=====Preparing models for printing using Blender=====

Before you can print a model successfully, you need to know it is manifold with all the surface normals pointing out.

Non-manifold objects will cause problems even before you get to the printing stage. If you apply a boolean operation, eg to join two objects or subtract object from another, you will get the wrong result if one of your objects is non-manifold. Objects downloaded from the web are very likely to be non-manifold, especially if they were not built specifically for 3d printing.

==== Definition of "manifold" ====


There are various definitions:
  * The model must be watertight. (have no holes)
  * The model must have an inside and an outside.
  * Each edge must connect exactly two faces.

My version is:
  * Each surface element must have an inside and an outside.

Or there is always the  [[https://en.wikipedia.org/wiki/Manifold|mathematical definition on wikipedia]].

==== Examples of non-manifold objects ====

  * Something with a hole, eg a cube missing one side. In this case, the inside and outside are connected so there is no real "inside".
  * Extra surfaces or extra vertices. The object may look OK but has doubled-up vertices. Or it may have an extra face internally. In this case we have surfaces that don't have an outside face.

====Fixing non-manifold objects====

  * You can import an STL file into netfabb to fix non-manifold problems, then bring it back into Blender to keep working. This is the best option for objects with lots of problems.
Alternatively you can try fixing the issues inside Blender.
This {{:repairingnon-manifoldmesheswithblender.pdf|tutorial by wickedwidgets}} goes into lots of detail on how to fix non-manifold objects in Blender. Its worth reading, even if you plan to use netfabb as your first choice.
The first couple of steps are very simple and may be all that are required:
  * Go into Edit mode and select all vertices
  * Choose remove doubles. You will get a message at the top of the screen showing how many vertices have been removed. This may be all that is required in many cases.
  * Deselect all vertices, then choose Select non-manifold from the Select menu. Any remaining non-manifold areas are then highlighted.
If the non-manifold area is an obvious hole, you can fill the hole. Select all the vertices around the hole, then fill.
If the area is not an obvious hole, it may be intersecting surfaces or something similarly ugly. In that case, its off to netfabb or following the wickedwidgets tutorial to fix it in Blender.
  * Finally, select your object and Recalculate Normals to ensure they are all pointing in the right direction. 

====Exporting the STL====
  * Select the object(s) to be printed and export as STL. Because Blender only exports the selected objects, you can include other working elements in the file without worrying about them being printed.