Upon further inspection, I think you're right, it's not a heat problem.
I think the problem occurs after the large retraction that happens at the end of a filament change, this causes the problem area to pull back, then it can't get back in again.
Here's what the tip looks like:
Thanks for the idea about drilling out the isolator. Should I bevel the top so the filament is fed into the printhead better?
That white teflon part is called the "isolator".
I also cut the tip of my filament to a point before inserting it but once it gets past all the lips it's good until the next time I decide to change filament.
Your issue is strange as the temperature shouldn't matter. I'm thinking your 3rd fan is broken. The 3rd fan is behind the print head. Make sure it is working and that it is blowing (not sucking) air towards the front. The fan comes on as soon as you turn on power - even before the lights come on.
If the fan was broken and you fix it, you probably need a new isolator as it has been too hot all this time.
Even if the fan wasn't broken you should probably remove the isolator and drill it out with a 3mm drill to get rid of that lip.
The isolator deforms under pressure especially with temperatures above 240C. So after you drill it out it might help to keep the head temperatures at 230C or lower.
edit: although I've occasionally printed 250C and 260C no problem.
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