A new process for manufacturing metal powder injection molded automotive turbocharger components

Metal Injection Molding (MIM) technology can be used to produce complex structural components from materials that are difficult to process using traditional methods. This characteristic makes it an ideal method for producing high-performance turbocharger components. BASF's unique Catamold process can help address a range of critical challenges in the development of turbocharger MIM components.

Turbocharger MIM Components

The core of a turbocharger lies in the turbine, which is driven by hot exhaust gas within the turbine housing, and the compressor impeller, which is located on the side of cold air. The compressor impeller only needs to tolerate relatively low temperatures, and aluminum impellers can fully meet these requirements. However, the high-temperature exhaust gas in the turbine housing requires the use of high-temperature resistant and high-quality steel for the turbine. Turbines are typically produced using precision casting techniques, but theoretically, they can also be manufactured using MIM technology.

Metal Injection Molding (MIM) technology has been applied to turbocharger components in the past. Due to its advantages in material selection and design freedom, MIM technology has seen a significant increase in the use of its components in recent years, and their performance has been proven in actual use.

Challenges Faced

Despite the progress made in MIM technology, there are still numerous challenges related to process optimization, component structure, and mold design when it comes to manufacturing components with stringent requirements. One such challenge is the accumulation of material in the central region of the turbine, which can lead to shrinkage porosity due to volumetric contraction during cooling. Both precision casting (using molten metal) and MIM (using molten feedstock) are susceptible to this defect when the mold is filled with material. Modern simulation technology can provide detailed analysis of this issue. For example, appropriate software can accurately predict the MIM injection molding process. Figure 1 illustrates the effect of turbocharger mold filling, where a conical sprue is used to inject molten feedstock into the component.

In addition to mold and melt temperatures, further adjustments to injection speed (cm3/s) can provide a highly realistic simulation of the mold filling process. Figures 1 and 2 show the mold filling process of the turbine over time. Under the set conditions, the component is filled within 1.1 seconds. The color temperature map shows the change in melt over time during mold filling, with the blue region being filled first and the red region last. Observing the cooling process of the component within the mold or after ejection can reveal subtle processes of solidification and defect formation. Figure 3 shows a cross-section of the solidification pressure of the turbine after cooling in the mold for 40 seconds. The larger blue region in the middle indicates very low pressure at the end of cooling, while the adjacent regions have already solidified, preventing more melt from entering. Therefore, the volumetric contraction caused by material cooling in the blue region leads to the formation of shrinkage porosity. Figure 4 clearly illustrates this issue, where material that has not solidified after the cooling time causes voids.

Lost Core Technology

In the Catamold process, after injection molding is completed, the polyoxymethylene binder decomposes in an acidic environment within a debinding furnace, allowing it to be rapidly removed from the component.

By first injecting a mold core made of polyoxymethylene and then overmolding the feedstock around it, complex hollow structures can be achieved as the polyoxymethylene mold core is removed during the debinding process.

The cross-sectional view in Figure 5 demonstrates how embedding a mold core during the injection molding process can transform a solid component into a hollow internal structure. As the mold core is removed after injection molding, a specific hollow structure is formed.

Figure 6 shows the improvement achieved by lost core technology in addressing defects in the turbine region. The color stripes represent the time required for solidification in different regions. The portion of the component outside the mold core fully solidifies after 27 seconds of cooling.

Compared to traditional MIM processes, the lost core method significantly improves component production efficiency. This is because, theoretically, the mold core can be made in any shape, and the internal structure can be adjusted based on the actual size and load of the turbine. Additionally, this technology can significantly reduce the weight of the turbine.

Sintering Process

The final step in metal injection molding technology is sintering, which removes the remaining binder and causes the component to shrink in size. The sintering temperature is slightly below the melting point of the alloy used, resulting in significant dimensional changes.

The shrinkage characteristics of MIM components are influenced by mold shape, long-term production stability, material batch variations, and processing windows. To achieve consistent shrinkage rates, mold production, especially for components with complex geometries, requires several rounds of optimization and size correction. Some of these dimensional changes may be difficult to predict ahead of time