Powder bed fusion (PBF) is one of seven technology modalities categorized under the term additive manufacturing (AM). Beyond the advantages of fabricating complex geometries and the "tool-less manufacturing" paradigm common to all types of AM, polymer PBF shows potential for significant industrial relevance through exploiting the technique's characteristic powder-filled bed (a.k.a. build piston) to utilize the full printer volume for batch-style production. Although PBF should be a suitable processing technique for all semi-crystalline polymers, the polyamide family currently occupies around 90% of the commercial market for polymer PBF. This commercial dominance of polyamides is mirrored in the focus of research publications.
The lack of chemical variety in published research questions the universality of reported Structure-Property-Process and Process-Structure-Property relationships for PBF. This dissertation presents the findings from identifying Structure-Property-Process relationships critical to fabricate multi-layer parts for poly(phenylene sulfide) (PPS) by PBF towards expanding PBF material selection and evaluating universality of relationship guidelines. PPS is an engineering thermoplastic used for its high strength, rigidity, dielectric properties, and chemical resistance at elevated temperatures. These properties are attributed to PPS' highly crystalline morphology. Its current use in the automotive and aerospace industries, which are early adopters of AM technologies, makes PPS a prime candidate for AM applications. Therefore, the goal of this work is to demonstrate PPS printing by PBF, study its behavior throughout the PBF lifecycle, and abstract general trends in polymer PBF relationships.
First, theoretical ranges for print parameter values are determined from properties of an experimental grade PPS powder feedstock. Successful printing of PPS by PBF is demonstrated in a way contrary to published empirical polymer-PBF relationships. Low temperature printing (i.e., bed temperature more than 15 °C lower than polymer peak melting temperature) of PPS successfully fabricated dimensionally accurate parts with reasonable mechanical properties compared against injection molding values. This distinct PPS behavior does not follow empirical guidelines developed for either polyamides or poly(aryl ether ketones).
The unique success of low-temperature PBF prompted further investigation into potential benefits of low-temperature printing. Structure-Property-Process relationships were characterized over the course of simulated powder reuse to show that low-temperature printing prolonged the time when PPS powder properties remained in the "printable" range. Significantly re-used PPS powder was shown to be printable when print parameters were adjusted to accommodate structure and property changes. Successful prints from reused powder is uncommon among published reports of PBF printing of high-performance engineering thermoplastics.
Observations of a change in molecular architecture through branching and crosslinking during simulated powder reuse motivated investigating if similar reactions occur in printed parts. PPS is commonly used at elevated temperatures in the presence of oxygen, which is the ideal environment for branching and crosslinking. Structural changes manifested in increased glass transition temperature and high temperature storage modulus. The relative change in structure when printed parts were thermo-oxidatively exposed was observed to be significant for parts printed from new powder, but minimal for parts printed from reused powder. This is a result of the structural changes occurring as powder feedstock during reuse over multiple builds.
The changing architecture of reused PPS exposed shortcomings with print parameter value selection based solely on polymer thermal properties. Branching and crosslinking reduced crystallinity, resulting in calculated less energy required to melt; however, it also increased melt viscosity. This negative impact on coalescence behavior was not reflected in the methodology for process parameter value determination because current guidelines neglect rheological properties. These observations motivated proposing a method for selecting print settings based on polymer coalescence behavior. Because it is based on coalescence, this method can predict the transition in governing physics from viscous coalescence to bubble diffusion, which is accompanied by a change in the dependence of mechanical properties on laser energy density.
Most work in polymer PBF has focused on "printed part triad'" Process-Property relationships. Work presented in this dissertation contributes to the "printability triad'" of Structure-Property-Process relationships and does so using the novel-to-PBF polymer, PPS. Additional polymers must be explored to continue to discern which polymer-manufacturing relationships are universal among all polymers and which are specific to one subset. The observations and connected interpretation to principles of polymer physics add to the body of knowledge for the polymer PBF field. These contributions will help pave the way for investigations into other polymer families and will re-shape the field's normative logic use when answering the question "what makes a polymer printable by PBF?" Understanding the connection between polymer properties and physical stimuli characteristic of PBF manufacturing will result in parts tailored for specific applications and more sustainable manufacturing, thus realizing additive manufacturing's full potential. / Doctor of Philosophy / Powder bed fusion (PBF) is one of seven distinct additive manufacturing (AM, also known as ``3D printing'') technologies. The manufacturing process creates solid, three-dimensional shapes through selectively heating, melting, and fusing together polymer powder particles in a layer-by-layer manner. Currently, organizations are interested in complementing existing manufacturing technology with PBF for one of three general reasons: (1) "complexity is free" PBF has the ability to make shapes that are difficult or expensive to fabricate using other manufacturing technologies. (2) "tool-less manufacturing" PBF only requires a digital design file to fabricate objects. This enables small changes to be easily made via computer-aided design (CAD) programs without the need to invest time and money into tooling (e.g., molds, jigs, fixtures, or other product-specific tools). This enables "mass customized" products (e.g., custom-fit medical devices and implants) to be economically feasible. (3) "material efficiency" AM is attractive as it often generates less waste than subtractive manufacturing techniques like milling. This is particularly a concern for organizations that manufacture parts from expensive, high-performance polymers, such as in the aerospace and medical industries. Despite these benefits, the state of the art for polymer PBF has room for improvement. Specifically, there are many details regarding material behavior during PBF manufacturing that are unknown; any unknown behaviors present challenges to building confidence in production quality. Additionally, approximately 90% of current PBF use is nylon-12 or else another material in the polyamide family of semi-crystalline thermoplastics. This limited selection of commercially available materials compared against other forms of manufacturing contributes to PBF's circular quandary: the manufacturing process physics are not robustly understood because most experimentation and research has been carried out on one family of polymers; however, a wider variety of polymers has not been developed because there is a limited understanding of the process physics.
This dissertation presents research toward answering both PBF challenge areas. The first three chapters present investigations into relationships between the properties of a novel, experimental grade poly(phenylene sulfide) (PPS) semi-crystalline thermoplastic polymer powder, the stimuli imposed on this polymer during PBF processing, and the resultant properties of printed parts (i.e., "property-process-property relationships"). The target polymer, poly(phenylene sulfide), is a high-temperature, high-performance polymer that is traditionally melt processed, but has not yet been commercialized for PBF. Prior literature has established mathematical representation for the interaction between manufacturing energy input and the thermal response of the polymer resulting in melting. This framework has been created through studying the polyamide family. Work presented in this dissertation evaluates existing guidelines for PBF process parameter selection using measured thermal behavior of PPS (i.e., a polysulfide, not a polyamide) to predict the range of manufacturing energies affecting geometrically accurate printed parts of high density and strength. In addition, the impact of thermal exposure from repeated PPS powder reuse over the course of multiple PBF prints was evaluated on powder, thermal, and rheological properties identified as critical for PBF printing. Changes to the molecular structure and properties of reused PPS powder were observed to follow different trends than those reported for other materials traditionally used. The effect of thermal exposure on printed parts was also investigated to determine if the observed changes in molecular structure occurring during thermal exposure of the powder would result in changes to mechanical performance properties of printed parts.
The importance of rheological flow properties in dictating printed part performance was observed to be a common theme throughout working with PPS. The final chapter presents a novel method for quantitatively predicting particle fusion during PBF and connecting the extent of particle fusion to mechanical properties of printed parts. The presented method is "polymer agnostic" and advances the state of the art in understanding the physics guiding polymer response to stimuli imposed during PBF AM.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/100053 |
| Date | 21 September 2020 |
| Creators | Chatham, Camden Alan |
| Contributors | Chemistry, Williams, Christopher B., Long, Timothy E., Bortner, Michael J., Moore, Robert Bowen, Johnson, Blake |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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