Metal molding 'injects' a new engineering twist

DN Staff

December 2, 1997

4 Min Read
Metal molding 'injects' a new engineering twist

Powder Metal MoldingTM offers engineers innovative solutions for complex metal parts, says Mark Rasmussen, president of Phillips Plastics Corp.'s Powder Metal Molding business unit.

Design News: What is metal injection molding?

Rasmussen: Metal injection molding is a process in which metal in a powdered form is combined with a polymer binder to produce a homogeneous mixture known as feedstock. The feedstock is processed in a manner similar to plastic injection molding, except that parts are designed slightly larger to account for shrinkage during the final sintering step. The molded part is called the "green" part.

Once molded, three methods are available to remove the binder from the green parts: solvent, thermal, and catalytic debinding. Differences in these methods affect the outcome of the product. The debinding phase can be the most time consuming aspect of the process. After de-binding, parts (known as "brown" parts), are sintered--a process in which the components are exposed to heat over an extended period.

Q: Could you go into more detail about traditional metal injection molding debinding processes?

A: Metal injection molded parts are traditionally debound using either a thermal or solvent method. With a thermal process, the part is heated above the wax's melting point to burn off the wax. In a solvent process, the wax is slowly dissolved by immersing the parts in a solvent bath. The resulting parts become very pliant. Both of these processes have a high potential for compromising the component integrity. The resulting soft parts can slump under their own weight, becoming distorted. Also, debinding using either of these techniques takes time--up to 20 or more hours--to minimize the distorting effects.

Q: What makes Powder Metal Molding (PMM) different?

A: PMM's formulated feedstock affords users the technological advantage of catalytic debinding. The catalytic process uses a chemical that reacts with resins in a less drastic way than wax binders. The process is accelerated with heat, but the temperature is maintained below the binder's melting point, eliminating the risk of parts slumping. And the process decreases debinding time to only three or four hours. Seeing the final product quicker can give customers a competitive edge.

Q: Is PMM unique?

A: Yes. Most metal injection molders use a batch furnace--generally the same one used to debind the parts. Due to the furnace's larger size, it is difficult to maintain a constant temperature inside. This is essential in producing consistent parts.

PMM's sintering phase is more controlled. Parts are carried conveyer-like through the furnace in a continuous series of narrow hot zones where the temperature remains very uniform. Because of this controlled process, every part is subjected to the same temperature profile.

Q: What are the advantages of PMM for engineers?

A: PMM serves those who have low- and high-volume needs for high-quality parts with reduced leadtimes. It allows for debinding that's 10 to 20 times faster than other metal injection molders, partially due to the use of special processing resins. This catalytic debinding process not only works faster than others, but also causes less distortion to the parts, ensuring the integrity of the product.

Q: What types of parts are we talking about?

A: Let me list some of the PMM's capabilities:

  • Small, complicated metal parts--the more complex and intricate the better.

  • Parts with tolerances of plus or minus 0.003 per inch.

  • Parts with weights up to 100 grams.

  • Parts that would fit within a tennis ball.

  • Parts with wall thicknesses from 0.02 to 0.3 inch.

  • Parts with a material density of 98% of theoretical or better.

  • Parts with a strength similar to wrought material.

  • And parts with a surface finish of 32 RMS.

Q: What kind of annual volumes can PMM run?

A: PMM will run volumes ranging from 5,000 to 500,000 parts per year. For certain applications, it can produce volumes of more than 1 million parts per year.

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