Composite forming technology is the development foundation and condition of composite industry. With the development of the application field of composite materials, the composite industry has developed rapidly, some molding processes have been improved, and new molding methods are emerging. At present, there are more than 20 molding methods of polymer matrix composite materials, which have been successfully used in industrial production, such as:
(1) Hand lay up process -- wet lay up process;
(2) Spray forming process;
(3) Resin transfer molding technology (RTM Technology);
(4) Bag pressing method (pressure bag method) forming;
(5) Vacuum bag press forming;
(6) Forming technology of autoclave;
(7) Forming technology of hydraulic kettle;
(8) Thermal expansion molding technology;
(9) Forming technology of sandwich structure;
(10) Molding material production process;
(11) ZMC injection molding material technology;
(12) Molding process;
(13) Laminated board production technology;
(14) Roll tube forming technology;
(15) Forming technology of filament winding products;
(16) Continuous plate making process;
(17) Casting technology;
(18) Pultrusion process;
(19) Continuous winding process;
(20) Manufacturing technology of braided composite;
(21) manufacturing technology of thermoplastic sheet molding plastics and cold die stamping process;
(22) injection molding process;
(23) extrusion process;
(24) centrifugal casting pipe forming process;
(25) other molding technologies.
Depending on the selected resin matrix materials, the above methods are applicable to the production of thermosetting and thermoplastic composite materials, and some processes are applicable to both.
Molding process characteristics of composite products: compared with other material processing processes, the composite molding process has the following characteristics:
(1) In general, the production process of composite materials, that is, the molding process of products, is completed at the same time of material manufacturing and product molding. The performance of the material must be designed according to the use requirements of the product, so when choosing the material, designing the proportion, determining the fiber layer and forming method, the physical and chemical properties, structural shape and appearance quality requirements of the product must be met.
(2) The products are easy to form. The resin matrix of general thermosetting composite materials is flowing liquid before forming, and the reinforcement material is soft fiber or fabric. Therefore, the process and equipment needed to produce composite products with these materials are much simpler than other materials. For some products, only one set of mold is needed.
1、 Contact low pressure forming process
The characteristics of contact low pressure forming process are to lay reinforcement materials by hand, soak resin, or use simple tools to assist laying reinforcement materials and resins. Another feature of the contact low pressure forming process is that the forming pressure (contact forming) does not need to be applied during the forming process, or only lower forming pressure (0.01-0.7mpa pressure is applied after contact forming, and the maximum pressure is not more than 2.0MPa).
In the process of contact low pressure forming, the material is first made into design shape on the female mold, male mold or counter mold, then solidified by heating or normal temperature, demoulded and then processed to obtain products. These molding processes include hand lay up molding, spray molding, bag press molding, resin transfer molding, autoclave molding and thermal expansion molding (low pressure molding). The first two are contact forming.
In the process of contact low-pressure forming, hand lay up is the first invention in the production of polymer matrix composite materials, which has the widest application range. Other methods are the development and improvement of hand lay up. The biggest advantage of contact forming process is simple equipment, wide adaptability, less investment and quick effect. According to the statistics in recent years, the contact low-pressure forming process still accounts for a large proportion in the composite industry production of all countries in the world, such as the United States accounts for 35%, Western Europe accounts for 25%, Japan accounts for 42%, and China accounts for 75%. This shows that the contact low-pressure forming process is very important and irreplaceable in the industrial production of composite materials. It is a process method that will never decline. But its biggest disadvantage is low production efficiency, high labor intensity, poor product repeatability and so on.
1. Raw materials
The raw materials of contact low pressure forming include reinforcement materials, resins and auxiliary materials.
(1) Reinforcement material
The requirements of contact forming for reinforcing materials: ① reinforcing materials are easy to be soaked by resin; ② there is enough deformation to meet the forming requirements of complex shapes of products; ③ bubbles are easy to be deducted; ④ the physical and chemical properties of products can be met; ⑤ the price is reasonable (as cheap as possible) and the source is rich.
The reinforcing materials used for contact molding include glass fiber and its fabric, carbon fiber and its fabric, aramid fiber and its fabric, etc.
(2) Base material
Contact with low pressure molding process for matrix materials: (1) under the condition of hand paste, it is easy to soak fiber reinforced materials, easy to exclude bubbles, and strong adhesion with fibers; second, it can gel and solidify at room temperature, and require less shrinkage and less volatilization; third, the viscosity is suitable: generally 0.2 to 0.5Pa s. Card.
The commonly used resins in production are: unsaturated polyester resin, epoxy resin, phenolic resin, bismaleimide resin, polyimide resin, etc.
Performance requirements of several contact molding processes for resin:
Requirements of molding method for resin performance
Gel coat making
1. No flowing and easy defoaming during forming
2. Uniform color, no floating
3. Fast curing, no wrinkle, good adhesion with the laying resin
Hand lay-up
1. Good impregnation, easy to soak fiber, easy to remove bubbles
2. Fast curing, less heat release and small shrinkage after laying
3. Less volatile matter, no stickiness on the surface of products
4. Good adhesion between layers
Spray forming
1. Ensure the requirements of hand lay up
2. Thixotropic recovery earlier
3. Temperature has little effect on resin viscosity
4. The application period of resin should be long, and the viscosity should not be increased after adding accelerant
Bag molding
1. Good wettability, easy to soak fiber, easy to discharge bubbles
2. Fast curing and low heat release
3. Not easy to flow, strong adhesion between layers
(3) auxiliary materials
Auxiliary materials in contact molding process mainly refer to filler and pigment, while curing agent, diluent and toughening agent belong to resin matrix system.
2. Mold and release agent
(1) Mold
Mold is the main equipment in all kinds of contact molding process. The quality of the mould 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 taken into consideration: ① meeting the accuracy requirements of product design, accurate mold size and smooth surface; ② sufficient strength and rigidity; ③ convenient demoulding; ④ sufficient thermal stability; ⑤ light weight, sufficient material source and low cost.
Mold structure contact molding mold is divided into three types: female mold, male mold and opposite mold. No matter which mold, it can be designed as a whole or assembled mold according to the size, molding requirements.
The mold material shall meet the following requirements when reversing the mold material:
① It can meet the requirements of dimensional accuracy, appearance quality and service life of products;
② The die material should have enough strength and rigidity to ensure that the die is not easy to deform and damage in use;
③ It is not eroded by resin and does not affect resin curing;
④ Good heat resistance, the mold will not deform when the product is solidified or heated;
⑤ Easy to manufacture, easy to demould;
⑥ Reduce the weight of mould in the day, convenient for production;
⑦ The price is cheap and the material is easy to get. The materials used for hand molding are wood, metal, gypsum, cement, low melting point metals, rigid foam plastics and FRP.
Basic requirements for release agent:
1. It does not corrode the mold and affect the resin curing, and the adhesive force to the resin is less than 0.01Mpa;
② Film forming time is short, thickness is uniform and surface is smooth;
③ It is safe to use and has no toxic effect;
④ It is heat-resistant and can be cured by heating;
⑤ Easy to operate, low price.
The release agents of contact molding process mainly include film release agent, liquid release agent, oil paste and wax release agent.
Hand Lay-up Molding
The technological process of hand lay up is as follows:
(1) Production preparation
The size of the site for hand lay up shall be determined according to the size of the products and the daily output. The site shall be clean, dry and well ventilated. The air temperature shall be kept between 15-35 ℃. The post processing and renovation section shall be equipped with air suction and dust removal and water spray devices.
Mold preparation includes cleaning, assembly and application of release agent.
Two problems should be paid attention to when preparing the resin glue. First, prevent bubbles from mixing in the glue. Second, the amount of dispensing should not be excessive.
Reinforcement material preparation the type and specification of reinforcement material shall be selected according to the design requirements.
(2) Paste and cure
There are two ways to lay the layer: wet method and dry method: ① dry method lay the layer with prepreg as the raw material, first cut the good prepreg (cloth) according to the template into the bad material, when laying the layer, heat and soften it, and then lay it on the mold layer by layer, and pay attention to eliminate the air bubbles between layers to make it compact. This method is mainly used for autoclave and bag forming. ② The wet lay layer directly leaches the reinforcement material on the mold, and clings to the mold layer by layer, and deducts the bubbles to make it dense. This method is commonly used in hand lay up process. The wet laid layer can be divided into gel coat layer paste system and structure layer paste system.
Hand pasting tools hand pasting tools have a great influence on the quality of products. There are wool roller, bristle roller, spiral roller, electric saw, electric drill, polishing machine, etc.
Curing products are divided into two stages: curing and aging. From gel to triangulation, 24h is generally required. At this time, the curing degree is 50% to 70% (the hardness of the bar is 15). It can be released from the mold, and then solidified for 1~2 weeks in natural environment. The curing strength of the product is more than 85% after curing for 1~2 weeks. Heating can promote the curing process. For polyester FRP, heating at 80 ℃ for 3 hours, for epoxy FRP, the post curing temperature can be controlled within 150 ℃. There are many methods of heating and curing, small and medium-sized products can be heated and solidified in the curing furnace, large products can be heated in the mold or infrared.
(3) Demoulding and finishing
The demoulding and demoulding shall ensure that the products are not damaged. The demoulding methods are as follows: ① the ejection device is embedded in the mould for ejection demoulding, when demoulding, the screw is rotated to eject the product. ② There is a compressed air or water inlet on the pressure stripping mold. When stripping, the compressed air or water (0.2MPa) is pressed between the mold and the product, and the product and the mold are separated by knocking with a wood hammer and a rubber hammer. ③ Large products (such as ships) can be demoulded by means of jacks, cranes, hardwood wedges and other tools. ④ For complex products, the manual demoulding method can be used to paste two or three layers of glass fiber reinforced plastics on the mold first, and then peel them off the mold after curing, and then put them on the mold to continue to paste until the design thickness, and it is easy to take them off the mold after curing.
There are two kinds of trimming: one is dimension trimming, the other is defect repairing. ① For the finished products, the excess parts shall be cut off according to the design size; ② defect repair includes perforation repair, bubble repair, crack repair, hole breaking and reinforcement, etc.
Spray forming technology
Spray forming technology is the improvement of hand lay up, semi mechanization degree. Spray molding technology accounts for a large proportion in composite molding process, such as 9.1% in the United States and 11.5% in Western Europe