Vinylester? Is that anything like “Epoxy fiberglass resin” sold by TAP plastics? ( They also sell “Polyester fiberglass resin” ). Any hints on making small fiberglass parts?
I’m not an expert on fiberglass construction. I have read about it quite a bit, I have some reference materials, and I built/assembled most of a Glasair III for some good friends (currently flying, 240 knot cruise). Prior to the GIII I did some work on a Glasair I, and I have tinkered (minor repair work) on a few others. People tend to develop preferences in composites based on their personal experience. I consider Ron Alexander (Sportair Workshops; some of which we have attended) to be quite expert in composites, but there are many others like him. I believe the following info to be accurate, but I will certainly accept and incorporate any corrections.
“Pure” Epoxy resin is not quite the same as Vinylester resin. You can get “promoted” Vinylester resin, and the required MEKP catalyst, from Aircraft Spruce. You can also get the tubes of clear MEKP at auto supply stores, but seldom the VE resin. Some local hobby shops or boat places may have it. If you can find it locally it may save you a HAZMAT fee (unless AS&S has stopped charging it on this particular resin). The Vinylester has a very low risk of allergic reaction, and lower flammability, compared to polyester or epoxy. It also has a low shrinkage rate, and is very strong in proper layups. The Polyester is a very poor choice for any work, let alone aircraft work. It is relatively weak, is quite flammable when cured, and has a very high rate of shrinkage (leading to warping, cracking, and “alligatoring” in the finish coating). The only advantage of Polyester is that it is cheap and readily available at an auto store.
Epoxy resin is plenty strong if you want to use it. It is quite a bit more sensitive to the proper mix ratio, and usually costs more than the Vinylester. It also carries a much higher risk or reaction. What’s more, the reaction exposure risk is cumulative. In other words, the third (or second or fifth) time you use the epoxy may be the time that you break out in severe hives or something. Once you develop the sensitivity, you’ll have it forever. It’s pretty discouraging to be halfway through a project, only to discover you can’t work with the materials any more. This has happened to many people in the homebuilding world, especially those working with epoxy resins.
There is nothing quick about this process; not even describing it. On the other hand, doing it right takes very little more time than does turning out crude junk. Making a very smooth and exact mold is key. Then you coat it with release agent and go to work. Note that the release compound I recommend is water soluble. That way if the part tries to stick to the mold, you can hold it under hot water to help pop it loose. Make sure that the mold shape is such that you can get it out of a cured part. When using this mold-method, I drape on a layer of glass first, to make sure that I don’t rub off the release agent with the epoxy brush. The resin will penetrate the cloth from the outside, and will stick the cloth to the mold. If you get the very thin hobby shop cloth, it will smooth around the corners quite well when you wet it with the resin.
There are many types, styles, and thicknesses of the glass cloth. Each has a different cost and intended use. For making small non-structural parts, especially those with complex shapes, you need very thin and flexible cloth. The cloth should have a universal weave (known as “Uni”, having the same threads running in both directions, perpendicular to each other). You actually cut the piece out “on the bias”. In other words, rather than cutting the piece out while following the thread lines, you cut it on the diagonal to the weave. If you look at a square or rectangular mold, the lines of the threads should point toward the corners, not the sides. That is what enables you to smoothly mold over the corners. Hard to explain, but easy to experience.
Have all the glass pieces cut and ready. Have a little food scale that shows grams (to weigh the resin), and the small chart that shows the number of drops of hardener for the number of grams of resin, for different working and curing times. With Vinylester, you can greatly vary the pot life by varying the mix ratio, as long as you don’t go below the minimum mix. While you are using a mix, at the first indication of thickening or gelling, finish your current task and discard it (mix a new batch). Several small medium-speed batches are much preferred to trying one large, slow batch. This is because of the risk of an “exotherm”, or thermal runaway. Larger batches generate enough of their own curing heat to dramatically accelerate their cure; they can get hot enough to smoke, if the depth of mix is an inch or more. If you keep trying to use gelling resin, it will make a huge mess. You can even wind up starting over from scratch, when the tip of the brush starts pulling the lay-up apart. Gelling resin also will not penetrate the glass cloth properly.
After wetting the first layer, lay on the next layer, dabbing it out smooth and wetting it out. The idea with the resin is to use just enough to color the glass cloth. Excess resin just adds weight, not strength, and little resin pools will crack easily. All the strength comes from the cloth, not the resin; the resin just helps hold the cloth in the desired shape. It the same principle as prestressed concrete bridge beams. All the strength is in the steel, as the concrete only has compressive strength (and little tensile or shear strength). The purpose of the concrete is to keep the steel in the right place, so it can bear the load. Due to the thin hobby shop glass, you’d probably be best off with at least four layers, and five or six would be better. Try to be sure that you work out any air bubbles or wrinkles with the tip of the resin brush, layer by layer. Getting good at that is where the”learning the art” comes into play.
Make a little sample layup on the side as you go. That’s the sample you use to tell when the cure has completed, as well as determining the final part strength. If you build a kit plane, you may make thousands of these little samples. That’s how you know (and can prove) that the content of the work on the interior of a wing (for example) will have developed the target strength.
After the final cure, pop the part off of the mold. You can use heavy duty shears to trim the perimeter. I prefer to use a Dremel cutoff blade for glass trimming and cutouts. The best blade is about an inch in diameter, with a segmented blade perimeter, and either a carbide or ceramic coating on the blade. You can use a round sanding drum on the Dremel to make nice contours at the ends of the slots, and to round the corners of the mounting flange. A sanding block will make nicely squared borders on the outside and on the edges of slots.
Do NOT use common auto body filler to finish out surface imperfections. Use the “Blue filler” called SuperFil (Aircraft Spruce) to finish off surface imperfections. This filler does not shrink, is very strong, weighs next to nothing, and sands well. Then you can use sandable primer and finish paint.
You will need your patience, but you can make very nice, strong, permanent parts this way. There is far more to the composites business than I have described here; this is just s simple primer for small parts using basic materials. Here are examples of my preferred materials from the online Aircraft Spruce catalog. I can’t include the pix in the Q&A:
Fiberglass Mold Release –
P/N 01-30600 $6.69 /Qt.
Plastilease 512B, a film-forming, water soluble parting agent, assures clean release of fiberglass parts from molds. For application by brush or spray.
Vinyl Ester Resin –
01-07350 VINYL ESTER RESIN 411-350PA 1G $31.75
01-07325 MEKP NORAC CATALYST 4 OZ $2.85
Dow Chemical’s Derakane 411-350 PA Vinyl Ester Resin is an epoxy-based Vinyl ester designed to provide superior toughness and high corrosion resistance. Many leading kit aircraft manufacturers use vinyl ester resins extensively due to its quality and ease of fabrication. We furnish medium “promoted” vinyl ester 411-350 PA resin which includes CONAP, DMA the resin is cured by adding the MEKP which is furnished with the kit. Gel times vary according to the amount of MEKP added and the ambient temperature. Shelf life of promoted vinyl ester resin is short at only a few months.
09-28250 POLY-FIBER SUPERFIL EPOXY A&B $16.45
09-28260 SUPERFIL INDUSTRIAL 3 GAL KT $147.00
Bondo for airplanes! Superfil is an ultra-lightweight epoxy filler for rough fill and contouring. Comes in two containers. Mix, apply with a squeegee, and let dry overnight. More consistent than hand-mixed micros. Weighs only 3.8 lbs. per gallon. Ships as NON-HAZMAT.