In the film processing industry, the traction rotating mechanism of haul-off rotary film machine is the core part to ensure uniform film stretching. The design combines mechanics, material science, thermodynamics, and so on, and realizes accurate control of both longitudinal and transverse directions uniform deformation of the film through multi-dimensional collaborative control. In this paper, the method of uniform stretching is analyzed systematically from four key dimensions: structure composition, motion control, temperature control and tension regulation.
1. Structural Composition: Multi-roller coordinated Mechanical Transmission System
The core of the traction rotating mechanism consists of several sets of rotating rolls, including preheating roll, stretching roll, cooling roll and flattening rolls. These rollers achieve precise mechanical transmission control through different diameters, speed matching, and spatial layout. The haul-off rotary film machine relies on this multi-roller system to maintain uniform tension distribution across the film width.
1.1 Graded Stretching Roll System
Typical configuration employs a ``small diameter-large diameter" rotary roll pairs. For example, the diameter of the first stage stretching roller is between 80 and 120 mm, and the diameter of the second stage stretching roller is between 150–200 mm. When the film passes through a roll systems of different diameters, a longitudinal stretching force is generated by the linear speed differential. If the first stage rolls at 50m/min, the second stage rolls at 80m/min, with the longitudinal stretching ratio of up to 1.6 times. This hierarchical design prevents stress concentration from a single stage of tension and ensures a uniform deformation gradient.
1.2 Three-Dimensional Spatial Layout
The roller system is staggered with a ``Z "or "S" pattern, and the height difference between adjacent rollers is maintained at 50 – 100 mm. This layout creates a wavy path for the film's journey, extending the length of the stretch path. For example, in the production of three-layer co-extruded LDPE film, the wavy path allows the film to be longitudinal stretching in 0.8 seconds, reduces the deformation time by 30% compared to a linear layout, and minimizes the risk of local overheating.
1.3 Specialized Design of Flattening Rolls
The terminal stage is equipped with bow-shaped flattening rolls with a curved shaft with a deflection of 2–5 mm and coated with silicone rubber. When the film touches the roll surface at a 15° inclination angle, the helical spring sheets produces a lateral component force of 0.5–1.2 N/cm, effectively eliminating edge curling. Experimental data show that bow-shaped flattening rolls of film can be increased by 92% and the edge loss can be reduced to below 3%.
2. Motion Control: A Dynamic Regulating System of Synchronized Speed Ratios
Dynamic matching of line speeds of multi-roll system is achieved by the coordinated control between servo motors and frequency converters. Its core technologies include:
2.1 Closed-Loop Control of Draw Ratio
Laser speed sensors continuously monitors the linear speed of the film and provides real-time feedback to PLC control system. The frequency of the drive motor is automatically adjusted when the speed fluctuations is greater than ± 0.5%. For example the PID algorithm PID algorithm maintains the tensile ratio at 5.2 ± 0.1 for the production of 20-μm-thick BOPP film, ensuring a standard deviation of longitudinal tensile strength ≤ 0.8 MPa.
2.2 Differential Speed Stretching Technology
Transverse stretching is achieved by the difference between the ultimate guide rail and the fixture. When the gap between fixtures widens from 100 mm to 400 mm, the guide width narrows simultaneously, resulting in a transverse stretching ratio of 4 times. The spring-connected fixture developed by Montedison (Italy) maintains clamping stability with spring forces of 0.3–0.5 N/mm and transverse thickness variation of + -± 1.5%.
2.3 Rotary Oscillation Mechanism
Some high-end models use 360-degree rotary disassembly devices that change the direction of the force as they stretch the film. Bayer's patented technology shows that 60 spins per minute can improve the uniformity of stress distribution within the membrane by 40%, especially for optical-grade films. Modern haul-off rotary film machine designs increasingly incorporate such rotary oscillation features to enhance film quality.
3. Temperature management: a gradient heating system with Deformation Control
Temperature uniformity directly affects the film's crystallinity and stretching performance. The traction rotating mechanism achieves accurate thermal management through a three-stage temperature field control system:
3.1 Infrared Heating of the Preheating Section
Short-wave infrared heaters with wavelengths of 2-10 microns and power density of 80–120 W/cm2 were employed. Reflective panels increases heat reflection efficiency to 95% and the film surface temperature to 120–140°C in 0.5 seconds. Experiments show that this heating method reduces thickness variation of the preheating section to ±0.8 microns.
3.2 Thermal 3.2 Hot Air Circulation in the extension
A zigzag hot Zigzag-shaped hot air ducts is arranged around the stretching roller, and the outlet of the hot air pipe is at ° to the direction of motion of the film. A thermal boundary layer of 0.5-1.0 mm thick can be formed by controlling the air velocity at 0.8 -1.2 m / s. Toray's (Japan) testing data indicate that this design effectively prevents crystal defects caused by local overheating by maintaining a standard deviation of membrane temperature ≤ 1.5°C in the stretch.
3.3 Rapid cooling to stabilize the shape of the cooling segment
The surface temperature of the film can be reduced to less than 60°C in 0.3 seconds by circulating water chromium plating cooling roller at 15 ° C. The cooling roller is slightly faster than the traction speed (1:1.02) to prevent wrinkles from forming as the film contraction. Brückner's (Germany) case study demonstrates that this rapid cooling technique reduces thermal shrinkage to below 0.3%.
4. Tension regulation: a stable control system with dynamic compensation
Tension fluctuations is the main cause of uneven film tensile. The traction rotating mechanism achieves dynamic equilibrium through multistage tension regulation:
4.1 Primary regulation through Magnetic Particle Brakes
Magnetic particle brakes is installed in the loosening device and controls the brake torque by adjusting the current. When film tension exceeds the set value, the system automatically reduces the brake current, limiting tension fluctuations to ±0.2 N/m. Hyosung's (South Korea) application shows that the technique can reduce the standard deviation of film's fracture elongation to 3.2%.
4.2 Real-time Monitoring of Ultrasonic Tension Sensors
Ultrasonic tension sensors installed in the stretch operate at a sampling frequency of 1,000 times per second. Once a sudden change in tension is detected, the system adjusts the speed of the drive motor in 20 milliseconds. For example, when tension increases by 0.5 N/m, PLC reduces servo motor speed by 0.3% to restore stable tension.
4.3 Auxiliary Control through Static Elimination Devices
The cooling section was equipped with a double sided electrostatic eliminator of ±7 kV, which neutralized the charge on the film surface and reduced static voltage from ±5 kV to ±0.5 kV. Tests by 3M (USA) show that electrostatic elimination improves winding neatness by 85% and reduces tension heterogeneity caused by electrostatic adhesion.
V. Application Case: Biaxial Orientation BOPET Films
One company utilized an improved traction rotating mechanism to produce 12-μm BOPET film, controlling key parameters as follows:
Longitudinal stretching: 130°C in preheating, 145°C in stretching and 3.8 times the tensile ratio.
Transverse stretching: 125°C pre-heated, 140°C tensile, 4.2x tensile ratio
Tension control: release N/m, stretch section tension N/m, coil tension 22N/m.
Production data showed that the standard deviation longitudinal tensile strength decreased from 1.2 MPa to 0.7 MPa, the variation of lateral thickness decreased from 3.2 μm to 1.8 μm, and the product qualification rate increased to 98.5%. This example verifies the effectiveness of multi-dimensional control system in landing gear rotation mechanism. The haul-off rotary film machine used in this case demonstrated exceptional performance in achieving uniform biaxial orientation.
Conclusion
Through structural optimization, motion control, temperature control and tension regulation, the technology system of uniform tensile film is constructed. As the haul-off rotary film machine continues to evolve, future iterations will develop in the direction of high accuracy, efficiency and intelligence, providing key technical support for the manufacture of high-end films.
