AC and DC Cure Monitoring Through Release Layers and Vacuum Bags

In composite manufacturing release films prevent the adhesion of parts to a tool. Vacuum Assisted Resin Transfer Molding (VARTM) employs vacuum bags so atmospheric pressure can compress a composite against a mold. Dielectric sensors typically must have direct contact with the material under test. By preventing contact, both release films and vacuum bags present challenges for dielectric cure monitoring (DEA).

In fact, a suitably designed sensor, such as the 1-inch Single Electrode Sensor from Lambient Technologies, can measure cure state through thin films and enable the use of DEA in a wider range of applications.

Release films and vacuum bags are made from PTFE or other plastics and are electrically insulating, blocking DC signals and allowing only AC measurements. Tests in this application note compare AC and DC cure monitoring through a release film with the 1‑inch Single Electrode Sensor. The results show that AC measurements with a release film have the same sensitivity as measurements with direct contact and confirm that DC measurements are not possible. As a result, even without direct contact, AC measurements can observe the important events that characterize thermoset cure:   

·         Flow front or resin contact

·         Minimum viscosity

·         Time of peak reaction rate, which may be related to gelation

·         User defined end of cure