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Parts must be manifold. Manifold is a term that means the surface of the part is contiguous and complete, and with regards to 3d printing, also not zero thickness. Imagine a soccer ball shape: that would be 'manifold,' but if the soccer ball were missing one polygon, it would not be manifold. In order to print such a shape, either the walls of the part need to be thickened, or the gaps or holes need to be patched. Some models created with common software, particularly SketchUp, can be impossible to print without serious repair and reworking, because the models include many holes or gaps and are often composed of zero-thickness planes rather than solid shapes with volume. 'Manifold' is also referred to as 'water-tight.'
Parts must be manifold. Manifold is a term that means the surface of the part is contiguous and complete, and with regards to 3d printing, also not zero thickness. Imagine a soccer ball shape: that would be 'manifold,' but if the soccer ball were missing one polygon, it would not be manifold. In order to print such a shape, either the walls of the part need to be thickened, or the gaps or holes need to be patched. Some models created with common software, particularly SketchUp, can be impossible to print without serious repair and reworking, because the models include many holes or gaps and are often composed of zero-thickness planes rather than solid shapes with volume. 'Manifold' is also referred to as 'water-tight.'
=== Designing 3D Models ===
Page in progress to describe [https://wiki.pumpingstationone.org/3D_Modeling 3D modeling.]
3D models for printing should be saved in the STL format (binary or ascii). Your model should be "watertight" (the mesh has a mathematically determinable interior), correct normals, and a manifold topology. Shapeways has a nice [http://www.shapeways.com/tutorials/things-to-keep-in-mind tutorial] explaining this.
All software suggestions in this section are open source, and are available for free.
*For individuals new to 3D modeling software, [http://www.blender.org/ Blender] is a good tool for designing models for 3D printing, as well as for debugging fussy models (Blender can highlight the parts of a model which make it non-manifold). Blender recently had a total overhaul of its codebase and user interface, and has become quite user friendly. Additionally, the Blender foundation produces and sells [http://www.blender3d.org/e-shop/default_dvds.php video tutorials]. The video tutorials are distributed on dvd data disks, but are creative commons licensed, and so you may legally download them for free online. The videos are created by artists, and cover a wide array of topics from basic usage of the program, modeling techniques, and the more advanced capabilities of Blender.
*[http://meshlab.sourceforge.net/ Meshlab] is an open source tool for repairing 3D models.
*[http://sourceforge.net/projects/free-cad/ FreeCad] is a general purpose feature-based, parametric 3D modeler for CAD, MCAD, CAx, CAE and PLM, aimed directly at mechanical engineering and product design but also fits a wider range of uses in engineering, such as architecture or other engineering specialties.
*[http://www.openscad.org/ OpenSCAD] is a different take to 3D modeling. OpenSCAD is a programming language that generates parametric 3D models.
After the part is modeled it needs to be turned into commands the 3D printer can recognize, a step called 'slicing' since it turns the solid shape into a series of paths and printer commands to move and start and stop the extruder, that build up the part layer by layer. The final step in 3D modeling, then, is to save the part in a format that the slicing software can recognize. Common formats include STL, but vary depending on the specific software and workflow you will be using.
After the part is modeled it needs to be turned into commands the 3D printer can recognize, a step called 'slicing' since it turns the solid shape into a series of paths and printer commands to move and start and stop the extruder, that build up the part layer by layer. The final step in 3D modeling, then, is to save the part in a format that the slicing software can recognize. Common formats include STL, but vary depending on the specific software and workflow you will be using.