Hey Reprappers,
So for the past four weeks or so, my team has been attempting to get an extruder working on our Darwin platform. The endeavour has been frustrating as, as soon as one challenge is overcome, a new problem has arisen. However, we've invested a lot of energy into troubleshooting our extruder problems, and have some thoughts to share on the subject.
First of all I'd like to make it clear - we have not yet solved our own extruder problems, although a promising new solution has been prepared and will be tested after the weekend.
For those of you with well-functioning extruders, you probably won't need to take note of this. My impression is that most people's extruders work well and ours does not, which means that we've been doing something wrong. Nonetheless, the problems that we've encountered could easily strike any person, so they are worth taking note of.
Problem 1: Insufficient torque
This is a common complaint. In our case, I machined a knurled shaft 7mm in diameter and mounted it on the standard Darwin extruder, with a high-torque NEMA 17 stepper from Lin Engineering. However, while the shaft got good grip, the torque was simply insufficient to drive the filament into the extruder, so the motor skipped even on maximum current.
Solution: Wade's geared extruder
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objects.reprap.org]
Wade kindly let us try out his geared extruder design, which can supply an incredible amount of force. It's also just a nice design in general, with variable spring force on the ball bearing and such.
Problem 2: Burn-out
We made some minor modifications to mount Wade's extruder on the Darwin, and it worked like a charm - until our heating element burned out, which was the day after our first print. The nichrome resistance shot up from 6.5 ohms to 10k ohms. The cause of this is still not known for sure, but it was clear that we were running the extruder too hot (240 C, according to our thermistor). However, much cooler than that and we could not extrude PLA.
Solution: Lower the required temperature
In an attempt to reduce the heat needed, I rebuilt the extruder barrel several times. Wondering if such high temperatures were needed because of a long sticky glass transition zone, I built our second extruder with the insulation focused at the end, and a long part of the brass screw exposed to the air. I also filed down the threads near the tip, mainly to remove the caked-on charred remains of the previous extruder, but partly to improve heat conduction to the core (no threads means the nichrome can be wrapped more tightly). This way, the temperature at the wires could be lower than before, and still achieve the same core temperature.
A fan was set up by the reprap to gently cool the exposed tube, in a further attempt to narrow the transition zone.
Problem 3: Backflow
Unfortunately, this extruder suffered from odd behaviour: Loading fresh PLA in, it would extrude perfectly for about ten minutes. After that, it would slow down, until eventually the motor would spin with no extrusion occurring at all (meaning the extruder had carved a hole into the PLA). Cooling the extruder to about 90C and pulling out the PLA allowed us to replace it with a fresh filament, but the result was unfortunately quite repeatable. (Cooling is important because very hot PLA will bond to the PLA extruder as it's removed). The mechanical resistance from the extruder was evidently increasing as a function of time. The problem worsened if the extruder was turned off for a short period of time after extruding.
Evidently, thermal effects were responisble for this. Based on the appearance of the PLA removed from the extruder, which ended in a thin tubular film, we hypothesized that the molten PLA was flowing back up the nozzle under pressure. When it reached the cooled section, it solidified. Turning the heat up to 250 in an attempt to melt the plastic led to a burn-out of this extruder as well. Time to try again...
To narrow down the problem, my teammate Bing conducted a series of experiments. Heating an extruder barrel with a lighter, she found it very easy to extrude filament with just gentle application of pressure from her hand. Evidently, the mechanical resistance of the extruder is not primarily from the viscosity of the liquid plastic. She found that the plastic tended to bond poorly to the brass tube, even when moderately cooled (though when left to cool to room temperature, it was impossible to remove - perhaps due more to the tight thermal contraction press-fit than any chemical reason). However, warm PLA bonded strongly to PEEK, the material we're using as an insulator. Furthermore, she observed that after a moderate amount of time, a significant quantity of PLA flowed back up out of the brass barrel and formed a growing plug above the barrel.
Thus, the problem could be that backflow was reaching a cold part of the barrel and causing a hardening plug there, but more likely, it was flowing all the way up the barrel and forming a plug at the insulator.
Anticipating the latter as the cause, I rebuilt the extruder with modifications. A series of aluminum heat sinks (C-shaped plates cut by hand from sheet metal) were applied to the extruder to assist with fan-powered cooling. I aimed to achieve a temperatue gradient steep enough to prevent any backflow out of the brass tube. Secondly, thermal-conductive paste was applied between the nichrome wire and the brass tube, to again reduce the chance of burnout.
The extruder did not burn out. However, it also did not extrude. The best we could get was a slow, gravity-fed drip, even at a variety of temperatures.
Solution(?): New extruder design
Consulting with Bernhard Zender, we decided to pursue an entirely new design, aiming for zero-backflow. Micrometer measurements showed our PLA filament to be about 2.88 mm in diameter. The PEEK insulator was replaced with a new one, made of Teflon and machined so that the diameter is just about 2.94 mm, a much tighter fit than before, and narrowing to about 2.5 mm at the very end of the nozzle. This narrow-diameter lip of teflon is intended to flow into the metal barrel, strained by the plastic filament, and forming a tight (but low-friction) seal to block backflow from any liquid.
The barrel itself is made from stainless steel, which conducts heat more poorly than brass. It screws onto an aluminum plate at the base of the Teflon, which acts as a heat sink to conduct any leftover heat away at the transition. The stainless steel is also drilled to a tight-fitting 2.94 mm, narrowing again slightly toward the tip at the melt zone before reaching the fine (~.5mm) nozzle.
Testing of the new configuration will be performed on Monday, Feb 22nd. I'll aim to put up illustrations to accompany this post, including drawings of the setup, before then.
I hope we can get all this extruder nonsense sorted out soon enough, so that I'll be able to go back to focusing on the SpoolHead.