One of the considerations when creating objects in most 3-D printers is that the laws of physics and gravity still apply to what you print (unlike what you can create in 3-D software). So supports and bridges still have to be built in to be removed later. Sometimes this is just because printing from the ground up creates gaps in support until the entire structure is finished. In many educational applications, this is a good problem because users have to think through problems, propose design solutions, etc. Some printers are overcoming this limitation in novel ways. A team at Harvard is using a novel combination of metal and lasers to create a printer that can solidify melted medal in midair, making lines of metal that can support more complex creations in midair. You kind of have to watch the video above to see it in action, but it is a pretty interesting process.