Know something about the molding process of composite materials (Ⅲ)

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Resin transfer molding

 

Resin Transfer Molding abbreviated as RTM. RTM began in the 1950s, is a closed die forming technology of hand paste molding process improvement, can produce a two-sided light products. In foreign countries, Resin Injection and Pressure Infection are also included in this category.

The basic principle of RTM is to lay the glass fiber reinforced material in the mold cavity of the closed mold. The resin gel is injected into the mold cavity by pressure, and the glass fiber reinforced material is soaked, then cured, and the molded product is demoulded.

From the previous research level, the research and development direction of RTM technology will include microcomputer controlled injection unit, enhanced material preforming technology, low-cost mold, rapid resin curing system, process stability and adaptability, etc.

The characteristics of RTM forming technology: (1) can produce two-sided products; (2) High forming efficiency, suitable for medium scale FRP products production (less than 20000 pieces/year); RTM is a closed mold operation, which does not pollute the environment and does not damage workers' health; (4) the reinforcement material can be laid in any direction, easy to realize the reinforcement material according to the stress state of the product sample; (5) less raw materials and energy consumption; Less investment in building a factory, fast.

RTM technology is widely used in construction, transportation, telecommunications, health, aerospace and other industrial fields. The products we have developed are: automobile housing and parts, recreational vehicle components, spiral pulp, 8.5m long wind turbine blade, radome, machine cover, tub, bath room, swimming pool board, seat, water tank, telephone booth, telegraph pole, small yacht, etc.

(1) RTM process and equipment

The whole production process of RTM is divided into 11 processes. The operators and tools and equipment of each process are fixed. The mold is transported by the car and passes through each process in turn to realize the flow operation. The cycle time of the mould on the assembly line basically reflects the production cycle of the product. Small products generally take only ten minutes, and the production cycle of large products can be controlled within 1h.

Molding equipment RTM molding equipment is mainly resin injection machine and mold.

 

 

Resin injection machine is composed of resin pump and injection gun. Resin pump is a set of piston reciprocating pumps, the top is an aerodynamic pump. When the compressed air drives the piston of the air pump to move up and down, the resin pump pumps the resin into the resin reservoir quantitatively through the flow controller and filter. The lateral lever makes the catalyst pump move and quantitatively pumps the catalyst to the reservoir. Compressed air is filled into the two reservoirs to create a buffer force opposite to the pump pressure, ensuring a steady flow of resin and catalyst to the injection head. Injection gun after the turbulent flow in a static mixer, and can make the resin and catalyst in the condition of no gas mixing, injection mold, and then the gun mixers have detergent inlet design, with a 0.28 MPa pressure solvent tank, when the machine after use, turn on the switch, automatic solvent, injection gun to clean clean.

 

Mold RTM mold is divided into glass steel mold, glass steel surface plated metal mold and metal mold. Fiberglass molds are easy to manufacture and cheaper, polyester fiberglass molds can be used 2,000 times, epoxy fiberglass molds can be used 4,000 times. The glass fiber reinforced plastic mold with gold-plated surface can be used more than 10000 times. Metal molds are rarely used in THE RTM process. Generally speaking, the mold fee of RTM is only 2% to 16% of that of SMC.

(2) RTM raw materials

RTM uses raw materials such as resin system, reinforcement material and filler.

Resin system THE main resin used in RTM process is unsaturated polyester resin.

Reinforcement materials General RTM reinforcement materials are mainly glass fiber, its content is 25% ~ 45% (weight ratio); Commonly used reinforcement materials are glass fiber continuous felt, composite felt and checkerboard.

Fillers are important to the RTM process because they not only reduce cost and improve performance, but also absorb heat during the exothermic phase of resin curing. Commonly used fillers are aluminum hydroxide, glass beads, calcium carbonate, mica and so on. Its dosage is 20% ~ 40%.

 

Bag pressure method, autoclave method, hydraulic kettle method and thermal expansion molding method

 

Bag pressure method, autoclave method, hydraulic kettle method and thermal expansion molding method known as low pressure molding process. Its molding process is to use the manual paving way, the reinforcement material and resin (including prepreg material) according to the design direction and order layer by layer on the mold, after reaching the specified thickness, by pressure, heating, curing, demoulding, dressing and obtain products. The difference between the four methods and the hand paste forming process only lies in the process of pressure curing. Therefore, they are just the improvement of hand paste forming process, in order to improve the density of products and interlayer bonding strength.

With high strength glass fiber, carbon fiber, boron fiber, aramong fiber and epoxy resin as raw materials, high performance composite products made by low pressure molding method have been widely used in aircraft, missiles, satellites and space shuttle. Such as aircraft doors, fairing, airborne radome, bracket, wing, tail, bulkhead, wall and stealth aircraft.

(1) Bag pressure method

Bag pressing molding is the hand paste molding of unsolidified products, through rubber bags or other elastic materials to apply gas or liquid pressure, so that the products under pressure dense, solidified.

The advantages of bag forming method are: (1) smooth on both sides of the product; Adapt to polyester, epoxy and phenolic resin; The product weight is higher than hand paste.

Bag pressure molding into pressure bag method and vacuum bag method 2: (1) pressure bag method pressure bag method is the hand paste molding not solidified products into a rubber bag, fixed the cover plate, and then through compressed air or steam (0.25 ~ 0.5mpa), so that the products in hot pressing conditions solidified. (2) Vacuum bag method this method is to hand paste shaped unsolidified products, with a layer of rubber film, products between the rubber film and the mold, seal the periphery, vacuum (0.05 ~ 0.07mpa), so that the bubbles and volatils in the products are excluded. Due to the small vacuum pressure, the vacuum bag forming method is only used for wet forming of polyester and epoxy composite products.

(2) hot pressure kettle and hydraulic kettle method

 

Hot autoclaved kettle and hydraulic kettle method are in the metal container, through compressed gas or liquid on the unsolidified hand paste products heating, pressure, make it solidified molding a process.

Autoclave method autoclave is a horizontal metal pressure vessel, uncured hand paste products, plus sealed plastic bags, vacuum, and then with the mold with the car to promote the autoclave, through steam (pressure is 1.5 ~ 2.5mpa), and vacuum, pressurized products, heating, bubble discharge, so that it solidifies under the conditions of hot pressure. It combines the advantages of pressure bag method and vacuum bag method, with short production cycle and high product quality. Hot autoclave method can produce large size, complex shape of high quality, high performance composite products. The size of the product is limited by the autoclave. At present, the largest autoclave in China has a diameter of 2.5m and a length of 18m. The products that have been developed and applied include wing, tail, satellite antenna reflector, missile reentry body and airborne sandwich structure radome. The biggest disadvantage of this method is the equipment investment, weight, complex structure, high cost.

Hydraulic kettle method Hydraulic kettle is a closed pressure vessel, the volume is smaller than the hot pressure kettle, upright placed, production through the pressure of hot water, on the unsolidified hand paste products heated, pressurized, so that it solidified. The pressure of hydraulic kettle can reach 2MPa or higher, and the temperature is 80 ~ 100. Oil carrier, heat up to 200. The product produced by this method is dense, short cycle, the disadvantage of hydraulic kettle method is large investment in equipment.

(3) thermal expansion molding method

Thermal expansion molding is a process used to produce hollow thin wall high performance composite products. Its working principle is the use of different expansion coefficient of mold materials, the use of its heated volume expansion of different extrusion pressure, the construction of the product pressure. The male mold of thermal expansion molding method is silicon rubber with large expansion coefficient, and the female mold is metal material with small expansion coefficient. The unsolidified products are placed between the male mold and the female mold by hand. Due to the different expansion coefficient of the positive and negative molds, there is a huge deformation difference, which makes the products solidified under hot pressure.


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