Sunday, 9 June 2013

3D PRINTING THE PAST: SOME ISSUES



Some weeks ago, Arc-Team and Kentstrapper (a Florentine startup that produces 3d printers) decided to collaborate, in order to make the Taung Child 3d model real, and possibly expose it in a museum.

But how does a 3d printer work exactly?
Basically a 3d printer uses the FDM (Fused Deposition Modeling) technique, an additive process where successive layers of material are laid down in different shapes; following a digital model of the object, the printer deposits layers of plastic material, automatically fused to create the final shape.
So, what we need first is a .stl model of the object. But (for now) not everything is 3d-printable: some spefic characteristics are required to be printed.

Which softwares can be used to locate and fix problems?
The most used software is Netfabb, that is not open-source neither free, but Netfabb Studio Basic can be freely downloaded.
In the Open-Source world, we can obviously use the 2 main 3d modeling softwares: MeshLab and Blender. In particular, in version 2.67 of Blender a 3d printing toolbox has been inserted as add-on: it's useful to check the mesh and see which are the problems. Pressing “Check All” a complete scan af the mesh will be done.

  1. Volume: the mesh must be solid. It cannot have holes, 2-point polygons or single sided polygon surfaces.
  2. Mainfoldness: the mesh must be completely and perfectly closed. The mesh must be “2-mainfold”: every edge must belong to 2 faces (not 1 or 3: only 2). Here are some reasons why a mesh cannot be 2-mainfold:
  • Holes: Automatic hole-fixig can be made with Meshlab (Edit-Fill hole) or Netfabb; in Blender, from version 2.63 just selecting the vertices that “compose” the holes and pressing F in Edit Mode the missing face will be created.
  • T-Edges: an edge cannot be on a board. In this case, the volume is considered open, even it seems closed. The face must be deleted and rebuilt (with the same method for closing holes).
  • Internal faces: internal faces must be deleted, because they make the mesh “3-mainfold”.
  1. Minimum wall thichkness: tipically a wall thickness of 2.5mm is required. The Blender toolkit can show the too-thin areas, that must be scaled till a proper dimension.
  2. Polygon number: with too few faces the figure will lose detail, but with too many faces the fill will be heavy and possibilities of error will increase. To reduce the number of polygons of a mesh we should use MeshLab, following this tutorial.
  3. Intersected faces: there may be 2 or more faces intersecting themselves, expecially in objects composed by two or more meshes. Even in this case, a solution should be remove the intersecting faces and then closing the hole with the method said.
  4. Zero volume faces/edges: faces/edges with no volume.


So, here you can find the .stl file of the mesh. It's a really complex mesh, with an enormous number of faces and several problems (thickness, distorted faces); our goal is making it 3d-printable, and that's why
we ask for your help. 

P.S. Thanks to David Montenegro for his suggestions.  

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