Mechanical Characterization of 3D Printed Core and Sandwich Composite

Abstract

Fused filament fabrication (FFF) is one of the most widely used additive manufacturing (AM) techniques to fabricate lightweight complex functional parts with zero tooling cost, lower energy, and reduced material consumption. Three-dimensional (3D) printed lightweight hollow particle-filled syntactic foam core, and sandwich composites are developed using the FFF process in the present work. Hollow glass micro balloons (GMBs) are used as filler particles, and high-density polyethylene (HDPE) is used as matrix material. Hollow GMBs have blended with HDPE matrix by 20, 40, and 60 volume % to form GMB/HDPE blends. These blends are extruded using a single screw extruder to develop lightweight feedstock filaments to be used as input in a 3D printer to print syntactic foam core and sandwich composites. The suitable extruder parameters are chosen to ensure a homogeneous mixture of constituent materials and develop syntactic foam filaments with minimum or no GMB particle breakage. Before printing of syntactic foam core and sandwich structures, the melt flow index (MFI), differential scanning calorimetry (DSC), coefficient of thermal expansion (CTE) and rheological properties of GMB/HDPE blends are studied to optimize the 3DP parameters. An increase in GMB content reduces MFI owing to the filler resistance to HDPE flow. MFI decreased by 23.29, 54.79, and 72.97%, increasing GMB by 20, 40, and 60 vol. %, respectively. A decrease in crystallinity (56.68%) for foam filaments is observed with increasing GMB % compared to HDPE. Compared to filaments, the corresponding prints have higher crystallinity and are anticipated to provide higher dimensional stability and reduce warpage-related issues. CTE values qualitatively exhibit warpage and dimensional stability information of 3D printed HDPE and foam samples. The addition of GMB in the HDPE matrix lowers CTE values. At higher printing temperatures, dimensional stability can be achieved by adding GMB into HDPE. This indicates that the warpage can be avoided to a greater extent in printed components with dimensional stability and lower residual thermal stresses. An increase in filler infusion increases the melt viscosity of the polymer and is observed in the entire frequency sweep during the rheological study of GMB/HDPE blends. At a higher frequency, HDPE shows a shear-thinning region. Similar behavior is observed in foams with a marginal increase in complex viscosity. With the increase in filler content and frequency, both storage and loss modulus are increased. All these properties act as a guideline for selecting appropriate process parameters for the printing of quality components. The performance and behavior of extruded foam filaments are influenced by the interaction of the filler−matrix, filler %, and matrix porosity. For filaments to be used in a 3D printer, adequate spooling stiffness and strength are needed. Hence, tests to find the density and morphology of the extruded filament and tensile properties are performed before printing to check the quality, stiffness, and strength necessary for filament feasibility to be used in a commercially available printer. HDPE filament's experimental and theoretical densities are very close, indicating lower void formations because of its hydrophobic nature. An increase in GMB content increases void content in filaments (0.84 - 7.70%) and prints (2.42 - 9.73%). Higher void content in print, as compared to filaments, indicate that matrix porosity is transferred from the filament to prints. Such porosity in prints amid 100% infill is because of air gaps between the raster (residual micro-porosity). These porosities form three-phase (HDPE, GMB, and raster gap) syntactic foams enhancing the damping capabilities. Tensile testing of extruded filaments is carried out to know its feasibility in a 3D printer. Stiffer intact GMB particles increase filament modulus by 8 - 47% in H20, H40, and H60, respectively, compared to neat HDPE. H20 exhibits more than 40% strain with the highest ultimate tensile strength (UTS) of 12.63 MPa among foams. In comparison, H60 exhibits the highest modulus because of a higher number of intact GMB particles. Strength decreases with increasing filler content as with increasing GMB content, HDPE volume decreases, lowering the ductile phase substantially. Pilot investigations are carried out to propose the suitable printing parameters for printing core (H20-H60) and sandwiches (SH20-SH60) by exploiting Nozzle - 1 and Nozzle - 2 available on the commercial FFF-based printers. GMBs presence in the HDPE matrix reduces the co-efficient of thermal expansion leading to lower warpage and samples with dimensionally closer tolerance. Several initial trials in the pilot investigations did not yield high-quality prints. The reasons for such observations and the possible solutions are discussed that result in sound quality core and sandwiches. Tensile testing of 3D printed samples exhibits similar behavior as that of respective filaments. Among foams, H60 displays the highest modulus and is 48.02% higher than the HDPE print. H20 shows up to 30.48% strain. In HDPE, a long necking region is observed due to raster fibrillation leading to the broom-like fibrous ends. A typical brittle fracture is observed in H40 and H60. 3D printed HDPE and foams modulus is better than respective filaments. The flexural testing of HDPE and syntactic foam core and sandwich composites are carried out in a three-point bending configuration. Foams displayed brittle fracture as compared to neat HDPE, which did not fail until 10% strain. GMB inclusion induces brittleness in the compliant HDPE matrix. An intact GMB particle increases the flexural modulus with higher filler loadings. The H60 modulus is 1.37 times higher than HDPE, while strength is observed to be decreased. Lower strength values are due to the poor interface bonding between constituent elements and raster gaps. Similar behavior is observed in flexural testing of syntactic foam-cored sandwich samples. SH20 did not fail until 10% strain and registered the highest strength as compared to other sandwiches. SH40 and SH60 showed a brittle fracture. SH60 showed the highest modulus compared to other sandwich compositions. The flexural strength of syntactic foam cored sandwich samples are higher than their respective cores. The mechanics of composite beam theory is used for theoretical calculations of critical load. The deviation between the experimental and theoretical loads is noted to be in very good agreement, up to half of the maximum load. The failure mode of sandwich structures is analysed, and it is observed that SH40 and SH60 showed indentation failure. None of the samples failed in shear. All the samples except SH20 fractured in an approximately straight line just below the loading point. Compressive responses of 3D printed core and sandwich samples are investigated at a constant crosshead displacement rate of 0.5 mm/min. The data is analysed using in-house developed MATLAB code to estimate yield strength and modulus for all the samples. HDPE exhibits a higher modulus and is 1.06 times higher than H60. The modulus of foam samples increases with GMB content. H60 displayed the highest modulus among foams due to the presence of intact GMBs at higher filler loading. HDPE displayed 1.23 times higher yield strength compared to H60 samples. The Yield strength of syntactic foams decreases with an increase in filler loading because of poor interface bonding between constituent elements and residual micro-porosities. Similar behavior is observed in sandwich samples as well. Among sandwiches, SH20 has higher yield strength, and SH60 has the highest modulus. The buckling and vibration response of 3D printed foams subjected to axial compression is investigated. The buckling load is estimated using Modified Budiansky Criteria (MBC) and Double Tangent Method (DTM) through the load-deflection plots. The first three natural frequencies and their mode shapes are computed as a function of axial compressive load. It is noted that the natural frequency reduces with an increase in axial compressive load. It is also observed that with an increase in GMB %, the natural frequencies and critical buckling load increase. Analytical solutions obtained from the Euler‐Bernoulli‐beam theory are compared with experimental results. Similar behavior is observed for sandwich samples that displayed global buckling mode during the buckling test, wherein the maximum deflection is reported at the mid-section with no signs of skin wrinkling, delamination, and skin micro buckling. The load-deflection data and frequency obtained experimentally are compared with numerical predictions deduced using finite element analysis (FEA), which is noted to match well. The comparative analysis of 3D printed samples is carried out with samples developed using other thermoplastic manufacturing routes through property maps for specified test conditions. The current work successfully demonstrated the development of lightweight feedstock filament intending to widen available material choices for commercially available 3D printers.