Filament Change Tips

I have a reasonably well tuned Prusa 2 setup and the kids are really enjoying CADing things up and printing them out, but it's time to try another color of filament. I have changed filaments a couple of times and each time it has been a pain so I'm looking for tips. When cold I could not take the old filament out so I got the tip hot and then pulled out the old filament. But instead of it just coming out, it would string out like when you step on hot chewing gum. Some of that hot "string" cooled off inside the tube in the top of the hot end and took much fiddling to get it out so I could feed the filament in again properly.

So what is the best method for changing filament? (1.75mm with a J-head) Thanks! --Bret

heat up the hotend just enough to soften the filament. I use 160 -170 for PLA and @ 180 for abs. That way it stays firm enogh to pull out nicely. may all depend on your hotend however. the other thing you could do is a long retract at the end of the print job, enough to move it to the cold zone of your hotend but not all the way out. that may allow you to remove it when it is cold.

The best way of cleaning your hot end is to heat it up to about 270 C, let it stabilise, then cool to just above the glass transition temperature of the filament you're currently using. For PLA this is about 70 C, although I find temperatures up to 100 work as well. Once the temperature has stabilised again, pull the filament out. If you've got the temperature right, all the filament will pull out and you'll see what looks like a straw at the end. This is also really good for pulling any debris out from your hot end.

Just a word of warning, the temperature recommendations in this thread seem to be on the high side. I can *extrude* PLA at about 160 degrees C. 270 degrees C will destroy your hot-end if it is PEEK-based. I'd say the safe way to do ths is to slowly heat up the hot end about 10 degrees C at a time, and pull gently until the filament releases. Start a little bit under the glass transition temperature for your material. Glass transition is around 60 for PLA and around 100 for ABS IIRC.

/Andreas

Thanks guys. 270 does seem a bit hot and it is PEEK-based so I'll avoid getting it that hot. The Temps from my setup are not calibrated and so could be off, but they are what the display says when it has been printing well. I run around 180 for this PLA and after a print I let it cool to 160 then pulled the filament out. It had a little "string" but wasn't as bad and didn't seem to cause a problem. Next time I'll go with the method of heating it 10 degrees more at a time until it releases and see how that goes (and note the temperature for future use).

Thanks for the tips! --Bret

My solution, starting from a cold hot-end, is to heat it to 80 (for PLA) then retract until you can pull it out. You need to enable Cold Extrusion M302 to retract. 80 degrees seems enough to release the plastic from the hot-end without melting it. This gives me a clean hot-end, ready for the next colour.

cheers
Bruce

I agree with bbowden, I'm used to change my PLA filament at 80°C