Last night I took apart Kurzweil's extruder in order to replace the insulator.
First I liberated the whole hot end from the extruder in order to pull the insulator out. I ended up with this (apologies for my crappy cellphone photography):
Now I know when building this extruder James and I pushed the insulator all the way into the nozzle, but check out the plug of ABS that formed between the insulator and the smaller nozzle canal. My hypothesis is that at some point the nozzle got clogged and a little bit of ABS was able to squeeze between the insulator and the nozzle. Then it cooled, and the next time I operated the extruder, that little plug-seed got melted again, and some of the pressure that should have been sending the filament out of the front of the nozzle got put towards increasing that plug with new melted filament. All this is exacerbated by the fact that the insulator just above the nozzle was able to buckle and bulge out.
I was curious what that crazy bulge looked like inside, so we cut it open.
What my badly-focused cellphone picture doesn't show is that there is clear evidence of back-flow up the insulator around the filament, or in the vernacular, extruder FAIL.
The replacement we'll be using for the insulator is a thinner PFTE tube surrounded on the nozzle-side by a PEEK tube and on the other end by a similar tube of what I'm pretty sure is aluminum.
After getting the insulator out, I took the nozzle apart so I could redo it, (1)to ensure proper thermistor placement and (2) so I could use Kapton tape, which is easier to maintain than fire cement, as the rest of this post will reveal. I started by cracking off the fire cement layer by layer with some pliers. The Ni-chrome heating element survived, but the thermistor was too tightly sealed into the hole on the side of the nozzle tip, and I ended up having to just tear the leads off and dig out what was left with a pointy implement. Then I put the nozzle in a cup of acetone for a while to soften the ABS still stick in the very tip, which worked like a charm.
So now we are in the market for a thermistor. I looked all over, but nowhere seems to have all of these qualities at once:
- 200k ohm
- shaped like this =O instead of this --
- operating temperature range high enough for ABS (235-240C°)
- a response curve compatible with the preexisitng software on Kurzweil
- being sold in reasonable quantities (no, digikey, I do not want to buy 2500 thermistors!)
Hopefully the fellas at BfB will be able to help out, but in lieu of that, any tips would be greatly appreciated.
Scott and I had our first RepRap session this evening. First off, we made sure the extruder was connected up correctly. When James and I were building Kurzweil a year ago, we lost the "12 Way Connector" that came with our BfB kit. We fudged a solution to get the extruder connected, but it's fragile, and got messed up during transportation across campus for the end-of-year poster session a month ago. Printing a new 12-way connector is an excellent candidate for a future project.
Once we were sure the extruder was connected properly, we performed the tedious but necessary rituals of bed-leveling and z-axis-setting (thus further fueling my desire for one of those well-constrained Mendels...).
We started noticing problems while printing the test rafts. The extruder was clicking, and I could see the filament jumping back, so I released the pressure bearings a bit. I may have gone too far, because the flow from the extruder became pretty feeble, so I tightened them back up half as much as I loosened them. That seemed to make the extruder run more smoothly but the flow was still pretty weak resulting in this test raft:
I think the problem is either in the tension of the pressure bearings, or some kind of clogging in the nozzle or the insulator, which, if you were wondering, looks like this:
I know that the bulge is a problem others have run into. The Bfb kit called for us to cut off a ring of the nozzle to restrain the insulator, but we didn't have the tools to do that and Ian at BfB assured us we'd be fine without it... I'll look around for solutions but if anyone reading this has a take on the situation I'd really appreciate the input.
While today is not the first time I've worked on the RepRap since I last posted 14 months ago, it is the first time since then I've documented my struggles. Let me bring you up to speed (I'll try to keep it short)...
In the Fall semester of 2009 I took a Computer Architecture course taught by Michael Siff at Sarah Lawrence College. For my independent project I attempted to design a system of modular mechanical logic gates. I seriously underestimated the scope of this endeavor. I was trying to simultaneously:
- learn Blender
- learn Skeinforge
- design complex pieces using Blender
- maintain and troubleshoot Kurzweil
All while also trying to keep up with an intense Psych class and a very full Music program. Needless to say, I did not succeed, though I learned the hard way that making an idea into reality never goes as smoothly as one first imagines.
Slightly discouraged, and reluctant to set myself up for disappointment again, in Spring 2010 I took a break from RepRap. And then I graduated.
Now, in what may be my last month and a half in the Westchester area, I've set another possibly preposterous goal: wherever I go next, I want to take pieces for a Mendel with me. But this time I have help!
Scott Calvin, an SLC Physics professor, and my excellent Don, wants to learn the RepRap so future SLC students may experience its agony and glory. And what better way to learn how a machine works than fixing one that don't work so well? Hopefully with our powers combined, we will be able to get this puppy running.
We went back to the Science Center to open our kit. So many shiny bits and pieces... We started right in on assembly. The acrylic parts smelled really bad, but looked really good. Putting all the little nuts and bolts was tricky but eventually we got the hang of it. We made most of the corner brackets, and James started in on the Z motor block. Some of the acrylic pieces don't fit quite right with the motor shaft, but it's nothing a little filing can't handle.