How to Prepare Your Model for 3D Printing

Hold your 3D model in your hands

Photo of a 3D resin print from Shapeways.
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Dolf Veenvliet.

3D printing is an incredibly exciting technology and getting to hold one of your digital creations in the palm of your hand is a fantastic feeling. 

If you want to print one of your 3D models so it is transformed into a real-world object you can hold in your hands, there are a few things you should do to prepare your model for 3D printing.

To ensure that the printing process goes as smoothly as possible and to save you time and money, follow this series of steps before you send your file off to the printer:

  • Make Sure the Model is Seamless
  • Hollow the Model for Lower Cost
  • Eliminate Non-Manifold Geometry
  • Check Surface Normals
  • Convert Your Model
01
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Make Sure the Model Is Seamless

When modeling for a static render, it's usually a lot easier to build your model out of dozens (or hundreds) of separate pieces. Hair is a perfect example. In traditional modeling packages like Autodesk Maya and Autodesk 3ds Max, an artist usually creates a character's hair as a separate piece of geometry. Same goes for buttons on a coat or the different components of a character's armor and weaponry.

This strategy doesn't work for 3D printing. Unless you intend to glue the parts together after the printing process is complete, the model needs to be a single seamless mesh.

For simple objects, this shouldn't be too painful. However, for a complex model, this step can take many hours if the piece wasn't created with 3D printing in mind.

If you're now starting a new model that you eventually plan to print, be mindful of the topology as you work. 

02
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Hollow the Model to Lower the Cost

A solid model requires significantly more material to print than a hollow one. Most 3D print vendors price their services by volume using cubic centimeters, which means it's in your financial interest to see that your model prints as a hollow figure instead of a solid one.

Your model will not print hollow by default.

Even though the model appears to be a hollow mesh while you're working in your 3D software application, when the model is converted for printing, it is interpreted as solid unless you prepare it otherwise.

How to Make Your Model Hollow:

  1. Select all the faces on the surface of the model.
  2. Extrude the faces along their surface normal. Either a positive or negative extrusion works, but negative is preferable because it leaves the appearance of the exterior surface unchanged. If you're using Maya, make sure you have the option keep faces together checked. It should be checked by default.
  3. Examine the surface. Make sure no overlapping geometry was created during the extrusion and fix any issues that may have arisen.
  4. Your model should now have an "inner shell" and an "outer shell." The distance between these shells will be the wall thickness when your model prints. Thicker walls are more durable but also more expensive. How much space you leave is up to you. However, don't go too small. Most vendors have a minimum thickness that they specify on their site.
  5. Create an opening in the bottom of the model so that excess material can escape. Create the opening without breaking the actual topology of the mesh—when you open a hole, it's important to bridge the gap between the inner and outer shell. 
03
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Eliminate Non-Manifold Geometry

If you're vigilant during the modeling process, this step should be a non-issue.

Non-manifold geometry is defined as any edge shared by more than two faces.

This problem can occur when a face or edge is extruded but not repositioned. The result is two identical pieces of geometry directly on top of each other. This situation ends up being confusing for 3d printing equipment.

A non-manifold model will not print correctly.

One common cause for non-manifold geometry occurs when an artist extrudes a face, moves it, decides against the extrusion, and attempts to undo the action. An extrusion is recorded by most software packages as two separate commands:

  • The extrusion itself
  • And the repositioning of the face or edge

Therefore, to undo an extrusion, the undo command must be given twice. Failure to do so result in non-manifold geometry and is a relatively common mistake for novice modelers.

It's a problem that's easy to avoid, but it's often invisible and therefore easy to miss. Fix it as soon as you're aware of the problem. The longer you wait to fix non-manifold issues, the harder they are to eliminate.

Spotting Non-Manifold Faces Is Tricky

If you're using Maya, make sure your display settings are such that a selection handle—a small square or circle—appears in the center of each polygon when you're in face selection mode.

If you spot a selection handle directly on top of an edge, you probably have non-manifold geometry. Try selecting the faces and clicking Delete. Sometimes this is all it takes. If it doesn't work, try the Mesh > Cleanup command, making sure the non-manifold is selected in the options box.

Although extrusion isn't the only cause of non-manifold issues, it's the most common. 

04
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Check Surface Normals

The surface normal (sometimes called a face normal) is the directional vector perpendicular to the surface of a 3D model. Every face has its own surface normal, and it should be facing outward, away from the model's surface.

However, this doesn't always prove to be the case. During the modeling process, a face's surface normal can accidentally be reversed by an extrusion or through the use of other common modeling tools.

When the surface normal is reversed, the normal vector points toward the interior of the model instead of away from it.

Fixing Surface Normals

It's easy to fix a surface normal problem once you know it exists. Surface normals are not viewable by default, so you'll most likely need to change some display settings to spot any issues.

  • In Maya, the easiest way to achieve this is to go up to View > Lighting and deselect Two-sided lighting. When two-sided lighting is disabled, any faces with their surface normals reversed are completely black.
  • Select these faces, go to the Polygons menu set, and select Normals > Reverse.
  • Checking your surface normals from time to time is a good habit to get into regardless of whether you ever plan on printing your models.

The instructions for fixing surface normals are similar in all 3D software packages. Check your software help files.

05
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Convert Your File and Other Considerations

The final step before you upload to one of the print services is to make sure your model is in an acceptable file format.

The most popular printer file types include STL, OBJ, X3D, Collada, or VRML97/2, but play it safe and contact your 3D print vendor before converting your file.

Notice that standard application formats such as .ma, .lw, and .max are not supported. From Maya, you either have to export as an OBJ or convert to STL with third-party software. 3DS Max supports both STL and .OBJ exporting, so you're free to take your pick, although keep in mind that OBJ files are typically pretty versatile.

Each of the vendors has a different range of file types that they accept, so now is a great time to explore your options and decide which printer you plan on using if you haven't already.

Popular 3D Print Service Providers

Popular online 3D print service companies include:

Before you decide which one to go with, it's a good idea to poke around each of the vendors' websites. Get a feel for the customer base they're targeting and look into which 3D printing technology they use. This may have bearing on where you decide to have your model printed.

When you've decided, read the printer's instructions carefully. One thing to look for is minimum wall thickness. Make sure to take into account the fact that if you're scaling your model down, its wall thickness will decrease. If the walls are acceptably thick in your Maya scene, but you set the measurements to meters or feet, there's a chance they'll be too thin when you scale the model down to inches or centimeters.

At this point, your model is ready for uploading. Assuming you've followed all five steps and any additional constraints from the vendor, you should have a good clean mesh in a format acceptable for 3D printing.