Page 13 - 3D Metal Printing Fall 2019

3DMPmag.com
FALL 2019 • 3D METAL PRINTING | 11
to make the particle more spherical and flow more smoothly.”
The gas purge also provides a solution for protecting oxygen-sensitive or volatile powders, which otherwise can compro-
mix. This is accomplished through a com- bination of macro and micro blending that produces a better distribution.
Macro blending, achieved by rotating the shaped vessel, allows the material bed to fall away from the vessel’s walls. The blender moves at a precise speed, with the vessel wall at a precise angle, so that the material cascades over itself. No additional
force is supplied by paddles, plows or spiral ribbons—just gravity.
While this occurs, micro mixing (if needed) simultaneously proceeds via agitator blades located in the center mixing zone of the vessel, where fine processing in the material tran- spires. Together, the macro and micro mixing evenly expose each particle to six times more active blending per revolution than tradi-
tional mixers.
Beyond metal powders, adding a heat
treatment step also can improve part qual- ity and manufacturing productivity for applications involving plastic or resin- based powders. 3DMP
Article supplied by Advanced Powder Solutions, the inhouse testing/engineering arm of Gemco, Middlesex, NJ; 800/654- 3626; wwwokgemco.com.
3D
melted in a crucible ejects from a nozzle into a high-pressure gas stream. This breaks the molten metal into fine particles, typically less than 50 to 150 microns in diameter. While this process typically pro- duces spherically shaped powders, surface porosity must be addressed in order to improve flowability. Otherwise, the powder can clog or slow during the process, affect- ing the speed and quality of printing.
“For reliable 3D printing production, the powder must flow continuously and smoothly,” explains George Paffendorf, director of operations at Advanced Powder Solutions (APS), a contract manufacturer with extensive powder expertise and the inhouse testing/engineering arm of Gemco, a manufacturer of tumble blend- ing and vacuum tumble drying equipment. “However, uniform powder flow can be inhibited if the particles are rough or porous, which creates more friction.”
The attractive force between tiny, sub- micron size particles also increases as the particles become smaller. So, finer pow- ders are typically less free flowing anyway, says Paffendorf.
From a quality standpoint, metal-par- ticle porosity also can reduce a finished part’s load-bearing, fracture-toughness and fatigue properties. Under cyclic stress conditions, porosity also can lead to cracks and part failure.
To eliminate porosity and enhance flowability, a growing number of pow- der suppliers and AM-machine manufacturers add a heat treatment step in the manufacturing process that involves tumble dryers to achieve a more consistent, high- quality product, says Paffendorf.
For example, APS utilizes advanced vacuum tumble dryers that provide sparging (gas injection) in addi- tion to heat application. With this approach, a perforated tube positioned under the bed of material distributes a flow of inert gas, such as nitrogen, to help circulate heat evenly amid the powder.
“A raw metal powder may have parti- cles with a porous, craggy outer surface,” says Paffendorf. “When we heat treat it in the tumble dryer, it closes up the pores
Optimizing metal powder via heat treatment can improve flowability to prevent clogging, thus speeding the process and producing a higher- quality part.
mise certain alloy chemistries. Using a blanket of inert gas to cover the material bed provides a protective barrier that prevents the powder from being exposed to atmospheric oxygen. This also improves operator safety. Because 3D printing with metal pow- der can involve working with some materials such as nickel, potentially dangerous to operators in powder form, containing these may be nec- essary to eliminate fire or explosion risk.
Reblend to Save on Costly Powder
Given the cost of metal powder, end users must reuse any left over at the bottom of the tray. Here, vacuum tumble blenders can reblend the pow- der with new material. Advanced tum- ble blenders are designed to apply even turbulence in all corners of the
Reblending enables metal printing companies to extend the life of costly powders, with micro and macro mixing together providing an improved reblended product.