Many different fabrication processes are available to make advanced composite structures. However, the available options begin to narrow if highly intricate components with superior performance need to be produced at higher volume production rates. Compression molding with long chopped fiber thermoplastics is one of the methods that can fulfill these requirements.
Compression molding involves forming of complex shaped components by molding of a charge of fiber reinforced prepreg bulk molding compound (BMC) under heat and high pressures. Unidirectional fiber reinforced thermoplastic tape is cut into long lengths of 6.4-50.8mm to create the BMCs (Figure 1).
Then, an exact amount of these loose chips or strands is weighed out to fill the volume of a specific tool located in a matched metal mold. The subsequent heating and compression of the material to pressures makes the fibers to flow and reach every complex feature in the mold cavity before cooling.
Thermoplastic BMC Compression Molding
Thermoset compression molding generally does not require a degree of mold temperature control. However, it is essential in the case of thermoplastic BMC compression molding.
Xpress is the tooling concept used by Toray Advanced Composites/CCS to achieve a tight control over the mold temperature in zones above the surface of the component.
This Xpress process has been used to mold a carbon fiber/thermoplastic bracket with intricate features and flat plates. Molding and testing of structural test coupons will be carried out to characterize strength and stiffness. Components can be designed using these BMC materials with the help of sophisticated design and analysis routines.
Thermoplastic Bulk Molding Compounds
Since the 1990s, long, discontinuous fiber, thermoplastic composites have been used to make compression molding components. Materials such as polypropylene and fiberglass were used in some automotive applications. Thermoset based sheet molding compounds (SMC) had been in use before this period and are still used.
For the past several years, SMCs have been used by the aerospace industry to make secondary structures such as covers. SMCs are different from BMCs, which are composed of chips, chopped flakes, or lengths of prepreg fibers instead of a continuous sheet of material consisting of continuous or discontinuous fibers.
The high strength and stiffness characteristics of carbon fiber make it suitable for the aerospace industry. Aircraft structures, as shown in Figure 2, have been fabricated using carbon fiber thermoset BMCs.