Thermoforming (Vacuum Forming) is a fast, easy, and cheap way to form sheet plastic into useful shapes. Widely used to produce costumes, masks, theatrical props, enclosures, and packaging, the heated plastic sheet is drawn down over/into a mold using a vacuum.
This mold can be made of many different materials. At the high end, metal is the longest lasting. But wood, high-temperature epoxy, and even plaster and modeling clay (for limited runs) can be used.
If you have access to a 3D milling machine (or 3D printer), plus skill and access to 3D software, you can create a 3D model and then an actual 3D mold directly from your model. You'll may need to add vent holes, but the mold should be largely useable as is.
However, and the point of this Instructable, If your design consists of basically flat surfaces at different heights, a fast way of accurately making thermoforming molds (especially multiple copies) is to use a laser engraver to cut layers out of acrylic, then glue them together in a stack. The result can even include vent holes.
In this Instructable, we'll use the example of a Power Supply for a Ferrous Gentleman (from the third motion picture). First, the design will be created in a vector graphics program. This will be separated by height to produce separate cut geometries for each layer (including vent holes). After cutting, the layers are glued. Finally, draft is added (if required).
Step 1: Create the (vector) design
Begin by creating a vector drawing of the model. Tools such as Inkscape, Corel Draw, Adobe Illustrator, and LibreCAD could all be used. In some cases you may be able to use a bitmap image (e.g., photo) as a guide when creating your 2D model.
Simplify when possible. For a positive mold--that is, where the plastic is formed down over the mold--you may not get a lot of detail on the outside. Consider too that you must have vent holes to create any features that are concave. If the air is not being drawn out, the plastic won't just droop into the feature.
Acrylic is most easily found in 1/8" (actually often a little thinner), and often 1/16", 3/32", and 1/4". I've found that it is often easier to just settle on one thickness, if possible, so you can cut a bunch of layers out of the same thickness.
If you have not used the Layers feature of a drawing or drafting tool, now is the time to use it. As you refine your design, you may move elements between layers, but it's useful starting off with an idea of the layers early on. The Layers feature will let you turn on and off visibility between layers, and this will be helpful in letting you locate vent holes (the tiny holes in the figure above).
What size vent holes? A common guide is that the diameter of vent holes should be no more than 1/2 the thickness of the plastic you are forming, but larger may be necessary. That's the diameter at the top of the mold--below the vent hole, the hole should be as wide as you can manage.
In the mold design here, I could probably have added a lot more vent holes in corners. It's not like it takes a lot of effort (each hole cuts very quickly on the laser), and probably made them larger.
I also experimented with using very large "vent holes" to create features (bolt pattern) on the top ring. It worked well.