How to check and fix an STL file ready for sending to a 3D printer

This is a common problem when sending files to a rapid prototyping bureau, errors can occur in the files depending on the software you have used to produce the STL file and how you modelled it.

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How to check and fix an STL file ready for sending to a 3D printer

Common issues are:

·  Reversed normal’s

· Bad edges

· Holes in the mesh

·  Noise shells

All of these will prevent your model from being 3D printed. These errors can normally be fixed using software freely available on the internet, one such program is Netfabb which is an STL file viewer and repair tool. This software is free to download (Scroll to the bottom of the page, you don’t need to fill in all of the information!) and use and will identify basic errors and repair them if possible. It is available for Windows, Linux and Mac.

Once you have installed Netfabb open up the software and it will look like the image below:


The important areas on the interface are highlighted above. You can rotate the model by pressing and holding the right mouse button and then dragging; clicking on one of the standard views will reset the viewpoint. To zoom you can use the scroll wheel on your mouse or you can use the icons shown above.

The basic information area of the interface shows the overall dimensions of the part (the bounding box), the volume, the surface area and the number of triangles in your model, this information is useful when you are selecting which 3D printing technology to use.

The analyse, repair and measuring area of the interface are the functions that we will be using to fix the STL file. Selecting the analyse button will generate additional information about your part including where and what the errors are if any. The repair function will analyse and repair the errors that it finds. There are a lot of free STL viewers out there but there are very few that will repair your file as well. The measuring icon will do just that and will allow you to take measurements around your model by selecting different triangles; this is useful for checking wall thickness's and minimum detail sizes.

The screen shot below shows a part that has been loaded into the software. You can see from the bottom right icon that there are errors in the file as soon as it has been loaded.


Fixing your STL file

Select the part so that it turns green and the bounding box is shown, now click on the Repair icon and this will open up another interface screen in the bottom right of the screen. Click on the auto-update tick box to ensure the data is up-to-date while you are working on it.


Click on the automatic repair button, this will open a popup box which will appear in the middle of the screen.


From this box you can choose either the default repair or simple repair option. The default option will run all of the repairs that Netfabb has so I would recommend that you use this one. Click on the execute button and Netfabb will carry out the repairs. Your statistics area should now update to show that there are no more errors in the 3D model. Border edges, invalid orientation and Holes should all be Zero and shells should preferably be equal to one although it won’t if there are multiple parts in the STL file.

All that is left to do is click on ‘Apply Repair’ and then confirm that you would like to remove the old part. To save your part you need to click on ‘Part>Export Part>as STL (binary)’ and that’s your STL file fixed.



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