The log Ion Viscosity data (solid line) during the cure of a polyester BMC sample is shown in Figure 1. The initial decrease in log Ion Viscosity is the result of the press closing and the material coming in contact with the sensor. The more gradual decrease is caused by the decrease in the viscosity of the compound as it heats toward the mold temperature and ion mobility decreases. As the temperature of the molding compound rises, the reaction is initiated (causing decreased ion mobility due to cross-linking) and reaction competes with the temperature effect. This results in a minimum in Ion Viscosity followed by a rapid increase in viscosity as reaction begins to dominate. Gradually, as the reaction slows down, the rate of increase in Ion Viscosity also slows; showing the cure is near completion.

The 1st derivative (slope) of the Ion Viscosity (dashed line) indicates the rate of reaction during the cure. Following the minimum in Ion Viscosity (slope = 0), the slope increases as the rate of reaction increases until the maximum rate of reaction is reached. Following the maximum, the slope gradually decreases as a function of the slower reaction rate.

Figure 1: Log Ion Viscosity and its first derivative during cure of polyester bulk molding compound

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Figure 2: Critical Points identified on the Log Ion Viscosity curing curve of a polyester BMC sample.

Among the Critical Points that can be identified on the Ion Viscosity and 1st derivative of Ion Viscosity curves are the Flow Point, Viscosity Minimum, Maximum Slope, and Cure Endpoint.

The Flow Point is defined as the Time at which the Log Ion Viscosity crosses a user defined threshold (Critical Viscosity). The Flow Point is often used to detect when material has made contact with the sensor.

The Viscosity Minimum is defined as the Time and Value of the minimum (lowest) Ion Viscosity value. The Viscosity Minimum is used to detect when the material has reached its maximum in flow. In the molding industry, this point is often referred to as the "gel time" since immediately after this point the viscosity increases and gelation occurs.

The Maximum Slope is defined at the Time and Value of the maximum (peak) in the slope of the Ion Viscosity. The Maximum Slope identifies the time and relative rate of the maximum rate of reaction. A higher maximum slope value indicates a higher rate of reaction and a later time indicates a more delayed reaction.

The Cure Endpoint is defined as the time when the slope of Ion Viscosity passes through a user-defined value (Critical Slope) associated with the material reaching the desired cure state. In QA/QC testing, the Cure Endpoint is used as a relative cure time indicator. For production control, the Cure Endpoint can be used to automatically trigger de-mold. Prior work has demonstrated a 17% reduction in average cure time in a production SMC molding operation by triggering de-mold at the Cure Endpoint.

Figure 3 demonstrates how the log IonViscosity (solid lines) and its 1st derivative (dashed lines) can identify differences in the curing behavior of polyester molding compound. In this example, three samples of a polyester BMC were stored for six weeks at temperatures of 1.5çC (35 çF), 21çC (70çF), and 32çC (90çF). As can be seen on the graph, all three samples show similar behavior through the viscosity minimum region. After the minimum there are clear differences in reaction rates with the samples stored at higher temperatures showing slower cure rates.

Figure 3: Log Ion Viscosity and 1st derivative during the cure of three samples of polyester BMC stored for 3 weeks at 3 different temperatures.