(1) 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 Maya and 3ds Max, an artist almost always 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 won’t work for 3d printing:
Unless you intend to glue the parts together after the printing process is complete, the model will need 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 originally created with 3D printing in mind.
Thinking back on some of the architectural and mechanical models I’ve made, there are a few that have so many pieces I’m not sure I’d even want to attempt to clean them up for the printing process. If you’ve got an old model that you absolutely want to print, you’ll have to bite the bullet and start merging vertices & edges until you’ve got a solid contiguous mesh.
If you’re just now starting a new model that you eventually plan on printing, remember to be mindful of your topology and everything should be a-OK.
(2) Hollow the Model to Lower the Price
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 will be interpreted as solid unless you prepare it otherwise.
To make your model hollow, follow these steps:
- Select all the faces on the surface of the model.
- Extrude the faces along their surface normal. A positive or negative extrusion will work, however negative is probably preferable as it will leave the appearance of the exterior surface unchanged. If you’re using Maya, make sure you have the option keep faces together checked (this should be checked by default).
- Examine the surface. Make sure no overlapping geometry was created during the extrusion and fix any issues that may have arisen.
- 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 be mindful you don’t go too small. Most vendors have a minimum thickness that they’ll specify on their site.
- Create an opening in the bottom of the model so that excess material can escape. Make sure you 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.
(3) Eliminate Non-Manifold Geometry
- Non-manifold geometry is technically defined as any edge shared by more than two faces.
This issue can occur when a face or edge is extruded but not re-positioned—the result is essentially two identical pieces of geometry directly on top of one another. As would be expected, this situation ends up being quite confusing for 3d printing equipment.
A non-manifold model will not print correctly.
One very common cause for non-manifold geometry is 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 re-positioning of the face or edge.
Therefore, to undo an extrusion, the undo command must be given twice. Failure to do so will 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 quite easy to miss. Be aware of the issue, know how to spot it, and be sure to 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 can be 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’ve probably got non-manifold geometry. Try selecting the faces and clicking delete—sometimes this is all it takes. If this doesn’t work, you can also try the Mesh > Cleanup command, making sure non-manifold is selected in the options box.
Although extrusion isn’t the only cause of non-manifold issues, it’s probably the most common.
(4) 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.
Fixing surface normals:
Thankfully, it’s quite 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 will be 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 are slightly different in every 3D software package, but should be readily available in your application help files.
(5) Convert Your File (and other considerations)
The most popular printer, 3Dmatic accepts the following file types: STL, OBJ, X3D, Collada or VRML97/2.
Notice that standard application formats like .ma, .lw, or .max are not supported. From Maya you’ll 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’ll accept, so now is a great time to explore your options and decide which printer you plan on using (if you haven’t already).
How to check if your model is water tight / manifold
Netfabb can also perform some automatic correction of small errors in the file. Select the part and click on Extras > Repair Part. Netfabb will then take you through a sequence of tasks that may result in the error being corrected.
Software to prepare files for 3D Printing
Autodesk Meshmixer is a good free software for creating 3d models.
Meshlab is an open-source not-for-profit software. Not recommended for beginners.
Pleasant3d is a home-grown free software for Mac users only.