How to Edit STL Files for 3D Printing: The Ultimate Guide

STL (stereolithography) files are one of the most common file formats used in 3D printing. STL files describe the surface geometry of a 3D object without any representation of color, texture or other common CAD model attributes.

While STL files are great for 3D printing, they can be difficult to edit and manipulate compared to native CAD files like CAD, STEP or IGES. However, with the right software and techniques, it is possible to edit STL files to fix problems, customize designs or prepare models for 3D printing.

In this ultimate guide, we will cover everything you need to know about working with STL files, including:

• What are STL files and how are they used in 3D printing?
• Why edit STL files vs. original CAD models?
• Manual vs. automated STL file repair
• Best software tools for editing STLs
• Tips and tricks for preparing STLs for 3D printing
• How to customize and remix STL files
• Advanced STL editing for complex modifications

Whether you’re a beginner looking to touch up 3D scans or a pro designer wanting more control over your 3D printing results, this guide will help you master STL editing and get the most out of your models. Let’s get started!

What Are STL Files?

STL stands for “stereolithography” and is a file format native to a CAD software called Standard Triangle Language (SLA) invented in the 1980s for use in 3D printing.

STL files describe a 3D object by approximating its surfaces with a mesh of triangular faces. Each triangle is defined by its three vertices and a unit normal vector indicating the direction the triangle is facing.

In addition to triangles, STL files can also contain other primitives like points and lines. However, triangles are by far the most common primitive used.

Here are some key things to know about STL files:

• Widely supported format for 3D printing applications
• Represents 3D geometry as a surface mesh, lacks color/texture/other attributes
• File size can be large due to approximating curved surfaces with many tiny triangles
• ASCII (text) and binary encoding possible
• Prone to issues like gaps, misalignments and degenerate triangles

STL files have become a standard in 3D printing because they provide a simple, vendor-neutral way of describing a printable 3D model. However, working with STLs can be tricky. Let’s look at why you might want or need to edit an STL file.

Why Edit STL Files?

For those used to working with CAD programs, editing an STL file can feel restrictive. Major CAD programs like Fusion 360, SolidWorks and Inventor are designed to work with higher-level CAD geometries like NURBS and parasolid.

With native CAD formats, designs are editable, parametric and associative. STL files, on the other hand, are simplified surface representations better suited for exporting a model for 3D printing versus continued design work.

Here are some common reasons you may need to edit an STL file rather than original CAD geometry:

• Only have access to the STL file from a client, marketplace or scanner
• Need to make edits specifically for 3D printability
• Want to customize or remix an existing 3D model from the internet
• Original CAD model is unavailable or inaccessible
• Don’t have access to high-end CAD software

For basic cleanup or preparing models for 3D printing, STL files provide all the geometric information you need. The trick is having the right tools to work with this limitation.

Another option beyond STL editing is to convert back to an editable CAD format. We’ll touch on options for STL to CAD conversion later on.

But first, let’s look at the manual versus automated options for repairing and editing STLs.

Manual vs. Automated STL File Repair

When exporting a CAD model to an STL, it’s not uncommon for issues like gaps, overlaps, inverted normals and degenerate triangles to occur. For a 3D print to succeed, these errors need to be addressed.

There are two main approaches:

1. Manual Repair: Manually inspecting a model and fixing errors as needed through modeling operations.
2. Automated Repair: Letting dedicated STL repair software detect and fix problems automatically.

Which route you take depends on the types of edits needed:

• Minor STL repairs – Quick automated repair is preferable
• Modification for 3D printability – Combination of manual and automated recommended
• Model customization – Manual modeling required
• Advanced editing – Convert to CAD format for greater flexibility

Next, let’s look at recommended software tools for both manual and automated STL editing techniques.

Best Software for Editing STL Files

When it comes to editing STL files, you’ll want CAD-like modeling capabilities, combined with STL-specific tools for analyzing and repairing models.

Here are some of the top options recommended for STL editing and repair:

MeshMixer

MeshMixer is a free STL editor from Autodesk focused on getting 3D models ready for printing. Capabilities include:

• STL repairs like hole filling, smoothing, remeshing
• Model orientation/layout tools for print bed optimization
• Vertex/face/volume selection tools for manual editing
• Sculpting brushes and transformation tools to modify geometry
• Analysis tools like inspecting for printability issues

Combined with its generous free offering and beginner-friendly tools, MeshMixer is one of the best places to start editing STLs. Just be aware that it lacks more advanced solid modeling capabilities.

3D Builder

Microsoft includes their basic 3D modeling app 3D Builder free with Windows. It provides essential STL editing tools like:

• Model repairs (gaps, normals, etc)
• Simplifying models into fewer triangles
• Vertex/face/model selection tools
• Basic transform tools (scale, rotate, move)
• Measurement and design analysis tools
• STL/OBJ/3MF import/export

3D Builder is another great starting point for new users to fix up STL’s for printing. Just don’t expect organic modeling or modification capabilities beyond the basics.

Blender

The free open-source 3D creation suite Blender has powerful organic modeling and sculpting capabilities making it great for customizing STL models. Key features include:

• Robust STL/OBJ/PLY/GLTF/FBX support
• Mesh modeling and sculpting tools
• Retopology and remeshing for optimizing STLs
• 3D printing toolbox for analyzing models
• Extensive materials, texturing and rendering support
• Python API for limitless customization

With Blender, you can push STL editing into more artistic modification territory. The tradeoff is Blender uses a complex interface that can intimidate new users.

Fusion 360

Autodesk Fusion 360 brings full-fledged CAD into the mix for advanced STL editing capabilities:

• Parasolid-based modeling for combining/altering CAD geometries
• Mesh editing tools like sculpting and remeshing
• Tools to convert STL back to CAD formats like NURBS
• Simulation tools for testing mechanical function
• Generative design to automate complex geometry
• Integrated CAM and 3D printing capabilities

With Fusion 360’s CAD foundation plus dedicated STL tools, you get maximum flexibility for editing. The downside is cost, with Fusion available only by monthly subscription.

Simplify3D

As a dedicated 3D printing and slicing software, Simplify3D includes handy tools for optimizing STL files:

• STL repairs like merging vertices, fixing normals and layer-by-layer slicing analysis
• Model checks for issues like self-intersections, holes, non-manifold edges
• Mesh export options like optimizing facets for 3D printing
• Slicing, support generation and GCode simulation for printing
• Advanced settings for dialing in high-quality prints

Simplify3D makes it easy to take an STL from concept to print-ready with all the tools in one place. The catch is the $149 price tag.

Netfabb

Netfabb by Autodesk specifically focuses on advanced STL file correction and preparation for 3D printing:

• Fully automated STL repair
• Tools to fix common mesh issues manually
• Extensive mesh diagnostic capabilities
• Mesh simplification, remeshing and optimization
• Specialized support for additive manufacturing workflows
• Available as paid software or subscription with Fusion 360

For extensive automated and manual STL editing capabilities, Netfabb is easily one of the top choices available. Expect to pay a premium for the pro-level features.

MeshLab

MeshLab is a free, open-source system with advanced analysis, editing and processing for STL files and other 3D meshes. Features include:

• Large library of mesh editing filters and modifiers
• Tools for evaluating mesh quality and geometry processing
• Capabilities for processing raw scan data into 3D models
• Support for many 3D file formats beyond STL
• Active development community behind ongoing improvements

MeshLab offers high-end STL editing tools for free. The interface and documentation can be technical, so expect a learning curve. But the capabilities are expansive.

Tinkercad

Tinkercad is a free, browser-based 3D design tool with handy STL conversion capabilities:

• Ability to open STL/OBJ files and convert to editable Tinkercad objects
• Easy interactive editing with drag-and-drop tools
• Repair mesh errors and simplify geometry through conversion process
• Export back to STL/OBJ format once modeling complete
• Great for beginners with simple drag-and-drop interactions

Tinkercad offers the path of least resistance for converting STLs into easy-to-edit CAD geometries. Just be aware of the limited customization tools.

This covers some of the most popular and fully-featured options. But there are also great online tools like MeshLab, MeshMixer, Sketchfab and Clara.io that provide free STL editing right in your web browser.

The best software comes down to your needs. For quick repairs, opt for automated tools. For artistic customization, choose organic modeling capabilities. And for complex revisions, convert to a CAD format.

Now let’s look at some best practices and tips for editing STL files.

Tips for Editing STL Files

When working with STL files, there are some key things to keep in mind:

• Triangulate models first – Any polygons beyond triangles will need to be triangulated as STLs only support triangles.
• Watch out for holes – It’s easy to accidentally create holes in a model when moving vertices and edges. Be methodical in moving geometry to avoid openings.
• Check orientations – Flipped normals can cause issues with rendering, slicing and print quality. Take time to validate no faces are inverted.
• Mind the manifold – STLs should be “water-tight” meaning every edge is connected to exactly two faces. Non-manifold edges can break 3D printing.
• Simplify complex meshes – Very complex STLs with millions of triangles slow down processing and editing. Simplify to speed up your workflow.
• Smooth wisely – While smoothing polishes models, it can also introduce non-manifold edges. Use smoothing features minimally or as a last step.
• Analyze before printing – Use tools like NetFabb or Meshmixer to analyze watertightness, thickness and intersections before printing.
• Retouch scan data carefully – Pulling and pushing vertices on scan data can distort and detach it. Use automated mesh mixing instead.

The more you work with STLs, the more adept you’ll get at avoiding pitfalls. Finding the right software that detects issues automatically can help you focus on modeling rather than troubleshooting.

Customizing and Remixing STL Files

Beyond just repairs and cleanup, editing STL files opens creative possibilities through customization and remixes. Some ideas include:

• Sculpting new shapes – Add unique contours and curves via sculpting tools in Blender or Meshmixer.
• Combining models – Merge multiple STL files together into more complex combinations using bool operations.
• Resizing models – Scale models to desired heights and dimensions. Great for custom miniatures.
• Hollowing models – Hollow out solid STLs to reduce material usage and weight. Must maintain wall thickness.
• Embossed text – Use mesh textures and bump maps to add text elements like serial numbers.
• Kitbashing – Mix and mashup elements from different kits to invent new designs.
• Cutting/Drilling holes – Remove geometry to create new cavities, slots, holes and openings.
• Thickening thin parts – Reinforce wafer-thin regions prone to breaking when printed.

The creative possibilities are endless! Just be careful about respecting any copyrights and giving attribution when remixing or customizing STLs you didn’t create yourself.

Some modeling skills go a long way towards custom STL editing. Fortunately there are some more automated ways to tweak models as well.

Advanced STL Modifications and Conversions

For making substantive changes to STL files beyond cleanup and customization, more advanced techniques come into play:

• Remeshing – Automatically optimize and generate a new mesh topology better suited for editing operations.
• Retopology – Manually reconstruct the mesh topology while maintaining the overall shape. Gives cleaner geometry.
• Boolean operations – Combine models by adding, subtracting and intersecting volumes together.
• Slicing – Intersect a model with a plane to divide it into two clean parts. Great for cutting away sections.
• Converting to CAD – Bring an STL into CAD programs to work with higher-level features like NURBS and parametric dimensions.
• Decomposing meshes – Break an STL down into basic primitives like boxes, cylinders and other predefined shapes.
• Mesh UV mapping – Layout mesh faces on a 2D plane to aid texturing, density control and analysis.

Don’t be afraid to move to more dedicated CAD software if an STL requires significant redesign. The modeling freedom of a program like Fusion 360 can save huge amounts of time versus wrestling with an STL.

Preparing STLs for 3D Printing

Once a model is repaired, customized and ready for fabrication, there are some final steps to optimize STLs for 3D printing:

• Check wall thickness – Validate no shells or walls are thinner than the minimum print thickness. Thicken thin regions as needed.
• Fix support structures – Auto-generate or manually add support structures to overhangs and islands to avoid drooping.
• Set build orientation – Position the model with short, well-supported spans across layers to reduce sagging and warping.
• Run analysis tools – Use specialized tools to check for issues like printability, clearances or collisions.
• Add helper features – Include rafts, brims, mouse ears or anchor points if needed for bed adhesion.
• Simplifyfile – Reduce model triangle counts for faster slicing without compromising results.
• Export optimized file – Choose higher-quality mesh export settings for cleaner part edges.

Taking these proactive steps during STL editing will minimize print failures and ensure the best quality final part.

Key Takeaways and Next Steps

Editing STL files certainly comes with challenges vs. working with original CAD source files. But with the right tools and techniques, modifying STL geometry can become an intuitive process.

The key takeaways from this guide include:

• Understand what STL files are and how they are used in 3D printing
• Automated and manual tools both play important roles
• Quality software designed for STL editing makes a big difference
• Take time to clean up and validate models before printing
• Don’t be afraid to move to CAD for substantive redesigns
• Customization and remixing offer creative possibilities

Ready to start editing your own STLs? Here are some suggested next steps:

• Try Meshmixer, 3D Builder or Blender to get started with free tools
• Look into automated repair options like NetFabb or Simplify3D for easy clean up
• Consider moving to CAD if models require significant modifications
• Print some test parts to validate your editing process
• Have fun customizing remixing models and making them your own!

With practice, you’ll be shaping STL files into print-ready models in no time. Happy editing!