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

Abstract

One of the main drawbacks of Fused Filament Fabrication is the often-inadequate mechanical performance of printed parts due to a lack of sufficient interlayer bonding between successively deposited layers. The phenomenon of interlayer bonding becomes especially complex for semi-crystalline polymers, as, besides the extremely non-isothermal temperature history experienced by the extruded layers, the ongoing crystallization process will greatly complicate its analysis. This work attempts to elucidate a possible relation between the degree of crystallinity attained during printing by mimicking the experienced thermal history with Fast Scanning Chip Calorimetry, the extent of interlayer bonding by performing trouser tear fracture tests on printed specimens, and the resulting crystalline morphology at the weld interface through visualization with polarized light microscopy. Different printing conditions are defined, which all vary in terms of processing parameters or feedstock molecular weight. The concept of an equivalent isothermal weld time is utilized to validate whether an amorphous healing theory is capable of explaining the observed trends in weld strength. Interlayer bond strength was found to be positively impacted by an increased liquefier temperature and reduced feedstock molecular weight as predicted by the weld time. An increase in liquefier temperature of 40 °C brings about a tear energy value that is three to four times higher. The print speed was found to have a negligible effect. An elevated build plate temperature will lead to an increased degree of crystallinity, generally resulting in about a 1.5 times larger crystalline fraction compared to when printing occurs at a lower build plate temperature, as well as larger spherulites attained during printing, as it allows crystallization to occur at higher temperatures. Due to slower crystal growth, a lower tie chain density in the amorphous interlamellar regions is believed to be created, which will negatively impact interlayer bond strength.

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

“…This may be indicative of the unsuitability of the crazing model to properly describe the failure of PLA welds with frozen-in orientation. Another possible reason is the existence of a low degree of crystallinity at the weld, which could effectively decrease the strength of the inter-layer bonding [ 31 , 46 ].…”

“…Along the same lines, in the study of poly-ether-ether-ketone (PEEK) printed samples, Xu et al showed that a reduction of molecular mass can increase fluidity, reduce internal defects and, consequently, significantly improve the mechanical properties of the printed product [ 30 ]. Moreover, by employing a trouser tear test, Vaes et al demonstrated that for nylon copolymers a decrease of the feedstock molar mass improved interlayer adhesion, due to the increased equivalent weld time [ 31 ]. However, the possible effect of residual orientation at the weld interface was neglected.…”

High-performance co-polyesters for material-extrusion 3D printing: A molecular perspective of weld properties

“…Due to the semi-crystalline nature of these co-polyester materials, the high-frequency and low temperature rheology data is inaccessible via traditional techniques. Thus, here we extrapolate to low temperatures via the WLF equation to calculate weld time, as is done broadly in the field [21,34]. We note that the WLF equation is known to be inaccurate in the vicinity of the glass transition temperature (e.g., see Ref [35,36]), thus here we report a qualitative comparison of the materials rather than an accurate prediction of the weld times; by comparing the two materials at the same print bed temperature the weld time of HT is limited compared to NGEN.…”

Laser sintering of self-healable and recyclable thermoset networks

“…It is expected that a chamber temperature above Tg could trigger the DA bonds formation between colliding particles, thus affecting the powder flow properties and particle size distribution for further reusage [48]. Regular amorphous and semicrystalline thermoplastics used in additive manufacturing commonly exhibit a weak interlayer bonding in the printing z-direction due to the hindered interlayer chain diffusion upon cooling [59].…”