In plastic film production, the cooling air ring is a core component ensuring film formation quality. Its air pressure control directly impacts bubble stability and product performance. When cooling air ring pressure is excessive, it not only induces film quality defects but also adversely affects equipment operation and production efficiency. Mastering countermeasures for excessive air pressure constitutes a critical aspect of film blowing process control.

 

(A) Film Formation Problems

Excessive air pressure severely disrupts bubble stability, causing violent shaking and deviation. Imagine the molten bubble behaving like an unsteady balloon in strong wind during the film blowing process – unable to maintain a fixed shape. This directly results in uneven film thickness. As noted in the Blown Film Machine Common Issues Analysis by Plastics Machinery Network, film exceeding thickness tolerance struggles to meet application requirements in packaging or agriculture, potentially leading to compromised sealing integrity or reduced load-bearing capacity.

Simultaneously, excessive cooling rates disrupt the crystallization process of polymer molecules. Research in Plastics Processing Technology indicates that rapid cooling under high airflow prevents adequate molecular chain alignment, resulting in incomplete crystallization. This structural defect significantly reduces the film's tensile strength and toughness, making it more prone to cracking or tearing during downstream processing and end use.

(B) Accelerated Equipment Wear

High air pressure intensifies friction between the air ring and the bubble, accelerating wear on the air ring outlet. Maintenance guides from Rui'an Xinye Packaging Machinery Co., Ltd. indicate that prolonged high-pressure operation causes thinning and deformation of the metal air ring outlet due to friction. This compromises uniform air distribution, further degrading film quality.

Blower fans also endure excessive stress under sustained high-load operation. According to the Industrial Fan Operation and Maintenance Manual, demanding excessive static pressure forces the fan motor into prolonged overload. This causes precipitous temperature rises, accelerating insulation aging and ultimately shortening motor lifespan while increasing maintenance costs and downtime risks.

(C) Reduced Production Efficiency

Bubble instability significantly extends start-up and adjustment time. Operators must repeatedly fine-tune parameters to stabilize the bubble shape, inevitably prolonging the transition from start-up to stable production and disrupting continuity. Field reports on plastic film production forums suggest that frequent adjustments caused by high pressure can reduce effective daily production time by 10-20%.

Furthermore, to counter issues from excessive pressure, operators frequently adjust other parameters like haul-off speed and extrusion temperature. This constant tuning not only increases operational difficulty but also lowers output per unit time, directly impacting production efficiency and profitability.

 

(A) Air Pressure Adjustment Devices

Blower and Variable Frequency Drive (VFD): The blower serves as the power source generating air pressure, while the VFD acts as the "intelligent switch" for pressure regulation. As detailed in Plastics Machinery Network's Blown Film Machine Air Pressure Adjustment Techniques, altering the VFD's output frequency enables precise control of the blower speed. Increasing the speed boosts the volume of air intake and discharge, consequently raising the air pressure; conversely, decreasing the speed lowers the pressure. This stepless speed regulation facilitates fine-tuning of air pressure.

Air Dampers: Dampers, available in manual and motorized types, are critical components for controlling airflow on/off and volume. Per the Generic Specifications for Blown Film Machine Operation Manuals, manual dampers adjust the duct opening size by rotating a handle to change the damper blade angle. Motorized dampers, driven by an electric motor, allow remote adjustment via the control system. Both types directly regulate the airflow volume entering the air ring, thereby achieving pressure adjustment.

Plenum Chamber and Air Outlet: The plenum chamber functions like a "pressure buffer," ensuring more uniform pressure distribution of the incoming air. The shape and dimensions of the air ring outlet directly influence how air pressure is distributed across the bubble surface. Plastics Molding Equipment emphasizes that a well-designed outlet structure is crucial for ensuring uniform pressure application on the bubble, preventing localized areas of excessive or insufficient pressure.

(B) Adjustment Principle

Air pressure regulation in the cooling air ring is based on fluid dynamics principles. Applications of Fluid Dynamics in Plastics Processing explains that air, as a fluid, exhibits pressure closely related to flow rate and duct cross-sectional area. With constant blower speed, reducing the duct cross-section increases air velocity and pressure, while enlarging it decreases pressure. Similarly, increasing blower speed raises the air volume flow rate; with a constant duct cross-section, this also results in higher pressure.

In practical operation, synergistic adjustment of the blower speed, damper opening, and optimization of the air ring structure allows precise control over both the magnitude and distribution of air pressure. This ensures the bubble maintains a stable shape during cooling, guaranteeing high-quality film formation.

(A) Operational Steps

Initial Inspection: Prior to adjustment, conduct a comprehensive inspection of the blower, dampers, and air ducts. As specified in the Blown Film Machine Routine Maintenance Procedures, verify blower blades are intact and rotate freely, dampers open/close properly, and ducts are clear of plastic debris or dust obstructions. Any component abnormality may compromise pressure adjustment or cause equipment failure.

Parameter Setting: Establish initial air pressure settings based on resin characteristics (e.g., PE, PP) and target film thickness, referencing the equipment manual. For instance: Lower pressures are typically required for thin food packaging films, while thicker industrial films may necessitate higher pressures. Critical note: Initial parameters must remain within the manufacturer's recommended range to avoid arbitrary settings.

Gradual Adjustment: Reduce pressure incrementally via the VFD or dampers, limiting adjustment increments to 5%-10% per step. During adjustment, closely monitor bubble behavior – observing stability, oscillation, or deviation. If stability improves, proceed with fine-tuning; if abnormalities occur, immediately halt adjustment, diagnose the cause, and revert partially.

Stability Confirmation: When the bubble maintains consistent form and produced film exhibits uniform thickness, smooth surface, and meets all quality specifications, pressure is considered optimized. Lock VFD/damper settings and document current parameters (pressure, blower RPM) for production reference.

(B) Precautions

• Safe Operation: Strictly adhere to Industrial Equipment Operation Safety Codes during adjustments: De-energize equipment and wear PPE. Never contact moving components (blower, dampers) during operation to prevent electrical/mechanical hazards.
• Avoid Sudden Changes: Abrupt pressure changes may cause sudden bubble rupture, resulting in material waste and production downtime. Therefore: Always follow the "gradual adjustment and stepwise optimization" principle, allowing adaptation time for both bubble and machinery.
• Real-time Monitoring: Operators must continuously observe bubble dynamics, film quality, and equipment parameters throughout adjustment. Beyond pressure, track haul-off speed and extrusion temperature to ensure overall system stability.
• Scheduled Maintenance: Per the Blown Film Machine Maintenance Manual, regularly clean air rings and ducts to remove accumulated dust and residue. Simultaneously, calibrate pressure gauges to ensure measurement accuracy. Consistent maintenance is fundamental for reliable pressure control.

 

Excessive air pressure in blown film machine cooling air rings is a common issue impacting film production quality and efficiency. By understanding its adverse effects on film formation, equipment operation, and production efficiency – coupled with mastering pressure regulation mechanisms and adhering to methodical operating procedures – operators can effectively resolve pressure abnormalities. In practical production, personnel must prioritize routine air ring maintenance and process optimization to maintain pressure within optimal ranges. This ensures consistent film quality, enhances equipment efficiency, and reduces operating costs.