Monitoring Resin Infusion in Composites Manufacture Using Heat Flux Sensors

Principle

Resin infusion in reinforcement in the manufacture of composite materials and components causes some thermal changes that can be detected with the help of highly sensitive heat flux sensors. Thus, material temperature and the entire cure cycle can be monitored from the outside of the vacuum layers.

Positioning of the Sensor

Heat flux sensors are appended on the top of a vacuum bag placed on the area of interest.

Non-intrusive heat flux sensors do not influence the processing cycle as they do not come into contact with the part.

Properties Measured

The properties measured are local heat flux (W/m²) and sensor temperature (°C). The heat flux sensor's temperature is close to the vacuum bag’s surface temperature. The heat flux measured is proportional to the difference in temperature between the surface and the part through several absorbing layers. If the proportional coefficient and surface temperature is known, the composite temperature can be calculated at any time (see figure below).

Figure 1. Heat flux, measured and computed composite temperature (in red) seen from outside of vacuum bag.

Resin flow front detection can be done with the help of changes in thermal conductivity between dry and wet reinforcement. However, infusion is mostly carried out under isothermal conditions. Hence, a small temperature gradient (1-2°C) should be induced across the composite so as to enhance the sensitivity of the system.

As the heat flux sensor detects the heat generated by the polymerisation reaction, cure monitoring can be done. Thus the reaction start, exothermic peak, and return to thermal equilibrium or end of cure can also be detected. For an isothermal reaction, the heat flux is directly proportional to the rate of the reaction and the cure cycle often gives a hint of changes in temperature. Hence, it is mandatory to process the signals so as to extract reactive heat flux from the global heat transfer, which contains heat flux induced by predominantly heating and cooling.

Kinetic Analysis

Kinetic analysis is almost the same as the DSC (differential scanning calorimetry) technique once the baseline is known. The rate of the reaction is then global through the thickness of all the parts.

Applications

The above measurements enable real savings in the development stage, by analyzing the true behaviour of resin infusion on real parts. The same measurement quality can be achieved in production throughout the product’s lifetime, and this proves very critical for controlling quality.

A post-processing tool is available for automatic cycle analysis. This tool extracts data such as flowability, maximum temperature reached in the part, and real heating rate which are usable for SPC control for transparent and simple process monitoring.

Major applications are in resin flow front detection, cure monitoring, R&D – cycle optimisation, process monitoring (temperatures, infusion and cure) in the aeronautics and nautical sectors.

About TFX-SA

TFX-SA design and implement in-line material control solutions.

Tfx-Technology® products, also known as Thermoflux are :

• Thermal sensors (Heat Flux sensors or Heat Flow sensors) : there is no mastership over the quality of your products without thermal control. Heat spreads and vehicle information! It is therefore possible to control a product without direct contact.
• Monitoring and control systems
• Industrial acquisition and post processing software
• Traceability of technical operations performed everyday

This information has been sourced, reviewed and adapted from materials provided by TFX-SA.

For more information on this source, please visit TFX-SA.