Process Monitoring Technology for Composites Manufacturing
Cure monitoring systems are being developed in the last 30 years with the aim to monitor and control industrial processes for Composites Manufacturing. Various technologies such as ultrasonics and fiber optics have been employed and some publications have reported their application to monitor the curing of a composite part. However, the majority is in a laboratory environment under strict conditions and have never provided any quantified results. In general, the dielectric techniques (DEA) are considered for many years as the most mature ones for cure monitoring. Synthesites is the only entity world wide that can provide a system to monitor the Tg online which has been also certified in real production conditions. As an example, our OptiCure systems are being used in every day production of wind turbine blades.
Comparison with conventional (AC) Dielectric Process Monitoring systems
The conventional AC Dielectric Analysis in which a series of AC signals is applied to the sensor's electrodes and its feedback is measured has been considered by the scientific community as the most matured technology to be used for cure monitoring. However, Synthesites’ unique DC Dielectric technology (Optimold) has taken the cure monitoring technology many steps forward, offering quite significant advantages over the conventional DEA/AC technique:
• DEA/AC systems require significant data processing before providing meaningful results while Optimold provides instantly the electrical resistance and temperature of the resin.
• In general, DEA/AC systems are using multiplexing to deal with multiple sensors while Optimold is connected only to a single sensor. In this way, speed and accuracy are ensured without compromises.
• DEA/AC technique requires sensor and cable calibration before each measurement session while Optimold can handle directly any sensor with any cable length.
• Synthesites DC monitoring technology is combined with the widest and range of proprietary durable and disposable sensors ensuring the use of the optimal sensor and best measuring quality for each application.
• DEA/AC technology is mainly for laboratory use needing highly qualified staff while our products are primarily focused on industrial applications and conditions but can also be ready for the laboratory.
• Synthesites offer the only reliable technology for the real-time Degree of cure and Tg-monitoring in CFRP production (proved at industrial conditions). Most DEA/AC systems provide just dielectric measurements while few claim that they provide Tg. However, Synthesites is the only provider that is in the position to provide scientific proof.
• Without wasting time scanning a wide frequency range, Optimold can monitor effectively the full cure cycle of minute-cure epoxy and polyurethane resins. Please refer to the relevant publication here.
• In CFRP production, as DC sensing is effective only at the surface of the part, only a very thin glass-fibre veil is necessary to avoid direct contact of the sensing element with carbon fibres or copper mesh. DEA/AC sensors are much more vulnerable to Carbon Fibres and thick glass-fibre protection is often necessary. Recently, Synthesites presented a new CF sensor that can be used in direct contact with Carbon Fibres
Last but not least, Synthesites DC technology is not affected by lengthy cables so the installation even in large autoclaves is not a problem (we manufactured in-house all the appropriate cabling and feed-throughs from outside to inside of the autoclave). You can check relevant publications with our equipment here.
Comparison with other Process Monitoring systems
Although Dielectric systems are considered the most mature technology for industrial process monitoring there are several other technologies that claim that they can also provide cure monitoring information.
Heat Flux sensors measure the heat flow during the exotherm reaction of a composite part claiming that they can reproduce conditions close to the DSC in order to estimate the curing of a part. However, from one side the temperature increase can be also measured by an accurate temperature sensor while the temperature evolution cannot be related to the degree of cure or Tg of the part but it is only an indication about the speed of reaction. Furthermore, when there is external heating to the part for curing it is not straightforward how someone distinguishes which part of the heat flux comes from the reaction and which comes from the external heating.
Fibre Bragg Ratings (FBG) have also been used in the process monitoring of composites mainly in bab-scale applications. Although they can provide information about resin arrival, for the curing their signal is affected significantly by temperature and its fluctuations making the extraction of curing information very challenging. Furthermore, the fibres stay with the part and in general, they are difficult to introduce during manufacturing especially in rigid moulds while they are very fragile and can be easily damaged.
AC Linear sensors have been also been proposed in the last three decades for providing information about flow and cure however, they have several drawbacks and few advantages. In most cases, the sensors not only stay with the part but they affect the resin flow due to their geometry creating racetracking conditions around them. So practically someone measures a flow pattern that is significantly distorted by the sensors. Also the information provided is an integral of an electric property along the wire so it is not clear either where the resin is at a specific time or which will be the final integral value over which the resin position will be calculated. Other drawbacks include the difficulty of introducing such sensors in rigid moulds. The existence of carbon fibres only makes things even more challenging because carbon fibres just cancel all the electric field around the sensors.
Flow monitoring
Synthesites DC-based flow monitoring system (Optiflow) offers the only dedicated system for reliable resin arrival and temperature monitoring.
• Optiflow system checks and ensures continuously the connection between the monitoring system and the sensors.
• Optiflow provides directly resin arrival time-stamp for each sensor
• Optiflow offers an inherent environment for process automation solutions
• A large range of sensors can be combined with Optiflow
• Optiflow can handle practically any number of sensors
There are a few independent publications with a comparison of our technology with other technologies available in the market by independent R&D centres and Universities where our technology was in favour in the specific test cases:
HP-RTM: Online Viscosity and Tg Measurement of CFRP manufactured using High-Pressure RTM, Matt Etchells, Cristian Lira, SAMPE Journal, January/February 2020.
Hybrid Thermoset/Thermoplastic Composites by Using Combination Technologies Including Injection Moulding, S.Schmitt, R.Schares, M.Emonts, K.Fischer, T.Henke, M.Würtele, E.Schuber, SAMPE Europe Conference 2017, Stuttgart, Germany.
ONLINE PROCESS MONITORING SYSTEMS – BENCHMARK AND TEST STUDY, R.Meier, S.Zaremba, F.Springl, K.Drechsler, F.Gaille and C.Weimer, FPCM 2012, N.Zealand.
