Te combination of additive and subtractive manufacturing processes is showing huge potential for the produc- tion of medical implants. Blending the two processes can create complete process chains, allowing medical OEMs and suppliers to significantly increase accuracy and reduce overall produc- tion times for highly complex surgical implants and instrumentation. For example, a start-to-finish addi-
tive/subtractive manufacturing pro- cess chain recently created tibial trays for surgical knee implants. Te setup included design soſtware that allows for osseonintegration through customized and randomized lattice structure and porosity, an EOS direct metal laser-sin- tering (DMLS) system, and wire EDM and five-axis milling machines. Te combined technologies en-
able medical manufacturers to quickly produce patient-specific implants. A DMLS system, for instance, can produce as many as 12 different-sized tibia trays for 12 different patients, and do so within the same build leſt untended. Tis capability could also be applied to patient-specific instrumentation, drill guides and cutting guides. Advancements in laser technology are driving increased interest in DMLS.
Te industry has transitioned from CO2 lasers to diode pump single-mode lasers that allow DMLS systems to process a wider range of alloys—including titanium, aluminum and Inconel—and do so more precisely. As a result, DMLS is being recog- nized as design-driven manufacturing, as opposed to manufacturing-driven design. Te combined additive/subtractive manufacturing process chain starts with
the design. In the tibial tray process chain, WITHIN medical soſtware cre- ated randomized porosity within a 3D CAD model. Te file was then loaded into an EOSINT M 280 that has a reservoir full of powdered metal, such as Ti64. Te system begins to build or ‘grow’ the part, layer by layer, by repeat- edly pushing the powdered material evenly across a build plate. In between each powder recoat—new layers that can range from 20 to 60-µm thick—the system exposes its laser to the surface of the powdered material to solidify it only where the part shape is.
By combining the two processes within one operation, shops can produce implants in a
virtually unbroken chain from design to finished product.
After each built layer, the system’s
table indexes downward so that the next layer solidifies upon the previ- ous one. The process is similar to that of stereolithography but uses metal powder instead of a photo-curable liquid resin. DMLS is essentially a welding
process, so finished parts are actually welded to the build plate. For the part cut-off operation, the workplate with attached parts is mounted into a wire EDM. Te machine removes only a very thin layer of material, so extra material doesn’t have to be added to the part in the DMLS system to account for the cut- off process. Tis also helps shorten part build times because the time involved with adding extra cut-off stock is elimi- nated. Plus, because of the wire EDM’s
high precision, parts can be cut off to finished size. Aſter cutoff, the separated individual
parts move to the CNC high-speed mill- ing operation where surfaces are finish machined to final dimensions. Because of the complex geometries produced by the DMLS system, finish machin- ing oſten requires a high-speed milling machine with full five-axis capability. For these subsequent surface-finishing
operations, the DMLS system builds parts with as little 0.005–0.010" (0.13–0.25 mm) of extra stock. Tese small amounts of stock help reduce cutting tool costs, while increasing tooling life, because parts do not require rough milling operations, only light finishing cuts. Tis is especially beneficial when parts are made from expensive materials such as titanium that require costly special tools for efficiently rough-machining the material. Material costs are further reduced because less raw material is needed to produce parts with the DMLS system. Tere are no resulting scrap or chips as would be the case if the parts were machined from solid blocks. Te blend of additive and subtractive
technologies goes far beyond what most other product developers are presently doing. Additive processes build parts layer by layer automatically in only a few hours with no need for tools. But additive processes are oſten dependent on subtrac- tive processes for finishing operations. By combining the two processes within one fully streamlined manufacturing opera- tion, medical shops can produce implants in a virtually unbroken manufacturing chain from design to finished product, and do so with as little human interven- tion as possible.
Medical Manufacturing 2013 33 Eric Ostini
Product Manager GF AgieCharmilles Lincolnshire, IL