What Designers Should Know about Metal Injection Technology

Medical Design Magazine - Apr 1, 2006 12:00 PM, Materials Technology, FloMet LLC, Deland, Fla., flomet.com

  
Metal injection technology (MIT) produces complex net-shape parts in a variety of metals including stainless steel alloys and at lower costs than traditional wrought metal and casting processes. Applications range from needles to hearing aids, as well as implantable, suturing, orthodontic, laparoscopic, and endoscopic devices. Typical parts weigh from 0.1 to 100 gram and are about 0.030-in. in diameter and 2.00-in. long. Some parts are even larger. The MIT abbreviation is intended to stress that the technology involves a special marriage of shape making (plastic injection molding) and material creating (powder metallurgy) processes.

MIT involves first mixing metal powders with plastic binders into a blend to make a proprietary feedstock. This goes into an injection molding machine that melts the binder into a fluid state and lets the homogeneous feedstock flow into and completely fill the mold. The part just out of the mold is referred to as “green.“ Commonly a thermal, or sometimes solvent, process removes the plastic binder from the green part making it a “brown“ part, which is then sintered.

Sintering heats the part to about 80% of its melting point under a controlled atmosphere or vacuum until the material's particles diffuse together into a high density metal or alloy. This forms a contiguous structure similar to that of wrought parts. After sintering, the part is usually ready for installation in a medical device. Scrap is eliminated, or significantly reduced, because the part typically requires no machining.

MIT eliminates inclusion defects, which result from unwanted material such as slag or ceramic chips that get into the molten material, caused by traditional casting. MIT parts never come into liquid contact with ceramics. Also, the large pores or gas pockets that often form during traditional casting are eliminated because the material fills the mold entirely. And since the parts are made of metal or alloy powders, purity and corrosion resistance are significantly better than with cast and wrought parts. Also, producing net-shape parts slashes costs by as much as 90% when compared to machining. Lastly, MIT often allows replacing an assembly of parts with one discreet component.