step 110 or 112 of FIG. 7 for the fabrication of the article 4 in a layer-by-layer manner. The method steps of FIGS. 7 and 8 are executed by the controller 28 acting upon components of system 2.

The disclosed system 2 can be used for a wide range of applications. One example of an application is the formation of a medical material or device requiring sterile surfaces and bulk materials. The maintenance module 26, with the maintenance tray 36 being removable and replaceable, minimizes or prevents a buildup of a bacterial growth on the maintenance tray 36. The effluent tray 38 captures excess fluid and prevents components of system 2 from being contaminated.

Another example of an application is for food printing. With food printing, the printing fluid can be a colored binder which can be difficult to dispense reliably. The use of the purge platform with a non-porous upper surface 66 allows the printhead to be fully purged. The colored binder requires a use of different primary colors that can be purged at once onto the upper surface 66. When this is done, the primary colors can mix upon the surface 66 and then be drawn into the nozzles by capillary action. The steps of wiping and then printing into a spittoon removes mixed colors from the nozzles.

Yet other examples of applications include the fabrication of non-food articles 4 using adhesive binding agents. The maintenance module 26 and the maintenance method maintain reliable operation and prevent sensitive printer components from being contaminated with the binding agents.

The specific embodiments and applications thereof described above are for illustrative purposes only and do not preclude modifications and variations encompassed by the scope of the following claims.

Claims

1. A three-dimensional printing system comprising: a printhead having an ejection face with a plurality of nozzles for ejecting a plurality of different printing fluids;a movement mechanism coupled to the printhead;a fluid supply containing the plurality of different printing fluids coupled to the printhead;a maintenance module including: a purge platform;a wiper; anda spittoon; anda controller configured to: (a) receive a new print job;(b) activate the movement mechanism to position the ejection face over the purge platform;(c) activate the fluid supply to flood a space between the ejection face and the purge platform with fluid, the nozzles receive a mixture of the different printing fluids;(d) activate the movement mechanism to translate the ejection face over the wiper to remove excess fluid;(e) activate the movement mechanism to position the ejection face over the spittoon; and(f) activate the plurality of nozzles to eject drops into the spittoon until the received mixture is expelled from the nozzles.

2. The three-dimensional printing system of claim 1 wherein the plurality of different fluids includes at least three different fluid types.

3. The three-dimensional printing system of claim 1 wherein the plurality of different fluids have a difference based upon a colorant, the colorants include at least two or more of black, white, cyan, light cyan, magenta, light magenta, yellow, blue, red, orange, green, violet, and a spot color.

4. The three-dimensional printing system of claim 1 wherein the purge platform is a raised platform with a nonporous surface.

5. The three-dimensional printing system of claim 1 wherein the controller is further configured to: after step (a) perform the following steps: halt operation to allow a user to remove a portion of the maintenance module;receive an indication concerning a presence of a maintenance tray within the servicing module; andproceed to step (b).

6. The three-dimensional printing system of claim 5 wherein after halting the operation the controller provides instructions to the user to remove and replace the maintenance tray from the maintenance module.

7. The three-dimensional printing system of claim 5 wherein the indication concerns one or more of removal of the maintenance tray, replacement of the maintenance tray, and presence of the maintenance tray.

8. The three-dimensional printing system of claim 5 including a sensor that generates a signal to provide the indication.

9. The three-dimensional printing system of claim 8 wherein the maintenance tray includes a magnet, the sensor is a hall effect sensor.

10. A method of manufacturing a three-dimensional object comprising: (a) receiving a new print job;(b) activating a movement mechanism to position a printhead ejection face having a plurality of nozzles over a purge platform, a space is defined vertically between the ejection face and the purge platform;(c) activating a fluid supply to force printing fluid out of the printhead and into the space;(d) activating the movement mechanism to translate the ejection face over a wiper to remove residual printing fluid from the ejection face;(e) activating the movement mechanism to position the ejection face over a spittoon; and(f) activating the ejection nozzles to eject drops into the spittoon until any purged fluid received by the nozzles is removed.

11. The method of claim 10 wherein during step (c) a plurality of different printing fluids are forced out of the printhead and are at least partially mixed in the space.

12. The method of claim 10 further comprising: after step (a), performing the following steps: halting operation to allow a user to remove a portion of a maintenance module;receiving an indication concerning a presence of a maintenance tray within the servicing module; andproceeding to step (b).

13. The three-dimensional printing system of claim 12 wherein after halting the operation the controller provides instructions to the user to remove and replace the maintenance tray from the maintenance module.

14. The three-dimensional printing system of claim 12 wherein the indication concerns one or more of removal of the maintenance tray, replacement of the maintenance tray, and presence of the maintenance tray.

15. The method of claim 12 wherein receiving an indication that a maintenance tray is present includes receiving a signal from a sensor.

16. The method of claim 12 wherein receiving an indication that a maintenance tray is present includes receiving a signal from a hall sensor.

17. A computer-readable storage medium for manufacturing a three-dimensional article, the computer-readable storage medium being non-transitory and having computer-readable code portions stored therein that in response to execution by a processor cause an apparatus to perform steps comprising: (a) receive a new print job;(b) activate a movement mechanism to position a printhead ejection face having a plurality of nozzles over a purge platform, a space is defined vertically between the ejection face and the purge platform;(c) activate a fluid supply to force printing fluid out of the printhead and into the space;(d) activate the movement mechanism to translate the ejection face over a wiper to remove residual printing fluid from the ejection face;(e) activate the movement mechanism to position the ejection face over a spittoon; and(f) activate the ejection nozzles to eject drops into the spittoon until any purged fluid received by the nozzles is removed.

18. The computer-readable storage medium of claim 17 wherein the steps further include: after step (a), perform the following steps: halt operation to allow a user to remove a portion of a maintenance module;receive an indication concerning a presence of a maintenance tray within the servicing module; and proceed to step (b).

19. The three-dimensional printing system of claim 18 wherein after halting the operation the controller provides instructions to the user to remove and replace the maintenance tray from the maintenance module.

20. The three-dimensional printing system of claim 18 wherein the indication concerns one or more of removal of the maintenance tray, replacement of the maintenance tray, and presence of the maintenance tray.