In this Instructable I'll share what I learned about how to draw a CAD for a mold and 3D printing it out. I will assume you have access to and moderate proficiency with a CAD system (it doesn't matter what kind). For the printing, if you want to make your own but don't have access to a 3D printer, there are many services which print for a fee, or Instructables may be able to do that for you. The actual casting I will leave out as I was using an expanding polyurethane foam, which is not a generally applicable material.
Unfortunately my print picture set and my mold CAD picture set do not show the same mold. The pictures of the print, in the second half of the instructable, are of a slightly more complex mold, with a two-plane parting line. The process I outline is the same for both, however.
Step 1: CAD prep
First, ensure you have a "solid" object, rather than abutting surfaces. The object must be theoretically water-tight. Save it as a new file (I just call mine 'originalname-MOLD.3dm'). Next, consider the orientation of the mold. It doesn't have to be printed such that the object is in its expected upright position, it can be at any angle that is suitable for casting. Do you have a flat bottom, such that you can make a one-piece mold, or do you need a parting line, the join where the mold will come apart to extract the casting? If so, where is the best place for it? In general this should be at the widest part of the object, and it is often easier to extract the casting if the parting line runs more or less along the length, rather than width or height, of the object. If you have undercuts in the object, significant concave areas, consider placing the parting line through or near these. Good parting line placement is a bit of an art.
If the parting line has to be highly shaped, the technique becomes more difficult; for this simple tutorial I am assuming a straight parting line, either in one plane or only on a few intersecting planes. If you are planning cast with a flexible material such as silicone or foam, you can get away with fudging some of these guidelines a small amount as I did in the example.
Step 2: Mold box and parting line
With the box selected, 'diff' or 'subtract' your original (you may need to hide the box to select the original depending on how your CAD app works).
Now create the parting line. Make a plane, or several, at the widest point(s) of your object. Each plane must be bigger than the box, at the area where it comprises the parting line, or it won't "cut" properly. You may have a section of the original that needs to pass through the parting line plane without being cut by it (see the pics). Simply make a hole in the plane around it.
Now use your intersection tool, or cut or subtract or diff or whatever it is called, to break up the box by the plane(s). If you got a complete cut (don't worry if you have more than two pieces, you can rejoin them later), delete the planes.
Step 3: Registration keys and trimming
When they are placed to your satisfaction, union the keys with one side of the mold. Subtract (diff) them from the other side of the mold (this can also be thought of as subtracting the unioned side of the mold from the other).
Arrange the two parts next to each other in a printable manner, taking into consideration the undercuts and height of each section.
If you are paying by weight for material you may wish to carve away some of the excess mold material. Make a big box, 20% or so bigger than the mold itself, and diff/subtract chunks off the mold, being careful not to leave any walls thinner than about a half centimeter or so. This depends a bit on what material you're printing with and what material you're going to cast. It doesn't matter how odd the outside looks!
Step 4: Pour hole and sprues
Step 5: Printing
The Connex basically just requires you to import your .stl file into its driver program, which if I can get a couple screenshots I will add here. You get an estimate of the time required to print, and of the amount of print material needed. The Connex handles undercuts by using a supporting material, which is a waxy substance that must be scraped and washed off the completed mold. There are many different printable materials the Connex can print, depending on the setup of the machine. I used their "ABS-like" plastic, as it is the most heat-resistant and my casting material uses an exothermic reaction to cure (in other words, it gets hot while setting).