Process Capability Study of Selective Laser Sintering for Plastic Components

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1 I J M S E Serials Publications 8(2) 2017 : July-December pp Process Capability Study of Selective Laser Sintering for Plastic Components Ru p i n d e r Si n g h 1 a n d Ra n v i j a y Ku m a r 2 1,2 Production Engineering Department, Guru Nanak Dev Engineering College, Ludhiana , India. 1 rupindersingh78@yahoo.com, 2 ranvijayk12@gmail.com Abstract: The purpose of the present study is to investigate process capability of selective laser sintering (SLS) process for plastic components. Starting from the identification of component, prototypes with polyamide plastic material were prepared under different conditions of orientations, fill laser power, and out-line laser power on SLS machine. Measurements on the coordinate measuring machine helped in calculating the dimensional tolerances of the plastic components produced. Some important mechanical properties were also compared to verify the suitability of the components. Final components produced are acceptable as per ISO standard UNI EN I (1995) and DIN The results of study suggest that SLS process lies in ±4.5 sigma (s) limit as regard to dimensional accuracy of polyamide plastic component is concerned. This process ensures rapid production of preseries technological prototypes and proof of concept at less production cost and time. Keywords: Selective laser sintering; process capability; polyamide plastic. 1. Introduction The Rapid manufacturing (RM) techniques are in transition stage, where manufacturing facilities are being used for low-volume and customized products [1]. It uses computer aided design (CAD) based automated-additive manufacturing process to construct parts that are used directly as finished products or components [2]. Selective laser sintering (SLS) is one of the forms of RM. The process is performed by machines called SLS systems. SLS technology is in wide use around the world due to its ability to easily make very complex geometries directly from digital CAD data [1, 2]. While it began as a way to build prototype parts early in the design cycle, it is increasingly being used in limited run production to produce end-use parts. In SLS a layer of powered material is spared out and leveled over the top surface of the growing structure. A laser then selectively scans the layer of fuse those areas defined by the geometry of the cross- section; the laser energy also fuses together [3]. The SLS process is shown in Figure 1 [1]. The infused material remains in place as the support structure after each layer is deposited, the platform lowers the part by thickness of the layer, and the next layer of powder is deposited [4]. Figure 1: Schematic of SLS process [1] When the shape is completely built up, the part is separated from the loose supporting powder [5]. For SLS process, several types of materials are in use, including plastics, waxes, and low melting metal alloys [6]. The literature review reveals that SLS is new and innovative technique in RP [3]. A lot of work has been reported for its commercial applications. But hitherto very less has been reported on process capability of SLS for fabrication of plastic components for batch type production. So in this research work plastic components were fabricated by using SLS in-order to find out the best setting of input parameters on SLS machine. Following are the objectives of present study:

2 136 Rupinder Singh and Ranvijay Kumar To find the process capability of SLS process (for fabrication of plastic components) for batch production. In the present work, the process of SLS has been used to manufacture the pen drive/ memory stick cover as a case study of micro mould fabrication, under different conditions of orientation and laser power for industrial applications. The tolerance grades of the specimens manufactured on best settings has been calculated as per ISO standard UNI EN I (1995) [7] and final verifications were made as per DIN16901 standard. from Table 2, experimental conditions at S. No. 8 show better dimensional accuracy and hardness. Table 1 Fixed Machine Input Parameters Name of parameters Value Part heater set point temperature 140 C Left feed heater set point temperature 115 C Right feed heater set point temperature 115 C Piston heater set point temperature 120 C STL units Slice thickness Power adding ratio 1 Roller speed Wait for temp duration mm mm 250mm/s STL scale 1 Material used 2. Experimentations 20s Polyamide The first step in this study was selection of the specific shape of the product. Figure 2 shows dimensions of benchmark. For manufacturing of the physical master model by SLS, first 3D-CAD model was created by CATIA software. After this, CAD model was converted into the STL format. The master pattern was produced by polyamide material. Table1 shows machine input parameters, which were fixed for experimental study. The output parameters of the study were dimensional accuracy and surface hardness of the plastic components prepared. The CMM machine was used to measure the critical dimensions of the specimens. Table 2 shows effect of input parameters on dimensional accuracy and surface hardness, during pilot experimentation stage. In the present study four critical dimensions (D1 = 60mm, D2 = 3mm, D3 = 10mm and D4 = 21.6mm) has been selected for comparison purpose (Ref. Figure 2). As observed Figure 2: Component/ benchmark dimensions S.No Fill laser power (w) Outline laser power (w) Table 2 Pilot Experimentation Orientation D1 D2 D3 D4 Shore Hardness (D) The result of the dimensional measurement have been used to evaluate the tolerance unit n that drives starting from the standard tolerance factor i, define in ISO standard UNI EN I(1995) [7]. The standard value of tolerance was evaluated

3 Process Capability Study of Selective Laser Sintering for Plastic Components 137 by considering the standard tolerance factor i (mm) as: i = 0.45 D 1/3 ± D where, D is the geometric mean of range of nominal size in mm. In fact, the standard tolerance are not evaluated separately for each nominal size, but for a range of nominal size, for the generic nominal dimension D JN, the number of tolerance unit n is evaluated as follows [8]: n = 1000 (D JN - D JM )/i where, D JM the measured dimension. Tolerance is expressed as a multiple of i [9]. Table 3 shows the classification of different IT grade according to ISO UNIEN I (1995) [7] in horizontal orientation for D1 = 60.00mm. Similarly IT grades for D2, D3 and D4 were calculated (Ref. Table 4) which are consistent according to ISO standard UNI EN I (1995) [7] and DIN for plastic materials [10-11]. Table 3 It Grades for D1 = 60mm S.No. D JM n IT Grades IT IT IT IT IT IT IT IT IT IT IT IT IT IT IT IT9 S.No. Table 4 It Grades for D2, D3 and D4 IT Grades for D2 IT Grades for D3 IT Grades for D4 1 IT10 IT11 IT10 2 IT11 IT10 IT11 3 IT12 IT11 IT8 S.No. 3. IT Grades for D2 IT Grades for D3 IT Grades for D4 4 IT12 IT9 IT9 5 IT12 IT9 IT9 6 IT13 IT9 IT7 7 IT9 IT9 IT8 8 IT10 IT11 IT9 9 IT11 IT9 IT8 10 IT12 IT10 IT9 11 IT10 IT9 IT8 12 IT9 IT7 IT9 13 IT60 IT9 IT7 14 IT10 IT8 IT7 15 IT13 IT9 IT7 16 IT10 IT8 IT8 Results and Discussion The observations of pilot experiment made it clear that plastic components prepared as per input parametric conditions at S.No. 8 of Table 2 was better for dimensional accuracy and surface hardness point of view. So these were further used in the final experimentation. For process capability analysis the dimensional data has been collected and analyzed; for the critical dimensions D1, D2, D3 and D4 by preparing 16 samples in horizontal orientation with Fill laser power = 33W and outline laser power = 3.4W. Table 5 shows summary of statistical analysis for nominal dimension D1, D2, D3 and D4. Table 5 Statistical Analysis for Nominal Dimensions Statistical analysis D1 D2 D3 D4 Cp Cpk Mean of data Lower specification limit (LSL) Upper specification limit (USL) Minimum value Maximum value Standard deviation Range Figure 3-5 shows R chart, X chart and process capability histogram for nominal dimension D3. As observed from Figure 3-5, for Cpk value of 1.5, the area under normal curve is and

4 138 Rupinder Singh and Ranvijay Kumar R Chart FOR D3 standard UNI EN I (1995) and are also acceptable as per DIN16901 standard Cp Cpk Histogram FOR D3 Range CL Range +2 Sigma +1 Sigma Average -1 Sigma -2 Sigma LCL FREQUENCY LSL 9.79 Mean Median Mode n 16 USL10.21 Cp Cpk CpU CpL Cpm Cr ZTarget/DZ Pp Ppk PpU PpL Skewness Stdev Min Max Z Bench % Defects0.0% PPM Expected Sigma Count Count2 Distribution PIECE NUMBER 1 Figure 3: R chart for nominal dimension D3 non conforming ppm is Similarly Cp and Cpk values for other dimensions (D1, D2 and D4) were calculated. The value of Cpk for all critical dimensions is >1.33. The results of study suggest that RSM process lies in ±4.5 sigma (s) limit as regard to dimensional accuracy of plastic component is concerned X CHART FOR D RANGES Figure 5: Process capability histogram for nominal dimension D3 III. The adopted procedure is better for proof of concept and for the new product, for which the cost of production for dies and other tooling is more and results are in line with the observations made by other investigators [12-18] Acknowledgement MEASUREMENTS CL Data1 +2 Sigma +1 Sigma Average -1 Sigma -2 Sigma LCL Authors are highly thankful to Mr. Puranjit Singh and Manufacturing Research Lab, GNDEC for providing technical and financial assistance to carry out the research work LCL PIECE NUMBER Figure 4: X-chart for nominal dimension D3 4. Conclusions On the basis of experimental observations following conclusions can be drawn: I. SLS is highly capable process. It is observed that the Cpk value for all the four critical dimensions in the present study is >1.33. As Cpk values of 1.33 or greater are considered to be industry benchmarks, so this process will produce conforming products as long as it remains in statistical control. II. The IT grades of the plastic components produced are consistent with the permissible range of tolerance grades as per ISO References [1] [2] [3] [4] [5] [6] J. Singh, Experimental investigations for statistically controlled rapid moulding solutions of plastics using SLS, M.Tech Thesis, P.T.U Jalandhar, 2011, pp R. Singh Three dimensional printing for casting applications: A state of art review and future perspectives, Advanced Materials Research, Vol , 2010, pp M. Chhabra, and R. Singh, Rapid casting solutions: a review, Rapid Prototyping Journal, Vol. 17(5), 2011, pp I. Gibson and S. Dongping, Material properties and fabrication parameters in selective laser sintering process, Rapid Prototyping Journal, Vol. 3(4), 1997, pp P. Mercelis, J-P. Kruth, Residual stresses in selective laser sintering and selective laser melting, Rapid Prototyping Journal, Vol. 12(5), 2006, pp N. Hopkinson, R. Hague, P. Dickens, Rapid Manufacturing: An Industrial Revolution for the Digital Age, Rapid Prototyping Journal, Vol. 11(4), 2009, pp

5 Process Capability Study of Selective Laser Sintering for Plastic Components 139 [7] [8] [9] [10] [11] [12] UNI EN , ISO system of limits and fits. Bases of tolerances, deviations and fits E. Bassoli, A. Gatto, L. Luliano and M.G. Violentte, 3D printing technique applied to rapid casting, Rapid Prototyping Journal, Vol. 13, 2007, pp J.P. Singh and R. Singh, Investigations for statistically controlled rapid casting solution of lead alloys using three dimensional printing, Journal of Mechanical Engineering Sciences (Proc. of IMechE Part C), Vol. 223, 2009a, pp J.P. Singh, and R. Singh, Comparison of statistically controlled rapid casting solutions of brass alloys using three dimensional printing, International Journal of Manufacturing Technology and Industrial Engineering, Vol. 1, 2010, pp V. Singh, Experimental investigations for statistically controlled rapid moulding solution of plastics using Polyjet rapid prototyping, M.Tech Thesis, P.T.U Jalandhar, pp R. Singh, An overview of three dimensional printing for casting applications, International Journal of Precision Technology, Vol. 2(1): 2011, pp [13] [14] [15] [16] [17] [18] C.W. Lee, C.K. Chua, C.M. Cheah, L.H. Tan and C. Feng, Rapid investment casting: direct and indirect approaches via fused deposition modeling, Int. Jol. Adv Manuf. Technology, Vol. 23, 2004, pp D.Y Chang and B.H. Huang, Studies on profile error and extruding aperture for the RP parts using the fused deposition modeling process, International Journal of Advanced Manufacturing Technology, DOI: / s J.P. Singh and R. Singh, Investigations for statistically controlled rapid casting solution of low brass alloys using three dimensional printing, International Journal of Rapid Manufacturing, Vol. 1, 2009, pp A. Singh, P. Kumar and S. Regalla, For Predicting Density of a Laser Sintered Part. Rapid Prototyping Journal, Vol. 16(6), 2010, pp A. Singh, P. Kumar and S. Regalla, For Predicting Density of a Laser Sintered Part. Rapid Prototyping Journal, Vol. 16(6), 2010, pp A.K. Sood, R.K Ohdar and S.S. Mahapatra, Parametric appraisal of mechanical property of fused deposition modeling processed parts, Material and Design, Vol. 31(1), 2010, pp

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