Friday, July 28, 2017

3D Printer Hot-end and Extruder Designs

Back when I started 3D printing, I had all the same problems every noob has.  Prints wouldn't stick and the extruder "jammed" more often than it fed filament.  It took about a year, but I eventually sorted out both problems. This post summarizes what I learned about extruders and hot-ends.

There are a few variations out there, but most extruders work by pinching the filament against a sharp toothed drive gear on a motor shaft.

The jamming I experienced early on was actually the extruder drive gear carving divots into the 1.75 mm filament (which was sort of a new thing, at that time).  Once that happens, the drive gear teeth have nothing left to grab and the extruder can't push filament any more.  I started researching extruders and found something interesting.  The people who used 3 mm filament almost never had problems with extruder jams, and the people using 1.75 mm filament almost always had problems.

I compared filaments.  3mm filament is pretty stiff and it takes some muscle to make it behave.  1.75 mm filament is much more flexible.

Next, I started looking at extruder designs. 3mm extruders all had gears to multiply the motor torque. Very few 1.75 mm extruders had such gears.  That got me thinking that at least part of the problem had to do with motor torque.  The other thing I noticed was that 3 mm extruders usually had some pretty strong springs pushing the filament pinch roller bearing against the drive gear.  The 1.75 mm extruders were usually pretty weak in that regard.

I eventually figured out that if you used strong springs on the pinch roller to push the filament hard against the drive gear, its teeth would bite deeply into the filament and the motor would not have enough torque to carve a divot into the filament.  So I modified my extruder with a stronger spring and preloaded it by compressing it with a screw.  That was the end of my filament divot carving problems, but now I still had problems with filament not extruding, which was either a hot-end problem or a motor torque problem, or both.

At some point during my quest I started experimenting with my own extruder drive concept.  I built a prototype and needed a hot-end to test it.  There was a Taz printer at the makerspace that had a Budaschnozzle hot-end and it seemed to work reliably, and on-line feedback indicated it worked pretty reliably, so I ordered one for my testing.  When it arrived I took a close look at it.  What I found was unbelieveable.

There was a laser cut wood part just a few mm away from the heater block.  Guess how long that part lasted after it charred black!  There was a large, threaded aluminum "heat-break" screwed into the aluminum heater block, impossible to disassemble without destroying the tube or the block, and there were what appeared to be heatsink fins on the body of the extruder, but upon disassembly, I found that the fins were really aluminum discs stacked on a teflon tube.  Teflon is plastic, a thermal insulator.  Why on earth would someone put a heatsink on a piece of plastic?  Those were the days when garage tinkering was sufficient "engineering" to produce a commercially viable product.  The design of the Budaschnozzle truly lived up to the ridiculousness of its name!

Since the 3 mm extruders all had gear boxes and seemed to work reliably with almost any hot-end, I figured that what I needed was more torque, so I started looking for an extruder that had a gear box.  I eventually settled on a BullDog XL, which has a 5:1 gear box.  The BullDog XL can push filament through just about anything going on inside a hot-end.  An additional benefit of a gearbox on an extruder is increased resolution in the filament extrusion which makes for very smooth print surfaces.

In a hot-end that has no real heat-break or cooling above the heat-break, PLA filament can get very sticky as heat creeps up the the hot end and softens the filament inside the tube.  This sort of problem usually shows up about 20 minutes or so into a print.  Everything will be going just fine and then the extruder will suddenly chew a divot into the filament for no apparent reason (if the extruder isn't properly adjusted), or the extruder motor will click as it starts skipping steps because it doesn't have enough torque to keep pushing the filament.

A lot of people think it's a problem to be solved by oiling the filament, presumably so it doesn't get sticky in the tube, while ignoring the problems that oil creates in getting prints to stick to the bed and/or print layers to stick together.  Others attribute the problem to dust on the filament jamming up the mechanism, so they put some sort of sponge or cloth in the filament path to wipe the filament clean before it goes into the extruder.  Neither solution addresses the real problem - heat creeping up the hot-end tube.

That experience got me looking at hot-end designs.  After some research, I came to the conclusion that hot ends should be actively cooled, especially for printing PLA which softens at very low temperatures.  I looked for designs that were actively cooled and otherwise made sense (no heatsinks on plastic, no wood parts, they had to have real heat-breaks, etc.) and found the E3D v6.  I've been using them for a few years and they just work.  The design makes sense (though I think they are as long as they are mostly to accommodate the 30 mm cooling fan- the new Aero version addresses that).

To summarize, reliable extrusion is most easily achieved with:

  • a high torque drive design that uses a gearbox to multiply motor torque (which prints smoother surfaces, too).
  • pinch roller pressure adjusted so that if the hot-end really jams, the extruder motor will skip steps without chewing a divot into the filament.
  • a hot-end that has an actively cooled section above a functioning heat-break.  
I've been operating a BullDog XL and E3D v6 combo on Son of MegaMax (SoM) for well over 2 years of almost daily printing and have had exactly one filament jam that occurred because of a foreign object embedded in the filament.  I don't have anything wiping dust off the filament, and no oil.  None of that sort of stuff is necessary.  If you have dust that's big enough to jam a 0.4 mm nozzle, you had better move to a place that will be safer for your lungs!

Foreign object embedded in the filament produced the only true jam in the hot-end in over two years of almost daily operation.

That extruder has never chewed a divot into the filament.  However, it has one design flaw.  There is a small gap between the bottom of the drive gear and the top of the guide tube that steers the filament down into the hot-end.  If you print with flexible filament, and try to extrude too fast, the filament will buckle in that gap and will then refuse to go down into the hot end, resulting in a failed print and filament wrapped around the drive gear.  The same can happen with more rigid filament if you set the pinch roller pressure so high that it squashes the filament.

This was a new (for me) failure mode for an ABS print.

After cutting away most of the bird's nest, I found this.  Filament isn't supposed to come out of the side of the extruder!

Removing the cover revealed this.  The filament had wrapped itself around the drive gear, but how/why?

This is how the filament was able to wrap itself around the drive gear.  That gap allows the filament to buckle in that space.
And this is why.  If you crank up the pinch roller pressure too high- it crushes the filament!

The crushed filament gets wider at the sides and thinner top-to-bottom, making it want to fold inside the gap between the drive gear and the guide tube.  This problem was fixed by backing off the pinch roller pressure.  There's still a gap between the guide tube and the drive gear making it tricky to set this extruder up for printing TPU filament (though I have successfully done so on several occasions), but it's proven extremely reliable for printing rigid filaments.
If the filament spool runs out during a print, once the end of the filament gets below the drive gear the extruder can no longer push or pull it.  If you try to feed in a new piece of filament, the stub in the gap will bend over and refuse to let you load the new filament.  You have to separate the extruder and hot-end to retrieve the stub of filament that stuck in the hot-end before you can feed fresh filament into the extruder.

The second problem is easily solved with proper printing "hygiene" which involves weighing the filament spool before starting a print to make sure it isn't going to run out, mid print.  That has always worked fine for me because I understand the problem, but Son of MegaMax is at the Milwaukee Makerspace and not everyone prints with the same attention to the process.  The result was a lot of down-time and a lot of extruder/hot-end disassembly.  I fixed the problem by adding a filament run-out sensor to the printer so that if the spool runs dry before the print is finished, the sensor will stop the printer before it pulls the end of the filament down into the extruder.

The run-out sensor created a new problem.  If there's no filament in the sensor and you power up the printer, all you get is a blank LCD screen.  I've had several people contact me reporting that the printer is "broken" because of it.  If you want to see if your 3D printer design is foolproof, leave it at a makerspace - you'll quickly find out all the flaws in your design!

I was updating the Taz and a Solidoodle printers at the makerspace and decided to see if there was an extruder that didn't have the same gap between the drive gear and guide tube.  I saw that E3D had recently released the Titan extruder that seemed to address that problem, so I ordered one to try it out.  It was about 1/2 the price of the BullDog XL and had a few obvious design advantages.  It was much lighter weight, more compact, properly fit on E3D hot-ends, and didn't have that gap between the guide tube and drive gear.  

When I got my first Titan extruder, I deliberately ran the filament out.  Then I tried loading fresh filament and it worked perfectly without any disassembly.  The Titan guide tube extends from the top of the hot-end all the way up to the bottom of the drive gear.  There's nowhere for the filament to buckle.  I like that!  Now I'm in the process of redesigning SoM's extruder carriage for a Titan extruder, and I put one on Ultra MegaMax Dominator.  I've also put one on the Taz printer at the makerspace.  The 3:1 drive gearing seems to have adequate torque when used with a "normal" sized motor.

A lot of people like to put low torque "pancake" motors on Titans to minimize weight so they can push their printer to print faster.  I think you have to make a choice.  You can use a pancake motor and operate at the very limits of performance to make relatively low quality prints, and occasionally lose one when the extruder jams up because it doesn't have enough torque.  Or you can put a more "normal" size motor on it and print a little slower, for higher quality prints that finish more reliably because the extruder has enough torque to keep pushing the filament even when things get less than ideal in the hot-end.

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