Blow molding machine Operating principle

Simply Overview

During the production process of blowing plastic film, the thickness of the film is a critical indicator that directly influences the quality and performance of the final product. Ensuring uniform thickness both vertically and horizontally is a key challenge. Vertical thickness uniformity can be controlled by maintaining the stability of extrusion and traction speeds. However, achieving uniform horizontal thickness requires the introduction of an automatic horizontal thickness control system, especially when production process parameters change.

The commonly used methods for horizontal thickness control include automatic mold adjustments (utilizing thermal expansion screws) and automatic air rings. These systems rely on precise mechanisms and advanced technologies to ensure consistent film quality. Below, we delve into the fundamental principles and control strategies behind these systems.

Fundamental Principles

The automatic air ring structure employs a dual air inlet method. This design allows for precise control over the air flow during the cooling process, which is critical for achieving uniform film thickness. The structure consists of two main components:

1. Lower Air Outlet: The air volume here remains constant, providing a stable foundation for the cooling process.

2. Upper Air Outlets: These are divided into multiple air ducts, each equipped with components such as air chambers, valves, and motors. The upper air outlets play a dynamic role in controlling the air volume at different points.

During operation, a thickness-measuring probe detects the film's thickness and sends the signal to a computer. The computer compares the measured thickness with the pre-set average thickness and analyzes the deviation and its trend. Based on this data, the computer sends signals to motor-driven valves to adjust the air volume at specific points:

• When the film is too thin, the motor reduces the air outlet size, decreasing the cooling effect.

• When the film is too thick, the motor increases the air outlet size, enhancing the cooling effect.

By adjusting the cooling rate at various points around the film’s circumference, the system effectively minimizes horizontal thickness deviations and maintains the film’s uniformity within the target range.

Control Plan

The automatic air ring system operates as an online real-time control system. Several motors distributed around the air ring manage the cooling air flow. The system’s process can be broken down as follows:

1. Air Flow Regulation: The cooling air from the fan is pressurized and distributed to each air duct. Motor-driven valves adjust the air outlet size and volume for each duct.

2. Thickness Adjustment: By modifying the cooling effect at specific points, the system ensures that the film’s thickness aligns with the target specifications.

One of the key challenges in this control process is the lack of a clear, linear relationship between motor adjustments and thickness changes. Factors such as film type, production speed, and environmental conditions introduce complexities. Additionally, adjustments at one point on the film often influence neighboring points, creating a coupled system with inherent delays.

Advanced Control Strategies

Traditional control methods struggle with the highly non-linear, strongly coupled, and time-varying nature of the system. To overcome these challenges, advanced algorithms such as fuzzy control are employed. This approach offers several advantages:

• Flexibility: Fuzzy control adapts to system parameter changes more effectively than traditional methods.

• Robustness: It handles uncertainties and non-linearities in the system without requiring an overly complex model.

• Dynamic Adaptation: By adjusting the fuzzy quantification factor, the system can respond more effectively to variations in production parameters.

Fuzzy control improves the system’s ability to achieve consistent film thickness and enhances overall production efficiency. It is particularly well-suited for systems where precise modeling is impractical or impossible.

Conclusion

The operating principle of blow molding machines, particularly in film production, revolves around maintaining uniform thickness. Automatic air rings and advanced control algorithms, such as fuzzy control, are essential for achieving this goal. These systems ensure high-quality film production by dynamically adjusting cooling rates and compensating for process variations. As the industry continues to evolve, integrating such advanced technologies will remain critical for meeting the ever-increasing demands for precision and efficiency in film production.