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Korean J. Chem. Eng., 19(6), 986-991 (2002)To whom correspondence should be addressed.E-mail: limjskist.re.krSupercritical Carbon Dioxide Debinding in Metal Injection Molding (MIM) ProcessYong-Ho Kim, Youn-Woo Lee, Jong-Ku Park*, Chang-Ha Lee* and Jong Sung LimNational Research Lab. for Supercritical Fluid, *Ceramic Processing Center,Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea*Dept. of Chem. Eng., Yonsei University, 134 Shinchon-dong, Sudaemoon-ku, Seoul 120-749, Korea(Received 4 March 2002 accepted 29 August 2002)AbstractThe conventional debinding process in metal injection molding (MIM) is critical, environmentally unfriendlyand time consuming. On the other hand, supercritical debinding is thought to be an effective method appropriate foreliminating the aforementioned inconvenience in the prior art. In this paper, supercritical debinding is compared withthe conventional wicking debinding process. The binder removal rates in supercritical CO2have been measured at333.15 K, 348.15 K, and 358.15 K in the pressure range from 20 MPa to 28 MPa. After sintering, the surface of thesilver bodies were observed by using SEM. When the supercritical CO2debinding was carried out at 348.15 K, all theparaffin wax (71 wt% of binder mixture) was removed in 2 hours under 28 MPa and in 2.5 hours under 25 MPa. Wealso studied the cosolvent effects on the binder removal rate in the supercritical CO2 debinding. It was found that theaddition of non-polar cosolvent (n-hexane) dramatically improves the binder removal rate (more than 2 times) for theparaffin wax-based binder system.Key words: Supercritical CO2Debinding, Metal Injection Molding (MIM), Binder, Diffusivity, CosolventINTRODUCTIONMetal injection molding (MIM) is used to make metallic partsthat cannot be readily produced by conventional material formingprocesses. It is a net-shape process and can be used to produce partswith complex geometries. In addition to being a cost-effective alter-native for cast or forged and machined parts, MIM enables one tomass produce complex-shaped parts that are difficult to machineby conventional methods German, 1987; Hens, 1990. Complexshapes that are produced using the MIM process can be formed in-expensively to nearly full-density through the use of a polymer-pow-der combination. Because high density can be achieved, the MIMprocess has the ability to mould high-performance engineering ma-terials Tam et al., 1997. Moreover, those parts are not necessaryto any secondary machining processes. Despite the numerous ad-vantages offered by the MIM process, several limitations exist thatincrease the complexity of the process. The debinding of MIM com-ponents is time-consuming and brings about defects, which affectproperties of the sintered parts because of capillary force Chartieret al., 1995, especially when the sizes of the powders used becomerelatively small (0In the case of a slab, the local content of solute remaining in thegreen body after some time t of extraction can be expressed by(2)It is often difficult to determine the concentration at various depths,and what is experimentally determined is the quantity of solute, whichhas been extracted, or the quantity remaining in the green part. Forthis purpose the average concentration is needed, and is obtainedby intergrating Eq. (2):(3)For a long duration of supercritical debinding (t0), the firstterm in the right hand side of Eq. (3) shows much larger than thesummation of the remaining terms, and hence, the fraction can beapproximated by Eq. (4).(4)The values of D are determined by using Eq. (4) by inserting ex-perimental values of average concentration ( ) at time t and ln( /c0)plotting with time. Then the diffusivities (D) can be obtained fromthe slope of the plot. Table 2 shows the densities and the diffusivi-ties determined in this work at supercritical conditions. The densityof pure CO2at a given temperature and pressure was calculated fromCt-= D2Cx2-cxt,() = 4c0pi-12n + 1- exp D2n + 1()2pi2tl2-sin2n + 1()pixl-n= 0cct() = 1l- cxt,()dx =0l8c0pi2-12n + 1()2-expD2n +1()2pi2tl2-n= 0cc0- =8pi2-12n +1()2-exp D2n + 1()2pi2tl2-n= 0cc0-=8pi2-exp Dpi2tl2-c cFig. 7. Effect of the cosolvent on binder removal rate in supercrit-ical debinding at 348.15 K, 25 MPa.Korean J. Chem. Eng.(Vol. 19, No. 6)990 Y.-H. Kim et al.the equation of state by Angus et al. 1976. This shows that dif-fusivity increases with an increase in pressure at constant tempera-ture. Fig. 5 and Fig. 6 show that theoretical curves calculated withEq. (3) by inserting diffusivities (D) obtained above are in good agree-ment with the measured data. In general, binder removal rate is af-fected by the diffusivity of wax, because solute diffusion will proba-bly govern the overall rate of mass transfer. Therefore, the diffu-sion of supercritical CO2might have relatively small effect on therate of mass transfer McHugh and Krukonis, 1994.6. The Analysis of the Sintered Parts SurfacesAfter debinding of the two samples-one is debinded by super-critical method and the other one is by wicking method-they weresintered at 1,673 K for 2 hours under vacuum conditions. The mi-crographs of two surfaces of the silver parts are compared in Fig. 8.As can be seen in these figures, the sample from wicking debind-ing has a few pores or cracks but that from supercritical debindinghas no defects.CONCLUSIONIn this paper, supercritical debinding is compared with conven-tional wicking debinding process. Wax-based binder system is usedin this experiment. The binder removal rate in supercritical CO2hasbeen measured at 333.15 K 348.15 K, and 358.15 in the pressurerange from 20 MPa to 28 MPa. After sintering, the surface of thesilver bodies was observed by using SEM. When the supercriticalCO2debinding was carried out at 348.15 K, almost all the wax (about71 wt% of binder) was removed in 2 hours under 28 MPa, and 2.5hrs under 25 MPa. We also studied the cosolvent effects (methanol,n-hexane) on the binder removal rate in the supercritical CO2 de-binding. It was found that the addition of non-polar cosolvent (n-hexane) dramatically improves the binder removal rate (more than2 times) for the paraffin wax-based binder system. The diffusivitiesof paraffin wax in supercritical CO2were calculated by Ficks dif-Table 2. Density-diffusivity relationship for binder removal insupercritical debindingTemp.(K)Pressure(MPa)Binderremoval for1 hr (wt%)Density ofsupercriticalCO2 (g/cm3)Diffusivity(m2/s)348.15 28 63.44 0.74595 3.651010348.15 25 59.93 0.71292 2.741010348.15 20 51.94 0.62824 1.821010358.15 25 60.19 0.66247 2.241010333.15 25 22.64 0.78774 5.291011Fig. 8. SEM micrographs for the surfaces of the silver parts sintered at 1,673 K for 2 hrs. The silver part debinded by (a) wicking method;(b) supercritical method.November, 2002Supercritical Carbon Dioxide Debinding in Metal Injection Molding (MIM) Process 991fusion model.In conclusion, supercritical CO2debinding may offer a short de-binding time and safe working environment as an alternative to thecurrent conventional debinding methods, such as the solvent extrac-tion or thermal debinding.ACKNOWLEDGMENTThis work was supported by Ministry of Science and Technol-ogy of Korea, the National Research Laboratory Program for Sup-ercritical Fluid. 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