Property-Process-Property Relationships in Powder Bed Fusion Additive Manufacturing of Poly(phenylene sulfide): A Case Study Toward Predicting Printability from Polymer Properties

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.