Broken STL files are usually a result of 3D modelling mistakes and are better fixed at source, but if you have an stl files and not the original CAD file, you may not have the option to fix it.
If you’re on Windows 10, the simplest fix to try first is using a free tool from the Microsoft Store called 3DBuilder. Whilst the tool is a pretty basic 3D modelling tool, it has a great ‘engine’ behind it which will identify broken models and often will fix them for you.
- Go to the Microsoft Store, search for 3DBuilder and install it.
NOTE: you may have it installed already but didn’t even know! - Open your stl. If the model has an issue, a red box will appear around the model and an option will be provided to automatically fix it.
- Fix the model and re-save. The default file format in 3D Builder is .3mf so you might need to change that back to .stl
