In this video I’ll highlight some of the
engineering design of this machine: the Ember
Precision Desktop 3D Printer.
It was manufactured and sold by Autodesk,
but now discontinued.
All its details, though, are open sourced
and so it might well live on.
There are many way to perform 3D printing,
but they all have in common this: they take
a 3D model, cut it into many thin, two-dimensional
slices.
And then . . . they print these slices one
at a time, one on top of the other to create
a three-dimensional object.
Three D printing promises a revolution in
manufacturing.
One company that uses 3D printing to manufacture
turbine blades reports that it reduces pollution,
take a quarter of the time to develop a new
blade — partly because of rapid prototyping
with 3D printing — and that they can repair
the blades 60 percent faster.
Futurists have imagined mobile production
plants rapidly deployed to disaster zones,
that can print arm splints, tent stakes, and
even buildings!
One company has pioneered 3D printing of concrete
buildings.
While this promise is enormous, today I’m
not creating anything exotic, but focusing
on printing this: Odile the Swan.
It’s one of several items used to test and
benchmark 3D printers.
On this printer the swan is created here,
on the lower side of what is called the “build
head” or “build platform.”
Odile will be printed upside down here — like
this.
The printer will first build the base and
then work its way to the swan’s head.
Let’s watch the printer in action.
The print head descends into this amber tray
. . . which contains a liquid, called resin,
which will solidify . . . there you can see
the print head settle into the resin … and
let’s watch the action right here on the
print head . . . here’s a close up . . . the
printer exposes the first layer — that’s
the flash of light — then the trays slides
sideways. . . and then returns so another
layer can be added.
. . You can’t see anything yet, but I’ll
speed everything up and then …. There . . . you
see in about thirty-five to forty minutes
the base of the swan appearing — remember
it is printing upside down… and by about
an hour the swan is done!
Let’s examine the key steps used to print
this swan.
Specifically, I’ll look at how that flash
of light creates a solid layer.
Why the machine’s build head moves upward,
and then explain why the tray moves sideways.
That will take us through the essentials of
the machine’s design.
After that I’ll look at a few details of
the chemistry of the resin.
Let’s start with that flash of light: the
flash creates a solid layer.
Here’s how.
The tray is filled with liquid resin.
It solidifies when exposed to blue light,
which appears green when photographed through
the orange tray.
That orange tray shields the resin from room
light, which might solidify the resin inadvertently.
To show you how this works . . . I’ve filled
this watch glass with a bit of the resin — I
printed the swan with clear resin, but here
I’ve mixed in some black resin to show solidification
better.
Watch what happen when I shine this blue laser
on it.
. . . almost instantly the resin solidifies
— its called curing.
What I’m doing with this laser pointer is
pretty crude, so to aim light precisely this
printer using a DLP Projector — DLP stands
for “Digital Light Processing.”
The tray has a clear window, so light flashes
can cure a layer.
This is the base of from a printer we took
apart so I can show you what’s inside.
Sitting on the base is the amber tray that
contains the resin.
Note that the silicone-coated window of the
tray is positioned over a window in the base
. . . And inside . . . there’s a bundle
of electronics that contains a powerful LED
— a light-emitting diode.
It produces blue light of a narrow range of
wavelengths . . . then some optics spread
that beam of light and shine it onto . . . a
device called a micromirror that creates the
light pattern appropriate for a particular
layer, then a mirror reflects the layer pattern
through the window in the tray and onto the
resin.
Micromirrors were developed by Texas Instruments
for use in projectors for computer.
This the optical train of the printer.
You can see the micromirror if I take off
this lens and look down the barrel.
It’s a small chip about three-quarters of
an inch by a quarter of an inch.
It is comprised of a bit over a million tiny
mirrors — each about 8 microns by 8 microns
square — recall that a human hair is some
eighty microns! — a micron is a thousandth
of a millimeter.
This drawing shows two of the mirrors.
Each of the mirrors can be controlled separately.
An electrostatic force generated by a small
voltage pivots a mirror plus or minus 12 degrees.
This directs reflected light either onto or
away from the resin.
Because these mirrors are so tiny light can
be directed to fifty micron sections of the
layer, so the printer has a high resolution
in the xy plane.
Underneath the mirrors is a slender post — it’s
a mere one micron or so — and below that
an elaborate, yet tiny hinge.
It would seem that such an assembly would
be fragile.
You picture a million mirrors clattering back
and forth as this machine is moved.
Yet, that’s not true: the mirrors are so
small that they don’t respond to the vibrations
and shakes from normal handling.
Nor, does their pivot wear out easily: tests
show that the mirrors could flop back and
forth for eleven years of continuous operation
before the pivot fails.
To see the layer images created by this micromirror,
I’ll put a yellow card right here where
the layers are created.
Remember that what we are looking at here
are the layers of the swan, which are printed
one-by-one on top of each other.
As we watch the layers, keep in mind the printed
swan: here the thin red line in the yellow
block on the swan indicates the cross section
being printed.
In the projected images the bright blue areas
are where light is reflected on the resin
and cures it; the dark areas reflect no light
on the resin and so it stays liquid.
Notice the round circles: these are the support
posts created when the swan prints — there’s
more easily seen here when highlighted in
blue.
These are later snapped off the swan.
Let’s sped up the printing and watch the
swan being formed … all the tiny circles
coming up are the support posts at the very
bottom … here the base prints … and then
the body … you can see the wings begin to
take shape …
these are the top of the wings … the round
blue dot on the right is the swan’s neck
… and then the blue flashes disappear as
we reach the top of the swan.
Now let’s turn to why the printer’s build
head move up rather than down.
It’s called “bottom-up” printing.
This dramatically reduces the amount of resin
needed to print the swan.
When using light to cure resin there are two
main ways of doing this: the bottom-up — used
here — and top-down printing.
In top-down, a platform moves down into a
vat of resin while light is projected down
from the top.
The great advantage of this top down is that
it is easy to expose a layer to light and
cure it, but it also has a very big drawback:
the vat must be as tall as the part to be
printed.
And this requires a large volume of resin
which can be very expensive particularly if
the resin has a finite lifetime.
In contrast, the bottom up method uses a shallow
tray that requires a much smaller amount of
resin, even for tall parts.
I find it stunning to watch a large object
being drawn out.
Next, let’s look at why the resin trays
slides sideways after each layer.
In a bottom-up print the layer is built on
a window.
This machine would fail if the cured resin
stuck to the window.
Most 3D printing resins have the property
that oxygen hinders the chemical reactions
that cause them to solidify.
To allow oxygen into the layer this window
is made from silicone -- a material highly
permeable to gas.
Oxygen residing in the window diffuses into
a thin layer of resin just above the window.
This layer is 5 to 50 microns thick.
But the concentration of oxygen in this layer
is enough to prevent the resin curing directly
on the window.
Yet even with this silicone window, the newest
layer will still partly adhere to the window.
If it does then no fresh, uncured resin will
be able to be added — and so no new layers.
So, the newest layer must be separated from
the window.
There are several ways to do this.
Some printers just pull the layer up.
Yet this “direct pull”, as it’s called,
can create problems.
This layer is like a suction cup.
Now, you can see that by putting a cup in
a puddle of liquid on a plate.
As I lift the glass I can feel some resistance
— you can even hear when it separates.
And so in a 3D printer to separate layer and
window requires a large motor to lift the
printhead.
This action might damage the layer.
I find it easy though to slide the glass to,
raising it slightly it is follows the contour
of the plate.
In general, the force from pulling up scales
as the fourth power of the radius of this
opening, while sliding scales as the square
of the radius.
So, typically sliding requires one-hundredth
the force of pulling up.
And for this reason this printer slides the
tray to separate layer and printhead.
Other printers peel the layer off with a rocking
motion.
The key takeaway here is that printing parts
with large cross sectional areas is very difficult.
This lattice has a fine, detailed structure
and looks impressive, but because it’s mostly
open space it’s actually one of the easiest
types of objects to 3D print.
That’s why demos from 3D printers rarely
show a thick, brick-sized object like this.
The forces are much greater, print speeds
are slower and it’s impossible to print
for many stereolithography 3D printers.
We’ve been looking at the mechanics of how
the printer works, but this printer relies
on both mechanics and chemistry.
The combination of precise motion, micromirror,
and fine-tuned chemistry creates the resolution
of this printer.
The resin contains three main ingredients.
First, there are two types of molecules that
will together form the rigid network.
These molecules come in two sizes: a monomer
and an oligomer — the latter is just three
or four monomers bonded together.
Second, a photoinitiator that, when struck
by blue or UV light, starts a chemical reaction
that links together the monomers and oligomers.
This solidifies the liquid resin.
It is the balance of the monomer and oligomer
that yields a piece that is rigid and well-printed.
For example, here’s a tiny boat — it’s
often called “Benchy” -- that we printed
using only momer, no oligomer.
If I squeeze it you see that it’s no longer
rigid, and in fact, it’ll even breaks.
And equally dramatically is leaving out the
the third chemical — a UV blocker.
This chemical prevents the blue light from
penetrating too much past the layer being
printed.
Here the boat is printed with the UV blocker.
You can see the resolved details in the boat.
Keep your eye on the circular hole in the
middle.
Here is the same boat printed without UV blocker
. . . compare the hole here and here …. Without
the UV blocker the light cures regions of
the boat that were meant to stay liquid and
lowers the resolution significantly.
There’s of course much more to be said about
an engineered object like this, so I’ve
linked to other videos that might interest
you.
Including several that describe the micromirror
in detail.
I’m Bill Hammack, the engineerguy.
