Session: Undergraduate Research in Polymer Science:

Parts manufactured via Fused Filament Fabrication (FFF) are subject to particular mechanical vulnerability in the z-axis. Enhanced diffusion of polymer chains between layers shortly after extrusion dictates the extent of interlayer adhesion for which controllable parameters such as print speed and temperature significantly affect. Elucidating patterns in interlayer adhesion and macroscopic characteristics such as overall crystallinity with respect to print parameters will allow for optimization of FFF manufactured parts. Specifically, there is a lack of available research focusing on print parameter effects on interlayer adhesion of semi-crystalline polymers which provides the impetus for our research.
Annular rectangles with single-extrusion-walls were printed at different print speed and temperature combinations using polylactic acid (PLA) (low degree of crystallinity) and polypropylene (PP) (a higher degree of crystallinity). Samples were subjected to trouser tear testing to directly measure interlayer adhesion force and energy and differential scanning calorimetry (DSC) to measure degree of crystallinity. Multiple replicates at a specific treatment were printed to evaluate the precision of the applied trouser tear tests and DSC.
PLA samples printed at a constant speed showed an increase in adhesion energy as the temperature increased from 190 to 210°C. Furthermore, adhesion energy was found to decrease marginally as the print temperature increased from 210 to 230°C. The increase in adhesion energy with respect to print temperature is consistent with the notion that existing layers remaining above the glass transition contributes to better interlayer diffusion. The data suggest that interlayer adhesion of PLA has little dependence on print speed. Degree of crystallinity data was found to fluctuate over a wide range for samples of the same treatment. This high variability would suggest that print conditions have minimal effect on the overall crystallinity of the FFF printed part of polymers having a low degree of crystallinity.

Simplified depiction of FFF, left, and adhesion energy as a function of print temperature at constant print speed, right.

Simplified depiction of FFF, left, and adhesion energy as a function of print temperature at constant print speed, right.

Arit Das

Oak Ridge National Laboratory Physical Sciences Directorate

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