If you are choosing a material for your next
3D printing project you might be thinking
about, which is the strongest material out
there.
Depending on the requirements in your project
ultimate strength of your part might not be
the most important you are looking for.
Stiffness is also something that could be
really important.
In order to see how different materials compare
in that regard, I built myself a testing apparatus
and analyzed the most common 3D printing polymers
for their rigidity.
Guten Tag everybody, I’m Stefan and welcome
to CNC Kitchen!
Before you start complaining and tell me that
we could have simply taken a look at the Young’s
Modulus in the datasheets let me tell you,
that data sheets often are a bit sugar coated
because they sometimes use test values not
from 3D printed samples but from injection
molded ones.
Then there is the point that 3D printed parts
have print direction dependent properties,
especially if we take a look fiber reinforced
once and at last, just taking a look at datasheets
doesn’t make a good video.
So for this video I’ve been taking a look
at the most common 3D printing materials which
cover PLA, ABS, PETG, Nylon and Polypropylene
but also included special ones like annealed
PLA, Carbon fiber reinforced PETG and something
pretty cool that is Nano Diamond reinforced
PLA from Tiamet3D where I was sent a roll
to play with.
So the stiffness of a material is characterized
by the Young’s Modulus which is the initial
slope in a stress-strain diagram and often
measured in a tensile test machine.
The Young’s modulus or tensile modulus is
the proportionality factor between the load
on a material and the resulting deformation
or strain.
A material with a higher Young’s Modulus
is stiffer or more rigid so a higher amount
of force is needed to deform a sample.
For comparison, the usual Moduli of the plastics
we commonly print are between 1000 and 3000
MPa, Aluminum is already by an order of magnitude
stiffer with around 70000 MPa, Titanium is
at 110000 MPa and Steel has a young’s modulus
of over 200000 MPa.
I’m usually analyzing that property for
my filament tests but with the test equipment
I built myself it’s quite hard to measure
the tiny displacements happening.
Another method is to use a bending or flexural
test.
For a three-point bending test you load a
sample that sits on two supports in the center
and measure the displacement for a given amount
of load and then calculate the bending modulus
with these values and the dimensions of your
specimen.
There are standard for that like the ISO 178
and I did my best to recreate that setup as
good as possible but in the end the dimensions
and setup I used doesn’t fully comply with
these standards so be aware how you use my
results.
I designed a jig in Fusion360 where I added
slots for dowel pins.
These pins are exactly spaced 100mm from each
other and will act as bearings.
The sample itself will be loaded with this
hook shaped part where also a dowel pin will
transmit the force to the sample.
The lower hole will be used to connect a bucket
where I’ll add weights step by step.
The sample itself is a simple bar that is
120mm long, 12mm wide and 4mm high.
All of the samples were printed on my CR-10
with the Titan Aero extruder and a 0.4mm nozzle
at 0.2mm layer height, 5 perimeters and 100%
infill.
All of the CAD files are by the way available
on Thingiverse if you want to build one on
your own.
If we take a look again at the formula that
will be used to calculate the bending modulus,
we’ll see that especially the thickness
is very important because it goes into the
equation by a power of 3, so a small deviation
from the nominal value will set off the results
a lot.
For this reason, I documented all the dimensions
of the samples and also weight them before
the tests.
This is especially interesting for the annealed
PLA specimens.
Not only do we see that it is less translucent
than the unannealed one but it also changed
it’s shape quite a bit during the crystallization
that happened during the annealing process.
It got shorter but also quite a bit thicker
which definitely needs to be taken into consideration
for the test evaluation.
The test setup is pretty simple.
I placed the jig on my vice and leveled it.
The specimen is placed on the dowel pins and
the U shaped hook will load it in the middle.
I placed a dial gauge above the hook that
will measure the deflection of the sample.
I attached a bucket in which I’ll successively
place soda cans that each weigh 353g so will
load the sample with 3.46N.
So I always added a can, noted the deflection
and then added another one up to 4 which worked
very nicely.
The only problem was the PP sample that was
so compliant that measurements were hard to
make and I had to stop after the addition
of 2 cans.
The results were very nice because you can
see the linear relation between load and deflection.
The more shallow the curve the more rigid
a material is.
If we now calculate the bending modulus which
also incorporates the dimensions of the samples
we get to the following results.
As expected PP is by far the least stiff material
with a modulus of only around 300 MPa, my
Nylon was at 1000 MPa.
ABS and PETG are at roughly 2000 MPa and PLA
is even an additional 50% stiffer with 3000
MPa in the as printed state and even a bit
stiffer if you anneal the material.
The carbon fiber reinforced PETG doubles the
stiffness of normal PETG and the new boy on
the block, nano diamond reinforced PLA is
the most rigid with almost 5000 MPa bending
modulus!
I told you in the beginning that I also weight
the samples to calculate the density.
If you want to have a lightweight structure
that is stiff you might be interested, which
material gives you the maximum amount of stiffness
with the least weight.
The orange bars in this plot show the weighted
stiffnesses by density where some small things
change because Nylon due to it’s lower density
gets a bit better and PETG loses in comparison
to ABS due to its higher density.
The rest stays pretty much the same and the
reinforced materials still win.
But keep in mind that especially for the reinforced
materials the stiffness can be highly dependent
on the printing orientation, so you need to
put some thought into their application otherwise
they are not better than their unreinforced
counterpart.
So I think I was able to give you a nice overview
about another important mechanical property
of 3D printed materials.
So if you have an application where stiffness
and not strength is important choose a suitable
material.
But also keep in mind that the stiffness of
a section under bending increases by the power
of 3 with its thickness, so just adding a
bit more thickness to your part might even
have a much higher impact!
Let me know down in the comments what you
think about the test and the results I got.
All right guys, thanks for watching and if
you have learned something hit the like button
and consider supporting my work on Patreon.
Subscribe if you aren’t yet and select the
bell to get notified when new videos are released.
I hope to see you in the next one, auf wiedersehen
and until next time!
