Space Saver for vaccuum press?

[crokett]

Is there any reason I can't use a space saver bag as a vaccuum press?

[maxparot]

None that I can think of

[SHADOWFOX]

I don't see why not. Use a small size space saver bag plus a hand pump, it's a cheap and easy way to get started in wood veneering without spending big bucks. These bags should be enough to handle small panels.

[drumpriest]

I have tried it, and it doesn't work well at all. They typically don't reach the PSI required for a good veneer bond. The cheap hand pump vac press kits from rockler and woodcraft work well though, so long as you can get the tape to seal properly.

A good vac press doesn 1500 lbs per sq foot of pressure. With the vac bags, it's easy to get glue bubbles under your veneer, and for the veneer to easily fail via chip out or worse.

[boblon]

I have used those bags for vacuum pressing and they hold up surprisingly well, but...

... I remove the valve that comes with them and add my own and I use my own bag closure.

I use an electric vacuum pump that cycles when needed as the seal is not perfect.

I was going to make my own vaccum bag out of heavy gauge vynyl but am still using the space saver bags. Maybe some day.

BobL.

[drumpriest]

Bob, that makes sense to me, if you have a pump on the thing to keep after the air that you are losing, it would probaby work fine. I somehow doubt that was David's intent, but you never know.

[boblon]

Yeah, he didn't specify his intended use. I wanted him to know it's possible, but I wanted to caveat it enough to avoid him trying it without a vacuum resivoir and ruining a project. I would have felt bad if that would have happened.

BobL.

[greencat]

I'm not a rocket scientist but doesn’t the 1500 pressure come from the surrounding atmosphere after the air in the bag is empty? So could a less expensive system work as long as it could pump as fast as any leaks into the bag?

[Tom Miller]

I'm not a rocket scientist but doesn’t the 1500 pressure come from the surrounding atmosphere after the air in the bag is empty? So could a less expensive system work as long as it could pump as fast as any leaks into the bag?

It's not just a matter of pumping as fast as any leaks, but ultimate base pressure. To keep up with the leaks, you could do the job just sucking through a straw. But you won't get enough clamping pressure that way.

(Why do I have a feeling that I opened the door for this thread to take a nasty turn?)

Regards,
Tom

[jerrye]

Has anyone tried using industrial vacuum bags/tubing? I sell that stuff to meat packing houses and it seems to do the job for them. IIRC my stuff is 1-2 mils thicker than the homeowner stuff.

David I might can get you some samples of my stuff to try. PM me if you're interested.

[crokett]

Well I'm misunderstanding things here then. AFIK the space saver bags are suck the air out, seal and they stay that way till you come back and open a valve. Is that not true? As far as PSI goes, that is produced by the air pressure outside the bag so I would think hooked up to my shop vac I should be able to pull enough air out to get the pressure I need. I mean once the bag is formed around the project, what other air is there that can be removed to increase the pressure?

[Tom Miller]

Well I'm misunderstanding things here then. AFIK the space saver bags are suck the air out, seal and they stay that way till you come back and open a valve. Is that not true?

Sure, except that when you say "suck the air out", what you should say is "suck some of the air out".

As far as PSI goes, that is produced by the air pressure outside the bag so I would think hooked up to my shop vac I should be able to pull enough air out to get the pressure I need. I mean once the bag is formed around the project, what other air is there that can be removed to increase the pressure?

Well, what you're really after is the pressure differential.

P(atm) - P(bag) = P(clamping force).

You can't do much about the first term, but the closer you can get P(bag) to zero, the better off you are. Keith mentioned a system that produces a differential of 1500 pounds per sq. ft., or 10 psi. Since P(atm) is about 15 psi (round #'s), that means there's still 5psi(a) in the bag. If you could get that down to ~2psi, that'd be even better. But much beyond that, you're return on investment is pretty low.

(As we "speak", I'm sitting at a vacuum chamber that's reading 5x10^-10 Torr. It takes lots of $$ of equipment to pump to that level, but it wouldn't really make any better vacuum bag.)

Regards,
Tom

[crokett]

Sure, except that when you say "suck the air out", what you should say is "suck some of the air out".

I still am not sure how much air could be left after the bag is formed around the work piece. Seems to me even a shop vac would pull most of the air out of that bag.

[drumpriest]

Tom, there is also a law of max required clamping pressure. As some point you are clamping too much.

David (crokett), if you feel that we are wrong, then you should go and do some veneering with your space saver bags. I'm not arguing from a theory standpoint, I've done it, and it didn't work nearly as well as my vacuum press. If you don't believe that, then you should just go and do it, and I wish you the best of luck with it.

I did get a piece to come out "ok" from the space saver one time, but it didn't stay that way, as the glue bond wasn't great. Bird's eye maple veneer on 1/2" MDF.

Perhaps you will have better luck, worst that you do is destroy some veneer and substrate material. I have found in general that Baltic birch is more forgiving than MDF, btw. And you want even glue, but fairly thin as well. It's easy to use too much glue.

[jerrye]

FWIW, I've tried Space Saver bags for their intended purpose and all of them have failed miserably. Not one maintained a good seal, and several never would pull down completely.

My.02