Joamin Gonzalez-Gutierrez 1, Igor Emri 1,2 *

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1 Izvirni znanstveni članek TEHNIKA ateriali Datu prejea: 27. arec 2015 ANALI PAZU 4/ 2014/ 2: Powder Injection Molding an Alternative Method in the Manufacturing of Parts for Vehicles Injekcijsko brizganje prahu, alternativna etoda za proizvodnjo delov za vozila Joain Gonzalez-Gutierrez 1, Igor Eri 1,2 * 1 Faculty of Mechanical Engineering, University of Ljubljana, Ljubljana, Slovenia 2 Institute for Sustainable Innovative Technologies isit, Ljubljana, Slovenia s: [email protected], [email protected] * Author for correspondence Abstract: Powder injection olding (PIM) is one of the ost versatile ethods for anufacturing sall coplex shaped coponents fro etal, ceraic powders for the use in any applications. Interesting applications of PIM include the autootive and aerospace industries. PIM can be used to cobine ultiple parts into a single part with coplex geoetry, and parts are lighter since porosity reains in the final part while the fatigue resistance increases. Therefore, it can be said that PIM can help reduce the weight of oving vehicles, which is a ajor concern fro the environental and econoic point of view. One of the ain liitations of current PIM technology is the long debinding tie, which is one of the steps within the process. This has been partially solved by introducing binder systes that undergo catalytic subliation. However, current catalytic binders have the ain proble of high viscosity. In this study, two ways to decrease the viscosity of PIM feedstock aterials with polyoxyethylene were investigated. The first way was to reduce the average olecular weight of the binder and the second one to select a polydisperse particle size distribution with high axiu packing fraction. Key words: powder injection olding, viscosity, feedstock, packing fraction. Povzetek: Injekcijsko brizganje prahu (PIM) je eden ized najbolj vsestranskih načinov za proizvodno ajhnih, kopleksnih delov iz kovinskega ali keraičnega prahu. Ena ized zaniivih aplikacij PIM-a je vesoljska industrija, saj se PIM lahko uporablja za združevanje več delov v skupen del s kopleksno geoetrijo. Z uporabo PIM tehnologije lahko torej dosežeo anjšo poroznost in povišano odpornost na utrujanje končnih izdelkov. Iz tega stališča je ogoče reči, da se lahko PIM poaga zanjšati težo vesoljska vozila, kar predstavlja velik proble z okoljskega in ekonoskega vidika v tej industriji. Eden ized glavnih oejitev sedanje PIM tehnologije je čas potreben za odstranjevanja veziva, ki je eden ized korakov v procesu. To je delno rešeno z uporabo veziva iz polioksietilena, ki katalitično subliira. Slaba stran uvedbe tega veziva pa je povišana viskoznost. V sklopu tega prispevka raziskujeo dva načina za zanjšanje viskoznosti PIM surovine na osnovi POM-a. Prvi način je zanjšati povprečno olekulsko aso veziva in drugi, da izbereo polidisperzno porazdelitev velikosti delcev z visoki deleže gostote pakiranja. Ključne besede: injekcijsko brizganje prahu, viskoznost, surovina, gostota pakiranja. 76

2 POWDER INJECTION MOLDING AN ALTERNATIVE METHOD IN THE 1. Introduction Powder injection olding (PIM) is a technology for anufacturing coplex, precision, netshape coponents fro either etal or ceraic powder. The potential of PIM lies in its ability to cobine the design flexibility of plastic injection olding and the nearly unliited choice of aterial offered by powder etallurgy, aking it possible to cobine ultiple parts into a single one [1]. Furtherore, PIM overcoes the diensional and productivity liits of isostatic pressing and slip casting, the defects and tolerance liitations of investent casting, the echanical strength of die-cast parts, and the shape liitation of traditional powder copacts [2]. Typical parts produced by PIM range fro illigras to a few hundred gras [3]. It is iportant to ention that this technology is recoended for the ass production of sall coplex parts, due to the high capital investent in aking high precision oulds with coplex geoetry. Therefore it is suitable for parts to be used in autootive or aerospace systes, and electronic devices, to ention a few. The PIM process presents several variations that are used in the industry today. Invariably, it consists of four steps: (1) ixing etal or ceraic powder and a polyeric binder, (2) injecting this ixture into old, (3) debinding to reove polyer fro ixture and (4) sintering to bring together the otherwise loose powder [4]. All these steps are scheatically shown in Figure 1. One of the interesting applications of PIM is in the anufacturing of parts for oving vehicles because PIM can be used to cobine ultiple parts into a single part with coplex geoetry, and parts are lighter since porosity reains in the final part, at the sae tie the fatigue resistance increases. Fatigue resistance increases since the sall pores act as crack stoppers, therefore preventing crack propagation and failure of a part loaded cyclically. Thus, it can be said that PIM can help reduce the weight of oving vehicles, which is a ajor concern fro the environental and econoic point of view. Ceraic injection olding has been around since 1940 s [5] and etal injection olding since the 1970 s [6], in fact, soe of the first applications of etal injection olding was for the aerospace industry. Soe exaples include flat screw seals and rocket burning syste, which were produced at the end of the 1970 s [7]. In the autootive industry etal injection oulding is being utilized for anufacturing lock caps, lock shafts, cable seals used in sunroofs, soft agnetic sensor housing parts, cas in electrical systes of seats, bonnet lock fixing bearings, turbocharger vanes, rocker ars in engines, rollers and adjustent rings. One notable exaple of a autootive part produced by ceraic injection olding is a turbine wheel [1]. Even though the technology has been around for quite soe tie, there is still roo for iproving the process and aterials utilized in PIM. One of the ain liitations of current PIM is the long debinding tie, which has been partially solved by introducing binder systes that undergo catalytic subliation (polyoxyethylene-based binders); thus the olded part is debound in the solid state without elting or dissolving the polyeric binder in a uch shorter tie. However, current catalytic binders have the ain proble of high viscosity. In this study, two ways to decrease the viscosity of PIM feedstock aterials with polyoxyethylene were investigated. The first way was to reduce the average olecular weight of the binder [8] and the second one to select a polydisperse particle size distribution with high axiu packing fraction. 2. Materials and Methods 2.1. POM-based binders Polyoxyethylene (POM) also known as polyacetal and less coonly as polyforaldehyde is a type of seicrystalline theroplastic, linear polyer produced by chain growth polyerization. POM is generally synthesized as a hoopolyer or copolyer. For this investigation, POM copolyers of different average olecular weight (Mw) were synthesized at BASF (Ludwigshafen, Gerany). The noenclature and average olecular weight of all the POM aterials used in this study is shown in Table 1. Molecular weights were easured by the supplier using gel pereation chroatography. Figure 1. Flow chart illustrating the ain stages of PIM process. 77

3 ANALI PAZU, 4/ 2014/ 2, str Joain GONZALEZ-GUTIERREZ, Igor EMRI Table 1. Average olecular weight of POM copolyers. Material ID Average Molecular Weight, Mw, [g/ol] MW MW MW MW MW MW MW MW Metal powders and feedstock aterials Description Virgin POM copolyer, laboratory scale synthesis Virgin POM copolyer, industrial scale synthesis Stainless steel 316 is a chroiu-nickelolybdenu austenitic stainless steel developed to provide corrosion resistance to traditional alloys such as 304. The grade 316LW has a lower content of carbon (0.03 wt%) copared to 316 (0.08 wt%), which provides added corrosion resistance. Therefore 316LW can be used in the anufacturing of parts to be used by the aritie, autootive and aerospace industry. For this investigation stainless steel 316LW powder with a broad particle size distribution (PSD) was supplied by BASF (Ludwigshafen, Gerany). The powder was produced via gas atoization. The supplier separated the etal particles of the coercial product in five different fractions, each of these with different PSD and different average particle size (Figure 2). Particle size distribution was easured by laser scattering at BASF. The atrix used in the suspension was polyoxyethylene (POM) copolyer with an average olecular weight of approxiately g/ol. The POM copolyer was also produced by BASF (Ludwigshafen, Gerany). Feedstock aterials with different solid content (20 to 45 vol %) were produced by extruding the steel powder with the POM copolyer at 190 C. Mixing of copolyers was done in a twin-screw extruder (Haake Polylab, Thero Scientific, Gerany). In order to ensure proper ixing, extrudate was pelletized after the first extrusion and extruded once again. Nuber of extrusions was liited to two in order to prevent echanical and theral degradation of POM copolyers. The teperature profile and screw rotational speed of the extruder were set in such a way as to ensure that the POM elt did not exceed 190 C to prevent theral degradation. In order to deterine the final content of solids (φ), the binder was reoved by heating up the feedstock aterial up to 300 C for two hours, and the reaining ass was used to estiate (φ) Shear viscosity easureents Viscosity easureents in oscillatory ode were perfored in a MARS-II rotational rheoeter (Thero Scientific, Gerany). Viscosity tests were perfored at 190 C, which is within the rage of teperatures at which POM is generally processed (180 to 230 C). A truncated cone-plate easuring-geoetry with a 20 diaeter and angle of 1 was used. Two frequency sweeps were perfored in each easureent; the first one increasing fro 0.01 Hz ( rad/s) to 100 Hz ( rad/s), and the second one decreasing fro 100 to 0.01 Hz. All viscosity easureents at a given teperature were perfored six ties per aterial. In this study, viscosity results are presented as the agnitude of the coplex viscosity ( η* ), which is related to the constant rotational viscosity (η) through the Cox-Merz rule [9]. Constant rotational shear viscosity of the resulting feedstock aterials was easured in a rotational rheoeter fitted with parallel plates of 20 diaeter at 180 C (MARS II, Thero-Scientific, Gerany). All easureents were perfored in triplicate. Shear rate was varied fro 0.1 to 100 s Figure 2. Differential voluetric particle size distribution for steel powder fractions.

4 POWDER INJECTION MOLDING AN ALTERNATIVE METHOD IN THE 3. Results and Discussion 3.1. Shear viscosity easureents of binder The agnitude of the coplex viscosity as a function of angular frequency for all POM copolyers was easured at 190 C. Fro the easured coplex viscosity data ( η* ) the Newtonian viscosity (η 0 ) was estiated fro the plateau at frequencies below 10 rad/ s. The results are presented as a function of the average olecular weight in Figure 3. As with other polyers, POM copolyers show a rapid decrease in viscosity as the average olecular weight decreases following a power function as proposed by Fox and Flory [10]. The propose equation for POM copolyers is shown inside Figure e16M w 2 R Figure 3. Newtonian viscosity of POM copolyers at 190 C; the solid line represents a power law fit to the experiental data. 3.6 With respect to the selection of an appropriate binder for PIM, one could choose between the first three olecular weights (MW010, MW024 and MW052), which are all around the recoended viscosity of 10 Pa s [5]. For the purpose of this investigation we selected the iddle one (MW024) for preparing feedstock aterials Shear viscosity easureents of feedstock Viscosity results for feedstock aterials as a function of voluetric solid content (φ) are shown in Figure 4. Please notice that relative viscosity (η r ) is used in the vertical axis, which is the ratio of the viscosity of the suspension over the viscosity of the polyeric atrix, both viscosities easured under the sae shear rate (100 s -1 ) and teperature (180 C). As expected, as φ increases so does the relative viscosity η r, but the increase is significantly different depending on the fraction used in the feedstock aterial, since each fraction has different particle size distribution (Figure 2). Using Fraction V, the highest φ was achieved without increasing uch η r. Five odels were selected fro the literature (Table 2) that predict the viscosity of concentrated acroscopic suspensions just by knowing the particle loading (φ) and the axiu packing fraction (φ ), which represents the particle loading at which the viscosity goes to infinity. Other odels were available in the literature, but they were ignored since they depend on other epirical paraeters or are not applicable for concentrated suspensions. Figure 4. Relative shear viscosity η r at 100 s -1 and 180 C as a function of particle load φ for prepared feedstock aterials containing powder fractions I-V. 79

5 ANALI PAZU, 4/ 2014/ 2, str Joain GONZALEZ-GUTIERREZ, Igor EMRI Table 2. Models to predict the relative viscosity of concentrated suspensions. Authors Year Equation Frankel & Acrivos [11] Chong et al. [12] Data shown in Figure 4 was fitted with the five odels selected and (φ ) for each powder fraction was estiated. The coefficient of deterination (R 2 ) was used to deterine which odel fits the data better overall. The results for the best (Zarraga et al) odel are shown in Table 3. Using the odel proposed by Zarraga et al, one can estiate the axiu loading needed to achieve a viscosity close to 1000 Pa s at the selected conditions for each fraction. Table 3 shows that Fraction V has the highest (φ ) and therefore it can be loaded the ost without increasing the viscosity beyond 1000 Pa s, approxiately to up to 83 %vol. 4. Conclusions Queada [13] 1976 Zarraga et al. [14] Mendoza & Santaaria- Holek [15] 9 nr 8 1 Reduction of viscosity of PIM feedstock is very iportant, since it will facilitate the injection olding of parts with coplex geoetry. In this study it was nr n 1 r n r e nr 1 c 1 c 2 observed that viscosity of POM copolyers increases with average olecular weight following a power law relationship. With these results on hand, we recoend to use a POM-based binder with a olecular weight around g/ol. Feedstock aterials were prepared with the recoended binder at different powder loadings. It was observed that viscosity of feedstock aterials for PIM can be further reduced by selecting the appropriate particle size distribution. In this study it was shown that using powder with a wide particle size distribution leads to higher axiu packing fraction that can reduce significantly the viscosity of feedstock aterials. It was also observed that fro the siple odels available in the literature the odel of Zarraga et al [14] best fits our experiental data. Using this odel the axiu loading for the feedstock to reach a viscosity of approxiately 1000 Pa s using the powder with the highest axiu packing fraction is approxiately 83 %vol. Acknowledgeents The authors would like to acknowledge the financial support of the Ad Futura Fund of the Republic of Slovenia, the Slovenian Research Agency and the Erasus Mundus Progra; as well as, the support of the staff at BASF Ludwigshafen, Gerany for the synthesis and olecular weight characterization of POM copolyers, as well as for fractioning the etal powder and easuring the particle size distribution. We also acknowledge the work of Aberto Baeza Capuzano and Sergio Carrillo De Hert during the preparation of feedstock, viscosity easureents and odel fitting. Literature 1. Hausnerova, B Powder injection oulding- An alternative processing ethod for autootive ites. In New Trends and Developents in Autootive Syste Engineering, Chiaberge M. (Ed.), InTech, Rijeka, Croatia, pp Table 3. Maxiu packing fraction (φ ) and coefficient of deterination (R2) obtained by fitting at 100 s -1 and 180 C by Zarraga et al odel [11]. Powder Fraction Maxiu packing fraction, Coefficient of deterination, R 2 Maxiu loading before reaching 1000 Pa s I II III IV V

6 POWDER INJECTION MOLDING AN ALTERNATIVE METHOD IN THE 2. Tandon, R. (2001). Metal injection oulding. In Encyclopedia of Materials: Science and Technology, Buschow, K.H.J., Cahn, R.W., Fleings, M.C., Ilscher, B., Kraer, E.J., Mahajan, S. & Veyssiere, P. (Ed.), Elsevier Science Ltd, Asterda, The Netherlands, pp Geran, R.M. (2008). PIM breaks the $1 bn barrier. Metal Powder Report,.63 (3): Stringari, G.B., Zupančič, B., Kubyshkina, G., von Bernstorff, B., and Eri, I Tie-dependent properties of biodal POM Application in powder injection olding, Powder Technology, 208 (3): Mutsuddy, B.C., and Ford, R.G Ceraic Injection Molding. Chapan & Hall, London, UK. 6. Willias, B Paratech Shapes Metals like Plastics, Metal Powder Report 44 (10): Applications for MIM IV: The Aerospace Industry, Powder Injection Moulding International. accept_cookies=1 (accessed ). 8. Gonzalez-Gutierrez, J., Oblak, P., von Bernstorff, B.S. and Eri, I Iproving powder injection oulding by odifying binder viscosity through different olecular weight variations. In Seliger G. (Ed), Proceedings of the 11th Global Conference on Sustainable Manufacturing- Innovative Solutions. Universitätsverlag der TU Berlin, Berlin, Gerany, p Cox, W. P., and Merz, E. H Correlation of dynaic and steady flow viscosities, Journal of Polyer Science, 28: Fox, T.G., and Flory, P.J Further studies on the elt viscosity of polyisobutylene, Journal of Physical and Colloidal Cheistry, 55: Frankel, N., and Acrivos, A On viscosity of a concentrated suspension of solid spheres, Cheical Engineering and Science, 22: Chong, J. S., Christiansen, E.B., and Baer, A.D Rheology of concentrated suspensions, Journal of Applied Polyer Science, 15: Queada, D Rheology of concentrated disperse systes and iniu energy-dissipationprinciple, Rheologica Acta, 16: Zarraga, I., Hill, D., and Leighton, D The characterization of the total stress of concentrated suspensions of noncolloidal spheres in Newtonian fluids, Journal of Rheology, 44: Mendoza, C., and Santaaria-Holek, I The rheology of hard sphere suspensions at arbitrary volue fractions: An iproved differential viscosity odel, Journal of Cheical Physics, 130: