Process defects and remedies

15 Snapshots of SMC flow showing void defect generation (a black dot inside the white circle in the right photo) (Odenberger et a ., 2004). 

As the internal pressure of volatiles (e.g., styrene monomer) or entrapped air is released at the moment of mold opening, interlayer cracks may be created near the part surface. These cracks are called blisters. They often appear to be dome-shaped bulges. They also can be created at elevated temperatures (e.g., in paint bake ovens) by the expansion of entrapped air. 

The formation of blisters can be decreased either by the vacuum application in the mold to minimize the entrapment of air or volatiles or by the use of coupling agents and contaminant reduction in the resin formulation to increase the interlaminar shear strength. 

Delamination can result from either poor fiber wetting with resin or residual stress that is caused by a non-uniform temperature distribution and/or incomplete curing. Hence, delamination is usually observed near the centerline of thick sections. To lessen this problem, it is most effective to use a relatively slow catalyst in order to obtain uniform curing reaction. This remedy, however, can increase the process cycle time. 

Sink marks are small depressions appearing on the part surface that usually are located opposite to the ribs or bosses (see Fig. 3.16). Even though they 

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