11. 3D Printers – Geometry restrictions

Here are presented in brief geometric restrictions that must be considered when determining whether a 3D model is suitable for 3D printing:

11.1 Minimum wall thickness

Often architects or game designers will produce elements in a 3D model that have an infinitesimal thickness (hair, capes, sails etc.). Thin features are impossible to 3D print, unless they are larger than the minimum printable feature size for each technology.

The table below summarizes the recommended minimum wall thickness for the most common 3D printing technologies. Note that in some cases, like SLA/DLP, it is possible to print smaller features, but this should be assessed in a case by case basis, after consulting with the machine operator.

11.2 Watertightness

Every 3D model that is intended for 3D printing should be completely manifold (watertight): every edge should be connected to exactly 2 polygons and the model must include no holes.

Models that are not manifold might get misinterpreted by the software that generates the instructions for the 3D printer (slicer). A non-manifold 3D model might cause inconsistent layers, holes or other errors, making the object unprintable.

Non-manifold issues are often not visible at the modeling stage. The simplest way to check whether a model is printable is to use an analyser software, like Netfabb or Meshmixer. These programs detect model features that will cause issues at the 3D printing stage and offer repair options (without impacting the overall geometry of the model).

11.3 Curved surfaces

Most CAD modeling software, such as Solidworks and Fusion360, use Non-uniform Rational Basis Splines (NURBS) to display the surfaces of a 3D model. When exporting your model to the STL file format for 3D printing it is important that an adequate number of polygons are used to represent its surfaces. This will ensure that part will be 3D print with a smooth appearance.

If the 3D model is exported with too few polygons, the edges connecting individual polygons will often be visible in the final 3D printed part. This effect is more prominent with large models (larger than 300 mm3), where the polygons becomes more visible on curved surfaces.

If the 3D model is exported with too many polygons, then its file size will be unnecessarily big, making it difficult to handle, and will have no effect in the final quality of the printed part, as tiny details cannot be 3D printed.

Luckily, most modelling software export 3D models with an adequate number of polygons using the preset, resulting in smooth 3D printed parts. If a higher polygon count is required, the export settings can be adjusted accordingly.

For a more in-depth exposition on file-fixing for 3D printing, please refer to the following articles:

– https://i.materialise.com/blog/en/preparing-files-for-3d-printing/
– https://www.3dhubs.com/knowledge-base/fixing-most-common-stl-file-errors

Other sources:
[1] https://www.3dhubs.com/knowledge-base/3d-printing-geometry-restrictions