PET transparent sheet production process and common problems （II）
Publish Time: 2021-05-10 Origin: Site
Continued
The fifth step is traction and reeling. The sheet comes out of the calender roller and enters the traction device by the guide roller. The components of the traction device are the active rubber roller and the passive rubber roller. The air pressure compresses the two rollers mainly to prevent the sheet material from passing through the two rollers to the reeling device, and there is material at the calender roller to produce a flattened sheet.
II. Analysis of common problems and production points of PET transparent film
Common quality problems in the production of PET transparent sheets and their solutions
（ⅰ）The sheet produces crystal point impurities. The reasons for this are raw materials and scraps. The PET chip itself does not produce crystal point impurities, but during the processing process, due to drying problems and more or less added scraps due to poor environment, impurities or low-quality raw materials are introduced. It cannot be eliminated during the sheet forming process.
（ⅱ）Horizontal lines and water lines (orange peel lines). Water streaks are caused by the fact that the material flow from the die of the extruder enters the calender roll because there is no residual material between the calender rolls, and the material is not compacted, and the surface of the sheet exhibits a poor finish like orange peel Lines. The solution is that there must be visible residual material between the calender rolls, and the residual material rotates evenly. The horizontal grain is the process defect of the extrusion method, just like the water ripple of the calendering method, the indentation caused by the speed difference between the two rollers of the calender roll, the solution is to require the three-roller calender The roller speed control accuracy is improved, and the synchronization accuracy must be improved to reduce the horizontal lines.
（ⅲ）Sheet yellowing, black spots or impurities, streamlines, uneven calendering, etc. The main reason for the occurrence of bubbles in the sheet is that the pellets are not completely dried and the moisture content exceeds 0.005%. If the moisture is not sufficiently dried, it will penetrate deep into the slice to indicate the formation of molecular bonds or remain deep inside the slice. If the drying temperature is too low or the time is too short, it will affect the drying effect. When blisters appear on the sheet, the drying temperature and time should be adjusted immediately. The main reason for the yellowing of the sheet is that the drying temperature is too high or the time is too long. At this time, the main measures are to reduce the drying temperature and reduce the drying time. Another reason for the yellowing of the sheet is that the temperature of the melt is too high. At this time, the melt temperature should be reduced quickly. The main reason for the black spots and impurities in the sheet is that the filter screen is broken or the PET decomposition material remains in the extruder.
Essentials of PET transparent sheet production
（ⅰ）The primary conditions for the production of PET transparent sheets are the drying time and the control of the hot air dew point. The drying effect directly determines the physical and mechanical properties and production of the sheet. Attention should be paid to the control of drying temperature and time when drying. The drying temperature and time can neither be too high nor too low. If the dew point cannot be lowered, check the molecular sieve. The aging molecular sieve cannot absorb the moisture in the air and cannot achieve the purpose of drying the slice. At this time, the molecular sieve must be replaced.
（ⅱ）If the temperature of the melt exceeds 280°C, the PET sheet will be discolored or decomposed, so the melt temperature must be controlled below 280°C.
（ⅲ）The die lip gap of the die head determines the flatness and thickness uniformity of the sheet. The three-roller temperature plays a key role in the transparency of the sheet and indicates the smoothness. The PET melt needs to quickly avoid the temperature zone with the fastest crystallization rate, and instantly change from the melting temperature. It drops below the crystallization temperature, so the distance between the die and the calender roll is particularly important.
Conclusion
The main advantages of PET transparent sheet are that it has a crystal clear appearance, good gas barrier properties, good protective properties, toughness and extensibility. Because PET transparent sheet does not contain any stabilizers and plasticizers, does not contain harmful elements, and its price is good, it is favored by consumers. PET transparent sheets can be recycled during production, transportation and use. Therefore, the PET transparent sheet has a higher use value. In the PET production process, it is necessary to strictly follow the production process requirements, master the drying process and the dew point of the hot air, control the gap between the calender rolls and the traction speed ratio of the subsequent units, so as to improve the production quality.