The components were created via an additive manufacturing process known as selective laser smelting. Unlike the commercially available 3D printers you can currently buy, which use spools of plastic, this process uses a laser to fuse together layers of metal powder. So if you were hoping to throw together a rocket engine in your garage, your RepRap or Makerbot isn’t going to be up to the task.
According to NASA, the 3D-printed engine is more of a proof of concept, to show that the individual 3D-printed components can work together, than a real, functioning rocket engine. This is why the engine doesn’t look like what you might expect a rocket engine to look like, even though it is capable of generating a staggering 20,000 pounds of thrust.