Sherman Builders
Advanced modeling techniques for
vulcanizing rubber tires and tracks

Copyright JR. BATES
Scratch model builders are always stymied by the problem of tires or tracks that use rubber. Most end up using one of silicon varieties or RTV types. Some of these are good but can be too hard or too soft, and more than a few of these compounds if any are cheap in price. With a metal mold, some virgin (unvulcanized) rubber and a kitchen oven the model builder can produce his own rubber tires or tracks. The process is actually very simple and cheap compared to that of RTV's. There isn't nothing special about this process and actually it is a quick,  generally safe and cheap way of making rubber tires. But there is a catch!  Always a catch.  You need to use metal molds , this will require either a lathe for turning the molds for tires or a milling machine for milling out the shapes needed such as cleats on a rubber track tread or groove in a tire mold. and you need to be able to buy the rubber!
Some things you need to know about rubber.
Metal molds and rubber tires.
True rubber tires or tracks can be easily made with the right equipment. You will need access to a lathe (A milling machine if making a track link) to turn the mold. I use a three piece mold to compress and heat the rubber around the wheel hubs. This includes a top and bottom plate along with a center ring. Aluminum, steel or iron can be used mold. Two types of molds I use can be seen in the pictures below. These were made from mild steel, they are not heat treated although it would extend the mold life for many, many thousands or more castings. The untreated steel mold is more than enough for a modeler's purpose. The mold needs to have a few small holes drilled to allow for excess rubber to be push out of the mold, remember this will leave sprues coming off the rubber which will need to be trimmed, so keep it small as not to mar the tire surface too much.
1 : Rubber needs to be vulcanized to be of any use.
( a:) Vulcanizing is accomplished by heat and pressure
(b:) The heat needed is approximately  325 degrees F
(c:) The pressure is provided by the use of C-clamps
2: Once vulcanized, rubber cannot be melted again into another useable form.
(a:) This pretty much means those spare balding tires you have in the garage are of no use.
(b) Chemical changes in the rubber during the heating process prevent  the material from reversing back.      
Think of it as a cake. You use eggs, flour among other  items to make the batter. Once this has heated in a oven the once liquid batter has transform into a solid as it were. The eggs used in the batter cannot be removed from the cake once cooked and placed back into the shell.
The rubber is purchased through a local supplier of rubber products to industry. The rubber I use is the same used by re-cappers of auto and truck tires. The box label refers to it as 'Pre Cure Cushion Gum'. Now I understand from a few who live in Europe that the idea of recapped tires is a no no and in some countries illegal thus they doubt their ability to get any sort of gum such as this. I can't help you there however it still may be worth a check with your local industrial rubber supply house to see if anything of the like is available.
3: Typically, rubber hardness is rated on a scale referred to as Durometer.
(a:) A soft rubber would be around 25 to 40, i.e. RTV rubber molds.  A hard rubber would be around 90 such as the hard rubber heel of  of a men's dress shoe. (Many of the silicon rubber materials scratch  builders use typically cure around 90. ) Automobile tires would be around a 60.
The rubber  comes in a 18 inch x 50 ft roll, which can be cut. It is only about 1/16 inch thick so it can be cut with scissors easily. For the wheels, I'll cut a strip long and wide enough to wrap around the wheel. The rubber has a sticky way about it and will stick to itself like tape. The trick here is to use just enough rubber to allow you to get the piece into the mold and still allow for compression. If you do not place enough rubber around the wheels, the mold will not compress the rubber and it will be left too soft as it has not cured properly. Next I use a bit of talc powder to keep the rubber from sticking to the mold and acts like a release agent. The wrapped wheel is placed into the preheated mold and clamped tightly using C-clamps. The mold is placed an oven set at 325 degrees F for roughly 15 minutes. You may be tempted to boost the oven temp and time in cook time up, I would recommend against this. The amount of time heat and pressure all play a part in the hardness of the rubber but then the added chemicals to the rubber makeup also play a parts into the durometer factor.Too much heat and time in the oven will simply push the rubber past the curing point and start breaking down the rubber. Nothing worst than having the kitchen smell like the local race track on a Saturday night.
The picture on the left shows the bottom of the vulcanizing mold with the cleats for the track milled in.

This picture shows the second half or ring of the mold with the frame for the track on top.

These are the three parts of the vulicanizing frame shown together.

This is the finished product , a true rubber with a duro of about 55.Much better than any of the RTV's used
For treads, the center ring needs to be cut into a pie shape of 3 or 4 pieces. This allow for the treads to be cut into the inside portion of the ring. You will need a way to clamp this back together during use or simply weld the pieces back into one ring. Keep in mind, the mold will be reversed such you will be removing the metal for the tread instead of that for the groove between the treads. Most smooth tire molds can be turned on a lathe in a fairly short time depending on experience and equipment. You can always turn to a machine shop with drawings in hand but expect to pay any where from 150 to 4 hundred dollars or more for the mold. Not bad though if you are expecting to make quite a few tires or just want the best for your model.
The picture on the left shows the middle ring of the mold. Note the two small holes drilled into the side for the release of excess rubber.
This picture shows the bottom plate with a center pin for the aluminum wheel to mount on. This is very important for the rubber to be centered onto the wheel. Note the slight ridge for compressing the rubber.
The mold together, ready to be clamped and placed into the oven for 15 minutes at 325 degrees F.
The wheel after the rubber molding process has been completed.
Take me back to Sherman Builders.
A complete wheel ready for final detailing. Much better than hard RTV or too soft silicon.