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KraussMaffei’s fibre composite spraying (FCS) process can cut costs in the production of large, high-quality fibre-reinforced parts. Coaxial fibre-feed via a closed tube opens up a wide range of design possibilities. The 4-component mixing head can be used to process different material systems and the one-part mould is a cheaper option that substantially reduces manufacturing costs for small and medium-sized production runs. Furthermore, the ability to use renewable raw materials, carbon fibres or aramid fibres as reinforcing materials is bound to open up interesting new applications in the future.
JOSEF RENKL, HEAD OF DEVELOPMENT, REACTION PROCESS MACHINERY DIVISION & ERICH FRIES, PRODUCT MANAGER, FIBRE COMPOSITES KRAUSSMAFFEI TECHNOLOGIES
PUR fibre spraying is a cost-effective, flexible technique for manufacturing large fibre-composite parts – and it is steadily gaining popularity. Blowing chopped fibres coaxially into the PUR spray jet makes it possible to spray complex geometries with the 4-component mixing head capable of handling several different material variations.
In many cases, large fibre-reinforced products continue to be laminated by hand. The obvious advantages of this solution are the minimal investment in machinery and the ability to produce parts with very complex geometries. The disadvantages are very high labour costs and long cycle times. Apart from this, manual processes are always associated with quality fluctuations. In addition, if the parts are made using unsaturated polyester resins, the resulting styrene emissions are a potential health hazard and mitigating this risk incurs additional operating costs.
KraussMaffei’s Fibre Composite Spraying (FCS) system is intended for the large fibre composite parts market. Typical applications are bodywork parts for utility vehicles such as buses or tractors and wind deflectors or sleeper cabins for trucks. There are also many potential applications in the construction and furniture industries.
The reinforcing filaments are cut from endless glass fibre rovings. The endless fibre is fed into a cutter and chopped to the specified length. Simultaneously, the PUR components are mixed in the mixing head and discharged using a special spray nozzle. The chopped glass filaments are blown into the spray jet, where they are wetted with the PUR mixture. The glass-reinforced PUR is applied layer by layer as the mixing head follows a preset spray pattern.
Fibre content and fibre length can be varied to suit product specifications. Glass-free layers can also be specified, for example when FCS is combined with in-mould painting. Combining the two processes produces reinforced parts with high-gloss surfaces in one manufacturing step. A glass-free PUR material is sprayed as the first layer on top of the paint layer in the mould. This barrier layer prevents fibres from coming into contact with the paint layer and ensures an impeccable paint surface.
Design freedom thanks to coaxial fibre injection
The cutter was located directly at the mixing head in firstgeneration FCS systems. The chopped glass fibres slid from the side, down a chute and into the spray jet. This system, however, had some disadvantages. One was the fly problem caused by the open design of the chute. Another was that the spray jet could only be directed vertically downwards.
First-generation systems were thus not suitable for producing parts with complex three-dimensional geometries.
The company has now completely reengineered the way glass fibres reach the mixing head. These second-generation systems are more flexible and cleaner. The fibres are blown coaxially into the PUR spray jet at the mixing head via venturi nozzles and a closed tube. This system now makes it possible to swing the spray jet as much as 90° from the vertical and spray the PUR/glass fibre mix onto vertical surfaces. This provides considerable design freedom for parts designers.
The closed feed system minimizes fly during processing. Another advantage is that the fibres are uniformly wetted with the PUR mix, which is important for product quality. Coaxial fibre feed also means that the cutter unit need not be located directly at the mixing head. This facilitates access to the mixing head and results in a more compact spray head design.
Four-component mixing head opens up new design options
The 4-component mixing head deployed in FCS processes opens up a wide range of material system options. For example, by using up to three different polyol components, compact layers and foam layers can be combined in the same product. The foamed layers reduce the product weight, and can also improve its acoustic properties.
Another variant is to process two different polyols and two isocyanates. In other words, two completely different PUR systems can be processed. For example, the outer layer of a product can be a UV-stable coating made of an aliphatic PUR system, while the inner layers can be made of conventional PUR.
The tooling technology for fibre spraying can be matched to product specifications. Depending on how and where the part will be installed, it may not be necessary to produce a perfectly contoured reverse side. In these cases, a one-part mould – made of resin, aluminium or steel – will be perfectly adequate. A one-part mould is a cheaper option that substantially reduces manufacturing costs for small and medium-sized production runs. If both the front and reverse sides of the part must meet high optical standards, then fibre spraying is perfectly feasible with two-part moulds. Once spraying is finished, the mould is closed and the part is pressed into shape.