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am in the process of completing my experimentsx with an induction heated borosilicate hot end here with quite amazing results! I wasn't targeting melting aluminum just yet but just any filament currently available.

Induction heating is the way forward I have concinved myself now and using an all borosilicate one piece hot end is also quite compelling,. The way I have approached it is to get 3.2mm ID borosilicate tubing with an 8mm ID and add a flare to the top so I can slide it into an extruder body and clamp on the flare while using it as a funnel for the filament to enter the hot end at the same time,.

As far as the heater and nozzle goes where this is where it gets cool. I have obtained 3mm ID tungsten washers with 4.5mm OD. I take a 40mm piece of the 3.2mm ID boro tubing and heat the lip and tungsten washer in my torch flame then carefully attach the tungsten washer on the end opening lip of the boro tube. Tungsten is one of the reasons why this whole thing will work as it is one metal that has the same coefficent of thermal expansion as borosilicate itself does so you can encase tungsten in boro and heat and cool them together without the glass cracking because of different expan sion profiles when heated.

After I have gotten the tungsten washer to stick to the lip of the boro tub and gotten the 3mm ID of the washer aligned with the 3.2mm ID filament path of the boro tube I take another piece of boro tubing and heat it as well as the tungsten washer tube end again and melt these together. Once melted together I encase a bit more glass on the outside so that the tungsten washer is not exposed to air at all on the outside of the tube and then I heat and pull down a taper for the exit nozzle. This whole assembly is then annealed in my kiln for 8 hours and slowly ramped down to room temp. Once cooled I chuck the assembly in my unimat micro lathe and slowly with lots of water and with a diamond tip lathe bit machine the glass away on the taper until I can measure with a feeler gauge the nozzle size 0.4mm I am aiming for. The end result is an 8mm OD boro tube with a flare/fluted top that mounts to the extruder body and a tapered nozzle on the tip. 

I have made a small homemade induction coil circuit with PWM controller that runs of 24V PSU and has 2mm OD copper tubing coiled about 3 times in a tight coild with a 12mm ID inside the coil. I have cold water pumping through the copper coil to keep the induction coil cool and there is + and - power hooked to the two sides(input and output) of the coil. The coolest part is that I do not need any thermistor or thermocouple to read the temperature. You can actually read the temperature of the hot end by reading the resistance in the copper coil you are electrifying and contolling with a PWM cycle.

When you fire it up the tungsten washer inside the boro tube  glows red hot and one of the other neat parts of this prototype is watching the filament through the clear glass as it moves down the barrel and passes through the 0.5mm thick tungsten washer and then out the nozzle. So the heat zone in this hot end is about 1mm to transition from cool filament to molten right before extrusion via the orifice.

Another property of boro which makes it perfect for this task is the fact that it is a very poor heat conductor. All the new stainless hot ends use stainless as their heat barriewrs because it does not transmit heat very well at all. Boro transmits heat even worse and I can actually ho9ld the top flare of this hot end in my fingers with the induction coil turned on and push filament through the hot end relatively easy compared to most hot ends,.

This is as far as I have gotten yet and my induction cuircuit needs work before it5 can run for hours on end as it gets hot after about 20 minutes right now,. And yes I am well aware that pics or it didn't happen but you are just gonna have to wait until I am ready to share photos and decide if you don't want to believe me or whatever until then ;P I will state that I have been a borosilicate glass artists and scientific glass worker for 20+ years if that helps with credability,.

In theory this hot end should be safe to run at temps up to around 700C before you start to come close to slumping or melting the glass itself,. So I am very happy with the experiment so far and I am working on making an affordable version to sell so I will update you when I h ave more to share(open source hardware design as always of course)

Cheers-
-Ezra