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Figure 1: 3D Printing illustrating Local Composition Control (LCC)

One of the great potential benefits offered by Solid Freeform Fabrication (SFF) technology is the ability to create parts that have composition variation within them. Such Local Composition Control (LCC) has the potential to create new classes of components. Material composition can be tailored within a component to achieve local control of properties (e.g., index of refraction, electrical conductivity, formability, magnetic properties, corrosion resistance, hardness vs. toughness, etc.). By such local control, monolithic components can be created which integrate the function of multiple discrete components, saving part count, space and weight and enabling concepts that would be otherwise impractical. Controlling the spatial distribution of properties via composition will allow for control of the state of the entire component (e.g., the state of residual stress in a component). Integrated sensors and actuators can be envisioned which are enabled by LCC (e.g., bimetallic structures, in-situ thermocouples, etc.). Devices which have as their function the control of chemical reactions are possible. The utility of Mesoscopic parts made by SFF will depend strongly on the ability to locally control composition.

Realizing the potential utility of LCC in SFF is a many-faceted challenge requiring developments in the: (1) Information technology and design tools required to support the design of parts with LCC; (2) Extension and characterization of the range of materials which can be deposited with local control (SFF technology specific); (3) Design of materials systems with locally varying composition which can be successfully treated in operations subsequent to the SFF process itself (e.g., densified in a furnace firing operation); (4) Exploration of specific applications of LCC.

The work reported here focuses primarily on the issue of Information Technology and Design Tools - (1) above. The absence of knowledge, methods and tools in this area presents an absolute bar to the exploration of materials systems and applications. Developments in these areas will allow a wider community to contribute to materials and applications. Information Technology and Design Tools may be divided into two categories: (1) tools which are generic, and (2) tools which are specific to a given SFF process. Generic electronic representations must be developed to allow for electronic specification within a component. There must be a suite of tools which allows a designer to communicate with this representation using high level features that are sensible to a designer. The designer must be able to visualize and interrogate the evolving model. The model must not allow the designer to request that which cannot be made. Process specific tools include methods to render desired continuous composition profiles in the discretized form required by a specific process and the generation of machine specific fabrication instructions.

Wherever possible, the work conducted under this project will be generic and applicable to a broad range of SFF technologies. However, in the cases where the outcome is process specific, Three-Dimensional Printing (3D Printing) will be used as the prototypical SFF technology. Among the SFF processes, 3D Printing is particularly well suited to the fabrication of parts with LCC. 3D Printing creates parts in layers by spreading powder, and then ink-jet printing materials into the powderbed. In some cases, these materials are temporary or fugitive glues, but in many cases, these materials remain in the final component. Examples of the latter include; ceramic particles in colloidal or slurry form, metallic particles in slurry form, dissolved salts which are reduced to metal in the powderbed, polymers in colloidal or dissolved form, and drugs in colloidal or dissolved form. 3D Printing has been extended to the fabrication of LCC components by printing different materials in different locations, each through its own ink-jet nozzle(s). Figure 1 illustrates this conceptually with two different colors, each representing the printing of a different material into the powder bed with local control of position. 3D Printing is thus capable of fully three dimensional control of composition.

Figure 2: Information Pathway
The LCC information pathway with 3D Printing begins with a designer interacting with a standard CAD system to define the shape of the object, see Figure 2. The solid model thus created is then exported from the CAD system as a standard exchange format such as STEP or IGES. In the course of our prior work, we implemented an LCC modeler based on tetrahedral mesh data structure. This finite-element based LCC modeler can be thought of as a special instance of our generalized cellular decomposition approach to LCC modeling. It was chosen as a convenient method to demonstrate the information pathway and to explore the issues associated with LCC. Once the geometry of the model is fully defined, it is loaded into a finite-element mesh generator via a neutral format, and meshed into a set of tetrahedra. This process is referred to as pre-processing in Figure 2. The composition of a part is established by specifying the composition values at the vertices of each tetrahedron and interpolating between them. As an exemplar of a design tool, we developed a method to specify a composition profile normal to the surface and apply this profile to an entire object.

Post-processing (see also Figure 2) then converts the designed LCC model into instructions for the 3D Printing machine. Post-processing takes place on a layer-by-layer basis along two parallel paths: (1) the accurate definition of the surface (Geometry Slice); and (2) rendering the composition of the body (Material Slice). The continuous-tone material composition is rendered into printable discrete information using haltoning (or dithering) algorithms. The boundary and composition information is recombined to produce the drop-by-drop instructions that are loaded onto the 3D Printing machine. Special attention is given to reconciling conflicts which occur at the boundary where the designer's intent in both composition and surface finish must be recognized.

The complete information and 3D Printing pathway have been tested and demonstrated with a part of representative complexity as shown in Figure 3. The part is an injection molding tool and the design challenge is to place hard phases in a designed composition profile near the surface. In this demonstration, two colors of ink were printed (magenta and cyan) with the condition that the sum of the materials was everywhere constant. The bottom image in Figure 3 shows a photograph of a layer of the actual printed part. This can be compared with the material and geometry information above it, which become merged to produce the instructions which led to the printed part.

Figure 3: Demonstration of Information Pathway