For applications requiring a more decorative surface the fibre-reinforced polyurethane layer may be coupled with a thermoformed plastic skin using film-moulding technology or with in-mould coating (IMC) processes. Both approaches can be used to eliminate costly and time-consuming secondary painting operations.
Thermoformed co-extruded skins with an acrylic top layer provide high-gloss Class-A surface with excellent weatherability. For less demanding surface quality applications, the IMC technology such as PU paints, polyurea coating and urethane acrylate gel coats can be used to produce a good finish part with a single mould tooling.
When a thin IMC is used, the application of a solid PU barrier layer is in general recommended between the in-mould paint and the LFI layer, in order to prevent the glass fibres from spoiling the smoothness of the paint surface. (Fig. 3)
The LFI technology is currently used for tractor bonnets and body panels for industrial vehicle; hulls and decks of personal water craft; spa panels, and for architectural or building products like door skins and decking.
The fabrication process typically uses a shuttle mould carrier, is highly automated and allows good control of processing conditions and reduced workforce.
The cycle time depends on the types of surface enhancement layers, the sizes of the parts and the chemical characteristics of the components. New-generation fast-cure PU LFI enable a demould time as low as 2 minutes, making the process suitable for volumes up to roughly 45 000 parts/year, with one tool operating on a two shift-basis. Compared with conventional polyester sheet moulding compound, PU LFI can be processed at far lower mould temperatures and clamping forces. This gives lower energy and investment costs, particularly for large parts.
Fibre composite spray (FCS) products are produced by blowing chopped fibres and added into the PU spray jet exiting the mixing head. The process of simply spraying onto a form liner (without the need of the two mould halves and a clamping unit) is very cost-effective for small series and/or large parts.
Material mechanical properties are definitely lower than with the LFI process, because the process only works with relatively short fibres and because there is no consolidation under pressure.
Part design can compensate to some extent for lower mechanical properties through multi-layered structures. Different layers may have different levels and types of fibre reinforcement for example. Lightweight multi-layered sandwich structures consisting of reinforced layers and expanded core layers provide adequate strength for many self-supporting non-structural applications. Similar to PU LFI, the FCS process can be coupled with film-moulding technology or in mould coating eliminating costly and time consuming secondary painting operations. (Fig. 4)
Structural-RIM for industrial and automotive applications
Moulded composite parts with continuous fibre reinforcement can be produced with Structural Reaction Injection Moulding (S-RIM). The process involves placing a pre-formed reinforcement in the mould followed by the injection of neat resin. The process enables the production of structural parts characterised by an excellent strength-to-weight ratio and, when desired, highly orthotropic mechanical properties. Among recent applications with glass fibres is worth again mentioning man-hole covers.
Carbon-fibre structural composites are today of high interest to the automotive industry. Such lightweight composites enable the reduction of CO2 emissions and this is driving an unprecedented development rush in innovating fabrication technology and formulation chemistry, both for PU and epoxy.