(392c) Understanding Flow and Stress Development in 3D Printing By Material Extrusion

Additive manufacturing of plastics offers a plethora of potential advantages associated with primarily mass customization, designs incompatible with injection molding, and distributive manufacturing. 3D printing of thermoplastics by Material Extrusion (MatEx) using filaments is well established, but the properties of the printed parts tend to be inferior to those of parts produced by conventional plastic manufacturing techniques. Defects (voids) and poor interface development between printed roads tend to be blamed for the poor performance. Additionally, the transient nature of the print leads to highly non-isothermal processing histories and potential chain alignment as the cooling rates are rapid and can be similar to relaxation times for the polymers. Moreover, the print path involves rapid acceleration and deceleration of the print head around corners, which will influence the viscoelastic properties of the polymer as it is printed. Here we directly visualize the flow in simple printed coupons using a plug of pigment in the filaments to examine the role of layer height (compression) and corners in the printed object. These results illustrate the formation of waves in the apparent streamlines from the flow indicator near the edges of even simple sample. These effects can extend over longer lengths than expected based on the simulated print path and suggest that elastic effects in the melt can be significant. The inclusion of transition metals in the pigments provides x-ray contrast to enable the visualization of the flow profiles with x-ray tomography in addition to optical observations, which provides 3D rendering of the flow profiles around the corners of the print. The complex temperature history will also influence the dimensional accuracy of the printed part through the stress development during solidification. We will briefly describe how the printed shape influences these effects and the reported mechanical properties.