Composite Material Molding Process – Lightweight Resin Transfer Molding Process (L-RTM)

 

Anyone who is engaged in the resin-based composites industry must be familiar with the lightweight resin transfer molding process. The lightweight resin transfer molding process (RTM-Light) is also known as LRTM, ECO, Vacuum Molding or VARTM. It is a low-cost manufacturing process that has developed rapidly in recent years.

Why is LRTM so popular among product manufacturers? First, it can produce double-sided products; second, the equipment investment is relatively small. Compared with traditional RTM, it only needs a vacuum pump and a mold, and does not require a large press and injection system; third, the closed mold process can control the resin content and improve product quality.

 

LRTM FRP Mould

 

Carbon Fiber Seat Made by LRTM Molding

 

Characteristics of LRTM

The L-RTM process retains the mold-matching process of the RTM process, thus retaining almost all the advantages of the RTM process. However, its upper mold is a semi-rigid fiberglass mold with a thickness of generally 6-8mm. It usually does not need to be reinforced with steel pipes. The mold has a rigid periphery with a width of about 100mm, and a double-channel sealing belt constitutes an independent sealing area.

As long as the vacuum is drawn, the mold is closed, which is very convenient and fast. Then the mold cavity is vacuumed, and the negative pressure in the mold and the lower injection pressure are used to inject the resin into the mold, so that the resin penetrates into the pre-laid reinforcing fibers or prefabricated parts.

The mold cost of RTM-Light is low, and because the pressure in the mold is reduced, its mold is similar to the open mold, and it is easy to transform from the mold of the open mold process.

In the L-RTM molding process, except for the vacuum pump, the injection equipment is optional, and the equipment requirements are basically the same as those used in RTM. If the resin injection equipment is not used, the same purpose can be achieved, but the efficiency is slightly different. The L-RTM mold cavity has a positive pressure of 0.2-0.8 kg, so the rigidity requirement for the inner surface mold of the product is not high, mainly to reflect the word “light”.

The L-RTM process is a low-cost production technology. Compared with traditional RTM technology, the mold cost is low, no complex clamping device or pressure system is required, the investment is small, the operation is simple, and human capital is saved.

Compared with the open mold process, it has the advantages of high product dimensional accuracy, low void ratio, low styrene volatilization during production, environmental friendliness, low material waste (resin utilization rate can reach 95%), low scrap rate, and high production efficiency.

 

LRTM Requirements for Resin

The resin used in the L-RTM process can be unsaturated polyester resin, vinyl or epoxy resin, and the main requirements are:

(1) low viscosity, which can flow, infiltrate, and penetrate in the high-density preform with the aid of vacuum. (Viscosity index 180-300 centipoise);

(2) Long application cycle, longer gel time, faster curing speed, which is conducive to penetration and exhaust;

(3) Can be cured at room temperature, and the resin working life meets the structural requirements;

(4) No additional pressure is required during curing, only vacuum negative pressure is required;

(5) It has good toughness and elastic modulus higher than general resins, as well as corrosion resistance.

 

Application of L-RTM Molding Technology

Many foreign companies use this technology to manufacture large structural parts, especially in the shipbuilding industry. In addition, space shuttle bulkheads, missile nose cones, missile automatic aiming head fairings, radar covers, minesweepers, thrusters, rocket launchers, etc. are all molded using L-RTM technology.

Since the L-RTM process uses a closed mold molding process, it is also particularly suitable for one-time molding of integral wind turbine blades (fibers, sandwich cores and joints can be co-molded in the mold cavity at one time) without the need for secondary bonding. Compared with the hand lay-up process, it not only saves various tooling equipment of the bonding process, but also saves working time and improves production efficiency. It reduces production costs.

At the same time, due to the use of low-viscosity resin to impregnate the fiber and the use of heating curing process, the quality of composite materials and production efficiency are greatly improved. L-RTM process production is less dependent on the technical level of workers, and the process quality only depends on the determined process parameters. The product quality is easy to guarantee, and the scrap rate of the product is lower than that of the hand lay-up process.

Therefore, at present, high-quality composite wind turbine blades abroad are often manufactured by molding, L-RTM, winding and prepreg/vacuum infusion process.

Among them, the molding process has a large investment and is suitable for mass production of small-sized fan blades (>5000 pieces/year); L-RTM process is suitable for medium-sized and small-sized fan blades. Medium-scale production (1000–3000 pieces/year); winding and prepreg/vacuum infusion process are suitable for large-scale production of large fan blades.