Colorized
plot of the light-assisted curing of a polymer over five seconds, as measured
with NIST's custom atomic force microscope with a nanocylinder probe. Darker
colors indicate a higher level of conversion from a liquid resin to a polymer.
The magenta block at left represents the light fixture that initiates the
reaction. Courtesy: NIST.
In a step
toward making more accurate and uniform 3D-printed parts such as personalized
prosthetics and dental materials, researchers at the National Institute of
Standards and Technology (NIST) have demonstrated a method of measuring the
rate at which microscopic regions of a liquid raw material harden into a solid
plastic when exposed to light.
NIST's
custom atomic force microscope (AFM) with a nanometer-scale, cylinder-shaped
tip revealed that the complex process of curing resins, as they react under
light to form polymers, requires controlling how much of the light's energy
goes into forming the polymer and how much the polymer spreads out, or
diffuses, during 3D printing.
Described
in a new paper (ACS Applied Polymer Materials, "Voxel-Scale Conversion
Mapping Informs Intrinsic Resolution in Stereolithographic Additive
Manufacturing"), the NIST experiments showed that overall light-exposure
conditions, not just the total optical energy as often assumed, control how far
the polymer diffuses. For example, increasing light intensity for a constant or
shorter duration reduced resin-to-polymer conversion and could distort the
shape of a printed part. The measurements required only a few microliters of
resin, offering a way to reduce the costs of making and testing novel resins.
"This
research really digs into the unique process and materials science insight
afforded by our new metrology techniques," project leader Jason Killgore
said.
The work
builds on the NIST team's prior development of a related AFM method --
sample-coupled-resonance photorheology (SCRPR) -- that measures how and where a
material's properties change in real time at the smallest scales during the
curing process. Those measurements were made with conventional, tapered AFM
probes, which have angled sides and therefore can't reliably measure localized
liquid flow or thickness, technically referred to as viscosity.
Now, NIST
researchers have quantified viscosity, conversion and diffusion by use of a
cylindrical AFM probe, which has straight sides surrounded by consistent liquid
flow. The probe's vibrations, as they perturb the resin, are reduced by an
amount that depends on cylinder length and liquid viscosity. The increase in
liquid resin viscosity is related to conversion, enabling measurements of the
polymer's evolution in space and time.
Researchers
used computational fluid dynamics to model the force slowing down, or damping,
the oscillating nanocylinder and the resulting changes in its speed to
determine the amount of resin affected by the motion. By relating SCRPR damping
to resin viscosity and conversion, researchers made spatial maps of conversion
versus time for different exposure conditions.
The AFM
was equipped with a light modulator that directed patterned light from an LED
to the resin sample. Measurements of the conversion of a fast-curing resin
showed polymer accumulating tens of micrometers away from the light source
within a few seconds of exposure, indicating the extent and speed of diffusion.
The size of the light pattern was important; wider features led to higher
conversion at a given light intensity and duration (see image above).
SCRPR has
attracted interest from industry. So far one company has visited NIST to use
the instrumentation, Killgore said.