Injection molding benefits metals.

Injection molding benefits metals

While the metalworking industry continues to fight the invasion of its conventional territories by plastics, it has also recently begun to borrow - and perfect - one of the plastics industry's most valued technologies: injection molding.

Battenfeld of America Inc, West Warwick, RI, is a manufacturer of injection-molding equipment. According to Heinz Rasinger, project manager, the relatively new area of metal injection molding (MIM) is one of the company's most rapidly growing new markets. "While powder metallurgy is a mature technology," says Rasinger, "only in the past few years have powder metal and resin binder systems been developed that are capable of being injection molded. For the processor of powder metal parts, this means that a high-volume, cost-efficient manufacturing process is now available to produce complicated molded metal parts requiring little, if any, secondary machining."

The primary advantages of MIM over conventional powder metallurgy are design flexibility and high part density, which provides better corrosion resistance and mechanical integrity. The materials, or feedstock, typically used include extremely fine (10 to 20 microns) metal powders (elemental or alloys) and a polymer binder, usually polyolefin or methylcellulose. After the blended materials are injection molded, the parts go through a debinding process (the binder polymer is extracted through heat) and high-temperature sintering.

Rasinger explains that while many facets of the MIM process are similar to thermoplastics injection molding, there are some significant differences that restrict the types of equipment that can be used. "Shrinkage rates in MIM are much higher than we see with thermoplastic molding - up to 20%, compared to 2% or 3%," he says. "So the process requires high precision in the injection and clamping unit to maintain tight tolerances and prevent part flaws, which are magnified in the debinding and sintering processes." Rasinger adds that other special equipment considerations in MIM include closed-loop control of injection speed, holding pressure and back pressure; sophisticated robotics; and quick screw changeover. Another consideration is wear and tear on the barrels, caused by abrasion from metal powders. This can normally be compensated for by selecting a special screw and barrel alloy, such as CPM-10-V.

According to Rasinger, current MIM technology limits most applications to smaller machines, usually from 11 to 85 tons. "The powders used in MIM are more expensive than the ones used for conventional P/M parts. As a result, the advantages of the process apply mostly to small parts, especially ones with complex geometries."

The cost reductions over conventional powder metallurgy processes can be striking - as much as 90% - but the MIM process is a complex one. "We've concluded that there are probably over 90 critical variables that we need to control for the process to be successful," says John Popken, executive VP, Advanced Forming Technology, Longmont, CO. "While we have many success stories, there has been a lot of development work involved, and there is much more to learn about improving the cost-efficiency of the process." Wolfgang Meyer, president of Battenfeld of America, adds that injection-molding technology for powder metals has come a long way. "As more molders become familiar with metal-injection molding and more efficient processes are developed, the potential for this market is tremendous."