Clogs while printing with PLA/ B-PET but prints ok with ABS

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?

J-head?
They are usually rated for PLA only as at ABS temps the teflon liner already softens and blocks the filament.
Most of them also have the problem of heat creeping up the clod end as there is not enough cooling possible on a plastic cold end.

In my experience PLA and PET have a much bigger problem with heat creep than ABS.
Reduce retract to a minimum, i use 1mm or less, and make sure the hotend is correctly cooled.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

I have a doubt about cooling the hot end:

I have: the noozle, then comes the block and then a tube (called heat break?) and then the cold end (the big body with cooling layers og the j-head)? the cooling need to be starting from the junction between the block and the heat break or in the junction of the heat break and the cold end?

The cooling should start right above the heatblock. The heatblock itself should not be in the airflow, or if it is it needs to be isolated.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

The best way to insulate the block? I have seen people useing capton tape? is that enough?

No, the tape is just there to hold the insulation in place. Usually it's some form of glass/ceramic fibre (and hopefully not asbestos). There are also some really nice silicon rubber insulators around.

I found a listing on ebay for insulation. It looks like it already has kapton tape stuck to it, but I'm not sure why that would be of any use, unless it's some sort of double sided adhesive tape.

[www.ebay.ca]

Edited 1 time(s). Last edit at 02/16/2016 08:42PM by JamesK.

Quote
Tinchus
The best way to insulate the block? I have seen people useing capton tape? is that enough?

I use high temperature silicone for this [b.bonkers.de]

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]