I wanted to match the shape of the bases to the real thing, but it would take too long to sculpt 12 bases, and casting them would be too expensive. It looked like it could be vacuum formed. I've worked with plastics in the past, but never with a vacuum former, though I've seen some in action. I couldn't spend a lot of time or money building a vacuum forming machine for this project; I figured if I liked what it was capable of, I would build a nicer one after this project based on what I learned.
After researching what I needed, I found I could build one for free. I already had the necessary components in my shop. I figured many prop makers will already have these parts as well, so I am sharing how I did it.
Step 1: Gather Your Materials and Tools
* a shop-vac
* a heat gun
* some scraps of MDF
* some scraps of lauan
* spring clamps
The tools I used were a drill press, a table saw and a small hand saw, a combination square and a pencil. I also used a bit of glue and some grid paper.
You can substitute some of the tools used as well as the precise materials needed. The shop-vac I had cost $25 new, and the heat gun was $25 new as well, both from the big-box hardware store. So if you had to buy everything for this, it would only be $50 for the equipment and a couple bucks for the materials. Even if you never vacuum form again, you can still use the shop-vac for cleaning your shop and the heat gun for other projects.
The plastic I was using was a thin sheet of Sintra, which is a brand of expanded PVC. It was about 2-3 mm thick, and I cut it into smaller squares using a utility knife and a straightedge.
Step 2: The Top of the Platen
I drilled a grid of holes through the MDF. I taped a piece of grid paper to the top to make the layout easy. I used a 1/8" drill bit; you do not want the holes to be too big. I left an inch undrilled around the border; this is where the frame will sit.
I then drilled a large hole with a spade bit in another 8" by 8" square of 1/2" MDF. This hole was just large enough so I could shove the end of my vacuum hose in and it would stay in place. I attached this MDF square to the other side of the 1/4" strips, making a "sandwich". The empty cavity in the middle of the sandwich allowed air to be sucked into each of the tiny holes when the vacuum cleaner was turned on.
Step 4: The Frame
I actually plunge-cut the inner squares on my table saw and finished the cuts with a small hand saw. You can use a jig saw or whatever other tool you feel more comfortable using.
Step 5: Testing
I was using some Sintra plastic (expanded PVC) for my vacuum forming I had left over from a few years ago. I think it's about 2-3mm thick. I clamped it inside the frame and heated it up with my heat gun for a few minutes until the whole piece was very flexible. (While writing this Instructable, I also learned that Sintra can be softened by submerging it in boiling water for 10 to 15 seconds, which seems like it would heat it more evenly).
When the Sintra is fully heated, turn on the vacuum cleaner and place the plastic over your objects onto the platen. The plastic will stretch as it lays over the objects; when the frame touches the platen and creates a seal, the vacuum will be able to suck the plastic tightly over the pieces. Make sure to bring the frame straight down rather than from the side, or you may stretch parts of the plastic and cause it to tear.
I found if the plastic does not pull as tightly over some parts as you would like (either because the plastic was not heated enough in that spot, or because the vacuum is not strong enough), you can heat it up some more with the heat gun as the vacuum is still running. With plastic as thin as I was using, it was really easy to burn holes through the plastic if I heated up a spot for just a split second too long.
Step 6: My First "pulls"
I wanted a clean edge along the bottom; I raised the model up a bit so the plastic would be pulled underneath it a little bit. I used 3 tiny pieces of MDF to hold the model up without covering any of the air holes.
The model I built was just a bit too big for this vacuum forming machine. If I were to do this again, I would make the platen 2 or 3 inches bigger on each side.
A vacuum cleaner is a really weak source of vacuum. If I were upgrading this machine, that is the first thing I would replace. I found that after I placed the plastic over the mold, I could use the heat gun to heat the spots which did not get sucked down all the way, so I was still able to get all 12 pieces out of this machine.
I also made a video which goes through the construction of this machine as well as letting you see it in action.