Aerospace Plastic Molding Process: Resin Transfer Molding

 

 

Resin Transfer Molding (RTM) is a process method that injects resin into a closed mold to infiltrate reinforcement materials and solidify. This technology does not require prepregs and autoclaves, effectively reducing equipment costs and molding costs. This technology has developed rapidly in recent years and has been increasingly used in the aircraft industry, automobile industry, shipbuilding industry and other fields. It has also developed multiple branches such as RFI, VARTM, SCRIMP, SPRINT, etc. to meet the application needs of different fields.

 

 

RTM started in the 1950s and is a closed mold molding technology that is an improvement on the hand lay-up molding process.

 

SMC, BMC compression molding, injection molding, RTM, and VEC technologies are all closed mold molding processes. Due to the formulation of environmental laws and the improvement of product requirements, open mold molding of composite materials has been increasingly restricted, which has prompted the application of closed mold molding technology, especially in recent years. The innovation and development of RTM technology.

 

Basic Principle

 

The basic principle of RTM is to pre-place reasonably designed, cut or mechanized preformed reinforcement materials in the designed mold. The mold needs to be sealed and tightened around the periphery, and the resin must flow smoothly; after the mold is closed, a certain amount of resin is injected, and the desired product can be demolded after the resin is cured.

 

 

In summary, through the RTM process, we can

① manufacture products with two sides;

② high molding efficiency;

③ closed mold operation, no pollution to the environment, no harm to workers’ health;

④ reinforcement materials can be laid in any direction, and it is easy to lay reinforcement materials according to the stress conditions of the product;

⑤ less consumption of raw materials and energy;

⑥ less investment in factory construction.

 

Origin of RTM process

 

The molding process of composite materials is the key to improving and enhancing the performance of advanced resin-based composite materials.

 

Traditional molding processes include hand lay-up molding, compression molding, winding molding, pultrusion molding, injection molding, injection molding, etc.

 

At present, my country mainly relies on hand lay-up molding, which requires manual work to alternately lay fiber reinforcement materials and resin in the mold. The early process is cumbersome, the production efficiency is low, the production safety is poor, and the labor cost is high, which is not suitable for large-scale production.

 

 

Hand Lay-Up Process Flow Chart

 

The rapidly developing Resin Transfer Molding (RTM) process can effectively avoid many disadvantages of hand lay-up. It has the advantages of efficient production of large-area complex components, low labor and consumption costs, high-efficiency rapid prototyping and environmental protection. In addition, the mold is easy to make, the designability is strong, the laminate size and structure are stable, the mechanical properties are excellent, and the surface of the manufactured laminate is smooth and beautiful. It provides conditions for the large-scale industrial production of advanced resin-based composite materials, thereby meeting the growing demand for composite materials applications

 

 

Traditional RTM process

 

The traditional RTM process is shown in the figure below. The presence of air in the mold will cause great defects in the finished product. In order to be able to output air from the inside of the mold, the RTM mold must have at least one inlet and one outlet for injecting resin, and sometimes there are multiple inputs and outputs. Depending on whether the curing agent and resin are mixed before injection or after injection, this molding process is divided into single-component injection and two-component injection, in which two-component injection is mixed in a mixer in a specific ratio before injection into the mold cavity.

 

Compared with traditional processes such as manual molding and compression molding, RTM has great advantages, simplifies the production process steps, improves production efficiency, greatly reduces labor costs, avoids safety problems caused by workers contacting raw materials such as resin, and provides a way for large-scale production of advanced composite materials.

However, the RTM molding process still has some disadvantages,

 

For Example:

 

1) The impregnation rate of resin to the reinforcing fiber is not high, and there are defects such as pores, dry spots, and resin-rich, which seriously affect the performance and quality of the product;

 

2) Since the reinforcing material has to go through the flow and filling process of pressurized resin in the mold cavity, it will drive or even disperse the fiber, causing fiber buckling in the composite molded product, uneven fiber distribution or even less fiber content, thereby greatly reducing the mechanical properties of the product

 

3) When making large products, the mold cavity area is large, and uneven resin flow may occur during the molding process. To a certain extent, it is difficult to predict and control the actual flow of resin and the degree of fiber impregnation.

 

Based on the above advantages and disadvantages of RTM, high pressure resin transfer molding (HP-RTM), vacuum assisted resin infusion process (VARTM), Siemens resin infusion technology (SCRIMP) molding process, lightweight RTM (LRTM) and other processes were improved and developed later.

 

Advanced Development of RTM Process

 

HP-RTM molding process is a derivative development process of RTM, which is mainly divided into high pressure injection molding process (HP-IRTM) and high pressure compression molding process (HP-CRTM). This type of process generally uses high pressure to produce products with low porosity and high fiber volume fraction. This type of process has high requirements on mold hardness, otherwise it is easy to deform, causing problems such as dry fiber and fiber precipitation.

 

 

HP-IRTM process flow

 

The HP-IRTM molding process first places the fiber preform into the mold cavity to form a completely closed mold cavity, and then evacuates the cavity. These steps are the same as the traditional RTM process. The difference is that during injection, the resin and curing agent must be mixed and then injected into the mold cavity under high pressure. This can greatly shorten the filling time, improve production efficiency, and maintain the surface quality and good morphology of the product, so that a product with a larger fiber volume fraction can be obtained.

 

 

HP-CRTM Process Flow

 

The HP-CRTM molding process places the fiber preform into the mold cavity, leaving a gap between the upper mold surface and the fiber preform, and then also evacuates. Therefore, the injection pressure of this process can be much lower than that of HP-IRTM, which is very harmful to the mold. The hardness requirement is also lower than that of HP-IRTM, but a compression process is required after injection. The mold cavity pressure is controlled to close the mold gap. As the gap decreases, the resin is completely squeezed into the fiber. After the gap is completely closed, the thickness of the final product is obtained. This This step will significantly affect the fiber volume fraction of the product, and the fiber volume fraction is a key factor characterizing the balance between mechanical properties and processing properties.

 

 

Working Principle of VARTM

 

VARTM is a new type of single-sided molding economical and efficient process, suitable for the production of large parts. After the preform is placed in the mold, the top is sealed with a vacuum bag. In the vacuum state, the resin is injected (the injection pressure is usually <0.6985 MPa) or the resin is directly sucked in by vacuum negative pressure, which effectively avoids defects such as bubbles in the resin-impregnated fiber preform. At this time, the resin has better fluidity and can fully impregnate the fiber preform. Finally, it is cured and demolded at room temperature to obtain a composite material product.

 

In the field of aerospace, the VARTM molding process can be used to obtain better satellite antenna reflectors. When manufacturing the reflector, it is required that the reflective surface should avoid deformation during the manufacturing process as much as possible, improve the dimensional accuracy and stability, and ensure the quality of the reflector. Some researchers have conducted experiments to verify that the rebound deformation of the reflector reflective surface manufactured using the autoclave manufacturing process is 0.37 mm; while the rebound deformation of the reflector reflective surface manufactured using the VARTM process is 0.35 mm, with a difference of nearly 5%, which fully demonstrates the performance advantages of the VARTM molding process in the manufacturing of satellite reflectors.

 

The VARTM molding process not only has low process costs and high economic benefits, but also has high mechanical properties and stability of the products. It also reduces VOC pollution to a certain extent, and is one of the effective ways to make large composite products in the future.

 

Prospects For The Application of RTM Process

 

Today, when composite materials are widely used, the derivative processes of RTM have been continuously improved and supplemented by scientific researchers, and are unique among various molding processes, showing enduring vitality.

 

Continuing to reduce waste, reduce VOC emissions and costs through improvements in molds and control technologies will not only benefit the large-scale commercial production of composite materials and meet the growing demand for materials, but also be conducive to green and sustainable development and achieve the environmental protection requirements of “green processes”.

 

Whether it is the automotive industry that is closely related to people’s livelihood, or the military and aerospace industries that are related to national strength, RTM and its derivative processes can meet the requirements.

 

In the future, the development of RTM and its derivative processes will inevitably be in line with the Internet era, and the support of computer simulation technology will bring unlimited possibilities to the RTM process.