Besides my casting box (http://www.thingiverse.com/thing:1240561) I used a casting ring for some models.

Now, as I had to change the FreeCAD file another time to print the ring at a different size, I decided to make an scad thing and to share it as a customizer.

Open the customizer (apps). Just change the hight and the diameter to your needs. Eventually you have to change the „tolerance“ if the two halves are too loose or too tight. There is no need to change the „wandstaerke“, which means thickness of the walls.

Print it and stick the two halves together. Mark them with a felt marker, so you can always put them together in the same position. Now you are good to go.

In short, what it´s good for: Put you model on a table, lay the first ring around it, fill it with molding sand, compress the sand, take up the form, stick the other half on and fill again with sand. Now take the halfes apart, remove the model, make a pour hole, maby annother smaler one for the air to come out. Stick the two halfs togeter again fix them with a clamp and pour in the hot metal :)