I put together my fourth heater on Friday and Saturday. It worked fine for a while; I was able to extrude both HDPE and ABS! However, after I allowed the plastic to cool in the extruder, I could not get the extruder to drive the plastic again, no matter how hot I melted it. I had to disassemble the heater to clean it out. As I was working to remove the ABS, one of the leads attached to the nichrome wire broke. I don’t have any way to fix those leads, so the fourth heater is lost. At least I was able to salvage most of the parts.
I now see a pattern in nearly all of the heaters I have built: the nichrome leads have broken every time, and re-heating plastic already in the extruder has never worked and led to major problems. The re-heated plastic simply refuses to move out the nozzle, no matter how hard the motor pushes; the plastic tends to find alternate paths to exit the heater!
I spent some time thinking about these problems. I intend to solve the nichrome lead problem by attaching a short metal bar to the heater barrel, the bar extended horizontally, then gluing the leads (bare copper wire) to the bar using JB weld (a hard insulator that can withstand about 310 degrees C). The extended end of the bar, which should be much cooler than the end attached to the heater, will have a 2 pin socket so I can detach the heater easily.
The problem with re-heated plastic is still a mystery. The third heater used a PTFE insulator mounted flush against the heater barrel, as specified by the bitsfrombytes diagrams. With that design, a lot of the plastic exited out the top of the barrel instead of the nozzle, although I can’t remember whether I was using re-heated plastic at the time. The fourth heater had an 8mm length of the M6 pipe threaded into the PTFE insulator. Although the fit between the heater and insulator seemed very tight, the ABS still found a way to exit out the top and made a nasty hard shell covering the threads. I now realize that the M6 tap I used was designed for tapping hard materials like metal, not soft PTFE; an M6 tap designed for soft material would have a slightly smaller diameter. Next time, I will make my own tap using brass or just force an M6 screw into the PTFE.
By the way, I have begun putting together a BoM for the hardware I actually used. Tres expressed interest in it and I imagine others will be interested, especially now that Vik has provided a way to buy the laser cut parts for around $400, a lot less than what I paid. Excellent work, Vik!