STL Repair Options (Online and Offline)

A typical STL file may contain tens of thousands of tiny triangles. Depending on the design, there are many different ways in which these triangles can be misrepresented (from a manufacturing perspective).

Let’s therefore take a 30,000-foot view of some common types of errors that might result in unprintable files and the problems that need to be fixed.

Holes

In order to exist in the real world, a 3D model needs to describe an enclosed or “watertight” volume, with all of its vertices’ edges and vertices connected. Yet, an STL can sometimes have small holes in the mesh, or perhaps triangles have edges that aren’t connected to their neighbors. Both can lead to issues when printing.

On a related note, each triangle notionally points into or away from the shape it describes, essentially having inside and outside faces. Occasionally, this orientation is flipped, causing problems in how a slicer interprets a complete surface.

Overlapping Geometry

3D models usually consist of combinations of different shapes. The way these shapes (or the “shells” of these shapes) are joined is important and, if done incorrectly, can hinder slicing. A related problem occurs when an edge or vertex is shared by one or more shape.

This can be a mind-bending concept, but consider how a modeling program might represent two intersecting cubes. Unless these two shapes are eventually interpreted as one object, your printer will have difficulty printing them together.

Noise

Some STLs can contain floating triangles that shouldn’t be present. Triangles may also overlap or intersect in a way that is hard or impossible to interpret.

Complex Geometry

There’s a thin line between an STL that has actual errors in its mesh and one that has a correct mesh but describes something that’s still hard or impossible to print. For example, attempting to represent hair or fur might result in an enormous STL (because of the many triangles required to represent such a “surface”), which is hard to process due to details smaller than a printer’s resolution.

Related to this, an STL may describe enclosed volumes that are too thin to be printed practically.

Fixing these types of issues is better described as an optimization step rather than repair, although both are often be handled by the same software.

Repairing STL files can be very similar to repairing photos. Photo editing tools often have an automatic option that fixes an image with a single click. This may be good enough in some cases, but certain images will need more in-depth improvements, and professionals may opt for very advanced editing software to get the quality they want (short of re-shooting).

The same situation exists for STLs. Repair tools are available to “auto fix” files, and while these can work, they often don’t, sometimes with unexpected results (like when holes that should be part of a model get filled in). Other tools provide detailed analysis and fine control over repairs, but in extreme cases, there may be no option but to go back and edit the original 3D model.

A typical STL repair and optimization workflow might look as follows:

1. Auto mesh repair: It always makes sense to at least try a simple, automatic fix. In many instances, this will be adequate, but in others, the original STL may be too large or too broken to process this way. Worse, the “repair” may alter important parts of the design.
2. Manual mesh repair: If the auto fix doesn’t work, look for a more advanced tool. Start with repairing holes or gaps in a way that preserves your 3D model. Then, choose options to fix other geometry issues. If problems persist, you can try re-meshing the entire model or use options such a “make solid” or “shrink wrap” (depending on the tool).
3. Re-modeling: If both of the above steps don’t work, you’ll probably need to re-edit the 3D model using CAD or other 3D modeling software.
4. Optimization: This step isn’t always necessary and may vary depending on the type of 3D printer you’re using. Typical actions include thickening thin sections, hollowing out otherwise solid shapes, or “resizing” an STL so that it has enough triangles to avoid obvious faceting on the 3D print but not so many that the file is too large to process.

Tools available for STL repair have changed significantly over the past few years. Once popular tools are no longer available, whereas others have become integrated with commercial workflow systems, especially those intended for professional 3D printing.

Generally speaking, STL repair software can be viewed under three broad categories:

• Dedicated, free, or open-source, tools are available to use online or offline, and these remain popular choices.
• Slicer software is also increasingly embedding analysis and repair tools that can handle many issues. For those that need more or better control (or faster processing), there are many more sophisticated offers that come at a cost.
• CAD and 3D modeling programs often provide repair functionality, and these are often the best options for those that do their own designs or remixes.

We’ll look at each of these types of software in turn, but it’s worth noting that virtually all commercial 3D printing services have their own built-in repair and optimization tools. These services have a vested interest in ensuring that STL files are printable and trouble-free, so investing in quality tools is important to them. In most cases, this functionality is part of a proprietary workflow, where you can’t download the repaired STLs, so we won’t be covering them here.

Microsoft’s 3D Tools is a simple cloud-based “3D object fixing tool”. It’s based on their own 3D modeling software and uses functionality originally developed by Netfabb (one of the oldest and most respected “STL repair” vendors). 3D Tools provides “Repair”, “Reduce”, and “Convert” services, which can make most STL files with simple errors printable.

The Repair service fixes geometry issues to create a closed, “water-tight” 3D mesh suitable for 3D printing. The Reduce feature simplifies the mesh to reduce STL file size and improve processing. And the third service converts various 3D file types into the relatively new 3MF file format. That last step is applied to all repaired STLs by default. In other words, the repaired file is stored as a 3MF file and not another STL.

There are no other options to fine-tune repair steps, and the entire process is conducted in the background with no print preview. You’ll also need to register or use an existing Microsoft ID. We found the service struggled a bit with very large STLs, but for a free and simple process, it generally does a great job.

• Platform: Browser (online)
• Cost: Free
• Best tool for: Anyone requiring simple STL repairs
• Available from: Microsoft

FormWare produces a commercial, full-featured slicing tool focused primarily on SLA and DLP printers, and it contains a powerful analysis and repair engine. That engine is also available online for free, and it’s very easy to use.

During analysis, the tool lists detailed results, highlighting how many of each error type have been found, before making appropriate repairs. As with most good repair software, it even identifies issues with the veritable Benchy STL file!

It has a timeout of four minutes of processing time, which should be more than enough for all but the largest and most damaged STLs.

• Platform: Browser (online)
• Cost: Free
• Best tool for: Anyone requiring high-quality STL repairs
• Available from: FormWare

3DPrinterOS is a comprehensive cloud-based toolset for managing multiple 3D printer workflows. It allows business enterprises or educational establishments to manage their own printers as well as contract printing services.

To ensure that objects are printable, 3DPrinterOS uses a function called “Magic Fix” to analyze STL files and make any necessary repairs. When doing so, it also takes your printer and other factors into consideration and produces a fixed version of your STL, properly scaled and oriented. This revised STL is saved in your default workflow and can be downloaded by clicking on the file name. 

• Platform: Browser (online, optimized for Chrome)
• Cost: Free repairs, trials for other functionalities
• Ideal for: 3D printing hobbyists interested in broader workflows
• Available from: 3DPrinterOS

MeshLab, in stark contrast to the simple services like 3D Tools, offers a fantastically rich toolset that enables detailed control over editing, cleaning, healing, inspecting, rendering, texturing, and converting 3D triangular meshes, including STL files.

It’s an open-source solution in constant development, and its focus is on processing models produced by 3D scanning and digitization tools. It can handle very large STLs, which often defeat other solutions, and it’s also useful for effectively combining more than one mesh.

• Platform: Windows, MacOS, Linux
• Cost: Free
• Ideal for: Experienced makers requiring advanced STL repairs
• Available from: MeshLab

Ultimaker’s Cura is the most established of the free-to-use slicers and has had the ability to at least identify many common STL issues for some time. Nowadays, it’s able to fix many of them, and the list continues to grow. Indeed, it’s worth periodically looking at the most up-to-date documentation or exploring the “Mesh Fixes” settings to see what’s current.

In addition to the slicer’s base functionality, a plug-in called “Mesh Tools” (available via “Marketplace” in the top right-hand corner of the UI) adds extra features to check STL files when they’re loaded. Handily, the plug-in also fixes simple issues.

If you’re using Cura, it’s at least worth experimenting with these options, as in many cases, they will eliminate the need to resort to other repair tools.

• Platform: Windows, MacOS, Linux
• Cost: Free
• Ideal for: Experienced Cura users
• Available from: Ultimaker

PrusaSlicer will identify most STL issues and automatically repair defective models to a certain degree. However, that functionality is heavily based on the 3D printing API built into Windows (which in turn is based on the original Netfabb).

If errors are detected and flagged when loading a new STL file, there are two options for making repairs: right-clicking on the displayed warning icon or selecting “Fix by Netfabb” on the model menu. This doesn’t offer the granularity of Cura, but on the other hand, it does make use of well-established functionality, which has proven to be effective most of the time.

Again, it’s worth keeping an eye on the most up-to-date technical details for the latest developments in this area.

• Platform: Windows (full repair functionality); MacOS and Linux (limited functionality)
• Cost: Free
• Ideal for: Windows PrusaSlicer users
• Available from: Prusa Research

Simplify3D explicitly highlights its capabilities for mesh analysis and repair. It has among the most sophisticated functionality to cover virtually every repair scenario.

This includes several built-in tools to help you identify and fix common mesh issues so that you can get back to printing quickly, usually without having to resort to even the more powerful, value-adding tools.

• Platform: Windows, MacOS, Linux
• Cost: $150; two-week free trial
• Ideal for: New and existing Simplify3D users
• Available from: Simplify3D

3D Builder is Microsoft’s free 3D modeling software for Windows and other Microsoft platforms. Primarily intended as an easy-to-use solution for simple modeling – and even 3D model scanning using a webcam – it also includes very capable STL repair functionality to ensure that your 3D models are printable.

Using the same basic functionality as 3D Tools, 3D Builder has the big advantage of allowing more granular control, along with other changes to be made to 3D models.

• Platform: Windows
• Cost: Free
• Ideal for: Anyone 3D modeling or editing simple parts
• Available from: Microsoft

The self-described “Swiss Army Knife” for 3D meshes is more than a simple STL repair tool. Meshmixer is a fully fledged modeling solution that’s also capable of hollowing, scaling, and mesh simplification.

This is no beginner’s tool, being consistently ranked among the best tools for STL repairs. It supports several highly regarded repair and fixing functions, including the famous “make solid”, and offers near-limitless ability to enhance and change a model’s original design.

Another big plus is the wealth of information available on the internet. Looking around, it’s not hard to learn about the various repair tools available in Meshmixer, including what their pros and cons are. And if you’re a fan of the CAD-CAM tool Fusion 360, Meshmixer’s (as well as Netfabb’s) mesh repair functionality is built-in.

• Platform: Windows, MacOS
• Cost: Free
• Ideal for: 3D printing hobbyists who 3D model
• Available from: Autodesk

FreeCAD is an open-source 3D modeling program that was initially designed with the needs of mechanical engineering and product design in mind. Among the many features of this program are very capable analysis and repair tools.

If you’re looking for a tool that gives you a high degree of control over your work, then FreeCAD may be the way to go. As with the other CAD programs, one advantage of this approach is the ability to not just repair STL files but also make other changes and modifications that may be required.

• Platform: Windows, MacOS
• Cost: Free
• Ideal for: Experienced makers who prefer classic CAD functionality
• Available from: FreeCAD

Blender has become the defacto standard tool for 3D modeling and animation. It should come as no surprise, then, that a tool focused on manipulating and managing mesh models offers very powerful facilities for fixing problematic STL files.

Yet, this sophistication comes at a price: Blender is not suited for beginners, and the learning curve can be steep. But for those looking for a well-supported, open-source tool capable of tackling almost any mesh modeling challenge, this program is worth considering.

• Platform: Windows, MacOS
• Cost: Free
• Ideal for: Experienced makers who prefer sophisticated software
• Available from: Blender

Netfabb is one of the best-known families of software for repairing STL files. It was one of the earliest solutions in the space and, in one form or another, has endured as either a standalone solution or as an embedded feature buried deep inside other solutions, including Microsoft’s 3D printing software.

Acquired by Autodesk in 2015, Netfabb continues to exist in its standard form, but it’s also offered in Premium and Ultimate versions, as outlined in this comparative table. These latter two offer advanced feature sets, like post-processing preparation and lattice optimization. Any of the three can be tested with a trial, after which it will revert to Netfabb Basic.

Apart from these standalone offerings, Netfabb is also integrated into the popular CAD and CAM tool Fusion 360, which also includes the mesh repair functionality of Meshmixer.

The original free version can still be downloaded from an unofficial GitHub repository. However, it hasn’t been updated since 2015.

• Platform: Windows
• Cost: $230 per year for Standard; ~$4,600 per year for Premium; price upon application for Ultimate; free trial
• Ideal for: Professionals requiring versatility and Fusion 360 integration
• Available from: Autodesk

MakePrintable does exactly what the name suggests: It takes STL (or other) files and identifies and optimizes features that might make them unprintable. For example, it can adjust the thickness of parts that are too narrow or thin to print, adjust clearances between parts, optimize polygon counts, hollow models for those processes that need it, and much more.

The popular tool can handle very large file sizes that may defeat other offerings, boasting processing times several times faster than rival services. It also provides users with a high level of control and specification over the repair process.

In keeping with its value proposition, it also supports seamless integration with tools such as Blender and SketchUp.

• Platform: Browser (online)
• Cost: $2 per repair; ~$15 to $100 for monthly volume plan
• Ideal for: 3D printing enthusiasts, makers, and businesses requiring quality and speed; infrequent users
• Available from: MakePrintable

LimitState:Fix is another professional STL repair tool with a significant price tag. According to the website, it “fixes the 3D models other tools can’t!”

Besides its auto-fix capability, the program allows granular control over repair options for closing solids, correcting non-manifold errors, and fixing the orientation of polygons. It also has features to enable users to merge STL files, remove noise shells and simplify meshes. 

LimitState uses robust Polygonica technology, a well-established professional 3D modeling solution used in industry. 

• Platform: Windows
• Cost: £499 (~$685) for Maker (3 activations), £2,999 (~$4,120) for Enterprise (10 activations)
• Ideal for: Professionals requiring reliability
• Available from: LimitState

Materialise’s well-regarded analysis, repair, and optimization tool has evolved rapidly in recent years. They continue to offer a free STL viewer, MiniMagics, which also performs detailed analysis of STL issues. However, their sophisticated repair and editing functionality requires a license.

Two very capable and advanced versions are on offer: Magics Essentials provides functionality best suited to small businesses and keen hobbyists, while the full Magics suite offers additional advanced tools (including support optimization) for busy professionals. Both of these are integrated within a wider suite of software supporting every aspect of modern, commercial 3D printing.

Both tools have three modes of STL repair: full auto, a fix wizard (which guides users through each step), and a full manual tool. Other features include a “shrink-wrap” function to provide minimal thickness to parts and fix other surface issues, and a host of other enhancing and optimization capabilities. The user interface is well designed and easy to use, and levels of user satisfaction are very high.

• Platform: Windows
• Cost: $150-$190 per month for Essentials; price upon application for Standard or Advanced; 30-day free trials of all versions
• Ideal for: Professionals and businesses requiring a powerful solution and the potential for expansion
• Available from: Materialise