Multi layer coextrusion hollow forming technology

2022-08-21
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Compared with metal fuel tank, plastic fuel tank has many advantages, such as light weight, large degree of freedom of shape, short development cycle, low cost, convenient for automobile design, manufacturing and modification, high safety and reliability, impact resistance, corrosion resistance, non explosion, etc

because the chemical molecular structure of gasoline is similar to that of HDPE, according to the principle of similarity and compatibility, its effective components will wet the surface of HDPE fuel tank, gradually diffuse to the inside of the container and penetrate into the outside world, resulting in gasification loss, which is the disadvantage of HDPE fuel tank. With the development of automotive industry technology and the continuous improvement of environmental protection and safety requirements, countries have higher and higher requirements for the impermeability of plastic fuel tanks. High impermeability has become the development trend of automotive plastic fuel tanks

there are three main methods to improve the impermeability of automotive plastic fuel tanks: fluorination, layered mixing and multilayer coextrusion. There are a series of environmental protection problems due to the recovery and treatment of fluorine gas in fluorination treatment. In the actual production operation, the mixing performance of extruder screw should be appropriate, which is very difficult to grasp; Moreover, the mechanical properties of layered mixed plastic fuel tank are low, and the impermeability is relatively poor and unstable. In comparison, the production of impermeable automotive plastic fuel tanks by multi-layer coextrusion has become the first choice. Judging from the molding equipment of automobile plastic fuel tank newly put into use in China in recent years, this can also be confirmed by the overwhelming majority of multi-layer co extrusion hollow machines

composition of multi-layer coextrusion hollow machine system

the multi-layer coextrusion hollow machine used for the molding of automobile multi-layer plastic fuel tank is different from the storage type hollow machine used for the molding of single-layer plastic fuel tank in terms of extrusion system and machine head. The typical structure of the head and parison of the multi-layer coextrusion hollow machine is shown in Figure 1. The parison has a total of six layers. From the outside, there are welding layer, recycled material layer, bonding layer, barrier layer, bonding layer and inner layer

from the beginning to the present, the basic composition of large hollow machines has not changed much, generally including: extruder, head, clamping device, blowing device, product taking out device, hydraulic station, electric control system. General external auxiliary equipment also includes: mixed feeding system, residual material crushing and recycling system, etc

large hollow machine can be divided into storage type and continuous extrusion type according to the way of extruding the parison, and can be divided into single-layer or multi-layer according to the structure of the parison. At present, the vast majority of Domestic 200 pairs of L all plastic industrial barrels are blown by a single-layer storage type hollow machine. The market has higher and higher requirements for the impermeability of automotive fuel tanks. Single layer plastic fuel tanks will be phased out and replaced by multi-layer plastic fuel tanks. Accordingly, the share of multi-layer hollow units in the molding equipment of plastic blow molding automobile fuel tank is increasing

typical structure of multi-layer coextrusion hollow head and parison

extrusion system

multi layer plastic fuel tank hollow machine generally has six extruders, plasticizing and extruding each layer respectively. The inner layer and welding layer are the main materials, which are the same as those used in single-layer fuel tank. The barrier layer is EVOH or pa6/66. Commonly used brands: EVOH is Eval f101a, soarnol dt2903 (Nippon gohsei); Pa6/66 is Ultramid C35 (BASF) and Durethan C38 (Bayer); The bonding layer is LDPE or HDPE grafted with maleic anhydride, and the commonly used brands are admer gt-5a (Mitsui), bynel4006 (Du Pont), etc; The recycled material layer uses the head of the parison and the crushed material of the waste products, including the above ingredients

Figure 2 the mixing element designed by Jew extruder screw head

the thickness ratio of each layer in the multi-layer plastic fuel tank is controlled by the extrusion system. The stable extrusion performance of each extruder is the basis for achieving this goal, so the extruder with IKv structure must be used for the inner layer, welding layer, recycled material layer and bonding layer, and the weight-loss feeding system is also widely used. The aspect that is easy to be ignored is the crushing state of recycled materials. If the crushing is uneven, the extrusion pressure of this layer will fluctuate greatly, which will affect the proportional relationship of each layer. Therefore, recycled materials are often granulated separately and then used

there are many raw material components in the recycled material layer. The extruder of this layer should have a good mixing capacity, so that EVOH or pa6/66 can be evenly distributed in HDPE in the state of particles less than 0.02mm. Various mixers are therefore used. Static mixer and new type of stretching mixer are placed between the extruder and the head. Jew company designed the mixing element as shown in Figure 2 at the screw head of the extruder

die

continuous extrusion multi-layer coextrusion die, the uniform extrusion of each layer of flow channel is particularly important. The flow passage design mostly adopts spiral flow passage design. The advantage of spiral flow passage is that no matter it is single-layer or multi-layer, there will be no obvious fusion joint area in the circumferential direction of each layer. However, the main problem is that with the increase of the number of layers, the design of spiral flow passage will become extremely difficult for hollow machines due to space constraints. Therefore, when there are many layers, the heart-shaped envelope flow passage design is also adopted, but the fusion joint areas of each layer need to be staggered

with the development of technology, the design of the head runner has also realized computer simulation. Large 2D, 3D and CAE software specially used for die head design has appeared. Among the six layer coextrusion head and 1000L storage head developed by Qinchuan group, flow2000, a large-scale computer design and analysis software of futuresoft company, is introduced. The software includes a variety of analyses from 2D flow, 3D flow, spiral flow channel, heart-shaped envelope flow channel, coextrusion flow and so on. It can simulate and display the temperature distribution, pressure distribution, shear stress distribution, flow rate distribution and residence time of the melt when flowing in the channel, Based on this, we can guide the design of runner and seek the best scheme. The use of large-scale analysis software not only solves many analysis and calculation that could not be carried out in the past, but also makes the design of the machine head based on reliable theory, improves the design quality of the large-scale machine head and reduces the design risk. It should be said that the emergence of such large-scale software is the most important technological progress in the field of head and die design and manufacturing

parison wall thickness control

parison wall thickness control of hollow machine head is a key technology. Parison wall thickness control is divided into axial control (AWDs) and radial control (PWDs). At present, large hollow machines generally have the function of axial parison control, and its control points are different from 24 points to 256 points. The function of axial wall thickness control is to make the injected parison obtain different thickness along the axis according to the different inflation ratio of the product, so as to basically ensure that the final product has a relatively uniform wall thickness distribution, It changes the blank thickness by changing the opening of the die head through the axial movement of the position of the core mold preset high-strength fastening bolt performance testing machine

although the axial wall thickness control can improve the wall thickness distribution of products, because the discharged parison is still in an equal thickness circle in the horizontal section, it is still not the best for products with large tensile requirements in a certain symmetrical direction, so the radial wall thickness control technology is produced. Radial control technology can make the extruded parison show the change of non-circular section in a required section (generally oval or rhombic circle). The combination of axial wall thickness control and radial wall thickness control can obtain the best parison and more ideal product wall thickness distribution

with the development of radial wall thickness control technology, there are basically two typical designs, one is called flexible ring type, and the other is lip repair type. The flexible ring type changes the thickness of the extruded parison by controlling the deformation of the thin-walled flexible ring in one or two symmetrical directions through the electrohydraulic servo who turns early. Its characteristic is that no matter what shape of products are blown, as long as the die diameter remains the same, the radial control can play a role. The main problem of this structure is that the fatigue life of the flexible ring is limited, it is easy to be damaged, and it is expensive to replace and make a new flexible ring parts. The change of parison wall thickness in the form of lip trimming is realized by the up and down movement of the trimming ring. Compared with the flexible ring structure, its biggest advantage is its long service life, and once it needs to be replaced, the factory with basic machining capacity can bear it. In some designs, the trimming part of the mouth ring is made into a movable block embedded, which facilitates replacement and reduces the cost of replacement. A British company has a patent for this technology

radial wall thickness control technology is undoubtedly an effective method to improve the quality of large hollow products. At the same time, it can ensure the quality and reduce the quality of products at the same time. However, the additional cost of adding a large radial wall thickness control device is also high. Taking the quotation of SIG company as an example, it is equipped with a set with two servo cylinders Φ 220mm diameter flexible ring radial controller, its quotation is 2e) result reproduction: the experimental results can be accessed at will for more than ten thousand German marks. The high cost limits the wide selection of radial wall thickness controllers, especially for large hollow machines. For hollow machines above 1000L, due to manufacturing difficulties, equipment manufacturers generally do not provide equipment options for radial wall thickness control devices

as an alternative, in order to make up for the lack of radial control devices, many users adopt a simple but effective method, namely core mold modification method. The specific method is to modify the specific position of the core mold on the machine head with the axial wall thickness control function, so that the radial wall thickness can be approximately obtained while the axial control is carried out, and the non-circular variation of the resulting increased weight can be offset by the composite material. Of course, this kind of modification requires some experience. The situation of the product should be analyzed in detail, so as to determine the location, orientation, amount and shape of the modification. Generally, strict symmetry should be paid attention to during the modification

large hollow machines can also be used in the production of a variety of products, such as outdoor hollow tables and chairs, removable hollow turnover boxes, highway intermediate isolation plates, etc. Some parts of the car can also be made by hollow blow molding, such as seats, ventilation pipes, spoilers, plastic fuel tank filler pipes, etc. The above products are made by the storage type hollow machine. If the impermeability of the fuel tank is required, the plastic fuel tank filler pipe will also be formed by multi-layer co extrusion hollow machine

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