Last Updated on: 7 7 月, 2025

 

Composite Materials Technology Issue 3 — Resin Transfer Molding Technology (Since There are Too Few Articles on this Technology in China, the Editor Updates it Slowly)

 

Resin Transfer Molding is referred to as RTM (Resin Transfer Molding). RTM started in the 1950s and is a closed mold molding technology improved from the hand lay-up molding process. It can produce products with two sides of the light. In foreign countries, resin injection and pressure injection also belong to this process category.

 

Although the introduction of RTM equipment in China began in the 1980s, its large-scale development began after the 10th Annual Academic Conference of FRP/Composite Materials organized by the China FRP Society in 1993.

 

The basic principle of RTM is to lay the fiber-reinforced material into the closed mold cavity, inject the resin glue into the mold cavity with pressure, soak the fiber-reinforced material, and then solidify it and demold the molded product. It is currently widely used in industrial fields such as construction, transportation, telecommunications, health, and aerospace.

 

The products that have been developed include: automobile shells and parts, recreational vehicle components, propellers, 8.5m long wind turbine blades, antenna covers, machine covers, bathtubs, showers, swimming pool boards, seats, water tanks, telephone booths, telephone poles, small yachts, etc.

 

1. RTM Resin Matrix

 

RTM special resin is different from hand-made resin, pultrusion resin and residual winding resin. RTM special resin should meet the process requirements of “one long”, “one fast”, “two highs” and “four lows”:

 

“One long” refers to the long gel time of the resin to meet the injection time, so that the resin system has a certain fluidity and permeability before the injection is completed.

 

“One fast” refers to the fast curing time of the resin (curing time refers to the time from gelling to the highest exothermic peak) to improve production efficiency.

 

“Two highs” refers to the high foaming and high wettability of the resin, reducing product defects (such as bubbles, voids, lack of glue, etc.) and improving product performance.

 

“Four lows” refers to the low viscosity of the resin (improving permeability and wettability), low volatile matter (reducing shrinkage and bubble generation), low curing shrinkage (reducing residual application force and cracking tendency, while ensuring the accuracy of the shape and size of the product), and low exothermic peak (reducing the curing temperature to reduce the requirements for the mold and save resources).

 

The resins currently used in RTM technology include vinyl resin, unsaturated polyester, epoxy resin, phenolic resin, cyanate resin, bismaleimide, etc.

 

2. Reinforcement Materials

 

The reinforcement materials used for RTM can be divided into glass fiber, graphite fiber, carbon fiber, silicon carbide fiber, aramid, etc. It must meet certain process requirements. Considering these requirements is very important for the reasonable selection of the preparation method of the preform, as follows:

 

① The most basic requirement that the reinforcement material should have is that it can be completely impregnated by the resin system, and ideally it can be achieved at high speed and low pressure.

 

② The resin must completely wrap each single filament in the reinforcement material to ensure a strong interface bond.

 

③ In order to ensure that the distribution of the fibers remains unchanged during the infiltration process, the reinforcement material must be able to withstand the scouring force applied when the resin flows through.

 

④ The reinforcement material must be able to be made into a product with a geometry close to the actual component, which depends on the manufacturing method of the preform.

 

⑤ The original geometry must be maintained during the laying of the reinforcement material into the mold.

 

⑥ In order to meet the requirements of RTM process for reinforcement materials, reinforcement materials are generally made into preforms. The specific preparation methods include manual laying, stitching, weaving, knitting, etc., which will not be introduced here one by one.

 

3. RTM Process Flow

 

 

4. Factors Affecting RTM Process

 

Mould

 

 

①Mold Alignment.

 

②No damage or deformation under injection pressure of 50~150KPa

 

③The main inlet must be perpendicular to the mold, and the resin must be injected vertically into the cavity during injection.

If the injection is not vertical, the resin will hit the injection port and reflect into the cavity, destroying the flow law of the resin in the cavity, and causing a large number of bubbles to accumulate in the cavity, resulting in injection failure.

 

④When making high-quality RTM products, a certain amount of resin will flow out of the outlet before the mold filling is completed.

The purpose is to improve the degree of impregnation of the resin on the fiber and discharge microscopic bubbles. That is, it improves the physical bonding strength between the fiber and the resin, and improves the density of the product. Generally, the amount of wasted resin can reach 1/3~2/3 of the weight of the molded product.

 

 

Injection Pressure

① The injection speed depends on the wettability of the resin to the fiber, the surface tension and viscosity of the resin, and is restricted by the resin activity period, the capacity of the pressure equipment, the mold rigidity, the size of the part and the fiber content.

② People hope to obtain a high injection speed to improve production efficiency and facilitate the removal of bubbles, but the increase in speed will be accompanied by an increase in pressure.

 

Injection Speed

① The injection speed depends on the wettability of the resin to the fiber, the surface tension and viscosity of the resin, and is restricted by the resin activity period, the capacity of the pressure equipment, the mold rigidity, the size of the part and the fiber content. People hope to obtain a high injection speed to improve production efficiency and facilitate the removal of bubbles, but the increase in speed will be accompanied by an increase in pressure.

 

Injection Temperature

① The injection temperature depends on the activity period and the minimum viscosity temperature of the resin system. Under the premise of not shortening the resin gel time too much, in order to make the resin fully infiltrate the fiber under the minimum pressure, the injection temperature should be as close as possible to the minimum resin viscosity temperature.

 

Too high a temperature will shorten the resin working cycle, and too high a temperature will increase the resin viscosity, which will increase the pressure and reduce the resin wettability. Higher temperatures will reduce the surface tension of the resin, causing the air in the fiber to rise due to heat, which is conducive to the discharge of bubbles.

 

5. Common Types Of Defects in RTM and Solutions

Although the RTM process has many advantages, various defects are prone to occur during the mold filling process, hindering its wider application. The specific manifestations of defects in the RTM process are: local roughness and dullness of the product surface, wrinkles, bubbles, dry spots, wrinkles and deformation, etc. The manifestations of the main defects are as follows:

 

Bubbles

This is a common defect of RTM products and a phenomenon that seriously affects product quality. The presence of bubble defects will lead to reduced fiber wettability and poor adhesion between the resin and the fiber interface, resulting in reduced strength and poor surface quality of composite products.

 

The RTM process is to lay the fiber preform in the mold in advance and then inject the resin. Therefore, the resin must quickly fill the mold cavity and penetrate the fiber during the mold filling process before the curing reaction occurs.

 

Since the injection of the resin is usually completed in a very short time, the time for the air in the mold cavity to be discharged is limited, making it very easy for air to be wrapped and form bubbles in the RTM process.

 

There are Four Reasons For The Phenomenon:

 

① The heat release of the resin curing reaction in the mold cavity is too high, and the curing time is too short, so the gas in the mold cavity is not completely discharged;

 

② When the resin is injected into the mold cavity, too much air is brought in, and it cannot be completely discharged during the injection time, and even the bubbles are always at the top of the mold cavity, and the resin from top to bottom cannot carry it out;

 

③ The viscosity of the resin is too high, and the bubbles cannot all overflow from the product during the infusion time. In addition, due to the high viscosity of the resin and the slow flow, the bubbles may even be carried to the middle area of ​​the side of the product (when the product height is high), so that the product after demolding contains more bubbles (or pit-shaped glue deficiency);

 

④ The pressure of the resin injected into the mold cavity is too high, causing the bubbles to be contained in the resin and difficult to discharge.

 

Dry Spots

The main reason for the dry spots inside the RTM process products is insufficient infiltration. Compared with bubble defects, dry spots have a more serious impact on product quality.

 

Dry spots refer to large areas with insufficient or no resin infiltration. If some spots appear at the key position of the structure bearing load, it will directly affect whether the product is qualified or even cause the product to be completely scrapped.

 

The cause of dry spots is often due to improper setting of the exhaust port position during the filling process. Generally, due to the complex geometric shape of the molded structural parts, multiple exhaust ports are usually set during filling. For the same exhaust port, if the resin does not reach it at the same time, the resin that reaches it first will close the exhaust port, thereby wrapping the air between the exhaust port and the subsequent resin flow front, forming dry spots.

 

At the same time, there are other reasons for the formation of dry spots. If the dry spots appear in a certain part of the product during the same period, it should also be considered whether it is caused by the contamination of the fiber cloth.

 

Preform Deformation

One form of deformation of the preform is wrinkles, which is also a common problem for many RTM products. Wrinkles are caused by wrinkles in the fabric layer. After demolding, the product appearance can be cleaned and the fabric wrinkles can be seen, but the space between the fabric wrinkles is filled with resin. The main reason for the wrinkles is that the resin flows in the mold cavity and squeezes the fabric to deform it, which may also be caused by the mold squeezing the fabric layer due to operational problems during mold closing. Another form of deformation of the preform is that the pressure used during resin injection is too high, which causes changes in the finished product structure and appearance near the injection, such as bumps.