> Here’s one for you aerospace guys.

> Can someone explain to me the vacuum bagging process. I know that the home
> built airplane guys use it a lot to get maximum strength/weight ratios out
> of a given thickness of cloth.

> As I understand it you apply the glass and epoxy in a normal way cover it
> in a “peel/ply” and/or “breather” layer, put the whole thing in a vacuum
> bag and then by evacuating the air, you get uniform pressure over the
> surface. My assumption is that less epoxy can be used and still get
> complete wet out and bonding to the surface.

> My questions are:

> 1. how do you peel off the layers after the epoxy cures? I remember trying
> to get my hatch covers off after building the lip for flush hatches. Even
> after liberal doses of past wax the epoxy was holding on pretty good.

There are a lot of things that epoxy won't stick to. Teflon is one of those. polyethylene is another. If you can get a sheet of plastic like that to lay smoothly on top of your glass/resin material, then you can get a smooth finish to the resin with minimal sanding. I understand a few Stitch and glue kits sugest you use Saran wrap and weights ( sand bags) to sandwich the scarph joints in their plywood panels. Same idea, but instead of sand bags, you take advantage of the weight of the atmosphere (Up to over 14 pounds per square inch) Sucking the air out of 20 square inches is a lot easier than playing with 300 pounds of sand! A household vaccuum cleaner or shop vac might give you half that suction -- and even that would be pretty powerful.

> 2. I have this vision of the vacuum pressure squeezing out the excess
> epoxy from your layup but where does it go. At best it would come to the
> surface to be sanded down but I don’t think that is right. If it squeezed
> into the peel/ply then I assume you would never be able to “peel” it off

Sounds about the same as what I have read. There is a porous layer of material (teflon or silicone coated fiberglass is sold for this, among other materials.) placed on top of the wet resin and fiberglass cloth AND mat.

We use squeegees to force the resin into the glass cloth. With bagging, you use the vaccuum to squeeze the resin into the cloth and mat. At the same time the pressure helps to compress the mat, which is a bit spongy. This compacts the glass fibers in the mat, giving you a thinner product with the proper glass to resin ratio. If you don't apply pressure in some way the glass cloth tends to float in the resin, and the glass mat tends to stay relatively swollen -- again like a sponge -- happily absorbing expensive, heavy resin.

The bag tends to keep the resin level, so there are few or no runs -- but wrinkles in the bag can be just as bad. If you avoid the wrinkles you cna put on enough resin to saturate AND fill the weave in the first batch. This would drastically cut time and sanding. The smoothing and flattening action of the plastic bag gives a smoother finish, so even if you do put on additional coats of resin, you should have less of a 'telegraphing' of the weave from the cloth.

> I have never heard of anyone on this board using it so I assume that it is
> either very expensive or provides only marginal strength / weight
> improvements.

The peel ply materials cost about as much per square yard as fiberglass cloth, so you would roughly double your costs in that area. If you use shrink wrap, or heat sealed materials for your bags you add in the cost of those onsumables. After that you add on the tools .

For the few square feet of glass cloth we conventionally apply to a hull, squeegees work well. this is a low tech-adaptation of industrial assembly techniques that have worked for hobbyists and amateurs for many years, so we save the money and build more boats.

American Science and Surplus has a venturi vaccuum 'pump' that operates from an air compressor. $15. Air compressors are far more common in work shops than vaccuum pumps. Same idea as an automotive intake manifold on a carbureted car. on a car, the air rushing in sucks some gas with it. In this case, the air rushing by sucks more air our of your bag.

If you have a compressor with an air tank, the motor will cycle on and off. With this design, there is no additional strain on the motor of a vaccuum pump or vaccuum cleaner when you suck all the air out of a closed bag. Nor is any excess resin heading into the intake of your pump.

http://www.sciplus.com/category.cfm?section=6&subsection=18&category=176

This company occasionally has surplus vaccuum pumps, which they may also sell as low pressure air compressors for artist's air brush use. I've seen prices in the $25 to $90 range depending on the particular model's specifications and availability.

The process of vaccuum bagging is also used in photography, where large photos are heat mounted or adhesive mounted using the same equipment ( bag and vaccuum pump. In the graphic arts industry vaccuum pumps are common pieces of equipment as they are used to insure that plate making materials are held together firmly during the manufacture of printing plates.

hope this helps

Paul G. Jacobson