How to control the thickness of thin films produced by three-layer co extrusion blown film machine

In modern plastic film production, the three-layer co extrusion blown film process is widely used due to its ability to composite materials with different properties. The uniformity of film thickness directly affects the mechanical properties, barrier properties, and appearance quality of products, and is a core indicator of production process control.

1、 The Influence of Raw Material Characteristics on Thickness Basis

Melt Flow Index (MFI) Matching

The three-layer co extrusion requires that the MFI difference of ABC three-layer materials be controlled within ± 2g/10min. For example, when producing PA/EVOH/PE barrier film, it is necessary to adjust the MFI of PA6 to 3.5-4.0 (230 ℃/2.16kg) to maintain flow consistency with the MFI 4.0 of LDPE. Excessive differences in MFI can lead to uneven interlayer flow velocity and thickness fluctuations.

Coordination of material shrinkage rate

The difference in cooling crystallization shrinkage rate of each layer of material should be less than 15%. Taking PP/PE/PP structure as an example, the shrinkage rate of PP is 1.5-2.0%, and the shrinkage rate of PE is 2.5-3.0%. It is necessary to adjust the crystallization rate of PP by adding nucleating agents to avoid film curling and uneven thickness distribution caused by shrinkage differences.

2、 Accurate control of key components of equipment

Design of multi-layer spiral distributor

The spiral channel structure optimized by Computational Fluid Dynamics (CFD) is used to control the pressure loss deviation within ± 5%. A certain type of distributor is equipped with an adjustable damping block at the outlet, and the resistance of each channel is adjusted through a fine adjustment bolt with a precision of 0.1mm, achieving accurate control of the layer thickness ratio within the range of ± 2%.

Automatic wind ring system

The dual outlet negative pressure cooling air ring is equipped with 32 independent air ducts, each of which is driven by a servo motor to open a valve with a response time of less than 0.5 seconds. When the online thickness gauge detects a deviation in a certain area, the system automatically adjusts the airflow rate of the corresponding air duct (with an adjustment accuracy of ± 0.5m ³/h) to achieve local control of the membrane bubble diameter.

Mold heating zone control

Adopting a 16 zone ceramic heater combined with PID temperature control, the temperature fluctuation of the working surface of the mold head is ≤± 0.5 ℃. Three sets of thermocouples are specially installed in the merging section for closed-loop control, ensuring that the temperature gradient of different melts at the interlayer interface is less than 3 ℃/cm.

3、 Optimization strategy for process parameters

Synchronization of extruder speed

The three extruders adopt a master-slave control mode, where the speed fluctuation of the main extruder (usually the middle layer) is less than 0.2%, and the subordinate extruders automatically compensate by tracking the real-time changes in the main motor current. When producing 0.1mm film, the extrusion deviation of each layer should be less than 1.5%.

Combination of inflation ratio and traction ratio

The product of inflation ratio (BUR) and traction ratio (DDR) determines the final thickness. The empirical formula shows that thickness (μ m)=(die gap x 1000)/(BUR x DDR). When producing 50 μ m thin films, the typical parameter combination is BUR=2.5 and DDR=8, and the stability of these two parameters needs to be maintained at>98%.

Parameters of Internal Cooling System (IBC)

The internal cooling air volume is controlled at 0.8-1.2 times the diameter of the membrane bubble (m ³/h · m), and the air pressure stability is ± 50Pa. Data from a certain production line shows that when the IBC air volume increases from 200m ³/h to 220m ³/h, the lateral thickness deviation of the film can be reduced from 8% to 5%.

4、 Implementation of closed-loop control system

Application of online thickness gauge

The beta ray thickness gauge collects data every 200ms, with a scanning speed of up to 1m/s. Collaborate with 128 point thickness matrix analysis to generate real-time thickness cloud maps. A certain enterprise reduced the thickness CV value from 5.8% to 2.3% by installing two cross scanning thickness gauges.

Adaptive control algorithm

Adopting fuzzy PID control strategy, triggering strong intervention mode when the thickness deviation is greater than 3%, and adjusting the mold head bolt; When the deviation is 1-3%, predictive control is enabled to adjust the extrusion parameters in advance. A certain case shows that the algorithm reduces the scrap rate by 42% when changing specifications.

Big data analysis optimization

Collect historical production data through MES system and establish a thickness process parameter correlation model. When the raw material batch is changed, the system automatically recommends a process window. For example, if a PE grade is changed, it is recommended to lower the extrusion temperature by 3-5 ℃ to maintain thickness stability.

5、 Common problem solutions

Horizontal thickness fluctuation • Phenomenon: presenting regular "thick thin" alternating stripes • Countermeasure: Check the uniformity of the mold head temperature, adjust the cooling curve of the air ring, and polish the mold lip if necessary

Longitudinal thickness unevenness • Phenomenon: Periodic thickness changes • Countermeasures: Check the stability of the extruder screw speed, replace the filter screen (recommended to replace every 8 hours), calibrate the eccentricity of the traction roller

Imbalance of interlayer ratio • Phenomenon: The proportion of the middle layer exceeds the set value ± 5% • Countermeasure: Clean the distributor flow channel, check the gear clearance of each layer's melt pump, and perform dynamic calibration if necessary

The thickness control of three-layer co extrusion blown film is a comprehensive reflection of mechanical accuracy, material science, and intelligent control. With the development of Industry 4.0 technology, thickness control is shifting from "post correction" to "predictive maintenance". It is recommended that production enterprises conduct regular simulation analysis of die flow channels (at least once a year), establish a raw material database, and cultivate a team of process engineers with interdisciplinary knowledge to meet the increasingly stringent requirements for film quality.