What is Roto-Molding?
Roto molding is a manufacturing process that is used to produce hollow, one-piece parts. The process involves rotating a mold on multiple axes while it is heated and filled with plastic powder or liquid resin. As the mold rotates, the resin evenly coats the inside surfaces of the mold, forming a hollow part. This method is often used to create large, complex-shaped parts that would be difficult to produce using other manufacturing processes.
Overview
The ultimate goal of the project was to roto mold a 4” diameter dodecahedron as hollow as we could. Instead of resin or plastic powder, we elected to use quickset concrete. The low cure time allowed us to test quickly. Had the dodecahedron been solid concrete, it would have weighed >2000g. The roto molder produced a hollow dodecahedron weighing less than 100g.
Lessons
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First Mold
This mold was two parts and was held together with tape during molding. This mold took too long to close for materials with short working times. When taking apart the mold, too much stress was put on the dodecahedron and it broke.
Second Mold
The second mold was held together with screws and was faster to seal. This mold was also 12 different pieces. Each face of the dodecahedron had its own mold part. Because of this, each face could be removed independently during demolding and this was no longer an issue.
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We were trying to understand roto molders and use one to make a hollow object. Therefore, we made the entire roto molder in just a few hours intending to fix issues as they came up.
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The molder has to orient the mold such that gravity pulls the material into every crevice.
First Gear Ratio
The first gear ratio we selected was n:n+1. This kind of ratio should have maximized the number of positions that were at the bottom of the mold.
(Ratio used 36:37 )
Once the material began curing, it cured quickly. Because the roto molder incremented a small offset each rotation, it was only ever 10ish degrees different from the previous rotation.
Second Gear Ratio
The second gear ratio was 2n:n+1 (36:17). This meant that the molder incremented 170ish degrees each rotation. This meant both sides of the mold would be at the bottom of each rotation while still incrementing.
I did this with my buddy and fellow tinkerer Jakob. I had been looking for an excuse to make a roto molder and we got the opportunity to make one for an art class led by Professor Rosenstock. Professor thank you for letting my curiosity rein free.