One of the most asked questions by those looking or purchasing a VAK press is "what glue to use".
The answer is the VAK press doesn't care, any glue will work in the press; however, you need to select the best glue for your job for they all have different characteristics. What I am about to present is general basic information regarding glues and VAKuum pressing. This report is not about the chemistry of glue, but more about what to expect or look out for when using the different glues in a VAK press.
There are exceptions to every rule so I do not consider anything below as the final word. New and improved products come on the market with improved formulations that make it a better glue. My information is based on some research and experimentation with different glues. In addition I have over 20 years experience working with a VAKuum press dealing with over 9000 customer world wide. This interaction has provided me with a wealth of information regarding glues and the experiences that my customers have had. I will share some of their stories to make a point or two about different glues.
What glues are generally used in woodworking: contact cement, yellow or white glue (PVA), urea glue (resin), polyurethane (PUR) glue and epoxies are the most common in woodworking.
Contact cement is used for many applications, however, it can be a problem down the road as contact cements, in general, never harden and is easily softened with heat or solvents. Contact cements are surface cements; they do not penetrate the substrate like other glues which provides a better bond. However, one advantage is that it does not have to cure for hours or a day. If you are gluing plastic laminate to make panels or countertops, a VAKuum press can be used to apply 1800 lbs/ft. of even and uniform pressure if you do not have a pinch roller. With plastic laminate you do not have the same problems or issues when using contact cement with wood veneers.
When used with wood veneers a couple of problems that have occurred are; bubbling when the finish was applied over the veneer, or softening when heat is applied. I have had countless calls from customers who wanted to know why the veneer bubbled when they applied their finish. This usually occurred at a seam (book matched panel, wedge designed table top, etc,) or when they used a burl veneer with it's many small holes. If you use water base contact cement and a water based finish, the moisture from the finish will reactivate the contact cement causing the bubbling. If you do use water based contact cement, it's best to use a solvent based finish and visa versa. If you use a paper backed veneer you normally will not have this problem unless there is a seam for the moisture to penetrate. Bubbling can also be caused by “green glue”. If the solvent did not evaporate to give the right amount of tack, the solvent can be trapped between the veneer and substrate. Using a paper back veneer and a birch board substrate (verses MDF which is porous), solvent can become trapped between the two. This will result in bubbling and require fixing.
I had a contractor call and purchase a VAKuum press because he had to redo a curved reception desk at a law firm. They called him in and when he stepped in the door, he could see a large blister at the base, directly in front of the receptionist. He walked around the counter and at her feet was a small heater – the heat softened the glue. You can argue who is at fault, the law firm for using a heater so close to the wood or the contractor for using a glue that didn't stand up to a typical office environment. Since it was a law firm, he charged more than normal, thinking they would probably sue if there were problems. He had enough money from the job to purchase the VAK press, redo the panel and everyone was happy. (Don't you just love stories with happy endings!)
Yellow and white glues are PVA [poly vinyl acetate] type glues and are water based. When used in a VAKuum press, the glues cure by the water moving away from the joint and when it reaches the edge of your substrate it will immediately vaporize in the vacuum. This is noticeable in the vacuum bag as you will see the glue squeeze out bubbling, this is the moisture from the glue. Using a VAK press also has the advantage of pulling the air out of the pores of your substrate, thus getting deeper penetration of the glue. Using a substrate like MDF, which is porous, will give a shorter dry time in the press as compared to using birchboard which is not very porous, thus a much longer drying time in the press. This is particularly important if you are doing something like a curved jamb made from a hardwood (oak, maple, etc.). With a PVA, it takes a long time for the moisture to migrate to the edge of the laminates and vaporize, unless the substrate is something like MDF which absorbs the moisture. I had a customer who purchased a system to make curved jambs and he used yellow glue. It took days to cure in the vacuum bag. Of course this is not cure in hours, the urea glue provides a very ridged glue line, thus much less or no spring back (Spring back not only is controlled by the type of glue, it's also affected by the thickness and number of your laminates as well as the type of wood). The water based PVA glues also have the same problem about a water base finish softening the glue at a seam or the holes in a burl.
Another issue with PVA glues is cupping of a panel. This can occur with other glues; however, most of the phone calls I received were with PVA's. The problem is due to moisture on one side that has not fully evaporated and within 15 minutes of removing a panel from the bag, it starts to cup. The immediate solution is to return it to the bag and press for a longer time. The next pressing should be left in the VAK bag for a longer period of time to prevent this from happening again. Another solution is to veneer both sides at the same time, thus you have balance. This is a good veneering practice as veneering the back side provides for long term balance against uneven moisture absorption.
Normally PVA's have a shorter cure time in the bag, than other glues, which can be from a half hour to several hours. In general, cure time depends on several factors: type of glue, temperature of the shop, size of the work piece and the substrate material. We discussed the substrate material (porous being better and therefore faster curing for PVA glues), the temperature of the shop (putting an electric blanket over the VAK bag can get the temperature up to 100° and cut the drying time in half or less. I have dried wood in the bag using this method.). Temperature also affects the open time. If it's 90° in the shop you normally need to apply the glue, assemble the pieces and get in the VAK bag and under pressure in less than 5 minutes. A urea or epoxy will give you more time.
With PVA's a small 15" x 24" panel will dry faster than a 4' x 8' sheet. I have had several calls from uses who took a 4x8 sheet out of the bag in an hour or so, just as they did with smaller cabinet doors. The 4x8 panel started to blister in the center after it was out of the bag for 15 minutes. The reason being, that the moisture in the center of a 4x8 sheet takes much longer to migrate to the edge and evaporate that it does on a 15" x 24" cabinet door.
Using the correct glue for the job is paramount for success. A customer, in the north, built a curved staircase for a job in California. It shipped by truck and when the staircase was unloaded, the laminates were peeling apart. Why? Yellow glue was used and the truck went thru the Arizona desert during the summer. The temperature inside truck gets well over 100° and the glue softened. This coupled with the fact that the laminates were under tension caused the delamination. This is not a happy ending when the job has to be done again at your expense. Glue is probably the least expensive component of any job. On a $25000 job, spending $100 for glue verses $50 doesn't make sense when the loss can be significant.
Our next e-mail will finish this article with polyurethane, epoxy and urea glues and their relationship with a VAK press along with some general notes.
If you have constructive comments or more information regarding the above, please send it to me via e-mail. (firstname.lastname@example.org) If deemed appropriate, I will modify this article as it is intended to be up-dated when new information becomes available.