Fused Deposition Modeling of Polyamides: Crystallization and Weld Formation

Abstract

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.

The Role of Molar Mass in Achieving Isotropy and Inter-Layer Strength in Mat-Ex Printed Polylactic Acid

“…However, the use of this technology for practical applications is significantly affected by the anisotropy of the mechanical properties found in the printed products [ 10 , 11 , 12 ]. In particular, it has been found that the ultimate tensile strength is higher in the direction parallel to that of the filament deposition, while it strongly decreases in the perpendicular direction [ 13 , 14 , 15 ]. It is therefore clear that the final mechanical properties of the manufactured articles are strongly correlated to the quality of the bond that is established between the adjacent layers [ 16 ].…”

“…Moreover, it is known that harsh printing conditions can largely influence the final mechanical properties. In previous works, it has been demonstrated that the use of low nozzle temperatures and higher printing speeds significantly reduce the quality of the weld [ 15 , 24 , 25 ]. By combining birefringence measurements and tensile tests with results from a continuum model, Costanzo et al highlighted that the cause of the low weld strength for stronger processing conditions is the molecular orientation present at the interface between the layers [ 26 ].…”

The Extent of Interlayer Bond Strength during Fused Filament Fabrication of Nylon Copolymers: An Interplay between Thermal History and Crystalline Morphology

“…The negative impact on bond strength can be attributed to the residual alignment due to orientation of polymer chains upon extrusion from the print nozzle. If the residual alignment cannot relax prior to solidification, insufficient entanglements can be formed leading to poorer interlayer adhesion [ 50 , 51 ]. The print speeds employed in this study are most likely too low to actually induce a significant change in interlayer bond strength through alignment effects upon extrusion.…”

“…However, a higher T build plate has been found to considerably enhance the attained degree of crystallinity and lead to thicker lamellae through an annealing effect by the heated build plate, which improves mechanical strength when loaded along the layer deposition direction [49]. Costanzo et al (2020) have reported a significant effect of print speed (v print ) through its impact on molecular orientation upon extrusion leading to a decrease in weld strength. Residual molecular alignment at the weld zone was found to be alleviated by increasing T liquefier or decreasing v print [50,51].…”

“…Costanzo et al (2020) have reported a significant effect of print speed (v print ) through its impact on molecular orientation upon extrusion leading to a decrease in weld strength. Residual molecular alignment at the weld zone was found to be alleviated by increasing T liquefier or decreasing v print [50,51]. Furthermore, the developed crystalline morphology can impact the final interlayer bond strength.…”