W.A.Y YUSOFF 1 A.J THOMAS 2. P O Box 10, Kuala Lumpur, Malaysia

Transcription

1 International Association for Management of Technology IAMOT 2008 Proceedings THE EFFECT OF EMPLOYING AN EFFECTIVE LASER SINTERING SCANNING STRATEGY AND ENERGY DENSITY VALUE ON ELIMINATING ORANGE PEEL ON A SELECTIVE LASER SINTERED PART W.A.Y YUSOFF 1 A.J THOMAS 2 1 International Islamic University Malaysia, Faculty of Manufacturing and Materials Engineering, P O Box 10, Kuala Lumpur, Malaysia wanyusoffwa@theiet.org 2 Innovative Manufacturing Research Centre (IMRC), Cardiff University, Aberconway Building, Colum Drive CF10 3EC Cardiff, Wales, United Kingdom, thomasaj3@cardiff.ac.uk This paper reports on an experimental study into the effects of using recycled Polyamide 12 (PA12) powder on the manufacture of a Selective Laser Sintered (SLS) part. It was found that through using recycled PA12 material, a poor and unacceptable surface finish was achieved. The aim of this paper is therefore to develop a strategy to ideally eliminate the coarse surface texture found on the product (Orange Peel effect) by controlling the most important SLS process parameters. The reason for employing the strategy is to ensure consistent surface quality of the resultant parts and more efficient use of the SLS recycled material. A suitable laser scanning strategy and energy density value along with an appropriate recycled material was chosen as the main process parameters. In this experiment 3 replications were experimentally tested. PA12 recycled materials with a Melt Flow Rate (MFR) of MFR and 15-16MFR were used in the experiments. An in-house surface texture scoring system was introduced to evaluate the surface quality produced on the parts. This paper extends the understanding of the SLS process and in particular the relationship between the SLS parameters and part quality. The experimental results suggest that through employing an effective laser scanning strategy and energy density value, the resulting part surface quality was improved. The proposed method provides a threshold MFR value for the PA12 powder and shows that materials with MFR values lower than 18 MFR created parts of poor and unacceptable surface quality. Keywords: Selective laser sintering, polyamide 12, surface finish, process parameters, melt flow rate, recycling. 1. Introduction One of the most effective and versatile Rapid Prototyping (RP) techniques available is Selective Laser Sintering (SLS). Recently, the SLS has been widely used not only for RP but also for Rapid Manufacturing (RM) or Direct Manufacturing (DM) of bespoke parts or even small batch manufacture. In this technology, an object is created layer by layer from heatfusible solid fine powdered materials with heat generated from a CO 2 laser (Wholers, 1999; Wholers, 2000; Naim, 2001). It is capable of producing very complex part geometries directly from three-dimensional CAD software by a quick, highly automated and flexible manufacturing process. (Au and Wright, 1993; Kai et al., 1998; King and Tanse, 2003; Pham and Dimov, 2000; Shi et al., 2004). At present, the most commonly employed materials are based on semi-crystalline Polyamide 12 (PA12) or Nylon 12 powders. Materials such as DuraForm (DF ) and PA2200 Fine Polyamide manufactured by 3D Systems Corp. (3D System) and Electro Optical Systems GmbH (EOS) have become standard in the RP community. The advantage of using PA12 as a SLS material is that it has good physical and mechanical properties with high melting temperatures. This material is suitable to produce prototypes and

2 W.A.Y Yusoff & A.J Thomas functional parts in particular for parts where fine feature details are required combined with durable, heat and chemical resistance properties. However, the PA12 powder deteriorates during the SLS process. This is due to the fact that once the part has been formed, the surrounding material close to the laser affected area of the SLS process is removed and reused. This reuse means that the semi-crystalline product breaks down and is less effective in subsequent operations thus imparting poor surface finish to the product (an Orange Peel effect is seen on the surface, Figure 1a). In order to fabricate parts with good surface quality, the users of the SLS machines apply a constant refresh rate with higher portion of new powder. As a result a huge amount of recycled powder has to be scrapped if high quality products are required (Gornet, 2002). This research therefore aims to develop a methodology which will ensure consistent and high quality products based upon the new settings SLS process parameters to be employed on the recycled PA12 material. This can be achieved by controlling the amount of energy density and laser scanning strategy, it is proposed that high quality surface textures will remain unaffected and the manufacturing cost of SLS parts (and the subsequent environmental impact) reduced through reusing SLS powder 1.1 The orange peel texture Figure 1a Orange peel texture Figure1b Good part surface Manufacturing parts using only new powder, although providing the best quality, is considerably more expensive than using recycled powder. On the other hand, using recycled powder creates the problem of the undesirable part surface finish and higher shrinkage which affects the part accuracy. Figure 1a shows an example of an SLS part affected by orange peel texture compared to a good SLS part surface shown in Figure 1b. Orange peel texture is clearly a rough, uneven, and coarse texture observed on the part surface due to PA12 powder deterioration. This problem must be addressed before the technology can gain wider acceptance.

3 IAMOT Proceedings Instructions for Typing Manuscripts 1.2 Literature review The issue of achieving high quality products from the SLS process has been studied by a number of researchers (Naim, 2001; 3D Systems, 2007; Ho et al., 1999; Ho et al., 2000; Yusoff and Thomas, 2008). The common problems related to manufactured SLS parts are as follows: (i) growth and bonus Z varying amounts of (ii) warp and curl (iii) shrinkage (iv) orange peel and poor surface textures To produce a high quality SLS part, the process parameters are set differently according to powder properties and the requirements of application. Some process parameters directly related to part quality are described in the following: (Naim, 2001; 3D Systems, 2007; Guo and Suiyan, 1996; Badrinarayan and Barlow, 1995; Gibson and Shi, 1997; Hardro et al., 2004). However, prior to this study, research on improving the surface finish of SLS sintered products using recycled PA12 by varying the setting level of process parameters has not been investigated to any real extent. This research aims to reduce or eliminate this unacceptable surface texture by controlling the most important process parameters which significantly affects the SLS process and the output of the process. 2 Laser Sintering Process Figure 2 shows a schematic view of the SLS process and the major components of the system which is commonly employed in the laser sintering process. Right feed heater Part heater Left feed heater Left Roller Over flow cartridge Right Over flow cartridge 170ºC 80ºC 80ºC Left Feed Cartridge Part Right Feed Cartridge Cylinder heater 150ºC Piston heater 130ºC part piston Figure 2 Schematic view of SLS process (Sinterstation 2500 HiQ)

4 W.A.Y Yusoff & A.J Thomas In general there are three stages of SLS process in any machines to produce sintered parts as follows. Warm-up stage Before the powder is sintered, the entire bed of the machine is preheated in a Nitrogen rich atmosphere between 100ºC and 120ºC or just below the melting point of the material used in the process. This is done in order to stabilise the material in the process chamber. Build stage The part heater is set just below the melting temperature of PA12 polymer powder. The purpose is to minimise additional laser energy needed to heat the powder to the fusing temperature. The CO 2 laser scans the surface and selectively draws a cross section of the part, it subsequently sinters the selected sections which it will heats and fuses together the relevance particles to produce the part. Then the roller spreads a new very thin (between 100 μm to 125μm) layer of new material across the part build cylinder in readiness for the next laser scanning process. Cooling down stage Once the process is complete and a part is produced, the chamber is cooled slowly to below 45ºC. This cooling stage takes approximately 4 to 8 hours after which the parts can be removed from the part cake. The left over powder can then be reused in the next process. The ability to reuse the un-sintered material significantly reduces the manufacturing cost. 2.1 SLS process parameters The powder properties and process parameters have a great influence on the surface quality of the SLS parts (Gibson and Shi, 1997). For this reason, the SLS machine manufacturers specify the default values for all process parameters based on the material used in the process. However, the user can adjust the process variables to suit the application which directly affects the quality of the manufactured part Energy density (ED) In order to produce a good functional SLS part, it is important that the powder on the part bed surface receives a sufficient amount of energy through the laser sintering process. The reason is that sufficient energy density is produced when the energy input increases and is applied to the part bed surface, this in turn causes a higher temperature at the powder interface and thus creates a better melt flow. However, too high an energy density causes a hard part cake, which in turn results in the operator having difficulty in taking parts out of the build as well as increased surface roughness, and a light brown colour being seen on the part surface due to overheating. The energy density is calculated by using the following equation (Ho et al., 2000; Badrinarayan and Barlow, 1995; Gibson and Shi, 1997). Energy density (J/mm 2 ) = P LS * SCSP Where P is a laser power, LS-laser speed, and SCSP - scan spacing.

5 IAMOT Proceedings Instructions for Typing Manuscripts Laser scanning strategy In the SLS process, the user can select the scanning strategy. This could be a fill only or a fill and outline option. In the most recently developed strategy, the laser beam does not only scan the entire cross section but also outlines its contour as shown in Figure 3. Outline Fill Laser beam movement Nominal dimension Figure 3 Laser scanning strategies Generally, the values used to set the process parameters for specific material come from configuration files that are developed by the SLS manufacturers. These settings usually, cannot be modified, except for small changes that are facilitated through part and build profiles (Hardro et al., 2004). The default process parameters of the Sinterstation 2500 HiQ machine recommended by the manufactures of PA12 material are shown in Table 1 and are illustrated in Figure 6a.

6 W.A.Y Yusoff & A.J Thomas Table 1 The Sinterstation 2500 HiQ specifications Sinterstation 2500HiQ Building envelope, mm 320 X 280 X 457 Volume, cm Material weight, kg 19 Material PA2200 Laser spot diameter, mm 0.45 Layer thickness, mm 0.1 Laser type CO 2 laser Laser speed, mm/sec 5080 Laser beam diameter, mm 0.4 Fill beam offset X and Y, mm Fill laser power, Watt 12 Outline beam offset X and Y, mm 0.2 Outline laser power, Watt 5 No.of Outline once

7 IAMOT Proceedings Instructions for Typing Manuscripts 3. Experimental results and discussion Objective To eliminate or reduce Orange Peel texture Design benchmark part Experiment 1 PA2200 (twice used) 18-19MFR Experiment 2 PA2200 (three times used) MFR SLS process 3 replications New process parameters Fill beam offset X and Y, mm = 0.38 Outline beam offset X and Y, mm = -0.1, 0, 0.1, and 0.2 No.of outline (scanning strategy) = 2 and 3 times Score response system, Rv Target is 70% Analysis and Results Physical part surface Microstructure Shrinkage SEM Figure 4 A new approaches to improve part surface finish of recycled PA12 in SLS process

8 W.A.Y Yusoff & A.J Thomas 3.1 Methodology and equipment used As shown in Figure 4, a new strategic approach which involves changing the default settings on the selected SLS process parameters in order to produce a benchmark part is proposed. The purpose of this experiment is to investigate the significance of using a different setting of scanning strategy and its effects on part surface quality when using partly deteriorated PA12 powder. The outcome of this experiment is to improve or to eliminate the orange peel texture which affects the SLS finish part surface Benchmark part The benchmark part was designed to contain features with different thickness, shape, orientation, and surfaces with different orientation to the vertical z direction (as shown in Figure 5 a, b) Zig-zag features Angled surfaces (outside) Angled surfaces (inside or hidden) Figure 5a Benchmark part (top view) Conical surfaces (outside) Vertical plane Surface Angled surfaces (inside or hidden) Angled surfaces (outside) Figure 5b Benchmark part (bottom view) The size of the benchmark part is 110mm (w) X 110mm (l) X 48mm (h).

9 IAMOT Proceedings Instructions for Typing Manuscripts Table 2 Benchmark part special features Features Description Function Pyramid Located on top of benchmark part sample Shrinkage measurement Angled surfaces Vertical plain surfaces Conical surfaces Different angles 50º, 54º,57 ºand 90º Located at the bottom of bench part. Different sizes of 1mm, 2mm, 3mm, 5mm and 7mm Different angles 50º, 54º,57º and 90º To study the affect of orange peel texture at different angles surfaces To study the affect of orange peel texture at different thickness and orientation To study the affect of orange peel texture cone shapes surfaces Response variable The outcome of an experiment that can be observed and measured is known as a response variable (Rv). It is used to evaluate the resulting responses from different combinations of process parameters. In this investigation, the surface quality is measured and compared by feel and touch, and observation. The advantage of this method is that it is simple and quick because the orange peel texture can be easily seen and measured The Surface Texture Scoring System As shown in Figure 4, the response score system is introduced in order to evaluate the Rv. Each part is individually inspected. The evaluation of the part quality from the point of view of the orange peel occurrence was done in accordance with a scoring system described in Table 3. Table 3 Scoring system for evaluation of the part surface finish Description Score Quality Good surface finish, NO orange 1 Acceptable peel Slightly rougher surface finish, 0.5 Acceptable NO orange peel Small signs of orange peel 0 Not Acceptable orange peel texture 0 Not Acceptable The three additional points are given to any benchmark part which shows good surface finish and its features. For example, if all features at the bottom of the benchmark part are perfect (no defects) especially on the 1mm thickness as shown in Figure 5b, the additional three point is given. The total score is calculated based on the total number of scores divided by 96 which is the total number of surfaces.

10 W.A.Y Yusoff & A.J Thomas Material The SLS material investigated in this study is PA12 based powder supplied by EOS GmbH. The 17-18MFR recycled PA12 powder had been recycled twice and was employed in the first experiment and the lower PA12 grade (15-16MFR) which has a higher melt viscosity was collected from a three times recycled process for the second experiment Scanning Electron Microscope (SEM) examinations For all examinations, a thin layer of gold was sputtered on substrates using auto sputter apparatus. Two pieces of equipment have been used consecutively. The first is a Bio-Rad SC500 for gold coating of the specimens and the second is an EMSCOPE SC500 for image capture. It is employed to characterise the individual powders and to analyse the surface morphology and microstructure of the sintered bench part. All SLS manufactured bench parts were examined under high vacuum conditions. A low voltage (10kV) was chosen to minimise heat damage to the sample (Ho et al., 1999). Outline (1X) Fill Laser beam movement 0.225mm 0.2mm 0.225mm Figure 6a The default setting of the laser scanning strategy process parameter 0.2mm Outline (2X) Fill Laser beam movement 0.38mm 0.2mm 0.38mm 0.2mm Figure 6b The best new setting of process parameter leads to elimination of orange peel texture

11 IAMOT Proceedings Instructions for Typing Manuscripts Outline (2X) 0.38mm -0.1mm 0.38mm -0.1mm Figure 6c The worst new setting of the process parameter cause the part delamination and weak 3.2 First experiment SLS process employing MFR powder As shown in Table 4, a list of new settings process parameters was employed to improve the Rv. In this experiment, a 2500 HiQ plus machine with the default setting 0.225mm for X and Y beam offset and one time (1X) outline scanning was changed to 0.38 mm and one time (1X), two times (2X), and three times outline scanning (3X). A powder with an MFR of was used first and a powder with a MFR was used for the second experiment. The Rv benchmark parts of using these old recycled powders was compared. Powder MFR No.of times outline scan Table 4 The experimental 1 results of employing MFR Beam offset Fill laser power X &Y,(mm) Beam offset Outline laser power X &Y,(mm) Rv (%) Energy Density (ED), J/mm 2 Shrinkage (%) Build time (%) X Delamination default X default X X X default X X As shown in Table 4, changes of the setting level of beam offset fill to 0.38 mm and outline 0.1mm and 0.2mm observed the average of Rv. Three repetition tests with one layer build were performed and the old used PA2200 with MFR was employed. As noted in Figure 4, the accepted score of Rv is 70%. The average of Rv for this optimisation experiment is 78%. This means the Rv target is achieved. The beam offset outline laser of the machine is 0.2 mm in the x and y axis. When the beam offset outline laser is set to a lower value, as it is in our experiments for trials

12 W.A.Y Yusoff & A.J Thomas with 0.1mm and -0.1mm, the processed parts will be slightly larger. In our case, the main reason was to study the effect of the beam offset on the orange peel texture regardless of the accuracy. The results suggest that the overall Rv decreases with the number of iterations of outline scan. However, the build time and part shrinkage slightly increase. Due to the fact that only a small portion of outline laser beam overlaps into fill laser beam in the sintering process, benchmark parts which set the beam offset laser power at -0.1mm (Figure 6c) setting level were found to be fragile and delaminated. Figure 7a, 7b, 7c, and 7d show the different surface finish position of the best Rv benchmark part (Table 4) after applying the new process parameters. Figure 7a Different plain surfaces thickness Figure7b Zig-zag surfaces Figure 7c Angled surfaces Figure 7d Vertical plain and cone surfaces No orange peel was found on the main part features, however the angled surfaces of the benchmarked part shown in Figures 7c and 7d were still found to be rough however, this roughness was considered as acceptable for the experimental analysis.

13 IAMOT Proceedings Instructions for Typing Manuscripts 3.3 Second experiment SLS process of employing MFR powder The second optimisation LS process used the very old recycled PA2200 powder with MFR powder quality. Powder MFR No.of times outline scan Table 5 The optimisation experiment results of MFR powder Beam offset Fill laser power X &Y,(mm) Beam offset Outline laser power X &Y,(mm) Rv (%) Energy Density (ED), J/mm 2 Shrinkage (%) Build time (%) X default X X X default X X As shown in Table 5, the overall Rv results using lower powder quality (15-16 MFR) show higher shrinkage and lower Rv compared to the results of using MFR powder (Table 4). This means that the PA12 powder deteriorates to very high levels at which the orange peel texture could not be reduced. The images of the benchmark part which obtained the highest Rv are shown in Figures 8a and 8b. Fig. 8a Cone and angled surfaces Fig. 8b Different plain surfaces thicknesses Figure 8a shows the conical and angled surfaces along the z axis at 90º and 57º affected by orange peel texture. The signs of orange peel and rough surfaces were observed along the z axis when angled at 50º and 54º. Figure 8b shows the different results for the different vertical plain thicknesses. For example, no orange peel texture was observed on the 1 mm and 2 mm thicknesses. At 3 mm vertical plain thickness, however, the signs of orange peel texture were noticed. The thicker vertical plain surfaces, such as 4 mm, 5 mm and 7 mm, displayed poorer orange

14 W.A.Y Yusoff & A.J Thomas peel texture. The surfaces of Zigzag features which were located on the top of the benchmark part were found to be rough. Figure 9a High dense microstructure Figure 9b Porus microstructure The results of comparing the microstructures of two different powder grades using SEM are shown in Figure 9a and 9b. The result suggests that the benchmark part with the highest Rv (78% in table 4) shows the powder polymer particles within the layers fusing easily which leads to a smooth surface. By contrast, the highest Rv (45% in Table 5) employing the 15-16MFR, agglomerates of melted particles at the region of high porosity can be observed and incomplete melting with the size approximately 100 µm found on the overall surface area which caused the orange peel texture. 4. Conclusions The orange peel phenomenon is one of the main constraints in the SLS process as it causes unacceptable surface quality. It happens when deteriorated PA12 powder is employed in the SLS process. This paper reported on the effect of the properties of the PA12 polymer on physical surface and microstructure of the parts which are affected by the orange peel phenomenon. The recommended new process parameters (changing the fill (0.38mm) and outside beam offset (0.2mm) x and y and applying twice the number of outline scanning cycles), the Rv of the benchmark parts was found to improve surface texture quality. This means that the problem of orange peel texture is significantly correlated between the selected process parameters which influence the sintering mechanism through the SLS process. The testing process also identified that the selection of a suitable Energy Density (ED) value applied to the SLS process is important, but a slight variation of ED applied to benchmark parts shown in Table 4 and 5 seems not to greatly to influence the Rv. This could be because the variation of melt viscosity still cannot improve the sintering mechanism due to deteriorated powder properties.

15 IAMOT Proceedings Instructions for Typing Manuscripts In this case, the rough and coarse surfaces shown in Figures 7c and 7d were properly sintered. It can be seen from the microstructures of the benchmark parts shown in Figure 9a, where applying the correct combination of the process parameters and ED could significantly improve the viscous flow mechanisms and powder fusion which leads to a reduction in the orange peel texture. In contrast, the same optimum process parameters applied to the older PA12 powder (15MFR-16MFR) could not overcome the orange peel texture problem. This could be due to the fact that the deteriorated powder has changed its thermal and sintering properties, which could not be easily improved. For instance, it could be due to the higher degree of chain molecules entanglement which influences the viscous flow to become higher. As a result, more efficient packing of the polymer chains leads to significant shrinkage, as shown in Table 5. This provides scope to develop further investigations into SLS process characterisation in the future These experiments suggest that the new process parameters could help the RP industry to reuse the recycled PA12 powders at specific powder grades. The new SLS process parameters could improve the part surface quality if the MFR of the recycled powder is 18g/10min-19g/10min or higher. The proposed method does not lead to improvement of the surface quality when the PA12 powder has deteriorated to very high levels (18-19 MFR or lower) Acknowledgement The author would like to thank to the European Regional Development Fund (ERDF) who funded this work under the "Improvement of industrial production integrating macro-, micro- and nanotechnologies for more flexible and efficient manufacturing (IPMMAN)". References 1.3D Systems Corporation, [15th February 2007]. 2.Au, S and Wright, PK (1993). A Comparative study of Rapid Prototyping Technology. In Proceedings of Solid Freeform Fabrication, pp Austin: Texas. 3.Badrinarayan, B and Barlow, JW (1995). Effect of processing parameters in SLS of Metal- Polymers, Proceedings of Solid Freeform Fabrication Symposium, pp Austin: Texas. 4. EOS, Electro Optical Systems GmbH, [15th February 2007]. 5.Gibson, I and Shi (1997). Material Properties and Fabrication Parameters in SLS Process. Rapid Prototyping Journal, 3(4), Gornet, TJ (2002). Characterisation of Selective Laser sintering TM to determine process stability. Proceedings of Solid Freeform Fabrication, pp Austin: Texas. 7.Guo and Suiyan (1996). Optimizing of Selective Laser Sintering; Laser Processing of Materials and Industrial Applications. Proceedings of SPIE The International Society for Optical Engineering, Beijing. Vol. 2888, pp Hardro, PJ, Wang, JH and Stucker, BE (2004). A design of experiment approach to determine the optimal process parameters for rapid prototyping machines. Proceedings of Solid Freeform Fabrication, pp Austin: Texas. 9.Ho, HC, Gibson, I and Cheung, LW (1999). Effects of energy density on morphology and properties of selective laser sintered polycarbonate. Journal of Materials Processing Technology, 89, Ho, HC, Gibson, I and Cheung, WL (2000). Effects of Energy density on Bonus Z, surface roughness and warpage of SLS sintered Polycarbonate. Rapid Prototyping 9 th International Conference, Tokyo Japan, June 12-13, pp

16 W.A.Y Yusoff & A.J Thomas 11.Kai, CC, Chou, SM and Wong, TS (1998). A study of the state of the Art Rapid Prototyping Technologies. The International Journal of Advanced Manufacturing Technology, 14, King, D and Tansey, T (2003). Rapid Tooling: selective laser sintering injection tooling. Journal of Materials Processing Technology, Naim J, (2001). Finite Element Analysis of Curl Development in the SLS process. Phd thesis, Leeds University. 14.Pham, DT and Dimov, SS (2000). Rapid Manufacturing, The Technologies & Applications of Rapid Prototyping & Rapid Tooling. Springer. 15.Shi, Y, Li Z, Huang S and Zeng F (2004). Effects of the properties of the polymer materials on the quality of selective laser sintering parts. In Proceedings of the IMECH E, part L Journal of Materials: Design and Applications, Vol. 218, no.3, pp Yusoff,W.A.Y and Thomas A.J, Effect of employing different grades of recycled Polyamide 12 on the surface texture of Laser Sintered parts, 9 th Cairo University International Conference on Mechanical Design and Production (MDP-9), January 8-10, 2008, Cairo, Egypt. 17.Wholers, T (1999). Rapid Prototyping & Tooling State of the Industry. Executive Summary, Worldwide Progress Report, Wohlers Associates Inc. 18.Wholers, T (2000). Executive Summary. Worldwide Progress Report, Wohlers Associates Inc.