100 research outputs found
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A Computer Model for Laser Photopolymerization
A computer model for a laser induced photopolymerization process has been established
which simulates stereolithography. The model couples irradiation, chemical reaction, and heat
transfer equations to provide insights into rate processes occurring in the volume element
contacted by the laser beam. Quantities predicted include the spatial variation in conversion of
monomer to polymer, depletion of photoinitiator, and local variations in temperature in and
around the spot contacted by the laser. This allows predictions to be made about the laser dwell
time, depth penetration and uniformity of the photopolymer formed in the process.Mechanical Engineerin
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Material and Process Parameters that Affect Accuracy in Stereolithography
Experimental real time linear shrinkage rate measurements simulating stereolithography
are used in an analysis of shrinkage during line drawing in stereolithography. While the amount of
shrinkage depends on the polymerization kinetics, shrinkage kinetics and overall degree of cure, it
also depends on the length of time to draw a line of plastic. A line drawn slowly will exhibit less
apparent shrinkage than one drawn very quickly because much of the shrinkage is compensated
for as the line is drawn. The data also indicates that a typical stereolithography resin in the green
state may shrink to only 65% of its maximum, thus retaining considerable potential for shrinkage
during post-cure. This infonnation can be used to predict the amount of shrinkage to be expected
under certain exposure conditions and to fonnulate overall strategies to reduce shrinkage and
subsequent warpage that causes shape distortion.Mechanical Engineerin
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Thermal-Expansion and Fracture Toughness Properties ofParts made from Liquid Crystal Stereolithography Resins
Liquid crystal (LC) resins are a new kind ofstereolithography material that can produce
parts with structured or ordered morphologies instead ofthe amorphous morphologies that result
from standard resins. The LC molecules can be aligned before cure resulting in an anisotropic
crosslinked network when the laser induced polymerization "locks-in" the alignment. Previous
papers have explored liquid crystal orientation dynamics [1], the effects of orientation on viscoelastic and mechanical properties [2,3], and the processing ofLC resins by stereolithography [4].
This paper considers the effects ofmorphology on fracture toughness and thermal-expansion
properties. Both toughness and thermal-stability continue to be important issues for
stereolithography parts. The use ofLC resins may provide a way to significantly improve
performance in both ofthese areas, and in addition result in parts with high upper use .
temperatures.Mechanical Engineerin
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Simulation of Laminated Object Manufacturing (LOM) with Variation of Process Parameters
A previously developed and verified thermal model for Laminated Object Manufacturing
(LOM) was used to investigate the effects of various processing parameters on the temperature
profile in a LOM part during the build cycle. The mathematical model, based on 3-dimensional
transient heat conduction in a rectangular geometry LOM part, allows calculation ofthe transient
temperature distribution within the part during the application of a new layer as well as during
other periods ofthe LOM build cycle. The parameters roller temperature, roller speed, chamber
air temperature, base plate temperature, and laser cutting time were independently varied, and the
LOM process response simulated. The results were analyzed in order to gain insight into
potential strategies for intelligent process control.Mechanical Engineerin
Development of polyisocyanurate pour foam formulation for space shuttle external tank thermal protection system
Four commercially available polyisocyanurate polyurethane spray-foam insulation formulations are used to coat the external tank of the space shuttle. There are several problems associated with these formulations. For example, some do not perform well as pourable closeout/repair systems. Some do not perform well at cryogenic temperatures (poor adhesion to aluminum at liquid nitrogen temperatures). Their thermal stability at elevated temperatures is not adequate. A major defect in all the systems is the lack of detailed chemical information. The formulations are simply supplied to NASA and Martin Marietta, the primary contractor, as components; Part A (isocyanate) and Part B (poly(s) and additives). Because of the lack of chemical information the performance behavior data for the current system, NASA sought the development of a non-proprietary room temperature curable foam insulation. Requirements for the developed system were that it should exhibit equal or better thermal stability both at elevated and cryogenic temperatures with better adhesion to aluminum as compared to the current system. Several formulations were developed that met these requirements, i.e., thermal stability, good pourability, and good bonding to aluminum
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A Simple Polymer Shrinkage Model Applied to Stereolithography
A simple polymer shrinkage model has been successfully applied to the stereolithography
process. The shrinkage model, which computes specific volume changes from the degree of
conversion of monomer to polymer, incorporates a lag between conversion and shrinkage. An
overall process model used to simulate the stereolithography process was modified by inclusion
of the shrinkage model. Use of the modified stereolithography process model allows prediction
of the shrinkage that might be expected to occur when fabricating a strand of plastic. By varying
the lag between conversion and shrinkage it is shown that faster shrinking resins should exhibit
lower overall shrinkage than slower shrinking resins. This is a direct result of the fact that less
shrinkage occurs after the strand has been scanned for the faster shrinking resins.Mechanical Engineerin
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Novel Liquid Crystal Monomers for Stereolithography: Reaction Rates and Photopolymerization Conversion
Liquid crystal (LC) monomers are a novel type ofresin for stereolithography that result
in polymers having unique physical and mechanical properties. These monomers consist ofrigid
central cores connected to acrylate functional groups by short aliphatic chains. Because ofthe
rigid-rod structure ofthe monomer the cross-linked polymer networks formed have high glass
transition temperatures (Tg). The high TglS result in particularly high upper-use temperatures for
stereolithography parts.
This paper reports on photopolymerization reaction rates and monomer conversion for
two acrylate monomers measured using reflectance real-time infra-red spectroscopy (RRTIR).
The RRTIR method measures the disappearance ofreactive acrylate groups in the monomer as a
function oftime while the monomers are being exposed to UV light. For the two new resins, UV
irradiation using an argon ion laser gives rapid photopolymerization with acrylate conversion as
high as 95 %. Conversion and polymerization rates in these monomers are dependent upon
photo-initiator selection and concentration. In addition, the results indicate that conversion
increases with increased laser intensity and elevated temperatures.Mechanical Engineerin
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Solid Freeform Fabrication of Functional Silicon Nitride Ceramics by Laminated Object Manufacturing 1
The processing of silicon nitride (Si3N4) structural ceramics by Laminated Object
Manufacturing (LOM) using ceramic tape preforms was investigated. The key processing stages
involved green shape formation (which used the LOM process), followed by the burnout of all
organics, and final densification by pressureless sintering. Two material systems were
considered. These were a) monolithic Si3N4 and b) a preceramic polymer infiltrated Si3N4. The
raw materials for the process were tape preforms of Si3N4, which were fabricated by standard
tape casting techniques.
Mechanical property data obtained for the LOM processed Si3N4 showed high strength and
fracture toughness values. The room temperature and high temperature (1260 o
C) flexural
strengths were in the range of 700-900 MPa and 360-400 MPa, respectively. The fracture
toughness averaged from 5.5-7.5 MPa.m1/2. These strength and fracture toughness values are
comparable to those reported for conventionally prepared Si3N4 ceramics. Thus, this research
demonstrated that the LOM technique is a viable method for preparing functional Si3N4 ceramics
with good physical and mechanical properties.Mechanical Engineerin
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Linear Shrinkage of Stereolithography Resins
The linear shrinkage of an acrylate and an epoxy based stereolithography resin was
measured during cure. A helium-cadmium (He-Cd) laser cured strands of resin as is done in the
stereolithography process using two exposures. The exposure time was held constant while the
delay time between exposures was varied. It was found for both resins that the final cure depth and
linear shrinkage were a function of delay timeMechanical Engineerin
Sex-Dependent Changes in miRNA Expression in the Bed Nucleus of the Stria Terminalis Following Stress
Anxiety disorders disproportionately affect women compared to men, which may arise from sex differences in stress responses. MiRNAs are small non-coding RNAs known to regulate gene expression through actions on mRNAs. MiRNAs are regulated, in part, by factors such as stress and gonadal sex, and they have been implicated in the pathophysiology of multiple psychiatric disorders. Here, we assessed putative sex differences in miRNA expression in the bed nucleus of the stria terminalis (BNST) – a sexually dimorphic brain region implicated in anxiety – of adult male and female rats that had been exposed to social isolation (SI) stress throughout adolescence. To assess the translational utility of our results, we assessed if childhood trauma in humans resulted in changes in blood miRNA expression that are similar to those observed in rats. Male and female Sprague-Dawley rats underwent SI during adolescence or remained group housed (GH) and were tested for anxiety-like behavior in the elevated plus maze as adults. Small RNA sequencing was performed on tissue extracted from the BNST. Furthermore, we re-analyzed an already available small RNA sequencing data set from the Grady Trauma Project (GTP) from men and women to identify circulating miRNAs that are associated with childhood trauma exposure. Our results indicated that there were greater anxiogenic-like effects and changes in BNST miRNA expression in SI versus GH females compared to SI versus GH males. In addition, we found nine miRNAs that were regulated in both the BNST from SI compared to GH rats and in blood samples from humans exposed to childhood trauma. These studies emphasize the utility of rodent models in studying neurobiological mechanisms underlying psychiatric disorders and suggest that rodent models could be used to identify novel sex-specific pharmacotherapies for anxiety disorders
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