LRTM FRP Closed Mold Forming Process

 

In recent years, due to the comprehensive requirements of quality, strength, cost, environmental protection, cleanliness, etc., my country’s hand lay-up FRP forming process has begun to transition from open hand lay-up process to closed mold forming process. This closed mold forming process has been popular abroad for many years; in recent years, domestic FRP enterprises have this technical demand, and foreign technologies have “come in one after another”. More than ten years ago, the RTM process was popular in China. In recent years, due to the manufacture of large blades and yachts, the vacuum forming process was introduced; for products with complex sizes and high strength requirements, the original RTM process is supplemented by vacuum forming, that is, light weight RTM process, and the obtained products are lighter, with lower material cost and higher strength. This article discusses a process.

 

The LRTM process is one of the closed mold forming processes. The types of closed mold processes are listed as follows:

 

 

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 positive pressure in the L-RTM mold cavity is 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 raw materials used in L-RTM are matrix resin and reinforced glass fiber material.

 

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

 

(2) Long application cycle, long gel time, and fast curing speed, which is conducive to penetration and exhaust.

 

(3) It 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 that of general resins, as well as corrosion resistance.

 

The main reinforcing material of L-RTM is glass fiber, with a content of 45% to 55%; the reinforcing materials include glass fiber continuous mat, composite mat and gingham cloth. The use of composite mat reduces the weight of the product. The composite mats of Changzhou Tianma and Zhejiang Lianze have been used in large quantities.

 

The L-RTM process is a semi-mechanized composite material molding process. Workers only need to put the designed dry fiber or preform into the mold and close the mold.

The subsequent process is completely completed and guaranteed by the mold and injection system.

There is no exposure of any resin, and therefore the requirements for workers’ skills and environment are far lower than the hand lay-up process and can effectively control product quality.

L-RTM molding technology is widely used abroad. Many 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 adopts a closed mold molding process, it is also particularly suitable for molding the entire wind turbine blade in one go (fibers, cores and joints can be co-molded in the mold cavity at one time) without secondary bonding.

Compared with the production of blades by hand lay-up, it not only saves various tooling equipment for the bonding process, but also saves working time and improves production efficiency.

Reduced production costs. At the same time, due to the use of low-viscosity resin to impregnate the fiber and the use of heating and curing process, the quality and production efficiency of the composite material are greatly improved.

 

The 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 induction processes.

Among them, the molding process requires a large investment and is suitable for mass production of small-sized fan blades (>5000 pieces/year); the L-RTM process is suitable for medium-sized and small-sized fan blades.

The medium-sized batch production (1000–3000 pieces/year); the winding and prepreg/vacuum infusion process is suitable for large-scale production of fan blades.

 

In addition to the LRTM process, a soft mold RTM process is also popular. The soft mold RTM process takes into account the advantages of RTM and vacuum infusion processes. The rigid upper mold of RTM is changed to a soft mold of silicone or rubber.

Its function is equivalent to a layer of vacuum bag film. The product is between the soft mold and the mold, the periphery is sealed, and vacuum is drawn (about 0.1MPa). While vacuuming, the resin is introduced into the mold from the other end of the mold through the pipeline to impregnate the reinforced glass fiber. Demolding after curing.

 

Features of soft mold RTM molding technology:

1. It can manufacture products with complex shapes and structures on both sides;
2. The molding efficiency is average, suitable for medium-scale FRP product production (less than 1,000 pieces/year);
3. The reinforcement material can be laid in any direction, and it is easy to lay the reinforcement material according to the stress condition of the product;
4. The service life of the soft mold is about 10-50 times, and the cost is between RTM and vacuum infusion process. It is currently used for boats, auto parts, engineering vehicle covers and fine relief processes.

 

In the soft mold RTM molding process, injection equipment is also not required, only a vacuum pump is needed; the mold needs a single-sided rigid mold. Generally, the mold cavity is negatively pressurized at 0.1Mpa, and the raw materials used are basically the same as those of vacuum infusion.

 

The efficiency of this process is between RTM and vacuum infusion. Its advantages mainly lie in the reusability of the vacuum-assisted materials used and the convenience it brings during the construction process.

Advantage 1: Consumables can be reused many times to reduce costs;

Advantage 2: It can be made into various complex shapes according to the mold, which is convenient for paving and can effectively avoid bridging and dry areas;

Advantage 3: It has excellent toughness. If there are dry spots during the injection process, they can be pulled vigorously to artificially create resin infusion routes without worrying about the vacuum bag being damaged.