Cheap@ss Extruder progress.
After a night’s sleep and a day where I couldn’t get into the garage (and the feedback from the group). I now know that a good extruder gets hot in some places, but stays cold in others.
Over the years, I’ve found if a part exists, it’s always cheaper to buy it…where a lathe comes in handy is when that part doesn’t exist, it’ll make it exist.
So I turned a steel sleeve that would fit the ID of the cooling tower and the OD of the PTFE. I’ll mount things up and see how well things stay hot and cold.
Read @nop_head 's ‘the story so far’ collected blogs if you really want to understand why ‘hotends’ are the way they are. Your time on this is a whole lot shorter because of those before you. It comes down to the dynamics of what happens to the filament from solid to liquid ! Press on !
Yup, this has turned out to be an interesting tangent. Got it all wired in and…well…brass is a GREAT conductor of heat. 
Poor little resistor couldn’t raise the environment above 150C…the top of the the assembly reached about 102C, and with the fan blowing on it, the temp at the hot end droopped to 130 or so.
Good thing all it cost me was time.
Get an E3D as they have produced a fine actively cooled unit with no PTFE. Important for when we get higher temp filaments as the PTFE won’t survive much past 250.
I’m pretty sure that’s where I’ll end up…however I’m suffering from a financial end of year, pre-tax return vacuum in funding. 
Still not convinced on stainless steel. Our Ubis hotends use peek / brass and are bulletproof if the thermistor does its job 
exotic filament is pushing us forward, our next experiment may be ceramic as the insulator. Manufacturability is our chief concern at this point. We make 1200 a month. Price is our next concern. I still think our $59 fully assembled Ubis hotend is a hard value to beat. How long it takes to build something is a huge part of the equation.
It’s an interesting math experiment. Building a part like this, especially in volume, is a balance between a lot of things…PEEK has it’s benefits, but the required additional components (PTFE), unit cost (it’s roughly three times SS’s cost) and machining, make all-comers pretty equivalent in cost.
The other thing to consider from an all metal hot-end is: lets just say the same thing happened there: the thermistor pops out, the heater shoots for the moon…how long does it take for the e3d to melt the printed base it’s sitting in? (Often it’s a case of finding the next weakest link down the line.)
All this is a moot point: I’m still extruderless.
@Mike_Miller Similar to the circle of life… find the next weakest link! My hot rod days were like that … more HP > tougher clutch > gearbox/u-joints > differential … then start over again ! There is an added point of failure with an actively cooled hotend … the fan ! Question is will the SS heat break xfer more than the aluminum heat sink dissipate before reaching the temps required to melt the base. My own experience melting down a J-head Mk 5 makes me wonder about a ‘watchdog’ safety that gets reset periodically by the heater shutting off.
As to the original question you posed… you would need the thermistor to fail AND the fan coincidentally.
@William_Frick it took 6 iterations before my 700r4 was no longer the weakest link.
Once the 700R4 made it into the C5 it got a lot better. Had no idea about high temps when I posted. See @Sanjay_Mortimer latest post … just a preview of what he is up to … 395 C !
Then it was a 4l65e…and pointing the wrong way round.
and 400C…I’m seriously interested in what’s taking those kinds of temps to extrude. That’s ALMOST Zinc territory (418C).
