Flexible calendering process for improving the quality of film sheets
For the molding of thermoplastics, the forming accuracy and the cooling setting process have a decisive influence on the quality of the final product, especially for the quality of the extruded film and sheet. Generally, the molding accuracy mainly affects the apparent quality of the film and the sheet, such as the flatness of the winding, the cleanliness of the end face, and the adaptability in use. The cooling and setting time, the cooling uniformity, and the cooling position determine the crystallization and molecular arrangement of the material to a large extent, the optical properties and mechanical properties of the film and sheet, and the subsequent thermoforming operation. Has a direct impact. Therefore, it is necessary to improve the molding precision and the cooling setting effect from the viewpoint of the process.
At present, a new production method is a flexible calendering process. The optical properties of the films and sheets produced by this process are comparable to those of the three-roll calendering process, and the physical and mechanical properties exceed the three-roll calendering and casting process. At the same time, the appearance quality of the product can achieve the effect of the casting process. Therefore, the flexible calendering process is very suitable for producing high quality diaphragms. Here, the characteristics of the three-roll calendering process, the casting process, and the flexible calendering process are compared using PP as a raw material for production.
For the three-roll calendering process, the basic structure and molding principle are basically the same whether it is a flat three-roller, a three-roller three-roller or a vertical three-roller. It is characterized in that the hot melt homogeneous melt which is plasticized and extruded is extruded through the slit of the die, calibrated and leveled between Roll1 and Roll2, and then cooled and formed in the Roll3 and the sixth zone. Since the roller surface of Roll1 and Roll2 is rigid, during the process of aligning and leveling the diaphragm, the circumferential dimension error of the roller surface is transferred onto the diaphragm and causes errors in the thickness direction, and this Errors are always present and do not change with changes in process conditions, levels of manipulation, material quality, etc. With the accumulation of finished product roll tolerances, the ultimate quality of the film is greatly reduced. In addition, the cooling of the diaphragm by the three-roll calendering process is carried out between the slits of the two rolls, and the cooling transfer of the rolls to the surface of the film allows the film to obtain good surface optical properties. However, in terms of the cooling conditions, since the diaphragm is double-sided at the same time in a small area of ​​the second region, it is rapidly cooled, and after entering the third region, it becomes single-sided cooling, and the other side of the cooling medium becomes The air causes uneven cooling, and the quality of the diaphragm is not perfect. The finished product may suffer from over-stretching, high heat shrinkage, poor flatness, and bending.
Casting process. As can be seen from the figure, the formation of the diaphragm is no longer calibrated and leveled by means of two rigid roller faces, so that the circumferential dimension error of the cooling roller is not transferred to the surface of the diaphragm. Here, the formation of the diaphragm error mainly comes from the lip gap error of the T die. It is well known that the lip of the casting die can be adjusted, so that despite the error of the diaphragm, this error can be improved by adjusting the die and using superior handling skills without causing tolerances. The accumulation, which is effective for improving the appearance quality of the diaphragm. From the viewpoint of the cooling and setting process of the diaphragm, the diaphragm is applied to the roller by a single blade on the side of the air knife, so that the roller only forms a good surface optical property on one side of the diaphragm, and the other side of the diaphragm The surface optical properties are very poor. Since the cooling of the second zone has been a single-sided roll cooling, the other side of the cooling medium is still air, which is basically similar to the conventional three-roll calender cooling process, so the film also exhibits high-strength stretching and high heat shrinkage. Rate and quality issues such as bending.
Flexible calendering process. In this process, the molding tolerance of the diaphragm does not depend on the two rigid roller surfaces for calibration and leveling, but like the casting process, the tolerance of the diaphragm is adjusted according to the gap of the lip, and the diaphragm is the ultimate. The appearance quality can reach the level that can be achieved by the casting process. The process is characterized in that the plasticized extruded hot-melt homogeneous melt is affected by the combination of the drum and the flexible steel strip, so that both surfaces of the ultimate membrane can obtain good surface optical properties. The advantage of flexible calendering is that in the calendering gap of the cooling zone of the second zone, both sides of the material can be sufficiently cooled, so that the diaphragm can obtain good optical properties and mechanical properties without excessive stretching and high heat. Problems such as shrinkage, poor flatness, and bending.
Now, compare the various properties of the film and sheet formed by the above three processes.
First, the crystallinity of the material is analyzed. In general, the performance of a PP film is largely determined by the crystallinity of the material. Due to the relationship between cooling temperature and speed, the materials have different crystallinity. In the flexible calendering process, the crystallinity progresses as the temperature of the rolls and calender belts increases. Compared with the three-roll calendering and casting process, the crystallinity and crystal grains of PP are relatively small and the grain density is high.
In terms of gloss, the flexible calendering and three-roll calendering process results in the same excellent gloss on both sides of the film, mainly because the gloss of the rolls and the rolled strip is transferred to both sides of the film. . In the casting process, only the gloss of the roll is transmitted to the single side of the film in a single direction, so that the gloss of the film of the casting process is generally performed.
The haze value reflects the degree of obscuration of the diaphragm, which is determined by a spectrometer. In addition to being affected by the material itself, haze is also affected by the cooling temperature. Generally, as the cooling temperature increases, the crystallinity of the material also progresses rapidly, thereby improving the haze of the finished product. In the above three processes, the three-roll calendering and casting process adopts a single-sided cooling method, and the cooling speed of the above two processes is slower than the double-sided cooling of the flexible calendering, thereby making the finished product haze Also progress accordingly.
The tensile strength in the longitudinal (MD) and transverse (TD) directions is inconsistent due to the cooling rate. Generally, the longitudinal (MD) tensile strength is greater than the transverse (TD) tensile strength. A comparison of the three processes shows that the tensile strength of the flexible calendering is worse than the others, which is due to the different crystallinity caused by the cooling rate. However, the anisotropy of the tensile strength of the finished product has a certain influence on the thermoforming properties of the subsequent channels. If the test sample has obvious retraction at the fracture, it indicates that the thermoforming performance is better, and the flexible calendered model is more obvious in this respect.
For the diaphragm produced by the flexible calendering, the impact strength is significantly improved as the temperature of the cooled steel strip and the calender roll increases. Compared to the other two processes, the flexible calendering process gives the film a better tensile strength, which has been confirmed in tests.
The controllability and maneuverability of the flexible calendering process is similar to the casting process in controlling the thickness deviation. Therefore, the diaphragm does not affect the apparent quality due to tolerance accumulation during the winding process. In terms of the gloss of the film, the flexible calendering process, like the three-roll calendering process, delivers high gloss from both sides, giving both surfaces a high gloss. During the cooling and setting process, the double-sided rapid cooling structure of the flexible calendering makes the quality of the finished product significantly improved, and the diaphragm does not bend or warp.
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