In one example, a decaking system for 3D printing includes a platform to support multiple green parts in unbound powder surrounding the green parts, a decaking tool to remove unbound powder from around the green parts, a camera to photograph green parts on the platform as unbound powder is removed from around the green parts, and a controller operatively connected to the camera. The controller is programmed to detect a pattern of light intensity in the photographs and, in response to a determination a detected pattern matches a reference pattern, modulate or stop the decaking tool.

BACKGROUND

3D printers convert a digital representation of an object into a physical object. 3D printers are used to manufacture objects with complex geometries using a variety of materials including thermoplastics, polymers, ceramics and metals. In powder based 3D printing, successive layers of a powdered build material are formed and portions of each layer solidified in a desired pattern to build up the layers of the 3D object. 3D printing is also commonly referred to as additive manufacturing.

The same part numbers designate the same or similar parts throughout the figures.

DESCRIPTION

Metal objects may be printed by selectively applying a liquid binding agent to portions of each of successive layers of metal powder to bind together those portions of the powder corresponding to a solid layer of the object. The binding agent is usually cured, for example using heat and/or ultra violet energy, to bind the powder more securely for subsequent handling. The bound object, commonly referred to as a “green part”, is heated in a sintering furnace to fuse the bound metal powder. Before sintering, however, green parts must be removed from the surrounding mass of unbound powder in a process commonly referred to as “decaking.” In a typical decaking process, the “build” platform that support the parts during printing is vibrated to break up the unbound powder to flow down and away from the parts through holes in the platform. Any unbound powder clinging to the green parts after decaking is removed in a cleaning process sometimes referred to as “depowdering.”

As the bulk of the unbound powder “cake” breaks up and falls away during decaking, the now unsupported green parts sometimes move around on the vibrating platform. Parts move faster and farther as more and more of the surrounding powder is removed. Moving parts that collide with each other and/or with adjacent structures sometimes break. A new decaking technique has been developed to automatically modulate and then stop the decaking process according to the amount of unbound powder removed. The pace of decaking is slowed as more powder is removed to reduce the risk of damaging part movements and then stopped when enough powder has been removed.

In one example, the decaking process includes vibrating a platform supporting green parts surrounded by a mass of unbound powder, photographing the green parts on the vibrating platform as the unbound powder is removed from around the green parts, detecting a pattern of light intensity in the photographs, determining a detected pattern matches a reference pattern, and then modulating or stopping the vibration. The decaking system may be calibrated based on the relative intensity of light reflected from the platform, unbound powder, and green parts. For a lighter powder on a darker platform, for example, the intensity of the light reflected off the “cake” may decrease as more powder is removed and more of the platform is visible, and the pattern of light intensity detected in the photographs includes progressively more darker pixels and fewer brighter pixels. For a darker powder on a lighter platform, for another example, the intensity of the light reflected off the “cake” may increase as more powder is removed and more of the platform is visible, and the pattern of light intensity detected in the photographs includes progressively more brighter pixels and fewer darker pixels.

The reference pattern may include a series of reference patterns each corresponding to an increasing amount of unbound powder that has been removed from around the green parts. The intensity of the vibration can then be reduced when the detected pattern matches each of the reference patterns and then stopped when the desired amount of powder is removed. Determining a detected pattern matches a reference pattern may be implemented, for example, by identifying progressively more darker pixels and fewer brighter pixels in the photographs (or the other way around) or by counting an increasing number of holes visible in the photographs.

These and other examples described below illustrate but do not limit the scope of the patent which is defined in the Claims following this Description.

As used in this document: “and/or” means one or more of the connected things; a “computer readable medium” means any non-transitory tangible medium that can embody, contain, store, or maintain information and instructions for execution by a processor and may include, for example, circuits, integrated circuits, ASICs (application specific integrated circuits), hard drives, random access memory (RAM), read-only memory (ROM), and flash memory; and a “green part” means a 3D printed object in which the powder is bound but not fully fused.

FIG.1illustrates an example system10for decaking green parts printed with a powder based 3D printer. Referring toFIG.1, decaking system10includes a platform12, a camera14to photograph green parts on platform12, and a decaking tool16. Decaking tool16may be implemented, for example, as a vibrator to vibrate platform12and/or a blower to blow unbound powder away from green parts on platform12. It is expected that platform12usually will be implemented with holes through which powder may flow away from the green parts during decaking. In some implementations, the green parts are printed and decaked on platform12. Any suitable digital still or video camera may be used for camera14. If a still camera14is used, the intensity patterns in a series of photographs taken periodically during decaking, at regular intervals for example, may be compared to one or multiple reference patterns to modulate and/or stop vibrator16. If a video camera14is used, the light intensity may be monitored continuously from the stream of photographs and compared to the reference pattern(s).

Decaking system10also includes a controller18operatively connected to camera14and vibrator16. Controller18includes the programming, processing and associated memory resources, and the other electronic circuitry and components to control the operative elements of system10. Controller18may include distinct control elements for different components. Decaking controller18may be part of or separate from the controller for a 3D printer. Controller18inFIG.1includes a processor20and a computer readable medium22with decaking instructions24that represent programming to photograph the green parts during decaking with a still camera or a video camera, detect patterns of light intensity in the photographs, determine that a detected pattern matches a reference pattern, and then modulate and/or turn off the vibrator, blower or other decaking tool16.

FIG.2illustrates an example decaking system10. Referring toFIG.2, decaking system10includes a platform12that supports green parts26for decaking. Platform12may also support green parts26for printing. In one example, green parts26are printed and decaked on platform12in the 3D printer and then transported for cleaning to a separate depowdering station. In another example, platform12is part of a portable build unit that is transported to a separate decaking station after green parts26are printed. A decaking tool16includes a vibrator16A and blowers16B. Vibrator16A vibrates platform12during decaking to break loose and separate unbound powder30from around parts26. Blowers16B blow air or another gas to remove unbound powder30from around green parts26. While decaking tool16includes a vibrator and a pair of blowers inFIG.2, other implementations are possible. For example, a decaking tool16may include only a vibrator16A or only a blower16B (or pair of blowers16B). Unbound powder30flows through holes28in the vibrating platform12, for example to a collection tank32for recycling or disposal. In other examples, unbound powder30is recycled directly to a printing supply. A vacuum may be applied to platform12, as indicated by arrows34, if desired to help remove powder30from platform12.

Controller18is operatively connected to camera14to photograph parts26during decaking and to decaking tool16to control the force and frequency of decaking, for example by modulating the force and frequency of vibrator16A vibrating and/or blowers16B blowing, based on photographs from camera14. Although only one camera14is shown inFIG.2, multiple cameras may be used. System10may also include a lamp36as a source of diffuse light to illuminate parts26while camera14photographs green parts26during decaking. Diffuse lighting may be desirable to enhance contrast for more distinct patterns of light intensity in the photographs from camera14.

FIG.3illustrates a process100to decake a group of green parts printed with a powder based 3D printer. Process100may be implemented, for example, with a processor20executing decaking instructions24on a controller18in a decaking system10as described above with reference toFIGS.1and2. Referring toFIG.3, process100includes vibrating a platform supporting green parts surrounded by a mass of unbound powder (block102), photographing the green parts on the vibrating platform as the unbound powder is removed from around the green parts (block104), detecting a pattern of light intensity in the photographs (block106), determining a detected pattern matches a reference pattern (block108), and based on the determining, modulating or stopping vibrating the platform (block110). Process100may also include illuminating the green parts on the vibrating platform with diffuse light while photographing the green parts, for example as described above with reference toFIG.2.

In one example, determining a detected pattern matches a reference pattern at block108includes identifying more darker regions and fewer brighter regions in a pattern of decaked green parts compared to a pattern of caked green parts. In another example, detecting a pattern of light intensity in the photographs at block106includes detecting a number of holes in a platform supporting the parts and determining a detected pattern matches a reference pattern at block108includes determining the number of detected holes exceeds a threshold number of holes.

FIG.4illustrates a group of green parts26in a mass of unbound build material powder30surrounding the green parts near the beginning of a decaking process, such as might be taken by a camera14inFIGS.1and2.FIG.5is a histogram illustrating one example of a pattern of light intensity near the beginning of a decaking process, for example as shown inFIG.4, such as might be detected by a controller18inFIGS.1and2.FIG.6illustrates a group of green parts26near the end of the decaking process, such as might be taken by a camera14inFIGS.1and2.FIG.7is a histogram illustrating one example of a pattern of light intensity near the end of a decaking process, for example as shown inFIG.6, such as might be detected by a controller18inFIGS.1and2. In each histogram, light intensity is represented in arbitrary units along the horizontal axis from lower intensity (i.e., darker) to higher intensity (i.e., brighter) and the number of pixels detected at each intensity is represented along the vertical axis from fewer pixels to more pixels.

As shown inFIG.4, green parts26are buried in a mass of unbound powder30near the beginning of the decaking process. The corresponding histogram of light intensity inFIG.5shows a pattern38with more higher intensity pixels and fewer lower intensity pixels. As shown inFIG.6, green parts26are exposed near the end of the decaking process as much of the unbound powder has been removed. The corresponding histogram of light intensity inFIG.7shows a pattern38with more lower intensity pixels and fewer higher intensity pixels compared toFIG.5.

FIGS.5and7also show a series of reference patterns40,42overlaid on the detected patterns38. In this example, each reference pattern40,42is a horizontal line representing a pixel count threshold for each of two corresponding levels of vibration (or other decaking technique). For example, a high level of vibration is applied when there are fewer lower intensity pixels (e.g., intensity<100) than higher intensity pixels (e.g., intensity>100) above threshold40, as shown inFIG.5, a reduced level of vibration is applied when there are more lower intensity pixels (e.g., intensity<100) than higher intensity pixels (e.g., intensity>100) above threshold40, and vibration is stopped when there are no higher intensity pixels (e.g., intensity>100) above threshold42, as shown inFIG.7.

Detected and reference patterns will vary depending on the printing materials and processes, including the type and color of powder and binders used to print the green parts, and the relative strength of the green parts. The histograms with detected and reference patterns38-42inFIGS.5and7illustrate just one of many possible scenarios as the light intensity shifts from lighter to darker (or from darker to lighter) as the decaking process progresses. AlthoughFIGS.4and6cover the entire platform, it may be desirable in some examples to photograph only part or parts of the platform to determine if and when to modulate or stop the decaking tool.

FIGS.4and6also show the emergence of an increasing number of holes28in platform12during decaking.FIG.4shows parts26earlier in the decaking process with fewer holes28exposed.FIG.6shows the parts26later in the decaking process with more holes28exposed. Detecting a pattern of light intensity at block106inFIG.3may be implemented by detecting the number of holes in the support platform, for example using a find contour function in OpenCV™, and determining that the detected pattern matches a reference pattern at block108inFIG.3may be implemented by determining that the number of detected holes exceeds a threshold.

The threshold may be a single threshold for modulating or turning off the decaking tool or a series of thresholds for modulating and turning off the decaking tool as more holes are detected during the decaking process. The threshold may represent an absolute number of holes or a percentage of the total number of holes in the platform. Other suitable threshold parameters may be possible. With only ambient lighting, each hole may be detected and counted as a dark spot in a photograph. With the platform lighted up from below (or from above), for example with a lamp36inFIG.2, each hole may be detected and counted as a bright spot in the photograph.

As noted at the beginning of this Description, the examples shown in the figures and described above illustrate but do not limit the scope of the patent. Other examples are possible. Therefore, the foregoing description should not be construed to limit the scope of the patent, which is defined in the following Claims. “A” and “an” in the Claims means one or more. For example, “a camera” means one or more cameras and subsequent reference to “the camera” means the one or more cameras.