Medical Design Briefs - December 2021 - 27

* The importance of optical coatings on
all optics to ensure maximum light
efficiency.
* The importance of optics-based realtime
closed-loop metrology to verify
part production.
Print resolution is a key driver in additive
manufacturing adoption. The ability
to print dimensionally accurate parts in a
wide variety of materials including titanium
and engineered polymers with prescribed
surface finish and density characteristics
minimizes post-processing steps
such as machining and polishing. This
allows printed parts customized to the
patient to become not only faster to manufacture
than traditional subtractive manufacturing
methods, but at lower costs and
delivered through a decentralized or distributed
supply chain. Directed laser energy
and projection systems are widely used
within optical systems for additive manufacturing
described in Table 1. With the
right optical components, these machines
tailor a beam to specific spot dimensions
and intensity uniformity to impart prescribed
energy profiles that yield higher
quality parts at faster build rates.
Metal additive manufacturing pro -
cesses based on powder bed fusion is one
example of how optical components and
system design play important roles in
overall part quality. The basic optical layout
for a laser powder bed fusion (LPBF)
system is presented in Figure 1. A highpower
laser source, typically a fiber laser
emitting 1 kW or greater power around
1070 nm wavelength is passed through
optical collimators and focusing elements
prior to being directed to the working
plane using multi-axis scan mirrors. The
quality and stability of the focused spot
are directly influenced by the selected
optical and mechanical components.
Processes utilizing higher power lasers
must be mindful of even small fractions
of scattered energy, which can result in
significant thermal effects in both the
optical elements and supporting
mechanics. Thermal expansion within
metal mechanics can cause element positions
to shift or drift, requiring special
attention to both material selection and
cooling using plumbed air or water.
Within the optics, a phenomenon known
as thermal lensing caused by heating of the
glass, affects both the lens shape and the
refractive index of the material. Thermal
lensing is a common issue at entrance
windows to the powder bed chamber as
well as elements near regions of high
optical intensity within the beam path.
Medical Design Briefs, December 2021
Cov
When materials and coatings are not
properly considered,
thermal
lensing
can cause the laser spot focus to drift
along with spatial and temporal changes
which result in lower than desired energy
profiles and excess porosity in the finished
part.
Design engineers should select substrate
materials with minimal inclusions
to avoid internal scatter and apply high
performance dielectric coatings with
exceptionally low absorption. High performance
fused silica glass such as
Corning 7980, is one example of a highly
pure glass and is often one of the only
options at critical locations within multikilowatt
laser systems. More sophisticated
lens designs are needed to reach high
performance and may require judicious
choices of glass materials to vary the
refractive index. Engineers may opt for
glass types with differing refractive index
temperature coefficients as one way to
balance out thermal effects, much in the
same way doublet lenses balance dispersion
profiles using crown and flint glasses.
Even perfect stability within the optical
design can still leave opportunity for
process variability during the melting
process. Powder particle size distributions,
alloy content and soot or spatter
settling within the chamber and on windows
can all impact the quality and porosity
of a printed part. To ensure that the
part being produced accurately matches
the design file, in-situ, real-time monitoring
of the build process is often required.
This is a relatively new technology tool
for 3D printing; however, it has been used
for decades in optical thin-film coating
deposition machines. Again, optics plays
a key role by observing the melting
process through emitted radiation signals
at very high acquisition rates to monitor
the signatures of the process as well as
overall machine health.
Deviations from nominal emitted signal
levels can flag issues in the process that
can potentially be corrected in real time.
Process errors that cannot be corrected
can instead stop the build early and avoid
unnecessary production time and flag
potential issues within the machine. As
shown in Figure 1, the use of a spectrally
complex dichroic beam splitter allows
blackbody radiation, emitted from the
powder bed and travels back through the
optical path (green line), and directed to
an optical detector, typically photodiodes
or an image sensor. Embedded software
allows one to calculate melt pool temperatures
using known multi-color pyrometry
www.medicaldesignbriefs.com
ToC
MEDICAL
SOLENOID
VALVES
DESIGNED &
BUILT-TO-SPEC
* FREE Design
& Engineering
* FAST Quotes
& Prototypes
Solenoid
Solutions
valves are
used in Air Beds,
Exam Chairs, O2
Devices,
Sterilizers,
Compression Therapy
and many more applications!
SolenoidSolutionsInc.com
888.825.8405
814.838.3190
Free Info at http://info.hotims.com/79420-764
http://info.hotims.com/79420-764 http://www.medicaldesignbriefs.com
Medical Design Briefs - December 2021

Table of Contents for the Digital Edition of Medical Design Briefs - December 2021
