Copper is an extremely interesting material due to its conductivity properties and for being highly malleable.
These very same properties hindered its success as a feasible option for 3D printing. However, that has changed.
The most common technique for 3D printing copper is Powder Bed Fusion. It uses electronic beams to melt material powders and "glue" the material. When the temperature drops, the material consolidates together.
The most popular processes of Powder Bed Fusion are known as Selective Laser Sintering (SLS), for plastics, and Selective Laser Melting (SLM), for metals.
Despite SLM being an excellent printing process for many metals, there was a particular challenge when it came to 3D printing with copper.
The conductivity properties of the material cause the heat of the electronic beams to be reflected instead of absorbed.
Another reoccurring problem was the fact that the printed part would crack when the temperature would drop too fast and/or too low.