How to Prepare Models for 3D Printing
A 3D-printable object must have a manifold shape — it must be completely without holes and (in essence) watertight to print successfully. A hole though the object is fine, so a 3D-printed doughnut is possible, but a hole in the surface of an object must be filled or patched to create a continuous outer surface before it can be printed.
When a scanner calculates a 3D object's shape, it does so using only the scanned surface (the outside) and so minor holes and other imperfections may require additional cleanup before the design is complete.
Some models need preparation before they're ready to be 3D-printed — whether they were created using CAD, scanned into the computer, or calculated from photographs through photogrammetry. Holes in the surface may need to be repaired; misaligned faces may need to be resolved (the results can be mixed up if two models intersect); extraneous details may need to be trimmed away, leaving only the part you want to print.
The model of King Richard III’s skull was prepared using the excellent commercial Geomagic FreeForm application, but a number of free software tools are also available to assist educators, home users, and hobbyists just getting into 3D printing.
3D model viewers
One of the easiest additions to your suite of tools will be some type of 3D object viewer, so that you can inspect your object before the 3D printing begins. The 3D printer's control interface will use a system like this to lay out items on the build plate before printing begins, but other options can make quick choices between models easier.
3D models can be viewed in 3D printer control software like MakerBot's MakerWare, the open source Repetier interface, or in standalone products like the free STL Viewer application.
Mesh is the term used to describe the surface of a 3D model, which is defined using numerous small triangles. Many tools can export 3D designs into formats such as PLY, STL, OBJ, COLLADA and other file-storage types, and a tool like the free, open-source MeshLab can help convert these into the file type your 3D printer needs.
Other mesh modelers — such as the free MeshMixer from Autodesk — can help cut away the parts of a scan or photogrammetric mesh you don't want, or close the holes in an incomplete mesh.
In addition to MeshMixer and MeshLab, there are tools that excel at helping to create a manifold object surface by extending a surface to fill in gaps — or to fill in overlapping areas where two manifold surfaces meet.
One of the more common tools is the commercial NetFabb Studio, which has a basic free version for noncommercial personal use. Such tools easily automate the preparation and repair of 3D objects, a very handy capability for users new to 3D modeling.
NetFabb also includes toolpath management for RepRap printers.