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Article updated on 02/03/20 by Clare Kiernan
Most composite applications can be divided into three categories, which are determined by performance requirements that are defined by reinforcement characterization. These three categories include:
- Structural: continuous aligned fibers
- Semi structural: continuous non-aligned fibers
- Non-structural: discontinuous fibers
Table 1 outlines the main areas of composite consumption in the United States between 1991 to 1994. While the majority of the industries identified cover more than one application type, they can be generally classified into one of the above. The major processing routes used to fabricate components within these performance bands are represented in Figure 1, together with the annual production quantities normally associated with these processes.
It is apparent that there is a lack of process technology available to address the volume markets in both the structural and semi-structural areas. This factor is a major contributor to the limited growth of composites in the industries requiring such components.
Table 1. Composite shipments for the period 1991-1994 from the USA in millions of kg.
Productivity increases are already being attained by the combined efforts of raw material suppliers, equipment manufacturers, trade molders and end users. Target productivities for four processing techniques are shown in Figure 2. Processing techniques can also be classified in terms of open mold processes, such as hand lay-up, and filament winding, closed mold processes, such as vacuum bag molding, RTM and SMC, as well as continuous processes, like pultrusion.
Sheet Molding Compounds
Until the mid-1980s, the molding of Sheet Molding Compounds (SMCs) was considered to be a slow process. Even so, this process was still viewed to be capable of delivering both small and large non-structural components with a surface finish that was superior to all other composite processing methods. SMC can refer to both the material and the process. Furthermore, complex shapes and details that may not be possible with sheet metal can often be achieved relatively easily with SMC.
SMC material usually comprises a filled polyester resin and glass fiber, which is either chopped, continuous or a mix of both. When ready to mold, SMC normally has a consistency similar to that of thick putty, as it will be partially cured (B staged). As the name suggests, SMC is supplied in sheet form and, in many respects, can be considered a low-performance prepreg. The sheets are essentially compression molded to final shape.
Commonly achieved cycle times were generally around four minutes, which resulted in processors operating multiple dies and ancillary equipment to keep up with production demands of the automotive industry, which is the main customer. Whilst there were cost implications with this, it also increases the risk of process or part variations.
A major deficiency of SMC components has been the structural inconsistency caused by uneven distribution of reinforcement during the flow of the charge material. Significant effort was focused on the SMC molding process at the time, which led to the one part per minute barrier being challenged. A key development to enable this improvement was vacuum-assisted molding.
In conventional SMC processes, charge loading and press closure speeds are selected with the primary aim of forcing out trapped air. Typically, `stacks’ of charge are placed in the mold, which cover only 40% of the mold surface, and closure rates are in the order of 0.1 m.min-1. With vacuum assistance, one-atmosphere vacuum is applied prior to closing the mold, which enables the charge to be spread and cover up to 90% of the mold surface.
Additionally, this process also achieves closure speeds at 0.9 m.min-1 without trapping the air.
The increase in mold surface coverage by the charge has several additional benefits associated with the reduced materials flow. For example, wave patterns and flow lines are eliminated and the localized strength of components is enhanced due to better retention of fiber orientation. Specially introduced orientation becomes feasible, and a high strength SMC process is available where unidirectional fibers are arranged at specific locations.
Raw material suppliers began to develop faster reacting resin systems designed to take advantage of the lower flow, higher speed requirements. Equipment manufacturers invested in automation and control of the process. These activities positioned SMC for a resurgence of interest in high volume semi-structural applications.
In the pultrusion process shown in Figure 3m dry reinforcements are impregnated with a specially prepared low viscosity liquid resin system and drawn through a die heated to about 120-150 °C where curing occurs. The solid laminate, which has assumed the shape of the die, is withdrawn by a series of haul-off grippers and cut to length or coiled.
Pultrusion is unique among the processes, under consideration that it is capable of producing complex components on a continuous basis. To this end, this process can produce any shape that can be extruded. It is also not allied to a single industry, thereby allowing its applications to range from civil engineering to electrical. These factors combine to give pultrusion one of the highest predicted growth rates of all composite processes.
The requirements of pultruders and their suppliers are to achieve stronger market development and higher productivity to capitalize on these projections. Faster production rates are one way of achieving this; however, the industry itself has recognized the need for greater consistency of raw materials and process operation, coupled with the ability to monitor the product quality more closely.
Polyester-Based Resin Systems
Production has been dominated by polyester-based resin systems, which have relatively wide processing windows. This has resulted in pultrusion remaining primarily an art rather than a science. Manufacturers concentrated on the provision of more sophisticated machinery, with the ability to run multiple tools.
The industry has sought to develop monitoring and control systems that permit a more efficient process, by reducing scrap levels and decreasing commissioning time that is normally associated with new dies or materials. A better understanding of the process has enabled more challenging resin systems to be considered and ongoing programs to include the evaluation of phenolics and thermoplastics.
Phenolic Resin Systems
Traditional phenolic materials pose three main problems to the pultrusion process, of which include volatile release, high reactivity and corrosion. These issues can result in high void contents, narrow processing windows and rapid tool wear. Process modifications have been implemented and new resins formulated to allow more 'user-friendly' and reproducible processing.
Phenolic pultruded profiles have since become available, in addition to further developments that have been made to enable the extension of range of profiles while simultaneously reducing production costs. The market demands the use of pultruded sections as replacement for beams, roof supports and modular building components. With tightening fire and toxic fume emission requirements, the future for phenolics looks bright.
The major restrictions on pultruded components, which previously existed, can be significantly reduced. With very few exceptions, products must be parallel-sided and have weaker transverse mechanical properties than those obtainable in the axial orientation, due to the inherent fiber alignment. Thermoplastics have previously shown dramatic productivity increases; however, these materials also offer the possibility of post-forming sections into non-parallel sided shapes. Similarly, the introduction of pull-winding variants provides the ability to accurately position transverse fibers, therefore redressing the balance with the axial performance.
High-speed precise laying down of resin-impregnated continuous fibers onto a mandrel is the basis of the filament winding process, as demonstrated in Figure 4. Pressure vessels, pipes and drive shafts have all been manufactured using filament winding. The mandrel can be any shape that does not have re-entrant curvature, although it is possible to remove the component from the mandrel before it has cured and use some other means of compaction to produce reverse curvature if required.
Multi axis winding machines can also be used. The process is usually computer-controlled and the reinforcement can be oriented to match the design loads. Components from small diameter tubes up to 40 meters long and 13-ton wind turbine blades have been manufactured using filament winding. One European company now manufactures commuter train carriages by a variation of the filament winding process.
The fibers may be impregnated with resin before winding, a process otherwise referred to as wet winding, pre-impregnated, which is also known as dry winding, or post-impregnation. Wet winding has the advantage of using the lowest cost materials with long storage life and low viscosity. The prepreg systems produce parts with more consistent resin content and can often be wound faster.
Glass fiber is one of the most frequently used materials for filament winding; however, carbon and aramid fibers are also used. Most high strength critical aerospace structures are produced with epoxy resins, with either epoxy or cheaper polyester resins being specified for most other applications. The ability to use continuous reinforcement without any breaks or joints is a definite advantage for filament winding, as is the high fiber volume fraction of 60-80% that is obtainable.
Only the inner surface of a filament wound structure will be smooth unless a secondary operation is performed on the outer surface. The component is normally cured at high temperature before removing the mandrel. Finishing operations, such as machining or grinding, are not normally necessary.
Prepreg molding is considered to be the next step up from hand lay-up. In this system, the resin content of the finished component can be accurately controlled, which cannot always true for hand layup. Additionally, woven or unidirectional fiber reinforcements are used, rather than chopped strand mat, as these fibers can be aligned in the required orientation.
Despite these advantages, prepreg materials are not cheap and usually require oven curing and vacuum bag or autoclave molding to take full advantage of their properties. The typical arrangements for vacuum bag molding is shown schematically in Figure 5. Prepreg molding is still used extensively in the aerospace industry; in fact, it is also the method used for the manufacturing of large one-off composite structures such as racing yachts and F1 racing car monocoques.
The prepreg plies, which contain the specified mix of resin, hardener, catalyst and reinforcing fibers, are cut to shape. The mold is treated with a release agent and can be coated with a gel coat layer for the best surface finish. When the gel coat is tacky and partly cured, the prepreg plies are laid down in the appropriate orientation. With complex curves, the prepreg may not exhibit sufficient drape; and therefore, some modification of the ply shape is required.
Once the plies are in position, the vacuum stack is put in place to produce a composite laminate. For the production of a sandwich structure, a film adhesive and the sandwich core material, which can be rigid foam or honeycomb, can be placed on top of the laminate, thereby allowing the laminate core joint to be cured in one hit.
The vacuum stack shown in Figure 5 consists of a peel ply, release film, air/resin bleed layer and the vacuum bag. The peel ply, which is generally nylon, is the final element to be removed from the cured laminate, thereby providing a clean and textured surface for bonding. The air/resin bleed layer not only allows the vacuum to be maintained over the laminate, but also allows for some resin to bleed out if necessary.
Once the vacuum has been applied, the bagged component is either put in an oven or an autoclave. Autoclave molding essentially involves vacuum bag molding in a pressure cooker that can reach pressures of up to 7 bar. As higher pressures are attained, thicker laminates and higher fiber volume fractions are possible.
The temperature/time cycle used to cure the laminate is critical to achieving the optimum properties, as this will determine the resin flow and degree of cure. The process can be semi-automated by using machines to cut the prepreg tape and robots to perform the lay-up.
Resin Transfer Moulding
In the Resin Transfer Molding (RTM) process, a low viscosity resin is transferred into a closed mold containing all the appropriate reinforcements and inserts as a preform. The air is normally evacuated from the mold, allowing the use of low resin injection pressures and epoxy molds.
Manhole covers, compressor casings, car doors and propeller blades have all been manufactured by RTM. Ford Motor Co. demonstrated that the entire 90-piece front end of the Ford Escort model could be replaced by a 2-piece RTM structure. Production cycle times were estimated to be less than 10 minutes. The RTM structure was stiffer and stronger than the steel structure, as well as 1/3 lighter.
Characteristics of RTM Processing
Traditionally associated with low volume parts manufacturing and low fiber contents, RTM research underwent significant investment. Cycle times of less than 3 minutes with fiber contents of over 50% by volume can be demonstrated, but, for complex or large components, a cycle time of one to two hours is typical. Continuous development is expected, with the intention to achieve a process capable of producing small components in less than one minute, with fiber contents approaching 60% by volume.
Resin suppliers are formulating low viscosity systems dedicated to enabling faster, more controllable mold fill. Reinforcement suppliers are constructing fabrics with high fiber contents, formability and minimum resistance to resin flow. Automotive components, which are more structural in nature than body panels, such as chassis and subframe members, can be produced on modified Resin Injection Molding (RIM) equipment using, RTM principles.
The production volumes will be an additional bonus to complement existing advantages such as the potential for molded inserts, foam cores, good quality surface finish and tight tolerances. Moving the process into higher performance applications involves redesigning resins to have lower viscosities to compensate for diffusion through higher densities of fiber. The development of fabric forms compatible with higher process speeds and injection pressures will also hold the key to expansion and growth of RTM technology.
The aerospace industry also has a keen interest in processing routes to compete with autoclaving, which is normally used for very high quality and low volume components. RTM may approach this requirement with considerable cost reductions, which could be around 40%. In response to this, autoclave technology has become more highly automated and less labor-intensive, as a result of the growing use of prepreg cutting, stacking and handling facilities.