Vat photopolymerization's (VP) use in large-scale industrial manufacturing is limited due to
poor scalability, and limited catalogue of engineering polymers. The challenges in scalability
stem from an inherent process paradox: the feature resolution, part size, and manufacturing
throughput cannot be maximized simultaneously in standard VP platforms. In addition,
VP's inability to process viscous and high-molecular weight engineering polymers limits the
VP materials catalogue. To address these limitations, the research presented in this work
was conducted in two stages: (1) Development and modeling of new VP platforms to address
the scalability and viscosity challenges, and (2) Investigating the influence of using the new
processes on the cured polymer network structure and mechanical properties.
First, a scanning mask projection vat photopolymerization (S-MPVP) system was developed
to address the scalability limitations in VP systems. The process paradox was resolved by
scanning the mask projection device across the resin surface while simultaneously projecting
the layer as a movie. Using actual projected pixel irradiance distribution, a process model
was developed to capture the interaction between projected pixels and the resin, and predict
the resulting cure profile with an error of 2.9%. The S-MPVP model was then extended
for processing heterogeneous UV scattering resins (i.e. UV curable polymer colloids). Using
computer vision, the scattering of incident UV radiation on the resin surface was successfully
captured and used to predict scattering-compensated printing parameters (bitmap pattern, exposure time , scanning speed). The developed reverse-curing model was used to successfully
fabricate complex features using photocurable SBR latex with XY errors < 1.3%.
To address the low manufacturing throughput of VP systems, a recoat-less, volumetric curing
VP system that fabricates parts by continuously irradiating the resin surface with a
movie composed of different gray-scaled bitmap images ( Free-surface movie mask projection
(FreeMMaP)) was developed. The effect of cumulative exposure on the cure profile
(X,Y,Z dimensions) was investigated and used to develop an iterative gray-scaling algorithm
that generated a combination of gray-scaled bitmap images and exposure times that result
in accurate volumetric curing (errors in XY plane and Z axis < 5% and 3% respectively).
Results of this work demonstrate that the elimination of the recoating process increased
manufacturing speed by 8.05 times and enabled high-resolution fabrication with highly viscous
resins or soft gels.
Then, highly viscous resins were made processible in VP systems by using elevated processing
temperatures to lower resin viscosity. New characterization techniques were developed
to determine the threshold printing temperature and time that prevented the onset
of thermally-induced polymerization. The effect of printing temperature on curing, cured
polymer structure, cured polymer mechanical properties, and printable aspect ratio was also
investigated using diacrylate and dimethacrylate resins. Results of this investigation revealed
increasing printing temperature resulted in improvements in crosslink density, tensile
strength, and printability. However, presence of hydroxl groups on the resin backbone caused
deterioration of crosslink density, mechanical properties, and curing properties at elevated
printing temperatures.
Finally, the lack of a systematic, constraint based approach to resin design was bridged
by using the results of earlier process-structure-property explorations to create an intuitive
framework for resin screening and design. Key screening parameters (such as UV absorptivity,
plateau storage modulus) and design parameters (such as photoinitiator concentration, polymer concentration, UV blocker concentration) were identified and the methods to optimize
them to meet the desired printability metrics were demonstrated using case studies.
Most work in vat photopolymerization either deal with materials development or process
development and modeling. This dissertation is placed at the intersection of process development
and materials development, thus giving it an unique perspective for exploring the
inter-dependency of machine and material. The process models, machines and techniques
used in this work to make a material printable will serve as a guide for chemists and engineers
working on the next generation of vat photopolymerization machines and materials. / Doctor of Philosophy / Vat Photopolymerization (VP) is a polymer-based additive manufacturing platform that uses
UV light to cure a photo-sensitive polymer into the desired shape. While parts fabricated
via VP exhibit excellent surface finish and high-feature resolution, their use for commercial
manufacturing is limited because of its poor scalability for large-scale manufacturing and
limited selection of engineering materials. This work focuses on the development of new VP
platforms, process models and the investigation of the process-structure-property relationships
to mitigate these limitations and enable fabrication of performance polymers.
The first section of the dissertation presents the development of two new VP platforms to address
the limitations in scalability. The Scanning Mask Projection Vat Photopolymerization
(S-MPVP)) was developed to fabricate large area parts with high-resolution features and
the Free-surface movie mask projection (FreeMMaP) VP platform was developed to enable
high-speed, recoat-less, volumetric fabrication of 3D objects. Computer-vision based models
were developed to investigate the influence of these new processes on the resultant cure
shape and dimensional accuracy. Process models that can: (1) predict the cure profile for
given input printing parameters (error < 3%), (2) predict the printing parameters (exposure
time, bitmap gray-scaling) required for accurate part fabrication in homogeneous and UV
scattering resins, and (3) generate gray-scaled bitmap images that can induce volumetric
curing inside the resin (dimensional accuracy of 97% Z axis, 95% XY axis), were designed
and demonstrated successfully.
In the second portion of this work, the use of high-temperature VP to enable processing
of high-viscosity resins and expansion of materials catalogue is presented. New methods to
characterize the resin's thermal stability are developed. Techniques to determine the printing
temperature and time that will prevent the occurrence of thermally-induced polymerization
are demonstrated. Parts were fabricated at different printing temperatures and the influence
of printing temperature on the resultant mechanical properties and polymer network structure
was studied. Results of this work indicate that elevated printing temperature can be
used to alter the final mechanical properties of the printed part and improve the printability
of the high-resolution, slender features.
Finally, the results of the process-structure-property investigations conducted in this work
were used to guide the development of a resin design framework that highlights the parameters,
metrics, and methods required to (1) identify printable resin formulations, and (2)
tune printable formulations for optimal photocuring. Elements of this framework were then
combined into an intuitive flowchart to serve as a design tool for chemists and engineers.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/111091 |
| Date | 07 January 2021 |
| Creators | Meenakshisundaram, Viswanath |
| Contributors | Mechanical Engineering, Williams, Christopher Bryant, Johnson, Blake, Zheng, Xiaoyu, Long, Timothy E., Acar, Pinar |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/x-zip-compressed |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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