Abstract:
A method is provided for making a workpiece in an additive manufacturing process in which a starting material is solidified in a layer by layer fashion, with each layer of the workpiece being solidified using a patterned image of radiant energy configured as a two-dimensional grid array of pixels. The method includes: for each layer of the workpiece, determining a preferred angular orientation of the grid array, relative to the layer; and orienting the patterned image to the preferred angular orientation before solidifying the starting material for that layer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to additive manufacturing, and more particularly to apparatus and methods for process control in pixel-based additive manufacturing. 
         [0002]    Additive manufacturing is a process in which material is built up layer-by-layer to form a component. Stereolithography is a type of additive manufacturing process which employs a vat of liquid ultraviolet (“UV”) curable photopolymer “resin” and an image projector to build components one layer at a time. For each layer, the projector flashes a light image of the cross-section of the component on the surface of the liquid, or just above a transparent lens at the bottom of the resin. The image is formatted as a grid array of pixels. Exposure to the ultraviolet light cures and solidifies the pattern in the resin and joins it to the layer below or above, depending on the specific build methodology. 
         [0003]    The pixels are inherently square or rectangular. The dimensions of the pixels are can be in the range of 20-100 μm, with 40-80 μm being more common. 
         [0004]    One problem with this method is that, no matter how small the pixels are, there will still be situations in which the edge of the area to be cured is not in alignment with a pixel, and the pixel protrudes past the intended border. This type of error is referred to as “stair stepping”. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    This problem is addressed by a method of pixel-based additive manufacturing in which an angular orientation of each build layer is set independently. 
         [0006]    According to one aspect of the technology described herein, a method is provided for making a workpiece in an additive manufacturing process in which a starting material is solidified in a layer by layer fashion, with each layer of the workpiece being solidified using a patterned image of radiant energy configured as a two-dimensional grid array of pixels. The method includes: for each layer of the workpiece, determining a preferred angular orientation of the grid array, relative to the layer; and orienting the patterned image to the preferred angular orientation before solidifying the starting material for that layer. 
         [0007]    According to another aspect of the technology described herein, a method is provided for making a workpiece. The method includes: placing an uncured resin in a vat; positioning a build platform in the resin at a selected location along a first axis, so as to expose a layer of resin; rotating at least one of the build platform and a projector about the first axis, so as to orient the projector at a predetermined angular orientation relative to the exposed layer of resin; using the projector, selectively curing the layer of resin by projecting a patterned image of radiant energy onto the layer of resin, wherein the patterned image is configured as a grid pattern comprising rows and columns of pixels arrayed along second and third mutually perpendicular axes respectively, wherein the second and third axes are perpendicular to the first axis, and wherein the grid pattern is aligned at the predetermined angular orientation. 
         [0008]    According to another aspect of the technology described herein, an apparatus for making a workpiece includes: a vat configured to contain a liquid resin; a platform movable along a build axis within the vat; a projector operable to project a patterned image of radiant energy comprising rows and columns of pixels; and means for changing a relative angular orientation of the platform and the projector about the build axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0010]      FIG. 1  is a schematic diagram illustrating a stereolithography apparatus; 
           [0011]      FIG. 2  is a schematic perspective view of an exemplary workpiece that can be constructed using the apparatus of  FIG. 1 ; 
           [0012]      FIG. 3  is a view taken along lines  3 - 3  of  FIG. 2 ; 
           [0013]      FIG. 4  is a view taken along lines  4 - 4  of  FIG. 2 ; 
           [0014]      FIG. 5  is a view of a layer of the workpiece shown in  FIG. 2  with a grid pattern overlaid thereon; 
           [0015]      FIG. 6  is a view of a layer of the workpiece shown in  FIG. 2  with a grid pattern overlaid thereon in a nominal orientation; and 
           [0016]      FIG. 7  is a view of a layer of the workpiece shown in  FIG. 2  with a grid pattern overlaid thereon in an optimized orientation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates schematically an stereolithography apparatus  10  suitable for carrying out an additive manufacturing method as described herein. Basic components of the apparatus  10  include a vat  12  containing a photopolymer resin  14 , a platform  16  connected to an actuator  18 , a projector  20 , and a controller  22 . Each of these components will be described in more detail below. 
         [0018]    The platform  16  is a rigid structure defining a planar worksurface  24 . For purposes of convenient description, the plane of the worksurface  24  is oriented parallel to an X-Y plane of the apparatus  10 , and a direction perpendicular to the X-Y plane is denoted as a Z-direction (X, Y, and Z being three mutually perpendicular directions). The Z-direction or Z-axis may also be referred to herein as a “build axis”. 
         [0019]    The actuator  18  is operable to move the platform  16  parallel to the Z-direction. It is depicted schematically in  FIG. 1 , with the understanding devices such as pneumatic or hydraulic cylinders, ballscrew or linear electric actuators, and so forth, may be used for this purpose. 
         [0020]    The projector  20  may comprise any device operable to generate a radiant energy patterned image of suitable energy level and other operating characteristics to cure the resin  14  during the build process, described in more detail below. In the illustrated example, the projector  20  comprises a radiant energy source  26  such as a UV lamp, an image forming apparatus  28  operable to receive a source beam B from the radiant energy source  26  and generate a patterned image P comprising an array of individual pixels to be projected onto the surface of the resin  14 , and optionally focusing optics  30 , such as one or more lenses. 
         [0021]    The radiant energy source  26  may comprise any device operable to generate a beam of suitable energy level to cure the resin  14 . In the illustrated example, the radiant energy source  26  comprises a UV flash lamp. 
         [0022]    The image forming apparatus  28  may include one or more mirrors, prisms, and/or lenses and provided with suitable actuators, and arranged so that the source beam “B” from the radiant energy source  26  can be transformed into a pixelated image in an X-Y plane coincident with the worksurface  24 . In the illustrated example the image forming apparatus  28  may be a digital micromirror device. 
         [0023]    The controller  22  is a generalized representation of the hardware and software required to control the operation of the apparatus  10 , including the projector  20  and actuator  18 . The controller  22  may be embodied, for example, by software running on one or more processors embodied in one or more devices such as a programmable logic controller (“PLC”) or a microcomputer. Such processors may be coupled to sensors and operating components, for example, through wired or wireless connections. The same processor or processors may be used to retrieve and analyze sensor data, for statistical analysis, and for feedback control. 
         [0024]    Generically, a build process begins by positioning the platform  16  just below the surface of the resin  14 , thus defining a selected layer increment. The projector  20  projects a patterned image P representative of the cross-section of the workpiece on the surface of the resin  14 . Exposure to the radiant energy cures and solidifies the pattern in the resin  14 . The platform  16  is then moved vertically downward by the layer increment. The projector  20  again projects a patterned image P. Exposure to the radiant energy cures and solidifies the pattern in the resin  14  and yet joins it to the previously-cured layer below. This cycle of moving the  16  and then curing the resin  14  is repeated until the entire workpiece is complete. 
         [0025]    Additionally, means are provided for rotating the projector  20  and the platform  16  relative to each other about the Z-axis. Rotation of either the projector  20 , or the platform  16 , or both are suitable to carry out the method described herein. In the illustrated example, an actuator  32  is provided which is operable to controllably rotate the projector  20 . 
         [0026]      FIG. 2  shows an exemplary workpiece  34  which takes the form of an elongated hollow structure having opposed side walls  36  and opposed end walls  38 , extending between a lower end  40  and an upper end  42 .  FIG. 3  illustrates a representative plan view of the workpiece  34  at the lower end  40 . The workpiece  34  is oriented such that the side walls  36  are parallel to a nominal X-axis direction (corresponding to the X-axis of the stereolithography apparatus  10  described above). The end walls  38  are perpendicular to the side walls  36 , and are therefore parallel to a nominal Y-axis direction (corresponding to the Y-axis of the stereolithography apparatus  10 ).  FIG. 4  illustrates a representative plan view of the workpiece  34  at the upper end  42 , with the workpiece in the same orientation as  FIG. 3 . At this location it can be seen that the side walls  36  are not parallel to the nominal X-axis direction, and the end walls  38  are not parallel to the Y-axis. Stated another way, the cross-sections of the workpiece at the lower and upper ends  40 ,  42  are rotated relative to each other about a nominal Z-axis direction. This type of structure may be described as “twisted”. 
         [0027]    In order to produce the workpiece  34  using the apparatus  10 , the workpiece  34  is modeled as a stack of planar layers arrayed along the Z-axis. It will be understood that the actual workpiece  34  may be modeled and/or manufactured as a stack of dozens or hundreds of layers. 
         [0028]      FIG. 5  illustrates a single representative layer at the lower end  40  of the workpiece  34 . The cross-sectional shape of the workpiece  34  overlaid with a grid array of pixels  44  comprising mutually perpendicular rows (X-direction) and columns (Y-direction). The pixels  44  may be quadrilateral shapes such as rectangles or more particularly squares. With the workpiece  34  in this particular orientation, each edge of the workpiece  34  is parallel to either the X-axis or the Y-axis. Thus it is apparent that the edge of a rectangular pixel parallel to the X-axis or Y-axis could always be aligned with the workpiece edge, given a suitably small pixel dimension. 
         [0029]    In contrast,  FIG. 6  illustrates a single representative layer taken at the upper end  42  of the workpiece  34 , again overlaid with a grid of pixels  44 . Because no edge of the workpiece  34  is parallel to either the X-axis or the Y-axis, it is not always possible to align the edge of a rectangular pixel parallel to a workpiece edge. Accordingly, no matter how small the pixel dimension, some degree of stair-stepping will occur, as described above. 
         [0030]    To counter this effect and improve part fidelity, the angular orientation of the projector  20  (and thus the X-Y grid orientation) relative to the workpiece  34  may be individually selected for each layer. 
         [0031]    For example, in the process of modeling and building the layer shown in  FIG. 4 , the layer orientation may be set at a nominal angular orientation, which is herein identified as being at 0° rotation. As noted above, flashing of the patterned image P may occur at the nominal angular orientation, with the expectation that the edges of pixels will be substantially aligned with the edges of the workpiece  34 . 
         [0032]    In order to model and build a layer shown in  FIG. 6 , the layer orientation may be set at a different angular orientation in order to achieve the best correspondence between the X-Y grid orientation and the workpiece edges. In the illustrated example, the workpiece cross-section at the upper end  42  is rotated approximately 30° counterclockwise relative to the workpiece cross-section at the lower end  40 . Accordingly, as shown in  FIG. 7 , the X-Y grid orientation may be rotated approximately 30° counterclockwise. During the build process, the projector  20  would be rotated approximately 30° counterclockwise and the flashing of the patterned image P may occur at this new angular orientation, with the expectation that the edges of pixels  44  will again be substantially aligned with the edges of the workpiece  34 . This new off-nominal orientation may be referred to as a “preferred orientation” or an “optimized orientation”. 
         [0033]    For a simple workpiece involving a twisted prismatic shape as described above, or other type of component having edges which are orthogonal to each other but rotated relative to the nominal X-Y axes, there is likely a specific angular orientation for each layer which provides exact correspondence between the pixel edges and the workpiece edges. 
         [0034]    For other types of workpiece sectional shapes, it is possible to use a software optimization algorithm to determine an optimized fit or best fit for each layer. For example, it is possible to compute for a given angular orientation how many pixels  44  in the layer would cross a part edge, or to compute for a given angular orientation the total surface area of pixels  44  crossing a part edge. The orientation may then be varied and the computations repeated until one or more of these values are minimized, resulting in an optimized angular orientation. 
         [0035]    In any case, because the pixels  44  are individually addressable along two mutually perpendicular axes, the optimum angular orientation for each layer may be achieved with only a 90° range of rotation. 
         [0036]    It is also possible when determining angular orientation of a layer to consider the fidelity and surface quality of the workpiece along the Z-axis. For example, considering the twisted prismatic workpiece  34  described above, it is apparent that if the optimum angular orientation is used for each layer individually, the exterior surface of the finished workpiece  34  may exhibit a “terraced” effect. In order to reduce or minimize this effect, the angular orientation of each layer may be alternated between a nominal (0°) orientation and an optimized orientation, or the orientation may be randomized for each layer. This would produce improved surface quality along the Z-axis, at the expense of incurring some amount of stair stepping in the individual layers. 
         [0037]    The method described herein has several advantages over the prior art. It method enhances fidelity for pixel-based layer manufactured components. This process ensures that various pixels are utilized with respect to the various geometries and their relative locations in the build to optimize the features at those respective locations in the component. It will provide increased ability to produce critical part geometry, reduce variability and increase yield, resulting in lower part cost. 
         [0038]    It is noted that the stereolithography apparatus  10  described above is merely an example used for the purposes of explanation. The method described herein is effective with any method of additive manufacturing in which a starting material is solidified, where the solidifying force is applied using a grid of pixels. 
         [0039]    The foregoing has described an apparatus and method for layer orientation control in a pixel-based additive manufacturing process. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
         [0040]    Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0041]    The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying potential points of novelty, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.