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Additive manufacturing (AM) opens the doors to designs not readily manufacturable by conventional means, including CNC machining and injection molding. Specifcally, designers can create parts, says James Berlin, DDM applications engineer for Stratasys ( stratasys.com ), that "look something like what you'd see in nature, with very complex and organic shapes, such as skeleton structures, rather than solid blocks"—parts that are not only strong, fexible, and rigid, but also lightweight. For designing prototypes that mimic nature, conventional computer-aided design (CAD) works just fne. But, continues Berlin, when talking about manufacturing rather than just proto- typing, specifc design considerations exist for making those parts cost- efective and the manufacturing efcient. Several players are ofering "AM-aware" design software—independent AM soft- ware vendors, AM machine vendors, and CAD vendors. The design tools typically reside in design software, but some are in the pre-processing software, which translates CAD geometry into machine code for the AM machine to produce the part. Conventional CAD doesn't easily support the new design opportunities that come from AM, explains Katrien Lenaerts, director of software for the Additive Manufacturing Business Unit of Materialise NV ( software. materialise.com ). For starters, CAD is typically aimed at subtractive manufacturing, not additive. Second, CAD describes models mathematically; in AM, the models are composed of layers. The layered data comes as massive sets of points and triangles (similar to FEA meshes). The surface tessellation (STL) fle format for AM contains this mesh data. These are very large fles and, for anything but STL editors, uneditable. When creating parts for AM, designers and engineers still typically start in conventional CAD. They then use various tools to reduce the amount of material to make the AM part, while still meeting the requirements for part strength, and they optimize the part designs for actual manufacture. At some point, though, this design process falls apart. Materialise's Lenaerts ofers some examples where AM-aware design software excels over traditional CAD. First, CAD models are generally—and literally—solid. Printing that solid would consume a colossal amount of material or make the part too heavy, probably both. Good AM practice is to hollow out the part, replacing the solid with a honeycomb structure covered by a "watertight" surface. Second, a part can simply be too large for an AM machine's build volume. AM-aware software can optimally "cut" the part into two pieces so that it can be (a) built and (b) easily assembled afterwards. Last, specifc to laser sintering AM, software can nest parts more efectively to minimize material usage and maximize the available build height. Not surprisingly, Materialise has AM-specifc software that can handle many common AM design problems. Magics connects a data source (such as an STL fle, CAD data, or scanned dataset) to an AM machine. Magics is both pre-processing software and STL editor. Users can modify the STL fle directly to adjust wall thicknesses and support structures, defne teeth on polyline cuts for better part quality and smooth reassembly, move points and delete redundant points in a polyline, specify distances between holes/pins and the outer edges of a part, produce a second surface with straight edges, and more. 3-matic STL, also from Materialise, helps in creating lightweight parts and porous designs, randomized structures, and surface textures. These features, explains Lenaerts, are very hard or impossible to create in traditional CAD or to create in giant, unmanageable datasets (meaning STL). Materialise has developed the Lightweight Structures module for 3-matic. This module helps create uniform and random internal structures, create internal structures that follow the AD&P; > June 2014 > FEATURE > When CAD Isn't Additive > Lawrence S. Gould > lsg@lsgould.com 34 0614ADP FEATURE CAD Additive.indd 34 5/21/2014 12:50:03 PM