Sander van Vliet

@Fablab Amsterdam


Week 14 (4 may - 11 may)


This weeks class is about composites and the lecture took us through the different types of materials which can be used to create composite structures. Composites are a two component system where the load on the final product is shared between the characteristics of the two components. A known example is concrete which consists of concrete which handles compression well and steel reinforcement bars/rebars which handle tension well. We also learnt about various processes to apply composites and the safety precautions that should be taken. See the class lecture file of the composites lecture to get an overview.
The main way to form a product of composite materials is by using molds. We will be making the mold using the shopbot or the lasercutter. This time we will use dense foam or cardboard to create a mold from.
We will use epoxy resin in combination with fabrics like burlap and linnen to create a light weight yet strong product.


What to make

As an example we have this frisbee in the Fablab which is made of epoxy resin and 3 layers of burlap.

At first I had the idea of making a kite, but after checking existing kite plans I concluded that the foam we have in the fablab is not thick enough for this idea. Also, a kite made out of epoxy resin combined with fabric like burlap or linnen could become too heavy.
My second idea was to make a boomerang and I have this memory of playing with a three winged boomerang when I was younger. Soon Í found myself browsing for existing designs and their dimensions. The wings of the boomerang need to. be exactly evenly shaped and the bigger the boomerang, the further it will fly.
I found a few examples like these.

and decided I would go for a boomerang with a similar shape.

Designing the mold

Using Rhino I modelled a boomerang and attached it to a solid box. This would be the mold. I had shaped it so that the wing tips are standing a little bit upwards and the wings each have a side which is thicker than the other side. This is for giving the boomerang lift when it spins in the air.

boomerang mold source file - rhino

Making the mold

I used the Shopbot to make the mold from a piece of dense foam.
During the first run which was the roughing toolpath the shopbot made a strange noise and I could see something went wrong. It seemed like the machine had missed some steps because it was continuing incorrectly after it made the sound. I ran a surface job with the big 2" milling bit to get a flat area again.

Now I could start over, so first start the roughing job again with a 0,5" (12,7mm) four flute flat end milling bit and then the finishing job with a 5mm two flute flat end milling bit. This time nothing unexpected happened and the mold came out nicely.

Making the fabric parts

I did some tests to figure out good speed and power settings for the lasercutter and then I cut the linnen, burlap and paper parts with it. The hard part was to lay the fabrics flat on the lasercutter bed because there were some folding lines. I used metal weights (and carefully ran tests with the laserbeam turned off first) but could not manage to fully flatten the fabric down everywhere. Two pieces came out bigger then I had designed because of this so I redid those two and so in the end all parts were cut out nicely. I stacked them on the mold the way they will form the boomerang.
boomerang fabric source file - rhino

Applying epoxy resin and vacuumbagging the product

We used the Super Sap® CLR Epoxy System which is a clear, general purpose liquid epoxy resin and we used it in combination with with fast hardener (CLF). See product information here and here. This combination has a pot life of 25 mins and a recommended full curing time of 7 days.
We prepared the room by opening the window and protecting the table with foil and ourselves with a lab coat, gloves and a respirator. On the right image you can see I did a dry test where I put my mold including the fabric and paper parts in the plastic vacuum bag. Creating a vacuum like this shows you how the bag reacts and this is a good test to anticipate issues like folding lines in the plastic material of the bag and misplacing of the layers of fabric and paper.

Then I put some vaseline on a piece of baking paper and placed it on the mold to be sure the resin would not get attached to the mold. I forgot it, but luckily our local instructor Mickael reminded me I should also put some vaseline on the inside of my vacuum bag. My fellow student Eva did this for me while I was finishing the other preparations.
Eva had mixed the epoxy and the hardener and I set a timer for the pot life so we could check how much time there was left while we would be applying the epoxy resin. Now I could start with soaking the fabric parts with epoxy by using a paint roller and carefully placing them on the mold and stacking them on top of each other.

Then I put the colored paper on top and now the uncompressed shape of the boomerang has been formed. On the image on the left you see a white boomerang shaped piece of baking paper on which I had put vaseline. I put this on top of the colored paper layer to prevent the resin from getting attached to the vacuum bag.
The colored pins you see are preventing the sheet of baking paper from moving while I was placing the layers of material.

Then the last step was placing the mold in the plastic bag and creating the vacuum using a strong vacuum cleaner. This compresses the layers of material evenly while the epoxy is curing. I put tape on the opening of the bag to make sure the vacuum would stay as long as possible.

Taking the boomerang out of the vacuumbag

The next day I opened the bag and was quite happy with the result! The weight felt good and the thickness turned out to be like I wanted it to. The boomerang is a little bit flexible but still rigid enough.

Sanding the sides

Then I sanded the sides of the boomerang to make them smoother. I also tried to create a little bit more of an airfoil shape on the three wings by sanding because my mold did not give enough of this shape to the boomerang. Hopefully this will create enough lift to fly and who knows, return?!

I could apply some epoxy on the sanded parts to make the surface smooth again.

Update: it does fly!

I went to a park and threw the boomerang for a while. It does fly like a boomerang but it is very sensitive to the way that it is thrown. I managed to almost catch it twice (missed it by a few meters) and even hit myself in the leg once! Which is a positive thing in this case =)

A hero's shot!