How To Adjust The Temperature, Screw Speed, And Traction Speed Of A Three-layer Blown Film Extruder To Optimize Film Thickness Uniformity?

In order to optimize the uniformity of film thickness of the three-layer fan, the temperature, screw speed and traction speed should be coordinated by layered control and dynamic feedback, combining raw material characteristics, equipment condition and on-line monitoring data. Specific adjustment strategies and principles are as follows:
I. Temperature Control: precise layered Adjustment to ensure stable Stable Melt Flowability
Temperature is the core parameter that influences melt viscosity and fluidity. Under the premise of uniform mold Differential temperature, differential temperature must be set according to the raw material characteristics of each layer.
Layered Temperature Settings
Outer layer: If a high strength material (e.g. PP or HDPE) is used, a higher temperature (180-230°C) is required to ensure sufficient plasticization and to prevent thickness fluctuations due to melt rupture. If the outer layer is required to be wearable, the temperature (e.g. 170-200°C) can be appropriately reduced to reduce shear heat and prevent molecular chain breakage.
Intermediate layer: If high barrier material (e.g. EVOH or PA) are used, temperatures must be strictly controlled between 200 ° C and250°C. Too low a temperature can lead to poor melt flowability and the formation of "melting points"; too high a temperature can lead to decomposition (for example, EVOH can easily degrade above 260°C), resulting in uneven thickness.
Inner layer: A commonly used thermalsealing material (such as LLDPE or LDPE) can be cooled slightly (160-200°C) to reduce the risk of adhesion between the thermalsealing layer and the die head, while avoiding uneven stretching due to the softness of the melt during traction. If the heat seal strength needs to be increased, the temperature (e.g. 180-210°C) can be appropriately increased to improve the uniformity of the melt.
Optimization of die head temperature gradient: lip temperature: should be uniformly distributed (temperature difference ≤2°C) to avoid local overheating or overcooling leading to differences in melt velocity. Infrared thermometer can monitor lip temperature real time and regulate heating coil power. For example, if the left side is 1.5°C hotter than the right side, the left side of the heating coil can reduce power by 5%-10%.
Flow temperature: The temperature from the inlet to the mold should be gradually reduced (e.g. 230 ℃ → 220 ℃ → 210) to ensure that the melt cools gradually in the flow and to reduce the accumulation of internal stress. If the flow temperature is too high, the melt shrinks unevenly; if the temperature is too low, the melt fluidity is poor, resulting in too thick.
Dynamic Adjustment of Cooling Air Ring
Airflow and temperature: too cold air flow or too low temperature can lead to rapid local solidification of the film, resulting in uneven stretching during traction; insufficient airflow can lead to incomplete film solidification and drooping of the film. Airflow (500-2000 m3/h) and temperature (10-25°C) need to be adjusted according to the thickness of the film (e.g. 20-100 microns). For example, when producing a a 50 μm film, the airflow can be set to 1200 m3/h, and the air temperature can be set to 18 degrees Celsius, ensuring that more than 80% of the film cools within 30-50 millimeters of forming.
Air Ring Height: The air ring height must match the traction speed. At high speeds (e.g. > 50m/min), the height of the sealing ring (e.g. 5080mm) should be reduced to enhance cooling; at low speeds (e.g. < 30m/min), the height of the sealing ring (e.g. 80-120mm) should be appropriately increased to prevent premature film curing of the film.
ii. Screw Speed matching: Layered Extrusion Control to Ensure Stable Flow Rate
screw speed directly affects the melt extrusion rate, which needs to be accurately matched according to material characteristics and layer thickness ratio to avoid uneven thickness due to flow fluctuation.
Layered Screw Speed Calibration Set
External thread: If the external material is (e.g., high-viscosity PP an appropriate increase in rotational speed (e.g., 40-60rpm) is required to compensate for melt resistance; if high strength is required, a decrease in rotational speed (e.g., 30-50rpm) is required to reduce shear heat. For example, the speed of the outer thread can be set to 45rpm to avoid excessive shearing of the molecular chain during the production of the piercing film.
Medium screw: High barrier materials (e.g. EVOH) are shear-sensitive and require tight speed control (e.g., 20-40 rpm) to prevent excessive melt temperature and decomposition. If the intermediate layer is small (e.g., 10%), the velocity (e.g., 25 rpm) can be further reduced to reduce melt residence time.
Internal Thread: Thermalsealing materials (e.g. LLDPE) good flowability and allows low speed (e.g. 30-50 rpm) to prevent melting. In order to increase heat seal strength, speed (e.g. 50-70rpm per minute) can be increased to improve the uniformity of the melt. For example, in high-speed packaging film production, screw speed can be set to 60 rpm to ensure uniform layer thickness heat seal.
Synergy of Screw Speed, braking and backpressure:
Back Pressure Adjustment: backpressure on the assembly will prolong melt residence time of the melt in the screw, leading to temperature rise and decomposition; insufficient backpressure will lead to incomplete melting of the melt and the appearance of unmelted particles. Back pressure needs to be adjusted to screw speed (for example, at 50 rpm, the back pressure should be set to 8-12 MPa). For example, if unmelted particles are found on the surface of the film, the backpressure can gradually increase to 10 MPa while the melt temperature is monitored for stability.
Melt Pressure monitoring: Install pressure sensor at die inlets to monitor melt pressure in real time (target value + 5%). If the pressure fluctuates by more than 10%, the screw speed or back pressure needs to be adjusted. For example, if the middle layer melt pressure fluctuation reaches 15%, the middle layer screw speed can be reduced by 2 rpm, the back pressure valve should be checked for blockage.
III. Traction Speed ​​Coordination: Control stretch ratio and cooling settings to ensure thickness stability.
traction speed determines the tensile ratio and cooling time of the film, which need to be matched dynamically with the extrusion rate and cooling efficiency to avoid thickness fluctuations due to uneven tensile strength.
Balance traction Speed, compensation rate and compression rate
Stretch Ratio Control: The stretch ratio (traction speed/extrusion line speed) needs to be determined according to the thickness of the film and the characteristics of the raw material. For example, when making a 50μm film, if the extrusion line speed is 0.5 m/min, the traction speed should be set at 1.5-2.0 m/min (stretch ratio 3-4 times). If the tensile multiples are too high (e.g. >5 times), the films become thin or even break; if stretch ratio are too low (e.g. <2 times), the films are too thick and raw materials are wasted.
Extrusion Rate Matching: When the traction speed changes, the screw speed needs to be synchronized to maintain a stable extrusion rate. For example, if the traction speed is increased by 10%, the screw speed needs to be increased by a corresponding 10%. If the adjusted film thickness is still too thin, check further to see if the mold seal is blocked or if the cooling airflow is too high.
Hinge Plate and Clamping Roller Adjustment
Hinge plate angle: If the herringbone plate angle is too high (if greater than 60 degrees), it causes uneven folding and thickness fluctuations of the film; if the angle is too small (if less than40 degrees), it tends to stick to the mold. Angles (45-55°) need to be adjusted according to the film width (e.g., 1000-2000mm). For example, when producing the herringbone plate 1500mm wide, it can be set at an angle of 50° to ensure a smooth transition to clamped rollers.
Grinder pressure and speed: Grinder pressure should be uniform (e.g. 0.2-0.5 MPa) to avoid film deformation due to excessive local pressure; grinder speed should be synchronized with traction speed (error ≤1%). For example, if the traction speed is 1.8 m/min, the grinder speed should be set to 1.8 m/min ± 0.018 m/min. If uneven film thickness is found the transverse direction, check the grinder for wear or pressure discrepancies.
IV. INTRODUCTION INTRODUCTION Online Monitoring and Dynamic Feedback: Real-time Parameter Adjustment for closed-loop control
Thickness of the film is monitored in real time using a thickness gauge (e.g. betaray or laser thickness gauge) and the data are fed back to the control system for automatic parameter adjustment:
Thickness Deviation Handling: if an area is found to be too thick (for example, more than 5 per cent of the target value), the cooling airflow may be increased in situ or the corresponding screw speed reduced. For example, if the left side is too thick, the left side of the airflow can be increased by 10%, while the external screw speed can be reduced by 2 rpm.
If thickness is insufficient (for example, less than 5 per cent of target value), cooling airflow may be reduced or screw speed increased. For example, if the thickness on the right side is too thin, the airflow from the right airlock can be reduced by 10%, while the speed of the inner thread can be increased by 2 rpm.
Closed loop control strategy: A PID control system is used to automatically adjust temperature, screw speed and traction speed according to thickness deviation. For example, when the thickness deviation is consistently greater than3%, the system automatically reduces the traction speed by 0.5 m/min and increases the corresponding layer screw speed by 2 rpm until the thickness returns to the target value.
Set alarm threshold (e.g. thickness deviation >8%). When the threshold is exceeded, production is suspended to check for mold lip wear, screw jamming, or cooling system malfunction.
V. Common Problems and Adjustment Directions
* Longitudinal film thickness fluctuation (periodic)
* Possible cause: Unstable screw speed or back pressure fluctuation.
Adjustment direction: check the wear condition of the screw reducer and coupling, adjust pressure valve opening to ensure the stability of melt pressure.
* Horizontal film thickness is uneven (thicker on one side)
* Possible cause: uneven temperature at die mouth or eccentricity of cooling wind rings.
Adjusting method: infrared thermometer is used to detect lip temperature, adjust heating coil power, calibrate gas ring liquid level, and ensure even airflow distribution.
'Filth 'on film surfaces
Possible reasons: EVOH is too hot or stays in the middle for too long.
Adjustments: Reduce shear heat by reducing the speed of the intermediate screw (e.g., from 40 rpm to 30 rpm); optimize mold flow design to shorten melt residence time.
Thin Film breakage during traction
Possible cause: Too fast traction or insufficient cooling.
Tweaks: Reduced traction speed (e.g. from 2.0m/min to 1.5m/min) and increase cooling airflow (e.g. from 1000 m3/h to 1500 m3/h).
Dynamic matching of temperature, screw speed and traction speed is the key to optimize the uniformity of the thickness of three-layer blowing film. This requires a "hierarchical control + closed-loop feedback" approach combining raw material characteristics, mold design and online monitoring data. In practice, it is recommended to determine the traction speed, adjust the screw speed to stabilize the extrusion volume, then optimize the temperature and cooling parameters, and then fine-tune the parameters using an online thickness gauge to gradually approach the target thickness uniformity of ± 1%.