Additive manufacturing of polyolefins via powder bed fusion

Abstract

Powder bed fusion (PBF) is an additive manufacturing (AM) process in which a fine polymer powder is selectively heated and melted using infra-red (IR) energy from a CO2 laser to melt the powder and coalesce the molten polymer together to form each layer. Semi-crystalline polymers are the most common thermoplastics processed in this AM process and powder is heated during the printing process to retard crystallization in the material. When crystallization happens too rapidly, warpage of a layer, which can lead to print failure is possible. Polyolefins represent a class of thermoplastic chiefly comprised of polyethylene and polypropylene. These materials are highly used in engineering applications however, their rapid crystallization kinetics generally make them much less suitable for PBF. They represent a material class in which traditional processing approaches with PBF are not always sufficient to enable printability. In this dissertation, printing of multiple polyolefins is investigated to both understand the processing of these materials and grow an overall understanding of processing in PBF for any polymer. A process chain which relies on fundamental polymer behavior is devised to process ultra-high molecular weight polyethylene (UHMWPE). This material has sufficiently high molecular weight that the viscosity is so high it is not considered to flow in the melt. The difficulty in coalescing this material was overcome by using melt explosion during processing to create some entanglements between adjacent powder particles and form a green part that could then be post-processed in the melt to develop final part properties. The viscosity of this material enabled shape retention during this post-processing. Though this process chain enabled printing of UHMWPE, the printed parts were highly porous even after post-processing. Post-processing under pressure was investigated to further densify printed parts to achieve the mechanical performance expected for UHMWPE. By employing both cold isostatic pressing (CIP) and hot isostatic pressing (HIP), fully dense UHMWPE samples were realized. Strain at break was on par with traditionally processed UHMWPE was achieved, and tensile strength was only slightly less than the traditional processed material. Copolymerization of polypropylene (PP) with polyethylene (PE) to create random PP-PE copolymers, and its impact on material properties and processing was investigated. Increases in ethylene content were expected to decrease crystallization kinetics, which would increase processability of the material. Here, increases in processability means decreases in the likelihood of warpage. Though increases in ethylene content did lower the crystallization kinetics, these increases also significantly shifted the onset of melting for the copolymer to much lower temperatures, which limited the temperatures in which the material could be printed. Together these two changes led to a polymer that was more processable when ethylene content was 2.2% and then processability decreased as the ethylene content was increased to 4.9%. Printed parts from each copolymer showed a decrease in crystallinity with increasing ethylene content. Strain at break increased while tensile strength decreased with increasing ethylene content. A method of emulation of the PBF process was created to enable prediction of crystallization during processing. This method used in situ thermal measurements of the printing process to inform a thermal model to generate temperature profiles for a printed layer and then used these temperature profiles in fast scanning calorimetry (FSC) to emulate a printed layer's thermal history. This emulation enabled prediction of the crystallinity and the shape and temperatures covered by the melting endotherm during the printing process. Investigations of printing UHMWPE and PP-PE copolymers helped expand processing knowledge of polyolefins and of polymers in PBF overall. Challenges in viscosity during printing were overcome by exploring unique processing and post-processing methods to enable PBF of UHMWPE. An understanding of the impacts of ethylene content on processing and properties of PP-PE copolymers was developed and this insight can be valuable to guide future development of polyolefins for PBF. A powerful methodology for emulating the PBF process to understand crystallization was developed. This emulation provides an alternative to crystallization modeling and characterizes the crystallinity in a printed layer rather than just determining an amount of crystallinity. Through each of these contributions, understanding of PBF of polyolefins and the PBF process in general has been furthered.