Me wearing ridiculous goggles

Using PLA with the MakerBot Plastruder MK5

Polylactic Acid is a compostable plastic that can be made from renewable resources — usually plants. Compared to ABS plastic, it’s much harder, warps much less, and can be crystal clear. When I first got my hands on some PLA filament in 2009, it had the potential to be my new favorite plastic…but getting it working took time.

Historical note: PLA on the MK3

Try as I might, with my original Plastruder MK3, I just could not get reliable PLA extrusion. I now believe this was the result of design flaws in the MK3’s hot end and filament drive that were uniquely exacerbated by PLA.

Hot end: At printing temperatures, PLA is oozier than ABS and flows through cracks more easily. At slightly lower temperatures, it becomes a high-friction goop. The MK3’s feed system has several gaps and cracks: where the heater barrel meets the nozzle, and again where it meets the PTFE insulator. When extrusion failed during my testing, these gaps would often be full of slightly-molten PLA residue. Because the heater’s temperature gradient was designed for ABS — which prints at higher temperatures and is more thermally forgiving — it was difficult to keep the business end within PLA’s narrow working range without allowing the “Goo of the Gaps” to cool and harden.

Other folks online reported more catastrophic failures, involving PLA oozing out of all sorts of places. I suspect these were unattended prints, where Goo of the Gaps accumulated and led to dangerously high barrel pressure.

Finally, these gaps made it difficult to switch cleanly between PLA and ABS. My preferred method involved acetone and a blowtorch — not something you want to undertake lightly.

Filament drive: The MK3’s filament drive also had a hard time with PLA. I suspect this was because PLA filament is so much harder than ABS. To extrude consistently, the filament drive must pinch the filament between the drive gear and idler wheel until the drive gear can “bite.” With harder filament, this requires much more force — force exerted on both the unsupported motor shaft and the notoriously fragile idler wheel.

Revisiting PLA

I care deeply about my technical work, and I carry failures like scars. I spent 2010 with a 5lb spool of PLA sitting sullenly on a shelf in my office. Short of acquiring a laser cutter, or getting into RepRap-style printable extruders, it wasn’t clear what I could do.

In January 2011, I peeked out of my cave long enough to notice that a new extruder had been released. (I skipped the MK4, which didn’t seem to provide significant improvements.) I spent a few days studying the design carefully.

The MK5 addresses most of my concerns about PLA in the MK3.

  • The hot end contains a chamfered teflon channel, ensuring a gap-free path from the filament drive to the nozzle’s tip — even though the nozzle is still interchangeable.
  • The filament drive uses Charles Pax’s inspired design to eliminate the idler wheel, and the motor shaft is now supported by a 608 bearing. This suggests that the MK5 can pinch much harder, safely.

(One remaining concern — that PLA seems more compressible than ABS, making it more difficult to regulate barrel pressure using a simple gearhead motor — has been addressed by the MK6. I have not yet upgraded.)

Now, MakerBot Industries doesn’t explicitly state that the MK5 is good for PLA. They seem to have set the PLA experiment aside, just as I did. They no longer even stock the stuff.

Which is unfortunate, because their new extruder turns out to be a spectacular match for the material. After about an hour of lab work, I had a temperature profile I was happy with, and the gorgeous transparent filament was a-flowin’.

How I Print PLA on the MK5

These are my notes on using PLA, collected from my lab notebooks.

Switching from ABS to PLA

The first step toward printing with PLA is putting some PLA in your printer. If your printer is currently loaded with ABS, this is a little more nuanced than it might seem. I recommend this process, which takes about five minutes:

  1. Loosen the filament tensioning bolt (the one that drives the delrin pusher).
  2. Heat your extruder to your normal ABS printing temperature (mine: 220C). Give the extruder a minute or so to stabilize at this temperature. It should be oozing a little worm of plastic.
  3. Gently push filament into your extruder by hand. (Your printer was not designed to take force in this direction. Push too hard and you risk breaking the extruder mounts.) This should cause ABS to flow from the nozzle.
  4. In one smooth, rapid motion, pull the filament backwards out of the extruder. If done correctly, the filament you’re holding will end in a cone with a tiny bump on the end — a cast of the inside of the nozzle. If it doesn’t, you still have ABS in your heater barrel; snip the filament end and repeat steps #3 and #4. If you can’t make this work, try slowly reducing temperature.
  5. Remove the ABS worm from the extruder nozzle.
  6. Set the extruder temperature to a compromise setting halfway between ABS and PLA. The goal is to keep ABS malleable, without burning PLA. I use 205C. Allow the system to reach this temperature and give it a minute to stabilize.
  7. Load PLA filament. You should be able to push gently and see extrusion from the nozzle, still looking suspiciously like ABS. It will be slow at first — do not push hard.
  8. Tighten the filament tensioning bolt.
  9. Run the motor forward. Check that the drive gear is biting. You should get a continuous but slow extrusion of ABS.
  10. Let it run. The extrusion will gradually become more clear and faster as the last of the ABS clears out. As soon as it becomes translucent, lower the temperature to your target PLA temperature (I use 185C).
  11. Run filament until the output is clear enough for your taste.

Switching from PLA to ABS

Basically, run the process above, but ramp the temperature up instead of down. I find switching to ABS is much faster than switching from it, because the PLA you’re clearing out has very low viscosity once you ramp the temperature up.

Calibrating PLA

There are two important aspects to calibrating your printer for PLA:

  1. The usual adjustments to Skeinforge’s extrusion width/diameter settings. The process you used for ABS will work fine, and I won’t discuss this further here.
  2. Getting the right temperature.

PLA is far more sensitive to temperature changes than ABS. I suggest running test extrusions at 5-degree intervals starting at 180C. Pay attention to three aspects: speed, viscosity, and clarity.

Speed : For easiest calibration, you want your PLA extrusion to go roughly as fast as your ABS extrusion. At higher temperatures, PLA will come out much faster. While this might seem great — higher head speeds, right? — it makes ooze control difficult because of the high pressures in the heater barrel.

Viscosity : PLA will tend to be less viscous than ABS. I test viscosity informally using the “tower test:” Raise the nozzle to a fixed height (say, 20mm) above the build platform, and run an extrusion, letting it pile up. It will tend to form a cylindrical tower or cone; the diameter of the tower is a function of your viscosity.

Clarity : Assuming you’re not using dyed filament, at the optimal temperature, PLA extrusions will be as clear as glass.

If at any point you notice the extrusion popping or smoking, lower the temperature, you have gone too hot.

In my case, my results were as follows:

Temp Speed Viscosity Clarity Notes
185 Similar to ABS Lower than ABS Very clear -
190 Faster than ABS Lower than ABS Very clear -
195 No change No change Very clear -
200 Increased Noticeably lower Very clear Very stringy
205 Increased Low, sagging visibly Has bubbles Seems too hot

Finally, note that the “Material” setting in ReplicatorG’s default version of SkeinForge (0006) affects only the raft settings. To adjust other settings, clone your ABS profile before editing it.

PLA Tips

  • PLA is harder than ABS — which makes it somewhat more complicated to drive a screw or bolt into it. While holes in ABS parts can stretch a little, an undersized hole in PLA means some quality time spent with a drill and tap.
  • Solid chunks of PLA can be quite clear. Experiment with your SkeinForge settings, such as the number of shells and thickness of exterior walls, to produce beautiful crystalline parts. I used this to good effect on my NuForce DAC case.

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