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The above is the crib sheet I use for making an extruder. The ingredients are: 18mm PTFE rod (can be whatever), 18mm SS hose clamp, 45mm M6 brass bolt, M6 brass dome nut (can be plated), 4 Ohms of nichrome wire, 100k thermistor, slightly diluted fire cement or fireproof rope cement from the solid fuel supplies house and PTFE "thread seal" tape.
Basically, you make it look like this:
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I'll add pertinent details as people ask awkward questions. Or I sell them for NZ$55 and ship worldwide :)
The main issue people have with PLA extruders is that they leak where the brass barrel screws in to the PLA. There are many complex machining options, but the easiest is to wrap the barrel with PTFE tape, screw it in, and then add a heavy stainless steel hose clamp over the join. That holds it!