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Continuous manufacturing processes like filament winding, pultrusion, and prepregging represent more than 30% of total composites manufacturing worldwide. All these processes share a key common feature: the continuous impregnation of fibre rovings.
ANGELOS MIARIS, RESEARCHER, RALF SCHLEDJEWSKI GROUP LEADER, MANUFACTURING SCIENCE, INSTITUT FÜR VERBUNDWERKSTOFFE GMBH (published on April 2010 - JEC Magazine #56)
Rovings, commonly referred to as tows, are bundles of continuous, untwisted aligned fibres. Carbon-fibre rovings are available in tex numbers ranging from 100 to 3,500, corresponding to rovings which include from 1,000 up to 50,000 individual filaments.
Impregnation, or prepregging, is the processing step where dry fibres are combined with the liquid resin to make a preform. It is a unique operation, as it combines the fundamentals of fluid flow (matrix) and elasticity (fibres) to produce a truly viscoelastic material. Impregnation is a processing step that strongly affects the process efficiency. Process parameters like speed, temperature, roving yield, and material waste, along with quality aspects like fibre volume fraction and laminate void fraction, are all directly affected by the impregnation process.
Although major development steps have taken place in the field of process optimization, the only industrially widespread method for the impregnation of continuous rovings is still open bath impregnation, exactly as it was 45 years ago.
Bath systems are the state of the art
Bath systems can be divided into two main techniques: drum-type resin baths and dip-type resin baths (Figures 1 & 2). In the drumtype bath system (Figure 1), the rovings are compacted over a roller on which a resin film is formulated by the doctor blade. In dip-type resin baths (Figure 2), the rovings dip below the resin level and pass over a series of pins to complete the impregnation.
One of the main drawbacks of open bath systems is that the resin’s viscosity changes over time as temperature and humidity affect the processing parameters. The doctor blade is set while the system is idle, and cannot generally be adjusted while the fibres are being drawn over the impregnation roller. In an open bath system, the pulling speed and fibre tension affect the impregnation roller speed, which varies the hydraulic pressure of the resin between the doctor blade and the resin impregnation roller, so there is no possibility to predefine the fibre/matrix ratio. A further and important disadvantage is that a large surface area of resin is exposed to air, causing a release of monomers into the plant environment. The monomers are usually styrene, which not only has an impact on the resin’s viscosity and pot life, but also raises concerns for workplace hygiene.
Closed impregnation systems
Over the past years, development works have focused on substituting resin baths with closed systems. The systems developed incorporate features like direct dosing of resin, active control of the fibre/matrix ratio, minimized labour time, and compact design.
Impregnating a roving involves a series of steps. The roving should be in a form that allows matrix penetration. The resin is brought into contact with the roving and a pressure difference then forces the resin to flow through the roving.
There are two main ways to impregnate a roving in a closed system, one using high pressure and the other, low pressure. A “high pressure” design consists in guiding the roving though a die that will compact the fibres and then injecting the resin under high pressure. The main disadvantages of such designs are the resin backflow, the fibre abrasion caused by the high compaction rate of the fibres, and the fixed fibre/matrix ratio – unlike a “low pressure” design, which enhances impregnation by applying a low hydrostatic pressure on the resin and accelerating fibre bed wetting through the action of capillary pressure.
IVW carries out active research work in the field of closed impregnation systems. One of its latest developments is the siphon impregnation mechanism, which is a closed, lowpressure impregnation system. Impregnation occurs as a single roving runs through three curves in a closed path as the required amount of resin is injected at the entry (Figure 3). The roving slides over the curved surface of the siphon and a thin resin film is created between roving and siphon surface. The roving tension causes an increase of the resin pressure on the film layer. The fibre pack is impregnated due to the pressure rise on the resin film. The pressure on the resin layer causes the resin to flow through the permeable roving.
The siphon was successfully tested with a wide spectrum of processing parameters. High-tenacity carbon fibres made of 6k, 12k, and 24k rovings were fully impregnated (Figure 4). One of the main advantages of this impregnation method is the ability to actively control the fibre/matrix ratio simply by controlling resin metering (Figure 5). The siphon was used in continuous operation for 8 hours without any gelation of the resin. The design also incorporates attractive features for industrial applications. The siphon’s die consists of a standard disposable PTFE tube, which eliminates the time and costs associated with the use and disposal of solvents like acetone. The siphons can be built in user-friendly, compact assemblies and placed in different angles, providing great process design flexibility (Figure 6).
The siphons have been successfully implemented in the ring winding head developed at IVW. Thirty-two siphons were incorporated in the assembly, and highend type-III vessels for hydrogen storage have been wound, achieving top quality standards. No problems associated with the impregnation process occurred during processing. The siphon has also been tested in the pultrusion process, where it delivered fully impregnated profiles consisting of multiple rovings.