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A new method and associated equipment has been developed for non-destructively following the progress of the cure of thermoset phenolic molding materials, during a molding operation. It is based on passing an ultrasonic pulse signal through the material as it is curing. The velocity of sound in the material increases monotonically with increases in modulus. Modulus, in turn, increases with increasing degree of cure. Through this method, it is possible to determine when a molded part is sufficiently cured to be removed from the mold. Cure development during compression molding is reported on four phenolic molding compounds, at three different cure temperatures.

Many factors influence SMC cure behavior including mold temperature, SMC temperature, formulation, and aging. Although most of these factors are controlled to some extent, small uncontrollable variations will always cause some degree of fluctuation in cure behavior. The recent development of in- mold cure sensors allows the analysis and control of these variations from part to part and from batch to batch. In this study, SMC molding was monitored for thousands of parts and analyzed using Statistical Process Control (SPC) methods. The standard deviation in part to part cure times was found to be on the order of 1 to 5 seconds depending on factory conditions. A strong correlation of cure time with mold surface temperature was found, however, other uncontrolled parameters were found to have an additional significant influence. The cure sensor and monitoring system was connected to the press controller thus implementing closed loop control.