Most 3D printed or additively made metal parts will need additional finishing or post-processing to achieve a suitable surface finish.
The media, compound, and equipment used affect how much material is removed. In general, most finishing operations will alter part tolerances, a factor to consider for parts that must maintain precise tolerances or that will mate with a tight fit. Also consider the cosmetic appearance desired before making a final choice in finishing options. The amount of material removed affects the smoothness or shine of the final part.
Three methods are popular; centrifugal disc, centrifugal barrel, and media blasting.
Centrifugal disc machines have a large drum with stationary sides and a rotating disc at the bottom. The rotating disc forces parts and media upward, efficiently de-burring and finishing the part or parts. These machines often use a variety of abrasive and polishing media mixed with a specialized liquid compound to agitate and tumble-finish the parts. Centrifugal disc machines are classified as high-energy mass finishing equipment.
Centrifugal barrel finishing is also a high energy form of finishing. It is one of the fastest methods of mass finishing. The parts are rotated around a horizontal main shaft in octagonal barrels containing media, water, and liquid compound. The circular drive plate connected to the shaft rotates in one direction while the connected barrels rotate in the opposite direction, creating a centrifugal force that increases gravitational pull by 15 to 20 times. Both abrasive and polishing media can be used in a wet process in centrifugal barrels, as well as dry granular polishing media. Barrels can be segmented to run multiple parts at once while avoiding part-to-part contact.
Media blasting uses compressed air to push loose, abrasive media through a nozzle directed at the surface of a part within a blasting cabinet. Depending on the desired surface quality and reflectivity, several types of media are available in various shapes and sizes. Round particles provide higher reflectivity, whereas angular particles provide more of a matte finish and better anchor patterns for adhesive bonding. There are also different types of equipment for various styles of media blasting, including suction, wet-blast, and direct-pressure—the most aggressive form of finishing. It is important to note that standard media blasting is not a mass finishing option. Only one part can be blasted at a time unless robotic automation is used.
Some points to keep in mind
–Mass finishing works equally on all sides and edges of the part, but it affects flat surfaces, curved surfaces, and edges differently.
— Know the difference between surface smoothness and shine. The average roughness (RA) measures the smoothness of a part and can be measured with a profilometer.
— Because surface finish can affect the design specifications, it’s important to consider the finishing method before fabrication. In some cases, parts may need to be “over-built” or “masked” in some areas to maintain tolerances during post-processing.
— There is no one-size-fits-all solution for metal finishing. For some applications, visible printing lines are acceptable and minimal post processing is needed. Different finishing techniques are better suited for applications that require a smooth and/or bright finish (<60 RA) versus those where a rougher surface finish is acceptable.
(some material for this article was supplied by Desktop Metal)
