BINDER JETTING

Binder jetting is another powder bed process, but it does so without the laser. 

If you ever built sandcastles on the beach on a summer’s day, you get a pretty good sense of what Binder Jetting is about. Dry sand can be cupped, but the moment your hands are removed, the sand collapses. Add a little water, or use damp sand instead, and suddenly the sand can retain the shape you designate. The water in this case acts like a binding agent.

As shown by steps a) to c) in the diagram below, like powder bed fusion binder jetting makes use of a powder bed. A recoating blade spreads a thin layer of powder over a build platform.  Binding agent is then selectively deposited per the requirements of the part design to hold the powder together. Once the layer has been fully processed, the build platform moves downwards and a recoating blade replenishes the surface with fresh powder and again binding agent is applied.

The process repeats until the part is complete.

So, Is that it? Well, what happens to the sandcastle when the water eventually evaporates? It crumbles.

As shown in steps d) to f) of the supplied graphic, BDJ metal parts require at least two additional steps not typically required by SLM or DED parts: Binder removal and/or infiltration or sintering.

  • Infiltration: After printing, the part is placed in a furnace, where the binder is burnt out leaving voids. At this point, the part is approximately 60% porous. Bronze is then used to infiltrate the voids via capillary action, resulting in parts with low porosity and good strength.
  • Sintering: After printing is complete, the parts are placed in a high temperature furnace, where the binder is burnt out and the remaining metal particles are sintered (bonded) together, resulting in parts with very low porosity.

Binding agents can range from water based to phenolic based, pending the material being printed, but require removal once the print process is complete. In addition, metal BDJ parts need to be sintered (heat treated) in order to ensure part integrity.

Machining may also be required for the final part.  Full-color prototypes are also infiltrated with acrylic and coated to improve the vibrancy of colors. Sand casting cores and molds are typically ready to use after 3D printing. It can handle metals and alloys, including aluminum, copper, iron, nickel, and cobalt-based alloys. In addition, BDJ can process ceramics, including glass, sand, and graphite. BDJ can work with any powder that allows for color printing.

BDJ is generally not suitable for structural applications due to porosity that may occur from the conventional debinding and sintering processes. Sintering also means green parts that are not near net tolerance. There is software that will compensate for shrinkage during processing, and scale green parts accordingly. However, it is imperative that a designer understand whether software they are using will scale for shrinkage or not. Below are some further pros and cons for BDJ.

PROS

  • No residual thermal stresses
  • Relatively cost effective
  • Can tune amount or ratio of binding agents

CONS

  • Post processing required
  • Mechanical properties not as strong

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