How to Make (almost) Anything

Week 12: Composites

This week we learned about composites and how to use relatively accessible items to generate a fiber composite part. As the name indicates, you need at least two materials: a fiber and a matrix which binds the fibers together. We were provided with burlap fabric and epoxy resin. I decided to make a skateboard deck and started by making a 3D model of the shape which was then processed into a shopbot toolpath file.

After milling the negative shape out of a block of 2" high density foam, I traced the deck outline onto a stack of six burlap layers and cut them out with fabric shears. Next was prepping the work surface with some plastic film and cutting two sheets of plastic release film for the mold. One of these release sheets, the bleeder layer, was perforated with the "torture device" which would serve the purpose of letting resin escape during vacuum compression. The other non perforated release sheet went into the mold first, to prevent the layup from sticking to the foam. I then also cut a rectangle of breather material which is just a thin layer of pillow stuffing. This layer absorbs the resin that is escaping through the bleeder layer and provides a cushion for air to pull through as the vacuum is compressing. Once all of the materials were prepped, I mixed a few cups of resin with help from Daniel. During his recitation, Sam suggested not trying to mix all of your resin in one large container as this will accelerate the exothermic reaction and reduce the pot life, which is spec'd to be about 25 min. for the Entropy "One" resin. I then went to work with the wet lay-up where you saturate each layer of burlap seperately (using a plastic trowel to scrape out excess resin) and then lay them into your mold. Once everything is in place, you place the bleeder layer, then the breather layer, then place the entire setup into a vacuum bag. We used large clothing storage vacuum bags, which work perfectly for this process. After making sure the seal is completely zip-locked, you turn on the vacuum until all of the air has been sucked out and lock the valve. These vacuum bags supposedly provide about 15psi of compressive force and you should be able to see the resin seeping through the perforations and into the breather layer during the compression. The tack-free time for this resin is 4 hours and 24 hours for full cure.