April 2

April 2, 2006

Glassing the PETG canopy bubble and a gravity blown rear wheel fairing

Since the 1/16" thick PETG canopy bubble was so thin, I thought that maybe coating the inside with a couple layers of fiberglass would be easier than making a new canopy out of thicker PETG. I was also concerned that I wouldn't have enough heat to draw the 1/8" PETG down to the depth I needed. In the end, the fiberglass reinforced bubble works great - nice and stiff and strong, but a bit ugly because you can see the fiberglass cloth through the clear PETG. I may try to make a 1/8" PETG dome and see how they compare.

The gory details on the process of glassing the PETG is below.

A blown rear wheel fairing

During a very brief and somewhat fuzzy brain storm, I thought that I would like to try to use our super canopy vacuum box to suck a rear wheel fairing. I think the idea has great merit, but things didn't work out quite perfectly. I've had this idea for a very long time and really want to take some time to explore it further. Basically, the idea is to use some method of gravity blowing - or vacuum forming PETG to use as a carbon composite MOLD. The big reason is that the PETG finish is PERFECT mold material. It's almost IMPOSSIBLE to get epoxy resin & carbon or fiberglass to stick to it. And when you pull the cured composite part off the PETG plastic, the finish is spectacular - perfect smooth and glossy like the PETG. Plus if I can figure out a way to really control the vacuum/gravity forming process, it makes for totally smooth and sexy aerodynamic shapes.

I printed out an airfoil shape that would work to enclose the rear wheel and cut out a top plywood sheet for the vacuum box. Then we tried to pull a deep, narrow wheel fairing, but the vacuum wanted to suck the hot PETG out around the perimeter of the cutout. Basically like a bubble rather than a sunken shape. We resorted to a simply gravity pull (no vacuum) and the deeper the plastic went, the further away from the heat source it got, and the longer it needed to stay under the heat lamps. This caused the edges of the plastic touching the plywood to melt into the plywood creating a real big mess. The answer is to provide some heat from BELOW, but we managed to get a pretty decent shape blown.

After an experiment with a vacuum bag, it was obvious that the thin PETG mold was going to deform way too much to use a vacuum bag. So, we filled the shape up with expanding foam. The chemical reaction from the two-part expanding foam melted the PETG and deformed it anyhow. So, we pulled the Styrofoam plug out of the PETG mold and we now have a very decent foam plug to use as a starting point. I'll further shape the plug to get a perfect wheel fender shape, then coat it with a few layers of glass, then pull some carbon fairings from the glass plug.

Again, the details are all below.

Glass reinforced canopy bubble
1. I marked off the area where the hard glass cover will go. I used an erasable marker for this.
2. I set the canopy bubble back into it's frame in the vacuum box and masked off the window area that is to be kept clear
3. Epoxy resins DO NOT bond to PETG. I experimented with a test part first and used a variety of sandpapers to scuff up the PETG before applying the resin, and in every case, the cured fiberglass easily popped off. However, sanding the surface was better than leaving the plastic smooth, so I scuffed the surface with some steel wool.
4. I was not going to vacuum bag this layup because the pressure of the vacuum would distort the thin PETG bubble. Instead, I used plenty of spray contact cement on the bubble surface as well as on the dry fiberglass cloth. Even still, it ended up peeling up a bit on some corners after it was fully wetted out.

I am still trying to find a full-proof way of getting composites to stick to their form without having to use a vacuum. I have had fairly good luck with spray adhesive, but once the fabric is fully wetted out, it will lift the glue up in some places.

I have also had fairly good luck with coating the mold surface with resin and letting the resin partially cure. Then stick the composite cloth on and complete the wet-out.

After the fiberglass cured, it was very hard and tough.
To hold the canopy bubble down to the fairing shell, I cut out two Sintra plastic brackets.
Then I used a heat gun to gently curve them to the exact shape of the canopy bubble.
Clamps held them in place as they were epoxied to the fiberglass on the inside of the PETG canopy bubble.
I mixed some micro with resin and bonded two aluminum bolts to the rear of the canopy. I will fasten an aluminum bracket to this which will be used to hold the rear of the canopy to the fairing.
One draw-back to using the vacuum to control the pull on the PETG bubble, is it tends to balloon the sides out. I'm not sure I like this at all, as extra width is NOT required and will end up being draggier.

I may try to blow another bubble, this time using 1/8" PETG to avoid having to coat the inside with fiberglass, and NOT use the vacuum so I can get straighter sides.

Gravity blown PETG wheel fairing
1. This is the vacuum box with a new template cut into the top to produce a shape that will fit nicely over the rear wheel.
2. Infrared heat melts the 1/8" thick PETG and a vacuum pulls it into the air tight box. The green plastic bag makes the box air tight so we can pull a decent vacuum.
3. One of the advantages of using a vacuum is that you can pull the hot plastic down to as far as you need it quickly, but we ended up making a whole bunch of plastic bags! Finally after some trial and error, we got a fairly decent shape out of a 1/8" thick sheet, but the edges were ballooning out just like the canopy bubble.
4. So we made a final wheel fairing by disconnecting the vacuum all together and letting gravity sag the hot PETG plastic down to it's final depth. This didn't work out that well because it took so long for gravity to do it's work that the plastic totally melted into the wood box at the flange.
5. Since we could not get the wheel fairing as deep as we needed it, we built a cardboard edge and filled the whole thing up with expanding foam
6. The chemical reaction from the expanding foam melted and distorted the plastic, so we had to rip it off the Styrofoam form. But, now I have a very good Styrofoam form that I can sand and carve into the perfect fairing shape.

I will cover this foam plug with a couple layers of fiberglass, then mold prep it with wax and PVA. Then I can pull off carbon wheel fenders off of the MALE foam/glass mold


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