Abbildung 11-3 zeigt einen Mini-Varicon-Sensor, der für die Aushärtungsüberwachung von "5-Minuten"-Epoxidharz mit dem LT-451 Dielektrischen Aushärtungsmonitor verwendet wurde. Dieser Sensor hat Elektroden mit gleicher Breite und Abstand von 0,004" (100 μm).
Lambient Technologies LLC, the leader in precision measurement solutions for the curing of advanced composite materials, announces the release of the LT-440 Dielectric Channel, the industry’s first cure measurement solution to combine AC and DC measurement capabilities. While Lambient Technologies has long provided AC measurement capabilities in its other instruments, the addition of DC makes the LT-440 Dielectric Channel the most versatile cure monitoring instrument available anywhere. The addition of DC measurement capabilities provides important benefits to users needing accurate data on certain extremely resistive materials, such as silicones, towards the end of cure. In this instance, DC measurements offer the advantage of faster and more accurate cure data. For users who currently rely on other DC measurement solutions, the LT-440 Dielectric Channel provides the added toolset of AC measurement capabilities, suitable for applications needing measurement through vacuum bags and release films, where DC measurements won’t work. In addition, DC users gain the ability to deploy AC measurements to more accurately follow the entire cure. The LT-440 Dielectric Channel offers the industry’s most comprehensive toolkit for both types of cure measurement, backed by our unmatched experience in dielectric cure monitoring.
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The Unitrode-1” sensor has a single electrode comprising one side of a parallel plate configuration. Embedded in a substrate that electrically insulates it from the tool, the electrode can withstand the temperatures and pressures of composite processing. This sensor makes a bulk measurement of material between the electrode and the grounded, opposite tooling and/or the surrounding surface, which make up the second electrode. Note the electric field reaches through the insulating substrate below the electrode as well as into the sample above the sensor. As with any parallel plate cell, the A/D ratio of the Unitrode-1” depends on the electrode separation. When the upper tooling is very close to the sensor, the electric field between them is uniform with very little fringing to the side, and the A/D ratio is simply electrode area A divided by separation D. When the upper tooling is far from the electrode (for this sensor, greater than about 5 mm) the fringing electric field predominately extends to the surrounding, grounded metal, which becomes the second electrode. In this case, the A/D ratio must be determined by experiment. For intermediate separations, the electric field reaches to both the upper tooling and the surrounding metal, and the A/D ratio must also be determined empirically.
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Electric Field Type: Fringe + parallel plate Electrode Design: Coated Disk 1-electrode Electrode Width: 0.50” dia. (12.7 mm) Electrode Spacing: 0.062” for test (1.6 mm) Substrate: High temp thermoplastic (εr = 4.2) A/D Ratio: 5 cm for 1.6 mm sample Relative Sensitivity: 1.0 Base Capacitance: ~20 pF Used for thermoset and composite cure monitoring. The Ceramicomb-1”, the Unitrode-1” and the Carbon+Unitrode-1” sensors were embedded in steel platens heated in an LTP-250 MicroPress. The Unitrode-1” and Carbon+Unitrode-1” sensors, however, have a parallel plate configuration, and their A/D ratios depend on the separation between their electrode and the upper heater platen, which acts as the second electrode. Because fringing electric fields complicate the calculation of a cell constant, the A/D ratio for a parallel plate sensor is usually determined experimentally. Measurements for a 1.6 mm sample thickness found A/D = 5 for the Carbon+Unitrode-1” sensor. Unlike the other sensors of Table 9-1, the Carbon+Unitrode-1” has a thin, insulating coating for use with carbon filled composites. This coating prevents carbon fibers from short circuiting the electrode and interfering with the measurement. The insulation, however, acts as a boundary layer, which is well understood to distort data during early cure when the Material Under Test is most conductive. Data for the Carbon+Unitrode-1” sensor deviates from the others during early cure because its insulating coating is a boundary layer that distorts measurements when the sample is most conductive. Even so, from mid-cure through end of cure, when the EMC is less conductive, data from the Carbon+Unitrode-1” agrees well with the other sensors. Carbon+Unitrode-1” sensor, manufactured by Lambient Technologies, Cambridge, MA USA.
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Sheet molding compound cures in less than five minutes, so the LTF-631 High Speed Dielectric Cure Monitor was used to test samples with a 10 Hz excitation under control of CureView 2 software. SMC curing behavior was observed with the LTF-631 High Speed Dielectric Cure Monitor, and the data clearly show: - Critical Points identify characteristic features of the cure such as minimum ion viscosity, maximum slope of log(ion viscosity) and the time to a chosen end of cure. - Cure time decreases as cure temperature increases, as expected for a thermally driven reaction. LTF-631 High Speed Dielectric Cure Monitor, manufactured by Lambient Technologies, Cambridge, MA USA.
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