Clogged while printing

Hi. I have this issue wich is driving me crazy... I can print useing ABS wiyh no problem.
Things go wrong when I chage to PLA (200C) or to B-Pet (240 - 260 C). Im very carefull cleaning al material before changeing type of filament.
When I start printing with PLA, after some minutes or sometimes after some seconds the noozle clogs. I was suspecting that may be some ABS was still there... but no, I cleaned the noozle, removed the clog and the problem still persist. If I go back to ABS I can print...
Then I tried B-Pet, wich has a higer temperature than ABS, so left overs of ABS should not be a problem. Well... the same problem arise. I printed a small model with no problems. Then I tried to print another one and the noozle clogged after some layers.
I have a j-head, the fan is working, a 40 mm fan. But somehow the melted material goes up too far in the hotend and clogs it?
Something I noticed: if the extrusion goes fast it looks like it helps, the problem start to build when the speed decreases.
Any ideas why is this happening?

Edited 1 time(s). Last edit at 01/20/2016 06:03AM by Tinchus.

Insulating your hot end block will help.

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when i had this kind of clogs ( in my case abs ) i realised after a lot of research that it was filaments humidity problem. i could print small parts but when somethng was more than 30minutes print then i was always had problem with clogs. So i learned my lesson the hard way. Now every time i print i put the filament in the oven at 50celcious for an hour or so and everything works just fine.

I will try putting the filament in the oven... Something I noticed is that with B-PET, there is a lot of popcorn noise when the temperature gets to 240 aprox... How humidity cause this problem (I want to learn).

Thanks. If anybody has other ideas... I will try everything, right now it seems I can only print useing ABS

whar mendel do you have and what hot end. give us some more details.

I have a prua I3, with a J-head, 0,4 noozle.

If someone has a similar problem... I think I found the cause of this clogging:

it look like humidity is responsible. While trying to print PLA, I notice the popcorn noise, That happened with the B-pet filament too. And I saw the filament comeing out of the extruder like "boiling". So I though that the temperature was too high and the filament was actually boiling, and I reduced the temperature... but to a point where this reduced temperature cause the clogging.
But actually this boiling is humidity makeing bubbles (exploding bubbles) when reaching certain temperature. I took some filament to the microwave... And I was able to do 2 prints with no problem!

May be the bubbles formin inside the melting chamber also made some kind of counterpressure pushing melted filament up through the extruder body where it was cold enough to be solid againg maken the coming filament to jam?

Good u found the problem. It is not a counter pressure . it is just boiled water that needs it way UP instead of the filament it looks it's way down. The problem is that no extruder is so powerful to beat steam.