Vacuum Bagging

Creating wooden laminates in your shop

AS A HOMEBUILDER WORKING on a Pietenpol Air Camper, I’ve had to contemplate a fair amount of woodwork. While some of the solutions are fairly straightforward and easy to resolve, others have taken a bit of research and have occasionally required me to learn a new technique, which is one of the best things about home building—the learning. For those familiar with the Air Camper, you know that many choose to construct the center struts (or cabane struts) out of wood. These vary in type from solid planks of walnut or Douglas fir to a combination of types.

After returning from our annual trip to Brodhead, Wisconsin, a friend and fellow Pietenpol builder, Jim Markle from nearby Pryor, Oklahoma, suggested that I try vacuum bagging the materials for my center struts. He had used this process in the past with excellent results, so I took him up on his offer and prepared some veneers of Sitka spruce and Brazilian cherry to see how they would look. The resulting assembly was very solid, straight, and nice-looking even before varnish was applied. I was so pleased with the outcome that we started to discuss wooden lift struts. I did a lot of research and found this to be a suitable option, so I set up my own vacuum bagging operation and pressed on. The following text will describe the necessary equipment and how easy it is to vacuum bag parts in your own shop.

Before I go any further, I feel like I should establish right up front that I am not an expert in wood, laminates, composite construction, or vacuum bagging principles. I am simply using techniques that have been established and proven by many others before me, including the vacuum bagging processes, equipment, and the use of wooden structures as components in a homebuilt aircraft. This is not intended to be a discussion or debate on the strength of wood or whether it is suitable in my application. I’ve done my research, and I am convinced that my approach is sound. If you would like more information on testing methods I would recommend that you review ASTM D143, “Standard Methods of Testing Small Clear Specimens of Timber,” and ASTM D2555, “Standard Test Methods for Establishing Clear Wood Strength Values.”

WHAT IS VACUUM BAGGING?

Vacuum bagging (or vacuum bag laminating) is a clamping method that uses atmospheric pressure to hold the adhesive-or resin-coated components of a lamination in place until the adhesive cures. The effectiveness of vacuum bagging permits the lamination of a wide range of materials from traditional wood veneers to synthetic fibers and core materials.

Benefits of vacuum bagging:

• No clamps required—even clamping pressure is provided by vacuum pump and vinyl film.

• Control of resin content.

• Far easier, cleaner, and more evenly applied than clamped by hand.

• Custom shapes.

• Materials are inexpensive and reusable.

You may recall that 1 atmosphere = 29.92 inches of mercury (inHg), which is equivalent to 14.7 psi. In rough terms, 2 inHg = ~1 psi. Therefore, if you were to vacuum bag a 1 square foot laminate (144 inches square) at 20 inHg (10 psi), your effective clamping force would be 1,440 psi evenly distributed over the entire surface of the assembly.

THE EQUIPMENT

At a minimum you will need a vacuum source and some type of film for bagging the parts and applying pressure. Most systems also include a vacuum gauge and a bleeder valve of some sort in order to fine-tune the applied pressure. Image 1 shows the components of a typical vacuum system as illustrated by West Systems. As you can see from this illustration, the system is fairly low-tech. This setup shows several layers— bag, breather, release film, etc. While this is one approach, my process used a simple vinyl bag or tube, which is suitable for bonding flat laminations.

My setup uses a Gast vacuum pump that is capable of a maximum of 24 inHg at 1.5 cubic feet per minute (cfm). I have the pump connected to a PVC cylinder, which acts as a buffer between the pump and the parts. This part is not entirely necessary, but the cylinder gives me a place to mount my vacuum gauge and bleeder valve, plus the volume in the cylinder acts as a buffer and smoothes out the delivery, which makes it easier to read the gauge and adjust the vacuum (i.e., pressure on the parts). Finally, a vacuum line is connected from the PVC cylinder to the vinyl bag, or tube. While it is not entirely clear in this photo, the tube gets clamped at both ends once the parts have been placed inside.

The pump unit is the most expensive part of the system, depending on your scrounging abilities. The Gast type pumps are fairly expensive if purchased new, but used models can be found for a significant savings. An alternative might be a pump unit found in a refrigerator—many have used these pumps with great success as they are capable of pulling 20 inHg or more. Others have had reasonable results using a Shop-Vac type vacuum for small projects. Another alternative is the kits or packages that can be purchased from specialty suppliers such as ACP Composites.

Most importantly, whatever pump you use should be designed for continuous duty, like the Gast type, or equipped with a pressure switch that will cycle the pump on or off as needed to maintain adequate vacuum levels. This is important because vacuum pressure will need to be applied until the adhesive or resin has cured, which could be several hours or days. Pumps not designed for continuous duty could burn up prematurely if not cycled properly.

Pump selection should be based on the size of your project. In my case, I was constructing assemblies made of flat veneers that did not require a high volume (high cfm) or high vacuum (high applied pressure) pump. Small and flat panels consisting of few layers of glass or flat veneers can be accomplished using a fairly small pump—1-2 cfm at 5-6 inHg (2.5-3 psi). Large panels or intricate molds (such as aircraft cowlings) require higher displacement pumps to apply adequate pressure, which is beyond the scope of this discussion.

THE BONDING PROCESS

My center section struts were constructed of five layers of Sitka spruce and Brazilian cherry material. The length (24 inches) and width (1-3/4 inches) were common with each plank, but thicknesses were 1/4 inch for the core (1) and 1/8 inch for each veneer

(4), for a total thickness of 3/4 inch.

In order to ensure good coverage and minimize voids, resin was applied to both surfaces of the materials to be joined using a small squeegee. In this case, we used a tongue depressor cut at an angle to work as a squeegee. No need to spread it too thin, but you don’t want to be wasteful either as most of the excess will simply be squeezed out of the joint under vacuum.

With all the glue applied the parts were placed in the bag, and vacuum pressure was slowly applied. This is where a good bleeder valve comes in handy. On small projects, a good pump will draw air out of the bag pretty quickly, which doesn’t allow much time to ensure proper alignment of parts. With a bleeder valve you can slow (or throttle) the drawdown and allow time to align or situate the materials. Once the majority of the air has been drawn out and the parts are aligned, close the valve a little more for a tighter squeeze and double-check alignment. When everything is looking good, adjust the valve to the desired level of vacuum and walk away. That’s it! This clamping method really is very quick, clean, and easy.

THE RESULT

When the struts were removed from the bag they were straight and solid—and I mean solid like a baseball bat. The glue joints were visible, but minimal and equal in width for the entire length. I was very impressed with this technique.

With the pieces squared up I was ready to work over the edges. This part doesn’t have much to do with the vacuum bagging process, so I’ll simply say that I rounded the leading edge using my table router and tapered the trailing edge using the table saw and some mild sanding.

As mentioned before, the appearance of my center struts motivated me to try to make a matched set of lift struts, which I did. The lift struts were a little more difficult than the center struts, but only in relation to time.

The lift struts required several operations in order to cut and assemble, but the fitting and gluing processes were essentially the same. Given their size and higher part count, the lift struts required several vacuum bagging sessions, but it was a lot more fun and way easier than using traditional clamps.

The lift struts are made with a Lloyd’s certified plywood core, the leading and trailing edges and outer caps are Sitka spruce, and the inlays are Brazilian cherry and African mahogany. From the normally visible edges (not the end cuts) these look like scaled-up versions of the center struts, but by using Cherry inlays rather than full-width planks, I’ve reduced the weight by a fair percentage. Just another example of what can be done using the vacuum bagging process. As with the center struts these came out of the bag looking straight and feeling very solid.

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