Composite coupon

For the last themed week, we first created a coupon composite mixing burlap and a matrix raisin. This was done in a group with Natalie, Robert and Max. Natalie wanted to do cup holders, and since this was a simple enough shape, we went with that as our coupon. It consisted in a simple extruded disk of 3.5in diameter. First, we milled a mold out of foam with the same diameter and depth 0.7in. Unfortunately, fabmodules are very slow, so even though the geometry was trivial, computing the actual mold took forever (i.e. 40min).

On the other side, we needed to laser cut slices of burlap, but the laser cutter in EDS was down (like the whole half of it because of some power issue during the weekend), and thus we went to the CSAIL shop where I had training. I designed a top cylinder with lots of holes to let any extra material through (in case, even though we also used a perforated bleeder film as well as a white buffer cloth). Cutting went fine for the acrylic. For the burlap, we used 50% power and 40% speed, which was slightly too fast, thus needed a second pass. On the later cuts, I tried various power variants (15%, 25%, 50%) and speeds (12.5%, 20%, 40%, 60%), and it was not clear which was best. In practice, it seems best to just use high power, but high speed (i.e. 50% power, 60% speed), and do multiple passes until the burlap visibly disconnected (it curls up). It takes a few passes (4-6), but this emits much less smokes than going slower.

When layout down everything, we used the desealant spray directly on the foam, which was our first mistake. The spray was corrosive and destroyed some of our mold. We added aluminum fold on top and re-sprayed that, which was fine.

Then we went on with mixing and applying the raisin (50g A, 21g B) onto the layers, one by one laid out in the mold.

Finally, we wrapped it into aluminum paper, clamped it, and let it rest for the night.

The next morning, we went to look at the result and unwrapped it, and the result was maybe slightly below our expectation. The green mass of material was not as shiny as we expected. At least, it seems solid.

Holy cross

I decided to cut my own design which I chose to be the next more complicated part beyond a circle or a rectangle: i.e. two overlapping rectangles, also known as a cross. I designed it in Onshape and created a similar holed top to the one for the coaster. Now, it's literally a holy cross.

I cut the corresponding pieces of burlap for a small version while also cutting the coaster. I eventually decided to also cut a much larger version (1 foot long) on Monday because I felt like it would be potentially nicer. And I was right. The small cross didn't really survive well whereas the large one ended up reasonable.

I didn't use any mold for that piece and clamped the layers between two pieces of de-sealed acrylic pieces with each one layer of perforated bleeder sheet. I also added one layer of breathing buffer, but I feel that this may not have been a good idea. Everything went nicely on the side without buffer and the side with buffer was a nightmare to clean afterwards.

When laying down the large cross, I mixed twice as much volume because I needed that much (i.e. 100g A, 43g B), and this ended up creating an overreaction. The material was getting way too hot, and it melted the cup it was in. This took some time (I was laying out the third layer of burlap when it became so hot I could not touch it anymore). It then solidified in a few seconds!

I then decided to go on with half of the volume and did two passes, thus avoiding too much reaction. I wrapped in aluminum fold and used several clamps.

The small cross had the same issue as the coaster with one acrylic layer getting stuck in it. Before this one was small and didn't any mold, the white buffer would be on all the sides.

The large cross came out much nicer than the small one. It has a few issues (some holes got filled with materials so it looks very bumpy), and the side with the buffer is messy, but the other bottom side is reasonable.