The blown film unit is mainly composed of an extruder, head, air ring cooling, traction, roll, and so on. A set of high-quality blowing film machines needs the whole process from extrusion to rolling. If any of the links between the design principle and manufacturing quality problems, the quality of the products produced will be greatly reduced, that is, affect the use and sales, this problem should be paid attention to.
1. Status quo of multilayer co-extruded films
In the global plastic packaging film market, the proportion of multilayer co-extrusion film is increasing. At present, film consumption in Europe, the United States, Japan, and other places increases by more than 10% % per year, the consumption in China increases by 30% % per year, nylon only accounts for the multi-layer coextruded high barrier packaging film composition of 10% % ~ 15%, it can be seen that the packaging film in daily life in the use of more and more, but also more and more popular, It can make a variety of food shelf life extended more than two times, at present and in the next ten years, the production of high-grade plastic packaging film fixed industry has a huge potential for development.
The technical level of film blowing machine affects product quality
As can be seen from the current situation of blowing film machinery in China, low technical content is the biggest problem. The technical level of the film blower directly affects the product quality. This is why many powerful manufacturers cannot buy advanced equipment from abroad, but high equipment prices raise production costs and affect market competitiveness. According to technical difficulty and importance, some developed countries in Europe and the United States put nose and roll technology first. In addition, they also strive to improve the blowing film technology, and constantly improve the quality of products.
However, there are big differences in national conditions between China and other countries. There are too many improvement projects, and the production cost will increase a lot, but the product quality will not improve. We should make necessary improvements according to China’s national conditions, and ensure that the cost is gradually reduced to expand the market.
2. Characteristics of multilayer co-extrusion blowing film technology
Many technologies in the multi-layer co-extruding unit have got good applications and development in our country, such as extruding technology. According to the current use conditions in our country, it has completely satisfied the requirements. It has been heavily invested and has achieved satisfactory results. In the industry, people call it the main engine, while other parts are called auxiliary machines. It is the improvement of this auxiliary machinery technology that greatly improves the quality of products.
(1) multi-channel plane superposition head
At present, the Chinese industry is aware of the variety of multi-layer co-extrusion heads, domestic literature is also a lot. However, due to geographical restrictions, the content is not detailed and accurate. This paper introduces a multi-channel plane superposition machine head.
1) Process characteristics of multi-channel plane superposition head
Five-layer coextruded double-flow plane superposition machine head, composed of disc superposition. Each layer of the disc has two feed ports, which can extrude two layers of film, so that each layer heats evenly, effectively improving the plasticizing performance. Therefore, the film blown by the five-layer co-extruded superposition machine head can reach ten layers, and the thickness error of the product is within 5 layers %. There is a heat insulation layer between plates, which can separately control the temperature of each layer of the head, and the temperature difference between adjacent layers can reach 80℃.
According to different production requirements. Each layer of the head can be removed, increased or rearranged, increase or decrease convenience, and cost-saving. The whole head is made of 38CrMoAIA material, which has good thermal stability. After a special heat treatment process and internal electroplating polishing, the strength of the film is guaranteed, the raw materials are saved, the plasticizing performance is improved and the product quality is improved. The packing film with the same thickness is 30 % higher than the similar composite film. Under the same strength condition, the multilayer co-extrusion film saves more than 20 kinds of raw materials.
2) Advantages of multi-channel plane superposition head
Compared with the conical superposition head, under the same conditions, the structure of the plane superposition head is compact, its actual height is only 60% of the conical superposition head, in the seven and nine layers of co-extruded film blowing unit, the effective height of the tower is improved, this head has absolute advantages;
Industry insiders know that in the process of multi-layer extrusion blowing film, LDPE and PA plasticization temperature difference is very big, can only be the independent heating head, conical superposition head surface can be independent heating, but some careful analysis is not appropriate, through the profile, the cylinder is relatively independent, but the conical part is inserted into another layer of the cylinder, theoretically affected by another layer of heating temperature, in addition, In the long production process, the heat conduction of the metal will interfere with the temperature of each cone layer and tend to be uniform. To ensure the temperature difference between two adjacent materials, insulation must be added. This design philosophy is evident in the flat overlay nose.
Because the flat superposition machine head adopts side feed, the internal flow channel of the machine head is less inflection point, no dead point, and stop and start frequently will never have a paste, especially when blowing nylon, its advantages are unmatched. This design theory of the nose has gone beyond traditional ideas and is a theoretical leap.
(2) Automatic winding machine
Nominally, there are two kinds of winding machines, one surface friction winding machine, the other is a turning center winding machine; For different varieties, there are surface friction coiler, gap surface friction coiler, center coiler, gap center transposition coiler, etc.
1) Technical points of the automatic winding machine
This kind of winding machine is the most widely used in the market, its technical key points are tension control and film flattening. The flattening technique is extremely important in winding. A reasonable flattening mechanism can even make up for some defects in the unit. It should be noted that the range of functions of the flattening mechanism is limited. In addition, the film will return to its pre-flattening state.
Therefore, the idea of having a flattening device between upper and lower traction is wrong. Not only does it increase the cost, but it also damages the film and ultimately doesn’t work. At present, the flattening mechanism of the domestic winder is unreasonable without exception. The technical point of the flattening mechanism is the Angle, position, and curvature of the film, which cannot be wrong. However, the folding diameter of the film has a great influence on the flattening effect. The smaller the folding diameter, the better the flattening effect. The flattening mechanism can be removed when the thickness and number of layers of the film are increased and wiped or folded.
2) Tension control of the automatic winding machine
The tension control requirements are related to the thickness, number of layers, and diameter of the film. Due to the improvement of mechanization level, the diameter of the full roll in European and American countries is generally 1200mm. As tension control theory improved, there were several rolls with a diameter of 1500 mm.
When the total coil diameter of the single film is less than 1000mm, constant tension control is generally adopted. If the diameter of a single or multi-layer crimp is too large, curve tension control should be adopted. The tension size varies with the diameter. Tension control mechanisms should be more sensitive and vice versa. When the thickness and number of layers of the film are increased and wiped or folded, the central auxiliary crimping mechanism of the surface friction crimping machine must be increased.