Monitoring cure viscosity of epoxy composite

Viscosity is an important process variable that should be monitored and controlled to ensure the consistent and reliable production of graphite‐fiber‐reinforced composite parts by the autoclave/vacuum bag process. Unfortunately, no technique is currently known for the direct measurement of viscosity in this process. However, d.c. conductivity and a.c. phase lag appear to correlate quite well with viscosity, and either electrical property may be followed in order to indirectly monitor viscosity. As the method of measuring d.c. conductivity is so inexpensive, simple, and almost free of electrical interference compared to the method of measuring a.c. phase lag, it appears to be the method of first choice for application in an autoclave environment.