A vacuum bag is a bag made of strong rubber-coated fabric or polymer film, open at one end, and used to bond or laminate materials. In some applications the bag encloses the entire material, or in other applications a mold is used to form one face of the laminate with the bag being single sided to seal the outer face of the laminate to the mold. The open end is sealed, and air is drawn out with a vacuum pump. As a result, uniform pressure approaching one atmosphere is applied to the surfaces of the object inside the bag, holding parts together while the adhesive cures. The entire bag may be placed in a temperature-controlled oven, oil bath or water bath and gently heated to accelerate curing.
Laminating of flat objects can be performed more efficiently in a heated laminating press, but when the objects are curved or have irregular shapes, a vacuum bag is normally used. This is true in large or commercial woodworking facilities, but recreational woodworkers rarely have access to a heated veneer press.
Typically,polyurethane or vinyl materials are used to make the bag, which is commonly open at both ends. This gives access to the piece, or pieces to be glued. A plastic rod is laid onto the bag, which is then folded over the rod. A plastic sleeve with an opening in it, is then snapped over the rod. This procedure forms a seal at both ends of the bag, when the vacuum is ready to be drawn.
These bags can be homemade, or purchased. They are available in many sizes.
A "platen" is used inside the bag for the piece being glued to lay on. The platen has a series of small slots cut into it, to allow the air under it to be evacuated. The platen must have rounded edges and corners to prevent the vacuum from tearing the bag.
All bags have access to the vacuum via a nipple which penetrates the bag.
When a curved part is to be glued in a vacuum bag, it is important that the pieces being glued be placed over a solidly built form, or have an Air Bladder placed under the form. This air bladder has access to to "free air" outside the bag. It is used to create an equal pressure under the form, preventing it from being crushed. 1
