Three processes of GMT materials：Wet Process、Dry Process、Fluidized Bed Process
1.1 Wet Process
Also known as the papermaking process, the wet process involves the following steps: feeding materials, mixing and flocculating, settling, vacuum forming, moisture removal, heating and plasticizing, shaping and compounding, and cutting8.
Process: Short-cut glass fibers and an additive suspension (resin, emulsifiers, etc.) are mixed and then settle onto a moving screen with a vacuum system to form a base felt9. This felt is then dried, heated to melt the resin, and finally compounded with non-woven fabric and adhesive film before being cut into sheets10.
Advantages: This method allows the glass fibers to be fully separated into single filaments and ensures good impregnation by the resin11.
Disadvantages: The process is only suitable for glass fibers 50-80mm in length, and it involves high equipment costs and significant energy consumption12.
1.2 Dry Process
Developed from non-woven fabric production techniques, the dry process is also called the carding process13.
Process: Glass fibers are mixed, opened, and carded into single filaments, then blended with hot-melt fibers14. This mixture is formed into a web, layered into a base felt, and reinforced with needle punching15. The felt is then heated, plasticized, and compounded with non-woven fabric and adhesive film before being cooled and cut16.
Advantages: It can process longer fibers (100-150mm) and is environmentally friendly as it produces no waste gas or water17.
Disadvantages: The carding process can damage the fibers, reducing their effective length to 50-80mm18. The resulting GMT sheets have noticeable anisotropy, meaning their mechanical properties differ significantly in the longitudinal and transverse directions19. To achieve desired mechanical performance, a higher resin content is often needed, which increases costs20.
1.3 Fluidized Bed Process
This is a newer technique that uses air as a fluidizing medium21.
Process: Hot air, glass fibers, and resin powder are fed into a fluidized bed and mixed thoroughly by a stirring paddle22. This mixture is then sprayed onto a conveyor belt to form a base felt, which is compounded with non-woven fabric and adhesive film, then cold-pressed and cut23.
Advantages: This method achieves a thorough pre-mixing of resin powder and individual glass fiber filaments24. It is environmentally friendly, producing no toxic or harmful waste25.
Recent advancements in dry processing include using air pressure to form the web, creating a uniform, three-dimensional fiber structure that improves GMT performance26. Another development involves using static electricity to impregnate the fiber mat with resin powder, ensuring more uniform distribution, reducing resin usage and cost, and enhancing material properties27.
