Infrared heat sources reduce energy waste in composite curing

Cermicx has joined forces with Comeragh Composites to develop a structured composite curing test program. Findhan Strain of Comeragh Composites reports.

Both partners have recognized the potential of infrared (IR) for a long time, having undertaken initial research into physical properties such as tensile strength, three point bending, impact resistance, and other factors key to validating composite curing methods.

To date, this research has fundamentally improved our understanding of how IR heat interacts with various fibers and thermostat matrices, most notably carbon fiber in an epoxy matrix.

However, engineers across the world must now understand that independent testing partners can replicate these results using standard processing techniques.

In effect, we have proven that the effectiveness of IR in curing composite components cannot be understated. Real processing efficiency gains can be made and measured. Also enhanced material properties can be achieved. Both Ceramicx and Comeragh are really looking forward to having these matters independently assessed over the coming weeks.

Composite Curing

There are key differences in composites when compared to metallic materials. For example, we create the properties of the composite during the manufacturing process and do not rely solely upon the geometry of the material structures.

In fact, curing is the most critical part of any thermoset composites process. The polymer chains are formed during curing, thereby ‘setting’ the resin matrix. Even before the formation of such cross-links, the resin viscosity profile, rate of reaction and many other factors that are directly influenced by the heat work must be precisely controlled in order to produce a high quality, void free composite.

The new Ceramicx test program aims to independently validate our approach to controlling these factors in carbon epoxy composite curing.

Figure 1 displays the relationship between time, temperature and viscosity of Cycom 5320-1; a typical Out of Autoclave (OOA) pre-preg system commonly used in the aerospace sector. Applying heat can control all of the key curves within this graph.

The blue line represents the programmed oven temperature of a typical convection oven.

In reality, however, the temperature of the part itself lags behind. Figure 2 shows a typical layout of a carbon fiber laminate curing in a convective oven. The thermocouple (T/C) at position 1 typically reads a lower value than the thermocouple at position 2.

Thermal conductivity through the part and tool causes this along with the overall mass of the materials. Therefore, the blue curve in Figure 1 usually contains more curves as the lagging thermocouple catches up with the remaining cure cycle. This can lead to lower processing times and potentially warped parts, primarily due to uneven heating and the resulting residual stresses.

Figure 2 – T/Cs at 1 remain at a lower temperature than at position 2 during heat-up

This lag also adds wasted energy to the system while the surroundings heat up and the heat eventually soaks into the part.

With IR-based heat sources, this problem still persists because the thermal conductivity of the composite remains the same. However, the process wastes less energy heating the surrounding walls, tooling fixtures, and tooling itself. A small degree of tool pre-heating also greatly reduces these differences, and IR-based energy can achieve that pre-heating more efficiently.

Viscosity Profiles

The red line in Figure 1 is the viscosity profile of the composite resin system. This starts off as a viscous system in the pre-preg state; applying heat to the composite then decreases the viscosity to a minimum point. This occurs prior to an exponential increase as the degree of cure (turquoise line) increases past the activation energy of the resin and cross-links begin to form. Applying heat to the component controls the slope of this viscosity drop and also the minimum value attained.

A fast heating rate, for example, can give rise to a very low viscosity. A slow heating rate can lead to a higher minimal viscosity. Low minimal viscosity results in good flow across the part and potentially easier void removal (particularly important in an OOA pre-preg package). However, excessive flow can occur, and this will result in a dry laminate.

When appropriate bagging techniques that throttle resin bleed are deployed, we can potentially achieve good flow, as well as high void removal and a fast processing time. With IR as a targeted energy source, we can achieve this more quickly and efficiently.

The glass transition temperature (Tg), shown by the green line, is more a function of final cure temperature and duration than of the initial application of heat. However, a better understanding of our process may still allow small efficiency savings and may shift the point of ‘Tg flat-lining’ earlier in the process, with the faster heating rate noted earlier achieving this shift.

All of these factors point to a greater need to understand heat-work within composites processing. The new Ceramicx composite curing testing program seeks to validate our approach. Testing also proves that IR energy can provide notable processing advantages in heating.

The program is already underway, and we plan to publish interim results in the next issue of HeatWorks magazine.