“Watertight” is a term used to describe a 3D mesh suitable for 3D printing. In short, it means that there are no holes, cracks or missing features on the mesh.
The easiest way to describe a good mesh for 3D printing is to think of it as a skin, and filling the inside with water. It is important to create watertight meshes, so that it is clear to the 3D printer what is the “inside” (which needs to be made up of 3D print material) and what is the “outside”.
1) This mesh will not print at all. It is made up of several separate components (the ball, and 14 spikes) and none of these are watertight - the ball has missing faces where the spikes intersect, and the spikes are open ended cones. Even though these parts intersect, creating what looks like a good mesh from the outside, this part will be rejected by the printer.
2) This mesh will print - but it will cost you! All of the components are water tight, but your pricing will not be the cheapest. The pricing calculator uses the total volume of all of your parts. This means that where the spikes and the ball overlap, you will be charged twice - once for the whole of the ball’s volume, and again for the whole of the spikes.
Since there are intersecting but watertight models, your model will turn out correctly. This can be used to your advantage - often it’s easier to pay a little bit extra rather than spend a lot of time resolving particularly complex intersections. Just make sure you overlap the two objects as little as possible.
3) Perfect! Not only is this mesh watertight, but it contains no unnecessary geometry, and is all contained in one single mesh. This will give you the cheapest result.
How should I do it?
The goal is to create a mesh that is manifold and watertight, but is still easy to work on. In some software packages it might be best to join your components together into a watertight mesh after you have made them, or you might find it easier to create all the intersections of parts first before you get stuck into the details.
It is best to create a watertight mesh as you go.
It can be very tricky trying to clean up a messy mesh. If your design is very complex it can often be easier to rebuild it from scratch.
Many software packages are what we call "Solid Modellers". These are programs which automatically create watertight meshes. These packages include: 123D, Solidworks, Alibre and Wings3D. As such they are excellent tools for modelling for 3D printing - but you certainly don't need them if you keep watertight modelling in mind when using your favorite piece of software :)