CAFEO 26 26-29 November 2008, Bangkok Thailand AN INFLUENCE OF SS316L’s PARTICLE SHAPE TO THE INJECTION MOLDING OF COMPOSITE BINDER FEEDSTOCK K. R. Jamaludin1, N. Muhamad2, M. N. Ab. Rahman2, S. Y. M. Amin2, S. Ahmad2, M. H. I. Ibrahim2, I. Murtadhahadi2, N. H. M. Nor2 1 Dept of Mechanical Eng., College of Science & Technology, University Technology Malaysia, Intl Campus, Kuala Lumpur, Malaysia 2 Precision Process Research Group, Dept. of Mechanical and Materials Engineering, Faculty of Engineering, National University of Malaysia, 43600 Bangi, Selangor Darul Ehsan, Malaysia *khairur@citycampus.utm.my ABSTRACT: Metal injection molding (MIM) has emerged as a viable method of producing complex shaped parts at a competitive cost. The MIM process uses a combination of powder metallurgy and injection molding technologies to produce net-shape parts and comprising of five main sub processes: raw materials selection (powder/binder), feedstock preparation, injection molding, debinding, and sintering. One of the advantages of MIM is its ability to produce parts with complex geometry without machining. However, most of the MIM parts are produced with fine and spherical shaped metal powder (gas atomized), which is expensive in cost. Thus, this paper presents an investigation to study the particle shape’s influence to the injection molding performance as well as the green body characteristics. The investigation is vital because the author attempts to seek a possibility of minimizing the production cost by substituting such existing metal powders with nearly spherical but still irregular shaped metal powder. The investigation begins with a rheological properties of the feedstock followed with the green body characteristics, and finally ends with analysis of variance of the processing parameters that influences the green strength and green density. The investigation discovered that the green characteristics of the green body produced with water atomized feedstock are satisfactory as its green characteristics are the equivalent as the one produced with gas atomized feedstock. KEYWORDS: Metal injection molding, Particle shape, Gas atomized powder, Water atomized powder 1. Introduction Metal injection molding (MIM) is a near-net shaping technology that combines the advantages of plastic injection molding and conventional powder metallurgy. It is a cost effective technology for fabrication of small intricate and precise parts in large quantities [1-3]. Four typical steps for the MIM process are mixing, injection molding, debinding and sintering. Initially, a binder with suitable formulation is mixed with the metal powder to form a feedstock. During molding, the feedstock is injection molded to produce green parts with required shapes. The molded parts then undergo a debinding step where the binder is removed. After debinding, the parts are subjected to a sintering step and final products with required properties are obtained [4-5]. In recent years, MIM has gained extensive popularity from the material science and industrial field due to its preponderances in fabrication area. Nowadays, a broad range of stainless steels are available as MIM powders. Most often they are produced by gas atomization or by water atomization. Figure 1 (a) shows the morphology of the spherical gas atomized powder while Figure 1 (b) 2 presents the irregular shaped water atomized powder. Table 1 gives the corresponding particle size distributions. Both kinds of powders have their advantages and disadvantages. Gas atomized powder has a higher packing density and thus needs less binder for injection molding leading to low shrinkage and distortion during sintering. The spherical morphology also guarantees a more isotropic shrinkage as the particles can not take a preferred direction during injection molding. The disadvantages of this kind of powder are its higher price and the lower strength of the molded part during debinding. After the stabilizing binder has been removed the spherical particles easily flow if sintering has not begun yet. Any handling or vibration can destroy the brown part in this state. The advantage of the water atomized powder lies in its lower price. Also, the brown strength of parts produced with this kind of powder is fairly high as the irregular particles cannot flow past each other. Disadvantages are the low tap density resulting in high sintering shrinkage as well as the tendency of the irregular particles to slightly align during injection molding and thus causing anisotropic shrinkage. This is why some companies ‘activate’ the water atomized CAFEO 26 26-29 November 2008, Bangkok Thailand powder by milling which produces a less irregular, slightly rounded powder [6]. The objective of this paper is to present the influence of SS316L’s powder particle shape to the green compact characteristics. The injection temperature and injection pressure has been indentified as factors that influences the green characteristics [7, 8]. An analysis of variance is used to investigate the significant and contributions of each factors to the green compact characteristics. Beside that, the rheological behavior of the feedstock is also presented. The rheological property of the feedstock is important because it will influence the green part characteristics as well as a molding performance. 2. Methodology 2.1 Materials Metal powder used in this study is SS316L gas atomized powder and water atomized powder which supplied by ANVAL Sweden and Epson Atmix Corporation, Japan respectively. The SEM image and particle size analysis of the powder is as shown in Figure 1 and Table 1 correspondingly; the D50 indicates the median particle size of the powder. Note that the gas atomized powder is more spherical than water atomized powder. a) Gas atomized powder b) Water atomized powder Figure 1 SEM image of the SS316L powder (2000 ×) Table 1 Particle size (µm) D10 Gas atomized powder 5.780 Water atomized powder 4.985 D50 D90 11.225 19.840 15.052 34.747 In order to facilitate the SS316L powder to flow easily into the mold cavity, polymer based binder is used to bind the powder particles together. Hence, the binder will act as a temporary vehicle for homogenously packing a powder into a desired shape and therefore hold the particles in that shape from the beginning of sintering. Although the binder should not dictate the final composition of molded material, it has a major influence on the success of MIM processing [9]. The major goal of the binder system is to provide the necessary flowability and to enable filling of a cavity during the injection molding process. The binder is subsequently removed from the mixture during debinding and the first stage of the sintering process [10]. The volumetric percentage (v/o) of powder in the mixture is termed as “powder loading” of the feedstock. This value has a large effect on virtually all properties of the feedstock. Consequently, a binder system based on polyethylene glycol (PEG) is used in the investigation. The minor component is polymethyl methacrylate (PMMA) and, stearic acid (SA) is added as the surface-active agent. The binder composition consists of 73 % PEG + 25 % PMMA + 2 % SA based on the weight fraction. 2.2 Experiment Procedure In the investigation, SS316L powders were mixed with binders in a sigma blade mixer for 190 minutes at 70 o C. After mixing, the paste was removed from the mixer and was fed into a strong crusher for granulation. The rheological characteristic of the feedstock was investigated using Shimadzu 500-D capillary rheometer with a die of L/D = 10, while the injection molding is performed with Battenfeld BA 250/50 CDC injection molding machine. 3. Results & discussion 3.1 Rheological behavior of the feedstock Viscosity is the most important parameter that judges the rheological behavior of the MIM feedstock. For example, high viscosity will make it hard to form molded components while low viscosity will make the binder separate from the powder. In this study, feedstock with a powder loading of 63 v/o is used. After granulating, viscosity of the feedstock was tested at the temperature of 130 °C and 140 °C and the results is as shown in Figure 2. It is acknowledged that viscosity is the internal friction force of running liquid and polymer melt is often considered as pseudo-plastic fluid. At definite temperature, viscosity decreases with the increase of the shear rate that can be expressed as follows: (1) where ߟ is the apparent viscosity, is the shear rate, K is a coefficient and n is a flow behavior index (< 1). Figure 2 demonstrates the relationship between shear rate and the apparent viscosity. The apparent viscosity is decreasing while the shear rate is increasing and this indicates the pseudoplastic behavior of the feedstock. The melt viscosity is also decreasing when the injection temperature is increasing due to more binders are able to wet the powder particle and thus minimizing the interparticle friction between powders. At the mean time, Figure 2 also shows that the water atomized powder is more viscous than of the gas atomized powder. This happens due to higher inter-particle friction generated by the non spherical and ligamental shaped of water atomized powder particles. CAFEO 26 26-29 November 2008, Bangkok Thailand This is due to the powder-binder separation occurred in the water atomized powder feedstock at high temperature. The reason is supported by the rheological result shown in Figure 2 that, there is a little increase of the water atomized melt viscosity at 140 °C especially at high shear rate. The increase of viscosity at high shear rate indicates the occurrence of powder-binder separation. Figure 2 Apparent viscosities versus shear rate of the SS316L feedstock at powder loading of 63 v/o. The pseudoplastic behavior of the melt is shown by the flow behavior index in Table 2. The pseudoplastic behavior index is associated with feedstock sensitivity to the shear stress. The small flow behavior index demonstrates higher melt sensitivity to the shear stress. Table 2 shows that the flow behavior index for both powders are increasing when the injection temperature is increases to 140 °C. This indicates that the shear sensitivity decreases when the injection temperature is increased. Moreover, the water atomized powder feedstock also demonstrates higher flow behavior index than of the gas atomized powder feedstock and this indicates water atomized powder feedstock is less sensitive compared to the gas atomized powder feedstock. Table 2 Flow Behavior index Powder type Temperature Gas atomized Water atomized 130 °C 140 °C 130 °C 140 °C Flow behavior index, n 0.57 0.69 0.68 0.84 3.2 Green body characteristics The green part characteristic presented in this paper is the green strength and green density. The inter-particle friction between metal powders and binders that hold the powder particles in its matrices is a major factor that influences the green strength. In addition, the irregular and ligamental shaped water atomized powder as shown in Figure 3 are stronger than of the gas atomized powders. This occurs due to the particle shape of the water atomized powder is providing more friction to the powder particle and thus improves the green strength. However, the water atomized compact become weaker when the injection temperature is increases and a vice versa happens to the gas atomized compact as it become stronger when the injection temperature was increased. Figure 3 Green strength versus injection temperature Furthermore, Figure 4 shows the injection temperature influence to the compact apparent density. The figure shows that the water atomized compact has higher green density than of the gas atomized compact. Mean while, water atomized compact demonstrates significant reduction of the apparent density due to powder-binder separation. The gas atomized compact exhibits an increase of the apparent density when the injection temperature is increases. At the mean time, Figure 3 and Figure 4 also show that the injection pressure has its influence to the green strength. Both figures demonstrate that the green strength and green density are proportional to the injection pressure. Figure 4 Green densities versus injection temperature 3.2 Analysis of variance for the green part characteristics An important technique for analyzing the effect of categorical factors on a response is to perform an analysis of variance. The analysis decomposes the variability in CAFEO 26 26-29 November 2008, Bangkok Thailand the response variable amongst the different factors. Depending upon the type of analysis, it may important to determine: (a) which factors have a significant effect on the response, and/or (b) how much of the variability in the response variable is attributable to each factor. Moreover, study of the analysis of variance table helps to determine which factors need control and which do not. The interactions between factors in the experiment can quantitatively determined by this analysis [11, 12]. As shown in Table 3, the interaction of injection pressure and injection temperature (A×B) to the gas atomized powder green strength is 23.88 %. This indicates that any changes to both factors will influence the gas atomized powder green strength. In addition, by comparing the variance ratio, Fn shown in Table 3 with the F statistics, it is indicates that only A×B is significant at moderate significant level of 5 %. However, the main factors, A and B do not show any significant to the green strength. Note that factors with a variance ratio, Fn less than the F statistic at 10 % significant level is considered not significant. Table 3 Analysis of variance for the injection temperature and injection pressure influence to the green strength of gas atomized powder Factor Sum of squares, Sn Variance, vn Variance ratio, Fn A (Inj. Pressure) B (Inj. Temp.) 0.323 0.323 0.282 7.020 2.340 2.038 AxB 12.596 4.199 3.656 E 18.375 1.148 T 38.314 F Contribution statistic percentage, Pn F(0.1)= 3.05 F(0.1)= 2.46 F(0.05)= 3.24 Table 4 Analysis of variance for the injection temperature and injection pressure influence to the green strength of water atomized powder Factor Sum of squares, Sn Variance, vn Variance ratio, Fn F statistic Contribution percentage, Pn A (Inj. Pressure) B (Inj. Temp.) AxB 0.133 0.133 0.147 F(0.1)=3.18 -5.34 0.601 2.857 0.301 1.428 0.332 1.578 F(0.1)=2.81 F(0.1)=2.81 -8.37 7.24 e 10.859 0.905 T 14.450 100 The analysis of variance for the green density is shown in Table 5 and Table 6. Table 5 shows that only main factor (injection temperature, B) is significant to the green density of the gas atomized powder compact while, in Table 6 the interaction of A × B is significant to the green density of the water atomized powder compact. Though, both factors are significant at 10 %, which is the lowest significant level. Additionally, Table 6 indicates that the interaction of A × B is 20.86 % compared to that shown in Table 5 for the main factor B is only 18.52 %. Table 5 Analysis of variance for the injection temperature and injection pressure influence to the green density of gas atomized powder Sum of squares, Sn Variance, vn Variance ratio, Fn F statistic Contribution percentage, Pn 0.013081 0.013081 0.978799 F(0.1)=3.05 -0.08348 2.570365 F(0.1)=2.46 18.52132 0.248365 F(0.1)=2.46 -2.15 Factor 9.33 A (Inj. Pressure) B (Inj. Temp.) 0.103051 0.03435 AxB 0.009957 0.003319 e 0.213831 0.013364 T 0.33992 23.88 68.94 100 Furthermore, Table 4 shows the analysis of variance for the green strength of water atomized powder compact. The table shows that none of the factors are significant but the interaction of A × B shows very little contribution (7.24 %) to the green strength of the water atomized powder compact. Although these factors are not contributed to the green strength of the water atomized powder, however it may influenced by some other factors such as powder loadings, mold temperature, holding pressure and etc. 106.463 -8.86558 90.42774 100 Table 6 Analysis of variance for the injection temperature and injection pressure influence to the green density of water atomized powder Factor Sum of squares, Sn Variance, vn Variance ratio, Fn F statistic Contribution percentage, Pn 0.0003813 0.0003813 0.2330953 F(0.1)=3.18 -3.7962595 -1.2191629 A (Inj. Pressure) B (Inj. Temp.) 0.0028691 0.0014345 0.8768518 F(0.1)=2.81 AxB 0.0101673 0.0050837 3.1073686 F(0.1)=2.81 e 0.0196319 0.001636 T 0.0330496 20.86353 84.151892 100 4. Conclusions The influence of the SS316L’s particle shape influence to the injection molding of MIM’s green compact is discussed in this paper. The authors have found that there are influences of the powder particle shape to the injection molding performance. CAFEO 26 The rheological investigation found that the water atomized powder feedstock exhibits higher viscosity than of the gas atomized powder feedstock. In addition, the flow behavior index of the feedstock shows water atomized powder feedstock demonstrates lower shear sensitivity. Moreover, water atomized powder compact was found to be stronger and denser than the gas atomized powder feedstock. This is for the reason that the irregular shapes of the water atomized powder provides more interparticle friction to the powder matrix and thus makes it stronger than the gas atomized powder compact. The analysis of variance also shows that the interaction of A×B is significant to the green strength of the gas atomized powder compact and green density of the water atomized powder compact respectively. Besides the main factor B is only significant to the green density of the gas atomized powder compact. Acknowledgements My deepest appreciation and gratitude to National University of Malaysia for the research grant, UKM-KK02-FRGS0013-2006 and University Technology of Malaysia and Ministry of Higher Education, Malaysia for the PhD scholarship. References [1] German, R. M, Hens, K. F. & Lin, S. T. 1991. Key issues in powder injection molding. Bulletin of American Ceramic Society. 70(1): 294-302. [2] Mutsuddy, B. C. & Ford, R. G. 1995. Ceramic Injection Molding. London: Chapman & Hall: 1-22. [3] German, R M. 1993. Technological barriers and opportunities in powder injection molding. Powder Metallurgy. 25(4): 165-169. [4] German, R.M. 1990. Powder Injection Molding. Metal Powder Industries Federation, Princeton, NJ. [5] Loh, N. H., Tor, S. B. & Khor, K. A. 2001. Production of metal matrix composite part by powder injection molding. Journal of Materials Processing Technology. 108(3): 398-407. [6] Hartwig, T., Veltl, G., Petzoldt, F., Kunze, H., Scholl, R. & Kieback, B. 1998. Powders for Metal Injection Molding. Journal of the European Ceramic Society 18: 121 l-1216. [7] Jamaludin, K. R., Muhamad, N., Rahman, M. N. Ab., Amin, S. Y. M. & Murtadhahadi . 2008. Analysis of Variance on the Metal Injection Molding parameters using a bimodal particle size distribution feedstock. Proceeding of the International Conference in Mechanical and Manufacturing Engineering 2008. Johor Bahru Malaysia. [8] Jamaludin, K. R., Muhamad, N., Rahman, M. N. Ab., Amin, S. Y. M., Murtadhahadi, Ismail M. H. 2008. Injection molding parameter optimization using the Taguchi method for highest green strength for bimodal powder mixture with SS316L in PEG and PMMA. Proceeding of the World Powder 26-29 November 2008, Bangkok Thailand Metallurgy and Particulate Materials 2008, Washington DC. [9] German, R.M., and Bose, A., 1997. Injection Molding of Metal and Ceramic, Metal Powder Industries Federation , Princeton, N.J. [10] L. Kowalski and J. Duszczyk, Specific heat of metal powder-polymer feedstock for powder injection molding. Journal of Materials Science Letters, Vol. 18 (1999) 1417-1420. [11] Ranjit, R. A primer on the Taguchi method, 1990, Society of Manufacturing Engineers, Michigan. [12] Park, S.H, Robust Design and Analysis for Quality Engineering, Chapman & Hall, London 1996.