3 research outputs found
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Fundamentals of Polymer Crystallization in Laser Powder Bed Fusion for New Material Screening
Although laser powder bed fusion (PBF/LB) was one of the first industrially viable additive
manufacturing (AM) methods for end-use part production, polyamides remain grossly dominant
at both the commercial- and research scale. The research community continues to develop and
refine “rapid screening” methods for evaluating the suitability of new polymers for PBF/LB. The
so-called “SLS Process Window,” which is the difference between melting and crystallization
temperature measured at 10 K min-1 as originally outlined in the patent literature, is perhaps the
most often reported screening method. Although perhaps appropriate as part of a larger study, the
simplistic guidelines put forth by the “SLS Process Window” are not sufficiently scientifically
rigorous to understand how crystallization kinetics affects successful 3D printing. The common
understanding of the SLS Process Window omits details from published theories of polymer
crystallization. as evidenced by published assumptions and methods in PBF/LB process modeling
papers. The authors explain polymer crystallization in the PBF/LB context and propose replacing
the “process window” with crystallization halftime and physical gelation for new material
screening. These measurements better represent behavior critical for ensuring a lengthy
coexistence of solid powder and molten polymer affecting warp-free parts.Mechanical Engineerin
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A comparison of mechanical properties from natural and process-induced interfaces in filament extrusion AM of polymer blends
Polymer blends are commonly tuned for specific applications to achieve desired properties
otherwise inaccessible or prohibitively expensive to obtain via homopolymers. The interfacial
characteristics of the polymer A-polymer B interface and resultant domain sizes govern key
performance properties. Micro- and meso-scale morphology forms through the interplay of
surface forces between the polymers and between each polymer and the surrounding atmosphere.
Analogously, the layer-layer and road-road interfaces of material extrusion (MEX) additive
manufacturing (AM) govern key performance properties of printed parts. This work explores the
effect of layer height on the thermomechanical performance of polystyrene (PS)-polycarbonate
(PC) blends. Filament is prepared from a 50/50 weight ratio of the two polymers and compared
against dual-nozzle printing where every layer alternates between PS or PC homopolymer forming
a part with an overall 50/50 polymer ratio. Typical indicators of polymer blend compatibility are
also studied.Mechanical Engineerin
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Fused Filament Fabrication of Polymer Blends with in situ Layerwise Chemical Modifications
The layerwise paradigm of additive manufacturing advertises voxel level control over both
geometry and material properties of parts although the latter is difficult to achieve. Recently the
Savannah River National Laboratory has demonstrated a new technique for voxel level material
property control via layerwise surface chemical modification of polycarbonate homopolymer with
UV and ozone during manufacturing. This technique can be utilized to modify each respective
phase of a blended polymer feedstock to increase chemical similarity in preparation for potential
in situ interphase crosslinking. Successful crosslinking of dissimilar polymers during
manufacturing could allow for further voxel level material property control than modification of a
homopolymer could allow. Test feedstock comprised of melt mixed polycarbonate and polystyrene
homopolymers, an immiscible polymer blend, were printed in an atmosphere containing ozone
and UV light. FTIR measurements indicate both phases of the blend may be modified
simultaneously in situ to form new oxygen functional groups, increasing chemical similarity.
Calorimetric and thermomechanical characterization show no indicators of increased
compatibilization due to the treatment. Uniaxial tension to failure experiments demonstrates
minimal loss of mechanical properties as the blend phase to phase interfacial properties dominates
the behavior despite the chemical modifications. Future work will focus on understanding the
complex relationships between configurable processing parameters (layer height, print speed,
temperature, etc), reaction site creation density, and blend degradation prior to further modification
while identifying a suitable crosslinker to improve mechanical and thermal properties of the blend.Mechanical Engineerin