Making Cast Mold Parts

I was looking around CD and notice old posts on how teams made custom wheel hubs either by 3D printing, machining them by hand and using cast molds . I am curious on how to make parts with cast mold and with what I read was Urethane and Fiberglass I think.

A couple years ago 192 experimented with molding Urethane wheels. We tried both with fiberglass, based on the same posts you were looking at I believe [edit: Andrew linked to it below], as well as without fiberglass, based on 118’s 2013 wheels. However neither project got to the point of testing the wheels on a robot.

We made the wheel masters first by machining a block of HDPE and then by 3D printing them. The molds were made by pouring silicone rubber around the masters in a mold frame. Then once the molds had cured we could pour in the urethane plastic. We used only single part molds so one face of the wheel would be rough and machined off after it was taken out of the mold.

We ordered all of our plastic from Smooth-On. They have a huge selection of different silicon rubbers, urethanes and anything else you need. We used Mold Max 30 silicon rubber for the molds, and Task 9 and task 3 for the bodies of the wheel. We also experimented with some urethane rubber for tread trying to copy 118 but we were less successful with that.

Smooth-on also has a bunch of videos about how to use their products that are really helpful when trying this stuff for the first time.

Let me know if you have more questions or would like more detail about what we did.

We made custom intake wheels this year using a casting process.

The original positive wheel model was 3D printed and then hot glued to a piece of foam board and then a ring from a 2L soda container was glued around it to create a mold frame. We then poured in Mold Star 16 fast from Smooth On into the mold frame creating a silicone negative mold.

To make the final pieces we used IE-3075 from Innovative polymers which is miles stronger than the Smooth-On products and is incredibly rigid. Looking back on it we should of used TP-4052 which has a higher impact strength but is slightly more flexible as we did have some issues with chipping on the thin side walls. We followed theprocess detailed here for pouring the urethane. I made my own vacuum chamber from a old pressure cooker to degas the urethane.

The tread in the picture is urethane made by a local urethane company. They were able to cast a long tube of urethane in a size and color of our choosing. The company then cut the tube into strips on a lathe using an exacto knife as a cutting tool. It ended up costing us about 6$ per 1" wide tread. While treads can be cast at home the hot pouring processes that the manufacture uses creates a more durable tread for the equivalent durometer.

Whats 118’s wheel from 2013?

In 2013 Team 118 used two part molded wheels with molded tread on their robot Apex. They also used molded wheels in 2012 with the tread molded directly to the body of the wheel. The 2013 wheel was an iteration designed so that when they wore out the soft tread they didn’t have to replace the whole wheel. They used Task 3 urethane from smooth-on for the body of the wheel and two different types of urethane rubber for the tread.

O I thought you were talking about their shooter wheel which I was curious of the material.

Oh, both their shooter wheels in 2013 were nasa packing foam. I think they wore out about every 10 matches. (I may be wrong on that lifetime)

where do you buy that stuff?

Did you mean nasal packing foam?

Thanks for the ideas!

I have posted about it before and am still planning on making a whitepaper about it, but here are some photos of my wheel making endeavors with a short description with each.

Can you explain how you made the molds? Were they also done on a mill?

I’d love to try doing this in the fall but I basically would need a step by step guide.
Like after doing a test model and understanding it then customizing and making other products would be doable.

The mold was made of silicone. The process was pretty similar to what Joey said. First you have to make a master wheel. The version 1 was 3D printed which is the orange one in the photos. It turned out to be not accurate at all so I then machined one out of a block of HDPE. That turned out to be nice and true. Once you have the master I like to glue it down to a piece of acrylic then pack an oil based clay half way up the mold (the 7th photo). After a coat of mold release the silicone (pink material) is poured in and allowed to cure. The clay is then removed and everything is flipped over inside the cylinder, I used a large piece of PVC (photo 8). After another generous coat of mold release, more silicone is poured in. Once cured, the two halves can be separated, the master removed, and urethane and be poured. Another thing to note is that all of the curing is done in a pressure chamber at 60 PSI to compress the air bubbles.

This is all very simplified and I will go into much more detail of how to use the process for more than just wheels in a whitepaper later this summer. I would be more than happy to answer any questions though.

This video might give you a good idea of how it’s done.

This is awesome! Thank you for sharing!

Check out theGuerrilla guide to CNC machining, mold making, and resin casting. Chapter 4 describes the resing casting process in a step by step.