Another Rambling thought on why heat sinks are a bad idea...

A lot of people have trouble with the filament getting "stuck" and not pushing through.  The filament pushing mechanism doesn't have a lot of push before it fails, temporarily, or permanently.

what happens, from what I can see, is that the filament melter opening gets hot, and melts the filament as it's going into the opening, and that allows the filament to mushroom around the barrel go down the threads of the PTFE which expands with heat anyway, and then contracts when it gets cold.

If we let the whole thing heat up a while, I'm pretty sure the filament would probably move through ok.

But then people started sinking the barrel to pull the heat away from the top end to prevent the mushrooming in the first place.   good idea, but I think it adds another problem.  --- metal, brass, is HIGHLY conductive, and ABS softens at even relatively cool temperatures.  Not to mention that it's going to soften up the length of the filament even without help from the brass.

well -
stop trying to sink the heat away, in fact, get as much heat as you can near the filament opening
stop trying to use PTFE which is a bad insulator, not dimensionally stable, and all around silly.

WOOD is a much better insulator - and with a smoke point well above (please someone correct me if I'm wrong) the 250°C that we're looking at, shouldn't be a risk.
WOOD is a much more stable material - takes threads easily, and holds them without stripping out so easily.

So - as an experiment, I'm about to try and use the makerbot heater barrel in a WOOD barrier.  It will eliminate the retainer rings, and hopefully insulate the filament pusher completely from any heat, and keep all the heat inside the block so that the filament will simply soften and push through.

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will let you know how things progress.... personally I still like the barrel I'm currently using that is PTFE lined, so there's not a single seam in the pathway.  That's right I said PTFE - but it's not structural, it's simply a lining.

There is one other experimenter I've seen using a PTFE lined heater barrel - and the only problem I had with their set up was that it was overly complicated and required a lathe - which I don't have.

anyway - onward and upward!

don't sink the heat - it's wasteful!
insulate the crap outta the barrel and make it EASY for the extruder controller to get it as hot as needed.