Principles of Liquid Composite Molding

Liquid Composite Molding (LCM) is the common name for several similar processes for the manufacturing of polymeric fiber reinforced composites. Widely used processes that belong to this class are Resin Transfer Molding (RTM), Vacuum-Assisted Resin Injection (VARI), and Structural-Reaction Injection Molding (S-RIM). 

Regardless of the variant of LCM the process can be subdivided into a number of steps. The first step is preforming, which means that dry reinforcement is tailored to the shape of the mold. This can be done in many different ways depending on the complexity of the geometry and the requirements on mechanical performance of the part. The preform is then placed in a mold cavity that is subsequently closed. The preform is usually compressed slightly by the mold. The next step is resin injection into the mold cavity until the preform is fully impregnated. The final step is in-mold cure (i.e., curing inside the mold until the part is sufficiently stiff to be demolded). A controlled postcure is sometimes performed to ensure that optimum properties are obtained. 

LCM has successfully been used to manufacture products ranging from cosmetic parts with moderate demands on structural properties up to highly load-bearing parts of military [1] and aerospace quality [2]. One example of the first type of product is body parts for trucks (see Fig. 12.1) that are produced in series typically above 10,000 parts per year. Another example of the latter type of product is a sail yacht rudder shaft (see Fig. 12.2) for which production volumes are typically below 100 parts per year. 

The extreme limits of the LCM process exhibit large differences and can almost be said to be a number of different processes. Thus, there is no single recipe for LCM that always leads to successful results. On the other hand, there are several general principles that can be applied in all cases, and that is what we will concentrate on in this chapter. 

In this review of LCM we will try to be general when possible, but we are forced to specialize to the RTM process in many cases. In particular the special features of the S-RIM process (high-speed mixing, materials, etc.) are almost completely left out of the discussion. For readers with a special interest in this topic we recommend the book by Macosko [3]. 

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