Effect of initial filament moisture content on the microstructure and mechanical performance of ULTEM® 9085 3D printed parts

Abstract

This study investigates the moisture absorption characteristics of the ULTEM® 9085 filament and how the uptake concentration affects the quality of material extrusion manufactured 3-D parts. The filament was exposed to humidity conditions to achieve various moisture concentrations (0, 0.05, 0.1, 0.16, 0.4 and 0.8 wt.%) and corresponding printed parts were evaluated for mechanical performance using tensile test dogbones. The rate of transport was modeled by Fickian diffusion and diffusion coefficients were obtained for various exposure conditions. Moduli, strain to failure and ultimate strength were evaluated in the XY (flat horizontal) and ZX(vertical) direction relative to the build plate orientation. Image analyses of cross-sections as well as their corresponding fracture surfaces were evaluated for consolidation, porosity distribution and failure behavior. Mechanical test data showed a significant decrease in tensile strength (>60%) and failure strain (>50%) over the range of filament moisture levels investigated. A decrease in failure strain of 41% was observed with moisture levels as low as 0.16%. This degradation was especially sensitive in parts printed in the vertical direction, which resulted in an ultimate failure strain of only 1%. The changes in mechanical performance are believed to be due to a combination of entrapped volatiles resulting in increased porosity at higher moisture levels as well as moisture induced pseudo-crosslinking at lower concentrations. Optical micrographs showed that specimens with 0.16% moisture or greater were filled with observable porosity and increased surface roughness. The rheological behavior of extruded material indicated plasticization as evidenced by melt flow index measurements and changes in the flow characteristics of moisture-exposed extrudate. DMA data show a distinct decrease in Tg with increased moisture content, which is consistent with plasticization. The absorption characteristics at room temperature lab conditions indicate that the material will reach an unacceptable level within one hour of room-temperature exposure. This investigation emphasized the need for awareness of the moisture sensitivities of ULTEM® 9085 when manufacturing high-quality material extrusion processed structures.