Optimal ratio of fresh to used nylon powder 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?
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