Improvement of the Dimensional Stability of Powder Injection Molded Compacts by Adding Swelling Inhibitor into the Debin
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TRODUCTION
THE two-stage debinding method, solvent debinding followed by thermal debinding, is one of the most widely adopted processes in the current powder injection molding (PIM) industry.[1] During the solvent debinding process, soluble binder components such as paraffin wax (PW) and stearic acid (SA) are extracted by the solvent and leave interconnected pores. This facilitates the removal of the decomposed gas molecules when nonsoluble backbone binders, such as polyethylene (PE), are subjected to the subsequent thermal debinding.[2,3] However, some defects and poor dimensional control are frequently encountered in the solvent debinding stage. For example, PIM parts with areas of different thickness will have different amounts of expansion in each of those different areas. This is because polymeric binders swell in the solvent and the amount of swelling depends mainly on three factors: thickness of the specimen, content and type of the binder, and solvent temperature.[4] Parts without flat supports or with cantilever sections may even slump or distort during debinding, when the solvent diffuses into the binder and causes swelling and softening of the part.[4,5] Such polymer swelling can be explained by thermodynamic YANG-LIANG FAN and SHAO-CHIN SU, Graduate Students, and KUEN-SHYANG HWANG, Professor The Department of Materials Science and Engineering, National Taiwan University, Taipei, 106, Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted March 31, 2007. Article published online December 28, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
parameters, particularly the change in free energy of the polymer solution.[6] DG ¼ DH TDS
½1
where DG, DH, and DS are the changes in the free energy, enthalpy, and entropy of mixing, and T is the absolute temperature. When a polymer is immersed in the solvent, entangled polymers are separated into many individual polymer chains and therefore the entropy of mixing of the polymer solution is usually positive (DS > 0). However, each segment on the single polymer chain is still linked by the covalent bond. Thus, the entropy of mixing for polymer solutions remains small. This suggests that the free energy of mixing in Eq. [1] is mainly dependent on the enthalpy change. Hildebrand derived an equation to relate the enthalpy change, DH, with the solubility parameter d as[7] DH ¼ V/s /p ðdp ds Þ2
½2
where V is the volume of the solution; /s and /p are the volume fractions of solvent and polymer, respectively; and ds and dp are the solubility parameters of solvent and polymer, respectively. The solubility parameter describes the affinity between molecules of the materials, and it can be quantified using the following equation:[6] DðDHv RTÞ 1=2 d¼ ½3 M where DHV, M, D, R, and T are heat of evaporation, molecule weight, density, gas constant, and absolute VOLUME 39A, FEBRUARY 2008—395
temperature, respectively. This equation, however, cannot be used to determine the solubility parameters for cross-linked polymers because they usually decom
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