Prepregs plies

Autoclave processing is a process in which individual prepreg plies are laid up in a prescribed orientation to form a laminate (Fig. 5.9). The process involves consolidation of the laminate, which generally results in a three-dimensional flow field. Similar to the IP process the fiber bed is not stationary in the AP process hence, its movement has to be specifically considered when the appropriate conservation equation for this process are developed. If it is assumed that the resin has a relatively constant density (i.e., the excess resin is squeezed out before the gel point is reached) then the appropriate conservation of mass equation for this consolidating system is Equation 5.12. 

Figure 7.3 Photomicrograph of two prepreg plies after consolidation...

Figure 7.6 A schematic representation showing two unidirectional prepreg plies in contact at the beginning of consolidation...

Equations 7.2 and 7.3 represent expressions for the degree of intimate contact of a single prepreg ply in contact with a smooth rigid surface (Fig. 7.5). 

One of the most often used production procedures for fabricating a high-performance structural laminate is the Autoclave/Vacuum Degassing (AC/VD) laminating process. In this process, individual prepreg plies are laid up in a prescribed orientation to form a laminate. The laminate is laid against a smooth tool surface and covered with successive layers of glass bleeder fabric, Mylar or Teflon sheets, and finally a vacuum... 

A butyl rubber sheet, or high temperature plastic film, is held taut aeross two vertieally stacked metal frames or rings, which are themselves sealed between the two platens of a press. The requisite number of prepreg plies is formed into a preform, which is sandwiched between sheets of PTFE coated glass cloth, or a non-perforated PTFE film with a perforated PTFE film next to the preform, and the paek placed on a steel plate sprayed with a fluorocarbon release agent and positioned on the bottom heated platen of the press. The preform... 

The difference between SQRTM and standard RTM is that, in place of a dry fiber preform, it substitutes a prepreg layup. Prepreg plies are arranged within the mold, the mold is closed, and then, somewhat counterintuitively, liquid resin is injected into the tool. The injected resin is the same as that used in the prepreg, and, therefore, those who adopt the process need not re-quaUfy materials. 

For obtaining a low -weight, high-bending-stiffness structure, sandwich constructions are a conunon choice for composite components. To make a sandwich, low-density materials are inserted as sandwich cores between two faces of the structural material itself (so in this case between two stacks of prepreg plies). Commonly used core materials are plastic foams (for example, made from PVC, PS, or PET) and balsa wood. Examples of core materials for more sophisticated, structured sandwich cores are honeycombs (made from aluminium, or resin-impregnated paper sheets), or fibre-reinforced foams. 

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. 

Previously cured bolt attachment lugs (typically with peel ply surfaces all over), inserted between prepreg plies during lay-up... 

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