Pict (above) showing most of the components making up my latest hot-end construction.
I identified some issues with the previous design that were addressed in the new version. Here's some of the issues I encountered.
Heat migration upwards through through the stainless pipe was taking place over longer print runs, despite the cooling fan and small heat-sink. When I increased the cooling (larger fan, closer to heat-sink) the hot-end could not reach it's target target temperature.
The heating of the stainless tube and subsequently the PTFE liner and filament was leading to an expansion of the PTFE and softening of the filament, too far from the hot end. It would loose it's rigidity, expand, even buckle and eventually jam.
I didn't want to have to go down the road of a more powerful heater, so I chose to address it from a different angle. I made some basic changes...
a) I narrowed the neck of the stainless tube just above where it meets the brass. This reduced the cross-sectional-area of the tube, reducing the heat bridge, and made a most amazing difference reducing the heat migration rate considerably. This was evident by a faster time to target temperature, due to less heat loss, and the cooling fan was now able to keep-up with cooling the stainless tube, while the heater block easily maintained targer temperature.
b) I cut off some of the upper portion of the brass nozzle, reducing contact area with the stainless steel.
c) I threaded the ss tube fully, drilled and tapped an aluminium block, and fitted some salvaged heat-sinks to the block. This is a bit over-kill in terms of extracting heat from the ss tube, but it does work well, and takes just a little air flow from the fan to keep everything cool.
d) A final problem I experienced, worth mentioning, was the PTFE liner tube moving within the stainless tube. If the PTFE moved away from where it met the brass, the soft PLA would enter that gap, and would eventually cause a jam. My observations are that PTFE expands and contracts significantly with changes of temperature, and this expansion and shrinkage, especially with the length of PTFE I'm using, is problematic. (I can see myself moving towards a fully stainless solution soon - no PTFE liner).
With the PTFE expanding due to heat migration upwards, I also found it pushed out of the top of the ss tube, so I devised a better way of capping the tube, to hold the PFTE in the tube. this is the purpose of the domed cap-nut you see in the photo above, and also below from a different angle.
Here's another photo of the domed nut which caps the ss tube, holding the PTFE liner in place.
I drilled a 2mm hole in the top of the domed nut. Also visible here is the much reduced and newly shaped aluminium plate, which is shaped to slot into my new x-carriage (I'm now using a "Gregs LM8UU X Carriage" ). The domed nut slots nicely into my extruder cold-end which is a "Gregs hinged accessible extruder". I printed that also, so it's been an amazing process printing enhancements! It just gets better and better! :-)
With the extruder fitted and working nicely, I then went on to print a better fan ducting, that would attach nicely to the x-carriage. Rich obliged by sharing his design... Here's my printed version.
That's it for now. Happy printing!