International newspapers and experts have called 3D printing the industrial revolution of this century. Among all its available variants, the fused deposition modeling (FDM) technique is of greater interest since its application is possible using simple desktop printers. FDM is a complex process, characterized by a large number of parameters that influence the quality and final properties of the product. In particular, in the case of semicrystalline polymers, which afford better mechanical properties than amorphous ones, it is necessary to understand the crystallization kinetics as the processing conditions vary, in order to be able to develop models that allow having a better control over the process and consequently on the final properties of the material. In this work it was proposed to study the crystallization kinetics of two different polyamides used for FDM 3D printing and to link it to the microstructure and properties obtained during FDM. The kinetics are studied both in isothermal and fast cooling conditions, thanks to a home-built device which allows mimicking the quenching experienced during filament deposition. The temperature history of a single filament is then determined by mean of a micro-thermocouple and the final crystallinity of the sample printed in a variety of conditions is assessed by differential scanning calorimetry. It is found that the applied processing conditions always allowed for the achievement of the maximum crystallinity, although in one condition the polyamide mesomorphic phase possibly develops. Despite the degree of crystallinity is not a strong function of printing variables, the weld strength of adjacent layers shows remarkable variations. In particular, a decrease of its value with printing speed is observed, linked to the probable development of molecular anisotropy under the more extreme printing conditions.

There has been extensive research in the field of material-extrusion (Mat-Ex) 3D printing to improve the inter-layer bonding process. Much research focusses on how various printing conditions may be detrimental to weld strength; many different feedstocks have been investigated along with various additives to improve strength. Surprisingly, there has been little attention directed toward how fundamental molecular properties of the feedstock, in particular the average molar mass of the polymer, may contribute to microstructure of the weld. Here we showed that weld strength increases with decreasing average molar mass, contrary to common observations in specimens processed in more traditional ways, e.g., by compression molding. Using a combination of synchrotron infra-red polarisation modulation microspectroscopy measurements and continuum modelling, we demonstrated how residual molecular anisotropy in the weld region leads to poor strength and how it can be eradicated by decreasing the relaxation time of the polymer. This is achieved more effectively by reducing the molar mass than by the usual approach of attempting to govern the temperature in this hard to control non-isothermal process. Thus, we propose that molar mass of the polymer feedstock should be considered as a key control parameter for achieving high weld strength in Mat-Ex.

Fused filament fabrication (FFF) is among the most accessible and rapidly developing additive manufacturing (AM) technologies. Even though amorphous thermoplastic polymers are the elected materials in FFF, semicrystalline polymers would enhance the strength and durability of 3D printed parts. The properties of the products depend on the crystalline structure and on the morphology, which develops during the printing process. However, to date, polymer crystallization during 3D printing is still largely unexplored. Then, fundamental knowledge of the crystallization process in realistic printing conditions is necessary to pave the way to semicrystalline polymers in FFF. This work focuses on the implementation of a simple optical setup to follow the crystallization process in situ. The new setup exploits light scattering generated during filament deposition and cooling. To this purpose, the beam scattered by a growing 3D printed wall geometry is collected on a semi-transparent screen during the deposition for both amorphous polylactide and a semicrystalline polyamide copolymer. While the amorphous polymer scatters light anisotropically and generates a vertical scattering pattern, the semicrystalline polyamide produces isotropic scattering. Moreover, the kinetics of scattering intensity increase follows closely that of deposition in the case of polylactide. Instead, it strongly deviates from that for the semicrystalline polyamide. These pieces of evidence suggest that simple light scattering measurements, which are quickly and cost-effectively implemented on a lab-scale, can be further developed to allow a better understanding of polymer structuring phenomena in the highly non-equilibrium conditions of 3D printing.