Fine-tuning, optimizing a MIM mold

As more markets and industries accept MIM technology, more expectations are put on MIM molders in terms of the dimensional capabilities and the sizes of the parts they produce. Chris Rista, product group sales manager at PCC-Advanced Forming Technology (AFT), a leading MIM and TXM molder, believes that the solution to meeting these demands lies in adopting a practical approach to fine-tuning each step of the MIM process, not the least of which is tooling. 

"It is interesting to note how often there is a failure to realize the significance of tool design and its effect on a part's ability to be processed through MIM," Rista says. Flow profiles, viscosity, and material densities are much different in MIM than in plastics molding. And since MIM is not the exact same process as molding plastics, the same is true for MIM molds. 

Yet, he continues, it is not uncommon to find MIM molds designed just as if MIM and plastics injection molding were the same. He cites a mold, inherited from another molder, as a good illustration of what it takes to fine-tune a mold design for MIM to produce acceptable parts. 

The mold in question was a four-cavity tool used to produce two sets of stainless steel jaws for a medical application. It was believed that previous yields from that design were approximately 25 percent, and of that fraction, none of the parts was dimensionally correct after sintering. The 3-D shape of the jaws was complex and the walls were very thin in some areas. After much fine-tuning, AFT was able to solve the problems. 

Powder/Binder Separation 
"The greatest constraint of this mold involved the use of a sprue bushing, square gates, and runners with sharp corners. In addition, turns and dual gating to one side of the part in an asymmetrical fashion intensified the problems," Rista explains. These features caused delamination of the binder from the powder and produced knitlines, which subsequently produced stress risers and distortion in the sintered parts. 

Delamination also led to high and low binder concentration in areas of the green material. This resulted in unacceptable voids in brown parts, Rista says. 

"The flow and knitline from the gating scheme created additional areas of high stress, ultimately resulting in damaged parts," Rista continues. "Overall, the result was low yield out of sintering with a majority of the parts turning out to be dimensionally unacceptable." 

Well-rounded Solutions 
AFT took a systematic approach to fine-tuning this tool. For example, the company conducted a series of tests in which one of the dual gates to each part was blocked off. After subsequent evaluation and processing, the blocked gates were reopened and the other gates were blocked. 

The best combination of the two gates to each part was used and the remaining gates were permanently blocked. "In general, it is best if a MIM part is gated in only one location," advises Rista. "If size or other considerations require a dual gate, placing the gates symmetrically is usually the best approach." 

The final process of fine-tuning included opening and rounding out the gates and runners, providing for smooth laminar flow into the four cavities. In the end, only one of the two gates to the part was used. 

The sprue bushing, which can sometimes lead to premature cooling of MIM feedstocks, could not be replaced due to cost considerations. "AFT's design uses proprietary technology to provide better control of material temperature," Rista concludes. Overall yields on the part were increased to 85 to 90 percent, a far cry from the 25 percent yields initially reported. 

Rista gave a presentation at an annual international conference and exhibit on emerging injection molding technologies, Molding 2001 (Jan. 29-31, New Orleans, LA). His paper covered fine-tuning of not only MIM mold designs, but also part design, material compounding, molding, SPC, and secondary operation options. To request copies of the proceedings, contact Executive Conference Management Inc., Plymouth, MI; (734) 420-0507; fax (734) 420-2280.