Need help printing open objects

Hey all,

I am attempting to print a toy Stormtrooper helmet to fit on a model of mine, but whenever I try to print it using Slicer it automatically fixes some errors and fill in the neck hole. I even tried to flip it upside down to pretend it was a vase or something but the top gets filled in no matter what! How can I print this without the top getting filled in? I'm super confused because I've seen people print objects with open tops all the time. Here is a link to the file I am using.

Thanks in advance!

That's just a mesh. Not a solid object. Slic3r filled the holes to make it solid. Try these tutorials [www.thingiverse.com] [www.405th.com].

Those tutorials worked like a charm! Thanks a million!