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Part 1
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The platen system inside the vacuum bag serves two purposes. First, it helps to keep the veneered panel flat while the glue is setting up. More importantly, the grooves on the platen allow air to flow out of the vacuum bag to the vacuum source thereby creating evenly distributed pressure inside. I have seen numerous ways of sandwiching the veneer to the substrate with the use of cauls and platens. I truthfully do not know which is better, so I will limit this article to the method I use which has never failed to bring anything less than optimal results. 
Platen Material If you are not using breather mesh (as described above), you'll need to make 2 platens from ¾" melamine. The use of a melamine-faced sheet good instead of plywood or MDF is that it will prevent the veneered panel and any excess adhesive from sticking to the platens. A coat of paste wax will ensure easy removal of the veneered panel from the press. The wax will last though many glue-ups. The platens should be at least 4 inches smaller than the inside measurement of the vacuum bag. This will prevent the vacuum bag from stretching at the seams which will cause the bag to split. You can use a much smaller platen system if you do a lot of smaller panels. For my 2 x 4 bag, I have a 23" x 48" platen setup and a smaller platen setup that I use for drawer fronts which is 8" x 24". The bottom platen canshould have a border (1/4" to 3/4" thick and approximately 1¾" wide) around the outside perimeter as shown in the picture below. This bordered bottom platen will help keep the top platen in place while in the press. The length and width of top platen should be 1/4" less than the bottom platen to allow easy removal after the pressing cycle is complete.
The sharp edges of the platen system will puncture the vacuum bag while under pressure. You can significantly increase the life of the vacuum bag by using a round-over bit in your router to ease the edges of the platens and border. Next, set your table saw blade to a height of 1/8", and cut grooves from the front to the back and side to side on the outside of the top platen. No grooves are needed on the bottom platen. The spacing of the grooves on the top platen is not critical but a 2 to 4 inch layout works well. These grooves will direct the air to the valve stem and are absolutely necessary. Remember, these grooves are on the outside when you place them in the vacuum bag.
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