Metal Injection Molding vs Metal Binder Jetting

Metal Binder Jetting apply inkjet nozzles to deposit binder agent onto a thin layer of metal powder, and builds green parts layer by layer finally. MBJ is raising in popularity, due to its batch production capability. As the printing process of MBJ is at room temperature, there is no thermal effect issue of warping and internal stresses. In addition, MBJ also never need support.

Metal binder jetting is a two-stage process, it involves a printing step and an essential post-processing (debinding & sintering) step.

• Spread a thin layer of metal powder (typical 50μm) over the build platform in binder jetting machine.
• The inkjet nozzles deposit a binding agent (polymer and wax) selectively, then bond the metal powder particles.
• Once a layer is finished, move down the build platform and repeat this process until the whole structure is complete.

This printing process will create the so-called ‘green’ part, it is similar to the greet part in metal injection molding process. Therefore, MBJ requires the post-processing step to remove the binding agent and create fully metal parts.

As the printed parts consist of metal particles and polymer adhesive, we apply different MBJ post-processing, there are two variations for this post-processing: infiltration & sintering. Normally, sintering is the major method in most applications, as infiltration creates parts with inferior properties like: mechanical and thermal behavior.

1. Infiltration: The green parts are placed in a furnace, and create brown parts with internal porosity of 60%. Then heat brown parts in industrial ovens with a low-melting-point metal like bronze. Finally, the internal voids are filled, resulting in a bi-metallic parts with low porosity and good strength.
2. Sintering: The green parts are placed in industrial ovens. The binder materials are burned off at first, then the remaining metal particles are sintered together with very low porosity. Finally, resulting the fully metal parts with 20% dimension smaller than the original green parts.

Metal Binder jetting is the only metal 3D printing technology, which is cost-effective for low-to-medium volume production of metal parts. MBJ is a great option once production volumes are well above prototyping.

• No support structures are required, MBJ can apply its whole build volume in mass production. This make it competitive in cost with traditional manufacturing, especially for low-to-medium volume production.
• MBJ produces metal parts with low surface roughness of Ra 6 μm, it can create a smoother finish and sharper edges than DMLS/SLM, there is no need of extra finishing operations. In addition, comparing DMLS/SLM, MBJ has lower cost of raw metal powder.
• Metal binder jetting is almost 10 times economical than other metal 3Dprinting technology, moreover, the build size of MBJ is considerably large. Furthermore, MBJ technology can build quickly by multiple head nozzles.

As MBJ technology applies sintering method to densify metal powder particles. There are internal voids and unexpected shrinkage in final metal parts. These elements result in undesired effect on final properties and tolerance.

• MBJ parts have internal porosity of 0.2 to 2%, sintering produces 97% dense parts, while infiltration approximately 90%. These internal voids will decrease the fatigue strength of metal parts greatly. However, may not affect the tensile strength in technical data.
• The sintering process is connected with significant shrinkage, and this shrinkage is non-homogenous. Therefore, it is difficult to predict with high precision. We need several trials to end up with desired final dimensions.
• Accuracy and tolerance of MBJ parts vary greatly depending on the geometry. It is difficult to predict, because of non-uniform shrinkage in different dimensions.