Hi all, thank you for all the great posts that I have been reading over the past year. this is my first post and I hope that you smart people out there can help me.

I am trying to build a large 8' x 24' vaccum bag press and am new to this type of project. I have looked at some commercial ones and don't have the cash to buy so looking for a diy type.

I know how to make the bag but I need help with sizing the vacuum, how to do it, I am leaning towards a venturi pump with my large air compressor but don't understand the how it all works. To start with I found a great veneering set of plans from joe wood worker .com and am building his v2 plus system. It will work for smaller projects but not for my larger projects.

I need to have the press hold 4 psi of for a couple of hours. Could someone please point me in the direction that I need to go.

Have a great day.
brett:)

If you have a big compressor (at least 60 gallon), the venturi pump should work. You'll probably want to use a shop vac to remove as much air as possible before starting the pump. It will greatly reduce the time needed to pull the vacuum.

The biggest problem you'll have is getting a perfect seal on something that large. If the seal is not perfect, then you won't want to use the venturi pump, because you're compressor will run nonstop.

Thanks for the help. I do have a compressor that big and the bag is going to seal to a table. I would post a picture of it but don't know how to do it. :) Would putting two ventri pumps on the bag also help or is that just overkill?
thanks agian
brett

Hi Brett,
The problem the you will face with a venturi and your project is size, or to be more correct volume. Given the size of the vacuum bag you will have a lot of volume to remove before the vacuum level can get anywhere.
It might be an idea to look at this in stages. First, get the bag deflated to a low vacuum level. A normal vacuum cleaner is very good for this. Second, see if you can find a "3 stage" vacuum generator (very fancy venturi) to increase the level of vacuum to where you want it.
The level of vacuum (-4psi) that you mention, is quite low and you may find that a powerful vacuum cleaner might do the trick. The only downside is that your time frame of 4 hours will probably burn out the vacuum cleaner unless you can cycle it on and off.
Errol at Vac-Clamp (http://www.vac-clamp.com)

I used to vacuum clamp 4' x8' sheets of carbon fiber. I used heavy "vis-queen" plastic sheeting over the carbon fiber with a length of 1/4" caulking putty surrounding the sheet under the plastic. I then rolled the plastic into the caulking putty with a flat roller. I used a small "Gast" vacuum pump and ran it for 8 hours with no problems. I made a "manifold" from a piece of copper tubing flattened with several holes drilled in it to prevent the plasitc sheeting from sucking into it and sealing it off. Just add some caulking where you slide it under the plastic and roll it in to seal it.You might need 2 or 3 pumps for your area although I held 24" to 26" inches of vacuum over the 32 sq ft area. The main thing is to get a good seal and then the pump will do its job.The run time shouldn't be a factor with one of these small pumps.