Composite

A composite is a material which is composed of two physically distinct and mechanically separable phases. One material serves as the fiber while a second serves as the matrix.

Matrix

The matrix binds the fibers together and transfers external loading to the fibers. Matrix type has a large influence on the service temperature, impact strength, toughness, and abrasion tolerance of the finished composite. The most common matrices used in large structural composites are the following:

Thermoset

Overview

Thermoset matrices are a "set and forget" matrix. Once cured they cannot be melted down and re-cured. Epoxy resins are the most commonly used thermosets for high performance composites in low to medium service temp ranges (up to ~250°F with a proper post cure). Bismaleimides are used in higher temperature environments such as hot structural sections of aircraft, and generally retain their properties at service temperatures up to ~500°F. Polyester and vinyl ester resins are rarely used in high performance composite structures, but are used where cost is one of the most important engineering requirements.

Epoxy Resins

Epoxy resins are by far the most commonly used thermoset resin in Aerospace and other high performance applications. Epoxy resin systems are generally 2 part systems, with a base and hardner being mixed together to start and complete the curing process. Epoxies are easy to process (little PPE is required when processing), have great strength and other mechanical properties.

Hygrothermal Effects on Epoxy Resin Systems

Epoxies can be highly susceptible to Hygrothermal effects; the more moisture absorbed by the matrix, the more it degrades in higher temperature environments. This property degredation is driven by the following parameters

Temp:

• Softens Matrix
  • Lower E
  • LowerσMax
  • Higherε Max

Moisture:

• Much Softer Matrix
  • • Very Low E
    • Very Low σMax
    • Much Higher ε Max

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Composite Forms and Processing Methods

Manufacturing of composite is an essential factor which should be considered when design composite components. The type of material, the form of material, mold design and processing method are all factors that need to be considered. Below are common forms and processing methods used by teams, but there are various other methods available.

Prepreg

Pre-impregnated material is purchased with the matrix already in the lamina. Prepreg can be applied to the mold directly without any additional resin. It is the easiest to work with and requires the most amount of equipment. Prepreg needs to be stored frozen at 0 degree F or lower to prevent the material from curing. There are several varieties of materials, weaves, out of autoclave (OOA) and autoclave only selections. Unlike wetlay and Infusion, Pre-preg will need elevated temperatures to cure and post cure. This requires an oven and oven recipe specific to the material data sheet.

Curing

Ramp

Dwell

Cool down

Dry Weave

Resin Infusion

Resin Infusion, the process of pulling resin through the laminate using vacuum. Infusion requires an inlet to the source of resin, and an outlet to the vacuum pump to exhaust. Depending on the size and complexity, multiple resin sources and exhaust tubes may be required. Like tradition wet lay, a vacuum pump and resin pot are required. Additional consumables are: polyethylene tubing, polyethylene spiral tubing (tubing wrap), flow mesh, peel ply and vacuum bag. It is recommended to use 3/8 or 1/2 ID polyethylene tubing and spiral tubing. The tubing is used for the resin inlet and exhaust port. The flow media is creates channels along the part to allow the resin to flow evenly and adequately. The Peel ply provides a barrier between the flow mesh and the laminate.

While regular lamination resin can be used, specially formulated infusion resins are highly recommended. Infusion resins are have lower viscosity for improved flow, and pot life & gel times designed for the infusion process. In addition to having the proper resin, warming mold surface will aid in the infusion flow.

The laminate is placed into the mold dry. Vertical walls or complex geometry may require adhesive to be used to tack the laminate in place. Super 77, Super 71 or generic adhesive spray are commonly used for this. When the lamination is complete, peel ply is applied over the entirety of the lamination. Flow mesh is then placed in the proper direction; where the diamond patterns are placed going along the intended direction of the resin flow. Failure to draw the resin in the same direction of the flow mesh will inhibit the flow severely. It is good practice to lay the flow mesh approximately 30mm from the all edges except where the inlet and outlet hoses are. This allows the peel ply to act as a resin break which prevents resin from being pulled into unwanted regions in the vacuum bag. For the outlet, leave a 30-50mm space between the outlet tube and flow mesh.

For the resin infusion process the vacuum bag needs to have a perfect seal. Ideally this is done by working the bag and leak testing until the drop the is as close to zero as possible. A part with a leaking bag or a part that is not fully debulked may allow air to introduced in to part. This can cause dryness on the mold surface or other impurities.

Wetlay

The wet layup method has several variations and techniques. It can be used to make parts, molds, repairs, and is great for bonding. The resin is applied directly onto the weave via squeegee, paint brush, or rollers. This method allows the user to control the resin ratio and it can be done in the mold or on a table. Wetlay can be done without vacuum bagging or an autoclave, rendering the best option for those whom lack the equipment.

If vacuum bagging is done, bagging, bag tape, release film (perforated), and breather cloth is required. Peel ply can be used in areas where bonding is to occur or in place of perforated release film. Caul plates can placed in areas where the user does not want texture (the breather and release film leave an undesirable texture on the bagged side of the part). Breather is used to distribute vacuum pressure evenly as the bagging alone can seal itself to regions of the part. Lastly, vacuum ports should be placed OFF the part, to the side, or in a "pleet" or area where there is excess bagging, and typically on a pad of breather cloth.

Wet layup can also be done by placing the weave onto a plastic film, pouring resin onto the weave, and then placing a second piece of film over top. The user can then use a squeegee to fully wet out the weave, visually seeing the areas that require more or less resin. Once fully wet, one side of the film can be slowly removed. The exposed side can now be placed into the mold and the other plastic sheet removed. This is an excellent way to avoid distorting the weave.