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Composite structure for public service buses using Long Fiber Injection process

News International-French

31 May 2016

The combination of the Long Fiber Injection (LFI) process from KraussMaffei and InMold primer allows production of large-format composite components with first-class surface.

High strength, dimensional stability and fit accuracy coupled with low weight are the hallmarks of large-format composite components produced using the Long Fiber Injection (LFI) process from KraussMaffei. First-class surfaces can also be achieved through a simple combination of processes - for example with the InMold primer. The Nanotechnology Center of Composites (NCC), the Russian lightweight construction research center in Moscow, is also relying on this combination and using it to produce a tailgate module in a self-supporting composite structure for a public service bus. The reward for the company's endeavor: the modular bus concept developed jointly by the NCC and the Evopro Group from Hungary won the 2016 JEC Innovation Award.

Quick color change and wide range of colors
One important feature of the public service bus is the very large two-tone tailgate. It is 2.2 meters wide and 1.4 meters high. NCC relies on the system competence of KraussMaffei in this case. "The customer's requirements were clearly defined. The customer asked for a substantial weight reduction and Class A surface quality together with a rigid, high-strength component. A quick color change and, thus, a wide range of colors were also requested. All of this had to be achieved in short cycle times and with cost-optimized production," said Wolfgang Hinz, Product and Sales Manager in the composites/surfaces business unit and the responsible project manager at KraussMaffei.

Ready to paint straight out of the mold
A joint decision was taken with the customer to use a combination of the Long Fiber Injection (LFI) process and the InMold primer process. This has one advantage: the surfaces prepared perfectly for individual painting are produced directly and fully automatically in the mold. The cycle times can therefore be minimized, and first-class surfaces from high gloss to matt and a wide range of colors are possible. Thanks to the use of self-releasing primers, there is also no need for external mold separating agents. In the Daimler-stone impact test components produced using the process perform up to 600 percent better than glass fiber-reinforced plastic (GFRP) components painted afterwards with a primer.

The system was delivered to NCC in 2015. The LFI system from KraussMaffei has been successfully used in production since then. It features a suitable shuttle mold carrier, a barrier coat spray machine, a mixing head, a 2-component paint spray machine and a LFI prototype mold. The primer is sprayed on in the first process step and the polyurethane barrier coat in the second process step. "The RimStar Compact spray mixing head from KraussMaffei ensures constant temperature control here and therefore produces the outstanding surface quality," said Hinz. LFI technology is then used in the third step. The glass fiber strands from the roving and the corresponding PU components are simultaneously discharged - controlled by a robot - into the open mold in a one-step process. This causes the glass fibers and the carrier matrix to bond into a high-strength composite. Since there is no need to cut and preshape fiber mats, no additional costs are incurred. "By comparison with the Sheet Mold Compounding (SMC) process or the Dicyclopentadien (DCPD) process which were also discussed by the customer, the LFI process offers considerable cost savings," added Hinz. In terms of component weight, the LFI process scores better than SMC with a low density of up to 55 percent. Other advantages of the LFI process are the high fiber volume content of up to 50 percent, which ensures high strength, and the high degree of flexibility in choosing fiber lengths. This permits a wide range of component designs, for example of ribs or function integration.

In the last step compression molding takes place without material flow and there is a reaction in the closed mold. The finished components are then removed and are now ready immediately for additional clear coating or final individual coating. Intermediate treatment or additional pretreatment is no longer necessary.

The system solution developed by KraussMaffei for NCC is also designed for other InMold coating processes of KraussMaffei. For example for InMold painting in which the final paint is discharged into the open mold in the first step. The spray system can consist of one or two components.

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