Fused Deposition Modeling (FDM) is a widely used additive manufacturing technology for fabrication of complex geometric parts using thermoplastic polymers. The quality issues and inferior properties of fabricated parts limited this process to manufacture parts for industrial level applications. Reinforcing the polymer with nanoparticles, short fibers or continuous fibers improve mechanical, thermal and electrical properties compared to the neat polymer. Several works have been carried out since last two decades to print quality products through FDM by using composite materials. The success of expanding this technique to industrial applications depends on the preparation of printable composite feedstock filament and printing without defects. This article reviews the challenges involved in the preparation of composite feedstock filaments and printing issues during the printing of nano composites, short and continuous fiber composites. The printing process of various thermoplastic composites ranging from amorphous to crystalline polymers is discussed. Also, detailed explanation is given about the analytical and numerical models used for simulating the FDM printing process and for estimating the mechanical properties of the printed parts. This critical review mainly helps the young researchers working in the area of processing of composite materials via 3D printing.

A critical review on the fused deposition modeling of thermoplastic polymer composites

3D Printing is widely used in scientific researches and engineering applications, ranging from aerospace to biomedicine. However little is known about the mechanical properties of 3D printing materials. In order to promote the mechanical analysis and design of 3D printing structures, the ultimate tensile strength of FDM PLA materials with different printing angles were studied theoretically and experimentally. A theoretical model was firstly established to predict the ultimate tensile strength of FDM PLA materials based on transverse isotropic hypothesis, classical lamination theory and Hill-Tsai anisotropic yield criterion, and then verified by tensile experiments. Compared with previous models, this model provided two kinds of in-plane shear modulus calculation methods, so the calculation results were more reliable. The specimens, designed according to the current plastic-multipurpose test specimens standard ISO 527-2-2012, were printed in seven different angles ( 0 ∘ , 15 ∘ , 30 ∘ , 45 ∘ , 60 ∘ , 75 ∘ , 90 ∘ ) with three layer thicknesses (0.1 mm, 0.2 mm, 0.3 mm) for each angle. The relative residual sum of squares between theoretical data and experimental data were all close to zero, so the results that the theoretical model can accurately predict the ultimate tensile strength of FDM materials for all angles and thicknesses were confirmed. It was also found that the ultimate tensile strength decreased as the printing angle becomes smaller or the layer becomes thicker. This theoretical model and experimental method can also be applied to other 3D printing materials fabricated by FDM or SLA techniques.

A method to predict the ultimate tensile strength of 3D printing polylactic acid (PLA) materials with different printing orientations

The increase in accessibility of fused filament fabrication (FFF) machines has inspired the scientific community to work towards the understanding of the structural performance of components fabricated with this technology. Numerous attempts to characterize and to estimate the mechanical properties of structures fabricated with FFF have been reported in the literature. Experimental characterization of printed components has been reported extensively. However, few attempts have been made to predict properties of printed structures with computational models, and a lot less work with analytical approximations. As a result, a thorough review of reported experimental characterization and predictive models is presented with the aim of summarizing applicability and limitations of those approaches. Finally, recommendations on practices for characterizing printed materials are given and areas that deserve further research are proposed.

https://www.mdpi.com/1996-1944/12/6/895/pdf

Characterization of the Mechanical Properties of FFF Structures and Materials: A Review on the Experimental, Computational and Theoretical Approaches

Previous studies have shown that 3D printed composites exhibit an orthotropic nature with inherently lower interlayer mechanical properties. This research work is an attempt to improve the interlayer tensile strength of extrusion-based 3D printed composites. Annealing was identified as a suitable post-processing method and was the focus of this study. Two distinct thermoplastic polymers, which are common in 3D printing, were selected to study the enhancement of interlayer tensile strength of composites by additive manufacturing: a) an amorphous polyethylene terephthalate-glycol (PETG), and b) a semi-crystalline poly (lactic acid) (PLA). It was determined that short carbon fiber reinforced composites have lower interlayer tensile strength than the corresponding neat polymers in 3D printed parts. This reduction in mechanical performance was attributable to an increase in melt viscosity and the consequential slower interlayer diffusion bonding. However, the reduction in interlayer tensile strength could be recovered by post-processing when the annealing temperature was higher than the glass transition temperature of the amorphous polymer. In the case of the semi-crystalline polymer, the recovery of the interlayer tensile strength was only observed when the annealing temperature was higher than the glass transition temperature but lower than the cold-crystallization temperature. This study utilized rheological and thermal analysis of 3D printed composites to provide a better understanding of the interlayer strength response and, therefore, overcome a mechanical performance limitation of these materials.

Enhancing the interlayer tensile strength of 3D printed short carbon fiber reinforced PETG and PLA composites via annealing

Additive manufacturing (commonly known as 3D printing) is defined as a family of technologies that deposit and consolidate materials to create a 3D object as opposed to subtractive manufacturing methodologies. Fused deposition modeling (FDM), one of the most popular additive manufacturing techniques, has demonstrated extensive applications in various industries such as medical prosthetics, automotive, and aeronautics. As a thermal process, FDM may introduce internal voids and pores into the fabricated thermoplastics, giving rise to potential reduction on the mechanical properties. This paper aims to investigate the effects of the microscopic pores on the mechanical properties of material fabricated by the FDM process via experiments and micromechanical modeling. More specifically, the three-dimensional microscopic details of the internal pores, such as size, shape, density, and spatial location were quantitatively characterized by X-ray computed tomography (XCT) and, subsequently, experiments were conducted to characterize the mechanical properties of the material. Based on the microscopic details of the pores characterized by XCT, a micromechanical model was proposed to predict the mechanical properties of the material as a function of the porosity (ratio of total volume of the pores over total volume of the material). The prediction results of the mechanical properties were found to be in agreement with the experimental data as well as the existing works. The proposed micromechanical model allows the future designers to predict the elastic properties of the 3D printed material based on the porosity from XCT results. This provides a possibility of saving the experimental cost on destructive testing.

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Effect of Porosity on Mechanical Properties of 3D Printed Polymers: Experiments and Micromechanical Modeling Based on X-ray Computed Tomography Analysis.

The present paper develops the constitutive material models of the 3D printed parts via fused deposition modeling. Additive manufacturing of a part results in a complex microstructure which depends on the process parameters and build orientation. Consequently, anisotropy is introduced into the material properties. The mechanical behavior of the printed parts is governed by the constitutive behavior of the material. Therefore, the stiffness matrix of the material of the final printed part needs to be estimated for accurately capturing their behavior. The constitutive material modeling of the printed parts using numerical homogenization procedure is emphasized in this work. The present simulation models can capture the influence of build orientation, printing direction and layer thickness on the material behavior of the printed parts. Then, the influence of layer deposition in printing of differently oriented parts of the structure on the material behavior is investigated. It is revealed that the material behavior of different parts of the structure is not same and is dependent on the build orientation of the parts and also their thickness. This work aids the computation of elastic moduli and also selecting of the correct constitutive material model of the printed parts for stress analysis.

Development of constitutive material model of 3D printed structure via FDM

Mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements

In the present study, the mesostructure of a fused deposition modeling (FDM) processed part is considered for the investigation of its mechanical behavior. The layers of FDM printed part behave as a unidirectional fiber reinforced laminae, which are treated as an orthotropic material. The finite element (FE) procedure to find elastic moduli of a layer of the FDM processed part is presented. The mesostructure of the part that would be obtained from the process is replicated in FE models in order to find stiffness matrix of a layer. Two distinctive architectures of mesostructure are considered in the study to predict the mechanical behavior of the parts. Also, influences of layer thickness, road shape and air gap on the elastic properties of a material are investigated. Further, the parts fabricated with FDM process are treated as laminate structures. From the numerical results, it is seen that the elastic moduli are governed by mesostructures. Our laminate results are validated with experimental to demonstrate the use of classical laminate theory (CLT) for FDM parts. The elastic moduli (E1, E2, G12, ν12) of a layer used in the analysis are calculated from FE simulations. This study establishes relationship between mesostructure and macro mechanical properties of the part.

Mechanical Characterization of Additively Manufactured Parts by FE Modeling of Mesostructure

Anisotropic material behavior of 3D printed composite structures – Material extrusion additive manufacturing

A comprehensive understanding of process–structure–property relationship of 3D printed parts is currently limited. In the present study, we investigate the influence of the mesostructure on the overall mechanical behavior of the parts synthesized via fused filament fabrication. In particular, characterization of anisotropic behavior is carefully studied by performing mechanical testing on the printed parts. The printed parts are treated as laminates and are characterized using laminate mechanics. Test coupons of thick layered and also thin layered unidirectional as well as bidirectional laminates are printed with polymeric material for tensile and bending tests. Test results revealed that the process parameters govern the mesostructure and therefore the material behavior of the parts. Mechanical behavior of the bidirectional printed laminates is studied in detail. The properties are significantly influenced by the layer thickness and layup order of the printed parts. Mechanical behavior of the printed parts can be characterized using laminate theory. The effect of lamina layup and layer thickness on the flexural properties of the laminates is significant. Furthermore, the first ply failure theory is employed for the finite element failure analysis of the printed parts. The results provide insights in the relationship between mesostructure–mechanical properties of the printed parts.

Madhukar Somireddy

Papers

Development of constitutive material model of 3D printed structure via FDM

Mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements

Mechanical Characterization of Additively Manufactured Parts by FE Modeling of Mesostructure

Anisotropic material behavior of 3D printed composite structures – Material extrusion additive manufacturing

Analysis of the Material Behavior of 3D Printed Laminates Via FFF