Assignment 8: Composites

This week, we learned how to make composite materials. I wanted to experiment with the type of material used for the fiber matrix. I really enjoy knitting and crocheting, so I wanted to see how a knitted or crocheted piece of fabric would hold up as a composite material.

UPDATE: Unfortunately, I failed to realize that part of this week's assignment was also to MAKE the mold! So I'll post an update when I've had another chance to practice using the ShopBot for large scale 3D machining.

UPDATE 2: For the final project, I used the ShopBot for large scale 3D machining of plywood.

Crocheting the Material

I decided to go with crochet rather than knitting, simply because I'm much faster at crochet. (In the future, a knitted fabric might actually be better since crochet produces large gaps between the "knots" of yarn, especially with a thicker yarn). I used a worsted weight wool yarn, since that's what I happened to have on hand.

For the sake of testing the yarn, I thought I'd make something simple. One advantage of being able to knit/crochet the fabric is that I'm not limited to zero Gaussian curvature. For that reason, I decided to make a bowl out of two crocheted layers.

The Mold

In the interest of time (this was a busy week) I decided to simply use an existing bowl as the mold, rather than machining one. I bought a cheap used glass bowl that I would stretch the fabric over. I wanted the inside of the crocheted bowl to have a smooth surface and the outside to have more of the original crocheted texture.

There were two things I was worried about using this method rather than a two sided mold:

  1. The release film is NOT a zero Gaussian curvature surface, so it would "scrunch" around the bowl shape. I was hoping that the crochet has enough texture that this would not be too noticeable.
  2. The bowl would not become unstuck from the composite. I sprayed the glass bowl with mold release, and I figured that if a foam surface covered in latex paint was slick enough to be released, then maybe so would the glass. In hindsight, this was pretty dumb, since epoxy is not only used for composites, but as a glue.

Vacuum Bagging It

Unfortunately this section is largely without photos since I had epoxy covered gloves on for most of this part. I used the Clear Coat Epoxy so that yarn would be more visible, and basically dunked the crocheted parts into my cup full of epoxy. I made my own vacuum bag (since I couldn't find the premade storage bags) using a piece of plastic and mastik tape. Since the only vacuum pump was in use, I thought I would use the shop vac.

DON'T DO THIS. The shop vac isn't meant for continuous use.

Unfortunately the shop vac is not meant for continuous use and after realizing this, I stuck my homemade bag into the storage bag, and tried to pull a vacuum on that with the one way valve. At that moment, Jessica saved the day by coming in and removing her part from the vacuum pump. So I cut a hole in my two layers of bag and stuck the vacuum tube in.

Unfortunately, the seal on the vacuum bag was awful, probably from all my MacGyvering with it, so when I release the vacuum pump, air went right back in. But it did have the vacuum on it for a few hours while the epoxy set.

The Result

First, the good parts:

Aaaand the bad parts.