What cfm for bag press?

[macomk]

Hi all.

I would like to build a bag press for our D&T department.
Could you please advise me what cfm vacuum pump should I be looking for? Are there any other features I should check before I purchase any pump?

Thank you

 

[rafezetter]

Hi - I can't comment on the CFM's or what to look for regarding possible pump problems, but on the few occasions I've seen this done on TV and net, the biggest issue they always spoke about was making sure all of the air is expelled uniformly - the way I've seen it done the most often is by drilling small holes through a support slab of wood / ply the work rests on about the same size as the bag within a few inches, and on the underside of the slab joining all the holes in a routed crisscross pattern to allow all the air to get sucked out from the bottom of the press. You could technically route out the entire underside where the holes are as long as you leave a raised edge with slots in to allow the air to escape, this would allow maximum pressure exerted to the support slab from the material above.

Also adding some sort of perforated "sock" over the end of tube inside the bag to allow it to continue to function once the bag flattens out

This also works for items that require pressure on all sides as the atmosphere on the outside of the bag exerts pressure in every direction trying to equalise with the vacuum inside. (thanks Mr Oaks)

 

[custard]

There are some clues here,

http://www.airpress.co.uk/bag-press.php

4 cubic metres/hour to run one bag (the Airpress "standard" pump), 8 cubic mtres/hour to run two bags or a very large bag (the Airpress "professional" pump)

 

[macomk]

Hi - I can't comment on the CFM's or what to look for regarding possible pump problems, but on the few occasions I've seen this done on TV and net, the biggest issue they always spoke about was making sure all of the air is expelled uniformly - the way I've seen it done the most often is by drilling small holes through a support slab of wood / ply the work rests on about the same size as the bag within a few inches, and on the underside of the slab joining all the holes in a routed crisscross pattern to allow all the air to get sucked out from the bottom of the press. You could technically route out the entire underside where the holes are as long as you leave a raised edge with slots in to allow the air to escape, this would allow maximum pressure exerted to the support slab from the material above.

Also adding some sort of perforated "sock" over the end of tube inside the bag to allow it to continue to function once the bag flattens out

This also works for items that require pressure on all sides as the atmosphere on the outside of the bag exerts pressure in every direction trying to equalise with the vacuum inside. (thanks Mr Oaks)

Thank you very much for your comment

 

[macomk]

There are some clues here,

http://www.airpress.co.uk/bag-press.php

4 cubic metres/hour to run one bag (the Airpress "standard" pump), 8 cubic mtres/hour to run two bags or a very large bag (the Airpress "professional" pump)

Thank you for your comment.
Does this mean that any pump which s pumping 4-8m3/h is good enough to be used with single bag press?

 

[macomk]

Could anyone advice about ultimate pressure for vacuum pump? i found one with listed ultimate pressure of 150 mbar. Would that be enough to use it for the air press/bag?

 

[Yetty]

Could anyone advice about ultimate pressure for vacuum pump? i found one with listed ultimate pressure of 150 mbar. Would that be enough to use it for the air press/bag?

In short, yes! That 150mbar figure is how close to absolute zero pressure (0 bar, a perfect vacuum, ultimate) that the pump can get.

Practically, since our general living environment is at about 1 bar, a connected vacuum bag would squeeze its contents by the difference in pressure i.e. 0.85 bar. With 1 bar pressure equaling about 10,000 kg/m2, the above vacuum bag squeeze is about 8,500 kg/m2 !

So, buying a less efficient pump, say pulling only a 250mbar absolute pressure, the bag squeeze would be about 7,500 kg/m2. To me that still seems like plenty...


PS, when comparing performance of vacuum sources, there's scope for loads of confusion, because there are so many different units used, plus, the figures given can be given as the vacuum achievable relative to atmospheric pressure, rather than absolute.

 

[custard]

I've read several times that a vacuum bag only just delivers satisfactory pressure levels for gluing. Certainly many veneering professionals prefer the much higher clamping pressures available from presses, either manual or hydraulic. I've used an airpress bag for several years without problems, but I wouldn't want to use a pump that was any less powerful. In other words you've got to aim for the most complete vacuum you can realistically achieve or you risk glue failure.

 

[macomk]

I've read several times that a vacuum bag only just delivers satisfactory pressure levels for gluing. Certainly many veneering professionals prefer the much higher clamping pressures available from presses, either manual or hydraulic. I've used an airpress bag for several years without problems, but I wouldn't want to use a pump that was any less powerful. In other words you've got to aim for the most complete vacuum you can realistically achieve or you risk glue failure.

What pump did you use? I have seen air and bag press are using dry pumps. Apparently they can achieve only approx.85% vacuum and if I want anything better than that I have to use oil pump. Is that right?