What is Prepreg?
"Prepreg" is the common term for a reinforcing fabric which has been pre-impregnated with a resin system. This resin system (typically epoxy) already includes the proper curing agent. As a result, the prepreg is ready to lay into the mold without the addition of any more resin. In order for the laminate to cure, it is necessary to use a combination of pressure and heat.
Advantages of Prepreg
"There are several advantages to using a prepreg rather than using traditional hand layup Maximum strength properties. In a hand layup, it is difficult to achieve 50% resin content. This means that the finished laminate weight is 50% fabric and 50% resin. Typical hand laminates, even when vacuum bagged, end up with a significant amount of excess resin. Excess resin increases brittleness and reduces overall properties. On the other hand, most prepreg contain around 35% resin. This is ideal for maximum cured properties and generally impossible to achieve in normal hand lamination.
1. Part uniformity and repeatability. Without the pitfalls of human lamination techniques, there will be neither resin-rich areas nor dry spots. Thickness will be uniform and every part that comes out of the mold has a theoretical likelihood of being identical. There is still a margin for error in vacuum bagging techniques, handling, etc., but prepreg reduce these problems significantly.
2. Less mess and less waste. Prepreg will bleed excess resin during the curing process but all of the excesses of hand layup – cups of resin, messy rollers, drips – are no longer a problem. Plus Fiber Glass prepreg are handled at room temperature so you are not fighting a clock trying to avoid your resin setting up before you are ready.
3. Less curing time. After the heat curing cycle is completed, the part is ready for service. You do not have to wait the standard 48 hours to allow a full cure as in a typical hand lamination.
4. Better cosmetics. Mold preparation and mold release is still required and will directly affect the cosmetics of the part just like a hand laminate. However, Fiber Glass prepreg virtually eliminate air bubbles and a smooth, glossy surface is more easily attainable.
Disadvantages of Prepreg
1. Cost. Prepreg are pricey. Even when you add up the cost of the resin, cure and fabric, prepreg still cost more.
2. Shelf life. This is less of a problem since Fiber Glass prepreg can be stored up to a year at room temperature. Nonetheless, heat cures prepreg and storage at warmer temperatures will reduce the shelf life. Keeping the material cooler will help and freezing will extend the life significantly.
3. Necessary heat cure. There will have to be a heat source and vacuum bagging at a minimum. You must be able to achieve a minimum of 270°F and sustain that temperature for a minimum of four hours. Many advanced fabricators use autoclaves but any source of heat will do.
How do Prepreg work?
Fiber Glass prepreg fabric will be shipped to you sandwiched between two layers of backing cloth. This cloth keeps the prepreg from sticking together. Peeling off the protective backing is easy and the material is immediately ready for layup.
To completely cure the prepreg, heat and pressure are required. Although the pressure could be achieved using a press, the more typical method is vacuum bagging. After the part is laid up, it must be vacuum bagged with all of the usual elements – a bag to hold the vacuum, peel ply so the bag does not stick to the part, and bleeder/breather cloth to absorb the excess resin. Additional information about vacuum bagging can be found in our white paper Vacuum Bagging Equipment & Techniques.
The ideal curing environment would be an autoclave which has both temperature and pressure controls, although prepreg are commonly cured otherwise.
The heat requirements generally require an oven in order to control temperature ramp up, ramp down, and to sustain a controlled, uniform temperature for the recommended duration. Ovens used for curing composites should never be used for cooking food as there will be contaminants released.
Heat lamps can be used to create enough heat but the trial and error associated with controlling the temperature will cause problems.
Is Prepreg right for you?
Racing Cycle Made with Prepreg
Prepreg are typically used by experienced fabricators who are interested in minimizing the weight of their part. Typical applications include aerospace, racing, sporting goods, pressure vessels, and commercial products.
Generally, prepreg are used by fabricators who have experience with hand layup and vacuum bagging. Although it is possible for a beginner to tackle prepreg to avoid the pitfalls of hand layup, it is typically not done.
