Blender's 3D Printing Toolbox
Blender is a popular 3D modelling suite that includes tools for modelling, animating, editing videos, creating games, and more. There isn't much Blender can't do - and that includes repairing your 3D models for printing.
Download the latest version of Blender for your operating system at Blender.org
If you encounter broken geometry out in the wild, or want to ensure your own models are good for printing, then Blender is more than capable of helping you create and repair models. It even includes its very own toolbox for 3D printing.
If you've never used Blender before you may want to look into some of the tutorials online. Blender can be a little different than other applications you may be familiar with. In particular, the left mouse button moves the 3D cursor while the right mouse button is what you use to select objects in the scene - this is probably opposite to what you may expect at first. The menus and buttons are selected with the left mouse button. If you accidentally move the 3D cursor you can ignore it - we won't be using features that rely on the cursor in this tutorial.
The first thing we'll need to do is enable the 3D Printing Toolbox. By default, the plugin is disabled but is included with the download of newer versions of Blender. We will need to go enable it. Mouse over to "File" and "User Preferences" or press Ctrl+Alt+U.
We're interested in the Add-Ons. Click the "Add-ons" tab at the top of the window. Use the search tool on the left to search for 'print'. You should find 1 result for Mesh: 3D Printing Toolbox. Click the box on the right of the search result to enable the add-on.
Once the add-on is enabled you should see a small tab on the left panel titled "3D printing". If the left panel is not visible press 'T' to toggle it.
Now that the 3D Printing Toolbox is enabled let's get started trying out some of the tools in our new toolbox. First, set Blender to "Edit Mode" mode by clicking "Object Mode" at the bottom panel and selecting "Edit Mode".
The cube is now an editable mesh. Edge, vertice and face indicators now appear on the model.
Since Blender's test cube is a complete and correct mesh we're going to have to make some small modifications to use it in our example. With the cube still selected (press A if it is not selected) look for "Subdivide" in the left menu. Press "Subdivide" a couple of times to liven up our cube.
Now that we have more faces to work with let's break the model a little. At the bottom of the screen near "Edit Mode" you should find 3 little cubes. One cube has only a corner vertex selected, the next has an edge selected and the final cube has a face selected. Click the cube with the face selected to put our selection cursor into face selection mode.
Once in face selection mode choose any face you'd like on the front of our cube.
We're going to intentionally break our geometry. 3D models are made up of vertices that are connected by edges and those edges create a face. The face is what you see when you look at a rendered 3D model. Faces in models tell our slicing software where the model ends (and where the plastic should go). A non-manifold model is a model that is incorrectly hollow. A manifold mesh is sometimes called "water tight" or similar to suggest that it has no holes.
To cause our mesh to become non-manifold (can't hold water) go ahead and press the delete key.
At the moment, we're using a basic example cube. It's very obvious to us just looking at the broken model we've created that there is something not quite right. In larger and more complicated meshes it can be extremely difficult to locate the holes in the mesh that will cause us headaches later. Luckily, Blender's 3D Toolbox has a tool for this.
Click the "3D Printing" tab on the left panel (press T if the panel has disappeared). There are several configurable checks available. We're going to click "Solid" at the top to check if our mesh is manifold (can hold water). It isn't. At the bottom of the panel (you may need to scroll the bar down) you should see an "Output" section with a button showing "Non-Manifold Edge: 4". Clicking that button should highlight the offending geometry.
Now that the problem area is clearly indicated there are a few options we can use to solve it. We could change to "Vertex Selection Mode" and merge the vertices together to close the hole. We could also merge two or more edges together. Since our problem is simply a missing face we will take the easy option and simply restore the missing face by pressing 'F'.
Well, that was easy! Our unprintable cube has now been restored to its full potential as a cube with help from the 3D Printing Toolbox!
Bonus Round - Overhangs
While we're here we can cover another common problem in 3D printing: overhangs. An overhang is an area of a model that extends a little too far out from the previous layer. Common results of problematic overhangs include "stringy" edges where plastic is extruded into open air unsupported by the previous layer.
Let's create an overhang. Select any face you'd like. Press the 'E' key to extrude the geometry. Move your mouse to extrude the geometry out. To make it a little easier you can constrain the movement to one axis - X (the red arrow), Y (the green arrow) or Z (the blue arrow). Press X, Y or Z to constrain the axis. You should see a helpful indicator showing which axis (or multiple axises) you have constrained the movement to. Press "enter" when you are happy with the results of the move.
Once you've successfully broken your model for a second time by giving it an unfortunate overhang click the "Overhang" check in the 3D Printing Toolbox. This option is configurable and, by default, is set to 45 degrees. Again, we should see some output and be able to click the error report to select the offending geometry.
There is a reason the default is 45 degrees: most printers can achieve pretty decent results with plastic that overhangs roughly 45 degrees (or less). To make this cube printable again we can reduce the overhang.
We have quite a few options. I'm going to remove the face below the overhang (select the face, delete it), then select the Vertex Selection Mode on the bottom tool bar. Select the vertex closest to the overhang and then, while holding shift, select a vertex at the bottom of where the face we deleted used to be. Merge the geometry (Alt+M) and select at last. Blender merges the vertices together. Repeat for the other side. When done, perform a second check for Overhangs in your model using the 3D Printing Toolbox.
You may notice we still have an error showing Overhang face: 16. This is fine - the 16 faces on my model are the ones pointing directly down. Depending on how many times you chose to subdivide your cube you may have fewer or more overhang faces.
Overhangs and non-manifold errors aren't the only errors in 3D printing. The 3D Printing Toolbox offers quite a lot of features. You can use it to calculate Volume and Area for a model you plan to have printed by someone else. Reducing edges that are too sharp can restore geometry to your print that mysteriously disappears during the print. A convenient "Export as STL" button is located right on the 3D Printing Toolbox panel providing a nice shortcut.
If you encounter broken geometry out in the wild, or want to ensure your own models are good for printing, then Blender is more than capable of helping you create and repair models. The 3D Printing Toolbox makes identification and repair of problem areas pretty easy. Don't forget to explore the mountains of features Blender offers.