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Noozle clogs with pla and B-pet but no with ABS

Posted by Tinchus 
Noozle clogs with pla and B-pet but no with ABS
February 12, 2016 03:49PM
Hi all! I have this weird problem. I have a prusa I3, witha j-head hot end.

From the beginning I started useing ABS. After some time I tried PLA and a material called B-PET (240-260 degrees)
I have the same issue with these 2 materials: the noozle clogs while printing. I did a lot of tests: cleaned the noozle, more temp, less temo, etc.
And this is the conclusions I got: I look lile it may be a combination of problems, 1 of them humidity on the filaments (when I rise the temperature I have like a popcorn factory running down there.) So I tried to dry the filaments. This reduced my problem, but the clogging was still there under some circunstaces.
For example: If I rise the temp more than I should I can start to print and then reduce the temp, BUT if I reduce the speed I do have a clog. So far I have discovered that I must print to a speed no lower than 35 mm/s, so basically that means I cant do it slowly for example for a nice a smooth surface finish, like 20 mm/s for example.
For some reason while printing faster than 30 mm/s it look no clogging happens. So, watching this behaviour it look like teh filament get too hot just before the hot-end block, makeing the filament to expand and block the entry to the melting chamber. If the speed is faster teh filament doesn't have time to get hot enough to expand and obstruct the access to the chamber.

Does anybody know how to solve this? I see averybody useing a 3 mm fan to cool the hotend, I have a 4mm fan for that and it seems is not enough?

Suggestions, ideas are welcome. May be the position of my fan is incorrect? too high or too low in the hotend?

This problem does not happend with ABS, only while useing PLA and B-PET.

Thnaks in advance
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