Forming Large Fiberglass Tubes
I was out in my garage working on my latest project when my wife walked in and asked, "Where do you intend to store that thing?" It was then that I realized she no longer had any interest in knowing, "What is that supposed to be?" I perceived this to be an ominous sign.
I had just started making working on our SS1 team project. This required an airframe tube that was 27 inches long and exactly 14.8125 inches in diameter to match the semi-ogive transitions I made using the bulbous rocket molds. The problem, of course, wasnt the required length. It was the diameter, because there is not a wide selection of tubes available in that size. I considered several options. I could use a 16-inch cement form if I reduced the diameter by removing a lengthwise strip and rejoining the edges (as described in [HERE!]) -- but those forms are pretty heavy, and this project needs the tube to be lightweight . Or I could make a stack of Styrofoam disks as described in [HERE!]) and fiberglass around the outside diameter -- but that wouldnt be very smooth and I didnt feel like filling and sanding. I had almost talked myself out of the project altogether when I came up with a simple way to construct a form that would enable me to shape the tube out of fiberglass without a foam or cardboard core.
I had scrounged up a large piece of Formica veneer -- free, I'd like to add -- since this has a great non-stick surface to use when working with resins. All I needed was a way to hold it in the shape of a cylinder while I fiberglassed around the inside. This turned out to be even easier than it sounds.
Using the technique described [HERE!] I cut round holes in the center of four 24 x 24 x 5/8 inch pieces plywood. To determine the exact inside diameter of the holes, I added the two times the thickness of the Formica to the desired diameter of the finished tube, because there will be a layer of Formica at each end of the diameter measurement. Using a set of calipers, I measured the thickness of the Formica at 0.031 inches; so my formula for the inside diameter of the hole is:
14.8125 + 0.031+ 0.031 = 14.8745 inches (or 14 & 3/4 inches)
After cutting the round holes in the plywood, I then had to trim the Formica to the proper size (the inside circumference of the form equals the outside circumference of the tube). The calculation for the circumference of the tube is diameter times pi, or
14.8125 x 3.1416 = 46.5348 inches (or 46 & 17/32 inches)
Before cutting the Formica, I used a builders square to make sure the corners were at right angles, and I measured twice. Then I marked the lines and cut the Formica with a construction knife and a straightedge. A quick test-fit showed that the holes and the Formica were matched perfectly. The Formica fit really well into the form except for right at the seam; the Formica formed a slight V where the two edges came together.
To fix this, and to prevent bulging at the seam, I calculated the distance required between each plywood brace and cut pieces of 2x4 to fit in between. I positioned the spacers against the edge of the seam to provide continuous reinforcement along the length of the form and secured them in place with construction screws. Then I taped the inside of the seam with aluminum tape (used for duct work and muffler repair). Finally, I inflated a LARGE party balloon -- 36 inches round, purchased at a party supply store -- inside the form. The cylinder trued up to round very nicely. I deflated the balloon and proceeded to prepare the surface with carnauba wax and PVA.
If this is unfamiliar territory for you, you can read up on using composite materials
[HERE!] and on using molds [HERE!].
Once the release agents were applied I cut the fiberglass material. Ive learned from experience that it is extremely difficult to apply a single piece of cloth or mat around the inside surface of any object, so I cut my pieces small enough to cover about one fourth of the inside diameter with about 3/4 inches of overlap at each seam, which in this example was 13 inches x 31 inches. I chose to use one layer of 1.5 ounce fiberglass mat and one layer of 6 ounce cloth, bonded with a really stiff polyester marine resin -- and this proved to be a good choice, very tough and still lightweight.
I sprayed the inside of the cylinder with two-part polyester sanding primer, allowed it to cure, then applied the mat, and applied the cloth last. After the entire inside circumference of the cylinder was covered, I re-inflated the large party balloon inside the form. The balloon simultaneously trued up the form, pressed the glass against the Formica, and forced any air bubbles and excess resin out the edges. Then I let the resin completely cure.
I had to trim some excess glass off one edge of the form to get the plywood braces off, but otherwise they slid off easily. I ran my knife up the seam to cut the tape on the inside surface of the form, and the Formica literally sprang off the newly formed tube. The tube I had just made was amazing -- perfect to the eye except for a shallow indentation of the tape at the seam and a slight keel where the two edges came together. This was erased with some Bondo® and light sanding.
I estimate that I spent about $30 on materials for the tube and the form. I got the Formicarap for free, but remnants are not difficult to come by at building supply stores. The big balloon cost me $6.
This method works best for large diameter tubes because Formica will break if it is rolled up too tight. (I'm the same way, to be completely honest.) It becomes less unyielding if you let it lay out in the sun for a while to warm up. (Also like me. Add an ice-cold Dos Equis Amber and I'm absolutely pliable.) You could potentially replace the Formica with sheet metal (steel or heavy aluminum flashing), particularly if you wanted to make a tube with a smaller diameter. With sheet metal you could overlap the edges and minimize or even eliminate the keel; just be diligent with the application of release agent. And remember you have to reach inside it, so avoid making the cylinder exceedingly long in relation to its diameter.