In modern manufacturing, these methods are rapidly becoming a thing of the past. Production systems now employ industrial robots and bespoke gantry machines to perform automated fiber placement and filament winding. Resin infusion methods are also used to inject resin into molds containing dry fiber preforms. This article provides an overview of the automated composite fabrication methods in use and considers the important process considerations that determine product quality.
Composite Fabrication Basics
Composite materials consist of a polymer matrix phase that holds reinforcing fibers. The fibers may be short strands or continuous fibers. Composites made using continuous fibers have greater strength and rigidity. They are known as advanced composites, which typically have a high fiber to resin ratio of 55-65 percent fiber by volume. Sandwich materials are also used where advanced composites are bonded over some other material, typically an aluminum honeycomb or polymer foam.
Individual filaments of reinforcing fiber are combined into bundles known as rovings (a slightly twisted bundle), yarns (a more tightly twisted bundle), strands or tows (untwisted bundles). Bundles of filaments are often woven into fabrics that are combined to make a preform, which is the complete fiber “skeleton” of a part without the cured resin matrix binding it together. Each section of fabric used to build a preform is known as a ply. Tackifier is a binder that holds the plys together in a preform. This makes it easier to lay-up the preform and reduces fiber wash—the movement of fibers during resin infusion.
There are two major types of polymer: thermosets and thermoplastics. They are both used as the matrix phase in polymer composites. Thermosets have a low viscosity, which makes processing easier, but they require curing, which increases cycle times. Thermoplastics are more viscous but do not require curing. Viscosity is important. A resin with low viscosity will flow more easily between fibers, ensuring a good “wetting-out” of the fibers.
The structural properties of a composite are largely determined by fiber orientation and the fiber to resin ratio (the volume fraction). The preform may be compressed a number of times during lay-up in order to improve the volume fraction and reduce problems, such as wrinkling. This process is known as debulking.
Bridging is a problem encountered on internal radii where the ply does not fully conform to the mold or to the previous ply, creating a void. Compaction is encountered on external radii when debulking and consolidation forces are concentrated onto a smaller area, resulting in a thinning of material over the corner.
In addition to the properties of the constituent materials, the structural properties are also affected by the interface between the matrix and the fibers. Of particular concern in this regard is that the resin fully infiltrates the fibers (wetting). Fabric pre-impregnated with resin (prepreg) is often used to make a preform that is both easier to lay-up and does not require subsequent infusion with resin. Disadvantages associated with the use of prepreg include increased costs due to the additional process of producing the prepreg material and storage costs since prepreg requires refrigeration to prevent it curing.
Advanced Composite Fabrication Processes
The different methods used to fabricate advanced composite structures may be classified in various ways. Most commonly, the processes are first classified into open-mold and closed-mold techniques. This is a useful distinction since closed molds will produce well-controlled surfaces on all faces of a part while open molds will only tightly control the surfaces in contact with the mold. The controlled surfaces that contact the mold are known as the outer mold line (OML). The uncontrolled surfaces are known as the inner mold line (IML). Composite fabrication processes may also be classified by the way resin flows through the preform. A complete fabrication process involves the following general steps: deposition of fibers, infusion of fibers with resin, debulking, trimming and, in the case of composites containing thermoset resin, curing. The order in which these process steps take place will vary between processes but, with the exception of curing, each step will always be present in some form.
Automated dry fiber preform stacking and through thickness stitching of preforms have a great deal of potential to reduce cycle times in liquid resin molding methods. These processes have the potential to produce the highest quality components by accurately controlling dimensional tolerances using closed molds and improving damage tolerance by the use of through thickness stitching. Additionally, the material costs are lower for these methods.
Although tooling costs have limited these methods to high-volume and relatively small components, GKN is pushing the boundaries by producing wing spars using RTM. Automated preform fabrication is likely to require sophisticated vision systems to verify ply placements on a ply-by-ply basis as the preform is built up.
Deposition rates are already being improved using machines with multiple gantries and both fiber placement and tape laying heads. Deposition rates are expected to increase further as these techniques mature. Efforts are likely to continue to eliminate the requirement for autoclave cure cycles. Possible technologies include thermoplastic resins and electron beam curing. In any case, the use of in-situ compaction to create a final structure from a single deposition process is likely to be a key process for the future.