![]() 5 Nanshan Road, Huli District, Xiamen City, Fujian Province, China Lambient Technologies application note AN2.Lambient Technologies application note AN2.16, “Electrode Polarization and Boundary Layer Effects”.and Senturia, S.D., Journal of Adhesion, V18, p.73 (1985) In addition, applications such as vacuum assisted resin transfer molding (VARTM), can employ dielectric sensors without introducing breaks in vacuum bags that both complicate sealing and risk leakage of pressure. This test also shows that DC measurements through the release film are not possible at all, indicated by the high ion viscosity at the measurement limit of the instrument.īecause AC signals can penetrate insulators, it is possible to implement dielectric cure monitoring (DEA) in manufacturing that uses release films to prevent adhesion of material to the mold. ![]() We have seen it is possible to correct distortion of AC data however, it is not possible to correct DC data. Comparison of AC and DC measurements of ion viscosityĪs expected from our understanding of electrode polarization, DC measurements, even with no release film, show significant distortion around the time of ion viscosity minimum. For reference, data from the 100 Hz AC measurements are also plotted.įigure 8. Inability to measure cure state through release films or vacuum bagsįigure 8 compares DC measurements of BMC using the 1-inch Single Electrode Sensor with and without the HTF-621 release film.Possible distortion of data due to electrode polarization.Comparison of raw ion viscosity and ion viscosity with EP (boundary layer) correctionĭC measurement of resistance is a simple method of probing cure state however, it has the following limitations: Note that for these two tests the minor differences between curves are largely due to differences in process temperatures.įigure 7. After EP correction, ion viscosity measured with the release film correctly follows ion viscosity measured without the release film. 1,2,3 Figure 7 shows how boundary layer correction-also called electrode polarization (EP) correction-recovers affected data. In many cases it is possible to mathematically correct this distortion and restore information about the cure. Comparison of BMC cure with and without release film, 100 Hz AC measurement The curves are substantially the same except around the time of minimum ion viscosity, when the boundary layer effect distorts measurements through the film.įigure 6. 1” Single Electrode Sensor in press platen with HTF-621 release filmįigure 6 compares 100 Hz ion viscosity measured with and without the HTF-621 release film during cure of bulk molding compound (BMC). 1” Single Electrode Sensor in press platenįigure 5 shows the same sensor covered with Northern Composites HTF‑621, a PTFE-based release film that is only 0.001” thick.įigure 5. A suitably designed sensor, such as the 1-inch Single Electrode Sensor from Lambient Technologies, takes advantage of this principle to enable cure monitoring through the film itself.įigure 4 shows how the 1-inch Single Electrode Sensor would be installed in a press or mold for direct material contact.įigure 4. To minimize this effect, an insulating film must be thin compared to the separation between electrodes. Electrical model of resin on sensor with insulating layerĪs I wrote previously, under certain conditions when the Material Under Test is very conductive, boundary layers may distort ion viscosity measurements. Capacitors pass only AC signals so cure monitoring is not possible with DC methods.įigure 3. Release films and vacuum bags are usually made from PTFE or other electrically insulating material, which introduces a pair of capacitors between the electrodes and the Material Under Test (MUT), depicted in the electrical model of Figure 3. Cross section of lay-up with insulating layer, showing electric field In each case the sensor’s electric field passes through an insulating layer into the composite, shown in Figure 2.įigure 2. Dielectric sensors with release film (top) and vacuum bag (bottom) Boundary layers also arise when dielectric sensors measure through release films and vacuum bags, and it is well worth understanding how AC signals probe cure state through them.įigure 1 illustrates a sensor in a closed mold with release films and on top of an open mold with a vacuum bag.įigure 1. In my last post I discussed boundary layers caused by electrode polarization and the resulting distortion of ion viscosity data. Guest Post by Huan Lee, Lambient Technologies
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