i cleaned my hot end the other day and am kicking myself for not taking a pic of this but i soaked it as usually in my paint stripper bucket. it loosened all the plastic in the nozzle and brass tube as usual but this time i didnt let it sit too long and i was able to actually pull the plastic out of the tube in one piece. it was kinda like a plastic straw lining the brass. i set this on the work bench and let it harden back up a bit then i took my xacto knife and sliced the end off. in the cross section you could actually see the rainbow layers of each color i had printed with over the past 3 weeks or so. this was the first actual hard evidence i found that many plastics dont intermix welll and each time you switch you are left with a thin film that stays in there cooking and it gets harder and harder making the hot end tighter.