Molding an Ergonomic Concrete Chair in 24 Hours

Concrete chair with the inside created with a foam product that is cut to form and then cast with concrete to make an ergonomic chair.

Chris Rousseau, manager of Hotwire Direct, shared with us his experience making a nicely molded concrete chair with the help of one of his company’s performance CNC foam-cutting machines. He measured different parts of his body to dial in the chair’s measurements and banged it out in just about 24 hours.

Hotwire Direct’s machines cut foam with, you guessed it, hot wire. Besides furniture, the cutters can make molds for architectural stucco shapes, sculptural work and 3-D signs, among other things.

A picture of the foam that has been cut for the structure of the concrete chair.

Here’s Rousseau’s story: “Morning coffee, if enjoyed properly, can be the moment of conception for many great ideas. A few months back I was sipping my mind into activity when I started to imagine a casual form. It was comfort. It was ease. It was man’s favorite medium of leisure: it was a chair.

“I recently returned from Arabia and wanted to keep the form as close to the floor without being directly in line with scorpions and rattlesnakes. I sketched that chair to the general shape, form and volume. When I realized coffee time was over, and I was satisfied with the sketch to the point that having it in my pocket made me proud, I knew I must take this a step further.

A close up look at the cut concrete foam.

“I got to work and didn’t want to do anything but make this chair, so I checked my responsibilities, decided they could wait, and didn’t clock in. I pulled the sketch out of my pocket going straight to work on a computer in an unused office, manipulating and stretching this continuous line in a drawing program until the general outline was achieved. I then took a tape and measured physical points on my body to make sure I was dealing with proper ergonomics. A bit of scaling and fine-tuning of curves and I was ready to cut a mold.

“I took the profile, translated it to machine code and produced the part in expanded polystyrene. Five minutes later I had my mold. I glued it to a sheet of plywood, shored up the outside of the mold (since I had cut the foam a bit thin), rubbed vegetable oil in the interior of the mold and started my mix.

Cut forms out of foam before the chair is created with concrete.

“I poured that day and demolded the next morning. (One thing nice about cutting a mold in EPS is that the waste of the mold is the form of the finished part. In this case the EPS was thick enough to actually sit on, actually, two grown women sat on it in Las Vegas at the same time!)

“I did a bit of surface work and stained it. Voila: chair! Now I have a new place to enjoy my morning coffee.”

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