Zoox is a leading automotive company in the race to deliver an entirely autonomous vehicle to therobo-taxi market. They also happened to be Common Fibers' very first fabrication contract! This post details our process for making a set of fenders for one of Zoox's early prototypes, however we later built an entire chassis for them as well.
We are lucky enough to own a large scale hobby laser. We used this to cut cross sections of the part every two inches. The cross sections were made in the CAD program SolidWorks. Without a hobby laser, these cross sections could be printed on paper at full scale and cut out with a jigsaw.
Here is the full lattice with the shape taking form. This took about 8 sheets of 1/4″ MDF from Home Depot ($6.50 each). All of these are glued to another sheet of MDF and glued at all the joints.
Once the lattice was complete, we used Great Stuff expanding foam to fill in all the slots. Although it wasn’t ideal, it worked! I would recommend a denser expanding foam if you have time to order online.
The foam will likely expand out of the top of the form and you will need to shave it down to just below the top of the wood. I would recommend a 1/4″ gap that can be filled with Bondo in the next step. This will provide a nice solid surface that won’t be effected by gradual changes in the foam over time.
We used almost two gallons of Bondo to cover the foam and fill in the surface of the mold. This was probably the longest part of the process and was certainly tedious. This adds a lot of weight to the molds, but also makes them very durable.
Once the mold has been covered in Bondo, multiple layers of sanding primer must be sprayed on the surface to create a nice smooth finish. I recommend Duratec or Orca grey sanding primers. You can use different dyes between each layer to distinguish how deep you have sanded.
After each layer of primer, we sanded the mold and patched it with more Bondo until we created a perfect surface. This process usually needs to be repeated 3 to 4 times to get the surface part ready.
When spraying the final primer coat I recommend mixing it 1:1 with Duratec Hi-Gloss Additive. This creates a layer that can be buffed to a reflective surface ready for laying up a carbon fiber part. Here is the mold ready to go!
With the mold complete, the fun part can begin! We applied 5 layers of wax to the molds letting them off gas for at least a half hour between each coat. While we waited for this we cut the carbon, vac-bag, breather bleeder, and release cloth materials using a stencil.
Using a $125 vacuum pump from Amazon we were able to pull 30 in Hg on both parts simultaneously. The parts will need to cure for at least 8 hours, but this is largely dependent on the type of epoxy resin used. Be sure to triple check for leaks as they will greatly affect the part’s surface finish. Pro tip: if the pump is creating lots of smoke and noise, there is a leak in the bag that needs to be fixed.
Fresh out of the mold, the parts should look ready to go! This is the sign of a good mold and layup. Here are our first four parts prior to getting trimmed. We ran tape lines to determine where to cut them down. We then used a “fart saw” or reciprocating air saw as well as a Dremel to cut the parts to shape.
Sand down all the edges to a bullnose with 400 grit sandpaper to avoid splinters and buff and polish the outer surface until it has a mirror finish. The pictures below are the final parts ready to go. All steps you saw took 13 days to complete with one to two people working 5-8 hours a day.