Investigation of Interlayer Interface Strength and Print Morphology Effects in Fused Deposition Modeling 3D-Printed PLA

Fused deposition modeling polymer 3D printing has become a popular versatile additive manufacturing technology. However, there are limitations to the mechanical properties due to the layer-by-layer deposition approach. The relatively low strength of the interface between layers is the cause for potential microstructural weak points in such printed components. The interface strength of 3D-printed Polylactic Acid (PLA) polymer was determined through physical tensile testing in combination with microstructural finite element method (FEM) simulations. A custom tensile specimen was created to isolate the interlayer interfaces for direct testing of interface strength. Tensile tests resulted in an average 2.4 GPa stiffness and an average 22.8 MPa tensile strength for printed specimens, corresponding to a 32.4% and 47.8% reduction from the bulk filament stiffness and strength, respectively. Sectioned tensile specimens were observed under a digital microscope to examine microstructural features such as inter-layer gaps, extrusion cross-section, and voids. These were measured to create accurate FEM microstructural model geometries. The brittle fracture that occurred during the tensile testing was due to debonding of the interfaces. This was represented in Abaqus by using cohesive surfaces. Interface strength was inferred by varying the strength of the cohesive surfaces until the simulation mechanical response matched the physical tests. The resulting interface strength of the PLA polymer was 33.75 MPa on average, corresponding to a 22.5% reduction from bulk properties. Potential improvements to the overall strength of the 3D printed PLA were investigated in simulation by parameterizing improved gap morphologies. As the size of the interlayer gaps decreased, the stiffness and strength of the printed parts improved, whereas completely eliminating gaps resulted in a potential 16.1% improvement in material stiffness and 19.8% improvement in strength. These models show that significant improvements can be made to the overall printed part performance by optimizing the printing process and eliminating inner voids.