Two RTM Processes for Large, High-performance Composites

 

Resin transfer molding (RTM) is a typical liquid molding process for fiber-reinforced resin-based composite materials. The main process is as follows:

(1) Design a fiber preform according to the shape and mechanical properties of the required part;

(2) Lay the pre-designed fiber preform in the mold, close the mold and compress it to obtain the corresponding volume fraction of the fiber preform;

(3) Under a dedicated injection device, inject resin into the mold at a certain pressure and temperature to exclude air and infiltrate the fiber preform;

(4) After the fiber preform is completely infiltrated with the resin, perform a curing reaction at a certain temperature until the curing reaction is completed, and then take out the final part.

 

 

Resin Transfer Mold is the main parameter that should be controlled in the RTM process. This pressure is used to overcome the resistance encountered when injecting into the mold cavity and soaking the reinforcement material. The time for the resin to complete the transfer is related to the system pressure and temperature. A short time can improve production efficiency. However, if the resin flow is too large, the glue will not have time to penetrate the reinforcement material, and it may cause accidents due to the increase in system pressure.

 

Therefore, it is generally required that the rising speed of the resin liquid level entering the mold during the transfer process is not more than 25mm/min. Monitor the resin transfer process by observing the discharge port. It is generally believed that the transfer process is completed when all the observation ports on the mold have glue overflowing and no more bubbles are discharged, and the actual amount of resin added is basically the same as the expected amount of resin added. Therefore, the setting of the discharge port should be carefully considered.

 

Resin Selection

 

The selection of resin system is the key to RTM process. To make the resin pour into the mold cavity and quickly infiltrate the fiber, the viscosity of 0.025-0.03Pa•s is the best. Polyester resin has a low viscosity and can be completed by cold injection at room temperature. However, due to different performance requirements of products, different types of resins will be selected, and their viscosities are not the same, so the size of the pipeline and injection head must be designed to meet the fluidity requirements of the special components. Resins suitable for RTM process include polyester resin, epoxy resin, phenolic resin, polyimide resin, etc.

 

Reinforcement Material Selection

 

In RTM process, glass fiber, graphite fiber, carbon fiber, silicon carbide and aramid fiber can be selected as reinforcement materials. The varieties can be selected according to design needs, such as chopped fibers, unidirectional fabrics, multiaxial fabrics, braiding, knitting, core materials or preforms.

 

From the perspective of product performance, the parts produced by this process have a higher fiber volume fraction, and the fiber local reinforcement design can be carried out according to the specific shape of the parts, which is conducive to improving product performance.

From the perspective of production cost, 70% of the cost of composite components comes from manufacturing cost, so how to reduce manufacturing cost is an important issue to be solved in the development of composite materials.

Compared with the traditional autoclave technology for manufacturing resin-based composite materials, the RTM process does not require expensive tanks, which greatly reduces the manufacturing cost.

In addition, the parts manufactured by the RTM process itself are not limited by the size of the tank, and the size range of the parts is relatively flexible, which can manufacture large, high-performance composite components.

In general, the RTM process has been widely used and rapidly developed in the field of composite material manufacturing, and is bound to become the dominant process for composite material manufacturing.

 

In recent years, composite products in the aerospace manufacturing industry have gradually developed from non-load-bearing components and small parts to main load-bearing components and large integrated parts. There is an urgent need for large, high-performance composite material manufacturing, so process technologies such as vacuum assisted-resin transfer molding (VA-RTM) and light-resin transfer molding (L-RTM) have been developed.

Vacuum Assisted Resin Transfer Molding (VA-RTM) Process

 

Vacuum Assisted Resin Transfer Molding (VA-RTM) is a process technology derived from the traditional RTM process.

The process mainly uses a vacuum pump and other equipment to evacuate the inside of the mold where the fiber preform is located, so that the resin is injected into the mold under the action of vacuum negative pressure to achieve the infiltration process of the fiber preform, and finally solidify and shape it inside the mold to obtain a composite material part with the desired shape and fiber volume fraction. The process device structure is shown in the figure.

 

Compared with the traditional RTM process, the VA-RTM process technology uses vacuum inside the mold, which can reduce the injection pressure inside the mold, and greatly reduce the deformation of the mold and fiber preform, thereby reducing the performance requirements of the process on the equipment and mold, and also enables the RTM process to use lighter molds, which is conducive to reducing production costs. Therefore, this technology is more suitable for manufacturing large composite parts, such as foam sandwich composite panels, which are one of the commonly used large parts in the aerospace field.

 

In general, the VA-RTM process is very suitable for the preparation of large, high-performance aerospace composite components, but the process is still semi-mechanized in China, the product manufacturing efficiency is low, and the process parameter design mostly relies on experience, and intelligent design has not yet been realized, and the product quality cannot be accurately controlled.

 

At the same time, many studies have pointed out that the process is prone to produce pressure gradients in the direction of resin flow, especially when using vacuum bags, there will be a certain pressure relaxation at the front edge of the resin flow, which will affect the resin infiltration, produce bubbles inside the part and reduce the mechanical properties of the product. At the same time, uneven pressure distribution will cause uneven thickness distribution of the part, affecting the appearance quality of the final part. This is also a technical problem that the technology still needs to solve.

 

Lightweight Resin Transfer Molding Process L-RTM Process

 

Lightweight Resin Transfer Molding Process L-RTM process is a new technology developed on the basis of traditional VA-RTM process technology. As shown in the figure, the main feature of this process technology is that the lower mold uses a rigid mold such as metal, the upper mold uses a semi-rigid lightweight mold, the mold is designed as a double sealing structure, the upper mold is fixed by vacuum on the outside, and the resin is introduced by vacuum on the inside.

 

Since the upper mold of this process adopts a semi-rigid mold and the inside of the mold is in a vacuum state, the pressure inside the mold and the manufacturing cost of the mold itself are greatly reduced, so that this technology can manufacture large composite parts. Compared with the traditional VA-RTM process, the thickness of the parts obtained by this process is more uniform and the upper and lower surface quality is superior. At the same time, the upper mold adopts semi-rigid material and can be reused, avoiding the waste of vacuum bags during the VA-RTM process. Therefore, this technology is very suitable for manufacturing aerospace composite parts with high surface quality requirements.

 

 

L-RTM Process Flow

 

However, in the actual production process, this process still has certain technical difficulties:

 

(1) Since the upper mold is made of semi-rigid material, if the rigidity of the material is not enough, it is very easy to cause collapse during the vacuum fixing process, resulting in uneven thickness of the part and affecting the surface quality of the part. At the same time, the rigidity of the mold also affects the life of the mold itself. How to choose a suitable semi-rigid material as the mold for L-RTM is one of the technical difficulties in the application of this process.

 

(2) Since the L-RTM process technology uses vacuum inside the mold, the sealing of the mold has a crucial impact on whether the process can proceed smoothly. If the sealing is insufficient, the resin infiltration inside the part will be insufficient, thereby affecting the performance of the part. Therefore, mold sealing technology is one of the technical difficulties in the application of this process.

 

(3) The resin used in the L-RTM process should maintain a low viscosity during the mold filling process to reduce the injection pressure and increase the service life of the mold. How to develop a suitable resin matrix is ​​one of the technical difficulties in the application of this process.

 

(4) During the L-RTM process, it is usually necessary to design flow channels on the mold to promote uniform flow of resin. If the flow channel design is unreasonable, defects such as dry spots and fat enrichment will appear on the parts, which will seriously affect the final quality of the parts. Especially for three-dimensional parts with complex shapes, how to reasonably design the mold flow channel is also one of the technical difficulties in the application of this process.