3D Printing Modeling Best Practices
Unlike 3D modeling for film or video, your models don't just have to look good, they need to be built a certain way so they can be physically 'replicated' in the real world with real world physics. Here are a few tips and best practices recommendations for when modeling items for 3D printing.
Models must be water tight. No holes. If the object is supposed to have a hole (like a piggy bank), then it must have an outside and an inside and a defined thickness (and that thickness must be closed water tight – no missing polygons or unconnected polygons).
The object must be what’s called manifold – which is an edge cannot share more than two faces. This sometimes happens during modeling when extruding edges vs faces, OR when extruding faces inwards but not up or down. This causes a shared edge among several polygons. 3d printers don’t know what to do with those.
Normals must be oriented in the correct (outward) position. In the piggy bank example – the outside edge normal must face OUT, and the inside surface normal must face IN.
Don’t make detail too thin. Everything must have a thickness that is more than 2-4 millimeters. When small thin details are printed, some printers will not print very fine detail. Especially if it is unsupported (sticking up by itself). So avoid antennae, hair, strings, or wires, etc. in your models.
It is good practice not to overlap/intersect geometry, but instead try to connect (Boolean) them. Example: if a gun barrel is sticking out of a tank top turret. Try to connect the gun to the tank top instead of just placing it through. 3D Printers sometimes get confused on what object its printing (the gun or the turret), and will sometimes build up plastic in that one area. Keep this practice to a minimum and you will have better 3D print results.
Another thing to be aware of is over-hangs. Because most 3d printers print like an ink jet (one layer at a time on top of the previous layer) – if you have an over-hang like a person sticking his arm out, it will require the printer to make a ‘support’ which is built into most slicing software. But, if you can keep that in mind when modeling it helps. Most 3D printers can print a 45 degree angle without support, they can also print spheres without the need for additional support structure.
Consider modeling something in 2 or more pieces that can be glued or connected together after printing to avoid having to use supports. ie: a Ship hull can be printed in 2 pieces and then glued together. Or in the example below, this chess piece would have needed supports to print the top. By separating them at printing stage, and gluing together after, you avoid the need for supports.
3D printers only print the data that is displayed as polygons. They don’t print Bump, Normal, or Displacement maps. If you create something in mudbox or zbrush, you will need to convert to polygons first. Some modeling software like Maya has a mode called Smooth Mesh Preview, which will show how it will "render". But, it will print without the smooth mesh preview and use the polygons.
Try to keep polygon count to a reasonable level (millions of polygons is very very heavy to print and will most likely crash most print software).
Finally, a note a bout size restrictions: Most desktop 3D printers can print an object at approx 6x6x6 inches, some a little larger, some a little smaller. Keep this in mind when creating a model. If you create a 10” action figure, very few desktop printers will print that size at the moment. If you need to print a larger item, consider splitting it up into several pieces and assembling after. NOTE - this is also a good idea for prints that take a very long time (over 10 hours) to print. Better to cut these up into smaller 2 hour prints. Nothing more frustrating then a fifteen hour print bombing in the fourteenth hour (power failure, etc) and having to start all over.
About the Author
Jerry Brown is a traditional illustrator turned 3D lighting and rendering veteran. He also provides complete creative production management services from concept to final product. Contact him to discuss your next creative project at firstname.lastname@example.org