So there are two main parts to this design. You need a mechanism to push the original 3mm filament through the heater and then you need the heater itself and a nozzle that will melt and reshape the ABS plastic into it's thinner form. For the first part I had chosen to go with a pinch-wheel style extruder. This basically works by sandwiching the 3mm filament between a drive gear attached to a stepper motor and a roller bearing. The teeth on the stepper gear bite into the ABS filament so you can control how much the filament moves. Here is a picture of this mechanism.
The stepper motor is a nema 23 style motor that is mounted on the underside of the sheet metal plate. I milled slots in the plate so that the motor could be adjustable from left to right. This means that you can change the space that the ABS gets sandwiched through from about 2mm to 3mm. My goal was around 2.5mm as this usually gives a good bite but doesn't apply too much stress on the components. As a bonus I also milled the slots for a nema 17 style motor in case I needed to reduce weight in the future. The guides for the filament are just 5/16" rods that I center drilled with a 1/8" bit and then hot glued to the surface. They don't undergo too high of lateral forces so hot glue should be fine for testing purposes. Here's a shot of the mechanism with the actual filament in place.
When the filament is pushed through it actually ends up with little teeth marks on the surface. This is good because it ensures that the teeth will have a solid hold. While it was running I couldn't stop the filament from being pulled in by hand and even while it was stopped (coils still energized) I couldn't pull it out. This should have more than enough power to force the filament through the nozzle and a nema 17 could probably be sufficient if needed.
So onto the nozzle and heater assembly. I used a 8 ohm length of insulated nichrome wire for the heater. It is wrapped around a piece of M6 brass studding with the inside drilled out to a 1/8" bore. The wrapped nichrome is then covered with JB weld which claims to withstand up to 600˚C. I also placed a thermocouple in a small indentation I filed out of the barrel near the tip. This should give accurate readings of what temp the plastic is actually encountering. Here is a shot of the step.
I used alligator clips to connect the nichrome to my power supply as my pin headers haven't arrived in the mail yet. I then had to use the alligator clips on my helping hands tool (http://www.amazon.com/Helping-Extra-Large-Magnifier-Soldering-Electronics/dp/B0016P7H1I/ref=pd_bbs_sr_4?ie=UTF8&s=hi&qid=1237294605&sr=8-4) to hold the thermocouple leads to the probes that my handheld multimeter has. I clearly need to get more alligator clips. Anyways, I switched the multimeter on and it read 16˚C. It's pretty cold out so I figured it was working fine. I then connected the nichrome wire the the +5V output from my new 500W power supply and turned it on. I watched the temperature readout on the multimeter, but the temp didn't change. I thought that maybe the barrel just took a while to warm up, but then I touched it and realized it was definitely above 50˚C easily. I turned off the power supply to investigate why the thermocouple wasn't working. After a good couple minutes of frustration it finally hit me. The two alligator clips on the helping hands tool that I was using to hold the probe leads from my multimeter to the leads of the thermocouple were connected via the metal bar that holds them both! The alligator clips were shorting out the thermocouple! I just wrapped some wire around both ends instead of using alligator clips and the problem was solved.
Now onto the exciting stuff. With the heater and the thermocouple working I tried hooking the nichrome wire up to the +12V leads on the power supply. This would give about 1.5 Amps of current through the wire. When I flipped it on the temperature instantly started racing up which is good. I wanted to see where it was going to plateau as it needed to be above 230˚C if I was going to have any success melting the ABS. Before I could get there I started seeing smoke coming off of the heater. Past experience tells me this is a good indication that something is wrong so I turned off the power supply. It looked as if the smoke was coming off of the nichrome wire itself. If you look at the picture above it appeared to be coming off of the nichrome wire that goes over to the black wired alligator clip. I'm not sure what was causing it. Maybe the nichrome insulation on the wire was melting, maybe there was a little JB weld that got on there that melted, or maybe the PTFE thread tape was melting. Maybe the heater just smokes a little at first. Not sure. If anyone has any thoughts feel free to post a comment!
Any exposed nichrome gets a lot hotter than the rest because it has nothing to take the heat away. People have mentioned it getting red hot. That is why I make my connections inside the heater insulation and bring out copper wires. A small amount of smoke may be caused by the grease from your finger marks burning off.
ReplyDeleteJB Weld claims to withstand 600C and set like steel. Both claims are fraudulent in my experience. It will go dark within minutes and with a few tens of hours of use it turns to dust.
Good to know. I am going to assume it was just the exposed Nichrome insulation or some residue on the wire burning off. I am still waiting on my pin headers for the connections, but once they get here I'll go with your advice of trying to get the connections completely inside the insulation so that nothing is exposed. Second, I had heard rumors about the JB weld breaking down before, but this really just intended for a short term solution. I will be making a once piece barrel (no screw-on nozzle) and using some fire cement if I can ever locate any on the second revision.
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