Patent Publication Number: US-2017368752-A1

Title: Non-contact control of layering for three-dimensional object printing

Description:
TECHNICAL FIELD 
     The device and method disclosed in this document relates to three-dimensional object printing and, more particularly, to leveling systems in three-dimensional object printers. 
     BACKGROUND 
     Digital three-dimensional object manufacturing, also known as digital additive manufacturing, is a process of making a three-dimensional solid object of virtually any shape from a digital model. Three-dimensional object printing is an additive process in which one or more ejector heads deposit material to build up a part. Material is typically deposited in discrete quantities in a controlled manner to form layers which collectively form the part. The initial layer of material is deposited onto a substrate, and subsequent layers are deposited on top of previous layers. The substrate is supported on a platform that can be moved relative to the ejection heads so each layer can be printed; either the substrate is moved via operation of actuators operatively connected to the platform, or the ejector heads are moved via operation of actuators operatively connected to the ejector heads. Three-dimensional object printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling. 
     In many three-dimensional object printing systems, a partially printed part is subjected to a leveling process after each layer of material is deposited. The leveling process ensures that each layer is a controlled thickness, and that the subsequent layer has a flat surface to be formed upon. By performing this leveling process between each successive layer, higher quality parts are manufactured within narrower tolerances. 
     In some three-dimensional object printing systems, a leveling roller flattens the upper surface of the part after each successive layer of material is deposited.  FIG. 6  shows a prior art three-dimensional object printing system  10  having a conveyor  14  and a leveling roller  18 . The conveyor  14  has a substantially planar surface  22  upon which printed parts, such as the partially formed part  26 , are built. The conveyor  14  is configured to convey the part  26  in a conveying direction X that is parallel to the surface  22  of the conveyor  14 . The roller  18  is arranged above the surface  22  of the conveyor  14  in a vertical direction Y that is normal to the surface  22  of the conveyor  14 . The roller  18  is cylindrical about a longitudinal axis that extends in a lateral direction Z, which is parallel to the surface  22  of the conveyor  14  and orthogonal to the conveying direction X. 
     After each successive layer of material is deposited, the conveyor  14  conveys the part  26  in the conveying direction X. The roller  18  is adjusted to an appropriate distance from the surface  22  of the conveyor  14 . The conveyor  14  feeds the part  26  between the conveyor  14  and the roller  18  to flatten an upper surface  30  of the part  26  that is opposite a bottom surface of the part  26  that sits upon the surface  22  of the conveyor  14 . 
     The printing system  10  is designed to handle parts, such as the part  26 , up to 20 inches wide in the lateral direction Z, but the roller  18  is intended to only remove about 3 microns of material from the upper surface  30  of the part  26 . This constraint imposes costly manufacturing tolerances for the roller  18 . For example, the roller  18  can be twenty inches long and two inches in diameter. This relatively large roller must be manufactured with tight tolerances for cylindricity. Particularly, the roller must be manufactured with tight tolerances for straightness and roundness. As used herein “straightness” refers to the variability of the roller&#39;s diameter across its length. As used herein “roundness” refers to the variability in diameter that depends on the angle on the circumference at which the diameter measured. A roller with perfect roundness has precisely the same diameter when measured from all angles. Conversely, a roller having imperfect roundness has variances in diameter that depend on the angle at which it is measured. This variance in diameter at different angles is referred to as “run-out.” 
       FIG. 7  shows a side view of the printing system  10  with a roller  18  having imperfect roundness, or run-out. A circular outline  34  shows an ideal roundness of the roller  18 . As can be seen, portions of the roller  18  extend beyond the circular outline  34 . The particular run-out of the roller  18  varies with each roller that is manufactured. Accordingly, the roller  18  is incapable of truly flattening the upper surface  30  of the part  26  unless the run-out of the roller is eliminated, but significant manufacturing costs must be incurred for the elimination of the run-out. 
       FIG. 8A  and  FIG. 8B  show the effect of the run-out of the roller  18  on the leveling process. As the roller  18  moves over the upper surface  30  of the part  26 , the longitudinal axis of the roller  18  maintains a fixed distance from the conveyor  14 . However, because the diameter of the roller  18  varies, a ripple is produced in the upper surface  30  of the part  26  as the roller  18  moves across the part  26 , as seen in  FIG. 8B . Accordingly, the run-out of the roller  18  adversely impacts the leveling process. 
     In current printing systems, such as the printing system  10 , the rollers  18  are ground to very tight tolerances on the order of one micron to minimize the effect of the run-out, which comes at great expense. Even when manufactured to the required precision, the rollers  18  risk contaminating or damaging the part  26 . Additionally, with each pass of the roller  18 , material is removed away from the part  26  and wasted. What is needed is a low cost method for leveling substrates in three-dimensional object printing. 
     SUMMARY 
     A three-dimensional object printer includes a platen; an ejector head having a plurality of ejectors configured to eject drops of material toward the platen; a sensor configured to measure heights of drops of material ejected onto the platen; and a controller operatively connected to the sensor and the ejector head. The controller is configured to operate the plurality of ejectors to eject drops of material toward the platen to form a first layer of material upon the platen; operate the sensor to measure a height profile of the first layer of material; and operate the plurality of ejectors to eject drops of material toward the platen to form a second layer of material upon the first layer of material with reference to the measured height profile. 
     A method of operating a three-dimensional object printer includes operating a plurality of ejectors of an ejector head to eject drops of material toward a platen to form a first layer of material upon the platen; operating a sensor to measure a height profile of the first layer of material; and operating the plurality of ejectors to eject drops of material toward the platen to form a second layer of material upon the first layer of material with reference to the measured height profile. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of the method and device are explained in the following description, taken in connection with the accompanying drawings. 
         FIG. 1  shows a three-dimensional object printer. 
         FIG. 2  shows a flow diagram for a method of operating a three-dimensional object printer. 
         FIGS. 3A-D  illustrate performance of the steps of the method of  FIG. 2  using the printer of  FIG. 1 . 
         FIG. 4  shows a plot including an exemplary measured height profile and an exemplary target height profile. 
         FIG. 5  shows an exemplary control system diagram for the printer of  FIG. 1 . 
         FIG. 6  shows perspective view of a prior art three-dimensional object printing system. 
         FIG. 7  shows a side view of the prior art printing system of  FIG. 6 . 
         FIGS. 8A and 8B  depict the ripple effect caused by run-out in the roller of the leveling assembly in the prior art printing system of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the printer and method disclosed herein as well as the details for the printer and method, reference is made to the drawings. In the drawings, like reference numerals designate like elements. 
       FIG. 1  shows a three-dimensional object printer  100 . The printer  100  includes a platen  104  and an ejector head  108 . The platen  104  has substantial planar upper surface  112  upon which three-dimensional objects, such as the part  116 , are formed by the printer  100 . The ejector head  108  has a plurality of ejectors  120  configured to eject drops of a build material to form three-dimensional objects upon the surface  112  of platen  104 . In many embodiments, the plurality of ejectors  120  are arranged in one or more rows in the cross-process direction Z. However, in other embodiments, the plurality of ejectors  120  may instead comprises only a single ejector  120 . In some embodiments, the plurality of ejectors includes a first plurality of ejectors configured to eject drops of a build material and a second plurality of ejectors configured to eject drops of a support material, such as wax. 
     The printer  100  includes a controller  124  operatively connected to the ejector head  108 . The controller  124  is configured to operate the ejector head  108  with reference to image data to form a three-dimensional object on surface  112  that corresponds to the image data. To form each layer of the three-dimensional object, the controller  124  operates the printer  100  to sweep the ejector head  108  one or more times in the process direction X, while ejecting drops of material onto the platen  104  from the ejectors  120 . In the case of multiple passes, the ejector head  108  shifts in the cross-process direction Z between each sweep. After each layer is formed, the ejector head  108  moves away from the platen  104  in the vertical direction Y to begin printing the next layer. The printer  100  may include rails  128  or other actuators known in the art configured to facilitate the aforementioned movements of the ejector head  108  in the X, Y, and Z directions. In alternative embodiments, the printer  100  includes actuators (not shown) configured to move the platen  104  in the X, Y, and Z directions to accomplish the same relative movements of the ejector head  108  and the platen  104 . 
     The printer  100  further includes a sensor  132  operatively connected to the controller  124  and configured to sense heights of the layers of material formed by the printer  100 . As discussed in greater detail below, the controller  124  is configured to operate the sensor  132  to measure a height profile of an upper surface of a layer of a partially formed part  116 . Based on this height profile, variances or errors in the height profile of the layer can be compensated by adjusting the thickness profile of the subsequent layer. In one embodiment, the sensor  132  is an optical profilometer configured to move with respect to the platen  104  in the process direction X to scan an entire part  116 , one line or row at a time. However, other configurations are possible in which the sensor  132  does not need to move to scan the part  116 . Additionally, as shown, the sensor  132  is attached to the ejector head  108 . However, the sensor  132  can be configured for movement independent of the ejector head  108  and is not attached to the ejector head  108  in such a configuration. 
     A method  200  for operating a three-dimensional object printer is shown in  FIG. 2 . In the description of the method, statements that the method is performing some task or function refers to a controller or general purpose processor executing programmed instructions stored in non-transitory computer readable storage media operatively connected to the controller or processor to manipulate data or to operate one or more components in the printer to perform the task or function. The controller  124  noted above can be such a controller or processor. Alternatively, the controller can be implemented with more than one processor and associated circuitry and components, each of which is configured to form one or more tasks or functions described herein. 
     When the method  200  is performed, it begins by operating a plurality of ejectors of an ejector head to eject drops of material toward a platen to form a first layer of material upon the platen (block  204 ). Particularly, as shown in  FIG. 3A , the controller  124  is configured to operate the ejectors  120  of ejector head  108  to sweep one or more times in the process direction X, while ejecting drops of material toward the platen  104  from the ejectors  120  to form a first layer of material  304  upon the surface  112  of the platen  104 . As shown, the first layer  304  is formed directly upon the platen  104 , but may similarly be a layer of material formed upon a previously formed layer of material on the platen  104 . 
     Next, the method  200  continues by operating a sensor to measure a height profile of the first layer of material (block  208 ). Particularly, as shown in  FIG. 3B , the controller  124  is configured to operate the sensor  132  to measure a height profile of the partially formed part  116  after formation of the first layer of material  304 . In one embodiment, the sensor  132  sweeps in the process direction X one or more times to scan the first layer  304  entirely. However, in some embodiments, the sensor  132  is configured to scan the entire first layer  304  without moving. As used herein, the term “height profile” refers to a plurality of height values or distance values that are associated with a plurality of relative positions in space and which represent the contours of the surface of a partially formed part. In one particular embodiment, the “height profile” represents the height of the partially formed part as a function of a position in the process direction X and a position in the cross-process direction Z, e.g. height=f(x,z). 
     Next, the method  200  continues by operating the plurality of ejectors to eject drops of material toward the platen to form a second layer of material upon the first layer of material with reference to the measured height profile (block  212 ). Particularly, as shown in  FIG. 3C , the controller  124  is configured to operate the ejectors  120  of ejector head  108  to sweep one or more times in the process direction X, while ejecting drops of material toward the platen  104  from the ejectors  120  to form a second layer of material  308  atop the first layer of material  304 . The controller  124  is configured to operate the ejectors  120  so as to form the second layer of material  308  such that it has a thickness profile that compensates for variances in the measured height profile after the formation of the first layer of material  304 . As used herein, the term “thickness profile” refers to a plurality of thickness values or length values that are associated with a plurality of relative positions in space. In one particular embodiment, the “thickness profile” represents the thickness of a layer of material as a function of a position in the process direction X and a position in the cross-process direction Z, e.g. thickness=f(x,z). 
     As described in greater detail below with respect to  FIG. 5 , in one embodiment, an adjusted thickness profile for the second layer of material  308  is determined based at least on the measured height profile of part after formation of the first layer of material  304 . The adjusted thickness profile is used to adjust the relative drop volumes or drop masses of the drops of material that are ejected to form the second layer of material  308  such that the second layer of material  308  has variations in thickness that compensate for measured variations in height after formation of the first layer of material  304 . 
     In one embodiment of the method  200 , the processing of blocks  208  and  212  are performed substantially simultaneously. Particularly, as shown in  FIG. 3D , the controller  124  is configured to, while operating the sensor  132  to measure the height profile, determine the adjusted thickness profile in real time and operate the ejectors  120  to form the second layer of material  308  according to the adjusted thickness profile. As shown in  FIG. 3D , the sensor  132  is physically arranged ahead of the ejectors  120  in the process direction X such that each portion of the second layer of material  308  is formed after the sensor  132  has scanned the height of the corresponding portion of the first layer of material  304 . In a further embodiment, the sensor  132  comprises a pair of sensors (not shown) arranged on opposite sides of the ejectors  120  in the process direction X. In this way, the printer  100  can operate bi-directionally in the process direction X, thereby enabling printing speed efficiencies. 
     In many embodiments, the method  200  is repeated for each layer of the part until the part is completely formed. Particularly, the controller  124  is configured to operate the ejectors  120  of the ejector head  108  to form a plurality of layers on the platen  104 . After forming each successive layer, the controller  124  is configured to operate the sensor  132  to measure a height profile of the previously formed layer of material. In forming each successive layer, the controller  124  is configured to operate the ejectors  120  with reference to the previously measured height profile in order to compensate for unintended variances in the height profile of the previously formed layer. 
     For the purpose of furthering the understanding of how the adjusted thickness profile is determined, an exemplary measured height profile is shown in  FIG. 4 . Particularly,  FIG. 4  shows a plot  400  including an exemplary measured height profile  404 . The measure height profile  404  is indicated by the solid line and corresponds to the measured height of part after the formation of the first layer of material  304 . The plot  400  also includes a target height profile  408 . The target height profile  408  is indicated by the dotted line and corresponds to an ideal or intended height of the part after the formation of the first layer of material  304 .We note that, for simplicity, the plot  400  only shows the profiles  404  and  408  with respect to a position (x) in the process direction X. However, in practice a height profile would include values defined with respect to both a position in the process direction X and a position in the cross-process direction Z. Additionally, as shown, a zero height value in the vertical direction Y essentially corresponds to the surface  112  of the platen. However, this correspondence is merely arbitrary for the purpose of the plot  400 . 
       FIG. 5  shows a control system diagram for one embodiment of the printer  100 . The control system is, in essence, a closed-loop feedback system that uses the sensor  132  to determine an error, for which the system compensates in a closed-loop manner by adjusting a thickness profile of a subsequent layer. In the illustrated embodiment, the controller  124  includes a position control component  504 , an ejector control component  508 , and a sensor control component  512 . We note that the particular arrangement shown and described with respect to  FIG. 5  is merely exemplary. One of ordinary skill in the art would understand that many alternative and equivalent arrangements could be employed to achieve similar functions. 
     The position control component  504  is configured to provide control signals for operating the rails  128  or other actuators responsible for providing relative motion of the ejectors  120  and the platen  104  and for providing relative motion of the sensor  132  and the platen  104 , as required. Additionally, in one embodiment, the position control component  504  provides relevant position information to the ejector control component  508  and the sensor control component  512 . In particular, the position control component  508  provides position information (X E , Z E ) to the ejector control component  508 , which indicates a position of the ejectors  120  in the process direction X and in the cross-process direction Z. The position control component  508  also provides position information (X S , Z S ) to the sensor control component  512 , which indicates a position of the sensor  132  in the process direction X and in the cross-process direction Z. 
     The sensor control component  512  is configured to operate the sensor  132  to measure heights of portions of the partially formed part  116 . The sensor control component  512  is configured to receive signals from the sensor  132  that correspond to a height of the partially formed part  116  at a particular position. The sensor control component  512  is also configured to receive the position information (X S , Z S ) regarding the position of the sensor  132  from the position control component  504 . Based on the signals from the sensor  132  and the position information (X S , Z S ), the sensor control component  512  is configured to generate a measured height profile for the partially formed part after formation of a layer of material, indicated as MH Layer (x,z) in  FIG. 5 . 
     The controller  124  is configured to compare the measured height profile MH Layer (x,z) with a target height profile for the next layer to be formed, indicated as TH Layer (x,z,) in  FIG. 5 . Based on the comparison, the controller  124  is configured to determine an adjusted thickness profile for the next layer to be formed, indicated as AT Layer+1 (x,z) in  FIG. 5 . In one embodiment, the controller  124  includes a comparator  516  configured to subtract the measured height profile MH Layer (x,z) from the target height profile TH Layer+1 (x,z,) in order to calculate the adjusted thickness profile AT Layer+1 (x,z). 
     As would be understood by a person having ordinary skill in the art, the particular mathematics can be expressed in many alternative but equivalent forms. For example, the target height profile for the next layer to be formed can also be represented as a summation of a target profile for the previously formed layer with a nominal thickness profile for the next layer to be formed. Additionally, a height error profile for the previously formed layer can be determined by comparing the measured height profile for the previously formed layer with the target profile for the previously formed layer. The adjusted thickness profile for the next layer to be formed can then be determined by modifying the nominal thickness profile for the next layer to be formed with the height error profile for the previously formed layer. 
     The ejector control component  508  is configured to receive the adjusted thickness profile AT Layer+1 (x,z) from the comparator  516 . Additionally, the ejector control component  508  is configured to receive the position information (X E , Z E ) regarding the position of the ejectors  120  from the position control component  504 . Based on the adjusted thickness profile AT Layer+1 (x,z) and the position information (X E , Z E ), the ejector control component  508  is configured to provide appropriate firing signals to the ejectors  120 . Particularly, the ejector control component  508  is configured to calculate a required drop mass or drop volume that should be ejected at a current position of the ejectors  120  in order to achieve a thickness according to the adjusted thickness profile AT Layer+1 (x,z). Based on the calculated drop mass or drop volume, the ejector control component  508  is configured to provide firing signals to the ejectors  120  that achieve the calculated drop mass or drop volume. In this way, the ejector control component  508  operates the ejectors  120  to form a subsequent layer with a thickness profile that compensates for the variations in height of the previously formed layer. 
     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, applications or methods. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.