Try using eSUN printer cleaning filament to remove filament from your print nozzle before using NinjaFlex.
If I don't clean the head, then during a print, I some times see specs of burnt plastic extrude with the NinjaFlex. It looks very ugly. I see this problem with NInjaFlex more than with other filaments.
The eSUN cleaning filament works very well. I have used this many times. I heat my print head to 230 C. Then insert the cleaning filament. I let it extrude just enough to know it's all the way inside the print nozzle (with about 10 mm of filament doing the cleaning job). I let it sit there for about 1 minute. When it heats up inside the nozzle, it expands, and it sticks to other plastics, but doesn't stick to the nozzle at all. Then I pull it out, and I will see pieces of plastic, and other burnt particles come out with it. I cut off the 10 mm used portion, and try again. If it comes out clean the next time, I know I'm done. Usually 1 time is enough.
I always clean the nozzle after using exotic composite filaments such as filaments containing wood, copper, bronze, etc. It does a very good job of preventing cross filament contamination by keeping your nozzle clean. Between PLA, ABS and HIPS usage, I have not needed to clean the nozzles at all. With things like NinjaFlex, and composite filaments I need to do this.
While it looks pricey at first, keep in mind that for each cleaning, you're only using about 10 mm of cleaning filament.
See here http://www.amazon.com/Printer-CLEANING- ... B00MG02WNK