New Multi Roll Stack prepreg and towpreg machine offers higher-rate composite production

Carbon fibre prepregs have been widely used in aerospace and high-performance automotive composites for many years because of their useful mechanical properties, which include strength, impact resistance and lightweighting capability. The ability to precisely determine the resin-to-fibre content, weight and thickness of a prepreg allows manufacturers to tailor the material to the exact needs of the end user, and this can be extremely useful in developing functional composite parts. However, as aircraft manufacturers explore higher production rates and lower costs, and motor vehicle manufacturers investigate new ways to exploit composites on a larger scale, there is increasing motivation in the market to develop a better, more efficient way to create prepregs that makes them a viable material in higher-volume applications.

Proven technology
The process of manufacturing prepregs is proven and qualified throughout the composites industry, and this is important, particularly for safetycritical markets such as aerospace. Therefore, while the design of Cygnet Texkimp’s prepreg processing lines has evolved in line with customers’ needs, including optimisation for precision, consistency and speed, the fundamental principles of the process, and the structure and layout of the machines, have remained constant. However, the company believed there was a place for more rigorous innovation in prepreg manufacture that would accommodate the established principles of the process while at the same time offering the industry more in terms of reducing machinery footprint, energy consumption and costs.

In 2021, Cygnet Texkimp became part of the ASCEND programme to develop the technical and supply chain capability to achieve high-grade, composites-intensive components at rate for high-volume markets. This four-year, €46 million (£40 million) UK-based programme is led by Tier-1 aerospace supplier GKN Aerospace with funding from the Aerospace Technology Institute (ATI) programme. The company’s role in the consortium is to develop the technology to make prepreg and towpreg materials viable in more mainstream markets including high-volume automotive. This gave them the mandate and the investment to show how prepreg manufacturing technology could better meet the industry’s need for cost, energy and space efficiency.

Challenging convention
The patent-pending Multi Roll Stack is a unidirectional (UD) and fabric prepreg impregnation machine comprising a series of compaction rollers configured vertically. It is a major departure from conventional in-line prepreg machines, which incorporate separate compaction stations arranged in a horizontal line. In contrast, the Multi Roll Stack features a single compaction module containing multiple compaction rollers arranged vertically.

In both cases, the UD fibre, or fabric substrate, is passed between the compaction rollers where it is squeezed together with the resin. The squeezing action forces the resin into the fibres to create the prepreg material. The point where two compaction rollers come together is known as a nip, and achieving the necessary level of impregnation usually requires successive nips. The level of impregnation increases with each pass as the gaps between material and resin are removed. To achieve the right level of impregnation at an acceptable processing speed, four, five or even six nips might be specified, particularly for very-high-tolerance applications where precise areal weight, thickness and uniformity are required.

In a conventional prepreg processing line, each nip is housed separately and is made up of two rollers. Creating four nips therefore requires eight rollers. In the Multi Roll Stack, however, the rollers are arranged in close succession, effectively stacked on top of each other, which means that two nips can be created using only three rollers. This economy is multiplied as more nips are added, with four rollers needed to create three nips and five rollers to create four nips –rather than the six or eight rollers needed in a conventional line. As well as saving space, this also reduces the cost of additional precision-machined rollers.

Roller configuration
As with Cygnet Texkimp’s conventional prepreg manufacturing technologies, each roller in the Multi Roll Stack is ground to achieve very high levels of accuracy, which in turn ensures precision and uniformity in the way the resin is applied to the fibre. This is further enhanced by the internal roller design, with a heating mechanism that creates uniform temperature across the entire roller.

Another advantage of stacking the rollers is that they take up less space. This leads to less deflection or bending of the rollers as a result of the high pressures created during impregnation. Without this protective sandwiching action, the variation in resin application and impregnation caused by roller deflection would be extremely small, but even tiny variations in uniformity can affect the performance of the finished part, particularly in extremely demanding applications such as lightweight, safety-critical parts for aerospace and automotive.

Reducing energy costs
With fewer rollers to heat, the Multi Roll Stack immediately delivers energy savings. Further savings are created because the material does not need to travel between compaction stations, or nips, and therefore is not subject to the heating-cooling cycle that is characteristic of conventional lines. The space between compaction stations in a conventional prepreg machine represents an opportunity for heat energy to be lost from the process. More energy is subsequently needed to heat the material up again at the next compaction station in order to reach the optimum temperature for successful resin impregnation. The design of the Multi Roll Stack means that the material maintains more surface contact with the heated rollers. The full extent of this enhanced contact is dependent on the chosen web path –the route in which the fibre is driven through the process– which can be varied according to the application. In some S-wrap configurations, for example, the material does not leave contact with the rollers from the moment it enters the process to the point at which the finished prepreg exits. The cumulative energy savings achieved by eliminating the coolingheating cycle and maintaining constant temperatures throughout the process are significant.

The compact size and stacked layout of the Multi Roll Stack also means that fewer drives are needed to feed the material at the correct speed and tension around the rollers and through the process. A single drive is sufficient to power a four-roll, three-nip version of the machine, whereas the equivalent conventional machine providing three compaction stations would require a total of six rollers and a single drive at each station to feed the material through the entire length of the process. Using one drive to power the entire process also guarantees consistent speed matching between the rollers.

Shortened web path
The configuration of the Multi Roll Stack gives manufacturers the opportunity to select the optimum web path for their application. This varies from S-wrap to straight-through nip and depends on the type of fibre or fabric being processed and the viscosity or thickness of the resin being applied. Regardless of the path selected, the fibre length through the compaction zone is typically half that of a conventional line. The implication of this is that up to 50% less fibre is needed to set up or thread up the machine ready for processing to begin. It is widely accepted in the industry that the…

Want to read the end of this article?
Subscribe for free now and access to the entire article published in the JEC Composites Magazine N°151.
Available in print, digital and via the mobile application.