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

Abstract

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.

Keywords: 3D printing; FDM interface strength; additive manufacturing; finite element modeling.

FIG. 1.

Left image shows a schematic of the FDM 3D printing process. Right image shows a 2D illustration of printed layers showing interfaces (dashed lines) and applied tensile stress (arrows). FDM, fused deposition modeling.

FIG. 2.

Custom slot design (top left) and close-up view of four-extrusion configuration layer preview showing the nozzle position where the layer seam occurs (top right). Dashed lines signify flat sections to be cut out for specimen fabrication that isolates extrusion interfaces. FDM tensile dog-bone specimen in various stages of the laser cutting preparation (bottom left). Experimental tensile test setup with USB camera video capture for DIC strain measurement (bottom right). DIC, digital image correlation.

FIG. 3.

Digital microscope images of three-extrusion (left) and four-extrusion (right) polished specimen cross-sections. The asymmetric gap morphology is created by the order in which the layers are deposited. In both images, the right-most extrusion is deposited first, followed sequentially from right to left. The extrusions overlap and rest over the previous one, while not fully filling the available space, leading to the void shapes observed.

FIG. 4.

FEM models of three-extrusion and four-extrusion FDM 3D printed tensile specimens (top), and five-extrusion model (bottom) to determine geometric of additional perimeters on the interface strength. FEM, finite element method.

FIG. 5.

Matching geometry model 3D view (left), and matching geometry model mesh (right), with dashed black lines show the locations of the cohesive surface definitions.

FIG. 6.

Original matching geometry model (top left), symmetric model made with similar gap dimensions and area (top right), improved microstructure model with a reduction in gap area (bottom left), and a perfect model with no gaps between extrusions while remaining exterior layer shape (bottom right).

FIG. 7.

Postfailure image of tensile test specimen (left), and failure surface microscopy showing planar fracture at interface between layers (right). The arrow shows the direction of the applied tension.

FIG. 8.

Tensile stress contours of matching geometry models. Left image shows the stress concentrations that form along the sharp corners, as indicated by the arrow. The right image shows the progressive failure of the interlayer interface where half of the interface has debonded. The crack tip and failure process zone are indicated by the arrow. The darker region of the stress contour on the right side of the model signifies low tensile stress due to the debonded interface along that portion of the model.