Study on the production process of the hottest bia

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Research on the production process of biaxially oriented polypropylene film (Part 2)

3 common problems and solutions in BOPP film production

the main goal to be achieved in the production of BOPP film is to achieve continuous production at the highest possible speed, to improve the performance of BOPP film, to ensure the quality, and to reduce energy consumption. However, in the actual production process, due to various reasons, BOPP film has a variety of problems, making the production goal difficult to achieve. In view of the common problems in production, the author puts forward the solutions in combination with the introduction of the 8.2mbopp film production line of French DMT company

3.1 common defects of cast sheet and solutions

3.1.1 transverse stripes

(1) the causes of large spacing transverse stripes mainly include unstable extrusion melt pressure, uneven speed or temperature of quench roll, and excessive fluctuation of air flow of air knife. Point 1 is common. There are many factors causing pressure instability. On the one hand, the production line speed is increased too fast, resulting in the rapid increase of metering pump speed. On the other hand, the screw speed of the main extruder is increased relatively slowly, resulting in insufficient discharge of die head and unstable pressure [9]. In such cases, it is better to appropriately extend the acceleration time. After the linear speed is stable, the transverse stripes will naturally disappear. Another common situation is the raw material factor. When the process conditions are well controlled and there is no obvious improvement after many adjustments, it is necessary to consider replacing raw materials [2]

(2) small spacing transverse stripe small spacing transverse stripe is not common in the actual production process. There are four reasons: inappropriate angle of the machine head, inappropriate angle of the air knife or air volume, influence of air flow near the machine head, and unstable speed of the quench roll. These four aspects can be solved

3.1.2 longitudinal stripes

during the casting process, continuous longitudinal stripes are sometimes seen at the local and fixed positions of the extruded cast sheet. If BOPP film is produced with this cast sheet, the transverse thickness of the film will be uneven; There are obvious protrusions (ribs) or longitudinal stripes on the appearance of the rolled and slitted films. The measures usually taken to eliminate the longitudinal stripes are as follows: ① select the die head with reasonable structure and good quality to ensure that the lip is clean and free of any mechanical damage. ② Strengthen melt filtration. ③ Timely remove the sundries on the lip and do a good job in the maintenance of the machine head. ④ Improve the uniformity of air blowing of air knife. ⑤ Reasonably control the temperature of each extrusion section. ⑥ Adjust the position of the head relative to the quench roll

3.1.3 warping on both sides

this phenomenon is mainly caused by poor attachment effect and excessive temperature difference on both sides during casting. The warping of the cast sheet will affect the flatness of the film. For PP, the uneven cooling and crystallization of the cast sheet will directly affect the film-forming property of the film. The edge of the casting sheet is usually warped to the side with lower temperature, so the temperature of the cooling water on the quench roll surface and in the water tank can be judged according to the warpage of the casting sheet during the production process, and then the solution can be considered.

3.1.4 bubbles appear.

if impurities are contained in the melt, the moisture content of the raw material is too high, the extrusion temperature is too high, the heating time of the material is too long, or there is air or degradation in the extruder and filter, Bubbles may appear in the cast sheet. If bubbles appear in the normal production process, carefully observe the bubble shape, color, etc., analyze the causes and solve them. If the air screw is started to extrude or a new melt filter is replaced, there may be air or degradation products in the extruder or filter, and bubbles will generally appear in the cast sheet. This problem can usually be solved by fully discharging

3.1.5 irregular edges

the irregular edges of the cast sheet may be caused by the edge leakage caused by the damage of the seals at both ends of the die lip, the abnormal blank holder system, or the unstable extrusion melt pressure. After finding out the cause, it is necessary to use corresponding methods to solve it in time, otherwise it is easy to cause horizontal pulling off of the clamp

3.1.6 other defects

defects such as crystal point, coke, unmelted material, inappropriate crystallinity, poor gloss and shark skin may also occur in the casting process. These defects generally do not appear on the production line with mature process and high technical level

3.2 causes and solutions of tensile film breaking

during the production process, the film may be broken in the whole process of materials from casting to winder, accessories, sheet metal, cards, sensors, etc. Generally, according to the location of membrane rupture, the membrane rupture can be divided into three types: before transverse tension, after transverse tension and after transverse tension

3.2.1 film breaking before transverse stretching

when the production conditions have changed significantly, the longitudinal thickness of the film has changed greatly, or the cast sheet has great defects, the local tensile stress of the cast sheet in the stretching process exceeds the allowable stress of the material, resulting in transverse film breaking. However, membrane rupture before transverse tension is rare in normal production process

3.2.2 transverse pull rupture

transverse pull rupture is the most common in the production process. The film is most likely to break when it is stretched laterally at high speed. Generally, the transverse tensile rupture is divided into three types: transverse rupture, longitudinal rupture and unclamp rupture. (1) There are many reasons for transverse film breaking, which can be divided into: ① raw materials contain impurities with large performance differences (low molecular weight substances, oil stains, etc.). ② There are obvious transverse stripes and bubbles on the cast sheet. ③ Various non obvious film breaking factors (longitudinal thickness fluctuation, etc.) before transverse tension are further expanded, resulting in excessive strain in local area. ④ The deviation of the crystallization and orientation of the cast sheet is too large. ⑤ The filter is damaged and the impurity content of the cast sheet is high. ⑥ Head leakage. ⑦ The roll surface is crushed. ⑧ Scraps and equipment scratch the film. ⑨ Improper setting of extrusion and longitudinal stretching temperature. ⑩ Various volatiles accumulated on the top of the oven and the air duct fall onto the film. In addition, chain clamp damage is also one of the important reasons

(2) if there is longitudinal film breaking, it can be analyzed from the following aspects: ① the lateral thickness deviation of the film is too large. ② The longitudinal and transverse stretching ratio is too large. ③ The edge temperature is too high during longitudinal stretching. ④ The longitudinal stretching setting temperature is too high, and the crystal orientation of the cast piece is not good. Today, I will tell you about the use steps of the metallographic microscope and the calibration steps of the equipment. ⑤ The chain clamp temperature is too high. ⑥ There are scraps scratching the film in the horizontal pulling oven. (3) It is mainly analyzed from three aspects: diaphragm, clamp and process. First, if the edge of the cast sheet is bad or the thickness deviation is large, it is easy to cause the clamping off. At this time, the casting process should be adjusted in time to eliminate the defects of the casting. Secondly, if the clamp is released during normal production and still released after manual reset, the equipment reasons should be considered. It may be that the chain clamp is damaged and cannot be closed, or there may be waste film hanging on the chain clamp, or the inlet edge guide may fail. In this case, stop the machine immediately and check carefully. In addition, when the film is stretched horizontally, the preheating, the stretching temperature is too low, and the inlet tension is not appropriate, etc. will also cause unclamp

3.2.3 film breaking after transverse tension (traction and winding film breaking) the film breaking after transverse tension is mainly caused by equipment failure or improper operation. It can be summarized as follows: ① excessive traction and winding tension. ② The distance between corona electrode and corona roll is too small, and the film is scratched. ③ The edge cutting knife is not sharp, resulting in uneven edges of the film. ④ The edge is not sucked in time. ⑤ Film wrapping roll. ⑥ Improper Throwing Knife, unable to change roll normally, etc

3.2.4 basic principles for preventing membrane rupture

(1) select and use raw materials in strict accordance with requirements and regulations. (2) Regularly overhaul the equipment to ensure that the equipment is in normal working condition

(3) formulate reasonable production process

(4) improve the technical level and heart of the staff

(5) find out the causes of membrane rupture in time and formulate reasonable solutions

4 conclusion

bopp film production technology has entered China for only a few decades. However, due to the active introduction of advanced production technology and equipment from the world's industrial developed countries and the efforts of Chinese professional technicians, China's BOPP film production has occupied an important position in the world's BOPP film production. At present, the BOPP film production process has become more and more mature, and the BOPP film market has remained relatively stable. Therefore, it has become a topic of common concern for all BOPP film manufacturers to timely solve problems in production and strive to improve product grade and quality

information source: Engineering Plastics Application

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