About Reinforcements
What is a reinforcement?
Resin by itself is very weak but combined with a fiber of some sort it can be made much stronger than either material alone. This is called a composite material, you may be familiar with fiberglass or carbon fiber, generally these are used with resins to make strong parts.
For most projects we generally suggest that you use fiberglass as your reinforcement material. It's inexpensive, requires little special consideration and is generally plenty strong enough for your project. We're not going to talk much here about Carbon Fiber, Kevlar, or other exotic reinforcement materials but they may be suitable to your project. Most of the nomenclature is the same with other types of reinforcements.
Chop Strand Mat
Mat, or CSM is exactly what the name implies, a mat of short fibers. Generally these are held together with a glue that dissolves in resin. Mats can be used alone or with other types of roll goods to reinforce your parts. Many parts will be built entirely out of mat. But often mat is used to fill space between rough surfaces, add a surface texture, or prevent the texture of other roll goods from "printing" though to the exterior surface. Mat is also easier to use in corners and is often used to prevent air bubbles in hard corners of parts.
Structural Reinforcments
The baisc building block of the fiberglass part is the structural reinforcment. These come in many different styles. You may be familiar with cloth, 1708, 1808, or woven roving. Both cloths and woven roving are what we call plain weave materials, and they follow an over-under weaving pattern to bind the fibers into a sheet. As a general rule cloths are thinner materials, and woven roving are heavier.
Modern materials include the 1708 and 1808 class materials, instead of weaving bundles of fiber, the bundles of fiber are instead laid next to and over each other in alternating layers and stitched together with thin material. They are non-woven and often called stitch mats because they usually have a mat stitched to the bottom of the material. These types of materials are perferred because the non-woven nature of the material is stronger; woven materials necessarily include kinks where the fiber bundles cross each other.
We generally suggest to our customers that they use a modern nonwoven material, typically 1808 or 1708.
But what does the 1708/1808 part mean you may ask? It is simply the weight of the material and the weight of the mat on the back; 1808 is 18oz per square yard, of nonwoven fiber, with 08 oz per square yard of mat on the back. 1708 and 1808 differ only by the direction of the fiber bundles, 1808 is along and across the roll, 1708 has it's fiber bundles at a +-45 degree angle to the roll. The BXM type materials can be useful for bonding stringers, and transoms because both sets of fibers can be made to cross the "bond-line".
By: Capt. Ken