This episode I'm goign to show you why your TPU prints are failing, we're going to look deeper
into the physics involved and why it makes TPU such a pain to print, what you can do about it,
and how to really understand what's going on between your extruder and your hotend.
This video is sponsored by PCBWay and we will talk about that later on.
TPU, the bendy wendy boingy woingy filament that we have all come to
hate and somehow we still want to print with it,
how fickle it is, with its stringing and its....houdini level escaping skills.
So without further ado....
Oh slight ado actually. As I always say on these engineeringy videos,
I'm conflating units as usual. When I say pressure I mean force,
when I say force I might even mean weight, who knows, feel free to complain in the commments,
but I'll still do it. And the other thing, a lot of this is going to be wrong,
I'm not an engineer, etc, etc. Ok anyway, let's go ahead and insult physics one more time.
[Elastic Plastic]
I've said this before but I want to flesh it out and show you an experiment in a moment, but we
need to talk about elastic potential energy. We'll start with this thing I made - I'm gonna call it
uh...a scale with a bowden tube going to it. It's not calibrated to give exact amounts but
you know what? It works perfectly well, hooking it up to an extruder means you
can extrude directly onto the scale, well directly through a bowden tube,
but what this lets us see is the kinds of forces we get with TPU. With the aid
of steve here who is a bowden printer we can load up 95A filament and turn on the scale.
As you can see as soon as the filament hits the scale it starts to rise. Not much thought is
given to the forces involved in tpu printing but they aren't small are they, a couple of
hundred grams is probably what I'd assume the normal force is applied to the filament.
What I wanted to show with this setup was what happens at the extremes and this is the
first point I want to make, why you shouldn't yell when TPU escapes. If I extrude a ton of
filament and this would simulate fast printing that's at the edge of overwhelming the extruder,
we see forces around 1kg at the nozzle, but interestingly it peaks around that mark and
then just won't go any higher. It's not a fault of the scale,
what is happening here is the tpu is slipping in the extruder. If we leave it,
it slips back down every time we extrude it, and once it starts slipping it seems to find
a path to less friction presumably because it's deformed the filament around the gears.
This is why in my opinion you should not be in this situation ever while printing TPU. If you
have enough force to be in this critical area of operation then you risk slipping
constantly which will mean underextrusion or worse, the extruder will chew enough of
the filament on the gears that it just won't have any grip to then move when the pressure
drops down. What does this mean in the real world? Slow down, but also, controversially,
and this is hugely controversial, but I think the extruder should be tuned to slip,
not to grip. You ideally want it to slip without damaging the filament if it hits an
undesirable level of force. So that thing where you want to dial up the tension for TPU? Don't.
[May a negligible amount of the force be with you]
If I load in some 70A filament into steve we can see what happens and get some insight into exactly
why you can't really print filament this soft on a bowden like this. It's literally impossible and
finally I can give you an exact answer why and honestly this surprised me as much as anyone.
We know from the previous experiment that the extruder is capable of forces in excess of 1kg,
I'm not saying newtons, I want you to understand clearly what
I mean, but we see in the case of this setup that the scale is only seeing a few grams.
It's not broken, this is literally all we can get at the end of the tube. And
it's happy to slip back down as soon as it gets chance. It's pretty easy to see that in
the case of a bowden tube setup this isn't enough to be able to push out the filament.
Where does all that extra force from the extrusion go? I cut a window in the side of a PTFE tube and
watched it under a macro lens. What you see here is the filament expanding into the tube, this is
presumably where it stores most of its elastic energy. Oh it's also probably why capricorn
tubing with its smaller internal diameter might be really worth the money. In theory at least.
[When retraction doesn't retract]
One thing this scale based experiment really showed that I didn't plan for
but makes total sense when you think about it, I'll show you.
In this TPU 95A scenario we're happily extruding away, the working pressure could be let's say,
whatever's on the screen now, a few hundred grams of force's worth of pressure. Yeah I'm
saying this on purpose, can't wait for the comments section. Everything is going fine,
the extruder isn't exploding TPU, nothing is slipping, but we need to do a travel move,
and so before we travel, we retract. Retracting 5mm of filament....
...doesn't retract the filament. If you think about it,
this is because you're just letting out some elastic potential energy from the system.
To actually retract you need in this case to retract ALL of the stored energy before
the filament even needs to think about moving, and then because duh it's flexible,
it can start storing energy the other way until it pleases itself and starts to actually move.
This is by the way why we use ptfe tube, less static friction means less of this, but it's still
a ridiculous amount for TPU - it takes in this case another 5mm to get to zero extrusion force.
And yes - this is why you get stringing on TPU. I know. Retraction isn't retracting _a lot of the
time_ Mind blown. But that's not all - this is why you need longer retraction length settings!
Speculatively I also feel like I could make a case for retraction settings being dependent on
higher working pressures, so higher speeds, but that's luckily...perhaps, solved in a moment.
[When deretraction doesn't deretract]
This is actually really interesting. If you think about all the elasticity we
had to unwind to get the filament to move you start to naturally think about what
needs to happen to make TPU flow again after a retraction. Obviously to add it all back again,
right? Yes. Exactly. So deretraction isn't just poking filament out of the nozzle either,
it has to recreate the same pressure level we had before retracting.
[So...] Right, so when you start printing from zero pressure,
you won't get the correct flow until you reach some kind of equilibrium.
This is really complicated stuff, but if you imagine you have a hole at the end of the
nozzle that filament is somewhat able to eject from, and you have pressure inside the nozzle,
the pressure will increase as a result of extruding but it's highly likely that the pressure
won't decrease from the nozzle at the same rate. So your extruded line will start underextruded.
As to how much this is a case is going to depend on a lot of things,
but this is kind of going into the territory of...linear advance,
a term you may have heard of. So let's go there. Linear advance. Let's do it.
[No Pressure]
Pressure advance and linear advance are the same thing.
I said it. haha. Honestly they kind of are,
or to be specific, they are solutions to the same problem so to all intents and porpoises
they are the same thing. What problem though? Well...the one we just mentioned. I will show you.
Here is some dodgy gcode I wrote in about half an hour that just makes some lines,
I'll get onto why I didnt use the marlin one in a minute. The lines do something a
slicer typically won't though, they speed up or slow down mid move for no reason. Why? To
show why we need linear or pressure advance, in a creative but not entirely real world way.
First line at the bottom...hang on...yeah why reprint when you can just use a uv light, right?
First one at the bottom is a line that doesn't vary in
width. It does, but we'll ....get back to that. Second line gets faster in the middle.
See that the change in speed causes a change in width? It shouldn't. There's no maths in the gcode
to tell it to do that, extrusion is calculated over the length not the speed, it should be the
same thickness but it's not. Same again for this one, but it's faster. Then the opposite, slowing
down in the middle, and what do you know it's fatter. Again it shouldn't be so what's going on?
Well I'll show you the same thing on a direct drive that managed to extrude a lot more by
not slipping and we'll talk about that in a moment too. While I show you this,
I'll explain what's going on. You see...an extruder, or rather, the extrusion system
as a whole, say it with me, is not a linear system. Especially not with TPU but with any
filament as it happens. You're pushing stuff in at one end and that's causing stuff to come
out the other and honestly with TPU it's a miracle you get even remotely what you ask
for in any kind of reasonable time frame. It's probably not a bad analogy to imagine a very
long train pulling away from a stop. Think about how long it takes for the rear carriage to start
moving, well, that's not a linear relationship between the speed of the engine at the front
and the distance the rear carriage moves is it? Eventually, sure. But this here isn't eventually.
Linear or pressure advance is a crude, am I allowed to say crude? It's condescending,
right? Anyway it approximates the filament, we'll do linear advance, which approximates it as, of
all things, a spring. And that's why there's a k value. The mystery k value, is a spring constant.
Hooke's law? F = -kx? coincidence? No. It's not a coincidence, this is exactly the principle that
linear advance is based on. When you push or pull an elastic material it obeys hooke's law,
assuming of course it obeys hooke's law, and so you can model that extra amount of
extrusion you have to do to get the exact amount of movement by subtracting the elastic...ation.
If that's all too much then we can use the easy explanation which is that you have to
push harder than the amount you would need to if the filament wasn't boingy,
and how much harder depends on how boingy the material is.
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[It's not just Linear Advance though is it?]
No. It's not.
This is what a speed change looks like on PLA and if you notice what happens here,
changing speed is recoverable pretty quickly.
TPU sorta does that if you use more normal parameters, but it's still quite evident
at least from these results that it will be very hard to truly tune TPU to behave
well for all speeds. (learning outcome: tune TPU for one narrow speed range, stick to it)
Now either I'm using the marlin linear advance tutorial wrong or it doesn't work well with TPU,
I suspect it's the latter. If you want my advice then just start with a value
like 0.2 and print cubes between that and 0.8 and pick the best one. You can
clearly see which one is best. Remember to print at your normal TPU printing speed.
As we just mentioned above the k value will increase if the material is more flexible,
so of course once we know that it makes perfect sense that a k value of 0.07 for PLA which is the
default on my sovol here, is not suitable for TPU. That's why it needs around 0.4 to 0.6.
So this video is about why TPU fails and how to fix it and in this case it's not that evident
what that has to do with linear advance, fine it improves quality , but if we think about it,
properly tuned linear advance is one really huge advantage against incorrect pressure at
the extruder especially at the end of a print move, and we'll get to why that's bad. But also
remember that having the correct pressure _at the end of a print move_ is key to tuning retraction.
[stop telling people not to retract] And for the last time, can we all stop telling people
not to retract tpu. Seriously. Just no. Why would you do that?
By the way, another thing that's obvious when you know it, a properly tuned linear or pressure
advance value will mean you can, and should, reduce your retraction length. Retraction is
about removing the nozzle's urge to ooze while moving and linear advance of course aims to
balance the pressure at the end of a printed line so it's all connected, everything is connected.
[Bowden?] Okay. It's 2022. The market for 3d printers has changed hugely in the last year and I will make
a video on that later but I feel like we need to have a talk about bowden. If we wind back to a
year or so ago, I was making videos on how to manage TPU in a bowden printer, and this is great,
of course it makes the seemingly impossible work, but I hope we've seen from the demonstrations in
this video that a lot of the problems with printing TPU are compounded way too much
by that foot or so of tube. Almost every new printer I get in for review now is some kind
of direct drive or another, even the last bastion of bowden the elegoo neptune range now has a pro
version with direct drive, looks cool doesn't it, review coming up on that. So my advice is,
and honestly this is hard because I know that a lot of you still have Steves or equivalent,
but if you want to print TPU well, and a lot, without frustration, you have to ditch the bowden.
Its not that bad though, I mean, a reel of TPU costs £20 and you'll waste a good amount of that
tuning and failing potentially, Around the cost of two reels and a couple hours could be spent
on something like this DD upgrade, to me it's a no brainer. Bowden has had its day generally,
and for TPU? It's just unnecessary pain that can be solved by using a direct drive. Of course,
for occasional TPU printers it doesn't make sense, but you get the idea.
[No linear advance?]
And the same advice goes for those of you on machines like the enders
with the board that doesn't have linear advance. If you have a steve, I would advise having a look
at mris.coc...'s firmware which actually has started enabling linear advance on a steve,
I know a lot of people think that these boards don't have UART connected so they can't use linear
advance but surprise! That has absolutely nothing to do with it. It's complicated,
but - linear advance on a steve is something I hugely recommend looking
at. It's a win win anyway, it hugely improves print corner quality and stringing on PLA too.
[So why DOES my tpu fail and what CAN I do about it?]
I think the main reason your tpu is failing is because you treat it like PLA. It's imperative
when printing TPU that you understand and manage the pressure in the extrusion system as a whole,
and the answer to what to do about it is to recognise what you're seeing and know why it's
doing what you are seeing. Filament escaping the extruder is a perfect example of this,
sure, some extrusion systems are literally impossible for achieving the right nozzle
pressure for printing TPU but in my experience, most are not, and the escaping filament,
that's on you. So I'm sorry to say that there's no quick fix, you have to become one with the
TPU. Every video I make on TPU I personally learn a tonne of new things about it, it's a huge topic.
[Bye]
I think the future is looking very good for TPU, some of the modern extruder
hotend combinations I've seen lately like the creality sprite and flsun are outstandingly
well engineered for dealing with the constraints of TPU. Not you bambu ams.
Anyway that was yet another TPU video, does your brain hurt? Mine does. Now go loosen
your extruder screw a bit, tune your linear advance if you have it and print something.
Stay where you are, theres an out take ;)