fibreglassing, vac bagging, and reducing weight

[disco stu]

In the spirit of Kaptain Zeros post here, I thought I would throw my questions up here as well.

When fibreglassing and vac bagging, the way I understand it you take off as much epoxy as possible via scraping and soaking up the excess. On a DLG balsa tail I did recently it turned out too dry and wasn''t adhered well, coming up in many parts. I did have problems with vac leaks so I wasn't pulling full vacuum, so maybe this made the difference??

What I was wondering about was using a peel ply that lets the epoxy seep through, paper towel on the other side of that. My thought was to throw the parts in the bag with this peel ply and pull vac for a few minutes to get out as much epoxy as possible, but still have enough epoxy there initially to ensure a good bond between the glass and balsa/foam. Once its all been pulled out, then wack on the mylars so you end up with a nice glassy surface as the peel ply has a texture to it.

Is this the idea, or wrong tree to bark up? Any better ideas then please direct me

Cheers, Stu

[kaptain_zero]

I'm not sure I'm understanding your question correctly..... However, I shall try to answer from my woodworking/vac bag background as I've not done fiberglass modeling this way. If your assembly is dry, it's because air was somehow allowed to enter after it being placed under vacuum. If there is no air entering the bag and you ensure that all air is pushed out of the assembly... as in squeegee the bubbles out of the part you are trying to glue together, there cannot then be a problem with adhesion.

Not knowing exactly what you were trying to accomplish this job is making it hard for me to formulate an answer, but I will say that any time I've used vacuum forming, it's always involved a mold coated with a release agent and then layers of material on top with an adhesive in between. This does not directly translate to fiberglass as the glass mats are porous and air can get entrained. I would consider a multi stage operation, doing one or two layers at a time over the mold, not necessarily using a vacuum technique.... much like boat building, or perhaps an inner/outer mold with the fiberglass in between, if that works better.....

On third thoughts... can you post some pix or explain in more detail? I'd love to help if I can....

Regards

Christian

[disco stu]

Thanks Christian. At work so dont have pics, but just trying to cover the balsa tail and tailplane with fibreglass to make them stronger and resist bending during launch forces. Same way as glassing a wing, not moulding but just covering.

I cut the cloth to size, layed it over newspaper to wet out with the resin (new technique I read about to reduce the amount of resin) then took as much of it off as possible, then transferred it onto the mylars (stiffer shiny plastic to have a nice glossy finish to the part, and to smooth over imperfections), then into the vac bag. As it was balsa I wasn't worried about squashing so pulled as much vac as possible, except there was a leak meaning it was lower than what I wanted.

My thinking is that the more vacuum you can pull on it, the less resin you can get away with. Is that correct?

Might have been teaching to suck eggs there, just thought I would outline my technique so you can spot any errors and guide to better techniques, if any

What I was hoping to find out was a way of getting the excess epoxy out of the lay up, but still getting that nice shiny surface from the mylars.

[kaptain_zero]

Thanks Christian. At work so dont have pics, but just trying to cover the balsa tail and tailplane with fibreglass to make them stronger and resist bending during launch forces. Same way as glassing a wing, not moulding but just covering.

That cleared it up for me.

I cut the cloth to size, layed it over newspaper to wet out with the resin (new technique I read about to reduce the amount of resin) then took as much of it off as possible, then transferred it onto the mylars (stiffer shiny plastic to have a nice glossy finish to the part, and to smooth over imperfections), then into the vac bag. As it was balsa I wasn't worried about squashing so pulled as much vac as possible, except there was a leak meaning it was lower than what I wanted.

The vacuum when properly applied should remove *all* air from the bag.... a leak will let new air in and hamper the removal of already entrained air.

My thinking is that the more vacuum you can pull on it, the less resin you can get away with. Is that correct?

Well, you want a smooth, glassy surface so you must have enough resin to fill the pores of the glass mat you are using. Placing mylar on top will attempt to smooth this out but getting a glass smooth surface will be neigh impossible this way because the flexible mylar will simply be pushed into the pores in the fiberglass. The only way to get a perfectly smooth surface is by using a mold which contains the mirror image of the surface you are trying to create. Think of fiberglass boats. The inside of the mold is the glass smooth outside of the boat's hull. The inside of the hull is very rough because there is no second mold used on the inside. To create a smooth flat surface on a flat sheet of balsa using fiberglass, I'd use two pieces of smooth board, say plywood or other substrate with a smooth facing layer such as a glossy arborite or some such (you could even sand and paint it smooth) and then sandwich the works between them, but don't forget a release agent such as mold release wax or you'll glue the whole works together. This would, when inserted into the vac bag or even between clamps give you two smooth surfaces but would leave the edges rough which should be fairly easy to sand smooth.

What I was hoping to find out was a way of getting the excess epoxy out of the lay up, but still getting that nice shiny surface from the mylars.

The above sandwich method would allow the excess resin to escape out to all sides... the only trick is to avoid any bubbles being caught up inside the fiberglass.... I suppose you could lay the fiberglass and resin on the balsa and then roll down the mylar on top of that using a hard rubber roller and then placing the works between the platens (remember that release agent anywhere you don't want the resin to stick).

There are more ideas that come to mind but I think I said enough for you to think about and then develop what works best for you. I'd seriously do some testing before going for broke though.... Getting this right may not be as easy as it seems....


Hope this helps

Christian

[disco stu]

Cheers. More to play with!!