Disentanglement effects on welding behaviour of polymer melts during the fused-filament-fabrication method for additive manufacturing

Abstract

Although 3D printing has the potential to transform manufacturing processes, the strength of printed parts often does not rival that of traditionally-manufactured parts. The fused-filament fabrication method involves melting a thermoplastic, followed by layer-by-layer extrusion of the molten viscoelastic material to fabricate a three-dimensional object. The strength of the welds between layers is controlled by interdiffusion and entanglement of the melt across the interface. However, diffusion slows down as the printed layer cools towards the glass transition temperature. Diffusion is also affected by high shear rates in the nozzle, which significantly deform and disentangle the polymer microstructure prior to welding. In this paper, we model non-isothermal polymer relaxation, entanglement recovery, and diffusion processes that occur post-extrusion to investigate the effects that typical printing conditions and amorphous (non-crystalline) polymer rheology have on the ultimate weld structure. Although we find the weld thickness to be of the order of the polymer size, the structure of the weld is anisotropic and relatively disentangled; reduced mechanical strength at the weld is attributed to this lower degree of entanglement.

Mechanical properties of 3D parts fabricated by fused deposition modeling: Effect of various fillers in polylactide

“…In 90° specimens, deposited filaments and their bond interfaces are perpendicular to the load direction. The layer‐to‐layer bond interface often cannot be comparable to bulk materials, since the thermal energy and time provided by the FDM process does not allow for the formation of sufficient diffusion thickness and entanglement at the interface . As a result, 90° specimens tend to facture at those weak bond interfaces (proved by the fracture morphology in Figure S2) and are used as a criteria for the interlayer bond strength of FDM parts .…”

“…When crystalline polymer filaments are used to prepare FDM parts, a square model (length as 100 mm, width as 100 mm, and height as 4 mm, and the width of frames and diagonals is 2 mm) was selected to evaluate the print quality of FDM parts. It is reported that the extrusion process can significantly deform and disentangle the polymer chains as high shear rates of 100–200 rad s −1 was generated in the liquifiers . After deposition, the printed layer cools; meanwhile, the disentangle deformation relaxes via reputation.…”

Entanglement dynamics at flat surfaces: Investigations using multi-chain molecular dynamics and a single-chain slip-spring model

“…The dynamics of confined polymers is an interesting subject that is particularly important for the behavior of nanocomposite materials 1,2 , the phenomenon of surface slip in polymer melts 3,4 , and additive manufacturing 5 . However the dynamics are difficult to characterise and predict.…”

Tuning the viscoelastic features required for 3D printing of PVC-acrylate copolymers obtained by single electron transfer-degenerative chain transfer living radical polymerization (SET-DTLRP)

“…In conjunction with adequate viscosity, polymers should show a sound layer adhesion leading to good weld strength of the manufactured object or piece [7,10,11]. As pointed out by Miegler and coworkers [24], this is related to the capacity of the polymer chains for interlayer diffusion, which is only possible in the terminal or flow viscoelastic region.…”

“…Thus, the understanding of the underlying rheological parameters of materials required for a successful printing, which are unclear at the moment and forces manufacturers to a trial-and-error process, can help to expand the number of materials suitable for these manufacturing techniques. Several studies focus on the modelling of melt-extrusion additive manufacturing, both fused filament fabrication [4,7], and extrusion-based printing techniques [8,9]. In essence, 3D printing using polymer melts implies at least three essential features: a) Adequate viscosity to ensure the envisaged printing velocity b) Immediate adhesion of deposed polymer layers and c) Adequate cooling process to guarantee final mechanical properties [10][11][12].…”