Additive Manufacturing

MAR 2018

ADDITIVE MANUFACTURING is the magazine devoted to industrial applications of 3D printing and digital layering technology. We cover the promise and the challenges of this technology for making functional tooling and end-use production parts.

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MARCH 2018 Additive Manufacturing FEATURE / Metal Additive Manufacturing 28 along the back wall is used by operators to ground themselves while working on the machines. Special filters in the HVAC system trap powder particles, and the room is also equipped with a halon gas system designed to suppress metal fires. Electron Beam Melting for Medical EBM systems build parts in a powder bed, using an electron beam rather than a laser as the energy source. But beyond this obvious fact, there are some key advantages to using EBM technology over laser-based systems for medical device applications. For one, the entire powder bed is heated during printing. This slows down the process somewhat, because the machines require a preheating period before printing can begin. But it also offers a major benefit: Parts are essentially heat-treated as they are being manufactured, avoiding the need for a separate heat-treating step. Another significant difference between EBM and laser-based systems is that the EBM process does not require that the build actually attach to the build plate. Instead, parts can "hover" above the plate and between each other, supported by unmelted powder. This means there is no need to EDM parts off the plate once complete. It also means that parts can be stacked, making fuller use of the space inside the build chamber. Support structures are still necessary in many cases, but they serve more as heat sinks than anything else, Rappach explains. Each of the Arcam EBM machines can produce approximate- ly 70 acetabular hip cups every five days. These implants feature a built-in trabecular texture on the outer surface to encourage bone growth, a feature made possible only through additive manufacturing. Competitive medical devices apply a bone-like texture to machined implants via plasma spray or a glued-on coating; however, this roughness is only surface level, and there is a danger that an external coating could delaminate inside the patient. The 3D printer also adds each hip cup's serial and part number as well as its layer and row within the build chamber to its outer diameter (shown in the photo on page 26). Once a batch has been printed, the parts and the unmelted material surrounding them are removed from the machines and transferred to the powder recovery room, adjacent to the room housing the EBM printers. Here, parts are cleaned of powder in a special blast cabinet and the unmelted powder is reclaimed and stored. This used powder is quarantined in a different part of the facility for sale back to AP&C, which recycles the used material. Because of the nature of EBM, elements are burned off during the process and it's difficult to know what's been lost; as a result, Slice relies only on virgin powder in its machines. Postprocessing with Automation After cleaning with proprietary equipment, the hip cups are finished blasted. This process is fully automated at Slice to elimi- nate operator error and enable unattended operation. Blasting is performed by a FANUC LR Mate 200iD robotic arm inside a blast cabinet, while custom fixtures hold the parts in place. Then the cups move to the main production space for processing on one of Slice's Willemin-Macodel multitasking machining centers with turning capabilities. These machines mill away the central support, smooth the interior of the cup, and drill and tap the holes where bone screws will fit. Automation is incorporated into the machining steps as well. One of the 508 MT machining centers is equipped with a robot and carousel system with rotating trays that can hold as many as 288 hip cups using custom plastic 3D-printed fixtures (see the photo on page 30). A laser system determines the diameter of each part. This information is relayed to the CNC to call the correct program for machining, enabling different implant sizes to be loaded on the carousel at the same time. This machining center can process around 100 hip cups per day, more than the Arcam printers can currently produce. Finishing Touches After machining, the hip cups get an unusual touch: They are anodized (one of the few procedures done by an outside firm), Michael Rappach, VP manufacturing, stands in front of Slice Mfg. Studios' Willemin 508 MT machining center used for finish machining the hip cups.

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