Netfabb is a free software used for cleaning STL files for 3D printing. In order for an STL to be printable, it must be "Water Tight". Water tight refers to the part being completely sealed. If there is a hole in your STL file, the slicing software will not be able to register it as an actual part or wall and will try to confiscate for it. This leads to either intentional openings disappearing or not being view able at all.
1) Visit netfabb.com and download Netfabb Basic for your machine.
2) Continue through the Setup process
3) To open netfabb, you will need to accept the terms of usage and wait 10 seconds. Click "Later"
For instruction on navigating through netfabb, please visit Netfabb - Cleaning up your STL File
Please open a Tech Ticket if you have any questions or need assistance with this process.