Enhancing the mechanical properties of fused filament fabricated parts by elevating the print room temperature

Abstract

Additive manufacturing (AM) is currently making a transition from mainly being used for prototyping to an alternative for conventional production methods. AM provides more freedom in design geometry combined with the possibility to produce (complex) products in low quantities. As a result, the need arises for printed parts with consistent mechanical properties that rival those of conventionally produced parts. Currently, the layer-by-layer process used by AM introduces weaknesses at the interlayer (weld) bonds. Polymer science suggests that raising weld bonds to temperatures above the glass transition temperature (Tg) can improve their mechanical properties up to bulk strength. This research aims to verify if fused filament fabrication (FFF) printing at elevated print room temperatures (Tenv) raises interlayer temperatures above Tg and as a result enhances mechanical properties. In order to measure the increase in interlayer properties double cantilevered beam (DCB) testing is used. Additionally, tensile testing is used to evaluate the effect of elevating Tenv on general mechanical properties. DCB and tensile testing of samples printed at elevated Tenv values has shown an increase of 109 % in interlayer energy release rate (G1c), up to 50 % in ultimate tensile strength and 106 % in tensile toughness. This has been determined by comparing samples printed at elevated Tenv with samples printed at room temperature (23 ◦C). To further understand the mechanisms behind the enhanced properties, the temperature history during printing has been determined. This has been done by running simulations and using IR imaging. The temperature history of printed parts has been related to data obtained from mechanical testing. This showed that mechanical properties increased for samples in which the interlayer bonds resided above Tg for prolonged periods of time. In addition, optical microscopy and micro CT has been used to monitor the meso-structure of printed samples. This showed that voids in the sample caused by printing defects contributed to an increased spread in measured values. Mechanical properties corrected for effective surface areas showed a roughly 10 % increase for average values. While significant this is not in the same order of magnitude as the increase in mechanical values reported for printing at higher Tenv values. Combining these results it has been found that elevating Tenv does not significantly affect the meso-structure, but it does cause bonds to spend an increased amount of time above Tg. Therefore, it has been concluded that the enhanced mechanical properties are caused by weld bond healing due to the part residing above Tg.