And then stretched vertically and horizontally. Due to the orientation of stretching molecules, such films have good physical stability, mechanical strength, air tightness, high transparency, luster, toughness and wear resistance. It is the most widely used printing film at present, the thickness is generally 20 ~ 40 microns, and the most widely used is 20 microns. The main disadvantage of biaxially oriented polypropylene is its poor heat sealability, so it is generally used as the outer film of composite film. For example, after being compounded with polyethylene film, its moisture resistance, transparency, strength, stiffness and printability are ideal, and it is suitable for loading dry food. Due to the nonpolar surface of biaxially oriented polypropylene, high crystallinity, low surface free energy, poor printing performance and poor adhesion to ink and adhesive, surface treatment is needed before printing and compounding.
Lateral movement
(1) The transverse stripes with large spacing are mainly caused by three points: unstable extrusion melt pressure, uneven speed or temperature of quenching roller, and excessive fluctuation of air volume of air knife. Among them, the point 1 is more common. There are many factors that cause pressure instability. The most important thing is that the linear speed of production line is too fast, which leads to the rapid increase of metering pump speed. On the other hand, the screw speed of the main extruder is relatively slow, which leads to insufficient die ejection and unstable pressure. In this case, it is best to extend the speed-up time appropriately, and the horizontal stripes will disappear naturally after the constant linear speed is stabilized. There is also a more common situation, which 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) Small-spacing horizontal stripes Small-spacing horizontal stripes are not common in the actual production process. There are four reasons: the head angle is not suitable, the air knife angle or air volume is not suitable, the air flow near the head is affected, and the quenching roller speed is unstable. It can be solved from these four aspects.
Longitudinal stripe
In the casting process, it is sometimes seen that there are continuous longitudinal stripes in the local and fixed positions of extruded castings. If BOPP film is produced by using this casting sheet, the transverse thickness of the film will be uneven; The appearance of rolled and cut films has obvious protrusions (ribs) or longitudinal stripes. The common measures to eliminate longitudinal stripes are as follows: ① Choose a mold with reasonable structure and good quality to ensure the smooth lip of the mold without any mechanical damage. ② Strengthen melt filtration. (3) Clean up the debris on the lips in time and maintain the nose. ④ Improve the uniformity of air knife injection. ⑤ Reasonable control of extrusion temperature. ⑥ Adjust the position of the head relative to the quenching roller.
Warp on both sides
This phenomenon is mainly caused by poor adhesion effect and excessive temperature difference between the two sides during pouring. The warpage of the casting will affect the flatness of the film. As far as PP is concerned, the non-uniformity of casting cooling crystallization will directly affect the film-forming performance. The edge of the casting sheet usually warps to the lower temperature side, so the temperature between the surface of the quenching roller and the cooling water in the water tank can be judged according to the warpage of the casting sheet in the production process, and then the solution can be considered.
Bubble appearance
If impurities are entrained in the melt, the water content of raw materials is too high, the extrusion temperature is too high, the heating time of materials is too long, or air or degradation products accumulate in the extruder and filter, bubbles may appear in the castings. If bubbles appear in the normal production process, carefully observe the shape and color of bubbles, analyze the reasons and solve them. If the empty screw is opened for extrusion or a new melt filter is opened again, there may be air or degradation products in the extruder or filter, and bubbles will generally appear in the casting. This problem can usually be solved by proper layout.
Irregular edge
The irregular edge of the casting sheet may be caused by the seal damage at both ends of the die lip, or the abnormal blank holder system, or the edge leakage caused by the unstable extrusion melt pressure. After finding out the reason, we should use the corresponding methods to solve it in time, otherwise it will easily lead to cross-pull clamping.
Other defects
Crystallizing point, paste, unmelted material, improper crystallinity, poor gloss, shark skin and other defects may appear in the casting process. These defects generally do not appear in production lines with mature technology and high technical level.
Causes and solutions of tensile film fracture
In the production process, film breakage may occur in the whole process from casting to winding. Usually, according to the position of rupture, rupture can be divided into three types: rupture before transverse pulling and rupture after transverse pulling.
Membrane rupture before transverse stretching.
When the production conditions change obviously during casting or longitudinal stretching, the longitudinal thickness of the film changes greatly, or the casting has great defects, the local tensile stress of the casting exceeds the allowable bearing stress of the material during stretching, resulting in transverse film breakage. However, in the normal production process, membrane rupture before transverse stretching is rare.
Transverse rupture of membrane
The transverse rupture of film is the most common in the production process. The film is most prone to rupture when stretched transversely at high speed. Generally speaking, the transverse rupture of fetal membranes can be divided into three types: transverse rupture, longitudinal rupture and peeling rupture. (1) transverse membrane rupture There are many reasons for transverse membrane rupture, which can be specifically divided into: ① raw materials contain impurities (low molecular weight, oil pollution, etc.). ) The performance varies greatly. ② There are obvious horizontal stripes and bubbles on the casting. (3) Before transverse stretching, all kinds of inconspicuous film breaking factors are further expanded (longitudinal thickness fluctuation, etc.). ), making the local strain too large. ④ The deviation of crystallization and orientation of the cast sheet is too large. ⑤ The filter is damaged, and the impurity content of the casting is high. ⑥ rhinorrhea. ⑦ The roll surface is broken. (8) Scrap materials and equipment scratch the film. Pet-name ruby extrusion, longitudinal stretching temperature is set improperly. Attending all kinds of volatiles gather at the top of the oven and on the air duct and fall on the film. In addition, the damage of chain clamp is also one of the important reasons.
(2) Longitudinal membrane rupture If longitudinal membrane rupture occurs, it can be analyzed from the following aspects: ① The transverse thickness deviation of the membrane is too large. ② The ratio of longitudinal and transverse stretching is too large. ③ The edge temperature is too high during longitudinal stretching. ④ When the set temperature of longitudinal stretching is too high, the crystal orientation of the cast sheet is not good. ⑤ The chain clamp temperature is too high. ⑥ Scrap in transverse stretching oven scratches the film. (3) Stripping, film breaking and stripping are mainly analyzed from three aspects: diaphragm, fixture and process. First, if the casting edge is not good or the thickness deviation is large, it is easy to cause peeling. At this time, the casting process should be adjusted in time to eliminate the defects of castings. Secondly, if the fixture is loosened in normal production and still loosened after manual reset, the reason of the equipment should be considered at this time. It may be that the chain clamp is damaged and cannot be closed, or there may be a waste film hanging on the chain clamp, or the guide rail at the entrance edge is faulty. In this case, stop the machine immediately and check it carefully. In addition, pre-heating, too low stretching temperature and inappropriate inlet tension during transverse stretching of the film will also lead to delamination.
Membrane rupture after transverse pulling (traction and winding) is mainly caused by equipment failure or improper operation. It can be summarized as follows: ① The tension of traction and winding is too large. ② The distance between corona electrode and corona roller is too small, which scratches the film. ③ The edge trimmer is not sharp, resulting in uneven film edge. (4) Edge suction is not timely. ⑤ Film wrapping roller. ⑥ The flying knife is not suitable, and the volume cannot be changed normally.
Basic principles of preventing membrane rupture
(1) Select and use raw materials in strict accordance with requirements and regulations. (2) Maintain the equipment regularly to ensure that the equipment is in normal working condition.
(3) Formulate a reasonable production process.
(4) Improve the technical level and sense of responsibility of employees.
(5) Find out the reason of membrane rupture in time and make a reasonable solution.