Processing prepreg moulding

Vacuum bag moulding (autoclave process) Prepreg is encased in flexible bag a vacuum is applied to draw the prepreg against a mould surface. Heat is applied. Process carried out in autoclave Very large structures prototypes short production runs high fibre content... 

The following period between 1975 and 1985 was characterized by activities related to the development of prepreg systems for low pressure autoclave moulding, i.e. modification of the resin chemistries to achieve flow, tack and non-volatile cure. These requirements were dictated by the industry to meet the processing techniques already in place for epoxy and polyester resins. 

In contrast to the planar reinforcement, the spatial reinforcement structures were produced in a discontinuous two-stage production process. In a first step, the fabric was coated with thermoset epoxy resin which remains in a so called and still shapeable b-stage . Afterwards, the prepreg-fabric was shaped with two moulds as shown in Fig. 6 (left). The moulds were placed in an oven and the fabrics were cured for 20 min at a temperature of about 180 C, Fig. 6 (right), resulting in an inherently stable reinforcement structure. 

As described in chapter 3.1 1ST and ITA already tested alternative ways to bring the B-staged prepreg inside of the concrete into the C-stage (Fig. 7). Thus, there is no further need to use a mould or autoclave process at all. This saves expenses. UV-, microwave or heat-curing systems are not favored by ISF and ITA, because cost intensive equipment would be needed as well as workers who have to deal with it on the construction side. 

For very large parts that cannot be accommodated in ordinary presses, the autoclave process can be used in conjunction with a vacuum bag [3]. In one example, prepreg is encased in a flexible plastic bag, which is situated in an autoclave (Fig. 2.2). A vacuum is applied to draw the bag against the mould surface and heat is applied. The process can be used to produce very large articles, but it can also be scaled down and has advantages for short... 

Vacuum bag moulding for retrofitting is a semi-automatic resin infusion under flexible tooling (RIFT) technique, which allows production of quality composites. In the RIFT process, dry GF are pre-formed in a mould and are taken to the side and attached to the structure. A resin supply is then channelled to the prepreg, and both are enveloped in a vacuum bag. The flow of resin into dry GF (preform) develops the composite material and the adhesive bond between composite and structure as well. 

The mechanical properties of FRP composites are dependent upon the ratio of fibre and matrix material, the mechanical properties of the constituent materials, the fibre orientation in the matrix, and ultimately the processing and methods of fabrication, which are the subject of Part II. Chapters discuss prepreg processing, liquid composite moulding (LCM), filament winding processes and pultrusion of advanced fibre-reinforced polymer (FRP) composites. 

Some part geometries, such as stiffener-reinforced panels, require preparing sub-structures (preforms) prior to lay-up of the part. These could, for example, consist of a C-channel stiffener made from several prepreg plies. Such a preform can be manufactured by the same lay-up techniques as described above, e.g. by depositing ply by ply into a C-shaped mould. As an alternative, preforms can also be made by the process of hot forming. 

A special, easy to use, process aid adhesive , fully miscible with a variety of resin matrices, specifically developed for tacking prepregs, fibres, or for example RTM preforms into a mould or on to a mandrel. Available in different widths. 

The advantage of knowing the process route and property level is attractive for many applications, but this has to be offset by either cost (prepregs) or tooling requirements (moulding compounds and GMT). 

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