Arguably one of the most important features of a BattleBot is the wheels. Without movement, your BattleBot is dead on arrival.
Caustic Creations decided to make custom wheels for Poison Arrow with a few goals in mind. Durability was paramount. Caster wheels have been commonly used on fighting robots because they are solid and usually coated in a grippy rubber, but the downside is that the hubs are usually a cast resin plastic not designed to take the abuse a BattleBot has to go through. Second, Poison Arrow needed more traction. Most caster wheels are designed to effortlessly glide across a surface, but they’re less effective at transferring power. Since traction in the Battle Box arena isn’t the greatest, Poison Arrow required a rubber wheel that would be softer. Using a soft rubber increases traction by allowing the rubber to deform more and make a greater contact with the ground. Lastly, style matters in BattleBots. Poison Arrow needed to stand out with a set of wheels that went with its mean green coloring.
Wheel construction starts at the core, which was made from solid nylon. Nylon can take a fair amount of abuse, it's easy to cut, and glues and resins can stick to it. The wheel geometry was chosen to maximize the grip of the rubber to the plastic. There is a channel milled into each wheel that lets the rubber wrap around the wheel and prevents it from coming off. This was an insurance policy in case the rubber lost adhesion to the plastic during a fight.
The next step was to build a mold. Poison Arrow’s mold was made from Delrin—a slippery plastic that holds its shape fairly well. Delrin machines like aluminum so it was possible to maintain the tight tolerances required to ensure the mold halves line up perfectly. Using Delrin proved to be a great decision because it didn’t require any mold release during casting. This was helpful in the next step: painting in the green rubber.
To get the green blob effect on the wheels two colors of rubber were used to mold. Each had the same base resin (PolyTek 75-70) and added dyes to give them green and black colors. At first, the green wasn’t allowed to fully cure because it had to bond with the black rubber. The sample was left alone to set until the consistency was similar to thick batter. It's always advisable to create a sample test batch to determine the correct curing times and hardness. By doing this you won't put finger prints on the finished product or disturb the actual part you intend to mold.
Once both halves had their fair share of green, it was time to close up the mold and pour in the black. At this point the green was pretty tacky and no longer of a consistency to move around in construction. If not enough time is allowed for curing, one color has a tendency to drip to the bottom, and more importantly, when the second color is poured on top, the colors will blend rather than keep well defined lines. That cool effect you were going for is ruined. Be sure to check your mold samples for consistency before pouring the second color!
Molding the second color presented real challenges. Once started, the show must go on. In this type of molding process, it's important to mix enough material ahead of time not only for the part you’re making, but also for a little extra to fill any voids that may appear. Luckily, Poison Arrow’s wheel mold geometry kept air pockets from becoming trapped, which made the process much easier. The material was poured in two sections: half at first, then the second half after a several minute pause to allow air pockets to escape. Sure enough, little cracks and small voids did appear where the mold meets, but this was expected, and the extra material filled in nicely. In molding this is called "flash". It's easy to trim after the part has cured. Each wheel was cured overnight in a warm environment, because heat helps accelerate the cure time.
Once everything was cured it was time to pop open the mold to reveal the finished product. You can see in this wheel there was a slight blending that happened between the green and black. It took some aggressive persuasion to open the mold, but patience is key to releasing the rubber without damaging it. Then it was time to make spares, spares, and more spares. Each set of tires only lasts about 2-3 matches, but in a competition as brutal as BattleBots, that’s something to be proud of.