Roto-Molding a Concrete Dodecahedron

For a college project, my friend Jakob and I experimented with various methods to manufacture dodecahedrons. We decided to try roto molding one out of concrete, aiming to make it as lightweight as possible. Rotomolders spin a mold containing a resin or other liquid material along two axes, ensuring even coating across the entire interior surface. For this project we made a few molds and iterated on a roto molder.

We 3d printed the mold and made the rotomolder out of things we found around Jakob’s dad’s shop in an afternoon. Most of the challenge of getting a successful run out of the molder was determining the correct gearing for it. We knew the material had to be rotated such that every portion of the mold had an opportunity to have the concrete flow into it. However it took a couple attempts to figure out what relationship between the spin rates of the axis aught to be.

First Gear Ratio

Our first gear ratio choice was 36 to 37 or n to n+1. We figured this was ideal because it would take 36 rotations of the molder to sync back up to the first orientation it started in. Unfortunately, this did not work. During the time it took for the mold to reorient to the starting position it would favor one section of the mold.

Second Gear Ratio

We then switched to a 36 to 17 or n to n/2-1. This changed the arrow’s trajectory so that it no longer favored one side of the mold as it moved through the rotation.

Conclusion

The goal of our project was to rotomold a 4-inch diameter dodecahedron that was as hollow and lightweight as possible. Rather than using traditional materials like resin or plastic powder, we opted for quickset concrete, which allowed for rapid testing. If the dodecahedron had been solid concrete, it would have weighed over 2000 grams. However, using the rotomolder, we made one that weighed less than 100 grams.