Length change and deformation of powder injection-molded compacts during solvent debinding
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I. INTRODUCTION
SOLVENT debinding is one of the most-widely adopted processes in the powder injection molding industry, because of its effectiveness in removing binders from compacts.[1,2,3] The first step in this process is to use solvent to extract soluble binder components from the compact. The remaining insoluble binders are then pyrolyzed or decomposed during a subsequent thermal heating process.[3,4,5] During the solvent extraction step, interconnected pore channels are left behind after soluble binders are dissolved into the solvent. As the solvent-debound compacts are subjected to thermal debinding, decomposed gas forms and escapes into the ambient air through the existing pore channels. Since there is little pressure built up from the gas, this process can, thus, eliminate defects like bubbles, cracks, exfoliation, and pin holes, which usually occur in the straight thermal debinding process.[6] However, some other defects such as cracking, distortion, and slumping are still frequently encountered in solvent debinding.[7–11] These defects have been shown to be related to the swelling and softening caused by the reaction between the solvent and the binders.[7,12,13] Lin and Hwang studied the effects of processing parameters and binder characteristics on the amount of swelling by measuring the in-situ dimensional changes of compacts during debinding.[13] Their results showed that the amount of expansion increases with an increase in the debinding temperature and the amount of insoluble binder and with the decrease in the molecular weight of the solvent and the binder. These previous studies have indicated that the tolerance control and defects of powder injection-molded (PIM) compacts are closely related to the swelling and softening behavior of binders during solvent debinding. However, few S.C. HU, Graduate Student, and K.S. HWANG, Professor, are with the Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, Republic of China. Manuscript submitted September 13, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
studies have been reported to date on the details of these phenomena. The purpose of this study was, thus, to further investigate the effects of debinding parameters on the dimensional control of PIM compacts. II. EXPERIMENTAL PROCEDURE Carbonyl iron powders with an average particle size of 4.6 mm were used in this study. The iron’s characteristics are shown in Table I. To prepare the feedstock, iron powders were kneaded with 6.5, 7, and 8 wt pct binders, respectively. The binder consisted of 40 wt pct low-density polyethylene (LDPE), 55 wt pct paraffin wax (PW), and 5 wt pct stearic acid (SA). Both PW and SA can be dissolved into heptane, while LDPE cannot; the former and the latter are, thus, categorized as soluble and insoluble binders, respectively. After being kneaded and granulated, the feedstock was molded into rectangular specimens of 2 3 10 3 100 mm in size. Solvent debinding was carried out by immersing molded compacts in heptane at various temperatur
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