Clamping Pressure for a Hydraulic Cold Press

      A woodworker seeks advice on getting good pressure in his shop-rigged press, and a consistent glue line while glueing up 6x6 turning blanks. May 26, 2008

I'm in the process of designing a hydraulic cold press for making large turning blanks for my turning business. I glue up lots of 6 X 6's and 8 X 8's 48"-60" long and need to know how much clamping pressure is needed (PSI) so that I can fit the right size hydraulic cylinders for my press. Anyone?

Forum Responses
(Adhesive Forum)
From contributor D:
That would depend on the glue you are using. I recommend contacting your glue manufacturer.

From the original questioner:
I'm using Titebond (PVA).

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
It also depends on the species of wood, so, as suggested, contact the glue manufacturer.

From contributor C:
The clamping pressure is not nearly as important as you might think. Do your components fit nicely, i.e. flat against each other without bows? No steps in the glue up. These conditions should be met - all things fit together well so that glue thickness can be a minimum so as not to waste glue and minimize glue up time. A Big Jorg bar clamp is good for 2K per sq. in. max and a good fitting joint squeezes out all the excess at 1/10th, about 200# per sq in, which spread out gets to the glue joint 20# plus or minus. It all depends on the fit and how cold the glue is and how long you want to wait while things are in clamps. Go large on the cylinders so they can be under maxed. Regulate them down if needed.

From the original questioner:
Thanks for your responses. I've gotten some good info from Titebond on figuring the pressure needed. I'm mostly gluing up domestics - hard and soft maple, oak, and poplar. Franklin is saying 100-300psi - lower pressures for soft woods and higher for hard woods.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
The role of pressure is to push the two pieces of wood close together (0.002 to 0.006 mils in most cases) but not too close, to spread the adhesive throughout the joint and into the small nooks and crannies, and to squeeze out any excess adhesive. Pressure does not squeeze adhesive into the wood. The thickness of the adhesive is therefore important. As temperature affects the thickness (called the viscosity) in the warm summertime, less pressure is needed than in the cold wintertime. Obviously, the adhesive should be at room temperature. In summary, pressure is a key element in developing a premium glue joint with the PVA adhesives mentioned here and with most other adhesives.

From the original questioner:
I'm not sure how I'd measure film thickness if I had to. Since I am attempting to build a cold press, I'm still trying to understand how to size the hydraulic jacks. If I'm gluing a 6 X 48" turning blank and need 150psi, that works out to 43,200 lbs of pressure. If I have two jacks with 2" diameter rams, their surface area = 6.28 in squared. So 43200/6.28 = 6,878 psi by the jacks that needs to be applied to achieve 43,200psi of clamping pressure. So one jack will be applying 3,439 psi (6878psi / 2). So am I looking for a pair of two ton jacks? Or am I over my head in this?

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
I am not an engineer. However, remember that the pressure must be applied uniformly along the joint.

From contributor F:
To the film measurement question: adhesive must be uniformly applied. A glue spreader, a paint roller, a hand held feeder consisting of a hopper and roller, even a brush. The film thickness or wet mills are measured with a mill gauge. A mill gauge is very inexpensive and available from paint stores and elsewhere. I have a couple that were free from my Sherwin-Williams dealer. The mill gauge and further information about applicators is available from C-P Industries.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
It is not unusual to have the glue applied in a thick bead or ribbon and then the pressure distributes the glue throughout the joint. The advantage of a thick application is that the glue does not pre-cure as easily.

From the original questioner:
I am familiar with the film gauge. I have never used one but a salesman at Sherwin Williams gave me one some time back. I had forgotten about it until your mention. As for glue applicators, I have a roller type that I bought from Veneer Systems. It works really well but you have to clean after every use. I have also ordered a pressure tank type with a 7" nozzle that has port holes for what I think you're talking about, Gene. It makes several beads of glue. The advantage, as you say, is that the glue won't pre-cure and also it will not have to be cleaned after every use.

From the original questioner:
By the way, I have a theory about face gluing 6" X 6" and wider with pipe and/or bar clamps. You get good squeeze out along the perimeter but the glue is trapped in the middle section of your lamination (probably more true with softer woods). So when I turn my 6 X 6 X 48 or 8 X 8 X 48's on the lathe, the glue line toward the center is thicker than the glue line on the perimeter. What do you think? This is one reason I'm interested in a cold press - So that the pressure is more uniform over the surface of the entire piece, not just along the perimeter.

From contributor R:
Wilsonart has an incredible group of technical folks that will come to your facility and help set up your machine provided you use the Wilsonart Adhesives.

From the original questioner:
I'm using PVA - Titebond to be exact. I suspect that Wilsonart adhesives are contact adhesives for laminates?

From contributor R:
This is a very common misconception. While Wilsonart does make contact adhesives for laminate, there is also an extensive line of PVA/EVA adhesive including those for cold press, hot press, post forming, and wood glues. I assure you that the products are good and the technical service is second to none.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor S:
I would calculate the pressure requirements at 300 PSI. 6 wide x 48 long x 300 PSI = 86,400 pounds of force. I would suggest that you use three actuators equally spaced along the length to distribute the pressure. The moveable platen will need to be structurally sound to prevent deflection to produce an even clamping pressure. Each actuator will be required to produce 14.33 tons. If you use typical hydraulic components with a 3,000 PSI operating pressure, then you will need 3 each 4 bore cylinders. This system will produce a maximum of 56.52 tons of force. An alternative would be bottle jacks. I would recommend that you have some type of gauge in the jacks so that you can see how much pressure you are applying and to see if the pressure drops during the curing cycle.

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