What Is a Composite Part?
A part that combines a resin and reinforcing strands can properly be referred to as a composite. This includes what is commonly referred to as fiberglass (technically, it should be called fiberglass reinforced plastic), but there are many other materials that are used in composite parts. To achieve desired properties in composites, the chemistry of the resin used, the type of reinforcing strands, and the ratio of resin to reinforcing strands can be varied. In general, the more strands in the mix, the stronger the final part becomes.
Why RTV Molding?
There are many desirable features of molded composite parts. Molded parts are almost always more durable, repairable, heat-resistant, and lighter than comparable strength carved wood parts, and they don’t soak up oil or moisture. In the case of a molded composite cowl, its thin wall property (about 0.05″…and you can’t carve a wood cowl to match!) gives much better air flow around the motor as a big side benefit. Molded composite wheel pants can handle bigger wheels and mount more conveniently. Other possibilities are wing tips, gear legs, dummy exhausts, spinners, etc., and all are made in a similar way.
Once a proper RTV molding is made, composite parts can be replicated quickly and easily, each one almost identical to its predecessors. So, you can make as many parts as you and your friends need, you can sell them, or , in the event of a crash, make a new one exactly like the original in just a fraction of the time it would take to carve and hollow another .
Molds and Plugs of RTV Molding
Actually, anything you can lay resin in or on and pull away a part with the opposite shape can be considered a mold. Male RTV molding can be used, but a part made from a male mold has a smooth interior and a rough exterior, which requires quite a bit of work to make ready for a finish. A female mold, on the other hand, produces parts with smooth exteriors, so every part molded in one will come out with a smooth exterior that is exactly the shape desired.
To create a female mold with a smooth, perfectly-shaped interior cavity, a male plug is carved and finished, and the female mold is cast around the male plug. This requires just a bit of extra work, but keep in mind that there is no need to hollow the plug, so making one goes quite quickly.
Plaster and even composite materials can be used to make a female mold, but in my experience, the absolute best overall material to use is RTV molding rubber (room temperature vulcanized). It’s easy to handle, and a female mold made from it is flexible enough to allow a flex and peel technique to free the molded part without doing any damage to the mold, so it can be used over and over again.
Polyester or Epoxy?
Polyester is cheaper and weaker than epoxy, but it’s certainly adequate for first attempts. Polyester gets drops of hardener, more will accelerate the cure. A downside is that finished parts often have pin holes. Polyester has a strong odor, and you need to wear eye protection because the catalyst can be highly dangerous if it gets in your eyes.
Reinforcing Fiber in RTV Molding
The material used for reinforcing fiber has great influence on the overall strength and ultimate weight of the finished part. Molding is best done with woven cloth, typically two to four ounces per square yard, in as many layers as needed to get the desired strength. Materials most used in composites for model applications are E-glass (fiberglass, white color), Kevlar (yellow), or carbon fiber (black).
E-glass is the least expensive and is fully adequate for most molding. Kevlar is much more expensive, difficult to bend and cut, and would be best to avoid if you’re just starting out. Carbon fiber is the strongest, but by far the most costly, hard to buy economically in small quantities, and is best left for after you’re a seasoned molder. Carbon veil is easy to handle, but it’s not well-suited to molding composite parts typically found in models.
In full-scale aircraft composite parts, the weight ratio of the fibers and the resin is carefully engineered and calculated for each part.
Fillers in RTV Molding
All epoxy suppliers sell fillers, such as a Cab-O-Sill, milled fibers, and Q-Cells. These are useful on some shapes to make a paste to get good edges and corners. Painted in corners, they help define small details, too. A paste made with filler is useful to fix flaws that may appear in the final part, also.