1. Overview
A film manufacturing company uses polyester film production equipment from Japan’s MHI with an annual output of 10,000 tons. During the trial production, problems such as transverse stretching and film tearing occurred. After transverse stretching, longitudinal stretching roller entanglement and traction tearing occur frequently. It has a greater impact on production and equipment. Commonly, the wrapping of the vertical stretching roller is typically attributed to the operator’s oversight. The assumption is that the operator fails to promptly address the film damage during the longitudinal stretching film-cutting process. However, upon thorough analysis and verification, the root cause lies in the mismatch of certain parameters in the speed control system with the process conditions. The following is an analysis and search process of the causes of winding of the vertical stretching roller. For reference by industries.
2. BOPET polyester film production line process overview
The BOPET polyester film production line mainly consists of raw material conveying device, pre-crystallizer, dryer, extruder, metering pump, filter, melt tube, die, cast machine, longitudinal stretching machine, transverse stretching machine, traction Machine, winding machine, and other equipment.
The process includes: raw materials are transported through conveying equipment and crystallized in a pre-crystallizer. Dry in dryer. Melt in extruder. Metering via metering pump. It reaches the mold through melt pipes. Formed on a casting machine. Stretch lengthwise on a lengthwise stretching machine. Stretch transversely on a transverse stretching machine. Towed by the tractor. Rewind on the rewinder. Finally, according to user requirements, the slitting machine cuts the film to the required specifications.
3. Introduction to vertical stretching equipment
Vertical stretching involves using multiple high-precision metal rollers heated at different speeds to elongate and thin the thick film produced by a casting machine. This process is beneficial to the longitudinal orientation of polymer molecules and cooling shaping. The equipment used for vertical stretching is called a vertical stretching device, or simply a vertical stretching device. It can be divided into single-point, two-point, and multi-point stretching. Each type of stretching usually consists of a preheating zone, a stretching zone, a setting zone, and a cooling zone.
Preheating area: composed of 3#~8# rollers. This zone gradually heats the thick film coming out of the casting machine to close to the stretching temperature. Consider the thermal expansion of the polyester sheet during preheating. There are small speed increments between preheating rollers. Tension control is maintained between the casting machine and the vertical stretching device. The tension controller is located at the exit of the casting machine.
Stretching zone: composed of 9#-10#, 11#-12#, 13#-14# rollers. These three sets of rollers facilitate continuous stretching. The stretch ratio and temperature at each point can be adjusted as needed. The first stretching point (9#-10# roller) is equipped with a set of infrared heaters. For additional heating during transverse stretching.
Shaping cooling area: composed of 15#-16# rollers. This region affects the crystallization state of the film.
4. Brief description of vertical stretching process
During production start-up, the thick film is initially pulled into the vertical stretcher at a low speed. After completing the vertical stretching, increase the line speed to the transverse stretching speed. And turn on the vertical stretching infrared light. After transverse stretching, the speed is further increased. The film is drawn out through traction and winding. Once stable, the speed is increased to normal production speed. When the film is torn in the transverse direction or in traction, the vertical stretching device speed is automatically reduced to the transverse stretching speed. After the film tearing problem is solved, traction and winding are carried out. And the film is pierced at normal production speed.
5. Problem presentation
During the trial production process, when the transverse or traction film is torn. The rollers (9#-10#) in the vertical stretching device are frequently wound and difficult to solve.
6. Confirmation Process
The longitudinal stretching equipment will decelerate rapidly when the transverse or traction film breaks. The film winding process takes place the instant the equipment is running. We took a closer look at zoomed-in historical running images of the device where the tangled issue occurred. The speed changes of the longitudinal rollers (9# and 10#). And the real-time load changes of the respective drive motors were repeatedly compared when the film ruptured. After the film ruptured, in the process of decreasing the longitudinal roller speed, the load on the 9# and 10# roller drive motors not only did not decrease but also showed an obvious upward trend over time. So after the film ruptures, the film begins to wrap around the 9# and 10# rollers during the deceleration process of the longitudinal rollers.
We carefully analyze the above situation. The stretching process of polyester film is an exothermic process. During film rupture, in the high-speed stretching process is concentrated in the 9# and 10# stretching areas. The heat generated cannot be immediately dissipated. As the 9# and 10# stretching roller speeds rapidly decrease, an excess of heat accumulates on the rollers. It causes the film to stick to the rollers resulting in wrapping issues.
So, the equipment supplier makes changes to the production line speed control parameters. They extend the time for the stretching roller speed to decrease. It effectively solves this type of longitudinal wrapping issue.
7. Conclusion
In normal production, after the film ruptures on the transverse and traction equipment, the rapid descent of the longitudinal stretching equipment’s speed prevents the immediate dissipation of excess heat. This leads to the wrapping of the film around the first-point stretching roller during longitudinal stretching.
