vacuum bag problem

General Discussion
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I’m wondering if someone can tell me what I’m doing wrong. The board I’m building is one and a half pound EPS that I cut from a large block. I hand lammed a layer of four ounce glass on the bottom and deck using a wetout table. This went pretty well and only added thirty ounces to the weight of the board (10 ft long board). I then put a layer of cordua, another layer of four ounce glass, some perf ply and a layer of breather fabric, the sort of wooly stuff from fiberglass supply, and put it in the vacuum bag. I may have momentarily pulled a slightly higher vacuum but when I got it settlled in, I adjusted it to about fourteen inches of vacuum. I noticed the surface of the bag was kind of dimpled into the breather cloth. When I pulled the board out, the dimples had gone into the surface of the board, apparently even into the already cured glass coat. Even though the cordua and glass still came out light (used the wetout table again) filling the dimples added a lot of extra weight and work with less than stellar results. Also, considering two layers of glass and the cordua, the bottom was still flexible to the thumb test and I wound up adding another layer of glass to stiffen it up. Any advice on how to achieve a smooth surface under vacuum would be much appreciated.

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Thanks Parthenon for the advice. I must have pulled the vacuum much higher than I remember. The cloth was streched tight but under vacuum formed into a lot of little beads (for lack of a better description) and those beads left the pebbled surface on the board. Thinking of making a bag from silicon sheet and, as you said, since there isn’t much resin to pull out, it might provide a better finish, under less vacuum, of course.

Lemat, thanks for the comment. I know you’re not a fan of less resin, but I’m trying at some point to construct a 9’ 6" longboard that weighs around 12 pounds and still has some duribility. I’m thinking of trying cork in the bottom layup. What’s your thought on that? I’ve been using wood veneer on the decks, including the board you commented on. and that seems to work pretty well for light but stiff.

Thanks for the comment Mark. Yeah, I too, was pretty impressed with the damage I did to an already lammed board. I didn’t realize how much pressure you could pull by momentarily running the guage up. Still going to try to find a better bagging material.

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Hard to say without seeing pictures, but if I were to guess, I would look at two areas. the wooly breather fabric from fiberglass supply; did it “bunch up” when you applied the vacuum? or more likely you pulled too much vacuum & crushed the foam. It’s always better to start with lower vacuum and increase; not the other way around.

My guess is that you pulled too many inches of vacuum. At 14 inches, you are pulling almost 1/2 Atmosphere (1 atmosphere= 29.92 inHg). That is the equivalent of almost 7 psi. Think about this : 7 psi x 12" x 12" = 1008 pounds per square foot ( alot of pressure). For reference, when I “bag” my laminates, I never exceed 11 inHg & try to keep it at 10 or slightly less (if possible).

Also, You said that you used a wetout table which should give you a fairly light resin to cloth ratio so theres no need to pull such a high vacuum.Dial it down on your next board & youl’ll soon figure out what works best for you.

Cheers!

 

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When you press the skin with tumb you make it flex while foam compress. If you want to increase stiffness you need to increase foam compression stiffness and/or skin flexural stiffness. Flexural stiffness dépend of specific stiffness of material and thickness of skin. You use a bit of stiff material, glass (70 GPa) and much mid stiffness material, resin and nylon (around 2 GPa), and you compress it with thin with vaccum bag, ended with a flexible skin. If you use adéquat élongation capibility resin, higher than nylon, you will have a though skin, hard to ding, but flexible, easy to buckle. Hope you have a good thick stringer.

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either the foam is really weak, or you pulled way too much vacuum.  You crushed a board that already had a cured hand lamination on it?  Either that, or the dents were already shaped into it and you didn’t notice them until it was bagged?my pump cycles on and off between 10 and 12

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And then about 15 when installing logo on top of textured finish

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