Semi-crystalline feedstock for filament-based 3D printing of polymers

Abstract

Additive Manufacturing (AM), and more specifically Fused Filament Fabrication (FFF), allow the production of highly customized parts, provide enormous freedom-of-design and can lead to material savings due to the layer-by-layer material deposition that is inherent to this family of production processes. FFF utilizes both amorphous and semi-crystalline thermoplastic filaments as feedstock materials, offering a wider range of materials compared to some other polymer-based additive manufacturing techniques. However, the current trend where FFF, and AM in general, are changing from a technique for rapid prototyping to the production of fully functional parts designed for high-end applications creates the inevitable need to incorporate more engineering and high-performance thermoplastics, which are most often semi-crystalline polymers, into the material palette. Crystallization provides semi-crystalline polymers with some distinct features that set them apart from their amorphous counterparts, yet it also can present difficulties regarding their processing. Understanding of the behavior of semicrystalline polymers during FFF processing is thus a prerequisite to exploit their full potential. This review provides a broad overview of FFF processing of semicrystalline polymers. Particular focus lies on the impact of processing conditions and feedstock modifications, such as the incorporation of fillers or the formation of blends, on crystallinity as well as the microstructure of printed parts, the impact of microstructure on the mechanical performance, and general part quality. Furthermore, attention is given to some specific phenomena that can occur during printing of semi-crystalline feedstock filaments which have shown to strongly impact the printing process. Examples are self-nucleation in the case of insufficient heat transfer and melting, flow-induced crystallization due to high shear deformations upon extrusion, and the negative impact of crystallization on chain mobility which is relevant for the development of interlayer strength and on dimensional accuracy due to excessive shrinkage. Finally, this review is concluded with a critical outlook on perspectives for future research to address the current challenges that are still faced when employing semi-crystalline polymers as FFF feedstock.