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


Composite material forming technology is the basis and condition of the development of composite material industry. With the widening of the application field of composite materials, composite industry has been developing rapidly, some molding process is improving, new molding methods continue to emerge, currently there are more than 20 polymer matrix composite molding methods, and successfully used in industrial production, such as:

(1) Hand paste forming process — wet lay-up forming method;

(2) Jet forming process;

(3) Resin transfer molding technology (RTM technology);

(4) Bag pressure method (pressure bag method) molding;

(5) Vacuum bag pressing molding;

(6) Autoclave forming technology;

(7) Hydraulic kettle forming technology;

(8) Thermal expansion molding technology;

(9) Sandwich structure forming technology;

(10) Molding material production process;

(11) ZMC molding material injection technology;

(12) Molding process;

(13) Laminate production technology;

(14) Rolling tube forming technology;

(15) Fiber winding products forming technology;

(16) Continuous plate production process;

(17) Casting technology;

(18) Pultrusion molding process;

(19) Continuous winding pipe making process;

(20) Manufacturing technology of braided composite materials;

(21) Manufacturing technology of thermoplastic sheet moulds and cold stamping molding process;

(22) Injection molding process;

(23) Extrusion molding process;

(24) Centrifugal casting tube forming process;

(25) Other forming technology.

Depending on the resin matrix material chosen, the above methods are suitable for the production of thermosetting and thermoplastic composites respectively, and some processes are suitable for both.

Composite products forming process characteristics: compared with other materials processing technology, composite materials forming process has the following characteristics:

(1) Material manufacturing and product molding at the same time to complete the general situation, the production process of composite materials, that is, the molding process of products. The performance of materials must be designed according to the requirements of the use of products, so in the selection of materials, design ratio, determine the fiber layering and molding method, must meet the physical and chemical properties of products, structural shape and appearance quality requirements.

(2) products molding is relatively simple general thermosetting composite resin matrix, molding is a flowing liquid, reinforcement material is soft fiber or fabric, therefore, with these materials to produce composite products, the required process and equipment is much simpler than other materials, for some products only a set of molds can be produced.

First, contact low pressure molding process

The contact low pressure molding process is characterized by manual placement of reinforcement, resin leaching, or simple tool-assisted placement of reinforcement and resin. Another characteristic of the contact low-pressure molding process is that the molding process does not need to apply molding pressure (contact molding), or only apply low molding pressure (0.01 ~ 0.7mpa pressure after contact molding, the maximum pressure does not exceed 2.0mpa).

Contact low-pressure molding process, is the first material in the male mold, male mold or mold design shape, and then through heating or room temperature curing, demoulding and then through auxiliary processing and products. Belong to this kind of molding process are hand paste molding, jet molding, bag pressing molding, resin transfer molding, autoclave molding and thermal expansion molding (low pressure molding). The first two are contact forming.

In the contact low pressure molding process, the hand paste molding process is the first invention in the production of polymer matrix composite material, the most widely applicable range, other methods are the development and improvement of hand paste molding process. The biggest advantage of contact forming process is simple equipment, wide adaptability, less investment and quick effect. According to statistics in recent years, contact low-pressure molding process in the world composite material industrial production, still occupy a large proportion, such as the United States accounted for 35%, Western Europe accounted for 25%, Japan accounted for 42%, China accounted for 75%. This shows the importance and irreplaceable of contact low pressure molding technology in composite material industry production, it is a process method that will never decline. But its biggest shortcoming is the production efficiency is low, the labor intensity is big, the product repeatability is poor and so on.

1. Raw materials

Contact low pressure molding of raw materials are reinforced materials, resins and auxiliary materials.

(1) Enhanced materials

Contact forming requirements for enhanced materials: (1) enhanced materials are easy to be impregnated by resin; (2) There is enough shape variability to meet the molding requirements of complex shapes of products; (3) bubbles are easy to deduct; (4) can meet the physical and chemical performance requirements of the use conditions of products; ⑤ Reasonable price (as cheap as possible), abundant sources.

Reinforced materials for contact forming include glass fiber and its fabric, carbon fiber and its fabric, Arlene fiber and its fabric, etc.

(2) Matrix materials

Contact low pressure molding process for the matrix material requirements: (1) under the condition of hand paste, easy to soak the fiber reinforced material, easy to exclude bubbles, strong adhesion with the fiber; (2) At room temperature can gel, solidify, and require shrinkage, less volatiles; (3) Suitable viscosity: generally 0.2 ~ 0.5Pa·s, can not produce glue flow phenomenon; (4) non-toxic or low toxicity; The price is reasonable and the source is guaranteed.

The commonly used resins in production are: unsaturated polyester resin, epoxy resin, phenolic resin, bismaleimide resin, polyimide resin and so on.

Performance requirements of several contact forming processes for resin:

Molding method requirements for resin properties

Gel production

1, molding does not flow, easy to defoaming

2, uniform tone, no floating color

3, fast curing, no wrinkles, good adhesion with the layer of resin

Hand lay-up molding

1, good impregnation, easy to soak the fiber, easy to eliminate bubbles

2, spread after curing fast, less heat release, shrinkage

3, volatile less, the surface of the product is not sticky

4. Good adhesion between layers

Injection molding

1. Ensure the requirements of hand paste forming

2. Thixotropic recovery is earlier

3, temperature has little effect on resin viscosity

4. The resin should be suitable for a long time, and the viscosity should not increase after the addition of the accelerator

Bag moulding

1, good wettability, easy to soak the fiber, easy to discharge bubbles

2, curing fast, curing heat to small

3, not easy to flow glue, strong adhesion between layers

(3) Auxiliary materials

Contact forming process of auxiliary materials, mainly refers to the filler and color two categories, and curing agent, diluent, toughening agent, belonging to the resin matrix system.

2, mold and release agent

(1) Molds

Mold is the main equipment in all kinds of contact forming process. The quality of the mold directly affects the quality and cost of the product, so it must be carefully designed and manufactured.

When designing the mold, the following requirements must be considered comprehensively: (1) Meet the precision requirements of product design, the mold size is accurate and the surface is smooth; (2) to have enough strength and stiffness; (3) convenient demoulding; (4) have enough thermal stability; Light weight, adequate material source and low cost.

Mold structure contact molding mold is divided into: male mold, male mold and three kinds of mold, no matter which kind of mold, can be based on the size, molding requirements, design as a whole or assembled mold.

When the mold material is manufactured, the following requirements should be met:

① Can meet the requirements of dimensional accuracy, appearance quality and service life of products;

(2) The mold material should have enough strength and stiffness to ensure that the mold is not easy to be deformed and damaged in the process of use;

(3) it is not corroded by resin and does not affect resin curing;

(4) Good heat resistance, product curing and heating curing, the mold is not deformed;

(5) Easy to manufacture, easy to demoulding;

(6) day to reduce mold weight, convenient production;

⑦ The price is cheap and the materials are easy to obtain. The materials that can be used as hand paste moulds are: wood, metal, gypsum, cement, low melting point metal, rigid foamed plastics and glass fiber reinforced plastics.

Release agent basic requirements:

1. Does not corrode the mold, does not affect the resin curing, the resin adhesion is less than 0.01mpa;

(2) Short film forming time, uniform thickness, smooth surface;

The use of safety, no toxic effect;

(4) heat resistance, can be heated by the temperature of curing;

⑤ It is easy to operate and cheap.

The release agent of contact forming process mainly includes film release agent, liquid release agent and ointment, wax release agent.

Hand paste forming process

The process flow of hand paste forming is as follows:

(1) Production preparation

The size of the working site for hand pasting shall be determined according to the product size and daily output. The site shall be clean, dry and well ventilated, and the air temperature shall be kept between 15 and 35 degrees Celsius. The post-processing refurbishment section shall be equipped with exhaust dust removal and water spraying device.

Mold preparation includes cleaning, assembly and release agent.

When the resin glue is prepared, we should pay attention to two problems: (1) prevent the glue from mixing bubbles; (2) The amount of glue should not be too much, and each amount should be used up before the resin gel.

Reinforcement materials The types and specifications of the reinforcement materials must be selected based on the design requirements.

(2) Pasting and curing

Layer-paste manual layer-paste is divided into wet method and dry method two: (1) dry layer-prepreg cloth as raw material, the pre-learn material (cloth) according to the sample cut into bad material, layer-softening heating, and then layer by layer on the mold, and pay attention to eliminate bubbles between layers, so that dense. This method is used for autoclave and bag molding. (2) Wet layering directly in the mold will strengthen the material dip, layer by layer close to the mold, deduct bubbles, make it dense. General hand paste process with this method of layering. Wet layering is divided into gelcoat layer paste and structure layer paste.

Hand pasting tool Hand pasting tool has a great impact on ensuring product quality. There are wool roller, bristle roller, spiral roller and electric saw, electric drill, polishing machine and so on.

Solidify products solidify cent sclerosis and ripe two stages: from gel to trigonal change want 24h commonly, right now solidify degree amount to 50% ~ 70% (ba Ke hardness degree is 15), can demolom, after taking off solidify below natural environment condition 1 ~ 2 weeks ability makes products have mechanical strength, say ripe, its solidify degree amount to 85% above. Heating can promote the curing process. For polyester glass steel, heating at 80℃ for 3h, for epoxy glass steel, post curing temperature can be controlled within 150℃. There are many heating and curing methods, medium and small products can be heated and cured in the curing furnace, large products can be heated or infrared heating.

(3) Demoulding and dressing

Demoulding demoulding to ensure that the product is not damaged. Demoulding methods are as follows: (1) The ejection demoulding device is embedded in the mold, and the screw is rotated when demoulding to ejection the product. The pressure demoulding mold has a compressed air or water inlet, demoulding will be compressed air or water (0.2mpa) between the mold and the product, at the same time with wood hammer and rubber hammer, so that the product and the mold separation. (3) Demoulding of large products (such as ships) with the help of jacks, cranes and hardwood wedges and other tools. (4) Complex products can use manual demoulding method to paste two or three layers of FRP on the mold, to be cured after peeling from the mold, and then put on the mold to continue to paste to the design thickness, it is easy to take off from the mold after curing.

Dressing dressing is divided into two kinds: one is size dressing, the other defect repair. (1) After shaping the size of the products, according to the design size to cut off the excess part; (2) Defect repair includes perforation repair, bubble, crack repair, hole reinforcement, etc.

Jet forming technique

Jet forming technology is an improvement of hand paste forming, semi – mechanized degree. Jet forming technology accounts for a large proportion in composite material forming process, such as 9.1% in the United States, 11.3% in Western Europe, and 21% in Japan. At present, domestic injection molding machines are mainly imported from the United States.

(1) Jet forming process principle and advantages and disadvantages

Injection molding process is mixed with initiator and promoter of two kinds of polyester, respectively from the spray gun out on both sides, and will cut off the fiberglass roving, by the torch center, mixing with resin, deposit to the mould, when the deposit to a certain thickness, with the roller compaction, make the fiber saturated resin, eliminate air bubbles, cured into products.

The advantages of jet molding: (1) using glass fiber roving instead of fabric, can reduce the cost of materials; (2) The production efficiency is 2-4 times higher than hand paste; (3) The product has good integrity, no joints, high interlayer shear strength, high resin content, good corrosion resistance and leakage resistance; (4) it can reduce the consumption of flapping, cutting cloth scraps and remaining glue liquid; The product size and shape are not restricted. The disadvantages are: (1) high resin content, low strength products; (2) the product can only do one side smooth; ③ It pollutes the environment and is harmful to workers’ health.

Jet forming efficiency up to 15kg/min, so suitable for large hull manufacturing. It has been widely used to process bath tub, machine cover, integral toilet, automobile body components and large relief products.

(2) Production preparation

In addition to meeting the requirements of hand paste process, special attention should be paid to environmental exhaust. According to the size of the product, the operation room can be closed to save energy.

Material preparation raw materials are mainly resin (mainly unsaturated polyester resin) and untwisted glass fiber roving.

Mold preparation includes cleaning, assembly and release agent.

Injection molding equipment injection molding machine is divided into two types: pressure tank type and pump type: (1) Pump type injection molding machine, is the resin initiator and accelerator are respectively pumped to the static mixer, fully mixed and then ejected by the spray gun, known as the gun mixed type. Its components are pneumatic control system, resin pump, auxiliary pump, mixer, spray gun, fiber cutting injector, etc. Resin pump and auxiliary pump are rigidly connected by rocker arm. Adjust the position of auxiliary pump on the rocker arm to ensure the proportion of ingredients. Under the action of air compressor, resin and auxiliary agent are evenly mixed in the mixer and formed by spray gun droplets, which are continuously sprayed to the surface of the mould with the cut fiber. This jet machine only has a glue spray gun, simple structure, light weight, less initiator waste, but because of mixing in the system, it must be cleaned immediately after completion, in order to prevent the injection blockage. (2) The pressure tank type glue supply jet machine is to install the resin glue in the pressure tank respectively, and make the glue into the spray gun to spray continuously by the gas pressure into the tank. It consists of two resin tanks, pipe, valve, spray gun, fiber cutting injector, trolley and bracket. When working, connect the compressed air source, make the compressed air pass through the air-water separator into the resin tank, glass fiber cutter and spray gun, so that the resin and glass fiber are continuously ejected by the spray gun, resin atomization, glass fiber dispersion, mixed evenly and then sink to the mold. This jet is resin mixed outside the gun, so it is not easy to plug the nozzle of the gun.

(3) Spray molding process control

Injection process parameters selection: ① Resin content spray molding products, resin content control at about 60%. When the resin viscosity is 0.2Pa·s, the resin tank pressure is 0.05-0.15mpa, and the atomization pressure is 0.3-0.55mpa, the uniformity of the components can be guaranteed. (3) The mixing distance of resin sprayed by different Angle of spray gun is different. Generally, an Angle of 20° is selected, and the distance between spray gun and mold is 350 ~ 400mm. To change the distance, the Angle of the spray gun should be high-speed to ensure that each component is mixed in the intersection near the surface of the mold to prevent the glue from flying away.

Spray molding should be noted: (1) the ambient temperature should be controlled at (25±5) ℃, too high, easy to cause the blockage of the spray gun; Too low, uneven mixing, slow curing; (2) No water is allowed in the jet system, otherwise the product quality will be affected; (3) Before forming, spray a layer of resin on the mold, and then spray the resin fiber mixture layer; (4) Before injection molding, first adjust the air pressure, control resin and glass fiber content; (5) The spray gun should move evenly to prevent leakage and spray. It can not go in an arc. The overlap between the two lines is less than 1/3, and the coverage and thickness should be uniform. After spraying a layer, immediately use roller compaction, should pay attention to edges and concave and convex surface, ensure that each layer is pressed flat, exhaust bubbles, prevent with fiber caused burrs; After each layer of spray, to check, qualified after the next layer of spray; ⑧ The last layer to spray some, make the surface smooth; ⑨ Clean the jet immediately after use to prevent resin solidification and damage to the equipment.

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); ③RTM 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; ⑥ 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.

② 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; ② 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.

Post time: 29-06-22