switching plastic types best practice?

I wondered if anyone had advice about the best practice for switch from one type of plastic to another. The question comes from an issue I ran into this week. I print in PLA but wanted to try,and have a need to print in ABS. So I pulled out the PLA and put in the ABS and turned the heat up. After my ABS run, I switched back to PLA. Shortly after my hotend started giving me problems. It started to jam and create enough back pressure push liquid pla out of the top of the brass insert into my peek/ptfe, and my output flow rate was off. Remembering some thing I read here last week about burnt crud in the hotend tip, I tore mine down, and sure enough there was crud. My thought is that the switch between ABS/PLA and the increase in heat burnt the PLA. Are there steps I should take to prevent this? Or is my assumption incorrect?

Edited 1 time(s). Last edit at 02/05/2014 09:22PM by cat.farmer.

I don't have the answer, only a related anecdote to share. I switched from black ABS to red ABS. After a bit of purging the red color seemed to be coming out nicely. Well three hours into a 5 1/2 hour print, several slightly darker layers were extruded into the shape! After that, the rest of the part was a very slightly purer red than before.

So it took three hours to get every bit of black out of the hot end... I imagine going from ABS to PLA would be even more touchy.

---

Yvan

Singularity Machine

I think the best way to switch feeds is to cut the current feed and follow it up with the new feed. On my extruder, I just push the new feed on top of the old feed while holding the feed about 6 inches or so away until the filament drive gear catches it. You need to round the new feed a bit so that it doesn't get caught as it goes from the cold end to the hot end.

As far as going back and forth between PLA and ABS, I also haven't had much luck with this. I've jammed j-heads in the past after switching back and forth a few times that I've stopped doing this. Other people on this forum claim that they can do it without a problem.

I've switched between PLA and ABS in prints several times and didn't run into any issues except forgetting to prime the hotend after I let it ooze too much. I just wait until I found where I needed to stop, paused the print, moved the head away from part, backed PLA out, raised temp to ABS temp, put ABS in, primed extruder, and un-paused the print. I did this with an E3D.

I find it easy to switch from pla to abs but not the other way round. I think it is mainly because the temperature needed to get the remaining abs out as a detrimental effect on the pla.

If I were to try this again, I would heat up the hot end for PLA and then push as much of the PLA out as possible by pushing the wrong end of the right size drill bit into the hotend. Then I would push in the ABS, push some more and then raise the temperature for ABS.

I go back and forth between PLA and ABS all the time. I heat the extruder up to ABS temperatures, loosen the idler, and then pull the filament out. Sometimes I need to push down a bit before I pull up, but this is mainly for PLA which, I think, swells a bit. After removing the old filament, and with the hot end still at ABS temperature, I push the new filament into place. I'll push down a bit to see if I see any extrusion from the hot end. I tighten up the idler, and then I manually extrude - hitting the button as many as 10 times (30mm). Then, if the new filament is PLA, I lower the temperature to 185, wait for it to reach temp, and start printing. I have never had a jam-up (fingers crossed)

Depending on the hotend internals, it is sometimes possible to pull out all of the old filament, while the hotend is cooling down or warming up, if done at the right time and with a bit of luck, you should see an imprint of the internal shape of the hotend in the filament. The sweetspot seems to be between 95 and 110 degrees C.

Pic:
[forums.reprap.org]

Quote
Ralf
Depending on the hotend internals, it is sometimes possible to pull out all of the old filament, while the hotend is cooling down or warming up, if done at the right time and with a bit of luck, you should see an imprint of the internal shape of the hotend in the filament. The sweetspot seems to be between 95 and 110 degrees C.

Pic:
[forums.reprap.org]

I'm guessing that you're probably removing the filament when it's about 5 to 10C below it's glass transition temperature.
Still soft and flexible, but strong enough to not break apart.

Quote
Ralf
Depending on the hotend internals, it is sometimes possible to pull out all of the old filament, while the hotend is cooling down or warming up, if done at the right time and with a bit of luck, you should see an imprint of the internal shape of the hotend in the filament. The sweetspot seems to be between 95 and 110 degrees C.

Pic:
[forums.reprap.org]

I've attempted that on a few occasions with three different hot end designs, no luck. It would be the best way to go though. I print with PLA, ABS and Nylon plus different colours. It would be best to break cleanly between each.

---

Yvan

Singularity Machine