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
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.
Step 2: Create the Layers
Since we're building up layer by layer, and need to glue the layers, keep the following in mind:
* do not have overhangs for the plastic to get under
* if you have a vent hole on an upper layer, you need an opening in layers below it (and aligned)
* if there is any issue of aligning layers with respect to each other, feel free to add tiny (engraved?) marks
The image above shows the different layers to be cut from 1/8" acrylic, from top (left) to bottom. The smaller image shows pieces to be cut from 1/16" acrylic. The bottom pieces (extreme right) of the mold are just rings for height with some support (probably not necessary for this tiny mold, only 3" or so in diameter), and are hollow because the stage the mold is set on has holes on perhaps 1" or 3/4" centers, which would be unlikely to line up with arbitrary vent holes if the lower pieces were essentially just copies of the middle piece. In addition, as noted earlier, vent holes should only be narrow at the mold face--below they should been as large as possible.
The PDFs attached are to the files shown, and include an alternative design for the bottom rings (thicker outside, easier to glue).
Step 3: Glue the Layers, Add Draft
This technique will work best with molds that are flattish. The taller/thinner, the more critical it will become that there is some degree of "draft" to the sides. No draft (0 degrees) means the walls of the mold are straight up and down. The problem is that with no draft, the mold may not be easy to release from the cooled plastic. For positive molds (plastic formed over the outside of mold), 5-7 degrees of draft is recommended. A negative mold (plastic drawn down into the mold) can have 2-3 degrees of draft.
We don't have control over the angle of the laser (you'll always get 90-degree cuts), but a quick pass on a disk sander could add draft to the outside. In some cases you may want to use a file to create draft on interior segments.
Step 4: Use It!
In general terms, you'll first locate the mold on the perforated platform. If the platform raises and lowers, drop the platform down. If the frame raises and lowers, raise the frame. Insert and clamp the plastic in the frame.
Turn on the heater (move the heater into position if needed). A variety of plastics can be used in thermoforming, but for your first tries hi-impact polystyrene ("HIPS") is a great choice--it is cheap, easily formed, requires no drying, and is widely available. With HIPS, it is often sufficient to use the droop of the plastic as a guide to when the plastic is ready to form. On a small thermoformer, a droop of perhaps 1/2" may be a good indication for thin HIPS, while a larger former may be associated with a larger droop. Each former will have a different sequence of steps for moving the plastic down over the mold, moving the heater out of the way, and turning on the vacuum or fan to draw the plastic down.
Just as we need vent holes on any concave features of the mold itself, there must be a way for air to be sucked out around the bottom of the mold, or plastic will not be drawn down around the mold. In this example, putting a few thin strips of acrylic under the mold raises it enough to allow the plastic to be pulled down well. If we didn't do this, the plastic would no be pulled down well, since early on the plastic would likely cover the vacuum holes on the platform leaving no where for the air to go.
Once pulled, remove the plastic. The mold may be retained in the plastic--gently push it out.
Since the molds can be made of scrap material, it's easy to make multiple units. If you need lots of thermoformed objects, forming over multiple molds at the same time makes a lot of sense.
That's it. For simple, "2-1/2D" objects, using the laser cutter to cut multiple layers is a fast and easy way to create thermoforming molds.