What is the Blow Ratio?

In the field of plastic film production, especially when using a film blowing machine, the concept of the blow ratio is of great significance. The blow ratio, also known as the blow - up ratio or blow - out ratio, plays a crucial role in determining the quality and characteristics of the final film product.

Definition of the Blow Ratio

For a film blowing machine, the blow ratio refers to the ratio of the diameter of the inflated film tube to the diameter of the mold. Mathematically, it can be expressed as: Blowing ratio = Membrane tube diameter ÷ Mold diameter. For example, if the diameter of the mold is 100 mm and the diameter of the film tube after inflation is 200 mm, then the blowing ratio is 2:1. This simple - to - calculate ratio is an important indicator for measuring the expansion degree of the bubble during the film production process.

Significance of the Blow Ratio
 

1. Thickness Distribution: The blow ratio has a direct impact on the thickness of the film. A larger blow ratio generally leads to a thinner film in the transverse direction. When the film tube is blown to a larger diameter relative to the mold diameter, the material is stretched more in the transverse direction, causing the film to become thinner. For instance, if the blow ratio is increased from 2:1 to 3:1, the film will be stretched more in the cross - section, resulting in a decrease in thickness.
    
2. Tensile Properties: It also affects the mechanical properties of the film, such as tensile strength and tear strength. Appropriate blow ratios can improve the mechanical properties of the film. When the film is blown to an appropriate extent, the polymer molecules in the film are oriented in a more favorable way, which can enhance the tensile strength of the film. However, if the blow ratio is too large, the film may become too thin and its strength will be reduced, making it more prone to tearing.
    
3. Transparency: The blow ratio is also related to the transparency of the film. In some cases, an appropriate blow ratio can help to obtain a more transparent film. A proper blowing process can make the internal structure of the film more uniform, which is beneficial to the transmission of light and thus improves the transparency of the film.

How to Judge the Blow Ratio
 

1. Direct Reading Method: This is the most basic method. During the production process using a film blowing machine, infrared or laser interferometers can be set up to observe the diameter of the film tube and the mold diameter in real - time, and then calculate the blow ratio according to the formula.
    

2. Pressure Method: Based on the relationship between the blow ratio, the internal air pressure of the film tube, and the external air pressure, the blow ratio can be determined by measuring the air pressure difference. Generally, the higher the blow ratio, the higher the internal air pressure of the film tube and the external air pressure.
    

3. Periodic Method: This method calculates the blow ratio by measuring the difference in the length and width of the film. Under certain temperature and pressure conditions, the periodic method can provide very accurate blow - ratio data.

Control of the Blow Ratio

1. Adjust the Material Ratio: The ratio of raw materials or the addition of additives can affect the blow ratio. By adjusting the material ratio, the blowing performance of the polymer can be changed, thereby controlling the blow ratio. For example, adding some plasticizers may change the fluidity of the polymer melt, which in turn affects the blowing process and the resulting blow ratio.
    
2. Adjust Film Tube Temperature: During the film production process, adjusting the temperature of the film tube can promote the diffusion of bubbles. Increasing the temperature of the film tube can make the polymer melt more fluid, which is conducive to the expansion of the bubble and thus helps to control the blow ratio.
    
3. Adjust Cooling Method: Changing the cooling method of the film tube can also adjust the film blow ratio. For example, using spray cooling or adding an appropriate amount of coolant can change the cooling rate of the film tube, which affects the expansion process of the bubble and finally controls the blow ratio.
    

In conclusion, understanding the blow ratio is essential for operating a film blowing machine efficiently and producing high - quality plastic films. By accurately judging and controlling the blow ratio, manufacturers can optimize the production process, improve product quality, and meet the diverse needs of the market.