Net Shape Manufacturing of Metal Components by Metal Injection Molding Improves Product Performance and Lowers Manufacturing Cost
Metal injection molding (MIM) is a powder metallurgy (PM) fabrication process employed by Raytheon to produce metal structures both simple and sophisticated. With MIM, metal or alloy parts can be formed in the same manner as plastic or ceramic parts at a fraction of the cost of machined parts, but with the same properties as wrought (hammered) materials (Figure 1). MIM also facilitates tailoring of alloy composition to satisfy unique performance requirements.
Although the defense industry has studied these processes since the 1980s, Raytheon leads the industry by successfully incorporating structural components and flight control surfaces produced using PM and MIM manufacturing methods into mature weapon systems.
The MIM Process
First, fine (≤ 44 µm ) metal powder is mixed with plastic and organic binders into raw material (feedstock) that behaves like a plastic. This is injection molded using standard techniques into a partially processed (green) part. The binder is then removed and the part is densified by sintering at temperatures of up to 2,300°F (1,260°C), depending on the alloy. Secondary operations such as coining (stamping) and final heat treating are completed to produce the strength, shape and surface quality required. Heat treating and secondary operations are standard wrought processes. Complex and sophisticated parts — some that would be difficult if not impossible to form using other methods — can be formed using the MIM process (Figure 2). The following are some examples of Raytheon’s use of MIM.
Use of MIM to Manufacture Control Fins
Excalibur is a guided artillery projectile that provides precision fire at extended ranges for all current and future 155 mm howitzers. The original Excalibur control fin (Figure 3A) was composed of 17-4PH precipitation hardened stainless steel. Machining was extremely time consuming, requiring tens of passes with a tool bit on both sides of the fin to create its complex shape. Each pass proceeded slowly to ensure accurate tracking of the tool. Machining of precisely located mounting bosses required numerous setups on sophisticated machine tools. Tolerances were difficult to maintain from part to part.
With the use of MIM manufacturing, the airfoil shape is molded in one piece (Figure 3B). The airfoil shape is injection molded in the green (unsintered) part. Because the hardened steel die cavity in the mold does not change from shot to shot of injected material and maintains it shape for a minimum of a quarter of a million shots, tolerances are easily achieved. In some cases, die cavities are not changed for over 10 million parts. After the part completes debinding, the fin is sintered to 2,300°F (1,260°C); the sintered part contains less than 2 percent porosity and is ready for secondary operations. All the pores are closed and located inside the grain — not at the grain boundary — thus eliminating any effect on the strength of the material. The airfoil is then coined to the final shape and attachment points are machined to their final dimensions. Final heat treating is completed to attain the strength required.
Table 1 compares the properties of MIM 17-4PH material with 17-4PH precipitation hardened stainless steel, showing that they have comparable performance. The cost of manufacturing the part using MIM, however, is on the order of 25 percent of the cost using traditional machining methods. (In the table, SAE AMS means “Society of Automotive Engineers Aerospace Material Specification.”)
Use of MIM to Manufacture RF Housings
The thin walls, unique hole design and large quantities of parts for some of today’s radio frequency (RF) electronics packages rule out casting, forging and machining. MIM is often the only process that can meet this need (Figure 4).
Electronics packages normally require a thermal management system attached with an adhesive bond. The thermal management system is limited in its ability to remove heat from the electronics package by the thermal properties of the adhesive and the bond thickness. With MIM, alloy compositions are adjusted so that the package and the thermal management system are co-molded and processed together. This provides intimate contact of the electronics package to the thermal management system for higher heat transport. Fewer parts and assembly steps reduce cost and enhance reliability.
With MIM, the rules are changed in favor of the designer and thermal management engineer. The Kovar™ ratio of iron and nickel can be changed to lower the coefficient of thermal expansion (CTE) of the alloy so the CTE of the alloy is closer to the CTE of glass. This lower alloy CTE reduces the probability of cracking the glass seal around package leads, a common cause for the loss of package hermeticity and of the failure of electronic components inside the package.
The MIM process facilitates producing components with unique properties, allowing designs that are significantly easier to incorporate into present and future systems. These attributes result in higher-value systems that have better performance at lower cost.
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