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Hollowing a model

3d print service providers typically charge by model volume, eventually in combination with bounding box, surface area and other factors. To minimize cost, it is often important to hollow a model, thus reducing the overall volume. This guide describes the essential steps.

During the process, we will reduce the volume and thus cost of a particular model by roughly 90%.
Your milage will vary depending on the geometry of your model. 

What you need:
This guide describes a generic process, written in form of a tutorial using a specific model file. If your model is already clean and/or free of intersecting faces, skip the irrelevant cleaning steps.

Step 1: Cleaning the mesh
  • Start Netfabb and open the seashell.gts file
You will recognize that it contains errors and also that the model is very small (0.89 mm long etc.) 
  • Resize the model by a factor of 100: Click Part -> Scale 
  • Run the automatic repair to close the hole in the model: Click Extras -> Repair part 
    • Click [Automatic Repair] followed by [Apply Repair]
  • Save the model: Click Part -> Export part -> as STL (binary)seashell (fixed).stl 
The model is now closed, but it still has intersecting faces which will later prohibit proper hollowing. Note the reported volume of 78.94 cm3.
  • Open in your browser and upload the newly created file 
  • When the cloud process is complete, download the repaired STL file: seashellfixed_fixed.stl 
The cloud process will remove all intersections, the mesh is now ready for hollowing. If you check in Netfabb, the repaired file will now report a volume of 59.80 cm3

Step 2: Creating the inner shell

  • Start Meshlab and import the seashellfixed_fixed.stl file: File -> Import Mesh
  • Decimate the number of triangles: Click Filters -> Remeshing ... -> Quadric Edge Collapse ... 
    • Check the "Preserve boundary ..." and "Planar Simplification" checkboxes, click [Apply] 
The decimation step is not strictly necessary, but it will often keep Meshlab from crashing during the next step. 
  • Create the inner shell: Click Filters -> Remeshing ... -> Uniform Mesh Resampling 
  • Set the Precision (perc.) value = 1.0 and the Offset value = -0.70, (for 0.7 mm wall thickness)
    • Check the "Clean Vertices" and "Multisample" checkboxes, click [Apply]
The remeshing may take a while depending on the complexity of your model. You may also need to select a smaller precision value to accomodate for tiny details in your model. 
  • Open the layer dialog: click View -> Show Layer Dialog 
  • Remove the (decimated) outer shell: Click the seashellfixed_fixed.stl layer, followed by [ - ]
  • Invert the orientation of the inner shell: Click Filters -> Normals, ... -> Invert Faces Orientation

Step 3: Finalizing the hollow model

  • Reimport the original seashellfixed_fixed.stl file: File -> Import Mesh
  • Combine the inner and outer shell into one: Click Layer ... -> Flatten Visible Layers 
  • Export the final mesh: Click Export Mesh As ... 
The model is now perfectly hollow, but automated and manual checks at the print service providers like Shapeways may not recognize it as such. It is usually required to add a little escape hole from the inside to the outside of the model. The minimum diameter of the escape hole varies by material.
  • Return to your modeling software of choice and add a little escape hole from the inside.
    For example, place a tiny cylinder with 0.2 mm diameter somewhere on the model boundary and apply a boolean subtraction. 

Hoorah !!!

If you now check the model in Netfabb, this hollowed model will report a lean volume of 6.45 cm3. You haved just reduced the cost of your model by roughly 90%.