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Compression RTM — An Efficient Method

News International-French

10 Feb 2016

When it comes to mass-producing components made from carbon-fiber-reinforced plastics, the HP-RTM method has proven particularly effective. With the introduction of an alternative process, it is now possible to achieve even shorter cycle times and improved component properties.

The HP-RTM method can be used to mass-produce components made from fiber-reinforced plastics such as CFRPs. At the start of the traditional RTM process, a preformed fiber layer—the preform—is inserted into the tool. The press then closes to produce the exact thickness required of the component. Next, resin is injected into the tool to impregnate the fiber layer and fill up the remaining space in the tool, at pressures up to 80 bar for HP-RTM. The high pressure is essential to ensure that the resin reaches every corner of the profile, even with more complex geometries, and to achieve a high level of fiber impregnation. The disadvantage of this method is that fibers shift under the high pressure, which can result in distortions. These distortions can have a negative impact on the distribution of forces within the component and reduce the overall component stability.

By developing the RTM method further, Dieffenbacher succeeded in finding a solution to this problem. Whereas traditionally the press closed to the desired thickness of the component, with the compression RTM method, the press initially moves to a gap impregnation position, which is 0.5 to 1 millimeter wider than the final thickness. The flow path for the injected resin within the tool is thus wider and offers less resistance to the flow of the resin. As a result, the injection pressure can be reduced to between five and ten bar, which does not distort the fiber composite of the preform The resin can also be injected more quickly because flow resistance is much lower.

The speed at which the resin can be injected is a key advantage of this process. The faster the resin fills the profile and the more quickly the fibers are impregnated, the more fast curing reactive resin systems can be used. By speeding up the curing process, shorter cycle times can be achieved.

However, the low injection pressure used in the compression RTM method is not yet sufficient to ensure that the fiber bundle of the preform is completely coated with resin. The liquid mixture of resin and curing agent is initially spread across the preform like a sheet and only penetrates the surface superficially. The high pressure required to ensure full penetration of the mixture with the conventional RTM method is generated by the clamping force of the press with the compression RTM method. The press closes to the desired thickness of the component and, at the same time, the compression stroke presses the resin through the preform. This ensures that all of the fibers are coated.

Another advantage of the compression RTM method is that it can be used to manufacture components with a higher fiber volume content in comparison to the conventional RTM method. The compression RTM method can easily produce components with fiber volume contents of up to 60%. These results can no longer be achieved using a simple injection process due to the extremely high levels of flow resistance. Increasing the fiber volume content maximizes the strength and rigidity of the component and improves its potential for use in lightweight construction.