MK8 extruding PLA but not ABS

[robmar]

Hi all,
I'm new to 3D printing, but after building my Geeetech I3 Pro B and using up some 1/2 kg of PLA filament for tests, I got to know the basics of a correct printer and extruder calibration and was able to get (almost) decent prints, coping with the many issues plaguing this printer, especially the clog-prone extruder, its tendency to oozing and stringing and the terrible Z-axis wobble, just to name a few.
But, being my experience so far confined to PLA, today I got myself some ABS and loaded it into the extruder.
Now, the procedure I usually follow to swap between PLA filaments is to preheat the hotend to 220°C, pull back the old filament, insert the new and push it all way in until the distinctive reaching of the melting point is felt, and the nozzle starts oozing. After which I keep pushing until all the remains of the previous color are extruded and the new color starts oozing.
Did the same procedure, this time preheating to 220°C, removing the PLA filament, raising temp to 250°C and then inserting the ABS.
This time, after successfully purging the PLA remainings, the ABS started to ooze but the extruded filament appeared much thinner than the PLA, and despite the greater force applied in pushing the wire, from the nozzle was coming out a very smaller quantity of ABS and much less fluid compared to the PLA filament.
But, blaming my inexperience with ABS, I managed to start the job anyway.
The result was:
1) I wasn't able to make the PLA extruded filament grip to the table, preheated to 90°C, not on my usual PLA surface (a layer of adhesive paper masking tape) nor directly on glass, both clean and dabbed with a glue stick as suggested around.
2) The extruded filament did not tend to stick to the nozzle, as PLA does, but instead formed curls and tangles that felt already solid the moment they left the nozzle's tip
3) The nozzle was oozing more and more feebly, and in a short time, despite the considerable pushing force exerted, came to a stop and never recovered, as if clogged once and for all.
4) I repeatedly pulled out all the filament back from the extruder inlet, cutting the deformed end and reinserting all way in. This usually worked for me with PLA clogs, but in this case to no avail. The end of the extracted filament was showing every time traces of melting, demonstrating that it actually did reach the melting chamber but without leading to effective extrusion.
5) The extruder remained clogged, I'll have to disassemble and clean it.
6) Also I have not clear in mind how to remove the nozzle. I mean, the hotend has to be heated to 250°C to soften any inner hardened plastic remains that could prevent unscrewing the nozzle, or it doesn't matter?

Being the first time I'm using ABS, it's very difficult for me to tell between normal and abnormal behaviour, thus I'm asking to more experienced users some suggestions about what I could be possibly doing wrong, as well as their experience with ABS .

And sorry for the long post, just trying to provide as many details as possible for better understanding.

Thanks in advance for any help!
Roberto

[Toy]

1.Here is ra video of assembling the extruder, you can check it to complete your disassembling: https://www.youtube.com/watch?v=aPvVUZQnOMM
If you can' remove the nozzle at at room temperature, I think the chilled jam is standing in the way of your disassembling, then heat the hotend and then remove the nozzle, Ensure your safety and you don't have to heat it to a very high temperature.
2.And here is a video to show how to clean the jam: https://www.youtube.com/watch?v=3iS8YU_TNEw
3.For your other problems, you need to check this link for troublrshooting: https://www.simplify3d.com/support/prin ... eshooting/