Why can't powder-based 3D printing techniques create enclosed or hollow structures? Metal powders are the fastest-growing segment within the 3D printing materials market, and 3D printing with metal offers a range of highly-sought out characteristics, including immense strength, reduced weight, biocompatibility and corrosion or thermal resistance, making it ideal for high-demand industries such as aerospace, medical, etc.

Conventional methods require focusing a very intense energy source, such as a laser or electron beam, across a bed of metal powder, fusing the powder particles together in a pre-determined pattern to create the final 3D structure. 

While this method does allow for incredibly strong metal 3D structures to be produced, it has its drawbacks, mainly: 


it is prohibitively expensive and time consuming; 
it does not allow for certain types of architectures, such as those that are hollow and enclosed, and;
it is limited by the types of compatible metals and alloys that can be used.


If we ignore the cost, why can't we do hollow or enclosed architectures to be printed with this technique?

Source: A closer look at the 12 biggest 3D printing tech innovations of the first half of 2016.
It's not hollow or enclosed structures that are a problem. It's structures that are hollow and enclosed. Think about it. The machine lays down a thin layer of powder, and then a laser fuses some of that powder together to make a shape. Then, it repeats the process for the next layer. If you try to build, for example, a hollow ball, the ball gets created in the middle of the powder, meaning that there will be powder around the outside of the ball and in the inside of the ball, and with the ball completely enclosed, there is no way for the powder that ends up inside the ball to get out. That's why hollow objects printed in this manner must have a hole somewhere to let out the powder contained in the object.

This is not only true of metal powder, but any 3D printing process that uses a bed of powder or a volume of resin and fuses some of it into a shape. Fused deposition modelling (FDM), the process used by most consumer-level 3D printers, build models in thin air, so only air is trapped inside the printed objects, making the printing of hollow, enclosed objects less of a problem. Of course, one has to deal with gravity. A hollow object might collapse before the filament hardens enough to support its weight.