Printing with nozzle not tight enough?

I beleive I've been printing with a loose nozzle as plastic leaked from the top of the hot end. Plus when I took the nozzle out the threads are full of plastic. 

Ever happened to anyone? I changed nozzles quite a few times and it never happened before so it could be something else...

Any suggestions on how to clean the hot end? Acetone?

Last 2 pictures were taken after I removed the plastic that leaked from the top of the hot end.

Thanks

Turns out one of the 2 screws  on top of the hot end became loose with time! 

Yea, I have done this. Acetone is indeed the answer, I also very carefully used dental picks to scrape away the plastic from the threads. But once it has been soaking then brushes usually used for cleaning an spraygun/airbrush are ideal for scrubbing the threads.

Like these:

5pcs-set-Tattoo-Machine-Tube-Tip-Cleanin