Tips and Tricks Repairing Your Problematic STL File

2023-11-22 | By Lulzbot

License: See Original Project 3D Printing

Courtesy of Lulzbot

Guide by Lulzbot

Are you dealing with a print that comes out surprisingly different than what you ‎expected, despite having ruled out more typical problems like improper slicing profile or ‎poor print bed adhesion? It’s possible that your STL file is to blame. Sometimes a model’s ‎‎“shells” contain errors that conflict with the intended geometry of the model. Cura ‎automatically compensates for some smaller errors, but larger errors may require ‎alternative methods in order to print correctly.‎

If you’re not sure what symptoms to look for or what tools to use, identifying these ‎model inconsistencies can seem like searching for a needle in a haystack. To alleviate the ‎search, this 3D printing STL file help tutorial shows you a few steps to help identify and ‎correct some problems in your STL file.‎

Identifying Problems with STL Files

Step 1. Open and Slice STL in Cura LulzBot Edition

Bring up the problematic STL on Cura LulzBot Edition’s virtual print bed, and then select ‎the material and the desired profile. Cura will automatically begin slicing the model. ‎Below are two example models to help illustrate common problems with STL files.‎

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Example A‎

Example B‎ ‎‎‎ ‎

Step 2. Previewing your 3D Print in Layers View

Select the drop-down toggle shaped like an eye on the left-hand side of the screen. This ‎toggle controls your view of the model, and the default is “Solid.” Select the 'Layers” ‎option instead, as this will give you a preview of how each layer will print. ‎

Slide the circles from top to bottom to show only the layers you want to see. As you ‎scroll down through the model, look for inconsistencies. If something stands out that ‎that doesn’t look how the model was designed, it’s possible that there are invisible shell ‎errors that are complicating the slicing process.‎

Example A) Manifold Error: A 3D model's shell is said to be 'manifold' when it can ‎theoretically hold water. If there are any holes in the shell, the object is said to be 'non-‎manifold,' and this may cause Cura to slice the model inaccurately. While some manifold ‎errors will require you to use modeling software such as Blender to fix the holes, Slic3r’s ‎manifold check and auto-repair tool can be used as an alternate method (outlined in Step ‎‎3.)

‎As you can see in this example, the model has been sliced inaccurately from a manifold ‎error. As a result, the middle section of the model is being ignored and will not print at all, ‎appearing invisible in the Layers view.‎‎ ‎‎ ‎‎ ‎‎

Example B) Reversed Normals: Every plane composing each shell of a 3D model has an ‎intended inside and an outside, called a 'normal.' When a plane’s orientation is reversed ‎in relation to those around it, it’s referred to as a 'reversed normal.' This becomes an ‎issue when it occurs unintentionally within a model, and ranges from a tiny section of a ‎shell, to intersecting shells of a complex multi-shell model, to an entire model with a ‎single shell that is all reversed.‎ ‎ ‎‎

‎Similar to the manifold error in Example A, this model has been sliced inaccurately, but ‎this time it's due to reversed normals in the cube on top of the cylinder. In Layers view ‎the cube appears invisible and intersects the cylinder below.‎‎ ‎‎ ‎

A second example of a reversed normal in the same model is shown here. Reversed ‎normals in the sphere on the side of the cylinder have caused inaccurate slicing that ‎creates an unintended void.‎‎ ‎‎‎

Fixing Problems with STL Files

Step 1. Open STL in Slic3r

Slic3r is a 3D model slicing software similar to Cura LulzBot Edition. Slic3r has a more ‎robust manifold error correction than Cura. ‎

Download and Install Slic3r. Start Slic3r and open the STL you want to check by ‎selecting “Add” in the top left corner.‎‎‎

‎Step 2. Check for Manifold Errors

With your model loaded in Slic3r, take notice of the information in the bottom right of ‎the screen. Next to “Manifold,” the exclamation point in the triangle warns you that ‎there are errors in the STL making the model non-manifold, and then displays how many ‎errors there are. This particular STL has 1 shell with 4 manifold errors still needing to be ‎corrected.‎

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‎Step 3. Attempt to Repair STL

On the top left of Slic3r, click on “File.” In the drop-down menu, select the button that ‎says “Repair STL file…,” then select the STL you want repaired, and choose a file name. ‎This is fairly effective as an algorithmic repair method, but it's far from perfect, and ‎sometimes the file is too damaged to fix without redesigning in modeling software.‎‎‎ ‎‎ ‎‎ ‎

To confirm in Slic3r the repairs have been made and the model is now manifold, right ‎click and delete the model on Slic3r's virtual print bed. Then select “Add” from the top ‎left and choose the file you have just repaired. Select the model on the virtual print bed ‎and verify there is a “Yes” next to 'Manifold.'‎

‎‎Step 4. Confirm Repairs in Cura

Open Cura, and visually inspect the repairs by opening the STL file and selecting Layer ‎view, as outlined in Steps 1 and 2. As shown below, the section of Example A that was ‎invisible before because of the manifold error is now repaired, resliced, and visible. ‎

If your model is manifold but still exhibiting slicing errors, reversed normals are likely to ‎blame. Unfortunately, this is not as simple to fix. Refer to the help section of your 3d ‎modeling software of choice for information on reversing normals.‎

‎Step 5. Ready to Print

Once you have repaired the portions causing errors and verified your STL file, you are ‎good to start printing!

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