In the world of 3D modeling and design, especially tri and quad poly model meshes (polygon mesh models) there is a chance that models can have gaps or holes. This is why we refer to the term "watertight". Essentially this means that your models must be sealed or closed. A good way to visualize this is to ask would it float or sink? A sphere will float but if you remove a small part of it, making a hole, it will take on water and sink.

In the world of modeling, models that have holes in them are called "non-manifold". This basically means they have a hole or holes in the model. 3D Printing requires that you produce a fully closed, watertight model. In Solid Modeling, having holes or non-manifold models is much less likely due to the design method. This means hat Engineering tools usually do not have this issue. Only models made in animation packages, 3D gaming packages and other mesh-modeling programs have this sort of issue.

BACKGROUND

CAD solid modeling has become the most popular method for preparing model data for RP. Surface modelers enable you to meet this requirement, but most users will agree that using a solid modeler is far easier. Without even trying, a good solid modeler will automatically produce water-tight models. The link between the CAD model and the RP machine is the STL file. An STL file is nothing more than a list of x, y and z coordinate triplets that describe a connected set of triangular facets. Also, it includes the direction of the normal vector for each triangle, which should point outward. CAD systems with an STL translator perform a surface tessellation and then output the facet information to either a binary or ASCII STL file. Binary STL files are much smaller and usually preferred, but ASCII STL files permit you view the contents of the file and even edit it if necessary. Certain RP systems, such as machines from 3D Systems and Stratasys, require support structures which support the RP part as it is being built. While it is possible to produce custom support structures with AutoCAD, don't. Most owners of RP systems use support generation software that automatically creates supports structures for an STL file. It is many times faster and it does a better job. 3D modeling and rapid prototyping go hand in hand. You cannot have a physical model from these machines without the virtual model. So it is important to prepare the model correctly. It must meet all of the following constraints:

Solid model. Solid modelers are better known in engineering environments. "Solid" means that the virtual model has real dimension or is not simply an open surface model.

Watertight .stl file. The .stl or stereolithography format is an ASCII or binary file used in manufacturing. It is a list of the triangular surfaces that describe a computer generated solid model. This is the standard input for most rapid prototyping machines. The model is required to be either a closed surface (sometimes called a "closed manifold surface") that is watertight (not missing any facets) or a solid model. In many cases, files created in standard animation and modeling packages (e.g., Maya, 3D Studio Max, AutoCAD, Lightwave) will need to be exported into a solid modeling environment (Rhino, Solidworks, Raindrop Geomagic) as an .obj, .dxf or .vrml and then saved as an .stl (binary).