These are the containers that the plyurethane mold making material comes in. I used Synair's Pour-A-Mold (PAM-111) which I purchased from Coast Fiber-Tek, but most places that specialize in plastics also carry this or similar materials. Industrial Paint & Plastics also carries a variety of mold making materials.

Quentin Cobb says that I used PAM-2020 and not PAM-111 (apparently the 111 is white and the 2020 is transparent, but I can't find any reference to PAM-2020 on Synair's website). Anyways, the containers say 2020 on them.

At the store they gave me SynLube 1000 as the release agent, and according to the web site, this isn't the correct stuff. The web site mentions SynLube 531 which is a wax based release agent. The SynLube 1000 didn't seem to work particularly well, so I think I'll need to look into this further.

See below. The 531 works much better. Bigger...

I went to Home Depot with the tire, looking for round things which might be used to make a mold. The diameter of the wheel that came with the Mark IIII is 2.58". I found a black end cap with an outside diameter just a bit bigger than the Mark III wheel. The black end cap has an inside diameter of 2.38" tapering down to 2.21", and the outside diameter is 2.72" tapering down to 2.70". I'm not sure exactly what size of pipe this corresponds to. I also found the white piece, which had an inside diameter a bit bigger than the outside diameter of the black piece. Bigger...

I cut a piece of clear plastic about 4" sqaure, and drilled a hole in the center and each of the 4 corners. The holes were big enough for a 4-40 screw. The corner screws were just to act like feet so that the whole thing would sit level when the center screw was installed.

In order to locate the center of the black piece, I created another piece of clear plastic, marked the center and drew a circle which matched the inside diameter of the black piece. After cutting this out and ensuring a snug fit, I drilled a very small hole in the center. I was then able to rotate the clear plastic with the black piece fitted to ensure that the hole was in the center. Once satisfied, I drilled the center out to accomodate a 4-40 screw.

Screwing the two clear plastic pieces together allows the black piece to be positioned. I then glued the white piece to the square plastic piece, leaving a space all around the black piece. Getting the white piece centered isn't important, and for that matter it could be any shape that is desired. The round clear plastic piece is missing from the picture (just in case you were wondering).

I sprayed the black piece with mold release and put some petroleum jelly on the portion that touches the clear square plastic piece. I then mixed some mold making material and put it between the black and white pieces.

Don't try to use heat to accelerate the drying process, it just causes lots of air bubbles to form! After about 6-8 hours the black piece (and the clear round piece) can be removed. I let the mold cure for a couple days after that. Bigger...

When mixing the two portions of the mold making material together, I found it easiest to use one of those small mixing cups that holds 1 fluid ounce (about 30 ml), and poured both components into the same container. If you use separate containers you need to compensate for the stuff that gets left behind. If you don't get the mixture at 1:1 then it won't set properly.

Due to the difficulty I encountered getting the black piece out of my mold (probably due to using the wrong mold release), I decided to line the inside of the mold with aluminum foil. You can see a piece of it here. I used some of the petroleum jelly to "stick" the foil to the side of the mold. The picture I took here was after the I finished both tires, so the foil was just placed there.

I also filed eight half round pieces around the edge of the wheel (not present in this picture, but visible in the next picture). I mixed the mold making material (it only takes 10-15 ml) and poured it using a toothpick across the top of the mixing container (the mixed material flows along the toothpick, allowing precise placement. When mixing the two portions, don't be too vigourous. You don't want to introduce air bubbles. Bigger...

This is the finished tire. The little ridges you see (visible in the lower left corner) are from me sawing excess material away with my X-acto knife. This stuff is hard to cut (not because it's hard, but because it's too flexible). You want a super sharp blade!

The tires pick up any dust they come in contact with, so I use rubbing alcohol to clean the tires between rounds.

The tires are sticky enough, that if I leave Marauder with clean tires on a flat surface for about half an hour, the tires "stick" to the surface and you need to carefully peel away the tires from the surface. I normally leave the tires dusty, or rest Marauder on it's back. Bigger...

The mold pictured above was created before I got my Sherline mill and lathe. Since I was going to make some more tires, I decided to make a new mold. I started using some purple UHMW that I picked up at Lee Valley. I cut the very rough circle out by hand using a scroll saw.

I knew the mold was going to be too big to put in the Sherline chuck, so I mounted it to the faceplate. Bigger...

Here you can see the rough circle mounted on the lathe. Notice how little clearance there is. Bigger...

After machining, it looks much nicer. Bigger...

I then mounted the face plate on the rotary table in order to drill six mounting holes around the outside edge. These were tapped for a 4-40 screw. Bigger...

I made a clear cover out of some plexiglass. Once the general round shape was roughed out on the mill, I cleaned it up on the lathe. Bigger...

Here are all of the pieces required for the mold. The four brass bolts are screwed in from the back into the 4 holes visible inside the mold. These allow the tire to be ejected once the polyurethane has set. Bigger...

This is the assembled mold with a tire blank inserted. Polyurethane is injected into one of the holes on the top, and the second hole allows the air to escape. Bigger...

This is the color of the polyurethane when it's mixed up. I typically only make 10-15 ml using little mixing cups like these. For these small quantities, I find it easier to measure by pouring both components directly into the mixing cup rather than trying to measure them separately and then mix them together. Bigger...

Here's what it looks like when the polyurethane has been poured. The cloudy appearance of the plexiglass is because both sides have been coated with the mold release. Coating both sides makes removing the overflow easier. Bigger...

This is the "proper" mold release and it works much better than anything else I've used. Bigger...

If you let the polyurethane in the syringe cure, you can pull the plug out and reuse the syringe later. Bigger...

Some pictures of other molds I and others have created. Details...