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K. Swarna Lakshmi and G. Arumaikkannu
K. Swarna Lakshmi (&) # G. Arumaikkannu
Department of Manufacturing Engineering, College of Engineering Guindy,
Anna University, Chennai, India
© Springer Science+Business Media Singapore 2017
D.I. Wimpenny et al. (eds.), Advances in 3D Printing & Additive
Manufacturing Technologies, DOI 10.1007/978-981-10-0812-2_1
Selective laser sintering (SLS) is a powder-based additive manufacturing technology in which powder particles fuse using CO 2 laser. In this work, the influence of various parameters at various levels is studied experimentally. In this work, the components were designed and fabricated as per ASTM standards. Experiments were designed based on Taguchi’s design of experiment. An L27 Orthogonal array of Taguchi design was used. In order to determine the signiﬁcance and contribution of each factor on the tensile strength, analysis of variance (ANOVA) was performed. The results determine that layer thickness and ﬁll scan spacing are signiﬁcant parameters that cause appreciable improvement in tensile strength.
Keywords Polyamide # Selective laser sintering # Taguchi
Additive manufacturing (AM) technology has been implemented in many industrial sectors – particularly in the ﬁeld of medicine and health care. SLS is one of the most popular AM process used for orthopedic and dental applications. Polymers have been widely used in the SLS process [1,2]. Process parameters are deﬁned variables that influence and control the SLS process. The net result of the SLS process is usually described by the geometrical and mechanical properties of the parts produced. The parts produced should possess sufﬁcient strength to ensure functional requirements. There are several parameters contributing the strength of an SLS product. Many trials were conducted to review the consequences of process parameters on the mechanical properties of SLS parts. William and Deckard used analytical and experimental methods to understand the outcome of density and strength in terms of flexural modulus by varying energy density, spot diameter, and delay . Gibson and Shi reviewed the consequence of laser power, hatch spacing, and scan size on strength and density of parts using ﬁne nylon material . Thompson and Crawford devised regression models to study the response of laser power, layer thickness, and build orientation on surface roughness and tensile strength . Caulﬁeld et al. investigated the importance of laser power, beam speed, and hatch spacing on the mechanical properties of polyamide components and identiﬁed that Young’s modulus, yield strength, and fracture strength increases with increases in energy density . Jain et al. investigated the influence of delay time on the tensile strength of SLS process keeping part bed temperature constant . Jain et al. investigated the orientation on the part strength by considering the delay time . Apart from the attempts made earlier in this work, various parameters were analyzed that can influence the tensile strength of the PA 12 parts. In the present work, a trial has been made to study the outcome of ﬁll scan spacing on the strength of SLS parts using polyamide (PA12) powder. Tensile test specimens are fabricated in various ﬁll scan spacing ranges. Fabricated test speci-mens are tested for ultimate tensile strength on universal testing machines.
2 Experimental Setup
Polyamide powder (PA12) tensile specimen, 115 × 6 × 4 mm 3 , as suggested by ASTM standard of D638 was used for the experiments. This material was selected for the experiments as it can be used for medical applications. Also this has some advantages such as increased stiffness and heat resistance.
2.2 Sintering Parameters
Layer thickness, ﬁll scan spacing, and orientation were the variables chosen for this work because from previous research  it was observed that these variables had the most influence on the surface roughness. A low, medium, and high level was selected for each of the sintering to have wide range of combinations.
2.3 Equipment Characteristics
SLS Sinter station 2500+ was used for the sintering operations whose build dimension is 381 × 330 × 457 mm. Selective laser sintering (SLS) is an additive manufacturing technique that uses a high power carbon dioxide laser which selectively fuses small particles to the desired 3D shape based on the 3D CAD model.
2.4 Design of Experiments
An efﬁcient method of experimental planning is design of experiments (DoE), which incorporates the orthogonal array developed by taguchi to collect statistically signiﬁcant data with the minimum possible number of repetitions. Here L 27 array was selected, and the levels of the parameter are displayed in Table 1.
2.5 Experimental Procedure
A tensile testing machine (Associated Scientiﬁc Engg. Works, New Delhi) with a testing load range of max 5 ton with a gear rotation speed (for gradual loading) of 0.25, 1.5, and 2.5 mm/min was used to measure the tensile strength of the specimen. Figure 1a–c shows a scheme of the experimental setup, the fabricated parts, and the testing conditions used in this work, respectively. As it is observed, once the trial concluded the change in length, area of cross section, and young’s modulus are measured and calculated. The values are tabulated in Table 1. The ANOVA was performed considering the process parameters as factors and tensile strength as response.
3 Results and Discussion
Eighty one parts were fabricated with different process parameters. The experi- mental results are displayed in Table 1. ANOVA was performed. Analysis of variance for the response variable (Tensile strength) is displayed in Table 2. The main effects plot for tensile strength is shown in Fig. 2. In the experimental setup, the building direction is normal to the platform (Z direction). The testing is performed parallel to the build layers. The orientation has no effect on the direction of testing. This assumption is also veriﬁed from the statistical analysis of the data. The influence of layer thickness has a greater effect on the tensile strength with statistical signiﬁcance. As explained above, CO 2 laser has a trivial penetration in this process. Therefore, an increase in layer thickness will tend to weaken the bonding between layers.
4 Analysis of Results
4.1 Statistical Analysis
After the collection of data, they are to be analyzed by means of calculating S/N ratio (quality indicator). Here, the performance of the process is evaluated based on changing a particular process parameter and is displayed in Table 1. The average effect of layer thickness for levels 1, 2, and 3 are calculated using the data of layer thickness from the experiments 1–9, 10–18, and 19–27, respectively, of Table 1. Table 3 summarizes that layer thickness and ﬁll scan spacing of the component have signiﬁcant influence over tensile strength.
In order to evaluate the signiﬁcance of ANOVA, computation was performed for evaluating the signiﬁcance of the process parameters over tensile strength. From Table 2 it is inferred that the F ratio values of parameters layer thickness and orientation are all greater than the F ratio values drawn from table (F2, 26 = 3.3690).
4.3 Response Graphs
It is an organized, graphical representation of the parameters. The performance and the variation of each parameter are represented pictorially when moving from one level to another. Figure 2a, b show tensile strength response for layer thickness, ﬁll scan spacing, and orientation. Here, level two for A3 = −1.357 has the highest S/N ratio value, which indicates that the sintering performance at such level produces greater variation of tensile strength This indicates that level 3 of C in the 12th sintered part produces larger variation in tensile strength when compared to level 1 and 2 of C. It may be because the incidence of laser energy is deeper and produces complete melting of the powder material. Therefore, there is a consistent dissemination of mechanical property across the layer thickness.
5 Prediction of Optimum Performance
Optimal tensile strength values are estimated as follows:
Predicted optimal tensile strength = A1 + C2 – Y = 3:673
where A1 is level 1 of layer thickness, C2 is the level 2 of orientation, and Y is the average tensile strength
The main intention of this work is to identify how the layer thickness, ﬁll scan pacing, and build orientation affect the mechanical property of parts manufactured by SLS technology. In terms of mechanical property, this process is somewhat sensitive to layer thickness and orientation. An increase in layer thickness tends to temper the part in terms of tensile strength. This study provides an integrated set of experimental data addressing the mechanical property. Further, a statistical approach was used to investigate the influence of the process parameters.
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2. Williams JD, Deckard CR (1998) Advances in modeling the effects of selected parameters on the SLS process. Rapid Prototyping J 4(2):90–100. doi:10.1108/13552549810210257
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