Molding Properties and Causes of Deterioration of Recycled Powder Injection Molding Feedstock
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INTRODUCTION
POWDER injection molding (PIM) is a relatively new process in fabricating structural parts with complicated shapes. Its main advantage over other competitive processes is the elimination of machining, particularly for parts with complicated shapes and using hard materials, such as ceramics and tool steels. The general process begins with kneading metal or ceramic powders with multicomponent binders. The kneaded feedstock is then molded into shapes and followed by debinding. One of the most widely used debinding processes is to immerse the molded part in a solvent bath to remove the soluble binders first. The solvent-debound parts are then heated to decompose the remaining binder and are subsequently sintered.[1] A significant amount of waste materials, including the runner, sprue, flash, gate, and defective green parts, are produced during the molding of powder injection molded products. The percentage of these waste materials increases as the size of the part decreases.[2] In order to lower the manufacturing cost and increase the competitiveness of the PIM process, these waste materials are usually recycled. Two methods are usually adopted by the PIM industries. The first one is to add 30 to 50 wt pct recycled feedstocks to fresh materials,[2] while the other is to use 100 pct recycled materials. When PIM feedstock is recycled, the metal powder and binder are subjected to a high shear rate and high L.H. CHENG, Graduate Student, and K.S. HWANG, Professor, are with the Department of Materials Science and Engineering, National Taiwan University, Taipei, 106, Taiwan, Republic of China. Contact e-mail: [email protected] Y.L. FAN, formerly Graduate Student, Department of Materials Science and Engineering, National Taiwan University, is Assistant Manager, Research and Development, Taiwan Powder Technologies Co., Taoyuan, 335, Taiwan, Republic of China. Manuscript submitted June 10, 2009. Article published online October 27, 2009 3210—VOLUME 40A, DECEMBER 2009
temperature again inside the barrel of the molding machine and at the gating area during remolding. Thus, oxidation or decomposition of the powder and binder occur. Sharma et al. showed that the stearic acid (SA) in the mixture of acetamide, paraffin wax (PW), and SA melted over a wide range of temperatures and had large variations in the latent heat after accelerated thermal cycles.[3] Camacho and Karlsson assessed the thermal properties of a blend of polypropylene and high-density polyethylene (PE).[4] It was noticed that the thermal degradation temperatures and melting points decreased after recycling due to chain scission during the extrusion cycles. From examination of the sintering characteristics of metal powder using recycled feedstocks, Manonukul et al. noticed that there was little change in the solid content and powder characteristics.[5] Thus, the sintered properties, including the sintered density, tensile strength, hardness, and microstructure, were similar to those attained using fresh feedstocks. However, Kulkarni and Kolts demonstrated th
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