Background of Study CONCLUSION AND RECOMMENDATION

1 CHAPTER 1 INTRODUCTION

1.1 Background of Study

Nowadays, major challenges in Powder Metallurgy PM research field are design complexity and flexibility of the fabrication process, with a low cost consideration. Researchers have tried to propose the best solution to fulfill the extensive industrial requirement. There are many improvisation and development in the field of powder metallurgy. One of the new invented methods is to introduce the very recent and innovative powder metallurgy route by combining the concept of plastic injection molding together with the utilization of the metal powder feedstock. Such a unique method mentioned is the Metal Injection Molding MIM. Basically, this process is more complicated compared to the plastic injection molding process. The process is based on the use of fine powder particles mixed with the small quantity of wax binders andor thermoplastic polymer to form the feedstock that can be molded German and Bose, 1997. MIM involves four main sequential processes, which are mixing, injection molding, debinding and sintering. Powder and binder mixed together to form the feedstock during the mixing process. Shape of the feedstock was developed through the injection molding process and followed by the debinding step. Debinding process involves the removal of the binder system. During the first stage of injection molding process, debinded product was heated up to the temperature great enough to form small, complex, precision parts and cost effective in high volumes, comparable to the material properties of conventional PM parts German, 1990. Every process involved in MIM has significant impacts to the 2 characteristics of the starting materials; the powder and the binder are critically important to the overall success of this process. The optimum powder loading should be applied as it will affect the whole process while the feedstock has to be homogenous as possible made by intensive mixing of metal powder and the binder. The optimum powder loading applied in this study is in accordance to the previous research done by Gulsoy et al., 2007 and Ye et al., 2008. Once the powder formulation has been established, further concern will be given to the ingredient mixing. The uniformity of the feedstock is crucial, since most inhomogenities condition cannot be corrected in the subsequent processing step. The best mixing occurs with high shear, but not to the point where the higher mixing shear damages the particles or overheats the binder. A properly mixed material will consist of homogenous powder dispersion in the binder with no internal porosity or agglomerates. Inhomogenities results in nonuniform viscosities, uneven molding and difficulties at the sintering stage Klar and Samal, 2007. Ideally, the feedstock design must be adequately considered the easiness of molding and strict control over the final dimensions. To achieve this situation, the feedstock used must have low molecular weight polymer binders as to reduce viscosity and to ease the molding process. If there are no voids or pores present in the feedstock, there are sufficient binders to fill in all interparticle spaces. Several defects that commonly occur are surface wrinkles, binder separation, weld lines, sink marks, flashing and unmature shot Norhamidi et al., 2002. After the molding, attentions are then given to the debinding stage. The binder use must be disposable component in the feedstock mixture. Failure to remove most of the binder before the sintering could results in component distortion, cracking and contamination. Removing the binder without disrupting the particles is a delicate process which can be successfully achieved in multiple steps. When the binder is heated, it softens and is unable to withstand the stress from the gravity, and temperature gradients or internal 3 vapor pockets. Alternatively, when the pores are partially open, the capillary forces resist distortion as the binder softens. Thus, progressive stepwise removal of the binder is mandatory to hold the component shape. Final step involved in MIM process is sintering, which take place at high temperatures in order to bond the particles together. Sintering is a thermal treatment for bonding the particles into a coherent, predominantly solid structure via mass transport events that often occur on the atomic scale German, 1996. The binder in the molded part that produces via injection molding process is removed through the solvent extraction prior to the thermal pyrolisis. The thermally debound part or the brown part still can retain the shape due to the friction among the powder particles even though all the binder has been removed at the earlier stage. Hence, this part is very fragile and needs to be handled carefully for the next sintering process which later performs to achieve desired final mechanical, physical and chemical properties. High final density is important for the optimization of the desired attributes. The final properties of the product can be further improved with additional heat and mechanical treatments.

1.2 Problem Statements