In PE flat bag production, precise control of the film blowing process to improve film uniformity requires coordinated optimization across multiple dimensions, including raw material selection, equipment commissioning, temperature control, blow-up ratio and draw ratio matching, cooling system optimization, online monitoring, and operational standards. Raw material selection is fundamental.
Blow-film-grade PE resin pellets must be selected. These raw materials contain an appropriate amount of lubricant to improve the film's opening and prevent sticking. Furthermore, the raw material's melt index (MI) must be controlled. A high MI results in low resin viscosity, a narrow processing window, poor film-forming properties, and insufficient strength. A low MI can lead to poor plasticization, affecting film transparency and surface gloss. Therefore, raw materials with an appropriate MI must be selected based on the specific performance requirements of the PE flat bag. The raw materials must be kept dry to avoid excessive moisture content, which can cause film misting or bubbles.
During equipment commissioning, particular attention must be paid to the design of the die and screw. The die flow path must be uniform to prevent leaks or accumulation, and the die lip gap must be precisely adjusted to ensure uniformity to prevent uneven film thickness due to gap deviations. The screw structure must be matched to the properties of the PE raw material. A screw with a mixing head can enhance plasticization and ensure uniform resin melting. Additionally, the mechanical transmission system must be checked to ensure a stable take-off speed to avoid longitudinal film thickness variations caused by speed fluctuations. Temperature control is a key component of the blown film process. The extruder temperature must be set in stages according to the melting characteristics of the PE raw material to ensure sufficient plasticization of the resin without decomposition. The die head temperature must be evenly distributed to avoid localized overheating or undercooling that can lead to crystal spots or stiff patches on the film surface. Cooling System design must balance efficiency and uniformity. For air ring cooling, the air outlet structure must be optimized to ensure uniform air flow distribution along the circumference to avoid transverse film thickness variations caused by uneven cooling. For thick PE flat bags, a negative pressure air ring or a double-inlet air ring can be used to improve cooling efficiency, shorten the cooling distance, and prevent thickness variations caused by film sagging due to its own weight.
Matching the blow-up ratio and take-off ratio is key to controlling film uniformity. The blow-up ratio (the ratio of bubble diameter to die diameter) must be controlled within a reasonable range. A too large ratio can easily lead to bubble instability and fluctuations in film transverse thickness; a too small ratio can reduce film transverse strength. The drawdown ratio (the ratio of drawdown speed to extrusion speed) must be adjusted in conjunction with the blow-up ratio to ensure uniform longitudinal stretching of the film and avoid film thinning or breakage due to excessive drawdown speeds. For PE flat bag production, the optimal blow-up ratio and drawdown ratio combination must be determined through experiments and maintained consistently throughout production.
Online monitoring and feedback mechanisms are the final line of defense for ensuring film uniformity. Laser or ultrasonic thickness gauges should be deployed to monitor film thickness in real time. A closed-loop control system should be used to automatically adjust extruder speed, drawdown speed, or cooling air volume to ensure thickness deviations remain within acceptable limits. Additionally, film samples should be regularly collected for physical property testing, such as tensile strength and tear strength, to verify the rationality of process parameters and optimize control strategies based on these test results.
Operational specifications and personnel training are crucial for process implementation. Standardized operating procedures should be established to clearly define key operational procedures for equipment startup, parameter setting, production monitoring, and downtime maintenance. Operators must receive professional training and be familiar with the equipment structure, process principles, and quality standards.
They must be able to promptly identify and resolve potential problems by observing parameters such as bubble morphology and cooling efficiency. For example, if wrinkles are observed in the film, the uniformity of the film thickness, cooling efficiency, and the herringbone angle should be checked. If the film's transparency is poor, adjustments should be made to the extrusion temperature, blow-up ratio, or cooling efficiency.
