JEC COMPOSITES MAGAZINE - Issue #57 - May 2010 - 33

on Automatica

Control

sensing and commercial dielectric systems measuring resin impedance showed the superiority of DC sensing, particularly after gelation, as can be seen in Fig. 1. Obviously, there is a deficiency of the dielectric system in measuring higher than 0.80 degree of cure, whereas the measurement quality of the DC monitoring system is very stable, even in the end-of-cure region and at temperatures as low as 20°C. Besides the end-of-cure area, the electrical measuring system can be used for monitoring resin viscosity in the cavity in order to verify the onset of viscosity rise, the quality of the resin matrix or other process deviations. The viscosity of a polymer is closely related to its electrical resistance. This was experimentally shown [6] using DC monitoring for a polyester with 2% MEKP (Fig. 2). Furthermore, DC sensing is cheaper than dielectric monitoring and uses more flexible, robust sensors which can be installed in several locations in the mould, in the die, in the feeding or evacuation lines, so that global process monitoring is feasible. All these advantages provided iREMO with appropriate tools to deal successfully with optimised process control of composite manufacturing in the whole range of industrial applications.

Fig. 3: Resistance (right axis) and temperature (left axis) history in curing an epoxy system with nominal mixing ratio (av22), 2.2% more hardener (Rout37 25) and 2.2% less hardener (Rout31 24)

under control for optimum output. At present, the industrial approach focuses, for example, on extending the cure cycle to ensure full consolidation, but this is obviously not the optimum approach. However, several deviations will emerge when we try to reduce the cure cycle. For example, the insitu mixing of two or three different components of a resin system just before injection may result in variations in the mixing quality or mixing ratio in some instances. In a fixed cure cycle, these variations, even if they are small, might cause some quality deviations. With the DC-based process monitoring system, as can be seen in Fig. 3, we can clearly see when the mixing ratio deviates from the set-point as little as 2.2%, so corrective action can be taken to maintain quality and productivity. Other deviations include temperature set-point deviations and expired resins. The challenge involved in the iREMO project is to deal with these deviations in real time.

Fig. 2: Simultaneous monitoring of electrical resistance (left axis) and viscosity (right axis) in two experiments with slight temperature variations for a polyester resin with 2% MEKP

Process control
A process control algorithm is under development to take full advantage of the process monitoring capabilities. The control platform will consist of several control algorithms, from fairly simplified ones based on a combination of events and rules [2] to very complex ones using real-time process models [3] that will cover the whole range of composites processing. The former can be applied in a straightforward manner using basic knowledge of the process and existing experience but it cannot be optimal. The rules need to be updated for each process and the specific rules need to be tested and verified. On the other hand, model-based control requires detailed information and process models as well as quantified targets and process constraints but it can be optimal and can be used in more complicated processes such as curing of thick components with significant exotherms. A rule-based control system was developed for the present

Process deviations
The widespread use of composites and the need for cost reduction have pushed materials and craftwork in composites manufacturing to the limits, so there is an evident need for automation to increase productivity and minimize rejection. The latter is especially important as materials and recycling are very costly, especially for carbonfibre parts. Changes to reduce costs and increase productivity have included moving production from autoclaves and prepregs to heated moulds, dry fabrics and low-viscosity resins. These changes introduced several deviations in the manufacturing process that need to be kept

No57 May 2010 / jec composites magazine

JEC COMPOSITES MAGAZINE - Issue #57 - May 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #57 - May 2010