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Composites have long been familiar materials in the development and construction of machines. They are very light and have mechanical advantages over conventional materials. Therefore, they are mostly found in industries where the lightweight construction philosophy is of higher importance. The application of products made of composite material therefore has mainly constructional backgrounds. This is because composite technology has a decisive disadvantage. Development and production of composite products are very demanding. As a result, the amount of work and time required increases, which in turn increases production costs. Composites are therefore significantly more expensive than conventional metallic materials. They are more likely to be found in industries where "light" is more important than "inexpensive".
In industry, the aerospace or automotive sectors have already taken some successful steps in research that reduce the production costs of composite products.
In addition to the economically strong sectors, other sectors - such as the wind power industry - are also building on this materials technology. For this reason, an inter-industry exchange is desirable in order to promote the state of the art technology. The research focus is currently on the automation of manufacturing processes.
The manufacturing processes for composites are very complex and, even with today's state-of-the-art technology, some can only be done manually. The correct handling of the fiber structures is difficult to define in terms of process technology. If these processes are automated by machines, the manufacturing costs can be reduced in the long run. The costs of manufacturing are the reason for the high price of composite products. If the price is reduced, this material technology can also become interesting for companies where weight and costs are equally important. The industry could thus also grow in the smaller sectors.
To understand why the production of composite products is so much more demanding, it is important to know the structure and composition of composite products. As the name suggests, it is a "composite" material that consists of different components. The way in which these components are combined is often the problem to be solved in manufacturing.
This material technology was inspired by nature, especially by the structure of plant tissue. These consist roughly of fibers, which are held together by an enveloping and protective matrix.
Two main components can be easily recognized in composite technology. The first is reinforced fibers, which usually consist of carbon (CFRP „Carbon Fiber Reinforced Plastic“) or glass (GFRP „Glass Fiber Reinforced Plastic“). The second is a plastic matrix, which provides stability and protection for the fibers.
The excellent mechanical properties that make composites attractive are due to efficient power transmission. Fiber-reinforced composites transmit forces along their fibers through the component in a targeted and effective manner. The force transmission in a single fiber depends on its direction (anisotropic). To keep the force influences on the product independent of direction, the individual fibers are strategically arranged.
The difficulty in production now lies in depositing the fibers in a structured manner and adding the plastic matrix. There are many possibilities for this, but they are still very complex and expensive. Especially the structured laying, or preforming, in particular generally requires manual operations. The aim of the industry is to automate the joining of the two components in an overall process.
In this way, manufacturing costs can be reduced in the long term.
Basically, as with all manufacturing processes, the material used is decisive. Therefore, it is important to know what kind of fibers are being used. Secondly, it depends on the selected plastic matrix. The designer decides which material is to be used depending on the application of the end product.
The fibers are usually carbon fibers (CFRP) or glass fibers (GFRP). There are alternative possibilities, but these are not as important in today's industry. If one compares individual fiber bundles in their handling with each other, there are small differences between carbon and glass. The important factor, however, is in which form the fibers are purchased. Often the fibers are already pre-woven into mats. The more closely they are braided, the less flexible they are in processing.
The nature of the fibers and their composition is therefore one of the main criteria by which production processes are determined.
Another criterion is the plastic matrix. There are many factors that determine which plastic is used. However, a basic distinction can be made between two types of plastics used. There are thermoplastic and thermoset composite plastics. Both have different advantages and disadvantages in their use.
The basic structure of plastics can be explained in the simplest form as follows: Several identical or even different monomers become chained polymers. In this case, a polymer is a macromolecule whose cohesion is based on the bonds between their atoms. The decisive factor for the plastic is then how the individual polymers bind with each other. In thermoplastics, the polymers are held together solely by interactions between the individual macromolecules. In thermosets, by contrast, the polymers are treated in a further step. Here, the macromolecules are influenced in such a way that they form real molecular bonds with each other and cross-link. The difference in cross-linking can be demonstrated in the following figure.
Since the thermosets cross-link closely during their formation, they are harder and more brittle. Thermoplastics, meanwhile, have a higher toughness. Once thermosets have finally hardened, they cannot be melted and re-formed. This requires special curing processes that must be integrated into the production chain. This gives the plastics a higher heat resistance.
Thermoplastics can be reshaped as required above their melting temperature, which has process-technical advantages. However, the heat resistance is not as high.
An established solution is so-called prepreg material. This fiber composite plastic can be purchased as a pre-impregnated semi-finished product with different mixing ratios of reinforcing fibers and matrix. These semi-finished products, also called tapes, consist of parallel continuous fibers impregnated on both sides with a resin film and wound up on reels. The prepreg is available in different widths and can also be processed in parallel, depending on the machine. Depending on the process, they can be cut and laid directly to size. As the fibers are unidirectional in the tapes, there are already various processes that strategically lay the tapes. Special care is taken to ensure that layers with different fiber directions are produced in order to obtain a quasi-isotropic end product. In recent years, many participants have dealt with automation and have developed well-known processes such as Automated Tape Laying (ATL) or Automated Fiber Placement (AFP). The figure shows a possible laying strategy that is pursued in AFP processes.
Automation in fiber composite technology is an important step towards manufacturing more cost-effective products from composites. However, there are still too many parameters and dependencies for a functioning production, which represent enormous obstacles in automated process chains. Currently there are several research projects dealing with these topics.
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