Aerospace_America_March_2020

14 | MARCH 2020 | aerospaceamerica.aiaa.org

explored yet: 1. A printer could extrude paste onto
woven carbon-fi ber mesh. Or, 2. Without a printer,
the mesh could be dipped into a less viscous version
of the paste or the liquidy paste could be poured into
a mold containing the mesh. With both concepts, the
combined mesh and paste would be fi red in a furnace
to create the composite ceramic.
Their big challenge was to make a paste that
would be accepted by the printer and harden into
parts that would be as dense as those they had made
earlier. Generally, denser ceramics are stronger and
more heat resistant.
They needed a binder to hold the mix together
while dispersing the metal and resin molecules
evenly throughout the paste.
The paste had to be liquid enough to fl ow through
the printer head, but once extruded it couldn’t be
too damp or too d r y. “There’s kind of a delicate
balance,” Butler says. “ Yo u don’t want it to dry too
fast, because it will induce cracking. But you want
it to dry quick enough that you can deposit multiple
layers to build taller structures. I t ’s something you
have to dial in.”
The key to achieving the right viscosity would be
the choice of binder, which is a polymer and plasti-
cizer that’s mixed in powdered form with the powdered
resin and metal. Liquid is added to create the paste.
Once a part is printed, i t ’s fi red in a furnace to trigger
the chemical reaction that turns the hardened paste
into a ceramic, after burning off the binder.
The NRL researchers tried 10 to 15 binders com-
mon in 3D printing. Some were water-soluble and
others alcohol-soluble. The scientists made pastes

with each and created test discs. One of the water-sol-

uble versions was chosen, because it proved best at

creating a homogeneous mix of the right viscosity.

Looking ahead

At the moment, the shapes they’ve made by robocast

printing are not as dense as those they’ve made with

the die-pressed technique. The NRL researchers

continue to search for the optimal heating rate for

the furnace, meaning one that burns off the binder

completely while fostering the resin and metal

Researchers have

3D-printed hollow

cylinders (shown) and

tapered and conical

discs several centimeters

high as they refi ne their

method.

U.S. Naval Research Laboratory

SpaceLiner is a

hypersonic passenger

craft concept created by

the German Aerospace

Center. In this illustration,

the SpaceLiner orbiter

separates from its

reusable booster stage.

German Aerospace Center