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I am operating a laser sintering machine, using polyamide 2200 powder (with a grain size of approximately 50 micrometers). During a print, a lot of powder goes unsintered and can theoretically be reused. However, using purely recycled powder degrades print quality to an unacceptable level.

Mixing a little used powder into a larger amount of fresh powder seems to work well though. What is the greatest ratio of used to fresh powder that still gives good results, and is there anything I can do (pre- postprocessing) to allow more powder to be reused?

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    $\begingroup$ This site seems to be getting almost exclusively FDM questions, I thought it would be nice to have a question about a different type of process. Unfortunately I do not actually own a million dollar printer :-( $\endgroup$
    – Tom van der Zanden
    Commented Jan 14, 2016 at 22:30
  • $\begingroup$ I noticed that your question has not had much activity lately, are you still looking for an answer to this question? How might we be able to close some gaps? $\endgroup$
    – tbm0115
    Commented Jul 3, 2016 at 20:18

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You'll find generally that mixing 40% new polyamide with 60% recycled polyamide will result in a reasonable finish and part. You will obviously want to use all new for parts requiring the best possible finish and mechanical properties, but this mixture will be very difficult to tell apart from a fully new mixture part:

http://www.paramountind.com/pdfs/eos_pa2200_mds.pdf

This is more detailed research showing how used powder changes and how that affects print quality here:

http://www.emeraldinsight.com/doi/abs/10.1108/13552540910960299

Searching for the research paper title may find a free source, but the linked resource does require a subscription or payment to that service.

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I currently use the 60/40 recycling mix ratio and find that it works very well. I do however wonder if there is an even more effective ratio in order to recycle used powder. I currently discard all "cake" powder (powder remaining in the build piston) and am only "recycling" the push off powder. I found this paper but it's unclear if they are reusing just the push off or both push off and cake. Any further opinions/ideas would be greatly appreciated.

http://www.internationaljournalssrg.org/IJME/2015/Volume2-Issue7/IJME-V2I7P106.pdf

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