Help! My holes closed up!
In this tutorial, I'm going to show how to fix the situation where you expecteda hole, but instead got a surface. I'm going to be using Netfabb Basic (available for free from

Click any image to zoom.

Step 1I have a model that I uploaded to Shapeways, and I was expecting a donut shape, but instead it turned out to be a closed cylinder.
Step 2 Load the STL file into Netfabb.  Observe the red red exclamation point - there is some kind of problem here. Click the red + sign to start Repair mode.
Step 3Note that the internal edges are outlined in Yellow, and that you can see the inside surfaces of the model in red. This means that there is a HOLE at that point, and it must be sealed up before you can 3d-print the model.   Several programs, including Shapeway's Mesh Medic will repair these two holes by taking the shortcut of sealing the circle - it won't take the option of making it a donut.

Click the Add Triangles icon.
Step 4 Select one of the vertexes by left-clicking on it. The line should turn from yellow to blue.
Step 5 Drag your mouse across to a matching spot on the other side. A green shape will be displayed connecting the two sides together.  Often, this produces two triangles to make a rectangular shape.
Step 6 Left click to add the triangle(s). The newly added triangles should show up as pale blue.
Step 7 Add a few additional triangles around the edge so that your intended geometry is obvious.
Step 8 Now, use the "Close Holes" tool - it should follow your "hints" and heal all the holes - producing the desired donut.
Step 9 Click "Apply Repair" and then right-click the model and select "Export as STL".  Once you've exported the file, you can upload it to Shapeways, and Mesh Medic will be happy with your model.