It is vital to determine the precise amount
of light energy, or exposure, needed to cure
a 3D printable resin.
Why?
Let’s say we have model with small positive
features.
These square posts range from 500 to 100 µm
in width.
And they’re all 4 mm tall.
Now we print it on an Ember with the proper
energy exposure: all of the features turned
out except for the very smallest one.
The 100 micron wide post.
It started to print but collapsed because
it was too thin.
When we print this model with only 2/3rds
the energy, neither the 150 or 200 micron
With half the light energy as the the original
print, only the 500 and 450 micron posts form
but they’re thinner than they should be.
For models with small negative features, too
much light energy can be a problem.
If we dial in the exposure the 500 µm and
larger gaps are clearly resolved.
The 250 µm gaps are partially resolved, and
the 125 micron features are completely filled
in.
If we add just 33% more light exposure, the
250 µm features are completely filled in,
and we effectively lowered resolution.
How easy is it to find the correct light energy
exposure?
These prints each take roughly 30 minutes
to print at 25 micron layers.
So if we vary the light energy by varying
the exposure time, we need to try 10 different
exposure before we can dial it in.
And that’s just for 25 micron layers.
What if we also want to print at 10, 50 and
100 micron layers?
So in total it could take upwards of 20 hours
worth of printing to find the right exposure
times for our resin.
But if we’re clever we can get the same
information from a single print!
We start with a diagnostic print file consisting
of 32 slices.
The slice images create a panel of 32 rectangles
where each rectangle receives a distinct amount
of light energy.
The rectangle that is illuminated in every
slice receives 32 times the energy as the
rectangle that is only illuminated in the
first slice.
The pattern is randomized to minimize the
effect of possible unequal illumination across
the build area.
Unlike a normal print, we don’t fill up
the tray with resin, simply pipette a layer
of liquid resin directly onto the PDMS window.
Also, the tray does not rotate between layers,
and we do not use a buildhead.
Then we run the print file, which only takes
a little over a minute.
After the print is complete, we wash off the
uncured resin and are left with a thin film
with distinct panels.
The darker panels received more light and
are thicker.
Ones that received less light are thinner.
And some panels never reached a threshold
amount of energy and did not solidify at all.
Next, we use a thickness gauge to carefully
measure the thickness of each panel, then
record the measurements onto a spreadsheet.
The spreadsheet creates a plot of thickness
vs light dosage.
We use a logarithmic fit to extrapolate into
the layer thicknesses we’re interested in
-- namely 10 to 100 µm.
 Then we locate the layer thickness we want
to print at, and find the dosage required
to cure.
For the Ember printer the irradiance is about
20 mW/cm2.
 This gives us an exposure time of 1.25 seconds.
But we’re not quite there yet.
Ideally, a 3D printed layer would solidify
all at once when exposed to light.
However, in practice, it is more complicated.
We wish the light intensity was perfectly
uniform throughout the thickness of the layer.
In reality, the light intensity drops off
as we move up through the resin So the layer
begins to form where the light intensity is
highest -- at the surface of the window.
And over time, it grows towards the previous
layer.
So the exposure time we calculated is only
the minimum time it takes to cure a film.
In practice, for the layer to fully adhere
to the previous layer, the exposure time is
usually between 20 and 40 percent more time
than is specified on the graph.
You can figure this out with a handful of
test prints.
Printing the panels, measuring their thicknesses
and calculating an exposure time takes about
30 minutes on Ember.
This process works well for a wide range of
3D printable resins.
