Virtually all 3D printers have adjustment
screws, knobs, or levers to change the pretension on the filament in the extruder. Though how
do you properly set it for your application, how can you get the most out of your extrusion
system, and why would you really lower the tension for flexibles? Let’s find out more!
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Extruder tension is something almost everyone who has been dealing with 3D printing was
already confronted with, but almost nobody really knows how to handle it properly. Even
if we look at manuals, they often just say to adjust it to the middle mark or a specific
distance from the extruder body and then adjust it to your liking. I honestly didn’t bother
in the past to tune or calibrate the tension simply because I always felt that it didn’t
have a huge impact on my parts. But there is actually more to it, and I, therefore,
tested the effect of extruder tension on a wide variety of extruders so that you will
know after this video what you need to look out for! Since I can’t test any extruder
and material combination, I’d be super happy if you shared your experience on that topic
in the comments so that others can learn. Do you ever touch this setting, and if you
do, how and why do you adjust it? On most filament-based 3D printers, the material
gets pushed into the hot nozzle by a set of gears that grip into the material and push
it forward. There are two variations. The simplest method is just having one driven
gear on one side and one freely rotating idler on the other side. Over the last years, dual
drive systems have become more and more established where you have gears on both sides of the
filament that pushes it forward. The idea of a dual-drive system is that you can exert
more force onto the material to feed filament more quickly and avoid stripping it because
every side only needs to push with half the force. In order to transmit force onto the
material, the gear needs to get pushed onto the filament so that the teeth grip into it.
A spring usually generates that force that you can set to a specific pretension via a
screw. But how much force do you need that an extrusion system properly works?
Let’s look at two extremes. First – not enough tension. In this case, the teeth of
the gears just barely dig into the filament. This way, only little force can be transmitted
via positive engagement, and most of it will come from friction between the gear and the
material. This causes grinding shows in grooves on the material.
On the other hand, the filament literally gets crushed if you apply too much pressure.
This doesn’t only deform the material and makes it harder to feed. Crushing, so permanent,
plastic deformation takes a lot of the motor’s energy, and you, therefore, lose power to
convey the material forward. Additionally, you’ll create more friction in the bearings
and might even break your idler arm if the pretension is too high! Ideal pretension in
most cases means that you have some indentation of the extruder gears on your filament so
that enough torque can be transferred but not so much that you crush the material and
waste energy at the extruder and make the deformed material harder to feed.
We already touched the first of three failure modes of an extruder with grinding filament.
Grinding happens if the teeth don’t dig deep enough into the material and basically
just slip over the surface and shave away the filament. However, grinding can also happen
if the extruder is too strong and at a jam simply shears away the material.
If your extruder is not strong enough to push the material forward or to shear away the
material, it will skip. Skipping means that the stepper motor loses a step. I usually
paint a small line on the motor shaft to quickly see if the extruder is skipping because that’s
when the shaft doesn’t turn continuously anymore. When only the filament jumps around,
but the shaft turns, it usually means grinding. In most you want a motor that skips before
it grinds on the filament. Why? Because when the extruder skips, you will get an inconsistent
flow for a bit, but there is a chance that your print recovers. If your extruder gears
severely grind, it’s hard to recover because the more material they grind away, the less
grip you will have – plus, the ground away material will clog your extruder gears, making
it even harder to grip into filament. Finally, there is material buckling which
usually happens with flexible materials. Here the column of filament buckles away under
the load, and when the filament path is not perfectly constrained, it will find a way
to get entangled in the extruder gears. Everyone who has already printed with flexibles will
know that phenomenon, which usually happens when printing too fast. Printer manuals will
often tell you to lower extruder tension when printing with these materials. Even though
one would think that this is due to the flexibility of the material and not squishing it too much,
another reason might be a fail-save feature. See, with lower tension on the gears, you
limit the maximum force you can apply onto the filament. So before the filament starts
buckling, it grinds and under extrudes a little. Due to the flexibility of TPU the teeth usually
don’t really bite into the filament so grinding often means just that the gears are slipping
but not really chewing away from the filament. It’s more acceptable for a user if you have
some under-extrusion on a TPU part rather than a complete print that fails because the
filament buckled. In terms of printing quality you want a feeding
force as constant as possible. This means that too little pretension is bad because
that can make the filament grind. When it’s too high you severely deform the material
which can make extrusions inconsistent, waste energy of the feeder and cause skipping. You
can either find your sweet-spot with a bit trial and error or by doing some extrusion
tests, where you’re looking for a pretension setting, that gives you very consistent results
over a broad range of flows. So which pretension setting gives us consistent
flows and the best hotend performance? I’ve done tests over the whole adjustment range
of the most common extruders. I tested Prusas MK3, which uses Bondtech gears, and that you
also find in many of the BMG clones. Then I tested the typical one-sided Ender-3 Extruder,
E3Ds Hemera extruder, the OmniaDrop, and Bondtechs new LGX, which is special because instead
of pretension, the LGX uses fixed distance gears which is something I’ve never worked
with before. I did my tests with regular DasFilament PLA. Of course, other materials behave slightly
differently. Still, many say the hard and brittle nature with a low temperature-resistance
makes PLA one of the more challenging materials to tune, and the overall results will be generally
comparable. I did a ton of flow tests with each of the extruders, from slow extrusion
speeds up to their limits to simulate increasing extrusion forces. After each test, I cleaned
the gears, adjusted the pretension from the lowest to the highest setting, and plotted
the achieved flow rates. Let’s start with my Prusa MK3S and its Bondtech
BMG gears. On the Prusa you adjust the pretension with an M3 screw. The lowest setting was when
the screw just cleared the nut on the other side. The maximum setting was eight more turns
in, which is 4 mm of travel. On the lowest setting, I got fairly constant extrusion up
to 8 mm³/s and then more and more under extrusion due to grinding. 2 turns in still wasn’t
perfect but definitely better. Interestingly, all other pretension levels resulted in the
same performance. Once the teeth properly bite into the filament, the system is very
robust and doesn’t crush PLA filament. The Hemera and the OmniaDrop extruder behaved
very similarly, with slightly lower performance levels at the lowest setting but then very
constant extrusion values if you slightly increase the pretension.
Before we take a look at the probably most common extruder used on millions of Creality
and Ender-Style printers, lets look at Bondtechs LGX. Here you don’t adjust the tension on
the filament but the gap between the two extruder gears. The gears are disengaged for loading
and unloading material in the first lever position. Setting 1 is where the gears are
the furthest apart from each other, recommended for PLA and all other non-flexible materials,
Settings 2 to 5, where the gap constantly decreases for increasingly soft materials.
In setting 1, I got really consistent results over a vast flow range until the extruder
started skipping. Setting 2 still was good, with a slightly worse performance at higher
flows. Anything more, and the results were devastating. When we look at the filament,
we can understand why this is the case. The lower settings nicely indent the teeth into
the part. The closer we bring the gears together, the more they bite into the filament until
they finally almost chop and severely deform it, which can be nicely seen in the whitening
marks when feeding ABS. The stepper motor uses most of its torque to deform the material
and therefore struggles to push it forward, which shows what too much pretension can result
in. Let’s finally look at the probably most
widespread extruder in desktop 3D printing – the one of the Ender-3. The old ones simply
had a lever that pushed the idler onto the filament without any adjustment, though you
can increase the spring tension by putting in some 3D printed spacers. Mine that I stole
from my Ender-3 V2 and under which I strapped a Mosquito Magnum on my testbench has an adjustment
screw that lifts a cap and allows a bit of variation of the pretension. The flow test
results were very interesting and worth all the efforts. Here I got fairly similar flow
performance over the whole adjustment range up to the point where the motor started skipping
and the dip starts. At these higher flows the underextrusion increased the higher the
pretension was. By removing the cap to futher decrease pressure I was even able to improve
the performance more, which goes along with some of the comments I found online where
people solved their extrusion issues by lowering and not increasing the pretension on their
Ender extruders. So keep that in mind and if you have problems try both increasing and
lowering the tension to find your sweetspot! Let’s also take a quick look at flexibles
to see if we really get better results for lower tension values. I tested some soft 30D
TPU from Fiberlogy with the Ender extruder and the Prusa that uses the Bondtech gears.
The Ender extruder didn’t perform very well because of the barely constrained filament
path but I had slightly better performance with low pretension because the gears started
grinding before they buckled the material and pushed it out to the side. On the Prusa,
the TPU results were very similar regardless of the extruder pretension. The material always
started buckling at 9 mm³/s with slightly better performance at the lowest pretension.
So after all of this how should you handle extruder pretension? If you don’t have any
problems, leave it as it is. If you see symptoms like a grinding or skipping extruder check
your filament path at first because I promise in 95% of cases, the problem comes after the
extruder! So, is your Bowden tube okay? Are your extruder gears clean and not worn down?
Is your nozzle not blocked? Is your hotend fan working and preventing heat creep? Is
your nozzle temperature high enough? Are you printing within the limits of your system?
Did you load the right filament? From my experience, all of these points were the cause of extrusion
problems in the past, and only in very rare cases have I touched extruder pretension.
But if you feel the need for changing that setting, I’d first extrude a bit of filament,
pull it out and check the surface. If you don’t or only barely see imprints of your
gears, increase the tension. If you see severe deformation instead, reduce it. Soft filaments
like ABS, ASA, or Nylons might need a little less, rigid filaments like PLA, or even fiber-reinforced
materials a little more. Flexibles shouldn’t be squished too much, and with less tension,
you might also have a fail save feature against buckling. Load and extrude some filament at
reasonable speeds. I’ve added some test g-codes for that on Printables.com if you
don’t know the G-Code commands by heart! Start with the default setting and then significantly
decrease or increase it. If your extruder skips, reduce pretension a little; if it grinds,
increase it. In my experience, extruder tension is not so touchy that you need to tune it
in, to a quarter of a turn of the set screw and all the tests I did here prove this. If
all of that doesn’t help, recheck the points I mentioned before and simply feed some filament
by hand to feel how much force you need to make sure that it flows easily! And that’s
it – I hope this helped you understand a little better when extruder pretension is
important and when not so much and how you need to adjust it in the future!
But what’s your experience with extruder tension and your method to find the right
level? Let me know down in the comments! Thanks for watching, everyone! I hope you
found this investigation interesting! If you want to support my work, consider becoming
a Patron or YouTube member and check out the other videos in my library! I hope to see
you in the next one! Auf wiedersehen and goodbye!
