Automobiles are the main flexible means of transportation on land, and the automobile industry is one of the most important basic industries in the national economy. With the mass production of automobiles, problems such as how to save energy and fuel, improve functions, save materials, simplify production procedures, and reduce costs have arisen.
1. The plasticization of automobiles is the general trend
It is based on three main reasons, one is to save energy, the other is to improve functions, and the third is to simplify manufacturing processes and processes.
Automobile fuel tank is one of the important functional and safety parts in automobile parts. The traditional fuel tank is made of metal. Due to the particularity of metal processing, it is difficult to form, and the strength of the welding seam is also low, the production qualification rate is low, and there are often problems in use. In recent years, in order to reduce the weight and cost of automobiles, the conversion from metal materials to plastic resins has attracted widespread interest from researchers.
2, the characteristics of plastic fuel tanks
Plastic fuel tanks can better solve the problems of metal fuel tanks because: the plastic has good molding processability, easy mass production, simplified production and manufacturing processes, and improved safe working conditions.
The weight of the plastic fuel tank is lighter than that of metal, and the relative density of plastic is only 1/8~1/7 of that of metal. Therefore, compared with the metal fuel tank of the same volume, its weight can be greatly reduced, which is beneficial to reduce the weight of the car and increase the speed. , Save fuel. According to statistics, for every 1kg reduction in vehicle weight, 1L gasoline can make the car travel 0.1km longer. The plastic fuel tank has a large degree of freedom in shape design, high space utilization, and can be processed into various complex shapes, which is conducive to making full use of the space of the car body. , Which can increase the load capacity of the fuel and improve the endurance of the car. For example, the PASSAT plastic fuel tank weighs 3.5kg and has a capacity of 51L plus a safety factor of 7 L. Compared with metal fuel tanks, the capacity is 6L and the weight is lighter. Plastic fuel tanks have better performance. Thermal insulation, gasoline and diesel will not heat up quickly when the vehicle is on fire, which can delay the explosion and increase the occupants’ hope of survival in an accident.
In Europe, plastic fuel tanks were first successfully developed and mass-produced by the German Volkswagen Automotive Industry Corporation in 1973. The car types started from PASSAT. After that, plastic fuel tanks developed from raw materials to processing and production equipment in countries with developed automobile industries. Very quickly, it became a specialized processing industry.
3, the current automotive plastic fuel tank molding process
At present, the use of automotive plastic fuel tanks has received widespread attention, and its processing and molding processes have been extensively researched and developed. In summary, the molding process of plastic fuel tanks is as follows.
The light metal mold for rotary molding can be installed on the frame of the rotary molding machine for three-dimensional rotation. The plastic powder is added to the hot mold. When it is rotated, the plastic powder is continuously melted and pasted on the inner wall of the hot mold. After it is completely plasticized to the required thickness , Inject cold water into the mold jacket for cooling, and then demold the product.
The disadvantage of this method is that it is difficult to ensure the uniformity of wall thickness at corners and narrow sections. The materials required by this method require that a tight and uniform melt is formed on the inner wall of the mold after being heated. However, the materials that meet the above conditions cannot meet the performance requirements of automobile fuel tanks. Cationic polymerization (monomer casting) cationic polymerization method is to inject caprolactam monomer into a heated rotary mold, cation polymerization, cooling demoulding. The advantage is that the mold is low in cost and easy to paint. Because this method is limited to cast nylon and cannot fully meet the requirements of automobile fuel tanks, this method is usually not used. Injection molding is limited due to demolding. The fuel tank produced by injection molding needs to be divided into two halves, and then the two halves are bonded into a whole by adhesive or hot melt welding. Adhesive strength tends to vary depending on the type of material, and the injection mold has to withstand high pressure (60~130MPa) injection, the mold structure is complex, and the manufacturing cost is expensive. The above are disadvantages. The advantage of injection molding is that the wall thickness of the finished product is easy to control, very uniform, and the required inserts can be assembled in the injection mold, and the fuel tank body and the accessory parts injection assembly can be fused into one. Vacuum blister molding heats the plastic sheet by vacuum blister molding to make two fuel tank halves, and then use adhesive or hot melt welding to bond the two halves into a whole. It is different from injection molding: the former cannot be made into a box with a complicated shape and structure and cannot be assembled with various inserts during molding. In addition, the molds are mostly aluminum alloy materials, with relatively low strength requirements, simple structure, and low cost. The disadvantage is that there are also adhesion problems.
The molding process of various plastic fuels, hollow blow molding is the best molding method for manufacturing fuel tanks. At present, plastic fuel tanks mainly adopt this method. When the fuel tank is blow-molded, the material is continuously heated, melted, and extruded, sent to the accumulator for storage, and then extruded from top to bottom through the core and sleeve to form the shape, and the parison is clamped by two halves of the (Hough) mold Then, the parison is blown and inflated to form firmly in the mold cavity, and the finished fuel tank is demoulded after cooling. This method is the best molding method, which can not only be mass-produced, but also simplifies the production process, and there is no adhesion problem.
4, currently commonly used barrier plastics, plastic fuel tank materials and processing technology
Not all plastics are suitable for the manufacture of automobile fuel tanks. First of all, the materials must be able to withstand the alternating load of high and low temperatures for a long time, and even can withstand sudden deformation at 40°C. After testing, they must meet the safety technical conditions and the standards promulgated by the government. It also has very good barrier properties. According to international standards, the current leakage of automobile fuel tanks does not exceed 2g per day and then decreases year by year. Therefore, barrier plastics must be used for automobile fuel tanks. Common barrier resin 47 are: polyvinylidene chloride (PVDC), ethylene-vinyl alcohol copolymer (EVOH), polyacrylonitrile (PAN), polyamide (PA), polyester (PET), new nylon (MXD6) , Liquid crystal OCP) and fluorine-containing plastics, among which liquid crystals and fluorine-containing plastics are not only costly but also difficult to process; although polyvinylidene chloride has high gas barrier properties, retort resistance, transparency, chemical resistance, heat It has the advantages of low shrinkage, safety and hygiene, and low cost, but its melting point (200C) is close to the decomposition temperature (210°C), forming and processing is very difficult, and the economic value is not large; although EVOH has better properties than PA and PA at normal temperature and dry state. PVDC has good barrier properties, but it does not have the physical properties of a single packaging material or container due to its insufficient properties. It must be compounded with other plastics. Compared with other barrier polymers, nylon has excellent low and high temperature performance and mechanical properties. Good, its barrier properties are better than those of EVOH and PVDC under high temperature and high humidity, so it is more and more widely used as a polymer barrier material. Considering barrier properties, processability and cost, through a series of experiments and attempts, high molecular weight, high density polyethylene is very suitable for manufacturing automobile fuel tanks, but its barrier properties are not good, and the oil leakage rate is much higher than nylon. Its advantages are: it does not lose its performance at low temperatures or even -40C, easy to blow molding, easy to friction welding, thermal element welding, high frequency welding, and hot air welding. Due to the low density, the plastic fuel tank is much lighter.于metal. However, the poor oil resistance of polyethylene limits it. At present, the commonly used polyethylene plastic fuel tanks mainly adopt the following production methods 8 10 to improve and improve the barrier properties of hollow containers: a single-layer plastic fuel tank with surface treatment on the inner wall.
Common methods:
①Epoxy spraying method, this method is relatively backward, the effect is also poor, and it has been basically eliminated;
②Sulphonation (2 gas) treatment method, which is relatively mature and is still used in the United States, Japan and other countries;
③ Fluorine gas treatment method, this method is to blow nitrogen containing 1% fluorine into the fuel tank at the same time during the blow molding process, so that the inner layer of the fuel tank forms a fluorine-containing layer that prevents fuel penetration. The fluorination treatment is carried out throughout the normal blow molding cycle. The inner surface of the container is chemically changed by replacing the hydrogen on the polymer chain with fluorine atoms, that is, in terms of polarity, cohesive energy, density and surface tension A change occurs, which reduces wetting, spreading, and ultimately the volatilization of non-polar liquids. Since this change only occurs on the surface (the depth is 5-10nm), there is no significant change in tensile and impact strength. After fluorination treatment, the reduction effect of the amount of penetrating gasoline in the fuel tank is more significant, which can be reduced from 16g/24h to 0.5g/24h. However, the last two of the above three methods will cause public hazards and cause environmental pollution. use.
(3) Another way is to use multi-layer co-extrusion technology, namely HDPE layer, bonding layer, barrier layer (PA or EVOH), bonding layer, HDPE layer 5-layer co-extrusion molding, where the barrier layer is Resins with barrier properties such as nylon or ethylene-vinyl alcohol copolymer, etc.; the adhesive used in the bonding layer has strong adhesion to the barrier material and HDPE, good bonding durability and processing performance; HDPE is used as an internal The layer and outer layer play the role of forming, strength, and skeleton.
The advantages of this method are: the quality of the finished product is excellent, especially the resistance to fuel oil penetration. However, this method requires high equipment, difficult process control, and requires a dedicated multilayer hollow blow molding machine. Now, plastic fuel tanks have been widely used in automobiles.
5, the current inspection of plastic fuel tanks
The raw materials are blow-molded into finished plastic fuel tanks, and the fuel tanks are tested according to the actual objects. The United States and Europe have established strict technical standards for automotive plastic fuel tanks. Currently commonly used test items are shown in Table 2. Table 1 Comparison of performance between domestic plastic fuel tank materials and similar foreign products Placed in a 35°C environment for 12h with a weight of 1t pendulum impact energy is 4k after impact, the box does not appear cracks and does not leak, the box is filled with refrigerant and placed in a 40°C environment for 12h with 14 7N pointed hammer impact energy It is 30. After the impact, there will be no cracks in the box body, no leakage. The box body is filled with 50% fuel. It is placed on direct and indirect flame for 120s. The box body will not break and explode. The box body is filled with refrigerant in a 40°C environment. Place it at room temperature for 12 hours and drop freely from a height of 10m. The box will not break and leak. Heat resistance test will be heated in an environment of 95°C for 1h. No leakage deformation, resistance to cold and heat alternating cycle test shall be 80°C (16h)-room temperature ( 1h)-40°C (6h)-room temperature (1h) is a cycle, after a total of 14 cycles, no leakage, deformation, aging resistance test, pressure test, vibration resistance test, air tightness test, penetration test, exposure to the atmosphere for one and a half years After that, the performance does not decrease significantly. The box is filled with liquid. Under the pressure of 0.03MPa and 53C, the box will not rupture, leak, normal temperature, vibration acceleration 28.4m/s2, vibration frequency 33.3Hz, vibration direction and time: up and down (4h), left and right (2h), before and after (2h), no rupture, no leakage, normal temperature, filled with a pressure of 0.03MPa (gauge pressure) for 30 seconds, and no leakage should occur. The pool has been exposed to a temperature of ten 40°C for 8 weeks, and the average leakage according to Western Europe and Japan standards is less than 20d; North America and the United States standard leakage is less than 2g/d
6, the current situation and prospects of plastic fuel tanks for automobiles
Plastic fuel tanks for automobiles began in Europe in the 1970s. The earliest countries developed were Germany and the United States. They were mostly used in cars, but their use in passenger cars and trucks continued to expand. Abroad, plastic fuel tanks have entered mass production. In recent years, due to the rapid development of my country’s automobiles, a number of vehicle and component manufacturing technologies have been introduced, which has greatly promoted the development of my country’s automobile industry and the advancement of car manufacturing technology, and has shortened the gap with the automobile industry of advanced countries in the world. As far as plastic fuel tanks are concerned, plastic fuel tanks have been gradually used instead of metal fuel tanks, and good achievements have been made in the work of localization. However, compared with the advanced level of foreign countries, there is still a big gap between our plastic fuel tanks for automobiles. In order to adapt to the competition in the international automobile market, vigorously developing and promoting the technology of plastic fuel tanks has become a topic that must be paid attention to and urgently to be solved in the development of my country’s automobile industry. With the rapid development of automobiles in the world and the prevention of air pollution, the U.S. Environmental Protection Agency has provided new strict hydrocarbon penetration limits, requiring that the emission of hydrocarbons be less than 0.2g/24h. For fuel tanks with high barrier properties, in view of the production situation of the fuel tanks, multi-layer co-extruded plastic fuel tanks are better.
Looking into the future, the application of plastic fuel tanks in automobiles has broad prospects. The widespread use of multi-layer composite plastic fuel tanks to replace metal fuel tanks and single-layer plastic fuel tanks will become the development trend of the world’s automobile industry in the 21st century.
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