Machinery's Handbook, 31st Edition
METAL ADDITIVE MANUFACTURING PROCESSES 1559 mirrors in SLM, the electron beam is moved by electromagnetic coils with no moving parts in EBM, facilitating high-speed scanning. EBM machines require a vacuum in the build chamber of <1 × 10 –4 mbar in order to prevent obstruction of the focused beam. Similar to SLM machines, EBM machines have a build platform that moves downward one layer thickness for each build cycle, and a pow- der distribution system, sometimes called a recoater , to spread a new layer of powder. But unlike SLM systems, the electron beam repulses powder particles in the build layer. To prevent powder dispersion, each layer of powder in the build box is first partially sintered by rapidly scanning the total surface with the electron beam at low power; this causes the powder particles to adhere to the previous layer and adjoining particles. Then the electron beam power is increased, and the layer profile is scanned, melting powder particles within the beam spot. Because EBM power is much greater than the laser power in SLM, the par- ticle size used is larger, between 0.002 and 0.004 in. (0.055 and 0.105 mm). Because each layer is partially sintered by quick scan of the electron beam over each powder layer, it is generally not necessary to print support structures for overhangs. Therefore, unlike SLM, the EBM process allows the production of multiple layers of parts in one build. In addition, because each layer is preheated—with the temperature of powder in the build box reaching temperatures around 1300°F (700°C)—temperature variation throughout the build box is very low and distortion due to thermal stress is negligible. Unlike the SLM process, the unused powder in the EBM process is not loose but is lightly sintered into a cake surrounding the finished part. After the build box cools down, which takes 12–18 hours, the powder cake is removed. An air gun, using the same-sized powder particles as those in the build, is used to impact the cake and break up the unused powder cake into fragments. Particles of the fragmented cake are reused in a future build. Binder Jetting (BJ).—Like SLM and EBM, binder jetting begins with an STL CAD file that is sliced in preparation for layer-by-layer printing (see Table 8), the part is built in a powder bed in which a build plate is lowered one layer thickness for each slice, and a recoater spreads a new layer of metal powder. Unlike the SLM and EBM processes, a printhead with one or more jets passes over the powder surface and deposit binder drop- lets within the 2D slice profile defined by the CAD file of the 3D part. This sequence is repeated layer by layer, until the 3D part is completed. In BJ, powder layer thickness ranges between 0.001 and 0.005 in. (0.0254 and 0.127 mm). A useful analogy is to think of the process as an inkjet printhead in a paper printer and each layer of metal powder as the paper sheet. Because binder jetting occurs at room temperature, there are no issues of heat transfer and supports are not needed. Therefore, as with EBM, multiple layers of parts can be made in each build box, increasing productivity. In BJ, each binder droplet agglomerates powder particles into a building block called a volume element or voxel —the 3D equivalent of a 2D picture element or pixel. The binder serves as an adhesive, bonding particles in the new powder layer into a voxel and to mate- rial in the previous layer (Fig. 27). This bond is not enough to transmit useful loads, but it is sufficient to handle the part during removal of excess power surrounding the part, removal of the part from the build box, and setup for the next process step. Printed parts are then sent to a sintering furnace, where the binder burns off and the powder particles sinter together at a temperature below the melting point.
Binder Droplet
Voxel
New Powder Layer
Previous Powder Layers
Fig. 27. Binder Jetting Process
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