A crease in my laminate

I have been doing a lot of vacuum bagging lately for changes to my big old cat. Latest was the first of two new daggerboards. It is a big and heavy board: NACA 0012 section, 450 mm chord, 3000 mm long.

The board is constructed with a corecell foam core: A1200 foam in the middle, with A500 foam in the nose and trailing edge. It was then covered with uni carbon and some biaxial glass to hold it all together. Most of the carbon in strips on each side, and a little of the carbon and all of the glass around the whole board. The last layer is glass starting at the trailing edge, wrapped around one side, over the nose, and back down the other side where the edges are sandwiched together. When it came out of the bag, there was this crease running along the nose of the board. It was a little cumbersome moving a 75 pound board of wet glass, but I am pretty sure it was smooth when I bagged it.

Any idea why this happened? About the only thing I can guess is that the vacuum squeezed enough resin out of the matrix to actually reduce the size of the board, and that the excess fabric gathered at the nose of the board. I was working pretty quickly and used enough resin that the breather material was quite full when I took it apart.

It appears to be only in the final outer layer. I can easily sand off the ridge, but of course that defeats the idea of a continuous layer around the whole board (but there are others).

So what am I doing wrong?
Thanks,
Larry Forgy

 

One little trick with vacuum bagging is to apply a very low 'pull' then feel with your hands through the bag along the work. Any small bump, overlap or defect can normally be felt with your fingers. If there is a problem, it is possible to release the vacuum, remove the bag and reposition the cloth/veneer whatever so that it is where you want it. I have done this quite a bit with cold moulded veneer construction on thin but long (9.5 meter) shells. You obviously repeat the feel test after altering things if needed, but it is very rare to have to alter things more than once in my experience. This feel test is very good, and works with opaque bags (ie black) too, so you cannot see the flaw visually, but your fingers will pick up very small problems indeed.

I have not dealt with such a large foil as the one you have, but on smaller ones when I glass sheath, I try to support the core and have the foil front edge vertical to drape the fabric around the 'nose'. Note although this gives the trailing edge to trailing edge overlap, it is slightly different in that the lamination starts at the 'nose' and the fabric weight drapes and pulls out tension so there is less loose cloth to buckle anywhere. So this starts by putting the middle of the fabric on the leading edge rather than one end, hopefully that makes sense. This helps by using gravity as you roller/squeegee the resin into the fabric and snug onto the core. If necessary with a long foil such as yours I would consider a 'sacrificial' area for a small rod support at the bottom which is later glassed over.

I take it the core foam was sealed with a thin coat first? to help reduce any air expansion and aid adhesion.

Hopefully someone else may have a little bit more experience on your specific problem especially perhaps supporting a long relatively heavy board for best results.

 

Larry, what you said was correct. I've had the same problem setting up an infusion of a rudder foil. When the bag pulls down, the circumference of the uncompressed layup reduces. This excess fabric has to crease or crimp somewhere and for some reason it likes to happen at the leading edge of the foil when using an envelope bag.

How to avoid it? Many ways to avoid it, depending on how you like to approach your jobs. One method might be to glass the trailing or leading edge first by wet layup allowing a recessed edge in the core so this first edge laminate finishes flush to save fairing out the overlap later ( just a small area using glass tapes wrapping the edge for example ) Then after cure, bag the rest of the entire wrap in 1 shot so that it doesn't wrap the complete circumference of the board, and the reduction in circumference length can be worked out in the overlap area as the bag slowly pulls down.

Knowing what caused it, means a solution can be found however you see fit.