DESCRIPTION OF THE AGAR BASED AQUEOUS BINDER

Traditional MIM binders are based on wax or polymeric systems that require the binder to be removed in a separate debind step, often taking many tens of hours. This debind step often requires acids or other noxious chemicals which present an environmental penalty and add significant process time. This binder is based on agar, a polysaccharide derived from seaweed and used as a common food additive. It is water soluble. Metal powder is mixed with the water, agar and minor additions to form feedstock pellets that can be easily fed into conventional plastic injection molding equipment. The metal powder employed is typically under about 20 micrometers and is made by gas or water atomization. Compounding into feedstock is done by twin screw extrusion. Typical solids loading of the feedstock is near 92wt% (61 vol%), the balance consisting of water. The feedstock is relatively fluid at 85°C, having the consistency of toothpaste, and is thus easily injection molded into a net shape mold in a manner similar to plastic injection molding. Primary differences are lower molding temperatures and pressures. Upon cooling in the mold to near room temperature, the now molded feedstock drops below its gelation temperature, setting into a green part and allowing it to be removed. Cycle times are on the order of half a minute, depending on part size.

After approximately 1 hour in ambient air, the green part will have dried and is now ready to be sintered. No separate debind step is required as is necessary for traditional MIM feedstocks. Rather, a dwell time of approximately 1 hour in air is incorporated into the beginning of the sintering cycle. This step pyrolizes the binder, allowing the carbon to be removed during the sintering cycle, which is typically done in hydrogen or vacuum depending on alloy. Sintering temperatures in the range of 1300-1400°C are typically employed for stainless steel alloys. Total debinding/sintering time is on the order of 14 hours using a large commercial batch furnace. Depending on part size, part loading quantities on the order of a thousand can be sintered in such furnaces, keeping per part sintering costs low.

Back | Next