Wednesday, January 18, 2012

Changing method of laying up carbon fiber in complex curve areas...

Over the last few days I have been laying up carbon fiber around the "U" shaped holder of the headtube -- and I have a lot of layers there in my layup diagram.  The shape consists of compound curves which make the laying up of carbon fiber difficult.  The reason for this is that the stresses -- deriving from the front wheel and fork -- that must be handled by the shape, call for many layers of carbon fiber. I doubt that a mere four layers will suffice.  In fact, I intend to lay up about 10 layers on the shape.

Steerer Tube/Head Tube Area

The 2x2 twill fabric will easily lay down around the compounded curves. But the peel ply fabric, whether nylon or polyester, does not adopt to the shape as does the twill carbon fiber.  Therefore, the peel ply must be cut into smaller pieces as compared to the carbon fiber.  As well, darts have to be cut in the peel ply. In the end, I found that the "wet" carbon fiber/peel ply will shift and bunch. Additionally, since there are many pieces of carbon fiber due to all the curves of the shape, the possibility of having a perfect layup is nearly impossible.  Therefore, I decided to try a technique that I originally did not have a lot of trust in.  That technique is to use a spray adhesive (3M's 77 General Purpose Adhesive) to tack the carbon fiber in place.

The use of the spray adhesive greatly improves (seemingly -- more about that later) the layup characteristics of the carbon fiber.  It stays in place through the tortuous cycle of adding more carbon fiber, resin, and peel ply.  Additionally, since during a layup I have to put down quite a number of pieces of carbon fiber, I can take my time in doing so since I am laying up "dry" carbon fiber.

After laying the many carbon fiber pieces, I then apply a VERY LIBERAL amount of wet resin with West System's 206 Slow Hardner to the dry carbon fiber. I then push the resin through the carbon fiber with many, many dabs of the brush.  I use 2" cheap Chinese model brushes with about 1/3 of the length of the bristles cut off.  The reduced length of the bristles allow the brush to push the epoxy better as compared to a longer-bristle brush.

The slow hardner gives me about double the amount of time to work (pot life) as compared to the fast hardner (West System's 205).  For flat areas and large curves, I usually use West System's 205 Fast Hardener verses the 206 Slow Hardener that I am using in the above situation.

After the layup of the carbon fiber and the wetting down of it with the resin, I can then layup the peel ply (many more pieces) over the wet carbon fiber.  I brush and dab the peel ply until it is throroughly wet. This takes the predominate share of time relative to the working time of the epoxy.  Matter of fact, it is during this stage of laying up the peel ply where I find that most problems appear.  If the peel ply does not completely lay down, then I can feel pretty assured that a problem exists. The problem can be that:

  • The peel ply is stretched past a concave curve;
  • The carbon fiber is not saturated with epoxy;
  • And/or there is an air bubbles.

These problems have to be resolved quickly since the pot life of the carbon fiber may be near its end. For me, it is imperative that I have all the tools right at hand:

  • Peel ply cut to different widths and lengths
  • SHARP scissors (keep the scissor sharpener handy!)
  • Extra clean brush (if the pot life is ending, the brush becomes nearly unworkable, so a clean one may help for a few more minutes)
Additionally, I usually mix the epoxy resin and hardeners in small quantities (2 pumps on the West System's pump system). I rather have a multiple mixing sessions in order that I can continue to work longer since each mixing has its own pot life.

Of course, there is a hitch to this  dry layup.  If the carbon fiber is laid up on a dry surface and the epoxy resin is never worked into and through the surface of the carbon fiber to the surface below, then I could end up with a dry area without adhesion to the underlying carbon fiber.  This is really bad and the bike will almost certainly have failure problems.  From what I read, manufacturers of carbon fiber bicycles expect about 3% of the area covered to have problems of adhesion. I hope mine is less, but I really will not know unless I have a failure of the handcycle for which I would perform an autopsy.  At that point, I can cut away and determine if there are indeed dry areas for which plys of carbon fiber did not adhere to its neighbor.  

You might ask, "Why chance it?"  The reason is, the layup of "wet" carbon fiber on these curves proves to have other problems:

  • Shifting carbon fiber
  • Bunching of carbon fiber
  • Dry spots due to shifting and bunching carbon fiber.

For long curves, this is not a problem.  For instance, I layup a layer of carbon fiber over the entire bottom in very little time and with no problems. There will be almost no air bubbles or dry spots.  If so, they may represent a aggregate of maybe a square inch in size over an area that is probably three square feet -- or a quarter of 1%.

A few tips on laying out and removing the peel ply:

  • Always, if possible, keep some part of each peel ply in an area that is "dry" of epoxy.  When pulling the peel ply from a finished layer, you need to be able to grab a portion of the peel ply in order to remove it.  If the whole of the piece of peel ply is wet in the epoxy, on the surface of the carbon fiber, and if the peel ply is laying flat to the surface as it should be, then you will have headaches in removing it.  So keep some of each piece of peel ply dry.
  • When removing the peel ply, attempt to remove it as soon as you can.  For instance, do not wait a couple of days. I attempt to remove it after four hours with the 205 hardener and eight hours with the 206 hardener.  The epoxy is still "green."
  • Attempt to pull the peel ply as parallel to the carbon fiber surface as possible in order to not to disrupt the carbon fiber layers beneath it.  The peel ply sticks quite well to epoxy.  So when you pull it hard, you could dislodge to carbon fiber on a microscopic level.

So, for those that are interested in designing and building something out of carbon fiber, think about limiting the areas of small complex curves. Those areas require infinitely more time as compared to the larger curves and flat areas.

In total, I expect the above shape will take about 15 hours of work to complete in terms of carbon fiber layup.  But I may just be ssssslllllloooooowwww.....


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