vacuum bagging vs. sandwich wall building

[Woody]

I got the overall priciple down on both. Does one have an advantage over the other? Are they structurally about the same strength I am not worried about the weight difference (should be minimal I am thinking) using the same materials

Woody

[BoilermakerFan]

Vacuum bagging is used to minimize air gaps and to ensure strong, solid contact of the glue surfaces with even force distribution across the panel or part.

I would use small screws to secure the two sides to the "joists" in the walls, then place the wall in the vacuum bag. After the epoxy has cured, the wall is removed from the bag and you can remove the screws. Patch the holes, and sand any excess expoxy off the edges.

[Woody]

Ok I'm with ya, but which is stronger verses weight. I had planned on installing a roof rack for light stuff and I like the look besides.

Woody

[BoilermakerFan]

IMHO, I would vacuum bag a 1/8" ply, 3/4" insulation, 1/8" ply sandwich layup. That's what I'm planning to do. The vacuum bagging forces a very strong, complete bond of the expoy to both sides of the layup materials.

That is probably the best compromise between, strength, weight, and costs if you have access to a larger shop type air compressor or vacuum pump. If you don't have a larger compressor, then I would probably use a level garage floor and small bags of concrete to press down the layup while the epoxy dries.

I'll post the link to a vacuum bag manf. that I have spoken to shortly when I can find it!

[angib]

Woody,

There are two applications of vacuum-bagging which can get mixed up:

1) Vacuum-bagging uncured laminate (eg, fiberglass) onto a core panel - here the vacuum bag will suck any excess resin from the laminate into a disposable breather cloth and will make a significant weight reduction in really lightweight panels.

2) Vacuum-bagging to bond solid material (eg, plywood, metal, cured laminate) onto a core panel - this is just a way of applying pressure while the glue sets and will not make a significant difference to the weight. It will produce the best quality bond, but if you aren't making aeroplane parts, that may not be necessary.

As alternatives to (2), you can use:
- temporary screws, as Larry sugested
- temporary staples (leave a strip of plastic packaging strap under the staple to help you pull out the staple afterwards)
- permanent nails
- cover with sheet of plastic and shovel any heavy material, like sand, on top.

The last one is traditional and, on a flat panel, probably works just as well as a vacuum bag.

Andrew

[BoilermakerFan]

Woody

I would use the plastic sheet and sand method over temporary fasteners only because the sand can be distributed over the entire surface.

Has anyone used a new, large children's wading pool? I have seen 12' diameter pools in the stores. A Pro of the pool would be that you don't have to figure out what to do with the sand or bags of concrete when you are finished. The Cons are that it wastes water unless you water the garden with it and if the pool has a leak...

[asianflava]

I hope that Andrew cleared it up. You vacuum bag a sandwich constructed wall. This is done to aid in the bonding of the panels. It is the same thing as putting several tons of weight on the wall. It also will pull out any air pockets between the panels.

My plan is to use my garage floor as the "caul plate" then I will lay some plastic vapor barrier down, place the wall down then fold it over the wall. Then I will use some rope caulk to seal the bag. I found this website: http://www.geocities.com/reystos/vacuum/to make the adapter to connect the bag to the hose. You also must put a porous substance on top so that the vacuum can flow without collapsing around the suction point (which is why Food Saver bags have that texture). I read that you can use bubble wrap on top of the piece to allow the vacuum to flow across the piece. I may also put a piece of 3/4 ply on top and park my garden tractor on top to insure that the wall is flat.

I have 2 vacuum pumps 1 is an El Cheapo Harbor Freight venturi pump. The other one is a Robinair that I use to evacuate airconditioning systems. I will try the cheap one first to see how it works.

[angib]

I have 2 vacuum pumps 1 is an El Cheapo Harbor Freight venturi pump. The other one is a Robinair that I use to evacuate airconditioning systems. I will try the cheap one first to see how it works.

I have been told that a regular vacuum cleaner makes a half-decent vacuum pump. If you are using a vacuum bag as a clamp for solid materials, then a very 'partial' vacuum is probably enough.

Two caveats:
- This is only what I have been told, though by people who knew what they were talking about, had used a regular vacuum pump, etc.
- My response has always been to wonder how long any vacuum cleaner can last in a 'stalled' condition before the motor burns out or some similar problem. Apparently this is not so. BUT I'm NOT volunteering to replace your vacuum cleaner if you burn it out...

Andrew

[asianflava]

I've seen the shop vac method but I don't think it will provide much "clamping force". Plus what you mentioned about leaving it in the "stalled" position. It may be a good way to get a majority of the air out since most vac pumps don't have a lot of volume.

[angib]

Yeah, you're right - no-one ever mentions in "vacuum bag instructions" that you must use an ordinary vacuum cleaner to suck 99% of the air out of the bag, before connecting the vac pump to it.
Andre

[fasttimes]

With epoxy, you don't want the super thin glue line like on older type glues. Epoxy likes some space between the two parts being glued.

Regarding using sand as a weight; you can also just use water. Sand is 100lbs/cu.ft. and water is 62.5 lbs/cu.ft. Lighter, but it's free. How much pressure do we want when gluing up with epoxy anyway? I can't find any real numbers, like "apply 1 lb/sq.ft. clamping pressure."

[asianflava]

Glue lines were discussed in this thread.

http://www.tnttt.com/viewtopic.php?t=1154