Why do Intersecting Shells Suck for 3D Printing 3D Printing 101 Video Lecture

recently in my gam body review of the
power armor model from fallout 4 I
mentioned in that review that
intersecting shells were bad and by
doing so confused quite a lot of people
who one didn't really understand what
shells were or two didn't really
understand why ten-second shells could
be bad for 3d printing and three
wondered why I would want to combine the
model into one shell instead of using
them as separate shells so in this video
I want to cover what shells actually are
when it comes to 3d printing and also
why intersecting shells are bad for most
3d printing applications and it's worth
mentioning that I am completely
self-taught when it comes to stuff so
the terms I use will probably not be
100% accurate so you probably shouldn't
use me as a reference for your paper at
University on intersecting shelves in
STL files for 3d printing or maybe you
should I don't care
let's get started already welcome back
to making these guys so what I have here
is mesh mixer I'm going to use mesh
mixer to demonstrate what shells are
when it comes to 3d printing and working
with mesh files particularly in this
case STL files so I have this bunny here
and the first thing you need to
understand with STL files is the fact
that an STL file is made up of a lot of
triangles in 3d space if I hit the W key
it's going to show the wireframe and you
can see this model is composed of
hundreds of triangles and this is the
backbone of the STL file format it's
basically a lot of coordinates which
make up a a file and those corners
pertain to where those triangles are in
3d space so you could have a single
triangle in 2d space with reference to
an origin that would be an STL file STL
files don't even remember what units
they were created in for example I could
create something in inches and export it
as an STL file and import it into a
slicer which imports in millimeter units
and the scaling would be completely
wrong there is no data in the STL format
to preserve that all you have are these
3d triangles in space a reference to an
origin so what does this have to do with
shells then well before I even get into
that let me show you the bottom of this
bunny so what you can see here you
as you can see a hollow cavity into this
shape which means this is not an
enclosed area so it's 3d printing it's
the jobs the slicer to take a STL file
of an enclosed area and then work out
it's going to slice that put in fill
perimeters that's the thing to make that
object in this case this object is
actually known as a surface and that is
actually specifically a zero thickness
surface so in the mathematical term for
zero thickness it has no thickness and
that means that this can't be physically
retreaded in its current form it's
impossible to make in the real world you
can't have saying that zero thickness so
I'm going to turn this surface into a
shell and to do this in mesh mixer I'm
basically just going to cap off that
bottom part so I'm going to repair it
and it's capped it off so what we have
now is a shell also known as a manifold
mesh or a watertight mesh now the
water-type analogy is actually quite
good matching your sinuses water and you
want that water to contain within the
object and not leak out so watertight
mesh means there's no holes or gaps
which is what you want for 3d printing
so we have a shell but what are multiple
shells well I can take this object here
in the object browser we see it and I
can literally just duplicate it
duplicate duplicate duplicate duplicate
I've now created multiple instances of
this shell
otherwise known as multiple shells and I
can demonstrate that even clearer by
moving some of them around so I'll move
one there and I'll move this one here
and then for example move this one here
and last one let's move him ah
what else okay there we go so what we
have here are multiple shells we have
multiple instances of our bunny but
let's have a look at the water-type
analogy would this still be watertight
well yes actually because they're all
watertight and are all separated from
each other which means that the 3d
printing slicer I can easily understand
what should be filled in with infill and
perimeters and what should be left with
nothing but what if we do this instead
of having the objects separated with a
gap between them we've just rotated them
around each
and they are still in the same area and
they are intersecting this is known as
intersecting shelves and this is where
things can start becoming a bit of a
problem for a slicer the reason being is
the STL format will not prevent you from
doing this so for example I can select
all of these objects and combine them
and now I can export this as an STL file
there's nothing preventing me from doing
that so to the STL file I just saved out
I brought it back into the mesh mixer it
is a single STL file which I called lots
of bunnies and it is indeed a lot of
intersecting shells so how could this
become a problem
for a slicer well I'll demonstrate it by
trying to plane cut this model so with
plane cut what it tries to do is work
out where it can cut an object and cap
it off so because these objects are
intersecting each other it doesn't know
what to do these member these triangles
are just in space they have zero
thickness all they're doing is trying to
define and in closed manifold shape and
by intersecting them like we've done it
makes things very complicated so as you
can see here the auto solved for the
plane cut isn't even working and if I
try to accept it's going to end up with
stuff like this which is the
intersecting shells all creating their
own faces over each other and this is
pretty nasty geometry again it's still a
legit STL file that you could save off
but it's going to lead to very nasty
files when you bring them into your
slicer so having said that let's bring
this file in to simplify 3d and I'll
show you where intersecting shells can
cause issues with your slicing right so
I've got a lots of bunnies STL file in
the slicer don't worry about support
material or anything like that I don't
care at this stage I've got my default
process for my mark 2 pressure mark 2
and I'm going to prepare to print so
first thing you'll notice is it does
resolve it will print something which is
fantastic which is what we want but
looking in closer you'll notice a few
anomalies if I let it play layer by
layer you can see what a mess it's made
so what the program is trying to do is
it's trying to work out what isn't
enclosed and enclosed space and
what should it fill with infill and what
should at least separates what it's
actually doing is discreetly separating
these different intersecting areas
intentionally and really you're going to
end up with a model that's going to fall
apart as soon as you print it and it's
not going to add all look like what you
intended because we haven't combined the
shells the shells are intersecting now
because this is such a prevalent problem
a lot of slices have come up with ways
to try to deal with intersecting shells
so it in simplify study for example if
you go into advanced there is slicing
behavior for non manifold segments where
we can discard or heal but also we can
merge all outlines into a single solid
model well that means it's going to
disregard the fact they're all
intersecting terrible but it's most
ignore that as going to merge all
outlines into a single solid model which
is actually a setting that can body
remain recommended I picked for their
model which had intersecting shells so
that's great it's going to fix all our
problems right well no we've actually
fix some things and introduced other
issues so if you remember the setting is
that it's going to merge all outlines
together which means we're losing
internal details as you can see here
zooming down it's kind of trying to
figure out what it should join together
and where it shouldn't and you've got
like a big fill there you've got like a
void there you've got these gaps and at
the top of the model you've got these
filled in areas where it's trying to
figure out I'm merging outline therefore
that area must be filled so that's a
poor solution as well so what should you
do if you find you have intersecting
shells and you need to print that model
well in simplify study at least you can
do it but it can become very tiresome if
that model has lots of intersecting
shells in this case we only have the
four bunnies what I can do is go to mesh
and separate connected surfaces which is
going to separate out all of our shells
so I've got that one that on that one
that one and now we can print it and
because we've told simplified show you
that these are separate shells it now
knows what to combine and what not to
combine and the model will be created as
it should be but what about the t60
Armour why is having multiple shells in
this just terrible well a lot of you
raised the point that having multiple
shells will make it easier to break
the model up for printing which could be
applicable in some cases but the problem
is these details intersect each other in
the actual model v I printed them
separately they wouldn't actually go
together because the physical objects
can't just phase through each other like
ghosts that's if you can't translate the
digital file to the real-world file by
using the shelves as they're provided
another is tricks to then use boolean
operations to cut away but the problem
is this model just has far too many
shells like every single screw on the
top of the helmet is a shell every
single detail added in is a shell every
you know they're not combined into
logical areas now you could do it
yourself but are you really going to do
that with over 200 individual parts I
don't think so so I guess the final
point is how should you fix intersecting
shells if you come across them well you
can use some free software to do it for
example you can use mesh mixer to join
together inspecting shells using the
Berlin Union if they're just a couple
it's not the best but it will work and
you can also use them make solid tool
which uses a lot of processing power but
it's great for really broken meshes with
lots of intersecting shells also you can
try the netfabb cloud service which I
believe is back online now or the 3d
builder service if you have Windows 10
those services will stitch shells
together with almost no damage to the
overall mesh and you'll end up with a
mesh that has no issues at all printing
because all the shells have been
combined so there you have it guys
that's what a shell is and that's what
intersecting shells are and that's why I
think they're a terrible idea that's
really printing I hope you found this
video useful guys here on make his muse
and he sort of see features 3d print
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it was meant a lot to me yeah it's
really hot today I've had to turn the
fan off
I put an industrial to do this video so
thank you so much for watching guys I'm
going to turn it back on and I look
forward to seeing again very shortly
here on makers view yep sorry guys bye
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