REPRINT FROM INDUSTRIAL HEATING

NEW BINDER SYSTEM

In addition, tight control of sintering parameters is key to producing net shape parts with high dimensional tolerance and free from impurity contaminants (such as carbon and oxygen). Carbon control, in particular, is extremely important since improper levels in stainless steels result in quantum reductions in strength and corrosion resistance. To provide for excellent control of sintering atmosphere and temperature and allow for repeatable processing of high quality net shape MIM components a batch furnace (Elnik Systems MIM 3000) has also been developed. The construction of this furnace guarantees that the parts are fully debound through even gas flow during the debind cycle and sintered to full density through the closed temperature uniformity during the sinter cycle.

The conventional MIM process is shown schematically in Fig. 1. These MIM materials were developed using wax, polymeric or methylcellulose binders that require separate and complicated debinding steps to remove the binder after molding but prior to sintering. Debinding of these systems is often time consuming, costly and result in enviromentally noxious residues that carry a disposal cost. These systems are also limited to thin cross sections since binder decomposition/removal from thick sections is impractical. Due to the complexity of debinding in these systems, they are often debound in a separate step from sintering.

The newly developed binder system is an agar-based, aqueous binder system that contains a minimum amount of volatiles as compared with traditional binders. Because no organic solvents are used, rapid debinding can be accomplished with no special handling or environmental issues. Contamination of the furnace or sintering environment is mitigated due to the small amount of volatiles present in a molded green component after drying.

Table I compares the properties of the agar-based binder system with conventional MIM binders. A major difference is that the separate solvent and/or thermal debinding step for conventional binders (shown schematically in Fig. 1) is not necessary for the agar-based system.
The agar-based binder system is compatible with both continuous furnaces, such as pusher furnaces, as well as batch furnaces. Each has its particular advantage. A batch furnace is both economical and robust for the flexible production of part runs ranging in volume from one to many thousand, depending on part size, and offers greater flexibility in thermal and atmospheric control during the debind and sintering cycles. For example, debinding in air, partial vacuum or an inert gas can be combined with hydrogen sintering in a single run.

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