How much vacuum to be effective?

[ColeyS1]

I'm having a fiddle with the vacuum bag and am wondering what reading I need to have for it to have sufficient pressure.
I've got a big venturi valve that shows nearly 0.8 bar when it running and a smaller one that reads only 0.4. I think I bought the smaller one so the compressor wouldn't keep kicking in as much.
Would 0.4 be sufficient or am I better using the larger one and have it 0.8 bar ? The readings mean nothing to me, looks like both seem to be squishing it though.
Thanks
Coley

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[Normancb]

The bar is a metric unit of pressure, though not a recommended SI unit. Normal atmospheric pressure at sea level is roughly 0.99 bar, which is 14.5 pounds force per square inch.

When your vacuum bag is open to air the air exerts the same pressure inside and out. Assuming your gauge is reading vacuum not pressure inside the bag, a reading of 0.4 bar vacuum corresponds to 0.6 bar of air pressure inside the bag and 1 bar outside so the difference is 0.4 x 14.5 = 5.8 psi acting on the veneer. A reading of 0.8 bar vacuum corresponds to 0.2 bar of air pressure inside the bag and 1 bar outside so the difference is 0.8 x 14.5 = 11.6 psi acting on the veneer.

A vacuum bag can never generate better than one atmosphere pressure. The reason they work with such low pressures is because the bag is flexible so it applies the pressure very evenly; hydraulic presses used in commercial veneering go up to much higher pressures. 6 psi is not a lot of pressure – I’d always go for the better vacuum and higher pressure. I run my vacuum bag with an old rotary vacuum pump which will still get down to 0.01 bar.

Also depends on the glue - try it and see if it works for you.

 

[worn thumbs]

The level of vacuum displayed may differ according to whether the gauge is reading the level of pressure remaining in the bag or whether it is displaying the level of pressure reduction within the bag.I have yet to have a job fail if more then 0.5 bar is applied to the job within,but it may well be that your 0.4 bar will suffice.We sometimes read of the perils of the "glue starved joint" if too much pressure is applied-again I have yet to experience a failure because of too much pressure.One of my vacuum pumps came with a label attached that said it had been tested to 0.01 mm of mercury and I haven't found a reliable gauge with that level of accuracy to test it with,nor do I think it matters too much.

Commercial presses typically run with more than 3000 psi in the hydraulic system.Looks impressive and then when you take into account the ram diameter,a vacuum bag is likely to exert more force and more uniformly.

 

[custard]

Titebond gave some data for recommended cramping pressure, the minimum recommended for their cold press veneer glue was, from memory, about 15 psi. In other words even if your system is absolutely pukka you only just get up to the bare minimum when vac press veneering. Having said that I've been vac veneering with PVA for years and, touch wood, I've never had a glue failure yet.

 

[ColeyS1]

Thanks for the insight guys. I think realistically I need to look at getting a pump instead of having a compressor running constantly. I switched the compressor off after an hour and a half earlier to try and keep noise to a minimum.
I'm sure I'd use it alot more if it was quieter.
Any suggestions for a pump that would do the job would be much appreciated. Seems to be a million and one different types on ebay ranging from £80-£800 !! Something old ugly but functional would suit me fine.

I think I read somewhere that some pumps turn off when a preset vacuum is reached. Would that be a better choice than a continuous running pump ?

Thanks for your help.
Coley

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[custard]

I think I read somewhere that some pumps turn off when a preset vacuum is reached. Would that be a better choice than a continuous running pump ?

I use an Airpress pump, it doesn't shut off when maximum vacuum is reached and runs continuously. I wouldn't get another one. One of the main reasons I use so much PVA for veneering is that with PVA I can switch the bloody thing off after 45 minutes! If I'm using UF glue I try and time it so it runs through the night and I don't have to listen to it.

 

[ColeyS1]

So the airpress pump is noisy aswell then ? I think the neighbours would kick off if I left the compressor going overnight.
I did think about trying to soundproof the compressor but was concerned it'd end up overheating and going bang. Nothings ever straightforward is it ! I use cascamite alot so another option would be an added hardener to get it to go off quicker. I'd much prefer to make it a quiter process if possible though.

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[RogerM]

Try East Coast Fibreglass Supplies. I got the forerunner of this pump about 5 years ago and it's been great. Not silent by any means, but the noise is not intrusive and I'll happily run it for 6 hours at a time without any adverse results. One day I'll get around to building a regulator like this one.

 

[ColeyS1]

Thanks for the info Roger. A small pump like that would fit in a cupboard under my bench easily. The valve thing you linked to is a little over my head to be honest. I think I understand what it's meant to do. Is it a simpler version of this ? https://www.joewoodworker.com/veneering ... enturi.htm

This is what I've managed to grasp so far.

The venturi valve needs to have the compressor running constantly for it to pull sufficient vacuum.
As soon as the compressors unplugged the air escapes from the bag and the vacuum is lost.
I'd need a way for the vacuum to remain in the bag. When the pressure drops a little I'd need it to switch the compressor back on again.

Does the valve you linked to do this ? If so I need and want !!! Lol

Cheers
Coley

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[RogerM]

I think the link you provided automates the switching on and off of a venturi pump, like your compressor, to hold a vacuum. A different set up is required for a vacuum pump, as explained here and here. Robert's gismo explained in my previous link achieves the same result for much less money, although Joe Woodworker always used to do a kit to build something similar. for around $120.

I have just kept the pump running for several hours, and sometimes just switch it off after turning off the isolation valve.