Brief Introduction of RTM Molding Technology

 

Since the 1990s, advanced industrial countries and regions in Europe and the United States have proposed concepts and technical development directions such as cost-effeetive composites and affordable composites for resin-based composites, and have invested heavily in a series of plans such as ACT (advised composite technology) and ATP (advanced technology plan) to vigorously develop composite liquid molding technology (LCM).

 

At present, the demand for advanced composite materials is increasing day by day, and its cost has become a bottleneck restricting the large-scale application of composite materials. Low-cost composite material technology represented by rapid resin transfer mold (RTM) molding technology has become one of the research hotspots in the field of composite materials in the world.

 

The cost of composite materials is mainly reflected in raw materials and manufacturing costs, of which manufacturing costs account for more than 50% of the total cost. There is an obvious nonlinear relationship between the performance and cost of composite products. In the past few decades, the research and development of composite materials has focused on material performance and process improvement. The current focus is on low-cost mass production technology of advanced composite materials. The development and use of various low-cost technologies will be the mainstream of composite material development.

 

RTM is one of the typical LCM processes. Its main process principle is to first lay fiber-reinforced materials or preforms designed according to structural and performance requirements in the mold cavity, and then use RTM injection equipment to inject a special low-viscosity resin system into the closed mold cavity through vacuum suction, fully infiltrate the fiber, and demould to obtain a composite product.

 

 

The RTM process has a series of advantages. It can quickly form large and complex components (including those with sandwich or plug-in) in one go; it can provide a smooth surface for the components without gel coat coating, and the post-processing workload is small; the size of the preform is easy to control and the designability is strong; the production cycle is moderate, semi-automatic or automatic production can be achieved, and the efficiency is high; the fiber content is high; computer-aided design can be used for mold and product design, and the filling process can be simulated; no secondary bonding is required.

 

In order to realize the automated molding of RTM technology, the RTM process is usually divided into several parallel sub-processes. The process mainly includes completely independent processes such as preforming of fiber-reinforced materials, injection of resin, curing and post-curing treatment, which can greatly reduce the mold occupancy time and improve production efficiency.

 

The RTM process originated from the cold mold casting process in the 1950s and has a history of 60 years. However, it has only been nearly a decade since the RTM process has been truly valued and rapidly developed. The RTM process has been successfully developed in developed countries. In the United States, RTM is growing at a rate of 20% to 25% per year. The development speed of RTM molding technology in Japan is even more amazing. From the early 1970s to the late 1990s, in just 20 years, the development of RTM has gone through three stages. The production efficiency of its third-generation RTM can match that of SMC (mold occupancy time is about 3 minutes), and the strength reliability of the resulting products has reached a higher level.

 

In January 2009, Toray Industries, Ltd. announced the establishment of a carbon fiber reinforced plastic research and development base in Europe, and set up a special research center for automotive composite materials. The center takes RTM as the core process and focuses on the industrial application technology of carbon fiber composite materials. In recent years, Dieffenbacher and KraussMaffei, experts in the field of processing machinery in Germany, have cooperated in the field of RTM technology and developed an automated production line for high-pressure resin transfer molding (HP-t). Compared with the traditional RTM process, the HP-RTM process reduces the number of resin injections, improves the impregnation quality of preforms, and shortens the molding cycle.

 

 

Domestic RTM molding technology started in the 1980s. Influenced by the rapid development of international RTM technology at that time, some small and medium-sized enterprises introduced many RTM injection equipment based on the idea of ​​changing the traditional manual operation situation, which once formed a “hot spot”. However, due to the imperfect raw material supporting system and the lack of process theory research at that time, the process basically remained in the research and development state between hand lay-up and RTM process, and did not form large-scale production.

 

The application of RTM process to the automotive industry and the systematic research on its technology started in the 1990s. In 2008, we began to study the application of continuous carbon fiber composite materials to automotive covers, structural parts and all-carbon fiber composite body, and developed rapid RTM molding technology, which shortened the molding cycle of composite parts for automobiles from several hours to dozens of minutes. During this period, we cooperated with domestic vehicle manufacturers to jointly develop all-carbon fiber composite electric vehicles.