1 4 A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 2 Aligning manufacturing supply chains to meet critical product development milestones is crucial to launching new products on time and on budget. With this result in mind, engineering and supply chain leaders need to be able to iterate through designs to optimize manufacturability and ensure their device or assembly performs as intended. This is especially true when it comes to sourcing small and complex metal components, which can be difficult to produce and hard to scale. Metal injection molding (MIM) has been widely adopted by a variety of industries. MIM manufacturing is positioned to produce medium to high-volume components through the efficiency of dedicated tooling. MIM is a cost-effective alternative to machined components due to its mechanical performance, dimensional stability, scalability, and cost competitiveness. While MIM is optimized for high-volume production, qualifying a MIM component can cost tens of thousands of dollars and take three to six months. This development time is often a major roadblock for design engineers who need to test components in weeks rather than months. The traditional solution is to do what has been done in the past—source machined prototypes. However, this approach does not optimize for high-volume MIM production and can lead to even more challenges down the road when converting from a high-cost machined component to a MIM component. Binder jet 3D metal printing is an additive manufacturing (AM) process that can be used to rapidly produce prototype components, allowing engineers to test their designs quicker than ever before. The MIM process and the binder jet 3D metal printing process share many similarities and complement each other as component manufacturing technologies. In this article, the technologies are discussed together. The purpose is to understand where each one fits, and how one technology can assist to accelerate adoption of the other. Topics discussed include powder types, distortion during sintering, unique capabilities of each method, process development acceleration, and shared capital equipment. Each process is described below. Metal injection molding: Metal injection molding (MIM) is a manufacturing process that combines the most useful characteristics of powder metallurgy and plastic injection molding to facilitate production of small, complex-shaped metal components with outstanding mechanical properties. As a general rule, MIM parts weigh less than 100 grams, feature complex geometries and tight tolerances, and can fit in the palm of a hand. The MIM process begins by mixing a combination of 5-to-25-μm metal powder with polymers to form a feedstock (Fig. 1). This feedstock is then molded into an oversized, geometry-specific tool at high temperatures (120°-250°C) and high pressures. The as-molded geometry is considered “green”—metal powders held together with polymers. The polymer is then removed using either a solvent or an oven incorporating a gas/solid reaction. The geometry is then placed in a furnace with support ceramic. The remaining polymers are removed, and the geometry is sintered and densified into final dimensions at temperatures as high as 1300°-1400°C depending on the alloy. The geometry can then be treated like a piece of solid metal: It can be worked, heat treated, and surface finished. Binder jet 3D metal printing: Metal 3D printing is an advanced AM technology that uses powdered metals to build a 3D component by applying evenly distributed layers. The approach of building a part layer-by-layer gives designers ultimate flexibility because they are not reliant upon machining or tooling to form complicated geometries. The binder jet process starts by vibrating a 10-to-20-μm metal powder—identical to the powder used in MIM feedstock formulation—onto a build platform. This layer of powder is “glued” together with an inkjet head that feeds a binder in a controlled 2D pattern. The bed of powder is lowered by 50-100 μm and a successive layer of powder is vibrated and spread onto the bed of powder, and then glued together. This is repeated until the geometry is built from successive layers (Fig. 2). A 1-cm-high part would consist of 100 to 200 layers of powder that are glued together. This glued part is then removed from the powder bed and the term “green” is used to describe its Fig. 1 — In the metal injection molding process, metal powder is mixed with polymers to form a feedstock, which is then molded into an oversized part and then further treated to achieve final dimensions.
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