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A small step for composites but a big step for my project

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oriol

Well-Known Member
Joined
Dec 31, 2009
Messages
1,473
Location
Barcelona, Spain.
Hi!


I just achieved a big milestone towards the completion of my current prototype!


It is a bicycle rack, together with the seat they make the only composite parts of the tricycle. Once the machine is finished, and well tested, I will like to make a small metal structure to fix a hang-glider. But for the moment that is another story that deserves another thread in its due time.


The building process of the part is quite similar to that of a fairing wheel; a closed structure with an opening. It is quite tricky to finish the interior, you have to get your head inside with one hand with the brush to put a final layer of resin and cloth to bond the two parts.

There a many ways to do it, I just learned myself through the whole process.


I could have bought a bicycle or motorbike carrier lugagge to do the job but it would haver resulted ugly as hell, especially after all the effort I have put in all the details and the structure itself. The carrier lugagge looks very similar in concept to the rear part of the tricycle that can be seen on picture two, but this one is much more oversized as well as the tricycle.

It is very difficult to apreciate the soft curves of the part on the pictures.


I guess that most HBAers familiar with composites can guess the whole process in this just two pictures.
However in case somebody not familiar with composites (like me!) but willing to learn to do small parts like those is reading, it is not very difficult to do it with a minimum of external guidance and patience.


I bought a 2 m^2 with 10 cm thickness block foam that was cutted in half to get inside my car.
I just could only cut a couple of complete part of the profile from the original block, the others were made joining smaller parts.
It took me four profiles of foam to achieve the required thickness.
The profiles were bonded together with polyester resin.
The final part was then sculpted with hotwire.

The top of the part was the most difficult to achieve because you have to get the symmetries by hand.
Of course you can draw the part with any 3D program and then ask someone to cut the final part with CNC. This is perhaps the most costwise option but I do not like 3D programs artificial curves. All modern cars look the same to me because of the CAD programs. Of course beauty lies on the eye of the beholder.

Once the part is sculpted, with a cutter and some sanding to round the edges, it is covered with cloth and polyester resin.
The hardest part is to cover the whole piece with putty to achieve the smoothest possible surface.
It is really difficult to achieve symmetry on top of the part with the naked eye but quite simple by using some reference lines draw with a pencil.
Once the sanding is finished then you can make the mold.

I made the mold on two halfs each made of three different parts to ease the demolding process.
To make the mold I had to build several ribs (seen on picture one) to make the lips in which the different molds are screwed together.
The mold is made with polyester resin because it is cheaper than epoxy resin, the final part is made with the later.

It takes some time to cut the clothe and adapt it to cover the half of the mold.
You have to avoid that the different clothes get superimposed at the same spots. That´s why are words written on the final part and the molds. I marked each piece of cloth to avoid putting them in the wrong spot.
Working with resin by hand can become quite stressfull because you have to apply several layers of cloth quick before the resyn dries.
There is no time to think where goes each layer of cloth and it is better to make no mistakes because resin is not cheap.
Like welding it is much more easy to work in favour of gravity than against it.

I applied two layers of thin clothe for the surface and then added some matt in the interior to achieve the required final thickness.
Matt is much more sympathetic to work with than clothe because you can cut it with your bare hands while applying the resin.
There are some people, obviously not experts, that cut matt with scissors. It is very dumb because you can cut it with your hands and because then the ends are sharp which makes them the perfect spot for future cracks. When cutting cloth you have to leave the final threads free to avoid sharp ends for the very same reason above.

So far the only work left is to apply a bit of putty in some minor spots and little else apart from painting.


The rack has costed me around 400E only in materials (Epoxy resin + Polyester resin + the foam block + several squared meters of cloth and putty).



original and mold plus ribs_opt.jpgfinal part and mold inside_opt.jpg



I just wanted with this thread to encourage other HBA members to share their acomplishements in composites for the rest of us to get inspired.



Thanks,


Oriol
 
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