Preimpregnated composite fabrics and tapes, better known as prepregs, are increasingly being used in composite manufacturing today. Although prepregs are excellent to work with from a quality and manufacturing point of view, selecting the correct prepreg can be difficult.
Determining what type of prepreg can be a challenge, as there are many variables in each type of prepreg. (As a quick side note, this article will be discussing thermoset prepregs, and not thermoplastic prepregs.)
Prepreg Variables:
In a composite prepreg system, on must select the following components. As there are many combinations possible, the guidance of an expert can help in the selection.
Fiber
First, one must determine what type of fiber is required for the end application. Carbon fiber, fiberglass, or aramid. Each has their own specific pros and cons. Usually, it is the trade-off between cost and properties. Additionally, not all fibers are the same. For example, the difference between aerospace grade carbon fiber and commercial grade carbon fiber can be dramatic on both a properties and pricing level.
Fiber Orientation
Once a type and grade of fiber is selected, it is important to determine how the fiber will be orientated in the laminate. Today, there is a wide range of software available to first model the laminate prior to building a part. The analysis can be very predictive and accurate.
Fiber orientation or fiber architecture is important in providing the correct amount of strength and stiffness in the specified areas. For example, a carbon fiber arrow shaft will want the majority of fibers to run along the length of the shaft, while a composite pressure vessel will want to be wrapped with fibers, providing "hoop strength" in the laminate wall.
Many different fabrics and cloths can be prepreged. Woven fabrics, stitched fabrics, and hybrids can be turned into a prepregs. Also common, is prepregging straight unidirectional fiber in parallel. These are often referred to as unidirectional tapes, or prepreg tapes.
Prepreg tapes can be used to wrap a mandrel. Or strips and sections can be cut and placed exactly where strength is required. In fact, many large aerospace structures are laid-up using computerized automated tape-laying machines, which precisely lay and cut prepreg unidirectional tapes where required.
Resin Matrix
Next, one must select the proper resin matrix to use in the composite. In most thermoset prepregs, epoxy is the dominate resin used. However, if special properties are required of the composite, such as extremely high heat, then different resin matrices can be used.
Resin Grade
Once the general resin type is determined, then the specific resin type is selected. There are a couple of specific factors which often determine what resin grade will be used. The most important is processing temperature, and operating temperature.
Processing temperature is the temperature your composite tool can operate at. This could be an autoclave, an oven, or the platens on a compression molding press. Most all prepregs require to be cured at a certain temperature, for a certain period of time. The capabilities of the composite processing equipment will dictate this.
Operating temperature is the temperature the finished composite will be required to withstand on both a continuous basis, and a maximum level. For example, a commercial aerospace component will need to withstand higher temperatures for safety purposes then a carbon fiber bicycle frame would.
Resin Content
Once the type of resin, fiber, and fabric have been determined, one must also specify how much resin will be in the prepreg. Usually, the resin content is referred to in a percentage, usually a percentage of weight, but sometime the percentage will be of volume.
Either way, how much resin to have in the prepreg must be specified. Usually, the weight target, surface finish, processing method, and computer analysis will help on deciding what percentage of resin to use.
