EFFECT OF LAYER THICKNESS, DEPOSITION ANGLE, AND INFILL ON MAXIMUM FLEXURAL FORCE IN FDM-BUILT SPECIMENS

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1 EFFECT OF LAYER THICKNESS, DEPOSITION ANGLE, AND INFILL ON MAXIMUM FLEXURAL FORCE IN FDM-BUILT SPECIMENS Ognjan Lužanin *, Dejan Movrin, Miroslav Plančak University of Novi Sad, Faculty of Technical Science, Serbia ABSTRACT As the ability to resist deformation under load, flexural strength is an important property of every material. Fused Deposition Modeling (FDM) technology allows users to control the density of models through parameter which is termed air gap or infill. Since lower infill values significantly increase the building speed, test specimens were used with infill between 10 and 30%. Therefore, this paper discusses experimental analysis of the influence of layer thickness, deposition angle and infill on the maximum flexural force in FDM specimens made of polylactic acid (PLA). A 2 3 factorial experiment without replication was used with three center points. The results indicate a dominant, statistically significant influence of extrusion speed, significant interaction between deposition angle and infill, as well as the nonlinearity of effects. Key words: 3D printing, FDM, layer thickness, deposition angle, infill, flexural force. 1. INTRODUCTION Additive manufacturing technologies, under various designations, have been in existence for about three decades now. However, it is only after the expiration of FDM patent rights in 2009, that the surge of consumer-grade FDM printers became available on the market and low-cost 3D printers are now available to a much wider number of researchers, professionals and enthusiasts. Similarly, expiration of patent rights for other popular additive manufacturing technologies, such as stereolythography and selective laser sintering shall further revolutionise the way in which modern society utilizes additive manufacturing. Research community already benefits from the availability of low-cost 3D printers in that the machines such as the Makerbot Replicator allow experimentation with a variety of easily programmable technological parameters. Also worth noting is the appearance of readily available, bio-degradable FDM filaments which are interesting from the investigative point of view, as their * Corresponding author s luzanin@uns.ac.rs

2 50 mechanical properties regarding the selection of various technological parameters have not been sufficiently explored. The study presented in this paper differs from the previously discussed investigations in two key points: (i) the material used in this study is polylactic acid (PLA), which, contrary to ABS, has not been extensively used in experiments of this kind, and (ii) the infill used to produce specimens ranges between 10 and 30%, which is most likely to be used in everyday FDM practice, since it radically influences the build time as one of the critical features of fused deposition modelling technology. With this in mind, the dependent variable in this study was maximum flexural force, since the cross-section of our specimens was not solid and would not allow the stress to be calculated in a conventional way. The paper is organized as follows: Section 2 presents investigations relevant for this study, in Section 3 the design of experiment and its practical realization are discussed. Section 4 presents experimental results which are analysed in Section 5. Concluding remarks are given in Section REVIEW OF LITERATURE A study conducted by Es Said et al. [1] examined tensile strength, modulus of rupture, and impact resistance for different layer orientations of ABS models. The samples were fabricated in five different layer orientations. According to their findings, 0 orientation where layers were deposited along the length of the samples displayed superior strength and impact resistance over all the other orientations. They concluded that anisotropic properties were probably caused by weak interlayer bonding and interlayer porosity. In a study on anisotropic properties of ABS material, Ahn et al. [2] used DOE to investigate the influence of five FDM process parameters on tensile and compressive strength of specimens. The process parameters were: raster orientation, air gap, bead width, colour, and model temperature. The results were compared with those of injection molded FDM ABS P400 material. Lee et al. [3] employed Taguchi method to find the optimal FDM process parameters to produce ABS-compliant prototype. They focused on the optimum elastic performance of a compliant ABS prototype, concluding that layer thickness, raster angle and air gap significantly affect the its elastic performance. Sood et al. [4] experimented with ABSP400 material to investigate the influence of layer thickness, part orientation, raster angle, air gap and raster width on dimensional accuracy of parts. They used Taguchi s parameter design to find optimum parameters setting to minimize percentage change in length, width and thickness of standard test specimens. They also used an artificial neural network (ANN) to predict an overall dimensional accuracy. Bagsik et al. [5] experimentally investigated the mechanical properties of FDM samples generated with the the FDM Fortus 400mc depending on the build direction. They conducted tensile and compression tests, also measuring part geometry for accuracy. Ziemian et al. [6] investigated the dependence of the mechanical properties of FDM parts on raster orientation and their ability to maintain integrity under service loading. They examined the effect of fibre orientation on a variety of important mechanical properties such as compressive, flexural and fatigue strength and compared their properties to those of injection molded ABS parts. Rayegani and Onwubolu [7] used the group method for data modelling and prediction to model the functional relationship between process parameters and tensile strength for FDM process. They conducted experiments varying part orientation, raster angle, raster width and air gap. They submitted the process parameters and the experimental results to the group method of data

3 51 handling (GMDH), and the predicted output values were found to correlate very closely with the measured values. In addition, they used the differential evolution (DE), to find optimal process parameters. 3. DESIGN AND REALIZATION OF EXPERIMENT 3.1. Design of experiment To reduce total time required for this experiment, a factorial experiment with no replication and three center points was conducted. The center points increased the number of degrees of freedom (dof), while allowing calculation of error and detection of possible non-linearity of effects. Table 1 shows the factors and respective levels. Table 1 - Factors and levels used in the experiment Factor Unit Low level (-1) Cent. level (0) Upper level (+1) Layer thickness (A) [mm] Deposition angle (B) [ o ] Infill (C) [%] The specimens were centered on the build platform, and oriented with their dominant dimension along the X printer axis Preparation and building of test specimens Geometry of the specimens used in the experiment was defined according to ISO 178:2001 [8]. Accordingly, specimens were built as prismatic parts, with following dimensions: 80x10x4mm. Makerbot Replicator 2 was used to print the eleven specimens according to the design of experiment. PLA (Polylactic Acid) filament was used to build the specimens. Extrusion temperature was 235 o C, no rafts were used, while the extrusion speed was kept at 60 mm/s. These were constant parameters throughout the experiment Experimental tests Three-point bend tests were completed according to scheme given in Fig.1. Table 2 presents the values of the relevant geometric parameters from Fig.1. Experiments were performed during a single session and in a randomized order. Environmental temperature was 24 o C throughout the experiment. Fig. 1 - Loading scheme used in experiment

4 52 Table 2 - Dimensional parameters related to loading scheme in Fig.1 Dimensional parameter designation Value Loading radius R1=5±0.1 mm Support radii R2=5±0.2 mm Test specimen Length l=80±2 mm Width b=10±0.2 mm Thickness h=4±0.2 mm Figure 2 illustrates the stages of experimental tests. Fig. 2 - Test stages - a) initial stage, b) flexion in progress, c) breaking of specimen 4. EXPERIMENTAL RESULTS Design table was generated and subsequent analysis performed using statistical software Minitab v16. The resulting factorial experiment is given in Table 3, in randomized order. Analysis of variance (ANOVA) assumptions were checked using diagnostic diagrams, of which two are shown in Fig.3 - normal probability plot and residuals versus fits. The residuals obviously follow normal distribution, while the random scatter of residuals about the zero indicates homogeneity of variance. Table 3 - Experimental results obtained from eleven runs Order of experiment Layer thickness [mm] Deposition angle [ ] Flexural Force [N] Infill [%]

5 53 Several regression models were tested and terms were eliminated based on significance and S, R 2 - adjusted, and R 2 -predicted parameters. The results of ANOVA for the final model are given in Table 4. Statistical significance of the effects of layer thickness, deposition angle and infill on the maximum flexural force, F [N], are shown in the Pareto diagram (Fig.4). The main effects diagram (Fig.5) illustrates the effect of layer thickness on F[N], as the only statistically significant factor. Also, the only significant interaction, between deposition angle and infill, is shown in Fig.6, while the contour plot in Fig.7 illustrates their relationship affects the maximum flexural force. 99 Normal Probability Plot (response is Force [N]) Percent Standardized Residual 2 3 a) ResidualsversusFits (response is Force [N]) 2 Standardized Residual Fitted Value b) Fig. 3 - Diagrams of residuals - a) Normal probability plot, b) Residuals v Fits

6 54 Table 4 - Results of ANOVA analysis pertaining to the final regression model Source DF SS MSS %TSS p-value Layer thickness Deposition angle Infill Deposition angle [ o ]*Infill [%] Curvature Error Total Pareto Chart of the Standardized Effects (response is Force [N], α = 0.05) Term A 2.57 Factor Name A Layer thickness [mm] B Deposition angle [ ] C Infill [%] BC C B Standardized Effect Fig. 4 - Pareto plot showing significance of factors Fig. 5 - Main effects plot

7 55 Fig. 6 - Influence of extrusion speed and temperature on surface roughness effects Fig. 7 - Influence of extrusion speed and temperature on surface roughness effects

8 56 5. ANALYSIS OF RESULTS Analysis of variance for maximum flexural force (F) (Tab.4), showed a dominant, statistically significant effect of layer thickness (p=.000). No significance was found for deposition angle (p=.247) and infill (p=.208). In addition, ANOVA showed that interaction between deposition angle and infill is statistically significant (p=.024). Curvature is also statistically significant (p=0.038), which indicates non-linearity of effects and the need to include quadratic terms in the model. In the column designated %TSS (Tab.4), values are shown which represent percentage contribution to the total sum of squares for each term. It is evident that the contribution of layer thickness (89.44%) is by far the largest contribution to variability, followed by the interaction (4.00%) and curvature (3.09%). Pareto chart (Fig.4) illustrates the findings of previous analysis. Main effects diagram (Fig.5) shows that minimal layer thickness radically affects maximum flexural force, i.e., when layer thickness is changed from -1 to +1 level, maximum flexural force drops more than three times. The same diagram shows that the center points are at some distance from the line representing mean values of F, indicating non-linearity. Interaction plot shown in Fig.6 shows that the level of infill significantly influences the effect of deposition angle on the dependent variable, F. The change in the level of the deposition angle results in a much larger effect on F when infill is at the low level. It is interesting to note that the center level yields highest flexural force. Finally, shown in Fig.7 is the contour diagram for the previously discussed interaction between deposition angle and infill. It confirms previous findings, showing that maximum flexural force in excess of 116 N can be achieved with both factors at their center levels. The contour diagram also shows that the infill of 10% can also yield higher F when combined with high level of deposition angle (60 o ), thus shortening the total building time while maintaining flexural strength within the ten percent of the maximum. 6. CONCLUSIONS In this paper the influence of layer thickness, deposition angle and infill on maximum flexural force in FDM-built specimens was investigated. The experiment was statistically designed and was conducted at two levels, without replication and with three center points. The introduction of center points added replication, allowed calculation of error term and check for the presence of curvature. Non-linearity of effects was confirmed by analysis of variance. Statistical analysis showed that layer thickness has a dominant, statistically significant effect on flexural force, F, while the interaction between deposition angle and infill was also significant. The issue of minimum detectable effect size (MDES) is also an interesting part of any DOE investigation. Given eight corner and three center points, an estimate of standard deviation of S= N, and the target test power of 0.8, we have calculated that the minimum detectable effect size for our experiment is 41.3 N. Considering that this force corresponds to approximately 2.15% of the average PLA flexural strength, we could consider this MDES acceptable for a preliminary study. It is interesting to note that the introduction of two additional center points would have allowed a substantially lower MDES of N. The introduction of center points enabled the detection of statistically significant curvature effect, which indicates that the next experiment should be conducted at three levels to approximate the values of quadratic terms in the model, as well as to perform optimization by Surface Response Analysis.

9 57 ACKNOWLEDGEMENT: Results of investigation presented in this paper are part of the research realized in the framework of the project Research and development of modeling methods and approaches in manufacturing of dental recoveries with the application of modern technologies and computer aided systems TR , financed by the Ministry of Science and Technological Development of the Republic of Serbia. 7. REFERENCES [1] Es-Said, O.S., Foyos, J., Noorani, R., Mendelson, M., Marloth, R.: Effect of Layer Orientation on Mechanical Propertiwes of Rapid Prottyped Samples, Materials and Manufacturing Processes, Vol. 15, No. 1, 2000, pp [2] Ahn, S.-H., Montero, M., Odell, D., Roundy, S., Wright, P. K.: Anisotropic material properties of fused deposition modeling ABS, Rapid Prototyping Journal, 8(4), 2002, pp doi: / [3] Lee, B. H., Abdullah, J., Khan, Z.: Optimization of rapid prototyping parameters for production of flexible ABS object, Journal of Materials Processing Technology, 169(1), 2005, pp.54 61, doi: /j.jmatprotec [4] Sood, A. K., Ohdar, R. K., Mahapatra, S. S.: Improving dimensional accuracy of Fused Deposition Modelling processed part using grey Taguchi method. Materials & Design, 30(10), 2009, pp doi: /j.matdes [5] Bagsik, A., Schöppner, V., Klemp, E.: FDM Part Quality Manufactured with Ultem *9085, 14th International Scientific Conference on Polymeric Materials 2010, Halle (Saale) [6] Ziemian, C., Sharma, M., & Ziemian, S.: Anisotropic Mechanical Properties of ABS Parts Fabricated by Fused Deposition Modelling, Mechanical Engineering, Dr. Murat Gokcek (Ed.), 2012, ISBN: , InTech, Available from: [7] Rayegani, F., & Onwubolu, G. C.: Fused deposition modelling (FDM) process parameter prediction and optimization using group method for data handling (GMDH) and differential evolution (DE). The International Journal of Advanced manufacturing Technology, 2014 pp.1-11, doi: /s [8] ISO, SREN. "178: 2001, Plastics.", Determination of flexural properties, Three point method, CEN, European Committee for Standadization, 2001, Bruxelles.

10 58 UTICAJ DEBLJINE SLOJA, UGLA DEPONOVANJA MATERIJALA I PROCENTA ISPUNE NA MAKSIMALNU SAVOJNU SILU KOD UZORAKA IZRAĐENIH U FDM TEHNOLOGIJI Ognjan Lužanin, Dejan Movrin, Miroslav Plančak Univerzitet u Novom Sadu, Fakultet tehničkih nauka, Srbija REZIME Kao sposobnost suprotstavljanja deformisanju pod dejstvom opterećenja, savojna čvrstoća predstavlja važnu karakteristiku svih materijala. Tehnologija modelovanja deponovanjem istopljenog filamenta (Fused Deposition Modeling - FDM) omogućava korisnicima da upravljaju gustinom ispune modela preko tehnološkog parametra koji se naziva vazdušni razmak (air gap) ili ispuna (infill). Budući da rad sa nižim vrednostima ispune značajno skraćuje vreme izrade modela, u ovom radu su izrađene epruvete kod kojih se ispuna kreće između 10 i 30%. Na osnovu toga, predmet rada je eksperimentalno ispitivanje uticaja debljine sloja, ugla deponovanja materijala i procenta ispune na maksimalnu savojnu silu kod uzoraka izrađenih FDM tehnologijom od PLA plastike. Korišćen je trofaktorni eksperiment na dva nivoa, sa tri centralne tačke, bez replikacije. Dobijeni rezultati ukazuju na izražen, statistički značajan uticaj debljine sloja. Takođe je pokazan statistički značajan uticaj nelinearnosti, kao i interakcije ugla deponovanja i procenta ispune. Ključne reči: 3D štampa, FDM, debljina sloja, ugao deponovanja, procenat ispune, savojna sila.