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Metal Binder Jet

Metal binder jetting is rapidly gaining popularity. Its unique properties make it particularly suitable for small to medium production runs.

What is metal binder jetting?

Metal binder jet in operation

Binder jetting builds parts by depositing binder on a thin layer of powder through an inkjet nozzle. It was originally used to make full-color prototypes and models out of sandstone. A variant of this process is currently gaining popularity due to its mass production capabilities.

The printing step in metal binder jet printing is performed at room temperature. This means that thermal effects (such as warpage and internal stress) are not an issue, as in DMLS/SLM, and no supports are required. Creating an all-metal part requires additional post-processing steps.

How does metal binder jetting work?

Schematic diagram of a typical metal binder jetting machine

Metal binder jetting is a two-stage process. It involves printing steps and necessary post-processing steps. Here's how the printing process works:

A thin layer of metal powder (usually 50µm) is spread over the build platform.

A carriage with ink jet nozzles passes through the bed, selectively depositing droplets of binders (polymers and waxes) that bind the metal powder particles.

When a layer is complete, the build platform moves down and repeats the process until the entire build is complete.

The result of the printing process is that the part is in a so-called "green" state. A post-processing step is required to remove the adhesive and create an all-metal part.

There are two variants of the post-processing step:

Penetration: The "green" part is first washed away from the adhesive, resulting in a "brown" part with significant internal porosity (~70%). The "brown" part is then heated in an industrial oven in the presence of a low melting metal (usually bronze). Internal voids are filled to form bimetallic components.

Sintering: The "green" part is placed in an industrial furnace. There, the binder is first burned off, and then the remaining metal particles are sintered together. The result is an all-metal part about 20% smaller in size than the original "green" part. To compensate for this shrinkage, parts are printed larger.

Today, sintering is used in most applications because infiltration produces parts with poor material properties and insufficiently documented mechanical and thermal behavior.

Binder jetting and metal injection molding (MIM)

After sintering, the binder jetted parts have very similar properties to those fabricated using MIM. MIM is a manufacturing process used to mass-produce almost every small metal part found in today's consumer electronics or automobiles.

MIM is a variant of the plastic injection molding process. Metal powder mixed in a plastic binder is injected into a mold to form a "green" part, which is then sintered into metal.

Therefore, metal binder jetting is built on the know-how of the MIM process.

Advantages and Limitations of Metal Binder Jetting

BinderJetting is the only metal 3D printing technology available today that can be used cost-effectively to produce metal parts in small to medium batches.

Since no support structure is required for printing, the binder jetting system can use its entire build volume. This enables it to compete on cost with traditional manufacturing, even for low to medium volume production.

Additionally, BinderJetted parts have smoother surfaces and sharper edges than DMLS/SLM, so additional finishing operations may not be required. The cost of raw metal powder is also lower compared to DMLS/SLM, which plays a big role in the unit price.

On the other hand, the internal porosity of parts produced with BinderJetting is always around 0.2% to 2%. Note that internal voids may not affect the tensile strength shown in the technical data sheet, but will greatly reduce the fatigue strength of the part.

Remember that the sintering step is associated with significant part shrinkage. This shrinkage is non-uniform and difficult to predict with high accuracy. In practice, multiple trial prints are required to end up with a CAD file that will result in a part with the desired final dimensions. However, the repeatability of the process is very good. This means that larger volumes of this part can be fabricated after successful calibration.

Cost-effective mass production

Printing without support

Smoother surface than DMLS/SLM

Lower performance than wrought metal

Exact dimensions can only be obtained by trial

Currently limited range of materials

Technical characteristics of metal bond injection

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