Thomas Forstner

Metal Injection Molding (MIM) is based on the processing of highly filled polymers via the well established polymer injection molding process. It offers a highly efficient processing route for the indirect manufacturing of especially small and complex metal parts. In this regard, polyoxymethylene (POM) is often used as a primary binder component in MIM feedstocks due to its high debinding rate through a time-saving catalytic debinding process, utilizing the acid-catalyzed degradation of POM for polymer removal. However, thermally induced degradation of POM under processing conditions can also lead to changes in processing behavior, which is particularly important in highly filled polymers due to their already challenging processability. In this context, the present work demonstrates the impact of POM homopolymers (POM-H) and copolymers (POM-C) with varying viscosities on feedstock characteristics, their influence on the thermal processing stability, and their significance for the properties of the green parts. Within the study, the thermal degradation of both material types was assessed by viscosity measurements and thermogravimetry, with POM-H exhibiting more significant degradation compared to the thermally more stable POM-C, especially at higher temperatures. Catalytic debinding performance was found to be adequate for all materials. However, lower viscosity POM-C grades are preferred to optimize processability in MIM.

Laser-based powder bed fusion of polymers (PBF-LB/P) is a layer-based additive manufacturing (AM) method that produces functional components with superior mechanical properties. However, the limitation of commercially available materials, primarily polyamides, reduces the range of applications for PBF-LB/P processed parts. The use of novel polymer feedstocks, such as polybutylene terephthalate (PBT) in PBF-LB/P is of great interest, as the properties of PBT allow for an extension of the application of AM-built parts in the automotive and electronics sectors. Studies on the flowability of the powder under process conditions at evaluated temperatures showed a homogeneous surface of the powder bed. In addition, the processability of PBT powders in the PBF-LB/P process was investigated with a thermal camera, showing process limits for low-viscosity materials. Component properties showed a higher Young’s modulus than injection molding, while fracture behavior showed brittle behavior for the homo-PBT tested.

Filament-based material extrusion (MEX) represents one of the most commonly used additive manufacturing techniques for polymer materials. In a special variation of this process, highly filled polymer filaments are used to create metal parts via a multi-step process. The challenges associated with creating a dense final part are versatile due to the different and partly contrary requirements of the individual processing steps. Especially for processing in MEX, the compound must show sufficiently low viscosity, which is often achieved by the addition of wax. However, wax addition also leads to a significant reduction in ductility. This can cause filaments to break, which leads to failure of the MEX process. Therefore, the present study investigates the influence of different ethylene–propylene copolymers (EPCs) with varying ethylene contents as a ductility-enhancing component within the feedstock to improve filament processing behavior. The resulting feedstock materials are evaluated regarding their mechanical, thermal and debinding behavior. In addition, the processability in MEX is assessed. This study shows that a rising ethylene content within the EPC leads to a higher ductility and an enhanced filament flexibility while also influencing the crystallization behavior of the feedstock. For the MEX process, an ethylene fraction of 12% within the EPC was found to be the optimum regarding processability for the highly filled filaments in MEX and the additional processing steps to create sintered metal parts.

The additive manufacturing of metals by material extrusion in a multi-step process (MEX-MSt/M) represents a special process variant of the commonly used material extrusion (MEX) and is based on the processing of highly filled polymer filaments. This technology uses the geometrical freedom and fast processing given by MEX to create individual metal parts after a debinding and sintering process in a cost and time-efficient way. The filaments for MEX-MSt/M are made by incorporating metal powders, such as aluminum, stainless steel, or bronze into a polymer matrix. Due to the challenges that are assigned to the processing of highly filled polymers, like the increased viscosity of the material or clogging of the nozzle, the binder materials have to meet several requirements. Therefore, waxes are often used to enable a better extrusion behavior for MEX; however, the addition of wax also affects other crucial processing properties of the filaments. In this work, the interactions of different types and amounts of waxes on thermal, mechanical, and rheological properties were investigated to create a better understanding of the alternating effects of wax addition into highly filled filaments for processing via MEX. The study demonstrated that an increase in wax contents resulted in both a significant decrease in ductility and an overall improvement in melt flowability. The choice of waxes also affected the particle–matrix interactions, partly leading to an improved wetting of the filler particles.

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