This solution article is for those who might be receiving errors when trying to print certain .STL files.

The first thing that many people do when a model doesn't print like they expected on their 3D printer is to blame the printer. Holes in printed models can often look like under extrusion. People a bit more familiar with 3D printing in general, understand that this could also be caused by the 3D slicing software. However, we'd like to tell you about a third possibility. The conversion of a .CAD model into an .STL model isn't perfect, anymore than making a digital copy of a song from a vinyl music record is perfect (appealing to all the music lovers out there). In both conversion processes, errors can occur.

What Kind of Errors Can Occur When a CAD Program Converts a Model to an .STL File?

[Note on the information in this section: We do not claim to be the experts on CAD related errors; if you want better explanations, for these errors, they are available in several places throughout the internet.]

.STL models are models made from "meshed" triangles that have no thickness in themselves:

◄Inverted Normals►

There are when a side of a triangle is flipped when the CAD program converts the model to a .STL. To us, it may not seem to matter but when the software tries to read the file code, it has a conflict.

◄Thin Walls (Shells)►

Look at the tail fin of the dolphin triangle mesh above. 3D printing software has a hard time printing a surface with no solid backing behind it. Though the tail fin in the .STL could be just made of 1 triangle (which would look okay to us when it is rendered into a 3D image), 1 triangle is only 2 dimensional, no matter how you turn it: if you lay it flat on a Z-axis plain it would look like a solid line and the thickness of that solid line is only to help us see it - the actual triangle would have no height on the z-axis. Therefore, when a designer makes a model for 3D printing, they have to keep in mind that all parts of the model need to have some sort of solid thickness and shouldn't be made out of a single plane.

◄Bad Edges►

Almost all 3D models aren't designed by free-hand drawing them. They are usually made from piecing a shape next to a shape next to a curve (and so on). When those shapes are placed together, it's okay for there to be a tiny gap or for the edges of two shapes to overlap. However, if two shapes share the same edge, the exported .STL will have what's called a bad edge - in short, the conversion won't know how to turn that into a triangle. From your youth, you know that triangles must have 3 sides; in this case the triangles must have it's own 3 sides.

◄Open Faces►

A less common error but one related to the thin walls explanation above is when you have open/missing faces. Think of a cube: when it has all of it's 4 faces, it looks solid. However, if you forget to add one of the 4 faces, you realize that the walls of the other faces are infinitely thin (because a cube in theory has four 2D faces - there is no thickness dimension on a face). Therefore, when the 3D slicing software tries to turn these infinitely thin faces into 3D sliced layers (even one layer of printed filament has a finite thickness).

All of this is just to tell you why the 3D slicing software might have an issue with a .STL model.

What You Might or Might Not See in the 3D Slicing Software

Note: the exact same .STL models may not register any errors in other 3D slicing software programs yet still could be causing issues when the printer is trying to print.

Creatr Software (powered by Materialise) will let you know usually if a model needs to be fixed:

(Materialise offers a repair service for a fee - after so many trial repairs.)

Notice that this model isn't so bad - it has 4 "bad edges." 

In Creatr Software, by having "Autofixing on Import" enabled, the slicing software will fix some small errors:


(The "Bad Edges" were fixed by the 3D slicing Software.)

Simplify3D fixes minor errors like this too by default upon importing a .STL file. However, some .STL models are beyond the many software programs' repair capabilities. Creatr software will still show a warning message if it could not fix the errors on import; Simplify3D will not show a warning message; Creatr software will prevent you from being able to print it, Simplify3D will do it's best to guess at the fixes not always yielding favorable results. If the error is big enough the Print Preview can often show various things like small-to-large holes in the model where there shouldn't be, gaps in layers, or maybe even size dimensional differences.

Okay, So How to Fix .STL File Errors

Materialise offers a Cloud Repair Service (with the first 10 repairs free, we believe) for a fee.

A good alternative to using Materialize's Cloud .STL Repair service is an free Microsoft Model Repair Service (powered by NetFab): *


*[Leapfrog TM is not responsible for the content of any third-party websites or services, any links contained in third-party websites or services, or any changes or updates to third-party websites or services. Where Leapfrog TM provides links and/or access to third-party websites and/or services it is only as a convenience to you, and the inclusion of any link or access does not imply an endorsement by Leapfrog TM of the third-party site or service.]

The only downside that we've seen to using this service is that you'll need to create a Microsoft Account (which is free, at least at the time of writing this article).

Once logged in, the service is as easy as clicking an Upload button, uploading your .STL file, waiting a short period (depending on size of the .STL), and then downloading the fixed .STL. Here's some screenshots:




If you have plans on printing multiple .STL models, you can see in the image above that they have an "Upload Another" button to the right of the Download button for fast repairs.

The whole process -from uploading to downloading - for a 2MB model took roughly 30 seconds when we tested the site for this solution article.

Are the Errors Really so Bad?

As mentioned, some errors in the .STL model have been known to cause massive holes (most common result) in the model upon printing. Other issues could lead to the printer stopping mid-print or the 3D slicing software reviving an error. We at Leapfrog Technical Support see this on occasion where a customers often contacts us saying "Your printer isn't working!" yet is later discovered through troubleshooting that the .STL model itself is the cause of the bad print quality. 

Therefore, fixing the model might help you resolve major print quality issues or prevent you from jumping to the wrong conclusion about hardware or software. It's certainly a quick and easy step in self-troubleshooting print quality issues.


We hope that this helps broaden your knowledge on .STL files, how they can contain errors, how those errors can effect the print, but more importantly that most of those errors can be fixed using online solutions.

We thank you for checking out our Leapfrog 3D Printer technical support solutions online! If you were unable to fine a solution in our database, please feel free to open a new support ticket by clicking the following link: I want to open a new support ticket.