As additive manufacture (AM) matures as a production process, it requires some serious considerations to enable clean, safe, and efficient handling of these dense metal powders through the complicated process of loading for use, re-use and screening. These include:
1. Loading new, virgin material into the machine. Dense metal powders complicate this powder transfer. Their weight dictates using forklift trucks to elevate the powder containers to connect to the inlet of the feed hopper and discharge under gravity. Another difficulty is the need to maintain inert conditions to prevent oxidation of the metal powder.
2. Unloading excess material from the build box overspill. As production volumes increase, the current method of “ploughing” the material to the collection hopper at the end of the build box will necessitate better collection and handling.
3. Unloading unused material from the build area around the finished component either from within the machine or from an unloading station. With unused material being as much as 85% of the powder volume within the build box, it is imperative to find a viable means of removing and repurposing it for future use. Currently this is done by simple suction of material and vacuum conveying from within the build box or removing the build box to an unloading station to remove the powder.
4. Screening the excess and/or unused material. All excess metal powder going through the build process needs to be screened to remove agglomerates to be suitable for re-use. This requires a means of powder transfer to the screening station and back to the build operation.
5. Blending of used and virgin materials. There are many ongoing studies on the effects of blending varying percentages of used materials with virgin materials but suffice it to say that the need for blending will increase, and methods of transfer to and from the blending unit need to be addressed.
6. Returning the blended and screened material to the AM machine. This process has the same requirements of the initial loading of material into the hopper.
Pneumatic vacuum conveying is the ideal solution for these processes for several reasons: The receivers are easily installed on the AM machine or in association with the screener system; they offer containment of the powders, keeping the environment clean; and advanced certified systems can accommodate material with Minimum Ignition Energy values as low as 1mJ without risk of explosion.
The conveying system design when conveying dense metal powders has several key requirements: ease of cleaning, minimal powder retention and high-quality filtration to the sub-micron range, as well as vacuum transfer type. Ideally the vacuum conveying process should be done with a low-velocity, dense phase transfer requiring a high level of vacuum. This method eliminates abrasion of the conveying lines and potential contamination of the metal power that can be caused by lean phase conveying. It requires levels of vacuum between 15 and 22 in. Hg, eliminating systems using side channel (regenerative) pumps or Roots-type units from consideration.
Vacuum conveyors also offer the advantage of maintaining a smaller footprint when production expands. One can establish a complete loop with one main conveying line supplying powder to several AM machines’ vacuum receivers, while keeping the bulk material supply in a separate room. The need for fork trucks within the build area is eliminated. The same is true of the powder recovered from the operation being transferred to the screening unit. Vacuum conveying keeps these processes simple as it does not require a metering valve. Rather it can collect material from the build box, unloading stations or overspill area and transfer it to screening and further convey it back to a bin for blending or re-use.
Finally, Increased production requires minimized unloading time to facilitate the next build. The physical nature of metal powders complicates this timing due to particle and screen aperture size. Screening is typically slower than the actual unloading time, thereby requiring incorporation of a buffer hopper system to allow rapid unloading independent of screening.
Nick Hayes is president of Volkmann Inc., which engineers and manufactures high-performance vacuum conveying systems, including the PowTReX line for additive manufacturing. For more information, call 609-265-0101 or visit www.volkmannusa.com.
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