JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009 - 105

curve reaches an upper boundary value. Alongside the ultrasound velocity, the ultrasound amplitude shows a characteristic run. The amplitude is calculated as a percentage and describes the ratio of the detected amplitude to the reference amplitude. characteristic sound velocities as a function of storage time is shown in Figure 3. Fig. 3: Characteristic sound velocities as a function of storage time. Fig. 2: Ultrasound signals for an SMC sample. During the curing reaction, the length of the molecular chains takes a value that matches the frequency of the emitted signal. This range, where resonance vibrations and consequently the maximum absorption occur, can be detected in the diagram as the minimum value. Beyond this point, the absorbability decreases and the amplitude increases as a result of the molecular chain growth until the curing reaction terminates [6]. During the tests, the minimum sound velocities appear at the lowest material viscosity and take nearly constant values of approx. 1,200 m/s. In contrast, the sound velocity for a sound acceleration of 1 m/s2 decreases from 2,040 m/s to 1,820 m/s after 18 weeks of storage. Correspondingly, the difference between the two values decreases over time. This decrease can be explained by the reactivity reduction of the peroxide component in the SMC [3]. Monitoring the compression moulding process Controlling the material reactivity These two parameters are used to monitor different material and process parameters. In a first test series, the properties of semifinished Class A SMC (SMC 0400 5321, Menzolit Compounds International, Heidelberg) are investigated. This kind of material is used for exterior applications in the automotive industry. Curing tests are performed to determine the development of the material reactivity. The tests are executed using a small testing device with a round cavity (diameter 40 mm) and a constant thickness of 4 mm. The device is equipped with ultrasound sensors. To determine the influence of the storage time (storage temperature +12°C), the tests are repeated after periods of several weeks. A characteristic reactivity value is required to compare the tests. The velocity difference between the minimum ultrasound velocity value and a value at a high degree of cross-linking is suitable in this case. Yet the maximum velocity cannot be definitely detected because the sound velocity grows very slowly over long periods of time. Therefore, the sound velocity c is differentiated and a point where only minor changes in velocity occur (dc/dt=1 mm/s2) is chosen as the upper limit value. The tests are executed for mould temperatures of 130°C, 140°C and 150°C. The test results are discussed for 140°C and are transferable to the other temperatures. The development of the Besides the properties of the semi-finished product, the compression moulding process parameters are of interest because of their decisive influence on the quality of the final part. Therefore, these parameters need to be traced and controlled to guarantee a constant quality. One of the most important parameters is the process temperature, which is the parameter that initiates and terminates the curing reaction. Figure 4 shows the ultrasound velocity of the Class A-SMC processed at 130°C, 140°C and 150°C. The diagram shows that the start of the curing reaction is offset due to different mould temperatures. The reaction starts earlier at higher temperatures. While a difference of around 60 seconds is recorded between mould temperatures of 130°C and 140°C, a 25-second delay can be detected between 140°C and 150°C. The reason for the different time lags is the faster inhibitor consumption at higher temperatures. Additionally, the complete reaction of the SMC requires more time at lower temperatures. The opportunity to monitor the curing reaction makes it possible to use ultrasound velocity as a control signal to terminate the moulding process. Hence, the process cycle time could be reduced to a minimum value by taking into account the actual temperature conditions inside the mould for each process. In addition to monitoring the process temperature, the start of the curing reaction gives information on the progress of the No47 March-April 2009 / jec composites magazine 105

JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009

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