'Water Tight' Meshes, File Fixing, & Inverted Triangles - What does it all mean?

Updated: Apr 25

Introduction:

In our previous article, we delved into the common file formats utilized with our 3D Printing Services. Understanding these file types is fundamental in grasping potential CAD errors that may arise.

Before 3D Printing all our parts must be a mesh (.STL format). If the original model isn't a mesh then a conversion must take place, which can either be an export from the original software or during an import into our software.

This conversion is the most common source for CAD Errors, though they are usually minor & easily fixed.

It's crucial to scrutinize your model for the following issues before submitting it for 3D printing:

Missing surfaces
Single thickness surfaces
Unwanted geometry (e.g. leaving bolts in the CAD)
Floating bodies / shells
Intersecting bodies / shells or surfaces

An image showing a cube with multiple CAD errors. Including: A bolt unintended for printing, a hole caused by a missing surface, an a surface with no thickness.

Presence of these issues in your CAD model during the conversion to mesh (.STL) will likely result in CAD errors which need fixing. Sometimes referred to as 'File Fixing' or 'CAD Fixing.'

Common CAD errors include:

Inverted Triangles:

Each triangle forming the mesh possesses an inside and outside face. Inverted triangles, or surfaces, occur when these orientations are reversed. Occasionally, the entire part may be inverted.

Holes/Non-Watertight Mesh:

As the name implies, holes occur when there are missing triangles or surfaces on the mesh. This may result from conversion errors or incomplete surfaces in the original CAD. Most CAD programs offer tools to address this, such as 'Knit Surface' in SOLIDWORKS or the 'Join' command in Rhino.

A cube with a missing triangle. It looks like a cube with a triangle shaped hole on it showing the inside of the cube visible through the hole.

Multiple Shells/Bodies:

Typically, files should contain only one shell; for multiple items, separate files are preferred. Overlapping shells are consolidated into a single shell, akin to the 'Combine' function in SOLIDWORKS. Alternatively, bodies in an assembly can be separated upon request, though our online portal assumes each file as a single part by default.

Exceptions for multiple shells include:

Parts designed with moving components, such as hinges or chains.
Hollow parts, where the internal surface is considered a separate shell if not connected to the exterior via a powder escape hole.

An example of a 3D printed chain, with a label to show that each chain link is a seperate shell.

How to repair CAD files?

The best way to avoid needing to repair files is by using a reputable CAD program & following the best practices for that software. Check for obvious things like missing surfaces & overlapping geometry before exporting the file.

If repair within the original software proves challenging, several options for repairing meshes are available. We utilize Materialise Magics, a robust but costly 3D printing software, for this purpose. Free options like Meshmixer are also viable alternatives.

Should all else fail, we offer file repair as part of our 3D Printing Services, with additional charges possible for particularly complex repairs.