Micropores in MIM provide material savings

While many in the plastics industry work to answer questions of MuCell's effects on material properties and solve the problem of poor surface finish, others are focused on opening new doors for MuCell applications. The most recent door to be unlocked is not for plastics, however. Rather, it involves the use of MuCell technology to produce microporous metal, ceramic, and intermetallic components with fully densified surfaces. 

In a paper developed by Ratnesh Dwivedi, principal engineer for the Technology Group at Southco Inc. (a MuCell licensee), the author discusses Southco's creation of microporous metal based on the MuCell process used with plastics, with the potential of "significant savings in feedstock consumption." 

In the process, a gas such as carbon dioxide or nitrogen is injected into molten feedstock, which then dissolves into the binder. As the feedstock with dissolved gas travels through the nozzle, the gas bubbles nucleate and grow to form pores. This growth halts when the melt freezes as it comes into contact with the mold's cool walls. The molded component is subsequently debinded and sintered with gas-generated pores preserved. 

The result is a metal part with a dense surface skin and pores ranging from 5 to 200 µm in diameter (Figure 1). (As a comparison, pores in plastic components formed with MuCell have reached sizes as small as .1 µm.) Dwivedi's belief is that this dense surface is achieved upon sintering because pore formation on the surface is minimized by rapid surface cooling when the feedstock hits the mold walls and releases gas. During sintering, the small, naturally occurring pores between metal particles are eliminated, leaving behind the larger gas-generated pores in the microstructure. 

In addition to gas injection, the formation of microporous metal is similar to MuCell in that both processes use the same type of molding machine. The barrel and screw, however, must be upgraded to accommodate materials used in MIM. Another requirement involves the use of small, uniformly sized powder particles in the feedstock in order to achieve small, evenly distributed pores, says Dwivedi. The chemistry of the binder is also important: It should have high gas solubility when liquid, and low solubility in solid state. 

Dwivedi suggests that this process can be used for applications such as jewelry, sporting goods, lightweight structures, and heat-insulating components. 

Work in Progress 
Southco's development in this area is continuing, according to Dwivedi. The company has filed patents covering the technology and is working closely with Trexel to devise a commercialization strategy. It has looked to outside organizations and universities to supplement its development efforts. "We don't have enough research facilities in-house for this purpose," he explains. 

Licensing details are not yet solidified, but Dwivedi speculates that they would involve a cross-licensing agreement; Trexel would hold the rights to commercialize the technology, and Southco would provide technical assistance.