17 Fabrication

1.7.1 Methods and Processes

1.7.1.1 Overview

The manufacture of a composite structure requires that the constituent fiber and resin be combined in a specified ratio, with the fibers in a chosen orientation and heated to cure or form the final product. The details of how this process is accomplished will ultimately determine the properties of the composite structure. Many of the techniques used have evolved from processing knowledge for plastic molding. Indeed, in the automotive sector, the composites manufacturing methods used most frequently are termed liquid molding and are similar to the resin transfer molding process used in the aerospace sector. The principal difference is the speed requirements for the product. And therein lies the challenge for modern advanced composites. The tolerable cost of manufacturing is dependent upon the end use. Low-volume application areas, such as aircraft or space, typically utilize the more expensive methods, and high-volume areas, such as automotive or infrastructure, require that costs be low. The processing methods that will be outlined in this section will follow manufacturing evolution from manual, labor-intensive methods to highly automated and rapid methods.

1.7.1.2 Processes

Manual Lay-up The simplest technique used to make a composite structure is the manual lay-up method. Fibers are laid on a form and liquid resin is added and distributed throughout the fibers by hand rolling. After the desired thickness is attained, the product is allowed to cure, either at room temperature or in an oven. This method is time consuming and produces composites of low quality. Much effort has been undertaken in the industry to improve the manual lay-up method. The development of prepreg materials was a significant advancement. Better control of the fiber-resin ratio and simpler lay-ups, combined with autoclave curing, produced better parts. Figure 3.15 shows the Filament winding technique used for composites.

Automated Tape Laying New machines have been developed that aid in the lay down of prepreg. These tape-laying machines are programmed to follow the contours of the mold, laying down prepreg tape in prescribed orientations and applying heat and pressure automatically. The head can follow reasonably gentle contours and, with some models, can automatically add or drop tape layer. The lay-up usually still requires vacuum bagging and autoclave curing.

Filament Winding The filament winding process can be a very cost-effective method for producing a composite part. As its name implies, the method consists of wrapping fibers around a mandrel in layers until the desired thickness is reached. A winding machine allows the fiber orientation to be varied thereby allowing the composite part to develop the design property profile. Matrix curing is most often done in an oven, although autoclave curing is occasionally used.

Resin Transfer Molding In resin transfer molding (RTM), a mold is filled with reinforcement and injected with resin. Cure takes place in the mold and the composite takes the shape of the mold. There are variations on this basic technique depending upon how and when the fiber and reinforcement are combined and cured. Reaction injection molding (RIM), structural reaction injection molding (SRIM), vacuum-assisted resin transfer molding (VARTM), and resin film infusion (RFI) are types that have been developed, usually first for a specific part need.

Pultrusion Pultrusion is the process where bundles of resin-impregnated fibers are cured by pulling them through a heated die. The addition of glass or carbon fiber to the pulling process yields a product that maximizes strength and stiffness in the pulling direction. When combined with part rotation and overwrap-ping techniques, pultrusion can produce a wide variety of structural composite shapes.

Advanced composites are formed on tools. The preceding process illustrations contain tooling adapted for the composite forming method used. Pressure and cure/forming temperatures are primary drivers for the design and materials chosen. Production quantity is also an important factor in tooling selection. Composites, themselves, are often used as tooling materials. As the cost of raw materials comes down, manufacturing costs, tooling, and speed became the barriers to the introduction of an advanced composite part into a high-volume application.

Machining The machining of polymer composites differs from both the machining of metals and plastics and requires consideration of techniques used in both.

TABLE 1.6 Adhesive Bonding vs. Mechanical Fastening

Property/Performance

Adhesive Bonding

Mechanical Fastening

Stress concentration/delamination

0 0

Post a comment