Bagging Composite Floats
We demystify vacuum mold composite construction. Part 1
By Cresswell Walker
Because of its incredible lightness and strength, composite construction can seem like chemical sorcery. Space-age materials, finished works of almost any shape, exotic materials such as carbon fiber, and mysterious processes such as vacuum bagging can make this kind of construction a bit intimidating for the uninitiated builder.
In the first part of this two-part series, we will go step by step through the process of creating vacuum bag molds for a set of ultralight floats. In Part 2, we’ll build a set of ultralight floats from these molds using vinylester and fiberglass composite materials.
On a keen, clear morning in mid September, my partner Irena and I find ourselves on the shores of Kootenay Lake in central British Columbia. We are with a friend and fellow pilot, Don Usher, as he flies for the first time to test our new composite floats. With a burst of throttle, he slips the miniMAX off the beach and into the still, beautiful waters. After a few moments of taxiing and testing the floats at various speeds, he lines up into the wind, puts the throttle down, pulls back on the stick, and slides off the water and into the sky in about 8 seconds.
How did we happen to be there watching? This venture really started about five years ago with a quest to build a set of affordable wood floats. Since then, my friends and I have had a lot of fun flying off of wood, but I wanted to see if I could build lighter, more durable floats with composite materials. What better way than to jump into a composite project?
This project has proceeded under the tutelage of the father-and-son team David and Paul Wheatley, proprietors of Pacific Oceaneering and Design in Nanaimo, British Columbia. Having worked with composites for 30 years, they made it possible for the project to take shape. The floats on Usher’s airplane culminated from 100 hours of work building the vacuum molds and another 30 hours building the floats.
Why Vacuum Bagging?
Being able to precisely control the ratio of resin to cloth is the main advantage of the vacuum process. In all composite work, the resin/fiber ratio is the critical strength factor, and it is well-studied and understood. Variations of as little as 5% can result in strength variations of 15-30%!
The target ratio is determined by many variables that boil down to being a function of the particular materials used and whether the composite needs to be strongest in compression or in tension. Vacuum bagging uses atmospheric pressure to press the cloth tightly against the mold surface by drawing the air out of the fiberglass mold before the composite mix cures. Through the vacuum film, a squeegee is used to accurately distribute the resin in the cloth and to remove excess resin. This technique requires a vacuum bagging film, a vacuum pump capable of drawing a vacuum of about 25 inches of mercury, and a mold adapted to vacuum lay-ups.
|The compound curves of the final float shape were made on top of the unfinished wood mold using auto-body fiberglass filler and fairing compounds.
Get the Plug Right
Most original composite projects begin with the process of building a male mold or plug unless one is fortunate enough to have a high quality original part that can simply be copied. Using our original wood float design, we chose Medite (a reconstituted wood composite) for its inexpensive, easily sanded, smooth manufactured finish. We used 1-inch Medite for the frames and quarter-inch medite for the outside panels. The additional lifting surfaces (complex curves) on the bottom profile were made using auto-body fiberglass fairing compounds.
Plug finishing is critical. A mold will precisely reproduce the shape and finish of the plug. What your plug looks like is what your finished project will look like. No magic here, just lots of elbow grease. It was quite a workout getting the plug sanded fair and true. Long boards were used to sand the float faces true, and progressively finer grit papers-from 40 to 240 grit-were used to smooth the surface.
Next we sprayed two coats of polyester finish (Duratec) on the plug, sanding and fairing between coats. You can use any material that will sand and polish to a high gloss finish, but Duratec fills quickly and is relatively easily sanded and buffed to a high-gloss shine. I tried (unsuccessfully) to avoid sanding through to the underlying wood between coats, which meant several touchups were needed.
We sanded the final coat with 120, 240, 400, 600, 800 and 1000 grit paper and then power buffed it with wet and dry polishing compounds to a high-gloss shine. It is possible to work with a less-than-perfect finish on the plug and to then correct the finish in the mold, but it can be more difficult to correct flaws because surfaces are often concave, and the tooling gelcoat in the mold is less easily sanded and polished. The advice I got proved to be correct: Get the plug right before going on to make the molds.
It may be obvious to some builders, but there are two other areas to keep in mind while designing and building a plug and planning how to take the molds off. First, be sure the shapes you are working with will come apart. Obviously, trapped concave shapes won’t come apart. Even right angles can be a challenge; they should be opened 1-2. Corner radii should be rounded, the more the better.
The second aspect to anticipate is how to rejoin the parts that come out of the mold. Butt joins, shoebox fits, H-shaped divider extrusions, and bonded or mechanical attachment are some of the choices to make before starting the molds. Each will have different strength and finishing requirements that you need to design for. Molds for our float project were taken from the plug in three parts: a top extending the first 2 inches down each side, and two side/bottom halves, each containing half the bottom (split down the center line) and the lower and mid portion of the side.
We chose a simple butt join along the bottom center seam, strengthened on the inside with tri-axial mat/roving. After the bottoms are joined and all of the interior bulkheads and other work in the float are complete, the top slides down over inside flanges on the bottoms and is epoxied in place. Just like a shoebox, but with a flush finish.
The Magic of Removing Molds
Once we had the molds planned and had decided how we would join the pieces, it was time to build the molds. This is where the magic of composite construction begins. The first step in creating each mold is to fit and attach temporary flanges to the plug. The separation lines were scribed into the plug and the flanges were hot-glued to a series of blocks attached to the plug with generous dollops of plasticine (no fooling).
Plasticine, the secret weapon of the process, has sufficient grip on the smooth surface of the plug to adhere during the lay-up process without damaging the hard-won surface underneath. For our flanges, we used quarter-inch Medite treated with two coats of shellac to seal the surface. For vacuum molding, a 6-inch-wide flange is necessary to provide room for the seal and air-evacuation tubes.
To create the mold flanges, the temporary Medite flanges were fitted to within 1/16 – 1/8 inch of the float face, and more plasticine was laid into the crack and smoothed with oil and grease remover to create a nice, clean right-angle surface on the flange at the plug surface. Lastly, a generous application of mold release agent was applied to the plug and flange faces. Carnuba wax compound has been used for years for this purpose, but we used another more reliable and semi-permanent agent called Zyvax.
After masking off the plug to protect it from overspray, we sprayed the polyester-based tooling gelcoat onto the plug. The tooling gel is a black gelcoat that becomes the tough inner working face of the finished mold. A delicate hand is necessary here to ensure an even application. Because it shrinks about 1% as it cures, applied too thickly, the gel may pre-release or pull away slightly from the mold before a layer of fiberglass can be applied on top to stabilize it. This can be a problem, especially in the corners.
As soon as the gelcoat is tacky, usually just a few hours after application, the still-curing gel is skinned out. Skinning out is the application of the first layer of mat and resin to stabilize the gelcoat and prevent pre-release.
Now for the second secret weapon of vacuum mold making: surgical tubing. After the tooling gel was tacky but before we applied the first lay-up of mat on the tooling gel, we superglued a continuous length of 1/2-inch outside diameter by 1/8-inch wall surgical tubing onto the tooling gel. This went almost all the way around the flange about 1 inch in from the outside edge.
The tubing acts as a form to create a channel in the inside face of the finished mold. The channel acts as a gasket groove with which to seal the vacuum bagging film (more about the vacuum system later). The tubing was then faired in to create an “ant hill” in the cross section with core bond, a lightweight, low-shrinking polyester resin filler.
While the core bond/tubing was still curing, the first layer of mat was applied to the whole gelcoated area on the flange and exposed plug. The first layer was 1-ounce mat in polyester resin. Over the next four or five days, we finished the mold lay-up by applying five layers of 1.5-ounce/square foot mat. The mold was left in place on the plug to cure more completely before removal. As mentioned, polyester resin shrinks about 1% to a full cure, so leaving the mold on the plug makes for a less distorted finished product.
The last step before taking the mold from the plug was to attach plywood frames to the mold. The plywood frames help stabilize the mold and allow its unusual shape to stand squarely on a worktable or a sawhorse. The plywood frame was tabbed only to the flange areas of the mold, away from the “part” surfaces. Part areas were avoided because the shrinking action of the tabs could pull on the mold and distort the inside finish. Tricky stuff, this polyester.
At last the mold was lifted from the plug and, like magic, the shiny black inside surface of the mold gleamed like the glint of satisfaction in a craftsman’s eye. Well…mostly. A few minor flaws had to be repaired. Where I had sanded (again) through the Duratec on the plug, I gave the tooling gelcoat a chance, despite the release agent, to reach into the surface of the plug and yank out some wood.
With the mold separated from the flange, the surgical tubing was pulled up and out of the flange through the thin layer of tooling gelcoat. The ragged broken edge of gel left behind was sanded to open the slot enough to make it easy to press the tube and vacuum film back into the groove as a gasket seal-a nifty bit of design. The seal works so well that it has not been necessary to charge the tube with air to create a perfect seal, although this is sometimes necessary.
Get the Air Out
The vacuum system is what really sets these molds apart for the production of high-quality parts. It wasn’t until we made our first part that I realized how well this system works. I had watched Dave Wheatley peel back the vacuum seal, lift a corner of the vacuum film and pour a precisely measured amount of catalyzed vinylester resin into the mold. I was surprised that so little resin would soak out of so much glass material.
After resealing the vacuum film, Dave cracked opened the vacuum hose cock and we watched the atmospheric weight of about 10 psi press hard on the resin/glass roving mix and begin to spread the resin like an invisible hand. As we squeegeed the mix evenly throughout the glass roving in the mold, I was also amazed at how clean this process was. Working through the vacuum film, there were no sticky fingers, slopping resin or odor. And the quality of finish on the first completed part lifted from the mold just hours later was to die for. But I am getting ahead of myself. The mold vacuum system consists of just a few parts.
Working from the top down, the vacuum film is draped over the whole work and is gasketed in place using the surgical tubing in the mold groove. The vacuum film is a remarkably thin but tough clear film made from polyvinyl acetate (PVA). Double-sided butyl tape seals the small area on the flange that’s not sealed by the surgical tube along the mold’s end.
To evacuate the air evenly from all areas of the mold, half tubes of 1-inch nylon pipe are laid around the flange. The flanges are placed over the four half-inch holes in the flange that feed into the vacuum system. Resin/air separators, another nifty bit of design, are connected to each of the four holes in the flange. These are made from a 1/2×2-inch steel nipple self-tapped into a half-inch hole drilled in the top of a 3-inch ABS sewer cleanout fitting. An ABS cap is then glued onto the cleanout fitting. This creates a sealed container that can be unscrewed and opened to remove accumulated resin.
Resin pouring into the separator drops in a paper cup, and the air is drawn out of a quarter-inch plastic fitting threaded into the top of the cleanout fitting. A bead of superglue seals the fitting. The quarter-inch plastic hose from each of the separators are T’d into an inline valve that controls the vacuum pressure. The vacuum line from the vacuum pump and reservoir connects to the other end of the shutoff valve.
When all three molds were completed and sitting shiny black on the workbench before me, I was nearly beside myself wanting to start experimenting with different lay-ups. As well, having come down the long and dusty trail of plug building and the messy business of hand lay-up techniques used in the molds, I was eager to start vacuum bagging. It seemed that my composite floats were all but finished…but that’s another story.