The melt viscosity of LLDPE is high, and its fluidity is poor. The extrusion cooling process is prone to film shrinkage, and there are many crystal points in the film during processing. These issues directly affect the quality of the product. After analysis and testing, the following methods can be adopted to solve them.
(1) Film shrinkage phenomenon.
① Check if the temperature of the rapid cooling shaping roller is appropriate. If the temperature is too low, it is easy to exacerbate shrinkage, and if it is too high, the cooling effect is poor.
② Check the position, direction, and even outlet of the air knife.
③ Increase the pressure and tension of the flattening roller to make the film adhere tightly to the quenching roller.
(2) Thin film crystal dots.
① Low or high temperature of the material barrel is not conducive. At low temperatures, the resin cannot be completely plasticized; Excessive temperature, low melt pressure, and low shear force are detrimental to the plasticization of LLDPE resin. Therefore, the appropriate barrel processing temperature is determined based on different resin grades.
② It is possible to increase the number of layers and mesh of the mesh appropriately to solve the problem of low shear force and poor visualization caused by low melt pressure
③ The screw speed is low and the shear force is insufficient. For shear sensitive LDPE resin, it is prone to poor plasticization, and the screw speed should be appropriately increased.
(3) Processing technology and film self-adhesion. The size of the self-adhesiveness of the film directly affects the performance of the product. The problem of poor self-adhesiveness in production can be solved through the following methods.
① In the flat film co extrusion process, the self-adhesiveness and flexibility of LLDPE winding film do not rely on the addition of viscosity enhancers, but on the rapid cooling of the shaping roller to produce self-adhesiveness of the film, so the quenching temperature is extremely critical.
② The thickness of the film also has a certain impact on its viscosity, such as excessive thickness, insufficient self-adhesion, and waste; The thickness is too small, and the film has poor strength and reduced puncture resistance, making it difficult to wrap and package. Therefore, it is advisable to control the film thickness between 18pm and 22pm.
③ Increasing the distance between the mold head and the cooling roller can also increase the viscosity of the film.
④ In order to ensure the viscosity and tightness of the film, adjust the contact pressure of the winding and the characteristic tension ratio of the winding and coiling traction appropriately.
