NUMERICAL AND EXPERIMENTAL ANALYSIS OF INFILL RATE ON THE MECHANICAL PROPERTIES OF FUSED DEPOSITION MODELLING POLYLACTIC ACID PARTS

Size: px

Start display at page:

Download "NUMERICAL AND EXPERIMENTAL ANALYSIS OF INFILL RATE ON THE MECHANICAL PROPERTIES OF FUSED DEPOSITION MODELLING POLYLACTIC ACID PARTS"

Carmella Ray
5 years ago
Views:

1 NUMERICAL AND EXPERIMENTAL ANALYSIS OF INFILL RATE ON THE MECHANICAL PROPERTIES OF FUSED DEPOSITION MODELLING POLYLACTIC ACID PARTS Övgü Yağız Çiçek, Atakan Altınkaynak, Istanbul Technical University, Istanbul, Turkey, Efe Can Balta, University of Michigan, Ann Arbor, MI, Abstract Material characterization of 3D printed PLA parts with different infill rates was performed by standard tensile tests. Three-dimensional finite element simulations of the tensile tests were also performed. For the simulations, a method for reconstructing the CAD model of the printed parts with different infill rates was proposed and numerical simulations were conducted on the reconstructed models. A reasonable prediction of the experiments in terms of material behavior and the stress-strain characteristics for infill rate of 100% is obtained from the numerical analysis. Discrepancies between the numerical predictions and experimental results especially in the plastic deformation region for other infill rates are discussed in the paper. Introduction Additive manufacturing methods with 3-dimensional (3D) printers have had great attention in the manufacturing industry over the last decade. Constraints related to specific traditional manufacturing methods are nearly eliminated with 3D printers, allowing the elaboration of flexible designs. In early stages, 3D printers were used to manufacture hobby products. However, 3D printers have been recently used in various industrial applications such as prototype manufacturing, structural models in architecture, specialized customer products, and medical applications (i.e., dental implants, prosthesis, and tissue, among others). The type of material, manufacturing tolerances, and the product application determine the type of additive manufacturing methods to be used. These methods can be stereolithography, inkjet printing, fused deposition modelling, and laminated object manufacturing [1]. The main disadvantage of these methods is a longer production time compared to traditional manufacturing methods. This is the main reason why they have not been used for mass production. Fused deposition modelling (FDM) is the most common additive manufacturing method used for rapid prototyping. In this method, a softened thermoplastic material is deposited layer by layer onto a table using extrusion to manufacture a 3D part [2-3]. In FDM, a thermoplastic filament is pushed into the temperaturecontrolled extruder nozzle by using two pulleys. The thermoplastic material is softened while passing through the extruder nozzle. The movement of the extruder nozzle and the table is computationally controlled. Thus, the extruder nozzle follows a pre-defined 2D path to extrude the softened material and to generate the first layer of the 3D part. Then, the table moves downwards to generate the second layer. This process continues until the 3D part is manufactured. One disadvantage of this method is that depending on the dimensional tolerances, the part surface may have a step-like appearance. In addition, inconsistencies on diameter and density of the filament may affect the extrusion of the material from the extruder nozzle which affects the overall quality of the part [4]. Several parameters such as layer orientation, layer height, extrusion angle, infill line orientation and pattern, and filament-related properties play an important role on manufacturing time, surface quality, dimensional tolerances and performance of the 3D printed parts in FDM [5-6]. Numerous experimental studies have been performed to determine the effect of these parameters on the behavior of FDM parts [5-11]. It was concluded that the orientation of the material deposition has an effect on the mechanical properties of the FDM parts, therefore, it should be considered in material models. It was also emphasized that having several parameters affecting the FDM part performance requires the development of numerical methods that are able to determine the optimum parameters since experiments are expensive and time consuming. These publications reported an infill rate of 100% (solid interior) and the effect of infill rate on mechanical properties was not investigated. Although the need for numerical models is essential for the characterization of FDM parts, a limited number of publications is available on this topic (see e.g., 6, 12-13). In these studies, researchers determined the material constitutive model parameters from the experiments and used these model parameters to perform the numerical analysis. Their results showed a good agreement in the elastic deformation region, however, discrepancies on the plastic deformation region were observed. In this work, the effect of infill rate on the mechanical properties of FDM parts manufactured using polylactic acid (PLA) polymer filament were investigated. Standard tensile test experiments were performed and the results SPE ANTEC Anaheim 2017 / 75

2 were used to verify the three-dimensional finite element method simulations. Experimental Procedure Tensile test experiments were performed for the mechanical characterization of FDM polylactic acid (PLA) parts. For additive manufactured parts, Standard procedures for tensile test are not available, thus, ISO 527 standard (Plastics - Determination of tensile properties) was followed for the tensile tests. PLA filament manufactured by Oo-kuma 3D Technologies and Ultimaker 2+ 3D printer was used to manufacture the tensile test samples. Five samples were manufactured and tested for each infill rate of 25, 50, 75 and 100%. The manufacturing orientation of the samples was on the same direction as the loading direction of the tensile tester. Samples consist of three layers of wall on the boundaries and 45 grid on the inside. The 3D printer parameters used for the manufacture of the samples are presented in Table 1. A Shimadzu AG-IS universal tensile testing machine was used for the tensile experiments. Experiments were performed at a speed of 10 mm/min. The elongation of the samples was measured using optical extensometer with a gauge length of 75 mm and load cell equipped on the tensile tester measured the force data. The force and elongation were recorded and used to generate the engineering stress/strain curves. The cross-sectional area of the manufactured part is required for the calculation of stress values, however, a standard procedure for the determination of the cross-sectional area with infill rate different than 100% has not been developed. The development of such method is important to generate the stress data since the cross-sectional area changes in the axial direction. In order to plot the stress-strain curves on the tested samples, the axial location of the ruptured crosssection was determined from the samples and the area of the same cross-section was measured with the CAD model. Numerical Simulations In the present work, finite element simulations of tensile testing were performed for FDM PLA parts with 25, 50, 75, and 100 % infill rates. ABAQUS software was used for dynamic explicit simulations of the deformation. The first step for the simulations was to generate the CAD model of the 3D printed samples. In FDM process, the solid CAD model is imported into an open-source slicer software that generates the appropriate G-code depending on infill rate, infill pattern, layer height, etc. The information in the G-code defines the movement of the printing head and specifies other parameters that are used in the printing process such as the printing head temperature and speed, and amount of material pushed through the nozzle. G-codes describe movements that are the actual physical behavior of the machine and contain information about the actual model that is manufactured rather than the designed model in CAD form. Due to this reason, G-codes were processed for samples with different infill rates to determine the printing head movement. Fig. 1(a) shows the path of the printing head for one layer during manufacturing of samples with different infill rates. The line width data given in Table 1 and the path information in Fig. 1(a) were used to generate the 3D models of the samples as shown in Fig. 1(b). Generated 3D models were used in the finite element simulations of tensile testing. In the simulations, one end of the samples was fixed and the other end was pulled with the same speed used in the experiments. The material properties needed for the simulations such as modulus of elasticity, tensile strength, and plastic deformation were obtained from experiments performed for the 100% infill rate. This material information was used for all simulations including infill rates other than 100%. Progressive failure element option in ABAQUS was used in the simulations. In this feature, stress values were tracked at each time step. Once the stress value for an element exceeds a pre-defined value, the stress for that element is reduced to zero and the element does not carry any load for the next time steps in the simulations. Tensile strength value obtained from the 100% infill rate experiments were used as the pre-defined value for this feature. For the simulations, tetrahedral mesh was used to discretize the domain. The total number of elements were 56375, 98080, and for infill rates of 25, 50, 75, and 100%, respectively. Results and Discussion Experimental results from the tensile test experiments are shown in Figs Pictures of selected samples with different infill rates are shown in Fig. 2. All tensile test experiments (i.e., for all infill rates) exhibited ruptures from two different locations of the narrow parallel-sided section: (a) close to the edge or (b) close to the middle. Data from the samples that broke close to the edge of the narrow parallel-sided section is also presented in this work since they have a distinct stress-strain characteristic as discussed later in this section. This distinct behavior may be observed in practical applications of 3D printed parts due to nature of layer by layer manufacturing and infill type. The failed cross-section for almost all samples was perpendicular to the loading direction of the sample. The exception was one sample with an infill rate of 75% which showed a ruptured surface with an irregular inclined surface as shown in Fig. 2(c). It can be observed from Fig. 3 that the thickness of the bottom wall was not uniform and evenly distributed for this sample. Although the outer edge of the bottom wall was flat, the inner edge of the bottom wall showed an inclination. This non-uniformity on the wall thickness SPE ANTEC Anaheim 2017 / 76

3 caused a notch at the inner bottom-right corner of the sample as marked in Fig. 3 which is believed to cause a natural crack initiation due to stress concentration. Later, crack propagation occurred on the maximum shear plane which was also the interior grid orientation during manufacturing of the samples. The reason of the nonuniformity on the wall thickness may be due to nonuniform cooling of the polymer caused by non-uniform temperature distribution on the polymer or on the print bed. Figures 4(a)-(d) show the average stress-strain results and their standard deviations for samples with infill rates of (a) 25, (b) 50, (c) 75 and (d) 100%. In Fig. 5, the average stress-strain data is given for all infill rates for comparison. In Fig. 4(b)-(c), the standard deviations are not available above specific strain values because only one sample exhibited higher elongation behavior. As shown in Fig. 4, there are discontinuities on the stress-strain curves at a strain of approximately for 25% and 50% infill rates, for 75% infill rate and for 100% infill rate. This behavior is due to the failure of some samples at these strain values. For these samples, common failure location was the edge of the narrow parallel-sided section. In general, samples that failed from the middle of the narrow parallelsided section exhibited higher elongation compared to those failed from the edge of that section. Similar behaviors were also observed by Tymrak et al. [5]. It is possible that the manufacturing tolerances and local stress concentration due to infill type caused some samples to fail at lower strain values. The abrupt change on the stress-strain curve for the 75% infill rate sample at approximately strain was due to the failure of one sample showing an inclined surface (e.g., see Fig. 2(c)). The surface angle suggests a shear failure for this specimen, which was not observed on any other samples. The possible reasons for this behavior was discussed at the beginning of this section. Average modulus of elasticity, tensile strength and strain for all infill rates are given in Table 2. Modulus of elasticity values were calculated using linear regression analysis in accordance with ISO 527 Standard. The results showed that modulus of elasticity in loading direction increases with increasing infill rate. This behavior might be due to less number of contours loaded in the axial direction for samples with smaller infill rate. In the literature, the average tensile strength, average strain at tensile strength, and average elastic modulus for 100% infill rate for PLA printed samples were reported as 56.6 MPa, and 3368 MPa, respectively, for extension rate of 5 mm/s in the experiments [5]. As presented in Table 2, results obtained in our work for 100% infill rate samples were higher than those presented in [5]. For example, strain at tensile strength reported in [5] was whereas our experimental results showed an average strain of which is approximately 20 % higher. One of the reasons for obtaining higher tensile strength and elastic modulus may be the higher extension rate applied in our tensile experiments (10 mm/s compared to 5 mm/s). It is shown in [14] that the tensile strength and elastic modulus for ABS fused deposition material increases with increasing strain rate. The material composition may be another reason for obtaining higher tensile stress, strain and elastic modulus since the exact composition of the filament material is rarely known to the customers. In general, tensile strength increased with infill rate, except for 50 % (see dashed line in Fig. 5). The 50% infill rate showed a lower tensile strength behavior. The reason for this behavior is not clear at this point. Tensile experiments with more number of specimens should be performed to investigate this behavior. However, it should be noted that the procedure for determining the crosssectional area of the samples may cause higher standard deviations on the stress calculations. Stress results from the numerical simulations for all infill rates are shown in Fig. 6 at the onset of the failure. Fig. 7 compares the stress-strain curves obtained from the simulations and experiments. In the numerical simulations, stress and strain values can be calculated on elemental basis. In order to generate the stress-strain curves, the stress and strain values of five elements on the narrow parallelsided section of the samples were averaged. As shown in Fig. 7(d), a reasonable prediction of the experiments for infill rate of 100% was obtained from the simulation. Beyond a strain of 0.010, the slopes (i.e., secant modulus) of the experimental and simulation curves showed an inverse behavior. The experimental curve showed a negative slope, whereas simulations showed a slight increase on the secant modulus. Both numerical simulation and experimental results for infill rate of 100% showed decreasing stress in the plastic deformation region. However, the nature of this decrease was nonlinear in the experimental case whereas an almost linear curve was observed from the numerical results. Better agreement between experimental results and numerical simulations may be achieved through an improvement of the material model for the plastic deformation region. As discussed in the Numerical Simulations section, material model of 100% infill rate was used for other infill rate simulations (i.e., 25, 50, and 75%). As shown in Fig. 7, the numerical simulations and experimental data had discrepancies especially on the plastic deformation region. In order to improve the results of the numerical simulations, the material model should be updated to include (i) the decay on the stress-strain curve on the elastic region and (ii) failure according to the fracture strain. Collection of additional experimental data is also needed to accurately capture the material behavior. SPE ANTEC Anaheim 2017 / 77

4 Conclusions Standard tensile experiments were performed to characterize the average tensile strength, strain, and elastic modulus of PLA printed parts with different infill rates. Three dimensional finite element simulations of tensile test were also performed. A reasonable prediction of the 100% infill rate experiments is obtained from the simulation results. Other samples with different infill rates exhibited different characteristics particularly in the plastic deformation region. In order to improve the agreement between experimental results and numerical predictions, additional tensile experiments and improved material model are necessary. Conducting an extensive set of experiments to derive an analytically regressed correlation data for different infill rates is within the scope of the future work for this research. Acknowledgments The authors would like to thank Prof.Dr. Doğan Gücer Mechanical Testing Laboratory in Mechanical Engineering Department at Istanbul Technical University for providing tensile testing machine. References 1. B.C. Gross, J.L. Erkal, S.Y. Lockwood, C. Chen, and D.M. Spence, Analytical chemistry, 86, 3240 (2014). 2. C.W. Ziemian, P.M. Crawn, Rapid Prototyping Journal, 7, 138 (2001). 3. A. Waldbaur, H. Rapp, K. Lange, and B.E. Rapp, Anal. Methods, 3, 2681 (2011). 4. R. Weeren, M. Agarwala, V.R. Jamalabad, A. Bandyophadadyay, R. Vaidyanathan, N. Langrana, A. Safari, P. Whalen, S.C. Danforth, and C. Ballard, Solid Freeform Fabrication Conference Proceedings, 314 (1995). 5. B.M. Tymrak, M. Kreiger, and J.M. Pearce, Materials & Design, 58, 242 (2014). 6. M. Domingo-Espin, J.M. Puigoriol-Forcada, A.A. Garcia-Granada, J. Llumà, S. Borros, and G. Reyes, Materials & Design, 83, 670 (2015). 7. L. Villalpando, H. Eiliat, H. and R.J. Urbanic, Procedia CIRP, 17, 800 (2014). 8. S.D. Kumar, V.N. Kannan, and G. Sankaranarayanan, International Journal of Current Engineering and Technology, 3, 93 (2014). 9. S.V. Raut, V.S. Jatti, and T.P. Singh, Applied Mechanics and Materials, 592, 400 (2014). 10. W. Wu, P. Geng, G. Li, D. Zhao, H. Zhang, and J. Zhao, Materials, 8, 5834 (2015). 11. V.N. Patel, and M.K.P. Kadia, International Journal for Innovative Research in Science and Technology, 1, 80 (2015). 12. A. Bellini, and S. Güçeri, Rapid Prototyping Journal, 9, 252 (2003). 13. R.H. Hambali, K. Celik, P.Smith, A. Rennie, and M. Ucar, International Conference on Design and Concurrent Engineering (idecon 2010), 20, (2010). 14. J.F. Rodriguez, J.P. Thomas, and J.E. Renaud, Rapid Prototyping Journal, 7, 148 (2001). Table 1 3D Printer parameters for sample manufacturing. Nozzle Diameter (mm) 0.4 Extruder Temperature ( C) 210 Print Bed Temperature ( C) 60 Layer Height (mm) Print Speed (mm/s) 60 Line Width (mm) 0.35 Wall Thickness (mm) Infill Type Grid Interior Grid Orientation ( ) 45 Wall Grid Orientation ( ) 0 Table 2 Average modulus of elasticity, tensile strength, and strain for samples with various infill rates. Values in parenthesis denote the standard deviation. Infill rate (%) Modulus of Elasticity (MPa) Tensile Strength (MPa) Tensile Strain (mm/mm) (±1.0) (± ) (±1.9) (± ) (±1.2) (± ) (±2.2) (± ) SPE ANTEC Anaheim 2017 / 78

Fig. 1 (a) The path of the printing head and (b) generated 3D CAD

rates of (a) 25%, (b) 50%, and (c) 75%. Fig.

5 Fig. 1 (a) The path of the printing head and (b) generated 3D CAD model of the samples for infill rates of 25, 50 and 75% from top to bottom, respectively. Fig. 2 Selected 3D printed samples after tensile testing for infill rates of (a) 25%, (b) 50%, and (c) 75%. Fig. 3 Ruptured cross-sections of two 75% infill rate samples; shear ruptured (top) and normal ruptured (bottom). SPE ANTEC Anaheim 2017 / 79

Fig. 4 Average stress-strain curves and their standard deviations for samples with various infill rates. Fig. 5 Comparison of average stress-strain curves for all samples.

6 Fig. 4 Average stress-strain curves and their standard deviations for samples with various infill rates. Fig. 5 Comparison of average stress-strain curves for all samples. Fig. 6 Stress values observed from the simulations at the onset of failure for infill rates of 25, 50, 75 and 100% from top to bottom, respectively. SPE ANTEC Anaheim 2017 / 80

7 Fig. 7 Comparison of numerical predictions and experimental results for various infill rates. SPE ANTEC Anaheim 2017 / 81

EXPERIMENTAL STUDY OF MECHANICAL PROPERTIES OF ADDITIVELY MANUFACTURED ABS PLASTIC AS A FUNCTION OF LAYER PARAMETERS

Proceedings of the ASME 2015 International Mechanical Engineering Congress and Exposition IMECE2015 November 13-19, 2015, Houston, Texas IMECE2015-52634 EXPERIMENTAL STUDY OF MECHANICAL PROPERTIES OF ADDITIVELY