The most important aspect in the production of polymer matrix composite parts is the combination methods of reinforcement and matrix. Also, this is the hardest thing in the production of composite parts. There are different types of combinations and methods in this manner.
Sometimes, matrix and reinforcements are combined before the production of parts. This is a preliminary form of composite compounds. Also, combination takes place in the production of exact parts like in the pultrusion and filament winding.
Molding compounds are very important in the production of exact shapes. Molding compounds are the pre-combinations of reinforcements and matrices. This preliminary form of composite will take the exact shape in shaping operations. For thermosets, the matrix of the molding compound is a semi-cured state that allows the forming before complete curing.
In molding compounds, short-sized fiber reinforcements, flakes, and particles are used as reinforcements.
There are two classifications of molding compounds according to the thickness and geometry;
In sheet molding compounds, the typical thickness is generally 6.5 millimeters. SMCs are generally produced from thermosetting resins which are mixed with chopped glass-fibers. Other fillers and additives are added also. They are produced between thin protective layers to prevent the evaporation of volatiles inside them.
The difference between bulk molding compounds from sheet molding compounds is that they are in billet instead of a sheet. Their thicknesses can be up to 20 millimeters. BMCs are also called dough molding compounds because of their dough-like physical characteristic.
Prepregs are special pre-form of fiber-reinforced composites. They include continuous fibers instead of chopped randomly oriented fibers as a reinforcing agent. Prepregs are produced as cross-plied sheets or fabrics. Prepregs are used in advanced composite applications.
To understand the PMC production or shaping processes, one would need to know the molding compounds and prepregs.
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