Polymer Filaments for Fused Filament Fabrication: Expanding the Current Material Palette to meet Requirements of Advanced Applications

Over the past decades, Additive Manufacturing (AM) technologies have become increasingly popular and widespread, which is reflected in the huge growth in revenues from AM-related products and astonishing surge in publications and research related to the field. This is a direct consequence of the inherent characteristics, including freedom-of-design, redundancy of part-specific tooling and huge material savings, shared by all AM techniques, which have led to an immense variety in promising applications encompassing fields such as medicine, aerospace and construction. Within the group of polymer-based AM processes, Fused Filament Fabrication (FFF), a technique based on the extrusion of a thermoplastic polymer filament, has emerged as one of the most rapidly growing technologies. Besides reduced energy consumption and a more extensive range of feedstock materials compared to other polymer-based AM methods, its simplicity and flexibility have definitely strengthened its popularity. However, as with any other emerging technology, FFF still faces some major issues, which might impede it to reach its full potential as a technique that can be utilized not only for rapid prototyping, yet simultaneously as a stand-alone production process for parts of outstanding quality for high-end applications. Improvement of the FFF process, so that it can distinguish itself as a fully realized manufacturing technique, can be tackled from either the point of view of the employed printer and a corresponding optimization of printing process control or from that of the printed feedstock polymer whose properties and behavior during printing are key to ensure excellent printed part quality to meet the requirements of more advanced applications. The work presented in this thesis follows the route of the feedstock filament as a means to gain new insights, open up exciting possibilities and pave the way for FFF to excel in future applications. In the first part of this dissertation, the use of semi-crystalline thermoplastics as feedstock materials is highlighted as most engineering and high-performance polymers are of semi-crystalline nature and generally tend to possess the properties desired to successfully fulfill the necessary prerequisites for cutting-edge applications. However, semi-crystalline polymers exhibit some distinct characteristics as a direct result of the crystallization phenomenon which set them apart from their amorphous counterparts in terms of processing thus complicating their adoption as feedstock materials for FFF, illustrated by the rather poor level of understanding pertaining to their crystallization behavior during printing. Moreover, the crystallization process is known to aggravate part shrinkage and distortion, and is expected to dramatically hinder macromolecular chain mobility which is key to establish sufficiently strong interlayer bonds between successively deposited layers. The control of interlayer adhesion, already a critical issue in FFF, is essential for mechanical strength of printed parts and can thus be handicapped by crystallization. Since both crystallization and interlayer bonding are strongly driven by temperature, the experienced thermal history forms the key to study both phenomena. The devised methodology described in this thesis thus originates from the thermal history of printed layers recorded by infrared thermography during FFF printing of a single-layer wall geometry. Both print settings as well as feedstock molecular weight of the employed polyamide (PA) 6/66 copolymers are varied. The recorded thermal profiles are then mimicked in a Fast Scanning Chip Calorimetry device to assess the impact of print settings and molecular weight on the evolution of the degree of crystallinity in the monitored layers at distinct points in time during printing. In the following chapter, the presented work attempts to associate the attained degree of crystallinity and crystalline morphology at the interlayer interface, which is visualized through polarized light microscopy, with the extent of interlayer bonding by executing trouser tear fracture tests on the printed wall specimens. Based on amorphous healing theory, the concept of an equivalent isothermal weld time is utilized as a predictive tool for bond strength. Liquefier temperature is observed to not impact the crystallization process. However, interlayer bond strength is positively influenced by increasing liquefier temperature as predicted by the weld time. The effect of print speed on both the extent of crystallization and interlayer adhesion is found to be negligible. A lower molecular weight of the PA feedstock copolymer is characterized by enhanced crystallization kinetics, especially at elevated build plate temperature, and dramatically improved macromolecular chain diffusion, reflected in the higher degree of crystallinity and significantly improved adhesion at the interface compared to the high molecular weight counterpart, again accurately reflecting the weld time prediction. Furthermore, the build plate, set at a temperature sufficiently above the feedstock's glass transition temperature, profoundly affects crystallization by increasing both the attained degree of crystallinity as well as leading to enlarged spherulites, since it allows crystallization to proceed at elevated temperatures. On the other hand, due to ensuing slower crystal growth, tie chain density, crucial for mechanical strength, in the amorphous interlamellar regions is lowered, which will be detrimental to interlayer adhesion. The work presented in this dissertation thus provides key insights into the impact of printing parameters and feedstock characteristics on the extent of crystallization and its interrelation to the ever so crucial development of sufficient interlayer bonds through macromolecular diffusion and re-entanglement of polymer chains across the layer-layer interface. The second part of this thesis features the industrially driven FLAMINCO research project which is aimed at the development of a printable polyvinyl chloride (PVC) compound to allow economically favorable fabrication of small series of highly tailored window and door profile segments, which otherwise depend on high costs and lead times of the necessary tooling for their production. Development of suitable PVC compounds occurs by variations in compound formulation through incorporation of various additives across multiple campaigns. The current material palette for FFF is thus expanded with a PVC feedstock material which should ideally fulfill all requirements in terms of ease of processing with FFF and achieving the quality standards associated with the intended application in the field of construction. The work presented in this dissertation attempts to discern the key properties a feedstock filament should ideally possess to reach this goal. A three-level screening methodology is established which allows to compare the performance of developed PVC compounds within one campaign and demonstrate the progression made with each consecutive campaign. Extrudability is assessed based on rheological measurements by capillary rheometry to determine compound viscosity at relevant processing temperatures which can be evaluated with reference to the established print zone, limited by a maximum allowable viscosity corresponding to filament feeding failure. Thermal stability of PVC is inherently problematic. Hence, stability of the developed compounds is determined, both quantitatively through thermogravimetric analysis and qualitatively with the employed capillary rheometer, so that a full image of thermal stability can be obtained under conditions that mimic those experienced during printing. Finally, by printing benchmark parts, printed part quality is checked. However, to meet quality standards for PVC window and door profiles, such as a sufficient level of impact strength, established in industry norms, mechanical tests are imperative. By continuous evaluation of the developed compounds through the devised screening methodology, PVC compound formulation is fine-tuned and improved to ultimately achieve the desired objective.

Jaar van publicatie:2021

Toegankelijkheid:Closed