1 5 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 PROCESS COMPARISON Binder jet 3D printing does not rely on custom tooling, so 3D-printed part prototypes can be produced rapidly and delivered in days rather than months. This freedom from tooling along with rapid delivery enables engineers to test much sooner with functional MIM components. Further, powder sharing enables the binder jetting process to take advantage of the bulk buying power of the MIM operation to reduce material costs. Baseline properties and downstream processes are already defined and stable. In addition, the MIM sintering profiles and infrastructure are in place to eliminate capital investment and development costs. Table 1 outlines the comparison in process and component properties. ALLOY PROPERTY COMPARISON 17-4 PH stainless steel is one of the commonly used alloys in both metal injection molding and binder jet 3D printing. In terms of mechanical properties, MIM and binder jet 3D-printed components can achieve comparable densities that exceed the MPIF Standard 35 density for 17-4 PH stainless steel of 7.5 g/cc. In a comparison done by Advanced Powder Products Inc. (APP), Philipsburg, Pa., the mechanical properties of MIM and binder jet 3D-printed components made from 17-4 PH stainless steel and heat treated to H900 were tested. With identical sintering processes, it was empirically proven that APP’s binder jet 3D-printed parts, under the trade name Printalloy, meets the chemical, mechanical strength, and yield point for MIM 17-4 PH per MPIF Standard 35, outlined in Table 2. DIMENSIONAL CAPABILITY Metal injection molded components offer highly precise and repeatable tolerances yet are slightly inferior to machined components. The general rule for MIM production capability is +/- 0.5% of the dimension to obtain a Cp greater than 1.33. This is due to the 20% shrinkage a MIM part experiences during sintering and needs to be factored in when designing for MIM. state. The green part is then ready to be placed in a furnace with support ceramic. The glue is removed thermally, and the geometry is sintered, shrinks, and densifies into final dimensions at temperatures as high as 1300°-1400°C depending on the alloy. Like its MIM counterpart, the 3D-printed geometry can then be treated similarly to that of a solid piece of metal: It can be worked, heat treated, and surface finished. MIM PROCESS DEVELOPMENT Because the binder jet process can use the same powder and sintering profile as MIM, the opportunity exists to perform sintering and secondary process development while the MIM tool is being built. Tooling may take six to 14 weeks to be designed and built. Printed parts can be fabricated immediately, and downstream processes developed and evaluated. In this way, knowledge of how the part should be fixtured during sintering can be explored in parallel to the tool build. Insight into how the part may warp or distort during the MIM process can be gained at this time. Printed components are available for test builds and the downstream processes can be defined with fixturing, methods, and work instructions in place while waiting for the MIM components to be fabricated. In some cases, the printed components may prompt a design change of the MIM process to aid in manufacturability, such as incorpor- ating a flat surface to prevent distortion. Fig. 2 — Schematic of binder jetting process. TABLE 1 — PROCESS COMPARISON Binder jet 3D printing Metal injection molding Investment $0 $25,000+ Lead time Days Weeks Powder 10-20 μm 5-25 μm Binder Dryable liquid Meltable polymer Shape forming Layer by layer (50-100 μm) Injection into custommold Debind No Solvent or gas phase Thermal debind and sintering Same Same Machinability Same Same Heat treat response Same Same
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