In the production of plastic bags using film blow molding machines, size, thickness, and tensile strength are the core indicators determining product quality and performance. These three parameters are interrelated and mutually influential, requiring coordinated control through precise equipment parameter adjustments, optimized raw material formulations, and standardized operating procedures. The following section provides a detailed explanation from three perspectives— "separate control methods,"  "key points for coordinated adjustment," and "essential precautions" —to serve as a practical reference for manufacturing operations.

I. Method for Controlling the Size of Plastic Bags

The size of plastic bags primarily refers to the width and length of the finished bags, achieved mainly through "blowing ratio adjustment,"  "traction speed control," and "die opening dimension matching," with the blowing ratio being the key parameter determining the width.

1. Core parameter adjustment: Blowing ratio

The inflation ratio refers to the ratio of the diameter of the blown film bubble to the die opening diameter, which directly determines the width of the plastic bag (the bag width is obtained by folding the film bubble in half). The formula is: Inflation Ratio = Film Bubble Diameter ÷ Die Opening Diameter.

Adjustment Method: The membrane bubble diameter is regulated by controlling the volume of compressed air entering the bubble. After equipment startup and once material discharge from the die is uniform and stable, slowly open the air inlet valve to inflate the bubble, monitor its diameter changes, and measure with a tape measure until the target width is achieved. The inflation ratios vary by material: PE (polyethylene) bags recommend an inflation ratio of 1.5–3.0, while PP (polypropylene) bags require 1.2–2.0. An excessively high inflation ratio may cause unstable bubbles and uneven film thickness, whereas an excessively low ratio fails to achieve the target width and results in insufficient lateral stretching and reduced tensile strength.

2. Auxiliary Control: Traction Speed and Die Size

Traction speed: With a fixed inflation ratio, a higher traction speed results in a greater length of film extruded per unit time, allowing indirect control over the final bag length (subsequent bag-making processes can then perform cutting as required). Excessively high traction speed may slightly reduce the diameter of the film bubbles (requiring simultaneous fine adjustment of air intake compensation), whereas excessively low speed can lead to excessive expansion and fluctuation of the film bubbles.

Mold opening dimensions: The diameter of the mold opening is fundamental and must be selected according to the target bag width. For example, when producing plastic bags with a width of 50 cm (for a film bubble diameter of approximately 100 cm), a mold opening with a diameter of 30–40 cm can be used, achieved by adjusting the inflation ratio to 2.5–3.3. This prevents the inflation ratio from exceeding the acceptable range due to an excessively large or small mold opening diameter.

II. Methods for Controlling the Thickness of Plastic Bags

The uniformity of plastic bag thickness directly impacts product strength and cost. Key control measures include adjusting die gap, synchronizing extrusion and traction speeds, and optimizing the cooling system, with particular emphasis on minimizing thickness variations.

1. Basic adjustment: Die gap

The die gap is critical in determining the initial thickness of the film; a larger gap results in a thicker extruded melt film, while a smaller gap yields a thinner film. During adjustment, rotate the regulating bolts on the die (uniformly distributed circumferentially) to precisely fine-tune the gap according to the target thickness. For example, when producing PE bags with a thickness of 0.02 mm, first set the die gap to 0.08–0.1 mm (accounting for subsequent stretching thinning), then refine the settings through further parameter optimization.

Note: The adjustment bolt should be rotated uniformly circumferentially to avoid excessive unilateral clearance causing uneven film thickness (if "one side thicker than the other" is observed, carefully check the symmetry of the die opening clearance).

2. Core Matching: Extrusion Speed and Traction Speed

The synchronization between extrusion speed and traction speed is pivotal for thickness control: With a fixed die gap, a higher extrusion speed yields greater melt output, resulting in thicker films; conversely, a faster traction speed ensures more complete stretching of the film, yielding thinner films. Both parameters must be adjusted in a proportional and coordinated manner.

Key operational guidelines: After startup, first stabilize the extrusion speed and initially achieve the target thickness by adjusting the traction speed. If the film is detected to be excessively thick, appropriately increase the traction speed (or decrease the extrusion speed); if it is too thin, reduce the traction speed (or increase the extrusion speed). It is recommended to use an online thickness gauge for real-time monitoring to ensure that thickness deviations remain within ±5%.

3. Auxiliary Optimization: Cooling System and Melt Temperature

Cooling System: The cooling rate of the film bubble significantly affects melt shaping performance. Insufficient cooling results in substantial thickness variations during film stretching, while excessive cooling makes the film brittle and reduces tensile strength. This is typically achieved by adjusting the cooling air flow rate and velocity of the air ring, ensuring uniform circumferential airflow for consistent cooling across the entire film bubble. For thick film production, a secondary cooling system can be added to enhance shaping stability.

Melt temperature: Excessively high temperatures increase melt fluidity, leading to excessive stretching of film bubbles, resulting in thin and uneven films; conversely, insufficient temperatures reduce melt fluidity, causing poor discharge and the formation of "bumps" or "striations" in the film, thereby compromising thickness uniformity. The barrel and die temperatures should be controlled according to the raw material type—for example, PE raw materials require a barrel temperature of 150–180°C and a die temperature of 180–200°C.

III. Methods for Controlling the Tensile Strength of Plastic Bags

The tensile strength of plastic bags (longitudinal and transverse tensile strength) primarily depends on the properties of the raw materials and the "tensile orientation" during processing. This can be achieved through "optimization of raw material formulations,"  "adjustment of the stretching ratio," and "control of processing temperature" to ensure balanced longitudinal and transverse tensile strength.

1. Basic Guarantees: Raw Material Selection and Formulation

The raw material is the key determinant of tensile strength: prioritize materials with an appropriate melt flow rate (MFR). For example, PE raw materials should have an MFR controlled between 0.5–2.0 g/10 min. An excessively high MFR results in overly fluid material and insufficient tensile strength, while an excessively low MFR increases processing difficulty and makes the film brittle.

Formula Optimization: Add modifiers as required. For example, incorporating an appropriate amount of LLDPE (linear low-density polyethylene) enhances the film's tensile strength and tear resistance; the addition of antioxidants and UV absorbers prevents stress loss due to material aging; note that the dosage of fillers (e.g., calcium carbonate) should not be excessive, as this would significantly reduce tensile strength.

2. Core Regulation: Stretch Ratio and Orientation Control

The stretching ratios include the longitudinal stretching ratio and the transverse stretching ratio, which directly influence the degree of molecular orientation: The longitudinal stretching ratio is determined by the ratio of the drawing speed to the extrusion speed; a higher ratio results in more pronounced longitudinal molecular orientation and stronger longitudinal tensile force. The transverse stretching ratio, also known as the blow ratio, indicates that a higher blow ratio leads to more pronounced transverse molecular orientation and stronger transverse tensile force.

Key practical considerations: Ensure balanced longitudinal and transverse stretch ratios to prevent excessive stretching on one side from causing force imbalance (e.g., excessive longitudinal stretch results in insufficient transverse force, making the plastic bag prone to longitudinal tearing). For example, when producing standard PE bags, the longitudinal stretch ratio should be controlled between 2–4, and the transverse stretch ratio (blow ratio) between 1.5–3.0. Through coordinated adjustments, both longitudinal and transverse forces meet operational requirements (typically requiring longitudinal tensile strength ≥15 MPa and transverse strength ≥12 MPa).

3. Auxiliary Control: Processing Temperature and Cooling Shaping

Processing temperature: Excessively high temperatures can damage the molecular structure of the raw material, resulting in reduced tensile strength; conversely, insufficient temperatures lead to inadequate molecular orientation and insufficient tensile strength. It is essential to precisely control the temperature at each section of the barrel to ensure complete plasticization without degradation. For example, the barrel temperature for PP raw material should be 170–220°C, while the die temperature should be 210–230°C.

Cooling and setting: Excessive cooling rate prevents molecules from achieving adequate orientation, leading to reduced tensile strength; conversely, insufficient cooling speed may result in adhesion and uneven thickness during film stretching, indirectly affecting tensile strength. The cooling air flow rate should be adjusted according to the raw material and film thickness to ensure stable film formation while maintaining adequate molecular orientation space.

IV. Integrated Control of the Three Components and Key Considerations

1. Key Points of Coordinated Regulation

Size, thickness, and tensile strength are interrelated and must be adjusted in coordination: For example, increasing the inflation ratio to expand the bag width (size adjustment) simultaneously enhances the lateral tensile force but may thin the film (thickness change); in such cases, the traction speed must be reduced (or the die gap increased) to compensate for the thickness variation and prevent exceeding the specified limit. Conversely, increasing the traction speed to reduce thickness increases the longitudinal tensile force; therefore, careful control of the longitudinal stretch ratio is essential to maintain balance with the lateral tensile force.

2. Key Considerations

2.1 Adequate equipment preheating: Before startup, raise the barrel and die temperatures to the set values and maintain them for 30–60 minutes to ensure uniform plasticization of the raw material, preventing uneven thickness and reduced tensile strength due to inadequate plasticization.

2.2 Raw Material Cleaning and Drying: Raw materials must be pre-screened to remove impurities (such as gravel and plastic fragments) to prevent clogging of the die opening and ensure uniform product output. Hygroscopic materials (e.g., PA) require prior drying to avoid bubble formation in the film and reduced tensile strength.

 

2.3 Regular equipment inspection: Regularly clean the mold opening of accumulated material (char), and check the uniformity of the mold opening gap; verify whether the traction roller pressure is uniform to prevent thickness and tension fluctuations caused by traction slippage.

 

2.4 Real-time Monitoring and Adjustment: During production, bag width, thickness, and tensile strength (measured using a tensile testing machine) should be regularly inspected. Parameters should be fine-tuned based on the inspection results to prevent persistent deviations that could lead to batch defects.

 

V. Summary

In the production of blow film machines, plastic bag size control is centered on the "blow ratio"; thickness control is based on the "match between die gap, extrusion, and traction speed"; and tensile strength control relies on the "raw material formulation and longitudinal/transverse stretch ratios." In practice, it is essential to understand the interrelationships among these three factors. By precisely adjusting parameters, optimizing the manufacturing process, and enhancing process monitoring, stable compliance with product specifications can be achieved. Additionally, parameters should be flexibly adjusted according to raw material type and product application to balance production efficiency with product quality.