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

Dries Vaes,Peter Van Puyvelde,Bart Goderis,Margot Coppens,Wim Zoetelief

doi:10.3390/polym13162677

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.

Highlights

Filament is continuously fed into the print head, moving at a set print speed in the xy-plane, which results in the solid portion of the filament acting as a plunger so molten polymer can be deposited according to a predefined pattern onto the heated build plate, making up the first layer of the envisioned 3D object
As a direct result of an ongoing trend where Fused Filament Fabrication (FFF) is evolving from a technique for rapid prototyping to a manufacturing process for the production of functional parts for high-end applications, the necessity to incorporate more engineering and high-performance thermoplastics, which are often semi-crystalline in nature, becomes evident in the current pool of feedstock polymers
One of the main drawbacks of the FFF process remains the often poor mechanical performance of the printed parts as a result of insufficient interlayer bonding between successively deposited layers, which can be hindered by the crystallization process due to the incorporation of macromolecular chains into growing crystals, drastically limiting molecular mobility

Summary

Introduction

Ongoing developments and improvements of AM technologies, including FFF, have sparked a clear transition over the last few years Where these techniques were often used as means for rapid prototyping in the past, nowadays, they have evolved into stand-alone production processes suited for the fabrication of functional products and parts of high quality for more high-end and technical applications. This trend is predicted to become even more prominent in the future [7]. A thorough understanding of the extent of crystallization during FFF and its relation to final printed part quality will become highly beneficial to successfully employ these polymers as feedstock materials in FFF for high-end applications in the future

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Results

Conclusion