Saved from the Junk Pile!
Part V: Vacuum Bagging the Core
Note from Mark C.: Well folks, it's been 3 years since the last part of this series was posted, and below is all I have for the rest of this project. I felt it was better to at least show what had been completed up until now rather than nothing at all. If it ever does get finished you can be sure that I will update you.
When I first started this project, it was to be a low buck rebuild I had always promised myself I would get to. The timing was right and I started off on my venture. Having done several HydroStream rebuilds I felt inclined to share the knowledge and experiences through this step-by-step rebuild. Most of the fiberglass work I have done was learned through trial and error (not necessarily in that order), and over the last twenty years I have gained a lot of practical know how. Sometimes in my haste to get the job done, even I make a rookie mistake. In the following pictures you will see a lot of duct tape on the seals. I'll get into that in a minute. The one thing I am sure of is that there are always several different ways to tackle the same project, each having its finer points, but some better than others. The pictures featured below are a demonstration of how I went about this rebuild, but in hindsight, I could have done it better. Life has slowed down the process; since the last article I have had two children and moved twice. Sorry for the wait.
All of the work has been performed in my backyard, inside a tent purchased at Costco. Not the ideal Scenario, but a great way to save ones garage. I would highly recommend purchasing a few Thru Bag vacuum connectors and the proper bagging film. I tried to use 3m clear masking film, after chasing all the leaks down, finally realized that the film was semi porous. Note all the Duct tape.
Working alone on the recore
requires that it be done in small sections. The stringers being placed in the
boat first, didn't make it any easier either.
That's another subject of debate. I
have my reasons for doing it this way. The
core is going to be laid in a bed of 1 1/2 oz matt and covered with the Biaxl,
all at the same time. Once the
sections being worked on are wetted out and in place, vacuum will be pulled.
An ideal fiberglass layup minimizes the amount of resin because resin by
itself adds weight without adding strength. When doing fiberglass layup by hand,
it is hard to get the minimum amount of resin because you need enough to soak
the cloth and the cloth doesn't lay completely flat against the surface. The
solution to this is vacuum bagging.
Vacuum bagging uses atmospheric pressure to press the cloth and Core tightly against the surface being covered so that the excess air and resin is squeezed out and soaked up in a disposable outer wrap. This technique requires a vacuum bag, vacuum pump capable of pulling a significant vacuum (at least 25 inches of mercury) and various accessories and supplies. The amount of force that vacuum bagging can produce is enormous. 25 inches of Mercury translates to roughly 12.25 PSI - for a 12" x 12" area, that's over 1700 lbs of force just on that one area of your core layup - try that with sand bags! There are numerous types of vacuum pumps available, and it is critical to choose the right one that works for you. Most people use either a mechanical pump (usually have an electric motor) or a venturi driven pump. The venturi style pump uses compressed air that creates vacuum by flowing the air through a restricted orifice or nozzle (Bernoulli's Principle). Mark Casper generously offered to supply me with a pump for my project. He chose to send me a multi-stage ejector pump (an extremely efficient venturi pump with multiple internal nozzles) manufactured by PIAB since it has the advantages of having no moving parts, no heat generation, and low noise. Set up properly, it is also more efficient and cheaper to run than a mechanical pump. Mark also set it up with an "energy saving" system which monitors the vacuum level in the sealed system and shuts off the air to the pump once the vacuum level reaches a certain maximum set point. Once the vacuum level drops below a minimum set point, the system automatically kicks on again to raise the level back up. So for a tightly sealed system, you can actually hold a user-set vacuum level range for hours without any energy consumption at all since the system is shut off until it needs to kick in again.
Unfortunately, it is difficult to achieve a sealed non-leaking system, so choosing the right model of vacuum pump is important since you have to address not only the vacuum level that it can reach, but also the vacuum flow as well. Vacuum flow is the rate at which atmospheric pressure is removed from the system, or the amount of outside atmosphere that flows through a pump. The more leaks you have, the higher the flow required from the pump. If there is one disadvantage to using a venturi style pump it is that you need the air to run it. The particular model pump I used requires around 4 SCFM of air to run it. I have a large air compressor, so this was no problem.
Vacuum bagging requires that the part being laminated be covered with cloth (as in hand layups). The part is covered with a thin film which is porous and will not stick to the resin, called "release," or “Peel Ply" and a thick layer of absorbent material, called "breather." The area is then covered with bagging film and the air inside is removed by a vacuum pump. Because the air inside the vacuum bag is removed, the air pressure from the atmosphere outside the bag pushes on the core, pressing the bag against the breather. The excess resin is squeezed out of the cloth, passes through the release and is soaked up by the breather. The breather also allows the air to flow away from the area and out of the bag. Peel ply is optional. Most often it is used to give the laminate a rough, rather than smooth, finish. Many engineers consider this a bondable finish, and it usually passes a wet-out test. I've seen tests, however, which show that sanding still gives a much better bond.
cure, the bagging film, the breather, and release are pealed off. Because resin
does not bond to the release, this generally comes off easily. At this point,
the breather has soaked up the excess resin…….
That's all I have, except for the following pictures which were going to be used in upcoming articles. As you can see, it looks like the boat turned out beautifully, which was an amazing transformation from where it all started from:
If you have any questions, please contact Chris at Boatmender@aol.com