Page 28 - 3D Metal Printing Spring 2016

3D Designing for Additive Manufacturing
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integrity, but for powder removal it pro- vides a challenge. During the preheat step, powder surrounding the solid material is partially sintered, sometimes making it difficult to remove from long cavities. This partially sintered powder does not flow, but the powder must be removed before
Fig. 1—Depicted are examples of line-of- sight powder removal: A) simple radius B) elongated radius C) straight center with radii on exits. The black lines show how far the compressed air in the powder-recovery system can reach into the cavity line of sight. It is not always intuitive as to which geometry would be best for powder removal.
Fig. 2—Designing large radii (2a) instead of corner points (2b) allows for easier powder removal due to a smooth surface rather than a surface with many discontinuities.
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any thermal part processing, such as HIP or heattreatment. Designing sweeping radii while maximizing line-of-sight cav- ities into a part makes powder removal more successful (Figs. 1 and 2).
Laser powder-bed fusion. Laser sys- tems offer a benefit in that the powder does not become sintered, and thus flows easily. When intricate interior cavities are required, the removal process in laser pro- vides an advantage. However, even though the powder flows more easily out of cavi- ties, the powder must be removed before stress relief and before the parts can be removed from the plate. Therefore, if the internal cavities are buried within the sup- port structure or cannot be reached by the vacuum, powder-removal options must be designed into the parts. If an ori- entation cannot be constructed to allow for full powder removal, it is common to add holes to the as-built model to ease the removal from cavities (Fig. 3). These holes then can be plugged by a weld.
In laser systems, parts and the plate essentially are welded together, adding complexity to part removal and adding time to the overall process. Parts must be removed by mechanical methods such as wire-EDM or a bandsaw. Wire-EDM is a more accurate process, so the additional stock needed underneath the parts may only be 3 mm thick, whereas with the band saw it is recommended to add at least 5 mm underneath the solid material or support structure.
Consider Thermal Processing
Thermal processing in most cases does not define or change many designs, unless significant distortion during the thermal process occurs due to drastically varying thicknesses compared to part length or height. This is another area where choos- ing the right additive technology proves beneficial. Large parts can be difficult to produce on laser systems due to the “cool” process that is challenged by residual stress. Most laser builds require stress
Fig. 3—If internal cavities are buried within the support structure of a part or cannot be reached by the vacuum, powder-removal options must be designed into the parts. If an orientation cannot be constructed to allow for full powder removal, it is common to add holes to the as-built model (upper left) to ease the removal from cavities. These holes then can be plugged by welds.
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26 | 3D METAL PRINTING • SPRING 2016
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