Myths and reality about AC and DC Dielectric cure monitoring
The DC -based dielectric cure monitoring was first presented in 2008 to overcome many of the drawbacks that prevented AC cure monitoring from penetrating into composites manufacturing as a reliable cure monitoring tool. Synthesites has developed a unique technology capable to measure accurately, repeatedly and reliably up to 100 TOhm (1014 Ohm) at the production floor and during moulding. This performance has been and is being proven continuously not only in leading Research Centres and Universities across Europe (contacts can be readily provided upon request) but also at our customers' industrial sites worldwide.
Myth Nr 1: DC is vulnerable to electrode polarisation
This may be the case with some DC measuring equipment when measuring liquids with low electrical resistivity but this is not the case in composites where we actually measure very low conductivity resins (insulators). On the other hand, AC may also polarise the sensor's electrodes while in this case that polarisation can be much more complicated as it may include also dipole polarisation across a wide frequency range.
Myth Nr 2. DC may be affected by the release agent
It is true that DC measures mostly on the surface between the two flat electrodes and this is an advantage against AC in the cure monitoring of carbon fibre composites. However, unless thick wax is used, no issues with release agents or any other mould maintenance liquids have been ever encountered. Actually, it is very interesting that in some cases Optimold can even measure the curing of the release agent itself before becoming neutral after some time.
Myth Nr 3. DC can 'only' provide electrical resistance.
The resistivity of the resin (which is directly related to its ion viscosity) has been widely accepted by experts as the main cure indicator for composites. So a DC system can measure 'only' resistivity but this is exactly what it is necessary (together with resin's temperature) to define online the cure of a resin. Everything else that is available by an AC dielectric system may be useful for other applications but at the same time, it adds to the system complexity requiring expert users and additional signal processing tools to deal with while DC is simple and straightforward providing what exactly the composite manufacturer needs. Furthermore, Synthesites cure sensors are always accompanied by an embedded temperature sensor which is absolutely necessary for the calculation of the useful resin properties.
Myth Nr 4. DC can not provide reliable absolute electric measurements.
DC has proven its ability to provide reliable absolute measurements in cases that neat resin is being measured and based on these measurements Optimold can provide information about resin ageing or wrong resin mixing and viscosity. However, what is also true is that for in-mould cure monitoring the absolute electrical properties is not the real target as composites manufacturers need the cure and not the resin's electrical properties which their absolute values change depending on the carbon fibres and other impurities inside the mould. For this reason, Synthesites has developed unique proprietary software to provide in combination with the resin temperature real-time information about the degree of cure and Tg of the resin as it cures.
Myth Nr 5. DC can not measure through another material e.g. a vacuum bag.
Indeed this is true, DC sensors should be in direct contact with the resin. However, it is an illusion that AC dielectric systems can measure in a reliable way through films. Naturally the film also primarily contributes to the dielectric measurement of the sensor which is the case at higher temperatures when the film softens. Furthermore, this measurement is highly affected by the thickness of the film. Our proposal to deal with vacuum bags is either to make a small hole in the bag and use our VacBag sensor or to use the Cure Simulator system which only works with an, attached to the bag, temperature sensor and some resin sample fed to the Cure Simulator.