Darryl Keil
Posts: 1455
Joined: 2003-05-22
Location: Maine
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In the spirit of Car talk’s puzzler (a quiz on a great radio talk show on public radio about car repair) I thought a vacuum veneer puzzler might be fun. It goes like this, I will pose a problem, and the first person to guess the correct cause will win a free gallon of Unibond 800.
So here it is. A customer who recently purchased a press called to describe a problem he was having. With his press set up and nothing in the bag except a 4’x 4’ platen, his press would recycle approximately every 5 seconds. Shutting off at 25”HG and back on at 21”HG. After about 20 times it would finally shut off and stay off for a while with the vacuum gauge showing just slightly above 21”HG. I knew this scenario to be what is called “flow restriction” not a leakage problem. A leak would recycle continuously where a flow restriction will stabilize eventually when the trapped air in the bag finally makes it to the pump. In this situation, air in the bag cannot reach the pump as fast as it wants to pull it due to some restriction. The switch, which is close to the pump, reads full vacuum and shuts the pump off. Immediately or within a few seconds, the air on the other side of the restriction makes it to the switch and turns the pump back on. The pump will recycle continuously like this until the air on the bag side of the restriction reaches the switches shut off point. Common causes are, something jammed in the hose, a clogged filter element, or glue in the grooves of the platen right around the exit hole. Because the customer was using the press for the first time, I knew it was not a clogged filter and I had him check the hose, which was clear. I even had him e-mail a picture of his platen to make sure it was done right. He had used a sheet of 3/4"MDF, which was nicely lacquered to prevent glue adhesion, and the 1/8” grid of table saw grooves were properly cut over the entire platen surface as well as through the platen sleeve as instructed.
At a complete loss to identify the problem I sent him another vacuum press and bag. A few days later he called back to say the problem was exactly the same. At this, I had him return both presses, bags, and the platen he had e-mailed me the picture of. I wanted to duplicate the exact problem he was experiencing.
When the packages arrived I opened up the two presses, bags, and platen. I took one look at the platen and knew exactly what the problem was. I set up one of the presses with his platen and sure enough, the results were exactly as he described.
What was causing the flow restriction, and why? Note, all the information you need to identify the cause is within this description.
Sincerely
Darryl Keil
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the bag could be drawn down into the grooves, reducing the effective cross section. It does not seem possible that the platen could be collapsing in the vacuum (which would reduce the working cross section of the grooves) unless it had an applied surface like cork or some such.