JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009 - 104

KNOW-HOW Process monitoring Process monitoring using ultrasound measurement technology Ultrasound measurement technology provides the opportunity to monitor the compression moulding process of SMC (Sheet Moulding Compound) materials. This technology can be used to detect changes in the curing properties of the semi-finished material as well as the influence of various parameters on the compression moulding process. PROF. DR-ING. DR-ING. E.H. WALTER MICHAELI, DIRECTOR, INSTITUTE OF PLASTICS PROCESSING, & DIPL.-ING. CHRISTOPH KREMER, SCIENTIFIC ASSISTANT, INSTITUTE OF PLASTICS PROCESSING, RWTH AACHEN UNIVERSITY Basic principles of ultrasound measurement technology Compared to other technologies such as those used to measure internal pressure or dielectric properties, ultrasound measurement technology is a contactless method. This means that no marks are made on the surface of the part. Figure 1 shows a compression mould assembly with piezoelectric ultrasound sensors. Since the transmission method is applied within the mould instead of using the impulse reflexion method, as is used in most non-destructive tests, sensors are necessary on both sides of the cavity. To measure the cavity height throughout the process, the mould is provided with a displacement transducer that is connected to the ultrasound data logging system. Two different properties of the ultrasound signal can be used to evaluate a moulding process: the velocity and the received amplitude of the signal. It has been shown in several papers that the longitudinal sound velocity signal that is calculated from the delay time of the signal and the actual thickness of the part can be used to monitor the progression of the curing reaction in thermoset materials [2, 3, 4, 5]. To understand sound velocity diagrams, it is important to know that sound velocity is proportional to the elastic modulus. Hence, an increasing modulus as a consequence of an ongoing curing reaction yields increasing sound velocity. Before the measurements can be made, a reference signal has to be recorded when there is no compound in the mould. Comparing this signal with the detected signals makes it possible to calculate the pure signal through the composite material. A typical graph for the sound velocity and amplitude of the signal for the SMC compression moulding process is shown in Figure 2. The decreasing sound velocity at the beginning of the process results from the lower material viscosity as a consequence of heating by the hot mould (approx. 145°C) at this stage. Once the curing reaction has started, the curve rises quickly due to the increasing degree of cross-linking. An indicator for the termination of the curing reaction is when the sound velocity MC is an established compression moulding material used to produce parts for automotive and electronic applications. In the automotive and truck industry, the material is used for exterior parts because of its excellent surface quality and online varnish ability. It also allows antenna systems to be built into car boot lids made out of SMC, for example [1]. Consequently, the material has to fulfil high quality demands. However, the properties of unprocessed SMC change as a function of time and storage conditions, influencing the quality of the manufactured part. So far, no established method has been available on the market to estimate the condition of the semi-finished product. Besides this challenge, the compression moulding process needs to be monitored and controlled in order to guarantee consistent quality. Ultrasound measurement technology is a method that can be used to characterise both the material quality and the moulding process. The possibilities of this technique are presented in this paper. S Fig. 1: Compression mould assembly with indirect ultrasound sensors. 104 jec composites magazine / No47 March-April 2009

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

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