Vacuum bag press

[Giff]

Has anyone any experience with Bag press or Air press. PVC bags, Polyureythene bags or Vinyl bags ? Mainly for forming and veneering flat panels.

 

[marcros]

i cant answer that, but have you considered making a setup? I am going to get the fittings for connecting to my vacuum pump this week, and make up some bags to give it all a run out. When done, i think it will be in the region of £200 including the (secondhand) pump.

There is an excellent thread on here if you have a look which takes it step by step.

 

[Steve Maskery]

I had an Airpress. It was one of the early ones. Very high quality, let down by the absence of an On/Off switch, which I thought was a tad mean given the considerable price tag.
I have my eye on a S/H vac pump, and if I do decide to get it, I will build my own set-up. Might even be the subject of a DVD one day.

I can tell you that once you have used one, a whole new field of designs open up to you.
S

 

[KevM]

No direct advice to add, except when I was looking at this a while back it did seem that some of the suppliers to the fibre-glassing and composites industry seemed more competitive on pricing.

http://www.easycomposites.co.uk/Categor ... plies.aspx
http://www.ecfibreglasssupplies.co.uk/c ... pment.aspx

Also worth looking at air-conditioning outfits for vacuum pumps.

 

[doctor Bob]

I have the large Air press, what do you want to know?

 

[Sgian Dubh]

Yes, both vacuum presses and air bags as well as different membranes. Like doctor Bob, what do you want to know? Slainte.

 

[Giff]

1.BagPress or AirPress ? and which bags. There seem to be vinyl, PVC and polyurethane.
2.Looking at the "build thread", which is really useful, it seems that the prices are a lot more than in the post (it was a few years ago).
3.Also does the pump have to be on until the adhesive has cured.
4.Are they noisy.
5.Would the workshop have to be at a certain temperature.

 

[gasman]

What about using those sealable plastic bags which you put clothes in and then an ordinary vacuum to remove the air? Would that do for a simple veneering set up?
Cheers
Mark

 

[doctor Bob]

1.BagPress or AirPress ? and which bags. There seem to be vinyl, PVC and polyurethane.
2.Looking at the "build thread", which is really useful, it seems that the prices are a lot more than in the post (it was a few years ago).
3.Also does the pump have to be on until the adhesive has cured.
4.Are they noisy.
5.Would the workshop have to be at a certain temperature.

I have an air press version, but I believe they are similar.
Bags are pretty much the same, one company does 300micron poly the other does 500micron vynyl, both say theirs are stronger. I buy from air press as they are cheaper. I do large curved work and the vinyl bags are fine.
Pumps have an auto mode so will switch off once pressure is reached.
They are a fairly constant buzz, but not noisy bit like a very very small compressor.
Depends on the glue you use, but if it's too cold put an electric blanket over the top.

 

[Sgian Dubh]

Currently I use a vacuum press from a company called Global, but I've also used a press from AirPress Solutions. Over the years I've used various vacuum bags as well. The presses are a much more efficient system saving a lot of fiddle-faddle. All the different manufacturers' systems are good, and you can get different sizes of press to suit your needs, space and budget. If you want to do a lot of curved work the vinyl membranes work better because as the vacuum is applied the membrane seldom gets tucked under or between the the laminates, whereas the heavy thick plastic membranes have a bit of a habit of doing this, which is a bit of a challenge, and sometimes a real pain in the pants, even occasionally ruining the lamination. I've seldom had this occur with the vinyl membrane.

Something that can be a real boon is the heated version of the press because a glue that might take hours to cure at room temperature such as some urea formaldehyde types will cure in minutes at 60º C, and this speeds up efficiency, but at a cost of course.

There's naturally a learning curve. There are some techniques you need to be aware of. For instance, veneering flat panels works best generally if you stack a series of diminishing sized boards on top of the panel as you press - this creates something resembling a pyramid over the top and ensures pressure is applied evenly over the whole panel and reduces chances of voids forming. You don't always need to do this pyramid thing, but it frequently helps.

Curved work with foam formers is a quick and lightweight way of doing the job, and saves a lot of time making up heavy MDF or chipboard ones that can be difficult to manoeuvre.

Apart from that I'd say doctor Bob has pretty much answered your questions already. Slainte.

 

[Giff]

Thanks for the really helpful replies which I think answer all the questions / doubts. Dr Bob I never thought of the electric blanket which I think is a brilliant idea……and thanks for the "pyramid technique" Slainte great tip as well.

 

[custard]

I use an Airpress with vinyl bags in different sizes for flat panels and some curved work. Generally speaking it works just fine, but if the design and budget permits for curved work I prefer both male and female formers with plenty of cramps. Remember that the pressure in any vac system only just meets the minimum requirements for most adhesives, and if the design means that you'll be cutting the workpiece after cramping with a vac bag you can find you have slightly thicker glue lines in some areas due to the glue "pooling".

 

[marcros]

if you are veneering one side of a piece of substrate, is it better to put veneer side up or down?

 

[Sgian Dubh]

If the ground is the exact finished size I put the veneer down. The order is: a board at the bottom to go on the press platen (any convenient but non-floppy thickness), sheet of plastic, veneer, glued face of the ground down on the veneer, a stack of 2- 5 boards in an acute angled approximately pyramid shape (number of boards on top dependent upon the size of the veneering job), then apply the pressure. The base board can be helpful in slipping the whole job into the press (or bag), and also takes care of any stray glue snots stuck to the platen or bag that might otherwise damage the veneer.

If the ground is oversize where it won't matter if the veneer rolls unsatisfactorily over the edge because the ground can be trimmed to size later, then either way will do, but in this case I usually put the veneer on top, then a sheet of plastic, and all topped by the stack of pyramided boards. This saves seeking out a base board to go on the platen if you do the job the other way about (veneer down). Slainte.

 

[Steve Maskery]

Richard
I've not come across the pyramid thingy before. Can you explain please how it evens out the pressure? I would have thought that 14 PSI (or whatever approximation the press achieves) will be 14PSI over the whole area. Clearly you find it beneficial, but I don't understand what it does.
Can you explain further please?

 

[Giff]

Custard could you explain what this means... "Remember that the pressure in any vac system only just meets the minimum requirements for most adhesives" I haven't come across this before. Thanks Geoff

 

[marcros]

I believe that it means when you pull a perfect vacuum, you are going to get a maximum of 14.7psi. It is impossible to get more than that, and a perfect vacuum itself is going to be a challenge to achieve.

Some glues will require a greater clamping pressure than 14.7, Sony using a vacuum bag you are not able to provide the clamping pressure that they require. Unfortunately I am struggling to find a data sheet that declares a clamping pressure.

 

[Sgian Dubh]

Richard
I've not come across the pyramid thingy before. Can you explain please how it evens out the pressure? I would have thought that 14 PSI (or whatever approximation the press achieves) will be 14PSI over the whole area. Clearly you find it beneficial, but I don't understand what it does.
Can you explain further please?

With a hydraulic press the pressure is applied in one plane, i.e., vertically. With a bag press, or a vac bag there's stretch in the bag. As the pressure is applied there's a tendency for the bag to suck down unevenly, often with wrinkles, and for the bag to drag sideways a bit as it's sucked into place. This can open up join lines or cause wrinkles as it drags the veneer one way or another. A board or boards over the top of the veneer reduce the chance of this sideways movement, and thus reduces the number of failures ... and I've seen plenty of failures, especially when inexperienced woodworkers go at the job and ignore my suggestion/instruction to stick a few boards over the top.

If the veneer is at the bottom side I've seen situations where there's more pressure towards the outer limits than in the middle of a panel, and the adhesion in the middle areas has been compromised.

The pyramidal jobby seems to take care of these problems about 95- 99% of the time. Slainte.

 

[Steve Maskery]

Excellent, thank you very much.
S

 

[custard]

Custard could you explain what this means... "Remember that the pressure in any vac system only just meets the minimum requirements for most adhesives" I haven't come across this before. Thanks Geoff

Pretty much as Marcros said. Bruce Hoadley in his book gives cramping pressures, as do Franklin International (Titebond) on their web site. From memory for PVA it's something like a maximum of 150 psi for softwoods and a maximum of 250 psi for hardwoods. Even if I'm a bit out on those pressures it's still way way more than you can ever get from a vac bag, and the vac bag just barely scrapes into the minimum required pressure.

However, vac bags still have loads of applications and I wouldn't be without one. Firstly other adhesives require less pressure, again from memory so please check but I think the optimum for UL39 is something like 35-50 psi. Secondly even with PVA a vac bag does make the grade (just)...but you can't afford to be sloppy and you have to get everything just right for reliable strong results.