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Low-cost, lightweight structures with excellent mechanical properties have become a sought-after concept not just in the aerospace industry, but also in automotive and the transportation sector in general. Letoxit Foil (LF) Technology, which was developed and patented by the 5M s.r.o. Company, is a very efficient technique for producing relatively cheap sophisticated composite structures.
(Published on January-February 2006 – JEC Magazine #22)
BY RICHARD PAVLICA, PROJECT MANAGER, R&D, 5M S.R.O.
Composites are the best materials for lightweight structures, because they can be tailored to specific applications. The biggest advantage is the wide choice of polymer-matrix and reinforcement materials, where the reinforcement is what mainly determines the mechanical properties/weight ratio.
To optimise the mechanical properties for every product, you have to start by defining the different reinforcements, their orientation and the number of layers required for all zones of a composite part as a function of the desired properties (including low weight). Embedded local sandwich structures are a big advantage.
How can one achieve these basic requirements? Prepreg materials have been used for years, but they are very expensive and many prepreg types are necessary, if composite structures should be well optimised. Big parts can be manufactured using RTM, a relatively cheap technology, but the main problem is the impregnation of reinforcements.
Only special types of reinforcement can be used, and the reinforcement content in the final composite part is relatively low. Resin Film Infusion (RFI) is a recently developed technology in which the production cycle usually takes place in an autoclave. Under the action of temperature and pressure, fibre infiltration and composite consolidation are achieved in a single-step process.
Typically, an RFI part consists of a thermoset resin film placed between one side of a metal tool and a dry textile fibre preform. Conventional RFI is not without its problems when manufacturing sandwich structures, however, because honeycomb is filled with resin and the foam cannot impregnate the sandwich skin on the opposite side of the resin layer.
In consideration of such disadvantages with conventional processes, and of the need for universal low-cost technology, 5M s.r.o. developed and patented a very efficient technique to produce relatively cheap and sophisticated composite structures. The technique is called Letoxit Foil (LF) Technology, and is based on laying down dry reinforcement and core material in the mould along with layers of foil polymer material Letoxit Foil™. The resulting structure is vacuum bagged and cured at elevated temperature (fig.1).
The finished part is released from the mould as a ready-to-use product with excellent surface finish. The flexibility of LF Technology provides the freedom to design composite parts at very competitive prices. While an autoclave can be used, it is not absolutely necessary, because the vacuumassisted impregnation is usually enough.
LF Technology’s basic component, Letoxit Foil, is a thermoset flexible film. The standard type is Letoxit Foil LFX 023, which is a composition of special epoxy resin and a hardener that is latent at room temperature.
Table 1: mechanical properties of LFX 023 cured at 120°C for 60 min.
The film colour is originally light yellow but can be set according to customer requirement. The thickness of Letoxit Foils varies between 0.1 and 0.7mm, and is determined by areal weight, which is usually between 100g/m² and 700g/m².
A big advantage of LF Technology is the variability of reinforcement due to very simple impregnation. Another is that it is possible to combine different types of reinforcement. The most common reinforcements used are glass, carbon, aramid, or basalt fabrics. Hybrids of these materials are also possible.
The most important fabric parameters for LF Technology are areal weight, thickness and sizing. The fabric/Letoxit Foil weight ratio depends on the specific geometry, which is determined by areal weight and thickness. All fabric voids must be filled with resin. The appropriate thickness of Letoxit Foil must be chosen. The minimum resin content is given by the equation:mLF /rLF + mR /rR = tR
where mLF is the areal weight of Letoxit Foil, rLF the density of Letoxit Foil, mR the areal weight of reinforcement (fabric, etc.), rR the density of reinforcement, and tR the thickness of reinforcement.
Usually, a layer of fabric is laid down in the release-agent-coated mould, then a layer of Letoxit Foil, and finally one or more layers of fabric together with next layers of Letoxit Foil. Some other reinforcement types are mats, stitched fabrics, multiaxial fabrics and direct or bulky rovings. The resin/ reinforcement ratio is defined using the same rule as for fabric.
The main difference between LF Technology and standard RFI lies in the potential for creating sandwich structures as with prepregs. Almost all core materials used for thermoset sandwich structures can be used. The most popular are honeycombs, foams and special core materials such as Soric® and Coremat®, among others. Core materials can be applied locally in the composite if a stiffness increase in certain areas of the composite part is required, eliminating the need for tape reinforcement. LF Technology also provides the possibility of applying local reinforcements in a single step, typically ribbon from metal or pultruded profiles, or steel, aluminium or composite inserts for screw and rivet fastenings.
Applications of LF Technology
LF Technology is suitable for any composite structure where a balanced weight/mechanical properties ratio is required. Typical applications are thinshell structures with high reinforcement volume, locally added reinforcements and/or different required local properties (for example reinforcement orientation), since manufacturing these types of structure is costly in terms of materials and labour. Other, more conventional composite structures that normally would be made using RTM, VARTM or hand lay-up are also possible but, due to lower mechanical-property requirements, the economy of LF Technology use must be taken into account.
Potential applications are bodies, covers and hoods in automotive; hoods, side panels, doors and racks in other land transportation segments; covers, doors and all type of panels in the aerospace industry; tables and eye-catchers in the building industry; and orthotic/ prosthetic devices in the medical field.
In late 2004, 5M s.r.o. started to introduce its young LF Technology through several carefully chosen companies. The first applications for LF Technology were chosen in the aviation and medical fields, where the advantages of the concept are the most significant. These appeared in early 2005 (samples of developed prototypes are shown).
The very first one was an engine hood for the CHS 701SP ultra-light airplane. The original composite part was made using unsaturated polyester resin and hand layup. Through the use of LF Technology, it became possible to reduce the weight by 60%, yet provide the same part stiffness using local Nomex honeycomb reinforcement and foam in the top section. Another important benefit was higher thermal stability. No gelcoat was necessary, as the part produced met the customer requirements for surface quality.
The second application concerned a carbon knee orthosis made by ING Corporation using Hexcel carbon prepreg. For the one made using LF Technology, material costs were reduced and the labour cost stayed roughly the same, yet the mechanical properties were still comparable to those for the prepreg device. In addition, the composite parts were securely bonded.
The third application was a baggage wall in a four-seat VUT airplane. Here, high stiffness and low weight were achieved using a sandwich manufactured from Rohacell foam and glass fabric with Letoxic Foil LFX 023. The experience gained from these applications has confirmed that LF technology is suitable not only for high-tech markets, but also for all types of composite structures where weight and mechanical properties play an important role.
LF Technology not only offers design flexibility, it also contributes to lowering the price of composite products even as it improves their properties and reliability. To judge by the customer response and the quick and relatively easy development of new products, all these advantages mean a bright future for LF Technology