Traditionally, building composite structures from materials such as carbon fiber has been a skilled manual process. The fiber reinforcement was first laid into the mold, and the resin was then worked into the fiber mat using brushes and rollers. 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. The advantage of regular debulking is illustrated below. The upper ply will wrinkle if the previous plys are not debulked before applying it.
Additional problems associated with ply placement include bridging and compaction on corners, illustrated below. Bridging is a problem encountered on internal radii where the ply does not fully conform to the mold or to 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.
In the following sections, I describe the most common advanced composite fabrication processes.
Automated Tape Laying
Automated tape laying (ATL) machines are robots, usually gantry type, with heads that deposit prepreg tape onto flat or slightly curved surfaces. The head contains rollers and heating elements that provide heat and pressure to debulk and tack plies together. Typically a 3-12mm wide tape with continuous fibers embedded in a thermoplastic tape is used, although tapes as wide as 300mm are used in some applications. The tape is initially placed onto tooling and then layers are built up by placing the tape onto previous layers. Cutting devices in the head cut each ply as it is deposited. Due to the width of the tape, this results in high material wastage.
Most commonly, thermoset resins are used. Although curing using a heated head is possible, autoclave curing is required to obtain the best performance. Thermoplastic tape laying is also possible, removing the need for autoclave processing. In this case, both the incoming tape and previously deposited material are preheated using laser, gas or induction heating immediately prior to placement to ensure good adhesion between layers. Advantages of thermoplastic resins include higher toughness and recyclability. Perhaps most importantly, components can be produced in a single automated process with no vacuum bagging or autoclave required.
The tape laying process is highly automated and able to achieve very high deposition rates of up to 45kg/h on flat surfaces. Its disadvantages include being limited to relatively flat or gently curved surfaces and the use of expensive prepreg materials.
Filament Winding
Filament winding uses a rotating mandrill as a single-sided mold. The rotation of the mandrill pulls fibers from a spool that moves along the axis of rotation to control fiber orientation. The method can be used with prepreg tape (dry winding) or with fibers impregnated by being passed through a resin bath (wet winding). This technique is only suited to the production of hollow and continuously convex parts. These may be axisymmetric or more complex forms, such as turbine blades. The mandrill used may be a reusable metallic or composite structure, or a soluble tool made from salt, sand, ceramic or plaster with a binder material that can be washed out of the cured component.
Debulking is carried out by tension applied to the filaments with the result that a compaction roller of the type used by tape laying machines is not required. If the fibers are orientated perpendicular to the axis of rotation, then tension is a highly effective way to debulk. As fibers are orientated at angles closer to the axis of rotation, it becomes increasingly difficult. If additional debulking is required, it may be achieved by using an expanding mandrill, vacuum bag or autoclave. Generally, autoclave processing is not required.
Filament winding is an automated and highly repeatable process able to achieve high fiber content and low material wastage. If wet winding is used, the material costs are also low. The major disadvantage is the limitation to axi-symmetric or convex parts.
Automated Fiber Placement
Automated fiber placement (AFP), also known as automated tow placement (ATP), combines the advantages of both filament winding and automated tape laying while avoiding many of the disadvantages. It has become widely used on cutting edge aerospace products, including sections of the F-18, V-22, C-17 III, F-22, 787 and A380. A number of narrow prepreg tapes are deposited simultaneously by a head similar to those of a tape laying machine. This allows the machine to conform to much tighter curvatures without encountering wrinkling while maintaining high deposition rates. It also reduces material wastage. Material may be deposited onto a fixed mold or rotating mandrill.
Depending on the configuration of the machine, the form of components is limited to continuously curved surfaces or hollow structures. More complex parts can be created by bonding a number of components together. For example, skin/stringer structures have been fabricated by first creating box sections on a rotating mandrill. These are then cut in half and bonded back-to-back to form I-beams. The I-beams are assembled into slots in an aluminum tool, and the skin is fiber placed over these. Bonding between previously formed components takes place in exactly the same way as it would occur between the plys of a single component.
Wet Lay-up and Autoclave Processing
Wet lay-up and autoclave processing are typically carried out using thermoset prepreg on an open mold. The tool is first covered with a release film to allow removal of the component. The prepreg is then cut and laid-up in the tooling. Automated tow placement and tape lay-up are possible at this stage. Peel ply, release fabric and bleeder/breather materials are used on the outside to give texture, facilitate complete curing and allow excess resin to drain. Finally, the lay-up is covered with a vacuum bag. Heat to cure the resin and pressure to drain excess resin are then applied in the autoclave. Finally, the mold is removed from the autoclave, the component is removed from the mold and excess material is trimmed.
The disadvantages of this process are the high cost of prepreg materials and the requirement to use an autoclave, which increases cycle times.
Liquid Composite Molding
Liquid composite molding (LCM) methods, also known as resin infusion methods, involve placing a dry fiber preform into a mold tool and then injecting a thermoset resin. This means that expensive prepreg materials are not required.
Resin transfer molding (RTM) uses a closed mold while vacuum-assisted RTM (VARTM), resin transfer infusion (RTI) and Seeman Composites Resin Infusion Molding Process (SCRIMP) use open molds and a vacuum bag to enclose the preform and allow it to be infused with resin. Depending on part size, typical cycle times for RTM are six to 20 minutes of cure time in the mold. The use of special low viscosity resins, such as used by structural reaction injection molding (SRIM), can reduce this to one to five minutes.
RTM is able to achieve complex forms to close tolerances but manual labor costs remain high due to difficulties in automating the lay-up of dry fiber preforms. Prototype automated systems have been developed, such as an industrial robot fitted with an electrostatic end-effector and a vision-based system that was able to achieve dry fiber lay-up rates of 15kg per hour in 1996. Despite this achievement, adoption by industry has been slow but automated preform manufacture is finally becoming a reality.
Techniques such as VARTM, which employ vacuum bags over single-sided tooling, are often used to fabricate large low-cost structures in which autoclave processing is not required. These techniques result in longer cycle times but reduced tooling costs when compared with a closed-mold RTM process. Since the cost of tooling increases rapidly with component size, open-mold processes are more suited to large parts. To achieve the highest performance components, autoclave processing is still required.
Autoclave and Mon-autoclave Methods
Many of the above processes require the use of an autoclave for debulking and curing. Autoclaves able to process large components are expensive and can become a production bottleneck. Various techniques have been developed to avoid the requirement to use an autoclave.
Electron beam (EB) curing is a promising technique that can greatly reduce the cure time and allow curing to take place at constant temperature, reducing tooling costs. This form of curing has been demonstrated with wet lay-up, filament winding, RTM, VARTM, tape laying and AFP. Currently, the strength of EB cured components is lower than for those thermally cured. The cost of radiation shielding is also substantial.
Initial EB cure processes required the complete component to be laid-up and debulked in a vacuum before EB radiation was emitted from a high power source so that it penetrated through the vacuum bag and the complete component. Developments of the process have allowed in-situ EB curing as part of the AFP process. This reduces the energy requirement since only a small number of plys must be penetrated, allowing the use of low-cost portable shielding and making the process more cost-effective than autoclave curing for some applications.
Through thickness stitching of preforms is an effective debulking method that also improves damage tolerance, giving benefits similar to a 3D-woven preform with reduced production costs. The technique has been used to produce preforms for use in VARTM, allowing high-fiber content without the use of an autoclave.
Future Trends
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.